TFA-MOD Method Research Articles

Enhancement of In-Field Critical Current Density of BaZrO3-Added (Y, Gd) BCO-Coated Conductors by Using a Multi-Coating TFA-MOD Method

We have investigated the in-field critical current properties in BaZrO3-added (Y, Gd) Ba2Cu 3O7-δ (YGdBCO)-coated conductor fabricated by using a multi-coating trifluoroacetates metal-organic deposition (MOD) method. We compared these properties with EuBCO-coated conductor fabricated by the pulsed laser deposition (PLD) method. The sample using the 30-nm once-coat-layer-thickness d once shows superior in-field J c down to 4.2 K than that of the previous standard coating using 170–nm-thick layer for each coating. From the analysis of E – J characteristics, an analytical expression for J c as a function T and B has been derived. It was confirmed that the high J c region in sample with d once = 30 nm is widely spread in high-temperature and high-magnetic-field region. By comparison with the J c properties of the BaHfO3 -added PLD CCs, the minimum J c, which is estimated from magnetic field angle dependence, shows even higher value up to 5 T of magnetic field at 65 K and up to 3 T at 77 K. From these results, the new MOD-YGdBCO process using the thin once-coat-layer-thickness is very promising for the practically important mid field region such as 3 to 5 T.

Refining Process of BaZrO3Particles in Coated Conductors by TFA-MOD Method

A wide range of coating thicknesses was utilized in the trifluoroacetate metal-organic decomposition process for (Y, Gd) Ba 2 Cu 3 O 7-δ (YGdBCO) coated conductor including BaZrO 3 (BZO) particles with a thin once-coated thickness(donce), which was developed for improving the critical current density in a magnetic field (Jc(B)). The value of Jc (B) increased as donce decreased to 30 nm, and a sample fabricated with donce = 30 nm exhibited high values of Jc(B) of 0.27 and 1.60 MA/cm 2 at 77 and 65 K, respectively, at 3 T (B//c). However, the value of Jc( B) of a sample with donce = 17 nm was almost the same as or slightly lower than that of the sample with donce = 30 nm. This tendency was explained by the change in the size of the BZO particles in the YGdBCO superconducting films. Additionally, the Cu elemental peak and the valleys of other elements were observed at the same position and at certain intervals in the elemental concentration line profiles along the depth direction of the precursors, fabricated under different donce conditions from 170 to 30 nm. However, the peaks and valleys were extremely suppressed, and the wave structure was barely observed in the sample with donce = 17 nm. The refining of BZO particles by reducing donce could be explained by the limited diffusion due to the existence of the concentration valleys/peaks. Moreover, the effect of the diffusion barrier due to the valleys/peaks might become weaker in the sample with donce = 17 nm.

Artificial control for nucleation and growth rate of YBa2Cu3O7-δ coated conductors prepared by low fluorine chemical solution deposition

The influence of various gas flow conditions on the nucleation and growth rate of YBa2Cu3O7-δ (YBCO) coated conductors in the high-temperature heat treatment stage of low fluorine chemical solution deposition (LF–MOD) process have been investigated, including the gas flow rate and gas flow direction. The results indicated that film-growth rate is faster while a-axis oriented phase coexisted with c-axis in the films at the higher gas flow rate. And YBCO (00l) grains grew slowly while a-axis oriented phase also nucleated easily on the end of the long films along gas flow direction. It could be caused by large quantities of HF gas accumulating from the chemical reaction for converting YBCO phase. Hence, the crystal mechanism of various LF–MOD methods can be supposed as still “BaF2” process as the TFA-MOD method, and orientation nucleation and growth rate of YBCO films can be artificial control by adjusting the gas flow conditions.

Another approach for controlling size and distribution of nanoparticles in coated conductors fabricated by the TFA-MOD method

A new process, which is conceptually different from that reported so far, was developed for positive and direct control of the size and distribution of BaZrO3 (BZO) nanoparticles acting as the effective magnetic flux pinning centers for coated conductors (CCs) fabricated on buffered metal substrates by the trifluoroacetate-based metal-organic decomposition method. The (Y,Gd)Ba2Cu3O6+δ (YGdBCO) phase crystallization heat-treatment process was applied one time to a multi-layered precursor prepared by repeating the coating/calcination process of the extremely thin layer of a few tens of nanometers. The size of BZO nanoparticles refined with thinning the once coated film thickness (donce) even in the same total film thickness of 0.6−0.7 μm, consequently the critical current density in magnetic-field (Jc(B)) was increased. A YGdBCO CC fabricated with donce of 30 nm revealed very high Jc(B∣∣c) characteristics of 0.26 and 1.60 MA cm−2 at 77 and 65 K, respectively, for 3.0 T.

High quality uniform YBCO film growth by the metalorganic deposition using trifluoroacetates

A need exists for the large-area superconducting YBa2Cu3O7-x (YBCO) films with high critical current density for microwave communication and/or electric power applications. Trifluoroacetic metalorganic (TFA-MOD) method is a promising low cost technique for large-scale production of YBCO films, because it does not need high vacuum device and is easily applicable to substrates of various shape and size. In this paper, double-sided YBCO films with maximum 2in diameter were prepared on LaAlO3 substrates by TFA-MOD method. Inductive critical current densitiy Jc, microwave surface resistance Rs, as well as the microstructure were characterized.A newly homemade furnace system was used to epitaxially grown YBCO films, which can improve the uniformity of YBCO film significantly by gas supply and temperature distribution proper design. Results showed that the large area YBCO films were very uniform in microstructure and thickness distribution, an average inductive Jc in excess of 6MA/cm2 with uniform distribution, and low Rs (10GHz) below 0.3mΩ at 77K were obtained. Andthe film filter may be prepared to work at temperatures lower than 74K. These results are very close to the highest value of YBCO films made by conventional vacuum method, so we show a very promising route for large-scale production of high quality large-area YBCO superconducting films at a lower cost.

Miniaturization of BaHfO3 nanoparticles in YBa2Cu3Oy-coated conductors using a two-step heating process in the TFA-MOD method

The critical current density (Jc) of YBa2Cu3Oy (YBCO)-coated conductors in a magnetic field can be enhanced by the doping of flux-pinning centers in a metal organic deposition (MOD) process with trifluoroacetates (TFA). The size of these flux-pinning centers should be less than 10 nm to achieve commercial use due to the coherence length of YBCO at 77 K. In this paper, BaHfO3 (BHO) nanoparticles were introduced into YBCO films using the TFA-MOD method. Microstructures and the Jc properties of the films prepared using a two-step heating process at crystallization were compared with film prepared using a conventional one-step heating process. The two-step heating process produced 15 nm average-sized BHO nanoparticles in the film compared to 19 nm nanoparticles in a film prepared using a one-step process. It was revealed that the size of nanoparticles in the films could be miniaturized by improving the heating processes in the MOD method, and the miniaturized nanoparticles could contribute to increased Jc in magnetic fields. The mechanism of miniaturization is also discussed based on microstructure observations of quenched films.

Phase Transition During Heat Treatment of Precursor Before YBa2Cu3O7−δ Nucleation in TFA–MOD Method

This work presents a study of the evolution of the phases in the intermediate heat treatment process (IHP) which was after the pyrolysis process and before the crystallization of YBCO superconducting thin films. It was shown that the evolution of the solid solution could always take place whether with or without water vapor and oxygen. When water vapor is present in the atmosphere, the evolution temperature of the intermediate phase was lower than the one without vapor. The intermediate phase of Y2Cu2O5 with the vapor was observed at 680 °C. By calculating the changes of Gibbs free energy of reactions possibly involved in the IHP, the priority reaction with water vapor was CuF2>YF3>BaF2 when CuF2, YF3, and BaF2 coexisted while CuF2 and YF3 prefer to react with BaO to form BaF2 while they coexisted. The appearance of intermediate phase features could be illustrated through the calculation.

Development of REBCO Coated Conductors by TFA-MOD Method With High Properties in Magnetic Fields

With technological advance of the REBaCuO (RE: Rare Earth elements, REBCO) coated conductors (CC), development of the superconducting applications for its practical use has been carried out recently in various fields. In addition, improvement of the magnetic properties of REBCO CCs makes superconducting devices feasible in the liquid nitrogen. The higher Ic values are required in high magnetic fields by the viewpoint of the requirement in conjunction with the superconducting CCs. Therefore the improvement of the Ic values in the magnetic fields is one of the most important problems to apply them to a device (e.g., NMR, MRI, others). In this work, we investigated the fabrication processing to improve the in-field properties of the YGdBaCuO (YGdBCO) CCs by introduction of artificial pinning centers (APCs). The TFA-MOD method using the batch type furnace for the conversion reaction as a cost effective process to fabricate REBCO CCs was improved by adopting the interim heat treatment during the ramp step of the conversion reaction. The improvement of the properties by the improved process was remarkable and the advanced CCs were named as nPAD-YBCO. A 124 m-long nPAD-YBCO consisted of YGdBCO CC with BZO nano particles was successfully fabricated, exhibiting the I cmin > 50 A/cm-w @77 K, 3 T (I c : Critical current) throughout the conductor. Analyzing the characteristics of the conductor, the BZO particles introduced into the YGdBCO layer worked as effective APCs in wide ranges of temperature and magnetic fields.

人工ピンニングセンター導入型 TFA-MOD REBCO 超電導線材の開発

人工ピンニングセンター導入型 TFA-MOD REBCO 超電導線材の開発

A low-fluorine solution with a 2:1 F/Ba mole ratio for the fabrication of YBCO films

In previously reported low-fluorine MOD-YBCO studies, the lowest F/Ba mole ratio of the precursor solution was 4.5. Further lowering the F/Ba ratio would bring benefits for the environment, thick film deposition, and an understanding of the heat treatment process. On the other hand, the F/Ba ratio must be at least 2 for full conversion of the Ba-precursor to BaF2 to avoid the formation of BaCO3, which is detrimental to the superconducting performance of YBCO films. In this study, a solution with a 2:1 F/Ba mole ratio was developed, and the fluorine content of this solution was approximately only 10.3% of that used in the conventional TFA-MOD method. Attenuated total reflectance-Fourier transform-infrared spectra (ATR-FT-IR) revealed that BaCO3 was remarkably suppressed in the as-pyrolyzed film—and eliminated at 700 °C. Thus, YBCO films with a critical current density (Jc) of over 5 MA cm−2 (77 K, 0 T, 200 nm thickness) could be obtained on lanthanum aluminate single-crystal substrates. In situ FT-IR spectra showed that no obvious fluorinated gaseous by-products were detected in the pyrolysis step, which indicated that all F atoms might remain in the film as fluorides. X-ray diffraction θ/2θ scans showed the presence of BaF2—but not of Y F3 or CuF2—in films quenched at 400–800 °C. The formation priority of BaF2 over Y F3 and CuF2 was interpreted by examining the chemical equilibrium of the potential reactions. Our study could enlarge the synthesis window of precursor solutions for MOD-YBCO fabrication, and serve as a foundation for continuously and systematically studying the influence of fluorine content in the precursor solutions.

Improvement of Production Rate of YBCO Coated Conductors Fabricated by TFA-MOD Method

The metal-organic deposition (MOD) method using trifluoroacetate (TFA) salts is considered to be an effective method for inexpensively fabricating YBa2Cu3O7-y (YBCO) coated conductors with high critical current density property. The long-length TFA-MOD YBCO coated conductors have been fabricated by multi-turn reel-to-reel system. Increasing the thickness per single coating in the multi-turn reel-to-reel system is a cost-effective technique for fabrication of the precursor films in the calcination process since it reduces the number of coatings and shortens the processing time. In this work, we have developed a new starting solution consisting of non-fluorine salts of yttrium propionate and copper 2-ethylhexanoate with focusing on increasing the thickness per single coating for a high-rate fabrication of the YBCO coated conductors by the TFA-MOD method. The critical thickness per single coating of the precursor film fabricated from the new starting solution was improved to 0.44μm/coat. Furthermore, the addition of diacetoneacrylamide in the new starting solution increased the critical thickness per single coating to 0.79μm/coat. High critical current of 791 A/cm-width with high critical current density of 2.7 MA/cm2 was obtained using the new starting solution with diacetoneacrylamide at the thickness per single coating of 0.49μm/coat.

Development of REBCO Coated Conductors by TFA-MOD Method With High Characteristic in Magnetic Field

YxGd(1 - x)Ba2Cu3 Oy coated conductors (CCs) with artificial pinning centers (APCs) were fabricated by the trifluoroacetates-metal organic deposition method using a batch heat-treatment process. This process has been industrially applied to fabricate long superconducting tapes that have high critical current (Ic). From the viewpoint of applications, the characteristics in magnetic fields become further important, and one solution has been shown previously as introduction of APCs such as BaZrO3. We applied the technique to our CCs fabrication process. The heat-treatment conditions, specifically total gas pressure, oxygen concentration, and crystallization temperature, were optimized for the batch heat-treatment process. As a consequence, fabricated CC showed extremely high characteristic Jc value of 3.5 MA/cm2 in self field, and 0.3 MA/cm2 at 3 T in liquid N2. Moreover, by cross-sectional transmission electron microscopy observation, it was confirmed that the APC was distributed uniformly. Furthermore, the anisotropy of the pinning center is also examined based on the evaluation result of the angular dependence of Jc in 3T.

Study on the oxygenation process during the heat treatment of TFA-MOD YBCO thin films by in situ resistance measurement

The oxygen content is one key factor to determine the properties of YBa2Cu3O6+y (YBCO) high temperature superconductors. In this study, YBCO thin films were produced by TFA-MOD method. The oxygenation process was carried out at 450°C for 40min, in various oxygen partial pressures from 0.01 to 1atm. An in situ resistance measurement system was built up to record the resistance evolution during the whole heat treatment process. It was found that the resistance decreased exponentially and reached a saturate value in a few minutes during oxygen annealing. It was also found both the balanced resistance and the c-axis length of YBCO decreased with increasing oxygen partial pressure. A defect reaction was found to control the mechanism of the oxygenation process. A porosity assisted oxygen diffusion mechanism was proposed to explain the fast diffusion kinetics of oxygen in MOD YBCO thin films.

Effect of holding temperature on microstructures and Jc properties of YBa2Cu3O7−X films fabricated by TFA-MOD method

In this study, we investigated the correlation between microstructures and critical current density (Jc) of YBa2Cu3O7−X (YBCO) films with BaZrO3 (BZO) pinning centers by introducing holding temperature during the crystallization process. Smaller BZO particles and higher in-field Jc were found in the YBCO film with holding temperature than the one without. As a result, films with improved microstructures were successfully fabricated by implementation of additional process, holding temperature, and highly in-field Jc were obtained by introducing smaller BZO particles as artificial pinning centers.

High-rate fabrication of YBCO coated conductors using TFA-MOD method

Abstract The YBa2Cu3O7-y (YBCO) coated conductors derived from metal-organic deposition (MOD) method using the metal salts of trifluoroacetic acid (TFA) have been developed with high critical current property. The long-length YBCO coated conductors have been fabricated by multi-turn reel-to-reel system. Increasing the thickness per single coating in the multi-turn reel-to-reel system is a cost-effective technique for fabrication of the precursor films in the calcination process. In this work, we have developed a new starting solution consisting of non-fluorine salts of yttrium 4-oxopentanoate and copper 2-ethylhexanoate with focusing on increasing the thickness per single coating for a high-rate fabrication of the YBCO coated conductors by the TFA-MOD method. The critical thickness per single coating of the film fabricated from the new starting solution was 0.54 μm/coat. High critical current of 377 A/cm-width with high critical current density (3.0 MA/cm2) was obtained in the YBCO film fabricated on CeO2 buffered LaMnO3/MgO/Gd2Zr2O-7/HastelloyTM substrates using the new starting solution at the thickness per single coating of 0.42 μm/coat.

Flux pinning properties of YBCO films with nano-particles by TFA-MOD method

Nano-particles were doped into YBCO films as pinning centers by a metal organic deposition (MOD) method using trifluoroacetates. Two types of initial solution with a cation ratio of Y: Ba: Cu = 1: 1.5: 3 were prepared; one with the dispersion of SnO2 particles with the size of 15-25 nm and the other one with the dispersion of smaller ZrO2 particles with the size of under 8 nm, then the solution was spin-coated on CeO2/Gd2Zr2O7/Hastelloy substrates. The coated films were calcined at 430 °C in oxygen atmosphere and crystallized at 780 °C in low oxygen atmosphere. From the results of X-ray diffraction analysis (XRD), peaks of BaSnO3 were observed clearly in the YBCO film by the starting solution with SnO2. On the other hands, little peaks corresponding to BaZrO3 were observed in the film by the solution with ZrO2. Many CuO segregations were recognized at the surface of SnO2 doped YBCO film in comparison to the YBCO film with ZrO2 doping. From these results, it is indicated that most of SnO2 particles in precursors are react with Ba during heating. Critical current density ( J C) of the YBCO films by both solutions showed higher performance than that of pure YBCO film in magnetic fields.

Growth process of BaZrO3 doped YBCO films by TFA-MOD method

Crystal growth process of YBa2Cu3O7-X (YBCO) films with BaZrO3 (BZO) pinning centers were investigated to enhance JC property by controlling microstructure of the films. The YBCO films were fabricated by a metal organic deposition (MOD) method using solutions with trifluoroacetates (TFA) and Zr-salts. Quenched films were prepared by cooling them rapidly during crystallization process and crystallized phases were identified by an X-ray diffraction (XRD) measurement. It is indicating that BZO forms at lower temperature than that of YBCO formation and that BZO and BaF2 are crystallized at the similar temperature range around 700°C. Then, we kept the heating temperature which is under 600°C before crystallization temperature of YBCO and investigated the effect of temperature keeping on film growth. In the film kept for more than 3 hours, BZO peak was detected by XRD measurement. However, BZO particles were not observed in the film even kept for 9 hours by transmission electron microscopy (TEM) and energy dispersive X-ray (EDS) analyses. It is indicated that growth rate of BZO is slow at 600°C. On the other hand, smaller YBCO particles and decreasing of surface roughness (Ra) were observed for the film which were kept at 600°C for 3 hours and then crystallized. This result suggests the density of YBCO film is higher than that for YBCO without that process. In summary, it can be considered that YBCO film density become high by temperature keeping process below 600°C before YBCO crystallization and that size of BZO particles are determined by heat treatment at the temperature of above 600°C

Influence of Gas Flow and Improvement of Homogeneity on the Distribution of Critical Current Density in YBCO Coated Conductor Processed by TFA-MOD Method

Abstract Using a scanning Hall-probe microscopy, we have investigated in-plane distribution of critical current density in TFA-MOD processed YBCO coated conductors. We compared the distributions of critical current density for two kinds of coated conductors processed with different directions of gas flow at the calcinations. As a result, it was found that the direction of the gas flow largely influenced the distribution of critical current density. For example, the maximum value of critical current density was 1.5 times higher than the average for a sample processed with a gas flow in width direction. On the other hand, the distribution of critical current density was relatively uniform for the one with a gas flow in axial direction perpendicular to the surface of the conductor. These findings will be very important information for the optimization of the manufacturer processes for the conductors. Actually, a very uniform distribution of critical current density has been observed for a coated conductor produced by an optimized process. This demonstrates a high potential of TFA-MOD processed YBCO coated conductors for practical applications.

Study of Modified TFA-MOD Method for YBCO Film Growth

Abstract The traditional all-TFA precursor solution for coated conductors has sensitivity to the heating rate in pyrolysis process. This sensitivity could be weakened by using a modified precursor solution, which was prepared by the mixture of yttrium trifluoroacetate, barium trifluoroacetate, and copper benzoate. The YBCO films were deposited on buffered NiW substrates (NiW/Y2O3/YSZ/CeO2) with the modified precursor solution. The texture, microstructure and superconducting properties of YBCO films were characterized by X-ray diffraction, scanning electron microscopy and four-probe method, respectively. The YBCO films prepared by modified TFA-MOD method demonstrated high performance.

Field Angle Dependence of Critical Current Density in YGdBCO Coated Conductor

The improvement of critical current density properties of REBCO coated conductors for applications in superconducting power devices is desired. It is known that the critical current density Jc of REBCO thin films made by the TFA-MOD method in a magnetic field is greatly improved by the introduction of BZO-nanoparticles of a non-superconducting phase as artificial pinning centers. In addition, the field angle anisotropy can also be reduced by isotropic flux pinning interactions of nanoparticles. However, a detailed report on the mechanism of flux pinning with a quantitative investigation in such coated conductors has not been given. In this study, the field angle dependence of the critical current density is measured for TFA-MOD-processed YGdBCO coated conductor with a superconducting layer of 0.65 μm thick, and the results are theoretically analysed to clarify the flux pinning mechanism.