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Date: Fri, 11 Apr 2008 22:44:30 -0400
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Subject: carbon-carbon composites
From: "bob mackey" <n103md@yahoo.com>
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> I think I missed something.
> What is carbon/carbon and what are the temp characteristics?

Craig:
I'm glad you asked. Most of the previous replies on this subject
have been misinformed.

Carbon (graphite fiber) retains its strength and stiffness well
up to about 2000 C. Yes... that's about 3600 degrees Fahrenheit.
It does start to oxidize at about 600 C (~1100 F) in air though,
so it is difficult to test to its full temperature range.

Some have mentioned that "carbon composites" lose strength at
120 F or some other low temperatures. Actually, the EPOXY softens
at temperatures varying with the type of epoxy. There is a wide
range, but typical values of Tg (glass transition temperature)
http://www.siliconfareast.com/tg.htm
are around 135 C. There are higher temperature resins like
phenolic or bis-maleimide with Tg of 200 C or more.
One of these days I'll rebuild my engine baffles with
carbon-bismaleimide composite instead of repairing the
cracked aluminum.


The highest temperature "resin" in common use is carbon itself.
Hence "carbon-carbon composite". That's carbon fibers with carbon resin,
forming a composite matrix.
http://www.composites-by-design.com/carbon-carbon.htm
The carbon resin can be pitch that is infiltrated into the carbon fiber
preform then fired to drive off the hydrogen. The pitch shrinks
leaving voids, so the process is repeated a few times. Other methods
use gas-phase carbon sources to carry the carbon into the matrix.
Because of the shrinkage, a final machining operation is often used
to make the net shape part.

The end result is lighter and stronger than steel, and suitable for
use at extreme temperatures except for oxidizing flames.
Space shuttle re-entry tiles and many rocket engine nozzles are
made of C-C.
http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_sys.html

If you want it to look metallic AND be made of lightweight
carbon-carbon, you could nickel-plate the carbon-carbon exhaust tunnels. :-)

-bob mackey