Lightweight, refractory ceramic/carbon thermal-insulation materials have been invented. These materials, consist of carbon, silicon, and oxygen in suitable pro- portions combined into molecular structures that are stable at high temperatures. Insulating tiles and other components made of these materials can retain their shapes and strengths at temperatures as high as 1,700 °C.
The production of a component of this type begins with the fabrication of a substrate (preform) of porous carbon (e.g., carbon felt or tile). This substrate serves as the source of carbon for the ceramic/ carbon material to be formed. The carbon substrate also serves as a framework that supports the other materials to be added (as described below) to the carbon to form the ceramic/carbon material and, hence, defines the size and shape of the component to be formed.
A sol-gel containing dialkoxy and trialkoxy silanes (and possibly tetralkoxy silanes, depending on the application) plus an alcohol and water is prepared, and gelling is initiated by adding an acid or base to the sol-gel. Before significant gelling occurs, the carbon substrate is impregnated by immersion in, or coating with, the sol-gel. Once impregnation has occurred, gelation is allowed to proceed. Gelation can occur at ambient temperature, but is preferably accelerated by heating to a temperature between 40 and 90 °C. Once gelation is complete, the excess gel is removed from the impregnated substrate, then the impregnated substrate is dried in a vacuum — typically overnight at a temperature between 70 and 100 °C — to remove volatiles. The dried ceramic substrate thus becomes a ceramic precursor.
The ceramic is formed by pyrolyzing the dried, gel-impregnated substrate in a vacuum or an inert gas (e.g., argon) at temperatures that can range from 800 to 1,500 °C. The carbon of the substrate enters into the pyrolysis reaction with the dried gel, thereby becoming part of the ceramic. The ceramic pyrolysis product contains –Si–C–Si– and –Si–O–C– bonds. The excess of C provided by the carbon substrate results in a predominance of –Si–C–Si bonds: this is fortunate because at high temperature, the –Si–C–Si– bonds are more stable than are the Si–O–C– bonds.
The substrate can be subjected to multiple cycles of impregnation, drying, and pyrolysis. Each cycle adds to the weight, strength, and high-temperature endurance of the finished product. Hence, one chooses the number of cycles in a tradeoff between light weight on the one hand and strength and high-temperature endurance on the other hand.
This work was done by Daniel B. Leiser of Ames Research Center and Ming-ta S. Hsu and Timothy S. Chen of HC Chem Research.
This invention has been patented by NASA (U.S. Patent No. 6,225,248). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel,
Ames Research Center,
Refer to ARC-14202.