Lightweight, monolithic ceramics that retain their shapes and strengths and resist oxidation at temperatures up to 1,200 °C have been invented. These ceramics are made of carbon, silicon, oxygen, and boron. These ceramics are made by (1) using a sol-gel process to infiltrate noncarbon ingredients into lightweight, porous carbon preforms; then (2) pyrolizing the infiltrated preforms.
A suitable carbon preform could be a piece of felt or boardstock, for example. Inasmuch as the finished monolithic ceramic article has the same size and shape as those of the preform, it is usually advantageous to start with a preform of the desired net size and shape.
In the sol-gel process used in this invention, the preform is immersed in a sol that comprises a mixture of silicon alkoxides and a borate ester (typically di- and tetrafunctional siloxanes and a boron alkoxide), then the sol is gelled in place. (Gelation comprises simultaneous hydrolysis and polymerization reactions.) The sol is prepared by mixing the siloxane and boron alkoxide reagents (see figure), preferably with an alcohol as a diluent. The alcohol prevents premature hydrolysis of the sol and ensures homogeneity of the sol.
Although the sol can be gelled by aging at ambient temperature or by heating, it is preferable to catalyze gelation by addition of an acid (e.g., HNO3) or a base (e.g., NH4OH) to the reaction mixture. Gelation of the catalyzed sol eventually occurs at ambient temperature, but it is further preferable to heat the impregnated preform gently to a temperature between 40 and 90 °C.
After gelation, the impregnated preform is removed from the gel and any surplus gel adhering to the preform is wiped off. The impregnated preform is then dried to form a ceramic precursor; preferably, the drying is done in a vacuum oven overnight at a temperature between 70 and 100 °C to ensure that all volatiles are removed before the pyrolysis step described next.
The dried, impregnated preform is heated in an inert gas (e.g., argon) or in a vacuum, preferably at a temperature between 900 and 1,200 °C. During this heating process, the carbon of the preform enters into pyrolysis reactions with the dried gel and thereby becomes part of the ceramic.
The Trapezoid on This Ternary Diagram encloses the range of proportions of three typical ingredients that can be used to make sols according to this invention. The range of optimum proportions is enclosed by the circle.
This work was done by Daniel B. Leiser, Ming-ta Hsu, and Timothy S. Chen of Ames Research Center.
This invention has been patented by NASA (U.S. Patent No. 5,618,766). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Ames Research Center; (650) 604-5104
Refer to ARC-12096