Zirconia particles and method for producing zirconia particles

Abstract

The present invention relates to zirconia particles containing molybdenum and each having a polyhedral shape. The molybdenum is preferably distributed non-uniformly in a surface layer of the zirconia particles. The present invention also relates to a method of manufacturing the zirconia particles. The method includes mixing a zirconium compound and a molybdenum compound to form a mixture and firing the mixture.

Description

Technical Field

[0001] The present invention relates to zirconium oxide particles and a method for producing zirconium oxide particles. Background Art

[0002] Zirconium oxide (i.e., zirconia) has many applications due to its mechanical strengths such as wear resistance and high toughness, as well as chemical stability, thermal insulation, heat resistance, and high refractive index, and is used in a wide range of fields such as pigments, paints, coatings, abrasives, electronic materials, thermal insulation materials, optical materials, cosmetics, decorations, biomaterials, and catalysts.

[0003] For example, PTL 1 discloses a method for producing zirconium oxide powder, comprising: (a) wet-milling an aqueous slurry containing zirconium hydrate; (b) applying shock waves to the wet-milled slurry to dry the slurry; and (c) firing the dried material.

[0004] PTL 2 discloses a method for producing ceramic powder. The method comprises: preparing a composite containing yttrium oxide fine powder or yttrium salt uniformly dispersed in zirconium hydroxide; heating the composite at a temperature ranging from 1100°C to 1400°C to form zirconium oxide; and grinding the zirconium oxide to produce ceramic powder.

[0005] [Citation List]

[0006] [Patent Document]

[0007] [PTL 1]

[0008] Japanese Unexamined Patent Application Publication No. 2007-106635

[0009] [PTL 2]

[0010] Japanese Unexamined Patent Application Publication No. 2013-075825 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] The manufacturing methods of zirconium oxide particles disclosed in PTL 1 and PTL 2 are based on the premise of controlling the properties of the particles by adding different metals such as yttrium oxide to an aqueous slurry containing zirconium hydrate.

[0013] However, these two approaches neither consider the hydrolysis and condensation behaviors nor stably control the particle shape.

[0014] The object of the present invention is to provide zirconium oxide particles with stable and controllable shapes and a method for producing the zirconium oxide particles.

[0015] Solutions for solving problems

[0016] The present invention includes the following embodiments.

[0017] [1] Zirconium oxide particles containing molybdenum and each having a polyhedral shape.

[0018] [2] The zirconium oxide particles as described in [1], wherein the molybdenum is unevenly distributed in the surface layer of the zirconium oxide particles.

[0019] [3] The zirconium oxide particles as described in [1] or [2], wherein the [11-1] plane crystallite size of the zirconium oxide particles is 90 nm or more.

[0020] [4] The zirconium oxide particles according to any one of [1] to [3], wherein the median diameter D of the zirconium oxide particles calculated by a laser diffraction / scattering method is 50 0.1 to 1000 μm.

[0021] [5] The zirconium oxide particles according to any one of [1] to [4], wherein the ZrO2 content (Z1) of the zirconium oxide particles relative to 100% by mass of the zirconium oxide particles as determined by XRF analysis of the zirconium oxide particles is 90.0 to 99.9% by mass, and the MoO3 content (M1) of the zirconium oxide particles relative to 100% by mass of the zirconium oxide particles as determined by XRF analysis of the zirconium oxide particles is 0.1 to 5.0% by mass.

[0022] [6] The zirconium oxide particles as described in any one of [1] to [5], wherein the ZrO2 content (Z2) of the zirconium oxide particles relative to 100% by mass of the surface layer of the zirconium oxide particles determined by XPS surface analysis of the zirconium oxide particles is 35.0 to 98.0% by mass, and the MoO3 content (M2) relative to 100% by mass of the surface layer of the zirconium oxide particles determined by XPS surface analysis of the zirconium oxide particles is 2.0 to 40.0% by mass.

[0023] [7] The zirconium oxide particles described in any one of [1] to [6], wherein the surface uneven distribution ratio (M2 / M1) of the MoO3 content (M2) relative to 100% by mass of the surface layer of the zirconium oxide particles determined by XPS surface analysis of the zirconium oxide particles to the MoO3 content (M1) relative to 100% by mass of the zirconium oxide particles determined by XRF analysis of the zirconium oxide particles is 2 to 80.

[0024] [8] The zirconium oxide particles according to any one of [1] to [7], wherein the zirconium oxide particles have an isoelectric point at a potential of 0 as determined by zeta potential measurement and a pH of 2.0 to 6.5.

[0025] [9] The zirconium oxide particles according to any one of [1] to [8], wherein the zirconium oxide particles have a specific surface area of ​​20 m 2 / g or less.

[0026]

[10] The zirconium oxide particles according to any one of [1] to [9], wherein the

[111] plane crystallite size of the zirconium oxide particles is 90 nm or more.

[0027]

[11] A method for producing zirconium oxide particles according to any one of [1] to

[10] , comprising:

[0028] mixing a zirconium compound and a molybdenum compound to form a mixture; and

[0029] The mixture is fired.

[0030]

[12] The method for producing zirconium oxide particles according to

[11] , wherein the molybdenum compound is molybdenum trioxide, lithium molybdate, potassium molybdate or sodium molybdate.

[0031]

[13] The method for producing zirconium oxide particles according to

[11] or

[12] , wherein the maximum firing temperature of the mixture is 800°C to 1600°C.

[0032] Effects of the Invention

[0033] The present invention can provide zirconium oxide particles with stable and controllable shapes and a method for manufacturing the zirconium oxide particles. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] [ Figure 1 ]

[0035] Figure 1 is a SEM image of the zirconium oxide particles of Example 1.

[0036] [ Figure 2A ]

[0037] Figure 2A is an SEM image of the zirconium oxide particles of Example 2.

[0038] [ Figure 2B ]

[0039] Figure 2B is a TEM image of the zirconium oxide particles of Example 2.

[0040] [ Figure 2C ]

[0041] Figure 2C is a TEM image of the zirconium oxide particles of Example 2.

[0042] [ Figure 3 ]

[0043] Figure 3 is an SEM image of the zirconium oxide particles of Example 3.

[0044] [ Figure 4 ]

[0045] Figure 4 is an SEM image of the zirconium oxide particles of Example 4.

[0046] [ Figure 5 ]

[0047] Figure 5 is an SEM image of the zirconium oxide particles of Example 5.

[0048] [ Figure 6 ]

[0049] Figure 6 is an SEM image of the zirconium oxide particles of Example 6.

[0050] [ Figure 7 ]

[0051] Figure 7 is an SEM image of the zirconium oxide particles of Example 7.

[0052] [ Figure 8 ]

[0053] Figure 8 is an SEM image of the zirconium oxide particles of Example 8.

[0054] [ Figure 9 ]

[0055] Figure 9 is an SEM image of the zirconium oxide particles of Example 9.

[0056] [ Figure 10 ]

[0057] Figure 10 This is an SEM image of the zirconium oxide particles of Comparative Example 1.

[0058] [ Figure 11 ]

[0059] Figure 11 This is an SEM image of the zirconium oxide particles of Comparative Example 2.

[0060] [ Figure 12 ]

[0061] Figure 12 It is a graph showing the XRD analysis results of the zirconium oxide particles of Examples and Comparative Examples.

[0062] [ Figure 13 ]

[0063] Figure 13 is a graph showing the results of XRD analysis of the zirconia particles of Example 2 before and after washing. DETAILED DESCRIPTION

[0064] [Zirconia particles]

[0065] The zirconia particles according to an embodiment contain molybdenum and each have a polyhedral shape.

[0066] The zirconia particles according to the present embodiment contain molybdenum. In the production method of the zirconia particles described below, controlling the amount and state of molybdenum allows the particle shape to be stably controlled so that the zirconia particles each have a polyhedral shape, and the particle size, shape, physical properties, and performance, etc. of the zirconia particles can be freely adjusted according to the purpose of use.

[0067] In the present specification, controlling the particle shape of the zirconia particles means that the produced zirconia particles are not amorphous. In the present specification, the zirconia particles having a controllable particle shape means the zirconia particles having a non-amorphous particle shape.

[0068] The zirconia particles according to an embodiment produced by the production method according to an embodiment can have a self-shape specific to a polyhedral shape, as described below in the examples.

[0069] In the present specification, "polyhedral shape" refers to a hexahedron or a polyhedron having more faces, preferably an octahedron or a polyhedron having more faces, more preferably a decahedron to triacontahedron.

[0070] With regard to the zirconia particles according to the present embodiment, molybdenum is preferably unevenly distributed in the surface layer of each zirconia particle.

[0071] "Surface layer" refers to a layer within 10 nm from the surface of each zirconia particle according to an embodiment. This distance corresponds to the detection depth of XPS used for measurement in the examples.

[0072] "Unevenly distributed in the surface layer" means that the mass of molybdenum or a molybdenum compound per unit volume in the surface layer is greater than the mass of molybdenum or a molybdenum compound per unit volume in a region other than the surface layer.

[0073] In the zirconia particles according to the present embodiment, the MoO3 content (M2) relative to 100 mass% of the surface layer of the zirconia particles determined by XPS surface analysis of the zirconia particles is greater than the MoO3 content (M1) relative to 100 mass% of the zirconia particles determined by XRF analysis of the zirconia particles, which can be determined to uneven distribution of molybdenum in the surface layer of the zirconia particles, as described below in the examples.

[0074] The uneven distribution of molybdenum or the molybdenum compound in the surface layer results in the zirconium oxide particles having high dispersion stability compared to the distribution of molybdenum or the molybdenum compound in both the surface layer and the region other than the surface layer (inner layer).

[0075] The [11-1] plane grain size of the zirconium oxide particles according to this embodiment is preferably 90 nm or more, more preferably 95 nm or more, and even more preferably 100 nm or more. The [11-1] plane grain size of the zirconium oxide particles according to this embodiment may be 450 nm or less, 400 nm or less, or 350 nm or less. The [11-1] plane grain size of the zirconium oxide particles according to this embodiment may be 90 nm or more and 450 nm or less, more preferably 95 nm or more and 400 nm or less, and even more preferably 100 nm or more and 350 nm or less.

[0076] In this specification, the [11-1] plane grain size of the zirconium oxide particles is a grain size calculated by using the Scherrer equation based on the full width at half maximum of the peak derived from the [11-1] plane (i.e., the peak near 2θ=28.2°) measured by using an X-ray diffraction method (XRD method).

[0077] The zirconium oxide particles according to the present embodiment can have a [11-1] plane grain size as large as 90 nm or more, can maintain high crystallinity, and can be easily controlled to have a polyhedral shape with a large average particle size.

[0078] The

[111] -plane crystal size of the zirconium oxide particles according to this embodiment is preferably 90 nm or more, more preferably 95 nm or more, and even more preferably 100 nm or more. The

[111] -plane crystal size of the zirconium oxide particles according to this embodiment may be 450 nm or less, 400 nm or less, or 350 nm or less. The

[111] -plane crystal size of the zirconium oxide particles according to this embodiment may be 90 nm or more and 450 nm or less, preferably 95 nm or more and 400 nm or less, and more preferably 100 nm or more and 350 nm or less.

[0079] In this specification, the

[111] plane grain size of the zirconium oxide particles is a grain size calculated by using the Scherrer equation based on the full width at half maximum of the peak derived from the

[111] plane (i.e., the peak near 2θ=31.5°) measured by using an X-ray diffraction method (XRD method).

[0080] The zirconium oxide particles according to the present embodiment can have a [11-1] plane grain size as large as 90 nm or more and a

[111] plane grain size as large as 90 nm or more, can maintain high crystallinity, and can be easily controlled to have a polyhedral shape with a large average particle size.

[0081] The average particle diameter of the primary particles of the zirconium oxide particles according to the present embodiment may be 0.10 to 400 μm, 0.10 to 200 μm, 0.10 to 100 μm, or 0.10 to 50 μm.

[0082] The average particle size of the primary particles of the zirconium oxide particles refers to the average primary particle size of at least 50 primary particles, wherein the primary particle size means the average value of the major axis (maximum Feret's diameter observed) and the minor axis (short Feret's diameter perpendicular to the maximum Feret's diameter) of each minimum unit particle (i.e., primary particle) forming an aggregate on a two-dimensional image of the zirconium oxide particles captured by a scanning electron microscope (SEM).

[0083] The ratio of the [11-1] plane grain size of the zirconium oxide particles to the average particle size of the primary particles of the zirconium oxide particles may be 0.01 to 1.00, 0.05 to 0.90, 0.10 to 0.80, or 0.20 to 0.60.

[0084] The median diameter D of the zirconium oxide particles according to the present embodiment calculated by the laser diffraction / scattering method is 50 It is preferably 0.1 to 1000 μm, preferably 0.5 to 600 μm, more preferably 1.0 to 400 μm, still more preferably 2.0 to 200 μm.

[0085] The median diameter D of the zirconium oxide particles calculated by laser diffraction / scattering method is 50 The median diameter D can be determined by measuring the particle size distribution of the zirconium oxide particle sample by a dry method using a laser diffraction dry particle size distribution analyzer, and obtaining the particle size at the point where the distribution curve of cumulative volume % intersects the horizontal axis at 50% as the median diameter D. 50 .

[0086] The median diameter D of the zirconia particles 50 The ratio to the average particle size of the primary particles of the zirconium oxide particles may be 0.01 to 1.00, 0.01 to 0.80, 0.01 to 0.60, or 0.01 to 0.40.

[0087] The zirconia particles according to the present embodiment preferably have a ZrO2 content (Z1) of 90.0 to 99.9% by mass relative to 100% by mass of the zirconia particles as determined by XRF analysis of the zirconia particles, and preferably have a MoO3 content (M1) of 0.1 to 5.0% by mass relative to 100% by mass of the zirconia particles as determined by XRF analysis of the zirconia particles.

[0088] The ZrO 2 content (Z1) and the MoO 3 content (M1) can be measured by, for example, X-ray fluorescence (XRF) analysis using an X-ray fluorescence spectrometer (Primus IV) available from Rigaku Corporation.

[0089] The zirconia particles according to the present embodiment preferably have a ZrO2 content (Z2) of 35.0 to 98.0% by mass relative to 100% by mass of the surface layer of the zirconia particles as determined by XPS surface analysis of the zirconia particles, and preferably have a MoO3 content (M2) of 2.0 to 40.0% by mass relative to 100% by mass of the surface layer of the zirconia particles as determined by XPS surface analysis of the zirconia particles.

[0090] The zirconium oxide particles according to the present embodiment may further contain lithium, potassium, sodium, or silicon.

[0091] In the zirconia particles according to the present embodiment, a surface uneven distribution ratio (M2 / M1) of the MoO3 content (M2) relative to 100 mass% of the surface layer of the zirconia particles determined by XPS surface analysis of the zirconia particles to the MoO3 content (M1) relative to 100 mass% of the zirconia particles determined by XRF analysis of the zirconia particles is 2 to 80.

[0092] The zirconium oxide particles according to the present embodiment have a more acidic pH at an isoelectric point when the potential is 0 as determined by zeta potential measurement than conventional zirconium oxide particles.

[0093] The pH of the isoelectric point of the zirconium oxide particles according to the present embodiment is, for example, in the range of 2.0 to 6.5, preferably in the range of 2.5 to 5, and more preferably in the range of 2.9 to 4. The zirconium oxide particles having an isoelectric point in the above pH range have high electrostatic repulsion and can improve their own dispersion stability when mixed with a dispersion medium.

[0094] The specific surface area of ​​the zirconium oxide particles according to the present embodiment determined by the BET method may be 20 m 2 / g or less, 15m 2 / g or less, or 10m 2 / g or less.

[0095] The specific surface area of ​​the zirconium oxide particles according to the present embodiment, as determined by the BET method, may be 0.01 to 20 m 2 / g, 0.1 to 15m 2 / g or 0.2 to 10m 2 / g.

[0096] [Method for producing zirconium oxide particles]

[0097] A production method according to one embodiment is a method for producing zirconium oxide particles, comprising mixing a zirconium compound and a molybdenum compound to form a mixture and firing the mixture.

[0098] A preferred method for producing zirconium oxide particles includes the steps of mixing a zirconium compound and a molybdenum compound to form a mixture (mixing step) and firing the mixture (firing step).

[0099] [Mixing step]

[0100] The mixing step involves mixing the zirconium compound and the molybdenum compound to form a mixture. The contents of the mixture will be described below.

[0101] [Zirconium compounds]

[0102] The zirconium compound is any compound that forms zirconium oxide when fired. Examples of zirconium compounds include, but are not limited to, zirconium oxide, zirconyl hydroxide, and zirconium hydroxide.

[0103] [Molybdenum compounds]

[0104] The molybdenum compound is, for example, a molybdenum oxide or a molybdate compound.

[0105] Examples of molybdenum oxides include molybdenum dioxide and molybdenum trioxide, with molybdenum trioxide being preferred.

[0106] Examples of molybdate compounds include, but are not limited to, salt compounds of molybdenum oxyanions, such as MoO4 2- 、Mo2O7 2- 、Mo3O 10 2- 、Mo4O 13 2- 、Mo5O 16 2- 、Mo6O 19 2- 、Mo7O 24 6- and Mo8O 26 4- The molybdate compound may be an alkali metal salt, an alkaline earth metal salt or an ammonium salt of a molybdenum oxyanion.

[0107] The molybdate compound is preferably an alkali metal salt of a molybdenum oxyanion, more preferably lithium molybdate, potassium molybdate or sodium molybdate, still more preferably potassium molybdate or sodium molybdate.

[0108] Since the alkali metal salt of the molybdenum oxyanion does not evaporate even within the firing temperature range and can be easily collected by washing after firing, the amount of molybdenum compound released from the firing furnace is low and the production cost can be significantly reduced.

[0109] The molybdenum compound may contain silicon. In this case, the silicon-containing molybdenum compound functions as both a flux and a shape control agent.

[0110] In the method for producing zirconium oxide particles according to the present embodiment, the molybdate compound may be a hydrate.

[0111] The molybdenum compound is preferably molybdenum trioxide, lithium molybdate, potassium molybdate or sodium molybdate, and more preferably molybdenum trioxide, potassium molybdate or sodium molybdate.

[0112] In the method for producing zirconium oxide particles according to the present embodiment, a molybdenum compound is used as a flux. In this specification, the production method using a molybdenum compound as a flux may be simply referred to as a "flux method". During firing, the molybdenum compound reacts with the zirconium compound at a high temperature to form zirconium molybdate, and then when the zirconium molybdate decomposes into zirconium oxide and molybdenum oxide at a higher temperature, the molybdenum compound is introduced into the zirconium oxide particles. Molybdenum oxide sublimes and is removed from the system. In this process, the reaction between the molybdenum compound and the zirconium compound causes the molybdenum compound to be formed in the surface layer of the zirconium oxide particles. Regarding the generation mechanism of the molybdenum compound contained in the zirconium oxide particles, more specifically, the reaction between molybdenum and Zr atoms forms Mo-O-Zr in the surface layer of the zirconium oxide particles, and firing at a high temperature causes the detachment of Mo, and forms, for example, molybdenum oxide or a compound having a Mo-O-Zr bond in the surface layer of the zirconium oxide particles.

[0113] The molybdenum oxide that is not incorporated into the zirconium oxide particles can be collected by sublimation and reused. Therefore, the amount of molybdenum oxide adhering to the surface of the zirconium oxide particles can be reduced, and the zirconium oxide particles can maintain their original properties to the greatest extent.

[0114] In the manufacturing method of the present invention described below, the sublimable substance is referred to as a flux, and the non-sublimable substance is referred to as a shape control agent.

[0115] [Shape control agent]

[0116] In order to form the zirconium oxide particles according to the present embodiment, a shape control agent may be used.

[0117] The shape control agent plays an important role in the growth of zirconium oxide crystals during firing of the mixture in the presence of the molybdenum compound.

[0118] Examples of shape control agents include alkali metal carbonates and silicon oxides. Specific examples include potassium carbonate, lithium carbonate, sodium carbonate, and silicon dioxide. Molybdate compounds function as both fluxing agents and shape control agents.

[0119] In the method for producing zirconium oxide particles according to this embodiment, the amounts of the zirconium compound and the molybdenum compound are not limited. Preferably, a mixture can be formed by mixing 35% by mass or more of the zirconium compound and 65% by mass or less of the molybdenum compound relative to 100% by mass of the mixture, and the mixture can be fired. More preferably, a mixture can be formed by mixing 40% by mass or more and 99% by mass or less of the zirconium compound and 0.5% by mass or more and 60% by mass or less of the molybdenum compound relative to 100% by mass of the mixture, and the mixture can be fired. Even more preferably, a mixture can be formed by mixing 45% by mass or more and 95% by mass or less of the zirconium compound and 2% by mass or more and 55% by mass or less of the molybdenum compound relative to 100% by mass of the mixture, and the mixture can be fired.

[0120] In the method for producing zirconium oxide particles according to this embodiment, using these compounds within the above-mentioned range allows the obtained zirconium oxide particles to contain an appropriate amount of molybdenum compound and to stably control the particle shape. The method for producing zirconium oxide particles according to this embodiment can produce zirconium oxide particles each having a polyhedral shape and having a large [11-1] plane grain size and a large

[111] plane grain size. The method for producing zirconium oxide particles according to this embodiment can unevenly distribute molybdenum in the surface layer of the zirconium oxide particles, can shift the pH of the isoelectric point of the zirconium oxide particles toward the acidic side, and can improve the dispersion stability of the zirconium oxide particles when mixed with a dispersion medium.

[0121] [Firing steps]

[0122] The firing step involves firing the mixture. The zirconium oxide particles according to the present embodiment are produced by firing the mixture. As described above, this manufacturing method is called a flux method.

[0123] The flux method is classified as a solution method. In more detail, the flux method is a crystal growth method that utilizes the crystal-flux binary system, which is a eutectic phase diagram. It is speculated that the flux method has the following mechanism. Specifically, heating the mixture of the melt and the flux causes the melt and the flux to form a liquid phase. Because the flux is a fusing agent, in other words, the melt-flux binary system shows a eutectic phase diagram, the melt melts at a temperature below the melting point of the melt to form a liquid phase. In this state, the evaporation of the flux leads to a low flux content, that is, the effect of the flux in lowering the melting point of the melt is reduced. This evaporation of the flux drives the crystal growth of the melt (flux evaporation method). Cooling the liquid phase of the solute and the flux can also cause the crystal growth of the solute (slow cooling method).

[0124] The flux method has advantages such as crystal growth at a temperature far below the melting point, precise control of the crystal structure, and formation of euhedral polyhedral crystals.

[0125] The mechanism of producing zirconium oxide particles using a flux method using a molybdenum compound as a flux is not entirely clear, but is presumed to be as follows, for example. Specifically, a zirconium compound is fired in the presence of a molybdenum compound to first form zirconium molybdate. In this case, the zirconium molybdate allows zirconium oxide crystals to grow at temperatures below the melting point of zirconium oxide, as will be understood from the above description. The zirconium molybdate is then decomposed by, for example, evaporation of the flux, causing crystal growth, to provide zirconium oxide particles. In other words, the molybdenum compound acts as a flux, and zirconium molybdate is formed as an intermediate in the production of zirconium oxide particles.

[0126] The mechanism of producing zirconium oxide particles by a flux method that further uses a shape-controlling agent is not entirely clear. For example, a flux method using a potassium compound as a shape-controlling agent is hypothesized to have the following mechanism. First, a molybdenum compound reacts with a zirconium compound to form zirconium molybdate. For example, the zirconium molybdate decomposes into molybdenum oxide and zirconium oxide. Simultaneously, the molybdenum compound containing molybdenum oxide formed by the decomposition reacts with the potassium compound to form potassium molybdate. In the presence of the molybdenum compound containing potassium molybdate, zirconium oxide crystals grow, thereby providing the zirconium oxide particles according to this embodiment.

[0127] The flux method can produce molybdenum-containing zirconia particles, in which the molybdenum is unevenly distributed in the surface layer of the zirconia particles.

[0128] The firing method is not limited and can be any conventionally known method. At a firing temperature above 600°C, the zirconium compound reacts with the molybdenum compound to form zirconium molybdate. At a firing temperature above 800°C, the zirconium molybdate (Zr(MoO4)2) decomposes due to the action of the shape-controlling agent to form zirconium oxide particles. The molybdenum compound is incorporated into the zirconium oxide particles as the zirconium molybdate decomposes into zirconium oxide and molybdenum oxide.

[0129] At a firing temperature of 1000° C. or higher, a molybdenum compound (eg, molybdenum trioxide) generated by decomposition of zirconium molybdate reacts with a potassium compound serving as an exemplary shape control agent to form potassium molybdate.

[0130] Specifically, as long as the zirconium compound and the molybdenum compound exist in the same space so that the molybdenum compound can react with the zirconium compound, the form of the zirconium compound and the molybdenum compound during firing is not limited. Specifically, the molybdenum compound and the zirconium compound can be simply mixed in the form of powder, mechanically mixed using a grinder or the like, or mixed using a mortar or the like, and can be mixed in a wet state or a dry state.

[0131] The firing temperature is not limited and is appropriately set based on, for example, the desired average particle size of the zirconium oxide particles, the formation and dispersibility of the molybdenum compound in the zirconium oxide particles. Regarding the firing temperature, the maximum firing temperature is preferably higher than or equal to 800°C, which is the temperature at which zirconium molybdate Zr(MoO4)2 decomposes, and more preferably higher than or equal to 900°C.

[0132] Generally, in order to control the shape of the zirconia obtained after firing, it is necessary to fire it at a high temperature of not less than 2400°C, which is near the melting point of zirconia. However, this firing poses problems for industrial application in terms of the burden on the firing furnace and energy costs.

[0133] The production method according to the present invention can be performed at high temperatures exceeding 1800°C. However, at temperatures below 1600°C, which is well below the melting point of zirconia, zirconia particles each having a polyhedral shape and having a large [11-1] plane grain size and a large

[111] plane grain size can be formed, regardless of the shape of the precursor. Because the zirconia particles have molybdenum unevenly distributed in the surface layer and have an isoelectric point at a more acidic pH than existing zirconia particles, they exhibit high electrostatic repulsion and excellent dispersion stability.

[0134] According to an embodiment of the present invention, zirconium oxide particles having good dispersion stability and having a large

[111] plane grain size and a large [11-1] plane grain size can be formed at low cost even under conditions of a maximum firing temperature of 800°C to 1600°C. More preferably, the particles are fired at a maximum firing temperature of 850°C to 1500°C, and most preferably, at a maximum firing temperature of 900°C to 1400°C.

[0135] In view of production efficiency and to reduce the possibility of damage to the preparation container (crucible or saggar) due to rapid thermal expansion, the heating rate is preferably 1 to 30°C / min, more preferably 2 to 20°C / min, and even more preferably 3 to 10°C / min.

[0136] Regarding the firing time, the temperature rise time to the predetermined maximum firing temperature is preferably in the range of 15 minutes to 10 hours, and the holding time at the predetermined maximum firing temperature is preferably in the range of 1 to 30 hours. In order to efficiently form the zirconium oxide particles, the holding time at the maximum firing temperature is more preferably about 2 to 15 hours.

[0137] By selecting the conditions of a maximum firing temperature of 800° C. to 1600° C. and a holding time at the maximum firing temperature of 2 to 15 hours, zirconia particles containing molybdenum unevenly distributed in the surface layer of the zirconia particles can be easily obtained.

[0138] As long as the advantageous effects of the present invention are obtained, the firing atmosphere is not limited. However, the firing atmosphere is preferably an oxygen-containing atmosphere such as air or oxygen, an inert atmosphere such as nitrogen, argon or carbon dioxide, and more preferably an air atmosphere to reduce costs.

[0139] The firing device is not necessarily limited and may be a so-called firing furnace. The firing furnace is preferably composed of a material that does not react with the sublimated molybdenum oxide. In order to effectively use the molybdenum oxide, the firing furnace is more preferably a highly sealed firing furnace.

[0140] Use of such a firing furnace can reduce the amount of molybdenum compounds adhering to the surface of the zirconia particles and allow the zirconia particles to retain their original properties to the greatest extent possible.

[0141] [Molybdenum removal step]

[0142] The method for producing zirconium oxide particles according to the present embodiment may further include a molybdenum removing step of removing at least a portion of the molybdenum as needed after the firing step.

[0143] As described above, since molybdenum sublimes during firing, controlling the firing time, firing temperature, etc. allows controlling the molybdenum content in the surface layer of the zirconia particles, as well as controlling the molybdenum content and state in the region other than the surface layer (inner layer) of the zirconia particles.

[0144] Molybdenum may adhere to the surface of the zirconium oxide particles. Molybdenum can be removed by washing with water, an aqueous ammonia solution, an aqueous sodium hydroxide solution, or an acidic aqueous solution, rather than by sublimation. While molybdenum need not be removed from the zirconium oxide particles, it is preferably removed from at least the surface of the particles. This is because when the zirconium oxide particles are dispersed in a binder-like medium, the original properties of the zirconium oxide can be fully exhibited without being affected by the molybdenum present on the surface.

[0145] In this case, the molybdenum content can be controlled by appropriately changing, for example, the concentration and amount of water, the ammonia aqueous solution, sodium hydroxide aqueous solution or acidic aqueous solution used, the cleaning site and the cleaning time.

[0146] [Grinding steps]

[0147] The fired material obtained after the firing step may contain aggregated zirconia particles and may not meet the average particle size range applicable to the present invention. To this end, the zirconia particles may be ground as needed to meet the average particle size range applicable to the present invention.

[0148] The method for grinding the combustion material is not limited, and may be a conventionally known grinding method using, for example, a ball mill, a jaw crusher, a jet mill, a disc mill, a SpectroMill, a grinder, or a mixer mill.

[0149] [Grading steps]

[0150] In order to adjust the average particle size to improve the flowability of the powder or to reduce the increase in viscosity when mixed with a binder for forming a matrix, the zirconium oxide particles are preferably classified. "Classification" is the process of grouping particles according to their size.

[0151] The classification may be wet or dry, but dry classification is preferred in terms of productivity.

[0152] Dry classification includes sieve classification and wind classification based on the difference between centrifugal force and fluid resistance. In terms of classification accuracy, wind classification is preferred and can be performed by using a classifier such as an air flow classifier utilizing the Coanda effect, a vortex air flow classifier, a forced vortex type centrifugal classifier, or a semi-free vortex type centrifugal classifier.

[0153] The grinding step and the classification step can be performed at necessary stages. By selecting whether to perform grinding and classification and the conditions for grinding and classification, for example, the average particle size of the obtained zirconium oxide particles can be adjusted.

[0154] The zirconium oxide particles according to the present invention or produced by the production method according to the present invention preferably exhibit little or no aggregation because such particles readily exhibit their original properties and are easy to handle, and exhibit good dispersibility when dispersed in a dispersion medium. When zirconium oxide particles produced without the grinding and classification steps exhibit little or no aggregation, the production method for zirconium oxide particles eliminates the need for the grinding and classification steps. This improves the productivity of zirconium oxide particles having desired properties, which is preferred.

[0155] [Example]

[0156] Next, the present invention will be described more specifically by way of examples, but the present invention is not limited to the following examples.

[0157] (Comparative Example 1)

[0158] As the zirconium oxide particles of Comparative Example 1, commercially available zirconium oxide (a reagent available from Kanto Chemical Co., Inc., ZrO2, purity: 99.0% or more) was used. The SEM image of the zirconium oxide particles of Comparative Example 1 is shown in FIG. Figure 9 The zirconium oxide particles of Comparative Example 1 are amorphous aggregated molten particles.

[0159] (Comparative Example 2)

[0160] [Production of Zirconium Oxide Particles]

[0161] Commercially available zirconium oxide (a reagent purchased from Kanto Chemical Co., Inc., ZrO2, purity: 99.0% or more) (10.0 g) was placed in a crucible and heated in a heating furnace SC-2045D-SP (available from Motoyama Co., Ltd.) from room temperature to 1100° C. at a heating rate of approximately 5° C. / min and fired at 1100° C. for 10 hours. After cooling, the crucible was removed to obtain 10 g of a white powder.

[0162] The SEM image of the zirconium oxide particles of Comparative Example 2 corresponding to the obtained white powder is shown in FIG. Figure 11 The SEM image shows that the zirconium oxide particles of Comparative Example 2 have a larger particle size than the zirconium oxide particles of Comparative Example 1 due to crystal growth. The zirconium oxide particles of Comparative Example 2 aggregate due to low dispersibility. The zirconium oxide particles of Comparative Example 2 remain amorphous.

[0163] [Example 1]

[0164] [Production of Zirconium Oxide Particles]

[0165] In a mortar, 9.5 g of commercially available zirconium oxide (ZrO₂, purity: 99.0% or higher, purchased from Kanto Chemical Co., Inc.) and 0.5 g of molybdenum trioxide (MoO₃, purchased from Nippon Inorganic Colour & Chemical Co., Ltd.) were mixed to form a mixture. The resulting mixture was placed in a crucible and fired in a ceramic electric furnace at 1100°C for 10 hours. After cooling, the crucible was removed to yield 9.5 g of a white powder.

[0166] Subsequently, 9.0 g of the obtained white powder was suspended in 300 ml of 0.5% ammonia water, and the slurry was stirred at room temperature (25° C. to 30° C.) for 3 hours, followed by separation by microfiltration and further washing with water and drying, thereby removing the molybdenum oxide remaining on the surface of the particles, thereby providing 8.7 g of white powder of the zirconium oxide particles of Example 1.

[0167] The scanning electron microscope (SEM) image of the zirconium oxide particles obtained in Example 1 is shown in Figure 1 The zirconium oxide particles of Example 1 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0168] [Example 2]

[0169] [Production of Zirconium Oxide Particles]

[0170] Firing was performed in the same manner as in Example 1, except that the amounts of the raw material reagents in Example 1 were changed to 9.0 g of zirconium oxide (a reagent purchased from Kanto Chemical Co., Inc., ZrO2, purity: 99.0% or more) and 1.0 g of molybdenum trioxide (MoO3, purchased from Nippon Inorganic Colour & Chemical Co., Ltd.). The resulting powder was washed in the same manner as in Example 1 to provide a white powder of zirconium oxide particles of Example 2.

[0171] The scanning electron microscope (SEM) image of the zirconium oxide particles obtained in Example 2 is shown in Figure 2A The transmission electron microscopy (TEM) images are shown in Figure 2B and Figure 2C The zirconium oxide particles of Example 2 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0172] [Example 3]

[0173] [Production of Zirconium Oxide Particles]

[0174] Firing was performed in the same manner as in Example 1, except that the amounts of the raw material reagents in Example 1 were changed to 8.0 g of zirconium oxide (a reagent purchased from Kanto Chemical Co., Inc., ZrO2, purity: 99.0% or more) and 2.0 g of molybdenum trioxide (MoO3, purchased from Nippon Inorganic Colour & Chemical Co., Ltd.). The resulting powder was washed in the same manner as in Example 1 to provide a white powder of zirconium oxide particles of Example 3.

[0175] The SEM image of the zirconium oxide particles obtained in Example 3 is shown in Figure 3 The zirconium oxide particles of Example 3 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0176] [Example 4]

[0177] [Production of Zirconium Oxide Particles]

[0178] Firing was performed in the same manner as in Example 1, except that the amounts of the raw material reagents in Example 1 were changed to 10.0 g of zirconium oxide (a reagent purchased from Kanto Chemical Co., Inc., ZrO2, purity: 99.0% or more) and 10.0 g of molybdenum trioxide (MoO3, purchased from Nippon Inorganic Colour & Chemical Co., Ltd.). The resulting powder was washed in the same manner as in Example 1 to provide a white powder of zirconium oxide particles of Example 4.

[0179] The SEM image of the zirconium oxide particles obtained in Example 4 is shown in Figure 4 Zirconia particles each having a polyhedral shape were observed. The zirconium oxide particles of Example 4 each had a polyhedral shape and were not significantly aggregated, thus having better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0180] [Example 5]

[0181] [Production of Zirconium Oxide Particles]

[0182] In a mortar, mix 10.0 g of zirconium oxide (ZrO₂, purity: 99.0% or higher, available from Kanto Chemical Co., Inc.) and 12 g of sodium molybdate dihydrate (Na₂MoO₄·2H₂O, available from Kanto Chemical Co., Inc.) to form a mixture. The resulting mixture is placed in a crucible and fired in a ceramic electric furnace at 1100°C for 10 hours. After cooling, the crucible is removed to yield 20 g of a white powder.

[0183] Subsequently, the obtained white solid was lightly crushed in a mortar and suspended in 300 ml of water, and the slurry was stirred at room temperature (25° C. to 30° C.) for 3 hours, and then separated by microfiltration and further washed with water and dried to remove lithium molybdate, thereby providing 9.4 g of white powder of zirconium oxide particles.

[0184] The SEM image of the zirconium oxide particles obtained in Example 5 is shown in Figure 5 The zirconium oxide particles of Example 5 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0185] [Example 6]

[0186] [Production of Zirconium Oxide Particles]

[0187] Firing was performed in the same manner as in Example 5, except that 12 g of sodium molybdate dihydrate was replaced with 10 g of potassium molybdate (K2MoO4, available from Kanto Chemical Co., Inc.). The resulting powder was washed in the same manner as in Example 5 to provide a white powder of zirconium oxide particles.

[0188] The SEM image of the zirconium oxide particles obtained in Example 6 is shown in Figure 6 The zirconium oxide particles of Example 6 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0189] [Example 7]

[0190] [Production of Zirconium Oxide Particles]

[0191] Firing was performed in the same manner as in Example 3, except that firing at 900° C. for 10 hours was changed from firing at 1100° C. for 10 hours. The obtained powder was washed in the same manner as in Example 3 to provide a white powder of zirconium oxide particles.

[0192] The SEM image of the zirconium oxide particles obtained in Example 7 is shown in Figure 7 The zirconium oxide particles of Example 7 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0193] [Example 8]

[0194] [Production of Zirconium Oxide Particles]

[0195] Firing was performed in the same manner as in Example 5, except that firing at 1300° C. for 24 hours was changed from firing at 1100° C. for 10 hours. The obtained powder was washed in the same manner as in Example 5 to provide a white powder of zirconium oxide particles.

[0196] The SEM image of the zirconium oxide particles obtained in Example 8 is shown in Figure 8 The zirconium oxide particles of Example 8 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0197] [Example 9]

[0198] [Production of Zirconium Oxide Particles]

[0199] Firing was performed in the same manner as in Example 8, except that 12 g of sodium molybdate dihydrate was replaced with 12 g of potassium molybdate (K2Mo2O7, available from Kanto Chemical Co., Inc.). The resulting powder was washed in the same manner as in Example 8 to provide a white powder of zirconium oxide particles.

[0200] The SEM image of the zirconium oxide particles obtained in Example 9 is shown in Figure 9 The zirconium oxide particles of Example 9 each have a polyhedral shape and are not significantly aggregated, and thus have better dispersibility than the zirconium oxide particles of Comparative Examples 1 and 2.

[0201] [Table 1]

[0202]

[0203] [Measurement of Average Particle Size of Primary Particles of Zirconium Oxide Particles]

[0204] Zirconia particles were captured using a scanning electron microscope (SEM). The major diameter (maximum Feret's diameter observed) and minor diameter (short Feret's diameter perpendicular to the maximum Feret's diameter) of each minimum unit particle (i.e., primary particle) forming an aggregate on a two-dimensional image were measured, and the average of the major and minor diameters was defined as the primary particle size. The same measurement was performed on 50 primary particles whose major and minor diameters could be measured. From the average primary particle size of these primary particles, the average particle size of the primary particles was calculated. The results are shown in Table 2.

[0205] [Measurement of grain size]

[0206] Powder X-ray diffraction (2θ / θ method) was performed under the following measurement conditions using an X-ray diffractometer (SmartLab, available from Rigaku Corporation) equipped with a high-intensity high-resolution crystal analyzer (CALSA) (available from Rigaku Corporation) as a detector. The analysis was performed using the CALSA function of the analysis software (PDXL) available from Rigaku Corporation. The [11-1] face grain size was calculated using the Scherrer equation from the full width at half maximum of the peak near 2θ=28.2°, and the

[111] face grain size was calculated using the Scherrer equation from the full width at half maximum of the peak near 2θ=31.5°. The results are shown in Table 2.

[0207] [Measurement conditions of powder X-ray diffraction method]

[0208] Tube voltage: 45kV

[0209] Tube current: 200mA

[0210] Scanning speed: 0.05° / min

[0211] Scanning range: 10° to 70°

[0212] Step size: 0.002°

[0213] βs: 20rpm

[0214] The standard width of the device is 0.026°, calculated using standard silicon powder produced by the National Institute of Standards and Technology (NIST, 640d).

[0215] [Crystal structure analysis: X-ray diffraction (XRD) method]

[0216] A sample of the zirconium oxide particles of any one of Examples 1 to 2, 5 to 7, 10, and Comparative Examples 1 to 2 was placed in a sample holder having a depth of 0.5 mm and set in a wide-angle X-ray diffraction (XRD) system (Ultima IV, available from Rigaku Corporation). Measurement was performed under the conditions of Cu / Kα radiation, 40 kV / 40 mA, a scanning speed of 2° / min, and a scanning range of 10° to 70°. The XRD analysis results of the zirconium oxide particles of Examples 1 to 2, 5 to 7, and Comparative Examples 1 to 2 are shown in FIG. Figure 12 middle.

[0217] The XRD analysis results of the zirconium oxide particles of Example 2 before and after cleaning are shown in Figure 13 middle.

[0218] The peaks marked with ● are derived from zirconium molybdate Zr(MoO4)2.

[0219] Figure 13 This indicates that the zirconium molybdate Zr(MoO4)2 detected before cleaning was removed by cleaning.

[0220] The [11-1] crystal peak of baddeleyite (ZrO2) was observed at approximately 2θ = 28.2°, while the

[111] crystal peak of baddeleyite (ZrO2) was observed at approximately 2θ = 31.5°. In other words, the patterns of these crystal peaks indicate that the zirconium oxide particles have a baddeleyite (ZrO2) crystal structure.

[0221] [Measurement of Particle Size Distribution of Zirconia Particles]

[0222] The particle size distribution of the zirconium oxide particle sample was measured by a dry method using a laser diffraction dry particle size distribution analyzer (HELOS (H3355) & RODOS, available from Japan Laser Corporation) under conditions of a dispersion pressure of 3 bar and a suction pressure of 90 mbar, and the particle diameter at the point where the distribution curve of cumulative volume % intersects the horizontal axis at 50% was obtained as the median diameter D 50 The results are shown in Table 2.

[0223] [Measurement of the isoelectric point of zirconia particles]

[0224] The zeta potential of the zirconium oxide particles was measured using a zeta potential analyzer (Zetasizer Nano ZSP, Malvern Panalytical Ltd). 20 mg of the sample was mixed with 10 mL of a 10 mM KCl aqueous solution in a THINKY mixer (ARE-310, Thinky Corporation) in mixing / degassing mode for 3 minutes. After the mixture was allowed to stand for 5 minutes, the supernatant was used as a sample for measurement. 0.1 N HCl was added to the sample using an automatic titrator, and the zeta potential was measured (applied voltage 100 V, single mode mode) until pH = 2, and the pH at the isoelectric point when the potential was 0 was determined. The results are shown in Table 2.

[0225] [Measurement of specific surface area of ​​zirconia particles]

[0226] The specific surface area of ​​the zirconium oxide particles was measured by using a surface area analyzer (BELSORP-mini, available from MicrotracBEL Corporation), and the surface area per gram of the sample measured from the amount of nitrogen absorbed by the BET method was calculated as the specific surface area (m 2 The results are shown in Table 2.

[0227] [Measurement of Zirconia Particle Purity: X-ray Fluorescence (XRF) Analysis]

[0228] About 70 mg of a zirconium oxide particle sample was placed on filter paper, covered with a PP film, and subjected to X-ray fluorescence (XRF) analysis using an X-ray fluorescence spectrometer Primus IV (available from Rigaku Corporation) under the following conditions.

[0229] Measurement conditions

[0230] EZ Scan Mode

[0231] Components of interest: F to U

[0232] Measuring time: Standard

[0233] Measuring diameter: 10mm

[0234] Remaining balance: None

[0235] Table 2 shows the results of the ZrO 2 content (Z1) relative to 100 mass % of the zirconium oxide particles and the MoO 3 content (M1) relative to 100 mass % of the zirconium oxide particles determined by XRF analysis.

[0236] [XPS surface analysis]

[0237] Surface elemental analysis of the zirconia particles was performed by X-ray photoelectron spectroscopy (XPS) using a Quantera SXM (available from ULVAC-PHI, Inc.) equipped with a monochromatic Al Kα X-ray source. The average atomic percentage (atom %) of each element was obtained by measuring an area (n=3) of 1000 μm square. To facilitate comparison with the XRF results, the zirconium content and molybdenum content of the surface layer of the zirconia particles were converted to oxide content to obtain the ZrO2 content (Z2) (mass %) and MoO3 content (M2) (mass %) relative to 100 mass % of the surface layer of the zirconia particles. The results are shown in Table 2.

[0238] The surface uneven distribution ratio (M2 / M1) of the MoO3 content (M2) relative to 100% by mass of the surface layer of the zirconia particles, as determined by XPS surface analysis of the zirconia particles, to the MoO3 content (M1) relative to 100% by mass of the zirconia particles, as determined by XRF analysis of the zirconia particles, was calculated. The results are shown in Table 2.

[0239]

[0240] In the zirconia particles of Examples 1 to 9, based on the fact that the MoO3 content (M2) relative to 100 mass% of the surface layer of the zirconia particles determined by XPS surface analysis of the zirconia particles was greater than the MoO3 content (M1) relative to 100 mass% of the zirconia particles determined by XRF analysis of the zirconia particles, it was confirmed that molybdenum was unevenly distributed in the surface layer of the zirconia particles.

[0241] Since the zirconia particles of Examples 1 to 9 have an isoelectric point at a more acidic pH than existing zirconia particles, the zirconia particles of Examples 1 to 9 have high electrostatic repulsion and good dispersion stability.

[0242] Unlike the existing zirconia particles, the zirconia particles of Examples 1 to 9 have molybdenum unevenly distributed in the surface layer thereof and have a relatively larger grain size than the existing zirconia particles.

[0243] [Evaluation of dispersibility in liquid]

[0244] The dispersibility in liquid was evaluated in the following manner.

[0245] (1) Add 20 mg of 35% hydrochloric acid and 0.1 g of zirconium oxide particle sample to 10 g of pure water.

[0246] (2) Shake the mixture by hand for 30 seconds and then let it stand for 5 hours.

[0247] (3) After the mixture was allowed to stand for 5 hours, the appearance was observed to evaluate the dispersibility in liquid based on the following criteria.

[0248] A: Almost no clear supernatant was observed.

[0249] B: The clear portion exists in the upper part of the liquid.

[0250] C: The liquid is clear because almost all the particles have settled.

[0251] Table 3 shows the results of the evaluation of dispersibility in liquid.

[0252] [Table 3]

[0253] Example 1 Example 2 Example 5 Example 6 Example 7 Comparative Example 1 Comparative Example 2 result A A B B A C C

[0254] Since the zirconia particles of the example have molybdenum unevenly distributed in the surface layer thereof, the zirconia particles of the example have better dispersibility in liquid than the zirconia particles of the comparative example.

[0255] Industrial applicability

[0256] The zirconium oxide particles according to the invention are expected to be used as ceramic raw materials for electrolytes in fuel cell applications, catalysts, cutting tool parts, as white pigments without photocatalytic activity or as thermal insulation materials for electric furnaces and rockets.

Claims

1. Zirconium oxide particles containing molybdenum and each having a polyhedral shape, The [11-1] plane grain size of the zirconium oxide particles is greater than 90 nm. 2 . The zirconium oxide particles according to claim 1 , wherein the molybdenum is unevenly distributed in the surface layer of the zirconium oxide particles.

3. The zirconium oxide particles according to claim 1, wherein the median diameter D of the zirconium oxide particles calculated by a laser diffraction / scattering method is 50 0.1 to 1000 μm.

4. The zirconium oxide particles according to claim 1 , wherein the zirconium oxide particles have a ZrO 2 content Z1 of 90.0 to 99.9 mass % relative to 100 mass % of the zirconium oxide particles as determined by XRF analysis of the zirconium oxide particles, and a MoO 3 content M1 of 0.1 to 5.0 mass % relative to 100 mass % of the zirconium oxide particles as determined by XRF analysis of the zirconium oxide particles.

5. The zirconium oxide particles according to claim 1 , wherein the zirconium oxide particles have a ZrO 2 content Z2 of 35.0 to 98.0 mass % relative to 100 mass % of the surface layer of the zirconium oxide particles as determined by XPS surface analysis of the zirconium oxide particles, and a MoO 3 content M2 of 2.0 to 40.0 mass % relative to 100 mass % of the surface layer of the zirconium oxide particles as determined by XPS surface analysis of the zirconium oxide particles.

6. The zirconium oxide particles according to claim 1, wherein a surface uneven distribution ratio M2 / M1 of a MoO3 content M2 relative to 100 mass % of a surface layer of the zirconium oxide particles as determined by XPS surface analysis of the zirconium oxide particles to a MoO3 content M1 relative to 100 mass % of the zirconium oxide particles as determined by XRF analysis of the zirconium oxide particles is 2 to 80. The zirconium oxide particles according to claim 1 , wherein the zirconium oxide particles have an isoelectric point at a potential of 0 as determined by zeta potential measurement, and have a pH of 2.0 to 6.

5.

8. The zirconium oxide particles according to claim 1, wherein the specific surface area of ​​the zirconium oxide particles determined by the BET method is 20 m 2 / g or less. 9 . The zirconium oxide particles according to claim 1 , wherein the [111] plane grain size of the zirconium oxide particles is 90 nm or larger.

10. A method for producing zirconium oxide particles according to any one of claims 1 to 9, the method comprising: mixing a zirconium compound and a molybdenum compound to form a mixture; as well as The mixture is fired. 11 . The method for producing zirconium oxide particles according to claim 10 , wherein the molybdenum compound is molybdenum trioxide, lithium molybdate, potassium molybdate or sodium molybdate. 12 . The method for producing zirconium oxide particles according to claim 10 , wherein a maximum firing temperature of the mixture is 800° C. to 1600° C.

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