Solidification Using a Baffle in Sealed Ampoules (SUBSA) Results

Results

The
SUBSA experiments got underway on July 11, 2002, with a 16-hour sample
processing run. SUBSA activities have been coordinated with troubleshooting
for the newly installed Microgravity Science Glovebox
(MSG), which has experienced difficulties with one of its fans and
downlinking data. Flight Engineer Peggy Whitson was able to restore the
MSG’s connection to the Ethernet during the week of July 15. They continue
to have problems with the Space Acceleration Measurement
System II (SAMS-II) sensor located inside the MSG. The sensor, designed
to record the vibration environment inside the rack, has not been sending
data to the SAMS-II control unit. The SUBSA run scheduled for July 18
was replaced with tests of SUBSA’s uplink connection and remote operation,
while the MSG team continued to troubleshoot SAMS-II.

Despite
the startup difficulties with MSG, Dr. Aleksandar Ostrogorsky declared
the first run a success: "I think my experiment went as nice as we
expected or hoped. . . . The molten semiconductor material was performing
as we wanted, without separating from ampoule walls or releasing undesirable
bubbles that have been reported in several previous microgravity investigations."

SUBSA
is built on a series of previous flight and ground experiments. Crystal
Growth Using a Baffle (CGB) was used to verify the SUBSA hardware and
to clarify the results the investigators expected to achieve with SUBSA.
More importantly, CGB was used to develop the method of directional solidification—Bridgman
growth with baffle—that would reduce the effects caused by acceleration
in microgravity. (Bridgman growth is initiated by reducing the applied
temperature when the sample in an ampoule has reached the solid-liquid
interface.)

Applications

Semiconductor
materials have electrical conductivity and insulating capabilities. This
means that they can efficiently conduct a range of signals, including
optical, without conducting heat. Tiny semiconductor crystals can be found
in computer chips, sensors, and wireless communication devices. They are
part of the reason that so many electronic devices today are so much smaller
and better than their predecessors. Improved semiconductor quality—well-formed
crystals with few or no imperfections—will reduce the size and increase
the quality of electronic devices even further.

Convection
and buoyancy caused by gravity interferes with materials processing on
Earth, making it difficult to create crystals that are free of pores and
other imperfections. The microgravity of Earth orbit seems a natural choice
for materials processing. But low gravity does not mean that Earth orbit
is completely quiescent. Acceleration, Station operations, and crew activities
creates mild vibrations that can disrupt crystal formation. SUBSA will
test a baffle system and encapsulation process that may solve this problem,
making it possible for future researchers to produce the high-quality
semiconductor crystals that are in demand on Earth.

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