To improve the ionic conductivity of ZrO2 stabilized by 8 mol% of Y2O3 (8YSZ), the SrTiO3 heterogeneously doped 8YSZ composite ceramics with different volumetric fractions of SrTiO3 were prepared through powder sintering at 1400 degrees C and 1450 degrees C in air. The phase compositions, microstructures, electrical conductivities and ion transference numbers of the composites were investigated by X-ray diffraction, scanning electron microscopy, complex impedance and Hebb-Wagner polarization method, respectively. The composite ceramics only consist of cubic YSZ and SrTiO3. The deviations of the typical diffraction peak positions in XRD spectra and the variations in unit cell parameters of both SrTiO3 and YSZ in composites have occurred due to the elemental inter-diffusion between both phases. The grain boundaries between these two heterogeneous phases do not exhibit any presence of a segregated impurity phase, and the heterogeneous doping of SrTiO3 can suppress the growth of YSZ grains at higher doping levels. The apparent conductivities of the composites show a peak at 5 vol.% of SrTiO3 addition. The activation energies for the oxygen ion transport of 8YSZ doped with 0-15 vol % of SrTiO3 all fall into the range of 101.71-104.93 kJ/mol. The ion transference numbers of the composites with 1-15 vol.% of SrTiO3 are all higher than 0.99, demonstrating that the electrical conductivity is almost pure ionic in nature. By analyzing the effects of SrTiO3 additions on the normalized conductivities of the composites, the SrTiO3/YSZ heterogeneous interfacial effects on the conductivity improvement can be proposed. (c) 2014 Elsevier B.V. All rights reserved.