Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Patent
1994-08-01
1996-02-20
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
264 65, 264 66, 264 121, C04B 3546, B29D 1100
Patent
active
054928729
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to ceramic mirrors for use in laser and X-ray apparatuses, cameras, and other precision optical instruments and the like and also to a method of manufacturing the mirrors.
Mirrors for use with precision optical instruments and the like are predominantly made of glass, ruby, sapphire and the like, and also of single crystal ceramics. In recent years alumina and SiC have partly come in use as polycrystalline ceramics.
Glass-based mirrors are made relatively thick to make up for the inadequate strength and fragility, and therefore the heavy weight and large size of the mirrors as a whole have been a problem. Polycrystalline alumina, SiC, and other ceramic materials are used in the form of sintered bodies after grinding and lapping. None of them have proved fully satisfactory in respect of the properties such as surface smoothness and optical properties, because the pores inherent to the ceramic materials remain exposed on the lapped surfaces.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a mirror having excellent surface smoothness with fewer pores than heretofore known and having superior optical properties due to lower irregular reflection of light.
The present inventor, in an effort to solve these problems, sintered a high-purity, fine titanium oxide powder at a relatively low temperature. This has been found to give a mirror having excellent surface smoothness with a smaller number of pores than before and having superior optical properties due to less irregular reflection of light. The discovery has led to the perfection of the present invention.
The invention resides, in essence, in a ceramic mirror characterized by a surface flatness of no more than 1 .mu.m and/or a centerline average height of no more than 1 nm as measured by the laser interference method, and also in a method of manufacturing a ceramic mirror characterized by the steps of compacting a titanium oxide powder having an average particle diameter of no more than 1 .mu.m and a purity of at least 99% and sintering the compact in an ordinary, inert, vacuum, or reducing atmosphere at a temperature between 1000.degree. and 1300.degree. C.
DETAILED DESCRIPTION
The invention will now be described in detail.
In accordance with the invention a fine titanium oxide powder of high purity is used as a material and is sintered at a lower temperature than the usual sintering temperatures to obtain a sintered body with excellent surface smoothness, inhibiting the particle growth and preventing the pore size increase while achieving an adequate sintered density.
The reason why a ceramic mirror having outstanding characteristics can be obtained by the aforesaid method is theoretically to be clarified yet. Possibly the low temperature sintering of a fine titanium oxide powder keeps the pores within the crystal grains (a phenomenon known as "ghosting"), suppressing the accumulation and growth to a grain boundary phase.
The expression "a lower temperature than the usual sintering temperatures" as used herein means a temperature range from 1000.degree. to 1300.degree. C. If the temperature is below 1000.degree. C. the sintering itself does not proceed. A temperature above 1300.degree. C., on the other hand, causes rapid grain growth and therefore a decrease in mechanical strength and pore migration and growth to a grain boundary phase. Consequent reduction of surface smoothness and irregular reflection of light renders it impossible to obtain a mirror of satisfactory optical characteristics.
If the material powder is not at least 99% pure titanium oxide or has a larger average particle diameter than 1 .mu.m, it is no longer easily sinterable at low temperatures. Sintering in the temperature range of 1000.degree. to 1300.degree. C. will not give a product with an adequate sintered density.
For this reason it is desirable that the titanium oxide powder have a purity of at least 99%, preferably at least 99.8%, and an average particle diameter of no more than 1 .mu.m, preferably
REFERENCES:
patent: 3459503 (1969-08-01), Roy et al.
patent: 3615759 (1971-10-01), Busdiecker et al.
patent: 3639132 (1972-01-01), Egerton et al.
patent: 4492669 (1988-09-01), Gould
patent: 5071596 (1991-12-01), Goela et al.
Review of Scientific Instruments, vol. 61, No. 2, Feb. 1990, pp. 728-731.
Journal of Optics, vol. 20, No. 5, Oct. 1989, pp. 219-223.
NTIS Tech Notes, Sep. 1989, pp. 728.
Applied Optics, vol. 19, No. 20, Oct. 15, 1980, pp. 3562-3584.
Bell Mark L.
Marcheschi Michael
Nihon Cement Co. Ltd.
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