Preparation And Physical Properties Analysis Of Bi₂Sr₂CaCu₂O_x+8 Superconducting Thick Film

Bi₂Sr₂CaCu₂O_8+x (Bi-2212) superconductors have several advantageous properties, such as it is possessed of high transition temperatures (Tc), and it can transport high currents in high external fields at low temperatures, thus Bi system superconductors have bright prospects in industry. At present Bi system superconducting material is one of the most promising high-temperature superconductors which applied to the electric power devices as well as strong magnetic fields, for example filters, fault-current limiter, superconducting magnets, nuclear magnetic resonance spectroscopy, and so on. Hence the preparation technology for low cost, high performance Bi system superconducting thick films becomes the current focus in the field of high temperature superconducting materials.In this thesis, we study on the phase equilibrium of the Bi-Sr-Ca-Cu-O system and perform a systematic research of the preparation of Bi-2212 superconducting thick films from commercial precursors by chemical solution method. Considering the demand for low-costs and large-scale production, chemical solution method is the most prospective techniques for industrialization in the preparation of the thick film, which mainly depends on partial melting maximum temperature, solidification temperature, process oxygen atmosphere, and cooling rate. By the measurements of X-ray diffraction, scanning electron microscopy, resistance-temperature curve,the structure and character of this thick film was discussed.Bi-2212 phase has a transition temperature (Tc) around 80-90K, which depends on precise control of synthesis conditions. The preparation of thick films uses spin coating method by spinning the prepared chemical solution on the single crystal substrates. The optimized heat treatment condition were screening through a series of comparison experiments and then optimize experiment, mainly including heat treatment parameters, oxygen partial pressure, and annealing temperature. Firstly, the effect of partial melting Tmax on transition temperature of Bi-2212 has been investigated. At highest temperatures, the thick film is partially molten to promote the Bi-2212 phase peritectic reaction, and upon cooling, Bi-2212 is re-formed from the liquid phase. Secondly, during the cooling from the partial melting Tmax to solidification temperature, the cooling rate must be slow enough to enable the film to take up oxygen from the atmosphere and the solid peritectic phases to dissolve in the liquid in order to deliver further actions.Through our research on the preparation process of Bi-2212 thick film, we found that the optimal maximum temperature and solidification temperature for thick film with highly c-axis orientation texture was 888°C and 845°C, respectively. And the cooling rate choose natural cooling in oxygen atmosphere (pO₂=0.1atm). Bi-2212 film has shown good superconducting properties derived from optimum conditions, and superconducting transition temperature point T_c^onsetwas improved to 93.3K. But it needs further study on reduce the second phase of thick film and prepare high quality Bi-2212 thick films with a sharp transition.