ionic conduction

In a copper wire, electric current is carried by electrons — tiny, almost weightless charges that drift through a fixed lattice of metal atoms. Ionic conduction is a stranger kind of current: here the charge is carried by whole ions, that is, by charged atoms physically picking themselves up and hopping through the solid. Imagine that instead of a bucket-brigade passing a message hand to hand, the people themselves walk across the room, each carrying a bucket. In certain ceramics this is exactly how electricity moves — oxygen ions or sodium ions or lithium ions shuffle through the crystal, and their motion IS the current.

An ion can only move if there is somewhere for it to go: it hops into a neighbouring empty site (a vacancy) or squeezes through the gaps between atoms, each jump costing a burst of thermal energy to get over an energy barrier. So ionic conduction is slow and cold at room temperature and rises steeply with heat. The conductivity is sigma = n times q times mu, where n is how many mobile ions there are, q is the charge on each, and mu is how easily they move (their mobility); mobility follows an Arrhenius law sigma times T = A exp(-Ea / kT), Ea being the migration energy barrier, k Boltzmann's constant and T absolute temperature. Yttria-stabilized zirconia conducts oxygen ions at about 0.1 S/cm at 1000 degrees C but is a near-perfect insulator at room temperature — a factor of a billion difference.

Ionic conduction is the reason a whole family of ceramic devices exists: fuel cells, oxygen sensors, and solid-state batteries all rely on a ceramic that lets ions through while blocking electrons. A material's usefulness here is measured by its transference number — the fraction of the current carried by ions rather than electrons; a good solid electrolyte has an ionic transference number very close to 1. The honest catch is that most useful ionic conductors only work when hot, which is why so much of the field is a hunt for materials that conduct ions fast at low temperature.