Warren W. Rhodes

Lead zinc niobate pyrochlore: Structure and dielectric properties

In Pb(Bx′B1−x″)O3 ceramic compositions, it is customary to find a mixture of cubic pyrochlore and perovskite phases after calcination. Based on X-ray diffraction analysis, we concluded that both phases are made up of the same structural unit of BO6 octahedra. The B′ and B″ ions occupy the B sites randomly to the extent that local charge equilibrium is maintained. Thus a general formula for the pyrochlore phase can be expressed as Pb(Bx′B1−x)O3 · Op where O ⩽p ⩽ 0.5. An extensive study of the Pb(ZnxNb1−x)O3.5−1.5x pyrochlore system was made by varying the zinc concentration. We interpret changes in the X-ray diffraction pattern and the lattice constant as indicative of the changing occupancy of the seventh oxygen sites in order to maintain local charge balance. The best combination of the dielectric properties, with a dielectric constant of130 and a σ factor greater than 1000 at10 MHz, is achieved at a composition of 0.3 ⩽x ⩽ 0.4 and a sintering temperature of 980° C. The temperature coefficient of the dielectric constant measures −0.75 × 10−3° C−1. It decreases to −0.54 × 10−3° C−1 when5 mol% of PbTiO3 was mixed with the nominal pyrochlore compositions and sintered. Thus, it may be possible to effect a larger change in the temperature coefficient by judiciously including selective amounts of a second phase which has the best compensating temperature coefficient.

Process-related problems of YBa2Cu3Ox superconductors

Abstract Ceramic processing of YBa 2 Cu 3 O x -based superconductors and process-related problems are described. Conditions for calcination, sintering and for microstructural development are important considerations for obtaining the superconducting properties. We examine different forming techniques for preparing superconducting components with different sizes, shapes and configurations. We also identify and discuss six problem areas related to (i) the incomplete decomposition of BaCO 3 precursor, (ii) the slow oxidation kinetics of YBa 2 Cu 3 O x ceramics, (iii) the sensitivity to water of the YBa 2 Cu 3 O x phase, (iv) the reactivity of the YBa 2 Cu 3 O x phase with substrates and co-firing materials, (v) the low critical-current density in YBa 2 Cu 3 O x bulk ceramics, and (vi) the low mechanical strength of YBa 2 Cu 3 O x ceramics.