| According to the classic rules of physics, substances melt at a lower temperature when their sizes decrease. But scientists at Indiana University Bloomington have found that at least one substance, gallium, breaks the rules, remaining stable as a solid at temperatures as much as 400 degrees Fahrenheit above the element's normal melting point. The discovery gives hope to some nanotechnologists and "nanocomputer" engineers, who have been worried that components will behave unpredictably at smaller sizes, possibly even melting at room temperature. "We expect this finding will interest nanotechnologists and the manufacturers of tomorrow's computers," said chemist Martin Jarrold, who led the National Science Foundation-funded research. "But we also believe chemists will find this phenomenon exciting -- it totally confounds their expectations." Jarrold and his collaborators showed that clusters of a few gallium atoms remain solid rather than becoming liquid near the element's normal melting point, 86 F. Just as surprisingly, the researchers showed that the tiny gallium clusters are actually more stable as solids when composed of 17, 39 and 40 atoms than a gallium slab containing trillions of atoms. Jarrold decided to test the stability of a substance that is especially sensitive to temperature changes near room temperature. Fitting that description is the metallic element gallium, which has an unusually low melting point. Placing a warm fingertip on a cube of gallium will cause it to melt. The researchers constructed a special device to shoot tiny gallium particles containing just a few atoms into gaseous helium. Collisions with the helium atoms broke the gallium clusters into small pieces. Two mass spectrometers measured the size of the intact and broken gallium clusters. By measuring the energy needed to break the clusters into pieces, the researchers were able to determine whether the clusters were in liquid or solid states. Jarrold and his team observed that gallium clusters with 39 and 40 atoms melt at around 531 F. Gallium clusters containing 17 atoms didn't melt at all across the minus 297 to 837 F temperature range the scientists surveyed. Why the gallium clusters retained such stability at high temperature is a mystery. Not all elements or compounds are likely to behave as gallium does. Using the same method, Jarrold previously learned that particles of sodium chloride -- table salt -- obey the classical rules of physics. Small salt particles with just a few atoms melt at low temperatures. Despite its potential implications for industry, Jarrold said his discovery and his general interest in atomic and molecular clusters, which began at Bell Laboratories, are mainly academic. "I just think it's fascinating to ask how small you can make something before its properties change," Jarrold said. |