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Epitaxy of buffer layer and superconducting performance development of YBCO on bi-layer buffers coated on Ni5W by all CSD
Journal of Alloys and Compounds 644 (2015) 554–561
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Epitaxy of buffer layer and superconducting performance development of YBCO on bi-layer buffers coated on Ni5W by all CSD Y. Wang a,⇑, C.S. Li a, L.H. Jin a, Z.M. Yu a, J.Q. Feng a, H. Wang a, P. Odier b, P.X. Zhang a a b
Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China Néel Institute, CNRS and UJF, 25 avenue des Martyrs, BP166, 38042 Grenoble Cedex, France
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 1 May 2015 Accepted 11 May 2015 Available online 15 May 2015 Keywords: Coated conductors Buffer layer Chemical solution deposition Texture Morphology
a b s t r a c t Textured La2Zr2O7 (LZO) films have been fabricated onto textured metallic Ni–5 at.%W (Ni5W) tapes using a reel-to-reel chemical solution deposition (CSD) system. Subsequently, CeO2 cap and YBCO superconducting layers have successfully deposited on the LZO buffered substrates in sequence by CSD method, indicating good structural transition and smooth surface morphology of such the buffer layer. We have evaluated the effect of the travel speed of the tape during the annealing process of the LZO layer on the texture and superconducting performance of the final YBCO layers deposited on Ni5W/LZO/CeO2 substrates by all chemical method. The appearance of outgrown grains and pinholes on the surface of CeO2 layer results in the formation a-axis grains and impurity, which leads to the decrease of superconducting performance of YBCO layer. The high c-axis-oriented YBCO film grown on the LZO/CeO2 buffered Ni5W metallic tape exhibits a high critical current density Jc of 2.1 MA/cm2 at 77 K and 0 T when the preferred travel speed of tape of 4.2 m/h is adopted to fabricate the underlying LZO film. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The second generation high-temperature superconducting coated conductors (CCs) composed of metal substrate/buffer layer/superconducting layer/protective layer is a very promising material for various electric and magnetic applications due to its high critical current densities in a magnetic at 77 K [1–3]. Buffer layer not only transfers the bi-axial texture of metallic substrate to superconducting layer but also prevents the oxygen and metal atoms diffusion [4,5]. For various approaches of manufacturing CCs, chemical solution deposition (CSD) method is promising because of its many advantages, such as easy to control the composition, a relatively low processing temperature and a short annealing time, cost effective [6–8]. As a very representative research enterprise of all CSD route Showa Electric Wire and Cable Co. Ltd. (SWCC) fabricated the Hastelloy/Al2O3/MgO/LaMnO3/CeO2/REBCO long tapes with 100 m, which exhibited a high critical current of 400 A/cm w at 77 K [9]. The zero external field critical current density Jc of 1.02 MA/cm2 was obtained for YBCO film with 400 nm thickness by the trifluoroacetates-metal organic deposition (TFA-MOD) on a chemical solution-derived buffer layer architecture of CeO2/La2Zr2O7 (LZO) grown on Ni–5 at.%W (Ni5W)
⇑ Corresponding author. Tel.: +86 29 86231079; fax: +86 29 86224487. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.05.091 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
substrate [10]. A rapid process of each functional layer fabrication is unquestionably beneficial to the commercial application of CCs [11,12]. On the other hand, a slow elevated temperature process and long dwelling heat-treatment time are necessary for the preparation of some functional layers in order to confirm the complete crystallization and the good epitaxial growth [5]. Therefore, the balance between these two factors is significant in a reel-to-reel CSD system by adjusting the travel speeds of the tape during the annealing process. Pyrochlore LZO film is structurally and chemically compatible with Ni5W substrate, and can provide a good barrier against oxygen diffusion during the annealing process of YBCO layer [5,13]. On the other hand, CeO2 is often used as a cap layer in multi-layer architecture of buffer films because of its small lattice mismatch and similar thermal expansion coefficient for YBCO layer [14,15]. In addition, the preparation procedures of LZO and CeO2 film on textured substrates by CSD method are readily controllable and rather mature for the short CCs tapes [5,13,14]. Therefore, we fabricate the LZO buffer layers with a pyrochlore structure on metallic Ni5W substrate by a reel-to-reel CSD method in a wide range of travel speed during annealing process. CeO2 and YBCO films are then deposited on the as-grown LZO by CSD method in sequence. The effect of traveling tape speed during annealing procedure on the property of CCs is explored by investigating the
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texture and morphology development of functional layers as well as the superconducting performance evolution. 2. Experiment All oxide films including buffer layers and superconducting layer were fabricated on textured Ni5W tapes in protective atmospheres by CSD method. And all the precursor solution preparations were carried out in ambient atmosphere. The Ni5W tapes (10 mm wide and 8 lm thick) in this study were used as received from EVICO. The length of each Ni5W substrate was approximately 150 mm. The volume fraction of cube texture for Ni5W tapes was larger than 98% and the root means square roughness (Rrms) value of tape was less than 5 nm. The Ni5W tapes were ultrasonically cleaned in acetone for 20 min before coating. The reagents Lathanum (III) 2, 4-pentanedionate and Zirconium (IV) 2, 4-pentanedionate were used as received from STREM. Cerium (III) propionate was prepared by mixing cerium carbonate (received from STREM) and propionic acid in our laboratory. Copper acetate, Barium acetate and Yttrium acetate were used as received from Alfa Aesar. The yellow-colored LZO precursor solution with the total metal concentration of 0.4 mol/L was prepared by mixing stoichiometric amount of Lathanum (III) 2, 4-pentanedionate and Zirconium (IV) 2, 4-pentanedionate into propionic acid with continuous stirring. Similarly, Cerium propionate was dissolved in propionic acid by heating with stirring to fabricate a colorless CeO2 precursor solution with the concentration of 0.3 mol/L. For the fabrication of YBCO precursor solution, the acetates of Y, Ba and Cu (Y:Ba:Cu = 1:2:3) were firstly dissolved into trifluoroacetic acid with continuously reflux to obtain a blue dry gel, and then the gel was re-dissolved into methanol to yield solution. Preparation of YBCO precursor solution was described elsewhere in details [16]. The long LZO buffer layers were fabricated using CSD method in a reel-to-reel system. The Ni5W tape was driven by a stepper motor and could freely move forward and backward. LZO precursor solution was used to dip-coated onto Ni5W
Fig. 3. h–2h scans of as-grown CeO2 films on Ni5W/LZO substrates fabricated at different traveling tape speeds.
substrates at a withdrawal speed of 5 m/h. The coated samples were annealed in a horizontal tube furnace at 950 °C in flowing Ar–4%H2. And the different travel speeds (including 5.25 m/h, 4.2 m/h, 3.15 m/h, 2.1 m/h and 1.05 m/h) of the tapes were chosen during all heat-treatment processes of LZO coatings. The Ni5W tapes with buffer layers of LZO were cut into pieces with dimensions of 10 mm 10 mm and were used as substrates. Subsequently, spin coating was used to deposit CeO2 layers at a spin rate of 2500 rpm for 30 s. The precursor films were then annealed at 950 °C for 30 min in a flowing forming gas of Ar–4%H2. At the end of the heat-treatment cycles, the samples were quenched to room temperature in the same atmosphere to obtain LZO/CeO2 buffer layer. YBCO precursor solution was finally spin coated onto the Ni5W tapes with buffer layers of LZO/CeO2 at a spinning rate of 4000 rpm for 2 min. The coatings were calcined at 400 °C in 3.1% humidified oxygen atmosphere and then were crystallized at 850 °C for 1 h in Ar–0.01%O2. X-ray diffraction (XRD) h–2h scans were measured to evaluate phase and texture analysis of the grown films. In addition, the as-prepared films and the underlying Ni5W substrates were also characterized by XRD, which was performed to carry out out-of-plan scan for texture degree. The thickness of all the as-prepared films was calibrated by a step apparatus. The homogeneity and microstructure analysis of the samples was performed by atomic force microscopy (AFM) in contact mode. The surface roughness, Rrms, was determined from Rq values of 10 lm 10 lm, 5 lm 5 lm and 1 lm 1 lm grid area of each film. The critical temperature Tc of the YBCO film was measured by a DC four-probe method. The critical current density Jc of the YBCO layer was determined by using the extended Bean model [17], with the formulas Jc = 20 DM/[Va(1 a/3b)], where DM is the width of the magnetization (M) loop, V (cm3) is the volume of the film, a and b are the width and length of the slab (a < b), respectively.
Fig. 1. XRD patterns of LZO films prepared on Ni5W tapes at different travel speeds of tape.
Fig. 2. Variation of DFWHM of the x and u scans on Ni5W and LZO at different travel speeds of tape.
Fig. 4. Development of DFWHM of the x and u scans on CeO2 and underlying LZO films as a function of the traveling tape speed.
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3. Results and discussion Fig. 1 shows XRD patterns of LZO films prepared on textured Ni5W tapes at the different travel speeds. It can be clearly observed that all the LZO coatings completely crystallized to form the c-axis-oriented films after annealed at 950 °C in Ar–4%H2. The thickness of the LZO crystallized films is nearly 130 nm. In order to reveal the improvement of in-plane texture and out-of-plane texture of the LZO films with the variance of the traveling tape speed, the development of the difference DFWHMu = FWHMLZO(222) FWHMNi5W(111) and DFWHMx = FWHMLZO(004) FWHMNi5W(002) as a function of the traveling tape speed is shown in Fig. 2. It could be observed that all the LZO crystallized films annealed at different travel speeds show the good bi-axial
texture in a broad annealing temperature range. The texture degree of LZO film fabricated only at the travel speed of 2.1 m/h is improved compared to that of underlying Ni5W tape. It indicates that the slower traveling tape speed is beneficial to the epitaxial growth of LZO film on textured Ni5W tape. However, the texture degree of LZO film becomes worse when a smaller traveling tape speed of 2.1 m/h is adopted during the annealing process. It may be explained by the inhomogeneous nucleation of the oxides of La and Zr at slower travel speed of the tape during the annealing process, which leads to the texture degradation of LZO film. On the other hand, the texture degree of LZO film has an increasing current with quickening the travel speed of the tape, which is very similar to the result of the static short samples.
Fig. 5. AFM images of the LZO films deposited on textured Ni5W tapes at various speeds (a) 1.05 m/h, (b) 2.1 m/h, (c) 3.15 m/h, (d) 4.2 m/h and (e) 5.25 m/h.
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Typical h–2h scans of CeO2 films deposited on Ni5W/LZO substrates are shown in Fig. 3. All the CeO2 coatings have also completely crystallized and formed the c-axial texture except that the sample under the travel speed of 2.1 m/h shows a weak CeO2 (1 1 1) diffraction peak. It indicates that the oriented growth of CeO2 film is dependent on not only the texture but also the morphology of the underlying LZO layer. The thickness of the CeO2 crystallized films is nearly 50 nm. Fig. 4 shows the DFWHMu = FWHMCeO2(111) FWHMLZO(222) and DFWHMx = FWHMCeO2(002) FWHMLZO(004) as a function of the travel speed of the tape. FWHM values of the x and u scans between LZO and CeO2 films are very near things. The DFWHM values are the smallest among all the samples when the travel speed of the tape is 4.2 m/h during the annealing process of LZO film. And the texture of CeO2 film is obviously sharpened compared to the underlying LZO layer. It indicates that the texture degree of CeO2 films reaches a maximum when the travel speed of tape increases to 4.2 m/h during the continuous deposition of underlying LZO films. In addition, the sharp bi-axis texture of the buffer layer is of great importance to the epitaxial growth of further YBCO superconducting layer [18]. AFM measurements were conducted for the LZO films annealed at different travel speeds of tape, as shown in Fig. 5. All pictures show very small grains distribution on the surface of all as-grown samples. LZO films fabricated at faster traveling tape speed exhibit smooth, hole-free and homogenous surface morphology (as seen in Fig. 5d and e). However, some pinholes and big grains are observed on the surface of Ni5W/LZO prepared at slower travel speed of the tape (as seen in Fig. 5a–c). With decreasing the traveling tape speed during the annealing process of LZO, grain growth proceeds, leading to the size decrease and the number increase of the bigger grains. It indicates that the decrease of the traveling tape speed during annealing process equals reducing the heating rate and prolonging the dwelling time. However, too long dwelling time during the annealing process may result in the abnormal growth of partial grains, which would negatively affect the epitaxy of the further YBCO superconducting layer. The Rrms values are calculated over three grid areas of each sample, including 10 lm 10 lm, 5 lm 5 lm, 1 lm 1 lm. Fig. 6 represents the Rrms values of Ni5W/LZO versus the travel speed of tape. The surface roughness of the LZO films slightly increases and then steadily dropping with the increase of traveling tape speed. However, the Rrms values of all the LZO buffer layers are less than 2 nm within the scanned areas except that LZO film annealed at 2.1 m/h. Such a very smooth surface can meet the requirements for the subsequent deposition of other oxide films. It illuminates that a rapid annealing process for a short dwelling time may benefit the uniform improvement of the nucleation and growth of LZO grains. And the relatively faster traveling tape speed is helpful to the improvement of the surface smoothness of the as-prepared film. On the other hand, there is a distinct decreasing trend of surface roughness when the traveling tape speed decreases from 2.1 m/h to 1.05 m/h. It may be explained by an average effect of roughness due to the size decrease and the number increase of the big grains. AFM investigations were carried out on CeO2 buffer layers on Ni5W/LZO substrates fabricated at different traveling tape speeds, as shown in Fig. 7. Several outgrown grains are observed on the surface of all the Ni5W/LZO/CeO2 samples. The emergence of these large grains may be correlated with the secondary recrystallization of partial CeO2 grains, which may occur mainly at the defect sites on the surface of Ni5W/LZO substrates. With decreasing the trave ling tape speed during the annealing process of LZO film, the number of outgrown grains increases obviously on the surface of Ni5W/LZO/CeO2 substrates. A more detailed roughness profile can be seen from AFM line scan analysis on 5 lm 5 lm regions
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Fig. 6. Variation of Rrms values of LZO films fabricated at different traveling tape speeds.
of the CeO2 film surface, which shows an average elevation and a maximum height difference. Some significant variations in all the surface profile along the line scan are observed, indicative of spatially uneven film growth. For CeO2 on the surface of NiW/LZO at the traveling speed of 2.1 m/h, the average elevation of outgrown grains on the surface reaches 30 nm, which was twice as high as that of other samples. Moreover, the AFM line scans of CeO2 surfaces coated on NiW/LZO at traveling speed of 1.05 m/h and 4.2 m/h show the smallest average elevation of 15 nm. The presence of these outgrown grains would have an influence on the property of YBCO further deposited on these bi-layer films. However, some obvious pinholes are observed on the surface of CeO2 x film covered on NiW/LZO at traveling speed of 1.05 m/h and the average diameter of pinholes is about 100 nm. These defects of pinholes would become alternative entryways of oxygen and Ni diffusion during YBCO layer deposition process. Dependency of the Rrms values on the traveling tape speed during the annealing process of LZO is plotted in Fig. 8. The varying trend of the surface roughness of CeO2 films is similar to that of the underlying LZO films with changing the traveling tape speed during the heat-treatment process of LZO films. Moreover, the observed typical Rrms values of all Ni5W/LZO/CeO2 samples are less than 5 nm within these scanned areas except the CeO2 film except the underlying LZO film fabricated at the tape travel speed of 2.1 m/h. It indicates that the large CeO2 grains are readily formed on the underlying LZO film with rougher surface, leading to the increase of the surface roughness of top CeO2 layer. Although the Rrms values of CeO2 layer are greater than that of the underlying LZO layer in general, the Rrms values less than 5 nm could meet the requirement of further deposition of other oxide films. Small Rrms values are beneficial for the further successfully growth process of other oxide [5]. Therefore, the fabrication of underlying LZO film with a smooth surface is essential for the decrease of surface roughness of the latter CeO2 layer. The characterization of the microstructure and growth orientation of buffer layers would affect orientated growth of final YBCO grains [18–20]. For instance, the weak bi-axial texture and rough surface of buffer layers could lead to the formation of a-axis oriented YBCO grains. The increase of the intensity of YBCO (2 0 0) diffraction peak implies the increase of the number of a-axis oriented grains. Fig. 9 represents the intensity ratio compounded through dividing the intensity of (0 0 6) diffraction peak of YBCO by the sum of the intensities of YBCO (0 0 6) and (2 0 0) peaks from XRD patterns obtained for Ni5W/LZO/CeO2/YBCO samples at
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Fig. 7. Surface microstructure of CeO2 films grown on Ni5W/LZO substrates obtained at different traveling tape speeds (a) 1.05 m/h, (b) 2.1 m/h, (c) 3.15 m/h, (d) 4.2 m/h and (e) 5.25 m/h investigated by AFM images and AFM surface profiles on 5 lm line scans.
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Fig. 8. Dependency of Rrms values of CeO2 films on the traveling tape speed.
different traveling tape speeds during the annealing process of LZO films. It shows that the relative intensity of YBCO (0 0 6) peaks, which gives an estimate of the c-axis oriented grain fraction in the YBCO films. The relative intensity of YBCO (0 0 6) peak reaches a maximum value when the traveling tape speed is adopted 4.2 m/h during the annealing process of underlying LZO layer. It suggests that the number of a-axis oriented grains reaches a minimum while that of the c-axis oriented grains reaches a maximum in our case. The thickness of the crystallized YBCO films is nearly 300 nm. A typical XRD h–2h spectrum for YBCO film on CeO2 covered Ni5W/LZO substrate at the traveling speed of 4.2 m/h is shown in Fig. 10. It indicates the presence of a strong (0 0 l) reflection of YBCO although small impurities of NiWO4, NiO and unreacted BaCeO3 were also present. On the whole, YBCO film with greater relative intensity of (0 0 6) peak is obtained on the CeO2 film with smaller Rrms value and least outgrown grains at the faster traveling tape speed (4–5 m/h). Moreover, the varying trend of the relative intensity of YBCO (0 0 6) peak basically agrees with that of the texture degree of LZO film as the change of the travel speed of the tape. It implies that the texture and surface morphology of the template layer have an important influence on the epitaxial growth of YBCO film. Fig. 11 shows the typical temperature dependence of electrical resistivity for YBCO film on the CeO2 covered Ni5W/LZO substrate
Fig. 9. Development of relative intensity of YBCO (0 0 6) as a function of the traveling tape speed.
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prepared at traveling tape speed of 4.2 m/h. a typical value of Tc = 88.3 K with a sharp transition range is obtained for the YBCO film. The relationship between critical current density Jc and the traveling tape speed during the annealing process of the underlying LZO film is sketched in Fig. 12. The varying trend of Jc is contrary to the content of a-axis grains in YBCO films as a function of the traveling tape speed during the annealing process of underlying LZO layer. It implies that the texture improvement of YBCO film is beneficial for the increase of the superconducting performance. Moreover, YBCO film on Ni5W/LZO/CeO2 shows a high Jc of 2.1 MA/cm2 at 77 K under 0 T when the traveling tape speed increases to 4.2 m/h during the annealing process of underlying LZO film. The YBCO nucleation density decreases and the lateral growth of the YBCO film is hindered, which results in the formation of impurity and dislocation, when some outgrown grains exist on the surface of CeO2 film. Moreover, there is much difference between the nucleation rates of YBCO film at the pinhole edges and that at other areas on the surface of buffer layers, which leads to the formation of a-axis grains. It indicates that a buffer layer with excellent bi-axis texture and less surface defects is important for the improvement of the superconducting performance of coated conductors. In the bi-layer structure of buffers, the lattice mismatch and transition at the interface between the metallic substrate and the first layer become the important factor to influence the epitaxial growth of the buffer layer. The role of the first layer in multi-layer buffers is to adapt the thermal, chemical and crystalline properties of metallic substrate to those of the oxide ceramic film [21]. Thus, the strain from lattice mismatch plays a significant role for the texture transfer. Because the intrinsic oxygen diffusion coefficient of LZO is much smaller than that of CeO2, LZO buffer layer plays a leading role in the prevention of oxygen diffusion through pores in film [22–24]. A LZO buffer layer with micro-defect is not efficient for the prevention of oxygen diffusion during the preparation of the YBCO film. Not only the micro-defect formation but also the texture degeneration of LZO layer has a negative effect on the superconducting performance of YBCO layer (as seen in Figs. 2 and 12) Ex-situ YBCO film grown by CSD have a layer-by-layer growth mode while this situation is very rare, in most case a Volmer–Weber mode dominates (3D nucleation). The epitaxial growth of YBCO film depends on several parameters, such as temperate, oxygen partial pressure, lattice misfit with the substrate, etc. In addition, both the degree the biaxial texture or the surface roughness are key parameters to optimize the critical currents in YBCO films grown by CSD [25–28]. In fact,
Fig. 10. A typical XRD h–2h spectrum for an YBCO film on the CeO2 covered Ni5W/ LZO substrate prepared at traveling tape speed of 4.2 m/h.
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structure as well as a smooth surface is beneficial for the fabrication of YBCO layer with high superconducting performance. 4. Conclusions
Fig. 11. Typical temperature dependence of the electrical resistivity of an YBCO film on the CeO2 covered Ni5W/LZO substrate prepared at traveling tape speed of 4.2 m/ h.
Bi-axial textured LZO buffer layers have been grown on moving textured Ni5W tapes at different traveling tape speeds by CSD techniques. And then CeO2 and YBCO films have deposited on Ni5W/LZO substrates in sequence. Our results indicated that not only the micro-defect formation but also the texture degeneration of LZO layer has a negative effect on the superconducting performance of YBCO layer. In addition, the existence of outgrown grains and pinholes on the surface of the cap layer results in the performance decrease of YBCO layer because of the formation of impurity and a-axis grains. However, all-chemical CCs show a high Jc value of 2.1 MA/cm2 at 77 K and 0 T by chemical solution deposited LZO/CeO2/YBCO multi-layers on textured Ni5W tape at an optimal travel speed of 4.2 m/h. Acknowledgements
the cap layer adjacent to superconducting layer is the genuine substrate for the growth of superconducting layer. The small lattice mismatch and no interface reaction between the cap layer and superconducting layer as well as sharp bi-axial texture of a cap layer become important for the epitaxial growth of superconducting layer. 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 [29]. Thus, the fabrication of a smooth cap layer becomes one of the crucial factors for the growth of further superconducting layer. For the sample deposited on NiW/LZO at the traveling speed of 2.1 m/h, the formation of a large quantity of outgrown grains on CeO2 film surface together with worst c-axis orientation of YBCO layer suggests that there exists a possible connection between the outgrown grains on surface of cap layer and the nucleation of a-axis oriented grains of YBCO layer. The number and size decrease of outgrown grains and pinholes on surface of the cap layer is a great help to the epitaxial growth of superconducting layer. In addition, the increase of texture degree of CeO2 cap layer is definitely beneficial to the improvement of superconducting property of YBCO layer (as seen in Figs. 4 and 12). It can be concluded that the buffer layer with the excellent bi-axis texture and dense
Fig. 12. Jc at 77 K for YBCO films dependent on the traveling tape speed during the annealing process of LZO.
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 Nos. 51302225 and 51202201), the Innovative Research Team of Shaanxi province (No. 2013KCT-07), the International Science & Technology Cooperation Program and the Natural Science Fund Program of Shaanxi Province (Grant Nos. 2013KW12-03 and 2014JQ6202), the Weiyang District Science and Technology Plan Program of Xi’an city in Shaanxi Province (Grant No. 201412). References [1] N.D. Khatri, G. Majkic, T. Shi, Y. Chen, V. Selvamanickam, Pre-fabricated nanorods in RE–Ba–Cu–O superconductors, Supercond. Sci. Technol. 26 (2013) 085022. [2] V. Narayanan, I.V. Driessche, Effect of polymer inclusion in preparation of thick LZO buffer layers for YBCO coated conductors, J. Mater. Sci. Technol. 29 (2013) 261–266. [3] S.P.K. Naik, N.D. Kumar, P.M.S. Raju, T. Rajasekharan, V. Seshubai, Effect of infiltration temperature on the properties of infiltration growth processed YBCO superconductor, Physica C 487 (2013) 72–76. [4] V. Roche, C. Jiménez, P. Chaudouët, R. Benaboud, F. Weiss, E. Sarigiannidou, Microstructural changes induced by the pyrolysis step on epitaxial La2Zr2O7 thin films grown by metalorganic decomposition, Thin Solid Films 520 (2012) 2566–2574. [5] K. Knoth, R. Hühne, S. Oswald, L. Schultz, B. Holzapfel, Detailed investigations on La2Zr2O7 buffer layers for YBCO-coated conductors prepared by chemical solution deposition, Acta Mater. 55 (2007) 517–529. [6] R.B. Mos, T. Petrisor Jr., M. Nasui, A. Calleja, T. Puig, L. Ciontea, T. Petrisor, Enhanced structural and morphological properties of Gd-doped CeO2 thin films obtained by polymer-assisted deposition, Mater. Lett. 124 (2014) 306– 309. [7] V. Cloet, P. Lommens, R. Hühne, K. De Buysser, S. Hoste, I. Van Driessche, A study of the parameters influencing the microstructure of thick La2Zr2O7 films, J. Cryst. Growth 325 (2011) 68–75. [8] S. Engel, K. Knoth, R. Hühne, L. Schultz, B. Holzapfel, An all chemical solution deposition approach for the growth of highly textured CeO2 cap layers on La2Zr2O7-buffered long lengths of biaxially textured Ni–W substrates for YBCO-coated conductors, Supercond. Sci. Technol. 18 (2005) 1385–1390. [9] T. Koizumi, The Latest Trends of MOD REBCO Superconducting Coated Conductors in SWCC, ISS2014, WT-9. [10] L. Molina-Luna, R. Egoavil, S. Turner, T. Thersleff, J. Verbeeck, Bernhard Holzapfel, Oliver Eibl, Gustaaf Van Tendeloo, Interlayer structure in YBCOcoated conductors prepared by chemical solution deposition, Supercond. Sci. Technol. 26 (2013) 075016. [11] W. Wu, F. Feng, K. Shi, W. Zhai, T.M. Qu, R.X. Huang, X. Tang, X.H. Wang, Q.Y. Hu, J.C. Grivel, Z.H. Han, A rapid process of YBa2Cu3O7 d thin film fabrication using trifluoroacetate metal–organic deposition with polyethylene glycol additive, Supercond. Sci. Technol. 26 (2013) 055013. [12] P. Zhao, A. Ito, T. Kato, D. Yokoe, T. Hirayama, T. Goto, High-speed growth of YBa2Cu3O7 d superconducting films on multilayer-coated Hastelloy C276 tape by laser-assisted MOCVD, Supercond. Sci. Technol. 26 (2013) 055020.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Epitaxy of buffer layer and superconducting performance development of YBCO on bi-layer buffers coated on Ni5W by all CSD Y. Wang a,⇑, C.S. Li a, L.H. Jin a, Z.M. Yu a, J.Q. Feng a, H. Wang a, P. Odier b, P.X. Zhang a a b
Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China Néel Institute, CNRS and UJF, 25 avenue des Martyrs, BP166, 38042 Grenoble Cedex, France
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 1 May 2015 Accepted 11 May 2015 Available online 15 May 2015 Keywords: Coated conductors Buffer layer Chemical solution deposition Texture Morphology
a b s t r a c t Textured La2Zr2O7 (LZO) films have been fabricated onto textured metallic Ni–5 at.%W (Ni5W) tapes using a reel-to-reel chemical solution deposition (CSD) system. Subsequently, CeO2 cap and YBCO superconducting layers have successfully deposited on the LZO buffered substrates in sequence by CSD method, indicating good structural transition and smooth surface morphology of such the buffer layer. We have evaluated the effect of the travel speed of the tape during the annealing process of the LZO layer on the texture and superconducting performance of the final YBCO layers deposited on Ni5W/LZO/CeO2 substrates by all chemical method. The appearance of outgrown grains and pinholes on the surface of CeO2 layer results in the formation a-axis grains and impurity, which leads to the decrease of superconducting performance of YBCO layer. The high c-axis-oriented YBCO film grown on the LZO/CeO2 buffered Ni5W metallic tape exhibits a high critical current density Jc of 2.1 MA/cm2 at 77 K and 0 T when the preferred travel speed of tape of 4.2 m/h is adopted to fabricate the underlying LZO film. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The second generation high-temperature superconducting coated conductors (CCs) composed of metal substrate/buffer layer/superconducting layer/protective layer is a very promising material for various electric and magnetic applications due to its high critical current densities in a magnetic at 77 K [1–3]. Buffer layer not only transfers the bi-axial texture of metallic substrate to superconducting layer but also prevents the oxygen and metal atoms diffusion [4,5]. For various approaches of manufacturing CCs, chemical solution deposition (CSD) method is promising because of its many advantages, such as easy to control the composition, a relatively low processing temperature and a short annealing time, cost effective [6–8]. As a very representative research enterprise of all CSD route Showa Electric Wire and Cable Co. Ltd. (SWCC) fabricated the Hastelloy/Al2O3/MgO/LaMnO3/CeO2/REBCO long tapes with 100 m, which exhibited a high critical current of 400 A/cm w at 77 K [9]. The zero external field critical current density Jc of 1.02 MA/cm2 was obtained for YBCO film with 400 nm thickness by the trifluoroacetates-metal organic deposition (TFA-MOD) on a chemical solution-derived buffer layer architecture of CeO2/La2Zr2O7 (LZO) grown on Ni–5 at.%W (Ni5W)
⇑ Corresponding author. Tel.: +86 29 86231079; fax: +86 29 86224487. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.05.091 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
substrate [10]. A rapid process of each functional layer fabrication is unquestionably beneficial to the commercial application of CCs [11,12]. On the other hand, a slow elevated temperature process and long dwelling heat-treatment time are necessary for the preparation of some functional layers in order to confirm the complete crystallization and the good epitaxial growth [5]. Therefore, the balance between these two factors is significant in a reel-to-reel CSD system by adjusting the travel speeds of the tape during the annealing process. Pyrochlore LZO film is structurally and chemically compatible with Ni5W substrate, and can provide a good barrier against oxygen diffusion during the annealing process of YBCO layer [5,13]. On the other hand, CeO2 is often used as a cap layer in multi-layer architecture of buffer films because of its small lattice mismatch and similar thermal expansion coefficient for YBCO layer [14,15]. In addition, the preparation procedures of LZO and CeO2 film on textured substrates by CSD method are readily controllable and rather mature for the short CCs tapes [5,13,14]. Therefore, we fabricate the LZO buffer layers with a pyrochlore structure on metallic Ni5W substrate by a reel-to-reel CSD method in a wide range of travel speed during annealing process. CeO2 and YBCO films are then deposited on the as-grown LZO by CSD method in sequence. The effect of traveling tape speed during annealing procedure on the property of CCs is explored by investigating the
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texture and morphology development of functional layers as well as the superconducting performance evolution. 2. Experiment All oxide films including buffer layers and superconducting layer were fabricated on textured Ni5W tapes in protective atmospheres by CSD method. And all the precursor solution preparations were carried out in ambient atmosphere. The Ni5W tapes (10 mm wide and 8 lm thick) in this study were used as received from EVICO. The length of each Ni5W substrate was approximately 150 mm. The volume fraction of cube texture for Ni5W tapes was larger than 98% and the root means square roughness (Rrms) value of tape was less than 5 nm. The Ni5W tapes were ultrasonically cleaned in acetone for 20 min before coating. The reagents Lathanum (III) 2, 4-pentanedionate and Zirconium (IV) 2, 4-pentanedionate were used as received from STREM. Cerium (III) propionate was prepared by mixing cerium carbonate (received from STREM) and propionic acid in our laboratory. Copper acetate, Barium acetate and Yttrium acetate were used as received from Alfa Aesar. The yellow-colored LZO precursor solution with the total metal concentration of 0.4 mol/L was prepared by mixing stoichiometric amount of Lathanum (III) 2, 4-pentanedionate and Zirconium (IV) 2, 4-pentanedionate into propionic acid with continuous stirring. Similarly, Cerium propionate was dissolved in propionic acid by heating with stirring to fabricate a colorless CeO2 precursor solution with the concentration of 0.3 mol/L. For the fabrication of YBCO precursor solution, the acetates of Y, Ba and Cu (Y:Ba:Cu = 1:2:3) were firstly dissolved into trifluoroacetic acid with continuously reflux to obtain a blue dry gel, and then the gel was re-dissolved into methanol to yield solution. Preparation of YBCO precursor solution was described elsewhere in details [16]. The long LZO buffer layers were fabricated using CSD method in a reel-to-reel system. The Ni5W tape was driven by a stepper motor and could freely move forward and backward. LZO precursor solution was used to dip-coated onto Ni5W
Fig. 3. h–2h scans of as-grown CeO2 films on Ni5W/LZO substrates fabricated at different traveling tape speeds.
substrates at a withdrawal speed of 5 m/h. The coated samples were annealed in a horizontal tube furnace at 950 °C in flowing Ar–4%H2. And the different travel speeds (including 5.25 m/h, 4.2 m/h, 3.15 m/h, 2.1 m/h and 1.05 m/h) of the tapes were chosen during all heat-treatment processes of LZO coatings. The Ni5W tapes with buffer layers of LZO were cut into pieces with dimensions of 10 mm 10 mm and were used as substrates. Subsequently, spin coating was used to deposit CeO2 layers at a spin rate of 2500 rpm for 30 s. The precursor films were then annealed at 950 °C for 30 min in a flowing forming gas of Ar–4%H2. At the end of the heat-treatment cycles, the samples were quenched to room temperature in the same atmosphere to obtain LZO/CeO2 buffer layer. YBCO precursor solution was finally spin coated onto the Ni5W tapes with buffer layers of LZO/CeO2 at a spinning rate of 4000 rpm for 2 min. The coatings were calcined at 400 °C in 3.1% humidified oxygen atmosphere and then were crystallized at 850 °C for 1 h in Ar–0.01%O2. X-ray diffraction (XRD) h–2h scans were measured to evaluate phase and texture analysis of the grown films. In addition, the as-prepared films and the underlying Ni5W substrates were also characterized by XRD, which was performed to carry out out-of-plan scan for texture degree. The thickness of all the as-prepared films was calibrated by a step apparatus. The homogeneity and microstructure analysis of the samples was performed by atomic force microscopy (AFM) in contact mode. The surface roughness, Rrms, was determined from Rq values of 10 lm 10 lm, 5 lm 5 lm and 1 lm 1 lm grid area of each film. The critical temperature Tc of the YBCO film was measured by a DC four-probe method. The critical current density Jc of the YBCO layer was determined by using the extended Bean model [17], with the formulas Jc = 20 DM/[Va(1 a/3b)], where DM is the width of the magnetization (M) loop, V (cm3) is the volume of the film, a and b are the width and length of the slab (a < b), respectively.
Fig. 1. XRD patterns of LZO films prepared on Ni5W tapes at different travel speeds of tape.
Fig. 2. Variation of DFWHM of the x and u scans on Ni5W and LZO at different travel speeds of tape.
Fig. 4. Development of DFWHM of the x and u scans on CeO2 and underlying LZO films as a function of the traveling tape speed.
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3. Results and discussion Fig. 1 shows XRD patterns of LZO films prepared on textured Ni5W tapes at the different travel speeds. It can be clearly observed that all the LZO coatings completely crystallized to form the c-axis-oriented films after annealed at 950 °C in Ar–4%H2. The thickness of the LZO crystallized films is nearly 130 nm. In order to reveal the improvement of in-plane texture and out-of-plane texture of the LZO films with the variance of the traveling tape speed, the development of the difference DFWHMu = FWHMLZO(222) FWHMNi5W(111) and DFWHMx = FWHMLZO(004) FWHMNi5W(002) as a function of the traveling tape speed is shown in Fig. 2. It could be observed that all the LZO crystallized films annealed at different travel speeds show the good bi-axial
texture in a broad annealing temperature range. The texture degree of LZO film fabricated only at the travel speed of 2.1 m/h is improved compared to that of underlying Ni5W tape. It indicates that the slower traveling tape speed is beneficial to the epitaxial growth of LZO film on textured Ni5W tape. However, the texture degree of LZO film becomes worse when a smaller traveling tape speed of 2.1 m/h is adopted during the annealing process. It may be explained by the inhomogeneous nucleation of the oxides of La and Zr at slower travel speed of the tape during the annealing process, which leads to the texture degradation of LZO film. On the other hand, the texture degree of LZO film has an increasing current with quickening the travel speed of the tape, which is very similar to the result of the static short samples.
Fig. 5. AFM images of the LZO films deposited on textured Ni5W tapes at various speeds (a) 1.05 m/h, (b) 2.1 m/h, (c) 3.15 m/h, (d) 4.2 m/h and (e) 5.25 m/h.
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Typical h–2h scans of CeO2 films deposited on Ni5W/LZO substrates are shown in Fig. 3. All the CeO2 coatings have also completely crystallized and formed the c-axial texture except that the sample under the travel speed of 2.1 m/h shows a weak CeO2 (1 1 1) diffraction peak. It indicates that the oriented growth of CeO2 film is dependent on not only the texture but also the morphology of the underlying LZO layer. The thickness of the CeO2 crystallized films is nearly 50 nm. Fig. 4 shows the DFWHMu = FWHMCeO2(111) FWHMLZO(222) and DFWHMx = FWHMCeO2(002) FWHMLZO(004) as a function of the travel speed of the tape. FWHM values of the x and u scans between LZO and CeO2 films are very near things. The DFWHM values are the smallest among all the samples when the travel speed of the tape is 4.2 m/h during the annealing process of LZO film. And the texture of CeO2 film is obviously sharpened compared to the underlying LZO layer. It indicates that the texture degree of CeO2 films reaches a maximum when the travel speed of tape increases to 4.2 m/h during the continuous deposition of underlying LZO films. In addition, the sharp bi-axis texture of the buffer layer is of great importance to the epitaxial growth of further YBCO superconducting layer [18]. AFM measurements were conducted for the LZO films annealed at different travel speeds of tape, as shown in Fig. 5. All pictures show very small grains distribution on the surface of all as-grown samples. LZO films fabricated at faster traveling tape speed exhibit smooth, hole-free and homogenous surface morphology (as seen in Fig. 5d and e). However, some pinholes and big grains are observed on the surface of Ni5W/LZO prepared at slower travel speed of the tape (as seen in Fig. 5a–c). With decreasing the traveling tape speed during the annealing process of LZO, grain growth proceeds, leading to the size decrease and the number increase of the bigger grains. It indicates that the decrease of the traveling tape speed during annealing process equals reducing the heating rate and prolonging the dwelling time. However, too long dwelling time during the annealing process may result in the abnormal growth of partial grains, which would negatively affect the epitaxy of the further YBCO superconducting layer. The Rrms values are calculated over three grid areas of each sample, including 10 lm 10 lm, 5 lm 5 lm, 1 lm 1 lm. Fig. 6 represents the Rrms values of Ni5W/LZO versus the travel speed of tape. The surface roughness of the LZO films slightly increases and then steadily dropping with the increase of traveling tape speed. However, the Rrms values of all the LZO buffer layers are less than 2 nm within the scanned areas except that LZO film annealed at 2.1 m/h. Such a very smooth surface can meet the requirements for the subsequent deposition of other oxide films. It illuminates that a rapid annealing process for a short dwelling time may benefit the uniform improvement of the nucleation and growth of LZO grains. And the relatively faster traveling tape speed is helpful to the improvement of the surface smoothness of the as-prepared film. On the other hand, there is a distinct decreasing trend of surface roughness when the traveling tape speed decreases from 2.1 m/h to 1.05 m/h. It may be explained by an average effect of roughness due to the size decrease and the number increase of the big grains. AFM investigations were carried out on CeO2 buffer layers on Ni5W/LZO substrates fabricated at different traveling tape speeds, as shown in Fig. 7. Several outgrown grains are observed on the surface of all the Ni5W/LZO/CeO2 samples. The emergence of these large grains may be correlated with the secondary recrystallization of partial CeO2 grains, which may occur mainly at the defect sites on the surface of Ni5W/LZO substrates. With decreasing the trave ling tape speed during the annealing process of LZO film, the number of outgrown grains increases obviously on the surface of Ni5W/LZO/CeO2 substrates. A more detailed roughness profile can be seen from AFM line scan analysis on 5 lm 5 lm regions
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Fig. 6. Variation of Rrms values of LZO films fabricated at different traveling tape speeds.
of the CeO2 film surface, which shows an average elevation and a maximum height difference. Some significant variations in all the surface profile along the line scan are observed, indicative of spatially uneven film growth. For CeO2 on the surface of NiW/LZO at the traveling speed of 2.1 m/h, the average elevation of outgrown grains on the surface reaches 30 nm, which was twice as high as that of other samples. Moreover, the AFM line scans of CeO2 surfaces coated on NiW/LZO at traveling speed of 1.05 m/h and 4.2 m/h show the smallest average elevation of 15 nm. The presence of these outgrown grains would have an influence on the property of YBCO further deposited on these bi-layer films. However, some obvious pinholes are observed on the surface of CeO2 x film covered on NiW/LZO at traveling speed of 1.05 m/h and the average diameter of pinholes is about 100 nm. These defects of pinholes would become alternative entryways of oxygen and Ni diffusion during YBCO layer deposition process. Dependency of the Rrms values on the traveling tape speed during the annealing process of LZO is plotted in Fig. 8. The varying trend of the surface roughness of CeO2 films is similar to that of the underlying LZO films with changing the traveling tape speed during the heat-treatment process of LZO films. Moreover, the observed typical Rrms values of all Ni5W/LZO/CeO2 samples are less than 5 nm within these scanned areas except the CeO2 film except the underlying LZO film fabricated at the tape travel speed of 2.1 m/h. It indicates that the large CeO2 grains are readily formed on the underlying LZO film with rougher surface, leading to the increase of the surface roughness of top CeO2 layer. Although the Rrms values of CeO2 layer are greater than that of the underlying LZO layer in general, the Rrms values less than 5 nm could meet the requirement of further deposition of other oxide films. Small Rrms values are beneficial for the further successfully growth process of other oxide [5]. Therefore, the fabrication of underlying LZO film with a smooth surface is essential for the decrease of surface roughness of the latter CeO2 layer. The characterization of the microstructure and growth orientation of buffer layers would affect orientated growth of final YBCO grains [18–20]. For instance, the weak bi-axial texture and rough surface of buffer layers could lead to the formation of a-axis oriented YBCO grains. The increase of the intensity of YBCO (2 0 0) diffraction peak implies the increase of the number of a-axis oriented grains. Fig. 9 represents the intensity ratio compounded through dividing the intensity of (0 0 6) diffraction peak of YBCO by the sum of the intensities of YBCO (0 0 6) and (2 0 0) peaks from XRD patterns obtained for Ni5W/LZO/CeO2/YBCO samples at
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Fig. 7. Surface microstructure of CeO2 films grown on Ni5W/LZO substrates obtained at different traveling tape speeds (a) 1.05 m/h, (b) 2.1 m/h, (c) 3.15 m/h, (d) 4.2 m/h and (e) 5.25 m/h investigated by AFM images and AFM surface profiles on 5 lm line scans.
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Fig. 8. Dependency of Rrms values of CeO2 films on the traveling tape speed.
different traveling tape speeds during the annealing process of LZO films. It shows that the relative intensity of YBCO (0 0 6) peaks, which gives an estimate of the c-axis oriented grain fraction in the YBCO films. The relative intensity of YBCO (0 0 6) peak reaches a maximum value when the traveling tape speed is adopted 4.2 m/h during the annealing process of underlying LZO layer. It suggests that the number of a-axis oriented grains reaches a minimum while that of the c-axis oriented grains reaches a maximum in our case. The thickness of the crystallized YBCO films is nearly 300 nm. A typical XRD h–2h spectrum for YBCO film on CeO2 covered Ni5W/LZO substrate at the traveling speed of 4.2 m/h is shown in Fig. 10. It indicates the presence of a strong (0 0 l) reflection of YBCO although small impurities of NiWO4, NiO and unreacted BaCeO3 were also present. On the whole, YBCO film with greater relative intensity of (0 0 6) peak is obtained on the CeO2 film with smaller Rrms value and least outgrown grains at the faster traveling tape speed (4–5 m/h). Moreover, the varying trend of the relative intensity of YBCO (0 0 6) peak basically agrees with that of the texture degree of LZO film as the change of the travel speed of the tape. It implies that the texture and surface morphology of the template layer have an important influence on the epitaxial growth of YBCO film. Fig. 11 shows the typical temperature dependence of electrical resistivity for YBCO film on the CeO2 covered Ni5W/LZO substrate
Fig. 9. Development of relative intensity of YBCO (0 0 6) as a function of the traveling tape speed.
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prepared at traveling tape speed of 4.2 m/h. a typical value of Tc = 88.3 K with a sharp transition range is obtained for the YBCO film. The relationship between critical current density Jc and the traveling tape speed during the annealing process of the underlying LZO film is sketched in Fig. 12. The varying trend of Jc is contrary to the content of a-axis grains in YBCO films as a function of the traveling tape speed during the annealing process of underlying LZO layer. It implies that the texture improvement of YBCO film is beneficial for the increase of the superconducting performance. Moreover, YBCO film on Ni5W/LZO/CeO2 shows a high Jc of 2.1 MA/cm2 at 77 K under 0 T when the traveling tape speed increases to 4.2 m/h during the annealing process of underlying LZO film. The YBCO nucleation density decreases and the lateral growth of the YBCO film is hindered, which results in the formation of impurity and dislocation, when some outgrown grains exist on the surface of CeO2 film. Moreover, there is much difference between the nucleation rates of YBCO film at the pinhole edges and that at other areas on the surface of buffer layers, which leads to the formation of a-axis grains. It indicates that a buffer layer with excellent bi-axis texture and less surface defects is important for the improvement of the superconducting performance of coated conductors. In the bi-layer structure of buffers, the lattice mismatch and transition at the interface between the metallic substrate and the first layer become the important factor to influence the epitaxial growth of the buffer layer. The role of the first layer in multi-layer buffers is to adapt the thermal, chemical and crystalline properties of metallic substrate to those of the oxide ceramic film [21]. Thus, the strain from lattice mismatch plays a significant role for the texture transfer. Because the intrinsic oxygen diffusion coefficient of LZO is much smaller than that of CeO2, LZO buffer layer plays a leading role in the prevention of oxygen diffusion through pores in film [22–24]. A LZO buffer layer with micro-defect is not efficient for the prevention of oxygen diffusion during the preparation of the YBCO film. Not only the micro-defect formation but also the texture degeneration of LZO layer has a negative effect on the superconducting performance of YBCO layer (as seen in Figs. 2 and 12) Ex-situ YBCO film grown by CSD have a layer-by-layer growth mode while this situation is very rare, in most case a Volmer–Weber mode dominates (3D nucleation). The epitaxial growth of YBCO film depends on several parameters, such as temperate, oxygen partial pressure, lattice misfit with the substrate, etc. In addition, both the degree the biaxial texture or the surface roughness are key parameters to optimize the critical currents in YBCO films grown by CSD [25–28]. In fact,
Fig. 10. A typical XRD h–2h spectrum for an YBCO film on the CeO2 covered Ni5W/ LZO substrate prepared at traveling tape speed of 4.2 m/h.
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structure as well as a smooth surface is beneficial for the fabrication of YBCO layer with high superconducting performance. 4. Conclusions
Fig. 11. Typical temperature dependence of the electrical resistivity of an YBCO film on the CeO2 covered Ni5W/LZO substrate prepared at traveling tape speed of 4.2 m/ h.
Bi-axial textured LZO buffer layers have been grown on moving textured Ni5W tapes at different traveling tape speeds by CSD techniques. And then CeO2 and YBCO films have deposited on Ni5W/LZO substrates in sequence. Our results indicated that not only the micro-defect formation but also the texture degeneration of LZO layer has a negative effect on the superconducting performance of YBCO layer. In addition, the existence of outgrown grains and pinholes on the surface of the cap layer results in the performance decrease of YBCO layer because of the formation of impurity and a-axis grains. However, all-chemical CCs show a high Jc value of 2.1 MA/cm2 at 77 K and 0 T by chemical solution deposited LZO/CeO2/YBCO multi-layers on textured Ni5W tape at an optimal travel speed of 4.2 m/h. Acknowledgements
the cap layer adjacent to superconducting layer is the genuine substrate for the growth of superconducting layer. The small lattice mismatch and no interface reaction between the cap layer and superconducting layer as well as sharp bi-axial texture of a cap layer become important for the epitaxial growth of superconducting layer. 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 [29]. Thus, the fabrication of a smooth cap layer becomes one of the crucial factors for the growth of further superconducting layer. For the sample deposited on NiW/LZO at the traveling speed of 2.1 m/h, the formation of a large quantity of outgrown grains on CeO2 film surface together with worst c-axis orientation of YBCO layer suggests that there exists a possible connection between the outgrown grains on surface of cap layer and the nucleation of a-axis oriented grains of YBCO layer. The number and size decrease of outgrown grains and pinholes on surface of the cap layer is a great help to the epitaxial growth of superconducting layer. In addition, the increase of texture degree of CeO2 cap layer is definitely beneficial to the improvement of superconducting property of YBCO layer (as seen in Figs. 4 and 12). It can be concluded that the buffer layer with the excellent bi-axis texture and dense
Fig. 12. Jc at 77 K for YBCO films dependent on the traveling tape speed during the annealing process of LZO.
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