Preflight diffusion experiments on liquid metals under 1g conditions for the FOTON-M2 mission

We measured diffusion of up to 5% Ag in liquid Pb, using the Foton shear cell (designed and used for the FOTON-M2 mission). Since the density change of the alloy with composition has been reported to be very small but is not reliably known, we performed two arrangements of layering: (a) with PbAg on top and Pb below and (b) with Pb on top and PbAg below. The Berlin group did interdiffusion experiments from a "thick layer" of PbAg into "semi-infinite" Pb and the Karlsruhe group did the typical "interdiffusion" experiments with a diffusion couple of PbAg versus Pb. The results of both groups were equivalent, showing much lower D values for arrangement (b) than for (a). The lower values were even definitely lower than reference microgravity values from MIR/MIM and could be fitted by D = AT(n) with n about 2.2. We discuss the stability/instability by density layering depending on the arrangement.