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Researchers at APS make graphite hard as diamond

DOE/Argonne National Laboratory : 04 April, 2007  (Technical Article)
Science has yet to achieve the alchemist's dream of turning lead into gold. But a group of researchers using the Argonne's Advanced Photon Source (APS) may have found a way to turn ordinary soft lead into a new, super-hard material that 'looks' just like diamond.
Using the high-brilliance X-ray beams from the APS, the researchers discovered that, under extreme pressure, graphite, among the softest of materials and the source of the lead found in pencils, becomes as hard as diamond, the hardest known material. What's more, the new super-hard material can be induced to return to its previous soft state.

The research group was led by Wendy Mao, a graduate-student at the University of Chicago. Mao's father, David, director of the High-Pressure Collaborative Access Team at the APS and a researcher with the Carnegie Institution of Washington, Geophysical Laboratory, was a member of the group. HP-CAT is a specialized facility integrating a wide range of techniques for high-pressure research.

Graphite is made of layers of loosely bound carbon atoms that are spaced far apart. Because the carbon atoms in, for instance, a graphite pencil lead are not tightly bound to each other, they can be scraped off onto a surface, leaving a mark. Diamond's atoms, which are also carbon, are tightly bound together, giving the material extreme hardness. These diamond-type atom bonds are difficult to achieve. Diamond in the earth is made by great pressures and intense heat over geological timescales. Many secrets remain about how carbon behaves under high pressure; the studies by Mao and her collaborators are shedding light on how diamond bonds form.

Mao's research team used a diamond anvil cell and inelastic X-ray scattering at the GeoSoilEnviroCARS and HP-CAT beamlines. These specialized research facilities are designed to create extreme pressures. The cell produced a pressure of approximately 17 gigapascals, or 170,000 times the atmospheric pressure at sea level. A pressure of 17 gigapascals would exist more than 300 miles beneath Earth's surface. The inelastic scattering signal is weak, but thanks to the high brilliance of the APS combined with the novel beamline x-ray focusing optics and a multi-element inelastic x-ray analyzer designed by scientists at GSECARS the data could be collected in only a few days.

Half of the weak, widely spaced bonds between the graphite layers were forced closer together, converting them to stronger, diamond-like bonds. In fact, the graphite became so hard that it cracked the diamond anvil. Moreover, the graphite became an optically transparent, super-hard insulator, much like diamond. But, while the known forms of naturally produced diamond retain their hardness, the graphite in this experiment reverted back to its original softness once the pressure was removed.

While experts point out that more studies are needed before this basic science can be carried over into practical applications, a better understanding of atomic bonding-structure fundamentals will help materials scientists understand how and why a material becomes super-hard. As this X-ray research technique evolves, it can be used to find answers to long-standing riddles about what transpires at the atomic scale, the key to understanding the behavior of materials. Researchers at the HP-CAT facility study how materials react and change under high pressure and varying temperatures. High-pressure researchers have discovered new materials and new physical properties, and have advanced understanding of what happens in highly compressed, hot planetary interiors.

HP-CAT focuses on experiments that take maximum advantage of the extremely high brilliance and X-ray energies available from the APS. Member institutions are the Carnegie Institution of Washington, Geophysical Laboratory; Lawrence Livermore National Laboratory, High-Pressure Physics Group; University of Nevada, Las Vegas, High Pressure Science and Engineering Center; and the University of Hawaii Institute of Geophysics and Planetology.

GSECARS, a national user facility in operation at the APS since 1998, is focused on advancing knowledge of the composition, structure and properties of earth materials, the geologic processes they control and the chemical and physical processes that produce them. GSECARS specializes in research that uses high-pressure and high-temperature crystallography and spectroscopy with the diamond anvil cell; high-pressure and high-temperature crystallography and imaging with the large-volume press; powder, single crystal and interface diffraction; inelastic X-ray scattering; X-ray absorption fine structure spectroscopy; x-ray fluorescence microprobe analysis; and microtomography.

Use of GSECARS was supported by the National Science Foundation, the U.S. Department of Energy, the W.M Keck Foundation, the U.S. Department of Agriculture and the State of Illinois. Use of the HP-CAT facility was supported by U.S. Department of Energy's Office of Science, Office of Basic Energy Sciences, and National Nuclear Security Administration; by the National Science Foundation; by the Department of Defense-Tank-Automotive and Armaments Command; and the W. M. Keck Foundation.
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