Melt-Resistant Metals: Carbon coating keeps atoms in order

May 14, 2003 at 2:16 pm

Scientists have long known that impurities and flaws in a metal’s crystal structure can lower the material’s melting point. In an unexpected twist, an international research team has dramatically boosted the melting points of metals by straightjacketing nanometer-scale crystals inside thin carbon shells.

TAKING THE HEAT. At 140C above lead’s normal melting point (top left), carbon-coated nanoclusters of the metal remain solid. At 270C (top right) above the normal melting point, the nanoclusters have become liquid. Simulations (bottom) show corresponding crystal structures. Banhart et al./Physical Review Letters

The findings may lead to microcircuits, sensors, and polymers that can function at higher temperatures than current ones do, says team member Mauricio Terrones of the Instituto Potosino de Investigación Científica y Tecnológica in San Luis Potosí, Mexico. Terrones and his colleagues describe their results in the May 9 Physical Review Letters.

Ordinarily, metal nanoclusters containing only hundreds to thousands of atoms melt at temperatures much lower than the metals’ larger-scale forms do. That relationship changed dramatically when Terrones and his colleagues enveloped tin or lead nanoclusters in graphitelike carbon layers a few atoms thick and then heated these assemblages while observing them with an electron microscope.

Although the scientists expected increases, they were startled by how high the metals’ melting temperatures rose. For tin, the melting point leaped as high as 265C above the bulk metal’s melting threshold. Likewise, lead clusters melted only at temperatures more than 140C above the melting point of bulk lead.

“The magnitude of the superheating is colossal,” comments materials scientist Robert W. Cahn of the University of Cambridge in England. “Nothing like that has ever been seen before.”

On a fundamental level, the researchers’ observations may clarify what happens on the finest scale as a solid melts, Terrones says. For lead clusters, the researchers found that a characteristic magnitude of vibration presaged melting, no matter what the pressure and temperature. This new finding suggests that vibration threshold is a fundamental property of the metal, not just a product of experimental conditions.

In this case, Terrones suggests, the carbon sheaths suppressed the nanocrystals’ vibrations compared with those in uncoated metal. Only at higher temperatures, therefore, did the vibrations reach the threshold for melting.

Some studies of crystals that had been essentially shrink-wrapped within ceramics and other materials have found more modest melting-point increases. These changes were attributed to restrictions in the motion of the crystals’ surface atoms.

That’s not enough to explain the huge melting-point boosts elicited by the carbon shells. In experiments and simulations, giant pressures–up to several thousand times atmospheric pressure–proved to be responsible, the scientists now report. The nanocapsules are “like pressure cookers,” Terrones says.

That pressure effect may have practical implications in nanowires where the loss of even a few atoms can spell failure, comments David L. Carroll of Wake Forest University in Winston-Salem, N.C. For instance, the pressure exerted by a carbon coating on metallic nanowires in future ultrasmall circuits could keep atoms in place despite heat and current flow.

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