Quick screening for new flux pinning materials in YBCO films with the combinatorial-PLD method

An effective way to improve the superconducting properties in REBa₂Cu₃O_y (REBCO) films under a magnetic field is to dope artificial pinning centers (APC). The pinning performance depends on the content of the APC materials. Usually, the optimal APC content is explored by preparing films one at a time from the REBCO target using different APC material content, which is an extremely time-consuming process.The combinatorial-PLD (C-PLD) method allowed us to prepare films that continuously changed in composition across a single substrate. In this study, we used the C-PLD method to prepare BaSnO₃ (BSO)-doped YBCO films. The films were deposited on SrTiO₃ substrate using a fourth-harmonic Nd:YAG laser. From the results of the J_c–B curves at 77 K and B//c, the film that contained 3.2 vol.% of BSO exhibited the best pinning performance in this study. We showed that the C-PLD method was efficient for quick screening of the optimal APC content with only one deposition. We also used the C-PLD method to explore new APC materials, and proved that it can quickly evaluate the new APC materials Ba₃Cu₃In₄O₁₂ and BaTbO₃.     

High performance GdBa2Cu3O7−z film preparation by non-fluorine chemical solution deposition approach

Biaxially textured GdBa₂Cu₃O_7?z (GdBCO) films with T_c above 93 K have been prepared on (0 0 l) LaAlO₃ substrate by self-developed non-fluorine polymer-assisted chemical solution deposition (PA-CSD) approach. The GdBCO films show smooth and crack-free morphology. Many nanoscale particles with homogeneous distribution are observed in the GdBCO films, which have not been observed yet in the YBa₂Cu₃O_7?z (YBCO) films prepared by the same processing technique. Besides a high J_c (77 K, 0 T) of 2.28 MA/cm², the optimized GdBCO films show a better J_c–B behavior and an improved high-field J_c, compared to the YBCO films.     

Strongly enhanced flux pining of GdBCO coated conductors with BaSnO3 nanorods by pulsed laser deposition

We report the effects of BSO addition on the crystallinity, texture, and the field dependency of critical current density (J_c) of GdBCO coated conductors (CCs) prepared by pulsed laser deposition (PLD). Undoped and BSO-doped GdBCO films showed only c-axis oriented growth, and the incorporated BSO nanorods exhibited epitaxial relationship with the GdBCO matrix. In comparison with undoped film, BSO-doped GdBCO film exhibited greatly enhanced J_c and higher pinning force densities in the entire field region of 0–5 T (H//c) at 77 and 65 K. The BSO-doped GdBCO film showed the maximum pinning force densities (F_p) of 6.5 GN/m³ (77 K, H//c) and 32.5 GN/m³ (65 K, H//c), ～2.8 times higher than those of the undoped sample. Cross-sectional TEM analyses exhibited nano-structured BSO nanorods roughly aligned along the c-axis of the GdBCO film, which are believed effective flux pinning centers responsible for strongly improved critical current densities in magnetic fields.     

Reduced crystallization time of YBCO in a fluorine-free MOD process using uv-lamp irradiation

We have studied the crystallization time dependence of the epitaxial YBCO films (t = 0.8 μm) grown on CeO₂-buffered SrTiO₃ substrates by fluorine-free metal–organic deposition using uv-lamp irradiation (uv-MOD). As increasing the time (T₀) for heat treatment at the reaction temperature (760 °C) from 0 to 90 min, J_c and the YBCO 0 0 l XRD intensity are steeply increased and reach their maximum values at T₀ = 10 min. This suggests that the heat treatment required for YBCO crystallization is significantly shortened in uv-MOD compared to conventional all-pyrolytic F-free MOD processes, which consume T₀ = 90–150 min for crystallizing 0.4–0.5-μm-thick films. Scanning electron microscope measurement revealed a drastic change in surface morphology between T₀ = 8 and 10 min, showing a good correspondence to the J_c and XRD data which suggest that the epitaxial growth reaches the film surface at the very early stage in the heat treatment.     

Attenuation of interfacial pinning enhancement in YBCO using a PrBCO buffer layer

Numerous experimental results have suggested that the J_c of YBa₂Cu₃O₇ (YBCO) films is significantly higher near the film–substrate interface than in the remainder of the film. We previously proposed that this effect is due to interfacial pinning enhancement caused by stress and the resulting misfit dislocations at the heteroepitaxial interface. To test this hypothesis we have used a non-superconducting PrBa₂Cu₃O_7?δ (PrBCO) buffer layer to minimize the lattice mismatch with YBCO. We find that the PrBCO layers lower J_c of the 0.4 μm YBCO films in a predictable way, and that, if sufficiently thick (～0.5 μm), they eliminate interfacial enhancement altogether. Our interpretation of this result is that the defects responsible for interfacial enhancement of flux pinning originate at the bottom of the non-superconducting PrBCO layer, which screens the pinning centers from vortices in YBCO. This result demonstrates that the pinning enhancement arises from stress at the film–substrate interface.     

Stable barium compounds in YBa2Cu3Oy superconductors

The stability of various amounts of Ba₃Cu₃In₄O₁₂ (334) or BaTbO₃ (BTO) in a sintered YBa₂Cu₃O_y (YBCO) matrix was examined. Samples with added 334 or BTO exhibited critical temperatures (T_c) above 90 K for up to 20 vol.% addition and improved critical current densities (J_c) under a magnetic field. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis indicated that 334 and BTO did not react with the YBCO matrix under the sintering conditions used. The normalized J_c under a magnetic field of 1 T reached a maximum at 14 vol.% of 334 addition and 20 vol.% BTO addition. YBCO thin films with added BTO showed a gradual decrease in the T_c with increasing BTO content. YBCO films with added 334 showed a constant T_c of 87 K up to a 334 content of 4 vol.%.     

Aqueous polymer–nitrate solution deposition of YBCO films

High critical current density YBa₂Cu₃O_7?x (YBCO) films were prepared by solution deposition of aqueous non-fluorine precursors. Non-fluorine polymer-assisted deposition (PAD) processes utilizing rheology modifiers and chelating agents were used to produce 50 nm films with a critical current density (J_c) over 3 MA/cm² and 400 nm films with J_c > 1 MA/cm²_. T_c measurements indicated that films have T_c values near 90 K. The total heat treatment time to produce these high performance films was less than 4 h. Rheology modifiers such as polyvinyl alcohol (PVA) and hydroxyethyl cellulose (HEC) were used to increase the thickness of deposited films independent of the solution cation concentration. Chelating agents such as polyethylene glycol (PEG) and sucrose increased the barium ion solubility. Nitrate crystallization during deposition was controlled through rapid drying with vacuum and coating with hot solutions.     

Homoepitaxial growth of MOD-YBCO thick films on evaporated and MOD templates

We have prepared metal organic deposition (MOD)-YBCO thick films by repeating the coating-pyrolysis-crystallization procedure onto ～100-nm-thick evaporated and MOD templates. Surface morphology of the template was found to strongly affect the homoepitaxial growth of MOD-YBCO layers on the template; namely, the epitaxial growth of MOD-YBCO on the evaporated template was much easier than that on the MOD template. A 220-nm-thick epitaxial MOD-YBCO film was successfully prepared on the 100-nm-thick evaporated-YBCO template to obtain a 320-nm-thick YBCO film, which exhibited J_c = 2.44 MA/cm² and I_c = 78 A/cm. The I_c value has significantly increased from 37 A/cm for the evaporated-template.     

Alumina nano-inclusions as effective flux pinning centers in Y–Ba–Cu–O superconductor fabricated by seeded infiltration and growth

The YBa₂Cu₃O_7?x (Y123) textured bulk superconductors with various amounts of nanometer alumina particles were fabricated by a seeded infiltration and growth process. The addition of nanometer alumina was found to be effective for an improvement of the superconducting properties. The critical current density (J_c) values were increased twice in self field with a slight addition amount of nanometer alumina particles (maximum J_c at 0.01 wt.% alumina addition). The present work suggests that the use of insulating inclusions in the nanometer sub-scale can stabilize the flux-line lattice and greatly enhance the pinning capabilities of the infiltrated samples. No refinement of Y211 particles was observed with alumina addition. The J_c improvement by nanometer alumina inclusions is likely rendered to the insulating nano-pinning centers that have been successfully embedded into superconducting Y123 matrix. On the other hand, we examined the effect of the pinning centers size on the superconducting properties of infiltrated YBCO bulk samples. To this effect insulating nano-pinning centers with two different size distributions has been successfully incorporated within YBCO matrix of bulk superconductor by slightly doping with nano-particle alumina dispersions. Two alumina nano-particle dispersions with mean size diameters of about 20 nm and 130 nm were used. It was shown that the size of the pinning centers can affect considerably the J_c performances and the pinning mechanism.     

Structural and optical properties of La-doped BaSnO3 thin films grown by PLD

In this study the structural and optical properties of lanthanum-doped BaSnO₃ powder samples and thin films deposited on fused silica were investigaed using laser ablation. Under an oxygen pressure of 5×10⁻⁴ mbar, phase pure BaSnO₃ films with a lattice constant of 0.417 nm and grain size of 21 nm were prepared at 630 °C. The band gap of BaSnO3 powder sample and thin films was calculated to be 3.36 eV and 3.67 eV, respectively. There was a progressive increase in conductivity for thin films of BaSnO₃ doped with 0~7 at% of La. The highest conductivity, 9 Scm⁻¹, was obtained for 7 at% La-doped BaSnO₃. Carrier concentration, obtained from Burstein-Moss (B-M) shift, nearly matches the measured values except for 3 at% and 10 at% La-doped BaSnO₃ thin films.     

Thickness dependence of Ic and Jc of LTG-SmBCO coated-conductor on IBAD-MgO tapes

We have reported SmBa₂Cu₃O_y (SmBCO) films on single crystalline substrates prepared by low-temperature growth (LTG) technique. The LTG-SmBCO films showed high critical current densities in magnetic fields compared with conventional SmBCO films prepared by pulsed laser deposition (PLD) method. In this study, to enhance critical current (I_c) in magnetic field, we fabricated thick LTG-SmBCO films on metal substrates with ion-beam assisted deposition (IBAD)-MgO buffer and estimated the I_c and J_c in magnetic fields.All the SmBCO films showed c-axis orientation and cube-on-cube in-plane texture. T_c of the LTG-SmBCO films were 93.1–93.4 K. J_c and I_c of a 0.5 μm-thick SmBCO film were 3.0 MA/cm² and 150 A/cm-width at 77 K in self-field, respectively. Those of a 2.0 μm-thick film were 1.6 MA/cm² and 284 A/cm-width respectively. Although I_c increased with the film thickness increasing up to 2 μm, the I_c tended to be saturated in 300 A/cm-width. From a cross sectional TEM image of the SmBCO film, we recognized a-axis oriented grains and 45° grains and Cu–O precipitates. Because these undesired grains form dead layers, I_c saturated above a certain thickness. We achieved that I_c in magnetic fields of the LTG-SmBCO films with a thickness of 2.0 μm were 88 A/cm-width at 1 T and 28 A/cm-width at 3 T.     

Reel-to-reel fabrication of meter-long YBCO coated conductor

YBa₂Cu₃O_7?δ (YBCO) superconductors were coated on the CeO₂/YSZ/Y₂O₃ buffered Ni-5at%W tapes by a reel-to-reel pulsed laser deposition (PLD). The process of a multi-layer deposition of YBCO film was explored. X-ray diffraction texture measurements showed good both in-plane and out of plane crystalline orientations in YBCO films. The average values calculated at a full width at half maximum (FWHM) of the peaks from phi-scans (φ) and omega (ω) scans for one meter-long YBCO tape were 7.49° and 4.71°, respectively. The critical current (I_c) was over 200 A/cm-width at 77 K and under self-field for meter-long YBCO tape. The critical transition temperature of the YBCO tape was typically as 90.1 K with 0.5 K transition widths.     

Effects on Jc of pinning center morphology for multiple-in-line-damage in coated conductor and bulk,melt-textured HTS

The properties of discontinuous aligned pinning centers (PCs) created by high-energy heavy-ions are compared for bulk melt-textured and coated conductor HTS. Properties of PCs, which increase J_c (pinning potential and entanglement), and negative properties which decrease J_c (e.g., decreased T_c and percolation paths) are evaluated. Mechanisms are proposed to explain the very large increases in J_c resulting from multiple-in-line-damage (MILD) compared to continuous columnar pinning centers (CCPC). In particular, a mechanism which results in fluxoid entanglement, even for parallel (unsplayed) PCs, is discussed. The same mechanism is found to also account for restoration of much of the pinning potential expected to be lost due to the gaps in MILD PCs. It also accounts for the fact that at high fluence, J_c increases as fluence is increased, instead of decreasing as expected. The very low self-field in coated conductor permits separation of the negative and positive effects of PCs. It is found that parameters developed to quantify the negative effects in bulk melt-textured YBCO, by 63 GeV U²³⁸ ions, successfully describe damage to 2.1 μm thick coated conductor by 1 GeV Ru⁴⁴ ions. Coated conductor at 77 K and self-field is generally known to have J_c about 100 times that of melt-textured YBCO. However, at 77 K and applied field of 1 T, when both forms of HTS are processed with comparable numbers of near-optimum MILD PCs, the difference in J_c is reduced to a factor of 1.3–2. Whereas J_c for melt-textured YBCO increased sharply, by a factor of up to 16.8 for high-fluence MILD PCs, J_c in coated conductor increased by a smaller factor of 2.5–3.0. Nevertheless, 2.1 μm thick coated conductor, with near-optimum MILD PCs, exhibits J_c = 543 kA/cm² at 77 K and applied field of 1.0 T, and I_c = 114 A/cm-width of conductor. This is the highest value we find in the literature. The phenomenology developed indicates that for optimum MILD PCs in coated conductor, J_c ～ 700 ± 70 kA/cm² should be achievable at 77 K, 1.0 T.     

YBCO films buffered by pyrochlore Nd2Mo2O7 layer on YSZ substrates by chemical solution deposition

A chemical solution deposition process was used to grow epitaxial Nd₂Mo₂O₇ (NMO) buffer layers on YSZ substrates to produce YBa₂Cu₃O_7?δ (YBCO) coated conductors. The NMO precursor solution prepared using metal acetylacetonates was spin-coated onto single crystal YSZ substrate of 10 mm × 10 mm in size at 3000 rpm for 30 s and heat-treated at 1000 °C for 2 h in Ar after calcinated at 550 °C for 1 h. The YBCO film was deposited by TFA-MOD route on top of the NMO/YSZ architecture. The phase purity and the crystalline orientation of NMO and YBCO films were evaluated by X-ray diffraction (XRD). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to observe their microstructure and their surface roughness. The critical current density (J_c) of YBCO film on NMO/YSZ is 1.8 MA/cm² at 77 K in self-field, which indicates that the Nd₂Mo₂O₇ is a potential buffer for YBCO coated conductor.     

Improved critical current density in Zn doped YBCO single crystals

Magnetic measurements were made using pure YBCO and Zn doped YBa₂(Cu_1?xZn_x)₃O_7?σ. Single crystals with Zn concentration of 0.5%, 1.5%, 3.0% and 4.3%. The magnetic hysteresis loops for these samples were measured in the temperature range 0.1 ? T/T_c ? 0.96 under magnetic fields of 5 T using SQUID. It was found that the critical current density J_c increased for low Zn content samples up to 3% Zn concentration compared to pure YBCO sample and decreased for the higher Zn content samples. These values varied consistently when compared at magnetic fields of 1 T and 3 T. Moreover Zn doped samples showed significant values of J_c in the temperature range of 0.7–0.9T_c, close to critical temperature compared to pure YBCO sample. The irreversibility field H_irr was also enhanced in this temperature range showing consistent decrease with increase of Zn concentration. The peak field H_p above H_c1 and irreversibility field H_irr, both show power law dependence of the form H = m₁(1 ? T/T_c)^m2 in the temperature range of 0.75–0.96T_c. The values of parameter m₂ increased from 1.44 to 1.95 for the samples up to 3% Zn content and decreased to 1.37 for higher Zn contents. The ratio H_irr/H_p was found to be 3–4 for the lower Zn content samples and was 7–8 for the sample with high Zn content indicating more disorder for higher Zn content samples. The region between peak field H_p and irreversibility field H_irr was broadened with the increase of Zn concentration. The strong effect of Zn substitution in modifying behavior of these samples even at elevated temperatures is possibly due to the changes in the anisotropy of our samples with the increase of Zn concentration and also due to the locally induced changes in magnetic moments by Zn substitution.     

Pinning potential in thick PrBa2Cu3Ox/YBa2Cu3O7−δ quasi-multilayers

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa₂Cu₃O_x (PrBCO) nano-dots and layers of YBa₂Cu₃O_7?δ (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 nm, and total thickness between 1.13 and 5.31 μm. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J_c at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J_c only in high fields. We have also investigated the frequency dependence of J_c from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nano-dots, both isotropic and c-axis correlated.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

Epitaxial growth of Ce2Y2O7 buffer layers for YBa2Cu3O7−δ coated conductors using reel-to-reel DC reactive sputtering

Biaxially textured Ce₂Y₂O₇ (CYO) films were deposited on Ni–5at.%W (Ni–5W) tapes by a DC reactive sputtering technique in a reel-to-reel system. Subsequent YBa₂Cu₃O_7?δ (YBCO) films were prepared using pulsed laser deposition leading to a simplified coated conductor architecture of YBCO/CYO/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CYO buffer layer with a texture spread down to 2.2° and 4.7° for the out-of-plane and in-plane alignment, respectively. Microstructural investigations showed a dense, smooth and crack-free surface morphology for CYO film up to a thickness of 350 nm, implying an effective suppression of cracks due to the incorporation of Y in CeO₂. The superconducting transition temperature T_c of about 90 K with a narrow transition of 0.8 K and the inductively measured critical current density J_c of about 0.7 MA/cm² indicate the potential of the single CYO buffer layer.     

Influence of Zr and Ce doping on electromagnetic properties of (Gd,Y)–Ba–Cu–O superconducting tapes fabricated by metal organic chemical vapor deposition

(Gd,Y)Ba₂Cu₃O_x tapes have been fabricated by metal organic chemical vapor deposition (MOCVD) with Zr-doping levels of 0–15 mol.% and Ce doping levels of 0–10 mol.% in 0.4 μm thick films. The critical current density (J_c) of Zr-doped samples at 77 K, 1 T applied in the orientation of H 6 c is found to increase with Zr content and shows a maximum at 7.5% Zr doping. The 7.5% Zr-doped sample exhibits a critical current density (J_c) of 0.95 MA/cm² at H 6 c which is more than 70% higher than the J_c of the undoped sample. The peak in J_c at H 6 c is 83% of that at H 6 a–b in the 7.5% Zr-doped sample which is more than twice as that in the undoped sample. Superconducting transition temperature (T_c) values as high as about 89 K have been achieved in samples even with 15% Zr and 10% Ce. Ce-doped samples with and without Ba compensation are found to exhibit substantially different J_c values as well as angular dependence characteristics.     

Modified Lanthanum Zirconium Oxide buffer layers for low-cost,high performance YBCO coated conductors

The pyrochlore Lanthanum Zirconium Oxide, La₂Zr₂O₇ (LZO), has been developed as a potential replacement barrier layer in the standard RABiTS three-layer architecture of physical vapor deposited CeO₂ cap/YSZ barrier/Y₂O₃ seed on Ni–5%W metal tape. The main focus of this research is to ascertain whether: (i) we can further improve the barrier properties of LZO; (ii) we can modify the LZO cation ratio and still achieve a high level of performance; and (iii) it is possible to reduce the number of buffer layers. We report a systematic investigation of the LZO film growth with varying compositions of La:Zr ratio in the La₂O₃–ZrO₂ system. Using a metal–organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of La_xZr_1?xO_y (x = 0.2–0.6) on standard Y₂O₃ buffered Ni–5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial LZO phase with only (0 0 1) texture can be achieved in a broad compositional range of x = 0.2–0.6 in La_xZr_1?xO_y. Both CeO₂ cap layers and MOD–YBCO films were grown epitaxially on these modified LZO barriers. High critical currents per unit width, I_c of 274–292 A/cm at 77 K and self-field were achieved for MOD–YBCO films grown on La_xZr_1?xO_y (x = 0.4–0.6) films. These results indicate that LZO films can be grown with a broad compositional range and still support high performance YBCO coated conductors. In addition, epitaxial MOD La_xZr_1?xO_y (x = 0.25) films were grown directly on biaxially textured Ni–3W substrates. About 3 μm thick YBCO films grown on a single MOD–LZO buffered Ni–3W substrates using pulsed laser deposition show a critical current density, J_c, of 0.55 MA/cm² (I_c of 169 A/cm) at 77 K and 0.01 T. This work holds promise for a route for producing simplified buffer architecture for RABiTS based YBCO coated conductors.