Date of Award:

12-2008

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Physics

Advisor/Chair:

Wilford N Hansen

Abstract

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.

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