Publication Date:
2016-04-02
Description:
The elastic properties of alloys between boron suboxide (B 6 O) and boron carbide (B 13 C 2 ), denoted by (B 6 O) 1− x (B 13 C 2 ) x , as well as boron carbide with variable carbon content, ranging from B 13 C 2 to B 4 C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B 6 O) 1− x (B 13 C 2 ) x is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational disorder in B 13 C 2 , where a part of the C atoms in the CBC chains substitute for B atoms in the B 12 icosahedra, drastically increase the Young’s and shear modulus, as compared to an atomically ordered state, B 12 (CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B 4 C to B 13 C 2 . The elastic moduli of the (B 6 O) 1− x (B 13 C 2 ) x system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B 6 O) 1− x (B 13 C 2 ) x . The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B 6 O-rich as well as ordered or disordered B 13 C 2 -rich domains in the material prepared through equilibrium routes is predicted.
Print ISSN:
0021-9606
Electronic ISSN:
1089-7690
Topics:
Chemistry and Pharmacology
,
Physics
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