Ultrasound creates magnetic powders for potential use in ferrofluidic sealing
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 114 KB
- Volume
- 1997
- Category
- Article
- ISSN
- 1350-4789
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✦ Synopsis
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. The collapse of these cavitating bubbles generates intense local heating, forming a hot spot in the cold liquid with a transient temperature of about 9000°F, the pressure of about a 1000 atmospheres and the duration of about a billionth of a second.
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. 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.' 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 materials. In the presence of molecules that will coat these particles, stable iron colloids are formed in which the metal particles stay suspended indefinitely.
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. By comparison, the new sonochemical synthesis technique is both simple and quick.