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.