The effects of gravity on combustion synthesis of advanced materials

Combustion synthesis (CS) is an attractive technique to synthesize a wide variety of advanced materials while utilizing relatively simple equipment and low energy. The process is characterized by fast heating rates, short reaction times and high temperatures. These high temperatures generate liquids and gases which are subject to gravity-driven flow. The removal of such gravitational effects is likely to provide increased control of the reaction front, with a consequent improvement in controlling the microstructure of the synthesized products. Therefore, microgravity (μg) experiments can lead to major advances in the understanding of fundamental aspects of combustion and structure formation under the extreme conditions of the CS wave. In addition, the specific features of μg environment allow one to produce unique materials which cannot be obtained under terrestrial conditions.; In this work, identical experiments were conducted in different conditions: terrestrial (1 g), μg (10−5–10−2 g) and overload (2 g) to make direct comparison and to identify the role of gravity on mechanism of combustion and microstructure formation. Different gravity conditions were generated during free fall in the 2.2 seconds drop tower (10−5 g) and parabolic flights of KC-135 aircraft (10−2 g; 2 g) at National Aeronautics & Space Administration (NASA) Glenn Research Center, Cleveland, OH. Quenching by massive copper block was utilized to investigate the mechanisms of microstructural transformation taking place during CS.; Research on the influence of gravity along the following three main directions was carried out: (1) Microstructural transformations during CS of metal-ceramic composites; (2) Mechanisms of heat transfer during reactions in heterogeneous media; (3) Reaction and phase separation mechanisms in thermite-type combustion reactions.; It was demonstrated that gravity has significant influence in CS along all the investigated directions: (1) Diffusion processes in melts under μg conditions occur slower as compared with 1 g conditions, therefore finer and more uniform microstructures were obtained. (2) Combustion of complex metal particle clouds in inert atmosphere has been observed for the first time. (3) Buoyancy is not the only controlling mechanism of phase separation in multiphase melts.; The results obtained can be used to optimize processing conditions for synthesis of advanced materials with desired microstructures and properties.