A Free Energy Model of Boron Carbide

TL;DR

This paper develops a thermodynamic model combining first principles calculations to revise the boron-carbide phase diagram, revealing two distinct phases with different symmetries and compositions, and explaining their stability across temperatures.

Contribution

It introduces a new thermodynamic model that accurately describes boron carbide phases and their stability, improving upon the existing phase diagram with first-principles insights.

Findings

Identification of two boron-carbide phases with different symmetries and compositions.

Explanation of temperature-dependent stability of these phases.

Revised phase diagram consistent with experimental observations.

Abstract

The assessed phase diagram of the boron-carbon system contains a single non-stoichiometric 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 600K 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.