Metallic hydrogen
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Metallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change, and it is an example of degenerate matter. Metallic hydrogen consists of a lattice of atomic nuclei (namely protons) with a spacing that 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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Discovery
Though topping the periodic table's alkali metal column, hydrogen is not, under ordinary conditions, an alkali metal itself. In 1935, however, Nobel Prize-winning physicist Eugene Wigner predicted that under immense pressure, hydrogen atoms would indeed join their first group kin, relinquishing their proprietary hold over their electrons.
The pressures required made experimental verification elusive. In March 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 kelvins and pressures of over a million atmospheres - the first identifiably metallic hydrogen, ending the 60-year search.
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 above the temperatures specified by metallization theory; furthermore, in previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 2.5 million atmospheres, detectable metallization did not occur. The team sought simply to measure the less extreme conductivity changes that they expected to take place.
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. First, at one end of the gun, the hydrogen was cooled to about 20 K inside a container that included a battery connected by wires to a Rogowski coil and an oscilloscope; the wires also touched the surface of the hydrogen in several places, so the apparatus could be used to measure and record its electrical conductivity. At the opposite end, up to 3 kg (7 lb) of gunpowder was ignited, and the resulting explosion pushed a piston through a pump tube, compressing the gas inside. Eventually the gas reached a pressure high enough to throw a valve at the far end of the chamber. Entering the thin "barrel", it propelled the plastic-covered metal impactor plate into the container at up to 8 km/s (18,000 mph), compressing the hydrogen inside.
The scientists were stunned to find that as pressure rose to 1.4 million atmospheres, the electronic energy band gap (a measure of electrical resistivity) fell to almost zero.
The electronic energy band gap of hydrogen in its uncompressed state is about 15 eV, making it an insulator, but as pressure rises to almost unimaginable heights, the band gap gradually falls to 0.3 eV. Because 0.3 eV are provided by the thermal energy of the fluid (the temperature became about 3000 K due to compression of the sample), the hydrogen can at this point be considered fully metallic.
Astrophysics
Metallic hydrogen is 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 be adjusted. The new data indicate that much more metallic hydrogen exists inside Jupiter than thought, that it comes closer to the surface, and therefore that Jupiter's tremendous magnetic field, the strongest of any planet in the solar system, is, in turn, produced closer to the surface.
Applications
One method of producing nuclear fusion involves pointing laser beams at pellets of hydrogen isotopes; the increased understanding of the behavior of hydrogen in extreme conditions could help to increase energy yields.
It may be possible to actually produce substantial quantities of metallic hydrogen, with practical benefit. The existence has been theorized of a form (called 'Metastable Metallic Hydrogen', abbreviated MSMH) that would not revert to ordinary hydrogen upon release of pressure, just as diamonds freed from the compression of the underground do not revert to ordinary graphite. With the tensile strength of aluminium and a third its weight, MSMH could be used to build extremely light, fuel-efficient cars. In addition, it would make an efficient fuel itself (and a clean one, with only water as an end product); 9 times as dense as standard hydrogen, it would give off considerable energy when reverting to that form. "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 metastability is possible.