The thermal stability of the nanograin structure in a weak solute segregation system

Solute segregation and thermal stability of nanocrystalline solid solution systems

A model coupling first principles and thermodynamics was developed to describe the thermal stability of a nanograin structure in solid solution alloys. The thermodynamic functions of solute segregation and conditions for thermal stabilization were demonstrated for both strongly and weakly solute-segregating systems. The dependence of segregation behavior on the grain size, solute concentration and temperature was quantified, where the parameters to control destabilization of the nanograin structure at a given temperature were predicted. For the first time it was found that there exists a transformation from the single-extreme to dual-extreme rule of the total Gibbs free energies of the solid solution systems with the decrease of solute concentration or increase of temperature. The model calculations were confirmed quantitatively by the experimental results, and a nanocrystalline W-10 at%Sc solid solution with a highly stable grain structure in a broad range from room temperature to 1600 K was prepared. The universal mechanism disclosed in this study will facilitate the design of nanocrystalline alloys with high thermal stability through matching of the doping element and the initial grain size.

Modeling of Li diffusion in nanocrystalline Li-Si anode material

To quantify the Li diffusion behavior in nanocrystalline anode materials for lithium-ion batteries (LIBs), a hybrid model of the first principles calculation and diffusion kinetics was developed. The dependence of the Li diffusion on the electronic structure, solute concentration, grain size and temperature was described for the nanocrystalline Li-Si system. In contrast to conventional polycrystalline materials in which the activation barrier for Li diffusion decreases with the increase of concentration before amorphization, there exists a coordination effect of the solute concentration and grain size on the Li diffusion in nanocrystalline materials. A maximum diffusion coefficient can be obtained in the nanocrystalline Li-Si by a combination of the concentration and grain size, which is increased by two orders of magnitude from that in the coarse-grained counterpart. The present work advanced the understanding of the Li diffusion mechanisms during lithiation/delithiation of LIBs and may facilitate the development of nanocrystalline anode materials.