Thomas D. Cabot Professor of the Natural Sciences Isaac Silvera and postdoctoral fellow Ranga Dias have long sought the material, called\u00a0atomic metallic hydrogen. In addition to helping scientists answer some fundamental questions about the nature of matter, the material is theorized to have a wide range of applications, including as a room-temperature superconductor. Their research\u00a0is described in a paper published today in Science.<\/p>\n
\u201cThis is the Holy Grail of high-pressure physics,\u201d Silvera said of the quest to find the material. \u201cIt\u2019s the first-ever sample of metallic hydrogen on Earth, so when you\u2019re looking at it, you\u2019re looking at something that\u2019s never existed before.\u201d<\/p>\n
In their experiments,\u00a0Silvera and Dias squeezed a tiny hydrogen sample at 495 gigapascal (GPa), or more than 71.7 million pounds per square inch, which is greater than the pressure at the center of the Earth. At such extreme pressures, Silvera explained, solid molecular hydrogen, which consists of molecules on the lattice sites of the solid, breaks down, and the tightly bound molecules dissociate to transforms into atomic hydrogen, which is a metal.<\/p>\n
While the work creates an important window into understanding the general properties of hydrogen, it also offers tantalizing hints at potentially revolutionary new materials.<\/p>\n
\u201cOne prediction that\u2019s very important is metallic hydrogen is predicted to be meta-stable,\u201d Silvera said. \u201cThat 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 are removed.\u201d<\/p>\n
Understanding whether the material is stable is important, Silvera said, because predictions suggest metallic hydrogen could act as a superconductor at room temperatures.<\/p>\n
\u201cAs much as 15 percent of energy is lost to dissipation during transmission,\u201d he said, \u201cso if you could make wires from this material and use them in the electrical grid, it could change that story.\u201d<\/p>\n
A room temperature superconductor, Dias said, could change our transportation system, making magnetic levitation of high-speed trains possible, as well as making electric cars more efficient and improving the performance of many electronic devices. The material could also provide major improvements in energy production and storage. Because superconductors have zero resistance, superconducting coils could be used to store excess energy, which could then be used whenever it is needed.<\/p>\n
Metallic hydrogen could also play a key role in helping humans explore the far reaches of space, as\u00a0a more powerful rocket propellant.<\/p>\n \u201cIt takes a tremendous amount of energy to make metallic hydrogen,\u201d Silvera explained. \u201cAnd if you convert it back to molecular hydrogen, all that energy is released, so that would make it the most powerful rocket propellant known to man, and could revolutionize rocketry.\u201d<\/p>\n The most powerful fuels in use today are characterized by a \u201cspecific impulse\u201d (a measure, in seconds, of how fast a propellant is fired from the back of a rocket) of 450 seconds. The specific impulse for metallic hydrogen, by comparison, is theorized to be 1,700 seconds.<\/p>\n \u201cThat would easily allow you to explore the outer planets,\u201d Silvera said. \u201cWe would be able to put rockets into orbit with only one stage, versus two, and could send up larger payloads, so it could be very important.\u201d<\/p>\n In their experiments, Silvera and Dias turned to one of the hardest materials on Earth, diamond.\u00a0But rather than natural diamond, Silvera and Dias used two small pieces of carefully polished synthetic diamond and treated them to make them even tougher. Then they mounted them opposite each other in a device known as a diamond anvil cell.<\/p>\n \u201cDiamonds are polished with diamond powder, and that can gouge out carbon from the surface,\u201d Silvera said. \u201cWhen we looked at the diamond using atomic force microscopy, we found defects, which could cause it to weaken and break.\u201d<\/p>\n The solution, he said, was to use a reactive ion etching process to shave a tiny layer \u2014 just five microns thick, or about a tenth the thickness of a human hair \u2014 from the diamond\u2019s surface. The diamond was then coated with a thin layer of alumina to prevent the hydrogen from diffusing into the crystal structure and embrittling it.<\/p>\n After more than four decades of work on metallic hydrogen, and nearly a century after it was first theorized, it was thrilling to see the results, Silvera said.<\/p>\n \u201cIt was really exciting,\u201d he said. \u201cRanga was running the experiment, and we thought we might get there, but when he called me and said, \u2018The sample is shining,\u2019 I went running down there, and it was metallic hydrogen.\u201d<\/p>\n \u201cI immediately said we have to make the measurements to confirm it, so we rearranged the lab \u2026 and that\u2019s what we did.\u201d<\/p>\n![\"Microscopic](\"https:\/\/harvardgazette.files.wordpress.com\/2017\/01\/metal_hydrogen_triptych_605.jpg?w=605&h=403\")