ELECTRONIC CONDUCTIVITY OF A SOLID OXIDE ELECTROLYTE IN THE LOW-TEMPERATURE RANGE

Using the Hebb-Wagner polarized-cell technique the electronic conductivity of yttria-doped thoria as a model system for high temperature solid electrolytes was determined in the temperature range from 230 to 660-degrees-C as a function of the oxygen partial pressure at the reversible electrode as well as the polarization voltage and the polarization time. In contrast to the behaviour expected from previous high temperature investigations, the total electronic conductivity consists of four rather than two component parts, viz. the conventional p- and n-conductivity branches and two time-dependent conductivities with p- and n-type behaviour. Initially the time-dependent conductivity branches predominate over the conventional ones, but decay over a long period of time. As a consequence, in the low temperature range the behaviour of the electrolyte is mainly determined by the properties of the newly found rather than the hitherto known electronic conductivities. The observations could be interpreted as resulting from a slow redistribution of the electronic charge carriers within a system of four traps with different capture cross-sections and different rate excitation-recombination constants.