Bi-2212 Powder Research Articles

AC Loss Properties in Bi2223 Multifilamentary Tapes with Enhanced Transverse Resistivity by Introducing Oxide Barriers

AbstractBi2223 multifilamentary tapes with enhanced transverse resistivity by introducing Ca2CuO3 and SrZrO3 as interfilamentary barriers were prepared and their AC loss properties were examined under AC external magnetic field. To improve the deformation properties of these oxide powders, 20-30wt% Bi2212 powders were mixed with them. The mixed oxide powders were introduced among the twisted Bi2223 filaments by using dip-coating method. AC loss properties under AC parallel or perpendicular field were measured and compared with the results for the tape without barriers. The loss measurements were carried out by changing both the amplitude and the frequency of external field. Based on the experimental results, the effect of barrier introduction on the loss reductions was discussed.

バリア入りBi2223多芯線材の作製と交流損失特性

Synopsis: Bi2223 multifilamentary tapes with interfilamentary oxide barriers were fabricated and their AC loss properties were examined under AC external magnetic field. In order to avoid the side effect on Bi2223 phase formation during heat treatment, Ca2CuO3 and SrZrO3 with average grain sizes of 1-2 μm were selected as barrier materials. Furthermore, an additional Bi2212 powder corresponding to 20wt% were mixed with them to improve their deformation properties for cold working. The mixed oxide powders were introduced among the twisted Bi2223 filaments. The critical current densities (Jc) in the barrier tapes were 10-20% lower than those in the tapes without barriers. In a parallel field, the loss reduction in the SrZrO3/Bi2212 barrier tape is most remarkable because the continuity of the SrZrO3/Bi2212 barriers is far superior to the Ca2CuO3/Bi2212 barriers. At 50 mT around power-grid frequencies, parallel field losses in a twisted tape with SrZrO3/Bi2212 barriers are 80% lower than the non- twisted tape. From the frequency dependence of AC losses in a perpendicular field, it was confirmed that the transverse resistivity in the SrZrO3/Bi2212 barrier tape is 8-9 times higher than the tape without barriers. However, around the power-grid frequencies, the introduction of barrier reduces AC losses only slightly in a perpendicular field. The twist pitch of the present barrier tape (= 16 mm) is probably too long to decouple the filaments in a perpendicular field.

A Study on the Processing Variables of the BSCCO-2212 Bulk Superconductors

Bi-2212/SrSO4 bulk superconductors were fabricated using a casting process, and the effects of the powder mixing method, annealing temperature, and SrSO4 content on the texture, microstructure, and critical properties were evaluated. The Bi-2212 powders were mixed with SrSO4 either by hand-mixing (HM) or planetary ball milling (PBM), followed by melting at 1100degC-1200degC, solidification, and annealing. The rod produced by PBM had a more homogeneous microstructure and smaller grain size of SrSO4 and minor phases, resulting in increased Ic, than the rod made by HM. The Ic of the rod was also dependent on the annealing temperature with the highest Ic of 200 A being obtained at an annealing temperature of 810degC. This was attributed to the moderate density and 2212 texture as well as the smaller and less minor phase than that observed at higher temperature. In addition, the optimum SrSO4 content was found to be 6 wt.% for an increased Ic. SEM, EPMA, and DTA analyses indicated that the differences in microstructural evolution caused the variations of the Ic with the powder mixing method, annealing temperature, and SrSO4 addition.

Fabrication of Bi-2212/SrSO<sub>4</sub> Composite Superconductors by Melting Powder Mixtures

We fabricated Bi-2212/SrSO4 composite superconductors by the melt casting process and evaluated the effects of the powder mixing method and melting temperature on their microstructure and superconducting properties. In the melt casting process, the Bi-2212 powders were mixed with SrSO4 by hand-mixing (HM) and planetary ball milling (PBM) and then the powder mixtures were melted at 1100°C~1200°C, solidified, and annealed. We found that the powder mixture prepared by PBM was finer and more homogeneously mixed than that prepared by HM, resulting in more homogeneous microstructure and smaller SrSO4 and second phases after annealing. The critical current (Ic) also varied significantly with the powder mixing method and the melting temperature. The Ic of the annealed rod prepared by PBM was 193 A at 77 K when melted at 1100°C, which is higher than that of the annealed rod prepared by HM (132 A). This enhancement in the Ic value for the former is considered to be due to its more uniform microstructure.

New size sorting technology for superconducting powders

A new technique is being developed to make it possible to achieve defect-free particle size sorting of the precursor powders for multi-filament superconductors, Nb/sub 3/Sn, Bi-2212 and Bi-2223. Sharp cutoff is critical in the effort to achieve small filament size in multi-filament composites of Nb/sub 3/Sn, Bi-2212 and Bi-2223. Accelerator Technology Corp. (ATC) has developed an approach using a slot-geometry virtual impactor to critically remove the coarse particles. Representative Fe/sub 2/O/sub 3/ and Bi-2212 powders were chosen to evaluate its performance. The results show that a sharp cutoff /spl sim/3 microns separation has achieved in both two powders.

Study on the growth mechanism of Bi-2212 ribbon-like thin films on flat Ag substrate by the atomization technique

The growth mechanism of Bi-2212 ribbon-like thin films on flat Ag substrate using an atomization technique was studied. The fine Bi-2212 powders were dispersed on the Ag substrate before the melting process, so as to avoid the change in the composition of the molten phase with distance, which would result in the formation of the impurity phases. Nearly reversible peritectic reaction on cooling causes the formation of well-defined Bi-2212 phase. Based on the results of in situ high temperature microscope observation, the wetting of the molten phase to the growing ribbons was identified as the essential condition for the growth of the ribbon-like thin films.

Electrical properties of superconducting Bi-2212 thick films

Bi 2Sr 2CaCu 2O 8+ x (Bi-2212) thick films were prepared by a partial melt technique on Al 2O 3 and Ag substrates. Well ground Bi-2212 powder with particle size of about 1 μm was used as starting material. The films have been prepared with melting temperatures T melt in the range from 860°C to 900°C. The critical temperature T c is determined from the temperature dependence of the film resistance, measured by a four-probe technique. The film structure and its critical current J c are significantly influenced by the melting temperature. The maximum J c≅10 3 A/cm 2 has been obtained on the sample prepared at T melt=880–885°C. The electrical properties of these materials are discussed by comparing the experimental data with results, calculated on the basis of the classical dirty limit model and the proximity effect model.

Radial changes in the microstructure of LFZ-textured Bi-2212 thin rods induced by stoichiometry modifications

Three different Bi-2212 powders, with stoichiometric deviations both in the Bi and Sr content as well as in the Bi/Sr ratio, have been used as precursors in the processing of laser floating zone-textured thin rods. The developed microstructure in the as-grown and annealed rods has been studied by ICP analysis, scanning electron microscopy and energy dispersive spectroscopy microanalysis. The marked differences in phase distribution, composition and concentration found among the rods have been correlated with the initial stoichiometry. Finally, the effect of these microstructural inhomogeneities on the critical temperature, critical current density at 77 K and pinning behaviour of the rods is discussed.

Addition of Ag, Pb, K, or Li to Bi 2Sr 2CaCu 2O y whisker

In order to examine the growth mechanism of a Bi 2Sr 2 CaCu 2O y (Bi2212) superconducting whisker, such elements as Ag, Pb, K, and Li were added to the Bi2212 powder in the molar ratio of 0.2 together with changes in sintering temperature. Potassium addition and sintering temperature at 865 °C achieved the fastest growth rate of the whisker. By varying the added elements, the diameter of whiskers is controllable. An Arrhenius plot of the growth rate of Bi2212 whiskers drew a straight line, indicating that its growth mechanism is correlated with the vapor–liquid–solid growth mechanism.

Formation processes of Bi-2212 films prepared on Ag( [formula omitted]) substrate by an atomization technique

We have studied in detail the growth of Bi-2212 ribbon-like thin film by melting Bi-2212 powders dispersed on flat, sputter-deposited Ag(0 0 1) films with order of tens of nm root-mean-square surface roughness. The formation processes of Bi-2212 ribbon-like thin films were studied by in situ high-temperature microscope observations. Because the powders melt incongruently, the liquid phase disperses with residual solid phases on the substrate. The residual solid phases act as the barrier for the melting phase to diffuse. Nearly monophasic Bi-2212 with excellent c-axis orientation in these thin films, proved by X-ray diffraction results, is the result of shortened diffusion length of the liquid phase. These techniques can be used especially to synthesize high quality Bi-based superconducting thin films for intrinsic Josephson junction devices.

Improved microstructure and critical current density of Bi-2212 double-layer tapes using an Ag/Ni/Ag clad substrate

We fabricated Bi-2212 double-layer tapes using an Ag/Ni/Ag clad tape as a substrate by a dip-coating method. It was found that the addition of CuO powder to the stoichiometric Bi-2212 powder precursor in a slurry was effective in the increase of critical current density ( J c) of the tapes. This is due to the decrease of non-superconducting Cu-free phase in the oxide layer by the addition. The presence of Cu was observed in the NiO layer which formed between Ag and Ni layers after heat treatment and therefore, the addition of the CuO successfully compensated the loss of the Cu in the oxide layer. The J c of the tapes reached to 150 kA/cm 2 at 4.2 K and 10 T when using the powder precursor of Bi-2212+20 mol% CuO. This value is comparable to that of the tape with a pure Ag substrate. Hence, it is expected that the Ag/Ni/Ag clad tape is a promising substitute for a pure Ag tape to improve the mechanical strength.

Fabrication of low cost Bi-2212 superconductors using Ag/Ni clad tape

With the intentions of the reduction of material cost and the improvement of mechanical properties of Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub x/(Bi-2212) superconducting tape, Ag/Ni and Ag/Ni/Ag clad tapes were successfully prepared by a diffusion and deformation process and used as substrates for the fabrication of Bi-2212 tape. The clad tape has extremely good adhesion between Ag and Ni layers. Bi-2212 powder precursor mixed with organic binder was mounted on the Ag layer of the clad tape and subjected to a heat treatment of melting and solidification. Bi-2212 formed on the Ag layer whose average thickness was more than 10 /spl mu/m showed good superconducting properties. The highest J/sub c/ (4.2 K, 10 T) value was 1.7/spl times/10/sup 5/ A/cm/sup 2/, which is comparable to the Bi-2212 tape fabricated using pure Ag tape. However, when the thickness of the Ag layer was reduced to 5 /spl mu/m, the tape showed deteriorated properties due to the transfer of Cu and Ni elements across the Ag layer. The J/sub c/ of the Bi-2212/Ag/Ni tape started to degrade at /spl epsiv/=0.2-0.3% in the bending test. A tensile test indicated that the Ag/Ni clad tape had about 3 times larger yield strength than pure Ag tape, From these results, it is expected that the Bi-2212/Ag/Ni tape has a much improved stress tolerance over the conventional Bi-2212/Ag tape.

Characteristics of HTS tube depending on heat-treatment conditions

Bi-2212 HTS tube was fabricated by centrifugal forming process (CFP). As a variation of melt casting process (MCP) or centrifugal casting technique, the CFP is a flexible method for manufacturing Bi-2212 bulk tubes and has been optimized to achieve smooth surface and uniform thickness. At this process, the slurry was prepared in the mixing ratio of 10:1 of Bi-2212 powder and binder. It was initially charged into the rotating mold at the speed of 300–450 rpm. Heat-treatment was performed at the temperature range 870–890°C in air for partial melting. The HTS tube fabricated by CFP at 890°C at the rotating speed of 450 rpm was highly densified and the plate-like grains with more than 20 μm in diameter were well developed along the rotating axis. The measured T c and J c at 10 K were around 85 K and 3000 A/cm 2, respectively.

Comparison of the Efficiency of the One and Two-Step Process for the Production of BSCCO (2212) Powders by Means of Analytical Techniques

The efficiency of the production of the high temperature superconducting powders Bi2Sr2CaCu2O8+x (BSCCO 2212) using the solid state reaction in one or two step processes under different thermal treatment was compared by means of different modern analytical techniques. Through the same techniques the optimization of the production of the production of Bi-2212 powder produced by the two step process, was achieved. X-ray diffraction analysis (XRD) and Raman spectroscopy were used to characterize the products for their stoichiometry and phase-purity. The grain size of the powders was observed by scanning electron microscopy (SEM), while their superconducting properties were tested by electrical DC-resistivity and magnetic susceptibility measurements using a superconducting quantum interference device (SQUID). It resulted that the two step process gives a high quality BSCCO 2212 superconducting powder with Tc = 85 K, in a shorter time and with a greater recovery rate than the one step process.

Fabrication of Bi-2212 superconductor using Ag/Ni clad tape as low cost substrate

Ag/Ni clad tape was successfully prepared by a diffusion and deformation process and used as a cost-effective substrate for the fabrication of Bi-2212 tape. The clad tape has extremely good adhesion between Ag and Ni layers. Bi-2212 powder precursor mixed with organic binder was mounted on the clad tape and subjected to a heat treatment of melting and solidification. After the heat treatment, a Bi-2212 layer with good grain alignment was produced on the Ag layer. J c for oxide exceeds 10 5 A /cm 2 at 4.2 K and 10 T, which is comparable to those of conventional Bi-2212 superconductors processed by using Ag sheath and Ag substrate.

Near-net-shape fabrication of continuous Ag-clad Bi-based superconductors

We have developed a near-net-shape process for Ag-clad Bi-2212 superconductors. This alternative to the powder-in-tube process offers the advantages of nearly continuous processing, minimization of processing steps, reasonable ability to control the Bi-2212/Ag ratio, and early development of favorable texture of the Bi-2212 grains. The powder is rolled into an Ag channel, a cap is applied, and the assembly is lightly rolled. To minimize distortion during heat treatment, it is especially important that the Bi-2212 powder be nearly free of volatile species such as carbon. A vacuum-calcination process was developed to control CO/sub 2/ evolution during heating. Superconducting properties are discussed.

Synthesis of bismuth-strontium-calcium-copper oxide powders for silver composite wires with uniform micron sized filaments

A chemical precipitation method is utilized to synthesize bismuth-strontium-calcium-copper oxide powder. Tetra-methyl ammonium hydroxide and tetra-methyl ammonium carbonate are used as a precipitant and for pH adjustment. Effect of processing parameters like carbonization time, and cation solution concentration on Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/ (Bi-2212) powders is reported. Powders with different particle size and distribution are used to fabricate Ag sheathed multifilamentary Bi-2212 round wires with diameter about 0.06 cm (corresponding to filament diameter about 3 /spl mu/m). Significant sausaging was observed for wires processed with larger particle size powders. Good filament uniformity was observed for wires produced from finer powders.

Relationship among synthesis, microstructure and properties in sinter-forged Bi-2212 ceramics

High quality Bi-2212 powders were prepared using three different synthetic methods (solid state, sol–gel and polymer matrix), showing that the polymer matrix method is very promising from the point of view of the synthesis time and grain size. The Bi-2212 sintered pellets were then submitted to sinter-forging process to obtain textured ceramic discs. The discs were characterized by XRD, pole figures and SEM to determine the phase purity, the texture degree and the microstructure, respectively. Following characterization, the discs were cut into bars and annealed for 12 h at different temperatures ranging between 845 and 873°C to determine the best thermal treatment, whereafter they were characterized using resistivity and transport J c measurements. It has been found that most of the bars had a T c around 92 K and that transport properties can be correlated to the samples' microstructure.

Effects of nanosize MgO additions to bulk Bi 2.1Sr 1.7CaCu 2O x

Nanosize MgO particles were added to Bi 2.1Sr 1.7CaCu 2O x (Bi-2212) powder in various molar fractions. Bulk samples were made from the MgO/Bi-2212 and heat treated by partial-melt processing. Scanning electron microscopy and energy-dispersive X-ray spectra were used to study the phases present in the samples. The effects of the MgO additions on the flux pinning behavior as a function of magnetic field and temperature were investigated by magnetization hysteresis measurements obtained with a SQUID magnetometer and a 33 T vibrating sample magnetometer. It was found that the nanosize MgO addition did not affect T c of Bi-2212, while significantly expanding the magnetization hysteresis loops and increasing the irreversibility field. X-ray and SEM analysis suggested that MgO is inert with respect to Bi-2212.

Non-uniform partial melting and effect of oxide/Ag contact on partial melting behavior in Bi-2212/silver tapes

The partial melting behavior in the electrophoretically deposited Bi 2Sr 2CaCu 2O 8+ δ /Ag tapes has been investigated. The results indicate for the first time that the partial melting is non-uniform in the beginning, that is, the partial melting initiates at a number of sites near the oxide/Ag interface and then spreads both across the broad surface and through the thickness. The initial partial melting temperature of Bi-2212 precursor powder depends on the intimacy of the contact between Bi-2212 powder and the silver substrate.