Correlation between microstructure and superconducting properties of MgB2 bulk samples with Mg addition and Mg/hBN co-additions

The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (Jc) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hexagonal boron nitride (hBN) doping improves intrinsic Jc by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic Jc by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1 wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature

Effect of cubic and hexagonal boron nitride additions on the microstructure and properties of bulk MgB2 superconductors

MgB2 is a promising material for intermediate temperature applications where conventional low temperature superconductors cannot be used, especially if the range of magnetic fields over which is has acceptable current carrying performance can be expanded. However, its applicability is limited by poor properties at elevated magnetic fields. Carbon-based dopants can be used to dramatically improve the high-field performance of MgB2, but at the cost of a reduction in the superconducting transition temperature (Tc) that limits the operation temperature to 20 K or below. Here we report an enhancement of superconducting performance of MgB2 with the addition of cubic and hexagonal boron nitride, without any significant reduction in Tc. Ex-situ bulk samples of MgB2 with two forms of boron nitride addition were manufactured by the field assisted sintering technique (FAST) after high energy ball milling of powder mixtures. We find that hexagonal BN (hBN) nanoparticles mixed homogenously with MgB2 powder react much more easily to produce Mg-N-B impurities than larger cubic BN (cBN) particles (~ 10 µm) under the same processing conditions. The addition of 1 wt% hBN or 5 wt% cBN combined with 6 h of milling has been demonstrated to improve the critical current density (Jc) of MgB2 over the entire magnetic field range. It is proposed that the nano-sized Mg-N-B impurities, that typically reside at MgB2 grain boundaries, increase pinning strength by introducing additional flux pinning centres. In addition, excess Mg may benefit the low-field performance by improving the connectivity. This work shows the significance of microstructural characterization on inhomogeneous superconducting materials to analyse their performance

Site‐specific specimen preparation for SIMS analysis of radioactive samples

Secondary Ion Mass Spectrometry is an important technique for the study of the composition of a wide range of materials because of the exceptionally high sensitivity that allows the study of trace elements and the ability to distinguish isotopes that can be used as markers for reactions and transport processes. However, when studying nuclear materials, it is often necessary to analyse highly radioactive samples, and only rather few SIMS facilities are available in active environments. In this paper, we present a methodology using focussed ion beam milling to prepare samples from radioactive specimens that are sufficiently large to undertake SIMS mapping experiments over microstructurally significant regions, but with overall activities small enough to be readily transported and analysed by a SIMS instrument in a normal laboratory environment. Radioactive samples prepared using this methodology can also be used for correlative SIMS analysis with other analytical microscopies. SIMS results showing the distributions of deuterium in oxides on in‐reactor corroded zirconium alloys are presented to demonstrate the potential of this sample preparation technique

Novel superconducting joints for persistent mode magnet applications

Persistent current joints are a critical component of commercial superconducting magnets.The standard jointing method widely used in the magnet industry for technological low temperature superconducting wires such as NbTi and Nb3Sn wires uses a superconducting solder (e.g. PbBi). In these joints the physical and superconducting properties of the solder materials inevitably play an important role in the overall performance of the joint. Key requirements for superconducting solders include low melting point to prevent degradation of the superconducting filaments during joining, good wettability of the superconducting filaments, suitable liquid phase viscosity, and finally adequate superconducting properties to enable sufficient supercurrent to pass through the joint under typical operating conditions (typically at 4.2K in a field of 1 T for an MRI magnet). PbBi solder satisfies all these criteria, but restrictions on the use of lead in the magnet industry are expected in the relatively near future, so new lead-free jointing techniques need to be developed. One approach is the development of superconducting lead-free solder materials. In our work, we have focussed on the In-Sn system and ternary systems involving In and Sn as two of the elements. Thermodynamic modelling has been used to produce ternary phase diagrams of potential alloy systems, and various formulations have been fabricated in order to explore how microstructure and phase chemistry influence the superconducting properties of the solders. Alternative approaches to fabricating lead-free joints, including spot welding and cold-pressing, have also been investigated. These methods have the potential advantage of achieving direct NbTi-NbTi joints with no intermediate, lower performance superconducting material. The spot welding method produced joints with the best superconducting performance, signifiantly better than the currently used PbBi solder, but the lack of reproducibility in this technique may be a problem from an industrial point of view.</p

Demonstration of better superconducting performance in a solder with low lead content

Lead-free SnIn solders are promising for superconducting magnet applications. However, their superconducting properties are not as good as lead solders. In order to improve the superconducting performance of the Sn-In solders, researchers have investigated the superconducting properties of ternary systems such as Sn-In-Bi for solder joints. In this study, powders of Pb, Nb, AgCu and grapheme nano pellets in the ratios of 0.5-5 wt% have been added into SnIn (35:65) to investigate their microstructural, thermal and superconducting properties. The added materials enhance the superconducting properties. We find that even low Pb additions show a dramatic improvement in superconducting properties, with an increase in both T c and J c values of up to 6.35 K and 1.47 × 104 A m−2, respectively. This shows that much lower Pb content superconducting solders can be effective and could be used to replace the PbBi solder commonly used with the superconducting properties T c =8.4 K, H C2 = 1.77 T, H C = 0.0909 T

Three-dimensional imaging of selenium and chlorine distributions in highly efficient selenium-graded cadmium telluride solar cells

Thin-film solar modules based on cadmium telluride (CdTe) technology currently produce the world's lowest cost solar electricity. However, the best CdTe modules now contain a cadmium selenium telluride (CST) alloy at the front of the absorber layer. Despite this, research characterizing the behavior of selenium in alloyed CdTe devices is currently very limited. Here we employ advanced secondary ion mass spectrometry measurements to map the three-dimensional distribution of selenium in a graded CST/CdTe device for the first time. We find significant interdiffusion of selenium between the CST and CdTe layers in the cell, primarily out of the CST grain boundaries and up into the CdTe grain boundaries and grain fringes above. This results in significant lateral variations in selenium concentrations across grains and hence also lateral fields, which we estimate using the measured selenium concentrations

The GPIHBP1-LPL complex is responsible for the margination of triglyceride-rich lipoproteins in capillaries.

Triglyceride-rich lipoproteins (TRLs) undergo lipolysis by lipoprotein lipase (LPL), an enzyme that is transported to the capillary lumen by an endothelial cell protein, GPIHBP1. For LPL-mediated lipolysis to occur, TRLs must bind to the lumen of capillaries. This process is often assumed to involve heparan sulfate proteoglycans (HSPGs), but we suspected that TRL margination might instead require GPIHBP1. Indeed, TRLs marginate along the heart capillaries of wild-type but not Gpihbp1⁻/⁻ mice, as judged by fluorescence microscopy, quantitative assays with infrared-dye-labeled lipoproteins, and EM tomography. Both cell-culture and in vivo studies showed that TRL margination depends on LPL bound to GPIHBP1. Notably, the expression of LPL by endothelial cells in Gpihbp1⁻/⁻ mice did not restore defective TRL margination, implying that the binding of LPL to HSPGs is ineffective in promoting TRL margination. Our studies show that GPIHBP1-bound LPL is the main determinant of TRL margination