High-quality YBCO Films Research Articles

Preparation of a/c-oriented YBa2Cu3O7−x homogeneous superconducting junctions via pulsed laser deposition

The growth orientation of YBa2Cu3O7−x (YBCO) is extremely sensitive to the growth atmosphere and the heat-treatment temperature. However, according to the existing literature, to obtain YBCO with preferred a-axis orientation, a buffer layer needs to be prepared on the substrate. In this work, YBCO films were grown via pulsed laser deposition, and the influence of N2 and O2 deposition atmospheres on the growth orientation of the YBCO films was studied. It was found that high-quality YBCO films with preferred c-axis orientation can be grown in O2 atmosphere, but it is difficult to grow a-axis-oriented YBCO films in O2 atmosphere without using a buffer layer; however, YBCO films with a high degree of preferred a-axis orientation can be grown in N2 atmosphere without using a buffer layer. Furthermore, an a/c-oriented homogeneous YBCO bilayer with c-axis orientation in the lower layer and a-axis orientation in the upper layer was prepared. X-ray diffraction and scanning transmission electron microscopy results show that the bilayer is epitaxial with a good a/c orientation, and the resistance–temperature curve shows the occurrence of two transitions at temperatures of 91 and 71 K. We believe that these temperatures correspond to the superconducting onset transition temperatures of YBCO in the c- and a-axis growth orientations, respectively. The superconducting current usually flows within the Cu–O plane. With its sudden change in the direction of the current at the interface, the homogeneous junction prepared in this work is expected to provide new ideas for the research of high-temperature superconducting junction devices.

Self-passivated freestanding superconducting oxide film for flexible electronics

The integration of high-temperature superconducting YBa2Cu3O6+x (YBCO) into flexible electronic devices has the potential to revolutionize the technology industry. The effective preparation of high-quality flexible YBCO films therefore plays a key role in this development. We present a novel approach for transferring water-sensitive YBCO films onto flexible substrates without any buffer layer. Freestanding YBCO film on a polydimethylsiloxane substrate is extracted by etching the Sr3Al2O6 sacrificial layer from the LaAlO3 substrate. In addition to the obtained freestanding YBCO thin film having a Tc of 89.1 K, the freestanding YBCO thin films under inward and outward bending conditions have Tc of 89.6 and 88.9 K, respectively. A comprehensive characterization involving multiple experimental techniques, including high-resolution transmission electron microscopy, scanning electron microscopy, Raman, and x-ray absorption spectroscopy, is conducted to investigate the morphology, structural, and electronic properties of the YBCO film before and after the extraction process where it shows the preservation of the structural and superconductive properties of the freestanding YBCO virtually in its pristine state. Further investigation reveals the formation of a YBCO passivated layer serves as a protective layer that effectively preserves the inner section of the freestanding YBCO during the etching process. This work plays a key role in actualizing the fabrication of flexible oxide thin films and opens up new possibilities for a diverse range of device applications involving thin-films and low-dimensional materials.

Transient liquid assisted growth of superconducting YBa2Cu3O7−x films based on pulsed laser deposition

We investigated the integration of transient liquid-assisted growth (TLAG) approach for epitaxial YBa2Cu3O7−x (YBCO) films by physical deposition methodologies (pulsed laser deposition (PLD)), as an additional opportunity for high-throughput growth of YBCO. As a prerequisite, highly flat and amorphous YBCO precursor films were deposited by PLD at temperatures below 400 °C on single-crystalline SrTiO3 (STO) and LaMnO3 (LMO)/STO, as well as industrial coated conductor architectures. Contrary to TLAG based on chemical solution deposition, where BaCO3 elimination is a key factor for the YBCO growth, TLAG-PLD growth is controlled by the transformation of Ba–Cu–O (s) to a transient liquid. High-quality c-axis YBCO films were successfully grown on different substrates, as demonstrated by high-resolution x-ray diffraction and transmission electron microscopy. In-situ resistance measurements revealed that the growth rates around 1000 nm s−1 can be achieved, outperforming the capabilities of standard PLD growth of REBa2Cu3O7 films by few orders of magnitude. Experimental conditions such as temperature, oxygen partial pressure, and heating ramp, were optimized to obtain critical temperature (T c) values up to 90 K. Critical current densities of 15 MA cm−2 at 5 K and 1.7 MA cm−2 at 77 K were obtained for YBCO films of 450 nm on LMO/STO.

Enhanced flux pinning of solution-derived YBa2Cu3O7−x nanocomposite films with novel ultra-small BaMnO3 nanocrystals

The demand for coated conductors with high cost performance has triggered the search for effective artificial pinning centers by the low cost and high-scalability route. In this work, YBa2Cu3O7−x (YBCO) nanocomposite films with BaMnO3 nanosized particles induced by manganese (Mn) addition were prepared by low-fluorine metal-organic deposition. A strategy of rapid heating during the medium-temperature stage was adopted to reduce the size of the BaMnO3 nanoparticles and improve the quality of the YBCO films. An enhanced critical current density and pinning force were found in the high-quality YBCO film embedded with extraordinarily small nanoparticles (average 3 ± 1 nm).

Effects of the Design Parameters on Characteristics of the Inductances and JJs in HTS RSFQ Circuits

We have investigated various geometrical design dependencies for rapid single-flux-quantum (RSFQ) circuits in high- $T_{c}$ superconductors (HTS) technology, including the characteristics of the intermediate inductances. Structural- and temporal- dependent combinations of ${C}_n$ – $R_{n}$ (capacitance and normal resistance of the Josephson junctions, JJs) in the HTS technology are also investigated. Relatively favorable combinations of normal resistance and low capacitance of the JJs needed in the HTS technology for RSFQ circuits have been made possible by recent fabrication methods of high-quality YBCO films. Obtaining these devices is made achievable by using the grain boundary structures and high critical current density ( JC ) of the HTS JJs. Mutual interactions of the intermediate inductances with other parts of the whole circuit are examined and optimized for the best operation situation. Here in particular, the operation of the HTS Josephson-transmission-line (JTL) cells utilizing the grain boundary Josephson junctions, including various structural inductances, is studied based on the reported parameters of the developed devices and thoroughly analyzed by simulation results.

Intermediate Phase Study on YBCO Films Coated by Precursor Solutions With F/Ba Atomic Ratio of 2

In the chemical solution deposition process of YBCO superconducting films, fluorine is widely regarded to be of significant importance in avoiding the formation of BaCO 3 , which hinders the growth of high-quality YBCO films. On the other hand, great efforts have been made to decrease the fluorine content in the precursor solution due to the environmental contaminations of the fluorinated gaseous by-products. In this study, we demonstrate that an F/Ba atomic ratio of 2 in the precursor solution is sufficient according to the results of energy dispersive spectrometry and attenuated Fourier transform infrared studies. The intermediate phase evolution prior to the end of the sintering stage is also investigated by X-ray diffractometry and scanning electron microscopy. Liquid-like structures are observed, which are proposed to be responsible for the film densification in the early stage of YBCO formation. The formation of YBCO is accompanied by the consumption of oxygen-doped BaF 2 (BOF), which is found to be highly textured. Moreover, in the early stage of YBCO formation, the crystallinity and texture of BOF enhance despite its consumption. After a full heat treatment, YBCO films with J c of 5 MA/cm 2 (77 K, 0 T) could be routinely obtained on lanthanum aluminate single-crystal substrates.

Effects of Laser Repetition Rate on the Structural and Superconducting Properties of YBCO films Deposited by PLD on NiW Tapes

YBa2Cu3O7−x (YBCO) films were fabricated at various laser repetition rates on CeO2/YSZ/CeO2 buffered Ni-W tapes by pulsed laser deposition (PLD). The dependence of structural and superconducting properties on the laser repetition rates was investigated. The microstructure and surface morphology of YBCO films were characterized by X-ray diffraction and atomic force microscopy. And the critical current (Ic) of YBCO films was measured by the conventional four-probe method. The results show that with increasing laser repetition rate the island density decreased and the island size increased. It was found that the texture and Ic of YBCO films were largely dependent on the laser repetition rate. And furthermore it was observed that the film thickness was not in directly proportional to laser repetition rate. Under optimum experimental condition, high quality YBCO films with critical current Ic above 500 A/cm at 77 K and 0T were obtained.

Carbon Dioxide Controlled Thermal-Decomposition Reactions for Preparation of High Quality YBCO Films Derived from Fluorine-Free Solution

We report the fabrication of high quality YBCO film through a carbon dioxide controlled thermal-decomposition reaction. During the pyrolysis process of the YBCO gel film, the low melting-point Ba(OH)2 phase was converted to a high melting-point BaCO3 phase when CO2 was introduced into the heat treatment atmosphere, thus resulting in a quite smooth surface morphology of the pyrolyzed YBCO film; Then the BaCO3 phase was converted to Ba(OH)2 phase used as the Ba resource to produce the YBCO phase during the subsequent crystallization and annealing oxidation process. It is found that YBCO film with smooth surface and excellent superconductivity can be obtained through controlling the conversion reaction between the Ba-contained compounds by introducing adequate carbon dioxide and water vapor.

Ultrafine nanocrystal precursor induced Jc increase of YBa2Cu3O7−x films prepared using advanced low-fluorine solution

Abstract High-performance YBa2Cu3O7−x (YBCO) films were obtained on LaAlO3 (LAO) substrate using an advanced low-fluorine solution with Y-TFA and fluorine-free Ba and Cu salts as precursors. Macropore-free precursor films were obtained using a rapid pyrolysis with the heating rate of 5 °C/min from 200 °C to 310 °C. In the precursor film, CuO and Ba1−xYxF2+x (BYF) were formed as intermediate phases. Ultrafine nanocrystalline CuO particles were found embedded inside the crystalline BYF matrix, leading to the formation of high-quality YBCO film. The obtained single-coated YBCO films showed a better in-plane and out-of-plane texture, and a higher critical current density (over 8 MA/cm2 at 77 K, 0T), compared with the YBCO films produced from precursor consisting of larger CuO particles.

Ethanol-based TFA–MOD method for preparation of YBCO films

Highly c-axis orientated YBa2Cu3O7–x films were prepared on LaAlO3 single crystal substrates via metal–organic deposition using trifluoroacetates using ethanol as solvent instead of the toxic methanol. The onset transition temperature is 91K and the critical current density under self field is over 1MA/cm2 at 77K, which indicates ethanol can be used as solvent for high-quality YBCO films using TFA–MOD.

Characterization of epitaxial YBa2Cu3O7−δ films deposited by metal propionate precursor solution

YBa2Cu3O7−δ(YBCO) filmswere deposited with a low-fluorine modified trifluoroacetate metalorganic deposition (TFA-MOD) methodon SrTiO3 single crystals and buffered Ni–W metallic tape with a thickness ranging from 450 to600 nm. The method consists in the substitution of yttrium and copper trifluoroacetateswith Cu and Y acetates dispersed in propionic acid. A reduced pyrolysis time with respectto the usual TFA method is obtained. Apart from CuO, no traces of second phasesare revealed by x-ray measurements. The films are compact without cracks, andexhibit a slight superficial porosity, but they still remain well connected, andtherefore the observed porosity does not affect either the critical current density orthe normal state resistivity values, which are indicative of high-quality YBCOfilms. Moreover, YBCO films were also obtained on Pd-buffered Ni–W, with aCeO2/YSZ/CeO2 buffer layer architecture. These films show good morphological, structural, and superconductiveproperties with high critical temperature (higher than 91 K) and critical current density higher than1 MA cm−2 at 77 K in self-field.

Stacks of YBCO Films Using Multiple IBAD Templates

In this paper we present initial results on growing YBCO/IBAD-MgO/YBCO stacks. Amorphous Y2O3 layers, deposited by sol-gel, are used to level the YBCO surface. Since the YBCO films prepared by pulsed laser deposition are relatively rough, preparing a smooth surface on top of YBCO for IBAD texturing is a major challenge for this structure. For all substrates studied-polished and unpolished metal tape and YBCO - we were able to reduce the roughness of the substrate using the sol-gel layer, with multiple coatings being more effective. First it is demonstrated that high quality YBCO films can be deposited on sol-gel-Y2O3/IBAD-MgO templates. Second it is shown that the sol-gel and the IBAD process do not affect the superconducting properties of the underlying YBCO film. Finally, a superconducting YBCO bilayer stack is demonstrated. Improvements in the properties of the top YBCO layer is expected when the surface roughness is reduced further for deposition of subsequent IBAD-textured layers. This new technology should enable novel designs of coated conductors for reduced AC losses and increased Je.

Engineered reactive cosputtered SmxZr1–xOythin films as buffer layers for YBa2Cu3O7−δcoated conductors

This study shows that biaxially textured SmxZr1−xOy(SZO) with a wide range of compositions (0.06 <x< 0.75) can be grown directly on ion-beam-assisted deposition (IBAD) MgO template using reactive cosputtering. The SZO crystal structure can be changed, and the lattice parameter can be tailored (from 5.23 to 5.49 Å) by changing the composition. We have developed a simplified high-temperature superconducting coated conductor using SZO as the buffer layer. YBa2Cu3O7−δ(YBCO) films grown by pulsed laser deposition on the SZO buffered IBAD MgO have self-field critical current densities (Jc) in the 2–4 MA/cm2range. The in-field measurements demonstrate that high-quality YBCO films can be grown on SZO buffered IBAD MgO. The present results are especially important because they were obtained on coated conductors with the simpler architecture by eliminating the additional homoepitaxial layer of MgO. This translates in faster production and lower manufacturing cost.

Epitaxial Growth of CeO2/YSZ/CeO2 Buffer Layers on Textured Ni Substrates for YBCO Conductors

CeO2/YSZ/CeO2 buffer layers were deposited on textured Ni substrates by in sitit pulsed laser deposition. The out-of-plane texture and in-plane texture of the buffer layers were characterized by X-ray diffraction (omega-scans and phi-scans. Using this CeO2/YSZ/CeO2 architecture as the buffer layers, high quality YBCO films with a zero-resistance T, about 90 K and a self-field critical current densities J(c) above 10(6) A/cm(2) at 77 K can be obtained on Ni substrates.

Nonlinear Meissner effect in a high-temperature superconductor

The lowest nonlinear correction to the penetration depth, i.e., the nonlinear Meissner effect, is calculated and compared to data from high-quality $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\mathit{\ensuremath{\delta}}}$ (YBCO) films. The calculation is based on the Green-function formulation of superconductivity, and the data consist of the intermodulation power as function of temperature and circulating power. At a low power level, the calculated temperature dependence compares very well with the data, including the divergence as ${T}^{\ensuremath{-}2}$ at very low temperatures. The calculated power dependence of the nonlinear penetration depth follows the data semiquantitatively and is enhanced due to the $d$-wave symmetry of the order parameter. These results support the assertion that the origin of nonlinearity in high-quality YBCO films is intrinsic. The analysis also implies that the nonlinear corrections to the penetration depth depend primarily on the total current carried by the strip and thus are insensitive to the edges. The comparison of the present approach with an alternative approach, based on quasiparticle backflow, is discussed.

The combined effect of thermal annealing of MgO substrate and Ca substitution on the surface resistance of YBa2Cu3Oz thin films

Single-layer Y1−xCaxBa2Cu3Oz (YCBCO) thin films (x=0.00, 0.02, 0.05, and 0.10) grown on annealed as well as unannealed MgO substrates have been systematically investigated for their carrier concentration, critical current density Jc, and microwave surface resistance Rs. For x⩽0.05, the grain growth follows a three-dimensional-spiral growth mechanism, while for x=0.10 we observed a mainly two-dimensional-like growth of grains. The results of Hall data reveal that the x=0.05 film is overdoped while films with x=0.02 and 0.10 are underdoped with respect to the x=0.00 film. However, the Hall mobility μH is highly enhanced for the x=0.02 film. Thermal annealing of MgO substrates prior to film deposition results to an improvement in the overall superconducting properties of the film such as suppression of normal-state resistivity, enhancement of Jc, and minimization of Rs both for pure as well as Ca-substituted films. Annealing of MgO substrates enhances the Jc value to a magnitude (i) nearly doubled for x=0.00 films and (ii) more than an order for x=0.02 films. Also it leads to a minimization of the Rs value to (i) more than half and (ii) nearly an order of magnitude higher, respectively, for x=0.00 and x=0.02 films. Furthermore, for the x=0.02 film, below 60K, we realized an enhanced Jc value in self- as well as in large-applied fields. For other than the Ca-2% substituted films, a suppression of Jc with a strong field dependency has been noticed. Furthermore, the Rs value of the x=0.02 film (0.1mΩ at 20K, 21.9GHz) was three times lower compared to that of the x=0.00 film (0.35mΩ at 20K, 21.9GHz). At 20K and 21.9GHz, the Rs value for the x=0.05 film is comparable to that of the x=0.00 film, whereas for the x=0.10 film it is twice that of x=0.00. The low normal-state resistivity, enhanced mobility, high Jc, and the minimized Rs observed for x=0.02 films firmly support the possible improvement of superconducting order parameters near the grain boundaries. Thus, the combined effect of thermal annealing of MgO prior to film deposition and substituting Y with 2% Ca may be readily applied to grow high-quality (high Jc and low Rs) YBCO films for microwave device applications.

The development of Y Ba2Cu3Ox thin films using a fluorine-free sol–gel approach for coated conductors

Despite great success in the TFA methods of depositingY Ba2Cu3Ox (YBCO) thin films for coated conductors, critical issues involved in removingBaCO3 have not entirely been settled. There could be other possible ways of dealing with carbonthat remains in the film. We have recently developed a fluorine-free sol–gel synthesis withseveral important advantages including precursor solution stability, improved filmdensity, and elimination of HF during processing. With this approach, high-qualityYBCO films have been developed on single crystal substrates with the transportJc s upto 106 A cm−2. In this study, the precursor solution stoichiometry was altered and its effects onsuperconducting properties were studied. The fluorine-free sol–gel-derived films on theLaAlO3 (LAO) substrate exhibited epitaxial growth with excellent in- and out-of-planetexture. Experimental details are reported on the sol–gel synthesis chemistryand XRD and TEM characterization of the YBCO thin films. Also discussed isthe underlying formation mechanism of the YBCO phase during the synthesis.

Reel-to-reel ex situ conversion of high critical current density electron-beam co-evaporatedBaF2 precursor on RABiTS

A reel-to-reel furnace equipped with seven transverse flow modules has been developed to continuously processex situ BaF2 precursor coated conductors. Experimental results have revealed that as the precursor thicknessincreases, the processing parameters need to be adjusted in order to obtain high quality YBCO films withJc’ s greaterthan 1 MA cm−2 at 77 K and self-field. For thin precursors, a relatively aggressive conversion environment of highP(H2O) can be used to obtain epitaxial YBCO with an extremely small amount of undesirable random ora-axis grains.Thin 0.3 µm YBCO was converted on a 2.3 m long by 1 cm wide Ni RABiTS substrate, yielding an end-to-endIc of18.9 A (Jc = 0.63 MA cm−2). 16 cm sectional measurements showed that the tape performance is uniform, with a meanIc of19.1 A (0.64 MA cm−2) and a 5.4% SD. In thick precursors, however, a less aggressive environment in the form of lowP(H2O) is better suited for epitaxial YBCO nucleation during the incubation period. By using a low initialP(H2O) with progressively higher values during later stages of conversion, an end-to-endIc of 92 A(1.02 MA cm−2) was obtained in 1.1 mlong by 1 cm wide 0.9 µm YBCO on better-textured Ni–W RABiTS. This conductor possesses uniformIc according to 1 cm sectional measurements with a meanIc of94.4 A (1.05 MA cm−2) and only 4.3% SD. Further adjustment in theP(H2O) schedule resulted in a 45 cm long sample with end-to-endIc of 111.8 A(1.24 MA cm−2), a meanIc of 114.8 A(1.28 MA cm−2) and 4.8% SD.The highest Ic of 138 A (1.53 MA cm−2) wasobtained in a short 0.9 µm YBCO sample on a pre-treated Ni–W RABiTS substrate.

Improving chemical solution deposited YBa2Cu3O7−δ film properties via high heating rates

Abstract The superconducting and structural properties of YBa 2 Cu 3 O 7− δ (YBCO) films grown from chemical solution deposited (CSD) metallofluoride-based precursors improve by using high heating rates to the desired growth temperature. This is due to avoiding the nucleation of undesirable a -axis grains at lower temperatures, from 650 to 800 °C in p (O 2 )=0.1%. Minimizing time spent in this range during the temperature ramp of the ex situ growth process depresses a -axis grain growth in favor of the desired c -axis orientation. Using optimized conditions, this results in high-quality YBCO films on LaAlO 3 (1 0 0) with J c (77 K) ∼ 3 MA/cm 2 for films thicknesses ranging from 60 to 140 nm. In particular, there is a dramatic decrease in a -axis grains in coated-conductors grown on CSD Nb-doped SrTiO 3 (1 0 0) buffered Ni(1 0 0) tapes.

Growth and characterization of SrRuO/sub 3/ buffer layer on MgO template for coated conductors

Recently, it has been shown that biaxially oriented MgO deposited by ion-beam-assisted-deposition (IBAD) is a significantly more cost-effective template for coated conductors. Nevertheless, the lack of good buffer layer materials for MgO has limited the accomplishment of high quality YBCO films. We, for the first time, have used SrRuO/sub 3/ as a single buffer layer directly on an MgO template (homo-epitaxial MgO/IBAD-MgO) for growth of thick YBCO films on polycrystalline metal substrates. By using SrRuO/sub 3/ as a buffer layer, we have routinely obtained YBCO films with an in-plane mosaic spread in the range of 3 - 6/spl deg/ full width at half maximum. A critical current density of 1.8 /spl times/ 10/sup 6/ A/cm/sup 2/ in self-field at 75 K for a film thickness of over 1.0 /spl mu/m has been achieved. We have further achieved a current per unit width of over 245 A/cm, which is the highest reported so far for a YBCO film on a polycrystalline metal substrate using MgO as a template.