Ranga Dias, I.F. Silvera
Scientists may have just given hydrogen a squeeze strong enough to turn it into a metal. The important point here is that they “may” have. In fact, some critics strongly dispute the new claim.
Under extremely high pressures hydrogen may become reflective. Researchers at Harvard University in Cambridge, Mass, have just claimed to have seen it. That reflectivity is a key trait of metals. Their feat required compressing hydrogen to 4.9 million times atmospheric pressure.
Isaac Silvera and Ranga Dias reported their findings online January 26 in Science.
If correct, this crushing work would end a decades-long search for a material that could have unusual properties, such as superconductivity. (That’s the ability to conduct electricity without resistance.)
Most superconductors work only at extremely cold temperatures. But some scientists have calculated that metallic hydrogen might prove a relatively high-temperature superconductor. It might have this trait even at room temperature — higher than any other known superconductor. If so, the new discovery would raise hopes that superconducting metallic hydrogen could be used in power lines. That could make transmission of electricity vastly more efficient.
“I think there’s a good chance that it’s correct,” David Ceperley says of the new claim. A theoretical physicist, he works at the University of Illinois at Urbana-Champaign. The pressure at which the hydrogen became reflective is about where theoretical physicists have calculated that a metal should form, he says.
What the new study did
To put the pressure on hydrogen, scientists capture it as a gas between the tips of two diamonds. Then they squeeze tips of that diamond “anvil” together. It’s not easy. “The problem in making metallic hydrogen has been that the predicted pressures have been very high,” says Silvera. “Diamonds always break before you can obtain those pressures.”
To prevent that, his team smoothed the surface of the diamond. This got rid of any defects. The scientists also covered the gems in a thin layer of aluminum oxide. That layer kept the hydrogen from diffusing inside the diamond and triggering cracks.
The researchers also cooled their lab setup to no more than 83 kelvins (−190° Celsius or -310° Fahrenheit). As the scientists ratcheted up the pressure, the hydrogen first turned black. That indicated a possible semiconducting phase. Eventually the hydrogen turned reflective. That pointed to it having become a metal. Metallic hydrogen could be either a solid or a liquid, Silvera notes.
Such experiments are tricky. Indeed, only a few research teams in the world can perform them. One risk: The hydrogen may leak from the chamber without the scientists realizing it. However, Silvera says, “We’re sure we have hydrogen in there.”
Some earlier attempts to make metallic hydrogen have monitored the element as the pressure was ramped up. The goal was to study the material’s transition — and to make sure the hydrogen did not escape along the way. To do this, scientists use a technique called Raman spectroscopy (RAH-mun Spek-TROS-koh-pee). It involves shining a laser through the diamonds, then observing how the light scattered. But at pressures this high, lasers could cause the diamonds to break, Silvera says. So the researchers used lasers only after the sample had reached the metallic state.
Silvera’s group is not the first to announce the discovery of metallic hydrogen. But earlier claims of finding the metal have been overturned. Even in this case, “It’s not the last word,” says Ceperley. However, he adds, “It should encourage all the other groups to come out and try to reproduce it.”
But not everyone is buying that the Harvard group achieved what it’s claiming it did. Among them is Eugene Gregoryanz at the University of Edinburgh in Scotland. This physicist works on similar experiments. And he claims that the new paper should never have been published. That it was, he argues, represents a failure of the journal’s review process.
Based on what is reported in the new paper, he even doubts that the claimed pressures were reached. He points out that the researchers gave data from only one experiment. “How is it possible to do only one experiment and claim such a big thing?” he asks.
Physicist Alexander Goncharov works at the Carnegie Institution for Science in Washington, D.C. He, too, challenges the Harvard team’s conclusions. “It's not shown whether they have hydrogen at all at high pressure,” he says.
Clearly, this story isn’t over.
(for more about Power Words, click here)
aluminum A metallic element, the third most abundant in Earth’s crust. It is light and soft, and used in many items from bicycles to spacecraft.
atmospheric pressure The pressure exerted by the weight of the atmosphere.
diamond One of the hardest known substances and rarest gems on Earth. Diamonds form deep within the planet when carbon is compressed under incredibly strong pressure.
diamond anvil A piece of equipment that researchers use to squeeze samples of material at very high pressure. Samples are typically sandwiched between tiny, flat pieces of diamond. Because diamonds are very hard, pressures inside the samples can reach very high levels. Scientists often squeeze tiny samples of minerals inside diamond anvils to see how they might behave deep inside Earth or on other planets.
electricity A flow of charge, usually from the movement of negatively charged particles, called electrons.
element (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.
journal (in science) A publication in which scientists share their research findings with the public. Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send out all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
kelvin A temperature scale that has units the size of those on the Celsius scale. The difference, 0 kelvin is absolute zero. By contrast, 0 kelvin is equal to -273.15 Celsius. So 0 Celsius is equal to 273.15 kelvins. NOTE: Unlike with the Celsius and Fahrenheit scales, there is no use of the term “degrees” for numbers on the kelvin scale.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
liquid A material that flows freely but keeps a constant volume, like water or oil.
metal A material that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure).
oxide A compound made by combining one or more elements with oxygen. Rust is an oxide; so it water.
physicist A scientist who studies the nature and properties of matter and energy.
pressure Force applied uniformly over a surface, measured as force per unit of area.
Raman spectroscopy A chemical tool that helps provide a chemical fingerprint of some sample.
reflective Having the quality of reflecting light strongly. Reflective objects can produce a strong bright glare when sunlight bounces off of them. Examples of reflective objects include a mirror, a smooth metal can, a car window, a glass bottle, ice, snow or the watery surface of a lake.
resistance (in physics) Something that keeps a physical material (such as a block of wood, flow of water or air) from moving freely, usually because it provides friction to impede its motion.
solid Firm and stable in shape; not liquid or gaseous.
superconductor Materials that have no resistance to the flow of electricity, typically only when they are cooled below a certain temperature. Superconductors also repel all magnetic fields, which allows them to float in the air when they are placed inside a strong magnetic field.
theoretical An adjective for an analysis or assessment of something that based on pre-existing knowledge of how things behave. It is not based on experimental trials. Theoretical research tends to use math — usually performed by computers — to predict how or what will occur for some specified series of conditions. Experimental testing or observations of natural systems will then be needed to confirm what had been predicted.
trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.
transition The boundary where one thing (paragraphs, ecosystems, life stage, state of matter) changes into another. Some transitions are sharp or abrupt. Others slowly or gradually morph from one condition or environment to another.