Introduction

Proton conduction in β- and (β″-Al2O3 is of interest because of potential use in steam-electrolysis/H2/O2 fuel-cells. Single crystal H30+/-β″-Al2O3 was reported by Farrington & Briant to be a fast ion conductor with a proton conductivity of 10–4–10–5 (Ω cm)–1 at 25 °C. These encouraging results led to the first fabrication, characterization and use of polycrystalline H3O + -β″ /β-A12O32.

A second electrolyte with a higher conductivity but a lower thermal stability is NH4+ -β″-Al2032. This system, NH4+/H30+ -β″-Al203, is the best conductor of protons (σ = 7 × 10–4 (Ω cm)–1 rising to 2 × 10–2 (Ω cm)–1 at 200 °C). All previous work has been on single crystals (see Chapters 13 & 23) or powders. This chapter describes the synthesis of NH4+O + -β″-Al2O3 polycrystals.

Ion conducting structure of β″- and β-aluminas

The β-A12O3 family of compounds is constructed from oxygen and aluminium ‘spinel blocks’ with intervening ‘conduction planes’. In βA12O3 there are two spinel blocks per unit cell and in β″-A12O3 there are three. The upper oxygen layer of the spinel block is mirrored across the conduction plane in β-Al2O3 but this symmetry is lost in β″-A12O3. The alkali ion ‘Beevers-Ross’ site in the β-Al2O3 conduction planes is octahedral. The equivalent site in β″-A12O3 is tetrahedral and smaller. Thus K+ (r = 0.28 nm) promotes formation of β-A12O3 and Na+ (r = 0.196 nm), β″- A12O3.