Zirconia sintered body and its manufacturing method

Abstract

The present disclosure provides a technology for realizing a zirconia sintered body having excellent translucency and hydrothermal degradation resistance. The manufacturing method of the zirconia sintered body disclosed herein includes a molded body preparation step of preparing a molded body containing zirconia and yttria and / or ytterbia, in which the total ratio of yttria and ytterbia is 3 mol% or more and less than 4 mol% when the total of zirconia, yttria, and ytterbia is 100 mol%, a first heating step of heating the molded body at 800°C or more and 1200°C or less, a second heating step of heating the molded body that has undergone the first heating step at 1600°C or more and 2000°C or less by microwave heating, and a cooling step of lowering the temperature of the molded body that has undergone the second heating step to 1300°C at a rate of 50°C / min or more.

Description

[Technical field]

[0001] The present disclosure relates to a zirconia sintered body and a manufacturing method thereof. This application claims priority based on Japanese Patent Application No. 2023-091593 filed on June 2, 2023, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] Yttria (Y) is used as a stabilizer. 2 O 3 Zirconia sintered bodies in which small amounts of rare earth elements such as ZnO, ZnCl, and ZnO are dissolved (hereinafter also referred to as "partially stabilized zirconia sintered bodies") tend to have improved mechanical properties such as strength and toughness compared to zirconia sintered bodies that do not contain stabilizers. Therefore, partially stabilized zirconia sintered bodies are used in machine structural materials such as cutting tools, bearings, and dispersing / pulverizing machines, and biomaterials such as dental materials, etc.

[0003] For example, Patent Document 1 discloses a translucent zirconia sintered body containing more than 4.0 mol % and not more than 6.5 mol % yttria as a stabilizer. Patent Documents 2 and 3 disclose that a zirconia sintered body having excellent resistance to hydrothermal degradation can be realized by setting the sintering temperature of the zirconia sintered body to at least 1350° C. or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2015-143178 [Patent Document 2] Japanese Patent Application Publication No. 2014-12627 [Patent Document 3] Japanese Patent Application Publication No. 2014-218421 Summary of the Invention [Problem to be solved by the invention]

[0005] By adjusting the content of the stabilizer contained in the partially stabilized zirconia sintered body, the mechanical properties of the zirconia sintered body can be adjusted. For example, when the content of the stabilizer is reduced, mechanical properties such as bending strength and fracture toughness tend to improve. However, in such a partially stabilized zirconia sintered body, the hydrothermal deterioration resistance and translucency tend to decrease.

[0006] The main object of the present disclosure is to provide a technique for realizing a zirconia sintered body having excellent translucency and resistance to hydrothermal degradation. [Means for solving the problem]

[0007] In order to achieve the above object, the present inventors have conducted research and found that a certain ratio of yttria and / or ytterbia (Yb 2 O 3 It was found that a zirconia sintered body with excellent translucency and resistance to hydrothermal degradation can be realized by sintering a molded body of partially stabilized zirconia having the above-mentioned structure by microwave heating and then rapidly cooling it to 1300°C. Analysis of the crystal phase of the zirconia sintered body revealed that the zirconia sintered body has a crystal phase in which the c / a axial length ratio of the unit lattice is in the range of 1.0055 or more and less than 1.010.

[0008] The present disclosure provides a method for producing a zirconia sintered body. The method for producing a zirconia sintered body includes a molded body preparation step of preparing a molded body containing zirconia and yttria and / or ytterbia, in which the total ratio of yttria and ytterbia is 3 mol% or more and less than 4 mol% when the total of zirconia, yttria, and ytterbia is 100 mol%, a first heating step of heating the molded body at 800°C or more and 1200°C or less, a second heating step of heating the molded body that has undergone the first heating step at 1600°C or more and 2000°C or less by microwave heating, and a cooling step of lowering the temperature of the molded body that has undergone the second heating step to 1300°C at a rate of 50°C / min or more. According to this manufacturing method, a zirconia sintered body having excellent translucency and resistance to hydrothermal deterioration can be manufactured.

[0009] In some preferred embodiments, the microwave heating is performed in a multi-mode. This allows heating while suppressing plasma generation. As a result, the generation of cracks in the zirconia sintered body is suppressed, and a zirconia sintered body having excellent translucency and hydrothermal deterioration resistance can be produced.

[0010] In some preferred embodiments, in the second heating step, the microwave heating is performed in an oxidizing atmosphere, which prevents the zirconia sintered body from darkening, and allows the production of a zirconia sintered body that is excellent in translucency, resistance to hydrothermal deterioration, and aesthetics.

[0011] In some preferred embodiments, in the second heating step, the microwave heating is performed in an atmosphere having an oxygen concentration of 30 vol% or more and 100 vol% or less. This effectively prevents the zirconia sintered body from darkening, and thus allows the production of a zirconia sintered body that is more aesthetically pleasing, translucent, and resistant to hydrothermal degradation.

[0012] In some preferred embodiments, in the second heating step, SiC susceptors are disposed so as to sandwich the molded body from both sides in a predetermined direction. This allows the sintering inside the molded body to proceed more favorably, so that a zirconia sintered body having better translucency and hydrothermal degradation resistance can be produced.

[0013] In some preferred embodiments, the molded body is prepared by molding a material containing zirconia-containing granules in the molded body preparation step, which can improve the shape stability of the molded body and improve the handling and workability.

[0014] The present disclosure also provides a zirconia sintered body. The zirconia sintered body contains zirconia, yttria and / or ytterbia, and when the total of zirconia, yttria and ytterbia is 100 mol%, the total ratio of yttria and ytterbia is 3 mol% or more and less than 4 mol%, and when the entire crystal phase is 100 mass%, the ratio of the crystal phase in which the c / a axis length ratio of the unit lattice is in the range of 1.0055 or more and less than 1.010 is 10 mass% or more. The zirconia sintered body having such a configuration has excellent light transmittance and hydrothermal deterioration resistance.

[0015] In this specification, the c / a axis length ratio and its ratio of the unit lattice of the zirconia sintered body can be obtained by Rietveld analysis of the profile of the X-ray diffraction pattern of the zirconia sintered body using RIETAN-FP as an analysis program.

[0016] In some preferred embodiments, when the entire crystal phase is taken as 100 mass%, the proportion of the crystal phase having a unit lattice c / a axis length ratio in the range of 1.007 or more and less than 1.010 is 10 mass% or more, which further improves the hydrothermal degradation resistance.

[0017] In some embodiments, the proportion of monoclinic crystals is 10% or less after immersion for 6 hours in hot water at 134° C. The zirconia sintered body disclosed herein can have such excellent resistance to hydrothermal deterioration.

[0018] In some embodiments, when the entire crystal phase is taken as 100 mass %, the proportion of the crystal phase having a c / a axial length ratio of the unit lattice in the range of 1 to less than 1.0055 is 20 mass % or less. With this configuration, the crystal phase becomes more homogeneous, and therefore, more excellent light transmittance and hydrothermal degradation resistance can be realized.

[0019] In some embodiments, the total light transmittance in the thickness direction of a 1 mm thick test piece for a D65 light source is 44% or more.

[0020] The present disclosure also provides a dental material containing the zirconia sintered body disclosed herein. Such a dental material can be used, for example, as a denture, a denture mill blank, or an orthodontic bracket. The zirconia sintered body disclosed herein is suitable for use as a dental material because of its excellent translucency and resistance to hydrothermal degradation. [Brief description of the drawings]

[0021] [Figure 1] 1 is a flowchart showing an outline of a method for producing a zirconia sintered body in one embodiment. [Diagram 2] FIG. 2 is a schematic diagram showing an example of a method for heating a molded body (preliminary sintered body) with microwaves. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, some embodiments of the technology disclosed herein will be described. Note that matters other than those specifically mentioned in this specification (for example, the temperature of microwave heating) that are necessary for carrying out this technology can be understood based on the technical content taught by this specification and the general technical common sense of a person skilled in the art in the relevant field. The contents of the technology disclosed herein can be carried out based on the contents disclosed in this specification and the technical common sense of a person skilled in the art in the relevant field. Note that in this specification, when a numerical range is described as "A to B (where A and B are any numerical values)", it means "A or more and B or less", and also includes the meanings of "more than A and less than B", "more than A and B or less", and "A or more and less than B".

[0023] The zirconia sintered body disclosed herein is made of at least zirconia (ZrO 2 The zirconia sintered body disclosed herein also contains yttria (Y 2 O 3 ) and Ytterbia (Yb 2 O 3) contains at least one of them. That is, the zirconia sintered body disclosed herein may be an embodiment containing both yttria and ytterbia, an embodiment containing yttria and not containing ytterbia, or an embodiment containing ytterbia and not containing yttria. The zirconia sintered body disclosed herein contains zirconia as a main component. Here, "containing zirconia as a main component" means that the proportion of zirconia is the largest among the compounds constituting the zirconia sintered body. When the entire zirconia sintered body is taken as 100 mass%, the proportion of zirconia is, for example, 70 mass% or more, preferably 80 mass% or more, and more preferably 90 mass% or more. A high proportion of zirconia can improve the strength, toughness, hydrothermal degradation resistance, etc. of the zirconia sintered body.

[0024] The yttria and / or ytterbia contained in the zirconia sintered body may be contained as a part of partially stabilized zirconia partially dissolved in zirconia (so-called stabilizer). Since the proportion of monoclinic crystals in partially stabilized zirconia is suppressed at room temperature, mechanical properties such as strength and toughness may be improved. Furthermore, since the proportion of monoclinic crystals is suppressed, the variation of the crystal phase constituting the zirconia sintered body is suppressed, and the translucency may be improved.

[0025] Generally, when the proportion of the stabilizer in partially stabilized zirconia is reduced, the mechanical properties improve, but the stability of the crystal phase tends to decrease. When the stability of the crystal phase is low, a phase transition from tetragonal to monoclinic is likely to occur, and the translucency decreases. In addition, when the proportion of monoclinic crystals increases, the hydrothermal degradation resistance tends to decrease. Therefore, it is desirable to improve the translucency and hydrothermal degradation resistance in partially stabilized zirconia with a low proportion of stabilizer.

[0026] The zirconia sintered body disclosed herein has a relatively low ratio of yttria and / or ytterbia contained as a stabilizer, but has excellent light transmittance and hydrothermal deterioration resistance. When the total of zirconia, yttria, and ytterbia contained in the zirconia sintered body is 100 mol%, the total ratio of yttria and ytterbia can be, for example, 4.2 mol% or less, 4 mol% or less, less than 4 mol%, or 3.9 mol% or less. In this way, excellent light transmittance and hydrothermal deterioration resistance are realized despite the low total ratio of yttria and / or ytterbia. In addition, the lower limit of the total ratio of yttria and ytterbia can be, for example, 3 mol% or more, 3.1 mol% or more, 3.3 mol% or more, or 3.5 mol% or more. This can stabilize the crystal phase. The "total ratio of yttria and ytterbia" also includes the case where the ratio of yttria or ytterbia is 0 mol% (i.e., the zirconia sintered body does not contain yttria or ytterbia). That is, in an embodiment that contains yttria but does not contain ytterbia, the ratio of yttria is the same as the range of the total ratio. Also, in an embodiment that contains ytterbia but does not contain yttria, the ratio of ytterbia is the same as the range of the total ratio.

[0027] When the zirconia sintered body contains both yttria and ytterbia, the proportion of yttria may be greater than that of ytterbia, or the proportion of yttria may be less than that of ytterbia. The yttria and / or ytterbia may be entirely dissolved in zirconia, or may include some that is not dissolved in zirconia.

[0028] In some embodiments, the zirconia sintered body further comprises alumina (Al 2 O 3). In a zirconia sintered body containing alumina, abnormal grain growth is suppressed, so that the strength and translucency of the zirconia sintered body can be improved. In addition, the resistance to low-temperature deterioration can be improved, so that the strength and translucency of the zirconia sintered body can be maintained for a long period of time. On the other hand, since alumina remains as an impurity inside the sintered body and acts as a light scattering factor, it is preferable that the alumina content is not too high. Therefore, when the entire zirconia sintered body is taken as 100 mass%, the alumina content may be, for example, 0.15 mass% or less, and may be 0.125 mass% or less, 0.1 mass% or less, or 0.05 mass% or less.

[0029] The zirconia sintered body may contain a conventionally known coloring agent within a range that does not significantly impair the effects of the technology disclosed herein. Examples of the coloring agent include transition metal elements and lanthanoid rare earth elements. Examples of such elements include iron, nickel, cobalt, manganese, niobium, praseodymium, neodymium, europium, gadolinium, erbium, etc. The coloring agent may be, for example, 2% by mass or less, 1% by mass or less, or 0.5% by mass or less, based on the entire zirconia sintered body.

[0030] In addition, the zirconia sintered body may contain elements that may be inevitably mixed in. For example, hafnium, magnesium, silicon, titanium, etc. are listed. The total content of these elements is preferably 2.5 mass% or less, more preferably 2 mass% or less, for example 1.8 mass% or less, calculated as oxide, based on the entire zirconia sintered body.

[0031] 1 is a flowchart showing a method for producing a zirconia sintered body in one embodiment. In some embodiments, the method for producing a zirconia sintered body may include a molded body preparation step S10 for preparing a molded body (work) containing zirconia and yttria and / or ytterbia, a first heating step S20 for heating the molded body, a second heating step S30 for heating the molded body (hereinafter also referred to as a "temporary sintered body") that has undergone the first heating step S20 by microwave heating, and a cooling step S40 for lowering the temperature of the molded body that has undergone the second heating step.

[0032] <Molded object preparation process S10> The molded body preparation process S10 may include preparing a material that constitutes the molded body (hereinafter also referred to as the "molded body material") (hereinafter also referred to as the "molded body material preparation process") and molding the molded body material (hereinafter also referred to as the "molding process").

[0033] In the compact material preparation step, first, a zirconia raw material is prepared. The zirconia raw material is not particularly limited, but for example, a zirconium salt or a hydrate thereof can be used. For example, the zirconium salt includes zirconium oxychloride, zirconium chloride, zirconium sulfate, zirconium nitrate, etc. These may be used alone or in combination of two or more.

[0034] Next, an aqueous solution of the zirconia raw material is prepared, and a hydrolysis reaction is carried out to prepare the zirconia sol. The hydrolysis reaction can be carried out by adding an alkali metal hydroxide, an alkaline earth metal hydroxide, an aqueous ammonia solution, or the like to the aqueous solution. Examples of the alkali metal hydroxide that can be used include lithium hydroxide, sodium hydroxide, and potassium hydroxide, and examples of the alkaline earth metal hydroxide that can be used include magnesium hydroxide and calcium hydroxide.

[0035] Next, the zirconia sol (ZrO 2 nH 2 O) is mixed with yttria and / or ytterbia, or a raw material thereof. The raw material of yttria is an yttrium-containing compound that can become yttria by firing. Examples of the yttrium-containing compound include yttrium chloride and yttrium nitrate. The raw material of ytterbia may be an ytterbium-containing compound that can become ytterbia by firing. Examples of the ytterbium-containing compound include ytterbium chloride and ytterbium nitrate.

[0036] When yttria and / or ytterbia are mixed with the zirconia sol, the ratio of yttria and / or ytterbia to be mixed may be the same as the total ratio of yttria and ytterbia in the zirconia sintered body described above. When the total of zirconia, yttria, and ytterbia is taken as 100 mol%, the ratio may be, for example, 3 mol% or more, 3.1 mol% or more, 3.3 mol% or more, or 3.5 mol% or more. In addition, the total ratio of yttria and ytterbia may be, for example, 4.2 mol% or less, 4 mol% or less, less than 4 mol%, or 3.9 mol% or less.

[0037] In addition, when the zirconia sol is mixed with an yttria raw material and / or an ytterbia raw material, the amount of yttria and / or ytterbia obtained by firing these raw materials should be in the above-mentioned range of the ratio of yttria and / or ytterbia. For example, yttrium chloride (YCl 3 )X moles (X is a positive number) of yttria (Y 2 O 3 ) can be obtained, so it is sufficient to mix yttrium chloride so that the amount of substance is twice as much as when yttria itself is mixed.

[0038] Next, the zirconia sol to which yttria and / or ytterbia or the raw materials thereof have been added is dried to obtain a dry powder in which each raw material is uniformly dispersed. The drying method is not particularly limited, and can be appropriately selected from, for example, natural drying, air drying, hot air drying, drying by heating using a heating furnace or the like, vacuum drying, suction drying, freeze drying, and the like.

[0039] The dried powder is calcined to obtain a calcined powder containing yttria and / or ytterbia partially stabilized zirconia. The calcination temperature is not particularly limited, but may be, for example, 800°C to 1200°C, preferably 1000°C to 1200°C. The calcination may oxidize the yttria raw material to yttria, and the ytterbia raw material to ytterbia. The heating device for calcination may be a conventional heating device, and examples of the heating device include an electric furnace, a muffle furnace, a tunnel heating furnace, and a microwave furnace.

[0040] The calcined powder contains particles having various shapes and particle sizes, and is therefore preferably pulverized. The pulverization method is not particularly limited, and for example, the powder can be pulverized by a known pulverizing device (for example, a ball mill, etc.). For the ball mill, it is preferable to use zirconia balls having a diameter of about 0.1 mm to 5 mm. In addition, it is preferable to select the powder after pulverization to have a desired particle size. For example, a zirconia powder having a desired particle size can be obtained by using a mesh sieve, and the size of the mesh openings may be appropriately selected according to the desired particle size.

[0041] The average particle size of the zirconia powder used as the compact material is preferably, for example, 100 nm to 300 nm, and more preferably 150 nm to 200 nm. If the average particle size is within this range, the sinterability is high, and the strength and translucency can be improved. In this specification, the "average particle size" refers to the particle size (D 50 For such measurements, for example, a particle size distribution measuring device LA950V2 (manufactured by Horiba, Ltd.) can be used.

[0042] The zirconia powder produced as described above mainly contains yttria and / or ytterbia partially stabilized zirconia particles. The proportion of yttria and / or ytterbia partially stabilized zirconia particles in the zirconia powder is 50% by number or more, preferably 60% by number or more, and may be 70% by number or more, 80% by number or more, 90% by number or more, or 95% by number or more. The zirconia powder may contain fully stabilized zirconia. The zirconia powder may also contain zirconia particles in which yttria and / or ytterbia are not solid-dissolved. Furthermore, the zirconia powder may contain yttria particles and / or ytterbia particles.

[0043] In this manner, zirconia powder can be obtained as a compact material, but the compact material is not limited to this zirconia powder.

[0044] In some embodiments, an aluminum compound may be mixed with the zirconia powder. The aluminum compound may be oxidized to alumina by heating in the first heating step S20 and / or the second heating step S30. Therefore, the amount of the aluminum compound to be mixed may be determined so as to be the content of alumina in the zirconia sintered body described above, assuming that all of the aluminum contained in the aluminum compound is oxidized to alumina. As the aluminum compound, alumina powder, alumina sol, hydrated alumina, aluminum hydroxide, aluminum chloride, aluminum nitrate, aluminum sulfate, etc. may be used. In addition, the zirconia powder and the aluminum compound may be dispersed in a solvent such as water to form a slurry. In the case of forming a slurry, the slurry can be dried to obtain a zirconia powder in which the aluminum compound is suitably dispersed.

[0045] The average particle size of the aluminum compound is preferably equal to or smaller than that of the zirconia powder. Although not particularly limited, the average particle size of the aluminum compound is, for example, preferably 300 nm or less, more preferably 200 nm or less, and may be 150 nm or less, 100 nm or less (for example, 20 nm to 50 nm). This allows the aluminum compound to be suitably dispersed in the zirconia powder. Therefore, alumina can be more uniformly distributed in the zirconia sintered body, and abnormal grain growth of the zirconia sintered body can be suitably suppressed.

[0046] In some embodiments, the molded body material contains granules. The average particle size of the granular molded body material is, for example, 10 μm to 100 μm, and may be 20 μm to 90 μm, or 40 μm to 80 μm. When the molded body material contains granules, the shape stability is improved, and the handling and workability can be improved. In addition, the residual stress during molding is alleviated, and the occurrence of hot spots due to the powder density difference during microwave heating can be suppressed. In addition, in the manufacturing method disclosed herein, since zirconia is sintered by heating with microwaves, even granules with an average particle size larger than that of the powder can be suitably heated to the inside of the granules.

[0047] The method for producing the granular compact material is not particularly limited, but for example, the granular compact material can be produced by spray drying zirconia powder. The zirconia powder may contain an aluminum compound and may further contain a binder. In the spray drying, the zirconia powder is mixed with a dispersion medium (e.g., water) to prepare a slurry, and the slurry is sprayed onto droplets and dried to obtain the granular compact material.

[0048] The binder may be a component that burns out at the heating temperature in the first or second heating step described below. Examples of the binder include acrylic resins, epoxy resins, phenolic resins, amine resins, alkyd resins, and cellulose polymers. Among them, it is preferable to include an acrylic resin. By including an acrylic resin, the adhesion between the zirconia powders is increased, and the zirconia granules can be suitably produced. In addition, the shape stability of the molded body is increased, and the molded body can be stably held. Examples of the acrylic resin include a polymer containing an alkyl (meth)acrylate as a main monomer (a component that accounts for 50% by mass or more of the total monomers) and a copolymer containing such a main monomer and a sub-monomer that is copolymerizable with the main monomer. In this specification, the term "(meth)acrylate" means acrylate and methacrylate.

[0049] If the amount of binder is too large, voids may easily occur in the zirconia sintered body after the binder burns through. If voids occur in the zirconia sintered body, the hydrothermal deterioration resistance may decrease. In addition, the voids may cause light to be easily refracted, and the translucency may decrease. Therefore, the content of the binder may be, for example, 10% by mass or less, and preferably 5% by mass or less, when the entire powder used for spray drying is taken as 100% by mass. In addition, if the amount of the binder is too small, the effect of the binder may be insufficient. Therefore, the content of the binder may be, for example, 0.5% by mass or more, and may be 1% by mass or more.

[0050] Next, the molding process will be described. The method for molding the molded body material is not particularly limited, and for example, pressure molding, injection molding, extrusion molding, cast molding, etc. can be adopted. As the pressure molding, for example, cold isostatic pressing (CIP), hot isostatic pressing (HIP), etc. are preferably adopted. By using CIP or HIP, a molded body (molded body) with high homogeneity and high density can be manufactured.

[0051] <First heating step S20> In the first heating step S20, the molded body is heated to pre-sinter the molded body to obtain a pre-sintered body. This heating can remove components such as moisture, binders, and impurities that may be contained in the molded body. In addition, pre-sintering can reduce voids that may be present in the molded body, so that cracks that may occur during sintering due to high temperature and high speed heating can be suitably prevented. Pre-sintering can be performed at a heating temperature of, for example, 800°C to 1200°C, preferably 1000°C to 1100°C. The time for pre-sintering can vary depending on the shape, size, composition, etc. of the molded body, so it may be adjusted appropriately, but may be, for example, 1.5 hours to 5 hours, or 2 hours to 4 hours. Heating of the molded body can be performed by a known method, and for example, a heating device such as a muffle furnace, an electric furnace, or a microwave furnace can be used.

[0052] The shape of the molded body is not particularly limited, and may be, for example, a plate, a disk, a rectangular parallelepiped, a cube, a column, or the like.

[0053] The heating rate in the first heating step S20 is not particularly limited, but can be, for example, 100° C. / h to 250° C. / h until 800° C. is reached, and 50° C. / h to 150° C. / h until a predetermined temperature (for example, 1000° C. to 1200° C.) is reached. This makes it possible to prevent rapid sintering and suppress the occurrence of cracks.

[0054] <Second heating step S30> In the second heating step S30, the molded body (preliminary sintered body) that has been subjected to the first heating step S20 is sintered by microwave heating to obtain a sintered product. By performing microwave heating, the inside of the preliminarily sintered body can be heated quickly, so that the difference between the progress of sintering on the surface side of the preliminarily sintered body and the progress of sintering on the inside side is reduced, and the voids inside the zirconia sintered body can be further reduced. Hereinafter, one embodiment of the second heating step S30 will be described with reference to the drawings. Note that the microwave heating method is not limited to the following example.

[0055] Fig. 2 is a schematic diagram showing an example of a method for microwave heating a pre-sintered body. The dimensional relationships (length, width, thickness, etc.) in Fig. 2 do not reflect the actual dimensional relationships. The directions of up, down, left, and right are indicated by arrows U, D, L, and R in the figure, respectively. Here, the directions of up, down, left, and right are determined merely for the convenience of explanation and do not limit the installation form.

[0056] 2, the microwave heating device 10 has a partition wall 12 and a heating space 14. A heat insulating container 20 is installed in the heating space 14, and a susceptor 40 and a provisionally sintered body 50 are accommodated in an accommodation space 22 of the heat insulating container 20. A gas supplier 30 is connected to the accommodation space 22 of the heat insulating container 20. A radiation thermometer 60 is installed at a position separate from the outside of the microwave heating device 10.

[0057] The microwave heating device 10 has a heating space 14 surrounded by a partition wall 12. The heating space 14 is a space for accommodating an object to be microwave heated. Although not shown, the side wall, ceiling and / or bottom wall of the heating space 14 have a microwave irradiation section, and the object accommodated in the heating space 14 can be irradiated with microwaves and heated. The microwaves may have a frequency that has been conventionally used for microwave heating, and for example, microwaves with a frequency of 0.3 GHz to 3 GHz (e.g., 2.45 GHz) can be used.

[0058] The partition wall 12 insulates the heating space 14 of the microwave heating device 10 from the outside, and a commercially available microwave device can be used. In order to improve the insulation, the heating space 14 side of the partition wall 12 may be lined with an insulating material.

[0059] The partition wall 12 is provided with a through hole 16 for measuring the temperature of an object in the heating space 14. The through hole 16 passes through the heating space 14 so as to connect the heating space 14 to the outside of the microwave heating device 10. As the microwave heating device 10 having such a configuration, for example, μ-Reactor EX or μ-Reactor Mx manufactured by Shikoku Keisoku Kogyo Co., Ltd. can be used.

[0060] The heat-insulating container 20 has a storage space 22 capable of storing the susceptor 40 and the provisionally sintered body 50 therein. As shown in FIG. 2, in this embodiment, the heat-insulating container 20 has a gas inlet 24 for connecting the storage space 22 to the gas supplier 30, a gas outlet 26 for communicating the storage space 22 to the heating space 14, and a through hole 28 for measuring the temperature of the object to be heated in the storage space 22. In this embodiment, the heat-insulating container 20 is a rectangular box-shaped container, but the shape is not particularly limited and may be, for example, cylindrical or prismatic. Although not shown, in this embodiment, the heat-insulating container 20 is designed to be separable into a lid portion and a case portion, and is designed to allow the object to be heated to be easily put in and taken out of the storage space 22. The material of the heat-insulating container 20 may be, for example, a ceramic fiber such as alumina-silica fiber.

[0061] The gas introduction hole 24 is a through hole that communicates the accommodation space 22 with the heating space 14, and is designed so that a pump 32 connected to a gas supplier 30 can be inserted therethrough. This allows a desired gas to be supplied to the accommodation space 22, and the atmosphere within the accommodation space 22 can be controlled.

[0062] The gas exhaust hole 26 is a through hole that communicates the accommodation space 22 and the heating space 14, and is designed so that the accommodation space 22 is not sealed. This makes it possible to prevent the accommodation space 22 from becoming a reducing atmosphere due to the consumption of oxygen in the accommodation space 22 as the sintering of the provisionally sintered body 50 proceeds. In addition, the gas exhaust hole 26 can prevent the gas supplied from the gas inlet hole 24 from stagnating in the accommodation space 22. Note that, although one gas exhaust hole 26 is provided in FIG. 2, a plurality (two or more) of gas exhaust holes 26 may be provided. In this embodiment, the gas exhaust hole 26 is provided in a wall opposite to the wall in which the gas inlet hole 24 is provided, but the position of the gas exhaust hole 26 is not particularly limited. The diameter of the gas exhaust hole 26 is not particularly limited, but may be, for example, about 5 mm to 50 mm, or, for example, about 5 mm to 20 mm.

[0063] As shown in FIG. 2, in this embodiment, a through hole 28 that communicates the storage space 22 and the heating space 14 is provided on the upper side of the heat-insulating container 20. Here, the through hole 28 and the through hole 16 of the microwave heating device 10 are arranged so as to be aligned in a straight line. This allows the temperature of the object to be heated placed in the storage space 22 to be measured by a radiation thermometer 60 installed outside the microwave heating device 10. The through hole 28 is not particularly limited as long as it is provided with a size that allows the radiation thermometer 60 to measure the temperature of the object to be heated, but for example, the diameter of the through hole 28 can be about 5 mm to 10 mm. In this embodiment, the gas exhaust hole 26 and the through hole 28 are provided, but even a single through hole can perform the functions of both the gas exhaust hole 26 and the through hole 28 described above, so a configuration in which only one of them is provided may be used.

[0064] The gas supplier 30 supplies a desired gas to the storage space 22 of the insulated container 20 via the pump 32, and can adjust the atmosphere of the storage space 22. The gas supplier 30 can be appropriately changed according to the desired gas, and a commercially available gas supplier (e.g., an oxygen supplier) can be used without particular restrictions. When adjusting the storage space 22 to an atmospheric atmosphere, a blower or the like may be used as the gas supplier 30.

[0065] When the oxygen concentration around the provisionally sintered body 50 decreases with the firing of the provisionally sintered body 50, the zirconia contained in the provisionally sintered body 50 may be reduced. This may cause the zirconia sintered body to darken and lose its aesthetics. Therefore, it is preferable that the microwave heating is performed in an oxidizing atmosphere. Examples of the oxidizing atmosphere include an air atmosphere and an atmosphere with a higher oxygen concentration than the air atmosphere. In particular, the oxygen concentration is preferably 30 vol% or more, and may be, for example, 50 vol% or more, 70 vol% or more. In such an oxidizing atmosphere, the darkening of the zirconia sintered body can be further suppressed. The upper limit of the oxygen concentration in the atmosphere is not particularly limited, and the oxygen concentration can be 100 vol% or less, but if the oxygen concentration is too high, abnormal heating due to oxygen plasma may occur. Therefore, the oxygen concentration is preferably, for example, 95 vol% or less, more preferably 90 vol% or less. It is sufficient that such control to an oxidizing atmosphere is performed at least in the storage space 22 of the insulating container 20 in which the provisionally sintered body 50 is placed.

[0066] During the firing of the pre-sintered body 50, in order to control the atmosphere to the above-mentioned oxidizing atmosphere, it is preferable to continue to supply air or a gas containing the above-mentioned oxygen concentration to the accommodation space 22 (specifically, the pre-sintered body 50). This makes it possible to suppress fluctuations in the atmosphere of the accommodation space 22 accompanying firing (e.g., a decrease in oxygen concentration, etc.). As shown by the arrows in FIG. 2, the gas supplied from the gas supplier 30 flows into the accommodation space 22 and is then discharged from the gas discharge hole 26 and / or the through-hole 28. By forming such an oxygen flow environment around the pre-sintered body 50, it is possible to suppress the occurrence of abnormal heating due to oxygen plasma.

[0067] The susceptor 40 is a heating auxiliary member that can increase the efficiency of microwave heating by efficiently converting microwave energy into thermal energy. Specifically, the susceptor 40 absorbs microwaves and reaches a high temperature more quickly than the pre-sintered body 50, so that it can assist in raising the temperature of the pre-sintered body 50 by thermal conduction. When the pre-sintered body 50 reaches a high temperature, the pre-sintered body 50 itself becomes more likely to absorb microwaves, and can act as a microwave absorber. When the pre-sintered body 50 becomes more likely to absorb microwaves, the internal heating mechanism of the pre-sintered body 50 is more likely to be promoted by microwave heating. This promotes sintering inside the pre-sintered body 50, making it difficult for voids to remain inside, and a zirconia sintered body with excellent strength and translucency can be manufactured.

[0068] From the viewpoint of raising the temperature of the pre-sintered body 50 in a shorter time, it is preferable to arrange the susceptor 40 at a position sandwiching the pre-sintered body 50 from both sides in a predetermined direction before microwave heating. For example, the susceptor 40 may be arranged on both sides (i.e., the upper and lower sides) of the pre-sintered body 50 in the vertical direction (up and down direction), or on both sides in at least one horizontal direction of the pre-sintered body 50. As a result, the surfaces of both sides in the predetermined direction of the pre-sintered body 50 are heated by the susceptor 40, so that the microwave absorption efficiency of the pre-sintered body 50 can be increased in a shorter time. As a result, the internal heating of the pre-sintered body 50 by microwave heating can be realized in a shorter time, so that the voids inside the zirconia sintered body can be further reduced. The susceptor 40 is typically arranged so as to be in contact with the surface of the pre-sintered body 50, but there may be a gap between the susceptor 40 and the surface of the pre-sintered body 50. The gap is not particularly limited, but is preferably 3 mm or less, more preferably 2 mm or less, and even more preferably 1 mm or less, for example.

[0069] In addition, it is preferable that the pre-sintered body 50 is not sealed by the susceptor 40. This makes it easier for microwaves to be directly absorbed by the pre-sintered body 50 without being hindered by the susceptor 40. Therefore, compared to when the susceptor completely encapsulates the pre-sintered body (for example, when the pre-sintered body is placed inside a closed box-shaped susceptor), the internal heating of the pre-sintered body can be induced from a lower temperature range. As a result, compared to the sintering mode caused by heat conduction from the surface, the number of pores remaining inside the zirconia sintered body can be reduced. In addition, since the pre-sintered body 50 is not sealed by the susceptor 40, it is possible to prevent the oxygen around the pre-sintered body 50 from being consumed and becoming a reducing atmosphere.

[0070] In addition, it is preferable that the susceptor 40 is not installed (open) on both sides of at least one direction different from the predetermined direction in which the susceptor 40 is arranged in the provisionally sintered body 50. This makes it easier for the microwaves to be directly absorbed by the provisionally sintered body 50, and the internal heating can be induced from a lower temperature range and more uniformly. Furthermore, by providing one direction in which the susceptor 40 is not installed, the provisionally sintered body 50 can be arranged in the flow of gas supplied from the gas supplier 30, so that the atmosphere around the provisionally sintered body 50 can be more suitably controlled.

[0071] 2, the pre-sintered body 50 is sandwiched between two plate-like susceptors 40 from above and below, and the horizontal direction of the pre-sintered body 50 is not covered by the susceptors 40. In this configuration, since the susceptors 40 are not arranged on either side of the pre-sintered body 50 in the horizontal direction, microwaves are particularly easily absorbed by the pre-sintered body 50, making it easier to manufacture a zirconia sintered body with reduced internal voids.

[0072] As the susceptor 40, a SiC susceptor mainly composed of silicon carbide (SiC) is preferably used. Here, "mainly composed of SiC" means that SiC accounts for 50 mass% or more of the compounds constituting the susceptor 40. Examples of the SiC susceptor include single crystal SiC, recrystallized SiC, reaction sintered SiC, nitride bonded SiC, oxide bonded SiC, silicon carbide fiber, and the like. From the viewpoint of increasing the microwave absorption efficiency, recrystallized SiC and silicon carbide fiber, which are materials with relatively high porosity, can be preferably used. Among these, recrystallized SiC is particularly preferably used because it has excellent heat resistance. Furthermore, even in the case of recrystallized SiC, the microwave absorption efficiency may decrease in dense recrystallized SiC, so the porosity of the recrystallized SiC may be, for example, 10% to 90%, and preferably 10% to 30%. The porosity can be measured by a conventionally known method, for example, by mercury intrusion porosimetry.

[0073] When the susceptor 40 is plate-shaped, the thickness of each sheet is preferably, for example, 1 mm to 4 mm, and more preferably 2 mm to 3 mm. If the susceptor 40 is too thin, the strength of the susceptor may decrease. If the susceptor 40 is too thick, the susceptor 40 may be difficult to heat, and the temperature rise rate may be slow. Therefore, within the above thickness range, a good balance is achieved between the strength of the susceptor 40 and the temperature rise rate of the susceptor 40. This makes it possible to more suitably reduce voids inside the zirconia sintered body.

[0074] 2, one plate-shaped susceptor 40 is disposed above and one below the pre-sintered body 50, but the number of plate-shaped susceptors 40 is not particularly limited as long as there are a plurality (two or more). For example, two or more susceptors 40 may be stacked on each of the upper and lower sides of the pre-sintered body 50. Also, different numbers of susceptors 40 may be used on the upper and lower sides of the pre-sintered body 50.

[0075] In this embodiment, the susceptor 40 is plate-shaped, but there is no particular limitation as long as the susceptor 40 is disposed on both sides in a predetermined direction of the pre-sintered body 50. For example, a box-shaped (e.g., hexahedral) susceptor having through holes on a pair of opposing surfaces, a columnar susceptor (e.g., cylindrical, prismatic), etc. may be mentioned.

[0076] The radiation thermometer 60 can measure the temperature of the object in a non-contact manner. As shown in FIG. 2, in this embodiment, the radiation thermometer 60 is installed at a position away from the microwave heating device 10, and measures the surface temperature of the susceptor 40 above the provisionally sintered body 50. In this specification, the heating temperature in the microwave heating in the second heating step S30 and the temperature used to calculate the temperature drop rate in the cooling step S40 described later refer to the temperature measured by the radiation thermometer 60. In addition, from the viewpoint of more accurately measuring the temperature change due to microwave heating, it is preferable to fix the radiation thermometer 60 at a predetermined position by a clamp or the like. As the radiation thermometer 60, for example, an OPTCTRF1MHSFVFC3 sensor (pseudo emissivity setting 1.0) manufactured by Optris can be used.

[0077] The microwave heating temperature is preferably 1600°C or higher (e.g., over 1600°C), more preferably 1630°C or higher, more preferably 1650°C or higher, even more preferably 1700°C or higher (e.g., over 1700°C), and particularly preferably 1720°C or higher. Although the mechanism is not particularly limited, by setting the microwave heating temperature to a high temperature of 1600°C or higher, a dense sintered body is produced in which voids that may occur inside the zirconia sintered body are suppressed. In addition, in the crystal phase of the zirconia sintered body, the variation of the crystal phase can be reduced, and the discontinuity of the crystal grain boundary can be reduced. As a result, it is presumed that the light passing through the zirconia sintered body is less likely to be reflected or refracted at the crystal interface, thereby improving the translucency. In addition, although not particularly limited, from the viewpoint of the heat resistance of the heating device, the microwave heating is suitably performed at 2000°C or less, and can be performed at 1900°C or less, 1800°C or less, 1750°C or less, or 1730°C or less. The holding time of the microwave heating is appropriately changed depending on the shape, size, composition, etc. of the provisionally sintered body 50, and can be, for example, about 1 minute to 20 minutes, or about 1 minute to 10 minutes. Note that the holding time here does not include the time required for the temperature to rise to the microwave heating temperature.

[0078] The microwave heating method is not particularly limited, and for example, either single mode or multi-mode can be used, but multi-mode is preferably adopted. In single mode, plasma may be generated in the pre-sintered body 50 depending on the arrangement position, size, etc. of the pre-sintered body 50, and cracks may occur in the zirconia sintered body. On the other hand, in multi-mode, concentration of the electromagnetic field in the heating space 14 is suppressed, making it difficult for plasma to be generated. This can suppress the occurrence of cracks in the zirconia sintered body.

[0079] The heating rate of microwave heating is not particularly limited since it is appropriately changed depending on the shape, size, composition, etc. of the presintered body. For example, the heating rate until reaching 1000°C to 1100°C can be 500°C / min to 900°C / min, preferably 500°C / min to 700°C / min. This allows the zirconia sintered body to be produced in a shorter time. Thereafter, until reaching 1100°C to 1200°C, for example, the heating rate can be 10°C / min to 50°C / min, preferably 15°C / min to 25°C / min. This can reduce the occurrence of cracks due to rapid sintering of zirconia. Thereafter, until reaching about 1600°C to 2000°C, for example, the heating rate can be 40°C / min to 100°C / min, preferably 40°C / min to 60°C / min. This allows the progress of sintering of the presintered body to be appropriately controlled, and a zirconia sintered body having better translucency and hydrothermal degradation resistance can be produced.

[0080] The shape of the pre-sintered body 50 is not particularly limited, but from the viewpoint of more uniform sintering by microwaves, it is preferably, for example, disk-shaped. The thickness of the pre-sintered body 50 is, for example, preferably 0.5 mm to 10 mm, more preferably 0.5 mm to 2 mm. Within this range, the strength of the pre-sintered body 50 can be maintained while sintering by microwaves can be efficiently performed. Furthermore, the maximum diameter of the pre-sintered body 50 is, for example, preferably 10 mm to 60 mm, more preferably 10 mm to 20 mm. Within this range, sintering by microwaves can be more uniformly performed.

[0081] <Cooling process S40> In the cooling step S40, the molded body that has been subjected to the second heating step S30 is rapidly cooled to 1300°C. The cooling step S40 is a step that is performed after the molded body that has been microwave-heated in the second heating step S30 has been heated to and maintained at a predetermined temperature (for example, 1600°C to 2000°C), and is typically performed continuously from the microwave heating. By such rapid cooling, the zirconia sintered body disclosed herein can be obtained.

[0082] The cooling rate when cooling the molded body to 1300 ° C. is, for example, 50 ° C. / min or more, preferably 100 ° C. / min or more, more preferably 200 ° C. / min or more. The faster the cooling rate, the more the hydrothermal deterioration resistance can be improved. The upper limit of the temperature drop rate is not particularly limited, but may be, for example, 1000 ° C. / min or less, 500 ° C. / min or less. The cooling method is not particularly limited as long as it can realize the temperature drop rate in the above range, but examples include control of microwave irradiation, natural cooling, and air blowing. In addition, in the cooling step S40, if a gas containing oxygen was supplied from the gas supplier 30 in the second heating step S30, the supply of the gas may be continued or stopped. According to the study by the present inventor, by stopping the supply of the gas during the temperature drop, the zirconia sintered body can be more stably manufactured. In this specification, the "temperature decreasing rate" in the cooling step S40 refers to the average temperature decreasing rate from when the temperature decrease starts until the temperature reaches 1300°C.

[0083] As described above, the manufacturing method disclosed herein sinters the molded body by microwave heating, so that a dense zirconia sintered body with few internal voids can be obtained. Furthermore, by performing microwave heating, the temperature of the heating space itself of the microwave heating device is not as high as that of the molded body and the susceptor, so that rapid cooling at the above-mentioned temperature drop rate is possible, and the proportion of the crystal phase in which the c / a axial length ratio of the unit lattice of the zirconia sintered body is in the range of 1.0055 or more and less than 1.010 can be increased. As a result, a zirconia sintered body with excellent translucency and resistance to hydrothermal degradation is realized.

[0084] The reason why excellent light transmittance and resistance to hydrothermal deterioration are realized is not particularly limited, but is presumed to be as follows. In a conventional compact sintered in a heating furnace where the heating space itself is at a high temperature, the cooling rate after sintering is slow, so that a crystal phase (e.g., tetragonal) with a unit lattice c / a axis length ratio in the range of more than 1.012 and not more than 1.017 and a crystal phase (e.g., cubic) with a unit lattice c / a axis length ratio in the range of 1 or more and less than 1.0055 are mixed and the yttrium concentration tends to segregate locally. It is generally believed that such segregation induces hydrothermal deterioration of zirconia sintered bodies. On the other hand, in this technology, it is presumed that the segregation of yttrium is suppressed by performing rapid cooling after sintering by microwaves. As a result, a phase that was previously precipitated as a crystal phase (e.g., cubic) in which the c / a axis length ratio of the unit lattice is in the range of 1 or more and less than 1.0055 is precipitated as a crystal phase (e.g., metastable tetragonal) in which the c / a axis length ratio of the unit lattice is in the range of 1.0055 or more and less than 1.010, or a crystal phase (e.g., tetragonal) in which the c / a axis length ratio of the unit lattice is in the range of more than 1.012 and less than 1.017, and it is considered that a more homogeneous crystal structure than the conventional one is realized. In a homogeneous crystal structure, hydrothermal deterioration is unlikely to be induced, and reflection and refraction of light at the crystal interface are suppressed. In addition, a zirconia sintered body with few voids and high density is realized by microwave heating, so that water is unlikely to penetrate into the zirconia sintered body. From the above, it is speculated that the zirconia sintered body disclosed herein has excellent translucency and resistance to hydrothermal deterioration.

[0085] In the zirconia sintered body disclosed herein, when the entire crystal phase is taken as 100 mass%, the proportion of the crystal phase in which the c / a axial length ratio of the unit lattice is in the range of 1.0055 or more and less than 1.010 is, for example, 10 mass% or more, preferably 15 mass% or more, and more preferably 20 mass% or more. The upper limit of such a proportion is not particularly limited, but may be, for example, 50 mass% or less, 40 mass% or less, 30 mass% or less, or 25 mass% or less. If the proportion of such a crystal phase is 10 mass% or more, the light transmittance and hydrothermal deterioration resistance tend to be improved.

[0086] In some embodiments, when the entire crystal phase is taken as 100% by mass, the proportion of the crystal phase in which the c / a axis length ratio of the unit lattice is in the range of 1.007 or more and less than 1.010 is, for example, 10% by mass or more, preferably 15% by mass or more, more preferably 20% by mass or more. The upper limit of such a proportion is not particularly limited, but may be, for example, 50% by mass or less, 40% by mass or less, 30% by mass or less, or 25% by mass or less. By having a c / a axis length ratio in such a range, the hydrothermal degradation resistance may be further improved.

[0087] In the zirconia sintered body disclosed herein, when the entire crystal phase is taken as 100 mass%, the total ratio of the crystal phase having a unit lattice c / a axis length ratio in the range of 1.0055 to less than 1.010 and the crystal phase having a unit lattice c / a axis length ratio in the range of more than 1.012 to 1.017 may be, for example, 75 mass% or more, preferably 85 mass% or more, more preferably 95 mass% or more, particularly preferably 98 mass% or more, or even 100 mass%. The higher this ratio, the more homogeneity of the crystal structure can be improved, and therefore better light transmittance and hydrothermal deterioration resistance can be realized.

[0088] In the zirconia sintered body disclosed herein, when the entire crystal phase is taken as 100 mass%, the proportion of the crystal phase having a unit lattice c / a axis length ratio in the range of 1 to less than 1.0055 may be, for example, 25 mass% or less, more preferably 15 mass% or less, even more preferably 5 mass% or less, particularly preferably 2 mass% or less, or even 0 mass% (not included or below the detection limit). The lower this proportion, the more the segregation of yttrium and / or ytterbium is suppressed, and better translucency and hydrothermal deterioration resistance can be realized.

[0089] In the zirconia sintered body disclosed herein, the proportion of monoclinic crystals after a hydrothermal degradation test in which the body is immersed in hot water at 134°C for 6 hours is, for example, 5% or less, preferably 2% or less, more preferably 1% or less, and particularly preferably 0.5% or less. The hydrothermal degradation test can be performed by immersing a zirconia sintered body with a mirror-polished surface in hot water at 134°C for 6 hours using a high-pressure microreactor (MMS-500, manufactured by O-M Labotec Co., Ltd.). The proportion of monoclinic crystals can be measured by obtaining an X-ray diffraction pattern of the polished surface of the zirconia sintered body after the hydrothermal degradation test.

[0090] The translucency of the zirconia sintered body disclosed herein is, for example, a total light transmittance of 44% or more, preferably 44.5% or more, more preferably 45% or more, and even more preferably 46% or more. Although not particularly limited, the total light transmittance may be, for example, 60% or less. In this specification, the term "total light transmittance" refers to the total light transmittance for a D65 light source in the thickness direction of a disk-shaped test piece having a thickness of 1 mm.

[0091] As described above, the zirconia sintered body disclosed herein has both excellent translucency and excellent resistance to hydrothermal deterioration, and therefore can be suitably used, for example, as a dental material, such as dentures for anterior teeth, dentures for posterior teeth, denture mill blanks, orthodontic brackets, dental prostheses, bridges, and the like.

[0092] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Section 1: A method for producing a zirconia sintered body, comprising the following steps: a molded body preparation step of preparing a molded body containing zirconia and yttria and / or ytterbia, in which the total ratio of yttria and ytterbia is 3 mol % or more and less than 4 mol % when the total of zirconia, yttria, and ytterbia is taken as 100 mol %; a first heating step of heating the compact at 800° C. or more and 1200° C. or less; a second heating step of heating the molded body having undergone the first heating step at a temperature of 1600° C. or more and 2000° C. or less by microwave heating; and a cooling step of lowering the temperature of the molded body that has been subjected to the second heating step to 1300°C at a rate of 50°C / min or more; The method for producing a zirconia sintered body includes the steps of: Item 2: The manufacturing method according to Item 1, wherein the microwave heating is performed in a multi-mode manner. Item 3: The method according to item 1 or 2, wherein in the second heating step, the microwave heating is carried out in an oxidizing atmosphere. Item 4: The production method according to Item 3, wherein in the second heating step, the microwave heating is carried out in an atmosphere having an oxygen concentration of 30 vol.% or more and 100 vol.% or less. Item 5: The manufacturing method according to any one of Items 1 to 4, wherein in the second heating step, SiC susceptors are disposed so as to sandwich the molded body from both sides in a predetermined direction. Item 6: The method according to any one of Items 1 to 5, wherein in the compact preparation step, the compact is prepared by molding a material containing zirconia-containing granules. Section 7: A zirconia sintered body containing zirconia and yttria and / or ytterbia, When the total of zirconia, yttria, and ytterbia is 100 mol%, the total ratio of yttria and ytterbia is 3 mol% or more and less than 4 mol%, When the entire crystal phase is taken as 100 mass%, the proportion of the crystal phase having a c / a axial length ratio of the unit lattice in the range of 1.0055 or more and less than 1.010 is 10 mass% or more. Zirconia sintered body. Item 8: When the entire crystal phase is taken as 100 mass%, the proportion of the crystal phase having a unit lattice c / a axial length ratio in the range of 1.007 or more and less than 1.010 is 10 mass% or more. The zirconia sintered body according to item 7. Item 9: The zirconia sintered body according to item 7 or 8, wherein the proportion of monoclinic crystals after immersion in hot water at 134 ° C. for 6 hours is 10% or less. Item 10: When the entire crystal phase is taken as 100 mass%, the proportion of the crystal phase having a c / a axial length ratio of the unit lattice in the range of 1 or more and less than 1.0055 is 20 mass% or less. The zirconia sintered body according to any one of items 7 to 9. Item 11: The zirconia sintered body according to any one of Items 7 to 10, wherein a test piece having a thickness of 1 mm has a total light transmittance of 44% or more for a D65 light source in the thickness direction. Item 12: A dental material comprising the zirconia sintered body according to any one of items 7 to 11. Item 13: The dental material according to item 12, which is a denture, a denture mill blank, or an orthodontic bracket.

[0093] Hereinafter, examples of the technology disclosed herein will be described, but such examples are not intended to limit the technology disclosed herein.

[0094] (Example 1) The zirconia sol produced by hydrolysis of the zirconium oxychloride solution was mixed with yttrium chloride. When the yttrium chloride was converted to yttria, the total of zirconia and yttria was 100 mol %, and yttrium chloride was mixed so that yttria was 3.3 mol %. After drying the mixture, it was calcined at 1120°C for 4 hours to obtain a partially stabilized zirconia powder. The zirconia powder was pulverized in a ball mill using zirconia balls with a diameter of 1 mm, and then selected with a mesh sieve to obtain a zirconia powder with an average particle size of 150 nm to 200 nm as a compact material. The zirconia powder was filled into a disk-shaped mold, and a pressure of 0.78 MPa was applied, after which the molded body was removed from the mold, and the molded body was subjected to CIP molding at 196 MPa. The obtained molded body was then heated at 1100°C for 2 hours to obtain a provisionally sintered body. The heating rate was 120°C / h up to 800°C and 100°C / h up to 1100°C.

[0095] The pre-sintered body was placed on a 2 mm thick plate-shaped SiC susceptor, and the pre-sintered body was placed on the 2 mm thick plate-shaped SiC susceptor, and the pre-sintered body was placed in a heat-insulating container. The heat-insulating container had the same structure as the heat-insulating container 20 shown in FIG. 2. The heat-insulating container was then placed in a microwave heating device. Recrystallized SiC was used as the SiC susceptor. The microwave heating device used was μ-Reactor EX manufactured by Shikoku Keisoku Kogyo Co., Ltd.

[0096] Next, gas with an oxygen concentration of 90 vol% was supplied into the thermal insulation container using M1O2 silent (manufactured by Kobe Medicare Co., Ltd.) as a gas supplying machine. Then, while supplying gas, microwave heating was started, and the temperature was raised to 1000°C at 600°C / min, 1100°C at 20°C / min, and 1730°C at 50°C / min, and was maintained at 1730°C for 1 minute. Thereafter, the temperature was lowered to 1300°C at a temperature lowering rate of 10°C / min, and when the temperature reached 1300°C, the microwave heating device was turned off to lower the temperature. During the temperature lowering, the supply of the gas was stopped. In this way, the zirconia sintered body of Example 1 was manufactured. The microwave heating method was a multi-mode. In addition, an OPTCTRF1MHSFVFC3 sensor manufactured by Optris was used for temperature measurement, and the temperature of the SiC susceptor on the upper side of the provisional sintered body was measured.

[0097] (Example 2) The rate of temperature drop to 1300°C after microwave heating was changed to 50°C / min from the manufacturing method of Example 1. A zirconia sintered body of Example 2 was manufactured in the same manner as in Example 1 except for this.

[0098] (Example 3) The rate of temperature drop to 1300°C after microwave heating was changed to 100°C / min from the manufacturing method of Example 1. A zirconia sintered body of Example 3 was manufactured in the same manner as in Example 1 except for this.

[0099] (Example 4) The rate of temperature drop to 1300°C after microwave heating was changed to 200°C / min from the manufacturing method of Example 1. A zirconia sintered body of Example 4 was manufactured in the same manner as in Example 1 except for this.

[0100] (Example 5) In the manufacturing method of Example 1, the heating by microwave was changed to heating by an electric furnace. Specifically, the temperature of the provisionally sintered body was raised to 1500°C at a rate of 120°C / h, and after being held for 120 minutes, the temperature was lowered at a rate of 5°C / min. The other operations were the same as those of Example 1, and the zirconia sintered body of Example 5 was manufactured.

[0101] (Example 6) The manufacturing method of Example 4 was modified by mixing yttrium chloride so that the yttria in the zirconia powder was 3.9 mol%, and a zirconia powder with an average particle size of 150 nm to 200 nm was obtained. A polyacrylic binder was mixed into the zirconia powder as a binder so that the binder was 3 mass% of the total. The mixture was granulated by partial drying to obtain zirconia granules with an average particle size of 70 μm. The zirconia granules were used as a molded body material, and the zirconia sintered body of Example 6 was manufactured in the same manner as in Example 4. However, the sintering temperature by microwave heating was changed from 1730°C to 1630°C.

[0102] (Example 7) In the manufacturing method of Example 6, the microwave heating was changed to heating in an electric furnace. Specifically, the temperature of the presintered body was raised to 1500°C at a rate of 120°C / h, held for 120 minutes, and then lowered at a rate of 5°C / min. The zirconia sintered body of Example 7 was manufactured in the same manner as in Example 6 except for these operations.

[0103] (Example 8) The zirconia sol produced by hydrolysis of the zirconia oxychloride solution was mixed with yttrium chloride and ytterbium chloride. When the yttrium chloride was converted to yttrium and the ytterbium chloride was converted to ytterbia, the total of the zirconia, yttria and ytterbia was taken as 100 mol%, and the yttrium chloride and ytterbium chloride were mixed so that the yttria was 1.7 mol% and the ytterbia was 1.8 mol%. The mixture was dried and then calcined at 1120°C for 4 hours to obtain a partially stabilized zirconia powder. The zirconia powder was pulverized in a ball mill using zirconia balls with a diameter of 1 mm, and then screened with a mesh sieve to obtain a zirconia powder with an average particle size of 150 nm to 200 nm as a molding material. Thereafter, zirconia granules were produced in the same manner as in Example 6. The zirconia granules were filled into a disk-shaped mold, and a pressure of 0.78 MPa was applied. The molded body was removed from the mold, and the molded body was subjected to CIP molding at 196 MPa. The molded body was then heated at 1100°C for 2 hours to obtain a provisional sintered body. The heating rate was 120°C / h up to 800°C and 100°C / h up to 1100°C. After that, microwave heating was performed in the same manner as in Example 1, and the temperature was lowered at a rate of 200°C / min up to 1300°C to obtain a zirconia sintered body of Example 8. However, the maximum sintering temperature of microwave heating was changed to 1640°C. In addition, the conditions of microwave heating were changed to heating at 600°C / min up to 1000°C, 20°C / min up to 1100°C, and 50°C / min up to 1730°C, and then maintained at 1730°C for 1 minute.

[0104] (Example 9) In the manufacturing method of Example 8, the heating by microwave was changed to heating by an electric furnace. Specifically, the temperature of the pre-sintered body was raised to 1500°C at a rate of 120°C / h, held for 120 minutes, and then lowered at a rate of 5°C / min. The other operations were the same as those of Example 8 to manufacture the zirconia sintered body of Example 9.

[0105] <Crystal phase analysis> The X-ray diffraction pattern profile of the zirconia sintered body produced in each example was obtained using an X'Pert Pro Alpha-1 (manufactured by Malvern PaNalytical) as an X-ray diffractometer. The measurement conditions were as follows: Radiation source: CuKα1 (45kV 40mA) Measurement range: 10°≦2θ≦90° Scan speed: 1.5° / min Step size: 0.0131°

[0106] The obtained XRD profile was subjected to Rietveld analysis using analysis software: RIETAN-FP to analyze the c / a axis length ratio of the unit cell and the proportion of crystalline phase (mass%). The analysis was performed on a mixed phase of tetragonal, metastable tetragonal, and cubic, and the temperature parameters of each element were the same. Note that metastable tetragonal here refers to a crystalline phase in which only the a-axis length and c-axis length of the tetragonal are different. The results are shown in Table 1.

[0107] <Evaluation of Translucency> The zirconia sintered body produced in each example was processed into a disk-shaped test piece having a thickness of 1 mm, and both sides of the test piece were mirror-polished using diamond slurry (average particle size 0.5 μm) as an abrasive, and then the total light transmittance of the D65 light source in the thickness direction was measured. For this measurement, a haze meter NDH4000 manufactured by Nippon Denshoku Kogyo Co., Ltd. was used. The results are shown in Table 1.

[0108] <Hydrothermal Deterioration Test> The monoclinic ratio (%) after hydrothermal treatment was measured for the zirconia sintered body produced in each example according to the method shown below. Specifically, first, the surface of the zirconia sintered body was mirror-polished with diamond slurry (average particle size 0.5 μm). Next, the polished sintered body was subjected to hydrothermal deterioration treatment at 140° C. for 100 hours in a device named: High Pressure Microreactor (manufactured by O.M. Labotec Co., Ltd.). Thereafter, the X-ray diffraction pattern of the polished surface was measured with an X-ray diffractometer (device name: Ultima IV, manufactured by Rigaku Corporation). Then, using the measurement results, the monoclinic ratio (%) was calculated according to the following formula. As can be understood from the following formula, the monoclinic ratio is expressed by the X-ray diffraction peak intensity [I m (111)], and the X-ray diffraction peak intensity corresponding to the monoclinic phase (11-1) plane [I m (11-1)], and the sum of the X-ray diffraction peak intensities corresponding to the (111) plane of a crystal phase other than the monoclinic phase [I o (111)]. The results are shown in Table 1. [Monoclinic crystal ratio (%)] ={I m (111)+I m (11-1)} / {I m (111)+I m (11-1)+I o (111)} ×100 The conditions for the X-ray diffraction device are as follows: Radiation source: CuKα (40kV 40mA) Measurement range: 26°≦2θ≦38° Scan speed: 2.0° / min Step size: 0.02°

[0109] [Table 1]

[0110] As shown in Table 1, in Examples 1 to 5, the rate of temperature drop from the maximum sintering temperature to 1300°C was set to 50°C / min or more, which resulted in a low monoclinic rate after hydrothermal degradation and improved hydrothermal degradation resistance of the zirconia sintered body. This is believed to be due to the fact that when the rate of temperature drop from the maximum sintering temperature to 1300°C is set to 50°C / min or more, the zirconia sintered body has a crystal phase in which the c / a axis length ratio of the unit lattice is in the range of 1.0055 or more and less than 1.010.

[0111] In addition, as shown in Examples 2 to 4, when the cooling rate to 1300 ° C. is improved, the crystal phase with a c / a axis length ratio of 1.0066 in Example 2 is shifted to a crystal phase with a c / a axis length ratio of 1.0070 in Example 3, and to a crystal phase with a c / a axis length ratio of 1.0074 in Example 4. In addition, in Examples 2 to 4, the monoclinic rate after hydrothermal degradation is lower in Examples 3 and 4 than in Example 2. From this, it can be seen that in the range of the c / a axis length ratio of the unit lattice being 1.0055 or more and less than 1.010, the zirconia sintered body having a crystal phase with a relatively high c / a axis length ratio (for example, a c / a axis length ratio of 1.0070 or more) has improved hydrothermal degradation resistance.

[0112] Examples 6 to 7 are test examples of zirconia sintered bodies in which the proportion of yttria is higher than in Examples 1 to 5. Even in this case, as shown in Example 6, it is found that a zirconia sintered body having excellent permeability and hydrothermal deterioration resistance can be realized.

[0113] Examples 8 to 9 are test examples of zirconia sintered bodies containing ytterbia. In these cases as well, as shown in Example 8, it is found that a zirconia sintered body having excellent permeability and resistance to hydrothermal deterioration can be realized.

[0114] Although specific examples of the technology disclosed herein have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0115] 10 Microwave heating device 12 Bulkhead 14 Heating space 16 Through hole 20 Insulated container 22 Containment Space 24 Gas inlet 26 Gas exhaust hole 28 Through hole 30 Gas supply machine 32 Pump 40 Susceptor 50 Pre-sintered body 60 Radiation thermometer

Claims

1. A method for producing a zirconia sintered body, comprising the following steps: a molded body preparation step of preparing a molded body containing zirconia and yttria and / or ytterbia, in which the total ratio of yttria and ytterbia is 3 mol % or more and less than 4 mol % when the total of zirconia, yttria, and ytterbia is 100 mol %; a first heating step of heating the molded body at 800° C. or more and 1200° C. or less; A second heating step of heating the molded body having undergone the first heating step at 1600° C. or more and 2000° C. or less by microwave heating; and a cooling step of lowering the temperature of the molded body that has been subjected to the second heating step to 1,300° C. at a rate of 100° C. / min or more; The method for producing a zirconia sintered body includes the steps of:

2. The method according to claim 1 , wherein the microwave heating is performed in a multi-mode manner.

3. The method according to claim 1 or 2, wherein in the second heating step, the microwave heating is carried out in an oxidizing atmosphere.

4. The method according to claim 3 , wherein in the second heating step, the microwave heating is performed in an atmosphere having an oxygen concentration of 30 vol % or more and 100 vol % or less.

5. The manufacturing method according to claim 1 or 2, wherein in the second heating step, SiC susceptors are disposed so as to sandwich the compact from both sides in a predetermined direction.

6. The method according to claim 1 or 2, wherein in the compact preparation step, the compact is prepared by molding a material containing granules having zirconia.

7. A zirconia sintered body containing zirconia and yttria and / or ytterbia, When the total of zirconia, yttria, and ytterbia is 100 mol %, the total ratio of yttria and ytterbia is 3 mol % or more and less than 4 mol %, When the entire crystal phase is taken as 100 mass%, the proportion of the crystal phase having a c / a axial length ratio of the unit lattice in the range of 1.0068 or more and less than 1.010 is 10 mass% or more. Zirconia sintered body.

8. 8. The zirconia sintered body according to claim 7, wherein the proportion of the crystalline phase having a c / a axial length ratio of the unit lattice in the range of 1.007 or more and less than 1.010 is 10 mass% or more when the entire crystalline phase is 100 mass%.

9. The zirconia sintered body according to claim 7, having a monoclinic ratio of 10% or less after immersion in hot water at 134°C for 6 hours.

10. 8. The zirconia sintered body according to claim 7, wherein the proportion of the crystalline phase having a c / a axial length ratio of the unit lattice in the range of 1 or more and less than 1.0055 is 20 mass% or less, when the entire crystalline phase is 100 mass%.

11. The zirconia sintered body according to claim 7, wherein the total light transmittance for a D65 light source in the thickness direction of a 1 mm thick test piece is 44% or more.

12. A dental material comprising the zirconia sintered body according to any one of claims 7 to 11.

13. 13. The dental material of claim 12 which is a denture, a denture mill blank, or an orthodontic bracket.

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