EXAFS investigation of the destabilization of the Mg–Ni–Ti (H) system

Ti doping destabilizes the Mg₂NiH₄ system, reducing the hydrogenation enthalpy from – 64 kJ/(mol H₂) to around – 40 kJ/(mol H₂). However, the equilibrium pressure is hardly affected, as also the entropy of reaction changes. To understand this thermodynamic behavior it is essential to understand the structure of the phases present in the Ti-doped Mg₂Ni system in metallic and hydrogenated state. We used Extended X-ray Absorption Fine Structure (EXAFS) to investigate the local coordination of Ni and Ti atoms in Mg–Ni–Ti thin films both in the as-prepared as well as in the hydrogenated state. In the as-prepared state two phases, Mg₂Ni and TiNi or TiNi₃, are formed, which transform in a single Ti-doped Mg₂NiH₄ phase in the hydrogenated state. These results are consistent with previous DFT calculations for this system.     

Thermodynamics, stress release and hysteresis behavior in highly adhesive Pd-H films

We investigate the role of clamping on the thermodynamics of highly adhesive metal hydride thin films. Using Pd as a model system, we add Ti as an intermediate adhesion layer to increase the interaction with the substrate. We show that Pd/Ti films remain clamped during (de-)hydrogenation while the stress release occurs by means of rearrangement and pile-up of the material. The compressive stress build-up reaches a value of about 1.5 GPa during hydrogen absorption. The enthalpy of hydride formation and decomposition, measured using Hydrogenography is found to decrease and increase by about 2.7 and 1.3 kJ/mol H₂ respectively, as compared to buckled Pd films. A simple model confirms that the change in the thermodynamics and the asymmetric expansion of the hysteresis correlate with the mechanical work needed to accommodate the stress induced plastic deformations in clamped Pd/Ti films during the (de-)hydrogenation cycle.     

An Investigation of the Linearity of Standard Thermistor Wattmeters and an Estimate of the Minimum Attainable Errors of Measurements of the Ratio of Microwave Powers

The results of experimental investigations of the nonlinearity of thermistor power converters are presented and the main components of the error in measuring nonlinearity in a differential microcalorimeter are analyzed. The resultant error in determining nonlinearity does not exceed 0.002 dB/10 dB at frequencies up to 18 GHz.     

Hydrogenography of PdHx thin films: Influence of H-induced stress relaxation processes

Hydrogenography is a new optical thin film combinatorial method that follows hydrogenation and determines its associated thermodynamic properties. Due to clamping to the substrate, stresses generated in thin films are larger than in bulk. This must be taken into account for a comparison between these two types of systems. In this article, we follow the microstructure, surface morphology and in-plane stress changes of thin polycrystalline PdH_x films upon several hydrogen ab/desorption cycles and correlate them to the evolution in shape and hysteresis of pressure–optical transmission isotherms (PTIs) recorded by hydrogenography. The in-plane stress in the first instance is relaxed inhomogeneously by buckling, and a more complete, homogeneous relaxation is only reached after the creation of a buckle-and-crack network that is the two-dimensional analogue of bulk decrepitated grains. This sequence of changes is clearly visible in the PTIs, demonstrating another useful facet of hydrogenography for characterizing metal–hydrogen systems.