The Fabrication, Mechanism And Doping Researches Of Bi-System Superconductor Materials By Sol-Gel Method

In this dissertation, the Bi-system high temperature superconducting powders and films were synthesized via soft chemical method. All the samples were prepared by metal nitrates as starting materials and ethylenediaminetetracetic acid (EDTA) as chelating agent. Different gel processes were devised to synthesize Bi2SnCaCu2O8+δ (Bi2212) powders by sol-gel method. The phase purity and crystallinity of Bi2212powders were dramatically improved by a modified sol-gel method called PSG method. Furthermore, effects of La-doping on the structural and electrical properties of Bi-2212compound and Ca-doping on that of Bi-2201powders were studied. Finally, Pechini method was applied to synthesize Bi-2212film on STO single crystal. The main contents of the dissertation are as follows:1. Effect of gel processes on the quality of Bi-2212superconducting powders was studied. The phase purity, grain size and crystallinity of samples were greatly improved. The superconductivity was improved due to the reduced grain boundary which is the result of larger size of Bi-2212. Effect of gel mechanism on the quality of Bi-2212powders demonstrated that the complexes stability of xerogel deriving from Pechini method was improved by the polymerization. The amount of free metal ions was reduced and the homogeneity of precursor gel was improved. It was a key step to maintain the ratio of metal element in precursor gel. So the quality of Bi-2212sample was dramatically improved and the sintering time for overcoming the ingredient segregation in precursor powder was remarkably decreased. Furthermore, the sintering time was reduced from10h to5h.2. The powders of La doped (Bi1-xLax)2Sr2CaCu2O8+δ superconducting compound were prepared from Bi-2212by sol-gel technique. The changes in the structure and electrical conductivity due to substitution of La for Bi were investigated. The range of La doping content varies from0.01to0.09. Thermal analysis (TG-DTA) and X-ray diffraction were employed to investigate the chemical reaction process and structural changes of the samples. The resistance measurement was carried out with a conventional four-probe method. Structural analysis showed that the powdered phase remains orthombic Bi-2212, even though the crystal axes a and b elongated and c shortened. And the electrical conductivity of Bi-2212phase changes obviously with increase in La doping content, as shown by the R-T curves.3. The powders of Ca doped Bi2Sr2-xCax-CuOy superconducting compound were prepared from Bi-2201by Pechini type Sol-Gel (PSG) method. We described here a large solid solution Bi2Sr2-xCaxCuOy for0≤x≤0.8, which was a single phase up to0.6. The changes in the structure and electrical conductivity due to the Ca doped were investigated. Results revealed that the sintering temperature was changed with Ca content x to obtain high phase purity of Bi2Sr2-xCaxCuOy. The parameters of the orthorhombic cell were affected by the Ca-Sr substitution. The superconducting transition temperature of Bi-2201was increased to86K according to the R-T curves. The structural model was proposed to explain Tc increase of Bi-2201phase.4. Spin-coating PSG method was employed to prepare Bi-2212film with c-axis orientation on single crystal STO substrate. The high phase purity of Bi-2212film was obtained by sintering the Bi2Sr2Ca1.05Cu2O8+δ precursor film at835℃for10min. The optimization of drying process showed that the rapid drying at150-190℃can improve the thin film quality by reduction of cracks created during pre-heating process. The Tc,onset of Bi-2212films have a little change with different synthesis conditions but Tc,0of these films have changed enormously.