IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017
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Engineering of Nanorods for Superior in Field Performance of 2G-HTS Conductor Utilizing Advanced MOCVD Reactor Goran Majkic, Rudra Pratap, Eduard Galstyan, Aixia Xu, Yuan Zhang, and Venkat Selvamanickam
Abstract—An advanced MOCVD (A-MOCVD) reactor has been developed with an aim of addressing the issues found in most superconductor deposition techniques such as a-axis grain formation, degradation of texture in thick films and poor source-to-film conversion efficiency. A record high-lift factor of ∼9 and critical current of 3346 A/12 mm at 30 K, 2.5 T, B||c have been demonstrated in films produced in a single pass in the A-MOCVD reactor. The performance results achieved are not possible in conventional MOCVD without resorting to two-pass deposition for films thickness 2 µm and up. Additionally, a precursor-to-film conversion efficiency increase by a factor of 2.5–4 has been achieved in 2-µmthick films deposited in a single pass. We also present results on the utilization of this system toward engineering of nanorods in thick films that are highly aligned over the entire film thickness and with very small diameter (2 nm), resulting in much higher nanorod density for the same molar fraction of Zr dopant. The results on increase in efficiency as well as on rapid nanorod characterization are also presented. Index Terms—2G HTS superconductors, CVD, critical current and flux pinning, superconducting tapes.
I. INTRODUCTION N RECENT years, great effort has been put into increasing the performance of Rare Earth Barium Copper Oxide (REBCO) coated conductors (CC) or 2nd Generation High Temperature Superconductors (2G-HTS) both in terms of the self-field critical current (Ic) and in-field performance. Two regimes of operation are of particular interest – intermediate temperatures and fields (∼3T, 30 K) and high field, low temperature (4.2 K, >9T), for rotating machinery (motors, generators) and high field applications, respectively [1], [2]. Introduction of c-axis oriented precipitates such as BaZrO3 has been demonstrated to drastically increase the in-field performance of 2G-HTS conductor. Two metrics for evaluating in-field performance have
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Manuscript received September 6, 2016; accepted December 5, 2016. Date of publication December 8, 2016; date of current version February 24, 2017. This work was supported in part by the Rare Earth Alternatives in Critical Technologies under Award DE-AR000196 and in part by the Office of Naval Research under Award N00014-14-1-0182. The authors are with the Department of Mechanical Engineering, Texas Center for Superconductivity and Advanced Manufacturing Institute, University of Houston, Houston, TX 77204-4006 USA (e-mail:
[email protected];
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[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2016.2637328
been targeted: the lift factor (LF), defined as the ratio of Ic at desired temperature/field and Ic at 77 K, self-field, as well as the Ic value at 77 K, self-field itself [3], [4]. The first metric provides insight into the efficiency of artificial pinning centers. The infield performance is then determined by the product LF∗Ic (77 K, s.f.). Significant progress in maximizing both metrics has been achieved in recent years utilizing various deposition techniques, with our focus being on Metal Organic Chemical Vapor Deposition (MOCVD) [5]–[11]. Optimization of film microstructure, dopant concentration, composition and growth conditions has resulted in a drastic increase in lift factor to LF∼7 by utilizing conventional MOCVD deposition [4], [12]. In a previous study under ARPA-E Grid-Scale Rampable Intermittent Dispatchable Storage (GRIDS) program, we developed a novel, Advanced Metal Organic Chemical Vapor Deposition (A-MOCVD) system with the goal of addressing the main issues found with conventional MOCVD reactors that utilize a susceptor for tape heating and a top-down showerhead for delivering the precursor. They include the inability to grow thicker films due to progressive increase in fraction of a-grains with thickness, poor temperature control, non-uniform flow distribution and therefore temperature and deposition rate, as well as poor precursor conversion efficiency of only ∼10% [13]. Growth of thick films with high critical current density (Jc ) has a high potential for increasing engineering current density and reducing cost per ampere. The main obstacle to achieving this is a remarkable degradation in Jc with thickness, due to increase in fraction of a-axis oriented grains in thicker films [14]–[18]. In conventional MOCVD, growing high-quality films 2 μm thick and higher in single pass has proven impossible due to excessive formation of a-grains, resulting in a need for a multi-pass technique (about 1 μm per pass) [19], which greatly complicates the process and price in terms of scale-up. The A-MOCVD is based on direct ohmic heating of a suspended substrate tape [20]. Direct tape temperature monitoring is achieved by non-contact optical probes. Furthermore, flow uniformity has been achieved by vapor path design resulting in a highly laminar parallel flow in the direction perpendicular to tape. In addition, the vapor path volume has been confined in order to increase precursor conversion efficiency. Previously, near-1000A/12mm critical current tapes have been achieved by MOCVD in single pass deposition of 1.8 μm thick undoped REBCO on IBAD-MgO/LMO substrates [20]. In addition,
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017
a-axis-free undoped REBCO films have been grown to thickness up to 4 μm thick, while the precursor conversion efficiency has been increased by 40%. In this study, we utilize the A-MOCVD reactor to grow heavily Zr-doped REBCO films for in-field applications. The primary motivation was to investigate whether thick (>2 μm) samples with the same high lift factors achieved with conventional MOCVD are also possible with A-MOCVD in single pass, thus combining the benefits of A-MOCVD and the expertise of growing high lift factor samples achieved with conventional MOCVD. If such goal can be achieved, the deposition process will be greatly simplified due to single pass deposition. The ability of A-MOCVD to grow very thick films with no degradation in texture (in terms of a-axis grain formation) can be combined with high LF to and optimize the process to maximize both metrics of interest: LF and self-field Ic, resulting in tape with high JE and reduced price in terms of $/kA-m metric. We have also investigated nanorod growth control in order to achieve very high nanorod density without increase in Zr dopa nt by controlling nanorod diameter and explore the potential for further increase in performance. Finally, an important aspect of A-MOCVD – increased precursor conversion efficiency, has been pushed to new limits. Further significant improvements in efficiency have been achieved, as will be presented.
Fig. 1. Angular in-field performance of 20% Zr containing REBCO film deposited in single pass using A-MOCVD, at 30 K, 2.5 and 3T.
II. EXPERIMENTAL A-MOCVD and conventional MOCVD were used for REBCO deposition on IBAD-MgO/LaMnO3 (LMO)/Hastelloy buffer [12], [13]. All ∼2 μm thick samples have been deposited in a single pass using A-MOCVD and two passes using conventional MOCVD. In-field Ic measurements were performed on a 9T solenoid system with variable temperature sample space and a rotating platform, enabling temperature, field orientation and field intensity scans [21]–[23]. Transmission Electron Microscopy (TEM) was performed on a JEOL 2000FX. Diffraction measurements were performed on a Bruker AXS General Area Detector Diffraction System (GADDS) Transport Ic measurements were done using four-point sensing and defining Ic at 1 μV/cm. III. RESULTS AND DISCUSSION A. In-Field Performance The main objective was to evaluate whether thick REBCO films with heavy Zr doping can be deposited in single pass utilizing A-MOCVD and produce the same performance as twopass, conventional MOCVD grown samples [3]–[6], [12], [19], [21]–[23]. Shown in Fig. 1 is the angular in-field performance of a 2μm, 20% Zr containing REBCO film deposited using AMOCVD in single pass, at 2.5 and 3T. The corresponding LFs at 2.5 and 3T are 9 and 8.5, respectively, which is a record high value among all MOCVD samples processed in our group, and to the best of the authors’ knowledge, highest yet reported in literature. In terms of Ic performance, the corresponding values at 2.5 and 3T are 3346 and 3175 A/12mm width, which until
Fig. 2. Ic performance at 30 K 3T vs 77 K 3T, B||c for two-pass (open symbols) MOCVD and single pass A-MOCVD samples.
recently [19] were also record high among A-MOCVD and MOCVD samples. Shown in Fig. 2 is a correlation plot of IC at 3T B||c, comparing the performance at 30 K to that at 77 K, for a number of samples processed by conventional MOCVD (squares) and by A-MOCVD (stars). The wide range of Ic values stems from optimization experiments. All MOCVD samples have 25% Zr addition, while A-MOCVD samples contain 15 and 20% Zr. For conventional MOCVD, samples in the upper right half of the graph (high IC ) are 2 μm thick and have been processed in two passes. A very good correlation between 77 K and 30 K performance can be observed. There appears to exist an upper and a lower envelope for the spread in IC performance at 30 K compared to 77 K, as indicated by the dashed lines. Clearly, for the same 77 K performance, there exists a significant window for tuning the 30 K performance with different LF which stems from nanorod and REBCO film characteristics. For example, samples with IC = 100 A at 77 K can have 30 K IC anywhere between 2000 and 3300 A. From the comparison between conventional and A-MOCVD performance, it is clear that both techniques produce samples that fall within the same envelope, even though all A-MOCVD samples have been produced in single pass. With A-MOCVD we were previously able to grow undoped
MAJKIC et al.: ENGINEERING OF NANORODS FOR SUPERIOR IN FIELD PERFORMANCE OF 2G-HTS CONDUCTOR UTILIZING ADVANCED
Fig. 3. TEM plane-view analysis of a 20% Zr REBCO sample processed by A-MOCVD: (a) (left) microstructure and (b) (right) plane-view electron diffraction pattern.
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Fig. 4. TEM plane-view micrographs of: (a) (left) 25% Zr containing sample, and (b) (right) 11% Zr containing sample. The 11% Zr sample features significantly higher nanorod density (8000-12000 vs 1500 rods/μm2 ) and matching field (17-26 vs ∼3T), despite ∼2.3x higher Zr content in 25% Zr sample, due to reduction in equilibrium nanorod diameter by process control.
REBCO films up to >4 μm thick in single pass deposition without a-grain formation and texture degradation. For the present heavily-Zr-doped samples grown up to 2 μm, no texture degradation with thickness at proper growth conditions was observed, indicating that even thicker heavily-Zr-doped samples can be grown. Therefore, A-MOCVD can be successfully used for production of heavily-doped REBCO samples of high thickness (2 μm demonstrated) in single pass deposition, with demonstrated potential growth up to 4-5 μm. B. Microstructure Control Using A-MOCVD, we have achieved repeatable control of nanorod diameter by controlling precursor composition and growth conditions. Shown in Fig. 3 are plane-view TEM micrograph and the corresponding electron diffraction pattern (ab plane) of the 20% Zr containing sample described in Fig. 1. The nanorod diameter is tuned to ∼4.5 nm, while density and rod-to-rod spacing is estimated at ∼3800-4000 rods/μm2 and ∼16 nm, respectively, with the corresponding matching field of ∼8T. In this case, in-plane texture is very sharp, indicating a good balance between the expected LF and Ic. It has been previously established that strong pinning and LF requires long continuous BZO nanorods that permeate through the whole film thickness [3], [4], [11], [12]. In contrast, samples that feature short discontinuous nanorods universally exhibit low lift factor and pinning force. In addition, long continuous nanorods can vary in diameter which in turn changes their average spacing (density). This leads to a great potential for performance improvement: if the diameter of nanorods can be reduced in half at constant Zr addition, the corresponding nanorod density and therefore matching field could be doubled. Therefore, a sample with 10% Zr addition could then have the same density of nanorods or pinning centers as a 20% Zr sample, with further benefit of increased superconducting (REBCO) fraction in the film. In the A-MOCVD system, we have identified several factors that can control the nanorod diameter and length, with the goal of pushing the limit on minimizing nanorod diameter and maximizing pinning center density while maintaining continuous growth throughout thickness. Shown in Fig. 4 are plane-view TEM micrographs of a sample grown in conventional MOCVD with 25% Zr addition and of a sample grown in A-MOCVD with only 11% Zr. A drastic increase in BZO nanorod density
Fig. 5. Electron diffraction pattern of 11% Zr REBCO sample: (a) plane-view (REBCO a–b plane) and (b) cross section (REBCO a-c plane).
has been achieved, despite the fact that Zr content is less than half in the A-MOCVD sample. Also, the nanorod diameter is considerably reduced in the 11% Zr, consistent with lower Zr content. The nanorod density in the 25% sample is estimated at ρ = 1500 rods/μm2 and between ρ = 8000-12000 rods/μm2 for the 11% Zr sample. This translates to average nanorod spacing of a = 25 nm and a = 9-11 nm for 25 and 11% Zr samples, respectively. It results in a remarkable change in matching field from ∼3T for the 25% Zr sample to 17-26 T for the 11% Zr sample. The nanorod diameter was more challenging to determine accurately with the resolution of the TEM instrument used especially for the small nanorods, and the statistical estimate over a number of rods gave an estimate of d = 7-8 nm and d ∼2-3 nm for 25 and 11% Zr samples, respectively. In order to have a high performance superconductor, the performance of the REBCO matrix should not be sacrificed at the expense of pinning center density. In other words, both Ic and LF should be high, and a right compromise between the two should be chosen depending on operating field and temperature. Shown in Fig. 5 are plane-view and cross-section electron diffraction patterns of the 11% Zr sample. It can be seen that the while out-of-plane texture is excellent, in-plane texture is degraded, leading to low IC . The amount of data is still limited to enable us to conclude whether this degradation is caused by high nanorod density or change in growth conditions. Further optimization of the process is underway, in order to achieve both high nanorod density and IC performance.
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017
Fig. 6. 2D-XRD diffraction pattern of two Zr doped REBCO samples: (a) nanorod diameter ∼20 nm and (b) nanorod diameter ∼ 2nm. Streaking of BZO (101) peak in the direction perpendicular to nanorod length is evident at very small nanorod size.
It becomes clear that there exists a great potential for further increasing pinning strength of BZO by controlling nanorod density via nanorod diameter rather than further increasing the Zr content if good REBCO film texture can also be preserved. It may also address the issue that the critical temperature [Tc ] is noted to decrease with Zr content [5]. C. Nanorod Quality Characterization Optimization of growth conditions, composition and other process parameters requires quick performance feedback. While Ic measurements and XRD provide quick feedback on REBCO film quality, characterization of nanorods is typically a much more involved task, using Focused Ion Beam (FIB) and TEM, or direct in-field IC measurements, all of which are heavily time consuming for routine characterization. This issue also translates to quality control in production. Therefore, a quick and reliable method for characterizing nanorod characteristics is needed. We have identified a rapid method to characterize nanorod quality using 2D-XRD. In-depth details will be published in a separate publication. In short, 2D-XRD scans provide tremendous amount of information in a short time frame, asa large fraction of reciprocal space is scanned, containing information about REBCO film, as well as BZO nanorod precipitates through the most notable BZO 101 peak. Shown in Fig. 6 are 2D-XRD scans of two Zr samples in χ = 90° configuration. Out of plane texture of REBCO can be readily assessed. Film thickness can also be estimated from the intensity of hastelloy substrate rings which diminish with REBCO thickness. But most importantly for this study, information on BZO nanorod diameter can be obtained. Fig. 6(a) is a typical scan for samples with larger nanorod diameter. The BZO peak is well defined and if any streaking exists, it will be in constant 2θ direction if the REBCO matrix also has measurable out-ofplane texture spread, which BZO will follow. In contrast, if the nanorod diameter is small, there are few scattering centers in the direction perpendicular to nanorod length. This leads to BZO 101 peak streaking in the direction perpendicular to REBCO 00L. This result is remarkable in the sense that it has enabled us rapid screening of multitude of samples and that it can be
Fig. 7. SEM micrograph of a cross section of a 1.25-μm-thick film deposited using A-MOCVD utilizing only 0.5 mL of precursor/solvent mixture per cm tape, which constitutes a 3.3-fold increase in efficiency compared to conventional MOCVD.
readily implemented as an in-line, real time quality control tool in production. D. Precursor Conversion Efficiency We previously reported an increase in efficiency of 40% over conventional MOCVD [20]. In conventional MOCVD, the efficiency (at fixed precursor concentration in liquid solvent) is about 0.75 μm thick film per cm tape for 1 mL of precursor/solvent mixture used. Shown in Fig. 7 is an SEM micrograph of a cross section of a film deposited using A-MOCVD (cross section is inclined 52o to image). For this example, the thickness of 1.25 μm is achieved using only ∼0.5 mL/cm, which results in efficiency of 2.5 μm/mL/cm. This is an improvement by a factor of 3.3x over conventional MOCVD. From a series of experiments, precursor-to-film conversion efficiency increases of 2.5 – 4x over conventional MOCVD have been achieved. Since efficiency in our conventional MOCVD is about 10% of theoretical, this translates to 25-40% of theoretical efficiency or 2.5 to 4-fold reduction in precursor cost. IV. CONCLUSION An advanced MOCVD reactor has been utilized to produce thick (2 μm), heavily Zr-doped REBCO films in single pass deposition. Record-high lift factor at 30 K, 2.5 T (LF∼9) and IC performance of 3346 A/12mm at B||c have been achieved. The performance of single pass A-MOCVD tapes matches that of the two pass samples processed by conventional MOCVD. The ability to grow same quality films in single pass constitutes a drastic simplification and cost reduction of the production process compared to conventional MOCVD. A-MOCVD has been utilized achieve good control of equilibrium nanorod diameter and length. By reduction of nanorod diameter, nanorod density has been increased from ∼4500 to 8000-12000 rods/μm2 while simultaneously reducing Zr concentration in film from 25 to 11%. This corresponds to increase in matching field from ∼3 to 17-26T. These results indicate that significant further progress in pinning performance is possible. A rapid characterization method for assessing nanorod morphology has been developed utilizing 2D-XRD and characterizing the streaking characteristics of BZO 101 peak. This method
MAJKIC et al.: ENGINEERING OF NANORODS FOR SUPERIOR IN FIELD PERFORMANCE OF 2G-HTS CONDUCTOR UTILIZING ADVANCED
results in drastic increase in speed of characterization compared to FIB and/or TEM. The method can be readily used as an in-line quality control tool in reel-to-reel production of 2G-HTS tapes. We have demonstrated that A-MOCVD can provide at least 2.5-4 fold increase in precursor conversion efficiency compared to conventional MOCVD. As precursor cost is a significant fraction of total production cost of 2G-HTS tapes by MOCVD, this constitutes a potential for significant reduction in tape cost. REFERENCES [1] A. B. Abrahamsen et al., “Feasibility study of 5 MW superconducting wind turbine generator,” Physica C—Supercond. Appl., vol. 471, no. 21–22, pp. 1464–1469, Nov. 2011. [2] P. Kummeth, M. Frank, W. Nick, G. Nerowski, and H. W. Neumueller, “Development of synchronous machines with HTS rotor,” Physica C— Supercond. Appl., vol. 426, pp. 1358–1364, Oct. 2005. [3] V. Selvamanickam et al., “Correlation between in-field critical currents in Zr-added (Gd, Y)Ba2Cu3Ox superconducting tapes at 30 and 77 K,” Supercond. Sci. Technol., vol. 27, no. 5, May 2014, Art. no. 055010. [4] V. Selvamanickam, M. H. Gharahcheshmeh, A. Xu, E. Galstyan, L. Delgado, and C. Cantoni, “High critical currents in heavily doped (Gd,Y)Ba2Cu3Ox superconductor tapes,” Appl. Phys. Lett., vol. 106, no. 3, Jan. 2015, Art. no. 032601. [5] V. Selvamanickam et al., “Enhanced critical currents in (Gd,Y)Ba2Cu3Ox superconducting tapes with high levels of Zr addition,” Supercond. Sci. Technol., vol. 26, no. 3, Mar. 2013, Art. no. 035006. [6] V. Selvamanickam et al., “The low-temperature, high-magnetic-field critical current characteristics of Zr-added (Gd, Y)Ba2Cu3Ox superconducting tapes,” Supercond. Sci. Technol., vol. 25, no. 12, Dec. 2012, Art. no. 125013. [7] V. Selvamanickam et al., “High performance 2G wires: From R&D to pilot-scale manufacturing,” IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 3225–3230, Jun. 2009. [8] V. Selvamanickam et al., “Progress in scale-up of second-generation HTS conductor,” Physica C—Supercond. Appl., vol. 463, pp. 482–487, Oct. 2007. [9] V. Selvamanickam, Y. Xie, J. Reeves, and Y. Chen, “MOCVD-based YBCO-coated conductors,” MRS Bulletin, vol. 29, no. 8, pp. 579–582, Aug. 2004. [10] G. Majkic et al., “Effect of high BZO dopant levels on performance of 2GHTS MOCVD wire at intermediate and low temperatures,” IEEE Trans. Appl. Supercond.,vol. 23, no. 3, Jun. 2013, Art. no. 6602605.
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