Ba6-3XNd8+2XTi18O54 Microwave dielectric resonators

The Ba6-3xNd8+2xTi18O54 solid solution (BNT) is well established as a useful microwave resonator material due to its high dielectric constant (epsilon(r)), high quality factor (Q = 1/tan delta), and tunable temperature coefficient of resonant frequency (tau(f)). Its crystal structure is similar to that of the tungsten bronzes, containing large pentagonal channels parallel to [001], which are only partly occupied by the divalent species. These pentagonal sites are separated by Nd+3 and TiO6 octahedra arranged in an essentially perovskite-like manner. Ceramics of this phase; have been processed in this study by a mixed-oxide route, and the variation in properties caused by doping with either Ca, Sr, or Pb was investigated. Most samples showed a multiphase character, with Nd4Ti9O24 (ss) being the main secondary phase. For x < 0.75, single-phase BNT was always obtained, while phase purity was only attained at 1400 degrees C when x = 0.75. Doping with PbO was found to usefully reduce tau(f) whilst leaving both epsilon(r) and Q at reasonable levels. Pb-doping lead to a moderately lower Q and, contrary to earlier reports, slightly lowered epsilon(r).