CaF2 and material compatibility issues

Hi folks,
I hope I can get some sage words from members of the group.
We have developed a new instrument for measuring the electrical
conductivity of slags and fluxes at elevated temperatures ( up to 2273 K), but I am having serious problems with measurements on CaF2( a key specification requirement), at only 1823 K. The testprobe is made of Molybdenum and alumina, the alumina is of course soluble in CaF2, but we THOUGHT since it wasn't immersed, there wouldn't be a problem. This appears to not be true. I thought that free fluorine wouldn't attack alumina ? Could anyone suggest alternatives ? Is Boron Nitride resistant to fluorine ?
On another, related point. Does molybdenum react in fuming nitric acid ? What we thought was pure, acid resistant Mo. develops a strong reddish, loose coating in seconds of exposure.
Thanks for any guidance.
Steve Taylor
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Steve Taylor wrote:

Pyrolytic boron nitride resists CaF2 at 1570K without any problems forever. But I have no idea what happens if you have higher temperatures.
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course soluble in CaF2, but we THOUGHT since it wasn't immersed, there wouldn't be a problem.

Just a wild ass guess..
Could it be that the hot CaF2 has a considerable vapor pressure? The vapor might be erroding your probe.
-scott
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aSkeptic wrote:

there
good guess...
at these high temperatures, considerable amounts of CaF2 evaporate and coat the probe. AFAIK CaF2 does not evaporate as free fluorine and calcium. It stays together in a molecular form when in vapor phase.
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I wouldn't rely on that assumption. There can be dissociation in the vapour phase.
I discovered with alkali glasses that you get sodium metal vapour and oxygen evolved in vacuum at melting temperatures (say 1600-1700K), where popular opinion had it that the oxide was evolved. In fact I always got a ring of alkali metal on the water-cooled silica tube furnace wall near the crucible mouth.
--
Terry Harper
http://www.terry.harper.btinternet.co.uk /
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