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Science Forum Index » Nanotechnology Forum » Nanotube Ceramic
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| sanman |
 Posted: Thu Sep 18, 2003 7:02 am |
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A nanotube ceramic developed at UC Davis has far greater conductivity,
superior mechanical properties and also has unique thermal diode
characteristics:
http://www.sciencedaily.com/releases/2003/09/030917072853.htm
Gee, I wonder if this could lead to a better turbine blade, or even
thermal re-entry paint shield for a spacecraft? I read that NASA may
be rushing to return to and revitalized the space capsule method of
space transportation, and a paint that could be re-applied after each
re-entry might be better than a heavy heat shield. Maybe even Burt
Rutan's SpaceShipOne could benefit.
A thermal diode type of material would help redistribute heat across
the entire surface of a re-entry craft, while preventing the heat from
penetrating towards the interior. |
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| Gordon D. Pusch |
| Posted: Thu Sep 18, 2003 9:00 am |
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manofsan@yahoo.com (sanman) writes:
Please go back and re-read the complete article more carefully before you
jump to unphysical and unjustified conclusions like this. It does =NOT= say
this material has "thermal diode characteristics" --- which BTW would violate
the Second Law of Thermodynamics.
What is says (in an admittedly clumsy and confusing way) is that this
material has ANISOTROPIC thermal conductivity: Heat diffuses more readily
_PARALLEL_ to the nanotubes than _PERPENDICULAR_ to them. (This is hardly
a surprise, since ballistic phonons transport along nanotubes was observed
years ago; hence, if reinforcing nanotubes can be given a common orientation
rather than a random orientation, the thermal conductivity of the composite
material will become a tensor quantity.)
Quote: Gee, I wonder if this could lead to a better turbine blade, or even
thermal re-entry paint shield for a spacecraft? I read that NASA may
be rushing to return to and revitalized the space capsule method of
space transportation, and a paint that could be re-applied after each
re-entry might be better than a heavy heat shield. Maybe even Burt
Rutan's SpaceShipOne could benefit.
Non sequitur: This stuff is not a "paint" that can be easily "re-applied" ---
it is a _CERAMIC_ grown from a powder by "spark plasma sintering." If you
google on this process, you will find that it requires that an electric
current be passed though the powder while it is under high applied pressure;
this strongly suggest to me that this material is going to need to be
manufactured as small tiles again, and could not be directly applied to the
entire vehicle in one piece and in situ.
Quote: A thermal diode type of material would help redistribute heat across
the entire surface of a re-entry craft, while preventing the heat from
penetrating towards the interior.
It could not "prevent" it from reaching the interior; at best, it would
slow down heat diffusion into the interior a bit, just like any other
thermal insulator.
Furthermore, to do what you suggest would require a material whose thermal
conductivity parallel to the skin is orders of magnitude larger than
perpendicular to the skin. However, I do not expect the difference between
the "parallel" and "perpendicular" thermal conductivities to be many orders
of magnitude, or even a full order of magnitude, because the nanotubes
constitute less than 10% of the ceramic matrix; I expect the conductivities
to differ by only a factor of a few, and the "perpendicular" conductivity
to not be much smaller than bulk Alumina. Hence, I would not be surprised
if even the "perpendicular" thermal conductivity of this material may well
be much larger than the ceramic foam NASA currently uses on the Shuttle.
Finally, note that the material is grown from a _POWDER_ by plasma sintering,
which strongly suggests that like most sintered materials, it will have a
polycrystalline structure composed of randomly oriented microcrystals.
Hence, the "bulk" material may well not even be thermally anisotropic!
For your application, you must instead presupposes that a method can be found
to orient all the individual microcrystals of the sintered material so that
they all point parallel to the desired direction; AFAIK, such a method does
not yet exist.
-- Gordon D. Pusch
perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;' |
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