Materials get a lift from sound.

Materials get a lift from sound

"Acoustic levitation' sounds more like a term you'd hear in a musical magic act than one that applies to making better glasses and other materials. But the decades-old idea of lifting and holding an object in place with intense sound waves traveling in a gas is becoming a technological reality for materials scientists. Their aim is to melt, mix, shape and cool new materials without having them touch the walls of a container, which may react chemically with a sample, introduce impurities or start unwanted crystal growth. At the recent Miami meeting of the Acoustical Society of America, researchers reported on progress with such containerless processing on earth and in space, where the absence of gravity requires less intense sound waves for levitation.

One group, led by Charles A. Rey of Intersonics, Inc., in Northbrook, Ill., is among the first to demonstrate that acoustic levitation at high temperatures is possible in a nearly weightless environment. In an experiment aboard the space shuttle Challenger, during its last successful flight two years ago, samples were levitated and heated to 1,900 kelvins. This is difficult to achieve on the ground because as the temperature rises, the density of the surrounding gas falls and it becomes harder to sustain and focus the high-intensity sound waves necessary to counteract gravity.

The experiments also demonstrated "that glass can be formed more readily without a container in zero gravity than it would be with a container,' says Rey. With no container to induce crystal growth, scientists will be able to make amorphous glasses out of a wide variety of compounds and carefully control their properties. Researchers estimate that 10 times as many glassy materials could be made in space as have been made so far on earth. The payoff, says Rey, may be improved fiber optics materials with more uniform optical properties, new types of glasses for lasers and improved glass lenses. With tighter control of glass compositions, some scientists expect, for example, that a typical camera zoom lens--which now uses more than 10 separate layers of glasses to produce the desired index of refraction and dispersion properties--might be made with only half as many layers.

Another space-based application might be the production of perfectly symmetric and uniform glass shells that would be used to hold fuel in inertial confinement fusion, a process being developed for energy and weapons research. According to Rey, no one has been able to make decent commercial-sized glass pellets on earth because with gravity, it's hard to keep a bubble smack in the center of a melted glass sphere. In the shuttle experiments his group nearly showed that the creation of perfect shells is possible, but because the temperature was too high the bubble migrated and escaped before the sphere cooled. The failure to make a shell, however, inadvertently yielded a positive answer to another troubling question: whether it is possible to get rid of glass bubbles in zero gravity.