dielectric strength of Alumina at high temperatures

[asimpson]

I am doing expirements with electric heaters by insulating heating wire with 95% Alumina fish spine beads wound around metal spool and running temperature up to 1100C. 230VAC 11A.
I noticed that above 600C there was some current leakage into metal spool.

Would this be due to current leakage through ceamic material or over the surface of ceramic or throuh the air. Or a combination of all three?

Any recommendations on better ceramic material to use? Would 99% Alumina be better?

Bead wall thickness is about 1 mm.

I solved the problem in the short term by using an isolation transformer. My longer term solution would be to use a ceramic spool.

I am sure a larger seperation of heater from any grounded conductor is also advisable using 230V.

Thanks

 

[racookpe1978]

95% alumina, right?
What is the other 5%, and does it conduct at high temperatures?
Does it "spark" or jump from inclusion to inclusion at high temperatures, if it is not uniformly mixed in the alumina?

 

[IRstuff]

Looks like your supposition is correct:

http://www.precision-ceramics.co.uk/macor-dc-volume-resistivity.htm

http://www.thevalvepage.com/valvetek/heater/heater.htm

http://zircarzirconia.com/technical-documents/electrical-resistivity-zircar-zirconia-fibers/

Looks like something that's fibrous or possibly porous would have the best resistivity performance


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[Compositepro]

Ceramics are electrolytes when they melt. For example molten glass conducts electricity while solid glass does not. Ingredients (impurities) are often added to aluminum oxide or silica in order to lower the melting point so that they can be fabricated economically.

 

[moltenmetal]

All ionic solids increase in electrical conductivity as temperature increases, since the ions become more and more mobile.

A little leakage current into a grounded metallic item being heated, or serving as the shell of a heater, is no problem from a safety perspective. If the item isn't grounded and is conductively coupled to a part which isn't thermally insulated and is cold enough to touch, then you can have an electric shock risk- but those are a lot of "ifs" that are rather easy to avoid by design.