Properties Of YBCO Films Research Articles

Effects of Firing Temperature and Film Thickness on the Critical Properties of YBCO Film by the '211 Process'

We fabricated by metal organic deposition (MOD) using trifluoroacetic acid (TFA) via the “211 process”, and then evaluated the phase formation, texture evolution, and critical properties as a function of the firing temperature and film thickness. In the fabrication process, Y2Ba1Cu1Ox and Ba3Cu5O8 powders were used as precursors instead of Y-, Ba- and Cu-based acetates. The films were fired in the temperature range of 750°C -800°C and the film thickness of films was controlled by repeating the number of coating cycles. The microstructure varied significantly with the firing temperature. The grains size increased, the film became denser, and the degree of the texture and phase purity varied with increasing firing temperature. The films fired at 775°C after calcining at 4005 showed the highest critical current (Ic) of 18 A/cm-width, which corresponds to a critical current density (Jc) of 0.9 MA/cm2. For multi-coated films, the Ic increased from 18 in the singly coated film to 100 A/cm-width with the triply coated film. In contrast, the corresponding Jc were in the range of 0.9-1.2 MA/cm2. Both Ic and Jc values decreased as the coating number repeated further as a result of the degraded microstructure.

The effect of film thickness on critical properties of YBCO film fabricated by TFA-MOD using 211-process

YBCO films were fabricated by the TFA-MOD method using the “211-process”, and the effects of the film thickness on phase formation, microstructure, texture evolution, and critical properties were evaluated. Various film thicknesses ranging from 0.41μm to 2.14μm were obtained by repeating the dip coating and calcining processes one to five times.The critical properties varied significantly with the film thickness. The Ic increased from 35 to 105A/cm-width with increasing the film thickness from 0.41μm to 1.17μm. On the other hand, the corresponding Jc remained almost constant in the range of 0.76–0.90MA/cm2. With further increases in thickness, these values decreased drastically, which was attributed to the degraded microstructure, i.e., the formation of BaF2 and a-axis grains and degraded texture and surface morphology arising from the insufficient heat treatment time. It is believed that the optimum thickness for improving both the Ic and Jc values is approximately 1.17μm.

Deposition process, microstructure and properties of YBCO film fabricated by advanced TFA-MOD method

The metal organic deposition (MOD) process using trifluoroacetates is expected as a promising method for producing YBCO coated conductors since the high JC performance can be provided by this low cost non-vacuum process. Investigation of the growth mechanism for the YBCO films is important to obtain the high crystal grain alignment for high JC in thick films. In order to clarify the YBCO growth mechanism in detail, we fabricated YBCO films on LaAlO3 (LAO) or CeO2/LAO substrate and investigated the microstructures of the films quenched during the crystallization process. The effects of the crystallization temperature and time on the microstructures of the films and its formation process as well as the JC values were studied. The YBCO growth model during crystallization was proposed according to the observation results of YBCO formation process.

Artificial chessboard like textured YBCO films

A chessboard like structure of in-plane textured superconducting film with 4 × 4 μm 2 elementary area has been fabricated by means of the homo-biepitaxial growing of YBCO film on yttria stabilized zirconia substrate. Regular arranged grain boundaries with fixed mismatch angle of 45° were realized. Each elemental square was connected to the four neighboring regions by π-Josephson junctions. Electro-physical properties of YBCO films with and without the chessboard like texture were measured. The magnetic field penetration into the chessboard like YBCO sample has been investigated by magneto-optical imaging at T = 4.2 K at different values of the applied magnetic field. The magneto-optical images have demonstrated the contribution of the grain-boundary superconducting transport to the formation of the magnetic field distribution inside the chessboard like sample.

Effects of heat treatment and film thickness on microstructure and critical properties of YBCO film processed by TFA-MOD

We fabricated YBCO film on an LAO substrate using the TFA-MOD method and evaluated the effects of the heat treatment temperature and film thickness on the microstructure, degree of texture, and critical properties. The calcining and firing processes were performed in the temperature ranges of 370–460°C and 750–800°C, respectively. We found that the phase purity, grain size and orientation, and degree of texture varied with the calcining and firing temperatures.The films fired at 775°C after calcining at 400–430°C showed the highest critical temperature (TC-onset) of 89.5K and critical current (IC) of 40A/cm-width, which corresponds to a critical current density (JC) of 1.8MA/cm2. According to the XRD, pole-figure, SEM images, and Raman analysis, these highest critical properties are probably due to the formation of a purer YBCO phase and stronger biaxial texture. In the multi-coated films, the IC value increased from 39 to 169A/cm-width as the number of coatings increased from one to four, while the corresponding JC was measured to be in the range of 0.8–1.2MA/cm2. Both the IC and JC decreased when a further coating was applied due to the degradation of the microstructure.

Influence of calcination conditions on superconducting properties of YBCO coated conductors by advanced TFA-MOD process

We have investigated influences of the H2O gas inlet temperature in the calcination process on both microstructures and superconducting properties of YBCO films fabricated by the advanced TFA-MOD process, from the viewpoint of the process control in the reel-to-reel system. Low Ic values were recognized in the films which were prepared by introducing the H2O gas from the lower temperature than 200°C. XRD measurements and cross-sectional TEM observations suggest that the YBCO films calcined with the H2O inlet temperature below 200°C revealed a random orientation without epitaxial growth. Consequently, it has been understood that control to keep low partial pressure of the H2O gas in a lower temperature region is important for fabrication of long-length YBCO tapes with high performance by the reel-to-reel system.

Investigation of magnetic properties of YBCO film with artificial pinning centers on PLD/IBAD metal substrate

In order to improve the critical current density (Jc) in magnetic fields, we have fabricated YBa2Cu3O7−x (YBCO) coated conductors with artificial pinning centers by pulsed laser deposition (PLD) method on a Hastelloy™ tape buffered by a self epitaxial PLD-CeO2 layer and an ion-beam assisted deposition (IBAD)-Gd2Zr2O7 (GZO) layer. Artificial pinning centers were introduced by the PLD deposition using YBCO target including YSZ particles. The Jc value was improved and abnormal microstructure was found to be effective for pinning. We call this new nano-epitaxial BaZrO3/YBCO structure “the bamboo structure” from its anisotropic growth and morphology. Furthermore, for the 1.16μm in thickness, Ic showed high value of 40.7A at 77K and 3T parallel to the c-axis. Namely, “the bamboo structure” also contribute to strong pinning in magnetic fields parallel to the c-axis in the film as thick as 1.16μm.

RF Power Properties of YBCO-Film/LAO Microstrip Disk Resonators for 4 GHz Band

This paper presents RF power properties of YBCO films on LaAlO 3 substrate examined using microstrip disk shape-patterned resonators for around 4 GHz. Characterization of the YBCO films for the resonators was carried out. For input-output power measurement, the resonators with the condition of strong I/O couplings were designed and fabricated. The RF power properties of the resonators at cryogenic temperatures were measured. The resonator condition to obtain the handling power over 10 W at 30 and 70 K was confirmed. The temperature dependence of the break-down powers at 4GHz for the resonators showed two broad peaks. We discuss the unique phenomenon with magnetic flux model, as referred to the YBCO films characteristics results.

Superconducting and normal state transport properties of epitaxial 10 at.% Ca substituted YBCO thin films

The improvement of the critical current density in YBCO coated conductor tapes necessarily concerns the study of grain boundary (GB) properties and the study of different pinning mechanisms involving both GB and the interior of the grain (IG). Recently, it has been proposed that the introduction of Ca substituting Y enhances Jc in YBCO grain boundaries (GB). On the other hand enhanced correlated pinning, as observed in YBCO thin films grown on CeO2 buffered substrates, has been ascribed to elongated defects along the c-axis of the film. In order to evaluate the role of calcium and CeO2 buffer layer on the superconducting transport properties of YBCO films, critical current density measurements as a function of temperature, magnetic field and angle between magnetic field direction and c-axis of the film have been performed on epitaxial thin films of unsubstituted and 10at.% Ca substituted YBCO grown on CeO2 buffered Al2O3 single crystal substrates. Results are compared with data obtained on 10at.% Ca substituted YBCO grown on SrTiO3 single crystal substrate.

Enhancement of flux pinning and critical currents in YBa2Cu3O7−δ films by nanoscale iridium pretreatment of substrate surfaces

We have acquired positive results in a controlled study to investigate the effects of substrate surface modification on the growth-induced flux-pinning nanostructures in YBa2Cu3O7−δ (YBCO) films. Nanoscale iridium (Ir) particles were applied to single-crystal SrTiO3 substrate surfaces using dc-magnetron sputtering. Superconducting properties of YBCO films grown on the Ir-modified substrates, measured by transport and magneto-optical imaging, have shown substantial improvement in the critical current densities (Jc) at 77 K over those on untreated, control substrates. Results also show a nearly uniform enhancement of Jc over all orientations of magnetic field. Present results are found to be consistent with cross-sectional transmission electron microscopy investigations. Ultimately, the objective of this approach is to produce enhancements in the properties of coated conductors by a simple pretreatment of the substrate surface.

Thickness effects of SrTiO 3 buffer layers on superconducting properties of YBa 2Cu 3O 7− δ coated conductors

A thin layer of SrTiO 3 has been successfully used as a buffer layer to grow high quality superconducting YBa 2Cu 3O 7− δ (YBCO) thick films on polycrystalline metal substrates with a biaxially oriented MgO template produced by ion-beam-assisted deposition (IBAD). Using this architecture, 1.5 μm-thick-YBCO films with an in-plane mosaic spread in the range of 2–3° in full width at half maximum, and a critical current density over 2 MA/cm 2 in self-field at 75 K have been achieved routinely. More interestingly, we found the thickness of SrTiO 3 buffer layers strongly affects the properties of YBCO films. The critical current density of YBCO films increases dramatically when the thickness of SrTiO 3 buffer layer reaches an optimum range of 40–120 nm. Microstructure studies, including transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and ion-milling experiments suggest that the YBCO texture evolution, STO outgrowths and STO surface dents are strongly related to STO thickness. They are the key factors for YBCO self-field J c variation.

Effect of annealing time on the structure and properties of YBCO films by the TFA–MOD method

We systematically investigated the effect of high-temperature annealing time on the structure and properties of YBCO films prepared by the TFA–MOD method. Critical current density ( J c) was measured using j c-scan Leipzig system to compare the electrical properties of the films. SEM and XRD were used to determine the microstructure and orientation of the films. The partially crystallized films have rough surfaces and impurity phases which attribute to poor electrical performance. In contrast, completely crystallized films have smooth surfaces, pure (0 0 1) YBCO phase, and exhibit high electrical performance with J c values over 3 MA/cm 2 (77 K, 0 T) and Δ T c lower than 1 K.

YBCO Thin Films Prepared by Fluorine-Free Polymer-Based Chemical Solution Deposition

We describe a new chemical solution deposition (CSD) route toward inexpensive and environmentally friendly preparation of YBCO coated conductors. This route is based on a fluorine-free polymer and exhibits some advantages compared to the TFA route, where trifluoroacetate precursors are used and aggressive HF is formed during the heat treatment. In this work, the main parameters influencing the structure and properties of YBCO films prepared by means of the polymer method are investigated. For the preparation of the YBCO films, stoichiometric amounts of Y, Ba and Cu nitrates were dissolved in a solution of polyacrylic acid in dimethylformamide. This metal polymer precursor solution was spin coated onto strontium titanate (STO) single crystals. After drying in air, the films were heat treated in an oxygen containing nitrogen atmosphere. The film structure was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). T/sub c/ and J/sub c/ were measured inductively. The preliminary results show that the amount of oxygen during heating and in the dwell time strongly influences the superconducting properties. Good superconducting properties, with T/sub c/ of 89.9 K (/spl Delta/T/sub c/=1.1 K) and J/sub c/ of 10/sup 6/ A/cm/sup 2/ at 77 K, have been obtained on epitaxially grown 250-nm-thick YBCO films using the polymer-based precursors.

Fabrication of High-<tex>$Jrm c$</tex>YBCO Films by the TFA-MOD Process Using YBCO Powder as Precursor

YBCO thin films were prepared on LaAlO/sub 3/ (100) single-crystal substrates using the metal-organic deposition of trifluoroacetates (TFA-MOD). Unlike the normal TFA-MOD process, TFA-based solutions were prepared dissolving YBCO powder into the trifluoroacetic acid solvent. The solutions were coated on the substrates by spin coating, fired at the temperature up to 400/spl deg/C in 4.2% humid and oxygen atmosphere, fired again at the temperature region of 725-850/spl deg/C for 2 h in 4.2% humidity and 1000 ppm oxygen partial pressure, and finally oxygenated at 450/spl deg/C for 1 h. Similar to the normal TFA-MOD process, both microstructure and superconducting properties of YBCO films were highly sensitive to the firing temperature. Optimally processed YBCO films of /spl sim/250 nm thickness exhibited T/sub C,zero/ of 90 K and J/sub C/ of 2.9 MA/cm/sup 2/ at 77 K in a self-field. The present results clearly evidence that our cost-effective modified TFA-MOD process is applicable to the fabrication of YBCO coated conductors.

Ion beam structure modification of YBa 2Cu 3O 7− x coated conductors prepared by solution techniques

Solution techniques are low cost non-vacuum methods for coated conductor development, which are promising in obtaining high quality YBa 2Cu 3O 7− x (YBCO) films. However, many pores and cracks are often formed in the YBCO film prepared by solution techniques, which degrades the film qualities. In this paper, we report a new method for modification of the structural and superconducting properties of YBCO film, namely, ion beam structure modification (ISM). Experimental results show that ISM is an effective way to eliminate the pores existed in solution deposited YBCO film. Denser and smoother films with improved c-axis grain alignment and grain connectivity have been obtained using ISM. The superconducting properties of the YBCO film are also improved.

Effect of Ca doping on the nonlinear microwave properties of YBCO thin films

We have investigated the microwave properties of a set of four identicalYBa2Cu3O7−δ (YBCO) films. One of the films has a 30% Ca-substituted YBCO overlayer, whereas thethree others are used as references to highlight the effect of the Ca substitution. Themicrowave characterization was carried out using the stripline-resonator technique. TheRs of the sample that has a Ca-rich overlayer increased slightly in the linear regime, whereasno significant effect on the nonlinear components and the intermodulation distortion hasbeen observed. We conclude that Ca doping does not significantly improve the nonlinearmicrowave properties of YBCO films, but the small changes in the linear surface resistancethat we observed are discussed in terms of the physics of Ca doping of YBCO.

Growth of YBCO film on SrRuO3-buffered MgO substrate

The superconducting, crystalline, and morphological properties of YBCO films deposited onSrRuO3-buffered MgO substrates were studied at various deposition temperatures. The film deposited atT = 770 °C had the best superconducting properties (critical current density of2.5 × 106 A cm−2 and critical transition temperature of 91 K). Scanning electron micrographs of this filmrevealed uniform and well-connected grains. X-ray analyses revealed that theY BCO/SrRuO3/MgO filmfabricated at 770 °C had good in-plane and out-of-plane textures. Raman spectroscopy showed that this film hadthe best out-of-plane texture and orthorhombic I domains.

Effects of deposition rate and thickness on the properties of YBCO films deposited bypulsed laser deposition

YBCO films with various thicknesses from 100 nm to1.6 µm were depositedon single crystal SrTiO3 substrates by pulsed laser deposition (PLD). The effects of thickness and depositionrate—by means of controlling the pulsed laser frequency—on the critical current density(Jc) were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were usedto examine the orientation, crystallization and surface quality. The amount ofa-axis YBCO component evaluated from the ratio of XRD chi-scan intensities of thea-axisand c-axis for the YBCO (102) plane increased as the YBCO film became thicker. SEM was usedto analyse the surface of YBCO film, and it was shown that the surface of YBCO filmbecame rougher with increasing thickness. There were many large singular outgrowths andnetworks of outgrowths on the surface of the YBCO films with thickness greater than0.4 µm. The increasedamount of a-axis YBCO component and the coarse microstructure of the thick YBCO film caused degradationof Jc with increasing thickness.

Effect of Ca doping in YBCO superconducting thin films

We investigate the effect of partial substitution of Y by Ca in YBCO superconducting thin films. The films were grown on (1 0 0) SrTiO3 single crystal substrates by pulsed laser deposition (PLD). The Ca doped film were ablated from Y1−xCaxBa2Cu3O7−δ targets with x=0, 0.05, 0.07, and 0.10. The dc transport properties of the films in applied magnetic field are analyzed to study the role of Ca on the superconducting properties of YBCO films. The irreversibility line for the samples with x=0 and 5 at.% Ca was derived from the E–J curves using a scaling theory for the vortex-glass transition.

The effect of MgO substrate roughness on YBa 2Cu 3O 7-δ thin film properties

We have investigated the effect of surface roughness of ‘as-received’ polished, single crystal (001) MgO substrates, from a range of commercial suppliers, on the physical properties of epitaxially-grown high-temperature superconducting YBa 2Cu 3O 7-δ (YBCO) thin films. In particular, we are interested in the effect of polishing grooves, present to some degree in nearly all commercially available substrates, on the transport properties of YBCO films. We have found that for substrate surfaces, which exceeded 1.6 nm roughness the film critical current densities, J c , were 2–3 times smaller than for films deposited on substrates with a surface roughness less than 1.0 nm. Physical analysis of the YBCO films using X-ray diffraction revealed a correlation between the width and relative intensity of the YBCO (006) peak and film J c which appears to be related to the film roughness and hence the underlying MgO roughness.