Effect of thermal treatment on TiO2 varistor properties in different atmospheres

TiO2 is a typical nonstoichiometric compound and defects such as oxygen vacancies in TiO2 grains and at TiO2 grain boundaries are very sensitive to the oxygen partial pressure in the environment. In this paper, commercial TiO2 doped with Nb2O5 and Bi2O3 was used as the raw material to prepare TiO2 varistors. The samples were thermally treated in three kinds of atmospheres: H2, vacuum, and O2 at 600 °C. In the two low oxygen partial pressure atmospheres of vacuum and H2, due to the volatilization of lattice oxygen in TiO2, the oxygen vacancy concentration and the number of semiconducting grains increase, while the height and width of the potential barrier and the varistor voltage decrease. After the oxygen atmosphere thermal treatment, the acceptor state density and the height and width of the potential barriers increase, the conductivity decreases, and the varistor voltage improves significantly as a result of oxygen accumulation at grain boundaries.