High temperature ceramic superconductors: oxalate co-precipitation procedure for synthesis and influence of compositional variations

Abstract

The objectives of the present thesis were mainly directed towards the development of new preparative procedures, rather than to prepare altoghether new high-T materials, and to study the effects of compositional variations using measurements such as X-ray Powder Diffraction (XFD), Scanning Electron Microscope (SEM), Energy Dispersive X-ray-Spectroscopy (EDS), and Resistance versus Temperature. Twenty-two superconducting samples of multi-component oxide having 21- and 23- structures, beside five non-superconducting samples, were prepared using oxalate co-precipitation method starting with reagent grade, rather than high-purity materials. A systematic subsituation of Sr by Ba and/or Ca ions was carried out in the 21-structure ternary oxides having stiochiometry La Sr Co , to produce 4- and 5-component metal oxides of the La Sr - AE C O series where AE = Ba and/or Ca. Similarly, a systematic substitution of Y by Gd ions in 123-structures was carried out to produce 4- and 5-component metal oxides of the type Y Gd Ba Cu O where 0<B<0.5. Similar procedure was used for all samples of both types of superconductors to generalize our preparation techniques. The conditions for co-precipitation, annealing, pulverization, effects of oxygen atmosphere, reaction with porcelain crucible and other processes were optimized for various compositions. X-ray powder diffraction (XFD) patterns confirmed the presence of a tetragonal phase all La-(Sr/Ba/Ca) CuO series and the non-superconducting RE-Ba-CuO samples. The presence of an othorhombic phase was confirmed for all superconducting RE-Ba -Cu O ceramic systems. Scanning Electron Microscopy (SEM) indicated the presence of grain sizes in the range of 0.3 to 15.0 mu, while Energy Dispersive X-ray Spectroscopy (EDS) indicated the presence of La:AE ration of 2:1 for La-(Sr/Ba/Ca)CuO and 123 for Y:Ba:Cu for the (Y/Gd)-Ba-CuO series. The resistance versus temperature measurements indicate Sr being the best alkaline-earth metal among Ca, Ba and Sr for the ternary La-containing ceramics since substitution or doping of Ca and/or Ba for Sr decreased the Tc of these 21-structures. The resistance versus temperature data for 123-strucgtures indicated that Y is the best choice among Gd and Y in the RE-Ba Cu O series. Substitution or doping of Gd for Y decreased the Tc to a lower value from 96K to about 91K.