Method for producing regenerated zirconia

Abstract

Provided is at least one of a method for producing easily pulverized regenerated zirconia from a zirconia sintered body and a method for regenerating a zirconia sintered body capable of regenerating the zirconia sintered body. This method for producing regenerated zirconia comprises an ammonia treatment step for heat-treating a zirconia sintered body in an oxygen-containing ammonia atmosphere at a temperature of 800-1200 DEG C inclusive, said ammonia treatment step satisfying either the following condition (a) or the following condition (b). (a) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.3 vol% or more. (b) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.2 vol% or more, and the cubic crystal ratio of the zirconia sintered body is 3.5% or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for producing recycled zirconia obtained by processing a zirconia sintered body, and further relates to a method for recycling a zirconia sintered body. BACKGROUND

[0002] A zirconia sintered body is excellent in mechanical properties, electrical conductivity, and aesthetics, and is therefore used not only as a crushing medium, a structural material, but also as a decorative material, an exterior member of an electronic device, a sensor, a biomaterial, and the like.

[0003] From the viewpoint of the sustainable development goals (SDGs) in recent years, recycling technology is attracting attention, and recycling and reuse of a zirconia sintered body are also required. The simplest recycling method is to finely divide the sintered body by crushing or the like, and to reuse it as a raw material powder. For example, as a recycling method for a zirconia sintered body in the related art, a method of crushing a zirconia sintered body in the form of a sheet having a thickness of less than 0.5 μ m into a powder and reusing it as a raw material for a zirconia sintered body has been proposed (for example, Patent Literature 1).

[0004] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Laid-Open No. 2009-263157 SUMMARY OF INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION A zirconia sintered body is generally dense, and has high mechanical strength. Therefore, powderization by crushing is very difficult. The method of Patent Literature 1 can be applied to a zirconia sintered body having an extremely small thickness. However, it cannot be applied to a zirconia sintered body having a shape other than a sheet shape and having a thickness of 0.5 μ m or more.

[0005] In view of such a technical problem, the present disclosure aims to provide at least either one of a method for producing a recycled zirconia that is easily crushed from a zirconia sintered body and a method for recycling a zirconia sintered body that enables recycling of a zirconia sintered body.

[0006] TECHNICAL SOLUTION FOR SOLVING THE PROBLEM In the present disclosure, recycling of a zirconia sintered body having a certain thickness was studied. As a result, it was noted that recycling of a zirconia sintered body can be achieved by temporarily porosifying the zirconia sintered body. Furthermore, it was found that porosification of a zirconia sintered body can be performed by heat-treating the zirconia sintered body in a specific atmosphere, and that a recycled zirconia can be obtained therefrom.

[0007] That is, the present application as recited in the claims, in addition, the gist of the present disclosure as shown below.

[0008] [1] A method of producing a regenerated zirconia, having an ammonia treatment process of heat treating a zirconia sintered body at a temperature of 800°C or higher and 1200°C or lower in an oxygen-containing ammonia atmosphere (i.e., "oxygen-containing ammonia atmosphere"), the ammonia treatment process satisfying either of the following condition (a) and the following condition (b).

[0009] (a) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.3% by volume or more.

[0010] (b) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.2% by volume or more, and the cubic crystal rate of the zirconia sintered body is 3.5% by mass or more.

[0011] [2] The method of producing according to the above [1], wherein the atmosphere gas of the oxygen-containing ammonia atmosphere at the start of the heat treatment of the zirconia sintered body is an atmosphere gas containing at least ammonia (NH3) and oxygen (O2).

[0012] [3] The method of producing according to the above [2], wherein the total concentration of ammonia (NH3) and oxygen (O2) in the atmosphere gas is 85% by volume or more and 100% by volume or less with respect to 100% by volume of the atmosphere gas.

[0013] [4] The method of producing according to any one of the above [1] to [3], wherein the heat treatment is performed in an oxygen-containing ammonia atmosphere in which an atmosphere gas is circulated in a system.

[0014] [5] The method of producing according to any one of the above [1] to [4], wherein the atmosphere gas of the oxygen-containing ammonia atmosphere during the heat treatment of the zirconia sintered body is an atmosphere gas containing at least either of an active species of ammonia and ammonia, and oxygen.

[0015] [6] The method of producing according to any one of the above [1] to [5], wherein the maximum thickness of the zirconia sintered body is 0.5 μ m or more.

[0016] [7] The method of producing according to any one of the above [1] to [6], wherein the relative density of the zirconia sintered body is 98% or more.

[0017] [8] The method of producing according to any one of the above [1] to [7], wherein the zirconia constituting the zirconia sintered body contains a stabilizing element.

[0018] [9] The method of producing according to the above [8], wherein the stabilizing element is yttrium.

[0019]

[10] The production method according to any one of the above [1] to [9], wherein the zirconia sintered body contains 1 or more selected from the group consisting of alumina, silica, and germania.

[0020]

[11] A regeneration method of a sintered body using the regenerated zirconia produced by the production method according to any one of the above [1] to

[10] as a raw material of zirconia.

[0021] Effects of Invention According to the present application, at least either one of a method of producing a regenerated zirconia that is easily pulverized from a zirconia sintered body and a regeneration method of a zirconia sintered body that can regenerate a zirconia sintered body can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is an SEM observation image of the surface of the regenerated zirconia obtained in Example 2.

[0023] Figure 2 is an SEM observation image of the surface of the zirconia sintered body used in Example 2.

[0024] Figure 3 is an SEM observation image of the surface of the regenerated zirconia obtained in Example 3. DETAILED DESCRIPTION

[0025] Hereinafter, an example of an embodiment will be described with respect to the production method of the present disclosure. It should be noted that the present disclosure should be considered to include any combination of the respective configurations and parameters disclosed in the present specification, and should also be considered to include any combination of the upper limits and lower limits of the values disclosed in the present specification.

[0026] The production method of the regenerated zirconia of the present embodiment has a step of heat-treating a zirconia sintered body at a temperature of 800°C or higher and 1200°C or lower in an oxygen-containing ammonia atmosphere (hereinafter, also referred to as "ammonia treatment step"). Moreover, the ammonia treatment step satisfies any one of the following condition (a) and the following condition (b).

[0027] (a) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.3% by volume or more.

[0028] (b) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.2% by volume or more, and the cubic crystal rate of the sintered body of zirconia is 3.5% by mass or more.

[0029] It should be noted that, in the present embodiment, the ammonia treatment step satisfying any one of the condition (a) and the condition (b) means that, in addition to the case where only any one of the condition (a) and the condition (b) is satisfied, the case where both the condition (a) and the condition (b) are satisfied is also included.

[0030] According to the present embodiment including the ammonia treatment process, even a dense zirconia sintered body can be made porous, and the strength thereof can be reduced. Thus, a regenerative zirconia that is easily pulverized, i.e., a zirconia composition that is easily powdered, and a zirconia composition that is easily recycled by horizontal recycling can be manufactured from a zirconia sintered body. Note that, in the present embodiment, recycling of the sintered body includes horizontal recycling and cascade recycling. Horizontal recycling is recycling in which the use before and after recycling is at least either of a sintered body and a precursor thereof, and is distinguished from cascade recycling in which the use after recycling is other than these. Examples include one or more selected from the group consisting of a precursor, a powder, a molded body, and a pre-sintered body of the sintered body.

[0031] First, the common elements of conditions (a) and (b) will be described below.

[0032] In the ammonia treatment process, the zirconia sintered body is heat-treated at a temperature of 800°C or higher and 1200°C or lower in an oxygen-containing ammonia atmosphere.

[0033] The oxygen-containing ammonia atmosphere in the ammonia treatment process is an atmosphere formed by heat treatment of an atmosphere gas at the start of heat treatment of the zirconia sintered body (hereinafter, also referred to as "initial atmosphere gas"). The initial atmosphere gas is an atmosphere in which at least ammonia (NH3) and oxygen (O2) are contained. The oxygen-containing ammonia atmosphere can have a composition different from that of the initial atmosphere gas during heat treatment of the zirconia sintered body, provided that at least ammonia (NH3) and oxygen (O2) are contained in the initial atmosphere gas. For example, the atmosphere gas in the oxygen-containing ammonia atmosphere during heat treatment of the zirconia sintered body is preferably an atmosphere gas containing at least one selected from the group consisting of an active species of ammonia and ammonia, and oxygen, and is further preferably an atmosphere gas containing an active species of ammonia and oxygen. As the active species of ammonia, one or more selected from the group consisting of NH2 - , NH 2- , and N 3- may be used, and at least N 3- is preferably contained. The active species of ammonia functions as active nitrogen. Furthermore, the oxygen-containing ammonia atmosphere can contain other gas components such as nitrogen (N2); as the other gas components, one or more selected from the group consisting of carbon monoxide, hydrogen, and nitrogen can be used, and nitrogen is further preferably contained.

[0034] The initial atmosphere gas in the oxygen-containing ammonia atmosphere is a gas containing at least ammonia (NH3) and oxygen (O2). Specific examples of the initial atmosphere gas include a gas containing ammonia and at least one of air and oxygen, and a gas containing ammonia and air (hereinafter, also referred to as "ammonia-air mixed gas") is further preferably used.

[0035] The total concentration of ammonia (NH3) and oxygen (O2) in the initial atmosphere gas can be, for example, 85% by volume or more and 100% by volume or less, relative to 100% by volume of the initial atmosphere gas. From the viewpoint of producing recycled zirconia that is more easily pulverized, the total concentration of ammonia (NH3) and oxygen (O2) in the initial atmosphere gas is preferably 90% by volume or more and 100% by volume or less, and more preferably 100% by volume (i.e., the initial atmosphere gas consists only of ammonia (NH3) and oxygen (O2)), relative to 100% by volume of the initial atmosphere gas.

[0036] Here, if the heat treatment in the ammonia treatment step is performed in an ammonia atmosphere containing no oxygen, or in an ammonia atmosphere containing oxygen at a concentration that does not satisfy the conditions (a) and (b) described below, the porosification of the zirconia sintered body hardly proceeds. It is believed that the main reason for this is that, when heat treatment is performed in such an atmosphere, the nitridation of zirconia is promoted, and the generation of nitrided zirconia proceeds preferentially. Therefore, when the heat treatment in the ammonia treatment step is performed in such an atmosphere, it is not possible to produce recycled zirconia that is easily pulverized.

[0037] The heat treatment in the ammonia treatment step can be performed in an ammonia atmosphere in which the atmosphere gas remains in the system, or in an ammonia atmosphere in which the atmosphere gas flows in the system (hereinafter, also referred to as "oxygen-containing ammonia flowing atmosphere"). In order to more effectively promote the porosification of the sintered body, it is preferable to perform the heat treatment in an oxygen-containing ammonia flowing atmosphere. The oxygen-containing ammonia flowing atmosphere is an atmosphere in which the atmosphere gas containing oxygen and ammonia flows in the system during the heat treatment, and, for example, an oxygen-containing ammonia atmosphere in which ammonia-air mixed gas flows in the system (hereinafter, also referred to as "ammonia-air mixed gas flowing atmosphere") can be given as the atmosphere gas.

[0038] The method of performing heat treatment on the zirconia sintered body in the oxygen-containing ammonia flowing atmosphere can also use a method known in the art. For example, a method in which the atmosphere gas is made to flow in a reaction tube in which the zirconia sintered body is disposed inside, and the reaction tube in which the atmosphere gas flows is heated, thereby performing heat treatment on the zirconia sintered body, can be used. As a specific heat treatment method, a method in which a setter in which the zirconia sintered body is disposed is disposed in the reaction tube, and heating is performed while the atmosphere gas flows, can be given. The reaction tube and the setter are each a reaction tube and a setter in which side reactions are less likely to occur during heat treatment. As the setter, for example, a ceramic setter can be given, and it is preferable to be one or more selected from the group consisting of a zirconia setter, a mullite setter, and an alumina setter, and more preferably a zirconia setter. In addition, as the reaction tube, for example, a reaction tube selected from the group consisting of a zirconia reaction tube, a quartz reaction tube, a mullite reaction tube, and an alumina reaction tube can be given, and it is preferable to be one or more selected from the group consisting of a zirconia reaction tube, a quartz reaction tube, and an alumina reaction tube, and more preferably a quartz reaction tube.

[0039] The flow rate of the atmosphere gas (e.g., ammonia-air mixed gas) as the atmosphere gas in the oxygen-containing ammonia atmosphere can be exemplified as 400 seem (= ml / minute. 1 atmosphere, 0°C) or more and 800 seem or less, for example. From the viewpoint of producing the recycled zirconia that is more easily pulverized, the flow rate of the atmosphere gas is preferably 450 seem or more and 600 seem or less.

[0040] As one of the reasons that the zirconia sintered body is made porous by the ammonia treatment process including any one of the conditions (a) and (b), it is considered that, by the heat treatment in the oxygen-containing ammonia atmosphere, the formation of nitrogen molecules due to the diffusion and association of nitrogen in an active state into the inside of the zirconia sintered body occurs, and the zirconia sintered body is made porous. That is, in the heat treatment under the oxygen-containing ammonia atmosphere, the active species of ammonia containing nitrogen in an active state generated from ammonia diffuses into the inside of the zirconia sintered body. Then, the nitrogen in an active state that has diffused into the inside of the zirconia sintered body becomes gaseous nitrogen (N2). It is considered that, by the generation of nitrogen (N2) in the inside of the zirconia sintered body and the detachment of the generated nitrogen (N2) from the zirconia sintered body, pores are formed, and thus the sintered body is made porous.

[0041] The temperature in the ammonia treatment process (heat treatment temperature) is 800°C or more and 1200°C or less. If the heat treatment temperature is outside the range of 800°C or more and 1200°C or less, the zirconia sintered body is hardly made porous, or the making porous takes a very long time. One of the reasons is considered to be that, if the heat treatment temperature is less than 800°C, the nitrogen in an active state generated from ammonia becomes less, and the nitrogen does not easily diffuse into the inside of the zirconia sintered body. Another reason is also considered to be that, if the heat treatment temperature exceeds 1200°C, the decomposition of ammonia proceeds, and the diffusion of the nitrogen in an active state into the inside of the zirconia sintered body hardly proceeds. Therefore, if the heat treatment temperature in the ammonia treatment process is outside the range of 800°C or more and 1200°C or less, the recycled zirconia that is easily pulverized cannot be produced.

[0042] The heat treatment temperature in the ammonia treatment process can be 800°C or more and 1200°C or less. From the viewpoint of producing the recycled zirconia that is more easily pulverized, the heat treatment temperature is preferably 900°C or more and 1100°C or less.

[0043] The time of the heat treatment in the ammonia treatment process can be appropriately set according to the amount of the zirconia sintered body subjected to the heat treatment in the ammonia treatment process and the performance of the heat treatment furnace, and can be exemplified as 5 hours or more, further 10 hours or more. The upper limit of the time of the heat treatment in the ammonia treatment process can be exemplified as 50 hours or less, further 20 hours or less. As the heat treatment time, 5 hours or more and 50 hours or less, or 10 hours or more and 20 hours or less can be exemplified.

[0044] The zirconia sintered body subjected to heat treatment in the ammonia treatment step (hereinafter, also referred to as "raw material sintered body") is a sintered body composed of crystal grains of zirconia, but as long as it is a sintered body mainly composed of crystal grains of zirconia, it can be a sintered body composed of crystal grains of zirconia and crystal grains of an additive component (described later).

[0045] The raw material sintered body can be a sintered body manufactured by sintering zirconia particles (powder) only. The zirconia sintered body can be a sintered body manufactured by sintering a molded body (powder compact) of zirconia particles (powder) molded into a prescribed shape by compression or the like, or a sintered body manufactured by sintering a pre-sintered body obtained by molding and pre-sintering zirconia particles (powder). Note that the molding conditions, sintering conditions, and pre-sintering conditions can use conventionally known conditions, and are not particularly limited. From the viewpoint of SDGs, the raw material sintered body is preferably a zirconia sintered body after circulation, that is, a used zirconia sintered body (used sintered body) that has once been used as a product on the market.

[0046] The crystal phase of the raw material sintered body can be a crystal phase containing at least any one of tetragonal crystal and cubic crystal, but is preferably a crystal phase containing at least cubic crystal. As the crystal phase of the raw material sintered body, a crystal phase composed of a mixed crystal of tetragonal crystal and cubic crystal or a crystal phase composed of cubic crystal can be given. For convenience, in the present embodiment, the crystal phase of zirconia can be considered to be monoclinic crystal, tetragonal crystal, and cubic crystal.

[0047] The raw material sintered body is preferably a dense zirconia sintered body that is difficult to be micronized by physical treatment such as pulverization. Specifically, a zirconia sintered body having a relative density of 98% or more is preferred, and a zirconia sintered body having a relative density of 99% or more is more preferred. The relative density can be 100% or less or 99.9% or less, and for example, a relative density of 98.5% or more and 100% or less or a relative density of 99.0% or more and 99.9% or less can be given.

[0048] In the present embodiment, the "relative density" is a density calculated from the measured density ρ [g / cm 3 ] obtained by the Archimedes method and the mass obtained by mass measurement ρ 0[g / cm 3 ] using the following formula (3). In the case where the raw material sintered body contains yttrium as a stabilizing element, the measured density of the sintered body ρ and the true density of the sintered body calculated from the following formulae (1) and (2) can be used. ρ 0[g / cm 3], the relative density of the sintered body is calculated by the following (3). Note that X in (1) and (2) is the yttrium content [mol%] in the zirconia (stabilized zirconia).

[0049] α = [0.5080 + 0.06980X / (100 + X)] 2 x [0.5195 - 0.06180X / (100 + X)] (1) ρ 0 = [124.25(100 - X) + 225.81X] / [150.5(100 + X) α ] (2) Relative density [%] = (0 ρ / ρ 0) x 100 (3) In addition, in the case where the raw material sintered body contains at least a stabilizing element other than yttrium (the case where the raw material sintered body contains a stabilizing element other than yttrium and yttrium as a stabilizing element, or the case where the raw material sintered body contains only a stabilizing element other than yttrium as a stabilizing element), instead of 0 calculated from (2), 0 calculated from (2') is used. ρ 0 [g / cm 3 ], the relative density is calculated from (3). In the case where 0 calculated from (2') is used, X in (1) can use the total content X [mol%] of the stabilizing elements in the zirconia (stabilized zirconia). ρ ρ

[0050] ρ 0 = [124.25(100 - X) + M1X1+ M2X2+... + M n X n ] / [150.5(100 + X) α ] (2') In (2'), M1, M2,..., M n are the amounts of substance [g / mol] of the respective stabilizing elements converted into oxides. X1, X2,..., X n are the contents [mol%] of the respective stabilizing elements, and X is the total content [mol%] of the stabilizing elements in the zirconia (stabilized zirconia), and is calculated from X = X1+... + X n Note that the numbers attached to M1, M2,..., M n and X1, X2,..., X n indicate the kinds of the stabilizing elements, and n is an integer of 1 or more, and is up to the number of kinds of the stabilizing elements contained in the raw material sintered body. ​​

[0051] Note that, in the case where the raw material sintered body does not contain a stabilizing element, X in the formulae (1) and (2) is replaced by 0, and the true density is calculated from the formula (2) ρ 0.

[0052] In addition, in the case where the raw material sintered body contains an additive component (described later), instead of the true density calculated from the formula (2) or (2'), the true density calculated from the formula (2") ρ 0 is used. ρ z [g / cm 3 ] (i.e., the relative density [%] = (z) x 100 as the formula (3)) is calculated. ρ ρ z) x 100 as the formula (3)) is calculated.

[0053] ρ z = 100 / [Y1 / M add1 + Y2 / M add2 +... + Y n / M addn + (100 - Y) / (100 - Y) ρ 0] (2") In the formula (2"), M add1 , M add2 ,..., M addn are the true densities [g / cm 3 ] of the respective additive components in the oxide form. Y1, Y2,..., Y n are the contents [mass %] of the respective additive components in the sintered body, and Y is the total content [mass %] of the additive elements in the zirconia (stabilized zirconia), which is calculated from Y = Y1 +... + Y n Note that the numbers attached to M add1 , M add2 ,..., M addn and Y1, Y2,..., Y n indicate the types of the additive components, and n is an integer of 1 or more, with the upper limit being the number of types of the additive components contained in the raw material sintered body. ρ 0 is the true density [g / cm 3 ] of the sintered body calculated from the formula (2) or (2').

[0054] Here, the true densities (M add ) of the respective additive components in the oxide form in the formula (2") are, for example, the true density (M add ) of aluminum oxide is 3.987 g / cm 3 , the true density of silicon dioxide is 2.334 g / cm 3 , and the true density of germanium oxide is 4.250 g / cm 3 .​

[0055] From the viewpoint of making it easier to pulverize the regenerated zirconia, the average grain particle diameter of the crystal grains of the raw material sintered body is preferably 10 μ or less, more preferably 5.0 μ or less, further preferably 3.0 μ or less, still more preferably 0.5 μ or less. The lower limit of the average grain particle diameter of the crystal grains can be exemplified by 0.1 μ or more. As the average grain particle diameter of the raw material sintered body, 0.1 μ or more and 10 μ or less, 0.1 μ or more and 5.0 μ or less, 0.1 μ or more and 0.5 μ or less.

[0056] The "average grain particle diameter" in the present embodiment can be obtained by the planimetric method from a scanning electron microscope (hereinafter, also referred to as "SEM") observation image obtained by the following measurement conditions using a conventional scanning electron microscope (device name: JSM-IT500, manufactured by JEOL Ltd.).

[0057] Accelerating voltage: 10 kV Observation magnification: 2000x to 40000x Before measurement by the planimetric method, the zirconia sintered body sample is mirror-polished so that the surface roughness Ra is 0.02 μ or less, and then heat-etched at a temperature at which the grain boundaries of the crystal grains can be confirmed (for example, a temperature 50°C lower than the sintering temperature) in an atmospheric atmosphere for 30 minutes, as a pretreatment.

[0058] In the planimetric method, a circle of a known area is drawn in the SEM observation image of the pretreated sintered body sample, the number of crystal grains within the circle (Nc) and the number of crystal grains on the circumference of the circle (Ni) are measured, and the "average grain particle diameter" can be obtained from the following equation (4) using the measured Nc and Ni.

[0059] Average grain particle diameter = (Nc + (1 / 2) x Ni) / (A / M 2 ) (4) In the above formula (4), Nc is the number of crystal grains within the circle, Ni is the number of crystal grains on the circumference of the circle, A is the area of the circle, and M is the magnification of the scanning electron microscope. In the examples described later, when a circle having a known area is drawn in a SEM observation image, the circle is drawn in such a manner that the total of Nc and Ni is 125 ± 25. Note that in the case where the total of Nc and Ni of the drawn circle does not satisfy 125 ± 25, the average of the values (provisional average crystal grain size) calculated for each SEM observation image is used as the "average crystal grain size".

[0060] The zirconia constituting the zirconia sintered body is preferably zirconia containing a stabilizing element, i.e., so-called stabilized zirconia, and more preferably zirconia containing at least yttrium. The stabilizing element is an element that stabilizes the crystal phase of zirconia, and examples thereof include one or more elements selected from the group consisting of calcium (Ca), magnesium (Mg), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), yttrium (Y), ytterbium (Yb), and lutetium (Lu), and preferably one or more elements selected from the group consisting of calcium, magnesium, cerium, and yttrium, and more preferably yttrium. Hereinafter, the stabilized zirconia in which the stabilizing element is yttrium or the like is also referred to as yttrium-stabilized zirconia or the like.

[0061] In the case where the zirconia constituting the zirconia sintered body is yttrium-stabilized zirconia, the content of yttrium (hereinafter, also referred to as "yttrium content") converted into Y2O3 is preferably 1.5 mol% or more and more preferably 2.5 mol% or more, with respect to 100 mol% of the yttrium-stabilized zirconia. The upper limit of the yttrium content is preferably 10 mol% or less and more preferably 8 mol% or less. In the case where the yttrium-stabilized zirconia has a tetragonal crystal as a main phase, the yttrium content can be 2 mol% or more and 6 mol% or less, and is preferably 2.5 mol% or more and 4 mol% or less. In the case where the yttrium-stabilized zirconia has a cubic crystal as a main phase, the yttrium content can be more than 6 mol% and 10 mol% or less, and is preferably 7 mol% or more and 9 mol% or less.

[0062] The raw material sintered body can be a sintered body in which zirconia is the main component (a sintered body in which the parent phase is zirconia), and can contain components other than zirconia (or stabilized zirconia) in addition to zirconia. As such other components, an additive component such as aluminum oxide, an unavoidable impurity of zirconia such as hafnium oxide (Hf02) can be exemplified, and one or two or more kinds thereof can be contained. As the additive component, one or more kinds selected from among aluminum oxide, silicon dioxide, and germanium oxide can be exemplified, and further, aluminum oxide can be exemplified. In the raw material sintered body, in the calculation based on the composition, the density, and the like of the zirconia sintered body, hafnium oxide is calculated as zirconia (Zr02).

[0063] In the case where the raw material sintered body contains an aluminum compound as an additive component, the aluminum compound is preferably aluminum oxide, and the preferable content of aluminum oxide can be exemplified as 0 mass% or more and 0.5 mass% or less, further more than 0 mass% and 0.5 mass% or less, and still further 0.1 mass% or more and 0.3 mass% or less. Note that the content of 0 mass% means that the substance is not contained.

[0064] The content of aluminum oxide in the present embodiment is the mass of aluminum converted into Al203 with respect to the mass of the zirconia sintered body (mass%). In addition, the mass of the zirconia sintered body is the total mass obtained by oxide conversion of the metal elements and semimetal elements contained in the zirconia sintered body. In the oxide conversion of the metal elements and semimetal elements, for example, zirconium (Zr) is converted into Zr02, calcium (Ca) is converted into CaO, magnesium (Mg) is converted into MgO, cerium (Ce) is converted into Ce02, praseodymium (Pr) is converted into Pr60 11 , neodymium (Nd) is converted into Nd203, samarium (Sm) is converted into Sm203, europium (Eu) is converted into Eu203, gadolinium (Gd) is converted into Gd203, terbium (Tb) is converted into Tb70 11 , dysprosium (Dy) is converted into Dy203, holmium (Ho) is converted into Ho203, erbium (Er) is converted into Er203, thulium (Tm) is converted into Tm203, yttrium (Y) is converted into Y203, ytterbium (Yb) is converted into Yb203, lutetium (Lu) is converted into Lu203, aluminum (Al) is converted into Al203, silicon (Si) is converted into Si02, and germanium (Ge) is converted into Ge203. Note that these oxide-converted substances are used for the conversion of M1, M2, …, M n , M add1 , M add2 , …, M addn in the formula (2') and the formula (2'').

[0065] The shape of the raw material sintered body is arbitrary, and examples include one or more shapes selected from the group consisting of a plate shape, a column shape, a cube shape, a polyhedral shape, a cone shape, a spherical shape, and a substantially spherical shape, in accordance with the intended use. The raw material sintered body can be a raw material sintered body having a non-uniform thickness, or can be a raw material sintered body having a uniform thickness. That is, the raw material sintered body can include portions having different thicknesses. In addition, the raw material sintered body can be, for example, a raw material sintered body having a maximum thickness of 0.5 μ mm or more. The raw material sintered body can be a zirconia sintered body having a maximum thickness of 100 μ mm or more. The raw material sintered body can be a zirconia sintered body having a maximum thickness of 1 mm or more, and further can be a zirconia sintered body having a maximum thickness of 1.5 mm or more and 5 mm or less. However, the raw material sintered body can be a zirconia sintered body having a maximum thickness of less than 0.5 μ mm. On the other hand, in the case where the raw material sintered body has a uniform thickness, the raw material sintered body can be a zirconia sintered body having a thickness of 0.5 μ mm or more. The raw material sintered body can be a zirconia sintered body having a thickness of 100 μ mm or more. The raw material sintered body can be a zirconia sintered body having a thickness of 1 mm or more, and further can be a zirconia sintered body having a thickness of 1.5 mm or more and 5 mm or less. However, the raw material sintered body can be a zirconia sintered body having a thickness of less than 0.5 μ mm. On the other hand, in the case where the raw material sintered body has a uniform thickness, the raw material sintered body can be a zirconia sintered body having a thickness of 0.5

[0066] Here, the thickness of the raw material sintered body refers to the raw material thickness, that is, the so-called wall thickness (distance between opposing surfaces), of the raw material sintered body, and is a value measured using a length measuring device (for example, a micrometer, a vernier caliper, a wall thickness measuring machine). For example, in a raw material sintered body having a plate shape, a column shape, a cube shape, a polyhedral shape, or a cone shape, a value obtained by measuring the dimension of the raw material sintered body in the perpendicular direction of the surface having the largest area (in the case where the surface having the largest area is a curved surface, the normal line direction of the surface having the largest area) of the raw material sintered body can be cited. Note that the plate shape, the column shape, the cube shape, the polyhedral shape, and the cone shape include shapes in which R processing has been performed on the surface. Also, for example, in a raw material sintered body having a spherical shape or a substantially spherical shape, a value obtained by measuring the dimension of the raw material sintered body in the perpendicular direction of the cross section having the largest area of the cross section of the raw material sintered body can be cited. In addition, the maximum thickness can be a value obtained by measuring the thickness of the portion having the largest thickness. For example, in a shape having a concave-convex shape, the thickness of the convex portion can be measured.

[0067] Next, the different configuration requirements between the ammonia treatment step satisfying condition (a) and the ammonia treatment step satisfying condition (b) will be described below.

[0068] In the ammonia treatment process satisfying the condition (a), the oxygen concentration in the oxygen-containing ammonia atmosphere is 0.3% by volume or more. From the viewpoint of producing recycled zirconia that is more easily pulverized, the oxygen concentration in the ammonia treatment process satisfying the condition (a) is preferably 0.4% by volume or more.

[0069] In the ammonia treatment process satisfying the condition (a), the upper limit of the oxygen concentration can be, for example, 10% by volume or less, 5% by volume or less, or 1% by volume or less.

[0070] The oxygen concentration in the ammonia treatment process satisfying the condition (a) can be any combination of the above-described upper and lower limits, and can be 0.3% by volume or more and 10% by volume or less, 0.3% by volume or more and 5% by volume or less, or 0.3% by volume or more and 1% by volume or less. From the viewpoint of producing recycled zirconia that is more easily pulverized, the oxygen concentration in the ammonia treatment process satisfying the condition (a) is preferably 0.4% by volume or more and 10% by volume or less, more preferably 0.4% by volume or more and 5% by volume or less, and still more preferably 0.4% by volume or more and 1% by volume or less.

[0071] The oxygen concentration in the oxygen-containing ammonia atmosphere is substantially the same as the oxygen concentration in the initial atmosphere gas. Therefore, by adjusting the oxygen concentration in the initial atmosphere gas, the oxygen concentration in the oxygen-containing ammonia atmosphere can be controlled. In the present embodiment, the oxygen concentration in the initial atmosphere gas (atmosphere gas containing at least ammonia and oxygen) in the oxygen-containing ammonia atmosphere can be regarded as the oxygen concentration in the oxygen-containing ammonia atmosphere. The oxygen concentration in the oxygen-containing ammonia atmosphere can be calculated, for example, from the volumes of the respective components (hereinafter also referred to as "introduced components") introduced to form the initial atmosphere gas. The volumes of the introduced components can be, for example, the volumes at 1 atm (1013 Pa) and 25°C. Note that the oxygen concentration in the oxygen-containing ammonia atmosphere can be calculated by measuring the oxygen concentration of the initial atmosphere gas using gas chromatography or an oxygen concentration meter. The measurement of the oxygen concentration can be performed, for example, at 1 atm (1013 Pa) and 25°C.

[0072] In the ammonia treatment process satisfying the condition (a), the cubic crystal rate of the raw material sintered body is not particularly limited, and the zirconia sintered body can not contain cubic crystals (i.e., the cubic crystal rate can be 0 mass%). From the viewpoint of producing recycled zirconia that is more easily pulverized, the cubic crystal rate of the raw material sintered body in the ammonia treatment process satisfying the condition (a) is preferably 1 mass% or more. In the ammonia treatment process satisfying the condition (a), the upper limit of the cubic crystal rate of the raw material sintered body can be, for example, 100 mass% or less, or 90 mass% or less. As the cubic crystal rate of the raw material sintered body in the ammonia treatment process satisfying the condition (a), 1 mass% or more and 100 mass% or less, or 1 mass% or more and 90 mass% or less can be given.

[0073] The "cubic crystal rate" in the present embodiment is the proportion of cubic crystals in the crystal phase of the zirconia sintered body, and is obtained by Rietveld analysis of an X-ray diffraction pattern measured under the following conditions using a conventional X-ray diffractometer (for example, X'pert PRO MPD, manufactured by Spectris).

[0074] Radiation source: Cu K α Radiation (λ) λ = 0.15418 nm) Scanning speed: 0.17° / min Measurement range: 2Θ = 10° to 140° Accelerating voltage • current: 40 mA • 45 kV Divergence longitudinal limiting slit: 10 mm Detector: High-speed detector (X' Celerator) The Rietveld analysis uses the analysis program "RIETAN-FP", and identifies the crystal phase under the following conditions, and takes the proportion [mass %] of cubic crystals in the total of the crystal phases obtained as the cubic crystal rate.

[0075] Profile function: Divided pseudo-Voigt function Range: 2Θ = 10° to 140° Tetragonal phase: Tetragonal system, space group P42 / nmc Cubic phase: Cubic system, space group Fm-3m In the ammonia treatment step, if the oxygen concentration is 0.3 vol% or more, the zirconia sintered body can be made porous regardless of the crystal phase of the zirconia sintered body (i.e., regardless of the cubic crystal rate of the zirconia sintered body). Therefore, if the production method of the present embodiment includes the ammonia treatment step satisfying condition (a), it is possible to produce a regenerated zirconia that is easily pulverized.

[0076] On the other hand, in the ammonia treatment step satisfying condition (b), the oxygen concentration is 0.2 vol% or more. From the viewpoint of producing a regenerated zirconia that is more easily pulverized, in the ammonia treatment step satisfying condition (b), the oxygen concentration is preferably 0.4 vol% or more.

[0077] In the ammonia treatment step satisfying condition (b), the upper limit of the oxygen concentration in the ammonia treatment step can be set to 10 vol% or less, 5 vol% or less, or 1 vol% or less, for example.

[0078] The oxygen concentration in the ammonia treatment step satisfying the condition (b) can be any combination of the above-mentioned upper and lower limits, and can be 0.2 vol% or more and 10 vol% or less, 0.2 vol% or more and 5 vol% or less, or 0.2 vol% or more and 1 vol% or less. From the viewpoint of producing a recycled zirconia that is more easily pulverized, the oxygen concentration in the ammonia treatment step satisfying the condition (b) is preferably 0.4 vol% or more and 10 vol% or less, more preferably 0.4 vol% or more and 5 vol% or less, and still more preferably 0.4 vol% or more and 1 vol% or less.

[0079] In the ammonia treatment step satisfying the condition (b), the raw sintered body has a cubic crystal rate of 3.5 mass% or more. From the viewpoint of producing a recycled zirconia that is more easily pulverized, in the ammonia treatment step satisfying the condition (b), the raw sintered body preferably has a cubic crystal rate of 4 mass% or more, more preferably 5 mass% or more, and particularly preferably 10 mass% or more, still more preferably 80 mass% or more, and even more preferably 90 mass% or more. In the ammonia treatment step satisfying the condition (b), the upper limit of the cubic crystal rate of the raw sintered body can be, for example, 100 mass% or less. As the cubic crystal rate of the raw sintered body in the ammonia treatment step satisfying the condition (b), there can be mentioned 3.5 mass% or more and 100 mass% or less, 4 mass% or more and 100 mass% or less, 5 mass% or more and 100 mass% or less, 10 mass% or more and 100 mass% or less, 80 mass% or more and 100 mass% or less, or 90 mass% or more and 100 mass% or less.

[0080] In the ammonia treatment step, when the oxygen concentration is less than 0.2 vol%, the zirconia sintered body cannot be made porous regardless of the cubic crystal rate of the zirconia sintered body. Also, when the oxygen concentration in the ammonia treatment step is 0.2 vol% or more and less than 0.3 vol%, the zirconia sintered body having a cubic crystal rate of less than 3.5 mass% cannot be made porous. It is considered that the reason for this is that, under such conditions, nitriding of zirconia proceeds, and the generation of nitride zirconia is preferentially promoted.

[0081] On the other hand, even when the oxygen concentration is 0.2 vol% or more and less than 0.3 vol%, if the cubic crystal rate of the raw sintered body is 3.5 mass% or more, the zirconia sintered body can be made porous. It is considered that the reason for this is that, if the cubic crystal rate of the raw sintered body is high, the ion conductivity of the zirconia sintered body becomes higher, and nitrogen (active state nitrogen) becomes easy to diffuse inside the zirconia sintered body. Therefore, according to the production method of the present embodiment, if the ammonia treatment step satisfying the condition (b) is included, a recycled zirconia that is easily pulverized can be produced.

[0082] According to the production method of the present embodiment described above, regardless of the shape of the zirconia sintered body, a regenerative zirconia that is easily pulverized can be produced from the zirconia sintered body. The "regenerative zirconia" in the present embodiment refers to a composition of zirconia obtained by the regeneration of a zirconia sintered body, and particularly refers to a composition of zirconia obtained from a zirconia sintered body and capable of being used as at least either one of a raw material for a sintered body and a precursor of a raw material. In particular, the regenerative zirconia produced by the production method of the present embodiment is zirconia in a state after a zirconia sintered body has been processed in a manner that enables regeneration (recycling) as a raw material for zirconia, and specifically is zirconia that has been made porous by the zirconia sintered body (hereinafter, also referred to as "porous zirconia"). As a specific example of the porous zirconia, as shown in the examples described later, a porous zirconia in which pores are formed in the crystal grains can be exemplified.

[0083] Here, the porous zirconia in which pores are formed in the crystal grains is zirconia in which intragranular pores (pores in the crystal grains) are formed by heat treatment, and is zirconia in which the intragranular pores formed in the crystal grains by heat treatment can be confirmed by comparing the crystal grains before and after heat treatment by SEM observation. The porous zirconia in which pores are formed in the crystal grains is different from zirconia in which only voids between the crystal grains can be confirmed (zirconia in which pores cannot be confirmed in the crystal grains), zirconia in which only the defects of the periphery of the crystal grains can be confirmed (zirconia in which pores cannot be confirmed in the crystal grains), and the like, in which intergranular pores (voids or defects of the interface (grain boundaries) of the crystal grains) are formed by heat treatment. Note that the porous zirconia in which pores are formed in the crystal grains can also have voids between the crystal grains or defects of the periphery of the crystal grains as long as pores can be confirmed in the crystal grains.

[0084] Note that the microstructure of the regenerative zirconia can be observed using a conventional scanning electron microscope (device name: S-4800, manufactured by Hitachi High-Tech) under the following conditions. The regenerative zirconia can be pre-processed by heat treatment at a temperature 50°C lower than the sintering temperature for 30 minutes in an atmospheric atmosphere before observation using a scanning electron microscope, and the regenerative zirconia subjected to the pre-processing can be used as the observation object.

[0085] Accelerating voltage: 20 kV Magnification: 10,000x, 30,000x, or 50,000x The shape of the regenerative zirconia produced by the production method of the present embodiment is arbitrary, and can have the same shape as the sintered body supplied to the ammonia treatment step. As a specific shape of the regenerative zirconia, one or more selected from a molded body, a block, and an aggregate can be exemplified.

[0086] The reclaimed zirconia produced by the production method of the present embodiment can be used as a raw material for zirconia. Therefore, according to the present application, a regeneration method of a zirconia sintered body in which the reclaimed zirconia produced by the production method of the present embodiment is used as a raw material for zirconia, and further, a level regeneration method of a sintered body in which the reclaimed zirconia produced by the production method of the present embodiment is used as a raw material for zirconia for sintered body production can be provided. In the regeneration method, the reclaimed zirconia produced by the production method of the present embodiment can be directly used as a raw material for zirconia, but in terms of production of a product, the zirconia as a raw material is preferably in the form of a particle (powder), and therefore, the produced reclaimed zirconia can also be micronized by pulverization or the like, and the micronized reclaimed zirconia (hereinafter, also referred to as "reclaimed zirconia powder") can be used as a raw material for zirconia. In particular, sintered bodies typified by a zirconia sintered body are produced by sintering of zirconia particles (powder). Therefore, in the level regeneration method of a sintered body, it is preferable to micronize the reclaimed zirconia produced by the production method of the present embodiment, and use the obtained reclaimed zirconia powder as a raw material for zirconia for sintered body production. The sintered body produced using the reclaimed zirconia is a sintered body containing zirconia, and examples thereof include a zirconia sintered body which is a sintered body of zirconia powder, a composite sintered body which is a sintered body of zirconia powder and another powder other than zirconia (for example, alumina powder).

[0087] Example Hereinafter, the present disclosure will be described using examples. However, the present disclosure is not limited to these examples.

[0088] (SEM observation) The microstructure of the zirconia sintered body (reclaimed zirconia) after ammonia treatment was observed using a scanning electron microscope (S-4800, manufactured by Hitachi High-Technologies Corporation) under the following conditions.

[0089] Accelerating voltage: 20 kV Magnification: 10,000x, 30,000x, or 50,000x Note that, before observation by SEM, the reclaimed zirconia was subjected to a heat treatment at a temperature 50°C lower than the sintering temperature for 30 minutes in an atmospheric atmosphere, and thereby was subjected to pretreatment as an observation object.

[0090] (Raman spectroscopic analysis) The Raman spectrum of the zirconia sintered body (reclaimed zirconia) after ammonia treatment was measured using a confocal Raman microscope (InVia, manufactured by Renishaw Corporation) under the following conditions, and the crystal phase was identified from the spectrum shape.

[0091] Excitation wavelength: 532 nm (Average crystal grain particle diameter) The average crystal grain particle diameter of the zirconia sintered body used in the examples and comparative examples was obtained by the following measurement conditions using a scanning electron microscope (device name: JSM-IT500, manufactured by JEOL Ltd.). Using the obtained SEM observation image, the average crystal grain particle diameter was calculated from the above (4) by the plane measurement method.

[0092] Accelerating voltage: 10 kV Observation magnification: 2000x to 40000x Note that, before measuring the average crystal grain particle diameter, the zirconia sintered body sample was mirror-polished so that the surface roughness Ra was ≤ 0.02 μ m, and then heat-etched for 30 minutes at a temperature at which the grain boundaries of the crystal grains could be confirmed (for example, a temperature 50°C lower than the sintering temperature), thereby performing pretreatment, and the pretreated zirconia sintered body was used as the measurement object.

[0093] (Relative density) The relative density of the zirconia sintered body sample was obtained from the measured density obtained by the Archimedes method and the mass obtained by mass measurement ρ , and the true density obtained from the above (1) to (2") formulae ρ 0 (in the case of using the (2") formula, the true density ρ z) was calculated from the above (3) formula.

[0094] (Crystal phase identification and cubic crystal rate) The crystal phase of the zirconia sintered body sample and the regenerated zirconia was measured for the X-ray diffraction pattern under the following conditions using an X-ray diffractometer (device name: X'pert PRO MPD, manufactured by Spectris).

[0095] X-ray source: Cu K α X-ray (λ = 0.15418 nm) λ = 0.15418 nm) Scanning speed: 0, 17° / minute Measurement range: 2θ = 10° to 140° Accelerating voltage · current: 40 mA · 45 kV Divergence longitudinal limiting slit: 10 mm Detector: High-speed detector (X'Celerator) For the XRD pattern of the obtained sintered body sample, using RIETAN-FP, the crystal phase was identified by Rietveld analysis based on the following conditions, and the cubic crystal rate was calculated from the proportion [mass %] of the cubic crystal in the total of the obtained crystal phases.

[0096] Profile function: Segmentation of pseudo-Voigt function Range: 2 theta = 10° to 140° Tetragonal phase: Tetragonal crystal system, space group P42 / nmc Cubic phase: Cubic crystal system, space group Fm-3m Example 1 A commercially available zirconia powder (powder of yttrium-stabilized zirconia with a yttrium content of 3 mol%. Product name: TZ-3YS, manufactured by Toyo Co., Ltd.) 5 g was filled into a mold with a diameter of 25 mm, and uniaxial press molding was performed at 19.6 MPa, followed by cold isostatic pressing at 196 MPa, whereby a molded body was obtained. The obtained molded body was subjected to atmospheric sintering at 1500°C for 2 hours in an atmosphere, whereby a sintered body of zirconia in the form of a round plate composed of yttrium-stabilized zirconia with a yttrium content of 3 mol% having a thickness of 3 mm, a relative density of 99.3%, and an average crystal grain size of 0.49 μ m was obtained. The obtained sintered body of zirconia had a crystal phase composed of tetragonal crystal and cubic crystal, and the cubic crystal rate thereof was 4 mass%.

[0097] The obtained sintered body of zirconia was disposed on an alumina plate, and then disposed in a tubular furnace provided with a reaction tube made of quartz glass. The zirconia sintered body disposed in the reaction tube was subjected to heat treatment (ammonia treatment) under the following conditions.

[0098] Heat treatment atmosphere: Ammonia-air mixed gas flowing atmosphere (Ammonia concentration: 96.2 vol%, oxygen concentration: 0.8 vol%) (Ammonia-air mixed gas flow rate: 520 seem) Heat treatment temperature: 1000°C Heat treatment time: 16 hours After the heat treatment (ammonia treatment), cooling to room temperature was performed, and the regenerated zirconia of the present example was obtained. It was confirmed by visual observation that the obtained regenerated zirconia was the same shape as the zirconia sintered body used as a raw material, but according to SEM observation before and after the heat treatment (ammonia treatment), it was confirmed to be a porous zirconia in which pores were formed in the crystal grains.

[0099] Example 2 As the heat treatment atmosphere, an ammonia-air mixed gas flowing atmosphere with an ammonia concentration of 98.0 vol% and an oxygen concentration of 0.4 vol% was used; otherwise, the sintered body was subjected to heat treatment by the same method as in Example 1, and the regenerated zirconia of the present example was obtained. The SEM observation image of the surface of the zirconia sintered body before the heat treatment is shown in Figure 2 The SEM observation image of the surface of the regenerated zirconia obtained by the heat treatment is shown inFigure 1 No hole (pore) was confirmed before the heat treatment, and a plurality of holes (pores) were confirmed in the crystal grains after the heat treatment, and the regenerated zirconia obtained was confirmed to be porous zirconia in which holes were formed in the crystal grains. It should be noted that the regenerated zirconia and the zirconia sintered body used as the raw material were confirmed to be the same shape by visual observation.

[0100] Example 3 A commercially available zirconia powder (a yttrium-stabilized zirconia powder containing alumina having an alumina content of 0.25 mass% and a yttrium content of 3 mol%. Product name: TZ-PX-172, manufactured by Tosoh Corporation) was used, and the sintering temperature was set to 1200°C; other than this, a zirconia sintered body in the form of a round plate having a thickness of 3 mm, a relative density of 99.5%, and an average crystal grain particle diameter of 0.18 μ m, which was composed of yttrium-stabilized zirconia containing alumina having an alumina content of 0.25 mass% and a yttrium content of 3 mol%. The zirconia sintered body obtained had a crystal phase composed of cubic crystals and tetragonal crystals, and the cubic crystal ratio thereof was 3 mass%. The zirconia sintered body obtained was subjected to heat treatment by the same method as in Example 1, and the regenerated zirconia of the present example was obtained. The regenerated zirconia obtained was confirmed to be the same shape as the zirconia sintered body used as the raw material by visual observation, but it was confirmed to be porous zirconia in which holes were formed in the crystal grains according to SEM observation before and after the heat treatment (ammonia treatment). Figure 3 An SEM observation image of the regenerated zirconia of the present example is shown. Figure 3 The SEM observation image shown is at a higher magnification than the SEM observation image shown in Figure 1 The SEM observation image shown is at a higher magnification than the SEM observation image shown in

[0101] Example 4 A zirconia sintered body produced by the same method as in Example 3 was used; other than this, the sintered body was subjected to heat treatment by the same method as in Example 2, and the regenerated zirconia of the present example was obtained. The regenerated zirconia obtained was confirmed to be the same shape as the zirconia sintered body used as the raw material by visual observation, but it was confirmed to be porous zirconia in which holes were formed in the crystal grains according to SEM observation before and after the heat treatment (ammonia treatment). It should be noted that the regenerated zirconia of the present example maintained the same shape as the sintered body immediately after the heat treatment was completed, but crumbled when taken out of the tubular furnace, and there were portions that became powdery.

[0102] Example 5 A commercially available zirconia powder (product name: TZ-8YS, manufactured by Tosoh Corporation) was used; other than this, a zirconia sintered body in the form of a round plate having a thickness of 3 mm, a relative density of 98.6%, and an average crystal grain particle diameter of 3.78 μA zirconia sintered body in the shape of a round plate composed of yttria-stabilized zirconia with a yttria content of 8 mol%. The obtained zirconia sintered body had crystals consisting of cubic crystals, and the cubic crystal rate was 100 mass%. The obtained zirconia sintered body was heat-treated in the same manner as in Example 1 to obtain regenerated zirconia of the present example. It was confirmed by visual observation that the obtained regenerated zirconia was the same shape as the zirconia sintered body used as a raw material, but according to SEM observation before and after heat treatment (ammonia treatment), it was confirmed to be porous zirconia in which pores were formed in the crystal grains.

[0103] Example 6 A zirconia sintered body obtained in the same manner as in Example 5 was used, and as the heat treatment atmosphere, an ammonia-air mixed gas flowing atmosphere with an ammonia concentration of 99.0 vol% and an oxygen concentration of 0.2 vol% was used; other than that, the zirconia sintered body was heat-treated in the same manner as in Example 1 to obtain regenerated zirconia of the present example. It was confirmed by visual observation that the obtained regenerated zirconia was the same shape as the zirconia sintered body used as a raw material, but according to SEM observation before and after heat treatment (ammonia treatment), it was confirmed to be porous zirconia in which pores were formed in the crystal grains.

[0104] Example 7 A zirconia sintered body obtained in the same manner as in Example 5 was used, and as the heat treatment atmosphere, an ammonia-air mixed gas flowing atmosphere with an ammonia concentration of 98.0 vol% and an oxygen concentration of 0.4 vol% was used; other than that, the zirconia sintered body was heat-treated in the same manner as in Example 1 to obtain regenerated zirconia of the present example. It was confirmed by visual observation that the obtained regenerated zirconia was the same shape as the zirconia sintered body used as a raw material, but according to SEM observation before and after heat treatment (ammonia treatment), it was confirmed to be porous zirconia in which pores were formed in the crystal grains.

[0105] Comparative Example 1 A zirconia sintered body obtained in the same manner as in Example 3 was used, and as the heat treatment atmosphere, an ammonia-air mixed gas flowing atmosphere with an ammonia concentration of 99.0 vol% and an oxygen concentration of 0.2 vol% was used; other than that, the zirconia sintered body was heat-treated in the same manner as in Example 1 to obtain regenerated zirconia of the present comparative example. The regenerated zirconia of the present comparative example had the same shape as before heat treatment, and the texture of its surface also did not change compared to before heat treatment (i.e., it was not porous zirconia).

[0106] [Evaluation of pulverization] Evaluation of the pulverizability of the obtained regenerated zirconia of the examples and the comparative examples was performed. That is, the regenerated zirconia in which the shape was confirmed to be destroyed when the regenerated zirconia after the treatment by the tweezers was gripped was judged to be pulverizable (evaluation: O), and the regenerated zirconia in which the shape was not destroyed was judged to be difficult to be pulverized (evaluation: X). The results are shown in Table 1.

[0107] [Table 1] As shown in Table 1, the result of the pulverization evaluation of the regenerated zirconia of the examples was O, and, on the contrary, the result of the pulverization evaluation of the regenerated zirconia of the comparative examples was X. From this result, it was understood that, according to the examples, the regenerated zirconia which was easy to be pulverized could be produced from the zirconia sintered body.

Claims

1. A method for manufacturing recycled zirconium oxide, characterized in that, It has an ammonia treatment process for heat-treating zirconia sintered bodies in an oxygen-containing ammonia atmosphere at a temperature of 800°C or higher and 1200°C or lower. The ammonia treatment process satisfies either condition (a) or condition (b) below. (a) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.3% by volume or more; (b) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.2% by volume or more, and the cubic crystallinity of the zirconia sintered body is 3.5% or more.

2. The manufacturing method according to claim 1, wherein, The oxygen-containing ammonia atmosphere at the start of the heat treatment of the zirconia sintered body is an atmosphere containing at least ammonia (NH3) and oxygen (O2).

3. The manufacturing method according to claim 2, wherein, The combined concentration of ammonia (NH3) and oxygen (O2) in the atmosphere is 85% to 100% of the total volume of the atmosphere.

4. The manufacturing method according to any one of claims 1 to 3, wherein, The heat treatment is carried out in an oxygen-containing ammonia atmosphere in which the atmosphere gas flows within the system.

5. The manufacturing method according to any one of claims 1 to 4, wherein, The oxygen-containing ammonia atmosphere in the heat treatment of the zirconia sintered body is an atmosphere containing at least one active species selected from ammonia and oxygen.

6. The manufacturing method according to any one of claims 1 to 5, wherein, The maximum thickness of the zirconia sintered body is 0.5 mm. μ m or more.

7. The manufacturing method according to any one of claims 1 to 6, wherein, The relative density of the zirconia sintered body is above 98%.

8. The manufacturing method according to any one of claims 1 to 7, wherein, The zirconia constituting the zirconia sintered body contains stabilizing elements.

9. The manufacturing method according to claim 8, wherein, The stabilizing element is yttrium.

10. The manufacturing method according to any one of claims 1 to 9, wherein, The zirconia sintered body contains one or more selected from alumina, silicon dioxide and germanium oxide.

11. A method for regenerating a sintered body, characterized in that, Regenerated zirconia manufactured by the manufacturing method according to any one of claims 1 to 10 is used as the raw material for zirconia.

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