Friday, January 27, 2017

Metallic Hydrogen at Last?

“We have produced atomic metallic hydrogen in the laboratory at high pressure and low temperature,” say Harvard scientists Isaac Silvera and Ranga Dias in a new article that appears today in the AAAS journal Science. This straightforward comment could mean the end of an 80-year quest...and the start of an energy revolution.

This image shows diamond anvils (green) compressing molecular hydrogen.
At high pressure the sample converts to atomic metallic hydrogen, as shown on the right.
Image Credit: R. Dias and I.F. Silvera.

Ordinarily, hydrogen atoms come in pairs, existing as an inert gas at standard temperatures. But in 1935, physicists Eugene Wigner and Hillard Bell Huntington predicted that hydrogen could adopt metallic properties under certain conditions. Numerous experiments have attempted to create solid metallic hydrogen by cooling down regular hydrogen and exposing it to extremely high pressures. None have succeeded. Along the way, we’ve learned that the pressure necessary to reach metallic hydrogen is much higher than Wigner and Huntington predicted. Modern predictions are for pressures so high that we haven’t been able to reach them.

Although it requires pushing the edges of our capacity, the potential payoff is huge. This long-sought-after form of hydrogen could revolutionize energy storage, transportation, space travel, and our fundamental understanding of the nature of matter. Theories suggest that solid metallic hydrogen is superconducting and likely meta-stable. If that is the case, metallic hydrogen would stay metallic after you remove the pressure and raise the temperature. This could lead to inexpensive superconducting wires, coils, and other extremely efficient systems for storing and transporting energy.

Whether it will live up to this potential remains to be tested, but the first step is creating the thing in the first place. Six months ago, Physics Buzz reported on new developments along the path to metallic hydrogen made by Silvera, Dias, and their colleague Ori Noked. In this new experiment, Silvera and Dias used a similar approach to squeeze hydrogen between two diamond tips at higher pressures than ever before.

The highest pressures on Earth are reached in a piece of equipment called a diamond anvil cell. In essence, the tool consists of two diamond tips and space between them for a small sample that is confined in a metallic gasket. Once a sample is loaded in the center, the tips are squeezed together as hard as possible, creating pressures greater than at the center of the Earth.

Exposing hydrogen to pressures as high as possible requires diamond tips that are as hard as possible. In this kind of experiment, the pressure you can reach is limited by the point at which the diamond tips break. To create ultra-strong diamonds, the researchers started with two tiny, polished pieces of synthetic diamond. Polishing a diamond can create defects on its surface that lead to weakening, so the researchers used a chemical process to shave a thin layer off of the surface of each diamond that took the defects with it.

Next, they coated each diamond with something called alumina, a chemical compound that doesn’t interfere with the sample. The alumina helps to prevent hydrogen from seeping into the diamond at high pressures, which makes the tips more brittle.

As a third step toward ensuring diamonds of the highest quality, the researchers limited the system’s exposure to lasers. Lasers are commonly used to make measurements in diamond anvil experiments, but they can also weaken diamonds under high pressure.

The experiment was conducted at a chilly 5.5 K, which is just shy of -450°F. As the tips squeezed tighter and tighter, the sample reached higher and higher pressures. Dias saw it turn from transmitting light to dark, and then back to reflective again—an exciting visual sign of metallic hydrogen.

Things get tricky and hard to measure under such extreme conditions, including the value of the pressure and the properties of the sample. However, it appears the transition happened somewhere between 465 and 495 gigapascals (this is equivalent to 70 million pounds of pressure per square inch), which is within the range of modern predictions. By measuring how well the sample reflects different kinds of light, the researchers confirmed that the hydrogen sample has the fundamental properties of a metal.

What’s next? Other scientists will work on replicating this experiment in their own labs, seeking independent confirmation of the results—some German scientists may have jumped the gun on this claim back in 2012, so many in the community are waiting to see the experiment repeated before they get too excited. Different techniques will be used to study the existing sample and its properties. Once we know a little more about what we’re dealing with, the scientific community will be better equipped to start talking details and possible applications. Physics Buzz will keep you updated along the way!

—Kendra Redmond

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