Microtexture and mesotexture in high-Jc Bi-2223

The microstructure of high-J_c Bi-2223 powder-in-tube tapes was studied using x-ray and electron diffraction. Although the c-axis is nominally aligned perpendicular to the tape surface (FWHM∼20‡), x-ray phi scans and pole figures show no evidence of any in-plane texture, either macroscopically or locally. Electron backscatter diffraction patterns acquired in a scanning electron miscroscope (SEM) were used to measure individual grain orientations. Grain boundary misorientation between adjacent grains was described by rotation angles and axes (i.e. the disorientation) and compared with theoretical values of ideal coincidence site lattices (CSLs). Data collected from over 113 spatially correlated grains resulting in 227 grain boundaries, show that over 40% of the boundaries are small angle. In addition, 8% of the boundaries were found to be within the Brandon criterion for CSLs (larger than ⌆1 and less than ⌆50). Grain boundary “texture maps” derived from the SEM image and orientation data reveal the presence of percolative paths between low energy boundaries.     

Effect of lead content on Bi-2223 formation by a two-powder process

Decreasing the total lead content from Pb_0.4 to Pb_0.3 significantly stunts Bi-2223 phase development and greatly reduces the critical current density (J_c) of powder-in-tube tapes made by a two-powder process. This effect can be explained on the basis of the solubility limit for lead in 2212. Pb_0.4 samples exceed the solubility limit for lead in 2212, so lead is rejected to create a lead-rich liquid that enhances the kinetics of 2223 formation during heat treatment. By contrast, a liquid does not form in the Pb_0.3 sample because its lead content is below the solubility limit. As a result, 2223 formation is much slower and J_c is much lower in the Pb_0.3 sample.     

On the manufacture of silver-sheathed Bi-2212 high temperature superconducting wires

High temperature superconductors (HTS) are presently being considered for a variety of defense and commercial applications. However, problems associated with low critical current density (J_c) in long length conductors still need to be addressed. It is realized that success in the fabrication of long lengths of wire and tape (up to 1 km) with optimum superconducting properties relies on improved manufacturing technologies. Unfortunately, there is no systematic study concerning the effect of processing parameters on the green properties of wires and tapes during the deformation processes. The focus of this investigation centered on silver (Ag) sheathed Bi-2212 wire forming processes and use of the two most common techniques, wire drawing and hydrostatic extrusion. Billets of 6.35 mm outside diameter were reduced separately by five different die sequences to a final diameter of 1.63 mm. Wires made by these procedures were characterized for properties such as Bi-2212 relative packing density, sheath material thickness, deformation force, elongation, drawing friction, and microstructural and mechanical properties. These results indicated that the amount of reduction per pass has a significant impact on the characteristics of non-heat treated wires and on the mechanical aspects of the wire drawing operations.     

Processing and transport properties of high-Jc silver-clad Bi-2223 tapes and coils

The powder-in-tube process has been used to fabricate long lengths of flexible, high-J_c, silver-clad Bi-2223 HTS conductors. By improving thermomechanical processing and precursor powder preparation, we have succeeded in achieving J_c values of≥4×10⁴ A/cm² at liquid nitrogen (77K) temperature and >10⁵ A/cm² at liquid helium (4.2K) and liquid neon (27K) temperatures in short tape samples. Detailed measurements with high applied magnetic fields are reported. Several long tapes up to 10 m in length have also been fabricated and cowound into small superconducting pancake coils by the “wind-and react” approach. Transport measurements at 77 and 4.2K for these coils are also reported.     

Current-voltage characteristics and transport current distributions in Ag/Bi-2223 monocore tapes

Current-voltage characteristics of Ag-sheathed Bi-2223 monocore tapes have been measured at 77 K in low d.c. magnetic fields. Results have been obtained with the c-axis of the sample both parallel and perpendicular to the applied field, together with the force-free orientation. The current–voltage characteristics have been analysed in terms of their n-value (V∝Iⁿ), which characterizes the sharpness of the resistive transition and the second differential (d²V/dI²), which shows the distribution in local critical current density values. The second differential has been fitted and analysed using the phenomenological Weibull distribution function, which is based on a weak-link model. The four parameters of this function have been used to investigate the microstructural nature of the sample, in particular, the factors influencing the critical current density and the transport current pathways. The results are consistent with the view that at low fields the critical current density is limited by weak-link grain boundaries and at high fields by intra-granular flux pinning.     

Self-field and geometry effects in transport current applications of multifilamentary Bi-2223/Ag conductors

This paper describes quantitatively the influence of the self-field and the cross-sectional geometry on the effective critical current and the ac losses in transport current applications of nontwisted multifilamentary Bi-2223/Ag conductors. The results are obtained with finite-element method simulations. The numerical implementation includes an anisotropic model for the dependence of the critical current density J/sub c/ and the power index n on the local parallel and perpendicular magnetic field components. The relation is given between the intrinsic critical current density and the effective critical current for different multifilamentary conductors. Shown are examples of the current and magnetic flux density distributions in order to demonstrate their effect on the ac losses in self-field.     

Fabrication and characteristics of tapes and test magnets made from Ag-Clad Bi-2223 superconductors

Pb_0.4Bi_1.8Sr₂Ca_2.2Cu₃O_x (Bi-2223) precursor powder was prepared by a solid-state reaction of carbonates and oxides of lead, bismuth, strontium, calcium, and copper, and the powder was then used to fabricate silver-clad tapes by the powder-in-tube technique. Transport critical current density (J_c) values>4×10⁴ A/cm² at 77K and 2×10⁵ A/cm² at 4.2 and 27K have been achieved in short tape samples. Long lengths of tape were tested by winding them into pancake coils. Recently, we fabricated a test magnet by stacking ten pancake coils, each containing three 16m lengths of rolled tape, and tested it at 4.2, 27 and 77K. A maximum generated field of 2.6 T was measured in zero applied field at 4.2K and the test magnet generated significant self-field in background fields up to 20 T. The results are discussed in this paper.     

Ultrasonic characterization of density and elastic properties in Bi-2212 powder compacts

This paper addresses the development and application of advanced sensing based on emerging nondestructive evaluation techniques, such as ultrasound, resonant frequency, x-ray, etc., for characterizing product quality at various stages of high temperature superconducting (HTS) conductor manufacturing. These techniques provide valuable information on the relationships between process variables and product properties, and enable new/emerging manufacturing process optimization. Such techniques are being explored for the characterization of critical product parameters at various stages of Bi-2212 powde-intube process. In addition, these techniques, when implemented in real-time, promise significant benefits for sensing and process control of HTS conductor processing. Specifically, this paper discusses the development of ultrasonic techniques for characterizing density and elastic properties in Bi-2212 powder compacts. Ultrasonic longitudinal and shear wave velocities in these materials were measured at 2.25 MHz. Ultrasonic velocity is a strong function of density (hence porosity) and provides a potential means of measurement of this quantity. Also presented are ultrasonic measurement of elastic properties of Bi-2212 compacts.     

Analysis of the operational characteristics of a high-Tc superconducting power supply with the Bi-2223 pancake load

This paper presents the design and fabrication of a high-Tc superconducting (HTSC) power supply with the Bi-2223 pancake load, as well as its characteristics as determined through experiments. HTSC power supply consists of two heaters, an electromagnet, a Bi-2223 solenoid, and a Bi-2223 pancake load. In this experiment, a 331-mH electromagnet and 0.8-A dc heater current were used, and 8.5 and 17 s were used for pumping periods, respectively. Mechanism of the superconducting switch is used for heater-trigger. In order to measure the pumping-current with respect to the magnet flux changes, a hall sensor was installed at the center of the Bi-2223 pancake load. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping-current has reached about 0.6 A with 0.01-V electromagnet voltage.     

Fabrication of long length Bi-2223 superconductor tape using continuous electrophoretic deposition on round and flat substrates

We have developed a continuous fabrication process for producing long lengths of Bi-2223 superconductor tapes. The process involves sequentially electrophoretically depositing and sintering superconductor and then silver layers on a substrate, followed by rolling and thermal processing. Both round and flat silver substrates have been used. Bi-2223 tapes made using flat silver substrates require only a few processing steps. Transport critical current densities at 77K in zero applied magnetic field exceeding 20000 A/cm² have been obtained.     

Next generation adduct purification techniques for low oxygen content metal alkyls

A new synthesis and adduct purification route has been developed for Group III metal alkyls which avoids the use of ether solvents. The R₃M compound is synthesized in a trialkylamine solvent such as NEt₃ which leads to adducts of the type R₃Ga(NEt₃). The NEt₃ ligand can be displaced by the addition of involatile tertiary amines (L) containing 2, 4, or 6 nitrogen donor sites to give adducts of the type (R₃Ga)_xL(x=2,4,6), from which base free R₃Ga compounds are obtained by mild thermal dissociation. The synthesis, characterization and thermal dissociation of these adducts is described and brief growth data are given for AlGaAs films grown by CBE using triisopropyl gallium prepared by this new route     

Mechanical Losses Due to Frictional Heat in Bi-2223 Tapes With Structural Materials of Various Thermal Expansion Properties

We have experimentally studied relation between mechanical losses and thermal expansion properties in superconducting coils. For the experiments, three kinds of structural materials (GFRP, DGFRP, and ZFRP) were used. A glass fiber reinforced plastic (GFRP) contracts with decreasing temperature; on the contrary, a Dyneema and grass fiber reinforced plastic (DGFRP) and a Zylon fiber reinforced plastic (ZFRP) expand with the decrease of temperature. We used sample holders made of those plastics and measured AC current losses of a Bi-2223 tape in liquid nitrogen. Even if the magnetic field applied to the tape and an AC transport current of the tape were same, the loss depended on the sample holder materials. When the sample holder is the GFRP, the loss was largest in the three kinds of plastics, and in case of the ZFRP, the loss was smallest. The ZFRP has the largest expansion property of the three materials, that is, the loss decreased with expansion of the sample holder's materials.     

Microstructure and properties of melt-processed Bi-2212 (Bi2Sr2CaCu2Ox)

Microstructural development and properties during melt processing of Bi-2212 were investigated with regard to the production of superconducting oxides in bulk shape for the application in electrical engineering. Oxygen loss during heating and melting leads to incongruent solidification on cooling and therefore multiphase microstructures. Phase compositions depend on oxygen stoichiometry, which is determined by oxygen partial pressure, maximum sintering temperature as well as cooling rate. During annealing, solid/liquid and subsequent solid/solid reactions yield high volume fractions of 2212. The oxygen absorption and the 2212 formation mechanism and its kinetics are strongly correlated. The11905→2212 transformation proceeds via intermediate states of high planar defect density and is promoted by frequent stacking faults, that allow diffusion of Ca- and Cu-atoms over short distance. Microstructures of the 2212 phase were also controlled by variations of the cation stoichiometry leading to an improvement of the superconducting properties.     

Geometry considerations for use of Bi-2223/Ag tapes and wires withdifferent models of Jc(B)

In typical power applications, Bi-2223 conductors carrying AC current will be subjected to external magnetic fields whose orientation and conductor's geometry are of major significance for the AC loss magnitude. This paper investigates the influence of the geometry and aspect ratio of nontwisted Bi-2223 conductors in reducing the AC loss for such applications. A numerical model of high-T_c materials has been used in finite-element-method (FEM) simulations. The model incorporates power-law E-J characteristics with J_c and n defined by both parallel and perpendicular local magnetic field components. It allows computations of field and current distributions with transport current and/or applied field of any orientation. Monofilamentary tapes of rectangular and elliptical geometry with anisotropic J_c(B), as well as square and round wires with isotropic J_c(B) have been used for simulations under various operating conditions. A comparison between AC losses, magnetic field, and current distributions in the tapes and wires is presented     

Determination of the AC losses of Bi-2223 HTS coils at 77 K at power frequencies using a mass boil-off calorimetric technique

A mass boil-off measurement system has been used to accurately measure and characterize the ac loss of high-temperature superconductor (HTS) coils at frequencies between 50 and 200 Hz, and in applied ac fields of up to 0.04 T. The mass boil-off calorimeter incorporated a glass cryostat, a copper field coil, and two mass flow meters. The response of the gas flow to a step change in the applied magnetic field was found to have a time constant of about 600 s. Under suitable experimental conditions, it was possible to measure the ac losses of coils with an accuracy of /spl plusmn/0.3 W. The ac loss characteristics of an HTS pancake and two HTS solenoid coils are presented and the accuracy of mass flow calorimetry in liquid nitrogen is reported on.     

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.     

On determination possibility of the total electron content of the interplanetary path from distortions of an ultrashort radio pulse

The character and features of dispersion distortions of an ultrashort radio pulse in the form of a sinusoidal train propagating along the spacecraft-Earth interplanetary path are theoretically analyzed. It is established that the instantaneous field oscillation frequency in the distorted pulse is completely determined by the value of the total electron content of the path. It is shown that, theoretically, one of the main characteristics of the solar wind can be measured from the frequency of the detected signal.     

蔬菜基地重金属含量检测系统设计

针对蔬菜基地存在的重金属污染,设计了一种重金属离子浓度检测系统。该系统基于离子选择膜技术,研制了微小型的Cu+-ISE,利用电势测定法研究ISE的电化学响应性能,并结合一元线性回归建立电极电势与离子浓度的数学模型。基于STC89C52单片机和AD620运放器,设计了硬件检测电路和检测程序,实现了对离子浓度的测量、显示和存储等操作。最后,通过对Cu+离子溶液的试验验证了该检测仪的精确性。     

The effect of forming gas anneal on the oxygen content in bonded copper layer

Using a combination of copper (Cu) thermocompression bonding and silicon wafer thinning, a face-to-face silicon bi-layer layer stack is fabricated. The oxygen content in the bonded Cu layer is analyzed using secondary ion mass spectrometry (SIMS). Copper-covered wafers that are exposed to the air for 12 h and 12 days prior to bonding exhibit 0.08 at.% and 2.96 at.% of oxygen, respectively. However, prebonding forming gas anneal at 150°C for 15 min on 12-day-old Cu wafers successfully reduces the oxygen content in the bonded Cu layer to 0.52 at.%.     

Influence of oxygen content on the crystallinity of MgO layers in magnetic tunnel junctions

With RF sputtering process, Si/Si02/Ta/Ru/Ta/CoFeB/MgO/CoFeB/Ta/Ru structure has been grown on Si （100） substrate. Attempting different targets and adjusting the oxygen dose, the crystallization quality of the MgO layer is studied. The X-ray diffraction measurements demonstrate that crystal structure and crystallization quality of MgO layers are related to the type of target and concentration of oxygen in sputtering process. With the method sputtering Mg in an ambient flow of oxygen, not only the crystallization quality of a normal MgO layer with lattice constant of 0.421 nm is improved, but also a new MgO crystal with lattice constant of 0.812 nm is formed and the perpendicular magnetic anisotropy of CoFeB is enhanced. Also it is found that crystallization quality for both the normal MgO and new MgO is more improved with MgO target and same oxygen dose, which means that this new method is helpful to form a new structure of MgO annealed at 400 ℃ in vacuum. with lattice constant of 0.812 nm. All of the samples were