Enhanced ionic transport in NaCl-Al2O3 heterogeneous electrolytes

Some ionic solids, such as the lithium halides, are known to acquire a strong enhancement of their electrical conductivity when they are thoroughly mixed with fine Al2O3 particles. We have started a direct study of the motion of the ions themselves in measuring their self-diffusion coefficients. We have chosen the material NaCl-Al2O3, never studied before, because of the availability of the radioactive isotopes Na-22 and Cl-36. The comparison of the electrical conductivity measured in NaCl single crystals and in NaCl-Al2O3 composites, respectively, shows the expected enhancement caused by the presence of the Al2O3 particles. The Na+ self-diffusion shows the same enhancement, in the extrinsic range as well as in the intrinsic high temperature region. The correlation factor is derived from the Nernst-Einstein relation. The value f = 0.78 +/- 0.1 shows that the mass transport in the composite proceeds by the same mechanism as in the unmixed ionic crystals, i.e. via Schottky defects in NaCl. The Nernst-Einstein relation is rather closely verified by considering all the ions of the alkali halide phase, in contrast to the space charge model. (C) 1999 Elsevier Science Ltd. All rights reserved.