Diamonds are known to be among the stiffest materials known to man. A new study finds a worthy competitor to the naturally-occurring cubic diamonds - in the form of lab-made hexagonal diamonds.
Hexagonal diamonds are named as such in reference to their crystal structure. Some samples were found at meteor impact sites, others were grown in laboratories, but all of them are too small or too short-lived to be precisely measured until now.
Researchers from the Institute for Shock Physics at the Washington State University have created hexagonal diamonds that are big enough to be measured in terms of stiffness using sound waves. They discovered that these hexagonal diamonds are even stiffer compared to naturally-occurring cubic diamonds, reporting their findings in the journal Physical Review B in the article "Elastic moduli of hexagonal diamond and cubic diamond formed under shock compression."
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The Search for Tougher, Stiffer Materials
"Diamond is a very unique material," said Yogendra Gupta, the corresponding author on the study and director of the Institute for Shock Physics, in a WSU press release. He notes the reason for its value aside from jewelry: its strength, optical properties, and high thermal conductivity.
"Now we have made the hexagonal form of diamond, produced under shock compression experiments, that is significantly stiffer and stronger than regular gem diamonds," Gupta announced.
The search for materials stronger than diamond has remained one of the longstanding materials science and engineering goals. These materials pave the way to the next generation of industrial and scientific applications. While the strength of hexagonal diamonds being previously theorized to be greater than cubic diamonds, the study from the Institute for Shock Physics provides the first experimental evidence supporting the theory.
Creating Hexagonal Diamonds in the Lab
According to the WSU press release, lead author Travis Volz focused his dissertation study on WSU on creating hexagonal diamonds from graphite. Volz is now a postdoc researcher with the Lawrence Livermore National Laboratory in California.
Volz and Gupta used compressed gas and gunpowder to send graphite disks - about the size of dimes - flying at speeds of 15,000 miles per hour toward a transparent material. The impact from these two materials created shockwaves that rapidly transformed the dime-sized graphites into hexagonal diamonds.
Shortly after the impact, researchers aimed a small soundwave at the material and used lasers to track its movement through the newly-formed hexagonal diamonds. As sound waves tend to travel faster through stiffer materials, researchers found that they moved faster in the hexagonal diamonds than the cubic diamonds. This process takes place in the range of only several billionths of a second - in the nanosecond scale - with the researchers taking measurements before the high-velocity impact shattered the diamonds shortly after.
The WSU release explains that stiffness refers to a material's ability to resist deformation from external forces applied on it, with Volz noting that generally, stiffer materials are harder materials. While the short-lived hexagonal diamonds can't be tested for hardness since they can't scratch it, the available data allows them to make inferences on its other properties - hardness included.
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