TFA-MOD Process Research Articles

Preparation of high performance YGdBCO films by low fluorine TFA-MOD process

YBCO films doped with different contents of gadolinium (Gd) were prepared by the low-fluorine (low-F) trifluoroacetate metal-organic deposition (MOD) method. The effects of flow rate and holding time of the firing (crystallization) stage on the superconducting properties of YxGd1–xBa2Cu3O7–δ (YGdBCO) films were investigated. The phase formation and texture were characterized by the X-ray diffraction (XRD), which indicate that severe degradation of the microstructure will be induced with the inappropriate flow rate. The surface morphology and element distribution were investigated by the scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that increasing the holding time of the firing stage is effective for the further decomposition of residual impurity phase on the surface. The mechanisms of the phase and surface evolution are also discussed. Finally, a high critical current density (Jc) value of 5.4 MA/cm2 was achieved in the Y0.9Gd0.1BCO film fabricated by the cooperative control of the flow rate and holding time of the firing stage, which are contributed to the formation of excellent texture, homogeneous microstructure and dense surface of the YGdBCO films.

Optimization of interim heat-treatment condition on TFA-MOD process for fabrication of Y0.77Gd0.23Ba2Cu3Oy coated conductors with BaHfO3

We have optimized the interim heat-treatment (IHT) time in trifluoroacetate based metal organic deposition process on fabrication of the BaHfO3 (BHO) doped Y0.77Gd0.23Ba2Cu3Oy (YGdBCO) coated conductor (CC) in order to further improve the in-field critical current density (Jc) performance. The in-field Jc values of 0.6 μm-thick BHO doped YGdBCO CCs increased with increasing the IHT time within 90 min. The highest critical current density (Jc) value of 0.86 MA/cm2 at 77 K in 3 T (B//c) was obtained for 0.6 μm-thick BHO doped YGdBCO CC with IHT time of 90 min. CCs with IHT time shorter than 90 min had relative low in-field Jc performance but high self-field Jc performance. It can be caused by the insufficient transformation to BaF2 in IHT which is a reason for the relatively large BHO particles. In the case of applying IHT for longer time than 90 min, coarsening of CuO particles in precursor films was suggested and degradation of self- and in-field Jc performances were observed.

Argon Ion Etching on Property of YBa2Cu3O7-x Thin Films Prepared by TFA-MOD Process

Argon Ion Etching on Property of YBa2Cu3O7-x Thin Films Prepared by TFA-MOD Process

Control of artificial pinning centers in REBCO coated conductors derived from the trifluoroacetate metal-organic deposition process

The metal-organic deposition (MOD) process using metal trifluoroacetate salts (TFA) has the advantages of low-cost and high-scalability for the fabrication of REBa2Cu3Oy (REBCO, RE: rare earth elements) superconducting coated conductors (CCs) with high critical current density, in principle, because of its non-vacuum process. For the magnetic applications of CCs such as motors, magnetic resonance imaging and superconducting magnetic energy storage, further improvement of superconducting performance under magnetic fields is required. However, the in-field superconducting performance of REBCO CCs derived from the TFA-MOD process had been inferior to those derived from the vapor-phase process. In order to improve the in-field performance, the size control of the artificial pinning centers has been known as an effective way. In the early stage, the BaZrO3 (BZO) material, which was one of the effective materials in the CCs by the vapor-phase process, was also introduced in the TFA-MOD-derived CCs. The unique feature of the BZO material in the TFA-MOD process is the shape. The BZO in the TFA-MOD process formed the particle shape, although in the vapor-phase process it has a rod shape with a long axis elongating along the thickness direction. In addition, a special heat treatment for refining the BZO particles was developed, which is called the ‘interim heat treatment’. This heating profile made the in-field characteristics higher, although they were still lower than those of the vapor-phase process. Then, the new MOD process including ‘ultra-thin once coating’ was recently developed for further refinement of the BZO particles. The characteristics of the new TFA-MOD-derived CCs in magnetic fields have become compatible with those of the CCs derived from the vapor-phase process.

Effect of Ba/Y Molar Ratio on Superconducting Properties of BaTiO3-Doped YBa2Cu3 O 7−δ Films

YBa2Cu3 O 7−δ (YBCO) films with BaTiO3 (BTO) nanostructures were prepared by using the precursor solutions with different cationic molar ratios of Y:Ba:Cu = 1.0:1.6–2.0:3.0 in the TFA-MOD process. These YBCO films were deposited on (00 l)-oriented LaAlO3 single-crystal substrates using a spin coater. The high superconducting critical current density (J C) (77 K, self-filed) of more than 10 MA/cm2 for the final BTO-doped YBCO film was obtained. Moreover, the effect of different Ba/Y molar ratios in the precursor solution on superconducting properties of BTO-doped YBCO films was investigated. Compared with the BTO-doped YBCO film deposited by using the precursor solutions with Ba/Y molar ratio of 2.0, an enhancement of J C in a magnetic field for the film from the solution with Ba/Y molar ratio of 1.9 was achieved. For Ba/Y molar ratios of 1.6 and 1.7, a reduction of J C in a magnetic field occurred. The J C enhancement may be mainly ascribed to the enhanced flux pinning by the Y2Cu2 O 5 nanostructures with the optimal number dispersing in YBCO matrix.

Development of BZO Doped YGdBCO High Performance Long CCs Using TFA-MOD Process

TFA-MOD (Metal Organic Deposition using Trifluoro-acetates) process is of considerable practical concern for future applications since it can fabricate high performance coated conductors (CCs) with low cost. We have previously reported the success in fabricating a short sample of BaZrO3 (BZO) nanoparticles dispersed Y0.77Gd0.23Ba1.5Cu3Oy (YGdBCO) thick films with high Ic values under magnetic field (67.8A/cm-w, @77K, 3T). To make thick CCs for long length, it is important to study the effect of process parameters varying with time. In this study, we investigated the effect of coating solution characteristics such as viscosity etc., which could vary with time due to evaporation of solvent, on the transverse film thickness distribution by using Reel-To-Reel (RTR)-dip coating method. We successfully fabricated BZO doped YGdBCO calcined films with 2.5μm thickness without crack and delamination over 200 m in length. The precursor films were crystallized by the batch type furnace at SWCC Showa Cable Systems Co., Ltd. resulting in the 124 m long CC with high Ic minimum value of 50 A/cm-w at 77K and 3 T.

Development of Low AC Loss TFA-MOD Coated Conductors

TFA-MOD process is expected to be promising for future applications since it can produce high performance YBCO coated conductors (CCs) with low cost. Applying YBCO CCs to the power electric devices such as transformers and power cables, the reduction of alternating current (AC) loss for long wire is necessary. Multi-filamentation process, which is one of the most effective approaches for AC loss reduction, has been developed by the scribing process. We have developed the filamentation process using chemical etching. MOD derived CCs are, however, easily damaged in the chemical etching process due to existence of pores in a YBCO layer, resulting in critical current (Ic)-degradation and weak delamination strength. Consequently, it is difficult to scribe MOD derived CCs into 1mm-wide filaments for long length using the chemical etching process. Accordingly, we have studied a scribing process using an excimer laser without chemical etching. We defined P’ value in this study as a function of irradiated laser power [J] divided by processing speed[m/s]. We studied relationship between the P’ value and the results of scribing. It was found that we could scribe the C.C. with a sufficient depth in the condition of large P’ value. Furthermore, we found that the Ic was degraded with further increase of the P’ value. A 5mm wide short sample was divided into 10 filaments by the excimer laser scribing process at the P’ of 9[J/(m/s)]. The sample revealed reduction of the hysteresis loss down to 1/10 which could be expected from a theoretical prediction using the numbers of the filaments. Ic-degradation was suppressed as 28%, which was smaller than that of the scribed sample using chemical etching (Ic degradation was 38%). Subsequently, we applied the technique to a 100m long YBCO CC. The hysteresis loss of the 100m long MOD derived CC was reduced down to 1/10 (1/the number of filaments) after the multi-filamentation.

Effect of Solution Composition on Superconducting Properties of YBCO Derived From TFA-MOD Process

The crystallization process of trifluoroacetate-metal-organic deposition derived YBa2Cu3O7-y (YBCO) crystals has been investigated to understand the influence of the starting solution composition on the crystal growth process. The starting solutions with molar ratios of Y: Ba: Cu = 1.0: 2.0: 3.0 (stoichiometry) and 1.0:1.5:3.0 (Ba-deficient) were examined. The highest Ic values were achieved at 100 min holding time of the conversion step for 0.8- μm-thick YBCO films, which were independent of the starting solutions. The degradation of Ic performance was observed only in the films prepared from the stoichiometric solution with longer holding time than 100 min. The longer holding times for disappearing of the X-ray diffraction peak of BaF2 derived from the stoichiometric solution is required compared to that from the Ba-deficient solution. On the other hand, the epitaxial growth rate measured from cross-sectional scanning electron microscopic observation for the YBCO films showed no difference between each other. X-ray diffraction analysis on the YBCO films fabricated from stoichiometric starting solution suggested that BaCuO2 and/or Y2O3 phases formed during the solid-state reaction process. The degradation of Ic performance might be caused by the formation of BaCuO2 phases.

Development of BZO Doped YGdBCO Thick Films Using TFA-MOD Process

Abstract TFA-MOD (Metal Organic Deposition using Trifluoro-acetates) process is of considerable practical concern for future applications since it can fabricate high performance coated conductors (CCs) with low cost. In this study, we developed the process for fabricating thick films in BZO nanoparticle doped Y 0.77 Gd 0.23 Ba 1.5 Cu 3 O y (YGdBCO) CCs in order to realize high I c values under magnetic field. The effect of RTR (Reel To Reel)-dip-coating conditions on microstructure, J c(- B -θ) properties, etc. was investigated in order to fabricate crack-free thick and high performance films. A BZO doped YGdBCO film with uniform 3.0 μm thickness showed the high in-field I c and J c values of 55.6 A/cm-w and 0.18 MA/cm 2 @77.8K, 3T, respectively.

Improvement of Film Thickness Uniformity in TFA-MOD Coated Conductors

Abstract TFA-MOD process is expected to be promising for future applications since it can produce high performance YBCO coated conductors with low cost. The dip-coating is adopted as the coating process because of its simplicity and controllability of the overall film thickness. Dip-coated films have uniform thickness along longitudinal direction, but not necessary in transverse direction. In the case of thicker films, the more cracks form during processing at the thicker region near the edges generate and propagate mainly due to tensile and bending strain. So we have to suppress the thickness distribution in transverse direction for thicker films for high I C values. In this study, we found that the thickness distribution was firstly given by meniscus shape and then the solution flew down till it's dried. The solution in the center region drops more since it is slowly dried compared with the edge region. Then, we developed a drying process, which accelerates the drying by blowing hot gas to prevent the coated solutions from dropping. As a result, the thickness uniformity was improved; the thickness ratio of the thick region (edge) to the flat one (center) was improved from 1.35 to 1.07. Furthermore, we successfully produced ~1.5 μm thick films with high critical current density values (> 2MA/cm 2 ) by the new coating process including the force drying step.

REBCO超電導テープ線材の交流損失特性(4)-TFA-MOD法で製作された人工ピンZrO2入りYGdBCO超電導テープ線材-

REBCO超電導テープ線材の交流損失特性(4)-TFA-MOD法で製作された人工ピンZrO2入りYGdBCO超電導テープ線材-

AC loss reduction of TFA-MOD coated conductors in long length by laser scribing technique

Abstract TFA-MOD process is expected to be promising for future applications since it can produce high performance YBCO coated conductors with low cost. Applying YBCO coated conductors to the power electric devices such as transformer, cable, motors, reduction of AC loss for long wire is necessary. Multifilamentation, which is one of the effective approaches for AC loss reduction, has been developed by the scribing process. YBCO coated conductors produced by our standard TFA-MOD process delaminated into two parts by the laser scribing. The delamination was clarified to occur within the superconducting layer caused by the defects such as pores in the superconducting layer. In order to reduce the defects in the superconducting layer, we modify the heat treatment profile performed on the decomposed precursor films by applying the interim annealing(550-600°C) before crystallization heat treatment(740-770°C). The interim annealed samples had much less and smaller pores than the standard processed ones. The peel strength measured by transverse tensile test was as high as the PLD derived coated conductors which was successfully scribed into five filaments resulting in 1/5 AC loss. A 50m long YBCO coated conductor with the characteristics of 398A/cmwidth was obtained and cut into 5 mm width, followed by the laser scribing process into five filaments. The multifilamentation process was successfully performed without delamination throughout the wire. The hysteresis loss was down to 1/N (N: number of filaments), as we aimed. The IC properties of the filaments were 29±4A, indicating the wire was uniformly fabricated.

Development of high-Ic processing for low cost YBCO coated conductors by multi-turn reel-to-reel crystallization large furnace for TFA-MOD process

Abstract Trifluoroacetates (TFA) –metal organic deposition (MOD) derived YBCO coated conductors are expected for many applications mainly due to cost-efficiency. High Ic and production rate are required for cost reduction in terms of $/Am. Improvement of Ic performance and production rate has been studied in this work, focusing on the conversion process of TFA-MOD. YBCO precursor films with desired thickness were prepared by using the multi-turn reel-to-reel (RTR) -system of dip-coating and calcination. The precursor films were heat treated under different conditions including the interim annealing. Crystallization process was performed on the interim annealed samples by using a multi-turn RTR system. The conversion furnace was modified so that improved conditions such as higher gas flow rate, lower total pressure can be used, which improved the production rate in crystallization process to 15 m/h by accelerating YBCO growth rate. Adopting the interim annealing, Ic property increased about 30% compared with that prepared without the interim annealing. X-ray θ-2θ diffraction analysis showed that; the peak intensities of YBCO (005) in interim annealed samples were 1.5 times stronger compared with those prepared without the interim annealing. The interim annealing affected the crystallinity and microstructure, resulting in high Ic. Then, the improved process for crystallization was applied to long tapes fabrication. A 205 m-long YBCO coated conductors fabricated at 15 m/h with interim annealing showed high and uniform Ic performance throughout the tape; Ic(min.)=372 A/cm-w@77 K, s.f., standard deviation = 15.2 A (3.7%). Uniformity along transverse direction was also analyzed by striating a 54 m long tape (199.5 A/5 mm-widths) into 5 filaments and, the Ic value of each filament (1 mm-width) was confirmed to be uniform; 24.3-32.9 A.

Effects of Sm-doping on Structures and Properties of YBCO Coated Conductors Fabricated by TFA-MOD Process

The Sm-doping YBCO films, Y1−xSmxBa2Cu3Oy and Y1SmxBa2−xCu3Oy (x=0.1, 0.3), were fabricated on LAO (00l) single-crystal substrates by TFA-MOD process in order to improve the superconducting properties of YBCO films. The XRD analyses indicated that these samples have a strong c-axis texture, but the secondary phase of BaCuO2 are found from Y1SmxBa2−xCu3Oy films with Sm3+ substitution on Ba2+ sites. High quality surface, better in-plane and out-of-plane texture of the Sm-doping films have been obtained compared with that of the undoped YBCO film. The Y0.9Sm0.1Ba2Cu3Oy films showed the highest value of Tc and Jc in self-field. Superconducting properties of all Sm-doping films are better than those of the pure film. These results strongly suggest that Sm-substitution on Y or Ba sites of YBCO phase is a powerful method to efficiently improve Jc and introduce artificial pinning centers in YBCO films.

Optimization of the TFA-MOD Process for Introduction of ${\rm BaZrO}_{3}$ Pinning Centers in YBCO Films

We present results of a study aimed at understanding the formation of BaZrO3 (BZO) precipitates in YBCO using the TFA-MOD process, as well as optimizing the BZO concentration with respect to the flux pinning performance. A reference YBCO sample containing no BZO, as well as three samples containing 4, 5 and 6% BZO were made and characterized with respect to in-field critical current (IC) performance and angular field IC dependence. Furthermore, the samples were analysed using transmission electron microscopy to identify the morphology of pinning precipitates and correlate it to the flux pinning behavior. The introduction of BZO does not result in formation of c-axis peaks in the angular field IC behavior, but rather in considerable widening of the ab-plane peaks in the field angular IC behavior with increasing BZO content, as well as in decreased field-angular anisotropy of IC. This behavior is analysed and correlated to the underlying microstructure of pinning precipitates.

Influence of BaZrO3 Amount on Microstructure and Properties in YBa2Cu3O7−x Films Prepared by TFA-MOD Process

Epitaxial YBa2Cu3O7−x/BaZrO3 films were prepared by complex metal–organic solutions including Y, Ba and Cu anhydrous trifluoroacetate precursors and Zr acetylacetonate precursors. The influence of the amount of BaZrO3 (BZO) on the structure and properties of YBa2Cu3O7−x (YBCO) films was systematically investigated. The YBCO films having 7 mol % BZO have a maximum critical current density (Jc) value (77 K, 0 T) of 6.0 MA/cm2. The enhanced Jc derives from a high density of BZO nanoparticles existing in the YBCO matrix. With increasing BZO amount, Jc of the YBCO films begins to decrease due to larger particles.

HTS current lead units prepared by the TFA–MOD processed YBCO coated conductors

Two superconducting current lead units have been prepared using ten coated conductors of the Tri-Fluoro-Acetate – Metal Organic Deposition (TFA–MOD) processed Y1Ba2Cu3O7−δ (YBCO) coated conductors with critical current (Ic) of about 170A at 77K in self-field. The coated conductors are 5mm in width, 190mm in length and about 120μm in overall thickness. The 1.5μm thick superconducting YBCO layer was synthesized through the TFA–MOD process on Hastelloy™ C-276 substrate tape with two buffer oxide layers of Gd2Zr2O7 and CeO2. The five YBCO coated conductors are attached on a 1mm thick Glass Fiber Reinforced Plastics (GFRP) board and soldered to Cu caps at the both ends. We prepared two 500A-class current lead units.The DC transport current of 800A was stably applied at 77K without any voltage generation in all coated conductors. The voltage between both Cu caps linearly increased with increasing the applied current, and was about 350μV at 500A in both current lead units. According to the estimated values of the heat leakage from 77K to 4.2K, the heat leakage for the current lead unit was 46.5mW. We successfully attained reduction of the heat leakage because of improvement of the transport current performance (Ic), a thinner Ag layer of YBCO coated conductor and usage of the GFRP board for reinforcement instead of a stainless steel board used in the previous study.The DC transport current of 1400A was stably applied when the two current lead units were joined in parallel. The sum of the heat leakages from 77K to 4.2K for the combined the current lead units was 93mW. In comparison with the conventional Cu current leads by gas-cooling, it could be noted that the heat leakage of the current lead is about one order of magnitude smaller than that of the Cu current lead.

Microstructure of YBCO thin films prepared by TFA-MOD method

The microstructure of the recently developed coated conductors was investigated by using electron back scatter diffraction pattern (EBSP). We prepared TFA (trifluoroacetates)-MOD (metal organic deposition) derived YBa2Cu3O7−x (YBCO) films on CeO2/LaMnO3/IBAD-MgO/Gd2Zr2O7/Hastelloy C276 substrates of 1cm-width. The EBSP observation showed that there was a difference of surface microstructure between the midsection and the end of TFA-MOD YBCO film layer in the direction of width. This is attributed not to the local difference of the biaxial texture of CeO2 top layer but to the local difference of growth condition during TFA-MOD process.

R&D of coated conductors for applications in Japan

On the research and development of coated conductors in Japan, the Ic characteristics and the length have been remarkably improved in the national project. Five hundred meter-long tapes with higher Ic values than 300A/cm-width were realized by the pulsed laser deposition (PLD) and the metal organic deposition using trifluoroacetates (TFA-MOD) processings for the superconducting layer on the IBAD-GZO buffered substrates. In order to realize the low cost by the increasing the production rate of the ion beam assisted deposition (IBAD) layer, the process of IBAD-MgO was developed and a 1000m-long IBAD buffer tape was fabricated at an extremely high production rate of 1km/h. On the other hand, the artificial pinning center has been introduced in both PLD and MOD processing. The BaZrO3 nano-rods were aligned along the c-axis of GdBCO superconducting films by the PLD process. The Jc value in the magnetic field parallel to the c-axis was remarkably improved. Additionally, the BaZrO3 nano-particles were uniformly dispersed in YGdBCO films by the TFA-MOD process and the Jc–B–θ property was confirmed to be isotropic.Based on the above-mentioned achievements, the new national project, whose main objective is the development of some electric power applications using a coated conductor, started in 2008. In this project, the improvement of the coated conductor includes the five sub-themes: (1) degradation of tapes, (2) high Ic under magnetic field, (3) low AC loss, (4) high mechanical strength and high Je and (5) low cost and high yield.

Effect of precursor composition on superconductivity properties of YBCO film prepared by TFA-MOD method

The YBCO films were prepared on LAO single crystal by TFA-MOD process. The structure and properties of YBCO films were investigated through changing the Y123 stoichiometry. It is found that higher JC values in self-filed of YBCO films are achieved by using the starting solution with the Ba ∶Y ratio of 1.5 ∶1. X- ray diffraction analysis revealed that Y2Cu2O5 phases are formed in the final YBCO films,which may lead to very effective vortex pinning and then enhance the JC of YBCO films.