β-alumina
/ BAY-tuh uh-LOO-mih-nuh /
Beta-alumina is one of the great misnamings in ceramics. When it was first found as a phase in aluminium-oxide production and studied by X-rays, it looked like just another crystal form of alumina (Al2O3), so it was labelled a 'beta' polymorph. It is nothing of the sort: it is a sodium aluminate, and its whole reason for fame is that it is a superb conductor of sodium ions. Its structure is the key — think of stiff, densely-packed slabs of alumina (called spinel blocks) stacked like floors of a building, with loosely-packed open 'conduction planes' sandwiched between them where sodium ions can slide almost freely in two dimensions.
The rigid spinel blocks hold the framework together and do not conduct; the sparse conduction planes, bridged by just a few oxygen columns, are so open that the sodium ions in them behave like a two-dimensional liquid. A common formula is roughly Na2O times 11 Al2O3 for the beta phase, and Na2O times 5 to 6 Al2O3 for the closely related, even better beta''-alumina, which is stabilized by a little lithia or magnesia. Sodium-ion conductivity reaches about 0.2 to 0.3 S/cm near 300 degrees C — high enough to carry real currents through a solid ceramic wall.
That is exactly what it is used for: beta''-alumina is the solid electrolyte of the sodium-sulphur battery and the ZEBRA (sodium-nickel-chloride) battery, both of which run hot, around 300 degrees C, with molten sodium on one side of a thin beta''-alumina tube and the other reactant on the far side. The ceramic must be dense and gas-tight so the two never mix except as sodium ions crossing the wall. It is a reminder that a functional ceramic's property comes from its microstructure and crystal architecture, not from a simple chemical name — and that historical names, once stuck, are almost impossible to unstick.
In a sodium-sulphur battery a thin tube of beta''-alumina holds molten sodium inside and molten sulphur outside. On discharge, sodium ions leave the molten metal, thread through the ceramic's conduction planes to the sulphur, while the electrons power the load — a battery whose electrolyte is a solid ceramic and whose electrodes are liquids.
Beta-alumina: rigid alumina slabs with sodium 'expressways' running between them.
Despite the name, beta-alumina is not a polymorph of Al2O3 — you cannot make it from pure alumina, it needs the sodium. The everyday conductor beta''-alumina is a distinct, better-conducting phase from the original beta-alumina, not just a typographic variant.