Ultrasound Creates Metal Powders With Potentially Wide Application

February 11, 1997

CHAMPAIGN, Ill. -- Loud sounds can break things -- a fact University of Illinois chemists are using to their advantage. They're making extremely fine metal powders through a technique called sonochemistry -- the chemical application of high-intensity ultrasound. The highly magnetic powders have potential uses in information storage, audio reproduction and magnetic sealing.

"Sonochemistry arises from acoustic cavitation -- the formation, growth and implosive collapse of small gas bubbles in a liquid blasted with sound," said Kenneth Suslick, a U. of I. professor of chemical sciences. "The collapse of these cavitating bubbles generates intense local heating, forming a hot spot in the cold liquid with a transient temperature of about 9,000 degrees Fahrenheit, the pressure of about a 1,000 atmospheres and the duration of about a billionth of a second." (For a rough comparison, these values correspond to the temperature of the surface of the sun, the pressure at the bottom of the ocean, and the lifetime of a lightning strike.)

When the bubbles collapse, the vapor of volatile metal-containing compounds inside the bubbles breaks down into individual metal atoms, which then clump into clusters, Suslick said. The clusters contain a few hundred atoms and are about one ten-millionth of an inch in diameter.

"These small magnetic metal clusters have attracted great scientific interest because they behave as 'superparamagnets,' " Suslick said. "In superparamagnetic materials, the magnetic moments consist of clusters of atomic spins, in which all the spins are aligned in the same direction. Each cluster behaves like a separate magnet, and magnetic moments of these clusters are more than 100 times higher than normal material.

"In the presence of molecules that will coat these particles, stable iron colloids are formed in which the metal particles stay suspended indefinitely," Suslick said.

Colloidal suspensions of fine magnetic particles are commercially produced as "ferrofluids" and have a wide range of applications, including magnets in stereophonic speakers, magnetic inks for bank checks, and magnetofluid seals, lubricants and bearings. Ferrofluids also have been applied widely in medicine -- to deliver drugs, to restrict blood flow to certain parts of the body, and to act as X-ray or magnetic resonance imaging materials for diagnostic imaging.

"Commercial ferrofluids are generally produced by exhaustively grinding iron oxide particles for many weeks," Suslick said. "By comparison, our new sonochemical synthesis technique is both simple and quick."

Suslick and graduate students Ming Fang and Taeghwan Hyeon reported their findings in the Nov. 27 issue of the Journal of the American Chemical Society.

University of Illinois at Urbana-Champaign

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