Metallic hydrogen
Metallic hydrogen is a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington.[1]
At
Theoretical predictions
Hydrogen under pressure
Though often placed at the top of the
In 1935, physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250,000 atm; 3,600,000 psi), hydrogen would display metallic properties: instead of discrete H2 molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons delocalized throughout.[1] Since then, producing metallic hydrogen in the laboratory has been described as "the holy grail of high-pressure physics".[3]
The initial prediction about the amount of pressure needed was eventually shown to be too low.[4] Since the first work by Wigner and Huntington, the more modern theoretical calculations point toward higher but potentially achievable metalization pressures of around 400 GPa (3,900,000 atm; 58,000,000 psi).[5][6]
Liquid metallic hydrogen
Geng predicted that the ZPE of protons indeed lowers the melting temperature of hydrogen to a minimum of 200 to 250 K (−73 to −23 °C) at pressures of 500–1,500 GPa (4,900,000–14,800,000 atm; 73,000,000–218,000,000 psi).[9][10]
Within this flat region there might be an elemental mesophase intermediate between the liquid and solid state, which could be metastably stabilized down to low temperature and enter a supersolid state.[11]
Superconductivity
In 1968,
As a rocket propellant
Metastable metallic hydrogen may have potential as a highly efficient rocket propellant, with a theoretical specific impulse of up to 1700 seconds (for reference, the current most efficient chemical rocket propellants have an Isp less than 500 s[13]), although a metastable form suitable for mass-production and conventional high-volume storage may not exist.[14][15] Another significant issue is the heat of the reaction, which at over 6000 K is too high for any known engine materials to be used. This would necessitate diluting the metallic hydrogen with water or liquid hydrogen, a mixture that would still provide a significant performance boost from current propellants.[13]
Possibility of novel types of quantum fluid
Presently known "super" states of matter are
Lithium alloying reduces requisite pressure
In 2009, Zurek et al. predicted that the alloy LiH6 would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiHn and possibly "other alkali high-hydride systems", i.e. alloys of type XHn, where X is an alkali metal.[19] This was later verified in AcH8 and LaH10 with Tc approaching 270 K[20] leading to speculation that other compounds may even be stable at mere MPa pressures with room-temperature superconductivity.
Experimental pursuit
Shock-wave compression, 1996
In March 1996, a group of scientists at
Other experimental research, 1996–2004
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
Pulsed laser heating experiment, 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.[27] Hydrogen-rich molecular silane (SiH4) was claimed to be metallized and become superconducting by M.I. Eremets et al..[28] This claim is disputed, and their results have not been repeated.[29][30]
Observation of liquid metallic hydrogen, 2011
In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and deuterium at static pressures of 260–300 GPa (2,600,000–3,000,000 atm).[31][32] This claim was questioned by other researchers in 2012.[33][34] It is recently proposed that the hydrogen in stars has an electric conductivity of 1.1×106 S/m.
Z machine, 2015
In 2015, scientists at the Z Pulsed Power Facility announced the creation of metallic deuterium using dense liquid deuterium, an electrical insulator-to-conductor transition associated with an increase in optical reflectivity.[35][36]
Claimed observation of solid metallic hydrogen, 2016
On 5 October 2016, Ranga Dias and Isaac F. Silvera of
In the preprint version of the paper, Dias and Silvera write:
With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a
Drude free electron model to determine the plasma frequency of 30.1 eV at T = 5.5 K, with a corresponding electron carrier density of 6.7×1023 particles/cm3, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.— Dias & Silvera (2016)[39]
Silvera stated that they did not repeat their experiment, since more tests could damage or destroy their existing sample, but assured the scientific community that more tests are coming.[40][41] He also stated that the pressure would eventually be released, in order to find out whether the sample was metastable (i.e., whether it would persist in its metallic state even after the pressure was released).[42]
Shortly after the claim was published in Science, Nature's news division published an article stating that some other physicists regarded the result with skepticism. Prominent members of the high pressure research community criticized the claimed results,[43][44][45][46] questioning the claimed pressures or the presence of metallic hydrogen at the pressures claimed.
In February 2017, it was reported that the sample of claimed metallic hydrogen was lost, after the diamond anvils it was contained between broke.[47]
In August 2017, Silvera and Dias issued an erratum[48] to the Science article, regarding corrected reflectance values due to variations between the optical density of stressed natural diamonds and the synthetic diamonds used in their pre-compression diamond anvil cell.
In June 2019 a team at the
W. Ferreira et al. (including Dias and Silvera) released a preprint in September 2022 claiming to have repeated the experiment, finding metallisation of hydrogen between 477 and 491 GPa. This time, the pressure was released to assess the question of metastability. They reported that metallic hydrogen was not found to be metastable to zero pressure, and that transformation to the molecular phase likely occurred between 113 and 84 GPa. The authors plan to study the metallisation and metastability of deuterium in the future.[50]
Experiments on fluid deuterium at the National Ignition Facility, 2018
In August 2018, scientists announced new observations[51] regarding the rapid transformation of fluid deuterium from an insulating to a metallic form below 2000 K. Remarkable agreement is found between the experimental data and the predictions based on quantum Monte Carlo simulations, which is expected to be the most accurate method to date. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[52][53]
See also
- Hydride#Interstitial hydrides or metallic hydrides
- Hydrogen safety#Cryogenics
- Juno (spacecraft)
- Metallization pressure
- Slush hydrogen
- Timeline of hydrogen technologies
References
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