STS-75 press kit

Edited by Richard W. Orloff, 01/2001/Page 24
Procedure. The Isothermal Dendritic Growth Experiment apparatus consists of a thermostat that contains
the dendrite growth chamber. The growth chamber will be filled with ultra pure succinonitrile (SCN), a
substance that mimics the behavior of metals, but is transparent, thus allowing the dendrites to be easily
photographed. Dendrite growth begins by cooling a tube, known as a stinger, which is filled with the
liquid and extends into the growth chamber. This causes the SCN to solidify, with a solidification front
moving down the tube to the tip of the stinger and emerging into the SCN volume as an individual
dendrite.
Two television cameras will allow scientists to watch for dendrites to emerge. The images of dendrites
growing in space will be viewed in near-real-time by scientists on the ground. When the experiment
computer detects dendrites, it will trigger two 35-millimeter cameras to photograph the samples.
Researchers will compare photographs of the space- grown dendrites to evaluate growth rate and dendrite
shape.
Materials for the Study of Interesting Phenomena of Solidification on Earth and in Orbit (MEPHISTO)
Principal Investigator: Dr. J. J. Favier,
Center for Nuclear Study, Grenoble, France
Objective. The investigation known as MEPHISTO is a cooperative program between NASA, the French
Space Agency and the French Atomic Energy Commission, with the goal of understanding how gravity-
driven convection affects the production of metals, alloys and electronic materials. MEPHISTO flew on
both previous USMP missions. Analyses of samples produced on orbit are being conducted by science and
technical teams to improve processes for making products ranging from alloys for airplane turbine blades
to electronic materials. This third flight of MEPHISTO will continue the investigation into how material
solidifies in microgravity. Ultimately, the MEPHISTO experiments may bring dramatic improvements in
materials production.
Researchers want to know what happens at the boundary between solid and liquid the solid/liquid interface
during solidification of a molten material, to better control this process on Earth. Temperature differences
at this boundary can cause fluid movements that affect the structure and properties of the solidified product
through convection and sedimentation. In microgravity, sedimentation and buoyancy-induced convection
are greatly reduced, so researchers can explore underlying processes that normally are masked by gravity.
Procedure. The MEPHISTO furnace aboard USMP-3 will repeatedly process three samples of a tin-
bismuth alloy using directional solidification, a common method for growing crystalline materials such as
metals and semiconductors. As the solidified region grows, the boundary between the solid and liquid
material will move from one end of the sample toward the other. Electrical measurements will gauge
temperature variations in the solidification front. These temperature variations are indicative of the stability
of the interface which is very important in controlling the properties of the material in its solid state. The
shape of the front will be marked in the growing crystal by subjecting the sample to electric-current pulses.
Researchers will compare results produced on orbit with those produced on the ground to better understand
and expand theories of materials, materials processing and the potential that the microgravity environment
offers for research in areas with down-to-Earth applications.