A Free Energy Model of Boron Carbide
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The assessed phase diagram of the boron-carbon system contains a single nonstoichiometric boron-carbide phase of rhombohedral symmetry with a broad, thermodynamically improbable, low temperature composition range. We combine first principles total energy calculations with phenomenological thermodynamic modeling to propose a revised low temperature phase diagram that contains two boron-carbide phases of differing symmetries and compositions. One structure has composition B4C and consists of B11C icosahedra and C-B-C chains, with the placement of carbon on the icosahedron breaking rhombohedral symmetry. This phase is destabilized above 600 K by the configurational entropy of alternate carbon substitutions. The other structure, of ideal composition B13C2, has a broad composition range at high temperature, with rhombohedral symmetry throughout, as observed experimentally.
KeywordsBoron carbide Third law First principles Thermodynamics
- 2.Springer materials (2012). http://www.springermaterials.com
- 6.Hecker, S.: Los Alamos Sci. 26, 244 (2000) Google Scholar
- 10.Widom, M., Huhn, W.P.: Solid State Sci. (2012). http://dx.doi.org/10.1016/j.solidstatesciences.2012.05.010. http://arxiv.org/abs/1111.1737
- 16.Landau, L.D.: On the Theory of Phase Transitions. The Collected Papers of L.D. Landau. Pergamon, Elsmford (1965) Google Scholar