YBCO Thick Films Research Articles

Pinning Centers Induced in YBCO Films by Nano-Dots in Substrate Decoration and Quasi-Superlattice Approaches

For power applications of superconducting films, the critical current density (J c) and the thickness of the film (d) should be as high as possible. Since J c decreases with both thickness and magnetic field, artificial pinning centers in addition to natural ones are required to keep J c high. The earliest cost-effective method used for introducing artificial pinning centers was the so-called substrate decoration, i.e., growing nanoscale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on other two approaches proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices) and distributing a secondary phase in the film from a compositionally changed target. Several materials have been used for the creation of artificial pinning centers. Here we report on the artificial pinning centers induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Ag, Au, Pd or non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.

Microstructural and Pinning Properties of ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$ Thin Films Doped With Magnetic Nanoparticles

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we report a strong enhancement in the in-field transport properties of the <formula formulatype="inline"><tex Notation="TeX">${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$</tex></formula> (YBCO) thin films doped with magnetic <formula formulatype="inline"><tex Notation="TeX">${\rm Fe}_{2}{\rm O}_{3}$</tex></formula> nanoparticles. We incorporated magnetic <formula formulatype="inline"><tex Notation="TeX">${\rm Fe}_{2}{\rm O}_{3}$</tex></formula> nanoparticles with two different architectures by laser ablation of the YBCO and dopant targets. YBCO film thickness was controlled at around 1 <formula formulatype="inline"> <tex Notation="TeX">$\mu{\rm m}$</tex></formula> for all the samples. We conducted a detailed microstructural characterization on all the doped samples by X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM) and compared with the pure YBCO films. A systematic study on the superconducting properties has been conducted on all the thin film samples. The critical current densities at both self-field and in-field (<formula formulatype="inline"> <tex Notation="TeX">$J_{c}^{sf}$</tex></formula> and <formula formulatype="inline"> <tex Notation="TeX">$J_{c}^{in-field}\ ({\rm H}//c)$</tex></formula>) and the critical transition temperature <formula formulatype="inline"><tex Notation="TeX">$(T_{c})$</tex> </formula> of the doped and un-doped YBCO sample were measured by a Superconducting Quantum Interference Device (SQUID). The <formula formulatype="inline"><tex Notation="TeX">$T_{c}$</tex></formula> of the doped YBCO films varies from 84 K–90 K and the <formula formulatype="inline"><tex Notation="TeX">$J_{c}$</tex> </formula> is in the range of 1.2–3.9 <formula formulatype="inline"> <tex Notation="TeX">${\rm MA/cm}^{2}$</tex></formula> (at 65 K) depending on the doping approach. The pinning properties of these doped YBCO films were explored at different temperatures (5 K, 40 K, and 65 K) and correlated with their microstructural characteristics. </para>

Modification and Conversion of E-Beam Co-Evaporated Precursors for Fabricating High Critical Current ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$ Films

Ex situ conversion of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</i> -beam co-evaporated precursors was studied in an effort to fabricate high critical current YBCO films using the BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> process. The precursors were deposited on CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffered single crystal YSZ substrates. It was shown that an intermediate oxygenation annealing prior to the conversion modifies the precursor crystallinity and promotes c-axis epitaxial growth while randomly-oriented film formation is suppressed. With the modified precursors, a critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , 77 K & 0 T) of 2.1 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> measured by SQUID magnetometry was obtained in 1.8 mum thick YBCO films. The mechanism of the pre-annealing effect was investigated by characterization of the precursors, quenched films, and fully converted films using XRD, SEM, and TEM. Cross-sectional TEM was used to study the early nucleation of YBCO film at the precursor/substrate interface. The significant effect of the precursor modification indicated that, in addition to optimizing conversion processing parameters, modifying the precursor is an effective way to achieve the desired epitaxial film structure and to obtain higher critical currents, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

Enhancing critical current in YBCO thick films: Substrate decoration and quasi-superlattice approach

For power applications of superconducting films, the critical current density ( J c ) and the thickness of the film ( d) should be as high as possible. Since J c decreases with both thickness and magnetic field, artificial pinning centres in addition to natural ones are required to keep J c high. The earliest cost-effective method used for introducing artificial pinning centres was the so-called substrate decoration, i.e., growing nano-scale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on another version of this approach proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices). Several materials have been used for the creation of artificial pinning centres. Here we report on the artificial pinning centres induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Pd and non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films. We observed an important shift of the position of the maximum in the thickness dependence of J c ( B) towards higher thicknesses compared with pure YBCO films by both approaches. A high J c ( B) in our quite thick films provides a very high total critical current per cm of the film width. Critical current as high as 800 A/cm width was achieved in a 2.4 μm thick quasi-superlattice film with non-superconducting YBCO nano-dots.

YBa 2Cu 3O 7 − δ thin films with enhanced film properties grown on sapphire using Y 2O 3/CeO 2 bi-layer buffer

Highly epitaxial, microcrack-free thin films of YBa 2Cu 3O 7 − δ (YBCO) have been grown by pulsed laser deposition on sapphire substrates with a double buffer of Y 2O 3/CeO 2. The Y 2O 3 layer, which has excellent compatibility with CeO 2 and YBCO, has been found to be beneficial in reducing the surface porosity of YBCO films as well as inhibiting a-axis-oriented epitaxial growth. The reduction in porosity is attributed to the presence of the Y 2O 3 layer which acts as a suitable barrier for the chemical reaction occurring at high deposition temperatures between YBCO and CeO 2. Due to the improvement in film quality and surface morphology, enhancement of the self-field critical current density ( J c, 77.3 K) by as high as 30% was obtained for a 650-nm thick YBCO film deposited on Y 2O 3/CeO 2 bi-layer buffer compared to simultaneously-deposited YBCO film on CeO 2 single-layer buffer only.

Effect of Melt Processing Programme on Microstructure and Properties of YBCO Thick Films

The screen printing technique was used in the fabrication of YBa2Cu3O7−σ (YBCO) superconducting thick films on yttria stabilised zirconia (YSZ) substrates. Different slow cooling rates were used in the preparation of YBCO thick films after fast cooling from the melt processing temperature. The effects of the melt processing programme on texturing, microstructure and superconducting properties of the melt processed YBCO films were studied. Slow cooling rates between 1005 and 990 °C were effective in increasing the interaction of viscous molten with reduced film/substrate, and hence a relatively large grain size has been obtained. Moreover, different c-axis texturing ratios and grain morphologies were observed.

Melt Processing of YBCO Thick Films Fabricated by Sol Gel Route Powder on YSZ

It is evident that the microstructure and superconducting properties of YBa2Cu3O7−σ (YBCO) thick films are highly dependent upon the route of the precursor. The precursor which was used in the preparation of YBCO thick films was produced using sol gel route technique. The screen printing technique was used in the fabrication of YBCO superconducting thick films on yttria stabilized zirconia (YSZ) substrates. The effect of slow cooling rate and slow cooling window on texturing, microstructure and superconducting properties of the melt processed films was carried out. Optimizing the melt processing program was effective in increasing the molten viscosity, keeping the liquid and hence relatively large grain size that has been obtained. Moreover, grain morphologies like which was found in YBCO bulk materials was observed.

Effect of the nano-structure Cu-carbon composite absorber layer on the response of superconductive YBCO TEBs

We have used a nanostructure copper composite layer as the absorber for infrared radiation on superconductive YBCO bolometers. The Cu composite has the advantage of having little destructive effects on superconductive YBCO material and its deposition process is done at low temperature. The bolometer is made of 200nm thick YBCO film on 1mm thick LaAlO3 and SrTiO3 crystalline substrates, deposited using pulsed laser deposition technique. The Cu composite layers were made by decomposition of C2H2 and co sputtering of Cu target in an RF system. The layer was deposited on an insulating buffer layer of diamond like carbon (DLC). The spectral absorption of the Cu composite layer was also measured through measurements of the spectral transmission and reflection of the layer on various substrates. The control of the absorption of the composite layer versus Cu composition has been achieved and presented here. In addition, optimization of the Cu component of the absorber layer for maximum increase of the response of the detectors as well as its effects on the response characteristics of the devices are presented in this paper.

Thick YBa2Cu3O7−x+BaSnO3 films with enhanced critical current density at high magnetic fields

The thickness dependence was studied for the critical current density (Jc) of YBa2Cu3O7−x(YBCO)+BaSnO3 (BSO) nanocomposite films. These films showed a significantly reduced decline of the Jc with thickness, especially at high magnetic fields. For example, a 2 μm thick YBCO+BSO film had a Jc∼3×105 A/cm2 at 5 T as compared to a typical Jc of 2.4×103 A/cm2 at 5 T for a 300 nm thick YBCO film. The thick YBCO+BSO films maintained high Tc (&amp;gt;88 K) and had a high density (2.5×1011/cm2) of continuous BSO nanocolumns that likely contributed for the observed Jc enhancements.

Deposition and characterization of high temperature superconducting YBa2Cu3O7-δ films obtained by DC magnetron sputtering and thermal annealing modification

C-axis oriented 100-nm thick YBCO films were deposited on LaAlO3 (100) substrates at substrate temperature of 780°C in a mixed oxygen/argon atmosphere (1:3) of 0.3 Torr by DC off-axis magnetron sputtering. The samples deposited were thermally annealed in oxygen ambient of 600 Torr at 530°C for 40 min. Superconductivity with zero resistance 89.1K was observed for the YBCO films after annealing. These results show that thermal annealing is an important technique for improving the parameters of thin superconducting films. A correlation between the YBCO layers properties before and after annealing was established.

YBCO superconducting tapes by melt-annealing method on metallic Ni%5W substrates

Superconducting tapes of YBCO-123 were produced by melt-annealing method on metallic Ni%5W substrates. YBCO thick films of about 1μm thick were deposited on Ni%5W tapes previously CeO2 coated using different thermal treatments. A final architecture of the tapes like Ni%5W/CeO2/YBCO/Au–Pd was achieved.Critical temperatures (Tc) of the superconducting tapes around 89K and critical current densities (Jc) of 104A/cm2 at 77K and B=0T were determined by resistive methods. All the samples displayed a granular character and the crystalline structure of the superconducting YBCO-123 with and preferential orientation along the c-axis as determined by SEM and XRD analysis, respectively. Both grain sizes and c-axis orientation are dependent of the thermal treatments.

MOD multi-layer YBCO films on single-crystal substrate

Several practical applications require relatively thickY1Ba2Cu3O7 (YBCO) coatedconductors (1–2 µm) to carrya high critical current (Ic). However, as the thickness of the YBCO film increases, its critical current density(Jc) decreases. In this work, we report the successful fabrication of thickYBCO films using a multiple coatings technique while maintaining a highJc. In particular, several YBCO films with different numbers of layers were fabricated on aLaAlO3 substrate by a multiple coatings process using a trifluoroacetate metal organic depositionmethod (TFA-MOD). No cracks or pores were observed in cross-sectional transmissionelectron microscope (TEM) images of these films; further, the layers were wellconnected with the smooth interface. The majority of the grains in the films werec-axis oriented. Full-width at half-maximum (FWHM) values forin-plane and out-of-plane alignments for a five-layer film were2.45° and1.2° respectively.Jc exceeding1.6 MA cm−2 and criticalcurrent (Ic) of160 A cm−1 have beenobtained for a 1 µm thick film with five layers at 77 K.

Metal propionate synthesis of epitaxial YBa2Cu3O7-x films

A modified TFA-MOD method, using only barium trifluoroacetate, is presented. The yttrium and copper triflouroacetates were replaced by the alcoholic solutions of Cu and Y acetates dispersed in propionic acid. Fourier transformed infrared spectroscopy (FT-IR), thermal analyses (DTA/TG) coupled with mass spectrometry (MS) and X-ray diffraction analyses were used to study the decomposition of the precursor. The method permits the shortening of the pyrolysis time by a factor 4, with respect to conventional TFA-MOD method, due to the smaller amount of evolved hydrofluoric acid. Using this method 600 nm thick YBCO films were grown both on (100)SrTiO3 and on CeO2/YSZ/CeO2/Pd buffered Ni-5at.%W substrates. The as obtained films exhibit good morphological, structural and superconducting properties with Tc (R=0) greater than 91K and with an out-of-plain texture of 0.240 and 1.90, respectively.

Enhanced flux pinning force and uniquely shaped flux pinning force plots observed inYBa2Cu3O7−x filmswith BaSnO3 nanoparticles

We demonstrate that the maximum of the bulk pinning force(Fp,max) of YBCO filmswith BaSnO3 nanoparticles(YBCO+BSO) can be more thantwice the value of Fp,max of regular YBCO films of similar thickness at 77 or 65 K. Also, inYBCO+BSO films, theposition of the Fp,max is shifted to higher fields occurring at more than 4 T at 77 K, as opposed to typical 1.5–2 T ingood-quality YBCO films. Further, we show that a unique, distinguishable dual-peak structure inFp versusH plotsin YBCO+BSO samples can exist, possibly due to the presence of simultaneously operative dual pinningmechanisms.

Effect of Substrate Texture on Superconducting Properties of YBCO Thick Films

On nontextured substrates, YBCO superconducting thick films were fabricated using a simple screen-printing method, from Cu-free powders (Y2O3 and BaCO3). However, the films have poor superconducting properties, such as the critical current density, Jc value due to the random orientation of the films. In this work, we investigate the influence of substrate texture on superconducting properties of the film. The films screen-printed on the nontextured and textured Cu substrates were heat-treated by 2-step procedure in air (980 °C 10 sec + 930 °C 90 sec) followed by oxygen annealing at 450 °C for 1 hr. We speculate that Cu ions diffusion from the textured Cu substrates is still faster than Cu ions diffusion from the nontextured Cu substrates that CuO formed excessively in the film deteriorates superconducting properties of the YBCO film.

Processing of Long-Length YBCO Coated Conductors Based on Stainless Steel Tapes

Recent progress is reported in the development of long lengths (up to 100 m) of YBCO coated conductors with good mechanical stability and improved critical current homogeneity. A non-magnetic CrNi stainless steel tape, 0.1 mm thick, is employed as a substrate tape. Prior to deposition, the tape is coated with yttria stabilized zirconia in the form of a bi-axially textured buffer layer, using a novel "alternating beam assisted deposition" technique. After deposition of a ceria cap layer onto the buffer, YBCO films are deposited employing a high-rate pulsed laser deposition method that allows higher critical current densities and incurs a lower YBCO loss (~10%) than other deposition techniques. The last processing step consists of coating the with a thin layer of silver or gold and a 2-40 mum thick copper shunt layer. In 40 m long tapes with a 1 mum thick YBCO film (at 77 K and zero external field), critical current, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , up to 235 A per cm-width were obtained. Higher critical currents, up to 574 A per cm, were achieved in short 0.2-7 m sections corresponding to current densities, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , of up to 4.4 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The critical current homogeneity and performance of our improved coated conductors under applied temperature cycling, multiple quench-recovery cycles, and repeated bending are reported.

Development of Modified MOD-TFA Approach for YBCO Film Growth

Low-cost coated-conductor fabrication methods are essential for various electric-power applications. Metal-organic-deposition (MOD) approach to grow both YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-delta</sub> (YBCO) and buffer layers on textured metal substrates is very promising towards fabrication of lower-cost second generation wires. YBCO coated conductors (CC) are being developed with high critical currents that should be sufficient for their extensive use in power applications. However, the present CC has high energy losses in ac magnetic field that are unacceptable. We have developed a modified MOD precursor route to deposit ~ 0.8 mum thick YBCO films in a single coat that requires less than one-fifth of the pyrolysis time compared to the traditional MOD approach. We have also developed a filamentization technique of CC using ink-jet printing to reduce ac losses due to applied ac fields. The preliminary results of YBCO films deposited on standard RABiTS template yielded an of 140 A/cm at 77 K and self- field. A modest reduction of ac loss was observed for the solution ink-jet printed filamentary conductor.

Fabrication of Thick $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-{\rm x}}$ Films on Optimized $\hbox{SrTiO}_{3}$ Buffered-MgO Single Crystals

In this work, we studied the optimum deposition conditions for SrTiO3 (STO) buffers deposited on MgO single crystals substrates by PLD method. We found that 120 nm thick STO films, deposited at 730 oC , present the smoothest surface and a minimum FWMH of the (002) reflection. High quality, smooth and free-crack (YBCO) thick superconducting films have then been fabricated using a deposition temperature of 800 oC on STO-buffered MgO substrates. A 373 nm thick YBCO/STO/Mg2O film can carry current densities (77 K, 0 T) of 1.4 MA/ cm2 and Jc (77 K, 5 T) of 1.4 MA/ cm2 The Jc in high field overcomes the current transported in same experimental conditions by 180 nm thick YBCO film grown on Y2O3 -decorated STO single crystal. For 1350 nm thick YBCO/STO/MgO film the Jc (77 K, 0 T) is about 0.8 MA/cm2 and Jc (77 K, 5 T) is 0.09 MA/cm2.

High rate deposition of thick YBa2Cu3O7−x superconducting films using low-fluorine solution

We have developed a new solution for fabrication ofYBa2Cu3O7−x (YBCO) films. This new solution contains only 30% of the fluorine content inan all-TFA solution, and the duration of the heat-treatment cycles is muchshorter than that when the traditional all-TFA solution is used. Pyrolyzedfilms with mirror-like surfaces can be obtained at a high heating rate of10 °C min−1. Therefore, thick YBCO films can be prepared rapidly using a multiplecoating–pyrolysis process. Using low-fluorine solution, Y and Ba fluorides(YF3 and BaF2) were formed during the pyrolysis process, which is the same as when an all-TFA solutionwas used. Hence, a liquid phase was formed prior to the YBCO nucleation, which enhancedthe epitaxial growth of the YBCO film as well as its superconducting performance.

High-resolution XRD study of stress-modulated YBCO films with various thicknesses

High-quality epitaxial YBa 2Cu 3O 7− δ (YBCO) superconducting films with thicknesses between 0.2 and 2 μm were fabricated on (0 0 l) LaAlO 3 with direct-current sputtering method. The influence of film thickness on the structure and texture was investigated by X-ray diffraction conventional θ–2 θ scan and high-resolution reciprocal space mapping (HR-RSM). The films grew with strictly c-axis epitaxial, and no a-axis-oriented growth was observed up to a thickness of 2 μm. Lattice parameters of the YBCO films with different thicknesses were extracted from symmetry and asymmetry HR-RSMs. The X-ray lattice parameter method was used to determine the residual stress in YBCO films by measuring the a-, b-, c-axis strains, respectively. The results showed that YBCO films within thinner than 1 μm were under compressive stress, which was relieved increasing of film thickness. However, beyond 1 μm in thickness, YBCO films exhibited a tensile stress. Based on the experimental analysis, the variety of residual stresses in the films is mainly attributed to oxygen vacancies with thickness of YBCO film increasing.