THERMAL-CONDUCTIVITY OF PMH COMPACTS, MEASUREMENTS AND EVALUATION

A metal hydride reaction bed consisting of PMH compacts constitutes a reaction bed of high thermal conductivity, k(eff), high and constant permeability and high packing density of the metal hydrides. In this investigation four series of PMH compacts were prepared from LaNi5 and MmNi4.15Fe0.85 with aluminum powders of two grain sizes: 6-9 and 34-42 mum in various weight fractional amounts, f, as matrix material. Some compacts were tested over 30,000 absorption -desorption cycles for stability. The effective thermal conductivity, k(eff), and the porosity, epsilon, of uncycled compacts were measured. An equation has been developed for the presentation of k(eff) as a function of the constituents: metal hydrides, aluminum powder as well as porosity. Only one fitting parameter, B1, is utilized. The k(eff) of the PMH compacts increases with f, which, however, diminishes the permeability, alpha'. Thus the choice of the fraction of the matrix, f, for a given application, must be optimized with regard to k(eff) and alpha' simultaneously. The polycrystalline bulk thermal conductivity, k, of the compacted metal hydrides and aluminum was established, by using extrapolation formulae: k(LaNi5) = 30.75 W m-1 K-1 k(MmNi4.5Fe0.85) = 15.7 W m-1 K-1 k(A1) = 145.7 W m-1 K-1 for 6-9 mum grain size k(A1) = 195.5 W m-1 K-1 for 34-42 mm grain size. For the simple case of "tube and shell" container heat exchanger the tubes of which are filled with metal hydrides, the density of hydride reaction beds of various morphologies are presented.