Nonuniform Distribution Of Hydrogen Research Articles

On modeling of stress‐induced diffusion within micropolar and classical approaches

AbstractThe paper continues and summarizes our previous investigation on a description of the local increase of hydrogen in the vicinity of the border of a metal specimen after hydrogenation. The local increase is observed after artificial saturation of metal specimens and after operation of metal details in corrosive environments. It can worsen significantly the material properties and its behavior at the macro‐level. A stress‐induced diffusion process in a cylinder is modeled. The paper accounts for the inner stresses and strains by means of chemical potential and by means of the dependence of the diffusion coefficient on elastic strain energy. The stress–strain state is determined within the micropolar theory of elasticity, the results are compared with the ones obtained within the classical theory. Nonuniform fields of stresses and strains result in a nonuniform distribution of hydrogen. Accounting for the couple stress interactions between continuum particles results in a faster saturation of the boundary layer with hydrogen.

Features of Thermal Diffusion Impregnation with Hydrogen of Porous Material Based on Titanium

The effect of bulk porosity on the kinetics of hydrogen thermal diffusion impregnation and on the structure of specimens prepared from different semiproducts, i.e., bars, wires, and fibers of commercially pure titanium, is studied. It is established that the rate of hydrogen absorption by porous titanium is at a maximum at 650°C and it increases with an increase in bulk porosity, pore size and a reduction in typical size of original semiproduct. Simultaneous thermal diffusion impregnation of monolithic and porous material leads to nonuniform distribution of hydrogen between them and correspondingly a change in structure.

Model of Carbon Corrosion in PEM Fuel Cells

This paper presents a mathematical model of the corrosion of carbon catalyst supports in polymer electrolyte membrane (PEM) fuel cells. The model describes how a maldistribution of hydrogen across the fuel electrode can induce both oxygen evolution and carbon corrosion on the positive electrode of the fuel cell in the fuel-starved region. Implications of this reverse-current mechanism are explored by simulating a cell with a nonuniform distribution of hydrogen along the fuel channel in both steady-state and transient operation.

Potential fluctuations, diffusion length and lateral photovoltage in hydrogenated amorphous silicon and silicon–germanium thin films

Abstract The lateral photovoltage (LPV) has been measured between coplanar electrodes by illuminating hydrogenated amorphous silicon samples at various positions with a red laser spot. We find that the LPV decreases at higher temperatures and increases upon light soaking. Similar results are obtained for hydrogenated amorphous silicon–germanium alloys. The diffusion length of carriers in our samples is measured by the steady-state photocarrier grating technique and the LPV is calculated. The calculated LPV is much smaller than that experimentally measured. We propose that the potential fluctuations present in the samples might be responsible for the large LPV. We have measured the ambipolar diffusion lengths in a-Si : H and a-SiGe : H samples by the SSPG technique. The ambipolar diffusion length decreases with increasing germanium incorporation in the films while keeping the other deposition parameters the same. This decrease in the ambipolar diffusion length with increasing germanium incorporation is attributed to the increase in the DOS as evident from our CPM measurements. Moreover, for all the samples studied, we found that the diffusion length decreases with LS and this is also explained on the basis of the rise in the DOS. These findings are as expected and are in agreement with the published results (Ritter et al. 1987, Weiser and Ritter 1989, Sakata et al. 1997). A LPV is observed in all the samples studied. The LPV decreases upon increasing the temperature of measurement but increases upon LS. In all cases the magnitude of the LPV is found to be much larger than expected from the measured L. The observations are explained on the basis of the potential fluctuations present in the material. These arise from the heterogeneities, that is non-uniform distribution of hydrogen and variation in silicon and germanium concentrations from point to point in the a-SiGe: H film. In our model, the effect of these potential fluctuations is twofold. The electrons and holes become separated spatially in the presence of the potential fluctuations. This tends to reduce their recombination probability and might increase L. Secondly, there is an accumulation of localized charges in the states that coexist above the percolation edge with the extended states. Although these charges do not participate in conduction, they will give a contribution to the LPV. Other factors, for example band bending at the surface might also contribute by separating the carriers. Hence larger potential fluctuations are likely to give a large LPV, since both N and L in equation (14) are expected to be larger. Since the presence of a bias light is expected to reduce the potential fluctuations, the LPV should also be reduced. This explanation of the LPV in terms of potential fluctuations agrees with the observation that the value of L measured by SSPG decreases as the light intensity increases (Weiser and Ritter 1989). Further, we note that SSPG is always carried out in the presence of light and, therefore, is expected to give a smaller L than its value in the dark. In order to test this, we measured the LPV in the presence of bias light and found that the LPV decreases when the bias light is on (see figure 12). The increase in LPV after LS in all the samples can also be explained by this model. Since LS increases the potential fluctuations (Hauschildt et al. 1982, Agarwal 1996, Agarwal et al. 1996), the increase in the LPV is understandable. Thus we can explain all our data by the potential fluctuation model. A more quantitative analysis is not possible at this stage. It is, however, clear that a more heterogeneous sample should show a larger LPV since the potential fluctuations are more. This is an interesting observation which could be useful in comparing the quality of samples prepared under different conditions. This requires more experimentation.

Comparison of the potential fluctuations in a-Si:H measured by the optical and transport methods

Hydrogenated amorphous silicon (a-Si:H) has potential fluctuations, which arise from the heterogeneities, variations in the bond angles, bond lengths, and nonuniform distribution of hydrogen and dopants, etc. We have estimated the potential fluctuations present in undoped and doped a-Si:H from the optical as well as from the transport measurements. In the optical measurements, the subgap absorption has been used to estimate the width of the potential fluctuations. In the transport method, the conductivity and the thermopower measured as a function of temperature are used to provide the width of the potential fluctuations, as shown by Overhof and Beyer. We find that in all cases, the width of the potential fluctuations measured by the optical measurements is about 20 times larger than that measured by the transport measurements on the same sample. This is explained by noting that optical absorption is a local process, which is sensitive to the short-range potential fluctuations by the order of 3 Å, whereas the transport measurements are sensitive to the long-range potential fluctuations (≈80 Å) which may have smaller width.

Diffusion of hydrogen in niobium under hydrostatic high pressure

To understand the kinetics of phase separation or hydrogen embrittlement, the influence of lattice deformations on the diffusion coefficient should be known. We have measured the diffusion coefficient of hydrogen in niobium under hydrostatic pressure up to 3.5 GPa. A non-uniform distribution of hydrogen in the sample, which is mounted in a high pressure cell, is generated by means of electrotransport. After reaching a stationary concentration gradient the electric field is switched off and the relaxation of hydrogen distribution towards equilibrium is measured. We found that the diffusion coefficient increased by about 10% after the application of 1 GPa at T = 285 K. We compared our results with recent calculations which predict a decrease of the diffusion coefficient with increasing pressure.

Hydrogen effects on phase transformations in austenitic stainless steels

The effects of hydrogen and stress (strain) on the phase transitions of a variety of stainless steels (316, 321, 347) were investigated. Hydrogen was introduced by severe cathodic charging at room temperature. X-ray diffraction was employed to reveal the transformations occurring in thin surface layers. After charging expanded ∈ phase is always present,α′ martensite content increases during ageing and the final content depends on the stability of the austenite. The broadening of diffraction peaks of austenite after cathodic charging is caused by nonuniform distribution of hydrogen. The state of hydrogen distribution in the steel and the relationship between internal stresses, surface cracking and phase transition is discussed.