W.P. Chen

Processing of Ag-doped Bi2212 bulks in high magnetic fields: a strong correlation between degree of texture and field strength

Abstract A series of high magnetic fields up to 9 T have been applied to the preparation of Ag-doped Bi2212 bulks and a strong correlation is observed between the field strength and the degree of texture developed in the bulks. Two powders of Bi 2.2 Sr 1.8 CuO x and CaCuO 2 were synthesized separately and then 10 wt.% Ag was doped to their equimolar mixture, from which the Bi2212 bulks were formed by a partial melt–solidification process in the high magnetic fields. The degree of texture in the bulks was evaluated quantitatively by X-ray diffraction and magnetization measurement. It is found that the applied magnetic fields tend to orient Bi2212 grains with their c -axis parallel to them and obvious texture exists in all samples prepared in the magnetic fields; while in samples prepared without magnetic field Bi2212 grains orient randomly. In the field range of 0–9 T, the degree of texture increases notably with increasing the magnetic field. In 9 T Bi2212, bulks with magnetization anisotropy factor more than four and a density of 6.15 g/cm 3 are obtained and SEM observation also shows a highly oriented grain alignment.

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.

Effect of magnetic field strength in melt-processing on texture development and critical current density of Bi-oxide superconductors

Abstract Vertical magnetic fields up to 15 T were applied to the magnetic melt-processing (MMP) of Bi2212 bulks and Ag-sheathed tapes with a core thickness above 80 μm, which were set horizontally. Texture with the c -axis along the direction of magnetic field applied during MMP is developed due to the anisotropy in magnetic susceptibility of Bi2212. The degree of texture and the anisotropy factor in magnetization increase almost linearly as the magnetic field strength H a during MMP is increased. The anisotropy factor in magnetization reaches 6.5 at a H a of 13 T for Ag-doped Bi2212 bulks, whereas for Bi(Pb)2212 in which Pb partially substitutes for Bi the factor is suppressed to a half of that of Ag-doped Bi2212. The transport critical current I c and the critical current density J c of Bi2212 tapes also increase with increasing H a due to the texture development and I c reaches above 1000 A in self-filed for the tapes with a core thickness of 180 μm. However, for further thicker tapes, I c decreases. The suppressions in the I c for the tapes and in the anisotropy factor in magnetization for Bi(Pb)2212 bulks are due to inhomogeneous melting, and inhomogeneous nucleation and growth of crystals during MMP. When crystal growth is restricted, crystal alignment is suppressed.

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 .

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.

Critical current density and a.c. losses of Ag-sheathed Bi2223 tapes with Sr–V–O barriers

Abstract Ag-sheathed Bi2223 multifilamentary tapes with 19 filaments with Sr–V–O (SVO) barriers have been prepared using powder-in-tube method. The SVO barriers are arranged inside and/or outside each filament. The critical current density J c at 77 K strongly depends on the preparation conditions such as the heat-treatment temperature and intermediate rolling. For the barrier tapes the J c values are higher than those of the non-barrier tapes for all the barrier configurations. Furthermore, for all the barrier tapes the transport a.c. losses at 77 K in self-field are lower than those of the non-barrier tapes. In particular, the inside-barrier tapes will give us an excellent possibility to reduce the over all J c and the ease to fabricate to tape shape.

Oriented crystal growth in Bi(Pb)2223 bulks prepared with magnetic melt processing

A containerless melting-solidification process has been developed for Bi-Pb-Sr-Ca-Cu-O system by using a mixture of Bi(Pb)2212 and CaCuO/sub 2/ as precursors. Almost no solute liquid is spread out from the pellet during the melt processing and Bi(Pb)2212 is reformed after solidification without changing the composition, therefore bulks of high Bi(Pb)2223 content can be formed by a long time sintering. When a magnetic field of 8 T is applied during the melting-solidification process, an obvious texture is introduced in the reformed Bi(Pb)2212. We found that an oriented crystal growth of Bi(Pb)2223 is achieved from the textured Bi(Pb)2212. This indicates that the magnetic melt processing can be applied to obtain texture development in Bi(Pb)2223 bulks by orienting Bi(Pb)2212 grains firstly.

Highly Oriented Crystal Growth of Bi-Based Oxide High- T c Superconductors in High Magnetic Fields

Grain alignment is of great importance for achieving high critical current density in high-T c superconductors. In recent years intensive research has been focused on obtaining texture in high-T c superconductors and several techniques have been developed. Among these techniques, magnetic melt processing (MMP), i.e. oriented crystal growth in high magnetic fields, can be applied to both Y-based and Bi-based superconductors. Due to the susceptibility anisotropy of high-T c superconductors along the crystal axes, some texture can be developed in high-T c superconductors by melting and solidification in a high magnetic field.

Fabrication and Properties of (Bi,Pb)-2223 Tapes with Different Lead Content

The (Bi,Pb)-2223/Ag monofilamentary tapes with different Pb-content have been prepared by using powder-in tube (PIT) method. Precursor powders were fabricated by mixing the (Bi,Pb)-2212 and CaCuO2 powders, which were prepared by solid state reaction method with different calcination conditions. The Pb-content was changed in Bi2-xPbxSr1.9Ca1Cu2Oy powder with x value of 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 and 0.5. The transport critical current (Ic) measurements indicated that the optimum sintering temperature is increased with the decrease of Pb-content. In the 8% oxygen atmosphere, sintering at 818 °C, the tape with x=0.3 has highest Ic of 23A (77K,0T), while the tape with 0.2 Pb-content exhibit higher Ic of 30A (77K,0T) when it was heat treated at 828 °C. The Jc-B dependence is almost same for all the tapes with different Pb-content in the case of applied field parallel to the tape surface.