H. Maeda

Growth and superconducting properties of Bi2Sr2CaCu2O8+δ single-crystal whiskers using tellurium-doped precursors

Bi2Sr2CaCu2O8+δ (Bi2212) single-crystal whiskers were grown by annealing sintered Te-doped Bi–Sr–Ca–Cu–O precursors that had not passed through a melt-quenched glassy state, a process that is known well at this time. The optimum nominal composition of precursors is Ca-rich Bi2Sr2Ca2Cu2Te0.5Ox with a Te content of 0.5. The whiskers contain no Te element. The precursors are mainly composed of Bi2212 and (SrCa)3TeO6 (STO) phases. The STO or the element Te acts as a catalyst in enhancing whisker growth. The critical current (Ic) with a multibranched structure of the current–voltage characteristics due to the intrinsic Josephson junctions along the c axis is quite uniform, showing that the whiskers are homogeneous and of good quality.

Phase development and kinetics of high temperature Bi-2223 phase

Abstract The two-dimensional nucleation (random)-growth mechanism were observed as a support for the related previous works, which is attributable to the growth of the Bi-2223 grain in the a–b plane direction of the Bi-2212 matrix is being much faster than in the c-direction, or that the early-formed plate-like 2212 phase confines the 2223 product. At the beginning of the reaction, the additional phases are partially melted. Because of the structure, composition and thermal fluctuation, the 2223 nucleates and grows in the phase boundary between the liquid phase and Bi-2212. It was shown here that the nucleation and the growth rate were relatively fast between 0 and 36 h. At the final stage, between 36 and 60 h, because of the impingement of the growth fronts of different nuclei, the high formation rate of 2223 is suppressed. The major reactant 2212 remains as a solid and its plate-like configuration determines the two dimensional nature of the reaction. The amount of liquid forms during reaction is small.

Microstructures and properties of Bi2212/Ag tapes grown in high magnetic fields

Abstract A comparison experiment has been carried out to study the effect of magnetic melt processing (MMP) on fabrication of Bi2212 tapes with reasonable thickness. Vertical magnetic fields up to 10 T were applied during the melt-processing of dip-coated monolayer Bi2212/Ag tapes (over 60 μm in thickness) which were set horizontally and vertically, respectively. It is found that in the horizontally set tapes the texture development has been obviously enhanced and the transport current density J c increases dramatically with the magnetic field strength H a up to 7.5 T. When H a increases from 7.5 to 10 T, however, J c decreases sharply and the reason is still unclear. The magnetic fields have quite different effects on the vertically set tapes and an interference between the Bi2212-silver interface effect and the magnetic field effect on texture development can be detected. These results show that MMP is effective to enhance the texture development in monolayer Bi2212/Ag tape with reasonable thickness and can be applied in its fabrication.

Effect of precursor composition on textured crystal growth of Bi2212 bulks in high magnetic field: investigations on Pb substitution and Ag doping

A comparison experiment has been carried out to study texture development in pure, Pb-substituted and Ag-doped Bi2212 bulks prepared by magnetic melt processing (MMP). X-ray diffraction and magnetization measurements show that no texture can be developed in pure Bi2212 bulk in 9 T magnetic field while obvious texture has been obtained in the Pb-substituted and Ag-doped bulks. Ag doping is found especially helpful for improving grain alignment and density of Bi2212 bulk prepared by MMP. It is proposed that during the crystal growth of pure Bi2212 there exists an obstacle to texture development in MMP. Some changes in precursor composition can overcome this obstacle and attention should be paid to precursor composition to enhance texture development in Bi2212 prepared by MMP.

Microstructure and critical current density of Bi2212 tapes grown by magnetic melt-processing

Abstract Vertical magnetic fields up to 9 T were applied to the magnetic melt-processing (MMP) of dip-coated and Ag-sheathed Bi2212 mono-core tapes with a core thickness of about 150 μm, which were set horizontally. SEM observation shows that a uniform high-degree of texture is formed throughout the thickness of the tapes by the magnetic field H a during MMP, whereas a large portion of the area is non-textured without H a . The critical current density J c of the tapes increases with increasing H a up to about 6 T due to the texture development. However, when H a >6 T some disturbance in J c appears; one is increased and the other is decreased. This may be due to the change of O 2 atmosphere around the tapes during MMP, which comes from the change of O 2 gas flowing by the magnetic field. The results suggest that a high magnetic field is very effective to enhance the texture development and to fabricate tapes with high critical current I c .

Growth and electrical transport characteristics of Bi2Sr2Ca1Cu2Ox and Bi2Sr2CuOx single-crystal whiskers using tellurium-doped precursors

Abstract Bi 2 Sr 2 CaCu 2 O x (Bi2212) and Bi 2 Sr 2 CuO x (Bi2201) single-crystal whiskers were grown by annealing sintered Te-doped Bi–Sr–Ca–Cu–O precursors. The growth of Bi2212 or Bi2201 whiskers definitely depends on the Ca content of precursors; the optimal composition is Ca-rich Bi 2 Sr 2 Ca 2 Cu 2 Te 0.50 O x for Bi2212 and Bi 2 Sr 2 CaCuTe 0.50 O x for Bi2201. In the precursors Te ions exist in the phase of (SrCa) 3 TeO 6 . The whiskers contain no Te element. The Bi2212 whiskers have a critical temperature of about 80 K and the critical current with a multibranched structure of the current–voltage characteristics due to the intrinsic Josephson junctions along the c -axis is quite uniform, showing that the whiskers are homogeneous and of good quality. The Bi2201 whiskers show semiconducting resistance with no superconducting transition down to 4.2 K.

Effect of pressing and Li doping on superconducting properties of Ag-sheathed Bi-2223 tapes

For Bi-2223 Ag-sheathed tapes, pressing conditions between sintering were optimized. It was found that with an increase in the pressure from 1 t cm -2 to 10 t cm -2 , I c of the tape was increased. To elucidate whether Li doping increases the I c of the tape as reported in the bulk sample, Li doping was carried out. It was found that doping of 0.1-0.3 at.% Li increased I c by 10%, which is consistent with the bulk data. As a result of I c -B measurement at a temperature of 4.2 K, it was found that I c /I c (O T) of the 0.3 at.% Li-doped tape under a magnetic flux density of 10 T was 20% higher than that of the non-doped tapes.

Powder production methods of Bi-Pb-Sr-Ca-Cu-O superconductors

In this work the production of Bi-Pb-Sr-Ca-Cu-O powder precursors by three different chemical route techniques, namely freeze-drying, spray-drying and thermal co-decomposition methods, plus the two-powder oxide route method, were investigated experimentally. The most reactive precursor made by the chemical route was made by the freeze-drying method. The reactivity of the powders was related to the particle size and stoichiometric changes during heat treatment. Sintering for 60 h at was sufficient to form nearly pure single-phase (91%) Bi-2223 when the freeze-drying method was employed, whereas the maximum amount of Bi-2223 phase produced by the thermal co-decomposition method was 77% for 83 h sintering at in air, and the maximum amount of Bi-2223 phase produced by spray drying was 55% for 150 h sintering at in air. The two-powder oxide route was found to be intermediate in efficiency among the other techniques used here, i.e. it is worse than freeze drying and thermal co-decomposition but better than spray drying.

Effect of ball milling materials and methods on powder processing of Bi2223 superconductors

Various milling systems consisting of agate and polypropylene grinding containers, agate and YSZ balls, and dry and wet milling were used in planetary ball-milling and YSZ balls and YSZ container were used in wet and dry attrition milling. The differently milled powders were then evaluated by measurements of particle size, surface area, porosity, size distribution and chemical analysis of the Si, Zr and C contents. The results show that dry milling is much more efficient for particle size reduction in planetary milling than wet milling, whereas wet milling and dry milling gave quite similar results in attrition milling. Meanwhile contamination was found in powder milled with an agate container with agate balls. Some C contamination from the polypropylene container was detected after milling, but negligible Zr from YSZ balls and C from the grinding carrier (hexane). It was found that after 1 h milling in the planetary mill fracture mechanisms transform from the elastic to the plastic region. Therefore, further milling is not very effective. It was also shown that the Bi2212 phase decomposes into several non-superconducting oxides such as , CuO and a main amorphous phase after extensive dry milling.

Texture development in Bi-based superconductors grown in high magnetic fields and its effect on transformation of Bi(Pb)2212 to Bi(Pb)2223

Abstract Ag-doped Bi2212 bulks with a magnetization anisotropy factor at 5 K as high as 6.5 have been obtained by magnetic melt processing (MMP) in a magnetic field of 13 T, which shows that high magnetic fields are of great importance to achieve extremely high texture in Bi2212 bulks. Some texture is also introduced in Bi(Pb)2212-based bulks by MMP. This texture can result in a texture development in the Bi(Pb)2223 phase when most Bi(Pb)2212 is transformed into Bi(Pb)2223 using a long sintering time, which suggests that there is a correlation between the grain alignment of Bi(Pb)2223 and its precursor Bi(Pb)2212. The atmosphere of 1 % O 2 + Ar is found especially useful for MMP of Bi(Pb)2212-based bulks from which Bi(Pb)2223 of high content can be formed.