GB2089777A - Process for heat-treating single crystal of tungstate - Google Patents
Process for heat-treating single crystal of tungstate Download PDFInfo
- Publication number
- GB2089777A GB2089777A GB8137633A GB8137633A GB2089777A GB 2089777 A GB2089777 A GB 2089777A GB 8137633 A GB8137633 A GB 8137633A GB 8137633 A GB8137633 A GB 8137633A GB 2089777 A GB2089777 A GB 2089777A
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- United Kingdom
- Prior art keywords
- crystal
- tungstate
- single crystal
- melting point
- oxygen
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
- C01G41/00—Compounds of tungsten
- C01G41/006—Compounds containing, besides tungsten, two or more other elements, with the exception of oxygen or hydrogen
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
- C01G41/00—Compounds of tungsten
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
- C30B29/10—Inorganic compounds or compositions
- C30B29/16—Oxides
- C30B29/22—Complex oxides
- C30B29/32—Titanates; Germanates; Molybdates; Tungstates
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2002/00—Crystal-structural characteristics
- C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
- C01P2002/84—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by UV- or VIS- data
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/32—Thermal properties
- C01P2006/33—Phase transition temperatures
- C01P2006/34—Melting temperatures
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2006/00—Physical properties of inorganic compounds
- C01P2006/60—Optical properties, e.g. expressed in CIELAB-values
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Inorganic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Crystallography & Structural Chemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Crystals, And After-Treatments Of Crystals (AREA)
- Measurement Of Radiation (AREA)
- Luminescent Compositions (AREA)
Abstract
The optical transmission of a single tungstate crystal of the formula MWO4,wherein M represents at least one of Mg, Zn and Cd, is improved by heating at a temperature below the melting point of the crystal, in an oxygen-containing atmosphere.
Description
SPECIFICATION
Process for heat-treating single crystal of tungstate
Background of the invention
This invention relates to a process for heattreating single crystal of tungstate.
Heretofore, Bi4Ge3O12 has been used as a scintillatorfor radiation detectors in an apparatus such as
X-ray computed tomography scanner, but has not always been satisfactory with respect to luminescence intensity, luminescence decay time, etc.
The present inventors have already filed a patent application for applying single crystal of tungstate represented by the general formula, MWO4, where
M stands for at least one divalent metal element of
Mg, Zn and Cd, and a solid solution of the divalent metal element and tungstate to a scintillator material for detecting radiation and ultraviolet rays. Single crystal of tungstate is grown by the Czochrolski technique (pulling technique), and then cut to predetermined sizes for scintillators and used. Strong luminescence is required for the scintillator, and the luminescence intensity of scintillator greatly depends upon luminescence efficiency with respect to an incoming radiation and absorption of luminescence by crystal itself, and a higher luminescence efficiency and zero absorption are desirable.The tungstate and solid solution containing at least one of Mg, Zn and Cd, and W is a scintillator material having a high luminescence efficiency at room temperature and the luminescence spectra have a peaks at a wavelength of 480 - 490 nm. For increasing luminescence intensity of tungstate, it is necessary to eliminate from the crystal or solid solution an impurities causing decrease in luminescence efficiency and absorption of luminescence, but even if the total impurity concentration is made lower than 5 ppm, the crystal shows a light brown color. For example, in the case of a ZnWO4 crystal plate having a thickness of 2 mm, light transmission at the wavelength of 480 nm is only 60%, and the low optical transmission causes absorption of luminescence generated by radiation and consequent decrease in luminescence intensity, as described above.
On the other hand, the following reference is also known to show the state of the art: i.e. Japanese
Published Unexamined Patent Application No.
130899/55.
Summary ofthe invention
An object of the present invention is to provide treatment of single crystal for obtaining a scintillator having a high luminescence intensity.
Another object of the present invention is to clarify the causes for the absorption of luminescence, thereby eliminating the absorption of luminescence and increasing the luminescence intensity.
These and other objects of the present invention can be attained according to a process for heattreating single crystal, which comprises heating single crystal of tungstate represented by the general fromula MWO4, wherein M represents at leaset one element selected from the group consisting of
Mg, Zn and Cd, at a temperature of less than the melting point but of higher than the temperature by 200"C lower than the melting point in an oxygencontaining atmosphere.
Brief description of the drawing
Single figure is a diagram showing optical transmission curves before and after the heat treatment of ZnWO4 single crystal.
Detailed description ofpreferred embodiments
Since tungstate is a crystal capable of readily generating the oxygen vacancy, and also since tungsten has a plurality of valencies, it seems that the coloring of the crystal is caused by the oxygen vacancy-tungsten ions and oxygen vacancy-impurity element ions. This can be confirmed by the fact that the optical absorption takes place in a broad range of wavelength and generally the melt of oxide has a high oxygen decomposition pressure, so that the crystal resulting from the melt is liable to be in an oxygen-deficient state.
According to the present invention, the grown crystal is heated under a high oxygen partial pressure, whereby the oxygen is diffused into the crystal from the crystal surface to eliminate the oxygen vacancy generated at the growth of the crystal. The absorption of luminescence (maximum luminescence wavelength: 480 nm) can be eliminated, and the luminescence intensity can be increased thereby.
That is, the present process for heat-treating single crystal is characterized by heating crystal of tungstate at a temperature of less than the melting point but of higher than the temperature by 200"C lower than the melting point in an oxygen-containing atmosphere.
Practically preferable heating temperature is as near to the melting point as possible, owing to more rapid diffusion of oxygen, but heating at a temperature a little lower than the melting point roughens the crystal surface, and thus it is more preferable to heat the single crystal at a temperature by at least 30"C lower than the melting point. On the other hand, no remarkable effect can be obtained at a temperature by at least 200"C lower than the melting point. That is, it is more preferable to heat the single crystal in a temperature range between the temperature by 30 C lower than the melting point and the temperature by200'C lower than the melting point.
The heating atmosphere must be an oxygencontaining atmosphere. When the single crystal is heated in a reductive atmosphere, the compound is reduced and decomposed. Thus, it is preferable to heat the single crystal in a mixed atmosphere of an inert gas and oxygen, particularly in an atmosphere containing at least 10% by volume of oxygen. An atmosphere of 100% oxygen is not objectionable.
The present invention will be described in detail below, referring to Examples.
Example 1
First of all, an example of zinc tungstate (ZnWO4) will be described. ZnWO4 has a melting point of 1,220 + 5"C (1,190"C according to J. C. Brice), and single crystal growth was carried out in a platinum crucible by the Czochralski technique. To prevent decomposition and evaporation of melt, oxygen was used for the growth atmosphere. Single crystal grown from high purity raw material showed slightly brown color, and optical transmission of the single crystal in the ultraviolet and visible range is shown by curve "A" in Figure. The sample had a thickness of 2 mm. As is evident from the transmission curve, broad absorption took place, and the transmission was 60% at the maximum luminescence wavelength of 480 nm.When the single crystal was heated in the air at 1,100"C for 30 hours, oxygen diffused into the crystal from the crystal surface, and the crystals turned light yellow from brown. The depth to the boundary of these two colors from the crystal surface, that is, distance of oxygen diffusion, is directional. Under the heating conditions at 1,100"C for 30 hour, it was 2.4 - 2.7 mm in the directions of axis a [100] and axis b [0101 and 3.2 mm in the direction of axis c [001]. The anisotropy of the diffusion depth is based on the anisotropy of crystal structure oftungstate (single crystal). Optical transmission of yellow-turned crystal is shown by curve "B" in Figure.As a result of the heat treatment, new absorption appeared at a wavelength of 340 - 420 nm, but the transmission by ZnWO4 at the maximum luminescence wavelength of 480 nm was 72%.
Correction of the transmission bythe surface reflection ofthe ZnWO4 having a high refractive index (n = 2.14) revealed thatthetransmission had no substantial absorption.
The absorption of luminescence was decreased and the luminescence intensity was 7% increased by eliminating the crystal color in the manner as described above.
The luminescence intensity was measured in the following manner. The crystal was irradiated with y rays (energy: 60 keV) from radioactive isotope 24arm, and the generated luminescence was connected to electrical pulses by a photomultipier. The height of the electrical pulses was analyzed by a wavelength analyzer to determine the pulse height in most frequent numbers (peak channel value) as the fluorescence intensity. The fluorescence intensity of CdWO4 crystal and (Zn, Mg)WO4 crystal was determined in the same manner as described above.
Investigation of the heat treatment conditions revealed that the diffusion coefficient of oxygen in ZnWO4was 1.4 x 10-10 m2/s at 1.180"C and 1.2 x 10-10 m2/s at 1,140"C. It was found that the activation energy determined from the temperature dependency of the diffusion coefficient was 4.0 x 10-19 j Practically desirable temperature is as near to the melting point as possible owing to rapid diffusion of oxgyen.
With heating at 1,0600C for 30 hours, on the other hand, the diffusion distance is as small as 1.7 mm, and heating below 1,000 C is practically not suitable.
Investigation of heat treatment atmosphere revealed that no decolorization took place when heated in a N2 atmosphere, whereas the same effect as in the air was obtained in the N2 atmosphere containing 10% by volume of 02. Needless to say, a good effect could be obtained in an 2 atmosphere.
However, no influence of oxygen partial pressure upon the diffusion distance was observed under an oxygen partial pressure above 10% by volume of 2 in the N2 atmosphere.
It is seen from the foregoing that preperable heat treatment condition for ZnWO4 single crystal is heating in a temperature of 1,020 to 1,1900C in an atmosphere containing at least 10% by volume of oxygen.
Example 2
An example of heat treatment of cadmium tungstate (CdWO4) single crystal will be described.
CdWO4 has the melting point of 1,325"C, which is by 1000C higher than that of ZnWO4. The single crystal grew by the Czochralski technique in the same manner as that for ZnWO4. The grown CdWO4 single crystal had a yellow color, but turned less yellow when heated under an oxygen partial pressure of at least 10% by volume in the same manner as that for
ZnWO4. Diffusion coefficient of oxygen from the crystal surface was 1.0 x 10-10 m2/s at 1,240"C and was substantially equal to that of ZnWO4. The luminescence intensity was 5% increased. No substantial diffusion was also observed at 1,000 C in the case of CdWO4. It can be said that the effect is less at a temperature by at least 200"C lower than the melting point also in the case of CdWO4.
Example 3
MgWO4 single crystal was not directly prepared from the melt. However, single crystal of solid solution of (Zn.Mg)WO4 could grow. That is, in a system of ZnWO4 and MgWO4, single crystals of solid solutions each containing 1%, 5% and 10% by weight of MgWO4 were grown by the Czochralski technique. The growth rate (pulling rate) of crystal was 3 mm/hr. The luminescence intensity of the respective grown crystals was substantially equal to that of ZnWO4. These single crystals had substantially equal color and transmission curve to those of
ZnWO4. The crystal containing 10% by weight of
MgWO4 had a melting point by about 20"C higher than that of ZnWO4, and the desirable heat treatment condition is heating at 1,040"C or higher.Diffusion coefficient was 1.5 x 10-10 m2/s at 1,180"C and was equal to that of ZnWO4. The luminescence intensity of the solid solutions was 7% increased, as compared with those before the heat treatment.
It is seen from the foregoing that in the present invention, the luminescence intensity can be 5 - 7% increased by heating single crystals of ZnWO4,
CdWO4 and (Zn.Mg)WO4 at a temperature of higher than 1,000"C, preferably at a temperature between the temperature by 30"C lower than the melting point and the temperature by 200"C lower than the melting point, under a high oxygen partial pressure.
In the foregoing Examples, three kinds of single crystals of ZnWO4, CdWO4 and (Zn.Mg)WO4 have been described. ZnWO4, MgWO4 and CdWO4 have the identical crystal structures and similar properties, and thus similar effect can be obtained in the heat treatment of solid solutions of these compounds.
Furthermore, the present heat treatment has an effect of removing residual strain in the crystal, and can facilitate processing of single crystal of tungstate with a high cleavage and an easy cracking property.
Claims (7)
1. A process for producing a tungstate crystal of improved optical transmission, which comprises heating a single crystal of tungstate of the formula
MWO4, wherein M represents at least one element selected from Mg, Zn and Cd, at a temperature lower than the melting point of the crystal but higher than 200"C below the melting point, in an oxygencontaining atmosphere.
2. A process according to claim 1, wherein the oxygen-containing atmosphere is an atmosphere containing 10% to 100% by volume of oxygen, the balance being an inert gas.
3. A process according to claim 1 or 2, wherein the heating temperature is between 30"C below the melting point and 200"C below the melting point.
4. A process according to any one of the preceding claims, wherein the tungstate is ZnWO4.
5. A process according to any one of claims 1 to 3, wherein the tungstate is CdWO4.
6. A process according to any one of claims 1 to 3, wherein the tungstate is (Zn.Mg)WO4.
7. A process according to claim 1 substantially as hereinbefore described with reference to any one of the Examples.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP55175807A JPS57100999A (en) | 1980-12-15 | 1980-12-15 | Heat treatment of single crystal of tungstic acid compound |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2089777A true GB2089777A (en) | 1982-06-30 |
| GB2089777B GB2089777B (en) | 1984-06-13 |
Family
ID=16002574
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8137633A Expired GB2089777B (en) | 1980-12-15 | 1981-12-14 | Process for heat-treating single crystal of tungstate |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPS57100999A (en) |
| DE (1) | DE3148988C2 (en) |
| GB (1) | GB2089777B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2003008676A1 (en) * | 2001-07-12 | 2003-01-30 | Furukawa Co., Ltd. | Method for preparing tungstate single crystal |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4560877A (en) * | 1982-12-29 | 1985-12-24 | General Electric Company | Solid state detector module |
| JP2003041244A (en) * | 2001-07-25 | 2003-02-13 | Furukawa Co Ltd | Scintillator |
| JP2005343753A (en) * | 2004-06-03 | 2005-12-15 | Mitsui Mining & Smelting Co Ltd | Zinc tungstate single crystal and its production method |
| JP2005263515A (en) * | 2004-03-16 | 2005-09-29 | Mitsui Mining & Smelting Co Ltd | Zinc tungstate single crystal and its manufacturing method |
| WO2005078171A1 (en) * | 2004-02-17 | 2005-08-25 | Mitsui Mining & Smelting Co., Ltd. | Zinc tungstenenate single crystal and method for preparation thereof |
| JP2005263621A (en) * | 2004-02-17 | 2005-09-29 | Mitsui Mining & Smelting Co Ltd | Zinc tungstate single crystal and its manufacturing method |
| JP2005272274A (en) * | 2004-03-26 | 2005-10-06 | Mitsui Mining & Smelting Co Ltd | Zinc tungstate single crystal and its manufacturing method |
| JP5087913B2 (en) | 2006-05-30 | 2012-12-05 | 日立化成工業株式会社 | Single crystal for scintillator and method for producing the same |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2240301A1 (en) * | 1972-08-16 | 1974-02-28 | Siemens Ag | PROCESS FOR MANUFACTURING SEMICONDUCTOR SINGLE CRYSTAL BARS WITH SPECIFIC RESISTANCE DROPPING DOWN TO THE CENTER OF THE BAR |
| DE2550154A1 (en) * | 1975-11-07 | 1977-05-12 | Alusuisse | THERMAL TREATMENT OF SUBSTRATE PLATES |
-
1980
- 1980-12-15 JP JP55175807A patent/JPS57100999A/en active Granted
-
1981
- 1981-12-10 DE DE3148988A patent/DE3148988C2/en not_active Expired
- 1981-12-14 GB GB8137633A patent/GB2089777B/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2003008676A1 (en) * | 2001-07-12 | 2003-01-30 | Furukawa Co., Ltd. | Method for preparing tungstate single crystal |
| CZ300221B6 (en) * | 2001-07-12 | 2009-03-18 | Furukawa Co., Ltd. | Method for preparing tungstate single crystal |
Also Published As
| Publication number | Publication date |
|---|---|
| DE3148988C2 (en) | 1986-01-02 |
| JPS646160B2 (en) | 1989-02-02 |
| JPS57100999A (en) | 1982-06-23 |
| GB2089777B (en) | 1984-06-13 |
| DE3148988A1 (en) | 1982-06-24 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 711A | Proceeding under section 117(1) patents act 1977 | ||
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20001214 |