Using Diamonds, Researchers Report Converting Hydrogen Into Metal

Fulfilling a decades-long dream of theoretical physicists, researchers will report today that they have converted hydrogen, the lightest of all the elements, into a metal by compacting it between two diamonds under a pressure millions of times higher than that of the Earth’s atmosphere.

When it becomes a metal, hydrogen reflects light like a mirror and should be a high-temperature superconducter of electricity, although the researchers at the Carnegie Institution in Washington, have not been able to measure that capability yet, they said in the journal Science.

Planets’ Cores

Physicists are extremely interested in the new finding because they believe that the cores of large gaseous planets such as Jupiter and Saturn are composed of metallic hydrogen, which explains their large magnetic fields. Demonstration of the existence of metallic hydrogen lends credence to that belief.

“It’s a very exciting development,” said Caltech astrophysicist David Stevenson. “It means, of course, that hydrogen is the most common metal in the solar system.”

Researchers also believe that metallic hydrogen will shed new light on the recently discovered ceramic high-temperature superconductors and have speculated about other potential uses for metallic hydrogen. If it could be stabilized, it might become a fuel for conventional nuclear fusion experiments or a highly compact rocket fuel that would generate extraordinary thrust for its weight and volume.

Soviet researchers, in the 1970s, erroneously claimed that they had converted hydrogen and other materials into metals under high pressures and stuck to the belief long after it was widely discredited. That incident is thought to have set Soviet high-pressure research back by at least a decade.

The new discovery was made possible by the earlier development at Carnegie of diamond anvils for producing high pressures--the concept of using diamonds had simply not occurred to researchers before. These anvils “have revolutionized the whole field of high-pressure physics,” according to theoretical physicist Marvin Cohen of UC Berkeley. “It used to take a seven-story building to do what you can now do with a device that you can fit in your shirt pocket.”

Diamond anvils are now used widely by other researchers to study a variety of materials under high pressures.

At the heart of the Carnegie device are two small, 1/3-carat diamonds that have each been polished so that one side is nearly flat. A diamond is the only material strong enough to withstand the tremendous pressures exerted over a very small area.

The Process

The sample to be studied is corraled between the two diamonds by a small hole in a stainless-steel plate between the diamonds. The diamonds are then forced together with a laboratory press. Because all the force of the press is focused on the very small area of the diamond, tremendously high pressures can be achieved--up to 5 million times atmospheric pressure (5 megabars).

The diamond anvil is also invaluable because the diamonds are transparent, allowing researchers to see what is happening to a sample inside the cell and to measure the optical properties of the sample. (Pressures are measured by observing the change in optical properties of a microscopic ruby placed inside the cell.

Geophysicist Ho-Kwang Mao and physical chemist Russell J. Hemley of Carnegie confined a small amount of hydrogen gas in the cell, cooled it to minus 321 degrees Fahrenheit and began applying pressure.

At a pressure of about 64,000 atmospheres, the hydrogen becomes a transparent, crystalline solid that is an insulator; it does not conduct electricity. It remains in that form over a very wide range of pressures.

But at a pressure of 1.5 megabars, the hydrogen undergoes a structural change and begins to absorb light, darkening visibly. At a pressure of 3 megabars, nearly the same pressure as at the center of the Earth, the hydrogen becomes “essentially an opaque solid,” Hemley said in a telephone interview. Electronic evidence obtained by optical methods suggests that it should conduct electricity, but the sample’s extremely small size precludes measuring it.

Mao and Hemley observed the same metalization process with gaseous nitrogen, which became a metal at about 1.8 megabars, but observed no changes in some other materials, including sodium chloride (table salt), aluminum oxide and silicon dioxide. Theory predicts, however, that all materials will become metals if they are squeezed hard enough.

Astronomers have long believed that gaseous planets such as Jupiter have metallic hydrogen at their core, where the pressure is 20 to 50 megabars, because that is the only way to explain their magnetic fields.

‘Scientific Process’

“Everybody believed it would be true,” said Caltech’s Stevenson. “But still, you have to adhere to the scientific process and show that metallic hydrogen can be formed.” One problem in the past, he said, was that such pressures could not be achieved in this type of experiment and that researchers did not know the pressure at which the transition to a metal occurred. “Now that we have this number, we can do more with it,” in theoretical modeling of the large planets’ interiors.

Theory also predicts that metallic hydrogen should be a superconductor--carrying electricity without any resistance. It will probably be impractical to make electronic devices out of metallic hydrogen, noted Cohen, but study of the material could help researchers find the mechanism by which recently discovered ceramic materials become superconducting, thereby leading to the discovery of even better superconductors.

The search for metallic hydrogen has proved embarrassing to scientists at the Institute of High-Pressure Physics near Moscow. More than a decade ago, researchers there built a 50,000-ton press, which extended 84 feet above the ground and 45 feet below it, at least partially for the purpose of making metallic hydrogen.

The device used one diamond, which pressed against a metallic plate. The Soviet diamond, however, was pointed rather than flat like the Carnegie gem. Because Soviet researchers could not see through the cell, they measured electrical conductivity between the diamond and the flat plate. When a current passed between them, they assumed that the sample had become a metal.

Over more than a decade, the Soviet researchers reported that they had converted hydrogen, rubies and a variety of other materials into metals. Subsequent studies showed, however, that the pointed diamond simply pierced the samples, making direct electrical contact with the metal plate.