What you need is a material with very high ultimate tensile strength. Carbon nanotubes might do it. Let's check.

To compute the burst pressure of a pipe, we can use Barlow's formula:

P_max = S * T /R

where T is wall thickness, R is radius of the outside of the pipe, and S is ultimate tensile strength. Here T=R is the limit where the inside radius is zero. So the best case is

P_max = S/1.0000001

Carbon nanotubes are the highest strength material, with a theoretical S=100 megabar. So not possible, sadly.

But 2 million bars = 2 Mbar. Surely 100 Mbar is enough, or did I miss something?

Bah, I messed up the Pascal-bar conversion in the text. Carbon nanotubes have S = 100 GPa = 1 Mbar. Too late to edit now. Thanks for catching this.

Thanks for these numbers! Can such pressures be attained by other means rather than permanent confinement? For example, the near wall pressure of a collapsing ultrasound bubble reaches about 100 MPa. Three orders of magnitude too little but can this technology be used to achieve much higher pressures? If so I wonder if maintaining an aggregate of material at high pressure (a high pressure foam as it were) would be sufficient to achieve bulk superconductivity.

This is far outside my wheel house, but... Surely it's the pressure differential that dictates the burst pressure? So if you had multiple sleeves, each decreasing in pressure as you get closer to the outside, this could be possible?

I'd expect that for a pipe under high pressure but small pressure differential compressive strength would matter much more than tensile strength.

Yes, it's the pressure differential that matters, but I don't think it helps in practice. Each pipe needs to have a ratio of inner to outer diameter of at least 100. If the innermost radius is 1 mm, at three layers the outside radius of the biggest pipe is 1 kilometer.

> Carbon nanotubes are the highest strength material

That we can produce.

I once asked my chemistry teacher if it's possible to make square carbon lattice, and he didn't have an answer.

      |   |   |
    ─ C ─ C ─ C ─  
      |   |   |
    ─ C ─ C ─ C ─  
      |   |   |
    ─ C ─ C ─ C ─  
      |   |   |

EDIT: looks like both Carbyne and boron nanotubes are stronger than carbon nanotubes.

A square carbon lattice would be very unstable. Carbon can support up to 4 single bonds, but they have to be arranged in a tetrahedral symmetry (sp3). 4-member carbon rings exist in some compounds, but they are unstable and reactive. Flat carbon sheets exist as graphene, which is a hexagonal grid rather than your square one.

Yes, it's possible but it requires a thick jacket. the formula for a thin-shell cylindrical pressure vessel is \sigma = rp/t, where sigma is tensile strength of the material, p is pressure, r is radius, and t is wall thickness. For thick-shell jackets, the formula is slightly more complicated [1] but you get the idea.

For Kevlar, max \sigma is 3.6GPa. So it's possible to maintain an internal pressure of 200GPa (2MBar), but the wall thickness must be much larger than the inside radius.

[1] https://tge.readthedocs.io/en/latest/content/mechanical/thic...

If you use the formula for thick walled cylindrical pressure vessel, you hit practical limit with outside diameter being twice the inside diameter. When this happens and you also take into account fatigue, usually about half of the yield strength of the material is usable.

Your best bet is using thin shells of maranging steel wrapped around each other with thin layer of pressurized non-compressible fluid in between. The pressure control of the fluid is then absolutely critical. Alternate with 0,5mm thick fluid and 1mm thick steel sections. Rinse and repeat. The total thickness would not be all that crazy, but the pressure control would be incredibly expensive.

EDIT: Did some calculations. Using roughly that system I described earlier you would only hit 10 GPa with roughly 50 meters thick piping system, that would have 75 alternating layers of pressurized oil. And each layer would have different pressure from all the other 74 layers of pressurized oil. So not possible.

If you try using single thick wall in your cylinder, increasing the wall thickness means that your max stress gets closer to the internal pressure reading. So you would need material that can deal with 200 GPa of stress.

Wouldn't that mean that we can get a 1 mm radius almost-room-temperature superconductor with a wall only 55 mm thick? That seems like it would almost be practical.

We quoted the same formula, but AFAIK the radius is the outside radius, right? So you cannot go above t/r = 1.0?

I think hope is that structures formed at such high pressures would be quasi-stable at atmospheric pressure. Such behavior is no unheard of.

Such pressures is not something our materials can handle on macro scopes. Yet. But they are naturally occurring outside diamond anvils, like deep I gas giants, etc.

It’s facing that such super high pressure chemistry may be more common than our low pressure one, because of the abandonment of raw materials in big planets.

while what you describe is imaginable, I don't think this is the case, as physicists would then speak of "superconductivity at temperature T for a material in phase X formed at pressure P", not "[ ... ] superconductivity at megabar"

Perhaps you also think the claim states room temperature conductivity, and one only needs 260K temperature to form this phase?

When the title is phrased like it is, it usually describes a point or region for the equation of state

Good point on the state. One good thing from such research results even if impractical itself can yield good targets for future research. If it’s a matter of geometry or crystal lattice spacing from the pressure an alternate formulation might be able to make a meta-stable structure that induces superconductance. Perhaps there’s another part of the state diagram that could be modified.

Indeed. It is important to pursue research even if there is no immediate benefit from it. In this case it is even touching on a gateway technology I would call marvellous. Stable high temperature superconductor material would change so much.

I believe these experiments are run in a diamond anvil (basically two diamonds pushed together onto a point). I'm skeptical that you could make a wire at this pressure with anything else -- even all the Kevlar in the world.

From what I understand this is required to create the pressure, but a sufficiently strong material may be able to maintain the pressure, although I may be incorrect.

Could the pressure be fairly neutral with current=0 and the current itself create the pressure?

Yep the article itself mentions that the tests are done in the anvil