Zhong-Liang Zhang

Propagation of shear bands in a Cu47.5Zr47.5Al5 bulk metallic glass

We report a novel finding of slither propagation of shear bands on the fracture surface of a Cu47.5Zr47.5Al5 bulk metallic glass (BMG). The nanoscale heterogeneities in the as-cast state are aggregated along shear bands with irregular morphology. Such heterogeneities create a fluctuating stress field during shear band propagation leading to a slither propagation mode. The slither propagation of 10 to 15 nm wide shear bands is effective to improve both the plasticity and the work-hardening-like behavior of BMGs if the size, the morphology, and the elastic properties of the heterogeneities are intimately intercalated during solidification.

Steady concentration distribution of hydrogen in elastic membranes during hydrogen diffusion

A critical discussion on non-linear steady-state concentration profiles for hydrogen diffusion in elastic metallic membranes, proposed by other authors, is provided based on mathematical analysis and numerical simulation. It is shown that the non-linear distribution is non-existent and the linear form is the only solution for the ideal solid solution phase.

Some problems on the resistance method in the in situ measurement of hydrogen content in palladium electrode

Some problems on the resistance method in determining the hydrogen content in PdHx electrodes are discussed. First, the resistivity ratio of PdHx , the temperature coefficient of resistivity and the resistance of PdHx , and the resistance of Pd having undergone hydriding � /dehydriding cycles are discussed. It is found that the resistivity ratio is somewhat higher than the resistance ratio with the same x value and their difference depends on the internal stress-state arising from hydrogen insertion. Another fact that has been omitted in past work is that the temperature coefficients of PdHx resistance and resistivity increase while x � /0.7. The Pd resistance decreases with hydriding � /dehydriding cycle number due to the shape deformation of the electrode, which occurs. Second, the effect of the non-uniform distribution of x in the electrode on determining the hydrogen content is discussed theoretically. It is proved that errors are particularly significant when the Pd� /H system is in the mixed a� /b phase or the resistance is near the maximum value. Finally, we calculate the additional potential drop and hence the apparent resistance of the PdHx electrode caused by the co-conduction of electrolyte, the concentration-cell effect, the collection of electrolysis current and other imperfect configurations of the electrode in the in situ measurement of electrode resistance, using a direct current. It is found that an electrode with a large ratio of length to radius, an active surface, a surrounding electrolyte with high conductance and a high electrolysis current will all induce substantial additional resistance. At the same time, some advice on measuring the PdHx resistance is presented. # 2002 Published by Elsevier Science B.V.

Effects of self-stress on the hydrogen absorption into palladium hydride electrodes of plate form under galvanostatic conditions

Effects of diffusion induced stress on the hydrogen absorption into plate form electrodes of b-phase PdHx are discussed numerically based on the Volmer‐Tafel route of the hydrogen evolution reaction, and thermodynamic considerations involving stress fields and non-ideal interactions of hydrogen in the electrode. It is found that the self-induced stresses enhance the absorption rate and may exceed the yield stress, especially when the thickness of the plate and:or charging current (or negative potential) increase. On the other hand, a plate with both sides exposed to electrolyte absorbs hydrogen more rapidly than that with only one side exposed to electrolyte under the same equivalent thickness and other conditions. Of course, the stresses developed in the former plate are always greater than those of the latter.

Effects of self-induced stress in tubular membranes during hydrogen diffusion

Various effects induced by self-stress during hydrogen diffusion across metallic tabular membranes are discussed. The up-hill diffusion in the initial time of permeation, the time course of inner pressure change, the steady distributions of hydrogen concentration and residual stress in membranes, and the acceleration of permeation rate on interruption of hydrogen charging are properly interpreted.

Effects of temperature on hydrogen absorption into palladium hydride electrodes in the hydrogen evolution reaction

Abstract On the basis of the Volmer–Tafel route of the hydrogen evolution reaction, and thermokinetic considerations involving H–H interactions and stress fields of hydrogen in β-PdH x , we discuss the analytical and numerical effects of temperature on the loading ratio, charging rate and self-stress for hydrogen absorption into electrodes of β-phase PdH x under galvanostatic and potentiostatic charging conditions. It was found that changes in the loading ratio, overpotential and current density with temperature can be expressed in terms of four parameters, i.e. enthalpies of adsorption and absorption of hydrogen on (into) palladium, and apparent activation energies of exchange current density of the Volmer and Tafel reactions, in addition to the usual physical quantities such as temperature and overpotential etc., although more factors are involved in the actual process. With increasing temperature, the absorption process tends to be determined by the surface reaction and the self-stresses decrease under the galvanostatic charging condition; however, the absorption time decreases but the self-stresses change slightly for the potentiostatic charging. Finally, our theory is consistent with the available experimental results.

Numerical simulation of hydrogen (deuterium) absorption into β-phase hydride (deuteride) palladium electrodes under galvanostatic conditions

Abstract The kinetics of H(D) absorption into a β-phase PdH x (PdD x ) electrode are discussed numerically, based on the Volmer–Tafel route of the hydrogen (deuterium) evolution reaction and thermodynamic and kinetic data of H(D) in the β-phase PdH x (PdD x ). It is found that the asymptotic loading ratio of H(D) is determined only by the Tafel step under galvanostatic conditions. The kinetics of H(D) absorption can be characterised by a parameter λ ∝ d Δ j /( D Δ x ) where d is the dimension of the electrode (thickness for plate, radii for cylinder or sphere); Δ j is the current density step; D is the average diffusion coefficient; Δ x is the loading ratio step of H(D) caused by the current step. If λ ⪢1 (large scale of dimension, high current density and/or low temperature), the absorption rate is controlled by diffusion; in contrast, if λ ≪1, the rate-determining step is the interface process and the charging efficiency approaches 100%; otherwise, the kinetics are under mixed control if λ ∼1.

Effects of hydrogen self-stress in thin circular-plates with clamped edges

A description of self-stress produced by hydrogen absorption into thin metallic circular-plates with clamped edges is presented. The plate deflects when the average hydrogen content exceeds a critical value, which depends mainly on the size of the plate, if no external load is applied; the plate distortion exhibits bistable characteristics, i.e. the deflection direction may be either upward or downward. On the other hand, a plate deflects prior to hydrogen insertion while an external force is imposed on it. The self-stresses and their effects on the solubility and diffusivity of hydrogen in plates are discussed as well.