Method for producing paramagnetic garnet-type transparent ceramic and method for producing magneto-optical device

Abstract

This method for producing a paramagnetic garnet-type transparent ceramic represented by formula (1) includes a step for adjusting a slurry containing a metal oxide powder raw material constituting formula (1) and a liquid medium, wherein the slurry has a median diameter D50 value of 250 nm to 1,500 nm in a particle size distribution at a concentration of 11±4 mass%, and a D95 value at a cumulative particle size distribution of 95% of 2,000 nm or less. In this method for producing a magneto-optical device, the paramagnetic garnet-type transparent ceramic is used as a Faraday rotator 110 to constitute an optical isolator 100.　(1): (Tb1-x-yRexScy)3(Al1-zScz)5O12 (in the formula, Re is Y and / or Lu, and 0≤x<0.45, 0<y<0.1, and 0<z<0.2

Description

Method for manufacturing paramagnetic garnet-type transparent ceramics and method for manufacturing magneto-optical devices

[0001] The present invention relates to a method for manufacturing paramagnetic garnet-type transparent ceramics and a method for manufacturing magneto-optical devices, and more particularly to a method for manufacturing paramagnetic garnet-type transparent ceramics containing terbium, which is suitable for constituting magneto-optical devices such as optical isolators, and a method for manufacturing magneto-optical devices using these paramagnetic garnet-type transparent ceramics as a magneto-optical material.

[0002] Optical isolators have the function of transmitting light in the forward direction and blocking light in the reverse direction, and are therefore used to prevent damage to the laser oscillation source and stabilize the output by being installed between the light source and the workpiece. Terbium-doped glass or terbium gallium garnet crystal (TGG crystal) is mounted inside the optical isolator as a Faraday rotor (for example, Japanese Patent Publication No. 2011-213552 (Patent Document 1)). The magnitude of the Faraday effect is quantified by the Verde constant, and the Verde constant of TGG crystal is 40 rad / (T・m) (0.13 min / (Oe・cm)), while that of terbium-doped glass is 0.098 min / (Oe・cm). Since the Verde constant of TGG crystal is relatively large, it is widely used as a standard Faraday rotor. In addition, there is terbium aluminum garnet crystal (TAG crystal), and since the Verde constant of TAG crystal is about 1.3 times that of TGG crystal, the length of the Faraday rotator can be shortened, making it a suitable crystal for use in fiber lasers (for example, Japanese Patent Publication No. 2002-293693 (Patent Document 2), Japanese Patent Publication No. 2004-539464 (Patent Document 3)).

[0003] In recent years, methods for fabricating TAG using transparent ceramics have been disclosed (for example, International Publication No. 2017 / 033618 (Patent Document 4), “High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics” (Non-Patent Document 1)). In addition, some terbium has been replaced with yttrium (Tb ｘ Y １－ｘ ) ３ Al５ O １２ (0.2 ≤ x ≤ 0.8, or 0.5 ≤ x ≤ 1.0)(YTAG) The manufacturing method of transparent ceramics has also been reported (for example, "Fabrication and properties of (Tb x Y 1-x )3Al5O 12 transparent ceramics by hot isostatic pressing" (Non-Patent Document 2), "Development of optical grade (Tb x Y 1-x )3Al5O 12 ceramics as Faraday rotator material" (Non-Patent Document 3)). Rare earth aluminum garnet containing Tb is said to be a material suitable for high-power applications because it has a small thermal lens effect compared to TGG.

[0004] As described above, in recent years, many reports on rare earth aluminum garnet containing Tb are by ceramics. This is due to the fact that since TAG has a decomposition melting composition, it is difficult to produce single crystals.

[0005] One of the performance requirements for a Faraday rotor is a high extinction ratio. The extinction ratio is the ratio that indicates the intensity of polarized light, and if the extinction ratio deteriorates, the amount of reflected light to the laser source increases, causing damage to the laser source and instability. It is known that the refractive index difference due to foreign matter such as different phases is an influence on the deterioration of this extinction ratio. For example, International Publication No. 2022 / 054593 (Patent Document 5) discloses that if the number of bubbles, foreign matter, different phases, microcracks, etc. inside the Faraday rotor exceeds a certain number, the extinction ratio deteriorates. It is stated that these different phases can be reduced by optimizing the sintering conditions such as pre-sintering, re-sintering, and annealing, as well as the crystal grain size of the sintered body, thereby reducing their optical influence, specifically the refractive index difference between the matrix phase and the different phase, and obtaining paramagnetic garnet-type ceramics with a high extinction ratio. Furthermore, Japanese Patent Publication No. 2012-206934 (Patent Document 6) discloses that in oxide ceramics, the extinction ratio deteriorates when there is a difference in refractive index between the main phase and the grain boundaries. Therefore, in order to obtain transparent ceramics with a high extinction ratio, especially for laser outputs of 100 W or more, it is necessary to minimize the difference in refractive index within the sintered body as much as possible.

[0006] Incidentally, Japanese Patent Publication No. 2014-88309 (Patent Document 7) describes how to produce high-quality transparent ceramics by mixing various raw material powders that constitute ceramics, molding them, and sintering them. In the above mixing process, the various raw material powders are mixed with a dispersion medium to form a slurry, and after achieving a certain particle size, the dispersion medium is removed to obtain the raw material powder for sintering, which is produced by a wet method. It is known that the state of this slurry greatly affects the performance of the sintered body. If the particle size of the raw material powder for sintering (in this invention, particle size refers to the actual particle size, and if the particles have aggregated into secondary particles, it refers to the particle size of the secondary particles) is larger than 2000 nm, the raw material powder may not mix thoroughly, which can cause compositional unevenness inside the sintered body. In addition, the densification rate during sintering may be slow, and it may not be possible to obtain high-quality paramagnetic garnet-type transparent ceramics. On the other hand, in the case of powders with a particle size of, for example, several tens of nm, the state of the powder is very unstable and easily forms aggregates, forming hard aggregates. Therefore, this can lead to adverse effects such as voids forming during molding or density unevenness occurring during sintering, causing the sintered body to crack. Consequently, it is desirable to evaluate the particle size of the sintering raw material powder used through particle size distribution measurement and manage it to ensure an appropriate particle size.

[0007] Furthermore, in the above-mentioned Patent Document 7, particle size distribution measurement is performed by dispersing the sample in ethanol and using a particle size analyzer (Microtrac MT3000II and Nanotrac UPA, manufactured by Nikkiso Co., Ltd.). Although the measurement concentration is not described, the measurement is performed using a dynamic light scattering method, in which laser light is irradiated onto the sample cell and the scattered laser light is measured at predetermined time intervals to calculate the particle size. With this device, if a concentrated solution is used as the sample, the incident and scattered light are multiple-scattered, making accurate measurement impossible, and it is necessary to dilute it significantly to a concentration in which light can be transmitted. On the other hand, in the field of ceramics in general, slurries with a high particle concentration are often used for production. Therefore, the above-mentioned laser scattering method, which performs measurements at a relatively lower concentration than the actual high-concentration slurry, cannot be said to truly represent the particle size of the slurry because the attractive and electrostatic repulsive forces between particles differ greatly from those of the actual slurry.

[0008] On the other hand, some particle size distribution measuring devices, such as the one described in Patent Document 8 (Japanese Patent Application Publication No. 2003-121338), can also measure high-concentration slurries by suppressing multiple scattering of incident and scattered light.

[0009] Japanese Patent Publication No. 2011-213552, Japanese Patent Publication No. 2002-293693, Japanese Patent Publication No. 2004-539464, International Publication No. 2017 / 033618, International Publication No. 2022 / 054593, Japanese Patent Publication No. 2012-206934, Japanese Patent Publication No. 2014-88309, Japanese Patent Publication No. 2003-121338, Japanese Patent Publication No. 2019-202916

[0010] “High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics”, Optical Materials Express, Vol. 6, No. 1 191-196 (2016) “Fabrication and properties of (TbxY1-x)3Al5O12 transparent ceramics by hot isostatic pressing”, Optical Materials, 72 58-62 (2017) “Development of optical grade (TbxY1-x)3Al5O12 ceramics as Faraday rotator material”, Journal of American Ceramics Society, 100, 4081-4087 (2017)

[0011] As described above, with the increasing power output of lasers, there is a growing demand for paramagnetic garnet-type ceramics with a high extinction ratio.

[0012] However, when the present inventors actually replicated the method described in Patent Document 5, they found no bubbles, foreign matter, different phases, or microcracks, and confirmed that highly transparent paramagnetic garnet-type transparent ceramics could be obtained. However, they discovered a problem with the extinction ratio, and it was confirmed that further problems needed to be solved.

[0013] When the inventors diligently investigated the above problem, they found that while particle size distribution measurements of dilute slurry with a slurry concentration of less than 0.1% could be controlled to be below 2000 nm, aggregates with a particle size exceeding 2000 nm existed at a slurry concentration of 11 ± 4 mass%. Further detailed investigation revealed that the aggregates were mainly aluminum oxide. When a molded body is produced using this slurry or the raw material powder obtained by removing the solvent from the slurry and allowing it to dry, uniformity cannot be achieved solely through thermal diffusion during the sintering process, and localized compositional irregularities may remain within the sintered body. Therefore, even when no scattering sources such as bubbles, foreign matter, different phases, or microcracks are observed, it is thought that localized refractive index irregularities occur within the sintered body, resulting in a deterioration of the extinction ratio.

[0014] In particular, dispersants are generally used to prevent particle aggregation in high-concentration slurries. The amount of dispersant added, the average molecular weight, and the structure are important factors that affect the particle dispersion effect. For example, if the average molecular weight of the dispersant is large, the dispersant may adsorb to multiple particles, conversely forming aggregates. On the other hand, if the average molecular weight is small, the dispersion effect may not be sufficiently exerted, and aggregation may not be suppressed. Also, if the amount of dispersant added is large, the crushability of the sintering raw material powder obtained by removing the solvent from the slurry and drying it may deteriorate, and cracks and voids may occur inside the sintered body, which may worsen the extinction ratio. On the other hand, if the amount of dispersant added is small, the dispersion effect may not be sufficiently exerted, and aggregation may not be suppressed. Furthermore, dispersants have structures that are linear or branched, and the dispersion effect and granule crushability also change depending on the structure. Therefore, it is necessary to select an appropriate dispersant according to the particle size, surface area, structure, etc. of the powder used, and in the case of transparent ceramics intended for Faraday rotators, it is necessary to add an appropriate amount that does not cause cracks or voids inside the sintered body. However, until now, information regarding the structure of dispersants that can solve the above problems, the appropriate amount to add, and the particle size distribution in concentrated slurries has not been clear.

[0015] Furthermore, Patent Document 9 (Japanese Patent Publication No. 2019-202916) describes how the composition inside the sintered material can be made uniform by producing the sintering raw material powder by coprecipitation. Specifically, a coprecipitation raw material powder in which all constituent elements are uniformly distributed is obtained by synthesizing precursor solutions of various elements by coprecipitation, and then the powder is molded and sintered to produce a uniform paramagnetic garnet-type transparent ceramic. With this method, a sintering raw material powder that is uniformly mixed at the primary particle level is obtained, so there is no concern about the compositional unevenness described above. In addition, there is an advantage that a uniform sintering raw material powder can be produced without using a dispersant.

[0016] However, even when rare-earth aluminum garnet containing Tb is weighed according to its composition, the actual composition may deviate due to the influence of surface-adsorbed water, etc., causing the perovskite phase to precipitate, and it may not be possible to obtain highly transparent rare-earth aluminum garnet containing Tb. Patent document 9 disclosed that it is extremely difficult to produce highly transparent garnet sintered bodies with good reproducibility due to the influence of differences in the solubility products of the constituent elements, errors in measuring the concentration of the precursor solution, and weighing errors. In fact, when the inventors conducted follow-up tests, they found that the composition differed for each raw material, and they were unable to stably achieve transparency.

[0017] Therefore, in view of the above problems, the present invention aims to provide a method for producing paramagnetic garnet-type transparent ceramics having a high extinction ratio of 35 dB or more by making the composition of the sintering raw material powder uniform, and a method for producing magneto-optical devices.

[0018] In one aspect, the present invention provides a method for producing a paramagnetic garnet-type transparent ceramic represented by the following formula (1): (Tb １-ｘ-ｙ Re ｘ Sc ｙ ) ３ (Al １－ｚ Sc ｚ ) ５ O １２(1) (wherein Re is Y and / or Lu, and 0 ≤ x < 0.45, 0 < y < 0.1, 0 < z < 0.2.) The process includes at least a step of preparing a slurry containing an oxide powder raw material of the metal constituting formula (1) and a liquid medium, wherein the slurry has a median diameter D50 value of the particle size distribution at a concentration of 11 ± 4 mass% of 250 nm or more and 1500 nm or less, and a cumulative particle size distribution D95 value of 95% of which is 2000 nm or less.

[0019] It is preferable to add a dispersant having anionic groups and graft chains to the slurry in an amount of 0.1 to 3.0% by mass relative to the total amount of oxide powder raw materials.

[0020] The above-mentioned anionic group is preferably selected from one or more of the following: a carboxylic acid group, a sulfonic acid group, or a sulfate ester group.

[0021] The above dispersant preferably has an average molecular weight of 1,000 to 50,000, and the above liquid medium is preferably water or a lower alcohol having 1 to 4 carbon atoms.

[0022] The graft chain described above is preferably a polyoxyalkylene chain.

[0023] The present invention preferably further includes the steps of: obtaining a molded body by drying and granulating the slurry and then molding it; obtaining a sintered body by pre-sintering the molded body at 1450°C to 1650°C; obtaining a pressurized sintered body by pressurizing the sintered body; obtaining a re-sintered body by heating the pressurized sintered body at a temperature exceeding the pre-sintering temperature or the pressurized sintering temperature; and obtaining a paramagnetic garnet-type transparent ceramic by annealing the re-sintered body in an oxidizing atmosphere at 1400°C or higher. The paramagnetic garnet-type transparent ceramic preferably has an extinction ratio of 35 dB or more when laser light with a wavelength of 1070 nm at an optical path length of 20 mm is incident with a beam diameter of 1.6 mm and an incident power of 100 W.

[0024] In another aspect, the present invention provides a method for manufacturing a magneto-optical device, which includes the step of constructing a magneto-optical device using paramagnetic garnet-type transparent ceramics obtained by the above-described method for manufacturing paramagnetic garnet-type transparent ceramics as magneto-optical elements.

[0025] The magneto-optical device described above is preferably an optical isolator usable at wavelengths of 0.9 μm to 1.1 μm, comprising the paramagnetic garnet-type transparent ceramic as a Faraday rotor, and polarizing materials positioned before and after the optical axis of the Faraday rotor.

[0026] According to the present invention, in the slurry preparation step, the median diameter D50 value of the particle size distribution at a concentration of 11 ± 4 mass% of the slurry is 250 nm or more and 1500 nm or less, and the D95 value at which the cumulative particle size distribution reaches 95% is 2000 nm or less, thereby making the composition of the raw material powder for sintering uniform. This provides a method for producing paramagnetic garnet-type transparent ceramics having a high extinction ratio of 35 dB or more at a wavelength of 1,064 nm and a pulse width of 5 ns. Furthermore, this paramagnetic garnet-type transparent ceramic can be used to provide a method for producing magneto-optical devices such as optical isolators.

[0027] This is a schematic cross-sectional diagram showing an example of the configuration of an optical isolator using paramagnetic garnet-type transparent ceramics according to the present invention as a Faraday rotor.

[0028] [1. Method for Manufacturing Paramagnetic Garnet-Type Transparent Ceramics] An embodiment of the method for manufacturing paramagnetic garnet-type transparent ceramics according to the present invention will be described below. First, the paramagnetic garnet-type transparent ceramics obtained by the manufacturing method of this embodiment are represented by the following formula (1). (Tb １－ｘ－ｙ Re ｘ Sc ｙ ) ３ (Al １－ｚ Sc ｚ ) ５ O １２ (1) (wherein Re is Y and / or Lu, and 0 ≤ x < 0.45, 0 < y < 0.1, 0 < z < 0.2.)

[0029] In the garnet crystal structure represented by equation (1), the site mainly occupied by Tb, i.e., the first half of equation (1), is called site A, and the site mainly occupied by Al is called site B. In site A of equation (1), terbium (Tb) is the element with the largest Verde constant among trivalent rare earth ions, and its absorption in the 1,070 nm region (wavelength band 0.9 μm to 1.1 μm) used in fiber lasers is extremely small, making it the most suitable element for use in optical isolator materials in this wavelength range. However, Tb(III) ions are easily oxidized to produce Tb(IV) ions. When Tb(IV) ions are produced in metal oxides, they absorb light over a wide range of wavelengths from ultraviolet to near-infrared, so it is desirable to eliminate them as much as possible. Also, rare earth elements have intrinsic f-f transition absorption, and it is preferable to select elements that do not have absorption in the wavelength band used. Specifically, it is particularly preferable to select from Tb, yttrium (Y), and lutetium (Lu), which do not exhibit absorption from the visible to near-infrared range. Furthermore, when the transparent ceramics of the present invention are used as a Faraday rotor, it is even more preferable that they contain 60 mol% or more of Tb from the viewpoint of having a high Verde constant.

[0030] At site B in equation (1), aluminum (Al) is the material with the smallest ionic radius among the trivalent ions that can stably exist in oxides having a garnet structure, and is the element that can make the lattice constant of the Tb-containing garnet-type oxide the smallest. It is preferable to be able to reduce the lattice constant of the garnet structure without changing the Tb content because it is possible to increase the Verde constant per unit length.

[0031] Here, in composite oxides where the constituent elements are only Tb, Y or Lu and Al, subtle weighing errors may result in the absence of a garnet structure, making it difficult to stably manufacture transparent ceramics usable for optical applications. Therefore, in this invention, by adding scandium (Sc) as a constituent element, compositional deviations due to subtle weighing errors are eliminated. Sc is a material with an intermediate ionic radius that can be solid-solved in both the A site and the B site in oxides having a garnet structure. When the mixing ratio of rare earth elements consisting of Tb and Y or Lu and Al deviates from the stoichiometric ratio due to variations in weighing, Sc is a buffer material that can adjust the distribution ratio to the A site (rare earth site consisting of Tb and Y or Lu) and the B site (aluminum site) to match the stoichiometric ratio and thereby minimize the crystallite formation energy. Furthermore, it is an element that can limit the abundance of the alumina phase relative to the garnet matrix to 1 ppm by mass or less, and also limit the abundance of the perovskite-type phase relative to the garnet matrix to 1 ppm by mass or less, and is an element that can be added to improve product yield.

[0032] Furthermore, in equation (1), the range of x is 0 ≤ x < 0.45, preferably 0.05 ≤ x < 0.45, more preferably 0.10 ≤ x ≤ 0.40, and even more preferably 0.20 ≤ x ≤ 0.40. When x is within this range, the Verde constant at room temperature (25°C) and a wavelength of 1064 nm is 30 rad / (T·m) or more, and it can be used as a Faraday rotor. Also, within this range, a larger x tends to result in a smaller thermal lensing effect, which is preferable. Furthermore, within this range, a larger x tends to result in a smaller diffuse transmittance, which is also preferable. In contrast, when x is 0.45 or greater, the Verde constant at a wavelength of 1064 nm is less than 30 rad / (T·m), which is undesirable. That is, if the relative concentration of Tb is excessively diluted, when using a general magnet, the total length of the Faraday rotor required to rotate a laser beam of 1064 nm by 45 degrees will exceed 30 mm, making manufacturing difficult, which is undesirable.

[0033] In equation (1), the range of y is 0 < y < 0.1, preferably 0 < y < 0.08, more preferably 0.002 ≤ y ≤ 0.07, and even more preferably 0.003 ≤ y ≤ 0.06. When y is within this range, the perovskite-type extraphase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, it is preferable that the amount of perovskite-type extraphase (typically granular with a diameter of 1 μm to 1.5 μm and appearing as a light brown color) in a 150 μm × 150 μm field of view under an optical microscope is 1 or less. At this time, the proportion of the perovskite-type extraphase relative to the garnet matrix is ​​1 ppm by mass or less.

[0034] When y is 0.1 or greater, in addition to substituting a portion of Tb with Y or Lu, Sc also substitutes a portion of Tb, resulting in an unnecessarily reduced solid solution concentration of Tb, which is undesirable as it leads to a smaller Verde constant. Furthermore, since Sc is an expensive raw material, unnecessarily over-doping with Sc is undesirable from a manufacturing cost perspective. Moreover, when y is 0.08 or greater, the risk of antisite defect absorption increases, where Tb and Y or Lu enter the B site and Al enters the A site.

[0035] In equation (1), when 0 ≤ x < 0.45 and 0 < y < 0.1, the range of 1-x-y is preferably 0.5 < 1-x-y < 1, more preferably 0.55 ≤ 1-x-y < 1, and even more preferably 0.6 ≤ 1-x-y < 1. When 1-x-y is within this range, a large Verde constant can be secured and high transparency can be obtained at a wavelength of 1064 nm.

[0036] In equation (1), the range of z is 0 < z < 0.2, preferably 0 < z < 0.16, more preferably 0.01 ≤ z ≤ 0.15, and even more preferably 0.03 ≤ z ≤ 0.15. When z is within this range, the perovskite-type extraphase is not detected by XRD analysis. Furthermore, it is preferable that the amount of perovskite-type extraphase (typically granular with a diameter of 1 μm to 1.5 μm and appearing as a light brown color) in a 150 μm × 150 μm field of view observed with an optical microscope is 1 or less. At this time, the relative abundance of the perovskite-type extraphase to the garnet matrix is ​​1 ppm by mass or less.

[0037] When z is 0.2 or higher, the effect of suppressing the precipitation of perovskite-type heterogeneous phases saturates and does not change. However, as the value of z increases, the value of y, i.e., the substitution ratio of Tb by Sc, also increases. As a result, the solid solution concentration of Tb unnecessarily decreases, and the Verde constant becomes small, which is undesirable. Furthermore, since Sc is an expensive raw material, unnecessarily over-doping with Sc is undesirable from a manufacturing cost perspective. Moreover, when z is 0.16 or higher, the risk of antisite defect absorption increases, where Tb and Y or Lu enter the B site and Al enters the A site.

[0038] The method for manufacturing such paramagnetic garnet-type transparent ceramics includes the steps of: preparing a slurry containing sintering raw material powder; obtaining a molded body using this slurry; degreasing the molded body to obtain a degreased body; pre-sintering the degreased body to obtain a densified pre-sintered body; then, pressurizing (HIP treatment) the pre-sintered body to obtain a pressurized sintered body; optionally re-sintering the pressurized sintered body to obtain a re-sintered body; and performing an oxide annealing treatment on the pressurized sintered body or the re-sintered body. Each step will be described below.

[0039] [1-1. Slurry Preparation Process] First, a sintering raw material powder corresponding to the composition of the garnet-type composite oxide of formula (1) described above is prepared. Suitable starting materials for the garnet-type composite oxide include oxide powders of metals such as terbium, yttrium, lutetium, scandium, and aluminum. The purity of the starting material is not particularly limited as long as it can be made transparent, but from the viewpoint of suppressing absorption due to impurities, a purity of 99.9% by mass or higher is preferred, 99.99% by mass or higher is more preferred, and 99.999% by mass or higher is most preferred. The average primary particle size of the starting material powder is not particularly limited as long as it can be made transparent, but from the viewpoint of easy sintering and mixability, 50 nm to 1000 nm is preferred, 100 nm to 800 nm is more preferred, and 100 nm to 600 nm is particularly preferred. The average primary particle size of the starting material can be referenced from the values ​​listed in the catalog, for example, values ​​measured by SEM, dynamic light scattering, or laser diffraction / scattering can be referenced. The shape of the primary particles can be selected from card-house-shaped, spherical, or rod-shaped, and is not particularly limited as long as transparency is possible. However, spherical aluminum oxide is preferable because it is stable even in concentrated slurries and aggregation is suppressed.

[0040] The raw material powder for sintering is prepared by a wet synthesis method. In the wet synthesis method, predetermined amounts of various oxide raw materials constituting formula (1) are weighed to obtain a composition corresponding to formula (1), and these are mixed with a liquid medium to form a slurry. The mixing (dispersion) method is not particularly limited and can be mixed (dispersed) by various methods such as a ball mill, bead mill, jet mill, homogenizer, or ultrasonic irradiation. After mixing (dispersing) this wet slurry, the solvent may be removed and calcination may be performed. However, the calcination temperature at this time is preferably 850°C or higher and less than 1100°C, more preferably 870°C or higher and 1050°C or lower, and particularly preferably 870°C or higher and 1000°C or lower. If the calcination temperature is below 850°C, the volume change due to the phase change of terbium oxide in the sintering process is large, increasing the risk of the sintered body cracking. Also, if the calcination temperature exceeds 1100°C, the activity (sinterability) of the sintering raw material powder decreases, and sufficient grain growth cannot be achieved in the subsequent resintering process, making it difficult to remove air bubbles. The calcination time should be at least one hour, and the heating rate at that time should preferably be between 100°C / h and 500°C / h. The calcination atmosphere should preferably be an oxygen-containing atmosphere such as air or oxygen, while nitrogen, argon, or hydrogen atmospheres are unsuitable. Examples of calcination equipment include vertical muffle furnaces, horizontal tubular furnaces, and rotary kilns, and are not particularly limited as long as the target temperature is reached and oxygen flow is established. Furthermore, after calcination, a grinding process may be followed by another calcination. Repeating calcination is preferable because it results in more uniform sintering raw material powder, but since repeating the process increases costs and time, it is preferable to limit the number of calcinations to three or fewer.

[0041] Furthermore, the liquid medium of the wet slurry is not particularly limited, but examples include alcohols such as lower alcohols with 1 to 4 carbon atoms, and pure water. When adding a dispersant, it is necessary to select a solvent with high solubility for the dispersant to be added. In addition to the dispersant, various organic additives may be added to this wet slurry for the purpose of improving quality stability and yield in the subsequent ceramics manufacturing process. The organic additives are not particularly limited, and various binders, lubricants, plasticizers, etc., can be suitably used. However, it is preferable to select high-purity types of these organic additives that do not contain unwanted metal ions. If calcination is performed after adding the dispersant or other organic additives, the dispersant and organic additives will decompose during calcination. Therefore, if calcination is to be performed, it should be done before adding the dispersant or other organic additives. Also, if coarse secondary particles or undisintegrated particles are present in the sintering raw material powder, voids may be formed inside the sintered body. Therefore, it is preferable to remove coarse secondary particles or undisintegrated particles by filtration or the like. The filtration method is not particularly limited, but for example, filtration can be performed by a total filtration method or a cross-flow method. In this case, it is preferable to remove particles of 20 μm or larger, more preferably 10 μm or larger, and particularly preferable to remove particles of 5 μm or larger.

[0042] In this embodiment, it is preferable to use a dispersant to suppress particle aggregation in the concentrated slurry. Examples of dispersants include cationic dispersants, nonionic dispersants, and anionic dispersants. The dispersant is not particularly limited as long as it can sufficiently exhibit a dispersion effect on aluminum oxide and has high dispersion stability, but anionic dispersants having anionic groups are most preferred because they can be efficiently adsorbed to aluminum oxide. Examples of anionic groups include carboxylic acid groups, sulfonic acid groups, sulfate ester groups, and organic compounds containing salts thereof. When using organic compounds containing salts, examples of counterions constituting the salt include alkali metal ions such as lithium, sodium, and potassium, alkaline earth metal ions such as calcium, magnesium, and barium, and ammonium ions. The anionic group is not particularly limited, but it is preferable that no unwanted metal ions remain in the paramagnetic garnet-type transparent ceramic sintered body in the end, and carboxylic acid groups, sulfonic acid groups, sulfate ester groups, and ammonium salts thereof are preferred. When using pure water, rare earth elements may dissolve in the acidic range (pH less than 5). Furthermore, in the basic range (pH above 9), the degree of ionization of functional groups such as carboxylic acids in anionic groups decreases, making efficient adsorption to aluminum oxide impossible. Therefore, it is preferable to set the pH of the slurry to above 5 and below 9.

[0043] The dispersant structure may be linear or branched, such as a graft chain, but a dispersant with a graft chain is preferred because it offers higher dispersion stability. The structure of the graft chain is not particularly limited, but a polyoxyalkylene chain is most preferred. Examples of polyoxyalkylene chains include polyoxyethylene chains, polyoxypropylene chains, polyoxybutylene chains, polyoxytetramethylene chains, copolymer chains of ethylene oxide and propylene oxide, copolymer chains of ethylene oxide and butylene oxide, etc. Furthermore, if organic matter remains during degreasing, it will be impossible to obtain highly transparent paramagnetic garnet-type transparent ceramics. For this reason, a structure having a single-bonded carbon chain that does not contain benzene rings or carbon-carbon double bonds is preferred. In addition, the elements constituting the dispersant are preferably carbon, hydrogen, oxygen, and nitrogen. The dispersant to be added is not limited to one type, and multiple types of dispersants may be combined. For example, multiple types of dispersants can be suitably used depending on the purpose, such as a combination of a low-molecular-weight dispersant with an average molecular weight of less than 1000 that has a large effect of imparting wettability to particles and a high-molecular-weight dispersant with an average molecular weight of 1000 or more that imparts steric repulsion. In this case, in the present invention, the average molecular weight of the above dispersant refers to the average molecular weight of the dispersant with the largest average molecular weight among the added dispersants. It is preferable to select a high-purity type of dispersant that does not contain unwanted metal ions.

[0044] The average molecular weight of the dispersant is preferably between 1,000 and 50,000, and more preferably between 1,000 and 30,000. If the average molecular weight is less than 1,000, the steric repulsion effect becomes small, making aggregation more likely in concentrated slurries, which is undesirable. On the other hand, if the average molecular weight is greater than 50,000, the dispersant forms a cross-linked structure and becomes an aggregate, which is also undesirable. Particularly preferred dispersants include, for example, the commercially available AKM-0531, SC-0708A, SC-1015F, and FA1160-08 manufactured by NOF Corporation, Aron T-50, Aron A-210, Aron A-6001, Aron A-6012, Aron A-6017, Aron A-30SL, Aron AC-10SL, Aron SD-10, Aron A-6330, and Aron A-6114 manufactured by Toagosei Co., Ltd.

[0045] Furthermore, the amount of dispersant added is preferably 0.1% by mass or more and 3.0% by mass or less relative to the total amount of oxide powder, more preferably 0.3% by mass or more and 2.0% by mass or less, and particularly preferably 0.5% by mass or more and 1.5% by mass or less. Here, the amount of dispersant added represents the actual amount of dispersant contained (the amount of dispersant when the active ingredient is 100%). For example, if 1.0% by mass of a dispersant with 50% active ingredient is added dropwise, the amount of dispersant added will be 0.5% by mass. If the amount of dispersant added is less than 0.1% by mass, the dispersion effect cannot be sufficiently exerted, and aggregation of aluminum oxide is likely to occur, which is undesirable. On the other hand, if the amount of dispersant added exceeds 3.0% by mass, the crushability of the granules during molding deteriorates, and voids are likely to occur inside the molded body, which is undesirable. Also, depending on the type of dispersant, the zeta potential of the particles to which the dispersant is adsorbed may approach 0, making aggregation more likely, which is undesirable. The amount of dispersant added needs to be adjusted within the above range according to the type of dispersant used and the average molecular weight.

[0046] Furthermore, it is preferable that the sintering raw material powder contains a sintering aid. For example, tetraethoxysilane (TEOS) may be added as a sintering aid along with the above starting material in an amount of Si equivalent of 100 ppm to 1,000 ppm in the total raw material powder (garnet-type composite oxide powder + sintering aid), or SiO ２ The powder should be added in an amount of 100 ppm to 1,000 ppm by mass in terms of Si equivalent to the total raw material powder (garnet-type composite oxide powder + sintering aid), mixed, and calcined to obtain the calcined raw material. A Si content of 1,000 ppm by mass or more in the total raw material powder is undesirable because the excess Si may cause trace amounts of light absorption due to crystal defects. The purity of the sintering aid should preferably be 99.9% by mass or higher, and particularly preferably 99.99% by mass or higher. The sintering aid may also be added during the preparation of the slurry. If a sintering aid is not added, it is preferable to select a sintering raw material powder (i.e., the above-mentioned composite oxide powder) whose primary particle size is nano-sized and which has extremely high sintering activity. Such selections may be made as appropriate.

[0047] (Particle Size Distribution Evaluation) The particle size distribution of the slurry obtained in this manner is measured and evaluated. In this invention, particle size refers to the particle size in a slurry with a concentration of 11 ± 4 mass%, and if the particles have aggregated to form secondary particles, it refers to the particle size of these secondary particles. The method for measuring particle size is not particularly limited as long as it can be used to evaluate in a concentrated slurry, but a method obtained by dynamic light scattering is preferred, and an example of a measuring device is the ELSZ-2000 manufactured by Otsuka Electronics Co., Ltd.

[0048] The concentration of the concentrated slurry to be measured is not particularly limited as long as the concentration difference between each sample is small. However, depending on the equipment used, if the slurry concentration exceeds 30% by mass, it may be affected by multiple scattering, making it impossible to obtain accurate particle size. Also, if the slurry concentration to be measured is less than 5% by mass, the actual interparticle interactions in the slurry will differ significantly, and it may not be possible to correctly evaluate the particle size in the actual concentrated slurry, which is undesirable. Therefore, it is preferable to adjust the slurry concentration to be measured within the range of 5 to 30% by mass, and even more preferable to adjust it within the range of 11 ± 4% by mass. In addition, some slurries will gradually settle if left standing before measurement. Therefore, it is preferable to sonicate the slurry for about 15 minutes before measurement and then start the measurement immediately.

[0049] In the obtained particle size distribution based on the scattering intensity distribution, the particle size at which the cumulative frequency is 5%, the particle size at which the cumulative frequency is 50% (median diameter), and the particle size at which the cumulative frequency is 95% are defined as D5, D50, and D95, respectively. A D50 value of less than 250 nm is undesirable because it increases the likelihood of compositional inconsistencies due to drying and agglomeration when removing the solvent from the slurry. Also, a D50 value of less than 250 nm is undesirable because it leads to a faster densification rate during sintering, making it easier for air bubbles to remain inside the sintered material. On the other hand, a D50 value greater than 1500 nm or a D95 value greater than 2000 nm is undesirable because it results in compositional inconsistencies inside the sintered material. Therefore, preferably the D50 value is 250 nm or more and 1500 nm or less, and the D95 value is 2000 nm or less, more preferably the D50 value is 300 nm or more and 1350 nm or less, and the D95 value is 1750 nm or less, and particularly preferably the D50 value is 300 nm or more and 1000 nm or less, and the D95 value is 1200 nm or less.

[0050] [1-2. Molding Process] The slurry obtained as described above is used to mold the product into a predetermined shape. Molding methods are broadly classified into dry molding and wet molding, but are not particularly limited as long as the predetermined shape can be reliably obtained. In the case of dry molding, an example is a method in which granules are made from the slurry using spray drying, the granules are filled into a jig, and then press molding is performed. As for wet molding, an example is the slip molding method in which the slurry is poured into a plaster mold and the solvent is evaporated. Other examples include extrusion molding, sheet molding, centrifugal slip molding, and cold isostatic pressing, but are not limited as long as the predetermined shape can be obtained.

[0051] In this embodiment, conventional press forming processes can be suitably used for press forming. Specifically, press forming processes in which a very common mold is filled and pressurized from a certain direction, or CIP (Cold Isostatic Pressing) and WIP (Warm Isostatic Pressing) processes in which the molded product is sealed in a deformable waterproof container and pressurized with hydrostatic pressure can be suitably used. The applied pressure can be adjusted as appropriate while checking the relative density of the resulting molded product, and is not particularly limited, but for example, controlling the pressure within a range of about 300 MPa or less, which can be handled by commercially available CIP equipment, can help reduce manufacturing costs. Alternatively, hot pressing processes, discharge plasma sintering processes, and microwave heating processes, which perform sintering in addition to the molding process at the same time, can also be suitably used.

[0052] [1-3. Degreasing Process] In this embodiment, a conventional degreasing process can be suitably used. That is, a heating degreasing process using a heating furnace can be performed. Furthermore, there are no particular restrictions on the type of atmospheric gas used at this time, and air, oxygen, hydrogen, etc., can be suitably used. There are no particular restrictions on the degreasing temperature, but 270°C or higher and 900°C or lower is preferred. It is difficult to completely remove organic additives, and at temperatures higher than 900°C, densification proceeds before the sintering process, making it difficult to obtain a transparent sintered body with low scattering.

[0053] [1-4. Pre-sintering process] In this process, a pre-sintered body is prepared as a sintered body before heating and sintering, preferably densified to a relative density of 93% or more, and preferably with an average sintered particle size of 5 μm or less. At this time, it is necessary to fine-tune the temperature and holding time conditions so that the sintered particle size falls within the desired range.

[0054] Here, general sintering processes can be suitably utilized. That is, heating sintering processes such as resistance heating and induction heating can be suitably utilized. The atmosphere at this time is not particularly limited, and various atmospheres such as air, inert gas, oxygen gas, hydrogen gas, and helium gas can be suitably utilized, but sintering under reduced pressure (vacuum) can be used more preferably. The vacuum level for pre-sintering is 1 × 10⁻⁶ －１ Preferably less than Pa, 1 × 10 －２ Less than Pa is preferable.

[0055] The sintering temperature in the pre-sintering process is preferably 1450 to 1650°C, and particularly preferably 1500 to 1600°C. A sintering temperature within this range is preferable because it promotes densification while suppressing the precipitation of different phases and grain growth. In the pre-sintering process of the present invention, a sintering holding time of several hours is sufficient, but it is preferable to densify the relative density of the pre-sintered body to 93% or higher. If the relative density of the pre-sintered body exceeds 99%, plastic deformation of the internal particles of the sintered body becomes difficult to occur during subsequent high-pressure sintering (HIP), and it becomes difficult to remove air bubbles remaining in the sintered body. Therefore, the relative density of the pre-sintered body is preferably 99% or less, and more preferably 98% or less.

[0056] The average sintered grain size of the crystal grains in the pre-sintered body is preferably 5 μm or less, more preferably 3 μm or less, and even more preferably 2.5 μm or less. The average sintered grain size of the crystal grains can be adjusted in conjunction with the type of raw material, atmosphere, sintering temperature, and holding time. If the sintered grain size is greater than 5 μm, plastic deformation will be less likely to occur during subsequent high-pressure sintering (HIP), and it may become difficult to remove air bubbles remaining in the pre-sintered body.

[0057] [1-5. Pressurized Sintering (Hot Isostatic Pressing (HIP)) Process] In this embodiment, after the pre-sintering process, the pre-sintered body can be subjected to hot isostatic pressing for pressurized sintering. The pressurized gas medium at this time can preferably be an inert gas such as argon or nitrogen, or Ar-O2. The pressure applied by the pressurized gas medium is preferably 50 to 300 MPa, and more preferably 100 to 300 MPa. Below 50 MPa, the transparency improvement effect may not be obtained, and above 300 MPa, even if the pressure is increased, no further improvement in transparency can be obtained, and the load on the equipment may become excessive and damage the equipment. The applied pressure is preferably 196 MPa or less, which can be processed by commercially available HIP equipment, for simplicity. The processing temperature (predetermined holding temperature) at this time is set in the range of 1000 to 1780°C, preferably 1100 to 1730°C. Heat treatment temperatures exceeding 1780°C are undesirable because they increase the risk of precipitation of non-uniform phases. Furthermore, heat treatment temperatures below 1000°C yield almost no improvement in the transparency of the sintered body. While there are no particular restrictions on the holding time of the heat treatment temperature, holding it for too long is undesirable as it increases the risk of precipitation of non-uniform phases. Typically, it is preferably set within the range of 1 to 3 hours.

[0058] The heater material, insulation material, and processing container used for HIP treatment are not particularly limited, but graphite, molybdenum (Mo), tungsten (W), and platinum (Pt) can be suitably used, and yttrium oxide and gadolinium oxide can also be suitably used as processing containers. In particular, when the processing temperature is 1,500°C or lower, platinum (Pt) can be used as the heater material, insulation material, and processing container, and the pressurized gas medium can be Ar-O2, which is preferable because it prevents the occurrence of oxygen deficiency during HIP treatment. When the processing temperature exceeds 1,500°C, graphite is preferred as the heater material and insulation material, but in this case, it is preferable to select one of graphite, molybdenum (Mo), or tungsten (W) as the processing container, and further select one of yttrium oxide or gadolinium oxide as a double container inside it, and fill the container with an oxygen-releasing material, as this minimizes the amount of oxygen deficiency that occurs during HIP treatment.

[0059] [1-6. Resintering Process] In this embodiment, after the HIP treatment is completed, the obtained transparent ceramics may be resintered for the purpose of grain growth. The resintering temperature is preferably higher than the pre-sintering temperature or the HIP treatment temperature. Specifically, 1650°C or higher is preferred, and 1700°C or higher is more preferred. Temperatures below 1650°C are undesirable because grain growth is insufficient. The average grain size of the crystal grains after resintering is preferably 10 μm or more, more preferably 15 μm or more, and particularly preferably 20 μm or more. The holding time for the resintering process is not particularly limited, but 5 hours or more is preferred, and 10 hours or more is more preferred. The temperature and holding time for the resintering process may be adjusted as appropriate after checking the average grain size. However, generally, if the sintering temperature is raised too high, unexpected abnormal grain growth will occur, making it difficult to obtain a homogeneous sintered body. Therefore, it is preferable to allow some margin in the resintering temperature and to adjust the size of the average grain size of the resintered body by extending the holding time. Furthermore, the processing temperature (predetermined holding temperature) is set in the range of 1,000 to 1,780°C, preferably 1,100 to 1,730°C. Processing temperatures above 1,780°C are undesirable because they increase the risk of precipitation of other phases. Also, processing temperatures below 1,000°C result in almost no improvement in the transparency of the sintered body. There are no particular restrictions on the holding time of the processing temperature, but holding it for too long is undesirable because it increases the risk of oxygen deficiency. Typically, it is preferably set in the range of 1 to 3 hours.

[0060] [1-7. Oxidation Annealing Process] In this embodiment, the resintered body or HIP body that has undergone the above series of processes is reduced, especially in the HIP treatment process, which can result in some oxygen deficiency and a gray to dark blue appearance. Therefore, an oxidation annealing treatment (oxygen deficiency recovery treatment) is performed in an oxidizing atmosphere (oxygen-containing atmosphere) such as air. The annealing temperature is 1400°C or higher, preferably 1450°C or higher. It is also preferable that it be 1500°C or lower. The holding time in this case is not particularly limited, but it should be performed for a time sufficient to recover the oxygen deficiency, preferably 10 hours or more, and more preferably 20 hours or more. A mild oxidation HIP treatment may also be performed. By performing these treatments, even if the resintered body has become discolored, the oxygen deficiency can be recovered, so the size and quantity of scattering sources (scattering contrast sources) can be controlled within a specified range, and a paramagnetic garnet-type transparent ceramic body with less absorption due to oxygen deficiency can be obtained. Of course, the inherent coloration (absorption) of the material due to the addition of colored elements such as dopants or impurities to impart functionality cannot be removed.

[0061] [1-8. Optical Polishing Process] In this embodiment, the transparent ceramic body that has undergone the above series of manufacturing processes is optically polished on both end faces on the optically utilized axis. The optical surface accuracy at this time is preferably λ / 2 or less, and particularly preferably λ / 8 or less, when the measurement wavelength λ = 633 nm. The surface roughness Sa (arithmetic mean height) of the polished surface is preferably 1 nm or less, and particularly preferably 0.7 nm or less. The surface roughness Sq (root mean square height) is preferably 1.5 nm or less, and particularly preferably 0.89 nm or less. It is also possible to further reduce optical loss by appropriately forming an anti-reflective film on the optically polished surface.

[0062] [2. Method for Manufacturing a Magneto-Optical Device] Next, an embodiment of the method for manufacturing a magneto-optical device according to the present invention will be described. The paramagnetic garnet-type transparent ceramic obtained by the above-described manufacturing method is intended to be used as a magneto-optical material, and the manufacturing method of this embodiment involves constructing a magneto-optical device using this paramagnetic garnet-type transparent ceramic. Specifically, it is preferable to apply a magnetic field parallel to the optical axis of the above-described paramagnetic garnet-type transparent ceramic, and then set a polarizer and an analyzer so that their optical axes are offset by 45 degrees from each other to construct and use a magneto-optical device. In particular, the above-described paramagnetic garnet-type transparent ceramic is suitably used as a magneto-optical device, especially as a Faraday rotator for an optical isolator with a wavelength of 0.9 to 1.1 μm.

[0063] Figure 1 is a schematic cross-sectional view showing an example of an optical isolator, which is a magneto-optical device equipped with a Faraday rotator made of paramagnetic garnet-type transparent ceramic as an optical element. As shown in Figure 1, the optical isolator 100 includes a Faraday rotator 110 made of the above-mentioned paramagnetic garnet-type transparent ceramic, a polarizer 120 and an analyzer 130 made of a polarizing material, inside its housing 102. These are arranged along the optical axis 104 of the Faraday rotator in the order of polarizer 120, Faraday rotator 110, and analyzer 130. The polarization oscillation plane of polarizer 120 and the polarization oscillation plane of analyzer 130 are arranged so that the relative angle is 45°. The optical isolator 100 also includes a magnet 140 for applying a magnetic field to the Faraday rotator 110 on at least one of the side surfaces of the Faraday rotator 110 inside the housing 102.

[0064] Such an optical isolator 100 can be suitably used in industrial fiber laser devices (not shown). The optical isolator can prevent the reflected laser light emitted from the laser light source from returning to the light source, which would otherwise cause unstable oscillation.

[0065] The present invention will be described in more detail below with reference to examples, reference examples, and comparative examples, but the present invention is not limited to these examples.

[0066] [Example 1] As Example 1-1 and Comparative Example 1-1, the case in formula (1) where Re is yttrium and x = 0.198, y = 0.004, and z = 0.03 is shown. Yttrium oxide powder, terbium oxide powder, scandium oxide powder from Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder from Daimei Chemical Co., Ltd. were obtained. Furthermore, tetraethyl orthosilicate (TEOS) from Kishida Chemical Co., Ltd., ethanol from Kanto Chemical Co., Ltd., and dispersant SC-0505K (average molecular weight approximately 5000 to 20000) from NOF Chemical Co., Ltd. were obtained. The purity of all powder raw materials was 99.9% by mass or higher. Using the above raw materials, terbium, yttrium, scandium, and aluminum were weighed so that the molar numbers were Tb:Y:Sc:Al = 2.394:0.594:0.162:4.850 (Tb ０．７９８ Y ０．１９８ Sc ０．００４ ) ３ (Al ０．９７ Sc ０．０３ ) ５ O １２ A mixed powder was prepared. Next, the powder was placed in a polyethylene pot, and ethanol was added to obtain a slurry so that the powder concentration was 30% by mass. Then, TEOS was added as a sintering aid, weighed out to an amount equivalent to 100 ppm by mass in terms of Si.

[0067] As Example 1-2, a slurry was prepared by weighing under the same conditions as in Example 1-1 and Comparative Example 1-1, except that the aluminum oxide powder was changed to NXA-150 (purity 99.99% by mass or higher) manufactured by Sumitomo Chemical Co., Ltd.

[0068] In Example 1-1, the dispersant SC-0505K was added to the obtained slurry at a concentration of 0.5% by mass relative to the total amount of oxide powder, and the slurry was dispersed and mixed using a ball mill for 24 hours. After that, undissolved particles were removed from the slurry using a nylon filter with a mesh size of 10 μm. Subsequently, for particle size distribution measurement, a portion of the obtained slurry was placed in a vial and diluted with ethanol to a powder concentration of 11 ± 4% by mass. The remaining slurry was spray-dried to produce granular raw material with an average particle size of 20 μm.

[0069] In Example 1-2, the obtained slurry was dispersed and mixed using a ball mill for 24 hours. After that, undissolved particles were removed from the slurry using a nylon filter with a mesh size of 10 μm. Subsequently, for particle size distribution measurement, a portion of the obtained slurry was placed in a vial and diluted with ethanol to a powder concentration of 11 ± 4% by mass. The remaining slurry was spray-dried to produce granular raw material with an average particle size of 20 μm.

[0070] As Comparative Example 1-1, the obtained slurry was dispersed and mixed using a ball mill for 24 hours. After that, undissolved particles were removed from the slurry using a nylon filter with a mesh size of 10 μm. Subsequently, for particle size distribution measurement, a portion of the obtained slurry was placed in a vial and diluted with ethanol to a powder concentration of 11 ± 4% by mass. The remaining slurry was spray-dried to produce granular raw material with an average particle size of 20 μm.

[0071] The granular raw materials obtained in the examples and comparative examples were subjected to uniaxial press molding and hydrostatic press treatment at a pressure of 198 MPa, respectively, to obtain CIP molded bodies. The obtained molded bodies were degreased in a muffle furnace at 1,000°C for 2 hours.

[0072] The degreased material obtained is placed in a vacuum furnace and 1.0 × 10 －２ A total of 11 types of pre-sintered bodies were obtained by pre-sintering at 1,600°C for 2 hours under reduced pressure below Pa. At this time, the relative sintering density of all samples was 93% or higher. Each of the obtained pre-sintered bodies was placed in a carbon heater HIP furnace and subjected to pressure sintering (HIP) treatment in Ar at 196 MPa, 1,600°C, and 3 hours. Subsequently, the pressure-sintered bodies were placed again in a vacuum furnace and subjected to 1.0 × 10 －２ A resintered body was obtained by resintering at 1,700°C for 20 hours under reduced pressure below Pa. Finally, the resintered body was annealed in air at 1,450°C for 30 hours.

[0073] As reference example 1-1, a TGG single crystal manufactured by Northrop Grumman was prepared.

[0074] Each of the transparent ceramics and TGG single crystals obtained in this way was cylindrically ground to a diameter of 5 mm, and then ground and polished to a length of 20 mm.

[0075] Furthermore, the optical ends of each sample were subjected to final optical polishing to achieve an optical surface accuracy of λ / 8 (when the measurement wavelength λ = 633 nm), a Sa of 0.7 nm or less, and a Sq of 0.89 nm or less. Subsequently, an anti-reflective coating designed to have a center wavelength of 1070 nm and a reflectivity of 0.1% or less was applied to both ends.

[0076] The following measurements were performed on each sample obtained in the manner described above.

[0077] (Particle Size Distribution Evaluation) The particle size distribution was measured according to JIS Z 8828:2019 "Particle Size Analysis - Dynamic Light Scattering Method" using a particle size distribution device (ELSZ-2000, manufactured by Otsuka Electronics Co., Ltd.) by dynamic light scattering (photon correlation method). The slurries for particle size distribution measurement obtained in Examples 1-1, 1-2 and Comparative Example 1-1 were irradiated with ultrasound for 15 minutes, and then 1.5 to 2.5 mL of the slurry was immediately placed in a four-sided transparent glass cell and set in the device. After confirming that the light intensity was 10,000 to 100,000 cps by automatic light intensity adjustment, the measurement temperature was set to 25°C, the number of integrations to 50, the number of repetitions to 1, the refractive index of the solvent to 1.3628, viscosity to 1.0917, and dielectric constant to 24.4, and the particle size distribution of each slurry was measured. The particle size distribution was determined by the histogram method from the autocorrelation function obtained by the photon correlation method. The obtained scattering intensity distribution criteria for cumulative particle size at 5%, 50%, and 95% are defined as D5, D50, and D95, respectively.

[0078] (Extinction Ratio Evaluation) The extinction ratio was measured in accordance with JIS C 5877-2:2012. The measurement was performed using a collimated CW laser beam with linear polarization, a wavelength of 1070 nm, an output power of 100 W, and a diameter of 1.6 mm. The sample, PBS, and power meter were placed on the optical axis of this laser beam. First, the PBS was placed parallel to the polarization of the laser beam, and the transmitted light intensity P was measured. / / The reading was taken. Next, the PBS was positioned perpendicular to the polarization and the transmitted light intensity P was measured. ⊥The following was read. The extinction ratio (dB) at an incident intensity of 100W was calculated using the following formula: Extinction ratio (dB) = -10log １０ (P ⊥ / P / / )

[0079] The results above are summarized in Table 1.

[0080]

[0081] From the above results, in Examples 1-1 and 1-2, the D50 value was between 300 nm and 1500 nm, and the D95 value was 2000 nm or less, resulting in an extinction ratio of 35 dB or higher. In contrast, in Comparative Example 1-1, the D50 value was greater than 1500 nm, and the D95 value was also greater than 2000 nm, resulting in an extinction ratio of less than 35 dB. Furthermore, it was confirmed that Examples 1-1 and 1-2 had higher extinction ratios than the TGG crystal of Reference Example 1-1. In other words, it was confirmed that when the D50 value is between 250 nm and 1500 nm, and the D95 value is 2000 nm or less, a paramagnetic garnet-type transparent ceramic with an extinction ratio of 35 dB or higher can be obtained.

[0082] [Example 2] As Example 2 and Comparative Example 2, the conditions of Example 1-1 are shown, but with changes to the amount and type of dispersant added. SC-0505K (average molecular weight approximately 5000 to 20000) from NOF Chemical Co., Ltd., AKM-0531 (average molecular weight approximately 10000 to 50000) from NOF Chemical Co., Ltd., FA-1160-08 (average molecular weight approximately 5000 to 20000) from NOF Chemical Co., Ltd., SP-0201 (average molecular weight less than 1000) from NOF Chemical Co., Ltd., and polyethylene glycol 400 (PEG400, average molecular weight 400) from Kanto Chemical Co., Ltd. were obtained. Subsequently, as in Example 1-1, terbium, yttrium, scandium, and aluminum were weighed so that the molar numbers were Tb:Y:Sc:Al = 2.394:0.594:0.162:4.850, respectively (Tb ０．７９８ Y ０．１９８ Sc ０．００４ ) ３ (Al ０．９７ Sc ０．０３ ) ５ O １２A total of 26 types of mixed powders were prepared. Next, each of the above powders was placed in a polyethylene pot, taking care to prevent mixing of them, and ethanol was added to bring the powder concentration to 30% by mass, obtaining a total of 26 types of slurries. Subsequently, TEOS was added as a sintering aid, and the amount of SiO ２ It was weighed and added so that the converted amount was 100 ppm by mass.

[0083] Next, a dispersant was added to each of the slurries obtained as Example 2 and Comparative Example 2. The type and amount of dispersant added are shown in Table 2. After that, each slurry was dispersed and mixed in a ball mill for 24 hours. Then, undissolved particles were removed from each slurry using a nylon filter with a mesh size of 10 μm. Subsequently, for particle size distribution measurement, a portion of each slurry was placed in a vial and diluted with ethanol to a powder concentration of 11 ± 4% by mass. The remaining slurries were spray-dried to produce granular raw materials with an average particle size of 20 μm.

[0084] The granular raw materials obtained in Example 2 and Comparative Example 2 were subjected to uniaxial press molding and hydrostatic press treatment at a pressure of 198 MPa, respectively, to obtain CIP molded bodies. The obtained molded bodies were degreased in a muffle furnace at 1,000°C for 2 hours.

[0085] The degreased material obtained in this way is placed in a vacuum furnace, and 1.0 × 10 －２ A total of 26 types of pre-sintered bodies were obtained by pre-sintering at 1,600°C for 2 hours under reduced pressure below Pa. At this time, the relative sintering density of all samples was 93% or higher. Each of the obtained pre-sintered bodies was placed in a carbon heater HIP furnace and subjected to pressure sintering (HIP) treatment in Ar at 196 MPa, 1,600°C, and 3 hours. Subsequently, the pressure-sintered bodies were placed again in a vacuum furnace and subjected to 1.0 × 10 －２ A resintered body was obtained by resintering at 1,700°C for 20 hours under reduced pressure below Pa. Finally, the resintered body was annealed in air at 1,450°C for 30 hours.

[0086] Each of the transparent ceramics and TGG single crystals obtained in this way was cylindrically ground to a diameter of 5 mm, and then ground and polished to a length of 20 mm.

[0087] Furthermore, the optical ends of each sample were subjected to final optical polishing to achieve an optical surface accuracy of λ / 8 (when the measurement wavelength λ = 633 nm), a Sa of 0.7 nm or less, and a Sq of 0.89 nm or less. Subsequently, an anti-reflective coating designed to have a center wavelength of 1070 nm and a reflectivity of 0.1% or less was applied to both ends.

[0088] For each sample obtained as described above, the particle size distribution and extinction ratio were evaluated in the same manner as in Example 1. The results are summarized in Table 2.

[0089]

[0090] From the above results, in Examples 2-1 to 2-12, where a dispersant with an average molecular weight of 1,000 to 50,000 was added in an amount of 0.1% to 3.0% by mass, the D50 value was between 250 nm and 1,500 nm, the D95 value was 2,000 nm or less, and the extinction ratio was 35 dB or higher. In contrast, in Comparative Examples 2-1, 2-3, and 2-5, where a dispersant with an average molecular weight of 1,000 to 50,000 was added in an amount of 0.05% by mass, the D50 value was greater than 1,500 nm and the extinction ratio was less than 35 dB. Furthermore, in Comparative Examples 2-2, 2-4, and 2-6, where a dispersant with an average molecular weight of 1,000 to 50,000 was added in an amount of 5.0% by mass, the D50 value was less than 250 nm or greater than 1,500 nm, and the extinction ratio was less than 35 dB. On the other hand, in Comparative Examples 2-7 to 2-14, in which a dispersant with an average molecular weight of less than 1000 was added in an amount of 0.1% to 3.0% by mass, the D50 value was greater than 1500 nm and the extinction ratio was less than 35 dB in all cases. In other words, it was confirmed that when a dispersant with an average molecular weight of 1000 to 50000 is added in an amount of 0.1% to 3.0% by mass, a paramagnetic garnet-type transparent ceramic can be obtained with a D50 value of 250 nm or more and 1500 nm or less, a D95 value of 2000 nm or less, and an extinction ratio of 35 dB or more.

[0091] [Example 3] Example 3 shows the case where the composition of formula (1) is changed. Yttrium oxide powder, terbium oxide powder, scandium oxide powder from Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder from Daimei Chemical Co., Ltd. were obtained. In addition, tetraethyl orthosilicate (TEOS) from Kishida Chemical Co., Ltd., ethanol from Kanto Chemical Co., Ltd., and SC-0505K (average molecular weight approximately 5000 to 20000) from NOF Chemical Co., Ltd. The purity of all powder raw materials was 99.9% by mass or higher. Using the above raw materials, a total of five types of raw materials with the final compositions shown in Table 3 were prepared by adjusting the mixing ratio as follows.

[0092] (Raw materials for Example 3-1) Terbium, scandium, and aluminum were weighed out so that the molar amounts were Tb:Sc:Al = 0.996:0.162:4.850, respectively (Y ０．１９９ Tb ０．７９７ Sc ０．００４ ) ３ (Al ０．９７ Sc ０．０３ ) ５ O １２ A mixed powder was prepared. Next, the powder was placed in a polyethylene pot, and ethanol was added to obtain a slurry so that the powder concentration was 30% by mass. Then, TEOS was added as a sintering aid, weighed out to an amount equivalent to 100 ppm by mass in terms of Si.

[0093] (Raw materials for Example 3-2) Terbium, yttrium, scandium, and aluminum were weighed out so that the molar amounts were Tb:Y:Sc:Al = 1.194:1.794:0.162:4.850 (Y ０．３９８ Tb ０．５９８ Sc ０．００４ ) ３ (Al ０．９７ Sc ０．０３ ) ５ O １２ A mixed powder was prepared. Next, the powder was placed in a polyethylene pot, and ethanol was added to obtain a slurry so that the powder concentration was 30% by mass. Then, TEOS was added as a sintering aid, weighed out to an amount equivalent to 100 ppm by mass in terms of Si.

[0094] (Raw materials for Example 3-3) The molar amounts of terbium, lutetium, scandium, and aluminum were weighed so that Tb:Lu:Sc:Al = 2.394:0.594:0.162:4.850, respectively (Lu ０．１９９ Tb ０．７９７ Sc ０．００４ ) ３ (Al ０．９７ Sc ０．０３ ) ５ O １２ mixed powder was prepared. Subsequently, the above powder was put into a polyethylene pot, and ethanol was added so that the powder concentration became 30% by mass to obtain a slurry. Then, TEOS was weighed and added as a sintering aid so that the added amount became 100 ppm by mass in terms of Si.

[0095] (Raw materials for Example 3-4) The molar amounts of terbium, lutetium, scandium, and aluminum were weighed so that Tb:Y:Al = 1.800:1.200:5.00, respectively (Tb ０．６００ Y ０．４００ ) ３ Al ５ O １２ mixed powder was prepared. Subsequently, the above powder was put into a polyethylene pot, and ethanol was added so that the powder concentration became 30% by mass to obtain a slurry. Then, TEOS was weighed and added as a sintering aid so that the added amount became 100 ppm by mass in terms of Si.

[0096] (Raw materials for Comparative Example 3-1) The molar amounts of terbium, yttrium, and aluminum were weighed so that Tb:Lu:Sc:Al = 1.194:1.794:0.162:4.850, respectively (Tb ０．５９８ Lu ０．３９８ Sc ０．００４ ) ３ (Al ０．９７ Sc ０．０３ ) ５ O １２ mixed powder was prepared. Subsequently, the above powder was put into a polyethylene pot, and ethanol was added so that the powder concentration became 30% by mass to obtain a slurry. Then, TEOS was weighed and added as a sintering aid so that the added amount became 100 ppm by mass in terms of Si.

[0097] Next, to the slurries obtained as Example 3 and Comparative Example 3, the dispersant SC-0505K was added at a concentration of 0.5% by mass relative to the total amount of oxide powder, and the mixture was dispersed and mixed in a ball mill for 24 hours. After that, undissolved particles were removed from each slurry using a nylon filter with a mesh size of 10 μm. Subsequently, for particle size distribution measurement, a portion of each slurry was placed in a vial and diluted with ethanol to a powder concentration of 11 ± 4% by mass. The remaining slurries were spray-dried to produce granular raw materials with an average particle size of 20 μm.

[0098] The granular raw materials obtained in Example 3 and Comparative Example 3 were subjected to uniaxial press molding and hydrostatic press treatment at a pressure of 198 MPa, respectively, to obtain CIP molded bodies. The obtained molded bodies were degreased in a muffle furnace at 1,000°C for 2 hours.

[0099] The degreased material obtained in this way is placed in a vacuum furnace, and 1.0 × 10 －２ A total of 26 types of pre-sintered bodies were obtained by pre-sintering at 1,600°C for 2 hours under reduced pressure below Pa. At this time, the relative sintering density of all samples was 93% or higher. Each of the obtained pre-sintered bodies was placed in a carbon heater HIP furnace and subjected to pressure sintering (HIP) treatment in Ar at 196 MPa, 1,600°C, and 3 hours. Subsequently, the pressure-sintered bodies were placed again in a vacuum furnace and subjected to 1.0 × 10 －２ A resintered body was obtained by resintering at 1,700°C for 20 hours under reduced pressure below Pa. Finally, the resintered body was annealed in air at 1,450°C for 30 hours.

[0100] Each of the transparent ceramics and TGG single crystals obtained in this way was cylindrically ground to a diameter of 5 mm, and then ground and polished to a length of 20 mm.

[0101] Furthermore, the optical ends of each sample were subjected to final optical polishing to achieve an optical surface accuracy of λ / 8 (when the measurement wavelength λ = 633 nm), a Sa of 0.7 nm or less, and a Sq of 0.89 nm or less. Subsequently, an anti-reflective coating designed to have a center wavelength of 1070 nm and a reflectivity of 0.1% or less was applied to both ends.

[0102] For each of the samples obtained as described above, the particle size distribution and extinction ratio were evaluated in the same manner as in Example 1 for the samples obtained as follows. The above results are summarized in Table 3.

[0103]

[0104] From the above results, in Examples 3-1 to 3-4 in which the composition of the paramagnetic garnet-type transparent ceramics satisfies formula (1), the D50 value was 250 nm or more and 1500 nm or less, the D95 value was 2000 nm or less, and the extinction ratio was 42 dB. On the other hand, in Comparative Example 3-1, a heterogeneous phase was precipitated and the sintered body became opaque (measurement of the extinction ratio was not performed). That is, it was confirmed that when the composition of the paramagnetic garnet-type transparent ceramics satisfies formula (1), a paramagnetic garnet-type transparent ceramics having an extinction ratio of 35 dB or more can be obtained.

[0105] [Example 4] As an example of a magneto-optical device, an example of constructing an optical isolator using the paramagnetic garnet-type transparent ceramics of the example will be shown. As Example 4-1 and Example 4-2, a paramagnetic garnet-type transparent ceramics (Example 1-1 and Example 1-2) having a D50 value of 250 nm or more and 1500 nm or less, a D95 value of 2000 nm or less, and an extinction ratio of 35 dB or more was used to fabricate an optical isolator having the configuration shown in FIG. 1. Further, as Comparative Example 4-1, an optical isolator was similarly fabricated using a paramagnetic garnet transparent ceramics (Comparative Example 1-1) having a D50 value exceeding 1500 nm and an extinction ratio of 34 dB.

[0106] (Isolation measurement) The isolation measurement was carried out referring to JIS5932-2:2019. The measurement was performed using a collimated CW laser beam having a wavelength of 1070 nm, an output power of 100 W, and a diameter of 1.6 mm. The intensity P １ [W] (in this case, 100 W) of this laser beam was read with a power meter. Subsequently, the transmitted light intensity P when light was inserted in the reverse direction of the optical isolator ２[W] was read from the power meter. The isolation (dB) at an incident intensity of 100W was calculated using the following formula: Isolation (dB) = -10log １０ (P ２ / P １ The evaluation results are shown in Table 4.

[0107]

[0108] From the above results, the isolation levels of both Example 4-1 and Example 4-2 were 43 dB or higher. In contrast, the isolation level of Comparative Example 4-1 was 33 dB. That is, it was confirmed that an optical isolator equipped with paramagnetic garnet-type transparent ceramics having a D50 value of 250 nm to 1500 nm and a D95 value of 2000 nm or less is an optical isolator with an isolation level of 35 dB or higher.

[0109] Although the present invention has been described using the above embodiments, the present invention is not limited to these embodiments. It can be modified in any way that is conceivable by those skilled in the art, including other embodiments, additions, changes, and deletions. Any embodiment that achieves the effects of the present invention is included within the scope of the present invention.

[0110] 100 Optical isolator 102 Housing 104 Optical axis 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims

1. A method for producing a paramagnetic garnet-type transparent ceramic represented by the following formula (1), (Tb １-ｘ-ｙ Re ｘ Sc ｙ ) ３ (Al １－ｚ Sc ｚ ) ５ O １２ (1) (wherein Re is Y and / or Lu, 0 ≤ x < 0.45, 0 < y < 0.1, 0 < z < 0.2.) A method for producing paramagnetic garnet-type transparent ceramics, comprising at least the step of preparing a slurry containing oxide powder raw materials of each metal element constituting formula (1) and a liquid medium, wherein the slurry has a median diameter D50 value of the particle size distribution at a concentration of 11 ± 4 mass% of 250 nm or more and 1500 nm or less, and a cumulative particle size distribution D95 value of 95% of 2000 nm or less.

2. The method for producing paramagnetic garnet-type transparent ceramics according to claim 1, wherein a dispersant having anionic groups and graft chains is added to the slurry in an amount of 0.1 to 3.0% by mass relative to the total amount of oxide powder raw materials.

3. The method for producing paramagnetic garnet-type transparent ceramics according to claim 2, wherein the above-mentioned anionic group is selected from one or more of carboxylic acid groups, sulfonic acid groups, and sulfate ester groups.

4. The method for producing paramagnetic garnet-type transparent ceramics according to claim 2, wherein the dispersant has an average molecular weight of 1,000 to 50,000, and the liquid medium is water or a lower alcohol having 1 to 4 carbon atoms.

5. The method for producing a paramagnetic garnet-type transparent ceramic according to claim 2, wherein the graft chain is a polyoxyalkylene chain.

6. A method for producing paramagnetic garnet-type transparent ceramics, further comprising the steps of: obtaining a molded body by drying and granulating the slurry above and then molding it; obtaining a sintered body by pre-sintering the molded body at 1450°C to 1650°C; obtaining a pressurized sintered body by pressurizing the sintered body above; obtaining a re-sintered body by heating the pressurized sintered body at a temperature exceeding the pre-sintering temperature or the pressurized sintering temperature; and obtaining a paramagnetic garnet-type transparent ceramic by annealing the re-sintered body in an oxidizing atmosphere of 1400°C or higher, wherein the paramagnetic garnet-type transparent ceramic has an extinction ratio of 35 dB or more when laser light with a wavelength of 1070 n at an optical path length of 20 mm is incident with a beam diameter of 1.6 mm and an incident power of 100 W.

7. A method for manufacturing a magneto-optical device, comprising the step of using a paramagnetic garnet-type transparent ceramic obtained by the method for manufacturing a paramagnetic garnet-type transparent ceramic described in claims 1 to 6 as a magneto-optical element to construct a magneto-optical device.

8. The method for manufacturing the magneto-optical device according to claim 7, wherein the magneto-optical device is an optical isolator usable at wavelengths of 0.9 μm to 1.1 μm, comprising the paramagnetic garnet-type transparent ceramic as a Faraday rotor and polarizing materials on the front and rear of the optical axis of the Faraday rotor.

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