USML-2 Public Affairs Status Report #10 6:00 a.m. CDT, Oct. 26, 1995 5/21:07 MET Spacelab Mission Operations Control Marshall Space Flight Center Crew activities were relatively quiet aboard the second United States Microgravity Laboratory overnight, as Mission Specialist Cady Coleman and Payload Specialist Fred Leslie took turns getting a few hours rest from their busy schedules. The mission's many crystal growth experiments continued uninterrupted, taking advantage of unique opportunities for discovery available only in the low-gravity environment of space. The Crystal Growth Furnace team finished melting Dr. David Matthiesen's gallium arsenide crystal and began slowly resolidifying the semiconductor sample. At almost 2,300 degrees Fahrenheit (1,255 degrees Celsius), the furnace is operating at the highest temperature it will reach during the mission. A new Crystal Growth Furnace feature for USML-2 will periodically mark the point where the melted material is solidifying with an electric pulse. When the crystal is analyzed after landing, the marks will indicate the exact growth rate of the crystal and the location of the solid/liquid boundary at each stage of solidification. Electronic devices using gallium arsenide semiconductors, such as high-speed digital circuits, operate at higher speeds and use less power than those using silicon crystals. Matthiesen's experiment investigates techniques for uniformly distributing a small amount of selenium within the crystal as it grows in microgravity. "There is less than one part per million of selenium in this sample," said furnace team member Dr. Frank Szofran of Marshall Space Flight Center. "Yet it greatly alters the electrical conductivity of the semiconductor." Trace materials, called dopants, are often added to semiconductors to improve or precisely control their electronic characteristics. To produce high quality crystals, scientists need to understand the process by which dopants are distributed within a compound during crystal growth. Growing the crystals in microgravity greatly reduces uneven dopant distribution caused by gravity on Earth, allowing more subtle influences to be identified. Leslie and Coleman spent most of their on-duty hours working with the Surface Tension Driven Convection Experiment (STDCE). Last night's runs were the first to use the experiment's largest chamber, almost 1-1/4 inches (3 centimeters) in diameter. As they have for the past five days, the science team in Huntsville and the Spacelab crew on orbit worked together to precisely adjust temperatures on the silicone oil surface. Again, they were able to pinpoint when surface-temperature-driven flows within the fluid became unsteady, or oscillatory. This was the first time oscillatory flows had ever been observed in such a relatively large container. Coleman reported seeing especially dramatic, wave-like oscillations near the center of the fluid flow during some of her experiment runs. Unwanted fluid flows affect the quality of materials solidified from a molten state on Earth. Understanding the subtle factors which control those flows gives researchers tools for eventually controlling them. Several factors are contributing to the success of the USML-2 surface-tension experiments. A new optics system developed by Dr. H. Philip Stahl and students from Rose Hulman Institute of Technology in Terre Haute, Indiana, gives the crew and ground controllers precise pictures of oil surface shapes and flow patterns. Spacelab's new six-channel Hi-Pac Television system is simultaneously downlinking video of those images, along with a three-dimensional view of the chamber and infrared temperature readings, giving the science team a complete representation of the experiment. STDCE team members also have been keeping a close eye on real-time data from the Three-Dimensional Microgravity Accelerometer, or 3-DMA. The low-frequency vibration detector has a sensor located in the rack next to the surface tension experiment. "3-DMA allows us to make a judgment as to whether to wait for external movements to settle down before beginning an experiment run," said Project Scientist Alex Pline. On its first Shuttle flight, 3-DMA was developed as a low- cost commercial accelerometer system by the University of Alabama at Huntsville's Consortium for Materials Development in Space. The instrument measures both the absolute level of microgravity acceleration (the difference between zero acceleration and what is experienced during the mission) and microvibrations which could affect the investigations onboard. Principal Investigator Jan Bijvoet worked in Huntsville for the European Space Agency when it was developing the Spacelab over a decade ago. "It's good to have an experiment aboard 'my' Spacelab," he said. The Geophysical Fluid Flow Cell Experiment team is wrapping up another six-hour solar atmosphere simulation, and growth continues in the Zeolite Crystal Growth Furnace and the many USML-2 protein crystal growth experiments. USML-2 crew members will go back to a full-time work schedule today. Their tasks include a Drop Physics Module experiment to study how an additive affects liquid surface behavior, as well as a photography session for the Glovebox Colloidal Disorder-Order Transition investigation. Status reports are issued from Johnson Space Center's Mission Control at 8 a.m. and 5 p.m.; and from Marshall Space Flight Center's Spacelab Mission Operations Control at 6 a.m. and 6 p.m. weekdays, 6 a.m. on weekends. For additional information, see the Internet USML-2 payload homepage, http://liftoff.msfc.nasa.gov/spacelab/usml2/welcome.html and the STS-73 Shuttle homepage, http://shuttle.nasa.gov