|About this Abstract
||Materials Science & Technology 2012
||Phase Stability, Diffusion, Kinetics and their Applications (PSDK-VII)
||First Principles Investigation of the Thermodynamically Improbable Phase Diagram of Boron-Carbon
||Michael Widom, William Paul Huhn
|On-Site Speaker (Planned)
Phase diagrams with composition ranges extending to low temperature are thermodynamically improbable, as pointed out by Okamoto and Massalski. Boron-carbon provides such an example, with the boron carbide phase of rhombohedral symmetry, denoted B4C, extending over the composition range 9% to 19.2% carbon, nearly independently of temperature. First principles total energy studies reveal the presence of two distinct optimal structures, one with rhombohedral symmetry and ideal composition B13C2 (13.3% carbon), the other with monoclinic symmetry and ideal composition B4C (20% carbon). Substitutional disorder destabilizes the monoclinic structure above T=600K, resulting in a single rhombohedral phase of broad composition range at high temperature, consistent with experimental observation. We propose a revised phase diagram in the low temperature regime.