Thermal Expansion of Micro- and Nanocrystalline ZrB2 Powders

Nano- and microcrystalline ZrB2 powders have been studied by high-temperature X-ray diffraction in the temperature range 300-1400 K, and their unit-cell parameters have been measured as functions of temperature. The thermal expansion coefficient (TEC) of ZrB2 has been shown to be a linear function of temperature, and its thermal expansion has been shown to be anisotropic: in the temperature range 300-600 K, both the micro- and nanocrystalline ZrB2 powders have anisotropic thermal expansion, with alpha(a) < alpha(c). Above 640 K, the a-axis TEC of ZrB2 exceeds its c-axis TEC. The thermal expansion of the nanocrystalline ZrB2 powder has been shown to be considerably smaller than that of the microcrystalline ZrB2. The anomalously small thermal expansion of the nanocrystalline ZrB2 is tentatively attributed to the effect of a boric anhydride layer on the surface of the nanoparticles.