Patent Description:
In recent years, laser processing using fiber lasers has started to emerge as laser output has increased. In order to stably perform laser processing, it is necessary to remove light from the outside so as not to disturb the oscillation state. Especially when light is reflected at the end face of a fiber, the reflected light reaches a laser light source. As a result, the oscillation is greatly disturbed. Therefore a part called an isolator is attached to the boundary connecting the fiber and the fiber in a common fiber laser to suppress the reflected light completely.

The isolator is constituted by a Faraday rotator, a polarizer disposed on the light incident side of the Faraday rotator, and an analyzer disposed on the light emission side of the Faraday rotator. In addition, the Faraday rotator applies a magnetic field parallel to the traveling direction of the light to be utilized. At this time, the polarized wave component of the light rotates only in a certain direction regardless of forward or backward traveling in the Faraday rotator. Furthermore, the Faraday rotator is adjusted to a length that the polarized wave component of the light is rotated by exactly <NUM> degrees. Herein, when the polarization planes of the polarizer and the analyzer are shifted by <NUM> degrees in the rotation direction of the forward traveling light, the polarized waves of the forward traveling light coincides at the position of the polarizer and at the position of the analyzer, and the forward traveling light is transmitted. On the other hand, the polarized waves of the backward traveling light is rotated by <NUM> degrees in the direction opposite to the deviation angle direction of the polarization plane of the polarizer shifted by <NUM> degrees from the position of the analyzer. Then, the polarization plane of the return light at the position of the polarizer is shifted by <NUM> degrees - (-<NUM> degrees) = <NUM> degrees from the polarization plane of the polarizer, and the return light cannot be transmitted. In this way, the optical isolator functions to transmit and emit the forward traveling light and block the return light traveling backward.

Examples of a material existing conventionally as a Faraday rotator include garnet based Tb<NUM>Ga<NUM>O<NUM> (<CIT> (Patent Document <NUM>)) and Tb<NUM>Al<NUM>O<NUM> (<CIT> (Patent Document <NUM>)), and C-type rare earth based (TbxRe(<NUM>-x))<NUM>O<NUM> (<CIT> (Patent Document <NUM>)). These materials all contain terbium, having a large Verdet constant (magneto-optical constant) with small light absorption at the wavelength <NUM>,<NUM> used by lasers.

However, the various magneto-optical materials have their own problems, as follows. The garnet based Tb<NUM>Ga<NUM>O<NUM> (TGG) has a small amount of terbium contained in the crystal so that the Verdet constant is small, the Faraday rotator needs to be lengthened, and the beam quality tends to be poor. On the other hand, the similar garnet based Tb<NUM>Al<NUM>O<NUM> (TAG) uses aluminum with an ionic radius shorter than that of gallium so that the amount of terbium contained in the crystal is increased, and the Faraday rotator can be shortened. However, since TAG is an incongruent melting crystal, there is a restriction that a perovskite phase is formed (precipitated) first at a solid-liquid interface at the time of crystal growth and then a TAG phase is formed (precipitated). In other words, the TAG crystal can be grown only in a state where the garnet phase and the perovskite phase are always mixedly present, and good-quality, large-size TAG crystal growth has not been realized. Finally, the C-type rare earth based (TbxRe(<NUM>-x))<NUM>O<NUM> can increase the terbium content as compared with other materials and contributes to the shortening of the isolator, but high-valent terbium is likely to occur, and the light absorption is also large as compared with the garnet based materials. If the light absorption is large, there is a problem that, for example, when a high power laser of <NUM> W or more is inserted, the isolator itself generates substantial heat due to the absorbed light energy, resulting in deterioration of the laser quality.

At present, the most commonly used Faraday rotator material is TGG, but the Verdet constant of TGG is small so that improvement has been demanded. Although TAG is expected as an alternative to the TGG, it has been difficult to grow TAG crystals due to the aforementioned incongruent melting. Therefore, TAG ceramics (<CIT> (Patent Document <NUM>)) and a Tb<NUM>Sc<NUM>Al<NUM>O<NUM> (TSAG) crystal (<CIT> (Patent Document <NUM>)) are exemplified for the purpose of making a substance similar to a TAG crystal. Since the former TAG ceramics can be produced at the incongruent melting temperature or less, different phases can be controlled to some extent. However, it is difficult to completely suppress the occurrence of different phases due to compositional deviation and the like, and the degree of scattering is still too large to be used for optical applications. With the latter TSAG, it is possible to suppress incongruent melting by adding Sc and further correct even subtle compositional deviations. Therefore, it is easy to grow the crystals. However since a large amount of expensive Sc is used, TSAG is costly and has not yet been practically used.

Recently, YTAG ceramics, in which yttrium is substituted for some of the terbium of TAG ceramics, have been disclosed (Non-Patent Document <NUM>).

Patent Document <NUM> discloses a method for producing a transparent ceramic containing terbium oxide and at least one other rare earth oxide selected from among yttrium oxide, scandium oxide and oxides of lanthanide rare earth elements (excluding terbium) as main components, and a transparent ceramic produced by the method.

By substituting yttrium for some of the terbium of unstable TAG, structure stabilization can be realized, and a Faraday rotator which has a potential to outperform the performance of TGG crystal has already been known. Note that the substitution with yttrium reduces the terbium content, and the Verdet constant becomes lower than that of the TAG ceramics. However, since TAG ceramics originally have a Verdet constant <NUM> times higher than that of TGG ceramics, the YTAG ceramics still have a Verdet constant the same as or slightly greater than that of TGG crystal. Thus, the YTAG ceramics are not disadvantageous as an isolator material. Furthermore, by substituting yttrium for some of terbium in the TAG ceramics, it is possible to suppress light loss derived from absorption by terbium. As a result, there is a possibility that the YTAG ceramics can be used as an isolator material for a high power fiber laser compared with the TAG ceramics. However, the structure stabilization of the YTAG ceramics is still insufficient. It will be difficult to practically use the YTAG ceramics unless high-quality transparent ceramics can be produced at a high yield.

The present invention has been made in the light of the above circumstances. Aspects of our proposals are to provide a method for preparing a sinterable complex oxide powder in which garnet-type rare earth complex oxide powder, in which four constituent elements, terbium, at least one other rare earth element selected from the group consisting of yttrium and lanthanide rare earth (excluding terbium), aluminum and scandium are all uniformly distributed, is synthesized by a coprecipitation method, to provide a method for further manufacturing a transparent ceramic material using the powder prepared by the method and a transparent ceramic material produced therein, and to provide a garnet-type complex oxide powder for sintering.

The inventors started to develop YTAG ceramics utilizing the transparent ceramics technology which has been conventionally known and found out that the crystal structure is stabilized by substituting Sc for part of the backbone of the YTAG ceramics, and that the quality of the YTAG ceramics as transparent ceramics is improved. Moreover, at this time, the amount of Sc can be small. Thus, this also does not cause a significant problem in terms of cost.

However, the conventional YTAG ceramics produced by mixing various kinds of oxides and solid phase reaction had a problem in that hard powder (i.e. hard aggregates of particles), which cannot be crushed by ball mill crushing or the like performed thereafter, is generated at the time of calcination (firing), and as it is, coarse cavities are formed inside the compact at the time of molding. Furthermore, when the hard powder was removed in order to avoid the problem, the yield was decreased, causing another problem which is not desirable from the viewpoint of the productivity.

The inventors investigated these problems and proved that such hard aggregates were not seen in the manufacturing of transparent ceramics so far but were problems unique to aluminum garnet containing Tb (TAG, including YTAG). The inventors found out that these problems are due to a change in density occurring when the raw material powder containing Tb oxide is subjected to solid phase reaction to form garnet structure and also found that, in the production of aluminum garnet based ceramics containing Tb, it is preferable to use a raw material powder which has already acquired garnet structure in production, instead of producing ceramics with the desired composition by the solid phase reaction. The inventors further found that, in order to obtain a ceramic raw material powder formed into garnet structure with the desired composition, it is important to synthesize particles by a build-up method rather than by a top-down method such as a crushing method, whereby the occurrence of hard powder problem described above, can be completely suppressed. Based on these findings, the inventors have made further improvements and achieved the present invention.

The present invention provides a method for preparing a sinterable complex oxide powder, a method for manufacturing a transparent ceramic material and a transparent ceramic material produced therein, and a garnet-type complex oxide powder for sintering, all as defined in the claims.

According to the present invention, it is possible to provide the sinterable garnet-type rare earth complex oxide powder, in which four constituent elements, terbium, at least one other rare earth element selected from the group consisting of yttrium and lanthanide rare earth (excluding terbium), aluminum and scandium, are all uniformly distributed, and to provide the garnet-type transparent ceramic material having uniform transparency by molding and sintering the powder.

Hereinafter, a method for preparing a sinterable complex oxide powder according to the present invention is described.

A method for preparing a sinterable complex oxide powder according to the present invention is characterized by adding in aqueous solution, to an aqueous co-precipitation solution, (a) terbium ions, (b) at least one other kind of rare earth ions selected from yttrium ions and lanthanoid rare earth ions (excluding terbium ions), (c) aluminum ions and (d) scandium ions; stirring the resulting solution at a liquid temperature of <NUM> or less to induce a co-precipitate of the components (a), (b), (c) and (d); filtering, heating and dehydrating the co-precipitate; and firing the co-precipitate thereafter at <NUM>,<NUM> or more and <NUM>,<NUM> or less, thereby forming a powder composed of garnet-type complex oxide represented by the following formula (<NUM>):.

(Tb<NUM>-x-yRxScy)<NUM>(Al<NUM>-zScz)<NUM>O<NUM>     (<NUM>).

wherein R is at least one element selected from the group consisting of yttrium and lanthanoid rare earth elements (excluding terbium), <NUM> ≤ x < <NUM>, <NUM> < y < <NUM>, <NUM> < <NUM>-x-y < <NUM>, and <NUM> < z <<NUM>.

The composition of the sinterable complex oxide (garnet-type rare earth complex oxide), which is a subject in the present invention, is represented by the above formula (<NUM>). Note that, in the garnet crystal structure represented by the formula (<NUM>), the Tb-coordinated side, that is, the side within the former parentheses in the formula (<NUM>) is referred to as an A site, and the Al-coordinated side, that is, the side within the latter parentheses in the formula (<NUM>) is referred to as a B site.

In the A site of the formula (<NUM>), Tb is the element having the greatest Verdet constant among the paramagnetic element group excluding iron (Fe) and is the element most suitable to be used as a material for an optical isolator for a wavelength region of <NUM>,<NUM> since the absorption does not occur in this wavelength region used by a fiber laser. However, Tb easily reacts with oxygen in the air, and high-valent Tb is generated. Since this high-valent Tb has light absorption properties, it is desirable to eliminate this Tb as much as possible. To eliminate this high-valent Tb, it is most preferable to employ a crystal structure that does not generate the high-valent Tb, that is, a garnet structure.

At the A site of the formula (<NUM>), R is at least one other rare earth element selected from the group consisting of yttrium and lanthanide rare earth elements (excluding terbium (Tb)). Specifically, R is at least one element selected from the group consisting of yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). Among them, R is preferably at least one element selected from the group consisting of Y, Ce, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, is more preferably Y and/or Lu from the viewpoint that the absorption does occur in the used wavelength band, and is still more preferably Y or Lu.

Moreover, in the B site of the formula (<NUM>), Al is the element having the shortest ionic radius among trivalent ions that can stably exist in oxide having a garnet structure, and is the element that can minimize the lattice constant of the Tb-containing paramagnetic garnet-type oxide. If the lattice constant of the garnet structure can be decreased without changing the Tb content, the Verdet constant per unit length can be increased, which is preferable. The Verdet constant of the actual TAG ceramic is improved to <NUM> to <NUM> times that of a TGG. Therefore, even when a relative concentration of terbium is lowered by substituting the above R ions for some of the terbium ions, the Verdet constant per unit length can be kept equal to or slightly lower than that of the TGG. Thus, these are constituent elements suitable in the present invention.

Herein, the complex oxide containing only the constituent elements of Tb, R (other rare earth) and Al may not have a garnet structure due to a slight weighing error, and it is difficult to stably produce a transparent ceramic usable for optical applications. Therefore, in the present invention, scandium (Sc) is added as a constituent element to eliminate compositional deviation due to a slight weighing error. Sc is the element that has an intermediate ionic radius and can be solid solution in both the A site and the B site in the oxide having a garnet structure. Sc is also a buffer element that can be solid solution by adjusting the distribution ratio thereof to the A site (rare earth site composed of Tb and R) and the B site (aluminum site) so as to exactly match the stoichiometric ratio and thereby minimize the generation energy of the crystallite when the compounding ratio of the rare earth elements of Tb and R to Al is deviated from the stoichiometric ratio due to variation at the time of weighing. Sc is also an element that can limit the abundance of the alumina different phase in the garnet parent phase to <NUM> ppm or less and limit the abundance of the perovskite-type different phase in the garnet parent phase to <NUM> ppm or less. Thus, Sc is an indispensable element in the present invention.

In the formula (<NUM>), the range of x is <NUM> ≤ x < <NUM>, preferably <NUM> ≤ x ≤ <NUM>, and more preferably <NUM> ≤ x ≤ <NUM>. If x is in this range, the perovskite-type different phase can be reduced to a level that cannot be detected by X-ray diffraction (XRD) analysis. Furthermore, the abundance of the perovskite-type different phase (which have a typical type size of <NUM> to <NUM> in diameter and are granular appearing to be colored by light brown) in a viewing field of <NUM> × <NUM> by optical microscope observation is one or less, which is preferable. The abundance of the perovskite-type different phase at this time in the garnet parent phase is <NUM> ppm or less. Similarly, if x is in the above range, the amount of pores (which have a typical size of <NUM> to <NUM> in diameter and become spherical gaps when subjected to HIP treatment) remaining in the ceramic sintered body in a viewing field of <NUM> × <NUM> by optical microscope observation is one or less in abundance, which is preferable. The abundance of the pores at this time in the garnet parent phase is <NUM> ppm or less.

When x is less than <NUM>, the effect of substituting R for some of Tb cannot be obtained, and this is not substantially different from the conditions for creating TAG. Thus, it becomes difficult to stably produce a high-quality ceramic sintered body with low scattering and low absorption, which is unpreferable. Moreover, when x is <NUM> or more, the Verdet constant for a wavelength of <NUM>,<NUM> becomes less than <NUM> rad/(T·m), which is unpreferable. Furthermore, if the relative concentration of Tb is excessively diluted, the overall length necessary to rotate laser light with a wavelength of <NUM>,<NUM> by <NUM> degrees becomes long, exceeding <NUM>. This makes the production difficult, which is unpreferable.

In the formula (<NUM>), the range of y is <NUM> < y < <NUM>, preferably <NUM> < y < <NUM>, more preferably <NUM> ≤ y ≤ <NUM>, and still more preferably <NUM> ≤ y ≤ <NUM>. If y is in this range, the perovskite-type different phase can be reduced to a level that cannot be detected by X-ray diffraction (XRD) analysis. Furthermore, the abundance of the perovskite-type different phase (which have a typical type size of <NUM> to <NUM> in diameter and are granular appearing to be colored by light brown) in a viewing field of <NUM> × <NUM> by optical microscope observation is one or less, which is preferable. The abundance of the perovskite-type different phase at this time in the garnet parent phase is <NUM> ppm or less.

When y = <NUM>, the perovskite-type different phase is highly likely to precipitate, which is unpreferable. Moreover, when y is <NUM> or more, R is substituted for some of Tb, and further Sc is substituted for some of Tb while the effect of suppressing the precipitation of the perovskite-type different phase is saturated and unchanged. As a result, the solid solution concentration of Tb is unnecessarily lowered, thereby decreasing the Verdet constant. This is unpreferable. Furthermore, Sc is expensive as a raw material, so unnecessary excessive doping of Sc is unpreferable from the viewpoint of the production costs.

In the formula (<NUM>), the range of <NUM>-x-y is <NUM> < <NUM>-x-y < <NUM>, preferably <NUM> ≤ <NUM>-x-y < <NUM>, and more preferably <NUM> ≤ <NUM>-x-y < <NUM>. If <NUM>-x-y is in this range, a large Verdet constant can be secured as well as high transparency can be obtained for a wavelength of <NUM>,<NUM>.

In the formula (<NUM>), the range of z is <NUM> < z < <NUM>, preferably <NUM> < z < <NUM>, more preferably <NUM> ≤ z ≤ <NUM>, and still more preferably <NUM> ≤ z ≤ <NUM>. If z is in this range, the perovskite-type different phase cannot be detected by XRD analysis. Furthermore, the abundance of the perovskite-type different phase (which have a typical type size of <NUM> to <NUM> in diameter and are granular appearing to be colored by light brown) in a viewing field of <NUM> × <NUM> by optical microscope observation is one or less, which is preferable. The abundance of the perovskite-type different phase at this time in the garnet parent phase is <NUM> ppm or less.

When z is <NUM> or less, the perovskite-type different phase is highly likely to precipitate, which is unpreferable. Moreover, when z is <NUM> or more, the value of y, that is, the substitution ratio of Sc for Tb also increases in conjunction with the increase in the value of z while the effect of suppressing the precipitation of the perovskite-type different phase is saturated and unchanged. As a result, the solid solution concentration of Tb is unnecessarily lowered, thereby decreasing the Verdet constant. This is unpreferable. Furthermore, Sc is expensive as a raw material, so unnecessary excessive doping of Sc is unpreferable from the viewpoint of the production costs.

The method for preparing the sinterable complex oxide powder according to the present invention is to create the garnet-type complex oxide powder represented by the formula (<NUM>) by the steps of preparing a co-precipitate of the aforementioned components (a), (b), (c) and (d); filtering and washing the co-precipitate; heat-drying (dehydrating) and crushing; and firing. The details are described below.

In the method for preparing the sinterable complex oxide powder according to the present invention, it is preferable that an aqueous solution containing the component (a), an aqueous solution containing the component (b), an aqueous solution containing the component (c) and an aqueous solution containing the component (d) are added together to a co-precipitating aqueous solution, and that the resulting solution is stirred to induce a state where the components (a), (b), (c) and (d) are coprecipitated. Herein, adding together to a co-precipitating aqueous solution is referred to simultaneously adding a plurality of target aqueous solutions to a co-precipitating aqueous solution, and preferably to mixing a plurality of target aqueous solutions separately prepared and adding (dropping) this mixed aqueous solution to a co-precipitating aqueous solution (hereinafter the same applies to this specification).

<FIG> shows the specific procedure. Herein, the description is given where the other rare earth (R) is yttrium as an example but the procedure can apply generally.

Prepare an inorganic acid aqueous solution (solution A) containing (a) Tb ions, (b) Y ions, (c) Al ions and (d) Sc ions.

Specifically, first, separately prepare an aqueous solution containing the component (a), an aqueous solution containing the component (b), an aqueous solution containing the component (c) and an aqueous solution containing the component (d). The aqueous solutions containing the components (a), (b), (c) and (d) are not particularly limited as long as the aqueous solutions contain the components (a), (b), (c) and (d) (i.e. containing the components as ions), but are each preferably an inorganic acid aqueous solution.

Herein, as the raw material for the component (a), a powdery material with preferably the purity of <NUM>% by weight or more, more preferably the purity of <NUM>% by weight or more, or still more preferably the purity of <NUM>% by weight or more, is preferable. At this time, the raw material is not particularly limited as long as the raw material can be dissolved to form the aqueous solution. For example, the raw material may be terbium oxide powder (Tb<NUM>O<NUM>) or Tb<NUM>O<NUM> powder. Alternatively, the raw material may be powder of other compounds, such as fluoride or nitride of terbium, as long as the raw material is dissolved in the acidic aqueous solution, does not form complex ions, and becomes terbium ions. Terbium oxide powder is more preferable since impurity ions may influence the reaction or firing.

As the raw material for the component (b), a powdery material with preferably the purity of <NUM>% by weight or more, more preferably the purity of <NUM>% by weight or more, or still more preferably the purity of <NUM>% by weight or more, is preferable. At this time, the raw material is not particularly limited as long as the raw material can be dissolved to form the aqueous solution. For example, the raw material may be yttrium oxide powder (Y<NUM>O<NUM>). Alternatively, the raw material may be powder of other compounds, such as fluoride or nitride of Y, as long as the raw material is dissolved in the acidic aqueous solution, does not form complex ions, and becomes Y ions. Yttrium oxide powder is more preferable since impurity ions may influence the reaction or firing.

As the raw material for the component (c), a powdery material with preferably the purity of <NUM>% by weight or more, more preferably the purity of <NUM>% by weight or more, or still more preferably the purity of <NUM>% by weight or more, is preferable. At this time, the raw material is not particularly limited as long as the raw material can be dissolved to form the aqueous solution. Examples of the raw material include aluminum nitrate, aluminum chloride, aluminum hydroxide, aluminum ethoxide and the like, and aluminum hydroxide is more preferable.

As the raw material for the component (d), a powdery material with preferably the purity of <NUM>% by weight or more, more preferably the purity of <NUM>% by weight or more, or still more preferably the purity of <NUM>% by weight or more, is preferable. In this case, scandium oxide powder is preferable. Alternatively, the raw material may be powder of other compounds, such as fluoride or nitride of Sc, as long as the raw material is dissolved in the acidic aqueous solution, does not form complex ions, and becomes Sc ions. The scandium oxide powder is more preferable since impurity ions may influence the reaction or firing.

Each of these raw materials is dissolved in an inorganic acid aqueous solution so as to have a predetermined concentration.

The inorganic acid aqueous solutions to be used are not particularly limited as long as the inorganic acid aqueous solutions can dissolve the raw materials of the four components without forming complex ions and contain ions of the components (a) to (d), and are preferably aqueous solutions to which strong acid is added. Moreover, the counter ions (i.e. anions) contained in the aqueous solutions are not particularly limited, and nitrate ions, sulfate ions, halide ions, phosphate ions and the like are available. Examples of the aqueous solutions include 5N nitric acid aqueous solution, sulfuric acid aqueous solution, hydrochloric acid aqueous solution, and the like. In this case, an acidic aqueous solution which dissolves each and every raw material for the four components is preferable, and a nitric acid solution is more preferable. When the nitric acid solution is used, the remaining amount of inorganic salts after firing is small. Each of the concentrations of the aqueous solutions is preferably <NUM> or more and <NUM> or less.

Note that the temperatures for preparing the aqueous solutions containing the respective components (a), (b), (c) and (d) are not particularly limited. However, for example, when aluminum hydroxide is dissolved, a temperature of <NUM> or more is not preferable because aluminum hydroxide is dehydrated and forms aluminum oxide which is difficult to dissolve. It is preferable to prepare the aqueous solutions under the temperature conditions suitable for the substances.

The aqueous solution containing the component (a), the aqueous solution containing the component (b), the aqueous solution containing the component (c) and the aqueous solution containing the component (d) obtained as described above are accurately weighted to form the composition (mole fraction) of the formula (<NUM>) and sufficiently stirred and mixed, thereby obtaining the inorganic acid aqueous solution (solution A) containing the (a) Tb ions, the (b) Y ions, the (c) Al ions and the (d) Sc ions. This fraction is directly applied to the weight ratio (parts by weight) in the raw material powder obtained by the coprecipitation method.

Alternatively, the inorganic acid aqueous solution (solution A) containing the (a) Tb ions, the (b) Y ions, the (c) Al ions and the (d) Sc ions may be obtained as follows: the raw material for the component (a), the raw material for the component (b), the raw material for the component (c) and the raw material for the component (d) are each weighed to form the composition (mole fraction) of the formula (<NUM>); then, these raw materials are mixed, and the mixed powder is dissolved in an inorganic acid aqueous solution thereafter; alternatively, each raw material is sequentially dissolved in the inorganic acid aqueous solution.

Note that, from the viewpoint of reproducibility, it is preferable to separately prepare the aqueous solution containing the component (a), the aqueous solution containing the component (b), the aqueous solution containing the component (c) and the aqueous solution containing the component (d).

The concentration of each aqueous solution can be determined by inductively coupled plasma mass spectrometry (ICP-MS), spectrophotometry, or gravimetry, and is preferably determined by ICP-MS which can measure the concentrations most conveniently with good reproducibility (up to this step included in step S11).

Next, add the resulting solution A to a co-precipitation aqueous solution (solution B). Herein, the co-precipitating aqueous solution is not particularly limited as long as the co-precipitating aqueous solution is a basic aqueous solution which can coprecipitate the ions of all the four components after the aqueous ions, especially inorganic acid aqueous solution containing the ions, of the four components (a), (b), (c) and (d) are added, remove ions from the co-precipitate by water washing and filtering, and synthesize particles that can be transparentized at the end. Examples of the co-precipitation aqueous solution include aqueous solutions of ammonium hydrogen carbonate (NH<NUM>HCO<NUM>), ammonia water (NH<NUM>OH), oxalic acid ((COOH)<NUM>), ammonium carbonate ((NH<NUM>)<NUM>CO<NUM>), ammonium oxalate and the like. Among them, carbonate aqueous solutions are preferable, and an ammonium hydrogen carbonate aqueous solution is more preferable. A precipitation aid such as ammonium sulfate may be added to the co-precipitating aqueous solution.

Note that Sc ion has poor reactivity in the presence of carbonic acid and may not always coprecipitate uniformly with the other components (a) to (c). Therefore, when carbonic acid is contained in the co-precipitation aqueous solution, an aqueous solution containing the (d) Sc ions may be added (dropped in) after the aqueous solution(s) containing the components (a) to (c) excluding the component (d) are added (dropped in) and have been stirred for a while after the reaction is started, and then the carbonic acid completely escapes, as an alternative to adding (dropping in) the aqueous solution containing the (d) Sc ions simultaneously with the aqueous solution(s) containing other components (a) to (c). The stirring time until the aqueous solution of the component (d) is dropped is preferably one hour or more from the viewpoint of a complete escape of the carbonic acid. This option is included herein in general references to adding or dropping in the solution A or the components (a) - (d), unless excluded by specific context.

The pH of the liquid (solution B + A ) after the solution A is added (dropped) is preferably <NUM> or more and less than <NUM>, and more preferably <NUM> or more and less than <NUM>. If the pH of the liquid is less than <NUM>, the precipitate once obtained is redissolved and the yield may be decreased. Further, if the pH is <NUM> or more, the dispersibility of the precipitate (co-precipitate precursor) of each component becomes different, and there is a possibility that a uniform co-precipitate cannot be obtained.

Herein, it is preferable to drop the solution A into the co-precipitation aqueous solution and more preferable to drop the solution A in while stirring.

After the solution A is added to the co-precipitating aqueous solution (solution B), stir the solution B + A. When the solution A is added and the resulting solution B + A is stirred, a white precipitate, which is the co-precipitate of the components (a), (b), (c) and (d), occurs. And then, sufficient stirring is performed so that the precipitation of the components (a), (b), (c) and (d) does not become non-uniform and so that the co-precipitate particles grow.

Herein, the co-precipitating aqueous solution (solution B + A) to which the solution A is added should typically be kept warm in a water bath heated to a liquid temperature of <NUM> or less, preferably <NUM> or more and <NUM> or less (i.e. from <NUM> to <NUM>), and more preferably <NUM> or more and <NUM> or less and be stirred by a rotor at a rotating speed of <NUM> rpm or more. If the liquid temperature exceeds <NUM>, the co-precipitate particles grow large, and the sinterability thereof is poor, and powder which is difficult to transparentize is formed in the subsequent sintering step. If the liquid temperature is less than <NUM>, the co-precipitate particles may not grow.

Moreover, a stirring (rotating) speed of <NUM> rpm or more is usually sufficient and is not particularly limited. Stirring is continued even after the total amount of the solution A is added, and the stirring time is preferably <NUM> hours or longer. If the stirring time is shorter than <NUM> hours, the co-precipitate particles may not sufficiently grow, excessively reactive fine powder may be formed, and it may be difficult to discharge bubbles during sintering.

After stirring for a designated period of time, filter and wash the resulting co-precipitate to filter and recover the resulting co-precipitate. As a filtering and washing method, suction filtration or pressure filtration is selected, and the filtration method should be selected in consideration of productivity and the like.

To wash the co-precipitate, ultrapure water with an electric conductance of <NUM>/cm or less is best used, and washing is repeated until the electric conductance of the filtrate becomes preferably <NUM>/cm or less and more preferably <NUM>/cm or less. If the electric conductance of the filtrate is high, light metals such as Na and ions such as ammonium remain in the recovered co-precipitate. The light metals such as Na may cause color centers (lattice defects) in the sintered body, while ions such as ammonium may cause strongly coagulated powder in the subsequent firing step. Note that Na is often contained in an aqueous solution containing the (c) Al ions, and this washing is an important treatment when an Al raw material containing a large amount of Na is used.

When the electric conductance of the filtrate is lowered to the minimum, recover and put the co-precipitate in a constant temperature drier at <NUM> or more for <NUM> hours or longer to dry.

Fire the resulting washed and dried co-precipitate. Specifically, the co-precipitate is put in a refractory oxide container typified by yttria or alumina and heated to <NUM>,<NUM> or more and <NUM>,<NUM> or less (i.e. from <NUM>,<NUM> to <NUM>,<NUM>) in an oxygen-containing atmosphere to fire. If the firing temperature is less than <NUM>,<NUM>, the crystal structure of the resulting fired powder does not become a garnet structure. If the firing temperature exceeds <NUM>,<NUM>, the primary particles of the fired powder grow too large, and the coagulation state also becomes strong. This fired powder is not suitable as powder for transparent ceramics.

The firing time may be one hour or longer, and the rate of temperature increase at that time is preferably <NUM>/h or more and <NUM>/h or less. The firing atmosphere is preferably an oxygen-containing atmosphere of atmospheric oxygen, and a nitrogen atmosphere, an argon atmosphere, a hydrogen atmosphere, or the like is unsuitable. Furthermore, the firing furnaces and kiln are exemplified by a vertical muffle furnace, a horizontal tubular furnace, a rotary kiln and the like, and are not particularly limited as long as the target temperature can be reached and an oxygen flow can be created. Note that firing unevenness occurs if oxygen is not sufficiently supplied into the refractory container containing the crushed co-precipitate. Thus, it is necessary to make a contrivance to uniformly distribute oxygen, such as providing a vent hole in the refractory container.

As described above, a sinterable garnet-type complex oxide powder according to the present invention is obtained. At this time, the primary particle size of the sinterable complex oxide powder is <NUM> or more and <NUM> or less.

The method for manufacturing a transparent ceramic material according to the present invention is characterized by molding a compact using the garnet-type complex oxide powder prepared by the method for preparing the sinterable complex oxide powder according to the present invention, according to any general or specific embodiment thereof described herein, and then sintering the compact. Subsequently, performing pressure sintering.

Herein, the sinterable garnet-type complex oxide powder (ceramic powder) obtained as described above is slurried by any of various dispersing methods using a ball mill, a bead mill, a homogenizer, a jet mill, ultrasonic irradiation or the like, and dispersed to primary particles. The solvent of this slurry is not particularly limited as long as the finally resulting ceramic can be highly transparentized, and examples thereof include alcohols, such as lower alcohols having <NUM> to <NUM> carbon atoms, and pure water.

For the raw material powder used in the present invention, to the slurry obtained as described above, it is preferable to add a sintering aid and especially to add tetraethoxysilane (TEOS) as a sintering aid in an SiO<NUM> conversion amount of more than <NUM> ppm to <NUM>,<NUM> ppm or less (more than <NUM>% by weight to <NUM>% by weight or less) in the entire raw material powder (garnet-type complex oxide powder + sintering aid), or to add SiO<NUM> powder in an amount of more than <NUM> ppm to <NUM>,<NUM> ppm or less (more than <NUM>% by weight to <NUM>% by weight or less) in the entire raw material powder (garnet-type complex oxide powder + sintering aid). If the addition amount exceeds <NUM>,<NUM> ppm, a slight amount of light absorption may occur due to crystal defects by Si excessively contained. Note that the timing of adding the sintering aid is most preferably at the time of slurrying, such as ball mill mixing described above, but the sintering aid may be added at the time of preparing the co-precipitate of the components (a), (b), (c) and (d). The slurry containing the garnet-type complex oxide powder and the sintering aid thus obtained is referred to as a raw material powder slurry.

In addition, various organic additives may be added to the raw material powder slurry for the purpose of quality stability and yield improvement in the subsequent step of producing the ceramic. In the present invention, these are also not particularly limited. That is, various dispersants, binders, lubricants, plasticizers and the like can be suitably utilized. However, for these organic additives, it is preferable to select high-purity types free of unnecessary metal ions.

A filtration treatment may be performed for the purpose of removing undisintegrated coarse particles remaining in the raw material powder slurry obtained as described above. If molding (described later) is performed in a state in which undisintegrated coarse particles remain, the optical quality of the transparent ceramic may be deteriorated due to these undisintegrated coarse particles. The filtration treatment method is not particularly limited as long as filtration to remove only the coarse particles is possible, but a filter is preferably a nylon filter from the contamination-free viewpoint. Further, the pore size thereof is preferably <NUM> or less, more preferably <NUM> or less, and still more preferably <NUM> or less.

In the present invention, the aforementioned raw material powder slurry is used to be molded into a predetermined shape, and then degreased and sintered thereafter to create a densified sintered body with a relative density of at least <NUM>% or more. It is preferable to perform a hot isostatic pressing (HIP) treatment as a subsequent step. Note that, if the hot isostatic pressing (HIP) treatment is directly conducted, the paramagnetic garnet-type transparent ceramic is reduced, and slight oxygen deficiency occurs. Therefore, it is preferable to recover the oxygen deficiency by performing a slight oxidation HIP treatment or an annealing treatment in an oxidizing atmosphere after the HIP treatment. Accordingly, a transparent garnet-type oxide ceramic material without defect absorption can be obtained.

In the present invention, the aforementioned raw material powder slurry is molded into a desired shape, by a dry molding method or a wet molding method. At this time, the molding method is not particularly limited as long as suitable form and dimensions, e.g. the diameter and the length which can be used for a Faraday rotator, are obtained, and the compact is not cracked or the like. Examples of the dry molding method include a pressure pressing method and a uniaxial pressing method. Examples of the wet molding method include pressure casting, centrifugal casting and extrusion. At this time, in the case of the dry molding method, it is preferable to use granules obtained by spray-drying the raw material powder slurry. Further, in the case of the wet molding method, it is preferable to use the raw material powder slurry directly or the raw material powder slurry in a state where the solvent has been removed to some extent.

In the production process of the present invention, it is preferable to further place and seal, in a deformable waterproof container, the compact formed by the dry molding method or the wet molding method using the raw material powder slurry and perform cold isostatic pressing (CIP) or warm isostatic pressing (WIP), which applies hydrostatic pressure. Note that the applied pressure is not particularly limited and should be adjusted as appropriate while the relative density of the resulting compact is checked. For example, if the applied pressure is managed in a pressure range of about <NUM> MPa or less, which can be handled by a commercially available CIP device or WIP device, the manufacturing costs can be suppressed. Alternatively, a hot pressing step, a discharging plasma sintering step, a microwave heating step, or the like, in which not only the molding step but also the sintering are carried out at once, can also be suitably utilized at the time of molding.

In the production process of the present invention, a common degreasing step can be suitably utilized. That is, for the purpose of removing organic substances such as a dispersant contained in the compact, the compact is degreased under the atmosphere or under an oxygen atmosphere. The degreasing temperature is preferably <NUM> or more and preferably <NUM>,<NUM> or less. If the degreasing temperature is less than <NUM>, organic substances may remain due to insufficient degreasing. If the degreasing temperature exceeds <NUM>,<NUM>, the subsequent sintering step is influenced and may adversely affect subsequent transparentization. This is known.

In the production process of the present invention, a conventional sintering step may suitably be utilized. That is, a heat sintering step, such as a resistance heating method and an induction heating method, can be suitably utilized. The atmosphere at this time is not particularly limited, and it is possible to sinter under various atmospheres of inert gas, oxygen gas, hydrogen gas, helium gas, and the like, or also under reduced pressure (in vacuum). However, since it is preferable to prevent the occurrence of oxygen deficiency at the end, an oxygen gas atmosphere and a reduced pressure oxygen gas atmosphere are exemplified as more preferable atmospheres.

The sintering temperature in the sintering step is preferably <NUM>,<NUM> to <NUM>,<NUM>, more preferably <NUM>,<NUM> to <NUM>,<NUM>, and still more preferably <NUM>,<NUM> to <NUM>,<NUM>. When the sintering temperature is in this range, densification is promoted while the precipitation of different phases is suppressed, which is preferable. If the sintering temperature is less than <NUM>,<NUM>, the densification of the sintered body is insufficient. A sintering temperature exceeding <NUM>,<NUM> may exceed the incongruent melting temperature of the ceramic.

The sintering retention time for the sintering step is sufficient for about several hours, but the relative density of the sintered body must be densified to at least <NUM>% or more. When the sintering retention time is long, ten hours or longer, and the relative density of the sintered body is densified to <NUM>% or more, the final transparency is improved, which is more preferable.

In the production process, it is possible to further provide an additional hot isostatic pressing (HIP) treatment step after the sintering step.

Note that, as for the type of the pressurized gas medium at this time, inert gas such as argon and nitrogen, or Ar-O<NUM> can be suitably utilized. The pressure applied by the pressurized gas medium is preferably <NUM> to <NUM> MPa, and more preferably <NUM> to <NUM> MPa. If the pressure is less than <NUM> MPa, the transparency improving effect may not be obtained. If the pressure exceeds <NUM> MPa, no further transparency improvement is obtained even when the pressure is increased. Moreover, the load applied to the device becomes excessive, and the device may be damaged. It is convenient and preferable that the applied pressure be <NUM> MPa or less, which can be handled by a commercially available HIP device.

Moreover, the treatment temperature (predetermined retention temperature) at that time is preferably set within the range from <NUM>,<NUM> to <NUM>,<NUM>, more preferably from <NUM>,<NUM> to <NUM>,<NUM>, and still more preferably from <NUM>,<NUM> to <NUM>,<NUM>. If the heat treatment temperature exceeds <NUM>,<NUM>, oxygen deficiency is likely to occur, which is nonpreferable. In addition, if the heat treatment temperature is less than <NUM>,<NUM>, an effect of improving the transparency of the sintered body is scarcely obtained. Note that the retention time of the heat treatment temperature is not particularly limited. However, if the heat treatment temperature is retained for a long time, oxygen deficiency is likely to occur, which is not preferable. Typically, the retention time is preferably set within the range from one to three hours.

Note that the heater material, the heat insulating material and the treatment container subjected to the HIP treatment are not particularly limited, but graphite, or molybdenum (Mo), tungsten (W), and platinum (Pt) can be suitably utilized, and yttrium oxide and gadolinium oxide can also be further suitably utilized as the treatment container. When the treatment temperature is <NUM>,<NUM> or less in particular, platinum (Pt) can be used as the heater material, the heat insulating material and the treatment container, and the pressurized gas medium can be Ar-O<NUM>. Thus, the occurrence of oxygen deficiency during the HIP treatment can be prevented, which is preferable. When the treatment temperature exceeds <NUM>,<NUM>, graphite is preferable as the heater material and the heat insulating material. In this case, any one of graphite, molybdenum (Mo) and tungsten (W) is selected as the treatment container, and one of yttrium oxide or gadolinium oxide is selected as a second or double container inside the treatment container. Then, if an oxygen release material is packed in the container, the occurrence amount of oxygen deficiency during the HIP treatment can be suppressed to the minimum, which is preferable.

In the production process, oxygen deficiency may occur in the resulting transparent ceramic sintered body after the HIP treatment is finished, and the transparent ceramic sintered body may exhibit a subtle light gray appearance. In that case, it is preferable to perform oxygen annealing treatment (oxygen deficiency recovery treatment) under an oxygen atmosphere at the HIP treatment temperature or less, typically <NUM>,<NUM> to <NUM>,<NUM>. If the annealing temperature is less than <NUM>,<NUM>, the annealing effect may be insufficient and oxygen defects may not be compensated. If the annealing temperature exceeds <NUM>,<NUM>, bubbles defoamed by the HIP treatment may be regenerated, and good optical quality may not be obtained. Moreover, the retention time in this case is not particularly limited, but is preferably selected within a time period which is not less than a time sufficient for oxygen deficiency to recover and does not waste electricity cost due to unnecessarily long treatment. By this oxygen annealing treatment, even those transparent ceramic sintered bodies which have exhibited a subtle light gray appearance following the HIP treatment step, can all become paramagnetic garnet-type transparent ceramic bodies which are colorless and transparent without defect absorption.

In the production process, it is preferable to process the paramagnetic garnet-type transparent ceramic material, which has undergone the above series of production steps, into a predetermined shape and optically polish both end faces which are on the optically utilizing axis. The optical surface accuracy at this time is preferably λ/<NUM> or less and particularly preferably λ/<NUM> or less, when the measurement wavelength λ = <NUM>. The conditions for the predetermined shape are that the paramagnetic garnet-type transparent ceramic material has a length so that the ceramic functions as an isolator (i.e. the incident light is rotated by <NUM>°), and the length is sufficiently longer than the diameter of the laser light. For example, when the diameter of the laser light is <NUM>, unless the transparent body has a diameter of about <NUM>, which is sufficiently longer than <NUM>, the laser light hits the edge of the transparent body and scatters as a result, and the laser light with high intensity leaks out, which is dangerous.

Note that the optical loss can be further reduced by forming antireflection films as appropriate on the optically polished surfaces.

Since a garnet-type complex oxide powder obtained as described above can already acquire the garnet structure at the stage of being powder, it is possible to solve the problem of the hard powder which arises in the conventional Tb-Al reaction. That is, since there is no hard powder, we find that coarse cavities do not occur inside the ceramic sintered body, high transparency is obtained, the extinction ratio of the entire ceramic is improved, and a paramagnetic garnet-type transparent ceramic material with excellent magneto-optical properties is obtained.

Furthermore, since the paramagnetic garnet-type transparent ceramic material obtained in the present invention is presumed to be utilized as a magneto-optical material, it is preferable to apply a magnetic field to the paramagnetic garnet-type transparent ceramic material parallel to the optic axis thereof and then set a polarizer and an analyzer such that their optical axes are shifted from each other by <NUM> degrees, thereby constituting a magneto-optical device to be utilized. That is, the magneto-optical material according to the present invention is suitable for magneto-optical device applications and is suitably used as a Faraday rotator of an optical isolator for a wavelength of <NUM> to <NUM> in particular.

<FIG> is a schematic cross-sectional view showing one example of an optical isolator which is an optical device having, as an optical element, a Faraday rotator made of the magneto-optical material according to the present invention. In <FIG>, an optical isolator <NUM> includes a Faraday rotator <NUM> made of the magneto-optical material (the paramagnetic garnet-type transparent ceramic material) according to the present invention, and a polarizer <NUM> and an analyzer <NUM>, which are polarization materials, are provided in front of and behind the Faraday rotator <NUM>, respectively. Further, in the optical isolator <NUM>, it is preferable that the polarizer <NUM>, the Faraday rotator <NUM> and the analyzer <NUM> be disposed in this order, and a magnet <NUM> be placed on at least one of the side faces thereof.

In addition, the above optical isolator <NUM> can be suitably utilized for industrial fiber laser devices. That is, the optical isolator <NUM> is suitable to prevent the reflected light of the laser light emitted from a laser light source from returning to the light source to cause unstable oscillation.

Hereinafter, the present invention is more specifically described with reference to Examples and Comparative Examples.

Sinterable garnet-type complex oxide powder was created as below.

High purity terbium oxide powder (Tb<NUM>O<NUM>, purity: <NUM>%), high purity yttrium oxide powder (purity: <NUM>%) and scandium oxide powder (purity <NUM>%) manufactured by Shin-Etsu Chemical Co. , and high purity aluminum hydroxide powder (purity: <NUM>%) manufactured by Nippon Light Metal Co. were prepared, and separately heated and dissolved in different 2N nitric acid aqueous solutions to obtain four respective aqueous solutions, that is to say an aqueous solution containing component (a), an aqueous solution containing component (b), an aqueous solution containing component (c) and an aqueous solution containing component (d). In these the concentrations of the (a) Tb ions, the (b) Y ions, the (c) Al ions and the (d) Sc ions respectively were each adjusted to about <NUM>.

Then, the precise concentrations of these aqueous solutions were determined by ICP-MS analysis. For each of the five target compositions shown in Table <NUM>, amounts of each of the four aqueous solutions was then weighed out so as to contribute the proportion of each component required for that composition, and the four solutions were mixed together to give a total volume of <NUM> (preparation of mixed aqueous solution).

Subsequently, the mixed aqueous solution was dropped into a co-precipitating aqueous solution containing <NUM> ammonium hydrogen carbonate and <NUM> ammonium sulfate while being heated and stirred. The liquid temperature of the water bath was <NUM>, and the stirring speed was <NUM> rpm at this time. Moreover, the amount of the co-precipitating aqueous solution was adjusted so that the pH after the mixed aqueous solution was dropped becomes <NUM>. After the completion of dropping the mixed aqueous solution, the mixture was stirred for <NUM> hours. At this time, the stirring speed was <NUM> rpm.

The resulting precipitate was filtrated and recovered (filtered) after stirring, and was washed with <NUM> of ultrapure water. At this time, the washing was repeated until the electric conductance of the filtrate became <NUM>/cm or less. Next, the recovered precipitate was dried in a dryer at <NUM> for <NUM> days and then fired under the conditions of an oxygen atmosphere at <NUM>,<NUM> and <NUM> hours to obtain a coprecipitated raw material powder.

Powder X-ray diffraction (XRD) analysis was performed to confirm whether the resulting fired powder had a garnet structure. Measurement was carried out at 2θ = <NUM>° to <NUM>° using a powder X-ray diffractometer (Smart Lab, manufactured by Rigaku Corporation). The resulting X-ray diffraction data was compared with the past reference data to confirm whether a garnet phase or a perovskite phase was present. Note that, when the diffraction peak of only the garnet phase appears, the existence of the perovskite phase is considered to be less than <NUM>%, and this peak represents a garnet single phase.

In order to measure the primary particle size of the resulting fired powder, a field emission scanning electron microscope (FE-SEM) is used. A carbon tape is stuck on a stage, the powder is sprinkled thereon, and gold evaporation is performed to avoid charge-up. One hundred or more primary particles were extracted from several FE-SEM photographs, and the sizes of all the particles were counted and averaged. This value was defined as the primary particle size.

The above results are summarized in Table <NUM>.

All the resulting powders had a garnet structure, and the primary particle size was also the same. Note that only a ratio of Tb to Y is changed in the composition, and the amounts of Al and Sc are all the same.

The resulting fired powder (white powder) was subjected to ball mill mixing using nylon balls with a diameter of <NUM> mmϕ and high purity ethanol as a solvent. At that time, polyethylene glycol was used as a dispersant, and a polyvinyl alcohol-based binder was used as a binder. In addition, tetraethoxysilane (TEOS) was added as a sintering aid in an amount of <NUM>,<NUM> ppm in SiO<NUM> conversion with respect to the raw material powder (fired powder + sintering aid). After the ball mill mixing, the resulting slurry was filtered using a nylon filter with a pore size of <NUM> to obtain a raw material powder slurry. Next, this raw material powder slurry was spray-dried to be granulated, and uniaxial pressing and subsequent CIP were performed using the granular raw material powder to obtain a compact with a relative density of about <NUM>%.

Then, the compact was subjected to degreasing treatment by heating at <NUM> in the atmosphere. Subsequently, the resulting degreased compact was placed in an oxygen atmosphere furnace and sintered under the conditions of <NUM>,<NUM> and <NUM> hours. Next, the sintered body was put in a tungsten container and subjected to HIP treatment under the conditions of Ar pressure of <NUM> MPa, <NUM>,<NUM>, and <NUM> hours. The resulting sample was somewhat dark, so the sample was annealed at <NUM>,<NUM> under an oxygen atmosphere.

The transparent sintered body thus obtained was ground and polished to have a diameter of <NUM> mmϕ × length of <NUM> mmL. Both end faces were optically polished so as to have surface accuracy (flatness) of λ/<NUM> (λ = <NUM>).

The optical properties (total light transmittance, extinction ratio and Verdet constant) and the thermal lens compatible output were evaluated as below for each of the samples obtained as described above.

The total light transmittance of the optically polished transparent sintered body with a length of <NUM> was measured with reference to JIS K7105 (ISO <NUM>-<NUM>: <NUM>).

That is, an inlet opening and an outlet opening, through which the light travels, are provided in an integrating sphere, and a sample is placed at the inlet opening portion. By attaching a reflector to the outlet opening portion, it is possible to detect all the light emitted from the sample with the integrating sphere, and the transmittance was measured from the ratio of the intensity of the detected emitted light to the intensity of the light incident on the sample. The measurement was performed using a spectrophotometer (V-<NUM>, manufactured by JASCO Corporation) with an attached integrating sphere. At that time, a pinhole was provided so that the spot diameter of the irradiation light became <NUM>. The measurement was performed by a double beam method using a halogen lamp as a light source, and a photomultiplier tube (wavelength of <NUM> or less) and a PbS photoelectric cell (wavelength of <NUM> or more) as detectors. For the total light transmittance, a value of <NUM>,<NUM> was used for the wavelength. The total light transmittance was measured for five samples from each Example and was evaluated with two significant figures and percentage as unit.

With reference to JIS C5877-<NUM>: <NUM>, the extinction ratio was measured by an optical system using and assembling a laser light source (manufactured by NKT Photonics), a power meter (manufactured by Gentec), a Ge photodetector (manufactured by Gentec) and polarizers (manufactured by Sigmakoki Co. The laser light used had a wavelength of <NUM>,<NUM> and a beam diameter of <NUM> to <NUM> mmϕ. The room temperature at the time of measurement was <NUM>.

First, two polarizers were rotated in the absence of the sample, the polarizers were fixed at positions where the power of light become maximum, and power P// of the light was measured. Thereafter, the sample was inserted between the two polarizers, the polarizer (analyzer) near the detector was rotated by <NUM>° to form crossed nicols, and power P⊥ of the light at this time was measured. The extinction ratio (dB) was determined based on the following equation: <MAT>.

Subsequently, the optically polished sample was coated with an antireflection film (AR coating) designed to have a center wavelength of <NUM>,<NUM>. For each resulting sample, the Verdet constant and the thermal lens compatible output were measured as below.

As shown in <FIG>, each resulting ceramic sample (corresponding to the Faraday rotator <NUM>) was inserted into the center of a neodymium-iron-boron magnet (magnet <NUM>) with an outer diameter of <NUM>, an inner diameter of <NUM> and a length of <NUM>, polarizers (the polarizer <NUM> and the analyzer <NUM>) were inserted at both ends thereof. Thereafter, high power laser beams with a wavelength of <NUM>,<NUM> were incident on the both end faces by using a high power laser (beam diameter: <NUM>) manufactured by IPG Photonics Corporation to determine the Faraday rotation angle θ. The Faraday rotation angle θ was defined as an angle that exhibits the maximum transmittance when the polarizer on the emission side was rotated.

A Verdet constant V was determined based on the following equation. Note that a magnetic flux density (B) applied to the sample was calculated by simulation based on the shape and dimensions of the measurement system, a residual magnetic flux density (Br) and coercivity (Hc). <MAT> wherein θ is the Faraday rotation angle (Rad), V is the Verdet constant (Rad/(T·m)), B is the magnetic flux density (T), and L is the length of the Faraday rotator (<NUM> in this case).

For comprehensive evaluation, ⊚ means that the following three target properties are all met, △ means that the two following target properties are met, and × means that the one or less following target properties are met. The target properties are the total light transmittance of <NUM>% or more, the extinction ratio of <NUM> dB or more, and the Verdet constant of <NUM> rad/T·m or more.

Note that, when the comprehensive evaluation was ×, measurement of the next thermal lens compatible output was not performed.

Laser irradiation was performed with a CW laser (wavelength: <NUM>,<NUM>, upper output limit: <NUM> W) manufactured by IPG Photonics Corporation, and the shape of the laser beam was evaluated by using a beam propagation analyzer (mode master manufactured by Coherent Inc. That is, when the laser focal position in the absence of the sample was f<NUM>, and the laser focal position when the sample was placed thereon was f, the laser intensity | f<NUM> - f | < <NUM> × f<NUM> was defined as the compatible laser intensity. That is, when the deviation (the maximum position variation amount) from the original focal position was less than <NUM>% due the presence or absence of the sample, the laser output at this time was defined as compatible. The output was measured up to <NUM> W, and the maximum possible output was determined.

The above results are summarized in Table <NUM> below.

From the above results of Examples <NUM>-<NUM> to <NUM>-<NUM>, when a certain amount or more of Tb was contained (when all the conditions of x, y, <NUM>-x-y and z in the formula (<NUM>) were met), all the above three properties were met, and the thermal lens compatible output was also <NUM> W (i.e. there was a potential of exceeding <NUM> W). On the other hand, when Y was not compounded (x = <NUM> in the formula (<NUM>)) as in Comparative Example <NUM>-<NUM>, the total light transmittance decreased because the light absorption amount increased. As a result, the thermal lens compatible output was also <NUM> W. Moreover, when the amount of Tb is too small (<NUM>-x-y = <NUM> in the formula (<NUM>)) as in Comparative Example <NUM>-<NUM>, the target properties of the total light transmittance and the extinction ratio were met, but the Verdet constant was small. Thus, even if the thermal lens compatible output is <NUM> W, it is undesirable from the viewpoint of increasing the size as an isolator.

The liquid temperature and the stirring time of the stirring treatment for the preparation of the co-precipitate of the components (a), (b), (c) and (d), and the firing temperature were changed from those in Example <NUM>-<NUM> as shown in Table <NUM>. Other than that, under the same conditions as in Example <NUM>-<NUM>, a sinterable garnet-type complex oxide powder was created, and a transparent ceramic was further produced by using this garnet-type complex oxide powder.

As shown in Table <NUM>, even if the liquid temperature and the stirring time of the stirring treatment for the preparation of the co-precipitate are changed, the sinterable garnet-type complex oxide powder has a garnet structure as long as the firing temperature is <NUM>,<NUM> or more, although the primary particle size changes. However, as in Comparative Example <NUM>-<NUM>, when the firing temperature was <NUM>, the sinterable garnet-type complex oxide powder did not have a garnet structure and became amorphous. Therefore, it can be seen that a firing temperature of <NUM>,<NUM> or more is necessary to obtain a garnet-type complex oxide powder.

The results of evaluating the transparent ceramic samples of this Example in the same manner as in Example <NUM> are shown in Table <NUM>.

As can be seen from the above results, good results were obtained when the liquid temperature of the stirring treatment at the time of preparing the co-precipitate was <NUM> or less and the firing temperature was <NUM>,<NUM> to <NUM>,<NUM>. However, when the liquid temperature of the stirring treatment at the time of preparing the co-precipitate was <NUM> (Comparative Example <NUM>-<NUM>) or when the firing temperature was <NUM>,<NUM> (Comparative Example <NUM>-<NUM>), the primary particles became large with the primary particle size exceeding <NUM>. Thus, sinterability was poor, and bubbles were difficult to be escaped, resulting in poor total light transmittance and extinction ratio. On the other hand, when the firing temperature was <NUM> (Comparative Example <NUM>-<NUM>), the primary particle size became small in turn. Thus, the sinterability was too good, and bubbles remained at the grain boundary after sintering, resulting in poor transparency.

The amount of the aqueous solution containing (d) Sc was mainly changed at the time of preparing the mixed aqueous solution to change the composition of the final complex oxide in Example <NUM>-<NUM>. Other than that, under the same conditions as in Example <NUM>-<NUM>, sinterable garnet-type complex oxide powder was created, and a transparent ceramic was further produced by using this garnet-type complex oxide powder.

From the above results of Examples <NUM>-<NUM> to <NUM>-<NUM>, when all the conditions of x, y, <NUM>-x-y and z in the formula (<NUM>) were met, all the sinterable garnet complex oxide powders exhibited a garnet structure. On the other hand, in Comparative Examples <NUM>-<NUM> and <NUM>-<NUM>, when y = z = <NUM> in the formula (<NUM>) or when z < <NUM>, the perovskite phase was precipitated.

From the above results, in Examples <NUM>-<NUM> to <NUM>-<NUM> in which the complex oxide powder had a single garnet structure, good optical properties were obtained, and the thermal lens compatible output was also <NUM> W. On the other hand, in Comparative Examples <NUM>-<NUM> and <NUM>-<NUM> in which complex oxide powder containing a perovskite phase in a garnet structure as a crystal structure was used, light scattering occurred due to the occurrence of the perovskite phase, and good results of the total light transmittance and the extinction ratio were not obtained.

The process for preparing the raw material powder slurry in Example <NUM>-<NUM> was changed as below. Thereafter, under the same conditions as in Example <NUM>-<NUM>, sinterable garnet-type complex oxide powders were created, and transparent ceramics were further produced by using the garnet-type complex oxide powders.

High purity terbium oxide powder (Tb<NUM>O<NUM>, purity: <NUM>%), high purity yttrium oxide powder (purity: <NUM>%) and scandium oxide powder (purity: <NUM>%) manufactured by Shin-Etsu Chemical Co. , and high purity aluminum oxide powder (purity: <NUM>%) manufactured by Taimei Chemicals Co. were weighed so as to have the composition shown in Table <NUM> (i.e. the same composition as in Example <NUM>-<NUM>) and the total amount of <NUM>. This mixed powder was subjected to ball mill crushing in ethanol by using nylon balls with a diameter of <NUM> mmϕ to be dispersed. Thereafter, the transparent ceramic was produced under the same conditions as in Example <NUM>-<NUM>. That is, polyethylene glycol was used as a dispersant and a polyvinyl alcohol-based binder was used as a binder in the ball mill crushing. In addition, tetraethoxysilane (TEOS) was added as a sintering aid in an amount of <NUM>,<NUM> ppm in SiO<NUM> conversion with respect to the raw material powder (fired powder + sintering aid). After the ball mill mixing, the resulting slurry was filtrated using a nylon filter with a pore size of <NUM> to obtain a raw material powder slurry. This was granulated by spray-drying. The yield of the raw material powder at this stage was <NUM>%.

High purity terbium oxide powder (Tb<NUM>O<NUM>, purity: <NUM>%), high purity yttrium oxide powder (purity: <NUM>%) and scandium oxide powder (purity: <NUM>%) manufactured by Shin-Etsu Chemical Co. , and high purity aluminum oxide powder (purity: <NUM>%) manufactured by Taimei Chemicals Co. were weighed so as to have the composition shown in Table <NUM> (i.e. the same composition as in Example <NUM>-<NUM>) and the total amount of <NUM>. This mixed powder was subjected to ball mill crushing in ethanol by using nylon balls with a diameter of <NUM> mmϕ to be dispersed. At that time, polyethylene glycol was added as a dispersant, and TEOS as a sintering aid was added in an amount of <NUM>,<NUM> ppm in SiO<NUM> with respect to the raw material powder (fired powder + sintering aid). For the resulting slurry, only the solvent was volatilized by a rotary evaporator, and the powder was recovered. Next, the recovered powder was dry-crushed in an agate mortar and then calcined at <NUM>,<NUM> under an oxygen atmosphere. The resulting calcined powder was again subjected to ball mill crushing with using nylon balls with a diameter of <NUM> mmϕ to be slurried, and then a polyvinyl alcohol-based binder was added as a binder and mixed again.

Subsequently, the resulting slurry was used directly as the raw material powder slurry and spray-dried to be granulated. This is Comparative Example <NUM>-<NUM>. The yield of the raw material powder at this stage was <NUM>%. Moreover, the resulting slurry was filtrated using a nylon filter with a pore size of <NUM> to form a raw material powder slurry, and this raw material powder slurry was spray-dried to be granulated. This is Comparative Example <NUM>-<NUM>. The yield of the raw material powder at this stage was <NUM>%.

The above results are summarized in Table <NUM>. Note that the result in Example <NUM>-<NUM> is also shown together with the measurement result of the yield of the raw material powder.

Using each granulated raw material powder, uniaxial pressing, CIP, degreasing, sintering, HIP, annealing, grinding and polishing, and optical polishing were sequentially performed under the same conditions as in Example <NUM>-<NUM> to obtain a ceramic sample.

The results of evaluating these samples in the same manner as in Example <NUM> are shown in Table <NUM>. The result of Example <NUM>-<NUM> is also shown.

As can be seen from the above results of Comparative Examples <NUM>-<NUM> to <NUM>-<NUM>, all of the transparent ceramics created by techniques different from the present invention (i.e. the preparation of the ceramic powder was performed by powder mixing - ball mill crushing) had some problems.

That is, in the case of using the raw material powder prepared only by powder mixing - ball mill crushing treatment without performing the calcination (firing) treatment as in Comparative Example <NUM>-<NUM>, the compact was frequently laterally cracked (compact cracking) at the completion of the molding. For example, in the case of using such raw material powder to mold a thin plate shape with a height (thickness) of about <NUM>, there is no problem of compact cracking. However, in the case of molding a thick shape requiring a height of <NUM> at the molding stage as in this Example, the press pressure is not conveyed to the center of the compact at the stage of the uniaxial pressing, the shape of the compact could not be maintained. Thus, this is considered as the reason for cracking. Note that wet casting was also examined as a molding method other than uniaxial pressing. However, the particles were too fine in the raw material powder not yet fired, and cracking occurred during drying after slip casting.

On the other hand, in Comparative Examples <NUM>-<NUM> and <NUM>-<NUM> no compact cracking occurred but coarse aggregates of particles (grade of <NUM> in diameter), which could not be crushed in slurrying (ball mill crushing) performed after the calcination, occurred during the calcination. As in Comparative Example <NUM>-<NUM>, when the raw material powder was formed with these coarse aggregates of particles remaining therein, the coarse aggregates of particles could not be crushed by the pressure applied at the time of molding and remained, so that coarse cavities were formed in the compact and good optical properties could not be obtained in the finally obtained ceramic. Meanwhile, when the coarse aggregates of particles were removed by filtration as in Comparative Example <NUM>-<NUM>, the optical properties of the ceramic were improved, but the yield of the raw material powder was about <NUM>%. This was not a favorable result from the viewpoint of productivity.

On the other hand, with the ceramic powders composed of the garnet-type complex oxide synthesized by the co-precipitation - firing method as in Example <NUM>-<NUM> according to the present invention, coarse aggregates of particles were not formed. Even if filtration was performed, the yield of the raw material powders exceeded <NUM>%, and transparent ceramics having good optical quality were obtained.

In respect of numerical ranges disclosed in the present description it will of course be understood that in the normal way the technical criterion for the upper limit is different from the technical criterion for the lower limit, i.e. the upper and lower limits are intrinsically distinct proposals.

For the avoidance of doubt it is confirmed that in the general description above, in the usual way the proposal of general preferences and options in respect of different features of the methods constitutes the proposal of general combinations of those general preferences and options for the different features, insofar as they are covered by the claims.

Claim 1:
A method of preparing a sinterable complex oxide powder, comprising the steps of:
adding (a) terbium ions, (b) ions of at least one other rare earth, selected from yttrium ions and lanthanoid rare earth ions excluding terbium ions, (c) aluminum ions and (d) scandium ions, in aqueous solution, to an aqueous co-precipitation solution;
stirring the resulting solution at a liquid temperature of <NUM> or less to induce co-precipitation of components (a), (b), (c) and (d);
filtering, heating and dehydrating the co-precipitate; and
firing the co-precipitate thereafter at from <NUM>,<NUM> to <NUM>,<NUM>, thereby forming a garnet-type complex oxide powder represented by the formula (<NUM>):

        (Tb<NUM>-x-yRxScy)<NUM>(Al<NUM>-zScz)<NUM>O<NUM>     (<NUM>)

wherein R is at least one element selected from yttrium and lanthanoid rare earth elements excluding terbium, <NUM> ≤ x < <NUM>, <NUM> < y < <NUM>, <NUM> < <NUM>-x-y < <NUM> and <NUM> < z <<NUM>.