W. A. Oates

Computer simulation of chemical diffusion in an ordered alloy

Abstract Chemical diffusion via the agency of vacancies in a simple cubic binary alloy with long- and short-range order has been simulated using the Monte Carlo method. Results for the diagonal and cross-correlation functions collected with the time-correlation method of Allnatt are in very good agreement with previous results collected with the in-field method of Murch. It is found that despite the existence of a maximum in the composition dependence of the thermodynamic factor in the ordered structure, other terms predominate, with the result that intrinsic and interdiffusion coefficients both show minima in their compositional dependence in a manner similar to that found previously for the tracer diffusion coefficient.

Thermodynamic properties of hydrogen and deuterium dissolved in palladium at low concentrations over a wide temperature range

The equilibrium pressures of the Pd/H2 and Pd/D2 systems have been measured in the temperature range from –100°C to 350°C for values of H(D)-to-Pd, atomic ratio, in the approximate range from 0.0005 to 0.005. These data were obtained in an ultra-high vacuum apparatus with large samples of low surface area. Partial thermodynamic data have been obtained from these equilibrium data; a significant temperature dependence is reported for the relative partial molar enthalpies and entropies.

Thermodynamic characterization of the Zr-Mn-H system part 1. Reaction of H2 with single-phase ZrMn2+χ C-14 Laves phase alloys

Abstract The reaction of hydrogen with ZrMn 2+χ has been investigated using reaction calorimetry for χ = −0.2, 0, 0.5 and 1.0. The enthalpies for hydride formation and decomposition are equal in magnitude over the two-solid-phase region; their magnitudes decrease with increasing χ. Hysteresis is large for all these intermetallics, approximately 3.0 kJ (mol H) −1 , and independent of χ.

A computer simulation study of manning's relations in a binary alloy with order

Abstract By means of Monte Carlo computer simulation we have explored the validity of the Manning relations which connect the phenomenological coefficients to the tracer diffusion coefficients in an alloy. The model chosen for calculation was the ordered binary alloy analogue of the king magnet. The vacancy mechanism was assumed. At temperatures above and below the order-disorder temperature, the Manning relations provide a good simulation, except at compositions approaching impurity levels. The relations then break down because they have built into them a requirement that ω4 = ω0 (four-frequency impurity model notation) in contrast to the model.

The effect of isotopic substitution on the thermodynamic properties of palladium-hydrogen alloys

Abstract An attempt has been made to predict the thermodynamic properties of the Pd-D and Pd-T systems from those of the Pd-H system. By considering only the isotopic dependence of the optic modes, a satisfactory interpretation of the infinitely dilute solutions is not possible. However, using only the isotopic dependence of the optic modes together with an empirical equation for the infinitely dilute solution yields calculated results for the concentrated solutions in both the single and two phase regions which are in excellent agreement with the experimental results for the deuteride and tritide systems.

A test-site method for the evaluation of the lattice-gas pressure in Monte Carlo simulations

Abstract A new method for the evaluation of the lattice-gas pressure from Monte Carlo simulations is proposed and is illustrated for a square lattice with nearest-neighbour interactions. The method involves the determination of the derivative of the partition function with respect to the change in the total number of sites by using the removal or addition of a test site. Estimates of the lattice gas pressure by this test-site approach are compared with those obtained from the exact solution (for the half-filled lattice), the thermodynamic integration method, and the method of Meirovitch which involves the calculation of the pressure from the integral entropy. Both above and below the critical temperature the new method is found to work very well. It has the advantage of speed over the other methods and is, therefore, the most suitable for use in phase equilibrium calculations.

Temperature-chemical potential diagrams for the representation of defect and phase equilibria in compound semiconductors—application to gallium arsenide

Abstract Diagrams which show the inter-phase (gas, liquid, solid) and intra-phase (atomic and electronic defect) equilibria during processing of a compound semiconductor, AB , are discussed. When complete equilibrium exists, T -μ B diagrams give the clearest representation but, when only partial (electron, e) equilibrium exists, T -μ e diagrams are more useful. In order to follow the solidification and subsequent cooling to room temperature, a composite T- sm B μ e , diagram is the most informative. The construction of these diagrams is described and their use discussed by using the Ga-As system as an example. A simple defect model for the non-stoichiometric solid is used. This model, which includes deep donor and shallow acceptor states in the energy gap, is sufficient to explain compensation and the generation of semiinsulating material.

Thermodynamic characterization of the Zr-Mn-H system Part 2. Reaction of H2 with two-phase alloys

Abstract The reactions of H 2 (g) with the two-phase alloys (a) α -Zr+ZrMn 2+ χ and (b) α -Mn+ZrMn 2+ χ , have been investigated. The homogeneity range of the Laves phase ZrMn 2+χ intermetallic compound has been determined from the dependence of its hydrogen absorption behavior upon χ and found to lie within the limits χ = −0.3 ± 0.1 and 1.0 ± 0.1. Normally H 2 (g) does not react with α-Zr below about 773 K, but it has been found that when it exists within the two-phase mixture, it reacts readily with hydrogen at room temperature. For alloys in the zirconium-rich two-phase field the hydrogen first reacts to form ZrH −2 and then it reacts with the ZrMn 2+χ phase. The former reaction leads to equilibrium pressures which are too small to measure, but the latter leads to measurable pressures. Results for alloys within the manganese-rich two-phase field, χ ≥ 1.0, are also reported. Calorimetric determinations of the enthalpies of reaction with hydrogen for several zirconium-rich alloys within the two-phase fields are reported for both the zirconium-rich and ZrMn 2+χ phases.

Thermodynamic properties of binary alloys from Monte Carlo simulations

Abstract The relations between the thermodynamic properties of the unary lattice gas and the equivalent binary substitutional and interstitial alloys are discussed. New methods for the estimation of the partial particle energy in a lattice gas from Monte Carlo simulations in both the petit and grand canonical ensembles are presented and evaluated. Taken together with well established procedures for estimating the particle chemical potential and more recent procedures for estimating the lattice gas pressure, it becomes possible, for the first time, to obtain reliable determinations of both the partial and integral quantities for internal energy, entropy and free energy at a given temperature and composition in a rigid binary alloy from the one Monte Carlo experiment in either ensemble. Some illustrative results are given for the square lattice with nearest neighbour interactions.

Computer simulation of the diffusion isotope effect in a binary ordered alloy

Abstract By means of Monte Carlo computer simulation, we have explored the extent of validity of the ‘impurity form’ of the tracer correlation factor in a model of the stoichiometric binary alloy (Ising magnet analogue). Above the order-disorder temperature the impurity form of the correlation factor appears to be valid. Below the order-disorder temperature, there is a deviation between the actual tracer correlation factor and that predicted by means of the impurity form, and this deviation increases substantially as the temperature decreases. These findings imply that the measured isotope effect in ordered alloys may not in fact be directly related to the usual tracer correlation factor.