The Oxygen Probe as a Ceramic System

Abstract

Oxide solid solutions which are based on the elements of Group IV and have the fluorite structure, can be prepared with vacant anion sites when an aliovalent cation oxide such as CaO or Y2O3 is mixed with ZrO2, HfO2 or Tho2. In the case of the last-named oxide, which has the fluorite structure in the pure state, it is possible to show that the electrical conductivity of this solid is markedly enhanced and also qualitatively modified when CaO or Y2O3 additions are made (1). The pure solid is a semi-conductor, having a p-type region over a wide range of temperatures which is the dominant conduction mechanism from about 10−6 to 1 atmos oxygen pressure. Below this pressure range, the oxide shows n-type conductivity which varies as p−l/4o2. In the solid solution range with CaO or Y2O3 additions, there is a range of oxygen pressures over which the electrical conductivity is independent of the oxygen pressure, from about 10−6 down to 10−30 atmos at 1000°C. The n-type component has thus been displaced as the major contribution to the total electrical conductivity in the low pressure region, by the addition of a larger electrolytic conductivity. This is the direct result of the introduction of oxygen ion vacancies. Although the vacancy concentration may readily be extended up to six atomic percent by solid solution formation, it is found that the electrical conductivity decreases when the vacancy concentration passes about 3.5 percent. In order to maximize the ionic transport number, ti, which is the fraction of the electrical conductivity due to ionic migration, it is necessary to use vacancy concentrations around this maximum, since the semi-conduction components are relatively unaffected by aliovalent cation additions to thoria (Fig.1).