In-situ monitoring of hydride formation in Pd thin film systems

In this work, the hydriding and dehydriding kinetics of Pd thin films has been monitored in-situ. The experimental technique consists of a high resolution curvature measurement setup, which continuously monitors the reflections of multiple laser beams coming off a cantilevered sample. After mounting the sample inside a high vacuum chamber, a H-containing gas mixture is introduced to instantaneously generate a given hydrogen partial pressure (pH2)(pH2) inside the chamber. The resulting interaction of H with the Pd layer leads to a volume expansion of the thin film system, which in turn induces changes in the sample curvature as a result of internal stresses developing in the Pd film as a result of hydride formation. This paper demonstrates how such high resolution curvature measurements, performed in real-time during hydriding/dehydriding cycles, allow to resolve in detail the kinetics of hydride formation. Moreover, the experimental setup also allows to address the reversibility of the hydriding mechanism. In this respect, our results show the existence of a residual curvature after a full hydriding/dehydriding cycle. When exposing the sample to multiple cycles, this residual value was found to remain constant, even when gradually decreasing the hydrogen partial pressure. Finally, it was demonstrated that during such multiple cycling experiments, the pH2pH2-dependence of the equilibrium stress level during hydriding quantitatively confirms Sievert's law.