Method for producing fine particles and apparatus for producing fine particles

Abstract

The present invention provides a method for producing fine particles with controlled particle size distribution, and an apparatus for producing fine particles. [Solution] The method for producing fine particles comprises a droplet formation step in which metal oxide powder with a particle size of 500 nm or less, as measured by the BET method, and a carbon source are supplied to a space with a temperature atmosphere above the reduction start temperature of the metal oxide powder but below the boiling point, thereby forming droplets of the metal oxide powder; a reduction step in which the metal oxide droplets formed by the droplet formation step are reduced by the carbon source; and a particle formation step in which the reduced metal oxide droplets are atomized.

Description

[Technical Field]

[0001] The present invention relates to a method for producing fine particles without putting the raw materials into a gaseous state, and more particularly to a method for producing fine particles and an apparatus for producing fine particles in which metal oxide powder is reduced after being liquefied into droplets. [Background technology]

[0002] Currently, nanoparticles such as silicon nanoparticles, oxide nanoparticles, nitride nanoparticles, and carbide nanoparticles are used in a wide range of fields. One method for producing such nanoparticles is the thermal plasma method. The thermal plasma method produces nanoparticles by instantly evaporating raw materials in a thermal plasma flame and then rapidly cooling and solidifying the evaporated material. The thermal plasma method has many advantages, including being clean, highly productive, capable of handling high-melting-point materials due to its high temperature, and relatively easy to combine with other gas-phase methods. For these reasons, the thermal plasma method is actively used as a method for producing nanoparticles.

[0003] For example, Patent Document 1 describes a method for producing non-stoichiometric titanium oxide fine particles. Patent Document 1 describes a method for producing non-stoichiometric titanium oxide nanoparticles using titanium oxide containing titanium dioxide, wherein titanium dioxide powder is dispersed in a liquid substance containing a carbon source, water is added to form a slurry, the slurry is formed into droplets and supplied into an oxygen-free thermal plasma flame, the carbon generated from the substance in the thermal plasma flame reacts with titanium dioxide to produce non-stoichiometric titanium oxide in a gaseous state, and the produced gaseous non-stoichiometric titanium oxide is rapidly cooled to produce non-stoichiometric titanium oxide nanoparticles. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6759246 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Patent Document 1 describes generating non-stoichiometric titanium oxide in a gaseous state and rapidly cooling the generated gaseous non-stoichiometric titanium oxide to produce non-stoichiometric titanium oxide fine particles. When the state is in a gaseous state, it is difficult to control the particle size distribution of the generated particles. In Patent Document 1, the particle size distribution of the generated particles is broad. The object of the present invention is to provide a method for producing fine particles with controlled particle size distribution, and an apparatus for producing fine particles. [Means for solving the problem]

[0006] To achieve the above objective, the invention [1] is a method for producing fine particles, comprising: a droplet formation step in which a metal oxide powder with a particle size of 500 nm or less, as measured by the BET method, and a carbon source are supplied to a space with a temperature atmosphere above the reduction initiation temperature of the metal oxide powder but below its boiling point, thereby forming droplets of the metal oxide powder; a reduction step in which the metal oxide droplets formed by the droplet formation step are reduced by the carbon source; and a particle formation step in which the reduced metal oxide droplets are atomized.

[0007] Invention [2] is a method for producing fine particles according to Invention [1], wherein the amount of a carbon source is changed relative to the amount of metal oxide powder supplied to a temperature atmosphere space during the droplet formation process. Invention [3] is a method for producing fine particles according to Invention [1] or [2], wherein the heat source forming the temperature atmosphere is a thermal plasma flame or a heater. Invention [4] is a method for producing fine particles according to any one of Inventions [1] to [3], wherein the particle formation step is a step of cooling and solidifying metal oxide droplets to form particles. Invention [5] is a method for producing fine particles according to any one of Inventions [1] to [4], further comprising a recovery step for recovering the particles obtained in the particle formation step after the particle formation step. Invention [6] is a method for producing fine particles according to any one of Inventions [1] to [5], wherein the metal oxide powder is titanium dioxide powder. Invention [7] is a method for producing fine particles according to any one of Inventions [1] to [6], wherein the particles formed in the particleization step after the reduction step are non-stoichiometric oxides of the metals constituting the metal oxide powder. Invention [8] is a method for producing fine particles according to any one of Inventions [1] to [7], wherein the atmosphere in the thermal plasma flame is a reducing atmosphere.

[0008] The invention [9] is a fine particle manufacturing apparatus for producing fine particles using metal oxide powder with a particle size of 500 nm or less as measured by the BET method and a carbon source, comprising: a chamber whose interior constitutes a space with a temperature atmosphere above the reduction initiation temperature of the metal oxide powder but below its boiling point; a material supply unit that supplies the metal oxide powder and the carbon source into the chamber constituting the space with the temperature atmosphere; and a fine particle manufacturing apparatus in which the metal oxide powder is formed into droplets in the chamber constituting the space with the temperature atmosphere, the generated metal oxide droplets are reduced by the carbon source, and the reduced metal oxide droplets are atomized. Invention

[10] is a fine particle manufacturing apparatus according to Invention [9], wherein the material supply unit supplies metal oxide powder in a dispersed state in a temperature-controlled atmosphere. Invention

[11] is a fine particle manufacturing apparatus according to Invention [9] or

[10] , wherein the material supply unit varies the amount of a carbon source relative to the amount of metal oxide powder supplied to a space of temperature atmosphere. Invention

[12] is a fine particle manufacturing apparatus according to any one of Inventions [9] to

[11] , comprising a plasma torch provided above a chamber and a plasma generating unit that generates a thermal plasma flame within the plasma torch, wherein the thermal plasma flame forms a temperature atmosphere.

[0009] Invention

[13] is a apparatus for producing fine particles according to any one of Inventions [9] to

[12] , wherein metal oxide droplets are cooled and solidified in a chamber to form particles. Invention

[14] is a fine particle manufacturing apparatus according to any one of Inventions [9] to

[13] , having a collection unit connected to a chamber for collecting particulated particles. The invention

[15] is a microparticle manufacturing apparatus according to any one of inventions [9] to

[14] , wherein the metal oxide powder is a titanium dioxide powder. The invention

[16] is a microparticle manufacturing apparatus according to any one of inventions [9] to

[15] , wherein the atomized particles are variable ratio oxides of the metal constituting the metal oxide powder. The invention

[17] is a microparticle manufacturing apparatus according to any one of inventions [9] to

[16] , wherein the atmosphere in the thermal plasma flame is a reducing atmosphere.

Advantages of the Invention

[0010] [[ID=ll]]According to the present invention, it is possible to provide a method for manufacturing microparticles with a controlled particle size distribution and a microparticle manufacturing apparatus.

Brief Description of the Drawings

[0011] [Figure 1] [[ID=lg]]It is a schematic diagram showing an example of a microparticle manufacturing apparatus according to an embodiment of the present invention. [Figure 2] It is an Ellingham diagram. [[ID=2j]] [Figure 3] It is a schematic diagram showing an SEM image of a titanium dioxide powder. [Figure 4] It is a schematic diagram showing an SEM image of the microparticles of Example 1. [Figure 5] It is a schematic diagram showing an SEM image of the microparticles of Example 2. [Figure 6] It is a schematic diagram showing an SEM image of the microparticles of Example 3. [Figure 7] It is a graph showing the results of analyzing the crystal structures of the titanium dioxide powder and the microparticles of Examples 1 to 3 by X-ray diffraction. [Figure 8] It is a graph showing an enlarged part of the results of analyzing the crystal structures of the titanium dioxide powder and the microparticles of Examples 1 to 3 by X-ray diffraction. [Figure 9] It is a graph showing the transmittances of the titanium dioxide powder and the microparticles of Examples 1 to 3.

Modes for Carrying Out the Invention

[0012] The method for producing fine particles and the apparatus for producing fine particles of the present invention will be described in detail below based on the preferred embodiments shown in the attached drawings. The figures described below are illustrative examples for illustrating the present invention, and the present invention is not limited to the figures shown below. In the following, the "~" indicating a numerical range includes the numbers written on both sides. For example, ε is the numerical value ε α ~Value ε β The range of ε is the numerical value ε α and the numerical value ε β This range includes ε α ≦ε≦ε β That is the case. Unless otherwise specified, angles and temperatures include tolerances generally accepted in the relevant technical field. Similarly, unless otherwise specified, other particle sizes and flow rates also include tolerances generally accepted in the relevant technical field. The following provides a detailed explanation of the method for producing fine particles and the apparatus for producing fine particles.

[0013] [Example of a microparticle manufacturing apparatus] Figure 1 is a schematic diagram showing an example of a fine particle manufacturing apparatus according to an embodiment of the present invention. The fine particle manufacturing apparatus 10 shown in Figure 1 (hereinafter simply referred to as manufacturing apparatus 10) is an apparatus that manufactures fine particles using metal oxide powder with a particle size of 500 nm or less, as measured by the BET method, and a carbon source. The fine particles manufactured are, for example, non-stoichiometric oxides of the metals that make up the metal oxide powder. More specifically, the fine particles manufactured are, for example, non-stoichiometric titanium oxide fine particles.

[0014] The manufacturing apparatus 10 includes a plasma torch 12 that generates a thermal plasma flame inside, a material supply unit 14 that supplies metal oxide powder with a particle size of 500 nm or less, as measured by the BET method, into the interior 16c (inside the chamber 16), a chamber 16 for generating fine particles 15, a cyclone 19 that removes coarse particles having a particle size greater than or equal to an arbitrarily defined particle size from the fine particles 15, and a recovery unit 20 that recovers the classified fine particles 18 having a desired particle size, which have been classified by the cyclone 19. The fine particles 15 and the classified fine particles 18 correspond to the "fine particles" of the present invention. The classified fine particles 18 are also simply called fine particles 18. For the material supply unit 14, chamber 16, cyclone 19, and recovery unit 20, for example, various devices described in Japanese Patent Application Publication No. 2007-138287 can be used. Furthermore, the manufacturing apparatus 10 includes a carbon source supply unit 32, a plasma generation unit 33, and a raw material supply unit 34.

[0015] The plasma torch 12 consists of a quartz tube 12a and a high-frequency oscillation coil 12b surrounding it. A supply pipe 14a, described later, for supplying metal oxide powder, which is the raw material, into the plasma torch 12 is located in the center of the upper part of the plasma torch 12. A plasma gas supply port 12c is formed around the supply pipe 14a (on the same circumference), and the plasma gas supply port 12c is ring-shaped. The plasma generation unit 33 is electrically connected to the high-frequency oscillation coil 12b. When a high-frequency voltage is applied from the plasma generation unit 33 to the high-frequency oscillation coil 12b, a thermal plasma flame 24 is generated inside the plasma torch 12.

[0016] The plasma gas supply unit 22 supplies plasma gas into the plasma torch 12. The plasma gas supply unit 22 is connected to the plasma gas supply port 12c via piping 22a. Although not shown in the diagram, the plasma gas supply unit 22 is equipped with a supply volume adjustment unit such as a valve for adjusting the supply volume. The plasma gas is supplied from the plasma gas supply unit 22 through the ring-shaped plasma gas supply port 12c into the plasma torch 12 from the directions indicated by arrows P and S.

[0017] For example, argon gas and hydrogen gas are used as plasma gases. In this case, argon gas and hydrogen gas are stored in the plasma gas supply unit 22. Argon gas and hydrogen gas are supplied from the plasma gas supply unit 22 through piping 22a and the plasma gas supply port 12c into the plasma torch 12 from the directions indicated by arrows P and S. The plasma gas used is not limited to the gases mentioned above, as it depends on the composition of the fine particles being manufactured.

[0018] The plasma generation unit 33 has a high-frequency power supply (not shown), which is connected to the high-frequency oscillation coil 12b. The plasma generation unit 33 has the function of supplying a high-frequency voltage to the high-frequency oscillation coil 12b. When a high-frequency voltage is applied from the plasma generation unit 33 to the high-frequency oscillation coil 12b, a thermal plasma flame 24 is generated inside the plasma torch 12. The thermal plasma flame 24 is an induced thermal plasma flame generated by the high-frequency voltage. In the plasma generation unit 33, the temperature of the thermal plasma flame 24 is adjusted by adjusting the high-frequency voltage supplied to the high-frequency oscillation coil 12b. The plasma torch 12, located above the chamber 16, generates a thermal plasma flame 24 inside the plasma generation unit 33. The thermal plasma flame 24 is a heat source that forms a temperature atmosphere above the reduction initiation temperature of the metal oxide powder described later, but below its boiling point. Furthermore, the pressure atmosphere inside the plasma torch 12 is preferably below atmospheric pressure. Here, the atmosphere below atmospheric pressure is not particularly limited, but for example, it is 0.5 to 100 kPa. Furthermore, the outside of the quartz tube 12a is surrounded by concentrically formed tubes (not shown), and cooling water is circulated between these tubes and the quartz tube 12a to water-cool the quartz tube 12a, preventing it from becoming too hot due to the thermal plasma flame 24 generated in the plasma torch 12.

[0019] The thermal plasma flame 24 inside the plasma torch 12 is used as a heat source to form metal oxide droplets from the metal oxide powder, and these metal oxide droplets are then reduced by a carbon source such as methane gas. As described above, when a high-frequency voltage is applied from the plasma generation unit 33 to the high-frequency oscillation coil 12b, a thermal plasma flame 24 is generated within the plasma torch 12. The temperature of the thermal plasma flame 24 can reach 6000°C, and theoretically, it is thought to reach around 10000°C. For this reason, when metal oxide powder is supplied into the plasma torch 12, some of the metal oxide powder evaporates, making it difficult to produce fine particles by liquefying the metal oxide powder without putting it into a gaseous state within the plasma torch 12.

[0020] Chamber 16 is located adjacent to the lower part of the plasma torch 12, and the temperature inside chamber 16c is lower than that inside the plasma torch 12. Therefore, the metal oxide powder can be dropletized inside chamber 16c without being turned into a gaseous state. The inside of chamber 16c constitutes a space with a temperature atmosphere that is above the reduction initiation temperature of the metal oxide powder, above its melting point, and below its boiling point. Inside chamber 16c, the metal oxide powder is reduced to droplets, and these droplets are reduced by a carbon source, causing the reduced droplets to become particles. Chamber 16 functions as a cooling tank. During particle formation, the metal oxide droplets are cooled and solidified inside chamber 16c, forming fine particles 15. Thus, chamber 16 has a space inside 16c with a temperature atmosphere above the reduction initiation temperature of the metal oxide powder but below its boiling point, and a space where the metal oxide droplets become particles. The space with the temperature atmosphere and the space where particles are formed are continuous. Furthermore, there is a temperature distribution inside the chamber 16c, with relatively high and low temperature regions. For this reason, a region in the longitudinal direction H of the chamber 16 is predetermined where the metal oxide powder can be liquefied without being put into a gaseous state, and where the temperature range is above the reduction start temperature of the metal oxide powder, above the melting point, and below the boiling point. The metal oxide powder and carbon source are then supplied to this region.

[0021] In this case, if the metal oxide powder is titanium dioxide, referring to the Ellingham diagram shown in Figure 2, the straight line 40 shown in Figure 2 corresponds to the oxidation reaction of titanium. Straight line 40 shows the reaction Ti + O2 = TiO2, and its slope is positive. Line 42 represents the oxidation reaction of carbon. Line 42 shows the reaction 2C + O2 = 2CO, and its slope is negative. Note that the vertical axis ΔG in Figure 2. 0 This represents the standard Gibbs free energy for the reaction. This shows that at temperatures above the temperature of the intersection point Pc between lines 40 and 42, carbon can reduce titanium dioxide to elemental titanium. Furthermore, at temperatures above the intersection point Pc, carbon itself is oxidized to carbon monoxide (CO). The temperature at the intersection point Pc corresponds to the reduction initiation temperature. In the case of titanium dioxide, the intersection point Pc of line 40 and line 42 is approximately 1650°C. Furthermore, the melting point of titanium dioxide is 1855°C, and the boiling point of titanium dioxide is 2972°C. From this, the temperature range of titanium dioxide that is above the reduction initiation temperature and below the boiling point is 1650°C to 2972°C. Considering the melting point of titanium dioxide, the temperature range is above the reduction initiation temperature and above the melting point, and below the boiling point. In this case, 1855°C to 2972°C is preferable. When the metal oxide powder is titanium dioxide, the above-mentioned temperature atmosphere, which is above the reduction initiation temperature but below the boiling point, is between 1650°C and 2972°C, preferably between 1855°C and 2972°C, which is above the melting point. In this case, the temperature atmosphere is above the reduction initiation temperature but above the melting point and below the boiling point.

[0022] As described above, the temperature of the thermal plasma flame 24 can be set to 6000°C. Therefore, the metal oxide powder and carbon source are supplied not to the center of the thermal plasma flame 24, but to a region within the aforementioned temperature range, below the longitudinal direction H of the tail 24b of the thermal plasma flame 24 inside the chamber 16c. Furthermore, the space with a temperature atmosphere above the reduction initiation temperature of the metal oxide powder but below its boiling point is not limited to being formed using a thermal plasma flame 24 as a heat source; a heater or the like can be used as a heat source without using a thermal plasma flame. In this case, the temperature atmosphere is adjusted to fall within the temperature range described above. The heater is not particularly limited, and an electric heater or the like can be used.

[0023] The material supply unit 14 is connected to the top of the plasma torch 12 via a supply pipe 14a. The material supply unit 14 supplies, for example, metal oxide powder, which is the raw material, to a region inside the chamber 16c where the temperature is above the reduction start temperature of the metal oxide powder but below its boiling point. More specifically, the material supply unit 14 supplies the metal oxide powder downwards in the longitudinal direction H of the tail 24b of the thermal plasma flame 24 inside the chamber 16c. As the material supply unit 14 for supplying the metal oxide powder, as described above, for example, one disclosed in Japanese Patent Application Publication No. 2007-138287 can be used. In this case, the material supply unit 14 includes, for example, a storage tank (not shown) for storing the metal oxide powder (raw material powder), a screw feeder (not shown) for quantitatively transporting the metal oxide powder (raw material powder), a dispersion unit (not shown) for dispersing the metal oxide powder (raw material powder) transported by the screw feeder into primary particles before it is finally scattered, and a carrier gas supply source (not shown).

[0024] The metal oxide powder (raw material powder) is supplied from the carrier gas supply source to the inside 16c of the chamber 16 via the supply pipe 14a, along with the carrier gas under extrusion pressure. The material supply unit 14 is not particularly limited in its configuration, as long as it can prevent the metal oxide powder (raw material powder) from agglomerating and disperse the metal oxide powder (raw material powder) into the interior 16c of the chamber 16 while maintaining its dispersion state. For example, argon gas can be used as the carrier gas. The carrier gas flow rate can be controlled using a flow meter, such as a float-type flow meter. The carrier gas flow rate value is the scale value of the flow meter.

[0025] The carbon source supply unit 32 is connected to the supply pipe 14a by a connecting pipe 32a. The carbon source supply unit 32 supplies the carbon source to a space with a temperature atmosphere, for example, the inside 16c of the chamber 16. The carbon source reduces metal oxide powder. Furthermore, when hydrocarbon gas is used as the carbon source, since the thermal decomposition of hydrocarbons is an endothermic reaction, it also serves to cool the dropletized metal oxide powder (metal oxide droplets (not shown)). Various hydrocarbon gases such as methane, ethane, propane, butane, acetylene, ethylene, propylene, and butene can be used as carbon sources. When the carbon source is hydrocarbon gas, the carbon source supply unit 32 includes, although not shown in the figure, a gas supply unit for storing hydrocarbon gas, a regulator (pressure regulator), a control valve for controlling the amount of gas supplied, etc.

[0026] The material supply unit 14 and the carbon source supply unit 32 supply the metal oxide powder and the carbon source to a space with a temperature atmosphere that is above the reduction initiation temperature of the metal oxide powder but below its boiling point. The material supply unit 35 is composed of a material supply unit 14 and a carbon source supply unit 32. The carbon source supply unit 32 can change the amount of carbon source relative to the amount of metal oxide powder supplied to the temperature-controlled atmosphere space. For example, it can change the amount of methane gas supplied relative to the amount of metal oxide powder. By changing the amount of carbon source, for example, the amount of methane gas, relative to the amount of metal oxide powder, the degree of reduction of the metal oxide powder can be changed, and the composition and crystallinity of the fine particles produced can be changed. Crystallinity refers to the presence or absence of a periodic arrangement of atoms and molecules that make up the fine particles. Low crystallinity means that the periodic arrangement of atoms and molecules is lost.

[0027] The material supply unit 35 may also have a raw material supply unit 34 connected to the material supply unit 14 by piping 34a. The raw material supply unit 34 stores metal oxide raw materials that will become metal oxide powder and supplies the metal oxide raw materials to the material supply unit 14. In a configuration that includes a raw material supply unit 34, the material supply unit 14 includes, for example, a jet mill (not shown) as a crusher. The jet mill crushes, grinds, and disperses the metal oxide raw material to obtain metal oxide powder with a particle size of 500 nm or less as measured by the BET method. The metal oxide powder is then supplied from the material supply unit 14 to the plasma torch 12 through the supply pipe 14a. In this case, the material supply unit 14, which constitutes the material supply unit 35, supplies the metal oxide powder dispersed in a temperature-controlled atmosphere. For example, a jet mill as described in Japanese Patent Publication No. 2009-173979 can be used. However, the method is not limited to a jet mill; pin mills and hammer mills can also be used as long as they can disperse and supply metal oxide powder in a temperature-controlled environment. The raw material supply unit 34 is not necessarily required, and it is not necessary to provide it.

[0028] A gas supply unit 28 is connected to the chamber 16 via piping 28a. The gas supply unit 28 supplies cooling gas into the chamber 16 via piping 28a. A thermal plasma flame 24 is used as the heat source to atomize metal oxide powder (raw material powder), and the gas supply unit 28 supplies cooling gas (rapid cooling gas) containing an inert gas to these droplets. The cooling gas (rapid cooling gas) can be, like the plasma gas, for example, argon gas or hydrogen gas. The gas supply unit 28 includes a gas supply source (not shown) in which cooling gas is stored, and a pressure-applying means (not shown) for applying extrusion pressure to the cooling gas supplied into the chamber 16. The gas supply unit 28 is also provided with a pressure control valve (not shown) for controlling the amount of gas supplied from the gas supply source. If the cooling gas is argon, for example, argon gas is stored in the gas supply source. The pressure-applying means is, for example, a compressor or a blower.

[0029] The gas supply unit 28 supplies argon gas as a cooling gas towards the tail 24b of the thermal plasma flame 24, that is, the end of the thermal plasma flame 24 opposite to the plasma gas supply port 12c, that is, the terminal end of the thermal plasma flame 24, at an angle of, for example, 45°, in the direction of arrow Q, and also supplies the aforementioned cooling gas along the inner wall 16a of the chamber 16 from top to bottom, that is, in the direction of arrow R shown in Figure 1.

[0030] The cooling gas supplied from the gas supply unit 28 into the chamber 16 has additional effects, such as contributing to the classification of fine particles 15 in the cyclone 19. When newly formed fine particle droplets collide with each other and form aggregates, the particle size distribution spreads, leading to a decrease in quality. However, the cooling gas introduced in the direction of arrow Q toward the tail 24b (terminal end) of the thermal plasma flame 24 prevents aggregation due to collisions between fine particle droplets. Furthermore, the cooling gas supplied in the direction of arrow R prevents the fine particles 15 from adhering to the inner wall 16a of the chamber 16, thereby improving the yield of the generated fine particles 15. For this reason, the cooling gas may be supplied only in the direction of arrow R, without supplying it in the direction of arrow Q.

[0031] Although argon gas was used as the cooling gas (rapid cooling gas), it is not limited to this, and any gas can be used depending on the composition of the fine particles being manufactured. If the metal oxide droplets can be atomized, that is, if the metal oxide droplets can be cooled and solidified into particles within the chamber 16, then a cooling gas (rapid cooling gas) may not be used. Thus, depending on the manufacturing conditions for atomizing the metal oxide droplets, a cooling gas (rapid cooling gas) may not be used. For this reason, the gas supply unit 28 is not necessarily required.

[0032] As shown in Figure 1, the chamber 16 is provided with a cyclone 19 for classifying fine particles 15 to a desired particle size. The cyclone 19 comprises an inlet pipe 19a for supplying fine particles 15 from the chamber 16, a cylindrical outer cylinder 19b connected to the inlet pipe 19a and located at the top of the cyclone 19, a frustoconical section 19c that extends downward from the bottom of the outer cylinder 19b and gradually decreases in diameter, a coarse particle recovery chamber 19d connected to the bottom of the frustoconical section 19c for recovering coarse particles having a particle size greater than or equal to the desired particle size, and an inner cylinder 19e that is connected to a recovery section 20 (which will be described in detail later) and protrudes from the outer cylinder 19b.

[0033] An airflow containing fine particles 15 is blown in from the inlet pipe 19a of the cyclone 19 along the inner circumferential wall of the outer cylinder 19b, thereby forming a downward-flowing swirling flow as this airflow flows from the inner circumferential wall of the outer cylinder 19b toward the frustoconical section 19c, as indicated by the arrow T in Figure 1. Then, when the aforementioned downward swirling flow reverses and becomes an upward flow, due to the balance between centrifugal force and drag, the coarse particles are unable to ride the upward flow and descend along the side of the frustum 19c, where they are collected in the coarse particle collection chamber 19d. In addition, fine particles that are more affected by drag than by centrifugal force are discharged from the inner tube 19e to the outside of the cyclone 19 along with the upward flow along the inner wall of the frustum 19c.

[0034] Furthermore, a negative pressure (suction force) is generated from the recovery unit 20, which will be described in detail later, through the inner tube 19e. This negative pressure (suction force) causes the fine particles separated from the swirling airflow to be sucked in, as indicated by the symbol U, and sent to the recovery unit 20 through the inner tube 19e.

[0035] An extension of the inner tube 19e, which is the outlet for the airflow within the cyclone 19, is provided with a recovery unit 20 for recovering fine particles 18 having a desired particle size on the order of nanometers. The recovery unit 20 comprises a recovery chamber 20a, a filter 20b provided within the recovery chamber 20a, and a vacuum pump 30 connected via a tube 20c provided at the bottom of the recovery chamber 20a. Fine particles sent from the cyclone 19 are drawn into the recovery chamber 20a by the vacuum pump 30, and are recovered while remaining on the surface of the filter 20b. Furthermore, in the manufacturing apparatus 10 described above, the number of cyclones used is not limited to one, but may be two or more.

[0036] [Method for producing fine particles] Next, an example of a method for producing fine particles using the manufacturing apparatus 10 described above will be explained. Note that the method for producing fine particles is not particularly limited to using the manufacturing apparatus 10 described above. The raw material is a metal oxide powder with a particle size of 500 nm or less, as measured by the BET method. For example, the metal oxide powder is titanium dioxide powder, with a particle size of, for example, 100 nm, as measured by the BET method. For example, argon gas and hydrogen gas are used as the plasma gas, and a high-frequency voltage is applied from the plasma generation unit 33 to the high-frequency oscillation coil 12b to generate a thermal plasma flame 24 inside the plasma torch 12. Next, using argon gas as the carrier gas, titanium dioxide powder is transported as a raw material and supplied via supply pipe 14a to a region inside chamber 16c where the temperature range is above the reduction start temperature of the metal oxide powder but below its boiling point. In addition, metal gas, for example, is supplied from the carbon source supply unit 32 via connecting pipe 32a and supply pipe 14a to the region inside chamber 16c where the temperature range is above the reduction start temperature of the metal oxide powder but below its boiling point. As described above, the region inside chamber 16c where the titanium dioxide powder (metal oxide powder) and methane gas (carbon source) are supplied, where the temperature range is above the reduction start temperature of the metal oxide powder but below its boiling point, is predetermined. In this way, the metal oxide powder and the carbon source are supplied to a region within the chamber 16c, which is a space with a temperature atmosphere above the reduction initiation temperature of the metal oxide powder but below its boiling point, to form droplets of the metal oxide powder (dropletization step). That is, the titanium dioxide powder supplied to the region within the chamber 16c, which is within the temperature range described above, is dropletized, and titanium dioxide droplets are formed. Since hydrogen gas is used as the plasma gas, the thermal plasma flame 24 is a reducing atmosphere.

[0037] Next, the metal oxide droplets formed by the droplet formation process are reduced by a carbon source (reduction process). In the reduction process, the titanium dioxide droplets formed by the droplet formation process come into contact with the carbon produced by the thermal decomposition of methane gas inside chamber 16c and are reduced by the carbon. More specifically, the O2 in titanium dioxide (TiO2) reacts with the C in methane gas (CH4), reducing the titanium dioxide (TiO2). This changes the composition of the titanium dioxide (TiO2).

[0038] Next, the reduced metal oxide droplets are atomized (atomization step). This yields the fine particles 15 described above. The particle formation process involves cooling and solidifying the reduced metal oxide droplets to form particles. The reduced titanium dioxide droplets cool and solidify to form particles. The reduced metal oxide droplets (reduced titanium dioxide droplets) are cooled and solidified to form particles in the chamber 16. Cooling in the chamber 16 can be done by natural cooling without a cooling gas or by forced cooling with a cooling gas.

[0039] The particles formed in the particle formation process after the reduction process described above, i.e., the fine particles 15, are, for example, non-stoichiometric oxides of the metal constituting the metal oxide powder. When titanium dioxide is used as the metal oxide powder, the fine particles 15 are non-stoichiometric oxides of titanium, and more specifically, non-stoichiometric titanium oxide fine particles. The fine particles 18 described above are also non-stoichiometric titanium oxide fine particles, just like the fine particles 15.

[0040] Next, after the particle formation process, the particles obtained in the particle formation process are recovered (recovery process). In the recovery process, the fine particles 15 obtained inside the chamber 16c are blown into the cyclone 19 from the inlet pipe 19a along the inner wall of the outer cylinder 19b, along with the airflow. As a result, this airflow flows along the inner wall of the outer cylinder 19b, as shown by arrow T in Figure 1, forming a swirling flow and descending. When the descending swirling flow reverses and becomes an upward flow, due to the balance between centrifugal force and drag, the coarse particles cannot ride the upward flow and descend along the side of the frustoconical section 19c, where they are recovered in the coarse particle recovery chamber 19d. Fine particles that are more affected by drag than centrifugal force are discharged from the inner wall of the frustoconical section 19c along with the upward flow on the inner wall, leaving the cyclone 19.

[0041] The discharged fine particles 18 are sucked in the direction indicated by the symbol U in Figure 1 by the negative pressure (suction force) from the recovery unit 20 by the vacuum pump 30, and sent to the recovery unit 20 through the inner tube 19e, where they are collected by the filter 20b of the recovery unit 20. In this way, the fine particles 18 are collected in the recovery process. In the recovery process, the internal pressure inside the cyclone 19 is preferably below atmospheric pressure. Furthermore, the particle size of the classified fine particles 18 can be any particle size on the order of nanometers, depending on the purpose. In the method for producing fine particles, argon gas may be supplied from the gas supply unit 28 to the tail 24b of the thermal plasma flame 24, i.e., the terminal end of the thermal plasma flame 24, in the direction of arrow Q, or argon gas may be supplied as a cooling gas in the direction of arrow R. Alternatively, argon gas may be supplied as a cooling gas only in the direction of arrow R.

[0042] In the above-described apparatus and method for manufacturing fine particles, the manufactured fine particles are formed into liquid droplets during the manufacturing process but are never in a gaseous state. Therefore, the particle size difference between the manufactured particles and the raw material, the metal oxide powder, is small, and the particle size and particle size distribution of the metal oxide powder are reflected in the particle size and particle size distribution of the manufactured fine particles. As a result, fine particles with a particle size of the same order as that of the metal oxide powder are obtained, and consequently, fine particles with controlled particle size are obtained. Furthermore, the particle size distribution of the metal oxide powder is also reflected in the particle size distribution of the manufactured fine particles; if the particle size distribution width of the metal oxide powder is narrow, the particle size distribution width of the manufactured fine particles will also be narrow.

[0043] <Metal oxide powder> Metal oxide powder is a raw material for fine particles. As described above, the metal oxide powder is a metal oxide with a particle size of 500 nm or less as measured by the BET method. The particle size of the metal oxide powder as measured by the BET method is preferably 10 to 200 nm, and more preferably 10 to 100 nm. Furthermore, the metal oxide powder is preferably spherical, but is not limited to spherical. For example, when the diameter of the short axis is α and the diameter of the long axis is β, the ratio β / α of the metal oxide powder is preferably less than 3. The particle size of the metal oxide powder is the particle size measured by the BET method, as described above. As mentioned above, metal oxide powders include, for example, titanium dioxide. In addition to titanium dioxide, other examples of metal oxide powders include boron oxide, magnesium oxide, aluminum oxide, silicon oxide, calcium oxide, vanadium oxide, chromium oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, zinc oxide, gallium oxide, germanium oxide, strontium oxide, yttrium oxide, zirconium oxide, niobium oxide, molybdenum oxide, ruthenium oxide, indium oxide, tin oxide, cesium oxide, barium oxide, hafnium oxide, tantalum oxide, and tungsten oxide.

[0044] <Fine particles> The fine particles to be manufactured are particles with a particle size of 500 nm or less, as measured by the BET method. The particle size measured by the BET method is preferably 10 to 200 nm, and more preferably 10 to 100 nm. Furthermore, the fine particles are preferably spherical, but are not limited to spherical. For example, when the diameter of the short axis is α and the diameter of the long axis is β, the ratio β / α is preferably less than 3. The particle size of the fine particles is the size measured by the BET method as described above. As mentioned above, the fine particles produced are, for example, non-stoichiometric oxides of metals that make up metal oxide powders, and more specifically, non-stoichiometric titanium oxide fine particles are an example.

[0045] The present invention is basically configured as described above. Although the method for producing fine particles and the apparatus for producing fine particles of the present invention have been described in detail above, the present invention is not limited to the embodiments described above, and various improvements or modifications may be made without departing from the spirit of the present invention. [Examples]

[0046] The features of the present invention will be further described in detail below with reference to examples. The processing content, processing procedures, etc., shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below. In the examples, the manufacturing apparatus 10 shown in Figure 1 was used to produce the fine particles of Examples 1 to 3. The manufacturing conditions for Examples 1 to 3 will be described below.

[0047] (Example 1) In Example 1, titanium dioxide powder with a particle size of 92.8 nm, as measured by the BET method and as shown in the SEM image in Figure 3, was used as the raw material. The titanium dioxide powder was dispersed using a crusher and supplied into a thermal plasma flame. A jet mill was used as the crushing machine. The crushing conditions were a crushing gas flow rate of 30 liters / minute (converted to standard conditions) and an ejector flow rate of 20 liters / minute (converted to standard conditions). Argon gas was used as the crushing gas. Argon gas was used as the carrier gas to transport titanium dioxide powder into a thermal plasma flame. The argon gas flow rate was set to 10 liters / minute (at standard conditions). The supply rate of titanium dioxide powder was set to 1.0 g / minute. Furthermore, the input to the high-frequency oscillation coil 12b (see Figure 1) for generating a thermal plasma flame was kept constant at 32 kW, and the internal pressure of the plasma torch was fixed at 85 kPa. Argon and hydrogen gases were used as plasma gases. The flow rate of argon gas was set to 240 liters / minute (at standard conditions), and the flow rate of hydrogen gas was set to 15 liters / minute (at standard conditions). Methane gas (CH4 gas) was used as the carbon source. The flow rate of methane gas was set to 0.5 liters / minute (converted to standard conditions).

[0048] (Example 2) Example 2 differed from Example 1 in that the methane gas flow rate was 2.5 liters / minute (converted to standard conditions), but otherwise it was the same as Example 1. (Example 3) Example 3 differs from Example 1 in that the methane gas flow rate is 12.5 liters / minute (converted to standard conditions), but otherwise it is the same as Example 1. Examples 1 to 3 vary the amount of methane gas, which is the carbon source, relative to the amount of titanium dioxide powder, which is a metal oxide powder.

[0049] Figure 4 shows the SEM image of the microparticles from Example 1. Figure 5 shows the SEM image of the microparticles from Example 2. Figure 6 shows the SEM image of the microparticles from Example 3. The particle size of the fine particles in Example 1 was 127 nm, as measured by the BET method. The particle size of the fine particles in Example 2 was 108 nm, as measured by the BET method. The particle size of the fine particles in Example 3 was 99.8 nm, as measured by the BET method. For the titanium dioxide powder with a particle size of 92.8 nm measured by the BET method as described above (see Figure 3), fine particles were successfully produced without coarsening. Fine particles in the same nano-order were produced from the metal oxide powder in the nano-order. It was confirmed that the particle size distribution of the metal oxide powder was reflected in the particle size distribution of the produced fine particles. In addition, as shown in the SEM images of Figures 4 to 6, the fine particles of Examples 1 to 3 had no noticeable aggregation and did not coarsen compared to the titanium dioxide powder shown in Figure 3. For the fine particles of Examples 1 to 3, the particle size tended to decrease as the supply amount of methane gas increased.

[0050] Crystal structure analysis by X-ray diffraction method was carried out for the titanium dioxide powder and the fine particles of Examples 1 to 3. The results are shown in Figures 7 and 8. The vertical axis in Figures 7 and 8 is the intensity, and the unit of the intensity is dimensionless. In Figure 7, ▲ indicates the diffraction peak of TiO x . ■ indicates the diffraction peak of TiO2 (rutile). ● indicates the diffraction peak of TiO2 (anatase). The XRD (X-ray diffraction method) spectrum 50 shown in Figure 7 represents the spectrum of the titanium dioxide powder. The XRD spectrum 52 represents the spectrum of the fine particles of Example 1. The XRD spectrum 53 represents the spectrum of the fine particles of Example 2. The XRD spectrum 54 represents the spectrum of the fine particles of Example 3. Figure 8 collectively shows the XRD spectra 50, 52, 53, and 54 in the range DL (2θ = 23° to 24.5°) of the diffraction angle (2θ) in Figure 7. In the range DL (2θ = 23° to 24.5°), the position of the diffraction peak of TiO x is included. Figure 8 shows the position P1 of the diffraction peak of Ti2O3 and the position P2 of the diffraction peak of Ti 1.85 O3. The diffraction peak of Ti2O3 is higher in the order of Examples 1 to 3. Regarding the diffraction peak of Ti 1.85 O3, among Examples 1 to 3, Example 2 has the highest peak. By changing the amount of methane gas as the carbon source, the composition of the produced non-stoichiometric titanium oxide can be changed.

[0051] Furthermore, compositional analysis was performed on the titanium dioxide powder and the fine particles of Examples 1-3 using XPS (X-ray photoelectron spectroscopy). The results are shown in Table 1 below. Table 1 also includes the particle size measured by the BET method.

[0052] [Table 1]

[0053] As shown in Table 1, in Examples 1 to 3, the proportion of TiO2 decreased relative to the raw material titanium dioxide powder, and Ti2O3, TiO, and Ti appeared, indicating that the titanium dioxide was reduced by methane gas, which is the carbon source. The degree of reduction, from least to most severe, was in Example 3, Example 2, and Example 1. Furthermore, it was shown that changing the amount of methane gas, which is the carbon source, alters the composition of the manufactured nanoparticles, and thus allows for control of the crystallinity of the manufactured nanoparticles.

[0054] The transmittance of titanium dioxide powder and the fine particles of Examples 1 to 3 was measured. The results are shown in Figure 9. In Figure 9, the horizontal axis represents wavelength and the vertical axis represents transmittance. In Figure 9, measurement line 60 shows the transmittance measurement results for titanium dioxide powder. Measurement line 62 shows the transmittance measurement results for Example 1. Measurement line 63 shows the transmittance measurement results for Example 2. Measurement line 64 shows the transmittance measurement results for Example 3. As shown in Figure 9, in Examples 1-3, Examples 1 and 2 have higher transmittance than Example 3 in the ultraviolet range below 400 nm and the visible light range between 400 nm and around 780 nm. This suggests that Example 3 has a higher TiO2 content than Examples 1 and 2. In the infrared range with wavelengths above 900 nm, Examples 1 and 2 exhibited lower transmittance than Example 3. This suggests that Examples 1 and 2 contained higher amounts of TiO and Ti2O3, which are known as heat shielding materials, than Example 3. The results of the compositional analysis corresponded to the transmittance measurement results.

[0055] The following explains how to measure transmittance. The transmittance measurement method involved first weighing 0.005 g each of the titanium dioxide powder and the fine particles from Examples 1-3, and preparing a solution by adding ethanol to make a total volume of 50 g. Each solution was then subjected to dispersion treatment for 5 minutes using an ultrasonic device (Tetora150, manufactured by Nikko Bios Co., Ltd.). Next, 10 g was taken from each solution after dispersion treatment, and ethanol was added to each solution to make a total volume of 50 g, followed by dispersion treatment for 1 minute using the ultrasonic device. This resulted in obtaining measurement solutions with a mass concentration of 20 ppm for the titanium dioxide powder and the fine particles from Examples 1-3. The transmittance of each obtained measurement solution at wavelengths of 200-1200 nm was then measured using an ultraviolet-visible-near-infrared spectrophotometer (V-770, manufactured by JASCO Corporation). [Explanation of Symbols]

[0056] 10. Manufacturing equipment for fine particles (manufacturing equipment) 12 Plasma Torch 12a quartz tube 12b High-frequency oscillation coil 12c Plasma gas supply port 14 Material supply section 14a Supply pipe 15 Fine particles 16 Chambers 16a Inner wall 16c internal 18 Fine particles 19 Cyclone 19a Inlet pipe 19b Outer cylinder 19th century frustum of a cone 19d Coarse Particle Recovery Chamber 19e inner tube 20 Recovery Section 20a Collection Room 20b filter 20c tube 22 Plasma gas supply unit 22a Piping 24 Thermal Plasma Flame 24b Tail 28 Gas supply unit 28a, 34a piping 30 Vacuum pumps 32 Carbon Source Supply Section 32a Connecting pipe 33 Plasma generation unit 34 Raw material supply department 35 Material supply unit 40, 42 straight line 50, 52, 53, 54 XRD spectra Measurement lines 60, 62, 63, 64 DL range Location of diffraction peaks in P1Ti2O3 P2Ti 1.85 Location of the diffraction peak of O3 Pc intersection

Claims

1. A droplet formation step involves supplying metal oxide powder with a particle size of 500 nm or less, as measured by the BET method, and a carbon source to a space with a temperature atmosphere above the reduction initiation temperature of the metal oxide powder but below its boiling point, and forming droplets of the metal oxide powder. A reduction step is performed in which the metal oxide droplets formed by the liquidization of the metal oxide powder in the aforementioned dropletization step are reduced by the carbon source, A method for producing fine particles, comprising a particle formation step of atomizing the reduced metal oxide droplets.

2. A method for producing fine particles according to claim 1, wherein in the droplet formation step, the amount of the carbon source is changed relative to the amount of the metal oxide powder supplied to the space of the temperature atmosphere.

3. The method for producing fine particles according to claim 1 or 2, wherein the heat source that forms the temperature atmosphere is a thermal plasma flame or a heater.

4. The method for producing fine particles according to claim 1 or 2, wherein the particle formation step is a step of cooling and solidifying the metal oxide droplets to form particles.

5. A method for producing fine particles according to claim 1 or 2, further comprising a recovery step of recovering the particles obtained in the particle formation step after the particle formation step.

6. The method for producing fine particles according to claim 1 or 2, wherein the metal oxide powder is titanium dioxide powder.

7. The method for producing fine particles according to claim 1 or 2, wherein the particles formed in the particle formation step after the reduction step are non-stoichiometric oxides of the metal constituting the metal oxide powder.

8. The method for producing fine particles according to claim 3, wherein the thermal plasma flame is a reducing atmosphere.

9. A fine particle manufacturing apparatus that produces fine particles using metal oxide powder with a particle size of 500 nm or less as measured by the BET method and a carbon source, The interior of the chamber constitutes a space with a temperature atmosphere that is above the reduction initiation temperature of the metal oxide powder but below its boiling point, A material supply unit that supplies the metal oxide powder and the carbon source into the chamber constituting the space of the temperature atmosphere, A fine particle manufacturing apparatus comprising the following steps: in the chamber constituting the space of the aforementioned temperature atmosphere, the metal oxide powder is formed into droplets, the generated metal oxide droplets are reduced by the carbon source, and the reduced metal oxide droplets are atomized.

10. The apparatus for producing fine particles according to claim 9, wherein the material supply unit supplies the metal oxide powder in a dispersed state into the space at the temperature atmosphere.

11. The apparatus for producing fine particles according to claim 9 or 10, wherein the material supply unit changes the amount of the carbon source relative to the amount of the metal oxide powder supplied to the space of the temperature atmosphere.

12. The system includes a plasma torch provided above the chamber and a plasma generating unit that generates a thermal plasma flame within the plasma torch. The apparatus for producing fine particles according to claim 9 or 10, wherein the thermal plasma flame forms the temperature atmosphere.

13. The apparatus for producing fine particles according to claim 9 or 10, wherein the metal oxide droplets are cooled and solidified in the chamber and formed into particles.

14. The apparatus for producing fine particles according to claim 9 or 10, further comprising a recovery unit connected to the chamber for recovering the atomized particles.

15. The apparatus for producing fine particles according to claim 9 or 10, wherein the metal oxide powder is titanium dioxide powder.

16. The apparatus for producing fine particles according to claim 9 or 10, wherein the atomized particles are an unstoichiometric oxide of the metal constituting the metal oxide powder.

17. The apparatus for producing fine particles according to claim 12, wherein the thermal plasma flame is a reducing atmosphere.

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