A Livermore team has determined the solid–liquid and solid–solid phase boundaries of carbon for pressures up to 20 million atmospheres and more than 10,000 kelvins. “Results of computer simulations show a consistent description of elemental carbon in a broad range of temperatures and pressures,” says Alfredo Correa, a University of California (UC) at Berkeley student who works in Livermore’s Physical and Life Sciences Directorate under the Student Employee Graduate Research Fellowship Program. The physical properties of carbon are of great importance for devising models of Neptune, Uranus, white dwarf stars, and extrasolar planets that are carbon-rich.
In its elemental form, carbon is found in materials such as coal, graphite, diamond, bucky balls, and nanotubes. These materials have very different properties, but, at the microscopic level, they differ only in their carbon atoms’ geometric arrangements. Experimental data on the phase boundaries and melting properties of elemental carbon are scarce because of difficulties in reaching megabar (one million atmospheres) pressures and temperature regimes of thousands of kelvins in the laboratory. “Our simulation results call for a partial revision of current planetary models, especially for the description of their core regions,” Correa said. “Our computational work also may help us interpret future experimental work.” Correa is the lead author of a report published in the January 31, 2006, online edition of the Proceedings of the National Academy of Sciences. The research team is composed of Correa, Stanimir Bonev, and Giulia Galli, all of whom were at Livermore at the time the work began. Galli is now a professor at UC Davis, and Bonev is an assistant professor at Dalhousie University in Canada.