The Effect of Included Hydrogen on the Motion Parameters of Edge Dislocations in Palladium Membranes (original) (raw)

Hydrogen permeation in stressed and strained membranes of palladium alloys

International Journal of Hydrogen Energy, 1992

ln experimental studies of hydrogen permeation through a Pd77Ag23 membrane at 25 and 50°C, forms of the time-dependent changes of hydrogen pressure within the membrane have been found to be clearly dependent on the catalytic activities of the membrane surface, initial hydrogen content of the membrane and conditions of lattice strain due to different processes of generation of lattice stress. This strongly supports the appreciation that hydrogen diffusion in metals involves non-Fickian features which are associated with Gorsky Effect phenomena. Correlations have been made between these effects and the forms of isothermal hydrogen pressure(p)-hydrogen content(n) relationships for the Pd77Ag23H, system.

Hydrogen Segregation in Palladium and the Combined Effects of Temperature and Defects on Mechanical Properties

2015

Atomistic calculations were carried out to investigate the mechanical properties of Pd crystals as a combined function of structural defects, hydrogen concentration and high temperature. These factors are found to individually induce degradation in the mechanical strength of Pd in a monotonous manner. In addition, defects such as vacancies and grain boundaries could provide a driving force for hydrogen segregation, thus enhance the tendency for their trapping. The simulations show that hydrogen maintains the highest localization at grain boundaries at ambient temperatures. This finding correlates well with the experimental observation that hydrogen embrittlement is more frequently observed around room temperature. The strength-limiting mechanism of mechanical failures induced by hydrogen is also discussed, which supports the hydrogen-enhanced localized plasticity theorem.

Study of Microstrain and Stress in Non-Planar Palladium Membranes for Hydrogen Separation

Advanced Materials Research, 2014

Palladiums tubular membranes are developed to operate up to 400 °C, for the synthesis of H2 and for the separation of CO2 in Water Gas Shift (WGS) processes and reforming gas of methane [. Palladium has FCC lattice that allows the separation of hydrogen from carbon dioxide through a solution-diffusion mechanism [. To ensure high selectivity in the separation process, the functional Pd layer on the porous substrate of the membranes must have a microstructure with low defects and free from residual stresses [.MicroXRD measurements were performed to evaluate the effect of the stress-relief heat treatment, carried out for different time and temperatures, on the palladium layer. Microstrains were assessed before and after stress-relief by the Williamson-Hall method [. The use of microdiffraction was mandatory considering the tubular shape of membranes. The data were corrected for elastic anisotropy of palladium and the altered Williamson-Hall method was successfully applied.The XRD two-d...

Fundamental Study of Hydrogen Segregation at Vacancy and Grain Boundary in Palladium

2015

We have studied the fundamental process of hydrogen binding at interstitial, vacancy and grain boundary (GB) in palladium crystals using Density-Functional Theory. It showed that hydrogen prefers to occupy the octahedral interstitial site in Pd matrix, however a stable H-vacancy complex with most H occupations would contain up to eight hydrogen atoms surrounding the vacancy at tetrahedral sites. Furthermore, H presence assists the pairing or formation of nearby vacancies, which in agreement with previous suggestions by both experiment and theory investigation. Also, this observation could imply about a hydrogen embrittlement (HE) mechanism through the connections of microvoid and cracks. The segregation of hydrogen at grain boundary, nevertheless, has shown a different effect. High H accumulation results in grain boundary extension, which is related the HE mechanism of grain decohesion observed by experiments.

The effect of lattice defects on hydrogen solubility in palladium

Journal of the Less Common Metals, 1976

It is proposed that the stress-field of the dislocation array is the principal cause of the solubility enhancement observed in the low hydrogen content, a-phase of cold-worked palladium. With an assumed uniform dislocation density of 9 X 1011 cmh2, the observed solubility enhancement of 1.65 (298 K) for heavily cold-worked palladium can be reproduced if the core radius is assumed to be 2 X Burgers vector. The temperature dependence of the solubility enhancement is also reasonably well predicted. The significance of measured relative partial molar enthalpies and entropies of absorption of hydrogen into cold-worked palladium is examined.

Hydrogen transport through thin layer Pd alloy membranes: kinetics and gas permeation studies

Separating hydrogen with high selectivity from a gas mixture needs a dense membrane through which only hydrogen can penetrate. Transport of hydrogen from one side of a dense membrane to the other side of the membrane is a sequence of different processes, such as adsorption and dissociation of hydrogen, absorption and diffusion of protons, recombination and desorption of hydrogen. Probably the most important step in this transport is the dissociation of hydrogen molecules in atoms at the membrane surface. This dissociation is necessary for absorption and diffusion to occur and can be realised with a catalytic metal such as palladium. Furthermore, both the solubility and the diffusivity of hydrogen in palladium are relatively very high, which makes palladium a good candidate material for hydrogen selective membrane films. However, palladium suffers from embrittlement by hydrogen that leads to deterioration of metal films. This phenomenon is caused by the formation of palladium hydride that can exist in an alpha and beta phase that co-exist at temperatures lower than about 310°C. To increase the lifespan of membranes made of this material the co-existence of alpha and beta palladium hydride has to be suppressed. One way of doing this is to alloy palladium with e.g. silver or copper that will hardly effect the migration of hydrogen through the membrane.