Thermodiffusion in liquid binary alloys computed from molecular-dynamics simulation and the Green-Kubo formalism

In the presence of a temperature gradient, the components of a binary liquid tend to segregate. This phenomenon, generally referred to as thermodiffusion or the Soret effect, is usually quantified by the heat of transport. We report heat of transport values Q(c)(*) for NiAl and NiCu melts computed using molecular dynamics simulation and the Green-Kubo formalism. Thermal conductivities are also reported. To develop a clear picture of the phenomena, we determined contributions to Q(c)(*) due to the convective and virial components of the heat current, which were then compared to the related terms in the partial enthalpy. It is shown that the contribution to Q(c)(*) from the convective component of the heat current is comparable to the average energy of the diffusing atoms, differing by an amount comparable to the activation energy for diffusion. The contribution to Q(c)(*) from the virial heat current is closely related to the pressure-volume term p(c)Omega in the partial enthalpy. It is established that the virial heat current plays a dominant role in determining the sign of the reduced heat of transport Q(c)(*') = Q(c)(*) - h(c) By comparing results obtained with different empirical potentials, a trend emerges. Specifically, it is found that the sign of the reduced heat of transport is correlated with the sign of the partial pressure associated with the low-mass component. It is also shown that two different empirical potentials for the NiAl system give vastly different results for Q(c)(*'). The results indicate that in developing a potential that might accurately predict Q(c)(*) the distribution of the partial energy and partial pressure between the two components is critical. Based on these observations, it would appear that existing empirical potentials may not be able to generate reliable predictions for Q(c)(*') without additional validation. (C) 2016 Published by Elsevier B.V.