Synthetic superdiamonds beat nature's best.

Synthetic superdiamonds beat nature's best

When asked how much money is enough, Nelson Rockefeller reportedly replied: "A little bit more." Scientists feel much the same about the unparalleled properties of diamond.

Last week, a research team announced it had made synthetic diamonds that conduct heat 50 percent more efficiently and can withstand 10 times more laser energy than the best natural diamonds, the previous world champions at these material skills.

"This is a whole new material that hopefully will enable technologies that we haven't even thought of yet," says William F. Banholzer, a chemical engineer who heads the diamond-making team at the General Electric Research and Development Center in Schenectady, N.Y. The group harvested its first carat-sized superdiamonds in 1988, and has since measured some of the gems' properties with scientists at Wayne State University in Detroit. They describe their findings in the July 15 PHYSICAL REVIEW B -- CONDENSED MATTER.

The synthetic diamond's heat-dissipating power makes it attractive for heat sinks that keep electronic components from overheating on chips -- a critical safeguard in hard-to-reach places such as satellites. In addition, the ability to withstand more radiation than any other transparent material could make the superdiamond ideal for mirrors and other components crucial to laser weaponry or laser-based machining of, say, tough superalloys.

The key to improving on nature's own gems emerged nearly 50 years ago when a Soviet physicist argued that material properties such as thermal conductivity depend on a crystal's isotopic composition. Elements come in several isotopes, or chemically identical forms having the same number of protons but different numbers of neutrons. Natural diamond--the coveted all-carbon crystal--contains about one carbon-13 atom for every 110 carbon-12 atoms.

Theoreticians had calculated that diamond would conduct heat better if it were made entirely of one carbon isotope. The different mass of the minority isotope dampens the heat-carrying "vibrations," or phonons, that course through a crystal lattice. An isotope-independent process in which phonons scatter off of each other also theoretically degrades heat transmission.

The GE researchers followed up on these tantalizing ideas by combining one old and one newer diamond-making technique to produce carat-sized diamonds with unnaturally low amounts of carbon-13. The gems' unexpectedly enhanced abilities to transmit heat and withstand more radiation than the best natural diamond reveal gaps in the earlier theories, Banholzer notes. To account for the record-breaking properties, other GE scientists are developing a theory in which both types of phonon scattering depend on isotopic composition.

To make the gems, the researchers first use a low-pressure technique, called chemical vapor deposition (CVD), that rearranges the carbon atoms of methane gas molecules into diamond films. By using isotopically purified methane, the scientists produce enough starting material for the second process, marked by temperatures over 2,500 [degrees]F and pressures nearing 1 million pounds per square inch. In the presence of a metal catalyst, such as nickel, and a diamond seed crystal, the CVD material dissolves into the catalyst and recrystallizes into gem-quality diamonds as big as pencil erasers. "By making isotopically pure diamond, something we never could do before, we have a product that is better than you can take out of the ground," says Banholzer.

"It's absolutely fascinating," comments Michael Pinneo, chief scientist at Crystallume, a diamond-film manufacturer in Menlo Park, Calif. Though the diamonds may help reveal how energy moves within crystals, their cost could keep them from finding a big market, he and others say. GE nonetheless predicts a multimillion-dollar market.