Heat Conduction in Metal Hydride Nano-Particles

Metal hydrides hold significant potential for use in solid-state hydrogen storage through reversible chemical reactions of metal constituents and hydrogen. Managing heat loads in the system is critical to controlling system performance because a substantial amount of the energy content in hydrogen gas is released during the exothermic hydrogen uptake process, and this process must occur in only a few minutes for vehicle applications. These materials often are used in a powder form in which the initial particle size is 50–100 micrometers. However, as the material is cycled by hydriding (M+H₂→MH) and dehydriding (M+H₂←MH), particle size can decrease by several orders of magnitude. For the solid metal hydride phase, relative contributions of the electronic and phononic thermal conductivities are quantified with varying composition and particle size. Particle size effects are approximated by a boundary scattering term in the phononic thermal conductivity formulation. Also, the electronic contribution to thermal conductivity is estimated as a function of hydrogen content. The results reveal that overall thermal conductivity is highly material-specific. Materials with large electronic contributions in the pure metal state are relatively unaffected by particle size, while those with lower electronic contributions exhibit a substantial decrease in thermal conductivity with particle size.