Showing papers in "Physica C-superconductivity and Its Applications in 2007"

Development of ex situ processed MgB2 wires and their applications to magnets

Abstract: In spite of the relatively short time dedicated to the development of magnesium diboride conductors since its discovery in early 2001, a substantial improvement was soon achieved in their manufacture and use. Unlike many others HTS and LTS materials, the MgB 2 conductor processing is more open to a number of improvements and modifications that help in making it more attractive for several DC and AC applications. Many kilometres of conductors were already produced throughout the world and it is now possible to start seriously thinking about a systematic industrial production of this material, as it is already possible to purchase it in reasonable lengths on the free market. These remarkable lengths of conductor were also wound in coils and their performance continuously improved in the past years. Here we will present a review of the recent results and a perspective for the future development of this “new” superconductor, starting from the optimisation of the precursor powders needed to improve the magnetic field behaviour of the tapes, to the conductor development, i.e. the production of multifilamentary Cu-stabilized tapes in lengths up to 1.78 km, to the realization of the first large-scale application devices such as MRI magnets and fault current limiters.

Limits of the upper critical field in dirty two-gap superconductors

Abstract: An overview of the theory of the upper critical field in dirty two-gap superconductors, with a particular emphasis on MgB2 is given. We focus here on the maximum Hc2 which may be achieved by increasing intraband scattering, and on the limitations imposed by weak interband scattering and paramagnetic effects. In particular, we discuss recent experiments which have demonstrated tenfold increase of Hc2 in dirty carbon-doped films as compared to single crystals, so that Hc2(0) parallel to the ab planes may approach the BCS paramagnetic limit, Hp [T] = 1.84Tc [K] ≃ 60–70 T. New effects produced by weak interband scattering in the two-gap Ginzburg–Landau equations and features of Hc2(T) in ultrathin MgB2 films are addressed.

Development of magnesium diboride (MgB2) wires and magnets using in situ strand fabrication method

Abstract: Since 2001 when magnesium diboride (MgB2) was first reported to have a transition temperature of 39 K, conductor development has progressed to where MgB2 superconductor wire in kilometer-long piece-lengths has been demonstrated in magnets and coils. Work has started on demonstrating MgB2 wire in superconducting devices now that the wire is available commercially. MgB2 superconductors and coils have the potential to be integrated in a variety of commercial applications such as magnetic resonance imaging, fault current limiters, transformers, motors, generators, adiabatic demagnetization refrigerators, magnetic separation, magnetic levitation, energy storage, and high energy physics applications. This paper discusses the progress on MgB2 conductor and coil development in the last several years at Hyper Tech Research, Inc.

Enhancement of Hc2 and Jc by carbon-based chemical doping

Abstract: In the past 5 years, various kinds of doping of MgB 2 , including single elements (metal and non-metal), silicates, various carbon sources, and other compounds have been investigated and reported. Most nanoparticle doping leads to improvement of critical current density, J c ( H ), and performance, but some types show a negative effect. In this paper, the effect of carbon doping on J c and the upper critical field, H c2 , of MgB 2 is reviewed. Carbon substitution effects make two distinguishable contributions to the enhancement of J c field performance: increase of H c2 and improvement of flux pinning, both because carbon substitutes for boron in the MgB 2 lattice. Among all the carbon sources so far, nano-SiC has been confirmed to be the most effective dopant to enhance the J c in magnetic fields and H c2 . An irreversibility field, H irr , of 10 T has been achieved with nano-SiC doping at 20 K, exceeding H irr of NbTi at 4.2 K. Besides that, H c2 of carbon alloyed MgB 2 film has reached the value of 71 T. The significant enhancement in J c ( H ) and H c2 via carbon substitution has provided great potential for practical applications of MgB 2 . The dual reaction model proposed by the authors’ group provides a comprehensive understanding of the mechanism of enhancement in J c and H c2 by chemical doping. Further improvement in self-field J c performance while maintaining the already achieved in-field performance remains as a major challenge in the development of MgB 2 .

MgB2 thin films by hybrid physical–chemical vapor deposition

Abstract: Hybrid physical–chemical vapor deposition (HPCVD) has been the most effective technique for depositing MgB2 thin films. It generates high magnesium vapor pressures and provides a clean environment for the growth of high purity MgB2 films. The epitaxial pure MgB2 films grown by HPCVD show higher-than-bulk Tc due to tensile strain in the films. The HPCVD films are the cleanest MgB2 materials reported, allowing basic research, such as on magnetoresistance, that reveals the two-band nature of MgB2. The carbon-alloyed HPCVD films demonstrate record-high Hc2 values promising for high magnetic field applications. The HPCVD films and multilayers have enabled the fabrication of high quality MgB2 Josephson junctions.

Physica c-superconductivity and its applications

Abstract: A concise and factual abstract is required. The abstract should state briefly the purpose of the research, the principal results and major conclusions. An abstract is often presented separately from the article, so it must be able to stand alone. For this reason, References should be avoided, but if essential, then cite the author(s) and year(s). Also, non-standard or uncommon abbreviations should be avoided, but if essential they must be defined at their first mention in the abstract itself.

A two-pole Halbach permanent magnet guideway for high temperature superconducting Maglev vehicle

Abstract: In order to improve the levitation performance of the high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle, a two-pole Halbach array’s permanent magnet guideway (PMG) is proposed, which is called as Halbach PMG. The finite element method (FEM) calculations indicate that Halbach PMG has a wider high-field region than the present PMG of equal PM’s transverse section. The levitation force of bulk HTSCs with the present PMG and Halbach PMG are measured. The results show that at different levitation gaps, the force ratios based on the Halbach PMG are about 2.3 times larger than that on the present PMG, which greatly increases the load capability of the system. Therefore, both the numerical analysis and experimental results have confirmed that the Halbach PMG will further enhance the performance of the vehicle and it is possible to decrease the total numbers of onboard HTSCs, reducing overall costs. So based on the Halbach PMG, we further study the width ratios between HTSCs and PMG for making the better use of the onboard HTSCs. Some preliminary results are given. These results are important for further HTS Maglev vehicle system designs using Halbach PMG.

Single crystals of MgB2: Synthesis, substitutions and properties

Abstract: Single crystals of MgB 2 , with a size up to 1.5 × 1 × 0.1 mm 3 and with a weight up to 230 μg, have been grown from flux with a high-pressure cubic anvil technique. Investigations of the P – T phase diagram prove that the MgB 2 phase is stable up to 2200 °C at high hydrostatic pressure. Specific band structure of MgB 2 with two bands (π and σ) involved in superconductivity is strongly influenced by chemical substitutions. Substitutions of Al for Mg and C for B lead to increase of scattering within both π and σ bands, however, with different rates for both substituents. Therefore, different changes of the upper critical field, H c2 , and its anisotropy, γ H c 2 , for Mg 1− x Al x B 2 and MgB 2− x C x are observed. Mg 1− x Al x B 2 crystals show a moderate decrease of the superconducting transition temperature, T c , for the samples with small x and, simultaneously, significant reduction of H c2 and its anisotropy at lower temperatures, as compared to the value for unsubstituted crystals. The temperature dependence of the anisotropy is less pronounced. MgB 2− x C x crystals exhibit only slight reduction of T c with substitution and, moreover, a significant increase of H c2 for an applied field oriented both parallel, H c2∥ ab , and perpendicular, H c2∥ c , to the ab -plane. For the single crystal with x = 0.13, H c2∥ c (0) ≈ 8.5 T is more than twice as large as that for an unsubstituted compound. The anisotropy of H c2 decreases from 6 (MgB 2 ) to about 4 ( x = 0.13) at low temperatures. The corresponding H c2∥ ab (0) ≈ 34 T is close to the maximum possible enhancement of H c2 due to the chemical substitutions. Hole doping with Li decreases T c , but in much slower rate than electron doping with C and Al. For MgB 2 crystals with simultaneously substituted Li for Mg and C for B, T c decreases more rapidly than in the case when only C is substituted. The T c reduction in co-doped crystals is a sum of T c reductions for separate C and Li doping. This means that holes introduced with Li cannot counterbalance electrons added with C. The possible reason of this can be that holes coming from Li occupy π band and do not compensate the addition of electrons which, coming from C, fill the σ band. Substitution of magnetic Mn for Mg strongly suppresses T c and H c2 due to the magnetic pair breaking. However, this is not the case for the substitution of Fe for Mg, at least for low Fe concentration.

BCS theory on a flat band lattice

Abstract: It is important to study the many-body effects of the electrons on the flat band lattice, since the flatness of the dispersion may lead to instabilities in the presence of interactions. To clarify the effects of flat dispersion for superconductivity, we investigate a simple two-band BCS Hamiltonian, where one of the bands has a flat dispersion, in the mean field approximation. A superconducting gap Δ and a critical temperature Tc strongly depend on the pairing coupling constant V. Especially, for small coupling constant, Δ and Tc are proportional to V, which are quite different from the single band BCS cases. Higher Tc, compared to the normal BCS system, are realized due to the existence of the flat band.

Progress in scale-up of second-generation HTS conductor

Abstract: Tremendous progress has been recently made in the achievement of high-performance, high-speed, long-length second-generation (2G) HTS conductors. Using ion beam assisted deposition (IBAD) MgO and metal organic chemical vapor deposition (MOCVD), SuperPower has scaled up tape lengths to 427 m with a minimum critical current value of 191 A/cm corresponding to a critical current × length performance of 81,550 m. Tape speeds up to 120 m/h have been reached with IBAD MgO, up to 80 m/h with buffer deposition and up to 45 m/h with MOCVD, all in single pass processing of 12 mm wide tape. Critical current value of 227 A/cm has been achieved in a 203 m long tape produced in an all-high-speed fabrication process. Critical current values have been raised to 721 A/cm, 592 A/cm and 486 A/cm in short, reel-to-reel processed tape, over 1 m length and over 11.1 m, respectively, using thicker MOCVD HTS films. Finally, over 10,000 m of copper-stabilized, 4 mm wide conductor has been produced and tested for delivery to the Albany Cable project. The average critical current of the 10,000 m lot was 81 A.

Chemical trends of superconducting properties in pyrochlore oxides

Abstract: Chemical trends of fundamental superconducting parameters and normal-state properties are described for a family of pyrochlore oxide superconductors. Particularly, the change of Tc from 1.0 K for α-pyrochlore Cd2Re2O7 to 3.3 K (A = Cs), 6.3 K (Rb), and 9.6 K (K) for β-pyrochlore AOs2O6 is discussed on the basis of the conventional BCS scheme. Enhanced Tc and anomalous features observed for KOs2O6 are ascribed to low-energy phonons probably coming from the rattling of the K cations.

Influence of starting solution composition on superconducting properties of YBCO coated conductors by advanced TFA-MOD process

Abstract: We have investigated the influence of the starting solution composition on both the microstructure and superconducting properties of YBa2Cu3O7−x (YBCO) films deposited on CeO2 buffered IBAD-Gd2Zr2O7/Hastelloy substrates and (1 0 0)-oriented LaAlO3 single crystals by the advanced TFA-MOD process. The starting solutions with different compositions of molar ratios of Y:Ba:Cu = 1.0:1.0–3.0:3.0 were studied. The critical current density (Jc) values of the YBCO films fabricated from Ba-rich (molar ratio of Ba/Y > 2) starting solutions were low. On the other hand, high Jc values were recognized, when the YBCO films were prepared from the Ba-poor (molar ratio of Ba/Y

Influence of Mg/B ratio and SiC doping on microstructure and high field transport Jc in MgB2 strands

Abstract: Improvements in B c2 , B irr , and high field transport J c s are crucial for many MgB 2 applications. As transport J c is inextricably linked with microstructure, a knowledge of the detailed influence of two of the most practical variables – Mg/B ratios and SiC additions – on microstructure, B c2 / B irr , and high field J c , is of interest. In this work, the influence of Mg/B ratios on microstructure, B irr , and transport J c was investigated in MgB 2 strands with and without SiC doping. The binary compositions chosen were Mg x B 2 where x = 0.85, 0.90, 1.0, 1.10, and 1.15 in monofilamentary strands. In general, high Mg molar percentages were seen to increase transport J c , especially at higher fields. At 8 T, for example, heat treatments at 700 °C for 40 min yielded J c s at 4.2 K which ranged from 5.8 × 10 3 A/cm 2 for x = 0.85 to 2.73 × 10 4 A/cm 2 at x = 1.15. Microstructures evolved from powdery (B-rich) to web-like (Mg-rich). In a second set of samples, Mg-rich stoichiometries were investigated with and without SiC. High field transport J c s increased, and the microstructure appeared to densify. Both excess Mg and SiC doping significantly increased high field J c . The highest transport result was seen for SiC dopant (5 mol%) added to 15% excess Mg strands, which yielded a transport J c at 4.2 K and 8 T of 5 × 10 4 A/cm 2 . Variable temperature analysis was performed on the 15% excess Mg strands with and without SiC in fields of 0–15 T. At temperatures below 20 K, excess Mg + SiC doping performed best, while above 20 K, excess Mg without SiC had better transport properties.

Flux-pinning characteristics as a function of density of columnar defects comprised of self-assembled nanodots and nanorods in epitaxial YBa2Cu3O7−δ films for coated conductor applications

Abstract: Pure YBa 2 Cu 3 O 7− δ (YBCO) and YBCO films with extended columnar defects comprised of self-assembled columns of nanodots and nanorods of BaZrO 3 (BZO), CaZrO 3 (CZO) and 8 mol.% Y 2 O 3 stabilized ZrO 2 (YSZ) were prepared by pulsed laser deposition on RABiTS templates of configuration CeO 2 /YSZ/Y 2 O 3 /Ni–W. Effects on increasing the size and density of columnar defects by varying the total volume fraction BZO, CZO or YSZ incorporations within the YBCO film are reported. BZO volume fraction was varied between 0% and 8%, CZO between 0% and 2% and YSZ 0–2%. The critical transition temperature ( T c ) was found to linearly decrease with increase in vol.% of BZO, CZO and YSZ, with the steepest decline for CZO additions. Nevertheless, YBCO films with these columnar defects exhibit significantly reduced J c dependence on applied field. At 77 K, best properties are obtained for 1–2 vol.% BZO incorporation in the form of columns of self-assembled nanodots and nanorods in fields of 0.1–1.5 T.

Effects of spin–phonon interaction within the CuO plane of high-TC superconductors

Abstract: The mechanism of spin–phonon coupling (SPC) in high-TC copper oxides is explored from band calculations on La(2−x)SrxCuO4 and HgBa2CuO4 systems. The LMTO band calculations, based on the local density approximation, are made for cells containing frozen phonon displacements and/or spin waves within the CuO plane. The virtual crystal approximation is used for studies of hole doped systems. The main result is that phonons are favorable for spin waves and vice-versa, and that pseudogaps appear naturally in the band structures of striped materials with strong SPC. The qualitative results are compatible with many observations showing that the properties of high-TC superconductors depend both on lattice interactions and magnetic fluctuations. The band results are used to model various properties, mainly of the normal state, such as isotope effects, pseudogaps, Fermi surface broadening, T-dependence of the pseudogap, phonon softening and some aspects of superconductivity. The possibility of perpendicular SPC is investigated, partly by the use of a nearly free electron model.

Progress in AMSC scale-up of second generation HTS wire

Abstract: American Superconductor has successfully scaled up its low-cost, high volume second generation (2G) HTS wire process into pre-pilot scale production, with performance approaching first generation (1G) HTS wire. AMSC’s manufacturing approach is based on RABiTS TM /MOD wide strip technology, with metal organic deposition (MOD) process for the YBCO layer and the Rolling Assisted Biaxially Textured Substrate (RABiTS) process for the template. In this paper, we review the status of the 2G manufacturing scale up at AMSC and describe the properties and architecture of the 2G wire being manufactured and developed for various applications.

Progress in development of advanced TFA-MOD process for coated conductors

Abstract: Recent progresses in R&D of the TFA-MOD process for coated conductors are reviewed. On the efforts for high I c tapes, the new process to prevent the crack formation in the thick film, which is the intermediate annealing between the calcination and the crystallization steps, was developed. Additionally, control of the starting composition, especially for the Ba-poor composition from the Y:Ba:Cu = 1:2:3 stoichiometric one, is found to be effective to improve the J c value of the films. Consequently, the extremely high I c value of 735 A was obtained. On the other hand, a reel-to-reel system has been applied to fabricate long tapes. A reasonable long tape of 25 m with 100 A was obtained by the increase of the gas flow rate which assists to enhance the conversion reaction rate. Additionally, it was found that the rigorous humidity control in the coating chamber is important to obtain the uniform characteristics in long tapes. The idea was applied to fabricate long tapes and a 56 m tape with the end-to-end I c value of 250 A was realized.

Fabrication and properties of powder-in-tube-processed MgB2 tape conductors

Abstract: MgB2 tapes are now fabricated by two powder-in-tube (PIT) methods. One is an in situ method, in which a powder mixture of Mg and B is used as a starting material. The other is an ex situ method, in which reacted MgB2 powder is used. Here, we report the recent progress of our PIT-processed MgB2 tapes. The superconducting properties of PIT-processed MgB2 tapes are sensitive to the quality of the starting powder, the porosity of the MgB2 core, the heat-treatment temperature, and impurity additions. In the case of the in situ method, a sub-micrometer Mg starting powder is quite effective for enhancing Jc. Some kinds of carbon compound additions to the starting powder introduce a carbon substitution for boron in MgB2 crystal and enhance Birr. At 20 K, Birr reached ∼10 T, a value which is nearly equal to that of commercial Nb–Ti at 4.2 K. This result clearly indicates that MgB2 tapes are promising as conductors of cryogen-free magnets. In general, ex situ-processed MgB2 tapes show lower Jc than in situ-processed tapes. The key factor to obtain a high Jc for an ex situ tape is the high quality of the MgB2 starting powder. Recently, we succeeded in the fabrication of high Jc ex situ MgB2/Fe tapes using MgB2 cores removed from in situ MgB2 tapes. Furthermore, the Jc of this ex situ-processed tape was less sensitive to the magnetic field than that of the in situ tape because of the higher Birr of the ex situ tape. Al-sheathed ex situ-processed tapes are interesting due to their light weight and the high thermal stability of conductor. By improving the tape fabrication process mentioned above, we have obtained a large increase of Jc values. The highest Jc values obtained so far are 27 kA/cm2 at 4.2 K and 10 T and higher than 10 kA/cm2 at 20 K and 5 T. However, these Jc values are still below a practical level. Further improvement of the processing of MgB2 tapes is essential in order to obtain a practical level of Jc values.

Processing and applications of bulk HTSC

Abstract: The author has been emphasizing the importance of adopting refrigerators to keep the superconductivity instead of using cryogen such as liquid nitrogen, since the handling of cryogen requires complicated skills and does not fit to the widespread industries. An intense field of 5.2 T has been obtained through the studies on the pulsed field magnetization process operated at 28 K. Several kinds of practical applications, for instance, the magnetron sputtering cathodes, the nuclear magnetic resonance NMR equipments, and magnetic separation systems, and so on are under way with use of the melt-processed large grain bulk magnets and cryo-coolers.

Flux pinning by Al-based nanoparticles embedded in YBCO: A transmission electron microscopic study

Abstract: A series of YBa2Cu3Oy (YBCO) samples with small amounts (0–0.6 wt.%) of nanosized alumina particles (50 nm) are synthesized in air by solid state reaction. The microstructure has been characterized by transmission electron microscopy (TEM) and the critical current density Jc has been measured by the standard four-probe method in the applied magnetic field at 77 K. TEM and energy dispersive X-ray spectroscopy (EDS) analysis have shown that alumina reacts with the YBCO matrix to form nanometric aluminium-rich inhomogeneities intergrown within the YBCO superconducting matrix. These inhomogeneities reduce the onset transition temperature T c onset and the zero resistance temperature Tc. In spite of the monotonic decrease of the superconducting temperature Tc with increasing alumina addition, the Jc(H) behaviour is remarkably improved. The characteristic behaviour of Jc can be explained in terms of the counterbalance of two effects simultaneously caused by the nanometric alumina addition in the system. One effect is the formation of the Al-rich nanometric inhomogeneities relevant for the flux pinning, and the other effect is the reduction of matrix superconducting volume, which is reflected by a decrease of the critical current density Jc at zero applied magnetic field.

High efficient DC power transmission using high-temperature superconductors

Abstract: DC power transmission systems are considered to be developed using high-temperature superconductors (HTS) with advantages of higher current carrying capability, energy loss reduction, lower system voltages, high stability with self-protection, and more compact systems. This paper will provide technical assessments of HTS DC electrical power transmission systems using HTS techniques, and analysis of the system performance in steady and dynamic states will also be presented.

Modelling method for critical current of YBCO tapes in cable use

Abstract: Superconducting YBCO cables are suitable to transfer large amounts of energy due to their small space requirements and low losses. The large energy transfer capacity is achieved by connecting several coated conductor (CC) tapes in parallel. The critical currents of the tapes will be changed when they are placed from self-field to the magnetic field of the cable. In this paper, the change was determined from the cable geometry and intrinsic J c ( B )-dependence with fast and robust integral element method exploiting thin elements. The critical current distribution and the magnetic fields of 1 kA cable were computed. The so-called gap effect causes the critical current of the individual tapes to rise from 86.5 to 88.8 A when they are moved from the self-field to the cable field. The gap effect was studied as a function of width of the superconducting layers, cable radius and the number of the tapes. It was shown that the cable critical current can be given as a function of the tape number and the layer fill factor defined in this paper. In addition, the gap effect was studied as a function of zero field critical current density. The results suggest that up to 16% rise can be obtained with a cable of 25 tapes with critical current per tape width 500 A cm −1 .

Current progress in the theoretical understanding of MgB2

Abstract: This review is devoted to the current theoretical understanding of MgB2, mainly focusing of the effect of impurities and doping on the physical properties of the material. We compare the theoretical predictions with experimental results in order to suggest an explanation for the behavior of the superconducting gaps and critical temperature with doping.

Joining of Bi-2212 high-Tc superconductors and metals using indium solders

Abstract: BSCCO tubes can be used as a base material for switching devices such as superconducting fault current limiters (SFCLs) that prevent an electrical problem from occurring in an electrical power system. To apply an BSCCO bulk tube to a switching device, the superconducting tube has to be joined with a metallic part to by the over current to the metal part when the FCL is quenched. In this study, joining between Cu–Ni alloy and BSCCO was accomplished by soldering using In–Sn and In–Bi solders. Additionally, an Sn–Ag–Cu/In–Bi solder was used for the soldering of a different kind. For a better joining of the BSCCO superconductor with the In–Bi solder, the surface of the BSCCO was pre-coated with Ag by electro-plating. From the experiments, an intermetallic compound (IMC) of AgxIny chain was observed to be mainly formed from In–Sn and In–Bi soldering process. In case of the soldering of a different kind, IMC of AgxIny and CuxSny was also developed. Finally, we confirmed that the properties of soldering were enhanced by Sn–Ag–Cu/In–Bi twice-soldering process.

Self-field reduces critical current density in thick YBCO layers

Abstract: The engineering current density in YBCO coated conductor applications can be improved in two ways. Either the critical current density should be improved or the superconducting films made thicker. Unfortunately, it has often been observed that the average critical current density decreases when the thickness of films increases. Suggested reasons for this behaviour include e.g. two dimensional pinning properties, microcracks and imperfect crystallographic alignment. However, it is often forgotten that the self-field effect unavoidably reduces the critical current density when the thickness of YBCO films increases and thereby total current rises. In this paper, the influence of self-field on the average critical current density is studied computationally as a function of film thickness. The situation is also scrutinized at different external magnetic fields in order to find ways to distinguish self-field effects from problems related to the manufacturing process. For this purpose, critical current measurements in external field perpendicular to the film surface are proposed.

Anharmonic and non-adiabatic effects in MgB2: Implications for the isotope effect and interpretation of Raman spectra

Abstract: We review the published calculations of the anharmonic effects in MgB 2 and show that different results are mainly related to the various degrees of approximation involved in the calculations. When all the leading order terms in anharmonic perturbation theory are included the magnitude of anharmonic effects is marginal. This result is in good agreement with the phonon dispersion measured by inelastic X-ray scattering showing weak anharmonic phonon frequency shift. However, Raman spectra display a feature having E 2 g symmetry at ∼12 meV above the available X-ray phonon dispersion near Γ . Raman data can be explained if dynamical effects beyond the adiabatic Born–Oppenheimer approximation and electron lifetime effects are included in the phonon self-energy, without invoking anharmonicity. Finally, we discuss the implications of weak anharmonicity for the interpretation of the isotope effect and conclude that the isotope effect is the most important unresolved issue in the physics of MgB 2 .

Simulations and experimental analyses of the active superconducting fault current limiter

Abstract: This paper presents the operation principle of a new type active superconducting fault current limiter (SFCL). The SFCL is composed of an air-core superconducting transformer, a PWM converter and a superconducting magnet. The primary winding of the air-core superconducting transformer is in series with AC main circuit, and the second winding is connected with the superconducting magnet through a PWM converter. In normal (no fault) operating state, the flux in air core is compensated to zero, so the SFCL has no influence on main circuit. In the case of short circuit, by controlling the amplitude and phase angle of the second winding’s current, the limiting impedance which is in series with the AC main circuit can be regulated and the fault current will be limited to a certain level. Using MATLAB SIMULINK, the simplified model of the active SFCL is created, and simulations validate this SFCL can suppress the fault current effectively. In addition, the current-limiting experiment is done with a small conventional transformer. Experimental results correspond well with simulation results.

Superconducting properties of bulk Bi1.6Pb0.4Sr2Ca2− xCdxCu3O10 system prepared via conventional solid state and coprecipitation methods

Abstract: The effect of Cd doping on the superconducting properties of BSCCO system with nominal starting compositions of Bi1.6Pb0.4Sr2Ca2−xCdxCu3O10 (x = 0.00–0.10) was studied. The preparation methods used to prepare the samples are the conventional solid-state oxide powder (SSR) and the coprecipitation (COP) techniques. Resistivity versus temperature measurements (R–T) showed that all doped samples exhibited metallic behaviour. For the SSR samples, existence of a two step feature was observed at x = 0.07 indicating the presence a lower temperature 2212 phase together with the higher temperature 2223 phase. This behaviour resulted in the shifting of the TC(R=0) towards lower temperature. However, the COP samples showed better superconducting properties probably due to higher homogeneity resulted from mixing of sub-micron particles during sintering. The R–T curve did not display any two step features due to the single phase nature of the samples. This is confirmed by the XRD data where Bi-2212 phase was minor. In addition, small amount of doping (x = 0.02 in COP and SSR samples) enhanced the phase formation and TC(R=0).

MgB2 tunnel junctions and SQUIDs

Abstract: Recent advances in the realization and understanding of MgB2 tunnel junctions and SQUIDs are surveyed. High quality MgB2 junctions with suitable tunnel barriers have been realized based on both oriented and epitaxial thin MgB2 films. Multiband transport properties, such as the existence of two energy gaps, phonon spectra and anisotropy have been investigated with these junctions. We review the suitability of different barrier materials and recent advances in obtaining reproducible all-MgB2 Josephson junctions for superconducting electronic circuitry. The development of epitaxial thin films has also led to high-quality multiband MgB2 SQUIDs and magnetometers that operate at high temperatures. The multiband nature of MgB2 provides new phenomena such as the Leggett mode. Manipulating the different phases of the condensates could lead to novel MgB2 devices with phase degrees of freedom.

Superconductivity at 20 K in yttrium metal at pressures exceeding 1 Mbar

Abstract: In experiments in a diamond anvil cell, yttrium metal is found to display a superconducting transition temperature in the ac susceptibility which increases monotonically from 35 K at 30 GPa to 195 K at 115 GPa, where the transition onset lies at 20 K This is the second highest value of T c ever observed for an elemental superconductor A fit to the Y data using the McMillan equation is consistent with electron–phonon superconductivity with moderately strong coupling