I don't have an exact numerical figure for temperature, but I did make a small quantity of white phosphorous in a borosilicate test tube under vacuum.
It used the usual mix of aluminum powder, sodium hexametaphosphate, etc. Soda lime glass was not robust enough to handle the heat. Boro was barely
enough. The reaction would proceed slowly with red heat, just as the test tube would begin slowly collapsing under vacuum. As a reward for my
efforts the result was a thin ring of yellowish/red phosphorus that spontaneously burst into flames when I let a bit of air inside the tube.
Anyway, one way to get a bit of extra temperature range out of boro under vacuum is to apply vacuum to both sides of the glass, inside and out.
Secondary containment would be needed around the outside, but only mild vacuum around the outside is needed to prevent collapse. Heating at that
point could be internal with resistance wire or inductive heating.
After many tries of heating molten salts in glassware over the years, the longevity of the glassware depends strongly on the molten salt. When I
dissolved titania into molten sodium bisulfate (a sodium salt that is strongly acidic), the borosilicate tubes were usually reusable. Alkaline melts
would always reduce the glassware to a single-use item as they would readily crack upon cooling.
Another thing is that under vacuum you don't have convective heat losses, so your temperature differentials across the glass can be less. Since you
mentioned vacuum earlier, if the process allows it then this can be used to an advantage.
[Edited on 22-08-01 by WGTR] |
