1. Field of the Disclosure
The present disclosure relates generally to high temperature superconductors (HTS), and more specifically to a multi-filament, AC tolerant superconductor and method of forming the same.
2. Background of the Disclosure
The potential for high temperature superconductors (HTS) to efficiently transmit, generate, transform, use, and store electrical energy is recognized. In particular, more efficient electric power systems depend on more efficient wire technology. Past advancements permit brittle HTS materials to be formed into kilometer-length wires capable of transmitting about two hundred times the current than conventional copper and aluminum conductors of the same physical dimensions. Recent research in HTS materials provides potential for the economically feasible use of such materials in the power industry, including applications for power generation, transmission, distribution, and storage. The use of HTS devices in the power industry would result in significant reduction in the size (i.e., footprint) of electric power equipment, reduced environmental impact, greater safety, and increased capacity over conventional technology.
Two generations of HTS wire materials have been explored previously. The first generation (hereinafter “1G”) of HTS wires included the use of BSCCO high-Tc superconductor, typically embedded in a matrix of noble metal (e.g. Ag). Without limitation, 1G wires are fabricated by a thermo-mechanical process wherein the superconducting powder is packed into silver billets that are drawn, rolled, and heat-treated to form the wire. The drawbacks of 1G wires are the high materials costs (e.g. Ag), elaborate processing operations, and generally poor critical current performance in high magnetic fields at high temperatures, which limit the lengths of the wires.
The second generation (hereinafter “2G) HTS wire processing involves thin film deposition of a multilayer stack on nickel alloy tapes. In order to achieve high critical currents, the maximal current of a superconductor, the superconducting film is grown epitaxially in a single-crystalline-like form on oxide buffer layers that provide a single crystalline-like template even when deposited on polycrystalline metal substrate. In certain instances, 2G HTS tape utilizes YBCO coated conductors.
Recently, the focus of the HTS industry has been to increase current carrying capacity, throughput of wire production, and decrease manufacturing cost. The objective is to fabricate commercially viable, high performance HTS wire that is available to the power industry to build devices, such as transmission cables and transformers, for the power grid. Recent prototypes have confirmed the great potential of HTS wire in electric power applications but have also shed light on deficiencies that pose risk to their widespread implementation.
Although superconductors have zero resistance to DC current, the architecture of the coated conductor has yet to be optimized for AC applications, such as motors, generators, and transformers. Hysteretic losses in the superconductor are the major component of AC losses and scale inversely as the width of the filament. In particular, the width to thickness ratio of HTS wire is high, which results in HTS coated conductors exhibiting very high hysteretic losses. The magnitude of the losses also varies with AC field amplitude and frequency and thus varies in different applications. AC losses in a typical 2G HTS wire with a copper stabilizer can be as high as 100 kW over 10 km in a perpendicular field of 100 mT at 60 Hz.
A significant reduction of hysteretic losses in HTS wires is a prerequisite for their use in AC power applications, such as transformers, generators, and motors. In practice, AC losses result in increased cryogenic burden and impose risks on electric power systems. To mitigate these risks, higher cooling capacities or redundant cooling equipment must be used, which greatly increases the overall system cost and is a significant deterrent to the adoption of this immature technology. For these reasons, the development of a commercially viable, high performance, AC tolerant HTS wire would be a transformational solution that would open up the application of superconducting products into electric power systems.
It known that hysteretic losses can be reduced if the superconducting layer is divided into many filament-like superconducting structures, segregated by non-superconducting resistive barriers. Therefore, to minimize AC hysteretic losses, it is desirable to subdivide the current-carrying HTS layer of a tape into long thin linear stripes, or filaments, thereby forming a multifilament conductor. Although these multifilament conductors have been shown to greatly reduce hysteretic losses, numerous engineering and manufacturing challenges remain prior to full commercialization of these HTS wires because the scale up of multifilamentary 2G HTS wire to industrial manufacturing is fraught with numerous barriers.
The method of fabricating low AC loss 2G HTS wire is to subdivide the superconducting and insulating material into multiple filaments by first depositing the superconducting layer and then etching the superconductor layer (by physical or chemical techniques) to create striations or continuous filaments. The use of an etchant inevitably results in damage to the superconducting material, such as edge rippling, undercutting, and broken filaments. The filament damage becomes more prominent when the gap between filaments is narrowed. More specifically, the filament damage greatly reduces the current carrying capacity of the HTS wire. Furthermore, if the etching process is modified to avoid filament damage, bridges or other incomplete separations between the filaments may be left behind which results in the coupling of filaments and negates the AC loss reduction. In fact any superconductor residue that remains in the gaps after etching can result in filamentary coupling. If the gap is widened to circumvent these problems, more superconducting material is removed which greatly reduces the current carrying capacity of the wire. Even at short lengths, such as meter lengths, these multi-filamentary 2G HTS wire contain the flaws described herein. As such, producing kilometer-long 2G HTS wires with fine filaments of continuous superconducting parallel lines, running end to end poses a technological barrier to scaling up the use of HTS wire in electric power systems.
As such, a commercially feasible method does not exist for fabricating an AC tolerant HTS wire by an etch-free process to produce a multi-filament HTS layers. Some etch-free techniques had been proposed, such as creating scratches on substrate prior to superconductor growth (U.S. Patent App. Pub. No. 2007/0191202, to Foltyn et al), or inkjet printing of superconducting filaments (R. C. Duckworth, M. P. Paranthaman, M. S. Bhuiyan, F. A. List, and M. J. Gouge, IEEE Trans. Appl. Supercond. 17, 3159 (2007)), or the dropwise deposition of superconducting material (U.S. Patent App. Pub. No. 2006/0040829, to Rupich et al). However, these techniques result in unintentional coupling of filaments that leads to poor and inconsistent AC loss reduction and tapes that cannot be utilized even in meter lengths, yet alone kilometer lengths. This highlights that in addition to providing a high degree of precision and control for producing a multi-filamentary HTS wire, any etch-free technique must be compatible with the current techniques of manufacturing HTS conductors, and it is also essential to control the flux and current distributions in the HTS tape.