Zeming Yu

YBCO bulk superconductors prepared by top-seed PMP process

Abstract Top-seeded powder melting process (TSPMP) is an effective melting processing technique for the preparation of high J c bulk YBCO. The remarkable advantage of TSPMP is that it is very convenient to control the size and distribution of 211 particles within bulk YBCO. We observed the microstructure of TSPMP-YBCO sample and the mean size of 211 particles is about 500 nm and well distributed. The thickness of 123 planes is about 2 μm. The density of twins in line is about (1.2–1.6)×10 4 cm −1 . We fabricated the YBCO bulk sample with dimensions of ∅30 mm ×15 mm in batch by the TSPMP process. The maximum levitation force was measured to be 85 N at 77 K in 0.5 T, by using ∅28 mm ×30 mm NdFeB magnet.

The relationship of levitation force between single and multiple YBCO bulk superconductors

The levitation forces have been measured for many individual and multiple single-domain YBCO bulk superconductors. It is found that the levitation force of multiple YBCO bulk is closely related with levitation force of each individual single-domain YBCO bulk (used for composing the multiple bulks). The resultant force of the multiple YBCO bulks is slightly less than the summation levitation force of each individual single-domain YBCO bulks, while each individual YBCO bulk is fixed in position the same as that in the multiple YBCO bulk. The difference is about 5%, so that we can predict the resultant levitation force of multiple YBCO bulks with various shape and complex configuration more easily. This is very important and useful for simplifying the experimental tests on superconducting devices during designing and constructions for practical applications.

Effect of precursor powder on microstructure and critical current density of (Bi,Pb)-2223 tapes

Abstract The phase assemblage of precursor powder was controlled by changing the calcination temperature from 830 to 860 °C. The microstructure and the critical current density of tapes were strongly dependent on the phase composition of the precursor powder. SEM analysis shows that large needles of 2212 with an angle of 30° were formed in the tape using the precursor powder calcined at 845 °C for 12 h. Some larger size CuO grains were presented in the tape with lower Jc value. Some secondary phases come from the precursor powder. It is important to obtain a reasonable phase composition in the precursor powder for the tapes with a good microstructure and high Jc.

The effect of processing parameters on the phase assemblage and critical current density in Bi(Pb)-2223 tapes

Abstract In this work, the phase assemblage of the precursor powder, especially the Ca 2 PbO 4 phase, was changed by adjusting the powder calcination temperature and the correlation between the properties of precursor powder and the heat treatment process of tapes, as well as the critical current has been analyzed. It was found that the tape heat treatment temperature has close relations with the content of the Ca 2 PbO 4 phase in the precursor powders. The optimum heat treatment temperature of the tape increases dramatically and the tape sintering processing window is enhanced when the relative content of Ca 2 PbO 4 phase decreasing in the precursor powders.

Growth and characterization of doped CeO2 buffers on Ni-W substrates for coated conductors using metal organic deposition method

CeO2 and Ce0.8M0.2O2−d films (M = Mn, Y, Gd, Sm, Nd and La) with (00l) preferred orientation have been prepared on biaxially textured Ni-W substrates by metal organic decomposition (MOD) method. The factors influencing the formation of cracks on the surface of these CeO2 and doped CeO2 films on Ni-W substrates were explored by X-ray diffraction (XRD), scanning electron microscopy (SEM) analysis, atomic force microscopy (AFM) and differential scanning calorimetry (DSC). The results indicate that many factors, such as the change of the ionic radii of doping cations, the transformation of crystal structure and the formation of oxygen vacancies in lattices at high annealing temperature, may be related to the formation of cracks on the surface of these films. However, the crack formation shows no dependence on the crystal lattice mismatch degree of the films with Ni-W substrates. Moreover, the suppression of surface cracks is related to the change of intrinsic elasticity of CeO2 film with doping of cations with a larger radius. SEM and AFM investigations of Ce0.8M0.2O2−d (M = Y, Gd, Sm, Nd and La) films reveal the dense, smooth and crack-free microstructure, and their lattice parameters match well with that of YBCO, illuminating that they are potentially suitable to be as buffer layer, especially as cap layer in multi-layer architecture of buffer layer for coated conductors.

Development of Low-Fluorine Solution for YBCO Film Growth

We developed a simple and stable precursor solution containing low fluorine content to fabricate YBa2Cu3Oy (YBCO) films on CeO2/YSZ/CeO2 buffered NiW substrates. The fluorine content in the precursor solution could be controlled at 23.1% by tuning the ratio of barium trifluoroacetate to barium acetate. Fast pyrolysis process and smooth surface of YBCO precursor films could be acquired by using the low-fluorine solution. The phase, texture, microstructure, and superconducting properties of YBCO film were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscope, and four-probe method, respectively. The crystallized YBCO film showed good out-of-plane and in-plane textures. A high critical current density Jc of ~3 MA/cm 2 (77 K, 0 T) could be obtained in the YBCO/CeO2/YSZ/CeO2/NiW sample derived from the low-fluorine solution. The results indicate that the controllable fluorine content solution could be useful for epitaxial growth of YBCO films on buffered NiW substrates.

Fabrication of Cube-Textured Ni/Cu bi-Metallic Substrate

Abstract The influences of electro-depositing parameters on the formation of cube-textured Ni layer on Cu tape were studied. Results show that the cube texture can be obtained when the cathode potential is low enough and the pH value of electrolyte is in an appropriate range from 3 to 4. The influences of electro-depositing parameters could be explained by the geometrical selection theory. Finally, the thermal stability of texture of Ni layers was also investigated.

Partial-melting process for (Bi,Pb)-2223/Ag tapes prepared using two-compound powders

Abstract The heat-treatment parameters of both the calcination and the reaction heat treatment play a crucial role in the achievement of high quality (Bi,Pb)-2223/Ag tapes. The melting process has been successfully used for the processing (Bi,Pb)-2212 in order to achieve good grain alignment and connection. In this paper, we report the partial-melting process to fabrication of (Bi,Pb)-2223/Ag tapes and discuss the results of the effects of the partial-melting treatment on microstructure and phase development. The results indicated that the tapes prepared by partial-melting process exhibit higher grain packing density and better texture than those prepared by the normal process. As a result, the critical current density is improved.

Improvement of superconducting properties by Y-211 particle size and oxygenation temperature control for bulk Y–Ba–Cu–O

Abstract The effects of CeO 2 adding on the reduction of Y-211 particle size and the oxygenation temperature on T c have been studied. It is shown that the decrease of pre-sintering temperature is more effective than adding CeO 2 for the Y-211 particle size reduction. For bulk YBCO, the oxygenation temperature affects not only the critical temperature T c and other superconducting property, but also its microstructure. Our results show the low oxygenation temperature is suitable for the oxygenation process of melt-textured bulk YBCO. If the oxygenation temperature is above 600 °C, the macrocrack will propagate.

Effect of the different precursor powder on the superconducting property of Bi-2223/Ag tapes

Abstract Two kinds of precursor powder were studied: one was a mixture of Bi-2212 and CaCuO 2 and the other was a mixture of Bi-2201 and CaCuO 2 . By comparing the formation rate of Bi-2223 synthesized by two kinds of precursor powder, results indicated that the mixture of Bi-2212 and CaCuO 2 was best phase assemblage of precursor powder; it could form the Bi-2223 phase rapidly. Concerning the mixture of Bi-2212 and CaCuO 2 , the content of CaCuO 2 would affect the superconducting property of Bi-2223/Ag tapes, the I c value (77 K, self-field) would increase with increasing the content of CaCuO 2 when the additional content of CaCuO 2 was smaller than 10%. If the additional content of CaCuO 2 was larger than 10%, the I c of Bi-2223/Ag tapes (77 K, self-field) would decrease. But it was very interesting that the I c – B curve would move to high magnetic field with increasing the content of CaCuO 2 , even if the additional content of CaCuO 2 was larger than 10%. It showed that adding the additional CaCuO 2 could enhance the flux pinning of Bi-2223/Ag tapes.