Abstract
Some of the nonstoichiometric carbides of the transition metals (e.g. VC x ) undergo transformations to ordered phases at specific values of x. In a thermodynamic study of such order-disorder transformations the latent heats of the reactions VC0.833=V6C5 and VC0.875=V8C7 were measured using differential thermal analysis (DTA) and gave values of 5.7±2.8 cal g−1 and 6.0±2.4 cal g−1 (1 cal=4.1855 J), respectively. For well-annealed specimens intermediate in carbon-to-metal ratio between these two compositions, DTA indicated the coexistence of domains of both ordered phases. The implication is that the nonstoichiometric, disordered, NaCl-structure phase VC x does not represent the lowest energy state of vanadium carbide at room temperature for any value of x. This result may apply to the other isomorphic carbides, i.e. TiC x , ZrC x , HfC x , NbC x and TaC x . The measured temperatures corresponding to the onset of the transformations were 1184±12‡ C for V6C5 and 1112±8‡ C for V8C7, but the centre of the distribution was approximately 50‡ C higher in each case. A rationalization of the existence of first-order transitions and an estimate of the latent heats expected from the measured transition temperatures were attempted using the Bragg-Williams theory of order-disorder transformations. The results are consistent with experiment, though the adequacy of this approximation for such a complex case is questionable.
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Emmons, G.H., Williams, W.S. Thermodynamics of order-disorder transformations in vanadium carbide. J Mater Sci 18, 2589–2602 (1983). https://doi.org/10.1007/BF00547575
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DOI: https://doi.org/10.1007/BF00547575