Dental oxide ceramic calcined body with good polishability and its manufacturing method

Abstract

The present invention provides: a partially sintered body comprising an oxide ceramic which has excellent polishing properties and has a favorable aesthetic appearance, since the polished surface of the partially sintered body and the surface of the sintered body thereof following sintering are very smooth; and a method for producing the same.　The present invention pertains to an oxide ceramic partially sintered body for dental use which contains oxide ceramic particles having an average roundness of the primary particles thereof of 0.81 or higher, and has a relative density of 43-63%.

Description

[Technical Field]

[0001] The present invention relates to a dental oxide ceramic calcined body containing an oxide ceramic that can be polished well with a tool, and a method for producing the same. [Background technology]

[0002] In recent years, sintered oxide ceramics have become popular as dental materials. Sintered dental materials are used with precise dimensions and surface processing to match the patient's clinical area. Machining such as CAD / CAM is used to process them into the desired shape.

[0003] Oxide ceramics such as aluminum oxide (alumina) and zirconium oxide (zirconia) are used in dental materials. Zirconia in particular is strong and has relatively good aesthetics, so demand for it is increasing, especially as prices have fallen in recent years.

[0004] However, zirconia sintered bodies have problems in terms of productivity and cost, such as being too hard to machine, cracking during machining, taking a long time to machine, and requiring frequent replacement of machining tools.

[0005] Therefore, in general, instead of machining a zirconia sintered body, a semi-sintered zirconia calcined body is machined into a cut or ground body that approximates a desired shape, such as a shape that resembles a tooth or a part of a tooth, and the resulting cut or ground body is fired at a temperature equal to or higher than the sintering temperature, thereby obtaining a zirconia sintered body having a desired shape. The zirconia calcined body is obtained by molding a raw material powder into a shape such as a disk or a rectangular parallelepiped, and firing the molded body at a temperature range that does not reach sintering (hereinafter also referred to as "calcination").

[0006] On the other hand, with regard to oxide ceramics other than zirconia, those using alumina have been proposed, for example, in Patent Documents 1 to 3. Alumina has a refractive index different from that of zirconia and is advantageous in terms of translucency after sintering, and is widely used as a sintered body, except for porous bodies such as heat insulating materials, so that a calcined body is not required, and therefore a sintered body is generally obtained by sintering a molded body.

[0007] The resulting sintered oxide ceramic body is generally subjected to a polishing operation to obtain surface smoothness from the viewpoint of dental aesthetics.

[0008] However, due to the high hardness of the sintered body, polishing takes a long time. Also, if the surface of the sintered body is chipped during polishing, it must be re-prepared. Therefore, there is room for improvement in terms of productivity and economy. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-37537 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-180275 [Patent Document 3] International Publication No. 2009 / 045940 Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, the inventors discovered that if the calcined body can be polished at the stage of calcination and the desired surface properties (for example, flatness after polishing) can be imparted, the time and risk involved in polishing after sintering can be reduced.

[0011] On the other hand, one possible way to improve the polishability of the calcined body is to reduce the resistance during polishing by lowering the relative density of the calcined body. However, if the relative density is lowered too much, the calcined body may become too soft and break, and / or its sintering ability may decrease, resulting in a problem that the relative density does not improve after sintering.

[0012] For example, Patent Document 1 discloses a manufacturing method for molding and polishing bioactive alumina, and describes surface polishing of an alumina sintered body. However, polishing a sintered body takes time, and there is a concern that some particles may fall off, which may actually reduce smoothness. Furthermore, as mentioned above, alumina is often used as a sintered body, except for porous bodies such as thermal insulation materials, and therefore does not need to be calcined. Therefore, Patent Document 1 does not consider smoothing the surface with a calcined body before sintering.

[0013] Furthermore, Patent Document 2 describes a method for producing an alumina sintered body, in which a compact obtained from an alumina powder having an average particle size of 0.2 to 1.0 μm is fired at 1480 to 1600°C. However, Patent Document 2 does not suggest dental applications, and does not consider calcined bodies with high abrasive properties. Furthermore, Patent Document 2 does not consider the circularity of the particles, and the calcined bodies produced were not good at abrasive properties due to the large particle size.

[0014] Furthermore, although Patent Document 3 describes the hardness and porosity of abrasive grains suitable for polishing, it does not consider the object to be polished, and even if the abrasive grain surface is considered to be a calcined body, it cannot be said that the appropriate conditions for smoothing the surface of the calcined body (abrasive grain surface) have been found.

[0015] Patent Documents 1 to 3 do not consider polishing the calcined body to make it smooth before sintering, and it was not possible to obtain a smooth sintered body in an economical and easy manner.

[0016] Therefore, an object of the present invention is to provide an oxide ceramic calcined body that has excellent polishability and is aesthetically pleasing because the polished surface of the calcined body and the surface of the sintered body after sintering have high flatness, and to provide a method for producing the same. [Means for solving the problem]

[0017] As a result of extensive research to solve the above problems, the inventors discovered that a calcined oxide ceramic body containing oxide ceramic particles having an average primary particle circularity of 0.81 or more and a relative density of 43 to 63% has high polishability. Based on this finding, the inventors conducted further research and completed the present invention.

[0018] That is, the present invention includes the following inventions. [1] A dental oxide ceramic calcined body containing oxide ceramic particles having an average primary particle circularity of 0.81 or more and having a relative density of 43 to 63%. [2] The dental oxide ceramic calcined body according to [1], wherein the particles have an average primary particle size of 30 to 600 nm. [3] The dental oxide ceramic calcined body according to [1] or [2], which has a three-point bending strength of 10 to 50 MPa. [4] The dental oxide ceramic calcined body according to any one of [1] to [3], which has a surface roughness Ra of 1.70 μm or less after polishing. [5] The dental oxide ceramic calcined body according to any one of [1] to [4], which has a surface roughness Rz of 54 μm or less after polishing. [6] The dental oxide ceramic calcined body according to any one of [1] to [5], wherein the oxide ceramic particles contain zirconia and / or alumina. [7] The dental oxide ceramic calcined body according to [6], wherein the alumina contains α-alumina particles with a purity of 99.5% or more. [8] The dental oxide ceramic calcined body according to [6] or [7], further comprising a sintering aid, wherein the sintering aid comprises at least one element selected from the group consisting of Group 2 elements, Ce, Zr, and Y. [9] A dental oxide ceramic calcined body according to any one of [1] to [8], which has a surface roughness Ra of 1.40 μm or less after being fired under atmospheric pressure to form a sintered body without using hot isostatic pressing.

[10] A dental oxide ceramic calcined body according to any one of [1] to [9], which has a surface roughness Rz of 51 μm or less after being fired under atmospheric pressure to form a sintered body without using hot isostatic pressing.

[11] A method for producing a dental oxide ceramic calcined body, comprising: The method includes a step of press-molding an oxide ceramic composition at a surface pressure of 20 to 600 MPa, and a step of firing the obtained molded body at 400°C or higher and lower than 1200°C under atmospheric pressure, A method for producing a dental oxide ceramic calcined body, wherein the dental oxide ceramic calcined body contains oxide ceramic particles having an average primary particle circularity of 0.81 or more and has a relative density of 43 to 63%.

[12] The method for producing a dental oxide ceramic calcined body according to

[11] , wherein the oxide ceramic particles contain zirconia and / or alumina.

[13] The method for producing a dental oxide ceramic calcined body according to

[12] , wherein the alumina contains α-alumina particles with a purity of 99.5% or more.

[14] A method for producing a dental oxide ceramic calcined body according to any one of

[11] to

[13] , further comprising a sintering aid, wherein the sintering aid comprises at least one element selected from the group consisting of Group 2 elements, Ce, Zr, and Y.

[15] A method for producing a sintered dental oxide ceramic body, comprising a step of polishing the calcined dental oxide ceramic body according to any one of [1] to

[10] with a dental polishing tool.

[16] A method for producing a dental oxide ceramic sintered body according to

[15] , comprising the step of sintering the dental oxide ceramic calcined body under atmospheric pressure without using a hot isostatic pressing treatment. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide an oxide ceramic calcined body that has excellent polishability and is aesthetically pleasing because the polished surface of the calcined body and the surface of the sintered body after sintering are highly flat, and a method for producing the same. Furthermore, according to the present invention, in the surface processing of dental materials, the surface of a calcined body that has been cut or ground by CAD / CAM or the like can be easily polished before sintering, and it is possible to provide a calcined body with good smoothness whose surface can be easily smoothed, as well as a sintered body with good smoothness, and a method for producing the same. In particular, alumina has a high hardness after sintering, and it is not easy to polish the surface after sintering. However, it is possible to provide a calcined body, a sintered body, and a method for producing the same that can be processed easily and precisely before sintering. According to the present invention, after the calcined body is processed into a crown shape, the surface is polished by polishing the side surface or the like, so that the sintered body also has excellent surface smoothness. Furthermore, the present invention can provide a zirconia calcined body having excellent machinability (cutting and grinding properties) and a method for producing the same. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is an optical microscope photograph of the surface of a polished calcined body according to Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0021] The dental oxide ceramic calcined body of the present invention has an average circularity of primary particles of 0.81 or more and a relative density of 43 to 63%.

[0022] The calcined body of the present invention will now be described. The calcined body can be a precursor (intermediate product) of a sintered body. In this specification, the term "calcined body" refers to a body in which oxide ceramic particles are necked (adhered together) and solidified in a state in which the oxide ceramic particles are not completely sintered together. The calcined body may have a predetermined shape (block shape (for example, disk shape, rectangular parallelepiped shape, etc.), dental product shape (for example, crown shape), etc.). The calcined body may be a processed body that has been processed into, for example, a tooth crown shape, and when processed, it is referred to as a "processed body" or a "cut or ground body." The processed body is obtained, for example, by processing a calcined oxide ceramic disk into a dental product (for example, a crown-shaped prosthesis) using a CAD / CAM (Computer-Aided Design / Computer-Aided Manufacturing) system.

[0023] The calcined body of the present invention contains an adhesion of particles made of oxide ceramics (hereinafter, sometimes simply referred to as "oxide ceramic particles"), and the polishability, machinability (cuttability and grindability), and surface properties (flatness of the calcined body and sintered body after polishing) change depending on the average circularity of the particles. If the average circularity of the oxide ceramic particles contained in the calcined body is 0.81 or more, the abrasiveness is high and the surface roughness Ra and / or Rz after polishing is low. On the other hand, if the average circularity is less than 0.81, the surface roughness Ra and / or Rz increases when the calcined body is polished, or the calcined body becomes too hard, extending the working time. The average circularity is preferably 0.82 or more, more preferably 0.83 or more, and even more preferably 0.84 or more. The method for measuring the average circularity is as described in the Examples below.

[0024] In the present invention, the surface roughness Ra means the arithmetic mean roughness Ra as defined in JIS B 0601:2013. The surface roughness Rz means the maximum height roughness Rz as defined in JIS B 0601:2013.

[0025] The alumina calcined body of the present invention is preferably one having a higher surface smoothness, since the aesthetic appearance after firing is enhanced. The surface roughness Ra of the alumina calcined body is preferably 1.70 μm or less, more preferably 1.65 μm or less, even more preferably 1.60 μm or less, and particularly preferably 1.50 μm or less. The surface roughness Rz of the calcined alumina body is preferably 54 μm or less, more preferably 50 μm or less, even more preferably 45 μm or less, and particularly preferably 35 μm or less. The surface roughnesses Ra and Rz refer to values ​​measured on the calcined alumina body after polishing. In the present invention, by adjusting the average circularity, it is possible to produce an alumina calcined body before polishing having a surface roughness Ra after polishing within the desired range (e.g., 1.70 μm or less), or an alumina calcined body before polishing having a surface roughness Rz after polishing within the desired range (e.g., 54 μm or less).

[0026] The surface roughness of the calcined alumina body of the present invention can be measured by a known method, such as a stylus method or laser interference method, or by taking a cross section and photographing it with an optical microscope or electron microscope and analyzing the image. Preferably, the surface roughnesses Ra and Rz are values ​​measured by the measurement method described in the Examples below.

[0027] The relative density of the calcined body of the present invention can be controlled by the manufacturing method described below. If the relative density is less than 43%, it means that the proportion of pores inside the calcined body is high, which reduces the number of contacts between particles inside the calcined body and makes the body too soft to be broken when polished, which is undesirable.Furthermore, the density variation inside the calcined body increases, which is undesirable. On the other hand, if the relative density exceeds 63%, it is not preferable because it is too hard and the working time is extended, and it is also not preferable because it may chip and the surface roughness Ra and / or Rz may increase.

[0028] When the relative density is 43 to 63%, the surface roughness Ra, Rz, and hardness when polishing the calcined body are appropriate, there is no increase in working time, the polished surface of the calcined body is highly flat, and the flatness remains high even after sintering, resulting in a sintered body with high aesthetic appeal. Furthermore, when the relative density is within the above range, the machinability (cuttability and grindability) is excellent, the probability of chipping (hereinafter referred to as "chipping rate") can be reduced, and the flatness of the sintered body can be maintained at a high level. The relative density is preferably 45 to 60%, more preferably 48 to 56%, in terms of achieving better effects of the present invention.

[0029] The relative density of the calcined body can be calculated from the porosity of the calcined body, and specifically, can be measured and calculated using a mercury porosimeter. The mercury porosimeter is preferably an apparatus capable of applying a mercury pressure of 15 to 30,000 psia, more preferably an apparatus capable of applying a mercury pressure of 0.5 to 60,000 psia. From the viewpoint of reducing measurement errors, the pressure resolution is preferably 0.1 psia or more. An example of a mercury porosimeter device is AutoPore (registered trademark) IV9500, manufactured by Micromeritics (USA).

[0030] The density of the calcined body refers to the density of the calcined body obtained by filling granules obtained by drying the raw materials into a specific mold (metal mold, etc.), forming a compact into a specific shape under pressure, heating it at a temperature at which organic components such as binders can be removed, and then heating it at a temperature at which yttria is suitably dissolved and necking (adhesion) is suitably formed. The temperature at which the organic components are removed is not particularly limited as long as it is a temperature at which the organic components such as binders can be removed, and may be selected depending on the type of organic components such as binders, and may be 150 to 500°C. The temperature at which necking (adhesion) is adequately formed is preferably 400°C or higher and lower than 1200°C. The firing temperature in the calcination step (hereinafter also referred to as "calcination temperature") will be described in detail.

[0031] Furthermore, the calcined body of the present invention contains adhered particles made of oxide ceramics (hereinafter, sometimes simply referred to as "oxide ceramic particles"), and the degree of adhesion varies depending on the average particle diameter of these particles, thereby changing the hardness of the calcined body.

[0032] The average primary particle size of the oxide ceramic particles contained in the calcined body is preferably 30 to 600 nm, more preferably 40 to 580 nm, further preferably 60 to 450 nm, and most preferably 80 to 350 nm. On the other hand, when the average primary particle diameter of the oxide ceramic particles is 600 nm or less, it is preferable because it is difficult to absorb small particles in the particle size distribution, so adhesion due to differences in particle size is unlikely to occur, hardness is unlikely to increase, working time is not increased, coarse particles are not present locally, chipping is minimal, and surface roughness Ra and / or Rz can be reduced. When the thickness is 30 nm or more, adhesion does not become too strong, the hardness of the calcined body does not increase, the working time does not increase, and the surface roughness Ra and / or Rz can be reduced, which is preferable. The method for measuring the average primary particle size in the calcined body is as described in the Examples below.

[0033] The calcined body of the present invention contains continuous pores (fine pores) inside, which ensures that when a polishing tool comes into contact with the calcined body, the pores provide room for particles to move, reducing polishing resistance and reducing the surface roughness Ra and / or Rz of the calcined body.

[0034] In the calcined body of the present invention, in terms of D10 and D90 in the cumulative pore distribution (cumulative distribution of pores), when D10 is 10 nm or more and D90 is 90 nm or less, tool wear and chipping rate can be reduced. In this specification, the median pore diameters corresponding to the 10% and 90% cumulative particle diameters from the smallest particle diameter side in the cumulative pore distribution are referred to as D10 and D90, respectively. The cumulative pore distribution including D10 and D90 can be measured by a method in accordance with JIS R 1655:2003.

[0035] In the cumulative pore distribution, when D10 is 10 nm or more, the voids do not become too small for particles having an average primary particle diameter of 30 to 600 nm, i.e., adhesion does not progress too much, the hardness of the calcined body can be reduced, and the working time does not increase. From the viewpoint of reducing the chipping rate and reducing the surface roughness, D10 is preferably 10 nm or more, more preferably 15 nm or more, and even more preferably 25 nm or more. Furthermore, when D90 in the cumulative pore distribution is 90 nm or less, there is no variation in density within the calcined body for particles with an average primary particle diameter of 30 to 600 nm, or the local presence of coarse particles can be suppressed, thereby further reducing the chipping rate during polishing and further reducing the surface roughness Ra and / or Rz. From the viewpoint of reducing the chipping rate, D90 is preferably 79 nm or less, more preferably 66 nm or less, and even more preferably 64 nm or less.

[0036] The BET specific surface area of ​​the calcined body of the present invention varies depending on the average circularity of the primary particle diameter of the oxide ceramic particles in the calcined body, the relative density, the average primary particle diameter, and the pore distribution. The BET specific surface area can be measured in accordance with JIS Z 8830: 2013. The BET specific surface area can be measured using a commercially available product such as a fully automatic specific surface area measuring device (trade name "Macsorb (registered trademark) HM model-1200", BET flow method (single-point method / multi-point method), manufactured by Mountech Co., Ltd.).

[0037] The BET specific surface area of ​​the calcined body of the present invention is 5 m 2 / g or more, and 7.5m 2 / g or more is more preferable, and 8m 2 It is more preferable that the saturation coefficient is 1 / g or more. 5m 2 When the average primary particle size is 1 / g or more, the average primary particle size is not too large, and an increase in the chipping rate can be suppressed, or excessive adhesion does not occur, and therefore an increase in working time can be suppressed. The BET specific surface area is 25m 2 / g or less, and 2 / g or less is more preferable, and 15m 2 It is more preferable that the saturation coefficient is 1 / g or less. The BET specific surface area is 25m 2When the average primary particle size is 0.1g or less, the calcined body does not become too hard, and the polishing time increases and / or the chipping rate is easily reduced, or the adhesion is not too small, and the occurrence of variations in density can be suppressed, and the chipping rate is easily reduced.

[0038] In the present invention, the "BET specific surface area" refers to a specific surface area measured without distinguishing between primary particles and secondary particles. In addition, the BET specific surface areas of the calcined body and a composition described later in the present invention are determined by subtracting the BET specific surface area of ​​the calcined body from the BET specific surface area of ​​the composition, and the difference is 10 m 2 / g or less is preferable because good polishing properties can be maintained.

[0039] A known device can be used to measure the BET specific surface area of ​​the calcined body. The BET specific surface area can be measured in accordance with JIS Z 8830:2013. The BET specific surface area can be measured using a commercially available product such as a fully automatic specific surface area measuring device (trade name "Macsorb (registered trademark) HM model-1200", BET flow method (single-point method / multi-point method), manufactured by Mountec Co., Ltd.).

[0040] The polishability of the calcined body of the present invention is also affected by the strength of the calcined body. The strength of the calcined body of the present invention can be evaluated, for example, by measuring the bending strength of the calcined body. The three-point bending strength of the calcined body of the present invention can be measured in accordance with JIS R 1601:2008.

[0041] The three-point bending strength of the calcined body is preferably 10 MPa or more, more preferably 18 MPa or more, and even more preferably 20 MPa or more, in order to ensure strength that allows machining. It is preferable that the three-point bending strength of the calcined body is 10 MPa or more, since this reduces the possibility that the calcined body will break during polishing. Furthermore, in order to facilitate polishing of the calcined body, the three-point bending strength of the calcined body is preferably 50 MPa or less, more preferably 45 MPa or less, even more preferably 40 MPa or less, and particularly preferably 35 MPa or less.

[0042] From the viewpoint of changing the polishability and / or hardness, the Vickers hardness of the calcined body of the present invention is preferably 350 HV 5 / 30 or less, more preferably 300 HV 5 / 30 or less, and even more preferably 100 HV 5 / 30 or less. When the Vickers hardness is 350HV 5 / 30 or less, the machinability (cutting and grinding properties) is excellent, the chipping rate is low, and the increase in tool wear can be suppressed. "HV 5 / 30" means the Vickers hardness when a load (test force) of 5 kgf is held for 30 seconds.

[0043] The calcined body of the present invention has a Vickers hardness within the above-mentioned range, thereby enabling a reduced chipping rate. The Vickers hardness is measured according to JIS Z 2244:2020.

[0044] Examples of methods for measuring Vickers hardness include the following. The calcined body is subjected to a load of 5 kgf for 30 seconds using a Falcon 500 manufactured by Innovatest, and the HV value can be calculated. For example, the average value of n=10 can be used.

[0045] The oxide ceramic particles used in the present invention are not particularly limited, and examples thereof include those containing zirconia, alumina, titania, silica, niobium oxide, tantalum oxide, yttria, etc. One type of oxide ceramic may be used alone, or two or more types may be used in combination. Among these, from the viewpoint of improving the aesthetics and strength of the sintered body as a dental material, those containing zirconia and / or alumina are preferred, and those containing zirconia and / or alumina as the main component are more preferred. Hereinafter, an embodiment in which the oxide ceramic particles contain alumina as a main component will be described, while also describing the case in which the oxide ceramic is zirconia as appropriate.

[0046] Among the oxide ceramics, compositions containing alumina as a main component are preferred because they enhance the aesthetics of the sintered dental material and have excellent chemical stability. Among them, α-alumina with a purity of 99.5% or more is more preferred because it contains few impurities, suppresses the formation of glass phases at grain boundaries due to impurities, prevents grain coarsening, and is less likely to reduce the aesthetics of the sintered dental material. Furthermore, using α-phase alumina (α-alumina), which is highly corrosive and stable at high temperatures, as the starting material is preferable because it allows the calcined body to be controlled homogeneously, making it easier to reduce the amount of tool wear or chipping rate, and also because it allows the grain size in the crystal structure within the sintered body to be densified. From the above viewpoints, it is particularly preferable that the alumina particles contained in the calcined body of the present invention include α-alumina particles with a purity of 99.5% or more.

[0047] This alumina raw material can be obtained by, for example, the alkoxide method, the modified Bayer method, the ammonium alum thermal decomposition method, the ammonium Dawsonite thermal decomposition method, etc., preferably the alkoxide method. The alkoxide method makes it easy to increase the purity of the alumina raw material powder and to uniformize the particle size distribution. Specifically, there is a method in which aluminum hydroxide obtained by hydrolyzing purified aluminum alkoxide is calcined in air at 1100°C or higher.

[0048] Examples of the alumina raw material include AA grade (α-alumina), AKP grade (α-alumina), or NXA grade (such as "NXA-100" and "NXA-150") (all of which are ultrafine α-alumina) with a purity of 99.99% or higher, manufactured by Sumitomo Chemical Co., Ltd.

[0049] In a preferred embodiment, the calcined oxide ceramic body of the present invention is an oxide ceramic calcined body containing alumina or zirconia.

[0050] The alumina calcined body of the present invention preferably further contains a sintering aid (an aid that promotes and stabilizes the sintering of alumina) from the viewpoint of increasing the strength after sintering and, in particular, achieving high aesthetic appeal.

[0051] The sintering aid contained in the alumina calcined body of the present invention preferably contains at least one element selected from the group consisting of Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra), Ce, Zr, and Y, more preferably contains at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Ce, Zr, and Y, and even more preferably contains at least one element selected from the group consisting of Mg, Ce, Zr, and Y. Of these, magnesium compounds are most preferred. Examples of magnesium compounds include oxides, nitrates, acetates, hydroxides, and chlorides. The magnesium compound is not limited to any particular compound as long as it becomes an oxide at temperatures below 1200°C when sintered in air, but the most preferred examples include magnesium nitrate, magnesium chloride, magnesium hydroxide, and magnesium acetate. Examples of sintering aids include MgCl2, Mg(OH)2, CeO2, ZrO2, and Y2O3.

[0052] Generally, the content of the sintering aid in the alumina raw material powder according to the present invention is preferably 10 ppm or more and 5000 ppm or less, more preferably 20 ppm or more and 3000 ppm or less, and even more preferably 50 ppm or more and 1500 ppm or less, in terms of the element (e.g., Mg element). In this specification, ppm means ppm by mass. If the content of the sintering aid (preferably a magnesium compound) is low, the color tone of the sintered body tends to be whiter than natural teeth, but if the content is too high, the sintered body may be too reddish. The mechanism by which the sintering aid increases the sintered density is thought to be that it exists as a different phase at the grain boundaries, suppressing the growth and development of the grain boundaries, and therefore the pores are excluded from the system without being incorporated into the grains. Furthermore, when a high purity sintered body, for example, 99.99 mass% or more, is required depending on the application, the content of the sintering aid in the alumina powder may be 10 to 100 ppm, or even 20 to 50 ppm, calculated as the element constituting the sintering aid (for example, Mg elemental equivalent). The content of the sintering aid in the alumina calcined body of the present invention and in the alumina composition described below is the same as the content of the sintering aid in the alumina powder.

[0053] Another preferred embodiment includes a dental oxide ceramic calcined body containing zirconia. The oxide ceramic contained in the calcined body of the present invention may be composed mainly of zirconia and a stabilizer capable of suppressing the phase transition of zirconia (hereinafter, sometimes simply referred to as "stabilizer"). The stabilizer is preferably one capable of forming partially stabilized zirconia. Examples of the stabilizer include calcium oxide (CaO), magnesium oxide (MgO), yttria, cerium oxide (CeO2), scandium oxide (Sc2O3), niobium oxide (Nb2O5), lanthanum oxide (La2O3), erbium oxide (Er2O3), and praseodymium oxide (Pr6O 11 , Pr2O3), samarium oxide (Sm2O3), europium oxide (Eu2O3), and thulium oxide (Tm2O3), among which yttria is preferred.

[0054] When the oxide ceramic is a calcined body containing zirconia as a main component (hereinafter, sometimes simply referred to as "zirconia calcined body"), the content of the stabilizer (suitably yttria) in the zirconia calcined body and its sintered body of the present invention is preferably 3.0 to 8.0 mol%, more preferably 3.2 to 6.5 mol%, even more preferably 3.5 to 6.0 mol%, and particularly preferably 3.9 to 5.4 mol%, based on the total moles of zirconia and stabilizer. If the stabilizer content is less than 3.0 mol%, the zirconia sintered body may have insufficient translucency. If it exceeds 8.0 mol%, the amount of phases that undergo a phase transition to a tetragonal and / or cubic system increases, resulting in an increased chipping rate and reduced polishability, and further, the strength of the zirconia sintered body may be reduced.

[0055] The content of the sintering aid or stabilizer in the calcined body of the present invention and its sintered body can be measured, for example, by inductively coupled plasma (ICP) emission spectroscopy, X-ray fluorescence analysis (XRF), scanning or transmission electron microscope (SEM or TEM) and energy dispersive X-ray analysis or wavelength dispersive X-ray analysis (EDX or WDX), or field emission electron microanalysis (FE-EPMA).

[0056] In dental products, surface smoothness is important from the viewpoint of aesthetics, and a low chipping rate of the calcined body is preferable. Furthermore, a low chipping rate is preferable in order to reduce the amount of work required to rework the cut or ground body after firing it as a dental material. The chipping rate is preferably 10% or less, more preferably 7% or less, and even more preferably 3% or less. The chipping rate was measured as follows. The side of a 1mm thick disk cut out to measure tool wear is photographed using an optical microscope, and the chipped areas are painted black, with the rest of the area being white (binarized). The chipping rate can be calculated as the percentage of the black area relative to the total area of ​​black and white. Image analysis software (product name "Image-Pro Plus", manufactured by Hakuto Co., Ltd.) can be used to measure the area.

[0057] Alumina and sintering aid, and zirconium oxide and stabilizer may be mixed together in small amounts within the range that does not impair polishability and aesthetic appearance after sintering. For example, the molar ratio of zirconium oxide to alumina (ZrO2 / Al2O3) is preferably 99.99-98, and more preferably 99.9-99. The molar ratio of zirconium oxide to alumina (ZrO2 / Al2O3) is preferably 0.01-2, and more preferably 0.1-1.

[0058] The oxide ceramic composition for producing the oxide ceramic calcined body of the present invention will be described below using an alumina composition as an example of a case where the oxide ceramic is aluminum oxide. Unless otherwise specified, the term "alumina composition" can be read as "oxide ceramic composition." When the oxide ceramic is zirconium oxide, the composition can be used as a zirconia composition in the same manner as the alumina composition, unless otherwise specified.

[0059] The alumina composition serves as a precursor for the above-mentioned calcined alumina body of the present invention. In this specification, the alumina composition and formed body refer to those before firing, and therefore those in which the alumina particles are not necked (adhered together). The contents of alumina and sintering aid in the alumina composition of the present invention are calculated from the contents of a predetermined alumina calcined body, and the contents in the alumina composition and the alumina calcined body are the same.

[0060] The form of the alumina composition is not limited, and the alumina composition of the present invention includes powder, a fluid in which the powder is added to a solvent, and a compact formed by molding the powder into a predetermined shape. When the alumina composition of the present invention has a powder form, it may be an aggregate of granules. The granules are formed by agglomeration of primary particles.

[0061] In this specification, the term "primary particle" refers to the smallest unit bulk. For example, a primary particle refers to a spherical particle as seen under an electron microscope (e.g., a scanning electron microscope). The primary particle includes alumina particles. When a particulate sintering aid is used, the primary particle includes alumina particles and sintering aid particles.

[0062] The particles constituting the granules made of the alumina composition are preferably primarily primary particles. Aggregates of primary particles are called secondary particles. For example, when visually inspecting an electron microscope image, the number of primary particles is preferably greater than the number of secondary particles. Because secondary particles typically have irregular shapes, an increase in the number of secondary particles leads to variations in density during press molding, as described below, leading to variations in density of the calcined body, which in turn increases chipping during polishing of the calcined body and increases the surface roughness Ra and / or Rz after polishing of the calcined body.

[0063] The average primary particle size of the particles constituting the granules made of the alumina composition affects the degree of adhesion during calcination and the hardness of the calcined body. When the average primary particle size of the particles is 30 nm or more, the surface area of ​​the primary particles contained in the calcined body does not decrease, adhesion does not become too strong, and hardness does not increase easily, which is preferable. On the other hand, when the average primary particle size of the particles is 600 nm or less, it is preferable because it is difficult to absorb small particles in the particle size distribution, and local adhesion due to differences in particle size is suppressed, making it difficult for coarse and dense particles to occur. The average primary particle size is preferably from 30 to 600 nm, more preferably from 40 to 580 nm, further preferably from 60 to 450 nm, and most preferably from 80 to 350 nm.

[0064] The primary particles constituting the granules made of the alumina composition may be a mixture of two types of alumina particles with different average primary particle diameters. For example, when the NXA is used as the primary particles of the alumina particles constituting the granules, the primary particles have an average primary particle diameter within the range of the primary particles constituting the granules, a mixture of "NXA-100" and "NXA-150" can be used. Any mixture may be used as long as the average circularity and relative density of the oxide ceramic particles in the calcined body are satisfied, and it is more preferable if the average primary particle diameter, strength of the calcined body, and cumulative pore distribution are satisfied.

[0065] The BET specific surface area of ​​the particles constituting the granules made of the alumina composition is 5 m when measured in accordance with JIS Z 8830:2013. 2 / g or more, and 7.5m2 / g or more is more preferable, and 8m 2 It is more preferable that the saturation coefficient is 1 / g or more. 5m 2 When the sintering temperature is 1 / g or more, the sinterable temperature can be easily lowered, sintering becomes easier, or the sintered body obtained after sintering can be easily prevented from becoming cloudy and losing translucency. The BET specific surface area is 25m 2 / g or less, and 2 / g or less is more preferable, and 15m 2 It is more preferable that the saturation coefficient is 1 / g or less. 25m 2 When the average primary particle size is 1 / g or less, the average primary particle size is not too small, the calcined body does not become too hard, the polishing time is reduced and / or the chipping rate during polishing is likely to be reduced, and the surface roughness Ra and / or Rz can be further reduced, or adhesion is not too small, and the occurrence of variations in density can be suppressed, and the chipping rate during polishing of the calcined body can be further reduced, and the surface roughness Ra and / or Rz can be further reduced, which is preferable.

[0066] Of the alumina composition of the present invention, 50% or more, preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more of the alumina can be in the form of granules.

[0067] When the alumina composition of the present invention does not take the form of granules, it is sufficient that the alumina particles constituting the powder have the above-mentioned average particle size and BET specific surface area.

[0068] The average particle size (secondary particle size, hereinafter also referred to as "average granule size") of the granules in the alumina composition of the present invention is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 14 μm or more. If the average granule size is less than 10 μm, air may be entrapped when the granules are placed in a mold, resulting in insufficient degassing during molding, and a uniform, dense molded body may not be produced. Furthermore, granules may be ejected from gaps during molding, resulting in a molded body that does not meet the required quantity. The average granule size is preferably 200 μm or less, more preferably 190 μm or less, even more preferably 180 μm or less, particularly preferably 150 μm or less, and most preferably 100 μm or less. If the average granule size exceeds 200 μm, cavities are likely to form inside the granules. Furthermore, gaps are likely to form when the granules are placed in a mold. These phenomena may result in insufficient degassing during molding, and a dense molded body may not be produced. Furthermore, shrinkage during molding may become so large that it may be impossible to produce a molded body of the desired size. It is preferable that 50% or more of the alumina in the alumina composition constitutes granules. It is preferable that the average granule particle size be measured using a method that does not destroy the granules. The average granule particle size can be measured, for example, by a dry sieving method or a wet sieving method. The dry sieving method can be measured according to the sieving test method described in JIS Z 8815:1994, and manual sieving or mechanical sieving can be used, with mechanical sieving being preferred. The sieves used in the sieving method may be those described in JIS Z 8801-1:2019 Test Sieves. The measuring device used in the sieving method can be, for example, a low-tap sieve shaker or a sonic vibration sieving measuring instrument. Examples of low-tap sieve shakers include the "RPS-105M" manufactured by Seishin Enterprise Co., Ltd. Examples of sonic vibration sieving measuring instruments include the "Robot Sifter RPS-01" and "Robot Sifter RPS-02" manufactured by Seishin Enterprise Co., Ltd.

[0069] The sphericity of the granules in the alumina composition of the present invention is preferably high. By increasing the sphericity of the granules, mixing at the interface between layers can be induced when alumina powders of different compositions are layered. Furthermore, when alumina powder is packed into a mold to produce a compact, a higher sphericity allows for a higher packing density even if the average particle size is the same. By packing alumina granules into a specific mold (such as a metal die) and increasing the packing density, which is the density of the compact formed into a specific shape by pressure, the strength and translucency of the sintered body can be increased. Furthermore, even if the mold has corners, the filling of the granules into the corners can be improved.

[0070] The sphericity of the granules in the alumina composition of the present invention can be expressed, for example, by loose bulk density, tight bulk density, or the like.

[0071] The lightly packed bulk density of the alumina composition of the present invention is 0.6 g / cm from the viewpoint of the good flow of granules (ease of clogging) to reduce the density of the resulting molded body. 3 It is preferable that the concentration is 0.7 g / cm or more. 3 More preferably, it is 0.8 g / cm or more. 3 More preferably, it is 0.9 g / cm or more. 3 More preferably, it is equal to or greater than this. The loose bulk density can be measured in accordance with JIS R 9301-2-3:1999.

[0072] The compacted density of the alumina composition of the present invention is 0.8 g / cm from the viewpoint of the good flow of granules (ease of clogging) to reduce the density of the resulting molded body. 3 It is preferable that the concentration is 0.9 g / cm or more. 3 More preferably, it is 1.0 g / cm or more. 3 More preferably, it is equal to or greater than this. The compacted bulk density can be measured in accordance with JIS R 9301-2-3:1999.

[0073] The alumina composition of the present invention preferably contains a binder.

[0074] Examples of the binder include organic binders. Examples of organic binders include commonly used acrylic binders, acrylic acid binders, paraffin binders, fatty acid binders, and polyvinyl alcohol binders. Among these organic binders, those having a carboxyl group in the molecular chain or carboxylic acid derivatives are preferred, acrylic binders are more preferred, and water-soluble polyacrylates are even more preferred. The polyacrylates may be copolymers of acrylic acid or methacrylic acid with maleic acid, or may contain sulfonic acid, and examples of the salt cations include sodium and ammonium.

[0075] Depending on the content of the binder contained in the alumina composition of the present invention, the distance between primary particles in the alumina composition can be adjusted, and the cumulative distribution of pores can be controlled, making it easier to increase or decrease the Vickers hardness or the strength of the calcined body. The binder content is preferably 1.2 to 2.8 mass% of the entire alumina composition, more preferably 1.5 to 2.5 mass%, and even more preferably 1.8 to 2.2 mass%. When the binder content is 1.2 mass% or more of the entire alumina composition, the strength of the calcined body is not too high, and there is no risk of the machined body becoming hard when removed. Furthermore, when the content is 2.8 mass % or less, the strength of the calcined body does not decrease too much, the possibility of the workpiece falling off during cutting can be reduced, and in addition, the chipping rate can be easily reduced.

[0076] The alumina composition of the present invention may contain, as needed, additives other than sintering aids (excluding CeO, ZrO, and YO), such as colorants (including pigments, composite pigments, and fluorescent agents), titanium oxide (TiO), silica (SiO), dispersants, and antifoaming agents. These components may be used alone or in combination of two or more. 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, Zr, Sn, Sb, Bi, Ce, Sm, Eu, Gd, and Er. Examples of the composite pigment include (Zr,V)O2, Fe(Fe,Cr)2O4, (Ni,Co,Fe)(Fe,Cr)2O4·ZrSiO4, and (Co,Zn)Al2O4. Examples of the fluorescent agent include Y2SiO5:Ce, Y2SiO5:Tb, (Y, Gd, Eu)BO3, Y2O3:Eu, YAG:Ce, ZnGa2O4:Zn, and BaMgAl 10 O 17 :Eu, etc.

[0077] The additives may be added during mixing or pulverization, or may be added after pulverization.

[0078] In one embodiment, a dental oxide ceramic calcined body can be given that has a surface roughness Ra of 1.40 μm or less after being sintered under atmospheric pressure without using hot isostatic pressing (HIP) treatment.

[0079] Another embodiment is a dental oxide ceramic calcined body that has a surface roughness Rz of 51.0 μm or less after being sintered under atmospheric pressure without using hot isostatic pressing.

[0080] Another embodiment is a dental oxide ceramic calcined body that has an average crystal grain size of 0.3 to 8.0 μm after being sintered under atmospheric pressure without using hot isostatic pressing. The method for measuring the average crystal grain size and the preferred range are the same as those for the average crystal grain size of the alumina sintered body described below.

[0081] In one embodiment, a method for producing a dental oxide ceramic calcined body includes the steps of: The method includes a step of press-molding an oxide ceramic composition at a surface pressure of 20 to 600 MPa, and a step of firing the composition at 400°C or higher and lower than 1200°C under atmospheric pressure, The method for producing a dental oxide ceramic calcined body includes oxide ceramic particles having an average primary particle circularity of 0.81 or more and a relative density of 43 to 63%.

[0082] In another preferred embodiment, the oxide ceramic particles contain zirconia and / or alumina, which are the same as those for the calcined body described above.

[0083] The method for producing an oxide ceramic calcined body of the present invention will be described below using a method for producing an alumina calcined body as an example in which the oxide ceramic is aluminum oxide. When the oxide ceramic is zirconium oxide, the method for producing a zirconia calcined body can be carried out in the same manner, unless otherwise specified.

[0084] Examples of methods for producing an alumina calcined body include a method including the steps of producing an alumina composition containing alumina particles and a sintering aid, and firing (calcining) the alumina composition (e.g., a molded body) to obtain an alumina calcined body having an average circularity of primary particles of 0.81 or more and a relative density of 43 to 63%. The content of the sintering aid is preferably 10 to 5000 ppm. First, the process for producing the alumina composition of the present invention will be described.

[0085] First, a mixture is prepared by mixing alumina and a sintering aid in a predetermined ratio (mixing step). For example, when the sintering aid is magnesium chloride, the alumina and magnesium chloride can be mixed in a ratio such that the above-mentioned content is achieved. The mixing may be dry mixing or wet mixing. In order to adjust the desired average circularity and relative density when producing a calcined body, the alumina composition can be pulverized (preferably crushed) to the above-mentioned average primary particle size (pulverization step).

[0086] The mixing step and the pulverization step can be carried out in the same step. The pulverization can be carried out, for example, using a ball mill, a bead mill, or the like after dispersing the composition and the binder in a solvent such as water or alcohol (dispersion step). The composition is pulverized (preferably crushed) so that the average primary particle size of the composition is, for example, 0.05 μm to 0.6 μm, so that the desired average circularity and relative density can be adjusted when the calcined body is produced. If necessary, the composition may be subjected to other treatments (classification, water treatment) to adjust the particle size.

[0087] The average primary particle size can be measured by a laser diffraction / scattering particle size distribution measurement method. For example, using a laser diffraction / scattering particle size distribution measurement device (product name "Partica LA-950") manufactured by Horiba, Ltd., a water-diluted slurry is irradiated with ultrasound for 30 minutes, and then the volumetric measurement can be performed while applying ultrasound. After the mixing step and / or the pulverization step, the mixture can be spray-dried using a spray dryer or the like to form the alumina composition into granular form as described above (drying step).

[0088] In the pulverization step, the average primary particle size of the alumina composition is preferably 30 to 600 nm, more preferably 40 to 580 nm, even more preferably 60 to 450 nm, and particularly preferably 80 to 350 nm. By setting the average particle size of the alumina composition to 30 to 600 nm, it is possible to achieve both surface roughness and hardness after polishing of the calcined body.

[0089] The alumina and the sintering aid may be prepared separately. For example, instead of simultaneously precipitating the alumina and the sintering aid (in the same process), the alumina preparation process (e.g., manufacturing process) and the sintering aid preparation process (e.g., manufacturing process) may be separate processes. This allows the aforementioned α-alumina to be obtained with high purity and a small primary particle size.

[0090] In the production of the alumina composition, a sintering aid may be reacted with alumina by heat treatment, and the resulting mixture may be used for the pulverization and drying steps.

[0091] As a result of the above, granules made of an alumina composition that will be used as a raw material for the alumina calcined body can be produced.

[0092] The granules or powder can be molded into a compact by applying an external force. The molding method is not limited to a specific method, and a suitable method can be selected depending on the purpose. For example, molding can be performed by press molding, injection molding, stereolithography, slip casting, gel casting, filtration, casting, etc. Multi-stage molding may also be performed. For example, the alumina composition may be press-molded and then further subjected to CIP treatment, or press molding and CIP molding may be repeated.

[0093] Examples of press molding methods include uniaxial pressing (hereinafter also referred to as "uniaxial pressure pressing"), biaxial pressing, CIP (Cold Isostatic Pressing), etc. These may be used in appropriate combination.

[0094] The molded body of the present invention can have a disk shape, a rectangular parallelepiped shape, or a dental product shape (for example, a dental crown shape).

[0095] In one embodiment, the pressure molding may be performed by uniaxial pressing. The product obtained by the pressure molding step may be, for example, a columnar molded body obtained by filling alumina granules into a mold and compacting them with a uniaxial press. The higher the surface pressure in the press molding, the higher the density of the molded body. This allows the relative density of the resulting calcined alumina body to be increased and the average circularity to be adjusted. On the other hand, if the density of the molded body is too high, the calcined alumina body becomes hard and poor machinability cannot be obtained. Therefore, in order to easily adjust the average circularity and relative density of the calcined alumina body to the desired range when combined with the average primary particle size of the primary particles constituting the granules made of the alumina composition as the raw material, the calcination temperature, etc., the surface pressure in the press molding (e.g., uniaxial pressing) is preferably 20 to 600 MPa, more preferably 25 to 400 MPa, and even more preferably 30 to 200 MPa. When the surface pressure in the press molding (e.g., uniaxial pressing) is 20 MPa or higher, the shape retention of the molded body is excellent. When the surface pressure is 600 MPa or lower, the density of the molded body does not increase too much, making it easier to prevent hardening. The surface pressure in press molding may be set to a suitable range of 50 MPa or more, 80 MPa or more, 100 MPa or more, or 150 MPa or more depending on the desired average circularity, relative density, and the like. For example, in one embodiment, the pressure may be 20 to 200 MPa, 25 to 190 MPa, or 30 to 180 MPa depending on the target average circularity, relative density, and the like.

[0096] The molded article of the present invention also includes a molded article densified by high-temperature pressure treatment such as CIP (Cold Isostatic Pressing). From the same viewpoint as above, the water pressure is preferably 50 to 1000 MPa, more preferably 100 to 600 MPa, and even more preferably 150 to 300 MPa.

[0097] The alumina calcined body according to the present invention serves as a precursor to the alumina sintered body described below. The alumina calcined body according to the present invention also includes a shaped product. For example, it includes a dental product (e.g., a crown-shaped prosthesis) made by processing a calcined alumina disk using a CAD / CAM (Computer-Aided Design / Computer-Aided Manufacturing) system.

[0098] The contents of alumina and sintering aid in the alumina calcined body of the present invention are the same as those in the alumina composition before the alumina calcined body is produced. From the viewpoint of the strength and translucency of a sintered body produced from the calcined body, which will be described later, it is preferable that the sintering aid in the alumina calcined body of the present invention is a sintering aid in which a magnesium compound is uniformly dispersed.

[0099] Next, the calcination step in which the obtained molded body is fired (calcined) under atmospheric pressure will be described. The calcination temperature in the calcination step affects the average circularity of the oxide ceramic particles contained in the calcined body, and also affects the Vickers hardness or strength of the calcined body. The abrasiveness and hardness of the calcined body change depending on the calcination temperature.

[0100] The calcination temperature (maximum calcination temperature) in the method for producing an alumina calcined body of the present invention is preferably 400°C or higher and lower than 1200°C, because by keeping the temperature at a level that does not cause sintering to proceed, the particle surface shape (spherical shape) is retained, the calcined body does not become too hard, and the surface roughness and hardness can be maintained during polishing. It is preferable that the calcination increases the average circularity of the oxide ceramic particles during calcination without excessive adhesion. From this viewpoint, the calcination temperature is preferably a temperature at which the average circularity of the oxide ceramic particles contained in the calcined body is 0.81 or more.

[0101] When the average primary particle size of the oxide ceramic particles contained in the alumina composition (e.g., a molded body) is small (e.g., 30 nm or more and 150 nm or less), adhesion begins at a low calcination temperature, and the Vickers hardness or strength of the calcined body is increased. For example, when the average primary particle size of the oxide ceramic particles contained in the alumina composition (e.g., a molded body) is about 95 nm, the calcination temperature is preferably around 750°C (700 to 850°C). When the average primary particle size of the oxide ceramic particles contained in the alumina composition is small, a calcination temperature of 1200°C or higher is not preferable because the average circularity decreases due to adhesion between the particles, making the composition too hard and requiring a long time for polishing.

[0102] When the average primary particle diameter of the oxide ceramic particles contained in the alumina composition (e.g., a molded body) is large (e.g., 380 nm or more and 600 nm or less), they do not stick at low calcination temperatures but start to stick at high temperatures, thereby increasing the Vickers hardness or the strength of the calcined body. For example, when the average primary particle diameter of the oxide ceramic particles contained in the alumina composition (e.g., a molded body) is about 580 nm, the calcination temperature is preferably around 1150°C (1100°C or more and less than 1200°C). When the average primary particle size of the oxide ceramic particles contained in the alumina composition (e.g., a molded body) is large, if the calcination temperature is 1000°C or less, the average circularity does not increase, adhesion between particles does not progress, the strength or Vickers hardness of the calcined body does not increase, chipping occurs during polishing, and the surface roughness Ra and / or Rz decreases, which is undesirable.

[0103] Although the average primary particle size of the primary particles constituting the granules made of the raw material oxide ceramic composition (e.g., alumina composition) and the average primary particle size of the primary particles of the oxide ceramic particles (e.g., alumina particles) contained in the calcined body may differ, as described above, the average primary particle size of the primary particles of the oxide ceramic particles (e.g., alumina particles) contained in the calcined body can be adjusted to a desired range by adjusting the degree of adhesion according to predetermined conditions including the average primary particle size of the primary particles constituting the granules made of the raw material oxide ceramic composition (e.g., alumina composition) and the calcination temperature.

[0104] Considering the relationship between the calcination temperature and the average particle size of the primary particles constituting the raw material alumina composition or granules made of the alumina composition, and from the viewpoint of preventing excessive adhesion and adjusting the average circularity of the primary particles of the oxide ceramic particles contained in the resulting calcined body to fall within a desired range, the calcination temperature is preferably a temperature at which the average circularity of the oxide ceramic particles contained in the calcined body is 0.81 or more and the relative density of the calcined body is 43 to 63%. The calcination temperature is preferably 600°C or higher and lower than 1200°C, and more preferably 750°C or higher and 1150°C or lower.

[0105] Holding the calcined body at the maximum calcination temperature for a certain period of time is preferable because the hardness of the calcined body falls within a preferred range and the chipping rate may decrease. The calcination conditions depend on the average primary particle size and density of the calcined body, but the holding time at the maximum calcination temperature is preferably 30 minutes to 6 hours. The rate of temperature increase up to the maximum calcination temperature and the rate of temperature decrease from the maximum calcination temperature are preferably 300°C / min or less.

[0106] The alumina calcined body of the present invention can be machined to produce a processed body. The processing method is not limited to a specific method, and a suitable method can be selected depending on the purpose. For example, an alumina disk, which is also the calcined body, can be cut or ground into the shape of a dental product (e.g., a crown-shaped prosthesis) using a CAD / CAM system to produce a processed body.

[0107] The processing machine used for machining the calcined body of the present invention is not particularly limited. For example, cutting or grinding machines may be desktop processing machines, large machining centers (general-purpose processing machines), etc., depending on the workpiece. Examples of cutting machines include benchtop processing machines "DWX-50," "DWX-4," "DWX-4W," "DWX-52D," and "DWX-52DCi" (manufactured by Roland DG Corporation). Grinding may also be used.

[0108] The tools used in the processing machine used for machining the calcined body of the present invention are not particularly limited. Milling burrs and grinding burrs recommended by the processing machine supplier can be suitably used. For example, a Katana (registered trademark) drill is an example of a milling burr used in a cutting processing machine.

[0109] The processed body obtained from the alumina calcined body of the present invention has a surface with machined steps due to machining. If the processed steps are sintered while they remain, the surface of the sintered body will have irregularities corresponding to the machined steps, and will therefore need to be polished before being used as a dental material. Therefore, it is preferable to finish the surface of the processed body obtained from the calcined body and having the machined steps smooth by cutting, grinding, or polishing.

[0110] The surface smoothness of the processed body obtained from the alumina calcined body of the present invention can be improved using a dental tool. For example, a bur and polisher kit suitable for modifying the shape of dental ceramics, including porcelain, by cutting or grinding, and improving the surface quality by polishing, can be used as the dental tool. Examples of tools include Noritake Protec diamond points, carbide burs, Meister cones, rubber points, felt wheels, and TWIST DIA COARSE, MEDIUM, and FINE types manufactured by Kuraray Noritake Dental Co., Ltd., and ceramic points for HP, including Cerapica disc-type medium polishing, teardrop-type medium polishing, disc-type finish polishing, and teardrop-type finish polishing, manufactured by PDR Corporation.

[0111] The surface smoothness of the processed body obtained from the calcined alumina body of the present invention may be improved using a tool. When the processed body obtained from the calcined alumina body of the present invention is polished with a tool, abrasive powder may be used. Examples include Pearl Surface (registered trademark) C and Pearl Surface (registered trademark) F manufactured by Kuraray Noritake Dental Co., Ltd. When an abrasive is used, it becomes a foreign substance during sintering, so it is preferable to wash it off.

[0112] When polishing the processed body obtained from the alumina calcined body of the present invention with a tool, the rotation speed of the tool is preferably 1000 to 7000 rpm. If the rotation speed is lower than 1000 rpm, it takes a long time, and the recoil of the rotating tool colliding with the calcined body makes it difficult to keep a steady hand, making it difficult to obtain a smooth surface. If the rotation speed is higher than 7000 rpm, the processing force of the tool increases, causing excessive polishing of the calcined body, making it difficult to obtain the desired shape. The rotation speed is more preferably 2000 to 6000 rpm, and even more preferably 3000 to 5000 rpm.

[0113] After the processed body obtained from the calcined alumina body of the present invention is polished with a tool, a sintering process is carried out. At this time, it is preferable to remove the processing waste, since if any processing waste adheres to the surface of the processed body, it will affect the shape and appearance after sintering. It is preferable to brush off the dust from the processed body that has been subjected to the finish polishing with a brush used for painting, etc., until the dust is visually removed.

[0114] Next, the method for producing an oxide ceramic sintered body of the present invention will be described below using a method for producing an alumina sintered body, taking as an example a case where the oxide ceramic is aluminum oxide.

[0115] The alumina sintered body of the present invention can be produced by sintering the alumina calcined body of the present invention and its cut or ground body at a temperature at which the alumina particles are sintered (sintering step). When the average primary particle size is about 100 nm, the sinterable temperature (e.g., the maximum sintering temperature) is preferably 1300°C or higher, more preferably 1350°C or higher, and even more preferably 1375°C or higher. Furthermore, the sinterable temperature is preferably 1500°C or lower, more preferably 1450°C or lower. The heating rate up to the sinterable temperature and the cooling rate from the sinterable temperature are preferably 300°C / min or lower.

[0116] In the sintering step, the holding time at the sinterable temperature (for example, the maximum sintering temperature) is preferably 120 minutes or less, more preferably 90 minutes or less, even more preferably 75 minutes or less, even more preferably 60 minutes or less, particularly preferably 45 minutes or less, and most preferably 30 minutes or less. The holding time is preferably 1 minute or more, more preferably 3 minutes or more, and even more preferably 5 minutes or more.

[0117] The alumina composition and alumina calcined body of the present invention can shorten the sintering process time for producing a sintered body without reducing the translucency and strength of the produced alumina sintered body. In particular, the holding time at the maximum sintering temperature for producing a sintered body can be shortened (short-time sintering). This improves production efficiency, and when the alumina calcined body of the present invention is used for dental products, it shortens the time from determining the dimensions of the dental product to be used for treatment, cutting or grinding, to making the dental product ready for treatment, thereby reducing the time burden on patients. It also reduces energy costs.

[0118] In the sintering step, the holding time at a sinterable temperature (for example, the maximum sintering temperature) can be, for example, 25 minutes or less, 20 minutes or less, or 15 minutes or less.

[0119] The temperature increase and decrease rates in the sintering step are preferably set so as to shorten the time required for the sintering step. For example, the temperature increase rate can be set so as to reach the maximum sintering temperature in the shortest time depending on the performance of the sintering furnace. The temperature increase 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, or 200°C / min or more. The temperature decrease rate is preferably set so as to prevent the sintered body from being deformed due to differences in shrinkage rate or from having defects such as cracks. For example, after heating is completed, the sintered body can be allowed to cool at room temperature.

[0120] In one embodiment, a method for producing a sintered oxide ceramic body for dental use includes a step of polishing any of the above-described calcined dental oxide ceramic bodies with a dental polishing tool. The dental polishing tool is not particularly limited, and known commercially available products can be used. The polishing conditions are not particularly limited.

[0121] Another embodiment of the present invention is a method for producing a sintered dental oxide ceramic body, which includes a step of sintering the calcined dental oxide ceramic body under atmospheric pressure without using a hot isostatic pressing treatment. In the method for producing a sintered dental oxide ceramic body of the present invention, no hot isostatic pressing (HIP) treatment is required, so no special equipment is required and a sintered dental oxide ceramic body can be produced easily.

[0122] The oxide ceramic sintered body will be described below using an alumina sintered body as an example in which the oxide ceramic is aluminum oxide.

[0123] An alumina sintered body is obtained by sintering a calcined alumina body or a processed body thereof. An alumina sintered body is a sintered body of alumina particles (powder). The relative density of the alumina sintered body is preferably 99.5% or more. The relative density of a sintered body can be calculated as the ratio of the actual density measured by Archimedes' method to the theoretical density. The relative density means the density d1 of a sintered body obtained by filling granules into a specific mold and pressing the molded body into a specific shape, and then sintering the molded body at a high temperature, divided by the theoretical density d2 of alumina (not including voids inside).

[0124] The alumina sintered body of the present invention includes not only sintered bodies obtained by sintering molded alumina particles under normal pressure or under no pressure, but also sintered bodies densified by high-temperature pressure treatment such as HIP (hot isostatic pressing) treatment.

[0125] The higher the relative density of the alumina sintered body of the present invention, the fewer internal voids there are and the less light scattering there is. This increases the translucency (ΔL), total light transmittance, and in-line light transmittance of the alumina sintered body, resulting in excellent aesthetics and improved strength. Therefore, a high relative density of the alumina sintered body of the present invention is preferable. The relative density of the alumina sintered body of the present invention is, for example, preferably greater than 95%, more preferably 98% or more, and even more preferably 99.5% or more. Furthermore, it is most preferable that the alumina sintered body of the present invention is substantially free of voids.

[0126] The average crystal grain size of the alumina sintered body of the present invention is preferably 0.3 to 8.0 μm, more preferably 0.4 to 6.0 μm, and even more preferably 0.5 to 3.0 μm, from the viewpoint of excellent translucency and strength. The average crystal grain size of the alumina sintered body can be measured by the following method.

[0127] For the alumina sintered body, an image of the surface was taken using a scanning electron microscope (product name "VE-9800", manufactured by Keyence Corporation). After marking the grain boundaries of each crystal grain in the obtained image, the average crystal grain size was calculated by image analysis. To measure the average crystal grain size, image analysis software (product name "Image-Pro Plus", manufactured by Hakuto Co., Ltd.) was used to binarize the captured SEM image, adjust the brightness range so that the grain boundaries were clearly visible, and identify the particles from the field of view (area). The crystal grain size obtained with Image-Pro Plus was the average of the lengths of the line segments connecting the outlines passing through the center of gravity determined from the outlines of the crystal grains, measured at two-degree intervals around the center of gravity. In the SEM photograph (3 fields of view) of the alumina sintered body, the crystal grain size of all particles not on the edge of the image was measured. The average grain size is calculated from the grain size of each particle and the number of grains obtained, and the obtained arithmetic mean diameter is used as the average grain size in the sintered body. "Grains not on the image edge" refers to particles excluding those whose outlines do not fit completely within the screen of the SEM photograph (particles whose outlines end at the top, bottom, left, and right boundary lines). The grain sizes of all grains not on the image edge are calculated by selecting the option to exclude all grains on boundary lines in Image-Pro Plus.

[0128] The contents of alumina and sintering aid in the alumina sintered body of the present invention are the same as those in the composition before the sintered body is produced and / or in the calcined body.

[0129] The light transmittance (ΔL) of the alumina sintered body of the present invention is preferably 5 or more, more preferably 10 or more, even more preferably 15 or more, and particularly preferably 20 or more.

[0130] Translucency (ΔL) is the value obtained by subtracting the second L* value from the first L* value, where the L* value of the lightness (color space) in the L*a*b* color system (JIS Z 8781-4:2013) is the L* value measured on a 1.2 mm thick sample (sintered compact) against a white background. The L* value measured on the same sample against a black background is the second L* value.

[0131] To prepare a sample for measuring the translucency (ΔL), first, the granules (composition) are press-molded so that the thickness of the sintered body is 1.2 mm, followed by CIP molding to prepare a disk-shaped compact with a diameter of, for example, 19 mm. Next, the compact is fired under predetermined firing conditions, and the surface is polished with #2000 grit to produce a sintered body with a thickness of 1.2 mm that serves as a sample. The L* value can be measured by applying a contact liquid to the surface of a sample and then measuring the L* values ​​on a black background and a white background using a color difference meter (e.g., CE100, analysis software "Crystal Eye" (manufactured by Olympus Corporation)). The contact liquid may have a refractive index nD of 1.60 measured at a measurement wavelength of 589 nm (sodium D line). The white background refers to the white part of the hiding power test paper described in JIS K 5600-4-1:1999 Part 4, Section 1, and the black background refers to the black part of the hiding power test paper.

[0132] The alumina sintered body of the present invention is preferably one having a higher surface smoothness because it appears more aesthetically pleasing. The surface roughness Ra of the alumina sintered body is preferably 1.40 μm or less, and from the viewpoint of more easily reproducing the surface roughness of natural teeth, is more preferably 1.25 μm or less, even more preferably 1.15 μm or less, and particularly preferably 1.10 μm or less. The surface roughness Rz of the alumina sintered body is preferably 51 μm or less, and from the viewpoint of easier reproduction of the surface roughness of natural teeth, is more preferably 48 μm or less, further preferably 42 μm or less, and particularly preferably 39 μm or less. The surface roughness Ra and Rz were measured by the method described in the Examples below.

[0133] The alumina sintered body of the present invention may have a predetermined shape. For example, the sintered body may have a disk shape, a rectangular parallelepiped shape, or a dental product shape (e.g., a dental crown shape).

[0134] The methods for producing the alumina composition, granules, powder, molded body, calcined body, cut or ground body, and sintered body described in the present invention are not limited to those described above unless otherwise specified, and various known production methods can be applied. The present invention includes embodiments in which the above-described configurations are combined in various ways within the scope of the technical concept of the present invention, as long as the effects of the present invention are achieved. In the present invention, the upper and lower limits of the numerical ranges (content of each component, each element (average primary particle size, etc.), each physical property, etc.) can be combined as appropriate. [Example]

[0135] Next, the present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples, and many modifications can be made by those skilled in the art within the scope of the technical concept of the present invention.

[0136] [Relative density measurement] A calcined body measuring approximately 20 mm long x 19 mm wide x 17 mm high was obtained by the method described in the following Examples and Comparative Examples, except that the size of the mold used to press the granules was changed. The relative density of the calcined body was measured at a distance of 1.2 cm from the calcined body. 3 A sample (10.8 mm diameter x 13 mm height) was cut out and measured using an automatic mercury porosimeter pore size distribution measuring device (AutoPore (registered trademark) IV9500, manufactured by Micromeritics, Inc., USA) in accordance with JIS R 1655:2003 at a pressure of 0.5 to 60,000 psia. The relative density was calculated using the measured porosity using the following formula. (Relative density) (%) = {1 - (porosity)} x 100

[0137] [Measurement of average primary particle size of calcined body] Using the calcined bodies obtained in the following Examples and Comparative Examples, images of the surfaces were taken with a scanning electron microscope (product name "VE-9800", manufactured by Keyence Corporation). Particle size was measured using image analysis software (Image-Pro Plus, manufactured by Hakuto Co., Ltd.) to binarize SEM images showing primary particles. The grain boundaries of each crystal particle were then plotted on the resulting image, and the particles were identified from the field of view (area). For areas where the grain boundaries were unclear, a degeneration filter was applied to the area, shrinking each area until it became one or more points. A Voronoi polygon was then constructed so that these points became the kernel points of the Voronoi polygon. A line connecting the midpoints of two adjacent kernel points was then drawn, and the line was then superimposed on the original particle image to separate adjacent particles. For example, in some cases, a single particle may appear gourd-shaped during image processing. In such cases, the particle was separated into two by assuming that two circular particles were touching and appearing as one. In the processing file that recognized the primary particle size, "Diameter" was selected in the "Count / Size Dialog" and the distribution was calculated (n = 4). Specifically, for four fields of view of one sample, the particle diameter (primary particle diameter) was measured in each field using image analysis software (Image-Pro Plus) and the average value was calculated.

[0138] [Measurement of average circularity of calcined body] Using the data obtained in the measurement of the average primary particle diameter, the circularity calculated by the following formula was taken as the "average circularity." (Circularity) = (4π × area) / (perimeter × perimeter)

[0139] [Measurement of sintering aid content] The samples used were the compositions, calcined bodies, or sintered bodies obtained in the following Examples and Comparative Examples. Measurements were performed using a field emission scanning electron microscope (FE-SEM Reglus8220, Hitachi High-Tech Corporation) and an energy dispersive X-ray analyzer (Aztec Energy X-Max50, Oxford Instruments) under the following conditions (average of n=3). Measurement magnification: 5,000 times Analysis mode: Area analysis Accelerating voltage: 5 kV Working distance: 15mm±1mm X-ray extraction angle: 30 degrees Dead time: 7% Measurement time: 100 seconds

[0140] [Surface polishing method] The alumina granules obtained in the examples and comparative examples were filled into a mold, uniaxially pressed at 200 MPa, and calcined at 750°C for 6 hours to produce a disk-shaped calcined body having a thickness of 14 mm and a diameter of 98.5 mm. Based on the 3D NC data, this disk-shaped calcined body was machined using a milling machine "DWX-52DC" manufactured by Kuraray Noritake Dental Co., Ltd. with an unused Katana (registered trademark) drill (manufactured by Kuraray Noritake Dental Co., Ltd., Φ2 mm, not diamond coated) to leave a thin disk with a thickness of 1 mm. The machining pattern was contour machining, from the center of the disk-shaped calcined body outward, with a spindle rotation speed of 30,000 rpm, a feed rate of 2000 mm / min, and a machining pitch of Z = 0.5 mm, XY = 1 mm. The resulting thin disk, 1 mm thick, was then cut into posts using a Noritake Protec Diamond Point DP-04 manufactured by Kuraray Noritake Dental Co., Ltd. Steps due to CAD / CAM processing were visible on the surface of the thin disk.

[0141] The entire surface of this thin disk was roughly polished using a polisher (KATANA (registered trademark) Zirconia TWIST DIA MEDIUM) manufactured by Kuraray Noritake Dental Co., Ltd. at a rotation speed of 4000 rpm and a load of 0.05 kgf for 5 minutes. Next, using another polisher (KATANA (registered trademark) Zirconia TWIST DIA FINE) manufactured by Kuraray Noritake Dental Co., Ltd., the entire surface was finish-polished at a rotation speed of 4000 rpm and a load of 0.01 kgf for 5 minutes. Next, the entire surface was further finish-polished for 5 minutes using a felt wheel attached to an abrasive (Pearl Surface (registered trademark)) manufactured by Kuraray Noritake Dental Co., Ltd., at a rotation speed of 4000 rpm and a load of 0.005 kgf. Finally, the abrasive dust was brushed off using a Kolinsky Taimir Sable No. 10 paintbrush manufactured by ESCODA.

[0142] [Surface roughness measurement] The calcined bodies or sintered bodies obtained in the examples and comparative examples were measured nine times under the following conditions using a stylus surface roughness meter ("DekTak-150" manufactured by Bruker Japan Co., Ltd.). Measurement distance: 10mm Scan Length: 15,000 μm Scan Duration: 100 seconds Meas. Range: 65.5 μm Stylus Force: 1.00mg Result Zn(X) Xn=0.0005[mm]×n(n=0,1,2...9)

[0143] From these results, a third-order fitting function (Z'n(X) = aX + b) was determined by the least squares method, and the tilt of the sample during measurement was removed by taking the difference between the measurement result and the fitting function.

[0144] Furthermore, the arithmetic mean roughness Ra was calculated using the following formula using the difference. The maximum height roughness Rz was taken as the maximum value of Zn(X).

[0145]

number

[0146] The measurement results of the surface roughness of the calcined body before polishing using the above-mentioned "surface polishing method" are listed under "before polishing" in Table 2, and the measurement results of the surface roughness of the calcined body polished using the above-mentioned "surface polishing method" are listed under "after polishing" in Table 2. Similarly, for sintered bodies, the measurement results of the surface roughness of sintered bodies that were not polished with a calcined body are listed under "without polishing with a calcined body" in Table 3, and the measurement results of the surface roughness of sintered bodies that were polished with a calcined body are listed under "polished with a calcined body" in Table 3.

[0147] [Measurement of the strength of the calcined body] Similar to the samples used in the surface polishing method described above, a disk-shaped calcined body with a thickness of 14 mm and a diameter of 98.5 mm was used. A sample measuring 5 mm x 10 mm x 50 mm was cut out from the disk-shaped calcined body in accordance with ISO 6872:2015, and the face and C-face of the sample (the surface with the corners of the sample chamfered at a 45° angle) (see 7.3.1.2.1 of ISO 6872:2015) were surface-finished in the longitudinal direction with 600-grit sandpaper. The specimens were positioned so that the widest surface faced vertically (in the direction of the load), and the three-point bending strength was measured using a universal testing machine (Shimadzu Corporation, "AG-I 100kN") with a span (distance between supports) of 30 mm and a crosshead speed of 0.5 mm / min (average value of n=3).

[0148] [Hardness evaluation] The calcined bodies obtained in the following Examples and Comparative Examples were machined into a dental crown shape using a CAD / CAM system. The machined bodies were integrated with the calcined bodies by a support (cylindrical shape with a diameter of 5 mm) made of a part of the calcined body. Using an ULTIMATE500 (manufactured by Nakanishi Co., Ltd.) equipped with a Diamond Point HP Model No. 25 (manufactured by Matsukaze Co., Ltd.), the diamond point HP was pressed against the support part perpendicular to the direction of the support at 10,000 rpm in the air with a load of approximately 500 g, and the cutting process was performed while visually checking according to the following evaluation criteria (n=10). If eight or more of the ten samples met the evaluation criteria of "A" and zero to two of the evaluation criteria of "B," the sample was judged as "Good." If eight or more met the evaluation criteria of "A" and one or two of the evaluation criteria of "C," the sample was judged as "Good." If three or more met the evaluation criteria of "B," the sample was judged as "Poor." If three or more met the evaluation criteria of "C," the sample was judged as "XX." Samples with a rating of "C" are likely to fall off the support and become unusable, whereas samples with a rating of "B" may be usable if they are machined over a long period of time. Therefore, a rating of "Good" is preferable to a rating of "Poor." <Evaluation criteria> A: More than 90% of the support was removed within 10 seconds B: The support could not be cut within 10 seconds (too hard) C: The support broke within 10 seconds before more than 90% of the support was worn away (too soft).

[0149] [Production of calcined body] <Examples 1 and 7> 1000 g of α-alumina raw material "NXA-100" (average primary particle size: 100 nm, manufactured by Sumitomo Chemical Co., Ltd.) and 0.1 g of magnesium chloride equivalent were weighed out, put into 10 L of ethanol, and ultrasonically dispersed. This and alumina beads were placed in a rotating container, and the alumina raw material containing agglomerated particles was pulverized in a ball mill, whereby the raw material was mixed and crushed until the desired average primary particle size was achieved. The primary particle size was measured on a volume basis using a laser diffraction / scattering particle size distribution analyzer (product name "Partica LA-950") manufactured by Horiba, Ltd., by irradiating the ethanol-diluted slurry with ultrasonic waves for 30 minutes, followed by ultrasonic irradiation. The desired slurry with almost no secondary agglomeration was obtained after approximately 1 hour of ball milling.

[0150] Here, the slurry was divided into 2 L beakers and stirred for 1 hour with a rotor at 200 rpm. The rotor was then immediately stopped and the mixture was left to stand for 15 minutes. White particles were visible at the bottom of the beaker, and the supernatant was cloudy. The top third of the slurry in the beaker was siphoned off and used as the slurry of Example 1, and the bottom third of the slurry in the beaker was used as the slurry of Example 7. In Table 1, the slurry was distinguished by the water treatment, with Example 1 listed as "NXA-100 water above" and Example 7 listed as "NXA-100 water below."

[0151] Next, an organic binder was added to this slurry. A water-based acrylic binder was used as the organic binder, and the amount added was 2.5 mass% (organic binder content relative to the total slurry) of the α-alumina raw material. The mixture was then stirred with a rotor blade for 24 hours. The stirred slurry was then dried and granulated using a spray dryer to obtain alumina granules. The average particle size of the granules was 40 μm. 350 g of this granule powder was poured into a cylindrical mold with a diameter of 100 mm and uniaxially pressed at a pressure of 150 MPa to obtain a compact. The compact was placed in an electric furnace, heated from room temperature at a rate of 3°C / min, and held at 500°C for 2 hours to degrease the organic components. The compact was then held at the maximum calcination temperature listed in Table 2 for 6 hours, and slowly cooled from the maximum calcination temperature at -0.4°C / min to obtain a calcined body.

[0152] <Examples 2 to 5, Comparative Example 7> 100 g of α-alumina raw material "NXA-100" (average primary particle size: 100 nm, manufactured by Sumitomo Chemical Co., Ltd.) or "NXA-150" (average primary particle size: 150 nm, manufactured by Sumitomo Chemical Co., Ltd.) and 0.1 g of magnesium chloride equivalent were weighed out, put into 1 L of ethanol, and ultrasonically dispersed. This and alumina beads were placed in a rotating container, and the raw materials were mixed and pulverized by ball milling until the desired primary particle size was achieved. The primary particle size was measured on a volume basis using a laser diffraction / scattering particle size distribution analyzer (product name "Partica LA-950") manufactured by Horiba, Ltd., by irradiating the ethanol-diluted slurry with ultrasonic waves for 30 minutes, followed by ultrasonic irradiation. The desired slurry with almost no secondary aggregation was obtained after approximately 1 hour of ball milling.

[0153] Next, an organic binder was added to this slurry. A water-based acrylic binder was used as the organic binder, and the amount added was 2.5 mass % (content of the organic binder relative to the total slurry) of the α-alumina raw material, and the slurry was stirred with a rotary blade for 24 hours. The stirred slurry was dried and granulated using a spray dryer to obtain granules. The average particle size of the granules was 40 μm. 350 g of this granule powder was poured into a cylindrical mold with a diameter of 100 mm and uniaxially pressed at a pressure of 150 MPa to obtain a compact. The compact was placed in an electric furnace, heated from room temperature at a rate of 3 ° C / min, and held at 500 ° C for 2 hours to degrease the organic components. The compact was then further heated at a rate of 3 ° C / min to the maximum calcination temperature shown in Table 2 for 6 hours, and slowly cooled from the calcination temperature at -0.4 ° C / min to obtain a calcined body.

[0154] <Examples 6, 8, and 9, and Comparative Examples 3 to 5 and 8> The same procedure as in Example 2 was carried out to obtain calcined bodies, except that AKP-53, AA-03, AA-05, and AA-07 manufactured by Sumitomo Chemical Co., Ltd. were used as the α-alumina raw material and the maximum calcination temperature during calcination was changed as shown in Table 2.

[0155] <Comparative Examples 1 and 2> A calcined body was obtained in the same manner as in Example 2, except that 0.12 g of magnesium chloride was used per 100 g of α-alumina raw material and the maximum calcination temperature during calcination was changed as shown in Table 2.

[0156] <Comparative Example 6> A calcined body was obtained in the same manner as in Example 2, except that the pressure of the uniaxial press was set to 15 MPa.

[0157] <Examples 10 to 13> The same procedure as in Example 3 was carried out except that the type and amount of the sintering aid were changed as shown in Table 1, to obtain calcined bodies.

[0158] [Manufacturing of sintered bodies] The calcined bodies produced in each of the above-mentioned Examples and Comparative Examples were cut into the thin disk shape, and the surface of each sample was polished. Using a Noritake Katana System Katana (registered trademark) F-1N (manufactured by Kuraray Noritake Dental Co., Ltd.), the temperature was raised from room temperature to the maximum sintering temperature in Table 3 at a rate of 3°C / min in an air atmosphere, and the sample was fired at the maximum sintering temperature for 2 hours. The sample was then slowly cooled from the maximum sintering temperature at a rate of -0.4°C / min to obtain a sintered body.

[0159] The results of each example and comparative example are shown in Table 3.

[0160] [Table 1]

[0161] [Table 2]

[0162] [Table 3] [Industrial Applicability]

[0163] The dental oxide ceramic calcined body of the present invention can be suitably used for machining such as CAD / CAM.

Claims

1. A dental alumina calcined body comprising alumina particles having an average primary particle circularity of 0.81 or more and having a relative density of 43 to 63%.

2. 2. The dental alumina calcined body according to claim 1, wherein the particles have an average primary particle size of 30 to 600 nm.

3. 2. The dental alumina calcined body according to claim 1, which has a three-point bending strength of 10 to 50 MPa.

4. 2. The dental alumina calcined body according to claim 1, wherein the surface roughness Ra after polishing is 1.70 μm or less.

5. 2. The dental alumina calcined body according to claim 1, wherein the surface roughness Rz after polishing is 54 μm or less.

6. 2. The dental alumina calcined body according to claim 1, wherein the alumina contains α-alumina particles having a purity of 99.5% or more.

7. 2. The dental alumina calcined body according to claim 1, further comprising a sintering aid, said sintering aid comprising at least one element selected from the group consisting of Group 2 elements, Ce, Zr, and Y.

8. 2. The dental alumina calcined body according to claim 1, which has a surface roughness Ra of 1.40 μm or less after being sintered under atmospheric pressure without using hot isostatic pressing to form a sintered body.

9. 2. The dental alumina calcined body according to claim 1, which has a surface roughness Rz of 51 μm or less after being sintered under atmospheric pressure without using hot isostatic pressing to form a sintered body.

10. A method for producing a dental alumina calcined body, comprising: The method includes a step of press-molding an alumina composition at a surface pressure of 20 to 600 MPa, and a step of firing the obtained molded body at 400°C or higher but lower than 1200°C under atmospheric pressure, The method for producing a dental alumina calcined body according to any one of claims 1 to 9, wherein the dental alumina calcined body contains alumina particles having an average primary particle circularity of 0.81 or more and has a relative density of 43 to 63%.

11. The method for producing a dental alumina calcined body according to claim 10, wherein the alumina contains α-alumina particles having a purity of 99.5% or more.

12. 11. The method for producing a dental alumina calcined body according to claim 10, further comprising a sintering aid, wherein the sintering aid comprises at least one element selected from the group consisting of Group 2 elements, Ce, Zr, and Y.

13. A method for producing a dental alumina sintered body, comprising a step of polishing the dental alumina calcined body according to any one of claims 1 to 9 with a dental polishing tool.

14. 14. The method for producing a dental alumina sintered body according to claim 13, further comprising the step of sintering the dental alumina calcined body under atmospheric pressure without using a hot isostatic pressing treatment.

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