JP7679782B2 - Manufacturing method for rare earth garnet type transparent ceramics - Google Patents

Description

The present invention relates to a method for producing rare earth garnet-type transparent ceramics.

Ceramics have long been widely used in everyday life, such as tiles and pottery. Most ceramics are opaque, but in recent years, it has become possible to produce transparent ceramics with an optically isotropic crystal structure, transparency comparable to that of single crystals, by thoroughly removing air bubbles and minimizing the occurrence of heterogeneous phases at grain boundaries. Transparent ceramics are being developed for use in laser materials, scintillator materials, phosphors, magneto-optical elements, highly durable windows, and more, and are expected to become a new optical material different from single crystals.

In addition, transparent ceramics must have an optically isotropic crystal structure, and examples of transparent ceramics include C-type rare earth structures, garnet structures, pyrochlore structures, and spinel structures. Among these, the garnet structure in particular is used in a wide range of fields, such as YAG:Ce phosphors, TGG magneto-optical elements, and Nb:YAG lasers. On the other hand, there is a document that states that the garnet structure cannot tolerate even a slight deviation from the composition of A ₃ B ₅ O ₁₂ , and that a deviation of about 0.2% from the stoichiometric composition reduces the light transmittance (X.W. Ba, et al., J. Eur. Ceram. Soc. 35 (2015) 3127-3136 (Non-Patent Document 1)). Weighing and mixing according to the composition of the garnet structure is the first step in creating highly transparent garnet-type ceramics.

Here, methods for producing garnet-type transparent ceramics can be roughly divided into two types.
In JP 2019-199386 A (Patent Document 1), high quality YAG ceramics are achieved by mixing various raw material powders and performing molding and sintering processes.
In addition, in Japanese Patent No. 6438588 (Patent Document 2), the synthesis of highly transmittance garnet-type transparent ceramics is achieved by synthesizing precursor solutions of various elements through a coprecipitation process, rather than by mixing powders.

JP 2019-199386 A Patent No. 6438588

X.W. Ba, et al., J. Eur. Ceram. Soc. 35 (2015) 3127-3136

However, in the production of ceramics in Patent Document 1, various raw material powders have their own specific surface areas and different amounts of moisture adsorbed on the surface. In addition to the amount of moisture, depending on the synthesis method of the powder, impurity components unrelated to the main components such as organic substances, nitrides, and sulfides may be contained. Furthermore, for example, Tb oxide is unstable in the state of Tb ₂ O ₃ at room temperature, and is sold in the state of Tb ₄ O ₇ , but there is no guarantee that the Tb oxide is truly Tb ₄ O ₇ , and the composition may change significantly depending on the manufacturer, production method, or storage method. Therefore, even if such raw material powder is weighed and used according to the target composition, the composition of the sintered body produced will deviate, resulting in a situation where the product is not transparent or does not have a high transmittance. In such cases, it is possible to manufacture highly transparent garnet-type transparent ceramics by determining the weighing amounts of various powders through trial and error; however, it is difficult to obtain the exact same powder, such as a powder with the same specific surface area, and when using raw material powders from a different production lot, such as when a certain type of raw material powder runs out of stock and is replaced with a new powder, it is necessary to adjust the weighing conditions again.
The advantage of the coprecipitation process in Patent Document 2 is that not only can a completely uniform powder be obtained, but also that there is no need to consider the effects of surface adsorbed water, as in the case of powder mixing. However, due to errors in measuring the concentration of the precursor solution and errors in weighing, there was no guarantee that the composition would be completely as desired. In the end, there was no way to avoid finding the conditions for achieving transparency through trial and error, just as in the case of production by mixing the raw material powders.

The present invention has been made in consideration of the above circumstances, and aims to provide a method for producing rare earth garnet-type transparent ceramics that can be made highly transparent with good reproducibility even in garnet-structure ceramics, which require strict composition control.

In order to achieve the above object, the present invention provides the following method for producing a rare earth garnet-type transparent ceramic.
1.
Using at least one oxide powder selected from Sc, Y, and a lanthanoid element and one oxide powder selected from a Group 13 element as starting materials, a powder represented by the following formula (1) was prepared.
(A ¹ _1-xy A ² _x Sc _y ) ₃ (B _1-z Sc _z ) ₅ O ₁₂ (1)
(In formula (1), A ¹ and A ² are different elements selected from Y and lanthanoid elements, B is one element selected from Group 13 elements, 0≦x≦0.4, 0≦y<0.08, 0≦z<0.16, and when z=0, y=0, and when z>0, y>0.)
A method for producing a rare earth garnet-type transparent ceramic, which is a sintered body of a complex oxide represented by the following formula:
The above A ¹ and A ² are different elements selected from Tb, Y, Ce, Lu and Dy, and B is Al;
The method for producing a rare earth garnet-type transparent ceramics uses, as the starting materials, oxide powders of different elements selected from Tb, Y , Ce, Lu and Dy, each having a purity of 99.9% by mass or more, weighed out in amounts of m _A1 and m _A2 calculated by the following formulas (2-1) and (2-2): an oxide powder of Al having a purity of 99.9% by mass or more, weighed out in an amount of m _B calculated by the following formula (2-3): and a scandium oxide powder having a purity of 99.9% by mass or more, weighed out in an amount of m _Sc calculated by the following formula (2-4):
m _A1 = 3/2 x (1-x-y) M _A1 / M _G x W x 100/(100-a _A1 ) (2-1)
m _A2 = 3/2×xM _A2 /M _G ×W×100/(100-a _A2 ) (2-2)
m _B =5/2×(1-z) M _B /M _G ×W×100/(100-a _B ) (2-3)
m _Sc = (3y/2+5z/2) (M _Sc /M _G )×W×100/(100-a _Sc ) (2-4)
(In the formula, M _A1 and M _A2 are the molar masses (g/mol) of oxides of elements different from each other selected from Tb, Y , Ce, Lu and Dy , M _B is the molar mass (g/mol) of an oxide of Al , M _Sc is the molar mass (g/mol) of scandium oxide, M _G is the molar mass (g/mol) of the composite oxide represented by formula (1), W is the planned total mass (g) of the starting materials, a _A1 and a _A2 are the mass reduction rates (%) when the starting material oxide powders of elements different from each other selected from Tb, Y , Ce, Lu and Dy are heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _B is the mass reduction rate (%) when the starting material oxide powder of Al is heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _Sc is the mass reduction rate (%) when the starting scandium oxide powder is heated from room temperature (20±10°C) to 1100°C in an inert gas atmosphere. x, y, and z are the same as above.
2 .
2. The method for producing a rare earth garnet-type transparent ceramic according to 1 , wherein the sintered body contains more than 0 mass % and 0.1 mass % or less of SiO ₂ as a sintering aid.
3 .
3. The method for producing a rare earth garnet-type transparent ceramic according to 1 or 2 , wherein A ¹ in the above formula (1) is Tb and 0.6≦1−x−y≦1.
4 .
4. The method for producing a rare earth garnet-type transparent ceramic according to any one of 1 to 3 , comprising molding the starting materials, degreasing the resulting molded body, and sintering it to obtain a sintered body of the composite oxide.

According to the present invention, when manufacturing transparent ceramics consisting of sintered composite oxides by mixing starting powders as raw materials, the required amount corresponding to the ceramic composition can be accurately weighed by correcting the mass used based on the amount of adsorbate not related to the ceramic composition measured in advance for each starting raw material powder, so that highly transparent rare earth garnet-type transparent ceramics can be manufactured with good reproducibility.

FIG. 1 is a schematic cross-sectional view showing an example of the configuration of an optical isolator using the transparent ceramic produced in accordance with the present invention as a Faraday rotator.

The manufacturing method for the rare earth garnet-type transparent ceramics according to the present invention is described below.

The method for producing a rare earth garnet-type transparent ceramic according to the present invention comprises using, as starting materials, a powder of at least one oxide selected from Sc, Y and a lanthanoid element, and a powder of one oxide selected from a group 13 element, and reacting the powder with an oxide represented by the following formula (1):
(A ¹ _1-xy A ² _x Sc _y ) ₃ (B _1-z Sc _z ) ₅ O ₁₂ (1)
(In formula (1), A ¹ and A ² are different elements selected from Y and lanthanoid elements, B is one element selected from Group 13 elements, 0≦x≦0.4, 0≦y<0.004, 0≦z<0.004, when z=0, y=0, and when z>0, y>0.)
A method for producing a rare earth garnet-type transparent ceramic, which is a sintered body of a complex oxide represented by the following formula:
The starting materials used are oxide powders of different elements selected from Y and the lanthanoid elements in amounts of m _A1 and m _A2 calculated by the following formulas (2-1) and (2-2): an oxide powder of one type selected from the Group 13 elements in an amount of m _B calculated by the following formula (2-3): and a scandium oxide powder in an amount of m _Sc calculated by the following formula (2-4):
m _A1 = 3/2 x (1-x-y) M _A1 / M _G x W x 100/(100-a _A1 ) (2-1)
m _A2 = 3/2×xM _A2 /M _G ×W×100/(100-a _A2 ) (2-2)
m _B =5/2×(1-z) M _B /M _G ×W×100/(100-a _B ) (2-3)
m _Sc = (3y/2+5z/2) (M _Sc /M _G )×W×100/(100-a _Sc ) (2-4)
(wherein, M _A1 and M _A2 are the molar masses (g/mol) of oxides of different elements selected from Y and a lanthanoid element, M _B is the molar mass (g/mol) of one oxide selected from Group 13 elements, M _Sc is the molar mass (g/mol) of scandium oxide, M _G is the molar mass (g/mol) of the composite oxide represented by formula (1), W is the planned total mass (g) of the starting materials, a _A1 and a _A2 are the mass reduction rates (%) when the starting material oxide powders of different elements selected from Y and a lanthanoid element are heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _B is the mass reduction rate (%) when the starting material oxide powder of one oxide selected from Group 13 elements is heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _Sc is the mass reduction rate (%) when the starting scandium oxide powder is heated from room temperature (20±10°C) to 1100°C in an inert gas atmosphere. x, y, and z are the same as above.

(Rare earth garnet type transparent ceramics)
The rare earth garnet-type transparent ceramics (hereinafter sometimes simply referred to as transparent ceramics) produced in the present invention are transparent sintered bodies of a complex oxide represented by the following formula (1).
(A ¹ _1-xy A ² _x Sc _y ) ₃ (B _1-z Sc _z ) ₅ O ₁₂ (1)
(In formula (1), A ¹ and A ² are different elements selected from Y and lanthanoid elements, B is one element selected from Group 13 elements, 0≦x≦0.4, 0≦y<0.004, 0≦z<0.004, when z=0, y=0, and when z>0, y>0.)

In the garnet crystal structure represented by formula (1), the side where the elements ^A1 , ^A2 , and Sc are coordinated, i.e., the side in the parentheses on the left side of formula (1), is referred to as the A site, and the side where the elements B and Sc are coordinated, i.e., the side in the parentheses on the right side of formula (1), is referred to as the B site.

Here, deviations in the composition ratio of the A site and the B site (i.e., 3:5) in the ceramics that are actually manufactured are not permitted. Even if the composition ratio in the manufactured ceramics deviates slightly, the ceramics may become transparent, but the transparency will be poor. The garnet structure of the ceramics cannot achieve a high degree of transparency unless the composition is as desired, so formula (1) must be satisfied.

The transparent ceramics produced by the present invention is preferably a sintered body of a garnet-type composite oxide containing at least terbium (Tb) and aluminum (Al). In this case, the element B in the above formula (1) is Al. Furthermore, it is more preferable that the sintering aid contains more than 0 mass% and 0.1 mass% or less of _SiO2 .

In addition, it is preferable that A ¹ in the above formula (1) is Tb, and 0.6≦1−x−y≦1. When Tb is contained at 40 mol% or more in the A site of formula (1), the obtained transparent ceramic is used as a magneto-optical element. Tb is an element with the largest Verdet constant among the paramagnetic elements except for iron (Fe), and since there is no absorption in the 1064 nm wavelength region used in fiber lasers, it is the most suitable element for use as an optical isolator material in this wavelength region. However, Tb easily reacts with oxygen in the air, generating tetravalent Tb. Since this tetravalent Tb has light absorption properties, it is desirable to eliminate it as much as possible. To completely eliminate this tetravalent Tb, it is necessary to aim for a crystal structure in which tetravalent Tb does not occur, and it is most preferable to aim for a composition that becomes a garnet structure.

When Tb is included in the A site of formula (1), transparency is possible with Tb alone, but Tb may be substituted with other elements to make transparency easier. The element substituted for Tb is preferably Y or at least one selected from the group of lanthanides, and Y, Lu, or La is preferred from the viewpoint of no absorption in the wavelength band used, and Y is the most preferred from the viewpoint of cost and stability. Y is an element that stabilizes the garnet structure, and the more it is substituted, the more stable it becomes and the easier it is to manufacture. However, if too much is added, the Verdet constant becomes too small, and the Verdet constant becomes smaller than that of TGG. Therefore, it is preferable to substitute Y in the range of 0 mol% to 40 mol% with respect to 100 mol% Tb.

At the B site of formula (1), one element (element B) selected from Group 13 elements is placed, with Al being preferred as mentioned above. Group 13 element Ga is highly volatile and volatilizes during sintering, reducing its amount, so special measures are required, but Al does not volatilize during sintering and has high thermal conductivity, making it the most preferred.

Here, in a composite oxide whose constituent elements are only Tb, Y, lanthanoid elements, and group 13 elements, subtle weighing errors of each raw material may cause the oxide to not have a garnet structure, making it difficult to stably manufacture transparent ceramics that can be used for optical applications. Therefore, in the present invention, it is preferable to add scandium (Sc) as a constituent element to eliminate the composition deviation caused by subtle weighing errors of each raw material. Sc is a material with an intermediate ionic radius that can be dissolved in both the A site and the B site in an oxide having a garnet structure, and is a buffer material that can adjust the distribution ratio to the A site (rare earth site consisting of Tb, Y, and lanthanoid elements) and the B site (site consisting of group 13 elements) to exactly match the stoichiometric ratio and thereby minimize the energy of crystallite formation when the compounding ratio of rare earth elements consisting of Tb, Y, and lanthanoid elements and group 13 elements deviates from the stoichiometric ratio due to variations in weighing.

The Sc content is preferably 0 mol% or more and less than 0.08 mol% at the A site, and 0 mol% or more and less than 0.16 mol% at the B site. If a high degree of transparency can be achieved without adding Sc, there is no particular need to add it, but from the viewpoint of stable production, it is preferable to add a certain amount. On the other hand, if the content is above the upper limit mentioned above, it will be easier to achieve transparency, but it will be undesirable because a large amount of expensive Sc will be used, which will not be cost-effective.

In view of the above, the transparent ceramic produced in the present invention is preferably a transparent sintered body of a complex oxide represented by the following formula (1').
(Tb _1-x'-y' Y _x' Sc _y' ) ₃ (Al _1-z' Sc _z' ) ₅ O ₁₂ (1')
(In the formula, 0.05≦x′≦0.4, 0≦y′<0.08, 0.52≦1−x′−y′<0.95, 0≦z′<0.16, and 0.001<y′+z′<0.2.)

In formula (1'), the range of x' is 0.05≦x'≦0.4, preferably 0.1≦x'≦0.4, and more preferably 0.2≦x'≦0.4. When x' is in this range, the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis.

In formula (1'), the range of y' is 0≦y'<0.08, preferably 0.001<y'<0.04, and more preferably 0.002<y'<0.04. When y' is in this range, it is preferable because the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because it is possible to prevent an excessive decrease in thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering.

In formula (1'), the range of 1-x'-y' is 0.52≦1-x'-y'<0.95, and more preferably 0.6≦1-x'-y'<0.8. When 1-x'-y' is in this range, a large Verdet constant can be ensured and high transparency can be obtained at a wavelength of 1064 nm.

In formula (1'), the range of z' is 0≦z'<0.16, more preferably 0.001<z'<0.004, and even more preferably 0.02≦z'<0.004. When z' is in this range, it is preferable because the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because it is possible to prevent an excessive decrease in thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering.

In formula (1'), the range of y' + z' is 0.001 < y' + z' < 0.2, more preferably 0.002 < y' + z' < 0.03, and even more preferably 0.003 < y' + z' < 0.025. When y' + z' is in this range, it is preferable because the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because it is possible to prevent an excessive decrease in thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering.

The transparent ceramics produced by the present invention contain the composite oxide represented by the above formula (1) or formula (1') as the main component, and contain _SiO2, which plays the role of a sintering aid, as a subcomponent in a range of up to 0.1 mass% in terms of metal Si. When a small amount of _SiO2 is added as a sintering aid, the precipitation of perovskite-type heterophases and alumina heterophases is suppressed, and the transparency of the paramagnetic garnet-type transparent ceramics is further improved. Furthermore, the small amount of _SiO2 added is vitrified during sintering at 1400°C or higher, resulting in a liquid phase sintering effect, and can promote the densification of the garnet-type ceramic sintered body.

Incidentally, "containing as a main component" means containing 90% by mass or more of the composite oxide represented by the above formula (1) or formula (1'). The content of the composite oxide represented by formula (1) or formula (1') is preferably 99% by mass or more, more preferably 99.9% by mass or more, even more preferably 99.99% by mass or more, and particularly preferably 99.999% by mass or more.

The transparent ceramics produced by the present invention are composed of the above-mentioned main components and subcomponents, but may further contain other elements within a range that does not impair the effects of the present invention. Typical examples of other elements include heavy metals such as iron (Fe) and chromium (Cr), and various impurities (unavoidable components) such as sodium (Na), potassium (K), and phosphorus (P).
The content of other elements in the sintered body is preferably 100 ppm or less.

By the way, when preparing raw material powders for transparent ceramics, there are two methods: mixing various oxide powders and synthesizing uniform oxide powders from an aqueous solution using a chemical reaction. The present invention relates to the former manufacturing method of mixing various oxide powders. Various oxide powders contain components other than the constituent oxides, such as adsorbed moisture according to their specific surface area, residual organic matter derived from the synthesis method of the starting materials, and nitrides, and the ratio of these components varies depending on the oxide powder. Due to the influence of components other than the constituent oxides, even if the oxide powder is weighed according to the target composition, the composition of the manufactured ceramics will vary, and a high level of transparency will not be achieved.
In the present invention, the above rare earth garnet-type transparent ceramics are produced as follows.

[Method of manufacturing rare earth garnet-type transparent ceramics]
(Starting material)
As the starting materials used in the present invention, single oxide powders of the elements constituting the above formula (1) are used. For example, in the case of a composite oxide represented by formula (1'), powders of oxides of terbium, yttrium, scandium, and aluminum (i.e., terbium oxide powder, yttrium oxide powder, scandium oxide powder, and aluminum oxide powder) are used. These are called raw material powders for garnet-type composite oxides. The purity of the raw material powders for garnet-type composite oxides is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. Hereinafter, an example will be described in which terbium oxide powder, yttrium oxide powder, scandium oxide powder, and aluminum oxide powder corresponding to the composite oxide represented by formula (1') are used.

In the present invention, the starting materials terbium oxide powder, yttrium oxide powder, scandium oxide powder, and aluminum oxide powder are precisely weighed to obtain the composite oxide composition represented by the above formula (1'). However, as described above, these starting oxide powders contain components other than the constituent oxides, so they are weighed taking into consideration the content of components other than the constituent oxides in each of the starting oxide powders. Specifically, the following is carried out.

(Measurement of mass reduction rate of oxide powder at 1100°C (content of components other than constituent oxides))
In the present invention, as a method for measuring the content (weight ratio) of components other than the above-mentioned constituent oxides in the starting oxide powder, it is preferable to measure the thermogravimetric change rate under an inert atmosphere. The thermogravimetric change rate can be measured in-situ using a thermogravimetric analyzer (TG device) or by measuring the weight before and after firing at 1000°C or higher using a firing furnace, but since it is necessary to accurately measure the weight of components other than the constituent oxides, it is most preferable to measure using a TG device. The atmosphere is preferably an inert atmosphere, and examples of the atmosphere include argon, nitrogen, and helium, but there is no particular limitation as long as the weight change rate can be accurately measured.

Here, the weight reduction rate (mass reduction rate) using a TG device is preferably calculated from the weight change (mass reduction) when heated from room temperature (20±10°C) to 1100°C. At temperatures lower than ₁₁₀₀ °C, residual adsorbed moisture or, in the case of _Tb4O7 , residual oxygen remains, making it impossible to capture the correct weight change, and the composition may not match. For most oxides, the mass reduction at temperatures higher than 1100°C, for example 1300°C, is not significantly different from that at 1100° _C , but for highly volatile oxides such as _Ga2O3 , an extra mass reduction due to volatilization may be observed at 1300°C. As a result of the inventors' investigation into the temperature at which the mass reduction rate of all oxides of the framework elements of rare earth garnets can be measured with high accuracy, it was found that 1100°C is the most preferable heating temperature. In other words, the "mass reduction rate at 1100°C" in the present invention refers to the percentage of mass reduction of the oxide powder, which is the starting material, when the oxide powder is heated from room temperature (20±10°C) to 1100°C under an inert atmosphere, relative to the mass (100%) of the oxide powder at room temperature (20±10°C).

In the present invention, the mass reduction rate at 1100°C is measured for each of the oxide powders used as starting materials (terbium oxide powder, yttrium oxide powder, scandium oxide powder, and aluminum oxide powder).

(Calculation of mass used)
The mass used is calculated using the mass reduction rate of each oxide powder used as _a starting material measured as above. Note that the molecular structures of the oxides in the oxide powders must be calculated as ^A12O3 _, ^A22O3 _{, B2O3} , and _{Sc2O3 .} For _example , Tb oxide is often sold in _the form of _Tb4O7 , but this _calculation does _not work with _Tb4O7 , so it is necessary to calculate with _{Tb2O3 .}

That is, the masses m _A1 and m _A2 of "powder of oxides of different elements selected from Y and lanthanoid elements" used as the starting materials are calculated from the following formulas (2-1) and (2-2). The mass m _B of "powder of one oxide selected from Group 13 elements" is calculated from the following formula (2-3). Furthermore, the mass m _Sc of "scandium oxide powder" is calculated from the following formula (2-4).

m _A1 = 3/2 x (1-x-y) M _A1 / M _G x W x 100/(100-a _A1 ) (2-1)
m _A2 = 3/2×xM _A2 /M _G ×W×100/(100-a _A2 ) (2-2)
m _B =5/2×(1-z) M _B /M _G ×W×100/(100-a _B ) (2-3)
m _Sc = (3y/2+5z/2) (M _Sc /M _G )×W×100/(100-a _Sc ) (2-4)
(wherein, M _A1 and M _A2 are the molar masses (g/mol) of oxides of different elements selected from Y and a lanthanoid element, M _B is the molar mass (g/mol) of one oxide selected from Group 13 elements, M _Sc is the molar mass (g/mol) of scandium oxide, M _G is the molar mass (g/mol) of the composite oxide represented by formula (1), W is the planned total mass (g) of the starting materials, a _A1 and a _A2 are the mass reduction rates (%) when the starting material oxide powders of different elements selected from Y and a lanthanoid element are heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _B is the mass reduction rate (%) when the starting material oxide powder of one oxide selected from Group 13 elements is heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _Sc is the mass reduction rate (%) when the starting scandium oxide powder is heated from room temperature (20±10°C) to 1100°C in an inert gas atmosphere. x, y, and z are the same as x, y, and z in the above formula (1).

In the composite oxide represented by formula (1), when the element A2 is not contained (x=0), m _A2 =0, and when Sc is not contained (y=z=0), m _Sc =0.

When terbium oxide powder, yttrium oxide powder, scandium oxide powder and aluminum oxide powder are used as the starting materials, the mass of terbium oxide powder used, m _Tb , the mass of yttrium oxide powder used, m _Y , the mass of aluminum oxide powder used, m _AL , and the mass of scandium oxide powder used, m _Sc , can be calculated from the following formulas.

m _Tb = 3/2 x (1-x'-y') M _Tb / M _G x W x 100/(100-a _Tb )
m _Y =3/2×x'M _Y /M _G ×W×100/(100-a _Y )
m _AL =5/2×(1-z') M _AL /M _G ×W×100/(100-a _Al )
m _Sc = (3y'/2+5z'/2) (M _Sc /M _G ) x W x 100/(100-a _Sc )
(wherein, M _Tb and M _Y are the molar masses of the oxides of Tb and Y (M _Tb = 365.8 (g/mol), M _Y = 225.8 (g/mol)), M _AL is the molar mass of the oxide of Al (M _AL = 102.0 (g/mol)), M _Sc is the molar mass of scandium oxide (M _Sc = 137.9 (g/mol)), M _G is the molar mass (g/mol) of the composite oxide represented by formula (1'), W is the planned total mass (g) of the starting materials, a _Tb and a _Y are the mass reduction rates (%) when the starting oxide powders of Tb and Y are heated in an inert gas atmosphere from room temperature (20±10°C) to 1100°C, a _Al is the mass reduction rate (%) when the starting Al oxide powder is heated in an inert gas atmosphere from room temperature (20±10°C) to 1100°C, a _Sc is the mass reduction rate (%) when the starting scandium oxide powder is heated from room temperature (20±10°C) to 1100°C in an inert gas atmosphere. x', y', and z' are the same as x', y', and z' in the above formula (1').

(Weighing)
Based on the masses of the starting oxide powders obtained as above, the starting oxide powders are precisely weighed using a precision balance capable of measuring to four decimal places.

(Mixing process)
The weighed oxide powder, which is the starting material, is mixed uniformly, and if necessary, the powder that has aggregated to become secondary particles is pulverized to the state of primary particles, so that a mixing process is performed. There are wet and dry mixing processes, but there is no particular limitation as long as the powder can be mixed uniformly. In addition, in the wet process, examples include ball mills, jet mills, bead mills, ultrasonic dispersion, and homogenizer processing, and examples of the dispersion medium include water, methanol, ethanol, propyl alcohol, or a dispersion medium obtained by mixing them in any ratio. In order to increase the efficiency of the mixing process, an organic dispersant may be added, and examples of the organic dispersant include polyethylene glycol-based dispersants, sulfonic acid-based dispersants, and phosphoric acid-based dispersants. A binder required to improve the shape retention in the subsequent molding process is added at the stage of this mixing process. It is essential to select a binder structure that dissolves in the dispersion medium. When the dispersion medium is water, a polyvinyl alcohol-based binder is exemplified, and when the dispersion medium is a lower alcohol such as ethanol, examples of the binder include polyvinyl butyral, polyvinyl acetate, or copolymers thereof. The amount of addition is not particularly limited as long as the shape retention is maintained in the subsequent molding process.

In addition, when _SiO2 is added as a sintering aid, the _SiO2 raw material is preferably a _SiO2 raw material such as tetraethyl orthosilicate (TEOS) or silicon oxide powder. The amount of tetraethyl orthosilicate (TEOS) added is an amount that is more than 0 ppm and less than 1,000 ppm (more than 0 mass% and less than 0.1 mass%) in terms of _SiO2 in the entire raw material powder (raw material powder for garnet-type complex oxide + sintering aid), and the amount of silicon oxide powder ( _SiO2 powder) is preferably more than 0 ppm and less than 1,000 ppm (more than 0 mass% and less than 0.1 mass%) in terms of SiO2 in the entire raw material powder (raw material powder for garnet-type complex oxide + sintering aid). If the amount of addition exceeds 1,000 ppm, there is a risk of slight light absorption occurring due to crystal defects caused by the excessive Si contained. In the case of silicon oxide powder (SiO ₂ powder), the primary particle size is not particularly limited, but it is preferable to use fine powder of less than 1 μm, since the sintering aid can be uniformly dispersed in the starting material.

(molding process)
After the mixing process, the mixture is molded into a predetermined shape. There are wet and dry molding methods, but the method is not particularly limited as long as the mixture is molded into a predetermined shape. Examples of wet molding include casting, centrifugal casting, extrusion, and tape molding, and examples of dry molding include dry uniaxial press molding and cold isostatic pressing (CIP (Cold Isostatic Pressing)) molding. Two or more of these molding methods may be combined, and CIP molding after dry uniaxial press molding or CIP molding after casting molding may be performed. The relative density to the true density after molding is preferably 45% to 65%, more preferably 50% to 60%.

In the present invention, it is preferable to wet mix the terbium oxide powder, yttrium oxide powder, scandium oxide powder and aluminum oxide powder as starting materials precisely weighed in the above mixing process with the _SiO2 raw material as the sintering aid to form a slurry, then spray dry the slurry to form a granular raw material, and use the granular raw material to form a molded product. In this case, a normal press molding process can be suitably used.

(Degreasing process)
The obtained molded body contains a large amount of organic matter such as binder, so it is subjected to a degreasing treatment. The degreasing is preferably performed in an atmosphere of 18 vol% or more of oxygen, and the degreasing temperature and time are not particularly limited as long as the conditions are selected so that the organic matter can be completely decomposed, but are preferably 250°C or more, more preferably 300°C or more, and even more preferably 400°C or more. The upper limit of the degreasing temperature is preferably a temperature at which densification does not progress, and is preferably 1000°C or less. The degreasing time is preferably 1 hour or more and 24 hours or less.

(Sintering process)
In order to make the degreased molded body transparent, a sintering process is performed. As a method of making the molded body transparent by sintering, there is a method of making the molded body transparent only by sintering under atmospheric pressure, and a method of making the molded body transparent by pre-sintering the molded body to a relative density of 93% to 99% under atmospheric pressure and then performing a hot isostatic pressing (HIP) process. Either method may be used in the present invention. The sintering temperature is not particularly limited as long as it is possible to achieve the desired sintered density or to make the molded body highly transparent. In addition, the sintering atmosphere is not particularly limited as long as no bubbles remain when the molded body is made transparent, and examples of the sintering atmosphere include a vacuum atmosphere, an oxygen atmosphere, and air. When performing the HIP process, the pressure medium is preferably argon, nitrogen, or argon containing 1 vol% or less of oxygen, and the pressure is preferably 100 MPa to 200 MPa.

(Annealing process)
The obtained sintered body may contain some oxygen deficiencies, so it is necessary to perform annealing treatment in an atmosphere with an oxygen content of 15 vol% or more. The annealing temperature is not particularly limited as long as the oxygen deficiencies are eliminated, but it is preferably 1200°C or more and 1500°C or less. If it is less than 1200°C, there is not enough heat to remove the oxygen deficiencies, while if it is more than 1500°C, bubbles may re-occur due to annealing, which is not preferable. The annealing time may be performed until the oxygen deficiencies are eliminated, and it is preferably 20 hours or more and 100 hours or less.

(Processing process)
In the manufacturing method of the present invention, it is preferable to optically polish both end faces on the optically utilized axis of the sintered body (rare earth garnet-type transparent ceramics) that has undergone the above-mentioned series of manufacturing steps. In this case, the optical surface accuracy is preferably λ/8 or less, particularly preferably λ/10 or less, when the measurement wavelength λ=633 nm. It is also possible to further reduce optical loss by appropriately forming an anti-reflection film on the optically polished surface.

In this manner, rare earth garnet-type transparent ceramics with high transparency can be obtained with good reproducibility. In addition, a transparent ceramic that can be used as a high-output Faraday rotator is obtained, which is made of a sintered body of a paramagnetic garnet-type composite oxide represented by formula (1') using terbium oxide powder, yttrium oxide powder, scandium oxide powder, and aluminum oxide powder as starting materials, and in which the absorption and scattering of light with a wavelength of 1064 nm at a mounting length of 20 mm is minimized.

[Magneto-optical devices]
Furthermore, since the rare earth garnet-type transparent ceramic obtained in the present invention is intended to be used as a magneto-optical material, it is preferable to apply a magnetic field parallel to the optical axis of the paramagnetic garnet-type transparent ceramic, and then set a polarizer and an analyzer so that their optical axes are shifted by 45 degrees from each other to form a magneto-optical device. That is, the magneto-optical material of the present invention is suitable for use in magneto-optical devices, and is particularly suitable for use as a Faraday rotator in an optical isolator with a wavelength of 0.9 to 1.1 μm.

FIG. 1 is a schematic cross-sectional view showing an example of an optical isolator, which is an optical device having, as an optical element, a Faraday rotator made of the transparent ceramic obtained according to the present invention.
1, an optical isolator 100 includes a Faraday rotator 110 made of the paramagnetic garnet-type transparent ceramic obtained by the present invention, and a polarizer 120 and an analyzer 130, which are polarizing materials, are provided in front of and behind the Faraday rotator 110. Moreover, it is preferable that the optical isolator 100 has the polarizer 120, the Faraday rotator 110, and the analyzer 130 arranged in this order on its optical axis, and that a magnet 140 is placed on at least one of the side surfaces of these elements.

The optical isolator 100 can also be used effectively in industrial fiber laser devices. In other words, it is effective in preventing reflected light from the laser light emitted from the laser light source from returning to the light source, causing the oscillation to become unstable.

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

[Test Example 1]
(Measurement of mass loss rate of starting material at 1100°C)
The mass reduction rate at 1100°C was measured for various oxide powders used as starting materials in this example using a thermogravimetric/differential thermal analyzer (Thermo Plus Evo) manufactured by Rigaku Corporation. That is, the oxide powder was placed in a platinum container, and the weight of the powder was measured using a precision balance capable of measuring to the fourth decimal point. After accurately measuring the weight, the platinum container containing the starting material was set in the thermogravimetric/differential thermal analyzer. The atmosphere was He, and the material was heated from room temperature (20°C) to 1200°C at a heating rate of 15°C/min, and then air-cooled to room temperature, during which thermogravimetric (TG) measurements were performed. The mass reduction rate at 1100°C was obtained from the measurement data, and the value was designated as a (%).

The details of the measured oxide powders are shown below.
Tb ₄ O ₇ (fine powder): manufactured by Shin-Etsu Chemical Co., Ltd., large specific surface area type, particle size 180 nm, purity 99.9% by mass
_Y2O3 (fine powder): manufactured by Shin-Etsu Chemical Co., Ltd., large specific surface area type, _particle size 180 nm, purity 99.9% by mass
_Sc2O3 : manufactured by _Shin -Etsu Chemical Co., Ltd., particle size 3 μm, purity 99.9% by mass
CeO ₂ : manufactured by Shin-Etsu Chemical Co., Ltd., large specific surface area type, particle size 180 nm, purity 99.9% by mass
_Lu2O3 : manufactured by Shin-Etsu Chemical Co., Ltd., large specific surface area type, _particle size 180 nm, purity 99.9% by mass
Dy ₂ O ₃ : manufactured by Shin-Etsu Chemical Co., Ltd., large specific surface area type, particle size 180 nm, purity 99.9% by mass
Al ₂ O ₃ : Taimicron, manufactured by Taimei Chemical Industry Co., Ltd., particle size 300 nm, purity 99.9% by mass
Tb ₄ O ₇ (coarse powder): manufactured by Shin-Etsu Chemical Co., Ltd., standard type, particle size 3 μm, purity 99.9% by mass
_Y2O3 (coarse powder): manufactured by Shin-Etsu Chemical Co., Ltd., standard type, _particle size 3 μm, purity 99.9% by mass

Table 1 shows the mass reduction rate at 1100°C of the starting material (oxide powder) used in the present invention. Table 1 shows that the mass reduction rate at 1100°C varies greatly depending on the type of starting material, and each has its own specific surface area and adsorbed components.

[Example 1]
Among the above starting materials _{, transparent ceramics} were produced _{using Tb4O7} (fine powder), _Y2O3 (fine powder), _{Sc2O3 , CeO2} , _Lu2O3 , _Dy2O3 , _{and Al2O3} in the following manner.

(Weighing and raw material preparation process)
For each target composition, the mass used was calculated as follows using the mass reduction rate a (%) at 1100°C measured in Test Example 1. In each case, the planned total mass W of the starting materials was 50 g.

(1) Target composition: Tb ₃ Al ₅ O ₁₂
(Example 1-1)
m _{Tb (fine powder)} = 3/2 x M _Tb / M _G1-1 x W x 100/(100-a _{Tb (fine powder)} ) = 36.63 (g)
m _AL =5/2×M _AL /M _G1-1 ×W×100/(100-a _Al )=16.24(g)
(Comparative Example 1-1)
m' _{Tb (fine powder)} = 3/2 x M _Tb /M _G1-1 x W = 34.14 (g)
m' _AL = 5/2 x M _AL /M _G1-1 x W = 15.86 (g)
(M _Tb = 365.8, M _AL = 102.0, M _G1-1 = 803.7, a _{Tb (fine powder)} = 6.8, a _Al = 2.2)

(2) Target composition: Y ₃ Al ₅ O ₁₂
(Example 1-2)
m _{Y (fine powder)} = 3/2 x M _Y / M _G1-2 x W x 100/(100-a _{Y (fine powder)} ) = 31.91 (g)
m _AL =5/2×M _AL /M _G1-2 ×W×100/(100-a _Al )=21.96(g)
(Comparative Example 1-2)
m' _{Y (fine powder)} = 3/2 x M _Y /M _G1-2 x W = 28.53 (g)
m' _AL = 5/2 x M _AL /M _G1-2 x W = 21.48 (g)
(M _Y =225.8, M _AL =102.0, M _G1-2 =593.6, a _{Y (fine powder)} = 10.6, a _Al = 2.2)

(3) Target composition: (Tb _0.8 Y _0.2 ) ₃ Al ₅ O ₁₂
(Examples 1 to 3)
m _{Tb (fine powder)} = 3/2 x 0.8 x M _Tb / M _G1-3 x W x 100/(100-a _{Tb (fine powder)} ) = 30.92 (g)
m _{Y (fine powder)} = 3/2 x 0.2 x M _Y /M _G1-3 x W x 100/(100-a _{Y (fine powder)} ) = 4.97 (g)
m _AL =5/2×M _AL /M _G1-3 ×W×100/(100-a _Al )=17.12(g)
(Comparative Example 1-3)
m' _{Tb (fine powder)} = 3/2 x 0.8 x M _Tb /M _G1-3 x W = 28.81 (g)
m' _{Y (fine powder)} = 3/2 x 0.2 x M _Y /M _G1-3 x W = 4.45 (g)
m' _AL = 5/2 x M _AL /M _G1-3 x W = 16.74 (g)
(M _Tb = 365.8, M _Y = 225.8, M _AL = 102.0, M _G1-3 = 761.7, a _{Tb (fine powder)} = 6.8, a _{Y (fine powder)} = 10.6, a _Al = 2.2)

(4) Target composition: (Tb _0.6 Y _0.4 ) ₃ Al ₅ O ₁₂
(Examples 1 to 4)
m _{Tb (fine powder)} = 3/2 x 0.6 x M _Tb / M _G1-4 x W x 100/(100-a _{Tb (fine powder)} ) = 24.54 (g)
m _{Y (fine powder)} = 3/2 x 0.4 x M _Y /M _G1-4 x W x 100/(100-a _{Y (fine powder)} ) = 10.53 (g)
m _AL =5/2×M _AL /M _G1-4 ×W×100/(100-a _Al )=18.11(g)
(Comparative Examples 1 to 4)
m' _{Tb (fine powder)} = 3/2 x 0.6 x M _Tb /M _G1-4 x W = 22.87 (g)
m' _{Y (fine powder)} = 3/2 x 0.4 x M _Y /M _G1-4 x W = 9.41 (g)
m' _AL = 5/2 x M _AL /M _G1-4 x W = 17.72 (g)
(M _Tb = 365.8, M _Y = 225.8, M _AL = 102.0, M _G1-4 = 719.7, a _{Tb (fine powder)} = 6.8, a _{Y (fine powder)} = 10.6, a _Al = 2.2)

(5) Target composition: (Tb _0.6 Y _0.38 Sc _0.02 ) ₃ (Al _0.998 Sc _0.002 ) ₅ O ₁₂
(Examples 1 to 5)
m _{Tb (fine powder)} = 3/2 x 0.6 x M _Tb / M _G1-5 x W x 100/(100-a _{Tb (fine powder)} ) = 25.08 (g)
m _{Y (fine powder)} = 3/2 x 0.38 x M _Y /M _G1-5 x W x 100/(100-a _{Y (fine powder)} ) = 10.22 (g)
m _AL =5/2×0.998×M _AL /M _G1-5 ×W×100/(100-a _Al )=18.47(g)
m _Sc = (3 x 0.02/2 + 5 x 0.002/2) (M _Sc / M _G1-5 ) x W x 100/(100-a _Sc ) = 0.35 (g)
(Comparative Example 1-5)
m' _{Tb (fine powder)} = 3/2 x 0.6 x M _Tb /M _G1-5 x W = 23.38 (g)
m' _{Y (fine powder)} = 3/2 x 0.38 x M _Y /M _G1-5 x W = 9.14 (g)
m' _AL = 5/2 x 0.998 x M _AL /M _G1-5 x W = 17.99 (g)
m' _Sc = (3 x 0.02/2 + 5 x 0.002/2) (M _Sc /M _G1-5 ) x W = 0.34 (g)
(M _Tb = 365.8, M _Y = 225.8, M _AL = 102.0, M _Sc = 137.9, M _G1-5 = 704.2, a _{Tb (fine powder)} = 6.8, a _{Y (fine powder)} = 10.6, a _Al = 2.2, a _Sc = 1.4)

(6) Target composition: (Y _0.999 Ce _0.001 ) ₃ Al ₅ O ₁₂
(Examples 1 to 6)
m _{Y (fine powder)} = 3/2 x 0.999 x M _Y /M _G1-6 x W x 100/(100-a _{Y (fine powder)} ) = 31.87 (g)
m _Ce = 3/2 x 0.001 x M _Ce /M _G1-6 x W x 100/(100-a _Ce ) = 0.04 (g)
m _AL =5/2×M _AL /M _G1-6 ×W×100/(100-a _Al )=21.95(g)
(Comparative Example 1-6)
m' _{Y (fine powder)} = 3/2 x 0.999 x M _Y /M _G1-6 x W = 28.49 (g)
m' _Ce = 3/2 x 0.001 x M _Ce /M _G1-6 x W = 0.04 (g)
m' _AL = 5/2 x M _AL /M _G1-6 x W = 21.47 (g)
(M _Y =225.8, M _Ce =328.2, M _AL =102.0, M _G1-6 =593.8, a _{Y (fine powder)} = 10.6, a _Ce = 5.9, a _Al = 2.2)

(7) Target composition: Lu ₃ Al ₅ O ₁₂
(Examples 1 to 7)
m _Lu = 3/2 x M _Lu /M _G1-7 x W x 100/(100-a _Lu ) = 36.31 (g)
m _AL =5/2×M _AL /M _G1-7 ×W×100/(100-a _Al )=15.30(g)
(Comparative Example 1-7)
m' _Lu = 3/2 x M _Lu /M _G1-7 x W = 35.04 (g)
m' _AL = 5/2 x M _AL /M _G1-7 x W = 14.97 (g)
(M _Lu =398.0, M _AL =102.0, M _G1-7 =851.9, a _Lu =3.5, a _Al =2.2)

(8) Target composition: Dy ₃ Al ₅ O ₁₂
(Examples 1 to 8)
m _Dy = 3/2 x M _Dy / M _G1-8 x W x 100/(100-a _Dy ) = 35.45 (g)
m _AL =5/2×M _AL /M _G1-8 ×W×100/(100-a _Al )=16.01(g)
(Comparative Example 1-8)
m' _Dy = 3/2 x M _Dy /M _G1-8 x W = 34.35 (g)
m' _AL = 5/2 x M _AL /M _G1-8 x W = 15.66 (g)
(M _Dy =373.0, M _AL =102.0, M _G1-8 =814.4, a _Dy =3.1, a _Al =2.2)

(9) Target composition: Tb _3.014 Al _4.986 O ₁₂
(Reference example 1)
m _{Tb (fine powder)} = 3.014/2 x M _Tb / M _G1-9 x W = 34.22 (g)
m _AL =4.986/2 x M _AL /M _G1-9 x W = 15.79 (g)
(M _Tb =365.8, M _AL =102.0, M _G1-9 =805.4)

For each target composition, the starting materials were weighed out in the amount determined as above. A precision balance capable of measuring to four decimal places was used for the weighing.
The weighed starting material (oxide powder) was placed in a ball mill container. Then, ethanol was added so that the concentration of the oxide powder was 30 mass%, and 0.01 mass% of polyethylene glycol was added as a dispersant, and TEOS (Kishida Chemical Co., Ltd.) was added as a sintering aid so that the concentration was 100 ppm in terms of Si. Next, alumina balls with a diameter of 2 mm were placed in the ball mill container, which was then set on a rotating table, and ball mill mixing was performed for 20 hours.
To the obtained slurry, a polyvinyl alcohol binder (manufactured by Kuraray Co., Ltd.) was added as a binder in an amount of 1.5 mass % based on the powder amount, and the mixture was further mixed in a ball mill for 4 hours.
Finally, the slurry was separated from the alumina balls, and the raw material was granulated by spray drying.

(molding process)
The granules obtained in the previous step were filled into a cylindrical mold and molded into a predetermined shape by dry uniaxial press molding. The molded body was a cylinder with a diameter of 8 mm and a length of 40 mm. The uniaxial press molded body was then subjected to CIP treatment at a pressure of 198 MPa, and the relative density of the molded body was set to 60%.
(Degreasing process)
The obtained CIP molded body was degreased at 270°C for 3 hours. The removal of organic matter was confirmed and the amount of oxygen absorbed by degreasing was measured by the above-mentioned TG analysis. The decomposition of organic matter was judged by the weight loss up to 400°C, and the amount of oxygen absorbed was compared by the weight increase around 400°C.
(Sintering process)
The sintering was carried out in a two-stage sintering process of pre-sintering and HIP. That is, the above-mentioned degreased compact was first set in a vacuum sintering furnace, and pre-sintering was carried out under the conditions of a vacuum degree of 1×10 ⁻⁴ Pa, a heating rate of 100° C./h, and a holding time of 1600° C. for 2 hours. The density of each sample after pre-sintering was 93% or more. Next, in order to achieve transparency, the pre-sintered samples were subjected to HIP treatment under the conditions of 1600° C. for 3 hours in an Ar atmosphere with a pressure of 198 MPa. Although the samples after HIP were transparent, blackening due to oxygen defects was observed, so oxidation annealing was carried out in the atmosphere at 1450° C. for 30 hours. All samples after oxidation annealing became colorless and transparent.
(polishing/processing process)
The annealed HIP sintered sample was cylindrically ground to an outer diameter of 5 mm, and then optically polished to a length of 20 mm. At that time, the processing conditions were adjusted so that the polished surface precision was λ/8 (λ=633 nm).

The transparent ceramic samples obtained as described above were evaluated as follows.
(Transmittance measurement)
The transparency of the optically polished transparent ceramic sample was measured with reference to JIS K7136:2000. That is, an entrance opening and an exit opening through which light passes are provided in an integrating sphere, and a sample (transparent ceramic sample) is placed at the entrance opening. By attaching a reflector to the exit opening, it becomes possible to detect all the light emitted from the sample with the integrating sphere, and the total light transmittance was measured from the ratio of the intensity of the detected emitted light to the intensity of the light incident on the sample. The device used was a spectrophotometer (V-670) manufactured by JASCO Corporation, and the measurement was performed using the attached integrating sphere. At that time, a pinhole was provided so that the spot diameter of the irradiated light was 3 mm. The measurement wavelength was fixed at 1064 nm.
The results are shown in Table 2.

As a result of the above, when the starting materials were weighed by the mass used corrected by the mass loss rate (a(%)) at 1100°C as in Examples 1-1 to 1-8, all were highly transparent. On the other hand, when the starting materials were weighed by the mass used calculated as in the conventional manner without correcting by the mass loss rate (a(%)) at 1100°C as in Comparative Examples 1-1 to 1-8, a decrease in transmittance due to deviation in the ceramic composition was observed. It should be noted that, as in Reference Example 1, it is possible to obtain highly transparent ceramics by shifting the target composition from the ideal ratio even without correcting by the mass loss rate (a(%)) at 1100°C for the starting materials. These results suggest that ceramics with a composition different from the target composition are produced due to the influence of moisture adsorbed on the oxide powder of the starting materials. Note that "highly transparent" here refers to a transmittance of 82% or more. Also, "hereinafter, there is a different phase" in the remarks column means that a different phase was observed by observation with an optical microscope at a magnification of 100 times.

[Example 2]
In Example 1-1 and Comparative Example 1-1, the starting Tb oxide powder was Tb ₄ O ₇ (coarse powder) and the mass used was calculated as follows, and transparent ceramic samples were produced in the same manner as in Example 1-1 and Comparative Example 1-1. In Reference Example 1, the starting Tb oxide powder was Tb ₄ O ₇ (coarse powder) and the transparent ceramic sample was produced in the same manner as in Reference Example 1. The results are shown in Table 3.
・Target composition: Tb ₃ Al ₅ O ₁₂
Example 2
m _{Tb (coarse powder)} = 3/2 x M _Tb / M _G2 x W x 100/(100-a _{Tb (coarse powder)} ) = 35.12 (g)
m _AL =5/2×M _AL /M _G2 ×W×100/(100-a _Al )=16.22(g)
(Comparative Example 2)
m' _{Tb (coarse powder)} = 3/2 x M _Tb /M _G2 x W = 34.14 (g)
m' _AL = 5/2 x M _AL /M _G2 x W = 15.86 (g)
(M _Tb = 365.8, M _AL = 102.0, M _G2 = 803.7, a _{Tb (coarse)} = 2.8, a _Al = 2.2)
・Target composition: Tb _3.014 Al _4.986 O ₁₂
(Reference example 2)
m _{Tb (coarse powder)} = 3.014/2 x M _Tb / M _G1-9 x W = 34.22 (g)
m _AL =4.986/2 x M _AL /M _G1-9 x W = 15.79 (g)
(M _Tb =365.8, M _AL =102.0, M _G1-9 =805.4)

As a result of the above, high transparency was achieved in Example 2, whereas the transmittance was low in Comparative Example 2. Moreover, in Reference Example 2, the transmittance was poor despite the target composition (weighing conditions) that achieved high transparency in Reference Example 1. It is believed that the difference between Reference Example 1 and Reference Example 2 was due to differences in the amount of adsorbed moisture in the oxide powder as the starting material.
As described above, according to the manufacturing method of the present invention, it is possible to produce highly transparent garnet-type ceramics with good reproducibility even if the type or lot of the starting oxide powder is changed.

[Example 3]
In the above Examples 1-3 and 1-4 and Comparative Examples 1-3 and 1-4, the combinations of Tb oxide powder and Y oxide powder were changed to Tb ₄ O ₇ (fine powder)-Y ₂ O ₃ (coarse powder) and Tb ₄ O ₇ (coarse powder)-Y ₂ O ₃ (fine powder), respectively, and the masses used were calculated as follows. Otherwise, transparent ceramic samples were prepared in the same manner as in Examples 1-3 and 1-4 and Comparative Examples 1-3 and 1-4.
(1) Target composition: (Tb _0.8 Y _0.2 ) ₃ Al ₅ O ₁₂
(Example 3-1)
m _{Tb (fine powder)} = 3/2 x 0.8 x M _Tb /M _G3(1) x W x 100/(100-a _{Tb (fine powder)} ) = 30.92 (g)
m _{Y (coarse powder)} = 3/2 x 0.2 x M _Y /M _G3(1) x W x 100/(100-a _{Y (coarse powder)} ) = 4.60 (g)
m _AL =5/2×M _AL /M _G3(1) ×W×100/(100-a _Al )=17.12(g)
(Comparative Example 3-1)
m' _{Tb (fine powder)} = 3/2 x 0.8 x M _Tb /M _G3(1) x W = 28.81 (g)
m' _{Y (coarse powder)} = 3/2 x 0.2 x M _Y /M _G3(1) x W = 4.45 (g)
m' _AL = 5/2 x M _AL /M _G3(1) x W = 16.74 (g)
(Example 3-2)
m _{Tb (coarse powder)} = 3/2 x 0.8 x M _Tb /M _G3(1) x W x 100/(100-a _{Tb (coarse powder)} ) = 29.64 (g)
m _{Y (fine powder)} = 3/2 x 0.2 x M _Y /M _G3(1) x W x 100/(100-a _{Y (fine powder)} ) = 4.97 (g)
m _AL =5/2×M _AL /M _G3(1) ×W×100/(100-a _Al )=17.12(g)
(Comparative Example 3-2)
m' _{Tb (coarse powder)} = 3/2 x 0.8 x M _Tb /M _G3(1) x W = 28.81 (g)
m' _{Y (fine powder)} = 3/2 x 0.2 x M _Y /M _G3(1) x W = 4.45 (g)
m' _AL = 5/2 x M _AL /M _G3(1) x W = 16.74 (g)
(M _Tb = 365.8, M _Y = 225.8, M _AL = 102.0, M _G3(1) = 761.7, a _{Tb (fine powder)} = 6.8, a _{Tb (coarse powder)} = 2.8, a _{Y (fine powder)} = 10.6, a _{Y (coarse powder)} = 3.4, a _Al = 2.2)

(2) Target composition: (Tb _0.6 Y _0.4 ) ₃ Al ₅ O ₁₂
(Example 3-3)
m _{Tb (fine powder)} = 3/2 x 0.6 x M _Tb /M _G3(2) x W x 100/(100-a _{Tb (fine powder)} ) = 24.54 (g)
m _{Y (coarse powder)} = 3/2 x 0.4 x M _Y /M _G3(2) x W x 100/(100-a _{Y (coarse powder)} ) = 9.74 (g)
m _AL =5/2×M _AL /M _G3(2) ×W×100/(100-a _Al )=18.11(g)
(Comparative Example 3-3)
m' _{Tb (fine powder)} = 3/2 x 0.6 x M _Tb /M _G3(2) x W = 22.87 (g)
m' _{Y (coarse powder)} = 3/2 x 0.4 x M _Y /M _G3(2) x W = 9.41 (g)
m' _AL = 5/2 x M _AL /M _G3(2) x W = 17.72 (g)
(Example 3-4)
m _{Tb (coarse powder)} = 3/2 x 0.6 x M _Tb /M _G3(2) x W x 100/(100-a _{Tb (coarse powder)} ) = 23.53 (g)
m _{Y (fine powder)} = 3/2 x 0.4 x M _Y /M _G3(2) x W x 100/(100-a _{Y (fine powder)} ) = 10.53 (g)
m _AL =5/2×M _AL /M _G3(2) ×W×100/(100-a _Al )=18.11g)
(Comparative Example 3-4)
m' _{Tb (coarse powder)} = 3/2 x 0.6 x M _Tb /M _G3(2) x W = 22.87 (g)
m' _{Y (fine powder)} = 3/2 x 0.4 x M _Y /M _G3(2) x W = 9.41 (g)
m' _AL = 5/2 x M _AL /M _G3(2) x W = 17.72 (g)
(M _Tb = 365.8, M _Y = 225.8, M _AL = 102.0, M _G3(2) = 719.7, a _{Tb (fine powder)} = 6.8, a _{Tb (coarse powder)} = 2.8, a _{Y (fine powder)} = 10.6, a _{Y (coarse powder)} = 3.4, a _Al = 2.2)

For the transparent ceramic sample obtained as described above, the transmittance was measured, and the Verdet constant was also measured on the assumption that an isolator was installed as described below.
(Verde constant)
Since the garnet-type transparent ceramics containing Tb have a magneto-optical effect, they can be used as materials for isolators. To investigate whether the obtained transparent ceramics can be used as an isolator, the Verdet constant was measured.
As shown in FIG. 1, each of the obtained ceramic samples was inserted into the center of a neodymium-iron-boron magnet with an outer diameter of 32 mm, an inner diameter of 6 mm, and a length of 40 mm, and polarizers were inserted at both ends. Then, a high-power laser beam with a wavelength of 1064 nm was incident from both end faces using a high-power laser (beam diameter 1.6 mm) manufactured by IPG Photonics Japan Co., Ltd., to determine the Faraday rotation angle θ. The Faraday rotation angle θ was defined as the angle showing the maximum transmittance when the polarizer on the output side was rotated. The Verdet constant was calculated based on the following formula. The magnitude (H) of the magnetic field applied to the sample was calculated by simulation from the dimensions of the measurement system, the residual magnetic flux density (Br), and the coercive force (Hc).
θ=V×H×L
(In the formula, θ is the Faraday rotation angle (Rad), V is the Verdet constant (Rad/(T·m)), H is the magnitude of the magnetic field (T), and L is the length of the Faraday rotator (0.020 m in this case).)
The above results are shown below together with the results of the Verdet constants measured for the transparent ceramic samples of Examples 1-1, 1-3, 1-4, 1-5, and 2 and Comparative Examples 1-1, 1-3, 1-4, 1-5, and 2.

In either case, the Verdet constant was not the same between the Example and the Comparative Example. This is thought to be because the comparative example did not correct for the mass reduction rate (a (%)) of the starting material at 1100°C, which caused the balance between Tb and other elements to be lost from the target composition, resulting in a difference from the Verdet constant of the Example. In other words, in the conventional manufacturing method, when the target composition is fixed to an ideal ratio, it is expected that the Verdet constant will vary due to the influence of the type (specific surface area, etc.) of the Tb oxide powder and other oxide powders in the starting material. If the Verdet constant varies, it is not preferable because it may affect the quality of the isolator and may deteriorate the extinction ratio of the return light.
As described above, the present invention is effective not only in making transparent ceramics highly transparent, but also in ensuring the performance of magneto-optical elements.

Although the present invention has been described above using the above-mentioned embodiment, the present invention is not limited to this embodiment, and can be modified within the scope of what a person skilled in the art can imagine, such as other embodiments, additions, modifications, deletions, etc., and any aspect is within the scope of the present invention as long as it provides the effects of the present invention.

100 Optical isolator 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims (4)

Using at least one oxide powder selected from Sc, Y, and a lanthanoid element and one oxide powder selected from a Group 13 element as starting materials, a powder represented by the following formula (1) was prepared.
(A ¹ _1-xy A ² _x Sc _y ) ₃ (B _1-z Sc _z ) ₅ O ₁₂ (1)
(In formula (1), A ¹ and A ² are different elements selected from Y and lanthanoid elements, B is one element selected from Group 13 elements, 0≦x≦0.4, 0≦y<0.08, 0≦z<0.16, and when z=0, y=0, and when z>0, y>0.)
A method for producing a rare earth garnet-type transparent ceramic, which is a sintered body of a complex oxide represented by the following formula:
The above A ¹ and A ² are different elements selected from Tb, Y, Ce, Lu and Dy, and B is Al;
The method for producing a rare earth garnet-type transparent ceramics uses, as the starting materials, oxide powders of different elements selected from Tb, Y , Ce, Lu and Dy, each having a purity of 99.9% by mass or more, weighed out in amounts of m _A1 and m _A2 calculated by the following formulas (2-1) and (2-2): an oxide powder of Al having a purity of 99.9% by mass or more, weighed out in an amount of m _B calculated by the following formula (2-3): and a scandium oxide powder having a purity of 99.9% by mass or more, weighed out in an amount of m _Sc calculated by the following formula (2-4):
m _A1 = 3/2 x (1-x-y) M _A1 / M _G x W x 100/(100-a _A1 ) (2-1)
m _A2 = 3/2×xM _A2 /M _G ×W×100/(100-a _A2 ) (2-2)
m _B =5/2×(1-z) M _B /M _G ×W×100/(100-a _B ) (2-3)
m _Sc = (3y/2+5z/2) (M _Sc /M _G )×W×100/(100-a _Sc ) (2-4)
(In the formula, M _A1 and M _A2 are the molar masses (g/mol) of oxides of elements different from each other selected from Tb, Y , Ce, Lu and Dy , M _B is the molar mass (g/mol) of an oxide of Al , M _Sc is the molar mass (g/mol) of scandium oxide, M _G is the molar mass (g/mol) of the composite oxide represented by formula (1), W is the planned total mass (g) of the starting materials, a _A1 and a _A2 are the mass reduction rates (%) when the starting material oxide powders of elements different from each other selected from Tb, Y , Ce, Lu and Dy are heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _B is the mass reduction rate (%) when the starting material oxide powder of Al is heated in an inert gas atmosphere from room temperature (20±10° C.) to 1100° C., a _Sc is the mass reduction rate (%) when the starting scandium oxide powder is heated from room temperature (20±10°C) to 1100°C in an inert gas atmosphere. x, y, and z are the same as above. 2. The method for producing a rare earth garnet-type transparent ceramic according to claim 1 , wherein the sintered body contains more than 0 mass % and 0.1 mass % or less of _SiO2 as a sintering aid. 3. The method for producing a rare earth garnet-type transparent ceramic according to claim 1, wherein A ¹ in the above formula (1) is Tb, and 0.6≦1−x−y≦1. The method for producing a rare earth garnet-type transparent ceramic according to any one of claims 1 to 3 , wherein the starting materials are molded, and then the obtained molded body is degreased and sintered to obtain a sintered body of the composite oxide.

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