MgB2 Superconducting Tips for Scanning Tunneling Microscopy Study

We report a simple method for the fabrication of reproducible, clean, and stable MgB₂ superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB₂ superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB₂, suitable for use as STM tips, is feasible.     

MgB2 Superconducting Tips for Scanning Tunneling Microscopy Study

We report a simple method for the fabrication of reproducible, clean, and stable MgB₂ superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB₂ superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB₂, suitable for use as STM tips, is feasible.     

Heat Treatment of Two-Turn Nb3Sn Superconducting Coil

A heat treatment system was built at High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL) for the heat treatment of large-scale Nb₃Sn superconducting coils and conductors. In order to evaluate the manufacturing technique of the joints for the Nb₃Sn CICC, a two-turn Nb₃Sn superconducting coil with two superconducting joints had been developed. A two-turn coil together with several Nb₃Sn witness samples was heat-treated. Experimental and finite elemental analysis was also discussed. The results of the measurements met the requirements of the magnet design.     

Effect of synthesis temperature on density and microstructure of MgB2 superconductor at ambient pressure

The experimental thermodynamic of MgB₂ synthesis process and phase compositions have been investigated by diffraction thermal analysis (DTA) technology and X-ray diffraction. The fabrication of MgB₂ bulks and superconducting properties at the temperatures range from 600 to 800°C were reported. And microstructure of MgB₂ bulks were observed by scanning electron microscope (SEM). A method was developed to determine the porosity of MgB₂ and the highest density can be obtained in MgB₂ prepared at 650°C at ambient pressure. It is found that the vapor pressure of Mg increases remarkably at high temperature, leading to the high porosity in MgB₂ samples. MgB₂ bulk with good superconducting property and fine microstructure was synthesized at 750°C.     

Monofilamentary In Situ Fe/MgB<Subscript>2</Subscript> Superconducting Wires Fabricated by Pellet-in-Tube Method

Thin monofilamentary Fe/MgB₂ superconducting wires without barriers are investigated by means of electrical transport measurements and surface and structural analysis methods. Small diameter wires are fabricated by pellet-in-tube method (PeIT) to obtain a high uniform initial filling density and heat treated as a function of various sintering temperatures and times. The results are discussed in terms of the grain connectivity, Fe₂B phase formation, and the relation between wire diameter and sintering conditions. We suggest that PeIT has a crucial importance to achieve homogeneous initial filling density, which leads to the fabrication of uniform long-length MgB₂ wires.     

Properties of MgB2 superconductors with regard to space applications

Thin MgB₂ conductors with diameters down to 50 μm have been developed for application as current leads on a satellite and as LH₂ level sensor. The high transition temperature of 39 K makes applications of MgB₂ at temperatures around 20 K possible, significantly above the temperature margin of low temperature superconductors like NbTi or Nb₃Sn. The absence of weak link behaviour and the fact that there is no need to texture MgB₂ to obtain high critical current densities is a crucial advantage compared to high temperature superconductors. We give an overview of the current status of MgB₂ conductor development and show the potential of these wires for space applications.     

Superconducting magnet development for fusion at JAERI

The development of superconducting magnets for fusion at the Japan Atomic Energy Research Institute (JAERI) is described. The objective of the project is the Fusion Experimental Reactor (FER) which will be constructed with superconducting toroidal and poloidal coils. For toroidal coils, JAERI has already developed the 8 T Japanese LCT coil and five other large coils (one NbTi and four Nb₃Sn coils, from 7 T to 11 T) for the Cluster Test Programme. For poloidal coils, JAERI has developed three 30–50 kA pulsed conductors. In addition to coil development, cryogenic technology and structural material development are also in progress.     

Imperfection of microstructural control in MgB2 superconducting tapes fabricated using an in-situ powder-in-tube process: toward practical applications

MgB₂ is a promising superconductor for future applications to superconduct wires and tapes used at the liquid hydrogen temperature (20 K). Because the maximal superconducting current (critical current) in MgB₂ depends on its microstructure, an “in-situ” process to fabricate MgB₂ based on a reaction between Mg and B is effective in controlling the microstructure. However, the critical current in the fabricated MgB₂ wires and tapes is not sufficiently high for practical use. This may be attributed to the imperfectness of the microstructural control in MgB₂ wires and tapes. In this paper, we will discuss the microstructural features of MgB₂ tapes fabricated using a typical in-situ powder-in-tube process. By observing the early stage of microstructural evolution in detail, we can gain insights into the next strategy for improving the microstructure of MgB₂ tapes with respect to their applications.     

Investigation of Nb–B Diffusion and the Superconducting Properties of MgB2/Nb/Cu Tapes

The Nb?CB diffusion behaviors and their effects on the superconducting properties of MgB₂/Nb/Cu tapes were investigated. Two relevant samples of the Nb?CB diffusion couples and monocore MgB₂/Nb/Cu tapes were prepared with the same standard in situ PIT method, respectively. And both the samples were sintered at 650, 750, 850, and 950°C for 2 hours, respectively. It has been found that Nb?CB interface is invisible in the diffusion couples sintered at low temperatures as 650°C, correspondingly the superconducting properties of MgB₂ tapes are superior to those of the ones sintered at the other three temperatures. With the heating temperature increasing, Nb?CB interface, mainly NbB₂ with a little NbB, appears and becomes thicker gradually in the Nb?CB diffusion couples. The corresponding superconducting properties of MgB₂ tapes follow the similar patterns: within the range from 650°C to 950°C, the higher the sintering temperature, the poorer the superconducting performances, because the Nb?CB diffusion in MgB₂ tapes could cause the property degradations of MgB₂/Nb tape. As extended to the actual fabrications of Nb sheathed MgB₂ tapes or wires, suitable sintering temperature range should be from 650°C to 750°C for the desired performances.     

Heat Treatment Study of Nb3Sn CICC

The design and fabrication of the model coil for the 40 T hybrid superconducting outsert has been completed at High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL). ??Wind-and-React?? method is adopted to ease subsequent handling and assembly and to protect the brittle material. The model coil which is made of Nb₃Sn CICC will undergo a high temperature diffusion reaction according to the optimum heat treatment schedule for the Nb₃Sn/RRP^? superconducting strand. The facility specialized in the heat treatment of Nb₃Sn superconducting coils has been established in CHMFL, and several kinds of monitoring instruments are equipped to monitor impurities in the argon gas during heat treatment. At present, the heat treatment of the model coil has been finished and the performance test results of the model coil meet our requirements. This paper mainly presents the introduction of the heat treatment facility, details of heat treatment process for the model coil, and some simulations of the states during heat treatment.     

Fundamental studies for the application of quench protection systems based on an active power method for cryocooled LTS coils

When the quenching occurs in a superconducting coil, excessive joule heating in normal area may damage the coil. It is necessary to detect quenching in the coil as soon as possible and discharge the magnetic energy stored in the coil. Therefore, we propose a superconducting coil protection system based on an active power method. The system is highly resistant to the noise and does not require cancel voltage taps, so it is useful for both AC and DC coils. We have presented the effectiveness of the system using some test coils cooled in LN₂ or LHe. However, we have not discussed the effectiveness of the proposed system for helium-free cryocooled magnets, in which a larger temperature rise occurs after quenching than in liquid-cooled magnets. In this paper, we verify the effectiveness and practicality of the proposed system through coil protection tests for a cryocooled Nb₃Al LTS coil.     

Nb3Sn material development in Russia

In the USSR and later in Russia, the main activities in technical superconductivity were concentrated in the institutes that belonged to the Ministry of Atomic Energy (Minatom). The development of new technologies shortly transferred to the large-scale industrial production of NbTi and Nb₃Sn superconductors in early 1970s. Two main technologies for multifilamentary Nb₃Sn strands were under investigation during that time – bronze-process and internal tin method. More than 25 ton of Nb₃Sn bronze-processed strands were produced for the fabrication of 90 ton of conductors for application in the magnet system of first in the world fusion facility (tokamak T-15) with magnet system based on the intermetallic compound. The characteristics of these strands and conductors have been briefly described. The requirements for the Nb₃Sn strands constantly increased and the main R&D on the enhancement of critical current density have been reviewed. For bronze-processed strands the increase of the tin content in large ingots was the crucial factor. The artificial doping of niobium filaments by niobium–titanium alloy was invented, which enabled to improve the workability of Nb₃Sn strands, with enhanced critical current density in high fields. For internal tin Nb₃Sn strands the main R&D were concentrated on the optimization of the layouts of the strand and on the multistage heat treatment because of the inevitable liquid phase formation which could result in severe distortion of the geometrical arrangement of the filaments and even in destruction of the whole strand. The main results of these investigations have been presented. The corresponding impact of these R&D on the design of bronze-processed and internal tin strands has been analyzed. The quantitative estimations of the grain size were made for bronze-processed and internal tin strands. It was shown that in bronze-processed and internal tin strands subjected to the standard ITER heat treatment characterized by two stages at 575 °C and 650 °C, the variation of Nb₃Sn grain size in the range of 30–300 nm could be observed. The correlations of microstructure and superconducting properties have been discussed. The ITER connected activities in Russia on the development of Nb₃Sn strands, which met the HP-II specification, have been outlined. The results of the ITER Model Coil Program have shown a degradation of the critical current of large cable-in-conduit conductors (CICC) built with Nb₃Sn strands. For this reason, the investigation on the strain dependence of critical current density in Nb₃Sn strands of different designs is of high interest and priority. The R&D on development of bronze-processed and internal tin Nb₃Sn strands with enhanced, by the nanostructured Cu–Nb material, mechanical strength have been reviewed.     

Improvement of the High-Magnetic-Field Critical Current Density of the <Emphasis Type="Italic">Ex-Situ</Emphasis> Annealed MgB<Subscript>2</Subscript> Thick Films by Oxygen Doping

This paper presents a very simple way to synthesis MgB₂ thick films with high critical current density in a magnetic field by ex-situ annealing precursor B films in air with excessive Mg in a sealed quartz tube. The films show a significant improvement of critical current density in a magnetic field compared to the high purity films annealed in vacuum, while its zero-resistance transition temperature T _c ^zero and normal state resistivity still maintain about 38 K and 17 μΩcm. The results demonstrate MgB₂ thick films have great potential applications in superconducting coated conductors.      

A novel high temperature superconducting magnetic flux pump for MRI magnets

This paper presents a kind of minitype magnetic flux pump made of high temperature superconductor. This kind of novel high temperature superconducting (HTS) flux pump has not any mechanical revolving parts or thermal switches. The excitation current of copper coils in magnetic pole system is controlled by a singlechip. The structure design and operational principle have been described. The operating performance of the new model magnetic flux pump has been preliminarily tested. The experiments show that the maximum pumping current is approximately 200 A for Bi2223 flux pump and 80 A for MgB₂ flux pump operating at 20 K. By comparison, it is discovered that the operating temperature range is wider, the ripple is smaller and the pumping frequency is higher in Bi2223 flux pump than those in MgB₂ flux pump. These results indicate that the newly developed Bi2223 magnetic flux pump may efficiently compensate the magnetic field decay in HTS magnet and make the magnet operate in persistent current mode, this point is significant to the magnetic resonance imaging (MRI) magnets. This new flux pump is under construction presently. It is expected that the Bi2223 flux pump would be applied to the superconducting MRI magnets by further optimizing structure and improving working process.     

Apparatus for calorimetric measurements of losses in MgB2 superconductors exposed to alternating currents and external magnetic fields

Inexpensive superconducting wires with low AC losses would open up for a large superconductor market in AC electrical power applications. One candidate for this market is the MgB₂ conductor. In the development of an AC optimized superconductor, high-quality measurements of the AC losses under application-like conditions must be available. This article describes an apparatus built for this purpose. The measurement method is calorimetric. The temperature increase of the superconductor sample is measured and compared to the temperature increase due to a heater with known heat input. The system is designed for measurements of losses due to magnetic fields combined with transport currents. Results from tests verifying the capabilities of the system are given, as well as from initial AC loss measurements on a tape-shaped MgB₂ superconductor.     

The propagation of the resistive region in high current density superconducting coils

A method is given describing the propagation phenomenon of the resistive region within the winding of superconducting coils wound from semi-stabilized filamentary conductors. The method permits the approximate calculation of the velocities of propagation of the normal conducting front in superconducting coils of high current density. The calculation is carried out assuming that the parameters of the conductor materials, the short sample data, and the heat transfer coefficient from the conductor into the ambient medium are given. The propagation velocities of the normal front have been measured in insulated wires freely immersed in liquid helium, in test samples using conventional interleaving material, and in a compact multilayer coil wound from the same wire. The calculated results are compared with experimental data.     

Fabrication of 6-filament MgB2 wires enhanced by high strength 91-filament Cu–Nb composite

We chose high strength and high conductive Cu–Nb composite as strengthening core to improve the mechanical properties of 6-filament MgB₂ wires. The Cu–Nb core become partially dispersion strengthened during the fabrications of the MgB₂ wires. It has been found that this Cu–Nb composite offers good promise of increased strength while maintaining the superconducting properties of the MgB₂ wire. The Young’s modulus of the best wire samples increased significantly to about 130 GPa, which is comparable to those of high strength ferromagnetic materials sheathed wires but without negative ferromagnetic effects. Those mechanical properties were enough to satisfy the low field application needs. The critical current I _c also achieves 200 A (engineering critical current density, J _ce above 1.30 × 10⁴ A/cm²) at 20 K 1 T field. The 91-filament Cu–Nb composite core reinforced wires were fabricated by in situ Powder In Tube method.     

Fabrication and Properties of MgB2 Coated Superconducting Tapes

We present the formation of MgB₂ coatings by simple and novel aerosol deposition technique which has a potential to escalate towards the fabrication of long superconducting tapes. The thin MgB₂ coatings were produced by using pre-synthesized MgB₂ powder. The ability of this technique to form a precursor powder in a thin film form has greatly reduced the intricacies involved in the synthesis of MgB₂ by other techniques like hybrid physical chemical vapor deposition etc. The as-synthesized thin films were characterized by the x-ray diffraction technique to study the structural properties. The thin films were found to be x-ray amorphous in nature depicting the formation of frustrated structure which showed a superconducting transition onset at around 36 K.     

Filamentary MgB2 Superconductors with Titanium Barriers

Ti diffusion barrier has been applied for several Cu stabilized MgB₂ wires. Pure Ti is well formable metal allowing formation of thin barrier layers which are not reacting with MgB₂. Instead of, Ti is able to purify MgB₂ filaments by absorbing some impurities during the final heat treatment. Ti has comparable coefficient of thermal expansion with MgB₂, which allows heat treatment at higher temperatures than for Nb barrier wires. Consequently, higher critical current densities can be obtained with Ti. Higher Ti resistivity offers a depressing of coupling currents in AC regime. One disadvantage of Ti is the inter-diffusion with copper during annealing and partial contamination of Cu stabilization. Benefits of Ti diffusion barrier have been utilized for the manufacture of fine-filamentary wires with minimal filament diameter of 10 μm. High critical current densities and high resistances to tensile stress and torsion stress at twisting have been demonstrated for these wires. AC loss measurements have shown reduced losses with decreased filament size and with shortened twist pitch.     

The role of the substrate surface morphology in enhancing the MgB2 superconducting temperature

We hereby report on the role of the surface morphology of various substrates in the enhancement of the superconducting critical temperature of MgB₂. MgB₂ thin layers were grown by hybrid physical–chemical vapour deposition on silicon carbide SiC substrates/fibers and several other substrates, characterized by diverse surface morphologies. By investigating the structural, morphological and transport properties of MgB₂ thin layers, the presented data show that the superconducting critical temperature T _c exceeds the bulk value only when the MgB₂ films are grown on atomically flat (0001) SiC single crystals and on MgB₂ bottom layers. These results further confirm the interpretation of the coalescence-driven tensile strain mechanism behind the enhancement of superconducting properties in MgB₂ thin films.