New nanodiamonds open up materials studies at terapascal pressures
By creating transparent, microscopic beads of nanocrystalline diamond, researchers have devised a technique to generate pressures of 1 TPa, far higher than any known method. The advance could pave the way for the experimental study of materials at ultrahigh pressures, which until now has only been done theoretically.
Scientists are interested in studying materials at extreme pressures because they behave differently under those conditions than they do at normal atmospheric pressure. Compression under pressures of hundreds of Gigapascals can, for example, spur some insulators to become metallic and some metals to superconduct. Probing materials at terapascal pressures could allow scientists to deepen their fundamental knowledge of materials and synthesize materials with novel properties. It could also help them to better understand the high-pressure interiors of planets.
Until now, terapascal pressures were produced by the use of shock waves or large lasers. These techniques not only require expensive facilities, they also produce the high pressures for very short times and then destroy the samples.
Materials physicist Natalia Dubrovinskaia at the University of Beyreuth in Germany and her colleagues were able to generate static pressures in the range of 0.9–1.2 TPa using a device called a double-stage diamond anvil cell (DAC). Diamond anvil cells have two single-crystal diamonds that squeeze material samples like a vice. In a double-stage DAC, 20–50 µm wide translucent beads of nanocrystalline diamond—diamonds composed of many crystal grains less than 50 nm in size—sit at the tip of each single-crystal diamond. As the larger diamonds are pushed together, the pressure transfers to the nanodiamonds, which compress and get harder, allowing them to withstand and generate super-high pressures.
Other researchers have in the past used double-stage DACs to generate static pressures of 600 GPa. Dubrovinskaia and her colleagues achieved pressures almost twice that by making a new type of nanocrystalline diamond microball with a unique microstructure and optical properties. The 10–20-µm wide microballs are transparent and have crystal grains only 3–9 nm in size linked together by a single layer of graphene-like carbon, which makes the material exceptionally strong, explains Dubrovinskaia.
The researchers made the balls by subjecting beads of glassy carbon—an advanced material that combines glassy and ceramic properties and is used in electrodes—to 18 GPa pressure and a temperature of 2000°C. A phase transformation converts the glassy carbon into nanocrystalline diamond.
The researchers fitted the DAC with an internal gasket that acts as a pressure chamber inside the cell. This allows them to study gases and liquids in addition to solids that are subjected to high pressures. They measure the high pressures in the double-stage DAC by measuring the change in x-ray diffraction spectrum from a tiny rhenium foil piece placed between the diamond tip and the nanodiamond microball.
The researchers now plan to rig the device to generate both ultrahigh pressure and temperature simultaneously, says Leonard Dubrovinsky, a co-author of the study published in the journal Science Advances. They should be able to do that by shining a high power pulsed laser through the diamond and the nanodiamonds to heat the sample when it is already under high pressure, he says.
Malcolm McMahon, a professor of high-pressure physics at the University of Edinburgh, UK who was not involved in the work says that the advance opens up the study of materials at high pressures. “Our community has talked for many years about core-core hybridization where the core electrons in the atoms and not just the valence electrons participate in chemistry and bonding,” he says. Such studies are limited because of the laser facilities they require. “A diamond anvil cell is a device that anyone can purchase for $5000. Once you have the nanodiamond anvils, anyone with a steady hand or good micromanipulator can do terapascal science.”