SELF-PROPAGATING HIGH TEMPERATURE SYNTHESIS UNDER MICROGRAVITY CONDITIONS

摘要

Self-propagating high temperature reactions are characterized by the fact that, once ignited by an external energy source, are able to propagate in the form of a combustion wave through the reacting mixture without requiring additional energy. This type of reactions have been exploited in the establishment of the technique referred to in the literature with the acronym of SHS (Self-propagating High temperature Synthesis) to obtain a variety of advanced materials such as ceramics, intermetallics, composites, solid solution, functionally graded materials, etc. The SHS method has received increasing attention for its simplicity, short reaction time, easy-to-build equipment, low-energy requirements and the possibility of obtaining complex or metastable phases. It is known that several parameters affect combustion synthesis reactions. For instance, reaction stoichiometry, green density, reactants particle size, thermal conductivity, ignition temperature, heating and cooling rates, physical states of reactants, as well as the presence of a gravitational field. are demonstrated to have an important effect on final product morphology and properties. Specifically, stoichiometry (including the use of diluents or inert reactants) affects the exothermicity of synthesis reaction and, therefore, the dynamics of the combustion process. In particular the addition of inert reactants leads to heat subtraction and, consequently, to a decrease of combustion temperature. The green density of the reacting sample also influences SHS processes, since it affects system reactivity as well as the thermal conductivity of the compact. Similar considerations can be made for other operating parameters. However gravity has been shown to play an important and specific role in self-propagating high temperature synthesis reactions. In fact, combustion synthesis and the related structure formation mechanisms involve several stages including melting of reactants and products, spreading of the melt, droplet coalescence, diffusion and convection, buoyancy of solid particles, and densification of the liquid product, most of which are affected by gravity. In particular generated liquid and gaseous species will be subject to gravity-driven fluid flow and vapour transport and convection, which are likely to significantly affect both SHS reaction stability and morphology of product phases. It is then apparent the importance of investigating the effect of gravity on the above mentioned phenomena in order to identify the detailed mechanism of reaction evolution and structure formation. Specifically, low gravity experiments are able to permit the general mechanism of combustion and structure formation to be revealed without disturbing effect of gravity, also through a direct comparison to data from equivalent ground based experiments Along these lines, interesting results have been recently obtained in USA, Japan, Canada and Russia. It has been shown that products with finer and more uniform micro-structure are obtained under low gravity conditions with respect to terrestrial ones. Finally, it should be noted that, since self-propagating combustion synthesis technology has the potential to prepare advanced materials and near net-shape articles with tailored physical and mechanical properties in one step, it is well-suited for direct use on space platform. The synthesis of the composites TiB_2-xTiAl and TiB_2-xTiAl_3, under both terrestrial and micro gravity (10~(-2)g where g is the terrestrial gravity acceleration) conditions are studied in this work. Experiments under micro gravity conditions have been performed during the 32~(th) ESA Parabolic Flights Campaign (Bordeaux, March 2002). An important objective of this experimental research is to provide a contribution towards the understanding of structure formation mechanism during SHS reactions, taking advantage of the so called Combustion Front Quenching (CFQ) technique. In facts, the latter one is based on the rapid exti