Manufacturing method of recycled zirconia

Abstract

The present invention provides at least one of a method for producing easily pulverizable recycled zirconia from a zirconia sintered body, and a method for recycling a zirconia sintered body, which enables the recycling of the zirconia sintered body. A method for producing recycled zirconia, comprising an ammonia treatment 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, wherein the ammonia treatment step satisfies either one of the following conditions (a) and (b): (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 volume % or more, and the cubic crystal fraction 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 treating a zirconia sintered body, and further to a method for recycling the zirconia sintered body. [Background technology]

[0002] Zirconia sintered bodies have excellent mechanical properties as well as electrical conductivity and aesthetic appeal, and are therefore used in a wide range of applications, including not only grinding media and structural materials, but also decorative materials, exterior components for electronic devices, sensors, and biomaterials.

[0003] In recent years, from the viewpoint of the Sustainable Development Goals (SDGs), recycling (regeneration) technology has been attracting attention, and there is a demand for the regeneration and reuse of zirconia sintered bodies. The simplest recycling method is to pulverize the sintered body by crushing or the like and reuse it as a raw material powder. For example, as a conventional method for recycling zirconia sintered bodies, a method has been proposed in which a sheet-shaped zirconia sintered body with a thickness of less than 0.5 μm is crushed into powder and reused as a raw material for zirconia sintered bodies (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-263157 A Summary of the Invention [Problem to be solved by the invention]

[0005] Zirconia sintered bodies are generally dense and have high mechanical strength. Therefore, it is very difficult to powder them by pulverization. The method of Patent Document 1 can be applied to zirconia sintered bodies with extremely thin thicknesses. However, it cannot be applied to zirconia having shapes other than plate-like shapes or zirconia sintered bodies with a thickness of 0.5 μm or more.

[0006] In view of these problems, the present disclosure aims to provide at least one of a method for producing easily pulverizable recycled zirconia from a zirconia sintered body, and a method for recycling a zirconia sintered body capable of recycling the zirconia sintered body. [Means for solving the problem]

[0007] In the present disclosure, recycling of a zirconia sintered body having a certain thickness was examined. As a result, it was noted that recycling of the zirconia sintered body can be achieved by making the zirconia sintered body porous once. Furthermore, it was found that the zirconia sintered body can be made porous by heat-treating the zirconia sintered body in a specific atmosphere, and that recycled zirconia can be obtained as a result.

[0008] That is, the present invention is as described in the claims, and the gist of the present disclosure is as follows. [1] A method for producing recycled zirconia, comprising an ammonia treatment 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, wherein the ammonia treatment step satisfies either one of the following conditions (a) and (b): (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 fraction of the zirconia sintered body is 3.5% by mass or more. [2] The manufacturing method according to the above [1], wherein the oxygen-containing ammonia atmosphere is an atmospheric gas containing at least ammonia (NH3) and oxygen (O2) at the start of the heat treatment of the zirconia sintered body. [3] The manufacturing method according to [2] above, wherein the total concentration of ammonia (NH3) and oxygen (O2) in the atmospheric gas is 85 vol% or more and 100 vol% or less with respect to 100 vol% of the atmospheric gas. [4] The method according to any one of [1] to [3] above, wherein the heat treatment is carried out in an oxygen-containing ammonia atmosphere with atmospheric gas flowing through the system. [5] The method according to any one of [1] to [4] above, wherein the oxygen-containing ammonia atmosphere is an atmospheric gas containing at least one of an active species of ammonia and ammonia, and oxygen during the heat treatment of the zirconia sintered body. [6] The method according to any one of [1] to [5] above, wherein the zirconia sintered body has a maximum thickness of 0.5 μm or more. [7] The method according to any one of [1] to [6] above, wherein the zirconia sintered body has a relative density of 98% or more. [8] The method according to any one of [1] to [7] above, wherein the zirconia constituting the zirconia sintered body contains a stabilizing element. [9] The method according to [8] above, wherein the stabilizing element is yttrium.

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

[11] A method for recycling a sintered body, which uses recycled zirconia produced by the production method according to any one of [1] to

[10] above as a raw material for zirconia. Effect of the Invention

[0009] The present disclosure makes it possible to provide at least one of a method for producing recycled zirconia that is easily pulverized from a zirconia sintered body, and a recycling method for a zirconia sintered body that can recycle a zirconia sintered body. [Brief description of the drawings]

[0010] [Figure 1] SEM observation of the surface of the recycled zirconia obtained in Example 2 [Diagram 2] SEM observation of the surface of the zirconia sintered body used in Example 2 [Diagram 3]SEM observation of the surface of the recycled zirconia obtained in Example 3 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The manufacturing method of the present disclosure will be described below with reference to an example embodiment. The present disclosure includes any combination of the configurations and parameters disclosed herein, and also includes any combination of the upper and lower limits of the values ​​disclosed herein.

[0012] The method for producing recycled zirconia according to the present embodiment includes a step of heat-treating a zirconia sintered body in an oxygen-containing ammonia atmosphere at a temperature of 800° C. to 1200° C. (hereinafter, also referred to as an “ammonia treatment step”). The ammonia treatment step satisfies either one of the following conditions (a) and (b): (a) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.3 volume % or more. (b) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.2% by volume or more, and the cubic fraction of the zirconia sintered body is 3.5% by mass or more.

[0013] In this embodiment, the ammonia treatment step satisfying either one of the conditions (a) and (b) means that the ammonia treatment step satisfies not only either one of the conditions (a) and (b), but also both of the conditions (a) and (b).

[0014] According to this embodiment including the ammonia treatment step, even a dense zirconia sintered body can be made porous, and its strength can be reduced. Therefore, recycled zirconia that is easy to crush, that is, a zirconia composition that is easy to powder, and further, a zirconia composition that is easy to recycle by horizontal recycling can be produced from the zirconia sintered body. In this embodiment, the 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 one of the sintered body and its precursor, and is distinguished from cascade recycling in which the use after recycling is other than these. One or more selected from the group consisting of a precursor of the sintered body, a powder, a molded body, and a calcined body can be mentioned.

[0015] First, the constituent elements common to condition (a) and condition (b) will be explained below.

[0016] In the ammonia treatment step, the zirconia sintered body is heat-treated at a temperature of 800° C. or more and 1200° C. or less in an oxygen-containing ammonia atmosphere.

[0017] The oxygen-containing ammonia atmosphere in the ammonia treatment step may be an atmosphere formed by heat treatment of the atmosphere gas at the start of the heat treatment of the zirconia sintered body (hereinafter also referred to as "initial atmosphere gas"). The initial atmosphere gas may be an atmosphere containing at least ammonia (NH3) and oxygen (O2). The oxygen-containing ammonia atmosphere may have a composition different from that of the initial atmosphere gas during the heat treatment of the zirconia sintered body, as long as the initial atmosphere gas contains at least ammonia (NH3) and oxygen (O2). For example, the oxygen-containing ammonia atmosphere is preferably an atmosphere gas containing at least one of active species of ammonia and ammonia, and oxygen, or further, an atmosphere gas containing active species of ammonia and oxygen, during the heat treatment of the zirconia sintered body. As the active species of ammonia, NH2 - , N.H. 2- and N 3-At least one selected from the group consisting of 3- It is preferable that the oxygen-containing ammonia atmosphere contains the above-mentioned. The active species of ammonia functions as nitrogen in an activated state. Furthermore, the oxygen-containing ammonia atmosphere may contain other gas components such as nitrogen (N2), and the other gas components may include one or more selected from the group consisting of carbon monoxide, hydrogen, and nitrogen, and further include nitrogen.

[0018] The initial atmosphere gas in the oxygen-containing ammonia atmosphere may be a gas containing at least ammonia (NH3) and oxygen (O2). Specific examples of the initial atmosphere gas include a gas containing ammonia, air, and / or oxygen, and a gas containing ammonia and air (hereinafter also referred to as "ammonia-air mixed gas").

[0019] The total concentration of ammonia (NH3) and oxygen (O2) in the initial atmosphere gas is, for example, 85% by volume or more and 100% by volume or less, based on 100% by volume of the initial atmosphere gas. From the viewpoint of producing recycled zirconia that is easier to pulverize, it is preferable that the total concentration of ammonia (NH3) and oxygen (O2) in the initial atmosphere gas is 90% by volume or more and 100% by volume or less, based on 100% by volume of the initial atmosphere gas, and furthermore, 100% by volume (i.e., the initial atmosphere gas is composed only of ammonia (NH3) and oxygen (O2)).

[0020] Here, when the heat treatment in the ammonia treatment step is performed in an oxygen-free ammonia atmosphere or an oxygen-containing ammonia atmosphere in which the oxygen concentration does not satisfy the conditions (a) and (b) described below, the zirconia sintered body is hardly porous. This is mainly because, when the heat treatment is performed in such an atmosphere, the nitridation of zirconia is promoted, and the production of zirconia nitride proceeds preferentially. Therefore, when the heat treatment in the ammonia treatment step is performed in such an atmosphere, recycled zirconia that is easy to crush cannot be produced.

[0021] The heat treatment in the ammonia treatment step may be performed in an oxygen-containing ammonia atmosphere in which the atmospheric gas remains in the system, or in an oxygen-containing ammonia atmosphere in which the atmospheric gas flows through the system (hereinafter also referred to as an "oxygen-containing ammonia flowing atmosphere"). In order to further promote the porosity 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 may be an atmosphere in which an atmospheric gas containing oxygen and ammonia flows through the system during the heat treatment, and for example, an oxygen-containing ammonia atmosphere in which an ammonia-air mixed gas flows through the system as the atmospheric gas (hereinafter also referred to as an "ammonia-air mixed gas flowing atmosphere").

[0022] A method for heat-treating the zirconia sintered body in an oxygen-containing ammonia flow atmosphere can also be used. For example, a method can be used in which an atmospheric gas is circulated in a reaction tube in which a zirconia sintered body is disposed, and the reaction tube through which the atmospheric gas flows is heated to heat the zirconia sintered body. A specific heat treatment method includes a method in which a setter in which a zirconia sintered body is disposed is disposed in a reaction tube, and then heated while circulating an atmospheric gas. Both the reaction tube and the setter may be ones that are unlikely to cause side reactions during heat treatment. Examples of the setter include ceramic setters, preferably one or more selected from the group of zirconia setters, mullite setters, and alumina setters, more preferably zirconia setters. In addition, examples of the reaction tube include one or more selected from the group of zirconia reaction tubes, quartz reaction tubes, mullite reaction tubes, and alumina reaction tubes, more preferably one or more selected from the group of zirconia reaction tubes, quartz reaction tubes, and alumina reaction tubes, and even more preferably quartz reaction tubes.

[0023] The flow rate of the atmospheric gas (e.g., ammonia-air mixed gas) in the oxygen-containing ammonia flow atmosphere can be, for example, 400 sccm (=ml / min. 1 atm, 0°C) or more and 800 sccm or less. From the viewpoint of producing recycled zirconia that is easier to pulverize, the flow rate of the atmospheric gas is preferably 450 sccm or more and 600 sccm or less.

[0024] One of the reasons why the zirconia sintered body is made porous by including an ammonia treatment step that satisfies either one of the conditions (a) and (b) is that the heat treatment in the oxygen-containing ammonia atmosphere causes the diffusion of active nitrogen into the zirconia sintered body and the formation of nitrogen molecules by association, which makes the zirconia sintered body porous. That is, in the heat treatment in the oxygen-containing ammonia atmosphere, the active species of ammonia containing active nitrogen generated from ammonia diffuses to the inside of the zirconia sintered body. Then, the active nitrogen diffused into the inside of the zirconia sintered body becomes gaseous nitrogen (N2). It is considered that pores are formed by the generation of nitrogen (N2) inside the zirconia sintered body and the desorption of the generated nitrogen (N2) from the zirconia sintered body, which makes the sintered body porous.

[0025] The temperature (heat treatment temperature) in the ammonia treatment step 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 it takes a very long time to make it porous. One of the reasons for this is that if the heat treatment temperature is less than 800°C, the amount of activated nitrogen generated from ammonia is small, making it difficult for nitrogen to diffuse into the zirconia sintered body. Another reason is that if the heat treatment temperature exceeds 1200°C, the decomposition of ammonia progresses and the diffusion of activated nitrogen into the zirconia sintered body hardly progresses. For this reason, if the heat treatment temperature in the ammonia treatment step is outside the range of 800°C or more and 1200°C or less, recycled zirconia that is easy to crush cannot be produced.

[0026] The heat treatment temperature in the ammonia treatment step may be 800° C. or more and 1200° C. or less. From the viewpoint of producing recycled zirconia that is easier to pulverize, the heat treatment temperature is preferably 900° C. or more and 1100° C. or less.

[0027] The time of the heat treatment in the ammonia treatment step may be appropriately set depending on the amount of the zirconia sintered body to be heat-treated in the ammonia treatment step and the performance of the heat treatment furnace, and may be, for example, 5 hours or more, or further 10 hours or more. The upper limit of the time of the heat treatment in the ammonia treatment step may be, for example, 50 hours or less, or further 20 hours or less. The heat treatment time may be, for example, 5 hours or more and 50 hours or less, or 10 hours or more and 20 hours or less.

[0028] The zirconia sintered body to be heat-treated in the ammonia treatment step (hereinafter also referred to as the "raw material sintered body") is a sintered body composed of zirconia crystal grains. However, it is sufficient that the sintered body is composed mainly of zirconia crystal grains, and it may be a sintered body composed of zirconia crystal grains and crystal grains of an added component (described later).

[0029] The raw material sintered body may be a sintered body produced by sintering zirconia particles (powder). The zirconia sintered body may be a sintered body produced by simply sintering zirconia particles (powder), but may also be a sintered body produced by sintering a compact (compressed powder) of zirconia particles (powder) formed into a predetermined shape by compression or the like, or a sintered body produced by sintering a calcined body obtained by molding and calcining zirconia particles (powder). The molding conditions, sintering conditions, and calcination conditions may be conventionally known conditions and are not particularly limited. From the viewpoint of the SDGs, the raw material sintered body is preferably a zirconia sintered body after distribution, that is, a used zirconia sintered body (used sintered body) that was once used as a product in the market.

[0030] The crystal phase of the raw material sintered body may be at least one of tetragonal and cubic, but is preferably a crystal phase including at least cubic. A preferred crystal phase of the raw material sintered body is a crystal phase consisting of a mixed crystal of tetragonal and cubic, or a crystal phase consisting of cubic. For convenience, in this embodiment, the crystal phase of zirconia may be considered to be monoclinic, tetragonal, and cubic.

[0031] The raw material sintered body is preferably a dense zirconia sintered body that is difficult to pulverize by physical treatment such as pulverization alone. Specifically, the raw material sintered body is preferably a zirconia sintered body having a relative density of 98% or more, and more preferably a zirconia sintered body having a relative density of 99% or more. The relative density may be 100% or less or 99.9% or less, for example, 98.5% or more and 100% or less, or 99.0% or more and 99.9% or less.

[0032] In this embodiment, the "relative density" refers to the actual density ρ [g / cm 3 ] obtained from the volume measured by the Archimedes method and the mass obtained by mass measurement. 3 ] and true density ρ0 [g / cm 3 When the raw sintered body contains yttrium as a stabilizing element, the measured density ρ of the sintered body and the true density ρ0 [g / cm 3 The relative density of the sintered body can be calculated by the following formula (3) using the above formulas: In addition, X in formulas (1) and (2) is the yttrium content [mol %] in zirconia (stabilized zirconia). α = [0.5080+0.06980X / (100+X)] 2 ×[0.5195-0.06180X / (100+X)] (1) ρ0 = [124.25(100-X)+225.81X] / [150.5(100+X)α] (2) Relative density [%] = (ρ / ρ0) × 100 (3)

[0033] In addition, when the raw material sintered compact contains at least a stabilizing element other than yttrium (when the raw material sintered compact contains yttrium and a stabilizing element other than yttrium as a stabilizing element, or when the raw material sintered compact contains only a stabilizing element other than yttrium as a stabilizing element), ρ0 [g / cm2] calculated from formula (2') is used instead of ρ0 calculated from formula (2). 3 When ρ0 calculated from formula (2') is used, the total content X [mol%] of the stabilizing elements in zirconia (stabilized zirconia) can be used for X in formula (1). ρ0= [124.25(100-X)+M1X1+M2X2+···+M n X n ] / [150.5(100+X)α] (2') In equation (2'), M1, M2, …, M n are the amounts of each stabilizing element converted into oxides [g / mol]. X1,X2,…,X n is the content [mol%] of each stabilizing element, X is the total content [mol%] of the stabilizing elements in zirconia (stabilized zirconia), and X = X1 + ... + X n It can be calculated from M1, M2, …, M n and X1, X2, …, X n The number appended to indicates the type of stabilizing element, and n is an integer of 1 or more and indicates the upper limit of the number of types of stabilizing elements contained in the raw material sintered compact.

[0034] When the raw material sintered body does not contain a stabilizing element, the true density ρ0 can be found from equation (2) by substituting 0 for X in equations (1) and (2).

[0035] In addition, when the raw sintered body contains an additive component (described later), the true density ρz [g / cm3] calculated from equation (2") is used instead of ρ0 calculated from equation (2) or (2'). 3 ] (i.e., relative density [%] = (ρ / ρz) × 100 using equation (3)) to calculate the “relative density”. ρz=100 / [Y1 / M add1 +Y2 / M add2 +...+Yn / M addn +(100-Y) / ρ0] (2”) In the formula (2"), M add1 ,M add2 ,…,M addn is the true density of each added component converted to oxide [g / cm 3 ]. Y1,Y2,…,Y n is the content [mass%] of each additive component in the sintered body, Y is the total content [mass%] of additive elements in zirconia (stabilized zirconia), and Y = Y1 + ... + Y n It can be calculated from M add1 ,M add2 ,…,M addn and Y1, Y2, …, Y n The number appended to indicates the type of additive component, and n is an integer of 1 or more, and the upper limit is the number of types of additive components contained in the raw sintered body. ρ0 is the true density [g / cm 3 ].

[0036] Here, the true density of each added component converted to its oxide (M add ) is, for example, the true density of alumina (M add ) is 3.987g / cm 3 , the true density of silica is 2.334g / cm 3 , and the true density of germania is 4.250 g / cm 3 The following points can be mentioned.

[0037] From the viewpoint of producing recycled zirconia that is easier to pulverize, the raw material sintered body preferably has an average crystal grain size of 10 μm or less, more preferably 5.0 μm or less, even more preferably 3.0 μm or less, and even more preferably 0.5 μm or less. The lower limit of the average crystal grain size of the crystal grains is, for example, 0.1 μm or more. The average crystal grain size of the raw material sintered body may be 0.1 μm or more and 10 μm or less, 0.1 μm or more and 5.0 μm or less, or 0.1 μm or more and 0.5 μm or less.

[0038] In this embodiment, the "average crystal grain size" may be determined by a planimetric method from a scanning electron microscope (hereinafter also referred to as "SEM") observation image obtained using a general scanning electron microscope (instrument name: JSM-IT500, manufactured by JEOL Ltd.) under the following measurement conditions: Accelerating voltage: 10kV Observation magnification: 2000x to 40000x

[0039] Prior to the measurement by the planimetric method, the zirconia sintered body sample is mirror-polished to a surface roughness Ra≦0.02 μm, and then thermally etched for 30 minutes in an air atmosphere 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). This is the pretreatment.

[0040] In the planimetric method, a circle of known area is drawn on the SEM image of a pretreated sintered sample, and the number of crystal grains within the circle (Nc) and the number of crystal grains on the circumference of the circle (Ni) are measured. The measured Nc and Ni are then used to calculate the "average crystal grain size" according to the following formula (4). Average grain size=(Nc+(1 / 2)×Ni) / (A / M 2 ) · · · (4) In the above formula (4), Nc is the number of crystal grains in a 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 drawing a circle with a known area on an SEM observation image, the circle was drawn so that the total of Nc and Ni was 125±25. If the total of Nc and Ni of the drawn circle is less than 125±25, multiple SEM observation images (for example, 2 to 5) were used, and the arithmetic mean value of the values ​​(provisional average crystal grain size) obtained for each SEM observation image was used as the "average crystal grain size."

[0041] The zirconia constituting the zirconia sintered body is preferably zirconia containing a stabilizing element, 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 can be exemplified by 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), preferably one or more elements selected from the group consisting of calcium, magnesium, cerium and yttrium, more preferably yttrium. Hereinafter, stabilized zirconia in which the stabilizing element is yttrium or the like is also referred to as yttrium stabilized zirconia or the like.

[0042] When the zirconia constituting the zirconia sintered body is yttrium-stabilized zirconia, the content of yttrium calculated as Y2O3 (hereinafter also referred to as "yttrium content") is preferably 1.5 mol% or more, more preferably 2.5 mol% or more, relative to 100 mol% of yttrium-stabilized zirconia. The upper limit of the yttrium content is preferably 10 mol% or less, more preferably 8 mol% or less. In the case of yttrium-stabilized zirconia having a tetragonal crystal as the main phase, the yttrium content is 2 mol% or more and 6 mol% or less, preferably 2.5 mol% or more and 4 mol% or less. In addition, in the case of yttrium-stabilized zirconia having a cubic crystal as the main phase, the yttrium content is more than 6 mol% and 10 mol% or less, preferably 7 mol% or more and 9 mol% or less.

[0043] The raw material sintered body may be a sintered body mainly composed of zirconia (a sintered body whose parent phase is zirconia), and may contain other components other than zirconia in addition to zirconia (or stabilized zirconia). Examples of such other components include additive components such as aluminum oxide and inevitable impurities of zirconia such as hafnia (HfO2), and one or more of these may be included. Examples of additive components include one or more selected from the group consisting of alumina, silica, and germania, and further alumina. In the raw material sintered body, when calculating values ​​based on the composition, density, etc. of the zirconia sintered body, hafnia is calculated as zirconia (ZrO2).

[0044] When the raw sintered body contains an aluminum compound as an additive component, the aluminum compound is preferably alumina, and the preferred alumina content is, for example, 0% by mass or more and 0.5% by mass or less, more preferably more than 0% by mass and 0.5% by mass or less, and even more preferably 0.1% by mass or more and 0.3% by mass or less. Note that a content of 0% by mass means that the substance is not contained.

[0045] The content of alumina in this embodiment is the mass (mass%) of aluminum converted into Al2O3 relative to the mass of the zirconia sintered body. The mass of the zirconia sintered body is the total mass of the metal elements and metalloid elements contained in the zirconia sintered body converted into oxides. When converting the metal elements and metalloid elements into oxides, for example, zirconium (Zr) is ZrO2, calcium (Ca) is CaO, magnesium (Mg) is MgO, cerium (Ce) is CeO2, and praseodymium (Pr) is Pr6O 11 , neodymium (Nd) is Nd2O3, samarium (Sm) is Sm2O3, europium (Eu) is Eu2O3, gadolinium (Gd) is Gd2O3, terbium (Tb) is TbO 11Dysprosium (Dy) is converted to Dy2O3, Holmium (Ho) is converted to Ho2O3, Erbium (Er) is converted to Er2O3, Thulium (Tm) is converted to Tm2O3, Yttrium (Y) is converted to Y2O3, Ytterbium (Yb) is converted to Yb2O3, Lutetium (Lu) is converted to Lu2O3, Aluminum (Al) is converted to Al2O3, Silicon (Si) is converted to SiO2, and Germanium (Ge) is converted to Ge2O3. These converted oxides are the M1, M2, ..., M in formula (2'). n And, M in Eq. (2) add1 ,M add2 ,…,M addn This can be used to convert

[0046] The shape of the raw material sintered body is arbitrary, and may be one or more selected from the group consisting of plate-like, columnar, cubic, polyhedral, pyramidal, spherical, and approximately spherical, or a shape according to the application. The raw material sintered body may be a raw material sintered body having a non-uniform thickness, or may be a raw material sintered body having a uniform thickness. That is, the raw material sintered body may include parts having different thicknesses. In addition, the raw material sintered body may be, for example, a zirconia sintered body having a maximum thickness of 0.5 μm or more, a zirconia sintered body having a maximum thickness of 100 μm or more, a zirconia sintered body having a maximum thickness of 1 mm or more, or even 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 may be a zirconia sintered body having a maximum thickness of less than 0.5 μm. On the other hand, when the raw material sintered body has a uniform thickness, the raw material sintered body may be a zirconia sintered body having a thickness of 0.5 μm or more, a zirconia sintered body having a thickness of 100 μm or more, a zirconia sintered body having a thickness of 1 mm or more, or even a zirconia sintered body having a thickness of 1.5 mm to 5 mm. However, the raw material sintered body may be a zirconia sintered body having a thickness of less than 0.5 μm.

[0047] Here, the thickness of the raw material sintered body is the material thickness of the raw material sintered body, so-called wall thickness (distance between opposing surfaces), and is a value measured by a length measuring device (for example, a micrometer, a vernier caliper, a wall thickness measuring device). For example, in the case of a plate-shaped, columnar, cubic, polyhedral, or pyramidal raw material sintered body, the value obtained by measuring the length of the raw material sintered body in a direction perpendicular to the surface with the largest area among the surfaces of the raw material sintered body (in the case where the surface with the largest area is a curved surface, the direction in which the normal to the surface with the largest area extends) can be mentioned. It should be noted that the plate-shaped, columnar, cubic, polyhedral, and pyramidal shapes include shapes in which the surface is subjected to R processing. In addition, for example, in the case of a spherical or approximately spherical raw material sintered body, the value obtained by measuring the length of the raw material sintered body in a direction perpendicular to the cross section with the largest area among the cross sections of the raw material sintered body can be mentioned. In addition, the maximum thickness can be a value obtained by measuring the thickness of the thickest part. For example, in a shape having irregularities, the thickness of the protrusions can be measured.

[0048] Next, the constituent elements that differ between the ammonia treatment process that satisfies the condition (a) and the ammonia treatment process that satisfies the condition (b) will be described below.

[0049] In the ammonia treatment step satisfying the condition (a), the oxygen concentration of the oxygen-containing ammonia atmosphere (hereinafter, simply referred to as "oxygen concentration") is 0.3% by volume or more. From the viewpoint of producing recycled zirconia that is easier to pulverize, the oxygen concentration in the ammonia treatment step satisfying the condition (a) is preferably 0.4% by volume or more.

[0050] In the ammonia treatment step that satisfies 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.

[0051] The oxygen concentration in the ammonia treatment step that satisfies condition (a) may be any combination of the upper and lower limits described above, but may be 0.3 vol% to 10 vol%, may be 0.3 vol% to 5 vol%, or may be 0.3 vol% to 1 vol%. From the viewpoint of producing recycled zirconia that is easier to pulverize, the oxygen concentration in the ammonia treatment step that satisfies condition (a) is preferably 0.4 vol% to 10 vol%, more preferably 0.4 vol% to 5 vol%, and even more preferably 0.4 vol% to 1 vol%.

[0052] The oxygen concentration in the oxygen-containing ammonia atmosphere does not substantially change from the oxygen concentration in the initial atmosphere gas. Therefore, the oxygen concentration in the oxygen-containing ammonia atmosphere can be controlled by adjusting the oxygen concentration in the initial atmosphere gas. In this embodiment, the oxygen concentration in the oxygen-containing ammonia atmosphere can be regarded as the oxygen concentration in the oxygen-containing ammonia atmosphere by the oxygen concentration in the initial atmosphere gas (atmosphere gas containing at least ammonia and oxygen). The oxygen concentration in the oxygen-containing ammonia atmosphere can be determined, for example, from the volume of each component (hereinafter also referred to as "introduced component") introduced to form the initial atmosphere gas. For the volume of the introduced component, for example, the volume at 1 atmosphere (1013 pa) and 25°C can be used. The oxygen concentration in the oxygen-containing ammonia atmosphere may be determined by measuring the oxygen concentration in the initial atmosphere gas by gas chromatography or an oxygen concentration meter. The measurement of the oxygen concentration can be performed, for example, under conditions of 1 atmosphere (1013 pa) and 25°C.

[0053] In the ammonia treatment process that satisfies the condition (a), the cubic crystal ratio of the raw material sintered body is not particularly limited, and the zirconia sintered body may not contain cubic crystals (that is, the cubic crystal ratio may be 0 mass%). From the viewpoint of producing recycled zirconia that is easier to pulverize, the cubic crystal ratio of the raw material sintered body in the ammonia treatment process that satisfies the condition (a) is preferably 1 mass% or more. In the ammonia treatment process that satisfies the condition (a), the upper limit of the cubic crystal ratio of the raw material sintered body is, for example, 100 mass% or less, and may be 90 mass% or less. The cubic crystal ratio of the raw material sintered body in the ammonia treatment process that satisfies the condition (a) may be 1 mass% or more and 100 mass% or less, or 1 mass% or more and 90 mass% or less.

[0054] The "cubic crystal ratio" in this embodiment is the ratio of cubic crystals to the crystal phase of the zirconia sintered body, and can be determined by Rietveld analysis of an X-ray diffraction pattern measured under the following conditions using a general X-ray diffraction device (for example, X'pert PRO MPD, manufactured by Spectris). Radiation source: CuKα radiation (λ=0.15418nm) Scan speed: 0.17° / min Measurement range: 2θ=10°~140° Acceleration voltage / current: 40mA / 45kV Divergence vertical limit slit: 10mm Detector: High-speed detector (X'Celerator)

[0055] Rietveld analysis can be performed using the analysis program "RIETAN-FP" to identify the crystal phases under the following conditions, and the cubic crystal ratio can be determined by the percentage [mass %] of cubic crystals in the total of the obtained crystal phases. Profile function: Split pseudo-Voigt function Range: 2θ=10°~140° Tetragonal phase: Tetragonal space group P42 / nmc Cubic phase: cubic space group Fm-3m

[0056] In the ammonia treatment step, if the oxygen concentration is 0.3 volume % 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 ratio of the zirconia sintered body). Therefore, if the production method of the present embodiment includes an ammonia treatment step that satisfies the condition (a), recycled zirconia that is easy to pulverize can be produced.

[0057] On the other hand, in the ammonia treatment step satisfying the condition (b), the oxygen concentration is 0.2% by volume or more. From the viewpoint of producing recycled zirconia that is easier to pulverize, in the ammonia treatment step satisfying the condition (b), the oxygen concentration is preferably 0.4% by volume or more.

[0058] In the ammonia treatment step that satisfies the condition (b), the upper limit of the oxygen concentration in the ammonia treatment step can be, for example, 10% by volume or less, can be 5% by volume or less, or can be 1% by volume or less.

[0059] The oxygen concentration in the ammonia treatment step that satisfies condition (b) may be any combination of the upper and lower limits described above, but may be 0.2 vol% to 10 vol%, may be 0.2 vol% to 5 vol%, or may be 0.2 vol% to 1 vol%. From the viewpoint of producing recycled zirconia that is easier to pulverize, the oxygen concentration in the ammonia treatment step that satisfies condition (b) is preferably 0.4 vol% to 10 vol%, more preferably 0.4 vol% to 5 vol%, and even more preferably 0.4 vol% to 1 vol%.

[0060] In the ammonia treatment step satisfying the condition (b), the cubic crystal ratio of the raw sintered body is 3.5% by mass or more. From the viewpoint of producing recycled zirconia that is easier to crush, in the ammonia treatment step satisfying the condition (b), the cubic crystal ratio of the raw sintered body is preferably 4% by mass or more, more preferably 5% by mass or more, particularly preferably 10% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. In the ammonia treatment step satisfying the condition (b), the upper limit of the cubic crystal ratio of the raw sintered body can be, for example, 100% by mass or less. The cubic crystal ratio of the raw sintered body in the ammonia treatment step satisfying the condition (b) can be 3.5% by mass or more and 100% by mass or less, 4% by mass or more and 100% by mass or less, 5% by mass or more and 100% by mass or less, 10% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, or 90% by mass or more and 100% by mass or less.

[0061] In the ammonia treatment step, when the oxygen concentration is less than 0.2% by volume, the zirconia sintered body cannot be made porous regardless of the cubic crystal fraction of the zirconia sintered body. In addition, when the oxygen concentration in the ammonia treatment step is 0.2% by volume or more and less than 0.3% by volume, the zirconia sintered body having a cubic crystal fraction of less than 3.5% by mass cannot be made porous. This is believed to be because, under such conditions, the nitridation of zirconia proceeds, and the formation of zirconia nitride proceeds preferentially.

[0062] On the other hand, even if the oxygen concentration is 0.2% by volume or more and less than 0.3% by volume, if the cubic crystal ratio of the raw material sintered body is 3.5% by mass or more, the porosity can be promoted. This is considered to be because, when the cubic crystal ratio of the raw material sintered body is high, the ionic conductivity of the zirconia sintered body becomes higher, and nitrogen (nitrogen in an active state) becomes more likely to diffuse inside the zirconia sintered body. Therefore, if the manufacturing method of this embodiment includes an ammonia treatment step that satisfies condition (b), recycled zirconia that is easy to crush can be manufactured.

[0063] According to the manufacturing method of the present embodiment described above, recycled zirconia that is easily pulverized can be manufactured from the zirconia sintered body regardless of the shape of the zirconia sintered body. In the present embodiment, "recycled zirconia" refers to a zirconia composition obtained by recycling the zirconia sintered body, and in particular, a zirconia composition obtained from the zirconia sintered body and capable of being used as at least one of the raw material of the sintered body and the precursor of the raw material. In particular, the recycled zirconia manufactured by the manufacturing method of the present embodiment is zirconia in a state in which the zirconia sintered body is treated so that it can be recycled (recycled) as a raw material of zirconia, and specifically, it is a zirconia sintered body that has been made porous (hereinafter also referred to as "porous zirconia"). As a specific example of porous zirconia, as shown in the examples described later, porous zirconia in which pores are formed in the crystal particles can be exemplified.

[0064] 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 the intragranular pores formed in the crystal grains by heat treatment can be confirmed by comparing the crystal grains before and after the heat treatment by SEM observation. The porous zirconia in which pores are formed in the crystal grains is different from the porous zirconia in which grain boundary pores (voids or defects at the interface (grain boundary) of crystal grains) are formed by heat treatment, such as porous zirconia in which pores are only confirmed between the crystal grains (pores not confirmed in the crystal grains) or porous zirconia in which defects can only be confirmed around the crystal grains (pores not confirmed in the crystal grains). Note that the porous zirconia in which pores are formed in the crystal grains may have voids formed between the crystal grains or defects around the crystal grains, as long as pores can be confirmed in the crystal grains.

[0065] The structural shape of the recycled zirconia can be observed under the following conditions using a general scanning electron microscope (instrument name: S-4800, manufactured by Hitachi High-Technologies Corporation): Prior to observation with the scanning electron microscope, the recycled zirconia is pretreated by heat treatment in air at a temperature 50°C lower than the sintering temperature for 30 minutes, and the pretreated recycled zirconia is used as the object of observation. Accelerating voltage: 20kV Measurement magnification: 10,000x, 30,000x or 50,000x

[0066] The recycled zirconia produced by the production method of the present embodiment may have any form, and may have the same form as the sintered body subjected to the ammonia treatment step. Specific examples of the recycled zirconia form include one or more forms selected from the group consisting of a molded body, a lump, and an aggregate.

[0067] The recycled zirconia produced by the manufacturing method of this embodiment can be used as a raw material for zirconia. Therefore, according to the present disclosure, a method for recycling a zirconia sintered body can be provided in which the recycled zirconia produced by the manufacturing method of this embodiment is used as a raw material for zirconia, and further, a horizontal recycling method for a sintered body can be provided in which the recycled zirconia produced by the manufacturing method of this embodiment is used as a raw material for zirconia for manufacturing a sintered body. In this recycling method, the recycled zirconia produced by the manufacturing method of this embodiment may be used as a raw material for zirconia as it is, but in manufacturing a product, it is preferable that the raw material zirconia is in the form of particles (powder), so that the recycled zirconia produced may be finely milled by pulverization or the like, and the finely milled recycled zirconia (hereinafter also referred to as "recycled zirconia powder") may be used as a raw material for zirconia. In particular, sintered bodies including zirconia sintered bodies are manufactured by sintering zirconia particles (powder). Therefore, in the horizontal recycling method for a sintered body, it is preferable to finely mill the recycled zirconia produced by the manufacturing method of this embodiment, and use the obtained recycled zirconia powder as a raw material for zirconia for manufacturing a sintered body. The sintered body produced using recycled zirconia is a sintered body containing zirconia, and examples of such a sintered body include a zirconia sintered body which is a sintered body of zirconia powder, and a composite sintered body which is a sintered body of zirconia powder and a powder other than zirconia (e.g., alumina powder). EXAMPLES

[0068] The present disclosure will be described below using examples, but the present disclosure is not limited to these examples.

[0069] (SEM observation) Using a scanning electron microscope (S-4800, Hitachi High-Technologies Corporation), the structure and morphology of the zirconia sintered body (recycled zirconia) after ammonia treatment was observed under the following conditions. Accelerating voltage: 20kV Measurement magnification: 10,000x, 30,000x or 50,000x Prior to SEM observation, the recycled zirconia was pretreated by heat treatment for 30 minutes in an air atmosphere at a temperature 50°C lower than the sintering temperature, and this was then used as the observation subject.

[0070] (Raman spectroscopy) Using a confocal Raman microscope (InVIA, manufactured by Renishaw), the Raman spectrum of the zirconia sintered body after ammonia treatment (recycled zirconia) was measured under the following conditions, and the crystal phase was identified from the spectrum shape. Excitation wavelength: 532nm

[0071] (Average grain size) The average crystal grain size of the zirconia sintered body used in the examples and comparative examples was measured by using a scanning electron microscope (device name: JSM-IT500, manufactured by JEOL Ltd.) to obtain SEM observation images under the following measurement conditions. Using the obtained SEM observation images, the average crystal grain size was calculated from the above formula (4) by the above-mentioned planimetric method. Accelerating voltage: 10kV Observation magnification: 2000x to 40000x Prior to the measurement of the average crystal grain size, the zirconia sintered body samples were mirror-polished to a surface roughness Ra≦0.02 μm, and then pretreated by thermal etching for 30 minutes in an air atmosphere 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). The pretreated zirconia sintered bodies were used as the measurement subjects.

[0072] (Relative Density) The relative density of the zirconia sintered body sample was calculated by the above formula (3) using the actual density ρ obtained from the volume measured by Archimedes' method and the mass calculated by mass measurement, and the true density ρ0 calculated from the above formulas (1) to (2") (true density ρz when formula (2") is used).

[0073] (Identification of crystal phase and cubic crystal ratio) The crystal phases of the zirconia sintered body samples and the recycled zirconia were measured for X-ray diffraction patterns using an X-ray diffractometer (device name: X'pert PRO MPD, manufactured by Spectris) under the following conditions. Radiation source: CuKα radiation (λ=0.15418nm) Scan speed: 0.17° / min Measurement range: 2θ=10°~140° Acceleration voltage / current: 40mA / 45kV Divergence vertical limit slit: 10mm Detector: High-speed detector (X'Celerator)

[0074] The XRD patterns of the obtained sintered samples were subjected to Rietveld analysis using RIETAN-FP under the following conditions to identify the crystal phases, and the cubic crystal ratio was calculated as the proportion [mass %] of cubic crystals in the total of the obtained crystal phases. Profile function: Split pseudo-Voigt function Range: 2θ=10°~140° Tetragonal phase: Tetragonal space group P42 / nmc Cubic phase: cubic space group Fm-3m

[0075] Example 1 5 g of commercially available zirconia powder (yttrium-stabilized zirconia powder with an yttrium content of 3 mol%, product name: TZ-3YS, manufactured by Tosoh Corporation) was filled into a die with a diameter of 25 mm, uniaxially pressed at 19.6 MPa, and then cold isostatically pressed at 196 MPa to obtain a green body. The green body obtained was sintered under atmospheric pressure at 1500°C for 2 hours in an air atmosphere to obtain a disk-shaped zirconia sintered body with a thickness of 3 mm, a relative density of 99.3%, an average crystal grain size of 0.49 μm, and a yttrium content of 3 mol% yttrium-stabilized zirconia. The obtained zirconia sintered body had a crystal phase consisting of tetragonal and cubic crystals, and the cubic crystal ratio was 4 mass%.

[0076] The obtained zirconia sintered body was placed on an alumina plate, and then placed in a tubular furnace equipped with a quartz glass reaction tube. The zirconia sintered body placed in the reaction tube was heat-treated (ammonia treatment) under the following conditions. Heat treatment atmosphere: Ammonia-air mixed gas flow atmosphere (Ammonia concentration: 96.2% by volume, oxygen concentration: 0.8% by volume) (Ammonia-air mixed gas flow rate: 520sccm) Heat treatment temperature: 1000℃ Heat treatment time: 16 hours

[0077] After the heat treatment (ammonia treatment), the product was cooled to room temperature to obtain the recycled zirconia of this example. The obtained recycled zirconia was confirmed by visual inspection to have the same shape as the zirconia sintered body used as the raw material, but SEM observation before and after the heat treatment (ammonia treatment) confirmed that the recycled zirconia was porous with pores formed in the crystal particles.

[0078] Example 2 The sintered body was heat-treated in the same manner as in Example 1, except that the heat treatment atmosphere was an ammonia-air mixed gas flow atmosphere with an ammonia concentration of 98.0 volume % and an oxygen concentration of 0.4 volume %, to obtain recycled zirconia of this example. SEM observation of the surface of the zirconia sintered body before heat treatment is shown in FIG. 2, and SEM observation of the surface of the recycled zirconia obtained by heat treatment is shown in FIG. 1. While no pores (air holes) were observed before heat treatment, many pores (air holes) were observed in the crystal particles after heat treatment, and it was confirmed that the obtained recycled zirconia was a porous zirconia in which pores were formed in the crystal particles. It was confirmed by visual inspection that the recycled zirconia had the same shape as the zirconia sintered body used as the raw material.

[0079] Example 3 A disk-shaped zirconia sintered body made of yttrium-stabilized zirconia containing alumina, having a thickness of 3 mm, a relative density of 99.5%, an average crystal grain size of 0.18 μm, an alumina content of 0.25 mass% and an yttrium content of 3 mol%, was obtained in the same manner as in Example 1, except that a commercially available zirconia powder (alumina-containing yttrium-stabilized zirconia powder with an alumina content of 0.25 mass% and an 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. The zirconia sintered body obtained had a crystal phase consisting of cubic and tetragonal crystals, and the cubic crystal ratio was 3 mass%. The zirconia sintered body obtained was heat-treated in the same manner as in Example 1 to obtain the recycled zirconia of this example. The obtained recycled zirconia was confirmed by visual inspection to have the same shape as the zirconia sintered body used as the raw material, but SEM observation before and after heat treatment (ammonia treatment) confirmed that it was porous zirconia with pores formed in the crystal particles. Figure 3 shows an SEM observation of the recycled zirconia of this example. The SEM observation shown in Figure 3 has a higher observation magnification than the SEM observation shown in Figure 1, so the state of the pores could be confirmed more clearly.

[0080] Example 4 The sintered body was heat-treated in the same manner as in Example 2, except that the zirconia sintered body produced in the same manner as in Example 3 was used, to obtain the recycled zirconia of this example. The obtained recycled zirconia was confirmed by visual inspection to have the same shape as the zirconia sintered body used as the raw material, but it was confirmed by SEM observation before and after the heat treatment (ammonia treatment) that it was porous zirconia in which pores were formed in the crystal particles. The recycled zirconia of this example maintained the same shape as the sintered body immediately after the heat treatment, but there were some parts that collapsed and became powdery when removed from the tubular furnace.

[0081] Example 5 A disk-shaped zirconia sintered body made of yttrium-stabilized zirconia having a thickness of 3 mm, a relative density of 98.6%, an average crystal grain size of 3.78 μm, and an yttrium content of 8 mol% was obtained in the same manner as in Example 1, except that a commercially available zirconia powder (product name: TZ-8YS, manufactured by Tosoh Corporation) was used. The obtained zirconia sintered body had crystals made of cubic crystals, and the cubic crystal ratio was 100 mass%. The obtained zirconia sintered body was heat-treated in the same manner as in Example 1 to obtain the recycled zirconia of this example. It was visually confirmed that the obtained recycled zirconia had the same shape as the zirconia sintered body used as the raw material, but it was confirmed that it was porous zirconia in which pores were formed in the crystal grains by SEM observation before and after the heat treatment (ammonia treatment).

[0082] Example 6 The zirconia sintered body obtained in the same manner as in Example 5 was used, and the zirconia sintered body was heat-treated in the same manner as in Example 1, except that the heat treatment atmosphere was an ammonia-air mixed gas flow atmosphere with an ammonia concentration of 99.0 volume % and an oxygen concentration of 0.2 volume %, to obtain recycled zirconia of this example. It was visually confirmed that the obtained recycled zirconia had the same shape as the zirconia sintered body used as the raw material, but SEM observation before and after the heat treatment (ammonia treatment) confirmed that it was porous zirconia in which pores were formed in the crystal particles.

[0083] Example 7 The zirconia sintered body obtained in the same manner as in Example 5 was used, and the zirconia sintered body was heat-treated in the same manner as in Example 1, except that the heat treatment atmosphere was an ammonia-air mixed gas flow atmosphere with an ammonia concentration of 98.0 volume % and an oxygen concentration of 0.4 volume %, to obtain recycled zirconia of this example. It was visually confirmed that the obtained recycled zirconia had the same shape as the zirconia sintered body used as the raw material, but SEM observation before and after the heat treatment (ammonia treatment) confirmed that it was porous zirconia in which pores were formed in the crystal particles.

[0084] Comparative Example 1 The zirconia sintered body obtained in the same manner as in Example 3 was used, and the heat treatment atmosphere was an ammonia-air mixed gas flow atmosphere with an ammonia concentration of 99.0 volume % and an oxygen concentration of 0.2 volume %, and the zirconia sintered body was heat treated in the same manner as in Example 1 to obtain recycled zirconia of this comparative example. The recycled zirconia of this comparative example had the same shape as before the heat treatment, and the surface structure was also unchanged from before the heat treatment (i.e., it was not porous zirconia).

[0085] [Crushability evaluation] The crushability of the recycled zirconia obtained in the examples and comparative examples was evaluated. That is, the recycled zirconia after the treatment was judged to be crushable (evaluation: ◯) if the shape of the recycled zirconia was not distorted when the recycled zirconia was held with tweezers, and was judged to be difficult to crush (evaluation: ×) if the shape of the recycled zirconia was not distorted. The results are shown in Table 1.

[0086] [Table 1]

[0087] As shown in Table 1, the recycled zirconia of the examples was evaluated as ◯ in terms of crushability, whereas the recycled zirconia of the comparative examples was evaluated as ×. From these results, it was understood that the examples enable the production of recycled zirconia that is easily crushed from a zirconia sintered body.

Claims

1. The method includes an ammonia treatment step of heat-treating the zirconia sintered body at a temperature of 800° C. or more and 1200° C. or less in an oxygen-containing ammonia atmosphere, The method for producing recycled zirconia, wherein the ammonia treatment step satisfies either one of the following conditions (a) and (b): (a) The oxygen concentration of the oxygen-containing ammonia atmosphere is 0.3 volume % or more. (b) the oxygen-containing ammonia atmosphere has an oxygen concentration of 0.2 volume % or more, and the zirconia sintered body has a cubic crystal fraction of 3.5% or more.

2. The oxygen-containing ammonia atmosphere is an atmospheric gas containing at least ammonia (NH ３ ) and oxygen (O ２ 2. The method according to claim 1, wherein the atmospheric gas contains

3. The atmospheric gas is ammonia (NH ３ ) and oxygen (O ２ 3. The method according to claim 2, wherein the total concentration of the above-mentioned elements is 85% by volume or more and 100% by volume or less with respect to 100% by volume of the atmospheric gas.

4. 3. The method according to claim 1, wherein the heat treatment is carried out in an oxygen-containing ammonia atmosphere in which atmospheric gas flows through the system.

5. The manufacturing method according to claim 1 or 2, wherein the oxygen-containing ammonia atmosphere is an atmospheric gas during the heat treatment of the zirconia sintered body, the atmospheric gas containing at least one of an active species of ammonia and ammonia, and oxygen.

6. The method according to claim 1 or 2, wherein the zirconia sintered body has a maximum thickness of 0.5 μm or more.

7. The method according to claim 1 or 2, wherein the zirconia sintered body has a relative density of 98% or more.

8. The method according to claim 1 or 2, wherein the zirconia constituting the zirconia sintered body contains a stabilizing element.

9. The method of claim 8 wherein the stabilizing element is yttrium.

10. The manufacturing method according to claim 1 or 2, wherein the zirconia sintered body contains one or more selected from the group consisting of alumina, silica and germania.

11. A method for recycling sintered bodies, which uses recycled zirconia produced by the method according to claim 1 or 2 as a raw material for zirconia.

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