A milestone in the hunt for metallic hydrogen
nature.com
nature.com
And if it were strong enough to contain the pressure, why not make the whole thing out of the same metal?
I'm a little sorry to have used such "uncivil" language ... but also, I'm not.
I'm not in this field, but I know several experts in it. I've helped out on experiments and have loaded DACs myself up to 20GPa or so.
I'll describe how it works in practice at that kind of pressure, and then what I understand of the different techniques needed for the extreme pressures used in this paper.
Loading the DAC is fiddly and done by hand. You start off looking through a microscope at the bottom side of the DAC. The lower diamond is pointing up at you. The tip of it is truncated at the point to give a flat end face around 100um across. With tweezers and a steady hand, you place your gasket - basically a rhenium washer with a maybe ~50um inner diameter - on top of that, with the outer edge of the gasket overhanging the edges of the diamond tip.
Then you need to drop two things into the hole in the middle. A little fragment of your sample material, and a tiny piece of ruby. The ruby is your pressure gauge! Ruby fluoresces, at a wavelength that varies with pressure in a known way. So when the cell is loaded, you can shine a laser through the ruby and look for the spectrographic line of the fluorescence to get the pressure.
Now you have your sample and your ruby sitting inside the gasket, but they're surrounded by air. That's not going to work. You need the space around them to be filled with a liquid, to act as a medium to distribute the pressure. So you take a little syringe and plop a drop of whatever your chosen liquid is into the hole. There are different things you might use depending on the experiment.
Now that you have everything in there ready, you can attach the top side of the DAC and bring the second diamond down onto the top of the gasket. The cell will need to have been carefully aligned beforehand to ensure that the diamond tips meet up flat in the middle.
With everything together, you can now start applying pressure. In simple DACs this is literally just a case of tightening screws around the outside by hand. For more precise/automated control, you can get cells with a built in gas membrane.
But your question was about the gasket - what role does it play? Well, firstly you need something there to hold your sample and your ruby and your liquid in place while you prepare and load the cell. If you just pushed the two diamonds together the liquid would just flow out, the diamonds would hit the sample or the ruby and crush them at a point, rather than compressing them under hydrostatic conditions.
But you need the gasket to give way when you start applying pressure, which is what it does. It gets thinner, and material is pushed out of the sides. When you eventually unload the cell, you can look at what's left of the gasket and see the imprints of the ends of the diamonds in it.
You still need the gasket to be made out of something very strong so that it doesn't pull apart completely. But it's playing a very different role to the diamonds. They need to be hard. The gasket needs to be tough.
The diamonds, by the way, usually sit in tungsten carbide seats, because that's hard enough to take the pressure transmitted through the wide end of the diamond, which is still pretty high. The carbide seats then widen out further to the point where the rest of the cell can be made from stainless steel.
But that's all at the measly 20GPa or so that I'm vaguely familiar with.
What I got from chatting to an expert about this paper was something like the following.
425GPa is crazy pressure. That's four million atmospheres. At this point you're actually beyond the compressive yield strength of diamond. If you tried to use flat-ended diamonds like I described, they'd shatter before you got half way to that pressure.
Instead, they use a focused ion beam to machine a recess in the face of each diamond, giving what's called a toroidal cell. You can actually see that in the compressed shape of the gasket in Figure 1a. The force is distributed in a ring around the outside, so that it stays within the range of what the diamonds can handle.
The sample in the middle is being compressed from above and below by the diamonds, and also from the sides by the gasket material being pushed inwards by the raised rings on the diamond faces coming together. So you're actually using it to help pressurise the sample even though you're way past the yield strengths of both materials.
It will have required some incredible capabilities to design, build, align and load that cell. Even aside from the question of metallic hydrogen, there's some serious showing off here just in terms of techniques.
You could say the small center area of pressure is acting on a very thick cross-section of gasket (like a big pipe with a tiny inside diameter), but isn't there a limit to this where the pressure doesn't care how much extra material you pile on?
Or for a more layman example, Jello has some amount of tensile strength, but you can't exactly build a cannon out of it regardless of how thick the barrel is. Metal at these pressures probably behaves more like jello than metal, so what's keeping it together in this case?
> I think a more refined version of your parent poster's question might be, why doesn't the gasket continue to extrude itself out of the gap, even though its ultimate strength has been exceeded by possibly several orders of magnitude?
It does continue to extrude itself out of the gap. The more you increase the pressure, the more gasket material comes out. And this isn't an elastic, reversible process - it's plastic deformation which leaves the gasket permanently thinner after you unload the cell. So the gasket material has failed, in terms of what we'd usually use "failure" to mean in an everyday mechanical structure.
But your expectation seems to be that beyond the point of failure, the gasket material should basically just flow freely, like a liquid. That's not what happens. Failure is just the point at which permanent damage starts to occur. There's a whole separate region of behaviour beyond that point, which depends on the material and the conditions.
I mentioned that in this work, they go beyond the yield strength of diamond. If you look at Fig 6 in the actual paper [0], they have an electron microsope image of the diamond tip after unloading the cell. It's permanently and severely damaged, with concentric ring cracks around the tip. So the diamond did "fail", in the formal sense of that word. But that doesn't mean it didn't do the job they needed it to do anyway.
> You could say the small center area of pressure is acting on a very thick cross-section of gasket (like a big pipe with a tiny inside diameter), but isn't there a limit to this where the pressure doesn't care how much extra material you pile on?
There probably is! But that's not what failure means, and so it's not what numbers like yield strength refer to. Failure for your big pipe is the point at which the internal pressure starts to permanently bulge the pipe. It's not necessarily the point at which it ruptures. That can happen at any point beyond failure, depending on the material and the conditions.
> Or for a more layman example, Jello has some amount of tensile strength, but you can't exactly build a cannon out of it regardless of how thick the barrel is. Metal at these pressures probably behaves more like jello than metal, so what's keeping it together in this case?
Let's take a layman's example that better fits the question at hand, and requires a bit less hypothesising about jello.
Rubber has some amount of tensile strength, but you can't build a car's engine block out of it.
It works just fine for the head gasket, though, where it has to successfully contain the extreme pressures that occur in the cylinders during the combustion cycle. It works in practice in the conditions where it's compressed between the head and the block.
And that's exactly the role that the rhenium gasket plays in a DAC.
In both cases, there is some pressure beyond which it will no longer do the job. But there probably isn't a simple number you can look up which will tell you when that will happen, based only on the material.
[0] https://sci-hub.se/https://doi.org/10.1038/s41586-019-1927-3
Pressurizing is done carefully so that nothing important is sacrificed to flowing or a flowable channel of metal! The ruby can help, with staged failsafes of pressure removal e.g. of hydraulic oil to mics pressurizing the anvil.
On Gaskets: https://pdfs.semanticscholar.org/1d2c/f4b386e2113f863d46cec5...
Doesn't cover doing it at microkelvin with a helium still co-located, which seems special! Can't one just be happy with experimental cosmology (to find a purified hydrogen body in telescope range...)
I'm not a physicist but I've long suspected that Jupiter has a metallic hydrogen core.
Lower mass elements would be displaced by higher mass elements like uranium, thorium, gold, platinum, etc...
Well you might want to go over to wikipedia and correct the articles: https://en.wikipedia.org/wiki/Jupiter https://en.wikipedia.org/wiki/Planetary_core
Like I said, I am not a I'm not a physicist or astrophysicist, I am just a software engineer. But in 2017 when I attempted to model the interior of gas giants... my model predicted a core surrounded by high density regions. The model resembled a close-packed lattice.
https://en.wikipedia.org/wiki/Close-packing_of_equal_spheres
I was just playing around with math so I am probably wrong.
Best Wishes, -David Delaune
Its abundance on a given body is a function of that overall abundance, and the gravitational pull of the body in question. At Earth's mass, free hydrogen (and helium) don't persist as the molecular speed exceeds Earth's escape velocity, and these gasses gradually bleed off into space.
(The Earth is thought to have lost about a quarter of its original water through this mechanism over 4.5 billion years.)
At Jupiter's scale, its ability to hold on to hydrogen, and the overall prevalence, dominates. Yes, there's probably some dense rocky / metallic core at the absolute centre, but Jupiter's core itself is thought to be substantially composed of extremely high-pressure hydrogen.
Why that's thought is an interesting question, though my understanding is that it's largely based on Jupiter's known diameter, its known mass (determinable through orbits of its moons), and chemical properties (including density). This is conjecture and may be wrong.