Preparation And Investigation On Second Generation High Temperature Superconducting Films And Its Buffer Layers Prepared By Laser Chemical Vapor Deposition

Since the discovery of the superconducting materials in 1911,hundreds of superconductors had been developed,including elementary substance,oxides,alloys,organic materials,etc,with their superconducting transition temperature been increased all along.The second generation high temperature superconductor YBa₂Cu₃O_7-x?YBCO?is promising because of its excellent electrical properties and high critical transition temperature.The main preparation methods for YBCO superconducting thin film include metal organic deposition,co-evaporation,magnetron sputtering,pulsed laser deposition and metal organic chemical vapor deposition?MOCVD?.Among these methods,the MOCVD has the advantages of fast deposition rate,pure film composition,uniform coverage,strong adhesion to the substrate and large deposition area.It is suitable for large-scale production of YBCO strips.In this paper,YBCO superconducting film and its buffer CeO₂thin films were prepared by laser chemical vapor deposition?LCVD?on the basis of MOCVD.LCVD can not only reduce the deposition temperature,but also promote the decomposition of precursor molecules and accelerate the reaction process,increasing the deposition rate of the deposited film.Meanwhile,LCVD also has the advantages of selective growth and precise controllable film thickness.In this study,YBCO thin films with pure composition and high quality were prepared on Al₂O₃substrates by LCVD with a solid precursor source supply system.Then the solid source supply system was upgraded to a spray atomizing and coprecipitating system.Then double-layer YBCO/CeO₂films were prepared on the Hastelloy C276 substrate.Effects of experimental conditions on the composition,structure,phase and electrical properties of the YBCO film were investigated.The results showed that the YBCO films contain the following compounds:YBCO,Cu YO₂,Cu₂O,Cu₄O₃,Y₂O₃,Cu O,Cu Ba O₂,Ba Y₂O₄and Ba₂Cu₃O_x.At laser power of 130W,deposition temperature of 1123 K,total pressure of 1 k Pa and precursors evaporating temperature of T_Y=453 K,T_Ba=603 K,T_Cu=453 K,a c-axis oriented YBCO thin films were prepared.The results also revealed that flow rate of the carrier gas Ar also had great effects on the diffusion of the precursor molecue,and finally affect the composition of the YBCO film.YBCO/CeO₂ films were prepared on Hastelloy C276 metal substrate with multi-buffer layers La Mn O₃/Mg O/Gd₂Zr₂O₇by LCVD with a spray atomizing and coprecipitating system.For CeO₂buffer layer thin film,it was prepared at the total pressure range of P_tot=0.2-1.4 k Pa.CeO₂film showed?100?preferred orientation at P_tot=0.2-1.0 k Pa,and it showed?100?-,?111?-,?220?-and?311?-coexisted orientations at P_tot=1.2-1.4 k Pa.When P_tot=0.8 k Pa,the maximum value of the texture coeffect?TC?was 5.9and the minimum FWHM value of the?-scan on the?200?reflection was 3.4o.When P_tot=0.8 k Pa,CeO₂thin film consisted small crystal grains in an average size of 150 nm uniformly arranged and covered with pyramidal top,the thickness of CeO₂thin films was560 nm and the deposition rate(R_dep)was 16.8?m h^-1.YBCO thin film was prepared under laser power of P_L=120 W(T_dep=841 K).YBCO thin film was?110?-oriented,and consisted of?110?oriented particles with a thickness of 510 nm and the bottom part of c-axis oriented grains with thickness of about 120 nm.