JP7472994B2 - Paramagnetic garnet-type transparent ceramics, magneto-optical device, and method for manufacturing paramagnetic garnet-type transparent ceramics - Google Patents

Description

The present invention relates to paramagnetic garnet-type transparent ceramics having translucency in the visible and/or near-infrared range, and more specifically to terbium-containing paramagnetic garnet-type transparent ceramics suitable for forming magneto-optical devices such as optical isolators, magneto-optical devices using the paramagnetic garnet-type transparent ceramics, and a method for producing the paramagnetic garnet-type transparent ceramics.

Industrial laser processing machines are equipped with optical isolators to prevent the return of light, such as reflected light, and inside these are mounted terbium-doped glass or terbium gallium garnet (TGG) crystals as Faraday rotators (for example, JP 2011-213552 A (Patent Document 1)). The magnitude of the Faraday effect is quantified by the Verdet constant, and the Verdet constant of TGG crystals is 40 rad/(T·m) (= 0.13 min/(Oe·cm)), while that of terbium-doped glass is 0.098 min/(Oe·cm). Since the Verdet constant of TGG crystals is relatively large, they are widely used as standard Faraday rotators. Another example is terbium aluminum garnet (TAG) crystal. The Verdet constant of the TAG crystal is about 1.3 times that of the TGG crystal, which allows the length of the Faraday rotator to be shortened, making it a good crystal that can be used in fiber lasers (for example, JP-A-2002-293693 (Patent Document 2) and JP-A-4107292 (Patent Document 3)).

In recent years, methods for producing TAG from transparent ceramics have been disclosed (for example, WO 2017/033618 (Patent Document 4), WO 2018/193848 (Patent Document 5), and “High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics” (Non-Patent Document 1)). Also, _{a method for producing transparent ceramics, YTAG: (TbxY1-x)3Al5O12 ( 0.2 ≦ x} ≦0.8, or 0.5≦x≦1.0, or x=0.6), in which part of the terbium is replaced by yttrium, has been reported (for example, “Fabrication and properties of ( _{TbxY1 -x )3Al5O12 transparent} ceramics _by hot isostatic pressing” ( _{Non-Patent Document 2), “Development of optical grade (TbxY1-x)3Al5O12 ceramics as Faraday rotator} material _” (Non-Patent Document 3), “Effect of (Tb+Y)/Al ratio on Microstructure Evolution and Densification Process of ( _{Tb0.6Y0.4 ) 3Al5O12 Transparent} Since Tb-containing rare earth aluminum garnet exhibits a higher thermal conductivity than TGG, it is expected to be used as a Faraday element with a smaller thermal lens effect.

The thermal lens effect is a phenomenon in which a Faraday element absorbs transmitted light and generates heat, causing a change in refractive index, resulting in a lens-like shape. If the focal position of a laser processing machine moves due to the thermal lens effect, the beam becomes defocused at the processing point, which is undesirable as it reduces processing accuracy. For this reason, efforts are being made daily to minimize the thermal lens effect of Faraday elements.

As mentioned above, many of the recent reports on rare earth aluminum garnets containing Tb have been based on ceramics. This is because TAG has an incongruent composition, making it difficult to produce single crystals. However, ceramics generally contain many scattering sources within the system, such as air bubbles, foreign phases, foreign matter, and microcracks. Therefore, in order to obtain highly transparent ceramics suitable for use in Faraday rotators, it is necessary to make every effort to thoroughly eliminate scattering sources such as air bubbles and foreign matter.

One method for reducing air bubbles and microcracks inside ceramics is pressure sintering (hot isostatic pressing (HIP)). In HIP, a sintered body (pre-sintered body) that has already been densified to a relative density of 94% or more is subjected to high temperature and high pressure treatment to induce plastic flow of the ceramic, compressing and removing defects. During HIP, many air bubbles are expelled from the system and removed, but some air bubbles often remain compressed within the system. For this reason, when the HIPed body is exposed to high temperatures and below normal pressure, the compressed and hidden air bubbles re-expand, and the scattering intensity is observed to increase.

A method for further reducing bubbles and heterogeneous phases inside ceramics that could not be removed by HIP treatment is to perform re-sintering after HIP treatment to remove them from the system by grain growth. Ikesue et al. have shown a method for pre-sintering YAG ceramics at 1,600°C for 3 hours in a vacuum, and then re-sintering transparent ceramics that have been HIP-treated at 1,500-1,700°C for 3 hours at 1,750°C, which is higher than the HIP treatment temperature, for 20 hours (for example, "Microstructure and Optical Properties of Hot Isostatic Pressed Nd:YAG Ceramics" (Non-Patent Document 5)). In addition, Patent Publication No. 2638669 (Patent Document 6) discloses a method for manufacturing a ceramic body in which green compacts having an appropriate shape and composition are formed, a pre-sintering process is performed in the temperature range of 1,350-1,650°C, a HIP treatment process is performed at a temperature of 1,350-1,700°C, and a re-sintering process is performed at a temperature exceeding 1,650°C, thereby removing pores.

Industrial laser processing machines require high beam quality to improve their processing accuracy. One example of an index of laser beam quality is the ^M2 value. The ^M2 value is a value that indicates the focusing ability of a beam, and while a theoretical Gaussian beam has ^M2 = 1, an actual laser beam has ^M2 > 1. When ^M2 = 1, the beam has the smallest spot at the focal point, and as the ^M2 value increases, the beam cannot be focused at the focal point. For this reason, it is considered preferable that the beam quality ^M2 value of the transmitted light of an optical isolator is not as large as possible relative to the beam quality ^M2 value of the incident light.

As described above, with the advancement of fine processing in pulsed laser processing machines, Faraday rotators with high beam quality and small thermal lens effect are required. In the above-mentioned circumstances, it has been recently disclosed that a dense ceramic sintered body having a composition of (Tb _x Y _1-x ) ₃ (Al _1-z Sc _z ) ₅ O ₁₂ (x = 0.5 to 1.0) has a higher extinction ratio (improved from the existing 35 dB to 39.5 or more) and can reduce insertion loss (improved from the existing 0.05 dB to 0.01 to 0.05 dB) compared to the existing TGG crystal (Non-Patent Document 3). The material disclosed in Non-Patent Document 3 is, first of all, a ceramic, so there is no precipitation of perovskite heterophase, which was a problem in the TGG crystal, and furthermore, by replacing a part of the Tb ions with Y ions, it is possible to further reduce loss, and it is a material from which a very high quality garnet-type Faraday rotator can be obtained.

JP 2011-213552 A JP 2002-293693 A Japanese Patent No. 4107292 International Publication No. 2017/033618 International Publication No. 2018/193848 Japanese Patent No. 2638669

“High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics”, Optical Materials Express, Vol.6, No. 1, pp.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) “Effect of (Tb+Y)/Al ratio on Microstructure Evolution and Densification Process of (Tb0.6Y0.4)3Al5O12 Transparent Ceramics”, Materials, 12, 300 (2019) “Microstructure and Optical Properties of Hot Isostatic Pressed Nd:YAG Ceramics”, Journal of American Ceramics Society, 79, 1927-1933 (1996) “Linear Intercept Technique for Measuring Grain Size in Two-Phase Polycrystalline Ceramics”, Journal of the American Ceramic Society, 55, 109 (1972)

However, when the inventors actually conducted follow-up tests using the material in Non-Patent Document 3 as a reference, they found that the rate of change in beam diameter when high-power output (e.g., 100 W) laser light with a wavelength of 1,070 nm was incident was large, at 13.1 to 17.0%, and confirmed that the thermal lens effect needed to be improved.

The present invention has been made in consideration of the above circumstances, and aims to provide a paramagnetic garnet-type transparent ceramic which is a transparent sintered body of a paramagnetic garnet-type oxide containing Tb and Al and has high laser beam quality and a small thermal lens effect, a magneto-optical device using the paramagnetic garnet-type transparent ceramic, and a method for manufacturing the paramagnetic garnet-type transparent ceramic.

As a result of the inventors' investigation into the above-mentioned problem, they discovered that (i) making the crystal grain size of the ceramics larger than a certain size and reducing the amount of bubbles, grain boundaries, foreign phases, and foreign matter inside the ceramics to a certain level, (ii) reducing the beam quality ( ^M2 ) of the laser transmitted through the ceramics to a certain value, and (iii) reducing the absorption of oxygen defects (e.g., F or F+ centers) by performing an oxidation annealing treatment are effective in improving the thermal lens effect. Based on this knowledge, the inventors conducted extensive investigations and have completed the present invention.

That is, the present invention provides the following paramagnetic garnet-type transparent ceramics, magneto-optical devices, and methods for producing the paramagnetic garnet-type transparent ceramics.
1.
The following formula (1)
(Tb1 _{-xyYxScy ) 3} (Al1 _{- zScz ) 5O12} (1 ₎
(In the formula, 0≦x<0.45, 0≦y<0.08, 0≦z<0.2, and 0.001<y+z<0.20.)
a sintered body of Tb-containing rare earth aluminum garnet represented by the formula (1) and having both end faces finished to an optical mirror surface, each end face being provided with an anti-reflection film, and being in the shape of a cylinder or prism of 20 mm in length; wherein when a laser beam of 1,070 nm in wavelength and having a laser intensity of ¹⁰⁰ W and a beam quality ^M2 value of m (1 < m ≦ 1.2) is incident on the sintered body, the ratio n/m is 1.05 or less when the beam quality M2 value of the transmitted light is n.
2.
2. The paramagnetic garnet-type transparent ceramic according to 1, wherein the rate of change in beam diameter due to thermal lensing is 10% or less when laser light having an optical path length of 20 mm, a beam diameter of 1.6 mm, and an incident power of 100 W is incident thereon.
3.
3. The paramagnetic garnet-type transparent ceramic according to 1 or 2, containing more than 0 mass % and 0.1 mass % or less of SiO ₂ as a sintering aid.
4.
4. The paramagnetic garnet-type transparent ceramic according to any one of 1 to 3, wherein the average sintered grain size is 10 μm or more and 40 μm or less.
5.
5. A magneto-optical device comprising the paramagnetic garnet-type transparent ceramic according to any one of 1 to 4.
6.
6. The magneto-optical device according to claim 5, which is an optical isolator that has the paramagnetic garnet-type transparent ceramic as a Faraday rotator and has polarizing materials in front of and behind the optical axis of the Faraday rotator and can be used in the wavelength range of 0.9 μm to 1.1 μm.
7.
5. A method for producing the paramagnetic garnet-type transparent ceramic according to any one of 1 to 4, comprising the step of:
(Tb1 _{-xyYxScy ) 3} (Al1 _{- zScz ) 5O12} (1 ₎
(In the formula, 0≦x<0.45, 0≦y<0.08, 0≦z<0.2, and 0.001<y+z<0.20.)
a Tb-containing rare earth aluminum garnet sintered body represented by the formula (1) is pressure-sintered, the pressure-sintered body is heated to a temperature higher than that of the pressure-sintered body, and the resintered body is subjected to an oxidation annealing treatment in an oxidation atmosphere of 1,400° C. or higher.
8.
8. The method for producing a paramagnetic garnet-type transparent ceramic according to 7, wherein the resintering is carried out under a reduced pressure of less than 1.0×10 ⁻³ Pa.
9.
9. A method for producing a paramagnetic garnet-type transparent ceramic according to claim 7 or 8, wherein the sintered body before pressure sintering has a relative density of 94% or more by pre-sintering.
10.
10. The method for producing a paramagnetic garnet-type transparent ceramic according to claim 9, wherein the pre-sintering is performed under a reduced pressure of less than 1.0×10 ⁻³ Pa.
11.
11. The method for producing a paramagnetic garnet-type transparent ceramic according to any one of 7 to 10, wherein the sintered body before pressure sintering has an average sintered grain size of 5 μm or less.
12.
12. The method for producing a paramagnetic garnet-type transparent ceramic according to any one of 7 to 11, wherein after the oxidation annealing treatment, both end faces are finished to an optical mirror surface, and then an anti-reflection film is formed on each of the end faces.

According to the present invention, it is possible to provide a paramagnetic garnet-type transparent ceramic having a small thermal lens effect, in which the beam diameter change rate is 10% or less when laser light having a wavelength of 1,070 nm and an optical path length of 20 mm is incident with a beam diameter of 1.6 mm and an incident power of 100 W, and which is suitable for constructing magneto-optical devices such as optical isolators.

FIG. 1 is a schematic cross-sectional view showing an example of the configuration of an optical isolator using the paramagnetic garnet-type transparent ceramic of the present invention as a Faraday rotator.

[Paramagnetic garnet-type transparent ceramics]
The paramagnetic garnet-type transparent ceramics according to the present invention will now be described.
The paramagnetic garnet-type transparent ceramic according to the present invention has the following formula (1):
(Tb1 _{-xyYxScy ) 3} (Al1 _{- zScz ) 5O12} (1 ₎
(In the formula, 0≦x<0.45, 0≦y<0.08, 0≦z<0.2, and 0.001<y+z<0.20.)
The present invention relates to a sintered body of Tb-containing rare earth aluminum garnet represented by the formula (1), which has both end faces finished to an optical mirror surface, and has a cylindrical or rectangular column shape with a length of 20 mm, each end face being provided with an anti-reflection film. When a laser beam having a wavelength of 1,070 nm and a laser intensity of 100 W and a beam quality ^M2 value of m (1 < m ≦ 1.2) is incident on the body, the ratio n/m is 1.05 or less, where n is the beam quality ^M2 value of the transmitted light.

In the garnet crystal structure represented by formula (1), the site primarily occupied by Tb, i.e., the first half of the parentheses in formula (1), is referred to as the A site, and the site primarily occupied by Al, i.e., the second half of the parentheses in formula (1), is referred to as the B site.

In the A site of formula (1), terbium (Tb) is an element with the largest Verdet constant among trivalent rare earth ions, and has extremely small absorption in the 1,070 nm region (wavelength band 0.9 μm to 1.1 μm) used in fiber lasers, making it the most suitable element for use as an optical isolator material in this wavelength range. However, Tb(III) ions are easily oxidized to generate Tb(IV) ions. When Tb(IV) ions are generated in metal oxides, they absorb light over a wide range of wavelengths from ultraviolet to near infrared, reducing the transmittance, so it is desirable to eliminate them as much as possible. One strategy to prevent the generation of Tb(IV) ions is to adopt a crystal structure in which Tb(IV) ions are unstable, i.e., a garnet structure.

Yttrium (Y) has an ionic radius that is about 2% smaller than that of terbium, and when it combines with aluminum to form a complex oxide, it can form a garnet phase more stably than a perovskite phase, making it an element that can be preferably used in the present invention.

In the B site of formula (1), aluminum (Al) is a material with the smallest ionic radius among trivalent ions that can exist stably in an oxide having a garnet structure, and is an element that can minimize the lattice constant of a paramagnetic garnet-type oxide containing Tb. If the lattice constant of the garnet structure can be reduced without changing the Tb content, it is preferable because the Verdet constant per unit length can be increased. Furthermore, since aluminum is a light metal, it has weak diamagnetism compared to gallium, and is expected to have the effect of relatively increasing the magnetic flux density generated inside the Faraday rotator, which is also preferable because it can increase the Verdet constant per unit length. In fact, the Verdet constant of TAG ceramics is improved to 1.25 to 1.5 times that of TGG. Therefore, even if the relative concentration of terbium is reduced by replacing some of the terbium ions with yttrium ions, it is possible to keep the Verdet constant per unit length equal to or slightly lower than that of TGG, making it a suitable constituent element in the present invention.

Here, a composite oxide whose constituent elements are only Tb, Y, and Al may not have a garnet structure due to a subtle weighing error, making it difficult to stably manufacture transparent ceramics that can be used for optical applications. Therefore, in the present invention, scandium (Sc) is added as a constituent element to eliminate the composition deviation due to a subtle weighing error. 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 and Y) and the B site (aluminum site) 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 and Y and Al deviates from the stoichiometric ratio due to variations in weighing. In addition, it is an element that can limit the presence ratio of the alumina heterophase in the garnet parent phase to 1 ppm or less, and can limit the presence ratio of the perovskite type heterophase in the garnet parent phase to 1 ppm or less, and can be added to improve product yields.

In formula (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 in this range, the Verdet constant at room temperature (23±15°C) and wavelength of 1,064 nm is 30 rad/(T·m) or more, and it can be used as a Faraday rotator. In addition, the larger x in this range is preferable because the thermal lens effect tends to be smaller. Furthermore, in this range, the larger x is preferable because the diffuse transmittance tends to be smaller. The same applies to laser light with a wavelength of 1,070 nm. On the other hand, when x is 0.45 or more, the Verdet constant at a wavelength of 1,064 nm is less than 30 rad/(T·m), which is not preferable. In other words, if the relative concentration of Tb is excessively low, when a general magnet is used, the total length of the Faraday rotator required to rotate laser light with a wavelength of 1,064 nm (or 1,070 nm) by 45 degrees exceeds 30 mm, which is undesirable because it makes manufacturing difficult.

In formula (1), the range of y is 0≦y<0.08, preferably 0<y<0.08, more preferably 0.002≦y≦0.07, and even more preferably 0.003≦y≦0.06. When y is in this range, the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, this is preferable because the number of perovskite-type heterophases (typically 1-1.5 μm in diameter and granular, light brown in color) present in a 150 μm x 150 μm field of view observed under an optical microscope is one or less. In this case, the ratio of the perovskite-type heterophase to the garnet parent phase is 1 ppm or less.

When y is 0.08 or more, in addition to substituting a portion of Tb with Y, Sc also substitutes a portion of Tb, resulting in an unnecessary decrease in the solid solution concentration of Tb, which is undesirable as it reduces the Verdet constant. In addition, since the raw material cost of Sc is expensive, it is undesirable from the viewpoint of manufacturing costs to unnecessarily over-do Sc. In addition, when y is 0.08 or more, there is an increased risk of antisite defect absorption, in which Tb and Y enter the B site and Al enters the A site.

In formula (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 in this range, the perovskite-type heterophase is not detected by XRD analysis. Furthermore, this is preferable because the number of perovskite-type heterophases (typically 1-1.5 μm in diameter and granular, light brown in color) present in a 150 μm x 150 μm field of view observed under an optical microscope is one or less. In this case, the ratio of the perovskite-type heterophase to the garnet parent phase is 1 ppm or less.

When z is 0.2 or more, the effect of suppressing the precipitation of perovskite-type heterophases saturates and remains unchanged, but as the value of z increases, the value of y, i.e., the substitution ratio of Tb by Sc, also increases, resulting in an unnecessary decrease in the solid solution concentration of Tb and a small Verdet constant, which is undesirable. Furthermore, since the raw material cost of Sc is expensive, it is undesirable from the viewpoint of manufacturing costs to unnecessarily over-do Sc. In addition, when z is 0.16 or more, there is an increased risk of antisite defect absorption, in which Tb and Y enter the B site and Al enters the A site.

In formula (1), the range of y+z is 0.001<y+z<0.20. When y+z is in this range, the perovskite-type heterophase is not detected by XRD analysis. Furthermore, it is preferable because the number of perovskite-type heterophases (typically 1-1.5 μm in diameter and granular, light brown in color) present in a 150 μm x 150 μm field of view in optical microscope observation is one or less. In this case, the ratio of the perovskite-type heterophase to the garnet parent phase is 1 ppm or less. Note that the effect of the present invention can be obtained even if the range of y+z is 0≦y+z≦0.001, but this is not preferable because the heterophase is more likely to occur due to weighing errors in the raw materials, resulting in a decrease in yield.

In addition, in the paramagnetic garnet-type transparent ceramic of the present invention, it is preferable that the sintered body further contains a sintering aid. Specifically, it is preferable to contain _SiO2 as a sintering aid in an amount of more than 0 mass% and 0.1 mass% or less (more than 0 ppm and 1,000 ppm or less). If the content exceeds 0.1 mass% (1,000 ppm), there is a risk of slight light absorption occurring due to crystal defects caused by the excessive Si contained, and when a 100W laser beam having a wavelength of 1,070 nm is irradiated onto a manufactured paramagnetic garnet-type transparent ceramic having a length (optical path length) of 20 mm, a thermal lens effect occurs, and when the value of the beam quality ^M2 of the incident light is m and the value of the beam quality ^M2 of the laser beam transmitted through the transparent ceramic is n, n/m becomes larger than 1.05, which is not preferable.

In addition, an oxide of magnesium (Mg) or calcium (Ca) can be added as a sintering aid. Both Mg and Ca are divalent ions, and can be suitably added because they are elements that can compensate for the charge balance shift inside the garnet structure caused by the addition of _tetravalent SiO2. The amount of addition is preferably adjusted to match the amount of _SiO2 added.

The paramagnetic garnet-type transparent ceramic of the present invention is a cylindrical or prismatic ceramic having a length of 20 mm, both end faces of which are mirror-finished and each end face is provided with an anti-reflection film, and when a laser beam having a wavelength of 1,070 nm and a laser intensity of 100 W and a beam quality ^M2 value of m (1<m≦1.2) is incident thereon, and the beam quality ^M2 value of the transmitted light is n, the ratio n/m is 1.05 or less, preferably 1.04 or less, and more preferably 1.02 or less. As a result, when the laser beam is transmitted, a high beam quality ^M2 is obtained.

In addition, the paramagnetic garnet-type transparent ceramics of the present invention preferably have a beam diameter change rate due to thermal lensing of 10% or less when a laser beam having a wavelength of 1,070 nm with an optical path length of 20 mm is incident with a beam diameter of 1.6 mm and an incident power of 100 W, preferably 9% or less, more preferably 8% or less, and even more preferably 7% or less. If the beam diameter change rate due to the thermal lens effect at a certain incident power is 10% or less, the system can be installed at that incident power, that is, the thermal lens characteristics are satisfactory. Since the paramagnetic garnet-type transparent ceramics of the present invention can control the beam diameter change rate due to the thermal lens effect to 10% or less when a high power of 100 W is incident, the material can be used in a high-power laser system for 100 W.

In addition, the paramagnetic garnet-type transparent ceramic of the present invention preferably has an average sintered grain size of 10 μm to 40 μm, more preferably 20 μm to 40 μm. If the average sintered grain size is less than 10 μm, the amount of scattering inside the ceramic increases, and as a result, when a cylindrical ceramic with a length of 20 mm is used as a Faraday rotator to be mounted inside a laser processing machine, when a laser beam with a wavelength of 1,070 nm and a laser intensity of 100 W and a beam quality ^M2 value of m (1<m≦1.2) is incident on the ceramic, and the beam quality ^M2 value of the transmitted light is n, the ratio n/m exceeds 1.05.

The average grain size (average sintered grain size) of the sintered grains in the paramagnetic garnet-type transparent ceramics (resintered body) is determined by measuring the grain size of the sintered grains of the target sintered body using a metallurgical microscope, and more specifically, is determined as follows.
That is, the transmission mode of the metal microscope is used for the resintered body, and a 50x objective lens is used to take a transmission open Nicol image of the sintered body sample with both end faces polished. In detail, the optically effective area of the target sintered body at a predetermined depth is photographed, a diagonal line is drawn on the photographed image, the total number of sintered grains that the diagonal line crosses is counted, and the value obtained by dividing the diagonal line length by this total count is defined as the average sintered grain size of the sintered grains in the image. Furthermore, the average grain sizes of each photographed image read in the analysis process are summed up, and the value divided by the number of photographs is defined as the average sintered grain size of the target sintered body (the same applies below in the manufacturing method and examples of paramagnetic garnet-type transparent ceramics).

[Method of manufacturing paramagnetic garnet-type transparent ceramics]
The method for producing the paramagnetic garnet-type transparent ceramics according to the present invention is a method for producing the paramagnetic garnet-type transparent ceramics according to the present invention described above, which comprises reacting a compound represented by the following formula (1):
(Tb1 _{-xyYxScy ) 3} (Al1 _{- zScz ) 5O12} (1 ₎
(In the formula, 0≦x<0.45, 0≦y<0.08, 0≦z<0.2, and 0.001<y+z<0.20.)
The present invention is characterized in that a sintered body of Tb-containing rare earth aluminum garnet represented by the formula (I) is pressure-sintered, the pressure-sintered body is further heated to a temperature higher than that of the pressure-sintered body, and the resintered body is further subjected to an oxidation annealing treatment in an oxidation atmosphere of 1,400° C. or more.

Here, the paramagnetic garnet-type transparent ceramics are manufactured according to the following procedure.
(raw powder for sintering)
First, a raw material powder for sintering corresponding to the garnet-type composite oxide composition of the above formula (1) is prepared.

The method for preparing the raw powder for sintering of the garnet-type complex oxide used in the present invention is not particularly limited, but may be such that a predetermined amount of metal oxide powder for each component element corresponding to the garnet-type complex oxide is weighed as the starting material so as to obtain a composition corresponding to formula (1), and mixed to obtain the raw powder for sintering. The starting material in this case is not particularly limited as long as it can be made transparent, but from the viewpoint of suppressing absorption from impurities, the purity is preferably 99.9 mass% or more, more preferably 99.99 mass% or more, and most preferably 99.999 mass% or more. In addition, the particle size of the primary particles of the raw powder is not particularly limited as long as it can be made transparent, but from the viewpoint of easy sintering, it is preferably 50 nm or more and 1,000 nm or less. The shape of the primary particles is selected from a card house shape, a spherical shape, and a rod shape, and is not particularly limited as long as it can be made transparent.

Alternatively, the method for producing the raw powder for sintering of the above-mentioned garnet-type complex oxide used in the present invention may be a coprecipitation method, a pulverization method, a spray pyrolysis method, a sol-gel method, an alkoxide hydrolysis method, a complex polymerization method, a homogeneous precipitation method, or any other synthesis method. In some cases, the obtained ceramic raw material of rare earth complex oxide may be appropriately processed by a wet ball mill, a bead mill, a jet mill, a dry jet mill, a hammer mill, etc. to obtain a desired particle size. For example, a solid-phase reaction method in which multiple types of oxide particles are mixed and fired to produce uniformity by thermal diffusion of ions, or a coprecipitation method in which hydroxides, carbonates, etc. are precipitated from an ion-containing solution in which oxide particles are dissolved, and then fired to produce oxides to produce uniformity, may be used to produce the raw powder for sintering.

In the case of the solid-phase reaction method in which multiple types of metal oxide particles are mixed and fired to produce uniformity by thermal diffusion of ions, metal powders consisting of terbium, yttrium, scandium, and aluminum, or the above metal powders dissolved in an aqueous solution of nitric acid, sulfuric acid, uric acid, etc., or oxide powders of the above elements can be suitably used as starting materials. The purity of the above raw materials is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. The starting materials are weighed in a predetermined amount so as to have a composition corresponding to formula (1), mixed, and then fired to obtain a fired raw material of the desired metal oxide, which can be crushed to obtain a raw material powder for sintering. However, the firing temperature at this time is preferably 1,100°C or less, more preferably 1,050°C or less, and even more preferably 1,000°C or less. If the temperature exceeds 1,100°C, the raw material powder will be sintered and may not be sufficiently crushed in the subsequent crushing process. The firing time may be 1 hour or more, and the heating rate is preferably 100° C./h or more and 500° C./h or less. The firing atmosphere is preferably air or an oxygen-containing atmosphere, and nitrogen atmosphere, argon atmosphere, hydrogen atmosphere, etc. are not suitable. The firing apparatus is not particularly limited as long as it can reach the target temperature and oxygen flow. Examples of the firing apparatus include a vertical muffle furnace, a horizontal tubular furnace, a rotary kiln, etc.

In addition, the raw powder for sintering preferably contains a sintering aid. For example, tetraethoxysilane (TEOS) is added as a sintering aid together with the above starting materials in an amount of more than 0 ppm and not more than 1,000 ppm (more than 0 mass% and not more than 0.1 mass%) in terms of SiO ₂ in the entire raw powder (garnet-type complex oxide powder + sintering aid), or SiO ₂ powder is added in an amount of more than 0 ppm and not more than 1,000 ppm (more than 0 mass% and not more than 0.1 mass%) in the entire raw powder (garnet-type complex oxide powder + sintering aid), mixed, and formed as necessary to obtain a raw powder for sintering. If the amount added exceeds 1,000 ppm, there is a risk of a slight amount of light absorption occurring due to crystal defects caused by the excessive Si contained. The purity is preferably 99.9 mass% or more. The sintering aid may be added when preparing the raw powder slurry. Note that Si element may be mixed in from the environment, such as glassware used in the manufacturing process, or may volatilize when sintering is performed under reduced pressure, so that the Si content in the final ceramic may be unintentionally increased or decreased when analyzed. In addition, when no sintering aid is added, it is advisable to select the raw material powder for sintering (i.e., the starting material mixed powder or composite oxide powder) to be used, which has a primary particle size of nano-size and has extremely high sintering activity. Such a selection may be made appropriately.

When the raw material to be fired is pulverized to obtain raw material powder for sintering, either a dry or wet pulverization method can be selected, but it is necessary to pulverize so that the target ceramics become highly transparent. For example, in the case of wet pulverization, the raw material to be fired is slurried by various pulverization (dispersion) methods such as a ball mill, a bead mill, a homogenizer, a jet mill, and ultrasonic irradiation, and then pulverized (dispersed) to primary particles. The dispersion medium of this wet slurry is not particularly limited as long as it can make the final ceramic highly transparent, and examples of such dispersion medium include alcohols such as lower alcohols with 1 to 4 carbon atoms and pure water. In addition, various organic additives may be added to this wet slurry for the purpose of improving the quality stability and yield in the subsequent ceramic manufacturing process. In the present invention, these are also not particularly limited. In other words, various dispersants, binders, lubricants, plasticizers, etc. can be suitably used. However, it is preferable to select high-purity types of organic additives that do not contain unnecessary metal ions. In the case of wet pulverization, the dispersion medium of the slurry is finally removed to obtain raw material powder for sintering.

[Manufacturing process]
In the present invention, a slurry containing the above-mentioned sintering raw material powder is used to form a desired shape, which is then degreased and pre-sintered to form a sintered body (pre-sintered body) made of a complex oxide densified to a relative density of 94% or more. The sintered body is then pressure sintered, and the pressure sintered body is further heated to a temperature exceeding that of the pressure sintering temperature and re-sintered, and the re-sintered body is then subjected to a specified oxidation annealing treatment.

(Molding)
The slurry thus obtained is subjected to solid-liquid separation and molded into a desired shape. The molding method is broadly divided into dry molding and wet molding, but is not particularly limited as long as the desired shape can be stably obtained. In the case of dry molding, a method of making granules from the slurry using spray drying, filling a jig with the granules, and then performing press molding is exemplified. In addition, an example of wet molding is a casting molding method in which the slurry is poured into a plaster mold and the solvent is volatilized. Other examples include extrusion molding, sheet molding, centrifugal casting molding, and cold isostatic pressing, but all of these methods are not limited as they can obtain the desired shape.

Before molding, a binder may be added to the slurry. The binder can increase the retention of the molded body and has the effect of making cracks and breaks less likely to occur. There are no particular limitations on the type of binder, but it is preferable to use one that is compatible with the solvent and does not leave a residue when heat treated. Examples of binders include polyvinyl alcohol, polyvinyl butyral, polyvinyl acetate, and polyacrylic acid, and polymers obtained by copolymerizing two or more of these may also be used. The amount of binder varies depending on the molding method or binder type, but a minimum of 0.5% by mass of the sintering raw material powder is required, and the upper limit is 8% by mass. It is most preferable to add the binder during wet grinding.

As the press molding, a normal press molding process can be suitably used. That is, a very common uniaxial pressing process in which the material is filled into a mold and pressed from a certain direction, a cold isostatic pressing (CIP) process in which the material is sealed in a deformable waterproof container and pressed with isostatic pressure, or a warm isostatic pressing (WIP) process can be suitably used. The applied pressure can be adjusted appropriately while checking the relative density of the resulting molded body, and is not particularly limited. For example, the manufacturing cost can be reduced by controlling the pressure within a pressure range of about 300 MPa or less that can be handled by commercially available CIP and WIP devices.

However, in the present invention, in order to control the size and amount of scattering sources such as foreign phases, foreign objects, dirt, and microcracks within specified ranges, it is preferable that the molding jigs and molding machines used are clean and dedicated ones that have been thoroughly washed and dried, and that the environment in which the molding work is carried out is a clean space of class 1000 or less.

(Degreasing)
In the manufacturing method of the present invention, a normal debinding process can be suitably used. That is, a heating debinding process using a heating furnace can be used. The type of atmospheric gas used at this time is not particularly limited, and air, oxygen, hydrogen, etc. can be suitably used. There is no particular limit to the debinding temperature, but if a raw material containing an organic additive is used, it is preferable to heat the raw material to a temperature at which the organic component can be decomposed and eliminated.

(Pre-sintering)
In this process, a pre-sintered body is prepared as a sintered body before heat sintering, which is preferably densified to a relative density of 94% or more and preferably has an average sintered grain size of 5 μm or less. At this time, it is necessary to determine the conditions of temperature and holding time so that the sintered grain size falls within the desired range.

Here, a general sintering process can be preferably used. That is, a heating sintering process such as a resistance heating method or an induction heating method can be preferably used. The atmosphere is not particularly limited, and various atmospheres such as air, inert gas, oxygen gas, hydrogen gas, and helium gas can be preferably used, but sintering under reduced pressure (in a vacuum) can be more preferably used. The degree of vacuum for pre-sintering is preferably less than 1×10 ⁻¹ Pa, more preferably less than 1×10 ⁻² Pa, and particularly preferably less than 1×10 ⁻³ Pa.

The sintering temperature in the pre-sintering step of the present invention is preferably 1,450 to 1,650°C, and particularly preferably 1,500 to 1,600°C. This range of sintering temperature is preferable because it promotes densification while suppressing heterophase precipitation and grain growth. A sintering holding time of several hours in the pre-sintering step of the present invention is sufficient, but it is preferable to densify the relative density of the pre-sintered body to 94% or more. If the relative density of the pre-sintered body exceeds 99%, it becomes difficult for plastic deformation of the internal particles of the sintered body to occur in the subsequent hot-press 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 at most, and more preferably 98% or less.

The average sintered grain size of the pre-sintered body of the present invention is preferably 5 μm or less, more preferably 3 μm or less, even more preferably 2.5 μm or less, and particularly preferably 1 μm or less. The average sintered grain size of the sintered grains can be adjusted by taking into account the raw material type, atmosphere, sintering temperature, and holding time. If the sintered grain size is larger than 5 μm, plastic deformation is less likely to occur in the subsequent hot-ip sintering, and it may be difficult to remove bubbles remaining in the pre-sintered body. The average sintered grain size of the pre-sintered body can be determined by observing the sintered grain size of the crystal grains on the surface of the transparent hot-ip sintered body obtained in the next process with an optical microscope.

(Pressure sintering (hot isostatic pressing (HIP)))
In the manufacturing method of the present invention, after the pre-sintering step, a step of pressure sintering (HIP treatment) is provided, preferably at a pressure of 50 MPa to 300 MPa and a temperature of 1,000 ° C to 1,780 ° C. The type of pressurized gas medium at this time is preferably an inert gas such as argon or nitrogen, or Ar-O _2. The pressure applied by the pressurized gas medium is preferably 50 to 300 MPa, more preferably 100 to 300 MPa. If the pressure is less than 50 MPa, the transparency improvement effect may not be obtained, and if the pressure exceeds 300 MPa, even if the pressure is increased, no further transparency improvement is obtained, and the load on the device may be excessive, which may damage the device. It is convenient and preferable that the applied pressure is 196 MPa or less, which can be processed with a commercially available HIP device. In addition, the processing temperature (predetermined holding temperature) at this time is preferably set in the range of 1,000 to 1,780 ° C., more preferably 1,100 to 1,700 ° C. Heat treatment temperatures higher than 1,780°C are not preferred because grain growth occurs during HIP treatment, making it difficult to remove bubbles. Heat treatment temperatures lower than 1,000°C may not provide much improvement in the transparency of the sintered body. There is no particular restriction on the time to hold the heat treatment temperature, but holding it for too long increases the risk of oxygen deficiency, which is not preferred. Typically, it is preferably set in the range of 1 to 3 hours. There are no particular restrictions on the heater material, heat insulating material, and treatment container used in the HIP treatment, but graphite (C), molybdenum (Mo), tungsten (W), and platinum (Pt) can be preferably used, and yttrium oxide and gadolinium oxide can also be preferably used as the treatment container. When the treatment temperature is 1,500° C. or higher, graphite is preferred as the heater material and heat insulating material. In this case, it is preferable to select either graphite, molybdenum, or tungsten as the treatment container, and further select either yttrium oxide or gadolinium oxide as a double container inside, and then fill the container with an oxygen release material, since this minimizes the amount of oxygen deficiency that occurs during HIP treatment.

(Resintering)
In the manufacturing method of the present invention, after the HIP treatment, the pressure-sintered compact is heated to a temperature higher than that of the pressure-sintering process, and re-sintered to grow grains to obtain a re-sintered compact. At this time, it is necessary to determine the temperature and holding time conditions so that the final sintered grain size falls within a desired range.

The type of atmospheric gas used in this process is not particularly limited, and air, oxygen, hydrogen, etc. can be suitably used, but it is more preferable to carry out the process under reduced pressure (under a vacuum of less than 1×10 ⁻³ Pa). The resintering temperature is preferably 1,650° C. to 1,800° C., more preferably 1,700° C. to 1,800° C. A temperature below 1,650° C. is not preferable because grain growth does not occur.
The average sintered grain size of the crystal grains by resintering is preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more. It is also preferably 40 μm or less. The holding time of the resintering process is not particularly limited, but is preferably 5 hours or more, more preferably 10 hours or more, and particularly preferably 20 hours or more. In general, the longer the holding time, the more the grain growth of the sintered body progresses. The temperature and holding time of the resintering process may be appropriately adjusted by checking the average sintered grain size. However, in general, if the sintering temperature is raised too much, unexpected abnormal grain growth occurs, making it difficult to obtain a homogeneous sintered body. Therefore, it is preferable to allow a certain amount of leeway in the resintering temperature, and to adjust the size of the average sintered grain size of the resintered body by extending the holding time.

(Oxidation annealing)
The resintered body that has undergone the above series of treatments may have some oxygen deficiency due to reduction, particularly in the HIP treatment process, and may have 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 1,400°C or higher, preferably 1,450°C or higher. It is also preferable that it is 1,500°C or lower. In this case, the holding time is not particularly limited, but it is preferable to select a time that is sufficient to recover the oxygen deficiency and is within a time that does not consume electricity by performing the treatment for a long time unnecessarily. In addition, a slight oxidation HIP treatment may be performed. By these treatments, even if the resintered body has become colored, the oxygen deficiency can be restored, so that the size and number of the scattering source (scattering contrast source) can be controlled within a specified range, and a paramagnetic garnet-type transparent ceramic with little absorption due to oxygen deficiency can be obtained. Of course, the essential coloring (absorption) of the material due to the addition of colored elements such as dopants and impurities for imparting functions cannot be removed.

If the oxidation annealing process is performed at too high a temperature for too long a period of time, the size and amount of bubbles remaining inside the sintered body may increase. This is not preferable because it makes it impossible to control the size and amount of bubbles and microcracks remaining inside the final sintered body within the specified range. In this case, it is preferable to perform HIP treatment on the sintered body again and then perform oxygen atmosphere annealing again, since this makes it possible to control the size and amount of bubbles and microcracks remaining inside the sintered body within the specified range.

In the manufacturing method of the paramagnetic garnet-type transparent ceramics of the present invention, after the above-mentioned oxidation annealing treatment, it is preferable to finish both end faces to an optical mirror finish and then form an anti-reflection film on each of the end faces.

(Optical Polishing)
In the manufacturing method of the present invention, the paramagnetic garnet-type transparent ceramics that have been subjected to the above-mentioned series of manufacturing steps preferably have a cylindrical or prismatic shape, and both end faces (optical end faces) on the optically utilized axis are preferably optically polished and finished (optically mirror finished). In this case, the optical surface precision is preferably λ/2 or less, particularly preferably λ/8 or less, when the measurement wavelength λ is 633 nm.

It is also possible to further reduce optical loss by forming an appropriate anti-reflection film (AR coating) on the optically polished surface. In this case, it is preferable to thoroughly clean the optical surface with a chemical solution and inspect the cleanliness with a stereoscope or microscope before applying the anti-reflection coating treatment so that no dirt remains on both optical end faces. If the cleanliness inspection determines that the cleanliness is low, it can be wiped clean. In order not to scratch the optical surface or rub dirt on it during the wipe-cleaning process, it is preferable to select a handling jig made of a soft material and a wiper that generates little dust.

In the above manner, a paramagnetic garnet-type transparent ceramic is obtained, which is a sintered body of a paramagnetic garnet-type complex oxide containing at least terbium-aluminum, has both end faces finished to an optical mirror surface, and is cylindrical or prismatic with a length of 20 mm, each end face being provided with an antireflection film, and has an n/m ratio of 1.05 or less when a laser beam having a wavelength of 1,070 nm and a beam quality ^M2 value of m (1<m≦1.2) is incident thereon with a laser intensity of 100 W and a beam quality ^M2 value of n of the transmitted light. In addition, it is possible to provide a paramagnetic garnet-type transparent ceramic in which the beam diameter change rate due to thermal lensing is 10% or less when a laser beam having a wavelength of 1,070 nm is incident with a beam diameter of 1.6 mm and an incident power of 100 W in an optical path length of 20 mm.

[Magneto-optical devices]
Furthermore, since the paramagnetic garnet-type transparent ceramic of the present invention is intended to be used as a magneto-optical material, it is preferable to use the paramagnetic garnet-type transparent ceramic to configure a magneto-optical device by applying a magnetic field parallel to the optical axis of the paramagnetic garnet-type transparent ceramic and setting a polarizer and an analyzer so that their optical axes are shifted by 45 degrees from each other. In other words, the paramagnetic garnet-type transparent ceramic of the present invention is suitable for magneto-optical device applications, 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 a Faraday rotator made of the magneto-optical material of the present invention as an optical element.
1, an optical isolator 100 includes a Faraday rotator 110 made of the paramagnetic garnet-type transparent ceramics of 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. In the optical isolator 100, the polarizer 120, the Faraday rotator 110, and the analyzer 130 are arranged in this order, and it is preferable 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. That is, it is effective in preventing the reflected light of 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.

[Example 1]
Example 1 shows a case where y=0.004, z=0.03, and y+z=0.034 in formula (1), and the value of x is set to 0≦x≦0.396.
Terbium oxide powder, yttrium oxide powder, and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. were obtained. In addition, tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. and polyethylene glycol 200 liquid manufactured by Kanto Chemical Co., Ltd. were obtained. The purity of the powder raw materials was 99.9% by mass or more, and the purity of the liquid raw materials was 99.999% by mass or more. Using the above raw materials, the mixing ratio was adjusted to prepare the following oxide raw materials having a total of four types of crystal structures with the final composition shown in Table 1.

(Raw materials for Example 1-1 and Comparative Example 1-1)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y:Sc:Al=1.794: _1.194 : _0.162 :4.850, respectively, to prepare a mixed powder for ( _{Tb0.598Y0.398Sc0.004} ) ₃ ( _Al0.97Sc0.03 ) _{5O12 .} Next, TEOS was added as a sintering aid, weighed out so that the amount added was 100 _ppm in terms of _SiO2 , to prepare a raw material.

(Raw materials for Examples 1-2 and Comparative Examples 1-2)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y:Sc:Al=2.091: _0.897 : _0.162 :4.850, respectively, to prepare a mixed powder for ( _{Tb0.697Y0.299Sc0.004} ) ₃ ( _Al0.97Sc0.03 ) _{5O12 .} Next, TEOS was added as a sintering aid, weighed out so that the amount added was 100 _ppm in terms of _SiO2 , to prepare a raw material.

(Raw materials for Examples 1-3 and Comparative Examples 1-3)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y:Sc:Al=2.391: _0.597 : _0.162 :4.850, respectively, to prepare a mixed powder for ( _{Tb0.797Y0.199Sc0.004} ) ₃ ( _Al0.97Sc0.03 ) _{5O12 .} Next, TEOS was added as a sintering aid, weighed out so that the amount added was 100 _ppm in terms of _SiO2 , to prepare a raw material.

(Raw materials for Examples 1-4 and Comparative Examples 1-4)
Terbium, scandium and aluminum were weighed out so that the mole numbers were Tb:Sc:Al=2.988: _0.162 :4.850 to prepare a mixed powder for ( _{Tb0.996Sc0.004} ) ₃ ( _Al0.97Sc0.03 ) _{5O12 .} Then, _TEOS was added as a sintering aid, weighed out so that the amount added was 100 ppm in terms of _SiO2 , to prepare a raw material.

Next, each was placed in a polyethylene pot, taking care to prevent mixing, and polyethylene glycol 200 was added as a dispersant to the oxide powder at 0.5% by mass. Each was dispersed and mixed in ethanol using a ball mill. The processing time was 24 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm.

Next, the four types of powder raw materials obtained were each subjected to uniaxial press molding and isostatic pressing at a pressure of 198 MPa to obtain CIP compacts. The obtained compacts were degreased in a muffle furnace at 1,000°C for 2 hours.

As an example, the degreased molded body was placed in a vacuum heating furnace and pre-sintered at 1,600°C for 2 hours under reduced pressure of less than 1.0 x ^10-3 Pa to obtain a total of four types of pre-sintered bodies. At this time, the sintered relative density of each sample was 94% or more and 99% or less. Each pre-sintered body obtained was placed in a HIP furnace made of a carbon heater and subjected to pressure sintering (HIP) treatment under conditions of 196 MPa, 1,600°C, and 3 hours in Ar to obtain a transparent pressure sintered body. When the crystal grains on the surface of this transparent pressure sintered body were observed with an optical microscope, no particles exceeding 5 μm were found. From this, it was determined that the sintered grain size of the pre-sintered body was also 5 μm or less. Next, the HIP-treated pressure sintered body was placed in a vacuum heating furnace again and re-sintered at 1,700°C for 20 hours under reduced pressure of less than 1.0 x ^10-3 Pa. Finally, the re-sintered body was subjected to oxidation annealing at 1,450°C for 30 hours under normal pressure atmosphere to obtain a total of four types of oxidation annealed bodies. After the oxidation annealing treatment, the ceramics all had a colorless and transparent appearance.

As a comparative example, the above-mentioned degreased molded body was placed in a vacuum heating furnace and treated at 1,600°C for 2 hours under a reduced pressure of less than 1.0 x ^10-3 Pa to obtain a total of four types of pre-sintered bodies. At this time, the sintered relative density of each sample was 94% or more and less than 99%. Each of the obtained pre-sintered bodies was placed in a HIP furnace made of a carbon heater and subjected to pressure sintering (HIP) treatment under conditions of 196 MPa, 1,600°C, and 3 hours in Ar to obtain a transparent pressure sintered body. When the crystal grains on the surface of this transparent pressure sintered body were observed with an optical microscope, no particles larger than 5 μm were found. In accordance with the prior art, the HIP-treated pressure sintered body was not subjected to re-sintering treatment and oxidation annealing treatment.

The thus obtained oxidized annealed body (Example) and pressure sintered body (Comparative Example) were each ground into a cylindrical shape with a diameter of 5 mm, and then each was cut to a length of 20 mm, and both end faces were optically polished with an optical surface precision of λ/8 (when the measurement wavelength λ=633 nm).
In addition, for an optically polished sample for evaluating the amount of change in beam quality and the rate of change in beam diameter due to thermal lensing, an anti-reflection film designed to have a center wavelength of 1,064 nm and a reflectance of 0.1% or less was applied to both end faces.
The samples thus obtained were subjected to the following measurements.

(Average sintered particle size D)
The average sintered grain size of the crystal grains of the sample was determined with reference to "Linear Intercept Technique for Measuring Grain Size in Two-Phase Polycrystalline Ceramics", Journal of the American Ceramic Society, 55, 109 (1972) (Non-Patent Document 6). Specifically, a mirror-polished transparent ceramic sample was treated in air at 1,300°C for 6 hours, and the grain boundaries of the thermally etched end surface were observed with an optical microscope to determine the average sintered grain size. At this time, the length of a line drawn arbitrarily on the end surface of the sample was C (μm), the number of particles on this line was N, and the magnification of the image was M, and the value with two significant digits of the value calculated from the following formula was taken as the average sintered grain size D (μm).
D = 1.56C/(M N )

(Beam quality ( ^M2 ) change amount (n/m) evaluation)
The beam quality was measured using a collimated CW laser light with a wavelength of 1,070 nm, an output power of 100 W, and a diameter of 1.6 mm. The beam quality ^M2 value of this laser light was measured using a ModeMaster PC ^M2 beam propagation analyzer manufactured by Coherent. Since the output power was high, the intensity of the laser light was attenuated to 1/1000 or less by a beam separator and then introduced into the beam profiler. The distance between the reference plane (full bezel) of the beam profiler and the sample holder was 1.9 m, and the distance between the reference plane and the collimator was 2.1 m. First, the ^M2 value of the original beam (incident light) was measured, and the value at this time was taken as m. Next, each sample with a length of 20 mm was placed in the optical path, and the ^M2 value of each transmitted light was measured and taken as n. The amount of change in beam quality in the present invention, n/m, was calculated. In addition, samples in which the beam shape of the transmitted beam was greatly deformed due to coarse foreign matter inside the ceramics of the sample were excluded from the evaluation. In addition, in this optical system, m = 1.12.

(Beam diameter change rate evaluation)
The beam diameter change rate was measured using an optical system with the same configuration as that used for the beam quality change measurement evaluation. That is, a collimated CW laser light manufactured by IPG Co., Ltd. with a wavelength of 1,070 nm, an output power of 100 W, and a diameter of 1.6 mm was used. The beam diameter of this laser light was measured at the reference plane (full bezel) of the profiler using a ModeMaster PC ^M2 beam propagation analyzer manufactured by Coherent. First, the beam diameter of the original beam (incident light) was measured, and the value at this time was taken as r _0. Next, each sample with a length of 20 mm was placed in the optical path, and the beam diameter of each transmitted light was measured and taken as r. As the beam diameter change rate in the present invention, (1-r/r ₀ )×100(%) was calculated, and a value of 10% or less was considered to be acceptable, and a value of more than 10% was considered to be unacceptable.
The above results and the measured values of a TGG single crystal (length 20 mm) manufactured by Northrop Grumman as Reference Example 1-1 are summarized in Table 1.

From the above results, the samples (Examples 1-1 to 1-4) in which the average sintered grain size was 22 μm or more and the beam quality change amount (n/m) when a 100 W laser was incident was 1.04 or less all had a beam diameter change rate due to thermal lensing when a 100 W laser was incident of 10% or less. On the other hand, the samples (Comparative Examples 1-1 to 1-4) in which the average sintered grain size was 4.5 μm or less and the beam quality change amount (n/m) when a 100 W laser was incident was 1.06 or more all had a beam diameter change rate due to thermal lensing when a 100 W laser was incident of more than 10%. In other words, it was confirmed that when the average sintered grain size of the paramagnetic garnet-type transparent ceramic is 22 μm or more and the beam quality change amount (n/m) when a 100 W laser is incident is 1.04 or less, a small thermal lens effect can be obtained. In particular, it was confirmed that the beam diameter change rate due to thermal lensing in Examples 1-1 to 1-4 is less than half that of TGG single crystal.

[Example 2]
As Example 2, the cases where x in formula (1) is fixed at 0.40, y = 0.001, z = 0.001, y + z = 0.002, y = 0.04, z = 0.08, y + z = 0.12, and y = 0.05, z = 0.13, y + z = 0.18 are shown. Also, as Reference Example 2-1, the case where y = z = y + z = 0 is shown.
As in Example 1, terbium oxide powder, yttrium oxide powder, and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. were obtained. In addition, tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. and polyethylene glycol 200 liquid manufactured by Kanto Chemical Co., Ltd. were obtained. The purity of the powder raw materials was 99.9% by mass or more, and the purity of the liquid raw materials was 99.999% by mass or more. Using the above raw materials, the mixing ratio was adjusted to prepare the following oxide raw materials having a total of four types of crystal structures with the final composition shown in Table 2.

(Raw material for Example 2-1)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y:Sc:Al=1.797: _1.200 :0.008:4.995, respectively, to prepare a mixed powder for ( _{Tb0.599Y0.4Sc0.001} ) ₃ ( _{Al0.999Sc0.001} ) _{5O12 .} Next _, TEOS was added as a sintering aid, weighed out so that the amount added was 100 _ppm in terms of _SiO2 , to prepare a raw material.

(Materials for Example 2-2)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y:Sc:Al=1.68: _1.20 : _0.52 :4.60, respectively, to prepare _a mixed powder for ( _{Tb0.56Y0.4Sc0.04} ) ₃ ( _Al0.92Sc0.08 ) _5O12 . Next, TEOS was added as a sintering aid in an amount of 100 _ppm calculated as _SiO2 , to prepare a raw material.

(Raw materials for Examples 2-3)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y:Sc:Al=1.65: _1.20 : _{0.80 :} 4.35, respectively, to prepare a mixed powder for ( _{Tb0.55Y0.4Sc0.05} ) ₃ ( _Al0.87Sc0.13 ) _{5O12 .} Next, TEOS was added as a sintering aid, weighed out so that the amount added was 100 ppm in terms of _SiO2 , to prepare a raw material.

(Raw material for Reference Example 2-1)
_A mixed powder for ( _Tb0.6Y0.4 ) _3Al5O12 was prepared by weighing out terbium, yttrium, and aluminum so that the mole numbers of Tb: _Y :Al were 1.8:1.2:5.0, respectively. Then, _TEOS was added as a sintering aid, weighing out the amount of TEOS added so that the amount was 100 ppm in terms of _SiO2 , to prepare a raw material.

Next, each was placed in a polyethylene pot, taking care to prevent mixing, and polyethylene glycol 200 was added as a dispersant to the oxide powder at 0.5% by mass. Each was dispersed and mixed in ethanol using a ball mill. The processing time was 24 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm.

Next, the four types of powdered raw materials obtained were each subjected to uniaxial press molding and isostatic pressing at a pressure of 198 MPa to obtain CIP compacts. The obtained compacts were degreased in a muffle furnace at 1,000°C for 2 hours.

The degreased molded body was placed in a vacuum furnace and pre-sintered at 1,600°C for 2 hours under reduced pressure of less than 1.0× ^10-3 Pa to obtain a total of four types of pre-sintered bodies. At this time, the sintered relative density of each sample was 94% or more and 99% or less. Each pre-sintered body obtained was placed in a HIP furnace made of a carbon heater and subjected to pressure sintering (HIP) treatment under conditions of 196 MPa, 1,600°C, and 3 hours in Ar to obtain a transparent pressure sintered body. When the crystal grains on the surface of this transparent pressure sintered body were observed with an optical microscope, no particles exceeding 5 μm were found. Next, the pressure sintered body was placed in a vacuum furnace again and re-sintered at 1,700°C for 20 hours under reduced pressure of less than 1.0× ^10-3 Pa to obtain a re-sintered body. Finally, the re-sintered body was subjected to oxidation annealing treatment at 1,450°C in air for 30 hours.

The oxidized annealed bodies thus obtained were each ground into a cylindrical shape and optically polished to give mirror finishes to both end faces, to prepare samples with a diameter of 5 mm and a length of 20 mm, in the same manner as in Example 1. In addition, for the optically polished samples for evaluating the amount of change in beam quality and the rate of change in beam diameter due to thermal lensing, antireflection films designed to have a central wavelength of 1,064 nm and a reflectance of 0.1% or less were applied to both end faces.
The samples thus obtained were evaluated in the same manner as in Example 1 for average sintered grain size, amount of change in beam quality, and rate of change in beam diameter due to thermal lensing.
The above results are summarized in Table 2.

From the above results, it was confirmed that the average sintered grain size of the samples of Examples 2-1 to 2-3 was 27 μm or more, the beam diameter quality change (n/m) when a 100 W laser was incident was 1.04 or less, and the beam diameter change rate due to thermal lensing when a 100 W laser was incident was 10% or less, and that paramagnetic garnet-type transparent ceramics with a small thermal lens effect could be obtained even when high-power laser light was incident.

[Example 3]
In Example 2-2, the resintering time was changed to 2 hours (Comparative Example 3-1), 6 hours (Example 3-1), and 40 hours (Example 3-2), but the other conditions were the same as those of Example 2-2 to prepare paramagnetic garnet-type transparent ceramic samples.
The evaluation results are shown in Table 3.

From the above results, the average sintered grain size of the samples of Examples 3-1 and 3-2 was 12 to 40 μm, the beam quality change (n/m) when a 100 W laser was incident was 1.05 to 1.01, and the beam diameter change rate due to thermal lensing when a 100 W laser was incident was 9.7 to 5.9%. In contrast, the average sintered grain size of the sample of Comparative Example 3-1 was 6.9 μm, the beam quality change (n/m) when a 100 W laser was incident was 1.07, and the beam diameter change rate due to thermal lensing when a 100 W laser was incident was 15.7%. In other words, it was confirmed that when the beam quality change (n/m) when a 100 W laser was incident was 1.05, a paramagnetic garnet-type transparent ceramic with a small thermal lens effect was obtained, in which the beam diameter change rate due to thermal lensing when a 100 W laser was incident was 10% or less.

[Example 4]
In Example 2-2, the temperature of the oxidation annealing treatment was set to 1,300° C. (Comparative Example 4-1), 1,400° C. (Example 4-1), and 1,500° C. (Example 4-2), and other conditions were the same as those in Example 2-2 to prepare samples of paramagnetic garnet-type transparent ceramics.
The evaluation results are shown in Table 4.

From the above results, the average sintered grain size of the samples of Examples 4-1 and 4-2 was 29 to 30 μm, the beam quality change (n/m) when a 100 W laser was incident was 1.02 to 1.01, and the beam diameter change rate due to thermal lensing when a 100 W laser was incident was 6.5 to 6.3%. In contrast, the average sintered grain size of the sample of Comparative Example 4-1 was 30 μm, but the beam quality change (n/m) when a 100 W laser was incident was 1.06, and the beam diameter change rate due to thermal lensing when a 100 W laser was incident was 13.8%. In other words, it was confirmed that when the oxidation annealing temperature was 1,400°C or higher, a paramagnetic garnet-type transparent ceramic with a small thermal lens effect was obtained, in which the beam diameter change rate due to thermal lensing when a 100 W laser was incident was 10% or less.

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

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

Claims (12)

The following formula (1)
(Tb1 _{-xyYxScy ) 3} (Al1 _{- zScz ) 5O12} (1 ₎
(In the formula, 0≦x<0.45, 0≦y<0.08, 0≦z<0.2, and 0.001<y+z<0.20.)
a sintered body of Tb-containing rare earth aluminum garnet represented by the formula (1) and having both end faces finished to an optical mirror surface, each end face being provided with an anti-reflection film, and being in the shape of a cylinder or prism of 20 mm in length; wherein when a laser beam of 1,070 nm in wavelength and having a laser intensity of ¹⁰⁰ W and a beam quality M2 value of m (1 < m ≦ 1.2) is incident on the sintered body, and the beam quality ^M2 value of the transmitted light is n, the ratio n/m is 1.05 or less. The paramagnetic garnet-type transparent ceramic according to claim 1, in which the rate of change in beam diameter due to thermal lensing is 10% or less when laser light having a wavelength of 1,070 nm and an optical path length of 20 mm is incident with a beam diameter of 1.6 mm and an incident power of 100 W. 3. The paramagnetic garnet-type transparent ceramic according to claim 1, further comprising more than 0% by mass and not more than 0.1% by mass of _SiO2 as a sintering aid. A paramagnetic garnet-type transparent ceramic according to any one of claims 1 to 3, having an average sintered grain size of 10 μm or more and 40 μm or less. A magneto-optical device constructed using the paramagnetic garnet-type transparent ceramics described in any one of claims 1 to 4. The magneto-optical device according to claim 5 is an optical isolator that has the paramagnetic garnet-type transparent ceramic as a Faraday rotator and has polarizing materials in front of and behind the optical axis of the Faraday rotator and can be used in the wavelength range of 0.9 μm to 1.1 μm. A method for producing the paramagnetic garnet-type transparent ceramics according to any one of claims 1 to 4, comprising the step of:
(Tb1 _{-xyYxScy ) 3} (Al1 _{- zScz ) 5O12} (1 ₎
(In the formula, 0≦x<0.45, 0≦y<0.08, 0≦z<0.2, and 0.001<y+z<0.20.)
a Tb-containing rare earth aluminum garnet sintered body represented by the formula (1) is pressure-sintered, the pressure-sintered body is heated to a temperature higher than that of the pressure-sintered body, and the resintered body is subjected to an oxidation annealing treatment in an oxidation atmosphere of 1,400° C. or higher. 8. The method for producing a paramagnetic garnet-type transparent ceramic according to claim 7, wherein the re-sintering is performed under a reduced pressure of less than 1.0×10 ⁻³ Pa. The method for producing paramagnetic garnet-type transparent ceramics described in claim 7 or 8, wherein the sintered body before the pressure sintering is densified to a relative density of 94% or more by pre-sintering. 10. The method for producing a paramagnetic garnet-type transparent ceramic according to claim 9, wherein the pre-sintering is performed under a reduced pressure of less than 1.0×10 ⁻³ Pa. A method for producing paramagnetic garnet-type transparent ceramics as described in any one of claims 7 to 10, in which the sintered body before the pressure sintering has an average sintered grain size of 5 μm or less. A method for producing a paramagnetic garnet-type transparent ceramic according to any one of claims 7 to 11, in which, after the oxidation annealing treatment, both end faces are optically mirror-finished, and then an anti-reflection film is formed on each of the end faces.

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