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We report the magnetic field and temperature dependence of the critical current density {Jc(H) and Jc(T)} flowing in a 5 μm thick sample of SmBa2Cu3O7−δ (SmBCO) film. The film is a coated conductor (CC) deposited on an IBAD-MgO textured metallic template. For a range of intermediate fields, we find J c α H - α with values of α between 0.44 and 0.49. These values are lower than those reported for other CC in the literature. The sample has a Jc of 13.6 MA/cm2 at 5 K and self-field. Such high-Jc wires should find applications in large-scale energy applications of high-temperature superconductors. The temperature dependence of Jc at self-field and different applied fields is also analyzed.

This paper reviews recent progress in research and development (R&D) of reactive co-evaporation for high performance REBCO coated conductors in Korea. Two types of reactive co-evaporation methods were developed for the deposition of SmBCO and GdBCO superconducting layers respectively on the IBAD (Ion Beam Assisted Deposition)-MgO template in the Korean coated conductor project. Batch type reactive co-evaporation equipment and its processing were developed for SmBCO coated conductors at Korea Electrotechnology Research Institute (KERI) in conjunction with the Korea Advanced Institute of Science and Technology (KAIST), and a very high critical current exceeding 1,000 A/cm at 77 K in the self field was achieved through the optimization of deposition parameters. Reel-to-reel type reactive co-evaporation processing with a high conversion rate was also developed, while long length GdBCO coated conductors have been routinely produced by SuNAM Co. The minimum critical current of 422 A/cm-w at 77 K in self field was confirmed for 1 km-long GdBCO tape.

Sm 1+ x Ba 2− x Cu 3+ y O 7− δ (SmBCO) films were directly deposited on the epi-MgO/IBAD-MgO/Y 2 O 3 /Al 2 O 3 /Hastelloy template by co-evaporation using the evaporation using drum in dual chambers (EDDC) system without the buffer layer in order to investigate the effect of the composition ratios on superconducting property, microstructure and texture of SmBCO film. The films with gradient composition ratios of Sm:Ba:Cu were deposited using a shield with an opening which was placed between the substrate and the boats. The highest I c of 52 A (corresponding to J c = 1.6 MA/cm 2 and a thickness of 800 nm) was observed at 77 K in self field at a composition x = 0.01–0.05 and y = −0.23 to −0.46. When the composition ratio is outside this range, the I c value rapidly decreased. The superconducting critical current was highly dependent on the composition ratio. As the composition ratio is farther away from that of the highest I c , the SmBCO (1 0 3) peak intensity increased and the amount of a -axis oriented parts increased. A dense microstructure with round-shape grains was observed in the region showing the highest I c . The optimum composition ratio can be found by analyzing films deposited with variable deposition rates of each depositing element.

The magnetic properties of high T c superconductor (HTS) SmBa 2 Cu 3 O 7− δ (SmBCO) coated conductors (CCs) which are based on IBAD (ion-beam-assisted deposition) templates were investigated. The SmBCO CCs were fabricated on LaMnO 3 (LMO)/epi-MgO/IBAD-MgO/Hastelloy substrates by Evaporation using Drum in Dual Chamber (EDDC) processing. The measurements of isothermal magnetizations M ( H ) for the prepared SmBCO-CCs were carried out at various temperatures in magnetic fields up to 8.5 T. The magnetic properties for the applied magnetic fields parallel to the ab -plane show non-conventional behaviors when compared to those parallel to both c -axis and c -axis + 45°. These normal and anomalous properties can be explained by introducing the magnetic flux pinning effects related to the direction of the applied magnetic fields.

The magnetic properties of SmBa2Cu3O7-x (SmBCO) coated conductor (CC) that was fabricated on LaMnO3(LMO)/IBAD-MgO substrates using Evaporation using Drum in Dual Chamber (EDDC), which is a batch type co-evaporation system, was investigated. LMO buffer layer was deposited on IBAD-MgO templates by PLD method. The measurements of both the magnetization M(H) and the resistance R(T) for the SmBCO CC, were carried out at temperatures between 10 and 120 K in fields up to 8.5 T, using a PPMS-9T (with both ACMS and DCR options, Quantum Design). The SmBCO CC exhibited Tc (onset temperature) of 94.7 K with a narrow transition width of 0.6 K, which indicates a good homogeneity of the superconducting properties. The upper critical field Hc2 near Tc of the SmBCO CC has been estimated by the resistance R(T) approaches. The magnetization curves M(H) are symmetric about the line M = 0 for the SmBCO CC with the field applied parallel to both the c-axis and c-axis +45°. For the SmBCO CC with the field applied parallel to the ab-plane, they are asymmetric. These symmetric and asymmetric curves can be explained by introducing the bulk pinning of 1-D flux lines and surface barrier-dominated system of 2-D pancake vortices.

To examine the effect of solvent treatments on the superconducting properties of MgB 2 , solvent-treated MgB 2 powders were prepared by mixing with water, acetone, or ethanol. No degradation of the superconductivity of MgB 2 by the acetone or ethanol treatments was observed. Appreciable change which accompanies degradation of superconducting properties was observed only when MgB 2 was mixed with water. In addition, to investigate the variation of the flux pinning effect due to the aging, the Ag 8 –MgB 2 , (SiC) 4 –MgB 2 , and sintered pure-MgB 2 pellets were kept at room temperature for two and half years in air. The aged Ag 8 –MgB 2 composite superconductor has showed degradation of flux pinning effect. The in-field superconducting properties of (SiC) 4 –MgB 2 , however, did not change after aging.

We have fabricated SmBCO coated conductor on IBAD-MgO substrates using unique co-evaporation method. The batch type co-deposition system was specially designed and was named EDDC (evaporation using drum in dual chamber) that is possible to deposit superconducting layer with optimum composition ratio of materials at temperature over 700 °C and several mTorr of oxygen. In this study, we have investigated the influence of SmBCO phase composition ratio on the critical current density of coated conductor. We have optimized the composition ratio by changing the deposition rates of Ba and Cu during co-evaporation for better critical current density. A higher critical current density was measured at the composition ratio of Ba deficiency (1.9–2.0) and Cu rich (3.2–3.4) compared to the Sm 1 Ba 2 Cu 3 O y stoichiometry. Especially, highest critical current density of 2.6 MA/cm 2 was measured at the composition of Sm 1 Ba 1.9 Cu 3.2 O y that has dense surface morphology.

Round shape Bi2212/Ag wire is isotropic, while Bi2212/Ag or Bi2223/Ag tape has anisotropic characteristics or performances with respect to magnetic field orientation, which is the only HTS wires that can be used to make Rutherford cable to transport high current. In this work, two different Bi2212/Ag round wires with different Ag ratio were fabricated using powder-in-tube method and processing factor at each step was investigated. Double stacked 385 (55 × 7) filamentary wires of various final diameter were heat treated at different melting temperatures. Microstructure after pre-annealing was investigated. Wires which have Ag ratio (silver area/superconductor area) of 0.3 and 0.42 after powder filling show similar critical current density. Higher Ag ratio wire at 0.74 mm diameter heat treated at melting temperature 890 °C shows critical current density of 2750 A/mm2 at 4.2 K, 0 T.

The local critical current along a sample length is different from position to position in a long sample, especially when the sample is damaged by externally applied strain. In the present work, we attempted to reveal the relation of the distribution of the local critical current to overall critical current and the sample-length dependence of critical current for slightly and significantly damaged Bi2223 composite tape samples. In the experiment, 48 cm long Bi2223 composite tape samples, composed of 48 local elements with a length of 1 cm and 8 parts with a length 6 cm, were bent by 0.37 and 1.0% to cause slight and significant damage, respectively. The V–I curve, critical current (1 µV cm−1 criterion) and n value were measured for the overall sample as well as for the local elements and parts. It was found that the critical current distributions of the 1 cm elements at 0.37 and 1.0% bending strains are described by the three-parameter- and bimodal Weibull distribution functions, respectively. The critical current of a long sample at both bending strains could be described well by substituting the distributed critical current and n value of the short elements into the series circuit model for voltage generation. Also the measured relation of average critical current to sample length could be reproduced well in the computer by a Monte Carlo simulation method. It was shown that the critical current and n value decrease with increasing sample length at both bending strains. The extent of the decrease in critical current with sample length is dependent on the criterion of the critical current; the critical current decreases only slightly under the 1 µV cm−1 criterion which is not damage-sensitive, while it decreases greatly with increasing sample length under damage-sensitive criteria such as the 1 µV one.

Superconductor is developed for applications in high-power devices such as power-transmission cables, transformers, motors and generators. In such applications, HTS tapes are subjected to various kinds of stress or strain. AC loss is also important consideration for many large-scale superconducting devices. In the fabrication of the devices, the critical current ( I c ) of the high temperature superconductor degrades due to many reasons including the tension applied by bending and thermal contraction. These mechanical loads reduce the I c of superconducting wire and the I c degradation affects the AC loss of the wire. The I c degradation and AC loss of Bi-2223 HTS tape were measured under tension and bending conditions at 77 K and self-field. Moreover, the frequency characteristics of AC loss was measured at the 30–480 Hz. As a result, self-field penetrates the deeper into the conductor at the lower frequency, which means higher self-field losses per cycle.

The distribution of the local critical current and the n-value along the sample length and its relation to the overall critical current were studied experimentally and analytically for the bent multifilamentary Bi2223/Ag/Ag–Mg alloy superconducting composite tape. Then, based on the results, it was attempted to simulate on a computer the dependence of the critical current on the sample length. The main results are summarized as follows. The experimentally observed relation of the distributed local critical current and n-value to the overall critical current was described comprehensively with a simple voltage summation model, in which the sample was regarded as a one-dimensional series circuit. The sample length dependence of the critical current was reproduced on the computer by a Monte Carlo simulation incorporating the voltage summation model and the regression analysis results for the local critical current distribution and the relation of the n-value to the critical current.

We report the upper critical field ( H c2 ), the irreversibility field ( H irr ), and the lower critical field ( H c1 ) of a dense polycrystalline MgB 2 superconductor which was prepared by the spark plasma sintering (SPS). The isothermal magnetizations M ( H ) of MgB 2 sample was measured at temperatures between 5 and 50 K in fields up to 8 T. The H c2 ( T ) data were well fitted to the BCS-type function H c2 ( T ) = H c2 (0)[1 − ( T / T c ) α ] β , with H c2 (0) = 8.5 T, α = 1.60 and β = 1.15. The H irr (melting line) could be described by the power-law relation H irr ( T ) = H irr (0)[1 − ( T / T c )] n with n = 1.10 and H irr (0) = 8.1 T. The deduced values of H c1 approximately followed a linear temperature dependence, not following a parabolic dependence of H c1 ( T ) = H c1 (0)[1 − ( T / T c ) 2 ].

Single-, multi-, and two kinds of coaxial-filament composite MgB 2 /SS–SS (stainless steel for both outer and inner tubes) and MgB 2 /SS–Cu (stainless steel for outer tube and copper for inner tube) wires were successfully fabricated using powder-in-tube (PIT) process with swaging only. The effect of the geometries of the MgB 2 PIT wires has been studied comparatively. The isothermal magnetizations M ( H ) for both the sintered and the as-rolled single-, multi-, and coaxial-filament MgB 2 wires were measured at temperatures between 5 and 50 K in fields up to 5 T. The critical current density was estimated by the M ( H ) data using Bean model. The coaxial-filament composite MgB 2 wires showed much better J c than both the single- and the multi-filament composite ones. The results of this study show that the superconducting properties of the MgB 2 PIT wires/tapes can improve by employing better geometries such as the coaxial-filament composite MgB 2 /SS–SS wires.

In order to investigate texture development of superconducting grains, multi-filamentary Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi-2223)/Ag tapes were fabricated with different pass reduction ratios of rolling. The deformation-induced-texture and reaction-induced-texture for Bi-2212 and Bi-2223 were evaluated by X-ray diffraction including pole figure analysis. It was found that the texture of Bi-2223 develops by the progress of thermo-mechanical treatment. The texture for both Bi-2212 and Bi-2223 was found to develop better along the rolling direction than the transverse direction. By scanning electron microscopy observation, the highly textured region of Bi-2223 grain was found to expand by the intermediate rolling and additional heat treatment.

We have studied the tensile behaviour of Bi2223 superconducting composite tapes at room temperature, and the influence of the tensile damages introduced at room temperature on the critical current Ic and the n values at 77 K. In the measurement of the Ic and n values, the overall composite with a gauge length 60 mm was divided into six elements with a gauge length of 10 mm in order to find the correlation of the Ic and n values of the overall composite to those of the local elements which constitute the composite. From the measured stress–strain curve of the composite and the calculated residual strain of the Bi2223 filaments, the intrinsic fracture strain of Bi2223 filaments was estimated to be 0.09–0.12%. When the applied strain was lower than the onset strain of the filament damage, the original Ic and n values were retained both in the overall composite and the elements. In this situation, while the overall voltage at the transition from superconductivity to normal conductivity of the composite was the sum of the voltages of the constituent elements, among all elements the overall voltage was affected more by the element with the lower Ic (higher voltage). The damage of the filaments arose first locally, resulting in a reduction of the Ic and n values in the corresponding local element, even though the other elements retained the original Ic and n values. In this situation, the voltage of the overall composite stemmed dominantly from that of the firstly damaged weakest element, and the overall Ic and n values were almost determined by the values of such an element. After the local element was fully damaged, the damage arose also in other elements, resulting in segmentation of the filaments. Thus, the Ic and n values were reduced in all elements. The correlation of Ic between the overall composite and the elements could be described comprehensively for non-damaged and damaged states from the voltage–current relation.

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.

The magnetic properties of high Tc superconductor (HTS) YBa2Cu3O7−δ (YBCO) coated conductors (CCs) which are based on RABiTS (rolling-assisted biaxially textured substrate) and IBAD (ion-beam-assisted deposition) templates were investigated. The measurement of isothermal magnetizations M(H) for the prepared YBCO-CCs was carried out at 77 K and 100 K in magnetic fields up to 3000 G. The magnetic properties for the applied magnetic fields parallel to the ab-plane are much smaller than that perpendicular to the ab-plane. Large magnetic property in the YBCO-CC based on the RABiTS template comes from the strong ferromagnetism of the textured Ni–W alloy substrate.

In order to deposit high quality high temperature superconducting (HTS) YBCO thin films with a large area, the pulsed laser scanning (PLS) method was tried and the thickness distribution of the film was investigated. Large area YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films on /spl phi/ 36 mm diameter LaAlO/sub 3/ and /spl phi/ 42 mm diameter Y-ZrO/sub 2/ (YSZ) single crystal substrates were obtained by rotating lenses with radii of 9 and 12.9 mm. The critical temperatures T/sub c/ of the films at the center and edge were all 90 K. The critical current density J/sub c/ at 77 K and zero magnetic field is over 10/sup 6/ A/cm/sup 2/. X-ray diffraction showed that the thin films are completely c-axis oriented. The V-I characteristics of the thin films were also investigated.

2.1 kA-class hybrid-type current leads for superconducting magnetic energy storage (SMES) magnets, consisting of conventional gas-cooled copper leads and high-T/sub c/ superconducting (HTSC) current leads, were designed. The HTSC parts of the leads are made of Bi-2223/99.8%Ag0.2%Au tape. For the design, numerical analysis was performed to find an optimum shape factor and an optimum varying rate of the cross-sectional area of HTSC lead which result in minimum refrigeration power consumption, and then the minimum evaporation rate of LHe. In the present study, the main results of the design are presented.

The superconducting Bi2223/Ag/Ag alloy composite tape specimens with a length 6 cm, composed of a series circuit of six local elements with a length 1 cm, were bent by 0.37% and 1.0% for measurement of distributions of the overall (6 cm) and local (1 cm) critical transport current and n-value. When the damage amount was small (0.37% bending strain), the distributions of the measured local and overall critical currents were described by the three parameter Weibull distribution function, while when the damage amount was large (1.0% bending strain), those were described by the bimodal Weibull distribution function. The distributions of n-value in the diagram of n-value versus critical current for both local elements and overall specimens were expressed by the regression curve of n-value as a function of critical current and the normal distribution function for the deviation of the measured n-values from the regression curve. The experimentally measured critical current distribution and the diagram of the n-value versus critical current of the overall specimens were reproduced successfully in the computer by inputting the formulated distribution functions for the critical current and n-value of the local elements into the proposed simulation method, which used a Monte Carlo simulation method and a one-dimensional series circuit model for the generated voltage near the transition of superconducting to normal state.

Mechanical and electromagnetic stresses are exerted on Bi2223/Ag/Ag alloy superconducting composite tapes during fabrication/winding and operation, which cause reduction in critical current when the Bi2223 filaments are damaged. In the damage process, the thermally induced residual strain and fracture strain of the Bi2223 filaments play a dominant role. The aim of the present work was to propose a comprehensive method for estimation of these strain values and a quantitative description method of the relation of critical current to the applied bending/tensile strain, and to examine the accuracy of the method in comparison with the experimental results. The residual strain of Bi2223 filaments in the composite tape was measured by the x-ray diffraction method. The measured residual strain value was used for analysis of the load-strain curve, from which the intrinsic fracture strain of filaments was estimated. The relation of critical current to applied bending/tensile strain was predicted by the proposed calculation procedure, in which the estimated strain values were input. The predicted critical current-applied strain relation agreed well with the experimental results, suggesting that the present method is a useful tool for prediction/description of tensile/bending applied strain dependence of critical current of multifilamentary-type conductors.

The residual strain change of Bi2212 and Ag during the cooling and heating process in the Bi2212/Ag/Ag alloy composite superconductor was studied. First, the residual strain of Bi2212 filaments at room temperature was measured by the x-ray diffraction method. Then, the Young's moduli of the constituents (Bi2212 filaments, Ag and Ag alloy) and yield strains of Ag and Ag alloy were estimated from the analysis of the measured stress–strain curve, based on the rule of mixtures. Also, the coefficient of thermal expansion of the Bi2212 filaments was estimated from the analysis of the measured thermal expansion curve of the composite wire. From the modeling analysis using the estimated property values and the residual strain of Bi2212 filaments, the changes of residual strain of Bi2212, Ag alloy and Ag with temperature during the cooling and heating process were revealed.

The change in n-value and critical current with bending strain and the relation of n-value to critical current of bending-damaged Bi2223 composite tape were studied experimentally and analytically. The n-value of the bending-damaged Bi2223 filamentary composite tape decreased very slightly with increasing bending strain and with decreasing critical current, in comparison with that of tension-damaged tape. To describe the experimental result for bending-damaged tape, a damage evolution model was applied in which the steep tensile-strain variation in the thickness direction, the shape of the core into which the superconducting filaments are bundled and the damage strain parameters obtained from the analysis of the tensile stress?strain curve were incorporated. The measured change in n-value and critical current with bending strain and the relation of n-value to critical current under applied bending strain were described satisfactorily by the present approach.

The current redistribution of a high Tc superconducting tape with increasing transport current in a magnetic field (Ha) was visualized. The external magnetic field was applied normal to the surface of the sample after zero-field cooling, and the transport current (Ia) was increased stepwise from 0 to 90% of the values of the critical current (Ic(Ha)) at Ha. The field distribution near the sample surface across the tape width (2w) was measured using the scanning Hall probe method. Applying an inversion to the measured field distribution, we obtained the underlying current distribution. The initial fieldlike distribution was gradually changed into a currentlike distribution with an increase of the transport current. The transition occurred when Ia and Ha approached the line f = c in the f–c plane where f = Ia/Ic and c = tanh(Ha/Hc). Hc is defined as Ic/(2πw).

The critical current at 77 K of multifilamentary Bi2223 composite tape was studied under applied tensile strain experimentally and analytically. Beyond the irreversible strain, the critical currents (IC) decreased significantly with increasing applied tensile strain (ec), due to the enhanced cracking of the Bi2223 filaments. The voltage generation in the voltage–current relation was calculated by the current share model in which the transport current is shared by the Bi2223 filament and Ag near the cracked portion. Then the critical current was estimated with a 1 µV cm−1 criterion. By the application of the current share model to the experimental result, the effective crack length responsible for the reduction in critical current was estimated, with which the change of critical current could be described satisfactorily as a function of applied strain.

In order to fabricate long-length high temperature superconducting coated conductors, we have employed a batch-type reactive co-evaporation process for the deposition of a SmBCO layer on the IBAD-MgO substrate of LaMnO3/epi-MgO/IBAD-MgO/Y2O3/Al2O3/Hastelloy. Critical current densities (Jc) of SmBCO coated conductors were investigated with respect to their composition ratios. Higher Jc was obtained in the Sm-rich, Ba-poor and Cu-rich composition region, compared to the stoichiometric region. The highest critical transition temperature (Tc) of the SmBCO coated conductor was ~94 K with a narrower transition width of about 0.5 K. A 93 m long SmBCO layer was deposited on a 4 mm wide IBAD-MgO substrate for 90 min. Although some differences in the appearance of pores and Cu oxide precipitates between both ends of the deposited tape exist, the deposited SmBCO layer was dense and biaxially textured. The growth of a highly textured SmBCO layer was maintained up to the thickness of 2.2 µm. A minimum Ic of 305 A/cm-w was achieved at the 27 m section of the 93 m long SmBCO coated conductor at 77 K in self-field.