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A new instrument will reveal how materials are transformed under extreme pressure. By squeezing and transforming matter in specially designed pressure cells and using neutron beams to tease out details of the altered structure, researchers hope to gain an understanding of how Earth's minerals behave when subjected to high pressures similar to those near Earth's core. The technique might also help researchers learn why some microbes survive under high pressure, and how to synthesize hydrogen-rich clathrates like those found on the ocean floor, new materials found in meteorites, and improved, artificial, single-crystal diamonds.
Geoscientists and other physical scientists seeking to uncover the effects of extreme pressures and temperatures on matter will soon add to their tool box a unique research facility being constructed at the Spallation Neutron Source. The new tool, known as the Spallation Neutrons and Pressure (SNAP) diffractometer, represents the next generation of neutron instrument because of a design that uses modern ultrahigh-pressure device technology. Chris Tulk, a researcher in the SNS Experimental Facilities Division, is guiding the construction of SNAP, which will be available to users in 2008. "In the SNAP instrument, we hope to raise the pressure bar to 100 gigapascals, or a million times atmospheric pressure, which is roughly five times that currently available at neutron sources," Tulk says. "That's close to the pressure of Earth's core mantle boundary, which is approximately 125 gigapascals. With SNAP, we hope to provide useful experimental data for computer models that simulate the pressure and temperature behavior of magnetic materials under 'core-like' conditions." At the heart of the SNAP instrument is the pressure cell, a small hydraulic press inside of which are cone-shaped anvils made of diamond, sapphire, or moissanite gemstones or tungsten carbide. The sample is held in a gasket and compressed as the anvils are forced together under loads of up to several hundred tons. According to Tulk, reducing the sample size generally yields higher obtainable pressures. Maximizing the neutron flux at the sample while minimizing neutron scatter from the pressure equipment presents several significant design challenges for the instrument team. A typical sample size at existing neutron sources is approximately a centimeter, or a half-inch, in diameter. "Our goal is a sample less than 100 microns, or several tenths of millimeters," Tulk says. "Our primary challenge is to focus the neutron beam to make it smaller and more 'needle-like' than previously achieved, while simultaneously locating the sample inside the cell and carrying out neutron diffraction experiments." At the NRU reactor at Chalk River Laboratories in Canada, Tulk and his colleagues recently "microfocused" a beam using prototype Kirkpatrick-Baez (K-B) neutron super-mirrors, designed specifically for the task by ORNL's Gene Ice. Originally designed to focus X rays, K-B mirrors are very compact. Prototype neutron mirrors are 10 times longer than X-ray mirrors and have a different reflective coating. Tulk and his associates microfocused a neutron beam down to 100 microns on single crystals of several minerals. "To ensure the beam was focused on our 100-micron sample, we developed a process that included very precise sample movement stages and comparisons of diffraction intensities collected at each step," Tulk says. "We are now trying to refine this process for use with pressure cells." The researchers had to be sure that the neutron beam was not striking the pressure cell's anvils, producing an overwhelming background. The scientists will then remotely control the anvils, making them squeeze the sample at pressures as high as 20 gigapascals, or 200,000 times atmospheric pressure. Tulk recently used high-pressure diffraction to study structural changes in glasses under pressure. He structurally characterized the octahedral form of oxide glasses structurally similar to those present in Earth and found that under high pressure, the central atoms in glass are surrounded by six oxygen atoms instead of the usual four. ORNL, Stony Brook University, and the Geophysical Laboratory of the Carnegie Institution in Washington, D.C., proposed to the Department of Energy an experiment in which SNAP would be used to produce and characterize solid hydrogen in pressure cells. Armed with new tools and new data, scientists believe the research could lead to new methods for storing and releasing hydrogen for fuel cells.
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