JP7578866B1 - Zirconia sintered body and its manufacturing method - Google Patents

Abstract

The present disclosure provides a technology for realizing a zirconia sintered body having excellent translucency and resistance to hydrothermal degradation. 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

This disclosure relates to a zirconia sintered body and a method for producing the same. This application claims priority to Japanese Patent Application No. 2023-091593, filed on June 2, 2023, the entire contents of which are incorporated herein by reference.

A zirconia sintered body in which a small amount of a rare earth element such as yttria (Y ₂ O ₃ ) is dissolved as a stabilizer (hereinafter also referred to as a "partially stabilized zirconia sintered body") tends to have improved mechanical properties such as strength and toughness compared to a zirconia sintered body that does not contain a stabilizer. Therefore, the partially stabilized zirconia sintered body is used in machine structural materials such as cutting tools, bearings, and dispersing/pulverizing machines, and biomaterials such as dental materials, etc.

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 with 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.

Japanese Patent Application Publication No. 2015-143178 Japanese Patent Application Publication No. 2014-12627 Japanese Patent Application Publication No. 2014-218421

By adjusting the amount of stabilizer contained in the partially stabilized zirconia sintered body, the mechanical properties of the zirconia sintered body can be adjusted. For example, lowering the amount of stabilizer tends to improve mechanical properties such as bending strength and fracture toughness. However, such partially stabilized zirconia sintered bodies tend to have reduced hydrothermal degradation resistance and translucency.

The main objective of this disclosure is to provide technology that realizes zirconia sintered bodies with excellent translucency and resistance to hydrothermal degradation.

In order to achieve the above object, the present inventors have conducted research and found that a zirconia sintered body excellent in light transmittance and resistance to hydrothermal degradation can be realized by sintering a molded body of partially stabilized zirconia containing a predetermined ratio of yttria and/or ytterbia (Yb ₂ O ₃ ) by microwave heating and then rapidly cooling it to 1300° C. Analysis of the crystal phase of such a zirconia sintered body has found 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.

The present disclosure provides a method for producing a zirconia sintered body. The method for producing the zirconia sintered body includes a molded body preparation step of preparing a molded body containing zirconia, 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.

In some preferred embodiments, the microwave heating method is a multi-mode method. This allows heating while suppressing plasma generation. As a result, the occurrence of cracks in the zirconia sintered body is suppressed, and a zirconia sintered body with excellent translucency and resistance to hydrothermal degradation can be produced.

In some preferred embodiments, the microwave heating in the second heating step is performed in an oxidizing atmosphere. This prevents the zirconia sintered body from darkening, and produces a zirconia sintered body that is excellent in translucency, resistance to hydrothermal degradation, and aesthetics.

In some preferred embodiments, in the second heating step, the microwave heating is performed in an atmosphere with an oxygen concentration of 30 vol% or more and 100 vol% or less. This effectively prevents the zirconia sintered body from darkening, making it possible to produce a zirconia sintered body that is more aesthetically pleasing, translucent, and resistant to hydrothermal degradation.

In some preferred embodiments, in the second heating step, SiC susceptors are arranged to sandwich the molded body from both sides in a predetermined direction. This allows the sintering inside the molded body to proceed more optimally, making it possible to produce a zirconia sintered body with superior translucency and resistance to hydrothermal degradation.

In some preferred embodiments, in the compact preparation step, the compact is prepared by molding a material containing zirconia-containing granules. This can improve the shape stability of the compact and improve the ease of handling and workability.

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 axial 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. In the zirconia sintered body of such a configuration, excellent translucency and hydrothermal deterioration resistance are realized.

In this specification, the c/a axial length ratio and its proportion of the unit cell 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.

In some preferred embodiments, when the entire crystal phase is taken as 100% by 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% by mass or more. This further improves hydrothermal degradation resistance.

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

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 axial length ratio of the unit lattice is in the range of 1 or more and less than 1.0055 is 20% by mass or less. With this configuration, the crystal phase becomes more homogeneous, and therefore better light transmittance and resistance to hydrothermal degradation can be achieved.

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

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.

1 is a flowchart showing an outline of a method for producing a zirconia sintered body in one embodiment. FIG. 2 is a schematic diagram showing an example of a method for microwave heating a molded body (preliminary sintered body).

Below, several embodiments of the technology disclosed herein are described. Matters other than those specifically mentioned in this specification (e.g., the temperature of microwave heating) that are necessary for implementing 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 content of the technology disclosed here can be implemented based on the content disclosed in this specification and the technical common sense of a person skilled in the art in the relevant field. In this specification, when a numerical range is described as "A to B (where A and B are any numerical value)," this 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."

The zirconia sintered body disclosed here contains at least zirconia (ZrO ₂ ). The zirconia sintered body disclosed here contains at least one of yttria (Y ₂ O ₃ ) and ytterbia (Yb ₂ O ₃ ). That is, the zirconia sintered body disclosed here 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 here 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 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, and the like of the zirconia sintered body.

The yttria and/or ytterbia contained in the zirconia sintered body may be included, for example, as part of partially stabilized zirconia partially dissolved in zirconia (so-called stabilizer). Partially stabilized zirconia has a suppressed proportion of monoclinic crystals at room temperature, which may improve mechanical properties such as strength and toughness. Furthermore, suppressing the proportion of monoclinic crystals suppresses the variation in the crystal phase that constitutes the zirconia sintered body, which may improve translucency.

Generally, when the proportion of 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 translucency decreases. Furthermore, as the proportion of monoclinic crystals increases, 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.

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.

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.

In some embodiments, the zirconia sintered body further contains alumina (Al ₂ O ₃ ). In the 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 low-temperature deterioration resistance 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 better 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.

The zirconia sintered body may contain a conventionally known coloring agent to the extent that the effect of the technology disclosed herein is not significantly impaired. Examples of coloring agents 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 of the entire zirconia sintered body, 1% by mass or less, or 0.5% by mass or less.

In addition, the zirconia sintered body may contain elements that may be unavoidably mixed in. Examples include hafnium, magnesium, silicon, titanium, etc. 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.

Figure 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 (workpiece) 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 "preliminary 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.

<Molded object preparation step S10>
The compact preparation step S10 includes preparing a material constituting the compact (hereinafter also referred to as a "compact material") (hereinafter also referred to as a "compact material preparation step") and compacting the compact material ( Hereinafter, this may include a molding process.

In the molding material preparation process, 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. Examples of zirconium salts include zirconium oxychloride, zirconium chloride, zirconium sulfate, zirconium nitrate, etc. These may be used alone or in combination of two or more types.

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.

Next, yttria and/or ytterbia, or a raw material thereof, is mixed with the zirconia sol ( _ZrO2.nH2O ) obtained by _hydrolysis . 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.

When yttria and/or ytterbia are mixed into 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%, it 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.

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 within the above-mentioned range of the ratio of yttria and/or ytterbia. For example, when X mol (X is a positive number) of yttrium chloride (YCl ₃ ) is used as the yttria raw material, 0.5X mol of yttria (Y ₂ O ₃ ) can be obtained, so that the amount of yttrium chloride is doubled compared to when yttria itself is mixed.

Next, the zirconia sol to which yttria and/or ytterbia or their raw materials 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, etc.

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. By such calcination, the yttria raw material may be oxidized to yttria, and the ytterbia raw material may be oxidized 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.

The calcined powder contains particles having various shapes and particle sizes, and therefore it is preferable to pulverize the powder. The pulverization method is not particularly limited, and the powder can be pulverized, for example, by a known pulverizing device (e.g., a ball mill, etc.). As 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.

The preferred average particle size of the zirconia powder used as the compact material is, for example, 100 nm to 300 nm, more preferably 150 nm to 200 nm. With an average particle size in 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 ) corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by the laser diffraction/light scattering method. For example, a particle size distribution measuring device LA950V2 (manufactured by Horiba, Ltd.) can be used for such measurement.

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 such 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.

In this manner, zirconia powder can be obtained as a molding material, but the molding material is not limited to such zirconia powder.

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 above-mentioned zirconia sintered body, 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.

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

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

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 process, the zirconia powder is mixed with a dispersion medium (e.g., water) to prepare a slurry, which is then sprayed onto droplets and dried to obtain the granular compact material.

The binder may be a component that burns out at the heating temperature of 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, "(meth)acrylate" is a term that means acrylate and methacrylate.

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 binder content 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. Furthermore, if the amount of binder is too small, the effect of the binder may be insufficient. Therefore, the binder content may be, for example, 0.5% by mass or more, and may be 1% by mass or more.

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 used. As the pressure molding, for example, cold isostatic pressing (CIP) and hot isostatic pressing (HIP) are preferably used. By using CIP or HIP, a molded body (molded body) with high homogeneity and high density can be produced.

<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. By this heating, components such as moisture, binder, impurities, etc. that may be contained in the molded body are removed. In addition, since the preliminary sintering can reduce voids that may be present in the molded body, it is possible to suitably prevent cracks that may occur during sintering due to high-temperature and high-speed heating. The preliminary sintering can be carried out at a heating temperature of, for example, 800° C. to 1200° C., preferably 1000° C. to 1100° C. The time of the preliminary sintering varies depending on the shape, size, composition, etc. of the molded body. The heating time may be appropriately adjusted to obtain the desired result, and may be, for example, 1.5 hours to 5 hours, or 2 hours to 4 hours. The molding may be heated by a known method, for example, a muffler. Heating devices such as a furnace, an electric furnace, a microwave furnace, etc. can be used.

The shape of the molded body is not particularly limited and may be, for example, plate-like, disk-like, rectangular, cubic, columnar, etc.

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.

<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. Since the inside can be heated quickly, the difference in the progress of sintering between the surface side of the pre-sintered body and the inside side is small, and the voids inside the zirconia sintered body are 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.

Figure 2 is a schematic diagram showing an example of a method for microwave heating a pre-sintered body. Note that the dimensional relationships (length, width, thickness, etc.) in Figure 2 do not reflect the actual dimensional relationships. The directions of up, down, left, and right are represented 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.

As shown in Figure 2, the microwave heating device 10 has a partition wall 12 and a heating space 14. An insulated container 20 is installed in the heating space 14, and a susceptor 40 and a pre-sintered body 50 are housed in the storage space 22 of the insulated container 20. A gas supplier 30 is connected to the storage space 22 of the insulated container 20. A radiation thermometer 60 is installed at a separate position outside the microwave heating device 10.

The microwave heating device 10 has a heating space 14 surrounded by a partition wall 12. The heating space 14 is a space that contains an object to be microwave heated. Although not shown, the side walls, ceiling and/or bottom wall of the heating space 14 have microwave irradiation sections, and the object contained in the heating space 14 can be irradiated with microwaves and heated. The microwaves need only have a frequency that is 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.

The partition 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 addition, in order to improve insulation, the heating space 14 side of the partition 12 may be lined with an insulating material.

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.

The insulated container 20 has a storage space 22 capable of storing the susceptor 40 and the pre-sintered body 50 therein. As shown in FIG. 2, in this embodiment, the insulated container 20 has a gas inlet 24 for connecting the storage space 22 to the gas supplier 30, a gas outlet 26 for connecting 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 insulated 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 insulated container 20 is designed to be separable into a lid portion and a case portion, and is designed to easily insert and remove the object to be heated into and from the storage space 22. The material of the insulated container 20 may be, for example, a ceramic fiber such as alumina silica fiber.

The gas introduction hole 24 is a through hole that connects the storage space 22 and the heating space 14, and is designed to allow the pump 32 connected to the gas supply device 30 to pass through. This allows the desired gas to be supplied to the storage space 22, and the atmosphere within the storage space 22 to be controlled.

The gas exhaust hole 26 is a through hole that communicates the storage space 22 and the heating space 14, and is designed so that the storage space 22 is not sealed. This prevents the oxygen in the storage space 22 from being consumed as the sintering of the provisionally sintered body 50 progresses, and the storage space 22 from becoming a reducing atmosphere. The gas exhaust hole 26 can also prevent the gas supplied from the gas inlet hole 24 from stagnating in the storage space 22. In FIG. 2, one gas exhaust hole 26 is provided, but multiple (two or more) holes 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 can be, for example, about 5 mm to 50 mm, or, for example, about 5 mm to 20 mm.

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 insulated 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 heated object 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 large enough to measure the temperature of the heated object by the radiation thermometer 60, 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 if there is only one through hole, it can perform the functions of both the gas exhaust hole 26 and the through hole 28 described above, so it may be configured to have only one of them.

The gas supply device 30 supplies the 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 supply device 30 can be appropriately changed according to the desired gas, and a commercially available gas supply device (e.g., an oxygen supply device) 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 supply device 30.

When the oxygen concentration around the provisionally sintered body 50 decreases due to 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 aesthetic appeal. 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, and 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 installed.

During the firing of the pre-sintered body 50, in order to control the above-mentioned oxidizing atmosphere, it is preferable to continue to supply air or a gas containing the above-mentioned oxygen concentration to the storage space 22 (specifically, the pre-sintered body 50). This makes it possible to suppress fluctuations in the atmosphere of the storage 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 supply device 30 flows into the storage space 22 and is then discharged from the gas exhaust 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.

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 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, it becomes easier for the pre-sintered body 50 itself to absorb microwaves, and it becomes possible to act as a microwave absorber. When the pre-sintered body 50 becomes easier to absorb microwaves, the internal heating mechanism of the pre-sintered body 50 is more easily promoted by microwave heating. This promotes sintering inside the pre-sintered body 50, making it difficult for voids to remain inside, and it is possible to produce a zirconia sintered body with excellent strength and translucency.

From the viewpoint of raising the temperature of the pre-sintered body 50 in a shorter time, it is preferable to place 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 placed 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 a 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 placed 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. Such a gap is not particularly limited, but is, for example, preferably 3 mm or less, more preferably 2 mm or less, and further preferably 1 mm or less.

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, it is possible to induce internal heating of the pre-sintered body from a lower temperature range than when the susceptor completely encapsulates the pre-sintered body (for example, when the pre-sintered body is placed inside a closed box-shaped susceptor). As a result, compared to the sintering mode caused by heat conduction from the surface, it is possible to reduce the number of pores remaining inside the zirconia sintered body. 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.

In addition, it is preferable that the pre-sintered body 50 has no susceptor 40 installed (open) on both sides of at least one direction different from the predetermined direction in which the susceptor 40 is arranged. This makes it easier for the microwaves to be directly absorbed by the pre-sintered body 50, and internal heating can be induced from a lower temperature range and more uniformly. In addition, by providing one direction in which the susceptor 40 is not installed, the pre-sintered body 50 can be placed in the flow of gas supplied from the gas supplier 30, so that the atmosphere around the pre-sintered body 50 can be more suitably controlled.

In this embodiment, as shown in Fig. 2, the pre-sintered body 50 is sandwiched between two plate-shaped 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 no susceptors 40 are 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.

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 SiC susceptors include single crystal SiC, recrystallized SiC, reaction sintered SiC, nitride bonded SiC, oxide bonded SiC, silicon carbide fiber, etc. 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 recrystallized SiC is preferably, for example, 10% to 90%, and preferably 10% to 30%. The porosity can be measured by a conventionally known method, for example, mercury intrusion porosimetry.

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. Also, if the susceptor 40 is too thick, the susceptor 40 may be difficult to heat, and the heating rate may be slow. Therefore, within the above thickness range, a good balance is achieved between the strength of the susceptor 40 and the heating rate of the susceptor 40. This makes it possible to more effectively reduce voids inside the zirconia sintered body.

In the embodiment shown in FIG. 2, one plate-shaped susceptor 40 is disposed above and below the pre-sintered body 50, but the number of plate-shaped susceptors 40 is not particularly limited as long as there are multiple (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.

In this embodiment, the susceptor 40 is plate-shaped, but there is no particular limitation as long as the susceptor 40 is arranged on both sides of the pre-sintered body 50 in a predetermined direction. 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 used.

The radiation thermometer 60 can measure the temperature of the object without contact. 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.

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 grain boundaries can be reduced. As a result, it is estimated 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, for example, 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.

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 position and size of the pre-sintered body 50, which may cause cracks in the zirconia sintered body. On the other hand, in multi-mode, the 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.

The heating rate of microwave heating is not particularly limited, as it is appropriately changed depending on the shape, size, composition, etc. of the pre-sintered 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 manufactured 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 provisionally sintered body to be appropriately controlled, making it possible to produce a zirconia sintered body having better translucency and resistance to hydrothermal degradation.

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

<Cooling step 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 is 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.

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 supply device 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.

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 does not become 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 crystalline phases 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.

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 molded body 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) in which the c/a axial length ratio of the unit lattice is in the range of more than 1.012 and not more than 1.017 and a crystal phase (e.g., cubic) in which the c/a axial length ratio of the unit lattice is in the range of 1 or more and less than 1.0055 are mixed and the yttrium concentration tends to segregate locally. In general, it is believed that such segregation induces hydrothermal deterioration of the zirconia sintered body. 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 axial 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 axial 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 axial length ratio of the unit lattice is in the range of more than 1.012 and 1.017 or less, 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, microwave heating realizes a zirconia sintered body with few voids and high density, so that water is unlikely to penetrate into the zirconia sintered body. From the above, it is presumed that the zirconia sintered body disclosed here has excellent light transmittance and hydrothermal deterioration resistance.

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 translucency and resistance to hydrothermal degradation tend to be improved.

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 axial length ratio of the unit cell 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, and 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 axial length ratio in such a range, the hydrothermal degradation resistance may be further improved.

In the zirconia sintered body disclosed herein, when the entire crystal phase is taken as 100 mass%, the total ratio of the crystal phase in which the c/a axial length ratio of the unit cell is in the range of 1.0055 to less than 1.010 and the crystal phase in which the c/a axial length ratio is 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 translucency and resistance to hydrothermal degradation can be achieved.

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 or more and 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 resistance to hydrothermal degradation can be achieved.

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.

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, "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.

As described above, the zirconia sintered body disclosed herein has both excellent translucency and excellent resistance to hydrothermal deterioration, and can therefore be suitably used, for example, as a dental material. Examples of dental materials include dentures such as front teeth dentures and back teeth dentures, denture mill blanks, orthodontic brackets, dental prostheses, bridges, and the like.

As described above, specific aspects of the technology disclosed herein include those described in the following sections.
Item 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 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 1600° C. or more and 2000° C. or less by microwave heating; and a cooling step of lowering the temperature of the molded body having undergone 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 manufacturing 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 arranged to sandwich the molded body from both sides in a predetermined direction.
Item 6: The manufacturing method according to any one of Items 1 to 5, wherein in the compact preparation step, a material containing zirconia-containing granules is molded to prepare the compact.
Clause 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 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. 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 the total light transmittance to a D65 light source in the thickness direction of a 1 mm thick test piece is 44% or more.
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.

Below, we describe examples of the technology disclosed herein, but these examples are not intended to limit the technology disclosed herein.

(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.

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

Next, gas with an oxygen concentration of 90 vol% was supplied into the insulated 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 maintained at 1730°C for 1 minute. After that, the temperature was lowered to 1300°C at a temperature lowering rate of 10°C/min, and when it reached 1300°C, the microwave heating device was turned off to lower the temperature. Note that the supply of the gas was stopped when the temperature was lowered. In this way, the zirconia sintered body of Example 1 was manufactured. Note that the microwave heating method was 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.

(Example 2)
The rate of temperature drop to 1,300° C. after microwave heating was changed to 50° C./min from the production method of Example 1. A zirconia sintered body of Example 2 was produced in the same manner as in Example 1 except for this.

(Example 3)
The rate of temperature drop to 1,300° C. after microwave heating was changed to 100° C./min from the production method of Example 1. A zirconia sintered body of Example 3 was produced in the same manner as in Example 1 except for this.

(Example 4)
The rate of temperature drop to 1,300° C. after microwave heating was changed to 200° C./min from the production method of Example 1. A zirconia sintered body of Example 4 was produced in the same manner as in Example 1 except for this.

(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 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 zirconia sintered body of Example 5 was manufactured in the same manner as in Example 1 except for these operations.

(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.

(Example 7)
In the manufacturing method of Example 6, 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 zirconia sintered body of Example 7 was manufactured in the same manner as in Example 6 except for these operations.

(Example 8)
The zirconia sol produced by hydrolysis of the zirconia oxychloride solution was mixed with yttrium chloride and ytterbium chloride. The yttrium chloride and ytterbium chloride were mixed so that the yttria was 1.7 mol% and the ytterbia was 1.8 mol% when the yttrium chloride was converted to yttrium and the ytterbium chloride was converted to ytterbia, with the total of the zirconia, yttria and ytterbia being 100 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 obtained molded body was then heated at 1100 ° C for 2 hours to obtain a provisional sintered body. The heating rate at this time was 120 ° C / h up to 800 ° C, and 100 ° C / h up to 1100 ° C. Then, microwave heating was performed in the same manner as in Example 1, and the temperature was lowered at a temperature lowering rate of 200 ° C / min up to 1300 ° C, to obtain the 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 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.

(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 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 9 was manufactured in the same manner as in Example 8 except for the above.

<Crystal Phase Analysis>
An 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 Pharmaceuticals) 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°

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 phases (mass%). The analysis was performed on a mixed phase of tetragonal, metastable tetragonal, and cubic crystals, with the same temperature parameters for each element. Note that metastable tetragonal crystals here refer to a crystalline phase in which only the a-axis length and c-axis length of the tetragonal crystal differ. The results are shown in Table 1.

<Evaluation of Translucency>
The zirconia sintered body produced in each example was processed into a disk-shaped test piece with 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.

<Hydrothermal Deterioration Test>
The monoclinic rate (%) after hydrothermal treatment of the zirconia sintered body produced in each example was measured 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 using a device named: High Pressure Microreactor (manufactured by O-M Labotec Co., Ltd.). Then, the X-ray diffraction pattern of the polished surface was measured using an X-ray diffractometer (device name: Ultima IV, manufactured by Rigaku Corporation). Then, using the measurement results, the monoclinic rate (%) was calculated from the following formula. As can be seen from the following formula, the monoclinic ratio can be calculated from the X-ray diffraction peak intensity [I _m (111)] corresponding to the monoclinic phase (111) plane, the X-ray diffraction peak intensity [I _m (11-1)] corresponding to the monoclinic phase (11-1) plane, and the sum of the X-ray diffraction peak intensity [I _o (111)] corresponding to the (111) plane of a crystal phase other than the monoclinic phase. 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°

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 reduced the monoclinic ratio after hydrothermal degradation and improved the hydrothermal degradation resistance of the zirconia sintered body. This is thought 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 crystalline 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.

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

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 clear that a zirconia sintered body with excellent permeability and resistance to hydrothermal degradation can be realized.

Examples 8 and 9 are test examples of zirconia sintered bodies containing ytterbia. Even in this case, as shown in Example 8, it is clear that a zirconia sintered body with excellent permeability and resistance to hydrothermal deterioration can be realized.

Specific examples of the technology disclosed herein have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

REFERENCE SIGNS LIST 10 Microwave heating device 12 Partition wall 14 Heating space 16 Through hole 20 Insulated container 22 Storage space 24 Gas inlet hole 26 Gas outlet hole 28 Through hole 30 Gas supplier 32 Pump 40 Susceptor 50 Pre-sintered body 60 Radiation thermometer

Claims (13)

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 having undergone the second heating step to 1300° C. at a rate of 100 ° C./min or more.
The method for producing a zirconia sintered body includes the steps of: The manufacturing method according to claim 1, wherein the microwave heating method is multi-mode. The method according to claim 1 or 2, wherein in the second heating step, the microwave heating is carried out in an oxidizing atmosphere. The manufacturing 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. The manufacturing method according to claim 1 or 2, wherein in the second heating step, SiC susceptors are arranged to sandwich the molded body from both sides in a predetermined direction. The manufacturing method according to claim 1 or 2, wherein in the compact preparation step, the compact is prepared by molding a material containing zirconia-containing granules. 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. The zirconia sintered body according to claim 7, in which 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 when the entire crystal phase is taken as 100 mass%. The zirconia sintered body according to claim 7, in which the proportion of monoclinic crystals is 10% or less after immersion in hot water at 134°C for 6 hours. The zirconia sintered body according to claim 7, in which the proportion of the crystal phase having a unit lattice c/a axial length ratio in the range of 1 or more and less than 1.0055 is 20 mass% or less, when the entire crystal phase is taken as 100 mass%. The zirconia sintered body according to claim 7, in which the total light transmittance for a D65 light source in the thickness direction of a test piece having a thickness of 1 mm is 44% or more. A dental material comprising the zirconia sintered body according to any one of claims 7 to 11. The dental material of claim 12, which is a denture, a denture mill blank, or an orthodontic bracket.

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