Hard as Diamond: A New Form of Carbon Created under Ultrahigh Pressure
OCTOBER 26, 2011
An amorphous diamond—one that lacks the crystalline structure that makes diamonds cleavable , but is every bit as hard—has been created by a team of researchers using a High Pressure Collaborative Access Team x-ray beamline at the U.S. Department of Energy Office of Science’s Advanced Photon Source (APS).
But a diamond that doesn't sparkle? Why would anyone want that?
"Sometimes amorphous forms of a material can have advantages over crystalline forms," said Yu Lin, a Stanford graduate student involved in the research.
The biggest drawback with using diamond for purposes other than jewelry is that even though it is the hardest material known, its crystalline structure contains planes of weakness. Those planes are what allow diamond cutters to cleave diamonds—they are actually breaking the gem along weak planes, not cutting it.
"With diamond, the strength depends on the direction a lot. It's not a bad property, necessarily, but it is limiting," said Wendy Mao, the Stanford mineral physicist who led the research. "But if diamond is amorphous, it may have the same strength, the same properties, in all directions."
That uniform super-hardness, combined with its light weight could open up whole new areas of application.
The researchers created the new, super-hard form of carbon using a high-pressure device called a diamond anvil cell. They did a series of experiments with tiny spheres of glassy carbon, an amorphous form of carbon stable at room temperature and pressure, which they compressed between the two diamonds of the anvil. The spheres were just a few tens of nanometers (billionths of a meter) in diameter.
They slowly cranked up the pressure on the spheres. When the pressure exceeded 40 gigapascals (GPa)—400,000 times atmospheric pressure—the arrangement of the bonds between the carbon atoms in the glassy spheres had completely shifted to a form that endowed the spheres with diamond-like strength. The experiments were conducted at room temperature.
The researchers detected the shift in internal bonding by probing the spheres with the x-rays from the HP-CAT 16-ID-D beamline at the Argonne National Laboratory’s APS.
They also did experiments in which the glassy spheres were simultaneously subjected to different pressures from different directions, to further assess the strength of the new form of carbon. While the diamonds in the anvil pressed down on the spheres with a pressure of 130 GPa—about 1.3 million times atmospheric pressure—the pressure on the sides of the spheres was held at 60 GPa.
"The amorphous diamond survived a pressure difference of 70 GPa, or 700,000 atmospheres, which only diamond has been able to do," Mao said. "Nothing else can withstand that sort of stress difference."
Although the bonds between atoms in the glassy spheres were altered by the extreme pressure, the amorphous, or disordered structure of the spheres was unchanged.
"The material doesn't get any more ordered as we compress it. It maintains its disorder," Mao said. The outer form of the original material was also retained. If the researchers started with a sphere, then even at the highest pressures, the sphere was still a sphere. The only change was in the type of bonds between the carbon atoms.
One advantage of the new amorphous diamond is that it is not always hard or always soft like most other materials. The hardness of the amorphous carbon is tunable; it is soft without pressure, but the stronger it was pressed, the harder it got. Once the pressure was released, it returned to its original form as simple glassy carbon, with strength no greater than it had to begin with.
For the amorphous diamond to find widespread application, Mao said, someone will have to find a way to either make the material at ambient conditions (surface pressure and temperature) or figure out how to preserve it once its assumed the super hard form under high pressure.
Even though the amorphous diamond returned to plain old glassy carbon when the pressure was released, there are still potential applications. The material could be used as a gasket in high-pressure devices where having a gasket that hardens with pressure would be beneficial. Or it could be used in further high-pressure experiments.
"We use a diamond anvil cell to compress samples for high-pressure research, but because this amorphous diamond phase hardens with pressure, it could be a second stage anvil inside the diamond anvil," Lin said.
"Having another anvil in sequence would let us create even higher pressures at the very tip."
Since the focus of Mao's research group is answering questions about the extreme environments in the deep Earth and other planetary interiors, a "double diamond" anvil could prove extremely useful. One can only speculate as to what exotic materials they might discover with such an amped-up anvil.