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Bi2Sr2Ca2Cu3O10+δ (Bi-2223) precursors for the fabrication of high temperature superconducting tapes were prepared by spray pyrolysis technique followed by a calcination process. The influences of precursor powder phase composition on the phase evolution dynamics and final current capacity of Bi-2223 superconducting tapes were systematically studied. It was interesting to notice that slight increase of Cu content in chemical composition could cause obvious variations of both the phase composition and the morphology of CuO particles in precursors. Therefore, the Bi-2223 phase formation process during the sintering process of tapes could be affected, and the current capacity changed accordingly. On the other hand, during the calcination temperature optimization process, the Bi-2223 phase content in precursors changed accordingly, thus the Bi-2223 phase formation dynamics were also studied. More Bi-2223 nuclei were formed due to the pre-existed Bi-2223 grains, which worked as inhomogeneous nuclei centers. Thus, although the final Bi-2223 phase contents were similar in all the tapes, the particle size and texture structure of Bi-2223 varied greatly, which caused a dramatic decrease of critical current in Bi-2223 tapes with the formation of Bi-2223 in precursors. Therefore, it could be concluded that both the morphology of CuO particles and the Bi-2223 phase content in precursors were the factors controlling the phase evolution dynamics during Bi-2223 tape sintering process.

FeSe superconducting bulks with different Fe contents were prepared by high energy ball milling aided sintering process followed by a post annealing process in low oxygen atmosphere. The influences of Fe content, annealing temperature and oxygen partial pressure have been systematically studied. It was found that both the critical temperature and superconducting phase volume ratio have been obviously improved with annealing. However, the effect of oxygen annealing process on FeSe polycrystalline bulks was different with that on FeSe single crystals. In polycrystalline bulks, the annealing process could further promote the phase transition from nonsuperconducting hexagonal δ-FeSe phase to superconducting tetragonal β-FeSe phase. Meanwhile, the chemical composition uniformity has also been enhanced with no obvious FeOδ phase detected by X-Ray Diffraction. With optimized annealing process under the oxygen partial pressure of 5% at 400 °C for 10 h, FeSe superconducting bulks have reached the critical temperature of 9.1 K.

Fluorine doping Fe1.2Se polycrystalline were prepared by solid-state sintering method. The influences of fluorine doping content and sintering temperatures on phase composition, lattice parameters, microstructure as well as superconducting properties were discussed in detail. It was found that fluorine doping could effectively reduce the content of non-superconducting hexagonal δ-FeSe phase. Besides, the content of hexagonal δ-FeSe phase and grain size of superconducting tetragonal β-FeSe phase were decreased with the sintering temperature increasing. Scanning electron microscopes (SEM) images indicated that the morphology of β-FeSe phase was a typical lamellar structure with the small particles of hexagonal δ-FeSe phase. Meanwhile, a higher critical temperature Tc of 8.3 K and larger shield volume fraction were obtained with high sintering temperature of 950 °C and proper F doping content.

Precursor powders of Bi2Sr2Ca2Cu3O10+δ (Bi-2223) high temperature superconducting tapes were prepared with spray pyrolysis technique. By tuning the oxygen partial pressure in calcination atmosphere as 1.0%, 7.5%, and 10.0%, respectively, the influences of calcination atmosphere on the phase evolution dynamics during the precursor powders calcination process have been discussed. Then the optimal calcination temperatures have been obtained correspondingly. Moreover, Bi-2223 multi-filament tapes have been fabricated with the precursor powders calcinated under different atmosphere. The effects of precursor powder calcination parameters on the phase composition, microstructures as well as the current capacity of final tapes have been systematically studied. Due to the proper secondary phase content and enhanced Bi-2223 texture structures, the maximum critical current of 109 A at 77 K, self-field, corresponding to the critical current density of 23.3 kA cm−2 has been obtained with the oxygen partial pressure of 7.5% in calcination atmosphere and the calcination temperature of 790 °C.

Bi 1.76 Pb 0.34 Sr 1.93 Ca 2.02 Cu 3.06 O 10+δ (Bi-2223) high temperature superconducting tapes were fabricated with Powder in tube (PIT) process. The traditional drawing process was partially replaced by groove rolling. The influence of cold working method on the filament density, the phase evolution mechanism as well as superconducting properties has been systematically studied. The groove rolling process could obviously enhance the filament density of Bi-2223 green wires, which thus led to the change of Bi-2223 formation rate. Meanwhile, although the heat treatment process has not been thoroughly optimized, the residual secondary phase content was still high, the critical current has been improved for more than 20% with groove rolling process, which should be attributed to the improved intergrain connections. On the other hand, the groove rolling process was also effective on the fabrication of Bi-2223 tapes with larger filament number, such as 85 or 121 to maintain the uniformity of filament deformation.

FexSe superconducting bulks with different norminal Fe content from x = 1.00 to 1.20 have been fabricated with high-energy ball milling (HEBM) aided sintering process. The influences of original Fe contents on the phase composition, microstructures and superconducting properties were systematically investigated. It was noticed that with the increase of original Fe content, the nonsuperconducting δ-FeSe phase content decreased, while the content of residual Fe increased correspondingly. Meanwhile, the increasing original Fe content could also cause the increase of FeSe crystal size, therefore, the (00l) texture of FeSe bulks was enhanced, which would be beneficial to the intergrain connections. Mossbauer spectra have been performed to study the magnetic orders in obtained FeSe bulks. A clear hyperfine magnetic sextet corresponding to the magnetic ordering with Hf of 333.0 kOe can be related to residual Fe in the bulks with Fe content higher than 1.10. The magnetic and superconducting properties of obtained bulks underwent a systematical change with original Fe content. The increasing Fe content can obviously enhance the superconducting performance of FeSe bulks and the optimal critical temperature, Tc of 9.0 K was obtained with original Fe content of 1.20.

Halogen doped Fe(Se, Te) superconducting materials were successfully synthesized through the eutectic transformation method. Both the distribution and morphology of the secondary hexagonal phase were tuned accordingly by substituting halogen elements. The morphology of the hexagonal phase was transformed from plate-like lamellar to agglomerated dots or bulk precipitates as the doping elements changed from F to Cl and Br. Moreover, the flux pinning mechanism, which represents the ability of materials to trap the penetrating magnetic field, was observed to change from Δκ to point pinning when doped by elements with heavier atomic mass. As a result, tremendous optimization of the critical current, upper critical field, and thermally activated flux flow activation energy were achieved in the as-prepared samples, with a specific value of 7 times, 2 times, and 3 times, respectively, in FeSe0.45Te0.5F0.05 compared with the values for the undoped sample. Our results reveal that the light-halogen element doping is a promising way to optimize the superconductivities in the Fe(Se, Te) systems and inspire us to manipulate the morphology of secondary phase to boost the superconductivities in iron-based superconductors.