Quantum chemistry study on boron carbides and Si-doped ones

The correlation among composition, structure, chemical bond and thermoelect ric boron carbide and Si doped ones was studied using DV-X. method, with 5 classes of 28 models. The representative structural unit of boron carbide i s [C-B-B(C)](delta-) [B11C](delta+). The models with C-B-B chains and B11C icosahedra are the stablest, and the charges of their icosahedra are the lo west, so the bipolaron forms easily, which is consistent with the experimen tal result that the conductivity has the largest value when the carbon cont ent is 13.3%, The representative structural unit of doping Si is [C-B-Si](t +)[B11C](t). As there are more types of structural units after the addition Si, the energy required by the disproportionation reaction decreases, and there are more paths for the bipolaron to hopping. At the same timet the co valent bond becomes weaker, and the thermal conductivity decreases. Therefo re, the thermoelectric property of Si-doped boron carbides is improved.