Yimin Chen

High Performance 2G Wires: From R&D to Pilot-Scale Manufacturing

Tremendous progress has been accomplished in 2008 in key metrics of YBa2Cu3Ox (YBCO) second-generation (2G) HTS wires at SuperPower. Using improved precursor chemistry in our metal organic chemical vapor deposition (MOCVD) process, critical currents (Ic) as high as 813 A/cm were reached over meter-long lengths, representing the highest reported Ic in meter long 2G HTS. By use of Zr doping of (Gd,Y)Ba2Cu3Ox, strong enhancements were achieved in in-field performance over a wide angular range. A Ic of 229 A/cm was achieved at 77 K and 1 T perpendicular to the wire which is about a factor of 2.5 times better than that of standard MOCVD-derived films. At 65 K and 3 T, a Ic of 340 A/cm was demonstrated perpendicular to the wire. Kilometer lengths of 2G HTS have been demonstrated for the first time. Over 35 tapes 1,000 to 1,500 m in length with a complete 5-layer buffer stack have been routinely produced in our pilot-scale manufacturing facilities. An excellent in-plane texture of 6 to 7 degrees with a uniformity of about 2% was reproducibly achieved in the kilometer-long fully-buffered tapes. Using these high-quality buffers, the longest 2G wire length to date of 1,311 m was produced with a minimum Ic of 153 A/cm corresponding to a Ic times length value of 200,580 m. Also, an 1,030 m long complete 2G HTS wire has been demonstrated with a minimum Ic of 227 A/cm corresponding to a record Ic times length value of 233,810 A-m. Further, a 630 m long segment of the wire sustained a minimum Ic of 302 A/cm, the longest 2G wire reported to date with a Ic over 300 A/cm. Also, a Ic of 337 A/cm was achieved over a 540 m long segment. All these values represent a 2 to 3 fold improvement in length and Ic of 2G wires compared to our achievements in 2007. In 2008, a 30 m long 2G cable made with nearly 10,000 m of 2G wire previously delivered by SuperPower was successfully installed and energized in the grid of National Grid in downtown Albany, marking the worlds first of 2G HTS device in the power grid.

Recent Progress in Second-Generation HTS Conductor Scale-Up at SuperPower

YBa2Cu3Ox (YBCO) second-generation (2G) HTS conductors have been produced in lengths over 300 m in pilot scale facilities established at SuperPower. High throughput pilot-scale manufacturing has been demonstrated with tape speeds at or above 30 m/h of 12 mm wide tape (corresponding to 90 m/h of 4 mm wide conductor) in all steps. A 322 m long conductor with a minimum critical current value of 219 A/cm has been produced, which corresponds to a critical current x length value of 70,520 A-m. A 270 m long, 4 mm wide conductor with an end-to-end critical value of 100 A has also been demonstrated. In a campaign to manufacture 2G conductor for the Albany cable project, SuperPower has produced 12,470 m of conductor that meets or exceeds the specification for piece length (42.4 m) and critical current (100 A/cm). In fact, more than 55% of the conductor produced is at least 100 m in piece length and more than 27% greater than 200 m in piece length. In addition to scaling up 2G conductors to high-throughput pilot-scale manufacturing, we have demonstrated high critical currents in short samples produced by Metal Organic Chemical Vapor Deposition (MOCVD). Critical current values of 557 A/cm have been achieved in 2.1 micrometer thick films in 12 mm wide, 10 cm long tapes. These samples exhibit a critical current value of 116 A/cm at 1 T and 76 K, in the orientation of field parallel to the c-axis. We have also constructed a 4-pancake coil that generated a magnetic field of 1.1 T at 77 K and 2.4 T at 64 K. A Fault Current Limiter (FCL) assembly has been successfully constructed and tested at high power levels. A prospective current of 90 kA was successfully limited to 32 kA within 1 ms without any HTS element failure.

Enhanced flux pinning by BaZrO3 and (Gd,Y)2O3 nanostructures in metal organic chemical vapor deposited GdYBCO high temperature superconductor tapes

We have formed BaZrO3 nanocolumns and (Gd,Y)2O3 nanoprecipitates in reel-to-reel metal organic chemical vapor deposition (MOCVD) processed (Gd,Y)Ba2Cu3O7−x coated conductors and increased the critical currents (Ic) of the conductors in applied magnetic fields to remarkable levels. A (Gd,Y)Ba2Cu3O7−x tape of 1 m in length with 6.5% Zr-additions and 30% composition rich in both Gd and Y showed Ic values of 813 A/cm width at (self-field, 77 K) and above 186 A/cm width at (1 T, 77 K). The strongly enhanced flux pinning over a wide range of magnetic field orientations can be attributed to the bidirectionally aligned defect structures of BaZrO3 and (Gd,Y)2O3 created by optimized MOCVD conditions.

The low-temperature, high-magnetic-field critical current characteristics of Zr-added (Gd,Y)Ba2Cu3Ox superconducting tapes

Critical current performances of state-of-the-art Zr-added (Gd, Y)BaCuO tapes have been investigated over a temperature range of 20–77 K, in magnetic fields up to 9 T and over a wide angular range of magnetic field orientations. The peak in critical current that is commonly observed in the field orientation perpendicular to the tape in BaZrO3 (BZO) containing superconducting tapes is found to vanish at 30 K in magnetic fields at 1–9 T. While the critical current of 15% Zr-added tapes was about 40% lower than that of 7.5% Zr-added tapes at 77 K, the pinning force values of the former were found to be 18–23% higher than those of the latter in the temperature range of 20–40 K and in magnetic fields of 3–5 T. The results from this study emphasize the importance of optimization of coated conductor fabrication processes for optimum performance not just in low magnetic fields at 77 K but also at the operating conditions of low temperatures and high magnetic fields that are of interest, especially for rotating superconducting machinery applications.

Angular dependence of Jc for YBCO coated conductors at low temperature and very high magnetic fields

We present very high field angle dependent critical current density (Jc) data for three recently obtained YBa2Cu3O7−x (YBCO) coated conductors used in the construction of high field solenoids. We find that strongly correlated pins, such as BaZrO3 (BZO) nanorods, while yielding strong c-axis peaks at 77 K, produce almost no measurable contribution at 4 K. Raising the field from <5 to 30 T at 4 K causes a marked transition from a Ginzburg‐Landau-like Jc(θ ) at low fields to a marked cusp-like behavior at high fields. Transmission electron micrographs show that all samples contain a high density of stacking faults which strengthen the plane correlated pinning parallel to the ab planes produced by the intrinsic ab-plane pinning of the Cu‐O charge reservoir layers. (Some figures in this article are in colour only in the electronic version)

High Throughput Processing of Long-Length IBAD MgO and Epi-Buffer Templates at SuperPower

SuperPower has successfully transferred from low throughput IBAD (Ion beam assisted deposition) YSZ technology to high throughput IBAD MgO technology. Pilot-scale IBAD system and pilot-scale epi-buffer deposition system, each with capabilities of producing single-piece lengths over 1000 m have been established. Helix tape handling approach instead of wide tape approach is chosen due to its immense advantages over wide tape approach. High-performance IBAD MgO buffer stack and their processes have been developed together with high throughput where each layer in the buffer stack is processed at 40 m/h tape speed or higher during our Phase I scale up. Both pilot systems are in routine production mode since March 2006. More than 14 km IBAD MgO buffer tapes of 12 mm width were produced for Albany cable project. 700-800 m IBAD MgO tape per run is routinely produced in pilot IBAD system with single piece tape length ~570 m. 400-550 m epi-buffer tape per run is routinely produced in pilot buffer system. High throughput IBAD MgO buffer gives better texture of YBa2Cu3O7 (YBCO) film compared with IBAD YSZ, and superior superconducting properties. A high critical current of 557 A/cm in short tapes and a world record critical current times length value of > 70, 000 A - m were obtained with the high throughput IBAD MgO buffers.

Enhanced critical currents in (Gd,Y)Ba2Cu3Ox superconducting tapes with high levels of Zr addition

The critical current and structural properties of (Gd,Y)BaCuO tapes made by metal organic chemical vapor deposition (MOCVD) with Zr addition levels up to 30 at.% have been investigated. The reduction in critical current beyond the previously optimized Zr addition level of 7.5 at.% was found to be due to structural deterioration of the (Gd,Y)Ba2Cu3Ox film. By a modified MOCVD process, enhanced critical current densities have been achieved with high levels of Zr addition, including 3.83 MA cm 2 in 15 at.% Zr-added 1.1 m thick film at 77 K in zero magnetic field. Critical currents as high as 1072 A/12 mm have been reached in (Gd,Y)BaCuO tapes with 15 at.% Zr addition at 30 K in a field of 3 T applied perpendicular to the tape, corresponding to a pinning force value of 268 GN m 3 . The enhanced critical currents achievable with a high density of nanoscale defects by employing high levels of second-phase additions enable the performance targets needed for the use of HTS tapes in coil applications involving high magnetic fields at temperatures below 50 K to be met. (Some figures may appear in colour only in the online journal)

Progress in Performance Improvement and New Research Areas for Cost Reduction of 2G HTS Wires

Second-generation (2G) HTS wires are now being produced routinely in kilometer lengths using Metal Organic Chemical Vapor Deposition (MOCVD) process with critical currents of 300 A/cm. While this achievement is enabling several prototype devices, in order to reach a substantial commercial market, the cost-performance metrics of 2G HTS wires need to be significantly improved in device operating conditions. Zr-doping has been found to be an effective approach to improve in-field critical current performance of MOCVD-based HTS wires. In this work, we have explored modifications to the Zr-doped precursor compositions to achieve three and two-fold increase in deposition rate in research and production MOCVD systems respectively. Production wires made with modified Zr-doped compositions exhibit a self-field critical current density of 50 MA/cm2 at 4.2 K and a 55 to 65% higher performance than our previous wires with Zr-doping, over magnetic field range of 0 to 30 T. We have also developed an alternate, low-cost technique, namely electrodeposition, to deposit silver overlayer on superconducting film. Wires made with electrodeposited silver are able to sustain the same level of overcurrent as sputtered silver layers. This process has been successfully scaled up to 100 m lengths.

Second-Generation HTS Conductor Design and Engineering for Electrical Power Applications

Besides critical current and conductor length, thermal stability, mechanical properties and ac loss characteristics of second-generation (2G) HTS wire are very important performance parameters to electric power device applications. Each application sets specific technical requirements on conductors. The properties of the practical 2G HTS wire manufactured at SuperPower, Inc. have enabled satisfactory performance in a number of demonstration devices including cable, high-field magnetic coil and fault current limiter. Yet, effort is being made to develop new conductor structures and configurations toward further enhanced performance in order to extend the potential of 2G HTS wire over a wider application range. This paper describes a variety of newly developed configurations: striation, narrow width, low-resistance joints and high quality insulation. The performance of wire with new configurations will promote the wide use of 2G HTS wire.

Composition Effects on the Critical Current of MOCVD-Processed Zr:GdYBCO Coated Conductors in an Applied Magnetic Field

Zr:GdYBCO films were grown by reel-to-reel metal organic chemical vapor deposition (MOCVD) on hastelloy tapes with IBAD-MgO-based buffer. The composition was varied systematically to investigate the effects of changes in (Gd+Y)/Ba, (Gd+Y)/Cu and Gd/Y ratios and Zr-doping concentration on the critical current density (Jc) of the films in an applied magnetic field (B). The magnetic-field-angle dependence of Jc measured at 77 K and 1 T showed that (1) for Gd+Y content ranging from 1.2 to 1.5, the minimum Jc for any angle did not vary significantly with the Gd+Y content; while the Jc at B//c varied significantly and took its maximum value at Gd+Y content of 1.2; (2) increasing Gd+Y could suppress or level off the Jc peak at B//c which was associated with the pinning from BaZrO3 nano-columns; (3) the optimum Zr-concentration for the highest Jc(77 K, 1 T) is in the range of 0.04-0.07 in the film; in this range under certain growth condition, the c-peak in the angular dependence of Jc could be higher than the ab-peak; (4) increasing Gd/Y ratio increased Jc_min(77 K, 1 T); (5) in self-field or low field, however, the optimized Gd/Y ratio was about 1.