Improvement Of Critical Current Density Research Articles

Critical current density improvement of RHQT Nb3Al superconducting wires by incorporating magnetic Dy2O3 nanoparticles as additional flux pinning centers

In this paper, we prepared magnetic Dy2O3 nanoparticles (n-Dy2O3) doped Nb3Al superconducting wires by rapid heating, quenching and transformation (RHQT) method, and investigated the micro-defects, superconducting properties and flux pinning mechanisms. TEM results showed high-density stacking faults (SF) with spacing of 5–25 nm located in Nb3Al (210) crystal planes, which is due to the absence of Nb atoms on the {100} crystal planes. Additionally, many intra-granular and inter-granular Dy2O3 nanoparticles with size of 4–40 nm were also observed in the Nb3Al superconducting phase. Jc was promoted in the doped samples, obtaining the best Jc of 1.95 × 105 A/cm2@ 4.2 K, 12 T in 1 at.% n-Dy2O3 doped Nb3Al wire, which was nearly 70 % higher than pristine Nb3Al wire. By doping n-Dy2O3, the fP(Fp/Fp,max)-h(B/Birr) peaks shift to larger h value (higher field). The flux pinning mechanism by fitting fP-h curves showed dominant surface pinning combined with point pinning, corresponding to the crystal defect of stacking faults and nanoparticles, respectively. The pinning mechanism verified that the nanoparticles acted as additional pinning centers to significantly elevate the Jc performance.

Solvent volume expansion inhibition during freeze casting and its application in composite solid lithium electrolytes

Composite solid electrolytes hold the promise to enable all-solid-state lithium-ion batteries to be practicable. One critical issue is the construction of uniform porous ceramic framework. The work proposed an innovative freeze casting-based technology to construct porous ceramic framework of Li1+xAlxGe2−x(PO4)3 (LAGP), which was further composited with polymeric electrolytes to assemble full cells. The pore structure of the ceramic framework was effectively tuned by the ratio of mixed water-ethanol solvents, which could offset the volume expansion of water during freeze casting, benefit powder compact and sintering, and thus improved the quality of the composite solid electrolytes. The unique solvent composition design strategy provided ample opportunity for microstructural modification and the suppression of short-circuiting lithium dendrites, as demonstrated by the improvement of critical current density and phase field modeling. This work offers new perspectives on creating different pore structure free of residual stress or volume expansion once freeze casting technique is adopted.

Improvement of critical current density J c in powder-in-tube rapid heating, quenching and transformation Nb3Al wires by doping with nano-SnO2

We prepared Nb3Al superconducting wires doped with nano-size SnO2 (n-SiO2) particles through a multi-time rapid heating and quenching process and investigated their microstructure and superconducting properties. All the samples showed a highly homogeneous A15 Nb3Al phase. Compared with pure Nb3Al, the n-SnO2 doped Nb3Al wires presented a larger ΔT c value and a higher J c value. The best J c at 4.2 K was found in the 1 wt% n-SnO2 doped Nb3Al sample, with 3.37 × 105 A cm−2, 2.55 × 105 A cm−2 and 1.80 × 105 A cm−2 at 8 T, 10 T and 12 T, respectively. These results were an improvement of about 60% compared with pure Nb3Al at the same applied fields. The maximum irreversible field value was obtained in the 1 wt% SnO2-doped Nb3Al wire, with a result of 29.5 T at 4.2 K. The improvement of J c performance in the n-SnO2 doped Nb3Al wires might be attributed to the formation of artificial nanoparticles in the grain, which act as extra effective flux pinning centers.

Improvement of critical current density of REBa2Cu3O7-δ by increase in configurational entropy of mixing.

REBa2Cu3O7-δ (RE123, RE: rare earth) is one of the high-temperature superconductors with a transition temperature (Tc) exceeding 90 K. Because of its high-Tc and large critical current density (Jc) under magnetic fields, RE123 superconductors have been expected to play a key role in superconductivity application. To accelerate application researches on RE123-based devices, further improvements of Jc characteristics have been desired. In this study, we investigated the effects of high-entropy alloying at the RE site on the superconducting properties, through the measurements of local (intra-grain) Jc () by a remanent magnetization method. We found that shows a trend to be improved when four or five RE elements are mixed at the RE site, which results in high configurational entropy of mixing (ΔSmix). All samples exhibited an order of few MA cm−2 which is a criterion for practical application and the highest resulted in a value of around 7.0 MA cm−2 at T = 2.0 K. Because high-entropy alloying can improve of RE123 superconductors, our entropy-engineering strategy introduced here would be useful for the development of RE123 superconducting materials available under high magnetic fields.

Critical current density improvement in CSD-grown high-entropy REBa2Cu3O7-δ films.

High-entropy oxide (HEO) superconductors have been developed since very recently. Different superconductors can be produced in the form of a high-entropy compound, including REBa2Cu3O7-δ (REBCO). However, until now, mainly bulk samples (mostly in polycrystalline form) have been reported. In this work, the first CSD-grown high-entropy (HE) REBCO nanocomposite films were successfully synthesized. In particular, high-quality Gd0.2Dy0.2Y0.2Ho0.2Er0.2Ba2Cu3O7-δ nanocomposite films with 12 mol% BaHfO3 nanoparticles were grown on SrTiO3 substrates. The X-ray diffraction patterns show a near-perfect c-axis oriented grain growth. Both T c and 77 K J sf c, 91.9 K and 3.5 MA cm-2, respectively, are comparable with the values of the single-RE REBCO films. Moreover, at low temperatures, specifically at 30 K, the J c values are larger than those of the single-RE samples. A transmission electron microscopy (TEM) study, including energy-dispersive X-ray spectroscopy (EDXS) measurements, reveals that the different RE3+ ions are distributed homogeneously in the matrix without forming clusters. This distribution causes point-like pinning centres that explain the superior performances of these samples at low temperatures. Although still seen as a proof-of-concept for the feasibility of preparing such films, these results demonstrate that the HE REBCO films are a promising option for the future fabrication of high-performance coated conductors. In the investigated B-T range, however, their J c values are still lower than those of other, medium-entropy REBCO films, which shows that an optimization of the composition of the HE REBCO films is needed to maximize their performance.

Investigation of the Correlation between Initial Microstructure and Critical Current Density of Nb-46.5 wt%Ti Superconducting Material

We have investigated the effect of initial microstructures on the change in critical current density (Jc) of Nb-46.5 wt%Ti (NbTi) superconducting material. It is well known that α-Ti phases distributed in NbTi material act as a flux pinning center, resulting in an improvement in critical current density. Therefore, it is crucial to obtain the grain-refined microstructure, which is strongly related with precipitation of uniformly distributed fine α-Ti phases and higher volume faction of α-Ti phases, as α-Ti phases are precipitated at the grain boundaries and triple points during heat treatments. Therefore, in order to characterize the effect of initial microstructure of NbTi on critical current density, different initial microstructures were obtained by applying equal channel angular pressing (ECAP) and hot rolling with different strains. It was revealed experimentally that hot rolling with a higher strain is efficient for obtaining the initial microstructure, which has equiaxed fine grains of β-NbTi with the aid of dynamic recrystallization, and which is helpful for precipitating fine α-Ti phases during intermediate heat treatment. Furthermore, it was confirmed that critical current density can be enhanced by obtaining a smaller α-Ti phase, a higher aspect ratio of α-Ti phase, a higher volume fraction of α-Ti phase and a ribbon-like folded α-Ti phase.

Improvement of critical current density of MgB2 bulk superconductor processed by Spark Plasma Sintering

AbstractSpark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and optimization of the sintering kinetics and therefore makes it possible to obtain MgB2 bulk superconductor with tailored microstructures consisting of grains with either spherical or elongated morphology. In this contribution, the role of the precursor powders on the superconducting properties of MgB2 is investigated. Three sets of bulk MgB2 material were processed from: (i) a commercially available MgB2 powder; (ii) a mixture of Mg metal and amorphous B using a single‐step solid‐state reaction process and (iii) a mixture of amorphous boron coated with carbon and Mg metal. The samples were prepared in the same SPS processing conditions. The microstructure of the samples was investigated by X‐ray diffraction, SEM and TEM and correlated to their superconducting properties. The critical current density of the best sample at 20K was Jc = 500 kA/cm2 in self‐field, which is one of the highest critical current density reported for MgB2 bulk superconductors.

Enhancement of Jc in C6H16N2 added MgB2 superconductor without carbon substitution for boron

The influence of C6H16N2 addition on the microstructural and superconducting properties of MgB2 was systematically investigated. Unlike previously reported carbon sources, the C6H16N2 additive had no effect on the lattice parameters of MgB2. However, the obvious improvement of critical current density (Jc) was obtained for C6H16N2 added MgB2 without carbon substitution for boron. The average grain size was reduced with the addition of C6H16N2. Additionally, different numbers of spherical MgB2 grains appeared inside the C6H16N2 added MgB2. As the added amount of C6H16N2 increased, the active cross-sectional area fraction (AF) values first increased then decreased. The possible reasons for these results have been discussed. The additive C6H16N2 resulted in a small depression in Tc and slightly increased the ΔTc of MgB2 at 0 T. The Hc2 (T) and Hirr (T) values of C6H16N2 added MgB2 samples are larger than those of pure MgB2 at temperatures below 25 K. It was concluded that the reduced grain size and the improved grain connectivity were mainly correlated with the enhancement of the Jc, Fp, Hc2, and Hirr performances in C6H16N2 added MgB2 superconductors.

Improvement of Critical Current Density in Bi-2223 Superconductor by Ag-Doping

High-Tc superconducting polycrystalline Bi-2223 undoped sample (A0) and 5wt.% Ag-doped sample (A5) have been prepared by solid-state reaction method at 8550C with sintering time of 8 days. X-ray powder diffraction (XRD) results shown that the major phase of these samples is Bi-2223. The volume ratio of Bi-2223 phase observed to be increased from 78% for undoped sample (A0) to 95% for 5wt.% Ag-doped sample (A5). The enhancement of the onset of high-Tc superconductivity (Tc,onset = 112.5 K) in silver doping sample was observed by DC-resistivity measurements. From the AC-susceptibility measurements combined with Bean critical state model, the temperature dependent parabolic law of inter-granular or matrix critical current density (Jcm) was fitted. Some Jcm values were estimated from these parabolic laws. The results showed that 5wt% Ag-doping can make the Bi-2223 sample with nearly double critical current density.

Performance comparison of 1X versus 4X HTS tapes toward the design a 10‐MW direct drive HTS generator for wind turbine applications

AbstractWith current developments underway at the University of Houston, 4× high‐temperature superconducting (HTS) yttrium barium copper oxide (YBCO) tapes are expected to reach a fourfold critical current density improvement at 30 K and 3 T as part of an Advanced Research Projects Agency‐Energy (ARPA‐E)‐sponsored project. The impetus for this project is to develop and deploy cost‐effective direct drive HTS generators for large offshore wind turbine generators. The improved conductor performance of 4× tapes over 1× tapes is projected to allow for a significant reduction in the amount of HTS tapes required to generate the excitation field and, thus, lead to a reduction of the overall generator cost. It was, however, deduced from simulation results that conventional methods of rotor field enhancement using rotor iron makes 1× conductor competitive enough compared with 4× conductors; hence, commercially available YBCO tapes can provide all the energy density requirements to make wind turbines a competitive energy resource based on energy forecasts. This paper is a presentation of design and optimization work to find the best generator topology from the point of view of both machine design and electricity supply. The generators are numerically modeled and simulated using 1× and 4× YBCO HTS conductors on either or of both the rotor and stator side of the generators.

Fast Lithium-Ion Conduction Solid State-Electrolytes

Interest in solid-state electrolytes has grown rapidly in recent years owing to the desire to utilize lithium metal anode for improved specific energy density and the inherent safety advantage of a non-flammable solid state electrolyte. A practical solid electrolyte for energy storage must be a fast ion conductor, have negligible electronic conductivity and adequate chemical and electrochemical stability with electrodes. For ease of manufacturing and potentially higher power capability we have focused research on oxide ceramic electrolytes. This work will cover our work on NASICON, perovskite, garnet and other potential structural families including how to stabilize the most conductive phase and maximize the ionic conductivity through substitutional chemistry. Our main focus on garnet will include how to understand important mechanical properties, the air and moisture stability and stability with cathode materials, improvement of critical current density and future challenges and opportunities for garnet based fast lithium ion electrolytes and potential new materials.

The effects of Mg doping on the microstructure and transport properties of internal tin-processed brass matrix Nb3Sn superconductors

The addition of Zn into Cu matrix has been proved to enhance the Sn activity and accelerate the formation of a Nb3Sn layer in internal tin-processed Nb3Sn wires. However, in our samples, Ti is doped in Sn cores, and during the reaction, Ti was observed to accumulate between the inner and outer sub-elements of the 1980-Nb-core multi-filamentary brass matrix wires. With the addition of Mg into the brass matrix, Ti accumulation between the sub-elements was significantly suppressed, which promoted Ti incorporation into the Nb3Sn layers. Meanwhile, the formed Nb3Sn layers exhibited smaller grain sizes. Compared to the brass (Cu-12wt%Zn) matrix wires, an improvement of critical current density (Jc) was found in the Cu-12Zn-0.2Mg matrix wires due to the incorporation of Ti into the Nb3Sn layer and the refinement of the Nb3Sn grain size. The irreversibility field (Birr) and maximum bulk pinning forces (Fp,max) were calculated from the Kramer plot by analyzing the Jc(B) curves. A small amount of Mg (0.2 wt%) doping into the Cu-12Zn matrix wires can increase the Birr value by about 0.5 T and the Fp,max value by about 3 ∼ 4 GN m−3 after the wires were heat treated between 670 °C and 730 °C. The non-Cu Jc of the Cu-12Zn-0.2Mg matrix wires reaches the maximum value of 1533 A mm−2 @12 T after heat treatment at 685 °C and decreases at higher temperatures.

Improvement of flux pinning ability by tungsten oxide nanoparticles added in YBa2Cu3Oy superconductor

In this study, series of superconductor-tungsten oxide (WO3) nanoparticles composites, YBa2Cu3O7-δ/(WO3)x, were produced via the solid-state reaction process. The structural, morphological, chemical compositions, electrical and magnetic properties were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM) along with EDX system and physical properties measurement system (PPMS), respectively. The XRD, SEM and EDX analyses showed the successful formation of the Y-123 orthorhombic phase. The electrical resistivity measurements proved the occurrence of superconductivity in different samples. The magnetic results showed an improvement of critical current density (Jc) and pinning ability in WO3 nanoparticles added Y-123 products. The dominant pinning mechanisms and the strength of pinning centers in various sintered products were examined and discussed. The measurements of zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature (M-T) indicated an increase in the magnitude of diamagnetic signal with the addition of WO3 nanoparticles in the Y-123 superconductor.

In situ hydrostatic pressure induced improvement of critical current density and suppression of magnetic relaxation in Y(Dy0.5)Ba2Cu3O7−δ coated conductors

We report on the effect of in situ hydrostatic pressure on the enhancement of the in-magnetic-field critical current density parallel to the crystallographic c-axis and vortex pinning in epitaxial Y(Dy0.5)Ba2Cu3O7−δ coated conductors prepared by metal organic deposition. Our results show that in situ hydrostatic pressure greatly enhances the critical current density at high fields and high temperatures. At 80 K and 5 T we observe a ten-fold increase in the critical current density under the pressure of 1.2 GPa, and the irreversibility line is shifted to higher fields without changing the critical temperature. The normalized magnetic relaxation rate shows that vortex creep rates are strongly suppressed due to applied pressure, and the pinning energy is significantly increased based on the collective creep theory. After releasing the pressure, we recover the original superconducting properties. Therefore, we speculate that the in situ hydrostatic pressure exerted on the coated conductor enhances the pinning of existing extended defects. This is totally different from what has been observed in REBa2Cu3O7−δ melt-textured crystals, where the effect of pressure generates point-like defects.

A comparative study on the effects of n C, n SiC and BRH on the structural and superconducting properties of MgB2 PIT wires

A comparative study on the structural and superconducting properties of MgB2 Powder-In-Tube (PIT) wires doped with various weight percentage of nano carbon (n C), nano SiC (n SiC) and Burned Rice Husk (BRH) has been carried out. X-Ray diffraction patterns of n SiC and BRH doped samples showed peaks of Mg2Si which is a useful intragrain inclusion for flux pinning. The Lattice parameter ‘a’ decreased with increasing doping concentration for all the doped samples, the maximum being for n C, indicating C substitution for B. The Full Width at Half Maximum (FWHM) of (hk0) peaks of all samples increased with doping. Doping caused a reduction in grain size and SEM images showed tightly packed smaller grains containing hexagonal crystallites in all the three types of doped samples. All the dopants reduced the transition temperature of the undoped sample and the effect was severe for n C doping. Unlike the pure sample, the transport critical current density (JC) of the doped samples didn't fall rapidly at high fields; but retained large fractions depending on the dopants. The best results were obtained for n SiC and BRH doped samples with BRH closely matching the performance of n SiC. Improvement in critical current density for these samples was an order higher than the undoped sample at 4.2 K and 8 T.

Strong flux pinning at 4.2 K in SmBa2Cu3Oy coated conductors with BaHfO3 nanorods controlled by low growth temperature

In order to apply REBa2Cu3Oy (REBCO, RE = rare earth elements or Y) coated conductors in high magnetic field, coil-based applications, the isotropic improvement of their critical current performance with respect to the directions of the magnetic field under these operating conditions is required. Most applications operate at temperatures lower than 50 K and magnetic fields over 2 T. In this study, the improvement of critical current density (Jc) performance for various applied magnetic field directions was achieved by controlling the nanostructure of the BaHfO3 (BHO)-doped SmBa2Cu3Oy (SmBCO) films on metallic substrates. The corresponding minimum Jc value of the films at 40 K under an applied 3 T field was 5.2 MA cm−2, which is over ten times higher than that of a fully optimized Nb–Ti wire at 4.2 K. At 4.2 K, under a 17.5 T field, a flux pinning force density of 1.4 TN m−3 for B//c was realized; this value is among the highest values reported for REBCO films to date. More importantly, the Fp for B//c corresponds to the minimum value for various applied magnetic field directions. We investigated the dominant flux pinning centers of films at 4.2 K using the anisotropic scaling approach based on the effective mass model. The dominant flux pinning centers are random pinning centers at 4.2 K, i.e., a high pinning performance was achieved by the high number density of random pins in the matrix of the BHO-doped SmBCO films.

YBa2Cu3Ox-SrTiO3 multilayer approach on textured Ni-W tapes: Morphology, structure and critical current density improvement

Multilayer epitaxial structure YBa2Cu3Ox–interlayer–YBa2Cu3Ox is a promising architecture for YBa2Cu3Ox (YBCO) superconductors. We formed single YBCO layers and multilayer epitaxial structures YBCO–interlayer–YBCO by pulsed laser deposition method on the cube textured Ni-W tapes. The use of SrTiO3 interlayers enabled to stop YBCO film morphology and microstructure degradation and maintain high critical current density (Jc). Jc in 1μm thick multilayer film composed of four 250nm thick YBCO layers is comparable to Jc in single-layer 250nm thick YBCO film. Application of this method allows overcoming such fundamental problem as degradation of Jc in thick films and multiplying critical current with number of YBCO layers.

Optimization of sintering conditions in bulk MgB2 material for improvement of critical current density

The present investigation focuses on methods to further improve the Jc values of disk-shaped bulk MgB2 superconductors by optimizing the sintering conditions. We prepared two sets of bulk MgB2 material from commercial high-purity powders of Mg metal and amorphous B using a single-step solid-state reaction process. To optimize the sintering time, a set of samples was sintered at 775 °C with sintering duration ranging between 1 and 10 h (pure Ar atmosphere). A second set of samples was produced similarly at 775, 780, 785, 795, 800 and 805 °C (3 h, pure argon atmosphere). X-ray diffraction analysis showed that both sets of samples were single phase MgB2. Magnetization measurements confirmed a sharp superconducting transition with Tc,onset ≈ 38.2 K–38.8 K. The critical current density (Jc) values for MgB2 samples produced for 1 h were the highest in all processed materials, i.e., the high Jc value of 270,000 A/cm2 and 125,000 A/cm2 (20 K, self-field and 1 T) were achieved in the sample produced at 775 °C, without any additional doping. In contrast, the second series of samples clearly indicated that at 805 °C (3 h) the highest Jc of 245,000 A/cm2 and 110,000 A/cm2 (20 K, self-field and 1 T) were achieved. AFM and EBSD observations indicated that largest amount of fine grains do exist in the sample sintered at 775 °C, but the narrowest distribution of grains does exist in the sample sintered at 800 °C. The present results clearly demonstrate a strong relation between the microstructure and the pinning performance. The optimization of the sintering conditions is crucial to improve the performance of bulk MgB2 samples.

Influences of Ca nonstoichiometry on the superconducting properties of Bi-2212 superconducting bulks

Polycrystalline bulks of Bi2Sr2Ca x Cu2.0O8+δ (Bi-2212) with x = 0.90, 0.95, 1.00 and 1.05 were fabricated by spark plasma sintering technique. The influences of Ca nonstoichiometry on the microstructures, carrier concentration, as well as the related superconducting properties were systematically investigated. XRD analyses revealed the corresponding change of lattice parameters of Bi-2212 phase with the Ca nonstoichiometry. The average particle radius in the sintered bulks underwent an obvious change with the increase of Ca content, which could be attributed to the variation of thermodynamic properties. AC susceptibility measurements exhibited the optimization of intergrain connections in Ca = 0.95 bulk, which lead to the optimization of both self- and in-field critical current density, J c of this system. The optimization of microstructures also caused the enhancement of surface pinning. Based on the enhancements of both intergrain connections and flux pinning properties, an obvious improvement of critical current density was obtained with the optimal doping content of Ca = 0.95.

Anisotropy Improvement of Critical Current Density in Bi-based Superconductor by Mg Impurity Doping

We investigated effects of Mg impurity doping on Bi 2 Sr 2 CaCu 2 O y high-temperature superconductor (Bi-2212 HTS). We used Bi-2212 HTS whisker as a sample and tried to improve its anisotropy of critical current density (J c ). For the Mg-doping, we adopted thermal diffusion process. In the process, nonsuperconducting phase is expected to appear on the surface of the whisker. From experimental results, we found that the anisotropy parameter of J c can be improved from 13.8 to 1.89 by doping Mg impurity with 400 °C annealing.