In this program, materials processing and synthesis capabilities of two techniques (enhanced vapor transport and self-propagating high-temperature synthesis or reactive sintering) will be combined to produce ceramics and ceramic- ceramic composites with unique microstructures and properties, including porous ceramics, foamed or expanded ceramics, fine ceramic powders, and ceramic whiskers. The research is aimed at attacking a major limitation of current combustion processing, namely lack of control over and uniformity of microstructures produced. Enhanced vapor transport, which can stem from either the ambient gas permeating the packed powder preform or the presence of vapor production within the powder compact during processing, has been shown to be a good way to control microstructure during conventional sintering. The general approach to be followed in this program will be centered around development of understanding of the microstructure development during reactive sintering/SHS processing in the presence of such enhanced vapor transport. Two model systems, a simple one-component TiB2 ceramic system, and a more complex TiB2-Al2O3 ceramic composite system, will be studied, with combustion synthesis of these materials being carried out both in argon and in a reactive gas (e.g., HCl); effects of gas composition, particle size, "green" density, and degrees of dilution of the reactants on product morphology will be examined. Complementary modeling studies will be included to help develop a better understanding of the underlying mechanisms controlling the resulting microstructure. Combustion synthesis has been demonstrated to be an effective process for producing a wide variety of both oxide and nonoxide ceramics. Under controlled conditions, ceramic- ceramic and metal-ceramic composites can be fabricated in- situ, with considerable energy savings and decrease in process complexity relative to alternate procedures for producing materials with desired properties. (In some cases, combustion synthesis actually offers the only route to certain desired products.) As indicated above, a current major limitation of combustion synthesis is the lack of control and uniformity of the microstructures produced; gaining understanding and control of the microstructure development will represent a major advance in this materials production technology.
