Better crystals? It's a matter of space.

Better crystals? It's a matter of space

Lead iodide crystals grown last year aboard the space shuttle Discovery show unexpected differences from those produced on Earth with identical techniques, reports Steven L. Suib of the University of Connecticut of Storrs. The shuttle crystals -- the first grown in space from an aqueous solution and the first in space using a template to direct growth -- proved not only purer and more symmetrical but also less dependent on a template to initiate crystallization. The reason for these differences remains unclear, says Charles W.J. Scaife, at Union College in Schenectady, N.Y., who, along with S. Richard Cavoli, designed the tests.

The University of Connecticut/Union College team produced five cyclindrical crystal-growth reactors. Each three-chambered reactor carred solutions of lead acetate and potassium iodide -- one chemical at each end -- initially separated from a chamber containing water with the template in the middle. In orbit, when valves isolating the innder chamber opened, each solution began diffusing toward the membrane. Video recordings of the 40-hour experiments show crystals beginning to form on the membrane 30 to 120 seconds later.

In two experiments on Earth, with the reactors oriented so that their lattice templates stood vertically, yellow lead iodide crystals formed only on the lower half of each lattice. A deep, horizontal, lead iodide shelf marked the upper boundary of crystal growth. Crystalline supports flared out beneath it, and a beard that grew from the shelf began dripping crystals onto the growth chamber floor within 20 minutes.

In contrast, a relatively even coating of crystals smothered the entire surface of the template membranes in each of the three space-borne reactors. Additional satellite growths (see photo) crystallized throughout the adjacent liquid -- a phenomenon that Scaife notes has never been witnessed in Earth-grown crystals offered a template on which to adhere.

For unknown reasons, significantly higher levels of carbon appear to have migrated out of the growth reactors on Earth, Scaife says. Shuttle-grown crystals contained only about one-tenth the carbon contamination of those produced on Earth. Purer lead iodide crystals could boost the sensitivity of X-ray and gamma-ray film -- potentially allowing physicians to reduce patient X-ray exposures without sacrificing image quality. Scaife says the Rochester, N.Y.-based Eastman Kodak Co. has already shown interest in the data. But the shuttle experiments also suggest space-grown crystals might yield significant quality improvements for "molecular sieves," a commercially important class of chemical filters made from tightly packed crystals.