Crystal Growth

Activation of USML-1 was complete by 9:00 pm and, while Dunbar busied herself with preparing it for research, Bowersox and DeLucas unstowed, initiated and photographed two samples in the Protein Crystal Growth (PCG) unit, housed in a locker on Columbia's middeck. It was hoped that data from these experiments, which had already flown on several occasions, would yield new knowledge about the proteins' molecular arrangement and assist in the production of more nutritious foods, highly resistant crops and better medicines with fewer adverse side effects. Moreover, the extended nature of USML-1 meant that slower-growing crystals could be produced.

After their return to Earth, the three-dimensional structures of such crystals would be carefully mapped. Crystals of proteins grown in terrestrial laboratories are large enough to study, but usually they include numerous flaws, caused by gravity-induced convection, buoyancy and sedimentation. Space-grown crystals, on the other hand, are of greater purity and have more highly ordered structures which significantly improves their X-ray analysis. For USML-1, protein crystal growth experiments were conducted both within an incubator in the middeck and inside the glovebox on the Spacelab module.

The importance of this research to the pharmaceutical industry was illustrated by the inclusion of DeLucas, a biochemist from the University of Alabama at Birmingham, on the crew. He was already recognised as one of the United States' foremost experts in protein crystal growth and USML-1 would be the first time such experiments had been conducted with a human expert in attendance to enhance the production process using a special controllable furnace. Previously, the experiments had largely been automated and the only astronaut involvement had been to mix the chemical solutions.

''Someday,'' DeLucas said before the flight, ''I feel confident we will find a drug that might help to prevent the complications of diabetes, maybe prevent hypotension, maybe find a cure for some types of cancer.'' It was considered possible that such research may eventually lead to treatments for emphysema and HIV-AIDS. Yet, despite the promising outlook for the experiments, DeLucas noted it would be a decade or more before a breakthrough could be made. ''We need a space station,'' he said. ''My investigators don't need one crystal so we can make a breakthrough. We need a constant supply of high-quality crystals.''

In addition to the protein crystal growth experiments, a Crystal Growth Furnace (CGF) in the Spacelab module was used to solidify a wide range of materials -mainly semiconductors - which form the basis of electronics devices, including computers, timepieces and audio, video and communications equipment. The furnace could process materials at temperatures of up to 1,260 Celsius and allowed researchers to investigate the factors affecting crystal growth and explore the best methods for producing better crystals. Post-flight analysis of the CGF crystals from USML-1 revealed distinguishable differences from terrestrial-grown samples and showed them to be of much higher purity.

During the mission, the CGF grew crystals using two different methods: 'directional solidification', whereby the solidification front proceeded in a specific direction along the sample, and 'vapour crystal growth', in which part of the sample was heated to make it sublime and the vapour was then allowed to flow towards, and condense upon, a substrate base in a cooler part of the sample ampoule. The USML-1 experiments grew crystals of cadmium telluride, mercury-zinc telluride, gallium arsenide and mercury-cadmium telluride, all of which also found applications in infrared detectors for medical equipment, night-vision goggles and telescope sensors.

The furnace itself, meanwhile, could handle up to six samples simultaneously,

Crystal growth 155

each of which was housed in its own quartz ampoule in a rotary changer; these samples were processed under computer control, although the experiments' principal investigators on the ground could uplink changes or adjustments where necessary. A flexible 'glovebox' provided access to the interior of the furnace if any of the ampoules failed, but handling of the samples was only done if absolutely necessary to prevent the risk of contamination to the Spacelab module's environment. It was, however, demonstrated successfully during the flight.

Dunbar activated the CGF during her first shift, late on 25 June, and started the first processing run of a mercury-cadmium telluride crystal by loading six samples into the rotary changer and setting it up for an 18-hour solidification run. ''This is the culmination of all those years of study,'' said Principal Investigator Heribert Weidemeier, whose team had spent a decade developing it. ''Nobody can grow as good a crystal on Earth as it is possible to grow in space.''

Unfortunately, a circuit breaker tripped early on 29 June and, although Meade followed a procedure to successfully revive it, a mercury-zinc telluride crystal had to be stopped halfway through its growth cycle. Instead, Meade inserted a sample of cadmium-zinc telluride for a 92-hour processing run. The furnace was later used to grow the longest-ever gallium arsenide crystal - measuring almost 16.5 cm - on 4 July. This latter compound contained a 'dopant' - a trace impurity deliberately added to a semiconductor to allow scientists to precisely engineer its properties.

''It is amazing to me that we can replace 10 atoms in a million with 10 atoms of a different substance and drastically change a crystal's electronic properties,'' said Principal Investigator David Matthiesen. ''But that tiny impurity makes electronic switching much faster, requiring less power and therefore allows more circuits to be packed into a given area.'' Gravity-driven convection on Earth makes it very difficult to control placement of these dopants and if too many are concentrated in one spot the crystal could have inconsistent material properties.

Another mercury-cadmium telluride sample was also removed from the CGF on 5 July after six hours of growth. Unlike earlier samples, it was only three-thousandths of an inch thick. Weidemeier told journalists that the wafer-thin sample would allow scientists to study the crystal with as little processing as possible. Several samples were loaded into the furnace using a flexible glovebox unit and, in total, the CGF ran for 286 hours during the course of the mission, processing seven semiconductor crystals - three more than planned - and reaching a maximum temperature of 870 Celsius.

Meanwhile, crystals of zeolites were being grown in the middeck. These complex arrangements of silica and alumina, which occur both naturally and synthetically, have an open crystalline structure that is selectively porous, enabling them to serve as molecular sieves, and they have found terrestrial applications as 'sponges' or 'filters' to selectively absorb elements or compounds. They are used as highly efficient catalysts, absorbents and ion-exchange materials. Scientists sought to grow them as part of efforts to refine gasoline, oil and other petroleum products more cheaply. The zeolites were grown both in middeck lockers and inside the Glovebox in the Spacelab module.

The aim of the USML-1 zeolite experiments was to study the effects on the crystals' morphology of subtle changes in the chemical solutions and the overall reduction of defects. On several occasions, demonstrating the usefulness of having a trained observer present, Dunbar spoke directly to Zeolite Crystal Growth (ZCG) Principal Investigator Al Sacco - who would himself fly on Columbia as a Payload Specialist on USML-2 in October 1995 - to discuss the best mixing procedures for a particular experiment run. ''A person up there optimising the mixing of the crystal solution,'' said USML-1 Mission Scientist Don Frazier, ''is a milestone in research.''

0 0

Post a comment