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Epitaxial growth of CSD modified lanthanum zirconium oxide buffer layer for coated conductors
Journal of Alloys and Compounds 682 (2016) 424e431
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Epitaxial growth of CSD modified lanthanum zirconium oxide buffer layer for coated conductors Yao Wang*, Chengshan Li, Jianqing Feng, Zeming Yu, Lihua Jin, Pingxiang Zhang Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2015 Received in revised form 14 March 2016 Accepted 18 April 2016 Available online 22 April 2016
Texture and morphology evolution of modified Lanthanum Zirconium oxide (LaZr1.1Oy) films grown on biaxially textured Nie5%W tapes by chemical solution deposition (CSD) method at different heattreatment conditions has been investigated. Results show that both the growth orientation and surface morphology of films are dependent on the heating rate and annealing temperature during heattreatment process. Compared with the annealing temperature, heating rate shows a greater influence on the orientation growth of CSD-LaZr1.1Oy film. Slow heating rate is beneficial to a single (00l) orientation growth of LaZr1.1Oy grains on textured Ni-5at.%W (Ni5W) substrate. However, the surface morphology analysis reveals a minor effect of heating rate on the grain size and surface roughness of LaZr1.1Oy film. We consider that a slow heating rate would lead to the homogeneity nucleation of LaZr1.1Oy grains and eventually forming the great textured film due to the narrow temperature interval of the complete decomposition for LaZr1.1Oy precursor gel. In any case, high-quality LaZr1.1Oy film has been achieved at annealing temperature of 900 C with heating rate of 2 C/min. Moreover, we have grown epitaxial CeO2 film on the textured Ni5W substrate covered by LaZr1.1Oy film using CSD approach. CeO2 precursor solution was spin-coated on short sample of Ni5W/LZO and then heat-treated at 1000 C for 30 min in Ar-4%H2. Detailed XRD and AFM studies indicate that CeO2 film has sharp bi-axial texture and smooth surface. These results demonstrate that a low-cost CSD LaZr1.1Oy film can act as a planarization template for the further production high Jc YBCO-superconducting film. © 2016 Elsevier B.V. All rights reserved.
Keywords: Coated conductors Buffer layer Chemical solution deposition Texture Morphology
1. Introduction The second generation high temperature superconducting coated conductor (CC) is one of the most promising materials for various electric and magnetic applications because of its high critical current density in a magnetic field at 77 K [1e3]. In the multi-layer architecture of CC, buffer layer plays a key role in improving the in-plane texture of superconducting layer as well as decreasing lattice mismatch and chemical reaction between superconducting and metallic substrate [4,5]. Thus, various buffer architectures with multi-layer have been developed for emerging the double functions discussed above. With the increase of layer amount of buffers, however, both their texture degree and surface quality would decrease, resulting in the increase of fabrication cost and the decrease of superconducting performance of CC [6]. Therefore, simplification of buffer architecture is important for
* Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.jallcom.2016.04.186 0925-8388/© 2016 Elsevier B.V. All rights reserved.
improving the quality of buffer layer. In terms of the manufactory approach for CC, rolling-assisted biaxial textured substrates (RABiTS)/chemical solution deposition (CSD) is still a promising route in respect of commercial viability and the simplification of buffer architecture, although CC tape with high performance have been obtained by using ion-beam-assisted deposition (IBAD)/ physical vapor deposition (PVD) route [7e9]. However, some nanoscale pores are often observed in buffer layers prepared by CSD method, promoting the oxygen diffusion during YBCO deposition and oxygenation [10,11]. The origin of the porosity in buffer layer is still a controversial issue. Possible reasons are the release of gas from buffer precursor film during heat-treatment process and redistribution of solid matter growth by three-dimensional mode during the crystallization process of buffer layer. But in any case, improvement of surface quality of buffer layers prepared by CSD method has been an area of great interest in order to realize the good epitaxial growth of YBCO layer and the inhabitation of impurity phase formation at grain boundary. Moreover, a smooth surface of buffer layer is mandatory to obtain a good interface with
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Fig. 1. DSC-DTG curve for the precursor gel of LaZr1.1Oy (a) and La2Zr2O7 (b) with heating rate of 10 K min1 in nitrogen.
Fig. 2. XRD q-2q patterns of LaZr1.1Oy films grown on biaxially textured Ni5W substrates at various annealing temperatures with heating rate of 20 C/min, (a) 850 C, (b) 900 C, (c) 950 C, (d) 1000 C, (e) 1050 C (a); the intensity ratio ILZO(004)/[ILZO(004)þILZO(222)] for all lanthanum zirconium oxide films annealed at different temperatures (b).
Fig. 3. XRD pattern of LaZr1.1Oy film annealed at 900 C with heating rate of 2 C/min, XRD q-2q scans for LaZr1.1Oy films annealed at 900 C with different heating rates (a) 2 C/min, (b) 5 C/min, (c) 10 C/min, (d) 20 C/min, (e) 50 C/min is plotted in the inset (a); the intensity ratio ILZO(004)/[ILZO(004)þILZO(222)] for all lanthanum zirconium oxide films annealed at 900 C with different heating rates (b).
the superconducting layer [12]. Therefore, the superconducting performance of CC can be greatly enhanced by improving the surface flatness and decreasing the pore density in textured buffer layer grown on metal substrate using CSD approach. It is essential to decrease the surface defects and improve the surface flatness degree of buffer layer fabricated by CSD method, which is suitable for large-scale production of buffer film at low cost. The overall
purpose of this work is to enable potentially low-cost and high surface flatness degree manufacturing process for buffer layer in coated conductors. Pyrochlore La2Zr2O7 (LZO) films have been used as a buffer layer in CC because its good structurally and chemically compatible with NiW and YBCO layer as well as its good barrier property against Ni and O diffusion between metallic substrate and YBCO layer.
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However, we have recently demonstrated that high-quality modified Lanthanum Zirconium oxide (LaZr1.1Oy) film has a great advantage as a barrier layer for coated conductors because barrier properties and surface flatness degree of CSD-LaZr1.1Oy film is superior to those of MOD-LZO film. Here, we expand upon our previous work in a systematic study of the texture and morphology evolution of LaZr1.1Oy films by changing the heating rate and annealing temperature during the heat-treatment process. Moreover, the orientation growth behavior of LaZr1.1Oy film on textured metallic substrate is also been explored. The CeO2 buffer film is deposited on as-prepared LaZr1.1Oy film by CSD method in order to verify the texture degree and surface flatness of LaZr1.1Oy film grown on textured NiW substrate at the optimal annealing condition. 2. Experimental procedure The precursor solution preparation was carried out in air. The starting reagents in this experiment were received from STREM. Lanthanum (Ⅲ) 2, 4-pentanedionate and Zirconium (Ⅳ) 2, 4pentanedionate were dissolved in a solvent of propionic acid by heating with continuous stirring to produce yellow-colored Lanthanum Zirconium oxide precursor solution. The total metal ion concentration of precursor solution was 0.8 M. In addition, the Cerium (Ⅲ) 2, 4-pentanedionate was dissolved in propionic acid by heating with stirring to fabricate a brown-colored CeO2 precursor solution with the concentration of 0.3 M. LZO precursor solution was spin-coated at 2500 rpm for 30 s onto short cube-textured Ni5at.%W (Ni5W) substrates ultrasonically cleaned in acetone for 20 min, and then the coatings were followed by heat-treatment. The annealing step was carried out at various temperatures
(850e1050 C) and different heating rates (2e50 C$min1) with a dwell time of 30 min in a flowing Ar-4%H2 mixture atmosphere. Finally, Ni5W/LaZr1.1Oy samples were cooled down to room temperature in the same reducing atmosphere. Subsequently, spin coating was also used to prepare CeO2 film on Ni5W/LaZr1.1Oy substrate with the optimal performance at a spin rate of 2500 rpm for 30 s. The precursor film was then annealed at 1000 C for 30 min in a flowing forming gas of Ar-4%H2 to obtain LaZr1.1Oy/CeO2 buffer layers. The thermal decomposition processes of the La2Zr2O7 and LaZr1.1Oy precursor gels were investigated by TG-DSC analysis, in which a heating rate of 10 C/min was used in flowing nitrogen at 100 mL/min. The phase purity and texture of samples were analyzed using X-ray diffraction (XRD), which was performed to carry out q-2q scans using CuKa radiation at 40 kV and 50 mA. In addition, in-plane and out-of-plane scans were also confirmed by XRD for the texture degree of as-grown films and underlying substrates. For further analysis, micro-texture and mis-orientation distribution information on the as-prepared film was acquired by the collecting data by means of electron backscattering diffraction (EBSD) (on a 500 mm 400 mm area). In order to gain information on the homogeneity of the elemental distribution within the asgrown LZO film on Ni5W substrate, X-ray photoelectron spectra (XPS) was used to make the depth profile analysis. A sputtering source providing Arþ was used to create the element depth profile. Dimension Icon Atomic force microscopy (AFM) in tapping mode documented the film surface morphology and roughness. 3. Results and discussion In order to obtain the difference of the thermal decomposition
Fig. 4. Typical u scans of the LZO (004) and Ni5W(002) reflection (a) and (c), f scans of LZO (222) and Ni5W(111) reflection for LaZr1.1Oy film annealed at 900 C with heating rate of 2 C/min and the underlying Ni5W substrate (b) and (d).
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Fig. 5. EBSD map of LaZr1.1Oy film grown on biaxially textured Ni5W substrate at 900 C within the scale of 400 mm 400 mm; {100} pole figure for Ni5W/LaZr1.1Oy is plotted in the right inset.
Fig. 6. XRD pattern of the Ni5W/LaZr1.1Oy (a) and Ni5W/LaZr1.1Oy/CeO2 (b) samples, where the LaZr1.1Oy layer was deposited at 900 C with heating rate of 2 C/min.
Fig. 7. XPS depth profiles of LaZr1.1Oy/CeO2 bi-layers on NieW substrate.
behavior between the LaZr1.1Oy and La2Zr2O7 precursor gels, DTG and DSC data were analyzed. Fig. 1 shows the DTG-DSC curves of as-
prepared gels, which were afforded by heating the precursor solutions at 120 C for several hours to remove solvents. The thermal decomposition behavior of LaZr1.1Oy gel is very similar to that of La2Zr2O7 gel. Each DSC curve shows four endothermic peaks (at about 155 C, 256 C 310 C and 360 C) and one exothermic peak (at above 1000 C). Endothermic peaks at below 200 C accompanied with obvious weight loss can be ascribed to the elimination of residual solvents and absorbed water. No weight change happens but endothermic peak appears at 256 C, which may be related to the melt of precursors. Rapid weight losses take place at 310 C and 360 C due to the decomposition of organic components. Both exothermic peak at higher temperatures of 1042 C (for LaZr1.1Oy gel) and 1130 C (for La2Zr2O7 gel) are not associated with any obvious signal of mass loss in DTG curves, which may be related to the formation of LZO phase out of La2O3 and ZrO2. On the other hand, the total weight loss at 1200 C attains 53.93% in LaZr1.1Oy and 56.3% in La2Zr2O7. Both of which are much less than the theoretical mass loss of around 70% for the stoichiometric precursors, which is related to the synthesis of propionic acid salts due to solution chemistry. It should be noted that the formation temperature of LZO phase in LaZr1.1Oy gel is less than that in La2Zr2O7 gel. It illustrates that the supply of excess ZrO2 can effectively reduce the synthesis temperature of lanthanum zirconium oxide phase and it may be beneficial to the formation of texture lanthanum zirconium oxide film at a lower annealing temperature. In order to study the orientation growth of LaZr1.1Oy films, we carried out different heating profiles to fabricate lanthanum zirconium oxide films on textured Ni5W substrates. Fig. 2 shows the XRD q-2q spectrum for a series of LaZr1.1Oy films grown on Ni5W substrates at different annealing temperatures ranging from 850 C to 1050 C. When the annealing temperature is lower than 900 C, no diffraction peaks of LZO are observed. It may be explained by the incomplete formation and crystallization of lanthanum zirconium oxide phase. At above 900 C, the films show extra weak lanthanum zirconium oxide (222) diffraction peaks along with sharp LZO (00l) reflections, indicating that c-axis oriented film can not be obtained at this range of annealing temperature with a heating rate of 20 C/ min. Fig. 2 also represents the intensity ratio computed through dividing the intensity of lanthanum zirconium oxide (004) peak by the intensity sum of its (004) and (222) peaks from XRD patterns for all lanthanum zirconium oxide films annealed at different temperatures. The results show that the intensity ratio I(004)/ I(004)þI(222) initially decreases and then increases with increasing annealing temperature ranging from 900 C to 1050 C. However, higher annealing temperature is unfavourable for the decrease of fabrication cost and it may result in the abnormal growth of grains.
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Fig. 8. AFM two-dimensional images and section profiles of LaZr1.1Oy film grown on NieW substrate at various annealing temperatures (a) 850 C, (b) 900 C, (c) 950 C, (d) 1000 C, (e) 1050 C.
Therefore, the optimal annealing temperature can be considered to be as 900 C. The XRD q-2q scans of the LaZr1.1Oy films annealed at 900 C with different heating rates ranging from 2 C/min to 50 C/min are shown in Fig. 3. When the heating rate is higher than 5 C/min, the as-prepared LZO film shows not only sharp LZO (00l) diffraction peaks but also LZO (222) peaks. LZO film with good (00l) orientation could be obtained only when the heating rate decreases to 2 C/min. This suggests that higher heating rate during the heattreatment process is unfavorable for the epitaxial growth of LZO film due to inhomogeneous nucleation of LZO grain. Moreover, the intensity ratio I(004)/I(004)þI(222) initially decreases and afterwards keeps unaltered with the increase of heating rate during the annealing process. It indicates that LZO crystallization film with single (00l) orientation could be obtained at the optimal deposition condition including low annealing temperature of 900 C and slow heating rate of 2 C/min during the heat-treatment process.
Compared with annealing temperature, the heating rate shows more effect on the orientation growth of LaZr1.1Oy film. A slower heating rate is beneficial to the (00l) oriented growth of LaZr1.1Oy grains on textured Ni5W substrate. We consider that the slower heating rate is helpful to the complete decomposition and homogeneous nucleation during a short annealing temperature interval, finally resulting in a highly c-axially textured LaZr1.1Oy film. The u (out-of-plane) and f (in-plane) scans of LaZr1.1Oy film annealed at 900 C with heating rate of 2 C/min and the underlying Ni5W substrate are shown in Fig. 4. It should be pointed out that the omega-scans were performed around the rolling direction of Ni5W tape. The full-width-at-half-maximum (FWHM) values of u and f alignment for LaZr1.1Oy film are 5.8 and 6.38 , respectively. These values are well comparable to those of the Ni5W substrates, which are 5.48 (Du) and 6.13 (Df). It indicates that highly textured LaZr1.1Oy film could be fabricated on textured NiW substrate at 900 C with heating rate of 2 C/min.
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Fig. 9. AFM micrographs and section profiles of Ni5W/LaZr1.1Oy samples prepared at 900 C with different heating rates (a) 2 C/min, (b) 5 C/min, (c) 10 C/min, (d) 20 C/min, (e) 50 C/min.
Fig. 5 shows the EBSD image and the corresponding pole figure, in which the LaZr1.1Oy film was prepared at 900 C with a heating rate of 2 C/min. LZO grows generally 45 rotated on Ni5W substrate due to a better lattice match in in-plane-alignment. It can be found that the texture in the surface of LaZr1.1Oy film is very great except some black spots. Moreover, the volume fraction of cube texture is larger than 93% within a tolerance angle of less than 13 . It implies that the biaxial texture of Ni5W substrate is transmitted until the surface of the LaZr1.1Oy film. The qualifying test for the suitability of LaZr1.1Oy buffer layer on Ni5W substrate is compatible with the CeO2 coating, which is deposited by CSD method. Typical q-2q spectra for Ni5W/LaZr1.1Oy sample fabricated at 900 C with heating rate of 2 C/min and CeO2 film grown on Ni5W substrate covered with LaZr1.1Oy film is shown in Fig. 6. A stronger (00l) diffraction peak at 33.2 and 69.5 in Ni5W/LaZr1.1Oy/CeO2 sample than that in Ni5W/LaZr1.1Oy indicates the presence of a perfect (00l) aligned CeO2 and LaZr1.1Oy films.
Moreover, the bi-axial texture of Ni5W substrate is transferred to CeO2 layer by CSD-LaZr1.1Oy film. In order to study the evolution of atomic concentrations c in depth, an XPS depth profile analysis for LaZr1.1Oy film grown on Ni5W substrate at 900 C with heating rate of 2 C/min was performed by Arþ bombardment for 27 min Fig. 7 illustrates the variation in composition of all elements over sputtering time. Aside from the expected elements of Ce, La, Zr and O, carbon atomic is detected in the buffer films. It possibly consists of little residual carbon in film and plenty of adsorbed carbon from air [13]. It is clear that the LaZr1.1Oy/CeO2 buffer layer is entirely removed from the surface of Ni5W substrate after approximately 23 min of sputtering. Considering that the bi-layer thickness is around 160 nm, as calibrated by a-step apparatus, it can be deducted that the average removal rate of the film is about 7 nm min1. After sputtering the surface of buffer layer, there is an obvious decreasing trend of Ce concentration as elongating sputtering time. Moreover, the slow
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Fig. 10. Dependency of the roughness (Rrms) of the LaZr1.1Oy films on the annealing temperature (a) and heating rate (b) measured by AFM within the scanning areas of 5 mm 5 mm.
Fig. 11. AFM picture and section profiles showing the typical surface morphology of CeO2 film deposited on Ni5W substrate covered by LaZr1.1Oy film.
drop in the CeO2 components and undefined raise of La and Zr signals at the CeO2/LaZr1.1Oy interface pointes out an indistinct interface having a very high roughness. It may be explained by serious interface diffusion between CeO2 and LaZr1.1Oy films during the preparation process of CeO2 film. Similar results have been reported by L. Molina-Luna et al. [7]. On the other hand, the atomic concentrations of La, Zr and O in LaZr1.1Oy buffer layer verses film thickness are nearly constant, showing a homogeneous elemental distribution. In addition, further Ce diffusion into LaZr1.1Oy layer, indicating that the intermixing layer of Ce, La, Zr and O at
CeO2eLaZr1.1Oy interface does not act as a Ce diffusion barrier. However, both the sharp drop of the LaZr1.1Oy components and the quick raise of the Ni signal at the LaZr1.1Oy-Ni5W interface illustrate a distinct interface. Thus LaZr1.1Oy buffer layer of 110 nm in thickness is sufficient to prevent Ni diffusion during the fabrication of CeO2 layer by CSD route. AFM investigations of the surface morphology of LaZr1.1Oy films annealed at different temperatures were carried out and are shown in Fig. 8. A profile through a typical line scan of each image is also given in Fig. 8. The scanning area of the films is 5 mm 5 mm. Both the grain size on the surface of LaZr1.1Oy film and the surface fluctuations of Ni5W/LaZr1.1Oy sample becomes larger with increasing annealing temperature, which results in the increase in the RMS values. It illuminates that a lower annealing temperature is beneficial to the surface flatness of LaZr1.1Oy film grown on Ni5W substrate. Fig. 9 represents the surface morphology and AFM line scans on a 5 mm 5 mm areas of LaZr1.1Oy films grown on textured Ni5W substrates at 900 C with different heating rates. It can be found that the maximum height difference in all the as-prepared films is less than 10 nm, indicating smooth surface for all as-prepared LaZr1.1Oy films. A threshold criterion of 2 nm was previously used to evaluate the degree of surface flatness in solution deposited oxide film [3,14,15]. The AFM image shows that LaZr1.1Oy film grown on textured Ni5W substrate at 900 C with a heating rate of 2 C/min has about 90% flat surface area with the surface roughness smaller than the threshold value. However, the flat surface area of LaZr1.1Oy films grown on textured Ni5W substrates shows a decreased trend with increasing the heating rate during the annealing process. It may be related to the inhomogeneous nucleation and growth of oxide grain under a rapid heating condition. Moreover, a slow heating rate improves grain refinement and size uniform of LaZr1.1Oy grains, resulting in a relative smooth surface of Ni5W/LaZr1.1Oy sample. Combined with the results discussed in Fig. 8, annealing temperature has a greater effect on the degree of surface flatness of LaZr1.1Oy film grown on textured Ni5W substrate than heating rate during the heat-treatment process. It may be explained by the outgrowth of LZO grains at high annealing temperature. The root-mean-square (RMS) roughness of the surface of LaZr1.1Oy films derived from the AFM observations are evaluated over scanning area of 5 mm 5 mm. Fig. 10 shows the RMS roughness values of LaZr1.1Oy films as a function of the annealing temperature and heating rate, respectively. As shown in the figure, the RMS roughness of LaZr1.1Oy films increases nearly linearly with increasing annealing temperature in the range of 850e1050 C.
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However, the RMS roughness of LaZr1.1Oy films grown on textured Ni5W substrates at 900 C with different heating rates are between 0.58 nm and 1.46 nm for the 5 mm 5 mm scanning areas. LaZr1.1Oy film is projected to have a minimum RMS roughness when a heating rate of 2 C/min is adopted during the annealing process. The AFM analysis of CeO2 buffer layer deposited on Ni5W substrate covered by as-prepared optimal LaZr1.1Oy by CSD method is shown in Fig. 11. It illustrates homogenous and crack-free CeO2 surface. In addition, AFM line scans reveals little height difference in the scanning area, illuminating a flat surface. A RMS roughness value of 2.16 nm is calculated over a scanning area of 5 mm 5 mm, which also illuminates that the smooth surface of the as-prepared LaZr1.1Oy/CeO2 bi-layer buffer structure deposited on Ni5W substrate using CSD method is suitable for further deposition of YBCO coating. The good epitaxial growth of YBCO layer and the inhabitation of impurity phase formation at grain boundary could be realized when the area of atomically flat surface region, at which YBCO nucleates preferentially, is larger than 70% of total area [16]. Therefore a dense and smooth surface of buffer layer is ideally desired for the subsequent growth of YBCO. A RMS roughness value of 2.16 nm indicates that the surface of LaZr1.1Oy/CeO2 buffer layers deposited on textured Ni5W substrate using CSD method is very suitable for further deposition of YBCO film. 4. Conclusion We have successfully fabricated LaZr1.1Oy buffer layer on bi-axial textured Ni5W substrate by CSD method. The preferred orientation of LaZr1.1Oy film is affected by heat-treatment temperature and heating rate during the annealing process. Moreover, annealing temperature has a greater effect on the degree of surface flatness of LaZr1.1Oy film grown on textured Ni5W substrate than heating rate during the heat-treatment process. XRD and AFM analysis has shown that the optimal annealing profile is 900 C and a heating rate of 2 C/min in flowing Ar-4%H2. In addition, CSD-CeO2 film with a low Rrms value of 2.16 nm has been grown on Ni5W substrate covered by optimal CSD-LaZr1.1Oy film. The microstructure and performance of YBa2Cu3O7-d films grown on the CSD-LaZr1.1Oy/ CeO2 bi-layer buffer films will be reported elsewhere. Acknowledgement This work was financially supported by the International Science & Technology Cooperation Program of China (Grant No. 2012DFA50780), the National Science Fund Program of China (Grant No. 51302225 and 51202201), the Innovative Research Team
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of Shaanxi province (Grant No.2013KCT-07), the Natural Science Fund Program of Shaanxi Province (Grant No. 2014JQ6202), the Weiyang District Science and Technology Plan Program of Xi’an city in Shaanxi Province (Grant No. 201412).
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Epitaxial growth of CSD modified lanthanum zirconium oxide buffer layer for coated conductors Yao Wang*, Chengshan Li, Jianqing Feng, Zeming Yu, Lihua Jin, Pingxiang Zhang Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2015 Received in revised form 14 March 2016 Accepted 18 April 2016 Available online 22 April 2016
Texture and morphology evolution of modified Lanthanum Zirconium oxide (LaZr1.1Oy) films grown on biaxially textured Nie5%W tapes by chemical solution deposition (CSD) method at different heattreatment conditions has been investigated. Results show that both the growth orientation and surface morphology of films are dependent on the heating rate and annealing temperature during heattreatment process. Compared with the annealing temperature, heating rate shows a greater influence on the orientation growth of CSD-LaZr1.1Oy film. Slow heating rate is beneficial to a single (00l) orientation growth of LaZr1.1Oy grains on textured Ni-5at.%W (Ni5W) substrate. However, the surface morphology analysis reveals a minor effect of heating rate on the grain size and surface roughness of LaZr1.1Oy film. We consider that a slow heating rate would lead to the homogeneity nucleation of LaZr1.1Oy grains and eventually forming the great textured film due to the narrow temperature interval of the complete decomposition for LaZr1.1Oy precursor gel. In any case, high-quality LaZr1.1Oy film has been achieved at annealing temperature of 900 C with heating rate of 2 C/min. Moreover, we have grown epitaxial CeO2 film on the textured Ni5W substrate covered by LaZr1.1Oy film using CSD approach. CeO2 precursor solution was spin-coated on short sample of Ni5W/LZO and then heat-treated at 1000 C for 30 min in Ar-4%H2. Detailed XRD and AFM studies indicate that CeO2 film has sharp bi-axial texture and smooth surface. These results demonstrate that a low-cost CSD LaZr1.1Oy film can act as a planarization template for the further production high Jc YBCO-superconducting film. © 2016 Elsevier B.V. All rights reserved.
Keywords: Coated conductors Buffer layer Chemical solution deposition Texture Morphology
1. Introduction The second generation high temperature superconducting coated conductor (CC) is one of the most promising materials for various electric and magnetic applications because of its high critical current density in a magnetic field at 77 K [1e3]. In the multi-layer architecture of CC, buffer layer plays a key role in improving the in-plane texture of superconducting layer as well as decreasing lattice mismatch and chemical reaction between superconducting and metallic substrate [4,5]. Thus, various buffer architectures with multi-layer have been developed for emerging the double functions discussed above. With the increase of layer amount of buffers, however, both their texture degree and surface quality would decrease, resulting in the increase of fabrication cost and the decrease of superconducting performance of CC [6]. Therefore, simplification of buffer architecture is important for
* Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.jallcom.2016.04.186 0925-8388/© 2016 Elsevier B.V. All rights reserved.
improving the quality of buffer layer. In terms of the manufactory approach for CC, rolling-assisted biaxial textured substrates (RABiTS)/chemical solution deposition (CSD) is still a promising route in respect of commercial viability and the simplification of buffer architecture, although CC tape with high performance have been obtained by using ion-beam-assisted deposition (IBAD)/ physical vapor deposition (PVD) route [7e9]. However, some nanoscale pores are often observed in buffer layers prepared by CSD method, promoting the oxygen diffusion during YBCO deposition and oxygenation [10,11]. The origin of the porosity in buffer layer is still a controversial issue. Possible reasons are the release of gas from buffer precursor film during heat-treatment process and redistribution of solid matter growth by three-dimensional mode during the crystallization process of buffer layer. But in any case, improvement of surface quality of buffer layers prepared by CSD method has been an area of great interest in order to realize the good epitaxial growth of YBCO layer and the inhabitation of impurity phase formation at grain boundary. Moreover, a smooth surface of buffer layer is mandatory to obtain a good interface with
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Fig. 1. DSC-DTG curve for the precursor gel of LaZr1.1Oy (a) and La2Zr2O7 (b) with heating rate of 10 K min1 in nitrogen.
Fig. 2. XRD q-2q patterns of LaZr1.1Oy films grown on biaxially textured Ni5W substrates at various annealing temperatures with heating rate of 20 C/min, (a) 850 C, (b) 900 C, (c) 950 C, (d) 1000 C, (e) 1050 C (a); the intensity ratio ILZO(004)/[ILZO(004)þILZO(222)] for all lanthanum zirconium oxide films annealed at different temperatures (b).
Fig. 3. XRD pattern of LaZr1.1Oy film annealed at 900 C with heating rate of 2 C/min, XRD q-2q scans for LaZr1.1Oy films annealed at 900 C with different heating rates (a) 2 C/min, (b) 5 C/min, (c) 10 C/min, (d) 20 C/min, (e) 50 C/min is plotted in the inset (a); the intensity ratio ILZO(004)/[ILZO(004)þILZO(222)] for all lanthanum zirconium oxide films annealed at 900 C with different heating rates (b).
the superconducting layer [12]. Therefore, the superconducting performance of CC can be greatly enhanced by improving the surface flatness and decreasing the pore density in textured buffer layer grown on metal substrate using CSD approach. It is essential to decrease the surface defects and improve the surface flatness degree of buffer layer fabricated by CSD method, which is suitable for large-scale production of buffer film at low cost. The overall
purpose of this work is to enable potentially low-cost and high surface flatness degree manufacturing process for buffer layer in coated conductors. Pyrochlore La2Zr2O7 (LZO) films have been used as a buffer layer in CC because its good structurally and chemically compatible with NiW and YBCO layer as well as its good barrier property against Ni and O diffusion between metallic substrate and YBCO layer.
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However, we have recently demonstrated that high-quality modified Lanthanum Zirconium oxide (LaZr1.1Oy) film has a great advantage as a barrier layer for coated conductors because barrier properties and surface flatness degree of CSD-LaZr1.1Oy film is superior to those of MOD-LZO film. Here, we expand upon our previous work in a systematic study of the texture and morphology evolution of LaZr1.1Oy films by changing the heating rate and annealing temperature during the heat-treatment process. Moreover, the orientation growth behavior of LaZr1.1Oy film on textured metallic substrate is also been explored. The CeO2 buffer film is deposited on as-prepared LaZr1.1Oy film by CSD method in order to verify the texture degree and surface flatness of LaZr1.1Oy film grown on textured NiW substrate at the optimal annealing condition. 2. Experimental procedure The precursor solution preparation was carried out in air. The starting reagents in this experiment were received from STREM. Lanthanum (Ⅲ) 2, 4-pentanedionate and Zirconium (Ⅳ) 2, 4pentanedionate were dissolved in a solvent of propionic acid by heating with continuous stirring to produce yellow-colored Lanthanum Zirconium oxide precursor solution. The total metal ion concentration of precursor solution was 0.8 M. In addition, the Cerium (Ⅲ) 2, 4-pentanedionate was dissolved in propionic acid by heating with stirring to fabricate a brown-colored CeO2 precursor solution with the concentration of 0.3 M. LZO precursor solution was spin-coated at 2500 rpm for 30 s onto short cube-textured Ni5at.%W (Ni5W) substrates ultrasonically cleaned in acetone for 20 min, and then the coatings were followed by heat-treatment. The annealing step was carried out at various temperatures
(850e1050 C) and different heating rates (2e50 C$min1) with a dwell time of 30 min in a flowing Ar-4%H2 mixture atmosphere. Finally, Ni5W/LaZr1.1Oy samples were cooled down to room temperature in the same reducing atmosphere. Subsequently, spin coating was also used to prepare CeO2 film on Ni5W/LaZr1.1Oy substrate with the optimal performance at a spin rate of 2500 rpm for 30 s. The precursor film was then annealed at 1000 C for 30 min in a flowing forming gas of Ar-4%H2 to obtain LaZr1.1Oy/CeO2 buffer layers. The thermal decomposition processes of the La2Zr2O7 and LaZr1.1Oy precursor gels were investigated by TG-DSC analysis, in which a heating rate of 10 C/min was used in flowing nitrogen at 100 mL/min. The phase purity and texture of samples were analyzed using X-ray diffraction (XRD), which was performed to carry out q-2q scans using CuKa radiation at 40 kV and 50 mA. In addition, in-plane and out-of-plane scans were also confirmed by XRD for the texture degree of as-grown films and underlying substrates. For further analysis, micro-texture and mis-orientation distribution information on the as-prepared film was acquired by the collecting data by means of electron backscattering diffraction (EBSD) (on a 500 mm 400 mm area). In order to gain information on the homogeneity of the elemental distribution within the asgrown LZO film on Ni5W substrate, X-ray photoelectron spectra (XPS) was used to make the depth profile analysis. A sputtering source providing Arþ was used to create the element depth profile. Dimension Icon Atomic force microscopy (AFM) in tapping mode documented the film surface morphology and roughness. 3. Results and discussion In order to obtain the difference of the thermal decomposition
Fig. 4. Typical u scans of the LZO (004) and Ni5W(002) reflection (a) and (c), f scans of LZO (222) and Ni5W(111) reflection for LaZr1.1Oy film annealed at 900 C with heating rate of 2 C/min and the underlying Ni5W substrate (b) and (d).
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Fig. 5. EBSD map of LaZr1.1Oy film grown on biaxially textured Ni5W substrate at 900 C within the scale of 400 mm 400 mm; {100} pole figure for Ni5W/LaZr1.1Oy is plotted in the right inset.
Fig. 6. XRD pattern of the Ni5W/LaZr1.1Oy (a) and Ni5W/LaZr1.1Oy/CeO2 (b) samples, where the LaZr1.1Oy layer was deposited at 900 C with heating rate of 2 C/min.
Fig. 7. XPS depth profiles of LaZr1.1Oy/CeO2 bi-layers on NieW substrate.
behavior between the LaZr1.1Oy and La2Zr2O7 precursor gels, DTG and DSC data were analyzed. Fig. 1 shows the DTG-DSC curves of as-
prepared gels, which were afforded by heating the precursor solutions at 120 C for several hours to remove solvents. The thermal decomposition behavior of LaZr1.1Oy gel is very similar to that of La2Zr2O7 gel. Each DSC curve shows four endothermic peaks (at about 155 C, 256 C 310 C and 360 C) and one exothermic peak (at above 1000 C). Endothermic peaks at below 200 C accompanied with obvious weight loss can be ascribed to the elimination of residual solvents and absorbed water. No weight change happens but endothermic peak appears at 256 C, which may be related to the melt of precursors. Rapid weight losses take place at 310 C and 360 C due to the decomposition of organic components. Both exothermic peak at higher temperatures of 1042 C (for LaZr1.1Oy gel) and 1130 C (for La2Zr2O7 gel) are not associated with any obvious signal of mass loss in DTG curves, which may be related to the formation of LZO phase out of La2O3 and ZrO2. On the other hand, the total weight loss at 1200 C attains 53.93% in LaZr1.1Oy and 56.3% in La2Zr2O7. Both of which are much less than the theoretical mass loss of around 70% for the stoichiometric precursors, which is related to the synthesis of propionic acid salts due to solution chemistry. It should be noted that the formation temperature of LZO phase in LaZr1.1Oy gel is less than that in La2Zr2O7 gel. It illustrates that the supply of excess ZrO2 can effectively reduce the synthesis temperature of lanthanum zirconium oxide phase and it may be beneficial to the formation of texture lanthanum zirconium oxide film at a lower annealing temperature. In order to study the orientation growth of LaZr1.1Oy films, we carried out different heating profiles to fabricate lanthanum zirconium oxide films on textured Ni5W substrates. Fig. 2 shows the XRD q-2q spectrum for a series of LaZr1.1Oy films grown on Ni5W substrates at different annealing temperatures ranging from 850 C to 1050 C. When the annealing temperature is lower than 900 C, no diffraction peaks of LZO are observed. It may be explained by the incomplete formation and crystallization of lanthanum zirconium oxide phase. At above 900 C, the films show extra weak lanthanum zirconium oxide (222) diffraction peaks along with sharp LZO (00l) reflections, indicating that c-axis oriented film can not be obtained at this range of annealing temperature with a heating rate of 20 C/ min. Fig. 2 also represents the intensity ratio computed through dividing the intensity of lanthanum zirconium oxide (004) peak by the intensity sum of its (004) and (222) peaks from XRD patterns for all lanthanum zirconium oxide films annealed at different temperatures. The results show that the intensity ratio I(004)/ I(004)þI(222) initially decreases and then increases with increasing annealing temperature ranging from 900 C to 1050 C. However, higher annealing temperature is unfavourable for the decrease of fabrication cost and it may result in the abnormal growth of grains.
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Fig. 8. AFM two-dimensional images and section profiles of LaZr1.1Oy film grown on NieW substrate at various annealing temperatures (a) 850 C, (b) 900 C, (c) 950 C, (d) 1000 C, (e) 1050 C.
Therefore, the optimal annealing temperature can be considered to be as 900 C. The XRD q-2q scans of the LaZr1.1Oy films annealed at 900 C with different heating rates ranging from 2 C/min to 50 C/min are shown in Fig. 3. When the heating rate is higher than 5 C/min, the as-prepared LZO film shows not only sharp LZO (00l) diffraction peaks but also LZO (222) peaks. LZO film with good (00l) orientation could be obtained only when the heating rate decreases to 2 C/min. This suggests that higher heating rate during the heattreatment process is unfavorable for the epitaxial growth of LZO film due to inhomogeneous nucleation of LZO grain. Moreover, the intensity ratio I(004)/I(004)þI(222) initially decreases and afterwards keeps unaltered with the increase of heating rate during the annealing process. It indicates that LZO crystallization film with single (00l) orientation could be obtained at the optimal deposition condition including low annealing temperature of 900 C and slow heating rate of 2 C/min during the heat-treatment process.
Compared with annealing temperature, the heating rate shows more effect on the orientation growth of LaZr1.1Oy film. A slower heating rate is beneficial to the (00l) oriented growth of LaZr1.1Oy grains on textured Ni5W substrate. We consider that the slower heating rate is helpful to the complete decomposition and homogeneous nucleation during a short annealing temperature interval, finally resulting in a highly c-axially textured LaZr1.1Oy film. The u (out-of-plane) and f (in-plane) scans of LaZr1.1Oy film annealed at 900 C with heating rate of 2 C/min and the underlying Ni5W substrate are shown in Fig. 4. It should be pointed out that the omega-scans were performed around the rolling direction of Ni5W tape. The full-width-at-half-maximum (FWHM) values of u and f alignment for LaZr1.1Oy film are 5.8 and 6.38 , respectively. These values are well comparable to those of the Ni5W substrates, which are 5.48 (Du) and 6.13 (Df). It indicates that highly textured LaZr1.1Oy film could be fabricated on textured NiW substrate at 900 C with heating rate of 2 C/min.
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Fig. 9. AFM micrographs and section profiles of Ni5W/LaZr1.1Oy samples prepared at 900 C with different heating rates (a) 2 C/min, (b) 5 C/min, (c) 10 C/min, (d) 20 C/min, (e) 50 C/min.
Fig. 5 shows the EBSD image and the corresponding pole figure, in which the LaZr1.1Oy film was prepared at 900 C with a heating rate of 2 C/min. LZO grows generally 45 rotated on Ni5W substrate due to a better lattice match in in-plane-alignment. It can be found that the texture in the surface of LaZr1.1Oy film is very great except some black spots. Moreover, the volume fraction of cube texture is larger than 93% within a tolerance angle of less than 13 . It implies that the biaxial texture of Ni5W substrate is transmitted until the surface of the LaZr1.1Oy film. The qualifying test for the suitability of LaZr1.1Oy buffer layer on Ni5W substrate is compatible with the CeO2 coating, which is deposited by CSD method. Typical q-2q spectra for Ni5W/LaZr1.1Oy sample fabricated at 900 C with heating rate of 2 C/min and CeO2 film grown on Ni5W substrate covered with LaZr1.1Oy film is shown in Fig. 6. A stronger (00l) diffraction peak at 33.2 and 69.5 in Ni5W/LaZr1.1Oy/CeO2 sample than that in Ni5W/LaZr1.1Oy indicates the presence of a perfect (00l) aligned CeO2 and LaZr1.1Oy films.
Moreover, the bi-axial texture of Ni5W substrate is transferred to CeO2 layer by CSD-LaZr1.1Oy film. In order to study the evolution of atomic concentrations c in depth, an XPS depth profile analysis for LaZr1.1Oy film grown on Ni5W substrate at 900 C with heating rate of 2 C/min was performed by Arþ bombardment for 27 min Fig. 7 illustrates the variation in composition of all elements over sputtering time. Aside from the expected elements of Ce, La, Zr and O, carbon atomic is detected in the buffer films. It possibly consists of little residual carbon in film and plenty of adsorbed carbon from air [13]. It is clear that the LaZr1.1Oy/CeO2 buffer layer is entirely removed from the surface of Ni5W substrate after approximately 23 min of sputtering. Considering that the bi-layer thickness is around 160 nm, as calibrated by a-step apparatus, it can be deducted that the average removal rate of the film is about 7 nm min1. After sputtering the surface of buffer layer, there is an obvious decreasing trend of Ce concentration as elongating sputtering time. Moreover, the slow
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Fig. 10. Dependency of the roughness (Rrms) of the LaZr1.1Oy films on the annealing temperature (a) and heating rate (b) measured by AFM within the scanning areas of 5 mm 5 mm.
Fig. 11. AFM picture and section profiles showing the typical surface morphology of CeO2 film deposited on Ni5W substrate covered by LaZr1.1Oy film.
drop in the CeO2 components and undefined raise of La and Zr signals at the CeO2/LaZr1.1Oy interface pointes out an indistinct interface having a very high roughness. It may be explained by serious interface diffusion between CeO2 and LaZr1.1Oy films during the preparation process of CeO2 film. Similar results have been reported by L. Molina-Luna et al. [7]. On the other hand, the atomic concentrations of La, Zr and O in LaZr1.1Oy buffer layer verses film thickness are nearly constant, showing a homogeneous elemental distribution. In addition, further Ce diffusion into LaZr1.1Oy layer, indicating that the intermixing layer of Ce, La, Zr and O at
CeO2eLaZr1.1Oy interface does not act as a Ce diffusion barrier. However, both the sharp drop of the LaZr1.1Oy components and the quick raise of the Ni signal at the LaZr1.1Oy-Ni5W interface illustrate a distinct interface. Thus LaZr1.1Oy buffer layer of 110 nm in thickness is sufficient to prevent Ni diffusion during the fabrication of CeO2 layer by CSD route. AFM investigations of the surface morphology of LaZr1.1Oy films annealed at different temperatures were carried out and are shown in Fig. 8. A profile through a typical line scan of each image is also given in Fig. 8. The scanning area of the films is 5 mm 5 mm. Both the grain size on the surface of LaZr1.1Oy film and the surface fluctuations of Ni5W/LaZr1.1Oy sample becomes larger with increasing annealing temperature, which results in the increase in the RMS values. It illuminates that a lower annealing temperature is beneficial to the surface flatness of LaZr1.1Oy film grown on Ni5W substrate. Fig. 9 represents the surface morphology and AFM line scans on a 5 mm 5 mm areas of LaZr1.1Oy films grown on textured Ni5W substrates at 900 C with different heating rates. It can be found that the maximum height difference in all the as-prepared films is less than 10 nm, indicating smooth surface for all as-prepared LaZr1.1Oy films. A threshold criterion of 2 nm was previously used to evaluate the degree of surface flatness in solution deposited oxide film [3,14,15]. The AFM image shows that LaZr1.1Oy film grown on textured Ni5W substrate at 900 C with a heating rate of 2 C/min has about 90% flat surface area with the surface roughness smaller than the threshold value. However, the flat surface area of LaZr1.1Oy films grown on textured Ni5W substrates shows a decreased trend with increasing the heating rate during the annealing process. It may be related to the inhomogeneous nucleation and growth of oxide grain under a rapid heating condition. Moreover, a slow heating rate improves grain refinement and size uniform of LaZr1.1Oy grains, resulting in a relative smooth surface of Ni5W/LaZr1.1Oy sample. Combined with the results discussed in Fig. 8, annealing temperature has a greater effect on the degree of surface flatness of LaZr1.1Oy film grown on textured Ni5W substrate than heating rate during the heat-treatment process. It may be explained by the outgrowth of LZO grains at high annealing temperature. The root-mean-square (RMS) roughness of the surface of LaZr1.1Oy films derived from the AFM observations are evaluated over scanning area of 5 mm 5 mm. Fig. 10 shows the RMS roughness values of LaZr1.1Oy films as a function of the annealing temperature and heating rate, respectively. As shown in the figure, the RMS roughness of LaZr1.1Oy films increases nearly linearly with increasing annealing temperature in the range of 850e1050 C.
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However, the RMS roughness of LaZr1.1Oy films grown on textured Ni5W substrates at 900 C with different heating rates are between 0.58 nm and 1.46 nm for the 5 mm 5 mm scanning areas. LaZr1.1Oy film is projected to have a minimum RMS roughness when a heating rate of 2 C/min is adopted during the annealing process. The AFM analysis of CeO2 buffer layer deposited on Ni5W substrate covered by as-prepared optimal LaZr1.1Oy by CSD method is shown in Fig. 11. It illustrates homogenous and crack-free CeO2 surface. In addition, AFM line scans reveals little height difference in the scanning area, illuminating a flat surface. A RMS roughness value of 2.16 nm is calculated over a scanning area of 5 mm 5 mm, which also illuminates that the smooth surface of the as-prepared LaZr1.1Oy/CeO2 bi-layer buffer structure deposited on Ni5W substrate using CSD method is suitable for further deposition of YBCO coating. The good epitaxial growth of YBCO layer and the inhabitation of impurity phase formation at grain boundary could be realized when the area of atomically flat surface region, at which YBCO nucleates preferentially, is larger than 70% of total area [16]. Therefore a dense and smooth surface of buffer layer is ideally desired for the subsequent growth of YBCO. A RMS roughness value of 2.16 nm indicates that the surface of LaZr1.1Oy/CeO2 buffer layers deposited on textured Ni5W substrate using CSD method is very suitable for further deposition of YBCO film. 4. Conclusion We have successfully fabricated LaZr1.1Oy buffer layer on bi-axial textured Ni5W substrate by CSD method. The preferred orientation of LaZr1.1Oy film is affected by heat-treatment temperature and heating rate during the annealing process. Moreover, annealing temperature has a greater effect on the degree of surface flatness of LaZr1.1Oy film grown on textured Ni5W substrate than heating rate during the heat-treatment process. XRD and AFM analysis has shown that the optimal annealing profile is 900 C and a heating rate of 2 C/min in flowing Ar-4%H2. In addition, CSD-CeO2 film with a low Rrms value of 2.16 nm has been grown on Ni5W substrate covered by optimal CSD-LaZr1.1Oy film. The microstructure and performance of YBa2Cu3O7-d films grown on the CSD-LaZr1.1Oy/ CeO2 bi-layer buffer films will be reported elsewhere. Acknowledgement This work was financially supported by the International Science & Technology Cooperation Program of China (Grant No. 2012DFA50780), the National Science Fund Program of China (Grant No. 51302225 and 51202201), the Innovative Research Team
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of Shaanxi province (Grant No.2013KCT-07), the Natural Science Fund Program of Shaanxi Province (Grant No. 2014JQ6202), the Weiyang District Science and Technology Plan Program of Xi’an city in Shaanxi Province (Grant No. 201412).
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