Metallic hydrogen

Metallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change; it is an example of degenerate matter. It consists of a crystal lattice of atomic nuclei (namely, protons), with a spacing which is significantly smaller than a Bohr radius. Indeed, the spacing is more comparable with an electron wavelength (see De Broglie wavelength). The electrons are unbound and behave like the conduction electrons in a metal.

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History

Predictions

Though topping the Periodic Table's alkali metal column, hydrogen is not, under ordinary conditions, an alkali metal. In 1935, however, physicists Eugene Wigner and H.B. Huntington predicted that under an immense pressure of two-hundred and fifty thousand atmospheres (~ 25 GPa), hydrogen atoms would display metallic properties, losing hold over their electrons.^[1] The pressures necessary made experimental proof elusive, but their prediction about the amount of pressure needed was proven to be too low.^[2]

Discovery

In March of 1996, however, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced, for about a microsecond and at temperatures of thousands of kelvin and pressures of over a million atmospheres (>100 GPa), the first identifiably metallic hydrogen.^[3]

The Lawrence Livermore team did not expect to produce metallic hydrogen, as they were not using solid hydrogen, thought to be necessary, and were working at temperatures above those specified by metallization theory. Furthermore, previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 2.5 million atmospheres (~253 GPa), did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes which were expected to occur.

The researchers used a 1960s-era light gas gun, originally used in guided missile studies, to shoot an impactor-plate into a sealed container containing a half-millimetre thick sample of liquid hydrogen. The liquid hydrogen was in contact with wires leading to a device capable of measuring electrical resistance.

The scientists were surprised to find that, as pressure rose to 1.4 million atmospheres (142 GPa), the electronic energy band gap, a measure of electrical resistance, fell to almost zero. The band-gap of hydrogen in its uncompressed state is about 15 eV, making it an insulator but, as the pressure increases significantly, the band-gap gradually falls to 0.3 eV and because the 0.3 eV is provided by the thermal energy of the fluid (the temperature became about 3000 K due to compression of the sample), the hydrogen may, at this point, effectively be considered metallic.

Liquid metallic hydrogen

The proton has one fourth the mass of 4He, which at normal conditions is a liquid even at lowest temperatures, a consequence of high zero-point energy. Similarly, zero-point energies of protons in a dense state are also high, and at elevated compressions there is expected to be a decline in the ordering energies from interactions relative to protonic zero-point energies. Arguments have been advanced by N.W. Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there may be a range of densities (at pressures around 400 GPa) where hydrogen may be a liquid metal even at lowest temperatures.^[4][5]

Superconductivity

Theory has been put forward by N.W. Ashcroft that metallic hydrogen may be a superconductor as high as room temperature (290K), far higher than any other known candidate material. This stems from its extremely high speed of sound and the expected strong coupling between the conduction electrons and the lattice vibrations.^[6]

Novel types of quantum fluid

Presently known "super" states of matter are superconductors, superfluid liquids and gases, and supersolids. It was predicted by Egor Babaev that if hydrogen and deuterium have liquid metallic states, they may have ordered states in quantum domain which cannot be classified as superconducting or superfluid in usual sense but represent two possible novel types of quantum fluids: “superconducting superfluid” and “metallic superfluid”. These were shown to have highly unusual reactions to external magnetic field and rotation which might represent a route for experimental verification of these possible new states of matter. It has also been suggested that under the influence of magnetic field the hydrogen may exhibit phase transitions from superconductivity to superfluidity and vice versa.^[7][8][9]

Contemporary research

Many experiments are continuing in the production of metallic hydrogen in laboratory conditions at static compression and low temperature. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998,^[10] and later Paul Loubeyre and René LeToullec from Commissariat à l'Énergie Atomique, France in 2002, have shown that at pressures close to those at the center of the Earth (3.2 to 3.4 million atmospheres or 324 to 345 GPa) and temperatures of 100 K–300 K, hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in laboratory at low temperature and static compression continues.

Astrophysics

Metallic hydrogen is thought to be present in tremendous amounts in the gravitationally compressed interiors of Jupiter, Saturn, and some of the newly discovered extrasolar planets. Because previous predictions of the nature of those interiors had taken for granted metallization at a higher pressure than the one at which we now know it to happen, those predictions must now be adjusted. The new data indicates much more metallic hydrogen must exist inside Jupiter than previously thought, that it comes closer to the surface, and that therefore, Jupiter's tremendous magnetic field, the strongest of any planet in the solar system is, in turn, produced closer to the surface.

Applications

Nuclear power

One method of producing nuclear fusion, called inertial confinement fusion, involves aiming laser beams at pellets of hydrogen isotopes. The increased understanding of the behavior of hydrogen in extreme conditions could help to increase energy yields.

Fuel

It may be possible to produce substantial quantities of metallic hydrogen for practical purposes. The existence has been theorized of a form called 'Metastable Metallic Hydrogen', (abbreviated MSMH) which would not immediately revert to ordinary hydrogen upon the release of pressure.

In addition, 'MSMH' would make an efficient fuel itself and also a clean one, with only water as an end product. Nine times as dense as standard hydrogen, it would give off considerable energy when reverting to standard hydrogen. Burned more quickly, it could be a propellant with five times the efficiency of liquid H₂/O₂, the current Space Shuttle fuel. Unfortunately, the 'Lawrence Livermore' experiments produced metallic hydrogen too briefly to determine whether or not metastability is possible.^[11]

References