Speaker:
Professor William G. (Bill) Fahrenholtz from Missouri University of Science and Technology
Synopsis:
Reactive hot pressing of zirconium hydride and boron powders was used to synthesize and densify zirconium diboride ceramics. Sintering mechanisms and kinetics were studied by analyzing displacement during the hot pressing cycle for temperatures ranging from 1800°C to 2100°C. Below 2000°C, the densification mechanism was identified as grain boundary diffusion with activation energy of 240 kJ/mole while at higher temperatures densification proceeded by lattice diffusion with an activation energy of about 700 kJ/mole. The change in mechanism corresponds to a significant change in densification rate and microstructure development for these ceramics. Reactive hot pressing was used to produce a series of ZrB2 ceramics with Hf contents ranging from 0.03 at% up to 0.39 at%. These ceramics showed increasing thermal conductivity and heat capacity as Hf content decreased. The effects of Hf content on lattice vibrations and electronic structure revealed the likely mechanisms responsible for the changes.