Microwave-assisted oxygenation of melt-processed bulk YBa2Cu3O7-delta ceramics

The properties (superconducting and non-superconducting) of YBa2Cu3O7-delta bulk ceramics (YBCO) are strongly dependent on the oxygen deficiency, delta. Unfortunately, whether sintering or melt-processing is used to form the material, the final oxygen content is usually far from that which gives the optimum superconducting properties (delta less than or similar to 0.1). Because of this, an additional oxygenation stage has to be carried out in which the material is heated and held at some elevated temperature (similar to 400-500 degrees C) for a significant time (often > 100 h) to allow oxygen to diffuse into the sample. Other diffusion-based processes (e.g. sintering) are known to be substantially enhanced in the presence of a microwave field, and it is reasonable to expect that the diffusion of oxygen into YBCO may also benefit from the application of such technology. The oxygenation of melt-processed YBCO using conventional and microwave-assisted heating has been compared. The diffusion coefficient at 400 and at 450 degrees C is shown to be enhanced by about 30% through the application of a high-frequency microwave field. Because the oxygenation time is inversely proportional to the diffusion coefficient, this represents a significant reduction in process time. The equilibrium value of the oxygen content is not affected by the application of a microwave field, and therefore the sample temperature must be the same for both microwave-assisted and conventional processing. Consequently, the observed enhancement is an example of a genuine non-thermal microwave effect.