thermal radiation shields

Hi folks,
I'd appreciate any guidance I can get on the suitability of a couple of
different choices of metals in a high temp inert atmosphere furnace
application.
We are building a new series of small 1750 C furnaces for our
instruments, to be used in measuring glass and metal properties in the
molten state. The furnace, which is roughly 2.5" wide and 3 inches
working length, is based around a graphite element, and that is
surrounded by radiation shields, to insulate the core. We are using
molybdenum sheet at the moment for the shields, but as you know this is
very sensitive to oxygen, creating large volumes of MoO3 (?) "snow", and
several other oxides, like dark violet Mo2O5. We have an inert gas purge
of argon or nitrogen feeding the furnace, and we don't even switch on
until we have well under 0.5% O2 present.
Still though, we get a trace of what can only be described as a soot
formation. The soot seems to be evolved from the moly, and while iits
is more an irritant than a serious problem, we wondered whether there
was any good reason NOT to use Tungsten instead ? Are there "snowy" or
"sooty" reaction products ?
We DID wonder whether we could actually use graphite as shields too.
Does anyone have any comments on that idea ?
Thanks for any pointers.
Steve
Reply to
steve
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Platinum just might take the temperature you cite. I remember use of platinum in oxygen containing furnaces as heating elements, but am uncertain of the max temp rating.
Graphite burns off, and helps to knock down the oxygen, but it will need replacement. Some grades are "dirty" with oxidizable inclusions.
An easily replaced "cheap" inner graphite radiation shield could be backed up by various metal shields which would be operating at lower temps, and would have fewer oxidation problems.
SiC type refractories may also be of some use.
My informal screening of refractory metals as heat shields 30 years ago, didn't turn up anything other than expensive platinum family alloys that would be good heat shields against oxygen at your temps. I am now uncertain as to how thorough that old search really was, however. Oxides tended to be poorly bound to the metal substrates, often powdery, and that led to contamination risks.
There aren't all that many oxides that cling tightly to their metallic substrates, or the use of metals at high temperatures in oxidizing atmospheres would be more common.
Reply to
jbuch
The oxygen we have is only "left over" from the purge - less than 0.5% during operation, so I rather hoped Tungsten might be acceptable.
Steve
Reply to
steve

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