The fault current limiting characteristics of a flux-lock type high-Tc superconducting fault current limiter using series resonance

We analyzed the fault current limiting characteristics of a flux-lock type high-T_c superconducting fault current limiter (HTSC-FCL) using series resonance between capacitor for series resonance and magnetic field coil which was installed in coil 3. The capacitor for the series resonance in the flux-lock type HTSC-FCL was inserted in series with the magnetic field coil to apply enough magnetic field into HTSC element, which resulted in higher resistance of HTSC element.However, the impedance of the flux-lock type HTSC-FCL has started to decrease since the current of coil 3 exceeded one of coil 2 after a fault accident. The decrease in the impedance of the FCL causes the line current to increase and, if continues, the capacitor for the series resonance to be destructed. To avoid this operation, the flux-lock type HTSC-FCL requires an additional device such as fault current interrupter or control circuit for magnetic field.This paper investigated the parameter range where the operation as mentioned above for the designed flux-lock type HTSC-FCL using series resonance occurred from the experimental results. In the design of the flux-lock type HTSC-FCL, the some methods to avoid the continuous increase of the line current were suggested and confirmed by the experiments that the suggested methods were available to prevent the continuous increase of the line current after a fault happened.     

Magnetic field enhancement of high-Tc superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T _c superconducting coils. The central magnetic fieldB ₀, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB ₀ for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB ₀ value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B ₀ was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T _c superconducting coils and magnets.     

High-pressure dc sputtering of High-Tc BPSCCO thin films withTc(0) above 100 K

High-T _c Bi(Pb)-Sr-Ca-Cu-O thin films have been made on single-crystal MgO substrates using high-pressure dc sputtering technique. X-ray studies confirm the crystallinity and highly oriented structure withc-axis perpendicular to the substrate. By optimizing the annealing schedule the formation of the high-T _c phase is stabilized. The best film exhibited superconducting transition temperature with zero-resistance temperature,T _c(0), as high as 101 K. Temperature dependence ofJ _c indicates the presence of Josephson-type weak links.     

Evaluation of loss of current leads for HTS power apparatuses

This paper evaluates the losses of current leads made of various metals used in high-T_c superconducting power apparatuses operating at liquid nitrogen temperatures. The heat flow into liquid nitrogen conducting adiabatic current leads was measured by the nitrogen boil-off method. The results were used to derive average Lorenz numbers for metals between room temperature and liquid nitrogen temperatures. The average Lorenz number is a good index of performance of current leads. A theory of loss evaluation of current leads using the average Lorenz number and a method for the optimum current lead design were described as well. In addition, the loss of current leads made of Bi2223/Ag tape conductors was evaluated above the liquid nitrogen temperature, and a higher efficiency was achieved than with pure metal current leads.     

Extended Domain of Existence for PSCs in Superconductors

By means of nanosecond pulse techniques, we have studied the current-induced dissipation in one-dimensional superconducting bridges, namely, metallic and high-T _c films. It is well known that narrow strips dissipate through phase-slip centers (PSC) close to T _c, or hot spots (HS) at low T, rather than by flux flow. When driven by step pulses of current, PSC give stable voltages, while HS produce a voltage linearly changing with time. By using two-step pulses of current, we have studied the decay of a HS into another HS, or a PSC, or into a zero-resistance state. It was thus found possible to reach the PSC state at arbitrary low temperatures.     

Simple and inexpensive technique for measuring the transition temperature of superconducting thick films from 60 to 300 K using liquid nitrogen and subcooled liquid oxygen

This paper describes a simple and low-cost experimental setup intended for AC susceptibility measurements between 60 and 300 K. A mutual inductance bridge for measuring the transition temperature of high-T_c superconductor thick films was constructed. The device is based on using liquid nitrogen and subcooled liquid oxygen; it allows temperatures down to 60 K to be achieved. Experimental details are described and illustrative measurements on high-T_c superconductor thick films are included.     

Effect of Bi/Pb ratio and annealing temperature onTc and formation of high-Tc phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onT _c and formation of the high-T _c ; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.80.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-T _c phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-T _c phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.     

Review of Applications of High-Temperature Superconductors

Since the discovery of high-T _c superconductor oxides in 1986, much research and development have been carried out, and much progress has been made. In the last ten years our efforts have been devoted to the development of materials technologies for these difficult materials, and remarkable progress has been made. This is a great contribution not only for application but also for fundamental research on high-T _c superconductors. In this paper, we will present a review of applications of high-T _c superconductors discovered in the last ten years. At present, it can be said that we are in the transition period from the period of growth to the period of specialization, looking for future applications of high-T _c superconductivity.     

Preparation of YBa2Cu3O7–x high-Tcceramic superconductors from melt

Fabrication of high-T _c ceramic superconductor in the system Y₂O₃-BaO-CuO by melting a mixture of component oxides has been investigated. The compositions of the resulting specimens and the effects of heat treatment have been investigated. It was determined that molten material was composed of phases including BaCuO₂, CuO, Y₂O₃, and Y₂BaCuO₅. A subsequent heat treatment in air produced a nominal amount of the high-T _c phase, while heat treatment in an O₂ atmosphere resulted in a significantly large percentage of the superconducting phase.     

Current distribution measurement in YBCO thin film for a superconducting fault current limiter

Current distribution in the superconducting film for a resistive fault current limiter is important, because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3 times of critical current (I_c). (2) The current in a superconducting film was uniform when the current was much higher than I_c. The current can be considered uniform when the film works as a fault current limiter, because the S/N transition starts about twice of I_c. (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.     

Characteristics of critical current of superconducting solenoid wound with the stacked tape

The high-T_c superconducting (HTS) magnet is an important element for developing HTS power equipments such as the dc reactor of the inductive type superconducting fault current limiter (SFCL). In order to use the HTS magnet for the large-scale power system, its critical current needs to be high enough. Generally, the double pancake HTS magnet has the severe decrease in the critical current because of magnetic field perpendicular to the tape surface. To fabricate a high critical current magnet, we wound a solenoid with the stacked tape. In this paper, the characteristics of the critical current of the HTS solenoid wound with the stacked tape were investigated. The results of this research can be used as the background data for the design of the large-scale HTS magnet.     

Electrostatic bonding of high-Tcsuperconducting films to any substrate

Complete integration of high-T _c thin-film superconducting technology with other electronic technologies requires hybrid structures with both technologies on the same substrate. This is difficult to do with direct growth of the superconductor on certain substrates (GaAs, InP) because of the high temperatures required for formation of the superconductor. A method is proposed to circumvent this problem by using electrostatic forces and appropriate thin-film materials to bond superconducting films to any substrate at 300°C. The same principle can be applied to the bonding of other devices on other substrates.     

Reduction of alternating current loss in a high-TC superconducting transmission cable by means of helical pitch adjustment

We have reported that the alternating current (ac) losses in a 66 kV_rms 3-core high-T_C superconducting (HTS) transmission cable fabricated by Tokyo Electric Power Company and Sumitomo Electric Industries Ltd. are calculated correctly by using an electric-circuit model. According to the calculated results, the circumferential field losses are dominant in the total ac losses in compared with the self-field losses and the axial-field losses. The helical pitches of each layer in the HTS cable are designed to obtain almost same layer currents, which gives the minimum self-field losses. We think that the optimum helical pitches giving the minimum total losses are different from the helical pitches designed by the companies and calculate the optimum values in the condition of the same helical direction of each layer in the cable. As a result, for example, it is found that the ac loss of 2.1 W m⁻¹ cc⁻¹ at transporting 1 kA_rms can be reduced to 1.8 W m⁻¹ cc⁻¹ (about 14% reductions) after redesigning the cable with the optimum helical pitches. The optimum helical pitches are obtained for each given transport current. After redesigning, the distribution of layer currents is not uniform and the circumferential fields are reduced.     

Superconductivity in epitaxial Bi2Sr2CuO6/Bi2Sr2CaCu2O8 superlattices: The superconducting behavior of ultrathin cuprate slabs

Utilizing atomic layer-by-layer molecular beam epitaxy (ALL-MBE), we have synthesized a series of high-quality superlattices in which ultrathin slabs (one-half unit cell thick) of the high-T _c superconductor Bi₂Sr₂CaCu₂O₈ alternate with up to five such layers of the low-T _c Bi₂Sr₂Cu₁O₆ phase. In all these superlattices we foundT _c to be essentially equal to that of the high-T _c Bi₂Sr₂CaCu₂O₈ phase itself, which indicates that this cuprate is a 2D superconductor insofar as the interslab coupling plays at best a secondary role. Furthermore, it is demonstrated thatT _c need not be reduced at heterostructure interfaces.     

Knight Shift Data and Superconducting Tc Modeling Based on an Effective Hole Model

The superconducting T _c of cuprates have been modeled on a linear scaling with hole concentration per CuO₂ plane and a deleterious influence of bond resonance with the apical system (effective hole formalism). In cases where distribution between various hole reservoirs is not trivial, Knight shift can provide actual hole concentrations. It is shown here that when Knight shift data are used in an effective hole algorithm, satisfactory T _c predictions can be made, corroborating the deleterious influence on T _c of apical O and earlier assumptions concerning hole distributions. For the case of stacking of more than two CuO₂ planes, the inner plane has to be treated as an infinite layer analog in the effective hole model. A separation into inner and outer planes with different dopings is indeed observed by Knight shift, with higher doping in the latter. This is explained here as being due to a tendency to equalize an effective doping degree, as the outer planes lose holes in resonance of bonding with the apical system.     

Video detection of microwaves using high-Tc Y-Ba-Cu-O thin films

Microwave video detection (demodulation) studies were carried out on high-T _c thin films at 64 K. The amplitudes of the video detection signal varies linearly with incident microwave power. The microwave response shows a peak at an optimum bias current. The shape analysis of the demodulated signal indicates the nonbolometric nature of the detector.     

On the characteristic voltage of high-Tc superconductors

The critical currents and normal resistances of the small bridges from yttrium-based high-T _c superconducting ceramics have been measured. The characteristic voltage of these bridges was found to be approximately 20 V. This effect can be explained if, between the ceramic grains, there are contacts of an order of one crystalline cell in size.The authors are grateful to V. N. Polushkin for his help in this investigation.     

High-temperature superconducting microwave devices: Fundamental issues in materials, physics, and engineering

High-T _c superconductivity has generated a great deal of interest because of the challenges it presents in the fields of material science, condensed matter physics, and electrical engineering, and because of the potential applications which may result from these research efforts. Thin-film passive microwave components may become the first high-temperature superconducting (HTS) devices available for widespread use and commercialization. In this article, we review aspects of material science, physics, and engineering which directly impact high-T _c superconducting microwave devices and discuss issues which determine the performance of these devices. Methods of growing HTS thin films on large-area substrates, techniques for fabricating single-level HTS passive microwave components, and the relevant properties of high-T _c superconducting films are discussed, with a focus on thin films of the HTS material YBa₂Cu₃O_7–. Several known mechanisms for microwave loss in both the superconductor and the dielectric substrate are presented. An overview of the general classes of superconducting passive microwave devices is given, and representative microwave devices which have been recently demonstrated are described in detail. Examples of a select few HTS active microwave components are also presented. Potential microwave applications are illustrated with comparisons to current technology.     

Effect of Interlayer Interaction on the Specific Heat of High-Tc Superconductors

The effect of interlayer interactions on the electronic specific heat of high-T _c layered superconductors has been studied within the framework of the Hirsch model. In the present paper we extend our previous work in order to evaluate the expression for specific heat. It is shown that the inclusion of interlayer interactions suppresses the height of the jump in the specific heat at T _c.