Crossover of thermal conductivity in SiC-reinforced ZrB2-HfB2 composites at elevated temperatures

Ultra-high temperature ceramics, especially di-borides, are regarded as potential candidates for thermal protection system in hypersonic vehicles by virtue of their high thermal conductivity. However, studies on thermal conductivity behaviour of ultra-high temperature ceramic di-borides exhibit scattered data at high temperatures. Herein, we report a systematic study of a range of di-boride compositions aiming at multiple aspects including the effect of temperature on thermal conductivity of monolithic di-borides (ZrB2 and HfB2), effect of SiC and carbon nanotubes (CNTs) addition on ZrB2 and HfB2 with increasing temperature till 1200 ?, and effect on thermal conductivity with systematic incorporation of ZrB2 and HfB2 into each another. Highest experimental thermal conductivity (63-83 W m(-1) K-1) observed in 20 vol% SiC reinforced ZrB2 composite (in the temperature range of 50-1200 ?) witnessed crossover (and drop) than that of monolithic sample (above 900 ?) due to increased phonon-phonon scattering in SiC with the rise in temperature. Concurrently, agglomerated CNT reinforcement further decreased thermal conductivity due to increased inhomogeneity in solid solutioning and enhanced phonon scattering.