Effects of Substituting Calcium for Yttrium on the Superconducting   Properties of YBa2Cu3Oz Bulk Samples
  We report systematic studies of AC magnetic susceptibility and transport properties of Y(1-x)Ca(x)Ba2Cu3Oz bulk samples with 0\leq x\leq 0.4. Single phase materials, reduction of carrier concentration and decrease of Tc to 85 K were obtained at doping levels up to 20%. For Y0.7Ca0.3Ba2Cu3Oz sample the improvement of grains boundary transport and screening capabilities has been observed as a result of the optimal ratio between carrier concentration and impurity phase BaCuO2 presence. The appearence of bulk pinning and nonlinear effects starting at the highest temperature were detected also. 
E. K. Nazarova, A. J. Zaleski, A. L. Zahariev, A. K.   Stoyanova-Ivanova, K. N. Zalamova