Mechanistic and product structure formation studies in the combustion synthesis of advanced materials.

摘要

Combustion synthesis is a novel technique that has been used to synthesize a wide variety of materials including ceramics, ceramic-metal composites, intermetallics and nanophase materials. It is based on the principle that once initiated by an external heat source, a highly exothermic reaction can self-propagate through the sample without requiring additional heat. The unique combustion synthesis features of extremely fast heating rates ({dollar}10sp5-10sp6{dollar} K/s), high temperatures (2500-3500 K) and short reaction times (on the order of a few seconds) have the potential to yield materials with novel structures and properties. However, in order to control this technique effectively, knowledge of combustion synthesis mechanisms is of critical importance.; The mechanisms of various systems were investigated using complementary techniques including systematic studies of processing variables, particle-foil experiments, quenching and time-resolved X-ray diffraction (TRXRD). In the particle-foil experiment, a lower melting particle reactant placed on the foil of a refractory reactant was heated rapidly by passing a current through the foil. This experiment provided a suitable experimental model for isolating the initial step in the mechanism of melting and spreading of one reactant. Quenching was achieved by reacting a wedge-shaped sample imbedded in a copper block in which the propagating combustion front progressively extinguished while traveling to the apex, resulting in regions reacted to different extents. Analysis of the quenched regions provided microstructural and phase composition information about the structure formation sequence. The TRXRD technique allowed a continuous in situ observation of phase composition during the reaction.; The utilization of the various complementary techniques has led to the identification of structure formation mechanisms of various silicide and intermetallic aluminide compounds. A general classification of the mechanisms based on the melting points of reactants and products relative to the maximum reaction temperature was obtained. The classification provides an experimental basis for developing a generalized theoretical model of the combustion synthesis process. This model will provide principles for scale-up and ultimately, lead to the design of advanced materials with tailored properties.