Bi-2212 Wires Research Articles

Synchrotron Microtomography Investigation of the Filament Microstructure in Differently Processed Bi-2212 Wires

The Bi-2212 microstructure and porosity distribution in entire state-of-the-art multifilament wires was characterized by high-energy synchrotron X-ray absorption microtomography after meltprocessing at ambient pressure, at 100 bar overpressure, and at ambient pressure after prior cold isostatic pressing of initially identical as-drawn wires. Beyond the visualization of the Bi-2212 microstructure, the porosity distribution in the different filament bundles has been quantified. Spherical granulometry and finite percolation probability calculations have been performed in order to provide a measure for the porosity distribution and to compare filament connectivity in the axial wire direction. Possible artifacts of the microtomography (μ-CT) experiment and the possibility to estimate the effect of the Bi-2212 microstructure as seen in the μ-CT images on critical current density are discussed.

Loss Measurement in a Cryogen-Free Bi-2223 HTS Dipole Magnet for Beam Line Switching

High-temperature superconducting (HTS) magnets have several advantages for ac and pulse operation due to having a high critical temperature. However, fast magnetic field changes generate severe ac losses and eddy current losses. Therefore, loss estimation and reduction is a significant issue for HTS magnet cooling design. For future cooling design, we have measured losses of a Bi-2223 HTS magnet for beamline switching in the cyclotron facility at the Research Center of Nuclear Physics, Osaka University. The time-varying magnetic field losses for triangular wave excitation with a perpendicular magnetic field amplitude of 0.8 T in wire and a minimum period of 42 s were obtained from measured cryocooler temperatures and preliminary measured cooling capacity curve. The eddy current losses of the metal components were calculated using transient electromagnetic finite element analysis. The ac losses of the Bi-2223 wire were derived from the measured time-varying magnetic field losses and the calculated eddy current losses. The frequency dependence of their losses indicates the importance of accurate evaluation and reduction of the eddy current losses for the future magnet with higher charging speed.

On the role of precursor powder composition in controlling microstructure, flux pinning, and the critical current density of Ag/Bi2Sr2CaCu2Ox conductors

Precursor powder composition is known to strongly affect the critical current density (Jc) of Ag/Bi2Sr2CaCu2Ox (Bi-2212) wires. However, reasons for such Jc dependence have not yet been fully understood, compromising our ability to achieve further optimization. We systematically examined superconducting properties, microstructural evolution and phase transformation, and grain boundaries of Bi-2212 conductors fabricated from precursor powders with a range of compositions using a combination of transport-current measurements, a quench technique to freeze microstructures at high temperatures during heat treatment, and scanning transmission electron microscopy (STEM). Samples include both dip-coated tapes and round wires, among which a commercial round wire carries a high Jc of 7600 A mm−2 at 4.2 K, self-field and 2600 A mm−2 at 4.2 K, 20 T, respectively. In the melt, this high-Jc conductor, made using a composition of Bi2.17Sr1.94Ca0.89Cu2Ox, contains a uniform dispersion of fine alkaline-earth cuprate (AEC) and copper-free solid phases, whereas several low-Jc conductors contain large AEC particles. Such significant differences in the phase morphologies in the melt are accompanied by a drastic difference in the formation kinetics of Bi-2212 during recrystallization cooling. STEM studies show that Bi-2212 grain colonies in the high-Jc conductors have a high density of Bi2Sr2CuOy (Bi-2201) intergrowths, whereas a low-Jc conductor, made using Bi2.14Sr1.66Ca1.24Cu1.96Ox, is nearly free of them. STEM investigation shows grain boundaries in low-Jc conductors are often insulated with a Bi-rich amorphous phase. High-Jc conductors also show higher flux-pinning strength, which we ascribe to their higher Bi-2201 intergrowth density.

Effects of filament size on critical current density in overpressure processed Bi-2212 round wire.

Bi2Sr2CaCu2Ox (Bi-2212) conductor is the only high temperature superconductor manufactured as a round wire and is a very promising conductor for very high field applications. One of the key design parameters of Bi-2212 wire is its filament size, which has been previously reported to affect the critical current density (Jc ) and ac losses. Work with 1 bar heat treatment showed that the optimal filament diameter was about 15 μm but it was not well understood at that time that gas bubbles were the main current limiting mechanism. Here we investigated a recent Bi-2212 wire with a 121×18 filament architecture with varying wire diameter (1.0 to 1.5 mm) using 50 bar overpressure processing. This wire is part of a 1.2 km piece length of 1.0 mm diameter made by Oxford Superconducting Technology. We found that Jc is independent of the filament size in the range from 9 to 14 μm, although the n value increased with increasing filament size. A new record Jc (4.2 K, 15 T) of 4200 A/mm2 and JE (4.2 K, 15 T) of 830 A/mm2 were achieved.

High critical current density Bi2Sr2CaCu2Ox/Ag wire containing oxide precursor synthesized from nano-oxides

Bi2Sr2CaCu2Ox (Bi2212)/Ag-alloy wires are manufactured via the oxide-powder-in-tube route by filling Ag/Ag-alloy tubes with Bi2212 oxide precursor, deforming into wire, restacking and heat treating using partial-melt processing (PMP). Recent studies propose several requirements on precursor properties, including stoichiometry, chemical homogeneity, carbon content and phase purity. Here, nanosize oxides produced by nGimat’s proprietary NanoSpray CombustionTM process are used as starting materials to synthesize Bi2212 oxide precursors via solid-state calcination. Oxide powders for wire fill (precursor powder) with precisely controlled stoichiometry and chemical homogeneity containing over 99 vol% of single Bi2212-phase are synthesized. Alkaline-earth cuprate are found to be the only impurity phase in the precursor powders. Phase transformation, carbon release and grain growth during calcination are studied through a series of quench studies. Effects of particle size, surface area, stoichiometry, chemical homogeneity and microstructures of the starting materials on Bi2212 formation and wire transport properties are discussed. Small particle size, high surface area and short diffusion length of the starting materials result in a rapid and homogeneous phase transformation to Bi2212, along with an early and rapid carbon release. The residual carbon in the precursor powder is between 50 and 90 ppm. The strong dependence of transport Jc on precursor stoichiometry indicates that compositional variations within precursor powders should be less than 1.5 mol%. Two Bi-rich and Ca-deficient stoichiometries give higher wire transport critical current density, with the highest being 2520 A mm−2 (4.2 K, 5 T) after 1 bar PMP and 4560 A mm−2 (4.2 K, 5 T) after 100 bar overpressure (OP) processing. The low residual carbon content results in smaller and fewer voids within an OP-processed wire filament. Bi-rich and Ca-deficient stoichiometries and small compositional variations within precursor powders may be a method for engineering uniformly-distributed and high-density Bi2201 intergrowths within Bi2212 grains after PMP.

AC Loss Characteristics of HTS Wires Carrying Currents With Different Waveforms

When applying high-temperature superconductor (HTS) devices in power systems, ac loss is a significant obstacle from an economic perspective. Many studies have been performed to investigate this issue, but the ac losses for different current waveforms have rarely been reported. However, in power systems, many nonlinear devices can distort current waveforms. For example, currents of converters have ripples, no-load currents of transformers are peaked waveforms, and currents of motors are normally nonsinusoidal. To investigate the influence of current waveforms on ac losses, three types of currents with different waveforms, namely, sine waves, peaked waves, and flat-topped waves, are studied using three types of HTS wires, namely, Bi2223 wire, YBCO wire with nonmagnetic substrate (Hastelloy C276), and YBCO wire whose substrate is ferromagnetic (Ni-5at.%W). The experimental results show that ac losses of HTS wires are independent of frequency when carrying sine currents and that they are dependent on frequency when carrying peaked and flat-topped wave currents. Moreover, for all types of measured wires, particularly YBCO wire with ferromagnetic substrate, measured ac losses are highly dependent on current waveforms. In the end, the authors provide a discussion for the experiment phenomenon from the point of magnetic flux creep.

Modeling effects of gas bubbles on the mechanical behaviors of Ag/Bi-2212 round wires using a double cantilever beam bridge model

Due to the larger current-carrying property, Bi2Sr2CaCu2Ox (Bi2212) superconductors have a great potential application in high field magnet. Bi2212 superconducting material can be fabricated as an isotropic round wire. However, there is 30% void space in the wire, such as gas bubbles. The void space has a larger influence on the property of the wire. In this paper, we will study the effect of gas bubble on the fracture behavior. Based on the double cantilever beam model and critical state theory, the mechanical behavior of Bi2212 wire is studied for decreasing field. Two different damage mechanisms are discussed using the strain energy release rate and strain of bridge. The results show that the large gas bubble can increase the strain of bridge. The central filaments with gas bubble are easier to be damaged than the edge filaments with gas bubble.

Comparison of Electromechanical Properties and Lattice Distortions of Different Cuprate High-Temperature Superconductors

The electromechanical properties of different cuprate high-temperature superconductors, notably two ReBCO tapes, a reinforced and a nonreinforced Bi-2223 tape, and a Bi-2212 wire, have been studied. The axial tensile stress and strain, as well as the transverse compressive stress limits at which an irreversible critical current degradation occurs, are compared. The experimental setup has been integrated in a high-energy synchrotron beamline, and the self-field critical current and lattice parameter changes as a function of tensile stress and strain of a reinforced Bi-2223 tape have been measured simultaneously. Initially, the Bi-2223 filaments exhibit nearly linear elastic behavior up to the strain at which an irreversible degradation is observed. At 77 K, an axial Bi-2223 filament precompression of 0.09% in the composite tape and a Bi-2223 Poisson ratio ν = 0.21 have been determined.

Influence of the Oxygen Partial Pressure on the Phase Evolution During Bi-2212 Wire Melt Processing

We have studied the influence of the oxygen partial pressure $\text{pO}_{2}$ up to 5.5 bar on the phase changes that occur during the melt processing of a state-of-the-art Bi-2212 multifilamentary wire. Phase changes have been monitored in situ by high energy synchrotron X-ray diffraction. We found that the stability of the Bi-2212 phase is reduced with increasing $\text{pO}_{2}$ . For $\text{pO}_{2} > \text{bar}$ , a significant amount of the Bi-2212 phase decomposes upon heating in the range of 400 °C–650 °C. The extent of decomposition strongly increases with increasing pO2, and at $\text{pO}_{2}=5.5 \text{bar}$ Bi-2212 completely decomposes in the solid state. Textured Bi-2212 can be formed during solidification when pO2 is reduced to 0.45 bar when the precursor is molten. Since the formation of current-limiting second phases is very sensitive to pO2 when it exceeds 1 bar, we recommend to reduce the oxygen partial pressure below the commonly used $\text{pO}_{2}=1 \text{bar}$ , in order to increase the pO2 margins and to make the overpressure process more robust.

New concept for the development of Bi-2212 wires for high-field applications

The first step towards high critical currents in Bi-2212 wires was recognizing that the supercurrent is blocked over long lengths by filament-diameter bubbles grown during the melt stage, which cause expansion of the wire diameter and dedensification of the superconducting filaments. While a succesful approach to reducing the problem of voids related to bubbles involved the application of a high overpressure during the heat treatment, we fabricated Bi-2212 wires by applying a new concept of suitably alternating groove-rolling and drawing techniques with the aim of densifying the phase during the working procedure prior to the heat treatment. We here for the first time were able to reach, in wires reacted with closed ends—i.e. with gas trapped in the wire as it happens in long length wires—the very same values of critical current shown in short wires reacted with open ends. This is the irrefutable evidence that, only by acting on the deformation technique, we were able to raise the critical current by properly densifying the superconducting powder inside the filaments already before the melt stage. Whole-conductor current densities in our long-length simulation wires already reach 400 A mm−2 at 4.2 K and 5 T, which can be still easily increased through architecture optimization. The actual breakthrough is that the densification is optimized without further complex treatments through a technique which can be straightforwardly applied to long length wires.

Quench degradation limit of multifilamentary Ag/Bi2Sr2CaCu2Ox round wires

Understanding safe operating limits of composite superconducting wires is important for the design of superconducting magnets. Here we report measurements of quench-induced critical current density Jc degradation in commercial Ag/Bi2Sr2CaCu2Ox (Bi-2212) round wires using heater-induced quenches at 4.2 K in self magnetic field that reveal a general degradation behavior. Jc degradation strongly depends on the local hot spot temperature Tmax, and is nearly independent of operating current, the temperature gradient along the conductor dTmax/dx, and the temperature rising rate dTmax/dt. Both Jc and n value (where n is an index of the sharpness of the superconductor-to-normal transition) exhibit small but irreversible degradation when Tmax exceeds 400–450 K, and large degradation occurs when Tmax exceeds 550 K. This behavior was consistently found for a series of Bi-2212 wires with widely variable wire architectures and porosity levels in the Bi-2212 filaments, including a wire processed using a standard partial melt process and in which Bi-2212 filaments are porous, an overpressure processed wire in which Bi-2212 filaments are nearly porosity-free and that has a Jc(4.2 K, self field) exceeding 8000 A mm−2, and a wire that has nearly no filament to filament bridges after reaction. Microstructural observations of degraded wires reveal cracks in the Bi-2212 filaments perpendicular to the wire axis, indicating that the quench-induced Ic degradation is primarily driven by strain. These results further suggest that the quench degradation temperature limit depends on the strain state of Bi-2212 filaments and this dependence shall be carefully considered when engineering a high-field Bi-2212 magnet.

高強度 Ag シース Bi-2223線材の開発

高強度 Ag シース Bi-2223線材の開発

Effect of epoxy impregnation on strain distribution of materials in Bi2223 superconducting coils by using synchrotron X-ray diffraction

Synchrotron X-rays have been used to observe strain distributions in composite materials such as superconducting wires with a thickness of less than about 2 mm. In applications that employ wound coils of superconducting wire, it is necessary to understand the strain distribution within the coiled wire. Superconducting wires such as Bi2223 and REBCO wires approximately 4–5 mm wide are commercially available. Coiled wires of this width are too thick to easily measure using conventional X-ray techniques, especially the inner strain, because the penetration depth tends to be around 2 mm. Therefore, the beam penetration must be improved, and it is known that the penetration depth of an X-ray beam depends upon the beam energy, beam intensity, measurement material, and measurement method. In this study, we used a white X-ray diffractometer at SPring-8 to develop a method of observing the strain distribution in Bi2223 superconducting coils winded by a 4.5 mm wide Bi2223 wire. We successfully observed a clear (400) peak of the Bi2223 phase by an appropriate measurement condition, and then observed the strains of each material in the Bi2223 coils with and without epoxy impregnation. This is the first time that we have obtained the strain of a Bi2223 phase in coiled wire using synchrotron X-ray diffraction. Further synchrotron-based study of superconducting coils will be useful in the development of advanced high-field magnets. The appropriate measurement method and the obtained measurement results are presented in this paper.

Development of HTS Conductors for Fusion Magnets

In view of the development of cables for next-generation fusion reactors, research activities are carried out on all HTS materials available from industrial production. Preliminary design of a react and wind cable using Bi-2212 wires is carried out, inspired by the Nb 3 Sn cable for European DEMO. The design and construction of a 60-kA prototype cable made of coated conductors have been carried out. The strands in the cable are composed of a stack of coated conductor tapes (4 mm wide) embedded in a copper profile of 6.3 mm in diameter. Tapes and copper profiles are soldered together in order to obtain a mechanically solid strand and to keep the inter tape resistance at a minimum, so that current can be easily redistributed among the tapes in a strand. The strands are fabricated in pieces 2 m long, but scaling up to industrial production should not present any major problem. A flat cable is manufactured by winding 20 strands around a central copper former; the cable will be inserted in a steel jacket for force flow cooling. The critical current of each strand was measured at liquid nitrogen in self-field just after the manufacturing process (twisted and straight) and after cabling: No reduction of the critical current was observed. Two pieces of cable, each 2 m long, are going to be prepared and assembled to form a sample that can be tested in the EDIPO facility.

Current Limiting Performance of Transformer-Type Superconducting Fault Current Limiter Made of BSCCO and REBCO Wires

Superconducting fault current limiters (SFCLs) are expected to be installed in the power system for fault current limitation and improvement of power system stability. Among various SFCLs, we have proposed the transformer-type SFCL that produces limiting reactance for smaller fault currents and, additionally, gives limiting resistance for larger fault currents. Therefore, the transformer-type SFCL shows better recovery performance than resistive-type SFCLs. In this paper, we fabricated two transformer-type SFCLs using different high-temperature superconducting wires [Bi-2223 (BSCCO) wire and GdBCO, which is a kind of rare-earth-barium-copper-oxide (REBCO) wire] for the secondary coils and compared their current limiting characteristics. They were designed under the condition that the specifications of the primary coils and their trigger current level are about the same, respectively. As a result of comparing their current limiting characteristics, the transformer-type SFCL using GdBCO wire for the secondary coil shows a larger limiting reactance and dissipates less energy. Therefore, the transformer-type SFCL using GdBCO wire for the secondary coil shows better recovery performance.

Reduced Magnetization and Loss in Ag–Mg Sheathed Bi2212 Wires: Systematics With Sample Twist Pitch and Length

Suppression of magnetization and effective filament diameter deff with twisting was investigated for a series of recent Bi2212 strands manufactured by Oxford Superconducting Technologies. We measured magnetization as a function of field (out to 14 T), at 5.1 K, of twisted and nontwisted 37 × 18 double restack design strands. The samples were helical coils 5-6 mm in height and approximately 5 mm in diameter. The strand diameter was 0.8 mm. The magnetization of samples having twist pitches of 25.4, 12.7, and 6.35 mm were examined and compared to nontwisted samples of the same filament configuration. The critical state model was used to extract the 12-T d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> from magnetization data for comparison. Twisting the samples reduced d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> by a factor of 1.5-3. The d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> was shown to increase both with L and L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> . Mathematical expressions, based upon the anisotropic continuum model, were fit to the data, and a parameter γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , which quantifies the electrical connectivity perpendicular to the filament axis, was extracted. The bundle-to-bundle connectivity along the radial axis was found to be approximately 0.2%. The d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> was substantially reduced with L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> . In addition, the importance of understanding sample length dependence for quantitative measurements is discussed.

Stability Analysis of the Cable Core of a 10 kA HTS DC Power Cable Used in the Electrolytic Aluminum Industry

High temperature superconducting (HTS) dc power cable shows a wide application prospect in the field of power transmission for its nearly lossless and rather high capacity. IEE has installed a 360-meter long high temperature superconducting (HTS) dc power cable at the self-supply power plant of Zhongfu Industrial Company Ltd. in Gongyi, Henan and the system has operated for two years. The cable connects a 19.5 MVA/1.5 kA silicon-controlled rectifier, which connects with a 110 kV/1 kV transformer, to the bus bar of an electrolytic aluminum cell. It is designed to carry 10-kA current and the voltage is 1300 V. The HTS dc power cable core consists of five conductor layers wound with the spliced Bi-2223 wires with the length of 40 km. The cable core has five layers and 23 HTS wires in each layer with the outer diameter of 45 mm. The HTS dc power cable is fabricated with the spliced superconducting wires which will have effect on the overall superconductivity. Also, since dc output of the rectifier contains a proportion of the ac harmonic ripple, the large dc and small ac will generate the loss in the cable core. In the operation of the 10 kA HTS dc power cable, anode effect will occur in electrolytic aluminum tank, which will lead to a large fault current in the cable and even lead to the power off protection. In this paper, stability of the spliced Bi-2223 wire, stability of the cable core under the cold shrinkage force, loss under the large dc and small ac ripple are analyzed by the theoretical and experimental methods. The test results of ac ripple loss, anode effect, and stable operation are also presented.

High strength kiloampere Bi2Sr2CaCu2Ox cables for high-field magnet applications

Multifilamentary Ag-sheathed Bi2Sr2CaCu2Ox (Bi-2212) wire can carry sufficient critical current density Jc for the development of powerful superconducting magnets. However, the range of its applications is limited by the low mechanical strength of the Ag/Bi-2212 strand. A potential solution is to cable Ag/Bi-2212 wire with high-strength materials that are compatible with the Bi-2212 heat treatment in an oxygen atmosphere. Past attempts have not always been successful, because the high-strength materials reacted with Bi-2212 wires, significantly reducing their Jc. We examined the nature of reactions occurring when Ag/Bi-2212 wires are heat-treated in direct contact with several commonly used high-strength alloys and a new Fe-Cr-Al alloy. INCONEL X750 and INCONEL 600 resulted in significant Jc loss, whereas Ni80-Cr caused little or no Jc loss; however, all of them formed chromium oxide that subsequently reacted with silver, creating cracks in the silver sheath. We found that Fe-Cr-Al did not show significant reactions with Ag/Bi-2212 strands. Scanning electron microscopy (SEM) and energy dispersive x-ray (EDS) examinations revealed that the Fe-Cr-Al alloy benefits from the formation of a uniform, crack-free, continuous alumina layer on its surface that does not react with Ag and that helps minimize the Cu loss found with INCONEL X750 and INCONEL 600. We fabricated prototype 6-around-1 cables with six Bi-2212 strands twisted and transposed around an Fe-Cr-Al alloy core coated with TiO2. After standard 1 bar melt processing, the cable retained 100% of the total current-carrying capability of its strands, and, after a 10 bar overpressure processing, the cable reached a total current of 1025 A at 4.2 K and 10 T. Tensile tests showed that Fe-Cr-Al becomes brittle after being cooled to 4.2 K, whereas INCONEL X750 remains ductile and retains a modulus of 183 GPa. We proposed new cable designs that take advantage of the chemical compatibility of Fe-Cr-Al and high strength of INCONEL X750 for various high-field magnet applications.

Optimization of Partial Melting Process for Bi-2212 Multi-Filament Wires

Abstract Bi-2212 precursor powder was synthesized by a co-precipitation process, and 19×(36+1) multi-filament wires were prepared by traditional powder in tube (PIT) process. Both the maximum melting temperature, T max and cooling rate, R c1 have been optimized during partial melting process (PMP). The texture structures of Bi-2212 grains and secondary phase distributions were observed with the scanning electron microscopy. Differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA) were used to evaluate the thermodynamic properties of Bi-2212 wires. The phase transition process during cooling was discussed based on the analysis of both microstructure evolution and the thermodynamic investigation. It can be concluded that both T max and R c1 have great influences on the distribution of pores and secondary phase, such as alkali earth cuprate (AEC). The optimized critical current density of ∼4400 A·cm −2 at 77 K and self field can be obtained with the T max of 892 °C and a two-step cooling process at the R c1 of 40 °C/h.

Comparison of growth texture in round Bi2212 and flat Bi2223 wires and its relation to high critical current density development.

Why Bi2Sr2CaCu2Ox (Bi2212) allows high critical current density Jc in round wires rather than only in the anisotropic tape form demanded by all other high temperature superconductors is important for future magnet applications. Here we compare the local texture of state-of-the-art Bi2212 and Bi2223 ((Bi,Pb)2Sr2Ca2Cu3O10), finding that round wire Bi2212 generates a dominant a-axis growth texture that also enforces a local biaxial texture (FWHM <15°) while simultaneously allowing the c-axes of its polycrystals to rotate azimuthally along and about the filament axis so as to generate macroscopically isotropic behavior. By contrast Bi2223 shows only a uniaxial (FWHM <15°) c-axis texture perpendicular to the tape plane without any in-plane texture. Consistent with these observations, a marked, field-increasing, field-decreasing Jc(H) hysteresis characteristic of weak-linked systems appears in Bi2223 but is absent in Bi2212 round wire. Growth-induced texture on cooling from the melt step of the Bi2212 Jc optimization process appears to be the key step in generating this highly desirable microstructure.