Harvard scientists get metallic hydrogen

    An image of diamond anvils compressing a sample of molecular hydrogen. At high pressure, hydrogen enters an atomic state, as shown on the right. Source: Dias & Silvera, 2017.

    In 1935, scientists Eugene Wigner and Bell Huntington predictedthe possibility of converting hydrogen to a metallic state under the influence of enormous pressure - 250 thousand atmospheres. A little later, this point of view was revised, experts raised the estimate of pressure required for the phase transition. All this time, the transition conditions were considered achievable, and scientists tried to "take the bar" necessary for the transition of hydrogen to a new phase. Metallic hydrogen was first tried in the 1970s. Repeated attempts were made in 1996, 2008 and 2011. Earlier it was reported that in 1996, scientists from Germany succeeded in transferring hydrogen to a metallic state to a fraction of a microsecond, although not everyone agrees with this.

    As for the pressure required to produce metallic hydrogen, with the development of quantum mechanics and physics, it became clear that the pressure should be about 20 times higher than previously thought - not 25 GPa, but 400 or even 500 GPa. It is believed that large quantities of metallic hydrogen are present in the cores of the giant planets - Jupiter, Saturn and large extrasolar planets. Due to the gravitational compression under the gas layer should be the core of metallic hydrogen. It is clear that in order to get a huge pressure, we need special technologies and methods. Achieve the desired result through the use of two diamond anvils.

    The strength of the anvil was enhanced by sputtering of aluminum oxide, which turned out to be impermeable to hydrogen atoms. The hydrogen sample was compressed between the pointed ends of two diamond anvils and at a pressure of 495 GPa, scientists achieved the transfer of the sample into the metal phase.

    Source: Dias & Silvera, 2017

    In any case, the sample first darkened and then began to reflect the light. With relatively low pressure, the sample was opaque; it did not conduct current. The experiment conducted by Isaac Silvera and Ranga Dias was repeated. For the first time, scientists managed to achieve the transition of hydrogen to the metal phase in mid-2016. But the results of the experiment needed confirmation, repeated experience. Since the results of the initial experiment were confirmed, they can be considered correct.

    Scientists walked to the current result for several years. Only to achieve the pressure at which the hydrogen breaks into individual atoms, it took three years for Silver and Diaz. The pressure in question is 380 GPa.

    After this increase in pressure implied the need to increase the strength of diamond anvils, which were used in the experiment. For this, they began to spray a thin film of aluminum oxide. Without reinforcing the strength, diamonds, which are the hardest minerals on Earth, begin to collapse when the pressure rises above 400 GPa.

    Scientists have done a lot of work on the study of diamonds. There could be several reasons for the destruction - from defects in the structure of the crystal to the influence of the most compressed to the high density of hydrogen. In order to solve the first problem, the specialists carefully checked the structures of the crystal under a high-magnification microscope. “When we looked at the diamond under a microscope, we found defects that make this mineral vulnerable to external factors,” said Silvera. The second problem was solved by spraying, counteracting the leakage of atoms and hydrogen molecules.

    It's hard to say so farWhat form of metal did the English get - solid or liquid. They themselves find it difficult to say, although they believe that hydrogen has passed into the liquid metal phase, since this is predicted by calculations. What they are sure of is that the hydrogen sample after compression has become 15 times more dense than before the start of this procedure. The temperature of hydrogen, which was placed in the diamond anvil, was 15K. After the transition of the element into the metal phase, it was heated to 83 K, and it retained its metallic properties. Calculations show that metallic hydrogen can be metastable, that is, to retain its properties even after the external factors that led to the transition of the element into the metallic phase will be weakened.

    Why does a man need metallic hydrogen? It is believed that in this state it exhibits the properties of a high-temperature superconductor. In addition, metastable metallic hydrogen compounds can be used as a compact, efficient and clean rocket fuel. Thus, the transition of metallic hydrogen to the molecular phase releases about 20 times more energy than when burning a kilogram of a mixture of oxygen and hydrogen — 216 ​​MJ / kg.

    “To get metallic hydrogen, we needed a huge amount of energy. And if you again translate atomic metallic hydrogen into a molecular state, all this energy will be released, so that we can get the most powerful rocket fuel in the world, which will revolutionize rocket production, ”said the authors of the study. In their opinion, the new fuel, provided it is used, will make it easy to reach other planets. Time to travel to them will be spent much less than at present, using modern technology.

    DOI: 10.1126 / science.aal1579

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