Metallic hydrogen

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Metallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change; it is an example of degenerate matter. Solid metallic hydrogen 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. As is the dihydrogen molecule H
2
, metallic hydrogen is an allotrope. In liquid metallic hydrogen protons do not have lattice ordering i.e. the system is a liquid of protons and electrons.

History

Theoretical Predictions

Metalization of hydrogen under pressure

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]. Since then metallic hydrogen was the holy grail of high-pressure physics. The initial prediction about the amount of pressure needed was proven to be too low.[2] Since the first work by Wigner and Huntington the more modern theoretical calculations were pointing toward higher but nonetheless potentially experimentally accessible metalization pressures. Professor Malcolm McMahon (Centre for Science and Extreme Conditions at Edinburgh University) states that they are currently developing techniques for creating pressures of up to five million atmospheres (ie, higher than the pressure at the center of the earth) in hopes of creating metallic hydrogen.[3]

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 Neil Ashcroft that metallic hydrogen may be a superconductor as high as 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.[6]

Possibility of 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]

Experimental pursuit

Metalization 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 and at temperatures of thousands of kelvin and pressures of over a million atmospheres (>100 GPa), the first identifiably metallic hydrogen.[10]

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.

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,[11] 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. Studies are also undergoing on deuterium.[12]. Shahriar Badiei and Leif Holmlid from the University of Goteborg have shown in 2004 that condensed metallic states made of excited hydrogen atoms (H Rydberg matter) are effective promoters to metallic hydrogen.[13]

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 innovative technique of pulsed laser heating.[14]

Hydrogen-rich alloy SiH4 was metalized in 2008 and found to be superconducting (by M.I. Eremets et al), confirming earlier theoretical prediction by N. W. Ashcroft.[15] In this hydrogen rich alloy, even at moderate pressures (because of chemical precompression) the hydrogen forms a sublattice with density corresponding to metallic hydrogen.

Metallic hydrogen in other contexts

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[citation needed] 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.[16]

References

  1. [1] E. Wigner and H. B. Huntington, On the Possibility of a Metallic Modification of Hydrogen J. Chem. Phys. 3, 764 (1935).
  2. [2] P. Loubeyre, R. LeToullec, D. Hausermann, M. Hanfland, R. J. Hemley, H. K. Mao, and L. W. Finger, X-ray diffraction and equation of state of hydrogen at megabar pressures Nature 383, 702 (1996).
  3. [3] BBC News Article: Peanut butter diamonds on display (27 June 2007)
  4. [4]. Ashcroft N.W., The hydrogen liquids, J.Phys. A 12, A129-137 (2000).
  5. [5]. Bonev, S.A., Schwegler, E., Ogitsu, T., and Galli, G., A quantum fluid of metallic hydrogen suggested by first principles calculations Nature 431, 669 (2004).
  6. [6]. N. W. Ashcroft Metallic Hydrogen: A High-Temperature Superconductor? Physical Review Letters 21 1748–1749 (1968).
  7. [7]. Egor Babaev, N.W. Ashcroft "Violation of the London law and Onsager–Feynman quantization in multicomponent superconductors".
  8. [8]. Egor Babaev, A. Sudbo, N.W. Ashcroft "A superconductor to superfluid phase transition in liquid metallic hydrogen" Nature 431 (2004) 666-668.
  9. [9]. Egor Babaev, "Vortices with fractional flux in two-gap superconductors and in extended Faddeev model" Phys.Rev.Lett. 89 (2002) 067001.
  10. [10] S. T. Weir, A. C. Mitchell, and W. J. Nellis, Metallization of Fluid Molecular Hydrogen at 140 GPa (1.4 Mbar) Physical Review Letters 76, 1860 - 1863 (1996).
  11. [11] C. Narayana, H. Luo, J. Orloff, and A. L. Ruoff Solid hydrogen at 342 GPa: no evidence for an alkali metal Nature 393, 46-49 (1998).
  12. [12]
  13. S. Badiei, L. Holmlid. Experimental observation of an atomic hydrogen material with H–H bond distance of 150 pm suggesting metallic hydrogen. J. Phys.: Cond. Matter 16 (2004) 7017-7023. [13]
  14. Shanti Deemyad and Isaac F. Silvera The Melting Line of Hydrogen at High Pressures http://arxiv.org/abs/0803.2321
  15. M. I. Eremets, I. A. Trojan, S. A. Medvedev, J. S. Tse, Y. Yao. "Superconductivity in Hydrogen Dominant Materials." Silane Science 14 March 2008 Vol. 319. no. 5869, pp. 1506–1509
  16. [14] W. J. Nellis Metastable Metallic Hydrogen Glass Lawrence Livermore Preprint (1996).


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