Ceramic material, optical member, head-mounted display, and method for producing ceramic material

Abstract

The present invention exhibits increased versatility while allowing visible light to appropriately transmit therethrough. A ceramic material (10) has a main surface having an area of 40 cm2 or more, exhibits, at a thickness of 1 mm, a total light internal transmittance of 10% or more with respect to light having a wavelength of 635 nm, and contains, as the main component, a compound having a perovskite structure represented by chemical formula AxBOz in which A includes Sr as the main component, B includes Ti as the main component, x represents 0.9-1.0, and z represents a numerical value that satisfies charge neutrality for the entire chemical formula.

Description

Ceramic material, optical member, head-mounted display, and method for manufacturing a ceramic material

[0001] The present invention relates to a ceramic material, an optical member, a head-mounted display, and a method for manufacturing a ceramic material.

[0002] In recent years, materials with high transmittance have been demanded. In particular, in wearable devices such as head-mounted displays that realize AR (Augmented Reality), VR (Virtual Reality), MR (Mixed Reality), etc., as shown in Patent Document 1 for example, as a light guide plate, high transmittance for visible light is required. Further, Patent Document 2 describes a translucent ceramic mainly composed of a perovskite compound.

[0003] International Publication No. 2022 / 234782, Patent No. 4747892

[0004] Here, in the ceramic material, it is required to appropriately transmit visible light while enhancing versatility.

[0005] An object of the present invention is to provide a ceramic material, an optical member, a head-mounted display, and a method for manufacturing a ceramic material that can appropriately transmit visible light while enhancing versatility.

[0006] The ceramic material according to the present disclosure is A

[0002] BO ｚ A compound having a perovskite structure represented by the chemical formula is used as a main component, the area of the main surface is 40 cm <00,00003> or more, the total light internal transmittance at a thickness of 1 mm for light with a wavelength of 635 nm is 10% or more, the A is mainly composed of Sr, the B is mainly composed of Ti, the x is 0.9 or more and 1.0 or less, and the z is a numerical value that satisfies the charge neutrality of the entire chemical formula.

[0007] The optical member according to the present disclosure uses the ceramic material as a light guide plate.

[0008] The head-mounted display according to the present disclosure uses the ceramic material as a light guide plate member.

[0009] The method for manufacturing ceramic materials relating to this disclosure is A x BO ｚ The process includes: preparing a powdered raw material of a perovskite compound represented by the chemical formula; molding the raw material; performing a primary sintering treatment on the molded raw material by heating it to a sintering temperature of 1300°C to 1450°C under a heating rate of 100°C / h or less to obtain a primary sintered body; performing a HIP treatment on the primary sintered body by heating it to a temperature of 1250°C to 1450°C under a pressure of 100 MPa or more; and performing a post-annealing treatment on the secondary sintered body, which is the primary sintered body that has been HIP treated, by heating it to a sintering temperature of 800°C to 1400°C under a heating rate of 200°C / h or less to obtain a ceramic material mainly composed of the compound. The above A is mainly composed of Sr, the above B is mainly composed of Ti, the above x is between 0.9 and 1.0, and the above z is a value at which the overall charge neutrality of the chemical formula is maintained. In the HIP treatment, a portion of the surface of the primary sintered body is in contact with a base portion having a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and the remaining portion of the surface of the primary sintered body is not in contact with other members when the HIP treatment is performed. In the post-annealing treatment, a portion of the surface of the secondary sintered body is in contact with a base portion having a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and the remaining portion of the surface of the secondary sintered body is not in contact with other members when the post-annealing treatment is performed.

[0010] According to the present invention, it is possible to appropriately transmit visible light while achieving high versatility.

[0011] Figure 1 is a schematic diagram of the ceramic material according to this embodiment. Figure 2 is a schematic diagram showing an example of a method for measuring the standard deviation. Figure 3 is a flowchart of the manufacturing method of the ceramic material according to this embodiment. Figure 4 is a schematic diagram of a furnace for primary sintering. Figure 5 is a schematic diagram of a furnace for HIP treatment. Figure 6 is a schematic diagram of a furnace for post-annealing treatment.

[0012] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. However, the present invention is not limited to these embodiments, and if there are multiple embodiments, they may be constructed by combining these embodiments. Furthermore, numerical values ​​include a range of rounding. In addition, in this embodiment, the lower and upper limits of the numerical range can be combined as appropriate.

[0013] (Ceramic Material) Figure 1 is a schematic diagram of the ceramic material according to this embodiment. The ceramic material 10 according to this embodiment is a translucent member that can transmit visible light. The ceramic material 10 is preferably a polycrystalline ceramic, and it can also be said that it is preferably a polycrystalline ceramic sintered body. As shown in Figure 1, the ceramic material 10 according to this embodiment is a plate-shaped member, but the shape of the ceramic material 10 is not limited to a plate and may be arbitrary. In this embodiment, the ceramic material 10 is used as an optical element. More specifically, in this embodiment, the ceramic material 10 is used as a light guide plate. That is, in this embodiment, an optical member using the ceramic material 10 as a light guide plate can be provided. The optical member here is a display device, etc. More specifically, the ceramic material 10 is used as a light guide plate for a head-mounted display. A head-mounted display is a display device (wearable device) that is worn on a person's head. However, the applications of the ceramic material 10 are arbitrary and are not limited to use as a light guide plate, nor are they limited to use in head-mounted displays. For example, the ceramic material 10 is also useful as a base substrate for growing other materials, a transparent substrate for capacitive sensors for remote sensing, a low-voltage driven electrostatic induction panel substrate, jewelry and furnishings, cover glass and housings for mobile terminals and wearable devices, interiors and non-contact panel covers for mobile bodies such as automobiles, dielectric antennas, window materials for surface microwave plasma generation, and droplet-controlled digital microfluidic substrates.

[0014] (A x BO ｚ ) The ceramic material 10 is A x BOｚ It contains, as a main component, a compound (oxide) having a perovskite structure represented by the chemical formula. That is, the compound as the main component has one or more kinds of element A located at the A site in the perovskite structure, one or more kinds of element B located at the B site in the perovskite structure, and O (oxygen element) located at the O site in the perovskite structure, and can be said to be an oxide. Here, the main component may refer to that the content rate with respect to the whole of the ceramic material 10 is more than 50% in terms of molar ratio. The ceramic material 10 is A as the main component x BO ｚ The content rate of is preferably 90% or more and 100% or less, more preferably 95% or more and 100% or less, in terms of molar ratio with respect to the whole of the ceramic material 10. Note that the content of the main component can be evaluated by a quantitative analysis method using the powder X-ray method. When two or more kinds of elements are included as element A, any of those elements is located at each A site of the perovskite structure, and the same applies to the description of the B site hereinafter

[0015] (Element A) (Main component) A x BO ｚ In, A is an element located at the A site in the perovskite structure. Element A has Sr as the main component. That is, it can be said that the main component (the element having the largest content among the elements located at the A site) among the elements located at the A site is Sr

[0016] For the ceramic material 10, A x BO ｚ In, the ratio (amount of Sr / total amount of element A) of the amount of Sr contained in element A to the total amount of each element A (each element located at the A site) is 50% or more in terms of molar ratio, preferably 95% or more and 100% or less, and more preferably 99.9% or more and 100% or less. When the amount of Sr contained in the A site is within this range, it becomes possible to appropriately transmit visible light and also possible to appropriately refract visible light

[0017] Furthermore, whether or not a particular element is present in a perovskite crystal can be analyzed using methods such as peak shift of diffraction lines by X-ray diffraction (XRD), structural analysis using the Rietveld method, or X-ray absorption fine structure (EXAFS) analysis. In addition, the elemental content can be measured by X-ray fluorescence analysis (XRF).

[0018] (Rare Earth Elements) Element A may contain rare earth elements as substitutional components. The rare earth elements contained in element A are preferably at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, more preferably at least one of La, Sm, Eu, Gd, Tb, Ho, Yb, and Lu, and more preferably at least one of La, Sm, Gd, Tb, Yb, and Lu. The inclusion of such rare earth elements in element A improves sinterability, making it possible to appropriately transmit visible light and to appropriately refract visible light.

[0019] A of ceramic material 10 x BO ｚ In this configuration, the ratio of the amount of rare earth elements contained in element A (rare earth elements located at site A) to the total amount of each element A (each element located at site A) (amount of rare earth elements / total amount of element A) is preferably 0% to 2.0% in molar ratio, more preferably 0% to 1.5%, even more preferably 0% to 1.0%, and even more preferably 0% to 0.1%. When the amount of rare earth elements contained at site A is within this range, it becomes possible to appropriately transmit visible light and to appropriately refract visible light. Note that, if element A contains multiple types of rare earth elements, the amount of rare earth elements contained in element A here refers to the total amount of those rare earth elements.

[0020] (Other components) A of ceramic material 10 x BO ｚ In this mixture, element A may contain elements other than alkaline earth metal elements and rare earth elements as other components (modifying agents). Examples of other components include at least one of Na, K, and Bi.

[0021] (Element B) (Main component) A x BO ｚ In this example, B is the element located at the B site in the perovskite structure. Element B is primarily composed of Ti. In other words, Ti is the main component among the elements located at the B site (the element with the highest content among the elements located at the B site).

[0022] A of ceramic material 10 x BO ｚ In this configuration, the ratio of the amount of Ti contained in element B (Ti located at the B site) to the total amount of each element B (each element located at the B site) (amount of Ti / total amount of element B) is preferably 50% or more in molar terms, preferably 90% or more and 100%, and more preferably 99.9% or more and 100% or less. When the amount of Ti contained in the B site is within this range, it becomes possible to appropriately transmit visible light and also to appropriately refract visible light.

[0023] (Rare Earth Elements) Element B may contain rare earth elements as substitutional components. The rare earth elements contained in element B are preferably at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and more preferably at least one of La, Sm, Eu, Gd, Tb, Ho, Yb, and Lu, and more preferably Sm, Gd, Tb, Yb, and Lu. The inclusion of such rare earth elements in element B improves sinterability, making it possible to appropriately transmit visible light and to appropriately refract visible light.

[0024] A of ceramic material 10 x BO ｚIn this configuration, the ratio of the amount of rare earth elements contained in element B (rare earth elements located at the B site) to the total amount of each element B (each element located at the B site) (amount of rare earth elements / total amount of element B) is preferably 0% to 2.0% in molar ratio, more preferably 0% to 1.5%, and even more preferably 0% to 1.0%. When the amount of rare earth elements contained at the B site falls within this range, it becomes possible to appropriately transmit visible light and to appropriately refract visible light. Note that, if element B contains multiple types of rare earth elements, the amount of rare earth elements contained in element B here refers to the total amount of those rare earth elements.

[0025] (Other components) A of ceramic material 10 x BO ｚ In this, element B may contain elements other than Ti and rare earth elements as other components (modifying agents). Examples of other components include at least one of Zr, Ge, Zn, Sc, Y, Ga, Si, Sn, Nb, Ta, Hf, and Al. A of ceramic material 10 x BO ｚ In this configuration, the ratio of the amount of other components (other components located at the B site) to the total amount of each element B (each element located at the B site) (amount of other components / total amount of element B) is preferably 0% or more and 1.0% or less in molar ratio, and more preferably 0% or more and 0.1% or less. When the amount of other components contained at the B site is within this range, it becomes possible to appropriately transmit visible light and to appropriately refract visible light.

[0026] (Value x) A of ceramic material 10 x BO ｚ In this equation, x represents the molar ratio of element A content, or more specifically, the molar ratio of element A content to element B content. Here, the content of element A refers to the total content of all elements if element A contains multiple types of elements, and similarly, the content of element B refers to the total content of all elements if element B contains multiple types of elements.

[0027] The numerical value x is between 0.9 and 1.0, preferably between 0.95 and 1.0, and more preferably between 0.98 and 1.0. A suitable crystal structure can be achieved when the numerical value x falls within this range.

[0028] (Value z) A of ceramic material 10 x BO ｚ In this formula, z is a numerical value that indicates the content (molar ratio) of O, and more specifically, it is the molar ratio of the O content to the content of element B. The numerical value z is derived from the chemical formula A x BO ｚ This is a value that maintains overall charge neutrality, and for example, it could be 3.

[0029] (Minor component) The ceramic material 10 is the main component A mentioned above. x BO ｚ The ceramic material 10 may also contain auxiliary components other than those listed above. The ceramic material 10 preferably contains at least one element selected from Li, Si, B, Y, Al, Ca, and Mg as an auxiliary component, and more preferably contains Si. This auxiliary component originates from the sintering aid used when sintering the raw materials to form the ceramic material 10. The ceramic material 10 may also contain the auxiliary components in the form of oxides or carbonates of the elements listed above. By introducing these auxiliary components, it is possible to adjust the sinterability while maintaining the transparency of the final material. The content of these auxiliary components is preferably 50 ppm to 50,000 ppm, more preferably 50 ppm to 20,000 ppm, and even more preferably 50 ppm to 5,000 ppm, in molar ratio relative to the total ceramic material 10. By setting the content of the auxiliary components within this range, a ceramic material 10 with high refractive index and high transmittance can be appropriately manufactured. The content of minor components can be measured by inductively coupled plasma mass spectrometry (ICP-MS).

[0030] The ceramic material 10 may contain auxiliary components other than those derived from the sintering aid. If the auxiliary components other than those derived from the sintering aid are considered auxiliary components, it is preferable that the ceramic material 10 contains at least one selected from Ce and Pr as an auxiliary component. By including such an auxiliary component, the auxiliary component can function as an oxidizing agent, appropriately receiving electrons and more favorably suppressing the decrease in transmittance. The ceramic material 10 may also contain the auxiliary component in the form of an oxide of the elements listed above. The content of the auxiliary component is preferably 50 ppm to 50,000 ppm, more preferably 50 ppm to 20,000 ppm, and even more preferably 50 ppm to 5,000 ppm, in molar ratio relative to the total amount of the ceramic material 10. By setting the content of the auxiliary component within this range, a ceramic material 10 with high refractive index and high transmittance can be appropriately manufactured. The content of the auxiliary component can also be measured by inductively coupled plasma mass spectrometry (ICP-MS).

[0031] In addition to the main component, minor component, and auxiliary component described above, the ceramic material 10 may contain any other component as an unavoidable impurity. Preferably, the ceramic material 10 does not contain any components other than the main component, minor component, and auxiliary component, excluding the unavoidable impurities.

[0032] (Properties of ceramic material) Next, the properties of the ceramic material 10 will be described.

[0033] (Shape) In this embodiment, the ceramic material 10 is plate-shaped, or more precisely, disc-shaped. However, the shape of the ceramic material 10 is arbitrary and can be any shape to match the product shape. For example, the ceramic material 10 may be elliptical, polygonal, polygonal, spherical, or ellipsoidal. Also, the ceramic material 10 may be curved.

[0034] (Size) The surface area of ​​the main surface of the ceramic material 10 is 40 cm². ２ That's all. 40 cm ２ More than 750cm ２The following is preferable: 40 cm ２ More than 340cm ２ The following is more preferable: 46 cm ２ 335cm or more ２ The following is more preferable: 46 cm ２ More than 100cm ２ The following is even more preferable. In particular, increasing the main surface area while transmitting visible light and maintaining the uniformity of the material are conflicting challenges in the prior art. However, in this embodiment, it is possible to increase the main surface area while transmitting visible light. Because the area of ​​the main surface is relatively large, it can be used in various applications that require visible light transmission, and the versatility of the ceramic material 10 can be increased. Here, the main surface refers to the surface with the largest area among the multiple surfaces of the ceramic material 10. For example, if the ceramic material 10 is in the form of a plate as shown in Figure 1, the area of ​​the main surface 10A, which is the largest surface of the plate, becomes the area of ​​the main surface. Also, if the ceramic material 10 is composed of a single surface, such as a sphere or ellipsoid, the area of ​​that single surface becomes the area of ​​the main surface.

[0035] (Thickness) The thickness D of the ceramic material 10 is preferably 1 mm or more and 50 mm or less, more preferably 2 mm or more and 30 mm or less, and even more preferably 5 mm or more and 20 mm or less. Having a thickness D within this range makes the versatility of the ceramic material 10 higher. Here, thickness D refers to the distance from the surface with the largest area (main surface) to the surface on the opposite side. For example, if the ceramic material 10 is in the shape of a plate as shown in Figure 1, the distance from the main surface 10A to the main surface 10B on the opposite side of the main surface 10A is the thickness D. Also, if the ceramic material 10 is composed of a single surface, such as a sphere or ellipsoid, the maximum value of the distance between any two points on the surface is the thickness D.

[0036] (Internal Transmittance) The ceramic material 10 has a total internal transmittance of 1 mm thickness for light with a wavelength of 635 nm of 10%, preferably 30% to 100%, more preferably 50% to 100%, and even more preferably 80% to 100%. The total internal transmittance of 1 mm thickness can be measured by using a spectrophotometer (PerkinElmer; LAMBDA950) with an integrating sphere unit (150 mm InGaAs Int. Sptere) as a detector, and bringing the ceramic material 10 into close contact with the integrating sphere. If the actual thickness of the ceramic material 10 is not 1 mm, the measured value can be converted to the total internal transmittance equivalent to a 1 mm thickness using the Lambert-Beer law. Furthermore, if the plate thickness t is greater than 1 mm, the plate thickness of the ceramic material 10 may be adjusted to 1 mm by polishing or etching before measurement. The total light transmittance is determined according to the composition of the ceramic material 10, but even with the same composition, it may differ depending on the manufacturing process of the ceramic material 10. Therefore, by manufacturing the ceramic material 10 using, for example, the manufacturing method described later, the total internal light transmittance can be set to the above range. The internal transmittance can be calculated using the measured values ​​T1 and reflectance R of two types of external transmittances with different plate thicknesses in the following procedure. External transmittance means transmittance including surface reflection loss. The converted internal transmittance can be calculated as follows. When the refractive index at 635 nm is n, the reflectance at the incident surface is calculated from the formula (n-1)² / (n+1)², and the theoretical transmittance T0 is obtained by subtracting twice the reflectance from 1, so T2 = T1 / T0 is the internal transmittance. The apparent absorption coefficient α (cm⁻¹) can be calculated by taking the natural logarithm of the internal transmittance T², adding a negative sign, and dividing by the sample thickness (in cm). By multiplying the apparent absorption coefficient by 0.1 (cm), adding a negative sign, and taking an exponent with base e, the converted internal transmittance T³ (%) for a 1 mm thickness can be evaluated. External transmittance can be measured using a spectrophotometer (Hitachi High-Technologies Corporation: U-4100) on samples with a plate thickness of 0.5 mm to 10 mm that have been mirror-polished on both sides. The converted internal transmittance may also be evaluated by the following method.When a laser light source with a spot diameter of 2 mm and a wavelength of 635 nm is incident perpendicularly on a mirror-polished parallel plate of ceramics, and a Si photodetector is placed 20 cm away from the sample on the side opposite the light source, the apparent absorption coefficient α'(cm-1) can be calculated by using the transmittance T1' from the ratio of the light intensity received by the Si photodetector to the output of the incident light source, taking the natural logarithm of T2' = T1' / T0 with a negative sign, and dividing by the sample thickness (in cm). The apparent absorption coefficient α'(cm-1) can be calculated by multiplying the apparent absorption coefficient by 0.1 (cm), adding a negative sign, and taking an exponent with base e to obtain the converted internal transmittance T3' for a 1 mm thickness, which can then be evaluated as the converted internal transmittance. Note that the sample polished surface is sufficiently large compared to the laser spot diameter, typically about 1 cm.

[0037] (Standard deviation of transmittance) The ceramic material 10 preferably has a standard deviation of total light internal transmittance for a thickness of 1 mm with respect to light of a wavelength of 635 nm at each position on the main surface (main surface 10A in this embodiment) of 0.5% or less, more preferably 0.4% or less, and even more preferably 0.3% or less. Having the standard deviation of total light internal transmittance within this range reduces the variation in transmittance depending on the position, allowing visible light to be transmitted appropriately.

[0038] Figure 2 is a schematic diagram illustrating an example of a method for measuring standard deviation. Figure 2 shows a ceramic material 10 viewed from a direction perpendicular to the main surface (thickness direction). As shown in Figure 2, the periphery of the main surface of the ceramic material 10 (in this case, the main surface 10A) is defined as the periphery 10C. A hypothetical edge (closed curve) on the main surface of the ceramic material 10, located 1 cm radially inward from the periphery 10C, is defined as edge 10D. In this case, the standard deviation of the total internal light transmittance at each position P within the region enclosed by edge 10D within the entire main surface of the ceramic material 10 may be used as the standard deviation of the total internal light transmittance of the ceramic material 10. Specifically, the standard deviation of the total internal light transmittance values ​​at positions P1 to P9 within the region enclosed by edge 10D may be used as the standard deviation of the total internal light transmittance of the ceramic material 10. Position P1 may be any position within the region enclosed by the edge 10D of the main surface of the ceramic material 10, position P2 is 2 cm away from position P1 in direction X (one direction along the main surface), and position P3 is 2 cm away from position P2 in direction X. Positions P4, P5, and P6 are 2 cm away from positions P1, P2, and P3 in direction Y (a direction perpendicular to direction X along the main surface). Positions P7, P8, and P9 are 2 cm away from positions P1, P2, and P3 in direction Y (a direction perpendicular to direction X along the main surface). The value of the total light transmittance inside the ceramic material 10 with a thickness of 1 mm for light with a wavelength of 635 nm, as described above, may be a measurement taken at any position within the region enclosed by the edge 10D of the main surface of the ceramic material 10.

[0039] (Refractive Index) The ceramic material 10 preferably has a refractive index of 2.30 or higher for light with a wavelength of 635 nm, more preferably 2.30 or higher and 2.45 or lower, and even more preferably 2.35 or higher and 2.40 or lower. A refractive index within this range allows for appropriate refraction of visible light. The refractive index can be measured by spectroscopic ellipsometry (JAWoollam Co., Inc.; M-2000 DI).

[0040] (Standard deviation of refractive index) The ceramic material 10 preferably has a standard deviation of refractive index for light with a wavelength of 635 nm at each position on the main surface (main surface 10A in this embodiment) of 0.003 or less, more preferably 0.002 or less, and even more preferably 0.001 or less. Having a standard deviation of refractive index within this range reduces the variation in refractive index depending on the position, allowing visible light to be transmitted appropriately. In this case, the standard deviation of refractive index at each position P within the region enclosed by the edge 10D of the main surface of the ceramic material 10 may be used as the standard deviation of the refractive index of the ceramic material 10. Specifically, the standard deviation of refractive index values ​​at positions P1 to P9 within the region enclosed by the edge 10D may be used as the standard deviation of the refractive index of the ceramic material 10. Note that the refractive index value of the ceramic material 10 for light with a wavelength of 635 nm described above may be a measured value at any position within the region enclosed by the edge 10D of the main surface of the ceramic material 10.

[0041] (Method for manufacturing ceramic material) The ceramic material 10 according to this embodiment may be manufactured by any method, but it is preferably manufactured by a solid-phase method, and an example of such a manufacturing method is described below.

[0042] (Preparation of raw materials) Figure 3 is a flowchart showing the method for manufacturing a ceramic material according to this embodiment. As shown in Figure 3, in this manufacturing method, powdered raw materials to be used in the manufacture of the ceramic material 10 are prepared (step S10). The raw materials here refer to the raw materials that are the main components of the ceramic material 10, A, B ｙ O ｚ It is a powdery compound with a perovskite structure represented by the chemical formula shown.

[0043] In this embodiment, a compound containing element A (preferably an oxide or carbonate of element A) and a compound containing element B (preferably an oxide of element B) are mixed and calcined, thereby producing AB ｙ O ｚ The raw materials are manufactured. For example, if the ceramic material 10 is SrTiO ３ In that case, a compound containing Sr (preferably in powdered form of SrCO) ３ ) and a compound containing Ti (preferably in the form of powdered TiO２ ) is mixed with and calcined, and the raw material, powdered SrTiO ３ The following can be manufactured. Also, if the ceramic material 10 contains rare earth elements in elements A and B, a compound of rare earth elements (preferably an oxide of a rare earth element) may be mixed in with the compound containing Sr and the compound containing Ti and then calcined. In addition, powdered SrTiO ３ It is manufactured using as a raw material, and in the subsequent molding process, this raw material (powdered SrTiO ３ ) may be mixed with a compound of a rare earth element (preferably an oxide of a rare earth element) and molded.

[0044] In the calcination process, it is preferable to add water to a compound containing element A and a compound containing element B to produce a slurry, and then add a dispersant to the slurry and mix to obtain a mixture to be calcined. The average particle size of the compound containing element A and the compound containing element B is preferably 1.0 μm or less, more preferably 0.05 μm or more and 0.9 μm or less, and even more preferably 0.08 μm or more and 0.8 μm or less. The average particle size can be measured by laser diffraction / scattering. Specifically, it can be measured using a particle size distribution analyzer (Shimadzu Corporation: SALD-2300). As the dispersant, it is preferable to use a polycarboxylate salt. When mixing, it is preferable to mix using a ball mill, and it is preferable to use alumina or zirconia balls. The mixing time is preferably 2 hours or more and 48 hours or less, more preferably 3 hours or more and 36 hours or less, and even more preferably 4 hours or more and 24 hours or less. By obtaining a mixture under the above conditions, the raw materials A and B ｙ O ｚ This allows for high sinterability, enabling the production of a highly versatile ceramic material 10 that appropriately transmits visible light.

[0045] In the calcination process, the mixture obtained as described above is dried and crushed, and then calcined to obtain the raw material AB ｙ O ｚIt is preferable to obtain the powder. The calcination temperature, which is the temperature at which the mixture is heated in the calcination process, is preferably 800°C to 1100°C, more preferably 820°C to 1070°C, and even more preferably 840°C to 1040°C. The calcination time, during which the mixture is held at the calcination temperature, is preferably 4 hours to 48 hours, more preferably 5 hours to 36 hours, and even more preferably 6 hours to 24 hours. Calcination is preferably carried out under an oxygen atmosphere (for example, under an atmospheric atmosphere). By calcining under the above conditions, the raw material AB ｙ O ｚ This allows for high sinterability, enabling the production of a highly versatile ceramic material 10 that appropriately transmits visible light.

[0046] If the ceramic material 10 contains auxiliary components, powdered auxiliary materials that serve as raw materials for the auxiliary components of the ceramic material 10 are also prepared. The auxiliary materials are materials containing elements included in the auxiliary components of the ceramic material 10, and are so-called sintering aids. The auxiliary materials may be any materials containing elements included in the auxiliary components of the ceramic material 10. As auxiliary materials, oxides of elements included in the auxiliary components (for example, Li ２ O, SiO ２ , B ２ O ３ Fluorides of elements contained in the minor components (e.g., LiF, MgF) ２ (e.g., carbonates of elements included in the minor components (e.g., Li) ２ CO ３ Examples include:

[0047] If the ceramic material 10 contains auxiliary components, powdered auxiliary raw materials that serve as raw materials for the auxiliary components of the ceramic material 10 are also prepared. The auxiliary raw materials are materials containing elements included in the auxiliary components of the ceramic material 10, and are so-called oxidizing agents. The auxiliary raw materials may be any materials containing elements included in the auxiliary components of the ceramic material 10. As auxiliary raw materials, oxides of elements included in the auxiliary components (for example, CeO) ２ , Pr ６ O １１ Examples include:

[0048] As described above, in this embodiment, the raw material (AB ｙO ｚ Prepare the powder (of AB), but the method of preparing the raw materials is not limited to this. For example, manufactured AB ｙ O ｚ The powder may be used as a raw material.

[0049] (Molding of raw materials) Next, the obtained raw materials are molded to obtain a molded body 10a (step S12). That is, the molded body 10a is made of powdered AB, which is the raw material. ｙ O ｚ It can be said that this is a molded body. The size and shape of the molded body 10a are set in accordance with the size and shape of the ceramic material 10 to be obtained. The molding conditions for the molded body 10a may be arbitrary, and for example, CIP (Cold Isostatic Pressing) molding may be used.

[0050] (Primary Sintering) Next, the obtained molded body 10a is subjected to primary sintering to obtain a primary sintered body 10b (step S14). That is, the primary sintered body 10b is a sintered body obtained by subjecting the molded body 10a to primary sintering treatment.

[0051] Figure 4 is a schematic diagram of a furnace used for primary sintering. As shown in Figure 4, in this manufacturing method, the molded body 10a is placed in the furnace F and the inside of the furnace F is heated to perform primary sintering of the molded body 10a.

[0052] The primary sintering temperature, which is the temperature at which the molded body 10a is heated in the primary sintering process (temperature inside the furnace F), is 1200°C or higher and 1450°C or lower, preferably 1250°C or higher and 1420°C or lower, and more preferably 1300°C or higher and 1400°C or lower. The heating rate (temperature increase inside the furnace F) when raising the temperature of the molded body 10a to the primary sintering temperature in the primary sintering process is 300°C / h or lower, preferably 30°C / h or higher and 200°C / h or lower, and more preferably 50°C / h or higher and 150°C / h or lower. The primary sintering time, during which the molded body 10a (inside the furnace F) is held at the primary sintering temperature, is preferably 2 hours or higher and 72 hours or lower, more preferably 4 hours or higher and 48 hours or lower, and even more preferably 6 hours or higher and 36 hours or lower. Primary sintering may be carried out under any atmosphere, but it is preferably carried out under an oxygen atmosphere (e.g., an atmospheric atmosphere) or a vacuum atmosphere (e.g., a total pressure of less than 10 Pa). Furthermore, the pressure used during primary sintering (pressure inside the furnace F) can be arbitrary, but in the case of an oxygen atmosphere, it may be set to 1 atm or more and 5 atm or less. By performing primary sintering under these conditions, even large molded bodies 10a can be sintered while suppressing cracking, making it possible to manufacture a highly versatile ceramic material 10 that appropriately transmits visible light.

[0053] Furthermore, in this manufacturing method, it is preferable to perform primary sintering in the furnace F with the molded body 10a placed on the base portion A1. That is, it is preferable that the molded body 10a is in contact with the base portion A1 without contacting the inner surface Fa of the furnace F during primary sintering. The base portion A1 is made of a material with a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and may be made of alumina, for example. The shape of the base portion A1 may be arbitrary, but may be plate-shaped, for example. In this case, the entire surface of the molded body 10a on the base portion A1 side may be in contact with the base portion A1. By performing primary sintering in this contact with the base portion A1, even a large molded body 10a can be sintered while suppressing cracking, so that a highly versatile ceramic material 10 can be manufactured while appropriately transmitting visible light.

[0054] (HIP) Next, the obtained primary sintered body 10b is subjected to HIP (Hot Isostatic Pressing) treatment to obtain a secondary sintered body 10c (step S16). That is, the secondary sintered body 10c is a sintered body obtained by subjecting the primary sintered body 10b to HIP treatment.

[0055] Figure 5 is a schematic diagram of a furnace used for HIP treatment. As shown in Figure 5, in this manufacturing method, the molded body 10a is placed in the furnace F, and the primary sintered body 10b is subjected to HIP treatment by heating the furnace F under pressure. The furnace F used for HIP treatment may be the same furnace F used for primary sintering, or it may be a different furnace.

[0056] The pressure applied to the primary sintered body 10b during the HIP treatment (pressure inside the furnace F) is 100 MPa or more, preferably 110 MPa to 190 MPa, and more preferably 120 MPa to 180 MPa. The HIP temperature, which is the temperature at which the primary sintered body 10b is heated during the HIP treatment (temperature inside the furnace F), is 1150°C to 1450°C, preferably 1200°C to 1400°C, and more preferably 1250°C to 1350°C. By setting the HIP treatment temperature to 1350°C or lower, the reduction of the material can be suppressed as much as possible, and the subsequent post-annealing process can efficiently restore the color. The heating rate (heating temperature inside the furnace F) when raising the primary sintered body 10b to the HIP temperature during the HIP treatment is preferably 400°C / h or less, more preferably 100°C / h to 380°C / h, and even more preferably 150°C / h to 370°C / h. The HIP time for holding the primary sintered body 10b (inside the furnace F) at the HIP temperature is preferably 0.5 hours or more and 8 hours or less, more preferably 0.5 hours or more and 6 hours or less, and even more preferably 1 hour or more and 4 hours or less. The HIP treatment may be carried out in any atmosphere, but it may be carried out in an oxygen atmosphere (for example, an atmospheric atmosphere) or in an atmosphere containing Ar (for example, an Ar atmosphere, or a mixed atmosphere of Ar and oxygen). By performing the HIP treatment under the above conditions, even a large molded body 10a can be sintered while suppressing cracking, so that a highly versatile ceramic material 10 can be manufactured while appropriately transmitting visible light.

[0057] Furthermore, in this manufacturing method, it is preferable to perform HIP treatment in the furnace F with the primary sintered body 10b placed on the base portion A2. That is, it is preferable that the primary sintered body 10b is in contact with the base portion A2 without contacting the inner surface Fa of the furnace F while undergoing HIP treatment. The base portion A2 is made of a material with a thermal conductivity of 5 W / m / L or more and 60 W / m / L or less, and may be made of alumina, for example. It is also preferable that the base portion A2 contacts only a part of the surface of the primary sintered body 10b on the base portion A2 side. In other words, it is preferable that the surface of the primary sintered body 10b on the base portion A2 side is in contact with the base portion A2 in a part of the area, and the remaining area is open in the furnace F (not in contact with other members). This allows the primary sintered body 10b to be uniformly pressed, suppressing cracking and enabling the production of large-sized ceramic materials 10. The base portion A2 may have any shape that contacts only a part of the surface of the primary sintered body 10b, but for example, it may be composed of multiple spherical members or multiple columnar members. This allows the surface of the primary sintered body 10b to be appropriately open between the spherical or columnar members.

[0058] Thus, the ceramic material 10 undergoes HIP treatment during the manufacturing process, and the primary sintered body 10b is in a reduced state after the HIP treatment. The degree of reduction of the primary sintered body 10b varies depending on the raw material composition, but the stronger the degree of reduction, the more it affects the heating time, holding time, and susceptibility to cracking in the oxidation process during the post-annealing process after the HIP treatment. The resistivity of the primary sintered body 10b is preferably 0.1 kΩ·cm to 20 MΩ·cm, more preferably 0.5 kΩ·cm to MΩ·cm, and even more preferably 1 kΩ·cm to 10 MΩ·cm. By having the resistivity of the primary sintered body 10b within this range, cracking due to rapid heat absorption and exothermic reaction between the sample and the material during the oxidation reaction during post-annealing when the size is increased can be suppressed, and the heat treatment temperature and heat treatment time can be reduced. The resistivity can be measured using a digital multimeter (HIOKI DT4256).

[0059] (Post-annealing) Next, the obtained secondary sintered body 10c is subjected to post-annealing treatment to obtain the ceramic material 10 (step S18).

[0060] Figure 6 is a schematic diagram of a furnace used for post-annealing. As shown in Figure 6, in this manufacturing method, the secondary sintered body 10c is placed in the furnace F and the inside of the furnace F is heated to perform post-annealing on the secondary sintered body 10c. The furnace F used for post-annealing may be the same furnace F used for primary sintering or HIP treatment, or it may be a different furnace.

[0061] The annealing temperature, which is the temperature at which the secondary sintered body 10c is heated in the post-annealing process (temperature inside the furnace F), is 700°C to 1400°C, preferably 750°C to 1270°C, and more preferably 800°C to 1250°C. The heating rate (temperature increase inside the furnace F) when raising the secondary sintered body 10c to the annealing temperature in the post-annealing process is preferably 200°C / h or less, more preferably 150°C / h to 200°C / h, and even more preferably 100°C / h to 200°C / h. The annealing time, during which the secondary sintered body 10c (inside the furnace F) is held at the annealing temperature, is preferably 5 hours to 120 hours, more preferably 6 hours to 96 hours, and even more preferably 7 hours to 72 hours. The post-annealing treatment may be carried out under any atmosphere, but it may be carried out under an oxygen atmosphere (for example, an atmospheric atmosphere) or under an Ar-containing atmosphere (for example, an Ar atmosphere, or a mixed atmosphere of Ar and oxygen). The pressure applied to the secondary sintered body 10c during the post-annealing treatment (pressure inside the furnace F) may be arbitrary, but it may be set to 1 atm or more and 5 atm or less. By performing the post-annealing treatment under the above conditions, even large molded bodies 10a can be sintered while suppressing cracking, so that a highly versatile ceramic material 10 can be manufactured while appropriately transmitting visible light.

[0062] Furthermore, in this manufacturing method, it is preferable to perform post-annealing treatment in the furnace F with the secondary sintered body 10c placed on the base portion A3. That is, it is preferable that the secondary sintered body 10c is in contact with the base portion A3 without contacting the inner surface Fa of the furnace F during post-annealing treatment. The base portion A3 is made of a material with a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and may be made of alumina, for example. It is also preferable that the base portion A3 contacts only a part of the surface of the secondary sintered body 10c on the base portion A3 side. In other words, it is preferable that the surface of the secondary sintered body 10c on the base portion A3 side is in contact with the base portion A3 in a part of the area, and the remaining area is open in the furnace F (not in contact with other members). This makes it possible to manufacture large-sized ceramic materials 10 while suppressing cracking. The base portion A3 may have any shape that contacts only a part of the surface of the secondary sintered body 10c, but for example, it may be composed of multiple spherical members or multiple columnar members. This allows the surface of the secondary sintered body 10c to be appropriately exposed between the spherical or columnar members.

[0063] (Total heating time) Here, the total heating time is defined as the time during which the molded body 10a is exposed to a temperature of 1000°C or higher from the time the molded body 10a is manufactured to the ceramic material 10. That is, for example, the total heating time is the sum of the time during primary sintering when the temperature is 1000°C or higher, the time during HIP when the temperature is 1000°C or higher, and the time during post-annealing when the temperature is 1000°C or higher. In this case, the total heating time is preferably 124 hours or less, more preferably 24 hours or more and 100 hours or less, and even more preferably 30 hours or more and 76 hours or less. By setting the total heating time within this range, discoloration due to excessive defects in the ceramic material 10 can be suppressed, and a highly versatile ceramic material 10 that appropriately transmits visible light can be manufactured.

[0064] (Effects) As described above, the ceramic material 10 according to the first aspect of this disclosure is A x BO ｚ The main component is a perovskite compound represented by the chemical formula, and the main surface area is 40 cm². ２The above conditions are met, and the total light transmittance within a 1 mm thickness for light with a wavelength of 635 nm is 10% or more. A is mainly composed of Sr, B is mainly composed of Ti, x is between 0.9 and 1.0, and z is the value at which the overall chemical formula maintains charge neutrality. Single crystal SrTiO ３ While it may be able to transmit visible light appropriately, its small size limits its versatility. In contrast, according to this disclosure, the ceramic material 10 has a total light-emitting internal transmittance and a main surface area within the above range. x BO ｚ By using a sintered body with this material as the main component, it can be made in a large size, thus enabling high versatility while appropriately transmitting visible light.

[0065] The ceramic material 10 according to the second aspect of this disclosure is the ceramic material 10 according to the first aspect, wherein the standard deviation of the total light internal transmittance for light with a wavelength of 635 nm at each position on the main surface of a 1 mm thickness is 0.5% or less. According to this disclosure, the variation in transmittance from position to position is reduced, and visible light can be transmitted appropriately.

[0066] The ceramic material 10 according to the third aspect of this disclosure is the ceramic material 10 according to the first or second aspect, wherein the refractive index for light with a wavelength of 635 nm is 2.30 or higher. According to this disclosure, visible light can be appropriately refracted.

[0067] The ceramic material 10 according to the fourth aspect of this disclosure is the ceramic material 10 according to the third aspect, wherein the standard deviation of the refractive index for light with a wavelength of 635 nm at each position on the main surface is 0.003 or less. According to this disclosure, the variation in refractive index from position to position is reduced, and visible light can be appropriately refracted.

[0068] The optical member according to the fifth aspect of this disclosure uses a ceramic material 10 according to any of the first to fourth aspects as a light guide plate. According to this disclosure, it is possible to achieve high versatility while appropriately transmitting visible light.

[0069] The head-mounted display according to the sixth aspect of this disclosure uses a ceramic material 10 according to any of the first to fourth aspects as a light guide plate member. According to this disclosure, it is possible to achieve high versatility while appropriately transmitting visible light.

[0070] A method for manufacturing the ceramic material 10 according to the seventh aspect of this disclosure is A x BO ｚ The process includes: preparing a powdered raw material of a perovskite compound represented by the chemical formula; molding the raw material; performing a primary sintering treatment on the molded raw material by heating it to a sintering temperature of 1300°C to 1450°C under conditions of a heating rate of 100°C / h or less to obtain a primary sintered body; performing a HIP treatment on the primary sintered body by heating it to a temperature of 1250°C to 1450°C under conditions of 100 MPa or more; and performing a post-annealing treatment on the secondary sintered body, which is the primary sintered body that has undergone HIP treatment, by heating it to a sintering temperature of 800°C to 1400°C under conditions of a heating rate of 200°C / h or less to obtain a ceramic material mainly composed of the compound, wherein A is mainly composed of Sr, B is mainly composed of Ti, y is between 0.9 and 1.0, and z is the value at which the charge neutrality of the entire chemical formula holds. x BO ｚ When manufacturing perovskite compounds represented by the chemical formula shown, coloring can prevent proper transmission of visible light, and large sizes can cause cracking. Therefore, there has been a need for a material that transmits visible light appropriately while being highly versatile. In contrast, the method disclosed herein allows for the production of a ceramic material 10 that has high transmittance and is less prone to cracking even when large in size, thus enabling the production of a highly versatile ceramic material 10 that transmits visible light appropriately.

[0071] The manufacturing method of the ceramic material 10 according to the eighth aspect of the present disclosure is the manufacturing method according to the seventh aspect. In the HIP treatment, a HIP treatment is performed in a state where a partial region of the surface of the primary sintered body 10b contacts a base portion A2 having a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and the remaining region of the surface of the primary sintered body 10b does not contact other members. In the post-annealing treatment, a post-annealing treatment is performed in a state where a partial region of the surface of the secondary sintered body 10c contacts a base portion A3 having a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and the remaining region of the surface of the secondary sintered body 10c does not contact other members. According to this, even if it has a large size, it can be made more crack-resistant, so that a ceramic material 10 with high versatility while appropriately transmitting visible light can be manufactured.

[0072] (Example) Next, examples will be described. Note that the embodiments may be changed as long as the effects of the invention are achieved. Table 1 is a table showing the ceramic materials of each example and their evaluation results.

[0073]

[0074] (Example 1) In Example 1, strontium carbonate as a compound containing element A and titanium oxide as a compound containing element B were mixed at a mixing ratio such that the stoichiometric ratio was SrTiO ３ The mixed compounds were wet-mixed under the condition of ball-mill mixing for 2 hours with alumina balls as the grinding medium, and then the mixture was dried under the condition of holding at 80°C for 10 hours. After the drying treatment, the mixture was calcined under the condition of holding at 900°C for 10 hours to obtain SrTiO ３A raw material was obtained. A binder (PVA aqueous solution, prepared from a raw material manufactured by Kanto Chemical Co., Ltd.) was added to the obtained raw material to obtain a raw material slurry. The obtained raw material slurry was dried at 80°C for 10 hours, and a disc-shaped molded body was obtained by using a uniaxial press and the CIP method. The obtained molded body was placed on a base made of an alumina plate (the entire surface of the molded body was in contact with the alumina plate) and subjected to primary sintering treatment under an oxygen atmosphere of 2 atm, a heating rate of 100°C / h, a primary sintering temperature of 1450°C, and a holding time (primary sintering time) of 2 hours to obtain a primary sintered body. The obtained primary sintered body was placed on a base made of multiple alumina balls (only a portion of the primary sintered body was in contact with the alumina) and subjected to HIP treatment under an argon atmosphere, an applied pressure of 150 MPa, a HIP temperature of 1300°C, and a holding time (HIP time) of 1 hour to obtain a secondary sintered body. The resistivity of the obtained secondary sintered body was measured using the method described in this embodiment. The obtained secondary sintered body was placed on a base plate covered with multiple alumina balls (so that only a portion of the secondary sintered body was in contact with the alumina), and post-annealing treatment was performed in an air atmosphere under the following conditions: heating rate of 200°C / h, annealing temperature of 1200°C, and holding time (annealing time) of 10 hours, resulting in a main surface area of ​​50.64 cm². ２ A ceramic material was obtained. The obtained ceramic material has the chemical formula SrTiO ３ A sintered body of a perovskite crystal with the composition represented by [formula] was obtained.

[0075] For the ceramic material in Example 1, the presence or absence of cracks, the area of ​​the main surface (the area of ​​the largest cracked sample if present), the transmittance to light at a wavelength of 635 nm (total light transmittance within a 1 mm thick sample), the standard deviation of the transmittance, and the refractive index and standard deviation of the refractive index to light at a wavelength of 635 nm were measured. Cracks were confirmed visually, the area of ​​the main surface was calculated from the captured image of the main surface, and the other measurement conditions followed the method described in this embodiment. The results of each measurement are shown in Table 1.

[0076] (Examples 2 to 9) In Examples 2 to 9, the ceramic material was manufactured using the same method as in Example 1, except that the manufacturing conditions were as shown in Table 1. The measurement results for each example are also shown in Table 1.

[0077] (Evaluation) For each example of ceramic material, the transmittance of visible light and its versatility were evaluated. In the evaluation of transmittance, when a 100W (10 mm diameter) white light source was placed behind the material, a circle (○) was used if the transmitted light was white light, and a cross (×) was used if the light was not transmitted or if the transmitted light was not white light. In the evaluation of versatility, a circle (○) was used if a large ceramic material could be obtained without cracking, and a cross (×) was used if a large ceramic material could not be obtained due to cracking.

[0078] As shown in Table 1, in the examples 1-2 and 7-9, which are embodiments, the evaluation of transmittance and versatility was ○, indicating that it is possible to transmit visible light appropriately while maintaining high versatility. On the other hand, in the comparative examples 3-6, the versatility evaluation was × because it broke, indicating that it is not possible to transmit visible light appropriately while maintaining high versatility.

[0079] Although embodiments of the present disclosure have been described above, the embodiments are not limited to those described herein. Furthermore, the aforementioned components include those that can be easily conceived by those skilled in the art, those that are substantially the same, and those that fall within the so-called equivalent range. Moreover, the aforementioned components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the gist of the embodiments described above.

[0080] 10. Ceramic Materials

Claims

1. A x BO ｚ The main component is a perovskite compound represented by the chemical formula, and the main surface area is 40 cm². ２ The above is true, and the total light transmittance within a 1 mm thickness for light with a wavelength of 635 nm is 10% or more, A is mainly composed of Sr, B is mainly composed of Ti, x is between 0.9 and 1.0, and z is a value that maintains charge neutrality for the entire chemical formula, a ceramic material.

2. The ceramic material according to claim 1, wherein the standard deviation of the total light internal transmittance for light with a wavelength of 635 nm at each position on the main surface of a 1 mm thick material is 0.5% or less.

3. The ceramic material according to claim 1, wherein the refractive index for light with a wavelength of 635 nm is 2.30 or greater.

4. The ceramic material according to claim 3, wherein the standard deviation of the refractive index for light with a wavelength of 635 nm at each position on the main surface is 0.003 or less.

5. An optical member comprising a ceramic material according to any one of claims 1 to 4 as a light guide plate.

6. A head-mounted display comprising a ceramic material according to any one of claims 1 to 4 as a light guide plate member.

7. A x BO ｚ The process includes: preparing a powdered raw material of a perovskite compound represented by the chemical formula; molding the raw material; performing a primary sintering treatment on the molded raw material by heating it to a sintering temperature of 1300°C to 1450°C under a heating rate of 100°C / h or less to obtain a primary sintered body; performing a HIP treatment on the primary sintered body by heating it to a temperature of 1250°C to 1450°C under a pressure of 100 MPa or more; and performing a post-annealing treatment on the secondary sintered body, which is the primary sintered body that has been HIP treated, by heating it to a sintering temperature of 800°C to 1400°C under a heating rate of 200°C / h or less to obtain a ceramic material mainly composed of the compound, wherein A is mainly composed of Sr, B is mainly composed of Ti, x is 0.9 or more and 1.0 or less, and z is a value that maintains charge neutrality for the entire chemical formula. A method for manufacturing ceramic materials, comprising: In the HIP treatment, a portion of the surface of the primary sintered body is in contact with a base portion having a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and the remaining portion of the surface of the primary sintered body is not in contact with other members, while the HIP treatment is performed; and In the post-annealing treatment, a portion of the surface of the secondary sintered body is in contact with a base portion having a thermal conductivity of 5 W / m / K or more and 60 W / m / K or less, and the remaining portion of the surface of the secondary sintered body is not in contact with other members, while the post-annealing treatment is performed.

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