University of Alabama at Birmingham researchers will use pressures greater than those found at the center of the Earth to potentially create as yet unknown new materials. In the natural world, such immense forces deep underground can turn carbon into diamonds, or volcanic ash into slate.
The ability to produce these pressures depends on tiny nanocrystalline-diamond anvils built in a UAB clean room manufacturing facility. Each anvil head is just half the width of an average human hair. The limits of their pressure have not yet been reached as the first 27 prototypes are being tested
“We have achieved 75 percent of the pressure found at the center of the Earth, or 264 gigapascals, using lab-grown nanocrystalline-diamond micro-anvil,” said Yogesh Vohra, Ph.D., a professor and university scholar of physics in the UAB College of Arts and Sciences. “But the goal is one terapascal, which is the pressure close to the center of Saturn. We are one-quarter of the way there.”
One terapascal, a scientific measure of pressure, is equal to 147 million pounds per square inch.
One key to high pressure is to make the point of the anvil, where the pressure is applied, very narrow. This magnifies the pressure applied by a piston above the micro-anvil, much like the difference of being stepped on by a spiked high heel rather than a loafer.
A more difficult task is how to make an anvil that is able to survive this ultra-high pressure. The solution for the Vohra team is to grow a nanocrystalline pillar of diamond — 30 micrometers wide and 15 micrometers tall — on the culet of a gem diamond. The culet is the flat surface at the bottom of a gemstone.
“We didn’t know that we could grow nanocrystalline diamonds on a diamond base,” Vohra said. “This has never been done before.”
Traditional diamond anvils in a diamond anvil cell (DAC) setting have been shown to generate pressures up to 416 GPa. Recent advances have made it possible to generate much higher pressures up to 1065 GPa in DACs by utilizing diamond anvils which have ‘second stage’ pressure generators on the culets of primary diamond anvils. These second stages have been developed by various methods. Researchers have taken the approach of synthesizing nanodiamond microballs from glassy carbon under high-pressures and high-temperatures and employing them as the second stage element. Others have fabricated their second stage by utilizing focused ion beam technology from both single crystal diamond as well as nano-polycrystalline diamond.
Previous work showed the feasibility of growth of a homo-epitaxial diamond micro-anvil on a single crystal substrate and static pressure generation of 86 GPa was achieved. However, for generation of ultra-high pressures, a second stage of nanocrystalline diamond (NCD) is preferred due to its higher yield stress under compression as compared to the bulk crystalline diamond. It is also well established from extensive literature on NCD coatings on metals that NCD thin-film material has higher fracture toughness and improved mechanical properties compared to their micro-crystalline diamond counterparts. They report in this letter on the successful synthesis of a NCD micro-anvil on top of a single crystalline diamond by combining mask-less lithography with microwave plasma CVD and demonstrate the application of CVD grown micro-anvils in ultra-high pressure studies on materials.
In the 264-gigapascal pressure test at Argonne National Laboratory in Lemont, Illinois, the nanocrystalline diamond showed no sign of deformation.
German researchers have already achieved terapascal pressure in a two stage anvil. Static pressure of 1 terapascal is three times higher than the pressure at the center of the Eart
This nubbin on the flat surface of a gem diamond (top) is a nanocrystalline diamond, half the width of an average human hair. Higher magnification (bottom) shows the granular structure of the nanocrystalline diamond.
“The structure did not collapse when we applied pressure,” Vohra said. “Nanocrystalline diamond has better mechanical properties than gem diamonds. The very small-sized grain structure makes it really tough.”
As more micro-anvils are tested and improved, they will be used to study how transition metals, alloys and rare earth metals behave under extreme conditions. Just as graphitic carbon that is subjected to high pressure and temperature can turn into diamond, some materials squeezed by the micro-anvils may gain novel crystal modifications with enhanced physical and mechanical properties — modifications that are retained when the pressure is released. Such new materials have potential applications in the aerospace, biomedical and nuclear industries.
Vohra says his research team wants to generate smaller grain sizes in the nanocrystalline diamond, which may make it even stronger; understand how the nanocrystalline diamond is bonded to the gem diamond; and use ion beams to machine the top of the micro-anvil to a hemispherical shape. That shape will mean an even narrower contact point, thus increasing the pressure.
ABSTRACT – Nanocrystalline diamond micro-anvil grown on single crystal diamond as a generator of ultra-high pressures
By combining mask-less lithography and chemical vapor deposition (CVD) techniques, a novel two-stage diamond anvil has been fabricated. A nanocrystalline diamond (NCD) micro-anvil 30 μm30 μm in diameter was grown at the center of a -oriented, diamond anvil by utilizing microwave plasma CVD method. The NCD micro-anvil has a diamond grain size of 115 nm and micro-focused Raman and X-ray Photoelectron spectroscopy analysis indicate sp3-bonded diamond content of 72%. These CVD grown NCD micro-anvils were tested in an opposed anvil configuration and the transition metals osmium and tungsten were compressed to high pressures of 264 GPa in a diamond anvil cell.
SOURCES- University of Alabama, AIP Advances