Enhanced ionic transport in NaCl-Al2O3 heterogeneous electrolytes

Some ionic solids, such as the lithium halides, are known to acquire a stro ng enhancement of their electrical conductivity when they are thoroughly mi xed 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 a vailability of the radioactive isotopes Na-22 and Cl-36. The comparison of the electrical conductivity measured in NaCl single crystals and in NaCl-Al 2O3 composites, respectively, shows the expected enhancement caused by the presence of the Al2O3 particles. The Na+ self-diffusion shows the same enha ncement, in the extrinsic range as well as in the intrinsic high temperatur e region. The correlation factor is derived from the Nernst-Einstein relati on. The value f = 0.78 +/- 0.1 shows that the mass transport in the composi te proceeds by the same mechanism as in the unmixed ionic crystals, i.e. vi a Schottky defects in NaCl. The Nernst-Einstein relation is rather closely verified by considering all the ions of the alkali halide phase, in contras t to the space charge model. (C) 1999 Elsevier Science Ltd. All rights rese rved.