Preparation Of YBa₂Cu₃O_7-x/interlayer/YBa₂Cu₃O_7-xTrilayers Using Sol-gel Method

YBa2Cu3O7-x(YBCO)/interlayer/YBCO trilayer architecture,which is a basic structure for superconducting electronics devices,could be used to prepare capacitor,Josephson junction and microwave components.However,several problems exist in preparing the trilayer heterostructure.For examples,the high heat-treatment temperature of YBCO film easily leads to the element interdiffusion during the period of growing the trilayer,the biaxial texture of the whole trilayer is difficult to achieve and the superconductivity of YBCO layer in the trilayer tends to degenerate.Especially that the formation mechanism of superconducting current through the interlayer in the YBCO/interlayer/YBCO trilayer is still ambiguous.In the thesis,based on the sol-gel process,effect of oxygen partial pressure PO2 and temperature T during the heat treatment on the critical current density Jc of YBCO film was firstly investigated.Relationship curve between the PO2 and T for obtaining high-Jc YBCO film was proposed.And it coincided with thermodynamic equilibrium line of YBa2Cu3O7-x corresponding to the oxygen content x=0.91.According to the PO2-T relationship curve,high-performance YBCO films could be obtained even at the lower heat-treatment temperature of 700 ℃.To improve the Jc of YBCO film under magnetic field,LaAlO3 nanodots array was prepared on LaAlO3 substrate using the sol-gel method,then YBCO film was grown on it.The LaAlO3 microarray inset into YBCO film acted as flux pinning sites,thus the Jc of YBCO film under magnetic field was enhanced.Furthermore,YBCO/LaAlO3/YBCO trilayer was prepared.It was found that when the average thickness of the interlayer LaAlO3 in the trilayer was 20 nm,nano-sized amorphous layer and nanocrystalline particles induced by the interlayer LaAlO3 behaved as flux pinning center,thereby the Jc of YBCO film was improved.Based on the above studies on the process conditions of YBCO and YBCO/LaAlO3/YBCO quasi-trilayer films,four kinds of trilayer films of YBCO/LaAl03/YBCO,YBCO/Y2O3/YBCO,YBCO/SrTiO3/YBCO and YBCO/LaNiO3/YBCO were prepared on LaAlO3 substrate by controlling the heat-treatment process of each layer appropriately.The XRD results showed that the trilayers presented outstanding biaxial-texture characteristics,and TEM analysis indicated that epitaxial growth took place among the film layers.Superconductivity with zero resistance was verified both in the top and the bottom YBCO layers in the four trilayers above.And supercurrent could transport through the interlayer in the trilayers when the interlayer thickness was within a certain range(e.g.nearby 30 nm).Nevertheless,their mechanisms were different.With the insulator(LaAlO3 and Y2O3)as the interlayer,the supercurrent was primarily derived from the weak links of point contact and microbridge;With the dielectric(SrTiO3)as the interlayer,the leading supercurrent mechanism transformed from point-contact and microbridge weak links to tunnel with increasing the interlayer thickness gradually;With the normal conductor(LaNiO3)as the interlayer,the supercurrent was dominated by proximity effect.In addition,as for the four trilayer structures,when the dielectric SrTiO3 was selected as the interlayer,the insulating character was optimal.And the requirement for isolating the superconducting electrode in the circuit could be met.