scholarly journals Fast Diffusion Process in Quenched hcp Dilute Solid 3He–4He Mixture

2009 ◽  
Vol 158 (3-4) ◽  
pp. 496-501 ◽  
Author(s):  
Y. O. Vekhov ◽  
A. P. Birchenko ◽  
N. P. Mikhin ◽  
E. Y. Rudavskii
Author(s):  
Logan S. McLeod ◽  
Levent F. Degertekin ◽  
Andrei G. Fedorov

Palladium and its alloys have long been used as hydrogen separation membranes due to their extremely high permeability and selectivity to hydrogen over all other gases [1]. The hydrogen permeation process begins with selective chemisorption of the gas onto the metal surface. As the adsorption process is the point in the permeation sequence where the majority of gases become excluded, it follows that a cleverly designed device could be created to take advantage of the so-called ‘fast’ diffusion paths of surface and grain-boundary diffusion to further enhance permeability without sacrificing selectivity. The contribution of grain-boundary diffusion to the overall permeation rate is dependent on the relative volume in the membrane occupied by grain-boundaries versus bulk material. Typically, grain boundaries only make up a miniscule fraction of the overall volume and therefore only contribute an appreciable amount to the overall diffusion process at temperatures low enough to make the bulk diffusion process nearly stagnant. However, in the case of a nanostructured membrane this paradigm is no longer valid. The fabrication methods associated with extremely thin membrane deposition typically lead to highly non-equilibrium microstructure with an average grain size on the order of tens of nanometers [2]. In order to exploit the potential advantages of grain boundary diffusion the nano-scale grains must persist throughout operation. To avoid the tendency for the grain structure to relax to a more equiaxed, coarse-grained morphology the self-diffusion of metal atoms in the film must be minimized by operating the membranes at a temperature much lower than the membrane melting temperature. Figure 1 shows the microstructural changes in a thin, sputtered, Pd/Ag alloy film before and after annealing. The initial fine-grained structure on the bottom surface of the membrane is due to a combination of low substrate temperature during deposition and the Ti adhesion layer onto which the Pd/Ag layer was deposited. After annealing at 400 C the grains have coarsened and the top and bottom structure are identical.


2007 ◽  
Vol 268 ◽  
pp. 15-36
Author(s):  
Dao Khac An

This paper summarizes some of the main results obtained concerning aspects of anomalous single-dopant diffusion and the simultaneous diffusion of multi-diffusion species in semiconductors. Some important explanations of theoretical/practical aspects have been investigated, such as anomalous phenomena, general diffusivity expressions, general non-linear diffusion equations, modified Arrhenius equations and lowered activation energy have been offered in the case of the anomalous fast diffusion for single-dopant diffusion process. Indeed, a single diffusion process is always a complex system involving many interacting factors; conventional diffusion theory could not be applied to its investigation. The author has also investigated a system of multi-diffusion species with mutual interactions between them. More concretely, irreversible thermodynamics theory was used to investigate the simultaneous diffusion of dopants (As, B) and point defects (V, I) in Si semiconductors. Some attempts at theory development were made, such as setting up a system of general diffusion equations for the simultaneous diffusion of multi-diffusion species involving mutual interactions between them, such as the pair association and disassociation mechanisms which predominated during the simultaneous diffusion of dopants and point defects. The paper then gives some primary results of the numerical solution of distributions of dopants (B, As) and point defects (V, I) in Si semiconductor, using irreversible thermodynamics theory. Finally, several applications of simultaneous diffusion to semiconductor technology devices are also offered.


2009 ◽  
Vol 283-286 ◽  
pp. 128-132
Author(s):  
Rodolfo A. Pérez

The diffusion process in hcp Zr with low amounts of Fe being in solution or forming very dilute Zr-Fe alloys is analysed and discussed. The enhancement of the diffusion coefficient in alloys with increasing amounts of Fe is studied using both experimental and theoretical results. In contraposition with the assumption made in the literature that the Fe in solution in the hcp Zr lattice is the responsible, this enhancement seems to be more related with the total amount of Fe present in the samples. This idea is supported by measurements of Au diffusion in Zr with 50 to 150 gr/gr of Fe which shows increments in the diffusion coefficients even at the lower temperatures where the reported Fe solubility in -Zr is negligible. Ab initio calculations using SIESTA and WIEN2k codes show several stable and meta-stable configurations for the Fe in the hcp Zr lattice in interstitial and off-centre positions, resembling the last ones a Zr3Fe like arrangement. These configurations are used in order to analyze the mechanism of both, self-diffusion enhancement and ultra-fast diffusion of Fe in -Zr.


2020 ◽  
Vol 92 (3) ◽  
pp. 31101
Author(s):  
Zahoor Iqbal ◽  
Masood Khan ◽  
Awais Ahmed

In this study, an effort is made to model the thermal conduction and mass diffusion phenomena in perspective of Buongiorno’s model and Cattaneo-Christov theory for 2D flow of magnetized Burgers nanofluid due to stretching cylinder. Moreover, the impacts of Joule heating and heat source are also included to investigate the heat flow mechanism. Additionally, mass diffusion process in flow of nanofluid is examined by employing the influence of chemical reaction. Mathematical modelling of momentum, heat and mass diffusion equations is carried out in mathematical formulation section of the manuscript. Homotopy analysis method (HAM) in Wolfram Mathematica is utilized to analyze the effects of physical dimensionless constants on flow, temperature and solutal distributions of Burgers nanofluid. Graphical results are depicted and physically justified in results and discussion section. At the end of the manuscript the section of closing remarks is also included to highlight the main findings of this study. It is revealed that an escalation in thermal relaxation time constant leads to ascend the temperature curves of nanofluid. Additionally, depreciation is assessed in mass diffusion process due to escalating amount of thermophoretic force constant.


1971 ◽  
Vol 32 (C1) ◽  
pp. C1-228-C1-230
Author(s):  
R. BALAKRISHNAN ◽  
R. V. LANGE

1978 ◽  
Vol 39 (C6) ◽  
pp. C6-115-C6-116 ◽  
Author(s):  
S. Maegawa ◽  
T. Mizusaki ◽  
Y. Hirayochi ◽  
T. Kusumoto ◽  
A. Hirai

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