T. Yamashita

The effect of Yb addition in Bi2Sr2Ca1−xY bxCu2Oy partial melted thick films

To study the phase relations in the Bi-2212 and Yb2O3 system, Bi2Sr2Ca1-xYbxCu2Oy thick films are prepared by partial melt processing via an intermediate reaction between Bi-2212 and Yb2O3. When Bi-2212 and Yb2O3 are partially melted and then slowly cooled, solid solutions of Bi2Sr2Ca1-xYbxCu2Oy form by reactions between liquid and solid phases which contain Yb. Following these reactions, Ca is partially replaced in Bi-2212 matrix and participates in the formation of secondary phases, such as Bi-free, (Ca, Sr)O-x and CaO. Variation of the Bi-2212-Yb2O3 ratios and processing parameters changes the balance between the phases and leads to different Yb:Ca ratios in the Bi-2212 matrix of processed thick films. When the partial melting process is optimized for each sample to minimize the growth of secondary phases, x = 0.42-0.46 for the samples prepared at pO(2) = 0.01 atm, x = 0.24-0.29 for the samples prepared at pO(2) = 0.21 atm, x = 0.18-0.23 for the samples prepared at pO(2) = 0.99 atm are obtained regardless to the starting compositions. It is found that superconducting properties of Bi2Sr2Ca1-xYbxCu2Oy thick films strongly depend on the processing conditions, because the conditions result in different Yb content in the Bi-2212 matrix and the volume fraction of the secondary phases. The highest T-c(0) of 77, 90 and 91 K were obtained for the samples processed at 0.01, 0.21 and 0.99 atm of O-2, respectively.

Microstructural characterization of quenched melt-textured YBa 2 Cu 3 O 7−δ materials

The microstructure of YBa2Cu3O7-δ (YBCO) materials, melt-textured in air and quenched from the temperature range 900-990°C, has been characterized using a combination of x-ray diffractometry, optical microscopy, scanning electron microscopy, transmission electron microscopy, and energy dispersive x-ray spectrometry. BaCu2O2 and BaCuO2 were found to coexist in samples quenched from the temperature range 920-960°C. The formation of BaCu2O2 preceded the formation of YBCO. Once the YBCO had formed, BaCu2O2 was present at the solidification front filling the space between nearly parallel platelets of YBCO. Large Y2BaCuO5 particles at the solidification front appeared divided into smaller ones as a result of their dissolution in the liquid that quenched as BaCu2O2.

Re-melting of Bi-2212/Ag laminated tapes by partial melting process

Abstract Bi-2212 tapes are prepared by a combination of dip-coating and partial melt processing. We investigate the effect of re-melting of those tapes by partial melting followed by slow cooling on the structure and superconducting properties. Microstructural studies of re-melted samples show that they have the same overall composition as partially melted tapes. However, the fractional volumes of the secondary phases differ and the amounts and distribution of the secondary phases have a significant effect on the critical current. Critical current of Bi-2212/Ag tapes strongly depends on the maximum processing temperature. Initial J c s of the tapes, which are partially melted, then slowly solidified at optimum conditions and finally post-annealed in an inert atmosphere, are up to 10.4×10 3 A/cm 2 . It is found that the maximum processing temperature at initial partial melting has an influence on the optimum re-heat treatment conditions for the tapes. Re-melted tapes processed at optimum conditions recover superconducting properties after post-annealing in an inert atmosphere: the J c values of the tapes are about 80–110% of initial J c s of those tapes.

Partial melt processing and electrical properties of Bi - Sr - Ca - Cu - O superconducting thick films on (100) MgO substrates

Superconducting thick films of (Bi-2212) on single-crystalline (100) MgO substrates have been prepared using a doctor-blade technique and a partial-melt process. It is found that the phase composition and the amount of Ag addition to the paste affect the structure and superconducting properties of the partially melted thick films. The optimum heat treatment schedule for obtaining high has been determined for each paste. The heat treatment ensures attainment of high purity for the crystalline Bi-2212 phase and high orientation of Bi-2212 crystals, in which the c-axis is perpendicular to the substrate. The highest , obtained by resistivity measurement, is 92.2 K. The best value for (transport) of these thick films, measured at 77 K in self-field, is .

The extrusion of continuous lengths of YBa2Cu3O7−x wire using hydroxypropyl methylcellulose

Wires of YBa2Cu3O7-x were fabricated by extrusion using a hydroxypropyl methylcellulose (HPMC) binder. As little as 2 wt.% binder was added to an oxide prepared by a novel co-precipitation process, to produce a plastic mass which readily gave continuous extrusion of long lengths of wire in a reproducible fashion. Critical temperatures of 92 K were obtained for wires given optimum high-temperature heat treatments. Critical current densities greater than 1000 A cm-1 were measured at 77.3 K using heat treatments at around 910°C for 10 h. These transport critical current densities, measured on centimeter-long wires, were obtained with microstructures showing a relatively dense and uniform distribution of randomly oriented, small YBa2Cu3O7-x grains.

Phase evolution of the quenched melt of with 20 mol% additions

Samples of mol% have been melt processed and quenched from temperatures ranging from 975 to 1100. The microstructure of the samples have been characterized via a combination of x-ray diffractometry, optical microscopy, scanning electron microscopy, energy dispersive x-ray spectrometry and wavelength dispersive x-ray spectrometry. (BC1) and (BC2) crystallize from the melt of samples quenched from temperatures between 985 and in air. The average yttrium content differs for BC1 and BC2, and it is 4.3 and 5.1 at.%, respectively. Holding times of 20 hours at temperatures above or equal to give rise to a dendritic pattern of BC1 surrounded by BC2. The complex changes of the nature of the melt as a function of temperature and time are likely to play a significant role in the mechanism of melt texturing.

Melt-textured YBa2Cu3O7 − x wires having plate-like grains

YBCO wires which consist of well oriented plate-like fine grains are fabricated using a moving furnace to achieve higher mechanical strength. Melt-texturing experiments have been undertaken on YBCO wires with two different compositions: YBa1.5Cu2.9O7-x, and YBa1.8Cu3.0O7-x. Wires are extruded from a mixture of precursor powders (formed by a coprecipitation process) then textured by firing in a moving furnace. Size of secondary phases such as barium cuprate and copper oxide, and overall composition of the sample affect the orientation of the fine grains. At zero magnetic field, the YBa1.5Cu2.9O7-x wire shows the highest critical current density of 1,450 Acm-2 and 8,770 Acm-2 at 77K and 4.2K, respectively. At 1 T, critical current densities of 30 Acm-2 and 200 Acm-2, respectively, are obtained at 77K and 4.2K. Magnetisation curves are also obtained for one sample to evaluate critical current density using the Bean model. Analysis of the microstructure indicates that the starting composition of the green body significantly affects the achievement of grain alignment via melt-texturing processes.

Melt textured Y123 bulk and thick film

Abstract YBa 2 Cu 3 O 7−δ - 25mol%Y 2 BaCuO 5 bars and thick films have been melt textured using a stationary furnace with a temperature gradient of 3 or 6 °C/cm. Samples are heated above the peritectic reaction temperature and quenched to just above the solidification temperature and then slowly cooled below the solidification temperature. All bar shaped samples consist of 2∼5 mm grains though the grain orientations strongly depend on the heat treatment conditions. The bar shows the maximum J c of above 3000 A/cm 2 , whereas the maximum J c of 200 A/cm 2 and T c zero of 88 K are obtained for the thick film on (100) LaAlO 3 single crystal.

Bi-2212/Ag laminated tapes : bending and joining effects

Abstract Superconducting composite Bi-2212/Ag tapes and their joints are fabricated by a combination of dip-coating and partial melt processing. The heat treated tapes have a critical current ( I c ) between 8 and 26 A, depending on tape thickness and the number of Bi-2212 layers. Current transmissions between 80% and 100% have been achieved through the joints of tapes. Different types of HTS joints of Bi-2212/Ag laminated tapes are made and their transport properties during winding operations are investigated. Irreversible strain values ( e irrev ) for laminated tapes and their joints are determined and it is found that the degradation of I c during tape bending depends on the type of joint.