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Interlayer structure in YBCO-coated conductors prepared by chemical solution deposition

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2013 Supercond. Sci. Technol. 26 075016

(http://iopscience.iop.org/0953-2048/26/7/075016)

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Supercond. Sci. Technol. 26 (2013) 075016 (8pp) doi:10.1088/0953-2048/26/7/075016

Interlayer structure in YBCO-coatedconductors prepared by chemical solutiondeposition

Leopoldo Molina-Luna1,2, Ricardo Egoavil1, Stuart Turner1,Thomas Thersleff3,4, Jo Verbeeck1, Bernhard Holzapfel3, Oliver Eibl5

and Gustaaf Van Tendeloo1

1 EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium2 Technische Universitat Darmstadt, Department of Material and Geosciences, Schnittspahnstraße 9,D-64287 Darmstadt, Germany3 IFW Dresden, PO Box 270116, D-01171 Dresden, Germany4 Department of Engineering Sciences, Division of Applied Materials Science, Uppsala University,Box 534 SE-751 21 Uppsala, Sweden5 Eberhard Karls Universitat Tubingen, Institute of Applied Physics, Auf der Morgenstelle 10,D-72070 Tubingen, Germany

E-mail: molina@geo.tu-darmstadt.de

Received 14 December 2012, in final form 8 April 2013Published 28 May 2013Online at stacks.iop.org/SUST/26/075016

AbstractThe functionality of YBa2Cu3O7−δ (YBCO)-coated conductor technology depends on thereliability and microstructural properties of a given tape or wire architecture. Particularly, theinterface to the metal tape is of interest since it determines the adhesion, mechanical stabilityof the film and thermal contact of the film to the substrate. A trifluoroacetate (TFA)—metalorganic deposition (MOD) prepared YBCO film deposited on a chemical solution-derivedbuffer layer architecture based on CeO2/La2Zr2O7 and grown on a flexible Ni5 at.%Wsubstrate with a {100}〈001〉 biaxial texture was investigated. The YBCO film had a thicknesswas 440 nm and a jc of 1.02 MA cm−2 was determined at 77 K and zero external field. Wepresent a sub-nanoscale analysis of a fully processed solution-derived YBCO-coatedconductor by aberration-corrected scanning transmission electron microscopy (STEM)combined with electron energy-loss spectroscopy (EELS). For the first time, structural andchemical analysis of the valence has been carried out on the sub-nm scale. Intermixing of Ni,La, Ce, O and Ba takes place at these interfaces and gives rise to nanometer-sized interlayerswhich are a by-product of the sequential annealing process. Two distinct interfacial regionswere analyzed in detail: (i) the YBCO/CeO2/La2Zr2O7 region (10 nm interlayer) and (ii) theLa2Zr2O7/Ni–5 at.%W substrate interface region (20 nm NiO). This is of particularsignificance for the functionality of these YBCO-coated conductor architectures grown bychemical solution deposition.

(Some figures may appear in colour only in the online journal)

1. Introduction

Coated conductors are of great interest for diverse applicationsin energy and magnet technology, such as power cables,transformers and current limiters [1]. In recent years,the chemical solution deposition (CSD) method has been

established as a viable route towards the fabrication oflow-cost, highly efficient coated conductor technology [2].Nevertheless, further improvement is still needed, andsuperconducting tape and wire fabrication remains agreat challenge [3]. For further improvement a thoroughunderstanding of the structural and chemical processes on the

10953-2048/13/075016+08$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Supercond. Sci. Technol. 26 (2013) 075016 L Molina-Luna et al

nanometer scale and a correlation with the physical propertiesis fundamental [4].

In this contribution we present results from a fullyprocessed YBCO-coated conductor prepared by chemi-cal solution deposition based on metal organic deposi-tion (MOD); trifluoroacetate (TFA)-YBCO deposited on aCeO2/La2Zr2O7/Ni-5 at.%W layer architecture. Extensivereports in the literature can be found on this particular archi-tecture and fabrication process [5, 6]. The goal of the presentcontribution is to use high-angle annular dark-field scan-ning transmission electron microscopy in combination withelectron energy-loss spectroscopy to investigate interlayerformation and possible interdiffusion/intermixing processes atthese interfaces. In a recent study on La2Zr2O7/Ni-5 at.%Winterfaces with STEM–EELS with a high spatial and energyresolution, it was shown that intermixing occurs at theseinterlayers and a nanometer sized oxide layer forms as aby-product (depending on the annealing conditions), beyondwhich no further diffusion could be measured. Neverthelessthe LZO buffer layer still acts as an efficient diffusionbarrier [7]. In order to obtain the full YBCO-coated conductorarchitecture, a sequential annealing process is needed todeposit the buffer layers and the final overlaying YBCOsuperconducting layer [5].

La2Zr2O7 buffer layers deposited on a Ni5 at.%Wsubstrate, prepared under reduced conditions, give rise toa gradual oxidation process leading to nanometer sizedintermediate oxide layer. La2Zr2O7 (LZO) thin films areused for the fabrication of low-cost buffer layers because oftheir scalability, compatibility with the high critical currentof YBCO and the small lattice mismatch of the a or baxis with that of YBCO (0.5% and 1.8%, respectively) [5].In order to analyze this process and the effect of furtherdeposition it is necessary to recognize two distinct thermaleffect conditions: (i) short range (less than 10 nm) and (ii)long range (several tens of nanometers) and relate these tothe annealing process steps. Samples with the full architecturehave been reported to have up to 100 nm sized oxide layersin the literature [8, 9], this implies that a further reactionprocess is responsible for the increased oxide layer thickness.Furthermore, the implications of depositing a CeO2 bufferlayer and the final superconducting YBCO layer towardsthe formation of reaction interlayers are investigated. Theformation of BaCeO3 has been previously reported [10].

2. Experimental details

Flexible cube textured Ni5 at.%W substrates of 80 µm inthickness and 1 cm in width were used for deposition [11].Details on substrate preparation can be found in [12]. There-crystallized substrates were cleaned in both an ultrasonicacetone and an isopropanol bath for 20 min. The substratewas dip-coated twice in a LZO precursor solution, followedby a CeO2 precursor solution. TEM cross-sections ofexample layer architectures have been previously studied [13].The sample preparation procedure is described in detailelsewhere [5, 14, 15]. The YBCO-coated conductor sampleinvestigated in this work has a critical current density (jc) of

Figure 1. Methodology.

1.02 MA cm−2 at 77 K and self-field, a Ic of 44.8 A cm−1 andan Tc of ∼90 K. The YBCO sample thickness is 440 nm.

A TEM cross-section electron transparent lamella wasprepared using a FEI Nova Nanolab 200 DualBeam SEM/FIBsystem by means of the ‘in situ lift-out’ technique. The samplewas cut parallel to the {100}〈001〉 oriented biaxially texturedNi5 at.%W substrate. HAADF-STEM and STEM–EELSexperiments were performed on a FEI TITAN ‘cubed’ 80-300transmission electron microscope operated at 300 kV, withaberration correctors for both image and probe forming lenses.Spectroscopy experiments were performed on a GIF Quantumspectrometer. The energy resolution for the EELS data infigures 3 and 5 was approximately 1 eV. For the EELS datain figures 7 and 8, the electron monochromator was usedto optimize the energy resolution for EELS measurementsto 400 meV. The STEM experimental conditions and thecorresponding relevant ionization edges are given in table 1.The sample was tilted into the [001] zone-axis of Ni tokeep the interface parallel to the electron beam for theSTEM–EELS measurements. EELS scans were performedacross the complete YBCO-coated conductor sample and atdistinct interfaces.

The EELS spectra were analyzed using Digital Mi-crograph and EELSMODEL software [16]. Where mapped,the EELS signal strength was extracted from the data byintegrating the intensity under the corresponding background-subtracted EELS edge using a suitable integration window inDigital Micrograph. To improve the signal to noise ratio inthe elemental maps, principal component analysis (PCA) wasapplied in the EELS data sets [18, 19] prior to mapping.

The plots in figures 3, 5 and 7 were generated in a similarmanner, but no PCA data treatment was used to generate theplots. To be able to extract the Ni L2 edge signal strength fromthe EELS dataset in figure 5, model-based fitting of the Ni L2edge was adopted to overcome the overlap between the NiL2,3 edge and the La M4,5 edge. The complete methodologyof the procedure is given in figure 1.

3. Results

In this section the results obtained on a fully processedYBCO/CeO2/La2Zr2O7/Ni-5 at.%W coated conductor sam-ple will be presented along with the corresponding detailed

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Table 1. Experimental details.

Ionization edges O K Ba M4,5 La M4,5 Ni L2,3 Ce M4,5532 eV 781 and 796 eV 832 and 849 eV 854 and 873 eV 883 and 902 eV

HAADF-STEMparameters

Convergence semi-angle (α) Collection semi-angle (β) Energy resolution22 mrad (figures 2–5), 18 mrad

(figures 6–8)94 mrad 1 eV (figures 3 and 5),

0.4 eV (figures 7 and 8)Probe size High tension

∼1.0 A (figures 2–5),∼1.5 A (figures 6–8)

300 kV

YBCO Interdiffusion layer 1 LZO Interdiffusion layer 2 Ni5 at.%WThickness 440 nm ∼10 nm ∼100 nm ∼20 nm ∼80 µm

Figure 2. HAADF-STEM image of the YBa2Cu3O7−δ/CeO2/La2Zr2O7/Ni-5 at.%W sample in cross-section.

analysis by scanning transmission electron microscopy incombination with electron energy-loss spectroscopy. Figure 2shows a HAADF-STEM image of a coated conductorsample with YBa2Cu3O7−δ/CeO2/La2Zr2O7/Ni-5 at.%Warchitecture viewed in cross-section. The box area in theimage indicates the region used for the STEM–EELSmeasurements. Two distinct interfaces are observed: labeledin the image as region (1), which corresponds to theLa2Zr2O7/Ni-5 at.%W interface area (bottom) and region (2),which includes the YBa2Cu3O7−δ/CeO2/La2Zr2O7 interfacearea (top).

Figure 3(a) shows a quantitative color map withBa (blue), La (green), Ni (purple) and Ce (red) EELSintensities extracted from the Ba-M4,5, La-M4,5 Ni-L2,3 andCe–M4,5 edges in the acquired dataset. Distinct sampleareas can be clearly identified and are indicated. Figure 3(b)is the corresponding background-subtracted EELS scan(horizontally integrated EELS datacube), the vertical axis isthe scan distance across the interfaces in nanometers andthe horizontal axis is the energy loss in electron volts. Theionization edges corresponding to O-K, Ba-M4,5, La-M4,5Ni-L2,3 and Ce–M4,5 are clearly identified. There is an

overlap between the La-M4 and Ni-L3 ionization edges,making it difficult to subtract a background for the Ni L2,3edge. Figure 3(c) shows normalized quantitative elementalprofiles for O, Ba, La, Ni and Ce obtained from an integratedsignal region across the interfaces shown in figure 3(a).

Figure 4 shows a HAADF-STEM image of region (1).The LZO layer can be clearly identified as the top layer.The dark contrast features are nanometer sized voids. At theinterface with the nickel tungsten substrate a ∼22 nm sizedoxide layer is observed. The thin dark contrast band abovethe nickel tungsten substrate suggests a relative lack of heavytungsten atoms at the interface. A gradually increasing changein the contrast is observed from top to bottom. Dark contrastareas can also be observed due to nickel deficiency.

Figure 5(a) is a quantitative color map of the sameregion shown in figure 4. The nickel tungsten substratecan be clearly distinguished from the oxide layer region. Aclear difference in this oxide layer can be observed withan intermixing region containing La, Ni and O. Figure 4(b)shows the background-subtracted EELS scan in this region.The O-K, La-M4,5 and Ni-L2,3 ionization edges can beclearly identified. Figure 4(c) shows normalized quantitativeelemental profiles for O, La and Ni obtained from anintegrated signal of this region. A 22 nm thick oxide layer isformed at the nickel tungsten–LZO interface. In a previousstudy it was demonstrated that a thin intermixing layer ofLa2Ni2O5 and ZrO2 forms [7].

Figure 6 is an HAADF-STEM image of the top interfacearea. The bottom area corresponds to the LZO buffer layer;again the dark contrast corresponds to nanometer sized voids.The rough, dark contrast layer corresponds to the CeO2film area on top of which an YBCO superconducting layeris deposited. The box indicates the region used for theSTEM–EELS measurement. This region is denoted as (2)in the figure. Figure 7(a) shows the quantitative elementalcolor map of this region. Intermixing of La, Ce and Ba canbe clearly observed in the waterfall EELS scan shown infigure 7(b). A small energy shift in the Ce M4,5 edge withinthe intermixing region (La, Ce, Ba) as compared to the CeO2layer area can also be observed in figure 7(b). The collectedCe M4,5 data is shown in more detail in figure 8, togetherwith references for Ce4+ and Ce3+. The small shift in theCe M4,5 edge onset together with the appearance of extrafeatures (indicated by arrows) point in the direction of a partialvalency change from Ce4+ to Ce3+ in the Ce/Ba intermixedregion [21].

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Supercond. Sci. Technol. 26 (2013) 075016 L Molina-Luna et al

Figure 3. (a) Quantitative color map obtained from the PCA-treated 2D EELS datacube. (b) STEM–EELS two-dimensional scan across theinterface area shown in figure 2. (c) EELS line profile (nm) as a function of the normalized core-loss signal (au).

Figure 4. HAADF-STEM image of the La2Zr2O7/Ni-5 at.%Wsample region 1 in cross-section. The green box indicates the areaused for STEM–EELS measurements.

4. Discussion

4.1. Methodology

A fully processed YBCO-coated conductor sample wasprepared by chemical solution deposition methods, as

described in detail in section 2 and references therein.Two-dimensional EELS spectrum images were acquired withscanning transmission electron microscopy combined withelectron energy-loss spectroscopy to investigate the interfacesfound in figure 2. Quantitative elemental maps correspondingto the intensity under the O-K, Ba-M4,5, La-M4,5, Ni-L2,3and Ce–M4,5 edges were generated. Prior to mapping, thedata was treated with PCA to improve the signal to noiseratio in the EELS data. This technique is a powerful toolto reduce the noise from STEM–EELS data sets, allowingone to extract the essential chemical information [18, 19].No PCA data treatment was used to generate the elementaldistribution plots. The complete methodology procedure isgiven in figure 1.

4.2. Interface structure

Nanovoids are observed in the La2Zr2O7 layers, are atypical feature found in LZO buffer layers derived fromchemical solution, and have already been studied in greatdetail in previous contributions [7, 8, 17]. From the HAADFimages, the interface towards the Ni substrate, interface region(1) appears as a dark layer, suggesting a tungsten and/ornickel deficiency at the interface, similar to that reported forLa2Zr2O7 buffer layers [7]. From our EELS profiles we canestimate approximately 22 nm of intermixing of La, Ni andO (region 1) and approximately 10 nm of intermixing La, Ce,O and Ba (region 2). The LZO layer is 100 nm in thickness.In order to analyze the intermixing process of each formed

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Figure 5. (a) Quantitative color map obtained from the PCA treated 2D EELS datacube for region 1. A region of intermixing is identified.(b) STEM–EELS two-dimensional scans across the interface area shown in 4(b). (c) EELS line profile (nm) as a function of the normalizedcore-loss signal (au).

interface layer in detail, each interface region was studiedseparately at higher magnification.

It is known that a nanometer sized oxide layer formsafter heat treatment and deposition of the buffer layers.Due to the repeated heat treatment to which the sampleis exposed after each buffer layer deposition and the finalYBCO superconducting layer, the oxide layer formed at thebottom interface area increases in size; from a few nanometers(2 nm in a case reported previously [7]) up to 22 nmfor a fully processed YBCO-coated conductor described inthis study. Beyond this thin oxide layer no further nickelcontribution could be measured by STEM–EELS. Previousresults showed that a thin intermixing layer of La2Ni2O5 andZrO2 forms [7]. This result was confirmed in the presentcase and an increase in the layer thickness was observed.Even though this increased layer thickness is a product of therepeated annealing procedure, the LZO layer still acts as anefficient diffusion barrier.

The EELS data suggest a partial change in valency fromCe4+ in the CeO2 region to a mixed valency of Ce4+ andCe3+ at the top intermixing region [20]. This interface isimportant, because it determines the epitaxy of the overlayingYBCO superconducting layer [22]. We observed that theBa concentration reaches within a few nanometers of theLZO buffer layer, beyond which no further signal could bemeasured. No remarkable changes are found for the La M4,5and Ba M4,5 edges. Figure 7(c) shows normalized quantitativeelemental profiles for Ba, La and Ce obtained from anintegrated signal of this region. A 13 nm thick oxide layer isformed at the YBCO/CeO2/LZO interface. This oxide layer

consists of two sub-oxide layers (4 and 9 nm thick) withdifferent Ce valency and Ba/La/Ce mole fraction ratio, asshown in the elemental maps in figure 7(a). To investigatethe cerium fine structure across the YBa2Cu3O7−δ/CeO2interface, background-subtracted EELS spectra taken fromdifferent positions (1–10) are plotted in figure 7(d). An energyshift of ∼0.6 eV is obtained and can be observed in figure 8,together with extra features in the Ce M4,5 edge at positions 2,3 and 4. These observations suggest a partial valency changefrom Ce4+ to Ce3+ at the top intermixed region [21].

4.3. Thermal treatment and relation to superconductingproperties

During the YBCO-coated conductor fabrication, sequentialannealing is carried out after depositing the subsequentbuffer layers and the final overlaying YBCO superconductinglayer [5]. The heat treatment for the buffer layers (La2Zr2O7and CeO2) occurs in three steps [12]. The buffered specimenis subject to temperature gradients corresponding to (i) apre-heating step at 180 ◦C, (ii) a pyrolysis step at 600 ◦Cand (iii) a crystallization step at around 1000 ◦C for eachbuffer layer. The TFA-YBCO layer is then obtained ontop by (i) pyrolysis (200–400 ◦C), (ii) phase formation(780 ◦C) and (iii) an oxygen treatment at around 460 ◦C. Themaximum annealing temperature reached during this processis 780 ◦C [14]. The Ba in the nucleating YBCO reacts with theCeO2 buffer at temperatures>700◦C [22]. The main productsof the chemical reaction of the nucleated YBCO and the CeO2layer are Y2BaCuO5, BaCeO3 and CuO, the amount of thereaction depends on the processing temperature and time [22].

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Figure 6. HAADF-STEM image of the YBa2Cu3O7−δ/CeO2/La2Zr2O7 sample region 2 in cross-section. The green box indicatesthe area used for STEM–EELS measurements.

Intermixing yields a 20 nm thick layer of composition, whichacts effectively as a barrier preventing Ce diffusion into thesuperconducting layer. The Ce found in the intermixing layeris mainly in a 4+ state, however a partial valency change toa 3+ state is observed at the top intermixing sub-layer. EELSanalysis does not indicate further Ce diffusion into the YBCOlayer beyond the nanometer sized intermixing region.

The analysis shown here indicated that the metallicsubstrate is oxidized during processing and the metal oxideinterface is Ni/NiLaO, yielding high mechanical strength andno visible degradation of the adhesion of the subsequentfilms. At the top layer of the LZO buffer 4 nm and 9 nmthick reaction layers were found which might contain Ce3+.The thickness of these layers is small compared to theYBCO layer thickness of 440 nm and might only affect thesuperconducting properties in the bottom part of the YBCOlayer, if at all. Good adhesion of the YBCO layer to thebuffer layer is preserved and the roughness of the surfaceis reduced due to the presence of these layers. Roughnesssignificantly affects the growth condition for the subsequentYBCO deposition. It has been shown in [8] that the grainsize of the LZO (100–200 nm) is much smaller than the Nisubstrate (40 µm). The formation of the intermediate layersmight be necessary for the change in grain size (from thesubstrate to the buffer layer and further to the YBCO layer).Coated conductors with improved critical current densities ascompared to the samples investigated here were grown byother groups. For similar samples, values of 4–5 MA cm−2

were reported in the literature for YBCO thicknesses of300–700 nm [23–25]. It is assumed that the misorientedgrowth in the YBCO layer is the most significant reasonfor the jc reduction [23]; beyond a certain film thicknessbiaxial texturing is degraded, yielding low jc areas hardlycontributing to the total Ic. Therefore, the results obtainedhere are important in understanding the biaxial growthprocess [26] , but are of little relevance for understanding thejc dependences. It is known that the deterioration of the YBCO

Figure 7. (a) Quantitative color map obtained from the PCA treated 2D EELS datacube for region 2. A region of intermixing is identified.(b) STEM–EELS two-dimensional scans across the interface area shown. Normalized quantitative elemental profiles are shown in (c).(d) EELS spectra corresponding to the positions 1–10 in figure 7(b). The Ba M4,5, La M4,5 and Ce M4,5 EELS edges are identified andinterdiffusion is observed.

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Figure 8. Ce M4,5 edge EELS signatures across the YBCO/CeO2interface. The spectra are taken from positions (2–7), as indicated infigure 7(d). A shift of 0.6 eV, together with extra features in the M4and M5 edge (arrows), are observed at positions 2, 3 and 4 in theBa/Ce intermixed region.

stoichiometry due to the interaction with CeO2 could be oneof the reasons for Tc and jc suppression; in particular, theincorporation of Ce into the YBCO unit cell has been studiedin detail in the literature [22].

5. Conclusions

A nanoscale spectroscopic assessment of a fully processedYBCO-coated conductor sample prepared by chemicalsolution deposition methods was realized by high-angleannular dark-field scanning transmission electron microscopyin combination with electron energy-loss spectroscopy.Scans over the entire coated conductor architecture wereobtained. Two regions of intermixing were identified. TheLZO–nickel tungsten interface denoted as region (1) containsan intermixing of La, Ni and O. An oxide layer of 22 nmin size is formed. Region (2), which contains the YBCOsuperconducting layer and the ceria- and lanthanum-basedbuffer layers, yields an intermixing area which is formed dueto the successive heat treatments. A layer of 13 nm in size is

formed and an intermixing of La, Ce and Ba is found. Resultsindicate a mainly 4+ state for Ce in the intermixing layer.No further Ce or La diffusion could be measured into theYBCO superconducting layer and the layer itself acts as a Cediffusion barrier.

Acknowledgments

This work was supported by funding from the EuropeanResearch Council under the 7th Framework Program(FP7), ERC grant No. 246791 COUNTATOMS, grantNMP3-LA-2010-246102 IFOX and under a contract for anIntegrated Infrastructure Initiative, Reference No. 312483-ESTEEM2, as well as the EC project EUROTAPES. STgratefully acknowledges the Fund for Scientific ResearchFlanders (FWO) for a post-doctoral fellowship. ST, JVand GVT acknowledge the FWO for project G.0044.13N.The microscope used for this research was partially fundedby the Hercules Foundation of the Flemish government.The fully processed YBCO-coated conductor sample wassynthesized under the framework and funding of the VirtualInstitute ‘Chemically deposited YBCO superconductors’ ofthe Helmholtz Gemeinschaft.

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