Transparent ceramics for magneto-optical elements, and magneto-optical elements

Abstract

The transparent ceramic for a magneto-optical element of the present invention contains: a paramagnetic garnet-type composite oxide that contains terbium, lutetium, and aluminum and that is represented by formula (1); and from 100 mass ppm to 1000 mass ppm of Si as a sintering aid.　(1) (Tb1-xLux)3Al5O12 (where 0.05≦x≦0.45.)　The magneto-optical element of the present invention comprises the transparent ceramic for a magneto-optical element as a Faraday rotator, and is an optical isolator that can be utilized in a wavelength band from 0.9 μm to 1.1 μm, and that is provided with a polarizer 120 and an analyzer 130 in front of and behind the optical axis 102 of the Faraday rotator.

Description

[Technical Field]

[0001] The present invention relates to transparent ceramics for magneto-optical elements and magneto-optical elements, and more particularly to transparent ceramics for magneto-optical elements containing a paramagnetic garnet-type complex oxide containing terbium, which is suitable for constituting magneto-optical elements such as optical isolators, and magneto-optical elements using this transparent ceramics for magneto-optical elements. [Background technology]

[0002] In recent years, as higher output has become possible, the use of laser processing machines using fiber lasers has become increasingly common. However, when external light enters the laser light source built into the laser processing machine, the resonance state becomes unstable, causing the oscillation state to be disrupted. In particular, when the oscillated light is reflected by the optical system along the way and returns to the light source, the oscillation state is significantly disrupted. To prevent this, an optical isolator is usually installed before the light source.

[0003] An optical isolator consists of a Faraday rotator, a polarizer placed on the light input side of the Faraday rotator, and an analyzer placed on the light output side of the Faraday rotator. The Faraday rotator is used by applying a magnetic field parallel to the light's direction of propagation. The polarization of the light rotates in only one direction, whether it is traveling forward or backward through the Faraday rotator. Furthermore, the Faraday rotator's length is adjusted to rotate the polarization of the light exactly 45 degrees. If the polarization planes of the polarizer and analyzer are offset 45 degrees from the direction of the forward light's rotation, the forward light's polarization will be identical at the polarizer and analyzer positions, allowing it to pass through. On the other hand, the backward light's polarization will be rotated 45 degrees in the opposite direction to the polarizer's polarization, which is offset 45 degrees from the analyzer position. In this case, the polarization of the returning light at the polarizer position will be offset by 45 degrees - (-45 degrees) = 90 degrees from the polarizer's polarization, preventing it from passing through the polarizer. In this way, it functions as an optical isolator that transmits and emits forward-moving light and blocks backward-moving returning light.

[0004] The material used for the Faraday rotator that constitutes the optical isolator has traditionally been TGG crystal (Tb3Ga5O 12 ) and TSAG crystal ((Tb (3-x) Sc x )Sc2Al3O 12 ) is known (Patent Document 1, Patent Document 2). TGG crystals are currently widely used in standard fiber laser devices, but due to their high absorption coefficient, the laser power that can be used is said to be limited to 80 W. On the other hand, TSAG crystals have higher rotation angle performance and a lower absorption coefficient than TGG crystals, making them compatible with lasers with higher output than TGG crystals. However, due to the use of large amounts of expensive Sc2O3 and poor production stability, TSAG crystals are not currently a widely used material.

[0005] Other Faraday rotator materials that have been developed include TAG ceramics (Patent Document 3), YTAG ceramics (Non-Patent Document 1), and KTF single crystal (Non-Patent Document 2). 12 ) ceramics have a higher Verdet constant than TGG single crystals and are said to be suitable for high power applications. YTAG ceramics, which replace some of the terbium in TAG ceramics with yttrium, have a lower Verdet constant than TAG ceramics, but because their absorption coefficient is lower than TAG ceramics, they have the potential to reach high power ranges that were previously impossible with TAG ceramics. The last one, KTF (KTb3F 10 ) single crystals have an absorption coefficient that is overwhelmingly lower than other materials, and are said to be capable of handling the highest powers of any Faraday rotator known to date. However, all three types of Faraday rotators have the problem of poor manufacturing stability. In addition, KTF single crystals have a low laser damage threshold, and may be damaged by short-pulse lasers.

[0006] We have previously investigated new magneto-optical materials, such as C-type rare earth (Tb,Y)2O3 (Patent Document 4) and garnet-type (Tb,Y,Sc)3(Al,Sc)5O 12(Patent Document 5) has been developed. The former C-type rare earth Faraday rotator has a high Verdet constant, but its absorption coefficient is also high, limiting its use in high-power applications. On the other hand, the latter garnet type has a Verdet constant 0.9 to 1.3 times that of a TGG single crystal, but its absorption coefficient is lower than that of a TGG single crystal, making it considered optimal for high-power applications. Furthermore, by adding a small amount of Sc, manufacturing stability is improved compared to YTAG.

[0007] However, (Tb,Y,Sc)3(Al,Sc)5O 12 Although YTAG has a small absorption coefficient, it also has low thermal conductivity, making it unsuitable for high-power applications exceeding 200W. When a Faraday rotator is irradiated with high-power laser light, a temperature distribution occurs in the Faraday rotator, resulting in a thermal lens effect. The thermal lens effect depends on the absorption coefficient and thermal conductivity; the smaller the absorption coefficient, the better, and the higher the thermal conductivity. A material with low absorption comparable to YTAG and high thermal conductivity is desirable, but until now, such a material has not existed.

[0008] Thermal conductivity is a value specific to a material and is affected by factors such as the crystal structure, composition, defects, and grain boundaries. In the case of transparent ceramics, the grain boundaries are very thin, less than 1 nm, so the effect of the grain boundaries on thermal conductivity is small. In addition, highly transparent ceramics that are not colored are thought to have very few defects, and in fact, at room temperature, thermal conductivity at the same level as that of a single crystal can be obtained. Therefore, it is the crystal structure and composition that determine thermal conductivity.

[0009] Non-patent literature 3 and 4 are examples of studies into the effect of composition on thermal conductivity. These non-patent literatures show that, for example, doping yttrium aluminum garnet with other rare earth elements causes a rapid drop in thermal conductivity. As such, it is clear that a single composition is preferable to increase thermal conductivity, and that adding so-called additives can cause a drop in thermal conductivity.

[0010] Non-Patent Document 5 describes a formula for the change in thermal conductivity when an additive is added. This formula shows that the thermal conductivity of an additive is affected by the thermal conductivity of each additive, the composition ratio, and the atomic weight difference of the constituent atoms. In the examples so far, rare earth elements are substituted for yttrium, resulting in an atomic weight difference of approximately 70 to 80. Therefore, it is thought that the thermal conductivity will drop sharply.

[0011] On the other hand, if the difference in atomic weight affects thermal conductivity, it is thought that the smaller the difference in atomic weight, the smaller the decrease in thermal conductivity. Non-Patent Document 5 shows the thermal conductivity when ytterbium is added to lutetium aluminum garnet. Since the atomic weight difference between lutetium and ytterbium is small, at 2, it can be seen that the decrease in thermal conductivity is kept to a minimum. Therefore, (Tb,Y,Sc)3(Al,Sc)5O 12 In order to increase the thermal conductivity of , it is thought that changing Y to Lu is effective.

[0012] Non-patent document 6 states that (Tb 0.72 Lu 0.28 )3AlO 12 The Verdet constant and thermal conductivity of the single crystal are shown. The Verdet constant is slightly lower than that of TGG single crystal, and the thermal conductivity is 1.4 times that of TGG single crystal. To increase the Verdet constant, the proportion of Tb must be increased, but it is known that it is difficult to create a single crystal with a composition other than the above (Non-Patent Document 7). Therefore, no LuTAG-based Faraday rotator has been found to date that has a Verdet constant equal to or greater than that of TGG single crystal, and a thermal conductivity equal to or greater than that of TGG single crystal.

[0013] Patent Document 6 describes (Tb, Lu, Sc)3(Al, Sc)5O 12 However, the atomic weight difference between Sc and Tb and Lu is large, which causes a significant decrease in thermal conductivity. In Patent Document 6, the amount of Sc added is at least 0.10 wt% (metallic Sc ​​equivalent), which is high, and there are concerns about a decrease in thermal conductivity. Therefore, even with the composition in Patent Document 6, the limit is 150 W, and further increases in power are essential. [Prior art documents]

Charter Documents

[0014] [Patent Document 1] Japanese Patent Publication No. 2011-213552 [Patent Document 2] Japanese Patent Publication No. 2002-293693 [Patent Document 3] Patent No. 6438588 [Patent Document 4] Patent No. 5397271 [Patent Document 5] Patent No. 6879264 [Patent Document 6] Patent No. 6881390

Non-licensed literature

[0015]

Non-patent document 1

Non-patent document 2

Non-patent document 3

Non-patent document 4

[0016] The present invention has been made in view of the above circumstances, and aims to provide a transparent ceramic for magneto-optical elements, which contains a paramagnetic garnet-type composite oxide containing terbium and lutetium and can be mounted in a high-power laser of 200 W or more, and a magneto-optical element. [Means for solving the problem]

[0017] In order to achieve the above object, one aspect of the present invention is a transparent ceramic for a magneto-optical element, which comprises a paramagnetic garnet-type composite oxide containing terbium, lutetium, and aluminum, represented by the following formula (1), and 100 ppm by mass or more and 1000 ppm by mass or less of Si as a sintering aid: (Tb 1‐x Lu x )3AlO 12 ···(1) (wherein 0.05≦x≦0.45.)

[0018] The transparent ceramic for a magneto-optical element may further contain 1000 mass ppm or less of Sc as a sintering aid.

[0019] The transparent ceramic for magneto-optical elements preferably has a thermal conductivity of 4.2 W / (m·K) or more at room temperature.

[0020] The transparent ceramic for magneto-optical elements preferably has an extinction ratio of 35 dB or more.

[0021] The above transparent ceramics for magneto-optical devices have a loss coefficient of 0.002 cm at 1064 nm. -1 It is preferable that:

[0022] The transparent ceramic for a magneto-optical element may have a ceramic crystal grain size of 1 μm or more and 40 μm or less.

[0023] The transparent ceramic for magneto-optical elements preferably has an extinction ratio of 35 dB or more when irradiated with a laser having a wavelength of 1064 nm and an output of 200 W.

[0024] The transparent ceramic for magneto-optical elements preferably has a Verdet constant of 30 Rad / (T·m) or more.

[0025] Another aspect of the present invention is a magneto-optical element that is constructed using the transparent ceramics for magneto-optical elements described above.

[0026] The magneto-optical element may be an optical isolator that has the transparent ceramic for magneto-optical elements 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. [Effects of the Invention]

[0027] According to the present invention, since the above-mentioned paramagnetic garnet-type composite oxide and sintering aid are contained, the thermal conductivity is high compared to conventional Faraday rotators, and the absorption coefficient is also in the best group. Therefore, even when irradiated with laser light of 200 W or more, the extinction ratio is 35 dB or more, which means that it is applicable to high-power laser applications. It is possible to provide a truly practical transparent ceramic for magneto-optical elements, and a magneto-optical element using the same. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a cross-sectional view showing a configuration example of an optical isolator using the transparent ceramic for a magneto-optical element according to the present invention as a Faraday rotator. DETAILED DESCRIPTION OF THE INVENTION

[0029] [Transparent ceramics for magneto-optical devices] First, an embodiment of a transparent ceramic for a magneto-optical element according to the present invention will be described, which contains a paramagnetic garnet composite oxide represented by the following formula (1) and sintering aids containing 100 ppm by mass to 1000 ppm by mass of Si and 1000 ppm by mass or less of Sc: (Tb 1-x Lu x )3AlO 12 ···(1) (wherein 0.05≦x≦0.45)

[0030] In formula (1), terbium (Tb) is the material with the largest Verdet constant of all paramagnetic elements except for iron (Fe). In particular, when contained in an oxide with a garnet structure, terbium is completely transparent at a wavelength of 1064 nm, making it the most suitable element for use in optical isolators in this wavelength range.

[0031] Lutetium (Lu) is an element that can be preferably used in this patent because it forms a garnet phase more stably than a perovskite phase when combined with aluminum to form a composite oxide. Furthermore, compared to other rare earth elements, lutetium does not have characteristic absorption (ff transition) in the visible to near-infrared region, and its atomic weight difference with terbium is only 16, making it an ideal element to add to develop a Faraday rotator with high thermal conductivity.

[0032] In the B site of formula (1), aluminum (Al) has the smallest ionic radius among trivalent ions that can stably exist in oxides with a garnet structure. It is the element that can minimize the lattice constant of Tb-containing paramagnetic garnet-type oxides. Reducing the lattice constant of the garnet structure without changing the Tb content is preferable because it increases the Verdet constant per unit length. Furthermore, aluminum is a light metal, and its diamagnetic properties are weaker than those of gallium. This is expected to relatively increase the magnetic flux density generated inside the Faraday rotator, which is also preferable because it increases the Verdet constant per unit length. In fact, the Verdet constant of TAG ceramics is 1.25 to 1.5 times that of TGG. Therefore, even when the relative concentration of terbium is reduced by substituting some of the terbium ions with lutetium ions, the Verdet constant per unit length can be maintained at or slightly lower than that of TGG, making it a preferred constituent element in the present invention.

[0033] In formula (1), the range of x is preferably 0.05≦x≦0.45, with 0.1≦x≦0.4 being more preferable. If x is less than 0.05, the effect of substituting a portion of the terbium with lutetium is not obtained, resulting in conditions essentially identical to those for producing TAG. This makes it difficult to consistently produce high-quality ceramic sintered bodies with low scattering and absorption, which is undesirable. Furthermore, if x exceeds 0.45, the Verdet constant at a wavelength of 1064 nm falls below 30 rad / (T·m), which is undesirable. Furthermore, if the relative concentration of terbium is excessively low, the total length required to rotate a 1064 nm laser beam by 45 degrees exceeds 25 mm, which is longer than that of a TGG single crystal, which is undesirable.

[0034] The transparent ceramic for magneto-optical devices of the present invention contains a composite oxide represented by the above formula (1) as its main component. Furthermore, Si, which acts as a sintering aid, is added as a secondary component in a range of 100 ppm by mass to 1000 ppm by mass. Adding Si as a sintering aid in this amount suppresses the precipitation of perovskite-type heterophases, thereby ensuring the transparency of the transparent ceramic for magneto-optical devices. Furthermore, the Si added in this amount vitrifies during sintering at 1400°C or higher, resulting in a liquid-phase sintering effect and promoting the densification of garnet-type ceramic sintered bodies. However, adding Si in excess of 1000 ppm by mass results in an extinction ratio below 35 dB when a 200 W laser beam with a wavelength of 1064 nm is irradiated onto a 20 mm long (optical path length) transparent ceramic for magneto-optical devices. Therefore, the amount of Si added must be 1000 ppm by mass or less.

[0035] The sintering aid Si can be added as an Si-based inorganic compound such as SiO2 or an Si-based polymer compound such as tetraethoxysilane (TEOS). In this case, it is preferable to adjust the amount added so that the amount is 100 mass ppm or more and 1000 mass ppm or less in terms of metallic Si. The lower limit of the amount of Si added is preferably 200 mass ppm or more, more preferably 500 ppm or more. The upper limit of the amount of Si added is preferably 800 mass ppm or less.

[0036] In addition to Si, Sc may be added as a sintering aid at 1000 ppm by mass or less. Adding Sc, like Si, suppresses the precipitation of perovskite-type heterophases, improving the transparency of transparent ceramics for magneto-optical devices. Furthermore, since Sc is an element that can form a solid solution in both the A site and the B site of a garnet-type structure, the greater the amount added, the easier it is to manufacture transparent ceramics. However, the atomic weight of Sc is 44.96, which is a large difference compared to the atomic weights of the main components Tb and Lu. Therefore, adding a large amount leads to a deterioration in thermal conductivity. Therefore, adding more than 1000 ppm by mass is not recommended.

[0037] Sc can be added as an Sc-based inorganic compound such as Sc2O3. The lower limit of the amount of Sc added is preferably 100 ppm by mass or more. The upper limit of the amount of Sc added is preferably 800 ppm or less. The total amount of Si and Sc is preferably 1000 ppm by mass or less.

[0038] In this specification, the term "added amount" refers to the amount of sintering aid intentionally added. Therefore, an added amount of 0 ppm by mass indicates that no sintering aid is intentionally added, and does not include cases where the corresponding element is contained as an impurity in the raw material powder.

[0039] Furthermore, "containing as a main component" means that the transparent ceramic for magneto-optical elements contains 90% by mass or more of the composite oxide represented by the above formula (1). The content of the composite oxide represented by formula (1) is preferably 99% by mass or more, more preferably 99.9% by mass or more, and more preferably 99.99% by mass or more.

[0040] The transparent ceramic for magneto-optical devices of the present invention is composed of the above-mentioned main and subcomponents, and may further contain other elements, such as rare earth elements such as yttrium (Y) and cerium (Ce), or various impurities such as sodium (Na), calcium (Ca), magnesium (Mg), phosphorus (P), tungsten (Ta), and molybdenum (Mo).

[0041] The content of other elements is preferably 10 parts by mass or less, more preferably 0.1 part by mass or less, and particularly preferably 0.001 part by mass or less, when the total amount of Tb is 100 parts by mass.

[0042] The transparent ceramic for magneto-optical devices of the present invention has a colorless and transparent appearance, and its loss coefficient at a wavelength of 1064 nm with an optical path length of 20 mm is 0.002 cm. -1 The lower limit of the loss is not particularly limited, but for example, it is 0.0001 cm -1 In the present invention, the "loss factor" is a coefficient that indicates the performance of a transparent ceramic, and is expressed by the following formula: Loss coefficient [cm -1 ]=10×log(I / I0) / (sample length [cm]) (In the formula, I is the transmitted light intensity (the intensity of light transmitted in a straight line through a 20 mm long sample), and I0 is the incident light intensity.)

[0043] The thermal conductivity of the paramagnetic garnet-type ceramics of the present invention is 4.2 W / (m K) or more. Thermal conductivity measurement methods are broadly divided into steady-state and transient methods. Steady-state methods include the heat flow meter method, and transient methods include the laser flash method, cyclic heating method, and hot wire method. In the present invention, either method may be used. Among these, the laser flash method is the most preferred method because it requires a smaller sample size than other measurement methods and facilitates the production of truly transparent ceramics. The upper limit of the thermal conductivity is not particularly limited, but it may be, for example, 8.0 W / (m K) or less.

[0044] The transparent ceramic for magneto-optical devices of the present invention preferably has a Verdet constant at a wavelength of 1064 nm of 30 rad / (T·m) or more, and more preferably 36 rad / (T·m) or more. A Verdet constant of 36 rad / (T·m) or more is particularly preferable because it allows for easy replacement with existing TGG single crystals without changing the component design. The upper limit of the Verdet constant is not particularly limited, but may be, for example, 60 rad / (T·m) or less.

[0045] The transparent ceramic for magneto-optical devices of the present invention has an extinction ratio of 35 dB or more as a ceramic device alone. Within the garnet composition range of the present invention, material defects such as strain and point defects are dramatically reduced, so the extinction ratio of the material device alone is stably controlled to 35 dB or more. The upper limit of the extinction ratio is not particularly limited, but may be, for example, 50 dB or less.

[0046] Furthermore, the transparent ceramic for magneto-optical elements of the present invention has an extinction ratio of 35 dB or more when a 1064 nm laser beam is incident at an input power of 200 W with an optical path length of 20 mm. When irradiated with a high-power laser, heat is generated in accordance with the amount of light absorbed by the transparent ceramic, and this heat generates a temperature distribution according to the thermal conductivity, resulting in thermal birefringence. Therefore, even if the extinction ratio is 35 dB or more at low power (e.g., 1 W), the extinction ratio decreases with high-power irradiation. An extinction ratio below 35 dB at high power is undesirable because it is likely to damage the laser light source. The upper limit of the extinction ratio at high power is not particularly limited, but may be, for example, 50 dB or less.

[0047] The changes in the focal position and diameter of the output beam are also affected by the same heat generation and thermal conductivity as the extinction ratio during high-power irradiation. These are called the thermal lens effect, and are caused by a refractive index distribution due to the temperature distribution caused by heat generation. In the case of a change in the focal position of the output beam, it is preferable that the rate of change between the beam focal position when no sample is placed and the beam focal position when the beam passes through the sample is less than 10%. In the case of a change in the diameter of the output beam, it is preferable that the rate of change between the initial beam diameter and the beam diameter when the beam passes through the sample is less than 10%. In the present invention, pass / fail judgment during a high-power irradiation test can be made based on the change in the extinction ratio, the change in the focal position, or the change in the beam diameter.

[0048] [Method for manufacturing transparent ceramics for magneto-optical devices] Next, we will explain one embodiment of a method for producing transparent ceramics for magneto-optical devices according to the present invention. In this embodiment, raw material powder is first press-molded into a predetermined shape, debound, and then sintered to produce a sintered body with a relative density of at least 95%. Hot isostatic pressing (HIP) is preferably performed as a subsequent step. If HIP is performed directly, the transparent ceramics for magneto-optical devices will be reduced, resulting in slight oxygen deficiency. Therefore, it is preferable to perform a slight-oxidizing HIP treatment or an annealing treatment in an oxidizing atmosphere after HIP treatment to recover the oxygen deficiency. This allows us to obtain transparent ceramics for magneto-optical devices containing a transparent paramagnetic garnet-type composite oxide with no defect absorption. The raw materials and each step are described below.

[0049] (1. Raw materials) As the raw materials used in this embodiment, metal powders of terbium, lutetium, scandium, and aluminum, aqueous solutions of nitric acid, sulfuric acid, uric acid, and the like, and oxide powders of the above elements can be suitably used.

[0050] Preparation of oxide powders for transparent ceramics can be broadly divided into two types: breakdown and build-up, but there are no particular limitations as long as they can be made transparent. The build-up method is a method of preparing oxide powders for transparent ceramics by pulverizing various powders, which has the advantage of being productive but has issues with compositional uniformity. On the other hand, the build-up method is a method of obtaining powder by nucleation and grain growth from a solution of various elements, which has the great advantage of being productive but has problems with productivity and reproducibility. In the present invention, there are no particular limitations as long as they can be made highly transparent.

[0051] In the breakdown method, it is most preferable to weigh out the various oxide powders and sintering aids and then perform wet or dry pulverization. The purity of the various oxide powders is preferably 99.9% or higher, more preferably 99.99% or higher. Furthermore, the primary particle size of the various powders is preferably 0.05 μm to 100 μm. A particle size less than 0.05 μm is undesirable because it is difficult to control the uniformity of the ceramic due to high particle cohesion, and rapid densification occurs during the sintering process, making it difficult to control the release of air bubbles. Furthermore, a particle size greater than 100 μm is unsuitable because it cannot be pulverized to fine particles by wet or dry pulverization. The pulverization process can be either wet or dry, and any of ball milling, bead milling, jet milling, and homogenizer processing can be suitably used. The pulverization process is preferably performed until the median value (D50) of the primary particle size distribution is less than 1 μm.

[0052] In the build-up method, a powder is preferably synthesized from a solution containing various elements (which may contain not only the main components but also sintering aids) and then fired at 1300°C or less. Precursors of the various elements are not particularly limited, but examples include chlorides, nitrates, carbonates, and sulfates. The powder synthesis method is not particularly limited, as long as highly transparent ceramics can be produced, but examples include coprecipitation, complex polymerization, and homogeneous precipitation. Regardless of the synthesis method, the primary particle diameter is preferably 0.05 μm or more, and the shape is not particularly limited. Depending on the properties of the obtained powder, it may be subjected to a wet or dry pulverization process after firing; as with the breakdown method, the pulverization method is not particularly limited.

[0053] In order to improve the stability of the production yield and the quality of the resulting product, organic additives such as dispersants, binders, plasticizers, lubricants, etc. may be added. In this case, the most stable method is to perform wet grinding and add the additives to the slurry. There are no particular restrictions on the amount of additives added, as long as the desired properties are obtained.

[0054] (2. Molding) In the manufacturing method of this embodiment, a typical press molding process can be suitably used. Specifically, a typical press process in which a material is filled into a mold and pressed from a certain direction, or a cold isostatic pressing (CIP) process or warm isostatic pressing (WIP) process in which the material is sealed in a deformable, waterproof container and pressed with hydrostatic pressure can be suitably used. The applied pressure can be adjusted appropriately while checking the relative density of the resulting green body, and is not particularly limited. Alternatively, a hot press process, spark plasma sintering process, or microwave heating process, which not only performs the molding process but also sintering at once, can also be suitably used. Furthermore, instead of press molding, green bodies can also be produced by slip casting. Other molding methods, such as pressure casting, centrifugal casting, and extrusion molding, can also be employed by optimizing the shape and size of the starting oxide powder and the combination of various organic additives.

[0055] (3. Degreasing) In the manufacturing method of this embodiment, a normal debinding process can be suitably used. That is, a temperature-raising debinding process using a heating furnace can be performed. Furthermore, there are no particular restrictions on the type of atmospheric gas used, and air, oxygen, hydrogen, etc. can be suitably used. The debinding temperature is also preferably 270°C or higher and 1000°C or lower. If the temperature is lower than 270°C, it is difficult to completely remove the organic additives. On the other hand, if the temperature is higher than 1000°C, densification will occur before the sintering process, making it difficult to obtain a transparent sintered body with low scattering.

[0056] (4. Sintering) In the manufacturing method of this embodiment, a general sintering process can be suitably used. That is, a heat sintering process such as a resistance heating method or an induction heating method can be suitably used. The atmosphere used is not particularly limited, and sintering can be performed in various atmospheres such as an inert gas, oxygen gas, hydrogen gas, or helium gas, or under reduced pressure (vacuum). However, sintering in a vacuum is most preferable, as it allows for a high degree of transparency.

[0057] The sintering temperature in the sintering step of this embodiment is preferably 1400 to 1780° C., particularly preferably 1450 to 1750° C. A sintering temperature in this range is preferable because it promotes densification while suppressing heterophase precipitation.

[0058] In the sintering process of this embodiment, a sintering holding time of several hours is sufficient, but the relative density of the sintered body must be densified to at least 93% or more. If it is less than 93%, it is not possible to obtain a transparent body in the subsequent HIP process, so this is unsuitable. The sintering holding time must be controlled so that the relative density of the sintered body is 93% or more.

[0059] The crystal grain size in the sintering process is preferably 1 μm or more and 40 μm or less, and more preferably 5 μm or more and 35 μm or less. A crystal grain size below 1 μm is unsuitable because it results in poor transparency due to minute compositional differences between the crystal grains. Also, a crystal grain size of 40 μm or more is not preferable because there is a risk of desulfurization occurring during the subsequent polishing process. It is preferable to set the sintering temperature and sintering holding time so that the crystal grain size falls within the appropriate range.

[0060] The average particle size of the sintered particles (average sintered particle size) is determined by measuring the particle size of the sintered particles of the target sintered body using a metallurgical microscope, and is determined in detail as follows. That is, a metallurgical microscope is used to photograph the pre-sintered body in reflection mode using a 50x objective lens to capture a reflection image of the sintered body surface. Specifically, the entire optically effective area of ​​the target sintered body is photographed, taking into account the effective image size of the objective lens, and the photographed images are then analyzed. First, a diagonal line is drawn on each photographed image, and the total number of sintered particles crossed by the diagonal line is counted. The diagonal line length is then divided by this total count to define the average particle size of the sintered particles in that image. The average particle sizes of each photographed image read in the analysis process are then added together, and the result divided by the number of photographs is used to determine the average particle size of the target sintered body.

[0061] (5. Hot Isostatic Pressing (HIP)) In the manufacturing method of this embodiment, an additional step of performing a hot isostatic pressing (HIP) treatment can be provided after the sintering step.

[0062] The type of pressurized gas medium that can be used here is preferably an inert gas such as argon or nitrogen, or Ar-O2. The pressure applied by the pressurized gas medium is preferably 50 to 300 MPa, 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, further improvement in transparency cannot be obtained even if the pressure is increased. For convenience, the applied pressure is preferably 196 MPa or less, which can be processed using a commercially available HIP device.

[0063] The treatment temperature (predetermined holding temperature) is set in the range of 1000 to 1780°C, preferably 1100 to 1730°C. Heat treatment temperatures above 1780°C are undesirable because they increase the risk of oxygen deficiency. Heat treatment temperatures below 1000°C are undesirable because they do not provide much transparency improvement to the sintered body. There are no particular restrictions on the holding time at the heat treatment temperature, but holding for too long is undesirable because it increases the risk of oxygen deficiency. Typically, the holding time is preferably set in the range of 1 to 3 hours.

[0064] The heater material, heat insulating material, and processing vessel used in the HIP process are not particularly limited, but graphite, molybdenum (Mo), tungsten (W), and platinum (Pt) are suitable, and yttrium oxide and gadolinium oxide are also suitable for the processing vessel. Platinum (Pt) can be used for the heater material, heat insulating material, and processing vessel, and Ar-O can be used as the pressurized gas medium, which is particularly preferred when the processing temperature is 1500°C or lower, since this prevents oxygen deficiency during the HIP process.

[0065] When the processing temperature is 1500°C or higher, graphite is preferred as the heater material and heat insulating material. In this case, it is preferable to select graphite, molybdenum (Mo), or tungsten (W) as the processing container, and then select either yttrium oxide or gadolinium oxide as a double container inside, and then fill the container with an oxygen release material, as this will minimize the amount of oxygen deficiency that occurs during HIP processing.

[0066] After the HIP treatment, the sintering treatment may be carried out again to further reduce scattering, and then the HIP treatment may be carried out again. There is no particular limitation on the number of times the sintering treatment and the HIP treatment are carried out, and they may be repeated until low scattering is achieved.

[0067] (6. Annealing treatment) In the manufacturing method of this embodiment, oxygen deficiency may occur in the obtained transparent ceramic sintered body after the HIP treatment, resulting in a faint light gray appearance. In such cases, it is preferable to perform an annealing treatment (oxygen deficiency recovery treatment) in an oxygen atmosphere or in the air at a temperature equal to or lower than the HIP treatment temperature, typically 1000 to 1500°C. In this case, the holding time is not particularly limited, but should be long enough to recover the oxygen deficiency, preferably 10 hours or more, and more preferably 20 hours or more.

[0068] By this oxygen annealing treatment, even transparent ceramic sintered bodies that have taken on a faintly light gray appearance in the HIP treatment process can be transformed into colorless, transparent, and defect-free transparent ceramic bodies for magneto-optical devices.

[0069] (7. Optical polishing) In the manufacturing method of this embodiment, it is preferable to optically polish both end faces of the transparent ceramic for magneto-optical elements that have undergone the above series of manufacturing steps, which are located on the optically utilized axis. In this case, the optical surface precision is preferably λ / 2 or less, and particularly λ / 8 or less, when the measurement wavelength λ is 633 nm. In addition, it is also possible to further reduce optical loss by appropriately forming an anti-reflection film on the optically polished surface.

[0070] In this way, it is possible to provide a transparent ceramic for magneto-optical devices that contains as its main component a paramagnetic garnet-type composite oxide containing terbium and lutetium, has a thermal conductivity of 4.2 W / mK or more at room temperature, has a Verdet constant of 30 rad / (T·m) or more at a wavelength of 1064 nm, and has an extinction ratio of 35 dB or more when a 1064 nm laser beam is incident on it with an optical path length of 20 mm, a beam diameter of 1.6 mm, and an incident power of 200 W.

[0071] [Magneto-optical elements] Next, an embodiment of the magneto-optical element according to the present invention will be described. The magneto-optical element according to the present invention is constructed using the transparent ceramic for magneto-optical elements described above. The transparent ceramic for magneto-optical elements described above can be used as a magneto-optical material. Specifically, it is preferable to construct and use a magneto-optical element by applying a magnetic field parallel to the optical axis of the transparent ceramic for magneto-optical elements and then setting a polarizer and an analyzer so that their optical axes are shifted by 45 degrees from each other. In particular, the transparent ceramic for magneto-optical elements according to the present invention is suitably used as a Faraday rotator for an optical isolator for wavelengths of 0.9 to 1.1 μm.

[0072] FIG. 1 is a cross-sectional view schematically illustrating an example of an optical isolator, which is a magneto-optical element having a Faraday rotator made of the transparent ceramic for magneto-optical elements of the present invention as an optical element. As shown in FIG. 1, the optical isolator 100 includes, inside its housing 102, a Faraday rotator 110 made of the above-described transparent ceramic for magneto-optical elements, a polarizer 120 made of a polarizing material, and an analyzer 130. These are arranged in this order along the optical axis 104 of the Faraday rotator: polarizer 120, Faraday rotator 110, analyzer 130. The polarization plane of the polarizer 120 and the polarization plane of the analyzer 130 are arranged so that the relative angle is 45°. The optical isolator 100 also includes a magnet 140 around the Faraday rotator 110 in the housing 102 for applying a magnetic field to the Faraday rotator 110.

[0073] Such an optical isolator 100 can be suitably used in an industrial fiber laser device (not shown). The optical isolator can prevent 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. [Example]

[0074] The present invention will be described in detail below with reference to examples. [Examples 1 to 4, Comparative Examples 1 to 4] (Manufacturing transparent ceramics) Terbium oxide powder (Tb4O7, manufactured by Shin-Etsu Chemical Co., Ltd.) with a purity of 99.999% or higher, lutetium oxide powder (Lu2O3, manufactured by Shin-Etsu Chemical Co., Ltd.), aluminum oxide powder (Al2O3, manufactured by Taimei Chemical Industry Co., Ltd., grade TM-DAR), tetraethoxysilane (Si(OC2H5)4, manufactured by Kishida Chemical Co., Ltd., hereafter referred to as TEOS) as a sintering aid, and scandium oxide powder (Sc2O3, manufactured by Shin-Etsu Chemical Co., Ltd.) were weighed in predetermined amounts and subjected to wet ball milling using ethanol (manufactured by Kanto Chemical Co., Ltd.) as a dispersion medium. 2 mm alumina balls (manufactured by Nikkato Corporation) were used as the ball mill media. 1 wt% polyvinyl alcohol (manufactured by Kanto Chemical Co., Ltd.) was added as a binder to the resulting slurry, which was then spray-dried to granulate it. The resulting granules were then uniaxially pressed and CIP-treated to form the desired shape, followed by air degreasing in a muffle furnace at 500°C. Then vacuum sintering process (10 -3 The transparent body was subjected to a pressure of 198 MPa and 1600°C, followed by HIP treatment (198 MPa and 1600°C), and then atmospheric annealing at 1450°C for 10 hours. The obtained transparent body was polished and processed to a diameter of 5 mm and a length of 20 mm, with an optical surface precision of λ / 8. Comparative Examples 1 to 4 were made transparent in the same manner as in the Examples, except that the lutetium oxide powder was either not added or was replaced with yttrium oxide powder (YO, manufactured by Shin-Etsu Chemical Co., Ltd.).

[0075] (Loss factor measurement) The loss coefficient was measured by measuring the light intensity when light with a wavelength of 1064 nm was transmitted through a beam diameter of 1 mm using an in-house optical system that included a light source manufactured by NKT Photonics, a power meter manufactured by Gentec, and a Ge photodetector, and was calculated using the following formula. Loss coefficient [cm -1 ]=10×log(I / I0) / (sample length [cm]) (In the formula, I is the transmitted light intensity (the intensity of light transmitted in a straight line through a 20 mm long sample), and I0 is the incident light intensity.)

[0076] (thermal conductivity measurement) Thermal conductivity was measured in accordance with JIS R 1611-1997 (Testing method for thermal diffusivity, specific heat, and thermal conductivity of fine ceramics using the laser flash method). A disc-shaped transparent ceramic sintered body with a diameter of 10 mm and a thickness of 2 mm was prepared, and laser irradiation was performed on one side. The difference in temperature rise between the laser-irradiated surface and the opposite surface was measured, and the thermal diffusivity α was determined using the half-time method. Density ρ was measured using the Archimedes method, and specific heat C was measured using differential scanning thermogravimetry. Thermal conductivity was determined as the product of thermal diffusivity α, density ρ, and specific heat C.

[0077] (Method for measuring extinction ratio) The extinction ratio was measured by transmitting light with a wavelength of 1064 nm through the sample with a large beam diameter of 3 mm using an optical system manufactured in-house using a light source manufactured by NKT Photonics, a collimator lens, a polarizer, a work stage, an analyzer, a power meter manufactured by Gentec, and a Ge photodetector. In this state, the light intensity I0' was measured when the polarization plane of the analyzer was aligned with that of the polarizer, and then the received light intensity I' was measured again with the polarization plane of the analyzer rotated 90 degrees to make it perpendicular to the polarization plane of the polarizer, and the extinction ratio was then calculated using the following formula. Extinction ratio (dB)=-10×log 10 (I' / I0')

[0078] (Extinction ratio evaluation under high power irradiation) The extinction ratio during high-power irradiation was measured with reference to JIS C 5877-2:2012. Measurements were made using a collimated CW laser beam with a linear polarization of 1064 nm wavelength, 200 W output power, and a diameter of 1.6 mm. The sample, PBS, and power meter were placed on the optical axis of this laser beam. First, the PBS was placed parallel to the polarization of the laser beam, and the transmitted light intensity P / / Then, the PBS was placed perpendicular to the polarized light, and the transmitted light intensity P ⊥ The extinction ratio (dB) at an incident power of 200 W was calculated using the following formula: Extinction ratio (dB)=-10×log 10 (P ⊥ / P / / )

[0079] (Isolator included) As shown in Figure 1, each ceramic sample 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. Polarizers were then inserted at both ends of the magnet, and a high-power laser beam with a wavelength of 1064 nm was incident on both end faces using a high-power laser (beam diameter: 1.6 mm) manufactured by IPG Photonics Japan, Inc., to determine the Faraday rotation angle θ. The Faraday rotation angle θ was defined as the angle at which maximum transmittance was achieved when the polarizer on the output side was rotated. The Verdet constant was calculated based on the following equation. The magnitude of the magnetic field (H) applied to the sample was calculated by simulation using the dimensions of the measurement system, the residual magnetic flux density (Br), and the coercivity (Hc) described above. 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).)

[0080] [Table 1]

[0081] Table 1 shows the results of property evaluation of various transparent ceramics. When the A site is substituted with Lu, it can be seen that the thermal conductivity is 4.2 W / mK or higher and the loss factor is reduced. As a result, the extinction ratio at 200 W irradiation is 35 dB or higher. On the other hand, when the A site is substituted with Y, the loss factor is about the same, but the thermal conductivity is significantly reduced, so the extinction ratio at 200 W irradiation is below 35 dB. These results show that Lu is a more preferable element to substitute for the A site than Y, and that it can be used as a Faraday rotator for high power applications exceeding 200 W.

[0082] [Examples 5 to 11, Comparative Examples 5 to 8] Transparent ceramics were fabricated in the same manner as in Example 2, except for varying the amounts of TEOS and Sc2O3 added. The results are shown in Table 2. When the amount of Sc added was 1000 mass ppm or less, there was no sudden decrease in thermal conductivity, and the extinction ratio at 200 W irradiation was 35 dB or higher. However, when Sc was added in amounts greater than 1000 mass ppm, the stability of transparency improved, but the thermal conductivity decreased, and the extinction ratio at 200 W irradiation fell below 35 dB. Therefore, it was found that the amount of Sc added was preferably 1000 mass ppm or less. It should be noted that reducing the amount of Sc makes transparency difficult, but transparency can be achieved by adjusting the amount of Si. It can be seen that the amount of Si in the range of 100 mass ppm to 1000 mass ppm did not affect the extinction ratio, particularly at high power irradiation, and at 0 mass ppm, transparency was not achieved due to the generation of a heterogeneous phase.

[0083] [Table 2]

[0084] Although the present invention has been described using the above-mentioned embodiments, the present invention is not limited to the above-mentioned embodiments, and can be modified within the scope of what a person skilled in the art can conceive, such as other embodiments, additions, changes, deletions, etc., and any aspect is included in the scope of the present invention as long as it achieves the effects of the present invention. [Explanation of symbols]

[0085] 100 Optical isolator 102 Case 104 Optical axis 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims

1. A transparent ceramic for a magneto-optical element, comprising a paramagnetic garnet-type composite oxide containing terbium, lutetium, and aluminum, represented by the following formula (1), and 100 ppm by mass or more and 1000 ppm by mass or less of Si as a sintering aid: (Ta １‐ｘ Lu ｘ ) ３ Al ５ O １２ ・・・(1) (Wherein, 0.05≦x≦0.45.)

2. 2. The transparent ceramic for a magneto-optical element according to claim 1, further comprising 1000 mass ppm or less of Sc as a sintering aid.

3. 2. The transparent ceramic for magneto-optical devices according to claim 1, which has a thermal conductivity of 4.2 W / (m·K) or more at room temperature.

4. 2. The transparent ceramic for a magneto-optical device according to claim 1, which has an extinction ratio of 35 dB or more.

5. Loss coefficient at 1064 nm is 0.002 cm －１ 2. The transparent ceramic for magneto-optical elements according to claim 1, wherein:

6. 2. The transparent ceramic for magneto-optical devices according to claim 1, wherein the crystal grain size of the ceramic is 1 μm or more and 40 μm or less.

7. 2. The transparent ceramic for magneto-optical devices according to claim 1, which has an extinction ratio of 35 dB or more when irradiated with a laser having a wavelength of 1064 nm and an output of 200 W.

8. 2. The transparent ceramic for a magneto-optical element according to claim 1, wherein the Verdet constant is 30 Rad / (T·m) or more.

9. A magneto-optical element formed using the transparent ceramic for magneto-optical elements according to any one of claims 1 to 8.

10. 10. The magneto-optical element according to claim 9, which is an optical isolator that has the transparent ceramic for magneto-optical elements 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.

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