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Looking for high temp. coating/ treatment for graphite crucibles

2004

I'm looking for a high temperature coating, or treatment, to apply onto graphite crucibles to prevent adhesion of SiO, or ZnS materials during 'evaporation' process.

The environment is high vacuum 4x10-4 Torr, temperature range is ambient up to 1800-1900 °C, time in process is 30-60 minutes. I have tried both fine powder and pellet forms of the SiO or ZnS materials. The graphite crucibles have a max. grain size of 0.0004" and withstand temps. up to 5000 deg F in vacuum.

During the process a hard crust of residual material forms on the interior of the custom ($$) made crucibles. The powdered materials are usually more consistent and easier to remove, the pellets last longer in process but leave a more 'lumpy' impenetrable crust. Brushing and spraying on BN (boron nitrate) solution has mixed results, sometimes the crust scrapes off fairly easily using a putty knife and other times it is nearly impossible to remove using a hammer and chisel!

Any suggestions offered will be appreciated.

Ken Martin
Industrial Mfr. - Bloomfield, Connecticut, USA

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I may be way off on the simplistic side here, but could you flux the crucible with bonami? That's what we do with our graphite crucibles for brass casting to keep the melt from sticking and for fluxing.

Guillaume Sudeterre
- Missouri
2004