The role of dopants in tailoring the microwave properties of Ba6-xR8+2/3xTi18O54 R = (La-Gd) ceramics

Microwave dielectric ceramics with a high dielectric constant need to satisfy very high technical demands. They should possess extremely low losses to achieve high Q-values (Quality factor) a small temperature coefficient of resonant frequency (tau(f)), and a relative permittivity (epsilon(r)) higher than 80. Industrial applications require very stringent electrical and dimensional tolerances, typically +/-0.5-1.0 ppm K-1 for a specified tau(f) and +/-0.25% for a specified epsilon(r). To meet such requirements ceramics based on BaO-R2O3-TiO2 (R=La-Gd) are used. The investigation of this type of ceramic was stimulated by the observation that ceramics based on compositions in the TiO2-rich region of the system exhibit highly temperature stable electrical properties. Especially interesting are compositions within the solid solubility region with the general formula Ba6-xR8+2/3xTi18O54. As the ionic radius of the rare earth decreases the extent of the solid solubility region becomes narrower, i.e., 0<x<3 for La and x=0.5 for Gd. Further improvements in the dielectric microwave properties can be achieved by combining different rare earth oxides, and by partial replacement of Ba2+ with other alkaline earth atoms such as Ca2+ and Sr2+. Typically such ceramics meet the requirements for Q and epsilon(r); however, tau(f) must be additionally adjusted by the use of dopants. Most commonly bismuth and lead oxides or titanates are used. In the present contribution the role of different dopants and their influence on the resulting microwave dielectric properties of Ba6-xR8+2/3xTi18O54 based ceramics are discussed.