Diffusion Coefficients of Metallic Melts Measured by Shear Cell Technique Under Microgravity and on the Ground

Abstract

Diffusion coefficients in liquid metals are very important parameters for both material processing and physics of liquid. However, we have not yet had enough amounts of accurate diffusion data to discuss the diffusion model in liquid metals, because several kinds of convection bother measurements. The sources of convections expected in diffusion experiments are classified into the followings: (1) natural convection caused by gravity, (2) Marangoni convection caused by difference of the surface tension, (3) convection induced by mechanical force, etc. As a result of existence of convection, the measured diffusion coefficients show values higher than the real values and the measured values are scattered. Until now several attempts were done to suppress natural convection during diffusion experiments. One of them is to perform diffusion experiments under microgravity conditions where no buoyancy force is generated. In 1980s–1990s Frohberg et al. performed diffusion experiments using the long capillary technique as explained later in the space shuttle missions [1–3] and demonstrated: both measured values of diffusion coefficients obtained under microgravity in the space shuttle; their scattering are smaller than that obtained on the ground. Furthermore, the temperature dependence of the diffusion coefficients was found to be proportional to the square of the temperature. Afterward Yoda et al. [4] and Itami et al. [5] performed concerning experiments in another space mission and obtained results that have consistency with the results of Frohberg's group. These results showed the effectiveness of microgravity conditions to suppress natural convection and are helpful to discuss diffusion mechanism.