Ageing of the Dielectric Properties of Barium Titanate Ceramics

The permittivity and power factor of ceramic barium titanate and barium titanate solid solutions have been measured as a function of time, starting from the time of cooling the material through the Curie point. Both parameters decrease linearly with log t over periods extending to several years. A theory is proposed which postulates that a large contribution is made to the permittivity by the motion of domain walls, and that the domain walls are gradually moved to more stable and less polarizable states by thermal agitation. A wide distribution of activation energies, reflecting the random character of obstacles to domain wall motion in a ceramic, yields the log t law. Domain walls moving over low-energy humps are considered responsible for the power factor decay.