ELECTRONIC CONDUCTIVITY OF GADOLINIA DOPED CERIA

The electronic conductivity of gadolinia doped ceria (Ce0.8Gd0.2O1.8) was measured as a function of oxygen activity (a(O2) = 1 corresponding to a gas with oxygen partial pressure p(O2) = 1.013 bar) in the range from a(O2) approximate to 10(1) to a(O2) approximate to 10(-13) for t emperatures between 600 and 750 degrees C. The Hebb-Wagner polarizatio n technique was used with an encapsulated Pt-microcontact in N-2 atmos phere and a Cu2O/CuO reference electrode. For instance, a minimum valu e of 3.10(-7) S/cm was found for the electronic conductivity at 600 de grees C and an oxygen activity of 9.10(-7). The minimum is shifted to higher activity with increasing temperature. The activity dependence o f the electronic conductivity in the n-type and p-type range agrees we ll with the theoretically expected values (p-type range sigma(P) appro ximate to a(O2)(1/4), n-type range sigma(n) approximate to a(O2)(-1/4) ). Extrapolation of the measured electronic conductivities, the electr olytic domain boundaries of Gd-doped CeO2 were obtained as log a(O2) = 13 to log a(O2) = -24.7 at 600 degrees C and log a(O2) = 11 to log a( O2) = -20.1 at 700 degrees C (the oxygen ion conductivity busing 4.05. 10(-3) S/cm at 600 degrees C and 1.29.10(-2) S/cm at 700 degrees C). T he activation energy was determined from the temperature dependence of the electronic conductivity giving E-A,E-e = (2.59+/-0.07) eV for ele ctrons in the n-type range and E-A,E-h = (1.16+/-0.04) eV for the hole s in the p-type range. The migration enthalpy of electrons was H-mg,H- e = 0.49 eV.