Since graphene material is the strongest manmade material, with "magical" properties, I wonder if it could be considered for use as a heat shield, since its melting point is 3000 to 5000 K stacked at 10 layers thick, which is higher than reentry temperatures.
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23$\begingroup$ I disagree, since melting is about the least important parameter. Thermal conductivity and flash point (once a little O2 is available) seem far more interesting. $\endgroup$– Carl WitthoftNov 9, 2021 at 13:58
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$\begingroup$ For "reentry" you would also need to consider abrasive resistance properties no? $\endgroup$– RobbBNov 10, 2021 at 2:29
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2$\begingroup$ "Cardboard -- cardboard is out..." (youtu.be/3m5qxZm_JqM?t=49) $\endgroup$– Peter - Reinstate MonicaNov 10, 2021 at 17:11
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1$\begingroup$ @Peter-ReinstateMonica brilliant! You've opened up a whole new world of videos I can watch to avoid doing what I'm supposed to :-) $\endgroup$– uhohNov 10, 2021 at 23:46
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$\begingroup$ @CarlWitthoft The melting point is of such paramount importance for a classic heat shield that we mentally screen out all materials that melt early before we even spend the first conscious thought on them. It only seems unimportant. There is a reason we don't use wax or PE (or water ice, whose flash point is non-existing). $\endgroup$– Peter - Reinstate MonicaNov 11, 2021 at 17:45
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2 Answers
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For a non ablative heat shield you need a material with a very high melting point and a very low thermal conductivity. It should not burn in hot air.
Unfortunately graphene seems to have a high thermal conductivity, higher than pyrolytic graphite. In-plane thermal conductivity could help to avoid hot spots, but through-planes conductivity should be low as commented by @uhoh.
It burns at very low temperature of 350 °C, the reaction with oxygen starts below 260 °C as commented by @GremlinWranger.
Source https://en.wikipedia.org/wiki/Graphene#Thermal_conductivity
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2$\begingroup$ From the same link we also have the fact at burns in oxygen above 260 degrees. $\endgroup$ Nov 9, 2021 at 13:05
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4$\begingroup$ For highly anisotropic materials like stacked graphene the in-plane thermal conductivity (what you'd want for heat redistribution and avoiding hot spots) may be much higher than conductivity through the stack; and it may be much harder to find an actual measurement of through-stack conductivity for stacked graphene material proper. $\endgroup$– uhohNov 9, 2021 at 13:08
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2$\begingroup$ @Gremlin yes hot oxygen will be its kryptonite! $\endgroup$– uhohNov 9, 2021 at 13:17
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1$\begingroup$ @uhoh yes, it is, but rather too thin for this at the moment. As it grows thicker, to stay graphene-like it would probably have to be a collection of thin stacks (of the type you linked) rather than one thick stack $\endgroup$– Chris HNov 9, 2021 at 13:19
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2$\begingroup$ Yes, the idea, ultimately, is to not pot roast the astronauts - not simply to ensure the structural integrity of the vehicle... hence the "shield" part of "heat shield". $\endgroup$– J...Nov 9, 2021 at 14:53
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“Sheets of stacked graphene”? Ok: look no further than graphite, AKA carbon. It very much is used for its thermal properties, in applications such as electrodes that can withstand molten aluminium.
And, indeed, heat shields. Specifically, carbon fibre reinforced carbon, which is what the Space Shuttle's leading edges were made of.
Now, ok, graphite isn't actually what you would understand by “stacked graphene”. It's only tiny flakes of graphene that are barely coupled together. But for a heat shield, that's actually a good thing, because heat conductance works excellent along the sheet but not so well between the layers. On the other hand, graphite has pretty rubbish mechanical properties because the flakes easily separate from each other, that why you need reinforced graphite.
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$\begingroup$ @uhoh graphite is stacked graphene, it's just too many stacked to display the quirks of the 2D geometry, and too weakly/unorderly coupled to see the mechanical and conductance superpowers. But it's still what it is, and indeed the thermal stability is one of the ways where graphite behaves just like any other arrangement of graphene. $\endgroup$ Nov 11, 2021 at 0:02
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1$\begingroup$ But stacked graphene is not necessarily graphite. Stacked graphene can be much more. They are not synonymous, a lump of graphite is not representative of all forms of stacked graphene. It's not superconducting for example! en.wikipedia.org/wiki/Twistronics and en.wikipedia.org/wiki/Bilayer_graphene $\endgroup$– uhohNov 11, 2021 at 0:19
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$\begingroup$ @uhoh definitely, but the question was not about some particular configuration of graphene, just about “stacked graphene”, of which graphite is an example. In many ways it's a “crippled” and boring example of stacked graphene, but heat shields happen to be an application where it works. — Admittedly my answer was phrased a bit bad as well. $\endgroup$ Nov 11, 2021 at 9:01