The fusion-solidification of a metal alloy can be studied partially inserting a metal bar into a furnace. The sample's outer end is solid whereas the other end in the furnace is liquid. If the furnace is displaced, the contact surface between solid and liquid (called solidification interface) is also moving about the sample. This motion creates an imbalance and so a temperature difference between solid and liquid (called undercooling). The shape of this mobile interface depends on the alloy concentration (in this case, tin and bismuth), the furnace's displacement speed about the sample (pulling rate) and the convection motions within the molten metal.
Microgravity eliminates these motions because it suppresses the action of density differences between hot and cold parts of the liquid which are responsible for these motions on Earth (hot parts tend to rise whereas cold ones tend to sink).
So, into space, two parameters can be adjusted: alloy sample initial concentration and pulling rate.
The metal alloys' solidifications are difficult to observe because they are totally opaque and the solidification interface's observation is only possible right after the process' end.
The MEPHISTO instrument, jointly developed and operated by CNES and the CEA, was designed to obtain information on the interface during the solidification thanks to the Seebeck effect: an electric potential difference is created when two different-temperatured bodies are in contact. This electric potential difference is called "thermocouple". The electrical voltage on the sample's ends is very low, in the microvolt and measuring it in the nanovolt was one of the challenges of the experiment. This real-time voltage, which constitutes the undercooling's measurement at the mobile interface, is transmitted directly to the ground. Since the undercooling depends on the interface structure, the scientists can thus confirm the solidification theories by detecting the pulling rate threshold after which the interface is no longer flat and become wrinkled. This is when, for pulling rate even higher, dendrites start forming like solid Christmas trees protruding into the liquid.
This experiment was the first real-time observation of the solidification phenomenon in the absence of convection since the previous ones required to bring the solidified samples back on the ground to analyse them afterwards. Now these research are still on studying in the International Space Station the solidification of transparent models' alloys. This transparency allows for direct images of the solid-liquid interface. These images are sent in near real-time to the scientists in the laboratories on the ground so they can accurately monitor the experiment and choose to redo it with different parameters.
- Condensed-Matter Physics program scientist: Bernard Zappoli