Classical Diffusion Mechanism Research Articles

Macroscopic Modeling of Water Uptake Behavior of PEDOT:PSS Films.

Poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is a widely used conductive polymer (CP) for applications in electronic devices. In the context of transparent electrodes or soft actuators, the uptake of water by PEDOT:PSS is an essential element in the performance of the physical system. We study the water uptake of pure films and films treated with ethylene glycol (EG) commonly used to enhance the electrical properties of PEDOT:PSS. Gravimetric analysis was used to investigate the water sorption–desorption of PEDOT:PSS and its change with the EG treatment for a wide range of configurations (thickness, temperature, and relative humidity). We demonstrate that a simple Fickian model cannot correctly represent the experimental results; we therefore introduce a fully coupled reaction–diffusion scheme. This model describes the transport of diffusing molecules into the polymer film, taking into account, in addition to the classical diffusion mechanism, the reaction between the reactive sites of the polymer network and the water molecules. We demonstrate that solvent treatments have a direct influence on the kinetics of the water uptake of PEDOT:PSS films in terms of diffusivity, solubility, and rate of reaction. The proposed model can be used to accurately predict the water uptake of CP films in the case of complex three-dimensional configurations that are needed for the design of complex actuators.

Cu diffusion in CdTe detected by nano-metal-plasmonic enhanced resonant Raman scattering

Copper is widely used as a p-dopant in CdTe thin film solar cells. However, Cu is not stable in CdTe and its diffusion is a big concern regarding the device stability when the CdTe solar cell is applied for long-term deployment. In this work, a new technique of nano-metal-plasmonic enhanced resonant Raman scattering (RRS) was demonstrated for quantitative study of Cu diffusion in CdTe along grain boundaries. The experimental setup was designed to directly detect Cu diffusion in CdTe. The experimental results showed that Cu diffusion in CdTe can be well described by the classical diffusion mechanism. The activation energy of Cu thermal diffusion was found to be 0.40 eV, which explains the easy diffusion of Cu in CdTe.

Relaxation of the electric resistance in YBa2Cu3O7−x single crystals at room temperature

In this paper, the relaxation of the electrical resistance of a YBa[Formula: see text]Cu[Formula: see text]O[Formula: see text] single crystal that is kept for a long time at room temperature in air atmosphere is investigated. It is shown that with increasing time of keeping, relaxation of the electrical resistivity at room temperature is observed, controlled by a single channel and the classical oxygen ion diffusion mechanisms. The faster transport of oxygen ions in the initial stage of the implantation process can take place along one-dimensional nonstoichiometric vacancies in a single-file diffusion mode. The final stage of oxygen transport in this compound is described by an ordinary classical diffusion mechanism.

Single-file diffusion of oxygen ions in the compound YBa2Cu3O7−x

This paper discusses experimental data which show that with increasing thermal processing time, oxygen doping of crystals of YBa2Cu3O7−x with x > 0.65 (tetragonal phase) is characterized by two stages with substantially different kinetics. In the first (initial annealing) stage, oxygen ions enter at a faster rate with an activation energy of approximately 0.4 eV. The second (end of annealing) stage is characterized by significantly slower transport of oxygen ions with an activation energy on the order of 1 eV. The kinetics of the oxygen doping process is analyzed in terms of the time variation in the electrical resistance of the compound. It is shown that the faster transport of oxygen ions in the initial stage of the implantation process can take place along one-dimensional nonstoichiometric vacancies in a single-file diffusion mode. The final stage of oxygen ion implantation in this compound is described by an ordinary classical diffusion mechanism.

A Novel Orthogonal Microscope Image Analysis Method for Evaluating Solvent-Swelling Behavior of Single Coal Particles

A novel orthogonal microscope image analysis method was developed to observe and evaluate quantitatively the dynamic solvent-swelling behavior and characteristics of coal particles separated using density gradient centrifugation technique. The important results obtained using the new method are as follows: (1) The maximum statistic probability of anisotropic swelling ratio from 1.1 to 1.3 measured in this work agrees very well with the data measured by Cody et al.; in addition, the samples with high vitrinite concentrate and high swelling ratios seem to have high anisotropic swelling feature. (2) The existence of a characteristic distribution of volumetric swelling ratio in the vitrinite concentrates strongly suggests the existence of a characteristic distribution of the molecular weight between cross-link points. (3) The average volumetric swelling ratios measured using the new method are comparable to that measured by the traditional packed bed method. (4) The simulated results using modified Painter's disinterspersion model based on the measured characteristic swelling ratios suggest that each cluster has aromatic and hydroaromatic rings from 1 to 3 in PDC-HV coal and from 1 to 5 in Witbank coal. Moreover, the relatively short average virtual bond length per carbon atom and the random walk of the chains are the main features of the coals in pyridine. (5) Although the case II and super-case II are dominant as for diffusion of pyridine in the coal particles, a few cases of Fickian and anomalous diffusion are also existing for two kinds of coal. In addition, because the value of diffusion exponents, n, of some PDC-HV coal samples changes from 0.22 to 0.39, which does not match to any classical diffusion mechanism, it seems to be the reflection of one diffusion mechanism.

The mechanism of hydrogen diffusion in zirconium dihydrides

Hydrogen diffusion in ZrHx (1.58<or=x<or=1.98) in the temperature range 600 K to 970 K has been measured by means of pulsed-field-gradient nuclear magnetic resonance. The activation enthalpy for hydrogen diffusion, Ha, obtained by fitting an Arrhenius expression D=D0exp(-Ha/kBT) to the diffusivities, increases sharply as x approaches the limiting value of two in good agreement with results deduced from the proton spin-lattice relaxation rate, Gamma 1, measured on the same samples. Hydrogen atoms jump predominantly between nearest-neighbour tetrahedral sites in ZrHx. The observed concentration dependence of both the effective value of H,and the pre-exponential factor D0 suggests, however, that at high hydrogen concentrations and high temperatures another interstitial site is occupied in addition. At x to 2 a small fraction of hydrogen atoms located on an interstitial site other than the tetrahedral site appears to contribute significantly to the diffusivity. The temperature and concentration dependence of the diffusion data can quantitatively be described by such a model. The activation enthalpies for all possible jumps in this system with two different kinds of site are shown to be independent of the hydrogen concentration x. The corresponding attempt frequencies nu a approximately=1013 s-1 are compatible with the picture of a classical diffusion mechanism.