Extending the compositional limit of combustion-synthesized B 4 C-TiB 2 composites by field activation
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I.
INTRODUCTION
THE feasibility of synthesizing composite materials directly from elemental reactants is one of the primary reasons for the current interest in the self-propagating combustion method. The use of this method to synthesize composite materials has been amply demonstrated in numerous investigations. These include the synthesis of metalmatrix and ceramic-ceramic composites) ~,2] An example of the latter is the system B4C-TiB2, which has been previously investigated as a preform for liquid metal infiltration in armor applicationsJ 3] In this regard, the specific gravity of the composite is of concern, and hence the molar ratio, y, of B~C/TiB: is an important factor. Using elemental reactants, such a composite can be synthesized by a selfsustaining combustion method for y < 0.5 when the reactants are at an initial temperature, To, of 298 K. [31 For y > 0.5, no self-sustaining combustion wave can be established without raising To. For example, for y values of 1.0 and 2.0, TO must be 800 and 1200 K, respectively. However, the preheating of the reactants to such temperatures has been shown to result in the formation of additional phases, in this case, TiC and TiB.I41 The formation of these phases is attributed to diffilsional processes occurring prior to combustion, as has also been observed in the synthesis of MoSi2. [5] The fact that composites of B4C-TiB 2 with y > 0.5 cannot sustain self-propagating combustion when To = 298 K is the consequence of a low adiabatic combustion temperature, T~, as seen in Figure 1. This is the same limiting factor for the lack of success of synthesizing B4C and SiC directly by the self-propagating high-temperature synthesis (SHS) method. I6:l For these and similar materials, an SHS process can be attained through thermal activation (preheating), as indicated previously, or through the use of an electric field. The earlier use of an electric field focused on providing Joule heating to the reactants to raise their temperature up to the ignition point.Is.9I This method, however, is restricted to systems in which the reactant's conductivity is relatively high at room temperature. More recently, a new method has been used in which the reactants are ignited in
the presence of an electric field. The method of field-activated synthesis was first demonstrated for the synthesis of /3_SiC[m.~.,21 and has since been used to synthesize MoSi2SiC and other composites/u.r4~ The result o f these studies showed that the effect of the field is largely confined to the combustion zone. The wave velocity increased linearly with an increase in field strength. On the basis of a combustionzone confinement, a model was developed to simulate field activation. Modeling and experimental results were in qualitative agreementY m21 In this work, we investigate the field activation of the combustion synthesis of B4C-TiB 2 composites with B~C/TiB 2 ratios as high as 8. II.
EXPERI[VIENTAL MATERIALS AND M E T H O D
Powders of boron, graphite, and titanium were used in the appropriate stoichiometric ratios
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