Did Harvard really create the "holy grail" of solid metallic hydrogen?
Researchers at Harvard University say they've managed to create a potentially revolutionary material that has only been imagined in theory for the past several decades: solid metallic hydrogen.
Back in 1935, scientists predicted that hydrogen could be transformed into a metal under immense amounts of pressure, similar to the way carbon atoms can form into diamonds. Back then it was thought that 25 gigapascals (about 250,000 times normal atmospheric pressure on Earth) should do the trick.
Harvard physicists Ranga Dias and Isaac F. Silvera say they had to find a way to subject hydrogen to nearly twenty times that much pressure, which is more intense than the pressure at the center of the Earth, before it finally underwent the transition that had been predicted over eighty years ago.
"This is the holy grail of high-pressure physics," Silvera said. "It's the first-ever sample of metallic hydrogen on Earth, so when you're looking at it, you're looking at something that's never existed before."
To create the material, a tiny liquid hydrogen sample was inserted into something called a diamond anvil cell in which the sample is basically squeezed between two specially-coated and strengthened diamonds at 495 gigapascal, or more than 71.7 million pounds-per-square inch. The extreme pressure breaks down the tightly bound molecules into atomic hydrogen, which Silvera explains is a metal, meaning it is a substance that is solid, shiny, ductile, malleable and a good conductor of heat and electricity.
While metallic hydrogen is the realization of a prophecy for physics, it could also have major implications as a useful new material.
"One prediction that's very important is metallic hydrogen is predicted to be meta-stable," Silvera said. "That means if you take the pressure off, it will stay metallic, similar to the way diamonds form from graphite under intense heat and pressure, but remain diamonds when that pressure and heat is removed."
If metallic hydrogen remains stable at room temperature, it could have great potential as a superconductor and lead to significant efficiency improvements for energy production, transmission and storage.
"That would be revolutionary," he said. "As much as 15 percent of energy is lost to dissipation during transmission, so if you could make wires from this material and use them in the electrical grid, it could change that story."
Of course, for right now it would take a lot of energy to create metallic hydrogen, so the notion of making all our wires from the stuff seems a ways off. But Silvera believes all that pent-up energy could have other applications off of our planet.
"If you convert it back to molecular hydrogen, all that energy is released, so it would make it the most powerful rocket propellant known to man, and could revolutionize rocketry."
He says that rockets fueled by metallic hydrogen could offer more blast for the buck, allowing for the launch of larger payloads with fewer rocket stages.
"That would easily allow you to explore the outer planets," Silvera said.
Silvera and Dias published a paper on the creation of metallic hydrogen in the latest issue of the journal Science. They explain the basics of the feat in the following promotional video. But is the accomplishment all they say it is?
Not everyone in the scientific community is sold on the claim that Silvera and Dias have really got their hands on that elusive metallic holy grail.
"From our point of view it's not convincing," Mikhail Eremets told Science in a separate article. "We see only one experiment. It should be reproduced."
Eremets is also working on creating metallic hydrogen at the Max Planck Institute for Chemistry in Germany. In 2012, he and his colleagues thought they had created the first bit of the stuff, but today says their evidence is not conclusive.
Researchers have managed to create liquid metal hydrogen though, including Professor Eugene Gregoryanz from the University of Edinburgh, who also speaks of Silvera and Dias' work in less than glowing terms to Science.
"The word garbage cannot really describe it," he said.
One of the parts of the dispute are the coatings that are put on the diamonds in the anvil. While they strengthen the diamonds, it also makes it more difficult to interpret laser measurements and figure out what is really going on in the experiment.
Past studies involving compressed hydrogen turn to an odd state somewhere between a gas and metal, but also turn so dark as to reflect no light at all.
Alexander Goncharov of the Carnegie Institution for Science told the news department of the journal Nature that he suspects the shiny material created at Harvard may actually be the aluminum oxide that is used to coat the diamond, which could behave differently under pressure.
"If they want to be convincing, they have to redo the measurement, really measuring the evolution of pressure," concurs Paul Loubeyre, a physicist at France's Atomic Energy Commission. "Then they have to show that, in this pressure range, the alumina is not becoming metallic."
Silvera and Dias stand by their results. Silvera explains that they wanted to announce them before they ran further tests because that could break their diamond vice. Eventually though, it seems, the controversy may resolve itself as the team plans to run more tests to confirm the sample has the atomic structure of a solid metal. They also plan to unscrew the vice to check and see if the metal is metastable as hoped.
"A looming challenge is to quench metallic hydrogen and if so study its temperature stability to see if there is a pathway for production in large quantities," reads the concluding line of the paper.