August 16, 2012

Superhard Diamond-Denting Material Created

A superhard mixture of crushed carbon spheres and a hydrocarbon solvent is the world’s first hybrid crystalline/amorphous material. Its creation by an international scientific team that included Wendy Mao, a Stanford University professor and SLAC researcher.

The new material is one of a class that is hard enough to dent diamond, the hardest known material. The team created it by squeezing a mixture of soccer-ball-shaped carbon-60 molecules (popularly known as “buckyballs”) and a xylene solvent to extremely high pressures – up to 600,000 times atmospheric pressure – in a device called a diamond anvil cell. The cell holds a tiny amount of material that is pressed between the flattened tips of two opposing diamonds. Scientists can shine lasers or X-rays through the transparent diamonds to observe and identify any atomic-scale changes caused by the rising pressure.

These new materials were created at Argonne National Laboratory's Advanced Photon Source by a team mostly associated with the Geophysical Laboratory of the Carnegie Institution of Washington.

Carnegie Institution of Washington - The team discovered that there is a narrow window of pressure, about 320,000 times the normal atmosphere, under which this new structured carbon is created and does not bounce back to the cage structure when pressure is removed. This is crucial for finding practical applications for the new material going forward.

The structure of carbon-60 solvate made with C-60 cages and m-xylene solvent.

Science - Long-Range Ordered Carbon Clusters: A Crystalline Material with Amorphous Building Blocks

Unlike earlier pressure-created carbon materials that Mao has created, this one retained its superhard compressed structure after the pressure was released, so it may ultimately find some industrial application, such as a wear-resistant protective coating.

“Although we haven’t yet tested its properties directly, we know it’s superhard because it dented the diamond anvil face,” Mao said. “Our next step is to test this new material’s properties. If they prove desirable, then we’d want to devise an economical way of making it. The diamond anvil cell is a great tool for discovery, but not for high-volume manufacturing.”

Most intriguing scientifically is that this material retained the long-range, regular molecular pattern that is characteristic of crystals even after the intense pressure crushed its major constituents – the buckyballs – into jumbled, amorphous blobs of carbon. The scientists determined that the solvent molecules played a crucial role in preserving the material’s crystallinity.

“Hybridization of crystalline and amorphous structures at an atomic level hasn’t been experimentally observed, although scientists believed such structures could be created,” said the paper’s first author, Carnegie scientist Lin Wang. “The finding in this paper should be the first of its kind.”

ABSTRACT - Solid-state materials can be categorized by their structures into crystalline (having periodic translation symmetry), amorphous (no periodic and orientational symmetry), and quasi-crystalline (having orientational but not periodic translation symmetry) phases. Hybridization of crystalline and amorphous structures at the atomic level has not been experimentally observed. We report the discovery of a long-range ordered material constructed from units of amorphous carbon clusters that was synthesized by compressing solvated fullerenes. Using x-ray diffraction, Raman spectroscopy, and quantum molecular dynamics simulation, we observed that, although carbon-60 cages were crushed and became amorphous, the solvent molecules remained intact, playing a crucial role in maintaining the long-range periodicity. Once formed, the high-pressure phase is quenchable back to ambient conditions and is ultra-incompressible, with the ability to indent diamond.

Close up of dented diamond

18 pages of supporting material

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