From: Marshall Space Flight Center
Posted: Wednesday, August 7, 2002
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 solidificationBridgman growth with bafflethat 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 qualitywell-formed crystals with few or no imperfectionswill 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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