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If we could get a material that could withstand temperatures of 7500 K, there would be a whole lot that we would be able to do that we aren't currently able to. Direct, winged entry by spaceplanes is one example. We could build nuclear rockets with higher Isps, we could build more efficient rocket engines, missions to the surface of the Sun, aerobrakes in the Jupiter and Saturn systems, light gas guns or magnetic launchers to launch directly into orbit from the surface of the Earth, and also vastly more efficient power conversion. Such an invention would be transformational.
So, how to do it? The material which melts at the highest temperature, Tantalum Hafnium Carbide, melts at 4500 K, so we have a long way to go.
In general, the math governing melting is the gibbs free energy. The relevant equation is:
delta G=delta H -T delta S
Where delta G is the change in gibbs free energy, and negative values indicate that a reaction is favorable and therefore will occur. delta H is the "enthalpy" of a reaction, which is how much heat it absorbs when it happens. A negative value means that a reaction releases heat, while a positive value means it absorbs heat. Enthalpy of melting and sublimation reactions are positive. T is the temperature, measured in Kelvin. delta S is the change in entropy. Entropy is related to the disorder of the system, and I don't really understand it, but it's typically positive for melting or sublimation reactions.
At the melting point, delta-G will be zero.
Basically, the challenge is that you need something that's very strongly held together, but has (relatively) high entropy as a solid and (relatively) low entropy as a liquid/vapor. Any ideas where we could go with this?
-Josh
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I have no clues about calculating such things from first principles. I do know that zirconia makes a semi-practical material, good to about 4000 F (near 2500 K).
There are some rare-earth oxides and carbides that have been identified as "ultra-high-temperature-ceramics", or UHTC's. The best of these will go to around 7000 F (near 4000 K). These are heavy, have some structural strength in shear and compression, seem resistant to thermal shock, but conduct heat in copious quantities. They're nose tips, not insulators at all. You have a truly enormous backside heat removal problem with them.
Remember my alumino-silicate heat shield paper in Denver? I'm looking pretty closely at fibrous zirconia in porous forms now, for another combustor insulator job. The problem is structural: nobody makes a zirconia cloth as coarse as the alumino-silicate fire curtain cloth I used so long ago, which resembled boat cloth in fiberglass work. The problem is also porosity for low thermal conductivity, the antithesis of structure, unfortunately.
I am iterating toward a two-layer/two-material approach, trying to get the 4000 F temperature resistance of the zirconia next to the fire, and the structural strength of the alumino-silicate composite on its backside, where things are much cooler. What works as a low-conductivity combustor insulator might also work as a retro-radiating refractive heat shield, if surface-coated black.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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I notice zirconium diboride is actually layers of hexagonal mono-atomic boron, with zirconium. Titanium diboride, tantalum diboride, hafnium diboride, are even magnesium diboride are the same. But zirconium carbide is face centred cubic. Could you make zirconium dicarbide with the same structure as zirconium diboride? That is a layer of graphene alternating with a single atom layer zirconium?
Or does the fact boron has 3 valence electrons create an ionic hexagonal layer, the forms an ionic bond with the metal? What if you alternated graphene with an electron donor on one side, an electron acceptor on the other? Such as sodium and chlorine? Or lithium and fluorine? So graphene, lithium, graphene, fluorine, repeat. Would that create a strong ionic bond holding layers together? Or would it be unstable?
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I have no clues about calculating such things from first principles. I do know that zirconia makes a semi-practical material, good to about 4000 F (near 2500 K).
There are some rare-earth oxides and carbides that have been identified as "ultra-high-temperature-ceramics", or UHTC's. The best of these will go to around 7000 F (near 4000 K). These are heavy, have some structural strength in shear and compression, seem resistant to thermal shock, but conduct heat in copious quantities. They're nose tips, not insulators at all. You have a truly enormous backside heat removal problem with them.
Remember my alumino-silicate heat shield paper in Denver? I'm looking pretty closely at fibrous zirconia in porous forms now, for another combustor insulator job. The problem is structural: nobody makes a zirconia cloth as coarse as the alumino-silicate fire curtain cloth I used so long ago, which resembled boat cloth in fiberglass work. The problem is also porosity for low thermal conductivity, the antithesis of structure, unfortunately.
I am iterating toward a two-layer/two-material approach, trying to get the 4000 F temperature resistance of the zirconia next to the fire, and the structural strength of the alumino-silicate composite on its backside, where things are much cooler. What works as a low-conductivity combustor insulator might also work as a retro-radiating refractive heat shield, if surface-coated black.GW
You might want to try the NASA solicitation described in this article:
NASA SEEKING ULTRA-LIGHTWEIGHT MATERIALS TO ENABLE MISSIONS TO MARS.
OCTOBER 28TH, 2014 J.D. TAYLOR
http://www.spaceflightinsider.com/organ … ions-mars/
The due date for the Notice of Intent has been modified a few times through appendices to the solicitation. Now it's March 20, 2015, with proposals due April 17, 2015:
http://nspires.nasaprs.com/external/vie … 15ECF).pdf
The full solicitation is here:
NASA RESEARCH ANNOUNCEMENT (NRA):
NNH15ZOA001N
Space Technology Research, Development,
Demonstration, and Infusion-2015
(SpaceTech-REDDI-2015)
http://nspires.nasaprs.com/external/vie … -29-14.pdf
Bob Clark
Last edited by RGClark (2015-03-02 16:07:11)
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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I want a material with a very high specific heat capacity. According to a quick glance at Wikipedia, Hydrogen has one of 14.3 kJ kg^-1 K^-1. Unfortunately, it's a gas...
A very high SHC would allow us to make very good heat sinks, which would enable much better thermal energy storage... and also significantly improve the prospects for stealth in space.
Use what is abundant and build to last
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I want a material with a very high specific heat capacity. According to a quick glance at Wikipedia, Hydrogen has one of 14.3 kJ kg^-1 K^-1. Unfortunately, it's a gas...
A very high SHC would allow us to make very good heat sinks, which would enable much better thermal energy storage... and also significantly improve the prospects for stealth in space.
Perhaps H2 could be used for the fill gas for a ballute. How much total energy would need to be dissipated by a heat shield on reentry?
Bob Clark
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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