Thermal and electrical transport in ZrB2-SiC-WC ceramics up to 1800 °C

The thermophysical properties of dense ZrB2-20 vol% SiC ceramics (ZS) with and without doping of 5 vol % WC (ZSW) were characterized up to 1800 degrees C. Because of the in-situ formed WB with low thermal conductivity and the strong phonon scattering effect from (Zr,W)B-2 solid solution, the thermal conductivity of ZSW (36.2 W/(m K)) at room temperature is much lower than that of ZS (100.4 W/(m K)), however, their differences at 1800 degrees C is very limited (42.2 W/(m K) for ZSW and 47.4 W/(m . K) for ZS). The detailed calculation indicates that contributions from lattice vibrations on the thermal conductivity decreased to nearly zero at 1800 degrees C in both samples, therefore, electrical conductivity of ZS and ZSW dominates their thermal transport behavior at higher temperatures. Furthermore, amorphous phase with compositions of Ca-Al-Si-O was occasionally found at the triple junction of ZS, its softening and rewetting of the grain boundary at higher temperature were confirmed from the internal friction curve and microstructural characterization at different levels, which was thought to be responsible for the tremendous increase of Interfacial thermal resistance and continuous drop of thermal conductivity of ZS up to 1800 degrees C. On the contrary, benefiting from the clean boundary and tight triple junction in ZSW, its thermal conductivity can stabilize between 36 and 47 W/(m K) in a broad temperature range from 25 to 1800 degrees C. Based on the measurement above, the type of carriers and thermoelectric figure of merit (zT) in ZrB2-SiC based ceramics are also firstly reported. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.