I'd like to melt alumina in a slow, controlled manner. So I thought a graphite crucible with a tungsten wire wrapped around it for heating would be a good start. To reduce heat losses, I put that crucible inside an outer crucible. Question 1 - can I get away with Al2O3 for the outer one, given that the radial temp profile will be something like 1/r . If not Al2O3 then what - Al2O3 would be nice as it has the lowest thermal conductivity that I can find of the refractory materials I looked at (W,SiC, C, BN) . And should I fill the annular space with e.g. YSZ powder (the quartz fiber I used to use for this kind of thing wont handle 2000C - is there some other fiber or 'wool' that can be used?) Finally, can I zets the melt from above with microwaves conducted thru a waveguide - will the electrical conductivity of the graphite be trouble or maybe help matters due to eddy currents?

  • $\begingroup$ A possible concern with the graphite crucible is contamination, sublimation and ignition. Would you be better off using zonal melting and horizontally directed crystallization. per crystaltechno.com/Al2O3_en.htm , which does not contact a crucible, with contamination issues? See also universitywafer.com/sapphire-wafers-al2o3.html $\endgroup$ Feb 3 '21 at 20:21
  • $\begingroup$ I'll read up on those methods - the crucible method seems more easily controlled than the Verneuil flame method - but I suppose crucible contamination may be possible. I don't know if there's any reactions expected bet. liquid Al2O3 and graphite. I don't think either material will ignite as both the Al2O3 and graphite are quite stable (and the Al2O3 of course is already 'burnt'). In principle I guess I could use a tungsten crucible (wow, even platinum is already liquid by 1770C) $\endgroup$ Feb 3 '21 at 22:09
  • $\begingroup$ Having become aware the Pt is also liquid at my temps of interest, how do I measure temperature - the Pt/PtRh thermocouple clearly won't suffice...maybe there's a W/W+something thermocouple ...yup there it is , tungsten/tungsten+rhenium , up to 3000C $\endgroup$ Feb 3 '21 at 22:31
  • $\begingroup$ If platinum thermocouples won't do it , the next step is optical/ infrared. /Thorium oxide fiber will tolerate your temperature . $\endgroup$ Feb 4 '21 at 2:17
  • $\begingroup$ Tungsten oxidizes at high temperature so must be in an inert atmosphere .Like little brother molybdenum , the oxide has some vapor pressure ; makes 316 SS a poor choice for high temperatures. $\endgroup$ Feb 4 '21 at 2:26

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