The hydriding kinetics of bulk metallic holmium under 1Â bar H2 and below 570Â K

The kinetics and topochemistry of the hydriding reaction of parallelepiped shaped holmium samples under 1 bar H2 at the temperature range 320-570 K were studied using pressure drop measurements and hot stage microscopy (HSM) direct observations. It is shown that the kinetic curve can be separated into two independent stages, namely a nucleation and growth stage followed by a contracting envelope mechanism, and that intrinsic kinetic parameters can be obtained for each stage. The hydride phase is nucleated in a final number of nucleation sites, N0, growing in a two-dimensional disc-like mode at constant radial velocity, Us. The thickness of the nucleation layer decreases with temperature from 20 μm at 320 K down to less than 2 μm at 500 K. Hot stage microscopy measurement at 523 K yielded values of Us = 3 ± 1 μm/s and of the nucleation site density N0/S0 = 3.2 × 104 cm−2 (S0 is the sample's surface area) the product of which is in good agreement with the result obtained from the kinetic curve measurements. From the contracting envelope stage values of the bulk hydride velocity, Ub, are obtained. The temperature dependence of Ub shows two different ranges which may reflect a change in the hydriding mechanism, namely a low temperature range below 370 K with activation energy of 33.3 kJ/mole H2 and a higher temperature range (370 K < T < 560 K), with activation energy of 15.6 kJ/mole H2.