Diffusion experiments in liquid Sn-Bi and Al-Ni systems with a stable density layering using the FOTON shear cell under 1g conditions

Abstract

In this study we investigated the possibility of reliable diffusion experiments in liquid metals under 1g conditions, instead of expensive μg-experiments. To minimise buoyancy convection we used thick layer diffusion from a binary alloy into apure metal. This can provide a stable density layering, which we have shown to be an important factor for successful 1g-experiments. To avoid the segregation problem and to minimize free surfaces a shear cell was used, which was specially developed for the mission FOTON-M2 and was equipped with reservoirs providing pressure on the liquid samples. Thick layer diffusion experiments from SnBi2.5wt% into Sn and from AlNi3.5wt% into pure Al were performed at 300°Cfor 8h and at 730°Cfor 5h respectively. For each set-up four parallel experiments were performed at the same time. The concentration profiles were obtained by AAS (atom absorption spectroscopy) and the diffusion coefficients were evaluated by fitting with the thick layer solution. For the evaluation a correction method was used for the shear convection and the AAS averaging effect inside a cell. As a result, the obtained concentration curves agreed well with the fitting function. The diffusion coefficients D_Bi=2.35x10^{− 9}m²/s andD_Ni=3.81x10^{− 9}m²/s agreed well within the error range with the μg-reference data obtained in the FOTON-12 mission and reference data obtained in the 1g diffusion experiment in a magnetic field. The reproducibility of the diffusion coefficients among four parallel experiments was very good with a standard deviation among four capillaries smaller than 3.1% including the standard temperature deviation. From these results we conclude that buoyancy convection was practically absent and thus the applied method was very effective.