High Temperature Superconductors (HTS) have the potential to provide multiple commercial solutions to a broad spectrum of sectors of the US economy such as energy, defense, industrial applications, communications, and medicine. In the energy sector, for example, HTS devices have the potential to benefit both renewable and non-renewable energy industries, accelerate introduction of smart grid hardware applications and improve sustainability through enhanced energy efficiency, high power density, less CO2 emission, better power quality and improved resiliency and security of the power grid. Superconducting devices do not simply provide improvements over conventional technologies; they provide unique solutions to challenges that cannot be achieved otherwise.
Nanoscale defects in epitaxial thin film superconductor tapes for improved flux pinning in high magnetic fields
Since superconductors are typically used in presence of a magnetic field, their critical current performance in a magnetic field is an important metric. In a thin film superconductor tape without any added dopants, the critical current density decreases by a factor of 7 to 10 when a magnetic field of 1 T is applied perpendicular to the tape plane. At the University of Houston, we have employed heavy doping (15% – 25%) of Zr to form a high density (1011 – 1012 cm-2) of BaZrO3 (BZO) nanoscale defects in the thin film superconductor matrix to pin the magnetic flux lines and achieve substantially improved critical current in high magnetic fields. The BZO defects are formed by a self-assembly process simultaneously during the epitaxial growth of the REBCO film. In a program funded by the Advanced Research Projects Agency-Energy (ARPA-e), we successfully developed a Metal Organic Chemical Vapor Deposition (MOCVD) process to achieve a high density of well-aligned BZO nanocolumns in heavily-doped REBCO films (up to 25 mol% doping) while maintaining excellent epitaxial growth .
Microstructures of a 25 mol.% Zr-doped thin film superconductor film synthesized by MOCVD showing abundant columnar structures of self-assembled BaZrO3.
By this process, we were able to improve the critical current of REBCO tapes by three-fold in high magnetic fields at 30 K. Record-high critical current density above 20 MA/cm2 at 30 K, 3 T  and record-high pinning force levels above 1.7 TN/m3  have been achieved in heavily-doped REBCO thin film superconductor tapes.
(Left) Improved critical current of thin film superconductor tapes at 30 K, 3 T with increasing levels Zr addition. (Right) Flux pinning force levels of thin film superconductor tapes made by institutions world-wide. Courtesy T. Puig, EUCAS 2015
We are working towards consistently achieving a high density of highly-aligned nanoscale defects over long tape lengths. The film composition and lattice parameter of the REBCO film and BZO nanoscale defects have been found to be key parameters in determining the alignment of the defects and hence the in-field critical current performance. In-line quality control tools such as X-ray Diffraction are being developed for real-time monitoring of the REBCO film characteristics to achieve a consistent nanoscale defect microstructure and in turn a consistent in-field critical current over long tape lengths.
Advanced Metal Organic Chemical Vapor Deposition (MOCVD) process for high performance thick film superconductor tapes
A challenge with achieving high critical current is the reduction in critical current density with thickness of the superconducting layer. We are working on improving critical current performance levels in thick films by addressing current-limiting mechanisms in superconducting films deposited by metal organic chemical vapor deposition (MOCVD). A main current-limiting mechanism is the increased growth of misoriented a-axis grains and deterioration of epitaxy in films thicker than 2 µm. This problem stems from inadequate temperature control during growth of thick films. The temperature control is even more critical in growth of thick films with high levels of dopant that is used for flux pinning.
Deterioration in microstructure of thin film superconductor tape beyond 2 µm with the formation of misoriented a-axis grains. These grains are also seen as (200) peaks in the X-ray Diffraction data
We have developed an Advanced MOCVD reactor to address the challenges in growth of high-quality thick superconductor films . Rather than a using a heater, the tapes are heated directly by ohmic heating wherein current is flowed directly into the tapes making use of the high resistivity of the Hastelloy substrate. Eliminating the heater also allows access to directly monitoring the temperature of the tapes using optical probes. Using the Advanced MOCVD system, 4 – 5 µm thick films have been growth with essentially no a-axis grains. Critical currents as high as 1600 A/12 mm have been achieved in these films, which is about 50% higher than that previously obtained in films grown in conventional MOCVD reactor .
The Advanced MOCVD reactor also has other new features such as a laminar flow channel which enables a high efficiency in the conversion of expensive metal organic precursors to film. This is especially important since only 15% of the precursors are converted to film in a conventional MOCVD reactor and the precursors are the most expensive component of the overall wire cost. High precursor conversion efficiency also leads to high throughput which is important for high-volume manufacturing. We used the Advanced MOCVD reactor to fabricate 4 – 5 µm thick REBCO films with record high critical current over 8700 A/12 mm at 30 K, 3 T  which is about seven times higher than the performance of commercial HTS tapes. The engineering current density of the thick film REBCO tapes made by the Advanced MOCVD method is about 5.5 kA/mm2 at 4.2 K, 14 T  which is over 5 times higher than the best reported performance of Nb3Sn superconductors which is the primary superconductor used now in high-field applications.
Our group recently received a $4.5 M award from the U.S. Department of Energy Advanced Manufacturing Office to utilize the Advanced MOCVD system to scale up high performance superconductor tapes for Next-generation Electric Machines. The goals of this program are to
Achieve a critical current of 1750 A/12 mm at 65 K, 1.5 T through combination of thick films and high lift factor in Advanced MOCVD tool (10X better performance than today’s production wire)
Increase precursor to film conversion efficiency by 3X in Advanced MOCVD tool to reduce cost to $20–30/m ($35–50/kAm at 65 K, 1.5 T)
Scale up process to demonstrate 50 m long tape with a critical current of 1750 A/12 mm at 65 K, 1.5 T
Design and fabricate and test sub-scale rotor coil at 65 K with high performance wire made by lower-cost, high throughput process
We have successfully fabricated 4 µm thick film REBCO tapes using Advanced MOCVD with excellent critical current performance. REBCO tapes with 4 µm thick films and 5% Zr addition exhibit critical currents of 1285 A/cm which is a factor of 4 higher than the performance of the best commercially-available REBCO tapes.
Critical current of GdYBCO tapes with 4 µm thick films and 5-15% Zr addition at 65 K, 1.5 T made in the program funded by the DOE- Advanced Manufacturing Office (AMO). The performance of the best commercially-available REBCO tape is included for comparison.
Engineering current density of UH REBCO tapes in high magnetic fields at 4.2 K, compared to other superconductor technologies (other data from National MagLab).
Advanced practical 2G HTS wire
Beyond critical current, in-field performance, lower cost and higher manufacturing throughput, it is important to develop superconductors which can meet other application requirements such as low AC losses, quench stability, robust mechanical properties, persistent joints, fault current limitation and recovery to name a few. Reduction in ac losses requires low hysteretic, eddy current, and coupling losses. Hysteretic losses can be minimized by subdividing the superconductor into fine filaments. At UH, we are working on dry etch and wet etch techniques to create multifilamentary 2G HTS wire architectures that lead to reduction in ac losses.
Multifilamentary second-generation high temperature superconducting tape and lower ac losses
We have developed a laser striation and a selective electroplating process to fabricate fully-stabilized multifilamentary thin film REBCO superconductor tapes. Every superconductor filament of these tapes is individually encapsulated with a copper stabilizer which provides the benefit of cryostabilization along with low AC losses. 10 – 20 m long tapes have been made with 12 filaments, 24 filaments and 46 filaments  as shown in Figure 1. The filament integrity was maintained uniformly over the long lengths. The copper electrodeposition was confined on the filaments without coupling between them.
Photographs of multifilamentary REBCO tapes made by laser striation followed by selective electroplating. (top-left) 12 filaments (top-right) 24 filaments (bottom-left) 46 filaments (bottom-right) 46 filaments after copper deposition
AC loss measurements at 100 Hz in AC fields up to 80 mT conducted on the 12, 24 and 46 filament tapes and a reference non-striated tape show that AC losses reduce proportional to the number of filaments in the tape. No evidence of coupling between the filaments was found.
We have developed another method for transposition of multifilamentary tapes by use of ultra-thin REBCO tapes. 2 mm wide, 25 µm thick tapes (thinnest produced to date) have been wound on formers with diameters as small as 0.51 mm without critical current degradation. 1.6 mm diameter round REBCO wires with a critical current of 495 A (engineering current density Je = 350 A/mm2) have been fabricated by this method. These round wires can be bent to a diameter of 3 cm without critical current degradation! We are working with industry to scale our round multifilamentary REBCO wire technology to long-length manufacturing. Modeling of the complex stress states in the round wire where the tapes are subjected to severe torsion and bending and accordingly optimizing the ultra-thin REBCO tape architecture is underway.
Current-voltage (I-V) curves of 2 mm wide, 25 µm ultra-thin REBCO tape by substrate abrasion when tested in a bent state over formers of different diameters. No degradation is observed in critical current even at a bend diameter of 0.51 mm!
(Left) Photograph of an 1.6 mm diameter round REBCO wire made by spiral winding six layers of 45 µm thick tapes over a 0.81 mm diameter former. (Right) Photograph of the round wire after bent to a diameter of 3 cm.
In collaboration with AMPeers in a Small Business Innovative Research (SBIR) program funded by the Department of Energy, we have developed a Symmetric Tape Round (STAR) wire. In the STAR wire, the REBCO film is placed close to the neutral plane. Such STAR wires exhibit excellent tolerance to bend strain with a critical current retention of more than 97% when bent to a radius of 15 mm. A 1.6 mm diameter REBCO STAR wire made with six 2.5 mm wide symmetric tapes reached an engineering current density (Je) of 454 A/mm2 at 4.2 K in a background field of 15 T at a bend radius of 15 mm . Such superior performance at a small bend radius can enable fabrication of future accelerator magnets, operating at magnetic fields above 20 T.
V. Selvamanickam, M. Heydari Gharahcheshmeh, A. Xu, Y. Zhang and E. Galstyan, “Critical current density above 15 MA/cm2 at 30 K, 3 T in 2.2 µm thick heavily-doped (Gd,Y)Ba2Cu3Ox superconductor tapes”, Supercond. Sci. Technol. 28, 072002 (2015).
V. Selvamanickam, M. Heydari Gharahcheshmeh, A. Xu, E. Galstyan, L. Delgado and C. Cantoni, “High critical currents in heavily doped (Gd,Y)Ba2Cu3Ox superconductor tapes”, Appl. Phys. Lett. 106, 032601 (2015).
A. Xu, N. Khatri, Y. Liu, G. Majkic, V. Selvamanickam, D. Abraimov, J. Jaroszynski and D. Larbalestier, “Broad temperature pinning study of 15 mol.% Zr-added (Gd, Y)-Ba-Cu-O MOCVD coated conductors”, IEEE Trans. Appl. Supercond. 25, 6603105 (2015).
G. Majkic, R. Pratap, A. Xu, E. Galstyan, H. C. Higley, S. O. Prestemon, X. Wang, D. Abraimov, J. Jaroszynski and V Selvamanickam, “Engineering Current Density over 5 kA/mm2 at 4.2 K, 14 T in Thick Film REBCO Tapes”, Supercond. Sci. Technol. 31 10LT01 (2018).
G. Majkic, R. Pratap, A. Xu, E. Galstyan and V. Selvamanickam, “Over 15MA/cm2 of critical current density in 4.8µm thick, Zr-doped (Gd,Y)Ba2Cu3Ox superconductor at 30 K, 3 T”, Nature Scientific Reports, 8, 6982 (2018)
S. Kar, W. Luo, A. Ben Yahia, X-F. Li, G. Majkic and V. Selvamanickam, “Je (4.2 K, 15 T) beyond 450 A/mm2 at 15 mm bend radius with REBCO Symmetric Tape Round (STAR) wire: A prospective candidate for future accelerator magnet applications”, Supercond. Sci. Technol. 3, 04LT01 (2018)