Improvement of high temperature stability of Pd coating on Ta by HfN intermediate layer

Hafnium nitride (HfN) was chosen as a material for non-porous intermediate layer to improve the high temperature stability of Pd-Ta composite membranes for hydrogen separation. A layer of dense HfN (70 nm) was prepared between Ta substrate and thin Pd film (300 nm), and the high temperature stability of Pd coating was examined by hydrogen absorption experiments at 573 K after the heat treatments at 873 and 973 K. The HfN layer showed obvious hydrogen permeability, though the permeation rate in HfN appeared to be smaller than that in Pd and Ta. In addition, the degradation in coating effects of Pd at elevated temperatures was substantially retarded by HfN layer. Such improved stability was ascribed to retardation of open porosity development in Pd films and interdiffusion between Pd and Ta. It was concluded that HfN is a potential candidate material for intermediate layer to improve high temperature stability of Pd-group 5 metal composite hydrogen separation membranes.