Abstract
The samples were prepared using the primary batch formula Y1−0.2xBa2−0.2xCu3+0.3x(Me)0.4xOy, where x=0–1.20 and Me denotes alkali metals (Na, K, Rb) introduced as carbonates. The presence of alkali carbonates during the process allows one to influence the YBCO grain size and improve the homogeneity of the structure. Despite the large concentration range of additives in the initial batch after the second treatment at temperatures higher than 900 °C, only traces of the alkali metals were revealed. The influence of Na, K, and Rb impurities in concentrations for Na about 5×10−1 wt %; for K to 4×10−2 wt %, and Rb to 2×10−2 wt % in polycrystalline YBCO was investigated. For Rb and K, the R(T) transitions were comparable to that of pure YBCO. The microwave surface resistance Rs was least influenced by the impurities, but strongly depended on the polycrystalline microstructure. Each impurity had a specific effect on χ(T) and Jc which are related to the grain-boundary structure.
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