Magnetic shields for SQUID applications were successfully fabricated using high Tc superconducting Bi–Sr–Ca–Cu–O (BSCCO). In order to produce shields with adequate superconducting properties and close dimensional control, it was necessary to develop several new processing techniques. Shields were produced by casting liquid BSCCO into molds, heat treating, and machining. A series of BSCCO alloys with different compositions were cast from the molten state into metal molds and subsequently heat treated to render the castings superconducting. The heat-treating cycles were studied with the aid of thermogravimetric analysis (TGA), differential thermal analysis (DTA), and dilatometer measurements. The phases and microstructures after various heat-treating cycles were monitored by x-ray diffraction (XRD), optical microscopy, and scanning electron microscopy (SEM). Superconducting properties were measured after various stages of heat treatment and machining. Prototype magnetic shields were machined from bulk castings and found to perform successfully. The most significant factor in shield quality was the nominal composition of the shield, which was shown by transmission electron microscopy (TEM) to affect the grain boundary chemistry.

Recently, Hg-based superconductors HgBa2Can−1CunO2n+2+x (n=l,2,3) were demonstrated to have remarkably high transition temperatures (Tc). The Hg-compounds are structurally analogous to the previously discovered Tlbased compounds TlBa2Can−1CunO2n+2+x. Oxygen site occupation in the rock-salt Hg-O layers and Tl-O layers differ, due to the difference in the valence states of Tl (III) and Hg (II). In the Hg-compounds the Hg-O layer is always single while in the Tl-compounds both single or double Tl-O layers are possible. Up to 4 Cu-O2 layers may be found in both families. Martin et al. investigated the mixed Hg-Tl system and reported a new structure akin to a single cell Hg-1201/Tl-2201 intergrowth, having a Tc about half that of either individual compound. We synthesized mixed Hg-Tl compounds corresponding to the higher order intergrowth formula Tl2Hg1 Ba4Ca4Cu6Ox (Hg-1223/Tl-2223) with the hope of producing new intergrowth structures with higher Tc. In this report, we only describe our structural characterization on one of the Tl-Hg compounds with nominal composition Tl2Hg1 Ba4Ca4Cu6Ox. A complete report on the synthesis and properties of these compounds will be published elsewhere.

The results of studies of new Hg based cuprate superconductors are reported. Several members of a new family of (Hg,Tl)(Ba,Sr)Can−2CunOx high temperature superconductors have been synthesized. These compounds, which are analogs to the Hg-Ba-Ca- and Tl-Ba-Ca-layered cuprates, are multi-phased and have superconducting transition temperatures above 100 K. Incorporation of Hg appears to stabilize several of the Tl-compounds, including a double layer. Tl/Sr system, in a manner similar to the role that Pb plays in the Tl/Sr- and Bi/Sr-systems. It has been suggested that recent reports of resistive Tc's above 200 K in Hg based samples are due to the presence of free Hg. Magnetization measurements of such a sample confirm this hypothesis.

A model incorporating the thermodynamic equilibrium oxygen content, oxygen in-diffusion, and oxide phases for the oxygenation of the YBa2Cu3O7−x is described. For sintered polycrystals, grain growth and the resulting grain size distribution are included. The model is used to calculate the volume percent of each oxide phase for several processing conditions of sintered specimens and is compared with available results on the quantification of phases present. Such a comparison indicates that as the concentration of impurity phases increases so does the concentration of the less oxygenated 123 phases, suggesting that impurities coating 123 grains act as oxygen indiffusion barriers. The model is also used to investigate the uniformity of the oxygen content in large twinned and detwinned single crystals such as have been used for measuring superconductivity parameters.

Highly oriented 2212 BSCCO thick films were prepared on polycrystalline MgO substrates using a melt/crystallization technique. Results compare very favorably with those on single crystal materials.1,2 The melting, quenching, and annealing processes were all found to be important in the development of a good microstructure and good superconducting properties. The best results (J c ∼ 2000 A/cm2 at 64 K and 6000 A/cm2 at 4.2 K) were obtained on films which had been melted, quenched to room temperature, heated to 860°C where they were annealed before slow cooling to room temperature in oxygen, lightly polished, and reannealed at 860°C.

We have machined cast BSCCO into SQUID and SQUID shield devices. Heat treatment of ascast samples results in a random three dimensional network of platelets. This microstructure is responsible for the machinability of BSCCO.

A detailed analysis of the intensities of valence-band photoelectron features of superconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ and semiconducting ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.05}$${\mathrm{CuO}}_{4}$ has revealed a resonance in the peak located at a binding energy of \ensuremath{\sim}9.5 eV for photon energies spanning the onset of O 2s excitations. This demonstrates conclusively that the feature is associated with oxygen excitations. The origin of the satellite is described and its disappearance on superconducting surfaces is explained.

A common mineralogical technique for identifying individual crystals in a heterogeneous, polycrystalline sample involves the observation at room temperature of their colors in reflected polarized light (colors of polarization). Here we present the observation that a brownish yellow (golden) color of polarization is not only characteristic of the YBa2Cu3O7 superconductor, but of the cuprate superconductors in general and is correlated with the occurrence of superconductivity.

Fundamental information about the structure of the valence band and the chemical valence states of the various constituents of the LaSrCu, YBaCu, BiSrCaCu and TlCaBaCu oxides have been obtained using photoelectron spectroscopy. These results show that the one-electron theories do not adequately describe the electronic structure of these superconductors. The atomic origins of the features observed in the valence bands have been investigated by studying photoemission resonances and changes in excitation cross-sections with photon energy. Results to date suggest that these materials have varying densities of states at the Fermi level, valence bands composed of O 2p and Cu 3d states, and display no significant changes in the band structure associated with the superconducting behaviour when the temperature is lowered below Tc. In addition, the complex surface chemistry of these oxides make it essential to study the surface stoichiometry and the interaction of simple molecules. O2 and CO are found to interact only weakly with the surfaces of the materials studied to date, while the H2O and CO2 react strongly, forming hydroxides and carbonates.

Bi-Sr-Ca-Cu-O (BiSCCO) was melted and cast into molds in order to render the superconductor into fully dense and useful shapes. Special conditions for casting and subsequent heat treatments were required to ensure structural integrity and a high transition temperature,T c .The history of the castings during heat treatments was studied by thermal analysis, X-ray diffraction, and microscopy.

Magnetic shields for SQUID applications were successfully fabricated using high Tc superconducting BSCCO. To produce shields with appropriate superconducting properties and close dimensional control, it was necessary to develop new processing techniques. Shields were produced by casting liquid BSCCO into molds, heat treating and machining. This technique is applicable to fabricating other types of superconducting devices requiring bulk superconductors with complex shapes and close dimensional control.

Resonant photoemission has been used to study the electronic states and electron‐electron interactions in a bulk sample of Tl‐Ba‐Ca‐Cu‐O high temperature superconductor. The electronic structure, i.e., broad peaks in the valence band at 3.2 and 5.5 eV and satellites at 10 and 13 eV binding energy, is similar to that observed for the 1‐2‐3 materials. This indicates that the electronic states and interactions are similar in these different classes of superconductor.The surface reactivity has been probed with photoemission using controlled exposures of atmospheric gases. Both H2O and CO2 react with a sticking probability of near 0.2, forming hydroxide, and carbonate species, respectively. O2 is non‐reacting while CO reacts only slightly.

Millimeter-sized crystals of ErBa2Cu3O7 were fortuitously grown while sintering a flat disk. It was possible to separate individual crystals from the sintered mass and measure their properties. The resistive transition and onset of magnetic transition of a single crystal was 93 K and the resistive transition width was < 1 K. Flux expulsion was observed and individual crystals could be levitated by a magnetic field. Crystals showed unusual growth features. They were heavily corrugated along the part of the crystal that grew parallel to the top surface of the disk and that coincided with thec axis of the unit cell. Crystals were characterized by X-rays, electron microscopy, and optical microscopy.

The shape of the normalized temperature derivative of the dc susceptibility provides useful information about superconducting materials. We propose that the position of the derivative peak be defined as the transition temperature Tc and the width of this peak define its sharpness.