Kinetics of the desorption of interstitial hydrogen in stable Nd2Fe14BHx, x-less-than-or-equal-to-5

Abstract

This paper reports the kinetics of desorption of interstitial H of peculiarly stabilized (washed in water) Nd2Fe14BHx, x less-than-or-equal-to 5, hydrides with passivated (amorphous) grain surfaces. These hydrides are stable at room temperature but release H on heating over high temperatures, usually between 450 and 750 K. The rate of desorption of H peaks at a certain temperature T(P), which is found to be a function of heating rate beta. The value of T(P) thus regularly increases from 533 to 657 K with raising the beta from 3 to 20 K/min, following a linear ln(T(P)/beta) vs T(P)-1 relation. The slope (E(a)/R, with R the gas constant) of the plot gives the average activation energy E(a) = 39.2 kJ/mol of the associated desorption process. More curiously, an isothermal heating, at a given temperature T(iso), exhibits four distinct maxima (H-1, H-2, H-3, and H-4) of the H desorption rate sequenced over the time intervals t(P). These are attributable to desorption of H from four different interstitial sites accompanied with four separate activation energies. The desorption maximum H-2 is the most prominent one and it was possible to study in detail by recording the isothermograms at several T(iso), in the range 483-543 K. An activation energy of E(a) = 38.2 kJ/mol is estimated from the slope of the Int(P) vs T(iso)-1 plot, which is fairly consistent with E(a) = 39.2 kJ/mol achieved by nonisothermal heating. The close similarity between the two E(a) values suggests that the same desorption process, i.e., attributed to H-2, dominates in either measurement