THE CHEMICAL DIFFUSION COEFFICIENT DURING ION EXCHANGE IN BETA ALUMINA MATERIALS

The rate of chemical diffusion during ion exchange on the fast-ion conductors (beta) and (beta)'' alumina has been investigated. The original mobile sodium ions in single crystals were partially exchanged with other cations. The ion distribution after exchange was measured by a novel technique called X-ray absorption mapping (XAM). XAM takes advantage of the two-dimensional nature of the ion motion in the (beta) aluminas to determine relative ion concentrations as a function of position in the planar sample. In homogeneous samples, XAM was used to measure the average extent of ion exchange. With inhomogeneous samples formed by partial ion exchange, the technique yielded three observations. First, XAM showed the effect of large defects in the crystal such as cracks which allow non-uniform ion exchange. Second, XAM contributed to the understanding of the behavior of certain ions in (beta)'' alumina. For example, mercury (beta)'' alumina decomposed when heated above 400(DEGREES)C in air. Third, XAM measured concentration profiles which were used to evaluate the chemical diffusion coefficients of the interdiffusing species. Average diffusion coefficients of silver and lead in sodium (beta)'' alumina were estimated as 2 x 10('-8)cm('2)/sec and 1 x 10('-9)cm('2)/sec at 25(DEGREES)C, respectively. Concentration profiles were also used to determine chemical diffusion coefficients of five ion pair systems as functions of the mobile ion concentration at the exchange temperatures. Two analysis methods were used to determine these functions: the Boltzmann-Matano analysis and a modeling analysis derived for this thesis. The Boltzmann-Matano analysis was subject to large experimental errors. The modeling analysis used a modified linear diffusion model to approximate the concentration dependence of the chemical diffusion coefficient. The five ion pair systems analyzed were silver exchanging into sodium (beta) or (beta)'' alumina, cadmium exchanging into sodium (beta) alumina, and strontium and neodymium exchanging into sodium (beta)'' alumina. In addition to the results from (beta) and (beta)'' alumina, multivalent ions were exchanged with the mobile sodium ions in (beta)'' gallate. Divalent ion exchanges were successful with lead, barium, and strontium ions. Trivalent ion exchanges were unsuccessful due to chemical attack by the exchange salts on the samples.