Transparent rare earth aluminum garnet ceramics

Abstract

Provided is a transparent rare earth aluminum garnet ceramic that has a highlight transmission rate and can be mass produced. The transparent rare earth aluminum garnet ceramic is represented by general formula R3Al5O12 (R is an element selected from the group consisting of rare earth elements having an atomic number of 65 to 71) and includes Si and Y as sintering aids, or is represented by general formula R3Al5O12 (R is an element selected from the group consisting of rare earth elements having an atomic number of 65 to 70) and includes Si and Lu as sintering aids.

Description

TECHNICAL FIELD

The present invention relates to a transparent rare earth aluminum garnet ceramic and specifically relates to a transparent rare earth aluminum garnet ceramic having an improved extinction ratio and an improved scattering coefficient.

BACKGROUND ART

The inventors of the present invention have put a Faraday rotator made of a polycrystalline garnet (Tb₃Ga₅O₁₂:TGG) ceramic having a Verdet constant substantially equivalent to that of the single crystal into practical use. The TGG, however, has a small Verdet constant in the visible light region, and thus downsizing of visible light lasers, which have been under aggressive development, is difficult. In addition, TGG is a comparatively expensive material, leading to a higher material cost.

To address these problems, single crystals of TAG (Tb₃Al₅O₁₂), which has a higher Verdet constant than TGG in a wavelength region from visible wavelengths to around 1,000 nm and does not contain Ga as an expensive material, have been studied. TAG is, however, an incongruent melting compound, and thus it is difficult to directly produce, from a melted composition of a starting material, TAG single crystals corresponding to the garnet phase. On this account, a Czochralski process that enables production of large single crystals is difficult to be applied, and thus TAG single crystals are produced by a floating zone method (Patent Literature 1, Non-Patent Literature 1). By the floating zone method, large single crystals are difficult to be produced. Thus, there are various problems in mass production.

Patent Literature 2 discloses a method of producing a transparent rare earth aluminum garnet ceramic by controlling the average particle diameter and the molding density of a calcined powder to improve the transparency. However, due to the low transparency and the small thickness of 0.15 cm of the sample for characteristic evaluation, the practicability is poor.

CITATION LIST

Patent Literature

• Patent Literature 1: JP 4107292 B
• Patent Literature 2: JP 2008-7385 A

Non-Patent Literature

• Non-Patent Literature 1: Journal of Crystal Growth, 267 (2004), 188-193 (Growth of terbium aluminum garnet (Tb₃Al₅O₁₂; TAG) single crystals by the hybrid laser floating zone machine)

SUMMARY OF INVENTION

Technical Problem

In view of the above problems in the conventional art, an object of the present invention is to provide a transparent rare earth aluminum garnet ceramic that has a high light transmission rate and can be mass produced.

Solution to Problem

An object of the present invention can be basically achieved by the transparent rare earth aluminum garnet ceramics according to [1] and [2].

[1] A transparent rare earth aluminum garnet ceramic represented by general formula R₃Al₅O₁₂ (R is an element selected from the group consisting of rare earth elements having an atomic number of 65 to 71), the transparent rare earth aluminum garnet ceramic comprising Si and Y as sintering aids.[2] A transparent rare earth aluminum garnet ceramic represented by general formula R₃Al₅O₁₂ (R is an element selected from the group consisting of rare earth elements having an atomic number of 65 to 70), the transparent rare earth aluminum garnet ceramic comprising Si and Lu as sintering aids.

When Si and Y are contained as sintering aids, it is preferable that Si is 5 ppm by mass to 250 ppm by mass in terms of metal, and Y is 20 ppm by mass to 600 ppm by mass in terms of metal.

When Si and Lu are contained as sintering aids, it is preferable that Si is 5 ppm by mass to 250 ppm by mass in terms of metal, and Lu is 20 ppm by mass to 600 ppm by mass in terms of metal.

It is preferable that the transparent rare earth aluminum garnet ceramic has an average crystal grain size of 0.8 to 30 μm. In the present invention, the average crystal grain size is a numerical value calculated by the following equation.d=1.56C/(MN)

d: average crystal grain size (μm)

C: the length of an arbitrary line on an image captured under a scanning electron microscope

N: the number of crystal grains on the arbitrary line

M: the magnification of the image

The transparent rare earth aluminum garnet ceramic of the present invention may be used as an optical component. When containing Si and Y as sintering aids, the transparent rare earth aluminum garnet ceramic of the present invention may be used as a magneto-optical component.

Advantageous Effects of Invention

The transparent rare earth aluminum garnet ceramic of the present invention contains Al in place of Ga, thus is inexpensive, comprises Si and Y as sintering aids, thus has a high light transmission rate, and can be mass produced by a burning process. In order to achieve practically excellent functions as an optical component, the ceramic preferably has, for example, an extinction ratio of 30 dB or more and a scattering coefficient of 0.4%/cm or less at wavelengths 633 nm and 1,064 nm. To provide an optical component having the above properties, the ceramic contains, as sintering aids, 5 ppm by mass to 250 ppm by mass of Si in terms of metal and 20 ppm by mass to 600 ppm by mass of Y in terms of metal, or 5 ppm by mass to 250 ppm by mass of Si in terms of metal and 20 ppm by mass to 600 ppm by mass of Lu in terms of metal. Also when the rare earth aluminum garnet ceramic has an average crystal grain size of 0.8 to 30 μm, an optical component having the above properties can be provided.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will now be described on the basis of specific examples, but various modifications and changes may be made without departing from the technical scope of the present invention, and it goes without saying that the present invention is not limited to the following examples.

EXAMPLES

Example 1

(Preparation of Material)

Terbium oxide having a purity of 99.9% or more was dissolved in nitric acid to prepare a terbium nitrate solution having a concentration of 1 mol/L. Aluminum chloride having a purity of 99.9% or more was dissolved in ultrapure water to prepare an aluminum chloride solution having a concentration of 1 mol/L. Next, 300 mL (milliliters) of the above terbium nitrate solution, 500 mL of the above aluminum chloride solution, and 150 mL of an aqueous ammonium sulfate solution having a concentration of 1 mol/L were mixed, and ultrapure water was further added to give a total volume of 10 L of the mixed liquid. While the obtained mixed liquid was stirred, an aqueous ammonium hydrogen carbonate solution having a concentration of 0.5 mol/L was added dropwise at a rate of 5 mL/min until the pH reached 8.0, and the whole was allowed to stand under stirring at room temperature for 2 days. After the 2-day standing, suction filtration and washing with ultrapure water were repeated several times, and the product was placed in a dryer at 150° C. and dried for 2 days. The resulting precursor powder was placed in an alumina crucible and was calcined in an electric furnace at 1,200° C. for 3 hours. Through the above procedure, a terbium aluminum garnet (TAG) material powder (purity: 99.9% by mass or more) having a specific surface area of 6.0 m²/g was prepared.

(Molding and Sintering)

To 75 g of the prepared TAG material powder, 50 g of ethanol as a solvent, 0.75 g of polyvinyl alcohol (PVA) as a binder, 0.75 g of polyethylene glycol (PEG) as a plasticizer, and 0.375 g of stearic acid as a lubricant were added. Next, SiO₂ and Y₂O₃ as sintering aids were added to the TAG material powder so as to give a Si content of 100 ppm by mass and a Y content of 200 ppm by mass relative to the sintered compact in terms of metal, giving a mixed material. The mixed material was further mixed with a nylon pot and nylon balls by a ball mill for 100 hours, giving a mixed powder. The mixed powder was spray dried by a spray dryer, giving dried spherical granules. The dried spherical granules were placed in a mold with a diameter of 5 mm, then was subjected to primary molding at a pressure of 20 MPa, and was molded by cold isostatic pressing (CIP) at a pressure of 125 MPa, giving a molded article having a relative density of 59.8% determined by the Archimedes method. The molded article was heated to 600° C. at a temperature increase rate of 10° C./h and was maintained at 600° C. for 20 hours to remove the solvent, the plasticizer, and the lubricant. In order to sufficiently remove the solvent and the like, the molded article was further heated to 1, 100° C. and was maintained at 1, 100° C. for 10 hours. Next, the molded article was burned in a vacuum furnace at a burning temperature of 1,600° C. for 8 hours, and then was cooled to room temperature, giving a transparent rare earth aluminum garnet ceramic (Tb₃Al₅O₁₂, having a solid cylindrical shape with a diameter of 3 mm and a length of 25 mm). The vacuum in the vacuum furnace was 10⁻¹ Pa, the temperature increase rate during the burning was 300° C./h, and the temperature drop rate was 300° C./h.

(Measurement of Extinction Ratio and Scattering Coefficient)

Both end faces of the ceramic were mirror polished with a diamond slurry (without an anti-reflective coating layer). When the transparent rare earth aluminum garnet ceramic of the present invention is, for example, used as a Faraday rotator for an optical isolator, an anti-reflective coating is preferably provided. The extinction ratios and the scattering coefficients of the ceramic having a solid cylindrical shape with a diameter of 3 mm and a length of 20 mm prepared as above were determined at a wavelength of 1,064 nm (YAG laser) and a wavelength of 633 nm (He—Ne laser). In the measurement of the extinction ratio, a polarizer was placed on each side of the ceramic, then the transmitted light amount was measured without application of a magnetic field, next one polarizer was rotated 90 degrees, and the extinction ratio was calculated from the ratio of a maximum value and a minimum value of the transmitted light amount.

(Calculation of Scattering Coefficient and Extinction Ratio)

The scattering coefficient was calculated by expression (1), and the extinction ratio was calculated by expression (3). For all the samples, the measurement methods and the calculation methods of the scattering coefficient and the extinction ratio are the same.Scattering coefficient=[(theoretical transmission rate of TAG)−((W₂/W₁)×100)]/sample length (cm)  (1)

W₁: laser intensity value without ceramic in optical path

W₂: laser intensity value with ceramic placed in optical pathTheoretical transmission rate of TAG=[((R−1)²/(R+1)²)−1]²×100  (2)(at a wavelength of 1,064 nm, the refractive index R is 1.844, and the theoretical transmission rate of TAG is 83.2%)(at a wavelength of 633 nm, the refractive index R is 1.863, and the theoretical transmission rate of TAG is 82.7%)Extinction ratio=10×log₁₀(W₄/W₃)  (3)

W₃: the minimum value of laser intensity during rotation of polarizer with ceramic in optical path

W₄: the maximum value of laser intensity during rotation of polarizer with ceramic placed in optical path

Examples 2 to 13 and Comparative Example 1

The same procedure as in Example 1 was performed except that the contents of Si and Y relative to the sintered compact in terms of metal were changed, giving transparent rare earth aluminum garnet ceramics of Examples 2 to 13 and Comparative Example 1. The contents (ppm by mass) of Si and Y, the extinction ratios, and the scattering coefficients of the ceramics of Examples 1 to 13 and Comparative Example 1 are shown in Table 1.

	TABLE 1
	Content	Extinction ratio (dB)	Scattering coefficient (%/cm)
	(ppm by	Measurement	Measurement	Measurement	Measurement
Sample	mass)	wavelength	wavelength	wavelength	wavelength
No.	Si	Y	633 nm	1064 nm	633 nm	1064 nm	Remarks
1	2	50	26	28	0.7	0.5	Example 2
2	5	50	31	31	0.4	0.3	Example 3
3	50	50	32	33	0.4	0.2	Example 4
4	100	10	27	30	0.9	0.3	Example 5
5	100	20	31	32	0.3	0.2	Example 6
6	100	50	33	34	0.3	0.2	Example 7
7	100	100	34	34	0.3	0.2	Example 8
8	100	200	35	35	0.3	0.2	Example 1
9	100	500	32	32	0.3	0.2	Example 9
10	100	600	30	31	0.4	0.3	Example 10
11	100	750	27	29	0.9	0.5	Example 11
12	250	50	33	35	0.3	0.2	Example 12
13	500	50	24	26	0.8	0.7	Example 13
14	100	0	26	30	0.9	0.3	Comparative
							Example 1

As shown in Table 1, in Comparative Example 1 where the ceramic did not contain Y, the extinction ratio was 30 dB or less, and the scattering coefficient was not small. In Example 2 having a Si content of 2 ppm by mass, Example 5 having a Y content of 10 ppm, Example 11 having a Y content of 750 ppm, and Example 13 having a Si content of 500 ppm by mass, the extinction ratio was 30 dB or less, and the scattering coefficient was not small. Based on Table 1, when having a Si content of 5 ppm by mass to 250 ppm by mass and a Y content of 20 ppm by mass to 600 ppm by mass, the ceramic can achieve an extinction ratio of 30 dB or more and a scattering coefficient of 0.4%/cm or less at wavelengths 633 nm and 1,064 nm.

Examples 14 to 19

The same procedure as in Example 1 was performed except that the element Tb was replaced with Dy, Ho, Er, Tm, Yb, or Lu, giving transparent rare earth element aluminum garnet ceramics of Examples 14 to 19. The extinction ratios and the scattering coefficients of the ceramics of Examples 14 to 19 are shown in Table 2. When a specific absorption was observed at a wavelength 633 nm or 1,064 nm, the extinction ratio and the scattering coefficient were not measured.

	TABLE 2
	Extinction ratio (dB)	Scattering coefficient (%/cm)
		Measurement	Measurement	Measurement	Measurement
Sample		wavelength	wavelength	wavelength	wavelength
No.	Ceramic	633 nm	1064 nm	633 nm	1064 nm	Remarks
15	Dy3Al5O12	35	—	0.3	—	Example 14
16	Ho3Al5O12	—	35	—	0.2	Example 15
17	Er3Al5O12	—	35	—	0.2	Example 16
18	Tm3Al5O12	35	35	0.3	0.2	Example 17
19	Yb3Al5O12	35	—	0.3	—	Example 18
20	Lu3Al5O12	35	35	0.3	0.2	Example 19

As shown in Table 2, Tm₃Al₅O₁₂ of Example 17 and Lu₃Al₅O₁₂ of Example 19 achieve an extinction ratio of 35 dB and a scattering coefficient of 0.3%/cm or less at wavelengths of 633 nm and 1,064 nm.

Examples 20 to 25

The same procedure as in Example 1 was performed except that the pressure at the time of molding by CIP was changed to alter the relative density of the molded article by the Archimedes method, giving transparent rare earth aluminum garnet ceramics of Examples 20 to 25. The molding pressures by CIP, the relative densities, the extinction ratios, and the scattering coefficients of the ceramics of Examples 20 to 25 are shown in Table 3.

	TABLE 3
	Extinction ratio (dB)	Scattering coefficient (%/cm)
	Molding	Relative	Measurement	Measurement	Measurement	Measurement
Sample	pressure	density	wavelength	wavelength	wavelength	wavelength
No.	(MPa)	(%)	633 nm	1064 nm	633 nm	1064 nm	Remarks
21	30	48.3	24	25	2.0	1.5	Example 20
22	50	50.7	25	27	1.2	1.1	Example 21
23	85	53.9	28	29	1.0	0.8	Example 22
24	100	55.3	34	34	0.4	0.3	Example 23
25	130	60.5	35	35	0.3	0.2	Example 24
26	150	63.2	35	35	0.3	0.2	Example 25

As shown in Table 3, in Examples 20, 21, and 22 having a relative density of 48.3 to 53.9(%), the extinction ratio was 29 dB or less, and the scattering coefficient was not small. In contrast, in Examples 23, 24, and 25 having a relative density of 55.3 to 63.2(%), the extinction ratio was 34 dB or more, and the scattering coefficient was 0.4%/cm or less at wavelengths of 633 nm and 1,064 nm.

Examples 26 to 35

The same procedure as in Example 1 was performed except that the burning temperature and the burning time were changed, giving transparent rare earth aluminum garnet ceramics of Examples 26 to 35. The burning temperatures, the burning times, the average crystal grain sizes, the extinction ratios, and the scattering coefficients of the ceramics of Examples 26 to 35 are shown in Table 4. The burning time is the holding time at a burning temperature. No dense ceramic was obtained at a burning temperature of 1,300° C., and thus the extinction ratio and the scattering coefficient were not measured. The average crystal grain sizes in Table 4 are numerical values calculated by the expression described in paragraph 0011.

	TABLE 4
	Average	Extinction ratio	Scattering coefficient
	crystal	(dB)	(%/cm)
	Burning	Burning	grain	Measurement	Measurement	Measurement	Measurement
Sample	temperature	time	size	wavelength	wavelength	wavelength	wavelength
No.	(° C.)	(h)	(μm)	633 nm	1064 nm	633 nm	1064 nm	Remarks
27	1300	8	0.4	—	—	—	—	Example 26
28	1400	8	0.6	18	21	5.0	3.5	Example 27
29	1500	8	0.8	32	30	0.4	0.4	Example 28
30	1600	0.2	0.6	23	25	2.2	1.6	Example 29
31	1600	0.5	1.2	30	30	0.4	0.4	Example 30
32	1600	1	2.0	31	31	0.4	0.4	Example 31
33	1600	5	7.2	33	35	0.4	0.3	Example 32
34	1650	8	15	35	35	0.3	0.2	Example 33
35	1700	8	30	34	35	0.3	0.2	Example 34
36	1750	8	35	27	29	1.0	0.8	Example 35

As shown in Table 4, in Example 27 in which the burning temperature was 1,400° C., Example 29 in which the burning time was 0.2 hour at 1,600° C., and Example 35 in which the burning temperature was 1,750° C., the extinction ratio was 29 dB or less, and the scattering coefficient was not small. In contrast, in Examples 28 and 30 to 34 in which the burning temperature was 1,500 to 1,700° C. and the burning time was 0.5 to 8 hours, the extinction ratio was 30 dB or more, and the scattering coefficient was 0.4%/cm or less at wavelengths 633 nm and 1,064 nm.

Examples 36 to 48

The same procedure as in Example 1 was performed except that hot isostatic pressing (HIP) was performed after the burning step, giving transparent rare earth aluminum garnet ceramics of Examples 36 to 48. The temperatures and the pressures by HIP, the extinction ratios, and the scattering coefficients of the ceramics of Examples 36 to 48 are shown in Table 5. The treatment time of HIP was 3 hours.

	TABLE 5
		Scattering coefficient
	Extinction ratio (dB)	(%/cm)
			Measurement	Measurement	Measurement	Measurement
Sample	Temperature	Pressure	wavelength	wavelength	wavelength	wavelength
No.	(° C.)	(MPa)	633 nm	1064 nm	633 nm	1064 nm	Remarks
37	1250	196	35	35	0.3	0.2	Example 36
38	1350	100	38	38	0.2	0.1	Example 37
39	1350	150	38	38	0.2	0.1	Example 38
40	1450	100	40	41	0.2	0.1	Example 39
41	1450	150	41	42	0.2	0.1	Example 40
42	1550	100	43	44	0.2	0.1	Example 41
43	1550	150	44	45	0.1	0.1	Example 42
44	1600	40	35	35	0.3	0.2	Example 43
45	1650	100	45	47	0.1	0.1	Example 44
46	1650	150	46	48	0.1	0.1	Example 45
47	1700	100	44	46	0.2	0.1	Example 46
48	1700	150	42	43	0.2	0.1	Example 47
49	1750	100	25	28	0.8	0.6	Example 48

As shown in Table 5, by HIP at a temperature of 1,250 to 1,700° C. at a pressure of 40 to 196 MPa for 3 hours (Examples 36 to 47), an extinction ratio of 35 dB or more and a scattering coefficient of 0.3%/cm or less at wavelengths 633 nm and 1,064 nm can be achieved. However, when HIP was performed at an excessively high temperature of 1,750° C. (Example 48), the extinction ratio was 28 dB or less, and the scattering coefficient was not small.

Examples 49 to 57 and Comparative Example 2

The same procedure as in Example 1 was performed except that the contents of Si and Lu relative to the sintered compact in terms of metal were changed, giving transparent rare earth aluminum garnet ceramics of Examples 49 to 57 and Comparative Example 2. The contents (ppm by mass) of Si and Lu, the extinction ratios, and the scattering coefficients of the ceramics of Examples 49 to 57 and Comparative Example 2 are shown in Table 6.

	TABLE 6
	Content	Extinction ratio (dB)	Scattering coefficient (%/cm)
	(ppm by	Measurement	Measurement	Measurement	Measurement
Sample	mass)	wavelength	wavelength	wavelength	wavelength
No.	Si	Lu	633 nm	1,064 nm	633 nm	1,064 nm	Remarks
50	2	50	25	27	0.9	0.6	Example 49
51	5	50	30	30	0.4	0.3	Example 50
52	100	10	28	30	0.9	0.4	Example 51
53	100	20	30	31	0.4	0.3	Example 52
54	100	100	34	34	0.3	0.2	Example 53
55	100	600	30	31	0.4	0.3	Example 54
56	100	750	27	28	0.9	0.4	Example 55
57	250	50	32	34	0.3	0.2	Example 56
58	500	50	25	27	0.9	0.6	Example 57
59	100	0	26	29	0.9	0.4	Comparative
							Example 2

As shown in Table 6, in Comparative Example 2 where the ceramic did not contain Lu, the extinction ratio was 29 dB or less, and the scattering coefficient was not small. In Example 49 having a Si content of 2 ppm by mass, Example 51 having a Lu content of 10 ppm, Example 55 having a Lu content of 750 ppm, and Example 57 having a Si content of 500 ppm by mass, the extinction ratio was 30 dB or less, and the scattering coefficient was not small. Based on Table 1, when having a Si content of 5 ppm by mass to 250 ppm by mass and a Lu content of 20 ppm by mass to 600 ppm by mass, the ceramics can achieve an extinction ratio of 30 dB or more and a scattering coefficient of 0.4%/cm or less at wavelengths 633 nm and 1,064 nm.

INDUSTRIAL APPLICABILITY

The transparent rare earth aluminum garnet ceramic of the present invention can be used as a Faraday rotator for an optical isolator.

Claims

1. A transparent rare earth aluminum garnet ceramic represented by molecular formula R3Al5O12 wherein R is an element selected from the group consisting of rare earth elements having an atomic number of 65 to 70, and wherein the transparent rare earth aluminum garnet ceramic comprises at least two sintering aids comprising Si and Lu, and wherein the transparent rare earth aluminum garnet ceramic contains 5 ppm by mass to 250 ppm by mass of Si in terms of metal and 20 ppm by mass to 600 ppm by mass of Lu in terms of metal.