What is Metallic Hydrogen?

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Metallic Hydrogen.

Hydrogen is the simplest and most abundant of elements. Composed of one proton and one electron, it makes up 90% of our universe (by number of atoms). Hydrogen, the most common element in the universe, is normally an insulating gas, but at high pressures it may turn into a superconductor. On Earth, hydrogen is commonly found as a diatomic molecular gas. However on Jupiter, where interior pressure is millions of times greater than that at our planet's surface, the hydrogen molecule is hypothesized to exist as a super hot liquid metal. Scientists predicted that it should be possible to form a metal from hydrogen, but the pressure that would be required to do so - some 4 million atmospheres - exceeds that at the center of the earth. Metallic hydrogen is a state of hydrogen which results when it is sufficiently compressed and undergoes a phase transition (from one phase or state of matter to another); it is an illustration of degenerate matter (degenerate matter is matter which has such extraordinarily high density).

 

Solid metallic hydrogen is predicted to consist of a crystal lattice of hydrogen nuclei (namely, protons). Crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry. In liquid metallic hydrogen, protons do not have lattice ordering, rather it is a liquid system of protons and electrons.

 

What are the Theoretical predictions?

  • Metallization of hydrogen under pressure: In 1935, physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of approximately 25 Pascal (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”.
  • Liquid metallic hydrogen: Helium is a liquid at normal pressure and temperatures near absolute zero, a result of its high zero-point energy (ZPE), the lowest possible energy. The ZPE of protons in a dense state is also high, and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Opinions have been advanced by Neil Ashcroft, Professor of Physics, and other 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 low temperatures.
  • Superconductivity: In 1968, Ashcroft put forward that metallic hydrogen may be a superconductor, up to room temperature and far higher than any other known candidate material. This stems from its extremely high speed of sound and the expected strong coupling (two systems are coupled if they are interacting with each other) between the conduction electrons and the lattice vibrations. The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium.  A periodic oscillation of the atoms in a crystal lattice about their equilibrium positions is known as lattice vibration.
  • Possibility of novel types of quantum fluid: It was predicted by Egor Babaev, a physicist, that, if hydrogen and deuterium (isotope of hydrogen) have liquid metallic states, they may have ordered states in quantum domains 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". Under the influence of magnetic field, hydrogen may exhibit phase transitions from superconductivity to superfluidity and vice-versa.
  • Requisite pressure reduced by Lithium doping: In 2009, it was found that the alloy LiH6 would be a stable metal at only 1/4 of the pressure required to metallize hydrogen.

 

What is the Experimental Quest for Metallic Hydrogen?

  • Experimental research since 1996: Several experiments are being conducted in the pursuit 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 France in 2002, have shown that at pressures close to those at the center of the Earth (3.2 to 3.4 million atmospheres) 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. Shahriar Badiei and Leif Holmlid from the University of Gothenburg, Sweden, have shown in 2004 that condensed metallic states made of excited hydrogen atoms are effective promoters to metallic hydrogen.
  • Metallization of hydrogen by shock-wave compression: In 1996, a team of researchers used a light gas gun, originally employed in guided missile studies, to shoot 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 (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 electronVolt (eV), making it an insulator but, as the pressure increases significantly, the band-gap gradually fell to 0.3 eV. Since the thermal energy of the fluid was above 0.3 eV, the hydrogen might be considered metallic.
  • Experimental breakthrough in 2008: The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by two scientists, Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating. Hydrogen-rich alloy Silane (SiH4) was metalized and found to be superconducting confirming earlier theoretical prediction by Ashcroft. In this hydrogen rich alloy, even at moderate pressures (due to the chemical pre compression) the hydrogen forms a sub-lattice with density corresponding to metallic hydrogen.

 

What are the Applications of metallic Hydrogen?

  • Nuclear power: One technique of producing nuclear fusion, called inertial confinement fusion, involves targeting laser beams at pellets of hydrogen isotopes. The increased understanding of the behavior of hydrogen in extreme conditions might help to increase energy yields.
  • Fuel: It might be possible to fabricate extensive quantities of metallic hydrogen for practical purposes. It has been theorized that it might be possible to get hydrogen into a form called "Meta-stable Metallic Hydrogen" (MSMH), which would not immediately revert to ordinary hydrogen upon the release of pressure. A system is in a meta-stable state when it is in equilibrium (not changing with time). MSMH would make an efficient fuel itself and also a clean one, with only water as an end product.
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