Influence of GeO2 on the microstructure and electrical properties of TiO2–Nb2O5–Ho2O3 –SiO2 varistors

TiO2 varistor ceramics co-doped with GeO2, Ho2O3, Nb2O5 and SiO2 were prepared by a conventional solid-state method. Meanwhile, the effects of GeO2 on the microstructure, varistor properties and dielectric properties were investigated in details. The SEM-EDS results revealed that the addition of GeO2 could induce more Nb5+ and Ho3+ ions to dissolve into TiO2 matrix and make Ho3+ ions well-distributed. Moreover, the incorporation of GeO2 significantly improved the dielectric properties. All samples showed a good temperature (−150–200 °C) and frequency(100 Hz–100 kHz) stability of dielectric properties and the sample with 0.9 mol% GeO2 obtained a giant dielectric constant about 2.68 × 105. Besides, the doping of GeO2 was found to be capable of reducing the breakdown voltage and increasing the nonlinear coefficient as well. Particularly, the 0.3 mol% GeO2-doped sample possessed a low breakdown voltage of 2.7 V/mm and the one doped with 0.6 mol% GeO2 showed the maximum nonlinear coefficient value of 6.7. •The breakdown voltage and dielectric constant are improved after doping GeO2.•Samples show a good temperature and frequency stability of εr and loss tanσ.•The effect of GeO2 on microstructure and electrical properties is explored.•Some electrical characteristics are calculated by J-E characteristics curves.•A grain-boundary barrier model for TiO2 varistor ceramic is established.