The manufacturing of an electroplated Ni layer on texturedcusubstratefor Cu-based HTS coated conductors

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1 INSTITUTE OF PHYSICS PUBLISHING Supercond. Sci. Technol. 18 (25) SUPERCONDUCTOR SCIENCE AND TECHNOLOGY doi:1.188/ /18/1/17 The manufacturing of an electroplated Ni layer on texturedcusubstratefor Cu-based HTS coated conductors YXZhou,LSun, X Chen, H Fang, P T Putman and K Salama Department of Mechanical Engineering, Texas Center for Superconductivity and Advanced Materials, University of Houston, TX 7724, USA Received 3 June 24, in final form 2 August 24 Published 16 November 24 Online at stacks.iop.org/sust/18/17 Abstract Asharpcube textured Ni overlayer on Cu substrates has been developed for the manufacturing of long-length RABiTS-based coated conductor tapes. Using a low-cost, non-vacuum and easily scalable technique of electroplating, smooth, crack-free and continuous Ni overlayers were deposited on cube textured Cu substrates without any intermediate layers. In addition, sharp cube textured Sm-doped CeO 2 buffer layers have been grown on the Ni-plated Cu substrates using pulsed laser deposition and found to exhibit in-plane and out-of-plane FWHM values of 6.5 and 5.25,respectively. This electroplating process promises an efficient route for manufacturing Cu-based HTS coated conductors. (Some figures in this article are in colour only in the electronic version) 1. Introduction Enormous strides have been made for the development of second-generation HTS wires (that focus mainly on YBCO coated conductors) for large scale applications operating at high magnetic fields and temperatures of 3 77 K such as generators, motors and magnets [1 4]. Since the current carrying capability of YBCO coated conductors is strongly affected by large-angle grain boundaries acting as weak links, a biaxial alignment of the YBCO films is required. A promising approach to this HTS wire development is the rolling assisted biaxially textured substrates (RABiTS) [5, 6] technique. To date, many reports have been published on coated conductors manufactured using the RABiTS technique where YBCO is deposited on high-purity Ni or Ni alloy textured substrates [7, 8], with intervening buffer layers for chemical separation and improved lattice matching. High critical current densities of 2.5 MA cm 2 with CeO 2 buffers have been reported for epitaxial YBCO films on substrates fabricatedbythe RABiTS techniques [9]. However, the YBCO coated conductors are still underdeveloped in terms of AC losses, low cost, enhanced stability and quench protection. Alternative Cu-based coated conductors have thus been proposed [1, 11] where textured Cu is used as the substrate. In general, such substrates have the advantages of easy formation of a sharp cube texture, no ferromagnetic contribution to hysteretic AC losses, low cost compared to Ni or Ni alloy substrates, and low resistivity. The primary disadvantage, however, is that Cu has poorer resistance to oxidation than Ni and Nialloy substrates, especially in an oxygen atmosphere at the high temperatures necessary for the fabrication of YBCO superconducting layers. One potential solution for overcoming this problem is to deposit a metal protective layer over the textured Cu substrate to reduce the Cu oxidation rate and protect its diffusion to YBCO [12]. Electroplating has been used for many years to fabricate decorative and protective metallic films [13, 14]. It is non-vacuum, low cost, fast and easily scalable, which gives the promise for the production of long-length Cu-based coated conductor tapes. In this work, we report the development of a textured Ni overlayer on textured Cu substrate using electroplating for the production of long-length RABiTS-based coated conductor tapes. The Ni overlayer is found to have the same texture as that of the Cu substrate while preventing any oxidation of copper during the deposition of CeO 2 buffer layers. 2. Experimental details The substrates used in this work were made of commercially oxygen-free pure Cu, which were mechanically deformed by rolling to a degree greater than 95% total deformation /5/117+5$3. 25 IOP Publishing Ltd Printed in the UK 17

2 YXZhou et al µm Cu(2) Ni(2) µm µm 5 1µm Figure 1. X-ray (111) pole figure of the cube textured Cu substrate. Figure 2. The θ 2θ x-ray diffraction patterns for different thicknesses of Ni overlayers electroplated on textured Cu substrate. Table 1. Bath compositions and conditions. Chemical Formula Comments Nickel sulfate NiSO 4-6H 2 O.25.5 M Nickel chloride NiCl 2.25 M Sodium chloride NaCl.15 M Boric acid H 3 BO 3.2 M ph 3 5 Current density.5.25 A cm 2 Temperature 3 35 K After Heat Treatment 8 6 Before Heat Treatment Cu(2) Ni(2) using about 1% reduction per pass and reversing the rolling direction during each subsequent pass. They were annealed at 8 Cfor about 6 min in flowing 5% H 2 in Ar. The procedure of rolling and annealing yielded the desired {1} 1 cubic textured Cu substrates. Electroplating the Ni was carried out using a low-cost, non-vacuum and easily scalable technique of electroplating. The plating bath compositions and conditions selected for this study are shown in table 1. Boric acid was introduced to adjust the ph value of the plating bath solution. To improve the conductivity of the solution, sodium chloride was added. A fresh plating bath was made for each experiment using analytical reagent grade chemicals and distilled water. The plating rate of the Ni layer was determined by weighing the substrate before and after plating. Pulsed laser deposition (PLD) was used for the deposition of buffer layers on the Ni-plated Cu substrate. This buffer layer is used to retard the oxidation of the metal substrate, reduce the lattice mismatch between substrate and YBCO, and also to prevent diffusion of the metal into YBCO. The deposition of an Sm-doped CeO 2 buffer layer was performed using a laser pulse frequency of 7 Hz, a pulse energy of 7 mj/pulse, and asubstrate temperature of 65 C. In order to minimize the Ni substrate surface oxidation during growth, a forming gas (4% hydrogen balanced with 96% argon) at a background pressure of 5 mtorr was introduced into the chamber. The Ni electroplated textured Cu substrate, and CeO 2 buffer layers were characterized using a Siemens D5 series x-ray diffractometer for phase purity and texture. The sharpness of the texture was measured using x-ray Figure 3. The θ 2θ x-ray scans for 3 µm Nioverlayers electroplated on textured Cu substrate after heat treatment at 75 C for 3 min. rocking curves. A Siemens general area detector diffractions system (GADDS) with a pole figure goniometer was used to obtain pole figure measurements. The pole figure is a stereograc projection, which shows the distribution of a particular crystallograc direction in the assembly of grains that constitutes the specimen. An enhanced scanning electron microscopy (SEM) based technique system was also used to examine the homogeneity of microstructure. AFM was used to determine the surface roughness. 3. Results and discussions 3.1. Cu substrate Cube texture is well known to develop directly from primary recrystallization in FCC metals. The texture is the result of the rotation of slip systems in the grains during plastic deformation and the subsequent growth of selected grains in the recrystallization process. Though Ni and Ni alloys are currently widely used for coated conductors, the copper substrate may be more suitable for several superconductor applications due to its non-ferromagnetism property, higher heat capacity, lower resistivity and better chemical compatibility with the HTS layers. Figure 1 shows a linear scale (111) pole figure for a cube textured Cu substrate that was annealed at 8 Cfor 18

3 1 8 FWHM=4.66 The manufacturing of an electroplated Ni layer on textured Cu substrate for Cu-based HTS coated conductors (a) Theta 2 FWHM=6.4 (b) 15 (a) Figure 4. (a) Rocking curve and (b) scans for 3 µm Nioverlayers on textured Cu substrate after heat treatment at 75 Cfor3 min. Phi (b) Figure 6. (a) SEM micrograph and (b) AFM image of 3 µm Ni overlayers on textured Cu substrate after heat treatment at 75 Cfor 3 min Electroplated Ni overlayer Figure 5. Thex-ray (111) pole figure from 3 µm Nioverlayers on textured Cu substrate after heat treatment at 75 Cfor3 min. 1hinflowing Ar 5% H 2. Only the peaks corresponding to {1} 1 cube orientations can be seen in the figure, indicating a fully textured Cu substrate. The (2) ω scans have a full width at half maximum (FWHM) of about 4.2 and the in-plane FWHM of (111) scans is6.5. Both ofthe in-plane and out-of-plane alignments are better than those of Ni and Ni alloy substrates [5, 6, 15], which indicates that Cu tapes more easily form asharpcube texture. Ni overlayers were deposited on the Cu substrates using electroplating without any intermediate layers. The electrolysis parameters, such as current density, ph values and temperature, were chosen to produce sharp textured Ni. In arandomly oriented polycrystalline Ni film the (111) peak corresponding to a 2θ angle of 44.4 has the highest intensity and the (2) peak has an intensity of 42% of the (111) peak. Figure 2 shows the θ 2θ x-ray diffraction pattern for different thicknesses of Ni overlayer electroplated on the textured Cu substrate. The θ 2θ scansexhibit only (h) peaks, indicating a good out-of-plane alignment of the Ni overlayers on the textured substrate. Furthermore, the consistent increase of intensity in the Ni(2) peaks indicates that the thickness of the film is increasing uniformly without any of the Ni(111) orientation showing in the 2θ patterns, which demonstrates that only (h) orientedgrains exist even in the film of 4 µm thickness. The thickness of the Ni overlayer is much less than that of the copper substrates. So the effect of nickel s magnetism on the AC losses is not significant. As subsequent depositions of buffer and superconducting layers require the substrate to be heated, it is important to ensure that the thermal stability, texture properties and the 19

4 YXZhou et al CeO2(2) theta Figure 7. The x-ray diffraction pattern of CeO 2 /Ni/Cu architecture. microstructure of the Ni-plated Cu substrate remain unchanged after high-temperature heat treatment. In this study, we focused on asampleof3µm Ni-plated Cu substrate, which was annealed at a temperature of 75 Cfor3 min in a reducing atmosphere of forming gas of 5% H 2 +95% Ar. Figure 3 shows the θ 2θ x-ray scans for the 3 µm Nioverlayer electroplated on the textured Cu substrate after heat treatment. The θ 2θ scans still exhibit only (h) peaks without the presence of a (111) peak for both the Ni overlayer and Cu substrate, indicating that the out-of-plane alignment is thermally stable. Furthermore, we examined the possibility of alloying between the Ni overlayer and Cu substrate using x-ray analysis and found that there is no shift for both the Cu(2) and Ni(2) peaks after the high-temperature heat treatment, confirming the chemical stability of the Ni layer on the textured Cu substrate. After heat treatment, texture analysis of the electroplated Ni layer (shown in figure 4) yielded the FWHM values of 6.4 and 4.66,forin-plane and out-of-plane alignments, respectively. These values are comparable to those of the textured substrate (6.5 and 4.2 ). A typical (111) pole figure for the Ni overlayer grown on the textured Cu substrate after heat treatment is shown infigure5. Fromthefigure, one can see that this overlayer still exhibits a single cube-oncube texture, confirming that the texture of the electroplated Ni layers is thermally stable. SEM analysis of the electroplated Ni (the micrograph is shown in figure 6(a)) reveals a dense, crack-free, and continuous microstructure that covers the textured Cu substrate very well. Also, AFM studies, shown in figure 6(b), reveal a smooth surface with an average roughness value of 4.6 nm, which iscomparable to that of the substrate and should be suitable for deposition of further layers. The AFM micrograph Cu(2) Ni(2) was taken over an area of 5 µm 5 µm onthesurface of the electroplated Ni overlayer Sm-doped CeO 2 buffer layer So far, we have shown that electroplated Ni overlayers on textured Cu substrate have a sharp cube texture, are thermally stable and have smooth surfaces. However, in order to deposit YBCO on these substrates with high current-carrying capability, it is necessary to develop oxide buffers capable of further preventing the metal diffusion, reducing the lattice mismatch between Ni and YBCO and avoiding the oxidation of the Ni-plated Cu substrate during the deposition of the YBCO layer. An Sm-doped CeO 2 buffer layer was deposited using the PLD technique. The x-ray diffraction pattern of the CeO 2 /Ni/Cu architecture, shown in figure 7, reveals that both the Ni overlayer and the CeO 2 buffer layerare(h)oriented. The out-of-plane crystallograc alignment of the textured CeO 2 /Ni/Cu structureisdetermined by x-ray ω-scans through the CeO 2 (2),Ni(2) and Cu(2) peaks. The values of the FWHM are 5.25,4.85 and 5.2,respectively. The in-plane alignment determined by CeO 2 (111), Ni(111) and Cu(111) pole figures is shown in figure 8. The presence of a {1} 1 cube texture is evident for the Cu substrate and electroplated Ni overlayer. The CeO 2 buffer has a {1} 1 texture, 45 rotated with respect to the Cu substrate and electroplated Ni overlayer. Figure 9 shows that the FWHM values of in-plane alignments are 6.5,6.82 and 6.3 for the textured CeO 2 /Ni/Cu structure respectively. YBCO has been deposited on CeO 2 /Ni/Cu and the texture is well comparable to that of the CeO 2 buffer. 4. Summary Astudy regarding the epitaxial oxide buffer and electroplated Ni overlayer on textured Cu substrate has been carried out. The Cu substrates were mechanically deformed by rolling followed by annealing at temperature 8 Cfor6 min. The FWHM values of these substrates were typically found to be 4.2 and 6.5 for out-of-plane and in-plane alignments, respectively. Alow-cost, non-vacuum and easily scalable technique of electroplating was developed for the production of long-length RABiTS-based coated conductor tapes. Smooth, crack-free and continuous 1 4 µm thickelectroplated Ni overlayers were deposited on cube textured Cu substrate without any intermediate layers. After high-temperature heat treatment, CeO 2 Ni Cu Figure 8. The CeO 2 (111), Ni (111) and Cu (111) pole figures for the CeO 2 /Ni/Cu textured structure. 11

5 The manufacturing of an electroplated Ni layer on textured Cu substrate for Cu-based HTS coated conductors FWHM=6.82 FWHM= Ni CeO 2 8 Cu 14 FWHM= Figure 9. The x-ray scans of CeO 2,NiandCufortheCeO 2 /Ni/Cu textured structure. both the Cu substrate and Ni overlayer retain the cube texture and chemical stability. The FWHM values of electroplated Ni for in-plane and out-of-plane alignments were 6.4 and 4.66, respectively. Sharp cube textured Sm-doped CeO 2 buffer layers have been grown on Ni-plated Cu substrate using pulsed laser deposition with in-plane and out-of-plane FWHM values of 6.5 and 5.25,respectively. This electroplating process promises a route for manufacturing Cu-based HTScoated conductors. Acknowledgments The authors wish to acknowledge Y Y Sun for the x-ray characterization ofsome of the copper samples. This work was financially supported by the Air Force Office of Scientific Research as part of project F , and the Texas Center for Superconductivity and Advanced Materials. References [1] Larbalestier D, Gurevich A, Feldmann D M and Polyanskii A 21 Nature [2] Selvamanickam V, Lee H G, Li Y, Xiong X, Qiao Y, Reeves J, XieY,Knoll A and Lenseth K 23 Physica C [3] Verebelyi D T et al 23 Supercond. Sci. Technol. 16 L19 22 [4] Watanabe T, Shihara Y and Izumi T 23 IEEE Trans. Appl. Supercond [5] Goyal A et al 1996 Appl. Phys. Lett [6] Zhou Y X, Rizwan T and Salama K 23 IEEE Trans. Appl. Supercond [7] Van Driessche I, Penneman G, De Meyer C, Stambolova I, Bruneel E and Hoste S 22 Eng. Mater. V [8] Zhou Y X, Naguib R, Fang H and Salama K 24 Supercond. Sci. Technol [9] Bindi M, Botarelli A, Gauzzi A, Gianni L, Ginocchio S, Holzapfel B, Baldini A and Zannella S 24 Supercond. Sci. Technol [1] Pinol S, Diaz J, Segarra M and Espiell F 21 Supercond. Sci. Technol [11] Aytug T, Paranthaman M, Thompson J R, Goyal A, Rutter N, Zhai H Y, Gapud A A, Ijaduola A O and Christen D K 23 Appl. Phys. Lett [12] Rutter N A, Goyal A, Vallet C E, List F A, Lee D F, Heatherly L and Kroeger D M 24 Supercond. Sci. Technol [13] Wood W A 1931 Proc. Phys. Soc [14] Goodall R, Moore J C, Pecz B, Grime G W, Salter C J and MGrovenor C R 21 Supercond. Sci. Technol [15] Zhou Y X, Bhuiyan S, Scruggs S, Fang H and Salama K 23 Supercond. Sci. Technol