The Research Of Crystal Growth Mechanism In Bi-based Superconducting Thick Films Synthetized By Sol-Gel Method

In this thesis, Bi-based superconducting thick films were synthesized by improved Sol-Gel method (Pechini method). All the films were prepared by metal nitrates as starting and ethylenediaminetetracetic acid (EDTA) as chelating agent. The experimental procession to prepare Bi2Sr2CaCu2O8+δ (Bi2212) thick films was investigated in detail by adjusting the sintering temperature and holding time. In addition, c-axis epitaxial character of Bi-based superconducting thick films was discussed. At last, the crystal growth mechanism in Bi-based superconducting thick films was analyzed using phenomenological chemical thermodynamics and phenomenological chemical kinetics. The detail contents of the thesis are as follows:1. Bi-based superconducting thick films were synthesized on the SrTiO3single crystal substrate by improved Sol-Gel method (Pechini method). The high phase purity and good surface morphology in Bi-based superconducting thick films were obtained by adjusting the sintering temperature and holding time. The experimental results reveal that Bi2212superconducting thick films with high phase purity could be obtained by sintering the Bi2212precursor films at821℃for55minutes,823℃for40to45minutes and825℃for23minutes. The surface morphology of Bi-based superconducting thick films was improved with the reduction of the sintering temperature and the extension of the holding time.2. The c-axis epitaxial character of Bi-based superconducting thick films synthesized on the SrTiO3single crystal substrate was analyzed by the mismatch principle. The results reveal that the mismatch with Bi2201or Bi2212lattice is less than3%while the SrTiO3lattice is rotated45degrees along [001], which is the reason why the Bi-based superconducting thick films have good c-axis epitaxial character. Furthermore, the mismatch will reduce and the better c-axis epitaxial character will be obtained with transformation from Bi2201phase to Bi2212phase and the rise of temperature. 3. The crystal growth mechanism in Bi-based superconducting thick films was analyzed using phenomenological chemical thermodynamics. The results reveal that four processes were experienced during the synthesis of Bi-based superconducting thick films:the transformation from metal nitrate to metal oxides, the synthesis from metal oxides to Bi2201phase, the transformation from Bi2201phase to Bi2212phase and the decomposition of Bi2212phase. The segregation, the reunification and the disappearance of metal oxides were avoided effectively by the improved Sol-Gel method (Pechini method), which aggrandizes the homogenization in the Bi-based superconducting thick films. The calculations of standard molar enthalpy reveal that Bi2212phase is in a relatively unstable state, easy to turn into activated state with the absorption of the energy and decomposes across the energy barrier finally.4. The crystal growth mechanism in Bi-based superconducting thick films was analyzed using phenomenological chemical kinetics. The results reveal that the synthesis reaction rate constant of Bi2212phase increase with the rise of temperature, which is the main reason why the Bi2212content is increasing with the rise of temperature. The decomposition reaction rate constant of Bi2212phase increases after the first reduce with the rise of temperature, which is the reason why Bi2212thick films could be obtained with relatively wide holding time.