Computer simulation of the diffusion behavior of an ordered adsorbate

This paper reports on new Monte Carlo results for the chemical diffusivity of an ordered adsorbate. The model used is the square planar lattice gas with repulsive nearest neighbor interactions and attractive next nearest neighbor interactions. The simulation is based on the recently formulated rigorous Darken equation. Within the ordered c (2 x 2) region a strong maximum in the diffusivity as a function of coverage is observed. It is demonstrated that this maximum is a direct consequence of the large gradient in the adsorption isotherm within the ordered region

Diffusion kinetics in dilute binary alloys with the h.c.p. crystal structure

In this paper, an extended version of the matrix method is derived in order to address diffusion kinetics for the full anisotropic three-dimensional h.c.p. structure. It is shown that the diffusion anisotropy can be properly addressed with a model of 13 atom-vacancy frequencies which is an extended version of the well-known 5-frequency model for the f.c.c. lattice. Both tracer and phenomenological diffusion coefficients are calculated using this new approach. Extended Monte Carlo simulations are performed in order to cross-check some of the results of the matrix method. Applications of the proposed model to experimental diffusion data are discussed

Tracer diffusion in an ordered alloy: application of the path probability and Monte Carlo methods

Tracer diffusion technique has been extensively utilized to investigate diffusion phenomena and has contributed a great deal to the understanding of the phenomena. However, except for self diffusion and impurity diffusion, the meaning of tracer diffusion is not yet satisfactorily understood. Here we try to extend the understanding to concentrated alloys. Our major interest here is directed towards understanding the physical factors which control diffusion through the comparison of results obtained by the Path Probability Method (PPM) and those by the Monte Carlo simulation method (MCSM). Both the PPM and the MCSM are basically in the same category of statistical mechanical approaches applicable to random processes. The advantage of the Path Probability method in dealing with phenomena which occur in crystalline systems has been well established. However, the approximations which are inevitably introduced to make the analytical treatment tractable, although their meaning may be well-established in equilibrium statistical mechanics, sometimes introduce unwarranted consequences the origin of which is often hard to trace. On the other hand, the MCSM which can be carried out in a parallel fashion to the PPM provides, with care, numerically exact results. Thus a side-by-side comparison can give insight into the effect of approximations in the PPM. It was found that in the pair approximation of the CVM, the distribution in the completely random state is regarded as homogeneous (without fluctuations), and hence, the fluctuation in distribution is not well represented in the PPM. These examples thus show clearly how the comparison of analytical results with carefully carried out calculations by the MCSM guides the progress of theoretical treatments and gives insights into the mechanism of diffusion

Energy cost of heat activating serpentinites for CO2 storage by mineralisation

In this contribution, we present fuel cost estimates based on a practical heat activation strategy for serpentinites, for large-scale mineralisation of CO2 in New South Wales, Australia. We have found the serpentinites from the Great Serpentinite Belt in New South Wales to be particularly suitable for heat activation, as opposed to the partly serpentinised ultramafic minerals of the Coolac Serpentinite Belt. The activation strategy comprises prograde heating to produce an active material with 20% OHres and the recovery of ~80% of the sensible heat from the dehydroxylated mineral. The strategy also involves direct combustion of natural gas to supply an energy input of at least 574MJ (tSerpentinite)-1, to minimise secondary CO2 emissions generated from the thermal activation of serpentinite. A CO2 penalty of about 7% yields 0.93 net tonne of available active serpentine per tonne of serpentinite feedstock. The cost for serpentinite heat activation for this practical heating strategy amounts to A$ 1.25 per tonne of available active serpentine

Simultaneous measurement of tracer and interdiffusion coefficients: an isotopic phenomenological diffusion formalism for the binary alloy

In this paper, a new development of the classic Onsager phenomenological formalism is derived using relations based on linear response theory. The development concerns the correct description of the fluxes of the atomic isotopes. The resulting expressions in the laboratory frame are surprisingly simple and consist of terms coming from the standard interdiffusion expressions and from Ficks first law, where the tracer diffusion coefficient is involved thus providing a better understanding of the relationship between the two approaches - Ficks first law and the Onsager phenomenological formalism. From an experimental application perspective, the new development is applied to the binary alloy case. The formalism provides the means to obtain the interdiffusion coefficient and tracer diffusion coefficients simultaneously from analysis of the interdiffusion composition profiles in a single experiment. © 2013 Copyright Taylor and Francis Group, LLC

Ab-initio electronic structure, optical, dielectric and bonding properties of lizardite-1T

The structural, electronic, optical, dielectric and bonding properties of lizardite-1T [Mg 3Si 2O 5(OH) 4] are investigated using plane wave pseudopotential density-functional theory (DFT) method taking the generalized gradient approximation (GGA) as the exchange-correlation energy functional. The structural properties are consistent with the earlier experimental and theoretical results. The direct electronic band gap at the Γ-point is estimated to be 3.34 eV, which is less than the optical band gap of ∼4.0 eV measured from the fundamental absorption edge. A remarkable optical anisotropy is observed in the optical spectra. The dielectric properties are consistent with previous theoretical calculations. Analysis of Mulliken charge and bonding population shows the coexistence of covalent and ionic bonding in the lizardite-1T and the results are also consistent with previous theoretical calculations and experimental results