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Supercomputing the liquid state of carbon.

Supercomputing the liquid state of carbon

The liquid state of carbon exists under such extreme conditions that its nature has long remained an enigma. Signs of melting on graphite and diamond surfaces blasted by intense laser pulses have spurred interest in this elusive liquid, but researchers have found it difficult to produce and contain liquid carbon long enough for study. Now, sophisticated computer calculations provide a glimpse of the liquid's key characteristics at pressures close to atmospheric pressure.

"It forms quite an interesting liquid," says physicist Richard M. Martin of the University of Illinois at Urbana-Champaign. According to a computer model developed by Martin and colleague Giulia Galli and described in the Aug. 28 PHYSICAL REVIEW LETTERS, carbon can exist in a liquid state at temperatures greater than 4,500 kelvins, even at relatively low pressures. Under those conditions, liquid carbon has the electrical conductivity of a metal. At the same time, each carbon atom, although free to move around, remains closely associated with two, three or four neighbors.

Such theoretical results provide information useful to researchers presently studying the formation of diamond films, which could be used in electronic devices. The findings also supply hints of what could happen to carbon that lies deep within the giant planets Uranus and Neptune or is scattered as dust across interstellar space.

Using a method developed by Roberto Car and Michele Parrinello of the International School for Advanced Studies in Trieste, Italy, Martin and Galli essentially start with Schrodinger's equation, which describes the relationships between the energies and positions of atoms and electrons. Their simulations, done on a supercomputer, represent the most exact calculations yet of a carbon system. "We think we can make the case that these calculations are accurate enough that they're real predictions of what should happen," Martin says.

"The only bad thing about [using this method] is that it takes so much computer time," says Jerry Tersoff of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y. "If it weren't for that, I think everybody would be using it by now."

In their computer model, Martin and Galli track the behavior of a set of 54 carbon atoms, along with 216 of their electrons. These atoms, representing a microscopic sample of the material, start in a particular arrangement, such as the tetrahedral diamond lattice in which each carbon atom bonds with four others. "We give the atoms some random displacements so they're not in a nice structure anymore, which is equivalent to putting in a lot of energy," Martin says. "We find the properties of the liquid by following the atoms and their motions for a sufficiently long time."

Martin and Galli's computer simulations show that at a sufficiently high temperature, the carbon atoms break out of a rigid structure but remain close to one another. The atoms continually switch partners, making and breaking bonds but always keeping two, three or four neighbors at hand. "They never get away from one another completely," Martin says. "They're always shuffling between neighbors." Because it takes a large amount of thermal energy to form and reform the bonds between neighboring atoms, such interactions explain why diamond and graphite are so hard to melt.

Despite intensive study for nearly a century, many of carbon's properties remain poorly understood. "Carbon has so many possibilities for what it can do that we can't claim to have investigated all the possibilities," Martin says. Martin and Galli are now studying how the properties of liquid carbon depend on pressure and temperature, especially at the high pressures encountered deep inside planets.
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Author:Peterson, Ivars
Publication:Science News
Date:Sep 9, 1989
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