Papers by Keyword: Hydrogen Diffusion

Abstract: In general, hydrogen permeabilityΦ of the alloy membrane is expressed as the product of the hydrogen diffusion coefficient D and the hydrogen solution coefficient K. Therefore, to improve the hydrogen permeability efficiently, the values of K and D should be separately considered. In the present study, hydrogen absorption and permeation behaviors of the Nb19Ti40Ni41 alloy consisting of the eutectic phase are investigated by measuring pressure-composition-isotherm (PCI) and by the hydrogen flow method and compared with those of palladium. The hydrogen absorption in the Nb19Ti40Ni41 alloy does not obey the Sieverts’ law in the pressure region of 0-1.0MPa at 523K, but it shows linear relationship between the difference in the square root of hydrogen pressure and hydrogen content between 0.1 and 0.4MPa. Although the value of D for the Nb19Ti40Ni41 alloy is considerably lower than that of palladium, its high K value enhances the hydrogen permeability Φ. It is suggested that the enhancement of D by microstructural control for Nb19Ti40Ni41 alloy is effective for improvement of Φ.

Abstract: Mg-rich alloys of the binary system Mg-Ni are prospective hydrogen-storage materials. In the present study, desorption characteristics of hydrided Mg2Ni intermetalic and hydrided Mg/Mg2Ni eutectic mixture were investigated. Structure of experimental materials during the hydrogenation was observed by SEM. Three modifications of (Mg2Ni)Hx (x ~ 4) were prepared differing in the ratio of two low-temperature phases f = LT2/LT1: with (i) f >1, (ii) f ~ 1 and with (iii) f <1. Evolution of the ratio f during hydrogen desorption was checked by XRD. It was found that the micro-twinned phase LT2 is not desirable in hydrogen-storage materials containing Mg2Ni intermetallic. Diffusion coefficient of hydrogen in LT2 is about 20 times lower than in LT1.

Abstract: Anodic polarisation curves of Mm(Ni3.6Co0.7Mn0.4Al0.3)x (Mm = mischmetal, 0.85  x  1.15) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen (H/M), potential sweeping rates (v), temperatures (T), and amounts of reducing agent (y = [NaBH4]) in alkaline solution. Anodic peak current (Ip) at the Mm(B5)x electrodes increased with an increase in T, x and y values. In addition, the Ip value depended linearly on initial hydrogen concentration and square root of potential sweeping rate, irrespective of T, x and y values. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in x and y values. From these results, it is considered that the surface reduction treatment of the Mm(B5)x alloys, performed in alkaline solution with sodium borohydride, the nonstoichiometry and the initial concentration of absorbed hydrogen, are important factors for improving the charge-discharge performance of negative electrodes for metal-hydrogen energy systems.

Abstract: The diffusion of hydrogen has been investigated in the AB2 Laves phase Compounds TiCr1.78 and TiCr1.4Mn0.4, by absorption techniques. It has been found that H at temperatures higher than 700 K diffuses through the classical over-barrier mechanism, while at low temperature (around 100 K) the diffusion is governed by phonon-assisted tunnelling. The activation energy for classical hopping is rather high and increases with the substitution of Mn for Cr. In the range of H contents nH investigated (nH=H/Me0.03) the chemical diffusion coefficient does not depend on the H concentration.

Abstract: Kinetics of hydrogen desorption from Mg2NiH4 was studied. Experimental material was prepared by two techniques – by melting and casting and by ball-milling and compacting into pellets. Experimental materials were hydrogen charged at elevated temperature and pressure. The pellets were charged in two different regimes resulting in structures with high fraction of twinned low-temperature phase LT2 and with low fraction of LT2. It was made an attempt to measure diffusion coefficients of hydrogen and its temperature dependence both in high-temperature (HT) and in low-temperature (LT) phases of Mg2NiH4. The measurement was carried out in temperature interval from 449 K to 576 K by the volumetric method. It was found that the LT2 slows-down the desorption rate considerably.

Abstract: Anodic polarisation curves of treated and untreated MmNi3.6Mn0.4Al0.3Co0.7 (Mm = mischmetal) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen (H/M), rates of potential sweep and temperatures. The alloy powders of the electrodes were treated with 6 M NaOH solution containing y M NaBH4 (y = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.05). The initial concentration of absorbed hydrogen in the electrodes varied between 0 and 0.06 H/M, and the working temperature range was 15 °C to 50 °C. Anodic peak current (Ip) at the MmNi3.6Mn0.4Al0.3Co0.7 electrodes increased with an increase in the y value. In addition, the Ip value depended linearly on initial hydrogen concentration and square root of potential rate irrespective of y value. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in y value. From these results, it is considered that the hydrogen diffusion in the MmNi3.6Mn0.4Al0.3Co0.7 alloys and the electrocatalytic activity at their surfaces influence the Ip value and that they are important factors for improving the charge-discharge performance of the negative hydride electrode. Its low temperature (15-30 °C) performances are also improved by the simple powder treatment with the 6 M NaOH solution containing NaBH4 as reducing agent.

Abstract: Hydrogen embrittlement is a common problem for the integrity of oil conducting pipes. In this work, we estimate the rate of hydrogen penetration into an API 5L steel pipe welded by electric arc. The hydrogen penetration was estimated by means of data taken from ultrasonic measurements. As expected, the steel pipe becomes more brittle as the hydrogen penetration rate does so. A simple diffusion model was developed. The model confirms the strong dependency between the rate of penetration and the mechanical damage to the material.

Abstract: The diffusion of H and the thermal decomposition of hydrides have been investigated at high temperatures in two PdCe alloys of composition 5% and 9% Ce. It has been found that the H diffusion coefficient obeys an Arrhenius-type of law with the following values of the activation energy W and diffusion constant D0, ( )     = ± × = ± − s m D W eV 2 7 0 2 2 10 0.20 0.02 (Pd95Ce5 alloy) ( )     = ± × = ± − s m D W eV 2 7 0 2 1 10 0.24 0.01 (Pd91Ce9 alloy) The high-temperature absorption data match the low-temperature ones deduced from internal friction measurements, indicating that Ce atoms do not act as strong trapping centres for H. Thermal decomposition of hydrides in the Pd95Ce5H0.008 alloy occurs in a single stage showing a homogeneous solid solution state of the H-Me system.

Abstract: Hydrogen permeation and diffusivity in V1−xNbx alloys (x = 0.25, 0.5 and 0.75) were investigated over a temperature range between 573 and 673 K. The alloy sheets surface-coated with Pd (100 nm) by ion-beam sputtering technique were subjected to the hydrogen permeation measurement with hydrogen pressures less than 3000 Pa. It was found that the hydrogen permeation is strongly suppressed especially at low hydrogen pressures; the hydrogen permeation is considered to be controlled by some surface processes. The V0.50Nb0.50 alloy showed the largest permeability 1.4×10−8 mol H2 m−1s−1Pa−1/2 at 673 K which is larger than that for pure Pd. The diffusivity which was determined from the fitting of the permeation curve decreases with the alloy concentration at both the Nb- and V-rich sides and takes a minimum value at x=0.75. The hydrogen potential obtained by a first-principles calculation showed that V atoms act as trapping centers for hydrogen in the Nb matrix and Nb atoms act as anti-trapping centers in the V matrix.

Abstract: Fick's laws were used to model the hydrogen diffusion in notched high-strength steel wires loaded in tension under elastic-plastic conditions. The plastic deformation at the notch tip has an effect on the peak distribution of the hydrostatic stress ( h σ ). So, in stress-assisted diffusion analysis, elastic-plastic material behavior should be considered. Coupled diffusion elastic-plastic finite element analysis was implemented in the finite element program ABAQUS using the user element subroutine (UEL) and the coupled temperature-displacement solver routine to solve the variational form of the diffusion equation in order to obtain the hydrogen concentration distribution ahead of the notch tip in high-strength steel wires under plane strain conditions. The analysis results are compared with those obtained from elastic analysis, which shows that, if a critical hydrogen concentration is regarded as a local fracture criterion, the elastic-plastic analysis results can be used to evaluate the hydrogen embrittlement of high-strength steel wires.