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Researchers develop amorphous diamond

A team of Stanford earth science researchers has pioneered the “amorphous diamond,” a new carbon material as hard as diamond but lacking its crystalline structure.

The group, led by assistant professor of geological and environmental sciences Wendy Mao, is part of the Stanford Extreme Environments Laboratory and collaborated with the Carnegie Institution of Washington to discover this new material.

(CAROLINE SHEN/The Stanford Daily)

The team began by investigating how increased pressure on materials changes their structure, bonding and, ultimately, their properties. After testing other crystalline structures, such as graphite and diamandoids, they decided to examine glassy carbon, an amorphous carbon material.

In order to put the glassy carbon under high pressure, it was placed in a device called a diamond-anvil cell.

“We compressed a sample between the tips of two diamonds, and since pressure is force times unit area, we could apply a pretty modest force between the tips of these small diamonds and get really high pressures,” Mao said. “At really high pressures, you can then change from one phase to another.”

Using techniques called X-ray diffraction and X-ray raman microscopy, the team was able to probe what was going on within the sample at high pressure and confirmed that it had not become crystalline. Instead of taking on the diamond’s long-range structure as expected, the material transformed from an amorphous, sheet-like substance to a super hard, amorphous, diamond-like material.

The next step was to test the hardness of the material by detecting the stress it could withstand and comparing it to the level the diamond could handle.

“The diamond can withstand stress of 100 gigapascals or over,” said Yu Lin, a graduate student in geological and environmental sciences who contributed to the research. “Here, with our final product, it can withstand a stress difference of 70 gigapascals. No other material except diamond can reach this kind of stress difference.”

Because the diamond is only hard in certain directions and has planes of weakness inherent in its structure, it is easily cleavable. According to Mao and Lin, the amorphous diamond may not have the same limitations.

“If something is amorphous, you can imagine that in all directions the hardness might be very similar, so it could have more uniform hardness,” Mao said. “The amorphous diamond has that very attractive potential property.”

Currently, amorphous diamond only maintains its form at high pressure and regresses to glassy carbon when pressure is released. However, the knowledge that the material’s hardness is tuned by pressure change can be incorporated into creating a stronger anvil cell.

With further research to discover methods that preserve the amorphous diamond in ambient conditions or create it at lower pressures, the uniformly hard material could have important applications. Because of the material’s hardness and light weight, for example, it could potentially be used for coding and cutting tools.

“Most of what we do is fundamental research for the Department of Energy which funds our research,” Mao said. “Thinking 20 years down the road, we do ‘discovery science’ to find these interesting properties that people can eventually use.”

According to Mao, the material may eventually generate a multibillion-dollar industry similar to that of the synthetic diamond, which is valued for its use in jewelry and for cutting tools.

“Its a matter of how much people will want to make it,” she said. “We are many steps behind the diamond, but the idea is that once you find a material with a special property, the industry may develop around it.”

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