LATTICE-VIBRATIONS OF BORON CARBIDES

The internal friction at low temperatures has been measured on polycry stalline samples of beta-B, B4C, B13C2, and B9C. As the carbon deficie ncy increases in the boron carbides, the internal friction approaches that of amorphous solids, For comparison, the internal friction of amo rphous B9C, produced as a thin film by e-beam evaporation on a Si doub le-paddle oscillator, has also been measured, It is very similar to th at of the polycrystalline B9C sample. The results are discussed in ter ms of glass-like lattice vibrations which have been noted before in hi ghly disordered crystals, e.g., Ba1-yLaxF2x. It is concluded that the low thermal conductivity of B9C near room temperature which has been o bserved previously is indeed equal to its theoretical lower limit. (C) 1997 Academic Press.