Ichiro Matsubara

Growth of Bi-Sr-Ca-Cu-O based superconducting whiskers

Abstract Superconducting whiskers of the Bi system have been grown by heating a glassy melt-quenched plate in a stream of O 2 gas. We have examined the growing phase and superconductivity of the whiskers grown at the different heating and cooling conditions. The Bi 2 Sr 2 CaCu 2 O 8 (2212 phase) whiskers are grown from a wide range of initial compositions when the melt-quenched plates are heated at 840°C. The Bi 2 Sr 2 CuO 6 (2201) phase is dominant in the whiskers grown at lower temperature, 820 and 810°C. The growing pure 2223 whiskers have not been obtained so far. For the superconductivity of the 2212 whiskers, high oxygen partial pressure ( P O2 , rapid cooling and higher heating temperature are preferable. Low P O2 and slow cooling are preferable for the 2223 phase contained in the 2212 whiskers as a minor part. Heating time does not give remarkable effects on the growing phase and superconductivity.

Growth of Li-doped and dopant-free Bi-Sr-Ca-Cu-O superconducting whiskers

Abstract The effects of dopant, Li, Na, K, Ga, Ge, Sb and Te, on the growing conditions and superconductivity in Bi-Sr-Ca-Cu-O whiskers have been examined and compared with dopant-free whiskers, synthesized by heating a glassy melt-quenched plate in a stream of O 2 gas. Every whisker has the 2212 structure as its main phase and a small amount of the 2223 phase, so that two resistance drops are observed in the R-T curve. Only Li is effective for increasing the T c of both the 2212 and the 2223 phase, and the T c s are enhanced with increasing Li content in the whiskers. The onset temperatures of Li-doped whiskers for the 2212 and 2223 phases are higher than those of the other samples by about 10 K and 6 K respectively. The T c,zero has a good correlation with the lattice parameter c of the 2212 phase. The length of the c -axis of the Li-doped sample is about 0.1 A longer than that of the other samples. A T c,zero value of 107 K is obtained and a T c,zero of 112 K is indicated for the 2223 phase by extrapolation of the R-T curve in the Li-doped whiskers. A J c of 34 000 A/cm 2 at 77 K and 300 000 A/cm 2 at 66 K in a zero magnetic field have been obtained for the Li-doped whisker. The J c decreases steeply in magnetic field above 0.2 T at 77 K, and an anisotropy of the J c value of samples in a magnetic field has been observed.

Flexible superconducting whiskers of the Li‐doped Bi‐Sr‐Ca‐Cu oxide

Flexible Li‐doped Bi‐Sr‐Ca‐Cu‐O superconducting whiskers have been prepared by heating a glassy melt‐quenched plate in a stream of O2 gas. The whiskers show a resistance drop at 110 K with a zero resistance at 81 K. These values are higher than those of the Pb‐doped Bi‐Sr‐Ca‐Cu‐O whiskers by 5 and 11 K, respectively. The doping of Li is effective to raise Tc for both the 2212 phase and the 2223 phase. The Jc values of the Li‐doped whiskers are 30u2009000 A/cm2 at 77 K and 300u2009000 A/cm2 at 66 K in a zero magnetic field.

Superconducting whiskers and crystals of the high Tc Bi2Sr2Ca2Cu3O10 phase

Whiskers of the high Tc Bi2Sr2Ca2Cu3O10u2009 (2223) phase have been prepared by annealing Bi2Sr2CaCu2O8 u2009(2212) whiskers in a Bi2Sr2Ca4Cu6Pb0. 5Ox powder. By the same method, single‐crystal sheets of the 2212 phase have also been converted to the 2223 phase, resulting in 2223 sheet crystals as large as approximately 1 mm2. Although the original sample outlines are retained after the phase conversion, the smooth surfaces were lost due to contact with the powder during annealing. A zero‐resistance temperature of 107 K and a maximum Jc value of 7.3×104 A/cm2 at 77 K in a zero magnetic field were obtained for the 2223 whisker.

Growth of superconducting whiskers in the Bi system

The preparation and properties of Bi 2 Sr 2 CaCu 2 O x (2212) and Bi 2 Sr 2 Ca 2 Cu 2 O x (2223) superconducting whiskers are presented. The 2212 whiskers have been grown by annealing a melt-quenched glass plate in a stream of O 2 gas. For the whisker growth, the glass precursor, AI addition to the glass, and a steady stream of O 2 are required. Each whisker, 2–10 μm thick, 10–500 μm wide and ~ 15 mm long, is composed of several plate-like single crystals which are stacked in a layered structure. The 2212 whiskers are flexible and are elastically bent up to a radius of curvature 0.2 mm corresponding to a bending strain of 0.5%. On the other hand, the 2223 whiskers have been prepared by a “conversion by annealing in powder” method (CAP method). They have been obtained by annealing the 2212 whiskers in a Ca- and Cu-rich Bi Sr Ca Cu Pb O calcined powder. The appropriate annealing temperature is in the partial melting region of the calcined powder, as determined by DTA analysis. The phase conversion is brought about by diffusion of Ca and Cu into the 2212 whiskers from the liquid phase. The CAP method has been applied also to single crystals prepared by a flux method, resulting in 2223 sheet crystals as large as 1 mm 2 .

Effects of Li doping on the superconducting properties of Bi-based superconducting whiskers

Abstract The effects of Li doping on the superconductivity of Bi-based superconducting whiskers have been investigated. The Li doping is effective in increasing T c of both the Bi 2 Sr 2 CaCu 2 O x (2223) whiskers. The Li-doped 2212 whiskers show a T c at 82 K, which is higher than that of the undoped 2212 whiskers by 5 K. The Li-doped and undoped 2223 whiskers have been prepared by the Conversion by Annealing in Powder (CAP) method. An enhancement of T c by 1.2 K is achieved by Li doping of the 2223 whiskers. The T c of the 2223 phase is commonly accepted to be lowered by a reduction of hole concentration, which is opposite to the situation of the 2212 phase. Therefore, the mechanism of the enhancement of T c is not due to the optimization of the hole concentration. From the compositional analysis and the measurement of the c -axis lattice parameter, the Li doping is thought to be effective in increasing the Sr/Ca ratio for both kinds of whiskers, which causes the enhancement of T c .

Conversion of Bi2Sr2CaCu2Ox whiskers to the Bi2Sr2Ca2Cu3Ox phase by annealing in powder

Abstract Bi 2 Sr 2 Ca 2 Cu 3 O x (2223) whiskers are prepared by annealing Bi 2 Sr 2 CaCu 2 O x (2212) whiskers in a Bi 2 Sr 2 Ca 4 Cu 6 Pb 0.5 O x calcined powder (CAP method: conversion by annealing in powder method). The effects of the composition of the calcined powder on the phase transformation of the whiskers have been investigated. The CAP method has been applied to 2212 whiskers using calcined powders with various compositions. To obtain pure 2223 whiskers, the composition of the calcined powder is important. The melting temperature of the calcined powder should be overlapping the temperature region, at which the whiskers can be recovered from the powder, and the region of the 2223 phase formation. The phase conversion is thought to proceed as follows. A liquid phase is generated in the powder around the 2212 whiskers. It reacts with the 2212 whiskers, resulting in the nucleation of the 2223 phase at the interface between the liquid phase and the whiskers. By supplying Ca, Cu, and Pb ions into the whisker through the liquid phase, the 2223 phase spreads into the whiskers but the outline of the original crystal shape remains. From a kinetic study on the CAP method, diffusion controlled two-dimensional growth with decreasing nucleation rate is proposed as the most probable model for the phase conversion from the 2212 to the 2223 phase.

Lower critical field and reversible magnetization of (Bi, Pb)2Sr2Ca2Cu3Ox superconducting whiskers

Abstract Magnetization measurements have been carried out on Pb doped Bi 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) whiskers with the applied field parallel to the c -axis. The temperature dependence of the first flux penetration field H p can be approximated by H p ( T ) = H p (0)[1 − ( T / T c ) 2 ] in the region T > 50 K, while it increases upward with decreasing temperature especially at T H p = H c 1(0) = 135 G is obtained by fitting the experimental data for T > 50 K. The penetration depth has depth also been evaluated from the reversible magnetization curves. λ ab − is a linear function of the H c (0) = 3300 G have been calculated from the H c1 (0) value together with the tupper critical field and coherence lenghts. penetration depth has also been evaluated from the reversible magnetization curves. λ ab −2 is a linear function of the temperature near T c .

Preparation and characterization of the Ga doped Bi-Sr-Ca-Cu-O system

Abstract The preparation and the characterization of the 2212 phase in the Bi2Sr1.7CaGaxCu2−xOy system prepared by a solid- state reaction is reported. The dependence of the thermal, structural and superconducting properties on the Ga content is discussed. XRD data and SEM-EDX observations revealed the presence of Bi-Sr-Ca-Ga complex oxides in the Ga richer compounds. The electrical and magnetic measurements showed a single step as well as a broadening of the transitions width at an increase of the Ga content. The higher critical current density value (3.8×106 A cm-2 at 5 K for H = 2 KOe and 5.3 × 106 A cm-2 at 5 K H = 2 KOe for x = 0 and x = 0.8, respectively) found in the Ga doped sample has been explained as a direct consequence of a stronger pinning due to the presence of the Ga containing complex oxides.

Li‐doped Bi three‐layered superconducting whiskers

Li‐doped Bi2Sr2Ca2Cu3O10 (2223) whiskers of single crystals with several millimeters length have been successfully prepared and it is revealed that Li‐doping is effective for increasing critical temperature (Tc) of the 2223 phase. By annealing Bi2Sr2CaCu2O8 (2212) whiskers in a calcined powder with a composition of Bi2Sr2Ca4Cu6Pb0.5LixOy (x=0.2, 0.3) only within the very narrow temperature region, the conversion from the 2212 phase to the 2223 phase and Li‐doping are achieved at the same time. The Tc value of the Li‐doped 2223 whiskers (108.1 K) is reproducibly 1.1 K higher than that of undoped 2223 whiskers prepared by the same method using a calcined powder without Li.