The enhanced electrical properties of TiO2-Nb2O5-ZnO varistor by sintering with the pre-synthesized B-Bi-O frit

In this work, H₃BO₃ and Bi₂O₃ with the ratio 3:2 were melted at 750 °C and then quenched in water to produce B-Bi-O frit, which was sintered and co-doped with TiO₂-0.60Nb₂O₅-0.50ZnO varistor ceramics. It is found that B-Bi-O frit can reduce the sintering temperature and improve the electrical properties of TiO₂ ceramics. The best comprehensive electrical properties with the nonlinear coefficient up to 8.9, the breakdown voltage down to 4.92 V/mm, the relative dielectric constant of 4.47*10⁵ and the leakage current of 0.102 mA are achieved by sintering the ceramics doped with 3 wt% B-Bi-O frit at 1400 °C. XRD analysis shows that B-Bi-O frit is an amorphous phase, and no second phase can be found in ceramics after the frit is doped. SEM morphologies display that B-Bi-O frit is beneficial to decrease the porosity while increase the grain size, and EDS mapping further presents no elements segregate on the grain boundary. XPS spectra demonstrate the coexistence of Ti³⁺ ions, Ti⁴⁺ ions and oxygen vacancies in TiO₂ ceramics. As a result, it can be concluded that the enhancement of the electrical properties of TiO₂ ceramics is mainly attributed to the following aspects: on the one hand, B-Bi-O frit helps to produce liquid phase sintering, which would reduce the porosity while increase grain size and promote solid solution of Nb₂O₅ and ZnO in TiO₂ ceramic. On the other hand, B and Bi elements can also act as acceptor dopants in TiO₂ ceramic to further promote grain semi-conductivity and increase grain boundary barrier height.