Dopant and substitution effects on YBCO based superconductors

The discovery of high-temperature superconductivity in La-Ba-Cu-0 system in 1986, attracts the attentions of the media all over the world. A large number of scientists expended and still expend excellent efforts in order to understand these unique materials. The excitement comes from the possibility for existance of a superconductor at room temperature, which will certainly change the human life. Although millions of papers have been published since the discovery, there are still a lot of mysteries, especially in theory- of high-temperature superconductivity. Substitutional studies are of great importance in solving the corresponding problems, so that these form a big part of the researches. In this thesis we tried to determine the effects of partial substitutions of Ni and Ag elements instead of Cu on superconducting properties of YBCO system. The superconducting samples were prepared by conventional solid-state reaction method and then characterized by resistivity-temperature variations, XRD patterns, SEM (Scanning Electron Microscope) images and density measurements. Electrical resistance was measured by standart dc four-point probe method. From these results we were able to determine the critical temperature, transition width and the character of normal-state resistivities of the samples. XRD patterns show the crystal phase as well as the impurity and unknown phases of the superconducting materials. Lattice parameters of the crystals were determined by the use of Powder Diffraction Package (PDP) programme. The oxygen content (y) which is an important parameter for YBa2Cu30y superconductors was estimated approximately by a well known equation required the c-axis length of the unit cell. SEM microphotographs give powerfull information about the microstructure, the sizes and orientation of grains as well as the pores between them. Densities of the samples were measured by the help of picnometer. The results showed smaller densities then theoric one reported as d=6,357g/cm for pure YBCO in all samples. This picture is typical for ceramic superconductors due to their granular structure. The degree of pores, defined as porosity, was easily determined for each sample by two simple methods including density values. The porosity results give information about the strength of connection between superconducting grains, which is of importance for superconductivity. In the light of these measurements and analyses we discussed the effects of both Ni and Ag dopants on superconducting properties such as critical temperature, transition width, normal resistivity, crystal as well as microstructure, oxygen content, density and porosity of YBa2Cu30y system.