Metallic hydrogen is a state of hydrogen which results when it is sufficiently compressed and undergoes a phase transition; it is an example of degenerate matter. Solid metallic hydrogen is predicted to consist of a crystal lattice of hydrogen nuclei (namely, protons), with a spacing which is significantly smaller than the Bohr radius. Indeed, the spacing is more comparable with the de Broglie wavelength of the electron. The electrons are unbound and behave like the conduction electrons in a metal. In liquid metallic hydrogen, protons do not have lattice ordering; rather, it is a liquid system of protons and electrons.
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 ~25 GPa (250,000 atm or3,500,000 psi), 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.
In 1968, Ashcroft put forward that metallic hydrogen may be a superconductor, up to room temperature (~290 K), 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
Experimental breakthroughs in 2008
The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating. Hydrogen-rich alloy SiH4 was metalized and found to be superconducting by M.I. Eremets et al., confirming earlier theoretical prediction by Ashcroft. In this hydrogen rich alloy, even at moderate pressures (because of chemical precompression) the hydrogen forms a sub-lattice with density corresponding to metallic hydrogen. However, the claimed high-pressure metallic and superconducting phase of SiH4 was later identified as platinum hydride, that formed after the decomposition of SiH4
Applications in 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.
Applications in 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.(if burned in pure oxygen). 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 up to five times the efficiency of liquid H2/O2, the current Space Shuttle fuel. Unfortunately, the above-mentioned Lawrence Livermore experiments produced metallic hydrogen too briefly to determine whether or not metastability is possible.
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