Table of contents

We report on the fabrication of all-chemical YBa₂Cu₃O₇ coated conductors on IBAD-YSZ (IBAD stands for ion beam assisted deposition; YSZ is yttrium stabilized zirconia) stainless steel substrates. YBCO films were grown by the trifluoroacetates route on top of CeO₂ buffer layers made by metal–organic decomposition. The achievement of atomically flat CeO₂ surfaces is found to be a key factor for obtaining clean interfaces with YBCO and high performance. Coated conductors with percolative critical currents of J_c^GB(65 K) = 1.8 MA cm⁻² were achieved. The determination of the intra-grain critical current J_c^G from inductive measurements suggests that the limiting factor for J_c^GB is the YBCO in-plane texture, which is already of higher quality than that of the IBAD-YSZ cap layer.

In situ nano-SiC doped MgB₂ wires were fabricated from MgH₂ and B powders. Hydrostatic extrusion, followed by rotary swaging and two-axial rolling, were applied as the forming processes. The critical current J_c of MgB₂ wires, made from MgH₂ and B powders, was significantly improved by nano-SiC doping. Nano-SiC doping substantially increased the upper critical (irreversibility) field B_{c 2} above 20 T. The maximum J_c values were measured for samples having 6 at.% SiC in low field and for those having 12 at.% SiC in high field, above 10 T. During the final sintering at 670 °C, the SiC decomposed and formed an Si-rich layer at the inner circumference of the Fe sheath. The composition of the core of SiC doped wires is more inhomogeneous in comparison to undoped ones, with MgO, Mg₂Si and probably Mg₂SiO₄ as the major segregated phases. Strong segregation of Si within the MgB₂ core was also observed. The highest T_c−mid = 39.3 K was measured for undoped wire. For the optimal SiC doping amount ∼6 at.%, at high field, there was no difference in J_c between hydrostatically extruded and hydrostatically extruded plus two-axially rolled wire. This can be attributed to the beneficial effect of hydrostatic extrusion, which causes higher density of the core in comparison to traditional deformation processes.

The dynamic property of a high temperature superconductor (HTS) coil which possesses short circuits between windings was investigated by magnetic measurement. Based on a general lock-in technique, the net magnetomotive force (mmf) provided by a short circuit coil carrying ac current is given in two parameters: the imaginary part of the mmf accounting for the power dissipation in the coil, and the real part of the mmf relating to the flux and attractive force in the magnetic circuit. These two terms are given in a broad range of current amplitude A and frequency f, which cover the main operational region for typical controllability requirements and other dynamic applications. An equivalent circuit model based on the assumption that the short circuits are formed by a group of closed circuits located coaxially with the coil was proposed. The parameters of the circuit are determined by fitting the experimental results in some specific cases. The feasibility and efficiency of the equivalent circuit concerning its applications at low and high frequency are discussed respectively.

The application of an electric current of 200 mA through the molten zone of BSCCO superconducting fibres during laser floating zone processing constitutes an upgrade for improving the grain alignment. When a direct electric current (positively polarized fibre) passes through the solidification interface, the solidification conditions approach equilibrium, favouring the development of a higher amount of 2212 and 14/24 stable phases. The new electrically assisted laser floating zone (EALFZ) technique also improves the 2223 phase formation in fibres heat treated at high temperatures (860 °C). However, the 2223 crystals grow perpendicularly to the fibre axis at the interface between the 2212 and 14/24 phases, crossing the crystals of the main phase responsible for the current transport, cancelling the alignment effect. The resultant current density and critical temperature values were J_c77 K = 230 A cm⁻² and T_c = 85 K, respectively. When a lower heat treatment temperature was accomplished (820 °C), the 2223 transverse crystals do not develop and a higher current density value of J_c77 K = 510 A cm⁻² was achieved, although with a critical temperature of T_c = 90 K.

The paper reports on a numerical calculation method of the recovery current of multifilamentary NbTi superconductors cooled by liquid helium. It is applied to the investigation of the influence of copper matrix purity on the recovery current. Our motivation was to estimate the range and limits for this parameter as an aid to the conductor selection process in the industrial manufacture of superconducting magnets.

A micrometre-size Josephson vortex flow transistor is demonstrated experimentally. The drain–source of this device is the output of a single-layer Nb superconducting quantum interference device (SQUID) with a nanometre-size hole, forming a nanoSQUID. The device input is a Nb strip, which is closely coupled to the nanoSQUID. The change of the input current induces a change in the magnetic flux coupling to the nanoSQUID, and hence modulates the current–voltage characteristics of the nanoSQUID. The current gain and transresistance of the device were investigated by evaluating the mutual coupling and the dynamic resistance of the nanoSQUID. A current gain of 0.3 and a transresistance >1 Ω were achieved.

It is of great interest for magnet designers to know the maximum current that a coil can be operated with, i.e. the critical current of the coil. Magnesium diboride is a newcomer in the field of practical superconductors. Iron is a possible matrix metal for use in MgB₂ conductors. Its ferromagnetic nature brings several new problems into the computation of the critical current of the coil. In this paper we show, by means of finite element method computations, that the measurement situation for the short-sample critical current in the case of an MgB₂/Fe wire differs considerably from that for a conventional superconductor. Also, we study a computation of the critical current of an MgB₂/Fe coil based on the engineering permeability. The computational model presented is applied to a small solenoidal MgB₂/Fe coil. On the basis of the results, we propose a non-linear, conductor geometry dependent shift for the measured short-sample critical current curve to give values for the critical current of the coil.

The chemical interactions between Bi₂Sr₃CaO₇ (Bi-2310) and Bi₂Sr₂Ca_0.8Dy_0.2Cu₂O₈ (Bi-2212(Dy)) at 965 °C were investigated by means of: (i) an interdiffusion couple and (ii) layers deposited by dip coating on oxidized nickel substrates. The samples were characterized by optical and electron microscopies, energy-dispersive x-ray (EDX) analysis and x-ray diffraction. It turns out that at the peritectic temperature of Bi-2212(Dy), the Bi-2310 phase reacts with the liquid phase resulting from the peritectic decomposition of the Bi-2212(Dy) phase. Dissolution of Bi-2310 leads to an enrichment in Sr and an impoverishment in Cu of the liquid phase, resulting in a shift of the composition of the insoluble phase towards the Ca-rich end of the (Ca, Sr)O solid solution.

We introduced high density columnar defects, as artificial pinning centres (APCs) for quantized vortices, into YBa₂Cu₃O_7−x (YBCO) films during the film deposition procedure. APCs were introduced perpendicular to the film surface using nanosized Y₂O₃ islands prepared on SrTiO₃(100) substrates by pulsed laser deposition. Varying the deposition parameters and the substrate annealing conditions allowed strong changes in the shape and density of the Y₂O₃ islands to be induced. The best performance among the APC samples as compared to the pure YBCO film was obtained for that grown on the Y₂O₃-deposited substrate with five laser pulses, corresponding to 0.2 Y₂O₃ monolayers (ML). Even when the 0.2 ML APC films were prepared using the same deposition conditions, the columnar defects enhanced J_c at 77 K from 1.8 to 2.7 MA cm⁻² (self-field) and from 0.06 to 0.10 MA cm⁻² (H = 5 T).

The integration of high-temperature superconductors into advanced semiconductor technology is rather intriguing in the microelectronics field. In this paper, we report on YBa₂Cu₃O_7−δ (YBCO) thin films deposited on silicon-on-insulator (SOI) substrates buffered by yttria-stabilized zirconia (YSZ) layers using the in situ pulsed laser deposition (PLD) technique. The strain in the YBCO films and the ageing effect caused by the thermally induced cracks are carefully studied. It is believed that because of the adoption of the SOI substrate, the strain in the YBCO films is greatly relaxed and the quality and stability of YBCO films are rather good, which is advantageous to the superconductive devices.

The suitability of Hall probe mapping for non-destructive experimental characterizations of YBa₂Cu₃O_7−δ bulk superconductors is discussed. Firstly, the flux penetration pattern, observed by Hall scans of the trapped field after activation in magnetic fields below the complete penetration field in the zero-field cooled (zfc) mode, provides valuable information on the homogeneity of the material. Secondly, the magnetoscan technique is presented, which was also applied to irradiated samples. Investigations of samples from the same batch (prior to neutron irradiation) allow us to draw conclusions about the quality of the batch process.

The transverse resistance and resistivity of a multifilamentary coated conductor is computed where the conductivity between filaments is largely due to the recently discovered damage produced by laser striations. The results when compared with measurements of Amemiya et al show that for one of their samples the resistance is due to the resistivity of the substrate, while in another sample it results from 'contact' resistance between the superconductor and substrate.

The effect of polycarbosilane (PCS) doping on the superconducting properties and microstructure of MgB₂ bulk has been studied in this paper. It is found that the lattice parameter a and the superconducting transition temperature decrease with increase of the doping level. However, the critical current density J_c is significantly enhanced and the J_c(B) properties are improved at low temperature. Microstructure analysis indicates that a lot of Mg₂Si nanoparticles, including those 5–10 nm in size, and some lattice defects induced by the PCS doping exist inside the PCS doped MgB₂ samples, which may be the main reason for the enhancement of the flux pinning property.

A series of Bi-2223 ([Bi, Pb]:Sr:Ca:Cu:O = 2:2:2:3) tapes with 37 superconducting core filaments was investigated in an attempt to correlate critical current and alternating current (AC) losses with twist-pitch. The twist-pitch of these multi-filamentary tapes which were produced by the powder-in-(Ag)tube (PIT) method varies from 8, 10, 13, 30, 50, 70 mm to  mm (non-twist). Critical current (I_c) measurements which were conducted in zero field by a four-probe method under liquid-nitrogen temperature showed that I_c is greater in the non-twist filament than that in twisted filaments. Among these tapes, three (twist-pitch of 10, 13, and 70 mm) were selected for AC loss experiments under a time-varying transport current. The results of AC loss measurements in general agree with that of the AC loss simulation using the ellipse model of the Norris equation. Simulation results show that the hysteretic AC loss is lowest in the non-twist tape and increases as the twist-pitch decreases. A much greater loss was found in tapes with small twist-pitch, i.e. 10 and 8 mm. Among different possible loss contributions to the total AC losses, the hysteretic loss was determined to be the main source. In addition, microstructural damage of tapes with small twist-pitch appears to contribute to the overall AC losses as well.

Stress–strain curves have been obtained for a 1.5 mm diameter bronze-process multi-filamentary composite Nb₃Sn wire at 77 and 300 K. Tensile test specimens were prepared with a variety of heat-treatment conditions, ranging from no heat-treatment to one with a duration exceeding that of the wire manufacturer's recommended full heat-treatment. The results show that for each heat-treatment condition, the mechanical properties are significantly different at 77 and 300 K: at the lower temperature, specimens have higher proof stress, are stronger and more ductile. The elastic modulus is not significantly different at 77 and 300 K, but rises from 102 GPa for specimens that were not heat-treated to 110 GPa for fully reacted ones. The proof stress is significantly higher for specimens that were not heat-treated compared to those that were. This is true even for heat-treatment durations that were too short to convert much of the niobium into Nb₃Sn. Extending the heat-treatment beyond these short durations, and thus increasing the Nb₃Sn volume fraction, makes the wires more brittle. However, despite having a dramatic effect on the composition, extending the heat-treatment duration does not have much of an effect on the stress–strain curve up to the point of failure.

Copper and Cu–Fe (Fe ∼2.35 wt%) alloy substrates were thermo-mechanically processed and the biaxial texture development, magnetic properties, yield strength, and electrical resistivity were studied and compared to determine their suitability as substrates for high-temperature superconducting coated conductor applications. Average full width half maximum (FWHM) of 5.5° in Phi scans (in-plane alignment), and 6.6° in omega scans (out-of-plane alignment) was obtained in copper samples. Cu–Fe samples showed 5.9° FWHM in Phi scans and 5.9° in omega scans. Even with the presence of 2.35% Fe in the Cu-alloy, the saturation magnetization (M_sat) value was found to be 4.27 emu g⁻¹ at 5 K, which is less than in Ni samples by an order of magnitude and comparable to that of Ni–9 at.% W substrates. The yield strength of the annealed Cu–Fe alloy substrate was found to be at least two times higher than that of similarly annealed copper substrates. The electrical resistivity of Cu–Fe alloy was found to be an order of magnitude higher than that of pure copper at 77 K.

The phase evolution inside Fe-sheathed wires containing precursor powders consisting of a mixture of Mg and B has been studied in situ by means of x-ray diffraction with hard synchrotron radiation (90 keV). Mg was found to disappear progressively during the heating stage. At 500 °C, the intensity of the Mg diffraction lines is reduced by about 20%. This effect is partly attributable to MgO formation. The MgB₂ phase was detected from 575 °C. Fe₂B was forming at the interface between the sheath and the ceramic core at sintering temperatures of 780 and 700 °C, but not at 650 °C. The formation rate of this phase is strongly dependent on the heat treatment temperature. Its presence can be readily detected as soon as the average interface reaction thickness exceeds 150–200 nm.

We explore optimal growth conditions for superconducting Nd_1+xBa_2−xCu₃O₇ (NdBCO) thin films deposited under various oxygen pressures in the range of 100–800 mTorr. In this study we address spatial inhomogeneity, growth orientation, impurity phases, cation disorder, and oxygen deficiency of NdBCO thin films by using micro-Raman scattering. The films grown in the low oxygen pressure range of 100–200 mTorr show predominantly a-axis orientation and degraded superconducting properties with a critical temperature (T_c) of ∼80 K. The degradation of the transition temperature of the films deposited at lower oxygen pressure is attributed to the cation disorder, on the basis of analysis of the apical oxygen Raman mode. On the other hand, the samples grown in the higher oxygen pressure range of 400–800 mTorr show strong c-axis orientation and much less cation disorder. These features correlate with their high values of T_c and J_c.

Melt processed superconductors containing mechanically drilled holes parallel to the c-axis were investigated by means of Hall scans of the trapped field distribution and the magnetoscan technique. We show that the remnant flux profiles are affected by the perforation in different ways. The location of the holes can be resolved by field mapping at activation fields below that for complete penetration of the bulk. The magnetoscan technique allows one to spatially localize the array of holes.

Inward unidirectional copper diffusion into a pellet and the resulting alloying of Cu–Mg during in situ reaction of MgB₂ were studied. The reaction between the copper and magnesium occurred at as low a temperature as 660 °C in a flowing argon atmosphere giving rise to the formation of a MgCu₂ layer at the interface between the Cu and the MgB₂. The kinetics of copper alloying in the core sample was much enhanced at 800 °C and above, as shown by energy dispersive x-ray analysis. Consequently, J_c was degraded drastically due to the decrease in the superconducting volume fraction. However, the alloying effect did not lower the T_c onset, but broadened the transition. A sharp T_c transition and J_c(6 K,4 T)∼2 × 10⁴ A cm⁻² was achieved when annealing was conducted at the lower temperature of 750 °C. Direct comparison between the pure and copper alloyed samples shows that optimization of the heat treatment and hence J_c rests on the competing effects of phase formation and grain connectivity versus MgCu₂ formation.

A series of double-doped La_1.85−1.5xSr_0.15+1.5xCu_1−xMn_xO₄ (0≤x≤0.2) samples with the Mn³⁺/Mn⁴⁺ ratio fixed at 1:1 were synthesized by the solid-state reaction method. The structural, transport and magnetic properties were studied. It is found that the doped Mn ions act as two kinds of steric states in the CuO₂ planes. One corresponds to the randomly distributed and isolated Mn ions which are in a paramagnetic state. The other corresponds to the Mn ions which become adjacent with other Mn ions and form clusters. For a slight doping level, we not only observed the coexistence of superconductivity and paramagnetism, but also observed superconductivity coexisting with ferromagnetism. A ferromagnetic spin-glass state occurs and the superconductivity was suppressed for the 0.08≤x≤0.15 doping level. For x>0.2, antiferromagnetism is observed.

The peculiar features of magnesium diboride make it suitable for many potential applications. In the last four years of investigation of MgB₂, the scientific community's research was mainly motivated by features such as low cost, compatibility with straightforward processing methods, relatively high critical temperature, and absence of weak links between grains. Other applications suggest the accentuation of other MgB₂ properties. In particular its very low mass density makes it attractive for space activities, where the cost per kilogram for orbiting is still a huge obstacle, e.g. the cost of transport to low Earth orbit can reach 15 k$ kg⁻¹. In order to promote the use of this compound for space activities, we tested titanium as a sheath material. Titanium is non-magnetic and its alloys are almost half the weight of steel but with yield stresses up to three times higher. We fabricated Ti-sheathed MgB₂ through the powder-in-tube process. These wires show similar results to those sheathed with Fe. At 4 K the critical current density J_c is well above 7 × 10⁵ A cm⁻². The interface between the superconducting intermetallic compound and the lightweight sheath does not show any evidence of reaction and diffusion up to 900 °C. An analysis was carried out using x-ray diffraction, scanning electron microscopy with an energy dispersive spectrometer, and electron microprobe analysis.

We investigated the effect of ZrSi₂ and SiC doping on the microstructure, critical current density J_c and flux pinning of Fe-sheathed MgB₂ tapes prepared by the in situ powder-in-tube method. Heat treatment was performed at a low temperature of 650 °C for 1 h. The phases, microstructures and flux pinning were characterized by means of x-ray diffraction, scanning electron microscope, magnetic and transport property measurements. It was found that the tapes doped with nanoscale SiC had the best pinning performance, while the ZrSi₂ powder showed a similar improved field dependence of J_c compared with undoped samples. J_c values for the SiC doped samples were enhanced by two orders of magnitude at 4.2 K in magnetic fields above 8 T. At 4.2 K and 10 T, the J_c reached ∼1.5 × 10⁴ A cm⁻². Moreover, the critical temperature for the doped tapes decreased slightly (<1.2 K). Microstructural analysis shows that very good grain connections or/and grain refinement were obtained for the doped tapes. The mechanism of the enhancement of the flux pinning is also discussed.

We have tuned the transition temperature of the AuPd/Mo bi-layer in the temperature range 100–800 mK and used models derived from the Usadel theory to predict and fit the behaviour of the transition temperature. As expected, the influence of the AuPd on the transition temperature is stronger than that found by others from pure Au, but weaker than that of pure Pd. The extracted coherence length of the Cooper pairs in the metal alloy points to the greatest influence coming from the Pd part of the AuPd. X-ray measurements show the films to be textured, with the Mo(110) and AuPd(111) peaks being dominant in the out-of-plane direction. The transition width is seen to increase with the spread of the orientation of the z direction of the individual grains.

The thermal stability of different Fe- and Ni-sheathed MgB₂ conductors has been studied experimentally and numerically, focusing on the estimation of the quench propagation velocities, v_p, minimum quench energies (MQEs) and minimum propagating zones (MPZs). The measurements have been done at self-field and under adiabatic conditions, at variable temperatures and transport currents. Energy pulses were deposited to the conductor by passing rectangular current pulses through a graphite-based epoxy heater. The measured voltage around the heater together with numerical simulation allows the estimation of the minimum propagating zone. Moreover, v_p was obtained by measuring multiple voltage taps and thermocouples attached along the conductor. The effect of the current sharing temperatures, I_c(T), of the superconductor, and thermal and electrical properties of the metal sheath have been analysed. The experimental results are in qualitative agreement with the simulated ones, obtained by solving the one-dimensional heat balance equation of the system.

The effect of different γ-irradiation doses on the critical current density, J_c, in a Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ superconducting polycrystal has been investigated under several values of applied magnetic field and at temperatures from 80 to 105 K using I–V measurements. Two doses of γ-irradiation were used: 50 and 300 MR. The specimen was prepared using the solid-state reaction technique. We found that for γ-irradiation dose of 50 MR the values of J_c were enhanced through the whole temperature range for all applied fields. This enhancement of J_c values is significant at low temperatures. We also found that for a γ-irradiation dose of 300 MR J_c values were suppressed at all temperatures for all applied fields. This reduction of J_c values is significant at low temperatures. At high temperatures both doses of γ-irradiation were found to have a very small effect on J_c values.

These results are discussed in terms of the effect of defects produced by γ-rays and grain boundaries on the vortex pinning forces in the samples.