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Better materials through clusters.

Better materials through clusters

Though limited to a cast of 90 or so atomic elements, chemists can build a veritable universe of molecules and materials. Materials scientists have been learning how to assemble tens to many thousands of molecules into clusters, which in turn could become the assembly units for another, virtually unexplored universe of new materials tailored for specific structural, electrical or optical applications.

"We want to be able to first decide what material properties we would like to have, and then actually make the material from building blocks of clusters," says Richard W. Siegel of argonne (Ill.) National Laboratory.

Compared with conventional bulk materials such as clay and metals, cluster-made materials have enormous proportions of their atoms on internal surfaces. In materials made of 5-nanometer-diameter (billionths of a meter) clusters, half the atoms are at the surfaces. In a hunk of metal -- an assemblage of much larger grains -- surface atoms represent only a tiny proportion of the total number of metal atoms.

Siegel's group uses a gas condensation chamber to make the clusters. For example, to make 12-nanometer clusters of the ceramic titanium dioxide (rutile), the researchers first place a piece of titanium on a heating element within a helium-filled chamber. As the metal atoms evaporate, they gather into nanometer-scale clusters and condense on a colder surface, also in the chamber. A dose of oxygen in the chamber then converts the titanium into rutile clusters, which are collected under a vacuum and mechanically pressed into ceramic pellets.

These clusters bake into a stable ceramic at far lower temperatures than do typical rutile grains 1,000 or more times larger, Siegel reports. Also, cluster-made rutile appears slightly more ductile, and therefore probably more formable, than its conventional counterpart, he says. With further development, the technique might enable researchers to fashion ceramics into intricate shapes unattainable with the normally brittle materials. The possibilities are so broad, Siegel remarks, that most of them have yet to be imagined.
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Title Annotation:Materials Science
Publication:Science News
Date:May 6, 1989
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