TETRAGONAL ZIRCONIA POLYCRYSTALS - A HIGH-PERFORMANCE SOLID OXYGEN ION CONDUCTOR

The ionic conductivity of sintered pellets of tetragonal zirconia polycrystals (TZP) was found to be higher than that of cubic stabilized zirconia (CSZ) below about 700-degrees-C in spite of the lower concentration of defects. At 400-degrees-C the bulk conductivity is 1.2 X 10(-4)OMEGA--1 cm-1 with an activation enthalpy of 0.92 eV. This shows that the mobility of the defects in TZP is about one order of magnitude higher than in CSZ. TZP pellets show high thermo-mechanical stability and may be readily sealed into glass tubes to provide gastight separate electrode compartments. Such an arrangement shows a fast response to variations of the oxygen partial pressure even at temperatures as low as 150-degrees-C. The correct Nernstian voltage is observed within about 1 s under reducing conditions and within a few minutes under oxidizing conditions. It may be assumed that the fast response even at low temperatures is related to the electronic properties of TZP. Investigations of the electronic behavior of the surface by work function measurements and of the minority charge carrier conductivities are reported as a function of the temperature and oxygen partial pressure. The evaluation of Hebb-Wagner polarization measurements has to be modified by considering the formation of internal electrical fields. It appears that TZP is very useful for fuel cells in view of both its thermo-mechanical stability and its favorable electrical properties at lower temperatures. Limiting-current oxygen sensors have been developed on the basis of this material which show superior performance compared with CSZ. Replacement of the gas diffusion barrier by a solid oxide allows high oxygen partial pressure to be measured.