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Abstract: We have developed a characterization method for two-dimensional imaging of critical current density in coated conductors (CCs) based on scanning Hall-probe microscopy (SHPM). The distributions of the magnetic field around a sample were measured for several different conditions of external magnetic fields, and then were converted to those of the sheet current density which flowed to shield the external magnetic field or to trap the penetrated magnetic field. As a result, it was found that the amplitude of the sheet current density corresponded to that of critical current density almost in all the area of the sample except for the region where current direction changed. This indicates that we could obtain an in-plane distribution of the critical current density with a spatial resolution of around 100μm in non-destructive manner by this method. We believe that this measurement will be a multifunctional and comprehensive characterization method for coated conductors. [Copyright &y& Elsevier]

SmBa2Cu3O7 (SmBCO) coated conductors were grown on IBAD-MgO templates the evaporation using a drum-in-dual-chambers (EDDC) deposition system. The material showed a critical temperature of 93 \sim 94 K and a critical current of 200 \sim 250 A/cm-width at 77 K in a self-field. The current-limiting properties and local structure of the films were examined by laser-induced thermoelectric (LITE) imaging and low temperature laser scanning microscopy (LTLSM). Surface morphology and chemical composition of the superconducting layers were examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. A 100\ \mum\times 1\ mm-microbridge was fabricated using a photolithographic technique and etching process. The measured LITE images of the SmBCO coated conductor showed that some grains on the surface, surface inhomogeneity, second phases, defects and tilted grains, cause different thermal gradients. The LTLSM images revealed two dimensional variations of local flux flow on the films as a function of the local critical current. The findings are consistent with the LITE images, indicating that both methods can visualize the local structures and phases. [ABSTRACT FROM AUTHOR]

Current transport properties in Y1Ba2Cu3O7δ (YBCO) coated conductors obtained by pulsed laser deposition process on a CeO2 capped Gd2Zr2O7-IBAD template have been studied by spatially resolved measurements. We utilized low temperature scanning laser microscopy along with laser induced Seebeck effect imaging and scanning SQUID microscopy that allow us to visualize 1) distributed flux flow dissipation, 2) current blocking obstacles and 3) local current flow, respectively. Combination of those measurements leads deep insights into current limiting mechanism in the coated conductor. Our results show that non-uniform current flow due to spatially distributed obstacles is responsible for the dissipation, whereas grain connectivity in each YBCO grains is not the limiting factor. Typical period of those obstacles is several tens μm to hundreds μm. Detailed correlation between local current flow and dissipation has been observed. Present methods have great potential as tools for basic understanding of current limiting mechanisms in the coated conductors. [ABSTRACT FROM AUTHOR]

Current transport properties in 200 A-200 m-class IBAD-YBCO coated conductor (CC) have been investigated over a wide range of temperature and magnetic flux density up to 26 T. YBCO CC fabricated by reel-to-reel pulsed laser deposition equipment with a multi-plume and multi-turn deposition system showed high critical current of 245 A in length of 212.6 m. Critical current density, Jc, in 5 T parallel to c-axis at 65 K and 4.2 K were 0.44 MA/cm2 and 5.5 MA/cm2, respectively. Moreover, even in very high magnetic flux density, the superior Jc property remains at lower temperature, e.g. Jc at 4.2 K in 26 T was 2 MA/cm2. We also showed analytical expressions of electric field vs. current density (E–J) characteristics based on a framework of percolation model and scaling law of the flux pinning properties. Using the expressions, E–J characteristics and Jc value at any conditions of temperature and magnetic flux density can be predicted quantitatively. [ABSTRACT FROM AUTHOR]

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters. [ABSTRACT FROM AUTHOR]

Extended E–J characteristics in a YBCO coated ion beam assisted deposition tape have been studied over a wide range of perpendicular magnetic field, B, and temperature, T. The statistical distribution of the critical current density, Jc, and the magnetic transition field have been analyzed within the frame work of the percolation model. It has been shown that the distribution of pinning strength in the coated tape is much sharper than that in an epitaxial thin film deposited on a SrTiO3 single crystalline substrate. This result suggests that effective pinning sites are introduced in the coated tape. Scaling behavior of the pinning force density will also be shown. The present analysis allows us to estimate the Jc value quantitatively at an arbitrary B, T and electric field criterion. [Copyright &y& Elsevier]

Critical current properties in a YBCO coated tape have been studied as a function of temperature, magnetic field and field angle under high magnetic field up to 18 T. 1 μm thick YBCO film was deposited on a bi-axially aligned Y2O3/YSZ/Hastelloy substrate fabricated by the IBAD technique. The value of critical current, Ic, of the 1 cm wide tape reached as high as 85 A at 77 K in self-field condition. After etching the tape into 100 μm wide and 1 mm long bridge, we measured extended E–J characteristics. The value of critical current density, Jc, is maintained higher than 105 A/cm2 at 70 K in 5 T perpendicular to the tape surface. Moreover, the power index n and the so called glass–liquid transition field, BGL, of the tape in perpendicular field are superior to those of epitaxial film deposited on a SrTiO3 single crystalline substrate. Jc vs. angle relationship shows broad peak around 90° where field is perpendicular to the tape surface, in addition to a sharp peak at 0°. These results suggest that the tape is highly uniform and correlated defects along c-axis direction contribute to flux pinning. [Copyright &y& Elsevier]

We have investigated the influence of sample width on the transport properties in YBCO thin film using 10, 120, and 1200 μm-wide bridges formed in a same c-axis oriented YBCO film deposited on a SrTiO3 substrate. We measured I–V characteristics of the bridges at various temperature, T, and magnetic field, B, perpendicular to the film surface. It has been found that the so called glass–liquid transition field, BGL, reduces as the sample width becomes narrower, while the critical temperature, Tc, and critical current density, Jc, in the glass region (B) are almost independent of the width of the bridges. The good agreement of Tc and Jc in three bridges indicates high uniformity of the bridges and that the reduction of BGL does not come from the degradation of the film. The measured width dependence of BGL can be scaled with a constant critical index as can be predicted by the percolation model at all measured temperature region from 70 to 85 K. The influence of sample width on Jc has also been discussed based on the scaling analysis. [Copyright &y& Elsevier]

Current transport properties in YBCO coated ion beam assisted deposition tapes have been studied as a function of temperature, magnetic field and field angle under high magnetic fields up to 18 T. The value of critical current, Ic, for 1 cm wide tape was 85 A at 77.3 K in self-field. Detailed properties of critical current density, Jc, were studied using 100 μm wide and 1 mm long bridge patterned in the tape. The results were compared with the properties of epitaxial YBCO thin films deposited on SrTiO3 single crystalline substrate. The value of critical current density, Jc, was maintained higher than 105 A/cm2 at 70 K in 5 T perpendicular to the tape surface. The value is ≈1/4 of Jc value in epitaxial thin film. In higher magnetic field, the difference becomes smaller. Power E–J relationship was observed over wide range of temperature and magnetic field. The power index n were superior to those of epitaxial thin films. And, the so-called glass–liquid transition field, BGL, of the coated tape was also superior to that of epitaxial thin films. Angle dependences of Jc showed a broad peak around 90° (∥c), in addition to a sharp peak at 0° (⊥c). These results suggest that correlated defects along c-axis direction contribute as pinning sites. [Copyright &y& Elsevier]

Abstract: We have investigated critical current property in YBa2Cu3O7− δ (YBCO) coated conductor fabricated by improved metal organic deposition using trifluoroacetates (TFA–MOD) process optimizing conditions such as solution compositions. The electric field vs. current density (E–J) characteristic measurement was carried out at wide range of temperature, T, and magnetic field, B. Critical current density, J c, reached 3.8×1010 A/m2 at 77K in self-field for 1.2μm thick YBCO layer. In-field J c is also improved by this new process. From the analysis of E–J characteristics, we have found out that the shape of statistical distribution of J c in the new process becomes sharper than that of the previous one. These results indicate that the uniformity of YBCO layer is improved by the new process. Furthermore, analytical expression of E–J characteristics, which is based on a percolation model and a scaling law of the pinning force density, shows good agreement with the experimental results. This allows us to predict the critical current property at arbitrary conditions of T and B even if we couldnot have measured data. For example, it is predicted that J c could be about 1×1010 A/m2 at 4.2K and 30T parallel to the c-axis. [Copyright &y& Elsevier]

Abstract: Establishment of processing technology for multifilamentary coated conductors (CCs) is a key issue for superconducting electric power applications from the viewpoint of AC loss reduction. On the other hand, CCs sometimes have local inhomogeneity in the superconducting layers, and that will cause a current blocking in a filament or inhomogeneity of critical current among filaments in a multifilamentary CC. Therefore, we need an assessment method for local electromagnetic behavior on AC loss properties of CCs. In this study, we developed a method for visualizing time-dependent AC loss distribution in CCs by using scanning Hall probe microscopy. We succeeded in visualizing local current density, electric field and loss density simultaneously with a spatial resolution of a few hundred micrometers. The measurement system has possible scanning area of 150×50mm2 and current capacity of 500A. This enables us to discuss the local electromagnetic behavior on a practical scale of CCs. We believe that this visualization method will be a very powerful tool to estimate the feasibility of processing technology for multifilamentary CCs. [ABSTRACT FROM AUTHOR]

Abstract: We have characterized current transport properties in a GdBa2Cu3O7−δ coated conductor fabricated by a combination of ion-beam assisted deposition (IBAD) method and pulsed laser deposition (PLD) method with reel-to-reel systems, and have applied them to a coil design for a high-field magnet. The J-E characteristics in the conductor are given as a function of temperature, magnetic field and the field angle based on percolation transition model, and its potential as a high-field magnet coil is discussed based on a finite element method analysis. As a result, we have found that the conductor with Ic =350A/cm-w at 77K and self-field has a possibility to realize a 40T magnet coil with very compact size (300mm in outside diameter) and small usage of the coated conductor (8km). Furthermore, we have also investigated the relationship between the scale of the magnet coil and the corresponding requirements for the conductor. Consequently, it has been suggested that the improvement of critical current characteristics of the conductor greatly advances the downsizing of the coil without large increment in hoop tensile force. In addition, it has also been found that such downsizing of the coil contributes to the reduction in stored energy and total magnetization loss of the coil. This means that the recent improvement of critical current characteristics in coated conductors is especially worthwhile for high-field magnet coils in terms not only of downsizing but also of quench protection and magnetizing/demagnetizing operation. [Copyright &y& Elsevier]

Abstract: We have visualized non-uniform current flow in RE123 coated conductors by using a scanning Hall-probe magnetic microscopy (SHPM). Newly developed SHPM system allows us to measure two-dimensional magnetic field distribution with high spatial resolution in micro-meter scale. Corresponding current density distribution can be obtained from the magnetic field image by solving inverted Biot–Savart’s law. One of the most important advantages of the present system is to visualize the current density distribution in practical high transport current and also in wide scanning area. For example, the system has current leads with large capacity up to 500A, and the operating distance can be 15cm by 15cm with a micro-meter step distance. Using the SHPM system, we have successfully visualized current density distributions in the coated conductor, and clarified different kinds of non-uniform current flow. Those insights are very useful to identify local defects as well as non-uniform tape quality. These results indicate that the SHPM system is a powerful diagnostic tool not only to observe spatial inhomogeneities of transport property but also to understand their reason in practical coated conductors. [Copyright &y& Elsevier]

Abstract: Critical current (I c) of a Bi-2223 tape fabricated by controlled-overpressure (CT-OP) sintering method has been attained up to 200 A-class in recent years. However, current limiting factors have not been fully clarified yet. In this study, we have investigated local current distributions in a CT-OP Bi-2223 tape by using the scanning Hall probe magnetic microscopy. As a result, it has been found that large sheet current flows in the center of the sample. Even though the influence of self-field is taken into account, there is a clear difference of local I c across the width. The maximum sheet current value in the center of the tape is 400–450A/cm when we applied the bias current of 100A, which is equal to the I c at around 87K. This value is equivalent to 800–900A/cm at 77K taking into account the value of I c. Furthermore, such current distribution continues uniformly in length direction. From these results, we have concluded that improvement of current uniformity across the tape width is the effective way to enhance critical current for the present CT-OP Bi-2223 tape. Our results show the feasibility for 300 A-class I c at 77K. [ABSTRACT FROM AUTHOR]

Abstract: We have carried out 2-dimensional (2D) measurement of local critical current in a Trifluoroacetates-Metal Organic Deposition (TFA-MOD) processed YBCO coated conductor using scanning Hall-probe microscopy. Recently, remarkable R&D accomplishments on the fabrication processes of coated conductors have been conducted extensively and reported. The TFA-MOD process has been expected as an attractive process to produce coated conductors with high performance at a low production cost due to a simple process using non-vacuum equipments. On the other hand, enhancement of critical currents and homogenization of the critical current distribution in the coated conductors are definitely very important for practical applications. According to our measurements, we can detect positions and spatial distribution of defects in the conductor. This kind of information will be very helpful for the improvement of the TFA-MOD process and for the design of the conductor intended for practical electric power device applications. [Copyright &y& Elsevier]

Abstract: To improve J C properties in a magnetic field (J C–B) of YBa2Cu3O7− δ (YBCO) films by a TFA-MOD (trifluoroacetates metal organic deposition) method, we fabricated YBCO films with SnO2-doping as artificial pinning centers and investigated the superconducting properties and the crystalline structures. TFA-MOD is expected as a cost-effective method with a non-vacuum system to fabricate YBCO films with high superconducting properties. However, YBCO films have problems that J C decreases in a magnetic field (B). In this study, the TFA solutions with SnO2 for pinning centers were used as starting materials. In the transmission electron microscope image, existence of second phase particles with size of 20–30nm has been observed in the YBCO film. These nano-particles contained Sn-element and distributed randomly in the film. The J C/J C (self field) values of the SnO2-doped YBCO films were enhanced in all magnetic field angles. Therefore it is considered that 3D pinning centers of Sn-compounds were introduced into YBCO film. [Copyright &y& Elsevier]

Abstract: YBa2Cu3O7− δ (YBCO) films were grown by using precursor films with barium concentration gradient in film thickness by an advanced metal organic deposition process using trifluoroacetates (TFA-MOD). We have reported previously that a lot of non-reacted particles such as Y- and Cu-oxides were remained for the YBCO film surface grown by the precursors using a starting solution with barium-poor (cation ratio as Y:Ba:Cu=1:1.5:3). Then, the barium concentration was increased in the film surface to complete the reaction among these Y and Cu residues and Ba and to increase the Y123 growth thickness for realizing higher J c performance. Transmission electron microscopy (TEM) observation showed the increase of film thickness of YBCO grown by the precursors with barium concentration gradient in film thickness, indicating that the reaction between these Y and Cu-oxides and Ba proceeded to form Y123 phase. Consequently, higher J c was obtained for the YBCO film by the precursors mentioned above than that of the YBCO film by the precursors with a constant concentration. Effects of barium concentration gradient in the precursors on the superconducting properties and microstructures in the YBCO film were discussed. [Copyright &y& Elsevier]

Abstract: YBa2Cu3O7− δ (YBCO) films were grown on LaAlO3 substrates by a metal organic deposition using trifluoroacetates (TFA-MOD). In this study, effects of calcination condition such as temperature on microstructures and J c properties for the YBCO films were studied. The TFA starting solution was coated on a substrate by a spin-coating method. Then, a two-step heat treatment was applied to the coated films. In the calcination step, the coated film was calcined to form a homogeneous amorphous precursor by increasing the temperature which was varied from 300°C to 500°C at a constant heating rate of 2°C/min. The prepared precursor films were crystallized at a constant condition. As a result, J c value depended on the calcination temperature, YBCO film calcined at 400°C showed the highest J c of 2.5MA/cm2. J c’s for the YBCO films calcined at 300°C and 350°C were 1.9MA/cm2 and 2.0MA/cm2, respectively. On the other hand, YBCO film calcined at 500°C showed the lowest J c of 1.4MA/cm2. Cross-sectional TEM images showed that the YBCO film calcined at 400°C had less pores and small size of pores in the crystallized film randomly throughout the film. Large pores were observed in the YBCO film calcined at 500°C. Also, these large pores were existed near substrate in the YBCO film. It is considered that the differences of these pore formation mechanisms are the main cause of the difference of J c characteristic. [Copyright &y& Elsevier]

The critical current density and the irreversibility field were measured for a coated Y-123 tape prepared by IBAD method. These results were compared with those of Y-123 thin film deposited on single crystal STO. It was found that Jc of the tape has a weaker magnetic field dependence, although its value at low fields is fairly lower than that of the thin film. This comes from the difference of the thickness. The lower Jc at low fields is due to the collective pinning property, and the better irreversibility at high fields can be attributed to a larger flux bundle volume. These results were compared with the calculated results using the flux creep-flow theory, and a good agreement was obtained. Jc was estimated with the aid of this theory assuming that a high-quality thick tape of 5 μm thick with strong pinning can be fabricated. [Copyright &y& Elsevier]