Does aluminium oxide ionize after melting point?

Question

My chem is very basic but I would like to know is there a temperature after melting point where aluminium oxide can ionize enough to pass electrical current through? Is there a temperature where the resistance of aluminium oxide deteriorates?

Answer 1

I'm going to try and answer a bit of confusion in the original question "Does aluminium oxide ionize after melting point?". Aluminium oxide is already ionised in the solid, or more pedantically the ionic model fits aluminium oxide sufficiently well that we can think of it comprised of $\ce{Al^{3+}}$ and $\ce{O^{2-}}$ ions and make reasonable predictions. The point is that in most solids, ionic or not, the constituent atoms and molecules are energetically constrained to vibrate about their "normal", so called equilibrium, position, and so never move very far, whereas in liquids they can move about reasonably freely. Now as conductivity involves the nett movement of charges most ionic solids are not conducting while the liquids are, as in the liquid the charged ions are free to move toward the appropriate electrode while in the solid they are not.

However it's not that quite that simple, but the above is probably good enough to university level. But for completeness I'll add that there are things called fast ionic conductors. These are "ionic" solids that conduct electricity well above a certain temperature. A well known example is silver iodide, which above 419 K start to conduct electricity pretty well. A simple model here is that at that temperature the iodide sub-lattice stay rigid, so providing a solid like framework, but the silver sub-lattice melts, and so the silver ions can move, and thus nett charge migration is possible, and so the solid conducts. For a bit more of an introduction, see Wikipedia article of fast ion conductor. Interestingly one form of aluminium oxide can act as a fast ion conductor, but in this case the charges that are moving are "impurities".

Answer 2

Yes. After molten, every ionic compound (including aluminium oxide, of course) turns into ion. This is how production of aluminium by Hall-Héroult process works.

However, it is very hard to reach aluminium oxide melting point (it is at 2,072 °C). Therefore, we need to put cryolite to decrease its melting point.