The Big Squeeze The Carnegie Institution for Science can observe how organic substances, like oxalic acid dihydrate, right, react under pressure, born of a mission to study Earths interior. Diamond anvils, top left, were used on ferrous iron oxide, xenon and oxygen, forming different structure WASHINGTON — In a recurring comic bit, David Letterman used to place household items — a plate of jelly doughnuts, a six-pack of beer — in an 80-ton hydraulic press, gleefully watching as the items squirted, exploded and disintegrated. That was but a light touch compared with the pressures Russell J. Hemley and his colleagues exert on molecules at the Carnegie Institution for Science here. When substances are pressed between two diamonds, they achieve a sort of alchemy. No, iron does not change to gold, but familiar atoms and molecules behave differently. Oxygen turns blue, then scarlet, and finally into a shiny metal. Peanut butter, as an early pioneer in the field demonstrated at General Electric in the 1950s, turns to diamond. So do roofing tar and wood. These are not just stupid science tricks. At the Carnegie Institution’s Geophysical Laboratory, the interest in high-pressure science grew out of the laboratory’s mission to study Earth’s interior. The research over the decades has broadened. Scientists use the high-pressure transformations to explore permutations of matter that do not exist in most of the universe, casting insight on what is going on near Earth’s core or within Jupiter. They also hope the experiments will lead to new materials that more efficiently capture sunlight in electricity-producing solar cells or serve as fuel tanks for hydrogen-powered cars. “It’s a new kind of chemistry,” Dr. Hemley said. It certainly gives new meaning to the term “high pressure.” At sea level, the air pressure is 14.7 pounds per square inch, or 1.03 kilograms per square centimeter. At the bottom of the Mariana Trench in the western Pacific, the deepest slice of the ocean, seven miles of water impose a pressure of about eight tons per square inch. With the diamond anvils at the Carnegie Institution, the pressure reaches 50 million pounds per square inch. In Germany, researchers have devised versions — essentially an anvil within an anvil — that can more than double that. Even so, certain places in the universe are far more crushing. The pressure at the center of Jupiter is more than a billion pounds per square inch. And then there are neutron stars, the collapsed remnants of burned-out suns, where gravity pulls atoms so closely together that the pressures are thought to reach a billion trillion times that of Jupiter’s core. In appearance, the anvils used at Carnegie and other laboratories around the world are rather unremarkable. Designs vary, but the housing is often a pancake-shaped metal cylinder about two inches wide and less than an inch high. To exert pressure, the scientists sometimes turn the screws on the top of the cylinder, pulling the top and bottom plates closer together. Inside, the bending of the cylinder plates presses together tips of two small diamonds, each a quarter to half a carat, typically no bigger than a quarter of an inch. On one diamond tip, a notch shaped like the caldera of a volcano has been carved to trap the material that is to be squeezed. The other tip presses down, like a stiletto heel crushing a bug. (For gases like oxygen or hydrogen, the apparatus is assembled in a box containing the gas, and some of it is trapped between the diamonds.) The screws apply only a few pounds of force. But those translate into tremendous pressure, because the diamond tips are so small. “Pressure is just force divided by area,” Dr. Hemley said. It is as if 100 elephants were pushing down on the point of a pencil, if one could find a pencil capable of holding all those elephants. If the diamonds have even a slight defect, they shatter — sometimes with a soft click, sometimes with a bang like a shotgun. One of the Carnegie scientists, Timothy A. Strobel, has been using these techniques to create a new form of silicon that could more easily turn sunlight into electricity. The usual form of silicon cannot directly absorb the photons of sunlight. “The atoms in the lattice need to shake a little bit to sort of kick the electron in the right position,” Dr. Strobel said. By squeezing a mixture of silicon and sodium, he has created a new tubelike structure. After chemically extracting the sodium, the tubes of silicon possess the desired electronic property of absorbing photons without the shaking. Dr. Strobel said the researchers were now exploring how to create the material without the high pressures so it could be used commercially in more efficient photovoltaic cells. “We think we’ve come close, maybe, to solving this problem,” he said. Created under pressure, the silicon structure is metastable. That is, it does not snap back into its original form when the pressure is removed. (Diamonds are the best known example of a metastable material.) “We’re not playing with the same set of materials everybody else is playing with,” Dr. Strobel said. One of the surprises is the changes that atoms undergo under pressure. At modest pressures, atoms stack neatly, like cannonballs, and scientists expected that the atoms would remain in this efficiently packed pattern as they were squeezed closer together. But while the distance between atoms does indeed narrow, they no longer remain in neat piles. Sodium, for example, shifts into wildly more complex arrangements. Nitrogen, which normally floats around in dumbbell-shaped pairs, ends up in a twisted lattice configuration. “That’s very counterintuitive,” Dr. Hemley said. As the atoms converge, the electrons are squirted into different locations, reconfiguring the molecules they are part of. The process turns some metals, which readily conduct electricity, into insulators, which do not. Some insulators turn into superconductors, ferrying current without resistance. “You have a new and different periodic table, in a sense,” Dr. Hemley said, “which is why it’s so fun and interesting.” Even noble gases like xenon, which rarely interact with other atoms, intertwine with hydrogen to form novel, complex structures. Malcolm McMahon, a physicist at the University of Edinburgh in Scotland, was curious about the red oxygen. Within an anvil, he and his collaborators managed to create a single ruby crystal of oxygen, off which they bounced X-rays to determine the structure. The oxygen atoms, usually bonded in pairs, had been pushed together into clusters of eight, and that structure absorbed the shorter blue wavelengths of light. The remaining wavelengths — red — passed through. At still higher pressures, oxygen turns into a metal. “It looks like steel,” Dr. McMahon said. “It’s indistinguishable from any other shiny metal.” Carbon is another element of particular interest. The Carnegie Geophysical Laboratory has a leading role in the Deep Carbon Observatory, a 10-year effort to better understand what happens to carbon in the high-pressure, high-temperature conditions within the planet. The unknowns include just how much carbon exists in Earth, Dr. Hemley said. The pressure could also alter chemical reactions important for life. “This could be relevant to questions associated with life, of the formation of life in extreme environments,” he said. Perhaps the biggest puzzle in high-pressure physics involves the simplest and most abundant atom — hydrogen. At the extreme pressures at the center of Jupiter, hydrogen is believed to turn into a fluid metallic state, with the churning flows generating the planet’s powerful magnetic fields. In the laboratory, that has been elusive. Theoretical calculations had predicted that the transition to metal would occur at pressures achievable in the anvils. Dr. Hemley’s and other groups found signs of the transformation at about 45 million pounds per square inch, but the evidence was not definitive. More recently, Dr. Hemley and his collaborators said it appeared that hydrogen had not followed the original predictions that it would turn into a simple metal, but had instead lined up into flat sheets in the hexagonal pattern of chicken wire and honeycomb, just as carbon atoms do in graphene. “It’s very different from the picture that people had for decades, really, that it would become a good metal,” said Ronald E. Cohen, a theorist at the Carnegie laboratory. “It’s probably not like that at all.” Dr. Cohen said this graphene-like state contained few conducting electrons, and unlike most metals, might be transparent instead of reflective. “It may be hard to detect unless you’re looking for poor metallic behavior,” he said. Dr. Hemley said that actually made hydrogen more interesting. “You have something that looks like a semiconductor, semi-metal,” he said. “There’s chemistry driving the system into classes of structures that were not expected.” NASA’s Juno spacecraft, which is on the way to Jupiter, is designed to take measurements that will reveal aspects of the planet’s hot, dense interior. The laboratory experiments could help interpret the Juno results and vice versa. One question is whether Jupiter has a rocky core. A core must have formed in order to generate the gravity that pulled in the hydrogen, but the highly reactive hydrogen could subsequently have dissolved it. That in turn opens the possibility of even more exotic chemical reactions between the hydrogen and the elements from the dissolved rocky core. Given the abundance of gas giant planets that have been discovered around nearby stars, this form of hydrogen may be a common one in the universe, even though it is hard to recreate on Earth. “Ultimately, we want a complete understanding of hydrogen in all conditions,”
Posted on: Mon, 11 Aug 2014 08:35:04 +0000
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