Showing papers in "Superconductor Science and Technology in 1997"
TL;DR: In this paper, the authors introduce the wide-band AC susceptibility, the so-called ''wide-band magnetic flux'' which refers to the magnetic flux and often offers an easier meeting between theory and experiment.
Abstract: A magnetic field harmonically varying in time (to probe the sample) and a lock-in technique (to register the sample response sensed by a pick-up coil) are widely used for characterizing superconductors. Measuring the temperature dependence of the complex AC susceptibility is the most common procedure of this type. This paper reviews these techniques, introducing in addition the complex AC susceptibility, the so-called `wide-band AC susceptibility'. The latter quantity refers to the magnetic flux and often offers an easier meeting between theory and experiment. Starting from models for linear flux diffusion, reversible screening, volume and surface flux pinning and the intermediate regime in a type II superconductor, the expressions for the complex AC susceptibility in different cases are presented and compared with those derived for the wide-band AC susceptibility. Derivation of the basic physical properties of high- superconducting materials from the AC data (resistivity, critical temperatures and fields, London and Campbell penetration depths, critical current density, granularity and content of superconducting phase, irreversibility line, pinning potential) is then thoroughly discussed.
277 citations
TL;DR: In this article, the influence of the angular dispersion (splay) of the tracks on vortex pinning and creep is discussed, and measurements of the temperature, field and defect density dependence of the persistent currents and their time relaxation in YBCO are presented.
Abstract: Columnar defects introduced by irradiation with very energetic heavy ions are the most effective pinning centres for flux lines in high-temperature superconductors. This correlated disorder generates large increases in the critical current densities and expansion of the irreversible regime in and the various Bi- and Tl-based compounds. In single crystals and thin films, the pinning enhancement is strongly angular-dependent, and maximizes when the applied magnetic field is parallel to the amorphous latent tracks. In contrast, in the much more anisotropic Bi- and Tl- based materials this unidirectional anisotropy is very small due to the quasi two-dimensional character of the vortices. Some of the extensive experimental studies on this topic are reviewed. Measurements of the temperature, field and defect density dependence of the persistent currents and their time relaxation in YBCO are presented. The analysis of these results based on recent theoretical models permits the identification of various pinning and creep regimes. Studies in other compounds, including technologically relevant Bi-based tapes, are summarized. The influence of the angular dispersion (splay) of the tracks on vortex pinning and creep is discussed.
183 citations
TL;DR: In this article, a three-phase superconducting fault current limiter with a rated power of 1.2 MVA has been built, tested and installed in a power plant.
Abstract: A three-phase superconducting fault current limiter with a rated power of 1.2 MVA has been built, tested and installed in a power plant. The device is based on the `shielded iron core concept'. The superconducting part consists of a stack of rings made of Bi-2212 ceramic. They were fabricated by a special partial melt process and have a diameter of 38 cm, a height of 8 cm and a thickness of 1.8 mm. The current - voltage characteristic obeys a power law with . The critical current density defined by the criterion is about . The nominal current and voltage of the device are 70 A and 10.5 kV, respectively. In three-phase short-circuit tests with a prospective fault current of 60 kA the current was limited to about 700 A in the first half-wave. After 50 ms the limited current was below 250 A. The test results are in excellent agreement with detailed simulations of both the normal operation and the behaviour under fault conditions. The current limiter has been installed in the auxiliary line of a hydropower plant for a 1 year endurance test. It is the first superconducting device tested in a power plant under actual operating conditions.
149 citations
TL;DR: In this paper, the effect of mixing three rare earth (RE) elements in the RE site on the superconducting properties of melt-processed, in which RE elements are selected from the group of Nd, Sm, Eu and Gd.
Abstract: We report the effect of mixing three rare-earth (RE) elements in the RE site on the superconducting properties of melt-processed , in which RE elements are selected from the group of Nd, Sm, Eu and Gd. The heat treatment profiles for melt processing were determined according to the peritectic decomposition temperatures obtained from the thermal analysis measurements. Microstructural observations and compositional analysis showed that the main phase was (, in which the ratio of three RE elements (R1, R2, R3) was equal to that of the nominal composition, indicating that mixing of RE elements was uniform. All the samples showed relatively high in the range 93.1 - 95.3 K and exhibited the secondary peak effect in the magnetization curves at 77 K with the irreversibility field higher than that of melt-processed Y - Ba - Cu - O. A high value of at 77 K and 2 T was achieved in (Nd, Eu, Gd) for field parallel to the c-axis.
118 citations
TL;DR: In this paper, the phase diagram studies with emphasis on the high temperature superconducting phases (2212 phase) and (2223 phase) are reviewed in detail at various temperatures and oxygen partial pressures.
Abstract: Phase diagram studies with emphasis on the high temperature superconducting phases (2212 phase) and (2223 phase) are reviewed in this article. The phase relations of the and the homogeneity regions of the 2212 and 2223 phases have been studied in detail at various temperatures and oxygen partial pressures. However, the Pb-doped system has been investigated less comprehensively. The studies of this system are mostly focused on the 2212 and 2223 phase and the experimental work regarding the 2223 phase is often concentrated on the processing of 2223 samples with high critical current density and not on the preparation of single phase 2223 ceramics. The investigations of the Ag-containing system have just started. Nevertheless, first results show that Ag significantly influences the phase relations and the temperature dependence of the cation solubilities of the 2212 and 2223 solid solutions. These results indicate that the obtained phase diagrams of the system cannot be applied without caution when Ag is present. Therefore, as Ag is widely used as a sheath material for the processing of 2212 and 2223 wires and tapes, the investigation of the entire Ag-containing system is inevitable.
107 citations
TL;DR: In this paper, basic concepts of the classical mechanical deformation theory are briefly discussed and simple descriptions of deformation processes like pressing, rolling, drawing and extrusion are also presented.
Abstract: The mechanical deformation of BiSrCaCuO/Ag composites made by the powder-in-tube method is a multi-step process. The main difficulty is that the mechanical properties of the ceramic powder are very different from those of the Ag sheath. A key parameter is the core density, which changes during mechanical deformation. In this review, basic concepts of the classical mechanical deformation theory are briefly discussed. Simple descriptions of deformation processes like pressing, rolling, drawing and extrusion are also presented. The term `freedom parameter', , is introduced to illustrate the influence of various constraint factors on the mass-flow behaviour. Simple pictures including mass redistribution and the powder-flow model are presented for interpreting the plastic deformation process of the composites. Experimental results are reviewed and our proposed pictures and models are applied for discussion.
106 citations
TL;DR: In this article, the precise formation conditions of the Pb-free phase Bi(2223) are described for the first time, and a new process including deformation by a four-roll machine is presented.
Abstract: The various fabrication and reaction processes leading to multifilamentary Bi,Pb(2223) tapes with high transport are described. In air, the Bi,Pb(2223) phase in pressed powders is found to be formed by nucleation, following the complete decomposition of the Bi,Pb(2212) intermediate phase. In this work, the precise formation conditions of the Pb-free phase Bi(2223) are described for the first time. It is shown that the presence of Pb leads to a lowering of the melting temperature from 879 to and of that of the `transient liquid' . The latter is formed after decomposition of the 2-layer compound and is necessary to the formation of the 3-layer compound. Thus Pb is found to have no influence on the formation mechanism of the 3-layer compound, which always starts by a nucleation process. Nucleation processes are also found to occur in the first phase of reaction inside Ag sheathed tapes, intercalation being possible at later stages. The highest values of the critical current density in Bi,Pb(2223) tapes reported at the present day are compared, taking into account the analysed tape lengths. The variation of in mono- and multifilamentary Bi,Pb(2223) tapes as a function of the applied magnetic field at 77 and at 4.2 K is described. A new process including deformation by a four-roll machine is presented, and the distribution of inside various tape configurations with similar critical current densities is analysed: for monofilamentary tapes, a maximum of is found at the borders, the ratio being 2.4, while for conventional multifilamentary tapes the maximum is found at the tape centre (, ratio 1.6). For the new four-rolled tapes, a ratio of only 1.1 was measured, indicating a more homogeneous current distribution. In order to lower the AC losses due to the highly conductive Ag matrix, the new concept of `oxide barriers' is introduced. A barrier of surrounds each filament, thus leading to an enhanced radial resistivity, which in turn reduces AC losses. Finally, the current transport mechanism inside Bi,Pb(2223) tapes and the various current limiting mechanisms are discussed.
84 citations
TL;DR: In this paper, three methods of increasing the growth rate of YBCO single crystals are reviewed, which are (i) employing high oxygen partial pressure, (ii) choosing rare earth elements with higher solubilities in the Ba- Cu-O solution, and (iii) growing REBCO crystals including several rare earth element elements.
Abstract: A low solubility of yttrium in the Ba - Cu - O melt and a steep liquidus slope near the peritectic temperature lead to a very slow growth rate of (YBCO or Y123) single crystals and this creates a problem in growth of large single crystals. To solve this problem, increasing the growth rate and extending the growth time are significant. Using the crystal pulling method, we have developed several processes and succeeded in growing large Y123 and (NdBCO) single crystals with an edge size over 25 mm in the a - b plane and up to 20 mm in the c-axis direction. In this article, three methods of increasing growth rate are reviewed. They are (i) employing high oxygen partial pressure, (ii) choosing RE (rare earth) elements with higher solubilities in the Ba - Cu - O solution, and (iii) growing REBCO crystals including several RE elements. Using these methods the growth rate was effectively enhanced from two to five times that of Y123. The critical temperature of 92.7 K was achieved from a Y123 single crystal grown under 1 atm oxygen partial pressure, indicating that is insensitive to the oxygen pressure of the growth atmosphere in the YBCO system. A high of about 95 K for NdBCO superconductors with a sharp transition was obtained by controlling the ratio of Ba to Cu (Ba/Cu) in the liquid, suggesting that the Ba/Cu ratio in the liquid composition has a significant importance in controlling . By partial substitution of Sm at the Y sites up to 30%, (Y(Sm)BCO) crystals show a of and do not display depression.
66 citations
TL;DR: For at least eight of the first ten years of high-temperature superconductivity, it has been clear that one of the earliest areas of application and commercialization of these materials is as passive devices for microwave electronics as discussed by the authors.
Abstract: For at least eight of the first ten years of high-temperature superconductivity it has been clear that one of the earliest areas of application and commercialization of these materials is as passive devices for microwave electronics. At the time of writing the first prototype products are entering service in the mobile communications industry and a wide range of other devices are at a relatively advanced stage of development. In the further future one can see a cryogenic integrated technology based on HTS materials and the related functional materials which have emerged within the last ten years.
62 citations
TL;DR: In this paper, a review of superconducting (HTS) materials is presented with special reference to their processing, their properties and the development of devices, including HTS magnetic shields, resonators for microwave filters, antennas and wire.
Abstract: High-temperature superconducting (HTS) thick films have undergone a period of rapid progress since their discovery in 1986. In this review thick-film materials are described with special reference to their processing, their properties and the development of devices. The devices include HTS magnetic shields, resonators for microwave filters, antennas and wire. The translation of science into applications has taken place in a relatively short time and already impressive performance is seen in HTS prototype devices.
56 citations
TL;DR: In this paper, single-domain ceramics with high and high critical currents have been prepared in air by means of directional solidification and the leading microstructural factors controlling the pinning of vortices have been analyzed.
Abstract: Single-domain ceramics of (Re = Y, Nd) with high and high critical currents have been prepared in air by means of directional solidification and the leading microstructural factors controlling the pinning of vortices have been analysed. The interface pinning mechanism at secondary inclusions has been found to be more effective in than in in agreement with HRTEM pictures showing sharper interfaces in the first case. The observation of high and a vanishingly small fishtail effect in Nd123/422 composites suggest that the concentration of Nd - Ba antisite defects is minimized during the directional solidification process in air. Finally, it is demonstrated by TEM analysis that Nd123/422 ceramics show distinctive microstructural features compared with Y123/211 which could enhance the high-field critical currents and the irreversibility line. Frequency-dependent ac susceptibility studies provide evidence, however, that the thermal activation of vortices is enhanced in Nd123 as compared with Y123. It is suggested that incomplete oxygenation reduces the effective pinning energies.
Journal Article•
TL;DR: In this paper, the formation mechanism of twins, stacking faults, microcracks, subgrain boundaries and dislocations in Y 2 BaCuO 5 (211 phase) inclusions and modifications of the microstructure generated through prolonged oxygen annealings and high-temperature plastic deformation of the ceramic are reviewed.
Abstract: Directionally solidified YBa 2 Cu 3 O 7 -Y 2 BaCuO 5 composites display a complex microstructure with several defects coexisting in the matrix and hence the identification of the flux-pinning mechanisms requires a detailed analysis of the critical currents. In this work we review the formation mechanism of twins, stacking faults, microcracks, subgrain boundaries and dislocations in connection with the refining strategies of Y 2 BaCuO 5 (211 phase) inclusions and modifications of the microstructure generated through prolonged oxygen annealings and high-temperature plastic deformation of the ceramic. The influence of these defects and post-processing treatments on the critical currents is then analysed focusing on their relative importance as flux-pinning centres at different temperatures and magnetic fields up to 20 T. A complete magnetic phase diagram is finally proposed as a basis for a systematic classification of the observed irreversible superconducting behaviour in single-grain melt-textured YBa 2 Cu 3 O 7 -Y 2 BaCuO 5 composites.
TL;DR: In this article, the top-seeding melt texture growth (TSMTG) process is investigated in a smallvertical-thermal-gradient furnace in order to better understand the conditions for growing large single-grain pellets.
Abstract: The top-seeding melt texture growth (TSMTG) process is investigated in a small-vertical-thermal-gradient furnace in order to better understand the conditions for growing large single-grain pellets. An crystal was used as seed in a Y123 - Y211 composite with additions. It was shown that the role of the seed is to initiate the sympathetic nucleation growth mechanism. Indeed, the additional driving force induced by the seed is clearly demonstrated by a decrease in the undercooling of the system. The seeding crystal introduces a heterogeneous nucleation centre, leading to a very well controlled nucleation rate. Furthermore, we discuss the relation between intrinsic anisotropic growth rate and the processing rate, showing the importance of the thermal conditions of the texturing process to stabilize a sympathetic growth. By careful control of the texturing parameters, we succeeded in producing single-grain pellets as confirmed by a pole figure experiment and neutron diffraction measurements. The levitation force for pellets of 2 cm in diameter reaches 16 N when an NdFeB magnet of the same diameter is used.
TL;DR: In this paper, the commonly observed linear sections in the voltage -current characteristics of Ag-sheathed BSCCO-2212 and BSCCo-2223 composites were studied and the authors concluded that all ohmic voltages are produced by current flow in Ag.
Abstract: The commonly observed linear sections in the voltage - current characteristics of Ag-sheathed BSCCO-2212 and BSCCO-2223 composites were studied. One source of such characteristics is due to ohmic losses in the silver at the current feed points. Characteristic current transfer lengths are of the order of millimetres at 4.2 K and tenths of millimetres at 77 K. However, linear components can also be observed well away from current feed points. These were found to be associated with regions of locally reduced critical current, which provoke local transfer of the excess current into the Ag, thus producing local, ohmic V - I characteristics. Because of the finite current transfer length, some ohmic voltage can be detected even outside regions of reduced critical current. When the silver was removed from lower critical current regions, ohmic voltages were no longer observed, even when the critical current was severely reduced by local damage with a laser. Within the range of electric field covered by this experiment , we conclude that all ohmic voltages are produced by current flow in Ag.
TL;DR: In this paper, the formation mechanism of twins, stacking faults, microcracks, subgrain boundaries and dislocations in connection with refining strategies of (211 phase) inclusions and modifications of the microstructure generated through prolonged oxygen annealings and high-temperature plastic deformation of the ceramic.
Abstract: Directionally solidified composites display a complex microstructure with several defects coexisting in the matrix and hence the identification of the flux-pinning mechanisms requires a detailed analysis of the critical currents. In this work we review the formation mechanism of twins, stacking faults, microcracks, subgrain boundaries and dislocations in connection with the refining strategies of (211 phase) inclusions and modifications of the microstructure generated through prolonged oxygen annealings and high-temperature plastic deformation of the ceramic. The influence of these defects and post-processing treatments on the critical currents is then analysed focusing on their relative importance as flux-pinning centres at different temperatures and magnetic fields up to 20 T. A complete magnetic phase diagram is finally proposed as a basis for a systematic classification of the observed irreversible superconducting behaviour in single-grain melt-textured composites.
TL;DR: In this paper, the current voltage characteristics and magnetic field dependence of a range of mono-core BSCCO 2223 tapes are presented, illustrating the complementary use of transport and magnetization experiments in determining and analysing the current-limiting dissipation processes in these HTS conductors.
Abstract: The current - voltage characteristics and the magnetic field dependence of the critical current of a range of mono-core BSCCO 2223 tapes are presented, illustrating the complementary use of transport and magnetization experiments in determining and analysing the current-limiting dissipation processes in these HTS conductors. Below a magnetic cross-over field the samples resemble a Josephson-linked current network, with the dominant dissipation at the weakest grain boundaries. In this regime, increasing field leads to a gradual fragmentation of the network. The network homogeneity and connectivity can be inferred from screening current length-scale measurements and comparison of transport and magnetization measurements. Above the cross-over field , flux motion within the surviving strongly linked backbone dominates the dissipation. The details of the dependences of the critical current and flux creep rate on magnetic field can be used to examine the intragranular pinning potential in the tapes. We show that, despite the wide range of transport critical-current values of the samples examined, the intragranular pinning proves to be remarkably sample independent. We conclude that while at low fields may be increased by improved processing that yields better intergrain connectivity, the high-field can be enhanced only by strengthening the pinning within the BSCCO 2223 crystallites themselves.
TL;DR: Theoretical analysis of the critical currents in solenoids wound with anisotropic superconductors was performed in this article, where it was shown that applying the external magnetic field to the cylindrical coils which are made of HTS tapes may lead to some increase in their critical current.
Abstract: Theoretical analysis of the critical currents in solenoids wound with anisotropic superconductors was performed. The results of the numerical calculations indicate that applying the external magnetic field to the cylindrical coils which are made of anisotropic HTS tapes may lead to some increase in their critical current. Taking into account the angle dependence of the of short Bi(2223)Ag tape samples measured at 77 K we have shown how the external homogeneous magnetic field parallel to the axis of cylindrical symmetry influences the value of the coil's critical current. We also present the analysis of the influence of the external magnetic field on the change in position of weak turns in the winding. It is shown that determination of the magnet critical current represents mathematically the solution of a non-linear equation. The methods as well as the computer procedure enabling the evaluation of the coil's expected critical current and the location of the position of weak places in the winding are described. Further, the expressions that can be applied to the calculation of critical currents of the magnets, made of superconducting wires and tapes, whose characteristics are isotropic, were derived. The possibility of obtaining analytical solutions for simplified linear and for the general case of is also discussed.
TL;DR: In this article, the authors summarized the different loss contributions in superconducting composites, some possible sources for differences in AC losses and time constants in measuring arrangements and their expected values in real magnetic systems are summarized.
Abstract: After summarizing the different loss contributions in superconducting composites, some possible sources for differences in AC losses and time constants in measuring arrangements and their expected values in real magnetic systems are summarized: induced currents in end portions, field variations in time and space, mixed contributions from substructures and normal parts, longitudinal field effects and inductive coupling of current loops. Along with uncertainties in determining the effective resistivities and effective volumes for the current loops, all effects are important for evaluating the expected losses, time constants and discharging process in superconducting cables. The size effects can be calculated in most cases and incorporated into theoretical models. The spatially changing fields and coupling of current loops on the cable seem to be the most important effects causing differences in cable measurements with respect to the situation in different magnetic systems. These could be decisive for the design of large superconducting magnets with required stability and quench behaviour.
TL;DR: In this article, it has been shown that the symmetry of the order parameter in the high-cuprates is dominated by a -component and the implications of the unconventional symmetry on devices and on large-scale applications of high-superconductors are discussed.
Abstract: It has been shown that the symmetry of the order parameter in the high- cuprates is dominated by a -component. In this contribution, the implications of the unconventional symmetry on devices and on large-scale applications of high- superconductors are discussed.
TL;DR: In this paper, a new rectangular configuration of the filaments conceived in order to reduce the effect of anisotropy on the transport properties at the liquid nitrogen temperature has been presented.
Abstract: Ag-sheathed multifilamentary Bi(2223) wires have been prepared with a new rectangular configuration of the filaments conceived in order to reduce the effect of anisotropy on the transport properties at the liquid nitrogen temperature. At present, critical current densities at 77 K of 20.5 kA have been achieved on rectangular rolled wires of cross-section. The transport properties have been measured as a function of the applied field and of its orientation with respect to the wire. The new rectangular wires have shown a clearly reduced anisotropy compared to that of monofilamentary Bi(2223) tapes with a value of 34.5 kA , which have been measured as a reference. With respect to standard tapes, where vanishes at a magnetic field of 0.5 T applied along the tape normal, the newly developed wires show considerable advantages: regardless of the orientation of the field, of rectangular wires vanishes at fields exceeding 0.8 T at 77 K.
TL;DR: In this article, a series of films were deposited on YSZ substrates by the on-axis pulsed-laser ablation technique, and various analytical techniques, including x-ray diffraction, scanning electron microscopy, micro-Raman spectroscopy and four-point probe method, were applied to characterize the films.
Abstract: In applications of superconducting devices, the crystalline texture of high-quality thin films is of primary importance. The preferred orientation of the films can be essentially controlled by means of the substrate temperature, . In order to study the dependence of the film texture on different , a series of films were deposited on YSZ substrates by the on-axis pulsed-laser ablation technique. The substrate temperature was varied from to while the rest of the growth parameters remained the same. Various analytical techniques, including x-ray diffraction, scanning electron microscopy, micro-Raman spectroscopy and the four-point probe method, were applied to characterize the films. At around , the best films with the smoothest surface morphology, lowest FWHM of the rocking curve and highest in-plane texture were obtained. The resultant in excess of at 77 K and around 91 K were also achieved at this temperature.
TL;DR: In this article, a multi-core Ag-sheathed Bi(2223) tapes have been prepared by OPIT technique using the two-axial rolling deformation by active Turk's head roller.
Abstract: Multicore Ag-sheathed Bi(2223) tapes have been prepared by OPIT technique using the two-axial rolling deformation by active `Turk's head' roller. In comparison to wire drawing, the presented technique allows one to avoid the tensile stress during composite deformation and to increase the BSCCO-core density. A better homogeneity of filaments has been obtained by two-axial rolling. The transport critical current and its angular dependencies have been measured for single and 36 filament rectangular wires and tapes. Total tape widening influences the transport current density considerably but anisotropy at 0.1 T and 77 K is affected only slightly. Reduced anisotropy has been measured in a single core wire which has texturing of grains in two perpendicular rolling planes.
TL;DR: In this article, it was shown that oxyanion groups can also be located on some cationic sites to provide a further means of controlling the electronic properties of the superconducting regions.
Abstract: After the discovery of high- superconductivity in multinary cuprates, a large number of different superconductors have been synthesized by varying the chemical composition and structural features of the insulating layers which connect the superconducting Cu - O planes. Although experimental studies initially focused on aliovalent cationic substitutions in these layers, it was subsequently discovered that oxyanion groups - - can also be located on some cationic sites to provide a further means of controlling the electronic properties of the superconducting regions. Even more recent developments have allowed the study of oxide - halide superconductors, synthesized by `soft chemistry' or high pressure techniques aimed at overcoming the thermodynamic instability of most of these materials. Superconductivity in these new families of oxide halides is of particular interest since critical temperatures appear to be at least as high as oxide analogues, and it provides a valuable guide to the importance of structural features, e.g. the apical anion, for superconducting properties.
TL;DR: In this article, a number of techniques by which effective pinning centers can be introduced to improve flux pinning in Bi - (Pb) - Sr - Ca - Cu - O HTSs are reviewed.
Abstract: 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.
TL;DR: In this article, a solenoid made of Bi(2223)Ag tape with a high anisotropy in the critical current -magnetic field characteristic at 77 K is determined in particular by the radial component of the magnetic field, which is large close to the coil flanges.
Abstract: The critical current of a solenoid made of Bi(2223)Ag tape with a high anisotropy in the critical current - magnetic field characteristic at 77 K is determined in particular by the radial component of the magnetic field, which is large close to the coil flanges. The original idea of utilizing supplementary (e.g. copper) windings to compensate partially the undesired radial magnetic field component of the high-temperature superconducting (HTS) coil is described. The results of extensive numerical calculations on the model coil indicate that an increase of the critical current of more than 50% can be expected.
TL;DR: In this paper, a method that leads to the Bi-2212 phase after a short thermal treatment of metal acetates is presented, where the thermal treatment consists in a calcination (2 h at + 2 h at ), pressing and sintering (1 or 3 h at ).
Abstract: A method that leads to the Bi-2212 phase after a short thermal treatment of metal acetates is presented. The thermal treatment consists in a calcination (2 h at + 2 h at ), pressing and sintering (1 or 3 h at ). TGA and DTA provide valuable data about the decomposition of the polymer and metal - polymer complex and the formation and decomposition of the carbonates in the intermediate compounds. In contrast with the most common sol - gel methods, the use of polyethylenimine yields relatively large particles (ca ).
TL;DR: In this paper, it is demonstrated that (NBCO) thin films, deposited by laser ablation, show extraordinary properties which are unprecedented in the 123 system, such as ease of oxidation, very high crystallinity, surface stability, and a relatively high.
Abstract: It is demonstrated that (NBCO) thin films, deposited by laser ablation, show extraordinary properties which are unprecedented in the 123 system. These include ease of oxidation, very high crystallinity, surface stability, and a relatively high . The properties of NBCO thin films are compared with those of the best reported films, and the origins of their differences are discussed. The implications of NBCO's superior thin-film quality in the context of device applications are also addressed throughout the paper.
TL;DR: In this article, the authors measured the transport critical currents at and exponents n of the power law characteristics of single-filament Bi(2223)/Ag tapes at 77 K in the self-field and in external magnetic fields up to 1.2 T.
Abstract: Transport critical currents at and exponents n of the power law characteristics of single-filament Bi(2223)/Ag tapes were measured at 77 K in the self-field and in external magnetic fields up to 1.2 T. The effects of tape orientation in a transverse magnetic field from to (related to the basic state at -axis) were investigated as well. The values of the transport critical current and exponent n as well their mutual relation reflect the quality of the Josephson-linked current network connectivity at ( is the cross-over field at which the weak-link currents cease to contribute to the transport current) and the ability of pinning centres to pin flux lines in the whole magnetic field region. The weak-link connectivity and the pinning potential are directly related to the structural state of the samples resulting from their different thermomechanical treatments. The better the structural quality, the higher the ratio. This ratio seems to reflect the effective number of intergrain connections and pinning centres.
TL;DR: In this paper, the authors discuss the development of high-temperature superconductors in terms of their physical basis and illustrated with selected examples of filters, stable oscillators and MRI antennas.
Abstract: The technology of high-temperature superconductors has become mature. Various methods to analyse and optimize the temperature and microwave field dependent surface impedance have been developed. Promising applications of passive microwave devices have been identified between 0.01 and 10 GHz. This progress profits from improved design and simulation tools, aimed at miniaturizing the circuits at optimum performance and integrating them into realistic systems. Important facets of these developments are sketched in terms of their physical basis and illustrated with selected examples of filters, stable oscillators and MRI antennas.
TL;DR: In this paper, the authors investigated the influence of different heat treatment steps on the critical current density of the films, and showed that the critical density strongly depends on the sample thickness.
Abstract: (Bi-2212) thick films were produced via tape casting and partial melting. The aim of the study was to investigate the influence of the different heat treatment steps on the critical current density of the films. Five processing parameters were studied: maximum densification temperature, cooling rate during crystallization, annealing time after crystallization, reduction treatment and processing atmosphere. It will also be demonstrated that the critical current density strongly depends on the sample thickness. In thick films we achieved at 77 K - 0 T and at 10 K - 0 T. The critical current density at 77 K - 0 T dropped to in thick films and levelled out at in bulk samples thicker than . These high critical current densities were reached only using a narrow processing window. The maximum densification temperature had to be within above the solidus temperature . Cooling from the maximum temperature to an annealing temperature of had to be around 5 to and the final annealing step was prolonged up to 70 h to optimize the critical current density. All processing steps were carried out in pure oxygen (1 atm) except the last step, reduction annealing at for 20 h, that was performed in nitrogen (). Processing in air () instead of oxygen leads to strongly decreased critical current densities in the high-temperature region above 30 K.