Mechanically activated solid-state synthesis of hafnium carbide and hafnium nitride nanoparticles

Nanocrystalline hafnium nitrides and hafnium carbides are synthesized, from powder mixtures based on partially hydrated hafnium tetrachloride, ph-HfCl4, and on magnesium, by a three-step process, namely a short mechanical activation step followed by a brief annealing step and a leaching step. Grinding of ph-HfCl4 + Mg leads to the formation of hafnium hydrides which decompose upon annealing in argon to give first metallic hafnium and then HfO2 at higher temperatures. Annealing of such ground ph-HfCl4 + Mg blends in flowing nitrogen yields HfN nanoparticles (average size between 10 and 30 nm). HfC carbides are directly mechanosynthesized from ph-HfCl4 + Mg + C powder mixtures. A subsequent heat treatment in flowing argon allows to better crystallize the grains of HfC (average size around 100 nm) and to decompose the intermediate products. The reaction mechanisms are discussed from results of thermogravimetric analyses and from infrared spectra of ph-HfCl4 + Mg based powder mixtures processed in various ways.