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We present a detailed analysis of the effects of excess Mg on the lattice parameters, the critical temperature (Tc), the critical current (Jc), the irreversibility field (Hirr), and the upper critical field (Hc2) of MgB2 bulks and wires produced by an in situ solid-state reaction method and the powder-in-tube method, respectively. It was found that Jc, Hirr, and Hc2 were significantly enhanced for Mg excess samples. All these properties were highly sensitive to the processing temperature for Mg excess samples, while there was only a weak dependence on processing temperature for normal ones. The Tc variation for the 10% Mg excess sample was 1.6 K (36.3–37.9 K) when the sintering temperature changed from 650 to 850 °C while it only varied by 0.5 K (37.2–37.7 K) for the normal sample. The low-field Jc for the 10% Mg excess samples sintered at 750 °C increased by a factor of 3, compared to that for the normal MgB2 samples, while the Hc2 for the 10% Mg excess samples sintered at 650 °C reached 8.7 T at 25 K, compared to 6.6 T for the normal sample. Rietveld refinement x-ray diffraction (XRD) analysis showed that the MgO content was reduced in the 10% excess Mg samples, leading to an increase in the effective cross section of the superconductor.

The effects on transition critical temperature, lattice parameters, critical current density, and flux pinning of doping MgB2 with carbon nanoparticles, were studied for bulk, wire and tape under a wide range of processing conditions. Under the optimum conditions, magnetic Jc was enhanced by two orders of magnitude at 5 K for a field of 8 T, and by a factor of 33 at 20 K for a field of 5 T for bulk samples, whereas enhancement by a factor of 5.7 was observed in the transport Ic at 12 T and 4.2 K for a wire sample. Samples sintered at high temperature (900 and 1000 °C) exhibited excellent Jc, approximately 10 000 A cm−2 in fields up to 8 T at 5 K. This result indicates that flux pinning was enhanced by the carbon substitution for B with increasing sintering temperature. Highly dispersed nanoparticles are believed to enhance the flux pinning directly, in addition to the introduction of pinning centres by carbon substitution. Nano-C is proposed to be one of the most promising dopants besides SiC and CNT for the enhancement of flux pinning for MgB2 in high fields.

Carbon nanotubes (CNTs) are an excellent candidate for introducing effective pinning centres and at the same time enhancing the upper critical field of MgB2. We report on the use of a low intensity ultrasonication as a method of dispersion of CNTs into precursor magnesium and boron powder. The ultrasonication improved homogenous mixing of CNTs with the MgB2 matrix. Ultrasonication of CNT doped MgB2 resulted in a significant enhancement in the field dependence of critical current density. The density of the sample increased due to the improved adherence between CNTs and MgB2 matrix. CNTs donate carbon that is substituted for boron in MgB2.

Af erromagnetic sheath around a superconducting wire results in an unusual transport Jc(H) .F or the f iel dp erpendicular to the current, there is a plateau in Jc(H) at high temperatures and intermediate fields. This plateau develops into a peak at lower temperatures—resembling a ‘peak effect’. A model based on cancellation of the self-fiel do f the current and the external field within the iron sheath was proposed for the explanation of the plateau in Jc(H) .W e test this model in three key experiments. Firstly, we show that the form of Jc(H) fo rr ound MgB2/Fe wires is strongly temperature dependent .T hi si si nc ontradiction with the model, because the properties of the iron sheath do not change in the measured temperature range. However, the temperature dependence of Jc might still account fo rt he change of Jc(H) .S econdly, the model requires a substantial component of the self-field to be parallel to the external field. Our measurements of Jc(H) for afi eld parallel to the current show a peak in Jc(H) at high temperatures and ap ronounced plateau at low temperatures. The model cannot explain this because the self-field and external field are perpendicular in this experiment. Thirdly, the iron sheath was made thinner on one side of the wire, which should produce an asymmetry in Jc(H) in the model for two different orientations of the external field. Such asymmetry was not observed. These experiments show that the effect of the self-field is of much lower importance than an as yet unknown effect that results in the observed plateau and peak in Jc(H) .S uch an effect is likely to be based on a specific interaction between the superconductor and ferromagnet, perhaps similar to the overcritical state effect.

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

Highly smooth and c-axis oriented superconducting MgB2 thin films were prepared by pulsed laser deposition (PLD) with off-axis geometry. The films were deposited on Al2O3-C substrates perpendicularly aligned to a stoichiometric MgB2 target in a 120 mTorr high purity Ar background gas. An in situ annealing was carried out at 650C for 1 min in a 760 Torr Ar atmosphere. Despite the short annealing time, an x-ray theta-2 theta scan shows fairly good crystallization, according to the clear c-axis oriented peaks for the films. Both atomic force microscopy and the x-ray diffraction results indicate that the crystallite size is less than 50nm. The root mean square roughness of our off-axis film is ~4 nm in a 5x5 square micron area. The Tc onset value of the best off-axis film reaches 33.1 K with a narrow transition width of 0.9 K. The films showed no anisotropy in Hc2-T curves when parallel and perpendicular fields were applied. The slope of Hc2-T curves in low field regime is 1 T/K, which is among the highest reported values., Comment: 7 pages 7 figures

Various amounts of Si up to a level of 18 wt% were added into MgB2 thin films fabricated by pulsed laser deposition (PLD). Si was introduced into the PLD MgB2 films by sequential ablation of a stoichiometric MgB2 target and an Si target. The precursor films were annealed in situ at 685 °C for 1 min. Up to an Si level of , the superconducting transition temperature (Tc) of the film does not change significantly, as compared to the undoped film. The magnetic critical current density (Jc) of the film at 5 K was increased by 50% with Si addition, as compared to the undoped film. The slope of Hirr(T) and Hc2(T) curves of the 3.5 wt% Si added MgB2 film is slightly higher than that for the undoped film.

Different precursor powders and procedures were used to fabricate MgB2 superconductor. It was found that the purity of the B powder had strong effects on the Tc and Jc of MgB2. The Jc of the sample made from 90% B powder decreased 40 times at 3 T and 20 K compared with the 99% B powder samples. The impurity phases might be the reason for the suppression of the Tc and the Jc. Variation of the Mg:B ratio also influenced the MgB2 properties. Jc decreased with fluctuations from the normal rate (Mg:B = 1:2). It was found that the effects of the oxidization of the Mg powder on Jc were not strong when the Mg powder was only slightly oxidized. However, when the Mg powder was severely oxidized, the Jc decreased significantly. A two-step sintering process was adopted to make MgB2. In this process, the first sintering stage was at a lower temperature. It was found that this pre-sintering did not have a good effect on the properties, despite the fact that the mass density was improved. The phase composition and microstructure were analysed to explain the different effects.

Polycrystalline samples of multi-walled carbon nanotube (CNT) doped MgB2 superconductors were synthesized by solid state reaction. The carbon nanotube substitution results in a small decrease in lattice parameter and Tc, as compared to ordinary carbon substitution. Enhancement of Hc2 and Hirr shown by CNT doped samples show that the effect of CNT doping is two-fold. For sintering temperatures higher than 900 °C, the CNTs partially decompose, allowing substitution of boron for carbon. This results in an increased Hc2. However, remains of the CNTs are incorporated into the crystal matrix of MgB2, and they act as pinning centres. For lower sintering temperatures, there is little substitution of boron for carbon, and CNTs are incorporated into the crystal matrix as a whole, acting as pinning centres without affecting Hc2.

Pure and 10 wt% nano-SiC doped MgB2/Cu tapes were fabricated using the coating and pressing method. Samples were sintered by an in situ reaction process. It was observed that the nano-SiC doped tapes were significantly reacted with the Cu sheath at 700 °C, while pure samples have less reactivity with Cu under the same conditions. However, for sintering at 667 °C for just 6 min, the reaction with Cu was significantly reduced for the nano-SiC doped samples and led to very high critical current densities of more than 1 MA cm−2 in zero field at K. The Jc values exceed 105 A cm−2 for 30 K in zero field, 20 K in 2 T and K in 4 T. These Jc values are of one to two orders of magnitude higher than those of the pure MgB2/Cu short tapes, are the best reported Jc values for Cu sheathed wires and tapes and are comparable to the Jc values reported for MgB2/Fe tapes. These nano-SiC doped MgB2/Cu tapes also exhibited very small flux jumping at 5 K. Such a high Jc value and its field performance together with its possible high thermal stability make the Cu-sheathed MgB2 tapes an attractive candidate for large scale applications.

La2CoMnO6 perovskite manganite was synthesized by solid-state reaction. Rietveld refinement of powder x-ray diffraction data indicated that this compound crystallized with an orthorhombic structure containing distorted (Co/Mn)O6 octahedra (Pnma; a = 5.5 A; b = 7.2 A; c = 5.4 A. The dc magnetization and real part (x') of the ac susceptibility were measured at dc fields up to 1 T and an ac field of 0.1 Oe at frequencies of 21, 217, 600 and 2000 Hz over a wide range of temperatures from 300 K down to 4.2 K. A ferromagnetic transition was observed at Tc = 220 K. A spin glass ground state was also determined from the shift of a peak at 220 K in the real part of the ac susceptibility with frequency and slow spin relaxation for T < Tc as indicated by the time dependence of the remanent magnetization. The spin glass state arises from a ferromagnetic state and might be caused by competition between the ferromagnetic and antiferromagnetic interaction.

La0.8Sr0.2Mn1−xZnxO3 perovskite manganite with x = 0, 0.01, 0.03, 0.05, 0.1, 0.15, 0.3, 0.5 and 0.75 has been prepared by the solid state reaction and investigated by structural, transport and magnetic measurements. The structural characterization of the samples has been done by x-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. The x-ray powder diffraction results indicate a single phase for 0 0.1 decreases with increasing zinc doping levels. The transport measurements show that the insulator-to-metal transition temperature, Tc, decreases with increasing zinc substitution on the Mn site, and the resistivity increases. The Tc variations agree with the magnetic measurement data for x 0.3. The possible mechanism behind the effects of zinc doping into the La0.8Sr0.2Mn1−xZnxO3 system involves degradation of the double exchange between Mn3+–O–Mn4+ in this compound.

The peak effect (PE) in Fe-doped Bi2.1Sr1.9Ca1.0(Cu1−yFey)2Ox crystals with different nominal y values of 0, 0.005, 0.013 and 0.022 grown by the floating zone method was studied by measuring M–H loops. The peak effect appears over a narrow temperature range between 20 and 40 K for y up to 2.2%, but with a small peak field of about 200 Oe. The appearance of the PE in all the Fe-doped Bi-2212 can be explained by the existence of Bi5+ cluster regions in the Bi3+ 2212 matrix. ΔMmax/ΔM0 versus T/Tc and (Hmax − Hmin)/Hmax versus T/Tc were used to characterize the peak effect. ΔMmax and ΔM0 represent the width of the hysteresis loop at the secondary peak (Hmax) and the central peak H0 (H = 0). Hmin represents the field at which the magnetization starts to increase, giving the anomalous peak. The secondary peak moves away from the central peak in a linear way as the temperature increases as indicated by (Hmax − Hmin)/Hmax versus T/Tc, in agreement with what is found in pure Bi-2212 single crystals.

Magnetic hysteresis loops and magnetic relaxation have been measured for pure and heavily lead-doped Bi2-xPbxSr2CaCu2Oy (Bi2212) single crystals. From the hysteresis loop measurements, the normalized critical current density Jc/Jc0 has been calculated, and from the relaxation measurements the activation energy, current-voltage curves and normalized relaxation rate have been obtained. The field and temperature dependences of Jc/Jc0 for Pb-doped crystals were much weaker than for the pure Bi2212 crystals. A large upward shift in the crossover temperature TCR, a temperature separating two different pinning regimes, has been observed for the first time with heavy lead doping. This was interpreted by improved inter-layer Josephson coupling of two-dimensional vortices, as a result of Pb doping. Heavily Pb-doped crystal had TCR = 35 K for the field that was within the secondary peak in its magnetic hysteresis loop. For pure Bi2212, TCR = 19 K.

Ag-sheathed Bi-2223 multifilamentary tapes were prepared with a hot-pressing technique to study its effect on grain connectivity. The critical current density after hot-pressing was significantly enhanced with the maximum increase reached more than double that before. Hot pressing raised the from to for 81-filament tape. However, the critical current density in an applied magnetic field was not improved by hot-pressing. The in a magnetic field of 1 T decreased from 33% of for the original tape to 21% of for the hot-pressed tapes before and after hot pressing. The fraction of strong links was improved from 64% to 73% for the 81-filament tape as a result of hot pressing. Microstructural analysis showed that hot pressing chiefly improved grain connectivity, increased core density, recovered microcracks and reduced secondary phase impurities.

The effect of the phase compositions after the final stage of heat treatment on the critical current density in Bi:2223/Ag tapes has been investigated. A high volume fraction of Bi:2223 has been considered the most critical factor in achieving a high ; however, it is extremely difficult to attain pure Bi:2223 phase via routine thermomechanical processing. Although a higher volume fraction of Bi:2223 phase can be attained using a higher processing temperature, it is always found to co-exist with Bi:2201 which resides at Bi:2223 colony boundaries. As these possess a plate-like morphology, the Bi:2201 joins weak links in a conductible chain. Bi:2201 can be eliminated using an annealing step at the end of the last thermal cycle, resulting in a significant improvement in . At annealing temperatures below , Bi:2223 decomposes into Bi:3221 and other cuprates, which degrade further. Magnetization, AC susceptibility and resistivity measurement results confirm that the Bi:2201 and Bi:3221 act as weak links.

Several recent developments in powder-in-tube (PIT) processing are presented. A cryogenic deformation process has been developed, involving rolling or pressing the wires and tapes in supercold conditions, such as in liquid nitrogen. Cryogenic deformation has been found to improve the density, grain alignment and Ag-oxide core interface and to increase dislocation density, thereby enhancing and flux pinning. By incorporating Pb into Bi-2212 phase the sintering temperature can be raised above , resulting in a significant reduction of total sintering time from several hundred hours to 100 h. Recently, a new process to eliminate the decomposition and recovery of Bi-2223 during cooling and heating has been developed that further reduces the heat treatment time for Ag/Bi-2223 tapes to 20-30 h, with and Bi-2223 volume fraction in the tapes comparable with those in tapes treated for 120 h. A two-stage annealing procedure in the final thermal cycle has been used to eliminate residual amorphous phase and Bi-2201, which has been identified to be one of the major causes of weak links in PIT tapes. By incorporating hot deformation in a two-step process not only can Bi-2201 be eliminated, but also texture and density are improved, resulting in a very high at 77 K in multifilamentary tapes.

A cryogenic deformation process has been developed and investigated for use in the processing of Ag/Bi-Pb-Sr-Ca-Cu-O superconducting wires. A typical cryogenic deformation process used comprised rolling or pressing the wires into tapes at 77 K and then heat treating the tapes at 838- for a period of 40-60 h. It was found that the cryogenic deformation improved the density, grain alignment, Ag/oxide core interface and critical current density. Critical current densities for cryogenically rolled and pressed Ag-clad mono- and multifilamentary tapes showed a 10 to 20% increase over those observed in normal processed tapes. In comparison with hot deformation and processed tapes, the cryogenic processed tapes showed improved flux pinning capability whereas the hot deformation was found to only improve weak links. During cryogenic deformation, the hardness of the silver sheath was significantly increased, which allowed a greater pressure to be applied during the deformation process. This large pressure was found to be the principal cause of the enhanced densification, texturing and increased dislocation densities.

High critical current density is essential for most large-scale applications of high-temperature superconductors (HTSs). In addition to the weak link problem, weak flux pinning is a major cause of the rapid decline of with magnetic field at high temperatures. Through intensive research in the past 10 years the weak pinning problem has been partially overcome and has reached a level approaching the requirement for some commercial applications. In this article, a number of techniques by which effective pinning centres can be introduced to improve flux pinning in Bi - (Pb) - Sr - Ca - Cu - O HTSs are reviewed. These include surface pinning through a spiral growth, fine precipitates produced through phase transformation or phase formation - decomposition, inclusions through chemical doping or addition, cascades and columnar defects created by fast-neutron and heavy ion beam irradiation and various defects induced by mechanical deformation. In particular, the results of Bi-2223 formation - decomposition and cold deformation versus hot deformation have a significant implication for HTS wire fabrication. Comparative studies with comprehensive electrical, magnetic and microstructural characterizations have been carried out to assess the effectiveness of these techniques.

Measurements of the transport critical current in a magnetic field (H), hysteresis loops and a.c. susceptibility were performed on Bi-2223/Ag tapes that were subjected to different degrees of mechanical deformation and subsequently sintered. against H for some of the tapes was also measured at different stages of thermo-mechanical treatment. All the measurements (the field of the peak of the pinning force, the irreversibility field obtained from hysteresis loops and the temperature of the peak of a.c. susceptibility for different d.c. fields) clearly show an increase of pinning with increase of the degree of mechanical deformation. Measurements of samples at different stages of thermo-mechanical treatment indicate that the pinning depends not only on the degree of mechanical deformation, but also on the conditions of the grain growth. Sintering can provide imperfect healing of micro- and nano-cracks, introduced upon mechanical deformation. It is proposed that the imperfections introduced by the healing are responsible for the improvement in flux pinning, and not the dislocations introduced upon mechanical deformation.

The nature of grain-boundary links in high-quality tapes is clarified. The investigations focus on the role that various types of links play in the restriction of the critical current. The multiple exponential function is found to be a good approximation for the magnetic-field dependence of critical current . The behaviour of at different temperatures gives clear evidence of the two-dimensional character of critical current. It is shown that at all temperatures higher than the Kosterlitz - Thouless transition temperature of isolated superconducting layers the critical current is totally defined by the Josephson interaction between the layers.

The critical current densities and microstructures of tapes prepared using various pre-sintering thermomechanical treatments were compared. The phase composition prior to sintering was found to have a strong effect on the rate of 2223 phase growth and texture and, consequently, final transport properties. The difference in optimum requisite tape sintering processes when using powder with the same stoichiometry from different powder processes or even using identical powder obtained from the same source is due to variations in the content and composition of the secondary phases in the calcinated powder and/or the different phase transformations that occur due to different thermomechanical treatments prior to the sintering procedure. The optimum phase composition before sintering is a mixture of the main phase (textured mature Pb-2212 and Pb free 2212) and a small amount of secondary phases consisting of SrCaCuO and with small particle size. BiSrCuO, CuO and CaCuO are all detrimental to final tape performance characteristics and should be minimized in precursor powder. The optimum phase composition can be prepared by appropriate pre-sintering preparation and thermomechanical procedures. The particle size of the oxide in the Ag sheath can be decreased to a large extent by the deformation process. A critical current density of and a high (1 T, 77 K)/ of 23.4% has been achieved reproducibly using short sintering times and one intermediate pressing procedure. The reproducibility is a direct consequence of controlling the phase composition of the oxide core prior to sintering.

Voltage-current characteristics were measured for Ag-clad (Bi, Pb)2Sr2Ca2Cu3O10 multifilamentary samples at 77 K, with magnetic field H parallel to the c-axis. The data were fitted by V=V0 exp[-(l*/l)mu ], where l is the critical current with no thermal excitations. The parameter mu was obtained from logd(In V)/d/3] versus log l plots, where the data give straight lines for each of the fields. The lines for the fields 30 mT

The influence of mechanical deformation on critical current has been studied, in order to assess the effect of sample handling on the current carrying capabilities of Bi-2223/Ag tapes. Transport critical current was measured at 77 K using a criterion. The effect of progressive bending of the tapes on grain connectivity and flux pinning were investigated by measuring against H. It was observed that bending can affect weak and strong links to a different degree. Damaged samples have a low , but often have a large critical current which is carried through strongly linked paths, compared to carried through weak links. The influence of tensile deformation on was also studied. was found not to degrade until the deformation of the tape approached its elastic limit.

Amorphous magnetic Ni–Co–B nanoparticles with an average size of 5 nm were added to precursor powders of MgB2 superconductor. The preparation procedure for MgB2 pellets was optimized for obtaining the best critical current density (Jc) at elevated magnetic fields. Addition of Ni–Co–B decreases the Jc for heat treatment of precursor powders at 650 ° C. Heat treatments at 770 ° C and higher improve Jc at 20 and 5 K. This improvement occurs at both temperatures through the increase of the effective connectivity between MgB2 crystals. Vortex pinning was enhanced at 5 K, but not at 20 K. Ni–Co–B nanoparticles reacted with Mg in heat treatments above 730 ° C, forming Mg2Ni and MgCo2 nanoparticles. Ni–Co–B addition was associated with lower oxygen content in MgB2, indicating that reduction of MgO content is the mechanism for improvement of grain connectivity. Decomposition of magnetic Ni–Co–B nanoparticles results mostly in non-magnetic nanoparticles, so magnetic pinning did not occur in our samples.

The measurement of transport critical current (Ic) for MgB2 wires and tapes has been investigated with two different techniques, the conventional four-probe arrangement with direct current (DC) power source, and a tailored triangle pulse at different rates of current change. The DC method has been widely used and practiced by various groups, but suffers from inevitable heating effects when high currents are used at low magnetic fields. The pulsed current method has no heating effects, but the critical current can depend on the rate of the current change (dI/dt) in the pulse. Our pulsed current measurements with varying dI/dt show that the same values of Ic are obtained as with the DC method, but without the artifacts of heating. Our method is particularly useful at low field regions which are often inaccessible by DC methods. We also performed a finite element method (FEM) analysis to obtain the time dependent heat distribution in MgB2 due to the electric potential produced at the current contacts to the superconducting sample and its gradient around the contacts. This gradient is defined as the current transfer length (CTL) of the samples and leads to Joule heating of the wire near the contacts. The FEM results provide further evidence of the limitation of the DC method in obtaining high transport critical current.

Direct comparison of the magnetic and transport critical current density (Jc) for the same pieces of copper-sheathed MgB2 wires shows a large discrepancy in magnitude and field dependence of the two. The value of magnetic Jc can differ from the value of transport Jc by a factor of 10 or more. This discrepancy does not occur merely because of the difference in the voltage at which the magnetic and transport Jc are measured, but mainly because of the specific microstructure of MgB2. Such microstructure results in superconducting screening on at least two different length-scales, despite the absence of weak links in MgB2, leading to erroneous magnetic Jc if a simple critical state model is applied to such a system. Consequently, the magnetic Jc cannot be used for analysis of physical processes where the accurate field dependence of Jc needs to be known, such as vortex pinning. The magnetic Jc can still be used for qualitative comparison of MgB2 samples if they are all of the same size and have the same microstructure, or if their size is large enough so that the magnetic Jc does not depend on the sample size.

The AC susceptibility and magnetisation were used to study the inter- and intragrain critical currents in a dense, low resistivity YBa2Cu3O7 ceramic in the temperature range 75-90 K. It is found that the magnetic-field dependence of the peaks in the imaginary part of the AC susceptibility can be used to determine both the inter- and intragrain critical currents. The relation between the field dependence of the intergrain peak in the imaginary susceptibility and the intergrain critical current has been verified by measuring the critical transport currents for two samples at 77.3 K. The magnetic-field dependences of the critical currents, lower critical fields and the flux penetration are briefly discussed.