Caixuan Xu

Flux pinning properties in a GdBa2Cu3O7−δ bulk superconductor with the addition of magnetic alloy particles

Pinning stability and the introduction of pinning centers are essential problems in the application of high temperature superconductors. A study of the role of the addition of a variety of oxides into Gd-123 bulk superconductors was pursued with respect to the effect of magnetic pinning centers on enhancement of critical current density. We have found the addition of magnetic alloy particles into Gd-123 bulk contributes to the enhancement of the critical current density under a magnetic field. The magnetic particles of Fe–B alloy with small amounts of Cu–Nb–Si–Cr were added into the Gd123 matrix and grown into single-domain bulk in air. The effects of Fe–B alloy magnetic particles on the superconducting pinning properties of Gd123 bulk are reported. The experimental results showed that the Tc of the bulk was suppressed significantly and the transition width became larger with an increase in the doping content of the magnetic particles. This was consistent with magnetic impurities suppressing superconductivity. The critical current density deduced from the magnetization as a function of applied external magnetic field was enhanced under low magnetic field and high magnetic field, which is different from the bulk doped with paramagnetic particles. Possible mechanisms and the role of magnetic materials from the viewpoint of increased flux pinning inside the bulk are discussed.

Effect of BaO2 and fine Gd2BaCuO7−δ addition on the superconducting properties of air-processed GdBa2Cu3O7−δ

Single-domain GdBa2Cu3O7−δ (Gd123) bulk superconductors were fabricated in air and studied with different BaO2 contents and Gd2BaCuO7−δ (Gd211) initial particle sizes. The experiments showed that BaO2 addition effectively suppressed the Gd/Ba substitution and therefore enhanced superconducting properties such as Jc and Birr. An optimum addition amount is Gd123:BaO2 = 1:0.1 molar. By employing fine Gd211, Jc at low magnetic fields is further enhanced. At B = 0 and 77 K, Jc reached 86 000 A cm−2, which is a record high Jc value for melt-processed Gd123 bulk samples in air.

Effect of Gd2Ba4CuMoOy addition on the band structure and spatial variation of superconducting properties in GdBa2Cu3O7−δ single domains

Gd2Ba4CuMoOy (Gd2411) single phase was synthesized in air and added into GdBa2Cu3O7−δ (Gd123) bulk superconductors with the addition amount ranging from 0.2% to 20%, in molar ratio to Gd123. Both microstructure and superconducting properties of each bulk were investigated from the seed position to the boundary of a single domain. Although a band structure with the alternative distribution of Gd2BaCuO5 (Gd211) and Gd2411 appeared in all samples, the range of this inhomogeneity only appeared within 5 mm from each side of the seed. The critical current density (Jc) shows that Gd2411 plays an important role in the control of the local superconducting properties. The highest Jc reached 84 500 A cm−2 at 77 K under the self-field in the sample with 0.4% addition of Gd2411.

Enhanced flux pinning of air-processed GdBa2Cu3O7−δ superconductors with addition of ZrO2 nanoparticles

The authors investigate the influence of ZrO2 nanoparticles on microstructure and superconducting properties of GdBa2Cu3O7−δ (Gd123) single domain superconductors grown in air. Experiments show that before Gd123 crystallization ZrO2 nanoparticles react with Gd123 and yield BaZrO3. Transmission electron microscopy evidences that BaZrO3 disperses in Gd123 crystals with a size less than 50nm. A high Jc value of 100000A∕cm2 has been achieved in Gd123 with 0.4M% ZrO2 at 77K and self-field. Compared to the flux pinning effect of submicrometer Gd2BaCuO5 particles, BaZrO3 nanoparticles embedded in Gd123 significantly enhance critical current densities in a wide magnetic field range.

Transmission Electron Microscopy and Atomic Force Microscopy Observation of Air-Processed GdBa2Cu3O7-δ Superconductors Doped with Metal Oxide Nanoparticles (Metal = Zr, Zn, and Sn)

Single-domain, c-axis-oriented 30-mm-diameter GdBa2Cu3O7-δ (Gd123) melt-textured bulk superconductors have been successfully grown by the top-seeded melt growth method from precursors of Gd123, Gd2BaCuO5, and Ag2O in air with doping of nanosized ZrO2, ZnO, or SnO2. Transmission electron microscopy (TEM) unveils a large amount of BaZrO3 or BaSnO3 particles with an average diameter of approximately 50 nm respectively embedded in ZrO2 or SnO2 doped samples, while no Zn-rich nanoparticles are observed in the ZnO-doped samples. The critical temperature Tc is almost unchanged up to a doping amount of 10 mol % for ZrO2- or SnO2-doped Gd123 melt-textured bulks, while ZnO-doped Gd123 becomes non-superconductive at this doping level. By atomic force microscopy (AFM), nanostripes with a wavelength of 15 nm are observed in the ZnO-doped Gd123 sample. Nanoscale particles in the grown Gd123 single domain, together with the micro-defects induced by nanoparticle doping, can account for the enhanced superconducting properties.

Materials processing, pulsed field magnetization and field-pole application to propulsion motors on Gd123 bulk superconductors

Gd123 bulk superconductor is one of the promising magnet materials. We studied the materials processing to grow high performance magnet with a doping of nano-sized metal oxides such as ZrO2 as a candidature of pinning centre. The enhancement of the critical current density was obtained. Growth of nano-sized particles of Gd211 in addition to BaZrO3 were observed by TEM. The formation of nano-sized particles appears a key to improve the integrated flux trapped inside the bulks and the TEM reveals an intriguing effect of the addition to the microstructure of bulk materials. Magnetization process is crucial especially for an extended machinery. Pulsed field magnetization was applied to the field-pole bulk on the rotor disk of the tested synchronous motor. The trapped flux density of 1.3 T for Gd123 bulk sample and of 60 mm diameter was reached in the limited dimension of the tested motor by a step cooling method down to 38 K with a closed-cycle condensed neon. The pulsed magnetic field was applied with a new type of split-armature coil. A large bulk of 140 mm diameter has also shown a potential flux trapping superior to other smaller specimens. The bulk magnet provides a strong magnetic field around the bulk body itself with high current density relative to a coil winding. A comparative drawing of a torque density of a variety of motors which is defined as the torque divided by the volume of the motor indicates a potential advantage of bulk motor as a super permanent magnet motor.

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Bulk GdBa₂Cu₃O_7-delta (Gd123) superconductors were fabricated in air by adding ZrO₂/SnO₂ nanoparticles with an average size of 50 nm. The effect of addition on microstructure and superconducting properties was investigated by scanning electron microscopy (SEM)/transmission electron microscopy (TEM) and SQUID. SEM results show that the distribution of Gd₂BaCuO₅ (Gd211) particles is inhomogeneous around the seeding site of single domain, induced by the addition of nano-par- ticles. Particles with size less than 50 nm were observed by TEM inside Gd123 single domain, while the typical size of fined Gd211 particles is about 100 nm. Critical current density (J_c) was enhanced by the proper amount addition of ZrO₂/SnO₂ additions. The highest J_c of 100,000 A/cm was presently achieved in the bulk with 0.4 mol % ZrO₂, under self-field at 77 K. Excess amount addition rather suppresses superconducting properties. The optimum addition amount is 0.2-0.8 mol % for ZrO₂and 0.4-0.8 mol % for SnO₂ , ratio to Gd123, respectively.

{\rm ZrO}_{2}/{\rm SnO}_{2}

We fabricated bulk GdBa2Cu3O7-? (Gd123) superconductors with additions of Gd2BaCuO5 (Gd211) and Gd2Ba4CuMoOx (Gd2411). We analyzed the relation between the addition amounts of Gd211/Gd2411 particles and critical current Jc. We found Gd2411/Gd211 particles formed a band structure which resulted in the inhomogeneous distribution of Jc in the Gd123 bulks. Gd2411 particles can stably exist in the vicinity of the seed site compared to Gd211 that is easy pushed out to edge area of single domain. This enhanced the critical current Jc in the vicinity area of bulks.

Nano-Particles on Superconducting Properties and Microstructure of Melt-Processed Gd123 Superconductors

Inclusion of the finely dispersed RE2BaCuO5 ( RE211 ) enhances flux pinning in bulk REBa2Cu3O7−δ. Recently, we tested the effect of Gd2Ba4CuMOx ( Gd2411; M= Zr, Mo) addition on the trapped field and critical current density ( Jc ) in single domain of bulk GdBa2Cu3O7−δ ( Gd123 ) with the additional amount ranging from 0.2 mol % to 20 mol %, ratio to Gd123. Around the seeding zone a novel band of the second phase distribution was observed for all Gd2411 additional levels. Second phase excess regions were followed by diluted second phase regions, those appeared alternatively, and formed an inhomogeneous band structure, while an uniform distribution of Gd211 appeared from 0.5 cm away from the seed to the edge of domain along 〈100〉/〈010〉 direction. Jc values strongly depend on the positions and the Gd2411 addition was proved to enhance Jc near the seeding site rather than edge‐periphery zone of the bulk disk.