Interface pinning causes the hysteresis of the hydride transformation in binary metal hydrides

Hydriding and dehydriding transitions in bulk and nanocrystalline binary metal hydrides were studied using the Pd-H model system by measuring pressure-composition isotherms with in situ x-ray diffractometry. Nanocrystalline Pd showed a smaller pressure hysteresis, solvus hysteresis, and hysteresis in lattice parameter, compared to bulk Pd. The time-dependence of pressure equilibration was measured after dosing with aliquots of hydrogen, giving equilibration times that were much faster in the single-phase regions than in the two-phase plateaus. In the broad two-phase plateaus, the pressure relaxations were exponential functions of time. An explanation of hysteresis is developed that is based on a dissipative potential barrier that impedes the motion of the interface due to interactions between lattice defects and the two-phase interface. The exponential pressure relaxations and hysteresis are consistent for this mechanism. For a simple model of the pinning potential, the potential barrier maximum is an order of magnitude less than typical grain boundary energies. These pinning effects are substantially different in the nanocrystalline Pd, suggesting differences in the hydriding mechanism.

Figure 1

High-resolution TEM image of the as-received nanocrystalline Pd.

Figure 2

(a) Pressure-composition isotherms for bulk (blue, red circles) and nanocrystalline (green trianges) Pd measured on a Sieverts apparatus. The inset shows the plateau region on a linear pressure scale. Closed symbols denote absorption, and open symbols correspond to the subsequent desorption. (b) Full and minor loop isotherms for bulk Pd measured in situ. After absorption to approximately 75% capacity, the isotherm was reversed and the sample was dehydrided.

Figure 3

Refined lattice parameters of the (a) bulk and (b) nanocrystalline Pd-H powders during hydrogen absorption and desorption. The red and blue curves correspond to the solid solution $\alpha$ phase and hydride $\beta$ phase, respectively. Error bars are indicated by the shaded region.

Figure 4

Kinetics of equilibration during absorption and desorption. (a) Equilibration time constants $\tau$ as a function of hydrogen concentration during cycling. Raw data from absorption cycle 2 and corresponding fits used to extract $\tau$ are plotted in (b) for the single-phase region and in (c) for the two-phase region. Reported error bars may be smaller than the associated data marker.

Figure 5

Absorption and desorption phase boundaries determined from terminal compositions of bulk and nanocrystalline Pd-H. Bulk and nanocrystalline Pd-H data from this study are plotted as red circles and green triangles, respectively. Additional bulk data from Wicke et al. is plotted as upside down triangles [32]. 6- and 4-nm nanoparticle Pd data from Vogel et al. is plotted as squares and diamonds, respectively [27]. Filled markers denote absorption, open markers denote desorption.