Metal oxide particles and the manufacturing methods.

Abstract

Provided is a method for producing core-shell oxide particles that makes it possible to efficiently and stably obtain core-shell oxide particles in which the entire surface of an oxide particle serving as a core is uniformly coated with an oxide serving as a shell. The method includes at least the following two steps: a step 1 in which the oxide particles serving as a core are precipitated within a mixed fluid obtained by mixing a core oxide raw material liquid and an oxide precipitation solvent; and a step 2 in which the mixed fluid is mixed with a shell oxide raw material liquid and the entire surface of the oxide particles serving as a core is uniformly coated with the oxide serving as a shell. (A) At least step 1 and step 2 are carried out continuously between at least two processing surfaces 1, 2 that are capable of approaching and separating from each other and that rotate relative to each other. (B) Step 2 is completed after step 1 during a predetermined time in which aggregation of the oxide particles serving as a core does not occur within the mixed fluid. (C) Steps 1 and 2 are controlled so that the primary particle size of the core-shell oxide particles is 190% or less of the primary particle size of the oxide particles serving as a core.

Description

Metal Oxide Particles and Method for Producing the Same The present invention relates to metal oxide particles and a method for producing the same. Oxide particles are materials used in a wide range of fields such as catalysts, conductive materials, magnetic materials, secondary electron emission materials, phosphors, heat absorbers, energy storage bodies, electrode materials, and color materials. Since the properties change depending on the particle size, oxide particles with different particle diameters and crystallinities are required depending on the purpose and requirements. In particular, by miniaturizing the particles, properties significantly different from the bulk state are exhibited, and oxide particles are materials that will continue to be widely demanded in the future. However, when there are multiple properties exhibited by miniaturization, depending on the application, it may be desired to exhibit only specific properties and suppress other properties. For example, in the case of metal oxides, both photocatalytic ability and ultraviolet absorption ability are exhibited by miniaturization. However, when using metal oxide fine particles as a color material or an ultraviolet protection agent, it is necessary to suppress their photocatalytic ability. As exemplified above, it is known that when suppressing specific properties exhibited by oxide particles, it can be solved by a method of coating the surface of the oxide particles with an oxide of a different element (for example, Patent Document 1). In the method described in Patent Document 1, oxide powder is dispersed in water containing an organic solvent, and a silicon compound is allowed to act thereon to coat the surface of the oxide particles with silica. However, it is very difficult to disperse powdery oxide to primary particles, and in the oxide suspension, a reaction solution for coating silica on the surface of the oxide needs to be added at a constant rate over about 6 to 8 hours, and then an aging treatment for about 12 hours is required. Therefore, the reaction conditions in the reaction vessel change sequentially, and it is difficult to make the concentration distribution of the reactants in the vessel uniform. Thus, it is even more difficult to coat the surface of each oxide particle with a silicon compound. In fact, in Patent Document 1, as shown in FIG. 2, the precipitated particles are aggregated, and as a result, the aggregated particles are coated with silica. Thus, in Patent Document 1, actually, a uniform coating treatment cannot be performed on individual particles. Moreover, since the treatment takes a long time, it cannot be said to be an efficient method. In particular, since the precipitation step and the coating step of the oxide particles are not associated, from the viewpoint of manufacturing core-shell type oxide particles, which are the final product, it has to be said that it is extremely inefficient industrially. In addition, Patent Documents 2 to 6 and 12 by the applicant of the present application disclose a method of precipitating particles between relatively rotating treatment surfaces that can approach and separate from each other. However, the method described in Patent Document 2 is a method for efficiently and stably producing oxide particles, and a method for producing core-shell type oxide particles whose surfaces are coated with oxides of different elements is not disclosed. On the other hand, Patent Document 3 discloses an example in which amorphous silica particles are discharged from between treatment surfaces and then the surfaces of the particles are uniformly coated with silver. However, it is a product obtained by retaining a discharge liquid containing amorphous silica and a silver nitrate solution in a tubular container for a predetermined time to coat the surface of the amorphous silica particles with silver. Even if this method is applied to coat the surface of the oxide particles obtained by Patent Document 2 with an oxide of a different element, it is impossible to perform a uniform coating on the entire particle surface. Note that Patent Document 4 discloses that, after depositing oxide particles, in order to adjust the dispersibility of the oxide particles, a fluid containing oxide particles and a fluid containing an acidic substance or hydrogen peroxide are mixed within 1 second. However, it is impossible to produce core-shell type oxide particles in which the entire surface of the oxide particles is uniformly coated with an oxide of a different element by this method. Moreover, Patent Document 4 does not even disclose suppressing specific properties exhibited by the oxide particles in the first place. Furthermore, Patent Document 5 describes that the photocatalytic activity of titanium dioxide particles can be controlled by covering the surface of the titanium dioxide particles with a substance such as aluminum oxide or silicon oxide between the surfaces for the above treatment. However, Patent Document 5 does not show any relevance between the deposition step of the titanium dioxide particles and the coating step of coating the deposited titanium dioxide particles with an oxide such as silicon oxide. Therefore, it has been difficult to stably apply a uniform oxide coating of a different element serving as a shell to the entire surface of each core oxide particle. Patent Document 6 discloses a method for producing metal fine particles. By introducing a plurality of fluids containing at least one type of metal and / or metal compound from separate introduction parts between the above treatment surfaces, it is disclosed that core-shell type alloy particles and the like can be produced. However, Patent Document 6 does not disclose a method for producing core-shell type oxide particles in which both the core and the shell are oxides, nor does it disclose the uniformity of the coating by the shell. In Patent Document 7, a method for producing a catalyst in which metal A is modified by metal B is disclosed using a device similar to the devices described in Patent Documents 2 to 6. Also disclosed are that the forms of both metal A and metal B may be oxides, that it is preferable that the surface of metal A is completely or partially covered by metal B, and that it is more preferable that it has a core-shell structure composed of a core part mainly composed of metal A and a shell part mainly composed of metal B, and metal A has a crystal structure different from that of metal B. However, the forms of the metal particles in the catalyst of the examples are not oxides for both metal A and B, and judging from the surface coverage rate described in Table 1, the entire surface of metal A is not uniformly covered by metal B. The inventor of the present application has found that by continuously depositing core oxide particles and coating the surface of the core oxide particles with shell oxide, core-shell type oxide particles uniformly coated with oxides of different elements over the entire surface of the oxide particles can be efficiently and stably produced, and thus the present invention has been achieved. Also, as one of the uses of oxide particles, it can be shown as a paint raw material. For example, paints used for the outer walls of building materials, signboards, vehicles, etc. are important not only for vividness of color and design, but also for durability such as weather resistance and light resistance. Therefore, a substance that protects ultraviolet rays is used to protect the paint and the components contained in the paint by a method of mixing it with the paint or coating it on the paint film. Generally, iron oxide, which is one of the metal oxides, is effective for such a substance. However, in order not to impair the color characteristics such as the color and chroma emitted by the paint and the transparency, and the design of the product, together with the ability to protect ultraviolet rays, it is required that the influence on visible light be reduced. In particular, iron oxide used for red paints for multilayer paint films needs to transmit red light as much as possible and absorb light other than red as much as possible. As iron oxide for protecting ultraviolet rays, Patent Document 8 describes a coloring pigment for a sunlight highly reflective paint containing red iron oxide or yellow iron hydroxide with an average particle diameter of 10 to 300 nm, and Patent Document 9 describes iron oxide as a needle-shaped silica-coated red iron oxide pigment with an average length of 500 nm and an average diameter of 100 nm. The iron oxide described in Patent Document 8 or 9 can be used by being blended in the paint described in Patent Document 10 or 11. However, the iron oxide or silica-coated iron oxide described in Patent Document 8 and Patent Document 9 has a region where the reflectance exceeds 18% in the wavelength range of 400 to 620 nm in the visible region, which impairs the color characteristics of the red paint and the design of the product. Therefore, it was not possible to achieve both ultraviolet protection and transparency. The applicant of the present application has described in Patent Document 12 a method for producing various nanoparticles such as iron oxide between relatively rotating processing surfaces that can approach and separate. However, the described iron oxide nanoparticles are black iron oxide (Fe ３ O ４: It is about nanoparticles of magnetite) or yellow iron oxide (FeOOH: goethite), but those iron oxide nanoparticles did not show the property of transmitting visible light along with the ability to defend against ultraviolet rays. International Publication No. WO98 / 047476 Pamphlet Patent No. 4868558 Gazette International Publication WO2013 / 128592 Pamphlet JP 2013-082621 A JP 2009-132596 A JP 2014-074222 A JP 2012-216292 A JP 2009-263547 A International Publication WO1998 / 26011 Pamphlet JP 2014-042891 A JP 2014-042892 A International Publication WO2009 / 008393 Pamphlet In the present invention, by continuously performing precipitation of oxide particles serving as a core and coating of an oxide serving as a shell on the surface of the oxide particles serving as the core, a core-shell type oxide particle in which the surface of the oxide particle is uniformly coated with an oxide of a different element can be efficiently and stably obtained. An object of the present invention is to provide a method for producing core-shell type oxide particles. Further, in view of such circumstances, an object of the present invention is to provide a silicon oxide-coated iron oxide composition for paint that has high transparency and is suitable for use in paint. In particular, a silicon oxide-coated iron oxide dispersion for paint that is effective for a red-colored coating body, has a transmittance of light with a wavelength of 200 to 420 nm of 2.0% or less, and a transmittance of light with a wavelength of 620 to 800 nm of 80% or more is provided. This is an issue. Further, the present invention provides a weather-resistant composition for paint that protects the coating body from ultraviolet rays, is used by being blended into the paint constituting the coating body, and is effective for coating bodies of various colors, particularly red-colored coating bodies. An object is to provide a weather-resistant composition for paint that can be used. As a result of intensive studies to solve the above problems, the inventors of the present application have found that the above object can be achieved by the method shown below, and have completed the present invention. The present invention relates to a method for producing core-shell type oxide particles in which the surface of core oxide particles is coated with a shell oxide, the method including at least the following two steps: Step 1 of precipitating the core oxide particles in a mixed fluid obtained by mixing a core oxide raw material liquid containing at least a core oxide raw material which is a raw material of the core oxide particles and an oxide precipitation solvent containing at least an oxide precipitation substance for precipitating the core oxide particles; and Step 2 of mixing the mixed fluid with a shell oxide raw material liquid containing at least a shell oxide raw material which is a raw material of the shell oxide to uniformly coat the entire surface of the core oxide particles with the shell oxide. At least the above two steps are continuously performed between at least two processing surfaces that rotate relative to each other and can approach and separate from each other. A method for producing core-shell type oxide particles is provided, characterized by this. The present invention also relates to a method for producing core-shell type oxide particles in which the surface of core oxide particles is coated with a shell oxide, the method including at least the following two steps: Step 1 of precipitating the core oxide particles in a mixed fluid obtained by mixing a core oxide raw material liquid containing at least a core oxide raw material which is a raw material of the core oxide particles and an oxide precipitation solvent containing at least an oxide precipitation substance for precipitating the core oxide particles; and Step 2 of mixing the mixed fluid with a shell oxide raw material liquid containing at least a shell oxide raw material which is a raw material of the shell oxide to uniformly coat the entire surface of the core oxide particles with the shell oxide. At least the above two steps are included, and after Step 1, Step 2 is completed within a predetermined time during which the core oxide particles do not aggregate in the mixed fluid. A method for producing core-shell type oxide particles is provided, characterized by this. The above predetermined time is preferably within 1 second. The present invention also relates to a method for producing core-shell type oxide particles in which the surface of the core oxide particles is coated with an oxide serving as a shell. The method includes at least the following two steps: Step 1 of precipitating the core oxide particles in a mixed fluid obtained by mixing a core oxide raw material liquid containing at least a core oxide raw material which is a raw material of the core oxide particles and an oxide precipitation solvent containing at least an oxide precipitation substance for precipitating the core oxide particles; and Step 2 of mixing the mixed fluid with a shell oxide raw material liquid containing at least a shell oxide raw material which is a raw material of the shell oxide to uniformly coat the entire surface of the core oxide particles with the shell oxide. The method is characterized in that Steps 1 and 2 are controlled such that the primary particle diameter of the core-shell type oxide particles is 190% or less relative to the primary particle diameter of the core oxide particles. A method for producing core-shell type oxide particles is provided. The present invention can also be implemented such that at least three fluids, namely the core oxide raw material liquid, the oxide precipitation solvent, and the shell oxide raw material liquid, are mixed between at least two processing surfaces that are arranged opposite to each other, can approach and separate from each other, and rotate relative to each other, to obtain the core-shell type oxide particles. Further, in the present invention, the center side of at least two processing surfaces is the upstream side, and the outer side is the downstream side. As the first fluid, either the oxide raw material liquid for the core or the oxide precipitation solvent passes from the upstream side to the downstream side between at least two processing surfaces while forming a thin film fluid. As the second fluid, the other of the oxide raw material liquid for the core and the oxide precipitation solvent passes through a second flow path independent of the first flow path into which the first fluid is introduced between at least two processing surfaces, and is introduced between at least two processing surfaces from an opening formed in at least one of at least two processing surfaces. The oxide raw material liquid for the core and the oxide precipitation solvent are mixed between at least two processing surfaces to precipitate oxide particles serving as the core. As the third fluid, the oxide raw material liquid for the shell passes through a third flow path independent of the first flow path and the second flow path, and is introduced between at least two processing surfaces from an opening formed in at least one of at least two processing surfaces. The opening of the second flow path is provided upstream of the opening of the third flow path, and the present invention can be implemented in this manner. Further, in the present invention, the flow rate F1 at which the first fluid is introduced between at least two processing surfaces, the flow rate F2 at which the second fluid is introduced between at least two processing surfaces, and the flow rate F3 at which the third fluid is introduced between at least two processing surfaces satisfy the relationship F1 > F2 and F1 + F2 > F3, and the present invention can be implemented in this manner. Further, in the present invention, the oxide particles serving as the core are zinc oxide particles or iron oxide particles, and the oxide serving as the shell is silicon oxide, and the present invention can be implemented in this manner. Further, in the present invention, the oxide particles serving as the core are zinc oxide particles, and the thickness of the oxide serving as the shell is 0.01% to 60% with respect to the diameter of the core-shell type oxide particles, and oxide particles can be obtained, and the present invention can be implemented in this manner. In addition, the present invention can be implemented as obtaining oxide particles in which the attenuation rate of the absorbance with respect to light near a wavelength of 660 nm derived from the absorption peak of the methylene blue dye is 10% or less by irradiating a dispersion liquid containing the methylene blue dye in which the core oxide particles are zinc oxide particles and the core-shell type oxide particles are dispersed with ultraviolet light of 365 nm for at least 2 hours. In addition, the present invention can be implemented as obtaining oxide particles in which the oxide particles serving as the core are iron oxide particles and the thickness of the oxide serving as the shell is 0.5% to 25% with respect to the diameter of the core-shell type oxide particles. In addition, the present invention can be implemented as obtaining oxide particles in which the attenuation rate of the absorbance with respect to light near a wavelength of 505 nm derived from the absorption peak of the congo red dye is 10% or less by irradiating a dispersion liquid containing the congo red dye in which the core-shell type oxide particles are dispersed with white light for at least 2 hours, where the oxide particles serving as the core are iron oxide particles. In addition, the present invention can be implemented such that the oxide serving as the shell contains an element different from the element contained in the oxide particles serving as the core. The present invention also provides core-shell type oxide particles in which the entire surface of the oxide particles serving as the core is uniformly coated with the oxide serving as the shell, where the oxide particles serving as the core are one zinc oxide particle, the oxide serving as the shell is silicon oxide, and the thickness of the oxide serving as the shell is 0.01% to 60% with respect to the diameter of the core-shell type oxide particles. The present invention also provides core-shell type oxide particles in which the entire surface of the oxide particles serving as the core is uniformly coated with the oxide serving as the shell, where the oxide particles serving as the core are one iron oxide particle, the oxide serving as the shell is silicon oxide, and the thickness of the oxide serving as the shell is 0.5% to 25% with respect to the diameter of the core-shell type oxide particles. In connection with the research on the above core-shell type oxide particles, the inventor of the present application has also found that silicon oxide-coated iron oxide can be applied to a paint composition, and has completed the following invention. That is, the present invention provides a silicon oxide-coated iron oxide composition for paints, which comprises iron oxide particles having a reflectance of less than 18% for light rays with a wavelength of 400 to 620 nm and a primary particle size of 1 to 50 nm, and at least a part of the surface thereof is coated with silicon oxide. The present invention can also be implemented as a dispersion containing the above iron oxide particles. The dispersion preferably has a transmittance of 2.0% or less for light rays with a wavelength of 200 to 420 nm and a transmittance of 80% or more for light rays with a wavelength of 620 to 800 nm. In the dispersion containing the above iron oxide particles, the haze value is preferably 2.0% or less when the concentration of iron oxide is 2% by weight. The present invention can also be implemented assuming that the silicon oxide is amorphous. The silicon oxide-coated iron oxide composition for paints according to the present invention can exhibit its performance as long as at least a part of the surface is coated with silicon oxide, and it may be in a form other than the core-shell type particle form, but it is preferably implemented as core-shell type oxide particles. That is, the present invention provides a new weather-resistant composition for paints that is used by being blended into a paint constituting a coated body, has a predetermined reflectance, transmittance, and transparency, and protects the coated body from ultraviolet rays. This weather-resistant composition for paints contains core-shell type iron oxide particles in which the surface of the core iron oxide particles is coated with shell silicon oxide. In these core-shell type iron oxide particles, the silicon oxide is preferably amorphous, and more preferably the primary particle size of the core-shell type iron oxide particles is 1 to 50 nm. Regarding the relationship between the core and the shell, in the core-shell type iron oxide particles, the primary particle size is preferably 100.5% or more and 190% or less of the primary particle size of the core iron oxide particles. Further, the core iron oxide particles are α-Fe ２ O ３It is preferable that it is. The core-shell type iron oxide particles having the above configuration may exhibit the following characteristics. That is, regarding the reflectance, the core-shell type iron oxide particles can have a reflectance of less than 18% with respect to light rays having a wavelength of 400 to 620 nm. Regarding the transmittance, the core-shell type iron oxide particles are in a state of a dispersion liquid in which the core-shell type iron oxide particles are dispersed in propylene glycol so that the concentration as iron oxide is 0.05% by weight, and the transmittance of the dispersion of light rays having a wavelength of 200 to 420 nm is 2.0% or less, and the transmittance with respect to light rays having a wavelength of 620 to 800 nm can be 80% or more. Regarding the transparency, the core-shell type iron oxide particles can have a haze value of 2.0% or less in a state of a dispersion in which the core-shell type iron oxide particles are dispersed in propylene glycol or water so that the concentration as iron oxide is 2% by weight. According to the present invention, it is possible to efficiently and stably produce core-shell type oxide particles having a uniform shell thickness. Further, when the core-shell type oxide particles are zinc oxide coated with silicon oxide or iron oxide coated with silicon oxide, they can be used as a raw material for a light-resistant coating film while suppressing the photocatalytic ability. Further, the present invention has been able to provide a silicon oxide-coated iron oxide composition for paints that has high transparency and does not impair the performance of the paints. Furthermore, the present invention provides a weather-resistant composition for paints that protects the coated body from ultraviolet rays, is used by being blended in the paint constituting the coated body, and can be effectively used for coated bodies of various colors, particularly red-colored coated bodies. (A) is a schematic cross-sectional view of a fluid treatment apparatus according to an embodiment of the present invention, and (B) is a schematic cross-sectional view of a fluid treatment apparatus according to another embodiment of the present invention. It is a schematic plan view of the first treatment surface of the fluid treatment apparatus shown in FIGS. 1(A) and (B). It is a TEM photograph of the core-shell type oxide particles obtained in Example 1 of the present invention. It is a STEM mapping of the core-shell type oxide particles obtained in Example 1 of the present invention. It is an XRD measurement result of the core-shell type oxide particles obtained in Example 1 of the present invention. It is an IR measurement result of the core-shell type oxide particles obtained in Example 1 of the present invention. It is an XRD measurement result of the particles obtained in Comparative Examples 2 to 4 of the present invention. It is a TEM photograph of the core-shell type oxide particles obtained in Comparative Example 5 of the present invention. It is a transmission spectrum of the core-shell type oxide particles obtained in Example 2 of the present invention and the oxide particles obtained in Comparative Example 7. It is an absorption spectrum of the core-shell type oxide particles obtained in Example 2 of the present invention and the oxide particles obtained in Comparative Example 7. It is an absorption spectrum before and after irradiating the core-shell type oxide particles obtained in Example 1 of the present invention dispersed in propylene glycol in which methylene blue dye is dissolved with ultraviolet light of 365 nm for 2 hours. It is an absorption spectrum before and after irradiating the oxide particles obtained in Comparative Example 7 of the present invention dispersed in propylene glycol in which methylene blue dye is dissolved with ultraviolet light of 365 nm for 2 hours. It is a TEM photograph of the core-shell type oxide particles obtained in Example 8 of the present invention. It is a STEM mapping of the core-shell type oxide particles obtained in Example 8 of the present invention. It is an XRD measurement result of the core-shell type oxide particles obtained in Example 8 of the present invention. It is a transmission spectrum of the core-shell type oxide particles obtained in Example 8 of the present invention and the oxide particles obtained in Comparative Example 12. It is an absorption spectrum of the core-shell type oxide particles obtained in Example 8 of the present invention and the oxide particles obtained in Comparative Example 12. It is an absorption spectrum before and after irradiating the core-shell type oxide particles obtained in Example 8 of the present invention dispersed in propylene glycol in which congo red dye is dissolved with white light for 2 hours. It is an absorption spectrum before and after irradiating the oxide particles obtained in Comparative Example 12 of the present invention dispersed in propylene glycol in which congo red dye is dissolved with white light for 2 hours.Transmission spectra of propylene glycol dispersions of silicon oxide-coated iron oxide particles obtained in Example 8 of the present invention, and iron oxide particles of Comparative Examples 12 and 13. Graph of the molar extinction coefficient against the measurement wavelength, calculated from the absorption spectra of propylene glycol dispersions of silicon oxide-coated iron oxide particles obtained in Example 8 of the present invention, and iron oxide particles of Comparative Examples 12 and 13. Reflectance spectra of powders of silicon oxide-coated iron oxide particles obtained in Example 8 of the present invention, and iron oxide particles of Comparative Examples 12 and 13. Reflectance spectra of aqueous dispersions of silicon oxide-coated iron oxide particles obtained in Example 8 of the present invention, and iron oxide particles of Comparative Examples 12 and 13. TEM photograph of iron oxide particles of Comparative Example 13 of the present invention. Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. (Core-shell type oxide particles) In the present invention, the core-shell type oxide particles refer to particles in which the entire surface of the oxide particles serving as the core is uniformly coated with an oxide serving as the shell. Here, the oxide particles serving as the core and the oxide serving as the shell are different substances, but it is preferable that the oxide serving as the shell contains an element different from the elements contained in the oxide particles serving as the core. Here, the "element different from the elements contained in the oxide particles serving as the core" means that the main component elements contained in the core and the shell are different between the core and the shell, and it does not prevent the elements contained in the oxide particles serving as the core from being contained in the oxide serving as the shell, nor does it prevent the elements contained in the oxide serving as the shell from being contained in the oxide particles serving as the core. Further, "serving as the main component" means that the main component element (excluding the oxygen element) contained in the oxide particles serving as the core or the oxide serving as the shell occupies 50% or more of all the elements (excluding the oxygen element) contained in the oxide particles serving as the core or the oxide serving as the shell. Further, the core-shell type oxide particles according to the present invention are preferably core-shell type oxide particles in which the oxide particles serving as the core are particles of zinc oxide or iron oxide, and the oxide serving as the shell is a silicon oxide. When the core-shell type oxide particles are used as a coloring material or an ultraviolet ray protecting agent, the photocatalytic ability among the exhibited properties (photocatalytic ability and ultraviolet ray absorption ability) can be suppressed. Also, from the viewpoints of improving dispersibility, transparency, and solvent resistance, it is preferable that the oxide serving as the shell is a silicon oxide. (Raw material for oxide) The oxide raw materials used for producing the core-shell type oxide particles in the present invention are not particularly limited. Any material that can be converted into an oxide by methods such as reaction, crystallization, precipitation, coprecipitation, etc. can be used. In the present invention, the method will hereinafter be described as precipitation. Here, the oxide raw materials used for producing the core-shell type oxide particles are the core oxide raw materials that are the raw materials of the core oxide particles and the shell oxide raw materials that are the raw materials of the shell oxide. For example, they are simple substances of metals or non-metals, metal compounds or non-metal compounds. The metals in the present invention are not particularly limited. Preferably, they are all metal elements on the periodic table of chemistry. Also, the non-metals in the present invention are not particularly limited, but preferably, non-metal elements such as B, Si, Ge, As, Sb, C, N, O, S, Te, Se, F, Cl, Br, I, At, etc. can be mentioned. Regarding these metals and non-metals, they may be a single element, or may be an alloy composed of a plurality of elements or a substance containing a non-metal element in a metal element. Also, in the present invention, the above metal compounds are referred to as metal compounds. The metal compounds or the above non-metal compounds are not particularly limited, but for example, salts, oxides, hydroxides, hydroxyoxides, nitrides, carbides, complexes, organic salts, organic complexes, organic compounds or their hydrates, organic solvates, etc. of metals or non-metals can be mentioned. The metal salts or non-metal salts are not particularly limited, but for example, nitrates, nitrites, sulfates, sulfites, formates, acetates, phosphates, phosphites, hypophosphites, chlorides, oxy salts, acetylacetonate salts or their hydrates, organic solvates, etc. of metals or non-metals can be mentioned, and as organic compounds, alkoxides of metals or non-metals can be mentioned. As described above, these metal compounds or non-metal compounds may be used alone or as a mixture of two or more. When the core oxide particles are zinc oxide or iron oxide, examples of the oxide raw material for the core include zinc or iron oxides, hydroxides, other compounds such as zinc or iron salts and alkoxides, and hydrates thereof. Although not particularly limited, examples include inorganic compounds such as zinc or iron chlorides, nitrates, and sulfates, and organic compounds such as zinc or iron alkoxides and acetylacetonates. As a specific example, zinc oxide, zinc chloride, zinc nitrate, iron(III) chloride, iron(II) chloride, iron(II) nitrate, iron(III) sulfate, zinc acetylacetonate, iron acetylacetonate, and their hydrates can be mentioned. When the shell oxide is a silicon compound, examples of the oxide raw material for the shell include silicon oxides, hydroxides, other compounds such as silicon salts and alkoxides, and hydrates thereof. Although not particularly limited, examples include phenyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-trifluoropropyl-trimethoxysilane, methacryloxypropyltriethoxysilane, tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and oligomeric condensates of TEOS, such as ethyl silicate 40, tetraisopropylsilane, tetrapropoxysilane, tetraisobutoxysilane, tetrabutoxysilane, and similar substances. Furthermore, as the oxide raw material for the shell, other siloxane compounds, bis(triethoxysilyl)methane, 1,9-bis(triethoxysilyl)nonane, diethoxydichlorosilane, triethoxychlorosilane, etc. may also be used. In the present invention, a core oxide raw material liquid containing at least a raw material for core oxide is used, and a shell oxide raw material liquid containing at least a raw material for shell oxide is used. When the above oxide raw material is solid, it is preferably used in a state where the above oxide raw material is melted or in a state where it is mixed or dissolved in a solvent (including a state of molecular dispersion) described later. Even when the above oxide raw material is liquid or gas, it may be used in a state where it is mixed or dissolved in a solvent (including a state of molecular dispersion) described later. Regarding the core oxide raw material or the shell oxide raw material, when a single oxide raw material is used for each, an oxide containing a single element other than oxygen can be produced for the element other than oxygen, so core-shell type oxide particles containing a single element other than oxygen can be produced for the core or the shell respectively. Further, when a plurality of oxide raw materials are used for the core oxide raw material or the shell oxide raw material respectively, a composite oxide containing a plurality of elements can be produced for the element other than oxygen, so it is also possible to produce core-shell type oxide particles composed of a composite oxide containing a plurality of elements other than oxygen for the core or the shell respectively. Furthermore, the above core oxide raw material liquid and shell oxide raw material liquid may include those in a state such as a dispersion or a slurry and still be practicable. The oxide precipitating substance in the present invention is a substance that can precipitate the core oxide raw material contained in the core oxide raw material liquid as core oxide particles, and can precipitate the shell oxide raw material contained in the shell oxide raw material liquid as shell oxide, and is not particularly limited as long as it can do so. For example, an acidic substance or a basic substance can be used. (Basic substance) Examples of the basic substance in the present invention include metal hydroxides such as sodium hydroxide and potassium hydroxide, metal alkoxides such as sodium methoxide and sodium isopropoxide, amine compounds such as triethylamine, diethylaminoethanol and diethylamine, and ammonia. (Acidic substance) Examples of the acidic substance in the present invention include inorganic acids such as aqua regia, hydrochloric acid, nitric acid, fuming nitric acid, sulfuric acid, and fuming sulfuric acid, and organic acids such as formic acid, acetic acid, chloroacetic acid, dichloroacetic acid, oxalic acid, trifluoroacetic acid, and trichloroacetic acid. (Solvent used for oxide precipitation solvent preparation) In the present invention, an oxide precipitation solvent containing at least an oxide precipitation substance is used, and it is preferable to prepare the oxide precipitation solvent by mixing, dissolving, and molecularly dispersing at least the oxide precipitation substance in a solvent. Examples of the solvent used for preparing the core oxide raw material liquid, the oxide precipitation solvent, and the shell oxide raw material liquid include water, an organic solvent, or a mixed solvent composed of a plurality of them. Examples of the above water include tap water, ion-exchanged water, pure water, ultrapure water, RO water, etc., and examples of the organic solvent include alcohol compound solvents, amide compound solvents, ketone compound solvents, ether compound solvents, aromatic compound solvents, carbon disulfide, aliphatic compound solvents, nitrile compound solvents, sulfoxide compound solvents, halogen compound solvents, ester compound solvents, ionic liquids, carboxylic acid compounds, sulfonic acid compounds, etc. The above solvents may be used alone or in combination of a plurality. Examples of the alcohol compound solvent include monohydric alcohols such as methanol and ethanol, and polyols such as ethylene glycol and propylene glycol. Further, within a range that does not adversely affect the production of the core-shell type oxide particles, the above acidic substance may be mixed with the core oxide raw material liquid or the shell oxide raw material liquid as needed. (Preparation apparatus) The core oxide raw material liquid, the shell oxide raw material liquid, or the oxide precipitation solvent in the present invention is preferably prepared using a preparation apparatus such as a rotary disperser that realizes homogeneous mixing by applying a shearing force to a fluid, such as one that rotates a stirrer of various shapes such as rod-shaped, plate-shaped, and propeller-shaped in a tank, or one equipped with a screen that rotates relative to the stirrer. As a preferable example of the rotary disperser, the stirrer disclosed in Japanese Patent No. 5147091 can be applied. The rotary disperser may be batch-type or continuous-type. When it is continuous-type, the supply and discharge of the fluid to and from the stirring tank may be continuous, or it may be carried out using a continuous mixer without using a stirring tank. With known stirrers and stirring means, the stirring energy can be appropriately controlled. Regarding the stirring energy, it is detailed in Japanese Patent Laid-Open No. 04-114725 by the applicant of the present application. The stirring method in the present invention is not particularly limited, but it can be carried out using various stirrers, dissolvers, emulsifiers, dispersers, homogenizers, etc., such as shear type, friction type, high-pressure jet type, ultrasonic type. As an example, there can be mentioned continuous emulsifiers such as Ultra Turrax (manufactured by IKA), Polytron (manufactured by Kinematic), TK Homomixer (manufactured by Primix), Ebara Milder (manufactured by Ebara Corporation), TK Homomicron Inline Flow (manufactured by Primix), Colloid Mill (manufactured by Kobe Steel Pan-Tech), Slasher (manufactured by Nippon Coke & Engineering Co., Ltd.), Trigonal Wet Fine Crusher (manufactured by Mitsui Miike Chemical Machinery Co., Ltd.), Cavitron (manufactured by Eurotech), Fine Flow Mill (manufactured by Pacific Machine Works Co., Ltd.), and batch-type or continuous-both-use emulsifiers such as Clear Mix (manufactured by M-Technique), Clear Mix Dissolver (manufactured by M-Technique), Film Mix (manufactured by Primix). Further, the stirring treatment is preferably carried out using a stirrer equipped with a high-speed rotating stirring blade, having a screen outside the stirring blade, and the fluid discharging as a jet flow from the opening of the screen, particularly using the above-mentioned Clear Mix (manufactured by M-Technique) or Clear Mix Dissolver (manufactured by M-Technique). (Dispersant, etc.) In the present invention, various dispersants and surfactants may be used as needed for the purpose, as long as they do not adversely affect the production of the core-shell type oxide particles. Although not particularly limited, various commercially available products generally used, products, or newly synthesized ones can be used as the dispersant and surfactant. As an example, anionic surfactants, cationic surfactants, nonionic surfactants, and dispersants such as various polymers can be mentioned. These may be used alone or in combination of two or more. The above surfactants and dispersants may be contained in at least one of the fluids of the oxide raw material liquid, the oxide precipitation solvent, and the shell oxide raw material liquid. Further, the above surfactants and dispersants may be contained in a fourth fluid, which will be described later, and is different from the oxide raw material liquid, the oxide precipitation solvent, and the shell oxide raw material liquid. (Reaction method: apparatus) The method for producing core-shell type oxide particles in the present invention or particles in which at least a part of the surface of the oxide particles is coated with an oxide includes, for example, producing oxide particles of the core part using a first microreactor, and then coating at least a part of the surface of the oxide particles serving as the core with an oxide serving as a shell by a second microreactor, or producing oxide particles by a method such as performing a reaction in a dilute system in a batch container and subsequently coating at least a part of the surface of the aforementioned oxide particles with an oxide by a reaction in a dilute system, or producing oxide particles by a pulverization method such as a bead mill and then coating at least a part of the surface of the oxide particles with an oxide in a reaction vessel. Further, an apparatus and a method as described in Japanese Patent Application Laid-Open No. 2009-112892 proposed by the applicant of the present application may also be used. The apparatus described in Japanese Patent Application Laid-Open No. 2009-112892 has a stirring tank having an inner peripheral surface with a circular cross-sectional shape and a stirrer attached with a slight gap from the inner peripheral surface of the stirring tank. The stirring tank is provided with at least two fluid inlets and at least one fluid outlet. From one of the fluid inlets, a first treated fluid containing one of the reactants among the fluids to be treated is introduced into the stirring tank, and from one of the fluid inlets other than the above, a second treated fluid containing one of the reactants different from the above reactant is introduced into the stirring tank through a flow path different from the first treated fluid. At least one of the stirring tank and the stirrer rotates at a high speed with respect to the other to make the fluid to be treated in a thin film state, and the reactants contained in at least the first treated fluid and the second treated fluid are reacted with each other in this thin film. In order to introduce three or more treated fluids into the stirring tank, it is described that three or more introduction pipes may be provided as shown in FIGS. 4 and 5. In the present invention, it is preferable to perform at least the mixing of the oxide raw material liquid and the oxide precipitation solvent using a microreactor, and among them, it is preferable to use the same one as the apparatus described in Patent Document 4 shown in FIG. 1. Hereinafter, the microreactor will be described in detail. In FIGS. 1(A)(B) and FIG. 2, R indicates the rotation direction. The microreactor (hereinafter also referred to as a fluid processing device) in the present embodiment includes two processing parts 10 and 20 that face each other, and the first processing part 10 rotates. The opposing surfaces of both processing parts 10 and 20 serve as processing surfaces, respectively. The first processing part 10 includes a first processing surface 1, and the second processing part 20 includes a second processing surface 2. Both processing surfaces 1 and 2 are connected to the flow paths d1, d2, and d3 of the first, second, and third fluids to be processed, and constitute a part of the sealed flow path of the fluid to be processed. The distance between both processing surfaces 1 and 2 is usually adjusted to a minute distance of 1 mm or less, for example, about 0.1 μm to 50 μm. As a result, the fluid to be processed passing between both processing surfaces 1 and 2 becomes a forced thin-film fluid forced by both processing surfaces 1 and 2. Then, in this device, fluid processing is performed between the processing surfaces 1 and 2 to mix and react the first, second, or third fluid to be processed to precipitate core-shell type oxide particles. More specifically, the above device includes a first holder 11 that holds the first processing part 10, a second holder 21 that holds the second processing part 20, a contact surface pressure applying mechanism 43, a rotation driving mechanism (not shown), a first introduction part d1, a second introduction part d2, a third introduction part d3, and fluid pressure applying mechanisms p1, p2, and p3. Compressors or other pumps can be employed for the fluid pressure applying mechanisms p1, p2, and p3. The first introduction part d1, the second introduction part, and the third introduction path each include a thermometer and a pressure gauge inside, and can measure the introduction pressure of the first, second, and third fluids to be processed and the temperature under the introduction pressure. Note that the opening of the third introduction part d3 on the second processing surface 2 is located outside the center of rotation of the first processing surface 1 compared to the opening of the second introduction part d2. That is, on the second processing surface 2, the opening of the third introduction part d3 is located downstream of the opening of the second introduction part d2. A gap is provided between the opening of the third introduction part d3 and the opening of the second introduction part d2 in the radial direction of the second processing part 20. In the above embodiment, the first processing unit 10 and the second processing unit 20 are ring-shaped disks. The materials of the first and second processing units 10 and 20 can be, in addition to metals, carbon, ceramics, sintered metals, wear-resistant steels, sapphires, other metals subjected to hardening treatment, or those with hard materials lined, coated, plated, etc. In the above embodiment, the first and second processing surfaces 1 and 2 of the two processing units 10 and 20 facing each other are mirror-polished, and the arithmetic mean roughness is 0.01 to 1.0 μm. In the above embodiment, the second holder 21 is fixed to the device, and the first holder 11 attached to the rotating shaft 50 of the rotation drive mechanism also fixed to the device rotates, and the first processing unit 10 supported by this first holder 11 rotates with respect to the second processing unit 20. Of course, the second processing unit 20 may be rotated, or both may be rotated. Also, in the present invention, the above rotation speed can be, for example, 350 to 5000 rpm. In the above embodiment, with respect to the first processing unit 10, the second processing unit 20 approaches and separates in the direction of the rotating shaft 50, and a part on the side opposite to the processing surface 2 side of the second processing unit 20 is housed in the housing portion 41 provided in the second holder 21 so as to be able to protrude and retract. However, conversely, the first processing unit 10 may approach and separate with respect to the second processing unit 20, or the two processing units 10 and 20 may approach and separate from each other. The housing portion 41 is a concave portion that houses a part of the second processing unit 20 on the side opposite to the processing surface 2 side, and is a groove formed in an annular shape. This housing portion 41 houses the second processing unit 20 with sufficient clearance to allow a part on the side opposite to the processing surface 2 side of the second processing unit 20 to protrude and retract. The joint surface pressure applying mechanism is a mechanism for generating a pushing force (hereinafter referred to as the joint surface pressure) in the direction in which the first processing surface 1 of the first processing unit 10 and the second processing surface 2 of the second processing unit 20 approach each other. By the balance between this joint surface pressure and the separating force (hereinafter referred to as the separating force) that separates the two processing surfaces 1 and 2 due to the fluid pressures of the first to third fluids to be processed, while maintaining the interval between the two processing surfaces 1 and 2 at a predetermined minute interval, a thin film fluid having a minute film thickness in the order of nm to μm is generated. In the above embodiment, the joint surface pressure applying mechanism applies the joint surface pressure by biasing the second processing unit 20 toward the first processing unit 10 by a spring 43 provided on the second holder 21. Furthermore, the pressure of a fluid for back pressure such as air can also be applied in addition to the spring 43. The sum of all these pressures is the above-mentioned joint surface pressure, and this joint surface pressure balances with the separating force due to the fluid pressures of the first to third fluids to be processed. When introducing and mixing each fluid to be processed above its boiling point between the processing surfaces, it is preferable to set the fluid pressure of each fluid to be processed high. Specifically, the fluid pressure of each fluid to be processed is set to exceed the standard atmospheric pressure, and the joint surface pressure that balances with this is also set high. Specifically, the pressure of the fluid for back pressure can be set to 0.020 to 0.350 MPaG, preferably 0.050 to 0.400 MPaG, more preferably 0.100 to 0.500 MPaG, and for the spring 43, it can be set to 0.007 to 0.030 MPa, preferably 0.010 to 0.20 MPa. The first fluid to be processed pressurized by the fluid pressure applying mechanism p1 is introduced from the first introduction part d1 into the space inside the two processing units 10 and 20 through the opening d10. In the present embodiment, the opening d10 is on the inner peripheral side between the processing surfaces 1 and 2. On the other hand, the second fluid to be processed pressurized by the fluid pressure applying mechanism p2 is introduced from the second introduction part d2 into the space inside the two processing units 10 and 20 from the opening d20 formed on the second processing surface through a passage provided inside the second processing unit 20. At the opening d20, the first fluid to be processed and the second fluid to be processed merge and mix. At that time, the mixed fluid to be treated becomes a thin film fluid forced by the two treatment surfaces 1 and 2 that hold the above-mentioned minute gaps, and attempts to move to the outside of the annular two treatment surfaces 1 and 2. Since the first treatment section 10 is rotating, the mixed fluid to be treated does not move linearly from the inside to the outside of the annular two treatment surfaces 1 and 2, but a composite vector of the movement vector in the radial direction of the annulus and the movement vector in the circumferential direction acts on the fluid to be treated, causing it to move in a substantially spiral shape from the inside to the outside. Here, as shown in FIG. 2, on the first treatment surface 1 of the first treatment section 10, a groove-shaped recess 13 extending from the center side to the outside of the first treatment section 10, that is, in the radial direction, may be formed. The planar shape of this recess 13 may be curved on the first treatment surface 1 or extend in a spiral shape, or, although not shown, may extend straight outward, bend or curve such as in an L shape, be continuous, intermittent, or branched. Further, this recess 13 can also be formed on the second treatment surface 2, or can be formed on both the first and second treatment surfaces 1 and 2. By forming such a recess 13, a micropump effect can be obtained, and there is an effect that the fluid to be treated can be transferred between the first and second treatment surfaces 1 and 2. It is desirable that the base end of the above-mentioned recess 13 reaches the inner circumference of the first treatment section 10. The tip of the recess 13 extends toward the outer peripheral side of the first treatment surface 1, and its depth gradually decreases from the base end toward the tip. A flat surface 16 without the recess 13 is provided between the tip of the recess 13 and the outer peripheral surface of the first treatment surface 1. The above-mentioned opening d20 is preferably provided at a position facing the flat surface of the first treatment surface 1. In particular, it is preferable to install the opening d20 at a position facing the flat surface 16 on the downstream side from the point where the flow direction of the first fluid to be treated when introduced by the micropump effect is converted to the spiral and laminar flow direction formed between the treatment surfaces. This enables the mixing of a plurality of fluids to be treated and the precipitation of fine particles under laminar flow conditions. It is preferable to give the second introduction part d2 a directionality. For example, the introduction direction from the opening d20 of the second processing surface 2 may be inclined at a predetermined elevation angle with respect to the second processing surface 2, or the introduction direction from the opening d20 of the second processing surface 2 has a directionality in the plane along the second processing surface 2. The introduction direction of this second fluid to be processed is an outward direction away from the center in the radial component of the processing surface, and may be a forward direction in the component with respect to the rotation direction of the fluid between the rotating processing surfaces. In this way, the flow of the first fluid to be processed at the opening d20 is laminar, and by giving the second introduction part d2 a directionality, it is possible to introduce the second fluid to be processed between the processing surfaces 1 and 2 while suppressing the generation of turbulence in the flow of the first fluid to be processed. Furthermore, as shown in Fig. 1(A), the third fluid to be processed pressurized by the fluid pressure applying mechanism p3 is introduced from the third introduction part d3 into the space inside both processing parts 10 and 20 through a passage provided inside the second processing part 20 from an opening d30 formed on the second processing surface. Since the opening d30 is provided on the downstream side of the opening d10 and the opening d20, the third fluid to be processed joins and mixes with the mixed fluid of the first fluid to be processed and the second fluid to be processed at the opening d30. Similar to the case where the first fluid to be processed and the second fluid to be processed join and mix, the mixed fluid to be processed becomes a thin film fluid forced by both processing surfaces 1 and 2 that maintain the above-mentioned minute gap, and moves in a substantially spiral shape from the inside to the outside of the annular both processing surfaces 1 and 2. It is preferable to provide the opening d30 at a position facing the flat surface of the first processing surface 1, and it is also preferable to install the opening d30 at a position facing the flat surface 16 on the downstream side from the point where the flow direction of the mixed fluid of the first fluid to be processed and the second fluid to be processed is converted into the spiral and laminar flow direction formed between the processing surfaces. Furthermore, it is also preferable to give the third introduction part d3 a directionality, which is the same as the case of the opening d20. In addition, the mixed fluid to be processed discharged outside the two processing sections 10 and 20 is collected in the beaker b as the discharged liquid via the vessel v. In the embodiment of the present invention, as will be described later, the discharged liquid contains core-shell type oxide particles. As described above, in the apparatus according to this embodiment, in the region between the two processing surfaces 1 and 2, the region between the opening d20 and the opening d30 becomes the deposition formation region of the oxide particles serving as the core related to Step 1. And in the region between the two processing surfaces 1 and 2, the region downstream (outside in the example of the figure) from the opening d30 becomes the deposition region of the oxide serving as the shell related to Step 2. However, since Step 1 and Step 2 are performed continuously, the two steps do not necessarily have to be completely separated. In other words, even after Step 2 is started, the deposition and growth of the oxide particles serving as the core may continue partially. Incidentally, the type of the fluid to be processed and the number of its flow paths are three in the example of FIG. 1, but may be four or more. Also, the shape, size, and number of the introduction openings provided in each processing section are not particularly limited and can be appropriately changed for implementation. For example, as shown in FIG. 1(B), the shape of the opening d20 may be a concentric annular shape surrounding the central opening of the processing surface 2 which is a ring-shaped disk, and the annular opening may be continuous or discontinuous. Similarly, the shape of the opening d30 may also be a concentric annular shape surrounding the central opening of the processing surface 2 which is a ring-shaped disk, and the annular opening may be continuous or discontinuous. Further, an introduction opening may be provided immediately before or further upstream between the first and second processing surfaces 1 and 2. In a preferred form in the present application, as will be described later, it is preferable that at least the opening d30 for introducing the shell oxide raw material is continuously opened in an annular shape, and the most preferable form is that the openings d20 and d30 are continuously opened in an annular shape. Since the opening d30 or the openings d20 and d30 are annular, all of the first to third fluids to be processed can surely be brought into contact between the processing surfaces 1 and 2. Thereby, only core-shell type oxide particles in which the entire surface of the oxide particles serving as the core is uniformly coated with the oxide serving as the shell can surely be produced. The apparatus having the above three openings (d10, d20, and d30) is suitable for performing steps 1 and 2 between continuous processing surfaces. However, in the practice of the present invention, it is also possible to perform one of steps 1 and 2 between the above-described processing surfaces and the other in a different apparatus. In this case, as shown in FIG. 1(B), the apparatus having the above-described processing surfaces only needs to have at least two openings (d10, d20), but it does not prevent the implementation with an apparatus having three or more openings. As an example of the case where one of steps 1 and 2 is performed between the above-described processing surfaces and the other is performed in a different apparatus, there is a method in which step 1 is performed between the above-described processing surfaces and step 2 is continuously performed in a batch or continuous mixing reactor following step 1. Specifically described, an inlet is provided in the vessel v of the apparatus described above (not shown), and the oxide raw material liquid for the shell is supplied into the vessel v from the inlet, and the oxide particles serving as the core immediately after being discharged from the processing surfaces 1 and 2 are mixed with the oxide raw material liquid for the shell. The mixed fluid containing the oxide particles serving as the core is released from the forced action by the processing surfaces 1 and 2 and discharged into a wider flow path space, and the oxide raw material liquid for the shell can be efficiently and continuously supplied to the above-described mixed fluid discharged while spreading. In the present invention, it is only necessary that the above processing can be performed between the processing surfaces 1 and 2. For example, the second fluid to be processed may be introduced from the first introduction part d1, and the first fluid to be processed may be introduced from the second introduction part d2. The expressions of the first to third in each fluid only have the meaning for identification that it is the nth of a plurality of existing fluids, and as described above, fluids of the fourth or higher order may also exist. In the present invention, in the fluid processing apparatus, it is preferable to introduce three fluids, i.e., an oxide raw material solution for the core, an oxide precipitation solvent, and an oxide raw material solution for the shell, from the first, second, and third introduction parts (d1, d2, d3) between the processing surfaces 1 and 2, respectively, and then carry out the process. First, introduce the oxide raw material solution for the core (the first fluid to be processed; hereinafter also referred to as the first fluid) from the first introduction part between the processing surfaces 1 and 2, and introduce the oxide precipitation solvent (the second fluid to be processed; hereinafter also referred to as the second fluid) from the second introduction part between the processing surfaces 1 and 2. Mix the two in the thin-film fluid formed between the processing surfaces 1 and 2, and precipitate oxide particles serving as the core in the mixed fluid of the two between the processing surfaces 1 and 2. Next, introduce the oxide raw material solution for the shell (the third fluid to be processed; hereinafter also referred to as the third fluid) from the third introduction part between the processing surfaces 1 and 2, and mix the mixed fluid containing the precipitated oxide particles serving as the core and the oxide raw material solution for the shell in the thin-film fluid formed between the processing surfaces 1 and 2, so that an oxide serving as the shell is precipitated on the surface of the oxide particles serving as the core, and core-shell type oxide particles can be produced. That is, after precipitating oxide particles serving as the core between the processing surfaces 1 and 2 using the fluid processing apparatus, before discharging the mixed fluid containing the oxide particles serving as the core from between the processing surfaces 1 and 2, by introducing the oxide raw material solution for the shell between the processing surfaces 1 and 2, the mixed fluid containing the oxide particles serving as the core and the oxide raw material solution for the shell are mixed between the processing surfaces 1 and 2, and an oxide serving as the shell is precipitated on the surface of the oxide particles serving as the core, so that core-shell type oxide particles can be produced. Since it is only necessary to mix the first fluid and the second fluid to precipitate oxide particles serving as the core, either one of the oxide raw material solution for the core and the oxide precipitation solvent may be the first fluid, and the other may be the second fluid. By the method as described above, step 1 of precipitating oxide particles serving as the core in the mixed fluid obtained by mixing the oxide raw material solution for the core and the oxide precipitation solvent, and step 2 of mixing the mixed fluid and the oxide raw material solution for the shell to uniformly coat the entire surface of the oxide particles serving as the core with the oxide serving as the shell can be continuously carried out.Before the oxide particles serving as the core, which are generated by mixing the first and second fluids, aggregate, it is necessary to mix the mixed fluid containing the oxide particles serving as the core with the oxide raw material solution for the shell and complete a uniform coating of the oxide that forms the shell over the entire surface of the oxide particles serving as the core. That is, after the above step 1, it is necessary to complete the above step 2 before the oxide particles serving as the core aggregate in the mixed fluid. Preferably, the time from when the first and second fluids are mixed until the precipitation of the oxide particles serving as the core and then the completion of the coating with the oxide that forms the shell, which is caused by mixing with the third fluid, is within 1 second. Therefore, it is preferable that the third fluid is continuously mixed following the mixing of the first and second fluids. Although not particularly limited, it is preferable to mix the third fluid within 1 second after the first and second fluids are mixed, and more preferably within 0.5 seconds. Thereby, it becomes possible to cause the oxide raw material for the shell contained in the third fluid to act on the oxide particles serving as the core before the oxide particles serving as the core, which are precipitated by mixing the first and second fluids, aggregate. In the prior art, since the oxide particles serving as the core aggregated after precipitation and the oxide raw material for the shell was made to act on the aggregated particles, for example, two or more of the oxide particles serving as the core were covered with the oxide that forms the shell, and thus the primary particle diameter of the produced core-shell type oxide particles could be larger than 190% of the primary particle diameter of the oxide particles serving as the core. In the method of the present invention, since the entire surface of each of the oxide particles serving as the core can be uniformly covered with the oxide that forms the shell, the primary particle diameter of the produced core-shell type oxide particles is 190% or less of the primary particle diameter of the oxide particles serving as the core. Further, if the coating of the oxide that forms the shell is too thin, there is a risk that the various effects of the produced core-shell type oxide particles cannot be exhibited. Therefore, it is preferable that the primary particle diameter of the produced core-shell type oxide particles is 100.5% or more of the primary particle diameter of the oxide particles serving as the core. In the present invention, when the flow rate of the first fluid introduced between the processing surfaces 1 and 2 is F1, the flow rate of the second fluid introduced between the processing surfaces 1 and 2 is F2, and the flow rate of the third fluid introduced between the processing surfaces 1 and 2 is F3, it is preferable that F1, F2, and F3 satisfy the relationship F1 > F2 and F1 + F2 > F3. Between the processing surfaces 1 and 2 that can approach and separate, it is important to introduce the second fluid from the second introduction part d2 into the first fluid introduced from the first introduction part d1 provided at the center of the processing part to precipitate oxide particles serving as the core, and then introduce the third fluid from the third introduction part d3 into the mixed fluid formed by mixing the first and second fluids. By satisfying the relationship F1 > F2, the second fluid can surely be mixed with the first fluid, so that by-products when the unreacted second fluid is mixed with the third fluid can be reduced. Further, by satisfying the relationship F1 + F2 > F3, the precipitation of the unreacted oxide raw material for the shell or the oxide serving as the shell alone can be reduced, and the precipitation of the oxide serving as the shell on the surface of the oxide particles serving as the core can be surely performed. That is, the inhibitory factors for the precipitation of the oxide serving as the shell on the surface of the oxide particles serving as the core are reduced, and the precipitation of the shell is surely performed, so that it becomes possible to form a shell uniformly over the entire surface of the core. Thus, by satisfying the relationship F1 > F2 and F1 + F2 > F3, it is possible to manufacture uniform core-shell type oxide particles. Usually, when F1 is 100 parts, F2 is preferably 99 parts to 1 part, more preferably 75 parts to 3 parts. Also, when F1 + F2 is 100 parts, F3 is preferably 99 parts to 1 part, more preferably 75 parts to 3 parts. Furthermore, regarding the combination of the first, second, and third fluids and the processing conditions, after mixing the first and second fluids to precipitate the oxide serving as the core, it is necessary that the mixing with the third fluid can be completed within 1 second to uniformly coat the entire surface of the oxide serving as the core with the oxide serving as the shell.For example, if the shell formation is too slow, the oxide particles serving as the core may aggregate and then be coated with the oxide serving as the shell, or if the precipitation of the oxide particles serving as the core after mixing of the first fluid and the second fluid is too slow, since the oxide particles serving as the core are not produced at the time of mixing of the third fluid, it is conceivable that the oxide particles serving as the core and the oxide serving as the shell are not in a coated form but in a state such as a composite oxide. The core-shell type oxide particles according to the present invention refer to particles in which the entire surface of the oxide particles serving as the core is uniformly coated with the oxide serving as the shell. Here, "uniformly coated" means that the thickness of the oxide serving as the shell is in the range of 0.01% to 60% with respect to the diameter of the core-shell type oxide particles, that is, the distance between the outermost peripheries of the core-shell type oxide particles, preferably in the range of 0.1% to 50%, and more preferably in the range of 0.2% to 40%. The core-shell type oxide particles according to the present invention can suppress specific properties exhibited by the oxide particles. When the oxide particles serving as the core are particles of zinc oxide or iron oxide and the oxide serving as the shell is silicon oxide, the photocatalytic ability can be suppressed. The suppression effect of the photocatalytic ability was confirmed by measuring the ultraviolet-visible light absorption spectrum. More specifically, light such as ultraviolet light or visible light was irradiated for a certain period of time to a dispersion containing a dye in which the core-shell type oxide particles according to the present invention were dispersed, the ultraviolet-visible light absorption spectrum before and after the irradiation was measured, and the photocatalytic ability was evaluated by the attenuation rate (Equation 1) of the absorbance at a specific wavelength derived from the absorption peak of the dye contained in the dispersion. Attenuation rate of absorbance = (A1 - A2) / A1 × 100 (%) (Equation 1) A1: Absorbance before light irradiation A2: Absorbance after light irradiation It was evaluated that the photocatalytic ability was suppressed when the attenuation rate of the absorbance was 10% or less. If the attenuation rate of the absorbance is 10% or less, it is preferable in that the light resistance can be maintained when the core-shell type oxide particles according to the present invention are used as a coloring material or an ultraviolet ray protecting agent. The attenuation rate of the absorbance is more preferably 7.5% or less, and even more preferably 5.0% or less. The ultraviolet light or white light in the present invention can be used to evaluate whether the particles of the present invention can suppress the photocatalytic ability with respect to the light rays contained in sunlight in nature or light such as fluorescent lamps in a room. The ultraviolet light is light rays in the wavelength range of 400 to 10 nm. In the evaluation of the catalytic ability in the present invention, it can be carried out using a device capable of outputting ultraviolet light rays or the like. Further, it may be carried out using a device that outputs ultraviolet light rays of a single wavelength. The white light in the present invention is light in which visible light rays in the wavelength range of 400 to 750 nm or 800 nm are mixed. In the evaluation of the catalytic ability in the present invention, it can be carried out using a device capable of outputting white light rays. Further, it may be carried out using a device that can output visible light rays of a single color. The above ultraviolet light and white light may be used alone to evaluate the photocatalytic ability of the core-shell type oxide particles produced by the production method of the present invention, or both lights may be irradiated simultaneously for evaluation. The above-mentioned dye in the present invention is not particularly limited as long as it can be decomposed by the photocatalytic effect of particles having photocatalytic ability. The above dye is mixed with the dispersion of the core-shell type oxide particles according to the present invention, and when the obtained dispersion is irradiated with the above ultraviolet light or the above white light for a certain period of time and the decomposition of the above dye is confirmed, it indicates that the particles have been recognized as having photocatalytic ability, and when it is not confirmed, it indicates that the photocatalytic ability has not been recognized. In the present invention, the decomposition of the dye was confirmed by the above-mentioned attenuation rate of absorbance. (pH region) In the present invention, the pH of the oxide raw material liquid for the core, the oxide precipitation solvent, and the oxide raw material liquid for the shell is not particularly limited. It can be appropriately changed depending on the oxide raw material used (oxide raw material for the core, oxide raw material for the shell), the type and concentration of the oxide precipitation substance, the purpose, and the type of the core-shell type oxide particles to be targeted. For example, when the oxide particles serving as the core are zinc oxide particles, it is preferable that the pH of the mixed fluid after completion of Step 2 exceeds 12. When the pH of the mixed fluid after completion of Step 2 is 12 or less, by-products other than zinc oxide (for example, 6Zn(OH) ２ ・ZnSO ４ ・4H ２ O) are produced, which is not preferable. Further, when the oxide serving as the shell is a silicon compound, when reacting the oxide raw material for the shell to obtain the oxide serving as the shell and coating the entire surface of the oxide serving as the core with the oxide serving as the shell, it is preferable that the fluidity of the reaction field is basic. In Step 2, under basic conditions, by reacting the oxide raw material for the shell by the sol-gel method, core-shell type oxide particles in which the entire surface of the oxide particles serving as the core is covered with the oxide serving as the shell can be produced. More specifically, after hydrolyzing the oxide raw material for the shell under acidic or basic conditions, and then performing dehydration polycondensation under basic conditions, when producing core-shell type oxide particles in which the entire surface of the oxide particles serving as the core is covered with the oxide serving as the shell, the uniformity of the shell can be improved. (Temperature) In the present invention, the temperature at which the oxide raw material liquid for the core and the oxide precipitation solvent are mixed is not particularly limited. Further, the temperature at which the mixed fluid obtained by mixing the oxide raw material liquid for the core and the oxide precipitation solvent and the oxide raw material liquid for the shell are mixed is not particularly limited. It can be carried out at an appropriate temperature depending on the type of oxide raw material and oxide precipitation substance used, the type of core-shell type oxide particles targeted, the pH of each fluid, and the like. Next, silicon oxide-coated iron oxide in which the oxide serving as the core is iron oxide particles and the oxide serving as the shell is silicon oxide will be taken up and described in detail. Hereinafter, the composition containing silicon oxide-coated iron oxide is also referred to as a silicon oxide-coated iron oxide composition for paint. The above silicon oxide-coated iron oxide exhibits its performance properly when used in a paint composition, particularly a paint for a multilayer coating film. Note that the same members are given the same reference numerals, and detailed descriptions are omitted. The silicon oxide-coated iron oxide composition for paints in the present invention is a composition containing silicon oxide-coated iron oxide powder, a dispersion obtained by dispersing silicon oxide-coated iron oxide in a liquid dispersion medium, or a dispersion obtained by dispersing silicon oxide-coated iron oxide in a solid such as glass or resin. The silicon oxide-coated iron oxide composition of the present application may be used alone or dispersed in a paint together with a pigment or a dye, or may be overcoated on a paint film. Furthermore, the silicon oxide-coated iron oxide composition of the present application can also be used as a single pigment. Examples of the liquid dispersion medium include water such as tap water, distilled water, RO water, pure water, and ultrapure water, alcohol solvents such as methanol, ethanol, and isopropyl alcohol, polyhydric alcohol solvents such as propylene glycol, ethylene glycol, diethylene glycol, and glycerin, ester solvents such as ethyl acetate and butyl acetate, aromatic solvents such as benzene, toluene, and xylene, ketone solvents such as acetone and methyl ethyl ketone, and nitrile solvents such as acetonitrile. These may be used alone or in combination of two or more. The silicon oxide-coated iron oxide dispersion of the present invention, specifically, the silicon oxide-coated iron oxide particles obtained in Example 8 described later were dispersed in propylene glycol at a concentration of 0.05 wt% as Fe ２ O ３ The transmittance spectrum in the wavelength range of 200 to 800 nm of the dispersion is shown in FIG. 20. From FIG. 20, the transmittance of light in the wavelength range of 620 to 800 nm is 80% or more, and the transmittance of light in the wavelength range of 200 to 420 nm is 2.0% or less. FIG. 22 shows the reflection spectrum of the silicon oxide-coated iron oxide powder of the present invention, specifically, the powder of the silicon oxide-coated iron oxide particles obtained in Example 8. As can be seen from FIG. 22, in the wavelength range of 400 to 620 nm, the reflectance of light is less than 18%. FIG. 23 shows the aqueous dispersion of the silicon oxide-coated iron oxide of the present invention, specifically, the silicon oxide-coated iron oxide particles obtained in Example 8 were dispersed in water as Fe ２ O ３The reflected spectrum of the dispersion liquid dispersed at a concentration of 0.31% by weight in the wavelength range of 400 to 750 nm was shown. As seen in FIG. 23, in the region of wavelength 400 to 620 nm, the reflectance of the light ray is less than 18%. When the reflectance of the light ray exceeds 18%, the color characteristics emitted by the red paint will be impaired. Therefore, in the present invention, in the region of wavelength 400 to 620 nm, it is preferable that the reflectance of the light ray is less than 18%, and more preferably less than 15%. The haze value of the dispersion liquid in which the silicon oxide-coated iron oxide particles obtained in Example 8 above were dispersed in propylene glycol at a concentration of Fe ２ O ３ at a concentration of 0.05% by weight was 0.00%, and also, the haze value of the dispersion liquid in which Fe ２ O ３ was dispersed in water at a concentration of 0.31% by weight was 0.08%. Both were dispersion liquids with very high transparency. The haze value is a numerical value indicating transparency. For example, when a composition with a haze value exceeding 2% is applied on the paint of a building or a vehicle, the color of the underlying paint will be impaired, and the intended coloring will be inhibited. Also, when applied to the human skin or the like, if a material with a haze value exceeding 2% and low transmittance is used, the texture and appearance will be impaired, which is not preferable. In the present invention, it is preferable that the silicon oxide-coated oxide particles have a haze value of 2% or less, and more preferably 1.5% or less, in a dispersion of 2% by weight as Fe ２ O ３ Such a silicon oxide-coated iron oxide dispersion liquid absorbs light rays in the ultraviolet region, reflects light rays in the near-infrared region, and further transmits light rays in the visible region. Therefore, in the use for the purpose of mixing with paint or protecting the clear layer for painting, it becomes a silicon oxide-coated iron oxide composition for paint that can defend against and protect from ultraviolet rays without impairing the vivid color development and designability of the coloring material. The silicon oxide-coated iron oxide of the present invention is iron oxide with its surface coated with silicon oxide. As an example, the TEM photograph of the silicon oxide-coated iron oxide particles prepared by the method of Example 8 described later is shown in Fig. 13, and the mapping results using STEM are shown in Fig. 14. As shown in Fig. 13, iron oxide particles with one core and a shell of silicon oxide, which uniformly coats the entire surface of the core with the shell, are observed, and a coating layer (shell) of silicon oxide with a thickness of about 1.37 nm is observed on the entire surface of the iron oxide particles serving as the core. Similarly, in Fig. 14, (a) is a dark field image (HAADF image), (b) is the mapping of oxygen (O), (c) is the mapping of iron (Fe), and (d) is the mapping of silicon (Si). For the particles observed in the HAADF image, it is observed that the respective elements are distributed throughout the particles for oxygen (O) and silicon (Si), and for iron (Fe), it is observed to be about 1.37 nm smaller in radius than the particles observed in the HAADF image. In particular, since iron oxide has photocatalytic ability, in a state where it is not coated with silicon oxide, it may decompose various components such as coloring materials, resins, and dispersants contained in paints and coatings by absorbing ultraviolet light and exhibiting photocatalytic ability. Therefore, in the present invention, silicon oxide-coated iron oxide with its surface coated with silicon oxide is used. Note that the coating can be carried out by coating at least a part of the particles serving as the core, not necessarily the entire particles serving as the core. Furthermore, when the surface of iron oxide is coated with crystalline silicon oxide, the reflectance of light rays with wavelengths of 400 to 620 nm may increase due to the influence on the refractive index and the like. In the present invention, by coating the surface of the above iron oxide with amorphous silicon oxide, there is an effect that the reflectance of light rays can be reduced to less than 18% in the region of the above wavelengths of 400 to 620 nm, and since reflection other than red can be reduced, the performance is improved when used for red paints, and furthermore, the transmission spectrum characteristics and transparency of the above dispersion can be achieved, which is preferable. The amorphous silicon oxide may be in the state of SiO ２ or may be in a state where a part of oxygen is deficient as in SiO ２―X . In the present invention, it is also considered that the molar extinction coefficient of the silicon oxide-coated iron oxide particles with respect to light rays having a wavelength of 200 to 420 nm is higher than that of conventional ones, which is one of the factors that led to the completion of the present invention. A graph of the molar extinction coefficient with respect to the measurement wavelength calculated from the measurement wavelength of the absorption spectrum of a dispersion obtained by dispersing the silicon oxide-coated iron oxide particles obtained in Example 8 described later in propylene glycol is shown in FIG. 21. In the present invention, the molar extinction coefficient of the above iron oxide particles with respect to light rays having a wavelength of 400 nm is preferably 500 L / (mol·cm) or more, and the molar extinction coefficient with respect to light rays having a wavelength of 220 nm is preferably 3000 L / (mol·cm) or more. The molar extinction coefficient can be calculated by the following formula 2 from the absorbance and the molar concentration in the measurement of the ultraviolet-visible absorption spectrum. ε = A / (c·l) (Formula 2) Here, ε is a constant inherent to the substance, called the molar extinction coefficient, and is the reciprocal of the ratio of the intensity of light when light passes through a 1 mol / L solution having a thickness of 1 cm, so the unit is L / (mol·cm). A is the absorbance in the measurement of the ultraviolet-visible absorption spectrum, c is the molar concentration (mol / L) of the sample. l is the length through which light passes (optical path length), which is usually the thickness of the cell when measuring the ultraviolet-visible absorption spectrum. In addition to the primary particle size of the silicon oxide-coated iron oxide used in the silicon oxide-coated iron oxide composition for paints according to the present invention being smaller than that of conventional ones that are generally in practical use, the molar extinction coefficient is higher than that of conventional ones. The molar extinction coefficient is the light absorption ability per unit mole of iron oxide. When the molar extinction coefficient of the silicon oxide-coated iron oxide particle dispersion with respect to light rays having a wavelength of 400 nm is 500 L / (mol·cm) or more, and the molar extinction coefficient with respect to light rays having a wavelength of 220 nm is 3000 L / (mol·cm) or more, the performance as a silicon oxide-coated iron oxide composition for paints can be properly exhibited, and when used by methods such as coating, the need for a large amount can be suppressed, and the possibility of impairing the haze value and transmittance can be reduced. In the silicon oxide-coated iron oxide of the present invention, not only is the surface area increased because the primary particle size is smaller than that of the conventional ones, but also the high crystallinity of the core iron oxide particles is considered to be a factor contributing to the high molar extinction coefficient. Note that the shape of the particles has a smaller influence compared to the above factors and may be of various shapes, but it is desirable to be substantially spherical because it has the effect of reducing birefringence in coating. The silicon oxide-coated iron oxide in the present invention is preferably substantially spherical particles with an aspect ratio (major axis / minor axis) in the range of 1.0 to 3.0, preferably 1.0 to 2.5, and more preferably 1.0 to 2.0. The silicon oxide-coated iron oxide in the present invention is preferably silicon oxide-coated iron oxide particles with a primary particle size of 1 to 50 nm, and more preferably 1 to 20 nm. In the present invention, in the silicon oxide-coated iron oxide where the core oxide is iron oxide particles and the shell oxide is silicon oxide, the iron oxide particles are preferably α-Fe ２ O ３ (hematite). Therefore, the iron ions contained in the core oxide raw material are preferably Fe ３＋ , and as the core oxide raw material, it is preferable to use a substance that generates Fe ３＋ ions in solution. However, a substance that generates Fe ２＋ ions is dissolved in a solvent, and Fe ２＋ ions are converted to Fe ３＋The core oxide raw material may be prepared by using means such as changing it into ions. In the present invention, regarding the silicon oxide-coated iron oxide particles constituting the silicon oxide-coated iron oxide composition for paints, it is only necessary to exhibit the characteristics of the silicon oxide-coated iron oxide composition for paints in the present invention, and it is not limited to the method for producing core-shell type oxide particles using the above microreactor. For example, iron oxide in the core part is produced using a first microreactor, and then at least a part of the surface of the iron oxide particles serving as the core is coated with silicon oxide serving as the shell using a second microreactor following this, or a method of producing iron oxide particles by performing a reaction in a dilute system in a batch container, and subsequently performing a coating of silicon oxide on at least a part of the surface of the above-described iron oxide particles by a reaction in a dilute system, or a method of producing iron oxide particles by a grinding method such as a bead mill, and then performing a coating of silicon oxide on at least a part of the surface of the iron oxide particles in a reaction vessel, and the like can be mentioned. Further, the apparatus and method described in Japanese Patent Application Laid-Open No. 2009-112892 proposed by the applicant of the present application may also be used. The paint composition that can incorporate the silicon oxide-coated iron oxide composition for paints of the present invention is not particularly limited other than those described in Patent Documents 10 or 11, and can be applied to various paint compositions such as solvent-based paints and water-based paints. In addition to various resin components, the paint composition can further appropriately contain additives such as pigments, dyes, wetting agents, dispersants, anti-color separation agents, leveling agents, viscosity modifiers, anti-skinning agents, anti-gelling agents, defoamers, thickeners, anti-sagging agents, anti-fungal agents, ultraviolet absorbers, film-forming aids, surfactants, etc. as necessary. Examples of the resin component may include polyester-based resins, melamine-based resins, phenol-based resins, epoxy-based resins, vinyl chloride-based resins, acrylic-based resins, urethane-based resins, silicon-based resins, fluorine-based resins, and the like. As the coated body to which the paint containing the silicon oxide-coated iron oxide composition of the present invention is applied, it may be a single-layer coated body composed of a single paint composition, or a multi-layer coated body composed of a plurality of paint compositions. Further, it can be carried out by including it in a paint containing a pigment, or it can also be carried out by including it in a paint such as a clear paint containing no pigment. As the color of the coated body, it can be suitably blended in a paint composition used for a coated body having a red color system, for example, a color having a hue of RP to YR in the Munsell color phase ring (including metallic colors), but it is not limited thereto, and other color phase colors may be used. Pigments and dyes that may be contained in the paint composition as necessary can be various pigments and dyes. For example, all pigments and dyes registered in the Color Index can be used. Among them, for example, for pigments and dyes that constitute red, in the Color Index, C.I. Pigments classified as C.I. Pigment Red, as well as dyes, C.I. Pigment Violet and C.I. Pigments classified as C.I. Pigment Orange and the like. More specifically, quinacridone pigments such as C.I. Pgment Red 122 and C.I. Pgment Violet 19, diketopyrrolopyrrole pigments such as C.I. Pgment Red 254 and C.I. Pigment Orange 73, naphthol pigments such as C.I. Pigment Red 150 and C.I. Pigment Red 170, perylene pigments such as C.I. Pigment Red 123 and C.I. Pigment Red 179, and azo pigments such as C.I. Pigment Red 144 and the like can be mentioned. These pigments and dyes that constitute red may be used alone or in combination of a plurality. In addition, the silicon oxide-coated iron oxide of the present invention can also be used alone without being mixed with the above-mentioned pigments and dyes that constitute red. Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples. In the following examples, the liquid A refers to the first fluid to be treated introduced from the first introduction part d1 of the apparatuses shown in FIGS. 1(A) and 1(B), and the liquid B refers to the second fluid to be treated introduced from the second introduction part d2 of the apparatuses (A) and (B). Also, the liquid C refers to the third fluid to be treated introduced from the third introduction part d3 of the apparatus (A). (Example 1) Using a high-speed rotary dispersion emulsifier, ClearMix (product name: CLM-2.2S, manufactured by M Technique), a core oxide raw material solution, an oxide precipitation solvent, and a shell oxide raw material solution were prepared. Specifically, based on the formulation of the core oxide raw material solution shown in Example 1 of Table 1, each component of the core oxide raw material solution was homogeneously mixed using ClearMix at a preparation temperature of 40°C and a rotor rotation speed of 20,000 rpm for 30 minutes to prepare the core oxide raw material solution. Also, based on the formulation of the oxide precipitation solvent shown in Example 1 of Table 1, each component of the oxide precipitation solvent was homogeneously mixed using ClearMix at a preparation temperature of 45°C and a rotor rotation speed of 15,000 rpm for 30 minutes to prepare the oxide precipitation solvent. Further, based on the formulation of the shell oxide raw material solution shown in Example 1 of Table 1, each component of the shell oxide raw material solution was homogeneously mixed using ClearMix at a preparation temperature of 20°C and a rotor rotation speed of 6,000 rpm for 10 minutes to prepare the shell oxide raw material solution. Regarding the substances indicated by the chemical formulas and abbreviations described in Table 1, MeOH is methanol (manufactured by Godo), 97 wt% H ２ SO ４ is concentrated sulfuric acid (manufactured by Kishida Chemical), KOH is potassium hydroxide (manufactured by Nippon Soda), 35 wt% HCl is hydrochloric acid (manufactured by Kanto Chemical), TEOS is tetraethyl orthosilicate (manufactured by Wako Pure Chemical Industries), 60 wt% HNO ３ is nitric acid (manufactured by Kanto Chemical), and ZnO is zinc oxide (manufactured by Kanto Chemical) were used. Next, the prepared oxide raw material liquid for the core, the oxide precipitation solvent, and the oxide raw material liquid for the shell were mixed using the fluid treatment apparatus shown in Fig. 1(A). Specifically, the oxide precipitation solvent was introduced as liquid A between the treatment surfaces 1 and 2, and while operating the treatment unit 10 at a rotational speed of 1130 rpm, the oxide raw material liquid for the core was introduced as liquid B between the treatment surfaces 1 and 2. The oxide precipitation solvent and the oxide raw material liquid for the core were mixed in a thin film fluid, and oxide particles serving as the core were precipitated between the treatment surfaces 1 and 2. Next, the oxide raw material liquid for the shell was introduced as liquid C between the treatment surfaces 1 and 2 and mixed with the mixed fluid containing the oxide particles serving as the core in the thin film fluid. As a result, the oxide for the shell was precipitated on the surface of the oxide particles serving as the core, and a fluid containing core-shell type oxide particles (hereinafter, core-shell type oxide particle dispersion liquid) was discharged from between the treatment surfaces 1 and 2 of the fluid treatment apparatus. The discharged core-shell type oxide particle dispersion liquid was collected in the beaker b via the vessel v. The time from when liquid B was introduced between the treatment surfaces 1 and 2 until the core-shell type oxide particle dispersion liquid was discharged from the treatment surfaces 1 and 2 was 0.5 seconds for most of the particles and within 1 second. Table 2 shows the operating conditions of the fluid treatment apparatus. The introduction temperature (liquid feeding temperature) and the introduction pressure (liquid feeding pressure) of liquid A, liquid B, and liquid C shown in Table 2 were measured using a thermometer and a pressure gauge provided in the sealed introduction paths (the first introduction part d1, the second introduction part d2, and the third introduction path d3) communicating between the treatment surfaces 1 and 2. The introduction temperature of liquid A shown in Table 2 is the actual temperature of liquid A under the introduction pressure in the first introduction part d1. Similarly, the introduction temperature of liquid B is the actual temperature of liquid B under the introduction pressure in the second introduction part d2, and the introduction temperature of liquid C is the actual temperature of liquid C under the introduction pressure in the third introduction part d3. For pH measurement, a pH meter of model number D-51 manufactured by HORIBA was used. Before introducing liquid A, liquid B, and liquid C into the fluid treatment apparatus, their pH values were measured at room temperature. Also, since it was difficult to measure the pH of the mixed fluid immediately after mixing the oxide raw material liquid for the core and the oxide precipitation solvent, and the pH of the fluid containing the oxide fine particles serving as the core and the oxide raw material liquid for the shell immediately after mixing, the pH of the core-shell type oxide particle dispersion liquid discharged from the apparatus and collected in the beaker b was measured at room temperature. A dry powder and a wet cake sample were prepared from the core-shell type oxide particle dispersion discharged from the fluid treatment apparatus and collected in the beaker b. The preparation method was carried out according to the conventional method of this kind of treatment. The discharged core-shell type oxide particle dispersion was collected, the core-shell type oxide particles were sedimented to remove the supernatant, and then, washing and sedimentation with a mixed solution of 100 parts by weight of pure water and 100 parts by weight of methanol were repeated three times, and then, washing and sedimentation with pure water were repeated three times to wash the core-shell type oxide particles. A part of the finally obtained wet cake of the core-shell type oxide particles was dried at 25 °C for 20 hours to obtain a dry powder. The rest was used as a wet cake sample. (Preparation of Sample for TEM Observation and Sample for STEM Observation) A part of the wet cake sample of the core-shell type oxide particles after the washing treatment obtained in the examples and comparative examples was dispersed in propylene glycol and further diluted 100-fold with isopropyl alcohol (IPA). The obtained diluted solution was dropped onto a collodion membrane or a microgrid and dried to obtain a sample for TEM observation or a sample for STEM observation. (Transmission Electron Microscope and Energy Dispersive X-ray Analyzer: TEM-EDS Analysis) For the observation and quantitative analysis of core-shell type oxide particles by TEM-EDS analysis, a transmission electron microscope, JEM-2100 (manufactured by JEOL), equipped with an energy dispersive X-ray analyzer, JED-2300 (manufactured by JEOL), was used. As the observation conditions, the acceleration voltage was set to 80 kV and the observation magnification was set to 10,000 times or more. The particle diameters (D) described in Tables 2 and 4 are the primary particle diameters, which were calculated from the distance between the outermost peripheries of the core-shell type oxide particles, and the average value of the results of measuring the particle diameters of 100 particles was shown. The core particle diameters (Dc) described in Tables 2 and 4 are the primary particle diameters, which were calculated from the distance between the outermost peripheries of the oxide particles that form the core of the core-shell type oxide particles, and the average value of the results of measuring the core particle diameters of 100 particles was shown. Also, EDS analysis was performed on each particle, and the molar ratio of the elements contained in the core oxide particles and the elements contained in the shell oxide was calculated. Further, the thickness of the shell oxide (hereinafter also referred to as the thickness of the shell layer) was measured. Four measurements were made for each particle, and the average value of 10 particles was described in the item "coating thickness" in Table 2 or Table 4. Hereinafter, the oxide particles that form the core are also referred to as the core, and the oxide that forms the shell is also referred to as the shell or the shell layer. (Scanning Transmission Electron Microscope and Energy Dispersive X-ray Analyzer: STEM-EDS Analysis) For the mapping and quantification of the elements contained in the core-shell type oxide particles by STEM-EDS analysis, an atomic resolution analytical electron microscope, JEM-ARM200F (manufactured by JEOL), equipped with an energy dispersive X-ray analyzer, Centurion (manufactured by JEOL), was used. As the observation conditions, the acceleration voltage was set to 80 kV, the observation magnification was set to 50,000 times or more, and analysis was performed using a beam diameter of 0.2 nm. (X-ray Diffraction Measurement) For the X-ray diffraction (XRD) measurement, a powder X-ray diffraction measurement device, EMPYREAN (manufactured by the PANalytical business unit of Spectris Co., Ltd.), was used. The measurement conditions were as follows: measurement range: 10 to 100 [°2Theta], Cu counter cathode, tube voltage 45 kV, tube current 40 mA, scanning speed 0.3° / min. XRD measurement was performed using the dried powder of the core-shell type oxide particles obtained in each example and comparative example. The molar ratio (shell / core) described in Table 2 is the value obtained by converting the molar ratio of the elements calculated by TEM-EDS analysis for one core-shell type oxide particle into the oxides of the respective elements. For example, the molar ratio (shell / core, SiO ２ / ZnO) in Example 1 of Table 2 is the value obtained by converting the molar ratio of Si / Zn calculated by TEM-EDS analysis for one core-shell type oxide particle into SiO ２ / ZnO, and the average value of the results of calculating the molar ratio (SiO ２ / ZnO) for 10 particles is shown together with the calculated value. The calculated value was obtained from the concentration and introduction flow rate of Zn in the oxide raw material solution for the core, and the concentration and introduction flow rate of Si in the oxide raw material solution for the shell. The TEM photograph of the core-shell type oxide particles obtained in Example 1 is shown in Fig. 3. Core-shell type zinc oxide particles in which the core is one zinc oxide particle and the shell is silicon oxide, and the entire surface of the core is uniformly coated with the shell, were observed, and a coating layer (shell layer) of silicon oxide with a thickness of about 1.8 nm was observed on the entire surface of the zinc oxide particles serving as the core. Also, for the silicon oxide-coated zinc particles obtained in Example 1, the mapping results using STEM are shown in Fig. 4. In Fig. 4, (a) is a dark field image (HAADF image), (b) is the mapping of oxygen (O), (c) is the mapping of zinc (Zn), and (d) is the mapping of silicon (Si). For the particles observed in the HAADF image, for oxygen (O) and silicon (Si), the distribution of the respective elements throughout the particles was observed, and for zinc (Zn), it was observed to be about 1.8 nm smaller in radius than the particles observed in the HAADF image. D / Dc was 122.1%. The XRD measurement results of the silicon oxide-coated zinc oxide particles obtained in Example 1 are shown in Fig. 5. In the measurement results, peaks derived from zinc oxide (ZnO) were observed, and no other peaks were observed. Also, Fig. 6 shows the IR (infrared absorption spectrum) measurement results of the silicon oxide-coated zinc oxide particles obtained in Example 1, for silicon dioxide (SiO ２), and is shown together with the IR measurement results of zinc oxide (ZnO). As seen in FIG. 6, for the silicon oxide-coated zinc oxide particles obtained in Example 1, a broad peak was observed around 900 cm －１ . This is a peak not seen in zinc oxide (ZnO), but is observed on the lower wavenumber side than the peak around 1000 cm ２ seen in SiO －１ . Therefore, the silicon oxide of the silicon oxide-coated zinc oxide particles obtained in Example 1 may be in a state where some oxygen is deficient, such as SiO ２ or SiO ２－ｘ . Note that for the IR measurement, a Fourier transform infrared spectrophotometer, FT / IR-4100 (manufactured by JASCO Corporation) was used. The measurement conditions were as follows: the ATR method was used, the resolution was 4.0 cm －１ , and the number of accumulations was 1024 times. (Examples 2 to 6, Comparative Examples 1 to 4: Using the apparatus of FIG. 1(A)) Similar to Example 1, using the apparatus of Fig. 1(A), the formulations of the oxide raw material liquid for the core, the oxide precipitation solvent, and the oxide raw material liquid for the shell, as described in Table 1, and the operating conditions (introduction flow rate, introduction temperature, introduction pressure) described in Table 2 were implemented, and core-shell type oxide particles were precipitated between the treatment surfaces 1 and 2. From the core-shell type oxide particle dispersion liquid discharged from the fluid treatment apparatus and collected in the beaker b via the vessel v, a dry powder and a wet cake sample were prepared. When TEM observation, XRD measurement, etc. were performed in the same procedure as in Example 1, the results as described in Table 2 were obtained. Regarding the conditions not described in Table 2, they were the same as in Example 1. The time from introducing the B liquid between the treatment surfaces 1 and 2 until discharging the core-shell type oxide particle dispersion liquid from the treatment surfaces 1 and 2 was within 1 second because it was 0.5 seconds for most of the particles. Fig. 7 shows the XRD measurement results of the particles obtained in Comparative Examples 2 to 4. In Examples 1 to 6, it was found that by changing each formulation and the operating conditions of the fluid treatment apparatus, the particle diameter, the core particle diameter, and the thickness of the oxide serving as the shell (coating thickness) can be changed. Also, as in Comparative Example 1, even when using the same substances as in Example 2, when the flow rate of the B liquid is larger than the flow rate of the A liquid, many zinc oxide particles not coated with silicon oxide are seen, and uniform coating treatment is impossible. Therefore, the particle diameter (D) was not measured. In Comparative Examples 2 to 4, cases were observed where it consisted only of the oxide particles serving as the core and only of the particles composed of the components that should have become the shell, and cases where such particles were included. Also, when the pH of the discharged liquid became 12 or less as in Comparative Examples 2 to 4, as seen in Fig. 7, substances other than zinc oxide were also produced as the oxide particles serving as the core, which is not preferable. However, the present application is not limited to the pH of the discharged liquid, and by changing the formulations of the A, B, and C liquids and the operating conditions of the fluid treatment apparatus, the particle diameter, the core particle diameter, and the thickness of the oxide serving as the shell (coating thickness) can be controlled. (Comparative Example 5) Using the same formulation as in Example 2, the oxide raw material liquid for the core, the oxide precipitation solvent, and the oxide raw material liquid for the shell were prepared using a high-speed rotary dispersion emulsifier, ClearMix (product name: CLM - 2.2S, manufactured by M Technique). Next, the prepared oxide raw material liquid for the core and the oxide precipitation solvent were mixed in the fluid treatment apparatus shown in FIG. 1(B). Specifically, as liquid A, an oxide precipitation solvent (MeOH 93.70 / 97 wt% H ２ SO ４ 6.30) (weight ratio) was introduced between the treatment surfaces 1 and 2 at 28° C. and 460 ml / min, and while operating the treatment unit 10 at a rotational speed of 1130 rpm, as liquid B, an oxide raw material liquid for the core (ZnO 3.0 / KOH 46.6 / pure water 50.4) (weight ratio) was introduced between the treatment surfaces 1 and 2 at 22° C. and 40 ml / min and mixed in the thin film fluid, and oxide particles (zinc oxide particles) serving as the core were precipitated between the treatment surfaces 1 and 2. The discharged liquid containing zinc oxide particles serving as the core (hereinafter, zinc oxide particle dispersion liquid) was discharged from between the treatment surfaces 1 and 2 of the fluid treatment apparatus (step 1). The discharged zinc oxide particle dispersion liquid was collected in the beaker b via the vessel v. The pH of the discharged liquid was 13.91 (measurement temperature 28.1° C.). The zinc oxide particles in the collected zinc oxide particle dispersion liquid had already aggregated. Regarding the operating conditions, they were made the same as in Example 2 except for the oxide raw material liquid for the shell. Next, the recovered zinc oxide particle dispersion and the oxide raw material solution for the shell (MeOH 94.61 / 35 wt% HCl 5.11 / TEOS 0.28) (weight ratio) were mixed using a high-speed rotary disperser, Clearmix (product name: CLM-2.2S, manufactured by M Technique). Specifically, 500 parts by weight of the dispersion containing the zinc oxide particle dispersion in the beaker was stirred at a rotor rotation speed of 10,000 rpm using Clearmix at a temperature of 25°C, and 125 parts by weight of the oxide raw material solution for the shell was added to beaker b and stirred for 30 minutes to uniformly mix the dispersion containing the zinc oxide particles and the oxide raw material solution for the shell, precipitate silicon oxide on the surface of the zinc oxide particles, and perform a treatment to coat the surface of the zinc oxide particles with silicon oxide (Step 2). The pH of the fluid after mixing was 13.79 (measurement temperature 28.1°C). In Comparative Example 5, the time required to transition from Step 1 to Step 2, that is, the time until 125 parts by weight of the oxide raw material solution for the shell was started to be added to the beaker b while stirring 500 parts by weight of the dispersion containing the zinc oxide particles in the beaker b was 2 minutes. A dry powder and a wet cake sample were prepared from the dispersion in beaker b. For the core-shell type oxide particles prepared by the method of Comparative Example 5, as a result of TEM observation, zinc oxide particles uniformly coated with silicon oxide over the entire surface of one zinc oxide particle, as obtained in Examples 1 to 6, were not observed, and many particles in which a plurality of zinc oxide particles were covered with a silicon oxide shell were also observed, and the D / Dc of the particles was 334%. A TEM photograph of the core-shell type oxide particles obtained in Comparative Example 5 is shown in FIG. 8. As seen in FIG. 8, an aggregate of primary particles of zinc oxide as the core is seen to be coated with silicon oxide as the shell. (Example 7) Under the same conditions as in Comparative Example 5, using the fluid treatment apparatus of FIG. 1(B), Liquid A (oxide precipitation solvent) and Liquid B (core oxide raw material liquid) were mixed in the thin-film fluid formed between the treatment surfaces 1 and 2, and zinc oxide particles were precipitated between the treatment surfaces 1 and 2, and a discharge liquid containing zinc oxide particles (hereinafter, zinc oxide particle dispersion) was discharged from between the treatment surfaces 1 and 2 of the fluid treatment apparatus. Next, as the third fluid, a shell oxide raw material liquid was introduced from an inlet (not shown) provided in the vessel v at 25° C. and 125 ml / min, and the zinc oxide dispersion immediately after being discharged from between the treatment surfaces 1 and 2 and the shell oxide raw material liquid were mixed inside the vessel v and collected in the beaker b. The pH of the dispersion collected in the beaker b was 13.92 (measurement temperature 28.4° C.). A dry powder and a wet cake sample were prepared from the dispersion collected in the beaker b. Incidentally, the time from when Liquid B was introduced between the treatment surfaces 1 and 2 until the zinc oxide particle dispersion was discharged from the treatment surfaces 1 and 2 and the mixed fluid of the discharged zinc oxide dispersion and the shell oxide raw material liquid was collected in the beaker b was 0.8 seconds for most of the particles and within 1 second. Regarding the core-shell type oxide particles produced by the method of Example 7, as a result of TEM observation, zinc oxide particles in which the entire surface of one zinc oxide particle was uniformly coated with silicon oxide, that is, silicon oxide-coated zinc oxide particles, as obtained in Examples 1 to 6, were observed. The particle diameter (D) was 26.5 nm, the thickness of the oxide serving as the shell (coating thickness) was 2.3 nm, and D / Dc of the silicon oxide-coated zinc oxide particles was 121.0%. (Comparative Example 6) Using the same formulation as in Example 2, a core oxide raw material liquid, an oxide precipitation solvent, and a shell oxide raw material liquid were prepared using a Clearmix (product name: CLM-2.2S, manufactured by M Technique), which is a high-speed rotary dispersion emulsification apparatus. Next, the prepared core oxide raw material liquid and the oxide precipitation solvent were stirred and mixed in a beaker using a magnetic stirrer. The stirring speed of the magnetic stirrer was 600 rpm. Specifically, the oxide precipitation solvent (MeOH 93.70 / 97 wt% H ２ SO ４While stirring 4600 ml of (weight ratio) 6.30 at 28°C, 400 ml of the oxide raw material liquid for the core (ZnO 3.0 / KOH 46.6 / pure water 50.4) (weight ratio) was introduced at 22°C over 10 minutes to precipitate zinc oxide particles, obtaining a zinc oxide particle dispersion. The pH of the zinc oxide particle dispersion was 13.89 (measurement temperature 28.1°C). The zinc oxide particles in the obtained zinc oxide particle dispersion had already aggregated. Next, using the fluid treatment apparatus of FIG. 1(B), the obtained zinc oxide particle dispersion was introduced as Liquid A at 33°C and 500 ml / min, and while operating the treatment section 10 at a rotational speed of 1130 rpm, the oxide raw material liquid for the shell (MeOH 94.61 / 35 wt% HCl 5.11 / TEOS 0.28) (weight ratio) was introduced as Liquid B between the treatment surfaces 1 and 2 at 25°C and 125 ml / min to mix the zinc oxide particle dispersion and the oxide raw material liquid for the shell in the thin film fluid. A discharge liquid containing core-shell type oxide particles in which the oxide for the shell was deposited on the surface of the zinc oxide particles (hereinafter referred to as the core-shell type oxide particle dispersion) was discharged from between the treatment surfaces 1 and 2 of the fluid treatment apparatus. The discharged core-shell type oxide particle dispersion was collected in beaker b via vessel v. The pH of the discharge liquid was 13.80 (measurement temperature 28.3°C). The time from setting the obtained zinc oxide particle dispersion in the fluid treatment apparatus until discharging the dispersion from the treatment surfaces 1 and 2 was about 5 minutes for most of the particles, and the treatment time of Comparative Example 6 was 10 minutes in total. A dry powder and a wet cake sample were prepared from the dispersion collected in beaker b. For the core-shell type oxide particles prepared by the method of Comparative Example 6, as a result of TEM observation, zinc oxide particles uniformly coated with silicon oxide over the entire surface of one zinc oxide particle, as obtained in Examples 1 to 6, were not observed, and many particles in which a plurality of zinc oxide nanoparticles were covered with a shell of silicon oxide were observed, and the D / Dc of those particles was 396%. (Comparative Example 7) For comparison with zinc oxide particles whose surfaces were coated with silicon oxide, zinc oxide particles whose surfaces were not coated with silicon oxide were prepared. Using the same formulation as the oxide raw material liquid for the core and the oxide precipitation solvent in Example 2, a CLEARMIX (product name: CLM-2.2S, manufactured by M Technique), which is a high-speed rotary dispersion emulsification device, was used to prepare the oxide raw material liquid for the core and the oxide precipitation solvent. Next, using the fluid treatment apparatus shown in Fig. 1(B), zinc oxide particles were produced under the following conditions with the prepared oxide raw material liquid for the core and the oxide precipitation solvent. Specifically, as Liquid A, an oxide precipitation solvent (MeOH 93.70 / 97 wt% H ２ SO ４ 6.30) (weight ratio) was introduced between the treatment surfaces 1 and 2 at 22°C and 460 ml / min, and while operating the treatment unit 10 at a rotational speed of 1130 rpm, as Liquid B, a zinc oxide raw material liquid (ZnO 3.0 / KOH 46.6 / pure water 50.4) (weight ratio) was introduced between the treatment surfaces 1 and 2 at 28°C and 40 ml / min. The oxide precipitation solvent and the oxide raw material liquid for the core were mixed in the thin film fluid, and zinc oxide particles were precipitated between the treatment surfaces 1 and 2. A discharge liquid (zinc oxide particle dispersion liquid) containing zinc oxide particles was discharged from between the treatment surfaces 1 and 2. The particle washing method, analysis / evaluation method, etc. were the same as in Example 1. The particle diameter measured by the same method as the core particle diameter in Example 1 was 10.1 nm, and from the XRD measurement results, only the peak of zinc oxide was detected. The pH of the discharge liquid was 13.92 (measurement temperature 28.2°C). The zinc oxide particles in the obtained zinc oxide particle dispersion liquid had already aggregated. The dispersion obtained by dispersing zinc oxide particles coated with silicon oxide obtained under the conditions of Example 2 in propylene glycol at a concentration of 0.00185 mol / L and the dispersion obtained by dispersing zinc oxide particles obtained under the conditions of Comparative Example 7 in propylene glycol at a concentration of 0.00185 mol / L were each prepared, and the ultraviolet-visible absorption spectroscopic measurement results of both are shown in Fig. 9 (transmittance) and Fig. 10 (absorbance). As can be seen from Figs. 9 and 10, the dispersion obtained by dispersing zinc oxide particles uniformly coated with silicon oxide over the entire surface obtained in Example 2 strongly absorbs light with wavelengths in the absorption region of 200 to 350 nm and is more likely to transmit light with wavelengths in the transmission region of 370 to 800 nm compared to the dispersion obtained by dispersing zinc oxide particles obtained in Comparative Example 8. It is considered that by uniformly coating the entire surface of one zinc oxide particle with silicon oxide, the affinity with the solvent is improved and the dispersibility of the particles is improved. (Photocatalytic ability evaluation) The photocatalytic abilities of the particles obtained in Examples 1 to 7 and Comparative Examples 1 to 7 were evaluated. Specifically, the particles obtained in each example or comparative example were dispersed in propylene glycol in which methylene blue dye was dissolved, irradiated with ultraviolet light of 365 nm for 2 hours, and the ultraviolet-visible absorption spectra before and after irradiation were measured. For the irradiation with ultraviolet light of 365 nm, a transilluminator (manufactured by VILBER LOURMAT: TFX20CL) was used. The photocatalytic ability was evaluated by the attenuation rate (Equation 1) of the absorbance with respect to light near the wavelength of 660 nm derived from the absorption peak of the methylene blue dye. The concentration of methylene blue in propylene glycol was set so that the absorbance at a wavelength of 660 nm was around 1 in the measurement before ultraviolet light irradiation, and the dispersion concentration of the particles was 5×10 －５It was set to mol / L. The measurement results of the absorption spectra of the particles obtained under the conditions of Example 1 before and after ultraviolet light irradiation are shown in Fig. 11, and the measurement results of the absorption spectra of the particles obtained under the conditions of Comparative Example 7 before and after ultraviolet light irradiation are shown in Fig. 12. For the zinc oxide particles coated with silicon oxide obtained in Example 1, there was almost no change in the absorbance at a wavelength of 660 nm before and after irradiating with ultraviolet light at 365 nm for 2 hours (the maximum absorbance after irradiation was 0.90 to 1.00 relative to the maximum absorbance before irradiation). However, in the case of the particles prepared in Comparative Example 7, the absorbance had decayed, and the maximum absorbance after irradiation had dropped to less than 0.90 relative to the maximum absorbance before irradiation. The particles prepared under the conditions of Comparative Examples 1 to 6 also showed the same tendency as Comparative Example 7. For the particles prepared under the conditions of Examples 1 to 7, by uniformly coating the entire surface of one zinc oxide particle with silicon oxide, the photocatalytic ability of the zinc oxide particles could be suppressed. However, for the particles prepared under the conditions of Comparative Examples 1 to 6, since there were those in which the surface of the zinc oxide particles was not coated with silicon oxide or those in which the surfaces of multiple zinc oxide particles were coated with silicon oxide, it was found that the photocatalytic ability of zinc oxide could not be suppressed. (Example 8) Using a high-speed rotary dispersion emulsification device, ClearMix (product name: CLM-2.2S, manufactured by M Technique), a core oxide raw material liquid, an oxide precipitation solvent, and a shell oxide raw material liquid were prepared. Specifically, based on the formulation of the core oxide raw material liquid shown in Example 8 of Table 3, each component of the core oxide raw material liquid was homogeneously mixed by using ClearMix and stirring at a preparation temperature of 40 °C and a rotor rotation speed of 20,000 rpm for 30 minutes to prepare the core oxide raw material liquid. Also, based on the formulation of the oxide precipitation solvent shown in Example 8 of Table 3, each component of the oxide precipitation solvent was homogeneously mixed by using ClearMix and stirring at a preparation temperature of 45 °C and a rotor rotation speed of 15,000 rpm for 30 minutes to prepare the oxide precipitation solvent. Further, based on the formulation of the shell oxide raw material liquid shown in Example 8 of Table 3, each component of the shell oxide raw material liquid was homogeneously mixed by using ClearMix and stirring at a preparation temperature of 20 °C and a rotor rotation speed of 6,000 rpm for 10 minutes to prepare the shell oxide raw material liquid. In addition, for the substances indicated by the chemical formulas and abbreviations shown in Table 3, 97 wt% H ２ SO ４ is concentrated sulfuric acid (manufactured by Kishida Chemical Co., Ltd.), NaOH is sodium hydroxide (manufactured by Kanto Chemical Co., Inc.), TEOS is tetraethyl orthosilicate (manufactured by Wako Pure Chemical Industries, Ltd.), and Fe(NO ３ ) ３ ・9H ２ O is iron(III) nitrate nonahydrate (manufactured by Kanto Chemical Co., Inc.). The particle washing method and the analysis / evaluation method are the same as those in Example 1. Next, the prepared core oxide raw material liquid, the oxide precipitation solvent, and the shell oxide raw material liquid were mixed in the fluid treatment apparatus shown in Fig. 1(A). Specifically, the core oxide raw material liquid was introduced between the treatment surfaces 1 and 2 as liquid A, and while operating the treatment unit 10 at a rotational speed of 1130 rpm, the oxide precipitation solvent was introduced between the treatment surfaces 1 and 2 as liquid B, and the core oxide raw material and the oxide precipitation solvent were mixed in a thin film fluid, and oxide particles serving as the core were precipitated between the treatment surfaces 1 and 2. Next, the shell oxide raw material liquid was introduced between the treatment surfaces 1 and 2 as liquid C and mixed with the mixed fluid containing the oxide particles serving as the core in the thin film fluid. The shell oxide was precipitated on the surface of the oxide particles serving as the core, and the discharge liquid containing the core-shell type oxide particles (hereinafter, the core-shell type oxide particle dispersion liquid) was discharged from between the treatment surfaces 1 and 2 of the fluid treatment apparatus. The discharged core-shell type oxide particle dispersion liquid was collected in the beaker b via the vessel v. The time from when liquid B was introduced between the treatment surfaces 1 and 2 until the core-shell type oxide particle dispersion liquid was discharged from the treatment surfaces 1 and 2 was 0.4 seconds for most of the particles and within 1 second. Table 4 shows the operating conditions of the fluid treatment apparatus. The introduction temperature (liquid feeding temperature) and the introduction pressure (liquid feeding pressure) of liquid A, liquid B, and liquid C shown in Table 4 were measured using the thermometer and the pressure gauge provided in the sealed introduction path (the first introduction part d1, the second introduction part d2, and the third introduction path d3) communicating between the treatment surfaces 1 and 2. The introduction temperature of liquid A shown in Table 4 is the actual temperature of liquid A under the introduction pressure in the first introduction part d1. Similarly, the introduction temperature of liquid B is the actual temperature of liquid B under the introduction pressure in the second introduction part d2, and the introduction temperature of liquid C is the actual temperature of liquid C under the introduction pressure in the third introduction part d3. The molar ratio (shell / core) described in Table 4 is the value obtained by converting the molar ratio of the elements calculated by TEM-EDS analysis for one core-shell type oxide particle into the oxides of the respective elements. For example, in Example 8 of Table 4, the molar ratio (shell / core, SiO ２ / Fe ２ O ３ ) is the value obtained by converting the molar ratio of Si / Fe calculated by TEM-EDS analysis for one core-shell type oxide particle into SiO ２ / Fe ２ O ３ . The average value of the results of calculating the molar ratio (SiO ２ / Fe ２ O ３ ) for 10 particles is shown together with the calculated value. The calculated value was obtained from the concentration and introduction flow rate of Fe in the oxide raw material solution for the core, and the concentration and introduction flow rate of Si in the oxide raw material solution for the shell. The TEM photograph of the core-shell type oxide particles obtained in Example 8 is shown in Fig. 13. Core-shell type oxide particles in which the core is one iron oxide particle and the shell is silicon oxide, and the entire surface of the core is uniformly coated with the shell, were observed. A coating layer (shell) of silicon oxide with a thickness of about 1.37 nm was observed on the entire surface of the iron oxide particle serving as the core. Also, for the silicon oxide-coated iron oxide particles obtained in Example 8, the mapping results using STEM are shown in Fig. 14. In Fig. 14, (a) is a dark field image (HAADF image), and (b), (c), and (d) are mappings of oxygen (O), iron (Fe), and silicon (Si), respectively. For the particles observed in the HAADF image, it was observed that the respective elements were distributed throughout the particles for oxygen (O) and silicon (Si), and iron (Fe) was observed to be about 1.37 nm smaller in radius than the particles observed in the HAADF image. D / Dc was 150.2%. From the XRD measurement results of the silicon oxide-coated iron oxide particles obtained in Example 8 shown in Fig. 15, iron oxide (Fe ２ O ３Peaks derived from ) were observed, and no other peaks were observed. (Example 9, Comparative Examples 8 and 9) Similar to Example 8, using the apparatus of Fig. 1(A), the formulations of the oxide raw material liquid for the core, the oxide precipitation solvent, and the oxide raw material liquid for the shell described in Table 3, and the operating conditions (introduction flow rate, introduction temperature, introduction pressure) described in Table 4 were used to carry out the experiment, and core-shell type oxide particles were precipitated between the treatment surfaces 1 and 2. The core-shell type oxide particle dispersion liquid discharged from the fluid treatment apparatus and collected in the beaker b via the vessel v was used to prepare a dry powder and a wet cake sample. When TEM observation, XRD measurement, etc. were carried out in the same procedure as in Example 8, the results as described in Table 4 were obtained. For the conditions not described in Table 4, they were the same as in Example 8. For most particles, the time from introducing the B liquid between the treatment surfaces 1 and 2 to discharging the core-shell type oxide particle dispersion liquid from the treatment surfaces 1 and 2 was 0.3 seconds and within 1 second. In Examples 8 and 9, it was found that by changing each formulation and the operating conditions of the fluid treatment apparatus, the particle diameter, the core particle diameter, and the thickness of the oxide serving as the shell (coating thickness) can be changed. The present application is not limited to the pH of the discharged liquid, and by changing the formulations of the A, B, and C liquids and the operating conditions of the fluid treatment apparatus, the particle diameter, the core particle diameter, and the thickness of the oxide serving as the shell (coating thickness) can be controlled. Also, as in Comparative Examples 8 and 9, when the flow rate of the B liquid was higher than that of the A liquid, many iron oxide particles not coated with silicon oxide were observed, and uniform coating treatment was impossible. Therefore, the particle diameter (D) was not measured. (Comparative Example 10) Using the same formulation as in Example 9, a Clear Mix (product name: CLM-2.2S, manufactured by M Technique), a high-speed rotary dispersion emulsification apparatus, was used to prepare the oxide raw material liquid for the core, the oxide precipitation solvent, and the oxide raw material liquid for the shell. Next, the prepared oxide raw material liquid for the core and the oxide precipitation solvent were mixed in the fluid treatment apparatus shown in Fig. 1(B). Specifically, as the A liquid, the oxide raw material liquid for the core (pure water 98.00 / Fe(NO ３ ) ３ ·9H ２O₂ (2.00) (by weight) was introduced between the treatment surfaces 1 and 2 at 142 °C and 400 ml / min. While operating the treatment unit 10 at a rotational speed of 1130 rpm, the oxide precipitation solvent as Liquid B (NaOH 9.00 / pure water 91.00) (by weight) was introduced between the treatment surfaces 1 and 2 at 86 °C and 50 ml / min, and the two were mixed in the thin-film fluid. Oxide particles (iron oxide particles) serving as the core were precipitated between the treatment surfaces 1 and 2. The discharge liquid containing the iron oxide particles serving as the core (hereinafter referred to as the iron oxide particle dispersion liquid) was discharged from between the treatment surfaces 1 and 2 of the fluid treatment device. The discharged iron oxide particle dispersion liquid was collected in the beaker b via the vessel v. The pH of the discharge liquid was 12.51 (measurement temperature 32.9 °C). The iron oxide particles in the collected iron oxide particle dispersion liquid had already aggregated. Next, the collected iron oxide particle dispersion liquid and the shell oxide raw material liquid (MeOH 93.70 / 35 wt% HCl 5.11 / TEOS 1.19) (by weight) were mixed using a high-speed rotary disperser, CLEARMIX (product name: CLM-2.2S, manufactured by M TECHNIC). Specifically, 450 parts by weight of the dispersion liquid containing the iron oxide particle dispersion liquid in the beaker b was stirred at a rotor rotational speed of 10,000 rpm using CLEARMIX at a temperature of 98 °C, and 100 parts by weight of the shell oxide raw material liquid was added to the beaker b and stirred for 30 minutes to uniformly mix the dispersion liquid containing the iron oxide particles and the shell oxide raw material liquid, and a treatment was performed to precipitate silicon oxide on the surface of the iron oxide particles and coat the surface of the iron oxide particles with silicon oxide (Step 2). The pH of the fluid after mixing was 11.98 (measurement temperature 32.91 °C). In Comparative Example 10, the time required from Step 1 to the transition to Step 2, that is, the time until 100 parts by weight of the shell oxide raw material liquid was started to be added to the beaker b while stirring 450 parts by weight of the dispersion liquid containing the iron oxide particles in the beaker b was 5 minutes. A dry powder and a wet cake sample were prepared from the dispersion liquid in the beaker b. Regarding the core-shell type oxide particles prepared by the method of Comparative Example 10, as a result of TEM observation, iron oxide particles in which the entire surface of one iron oxide particle was uniformly coated with silicon oxide as obtained in Examples 8 to 9 were not observed, and many particles in which a plurality of iron oxide particles were covered with a silicon oxide shell were also observed, and the D / Dc of the particles was 412%. (Example 10) Under the same conditions as in Comparative Example 10, using the fluid treatment apparatus shown in Fig. 1(B), liquid A (core oxide raw material liquid) and liquid B (oxide precipitation solvent) were mixed in the thin film fluid formed between the treatment surfaces 1 and 2, and iron oxide particles were precipitated between the treatment surfaces 1 and 2, and a dispersion liquid containing iron oxide particles (hereinafter referred to as iron oxide particle dispersion liquid) was discharged from between the treatment surfaces 1 and 2 of the fluid treatment apparatus. Next, as the third fluid, a shell oxide raw material liquid was introduced from an inlet (not shown) provided in the vessel v at 89 °C and 100 ml / min, and the iron oxide dispersion liquid immediately after being discharged from between the treatment surfaces and the shell oxide raw material liquid were mixed inside the vessel v and collected in the beaker b. The pH of the dispersion liquid in the beaker b was 13.89 (measurement temperature 29.6 °C). A dry powder and a wet cake sample were prepared from the dispersion liquid collected in the beaker b. Note that, after introducing liquid B between the treatment surfaces 1 and 2, the iron oxide particle dispersion liquid was discharged from the treatment surfaces 1 and 2, and the time from when the discharged iron oxide particle dispersion liquid and the shell oxide raw material liquid were mixed until the mixed fluid was collected in the beaker b was 0.8 seconds for most of the particles and within 1 second. Regarding the core-shell type oxide particles produced by the method of Example 10, as a result of TEM observation, core-shell type oxide particles in which the entire surface of one iron oxide particle was uniformly coated with silicon oxide, as obtained in Examples 8 to 9, that is, silicon oxide-coated iron oxide particles were observed. The particle diameter (D) was 9.90 nm, the thickness of the oxide serving as the shell (coating thickness) was 1.17 nm, and D / Dc of the silicon oxide-coated iron oxide particles was 131.0%. (Comparative Example 11) With the same formulation as in Example 9, using a Clear Mix (product name: CLM-2.2S, manufactured by M Technique), a high-speed rotary dispersion emulsification apparatus, a core oxide raw material liquid, an oxide precipitation solvent, and a shell oxide raw material liquid were prepared. Next, the prepared core oxide raw material liquid and the oxide precipitation solvent were stirred and mixed using a stirrer in a pressure vessel. The stirring speed of the stirrer was 600 rpm. Specifically, as the first fluid to be treated, a core oxide raw material liquid (pure water 98.00 / Fe(NO ３ ) ３ ·9H ２2.00) (by weight) 4000 ml was stirred at 142 °C, and 500 ml of an oxide precipitation solvent (NaOH 9.00 / pure water 91.00) (by weight) as the second fluid to be treated was introduced at 86 °C over 10 minutes to precipitate iron oxide particles, obtaining an iron oxide particle dispersion. The pH after mixing was 12.64 (measurement temperature 33.2 °C). The iron oxide particles in the obtained iron oxide particle dispersion had already aggregated. Next, using the fluid treatment apparatus of Fig. 1(B), the obtained iron oxide particle dispersion was introduced as liquid A at 142 °C and 450 ml / min between treatment surfaces 1 and 2, and while operating treatment section 10 at a rotational speed of 1130 rpm, as liquid B, a shell oxide raw material liquid (pure water 93.70 / 97 wt% H ２ SO ４ 5.11 / TEOS 1.19) (by weight) was introduced between treatment surfaces 1 and 2 at 89 °C and 100 ml / min, and the iron oxide particle dispersion and the shell oxide raw material liquid were mixed in the thin film fluid. Shell oxide was precipitated on the surface of the iron oxide particles, and a discharge liquid containing core-shell type oxide particles (hereinafter referred to as a core-shell type oxide particle dispersion) was discharged from between treatment surfaces 1 and 2 of the fluid treatment apparatus. The discharged core-shell type oxide particle dispersion was collected in beaker b via vessel v. The pH of the discharge liquid was 11.88 (measurement temperature 32.4 °C). The time from setting the recovered iron oxide particle dispersion in the fluid treatment apparatus until discharging the dispersion from treatment surfaces 1 and 2 was 10 minutes for most of the particles, and the total treatment time of Comparative Example 11 was 20 minutes. A dry powder and a wet cake sample were prepared from the dispersion collected in beaker b. Regarding the core-shell type oxide particles prepared by the method of Comparative Example 11, as a result of TEM observation, iron oxide particles uniformly coated with silicon oxide over the entire surface of one iron oxide particle, as obtained in Examples 8 to 9, were not observed, and many particles in which a plurality of iron oxide particles were covered with a silicon oxide shell were also observed, and the D / Dc of those particles was 433%. (Comparative Example 12) For comparison with iron oxide particles whose surface was coated with silicon oxide, iron oxide particles whose surface was not coated with silicon oxide were prepared. Using the same formulation as the oxide raw material liquid and the oxide precipitation solvent for Examples 8 and 9, a Clear Mix (product name: CLM-2.2S, manufactured by M Technique), a high-speed rotary dispersion emulsifier, was used to prepare the oxide raw material liquid for the core and the oxide precipitation solvent. Next, using the fluid treatment apparatus shown in Fig. 1(B), the prepared oxide raw material liquid for the core and the oxide precipitation solvent were used to produce iron oxide particles under the following conditions. Specifically, as Liquid A, the oxide raw material liquid for the core (pure water 98.00 / Fe(NO ３ )) ３ ・9H ２ O 2.00) (weight ratio) was introduced between the treatment surfaces 1 and 2 at 142°C and 400 ml / min. While operating the treatment unit 10 at a rotational speed of 1130 rpm, as Liquid B, the oxide precipitation solvent (NaOH 9.00 / pure water 91.00) (weight ratio) was introduced between the treatment surfaces 1 and 2 at 86°C and 50 ml / min. The oxide precipitation solvent and the oxide raw material liquid for the core were mixed in the thin-film fluid, and iron oxide particles were precipitated between the treatment surfaces 1 and 2. The discharge liquid (iron oxide particle dispersion liquid) containing the iron oxide particles was discharged from between the treatment surfaces 1 and 2. The particle washing method, analysis / evaluation method, etc. were the same as those in Example 8. The particle diameter measured by the same method as the core particle diameter in Example 8 was 6.40 nm. From the XRD measurement results, only the peak of iron oxide was detected. The pH of the discharge liquid was 13.89 (measurement temperature 29.6°C). The iron oxide particles in the obtained iron oxide particle dispersion liquid had already aggregated. (Example 11) As Example 11, silicon oxide-coated iron oxide particles were produced under the same conditions as in Example 8, except that the apparatus described in JP-A-2009-112892 and the mixing and reaction method of Liquid A (oxide raw material liquid for core), Liquid B (oxide precipitation solvent), and Liquid C (oxide raw material liquid for shell) were used. Here, the apparatus described in JP-A-2009-112892 is the apparatus described in FIG. 4 of the same publication, and an apparatus with a uniform inner diameter of the stirring tank was used. The inner diameter of the stirring tank was 420 mm, the gap between the outer end of the stirrer and the inner peripheral surface of the stirring tank was 1 mm, and the rotational speed of the stirring blades was the same as the rotational speed of the treatment section of the fluid treatment apparatus shown in FIG. 1(A) used in Example 8 (1130 rpm). Further, Liquid A was introduced into the stirring tank, Liquid B was added and mixed and reacted in a thin film composed of Liquid A pressed against the inner peripheral surface of the stirring tank, and Liquid C was added and mixed and reacted in a thin film composed of a mixed liquid of Liquid A and Liquid B pressed against the inner peripheral surface of the stirring tank. As a result of TEM observation, silicon oxide-coated iron oxide particles in which the core was a single iron oxide particle and the shell was silicon oxide and a part of the surface of the core was covered with the shell were observed, and a coating layer (shell) of silicon oxide with a thickness of about 1.0 to 2.0 nm was observed on the surface of the iron oxide particle serving as the core. Similarly, for the silicon oxide-coated iron oxide particles obtained in Example 11, as a result of mapping using STEM in the same manner as in Example 8, for the particles observed in the HAADF image, for oxygen (O), it was observed that each element was distributed throughout the particles, for iron (Fe), it was observed to be about 1.0 to 2.0 nm smaller in radius than the particles observed in the HAADF image, and for silicon (Si), it was observed that it was mainly distributed only in the coating layer. The particle diameter (D) was 16.9 nm, the thickness of the oxide serving as the shell (coating thickness) was 1.0 to 2.0 nm, and D / Dc of the silicon oxide-coated iron oxide particles was 111.8 to 123.7%. From the XRD measurement results of the silicon oxide-coated iron oxide particles obtained in Example 11, peaks derived from iron oxide (Fe ２ O ３ ) were observed, and no other peaks were observed. Regarding the silicon oxide-coated iron oxide particles obtained under the conditions of Example 8 and the iron oxide particles obtained in Comparative Example 12, the transmission spectrum was obtained by adding silicon oxide-coated iron oxide to propylene glycol at Fe ２ O３ A dispersion was prepared by dispersing at a concentration of 0.05% by weight, and regarding the absorption spectrum, iron oxide coated with silicon oxide in propylene glycol was used as Fe ２ O ３ A dispersion dispersed at a concentration of 0.007% by weight (0.0005 mol / L) was prepared as a measurement sample, and each spectrum was measured. The ultraviolet-visible absorption spectroscopic measurement results of both are shown in Fig. 16 (transmittance) and Fig. 17 (absorbance). As can be seen in Figs. 16 and 17, the dispersion in which iron oxide particles uniformly coated with silicon oxide over the entire surface obtained in Example 8 strongly absorbs light with a wavelength of 200 to 400 nm in the absorption region and easily transmits light with a wavelength of 450 to 800 nm in the transmission region compared to the dispersion in which iron oxide particles obtained in Comparative Example 12 were dispersed. It is considered that by coating the entire surface of one iron oxide particle with silicon oxide, the affinity with the solvent is improved and the dispersibility of the particles is improved. Also, regarding the silicon oxide-coated iron oxide particles prepared under the conditions of Example 11, although the characteristics of the particles obtained in Example 8 were not obtained, it was found that they strongly absorb light with a wavelength of 200 to 400 nm in the absorption region and easily transmit light with a wavelength of 450 to 800 nm in the transmission region. (Photocatalytic activity evaluation) The photocatalytic activities of the particles obtained in Examples 8 to 10 and Comparative Examples 8 to 12 were evaluated. Specifically, the particles obtained in each example or comparative example were dispersed in propylene glycol in which Congo red dye was dissolved, irradiated with white light for 2 hours, and the ultraviolet-visible absorption spectra before and after irradiation were measured. For the irradiation with white light, a 250 W metal halide light source device (USHIO Lighting Co., Ltd.: UF3250NAC) equipped with a metal halide lamp (USHIO Lighting Co., Ltd.: ULRD-250ST / 9H2) was used. The photocatalytic activity was evaluated by the attenuation rate (Equation 1) of the absorbance with respect to the light with a wavelength of 505 nm derived from the absorption peak of Congo red dye. The concentration of methylene blue in propylene glycol was set so that the absorbance at a wavelength of 505 nm was around 1, and the dispersion concentration of the particles was 5×10 －５It was set to mol / L. The measurement results of the absorption spectra before and after white light irradiation of the particles obtained under the conditions of Example 8 are shown in Fig. 18, and the measurement results of the absorption spectra before and after white light irradiation of the particles obtained under the conditions of Comparative Example 12 are shown in Fig. 19. For the silicon oxide-coated iron oxide particles obtained in Example 8, there was almost no change in the absorbance at a wavelength of 505 nm before and after irradiating white light for 2 hours (taking the value before irradiation as 1, the value after irradiation was 0.98 - 1.00). However, for the particles prepared in Comparative Example 12, the absorbance had decayed, and taking the value before irradiation as 1, it had dropped to less than 0.90 after irradiation. The particles prepared under the conditions of Comparative Examples 8 - 11 also showed the same tendency as Comparative Example 12. For the particles prepared under the conditions of Examples 8 - 10, by uniformly coating the entire surface of one iron oxide particle with silicon oxide, the photocatalytic ability of the iron oxide particles could be suppressed. However, for the particles prepared under the conditions of Comparative Examples 8 - 11, since some particles had the surface of the iron oxide particles not coated with silicon oxide or some particles had the surfaces of multiple iron oxide particles coated with silicon oxide, it was found that the photocatalytic ability of iron oxide could not be suppressed. Next, further experiments were conducted on the silicon oxide-coated iron oxide particles obtained in Example 8 and Example 11 above and the dispersions in which the silicon oxide-coated iron oxide particles were dispersed in a dispersion medium. (Transmission spectrum and absorption spectrum) The transmission spectrum and absorption spectrum were measured using a visible ultraviolet absorption spectrophotometer (product name: UV-2450, manufactured by Shimadzu Corporation). The measurement range was set to 200 - 800 nm, the sampling rate was 0.2 nm, and the measurement was performed at a low speed. For the transmission spectrum, a dispersion liquid in which silicon oxide-coated iron oxide was dispersed at a concentration of 0.05 wt% with Fe ２ O ３ in propylene glycol was used as the measurement sample. For the absorption spectrum, after measuring the absorption spectrum using a dispersion liquid in which silicon oxide-coated iron oxide was dispersed at a concentration of 0.007 wt% (0.0005 mol / L) with Fe ２ O ３ in propylene glycol as the measurement sample, the absorbance obtained from the measurement results and the iron oxide concentration (Fe ２ O ３From the absorbance (as absorbance per unit path length), the molar extinction coefficient was calculated, and a graph was made with the measurement wavelength on the horizontal axis and the molar extinction coefficient on the vertical axis. For the measurement, a cell for liquids with a thickness of 1 cm was used. (Haze value measurement) For the haze value measurement, a haze meter (model: HZ-V3, manufactured by Suga Test Instruments Co., Ltd.) was used. Under optical conditions corresponding to JIS K 7136 and JIS K 7361, a double-beam method was used, and D65 light was used as the light source. The measurement was performed on the same dispersion liquid as that used for the ultraviolet-visible absorption / transmission spectrum measurement in a cell for liquids with a thickness of 1 mm. (Reflection spectrum) For the reflection spectrum, an ultraviolet-visible near-infrared spectrophotometer (product name: SolidSpec-3700, manufactured by Shimadzu Corporation) was used. The measurement range was set to 400 to 750 nm, the sampling rate was set to 2.0 nm, and the measurement was performed at a medium speed. For the background measurement when measuring the powder, a standard white plate (product name: Spectralon (trademark), manufactured by Labsphere) was used, and for the background when measuring the dispersion liquid, barium sulfate was used. As the measurement samples, as described later, powders of iron oxide coated with silicon oxide and a dispersion liquid in which iron oxide coated with silicon oxide was dispersed at a concentration of 0.31% by weight as Fe ２ O ３ were used. (Comparative Example 13) As Comparative Example 13, iron(III) oxide (α-Fe ２ O ３ ) manufactured by Wako Pure Chemical Industries, Ltd. was dispersed in propylene glycol or water in the same manner as in Example 8, and TEM observation, haze value, ultraviolet-visible transmission absorption spectrum, reflection spectrum, and XRD were measured in the same manner as in Example 8. Fig. 24 shows a TEM photograph of the iron oxide particles of Comparative Example 13. The average primary particle size was 119.6 nm. In the preparation of the TEM observation sample of Comparative Example 13, the above-mentioned commercially available iron(III) oxide (α-Fe ２ O ３ ) was used without washing. For comparison with the iron oxide particles whose surfaces were coated with silicon oxide, Comparative Example 12 was used as the iron oxide particles whose surfaces were not coated with silicon oxide. As a result of XRD measurement, α-Fe ２ O ３ (hematite) peaks were clearly detected in both Example 8, Example 11, Comparative Example 12, and Comparative Example 13. Also, in Example 8 and Example 11, as described above, no peaks were detected for the silicon oxide coated on the surface of the particles, and it is considered to be amorphous. The transmission spectrum measurement results of the dispersion obtained by dispersing the silicon oxide-coated iron oxide particles obtained in Example 8 in propylene glycol and the dispersions obtained by dispersing the iron oxide particles of Comparative Examples 12 and 13 in propylene glycol are shown in FIG. 20. The silicon oxide-coated iron oxide particle dispersion obtained in Example 8 scarcely transmitted ultraviolet light with a wavelength of 200 to 400 nm, and the transmittance at a wavelength of 420 nm was 1.64%. Also, for the silicon oxide-coated iron oxide particle dispersion obtained in Comparative Example 12, it scarcely transmitted ultraviolet light with a wavelength of 200 to 400 nm, and the transmittance at a wavelength of 420 nm was 1.73%. It was found that the silicon oxide-coated iron oxide particle dispersions obtained in Example 8 and Comparative Example 12 had a transmittance exceeding 80% at wavelengths of 620 to 800 nm. That is, it was found that the light with a wavelength of 200 to 420 nm was absorbed and the light other than that, particularly the light with a wavelength of 620 to 800 nm, was transmitted. On the other hand, the transmittance of the iron oxide particle dispersion of Comparative Example 13 was generally 10% throughout the measurement range, and no clear difference was observed in the absorption region and the transmission region. Also, the silicon oxide-coated iron oxide particle dispersion obtained in Example 11 scarcely transmitted ultraviolet light with a wavelength of 200 to 400 nm, and the transmittance at a wavelength of 420 nm was 1.89%. Regarding the transmittance at wavelengths of 620 to 800 nm, although it did not reach the characteristics of the particles obtained in Example 8, it was found to exceed 80%. The absorbance obtained from the absorption spectrum measurement results of the dispersion obtained by dispersing the silicon oxide-coated iron oxide particles obtained in Example 8 in propylene glycol and the dispersions obtained by dispersing the iron oxide particles of Comparative Examples 12 and 13 in propylene glycol and the iron oxide concentration of the measurement solution (Fe ２ O ３From the absorbance (as described above), the molar extinction coefficient was calculated, and a graph showing the measurement wavelength on the horizontal axis and the molar extinction coefficient on the vertical axis is shown in FIG. 21. As can be seen in FIG. 21, the molar extinction coefficient of the iron oxide particles obtained in Example 8 was 1193 L / (mol·cm) for light with a wavelength of 400 nm and 5479 L / (mol·cm) for light with a wavelength of 220 nm. The molar extinction coefficient of the iron oxide particles obtained in Comparative Example 12 was 952 L / (mol·cm) for light with a wavelength of 400 nm and 4488 L / (mol·cm) for light with a wavelength of 220 nm. On the other hand, the molar extinction coefficient of the iron oxide particles of Comparative Example 13 was 50 to 150 L / (mol·cm) throughout the measurement range. The molar extinction coefficient of the iron oxide particles obtained in Example 11 was 595 L / (mol·cm) for light with a wavelength of 400 nm and 3112 L / (mol·cm) for light with a wavelength of 220 nm. FIG. 22 shows the reflection spectra of the silicon oxide-coated iron oxide particle powder obtained in Example 8 and the iron oxide particle powders of Comparative Examples 12 and 13. As can be seen in FIG. 22, for the silicon oxide-coated iron oxide particles obtained in Example 8, the reflectance of light was less than 18% in the wavelength range of 400 to 620 nm, whereas for the iron oxide particles of Comparative Examples 12 and 13, it exceeded 18%. FIG. 23 shows the reflection spectrum of a dispersion in which the silicon oxide-coated iron oxide particles obtained in Example 8 were dispersed in water at a concentration of 0.31 wt% as Fe ２ O ３ in the wavelength range of 400 to 750 nm. As can be seen in FIG. 23, the reflectance of light was less than 18% in the wavelength range of 400 to 620 nm, whereas for the iron oxides of Comparative Example 1 and Comparative Example 2, it exceeded 18%. Also, for the silicon oxide-coated iron oxide particles obtained in Example 11, the reflectance of light was less than 18% in the wavelength range of 400 to 620 nm. The haze value of a dispersion in which the silicon oxide-coated iron oxide particles obtained in Example 8 above were dispersed in propylene glycol at a concentration of 0.05 wt% as Fe ２ O ３ was 0.00%, and also, when dispersed in water at a concentration of 0.05 wt% as Fe ２ O ３The haze value of the dispersion liquid dispersed at a concentration of 0.31% by weight was 0.00%, and both were highly transparent dispersion liquids. Furthermore, the silicon oxide-coated iron oxide particles obtained in Example 8 above were added to water as Fe ２ O ３ The haze value of the dispersion liquid dispersed at a concentration of 2.0% by weight was 0.89%, and it was a highly transparent dispersion liquid. On the other hand, the haze value of the dispersion liquid in which the iron oxide particles of Comparative Example 13 were dispersed in propylene glycol at a concentration of 0.02% by weight was 21.9%, and the haze value of the dispersion liquid dispersed in water at a concentration of 0.31% by weight was 15.9%, and the haze value of the dispersion liquid dispersed in water at a concentration of 2.0% by weight was 23.4%, and obvious turbidity was observed. Also, the haze value of the dispersion liquid in which the iron oxide particles obtained in Comparative Example 12 were dispersed in water at a concentration of 2.0% by weight was 2.56%, and turbidity was observed. The silicon oxide-coated iron oxide particles obtained in Example 11 above were added to propylene glycol as Fe ２ O ３ The haze value of the dispersion liquid dispersed at a concentration of 0.05% by weight was 0.91%, and the haze value of the dispersion liquid dispersed in water as Fe ２ O ３ at a concentration of 0.31% by weight was 1.46%. Although it was not as good as the silicon-coated iron oxide particles obtained in Example 8, it was a highly transparent dispersion liquid. Furthermore, the silicon oxide-coated iron oxide particles obtained in Example 11 above were added to water as Fe ２ O ３ The haze value of the dispersion liquid dispersed at a concentration of 2.0% by weight was 1.64%. Although it was not as good as the silicon-coated iron oxide particles obtained in Example 8, it was a highly transparent dispersion liquid. From the above, the transmission / absorption spectrum, reflection spectrum, primary particle diameter, and haze value of the silicon oxide-coated iron oxide particles or their compositions obtained in Example 8 and Example 11 can be preferably used without impairing the original color development, color characteristics, and designability of the product when used particularly in red paints. However, for the iron oxide particles of Comparative Example 13, no clear difference was observed between the transmission region and the absorption region in the ultraviolet-visible region, and since it has strong reflection characteristics in the red region, it impairs the original color development, color characteristics, and designability of the red paint. Also, for the iron oxide particles of Comparative Example 12 whose surfaces were not coated with silicon oxide, in the region of wavelengths 400 to 620 nm, the reflectance of light exceeded 18%, reflecting light other than red, and the iron oxide particle powder appeared yellowish compared to the silicon oxide-coated iron oxide particles obtained in Example 8. Therefore, it impairs the original color development, color characteristics, and designability of the red paint. 1 First processing surface 2 Second processing surface 10 First processing part 11 First holder 20 Second processing part 21 Second holder d1 First introduction part d2 Second introduction part d3 Third introduction part d10 Opening d20 Opening d30 Opening