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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]

Abstract: Recent process optimization allows improving homogeneity and a significant increase of Jc in GdBa2Cu3O7- δ deposited on ion-beam assisted deposited MgO template (GdBCO/IBAD-MgO). We applied low-temperature laser scanning microscopy and laser scanning thermo-electric microscopy (LSTEM) to investigate local dissipation and defects simultaneously in recent GdBCO/IBAD-MgO coated conductor. By using high-resolution LSTEM, we could detect current blocking obstacles which are responsible for the large scale local dissipation. Data on the present GdBCO/IBAD-MgO coated conductors point out a significant reduction of current blocking obstacles compared to the previous process. We have shown the improvement of spatial homogeneity in the recent GdBCO/IBAD-MgO coated conductors. Current blocking obstacles are much less densely distributed as compared to previous YBCO/IBAD-GZO coated conductor. Obstacles larger than several micrometer significantly increase local dissipation whereas smaller defects do not influence it noticeably. 2D map of the phase delay component of the thermoelectric voltage lock-in signal is effective to analyze current blocking obstacles having action upon local dissipation. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the improvement of critical current in YBCO coated conductors in association with c-axis correlated artificial pinning centers (APCs). Current carrying property has been measured in detail at various conditions of temperature, T: 65–83K, magnetic field, B: self-field to 17T, and field’s angles, θ. The APCs were introduced by pulse laser deposition process using 2vol% YSZ doped target. The enhancement of J c in parallel field along the c-axis direction was observed experimentally over wide range T, B, and θ. Moreover, improvements of transition field and pinning force density in B∥c were also evaluated by pinning analysis based on percolation model. [Copyright &y& Elsevier]

Abstract: We studied the barrier formation process in YBa2Cu3O x (YBCO) vertically-stacked interface-treated Josephson junctions using some variations of the fabrication process, focusing on two points, i.e., the amorphous state before annealing and the effective structure as barrier at the interface after annealing. First, we simulated a surface amorphous layer in the interface treatment technique by depositing an amorphous Y–Ba–Cu–O layer with deviated composition artificially on the base YBCO electrode and succeeding annealing. However, we could not obtain Josephson junctions by this method. It was suggested that the amorphous state is not sufficient condition for the ion-bombarded layer in the interface-treated junctions. Next, we prepared interface-treated junctions using different electrode materials, i.e., YBCO as a base electrode and YbBa2Cu3O x (YbBCO) which has the same structure as YBCO and has lattice constants a little smaller than YBCO, as a counter electrode. The yield of the junction increased by using YbBCO counter electrode. Some kind of disorder between the base and counter electrodes may contribute to the barrier. [Copyright &y& Elsevier]

Abstract: We investigated the local inhomogeneity of flux flow dissipation by use of low temperature scanning laser microscopy (LTSLM) in a Y1Ba2Cu3O7−δ (YBCO) coated conductor obtained by the Metal-Organic Deposition process using metal trifuluoroacetate (TFA-MOD process). The LTSLM images showed that the highest J c value in local region reached almost two times higher than the minimum value, while the global J c was limited by the weakest spot. Typical length scale of the spatial inhomogeneity of J c was about several tens to hundreds μm. [Copyright &y& Elsevier]

Abstract: We investigated trapped vortices in Y1Ba2Cu3O7−δ (YBCO) coated conductor by using a scanning SQUID microscope (SSM). Several peculiar properties of the vortices were observed such as a large bundle of vortices at a certain position and a coexistence of positive- and negative-sign vortices. Comparing the SSM images with that of low temperature scanning laser microscopy (LTSLM) and SEM, we identified the relationship between local dissipation and position of current blocking defect. Furthermore, regular vortex images were also observed by the SSM in the region where higher J c has been identified by the LTSLM. [Copyright &y& Elsevier]

E–J characteristics in a YBCO coated conductor at low temperature and very high magnetic field up to 27 T have been estimated using experimental E–J characteristics below 12 T along with a scaling analysis based on the percolation model. 1 mm thick YBCO films was deposited on a biaxially aligned Y2O3/YSZ/Hastelloy substrate fabricated by the IBAD technique. The critical current Ic for the 1 cm wide tape was 85 A at 77 K in self-field. We measured E–J characteristics in perpendicular field up to 12 T using 100 μm wide and 1 mm long bridge patterned in the tape. Statistic Jc distribution and pinning properties were analyzed from the E–J characteristics. Using the pinning parameters determined by the scaling analysis, we can calculate the critical current properties as a function of temperature and magnetic field. E–J characteristics at low temperature and very high magnetic field up to 27 T were estimated analytically and compared with the experimental data. Estimated value of Jc and n-value, and their error analysis were also discussed. [Copyright &y& Elsevier]

The author’s recent studies on the critical current properties in Bi-2223 multifilamentary tapes as well as YBCO coated IBAD tapes have been reviewed. Extended electric field vs. current density (E–J) characteristics were studied over wide range of temperature, T, magnetic field, B, and angle, θ. Based on the analysis of statistic Jc distribution and scaling, we derived an analytical expression for J as a function of E, T, B, and θ. This method allows us to describe and extrapolate the nonlinear E–J characteristics even in extremely low E region and/or high B region based on rather simple measurements. [Copyright &y& Elsevier]

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 current transport property in Gd1Ba2Cu3O7− δ coated conductor with artificial pinning centers in a wide range of temperature, magnetic field, B up to 27T, and field angle. Due to the additional c-axis correlated pins, critical current density, J c in B//c was enhanced and the improvement was observed in wide range of B. On the other hand, around B⊥c below 65K, the angular dependence of n-value showed a valley-like behavior, although the J c was increasing. In addition, the temperature dependence of the pinning force density defined as J c × B was not scaled on an expected master curve. These results indicate the pinning in B⊥c is governed by different mechanism below 65K and high magnetic field. [ABSTRACT FROM AUTHOR]

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: Prior to the development of fabrication technique for the chemical vapor deposited (CVD) YBa2Cu3O7-δ coated conductor on a IBAD-Gd2Zr2O7, investigations on the improvement of spatial homogeneity have been done. By using spatially resolved measurements and combined multiple microanalysis techniques with length scale of several μm, physical and transport properties of the CVD samples have been investigated before and after fabrication modification. Structural inhomogeneity was visualized using thermoelectric voltage imaging (TVI) technique using a laser scanning microscope. Laser scanning microscopy at superconducting temperature is used to visualize flux flow dissipation; furthermore, mappings of 2D local current flow density distribution have been done using a scanning SQUID microscopy. It has been shown that the superconducting layer consisted of YBCO matrix with localized defects originating from the buffer layer. This led to current non-uniformity and caused high flux flow dissipation within the vicinity of the defects. Process conditions have been modified effectively based on those insights. After fabrication modification, our measurement analyses shows that the texturing of the YBCO layer improved significantly and the appearance of spatially distributed obstacles that are responsible for non-uniform current distribution and localized dissipation are reduced. Our complementing, quick yet non-invasive technique not only can quantify the improvement of YBCO homogeneity but also shed light on the basic understanding of the current limiting mechanism in the IBAD based coated conductors. [Copyright &y& Elsevier]

Abstract: Nonlinear current flow around artificial defects in YBCO epitaxial film has been investigated by use of scanning SQUID microscope. Two-dimensional local current flow around the transverse defects in the YBCO film has been visualized successfully from the magnetic self-field image. Experimental results showed good agreement with theoretical solution based on the Maxwell’s equation along with nonlinear current–voltage characteristics. Present method allows us to measure the non-uniform current flow quantitatively under the influence of obstacles in the micro-meter length scale. [Copyright &y& Elsevier]

We applied OFC-magnetometer (open-flat coil-based tunnel diode oscillator) to study fine peculiarities of vortex flow and magnetic phase (MP) transitions between different phases of the vortex-matter in a high quality YBa2Cu3Oy crystal. The method is based on a detection of the inductance and Q-factor value changes of a single-layer flat coil, which is placed at a face of flat sample. These finally lead to frequency and/or amplitude changes of the oscillator. Simultaneous frequency and amplitude tests enabled to construct the vortex MP-diagram for untwinned YBa2Cu3Oy crystal with (0.25×0.6) mm2 area and ∼50 μm-thick in the temperature range of 65 K to Tc and the field of 0–12 T. In outlines, the plotted diagram is similar to the one defined by several other methods such as SQUID and cantilever magnetometers, etc. for the same crystal. However, our technique revealed clear separation of the lines on a diagram below the upper critical point Hucp i.e., no kink point was observed in our method. The amplitude tests revealed also some new lines on a diagram below the lower critical point Hlcp (close to Tc). The obtained results may help us to understand the behavior of the vortex-mater and its dynamics in HTS materials. The problem how to improve resolution of the method for very small-size HTS objects is also discussed. [Copyright &y& Elsevier]

Single-layer open-flat coil-based tunnel diode oscillator (OFC-magnetometer) is sensitive to small changes of penetration depth of MHz-range magnetic field into plate-like high-Tc superconductivity (HTS) materials. Δλ∼1–3 A˚ absolute and better than Δλ/λ∼10−6 relative resolution is reached, which corresponds to changes ∼1 pH of HTS film's magnetic inductance. It operates in high magnetic fields and measures the linear changes of λ in a wide dynamic range. Owing to pick-up coil's flat design, relatively low operation frequency and high relative resolution this method has advantages, compared to others, which become crucial at non-destructive studies with thin flat HTS materials (with a small signal), especially near Tc (at the start of “Cooper” pairs' formation). Due to these, a new “paramagnetic” (PM) peculiarity of superconductive transition was detected in YBaCuO film recently, which precedes well-known “diamagnetic” ejection. It specifies the details of an actually complicated shape of the N/S transition curve. In this report we discuss influence of the magnetic field on PM effect and relationship between the quality of HTS film and the amplitude of the effect. Differences between the PM effect and the known, externally similar, resistive “peak” as well as “paramagnetic Meissner” effects is shortly discussed too. [Copyright &y& Elsevier]

We applied OFC-magnetometer (open-flat coil-based tunnel diode oscillator) to study fine peculiarities of vortex flow and magnetic phase (MP) transitions between different phases of the vortex-matter in a high quality YBa2Cu3Oy crystal. The method is based on a detection of the inductance and Q-factor value changes of a single-layer flat coil, which is placed at a face of flat sample. These finally lead to frequency and/or amplitude changes of the oscillator. Simultaneous frequency and amplitude tests enabled to construct the vortex MP-diagram for untwinned YBa2Cu3Oy crystal with (0.25×0.6) mm2 area and ∼50 μm-thick in the temperature range of 65 K to Tc and the field of 0–12 T. In outlines, the plotted diagram is similar to the one defined by several other methods such as SQUID and cantilever magnetometers, etc. for the same crystal. However, our technique revealed clear separation of the lines on a diagram below the upper critical point Hucp i.e., no kink point was observed in our method. The amplitude tests revealed also some new lines on a diagram below the lower critical point Hlcp (close to Tc). The obtained results may help us to understand the behavior of the vortex-mater and its dynamics in HTS materials. The problem how to improve resolution of the method for very small-size HTS objects is also discussed. [ABSTRACT FROM AUTHOR]

Single-layer open-flat coil-based tunnel diode oscillator (OFC-magnetometer) is sensitive to small changes of penetration depth of MHz-range magnetic field into plate-like high-Tc superconductivity (HTS) materials. Δλ∼1–3 A˚ absolute and better than Δλ/λ∼10−6 relative resolution is reached, which corresponds to changes ∼1 pH of HTS film's magnetic inductance. It operates in high magnetic fields and measures the linear changes of λ in a wide dynamic range. Owing to pick-up coil's flat design, relatively low operation frequency and high relative resolution this method has advantages, compared to others, which become crucial at non-destructive studies with thin flat HTS materials (with a small signal), especially near Tc (at the start of “Cooper” pairs' formation). Due to these, a new “paramagnetic” (PM) peculiarity of superconductive transition was detected in YBaCuO film recently, which precedes well-known “diamagnetic” ejection. It specifies the details of an actually complicated shape of the N/S transition curve. In this report we discuss influence of the magnetic field on PM effect and relationship between the quality of HTS film and the amplitude of the effect. Differences between the PM effect and the known, externally similar, resistive “peak” as well as “paramagnetic Meissner” effects is shortly discussed too. [ABSTRACT FROM AUTHOR]

The behavior of current–voltage (I–V) characteristics in Bi-2223 AgAgMg sheathed tape at various bending strain has been studied. The measured results were analyzed using the percolation model, and then the variation of critical current density, Jc, has been analyzed. As increasing of bending strain, Jc in the tape is reduced irreversible way. The irreversible degradation indicates that the Jc reduction is caused mainly by crack formation. However, if we extract statistic Jc distribution taking into account the cracking, it can be seen that the value of local Jc is enlarged in the remained region as the strain is increased. Namely, the pinning strength in the remained region is enlarged as the bending strain is increased. Present analysis allows us to separate the influence on grain connectivity and pinning strength originated from the strain. [Copyright &y& Elsevier]

Abstract: We have investigated electric field criterion of in-plane critical current density in a coated conductor characterized by scanning Hall-probe microscopy (SHPM). From remanent field distribution and its relaxation measurements, we could obtain critical current distribution and induced electric field simultaneously by considering the Biot-Savart law and the Faraday’s law, respectively. These results lead us to evaluate a distribution of local critical current density and the corresponding criterion of electric field. As a result, it was found that the electric field criterion for the SHPM analysis was several orders lower than that used in the conventional 4-probe resistive method. However, the data point obtained by the SHPM shows good agreement with E–J curve analytically extended from the measurements by the 4-probe method. This means that we could characterize in-plane distribution of critical current density in a coated conductor at an electric field criterion quantitatively by this method in a nondestructive manner. These findings will be very important information since the uniformity of local critical current density in a coated conductor at extremely low electric fields is a key issue (1) especially for DC applications, (2) for quality control of coated conductors, and (3) for the standardization of the characterization of critical current among different methods. [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]

Abstract: Metal organic deposition using trifluoroacetates is expected as a method for YBCO coated conductors since this method can provide high superconducting properties and is a cost-effective process with non-vacuum system. Recently, REBCO coated conductors, which has high critical properties, have attracted considerable attention. However, there are some problems to optimize fabrication conditions of REBCO films with this method. In this study, we fabricated SmBCO films by TFA-MOD method and investigated the effect of fabrication conditions on crystalline. We focused especially on a starting temperature (T 1) of inlet moisture in calcination step and O2 gas concentration in crystallization step. Flat surfaces were obtained for the calcined films at both T 1 =300°C and 350°C compared with that at 200°C. One possible reason for this worse morphology at T 1 =200°C might be a length of a reaction time between moisture and coated film. When moisture is supplied from low temperature for long time, water could react easily to the fluoride which is contained a lot in the coated film; as a result, more HF gas is generated from the film. The crystalline was also improved for the SmBCO films calcined at over T 1 =300°C. Furthermore, the film calcined at T 1 =350°C was crystallized under the condition that O2 gas concentration was varied between 0.2ppm and 1000ppm. The degree of c-axis orientation depends on oxygen partial pressure and the highest c-axis oriented SmBCO film was obtained in the case of 10ppm. [Copyright &y& Elsevier]

Abstract: Metal organic deposition process using trifluoroacetates (TFA-MOD) is one of the most promising processes to fabricate YBCO film. It has been reported that YBCO films grown by the starting solution with Ba-poor (cation ratio as Y:Ba:Cu=1:1.5:3) have higher JC value and has smaller and less pores than those of the YBCO film with stoichiometric composition. It is important to investigate the growth mechanism of YBCO crystals to obtain a high JC film by controlling the crystal structures. In this study, YBCO films were fabricated under various Ba concentrations in the TFA starting solution, and the influences of Ba composition on the growth process and microstructures were investigated. As a result, the Ba-poor YBCO film with Ba/Y=1.5 and high JC had less a-axis oriented Y123 in comparison with the film with Ba/Y=2. Furthermore, pores in the Ba-poor film were less than that in the stoichiometric composition film. This decrease of pores in the Ba-poor film was considered to be caused by the smaller size of non-reacted phases especially such as Ba–F rich particles entrapped by growing Y123 in the growing Y123 layer. It is considered that both the reductions of a-axis oriented Y123 and pores were the reasons of improving JC values in Ba-poor film. [Copyright &y& Elsevier]

Abstract: YBCO films were grown on LaAlO3 substrates by a metal organic deposition using trifluoroacetates (TFA-MOD). In the TFA-MOD method, a starting solution is coated on a substrate firstly, and then the coated film is calcined to form precursor and finally the precursor film is crystallized to form superconducting YBCO film. In this study, the effects of calcination conditions such as temperature on the microstructures and J c properties of the YBCO films were studied. In the calcination step, spin coating and calcination were repeated three times for obtaining thick film under the maximum temperatures of 400–500°C. The prepared precursor films were crystallized at a constant condition. The J c values of the films calcined at 500°C after at 400°C×2 times, 400°C×3 times and 500°C×3 times were about 1.49MA/cm2, 2.52MA/cm2 and 1.47MA/cm2, respectively. J c depended strongly on the calcination temperature. The cross-sectional TEM images showed that the YBCO film calcined at 400°C×3 times had less pores than the others. Optimization of the calcination temperature was important to suppress the formation of large precipitates in precursor film, resulting in the suppression of the large pores in the YBCO film. [Copyright &y& Elsevier]

Abstract: We have developed a new in situ preparation process for trilayer structures of vertically-stacked Josephson junctions. We adopted YBa2Cu3O7−x (YBCO) as a base electrode, and YbBa2Cu3O7−x (YbBCO) as a counter electrode in these junctions. The barrier was formed by sputtering in a reduced total gas pressure and subsequent annealing. As the gas pressure decreases, the mean free path of the negative ions increases, which results in the bombardment of higher energy ions to the substrate and in etching the base electrode surface. The critical current I c of the obtained junction was modulated up to 50% by applying an external magnetic field. The difference of the characteristics between the junctions fabricated by this new technique and conventional interface-treated junctions would be attributed to the difference in the species and their energy distributions of the particles incident to the sample during the barrier formation. [Copyright &y& Elsevier]

The E–J characteristics were measured for a YBCO coated-conductor by using AC inductive method of third-harmonic voltage. The results are compared with the measurements obtained by the four probe method and DC relaxation method of magnetization. The results by three kinds of measurements meet approximately on a single curve and E–J curve in a wide range of electric field is obtained. Hence, the third-harmonic voltage method is useful for a continuous measurement of Jc property of long coated tape. The obtained E–J characteristics are argued. [Copyright &y& Elsevier]

The E–J characteristics were measured for a YBCO-coated conductor by using the four probe method and the relaxation method of DC magnetization. The result on the latter measurement at extremely low electric fields was theoretically analyzed based on the flux creep-flow model. From the result on n-value the flux lines in YBCO are considered to be in the glass state even at high fields. The apparent pinning potential U0* estimated from the relaxation of DC magnetization is much larger than that of Bi-2223. The present theoretical analysis clarifies a much stronger flux pinning and a higher dimensionality of YBCO in comparison with Bi-2223. [Copyright &y& Elsevier]

It is known that the critical current density vs. field angle of an Y-123 coated tape has a broad peak around the field direction normal to the tape surface. In this work, the angular dependencies of the irreversibility field and the critical current density are analytically investigated using the flux creep-flow theory with an assumption of the angular dependence of flux pinning strength, and these results are compared with the experimental results. The angular dependence of flux pinning strength obtained so as to get a good agreement between the experimental and theoretical results suggests an existence of long pinning centers along the c-axis. [Copyright &y& Elsevier]

We have successfully demonstrated the elimination of excess currents in interface-treated Josephson junctions (ITJs) having vertically stacked trilayer structure based on c-axis oriented YBa2Cu3O7−δ (YBCO) films. Several experimental results obtained so far suggest that the excess currents are transported via superconducting short paths formed in the tunnel barrier. Considering that short paths are made of YBCO, we intend to patch the paths into insulators by doping PrGaO3, after the amorphous layer on the surface of the base YBCO electrode was formed by etching process. The other fabrication sequences are the same as the previous ones. The obtained ITJs showed excellent resistively shunted-junction type current–voltage (I–V) characteristics and almost 100% of modulation depth of the critical current for an external magnetic field at 4.2 K. Even for smaller value of Jc, the vertically stacked type ITJs showed almost the same IcRn value as that obtained for ramp-edge type IMJs with higher Jc. Furthermore, the yield is remarkably improved by using this technique. [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]

Abstract: We investigated joint properties of REBCO (REBa2Cu3O7− X , RE=rare earth element) coated conductors using a diffusion joint process. Two pieces of the coated conductor tapes with stabilizing silver layer on REBCO were overlapped in a face to face manner, which were pressurized and heat-treated at about 623–673K in oxygen atmosphere. The joint resistance for YBCO tape was about 6.5nΩcm2 at 77K, on the other hand, that for GdBCO tape was about 37nΩcm2 which was higher than YBCO. From the results of both cross-sectional transmission electron microscope (TEM) and energy dispersive X-ray spectrometry (EDS) analysis, diffusion of Gd-element into Ag layer was observed for the joined sample of GdBCO tape, it is suggested to be a cause of higher joint resistance for GdBCO tape. [Copyright &y& Elsevier]