Effective thermal conductivity of the Mg-Ni metal-hydrogen system under operating conditions

Abstract Effective thermal conductivity (ETC) of dimagnesium nickel tetrahydride (Mg ₂ NiH ₄ ), prepared via temperature driven cycles under hydrogen atmosphere of a 2 Mg + Ni powder mixture (activation at 350 °C, 18.4 bar of hydrogen pressure for 24 h) was measured by applying the transient plane source method. In each cycle, measurements were carried out in hydrogenated and dehydrogenated state during 310 hydrogenations and 309 dehydrogenations. Before and after activation ETC isotherms were measured from 1 bar to 90 bar H ₂ -pressure, in a temperature range from 30 °C to 100 °C before activation and from 30 °C to 350 °C after activation. The H ₂ -release/uptake for all cycles was calculated using the pressure increase/drop during de-/hydrogenations. The hydrogen amounts are in good correlation with TGA-DSC measurement results of the sample, taken after the test. The effective thermal conductivity of the initial powder mixture increased with temperature and pressure, varied between 0.36 Wm ⁻¹ K ⁻¹ (at 1 bar, 30 °C) and 0.56 Wm ⁻¹ K ⁻¹ (at 70 bar, 100 °C). During activation at 350 °C the effective thermal conductivity increased from 0.25 Wm ⁻¹ K ⁻¹ to 0.46 Wm ⁻¹ K ⁻¹ while pressure in the whole set-up decreased by ~ 6.5 bar due to hydrogenation. Varying ETC and dehydrogenation pressures over the first ~ 70 cycles gave hint to a formation of Mg ₂ NiH ₄ /Mg ₂ Ni from 2 MgH ₂ + Ni/2 Mg + Ni + 2 H ₂ . XRD measurements before and after activation of an equally treated sample and of the cycled sample after the tests’ end support those findings.