| YBa2Cu3O7-δ (YBCO) thin films have promising electronic and electric applications because of its excellent electrical properties, especially the high critical current density (Jc) in external magnetic fields. Metal-organic deposition using trifluoroacetates (TFA-MOD) process is attractive for its low cost, the ease of scalability and composition control among all the methods. In this dissertation, the structure and properties of the YBCO films with TFA-MOD process were systematically studied.The effect of growth parameters of the TFA-MOD, such as, the annealing temperature, annealing time, oxygen partial pressure, the precursor solution concentration, dipping rate, and the number of coating layers, on the microstructure and properties of YBCO thin films were systematically studied in this dissertation. Under optimized process conditions, high quality YBCO films with Jc of the order of 106 A/cm2(77K,0T) and microwave surface resistance (Rs) of 0.75mΩ(77K, 10GHz) had been grown on LaAlO3 (LAO) single crystal substrates with diameter of 30 mm. Studies of the compatibility of the TFA-MOD process with the MOD buffer showed that current density of the order of 106 A/cm2 (77K,0T) can be obtained for YBCO films on SrTiO3 (STO) buffered LAO single crystals, and the result gives promise to the applicability of the solution deposition techniques to coated conductor fabrication. Films were also deposited on RABiTS Ni substrates with buffers of CeO2/YSZ/CeO2 and CeO2, respectively. And the results indicated that too high temperature and rough surface of the buffer will help the formation of BaCeO3 and deteriorate to the epitaxial growth of the YBCO films. Under optimized process conditions, YBCO films with Tc of 84 K and△Tc of 1.2K can be obtained on CeO2/Ni-5%W.Nanoscale CeO2 particles were decorated on single-crystal STO substrate surfaces using pulsed laser deposited before YBCO precursors coating. Superconducting properties of the YBCO films grown on well controlled CeO2-modified substrates have shown substantial improvement in Jc over those on untreated substrates in almost all the magnetic field (78% increment at 1T, 77K). At the same time, YBCO films were doped with rare-earth element (RE=Y,Ho,Er and Nd) in the TFA-MOD process, and the results indicated that the rare-earth elements in the YRExBa2Cu3O7-δ films are in the form of nanoscale (Y, RE)2O3. The field dependence of Jc demonstrated that YBCO films with Y and Ho doping had enhanced Jc in comparison with stoichiometric YBCO films in the magnetic fields (43% and 240% increment, respectively, at 1T, 77K.). While the YBCO films doped with Nd have poor crystallinity and electrical property.Based on the normal TFA-MOD, fluorine-reduced metal-organic deposition using trifluoroacetates was developed. In this routes, stoichiometric amounts (1:2:3) of yttrium trifluoroacetate, barium trifluoroacetate and copper acetate were chosen as starting substances, methanol as solvent, and triethanolamine as lacquer solvent, the content of fluorine is half of that in the normal TFA-MOD process. The calcining time of the precursor films during 200～250℃was reduced to 5h from 10h and high quality YBCO films with Jc of 2×106 A/cm2(77K,0T), Tc of 88.3 K and△Tc of 0.9 K on LAO substrates were also fabricated.Based on the chemical reaction mechanism during the process of forming YBCO, the models of critical nucleus size, energy barrier and driving force of the chemical reaction from precursor to YBCO phase were founded with thermodynamics. Consequently, an energy-controlled nucleation mode has been suggested and factors of avoiding body nucleation and adjusting interfacial nucleation had been discussed. Also, the nucleation rate is calculated. These models give the informations how to get high quality YBCO films. Our research also shows that the growth and orientation of the YBCO films were affected by the oxygen partial pressure, thickness and number of coating layer of the films in addition to the main factors, T, PH2O and PHF. Through researching on the crystallization dynamics process of the YBCO films, we got the growth rate of the YBCO films which could help us to control the growth of YBCO film with different film thickness.
|
PDF Full Text Request