Zirconia calcined body and method for producing same

Abstract

The present disclosure provides: a zirconia calcined body that makes it possible to suppress variation in mechanical strength in an obtained zirconia sintered body; and a method for producing the zirconia calcined body. The present disclosure relates to a zirconia calcined body containing zirconia and a stabilizer capable of suppressing zirconia phase transition, wherein the zirconia calcined body is heated from room temperature to maximum sintering temperatures at a rate of 10°C / minute, the maximum sintering temperatures are set to three different temperatures of 1450°C, 1500°C, and 1550°C, and the zirconia calcined body has a coefficient of variation of less than 0.10 in a tetragonal ratio measured by Rietveld analysis by X-ray diffraction in three kinds of sintered bodies obtained when the zirconia calcined body is fired with a two-hour hold at each maximum sintering temperature.

Description

Zirconia calcined body and method for manufacturing the same

[0001] This disclosure relates to a zirconia calcined body and a method for manufacturing the same. More specifically, this disclosure relates to a zirconia calcined body in which variations in mechanical strength are suppressed, and a method for manufacturing the same.

[0002] Zirconia sintered bodies are widely used industrially. In particular, in recent years they have been used in dental materials such as dental prostheses. These dental prostheses are often manufactured by forming zirconia molded bodies into desired shapes such as discs or prismatics by press molding zirconia particles or by molding using a composition containing zirconia particles, then calcining these bodies to form calcined bodies (mill blanks), cutting (milling) them into the shape of the intended dental prosthesis, and finally firing them again.

[0003] Zirconia is a compound in which phase transitions occur between multiple crystal systems. Therefore, partially-stabilized zirconia (PSZ) and fully-stabilized zirconia (FSZ), which suppress phase transitions by solid-solving stabilizers such as yttria (yttrium oxide; Y2O3) into zirconia, are used in various fields.

[0004] In the dental field, zirconia has been used as a frame material due to its high strength but low translucency. However, in recent years, with improvements in the translucency of zirconia, it has become increasingly common to fabricate dental prostheses using only zirconia.

[0005] Examples of dental materials using such zirconia materials include Patent Document 1.

[0006] International Publication No. 2018 / 056330

[0007] However, according to the present inventors, when a zirconia sintered body is manufactured by firing a zirconia molded body or zirconia calcined body according to Patent Document 1, if the maximum sintering temperature inside the firing furnace unintentionally deviates from the set temperature to a higher or lower temperature, the ratio of tetragonal crystals changes, and it has been confirmed that the mechanical strength of the resulting zirconia sintered body varies. Therefore, in order to obtain a large quantity of zirconia sintered bodies with a certain mechanical strength, strict temperature control is required so as not to deviate even slightly from the set firing temperature. Even if the firing temperature inside the firing furnace changes unintentionally, a zirconia calcined body that guarantees a certain mechanical strength has not been realized, and from the viewpoint of industrial production, there is room for further improvement.

[0008] This disclosure aims to provide a zirconia calcined body that can suppress variations in the mechanical strength of the resulting zirconia sintered body, and a method for manufacturing the same.

[0009] The present inventors have conducted extensive research to solve the above problems and have found that by using a zirconia calcined body containing zirconia, in which the zirconia calcined body is heated from room temperature to a maximum sintering temperature at a rate of 10°C / min, and the maximum sintering temperature is set to three different temperatures of 1450°C, 1500°C, and 1550°C, and the zirconia calcined body is fired at each maximum sintering temperature for 2 hours, the coefficient of variation in the tetragonal ratio measured by Rietveld analysis using X-ray diffraction is less than 0.10 in three types of sintered bodies, it is possible to suppress variations in the mechanical strength of the resulting zirconia calcined body and obtain uniform mechanical strength. Based on this finding, the inventors have conducted further research and completed this disclosure.

[0010] This disclosure encompasses the following inventions: [1] A zirconia calcined body comprising zirconia and a stabilizer capable of suppressing the phase transition of zirconia, wherein the zirconia calcined body is heated from room temperature at a heating rate of 10°C / min to a maximum sintering temperature, and the maximum sintering temperature is set to three different temperatures: 1450°C, 1500°C, and 1550°C, and the sintered body is fired at each maximum sintering temperature for 2 hours, and the coefficient of variation of the tetragonal ratio measured by Rietveld analysis by X-ray diffraction is less than 0.10 in three types of sintered bodies obtained. [2] The zirconia calcined body according to [1], wherein the stabilizer is yttria. [3] The zirconia calcined body according to [2], wherein the yttria content is 2.0 to 10 mol% with respect to the total mol of zirconia and yttria. [4] A method for producing a calcined zirconia body according to any one of [1] to [3], comprising the steps of: producing a zirconia composition containing zirconia particles and stabilizer particles capable of suppressing the phase transition of zirconia; a grinding step of grinding the zirconia composition; a molding step of molding the zirconia composition after grinding to obtain a molded body; and a calcination step of calcining the molded body, wherein the grinding step is (i) using grinding media with a diameter of less than 1 mm and a grinding time of 10 minutes or more, and / or (ii) using grinding media with a diameter of 1 mm or more and a grinding time of 20 hours or more (except in cases where only the grinding treatment of (ii) is performed on the zirconia composition for 20 hours or less as the grinding treatment), and the maximum calcination temperature in the calcination step is greater than 900°C and 1200°C or less. [5] The method for producing a calcined zirconia body according to [4], wherein the grinding step includes a step of grinding the zirconia composition and further grinding the stabilizer particles, and the grinding step of grinding the stabilizer particles is (i) using a grinding medium with a diameter of less than 1 mm and a grinding time of 10 minutes or more, and / or (ii) using a grinding medium with a diameter of 1 mm or more and a grinding time of 20 hours or more. [6] The method for producing a calcined zirconia body according to [5], wherein the step of grinding the stabilizer particles is performed before the step of producing the zirconia composition.[7] A method for producing a zirconia calcined body according to [4] or [5], wherein the calcination time in the calcination step is (i) 2.5 hours or more when the maximum calcination temperature is greater than 900°C and less than 1000°C, or (ii) 1.5 hours or more when the maximum calcination temperature is greater than 1000°C and 1200°C or less. [8] A method for producing a zirconia sintered body, wherein the zirconia calcined body according to any one of [1] to [3] is fired under normal pressure at a maximum sintering temperature greater than 1200°C and 1650°C or less.

[0011] This disclosure provides a zirconia calcined body that can suppress variations in the mechanical strength of the resulting zirconia sintered body, and a method for manufacturing the same. Furthermore, this disclosure provides a zirconia calcined body that ensures a constant mechanical strength regardless of changes in the maximum sintering temperature in the firing furnace, and a method for manufacturing the same. In particular, this disclosure provides a zirconia calcined body that can suppress variations in the mechanical strength of the resulting zirconia sintered body, especially when firing at a maximum sintering temperature of 1400°C or higher, and a method for manufacturing the same, which is industrially advantageous.

[0012] In this specification, "zirconia composition" is a composition containing at least zirconia particles and particles of a stabilizer capable of suppressing the phase transition of zirconia (hereinafter also simply referred to as "stabilizer"). In this specification, "molded body" means a body that has not reached either a semi-sintered state (calcined state) or a sintered state. That is, a molded body is distinguished from a calcined body and a sintered body in that it has not been calcined after being molded. In this specification, "calcined zirconia body" means a semi-sintered state in which the zirconia particles are necking (fixed) to each other and the zirconia particles are not completely sintered. In this specification, "sintered zirconia body" means a sintered state in which the zirconia particles are completely sintered. In a sintered zirconia body, the zirconia particles solidify together through sintering, and the relative density increases with sintering, resulting in a fully sintered state with a relative density of 95% or more. In this specification, "zirconia" means zirconium(IV) oxide (ZrO2), and ZrO2 particles may contain trace amounts of HfO2 (0.5% to 3% by mass) relative to the amount of ZrO2. Since HfO2 is difficult to separate, terms such as "zirconia," "zirconia particles," and "zirconia powder" refer to materials containing both ZrO2 and HfO2. Furthermore, particles and powders in which a stabilizer is solid-dissolved in zirconia are also included in "zirconia particles" and "zirconia powder," respectively. In this specification, the stabilizer content (mol%) means the content calculated by converting zirconia and stabilizer to oxide values. In this specification, "atmospheric pressure" means standard atmospheric pressure (1 atm). In this specification, "calcination time" means the holding time (mooring time) at the maximum calcination temperature. In this specification, the upper and lower limits of numerical ranges (temperature range, content of each component, abundance of crystal systems, values ​​calculated from the components, and each physical property, etc.) can be combined as appropriate.

[0013] [Zirconia Calcined Body] The zirconia calcined body of this disclosure contains zirconia, and is a zirconia calcined body obtained by heating the zirconia calcined body from room temperature at a heating rate of 10°C / min to a maximum sintering temperature, and setting the maximum sintering temperature to three different temperatures of 1450°C, 1500°C, and 1550°C, and firing at each maximum sintering temperature for 2 hours, wherein the coefficient of variation of the tetragonal ratio measured by Rietveld analysis by X-ray diffraction is less than 0.10.

[0014] Regarding the zirconia calcined bodies of this disclosure, the reason why variations in the mechanical strength of the resulting zirconia sintered bodies can be suppressed by satisfying a coefficient of variation of less than 0.10 in three different types of sintered bodies, measured by Rietveld analysis using X-ray diffraction, when the maximum sintering temperature is set to three different temperatures above 1400°C (1450°C, 1500°C, and 1550°C), is not clear, but it is presumed to be as follows. In the prior art, in the grinding process of the manufacturing method of zirconia calcined bodies, the stabilizer particles are not sufficiently ground, so even when sintering is performed at a maximum sintering temperature of 1400°C or higher, the diffusion of the stabilizer (preferably yttria) becomes uneven, the phase transition between tetragonal zirconia and cubic zirconia proceeds irregularly, the ratio of tetragonal zirconia is unstable, and the mechanical strength varies. In contrast, in the grinding process for manufacturing a calcined zirconia body, the stabilizer particles are ground more thoroughly than in the prior art (for example, Patent Document 1), thereby obtaining a zirconia composition in which the stabilizer (preferably yttria) is finely dispersed. By calcining a molded body formed from this zirconia composition at a maximum calcination temperature of over 900°C and 1200°C or less, a calcined zirconia body is obtained in which the diffusion state of the stabilizer (preferably yttria) into the zirconia particles is uniform. As a result, when sintering the calcined zirconia body, the phase transition between tetragonal zirconia and cubic zirconia is completed by the time the heating temperature during sintering reaches about 1400°C, and the phase transition does not proceed further, resulting in a thermodynamically stable structure. Furthermore, in this disclosure, by adjusting the calcination conditions, for example, by extending the holding time at the maximum calcination temperature and / or selecting a maximum calcination temperature in a higher temperature range, the diffusion of the stabilizer (preferably yttria) into the zirconia particles can be further promoted, and grain growth during calcination is accelerated, resulting in larger particle size. As a result, the number of grain boundaries decreases, and the factors that promote phase transitions also decrease. Moreover, with the reduction in the number of grain boundaries, the movement of atoms necessary for the phase transition is also suppressed, and the rate of the phase transition slows down, making the phase transition less likely to occur.Furthermore, grain growth relieves strain (stress) within the lattice of the crystal grains, making it less likely for phase transitions to occur due to thermal expansion or cooling. As described above, the combined effect of the stabilizing agent (preferably yttria) in the zirconia particles at the start of sintering and the suppression of phase transitions due to grain growth allows for a stable abundance of tetragonal zirconia in the temperature range of 1400°C or higher, resulting in less variation in mechanical strength and the maintenance of high mechanical strength.

[0015] In the calcined zirconia body of this disclosure, the temperature is raised from room temperature at a heating rate of 10°C / min to the maximum sintering temperature. For three types of sintered bodies with different maximum sintering temperatures, the coefficient of variation in the tetragonal ratio measured by Rietveld analysis using X-ray diffraction (XRD) (hereinafter referred to as Rietveld analysis of each of the three types of sintered bodies) is preferably 0.090 or less, more preferably 0.080 or less, even more preferably 0.070 or less, and particularly preferably 0.065 or less, in order to further suppress variations in the mechanical strength of the resulting zirconia sintered body. The coefficient of variation in the tetragonal ratio measured by Rietveld analysis using X-ray diffraction (XRD) is defined as the one measured and calculated by the method described in the examples.

[0016] The three zirconia sintered bodies used as samples for the Rietveld analysis by X-ray diffraction (XRD) are prepared by heating a calcined zirconia body from room temperature at a heating rate of 10°C / min to the maximum sintering temperature (1450°C, 1500°C, and 1550°C), with a holding time of 2 hours at each maximum sintering temperature. After holding for 2 hours at the maximum sintering temperature, cooling is started, and after cooling to room temperature, the Rietveld analysis by XRD is performed. The cooling conditions are not particularly limited; for example, the cooling rate may be 10°C / min. Cooling may also be done by air cooling. The processing time during cooling is not particularly limited. Using the three sintered bodies obtained by the above method, the crystalline phase of the sintered bodies is analyzed by XRD using the Rietveld method.

[0017] XRD and Rietveld analysis can be performed, for example, at room temperature using an X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation) and integrated X-ray analysis software (SmartLab Studio II, manufactured by Rigaku Corporation). The conditions for using these are as described in the examples below.

[0018] The calcined zirconia body contains a stabilizer capable of suppressing the phase transition of zirconia in order to obtain excellent translucency intended for use in dental materials.

[0019] The stabilizer content in the calcined zirconia body is preferably 2.0 mol% or more, more preferably 3.0 mol% or more, even more preferably 3.5 mol% or more, and particularly preferably 4.0 mol% or more, relative to the total mol of zirconia and stabilizer. When it is 2.0 mol% or more, it is preferable in that the cubic crystal system is more abundant in the sintered body, improving light transmittance. Furthermore, the stabilizer content is preferably 10 mol% or less, more preferably 9.0 mol% or less, even more preferably 8.5 mol% or less, and particularly preferably 8.0 mol% or less. When it is 10 mol% or less, it is preferable in that it prevents a decrease in the mechanical strength of the zirconia sintered body. In other words, the stabilizer content in the calcined zirconia body is preferably 2.0 mol% to 10 mol%, more preferably 3.0 mol% to 9.0 mol%, even more preferably 3.5 mol% to 8.5 mol%, and particularly preferably 4.0 mol% to 8.0 mol%. Furthermore, in one embodiment, from the viewpoint of obtaining higher mechanical strength in the zirconia sintered body, the stabilizer content may be 7.5 mol% or less relative to the total mol of zirconia and stabilizer.

[0020] Examples of stabilizers capable of suppressing the phase transition of zirconia include calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y₂O₃), cerium oxide (CeO₂), scandium oxide (Sc₂O₃), niobium oxide (Nb₂O₅), lanthanum oxide (La₂O₃), erbium oxide (Er₂O₃), praseodymium oxide (Pr₂O₃, Pr₆O 11 ₁₁), oxides such as samarium oxide (Sm₂O₃), europium oxide (Eu₂O₃), thulium oxide (Tm₂O₃), gallium oxide (Ga₂O₃), indium oxide (In₂O₃) and ytterbium oxide (Yb₂O₃), among which yttria (Y₂O₃) is preferred. One type of the stabilizer may be used alone, or two or more types may be used in combination. In the zirconia calcined body of the present disclosure, the stabilizer may be only yttria (Y₂O₃), and may further contain yttria and a stabilizer other than yttria that is capable of suppressing the phase transition of zirconia.

[0021] When the zirconia calcined body of the present disclosure is sintered at a predetermined sintering temperature (for example, 1450°C), the tetragonal crystal ratio is preferably 10% or more, more preferably 15% or more, and still more preferably 18% or more from the viewpoint of increasing mechanical strength. Further, from the viewpoint of improving transparency, the tetragonal crystal ratio is preferably 70% or less, more preferably 65% or less, and still more preferably 60% or less. In other words, the tetragonal crystal ratio is preferably 10% or more and 70% or less, more preferably 15% or more and 65% or less, and still more preferably 18% or more and 60% or less. One embodiment includes a zirconia calcined body in which the tetragonal crystal ratio of the sintered body obtained at each sintering temperature (1450°C, 1500°C, and 1550°C) falls within the predetermined range described above.

[0022] In the zirconia calcined body of this disclosure, it is preferable to include primary particles having an average primary particle diameter of 60 nm or more and less than 250 nm. The average primary particle diameter is more preferably 61 nm or more, even more preferably 62 nm or more, and particularly preferably 63 nm or more, from the viewpoint of suppressing a decrease in light transmittance while the resulting zirconia sintered body has excellent mechanical strength and light transmittance. Furthermore, the average primary particle diameter is more preferably 220 nm or less, and even more preferably 200 nm or less. In the zirconia calcined body of this disclosure, the average primary particle diameter can be appropriately selected as long as it falls within the above-mentioned preferred range. In one embodiment, a zirconia calcined body containing primary particles having an average primary particle diameter of 60 nm or more and less than 80 nm is mentioned, as it is easier to obtain a zirconia sintered body with excellent mechanical strength and light transmittance even when the maximum sintering temperature is lower than conventional temperatures (for example, 1450°C or less). In another embodiment, a zirconia calcined body containing primary particles having an average primary particle diameter of 80 nm or more and less than 200 nm is mentioned. The average primary particle diameter can be evaluated using, for example, the following method: An ultra-high-resolution field emission scanning electron microscope (product name "SU8200", manufactured by Hitachi High-Tech Corporation) is used to image the surface of a zirconia calcined body with an acceleration voltage of 5.0 kV, a working distance (WD) of 8.7 mm, and a magnification of 100,000x. The average particle diameter is calculated from the obtained image using image analysis. Image analysis software (product name "Image-Pro Plus", manufactured by Hakuto Co., Ltd.) is used to measure the particle diameter. The acquired SEM image is binarized, the brightness range is adjusted so that the grain boundaries are clear, and particles are recognized from the field of view (region). In the processed file in which the primary particle diameter has been recognized, the "Diameter" is selected in the "Count / Size dialog" to find the distribution (n=4). Specifically, for four fields of view of one sample, the arithmetic mean of the average particle diameter (primary particle diameter) measured in each field of view using the image analysis software is calculated.

[0023] The zirconia calcined body of the present disclosure may contain additives in addition to zirconia and the aforementioned stabilizer, as long as the effects according to the present disclosure are achieved. Examples of such additives include colorants (including pigments, composite pigments and fluorescent agents), alumina (Al₂O₃), titanium oxide (TiO₂), silica (SiO₂), and the like.

[0024] Examples of the pigment include oxides of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Sn, Sb, Bi, Ce, Pr, Sm, Eu, Gd, Tb and Er (specifically, NiO, Cr₂O₃, etc.) (excluding Y₂O₃ and CeO₂), oxides of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Sn, Sb, Bi, Ce, Pr, Sm, Eu, Gd, and Tb are preferred, and oxides of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Sn, Sb, Bi, Ce, Sm, Eu, Gd, and Tb are more preferred. In addition, the zirconia calcined body of the present disclosure may be free of erbium oxide (Er₂O₃). Examples of the composite pigment include (Zr,V)O₂, Fe(Fe,Cr)₂O₄, (Ni,Co,Fe)(Fe,Cr)₂O₄·ZrSiO₄, (Co,Zn)Al₂O₄, and the like.

[0025] The zirconia calcined body of this disclosure may contain a fluorescent agent. The inclusion of a fluorescent agent in the zirconia calcined body gives the zirconia sintered body fluorescence. There are no particular restrictions on the type of fluorescent agent; one or more types that can emit fluorescence at any wavelength of light can be used. Examples of fluorescent agents include those containing metal elements. Examples of such metal elements include Ga, Bi, Ce, Nd, Sm, Eu, Gd, Tb, Dy, and Tm. The fluorescent agent may contain one of these metal elements alone, or two or more. Among these metal elements, Ga, Bi, Eu, Gd, and Tm are preferred, and Bi and Eu are more preferred. Examples of fluorescent agents include oxides, hydroxides, acetates, and nitrates of the above metal elements. Furthermore, the fluorescent agents are Y2SiO5:Ce, Y2SiO5:Tb, (Y,Gd,Eu)BO3, Y2O3:Eu, YAG:Ce, ZnGa2O4:Mn, BaMgAl 10 O 17 EU, etc., would also be acceptable.

[0026] The content of the fluorescent agent in the zirconia calcined body is not particularly limited and can be adjusted as appropriate depending on the type of fluorescent agent or the intended use of the zirconia sintered body. However, from the viewpoint of suitability for use as a dental prosthesis, it is preferable that the content of the fluorescent agent, in terms of oxides of the metal elements contained in it, be 0.001% by mass or more, more preferably 0.005% by mass or more, and even more preferably 0.01% by mass or more, based on 100% by mass of zirconia contained in the zirconia calcined body. Furthermore, the content of the fluorescent agent, in terms of oxides of the metal elements contained in it, is not particularly limited as long as suitable fluorescence is achieved, but it can be 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less. By having a content above the lower limit, the fluorescence is not inferior to that of natural human teeth, and by having a content below the upper limit, a decrease in the mechanical strength and light transmittance of the zirconia sintered body can be suppressed.

[0027] Furthermore, the bending strength of the zirconia calcined body of the present disclosure is preferably 15 MPa or more in order to secure mechanical strength that enables mechanical processing in the state of the calcined body. In addition, in order to facilitate mechanical processing in the state of the calcined body, the bending strength of the zirconia calcined body is preferably 70 MPa or less, and more preferably 60 MPa or less. In other words, the bending strength of the zirconia calcined body is preferably 15 MPa or more and 70 MPa or less, and more preferably 15 MPa or more and 60 MPa or less.

[0028] The bending strength can be measured in accordance with ISO 6872:2015 (Dentistry-Ceramic materials), except that only the size condition of the test piece is changed, and the measurement is performed using a test piece with a size of 5 mm×10 mm×50 mm. The surface and C-face of the test piece (the surface obtained by chamfering the corner of the test piece at an angle of 45°) is surface-finished in the longitudinal direction with No. 600 sandpaper. The test piece is placed such that its widest surface faces the vertical direction (load direction). In the measurement of the three-point bending test, the distance between fulcrums (span) is 30 mm, and the crosshead speed is 0.5 mm / min.

[0029] Furthermore, the density of the zirconia calcined body of the present disclosure is 2.7 g / cm 3 or higher, preferably 3.0 g / cm 3 or higher, more preferably 3.2 g / cm 3 or higher, even more preferably. In addition, the density of the zirconia calcined body is 4.0 g / cm 3 or lower, preferably 3.8 g / cm 3 or lower, more preferably 3.6 g / cm 3 or lower, even more preferably. When the density is within the above range, processing can be easily performed. In other words, the density of the zirconia calcined body is 2.7 g / cm 3 or more and 4.0 g / cm 3 or less, preferably 3.0 g / cm 3 or more and 3.8 g / cm 3 or less, more preferably 3.2 g / cm 3 or more and 3.6 g / cm 3The following is even more preferable. The density of the zirconia calcined material can be calculated as (mass of zirconia calcined material) / (volume of zirconia calcined material). For example, the density of the zirconia calcined material can be calculated by cutting out 10 mm square test specimens at arbitrary locations (n=3) while changing the cutting position of the zirconia calcined material, measuring the mass and volume of the obtained test specimens, calculating the arithmetic mean of the measured values, and using the arithmetic mean, calculating the density using the above formula.

[0030] [Method for Manufacturing Zirconia Calcined Bodies] A method for manufacturing zirconia calcined bodies includes the steps of: manufacturing a zirconia composition containing zirconia particles and stabilizer particles capable of suppressing the phase transition of zirconia (hereinafter also simply referred to as "stabilizer particles"); a grinding step of grinding the zirconia composition; a molding step of molding the zirconia composition after grinding to obtain a molded body; and a calcination step of calcining the molded body, wherein the grinding step includes a predetermined grinding process described later.

[0031] One embodiment of this method involves a manufacturing method in which the stabilizer particles capable of suppressing the phase transition of zirconia are yttria particles.

[0032] In the following, a method for manufacturing a zirconia calcined body will be described using the case where yttria is used as the stabilizer as an example. The zirconia calcined body of this disclosure can be manufactured by grinding the zirconia composition, and if necessary, grinding the yttria particles used in the manufacture of the zirconia composition to sufficiently grind the yttria particles, and by selecting calcination conditions (such as selecting a range with a high maximum calcination temperature and extending the holding time at the maximum calcination temperature) to form a molded body from the pulverized zirconia composition and then firing (calcining) the molded body to the extent that the zirconia particles do not sinter with each other.

[0033] The zirconia composition before pulverization can be produced by mixing zirconia powder and yttria powder, which are the raw material powders. The mixing method is not particularly limited, and known methods and apparatus can be used. Furthermore, as described below, in addition to the pulverization of the zirconia composition, if necessary, the yttria particles used as raw materials for the zirconia composition may also be pulverized before producing the zirconia composition, and the yttria powder obtained after pulverization may be mixed with the zirconia powder. Hereinafter, the pulverization of the yttria particles used as raw materials will also be referred to as "pre-pulverization of yttria particles."

[0034] As a method for preparing zirconia particles constituting zirconia powder and yttria particles constituting yttria powder, for example, a breakdown process in which coarse particles are crushed or broken down into fine particles, or a building-up process in which they are synthesized from atoms or ions through nucleation and growth processes, can be employed. However, even when manufactured by the building-up process, a predetermined crushing treatment is required to obtain the coefficient of variation at the desired tetragonal ratio measured by Rietveld analysis in the zirconia calcined body, and from the viewpoint of being easy to manufacture, the breakdown process is preferred.

[0035] The following describes a method for manufacturing zirconia compositions, using a breakdown process as an example.

[0036] The zirconia powder preferably contains monoclinic zirconia. The zirconia powder may also contain tetragonal zirconia and / or cubic zirconia. The zirconia powder may contain a stabilizer that can suppress the phase transition of zirconia, with a content of 0 to 2.7 mol% relative to the total mol of zirconia and the stabilizer. In the zirconia powder, as long as it contains zirconia, one type of zirconia crystal system may be used alone, or two or more types may be used in combination.

[0037] As the zirconia powder, commercially available zirconia particles may be used, or commercially available powder may be used after being ground in a known grinding and mixing device (ball mill, bead mill, etc.). As the yttria powder, commercially available yttria particles may be used, or commercially available powder may be used after being ground in a known grinding and mixing device (ball mill, bead mill, etc.).

[0038] Examples of commercially available zirconia powders include: Zpex® (product name) (Y2O3 content: 3 mol%), Zpex® 4 (Y2O3 content: 4 mol%), and Zpex® Smile (registered trademark) (Y2O3 content: 5.5 mol%), TZ-3Y (Y2O3 content: 3 mol%), TZ-3YS (Y2O3 content: 3 mol%), TZ-4YS (Y2O3 content: 4 mol%), TZ-6Y (Y2O3 content: 6 mol%), TZ-6YS (Y2O3 content: 6 mol%), TZ-8YS (Y2O3 content: 8 mol%), TZ-10YS (Y2O3 content: 10 mol%), TZ-3Y-E (Y2O3 content: 3 mol%), TZ- Examples include "3YS-E" (Y2O3 content: 3 mol%), "TZ-3YB-E" (Y2O3 content: 3 mol%), "TZ-3YSB-E" (Y2O3 content: 3 mol%), "TZ-3YB" (Y2O3 content: 3 mol%), "TZ-3YSB" (Y2O3 content: 3 mol%), "TZ-3Y20AB" (Y2O3 content: 3 mol%), "TZ-8YSB" (Y2O3 content: 8 mol%), and "TZ-0" (Y2O3 content: 0 mol%; monoclinic zirconia); all manufactured by Tosoh Corporation).

[0039] A zirconia composition containing zirconia particles and yttria particles is subjected to a grinding process. During the grinding process, the zirconia composition may be mixed with a solvent as appropriate before grinding. In addition to grinding the zirconia composition, the yttria particles used in the production of the zirconia composition may also be subjected to a grinding process as needed. By grinding the yttria particles thoroughly in the grinding process under predetermined conditions, a zirconia calcined body can be obtained in which the coefficient of variation in the tetragonal ratio measured by the Rietveld analysis is adjusted to a desired range when the zirconia calcined body obtained in which the zirconia composition after grinding is used and combined with predetermined calcination conditions. In this disclosure, it is presumed that by thoroughly grinding the yttria particles, the coefficient of variation in the tetragonal ratio measured by the Rietveld analysis of each of the three types of sintered bodies can be adjusted to a desired range when the zirconia calcined body is obtained in which the zirconia composition after grinding is used and combined with predetermined calcination conditions, thereby suppressing variations in mechanical strength regardless of changes in the maximum sintering temperature in the firing furnace when manufacturing zirconia sintered bodies.

[0040] In order to further suppress variations in the mechanical strength of the final sintered body by adjusting the coefficient of variation in the tetragonal ratio measured by Rietveld analysis of each of the three types of sintered bodies in the zirconia calcined body to a desired range, it is preferable to use the following methods as specific operations for the grinding treatment of the zirconia composition, and, if necessary, for the grinding treatment of the yttria particles used in the manufacture of the zirconia composition, in order to sufficiently grind the yttria particles. (i) Using a grinding medium with a diameter of less than 1 mm and a grinding time of 10 minutes or more, and / or (ii) Using a grinding medium with a diameter of 1 mm or more and a grinding time of 20 hours or more. However, in the case of method (ii), if only the grinding treatment of (ii) is performed on the zirconia composition for 20 hours or less, the disintegration and grinding will not proceed sufficiently, and it will not be possible to obtain a zirconia composition in which yttria is finely dispersed. Furthermore, because the grinding treatment of yttria particles in the overall manufacturing method of the zirconia calcined body is insufficient, it is not possible to obtain a zirconia calcined body in which the diffusion state of yttria into the zirconia particles is uniform. Therefore, the entire method for manufacturing a calcined zirconia body according to this disclosure does not include cases where only the pulverization treatment described in (ii) is performed on the zirconia composition for 20 hours or less as a pulverization treatment. The pulverization treatment for the yttria particles will be described later. By pulverizing the zirconia composition in this way, and further combining the pulverization treatment of the zirconia composition with the pre-pulverization treatment of the yttria particles as necessary, a zirconia composition in which yttria particles are finely dispersed can be obtained. By calcining a molded body formed from this zirconia composition under conditions where the maximum calcination temperature is greater than 900°C and less than or equal to 1200°C, a calcined zirconia body in which the diffusion state of yttria into the zirconia particles is uniform can be obtained. When sintering the calcined zirconia body, the phase transition between tetragonal zirconia and cubic zirconia is completed by the time the heating temperature during sintering reaches about 1400°C, and the phase transition does not proceed any further, resulting in a thermodynamically stable structure.Furthermore, by selecting and combining the aforementioned pulverization process with calcination conditions (such as increasing the maximum calcination temperature and / or extending the calcination time), the coefficient of variation in the tetragonal ratio measured by Rietveld analysis of each of the three types of sintered zirconia calcined bodies can be adjusted to a desired range.

[0041] When using grinding media with a diameter of less than 1 mm, using grinding media with a fine diameter allows for the production of a zirconia composition in which yttria particles are finely dispersed. Therefore, when a molded body formed from this composition is calcined, a calcined zirconia body with uniform diffusion into the zirconia particles is obtained. From this viewpoint, grinding media with a diameter of 0.1 to 0.5 mm is preferred. Commercially available grinding media with a diameter of less than 1 mm may be used. Examples of grinding equipment that uses grinding media with a diameter of less than 1 mm include bead mills.

[0042] When using grinding media with a diameter of less than 1 mm, the grinding time is not particularly limited as long as the coefficient of variation in the tetragonal ratio measured by Rietveld analysis of each of the three types of sintered bodies in the resulting zirconia calcined body can be adjusted to a desired range. However, from the standpoint of easily adjusting the coefficient of variation in the tetragonal ratio measured by Rietveld analysis to a desired range, it is preferable that the grinding time be 10 minutes or more, more preferably 15 minutes or more, even more preferably 20 minutes or more, and particularly preferable that it be 30 minutes or more. Furthermore, from the standpoint of easily adjusting the coefficient of variation in the tetragonal ratio measured by Rietveld analysis of each of the three types of sintered bodies in the resulting zirconia calcined body to a desired range, it is preferable that the grinding time be 20 hours or less, more preferably 10 hours or less, even more preferably 5 hours or less, and particularly preferable that it be 2 hours or less. Note that when using a grinding device that grinds while circulating the slurry (for example, a circulating bead mill), the grinding time refers to the residence time during which the slurry remains in the vessel (grinding chamber).

[0043] When using grinding media with a diameter of 1 mm or more, the diameter of the grinding media is preferably 1.5 mm or more, and more preferably 2.0 mm or more, because it is easier to increase the amount of energy used in the grinding process by combining it with a long grinding time. Commercially available grinding media with a diameter of 1 mm or more may be used. Examples of grinding devices that use grinding media with a diameter of 1 mm or more include ball mills.

[0044] When using grinding media with a diameter of 1 mm or more, the grinding time (grinding treatment time) is preferably 20 hours or more, more preferably 55 hours or more, even more preferably 80 hours or more, and particularly preferably 100 hours or more, as this makes it easier to adjust the coefficient of variation in the tetragonal ratio measured by Rietveld analysis of each of the three types of sintered bodies in the resulting zirconia calcined body to a desired range. Furthermore, the grinding treatment time is preferably 1000 hours or less, more preferably 800 hours or less, even more preferably 500 hours or less, and particularly preferably 300 hours or less.

[0045] In this disclosure, by supplying a pulverization energy increased by adjusting the combination of pulverization time and pulverization media diameter to the zirconia composition and by selecting calcination conditions, a zirconia composition can be obtained in which the coefficient of variation of the tetragonal ratio measured by Rietveld analysis of each of the three types of sintered bodies in the resulting calcined zirconia body is adjusted to a desired range.

[0046] One embodiment of a method for producing a calcined zirconia body is described, wherein the grinding step includes grinding the zirconia composition and further grinding the stabilizer particles (preferably yttria particles) used in the production of the zirconia composition, and the grinding step for grinding the stabilizer particles is described as (i) using grinding media with a diameter of less than 1 mm and a grinding time of 10 minutes or more, and / or (ii) using grinding media with a diameter of 1 mm or more and a grinding time of 20 hours or more. Another embodiment of a method for producing a calcined zirconia body is described, wherein the step of grinding the stabilizer particles is performed before the step of producing the zirconia composition.

[0047] In this embodiment, the yttria powder raw material is thoroughly pulverized before mixing with the zirconia powder, and further pulverization is performed on the zirconia composition after mixing. This process thoroughly pulverizes the yttria particles, resulting in a zirconia composition in which yttria is finely dispersed. Furthermore, by calcining the molded body formed from this zirconia composition at a maximum calcination temperature of over 900°C and up to 1200°C, a calcined zirconia body with a uniform diffusion state of yttria into the zirconia particles is obtained. When the calcined zirconia body obtained in this way is sintered, the phase transition between tetragonal zirconia and cubic zirconia is completed by the time the heating temperature during sintering reaches approximately 1400°C, and the phase transition does not progress further, making it easier to obtain a thermodynamically stable structure. By manufacturing a calcined zirconia body using the zirconia composition obtained as described above, the yttria is finely dispersed in the resulting calcined zirconia body. Therefore, it is presumed that when the zirconia calcined body is fired, the diffusion state of yttria into the zirconia particles becomes uniform, the abundance ratio of tetragonal zirconia stabilizes in the temperature range of 1400°C or higher, and variations in the mechanical strength of the resulting zirconia sintered body can be suppressed.

[0048] In the grinding process for pulverizing yttria particles, the grinding time can be selected according to the size of the grinding media. Separately from the raw material yttria powder, the raw material zirconia powder may also be subjected to the grinding process before mixing, in the same manner as the yttria powder.

[0049] The zirconia composition before and after the grinding process may be in the form of granules, slurry, etc. If the zirconia composition is a slurry, the slurry can be produced by mixing the mixed powder obtained by the grinding process with a solvent (preferably water). Alternatively, the slurry can be produced by mixing the zirconia composition with a solvent and then performing a grinding process. Furthermore, in the method for producing a calcined zirconia body according to this disclosure, it is preferable not to perform a heat treatment of 900°C or higher on the zirconia composition, as this makes it easier to suppress variations in the mechanical strength of the resulting zirconia sintered body. It is preferable not to perform a heat treatment of 900°C or higher before or after the grinding process. One embodiment of a method for producing a calcined zirconia body includes the steps of: producing a zirconia composition containing zirconia particles and stabilizer particles capable of suppressing the phase transition of zirconia; a grinding step of grinding the zirconia composition; a molding step of molding the zirconia composition after grinding to obtain a molded body; and a calcining step of calcining the molded body, wherein the grinding step is (i) using grinding media with a diameter of less than 1 mm and a grinding time of 10 minutes or more, and / or (ii) using grinding media with a diameter of 1 mm or more and a grinding time of 20 hours or more (except in cases where only the grinding treatment of (ii) is performed on the zirconia composition for 20 hours or less as the grinding treatment), and does not include a heat treatment of 900°C or higher on the zirconia composition, and the maximum calcination temperature in the calcination step is greater than 900°C and less than or equal to 1200°C.

[0050] Median diameter D of the zirconia composition after grinding treatment 50 The sintered body is preferably 0.2 μm or less in terms of excellent mechanical strength and light transmittance. Furthermore, it is preferably 20 nm or more in terms of excellent moldability.

[0051] Median diameter D of zirconia particles and yttria particles in the zirconia composition after grinding treatment 50This can be measured by a dynamic light scattering particle size distribution method. For example, using a dynamic light scattering particle size distribution analyzer (product name "SZ-100V2", manufactured by Horiba, Ltd.), a slurry diluted with water to about 0.1% by mass is irradiated with ultrasound for 30 minutes, and then measured by volume while applying ultrasound.

[0052] Furthermore, the zirconia composition may contain additives such as binders, dispersants, emulsifiers, plasticizers, defoamers, pH adjusters, lubricants, and light transmittance adjusters. These additives may be used individually or in combination of two or more.

[0053] Examples of binders include polyvinyl alcohol, methylcellulose, carboxymethylcellulose, acrylic binders, wax binders, polyvinyl butyral, polymethyl methacrylate, and ethylcellulose.

[0054] The binder content in the zirconia composition of this disclosure is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less, based on 100% by mass of zirconia, in order to improve light transmittance.

[0055] Examples of plasticizers include polyethylene glycol, glycerin, propylene glycol, and dibutylphthalic acid.

[0056] Examples of dispersants include ammonium polycarboxylate (such as triammonium citrate), ammonium polyacrylate, acrylic copolymer resin, acrylic ester copolymer, polyacrylic acid, bentonite, carboxymethylcellulose, anionic surfactants (such as polyoxyethylene alkyl ether phosphates like polyoxyethylene lauryl ether phosphate), nonionic surfactants, oleic glycerides, amine salt type surfactants, oligosaccharide alcohols, and stearic acid.

[0057] Examples of emulsifiers include alkyl ethers, phenyl ethers, and sorbitan derivatives.

[0058] Examples of defoaming agents include alcohol, polyether, polyethylene glycol, silicone, and wax.

[0059] Examples of pH adjusting agents include ammonia and ammonium salts (including ammonium hydroxide such as tetramethylammonium hydroxide).

[0060] Examples of lubricants include polyoxyethylene alkyl ethers and waxes.

[0061] Examples of light-transmitting agents include aluminum oxide (Al2O3), titanium dioxide (TiO2), silicon dioxide (SiO2), zircon, lithium silicate, and lithium disilicate.

[0062] The BET specific surface area of ​​the particles constituting the zirconia composition was 7.0 m² when measured in accordance with JIS Z 8830:2013. 2 It is preferable that the amount be 7.5 m or more. 2 It is more preferable that the amount is 8.0 m or more per gram. 2 It is even more preferable that it be 7.0 m or more. 2 When the amount is greater than or equal to 1g, it is easier to suppress the clouding of the sintered body during sintering. Furthermore, the BET specific surface area is 50m². 2 Preferably, it is less than / g, and 45m 2 It is more preferable that it be less than or equal to 40m 2 It is even more preferable that it be less than or equal to / g. 50m 2 If the value is less than / g, it becomes less susceptible to temperature variations within the firing furnace. In other words, the BET specific surface area is 7.0 m². 2 / g or more 50m 2 It is preferable that the amount be less than or equal to 7.5 m 2 / g or more 45m 2 It is more preferable that it be less than or equal to 8.0 m 2 / g or more 40m 2It is even more preferable that the amount is less than or equal to / g. The BET specific surface area can be measured using commercially available products such as a fully automatic specific surface area measuring device (product name "Macsorb® HM model-1200", BET flow method (single-point method / multi-point method), manufactured by Mountec Co., Ltd.). For example, it can be measured using the BET flow method (single-point method) with the aforementioned fully automatic specific surface area measuring device. Furthermore, if the BET specific surface area is within the above range, the light transmittance of the sintered body is less likely to decrease even if the firing time for sintering is shortened. The "BET specific surface area" referred to here is the specific surface area measured without distinguishing between primary and secondary particles.

[0063] Next, a molded body is produced by a molding process in which the zirconia composition after the pulverization treatment is shaped. The molded body is obtained by shaping the zirconia composition by applying external force in a known manner.

[0064] The molding method is not particularly limited, and for example, the following methods can be used: (a) a method comprising the step of slip casting a slurry containing the zirconia composition after grinding; (b) a method comprising the step of gel casting a slurry containing the zirconia composition after grinding; (c) a method comprising the step of press molding the zirconia composition after grinding; (d) a method comprising the step of molding a zirconia composition containing zirconia particles, yttria particles, and resin; (e) a method comprising the step of polymerizing a zirconia composition containing zirconia particles, yttria particles, and polymerizable monomers or oligomers; (f) a method comprising the step of additively manufacturing granules containing zirconia particles and yttria particles.

[0065] (a) When a zirconia molded article is manufactured by a method having a step of slip casting a slurry containing a zirconia composition after slip casting grinding treatment, there are no particular restrictions on the specific slip casting method, and for example, a method of pouring the slurry into a mold and then drying it can be adopted. The content of the dispersion medium in the slurry used is preferably 80% by mass or less, more preferably 50% by mass or less, and even more preferably 20% by mass or less, in order to facilitate pouring the slurry into the mold, prevent the drying from taking a great deal of time, and increase the number of times the mold can be used. The slurry may be poured into the mold under normal pressure, but it is preferable to do so under pressurized conditions from the viewpoint of production efficiency.

[0066] (b) When a zirconia molded article is produced by a method having a step of gel casting a slurry containing a zirconia composition after gel casting grinding, there are no particular restrictions on the specific method of gel casting. For example, a method can be employed in which a wet body is formed by gelling the slurry in a mold and then drying it. The content of the dispersion medium in the slurry used is preferably 80% by mass or less, more preferably 50% by mass or less, and even more preferably 20% by mass or less, in order to prevent the drying from taking a long time and to suppress the occurrence of cracks during drying. Gelation may be performed, for example, by adding a gelling agent, or by adding a polymerizable monomer and then polymerizing it.

[0067] There are no restrictions on the type of gelling agent; for example, a water-soluble gelling agent can be used, and specifically, agarose, gelatin, etc., are preferably used. One type of gelling agent may be used alone, or two or more types may be used in combination. The amount of gelling agent used is not particularly limited as long as no problems such as cracks occur during sintering, but it can be 10% by mass or less, 5% by mass or less, or 1% by mass or less, based on the mass of the slurry after the gelling agent has been added.

[0068] Furthermore, there are no particular restrictions on the type of polymerizable monomer. Examples include (meth)acrylate monomers such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, propylene glycol mono(meth)acrylate, glycerol mono(meth)acrylate, and erythritol mono(meth)acrylate; and (meth)acrylamide monomers such as N-methylol (meth)acrylamide, N-hydroxyethyl (meth)acrylamide, and N,N-bis(2-hydroxyethyl) (meth)acrylamide. A single polymerizable monomer may be used, or two or more may be used in combination. The amount of polymerizable monomer used is not particularly limited as long as no problems such as cracks occur during sintering, but it can be 10% by mass or less, 5% by mass or less, or 1% by mass or less, based on the mass of the slurry after the polymerizable monomer has been added.

[0069] When gelation is performed by polymerization of polymerizable monomers, it is preferable to use a polymerization initiator. There are no particular restrictions on the type of polymerization initiator, but photopolymerization initiators are particularly preferred. As a photopolymerization initiator, one can be appropriately selected from those used in general industry, and among these, photopolymerization initiators used in dental applications are preferred.

[0070] Specific examples of photopolymerization initiators include (bis)acylphosphine oxides (including salts), thioxanthones (including salts of quaternary ammonium salts, etc.), ketals, α-diketones, coumarins, anthraquinones, benzoin alkyl ether compounds, and α-aminoketone compounds. One photopolymerization initiator may be used alone, or two or more may be used in combination. Among these photopolymerization initiators, it is preferable to use at least one selected from the group consisting of (bis)acylphosphine oxides and α-diketones. This allows polymerization (gelation) to be carried out in both the ultraviolet region (including the near-ultraviolet region) and the visible light region, and in particular, polymerization (gelation) can be carried out sufficiently regardless of the light source used, such as lasers such as Ar lasers and He-Cd lasers; or lighting such as halogen lamps, xenon lamps, metal halide lamps, light-emitting diodes (LEDs), mercury lamps, and fluorescent lamps.

[0071] There are no particular restrictions on the drying method used to dry the formed wet material. Examples include natural drying, hot air drying, vacuum drying, dielectric heating drying, induction heating drying, and constant temperature and humidity drying. One of these methods may be used, or two or more may be used. Among these, natural drying, dielectric heating drying, induction heating drying, and constant temperature and humidity drying are preferred because they can suppress the occurrence of cracks during drying.

[0072] There are no particular restrictions on the type of mold used in slip casting and gel casting. For example, porous molds made of gypsum, resin, ceramics, etc., and non-porous molds made of metal, resin, etc., can be used.

[0073] (c) Press forming: In the step of press forming the zirconia composition after the crushing treatment, there are no particular restrictions on the specific method of press forming, and it can be carried out using a known press forming machine. Specific methods of press forming include, for example, a uniaxial press.

[0074] Furthermore, multi-stage molding may be performed. For example, after press molding the zirconia composition, a CIP (Cold Isostatic Pressing) treatment may be applied.

[0075] The shape of the molded body is not particularly limited and may be disc-shaped, rectangular parallelepiped-shaped, or in the shape of a dental product (e.g., a tooth crown). The molded body may be, for example, a columnar zirconia molded body obtained by filling a mold with a zirconia composition (e.g., granules) and pressing it with a uniaxial pressure press. The higher the surface pressure during press molding, the higher the density of the molded body. On the other hand, if the density of the zirconia molded body is too high, the zirconia calcined body becomes hard. Therefore, the surface pressure during press molding when producing a zirconia molded body is preferably 30 to 200 MPa. When the surface pressure of the press is 30 MPa or higher, the shape retention of the zirconia molded body is excellent, and when it is 200 MPa or lower, the density of the zirconia molded body does not increase too much, making it easier to prevent it from becoming hard.

[0076] The aforementioned molded articles also include molded articles that have been densified by high-temperature and high-pressure treatments such as CIP (Cold Isostatic Pressing). From the same viewpoint as above, the pressure for CIP is preferably 30 to 200 MPa.

[0077] (d) Molding of zirconia composition containing resin When a zirconia molded article is manufactured by a method having a step of molding zirconia particles, yttria particles, and a zirconia composition containing resin, there are no particular restrictions on the specific method for molding the zirconia composition, and for example, injection molding, casting, extrusion molding, etc. may be used. Alternatively, a method of fabricating the composition by fused deposition modeling (FDM), an inkjet method, a powder / binder deposition modeling method, or other additive manufacturing methods (3D printing, etc.) may be used. Among these molding methods, injection molding and casting are preferred, and injection molding is more preferred. There are no particular restrictions on the type of resin, but it is preferable to use the binder described above.

[0078] (e) Polymerization of a zirconia composition containing zirconia particles, yttria particles, and polymerizable monomers or oligomers By polymerizing a zirconia composition containing zirconia particles, yttria particles, and polymerizable monomers or oligomers, the polymerizable monomers or oligomers in the composition polymerize and the composition can be cured. When a zirconia molded body is manufactured by a method having the step of polymerization, there are no particular restrictions on the specific method, and for example, a method of polymerizing the zirconia composition in a mold; a stereolithography (SLA) method using the zirconia composition can be employed. Among these, stereolithography (SLA) is preferred. With stereolithography, a shape corresponding to the desired shape of the zirconia sintered body that is finally obtained can be imparted to the zirconia molded body at the time of manufacture. For this reason, stereolithography may be particularly suitable when the zirconia sintered body is used as a dental material such as a dental prosthesis. There are no particular restrictions on the type of polymerizable monomer; it may be any monofunctional polymerizable monomer such as monofunctional (meth)acrylate or monofunctional (meth)acrylamide, or any polyfunctional polymerizable monomer such as a bifunctional aromatic compound, a bifunctional aliphatic compound, or a trifunctional or more functional compound. One polymerizable monomer may be used alone, or two or more may be used. Among these, it is particularly preferable to use a polyfunctional polymerizable monomer when employing photopolymerization. The oligomer is not particularly limited as long as it is a compound that is polymerizable by bonding two or more of the polymerizable monomers.

[0079] Examples of monofunctional (meth)acrylates include hydroxyl group-containing (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, propylene glycol mono (meth)acrylate, glycerol mono (meth)acrylate, and erythritol mono (meth)acrylate; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, and sec-butyl (meth)acrylate. Examples include alkyl (meth)acrylates such as t-butyl (meth)acrylate, isobutyl (meth)acrylate, n-hexyl (meth)acrylate, lauryl (meth)acrylate, cetyl (meth)acrylate, and stearyl (meth)acrylate; alicyclic (meth)acrylates such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; aromatic group-containing (meth)acrylates such as benzyl (meth)acrylate and phenyl (meth)acrylate; and (meth)acrylates having functional groups other than polymerizable groups, such as 2,3-dibromopropyl (meth)acrylate, 3-(meth)acryloyloxypropyltrimethoxysilane, and 11-(meth)acryloyloxyundecyltrimethoxysilane. Examples of monofunctional (meth)acrylamides include (meth)acrylamide, N-(meth)acryloylmorpholine, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-di-n-propyl(meth)acrylamide, N,N-di-n-butyl(meth)acrylamide, N,N-di-n-hexyl(meth)acrylamide, N,N-di-n-octyl(meth)acrylamide, N,N-di-2-ethylhexyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide, and N,N-bis(2-hydroxyethyl)(meth)acrylamide.Among these monofunctional polymerizable monomers, (meth)acrylamide is preferred due to its excellent polymerizability, and N-(meth)acryloylmorpholine, N,N-dimethyl(meth)acrylamide, and N,N-diethyl(meth)acrylamide are more preferred.

[0080] Examples of bifunctional aromatic compounds include 2,2-bis((meth)acryloyloxyphenyl)propane, 2,2-bis[4-(2-hydroxy-3-acryloyloxy-2-hydroxypropoxy)phenyl]propane, 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane (commonly known as "Bis-GMA"), 2,2-bis(4-(meth)acryloyloxyethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxypolyethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxydiethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxytetraethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxypentaethoxyphenyl)propane, 2, Examples of (meth)acrylates include 2-bis(4-(meth)acryloyloxydipropoxyphenyl)propane, 2-(4-(meth)acryloyloxydiethoxyphenyl)-2-(4-(meth)acryloyloxyethoxyphenyl)propane, 2-(4-(meth)acryloyloxydiethoxyphenyl)-2-(4-(meth)acryloyloxytriethoxyphenyl)propane, 2-(4-(meth)acryloyloxydipropoxyphenyl)-2-(4-(meth)acryloyloxytriethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxypropoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxyisopropoxyphenyl)propane, and 1,4-bis(2-(meth)acryloyloxyethyl)pyromellitate. Among these, Bis-GMA and 2,2-bis(4-(meth)acryloyloxypolyethoxyphenyl)propane are preferred due to their excellent polymerizability and the mechanical strength of the resulting zirconia molded articles. Among 2,2-bis(4-(meth)acryloyloxypolyethoxyphenyl)propane, 2,2-bis(4-methacryloyloxypolyethoxyphenyl)propane (average number of moles of ethoxy groups added: 2.6, commonly known as "D-2.6E") are preferred.

[0081] Examples of bifunctional aliphatic compounds include glycerol di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, butylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, and 1,4-butanediol di(meth)acrylate. Examples of (meth)acrylates include ethyl dimethacrylate, 1,6-hexanediol dimethacrylate, 2-ethyl-1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,2-bis(3-methacryloyloxy-2-hydroxypropoxy)ethane, and 2,2,4-trimethylhexamethylenebis(2-carbamoyloxyethyl) dimethacrylate (commonly known as "UDMA"). Among these, triethylene glycol dimethacrylate (commonly known as "TEGDMA") and UDMA are preferred in terms of their excellent polymerizability and the mechanical strength of the resulting zirconia molded articles.

[0082] Examples of compounds with three or more functionalities include (meth)acrylates such as trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, trimethylolmethane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, N,N-(2,2,4-trimethylhexamethylene)bis[2-(aminocarboxy)propane-1,3-diol]tetra(meth)acrylate, and 1,7-diacryloxy-2,2,6,6-tetra(meth)acryloyloxymethyl-4-oxaheptane. Among these, N,N-(2,2,4-trimethylhexamethylene)bis[2-(aminocarboxy)propane-1,3-diol]tetramethacrylate and 1,7-diacryoyloxy-2,2,6,6-tetraacryloyloxymethyl-4-oxaheptane are preferred in terms of their excellent polymerizability and the mechanical strength of the resulting zirconia molded articles.

[0083] In any of the methods described above, polymerization of the composition is preferably carried out using a polymerization initiator, and the composition is preferably further comprising a polymerization initiator. There are no particular restrictions on the type of polymerization initiator, but photopolymerization initiators are particularly preferred. As the photopolymerization initiator, it can be appropriately selected from photopolymerization initiators used in general industry, and among these, photopolymerization initiators used in dental applications are preferred. Specific examples of photopolymerization initiators are the same as those described above in the explanation of gel casting.

[0084] When manufacturing a zirconia molded body using a stereolithography method with a zirconia composition after pulverization, there are no particular restrictions on the specific method of stereolithography, and known methods can be appropriately employed for stereolithography. For example, a method can be employed in which a target zirconia molded body is obtained by sequentially forming each layer having the desired shape by photopolymerizing a liquid composition with ultraviolet light, a laser, etc., using a stereolithography apparatus.

[0085] To further improve the density of the cured zirconia molded body, the zirconia molded body may be subjected to humidification treatment followed by CIP treatment. If press molding is performed, the zirconia composition may be subjected to humidification treatment before press molding. The humidification treatment can be performed using any known method without limitation, including spraying water with a spray bottle or using a constant humidity chamber or constant temperature and humidity chamber. The amount of moisture increase due to the humidification treatment depends on the median diameter D of the zirconia particles contained. 50 , median diameter D of stabilizer particles 50 Depending on various factors, the moisture content is preferably more than 2% by mass, more preferably more than 3% by mass, even more preferably more than 4% by mass, particularly preferably more than 5% by mass, and preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 11% by mass or less. The increase in moisture content due to the humidification treatment can be calculated as a percentage by subtracting the mass of the powder before wetting and the molded body from the mass of the powder after wetting and the molded body, and then dividing the result by the mass of the powder before wetting and the molded body. The pressure for the CIP treatment is the same as described above in the explanation of press molding.

[0086] (f) Process for additive manufacturing of granules containing zirconia particles and yttria particles. There are no particular restrictions on the specific method for manufacturing granules containing zirconia particles and yttria particles. For example, a method can be employed in which a slurry is obtained and then dried in a spray dryer to form granules, and the resulting granules can be used for powder bed additive manufacturing. There are no particular restrictions on the powder bed additive manufacturing method, but examples include the powder bed method, SLS method (selective laser sintering method), SLM method (selective laser melting method), electron beam method, arc discharge method, and binder jet method. For methods in which it is preferable not to include organic matter during additive manufacturing, it is preferable not to include organic matter in the granule manufacturing stage either.

[0087] Next, a calcination process is performed to calcin the molded body, thereby obtaining the calcined zirconia body of this disclosure.

[0088] The calcination temperature (maximum calcination temperature) in the calcination process is preferably above 900°C, more preferably above 920°C, even more preferably above 940°C, and particularly preferably above 950°C, in order to ensure a semi-sintered state using the specific zirconia composition described above, promote grain growth, increase particle size, or make phase transitions less likely. Furthermore, the calcination temperature is preferably 1200°C or lower, more preferably 1150°C or lower, even more preferably 1100°C or lower, and particularly preferably 1050°C or lower, in order to ensure processability. In other words, the calcination temperature in the method for producing a calcined zirconia body according to this disclosure is preferably above 900°C and 1200°C or lower.

[0089] The holding time (anchoring time) at the maximum calcination temperature is not particularly limited as long as a zirconia calcined body can be obtained that is in a semi-sintered state and has a coefficient of variation in the tetragonal ratio measured by Rietveld analysis within a predetermined range, but it is preferable to hold it at the maximum calcination temperature for 1.5 to 6 hours. Furthermore, the heating rate to the maximum calcination temperature and the cooling rate from the maximum calcination temperature are preferably 300°C / min or less. In one embodiment, there are no particular restrictions on the heating rate to the maximum calcination temperature when calcining the zirconia molded body of this disclosure, but it is preferably 0.1°C / min or more, more preferably 0.2°C / min or more, even more preferably 0.5°C / min or more, and also preferably 50°C / min or less, more preferably 30°C / min or less, and even more preferably 20°C / min or less. Productivity is improved when the heating rate is above the above lower limit. Furthermore, by keeping the heating rate below the above upper limit, the volume difference between the inside and outside of the zirconia molded body or calcined zirconia body can be suppressed, and if the zirconia molded body contains organic matter, the rapid decomposition of such organic matter can be suppressed, thereby preventing cracks and fractures.

[0090] [Zirconia Sintered Body] Next, we will explain the zirconia sintered body. The zirconia sintered body is obtained by firing the calcined zirconia body obtained as described above.

[0091] The content of the stabilizer (preferably yttria) in the zirconia sintered body of this disclosure is the same as the content of the stabilizer in the calcined zirconia body.

[0092] In the zirconia sintered bodies of this disclosure, the coefficient of variation in the tetragonal ratio measured by Rietveld analysis of each of the three types of sintered bodies is within a specific range. In particular, variations in mechanical strength are suppressed in zirconia sintered bodies produced by firing at a maximum sintering temperature of 1400°C or higher (for example, 1550°C), thus ensuring a consistent mechanical strength.

[0093] The zirconia sintered body of this disclosure may contain a fluorescent agent. The fluorescent agent is the same as the fluorescent agent in the calcined zirconia body. In this specification, the phrase "for 100% by mass of zirconia contained in the calcined zirconia body" can be read as "for 100% by mass of zirconia contained in the zirconia sintered body."

[0094] The zirconia sintered body of this disclosure may contain a coloring agent. Examples of coloring agents include those used in calcined zirconia bodies. There are no particular restrictions on the coloring agent content in the zirconia sintered body, and it can be appropriately adjusted depending on the type of coloring agent and the intended use of the zirconia sintered body. However, from the viewpoint of suitability for use as a dental prosthesis, the coloring agent content is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, and even more preferably 0.01% by mass or more, based on 100% by mass of zirconia contained in the zirconia sintered body, in terms of oxides of the metal elements contained in the coloring agent. Furthermore, the coloring agent content is preferably 5% by mass or less, more preferably 1% by mass or less, even more preferably 0.5% by mass or less, 0.1% by mass or less, and even more preferably 0.05% by mass or less, based on oxides of the metal elements contained in the coloring agent.

[0095] To adjust the translucency of the zirconia sintered body of this disclosure, the zirconia sintered body of this disclosure may contain a translucency modifier. Examples of translucency modifiers include those similar to those used in calcined zirconia bodies. There are no particular restrictions on the content of the translucency modifier in the zirconia sintered body, and it can be appropriately adjusted depending on the type of translucency modifier and the intended use of the zirconia sintered body. However, from the viewpoint of being suitable for use as a dental prosthesis, it is preferable that the content is 0.1% by mass or less relative to 100% by mass of zirconia contained in the zirconia sintered body.

[0096] [Method for Manufacturing a Zirconia Sintered Body] A method for manufacturing a zirconia sintered body according to the present disclosure includes firing the zirconia calcined body described above. A preferred manufacturing method includes a step of firing the zirconia calcined body under normal pressure at a maximum sintering temperature of over 1200°C and up to 1650°C. The zirconia calcined body according to the present disclosure is within the range of coefficients of variation for the predetermined tetragonal ratio described above, thereby suppressing variations in mechanical strength and making it possible to easily manufacture a zirconia sintered body according to the present disclosure that has excellent mechanical strength.

[0097] When manufacturing a sintered body by firing the zirconia calcined body of this disclosure, the maximum sintering temperature is preferably such that both light transmission and mechanical strength are achieved in the zirconia sintered body. From this viewpoint, the maximum sintering temperature is preferably above 1200°C, more preferably above 1250°C, and even more preferably above 1300°C, from the viewpoint that the desired zirconia sintered body can be easily obtained under normal pressure. Furthermore, the maximum sintering temperature is preferably 1650°C or lower, more preferably 1600°C or lower, even more preferably 1550°C or lower, particularly preferably 1500°C or lower, and most preferably 1450°C or lower. One embodiment is a method for manufacturing a zirconia sintered body that includes a firing step in which the maximum sintering temperature is between 1450°C and 1650°C, from the viewpoint that it is easy to suppress variations in the mechanical strength of the zirconia sintered body. By having the maximum sintering temperature above the lower limit and below the upper limit, sintering can proceed sufficiently, and a dense zirconia sintered body can be easily obtained. Furthermore, by keeping the maximum sintering temperature below the above upper limit, deactivation of the fluorescent agent can be suppressed.

[0098] The holding time at the maximum sintering temperature is preferably 15 minutes or more, more preferably 30 minutes or more, and even more preferably 40 minutes or more. Furthermore, the holding time is preferably 20 hours or less, more preferably 5 hours or less, and even more preferably 2 hours or less.

[0099] In the firing process, the heating rate and cooling rate should preferably be set to minimize the time required for the firing process. For example, the heating rate can be set to reach the maximum sintering temperature in the shortest possible time, depending on the performance of the firing furnace. The heating rate to the maximum sintering temperature can be, for example, 10°C / min or more, 50°C / min or more, 100°C / min or more, 120°C / min or more, 150°C / min or more, 200°C / min or more, 250°C / min or more, 300°C / min or more, or 350°C / min or more. The cooling rate should preferably be set to a rate that prevents defects such as cracks from occurring in the sintered body. For example, after heating is complete, the sintered body can be allowed to cool at room temperature.

[0100] Calcination and firing in this disclosure can be carried out using a firing furnace. There are no particular restrictions on the type of firing furnace; for example, electric furnaces and degreasing furnaces used in general industry can be used. Commercially available dental firing furnaces (for example, trade name "Sintra CS" (manufactured by Shenpaz)) may also be used.

[0101] The zirconia sintered body of this disclosure can be manufactured without HIP treatment, but as an optional step, further improvements in light transmittance and mechanical strength can be achieved by performing HIP treatment after sintering under atmospheric pressure. In the following, the sintered body obtained by sintering at the above-mentioned maximum sintering temperature (sintered body before HIP treatment) will be referred to as the "primary sintered body," and the sintered body after HIP treatment will be referred to as the "HIP-treated sintered body." HIP treatment can be performed using a known hot isohydrostatic press (HIP) apparatus.

[0102] When performing HIP treatment on a primary sintered body, the HIP treatment pressure is not particularly limited, but a dense sintered body with high mechanical strength can be obtained. Therefore, the HIP treatment pressure is preferably 100 MPa or higher, more preferably 125 MPa or higher, and even more preferably 130 MPa or higher. Furthermore, there is no particular upper limit to the HIP treatment pressure, but it can be, for example, 400 MPa or less, 300 MPa or less, or even 200 MPa or less.

[0103] When the primary sintered body is subjected to HIP treatment, the heating rate is not particularly limited, but is preferably 0.1°C / min or higher, more preferably 0.2°C / min or higher, and even more preferably 0.5°C / min or higher. Furthermore, the heating rate is preferably 50°C / min or lower, more preferably 30°C / min or lower, and even more preferably 20°C / min or lower. Productivity is improved when the heating rate is above the above lower limit.

[0104] When the primary sintered body is subjected to HIP treatment, the HIP treatment time (the time during which the maximum pressure and temperature are maintained) is not particularly limited. Since a dense zirconia sintered body with high mechanical strength can be obtained, the HIP treatment time is preferably 5 minutes or more, more preferably 10 minutes or more, and even more preferably 30 minutes or more. Furthermore, the HIP treatment time is preferably 10 hours or less, more preferably 6 hours or less, and even more preferably 3 hours or less.

[0105] In the method for manufacturing a zirconia sintered body according to the present disclosure, when the primary sintered body is subjected to HIP treatment, the pressure medium is not particularly limited, and from the viewpoint of minimizing the impact on zirconia, at least one can be selected from the group consisting of oxygen gas, oxygen mixed gas, air, and inert gas (e.g., nitrogen gas, argon gas, etc.). When the primary sintered body is subjected to HIP treatment under an oxygen mixed gas atmosphere, the oxygen concentration is not particularly limited, but can be, for example, greater than 0% and 20% or less. When using an oxygen mixed gas, at least one inert gas (e.g., nitrogen gas, argon gas, etc.) can be selected as the gas other than oxygen.

[0106] If the HIP treatment is performed in a reducing atmosphere, such as by using an inert gas, blackening may occur due to oxygen vacancies. In that case, in order to remove the blackening, it is preferable to include a step of heat treatment at 1650°C or lower in air or an oxygen-rich atmosphere (hereinafter also referred to as "tempering treatment") after the HIP treatment step, and it is more preferable to perform the heat treatment in an oxygen-rich atmosphere from the viewpoint of efficient heat treatment. An oxygen-rich atmosphere means that the oxygen concentration is higher than the oxygen concentration in the air. The oxygen-rich atmosphere is not particularly limited as long as the oxygen concentration is between 21% and 100%, and can be appropriately selected from this range. For example, the oxygen concentration may be 100%.

[0107] The zirconia sintered body of this disclosure is not particularly limited as long as it achieves the effects of this disclosure, and may be a primary sintered body, a HIP-treated sintered body, or a sintered body after tempering treatment.

[0108] Depending on the aesthetics of the zirconia sintered body (e.g., the shade of the dental prosthesis), the temperature of the heat treatment in air or an oxygen-rich atmosphere can be appropriately changed. In one embodiment, from the viewpoint of aesthetics of the zirconia sintered body, the temperature of the heat treatment in air or an oxygen-rich atmosphere is preferably 1650°C or lower, more preferably 1600°C or lower, and even more preferably 1550°C or lower. In another embodiment, from the viewpoint of aesthetics of the zirconia sintered body, the temperature of the heat treatment in air or an oxygen-rich atmosphere is preferably 1400°C or lower, more preferably 1300°C or lower, and even more preferably 1200°C or lower. Furthermore, in all embodiments, the temperature of the heat treatment is preferably 500°C or higher, more preferably 600°C or higher, and even more preferably 700°C or higher.

[0109] For the tempering process, a general-purpose furnace for dental zirconia can be used. Commercially available furnaces for dental zirconia may also be used. Examples of commercially available furnaces include Noritake Katana (registered trademark) F-1, F-1N, F-2, and F-2N (all manufactured by SK Medical Electronics Co., Ltd.).

[0110] The zirconia sintered body obtained by firing the zirconia calcined body of this disclosure can be suitably used in dental products. Examples of dental products include copings, frameworks, crowns, crown bridges, abutments, implants, implant screws, implant fixtures, implant bridges, implant bars, brackets, denture bases, inlays, onlays, orthodontic wires, and laminate veneers. By using the zirconia calcined body of this disclosure for components such as implant screws and implant fixtures, discoloration of the gingiva caused by metal when using metal materials can be suppressed, resulting in superior aesthetics. Furthermore, while an appropriate manufacturing method can be selected depending on the application, for example, dental products can be obtained by machining the zirconia calcined body of this disclosure and then firing it. It is preferable to use a CAD / CAM system in the machining process. The CAD / CAM system is not particularly limited, and known devices can be used. Examples of known devices include CAD / CAM systems (such as the "Katana® CAD / CAM System," manufactured by Kuraray Noritake Dental Co., Ltd.).

[0111] This disclosure includes, to the extent that it achieves the effects relating to this disclosure, embodiments that combine all or part of the above-described configurations in various ways, within the scope of the technical idea of ​​this disclosure. References to “one embodiment / aspect” or “embodiment / aspect” in this specification mean that certain features, structures, or characteristics described in relation to that embodiment / aspect are included in at least one embodiment / aspect of this disclosure. The use of the phrases “one embodiment / aspect” or “another embodiment / aspect” in various parts of this specification does not necessarily refer to the same embodiment / aspect, nor are separate or alternative embodiments / aspects mutually exclusive with other embodiments / aspects. Furthermore, various features are described that are shown in some embodiments / aspects but not in others. Similarly, various requirements are described that are required in some embodiments / aspects but not in others. Embodiments and aspects are, in some cases, interchangeable.

[0112] Next, the present disclosure will be described in more detail with reference to examples, but the present disclosure is not limited in any way by these examples, and many modifications are possible by those who are ordinary skill in the art within the scope of the technical idea of ​​the present disclosure.

[0113] [Example 1] Commercially available yttria powder was added to water. This and zirconia grinding media (diameter: 2 mm) were placed in a ball mill container, and the grinding process was carried out in the ball mill for 20 hours to obtain an yttria slurry. The median diameter D of the obtained slurry 50 The yttria slurry was less than 300 nm. The yttria slurry and commercially available zirconia powder (Y2O3: 0 mol%; monoclinic zirconia) were collected in a mol ratio of 4.5:95.5. These, along with water and zirconia grinding media (diameter: 2 mm), were placed in a ball mill container, and the grinding process was performed in the ball mill for 20 hours to obtain a slurry. The median diameter D of the obtained slurry was... 50 The wavelength was less than 200 nm.

[0114] Next, an organic binder was added to the obtained slurry and mixed and stirred. The stirred slurry was dried and granulated using a spray dryer to obtain a powder. This powder was poured into a columnar mold and subjected to uniaxial pressure pressing at a pressure of 33 MPa, followed by further CIP treatment at 190 MPa to obtain a disc-shaped molded body. The obtained molded body was placed in an electric furnace, heated from room temperature at 10°C / min, held at 950°C for 2 hours, and slowly cooled at 10°C / min to obtain a calcined zirconia body.

[0115] Furthermore, by sintering this calcined zirconia body under atmospheric pressure at a heating rate of 10°C / min and at three maximum sintering temperatures of 1450°C, 1500°C, and 1550°C, with a holding time of 2 hours in each case, zirconia sintered bodies containing 4.5 mol% of three types of yttria were obtained. All of the obtained zirconia sintered bodies were white.

[0116] [Example 2] An yttria slurry similar to that prepared in Example 1 and commercially available zirconia powder (Y2O3: 0 mol%; monoclinic zirconia) were added to water. These, along with zirconia grinding media (diameter: 2 mm), were placed in a ball mill container, and the grinding process was carried out in the ball mill for 20 hours to obtain a slurry. Subsequently, the obtained slurry and zirconia grinding media (diameter: 0.1 mm) were placed in a bead mill container, and the grinding process was carried out in the bead mill for 0.5 hours to obtain a slurry. The median diameter D of the obtained slurry 50 The wavelength was 200 nm or less. The method for obtaining a sintered body from the obtained slurry was the same as in Example 1.

[0117] [Example 3] Commercially available zirconia powder (Y2O3: 0 mol%; monoclinic zirconia) and commercially available yttria powder were added to water. These, along with zirconia grinding media (diameter: 2 mm), were placed in a ball mill container, and the grinding process was carried out in the ball mill for 20 hours to obtain a slurry. Subsequently, the obtained slurry and zirconia grinding media (diameter: 0.1 mm) were placed in a bead mill container, and the grinding process was carried out in the bead mill for 0.5 hours to obtain a slurry. The median diameter D of the obtained slurry 50 The nanoscale was less than 200 nm. Next, an organic binder was added to the obtained slurry and mixed and stirred. The stirred slurry was dried and granulated using a spray dryer to obtain a powder. This powder was poured into a columnar mold, uniaxially pressed at a pressure of 33 MPa, and then further subjected to CIP treatment at 190 MPa to obtain plate-shaped or disc-shaped molded bodies. The obtained molded bodies were placed in an electric furnace, heated from room temperature at a rate of 10°C / min, held at 500°C for 2 hours to degrease the organic components, held at 950°C for 2 hours, and slowly cooled at a rate of 10°C / min to obtain calcined zirconia bodies. Furthermore, these calcined zirconia bodies were sintered under atmospheric pressure at a heating rate of 10°C / min, at three maximum sintering temperatures of 1450°C, 1500°C, and 1550°C, with a holding time of 2 hours, to obtain zirconia sintered bodies containing 4.5 mol% of three types of yttria. All of the obtained zirconia sintered bodies were white.

[0118] [Examples 4-6] Example 4 was carried out in the same manner as Example 3, except that the calcination conditions were changed to those described in Table 1. Examples 5 and 6 were carried out in the same manner as Example 3, except that the yttria content was changed to those described in Table 1.

[0119] [Comparative Example 1] Commercially available yttria powder and commercially available zirconia powder (Y2O3:0 mol%; monoclinic zirconia) were collected in a mol ratio of 3.8:96.2, respectively. These were placed in a ball mill container along with water and zirconia grinding media (diameter: 2 mm), and the grinding process was carried out in the ball mill for 20 hours to obtain a slurry. The median diameter D of the obtained slurry was... 50The nanoscale was less than 200 nm. Next, an organic binder was added to the obtained slurry and mixed and stirred. The stirred slurry was dried and granulated using a spray dryer to obtain a powder. This powder was poured into a columnar mold, uniaxially pressed at a pressure of 33 MPa, and then further subjected to CIP treatment at 190 MPa to obtain a plate-shaped or disc-shaped molded body. The obtained molded body was placed in an electric furnace, heated from room temperature at a rate of 10°C / min, held at 500°C for 2 hours to degrease the organic components, held at 950°C for 2 hours, and slowly cooled at -10°C / min to obtain a calcined zirconia body. Furthermore, this calcined zirconia body was sintered under atmospheric pressure at a heating rate of 10°C / min, at three maximum sintering temperatures of 1450°C, 1500°C, and 1550°C, with a holding time of 2 hours, to obtain a zirconia sintered body containing 3.8 mol% of three types of yttria. All of the obtained zirconia sintered bodies were white. [Comparative Example 2] A zirconia sintered body containing 4.5 mol% yttria was obtained by mixing yttria powder and zirconia powder in a mol ratio of 4.5:95.5, except that the mixture was the same as in Comparative Example 1. [Comparative Example 3] A zirconia sintered body containing 6.0 mol% yttria was obtained by mixing yttria powder and zirconia powder in a mol ratio of 6.0:94.0, except that the mixture was the same as in Comparative Example 1. [Comparative Examples 4 and 5] In Comparative Example 4, "Zpex® 4" (Y2O3 content: 4 mol%, manufactured by Tosoh Corporation) was used as the raw material zirconia composition. In Comparative Example 5, "Zpex® Smile®" (Y2O3 content: 5.5 mol%, manufactured by Tosoh Corporation) was used as the raw material zirconia composition. These powders were poured into a columnar mold, uniaxially pressed at a pressure of 33 MPa, and then further subjected to CIP treatment at 190 MPa to obtain plate-shaped or disc-shaped molded bodies. The obtained molded bodies were placed in an electric furnace, heated from room temperature at a rate of 10°C / min, held at 500°C for 2 hours to degrease organic components, held at 950°C for 2 hours, and slowly cooled at a rate of 10°C / min to obtain calcined zirconia bodies. Furthermore, these calcined zirconia bodies were sintered under atmospheric pressure at a heating rate of 10°C / min, at three maximum sintering temperatures of 1450°C, 1500°C, and 1550°C, with a holding time of 2 hours, to obtain sintered zirconia bodies containing 4.5 mol% of three types of yttria.All of the obtained zirconia sintered bodies were white. [Comparative Example 6] Commercially available yttria powder and commercially available zirconia powder (Y2O3:0 mol%; monoclinic zirconia) were taken in a mol ratio of 4.5:95.5, respectively. These, along with water and zirconia grinding media (diameter: 0.1 mm), were placed in a bead mill container, and the grinding process was carried out in the bead mill for a grinding time of 1 hour to obtain a slurry. The median diameter D of the obtained slurry was... 50 The nanoscale was less than 200 nm. Next, an organic binder was added to the obtained slurry and mixed and stirred. The stirred slurry was dried and granulated using a spray dryer to obtain a powder. This powder was poured into a columnar mold, uniaxially pressed at a pressure of 33 MPa, and then further subjected to CIP treatment at 190 MPa to obtain plate-shaped or disc-shaped molded bodies. The obtained molded bodies were placed in an electric furnace, heated from room temperature at a rate of 10°C / min, held at 500°C for 2 hours to degrease the organic components, held at 875°C for 2 hours, and slowly cooled at a rate of 10°C / min to obtain calcined zirconia bodies. Furthermore, these calcined zirconia bodies were sintered under atmospheric pressure at a heating rate of 10°C / min, at three maximum sintering temperatures of 1450°C, 1500°C, and 1550°C, with a holding time of 2 hours, to obtain zirconia sintered bodies containing 4.5 mol% of three types of yttria. All of the obtained zirconia sintered bodies were white.

[0120] The following evaluations were performed on each slurry and the zirconia sintered bodies obtained in each example and comparative example.

[0121] <Median diameter D 50 Measurement method > Median diameter D of zirconia slurry and yttria slurry 50 The particle size distribution was measured using a dynamic light scattering laser diffraction / scattering particle size distribution analyzer (product name "SZ-100V2", manufactured by Horiba, Ltd.). A slurry diluted to approximately 0.1 wt% with a 0.2 wt% sodium hexametaphosphate aqueous solution was irradiated with ultrasound for 30 minutes, and the particle size distribution was measured based on volume to determine the median diameter D 50 The result was calculated.

[0122] <Evaluation of the abundance of crystalline systems in zirconia sintered bodies> The abundance of tetragonal zirconia in the zirconia sintered bodies obtained in each example and comparative example (tetragonal ratio) was determined by Rietveld analysis using XRD measurement and calculation. Specifically, for X-ray diffraction, measurements were performed using a fully automated horizontal multi-purpose X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation) and integrated X-ray analysis software (SmartLab Studio II, manufactured by Rigaku Corporation) under the following conditions, and the area intensity of each peak (peak area intensity I) was determined. X-ray source: Cu Kα (λ = 1.54186 Å) Goniometer length: 300 mm Optical system: Focused method Detector: High-speed 1D X-ray detector (D / teX Ultra250) Monochromatorization: Kβ filter Tube voltage: 40 kV Tube current: 30 mA Scan axis: 2θ / θ Measurement range (2θ): 5 to 90° Scan speed: 0.2° / min Sampling step: 0.01°

[0123] Furthermore, Rietveld analysis was performed using SmartLab Studio II to determine the crystalline phase of each sintered body. Then, the tetragonal ratio was calculated based on the following formula. Here, the area intensity of each peak was calculated by processing the background using polynomial approximation. f t (%) = I t / I total ×100 (1-1) (where f t The value represents the abundance (%) of the tetragonal crystal system, and I t This represents the area intensity of the peak around 2θ = 30.3° where the peak top of the main peak of the tetragonal system appears in XRD measurements, and I total This represents the sum of the area intensities of all peaks detected in the range of 2θ = 5 to 90° in the XRD measurement. The lattice constants for the tetragonal and cubic crystal systems used in the Rietveld analysis are as follows: Tetragonal system: Lattice constants a = b = 5.116 (Å), c = 5.157 (Å) α = β = γ = 90° Cubic system: Lattice constants a = b = c = 5.130 (Å) α = β = γ = 90°

[0124] <Coefficient of Variation of Tetragonal Ratio in Zirconia Sintered Bodies> Ten samples were prepared for each of three types of sintered materials, each sintered at 1450°C, 1500°C, and 1550°C. X-ray diffraction was performed at three randomly selected locations on each sample to measure the tetragonal ratio. In this way, the tetragonal ratio was measured at 30 points at each temperature, and the arithmetic mean (M) and standard deviation (σ) were calculated. From these arithmetic mean (M) and standard deviation (σ), the coefficient of variation (CV) of the tetragonal ratio was calculated using the following formula. The results are shown in Table 1. CV = σ / M

[0125] <Evaluation of the coefficient of variation of biaxial bending strength in zirconia sintered bodies> The biaxial bending strength of zirconia sintered bodies was measured in accordance with ISO 6872:2015. Based on the methods of each example and comparative example, a Φ18 mm × 1.4 mm disc-shaped molded body was calcined to obtain a Φ17 mm × 1.3 mm calcined zirconia body. The obtained calcined bodies were sintered at temperatures of 1450°C, 1500°C, and 1550°C to obtain three types of sintered bodies with different sintering temperatures and a diameter of Φ15 mm × 1.2 mm. These sintered bodies were measured using a universal testing machine (product name "AG-I 100kN", manufactured by Shimadzu Corporation) under conditions of a crosshead speed of 0.5 mm / min. The results are shown in Table 1.

[0126] Ten samples were prepared for each of three different sintering temperatures: 1450°C, 1500°C, and 1550°C. The arithmetic mean (M) and standard deviation (σ) of the biaxial bending strength at each temperature were calculated. The coefficient of variation (CV) was calculated from these arithmetic mean (M) and standard deviation (σ) using the following formula. The results are shown in Table 1. CV = σ / M

[0127] In the table, the yttria content in the calcined zirconia represents the ratio (mol%) of moles of yttria to the total moles of zirconia and yttria.

[0128] From the above results, it was confirmed that the zirconia calcined body of this disclosure yields a zirconia sintered body with suppressed variations in mechanical strength. In the comparative examples, the coefficient of variation in the tetragonal ratio measured by Rietveld analysis was not within the desired range, and therefore the desired physical properties could not be obtained. In Comparative Examples 1 to 3, the grinding treatment was insufficient, and the phase transition could not be suppressed. As a result, the coefficient of variation in the tetragonal ratio measured by Rietveld analysis was 0.10, 0.40, and 0.24, and it was confirmed that the mechanical strength also varied.

[0129] The zirconia calcined material of this disclosure is useful for the manufacture of dental products (particularly for the manufacture of dental products used in dental treatments).

Claims

1. A zirconia calcined body comprising zirconia and a stabilizer capable of suppressing the phase transition of zirconia, wherein the zirconia calcined body is heated from room temperature at a heating rate of 10°C / min to a maximum sintering temperature, and the maximum sintering temperature is set to three different temperatures: 1450°C, 1500°C, and 1550°C, and the sintered body is fired at each maximum sintering temperature for 2 hours, and the coefficient of variation of the tetragonal ratio measured by Rietveld analysis using X-ray diffraction is less than 0.10 in three types of sintered bodies obtained.

2. The zirconia calcined body according to claim 1, wherein the stabilizing agent is yttria.

3. The calcined zirconia body according to claim 2, wherein the yttria content is 2.0 to 10 mol% relative to the total mol of zirconia and yttria.

4. A method for producing a calcined zirconia body according to claim 1, comprising the steps of: producing a zirconia composition containing zirconia particles and stabilizer particles capable of suppressing the phase transition of zirconia; a grinding step of grinding the zirconia composition; a molding step of molding the zirconia composition after grinding to obtain a molded body; and a calcination step of calcining the molded body, wherein the grinding step is (i) using grinding media with a diameter of less than 1 mm and a grinding time of 10 minutes or more, and / or (ii) using grinding media with a diameter of 1 mm or more and a grinding time of 20 hours or more (except in cases where only the grinding treatment of (ii) is performed on the zirconia composition for 20 hours or less as the grinding treatment), and the maximum calcination temperature in the calcination step is greater than 900°C and less than or equal to 1200°C.

5. The method for producing a calcined zirconia body according to claim 4, wherein the grinding step includes grinding the zirconia composition and further grinding the stabilizer particles, and the grinding step for grinding the stabilizer particles is (i) using grinding media with a diameter of less than 1 mm and a grinding time of 10 minutes or more, and / or (ii) using grinding media with a diameter of 1 mm or more and a grinding time of 20 hours or more.

6. The method for producing a calcined zirconia body according to claim 5, wherein the step of pulverizing the stabilizer particles is performed before the step of producing the zirconia composition.

7. A method for producing a calcined zirconia body according to claim 4 or 5, wherein the calcination time in the calcination step is (i) 2.5 hours or more when the maximum calcination temperature is greater than 900°C but less than 1000°C, or (ii) 1.5 hours or more when the maximum calcination temperature is greater than 1000°C but 1200°C or less.

8. A method for producing a zirconia sintered body, comprising firing the zirconia calcined body according to claim 1 or 2 at atmospheric pressure and a maximum sintering temperature of over 1200°C and up to 1650°C.

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