[Show abstract][Hide abstract] ABSTRACT: Applications of (RE = Y, Gd)BCO coated conductors for the generation of high magnetic fields are increasing sharply, this while (RE)BCO coated conductors themselves are evolving rapidly. This article describes and demonstrates recently developed and applied mathematical models that systematically and comprehensively characterize the transport critical current angular dependence of a batch of (RE)BCO coated conductor in high magnetic fields at fixed temperatures with an uncertainty of 10% or better. The model development was based on analysis of experimental data sets from various published sources and coated conductors with different microstructures. These derivations directly are applicable to the accurate prediction of the performance in high magnetic fields of coils wound with (RE)BCO coated conductors. In particular, a nonlinear fit is discussed in this article of transport critical current at T = 4.2 K versus field and angle data. This fit was used to estimate the hysteresis losses of (RE)BCO coated conductors in high magnetic fields, and to design the inserts wound with such conductors of the all-superconducting 32 T magnet being constructed at the NHMFL. A series of such fits, recently developed at several fixed temperatures, continues to be used to simulate the quench behavior of that magnet.
[Show abstract][Hide abstract] ABSTRACT: We have developed TiO2 coating on Ag-alloy sheathed Bi2Sr2CaCu2O8−x (Bi-2212) round-wire conductor for electrical insulation in Bi-2212 magnets. The green coating has a base layer comprised of TiO2, polyvinyl butyral (PVB) and a small amount of polysilicate and a top layer made of polyacrylic. The coating was applied on the conductor using a continuous reel-to-reel dip coating process and showed very good adherence and flexibility that is suitable for magnet coil winding. The thickness of the coating is a function of slurry viscosity, wire withdrawal speed and wire radius. Small test coils were built with the coated Bi-2212 round-wires and were heat treated at 100 atm pressure. During the heat treatment, the PVB and polyacrylic were removed from the green coating and the polysilicate decomposed to SiO2 that served as a sintering aid for TiO2. After the heat treatment, the coating remained strongly adhered to the conductor and did not have a detrimental effect on the critical current (I c) values. The breakdown voltage was about 150 V across a 7 μm thick heat treated coating on Bi-22112 round-wire conductor, corresponding to a dc dielectric strength of about 21 MV m−1.
[Show abstract][Hide abstract] ABSTRACT: Second-generation high-temperature superconductors (2G HTS) have high current density in very high magnetic fields. They are good candidates for high field magnets, especially when the magnetic field exceeds the critical fields of low-temperature superconductors. However, the thin and flat geometry of these conductors allows persistent screening currents (or shielding currents) to flow in the conductors. The screening currents caused by the ramping of applied current to the coil is identified as the self-field screening effect. The screening-current-induced magnetic field changes the magnetic field distribution of the magnet, and it also generates drift. This paper employs both experimental and numerical methods to study the mechanism of self-field screening currents for 2G HTS magnets. A 2G HTS magnet was constructed and tested, and a finite element model was built based on the magnet. The comparison between calculation and measurement is presented with detailed analysis. Current distributions inside the HTS magnet are calculated to illustrate the effects of screening. The screening-current-induced magnetic field is quantified by comparing the magnetic field distribution with a baseline copper model. The model is also used to explain the mechanism of the current sweep strategy, which can be used to effectively eliminate screening currents.
[Show abstract][Hide abstract] ABSTRACT: Magnets are the principal market for superconductors, but making attractive conductors out of the high-temperature cuprate superconductors (HTSs) has proved difficult because of the presence of high-angle grain boundaries that are generally believed to lower the critical current density, Jc. To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O7-x and (Bi, Pb)2Sr2Ca2Cu3O10-x are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O8-x (Bi-2212) can be made in the much more desirable isotropic, round-wire, multifilament form that can be wound or cabled into arbitrary geometries and will be especially valuable for high-field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology. An appealing attribute of this Bi-2212 conductor is that, being without macroscopic texture, it contains many high-angle grain boundaries but nevertheless attains a very high Jc of 2,500 A mm(-2) at 20 T and 4.2 K. The large potential of the conductor has been demonstrated by building a small coil that generated almost 2.6 T in a 31 T background field. This demonstration that grain boundary limits to high Jc can be practically overcome underlines the value of a renewed focus on grain boundary properties in non-ideal geometries.
Nature Material 03/2014; 13(4). DOI:10.1038/nmat3887 · 36.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Conductor insulation is one of the key components needed to make Ag-alloy clad Bi2Sr2CaCu2O8+x (Bi-2212/Ag) superconducting round wire (RW) successful for high field magnet applications, as dielectric standoff and high winding current densities (Jw) directly depend on it. In this study, a TiO2–polymer insulation coating developed by nGimat LLC was applied to test samples and a high field test coil. The insulation was investigated by differential thermal analysis (DTA), thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), dielectric property measurement, and transport critical current (Ic) property measurement. About 29% of the insulation by weight is polymer. When the Bi-2212/Ag wire is fully heat treated, this decomposes with slow heating to 400 ° C in pure O2. After the full reaction, we found that the TiO2 did not degrade the critical current properties, adhered well to the conductor, and provided a breakdown voltage of more than 100 V, which allowed the test coil to survive quenching in 31.2 T background field, while providing a 2.6 T field increment. For Bi-2212/Ag RW with a typical diameter of 1.0–1.5 mm, this ~15 μm thick insulation allows a very high coil packing factor of ~0.74, whereas earlier alumino-silicate braid insulation only allows packing factors of 0.38–0.48.
[Show abstract][Hide abstract] ABSTRACT: We report here that magnetic fields of almost 34 T, far above the upper 24 T
limit of Nb3Sn, can be generated using a multifilament round wire conductor
made of the high temperature cuprate superconductor Bi2Sr2CaCu2O8-x (Bi-2212).
A remarkable attribute of this Bi-2212 conductor is that it does not exhibit
macroscopic texture and contains many high angle grain boundaries but
nevertheless attains very high superconducting critical current densities Jc of
2500 A/mm2 at 20 T and 4.2 K. This Bi-2212 conductor does not possess the
extreme texture that high Jc coated conductors of REBa2Cu3O7-x (REBCO) require,
avoiding also its high aspect ratio, large superconducting anisotropy and the
inherent sensitivity to defects of a single filament conductor. Bi-2212 wires
can be wound or cabled into almost any type of superconducting magnet and will
be especially valuable for very high field NMR magnets beyond the present 1 GHz
proton resonance limit of Nb3Sn technology. This demonstration that grain
boundary limits to high Jc can be practically overcome suggests the huge value
of a renewed focus on grain boundary properties in non-ideal geometries,
especially with the goal of translating the lessons of this Bi-2212 conductor
into fabrication of multifilament round wire REBCO or Fe-based superconductors.
[Show abstract][Hide abstract] ABSTRACT: It is well known that longer Bi-2212 conductors have significantly lower
critical current density (Jc) than shorter ones, and recently it has become
clear that a major cause of this reduction is internal gas pressure generated
during heat treatment, which expands the wire diameter and dedensifies the
Bi-2212 filaments. Here we report on the length-dependent expansion of 5 to 240
cm lengths of state-of-the-art, commercial Ag alloy-sheathed Bi-2212 wire after
full and some partial heat treatments. Detailed image analysis along the wire
length shows that the wire diameter increases with distance from the ends,
longer samples often showing evident damage and leaks provoked by the internal
gas pressure. Comparison of heat treatments carried out just below the melting
point and with the usual melt process makes it clear that melting is crucial to
developing high internal pressure. The decay of Jc away from the ends is
directly correlated to the local wire diameter increase, which decreases the
local Bi-2212 filament mass density and lowers Jc, often by well over 50%. It
is clear that control of the internal gas pressure is crucial to attaining the
full Jc of these very promising round wires and that the very variable
properties of Bi-2212 wires are due to the fact that this internal gas pressure
has so far not been well controlled.
[Show abstract][Hide abstract] ABSTRACT: Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable
property of powder-in-tube conductors is that there is about 30% void space in
the as-drawn wire. We have recently shown that the gas present in the as-drawn
Bi-2212 wire agglomerates into large bubbles and that they are presently the
most deleterious current limiting mechanism. By densifying short 2212 wires
before reaction through cold isostatic pressing (CIPping), the void space was
almost removed and the gas bubble density was reduced significantly, resulting
in a doubled engineering critical current density (JE) of 810 A/mm2 at 5 T, 4.2
K. Here we report on densifying Bi-2212 wire by swaging, which increased JE
(4.2 K, 5 T) from 486 A/mm2 for as-drawn wire to 808 A/mm2 for swaged wire.
This result further confirms that enhancing the filament packing density is of
great importance for making major JE improvement in this round-wire magnet
[Show abstract][Hide abstract] ABSTRACT: Wind-and-react coils using alumino-silicate-braid-insulated Bi2Sr2CaCu2O8+x (Bi-2212) round wire consistently show reduced critical current densities (J c) compared to short samples heat treated under nominally identical conditions. This is a significant detriment to the design of Bi-2212 insert coils for high field magnets. Here we report on the superconducting properties of a series of wind-and-react test coils and short conductor samples systematically extracted from various sections of some of these wind-and-react coils. Overall we find remarkable uniformity of the superconducting properties throughout a majority of each coil, even though the J c of the coil is markedly below that of short samples processed at the same time. Analysis of the critical temperature (T c) and the irreversibility field (H irr) shows that these J c variations are solely related to changes in connectivity within the Bi-2212 filaments, rather than to any variations of oxygen uptake within the coil winding during heat treatment. We conclude that the reduced J c of the coils is related to a sample length dependence of J c rather than to any lack of full oxygenation of even a 20-layer coil. Raising the J c of the coils thus appears to be a problem that needs addressing chiefly at the strand rather than the coil level.
[Show abstract][Hide abstract] ABSTRACT: It is well known that the critical current density J c of Ag-sheathed Bi 2 Sr 2 CaCu 2 O x (2212) varies strongly with heat treatment details, particularly the maximum processing temperature T max , but the mechanism for such J c variations and how the processing window can be widened remain unknown. We systematically measured the J c and electromagnetic properties of a powder-in-tube Ag-sheathed multifilamentary Bi 2 Sr 2 CaCu 2 O x (2212) round wire processed with the maximum processing temperature T max ranging from 887 to 900 • C and the time at the maximum temperature t max from 0 to 3 h using three representative heat treatment schedules. We found that J c correlates weakly to T max , but it correlates strongly to the time in the melt t melt , a processing parameter that has not been explicitly considered before. J c is rather insensitive to T max in the temperature range 887–900 • C and the true cause of J c declining with high T max appears to be the long t melt that leads to collapse of filament structure. By tuning t melt we were able to widen the T max window to 10 • C. The J c –t melt correlation, as well as quench studies, indicate that J c is controlled by complex diffusion processes occurring in the melt (filament bonding, bubble agglomeration, and perhaps Cu loss). Our findings highlight t melt as an important processing parameter for optimizing J c and may serve as a general guide for heat treating 2212 coils.
[Show abstract][Hide abstract] ABSTRACT: To explore the limits of layer wound (RE)Ba2Cu3O7-x (REBCO, RE = Rare Earth)
coils in a high magnetic field environment > 30 T, a series of small insert
coils have been built and characterized in background fields. One of the coils
repeatedly reached 35.4 T using a single ~100 m length of REBCO tape wet wound
with epoxy and nested in a 31 T background magnet. The coil was quenched safely
several times without degradation. Contributing to the success of this coil was
the introduction of a thin polyester film that surrounded the conductor. This
approach introduces a weak circumferential plane in the coil pack that prevents
conductor delamination that has caused degradation of several epoxy impregnated
coils previously made by this and other groups.
[Show abstract][Hide abstract] ABSTRACT: The uniaxial strain dependence of critical current was measured both in tension and compression in Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> (Bi-2212) high-temperature superconducting round wires. Permanent damage to the critical current easily occurred due to strain. To improve the electromechanical properties of Bi-2212 wires, development of stronger sheathing materials is needed. Ideal materials would be not only mechanically strong, but also chemically compatible with Bi-2212 during the final heat treatment. To identify such materials, we measured stress-strain properties of some new Ag alloys and extracted their respective Young's modulus values and yield strength. The database may be useful for development of new Bi-2212 strands for fabricating high-field superconducting magnets above 20 T.
[Show abstract][Hide abstract] ABSTRACT: It is well known that the critical current density J c of multifilamentary Bi-2212 wires tends to decline as the wire length increases, but the reasons for and the magnitude of this decline remain obscure and quantitatively unpredictable. Here we report on the J c and mass density variation with length on ∼1 m long samples taken from two recent and representative wires, in which we find a strong decrease of J c with distance from the end and a strong correlation between J c and the local mass density. The mass density variations occur on length scales of centimeters, many times the nominal 15 μm filament diameter. The cause of the mass density variation appears to be internal gas pressure that generates bubbles which almost fill the filament diameter when the Bi-2212 melts. Control of this internal pressure seems to be vital to moderating or avoiding the length dependence of J c .
[Show abstract][Hide abstract] ABSTRACT: We have recently shown that the gas present in the only ∼70% dense filaments of as-drawn Bi-2212 wire agglomerates into large bubbles that fill the entire filament diameter during the melt phase of the heat treatment. Once formed, these bubbles never disappear, although they can be bridged by 2212 grains formed on cooling. In order to test the effect of these bubbles on the critical current I c , we increased the density of the filaments after drawing using 2 GPa of cold isostatic pressure, finding that the bubble density and size were greatly reduced and that I c could be at least doubled. We conclude that enhancement of the filament packing density is of great importance for making major I c improvements in this very useful, round superconducting wire.
[Show abstract][Hide abstract] ABSTRACT: A dual-solenoid magnet system was designed and fabricated for characterizing high temperature superconducting tapes and coils. The magnet coils were made of copper wire and were fabricated using a new winding approach, which enables multi-layer and epoxy-free coils. It was demonstrated that the magnet system provided a uniform AC magnetic field in radial and axial orientations on superconducting rings fabricated from 40 mm wide second generation (2G) high temperature superconducting tape. AC losses were measured on the superconducting rings with variable frequency AC magnetic field applied in both the radial and axial orientations. The versatility of the magnet and the AC loss measurement system is useful for generating the necessary AC loss data for applications such as transformers, inductive fault current limiters, and magnetic shields.
[Show abstract][Hide abstract] ABSTRACT: The NHMFL has had a long running program to develop Bi 2 Sr 2 CaCu 2 O x (Bi2212) for high field magnets. The recent development of round wire Bi2212 (RW2212) has strengthened the effort to develop solenoid magnets with fields substantially greater than can be achieved with Nb 3 Sn. The present paper briefly summarizes some of the results obtained at the NHMFL in the past 12 months. It summarizes the talk given by David Larbalestier at WAMSDO on May 24, 2008. Much of the work is ongoing and will be reported in the normal peer reviewed literature in late 2008.
[Show abstract][Hide abstract] ABSTRACT: The critical current of a short YBa2Cu3O7 − δ (YBCO) coated conductor sample degrades in an unprotected quench performed in a nearly 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 of the transport current is fixed as 91% of the sample's initial critical current. The duration of the current is increased to simulate an unprotected quench and to reach increasing and controlled voltage and temperature levels. A peak temperature of 490 ± 50 K and a heating rate of 1800 K s − 1 are measured when the critical current degrades by ~ 5%. The applied thermal strain on the YBCO layer from 30 to 490 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, determined by the current density in the Cu stabilizer, are estimated assuming adiabatic conditions based on the short sample case. For a Cu stabilizer current density ranging from 1000 to 2000 A mm − 2, achieved in commercial conductors currently available, the quench detection and protection requires a response time < 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 current density to gain more time for quench detection and protection while maintaining the engineering current density.
[Show abstract][Hide abstract] ABSTRACT: Development of high-field magnets using high temperature superconductors (HTS) is a core activity at the NHMFL. Magnet technology based on both YBCO-coated tape conductors and Bi-2212 round wires is being pursued. Two specific projects are underway. The first is a user magnet with a 17 T YBCO coil set which, inside an LTS outsert, will generate a combined field of 32 T. The second is a 7 T Bi2212 demonstration coil set to be operated in a large bore resistive magnet to generate a combined magnetic field of 25 T. Owing to the substantial technological differences of the two conductor types, each project faces different conductor and magnet technology challenges. Two small coils have been tested in a 38-mm cold bore cryostat inserted in a 31 T resistive magnet: a Bi2212 round-wire layer-wound insert coil that generated 1.1 T for a total of 32.1 T and a YBCO double-pancake insert that generated 2.8 T for a total central field of 33.8 T. Four larger layer-wound coils have been manufactured and tested in a 20 T, 186-mm cold bore resistive magnet: a sizeable Bi-2212 coil and three thin large-diameter YBCO coils. The test results are discussed. The current densities and stress levels that these coils tolerate underpin our conviction that >30 T all-superconducting magnets are viable.
[Show abstract][Hide abstract] ABSTRACT: A small coil was made and tested for the purpose of magnet technology development for YBa2Cu3O7−x (YBCO) high field coils. The coil has a winding inner diameter of 24 mm, outer diameter of 36 mm, and length of 46 mm in a pancake wound construction of 5 double pancakes. The YBCO conductor is SuperPower SCS4050 with 2×20 μm of copper stabilizer. The coil was tested in the high field user facility of the NHMFL in a 31 T background field magnet. At this background field, the coil was operated to a current of 325 A and produced a field increment of 2.8 T for a record total field of 33.8 T. The average current density in the windings at full current was very high at 446 A/mm2. The test was complicated by the accumulation of trapped helium gas about the coil due to the diamagnetic effect of helium at high field. Even though the coil operated at elevated temperature during the high field test, the coil performance was limited by mechanical degradation and not a critical current limit.
[Show abstract][Hide abstract] ABSTRACT: Presentamos un trabajo de investigación en colaboración con la Universidad de Texas en El Paso y El Laboratorio Nacional de Altos Campos Magnéticos. Este trabajo presenta una serie de mediciones de pérdidas de potencial (AC) en una cinta multi-filamentaria de Bi2223/Ag utilizando un método calorimétrico descrito por Ashworth and Suenaga. Las pérdidas de corriente (AC) son investigadas en bajas temperaturas usando nitrógeno líquido (77 K) para diferentes corrientes AC en un rango de frecuencia de 50Hz a 500 Hz. Los resultados son comparados con el modelo de elíptico Norris para calcular las pérdidas por campo directo en cintas superconductivas de alta temperatura.
Revista Mexicana de Fisica 01/2010; · 0.34 Impact Factor