Trifluoroacetates-metal Organic Deposition Process Research Articles

J c and Composition Distribution of YBCO Coated Conductors Fabricated by the TFA-MOD Process

The Trifluoroacetate-Metal Organic Deposition (TFA-MOD) process is one of the most promising fabrication processes for low-cost superconducting coated conductors (CCs) because of its high yield and non-vacuum system. However, in the doping and multi-coating process for the enhancement of critical current (<i>I</i>_c), degradation of <i>I</i>_c often occurs. In this study, to clarify the origin of <i>I</i>_c degradation, we have investigated the local critical current density (<i>J</i>_c) distribution, in-field <i>J</i>_c properties and the variation of composition for TFA-MOD processed YBa₂Cu₃O_y (YBCO) CCs. The local <i>J</i>_c distributions were measured by Scanning Hall-Probe Microscopy for confirming the location of the high&#x002F;low-<i>J</i>_c regions. The CCs were then cut into 3 mm&#x00D7;3 mm or 3 mm&#x00D7;4 mm for dc magnetization measurement. The <i>T</i>_c of the all samples were about 87 K to 90 K. In-field <i>J</i>_c at 77 K was obtained from <i>M</i>-<i>H</i> measurements and all the samples were compared with each other. The in-field <i>J</i>_c of high-<i>J</i>_c sample showed about 100 times higher value than that of the low-<i>J</i>_c sample. Then we have observed the microchemistry and the variation of composition by SEM-EDS. In the high-<i>J</i>_c sample, the grains of the superconducting phase were large in size and were connected to each other. This means that superconducting current could form through the whole sample area and generate a large magnetic moment. On the other hand, in the low-<i>J</i>_c sample, though the superconducting phase could be observed, the grains were small in size and sparse. Therefore, the magnetic moment measured was small. However, it was also suggested that the intra-grain <i>J</i>_c would be almost the same as that of high-<i>J</i>_c sample.

High homogeneity 10 cm long BaTiO3-doped YBa2Cu3O7-δ films by the trifluoroacetate metal-organic deposition process

The trifluoroacetate metal-organic deposition (TFA-MOD) is one of the most appealing routes for the growth of high-quality YBa2Cu3O7-δ (YBCO) coated conductors (CCs) due to its advantages of low cost and high efficiency. However, the puzzle of the inhomogeneities in microstructures and superconducting properties of YBCO CCs prepared based on the TFA-MOD route remain still a challenge. In this paper, 10 cm long YBCO films were prepared under different low-pressure conditions. These YBCO films were deposited on buffered Hastelloy C276 tapes using a dip coater. The effect of different ambient pressures on the microstructure and critical current density of YBCO films was investigated. High critical current density (JC) YBCO coated conductors with good homogeneity were obtained. The JC values (77 K, self-field) at different positions along the length direction of YBCO coated conductor were around 3.5–4 MA/cm2. The results showed that only under a suitable pressure the homogeneities of the microstructure and critical current density along the length direction of coated conductor can be greatly improved. The improvement was mainly ascribed to good texture and the absence of impurity phases BaF2 and Ba4Y2O7 in the films.

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.

Microstructures Property and Improved J c of Eu-Doped YBa2Cu3.6O7−δ Thin Films by Trifluoroacetate Metal Organic Deposition Process

YEuxBa2Cu3.6O7−δ(x = 0, 0.1, 0.3, 0.5) thin films were prepared on buffered substrates by trifluoroacetate metal organic deposition (TFA-MOD). The europium-doped film showed better out- and in-plane texture than the pure YBa2Cu3.6O7−δ thin film and special topography. In the meantime, the superconducting transition temperature Tc was improved with the increase of europium content and spanned a small range when x= 0.5 doping levels. Besides, the Jc value was obviously enhanced with europium addition, and YEu0.3Ba2Cu3.6O7−δ showed the best superconducting properties under self-field. Furthermore, the Jc value was obviously improved under magnetic field, and the enhanced flux-pinning properties was also observed in europium-substituted samples which may be attributed to the good grain connections and the effective flux pinning centers introduced by europium addition. The thickness of prepared film samples were approximately 400 nm. The structure of samples were characterized by x-ray diffraction (XRD), Raman spectrum was used to investigate the crystallization and growth orientation, and the surface morphology of films were got by field-emission scanning electronic microscopy (FE-SEM). Moreover, energydispersive spectroscopy (EDS) was utilized to determine element composition. Spatial distribution of Jc of the film was obtained by mapping analysis of inductive measurement at 77 K. And the superconducting properties under magnetic fields were measured by magnetic property measurement system with a SQUID magnetometer.

Three-Dimensional Characterization of BaZrO&lt;sub&gt;3&lt;/sub&gt; Precipitates in Y&lt;sub&gt;1-x&lt;/sub&gt;Gd&lt;sub&gt;x&lt;/sub&gt;Ba&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;7-y&lt;/sub&gt; Prepared by TFA-MOD Using STEM-Tomography

Y1-xGdxBa2Cu3O7-yfilm with BaZrO3was fabricated on CeO2buffered LaMnO3/ion beam assisted deposition MgO/Gd2Zr2O7/Hastelloy C276TMsubstrates by the trifluoroacetates metal organic deposition process, whose microstructural and elemental analyses were performed by transmission electron microscopy. Y1-xGdxBa2Cu3O7-yfilm with the thickness about 700 nm was found composed of c-axis oriented grains and large numbers of randomly oriented precipitates, such as (Y,Gd)2Cu2O5, CuO and BaZrO3. (Y,Gd)2Cu2O5and CuO precipitates were heterogeneously dispersed in the Y1-xGdxBa2Cu3O7-ymatrix with their sizes ranging between 100 and 200 nm, and BaZrO3precipitates were uniformly dispersed with their sizes ranging between 10 and 20 nm. Electron tomography with elemental information was performed further to reveal the three-dimensional information of BaZrO3 precipitates.

Improvement of magnetic properties for long YGdBCO coated conductors using TFA-MOD process

Abstract YxGd(1-x)Ba2Cu3Oy(YGdBCO) coated conductors (CCs) containing artificial pinning centers (APCs) were fabricated by the trifluoroacetate-metalorganic deposition (TFA-MOD) method using a batch heat-treatment process, which have been industrially applied to fabricate long superconducting tapes which have high critical current (Ic) have been successfully manufactured. From the viewpoint of applications, the characteristics in magnetic fields become further important and one solution has been showed by Miura et al. [1] , [2] as introduction of APCs such as BaZrO3 (BZO). We applied their 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. A 40 m-long YGdBCO CC in which APCs are introduced was fabricated, and it has Ic value of 320 A/cm-width (critical current density (Jc) value of 2.2 MA/cm2) in self-field and 20 A/cm-width at 3 T in Liq. N2.

Measurement of local critical currents in TFA-MOD processed coated conductors by use of scanning Hall-probe microscopy

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.

Fabrication of GdBa 2Cu 3O 7− x films by TFA-MOD process

GdBa 2Cu 3O 7− x (GdBCO) films have been deposited on LaAlO 3 (LAO) (0 0 l) single crystal substrates by trifluoroacetate metal organic deposition (TFA-MOD) method. The effects of oxygen partial pressure and firing temperature on microstructure and critical properties of GdBCO films were discussed. The phase formation, texture and microstructure of films were characterized by X-ray diffraction and scanning electron microscopy. The oxygen partial pressure was considered to play a great role for formation of impurity phase and a-axis oriented grains. The degree of c-axis orientation was also influenced by the firing temperature. The highly c-axis oriented GdBCO film obtained at 815 °C under an oxygen partial pressure of 100 ppm has a high performance critical current density J c (77 K, self field) = 1.8 MA/cm 2.

Transport performance of a HTS current lead prepared by the TFA-MOD processed YBCO tapes

A superconducting current lead has been prepared using 12 tapes of the trifluoroacetates – metal organic deposition (TFA-MOD) processed Y 1Ba 2Cu 3O 7- δ (YBCO) coated conductors with critical current ( I c ) of about 100 A at 77 K in self-field. The tapes are 4.5 mm in width, 220 mm in length and about 120 μm in overall thickness. The 1 μm thick superconducting YBCO layer was formed through the TFA-MOD process on Hastelloy TM substrate tapes with two buffer oxide layers of Gd 2Zr 2O 7 (GZO) and CeO 2. The 12 YBCO tapes were arrayed on the both sides (six tapes on each side) of a stainless steel board with 3 mm in thickness for a board type shape. They were similarly soldered to copper caps at the both ends. The transport current of 1000 A was stably applied for 10 min in the liquid nitrogen temperature without any voltage generation in all tapes. Although some voltage in some YBCO tapes generated at the applied currents of about 1100 A, the transport current of 1200 A was successfully applied without quenching. The voltage between both copper caps linearly increased with increasing the transport current, and it was about 300 μV at an applied current of 1000 A. A low joint resistance between the YBCO tapes and the copper caps resulted in small amounts of the Joule heating at the joints when 1000 A was applied. The overall (effective) thermal conductivity of the current leads composed of YBCO tapes and the stainless steel board was much lower than that of Non-superconducting current leads. Therefore, the present current leads with small heat leakage seemed to be practically promising for superconducting magnets.

Development of multi-turn reel-to-reel crystallization large furnace for high production rate of YBa 2Cu 3O y coated conductors derived from TFA–MOD process

YBa 2Cu 3O y (YBCO) coated conductors were fabricated by trifluoroacetates (TFA)–metal organic deposition (MOD) previously using a single reel-to-reel (RTR) crystallization furnace with a gas flow system parallel to the tape surface which could realize only a low production rate. In this study, we developed a new multi-turn (MT) RTR furnace with a vertical gas flow system to increase the production rate of the crystallization step and attained the uniform reaction in the tapes of the multi-turn system. We fabricated YBCO films with different conditions partial pressure of water vapor, the total pressure and the gas flow volume using the MT-RTR furnace with the vertical gas flow system, in order to investigate the influence of the processing parameters on the growth rate and the superconducting properties of the YBCO films. It was found that the growth rate of the YBCO phase film increased under the conditions of the high gas flow rate, the low total pressure and the high water vapor partial pressure. As a result, 21 times higher J c value was attained at 17 times faster growth rate than that without controlling the processing parameters in the crystallization step. This is thought to be due to the suppression of coarsening of Y 2Cu 2O 5 and CuO particles in the precursor film. Consequently, the high I c,end-to-end value of 250 A was achieved in a 5 m long tape fabricated at the production rate of 3 m/h using the only two lanes of the entire MT-RTR crystallization furnace with 10 lanes.

Effect of holmium additions on microstructure in YBa 2Cu 3O 7−δ

The microstructure of Y–Ho–Ba–Cu–O samples with two different levels of Ho, Ho/Y = 0.5 and 1.0, processed by the trifluoroacetate metalorganic deposition process, have been studied by transmission electron microscopy (TEM) techniques, including high-resolution TEM, conventional TEM and fine-probe energy-dispersive spectroscopy. Oxide nanoparticles with varying levels of Ho have been characterized in detail, and related to other microstructural features. At a lower level of Ho, the (1 1 0) coarse twins with the boundary spacing of 30–60 nm form. Further increase in Ho content considerably produces fine microtwins, and increases the density of nanoparticles and microtwins. Ho additions increase the critical current for the magnetic field parallel to the c-axis, and decrease it for the field parallel to the a– b plane, resulting in a decreased anisotropy of critical current with field angle compared with undoped Y–Ba–Cu–O samples processed under similar conditions. These trends are discussed in terms of the detailed microstructural observations.

The influence of the heating rate on YBCO films prepared by the trifluoroacetatemetal–organic deposition process

A promising approach used to fabricateYBa2Cu3O7−δ (YBCO) thin films is the metal–organic deposition (MOD) method usingtrifluoroacetate (TFA) solution. In this method, the heating rate is known to be acrucial parameter and an important variable for optimization. However, theredoes not seem to be an in-depth understanding of the materials issues associatedwith different heating rates. Some aspects of this correlation have been addressedin this paper, where the influence of the heating rate during calcination in the200–250 °C temperature range on the surface chemistry, morphology, and electrical properties has beenstudied. X-ray photoelectron spectroscopy reveals similar chemical compositions and almostcomplete decomposition of metal trifluoroacetates at all heating rates. However, the heatingrate is seen to have a significant influence on the morphology of the calcined film, andleads to great changes in the final film. When a TFA film is heated through the200–250 °C step at 3 °C h−1, it has a smooth and uniform surface. On the other hand, a slower heating rate(1.5 °C h−1) results in phase separation during calcination, and a faster heating rate(10 °C h−1) leads to a rough film decorated with micron-scale pores. This leads tothe final reacted films having very different microstructures. A uniform,c-axis oriented microstructure is observed in the3 °C h−1 heated films. A slower heating rate results in a large density ofa-axis oriented grains and a faster heating rate causes higher pore density together with a biggeraverage pore size. Although all films exhibit high phase purity YBCO with noticeablec-axis orientation, theelectrical resistivity (ρ) for the normal state (100 K) shows an increasing sequence ofρ(3 °C h−1)<ρ(10 °C h−1)<ρ(1.5 °C h−1). In this experimentset-up, the highest Jc value of 1.3 × 106 A cm−2 (77 K, self-field)was achieved with a 3 °C h−1 heating rate, which can be correlated with texture and microstructural observations.

High Magnetic Field Properties of Critical Current Density in&amp;lt;tex&amp;gt;$rm Y_1rm Ba_2rm Cu_3rm O_7-delta$&amp;lt;/tex&amp;gt;Coated Conductor Fabricated by Improved TFA-MOD Process

Current transport characteristics in a Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductor fabricated by a trifluoroacetates-metal organic deposition (TFA-MOD) process were investigated over a wide range of temperature and magnetic field up to 25 T. Improved TFA-MOD process was successfully introduced for the better property of critical current density, J/sub c/, at high magnetic fields. The J/sub c/ and critical current, I/sub c/, values of multi-coated film with 1.2 /spl mu/m thickness and 1 cm width were 2.1 MA/cm/sup 2/ and 251 A at 77 K in self-field. Additionally, the superior J/sub c/ properties remained even at high magnetic fields over 20 T and lower temperature, e.g. J/sub c/ value at 30 K in 25 T was 1.0 MA/cm/sup 2/. The J/sub c/ value was about 2.5 times higher than those of previous TFA-MOD process at wide range of magnetic field. Moreover, the statistical distribution of J/sub c/ in the conductor was also estimated within a framework of the percolation model. The uniformity in the YBCO coated conductor was improved by optimizing the TFA-MOD process.

Fabrication and growth mechanism of YBCO coated conductors by TFA-MOD process

In the development for the coated conductors, the trifluoroacetates-metal organic deposition (TFA-MOD) process using the multi-coating method was applied to form thicker YBa 2Cu 3O y (YBCO) films on the CeO 2 buffered IBAD(Zr 2Gd 2O 7)/Hastelloy tape. As a result, in the 5 times coated film with 1.38 μm in thickness, the J c and I c values achieved to be 1.5 MA/cm 2 and 210 A, respectively. Then, we fabricated a 1 m long YBCO tape by the continuous reel-to-reel system. As a result, we confirmed the tape to be homogeneous by XRD analysis. Additionally, we report the theoretical analysis of YBCO growth during post-annealing in the TFA-MOD process considering both the diffusion in the gas boundary layer and the growth kinetics at the precursor/YBCO interface. This model reveals a basic idea of the growth mechanism to define the steady state growth rate, and could explain the experimental results. In addition, it predicts that the growth rate could be estimated from the partial pressure of the water vapor of the inlet gas.

Preparation of double-sided CeO/sub 2/ buffer layers on MgO substrates for YBa/sub 2/Cu/sub 3/O/sub 1-X/ films

Double-sided CeO/sub 2/ buffer layers were fabricated by pulsed-laser deposition on 20 mm /spl times/ 20 mm MgO[100] substrates to apply YBCO films using the TFA-MOD (trifluoroacetate-metalorganic deposition) process for microwave devices. The double-sided CeO/sub 2/ films had the characteristics of a smooth surface (R/sub ms/ < 1 nm) and a highly [100] aligned orientation, which is almost independent of thickness. We successfully obtained a YBCO film deposited by the TFA-MOD process on CeO/sub 2//MgO[100], which had a critical current density (J/sub c/) of 6 MA/cm/sup 2/ (77 K, 0 T) with good reproducibility.

Deposition of CeO 2/YSZ buffer layer on Hastelloy substrates for MOD process of YBa 2Cu 3O 7− x film

Trifluoroacetate metalorganic deposition (TFA-MOD) process is expected as a low cost process for mass production of coated conductors because it is a non-vacuum process. In order to apply the technique to fabrication of coated conductors, suitable buffer layers have to be considered to achieve a high orientation of superconducting layer and prevention of the reaction with metal substrate. The combination of CeO 2 on IBAD-YSZ is considered as an effective buffer for TFA-MOD process expecting to satisfy a high acid resistivity and high crystal grain alignment. The CeO 2 buffer layer was deposited on IBAD-YSZ/Hastelloy substrates by RF magnetron sputtering. From XRD analysis, the CeO 2 buffer layer showed very good in-plane alignment on YSZ-IBAD buffer layer. In a holding time of 1 h, the suitable maximum heat treatment temperature was found to be from 750°C to 775°C for TFA-Y123 on metal substrate. The J c– B property of Y123 on CeO 2/YSZ/Hastelloy shows the J c values of 1.4 MA/cm 2 at 77.3 K, 0 T and more than 10 5 A/cm 2 at 77.3 K, 2 T. The high performance under high magnetic field was confirmed.