Translucent alumina ceramics and method for producing the same

Abstract

The present invention provides materials with optical properties in which birefringence is extremely minimal or undetectable in the visible to near-infrared and near-infrared to mid-infrared wavelength ranges. Furthermore, it provides ceramics having optical and mechanical properties equivalent to or better than those of c-axis sapphire single crystals. [Solution] The present invention relates to alumina ceramics having an alumina content of 99.7% by weight or more, wherein (1) the total content of one or more additives selected from MgO, ZrO2, Sc2O3, Y2O3, and lanthanide rare earth oxides is 100 to 2500 ppm by weight, (2) the residual pore volume is 100 ppm by volume or less, and (3) the translucent alumina ceramics in which segregation of the additives between alumina particles cannot be confirmed by observation at 100,000x magnification using a scanning electron microscope or a transmission electron microscope.

Description

[Technical Field]

[0001] This invention relates to novel translucent alumina ceramics and a method for producing the same. [Background technology]

[0002] In the 1950s, RL Coble successfully synthesized polycrystalline alumina ceramics with light-transmitting properties for the first time (Non-Patent Document 1), and these alumina ceramics have been put into practical use as discharge tubes for high-pressure sodium discharge lamps, etc. These light-transmitting ceramics have undergone various improvements to their transmission properties and are still in use today (Patent Document 1).

[0003] While the translucent alumina ceramics developed to date have relatively high total transmittance, their linear transmittance is low due to the presence of birefringence inherent in the hexagonal crystal system of alumina. As a result, their applications are extremely limited, mainly being used in discharge lamps.

[0004] In other words, because alumina ceramics are sintered bodies in which hexagonal particles are randomly oriented, the direction and intensity of birefringence differ for each particle constituting the sintered body. Therefore, when light is irradiated, linear transmittance is low, and diffuse transmission is dominant, but even this performance is not sufficient. As a result, although light transmission through alumina ceramics can be confirmed, the image seen through this material is not clear at all. Furthermore, because diffuse transmission is dominant, when used as a window, the image resolution is low, and in some cases, an image cannot be obtained at all due to enormous birefringence. Therefore, it is fundamentally unusable as a window for visible to infrared cameras, and there are no reported examples of it actually being used.

[0005] In the case of a sapphire single crystal, while birefringence is not apparent in material cut along the c-axis, birefringence is always present in other directions. When used as a window, the image appears doubled due to birefringence (extraordinary light).

[0006] Polycrystalline translucent alumina ceramics obtained by sintering exhibit significant scattering because, each time incident light passes through the randomly oriented alumina particles, scattering and birefringence occur due to differences in refractive index. Therefore, even with a thin sample thickness (e.g., around 0.3 mm), the coherence of light is lost due to refractive index and birefringence, making it difficult to even see the surrounding scenery through the translucent alumina ceramics.

[0007] Sapphire single crystals are known as windows for the visible to mid-infrared region. This material is extremely hard, with a Vickers hardness of around 2200, making it resistant to surface scratches and relatively high fracture strength (300-450 MPa). However, it has poor fracture toughness and cannot withstand strong impact fracture. Moreover, fabricating optical windows from sapphire single crystals involves significant processing costs, including cutting along a specific plane orientation (c-axis), slicing from a high-hardness sapphire single crystal block, laser processing for microphone, speaker, and camera mounting areas, and lapping and polishing of the screen surface. In particular, for optics requiring domes or complex shapes, this method is not practical in terms of production costs.

[0008] Incidentally, to address the problems inherent in conventional translucent alumina ceramics, A. Krell et al. have reported successfully improving the transmittance of alumina ceramics by fabricating them from fine particles (Non-Patent Literature 2).

[0009] However, Non-Patent Document 2 does not mention any specific means to resolve the birefringence problems described above.

[0010] Furthermore, while alumina ceramics with only a small amount of MgO or ZrO2 added can achieve fracture strength exceeding that of single-crystal sapphire, they fail to meet the properties required for infrared-transmitting materials such as windows.

[0011] Regarding such translucent alumina ceramics, the applicant has previously developed translucent alumina ceramics in which zirconia is segregated within and / or between alumina particles in at least a portion of the space between alumina particles, and has filed a patent application for this (Patent Document 2).

[0012] However, while the above-mentioned translucent alumina ceramics can exhibit excellent hardness and mechanical properties along with high transparency when used in thin sheet form, such as in mobile screens, there is still room for further improvement in areas such as birefringence. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 2014-15362 [Patent Document 2] Japanese Patent Publication No. 2023-113369 [Non-patent literature]

[0014] [Non-Patent Document 1] RL Coble, “Preparation of Transparent Ceramic Al2O3,” Am. Ceram. Soc. Bull., vol. 38, p507 (1959). [Non-Patent Document 2] A. Krell, P. Blank, H. Ma, T. Hutzler, Michel PB van Bruggen, R. Apetz, “Transparent Sintered Corundum with Hardness and Strength”, J. Am. Ceram. Soc., 86 [1] 12-18 (2003). [Non-Patent Document 3] T. Ashikaga, B. N. Kim, H. Kiyono, T. Suzuki “Effect of Crystallographic Orientation on Transparency of Alumina Prepared using Magnetic Alignment and SPS”, J. Eur. Ceram. Soc., 38 [7] 2735-41 (2018). [Non-Patent Document 4] Y. L. Aung, A. Ikesue, “Importance of Optical Homogeneity for High-Quality Transparent Ceramics”, J. Eur. Ceram. Soc., DOI. org. / 10.1016 / j.jeurceramsoc.2022.05.067 [Summary of the Invention] [Problems to be Solved by the Invention]

[0015] In general, for optics, except for applications such as smartphone screens, the thickness is 1 mm or more. At this time, not only is a high linear transmittance characteristic close to the theoretical transmittance required, but it is also difficult to use as a window or the like unless optical homogeneity can be guaranteed.

[0016] In the case of sapphire single crystals, since birefringence exists in the same material, only materials processed in the c-axis orientation can be used. This is because sapphire belongs to a hexagonal crystal structure and has optical anisotropy. When light is irradiated, birefringence occurs, and extraordinary light (birefringence) is generated in addition to the main beam. Therefore, when an image is obtained through this material, a double image will result, and a clear image cannot be obtained.

[0017] On the other hand, for polycrystalline ceramics with a cubic crystal structure produced by the sintering method, if residual pores and grain boundary phases, which are scattering sources, can be completely removed and clean grain boundaries can be formed, most materials can obtain high transparency.

[0018] However, in anisotropic materials like alumina, birefringence occurs each time light passes through the crystal grain, and scattering also occurs due to the difference in refractive index resulting from the difference in crystal orientation between adjacent crystal grains. Therefore, it is considered fundamentally impossible to obtain the excellent optical performance seen in polycrystalline materials with a cubic crystal structure. In fact, the polycrystalline alumina ceramics described in Non-Patent Documents 2 and 3 have inferior properties compared to single-crystal materials, and even thin samples of 0.8 mm or less have low transmittance, and none are currently available for use as optics.

[0019] On the other hand, polycrystalline alumina is a birefringent material and has been widely used as a discharge tube in high-pressure sodium discharge lamps since around 1960. The linear transmittance of a translucent alumina discharge tube that emits visible light is about 30-40%, and the total transmittance is about 50-60%, which are inferior to those of single-crystal sapphire. Since the role of a discharge tube is to emit light generated inside the discharge tube to the outside, high optical performance is not required, as long as the light inside the discharge tube is scattered and reflected and eventually emitted to the outside, even if the linear or diffuse transmission characteristics are not very high.

[0020] Thus, transparent alumina ceramics that exhibit high linear transmittance in the visible to near-infrared region (0.4 to 0.9 μm) and the near-infrared to mid-infrared region (0.9 to 5 μm), which are suitable for optical applications, have yet to be developed for use as optics.

[0021] Therefore, the main objective of the present invention is to provide a material with optical properties in which birefringence is extremely minimal or undetectable in the visible to near-infrared and near-infrared to mid-infrared wavelength ranges. Furthermore, the present invention also aims to provide a ceramic that possesses optical and mechanical properties equivalent to or better than those of a c-axis sapphire single crystal. [Means for solving the problem]

[0022] In light of the problems of these prior art, the inventors conducted extensive research and, as a result, discovered that the above objectives could be achieved by developing alumina seracus having a specific composition and structure, thus completing the present invention.

[0023] In other words, the present invention relates to the following translucent alumina ceramics and a method for producing the same. 1. Alumina ceramics having an alumina content of 99.7% by weight or more, (1) The total content of at least one additive, MgO, ZrO2, Sc2O3, Y2O3, and lanthanide rare earth oxides, is 100 to 2500 ppm by weight. (2) The residual pore volume is 100 ppm or less, (3) When observed at 100,000x magnification using a scanning electron microscope or transmission electron microscope, no segregation of the additive between the alumina particles can be confirmed. Translucent alumina ceramics characterized by the following features. 2. The three-point bending strength is 350 MPa or higher, and the fracture toughness (K) 1c A translucent alumina ceramic as described in item 1 above, wherein the value of ) is 3.5 or higher. 3. Translucent alumina ceramics as described in item 1 above, having an average particle size of 0.1 to 0.8 μm. 4. In a sample of alumina ceramics with a thickness of 1.0 mm and a mirror-polished surface, (1) The linear transmittance at a wavelength of 600 nm must be 55% or higher. (2) The linear transmittance at a wavelength of 1200 nm is 70% or more, (3) The linear transmittance at a wavelength of 3500 nm must be 80% or higher. A translucent alumina ceramic as described in item 1 above, which satisfies all of the conditions. 5. In a sample of alumina ceramics with a thickness of 1.0 mm and a mirror-polished surface, (1) The extinction ratio at a wavelength of 600 nm is 15 dB or more, (2) The extinction ratio at a wavelength of 1200 nm is 25 dB or more. A translucent alumina ceramic as described in item 1 above, which satisfies all of the conditions. 6. An optical material comprising a translucent alumina ceramic as described in any one of items 1 to 5 above. 7. A device for detection in the near-infrared to mid-infrared wavelength region of 0.9 to 5 μm, comprising a translucent alumina ceramic as described in any one of items 1 to 5 above. 8. A method for producing translucent alumina ceramics, (1) A process to obtain a mixture by wet mixing in alcohol a raw material comprising (a) alumina powder with a purity of 99.9% by weight or more and an average primary particle size of 0.05 to 0.5 μm as an alumina starting material, and (b) an additive source comprising (b1) ZrO2, MgO, Sc2O3, Y2O3 and lanthanide rare earth oxides having an average primary particle size smaller than the average primary particle size of the alumina powder, (b2) an organic compound containing Zr, Mg, Sc, Y and lanthanide rare earths, and (b3) at least one inorganic salt containing Zr, Mg, Sc, Y and lanthanide rare earths. (2) A step of obtaining a compacted powder by press molding the mixture or the dried product thereof, (3) A step of obtaining a pre-sintered body with a relative density of 96-99% by pre-sintering the compacted powder at a temperature of 1150-1300°C. (4) A step of performing HIP treatment on the pre-sintered body at a temperature of 1100 to 1400°C and a pressure of 98 to 396 MPa and (5) A step of annealing the sintered body obtained by the HIP treatment at 700 to 1000°C. A method for producing translucent alumina ceramics, characterized by containing [a specific substance]. 9. The manufacturing method according to item 8, wherein in the HIP treatment, the cooling rate in the temperature range of 1350 to 800°C exceeds 600°C / hr. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide materials with optical properties in which birefringence is extremely minimal or undetectable in the wavelength range of visible to near-infrared and near-infrared to mid-infrared. Moreover, according to the present invention, it is also possible to provide ceramics having optical and mechanical properties equivalent to or better than those of a c-axis sapphire single crystal.

[0025] In particular, the present invention provides a material with excellent linear transmittance in the visible, near-infrared, and mid-infrared regions by selecting additives and controlling the microstructure. Therefore, the translucent alumina ceramics of the present invention are optics with excellent performance in the visible, near-infrared, and mid-infrared regions with wavelengths of 0.4 to 5 μm, and can be used not only in thin plate form of about 0.2 to 0.4 mm for smartphone screens, but also in thicknesses of 0.4 to 10 mm (especially 1 to 10 mm).

[0026] More specifically, by reducing the amount of residual pores to 100 ppm or less (preferably substantially zero) and preferably controlling the particle size to be smaller than the wavelength used, birefringence within the particles can be effectively reduced, and high linear transmittance in the visible to infrared range can be obtained. More specifically, when the sample thickness is 1 mm, it is possible to provide alumina ceramics having a linear transmittance of 55% or more in the visible range (representative wavelength 600 nm), 70% or more in the near-infrared range (representative wavelength 1200 nm), and 80% or more in the mid-infrared range (representative wavelength 3500 nm).

[0027] Furthermore, with regard to additives in particular, the present invention makes it possible to achieve excellent optical properties by incorporating specific additives in a specific amount into alumina ceramics. Regarding such optical properties, in recent years, polycrystalline ceramics with excellent optical properties have been reported and are being applied to ultraviolet or visible window materials, as well as achieving sufficient results in lasers, which require the highest level of optical technology. However, materials that have achieved such optical technology are limited to those with a cubic crystal structure, and there have been no reports of advanced optical applications being possible with anisotropic materials such as orthorhombic or hexagonal crystals.

[0028] As previously mentioned, in the 1950s, RL Coble proposed translucent alumina by removing residual pores present in hexagonal alumina ceramics, which were then considered opaque. However, although this alumina transmits light, it is mainly diffuse transmission (with only a small linear transmission component), and therefore its use is limited to discharge tubes for high-pressure sodium discharge lamps. In this ceramic, reducing residual pores can create paths through which light can pass, but due to the random orientation and the presence of birefringence inherent in hexagonal materials, light cannot travel in a straight line through the material and can only pass through while repeatedly diffusing due to birefringence. On the other hand, sapphire single crystals, especially c-axis oriented sapphire single crystals, which have a single crystal orientation and no grain boundaries, have excellent linear transmittance and a high extinction ratio, properties that cannot be achieved with polycrystalline alumina, because they are in an orientation without birefringence.

[0029] Thus, the prevailing scientific consensus has been that hexagonal, unoriented polycrystalline alumina cannot be given properties similar to those of a sapphire single crystal.

[0030] As mentioned earlier, about 40 years after RL Coble's report on translucent alumina, A. Krell succeeded in developing alumina with higher linear transmittance by creating polycrystalline alumina with smaller particle sizes. When similar ceramics are fabricated, linear transmittance is indeed improved due to the "particle size effect," and the surrounding scenery can be seen through the polished material (it is impossible to see the surrounding scenery with the technology developed by RL Coble, because the control of light scattering and birefringence is insufficient), but it is not very clear.

[0031] In contrast, the translucent alumina ceramics of the present invention, in addition to reducing the size of its crystal grains, further densifies the sintered body by adding at least one additive, such as an oxide of Mg, Zr, Sc, Y, and a lanthanide rare earth element, in a specific amount. At the same time, by controlling the additive so that it does not substantially segregate at the grain boundaries of the alumina crystal grains, it is possible to achieve very low birefringence in the long wavelength range above near-infrared, and to control (suppress) light scattering. This is thought to be because there is some effect that mitigates the difference in refractive index between alumina particles with different crystal orientations, thereby reducing scattering near grain boundaries. As a result, it is possible to provide alumina ceramics that combine a high extinction ratio (low leakage light (birefringence) when light is transmitted with a polarizing plate orthogonal) and high transmittance, comparable to that of a c-axis sapphire single crystal.

[0032] Furthermore, alumina ceramics to which at least one of Mg, Zr, Sc, Y, and lanthanide rare earth element oxides are added not only exhibit a large extinction ratio, but also significantly improve transmittance not only in the visible range but especially in the infrared range above 0.9 μm, so that images comparable to those obtained with a c-axis sapphire window can be obtained with an infrared camera through a relatively thick window. In particular, by making the average particle size of the translucent alumina ceramics of the present invention even smaller (especially approaching 0.1 μm), birefringence in the visible range can be drastically reduced, thereby dramatically improving transparency (transmittance).

[0033] The extinction ratio mentioned above is given by the following equation (1). Extinction ratio (dB)=10×log(T H / T L ) …(1) (In the formula, T H This shows the light intensity measured with two polarizing plates placed parallel to each other (without the sample). L This indicates the intensity of light measured by placing the sample with polarizing plates perpendicular to each other (the intensity of stray light due to birefringence). This means that the greater the light leakage, the lower the extinction ratio (i.e., the less light leakage, the higher the extinction ratio).

[0034] In the ceramics of the present invention, although the additives include oxides of Mg, Zr, Sc, Y, and lanthanide rare earth elements, the reason why the extinction ratio of alumina ceramics can be increased is not entirely clear. However, it is thought that this is because the generation of segregation of additives between alumina crystal particles is effectively suppressed, scattering due to grain boundary phases is essentially eliminated, and even if there is scattering, it is only from particle interfaces with slight refractive index differences due to differences in crystal orientation.

[0035] These additives are present at and near the grain boundaries during sintering, effectively suppressing grain growth of alumina particles. In the final stages of sintering, the cations of these additives dissolve into the alumina particles and diffuse from the inside of the particles to the vicinity of the grain boundaries during the cooling process. In particular, during the cooling process of the HIP treatment, which is the final step of sintering, the cations move from grain boundary to grain boundary direction, so by controlling the cooling rate up to a certain temperature, most of the cations can be kept near the grain boundaries. As a result, grain boundary segregation is virtually eliminated, and segregation occurs only within the grains (near the grain boundaries). For example, Mg segregated near the grain boundaries does not form compounds, but because the matrix material is Al2O3, it becomes an MgO-Al2O3 system composition similar to cubic spinel and exists near the grain boundaries of the alumina particles. It is presumed that this alumina grain segregation mitigates the refractive index difference between alumina particles with different orientations and simultaneously leads to a reduction in birefringence, but currently there is no scientific technology that can elucidate this.

[0036] Furthermore, in the translucent alumina ceramics of the present invention, by setting the average particle size of the alumina crystal particles to 0.1 to 0.8 μm, even better optical properties can be achieved. Although the translucent alumina ceramics of the present invention have a hexagonal crystal structure, by controlling the average particle size to be small within the above range, birefringence can be further reduced optically and isotropic optical properties can be imparted when the wavelength is smaller than the application wavelength (>0.9 μm).

[0037] In particular, when the translucent alumina ceramics of the present invention are applied to windows, domes, etc. in the near-infrared to mid-infrared region, they exhibit very little birefringence, resulting in very little distortion of transmitted light. Therefore, they can capture images with a resolution equivalent to that of a c-axis sapphire single crystal.

[0038] For example, when the light leakage of the 1 mm thick translucent alumina ceramic of the present invention is measured under crossed nicols (two polarizers orthogonal), in the near-infrared region, and in linear transmission mode, the detected light leakage is less than 1% even in the visible region. The magnitude of this light leakage is related to the extinction ratio; materials with low light leakage exhibit a large extinction ratio as shown in equation (1) above, and such materials exhibit high optical quality not only for mobile screens but also for other applications. Conventional translucent alumina developed by Coble shows significant birefringence (light leakage), and the extinction ratio remains at the level of a few dB.

[0039] Materials with significant light leakage due to birefringence, even single crystals, cannot be used as optics. To obtain a clean and sharp image through optics, the light leakage due to birefringence must be below a certain level, and this discussion cannot be applied to the properties of translucent alumina as previously understood.

[0040] One of the features of the translucent alumina ceramics of the present invention is that they are polycrystalline. YAG(Y3Al5O) has a cubic crystal structure. 12Transparent polycrystalline ceramics such as spinel (MgAl2O4) and yttria (Y2O3) have been developed, and some have been reported to be promising as optical materials. In principle, birefringence should not exist in single crystals or polycrystalline ceramics with a cubic crystal structure, but most commercially available single crystals exhibit birefringence. Since polycrystalline ceramics are aggregates of particles with different crystal orientations, each time light passes through the constituent particles, abnormal light (birefringence) with a different direction of propagation is generated, which is a very significant problem. The inventors of this invention have created transparent polycrystalline ceramics a) birefringence-free and b) birefringent using yttria, which has a cubic crystal structure, as an example. Even if the transmittance (optical loss) of both is close, if there is a relatively large amount of birefringence, even if this material is polished to high precision, the surrounding scenery cannot be seen clearly (see Non-Patent Literature 4, etc.). This is because the transmitted light is distorted by birefringence, causing "image blurring". A similar phenomenon occurs in polycrystalline alumina ceramics, but since alumina is inherently anisotropic (hexagonal crystal structure), a large amount of birefringence is definitely present within the material in principle.

[0041] In contrast, the translucent ceramics of the present invention, despite being polycrystalline, exhibit a reduction to almost complete disappearance of birefringence, although this is wavelength-dependent, thus achieving optical properties equivalent to those of a c-axis sapphire single crystal. Moreover, the polycrystalline alumina ceramics of the present invention have no dependence on crystal orientation and exhibit the same optical properties from any direction, offering a significant advantage as they can be handled in virtually the same way as cubic structure ceramics, which are optically isotropic.

[0042] The manufacturing method of the present invention allows for the more efficient and reliable production of the translucent ceramics of the present invention. In particular, molded bodies with desired shapes can be produced relatively easily. For example, in applications such as smartphone screens, a mold can be prepared in advance using an injection molding method, eliminating the need for laser drilling for microphones, speakers, etc., and corner rounding of edges. The translucent alumina ceramics of the present invention obtained by molding, degreasing, calcining, pre-sintering, and HIP treatment using this mold can then be cut with a wire saw and polished to obtain the desired screen (in this case, only the visible spectrum can be used).

[0043] Furthermore, because the translucent ceramics of the present invention are polycrystalline, they offer superior processability and other advantages compared to single crystals. For example, when using sapphire single crystals as optics, the crystal is generally grown in the a-axis direction, and if it is not cut from the grown ingot in the c-axis direction, an optical problem called "birefringence" occurs. Optics using sapphire single crystals are the most commonly used material in the visible to mid-infrared region, but they have the drawback of being limited to the c-axis direction for optical applications. Methods for manufacturing sapphire single crystals include the Czochralski method, HEM (heat exchange method), and Bernoulli method, but methods that produce low birefringence and high optical uniformity are limited. Even if high-quality sapphire single crystals are obtained, they can only be applied optically in the c-axis direction, leaving challenges in productivity, including processing. In contrast, with the translucent alumina ceramics of the present invention, for example, when a disc with a thickness of 1 to 10 mm and a diameter of 100 mm, with both sides polished, is formed by machining and used as a window for an infrared camera, the polycrystalline alumina ceramics are composed of microcrystalline particles with random orientations, so there is no need to consider the cutting orientation, and there is also the economic advantage that only the required quantity can be manufactured by near-net molding.

[0044] Thus, the translucent alumina ceramics of the present invention, which have excellent linear transmittance, can be used in the visible, near-infrared, and mid-infrared regions. In particular, in the near-infrared to mid-infrared region, they have optical properties similar to sapphire single crystals, and their mechanical strength can far surpass that of sapphire single crystals. Therefore, they can be suitably used in applications where at least one of these properties is required.

[0045] For example, it can be applied to infrared detection windows, rocket domes, and radar windows for collision avoidance in autonomous driving systems. Not only is it less prone to damage from drops, collisions, etc., but when observing subjects through the window using CCD cameras, near-infrared cameras, mid-infrared cameras, etc., in the visible, near-infrared, and mid-infrared wavelength range, high-resolution and bright images can be captured. This is because the material has high optical uniformity and transmittance.

[0046] Furthermore, for example, the present invention can provide a material in the form of a thick plate several millimeters thick, which can be used in security or surveillance INGAS cameras that utilize near-infrared to mid-infrared light, and which has optical properties and hardness equivalent to those of a sapphire single crystal, as well as mechanical properties equivalent to or better than those of a sapphire single crystal.

[0047] In addition to other applications, the ceramics of the present invention are also useful as engineering ceramics (especially structural materials) where transparency is not particularly required but high mechanical properties are necessary. For example, they can be suitably used as wear-resistant materials or sliding members such as grinding balls, nozzles, and valves used in ceramic manufacturing processes, as well as bonding capillaries for semiconductors, semiconductor substrates, battery materials, IC packages, dental materials, and implant materials (e.g., artificial bone, artificial joints). [Brief explanation of the drawing]

[0048] [Figure 1] This is a scanning electron microscope (SEM) image of the translucent alumina ceramics of the present invention. [Figure 2]These are the results of observing fluorescent lamps through conventional alumina ceramics with a thickness of 0.2 mm and a thickness of the present invention's translucent alumina ceramics with a thickness of 1.0 mm, both of which were mirror-polished on both sides. [Figure 3] (a)(a1) Linear transmittance measured by a spectrophotometer for a 1.0 mm thick c-axis sapphire single crystal, (a2) a 1.0 mm thick translucent alumina ceramic of the present invention, and (a3) ​​a 0.4 mm thick conventional alumina. (b) Wavelength dependence of the leaked light (birefringence) (amount of leaked light due to birefringence during polarization measurement) when two polarizing plates are orthogonal (crossed nicols) is shown for the samples of (a1) and (a2). [Figure 4] The images show the results of observing conventional translucent alumina (0.5 mm thick) with an average particle size of 10 μm (top row, (a1) one polarizing plate and (a2) two polarizing plates) and the translucent alumina ceramics of the present invention (0.5 mm thick) with an average particle size of 0.5 μm (bottom row, (b1) one polarizing plate and (b2) two polarizing plates) using a transmission polarizing microscope (wavelength visible) (camera: CCD). [Figure 5] The optical quality of conventional translucent alumina (thickness 0.5 mm) (top row, (a1) one polarizing plate and (a2) two polarizing plates) and the translucent alumina ceramics of the present invention (thickness 0.5 mm) (bottom row, (b1) one polarizing plate and (b2) two polarizing plates) was investigated using a different INGAS camera than the one shown in Figure 4, with the measurement wavelength limited to 1.2 to 1.7 μm. [Figure 6] Figure 6 shows images (b1) and (b2) of a subject (human hand) captured by a near-infrared camera (wavelength sensitivity 0.9-1.7 μm) and a mid-infrared camera (wavelength sensitivity 3-5 μm) through the translucent alumina ceramics of the present invention, measuring 60 mm in diameter × 2 mm in thickness and 60 mm in diameter × 4 mm, respectively. Results of similar images taken through conventional translucent alumina (a1,)(a2) of 60 mm in diameter × 2 mm in thickness and 60 mm in diameter × 4 mm, and c-axis sapphire single crystals (c1) and (c2) of 60 mm in diameter × 2 mm in thickness and 60 mm in diameter × 4 mm are also shown. [Figure 7]Figure 7 shows images (B1), (B2), (C1), and (C2) of a subject (human hand) captured by a near-infrared camera (wavelength sensitivity 0.9-1.7 μm) and a mid-infrared camera (wavelength sensitivity 3-5 μm) through the translucent alumina ceramics of the present invention, measuring 60 mm in diameter × 2 mm in thickness and 60 mm in diameter × 6 mm, respectively. The results of similarly capturing images with a blank (without the sample) are also shown (A1) and (A2). [Figure 8] Figure 8 shows the beam shape (laser source) obtained by irradiating a 20 mm diameter × 1.5 mm thick c-axis sapphire single crystal (C), a 20 mm diameter × 1.5 mm thick translucent alumina ceramic of the present invention (B), and a 20 mm diameter × 0.4 mm thick conventional alumina with a wavelength of 1064 nm, and photographing the emitted laser light with a beam profiler. The beam shape of the original beam (A) is also shown. [Modes for carrying out the invention]

[0049] 1. Translucent alumina ceramics The translucent alumina ceramics of the present invention (ceramics of the present invention) are alumina ceramics having an alumina content of 99.7% by weight or more. (1) The total content of at least one additive, MgO, ZrO2, Sc2O3, Y2O3, and lanthanide rare earth oxides, is 100 to 2500 ppm by weight. (2) The residual pore volume is 100 ppm or less, (3) When observed at 100,000x magnification using a scanning electron microscope or transmission electron microscope, no segregation of the additive between the alumina particles can be confirmed. It is characterized by the following:

[0050] The ceramic material of the present invention is an alumina ceramic with an alumina content of 99.7% by weight or more, and contains at least one additive, MgO, ZrO2, Sc2O3, Y2O3, and lanthanide rare earth oxides, in a total content of 100 to 2500 ppm by weight.

[0051] As mentioned above, the alumina content is usually sufficient if it is 99.7% by weight or more, so it can be, for example, 99.99% by weight or more.

[0052] In the ceramics of the present invention, the additive plays a particularly important role in promoting densification. On the other hand, since the additive is not precipitated at the grain boundaries of the alumina crystal particles in the ceramics of the present invention, the occurrence of birefringence can be suppressed or prevented.

[0053] From this viewpoint, the smaller the total amount of additive, the better. In this invention, the amount is 100 to 2500 ppm by weight, and within that range, 100 to 1000 ppm by weight is preferable. If the total amount of additive is too small, the desired light transmittance, mechanical properties, etc., cannot be obtained. Conversely, if the total amount of additive is too large, the optical properties will deteriorate.

[0054] The additives can be MgO, ZrO2, Sc2O3, Y2O3, and at least one of the lanthanide rare earth oxides. The lanthanide rare earth oxides can be oxides containing La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu. Examples include, but are not limited to, La2O3, Gd2O3, and Lu2O3. Commercially available products can also be used.

[0055] The ceramics of the present invention have a polycrystalline structure as described above, and their average crystal grain size is 0.1 to 0.8 μm, and is particularly preferably 0.1 to 0.4 μm. If the average crystal grain size is less than 0.1 μm, it may be difficult to manufacture due to the limited particle size of the starting material. Also, if the average crystal grain size exceeds 0.8 μm, significant birefringence may occur.

[0056] The amount of residual pores in the ceramics of the present invention is usually 100 volume ppm or less, and particularly preferably 10 volume ppm or less. If the amount of residual pores exceeds 100 volume ppm, scattering becomes significant and transmittance becomes insufficient. The lower limit of the amount of residual pores is most preferably substantially 0 volume%, but usually it can be around 1 volume ppm.

[0057] The ceramics of the present invention exhibit extremely minute or undetectable levels of birefringence in the visible to near-infrared and near-infrared to mid-infrared wavelength ranges. Specifically, when observed at 100,000x magnification using a scanning electron microscope or transmission electron microscope, the segregation of the additive between alumina particles cannot be confirmed.

[0058] More specifically, when measuring a 1.0 mm thick sample of the ceramics of the present invention, with both sides mirror-polished, the sample and polarizing plates are placed orthogonally in a spectrophotometer (spectrometer) before and after the light passes through the sample, and the amount of leak light (%) due to birefringence is measured, it is preferable that the amount of leak light at a wavelength of 600 nm is 3% or less, and particularly 2% or less. In a similar measurement method, it is preferable that the amount of leak light at a wavelength of 1200 nm is 1% or less.

[0059] The ceramics of the present invention are translucent, and in particular, a sample of the ceramics of the present invention with a thickness of 1.0 mm and a mirror-polished surface can be considered for applications in the visible range (400-900 nm). At a wavelength of 600 nm, the linear transmittance is 55% or more, and the extinction ratio is 15 dB or more. A typical application in this wavelength range is smartphone screens. In this case, the thickness of the screen used is about 0.2-0.3 mm, so even with a thickness of 1 mm, a linear transmittance of about 55% is sufficient transparency. However, if the resulting birefringence is not reduced, the image quality will be affected, especially when polarized, as the image will not be sharp unless it is dark-field (black). Therefore, an extinction ratio of at least 15 dB is necessary. As for other optical applications in the visible range, if it can be used for smartphone screens, it can be used in almost all cases.

[0060] Furthermore, the ceramics of the present invention preferably have a linear transmittance of 70% or more in the near-infrared wavelength band of 900 to 1700 nm (with 1200 nm being the representative wavelength for the visible wavelength used), and an extinction ratio of 25 dB or more at 1200 nm (to satisfy 25 dB, it is desirable that the leak light is small, such as 0.3% or less when measuring the linear transmittance of a 1 mm thick ceramic under orthogonal nicols). Due to these physical properties, when an object is viewed through the ceramics of the present invention, the display screen can be seen more clearly. Furthermore, if observed using near-infrared and mid-infrared cameras, an even clearer image can be obtained.

[0061] When using the ceramics of the present invention in the infrared region, it is preferable that a 1 mm thick sample of the ceramics of the present invention has a linear transmittance of 80% or more at wavelengths of 2.5 to 5 μm (with 3500 nm as the representative wavelength for the infrared wavelength used) and a high extinction ratio. In this wavelength range, polarizing plates for measuring the extinction ratio are not commercially available, so the extinction ratio cannot be specifically defined. If a polarizing plate usable in the mid-infrared region exists, it would suffice to satisfy the same requirements as in the near-infrared region: 0.3% or less of stray light and an extinction ratio of 25 dB or more at a representative wavelength of 3500 nm. By possessing such physical properties, the ceramics can be applied as optical materials such as windows even in the infrared region.

[0062] Furthermore, when using the ceramics of the present invention in applications where high light transmittance is not required, it is not necessarily necessary for them to possess the above-mentioned linear transmittance and extinction ratio.

[0063] The mechanical properties of the ceramics of the present invention are equivalent to or better than those of a sapphire single crystal. Specifically, the three-point bending strength is preferably 350 MPa or higher, and more preferably 500 MPa or higher. The upper limit of the three-point bending strength can be, for example, around 1000 MPa, but is not limited to this.

[0064] Furthermore, in the ceramics of the present invention, the fracture toughness value (K 1c ) by the SEPB (Single Edge Precracked Beam) method specified in Japanese Industrial Standard JIS R1607 is preferably 3.5 MPam 0.5 or more, more preferably 4.0 MPam 0.5 or more, and most preferably 4.5 MPam 0.5 or more. Thereby, it is possible to exhibit higher performance than sapphire single crystal (K 1c : about 3.0). Note that the upper limit value of the fracture toughness value (K 1c ) can be, for example, about 6.0 MPam 0.5 , but is not limited thereto.

[0065] The hardness (Vickers hardness) of the ceramics of the present invention is not particularly limited as long as it is equivalent to that of sapphire, and is usually 2100 or more, and particularly preferably 2150 to 2300.

[0066] 2. Method for manufacturing translucent alumina ceramics The ceramics of the present invention can be preferably manufactured by the following manufacturing method. That is, a method for manufacturing translucent alumina ceramics, (1) (a) As an alumina starting material, alumina powder having an average primary particle diameter of 0.05 to 0.5 μm and a purity of 99.9% by weight or more, and (b) as an additive source, (b1) ZrO2, MgO, Sc2O3, Y2O3 and lanthanoid rare earth oxides having an average primary particle diameter smaller than the average primary particle diameter of the alumina powder, (b2) an organic compound containing Zr, Mg, Sc, Y and lanthanoid rare earths, and (b3) an inorganic acid salt containing Zr, Mg, Sc, Y and lanthanoid rare earths. A step of obtaining a mixture (mixing step) by wet-mixing the raw material containing at least one kind in alcohol, (2) A step of obtaining a compact (forming step) by press-forming the mixture or its dried product, (3) A step of obtaining a pre-sintered body having a relative density of 96 to 99% by pre-sintering the compact at a temperature of 1150 to 1300 ° C. (pre-sintering step), and (4) A step of performing HIP treatment on the pre-sintered body at a temperature of 1100 to 1400°C and a pressure of 98 to 396 MPa (HIP treatment step) The ceramics of the present invention can be suitably produced by a method for producing translucent alumina ceramics characterized by containing [the specified element].

[0067] Mixing process In the mixing step, a mixture is obtained by wet mixing in alcohol the following raw materials: (a) alumina powder with a purity of 99.9% by weight or more and an average primary particle diameter of 0.05 to 0.5 μm as an alumina starting material, and (b) at least one of the following as an additive source: (b1) ZrO2, MgO, Sc2O3, Y2O3 and lanthanide rare earth oxides having an average primary particle diameter smaller than the average primary particle diameter of the alumina powder, (b2) an organic compound containing Zr, Mg, Sc, Y and lanthanide rare earths, and (b3) an inorganic salt containing Zr, Mg, Sc, Y and lanthanide rare earths.

[0068] As a raw material, it can be prepared by mixing alumina starting material and an additive source, as described above.

[0069] As the alumina starting material, alumina powder with an average primary particle size of 0.05 to 0.5 μm is used. Typically, α-alumina powder is used, and commercially available products can also be used. Furthermore, the purity of the alumina powder (raw material) is not limited as long as it does not hinder the effects of the present invention, but it is generally preferable to use a powder with a purity of 99.9% by weight or higher, and more preferably 99.99% by weight or higher.

[0070] As sources for the additives MgO, ZrO2, Sc2O3, Y2O3, and lanthanide rare earth oxides, (b1) ZrO2, MgO, Sc2O3, Y2O3, and lanthanide rare earth oxides having an average primary particle diameter smaller than the average primary particle diameter of the alumina powder, (b2) an organic compound containing Zr, Mg, Sc, Y, and lanthanide rare earths, and (b3) at least one inorganic salt containing Zr, Mg, Sc, Y, and lanthanide rare earths are used.

[0071] The powders of MgO, ZrO2, Sc2O3, Y2O3, and lanthanide rare earth oxides mentioned in (b1) above shall have an average primary particle size smaller than that of the alumina powder used.

[0072] Examples of the organic compounds in (b2) above include alkoxide compounds containing each cation, acetylacetonate compounds, and organic acid salts. More specifically, in the case of zirconia, examples include tetra-n-butoxyzirconium, zirconium propoxide, zirconium butoxide, and zirconium tetraacetylacetonate.

[0073] Examples of the inorganic salts in (b3) above include magnesium nitrate, magnesium chloride, zirconium oxychloride, zirconium oxynitrate, yttrium sulfate, and other lanthanide rare earth salts.

[0074] The mixing ratio of the alumina raw material and the additive source should be adjusted so that the total content of the additives in the final ceramic of the present invention is 100 to 2500 ppm by weight in terms of oxides, and the alumina content is 99.7% by weight or more.

[0075] The mixing of the alumina starting material and the additive source is carried out by wet mixing in alcohol. While industrial alcohol primarily composed of ethanol (e.g., product name "Solmix" (registered trademark, Nippon Alcohol Sales Co., Ltd., etc.)) is economically effective as the alcohol, it is not particularly limited; at least one of the following may be used: ethanol, methanol, isopropyl alcohol, etc., and pure water can also be used in combination. The amount of alcohol used is approximately 150 to 300 parts by weight per 100 parts by weight of the total weight of the alumina starting material and additive source, but is not limited to this range.

[0076] In this invention, the term "alcohol" includes not only 100% by weight alcohol but also alcohol diluted with water. The concentration when diluting alcohol with water can be appropriately set to, for example, an alcohol concentration of approximately 50-100% by weight, but is not limited to this range.

[0077] Organic binders may be added to the above raw materials as needed. While the organic binder is not particularly limited, at least one type of synthetic resin binder, such as an acrylic resin binder or a polyvinyl alcohol binder, can be suitably used. The amount of organic binder used can be approximately 0.1 to 1.0 parts by weight per 100 parts by weight of the total weight of the alumina starting material and additive source, but is not limited to this amount.

[0078] When wet mixing, various grinding media can be used as long as they do not hinder the effects of the present invention, but it is basically preferable to use alumina balls. For example, alumina balls with a purity of 99.5% by weight or higher can be suitably used. This minimizes contamination from the balls. Similarly, when using grinding media, it is preferable to use a container made of synthetic resin or an alumina container with a purity of 99.5% by weight or higher. Therefore, for example, 500 to 2000 parts by weight of alumina balls with a purity of 99.5% by weight or higher and a diameter of 0.5 to 3.0 mm can be used per 100 parts by weight of the total weight of the alumina raw material and additive supply source, these can be placed in an alumina pot with a purity of 99.5% or higher, placed on a rotating stand, and mixed for about 15 to 30 hours to obtain the mixture in the form of a slurry.

[0079] The slurry can also be subjected to drying, granulation, and other processes before being used in the molding process. For example, the drying process can include a method that includes 1) a step of obtaining a dried powder by evaporating the solvent from the slurry at a temperature of 70 to 90°C, and 2) a step of crushing the dried powder and then obtaining a powder that has passed through a sieve of about 100 mesh.

[0080] As for the granulation process, for example, a method including the steps of granulating and drying the slurry can be employed. Granulation and drying can be carried out by known methods such as spray drying. In this case, the average particle size of the granules is not limited, but it is usually desirable to be around 10 to 50 μm.

[0081] In the manufacturing method of the present invention, any mixture that has undergone at least a drying treatment is referred to as a dried product. Therefore, granulated materials as described above are also included in the dried product. The amount of solids in the dried product can be appropriately adjusted depending on, for example, the types of components that make up the dried product.

[0082] Molding process In the molding process, a compacted powder is obtained by molding the mixture or its dried form. The molding method is not particularly limited, and various methods such as uniaxial press molding, cold isohydrostatic pressing (CIP molding), casting, extrusion molding, and injection molding can be employed, either individually or in combination of two or more. As an example of a combination of two or more molding methods, a compacted powder can be obtained by uniaxial press molding as the primary molding, followed by the CIP method as the secondary molding.

[0083] Furthermore, in the molding process, the molding method and its conditions can be appropriately set according to the desired product shape, etc. For example, when forming optics with complex shapes, it is possible to knead wax into the raw material without using the aforementioned dried powder and perform near-net molding by injection molding. Also, for example, when forming infrared lenses, a mold for forming a disc can be prepared, and if it is a dome shape, a dome-shaped rubber mold can be prepared and then molded.

[0084] The pressure applied during molding can be appropriately set depending on the particle size and composition of the raw material powder used, but generally, it is preferable to apply pressure so that the resulting molded body has a density of approximately 40-65% of its theoretical density.

[0085] In this invention, CIP molding is desirable because it can form a relatively uniform density distribution. The molding pressure when using CIP molding can usually be set appropriately within the range of 98 to 396 MPa (preferably around 100 to 250 MPa), but is not limited to this range. Therefore, for example, it may be 98 to 392 MPa. Also, when molding in two stages as described above, uniaxial press molding can be performed using a mold in the range of 5 to 20 MPa, followed by CIP molding in the range of 98 to 396 MPa.

[0086] The resulting compacted material is subjected to a pre-sintering process. However, if an organic binder is used in the molding process, or if a grinding media (especially a synthetic resin media) is used, it is preferable to remove the organic components by calcining the resulting compacted material. The calcination (deorganic matter removal) conditions in this case are not limited and can be, for example, around 600-800°C in an oxidizing atmosphere.

[0087] Pre-sintering process In the pre-sintering process, the compacted powder is pre-sintered at a temperature of 1150 to 1300°C to obtain a pre-sintered body with a relative density of 96 to 99%.

[0088] The pre-sintering temperature is usually 1150-1300°C, and preferably 1170-1250°C, but it can be adjusted as appropriate depending on the sintering characteristics of the raw materials used. If the temperature is too low, even if the pre-sintered body (sintered body with many open pores) is subjected to HIP, the pressure cannot be transmitted to the interior, resulting in insufficient densification and inadequate transparency. If the temperature is too high, the constituent particles are too large relative to the size of the residual pores, so the residual pores cannot be efficiently removed, resulting in insufficient densification and transparency. The pre-sintering time can be set as appropriate depending on the density and particle size of the pre-sintered body, the size of the compacted powder, etc.

[0089] Furthermore, the heating rate during pre-sintering can be appropriately set according to factors such as the sinterability and size of the compacted powder, but it is preferable to set it to 100°C / hr or less, especially in the temperature range of 900°C or higher. This allows for a more effective reduction in the amount of residual pores.

[0090] The pre-sintering atmosphere is preferably a hydrogen atmosphere, an oxygen atmosphere, or a vacuum, but a vacuum is particularly preferred. The degree of vacuum is not limited, but is particularly 10 -1 ~10 -5 It is preferable to set it to around Pa.

[0091] The pre-sintered body obtained in this way is subjected to the HIP treatment process. In this case, the relative density of the pre-sintered body is preferably 96-99%, and more preferably 98-99.9%. Furthermore, it is desirable that the average grain size of the pre-sintered body be 0.5 μm or less, and more preferably 0.3 μm or less. By pre-sintering within the range of the above conditions, a material having the properties of the present invention can be obtained more reliably. The pre-sintered body may be cooled once prior to the HIP treatment.

[0092] HIP treatment process In the HIP treatment process, the pre-sintered body is subjected to HIP treatment at a temperature of 1100 to 1400°C and a pressure of 98 to 396 MPa.

[0093] The temperature used in the HIP treatment is usually 1100 to 1400°C, but it is particularly preferable to set it to 1150 to 1350°C, even more preferably to 1150 to 1300°C, and even more preferably to 1150 to 1250°C. By setting the temperature within this range, the amount and size of pores remaining in the resulting sintered body can be effectively reduced, and at the same time, a microstructure with suppressed grain growth can be obtained, resulting in a material with excellent optical properties and mechanical strength.

[0094] The pressure for HIP treatment is typically set to 98-396 MPa, and preferably to 176-396 MPa. Therefore, it can also be set to, for example, 98-392 MPa.

[0095] The atmosphere for the HIP treatment is not particularly restricted, and any of the following can be used, for example, an oxygen-containing atmosphere or an inert gas atmosphere. As an inert gas atmosphere, for example, N2 gas or Ar gas can be used. As an oxygen-containing atmosphere, an O2-Ar mixed gas or an O2-N2 mixed gas with an oxygen concentration of 20% by volume or less can be preferably used. In the present invention, it is more preferable to use an atmosphere of an O2-Ar mixed gas or an O2-N2 mixed gas with an oxygen concentration of 20% by volume or less.

[0096] The HIP treatment time can be adjusted as needed depending on the HIP treatment temperature, etc., but it is usually sufficient to keep it within the range of 1 to 3 hours.

[0097] Furthermore, in HIP treatment, the heating rate and cooling rate are not particularly limited, but both are generally suitable at a rate of around 400-800°C / hr. In particular, if the cooling rate is relatively fast from the maximum temperature to around 800°C, grain boundary segregation is less likely to occur, and the slower the cooling rate, the stronger the segregation. From this point of view, in the present invention, the cooling rate in the temperature range from the maximum temperature (especially below 1400°C) to 800°C is preferably greater than 600°C / hr, and more preferably 610°C / hr or higher. However, considering the yield associated with rapid cooling, it is preferable to set the upper limit to around 900°C / hr. By adopting such a cooling rate, grain boundary segregation can be suppressed more effectively, resulting in the more reliable production of alumina ceramics with very little or no birefringence.

[0098] Annealing process In the present invention, the process may optionally include a step of annealing the sintered body obtained by the HIP treatment at 700 to 1100°C. In the manufacturing method of the present invention, annealing is basically unnecessary, but sometimes the material may be reduced and turn gray or other colors (especially when N2 gas or Ar gas without oxygen is used in the HIP treatment). In this case, it is preferable to anneal the sintered body obtained by the HIP treatment at about 700 to 1000°C. By performing the annealing step, lattice defects, distortions, etc. that may exist inside the sintered body can be removed more reliably.

[0099] The annealing temperature should normally be around 700-1000°C, and preferably 700-900°C. The processing atmosphere should be an oxygen-containing atmosphere, but it is preferable to perform the process at a lower temperature than the HIP processing temperature. The pressure is not particularly limited, but it is usually fine to perform the process under atmospheric pressure (normal pressure). The annealing time can be appropriately changed depending on factors such as the size of the sintered body and the processing temperature, but it is usually within the range of 3 to 10 hours.

[0100] The annealed sintered body can be used as is, or after being processed into a predetermined shape as needed, as a predetermined optical material. The processing method is not particularly limited and can be carried out according to known cutting methods, polishing methods, etc.

[0101] 3. Use of the ceramics of the present invention The ceramics of the present invention can be used as optics in the near-infrared to mid-infrared range. In particular, they are suitable for use as window materials for security-related surveillance, rocket domes, infrared (thermal sensing) windows, and even as window materials for laser radars used in autonomous driving, and possess excellent scratch resistance and fracture resistance.

[0102] Unlike glass, the ceramics of the present invention have advantages in thermomechanical properties such as high heat resistance (melting point 2100°C), Vickers hardness around 2200, and high thermal conductivity near 30 W / mK. Furthermore, while the crystal structure of alumina ceramics generally belongs to the hexagonal system, if it is a single crystal, strong birefringence occurs, and this birefringence cannot be eliminated in any wavelength range, posing an optical problem. In contrast, with the ceramics of the present invention, by controlling the particle size, birefringence can be minimized at wavelengths longer than the near-infrared region, thus providing high linear transmittance and low birefringence, similar to cubic materials. Due to this unique performance, even though it is an anisotropic polycrystalline material, its optical properties can be altered to resemble those of isotropic materials, making it comparable to c-axis sapphire single crystals (the only material that does not exhibit birefringence), cubic spinel (MgAl2O4), and YAG (Y3Al5O4). 12 ) and similarly, clean images can be obtained through the material.

[0103] <Embodimentary form of the ceramics of the present invention> To produce the ceramics of the present invention, an alumina raw material is prepared, having a crystalline phase of α (hexagonal), a purity of 99.9% by weight or more, and an average particle size of 0.4 μm or less. Additives are added to this alumina raw material. For example, when zirconia is used as an additive, a) a zirconia sol or zirconia powder with an average particle size of 1 / 2 or less of that of alumina is added, and finally it is added to the alumina ceramics as an oxide.

[0104] However, an additive configuration in which zirconia is coated around alumina particles is preferred in order to improve the optical properties (especially birefringence) and mechanical strength of the material.

[0105] In case a) above, a zirconia sol with the smallest possible average particle size must be used. For this purpose, it is preferable that the size be at least half or less of the size of the alumina particles, which are the main raw material. For example, alumina particles with a purity of 99.99% by weight and a particle size of 0.3 μm are used, and the required amount of zirconia sol with a particle size of 150 nm or less is added. If the alumina particles are 0.3 μm, commercially available zirconia powder can be used as long as the size of the zirconia particles is 150 nm or less.

[0106] After adding the additive sources shown in (b1) to (b3) above, an appropriate amount of organic binder and ethyl alcohol are added, and the mixture is crushed and mixed for 15 to 30 hours using 99.9% high-purity alumina balls to obtain a uniform slurry. The obtained slurry is sprayed and dried with a spray dryer to obtain spherical granules with a diameter of 20 to 50 μm. To balance the optical properties and mechanical strength of the ceramics of the present invention, the total amount of additives added is preferably in the range of 100 to 2500 ppm (especially 100 to 2000 ppm). If the amount of additive is too small, the light transmittance will be poor and the mechanical properties will be those of a typical alumina ceramic. Conversely, if the amount of additive is too large, the mechanical strength will be maintained, but the optical properties will be significantly reduced.

[0107] The obtained granules are molded in a mold, and a cylindrical mold with a diameter of about 80 mm or a mold of another shape is used to uniaxially mold at a pressure of 10-20 Pa to create a molded body with a thickness of about 10 mm. After the molded body is CIP molded at a pressure of 98-392 MPa, it is degreased in air at a temperature of 700-900°C for 3 hours to remove organic components such as organic binders. -2 ~10 -5 The material is sintered in a vacuum of Pa at 1150-1300°C for 2-5 hours to obtain a pre-sintered body with a density of 96-99%. The pre-sintered body is then subjected to HIP treatment at 1150-1350°C for 1-5 hours (gas pressure of 98-392 MPa) to obtain an ultra-dense sintered body with small particle size and residual pores of 100 ppm or less, resulting in a transparent sintered body.

[0108] When the fabricated sintered body is to be used as a window, the outer circumference is processed to the specified size as needed, and then adjusted to the specified thickness, after which both sides are mirror-polished or optically polished.

[0109] In methods other than press molding, the aforementioned raw materials (alumina, oxides that serve as additive sources, etc.) are mixed with heated wax to impart fluidity, and this raw material is then molded using an injection molding machine. In this case, the mold is made in a near-net shape to eliminate the need for post-processing such as drilling holes and adjusting the curvature of corners, and the necessary surfaces are polished.

[0110] In this invention, α-alumina with a purity of 99.9% by weight or higher and an average primary particle size of 0.1 to 0.4 μm is used as the starting material. An appropriate amount of oxide additive source, either alone or in combination, is added, until the final oxide content is 100 to 2500 ppm by weight. Subsequently, the material is synthesized by wet mixing, drying, uniaxial press molding, cold isostatic press molding (CIP molding), degreasing, pre-sintering, and hot isostatic pressing. The resulting transparent sintered body is annealed at 700 to 1000°C in an oxygen-containing atmosphere to remove lattice defects, distortions, etc., from the inside of the material, and then machined into a predetermined shape. In this case, the optical quality can be checked by finishing the required surfaces to a general mirror finish. The three-point bending strength and fracture toughness conform to JIS R1601 and R1607, but the sample size is 3 mm × 4 mm × 40 mm, and fracture toughness can be measured by inserting a notch with a width of 100 μm and a depth of 200 μm.

[0111] Figure 1 shows a scanning electron microscope (SEM) image of the ceramic material of the present invention (alumina of the present invention). This is an example of an embodiment of the present invention, but the average particle size is 0.4 μm and no segregation of sintering additives is observed at the grain boundaries.

[0112] Figure 2 shows the observation of a fluorescent lamp through a conventional alumina ceramic measuring 12 mm x 29 mm x 0.2 mm thick, with both sides mirror-polished, and through the ceramic of the present invention measuring 20 mm in diameter x 1.0 mm thick. With conventional alumina ceramics, even a thin sample cannot detect the fluorescent lamp, but the ceramic of the present invention has excellent transmission performance even in the visible range, and can clearly capture the fluorescent lamp. The camera used in this demonstration was a visible-vision CCD.

[0113] Figure 3 shows (a) the linear transmittance at wavelengths of 400 to 2200 nm for a 1 mm thick c-axis sapphire single crystal, the ceramics of the present invention, and a 0.4 mm thick conventional translucent alumina, respectively. (b) The amount of leak light (birefringence) under crossed nicols for a 1 mm thick ceramic of the present invention and a 0.4 mm thick conventional translucent alumina. Conventional alumina ceramics can only obtain a maximum linear transmittance of 30-40% even when the sample thickness is as thin as 0.4 mm. In contrast, although the transmission characteristics of the ceramics of the present invention are inferior to those of the c-axis sapphire single crystal in the visible region, they approach each other from the near-infrared region onward, and ultimately reach the same level.

[0114] The ceramic material of the present invention shown in Figure 3 exhibited higher linear transmittance at longer wavelengths, while conversely, birefringence decreased. Around a wavelength of 1200 nm, the linear transmittance approached that of a c-axis sapphire single crystal, and the leakage light associated with birefringence became extremely small. Microstructural observation using a polarizing microscope was consistent with the results of the spectroscopic measurements, clearly explaining why the ceramic material of the present invention has optical properties similar to or comparable to those of a c-axis sapphire single crystal.

[0115] Currently available polarizing plates only cover wavelengths up to around 2200 nm, making it impossible to observe birefringence in wavelengths above 2200 nm. However, in the mid-infrared region (3-5 μm band), the ceramics of this invention undergo further optical isotropy, and the reason why their optical properties are comparable to those of c-axis sapphire is clear.

[0116] Figure 4 shows the results of observing conventional translucent alumina with an average particle size of 10 μm (top row) (using one polarizing plate (a1) and two polarizing plates (a2)) and the ceramics of the present invention with an average particle size of 0.5 μm (bottom row) (using one polarizing plate (b1) and two polarizing plates (b2)) with a transmission polarizing microscope (CCD camera). In both cases, the sample thickness is 0.5 mm. A xenon lamp was used as the light source. The left photograph shows observation with one polarizing plate, and the right photograph shows observation using two polarizing plates (both orthogonal). The difference between conventional translucent alumina and glass material was also observed on a glass plate.

[0117] Observations using a single polarizing plate show that conventional translucent alumina exhibits strong birefringence (strong black and white contrast) (a1), but the ceramics of the present invention significantly reduce the birefringence inherent to alumina (anisotropic material) (b1). Furthermore, the glass is amorphous (optically isotropic), and virtually no strain is detected, resulting in a uniform appearance.

[0118] When two polarizing plates are crossed and observed (hereinafter simply referred to as "observation of two polarizing plates"), a large amount of birefringence is observed in conventional translucent alumina (a2), but the ceramics of the present invention are dark-field (b2) and exhibit the same behavior as glass. This indicates that the ceramics of the present invention have optically isotropic properties, similar to glass.

[0119] Materials without optical anisotropy exhibit no stray light, resulting in a dark-field observation. Glass, lacking optical anisotropy, exhibits a dark-field observation. However, the ceramics of this invention, being a hexagonal and unoriented polycrystalline material that should exhibit optical anisotropy, exhibit a near-dark-field observation as shown in Figure 4(b2). Conventional alumina, as theoretically expected, exhibits a bright-field observation, allowing for the detection of a large amount of stray light. Furthermore, it exhibits an optical microstructure where each constituent particle has different stray light, meaning that each particle with a different crystal orientation has a different amount of polarization.

[0120] Figure 5 shows the results of examining the optical quality of materials using a different INGAS camera than in Figure 4, with the measurement wavelength limited to 1.2 to 1.4 μm for conventional translucent alumina with an average particle size of 10 μm (top row) (one polarizer (a1) and two polarizers (a2)) and the ceramics of the present invention with an average particle size of 0.5 μm (bottom row) (one polarizer (b1) and two polarizers (b2)). In both cases, the sample thickness was 0.5 mm.

[0121] As shown in Figure 5, when conventional translucent alumina is observed with a single polarizing plate, numerous optical distortions are observed (a1), and even when two polarizing plates are crossed, significant light leakage due to birefringence can be detected (a2). On the other hand, the ceramics of the present invention show even less optical distortion than when observed in the visible range, reaching a level indistinguishable from glass (b1). Furthermore, when observed with two polarizing plates crossed, it is almost a perfect dark field, similar to glass (b2), indicating that birefringence has almost disappeared and that it has optically isotropic properties similar to glass.

[0122] Figure 6 shows images of a subject (human hand) captured using a near-infrared camera (wavelength sensitivity 0.9-1.7 μm) and a mid-infrared camera (wavelength sensitivity 3-5 μm), respectively, of conventional translucent alumina with a diameter of 60 mm and a thickness of 2 mm (a1, a2), the ceramics of the present invention with a diameter of 60 mm and a thickness of 4 mm (b1, b2), and a c-axis sapphire single crystal with a diameter of 60 mm and a thickness of 4 mm (c1, c2). (a1), (b1), and (c1) are the observation results obtained with the near-infrared camera, and (a2), (b2), and (c2) are the observation results obtained with the mid-infrared camera.

[0123] Subjects photographed through the ceramics of the present invention (b1, b2) and c-axis sapphire single crystals (c1, c2) can be clearly viewed in both near-infrared and mid-infrared. However, subjects photographed through conventional translucent alumina (a1, a2) cannot be captured by a near-infrared camera, and even in mid-infrared, the image quality is poor, resulting in very dark images.

[0124] Figure 7 shows images of the ceramics of the present invention, measuring 60 mm in diameter x 2 mm in thickness and 60 mm in diameter x 6 mm in thickness, captured with a near-infrared camera (wavelength sensitivity 0.9-1.7 μm) and a mid-infrared camera (wavelength sensitivity 3-5 μm) on a subject (a human hand). Blanks (A1, A2) are images of the subject directly captured with each camera. (A1), (B1), and (C1) are observation results from the near-infrared camera, and (A2), (B2), and (C2) are observation results from the mid-infrared camera.

[0125] As shown in Figure 7, when photographing an object through the ceramics of the present invention, a bright and clear image is obtained in both near-infrared and mid-infrared fields, similar to the blank (A1, A2), even when the thickness reaches 6 mm, just as in the case of a thickness of 2 mm (B1, B2) (C1, C2).

[0126] Figure 8 shows the results of mirror polishing both sides of a c-axis sapphire single crystal with a diameter of 20 mm and a thickness of 1.5 mm, the ceramic material of the present invention with a diameter of 20 mm and a thickness of 1.5 mm, and a conventional alumina ceramic with a diameter of 20 mm and a thickness of 0.4 mm, and then irradiating them with laser light of a wavelength of 1064 nm and capturing the emitted light with a beam profiler.

[0127] As is clear from Figure 8, the beams obtained from the ceramics of the present invention (B) and the c-axis sapphire single crystal (C) maintain the original shape of the original beam (A). In contrast, the light emitted from conventional alumina ceramics (D) has a very weak main beam, losing its original shape, and the rest is scattered at a high angle. Since the radar light source used for autonomous driving uses a 1.5 μm eye-safe laser, the light emitted from the ceramics of the present invention is even more coherent and has less loss, making it a suitable material for use as a window in laser radar for autonomous driving. [Examples]

[0128] Examples and comparative examples are shown below to give a more detailed explanation of the features of the present invention. However, the scope of the present invention is not limited to the examples.

[0129] Example 1 Commercially available α-Al2O3 powder (purity >99.99% by weight, average primary particle size 0.2 μm) was used as the alumina starting material. To this powder, 200 ppm of ZrO2 sol with an average primary particle size of 30 nm was added as an additive source. For every 100 parts by weight of this mixed powder, 2 parts by weight of a commercially available acrylic resin binder (20% by weight solution of acrylic resin) was added and placed in a synthetic resin container. A slurry was prepared by wet mixing 500 g of the total mixed powder with 5 kg of high-purity Al2O3 balls for grinding (particle size approximately 2 mm) and 1200 ml of ethanol over 15 hours. The recovered slurry was spray-dried using a spray dryer to obtain a powder consisting of spherical granules approximately 30 μm in size. This powder was sieved through a nylon sieve (150 mesh). Next, the powder that passed the sieve was uniaxially press-formed under a pressure of approximately 10 MPa, and then CIP molding was performed under a pressure of 147 MPa. After degreasing the molded body at 700°C, the resulting molded body was subjected to a 1 × 10⁻¹⁶ treatment. -3 Pre-sintering was performed in a vacuum of Pa at a heating rate of 150°C / hr, reaching a maximum temperature of 1200°C. The relative density of the pre-sintered body was measured by the Archimedes method and found to be 98%. Next, the preliminary sintered body was subjected to HIP treatment. The treatment conditions were: treatment atmosphere: Ar gas, pressure: 98 MPa, temperature: 1300°C, treatment time: 2 hours. During cooling, the cooling rate was controlled to 620°C / h in the temperature range of 1300 to 850°C. The HIP-treated sintered body was transparent but gray, so it was further annealed in air at 900°C for 5 hours to remove discoloration and internal lattice defects. The sintered body was thus produced.

[0130] Examples 2-27 A sintered body was prepared in the same manner as in Example 1, except for the conditions shown in Tables 1 to 3.

[0131] Comparative Example 1 As a candidate material to replace existing glass screens, and also as a candidate material for windows used in laser radar for autonomous driving, we used c-axis sapphire single crystals fabricated by the CZ method as a sample.

[0132] Comparative Example 2 Translucent alumina of the same grade used in conventional high-pressure sodium discharge lamps was used as a sample.

[0133] Comparative Examples 3-8 A sintered body was prepared in the same manner as in Example 1, except for the conditions shown in Table 4.

[0134] Test Example 1 The following characteristics were evaluated for each example and comparative example sample. The results are shown in Tables 1 to 4. Tables 1 to 4 also show the composition of each sample.

[0135] (a) Analysis of additives added to alumina Additives added to the material can be detected in ppm units using a general instrumental analyzer, ICP (Inductive Coupled Plasma)-MASS, to determine the content of MgO, ZrO2, Sc2O3, Y2O3, and lanthanide rare earth oxides in the material.

[0136] (b) Average particle size of the sintered body The surface of the prepared sample is mirror-polished, and the grain boundaries are thermally etched at a temperature 50-100°C lower than the sintering temperature. At least three arbitrary locations are observed at 10,000x magnification using a commercially available scanning electron microscope, and the particle size is measured using an image analysis device.

[0137] (c) Presence or absence of grain boundary phase and composition of grain boundary phase (confirmation of segregation of added oxide) The grain boundary phase precipitated between Al2O3 particles is observed at a magnification of 100,000x using a commercially available transmission electron microscope. At least five grain boundaries are examined at the specified magnification to determine the presence or absence of the grain boundary phase.

[0138] (d) Measurement of linear transmittance in the visible to infrared region The prepared sample is adjusted to a thickness of 1.0 mm and both sides are mirror-polished. The transmittance of the polished sample is measured using a commercially available spectrophotometer at wavelengths of 400 to 2500 nm, and using an infrared spectrophotometer at wavelengths exceeding 2500 nm. It is confirmed that the linear transmittance at wavelengths of 600 nm, 1200 nm, and 3500 nm is 55% or higher, 70% or higher, and 80% or higher, respectively.

[0139] (e) Measurement of optical strain and birefringence state inside the material from microstructural observation The prepared sample is adjusted to a thickness of 1.0 mm and both sides are mirror-polished. A single polarizing plate is placed in a transmission microscope, and the transmitted image of the sample is photographed. The state of internal distortion is observed from the difference in color, and the state of birefringence is observed from the leaked light passing through two polarizing plates placed orthogonally. When observing (photographing) with a microscope, a general CCD camera is used in the visible range, and in the near-infrared range, a filter that can cut out wavelengths below 1200 nm is attached to the light emitted from the lamp (xenon lamp) that serves as the light source, and observation is performed with a near-infrared camera (INGAS, detection wavelength ~1400 nm), making it possible to observe in a wavelength range of 1200 to 1400 nm.

[0140] (f) Quantitative measurement of birefringence present within the material from spectroscopic measurements The prepared sample is adjusted to a thickness of 1.0 mm and both sides are mirror-polished. Two polarizing plates are placed parallel to each other in a spectrophotometer, and the amount of light passing through is measured in the wavelength range of 400 to 2200 nm. This measurement serves as the baseline. Subsequently, polarizing plates are placed orthogonally in front of and behind the sample through which light passes, and the amount of transmitted light in the wavelength range of 400 to 2200 nm (this is the leak light due to birefringence, i.e., stray light) is measured. If there is no birefringence inside the material, the amount of transmitted light will be zero, and leak light will be generated according to the amount of birefringence present, and this light can be detected. The extinction ratio is calculated using equation (1) from the ratio of the amount of leak light to the amount of light measured with the polarizing plates placed parallel to each other. In this invention, the representative wavelength for the visible region is set to 600 nm, and the representative wavelength for the near-infrared region is set to 1200 nm. Measurement is not possible in the mid-infrared region because no commercially available polarizing plates capable of measurement exist, but theoretically, the wavelength is smaller than that of the near-infrared region.

[0141] (g) Measurement of residual porosity in the material The prepared sample is adjusted to a thickness of 1.0 mm and both sides are mirror-polished. Using a transmission microscope, photographs are taken at 400x magnification at five arbitrary locations, and the amount of residual pores can be determined by measuring the size and number of bubbles that can be observed in an observation volume of 1.0 mm × observation area. In this invention, residual pores refer only to the total amount of pores that can be observed at 400x magnification, and bubbles that cannot be detected by this method are not counted.

[0142] (h) Measurement of mechanical strength (three-point bending strength, fracture toughness) The three-point bending strength is determined according to the Japanese Industrial Standard JIS R1601 and the SEPB method. The fracture toughness is calculated using the method and formula described in the Japanese Industrial Standard JIS R1607.

[0143] [Table 1]

[0144] [Table 2]

[0145] [Table 3]

[0146] [Table 4]

[0147] (a) The properties of commercially available c-axis sapphire single crystals prepared by the Czochralski method, (b) conventional translucent alumina, and (c) alumina with an average particle size of 600 nm without additives are shown in Table 4, and (d) the properties of each sample of the ceramics of the present invention are shown in Tables 1 to 3, respectively.

[0148] Although the Vickers hardness of the alumina-based materials shown in the examples and comparative examples is similar at approximately 2200, and therefore data is not shown, as is clear from the results in Table 4, the three-point bending strength of the ceramics of the present invention is significantly higher than that of the comparative example, which is conventional translucent alumina, and consistently exceeds the values ​​for c-axis sapphire single crystals.

[0149] Regarding fracture toughness, the K1c of conventional translucent alumina is 2.1, while that of c-axis sapphire single crystal is 2.8 MPam. 0.5 In contrast, the ceramics of the present invention have a pressure of 3.7 to 5.5 MPam 0.5 This shows a very high value.

[0150] From the above, it can be seen that the ceramics of the present invention have high linear transmittance and, even if birefringence occurs at wavelengths above near-infrared, it is minimal, making them suitable for use as optics.

[0151] Furthermore, Figure 7 shows the window characteristics in the infrared region for samples up to 6 mm thick. The results also overturn the conventional wisdom regarding polycrystalline and anisotropic materials, showing that clear and high-brightness images can be captured through the ceramics of the present invention when the medium thickness is within 10 mm. This is because the ceramics of the present invention possess the unique characteristic of changing from optical anisotropy to optical isotropy as the wavelength increases, a phenomenon that is not present in the conventional wisdom of materials science.

[0152] It was also demonstrated that when the subject is photographed using a near-infrared to mid-infrared camera, it exhibits windowing characteristics comparable to those of a c-axis sapphire single crystal or optically isotropic Y2O3 ceramics (not shown in the figure).

[0153] Of course, as shown in Figure 2, even in the visible spectrum, if the thickness is about 1 mm, the transmittance is high and a very clear image can be obtained. For example, in the case of thin plate shapes such as mobile screens, the leakage light due to birefringence is minimal and the extinction ratio is very high, so optical applications are possible even in this wavelength range. It has been theoretically thought that it is impossible to obtain a transparent material from anisotropic and polycrystalline ceramics because birefringence occurs each time light passes through the particles (because the particles themselves become scatterers that generate birefringence). In the ceramics of the present invention, by limiting the particle size and adding a specific amount of specific additives, the generation of birefringence from the particles themselves can be minimized, thereby obtaining a more transparent ceramic. Until now, it was thought that a necessary condition for fabricating optical materials from polycrystalline materials was that the crystal structure of the material had to be "cubic (optically isotropic)", but with the present invention, it can be said that a "new door to materials science" has been opened, showing that anisotropic polycrystalline materials other than cubic can also be used as optical materials. [Industrial applicability]

[0154] This invention can provide polycrystalline alumina ceramics that exhibit unprecedentedly high linear transparency.

[0155] In particular, the ceramics of the present invention exhibit linear transmittance comparable to that of c-axis sapphire single crystals in the near-infrared to mid-infrared wavelength range, while minimizing birefringence. Furthermore, its excellent mechanical properties (especially its high fracture toughness) realize an "extremely crack-resistant transparent alumina," making it useful not only for screens for display devices but also for windows, domes, and other applications where thickness is required in the near-infrared to mid-infrared range. From a different perspective, the ceramics of the present invention, with their hardness, fracture strength, and superior fracture toughness, are also expected to serve as a new alternative to zirconia, silicon carbide, and other materials.

Claims

1. Alumina ceramics having an alumina content of 99.7% by weight or more, (1) MgO, ZrO ２ , Sc ２ O ３ , Y ２ O ３ The total content of at least one additive, a lanthanide rare earth oxide, is 100 to 2500 ppm by weight. (2) The residual pore volume is 100 ppm or less, (3) When observed at 100,000x magnification using a scanning electron microscope or a transmission electron microscope, the segregation of the additive between the alumina particles is not observed. Translucent alumina ceramics characterized by the following features.

2. The three-point bending strength is 350 MPa or higher, and the fracture toughness (K １ｃ The translucent alumina ceramics according to claim 1, wherein the ratio is 3.5 or higher.

3. The translucent alumina ceramics according to claim 1, having an average particle size of 0.1 to 0.8 μm.

4. In a sample of alumina ceramics with a thickness of 1.0 mm and a mirror-polished surface, (1) The linear transmittance at a wavelength of 600 nm is 55% or more. (2) The linear transmittance at a wavelength of 1200 nm is 70% or more, (3) The linear transmittance at a wavelength of 3500 nm is 80% or higher. A translucent alumina ceramic according to claim 1, satisfying all of the conditions.

5. In a sample of alumina ceramics with a thickness of 1.0 mm and a mirror-polished surface, (1) The extinction ratio at a wavelength of 600 nm is 15 dB or more, (2) The extinction ratio at a wavelength of 1200 nm is 25 dB or more. A translucent alumina ceramic according to claim 1, satisfying all of the conditions.

6. An optical material comprising a translucent alumina ceramic according to any one of claims 1 to 5.

7. A device for detection in the near-infrared to mid-infrared wavelength region of 0.9 to 5 μm, comprising a translucent alumina ceramic according to any one of claims 1 to 5.

8. A method for producing translucent alumina ceramics, (1)(a) As the alumina starting material, alumina powder with a purity of 99.9 wt% or more and having an average primary particle diameter of 0.05 to 0.5 μm, and (b) as the additive supply source, a zirconia (ZrO) having an average primary particle diameter smaller than the average primary particle diameter of said alumina powder, magnesia (MgO), scandia (Sc<O>), yttria (Y<O>), and lanthanoid rare earth oxides, (b2) an organic compound containing Zr, Mg, Sc, Y, and lanthanoid rare earths, and (b3) at least one of inorganic acid salts containing Zr, Mg, Sc, Y, and lanthanoid rare earths are wet-mixed in alcohol to obtain a mixture. ２ , MgO, Sc ２ O ３ , Y ２ O ３ And a step of obtaining a mixture by wet-mixing a raw material containing at least one of (b2) an organic compound containing Zr, Mg, Sc, Y, and lanthanoid rare earths and (b3) an inorganic acid salt containing Zr, Mg, Sc, Y, and lanthanoid rare earths in alcohol. (2) A step of obtaining a compacted powder by press molding the mixture or the dried product thereof, (3) A step of obtaining a pre-sintered body with a relative density of 96 to 99% by pre-sintering the compacted powder at a temperature of 1150 to 1300°C, and (4) A step of performing HIP treatment on the pre-sintered body at a temperature of 1100 to 1400°C and a pressure of 98 to 396 MPa and A method for producing translucent alumina ceramics, characterized by containing [a specific substance].

9. The manufacturing method according to claim 8, wherein in the HIP treatment, the cooling rate in the temperature range of 1400 to 800°C exceeds 600°C / hr.

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