Creating the Heart of a Planet in the Heart of a Gem
OCTOBER 27, 2011
Although materials scientists have theorized for years that a form of super-dense aluminum exists under the extreme pressures found inside a planet’s core, no one had ever actually seen it.
Until now, that is.
Using a new table-top laser device at Shizuoka University in Japan that penetrates crystals and sets off micro-explosions inside them, an international team of researchers blasted tiny bits of sapphire creating powerful shock waves that compressed the surrounding material. Under these extreme conditions—terapascals of pressure and temperatures of 100,000 Kelvin—warm dense matter forms, the state of matter between a solid and a plasma.
The researchers, from Stanford University, SLAC National Accelerator Laboratory, The Australian National University, Shizuoka University, Argonne National Laboratory, the Carnegie Institution of Washington, and Swinburne University of Technology then examined the interior of the sapphire, utilizing a beam of x-ray light from the High Pressure Collaborative Access Team (HP-CAT) beamline 16-BM-D at the U.S. Department of Energy Office of Science’s Advanced Photon Source at Argonne National Laboratory, and found a novel form of aluminum.
Because sapphire is a form of aluminum oxide, or alumina, the researchers expected to find evidence of various phases of high-pressure alumina inside the gem. Instead, they observed minuscule amounts of a surprisingly stable, highly-compressed form of elemental aluminum called body-centered cubic aluminum.
“High pressure experiments generally are done either with big equipment or with a diamond anvil cell where two diamonds are squeezing a tiny bit of material, but even diamond gives up at a certain point with those pressures,” said Arturas Vailionis, a researcher with the Geballe Laboratory for Advanced Materials and Stanford Institute for Materials & Energy Sciences and a co-author on the paper. “But this very short pulse of laser light actually ionizes all the material within a very small volume over a short time and creates a plasma which is under enormous pressure and temperature” without fracturing the outer shell of the sapphire.
Of particular interest is that the researchers “demonstrated that high-energy density produced in a simple tabletop experiment makes it possible to form an exotic high-density material phase which could not be produced by other means,” as noted in the nature communications paper.
While scientists long have predicted that new classes of materials with unusual combinations of physical properties should exist under extreme pressure and temperature conditions, only hcp-Al, the hexagonal close-packed aluminum phase, had been observed previously. The form made in this experiment is bcc-Al, or body-centered cubic aluminum.
Vailionis said he doesn’t expect the research to result in production of new materials in large quantities any time soon, but he believes the equipment used in the experiment offers a new strategy for synthesizing nanoscale amounts of new materials in the laboratory, and opens new possibilities for tabletop research into warm dense matter—the state of matter between a solid and a plasma. Such work could bring scientists a step closer to understanding Earth’s early history.
“Now we’re thinking about different materials we could use to recreate some of the environments that existed deep in the Earth’s core when the planet was forming,” he said.