Structure and single-phase regime of boron carbides
The boron carbides are composed of twelve-atom icosahedral clusters which are linked by direct covalent bonds and through three-atom intericosahedral chains. The boron carbides are known to exist as a single phase with carbon concentrations from about 8 to about 20 at. %. This range of carbon concentrations is made possible by the substitution of boron and carbon atoms for one another within both the icosahedra and intericosahedral chains. The most widely accepted structural model for B/sub 4/C (the boron carbide with nominally 20% carbon) has B/sub 11/C icosahedra with C-B-C intericosahedral chains. Here, the free energy of the boron carbides is studied as a function of carbon concentration by considering the effects of replacing carbon atoms within B/sub 4/C with boron atoms. It is concluded that entropic and energetic considerations both favor the replacement of carbon atoms with boron atoms within the intericosahedral chains, C-B-C..-->..C-B-B. Once the carbon concentration is so low that the vast majority of the chains are C-B-B chains, near B/sub 13/C/sub 2/, subsequent substitutions of carbon atoms with boron atoms occur within the icosahedra, B/sub 11/C..-->..B/sub 12/. Maxima of the free energy occur at the most ordered compositions: B/sub 4/C,B/sub 13/C/sub 2/,B/sub 14/C. This structural model, determined by studying the free energy, agrees with that previously suggested by analysis of electronic and thermal transport data. These considerations also provide an explanation for the wide single-phase regime found for boron carbides.
- Research Organization:
- Sandia National Laboratories, Albuquerque, New Mexico 87185
- OSTI ID:
- 6845198
- Journal Information:
- Phys. Rev. B: Condens. Matter; (United States), Journal Name: Phys. Rev. B: Condens. Matter; (United States) Vol. 38:9; ISSN PRBMD
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360202* -- Ceramics
Cermets
& Refractories-- Structure & Phase Studies
BORON CARBIDES
BORON COMPOUNDS
CARBIDES
CARBON
CARBON COMPOUNDS
ELECTRONS
ELEMENTARY PARTICLES
ELEMENTS
ENERGY
ENTROPY
FERMIONS
FREE ENERGY
LEPTONS
NONMETALS
PHYSICAL PROPERTIES
QUANTITY RATIO
STRUCTURAL MODELS
THERMODYNAMIC PROPERTIES
TRANSPORT THEORY