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 permittivit y (epsilon(r)) higher than 80. Industrial applications require very st ringent electrical and dimensional tolerances, typically +/-0.5-1.0 pp m 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) ar e used. The investigation of this type of ceramic was stimulated by th e observation that ceramics based on compositions in the TiO2-rich reg ion of the system exhibit highly temperature stable electrical propert ies. Especially interesting are compositions within the solid solubili ty region with the general formula Ba6-xR8+2/3xTi18O54. As the ionic r adius of the rare earth decreases the extent of the solid solubility r egion becomes narrower, i.e., 0<x<3 for La and x=0.5 for Gd. Further i mprovements in the dielectric microwave properties can be achieved by combining different rare earth oxides, and by partial replacement of B a2+ with other alkaline earth atoms such as Ca2+ and Sr2+. Typically s uch 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 contribu tion the role of different dopants and their influence on the resultin g microwave dielectric properties of Ba6-xR8+2/3xTi18O54 based ceramic s are discussed.