Kathleen Amm

Synthesis of (Hg,Re)Ba2Can−1CunOy superconductors

Abstract High phase purity (Hg,Re)Ba 2 Ca 1 Cu 2 O y and (Hg,Re)Ba 2 Ca 2 Cu 3 O y superconductors were prepared from commercially available Ba 2 Ca n −1 Cu n O x precursors, HgO, and Re 2 O 7 . The addition of Re was found to have beneficial effects on the phase formation and chemical stability of the superconducting materials. Optimal annealing temperatures were found to be above 880°C for the (Hg,Re)Ba 2 Ca 2 Cu 3 O y phase and below 850°C for the (Hg,Re)Ba 2 Ca 1 Cu 2 O y phase. The microstructure of the samples is dominated by high density island-like superconducting areas interconnected by low density superconducting material and second phases. The randomly oriented platelike superconducting grains are typically around 5 μm thick and up to 100 μm long. Investigations of (Hg,Re)Ba 2 Ca 2 Cu 3 O y samples quenched from various annealing stages indicate that the (Hg,Re)Ba 2 Ca 2 Cu 3 O y phase forms via the (Hg,Re)Ba 2 Ca 1 Cu 2 O y phase and is assisted by a liquid phase on a microscopic length scale.

Synthesis and processing of (Hg,Pb)1Ba2Ca2Cu3Oy superconductors

Abstract Synthesis and processing of bulk (Hg,Pb)Ba2Ca2Cu3Oy (HgPb1223) superconductor using commercial BaCaCuO precursor powders is reported. HgPb1223 is shown to form over a wide range of temperatures, 750–880°C. The microstructure and intergrain connectivity of samples depends strongly on heat treatment and Hg-pressure during synthesis. Use of an external Hg-source during the reaction improves the density of the HgPb1223 products. The chemical nature of the external Hg-source changes the Hg-pressure and influences melting characteristics and grain growth. CaHgO2 is shown to be a particularly effective external Hg-source for producing dense HgPb1223 materials with improved grain growth and texture. Microstructure of HgPb1223 synthesized using CaHgO2 as the external Hg-source is characterized by large colonies of well-aligned superconducting grains. As-synthesized HgPb1223 samples were superconducting with Tc values in the range 132–134 K without post annealing in oxygen or argon. Phase purity and microstructural characteristics are correlated with magnetization measurements.

Flux pinning and magnetic anisotropy in neutron irradiated Hg-Ba-Ca-Cu-O

Magnetic studies are performed on randomly oriented and aligned polycrystalline samples of HgBa/sub 2/CaCu/sub 2/O/sub 6+x/ (Hg-1212) and HgBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub 8+x/ (Hg-1223). The measurements of magnetic hysterisis and magnetic relaxation confirmed the existence of a strong surface barrier. The decay of the critical current density J/sub c/ with temperature is much slower than the decay of their single-layered counterpart, Hg-1201. Measurements of aligned Hg-1212 revealed that the dependence of J/sub c/ on field orientation is relatively weak and the irreversible area in the H-T plane is much wider than other high temperature superconductors. Although neutron irradiation only enhances the critical current values moderately and severely suppresses the surface pinning, neutron irradiation improves all of the magnetic properties mentioned above, and at T=100 K, the hysteresis loop remains open up to H=6 kG.<<ETX>>

HgBaCaCuO superconductors : Processing, properties and potential

Abstract HgBaCaCuO superconductors exhibit superior superconducting properties especially at temperatures above 77K. Irradiation studies have proven that there is a great potential even for further enhancements. Progress in the synthesis of Hg-based superconductors has been achieved by the development of a two-zone technique, which has opened the window for applying a variety of materials processing techniques successfully used in other high temperature superconductors. HgBaCuO—silver composites have been prepared by the powder in tube method and reacted in the two-zone assembly. Silver, which has successfully been used as sheath material for most other high- T c superconductors, appears to be compatible with superconductors of the HgBaCaCuO family.

Experimental Layer-Wound Mock-Up Coil for HTS MRI Magnet Using BSCCO Tape

Typical coils with BSCCO tape are wound in a pancake or double-pancake style to minimize the strain in the tape by reducing or eliminating the edge-wise bending. Stainless steel reinforced tapes are frequently used in the winding process to increase the strength and reduce the strain due to winding tension and handling. However, an MRI magnet requires high current density in the winding pack. This high current density in the winding pack gives a higher field in the imaging volume and also allows for a reduction in the overall magnet size. Layer winding was preferred for a better tolerance control and for a reduction in the number of joints, which are known sources of resistance and therefore locations of instability in the coil. A mock-up coil was wound using a high-current-density type of BSCCO tape without the typical stainless steel reinforcement. The coil was layer-wound which involved a few inline lap joints embedded in the winding pack. The test of the coil reveals a few issues that need to be addressed. Investigations and analysis lead to a better understanding of the issues. This paper discusses the lessons learned and solutions for using non-reinforced tape in a layer-wound coil, while controlling insulation dimensions within the build.

Iterative EM Design of an MRI Magnet Using HTS Materials

Conventional superconducting MRI magnet electromagnetic (EM) design involves the critical parameters of the magnet field and dimensional requirements, plus the low temperature superconductor (LTS) wire properties. When using HTS material for an MRI magnet, in addition to the conventional characteristics of a superconductor, the wire and the joint resistance need also to be considered. This is mainly due to the relatively low n-values of the BSCCO tape when compared to that of a conventional LTS wire, and it is also due to the more resistive joint nature for HTS joints. This paper discusses an iterative EM design procedure that calculates the resistance of the HTS wire/tape for the entire magnet, which is a strong function of the magnetic fields at different locations in the coils. The resistance is fed back to the regular MRI EM design for optimization, in order to meet resistance target and the ordinary MRI design goals. This iterative design process results in an optimized HTS MRI design with overall low resistance for magnet drifting and power supply requirements.

Growth of Hg/sub 0.9/Re/sub 0.1/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 8+x/ on a metallic substrate

The large-scale application of any high temperature superconductor will require a metallic sheath to satisfy the mechanical and thermal requirements of superconducting magnets. Thus, an important step in the development of a high temperature superconductor is to study the properties of the superconductor in contact with a metallic surface. Due to the highly corrosive atmosphere necessary for the synthesis of Hg/sub 0.9/Re/sub 0.1/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 8+x/ ((Hg,Re)1223) the selection of a suitable metal substrate is critical. In this study, we investigate the synthesis of(Hg,Re)1223 samples on Ag, AgHg, Au, and Pt substrates. The samples were prepared by mixing precursor powders in a dry methanol solution which was then pipette dropped or centrifuge coated onto the metal substrates. The samples were encapsulated in quartz and annealed. Microstructural analysis of phase growth and grain alignment at the superconductor/metal interface were carried out on multiple length scales via x-ray diffraction, scanning electron microscopy, and x-ray microanalysis. Magnetic characterization has been carried out in a SQUID magnetometer.

The influence of metallic interfaces on the properties of superconductors

The influence of AgPd and Au interfaces on the formation of the (HgBi-1223) superconductor was investigated. Variations in the BaCaCuO precursor phase assemblage and the use of as the Bi source were examined in an attempt to lower the reaction temperatures suitable for the growth of HgBi-1223 in conjunction with AgPd and Au substrates. Phase pure samples of HgBi-1223 were synthesized at temperatures below . Use of as a Bi source combined with variations of the BaCaCuO precursor phase assemblage allowed the formation of some liquid phase during the reaction and resulted in dense HgBi-1223 samples with aligned colonies of grains. Magnetization measurements in fields up to 30 T indicated good intergrain coupling within the large grain colonies. Comparison of the microstructural features of bulk samples and the metal-superconductor interface showed that the interface has a positive influence on grain growth and alignment. The metallic interfaces, however, were found to have a detrimental effect on the purity of the superconducting phase due to the formation of amalgams.

Enhanced flux pinning in HgBa2CuO4+x by neutron irradiation and its relationship to magnetic anisotropy

Our previous results indicate that neutron irradiation is an effective method for improving the pinning strength and critical current density of HgBa2CuO4+x, demonstrating that the mercury‐based materials are not intrinsically limited to low current density. Here we expand our investigations of random HgBa2CuO4+x powders with a study of the effects of further neutron irradiation and annealing on the magnetization hysteresis and flux creep of powder samples. We also report the effects of irradiation on aligned powder samples and thus understand the relationship between flux pinning and magnetic anisotropy. We find that the critical current density continues to increase with increasing neutron fluence but at a decreasing rate. The irradiation enhancements in ground‐then‐aligned powders are not as large as in the as‐sintered material, which we attribute to small defects introduced during the grinding process that inhibit the formation of defect clusters during irradiation. Lastly, we find that the neutron ir...

Bulk processing of HgBaCuO high T/sub c/ superconductors by a two-zone technique

Hg1201 superconductors have been synthesized from Ba-Cu-O precursors in a two zone assembly. Critical temperatures of 92 K as prepared and 95 K after oxygen annealing resulted. The Hg1201 phase forms at mercury pressures above 6 bar (800/spl deg/C) and at temperatures between 750/spl deg/C and 850/spl deg/C. The effect of dopants on melting and annealing temperatures of the precursor materials has also been studied. After Li doping of up to 20%, melting peaks shift to lower temperatures and broaden significantly.<<ETX>>