Combustion synthesis of advanced materials: Studies of the influence of gravity and reaction kinetics

Combustion synthesis is an attractive technique to synthesize a wide variety of advanced materials that include powders and near-net shape products of ceramics, intermetallics, composites and functionally gradient materials. It is also considered to be a valuable method for space applications, because of low energy requirements and simple equipment. However, it is necessary to understand how microgravity influences the combustion mechanism and properties of the synthesized materials.; In this work, combustion synthesis experiments were conducted both in normal and in low-gravity conditions, using a unique setup designed and developed for this purpose. Microgravity experiments were done in NASA Lewis Research Center using Drop Tower which provided 2.2 s of 10−5 g level, as well as on-board DC-9 aircraft (20 s of 10−2 g). It was clearly demonstrated that gravity plays an important role during combustion synthesis. It significantly influences both the combustion and structure formation processes. It was also shown that microgravity conditions allow the synthesis of materials with improved micro- and macrostructures.; The study of chemical reaction kinetics in combustion synthesis systems is of critical importance. The measurement of kinetic parameters (especially activation energy) and a comparison with known elementary processes provides an insight into the controlling step of the mechanism.; In this work, a computer-assisted electrothermography method was developed to determine the intrinsic kinetics of reactions under conditions similar to those realized during combustion synthesis of materials. This technique was applied to investigate the kinetics and other features associated with the reaction of titanium with nitrogen at 1 atm pressure. It was shown that at temperatures below the melting point of titanium, the reaction follows parabolic rate law. The obtained activation energy value is in good agreement with literature data. At higher temperatures, however, the reaction mechanism is different due to faster diffusion of nitrogen in molten titanium. Mathematical treatment of experimentally obtained heat generation curves allowed to estimate the diffusion coefficient of nitrogen in molten titanium. In addition, the reaction rate was shown to depend strongly on the heating rate.