C.R.M. Grovenor

A trapped field of >3 T in bulk MgB2 fabricated by uniaxial hot pressing

A trapped field of over 3 T has been measured at 17.5 K in a magnetized stack of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness 5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which is a readily scalable, industrial technique, to 91% of their maximum theoretical density. The macroscopic critical current density derived from the trapped field data using the Biot–Savart law is consistent with the measured local critical current density. From this we conclude that critical current density, and therefore trapped field performance, is limited by the flux pinning available in MgB2, rather than by lack of connectivity. This suggests strongly that both increasing sample size and enhancing pinning through doping will allow further increases in trapped field performance of bulk MgB2.

Study of the structural, electric and magnetic properties of Mn-doped Bi2Te3 single crystals

Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi2Te3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi2Te3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with TC varying between 9 and 13 K (bulk values) and a saturation moment that reaches 4.4(5) μB per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab-plane. The electrical transport data show an anomaly around TC that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n-doped with carrier concentrations of ~ 0.5–3.0 × 1020 cm−3. X-ray magnetic circular dichroism (XMCD) at the Mn L2,3 edge at 1.8 K reveals a large spin magnetic moment of 4.3(3) μB/Mn, and a small orbital magnetic moment of 0.18(2) μB/Mn. The results also indicate a ground state of mixed d4–d5–d6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga1−xMnxAs. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c-axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.

Studies regarding corrosion mechanisms in zirconium alloys

Understanding the key corrosion mechanisms in a light water reactor primary water environment is critical to developing and exploiting improved zirconium alloy fuel cladding. In this paper, we report recent research highlights from a new collaborative research programme involving 3 U.K. universities and 5 partners from the nuclear industry. A major part of our strategy is to use the most advanced analytical tools to characterise the oxide and metal/oxide interface microstructure, residual stresses, as well as the transport properties of the oxide. These techniques include three-dimensional atom probe (3DAP), advanced transmission electron microscopy (TEM), synchrotron X-ray diffraction, Raman spectroscopy, and in situ electro-impedance spectroscopy. Synchrotron X-ray studies have enabled the characterisation of stresses, tetragonal phase fraction, and texture in the oxide as well as the stresses in the metal substrate. It was found that in the thick oxide (here, Optimized-ZIRLO, a trademark of the Westinghouse Electric Company, tested at 415°C in steam) a significant stress profile can be observed, which cannot be explained by metal substrate creep alone but that local delamination of the oxide layers due to crack formation must also play an important role. It was also found that the oxide stresses in the monoclinic and tetragonal phases grown on Zircaloy-4 (autoclave testing at 360°C) first relax during the pre-transition stage. Just before transition, the compressive stress in the monoclinic phase suddenly rises, which is interpreted as indirect evidence of significant tetragonal to monoclinic phase transformation taking place at this stage. TEM studies of pre- and post-transition oxides grown on ZIRLO, a trademark of the Westinghouse Electric Company, have used Fresnel contrast imaging to identify nano-sized pores along the columnar grain boundaries that form a network interconnected once the material goes through transition. The development of porosity during transition was further confirmed by in situ electrochemical impedance spectroscopy (EIS) studies. 3DAP analysis was used to identify a ZrO sub-oxide layer at the metal/oxide interface and to establish its three-dimensional morphology. It was possible to demonstrate that this sub-oxide structure develops with time and changes dramatically around transition. This observation was further confirmed by in situ EIS studies, which also suggest thinning of the sub-oxide/barrier layer around transition. Finally, 3DAP analysis was used to characterise segregation of alloying elements near the metal/oxide interface and to establish that the corroding metal near the interface (in this case ZIRLO) after 100 days at 360°C displays a substantially different chemistry and microstructure compared to the base alloy with Fe segregating to the Zr/ZrO interface.

High-resolution microchemistry and structure of grain boundaries in bulk Y1Ba2Cu3O7-x

A simple chemical thinning process has been developed which allows high-quality TEM specimens to be prepared from bulk Y1Ba2Cu3O7-x (YBCO) material. This enabled the microchemistry of a statistically significant number of grain boundaries, with a wide range of misorientations, to be studied using TEM, STEM and EDX techniques. In YBCO samples where the overall stoichiometry is exactly 1:2:3 no unreacted CuO, BaCO3 or Y2O3 are present as grain boundary phases. Significant traces of segregated impurity elements (Mo, Zr and Sr in particular) were detected, and the presence of these impurities resulted in the formation of non-stoichiometric phases containing mostly Y, Ba and Cu. Segregation of these impurities was found to occur most often at grain boundaries where no high-symmetry relationship exists between the two grains. There is a direct correlation between the observed amount of impurity-induced grain boundary phases, the grain boundary orientations and the measured values of Jc.

Characterization of melt grown Y1Ba2Cu3O7−x containing barium titanate inclusions II

Abstract Transmission electron microscopy has been used to study the microstructure of melt textured Y 1 Ba 2 Cu 3 O 7−x (YBCO) dope d with 5 wt.% BaTiO 3 . A higher density of dislocations, microcracks, and strain was observed in the YBCO matrix surrounding the titanate inclusions than found in undoped melt textured YBCO grown by the same treatment. The enhanced critical current density J c and the flux pinning behavior of the YBCO-BaTiO 3 composite was correlated to the deformation of the YBCO matrix introduced by the dopant.

Highly reproducible high critical current density in partial-melt Bi2Sr2CaCu2Oy/Ag tapes fabricated by electrophoretic deposition

Highly c-axis-oriented Bi2Sr2CaCu2Oy/Ag tapes with high critical current density (Jc) were prepared by electrophoretic deposition followed by partial-melt processing. Slow cooling at 0.2 degrees C min-1 from the partial-melt temperature and a subsequent prolonged annealing at 825-830 degrees C resulted in an almost phase-pure highly textured microstructure in the samples fabricated from partly reacted precursor powder. The transport Jc can be improved to 1.7*104 A cm-2 at 77 K, and, at 4.2 K, 3.0*105 A cm-2 (Ic=470 A for a 6 mm wide sample) and 1.3*105 A cm-2 with zero and 15 T applied magnetic induction, respectively. Values of Jc were reproducible to within 10% for samples that experienced the same heat treatment and were of similar thickness.

Grain boundary misorientation and thermal grooving in cube-textured Ni and Ni-Cr tape

AFM and EBSD characterization has been carried out to measure the degree of grain boundary grooving in cube-textured Ni and Ni-10wt%Cr substrates. Low angle grain boundary grooves are found to be consistently shallower than grooves at high angle boundaries. Our results suggest that the recrystallization of pure Ni substrates in a Ar-H/sub 2/ atmosphere rather than vacuum may be beneficial in minimizing groove depth. Grain boundary grooves in Ni-10wt%Cr tape were found to be deeper than in pure Ni, consistent with the higher temperature used for recrystallization.

Self-field effects on critical current values in Tl-1223 tapes

Abstract We have measured the properties of a large number of Tl-1223 tapes as part of a programme to optimise the fabrication procedures. During this work we have noticed that the highest I c values are not proportionally dependent on the cross-sectional geometry of the tapes. There seems to be a limiting I c value of around 10–13 A in our samples (and those of many other groups). We have performed a series of experiments on self-field effects in Tl-1223 tapes and compared the results with calculated field distributions in tape-shaped conductors in different configurations. We conclude that the I c are self-field limited in these tapes when the currents being carried are around 10 A.

The fabrication of Bi-based superconductor tape by electrophoretic deposition and melt-texturing techniques

A processing technique has been developed for the fabrication of high-current Bi-2212 superconductor tapes. The technique comprises three steps: (i) treating the particulate ceramic superconductor with trichloroacetic acid in 2-butanol to generate a negative charge on the particle surface; (ii) electrophoretic deposition of the particulate ceramic onto an Ag-based substrate and (iii) melt-texturing of the deposit to form a superconductor coating on the substrate. It has been found that the 2212 superconductor can only be melt textured on an Ag substrate in a narrow temperature window around 885 degrees C. Slow cooling from the molten state not only allows the development of superconductor phase texture, but also suffers from the growth of Ca-Cu-O oxides. These oxides can be transformed into 2212 phase by annealing at 850 degrees C. However, the 2212 grains formed from the Ca-Cu-O precipitates are invariably oriented at right angles to the 2212 phase formed in the course of solidification. The presence of such large-angle grain boundaries is detrimental to the superconductor properties. An optimal processing procedure has been proposed and some important processing parameters discussed. A pilot plant has been set up and semi-continuous tape of Bi-2212 superconductor on Ag has been produced. Critical currents as high as 155 A have been measured over a length of 1 m of multilayer coil at 4.2 K. The performance corresponds to a critical current density of about 70000 A cm-2 in the superconductor layer in a self-field up to about 64 mT. The uniformity of Jc measured on adjacent 10 cm lengths in the multilayer coil is much better than that in a single layer coil, about 30% variation in the former compared with 80% in the latter. The results show that electrophoretic tapes have a promising future for integration into small magnet windings.

Fabrication and properties of TBCCO films via air-atomising spray deposition of thallium-free precursors

Abstract The high-field properties of polycrystalline superconducting TlBaCaCuO films fabricated by the incorporation of thallium vapour into air-atomised BaCaCuO precursors are described. Thick films with T c values in the range 106–111 K have been prepared on polycrystalline yttria-stabilised zirconia substrates. The surface morphology, crystal structure and composition of the films are related to their high-field transport and magnetisation properties. Typical 10 mm × 9 mm films show J c values > 1×10 4 A/cm 2 at 77 K (0 T). The best film has a J c =1.3×10 4 A/cm 2 ( I c =3.6 A) at 77 K (0 T). Films prepared on 26 mm×9 mm substrates show typical large-area J c values > 0.5×10 4 A/cm 2 (77 K, 0 T). A square planar specimen of dimensions 4.3 mm ×4.3 mm exhibited magnetisation J c values=1.2×10 5 A/cm 2 at 4.2 K (0.1 T), 9.3×10 4 A/cm 2 at 10 K (0.1 T), 3.3×10 4 A/ cm 2 at 4 K (8 T), and 1.6×10 4 A/cm 2 at 10 K (8 T).