Second generation (2G) high temperature superconductor (HTS) wires are based on a coated conductor technology. They follow on from a first generation (1G) HTS wire consisting of a composite multifilamentary wire architecture. During the last couple of years, rapid progress has been made in the development of 2G HTS wire, which is now displacing 1G HTS wire for most if not all applications. The engineering critical current density of these wires matches or exceeds that of 1G wire, and the mechanical properties are also superior. Scale-up of manufacturing is proceeding rapidly, with several companies already supplying the order of 10 km annually for test and demonstration. Coils of increasing sophistication are being demonstrated. One especially attractive application, that relies on the specific properties of 2G HTS wire, is fault current limitation. By incorporating a high resistivity stabilizer in the coated conductor, one can achieve high resistance in a quenched state during a fault event and at the same time provide significant heat capacity to limit the temperature rise. A test of a 2.25 MVA single phase system at 7.5 kV employing such wire by the Siemens/AMSC team has demonstrated all the key features required for a cost-effective commercial system. A novel approach to providing fault current limiting functionality in HTS cables has also been introduced.

It has been well established that the critical current density J c across grain boundaries ͑GBs͒ in high-temperature superconductors decreases exponentially with misorientation angle beyond ϳ2°-3°. This rapid decrease is due to a suppression of the superconducting order parameter at the grain boundary, giving rise to weakly pinned Abrikosov-Josephson ͑AJ͒ vortices. Here we show that if the GB plane meanders, this exponential dependence no longer holds, permitting greatly enhanced J c values: up to six times at 0 T and four times at 1 T at ϳ 4°-6°. This enhancement is due to an increase in the current-carrying cross section of the GBs and the appearance of short AJ vortex segments in the GB plane, confined by the interaction with strongly pinned Abrikosov ͑A͒ vortices in the grains.

We report a detailed study of the grain orientations and grain boundary (GB) networks in YBa2Cu3O7-δ (YBCO) films ∼0.8 μm thick grown by both the in situ pulsed laser deposition (PLD) process and the ex situ metalorganic deposition (MOD) process on rolling-assisted biaxially textured substrates (RABiTS). The PLD and MOD growth processes result in columnar and laminar YBCO grain structures, respectively. In the MOD-processed sample [full-width critical current density Jc(0 T, 77 K) = 3.4 MA/cm2], electron back-scatter diffraction (EBSD) revealed an improvement in both the in-plane and out-of-plane alignment of the YBCO relative to the template that resulted in a significant reduction of the total grain boundary misorientation angles. A YBCO grain structure observed above individual template grains was strongly correlated to larger out-of-plane tilts of the template grains. YBCO GBs meandered extensively about their corresponding template GBs and through the thickness of the film. In contrast, the PLD-processed film [full width Jc(0 T, 77 K) = 0.9 MA/cm2] exhibited nearly perfect epitaxy, replicating the template grain orientations. No GB meandering was observed in the PLD-processed film with EBSD. Direct transport measurement of the intra-grain Jc(0 T, 77 K) values of PLD and MOD-processed films on RABiTS revealed values up to 4.5 and 5.1 MA/cm2, respectively. As the intra-grain Jc values were similar, the significantly higher full-width Jc for the MOD-processed sample is believed to be due to the improved grain alignment and extensive GB meandering.

The density n of stacking faults (SFs) in epitaxial YBa2Cu3O7−x (Y123) films, consisting of extra CuO planes, is measured by fitting x-ray diffraction patterns using a random stacking model. The SF density is n=0.068nm−1 in films grown by metal-organic deposition on textured templates and optimized for high Ic. The presence of SF is correlated with pinning of magnetic field (H) applied in the Y123 ab plane. SF can be nearly eliminated by a high temperature anneal, or by adding excess Dy, resulting in Ic which is nearly independent of the orientation of H.

AMSC has established a Second Generation (2G) High-Temperature Superconductor (HTS) wire manufacturing technology based on the Rolling Assisted Biaxially Textured Substrate and Metal Organic Deposition processes. AMSC's 2G wire (Amperium) has been used by a wide range of customers for development and testing of initial commercial HTS-based applications. Although the Amperium wire properties and quantities satisfy the requirements for these initial projects, improvements in critical current, field performance, and cost are beneficial for large-scale commercial and military applications. As Amperium wire manufacturing continues to ramp up, AMSC's R&D program has focused on increasing critical current, and the development of nonmagnetic substrates. The R&D process developed for a single-coat, 1.2 μm YBCO film has been transferred to production-scale equipment, resulting in the first Amperium wires with critical currents reaching 500 A/cm-w (77 K, self-field) in production length. A nonmagnetic substrate, which minimizes ferromagnetic substrate losses in ac cable applications, has been produced in R&D lengths and demonstrated in an Amperium cable wire.

We report on the thickness dependence of the superconducting characteristics including critical current I c , critical current density J c , transition temperature T c , irreversibility field H irr , bulk pinning force plot F p (H), and the normal state resistivity curve ρ(T) measured after successive ion milling of ~ 1 µm thick high I c YBa 2 Cu 3 O 7-x films made by an ex situ metal-organic deposition process on Ni-W rolling-assisted biaxially textured substrates (RABiTS TM ). Contrary to many recent data, mostly on in situ pulsed laser deposition (PLD) films, which show strong depression of J c with increasing film thickness t, our films exhibit only a weak dependence of J c on t. The two better textured samples had full cross-section average J c,avg (77K,0T) ~ 4 MA/cm 2 near the buffer layer interface and ~3 MA/cm 2 at full thickness, despite significant current blocking due to ~30% porosity in the film. Taking account of the thickness dependence of the porosity, we estimate that the local, vortex-pinning current density is essentially independent of thickness, while accounting for the additional current-blocking effects of grain boundaries leads to local, vortex-pinning J c values well above 5 MA/cm 2 . Such high local J c values are produced by strong three-dimensional vortex pinning which subdivides vortex lines into weakly coupled segments much shorter than the film thickness. which levels off above a critical thickness t c ~ 1 µm [2-9]. Such a thickness dependence is suggestive of the transition from the 2 dimensional (2D) pinning of rigid vortex lines in films thinner than the longitudinal pinning correlation length l c to the 3-dimensional (3D) pinning of deformable vortices at t > t c [14]. It was recently pointed out [15] that t c can indeed approach a few µm if the collective pinning model incorporates a multi-scale pinning potential appropriate for the strong-pinning second phase precipitates, pores, and correlated defects found in CCs. Strong-pinning defects have a pin interaction range, r p much greater than the coherence length ξ and produce large plastic deformations of vortices, rather than the small elastic deformations produced by weak, point pins. This strong-pinning model predicts a crossover thickness t c as large as 1-2 µm, in agreement with the observed J c (t) dependence of many PLD films [2-8] and qualitatively consistent with many recent studies of the angular dependence of 3 J c in CCs, which also reveal much evidence for correlated pinning along the c-axis in PLD films [16]. This multiscale pinning model also predicts the t -1/2 thickness dependence of J c (t), but the magnitudes of J c and t c can be very dependent on the specific pinning microstructure and thus on the film growth process.Another interpretation of the t -1/2 thickness dependence of J c (t) in PLD films has recently been advanced by Foltyn et al. [17]. They attribute this to a local J c (z) profile (where z is distance from substrate) across the film, such that J c is ~ 7.2 MA/cm 2 at the substrate and then linearl...

YBa2Cu3O7-δ coated conductors (CCs) have achieved high critical current densities (Jc) that can be further increased through the introduction of additional defects using particle irradiation. However, these gains are accompanied by increases in the flux creep rate, a manifestation of competition between the different types of defects. Here, we study this competition to better understand how to design pinning landscapes that simultaneously increase Jc and reduce creep. CCs grown by metal organic deposition show non--monotonic changes in the temperature--dependent creep rate, S(T). Notably, in low fields, there is a conspicuous dip to low S as the temperature (T) increases from 20 K to 65 K. Oxygen--, proton--, and Au--irradiation substantially increase S in this temperature range. Focusing on an oxygen--irradiated CC, we investigate the contribution of different types of irradiation-induced defects to the flux creep rate. Specifically, we study S(T) as we tune the relative density of point defects to larger defects by annealing both an as--grown and an irradiated CC in O2 at temperatures TA = 250°C to 600°C. We observe a steady decrease in S(T > 20 K) with increasing TA, unveiling the role of pre--existing nanoparticle precipitates in creating the dip in S(T) and point defects and clusters in increasing S at intermediate temperatures.

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