Structural studies of various β-aluminas

Thesis

This thesis describes results obtained using high resolution electron microscopy, acoustic microscopy and chemical analysis to study the structure and properties of the superionic β-aluminas.

The acoustic microscopy and chemical analysis results relate solely to sodium β- and β" -alumina, which are used as the solid state electrolyte in the sodium/sulphur cell.

The high resolution electron microscopy results cover sodium β- and β"-alumina as well as a number of ion-exchanged β"-aluminas.

The β-alumina structure consists of spinel-like blocks separated by the so-called conduction planes. The conduction planes have a low density, and contain all the mobile cations. Lattice images of sodium β- and β"-alumina, silver β-alumina, ammonium/hydronium β"-alumina, gadolinium β" -alumina and divalent and trivalent europium β" -alumina are presented and discussed.

A hitherto unreported long-period structure in sodium β-alumina is shown, as is superlattice ordering in the divalent and trivalent β"-aluminas.

Defects in these materials are also discussed. The most common damage mode in the β" -aluminas, due to electron beam irradiation, is the loss of the mobile-ion containing planes, and the subsequent collapse and shear of the structure to form broad defect spinel blocks. It is shown that collapse vectors determined for sodium β"-alumina can also be applied to ammonium/hydronium β"-alumina. Two further damage modes observed in this β"-alumina are also discussed. A damage mode has been observed in sodium B-alumina and silver β-alumina which involves the extrusion of material to the crystal surface. Electron diffraction patterns from the extruded material have been indexed.

The acoustic microscope has been used to examine bulk sodium β/β"-alumina electrolyte tube specimens. Images of rectangular features present in the tubes (approximately 40um in length) are presented and the possible nature of the features is discussed.

Authors: Petford-Long, A; Petford, Amanda

• Publication date: 1984
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Materials; Defects; Superionic conductors