Junichiro Kase

Preparation of the textured Bi-based oxide tapes by partial melting process

Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub x/ textured tapes with an excellent J/sub c/ were prepared by doctor-blade casting and a partial melt process. A green tape, composed of Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub x/ powder and an organic formulation, was laid on a silver foil and heat-treated. A highly textured microstructure was formed during slow cooling between 890 degrees C and 870 degrees C. X-ray diffraction and scanning-electron-microscope observation indicated that initially an oriented Bi/sub 2/Sr/sub 2/Cu/sub 1/O/sub x/ structure forms by growth from the melt, and it subsequently transforms to the Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub x/ phase. A grain alignment was significantly enhanced in the oxide layer with a thickness less than 20 mu m. Silver dissolved in the oxide from the substrate also plays an important role in the grain alignment by lowering the melting point of the oxide. The T/sub c/ of a tape could be increased to 89 K by quenching from 870 degrees C. The J/sub c/ was 1.3*10/sup 4/ A/cm/sup 2/ at 77 K, 0 T and 1.4*10/sup 5/ A/cm/sup 2/ at 4.2 K in a high magnetic field of 25 T.

Partial melt growth process of Bi2Sr2Ca1Cu2Ox textured tapes on silver

Bi2Sr2Ca1Cu2Ox textured tapes with an excellent Jc were synthesized by doctor-blade casting and a melt process. Green tape was laid on a silver foil and oxide/silver composite tape was heat treated. A highly textured microstructure was formed by slow cooling from 890°C to 870°C with the oxide in a partially melted state. The highest Tc and Jc (77 K, 0 T), 89 K and 1.3×104 A/cm2, respectively, were obtained by quenching from 870°C. However, Jc at 4.2 K was higher for the sample slowly cooled to room temperature and reached 1.4×105 A/cm2 at 25 T. These results indicate the potential for the practical application of Bi-based oxide superconductors at both 77 K and 4.2 K.

Improvement in critical current density of Bi2Sr2Ca1Cu2Ox tapes synthesized by doctor‐blade casting and melt growth

Bi2Sr2Ca1Cu2Ox superconducting tape with an excellent Jc at 4.2 K has been synthesized by a modified doctor‐blade process. A green tape prepared by doctor‐blade casting is laid on a silver foil and heat treated at various temperatures below and above its partial melting temperature. The Jc of the tape increases dramatically when heat treated in a narrow temperature range just above the melting temperature. A highly oriented layer structure with c‐axis alignment is obtained by the growth of aligned Bi2Sr2Ca1Cu2Ox grains in the partially melted state. This oriented microstructure produces an exceptional increase in Jc . Typical Jc ’s of the tape at 4.2 K are 50 000 A/cm2 at 0 T, 22 000 A/cm2 at 10 T, and 17 000 A/cm2 at 23 T. Note the small magnetic field dependence in Jc above 10 T.

Textured Thick Films of Bi2Sr2Ca1Cu2Ox

Thick textured films of Bi2Sr2Ca1Cu2Ox are obtained by partially melting and slow cooling doctor-blade-processed tapes. Heat treatment between 890 and 870°C produces aligned microstructures on substrates of either Ag or Ag coated MgO. During heat treatment dissolution of Ag into the liquid oxide occurs. Because silver lowers the melting point of the oxide, the processing temperatures must be increased by about 10°C when an MgO substrate is used. The resulting films show that the oxide surface plays an important role in grain alignment. The Bi2Sr2Cu1Ox phase, stable for temperatures greater than about 870°C, always transforms to the desired Bi2Sr2Ca1Cu2Ox phase regardless of the heat treatment history (quenched from 890°C and annealed at 870°C or slowly cooled from 890°C to 870°C). The texture of the microstructure, however, is dependent upon the heat-treatment path, with a higher texture obtained with slower cooling.

J/sub c/ characteristics of textured Bi-based oxide tapes

Critical current densities J/sub c/ have been measured by both the resistive and the magnetic methods at various temperatures and magnetic fields for grain-oriented Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub x//Ag composite tapes which were prepared by the combined process of doctor blade casting and crystal growth from the partially molten state. At 77 K, J/sub c/ of the tapes was greater than 10/sup 4/ A/cm/sup 2/ in zero magnetic field. However, J/sub c/ at 77 K is very sensitive to an applied magnetic field due to the large flux creep. This sensitivity of J/sub c/ to the magnetic field is significantly reduced at lower temperatures such that J/sub c/s of a practical level ( approximately 10/sup 5/ A/cm/sup 2/) are obtained at fields above 25 T at 4.2 K. The textured tape has an anisotropy in J/sub c/ with respect to magnetic field direction. This anisotropy increases with increasing temperature. However, the anisotropy factor at 4.2 K is small ( approximately 1.5) and should not limit practical application.

Preparation of Y-Ba-Cu-O Superconductors with High Critical Current Density by Unidirectional Melt Solidification

Y-Ba-Cu-O superconductors with high Jc have been prepared by a unidirectional melt solidification method. Uniformly oriented plate-shaped YBa2Cu3O7-y crystals containing small particles of Y2BaCuO5 have been grown successfully using the material with the nominal composition of Y1+xBa2-xCu3-2xOz (x0.4) as a starting material. The Jc values of the unidirectionally solidified Y-Ba-Cu-O samples, measured at 77 K using a four-probe method with continuous direct current, were 4600 and 12000 A/cm2 in 1 and 0.25 T, respectively. These values nearly coincided with those estimated from magnetization hysteresis loops using Beans critical state model.

Addition of New Pinning Center to Unidirectionally Melt Solidified Y-Ba-Cu-O Superconductor

Y-Ba-Cu-O bulk superconductor including small particles of BaSnO3 was synthesized by the unidirectional melt solidification method. Finely dispersed Y2BaCuO5 precipitates and BaSnO3 particles were observed in YBa2Cu3Oy matrix. The critical current density at 77 K in 8 kOe was 1.8×104 A/cm2, which was about four times higher than that of the BaSnO3-free specimen. Fine particles of BaSnO3 (0.1–1 µm) can act as effective flux pinning centers in the specimen and result in an increase of the critical current density.

Superconducting properties of textured Bi - Sr - Ca - Cu - O tapes prepared by applying doctor blade casting

Abstract Superconducting properties are discussed in connection with microstructure for grain oriented Ag/Bi 2 Sr 2 Ca 1 Cu 2 O x (2212) and Bi 2 Sr 2 Ca 2 Cu 3 O y (2223) tapes prepared by applying a doctor blade casting method. A.c. susceptibility and d.c. magnetization measurements indicate that the coupling between grains is significantly improved by grain orientation for both tapes. In particular, for the 2212 tape, weak coupling is not observed, and the tape has excellent transport J c > 10 4 A cm −2 at 77 K and zero magnetic field. However, at 77 K, the magnetic field dependence of J c is very large. At temperatures J c is much less sensitive to magnetic field and excellent J c -B properties were obtained. The 2212 tapes show transport J c values well above 10 5 A cm −2 at 4.2 K in 25 T and 5 × 10 4 A cm −2 at 20 K in 12 T. The results indicate that the tapes are promising for practical application with liquid helium or liquid hydrogen as coolant.

Effect of Cooling Rate on Critical Current Density for Bi2Sr2CaCu2O8+δ/Ag Composite Tapes

Bi2Sr2CaCu2O8+?(Bi-2212)/Ag composite tapes were prepared by melt-solidifying method and the relationship between cooling conditions after the melt-solidification and superconducting properties were investigated. A high cooling rate was found to be effective in suppressing the decomposition of Bi-2212 and in increasing Jc. A tape with high Jc of 2.4?105 A/cm2 at 4.2 K in 12 T was obtained by fast cooling. With annealing of this tape in N2 atmosphere, Jc of 3.1?104 A/cm2 at 77.3 K in 0 T was attained due to the enhancement of Tc.

Influence of the Flux Creep on the Critical Current Density in Bi2Sr2Ca2Cu3Ox and Y(Ho)Ba2Cu3Oy Superconductors

Intragrain Jc values of the textured Bi- and Y-based oxides were obtained at 4.2 K–77 K by the D.C. magnetization measurements with various sweep rates of the magnetic field. The Jcs at 4.2 K were comparable for both oxides. However, with increasing temperature, the Jc of the Bi-based oxide became much more sensitive to the electric field criterion Ec related to the sweep rate of the magnetic field than that of the Y-based oxide. As a result, Jc at 77 K of the Bi-based oxide was significantly decreased in magnetic fields for small Ec. The dependence of Jc on Ec for both oxides can be well understood using the flux creep model.