Properties can be tailored in many different ways.
The term “ecoceramics” (a contraction of “environment-conscious ceramics”) denotes a class of ceramics made partly from wood-based products, which can include natural wood, sawdust, cardboard, and/ or paper. In addition to the environmental advantage of renewability of the carbonaceous ingredients, the concept of ecoceramics offers an advantage of tailorability of the properties of the ceramic end products.
In the case of natural wood, the natural open cellular structures can serve as templates. Wood preforms can be laminated to optimize the toughnesses and strengths of the resulting ceramics. Alternatively or in addition, wood and sawdust layers can be alternated to obtain desired properties.
The wood-based preform of an ecoceramic object is pyrolized at a temperature of as much as 1,000 °C in a chemically inert atmosphere (nitrogen or argon). If the intended product is a silicon carbidebased ceramic, then the pyrolyzed preform is infiltrated with molten silicon or a molten silicon alloy. The temperature and time of infiltration depend on the type of preform and the composition of the infiltrating material: Typical infiltration temperatures are 40 to 50 °C higher than the melting temperatures of the infiltrating materials, and typical infiltration times range from 15 minutes to an hour. For example, infiltration by silicon at 1,450 °C for 30 minutes is sufficient for making ecoceramics from preforms derived from natural oak.
To make a porous silicon carbide-based ceramic, one can begin by dipping a preform derived from natural wood in an aqueous slurry that contains 40 to 60 percent of silicon or a silicon alloy powder, the balance comprising organic binders and water. After dipping in the slurry, the preform is dried, then heated in a furnace to a temperature above the melting temperature of the silicon or the silicon alloy. The silicon or silicon alloy melts and reacts with cell walls of the wood, yielding a porous structure.
It is also possible to make nonceramic carbon-matrix composite materials through infiltration of thermoplastic or thermoset resins into preforms derived from wood. In other variations on the basic theme, preforms derived from wood can be infiltrated with such diverse materials as organometallic polymers; oxide precursors (sols and slurries); molten binary, ternary, and higher oxides; metals; intermetallic compounds; ferro silicon; and combinations of the aforementioned materials. The infiltrants can be selected to provide hard and soft phases with desired thermomechanical, magnetic, electronic, piezoelectric, and/or wear properties. In yet another variation, small and simply shaped ecoceramic parts can be joined to make larger, more complexly shaped, and/or functionally graded parts.
This work was done by Mrityunjay Singh of Dynacs Engineering Co., Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Commercial Technology Office,
Attn: Steve Fedor,
Mail Stop 4–8,
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Refer to LEW-17043.