Showing papers in "Superconductor Science and Technology in 1998"
TL;DR: In this paper, a comprehensive review of the work on ferroelectric materials is presented, including models of the Ferroelectric permittivity and loss tangent as well as methods of measurement of these properties.
Abstract: When an electric field is applied to a ferroelectric material, the microwave permittivity undergoes a substantial change. This change in permittivity can be utilized in microwave devices to produce frequency-agile functions. This paper is a comprehensive review of the work on ferroelectric materials; this includes models of the ferroelectric permittivity and loss tangent, as well as methods of measurement of these properties. New measurements are presented on thin-film strontium titanate and single-crystal strontium barium titanate substrates. These results are compared with the model. A brief discussion is given of the applications of ferroelectric material in microwave devices.
235 citations
TL;DR: In this paper, large c-axis-oriented Sm - Ba - Cu - O bulk superconductors were prepared using the melt-process technique in a reduced oxygen atmosphere, and single-domain samples up to 36 mm in diameter were successfully and reproducibly synthesized with no severe weak-link within the samples.
Abstract: Large c-axis-oriented Sm - Ba - Cu - O bulk superconductors were prepared using the melt-process technique in a reduced oxygen atmosphere. Single-domain samples up to 36 mm in diameter were successfully and reproducibly synthesized with no severe weak-link within the samples. The trapped magnetic field at the open surface of the sample was recorded as 8.0 T at 40 K when measured immediately after the external field was swept down to zero. This value far exceeds that of a melt-processed Y - Ba - Cu - O with a similar size.
150 citations
TL;DR: In this paper, a process for the fabrication of high-quality and near-net-shaped superconducting specimens of (Y-123) with uniformly distributed fine (Y -211) is discussed, where a compact of 123 or 123 rich in liquid phases acting as a source of liquid phases, is placed in contact with the 211 preform, and heated above the peritectic temperature of 123.
Abstract: A process for the fabrication of high-quality and near-net-shaped superconducting specimens of (Y-123) with uniformly distributed fine (Y-211) is discussed. The process involves the fabrication of 211 preforms by conventional ceramic routes such as uniaxial and isostatic pressing, injection moulding and slip casting, and pressureless infiltration basically from a reservoir containing liquid phases. A compact of 123 or 123 rich in liquid phases(s), acting as a source of liquid phases, is placed in contact with the 211 preform, and heated above the peritectic temperature of 123. The liquid from the source compact infiltrates the 211 filler and the peritectic reaction occurring between the preform material (211) and the matrix (liquid phases) during slow cooling from the peritectic temperature results in the growth of 123 with uniformly distributed fine 211 particles. We have also demonstrated that the present process can be extended to the fabrication of other rare earth (RE) superconductors where a solid solution of the kind occurs due to comparable size of the RE and Ba atoms, by choosing the RE = Gd system as an example. Herein the advantages and bounds of the process are reviewed; the effect of various cooling rates on the growth kinetics of 123 and the resulting macro- and microstructures are discussed. Fabrication of composites containing fine and very uniformly distributed Ag is also demonstrated by the process. Comparisons are made between the characteristics of the samples produced in the present process and the existing melt texturing process. The utility of the process is demonstrated by the fabrication of a three-dimensional component - a hollow cylinder.
114 citations
TL;DR: In this paper, the effects of NEG-211, Pt and additions on OCMG-processed (NEG') composites have been investigated in detail using a combination of scanning electron microscopy and measurements of critical current densities.
Abstract: The effects of (NEG-211), Pt and additions on OCMG-processed (`NEG') composites have been investigated in detail using a combination of scanning electron microscopy and measurements of critical current densities, . Microstructural observations clearly indicated that the platinum addition was effective in refining the NEG-211 secondary-phase particles, which led to a dramatic increase in in the low-field as well as in the high-field region. TEM-EDX analyses revealed that the small secondary-phase particles (diameter ) consist mainly of Gd in the rare earth site, while the large particles contain Nd, Eu and Gd in the rare earth site with an even ratio, which is identical to the nominal composition of the precursor powder. For an NEG sample with 40 mol% NEG-211 additions, a large critical current density of 60 000 A at 77 K was observed in a magnetic field of 3 T applied parallel to the c-axis of the sample. The critical current density measurements showed that the zero field and peak values are similar in samples with 30 and 40 mol% of NEG-211 added. In contrast to this, the combination of and Pt addition was not as effective in refining the NEG-211 particles, and also the values were found to decrease in the entire field region.
110 citations
TL;DR: In this article, the effect of rolling reduction and annealing conditions on the sharpness of the cube texture, the incidence of other orientations, the grain size and the surface topography was investigated.
Abstract: The biaxial textures created in metals by rolling and annealing make them useful substrates for the growth of long lengths of biaxially textured material. The growth of overlayers such as high-temperature superconductors requires flat substrates with a single, sharp texture. A sharp cube texture is produced in high-purity Ni by rolling and annealing. We report the effect of rolling reduction and annealing conditions on the sharpness of the cube texture, the incidence of other orientations, the grain size and the surface topography. A combination of high reduction and high-temperature annealing in a reducing atmosphere leads to >99% cube texture, with a mosaic of about the rolling direction, about the transverse direction, and about the normal direction.
94 citations
TL;DR: In this article, a review of the effects of vortex-lattice melting, flux pinning, flux creep and critical current density on superconducting materials is presented, and a number of consequences are discussed.
Abstract: The electrodynamic behaviour of the high-temperature cuprate superconductors is strongly affected by the anisotropy of these layered materials. Their transport, magnetic and pinning properties are determined by the properties of vortices, which can be regarded as stacks of two-dimensional (2D) pancake vortices (with cores localized in the layers, bilayers or trilayers) connected by Josephson vortices (with cores localized in the nonsuperconducting charge-reservoir layers). When the Josephson coupling between the superconducting layers is strong, as in YBCO, the line energy associated with the Josephson vortices is large, the 2D pancake vortices in adjacent layers are strongly coupled and the magnetic anisotropy of the superconductor is relatively weak. When the Josephson coupling is weak, as in BSCCO, the line energy associated with the Josephson vortices is small, the 2D pancake vortices in adjacent layers are weakly coupled and the magnetic properties are highly anisotropic and nearly 2D. The transport properties for currents flowing parallel to the layers are dominated by 2D pancake vortex motion, while those for currents flowing perpendicular to the layers are dominated by Josephson vortex motion. In this paper, I review these effects and discuss a number of consequences for vortex-lattice melting, flux pinning, flux creep and the critical current density.
58 citations
TL;DR: In this article, a review of various aspects of the problem of current transfer in high-superconductors (HTSs) is presented, and the role of grain boundaries in transport features of the samples is discussed.
Abstract: Various aspects of the problem of current transfer in high- superconductors (HTSs) are reviewed. The spatial inhomogeneities of various types are identified as a primary cause of non-uniformity of both normal currents and supercurrents in real samples of HTSs. The role these inhomogeneities play in transport features of the samples is discussed. The case of grain boundaries in polycrystalline samples is elaborated in detail. The local structural and transport properties of isolated grain boundaries are first reviewed and then integrated into the knowledge of global (macroscopic) charge transport. The paper emphasizes the common ingredients characterizing the transport in various forms and families of HTS samples in small magnetic fields. The phenomenon of percolation is identified as the most obvious one and is shown to dominate a large number of observations covered by this report. The experimental results focused on by this report elaborate primarily the problems of critical currents, initial dissipation and current-voltage characteristics, penetration depth, resistive and metal-insulator transition, resistance noise and magneto-optical studies of current paths. Various models for current transfer (disordered bonds, brick wall and railway switch) are also reviewed and discussed.
57 citations
TL;DR: In this paper, Y-Ba-Cu-O melt processed samples were prepared from mixtures of powders (starting average particle size ranging from to ) with and 1 wt% or 1 t%, and the resulting microstructure of the samples was composed of network and low concentration regions with size and morphology correlating with the starting powder size.
Abstract: Y-Ba-Cu-O melt processed samples were prepared from mixtures of powders (starting average particle size ranging from to ) with and 1 wt% or 1 wt% . The resulting microstructure of the samples was composed of network and low concentration regions with size and morphology correlating with the starting powder size independently of the holding time in the melted state. The 211 particle size was smaller for Ce doped samples than for Pt doped samples. The differences in porosity of Pt and Ce doped samples were related to the changes in the interfacial energy of the constituent phases.
47 citations
TL;DR: In this article, the temperature dependence of the 3D paraconductivity in the vicinity of a superconducting phase transition is derived using the linearized Ginzburg-Landau equations for a two-band model taking into account interband scattering of pairs within the Kubo formalism.
Abstract: The formulae for the temperature dependence of the 3D paraconductivity in the vicinity of a superconducting phase transition are derived using the linearized Ginzburg-Landau equations for a two-band model taking into account interband scattering of pairs within the Kubo formalism. In general there exist two correlation lengths. However, only one of them is critical and determines the fluctuation conductivity . The Aslamazov-Larkin formulae are modified in the two-band model. After calculating from available data the various terms and amplitudes, our theory is compared with experimental data on La-123, Y-123 and Bi-2212 based films.
44 citations
TL;DR: In this paper, the authors show that when a magnetic field is applied during melt processing, the bulk Bi2212 is textured with the c-axis parallel to the field direction, and the addition of MgO induces an improvement in the material properties.
Abstract: Bulk samples of Bi2212 pure and doped with MgO or were prepared by melt processing in a high magnetic field. The structure of the samples was analysed using XRD and SEM and their transport properties were studied. The results show that when a magnetic field is applied during melt processing the bulk Bi2212 is textured with the c-axis parallel to the field direction. The addition of MgO induces an improvement in the material properties. This is not observed using . A significant improvement is observed at 77 K in the critical current density in self-field and in an external magnetic field. This improvement can be related to the better texturation obtained when MgO is added: it maintains high viscosity during melt processing and thus the texturation induced by the magnetic field is not weakened by liquid-phase spreading. The addition of MgO also induces a decrease in volume of residual phases. The improvement in properties in an external magnetic field can be attributed to a better indirect vortex pinning due to submicron MgO-rich inclusions in the superconducting matrix.
41 citations
TL;DR: In this article, the conditions leading to a-b microcracking were investigated in melt-processed RE-Ba-Cu-O superconductors. And the microstresses, introduced to the (123) matrix by different thermal expansions and different elastic constants of 211 (422) and 123 phases, were calculated and illustrated by the observed microcrack pattern around 211(422) particles.
Abstract: Microcracking is an important phenomenon in melt-processed RE-Ba-Cu-O superconductors. The conditions leading to a-b microcracking are the objective of this paper. The microstresses, introduced to the (123) matrix by different thermal expansions and different elastic constants of 211 (422) and 123 phases, are calculated and illustrated by the observed microcrack pattern around 211 (422) particles. The critical 211 particle size for a-b microcracking is estimated using a simplified model.
TL;DR: In this paper, the effects of porosity on the elastic modulus, the flexural strength, the hardness and the fracture toughness of (Bi, Pb)SrCaCuO (BSCCO) are examined.
Abstract: The mechanical properties of (Bi, Pb)SrCaCuO (BSCCO) are examined. Particular attention is paid to the effects of porosity on the elastic modulus, the flexural strength, the hardness and the fracture toughness. A study of the effects of porosity on these properties is crucial in developing a clearer idea of the behaviour of BSCCO conductors in service and is a prerequisite for modelling using, for example, finite element techniques. Experiments are carried out which show the effect of pore volume on the mechanical properties and the effect of processing on the fracture toughness. It is found that the critical defect length in BSCCO is surprisingly large and this has important implications for the processing of the materials and for the limits of application.
TL;DR: In this paper, the microwave melt textured growth process (MMTG) is used to investigate the influence of and doping and their combination with two different Sn/Ce initial ratios on the (Y123) growth mechanism, microstructure and superconducting properties.
Abstract: The microwave melt textured growth process (MMTG) is used to investigate the influence of and doping and their combination with two different Sn/Ce initial ratios on the (Y123) growth mechanism, microstructure and superconducting properties. It was shown that the combination of `' allows us to increase the Y123 crystal growth velocity compared with used alone. Furthermore, this combination reduces the particle coarsening observed when doping is added alone. Secondary phase SEM observations show that square submicrometric precipitates are homogeneously distributed in the matrix when `' are used. Thus, the identified phase appears to be a solid solution derived from the phase . In this article, the initial Sn/Ce ratio was found to be a crucial parameter that affects the matrix purity. In terms of superconducting properties, `' doped samples exhibit the highest magnetic critical current density in zero field , even under 1 Tesla , depending on the Sn/Ce ratio. The superconducting properties are discussed in relation with the microstructure and the EDS analysis results.
TL;DR: In this article, the authors investigated the mechanisms leading to a maximum of as a function of reaction time in monofilamentary tapes and found that the observed maximum of at about 180 h is the result of two competing effects: (a) the improvement of the quality of grain boundaries and (b) the decrease of the pinning efficiency in the grains.
Abstract: The mechanisms leading to a maximum of as a function of reaction time in monofilamentary tapes are investigated. Various properties of tapes previously reacted for 40 h in air at , rolled and submitted to additional reaction during times varying from 40 to 240 h were measured. The evolution of the morphology, phase assembly, stoichiometry, weight, critical temperature, critical current and irreversibility line is described. XRD and SEM analysis reveal no decomposition of the Bi(2223) phase and no noticeable grain growth after the 40 h additional time. Pb losses up to 40% were detected by means of thermogravimetry and EDX as function of reaction time, while an increase of from 107.25 to 109 K was observed. On transport and magnetic measurements, it is found that the observed maximum of at about 180 h is the result of two competing effects: (a) the improvement of the quality of grain boundaries and (b) the decrease of the pinning efficiency in the grains.
TL;DR: When uranium (U) is added to Y123 powders, after texturing the deposits containing U are found to be nm. These deposits act as pinning centres, and increase by 90% per 0.8 wt% U.
Abstract: When uranium (U) is added to Y123 powders, after texturing the deposits containing U are found to be nm. These deposits act as pinning centres, and increase by 90% per 0.8 wt% U. Neutron irradiation further increases . Total increases of by factors of e.g. (at 77 K, 0.25 Tesla), and (at 50 K, 10 Tesla) result. Magnitudes of are (at 77 K, 0.25 T) and (at 50 K, 0.25 T). The U/n process is not yet optimized. Chemistry, microstructure, radiation effects and cost are discussed.
TL;DR: In this paper, single-phase Hg1223 and Cu-1223:P samples in wide hole-doping ranges were successfully synthesized by a high-pressure (5 GPa) technique utilizing internal and/or external oxidizing agents and by subsequent post-annealings performed in a thermobalance.
Abstract: Essentially single-phase Hg-1223 and Cu-1223:P samples in wide hole-doping ranges were successfully synthesized by a high-pressure (5 GPa) technique utilizing internal and/or external oxidizing agents and by subsequent post-annealings performed in a thermobalance. The hole-doping level of the as-synthesized samples was found to be in the overdoped side for both of the phases by thermoelectric power measurements. values as low as 107 K and 67 K were obtained for the overdoped Hg-1223 and Cu-1223:P samples, respectively. The values of and were calculated from the measured M versus H data employing, respectively, the Bean model and two different criteria: (at ) and (as ). It was found that the heavier the doped hole concentration was, the more improved were the versus characteristics for both the Hg-1223 and the Cu-1223:P phases. This tendency was the same for versus characteristics calculated using either criterion. The improvement with increasing hole-doping level was especially profound in the case of Cu-1223:P: the versus characteristics of a Cu-1223:P sample with K, probably being close to the optimally doped state, were as poor as that reported for Bi-2212, while an overdoped Cu-1223:P sample with K was superior to an overdoped Hg-1223 sample with K. The mechanisms as well as the usefulness of the observed phenomena are discussed.
TL;DR: In this paper, the phase formation rate depends sensitively on the established oxygen partial pressure with an optimum partial pressure being approximately 10 vol.% oxygen in the furnace and the total pressure was always kept at and the volume fraction of was varied.
Abstract: phase formation has been studied in Ag-sheathed multifilamentary tapes processed in different oxygen partial pressures during sintering. In the furnace the total pressure was always kept at and the volume fraction of was varied. The samples prepared for the series were characterized by the critical current density , ac susceptibility and x-ray diffraction. Detailed investigations of the microstructure were carried out by SEM and TEM for samples sintered at 8 and 10% oxygen. The formation of the Bi(Pb)-2223 phase occurs at initial stages of the heat treatment, in which , alkaline earth cuprates and Pb compounds act as the precursor phases. The phase formation rate depends sensitively on the established oxygen partial pressure with an optimum partial pressure being approximately 10 vol.% oxygen. The critical current density varies more sensitively with the applied oxygen partial pressure and temperature than the Bi(Pb)-2223 phase fraction as determined by x-ray diffraction. Optimum values can only be obtained in a fairly small temperature/oxygen partial pressure window. One explanation for the strong decrease in is the poor crystalline quality of the grains. By TEM columnar defects were observed with diameters of typically 50 nm extending parallel to the c-axis of the grains. The individual grains contain a high density of these defects yielding still strong x-ray diffraction peaks but obviously poor superconducting properties. The formation rate and defect density depends sensitively on both the temperature and the oxygen partial pressure during sintering. The phase composition at initial stages of the heat treatment, particularly the phase fraction of the lead oxide compounds, can be controlled by the applied oxygen partial pressure. The reason for this is that Pb changes its oxidation state from 4+ to 2+. A stability line was determined for lead oxide compounds with lead being , e.g. .
TL;DR: In this paper, a new rolling technique, called eccentric rolling, was developed for texture and density improvement in Bi(2223)/Ag tapes made by PIT, which allows to deform the BSCCO/Ag composite in a way similar to flat rolling using rollers of very large radius or to a continuous pressing without any overlapping parts.
Abstract: A new rolling technique, so called `eccentric rolling', has been developed for texture and density improvement in Bi(2223)/Ag tapes made by PIT. It consists in rolling deformation between the outer surface of the inner roller and inner surface of the outer hollow cylinder. This technique allows to deform the BSCCO/Ag composite in a way which is similar to flat rolling using rollers of very large radius or to a `continual' pressing but without any overlapping parts. In comparison to classical rolling with roller of a similar radius, a higher core density and widening of tape have been obtained. The transport critical current densities have been measured for single-filament and 19-filament tapes deformed by the roller presented and a classical roller. Current densities which are two times higher than in the case of classical roller (at T = 77 K, B = 0 T) have been measured for both types of tapes deformed using the new technique presented.
TL;DR: In this article, the authors investigated planar gradiometers on the basis of galvanically coupled high-DC SQUIDs for application in non-destructive testing (NDT) and showed measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as well as the influence of sensor position and experimental environment.
Abstract: We investigated planar gradiometers on the basis of galvanically coupled high- DC SQUIDs for application in non-destructive testing (NDT). The stability of the sensor of better than 1% permits NDT investigations in unshielded environments. The layout of the gradiometer sensors and a corresponding determination of their effective areas and baselines will be discussed as well as the properties of the Josephson junctions in the DC SQUID based on step-edge and bicrystal grain boundaries. We show measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as well as the influence of sensor position and experimental environment such as the dewar material on the performance of the sensor and the whole testing system. As an example first investigations of the permanent magnetization of hardened valves will be presented where we look for a correlation between hardening and magnetic field distribution.
TL;DR: In this article, a small coil made by winding a superconducting Ag/Bi-2223 multifilamentary tape with eight pancakes was used to measure the critical current.
Abstract: A small coil (35 mm bore) was made by winding a superconducting Ag/Bi-2223 multifilamentary tape. The coil, composed of eight pancakes, showed reduced performance in the pancakes placed at the sides with respect to central ones. This effect was attributed to the radial field at the coil ends. In order to explain this behaviour, a general method for predicting the critical current of a coil, starting from basic information on short samples, was developed. The method was used to understand the possible advantages from a reduction of radial field in the coil through a suitable iron yoke for flux return. The measured performance (in terms of critical current) of the coil with iron improved by 10%. The central field at 77 K was 1100 Gauss and 6000 Gauss at 4.2 K.
TL;DR: In this article, the dissolution behavior of Sm123 seed was explained in terms of the solubility of samarium in the liquid and the dissolution kinetics at the seed/liquid interfaces.
Abstract: During top-seeded melt growth processing of (Y123), dissolution of (Sm123) seed crystals was observed at temperatures below the melting point of the seed material. The dissolution was dependent on factors such as the seeding method (cold seeding or hot seeding), the holding temperature and the composition of the seed and compact. Applying the hot seeding method, lowering the seeding temperature and using (Sm211) excessive Sm123 seed and (Y211) excessive Y123 compact decreased the dissolution rate of the seed crystal, resulting in a well-grown Y123 single crystal. The dissolution behaviour of the Sm123 seed was explained in terms of the solubility of samarium in the liquid and the dissolution kinetics at the seed/liquid interfaces.
TL;DR: In this article, it is shown that there is a close relationship between the formation mechanisms of Pb free and the Pb substituted phases, which both can be described as a nucleation and growth process.
Abstract: The reaction leading to the formation of the Pb free phase from calcined precursor powders has been studied by means of scanning electron microscopy, x-ray diffraction and differential thermal analysis. The observation of the transformations occurring at a fixed location on the surface of a pellet has shown that the formation of the phase crystallites occurs simultaneously with a gradual decomposition of the platelets initially present in the mixture. The present investigations are discussed in relation with the formation of the Pb substituted phase. It is shown that there is a close relationship between the formation mechanisms of the Pb free and the Pb substituted phases, which both can be described as a nucleation and growth process.
TL;DR: In this paper, the fabrication process of long Bi-2223/Ag tapes by the powder-in-tube technique is presented and discussed with the aim of surveying the influence of some selected process parameters on the properties of final conductors.
Abstract: At present, rolled multifilamentary Bi-2223/Ag tapes exhibiting at 77 K are fabricated, making them the most favourable candidates for the fabrication of high-temperature superconductor (HTS) conductors for power applications. In this paper the fabrication process of long Bi-2223/Ag tapes by the powder-in-tube technique is presented and discussed with the aim of surveying the influence of some selected process parameters on the properties of final conductors. limitations in HTSs are discussed and some of the possible applications of Bi-2223/Ag or Bi-2212/Ag tapes are briefly presented.
TL;DR: In this paper, the role of Ag additions in the microstructural evolution and related mechanical properties of YBCO has been studied, and various amounts of Ag have been added to YBCOs by two different mixing techniques, i.e. in the form of metallic Ag powder and solution.
Abstract: The role of Ag additions in the microstructural evolution and related mechanical properties of YBCO has been studied. Various amounts of Ag have been added to YBCO by two different mixing techniques, i.e. in the form of metallic Ag powder and solution. The strength and fracture toughness of YBCO specimen increased with increasing Ag content, probably because of strengthening mechanisms resulting from Ag addition. In addition, YBCO-Ag composites showed higher values of strength and fracture toughness when Ag was added in the form of solution compared with those when it was added in the form of metallic Ag powder. Specifically, an addition of 10 vol% Ag, made by mixing with metallic Ag powder, increased strength from 88 to 116 MPa and fracture toughness from 1.6 to . Corresponding improvements in strength and fracture toughness were 128 MPa and , respectively, when YBCO was mixed with solution. The better mechanical properties of the YBCO-Ag specimen mixed by the solution method are believed to be due to the microstructure of more finely and uniformly distributed Ag particles. Furthermore, addition of Ag did not have an adverse effect on the critical current density of YBCO superconductors.
TL;DR: In this article, a new technology process in LTS implementation to fabricate intrinsically shunted tunnel junctions has been reported in SINIS multilayer thin-film technology.
Abstract: The paper reports on recent developments in a new technology process in LTS implementation to fabricate intrinsically shunted tunnel junctions. The process has been realized in SINIS multilayer thin-film technology. In various test series, circuits containing a large variety of single junctions and junction arrays of different contact areas and sizes were fabricated and measured. By variation of the oxidation parameters the fabrication process has been optimized for application in integrated circuits operating in RSFQ impulse logic. The junction parameter values realized for the critical current density range to up to about , those for the characteristic voltage to up to about . The junctions show nearly non-hysteretic current-voltage characteristics; the intra-wafer parameter spread is below 10%. The junctions realized fulfil the requirements imposed for digital RSFQ circuit operation at clock frequencies in the lower GHz frequency range.
TL;DR: In this paper, the excess conductivities of polycrystalline Hg-1223 superconductor have been studied under high-pressure conditions and the experimental results were analyzed within the framework of Aslamazov-Larkin theory.
Abstract: Excess conductivities of polycrystalline Hg-1223 superconductor have been studied under high-pressure conditions. The critical temperature was chosen as the mean field transition temperature . Above this temperature, the normal state resistivity decreases with increasing pressure. The relative change in resistivity w.r.t. pressure, namely , is about . It is much smaller than the typical value of for most high- superconductors. As a consequence of the change in the normal state resistivity due to the applied pressure, the fluctuation-induced conductivity is also changed. The experimental results were analysed within the framework of Aslamazov-Larkin theory. The logarithmic plots of the excess conductivity versus reduced temperature reveal that there is a cross-over temperature which determines the transition from two- to three-dimensional conductivity of charge carrier transportation. With the help of the cross-over temperature , the values of the coherence length for the Hg-1223 superconductor could be estimated. The change in the fluctuation conductivity is discussed in terms of the coherence length and the thermodynamical critical field .
TL;DR: In this paper, the correlation between reaction kinetics of the Bi(Pb)-2223 phase and critical current density in Bi(pb)-2223/Ag tapes has been analyzed.
Abstract: The correlation between reaction kinetics of the Bi(Pb)-2223 phase and critical current density in Bi(Pb)-2223/Ag tapes has been analysed. It has been found that a slow formation rate of the 2223 phase allows a higher value, while a faster formation rate of the 2223 phase corresponds to a lower value. A mechanism for the Bi(Pb)-2223 formation has been proposed, in which two reaction routes control the formation of the 2223 phase. One is a liquid-assisted nucleation and growth; the other is an intercalation process. The reaction rates of the two routes depend on the phase assemblage of the precursor powder, determined by the start composition and calcination conditions. The new mechanism has successfully described the correlation between the reaction kinetics and critical current density in Bi(Pb)-2223/Ag tapes and can be used as a guide for preparing the precursor powder for Bi(Pb)-2223/Ag tapes.
TL;DR: In this paper, an improved travelling solvent floating zone (TSFZ) method was used to grow reproducibly over a wide doping range using an improved lamp-image focusing.
Abstract: We grew large single crystals of reproducibly over a wide doping range using an improved travelling solvent floating zone (TSFZ) method. We performed a simulation on the lamp-image focusing to find an optimum condition for a steep temperature gradient along the rod. Based on the results of simulation, we adapted smaller lamps and shielded the partial area of the mirror surfaces. The improvement of the focusing had a great effect on keeping the molten zone stable for more than 100 h. Stabilization of the molten zone improves both the reproducibility of the crystal growth and the quality of the grown crystals.
TL;DR: In this paper, the irreversibility lines in superconductors containing various amounts (0.3-0.7 wt%) of uranium and various amounts of the fissionable isotope were investigated.
Abstract: Results on the enhancement of critical current densities in melt-textured bulk (Y-123) superconductors, which were doped with and exposed to thermal neutron irradiation to create fission tracks, are presented. These tracks, which all originate from U-containing clusters of a stable UPtYBaO compound, form isotropic `starlike' defects with diameters of approximately 10 nm and lengths of . However, if we consider projections onto the symmetry plane of the flux line lattice and allow for a certain tilt modulus , their pinning-effective size is mostly (90%) between 10 and 20 nm, i.e. only twice as large as the characteristic defect structure (amorphous collision cascades with diameters of nm) introduced into undoped superconductors by fast neutron irradiation. We report on studies of and the irreversibility lines in superconductors containing various amounts (0.3-0.7 wt%) of uranium and various amounts of the fissionable isotope . In the best case (0.3 wt% U, thermal neutrons), we find (77 K) to be around (0 T) and (5 T). Control experiments on samples containing (nominally) only and exposure to thermal or to thermal plus fast neutrons confirm the above flux pinning considerations. As reported before for undoped melt-textured Y-123, the irreversibility lines remain practically unchanged due to the presence of strongly pinning Y-211 precipitates.
TL;DR: In this paper, the melting process of Sm-123 is done by infiltrating liquid phases into an Sm-211 preform and allowing the growth of SM-123 to take place.
Abstract: Melt processing of (Sm-123) is done by infiltrating liquid phases into an Sm-211 preform and allowing the growth of Sm-123 to take place. The melt-textured samples are found to be free of macrodefects, and the microstructure contains uniformly distributed fine-sized Sm-211 particles. The behaviour of the critical current density of the samples in high fields is reported here.