ALKALI-METALS IMPURITIES INFLUENCE ON THE MAGNETIC AND ELECTRICAL-PROPERTIES OF YBCO

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 d uring the process allows one to influence the YBCO grain size and impr ove 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-degrees-C, only traces of the alkali meta ls were revealed. The influence of Na, K, and Rb impurities in concent rations for Na about 5 x 10(-1) wt %; for K to 4 x 10(-2) wt %, and Rb to 2 x 10(-2) wt % in polycrystalline YBCO was investigated. For Rb a nd K, the R(T) transitions were comparable to that of pure YBCO. The m icrowave surface resistance R, was least influenced by the impurities, but strongly depended on the polycrystalline microstructure. Each imp urity had a specific effect on chi(T) and J(c) which are related to th e grain-boundary structure.