Claims
- 1. A ceramic preform which will withstand molten metal infiltration comprising monocrystalline hexagonal platelets of .alpha.-alumina having a porosity of at least 70% and essentially consisting of pores having a radius larger than 0.1 .mu.m.
- 2. The ceramic preform as defined by claim 1, essentially consisting of said hexagonal platelets of .alpha.-alumina.
- 3. The ceramic preform as defined by claim 1, comprising at least 90% by weight of such .alpha.-alumina.
- 4. The ceramic preform as defined by claim 1, further comprising a reinforcing amount of at least one reinforcing material.
- 5. The ceramic preform as defined by claim 4, said at least one reinforcing material comprising ceramic whiskers, fibers or fine particles.
- 6. The ceramic preform as defined by claim 5, comprising a predominant amount by weight of said hexagonal platelets of .alpha.-alumina.
- 7. A ceramic preform which will withstand molten metal infiltration comprising monocrystalline hexagonal platelets of .alpha.-alumina, porous in nature and having a pore radius distribution such that .sigma..sup.2 =R 84/R 16.ltoreq.3, wherein R 84 is the pore radius of 84% of the cumulative pore volume, R 16 is the pore radius of 16% of the cumulative pore volume, with R 84/R50=G, and G being the standard deviation of the pore radius distribution.
- 8. The ceramic preform as defined by claim 1, said hexagonal platelets essentially consisting of macrocrystals having a diameter ranging from 2 to 50 .mu.m, a thickness ranging from 0.1 to 2 .mu.m and a diameter/thickness ratio ranging from 5 to 100.
- 9. A process for the preparation of the ceramic preform as defined by claim 1, comprising crystallizing a settled powder of amorphous alumina, transition alumina or alumina hydrate into a coherent mass of said hexagonal platelets of .alpha.-alumina.
- 10. The process as defined by claim 9, carried out in the presence of a flux which comprises chemically bonded fluorine and which, in the molten state, is a solvent for said alumina.
- 11. The process as defined by claim 10, said flux comprising at least one non-hydrolyzable fluorine-containing compound, or a system comprising a non-hydrolyzable fluorine-containing compound phase and a hydrolyzable fluorine-containing compound phase, one of said phases being dissolved in the other.
- 12. The process as defined by claim 10, carried out at a temperature higher than the melting point of the flux.
- 13. The process as defined by claim 12, carried out at a temperature ranging from 900.degree. to 1,200.degree. C.
- 14. The process as defined by claim 10, said settled powder further comprising hexagonal platelets of .alpha.-alumina.
- 15. A refractory brick shaped from the ceramic preform as defined by claim 1.
- 16. A ceramic shaped article comprising the ceramic preform as defined by claim 1.
- 17. The ceramic preform as defined by claim 1, wherein the preform is porous and the platelets are randomly aligned.
- 18. The preform as defined in claim 1, said preform being stable at temperatures above 1,500.degree. C.
Priority Claims (1)
Number |
Date |
Country |
Kind |
90 06448 |
May 1990 |
FRX |
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Parent Case Info
This application is a continuation of application Ser. No. 08/335,953, filed Nov. 3, 1994 (now abandoned), which is a continuation of application Ser. No. 08/071,038, filed Jun. 2, 1993 (now abandoned), which is a continuation of application Ser. No. 07/915,757, filed Jul. 21, 1992 (now abandoned), which is a continuation of application Ser. No. 07/704,368, filed May 23, 1991 (now abandoned).
US Referenced Citations (15)
Continuations (4)
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Number |
Date |
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Parent |
335953 |
Nov 1994 |
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Parent |
71038 |
Jun 1993 |
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Parent |
915757 |
Jul 1992 |
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Parent |
704368 |
May 1991 |
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