Molecular Dynamics Analysis of Temperature Dependence of Liquid Metal Diffusivity
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WLEDGE of the diffusivities of liquid metals, obtained through either experimental measurement or theoretical prediction, is essential for the understanding of the dynamic properties of liquid metals. The diffusion coefficients have wide practical applications in engineering and science, such as metal processing, interfacial reactions, crystal growth, phase separation, and corrosion. Some efforts have been devoted to experimental and theoretical studies of the diffusivity of liquid metals. However, data on self-diffusion and binary diffusion for liquid metals and metallic alloys are scarce in the open literature,[1,2] and the temperature range explored in these studies was frequently rather narrow. Gravity and Marangoni convection disturb the experimental measurement of the diffusion coefficient. To minimize their disturbances, some methods were developed, including the shear cell technique[3] and the trilayer film technique.[4] Some investigations were carried out under a microgravity environment in space at a great cost. Furthermore, improving the accuracy of the concentration measurement of the diffusing species is also a nontrivial task for deducing the diffusion coefficient. The development of theoretical study of the atomic transport coefficient in liquid metals follows mainly two approaches: semiempirical analytical formulations[5,6] SUI YANG, Doctor, and XUPING SU, JIANHUA WANG, and FUCHENG YIN, Professors, are with the Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University, Xiangtan, 411105 Hunan, People’s Republic of China. Contact e-mail address: [email protected] NAI-YONG TANG, Doctor, is with the Teck Metals Ltd. Product Technology Centre, Mississauga, ON, Canada L5k 1B4. Manuscript submitted February 9, 2009. Article published online September 17, 2009 3108—VOLUME 40A, DECEMBER 2009
and molecular dynamics (MD) simulation.[7–9] The former contains several adjustable parameters for fitting the experimentally measured diffusion coefficients; the Stokes–Einstein equation[5] and the Enskog expression[6] are two well-known examples. However, the atomic radius in the Stokes–Einstein equation could assume different values under different circumstances simply for achieving a better fit to the experimental results. As such, this equation lacks general satisfaction. The Enskog expression is deduced from the kinetic theory, treating atoms as hard spheres. The interatomic potential employed in the hard sphere method is a rudimentary approximation for predicting the diffusivity of a liquid metal, because only repulsive interaction is considered between the constituents of the liquid. Some physical properties predicted by the pair potential models were not confirmed by experimental work, including surface properties, vacancy formation energy, melting point, and the change in entropy at the melting point. To overcome these shortcomings associated with the pair potentials, the many-body semiempirical potentials have been developed, such as the embedded atom method (EAM),[10] the tight-binding method
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