The beta alumina structures are known for their high ionic mobility within the lattice. This lead to the development of the Na-β”-alumina polycrystal as solid electolyte in Na/S and Na/NiCl2 batteries. The K-β”-alumina compound is a suitable precursor material to establish proton conducting materials by ion exchange. Tests with single crystal and polycrystalline samples showed the possible application in fuel cells operating between 150 - 250 °C.
One of the main problems to be solved is the correlation between composition and phase evolution of either β- or β”-phase, another problem occuring during sintering is the high vapor pressure of the alkaline oxide. This leads to the decomposition of the highly conductive β”-alumina phase into β-alumina or corundum phases and lowers significantly the ionic conductivity.
We investigated the beta alumina phase evolution using alumina raw materials with different crystallographic structure and grain size. The influence of initial alkaline content and dopant concentration on phase formation and phase stability under sintering conditions has been investigated. A refined phase diagram for Na- and K-beta aluminas will be presented.