Abstract
The influence of the modification of some preparational steps on the microwave properties of polycrystalline YBa 2Cu 3O 7 − γ was investigated. The surface resistance R s was measured in the temperature range between 300 and 4.2 K and at 3, 21.5 and 86.5 GHz. Grinding steps during calcination and long sintering times were found to be of significant importance for the attainment of low R s (100 K) values and a steep reduction in R s below T c (which results in a low surface resistance at 77 K). An increase in the sintering time up to 200 h resulted in a decrease in R s at 4.2 K. For longer sintering periods a saturation behaviour was observed. Mechanical polishing of the bulk samples influenced R s (4.2 K) significantly. At 86.5 GHz an improvement by a factor of about four was observed after polishing compared with the as-sintered sample. The data on polycrystalline YBa 2Cu 3O 7 − δ given in this paper show a lower surface resistance at 77 K than that of copper for frequencies up to about 20 GHz.
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