Variation Of Diffusion Length Research Articles

Experimental investigation of the constant and time-dependent dynamic diffusion coefficient: Implication for CO2 injection method

Gas diffusion in coal is an important transport mechanism that plays a crucial role in carbon dioxide (CO2) storage in and methane (CH4) extraction from coal seams. Studies on coal diffusion are largely based on experimental work on different coal particles and the establishment of uni-pore or bidisphere theoretical models. However, problems remain in both experimental and theoretical analyses. Therefore, the initial desorption time of adsorbed gas, something that is not done using conventional experimental devices, was revealed in this study. The initial desorption time was obtained by calculating the relationship between the amount of gas collected and the theoretical amount of free gas. An improved diffusion model considering lost gas was established based on the initial desorption time of adsorbed gas. The mpdel provides diffusion coefficients for different equilibrium pressures and particle sizes on the basis of a unipore diffusion model. By correlating diffusion coefficient and time, an empirical time-dependent diffusion model was established to solve the dynamic diffusion coefficient under different conditions. Finally, in order to substantiate that the variation in diffusion length is the primary cause of the variation of diffusion coefficients with time, the variation of diffusion length with desorption time was obtained. As a result, a stepwise pressure-rasing method was proposed. This new injection method can effectively improve CO2 storage and CH4 recovery from coal seams. The study has a certain guiding significance in engineering practice.

Diffusion Profiles Around Quartz Clasts as Indicators of the Thermal History of Pseudotachylytes

AbstractPseudotachylytes are generated by the cooling and solidification of frictional melt produced along a fault surface during seismic slip. Pseudotachylytes can, therefore, provide important constraints on thermal histories of faults during coseismic slip: survivor clast mineralogies and quenched crystallite morphologies have previously been used to constrain the peak temperatures during slip. Here we show that silicon‐diffusion gradients are preserved around quartz survivor clasts and that these can be used to constrain the immediate cooling histories of pseudotachylytes after the cessation of slip. The variation of diffusion length with position in pseudotachylyte veins can be well reproduced by combining simple thermal history models with Arrhenius parameters for diffusion of appropriate magma compositions.

A First-Principles Study on Water Flow Through Single-Walled Carbon Nanotubes Using Artificial Neural Network Method

Ab initio molecular dynamics simulations are done to elucidate the electronic structure and properties of water/single-walled carbon nanotubes (SWCNTs) systems. The artificial neural network (ANN) approach and statistical methods are then used to model and analyze these properties. The ANN method substantially speeds up the ab initio electronic structure calculations and has superior accuracy in mimicking the results of such calculations. We aim to understand the effects of CNT chirality, temperature, and CNT flexibility on the water diffusion inside SWCNTs. In this regard, the CNT is fixed implies that the position of CNT is kept constant during the diffusion process. Statistical analysis of results shows that there is a nonmonotonic variation of diffusion length of water with respect to the CNT chirality. However, an increase in temperature and rigid CNTs accelerate the water diffusion.

Photoelectrochemical Diffusion Length Measurements on p‐Type Multicrystalline Silicon for Photovoltaic Applications

We report photoelectrochemical measurements of diffusion lengths in p‐type multicrystalline silicon used for photovoltaic solar cells. We show that, under some conditions, this method provides a very practical means for rendering an account of the quality nature of this material through diffusion length variations. The results obtained during two years of use of this method in the industrial processing of photovoltaic cells are presented and discussed.

Simulating solar cells with a microcomputer

Designing modern silicon solar cells in order to get the maximum efficiency requires the inclusion of effects such as the variation of diffusion length, and electric field as a function of impurity concentration in nonhomogeneous silicon. In this paper, it is shown that it is possible to develop an efficient computer program to simulate the I–V curves of solar cells, which takes into account such effects, when low-injection conditions apply to the semiconductor. The program can be run in any PC microcomputer, without any coprocessor, giving results similar to larger programs, in a fraction of the time usually required for this purpose. The program allows the inclusion of internal resistances (series and shunt) in order to compare the ideal and real behavior of a given solar cell. Conversion efficiency, open circuit voltage, short circuit current, and filling factor are obtained for a set of fabrication parameters, such as substrate and emitter dopings, thicknesses, and surface recombination velocities. The results are in agreement with recent experimental results.

Evaluation of the diffusion length of minority carriers in bulk GaAs

The diffusion length of minority charge carriers has been investigated in LEC GaAs, silicon-doped with doping density N D - N A ranging from 10 16 to 10 18 cm -3, by surface photovoltage (SPV) and electron-beam-induced current (EBIC) of scanning electron microscopy (SEM) measurements. Au Schottky diodes have been evaporated along the diameter of wafers cut from different doping density ingots to determine the variation of minority carrier diffusion length with both the radial position on the slice and the carrier concentration. The diffusion length values obtained by optical and electron excitation enhance systematic differences, which can be explained by the different surface recombination weight in the carrier generation volume and by the injection level, too. In all the examined samples an M-shaped radial variation of the diffusion length has been observed; on the other hand, the mean value of L p increases from 0.5 to 7 μm when the doping concentration increases. The authors correlate this distribution to the electrical inhomogeneity induced by native defects and associated recombination centres. The role of the dislocations, which induce two competitive effects, i.e. an enhanced recombination probability and a precipitate condensation, is here discussed.

3D-resolved determination of minority-carrier lifetime in planar silicon solar cells by photocurrent decay

In modern production schemes for Si solar cells, defect passivation and impurity gettering are often used to improve material quality and thereby cell efficiency. These processes generally alter the spatial uniformity of minority-carrier transport parameters over the wafer and may result in minority-carrier lifetime and diffusion length variations both lateral and in depth. In polycrystalline Si these spatial dependences are already present due to the nature of the material. We present an extension of the photocurrent decay method to determine the diffusion length in a three-dimensionally resolved fashion. From a single photocurrent decay curve the back-surface recombination velocity, the average minority-carrier diffusion length, and an asymmetry factor which qualitatively describes the depth dependence of the diffusion length are determined. This is done using the three observables: quantum efficiency, fundamental decay time, and intercept of the extrapolated decay curve with the time-zero axis. Lateral resolution is obtained by focusing the light beam to a small spot on the cell and measuring the current decay curve as a function of position on the cell. It is shown that light-pulse durations longer than the minority-carrier lifetime and wavelengths longer than 950 nm are required. These conditions are met by using modulated wavelength-tunable light from a Ti:sapphire laser. Measurements on monocrystalline cells show that the decay time is independent of wavelength and light-pulse duration, as predicted by theory. Furthermore, the intercept with the time-zero axis was shown to increase with increasing pulse duration and wavelength. Measurements on a set of polycrystalline Si cells were performed showing that gettering treatments during cell production result in depth-dependent lifetimes.

Variation of diffusion length with processing in dielectrically isolated π-silicon tubs

The variation of minority-electron diffusion length with processing in dielectrically isolated π-silicon tubs is investigated using electron-beam-induced current measurements. The analysis of these measurements is complicated by the six boundary conditions on the π-silicon. The diffusion lengths measured in virgin π-tub silicon are ≤ 250 µm while those in π-silicon undergoing a phosphorus getter are >> 250 µm. The determination of exact values for the diffusion lengths as a function of processing are not possible without a more complete understanding of the parasitic effects of the inversion region at the tub walls. Trends in the diffusion lengths are indicative of trends in the carrier lifetime, which are critical in the high-voltage applications for silicon tubs.

Defect analysis in polycrystalline silicon solar cells

Localized bulk defects like diffusion length variations and structural defects like grain boundaries are analyzed in polycrystalline silicon solar cells using laser scanning and deep level transient spectroscopy techniques. The effect of hydrogen passivation on the role of grain boundaries has been studied.

Mobility, lifetime and diffusion length in polycrystalline materials

Theoretical investigations of the influence of grain size on mobility, lifetime and diffusion length in a polycrystalline material are made. it is found that the variation of diffusion length with grain size can be accounted for if the dependence of mobility and lifetime on grain size are considered. The calculations reasonably represent the experimentally observed variations of diffusion lengths with grain size.