Experimental measurements and general conclusions on the effective thermal conductivity of powdered metal hydrides

The effective thermal conductivity K_e of Mg₂NiH₄ and MmNi₄FeH_5.2 (Mm ≡; misch metal) was measured under steady state conditions as a function of the hydrogen pressure and the temperature. The effective thermal conductivity of the two hydrides increases significantly as a function of the hydrogen pressure up to approximately 40 atm and then remains almost constant. The K_e values for Mg₂NiH₄ and MmNi₄FeH_5.2 are 0.83 W m^-1 K^-1 and 1.05 W m^-1 K^-1 at 373 K and 273 K respectively and a hydrogen pressure of 40 atm. The results were analysed using the Yagi-Kunii model for the effective thermal conductivity in powder-fluid beds. The solid thermal conductivity K_s of the above two hydrides was then estimated. The most important and significant result of the present work is the conclusion that K_e "saturates" at relatively low values of K_s. For example, the maximum K_e values (above the hydrogen breakaway pressure) at 300 K and a typical hydride void fraction (ε{lunate} = 0.5) change from about 0.4 to about 1.5 W m^-1 K^-1 for a corresponding K_s change between 1 and 500 W m^-1 K^-1. A survey of the existing data in the literature for the effective thermal conductivities of powdered metal hydrides supports the above conclusion. The practical meaning of this result is that, in engineering applications of hydrides in which a knowledge of K_e is necessary, it can safely be assumed that K_e ≈ 1-2 Wm^-1K^-1.