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Scientists Seem Nearer Their Dream for Hydrogen: Making It Metal
By Malcolm W. Browne
The New York Times
March 16, 1990
ANAHEIM, Calif., March 15 — Since 1935, scientists have dreamed of converting hydrogen into a metal with wonderful properties. Now, physicists have reported here that they are a little closer to that elusive goal.
Hydrogen, the simplest of all elements, is a light gas that forms a explosive mixture with air under ordinary conditions. When hydrogen is cooled to near absolute zero it is a liquid, and at still lower temperatures it freezes solid.
But theorists studying hydrogen's atomic structure predicted 55 years ago that under enormous pressure, hydrogen should become a metal. They believe that metallic hydrogen could remain metallic even after the pressure is released and that it should be a room-temperature superconductor.
Unearthly Pressured Required
Ordinary superconductors must be cooled to near absolute zero, but in the past few years several compounds have been discovered that conduct electricity without resistance at somewhat higher temperatures. The possibility of creating a superconductor that would work at room temperature seems remote, unless scientists should finally succeed in making metallic hydrogen.
The pressures needed to do this are literally unearthly; metallic hydrogen may exist in nature only in such ultrahigh-pressure regions as the mantle of Jupiter.
To achieve such pressures, physicists at the Carnegie Institution of Washington in the late 1970's pioneered development of the ''diamond anvil,'' a hydraulic press that squeezes the faces of two gem-quality diamonds together.
Trapped by a metal gasket between the two diamonds, tiny samples of hydrogen or other gases are squeezed to pressures that recently reached a record 2.5 million times the pressure of the atmosphere.
The Beginning of Hope
Scientists at a meeting of the American Physical Society this week heard reports that hydrogen at this pressure begins to show some metallic features.
Dr. Ho-Kwang Mao and Dr. Russell J. Hemley of the Carnegie Institution of Washington said that in recent experiments, as pressure increased to 2.5 million times atmospheric pressure, from 1.5 million, hydrogen remained transparent to visible light but began to reflect infrared radiation, as metals do.
Dr. Isaac F. Silvera said he had obtained similar results at Harvard University in the past several months. He said these experiments produced a crystalline form of hydrogen (called Hydrogen A) that appears to be a kind of metal consisting of a crystal lattice of hydrogen molecules. Higher pressures would be needed to convert this into a lattice of hydrogen atoms rather than molecules, a substance that would more truly resemble a conventional metal.
In the course of his high-pressure experiments, Dr. Silvera said he had compressed heavy hydrogen (deuterium) and the metal palladium to a pressure of 100,000 times atmospheric pressure, thereby achieving the highest density of deuterium ever absorbed by palladium. Despite this, he saw no evidence of ''cold fusion,'' a phenomenon two chemists at the University of Utah said they achieved last year.
The two chemists said that an electrical current passed to a palladium electrode immersed in heavy water (deuterium oxide) caused the fusion of hydrogen atoms and the release of enormous energy, a finding that has not been verified by other investigators. The Utah group contended that fusion had occurred because of a very high density of deuterium packed together within the crystal structure of the palladium.
But the Harvard experiment, which achieved a greatly higher density, produced no fusion.
Possible Success Reported
In a separate report at this week's meeting, Dr. Arthur L. Ruoff of Cornell University said his research group had probably transformed oxygen to a metallic state by compressing it in a diamond anvil.
''The reflection of infrared radiation by our sample increased substantially, and we are about 60 percent confident that we produced metallic oxygen at a pressure of 1.32 million times atmospheric pressure,'' he said. In earlier work, the Cornell group compressed the elemental gas xenon and was even more confident that it had created a metallic form.
Because the samples squeezed between the faces of diamonds are vanishingly small, efforts to measure their electrical conductivity, a key test of metallization, have failed, the scientists said.
Moreover, as pressures are increased experiments become much harder. Unless the diamonds used as presses are structurally flawless and are perfectly aligned, they are easily destroyed by the huge pressure. The metal gaskets give way frequently. As pressures increase, the diamonds begin to fluoresce, obscuring the observers' views into the cell interior.
''It's quite possible that hydrogen becomes a superconductor only at pressures around four million times that of the atmosphere,'' Dr. Silvera said. ''At that pressure diamonds may not hold together, or may become opaque. That would certainly limit their usefulness in experiments at higher pressures.''
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