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Metallic Hydrogen

28/02/2017

A research team is claiming that they have synthesised metallic hydrogen in a lab. If true, this will have large implications for the field of rocketry…


Hydrogen is the most common element in the universe. Roughly 75 percent of the universe is comprised of this simple molecule, which mostly exists as a proton-electron pair, a lone proton, or in a plasma. To most of us here on Earth, w​e typically think of hydrogen as a gas. However, humans have been able to synthesise liquid hydrogen since James Dewar first liquefied hydrogen in 1898. Dewar later synthesised solid hydrogen in 1899. However, there is a key difference between solid hydrogen and metallic hydrogen. Solid hydrogen is comprised of​ discrete H2 molecules arranged to form a solid. In metallic hydrogen, these H2 molecules have dissociated into a solid lattice of protons with delocalised electrons between the protons – the structure of a metal. This difference means that metallic hydrogen requires immense pressures to create – it is theorised that in our solar system, metallic hydrogen might be found at the core of Jupiter. Scientists have long tried to recreate these conditions in labs here on Earth, but now it seems that they may have had some success.​

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An artist’s impression of a diamond anvil, the apparatus used by researchers to create very high pressure environments (Source: P.Dalladay-Simpson/E.Gregoryanz)

 

Metallic hydrogen is theorised to be “metastable”. Metastability is a physics concept relating to the idea of entropy and energy states. It is a known principle that an object’s entropy tends to naturally increase, which causes the energy state of the object to decrease. A metastable object is not at its lowest possible energy state, however it will not change any further without an input of energy. A good analogy is to imagine a ball sitting at the top of a hill as being an object in its highest energy state. When the ball begins rolling down the hill, its entropy is increasing and its energy state is decreasing. However, if the hill has a hollow, as shown in the diagram below, the ball will come to rest before reaching the bottom of the hill – energy is required to push the ball into reaching the lowest energy state.

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A diagram of an object reaching its stable and metastable states (Source: Georg Wiora, Licence: CC BY-SA 3.0)


This has led some to theorise that metallic hydrogen could remain in its metallic form at room temperature and pressure. This would have massive implications for various fields. For one, metallic hydrogen is hypothesised to be a superconductor at room temperature. A superconductor is a material that has an electrical resistance of 0, allowing massive amounts of current to be transferred across it. However, the only superconductors we know of at the moment need to be cooled close to absolute zero in order to attain 0 resistance. Metallic hydrogen being a superconductor at room temperature would enable the construction of massive nation or world-wide electricity grids due to there being no transmission loss of power. It would also make procedures that involve superconducting magnets like magnetic resonance imaging (MRIs) cheaper, as liquid nitrogen and oxygen are no longer required to cool the circuitry.

But metallic hydrogen could impact fields outside of electronics. In particular, it is theorised that metallic hydrogen could be a very efficient rocket fuel. Modern rockets work by combining chemicals in reactions that release large amounts of energy. This energy drives the exhaust out of the rear of the rocket, providing thrust. In our current rockets, molecules such as O2 and H2 have to be split into their component atoms before they can combine into the product chemicals. This initial decomposition takes energy, which draws power away from the net power driving the rocket. Metallic hydrogen, on the other hand, is already in the form of its component atoms. As such, it can simply combine, without needing to split. This would result in a massive increase in rocket power; a modern hydrogen-oxygen rocket has a specific impulse – a measure of the change in momentum per unit of propellant provided by the rocket – of roughly 460 seconds. Theoretical modelling places the specific impulse of a metallic hydrogen-fuelled rocket at 1700 seconds; more than 3.5 times greater than the hydrogen-oxygen rocket.

Increasing the efficiency of the rocket in this manner could expand the scope of space exploration through reduction of the “tyranny” of the rocket equation. This is an often-referred-to characteristic of the Tsiolkovsky rocket equation, which states that the more fuel you need to reach your destination in a rocket, even more fuel is required due to the increased mass of the craft. By making a highly-efficient rocket fuel, the impact of this effect is reduced, allowing spacecraft to reach further on smaller tanks, or reach the same destinations faster or more economically. If metallic hydrogen were to be metastable, it could potentially revolutionise the field of space travel.

Metallic hydrogen could potentially be a revolutionary discovery for the field of rocketry, providing an incredibly powerful and abundant source of fuel for spacecraft. However, it is important to note that this is all still theoretical. The metallic hydrogen sample that was reportedly created by physicists at Harvard was lost or destroyed when the diamond anvil containing it shattered, and there are now doubts that the sample had even become metallic in the first place. However, work is still ongoing to create what is called the “holy grail” of high-pressure physics, and it seems that humanity is closer than ever to finding it.​