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A grazing view of melting.

A grazing view of melting

Although freezing is a complicated process, scientists generally agree on its essential features. Melting, on the other hand, is still very much a mystery. For example, water can be cooled well below its freezing point of 0 [deg.] C if the water contains no impurities around which ice crystals can begin to form. But ice always melts at 0 [deg.] C; it can't be "superheated." One possible explanation is that solids begin to melt before they reach their melting points. Researchers have recently found experimental evidence for just such an effect, known as "premelting," in lead at temperatures well below its melting point.

The experiments were done by Sean Brennan of Stanford University and his collaborators at AT&T Bell Laboratories in Holmdel, N.J. The researchers used a special technique that strictly limits the depth to which X-rays, coming in at a grazing angle, penetrate a solid. By observing any scattered radiation, they could detect the presence of a liquid lead layer on the surface of a single crystal of lead.

Brennan and his colleagues found that the initial stages of premelting may occur at temperatures well below a material's melting point. Because the arrangement of lead atoms on different surfaces of a single crystal looks somewhat different, the researchers also noted that these surfaces behave somewhat differently. Although both of the two types of surfaces studied showed traces of premelting at similarly low temperatures, one surface ended up with a thicker liquid layer than the other at temperatures closer to the melting point.

Brennan has also used "grazing-incidence" X-ray scattering to study the structure of thin iron-oxide films on various surfaces (a matter of interest to the magnetic recording industry) and to follow the growth of alternating layers of zinc and selenium atoms on a gallium-arsenide base. "We can watch while we're growing the material how its surface structure changes," Brennan says. "It's exciting to know what that structure is under varying conditions."
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Author:Peterson, Ivars
Publication:Science News
Date:Feb 4, 1989
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