Abstract
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 Al 2O 3 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–Al 2O 3, never studied before, because of the availability of the radioactive isotopes 22Na and 36Cl. The comparison of the electrical conductivity measured in NaCl single crystals and in NaCl–Al 2O 3 composites, respectively, shows the expected enhancement caused by the presence of the Al 2O 3 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.
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