| REBa2Cu3O7-x(RE=rare earth,REBCO)based second generation high-temperature superconducting tape is also called“coated conductor”,for having a metal substrate and multi-layer oxides.Comparing with first generation Bismuth-based high-temperature superconductor,REBCO superconducting material has higher critical current density and higher upper critical field.The upper critical field of second generation high-temperature superconducting tape at 77 K is up to 7 T,while at 4.2 K,the upper critical filed can be more than 100 T.What's more,second generation high-temperature superconducting tape has smaller AC loss.Therefore,it has a broader prospect in the field of strong electricity application.In practical application for the field of strong electricity,critical current(Ic)is used to present the capacity of the tape to carry the current.The value of Ic is equal to the product of the critical current density(Jc)and the cross-sectional area of the superconducting layer.Therefore,one fundamental way to improve Ic is to increase the thickness of the superconducting layer.However,in previous studies,the“thickness effect”of REBCO film was found,that is,with the increase of film thickness,the increase of Ic would slow down.Another hot spot in research is that the tapes are often applied under magnetic field.As a result,in preparation of REBCO film,the electrical transport properties under magnetic field is also a vital parameter.According to the references above,this paper mainly studies the production process,growth mechanism and physical characteristics of superconducting layer in the following aspects.(1)In the preparation of the second generation high-temperature superconducting tape,different laboratories and companies adopt different process routes.In order to obtain high-quality film,we need to find suitable process window in advance.In this text,we chose Y0.5Gd0.5Ba2Cu3O7-x(YGBCO)as the material of superconducting layer.And a high-efficient reel-to-reel pulsed laser deposition system is used to prepare the superconducting layer.We determined the parameters of obtaining the superconducting layer according to our former experience since 2007,while some other parameters were determined by means of controlling variate method.As a result,we obtain a pure c-axis oriented YGBCO film with a smooth surface.The YGBCO films were deposited using the following experimental parameters:laser repetition frequency of 160 Hz,laser power density of 1 J/cm2,distance between the substrate and the target of 4 cm,substrate temperature of 810?to 830?,and oxygen partial pressure of 200 m Torr.(2)As mentioned above,for higher Ic,increase of the thickness of superconducting layer is a simple approach.However,due to the“thickness effect”,Jc will decrease with the increase of the film thickness.In this study,in order to obtain higher Jc,we adopted a method called multi-step deposition process,which means every deposition cycle we deposited a 240 nm-thick film and got thicker film through the repeat of the deposition cycle.The advantage of this method is that we can control and optimize the process parameters of each cycle.Experimental results show that,compared with previous results,this method can smoothen the film surface,and to some extent inhibit“thickness effect”in thick YGBCO film.Jc of a 1.2?m-thick YGBCO film at 77 K and self-field is still up to3.3 MA/cm2.In addition,in the study of the“thickness effect”,we found that at 77K and self-field the Jc of 4.3 MA/cm2 in the 480 nm-thick film was higher than Jc of 4.17 MA/cm2 in the 240 nm-thick film.This result was not reported before.After we calculated the internal residual strain according to X-ray diffraction patterns of the samples,we found that the internal residual strain in the 240 nm-thick film was around 0.232%,while in the films thicker than 480 nm the internal residual strain was almost 0.We concluded that by using multi-step deposition process,the internal residual stress caused by the lattice mismatch between the buffer layer and the superconducting layer was no longer observed.And the release of the internal residual stress in the 480 nm-thick film is the main reason for its high Jc.Further improving the thickness of the film can lead to the degradation of the surface morphology and crystallization of the samples,then resulting the decrease of Jc.(3)In previous reports,a structure of superconductor/insulator/superconductor is treated as an effective way to inhibit the“thickness effect”.However,in the past research,the thickness of the insulator inter-layer is generally controlled less than 20 nm.In this study,we chose YGBCO as superconducting material,and strontium titanate(Sr Ti O3,STO)as the insulator material for its small lattice mismatch to YGBCO.We prepared a series of samples with different STO inter-layer thickness ranging from 10 nm to 80 nm.The results show that the surface of the film become smoother,and the electrical transport performance under magnetic field is improved by adding the inter-layer.We also found that when the thickness of the inter-layer was 80 nm,the surface morphology and electrical transport performance were the best,Jc was improved from 4.0MA/cm2 to 4.4 MA/cm2 at 77 K and self-field.In addition,we present a reasonable explanation to these observed experimental results.(4)After getting the best thickness of the inter-layer,we prepared a set of YGBCO/STO/YGBCO tri-layer films asymmetrically.Which means,the total thickness of the superconductor films are the same while the thickness of YGBCO bottom-layer differs from each other.It is found that the added inter-layer can still improve the electrical transport performance of the samples under magnetic field.Jc is higher in the sample with a thicker bottom-layer.We believe that internal residual stress should be reduced before the deposition of the STO inter-layer.(5)Through the research above,we conclude that the internal residual stress caused by the lattice mismatch between buffer layer and superconducting layer plays a vital role in the effect on the film's electrical transport properties.Furthermore,the internal residual stress can be removed by multi-step deposition process.Therefore,in the last part of this text,we attempted to introduce a YGBCO“seed layer”into the preparation process.That is,depositing a thin layer of YGBCO on the Ce O2 buffer layer as a growth template before the homoepitaxial growth of the further deposition.Through the comparison of samples with different thick“seed layer”,we found that a 15 nm-thick“seed layer”was the most ideal,Jcincreased from 3.17 MA/cm2 to 4.17 MA/cm2 at 77 K and self-field.In conclusion,through research,we have successfully prepared a YGBCO superconducting film with pure c-axis orientation and high-quality surface morphology.For thin YGBCO film,Jc at 77 K and Self-field was above 4 MA/cm2.When the thickness was over 1?m,Jc at 77 K and self-field was also high of 3.3 MA/cm2.The results are no less impressive than the world's peers. |
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