Texture analysis of RF-sputtered CeO2 buffer layers and superconducting MOD-YBCO films

CeO₂ buffer layers were deposited on YSZ single-crystal substrates using an RF-sputtering method. The development of crystalline textures of sputtered CeO₂ films at different sputtering pressure and their effects on YBCO films, deposited by Metal Organic Deposition (MOD), were investigated. Both CeO₂ and subsequent YBCO films grew well epitaxially. The relative XRD peak intensities of CeO₂ (2 0 0) to substrate YSZ (2 0 0) increased with deposition pressure in the range of 3–5 mTorr and were inversely proportional to the θ–2θ scan FWHM values of CeO₂ (2 0 0). Also, the reaction layers of BaCeO₃ were thicker in the samples with lower CeO₂ (2 0 0) intensities and poor out-of-plane alignment when CeO₂ were deposited at the lower pressure of 3.3 mTorr. It is noted, however, that the superconducting layer grew well epitaxially on these BaCeO₃ layers, possibly due to the epitaxial relation between CeO₂ and YBCO. The superconducting critical currents of MOD-YBCO films showed an increasing tendency as both the Δ2θ (CeO₂) and BaCeO₃ peak intensities decreased.     

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.     

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.     

Effect of the composition on the superconducting properties of IBAD-MgO SmBCO coated conductors with superconducting film directly deposited on epi-MgO layer

Sm_1+xBa_2?xCu_3+yO_7?δ (SmBCO) films were directly deposited on the epi-MgO/IBAD-MgO/Y₂O₃/Al₂O₃/Hastelloy template by co-evaporation using the evaporation using drum in dual chambers (EDDC) system without the buffer layer in order to investigate the effect of the composition ratios on superconducting property, microstructure and texture of SmBCO film. The films with gradient composition ratios of Sm:Ba:Cu were deposited using a shield with an opening which was placed between the substrate and the boats. The highest I_c of 52 A (corresponding to J_c = 1.6 MA/cm² and a thickness of 800 nm) was observed at 77 K in self field at a composition x = 0.01–0.05 and y = ?0.23 to ?0.46. When the composition ratio is outside this range, the I_c value rapidly decreased. The superconducting critical current was highly dependent on the composition ratio. As the composition ratio is farther away from that of the highest I_c, the SmBCO (1 0 3) peak intensity increased and the amount of a-axis oriented parts increased. A dense microstructure with round-shape grains was observed in the region showing the highest I_c. The optimum composition ratio can be found by analyzing films deposited with variable deposition rates of each depositing element.     

Thermal diffusivity measurement of YBa2Cu3O7−x thin film with a picosecond thermoreflectance technique

The thermal diffusivity of YBa₂Cu₃O_7?x (YBCO) film was measured using the optical pump–probe method. A theoretical finite-difference model was employed to calculate the diffusivity value, and the best fit for the c-axis oriented YBCO film showed an average thermal diffusivity of 0.25 ± 0.05 mm² s^?1. The obtained result is compared to previous reports measured using various methods.     

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.     

Preparation of solution-based YBCO films with BaSnO3 particles

The YBCO films with BaSnO₃ (BSO) particles were prepared on LAO (0 0 1) substrates by metal organic deposition using trifluoroacetates (TFA-MOD) via introducing SnCl₄ powders into the YBCO precursor solution. It was found that with the increase of the SnCl₄ contents, the slower decomposition and higher temperature for nucleation during the reaction were requested compared to that of pure YBCO film. The YBCO films with different contents of Sn with dense surface and well c-alignment were obtained under optimized heat treatment, and the BaSnO₃ phases were detected by XRD analysis. Litter effect of BSO particles on the T_c and J_c values of YBCO films was found. All YBCO films with BSO particles had T_c values over 90 K and J_c values over 1 MA/cm². A significant enhancement of J_c was observed for YBCO films with BSO particles compared to that of pure YBCO film by the field dependence of J_c values. The best property was obtained for YBCO film with 6 mol.% Sn at 77 K under magnetic field. The results showed that the J_c value of YBCO film with 6 mol.% Sn was enhanced by a factor of 2 in 2 T, and over a factor of 10 beyond 4 T compared to that of pure YBCO film.     

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.     

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.     

Epitaxial growth of La2Zr2O7 buffer layers for YBa2Cu3O7−δ coated conductors on metallic substrates using pulsed laser deposition

In the present work, La₂Zr₂O₇ (LZO) buffer layers were deposited using pulsed laser deposition (PLD) on various metallic substrates including epitaxial pure Ni on a LaAlO₃ (LAO) substrate as well as highly textured Ni–5 at.%W tapes. It is shown that the LZO deposited on pure Ni-buffered LAO exhibits a mixed orientation while LZO on Ni–5 at.%W grows epitaxially. This difference may be explained by the existence of a sulphur superstructure on the surface of Ni–5 at.%W tapes, promoting the epitaxial (0 0 l) nucleation of seed layers. Highly textured YBa₂Cu₃O_7?δ layers were prepared either by using a single buffer layer of LZO or bilayer buffers of CeO₂/LZO on Ni–5 at.%W. The superconducting transition temperature (T_c) increases with the LZO thickness, reaching a value of 90 K with a very narrow transition width (1.5 K) for 240 nm thick LZO layers. Inductive J_c measurements at 77 K in self-field show a value of about 0.96 MA/cm² for the thickest LZO layers, which is comparable to the value observed on standard buffer architectures such as CeO₂/YSZ/Y₂O₃.     

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.     

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.     

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.     

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.     

Growth evolution of Bi2Sr2CaCu2O8+δ thin films deposited by infrared (1064 nm) pulsed laser deposition

Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) thin films were grown on MgO (1 0 0) using 1064 nm infrared pulsed laser deposition with post heat treatment. The material arrives at the substrate surface with characteristic spheroidal morphology. Smooth, homogeneous and highly c-axis oriented films were obtained after heat treatment. Using infrared laser as excitation, the stoichiometry of the target is preserved on the film. However, the films were apparently under-doped.     

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.%.     

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.     

c-Axis orientation control of YBa2Cu3O7−x films grown on inclined-substrate-deposited MgO-buffered metallic substrates

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on non-textured metal substrates with inclined-substrate-deposited (ISD) MgO as template. The biaxial texture feature of the films was examined by X-ray pole-figure analysis, φ-scan, and 2θ-scan. A tilt angle of 32° of the MgO[001] with respect to the substrate normal was observed. Epitaxial growth of YBCO films with c-axis tilt angle of 32° with respect to the substrate normal was obtained on these substrates with SrTiO₃(STO) as buffer layer. Whereas, by choosing yttria-stabilized ZrO₂ and CeO₂ instead of STO as buffer layer, a c-axis untilted YBCO film was obtained. Higher values of T_c=91 K and J_c=5.5×10⁵ A/cm² were obtained on the c-axis untilted YBCO films with 0.46 μm thickness at 77 K in zero field. Comparative studies revealed a unique role of CeO₂ in controlling the orientation of the YBCO films grown on ISD-MgO buffered metal substrates.     

Ion irradiation induced nano pattern formation on TiO2 single crystal

Nanodots have been fabricated on rutile TiO₂(1 1 0) single crystals using Ar ion beam. Ion beam sputtering creates oxygen vacancies, leading to a 45 nm thick Ti rich layer, on the surface. Post-sputtering, rutile TiO₂ also exhibits a decrease in the inter planar separation along [1 1 0] direction. Additionally, blueshift in the E_g Raman mode, representing the vibrations of oxygen atoms along c-axis, is also observed. Both these results suggest the development of a compressive stress along c-axis upon sputtering. Enhancement in intensity of A_1g Raman mode also indicates modification in TiO vibrational influence.     

Effective utilization of spray pyrolyzed CeO2 as optically passive counter electrode for enhancing optical modulation of WO3

Nanocrystalline cerium oxide (CeO₂) thin films were deposited onto the fluorine doped tin oxide coated glass substrates using methanolic solution of cerium nitrate hexahydrate precursor by a simple spray pyrolysis technique. Thermal analysis of the precursor salt showed the onset of crystallization of CeO₂ at 300 °C. Therefore, cerium dioxide thin films were prepared at different deposition temperatures from 300 to 450 °C. Films were transparent (T ~ 80%), polycrystalline with cubic fluorite crystal structure and having band gap energy (Eg) in the range of 3.04–3.6 eV. The different morphological features of the film obtained at various deposition temperatures had pronounced effect on the ion storage capacity (ISC) and electrochemical stability. The larger film thickness coupled with adequate degree of porosity of CeO₂ films prepared at 400 °C showed higher ion storage capacity of 20.6 mC cm^? 2 in 0.5 M LiClO₄ + PC electrolyte. Such films were also electrochemically more stable than the other studied samples. The Ce⁴⁺/Ce³⁺ intervalancy charge transfer mechanism during the bleaching–lithiation of CeO₂ film was directly evidenced from X-ray photoelectron spectroscopy. The optically passive behavior of the CeO₂ film (prepared at 400 °C) is affirmed by its negligible transmission modulation upon Li⁺ ion insertion/extraction, irrespective of the extent of Li⁺ ion intercalation. The coloration efficiency of spray deposited tungsten oxide (WO₃) thin film is found to enhance from 47 to 53 cm² C^? 1 when CeO₂ is coupled with WO₃ as a counter electrode in electrochromic device. Hence, CeO₂ can be a good candidate for optically passive counter electrode as an ion storage layer.