ULTRASONIC STUDY OF SINTERED SiC AT LOW TEMPERATURES

Abstract

Abstract We investigated the temperature dependence, at low temperaturs, of sound velocity in sintered silicon carbide samples, containing three different sintering additives, boron or beryllium oxide or aluminum nitride. It was found that samples containing AlN or B showed lnT dependence (T is the temperature) of sound velocity but the one with BeO did not. The slope of the lnT dependence, which is proportional to the density of states of two level tunneling systems (TLS), is much larger for SiC with AlN than that with B. This sequence and no lnT dependence in BeO, correlate with the solubility of these additives into SiC, suggesting that the TLS, at least in this case, are created by the additives in solid solutions in the crystals. The samples containing B were heat treated to allow grain growth as well as densification. The density of states of TLS in these samples (measured by the slope of the lnT dependence of velocity) increased as the grain size increased. This suggests that the TLS do not reside in grain boundaries and that B migrates from the grain boundaries (where it originally segregated) into the bulk, thus creating more TLS inside the grains.