The structural properties of silver clad high-Tc superconducting ceramic tapes of (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) have been investigated by means of synchrotron X-ray diffraction, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS).
By synchrotron X-ray diffraction in situ studies of the phase development during the transformation of (Bi,Pb)2Sr2Ca1Cu2Ox (Bi-2212) into Bi-2223, the stoichiometry changes and the texture have been performed during annealing in 8% O2 and in air. Furthermore, an annealing with a slow ramping (from 750 °C) followed by two high temperature cycles has been performed to study the equilibrium phenomena. In addition, a tape beforehand fully converted into Bi-2223 was studied in situ during a last annealing. During heating the (Ca,Sr)2PbO4 phase is observed to decompose at temperatures between 700 °C and 840 ◦C. Simultaneously, the Bi-2212 lattice contracts, indicating an incorporation of Pb. Moreover, the grain mis-alignment decreases significantly. In air we have observed that Bi-2212 partly dissociates into (Ca,Sr)2CuO3 and a liquid phase at temperatures above 812 °C. At the annealing temperature Bi-2212 and the (Ca,Sr)2CuO3 phase react with the liquid to form Bi-2223. Both in 8% O2 and in air we have observed that the final texture of Bi-2223 and Bi-2212 is identical, the Avrami plot exponents are in the range of 1–2 and the linewidth of Bi-2212 is constant during conversion into Bi-2223, indicating that no strain and finite-size broadening of the diffraction peaks occur. The transformation mechanism is discussed. During cooling in air below ∼750 °C the (Ca,Sr)2CuO3 phase and the liquid mainly transform into Bi-2201. Below ∼780 °C Bi-2223 decomposes to 3221. During cooling in 8% O2 the Bi-2201 phase and (Ca,Sr)2PbO4 or 3221 are also observed to appear. Using synchrotron radiation and the 3DXRD microscope setup the dynamic behavior of the individual grains within a ceramic was observed for the first time. In addition, a two-step cooling experiment and a decomposition study have been performed in 8% O2. The tapes were afterwards investigated by transport current measurements, SEM and synchrotron radiation. Our interpretation of the cooling experiments is that a slow cooling rate to 800 °C provides time for precipitation of a liquid phase used for mass transport when forming the Bi-2223 phase. A decomposition experiment has shown a very slow reaction where Pb-rich Bi-2223 transforms to Pb-poor Bi-2223 during post annealing at 650 °C. Simultaneously, the amount of a 3222 (Bi = 1.6 and Pb = 1.4) phase increases. By TEM the grain and colony size in the c-axis direction and the angles of caxis tilt grain boundaries are investigated. By high-resolution TEM the grain and grain boundaries are investigated on the atomic scale. The intergrowth content and distribution are examined. A fully processed tape has on average 50% thicker grains than a tape after the 1st annealing. The angles of c-axis tilt grain boundaries are on average 14° and 26° for the fully processed tape and the tape after the 1st annealing, respectively. The intergrowth content (15%) and distribution are similar in these two tapes.
|Place of Publication||Roskilde|
|Publisher||Risø National Laboratory|
|Number of pages||79|
|Publication status||Published - 2001|
|Series||Denmark. Forskningscenter Risoe. Risoe-R|