Optimization of microwave response in barium nickel and zinc tantalate perovskites
Complex perovskites, such as BaZn1/3Ta2/3O 3 (BZT), are currently used in mobile phone base stations. To increase the system efficiency, these materials are required to possess low dielectric losses or high quality factors (Q). Q's have been reported to be associated with the degree of cation order, which can be induced through extended high-temperature processing. ZnO volatilization often complicates the resultant crystal chemistry and the relationship of the structure to Q.^ BaNi1/3Ta2/3O3 (BNT) and Ni-doped BaZn 1/3Ta2/3O3 (BNZT) were employed in this study. The ordering kinetics in BNT powders and pellets was studied by annealing treatments at various temperatures and times. The effect of non-stoichiometry and processing atmosphere on the ordering, sintering and dielectric properties was investigated.^ This study resulted in a better understanding of correlations among different order variables. The lattice distortion (c/a) has been shown to be more influenced by the ordered domain size (D) than by the degree of order (S). The domain growth was also found to be anisotropic; the domain size along  or D 100 is always larger than that along  (D 001). In addition, the uncovering of an ordered domain size gradient in dense pellets provides an explanation to the inferior Q values commonly encountered in large, commercial pucks. Different types of non-stoichiometry and processing atmosphere were demonstrated to influence the order stability, sinterability and microwave response. Large improvement in as-sintered Q (∼ 10,000 to ∼ 12,000) was observed in the samples located along BNZT-(Ni0.13Zn0.87)Ta 2O6, BNZT-BaTa2O6, BNZT-Ta2O 5 and BNZT-Ba8(Ni0.13Zn0.87)Ta 6O24 pseudo-binaries. The highest Q ∼ 14,000 was obtained in a BNZT-Ba8(Ni0.13Zn0.87 )Ta6O24 sample sintered at 1500°C for 24 hours. These high-Q, fully-ordered samples were always accompanied by large lattice distortion and extensive domain growth. In addition, sintering temperatures could be lowered by ∼ 75°C (to 1375°C, 12 hours for BaO-deficient BNZT) when processing in a ZnO vapor-rich atmosphere, and its resultant Q could be further improved up to ∼ 12,000 by post-sintering annealing at 1350°C.^ These findings enable further improvement in the microwave response in the existing dielectric perovskite materials. A large reduction in the annealing time to reach a high-Q state can be achieved through controlled manipulations of the stoichiometry. The lowering in the sintering temperature and time, by alterations in the sintering atmosphere, minimizes complications of ZnO volatilization commonly encountered in BZT-based ceramics.^
Chemistry, Inorganic|Engineering, Materials Science
"Optimization of microwave response in barium nickel and zinc tantalate perovskites"
(January 1, 2007).
Dissertations available from ProQuest.