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The Effect of Gravity on the Combustion Synthesis of Porous Ceramics and Metal Matrix CompositesCombustion synthesis (self propagating, high temperature synthesis-SHS) is a novel technique that is capable of producing many advanced materials. The ignition temperature (Tig) of such combustion synthesis reactions is often coincident with that of the lowest melting point reactant. The resultant liquid metal wets and spreads around the other solid reactant particles of higher melting points, thereby improving the reactant contact and kinetics, followed by formation of the required compounds. This ignition initiates a combustion propagating wave whose narrow reaction front rapidly travels through the reactants. Since this process is highly exothermic, the heat released by combustion often melts the reactant particles ahead of the combustion front and ignites the adjacent reactant layer, resulting in a self-sustaining reaction. Whenever a fluid phase (liquid or gas) is generated by the reaction system, gravity-driven phenomena can occur. Such phenomena include convective flows of fluid by conventional or unstable convection and settling of the higher density phases. A combustion process is often associated with various kinds of fluid flow. For instance, if the SHS reaction is carried out under inert or reactive gas atmospheres, or a volatile, e.g., B2O3, is deliberately introduced as a reactant, convective flows of the gas will occur due to a temperature gradient existing in the atmosphere when a combustion wave is initiated. The increased gas flow will produce a porous (or expanded) SHS product. Owing to the highly exothermic nature of many SHS reactions, liquid phase(s) can also form before, at, or after the combustion front. The huge temperature gradient at the combustion front can induce convective flows (conventional or unstable) of the liquid phase. Each of these types of convective fluid flow can change the combustion behavior of the synthesizing reaction, and, therefore, the resultant product microstructure. In addition, when two or more phases of different density are produced at or ahead of the propagating combustion front settling of the higher density phase will occur resulting in a non-uniform product microstructure and properties.
Document ID
19970020551
Acquisition Source
Legacy CDMS
Document Type
Conference Paper
Authors
Moore, J. J.
(Colorado School of Mines Golden, CO United States)
Woodger, T. C.
(Colorado School of Mines Golden, CO United States)
Wolanski, T.
(Colorado School of Mines Golden, CO United States)
Yi, H. C.
(Guigne International Ltd. Paradise, Newfoundland Canada)
Guigne, J. Y.
(Guigne International Ltd. Paradise, Newfoundland Canada)
Date Acquired
August 17, 2013
Publication Date
May 1, 1997
Publication Information
Publication: Fourth International Microgravity Combustion Workshop
Subject Category
Materials Processing
Accession Number
97N21824
Funding Number(s)
CONTRACT_GRANT: CSA-4700468-1
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.
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