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RESEARCHERS WRITE RUDIMENTS OF DIAMOND RECIPE
STANFORD - Diamond making is still a bit like baking a cake without a recipe or building a house with bricks that you can't always lay straight. If you're unlucky, you get something that nobody wants for the price - in this case, graphite, the stuff of pencil lead.
"Everybody can make diamond, but it's a slow process," says Stanford University mechanical engineering Professor Mark Cappelli. "Diamond film won't be cost effective until we figure out how to make it faster and more efficiently."
There are hundreds of potential applications for thin diamond films, but those produced with current technology cost $10 to $100 a karat, Cappelli said.
"The economists tell us we have to improve that to $1 to $10 if diamond is to be used to coat tools and blades or be used as an electronics component or heat spreader," he said.
"We're making progress, but we still need technical breakthroughs," Cappelli said. "I think it will be at least five years before we see diamond coatings on commonplace tools."
Cappelli's research team in the High-Temperature Gas Dynamics Laboratory has made progress recently toward writing a chemical formula - a recipe, so to speak - for cooking high-quality diamond film or coatings out of the purified natural gas methane.
He and graduate student Michael Loh recently found they could have "too much of a good thing" - their raw material, methane - for efficient, uniform diamond growth.
"If you have to bake a cake and you don't measure the ingredients, you often don't quite get a cake," Cappelli said. "The situation with diamond is particularly sensitive to getting things just right, which tells you there's some very interesting chemistry."
There are three distinct chemical processes involved in diamond making, he said. First, a gas mixture must be activated to produce the mysterious molecules that are a precursor to diamond. Those molecules then must be transported to the surface where the diamond is to be grown, and finally, they must attach to and share their electrons in a diamond arrangement with that surface.
"We go through a painstaking process of sampling the gas before it hits the surface, and of looking at it with lasers and other optical methods to try to get a sense for what the chemistry is very close to where the diamond is growing," Cappelli said. "This has allowed us to try to speculate on the proper formula."
Previous speculation has been that two different types of precursor molecules were responsible for producing graphite and diamond. The Stanford team's measurements indicate one type of precursor molecule may be able to produce both, Cappelli said.
"It looks like if you have too much of it, you start to contaminate the diamond growing surface and grow something other than diamond," he said. The team has found it has the best results with low ratios of methane to hydrogen in its high-temperature gas plasma mixture, even though it is the methane that provides the carbon from which diamond is built.
"More important, however, we've also found that there is pretty much a family of molecules that can be used to grow diamond, and we're just starting now to unravel some of the chemistry that differentiates those that are favorable from those that aren't."
One of six research teams studying diamond growth at Stanford, Cappelli's group focuses primarily on the transport and surface chemistry. Like most diamond growers, they start with a carbon-based gas and try to get it to "rain out" as diamond rather than as the soot normally seen when fuel is burned in autos or fireplaces.
They begin by mixing methane and hydrogen, or acetylene and hydrogen, into a high-temperature plasma injected into a chamber. In their case, the chamber is a supersonic satellite engine.
"Just around the corner in another one of my laboratories was a program to build arc jet engines that will be used on communication satellites. It's a low-thrust hydrogen engine that keeps the satellite pointed right. We decided to try it because of its clean environment," Cappelli said.
The first step - activating the plasma with an electrical discharge - breaks the hydrogen atoms free from methane, which is a carbon atom with four hydrogen atoms attached. Once free of some hydrogen, the carbon atom goes looking for electrons to share with other atoms because it has unused spaces in the second shell of its electron cloud.
"If it bonds to the electrons of four other carbon atoms, it is, in fact, diamond," Cappelli said. "But it may also bond to three neighboring carbons, which produces graphite."
Activating the gas also takes some of the hydrogen atoms off the seed crystal's surface, where they have temporarily shared an electron with the carbon atoms.
"That leaves some spaces on the surface for the 'bricks' to come down and attach," forming diamond, Cappelli said.
"All we have to do is build the diamond like the wall of a house, where you just keep laying on bricks," Cappelli said, "but the trick is getting them to lay down right."
Once the molecules are positioned correctly on the surface, still other chemical reactions are needed to "lock [them] into place," he said. "It's almost like you need to prepare the brick wall with mortar, add the brick correctly and then clean it up."
People still characterize these tricks as alchemy, he said, because "we adjust the chemistry, not exactly knowing what we're doing, finding out if we did the right thing, then going back to think about what it was we did."
Many researchers believe that the so-called "brick" precursor to building diamond is an unstable methyl radical - a carbon atom attached to three hydrogen atoms - not just carbon alone, Cappelli said.
"Our results also support this hypothesis. However, we have evidence to believe that more complicated molecules such as acetylene - two carbon atoms bonded to two hydrogen atoms - also participate as precursor molecules," he said.
"This suggests that there may be some very interesting and not-so-well-understood surface chemistry. It is often thought that acetylene is too stable a molecule to participate."
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