Evaluation of a solid nitrogen impregnated MgB2 racetrack coil

To develop powerful wind turbine generators using superconducting technology, high-performance superconducting racetrack coils are essential. Herein, we report an evaluation of a multifilamentary magnesium diboride (MgB2) conductor-based racetrack coil cooled and impregnated simultaneously by solid nitrogen (SN2). The coil was wound on a copper former with 13 mm winding width, an inner diameter of 124 mm at the curvature, and 130 mm length of the straight section. An in situ processed S-glass-insulated 36-filament MgB2 wire was wound on the former in two layers with 19.5 turns, and heat treated via the wind and react method without any epoxy resin. The coil was evaluated for critical temperature and transport critical current in the SN2 environment at different temperatures up to 31.3 K in self-field. The coil was able to carry 200 A transport current at 28.8 K in self-field. During coil charging and operation, SN2 effectively acted as an impregnation material. The test results demonstrate the viability to use MgB2 racetrack coil potentially with SN2 impregnation in advanced rotating machine applications

Bioelectromagnetics research within an Australian context: the Australian centre for electromagnetic bioeffects research (ACEBR)

Mobile phone subscriptions continue to increase across the world, with the electromagnetic fields (EMF) emitted by these devices, as well as by related technologies such as Wi-Fi and smart meters, now ubiquitous. This increase in use and consequent exposure to mobile communication (MC)-related EMF has led to concern about possible health effects that could arise from this exposure. Although much research has been conducted since the introduction of these technologies, uncertainty about the impact on health remains. The Australian Centre for Electromagnetic Bioeffects Research (ACEBR) is a National Health and Medical Research Council Centre of Research Excellence that is undertaking research addressing the most important aspects of the MC-EMF health debate, with a strong focus on mechanisms, neurodegenerative diseases, cancer, and exposure dosimetry. This research takes as its starting point the current scientific status quo, but also addresses the adequacy of the evidence for the status quo. Risk communication research complements the above, and aims to ensure that whatever is found, it is communicated effectively and appropriately. This paper provides a summary of this ACEBR research (both completed and ongoing), and discusses the rationale for conducting it in light of the prevailing science.Sarah P. Loughran ... Jim Manavis ... Robert Vink ... et al

Enhancement of MgB2 superconductor by magnetic nanoparticle doping

Magnesium diboride superconductor is easy and cheap to produce, and it can be operated at temperatures that can be maintained by inexpensive cryocoolers, requiring only electrical power input. In order to make it competitive with classical superconductors, its performance in high magnetic field needs to be improved. This is done by improvement of vortex pinning through including non-superconducting nano-defects. Theoretical work shows that magnetic nanoparticles could give better vortex pinning than the non-magnetic ones. Existing experimental reports on this topic are patchy and inconclusive. This work presents a systematic study on the preparation of various magnetic nanoparticles, their incorporation into MgB2, and mechanisms for improvement of current carrying capabilities for the most successful nanoparticles. A number of different nanoparticles were prepared and tried: carbon coated Co, carbon coated Fe, carbon coated Ni, CuFe2O4, Fe2B, Co2B, NiCoB, Fe3O4, CoB, SiO2 coated Fe2B, Co2B, and commercial Eu2O3 and Dy2O3. Most of them resulted in marginal improvement of MgB2, or even in degradation of its properties. The most successful results were obtained with NiCoB nanoparticles 5 nm in size. They are the focus of this project. The best NiCoB nanoparticles were prepared by the wet method of chemical reduction of metallic salts, which yielded nanoparticles small enough to be successful pinning centres in MgB2.The success of this method is due to the medium level alkaline environment with surfactants which supported their growth. The nanoparticles were clearly superparamagnetic. Optimization of the preparation procedure for nano-NiCoB doped MgB2 gave the best performance for 2.5 wt% NiCoB doping, the use of amorphous 500 nm-size boron with micron-sized Mg, and heat treatment at 850℃ for 30 minutes. Detailed analysis of the improvement of critical current density, Jc, by this doping revealed that the connectivity between superconducting crystals and the vortex pinning were both improved as a result of this procedure. The connectivity improved due to refinement of crystals and, at the same time, due to removal of MgO from the crystals due to the reaction between Mg and the dopant nanoparticles. The vortex pinning improved only at low temperatures. High temperature heat-treatment was required to obtain the vortex pinning. A reaction between the NiCoB nanoparticles and Mg led to formation of Mg2Ni and Co2Mg nanoparticles that were incorporated into the MgB2 matrix, where they acted as pinning centres. Competition of NiCoB for Mg with MgO was the most likely cause of the decrease in MgO content in the samples, leading to better connectivity. These new nanoparticles did not have a spontaneous magnetic moment. No effects of magnetic pinning were observed in this project, although a large number of different magnetic nanoparticles were used as dopants. Therefore, this project casts doubt on the effectiveness of magnetic nanoparticles as a means of improvement of vortex pinning in MgB2 through the interaction of magnetic vortices with the magnetic moment of nanoparticles

Flux pinning and inhomogeneity in magnetic nanoparticle doped MgB2/Fe wires

The effects of magnetic nanoparticle doping on superconductivity of MgB2/Fe wires have been investigated. Fe2B and SiO2-coated Fe2B particles with average diameters 80 and 150 nm, respectively, were used as dopands. MgB2 wires with different nanoparticle contents (0, 3, 7.5, 12 wt.%) were sintered at temperature 750°C. The magnetoresistivity and critical current density Jc of wires were measured in the temperature range 2–40 K in magnetic field B ≤ 16 T. Both transport and magnetic Jc were determined. Superconducting transition temperature Tc of doped wires decreases quite rapidly with doping level (~ 0.5 K per wt.%). This results in the reduction of the irreversibility fields Birr(T) and critical current densities Jc(B,T) in doped samples (both at low (5 K) and high temperatures (20 K)). Common scaling of Jc(B,T) curves for doped and undoped wires indicates that the main mechanism of flux pinning is the same in both types of samples. Rather curved Kramer\u27s plots for Jc of doped wires imply considerable inhomogeneity

The influence of CuFe2o4 nanoparticles on superconductivity of MgB2

The influence of CuFe2O4 nanoparticle doping on superconducting properties of Fe-sheated MgB2 wires has been studied. The wires containing 0, 3 and 7.5 wt.% of monodisperse superparamagnetic nanoparticles (∼ 7 nm) were sintered at 650◦C or 750◦C for 1 hour in the pure argon atmosphere. X-ray diffraction patterns of doped samples showed very small maxima corresponding to iron boride and an increase in the fraction of MgO phase indicating some interaction of nanoparticles with Mg and B. Both magnetic and transport measurements (performed in the temperature range 2−42 K and magnetic field up to 16 T) showed strong deterioration of the superconducting properties upon doping with CuFe2O4. The transition temperatures, Tc, of doped samples decreased for about 1.4 K per wt.% of CuFe2O4. Also, the irreversibility fields Birr(T) decreased progressively with increasing doping. Accordingly, also the suppression of Jc with magnetic field became stronger. The observed strong deterioration of superconducting properties of MgB2 wires is at variance with reported enhancement of critical currents at higher temperatures (determined from magnetization) in bulk MgB2 samples doped with Fe3O4 nanoparticles. The probable reason for this discrepancy is briefly discussed

Novel synthesis of superparamagnetic Ni-Co-B nanoparticles and their effect on superconductor properties of MgB2

A new procedure for the preparation of amorphous Ni-Co-B nanoparticles is reported, with a detailed investigation of their morphology by X-ray diffraction and transmission electron microscopy, as well as their magnetic properties. Many factors, such as chemical composition, anisotropy, size and shape of the particles, were controlled through chemical synthesis, resulting in the control of morphological and magnetic properties of the nanoparticles. Controlling pH values with ethylenediamine and using sodium dodecyl sulfate surfactant lowered the size of the nanoparticles to below 10 nm. Such a small structure and chemical disorder in nanocrystalline materials lead to magnetic properties that are different from those in their bulk-sized counterparts. The obtained nanoparticles can be used for different purposes, from pharmaceutical applications to implementations in different materials technology. The focus of this research is the synthesis of Ni-Co-B nanoparticles in a new way and studying the reaction of Ni-Co-B nanoparticles with Mg and B precursors and their effect on MgB2 properties. New nanostructures are formed in the reaction of Ni-Co-B nanoparticles with Mg: Mg2Ni, Co2Mg and possibly Mg2Co

Interplay between cold densification and malic acid addition (C4H6O5) for the fabrication of near-isotropic MgB2 conductors for magnet application

© 2020 The effect of cold high pressure densification (CHPD) on anisotropy of the critical current density (Jc) in « in situ » single core binary and alloyed MgB2 tapes has been determined as a function of temperatures at 4.2 K, 20 K and 25 K as well as at applied magnetic fields up to 19 T. The study includes binary and C4H6O5 (malic acid) doped MgB2 tapes before and after CHPD. It is remarkable that the CHPD process not only improved the Jc values, in particular at the higher magnetic fields, but also decreased the anisotropy ratio, Γ = Jc///Jc┴. In binary MgB2 tapes, the anisotropy factor Γ increases with higher aspect ratios, even after applying CHPD. In malic acid (C4H6O5) doped tapes, however, the application of CHPD leads only to small enhancements of Γ, even for higher aspect ratios. This is attributed to the higher carbon content in the MgB2 filaments, which in turn is a consequence of the reduced chemical reaction path in the densified filaments. At all applied field values, it was found that CHPD processed C4H6O5 doped tapes exhibit an almost isotropic behavior. This constitutes an advantage in view of industrial magnet applications using wires with square or slightly rectangular configuration

Improvements in the dispersion of nanosilver in a MgB2 matrix through a graphene oxide net

The effects of graphene oxide (GO) addition on the dispersion of nanosilver (Ag) in an MgB2 matrix were studied using bulk samples prepared through a diffusion process. The influence of the dispersion of Ag and Ag/GO particles on the critical current density (Jc) of MgB2 was also investigated. GO has emerged as an excellent dopant which can significantly improve both the low- and high-field performance of MgB2 due to its capability to improve intergrain connectivity (GO) and inter- and intragrain pinning (GO and AgMg). The addition of nanosize Ag particles also results in an improvement of vortex pinning, and at the same time, it offers the advantage of preventing the loss of Mg during the sintering process. It is found that the dispersion of nanosilver in the presence of GO results in significant improvements in the critical current density in MgB2, particularly at high magnetic fields, due to improved intergrain connectivity and flux pinning. The use of the GO net as a platform for doping MgB2 in our case with Ag yielded a 10-fold-better critical current density (Jc) than standard Ag doping at 9 T and 5 K. Even without sophisticated processes, we obtained a Jc result of 104 A/cm2 at 9 T and 5 K, which is one of the best ever achieved

Significant reduction of critical current anisotropy in malic acid treated MgB2 tapes

The anisotropy of the critical current density, the n-factor and the irreversibility field of mono-core in-situ MgB2 tapes have been studied at various magnetic field orientations and temperatures. Undoped as well as silicon carbide (SiC) and malic acid (C4H6O5) added tapes were studied. The anisotropy is noticeably influenced by the additives due to different carbon contents and fabrication processes. The malic acid added tape exhibits almost isotropic behavior compared to SiC doped MgB2 tapes due to the homogeneous carbon substitution through the chemical solution route, which paves the way to design MgB2 tapes to be applied for industrial magnet application