YBCO Layer Research Articles

Observation of Transient Behavior of Magnetic Flux in Inductive-type Fault Current Limiter with YBCO Thin Film Disc

Abstract Recently, the installation of fault current limiters (FCLs) in power systems is expected for controlling large short circuit currents. In this study, we focus on inductive-type FCLs having a YBCO superconducting thin film disc that is fabricated by metal-organic deposition. AC currents were injected into the FCL so that the periodic transient behavior of the apparent magnetic flux density around the FCL could be measured by using a pick-up coil. The magnetic flux density exhibited hysteresis when AC current was injected into the FCL. The transition between the conducting states in the YBCO layer was explained by the hysteresis relationship between the magnetic flux density and current.

EPR and speciation simulation study of Cu2+ complexes in an amine-based aqueous precursor system used for preparation of superconducting YBCO coatings

In this work, we investigate the chemistry for an aqueous acetate-triethanolamine-ammonia based YBa(2)Cu(3)O(7-δ) (YBCO) precursor system. These precursor solutions are suited for the chemical solution deposition of superconducting YBCO layers on top of single crystal SrTiO(3) or buffered NiW tapes. The development of this kind of precursor inks often involves trial-and-error experimenting and thus is very time-consuming. To reduce labwork to the minimum, the theoretical prediction of pH stability limits and the complexation behaviour of the different metal ions and complexants in the inks are very important. For this purpose, we simulated, based on literature values, the complexation behaviour of Cu(2+) in the aqueous precursor solutions as a function of pH. To validate the used model, we performed potentiometric pH titrations for solutions with similar composition and checked the correctness of fit between experiment and model. The generated complexometric results are coupled with X-band EPR spectra to further confirm the results. EPR spectra for fully prepared precursor solutions as well as for Cu(2+) reference solutions containing only one type of ligand (acetate, triethanolamine or ammonia) were investigated as a function of pH. We find that, in line with speciation simulation, only acetates are actively complexing the Cu(2+) ions at pH values below 7, while when reaching higher pH levels mainly triethanolamine complexes are formed. Over the entire pH range, no trace of free Cu(2+)or Cu(OH)(2), possibly creating precipitation during gelation and thus complicating further processing, could be found.

Three-Dimensional Micrometer-Scale Modeling of Quenching in High-Aspect-Ratio $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta}$ Coated Conductor Tapes—Part II: Influence of Geometric and Material Properties and Implications for Conductor Engineering and Magnet Design

YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (YBCO) coated conductors (CCs) show great promise for applications, but due to a very slow normal-zone propagation velocity (NZPV), quench detection and protection in YBCO magnets may be difficult. Present YBCO CCs have been developed with a primary focus on maximizing the critical current density for elevated-temperature low-field or low-temperature high-field applications. As the market for magnet applications progresses, it becomes important to consider design parameters such as the thicknesses and properties of all YBCO CC components, with the intent of considering quench-related behaviors as an integral part of the conductor and magnet design processes. Thus, it is important to know the impacts of conductor parameters on quench behavior. Considering that the YBCO layer itself is on the order of a micrometer in thickness, quench behavior must also be considered at this scale length. Here, the highly accurate experimentally validated micrometer-scale 3-D tape model reported in Part I is used to study how variations in CC geometry and material properties affect quench behavior, including the NZPV, hot-spot temperature, and minimum quench energy. The parametric variations focus on quantities that can be most readily modified by CC manufacturers. Based on simulation results, the relative sensitivities of the quench quantities to the parametric variations are calculated to identify which CC design parameters are most impactful on quench behavior. The implications of these results for quench detection and protection are discussed.

Sensitivity of Traveling Wave Photodetectors in Superconducting Fiber Plasmon–Polariton Optical Waveguides

By using an analytical model and a finite element method, we investigate a new, very sensitive, superconducting traveling wave photodetector made by a fiber waveguide, which includes a high index layer, a metallic layer, and an active superconducting layer. A comparison with the corresponding superconducting box shaped waveguide shows that a larger number of modes (HE11, TM01 and HE12) are obtained in optical fiber due to the surface plasmon–polariton modes at the interfaces between gold and air layers or between gold and YBCO layers. The radial component of the electric field is perpendicular to the metal surface and has sign changes at the gold boundaries as in the simple case of surface plasmon polaritons on metal cylinder with dielectric core. In a structure of the fiber with six layers, the imaginary parts of the TM01, HE12 modes, and the power absorption efficiency in superconducting layer are larger in comparison with that of the fiber with five layers. The confinement regimes of the light and the power absorption efficiency in superconducting layer can be optimized by only acting on the fiber geometry.

Internal Strain Behavior Exerted on YBCO Layer in the YBCO Coated Conductor

The stress/strain dependence of elastic property of the surround Cu stabilized YBCO coated conductor was precisely investigated by means of white X-ray and pulsed neutrons. Multiple diffraction peaks were observed along axial and lateral directions with the tape axis as a function of tensile load. The following unusual stress/strain behaviors exerted on the YBCO layer were made clear. Pairs of (020)/(200) and (040)/(400) were observed side by side in two rows and their diffraction intensity was almost constant with increasing tensile load. The ratio of diffraction strain to external strain became less than unity. The diffraction elastic constants estimated from (0 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> 0) diffraction peaks were larger than from ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</i> 00) peaks. Their observations strongly suggest that the micro twin structure is key nanostructure to understand the microscopic elastic constant and strain obtained from the diffraction experiments.

Pushing Coated Conductor Critical Currents Beyond 1 kA per cm Width: Stacks of YBCO Layers

For a number of superconducting power applications, a high value of the engineering critical current density (Je) for the wire is crucial. The superconducting layer in the coated conductor is typically a small portion of the overall cross-section, so increasing the superconductor fraction will directly result in an increase of Je . However, as the thickness of the superconductor is increased, Jc eventually drops. We describe a way to increase Je by making a stack of superconducting layers using sequential Ion-Beam Assisted Deposition (IBAD)/Superconductor deposition. Reactive Co-Evaporation by Cyclic Deposition and Reaction (RCE-CDR) is used for superconductor. An IBAD-textured layer resets the crystalline structure after each superconducting layer and we use IBAD-MgO for this purpose. However, IBAD-MgO texturing requires an extremely smooth starting surface (about 1 nm root mean square roughness), whereas the YBCO layer is typically 10-100 times rougher. We employ the Solution Deposition Planarization (SDP) process to planarize the rough surface of YBCO. The SDP layer is insulating and it provides for an easy way to separate the superconducting layers electrically. We discuss unique features of the stacking structure that allow for high Ic, low ac-losses in applied fields, as well as high Je .

Degradation of YBCO Coated Conductors Due to an Over-Current Pulse

YBCO coated conductors are candidate materials for future electric power devices such as transmission cables, transformers, and fault current limiters. In practical applications, YBCO coated conductors are subject to short-circuit fault currents greater than the operating current. These fault currents cause heat generation, resulting in degradation of the characteristics of YBCO coated conductors. It is therefore necessary to identify the maximum temperatures under which YBCO coated conductors can operate without suffering degradation. To date, we have carried out preliminary experiments on the degradation of YBCO coated conductors due to an over-current pulse. We also focused on the relationship between increases in temperature and Ic degradation. In this study, we prepared YBCO coated conductors fabricated by TFA-MOD method. We also carried out over-current tests to investigate the Ic degradation and performed numerical simulations on thermal stress to estimate the stress and strain acting on the YBCO layer.

All-MOCVD Technology for Coated Conductor Fabrication

The deposition of all buffer and YBCO layers for coated conductors via MOCVD represents one of the lowest possible capital and operational cost approaches to coated conductor manufacturing. Modular design of our prototype MOCVD system allows for either in-line or parallel fabrication of any number of buffer layers and YBCO layers. At 77 K in self-field Ic >; 200 A/cm has been reproducibly demonstrated on all-MOCVD architecture, the best up-to-date result being Ic = 236 A/cm and Jc = 2.13 MA/cm2. Measurements performed at 4.2 K in 12 T magnetic field have shown Je = 278 A/mm2. The results achieved, combined with the low cost, make the all-MOCVD coated conductors very promising materials for application both at liquid nitrogen and liquid helium temperatures.

Influence of Buffer Layer Surface Morphology on YBCO Critical Current Density Deposited on NiW Tapes

YBCO thin films were grown on NiW tapes under continuous moving deposition process. All of YBCO layer, buffer layer, and cap layer were deposited in a compact reel-to-reel pulsed laser deposition system. Since high critical current density is the most important and effective factor to improve performance/price ratio of coated conductor for large scale applications, we focused our research work on enhancing critical current density of YBCO layers on NiW substrates. It was found that superconducting transport properties of YBCO layers were dependent on not only in-plane texture but also surface morphology of buffer layers, especially surface structure and large particles along grain boundary in NiW substrates. High quality YBCO layers with >; 4.0 × 106 A/cm2 (at 77 K, in zero magnetic field) were fabricated on cap layers with nano-scale surface roughness.

MOCVD of YBCO and Oxide Buffer Layers on Textured Ni-Alloy Tapes

MOCVD technique was applied for deposition of oxide buffer layers onto magnetic and nonmagnetic Ni-alloy tapes at low oxygen partial pressure. YBCO was produced by MOCVD on CSD- and MOCVD-buffer layers. As a result, 1 μm thick YBCO layers were obtained with a critical current density of max. 1.5 MA/cm2, which corresponds to 150 A/cm-width. The new MOCVD setup with of several deposition reactors in a one reel-to-reel line was constructed for the single-pass low-cost production of up to 100 m long and 1 cm wide coated conductors with the tape velocity of 10-20 m/h.

Coated conductor architectures based on IBAD-TiN for high-Jc YBCO films

Ion-beam assisted deposition (IBAD) is one of the major approaches to realise highly textured templates for YBa2Cu3O7−x (YBCO) coated conductors. Whereas IBAD-MgO is mostly used so far in such buffer architectures, TiN is a suitable alternative for this approach as it is showing a similar textured nucleation required for fast processing. Highly textured TiN films were prepared by ion beam assisted pulsed laser deposition on Hastelloy as well as on stainless steel using a Ta0.75Ni0.25 or Y2O3 bed layer. The biaxial texture was preserved to a higher thickness using homoepitaxial growth at higher temperatures and transferred to a second SrZrO3 buffer layer. Finally, YBCO layers with an in-plane and out-of-plane alignment of less than 6° and 3°, respectively, were prepared on the buffered metal tapes leading to critical current densities well above 1MA/cm2. The microstructure of the different architectures was studied in detail.

Advanced development of TFA-MOD coated conductors

American Superconductor is manufacturing 2G wire for initial commercial applications. The 2G wire properties satisfy the requirements for these initial projects; however, improvements in the critical current, field performance and cost are required to address the broad range of potential commercial and military applications. In order to meet the anticipated the performance and cost requirements, AMSC’s R&D effort is focused on two major areas: (1) higher critical current and (2) enhanced flux pinning.AMSC’s current 2G production wire, designed around a 0.8μm thick YBCO layer deposited by a Metal Organic Deposition (MOD) process, carries a critical current in the range of 200–300A/cm-w (77K, sf). Achieving higher critical current requires increasing the thickness of the YBCO layer. This paper describes recent progress at AMSC on increasing the critical current of MOD-YBCO films using processes compatible with low-cost, high-rate manufacturing.

Fabrication of highly textured Ni–7at.%W substrates and further application for coated La2Zr2O7 buffer layer

In this work, highly cube textured Ni–7at.%W (Ni7W) substrate was fabricated from the ingots synthesized by Spark Plasma Sintering (SPS) technique. Both the La2Zr2O7 (LZO) buffer layer and the YBCO superconducting layer were coated using chemical solution deposition (CSD) and pulsed laser deposition (PLD) layer by layer. The full-width-at-half-maximum (FWHM) values of the (111) phi-scan and the (200) ω-scan in rolling direction of the Ni7W substrate were 6.30° and 5.08°, respectively. The EBSD analyses indicated that the percentage of the cube texture component on the surface of the as-obtained Ni7W substrate was 99.4% within a tolerance angle of less than 10°. Highly epitaxial growth of homogenous crack-free LZO film was obtained cube-on-cube relative to the Ni7W substrate, with FWHM values of (222) phi-scan and (400) ω-scan of 7.57° and 5.73°, respectively. The critical temperature of the YBCO layer prepared on LZO/Ni7W substrate is about 90.5K, while the critical current density is 1.25MAcm−2 (77K, sf).

Mode I type delamination fracture toughness of YBCO coated conductor with additional Cu layer

Although interlaminar fracture at a YBa2Cu3O7−δ (YBCO)/CeO2 interface was reported for YBCO coated conductors, this has not yet been investigated by a fracture mechanical approach. In the present study, we developed a mode I type fracture toughness test method for a YBCO coated conductor with an additional Cu layer using double cantilever beam (DCB) specimens. Fracture mechanism was investigated by microscopic observation by a scanning electron microscope (SEM), together with composition analysis by an energy dispersive X-ray spectroscope (EDS). A pre-crack introduced at the YBCO/CeO2 interface deviated from the interface, and propagated into the YBCO layer, and sometimes reached the Ag/YBCO interface. The fracture toughness, GR, for YBCO and the Ag/YBCO interface was evaluated to be 7–10J/m2 and 80–120J/m2, respectively. The complex stress intensity factor ratio, K2/K1, at YBCO/CeO2 interface was evaluated to be −0.19, and this ratio controlled the formation of microcracks in the YBCO layer. The main crack propagated into the YBCO layer accompanied with the formation of microcracks.

Microstructural defects associated with enhanced magnetic flux pinning inYBCO/DyBCO/YBCO multilayered thin films

We investigated the nature of microstructural defects existing inYBCO/DyBCO/YBCO multilayers deposited by pulsed laser deposition onCeO2-buffered sapphire substrates to clarify the origin of magnetic flux pinning enhancementobserved in these films. A relatively thin YBCO film contains high concentrations ofantiphase boundaries (APBs) and short-range stacking faults (SFs) oriented along theab plane. Multilayering thin layers of YBCO with DyBCO results in a similar highly faultedmicrostructure, with APBs and SFs homogeneously distributed throughout thethickness. In addition, multilayers have incorporated elongated defects, parallel to thec-axis direction, emanating from the DyBCO intermediate layer and extending towards thetop YBCO layer. Multilayers have also developed long-range SFs which intersect thec-axis-oriented elongated defects and are mostly concentrated in the region of the DyBCOand the upper YBCO layer, indicating a correlation between the formation of the two typesof defects. The microstructural features of the YBCO/DyBCO/YBCO multilayerscorrelate well with the evaluation of the magnetic field angular dependence ofJc, which revealed a significant enhancement when the field is aligned along theab plane .

Influence of the ferromagnetic layer on the pair breaking and low alternating current field magnetic response in superconductor/ferromagnet bilayer

We investigate the influence of the ferromagnetic layer on the magnetic and transport properties of YBa2Cu3O7−δ in YBa2Cu3O7−δ (YBCO)/La0.7Sr0.3MnO3 (LSMO) bilayers. The temperature dependent dc magnetization study reveals the presence of magnetic anisotropy in YBCO/LSMO bilayer as compared to the pure YBCO layer. The ac susceptibility study on YBCO/LSMO bilayers reveals stronger pinning and the temperature dependent critical current is found to be less prone to temperature. Besides, the current (I) dependent electrical transport studies on YBCO/LSMO exhibit a significant reduction in the superconducting Tc with increase in I and it follows I2/3 dependence in accord with the pair breaking effect. The higher reduction of superconducting Tc in YBCO/LSMO is believed to be due to the enhanced pair-breaking induced by the spin polarized carriers being injected into the superconductor.

Stray field and the superconducting surface spin valve effect in La0.7Ca0.3MnO3/YBa2Cu3O7−δ bilayers

Electronic transport and magnetization measurements were carried out on La0.7Ca0.3MnO3/YBa2Cu3O7−δ (LCMO/YBCO) bilayers below the superconducting transition temperature in order to study the interaction between magnetism and superconductivity. This study shows that a substantial number of weakly pinned vortices are induced in the YBCO layer by the large out-of-plane stray field in the domain walls. Their motion gives rise to large dissipation peaks at the coercive field. The angular dependent magnetoresistance (MR) data reveal the interaction between the stripe domain structure present in the LCMO layer and the vortices and anti-vortices induced in the YBCO layer by the out-of-plane stray field. In addition, this study shows that a superconducting surface spin valve effect is present in these bilayers as a result of the relative orientation between the magnetization at the LCMO/YBCO interface and the magnetization in the interior of the LCMO layer that can be tuned by the rotation of a small H. This latter finding will facilitate the development of superconductive magnetoresistive memory devices. These low-magnetic-field MR data, furthermore, suggest that triplet superconductivity is induced in the LCMO layer, which is consistent with recent reports on triplet superconductivity in LCMO/YBCO/LCMO trilayers and LCMO/YBCO bilayers.

Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetizationrelaxation measurements

We investigated the relaxation of the irreversible magnetization in a series of 200 nm thickYBa2Cu3O7/PrBa2Cu3O7 [(YBCO)n/(PrBCO)m] superlattices, wherethe thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept tom = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thicknessn of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standardzero-field-cooling dc magnetization relaxation data obtained in the low temperatureT region with the appliedmagnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at lowT–plastic vortexcreep at high T,generated by the T dependent macroscopic currents induced in the sample during measurements. For thin superlattices(n < 20) the creep crossover temperature , and Tcr decreaseslinearly with increasing ln(H) for a fixed n. This crossover represents an alternative to the elastic vortex glass behaviorreported for superlattices, as well as to ‘quantum vortex creep’ at unexpectedly highT inferred for thin films. We also discuss the absence of an increase of the magneticallydetermined critical current density with decreasing YBCO thickness in our superlattices,which apparently contradicts the collective pinning theories.

Macroscopic Elastic Constant Analysis and Strain Behavior of Crystal on the YBCO Layer in a YBCO-coated Conductor

YBCO-coated conductor tape is a composite material that consists of two or more layers. The superconducting layer is generally a thin film. In this study, the YBCO consisted of columnar aggregates of orthorhombic crystals with a texture whose axis was 001 to the film surface. Due to the difficulty of obtaining such a thin film for mechanical testing, the mechanical elastic constant is unknown. The elastic constant of the thin film can be obtained only via analysis. In this study, finite element models of bi-textured thin films were constructed. Through the analysis, the uniaxial mechanical elastic constant and in-plane Poisson's ratio were obtained. The strain dependence of the elastic properties of the YBCO-coated conductor with surrounding Cu stabilized were investigated precisely using white X-ray diffraction. The diffraction elastic constants estimated from (0k0) were larger than those estimated from (h00), and this tendency corresponded to the FEM analysis. Using X-ray diffraction, the obtained diffraction strain was smaller than the macroscopic strain. The diffraction strain - macroscopic strain slope theoretically become one. Strain relaxation will result if the slope does not become one. Our observations strongly suggest that strain relaxation is caused in the interface of twin crystals. The effective mechanical elastic constant was predicted by correcting the calculated mechanical elastic constant.

Critical current degradation of shortYBa2Cu3O7 − δ coated conductor due to an unprotected quench

The critical current of a short YBa2Cu3O7 − δ (YBCO) coated conductor sample degrades in an unprotected quench performed in anearly adiabatic environment at 30 K. The conductor has Cu stabilizers on both surfaces.The quench is initiated by a heater attached to the sample surface. The amplitude ofthe transport current is fixed as 91% of the sample’s initial critical current. Theduration of the current is increased to simulate an unprotected quench and to reachincreasing and controlled voltage and temperature levels. A peak temperature of490 ± 50 K and a heatingrate of 1800 K s − 1 are measured when the critical current degrades by ∼ 5%. The applied thermal strain on the YBCO layer from 30 to490 K is estimated to be 0.31% and is applied at a strain rate of ∼ 1% s − 1.The rate of temperature change and the time to reach a certain peak temperature, determinedby the current density in the Cu stabilizer, are estimated assuming adiabatic conditions basedon the short sample case. For a Cu stabilizer current density ranging from 1000 to 2000 A mm − 2, achievedin commercial conductors currently available, the quench detection and protection requires a responsetime < 200 ms to limit the peak temperature below 200 K. A Cu stabilizer current density higher than 3000 A mm − 2 may challenge the existing detection and protection techniques for the same 200 K limit.Integrating the substrate as part of the stabilizer may help reduce the stabilizer currentdensity to gain more time for quench detection and protection while maintaining theengineering current density.