The reactive liquid Mg infiltration process to produce large superconducting bulk MgB2 manufacts

An alternative and simple manufacturing process is presented to produce high density bulk MgB₂ superconducting objects of large dimensions. The process avoids the use of high pressure apparatus and consists in the reactive infiltration of liquid Mg in B powders preforms. With an appropriately designed stainless steel container for the reactants, several manufacts of different shape have been obtained, including tubes, cylinders, rings or disks, of dimensions of the order of the tens of centimeters.The MgB₂ material, unlike high temperature oxide superconductors, allows an easy percolation of the supercurrents across the grains boundaries, even if it is in the polycrystalline form. Due to this property, the large MgB₂ manufacts obtained by the reactive liquid infiltration present high superconducting characteristics, as demonstrated by transport and magnetic measurements up to 35 K. In particular the magnetic levitation and the magnetic shielding capability appear as the most promising applicative fields in which there is a need for large and homogeneously superconducting bulk pieces.For space applications, the use of MgB₂ superconductors may enable a substantial improvement in the compactness and weight of cryogenic systems, with respect to the actual systems based on the liquid He temperatures. Furthermore, with respect to other high temperature oxide superconductors, MgB₂, besides the disadvantages of the need of a 20–30 K cryogenic system, presents the advantages of a relative lower density (2.4 g/cm³), higher mechanical strength and easier processability, like that here described.     

Analysis of the levitation force of pure and starch/polystyrene/MWCNT added bulk MgB2 superconductors using frozen image model under zero field cooling condition

The measurement of superconducting levitation force between permanent magnet and polycrystalline samples of pure and MgB₂ added with starch, polystyrene (PS) and multiwall carbon nanotube (MWCNT) have been performed under zero field cooling (ZFC) condition at 20 K in both descending and ascending modes. For this, the bulk pellets were synthesized by conventional solid state sintering technique. The XRD data indicate well developed MgB₂ phase. However, a decrease in lattice parameter ‘a = b’ have been observed for doped MgB₂ samples. Superconducting transition temperature of MgB₂ also decreases with starch/PS/MWCNT addition. Unlike MWCNT, the addition of starch/polystyrene is found to enhance the levitation force of MgB₂ superconductor. The levitation force between PM and investigated pellets in ZFC condition is explained well in terms of the updated version of modified frozen image model and the magnetic moment originated due to vertical motion of the superconductors have been estimated. It may be noted that except for MWCNT, addition of starch/PS in MgB₂ improves the magnetic moment generated by vertical movement of pure MgB₂. However, this improvement is more pronounced for 1 wt.% of PS added MgB₂, which indicates more flux trapping and hence better levitation properties in 1 wt.% of PS added MgB₂. The vertical stiffness estimated for pure and starch/PS/MWCNT doped MgB₂ samples indicate that the levitation force are more sensitive in the region close to the PM.     

The role of various boron precursor on superconducting properties of MgB2/Fe

The superconducting properties of Fe sheathed MgB₂ wire has been studied as a function of precursor B powder particle size. The in situ processed MgB₂ samples were prepared by means of conventional solid state reaction method with magnesium powder (99.8%, 325 mesh) and three different types of amorphous boron powders (purity; 98.8%, >95% and 91.9%) from two sources, Pavezyum (Turkish supplier) and Sigma Aldrich. The particle sizes of Turkish boron precursor powder were selected between 300 and 800 nm. The structural and magnetic properties of the prepared samples were investigated by means of the X-ray powder diffraction (XRD) and ac susceptibility measurements. The XRD patterns showed that the diffraction peaks for our samples belong to the main phase of the MgB₂ diffraction patterns. The highest critical temperature, T_c = 38.4 K was measured for the MgB₂ sample which was fabricated by using the 98.8% B. The critical current density of this sample was extracted from the magnetization measurements and J_c = 5.4 × 10⁵ A cm⁻² at 5 K and B = 2 T. We found that the sample made by using the 98.8% boron showed almost 2 times higher J_c than that of obtained from 91.9% B powder.     

Overview of the recent performance of DI-BSCCO wire

Sumitomo Electric has been developing the silver-sheathed Bi2223 multi-filamentary wires since the discovery of Bi-based superconductors. DI-BSCCO (Dynamically-Innovative BSCCO) is the high performance wires produced with the controlled-overpressure (CT-OP) sintering technique. The recent R&D activities have enabled to produce the 1000 m-long wires with I_c of 200 A and the maximum I_c reached 250 A at 77 K by short sample. Besides, the fine control of the carrier density with the non-stoichiometric oxygen enhanced the in-field I_c performances at lower temperature. At a magnetic field of 3 T applied normal to wire surface, the I_c of 280 A at 30 K, and 420 A at 20 K were successfully achieved. To meet the growing needs for various high temperature superconducting applications, Type HT wire with high mechanical strength and Type G wire with low thermal conductivity have been developed.     

MgB2 wires with Ti and NbTi barrier made by IMD process

MgB₂ wires with Ti and NbTi barriers have been made by internal magnesium diffusion (IMD) into boron process. Critical currents, strain tolerances and AC loss of wires with Ti and NbTi barriers have been compared. It was shown that worse uniformity of NbTi barrier affects the creation of regular MgB₂ layer and consequently influences (reduces) also the current densities. Positive effects of NbTi barrier are in improved strain tolerance and reduced coupling losses. The maximum AC loss of not twisted wire with Ti barrier is measured at frequency 9 Hz, but it is shifted up to 60 Hz for NbTi due to considerably increased barrier resistance at 20 K.     

Synthesis of RuSr2GdCu2O8 − δ superconductors by the sol–gel route

The RuSr₂GdCu₂O_8 − δ (Ru-1212) superconductors have been prepared through the sol–gel route. We found that 1030 °C is the optimum annealing temperature for the formation of the Ru-1212 superconductors synthesized by the sol–gel route. X-ray powder diffraction analysis indicates that nearly all the peaks from the samples can be indexed to a single phase of Ru-1212, tetragonal symmetry with lattice parameters a = b = 3.382 Å, c = 11.478 Å and space group I4/mmm. The RuSr₂GdCu₂O_8 − δ superconductors prepared by the sol–gel method exhibit onset transition temperature T_c-onset near 55 K and zero resistance temperature T_c-zero at 45 K.     

Effect of negative pressure on superconducting transition temperature of MgB2

Atomistic simulation has been performed with interatomic potentials to investigate the effect of negative hydrostatic pressure on the superconducting transition temperature (T_c) of MgB₂. The calculation reveals that T_c can be greatly enhanced by applying negative pressure and it can reach up to 52.2 K at ?13 GPa, about 13 K higher than that at ambient pressure. The mechanism for the enhancement of T_c is attributed to that negative pressure reduces high-frequency phonon vibrations of B and thus dramatically enhances the electron–phonon coupling. Our result seems to open up a possible way for the enhancement of T_c in MgB₂.     

Improved critical current density in ex situ processed MgB2 tapes by the size reduction of grains and crystallites by high-energy ball milling

We have fabricated Fe-sheathed MgB₂ tapes through an ex situ process in a powder-in-tube (PIT) technique using powders ball milled under various conditions. Although the ex situ processed wires and tapes using the high-energy ball milled MgB₂ powders have been studied and the decrease of grain and crystallite sizes of MgB₂ and the critical current density (J_c) improvement of those conductors were reported so far, the use of filling powders milled at a higher rotation speed than previously reported further decreases the crystallite size and improves the J_c properties. The improved J_c values at 4.2 K and 10 T were nearly twice as large as those previously reported. Those milled powders and hence as-rolled tapes easily receive contamination in air. Thus, the transport J_c properties are easily deteriorated and scattered unless the samples are handled with care. The optimized heat treatment temperature (T_opt) of those tape samples at which best performance in the J_c property is obtained decreases by more than 100 °C, compared with that of tapes using the as-received MgB₂ powder.     

Cryogenic design and test results of 30-m flexible hybrid energy transfer line with liquid hydrogen and superconducting MgB2 cable

In this paper we present the development of a new hybrid energy transfer line with 30 m length. The line is essentially a flexible 30 m hydrogen cryostat that has three sections with different types of thermal insulation in each section: simple vacuum superinsulation, vacuum superinsulation with liquid nitrogen precooling and active evaporating cryostatting (AEC) system. We performed thermo-hydraulic tests of the cryostat to compare three thermo-insulating methods. The tests were made at temperatures from 20 to 26 K, hydrogen flow from 70 to 450 g/s and pressure from 0.25 to 0.5 MPa. It was found that AEC thermal insulation was the most effective in reducing heat transfer from room temperature to liquid hydrogen in ∼10 m section of the cryostat, indicating that it can be used for long superconducting power cables. High voltage current leads were developed as well. The current leads and superconducting MgB₂ cable passed high voltage DC test up to 50 kV DC. Critical current of the cable at ∼21 K was 3500 A. It means that the 30 m hybrid energy system developed is able to deliver ∼50–60 MW of chemical power and ∼50–75 MW of electrical power, i.e. up to ∼135 MW in total.     

Synthesis of As-grown superconducting MgB2 thin films by sputtering method

As-grown superconducting MgB₂ thin films were prepared by very simple RF sputtering method using a single target without post-annealing process. A two-component target comprising of many small chunks of B on a Mg disk was used. The films were deposited onto polished single crystal sapphire (R-plane) or SrTiO₃ (1 1 1) substrates at rather low temperatures ranging from 100 to 110 °C. Five samples on SrTiO₃ substrate and one on sapphire showed zero resistivity at about 15 K and onset at about 20 K with measuring the resistance versus temperature (R–T curves) and the current versus voltage curves (I–V curve).     

Design and experimental investigation of the high efficiency 1.5 W/4.2 K pneumatic-drive G-M cryocooler

The development of a high cooling power and high efficiency 4.2 K two stage G-M cryocooler is critically important given its broad applications in low temperature superconductors, MRI, infrared detector and cryogenic electronics. A high efficiency 1.5 W/4.2 K pneumatic-drive G-M cryocooler has recently been designed and developed by ARS. The effect of expansion volume rate and operation conditions on the cooling performance has been experimentally investigated. A typical cooling performance of 1.5 W/4.2 K has been achieved, and the minimum temperature of the second stage is 2.46 K. The steady input power of the compressor at 60 Hz is 6.8 kW, while the operation speed of the rotary valve is 30 rpm. A maximum cooling power of 1.75 W/4.2 K has been obtained in test runs.     

Low temperature physical properties of Co-35Ni-20Cr-10Mo alloy MP35N®

Multiphase Co-35Ni-20Cr-10Mo alloy MP35N® is a high strength alloy with excellent corrosion resistance. Its applications span chemical, medical, and food processing industries. Thanks to its high modulus and high strength, it found applications in reinforcement of ultra-high field pulsed magnets. Recently, it has also been considered for reinforcement in superconducting wires used in ultra-high field superconducting magnets. For these applications, accurate measurement of its physical properties at cryogenic temperatures is very important. In this paper, physical properties including electrical resistivity, specific heat, thermal conductivity, and magnetization of as-received and aged samples are measured from 2 to 300 K. The electrical resistivity of the aged sample is slightly higher than the as-received sample, both showing a weak linear temperature dependence in the entire range of 2–300 K. The measured specific heat C_p of 430 J/kg-K at 295 K agrees with a theoretical prediction, but is significantly smaller than the values in the literature. The thermal conductivity between 2 and 300 K is in good agreement with the literature which is only available above 77 K. Magnetic property of MP35N® changes significantly with aging. The as-received sample exhibits Curie paramagnetism with a Curie constant C = 0.175 K. While the aged sample contains small amounts of a ferromagnetic phase even at room temperature. The measured MP35N® properties will be useful for the engineering design of pulsed magnets and superconducting magnets using MP35N® as reinforcement.     

Electroresistance and magnetoresistance in La0.9Ba0.1MnO3 thin films

The electroresistance and magnetoresistance effects have been investigated in La_0.9Ba_0.1MnO₃ epitaxial thin films. Tensile strain caused by substrate mismatch makes the Curie temperature T_C of the film at ∼300 K. The influence of an applied dc-current on the resistance in the absence of a magnetic field was studied. Significant change of the peak resistance at different currents was found. The reduction of the peak resistance reaches ∼27% with an electric current density up to 1.3 × 10⁵ A cm⁻². We also studied colossal magnetoresistance (CMR) effect in the films. Applying a magnetic field of 2 T could lead to a magnetoresistance as large as 42%. The reduction of resistance caused by a current density ∼1.3 × 10⁵ A cm⁻² was found to be equivalent to the CMR effect caused by 1.5 T near T_C. The phenomenon that the resistance in CMR manganites could be easily controlled by the electric current should be of high interest for both fundamental research and practical applications.     

Thick REBaCuO superconducting films through single-coating of low-fluorine metallorganic solution

A high critical current I_c is crucial for the application of high temperature superconductors YBa₂Cu₃O_7-δ in energy efficient power devices and wires.In this paper we report the fabrication of thick (YGd)_1.3Ba₂Cu₃O_7−x films on a metal substrate using low-fluorine metal organic deposition method. The effects of the film thickness on the microstructure, texture and superconductivity properties of the films were evaluated. In order to increase the film thicknesses by single coating, the influence of withdrawal speed during the dip coating on resulting thickness are investigated with the other processing parameters fixed. It is revealed that there is a maximum thickness for a certain starting solution. Here we used 3 different solutions, Conventional Low Fluorine solutions with 2 M and 2.5 M, and super low-fluorine solution with 2.5 M. The maximum thicknesses of about 710 nm, 1280 nm and 1460 nm were obtained, respectively.     

Forced flow He vapor cooled critical current testing facility for measurements of superconductors in a wide temperature and magnetic field range

As superconducting materials find their way into applications, there is increasing need to verify their performance at operating conditions. Testing of critical current with respect to temperature and magnetic field is of particular importance. However, testing facilities covering a range of temperatures and magnetic fields can be costly, especially when considering the cooling power required in the cryogenic system in the temperature range below 65 K (inaccessible for LN₂). Critical currents in excess of 500 A are common for commercial samples, making the testing of such samples difficult in setups cooled via a cryocooler, moreover it often does not represent the actual cooling conditions that the sample will experience in service. This work reports the design and operation of a low-cost critical current testing facility, capable of testing samples in a temperature range of 10–65 K, with magnetic field up to 1.6 T and measuring critical currents up to 900 A with variable cooling power.     

The effects of annealing temperature on the in-field Jc and surface pinning in silicone oil doped MgB2 bulks and wires

The effects of sintering temperature on the lattice parameters, full width at half maximum (FWHM), strain, critical temperature (T_c), critical current density (J_c), irreversibility field (H_irr), upper critical field (H_c2), and resistivity (ρ) of 10 wt.% silicone oil doped MgB₂ bulk and wire samples are investigated in state of the art by this article. The a-lattice parameter of the silicone oil doped samples which were sintered at different temperatures was drastically reduced from 3.0864 Å to 3.0745 Å, compared to the un-doped samples, which indicates the substitution of the carbon (C) into the boron sites. It was found that sintered samples at the low temperature of 600 °C shows more lattice distortion by more C-substitution and higher strain, lower T_c, higher impurity scattering, and enhancement of both magnetic J_c and H_c2, compared to those sintered samples at high temperatures. The flux pinning mechanism has been analyzed based on the extended normalized pinning force density f_p = F_p/F_p,max scaled with b = B/B_max. Results show that surface pinning is the dominant pinning mechanism for the doped sample sintered at the low temperature of 600 °C, while point pinning is dominant for the un-doped sample. The powder in tube (PIT) MgB₂ wire was also fabricated by using of this liquid doping and found that both transport J_c and n-factor increased which proves this cheap and abundant silicone oil doping can be a good candidate for industrial application.     

Effect of magnetic fields on the abnormal electroresistance behavior in epitaxial thin films of La0.8Ca0.2MnO3

We report an unusual electroresistance (ER) behavior induced by a current and its response to magnetic fields in La_0.8Ca_0.2MnO₃ epitaxial thin films. These thin films were fabricated on SrTiO₃ (1 0 0) substrate using pulsed laser deposition (PLD) technique. It is found that the electric resistivity in these films is significantly enhanced by applying a dc current over a threshold value. Simultaneously, an abnormal electroresistance behavior appears in the temperature range from 10 to 300 K. The enhanced resistance turns out to be very sensitive to a weak current. Even a very small dc current can remarkably depress the high resistance, showing an unusual colossal ER effect. The ER reaches ∼1175% at temperatures lower than ∼50 K, and ∼705% at 300 K for a current changing from 0.72 to 10.5 μA. The influence of magnetic fields on the transport was also studied. The I–V curves can be strongly influenced by a low magnetic field even at room temperature. The deduced magnetoresistance (MR) reaches 120% at 300 K upon applying a magnetic field of 0.25 T. An interesting phenomenon is that the observed MR is current dependent.     

Microstructural factors important for the development of high critical current density Nb3Sn strand

Nb₃Sn is the primary candidate for the next generation of accelerator magnets as well as for NMR and other applications that require magnetic fields between 11 and 20 T. Since 1999 the layer critical current density available in long length accelerator quality strand has almost doubled. The microstructural and microchemical factors that are important for high critical current density Nb₃Sn are reviewed. The highest critical current density strands have a Nb₃Sn layer that minimizes chemical and microstructural inhomogeneities and has a high fraction of the layer close to stoichiometric Sn content. Only the internal Sn process has yielded critical current densities beyond 3000 A/mm² at 12 T (4.2 K) and only with interfilamentary Cu thicknesses that are too low to separate the filaments after the final reaction heat treatment. The result of the reaction heat treatment is to produce a continuous ring of Nb₃Sn from hundreds of Nb or Nb-alloy filaments and thus a major ongoing challenge of Nb₃Sn conductor design is to reduce the effective filament diameter to acceptable levels for intended applications. Recent successful attempts to reduce the cost of alloying the Nb₃Sn for high field application are also examined and the potential for future improvements discussed.