http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:11956 First discovered by the late Dr John Manning, the vacancy-wind effect is a subtle phenomenon that occurs when two or more atomic species compete for vacancies in a net vacancy flux. The vacancy-wind effect is incorporated in (for example) the vacancy-wind or Manning factor that appears in the Darken-Manning Equation relating the interdiffusivity, the tracer diffusivities and the thermodynamic factor. The mechanism of the vacancy-wind phenomenon has long been very poorly understood. Recently, a moving reference frame Monte Carlo method was used to illustrate graphically how the vacancy-wind effect operates in both ionic conductivity in an ionic solid with a dilute solute and chemical interdiffusion in concentrated alloys and ionic compounds. That strategy is extended in this paper to show graphically how the vacancy-wind effect operates in interdiffusion in a stoichiometric intermetallic taking the B2 structure. A simple 4-frequency vacancy diffusion model is used. In previous work, it was shown that depending on composition and temperature, this model can exhibit the six-jump-cycle mechanism. It is shown that in the limit of perfect order that there is no vacancy-wind effect associated with this mechanism when both types of cycle operate equally (zero net vacancy flux). The non-unity value of the vacancy-wind factor found for this mechanism under zero vacancy flux conditions is purely a consequence of a particular geometric mix of tracer and collective atom displacements. The concept that a non-zero off-diagonal phenomenological coefficient provides the vacancy-wind effect is verified. 2012-11-09T00:27:15.549Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8316 Results of kinetic Monte Carlo simulation of the formation of a hollow nanosphere by interdiffusion from a core-shell binary system are presented for the first time. The faster diffusing species is located in the core whilst the slower diffusing species form the shell. With its self-generated vacancy composition all stages of the hollow sphere formation process are observed in our model: interdiffusion, the supersaturation of the core of the nanosphere by vacancies, precipitation of pores and eventual void formation. Results of this simulation confirm the experimental conclusions that interdiffusion accompanied by the Kirkendall effect and Kirkendall porosity is one of the mechanisms responsible for the formation of hollow nano-objects. 2011-07-19T02:10:29.048Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:805 C1 2010-04-27T06:49:08.997Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:804 In this paper, the vacancy-wind factors which appear in the well known expressions relating the interdiffusion coefficient and intrinsic diffusion coefficients to the tracer diffusion coefficients are examined in the context of the random alloy model. It is shown by Monte Carlo simulation that the formalism of Moleko et al. provides a much better description of these factors than the well known and commonly used formalism of Manning. Even when the ratio of the tracer diffusion coefficients is relatively close to unity, it is preferable to use the Moleko et al. formalism. The latter formalism is re-expressed in this paper to be suitable for direct experimental use in determining vacancy-wind factors. The limiting behaviour of the ratios of the tracer diffusivities and intrinsic diffusivities is also discussed. In the latter case the ratio is given simply (and generally) by the ratio of the exchange frequencies. 2010-04-27T06:48:49.106Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:15 In this overview, we discuss the sum-rule relating the phenomenological coefficients in random systems. First, we discuss the random alloy model itself Next, we present the basic sum-rule for diffusion via the vacancy mechanism in the random alloy model. We then describe two applications of this sum-rule, one to chemical diffusion in a multicomponent alloy, the other to chemical diffusion in strongly ionic mixed cation crystals. In the latter, we also present expressions linking the interdiffusion coefficient and the tracer diffusion coefficients and discuss the relationships with the Darken-Manning and Nernst-Platick equations. Next, we discuss the sum-rule for a vacancy-pair mechanism in strongly ionic mixed cation crystals. We then give an application of this sum-rule by describing chemical diffusion in that model. Again, we present expressions linking the interdiffusion coefficient and the tracer diffusion coefficients. 2010-04-27T05:35:29.488Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:4809 Net fluxes of vacancies commonly occur during chemical interdiffusion in alloys, ionic conductivity and the annealing out of radiation damage. When atoms with different jump rates diffuse in a net flux of vacancies the phenomenon of the vacancy-wind effect will occur. This effect, first discovered by the late Dr John Manning, is a subtle phenomenon arising from a disturbed distribution of vacancies with respect to a given moving atom or species of atom. In this paper, the vacancy-wind effect is discussed and its visualization, performed for the first time by computer simulation, is demonstrated. 2010-04-27T05:33:01.977Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3241 In this paper we explore the behaviour of the vacancy-wind factor or Manning factor for chemical diffusion in antistructurally disordered intermetallics taking the B2 structure. The analytical formalism is based on generalized six-jump- cycles providing the vehicles for diffusion of both components. Comparison of the analytical results is made with results from Monte Carlo calculations based only on single vacancy jumps. Very good agreement is found at low temperatures/ high levels of order where it can be anticipated that six-jump-cycles dominate the diffusion process. In such circumstances, a first (rough) approximation, it would be reasonable to put the vacancy-wind factor/Manning factor equal to 0.5 rather than unity as is often done. 2010-04-27T05:23:40.011Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3230 The non-random interaction of vacancies with atoms during interdiffusion and ionic conductivity is referred to as the vacancy-wind effect. This effect, first discovered by the late Dr John Manning, is a subtle phenomenon arising from the non-random distribution of vacancies with respect to a given moving atom within a net flux of vacancies. Recently, a good deal of progress has been made in determining accurate expressions for vacancy-wind factors in binary and ternary alloys, and in mixed cation ionic systems. The present paper provides an overview of these recent findings and puts them into a broader and historical context. 2010-04-27T05:23:37.665Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3277 The six-jump-cycle (6JC) diffusion mechanism is used to analyze the behavior of vacancy-wind factors and collective correlation factors in partially ordered B2 intermetallic compounds at stoichiometric and near- stoichiometric compositions. Expressions for the vacancywind factors are obtained in the framework of the four-frequency model where the two sublattices exist a priori. The phenomenological coefficients on the two sublattices that remain hitherto independent in 6JC mechanism are connected through a microscopic detailed balance condition. The present results for collective correlation factors when compared with our earlier calculation based on taking the harmonic mean of the sublattice correlation factors show much better agreement with Monte Carlo simulation results. The collective correlation factors and tracer correlation factors are used to calculate the vacancy-wind factors. Our results for vacancy-wind factors agree qualitatively with the simulation data when the frequency ratio (α) of structural and antistructural atoms jumps decreases up to the order of unity. 2010-04-27T05:07:02.642Z ]]>