Hydrogen squeezed into solid metal,could revolutionize rocket fuel

Image credit: R. Dias and I.F. Silvera

Researchers at Harvard University have managed to create a potentially revolutionary material: solid metallic hydrogen in the lab for the first time.

The highly elusive, electrically conductive (even at low temperatures) material, which was only imagined in theories by physicists over the past several decades is finally a reality.

Harvard physicists Ranga Dias and Isaac F. Silvera found a way to develop Metallic Hydrogen, a bizarre form of the hydrogen element simply by squeezing a sample of the element to incredibly high pressures between two ultra-pure diamonds. The applied pressure was close to 1.5 times the pressure that exists at the center of the Earth, before it finally underwent the transition that had been predicted several decades ago.

The creation of the solid metal hydrogen could revolutionize rocket fuels or producing magnetic-levitating trains/MRI machine one day.

Earliest Predictions: Long-sought material

In 1935 Princeton University physicists Eugene Wigner and Hillard Bell Huntington predicted that beyond high pressures of around 25 gigapascals (GPa) (about 246,000 times atmospheric pressure) non-conductive solid molecular hydrogen would would become metallic similar to the way carbon atoms can form into diamonds under high pressure conditions.

As per the prediction, high pressures could force the normal bonds between solid hydrogen atoms to break down, freeing electrons to move around. In simple terminology, the normally transparent material would become shiny and reflective, and have other metals associated properties.

Since 25 GPa is a pressure over 200 times that at the deepest point of Earth: bottom of the Mariana trench, their claim was impossible to verify.

With increase in the knowledge of the quantum world, physicists found ways to produce higher and higher threshold pressures, still with no sign of a solid metal.

More recent research found that the pressure needed for hydrogen transition was even higher at between 450- 500 GPa— pressure that is even more only deep at the core of dense planets.

“This is the holy grail of high-pressure physics,” said Silvera. “It’s the first-ever sample of metallic hydrogen on Earth, so when you’re looking at it, you’re looking at something that’s never existed before.”

This is how they created the material

They pushed their diamond anvil cell into an previously unexplored realm of low temperature and extreme pressure conditions, avoiding use of monitoring by laser illumination out of the fear to cause an anvil’s diamonds to fail and heating of the black sample they obtained.

Chart of transition involving three types of hydrogen, ranging from insulating to a metallic conductor

Stages of creation of metallic Hydrogen, R. Dias and I.F. Silvera,Image credit: Harvard Universit and New Scientist

To create the material, a very small amount of liquid hydrogen sample was crushed between the flattened tips of two specially-coated synthetic diamonds by inserting into a diamond anvil cell. As they neared 495 gigapascal, or more than 71.7 million pounds-per-square inch, extreme pressure breaks down the tightly bound molecules into atomic hydrogen.

The initially obtained black sample became shiny and slightly reddish. Silvera explains the obtained result is a metal, means it is a substance with all the basic properties of a metal i.e. that is solid, malleable, ductile, shiny and a good conductor of heat and electricity.

But how did they achieve such high pressures?

Silvera and his postdoctoral researcher, Ranga Dias say they’ve followed a rigorous strategy to master diamond failure which is the principal limitation under the exceedingly higher pressures required to observe metal conversion.

For this process instead of diamonds dug from the earth which have inconsistencies in their structure, synthetic diamonds were used which can be produced without such inhomogeneities.

These microscopic defects can be failure points where diamonds start to crack, they exclaimed.

To make their diamonds more robust and prevent from cracking, the tips were carefully etched using reactive ion etching to remove surface defects, and vacuum annealed(heated) to higher temperatures to remove any residual stresses.

A second point of failure in these experiments is caused due to hydrogen diffusion into the diamonds due to the insanely high pressures or temperature . This results in diamond embrittlement or cracking from hydrogen diffusion. So the team used diamond anvils coated in alumina, the same material which is found in sapphire, a compound of aluminium and oxygen prevents diffusion of hydrogen through.

Also, to suppress the diffusion, the whole system was maintained at low temperatures using liquid nitrogen or liquid helium during the experimental runs.

Meta-stability

“The obtained metallic hydrogen is predicted to be meta-stable,” Silvera said

That means it will stay metallic, even if you take the pressure off, very similar to how diamonds form from graphite under extreme heat and pressure, but remain diamonds once the heat and pressure are removed.

Right now, scientists don’t know much about the material’s properties. The whole experimental setup is still sitting under high pressure in the lab, waiting for the next tests.

Extraordinary Applications

If the metallic hydrogen maintains its properties even after the high pressure is removed, it’s possible it could be used to make a room-temperature superconductor, Silvera said.

This could be helpful in producing magnetic-levitating trains or MRI machines that do not require the material to be cooled to liquid helium temperatures.

The researchers confidently predict that metallic hydrogen will behave as a superconductor at room-temperature  and could possibly sustain, once created, at normal pressure conditions.This would mean it could be could potentially be used to make electric wires out of it that carry electricity to far distances without any power dissipating.

Revolutionizing rocket Industry

Basically because the whole process to squish hydrogen into its solid metallic state takes so much energy- “If you cause it to convert back to molecular hydrogen, they release huge amounts of energy, it’s also predicted to be the most powerful rocket propellant known to man, and could revolutionize rocketry”, Silvera believes.

And because hydrogen is the lightest known element, it would be a lot more lighter than existing rocket fuels.

The whole research is published in the latest issue of the journal Science. The very basics of the achieved feat  is explained in the following promotional video.

Source Science mag Harvard university

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