Study On Microstructure Of Bi-based Superconductor Materials

In the copper oxide family of superconductors, Bi-based high temperature superconductors with non-toxic system elements and non-rare elements, intrinsic Josephson junction and better preferred orientation, etc., has been one of the hot spots in condensed matter physics. Compared with Bi2Sr2CaCuOy(Bi-2212) phase and Bi2Sr2Ca2Cu3Oy (Bi-2223) phase, Bi2201 phase takes on some behaviors, such as simple structure, lower calcination temperature, wider calcination temperature range, high stability and so on, but its superconducting critical transition temperature (Tc) is very low. Have been reported in the literature, after doped into unit cell of Bi-2201 phase by other element, superconducting critical transition temperature increases due to the atomic radius difference arising from the lattice distortion. Currently, it is an effective way to improve Tcby doping Ca into Bi-2201 phase.In this thesis, the HTSC system of Bi2CaxSr2-xCuOy(x=0-1.0) were considered as the research objects by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray spectroscopy (EDS). The influence of Ca doping on the microstructure, lattice constant, modulation period and distribution of elements were investigated in detail. Occupancy of Ca elements in Bi2CaxSr2-xCuOy was explored by X-ray refinement method. The following results were obtained in three areas:(1) XRD results showed that:With the increase in the amount of Ca doping, Bi-2201 phase lattice constants a and b changed little, the lattice constant c decreases.(2) TEM and EDS results showed that:The results showed:Bi-2201 phase lattice modulation wavelength qb in the doping concentration x=0.2-0.6 between increases; in the doping concentration x=0.8-0.9, qb tends to decrease. In Ca0.2-0.9 cross-section samples, monoclinic modulation vector was found in diffraction pattern of [100] zone axis. Spectroscopy (EDS) results show that:the actual stoichiometric of sample Ca0.5 is simlar to the nominal stoichiometry of sample Ca0.5, and Bi (at%)/(Sr+Ca) (at%)= 1/1, and the Sr (at%)/Ca (at%)=3/1. Then we speculated:in Bi2Sr1.5Ca0.5CuOy unit cell, Sr (at%)/Ca (at%) =3/1.(3) X-ray refinement results showed that:if Ca atoms replaced Sr atoms in Bi-2201 phase, we simulated calculated spectrums of X-ray diffraction based on a model structure of Sr (at%)/Ca (at%)=3/1 in Bi-2201 unit cell. We found that the X-ray calculated spectrum is similar to the X-ray experimental spectrum of Bi2Sr1.5Ca0.5CuOy, and (hkl) peaks of calculated diffraction spectrum is close to the corresponding (hkl) peaks of actual spectrum; In addition, with the increase in the amount of Ca substitution, (hkl) peaks of calculated diffraction spectrum is close to the corresponding (hkl) peaks of actual spectrum.