Advanced development of TFA-MOD coated conductors

The performance of Second Generation (2G) high temperature superconducting wire manufactured by continuous reel-to-reel processes is nearing the 300 A/cm-width (77 K, self field) performance threshold for commercial power cable applications. The 2G manufacturing approach under development at American Superconductor is based on the combination of the RABiTS substrate-buffer technology with metal organic deposition (MOD) of the YBCO layer. The capability of this process has been demonstrated in multiple 10 meter lengths with critical currents exceeding 250 A/cm-width with high uniformity and reproducibility. Critical currents of 380 A/cm-width have been achieved in short length samples prepared by the same basic process. The incorporation of nanoparticles (nanodots) into the YBCO layer using the MOD process has resulted in a 2-fold improvement in the critical current at 65 K in a 3 T field. The research and development focus at ASMC is now directed toward the economical scale-up of the RABiTS/MOD process, optimization of the conductor properties for targeted applications and the use of 2G wire in initial demonstration applications.

The RABiTS™/MOD-YBCO (rolling assisted biaxially textured substrate/metal–organic depositionof YBa2Cu3O7−δ) route has been established as a low-cost manufacturing process for producing highperformance second generation (2G) wire. American Superconductor Corporation (AMSC)has used this approach to establish a production scale manufacturing line based on awide-web manufacturing process. This initial production line is currently capableof producing 2G wire in lengths to 500 m with critical currents exceeding 250 A cmwidth−1 at 77 K, in the self-field. The wide-web process, combined with slitting and laminationprocesses, allows customization of the 2G wire width and stabilizer composition to meetapplication specific wire requirements. The production line is currently supplying 2G wirefor multiple cable, fault current limiter and coil applications. Ongoing R&D is focused on thedevelopment of thicker YBCO layers and improved flux pinning centers. This paper reviewsthe history of 2G wire development at AMSC, summarizes the current capability ofthe 2G wire manufacturing at AMSC, and describes future R&D improvements.

The metal organic deposition (MOD) of YBCO high temperature superconducting films on RABiTS (rolling assisted biaxially textured substrates) templates has been developed at American Superconductor as a low-cost, scalable manufacturing process for the commercialization of the second generation (2G) HTS wire. The MOD process is based on the deposition of a triflu-oroacetate (TFA) based metal organic precursor film which is converted, in an ex-situ process, to the superconducting YBCO film. A major goal of the development has been achieving high critical currents. This paper reports the preparation and characterization of MOD-YBCO films with critical currents exceeding 500 A/cm-w (77 K, self-field) using a scaleable thick film approach on RABiTS templates. The high critical current films were obtained through optimization of the precursor composition, nucleation and growth conditions. The through-thickness dependence of the critical current density of MOD Alms as a function of film thickness and a correlation of the through-thickness transport properties and microstructure of the thick MOD/RABiTS samples is reported.

Progress in AMSC scale-up of second generation HTS wire

Significant enhancements in flux-pinning were obtained for Dy-doped, YBCO films via a metalorganic deposition (MOD) process on rolling-assisted biaxially textured substrates (RABiTS). It has been reported previously that incorporation of excess rare-earth ions during the MOD process, results in improvement of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for H//c, however, a decrease in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for H//ab is observed. We report here that by altering the processing conditions the reduction in the magnitude of the current peak for H//ab can be minimized while significantly enhancing the random pinning at all field orientations. The result is a YBCO film with significantly reduced anisotropy compared to the typical YBCO films prepared by the MOD process. This is accomplished by incorporating both a high density of stacking faults and (Dy, Y) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanoparticles which result in the strong pinning for H//ab and a broad pinning peak for H//c respectively.

Normal-incidence irradiation by 100 MeV Ag ions is used to improve flux pinning in previously optimised commercial REBCO tapes from the American Superconductor Corporation. We observe distinct critical-current anisotropy enhancements below and above 40 K. Above 40 K a strong c-axis peak appears in the angular dependence of the critical current, as is usually expected upon the introduction of columnar defects. The critical current is enhanced significantly but only for a limited range of field angles. Close to the parallel-field direction there is no enhancement or even a reduction in critical current. Below 40 K, on the other hand, the enhancement is much broader with respect to field angle, creating an almost isotropic response at 20 K, 3 T. The absence of a prominent c-axis peak does not indicate a lack of pinning, since the absolute value of the critical current still increases by a factor of 2.8 compared to an unirradiated sample. Instead, we postulate that pre-existing point-like pinning centres act to mediate an interaction between the existing planar and newly-introduced columnar pins, broadening both contributions. The point-like pins become less effective with increasing temperature as the coherence length increases, leading to a reduction in this interaction and a separation of the individual peaks relating to planar and columnar pins. At 20 K, we achieve an enhancement in the angular-minimum critical current by a factor of 2.7, in a material that had already been process-optimised for low-temperature pinning.

Non-hysteretic metal–insulator transition of VO2 films grown by excimer-laser-assisted metal organic deposition process

The critical current of an YBCO tape is determined by the magnetic field inside the YBCO layer and the quality of YBCO material. In thick YBCO layers the average critical current density is reduced by the self-field and decreased material quality. In this paper the combined influence of the material nonhomogeneities and self-field on the critical current of YBCO tapes is scrutinised. First, the zero field critical current density was assumed to decrease along the YBCO thickness. Secondly, the possible defects created in the cutting of YBCO tapes were modelled as a function of lowered critical current density near the tape edges. In both cases the critical current was computed numerically with integral element method. The results suggest that the variation of zero field critical current density, Jc0, along the tape thickness does not effect on the critical current if the mean value of Jc0 is kept constant. However, if Jc0 is varied along the tape width the critical current can change due to the variated self-field. The computations can be used to determine when it is possible to evaluate the average zero field critical current density from a voltage-current measurement with an appropriate accuracy.

DyBa2Cu3O7−δ/YBa2Cu3O7−δ multilayer thick films were prepared on oxide-buffered Hastelloy substrates by low fluorine metal organic deposition (MOD) process with a thickness ranging from 0.7 μm to 2.8 μm. In the thickness of 1.4 μm sample, transition temperature (Tc) and critical current density (Jc) are 91.2 K and 2.5 MA/cm 2 (at 77 K, self field) respectively. With increasing thickness, Tc drops as well as Jc. Unfortunately, the critical current (Ic) does not reach an ideal value when the thickness is 2.8 μm just only 365A/cm width as thickness is 2.1 μm. Rough surface structure, compressive, and tensile stress in the interlamination and Ba-Cu-O mixed phase were found in layer thickness of 2.1 μm and 2.8 μm are assumed to be the main reason for the limitation of the current transport.

Electrical Measurements were performed on two high-temperature superconducting coils made by American Superconductor Corporation. One coil measured 24-mm i.d., 59-mm o.d, 50-mm long, and used 85-m long Y-124 tape conductor. The other coil measured 29-mm i.d., 44-mm o.d., 43-mm long, and used 35-m long Bi-2223 tape conductor. We obtained V-I curves from room to helium temperature by using a variable temperature cryostat cooled by helium gas in external fields up to 4 T. Without an external field, the better performing Bi-2223 coil had a critical current, I c , of 14.1 A (2,820 A/cm2 over the conductor) at 4.2 K and 1.8 A (360 A/cm2) at 77 K. At 4 T, the I c was 4.9 A (980 A/cm2) at 4.2 K and 2.0 A (400 A/cm2) at 50 K. Reduced critical current, I c (B)/I c (0) vs. field plots indicated that a single smooth curve could fit all the data up to 50 K. The reduction in critical currents with external fields for the Y-124 coil was more than 80% at i T. For the Bi-2223 coil, it was about 42% at 1 T and about 61% at 4 T.

High critical current YBCO thick films by TFA–MOD process

Heavy-ion irradiation of solids produces damage tracks with radii typically of the order of 1 nm, depending on the ion species and energy. In cuprate superconductors this is close to the coherence length, which makes these defects highly effective flux pinning centers. Varying the ion-beam energy allows tuning of the dimensionality of the defects created, with higher-energy ions tending to produce columnar tracks and lower-energy ions tending to produce point-like defects. Starting with consistent production-standard REBCO tape from American Superconductor we have explored the energy-dependence of silver-ion irradiation and characterized the irradiated samples with angle-dependent transport critical current measurements. Using silver ions with energies in the range 50 MeV to 150 MeV and fluence of 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> we have been able to tune the irradiation-induced damage from point-like defects to columnar tracks, manifesting in changes to the pinning landscape ranging from isotropic critical current enhancement to the production of strong peaks in the angle dependence of critical current.

Draper Laboratory and Boeing North American have formed an alliance to develop small, low cost rate sensors for commercial and military applications. Advanced silicon micromachining techniques produce sensors of high performance, ruggedness, and inherent symmetry. When integrated with Applications Specific Integrated Circuits (ASIC's), the rate sensor will fit in a 3 cm per side flat pack and operate from a single 5 Vdc supply.

American Superconductor (AMSC), a leading producer of high-temperature superconducting wire, is joining with BASF to develop a new, low-cost manufacturing process for the wire. The companies hope to combine AMSC’s expertise in superconducting wire manufacturing with BASF’s solution deposition technology. AMSC has been developing superconducting wire applications for electric utilities and the U.S. Navy. Such wires can carry 200 times the electric current of conventional wires, the firm says.

Microstructure of metal-organic deposited YBa2Cu3O7− wires with Dy and Zr additions observed by TEM