Metallic hydrogen is a phase of hydrogen in which it behaves as an electrical conductor.
This phase was predicted theoretically in 1935, but has not been reliably produced in
laboratory experiments due to the requirement of high pressures, on the order of hundreds
of gigapascals. At these pressures, hydrogen might exist as a liquid rather than solid.
Liquid metallic hydrogen is thought to be present in large amounts in the gravitationally
compressed interiors of Jupiter, Saturn, and in some of the newly discovered extrasolar
planets.
History Theoretical predictions
Metallization of hydrogen under pressure Though at the top of the alkali metal column
in the periodic table, hydrogen is not, under ordinary conditions, an alkali metal. In 1935
however, physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense
pressure of around 25 GPa, hydrogen atoms would display metallic properties, losing
hold over their electrons. Since then, metallic hydrogen has been described as "the holy grail
of high-pressure physics". The initial prediction about the amount of
pressure needed was eventually proven to be too low. Since the first work by Wigner and
Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless
potentially accessible metallization pressures. Techniques are being developed for creating
pressures of up to 500 GPa, higher than the pressure at the center of the Earth, in hopes
of creating metallic hydrogen. Liquid metallic hydrogen
Helium-4 is a liquid at normal pressure near absolute zero, a consequence of its high zero-point
energy. The ZPE of protons in a dense state is also high, and a decline in the ordering
energy is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others
that there is a melting point maximum in compressed hydrogen, but also that there may be a range
of densities where hydrogen may be a liquid metal, even at low temperatures.
Superconductivity In 1968, Neil Ashcroft put forward that metallic
hydrogen may be a superconductor, up to room temperature, 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.
Possibility of novel types of quantum fluid Presently known "super" states of matter are
superconductors, superfluid liquids and gases, and supersolids. Egor Babaev predicted that
if hydrogen and deuterium have liquid metallic states, they may have quantum ordered states
which cannot be classified as superconducting or superfluid in the usual sense. Instead,
they may represent two possible novel types of quantum fluids: "superconducting superfluids"
and "metallic superfluids". Such fluids were predicted to have highly unusual reactions
to external magnetic fields and rotations, which might provide a means for experimental
verification of Babaev's predictions. It has also been suggested that, under the influence
of magnetic field, hydrogen may exhibit phase transitions from superconductivity to superfluidity
and vice-versa. Lithium doping reduces requisite pressure
In 2009, Zurek et al. predicted that the alloy LiH6 would be a stable metal at only 1⁄4
of the pressure required to metallize hydrogen, and that similar effects should hold for alloys
of type LiHn and possibly other alloys of type ?Lin.
Experimental pursuit before 2011 Metallization of hydrogen in shock-wave compression
In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that
they had serendipitously produced, for about a microsecond at temperatures of thousands
of kelvins, pressures of over a million atmospheres and density of approximately 0.6 g/cm3, the
first identifiably metallic hydrogen. The team did not expect to produce metallic hydrogen,
as it was not using solid hydrogen, thought to be necessary, and was working at temperatures
above those specified by metallization theory. Previous studies in which solid hydrogen was
compressed inside diamond anvils to pressures of up to 2,500,000 atm, 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 employed in guided missile studies, to shoot an impactor plate into a sealed container
containing a half-millimeter thick sample of liquid hydrogen. The liquid hydrogen was
in contact with wires leading to a device measuring electrical resistance. The scientists
found that, as pressure rose to 1,400,000 atm, 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 fell to 0.3 eV. Because the thermal energy of the fluid was above
0.3 eV, the hydrogen might be considered metallic.
Other experimental research since 1996 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, 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 and temperatures of
100–300 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. Studies are also ongoing on deuterium. Shahriar Badiei and Leif Holmlid from the
University of Gothenburg have shown in 2004 that condensed metallic states made of excited
hydrogen atoms are effective promoters to metallic hydrogen.
Experimental results in 2008 The theoretically predicted maximum of the
melting curve was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser
heating. Hydrogen-rich molecular SiH4 was claimed to be metallized and become superconducting
by M.I. Eremets et al.. However this claim is disputed, and their results have not been
repeated. Metallization of hydrogen in 2011
In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and
deuterium at static pressures of 260–300 GPa. This claim was questioned by other researchers
in 2012. Metallic hydrogen in other contexts
Astrophysics Liquid metallic hydrogen is thought to be
present in large 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 Fuel and engineering
Metastable metallic hydrogen would make an efficient fuel and also a clean one, with
only water as an end product. About twelve times as dense as ordinary liquid molecular
hydrogen, when recombining to molecular form it would release about 20 times the energy
of hydrogen burning in oxygen. Burned more quickly, it could be a propellant with up
to five times the efficiency of liquid H2/O2, the fuel that the Space Shuttle used. Unfortunately,
the above-mentioned Lawrence Livermore experiments produced metallic hydrogen too briefly to
determine whether metastability is possible. See also
Slush hydrogen Solid hydrogen
Timeline of hydrogen technologies References