Pii: S0008-6223(97)85361-4
نویسندگان
چکیده
Oxidation is a serious problem for C/C composites above 4OOT, which limits their application at high temperatures. It is experimentally shown that boron (B), when substituting for C atoms in the graphite structure, may significantly improve the oxidation behavior of a C/C composite [l-4]. However, there is very little known about the role of boron in reducing the oxidation rate of graphite although several oxidation-inhibition mechanisms has been proposed [3,5]. One of the proposed mechanisms involves the alteration of reactivity at active sites on the edges of graphene layers by boron atoms. The main objective of this paper is to examine the possibility and validity of this particular mechanism by investigating the effect of boron atoms on the electronic properties of graphite using the semi-empirical Molecular Orbital Package (MOPAC). One main advantage of MOPAC is that one can easily obtain accurate geometric optimization and electronic structural parameters of atom clusters. Because of the weak interlayer interactions in graphite, we consider graphene monolayer with edges; a schematic representation of the atomic arrangement of which is shown in Fig. 1. It is generally believed that the edge sites are terminated with hydrogen atoms. However, since we are mainly interested in the electronic density distribution along the edge of graphene layer during oxidation, we assumed that the hydrogen atoms which were bonded to the carbon atoms were oxidized and removed from the edges, prior to the oxidation of
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