Effect of structural defects on the hydriding kinetics of nanocrystalline Pd thin films

Abstract

MicrostructureInternal stress a b s t r a c t While the microstructure of a metal is well-known to affect its equilibrium hydrogen uptake and therefore the hydriding thermodynamics, microstructural effects on the hydriding kinetics are much less documented. Moreover, for thin film systems, such microstructural effects are difficult to separate from the internal stress effect, since most defects generate internal stresses. Such a decoupling has been achieved in this paper for nanocrystalline Pd thin film model systems through the use of a high-resolution, in-situ curvature measurement set-up during Pd deposition, annealing and hydriding. This set-up allowed producing Pd thin films with similar internal stress levels but significantly different microstructures. This was evidenced from detailed defect statistics obtained by transmission electron microscopy, which showed that the densities of grain boundaries, dislocations and twin boundaries have all been lowered by annealing. The same set-up was then used to study the hydriding equilibrium and kinetic behaviour of the resulting films at room temperature. A full quantitative analysis of their hydriding cycles showed that the rate constants of both the adsorption-and absorption-limited kinetic regimes were strongly affected by microstructure. Defect engineering was thereby shown to increase the rate constants for hydrogen adsorption and absorption in Pd by a factor 40 and 30, respectively.

Mechanical stress and stress release channels in 10–350 nm palladium hydrogen thin films with different micro-structures

“…Thus, the H-induced change of the films' elastic energy density is by orders of magnitude too small to generate edge dislocations in the early DSR events. This conclusion is supported by results of Delmelle et al [7], who showed that the dislocation density of as deposited nanocrystalline…”

“…According to TEM investigations on nanocrystalline Pd films reported in Refs. [6,7,31,32] these films contain pre-existing dislocations and presumably nano-twins, next to the large content of grain boundaries [22]. Deposition at 673 K leads to epitaxial Pd films with [111] out-of-plane orientation and domain sizes in the mm range, on sapphire substrates [29,30].…”

“…1. Furthermore, stress can be reduced by the formation of growth twins [6,7]. Due to the different thermal lattice expansions of Pd and Al 2 O 3 the lattices match less at 673 K. Then the critical thickness for the implementation of misfit dislocations is reduced to 8 nm for the threefold epitaxial film [44].…”

“…3 can be interpreted as follows. In region I, hydrogen is suggested to enter pre-existing microstructural defects such as surface and subsurface sites, vacancies, grain boundaries and dislocations present in the films after film preparation [7,11,47]. Defect sites yield a lowered lattice expansion [11], causing smaller inplane stress changes.…”

“…[13,15e21]. For the case of nanocrystalline Pd-H thin films the evolution of microstructural defects upon hydrogen uptake has recently been studied by Delmelle et al [7], in the solid solution region of as-prepared and annealed Pd-H films. While the number densities of pre-existing dislocations and growth-twins were reported not to change considerably with hydrogenation in the asprepared films, their number densities are reduced with annealing and strongly resurge with sub-sequent hydrogenation [7].…”