We report changes in the intergranular material and grain morphology of YBa2Cu3O7-x during cyclic anneals between 780 and 980 ºC in oxygen at atmospheric pressure. Two endothermic reactions were detected: (a) the eutectic reaction of YBa2Cu3O7-x with CuO and BaCuO2 at 900 ºC (enthalpy ΔHa) and (b) the peritectic reaction of YBa2Cu3O7-x with CuO at 950 ºC (ΔHb). During the first anneal, only reaction (b) is detected, and although it should only occur if there is an excess of CuO, its signature is present in all published data. Cyclic annealing causes a monotonic decrease in ΔHb and an increase in ΔHa, larger average grain sizes, and greater volume fraction of the superconducting phase. A steady state is reached after 10 cycles at which point ΔHb = 0. We propose a model that explains the origin of the intergranular CuO and the changes in the intergranular material composition with cyclic annealing.
| Identifer | oai:union.ndltd.org:UTAHS/oai:digitalcommons.usu.edu:etd-1087 |
| Date | 01 December 2008 |
| Creators | Clarke, Andrew Peter |
| Publisher | DigitalCommons@USU |
| Source Sets | Utah State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | All Graduate Theses and Dissertations |
| Rights | Copyright for this work is held by the author. Transmission or reproduction of materials protected by copyright beyond that allowed by fair use requires the written permission of the copyright owners. Works not in the public domain cannot be commercially exploited without permission of the copyright owner. Responsibility for any use rests exclusively with the user. For more information contact Andrew Wesolek (andrew.wesolek@usu.edu). |
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