New material breakthrough: Super-hard graphite cracks diamond

It is hard to imagine that graphite, the soft “lead” of pencils, can be transformed into a form that competes in strength with its molecular cousin diamond. Using a diamond anvil to produce extreme pressures and the ultra-brilliant X-ray beams at the Advanced Photon Source in Illinois, scientists with the High-Pressure Collaborative Access Team (HPCAT)* have surmounted experimental obstacles to probe the changes that graphite undergoes to produce this unique, super-hard substance. The study is reported in the October 17, issue of Science.

“Researchers have speculated for years on the extreme conditions that might change the molecular structure of graphite into a super-hard form that rivals diamond,” said Wendy Mao, the study’s lead author from the Carnegie Institution’s Geophysical Laboratory in Washington, D.C., and the University of Chicago. “This experiment is the first to determine quantitatively how the bonding in graphite changes under high-pressure conditions. Conventional methods limited our observations to surface studies of the material,” she stated. “Now, with the super high-intensity X-rays of the Argonne facility and with our team’s technology to focus the entire beam to a small spot, we’ve been able to look at the material in the diamond-anvil cell while under high pressure. We’ve overcome the obstacles of the past,” she concluded.

Graphite and diamond are both made of carbon. The geometric arrangement and spacing of the carbon atoms is what makes the materials differ in appearance and strength. The atoms in graphite are arranged in layers that are widely spaced. The atoms in diamond, on the other hand, are tightly linked producing a strongly bonded structure. The HPCAT scientists subjected graphite to pressures that are equivalent to 170,000 times the pressure at sea level ( 17 gigapascals). “We were able to see how the structure changed at the atomic level when the graphite was squeezed into the super-hard form,” remarked co-author Dave Mao of Carnegie’s Geophysical Laboratory. “The graphite that resulted from our experiment was so hard that when we released the pressure we saw that it had actually cracked the diamond anvil.”

The super-hard from of graphite opens the door to a myriad of applications in industry particularly as a structural component.

* HPCAT is made up of researchers from the Carnegie Institution’s Geophysical Laboratory, the High-Pressure Physics Group of the Lawrence Livermore National Laboratory, the High Pressure Science and Engineering Center of the University of Nevada, Las Vegas, and the University of Hawaii Institute of Geophysics and Planetology. Use of the HPCAT facility at Argonne National Laboratory for this work was funded by the Department of Energy, the National Nuclear Security Administration, the National Science Foundation, the Department of Defense, the W.M. Keck Foundation, and the Carnegie Institution of Washington.

The Carnegie Institution of Washington (www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments in the U.S.: Plant Biology, Global Ecology, The Observatories, Embryology, the Department of Terrestrial Magnetism, and the Geophysical Laboratory.