Activation Energy For Hydrogen Diffusion Research Articles

Hydrogen diffusion in natural and synthetic orthopyroxene

Hydrogen diffusion coefficients in natural orthopyroxenes and synthetic enstatite were determined by dehydration and hydration experiments at 700 and 900 °C. In natural Opx (approximately En90Fs10) small but significant differences in diffusivities along the three crystallographic axes were observed, [001] being the fastest direction, followed by [100] and [010]. Hydrogen diffusion in pure enstatite proved to be about 2 orders of magnitude slower and isotropic. The activation energy for hydrogen diffusion in pure enstatite was determined to be −295 (±55) kJmol−1, and −213 (±47) kJmol−1 for orthopyroxene from Kilbourne Hole. Long-term hydration experiments did not lead to saturation in hydrogen. Instead, after an initial increase in hydrogen concentration, a slow but continuing decrease could be observed in all cases. It is suggested that the investigated samples lose their ability to store hydrogen even when heated in a hydrogen atmosphere. This loss in storage ability can itself be described by a diffusion equation, its diffusion coefficients being more than 1 order of magnitude slower than the diffusion of hydrogen.

Mechanism of Hydrogen Trapping and Transport in Carbon Materials

Bulk hydrogen retention and hydrogen diffusion in graphite and carbon materials have been studied to estimate hydrogen recycling and tritium inventory in a fusion reactor environment. Hydrogen may permeate into a filler grain in the form of a hydrogen molecule, diffuse through crystallite boundaries and finally be trapped as hydrogen atoms at the edge surface of a crystallite. In the estimation of hydrogen transport, the activation energy of hydrogen diffusion can be determined from absorption experiments, and the activation energy of detrapping can be obtained from desorption experiments. The activation energies of hydrogen trapping are 2.6 eV in ordinary graphite and 4.4 eV for irradiated or mechanically milled graphite samples.

Diffusion of hydrogen in different charge states in realistic models ofa-Si:H

In order to determine the mechanism or mechanisms for hydrogen transport in a-Si:H one must identify the reactions that bring deeply bonded H (on dangling bonds) to the transport band which is some form of interstitial hydrogen. Further, one must have reliable values of the energy difference between these deep states and transport states, as well as the barrier energies between neighboring transport sites. Since experiments clearly show a dependence of hydrogen diffusion activation energy on the charge of the mobile hydrogen and on the Fermi level, different charge states for the hydrogen must be included. In this paper we consider all of these ingredients in order to obtain an explanation of how hydrogen migrates in a-Si:H.

Determination of hydrogen diffusion coefficient in metal hydride electrode by modified Warburg impedance

Hydrogen diffusion coefficients in MlNi 3.75Co 0.65Mn 0.4Al 0.2 metal hydride electrode as a function of depth of discharge (DOD) and temperature were evaluated with modified Warburg impedance which describes more precisely the practical diffusion behavior. It was found that hydrogen diffusion coefficient in this electrode increases with the increase in DOD at ambient temperature, and for this electrode with 50% DOD, hydrogen diffusion coefficient increases with the increase of temperature and the activation energy for hydrogen diffusion in it is 35 kJ mol −1.

Hydrogen diffusivity in single crystal stoichiometric NiAl

The hydrogen diffusion behavior of single crystal stoichiometric NiAl was investigated. The results show that the hydrogen diffusivity and permeabilty of single crystal stoichiometric NiAl obey Arrhenius relationship in the experimental temperature range. The activation energy of hydrogen diffusion in single crystal stoichiometric NiAl is about 45 kJ/mol.

Hydrogen permeation behavior in Fe3Al-based alloy

Ultra-high vacuum gaseous hydrogen permeation experiments on Fe3Al-based alloy were performed in the temperature range of 330∼450°C with an upstream hydrogen pressure between 3. 38×104 Pa and 7.28×104 Pa. The results show that the hydrogen diffusivity and permeability in Fe3Al-based alloy obey Arrhenius relationship in the experimental temperature range and the hydrogen permeation process is controlled by the lattice diffusion of hydrogen at relative high temperature. The activation energy of hydrogen diffusion in the Fe3Al-based alloy was found to be 75 kJ/mol.

Some theoretical aspects of hydrogen motion ina-Si:H

The mechanism (or mechanisms) of hydrogen transport in a-Si:H is of great intrinsic interest but is also important for the understanding of a number of phenomena such as light induced defects. The standard picture for hydrogen migration is that a hydrogen atom leaves a trap site for a higher energy transport site, migrates among the transport sites, and eventually falls back into a trap site. Presumably the transport sites are bond centered sites and the trap sites are H passifying dangling bonds although other possibilities do exist. We find that the literature includes many calculations for the activation energy for hydrogen diffusion that are at best misleading and often interpreted incorrectly. In this paper we exhibit a number of calculations that apply to particles hopping in complex medium and apply the results to a-Si:H. These calculations, coupled with existing estimates of the energy of various H configurations in a-Si:H, show that the standard picture is likely to be incorrect. However, some preliminary calculations and arguments presented in this paper suggest that the mobile species of hydrogen in a-Si:H may well be molecular.

Hydrogen Permeation through an Al-0.2 at% Mg Alloy

The diffusion of hydrogen in an Al-0.2 at% Mg alloy at temperatures between 625 K and 773 K has been measured by a gas permeation technique, in which the permeation run for the sample was repeated for many times at high temperatures. Because Mg trapped hydrogen, the permeation rate for the Al-0.2 at% Mg alloy was lowered than that for an pure Al for early permeation runs. After heavily repeated runs, however, Mg was transported to the sample surface at the hydrogen reservoir side and was no loner present in the alloy except on the surface, and the permeation rate abruptly increased. The state of Mg on the surface could be a ternary oxide which consists of Mg, Al and 0. The diffusivity of the Al-0.2 at% Mg after heavily repeated permeation runs were also determined and the activation energy of hydrogen diffusion was estimated to be 0.25 eV.

Diffusion and thermal stability of hydrogen in homoepitaxial CVD diamond films

Abstract The diffusion and out-diffusion properties of hydrogen have been investigated in homoepitaxial layers of CVD boron-doped diamond which have been exposed to a deuterium microwave plasma. The diffusion experiments show that the onset of (B, H) pair dissociation occurs at 550°C. The hydrogen diffusion activation energy is 1.4 eV. Hydrogen out-diffuses at 880°C and the diamond surface barrier limits the out-diffusion process. In r.f. plasma exposed diamond epilayers, the hydrogen out-diffusion spectra are composed of a peak at 720°C related to the subsurface hydrogen accumulation layer and another one at 840°C attributed to the hydrogen adsorbed at the surface.

A Case for Molecular Hydrogen Being the Mobile H Species in a-Si:H

ABSTRACTThe mechanism (or mechanisms) of hydrogen transport in a-Si:H is of great intrinsic interest but is also important for the understanding of a number of phenomena such as light induced defects. We find that the literature includes many calculations for the activation energy for hydrogen diffusion that are at best misleading and often interpreted incorrectly. In this paper we summarize a number of calculations that apply to particles hopping in complex media and apply the results to a-Si:H. These calculations, coupled with existing estimates of the energy of various H configurations in a-Si:H, show that the standard picture is likely to be incorrect. However, previous work on the mobility of molecular hydrogen in crystalline Si coupled with some molecular dynamics simulations presented in this paper show that the mobile species of hydrogen in a-Si:H may well be molecular.

A new model for the diffusion behavior of hydrogen in metallic glasses

Both experimental and simulation results have indicated the deviation from Arrhenius law of the diffusion coefficient of hydrogen in metallic glasses. This deviation was traditionally explained by the existence of various kinds of jumps or in terms of continuous distributions of activation energies due to different kinds of disorder. A new model is suggested, which relates this deviation to the temperature dependence of the short-range order (SRO). The suggested model is applied to simulate the diffusion behavior of hydrogen in dilute amorphous Fe–H, Fe–Si–H and Fe–B–H alloys. The effect of the alloying elements on the activation energy of hydrogen diffusion in amorphous iron is discussed in terms of their electronic structure and mean volume.

Hydrogen in Monocrystalline CVD Boron Doped Diamond

The diffusion properties of hydrogen have been investigated in homoepitaxial layers of CVD boron doped diamond as a function of the diffusion temperature, the boron dopant concentration and the nature of the hydrogen source (rf or microwave plasma). Preliminary hydrogen effusion experiments on these hydrogenated samples are also reported. For both kinds of plasma we show that, below diffusion temperatures of 500 °C, the hydrogen diffusion is limited by trapping on boron acceptors. The onset of dissociation of B, H pairs occurs at 550 °C which implies a significantly higher stability of these pairs in diamond compared to silicon. However, a higher hydrogen diffusion activation energy is found in the microwave plasma exposed samples compared with the rf plasma exposed samples together with the absence of the subsurface hydrogen accumulation layer which is observed after rf plasma. Hydrogen effusion experiments show that in the rf plasma exposed samples the main effusion peak at 720 °C is attributed to the subsurface hydrogen accumulation layer. Another effusion peak at 850 °C is ascribed to the desorption of hydrogen adsorbed at the surface.

Hydrogen Internal Friction and Interaction of Solute Atoms in Niobium- and Vanadium-Based Alloys

A computer model has been proposed to be used to calculate the internal friction spectrum, caused by the “diffusion under stress” of hydrogen atoms in a solid solution with a b.c.c. lattice containing substitutional atoms. The model takes into account the long-range pair interaction of dissolved atoms. It is suggested that such interaction acts on diffusion by producing short-range order of mobile hydrogen atoms and by changing their energy. These changes occur in the tetrahe- dral (before the jump) as well as in the octahedral (at the saddle point of the diffusion barrier) interstitial sites and, therefore, produce local changes of the hydrogen diffusion activation energy (the activation energy of internal friction). The relaxation strength is calculated from the local fields of atomic displacements around every atom that participates in diffusion. The model has been used to study the nature of hydrogen relaxation in Ti- and Zr-containing Nb- and V-based alloys and to calculate the “chemical” interaction energy of the H(D)±Ti(Zr) pairs. It was shown that the hydrogen relaxation mechanism in Nb(V)–Ti(Zr)–H(D) alloys consists in diffusion under stress of hydrogen or deuterium atoms in the vicinity of single substitutional atoms at low concentration of substitutional atoms and high hydrogen or deuterium concentration, and in the vicinity of substitutional pairs — at high concentration of substitutional atoms and low hydrogen or deuterium concentration. The “chemical” interaction H(D)–Ti(Zr) in niobium and vanadium is stronger or is of the same order, as the strain-induced (elastic) interaction.

The kinetics of hydrogen diffusion in Ti3Al-based alloy

The process of gaseous hydrogen charging into a Ti3Al-based alloy in the temperature range of 500–650°C is investigated. The results showe that the relationship between the average hydrogen concentration at constant temperature and charging time reveals a parabolic rate law. Applying the theory of lattice diffusion to analyze the hydrogen diffusion in the alloy, we find that the apparent activation energy of hydrogen diffusion is 90.40 kJ/mol, and the equilibrium hydrogen content in the alloy depends on the temperature of the gaseous hydrogen charging process.

Hydrogen isotope fractionation and hydrogen diffusion in the tourmaline-water system

Hydrogen isotope exchange between tourmaline and water has been studied experimentally over the temperature range of 800-350°C. The hydrogen isotope equilibrium fractionation factor between tourmaline and water may be expressed as a function of temperature by the following linear equation: 10 3 ln α tourmaline-water e = −27.9(10 6/ T 2) + 2.3 A study of the kinetics of hydrogen isotope exchange between tourmaline and water indicates that exchange is dominated by hydrogen diffusion. Over the temperature range of 800-450°C, the relation between the hydrogen diffusion coefficient and temperature for the tourmaline-water system is as follows: Cylinder model: log D cyl = −5.96 − 6.68(10 3/ T) Plate model: log D plate = −5.64 − 6.42(10 3/ T) The activation energies for hydrogen diffusion are 128.0 kJ/mol (cylinder model) and 123.1 kJ/mol (plate model), respectively. For the temperature range 800-450 C, the relation of the hydrogen isotope equilibrium fractionation factor to the temperature (10 6/ T 2. chemical composition ( M), and hydroxyl-stretching frequency (ƒ) for systems of nonhydrogen-bonded hydroxyl minerals and water can be expressed as 10 3 ln α mineral-water e = − 14.7 10 3 × f − 3 (10 6/ T 2) + 26.4 + (2 M Al − 4 M Mg − 68 M Fe ) where M represents the mole fraction of designated cation in the mineral.

Effect of stoichiometric ratio on discharge efficiency of hydrogen storage alloy electrodes

Discharge efficiencies were evaluated for nonstoichiometric and stoichiometric Mm(B5), (B5 = Ni3.6Mn0.4Al0.3Co0.7, x = 0.88, 0.96, 1.00, 1.12) alloys. The discharge efficiency decreased with an increase in activation energy for hydrogen diffusion in the alloys. X-ray diffraction data showed the appearance of Ce2Ni7-type second phase, which formed a very stable hydride, for the Mm(B5)0.88 alloy and Ni3Al-type one, which had high catalytic activity, for the Mm(B5)1.12 alloy, respectively. The Mm(B5), alloys with larger x values or higher electrocatalytic activity showed higher discharge efficiency. It was therefore considered that not only hydrogen diffusion in the alloys but also charge transfer at the alloy/solution interface are responsible for the improvement of the discharge efficiency.

Electrochemical characterization of absorbed hydrogen in stoichiometric and nonstoichiometric hydrogen storage alloys

Anodic polarization curves of Mm(Ni 3.6Mn 0.4Al 0.3Co 0.7) x (Mm = misch metal, 0.88 ≦ x ≦ 1.12) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen ( H M ), rates of potential sweep and temperatures. Anodic peak current ( I p ) at the Mm(Ni 3.6Mn 0.4Al 0.3Co 0.7) x electrodes increased with an increase in x value. In addition, the I p value depended linearly on initial hydrogen concentration and square root of potential sweep rate irrespective of x value. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in x value. From these results, it is considered that the hydrogen diffusion in the Mm(Ni 3.6Mn 0.4Al 0.3Co 0.7) x alloys and the electrocatalytic activity at their surfaces influence the I p value and that they are important factors for improving the charge-discharge performance of negative electrode.

1H NMR study of local structure and proton dynamics in β-Ti1−yVyHx

The local structure of the metal atoms around a hydrogen atom and the dynamics of hydrogen atoms in disordered metal hydrides β-Ti1−yVyHx (x ∼ 1 and 0.2 ⩽ y ⩽ 0.8) were studied by proton nuclear magnetic resonance (1H NMR). The temperature and frequency dependences of the 1H spin-lattice relaxation time (T1) were measured over the temperature range 105–400 K and at frequencies of 9, 22.5, 52 and 90 MHz. All the hydrides showed a minimum value in the temperature dependence curve of T1, which originated from the fluctuations of 1H1H and 1H51V dipolar interactions caused by the hydrogen diffusive motion in the TiV alloys. The activation parameters for hydrogen diffusion were determined from data fitting using the Bloembergen-Purcell-Pound (BPP) theory taking into account the distribution of the correlation time. With increasing vanadium concentration, the apparent activation energy for hydrogen diffusion gradually decreased from 24 kJ mol−1 for y = 0.2 to 21.8 kJ mol−1 fory = 0.6, and then rapidly decreased to 17.5 kJ mol−1 for y = 0.8. The local structure of the metal atoms around a hydrogen atom was obtained from this composition dependence using a cluster model which allowed the short-range ordering of metal atoms. The short-range order parameter in this work was σ = 0.4 and was in good agreement with that determined from incoherent inelastic neutron scattering (σ = 0.43 ± 0.05).

Modeling hydrogen transport causing environmental embrittlement in Co3Ti alloys

Abstract Effects of deformation rate on the fracture energy of Co-21.5 at.% Ti, Co-23.1 at.% Ti and Co-21 at.% Ti-3 at.% Fe were investigated at temperatures ranging from 77 to 573 K. Environmental embrittlement was observed at temperatures between 200 and 420 K in Co-21.5 at.% Ti and Co-23.1 at.% Ti. The lower critical temperature decreases with decreasing deformation rate. The deformation rate dependence of fracture mode is accounted for on the basis of dislocation transport of hydrogen, which results in the estimation of the activation energies of hydrogen bulk diffusion to be 0.39 eV for Co-21.5 at.% Ti and 0.37 eV for Co-23.1 at. % Ti. Apparent activation energy of hydrogen diffusion was also estimated by measuring the depth of intergranular cracking after deformation at 77 K following hydrogen charging under various conditions of the charging temperature and period. The resulting activation energy is about 0.4 eV for Co-21.5 at.% Ti, which is similar to that obtained by modeling dislocation transport of hydrogen.

Experimental studies of alunite: II. Rates of alunite-water alkali and isotope exchange

Rates of alkali exchange between alunite and water have been measured in hydrothermal experiments of 1 hour to 259 days duration at 150 to 400°C. Examination of run products by scanning electron microscope indicates that the reaction takes place by dissolution-reprecipitation. This exchange is modeled with an empirical rate equation which assumes a linear decrease in mineral surface area with percent exchange (f) and a linear dependence of the rate on the square root of the affinity for the alkali exchange reaction. This equation provides a good fit of the experimental data for f = 17% to 90% and yields log rate constants which range from −6.25 moles alkali m −2s −1 at 400°C to − 11.7 moles alkali m −2s −1 at 200°C. The variation in these rates with temperature is given by the equation log k∗ = −8.17(1000/T(K)) + 5.54 (r 2 = 0.987) which yields an activation energy of 37.4 ± 1.5 kcal/mol. For comparison, data from O'Neil and Taylor (1967) and Merigoux (1968) modeled with a pseudo-second-order rate expression give an activation energy of 36.1 ± 2.9 kcal/mol for alkali-feldspar water Na-K exchange. In the absence of coupled alkali exchange, oxygen isotope exchange between alunite and water also occurs by dissolution-reprecipitation but rates are one to three orders of magnitude lower than those for alkali exchange. In fine-grained alunites, significant D-H exchange occurs by hydrogen diffusion at temperatures as low as 100°C. Computed hydrogen diffusion coefficients range from −15.7 to −17.3 cm 2s −1 and suggest that the activation energy for hydrogen diffusion may be as low as 6 kcal/mol. These experiments indicate that rates of alkali exchange in the relatively coarse-grained alunites typical of hydrothermal ore deposits are insignificant, and support the reliability of K-Ar age data from such samples. However, the fine-grained alunites typical of low temperature settings may be susceptible to limited alkali exchange at surficial conditions which could cause alteration of their radiometric ages. Furthermore, the rapid rate of hydrogen diffusion observed at 100–150°C suggests that fine-grained alunites are susceptible to rapid D-H re-equilibration even at surficial conditions.