The effect of carbon morphology on the combustion synthesis of titanium carbide
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Erica Riley Chemical Research, Development, and Engineering Center, Aberdeen Proving Grounds, Maryland 21010 (Received 15 April 1988; accepted 25 October 1988)
Titanium carbide may be readily produced in high purity via direct reaction between the solid phases of titanium and carbon in the process of combustion synthesis, also known as self-propagating high-temperature synthesis (SHS). The high temperatures generated by this exothermic reaction ( TiC
'Work performed while at Research Triangle Institute, Research Triangle Park, North Carolina 27709.
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Titanium Carbide Synthesis Mechanism
AH = -44.1 kcal/mol.
The actual procedure for making this material is fairly simple. The two reactant powders are mechanically mixed in stoichiometric portions and cold-pressed into a green compact. A heat source (such as a tungsten heating coil, oxy-acetylene torch, electron beam, etc.) is applied to the surface of the compact until an ignition temperature of 1500° to 1600 °C is attained. Once ignited, the heat of reaction between titanium and carbon generates an adiabatic temperature of over 3000 °C, sufficient to propagate a
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combustion wave through the remainder of the unreacted powder. A simple diagram of this action is shown in Fig. 1. Although the overall process seems straightforward, the combination of events occurring in the propagation zone is actually quite complex. Hardt and Phung report one of the very few mathematical models of the SHS process in this region.1 Several details of the overall reaction are described as follows. At the reaction temperature (in excess of 3000 °C), titanium becomes molten, flows to the carbon surface, and subsequently diffuses into and reacts with the carbon substrate. The reaction temperature is still below the melting temperature of carbon (>38OO °C) and, hence, much of the original carbon particle morphology is preserved. Sintering of the ceramic product does occur at temperatures above 2000 °C and, thus, very small physical features may not survive. Gases adsorbed on the reactant particles or trapped in
J. Mater. Res., Vol. 4, No. 2, Mar/Apr 1989
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Direction of Propagating Wave Front
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.Concentration of Product
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Heat Reactants Transfer I Synthesis (Ti + C) Zone
Temperature
Products (TiC)
FIG. 1. Diagram of the combustion synthesis mechanism. 1989 Materials Research Society
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M. E. Mullins and E. Riley: Effect of carbon morphology
the interstices expand in volume several-fold due to the rapidly rising temperatures and are forced from the compact. The heat of reaction is transferred to the adjacent reactant materials, and the process continues. Since the reaction itself is quite vigorous, it seems likely that the rate of propagation is dependent upon either the rate of heat transfer within the green compact or the rate of titanium transport (both flow and diffusive) to the carbon. The resulting products are therefore quite independent of the titanium powder morphology, but retain some of t
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