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YBa₂C₃O 7-x films were fabricated on a SrTiO3 (100) substrate using a trimethylaceate propionic acid (TMAP)-based MOD process by controlling the precursor solution viscosity, firing temperature, and by using various coatings. The viscosity of the precursor solution was controlled by the addition of Xylenes. The films were heat treated with different temperatures from 750 to 800 ℃. c-axis oriented films were obtained. After adding 9 ㎖ of Xylene into the precursor solution, the T c of the YBCO film, which was coated 2 times and heat treated at 800 ℃, was 86 K and the measured J c was above 2.5 MA/㎠ at 77 K in a zero-field.

Performance of a resistive superconducting fault current limiter (SFCL) component is usually limited by temperature rise associated with energy input by fault current application during a fault. Therefore, it is expected that short application of the fault current may enhance the power ratings of the component. This can be accomplished by a combination of a HTS component and a mechanical switch. The fast switch (FS) developed recently enables the fault duration to be as short as 1/2 cycle after a fault. Various second-generation (2G) high temperature superconductors (HTS) and YBCO thin films have been tested. The relation between the rated voltage V and the fault duration time t was found to be V 2 ∼ t − 1 . Based upon the relation, we predict that when the FS break the fault current within 1/2 cycle after a fault, the amount of HTS components required to build an SFCL can be reduced by as much as about 60%, of that when breaking the fault current at three cycles.

We have constructed and completed the preparation for a long-term operation test of a superconducting fault current limiter (SFCL) in a Korea Electric Power Corporation (KEPCO) test grid. The SFCL with rating of 22.9 kV/630 A, 3-phases, has been connected to the 22.9 kV test grid equipped with reclosers and other protection devices in Gochang Power Testing Center of KEPCO. The main goals of the test are the verification of SFCL performance and protection coordination studies. A line-commutation type SFCL was fabricated and installed for this project, and the superconducting components were cooled by a cryo-cooler to 77 K in the sub-cooled liquid nitrogen pressurized by 3 bar of helium gas. The verification test includes un-manned – long-term operation with and without loads and fault tests. Since the test site is 170 km away from the laboratory, we will adopt the un-manned operation with real-time remote monitoring and controlling using high speed internet. For the fault tests, we will apply fault currents up to around 8 kArms to the SFCL using an artificial fault generator. The fault tests may allow us not only to confirm the current limiting capability of the SFCL, but also to adjust the SFCL – recloser coordination such as resetting over-current relay parameters. This paper describes the construction of the testing facilities and discusses the plans for the verification tests.

The stabilizer of a YBCO coated conductor (CC) plays a very important role in bypassing the over-current and transferring the heat generated at the moment of a fault. Therefore, one of big issues is to determine the best material for the stabilizer and its dimensions for high performance in a HTS power application system. In the case of a superconducting fault current limiter (SFCL), which requires it to react immediately to any fault, the characteristics of the stabilizer are decisive in limiting the fault current and recovering the superconducting properties during and after quenching. This study examined the electrical and thermal behavior of YBCO CCs with various stabilizer thicknesses in fault mode to determine the effects of the stabilizer thickness on the quench/recovery characteristics of YBCO CCs. In particular, the electric field intensity (E) of YBCO CCs in fault mode were calculated and compared with the experimental results.

In the first-peak non-limiting type superconductor-triggered fault current limiter (STFCL), which was developed by the collaborative work of KEPRI and LSIS, the first half cycle of fault currents is passed, unlimited. Due to these characteristics, information needed for the fault judgments can be provided to the protection relays, which is useful for the protective coordination of power system. In this fault current limiter, superconductors are used as a fault detector that does not generate resistive loss in normal operation state. Therefore, high J c and fast occurrence of large resistance are the essential properties of superconductors for the application to fault detectors. In this study, for the design of superconducting fault detector (SFD), we investigated the quench behavior of two kinds of superconductors, stabilizer-free coated conductors (SF-CC) and Au/YBCO/sapphire substrate, during the operation of the first-peak non-limiting type STFCL. The dimension of SF-CC was 12 mm wide × 1000 mm long and that of Au/YBCO/sapphire was 20 mm wide × 100 mm long. Their critical current ( I c ) values were 250 A and 200 A, respectively, and the critical temperature ( T c ) was both 90 K for both superconductors. When fault currents of over 3 kA were applied, the STFCL carried out the current limitation successfully. At the beginning of the quench, SF-CC generated a resistance of 150 mΩ, and 394 mΩ was generated in the Au/YBCO/sapphire substrate, when 150 V rms was applied. After the first half cycle, the resistance of the superconductors increased up to 263 mΩ, and 846 mΩ, respectively, and the temperature reached 132 K and 127 K. In addition, we also investigated the recovery characteristics of the SFDs. The SF-CC and Au/YBCO/sapphire substrate recovered the superconductivity by 353 ms and 357 ms, respectively. Based on the results of the analysis, we determined the required length of the superconductors for the SFD application.

Resistive superconducting fault current limiters (SFCL's) are assumed to be a prospective solution for commercialization. But in spite of excellent current limiting performances realized by resistive SFCL's, commercialization and installation of SFCL's have been delayed due to difficulties in overcoming several technical problems such as a coordination with conventional relays. Resistive SFCL's may have difficulty in controlling an initiation of current limiting operation that sometimes is very important for coordination with relays, because the superconductor in the SFCL will quench immediately after the fault current reaches its critical current. In order to solve these problems, a novel hybrid SFCL of the first half cycle non-limiting type was developed. This paper presents the characteristics of this hybrid SFCL, its configuration and test results.

We fabricated BSCCO-2212 (2212) rods using a casting process and evaluated the effects of annealing time and size to improve critical current ( I c ). Diameters of rods were 1.0 cm and 1.6 cm and annealing was performed from 60 h to 200 h at 800 °C. The I c of rods increased with increasing annealing time, probably due to increased grain size of 2212. Annealing time of the highest I c for the smaller rod was shorter (150 h) than that of the larger rod (200 h). This size effect seems to be related to different grain sizes of the intermediate phases such as 2201 and secondary phases in the ingot. In addition, we successively fabricated 2212 tubes from the rod by making a hole in the center region which contained inhomogeneous microstructures. This method was simple and I c did not significantly decrease. However, critical current density ( J c ) improved when as much as 60% of the diameter in the center region was machined out of the rod.

We have designed and tested a superconducting fault detector (SFD) for a 22.9 kV superconductor triggered fault current limiters (STFCLs) using Au/YBCO thin films. The SFD is to detect a fault and commutate the current from the primary path to the secondary path of the STFCL. First, quench characteristics of the Au/YBCO thin films were investigated for various faults having different fault duration. The rated voltage of the Au/YBCO thin films was determined from the results, considering the stability of the Au/YBCO elements. Second, the recovery time to superconductivity after quench was measured in each fault case. In addition, the dependence of the recovery characteristics on numbers and dimension of Au/YBCO elements were investigated. Based on the results, a SFD was designed, fabricated and tested. The SFD successfully detected a fault current and carried out the line commutation. Its recovery time was confirmed to be less than 0.5 s, satisfying the reclosing scheme in the Korea Electric Power Corporation (KEPCO)’s power grid.

In order to apply resistive superconducting fault current limiters into electric power systems, the urgent issues to be settled are as follows, such as initial installation price of SFCL, operation and maintenance cost due to ac loss of superconductor and the life of cryostat, and high voltage and high current problems. The ac loss and high cost of superconductor and cryostat system are main bottlenecks for real application. Furthermore in order to increase voltage and current ratings of SFCL, a lot of superconductor components should be connected in series and parallel which resulted in extreme high cost. In addition, the method to quench all components at the same instant needs very sophisticated skill and careful operation. Due to these problems, the practical applications of SFCL were pending. Therefore, in order to make practical SFCL, the price of SFCL should be lowered and should meet the demand of utilities. LSIS and KEPRI designed novel hybrid SFCL which combines superconductor and conventional electric equipment including vacuum interrupter, power fuse and current limiting reactor. The main purpose of hybrid SFCL is to drastically reduce total usage of superconductor by adopting current commutation method by use of superconductor and ultra fast switch. Consequently, it was possible to get the satisfactory test results using this method, and further works for field tests are in the process.

BSCCO tubes can be used as a base material for switching devices such as superconducting fault current limiters (SFCLs) that prevent an electrical problem from occurring in an electrical power system. To apply an BSCCO bulk tube to a switching device, the superconducting tube has to be joined with a metallic part to by the over current to the metal part when the FCL is quenched. In this study, joining between Cu–Ni alloy and BSCCO was accomplished by soldering using In–Sn and In–Bi solders. Additionally, an Sn–Ag–Cu/In–Bi solder was used for the soldering of a different kind. For a better joining of the BSCCO superconductor with the In–Bi solder, the surface of the BSCCO was pre-coated with Ag by electro-plating. From the experiments, an intermetallic compound (IMC) of AgxIny chain was observed to be mainly formed from In–Sn and In–Bi soldering process. In case of the soldering of a different kind, IMC of AgxIny and CuxSny was also developed. Finally, we confirmed that the properties of soldering were enhanced by Sn–Ag–Cu/In–Bi twice-soldering process.

Several verification tests are being underway world widely in order to apply HTS cable to real grid. KEPCO (Korea Electric Power Corporation) has initiated HTS cable project since 2002 with the aid of MOCIE (Ministry of Commerce, Industry and Energy), Korea. The HTS cable system with the specification of three-phase, 1.25 kArms, 22.9 kV and 100 m was installed at the KEPCO’s Gochang test yard. In consideration of economical efficiency on retrofit for typical underground cable, the HTS cable was designed to be applicable for 175 mm ELP duct and 55 m-long cable was laid on underground trench in the same manner as the conventional pulling method. Cool-down to LN2 temperature and warm-up to room temperature were performed several times to investigate the influence of thermal cycle. The main objectives of this project are to verify performance of the HTS cable system and evaluate the economical and technical feasibility from all angles. Expected pros and cons of HTS cable including reliability and maintainability will be assessed with the view point of utilities. This paper summarizes some results of installation and after-laying tests.

In this study, the Bi 2212 superconductor and Cu–Ni metal were soldered with In–Bi alloy at low temperature. In order to improve the contact properties of the solder Cu/Ag precoating layers were applied on the surface of the superconductor by electroplating process. The shear strength of the contact was correlated with the electroplating current density and the Ag layer thickness. The strength increased as the electroplating current density and the Ag layer thickness increased. In the case of Cu/Ag multi-precoating, the contact strength was higher than that of the single Ag precoating layer case. The contact strength also increased when the Ag precoated layer was annealed at the temperature range of 500 °C due to better connectivity at the interface layers.

Although, a superconducting fault current limiter (SFCL) guarantees the fast limiting operation, it usually needs a considerably long time to recover to superconducting state after the quench. Considering the reclosing time in the protection coordination of power systems, the time required for the recovery should be investigated clearly. In this study, the quench and recovery characteristics of Au/YBCO thin films designed as an SFCL element with a bi-spiral pattern were investigated. The quench development of the SFCL was measured by two kinds of methods. Firstly, after applying the fault current of 5.5 cycles, we measured the resistance of the YBCO by a small current flowing through the pattern of Au/YBCO thin film. The temperature variation above the critical temperature, 85 K, was investigated indirectly from the resistance variation. Secondly, in order to measure the temperature from 85 K to 77 K, a meander line shape of Au thin film was evaporated on the back side and used as a temperature detecting sensor. The temperature variations detected by both methods were compared and analyzed. For the investigation of the recovery characteristics, the required time for the recovery of the superconductivity was measured for various magnitude and duration of the applied voltages. In addition, for the purpose of examining the dependence of the line impedance on the recovery time, resistors of various resistances were inserted in the fault current testing circuit and the recovery time was measured and analyzed.

As high temperature superconducting (HTS) power cables have some merits over conventional cables, several demonstration projects on the HTS cable system are presently under way around the world. Korea Electric Power Corporation (KEPCO) also initiated an HTS cable project in 2002 with the Korean government's support. A three phase 100 m HTS cable system with a capacity of 50 MVA has been installed at Gochang test yard, located in Chonnbuk province, Korea. The HTS cable system is composed of a 100 m-long cable, two terminations and a cooling system. The rated current is 1,250 Arms and the rated voltage is 22.9 kV considering compatibility with the conventional power distribution system in Korea. Main purposes of this project are to verify the performance of an HTS cable system and to evaluate the potential of the HTS cable system from the viewpoint of power utilities. The real grid application of the HTS cable system requires the demonstration of system reliability, accumulated operation experiences, and it has to meet the practical needs of the utilities. In such a meaning, this project provides various challenges for KEPCO, and the feedback will be delivered to cable manufacturers. This user initiative test will facilitate the introduction of HTS cable systems into a real grid network. The installation process of the HTS cable system and some results of the preliminary test were presented in this paper.

We present the fabrication and short circuit test results of a 14 kV single-phase resistive superconducting fault current limiter (SFCL) based on YBa 2 Cu 3 O 7 (YBCO) films. Individual components were processed using the 4″ YBCO films and have the rated voltage and current of 600 V and 35 A at 77 K, respectively. Twenty four components, eight components in series and three lines in parallel, make a module having the rated voltage and current of 4.8 kV and 105 A, respectively. Three modules were assembled in series to produce the SFCL working at 77 K, a 14 kV single-phase machine for the 22.9 kV Y–Y grid. short circuit tests were successfully conducted in an accredited test facility with the maximum fault currents up to 14.1 kA P . All components quenched together upon faults and shared the rated voltage evenly without any supplementary device between the modules. This proves that the SFCL based on YBCO films may not only work reliably at 22.9 kV, but also provide technical feasibility for higher voltage application including the transmission grids.

We have been developing a magnetic field measurement system with two-axis hall-probes to study the critical current density distribution or magnetic properties of high- T c superconductors. In this study, the current distributions at corners of the meander type resistive fault current limiters were measured by hall probe with high spatial resolution. The both of axial and radial magnetic field components ( B x and B z ) according to the direct (DC) and alternating (AC) transport currents were measured by hall probe to estimate the current density distribution with high accuracy. And, it is confirmed that the measured radial components are very useful to estimate the current distribution.

We fabricated 2212-SrSO 4 composites by the melt casting process and evaluated the effect of melt flowing on the critical current ( I c ) by using two different pouring methods, i.e., the conventional vertical casting and the tilt casting methods. It was observed that the 2212-SrSO 4 rod processed by the tilt casting method had a higher I c value than that processed by the vertical casting method. This improved I c is considered to be related to the more homogenous microstructure due to the laminar flow of the melt. In addition, we evaluated the variation of the I c with the SrSO 4 content. The I c value was found to be significantly dependent on the SrSO 4 content: the I c of the 2212 increased as the SrSO 4 content increased and reached a peak value (260 A at 77 K) at an SrSO 4 content of 6 wt.%, and then decreased when the SrSO 4 content was further increased. This increase is probably due to the lower porosity, larger grain size and stronger texture, resulting from the enhanced diffusion kinetics. The possible causes of the variation in the I c with the SrSO 4 content based on the microstructural observations were discussed in detail.

Based on the electrical behavior of Bi-2223/Ag tape in transient state, the possibility of SFCL application was investigated as a fundamental study for the protection of HTS power apparatus. For the experiments, the Bi-2223/Ag tape having the critical current of 56 A and the resistive type SFCL fabricated using three elements of YBCO thin films were examined. When the applied current exceeds the rated current of the SFCL, the current was limited to sufficiently low level, however, higher than 40 Vrms was not allowed considering power density of SFCL. On the other hand, as a method to increase the power density, a shunt resister across the SFCL was adopted to increase the allowed power density. In this case, the results showed that the over current was limited below the safe range for HTS tape and the SFCL was safe, although 100 Vrms was applied to the circuit. In conclusion, the switching power capacity of a SFCL could be increased by using a shunt resistor, and HTS tape could be protected safely within the quench margin

Starting from investigation of feasibility and basic studies, KEPCO/KEPRI has carried an enterprising project for HTS cable system since 2002. The objectives of this project are to demonstrate HTS cable system and evaluate the feasibility in electricity utility's view. The installation of 100m/3phase/22.9kV/50MVA HTS cable system is undergoing in KEPCO's test yard, located in Gochang, Korea. The HTS cable system consists of 100m long cable, two terminations and a cooling system. To simulate the actual cable installation, the 55m section of the 100m long cable was installed into an underground tunnel. Commissioning test is scheduled at the end of September, 2005 and followed by long term operation test. In this paper, the installation of HTS cable system and initial results of system operation will be summarized

We investigated temperature distribution in SFCLs based on Au/YBa 2 Cu 3 O 7 (YBCO). SFCLs were fabricated by patterning Au/YBCO thin films grown on sapphire substrates into meander lines by photolithography. A gold film grown on the back side of the substrate was patterned into a meander line, and used as a thermometer. The front meander line was subjected to simulated AC fault currents, and the back line to DC current. Resistance of the front and back meander lines were measured and analyzed. The SFCLs were immersed in liquid nitrogen during the experiment for effective cooling. The temperature at the back side was close to that at the front side, and was closer at lower temperatures. This was observed at all stripes. The oscillatory component of the resistance of the back meander line is smaller than, and out-of-phase with that of the front meander line, which was more pronounced at higher temperatures. These results were analyzed quantitatively with the concept of heat transfer within the SFCL and to surroundings. Solutions for a heat equation explained the temperature distribution in SFCLs quantitatively: data coincided well with the solutions. In addition, quench development near the quench start point could be understood better than before, using the results.

Korea's superconductivity technology in the power sector has progressed rapidly since the launch of the 21st Century Frontier R&D Program for superconductivity. Under this program, large-scale active research has been conducted. The program supports HTS cable, motors, transformers, fault current limiters, and basic materials for 2G wire development. Current technology is somewhat less advanced compared to developed countries such as the USA, Japan and Europe. It is expected that the efforts and strong motivation of the Koreans will soon make the country competitive with advanced nations. In addition, the power industry has also begun to support the research and development of superconducting power technology via ETEP. Including such support, we review the recent progress of applied superconductivity research and development in Korea, mainly in the power sector.

We investigated the time dependence of the resistance of superconducting fault current limiters (SFCLs) based on YBa 2 Cu 3 O 7 thin films after quench completion. The 0.3 μm thick film was coated with a gold layer of 0.2 μm thickness and patterned into pairs of 1 mm wide and 26 cm long meander lines by photolithography. The SFCL was subjected to simulated AC fault currents of various peak values. The resistance of the SFCL consisted of an oscillatory component and a slow varying one. The slow varying component of the resistance increased exponentially with time after the initial rapid increase due to phase transition. The magnitude of the oscillatory component was small compared with the slow varying one. These results could be explained quantitatively with the concept of heat transfer within the film and to the surroundings.

We investigated the quench characteristics of resistive superconducting fault current limiters (SFCLs) based on YBa2Cu3O7 (YBCO) films. The SFCL consists of meander-type YBCO stripes covered with 200 nm Au layer grown in situ for current shunt and heat dispersion at hot spots. The SFCL quenched in a 100 Vrms circuit to produce resistance of 29 Ω at 55 ms. This resistance indicates the gold film temperature of about 200 K. The SFCL showed reproducible characteristics during more than a hundred times of repeated experiments. Two nearly identical SFCLs were connected in series to investigate simultaneous quench. A slight difference in the rate of voltage increase between two SFCL units produced significantly imbalanced power dissipation between units. The imbalance of power was matched by a shunt resistance connected in parallel. The appropriate values of shunt resistance here were 80 Ω at 75 Vrms, 100 Ω at 100 Vrms and 110 Ω at 120 Vrms, respectively. Increased power input at high voltages also tends to balance the power dissipation, but with increasing in film temperature to higher than 200 K.

We investigated the quench distribution in resistive superconducting fault current limiters (SFCLs) based on YBa2Cu3O7 thin films at various source voltages. The film was grown on sapphire substrates, coated in situ with a gold layer and patterned into pairs of 1 mm wide and 26 cm long meander lines by photolithography. Fabricated limiters were tested with simulated AC fault currents. The resistivity of Au/YBa2Cu3O7 meander lines was relatively uniform in the center stripes at all voltages. This result is important because it means the dissipated power was distributed relatively evenly among most of stripes. The resistivity of the stripes near electrodes was significantly lower than that of center stripes, reflecting the cooling power of the electrode area. The difference increased with the source voltage. These results could be explained quantitatively in terms of heat transfer from limiter meander lines to surroundings. Data fit reasonably well to the solution of heat balance equation obtained with appropriate boundary conditions.

We have investigated various operating properties of superconducting fault current limiters (SFCLs) based on 300 nm thick YBa2Cu3O7 (YBCO) films to determine the voltage ratings of the SFCLs as applied to Korean power system. The SFCL consists of 1 mm wide and 26 cm long meander-type YBCO stripes covered with a 200 nm gold layer grown in situ for heat dispersion at hot spots. Measured operating properties included maximum voltage, maximum limited current, response and recovery time under various conditions. Maximum voltage across the SFCL during the quench was independent of fault angle between 0° and 90° while maximum limited current was the highest at fault angles between 60° and 120° due to high current value at the fault instant. The response time showed reverse behavior to maximum limited current. When inductance was inserted into the circuit, the response time rapidly increased with decreasing power factor down to 0.985 while the maximum limited current varied little. Recovery time increased linearly with applied voltage. Considering the above characteristics, particularly the recovery time, it was determined that optimal voltage rating of our SFCL units was below 100 V rms .

Bi 2 Sr 2 Ca 1 Cu 2 O x (BSCCO-2212) tubes were fabricated by melt and solidification process. BSCCO-2212 powder compact was melted in a resistance furnace using a Pt crucible and poured in a rotating cylindrical mold. The solidified BSCCO-2212 tube samples were heat-treated at various temperatures in oxygen atmosphere. The current–voltage curves at 77 K of the samples were obtained by transport measurement, and the microstructure was investigated by scanning electron microscope. It was found that the critical current ( I c ) of the BSCCO-2212 samples was dependent on the heating schedule regarding the grain growth of the BSCCO-2212 plates and the formation of the second phase.

We present the magnetic properties of YBa2Cu3O7−δ crystals prepared by top-seeded solution growth (TSSG) [1]. The crystals showed the peculiar peak effect (fish-tail) for fields parallel to the c-axis, for which the hysteresis width in the magnetic isotherm M(H) showed a single maximum at |Hp| > 0. This M(H) was observed up to 85 K, just below the maximum Tc = 86 K, and was found to be insensitive to various oxygen annealings. On the other hand, oxygen reduction, which resulted in a decrease in Tc by up to 4 K, showed a scaling behavior of the normalized (M · H) for both temperatures and oxygen contents, indicating a single pinning mechanism. This implies that the oxygen vacancies or twin planes are unlikely to account for this peak effect. We also discuss the effects of defects such as twisted boundaries, localized stacking faults, and impurities on the peak effect. Together with the fish-tail associated with oxygen vacancies, we suggest that the peak effect is not related to specific ions, but to material processing.

We present transport measurements for ${\mathrm{Tl}}_{\mathit{n}}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ (n=1,2) films. The current-voltage characteristics clearly show a curvature change in the ln(E)-ln(J) plot, presence (T\ensuremath{\gtrsim}${\mathit{T}}_{\mathit{g}}$) and absence (T${\mathit{T}}_{\mathit{g}}$) of linear resistivity, and scaling behavior with measured critical exponents z=7\char21{}8 and \ensuremath{\nu}=1\char21{}1.3, suggesting a vortex-glass\char21{}liquid phase transition. The critical exponents reported so far are lacking in universality, but dependent upon both systems (${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$, etc.) and samples (crystal, film, etc.). We discuss the possibility of an anisotropic correlation length of a vortex glass for an anisotropic superconductor, a finite correlation length due to inhomogeneous pinning for defective samples, and technical problems in the scaling procedure.

The preparation of thick films of the LaBa2Cu3Oy (La-123) phase by using the partial melting technique (PMT) of powders deposited on MgO substrates by drying alcoholic suspensions is reported. The effects of the starting composition, processing time (t), and temperature (T) on the superconducting properties of the La1+xBa2Cu3Oy (−0.25 ⋚ x ⋚ 0.5) system were investigated. The presence of the La4Ba2Cu2O10 (La 422) phase was detected in the samples quenched in liquid N2 after heating up to 1200 °C. The crucial step of the processing is the cooling rate down to the crystallization temperature, Tp, determined by DTA analysis, in order to favor the formation of the La-123 phase from the reaction between the La-422 solid phase and the liquid. The superconducting properties, tested by magnetization measurements, showed a diamagnetic onset temperature (Tc,diam) as high as 89 K.

The transport measurements were performed for the TlnBa2Ca2Cu3Ox (n = 1, 2) thin films in magnetic fields. The resistive state could be approximately described by the thermally activated form, ϱ ∼ exp [- U(H,T) k B T ] , where U(H,T) ∝ (const. + T)H−α with α ≈ 0.5 ∼ 0.6. This form, however, appreciably deviates from the data for low dissipation and high fields. As for the viscosity of the supercooled liquid, the divergence of U well described the behavior, particularly, of the low dissipation state for all fields, implying the presence of the vortex solid. The log(V) − log(I) characteristics revealed the curvature change, which defines the vortex solid (glass) - liquid melting temperature Tm. Both the irreversibility line and the melting line showed approximately the same T dependence, Hα ∼ T−1, which is from the thermally activated form. This correlation suggests that the irreversibility line appropriately figures the vortex solid (glass) - liquid melting line.

The preparation of thick films of the NdBa 2 Cu 3 O y (Nd-123) phase by using partial-melting techniques (PMT) of powders deposited on MgO substrates by drying alcoholic suspensions is reported. The effects of the starting composition and processing conditions (temperature and annealing time) on the superconducting properties of the Nd 1+ x Ba 2 Cu 3 O y (0.0≤ x ≤0.50) system were investigated. An improvement of the quality of the films in the NdBaCuO system, as far as the microstructure and magnetic properties are concerned, was observed in the samples with nonstoichiometric starting compositions; the growth of the Nd-123 phase with tetragonal symmetry was detected for x = 0.5. The magnetization measurements, performed on samples annealed in O 2 , showed a T c as high as 91.5 K and a weak field dependence due to the effectiveness of the pinning centers.

We have investigated the transport properties of the grain boundaries of TlBa2Ca2Cu3Ox (Tl-1223) and Tl2Ba2CaCu2Ox (Tl-2212) epitaxial thin films. Both Tl-1223 and Tl-2212 thin films were grown on SrTiO3 bicrystal substrates with various tilt angles (5 degrees , 10 degrees , 15 degrees , 24 degrees and 36.8 degrees ) of (001) direction by a laser ablation and post-annealing method. For both Tl-1223 and Tl-2212 phases, the grain boundary Jc for large tilt angles ( theta >or=15 degrees ) at 5 K dropped by almost one order of magnitude, compared with the intragrain Jc. We also observed hysteretic magnetic-field dependence of grain boundary Jc for large tilt angles in both phases. On the other hand, the grain boundaries for low tilt angles ( theta

High-J/sub c/ YBa/sub 2/Cu/sub 3/O/sub y/ thick films were prepared by the liquid phase epitaxy (LPE) method. The growth rate was typically about 2 /spl mu/m/min, which was 10-10/sup 2/ times larger than that by conventional vapor growth techniques. The T/sub c/ of the best film exceeded 88 K after oxygen annealing, and the transport zero-field J/sub c/ was over 10/sup 5/ A/cm/sup 2/ at 77 K. The peak effect in J/sub c/-B characteristics was observed at several tesla by magnetic and transport measurements. Based on the micro-structure, the relevant peak effect is considered to be caused by stacking faults which act as a field induced pinning center. >

Heat capacity, magnetization, and electrical resistivity measurements were carried out on ${\mathrm{CeCoGe}}_{3}$ and ${\mathrm{LaCoGe}}_{3}$. ${\mathrm{CeCoGe}}_{3}$ was found to be a heavy-fermion system with \ensuremath{\gamma}=111 mJ/mole Ce ${\mathrm{K}}^{2}$. Two magnetic transitions were found at \ensuremath{\sim}21 and \ensuremath{\sim}18 K in the absence of a magnetic field. The upper ordering temperature is believed to be a transition from the paramagnetic state to a c-axis ferrimagnetic (FERRI) state, which in turn transforms a colinear antiferromagnetic (AF) state at \ensuremath{\sim}18 K. The AF phase undergoes an irreversible phase transition to the FERRI state under an applied magnetic field. Consequently, magnetic glasslike behaviors such as thermal and magnetic hysteresis, frozen moment, and magnetic relaxation were observed. A nonhysteretic metamagnetic transition to a c-axis ferromagnet (FERRO) was observed at high magnetic fields. The critical spin-flip field, ${\mathit{H}}_{\mathit{m}}$, increases in the FERRI state, and decreases in the AF state as temperature decreases. The magnetic data also suggest that the Ce moments in the ab plane are antiferromagnetically aligned in all three magnetic phases. The existence of both ferromagnetism and antiferromagnetism implies that there may be close competition of ferromagnetic and antiferromagnetic exchange interactions between Ce moments leading to possible spin frustration.

We report the peculiar microwave properties of high-Tc superconducting (HTS) microstrip antennas without impedance matching circuits near the critical temperature (Tc), where impedance mismatch is usually expected. The superconducting thin films used in this report were YBa2Cu3O7−x films (Tc=88 K) with various thicknesses deposited on MgO substrates by the pulse laser deposition technique. At the temperature of 86 K the reflection coefficient rapidly drops down below −62.52 dB and the standing wave ratio approaches almost unity 1.0015 at the resonant frequency of 11.812 GHz. This implies that a HTS microstrip antenna somehow tunes itself for the optimal performance at a specific temperature, indicative of possible exemption from impedance matching circuits.

The superconducting free-energy surface of grain-aligned Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4{minus}{ital y}} was measured to determine whether these electron-carrier materials have thermodynamic variables similar to the other high-temperature superconductors. There is a wide area in the {ital H}-{ital T} plane near {ital T}{sub {ital c}}, where the magnetization is reversible, just as was found for the Y, Bi, and Tl cuprates. The magnetic-field dependence of the specific heat derived from these data, {ital C}{sub 0}{minus}{ital C}{sub {ital H}}, shows a peak in the fluctuation region near {Tc} of about 1.2 mJ/cm{sup 3} K for {ital H}=2 T and {bold H}{parallel}{bold c}. At lower temperatures, below 16 K, the magnetically derived specific heat is nearly independent of field just as was found for the other high-{Tc} materials. A study of the transformation from the reversible to the irreversible regime using flux-creep and critical-current studies shows that the pinning potential to thermal energy ratio, {ital U}{sub eff}/{ital kT}, gradually rises from 2 to 20 at roughly the same value of reduced temperature, {ital T}/{ital T}{sub {ital c}}, as the Bi and Tl compounds.

In order to investigate weak link properties in the Tl1Ba2Ca2Cu3Ox[Tl‐(1223)] system, we measured the transport properties of artificially grown grain boundaries in Tl‐(1223) films. Epitaxial thin films were grown on SrTiO3 bicrystal substrates with five tilt angles of 5°, 10°, 15°, 24°, and 36.8°. We found that the grain boundaries Jc (Jcg.b.) of large tilt angles (θ≥15°) at 77 and 5 K were substantially less than intragrain Jc(Jcg) and limited by a weak link. However, the Jcg.b./Jcg values for low tilt angle grain boundaries (θ≤10°) were almost unity. Moreover, magnetic‐field history dependent Jc values for low tilt angle grain boundaries were not observed in a low magnetic field (

A study of critical currents, flux creep and vortex line energy was undertaken for a variety of materials to determine some of the factors that control the intragranular critical current. Creep measurements for Nd 1.85 Ce 0.15 CuO 4 and TI 2 Ba 2 Ca 2 Cu 3 O 10 , plotted as a function of reduced temperature, t = T / T c , show essentially the same behaviour. This implies that the Nd compound, which has a much longer coherence distance and may have many electron carriers, has the same effective pinning potential to kT ratio at the same reduced temperature. Hence these two factors do not seem to be dominant in controlling J c . A change in the vortex line energy, however, seems to be very important. With the addition of small amounts of Fe, the line energy and J c both fall exponentially with Fe content in Y 1 Ba 2 (Cu 1− x , Fe x ) 3 O 7−σ . Hence the free energy per unit length of vortex is a very important factor controlling the intragranular critical currents.

We have investigated irreversibility magnetic fields (H*) and vortex-glass transition perpendicular to the a-b plane of Tl1Ba2Ca2Cu3Ox (Tl-(1223)) and Tl2Ba2Ca2Cu3Ox (Tl-(2223)) thin films. Temperature dependence of the H* in both Tl-(1223) and Tl-(2223) thin films were consistent with the formula assuming a flux creep; √H*∝1/kT-1/kTc1). These results suggested that the H* occurred from thermally activated plastic motion of pinned flux liquid. Moreover, we also discussed the current density-voltage (J-E) curves in various magnetic fields and found vortex-glass-like behavior of both the Tl-(1223) and Tl-(2223) thin films in magnetic fields. It was found that a finite linear resistivity ρlin=(E/J)J→0 was observed in the vortex-liquid regime and vanished in the vortex-glass regime in both phases.

Experimental aspects of flux expulsion in ${\mathrm{Nb}}_{3}$Sn and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ type-II superconductors are presented. There is a clear distinction in magnetization between field-cooled measured-upon-cooling (FCC) and field-cooled measured-upon-warming (FCW) results. This thermal hysteresis, predicted in the temperature-dependent critical-state model at low fields by Clem and Hao, was observed for measuring fields up to about 0.5 T. The model explains the observation of increases in diamagnetism after field cooling and thermal cycling. The thermal hysteresis, together with weak links, accounts for the occurrence of a negative peak in FCW magnetization. The FCC-FCW bifurcation observed for a 0.1 mT field down to 5 K might imply that flux lines are not completely frozen below ${\mathit{T}}_{\mathit{c}1}$, the temperature at which the lower critical field is equal to the measuring field, but are expelled from the sample even in the Meissner state.

The increased use of distributed power generation has led to increasingly high fault current levels. A superconducting fault current limiter (SFCL) is a potential solution to prevent the problem of short-circuit currents. YBCO-coated conductors (CCs) are one of the most promising superconducting materials for SFCLs. Most YBCO CCs have stabilizers, which play a significant role in limiting the fault current in the SFCL. Therefore, the selection of the appropriate material and the thickness of the stabilizer of the CC used for the SFCL may affect its quench/recovery characteristics. In this paper, the quench/recovery characteristics of YBCO CC tapes having stabilizers with various thicknesses were investigated. The quench/recovery test results showed that, as the thickness of the stabilizer decreased, both the final approach temperature and the recovery time decreased.

X-ray absorption spectroscopy based on the polarized nature of synchrotron radiation is considered. It is concluded that linearly polarized radiation can be used for studying the anisotropic nature of electronic and atomic structure by measuring X-ray absorption near edge structure and extended X-ray absorption fine structure spectra of single crystals or oriented powders. Circularly polarized radiation can be used for studying the spin density of final electronic states in a magnetic system. Particular attention is given to experimental results on absorption of linearly polarized radiation through a highly anisotropic medium of oriented copper oxides at the Cu K-edge.

Linearly polarized synchrotron radiation is used to study the structure of magneticallyaligned layered copper oxide high temperature superconductors (HTSC). We report polarizedextended x-ray absorption fine structure (EXAFS) measurements of oriented powders ofYBa2Cu3O69 at temperatures ranging from 20-300 K. Measurements made at the Cu K-edge with the x-ray polarization vector oriented perpendicular and parallel to the c axis can be used toeffectively probe the structure around the Cu-atoms in the ab (horizontal layer) plane as well as inthe c (vertical layer) plane. 1. INTRODUCTION Since the discovery of layered copper oxide as HTSC's [1] the structure and compositionof La2CuO4, Nd2CuO4, YBa2Cu3O69 and have been extensively studied via XRD [2], neutron diffraction[3] and EXAFS [4,5]. The first two techniques give more accurate results for long range order, while the EXAFS probes the local structure around the absorbing atom. Moreover due to the linearly polarized nature of the synchrotron radiation, the EXAFS will show anorientation dependence for an anisotropic sample, thereby yielding a determination of the

In general, conventional high-Tc superconducting fault current limiters (SFCLs) are operated by cooling systems with a liquid cryogen, such as liquid nitrogen (LN2). However, in the fault mode, LN2 evaporates because of joule heating in the SFCL module, so the SFCL system experiences an enormous increase in nitrogen gas volume. In this case, the thermal stability and protection of the system become the primary concerns for the design of the SFCL cooling system. In order to enhance the thermal stability and safety of the system, an SFCL cooled by solid nitrogen (SN2) as a large heat capacitor has been proposed as an alternative. In this paper we report the quench/recovery characteristics of a YBCO-coated conductor (CC) evaluated in an SN2 cooling system for the SFCL. A feasibility study is also conducted on the reference design codes and thermal requirements for the optimal design of the SN2 cooling system. The results demonstrate that the improved thermal contact obtained between the SN2 cooling system and the SFCL module renders the proposed system a suitable cryogen for the SFCL module. Detailed experimental results for the LN2 and SN2 cooling systems are presented and discussed.

The non-destructive measurement of charge on an ion in a solid has always been a source of great controversy. In this chapter, we will review how x-ray absorption spectroscopy addresses this issue. We will describe the underlying principles of the technique, list some recent experiments, and try to explain the controversial aspects. A new method of evaluation of the position of the absorption edge will be introduced, and its relation to the Mulliken populations determined by discrete variational method for calculating orbital states of a molecular cluster will be discussed. We will then show how x-ray absorption spectroscopy can probe variations in effective charge on atoms as a result of chemical doping. Finally, we will point out some newer developments as a result of using polarized x-rays and oriented crystals.

Magnetization studies were undertaken to determine the change in free energy of the superconducting state of ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7}$ with changes in magnetic field and temperature. Close to the transition temperature ${T}_{c}$ magnetic flux moves easily and reversibly in these materials so that magnetization is a thermodynamically reversible variable. Hence, the free-energy surfaces can be measured and quantities such as entropy and specific heat can be derived. Magnetization is found to be linear in ${({T}_{c}\ensuremath{-}T)}^{2}$ near ${T}_{c}$. The free-energy surface shows substantial changes with magnetic field all the way up to 92 K, well outside the usual ${H}_{c2}$ vs $T$ plots derived from resistivity or ac susceptibility data.

Procedures have been developed to trap a single quantized vortex in a superconducting thin film and systematically move it around under the influence of an applied Lorentz force. The location of the vortex is determined from the diffraction pattern of a Josephson junction formed by superconducting cross strips and a normal-metal barrier; the force is applied by a current in the superconducting film. Vortices move across the junction in a reproducible series of discrete steps, typically a few micrometers long. The elementary pinning force is found to be approximately ${10}^{\mathrm{\ensuremath{-}}6}$ N/m in a Pb-Bi film and to vary as (1-t${)}^{3/2}$, where t is the reduced temperature. By controlling the pinning sites along a symmetry axis of the junction, one can perform rather simple read and write operations for logic elements.

Procedures have been developed to make Nd1.85Ce0.15CuO4 powders which have thec-axis of all the grains aligned in one direction. By rotating a powder sample mixed with liquid epoxy about an axis perpendicular to a strong applied magnetic field, thec-axis of the grains aligns with the axis of rotation. The procedure works for any material having the maximum room-temperature susceptibility perpendicular to thec-axis. Using this technique, we measured the magnetic anisotropy of this electron-carrier high-temperature superconductor.

The elementary pinning force for an isolated yortex has been measured, and procedures have been developed for systematically moving the vortex back and forth across a Josephson junction. Changes in the current-voltage curves for the Josephson junction as the vortex changes position provide an easy way to read out the location of the vortex.

The elementary pinning force ${\mathrm{f}}_{\mathrm{p}}$ has been measured for a single vortex trapped in one of the superconducting layers of a cross-strip Josephson junction. At temperatures close to the transition temperature the vortex can be pushed across the junction by a transport current. The vortex is found to move in a small number of discrete steps before it exits the junction. The pinning force for each site is found to be asymmetric and to have a value of about ${10}^{\mathrm{\ensuremath{-}}6}$ N/m at the reduced temperature, t=T/${\mathrm{T}}_{\mathrm{c}}$=0.95. As a function of temperature, ${\mathrm{f}}_{\mathrm{p}}$ is found to vary approximately as (1-t${)}^{3/2}$. .AE