Peter C. Talbot

Continuous production of Bi-2212 thick film on silver tape

Bi-2212 thick film on silver tapes are seen as a simple and low cost alternative to high temperature superconducting wires produced by the Powder In Tube (PIT) technique, particularly in react and wind applications. A rig for the continuous production of Bi-2212 tapes for use in react and wind component manufacture has been developed and commissioned. The rig consists of several sections, each fully automatic, for task specific duties in the production of HTS tape. The major sections are: tape coating, sintering and annealing. High temperature superconducting tapes with engineering critical current densities of 10 kA/cm/sup 2/ (77 K, self field), and lengths of up to 100 m have been produced using the rig. Properties of the finished tape are discussed and results are presented for current density versus bend radius and applied strain. Depending on tape content and thickness, irreversible strain /spl epsi//sub irrev/, varies between 0.03 and 0.1%. Cyclic bending tests when applied strain does not exceed /spl epsi//sub irrev/, showed negligible reduction in J/sub c/ along the length of the tape.

Sol-Gel synthesis and characterization of cubic bismuth zinc niobium oxide nanopowders

Bismuth zinc niobium oxide (BZN) was successfully synthesized by a diol-based sol-gel reaction utilizing metal acetate and alkoxide precursors. Thermal analysis of a liquid suspension of precursors suggests that the majority of organic precursors decompose at temperatures up to 150°C, and organic free powders form above 350°C. The experimental results indicate that a homogeneous gel is obtained at about 200°C and then converts to a mixture of intermediate oxides at 350–400°C. Finally, single-phased BZN powders are obtained between 500 and 900°C. The degree of chemical homogeneity as determined by X-ray diffraction and EDS mapping is consistent throughout the samples. Elemental analysis indicates that the atomic ratio of metals closely matches a Bi1.5ZnNb1.5O7 composition. Crystallite sizes of the BZN powders calculated from the Scherrer equation are about 33–98 nm for the samples prepared at 500–700°C, respectively. The particle and crystallite sizes increase with increased sintering temperature. The estimated band gap of the BZN nanopowders from optical analysis is about 2.60–2.75 eV at 500-600°C. The observed phase formations and measured results in this study were compared with those of previous reports.

Computational prediction and experimental confirmation of rhombohedral structures in Bi1.5CdM1.5O7 (M = Nb, Ta) pyrochlores

In this study, computationally predicted band gaps and structures using density functional theory (DFT) in Bi1.5CdM1.5O7 (M = Nb, Ta) pyrochlores are confirmed by experimental data on synthesized samples. Ordered Cd substitutions in the B-site of the pyrochlore structures are required to achieve electronic band gaps in the calculated energy band structures, when using full plane waves for DFT calculations, which are supported by a significantly lower total enthalpy. The computationally predicted band gap values are closely matched to experimental band gaps estimated from optical absorption spectra in the UV-Vis. In addition to the prediction of electronic structures, the models also indicate that the large ionic radius of the Cd-cation leads to symmetry modification from the archetypal cubic pyrochlore structure in Bi1.5CdM1.5O7 (M = Nb, Ta). A rhombohedral structure and localized superlattice order are confirmed using X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. Energy dispersive X-ray spectroscopy profiles across the superlattice domain interfaces, which are constant within experimental uncertainty, indicate that domain formation is not compositionally driven but likely a mechanism to alleviate strain build up. Raman and FTIR spectroscopy analyses on these two compounds display strong similarities suggesting that peaks and activities belong to the same structure type.

Phonon anomalies predict superconducting T(c) for AlB2-type structures.

We show that the well-known Kohn anomaly predicts Tc for ordered AlB2-type structures. We use ab initio density functional theory to calculate phonon dispersions for Mg1-xAlxB2 compositions and identify a phonon anomaly with magnitude that predicts experimental values of Tc for all x. Key features of these anomalies correlate with the electronic structure of Mg1-xAlxB2. This approach predicts Tc for other known AlB2-type structures as well as new compositions. We predict that Mg0.5Ba0.5B2 will show Tc = 63.6 ± 6.6 K. Other forms of the Mg1-xBaxB2 series will also be superconductors when successfully synthesised. Our calculations predict that the end-member composition, BaB2, is likely to show a Tc significantly higher than currently achieved by other diborides although an applied pressure ∼16 GPa may be required to stabilise the structure.

Electronic Structure Studies and Photocatalytic Properties of Cubic Bi1.5ZnNb1.5O7

The photocatalytic ability of cubic Bi1.5ZnNb1.5O7 (BZN) pyrochlore for the decolorization of an acid orange 7 (AO7) azo dye in aqueous solution under ultraviolet (UV) irradiation has been investigated for the first time. BZN catalyst powders prepared using low temperature sol-gel and higher temperature solid-state methods have been evaluated and their reaction rates have been compared. The experimental band gap energy has been estimated from the optical absorption edge and has been used as reference for theoretical calculations. The electronic band structure of BZN has been investigated using first-principles density functional theory (DFT) calculations for random, completely and partially ordered solid solutions of Zn cations in both the and sites of the pyrochlore structure. The nature of the orbitals in the valence band (VB) and the conduction band (CB) has been identified and the theoretical band gap energy has been discussed in terms of the DFT model approximations.

Synthesis, Characterization, and Electronic Structure Studies of Cubic Bi1.5ZnTa1.5O7 for Photocatalytic Applications

Bi1.5ZnTa1.5O7 (BZT) has been synthesized using an alkoxide based sol-gel reaction route. The evolution of the phases produced from the alkoxide precursors and their properties have been characterized as function of temperature using a combination of thermogravimetric analysis (TGA) coupled with mass spectrometry (MS), infrared emission spectrometry (IES), X-ray diffraction (XRD), ultraviolet and visible (UV-Vis) spectroscopy, Raman spectroscopy, and N2 adsorption/desorption isotherms. The lowest sintering temperature (600°C) to obtain phase pure BZT powders with high surface area (14.5 m2/g) has been determined from the thermal decomposition and phase analyses. The photocatalytic activity of the BZT powders has been tested for the decolorization of organic azo-dye and found to be photoactive under UV irradiation. The electronic band structure of the BZT has been investigated using density functional theory (DFT) calculations to determine the band gap energy (3.12 eV) and to compare it with experimental band gap (3.02 eV at 800°C) from optical absorption measurements. An excellent match is obtained for an assumption of Zn cation substitutions at specifically ordered sites in the BZT structure.

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.

Bi-2223 precursor billets for PIT wire production

High temperature superconductor precursor billets (feed rods) have been developed for loading into silver tubes. The billets are loaded prior to rolling or drawing operations in PIT wire manufacture. Investigations have shown that wires and tapes prepared from feed rod loaded tubes show enhanced uniformity of electrical transport properties when compared with conventional powder packing, especially in wires drawn to long lengths. Analysis on production feed rods have shown carbon content to be as low as 110 ppm.