Compositions: ceramic – Ceramic compositions – Yttrium – lanthanide – actinide – or transactinide containing
Reexamination Certificate
2003-02-28
2004-11-30
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Yttrium, lanthanide, actinide, or transactinide containing
C264S681000
Reexamination Certificate
active
06825144
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a transparent sintered rare earth metal oxide body represented by R
2
O
3
(R being at least one element of a group comprising Y, Dy, Ho, Er, Tm, Yb and Lu) and a production method thereof. The sintered body of this invention can be used satisfactorily, for example, as materials for infrared rays transmission windows, polarization plates, discharge lamp envelopes, optical parts, and laser oscillators.
PRIOR ART
The rare earth metal oxides represented by a general formula R
2
O
3
(R being at least one element of a group comprising Y, Dy, Ho, Er, Tm, Yb and Lu) have a cubic crystal structure and show no double refraction. Hence they can provide sintered bodies of excellent transparency when pores and segregation of impurities are completely eliminated from them.
Among them, yttria (Y
2
O
3
) has a melting point of 2415° C. being the highest of those of rare earth metal oxides, has a good heat resistance and a good alkali resistance, and exhibits high transparency in the infrared region. Moreover, as yttria has high thermal conductivity, it is expected as a host material for a solid-state laser. However, as yttria has a very high melting point and exhibits phase transition (between cubic crystal and hexagonal crystal) in the neighborhood of 2280° C., it is difficult for the existing single crystal synthetic techniques to synthesize large crystals of excellent optical properties. On the other hand, as its ceramics (polycrystals) can be synthesized at relatively low temperatures below its melting point, efforts have been made extensively to apply its ceramics as high temperature window materials for infrared rays, discharge lamp envelopes, corrosion resistant members, etc.
In preparing transparent sintered bodies, not limited in those of rare earth metal oxides, what is most important is whether elimination of pores can be well done during grain growth in the sintering stage. A technique of adding a sintering additive is normally used to control the velocity of grain growth. In the greater part of the production methods of yttria which have been reported up to the present, a sintering additive is added.
The following methods are known as production methods of transparent yttria using a sintering additive:
(1) A method of adding ThO
2
and sintering in a hydrogen atmosphere at 2100° C. or over (Ceramic Bulletin Vol. 52, No5(1973));
(2) A method of sintering Y
2
O
3
powder, to which AlF
3
is added, by a vacuum hot press (Japanese Provisional Patent Sho 53-120707);
(3) A similar method of hot pressing Y
2
O
3
powder to which LiF or KF is added (Japanese Provisional Patent Hei 4-59658); and
(4) A method of adding La
2
O
3
or Al
2
O
3
and sintering in a low O
2
atmosphere (Japanese Provisional Patent Sho 54-17911, Japanese Provisional Patent Sho 54-17910).
In the method of (1), radioactive thoria is added as a sintering additive, which is not easy to obtain and handle, although its addition will provide a sintered body of a relatively high transparency. Moreover, as sintering is carried out at a high temperature for a long period of time, the mean grain size is as large as 100 &mgr;m or over, and the strength of the material is extremely low. Hence the sintered body is not applicable to daily use. The hot pressing method of (2) allows sintering at a relatively low temperature. It, however, can only provide sintered bodies of which in-line spectral transmittance in the visible region is about 60%.
According to the method of (3), sintered bodies of which in-line spectral transmittance in the infrared region at a wavelength of 2 &mgr;m or over is about 80% can be produced by hot press at a temperature of 1500° C. or over. The transmittance in the visible region is not certain because it is not indicated therein. However, the fluorides which are added as sintering additives have low melting points (LiF: 842° C.; KF: 860° C.) and may evaporate in the sintering process to generate a gap in the velocity of grain growth between the circumferential portion and the internal portion of the sample. Therefore, it is estimated to be difficult to produce a homogeneous sintered body when the sample is thick. According to Majima, et al. (Journal of Japan Inst. Metals Vol. 57, No. 10 (1993) P.1221-1226), it is reported that when LiF is used as an additive and hot pressing is used, even if the amount of additive is optimized, fluorine will remain in the central portion of the sample, and the transmittance thereof will be lower in comparison with that of the peripheral portion of the sample. Accordingly, it is not easy to use fluorides as sintering additives to produce large-sized and thick sintered bodies.
According to the method of (4), La
2
O
3
is added by about 6 to 14 mol %, and La
2
O
3
which can not be solid dissolved tends to form a segregation phase (refer, for example, to Journal of Materials Science 24 (1989) 863-872), hence it is not easy to prepare an optically homogeneous sintered body. According to the method of Al
2
O
3
addition, the amount of the additive is from 0.05 wt % to 5 wt %, and high density bodies are prepared by liquid phase sintering at a temperature not lower than the eutectic temperature between Y
4
Al
3
O
9
and Y
2
O
3
(1920° C.). However, in spite of the sintering at a high temperature, the transmittance of the sintered bodies thus obtained is only 80%, at the highest, of the theoretical transmittance.
On the other hand, production methods of yttria with no sintering additive are disclosed in Japanese Patent No. 2773193 and Japanese Provisional Patent Hei 6-211573. According to Japanese Patent No. 2773193, yttria powder having BET value of 10 m
2
/g or over is hot pressed to achieve maximum density of 95% or over of the theoretical density, and after that, HIP treatment is given. The transmittance of the sintered body thus obtained is as good as about 80% in the infrared region of 3-6 &mgr;m of wavelengths, but the transmittance in the region of 0.4-3 &mgr;m in wavelength remains to be about 75% in average. This insufficient transparency in the shorter wave region in spite of the HIP treatment may be attributed to the use of ultra-fine powder, which is difficult to handle, as the starting material; although the surface of the sintered body is densified by hot pressing, large voids which are difficult to be removed even by the HIP treatment tend to remain in the inner part of the sample.
According to the method of Japanese Provisional Patent Hei 6-211573, transparent bodies are prepared by CIP-molding easily sinterable powder having a mean particle size of 0.01-1 &mgr;m and vacuum sintering at 1800° C. or over or giving HIP treatment at 1600° C. or over. It is stated that the sintered bodies obtained by this method have a mean in-line spectral transmittance as high as 80% or over in the visible region, and that it is possible to prepare a sintered body which can make laser oscillation by adding a luminiferous element. However, to prepare a sample of high transparency, it is necessary to execute sintering at a high temperature around 2000° C. in either case of vacuum sintering or HIP treatment. In the case of industrial continuous production, the degradation of the sintering furnace is excessive and the maintenance of the furnace is troublesome. Moreover, when the wavelength gets shorter, the transmittance will drop markedly (when the wavelength is reduced from 1000 nm to 400 nm, the transmittance decreases by 10 or more percent). Hence this method is not appropriate for producing optical parts of which transparency in the visible region is important.
Generally, mother salts of rare earth metal oxide material powders used in the conventional methods are oxalates. The material powders which are obtained by calcining oxalates are composed of highly aggregated secondary particles and their particle size distributions are inhomogeneous. Hence packing by molding can not be done sufficiently, and it is not easy to prepare high density bodies. To improve this point, methods of preparing transparent bodies, which us
Hideki Yagi
Takagimi Yanagitani
Group Karl
Konoshima Chemical Co. Ltd.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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