Enhanced superconducting performance in solution-derived YBCO-BaZrO3 composite film

Li Lei,Limin Li,Xiuting Wng,Zho Zhng,Goyng Zho,c,∗∗,Jiqing Ji,Fuxue Yn,Zongfn Dun,Lihu Jin,Chengshn Li,Pingxing Zhng

a Advanced Material Analysis and Test Center,Xi'an University of Technology,Xi'an,Shaanxi,710048,China

b School of Material Science and Engineering,Xi'an University of Technology,Xi'an,Shaanxi,710048,China

c Key Laboratory of Electrical Materials and Infiltration Technology of Shaanxi Province,Xi'an,Shaanxi,710048,China

d Northwest Institute for Nonferrous Metal Research,Xi'an,Shaanxi,710016,China

ABSTRACT YBa2Cu3O7-δ/BaZrO3 (YBCO/BZO) composite film with enhanced flux-pinning performance was prepared on LaAlO3 (LAO) substrate via a fluorine-free sol-gel method in combination with a novel photosensitive sol-gel lithography technique.Owing to the difference of lattice mismatch between the interfaces of YBCO/LAO and YBCO/BZO,high-density stacking faults were successfully introduced into YBCO matrix,which has been confirmed by XRD and HRTEM analysis.Magnetization measurement results indicate that YBCO/BZO composite film has a stronger current-carrying capacity than the pure YBCO film especially under applied magnetic fields.However,the critical current density of YBCO/CeO2 composite film is much lower than that of the pure YBCO film at all the applied magnetic fields due to the chemical reaction between YBCO and CeO2.Therefore,it can be concluded that the flux-pinning performance of YBCO film can be improved by the lattice distortions within YBCO matrix as long as the metal oxide microarrays cannot react with YBCO films.

Keywords:Epitaxial growth Sol-gel preparation Superconductors Flux pinning YBCO

1.Introduction

In recent years,REBCO Coated Conductors (CCs) have been receiving much attention in electrical power applications such as superconducting transmission cables,superconducting fault current limiter and superconducting transformer due to their better electrical and magnetic properties than any other superconductors[1-6].However,it has been well known that the superconducting performance of YBCO films especially critical current density will suffer from a dramatic decrease when they are exposure to a high magnetic field [7].To overcome this problem,many researchers have attempted to introduce artificial pinning centers into YBCO matrix using different approaches such as nanoparticle addition [8],substrate decoration [9],element substitution [10]and artificial crystal defects [11].Whichever method is used to do that,the purpose is the same,that is,we are trying to introduce a certain amount of non-superconducting regions with nanoscale (several nanometers) into YBCO film as pinning centers to enhance their flux-pinning force and thus significantly improve the critical current density under magnetic field.A.Llordés and X.Obradors et al.reported that an outstanding vortex pinning enhancement can be produced by nanoscale strains within YBCO film.This new vortexpinning mechanism is based on bond-contraction pairing model,where pair formation is quenched under tensile strain,forming new and effective core-pinning regions [12].This result enlightened us that the flux pinning performance of YBCO film could be significantly improved as long as the lattice distortion can be introduced into the YBCO matrix.In our previous work,we have obtained strongly enhanced flux-pinning performance induced by nanoscale lattice strains in YBCO films grown on Ba1-xSrxTiO3(BST) microarrays buffered LaAlO3(LAO) substrate[13].However,the influence of lattice constant of the metal oxide microarray on the formation of lattice strains has not been discussed yet.

In this present work,we selected three kinds of metal oxides(SrTiO3(STO),BaZrO3(BZO)and CeO2)with different lattice constant to prepare the corresponding microarrays and serve as the surface decorators on LAO substrates prior to the deposition of YBCO films.And then the difference of superconducting performance especially the current-carrying capacity under magnetic field was investigated.The critical current density of YBCO film decorated by BZO microarrays was improved remarkably at both self-field and magnetic field.At the same time,it was accidentally found that the superconducting performance of YBCO film was degraded when the microarrays (CeO2) used to decorate substrate can react with YBCO film during the film growth.

2.Experimental

Three kinds of metal oxide (BZO,STO and CeO2) microarrays were prepared by a distinctive photosensitive Sol-Gel lithography.Take BZO for example,the patterning process of this photolithography is shown in Fig.1(a).The photosensitive BZO gel film was irradiated by UV light passing through a mask,then the unirradiated areas of the gel film were dissolved in organic solvent while the irradiated areas were retained.Thus,the BZO microarray in the gel state which has the same pattern as the negative image of the mask was obtained.Then,the gel-state microarray was converted to oxide microarray after a proper heat treatment.The surface micro-morphologies of BZO microarray before and after heat treatment are shown in Fig.1(b)and Fig.1(c),respectively.The profile lines of BZO microarray are measured by a 3D measuring laser microscope and the results are shown in Fig.1(d) and (e) as well as the tables under them,respectively.As presented in the tables,the average diameter of one circular array before heat treatment is calculated to be 49.8 μm which is slightly larger than that (47.6 μm) after heat treatment,while the average thickness (21.3 nm) of the latter is much thinner than that(215 nm)of the former.Finally,YBCO film was deposited on BZO microarray buffered LAO substrate through a fluorine-free sol-gel approach.The detailed preparation procedures can be learned from our previous works [14,15].In addition,a pure YBCO film was directly prepared on a bare LAO substrate by the same process as that for the YBCO/BZO composite for comparison.The thicknesses of the pure YBCO film and the YBCO/BZO composite are measured to be around 235 nm and 206 nm,respectively.

An Olympus OLS4000 type 3D measuring laser microscope was employed to observe the micro-morphology and measure the geometric dimensions of YBCO/BZO composite film.The growth orientation and in-plane and out-of-plane textures of YBCO films were evaluated by a Shimadzu 7000S type X-ray diffractometer (XRD) and a Rigaku SmartLab XRD,respectively.The internal microstructure of the nanocomposite films was analyzed by a 3010F-type high-resolution transmission electron microscope (HRTEM).Magnetization critical current density measurement was performed on a VersaLab-VSM system(Quantum Design)to analyze the flux pinning properties of YBCO films.The applied magnetic field with a strength of 0-3 T was paralleled to the surface of YBCO films during the magnetization measurement.

3.Results and discussion

As shown in the upper part of Fig.2(a),YBCO/BZO composite film only presents seven diffraction peaks corresponding to (001)～(007)planes except for the reflections from(001),(002)planes of BZO,which indicates that YBCO film has excellent (00l) preferential orientation without any negative impact from BZO microarrays.In the lower part of Fig.2(a),YBCO/STO composite film also presents perfect(00l)epitaxial growth along with the overlapped diffraction peaks from (001)STO/(003)YBCOand (002)STO/(006)YBCO.It is obviously learned that YBCO films realize good epitaxial growth on (001)-oriented BZO and (001)-oriented STO microarrays buffered LAO single crystal substrates,that is,the BZO and STO microarrays play an essential role of transmitting the texture of LAO substrates to YBCO films.To evaluate the growth texture of YBCO films,XRD φ-scan pattern of (103) plane of YBCO/BZO composite film was shown in Fig.2(b).There are four diffraction peaks appeared with a four-folder symmetry,i.e.,the diffraction peaks appear every 90° in Fig.2(b),and the average FWHM is around 1.26°,which indicates that YBCO film exhibits a good in-plane texture,that is,the intergranular angle of all YBCO grains which is formed by a rotation around the c-axis of YBCO is quite small.XRD ω-scan pattern of (005)plane of YBCO/BZO composite film was shown in Fig.2(c).There is only one diffraction peak appeared at the omega angle of 19.23°with a full width at half maximum (FWHM) of around 0.29°,which suggests that YBCO film has a good out-of-plane texture,that is,the c-axis directions of all YBCO grains tend to be perpendicular to the surface of YBCO film.However,it should be noted that the top of the peak does not look very sharp but relatively flat,which indicates that the interplanar spacing of YBCO (005) plane has changed within a quite small range.That is,a certain amount of lattice distortions or strains probably exist in the matrix of YBCO film.Williamson-Hall method is an effective method to quantitatively analyze the internal strains of YBCO films with X-ray diffraction line-broadening [16,17].The relationship between the full width at half maximum (FWHM) of X-ray diffraction peaks and the lattice strains is given by the following equation (1),

where B is the FWHM of diffraction peaks of(hkl)planes,θ is the Bragg angle of the corresponding (hkl) plane,ε is the lattice strain,λ is the wavelength,and D is the size of the YBCO grain (here it is approximately equal to the thickness of YBCO film).A linear function was obtained by plotting B2cos2θ against sin2θ.Thus,ε can be easily calculated through the slope 16ε2of the linear curve.The typical Williamson-Hall plots from the (00l) reflections for the YBCO/BZO,YBCO/STO composite films and the pure YBCO film (for comparison)are presented in Fig.2(d).It can be learned from Fig.2(d) that the lattice strains of the YBCO/BZO,YBCO/STO composite films and the pure YBCO film are 0.32%,0.15% and 0.13%,respectively.It is obviously learned that the YBCO/BZO composite film contains more crystal defects than the other two.These defects may serve as pinning centers to enhance the flux pinning force of YBCO/BZO composite film.The almost same amount of lattice strains for YBCO/STO composite and pure YBCO film can be attributed to small lattice mismatch between STO and YBCO (LAO) which is much smaller than that between BZO and YBCO (LAO).Typically,more lattice strains within YBCO matrix will lead to much stronger flux-pinning performance.It is well confirmed by the cross-sectional high-resolution TEM analysis of YBCO/BZO composite film presented in Fig.2(e).The TEM image was taken at the area which locates nearby and above one of the BZO microarrays.As seen in Fig.2(e),a considerable amount of short stacking faults(denoted by red arrows) appeared in the YBCO matrix and several crystal planes have bent significantly (marked with blue arrows) close to the top surface of YBCO film.It is believed that the stacking faults and the bent planes caused the lattice distortions,i.e.,a slight change of the interplanar spacing of YBCO (005) plane,thus resulting in the flatted top of the diffraction peak in ω-scan pattern (Fig.2(c)).

The magnetic-field dependence of critical-current density (Jc-H curves) is shown in Fig.3(a).Jc-H curves can be obtained from M-H loops according to the Bean Model [18].As seen from Fig.3(a),the Jcvalues of the YBCO/BZO composite film are much higher than those of the pure YBCO film under almost all the applied magnetic fields at both 50 K and 70 K.The magnetic field dependence of the pinning force Fp-H(where Fp=Jc(H) × H) of these two films is shown in Fig.3(b).As shown in Fig.3(b),the maximum pinning force of the YBCO/BZO composite film is up to 52.5 GN m−3for 50 K and 5.6 GN m−3for 77 K,which is much higher than that of the pure YBCO film at the same temperature.These results confirm that the lattice distortions caused by the stacking faults and the bent planes observed in Fig.2(e) indeed provide a gigantic flux pinning force for the YBCO/BZO composite film.

To verify whether the other two metal oxide microarrays can also play the same role in enhancing the flux-pinning performance of YBCO film,the flux-pinning performance measurement were also carried out on YBCO/STO and YBCO/CeO2composite films.Fig.3(c) shows the magnetic-field dependence of Jcat 50 K for the pure YBCO film (for comparison),YBCO/STO and YBCO/CeO2composite films.There is no distinct Jcimprovement found in the Jc-H curve of YBCO/STO composite compared with the pure YBCO film probably because of the small lattice mismatch and the close amount of lattice strains within them.However,the results are surprising,that is,the Jcvalues of the YBCO/CeO2composite film are much lower than that of the former two prepared under the same condition at all the applied magnetic fields(0～3 T).The degraded superconducting property of YBCO/CeO2composite film can be attributed to the formation of the second phase BaCeO3through the chemical reaction between YBCO and CeO2microarrays and the appearance of (103),(020)-oriented YBCO grains,which is presented in the XRD result shown in Fig.3(d).The internal microstructure of YBCO/CeO2composite film was further analyzed by HRTEM and the results are shown in Fig.3(e).It is found that not only the stacking faults were not produced within YBCO matrix but also a second phase BaCeO3was generated by a chemical reaction between YBCO and CeO2.The intergranular spacing of(110)plane of BaCeO3is measured to be around 0.31 nm after applying a Fourier filter on the TEM image shown in the inset of Fig.3(e) marked with the dark blue dash line.The above results reveal that the metal oxide microarray which has a moderate lattice mismatch with YBCO can introduce a certain amount of crystal defects within YBCO matrix and thus effectively enhance the flux-pinning performance of YBCO film provided that the selected metal oxide cannot react with YBCO film.

4.Conclusions

YBCO/BZO composite film with strong flux-pinning performance was fabricated by chemical solution-based process in combination with a novel lithography technique.XRD and HRTEM analysis show that YBCO/BZO composite film exhibits highly (00l)-preferred orientation and the high-density stacking faults were introduced into YBCO matrix.It is found that the stacking faults can effectively promote the enhancement of the flux-pinning performance of YBCO/BZO composite film,while the superconducting property of YBCO/CeO2microarray was greatly degraded owing to the reaction between YBCO and CeO2.Therefore,it can be learned that the crystal defects will not be produced within YBCO film if the selected metal oxide can react with YBCO,which will lead to deterioration of superconductivity.

Declaration of competing interest

The authors declare that they do not have any commercial or associative interest that represents a conflict of interest in connection with the manuscript submitted.

Acknowledgements

This project is supported by the National Natural Science Foundation of China(No.51202190 and 51672212)and also supported by the Project of Science and Technology of Shaanxi Province (No.2014KJXX-63).The author would like to acknowledge China Scholarship Council (CSC) for sponsoring my study abroad.