Pair and Quasiparticle States of YBa2Cu3O7?x Films Deduced from the Surface Impedance and a Comparison with Nb3Sn

Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb₃Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance R _s and penetration depth of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance R _res(19 GHz)90 constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting R _res from R _s (T) revealed activated behavior with a reduced energy gap ₀/k _B T _c0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T0.5 ·T _c, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of (T) at T0.5 · T _c was related to qp scattering, which also caused an extremal conductivity ₁(T). A shoulder in (T) at T=(0.6–0.7) · T _c confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of (B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.     

Microwave Surface Impedance of Nd-rich Nd 1+x Ba 2−x Cu 3 O 7−δ Thin Films

Surface impedance measurements on highly c-axis epitaxial Nd _1+x Ba _2–x Cu ₃ O ₇ (x=0, 0.09 and 0.12) films grown by d.c. magnetron sputtering on LaAlO ₃ substrates are presented. It is found that the zero temperature London penetration depth correlates well with the critical temperature of the films and with the corresponding number of carriers. The low temperature penetration depth follows a linear T law for optimally doped Nd123 sample and a T ² law in Nd-rich samples. In the case of the heavily underdoped samples (T _c < 60K) the T ² law extends to temperatures higher than T _c/2. The possible role of the Nd/Ba ions substitution on the penetration depth and surface resistance is discussed.     

Surface Impedance of YBa2Cu3O7−δ Films Grown on MgO Substrate as a Function of Film Thickness

The surface impedance characteristics of epitaxial YBa₂Cu₃O_7?δ films of thickness d _f = 75, 150, 300, 600 nm, produced by magnetron thermal co-evaporation onto single crystal MgO substrates was studied using measurement technique based on Ka-band whispering gallery mode (WGM) dielectric resonator (DR) fabricated from single crystal sapphire. Characterization of the unpatterned films was carried out in temperature interval from 20 K to 90 K. It was shown that the effective surface resistance approaches the minimum value for d _f >300 nm. At the same time, intrinsic impedance properties are practically independent on d _f in the studied interval of d _f values. The temperature dependence of London penetration depth was estimated experimentally and approximated with the model expressions. Effect of reducing the surface resistance of approximately two times at low temperatures one year later after their manufacture was registered for all films(except the film of 75 nm thickness). The effect may be explained by changes of the film parameters in time after the film light overdoping.     

Nonlinear Power Handling of YBa2Cu3O7−x Films and Microwave Devices

Physical mechanisms which limit the power handling of YBa₂Cu₃O_7?x films and devices are discussed in terms of a quantitative classification scheme. The possible limitations are devided into magnetic or thermal, and global or local in nature. Analytical estimations are compared with measurements of YBa₂Cu₃O_7?x films (Ø = 1″–2″) using a niobium-shielded sapphire resonator at 19 GHz, and disk resonators at 2 GHz. Magnetic effects are found to play an essential role in nonoptimized films in terms of weak-links, and in high-quality films if the lower critical field Bcl is reached. The majority of films and disk resonators appear to suffer from microwave heating. Global heating appears predominantly at CW operation. Local heating results mainly from defects in films of medium quality. Defect-induced quenches are observed at moderate field levels, sometimes resulting in an irreversible degradation of the power handling.     

Fabrication and characterization of YBa2Cu3O7−x /Sn composite superconductors

YBa₂Cu₃O_7–x /Sn (YBCO/Sn) composites sintered at 230 °C exhibited a percolation threshold for electrical conductivity at 20 vol% Sn, with semiconducting and metallic behaviour below and above it, respectively. The simultaneous decrease in =d/dT and ₀ with increase in Sn content was related to formation of defect-free interfaces. The diamagnetic shielding property of the composites weakened with Sn content, as deduced from magnetic levitation experiments.     

Sintering of YBa2Cu3O7−x compacts

Heating of YBa₂Cu₃O_7−x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm²) have been obtained in these solid-state sintered samples.     

YBa2Cu3O7−x Thin Films by Citrate-Based Non-fluorine Precursor

A new fluorine-free chemical solution deposition route has been developed for YBa₂Cu₃O_7−x (YBCO) thin films. The precursor was formed by dissolving metal nitrates and citric acid in ethylene glycol. Polyethyleneimine was used to modify the solution. The preparation of YBCO thin films consisted of repeated dip coating, drying, heat-treating in air and annealing in flowing argon with a small addition of oxygen and with final treatment in pure oxygen. The films were characterized by XRD and SEM and show dominating and well-textured YBCO phase. The AC susceptibility and magnetization measurements reveal a critical temperature about 90 K and a high critical current density, comparable with that of pulse laser deposited films. The simplicity and high performance of the method may be attractive for large-scale superconducting applications.     

Shadowed Off-Axis Pulsed Laser Deposition of YBa2Cu3O7−x Thin Films

Using a shadowed pulsed laser deposition allows us to reduce the number of droplets on the YBa₂Cu₃O_7?x film surface obtained by pulsed laser deposition method. In this study, we present a new modification of pulsed laser deposition method: a shadowed off-axis pulsed laser deposition. The substrate surface lies in the plain defined by the laser beam. A rectangular shadow mask located between target and substrate reduce the quantity of large particles deposited from the diffusion flow. Such geometry allows us to carry out the laser plume scanning on the target surface for simultaneous one-step double-sided deposition films with a droplet-free surface. Both films had good superconducting properties.     

High-temperature deformation behaviour of YBa2Cu3O7−x

High-temperature compression tests were performed in air for YBa₂Cu₃O_7–x polycrystals with grain sizes of 3 and 7 m at various strain rates between 1.3×10^–5 and 4×10^–4s^–1 and at temperatures between 1136 and 1253 K. Steady state deformation appeared above 1203 K for both samples. A stress exponent of 1.3 and an activation energy of 150 kJ mol^–1 were evaluated. The compression tests and microstructural observations revealed that there was a difference in deformation mechanism above and below 1203 K. The dominant mechanism was diffusional creep associated with grain-boundary sliding above 1203 K, and dislocation glide accompanied with grain-boundary sliding below 1203 K. The growth of anisotropic grains and their preferred arrangement were enhanced by deformation.     

Microstructure and properties of K2O doped superconducting YBa2Cu3O7−x

K doped YBa₂Cu₂O _x powders have been obtained by incorporating K₂CO₃ to previously coprecipitated amorphous hydroxide mixtures. Synthesis was performed at 900C and an orthorhombic 1 2 3 phase was the final product. Sintering of isopressed bars was carried out from 920 to 940C. K doping up to 5 at % led to the development of dense bodies with good superconducting behaviour. Oxygen pick-up during the cooling step following sintering was well accomplished despite the high density of the samples. Transition temperatures 90 K were measured. Doping above that percentage led to samples with lower density values. The presence of non-superconducting phases, such as CuO and a green phase, was very significant, and suppressed the superconducting transition.     

Effect of Firing Temperature on the Structure and Properties of YBa2Cu3O7−x Films by TFA-MOD Method

YBa₂Cu₃O_7−x (YBCO) films were fabricated on LaAlO₃ (LAO) substrate under various firing temperatures (760–870 °C) in the crystallization process by metalorganic deposition (MOD) method using trifluoroacetates. The effect of firing temperature on the structure and properties of YBCO films was systematically investigated. According to the XRD and SEM images, the films fired at low temperature (760–800 °C) showed poor electrical performance due to rough surfaces and impurity phases. However, the films fired at 850 °C showed the highest critical temperature of 90 K and the highest J _c of 3.1 MA/cm² which attribute to the formation of a purer YBCO phase, fewer pores, and stronger biaxial texture.     

Significant Change in Mechanical Properties of YBa2Cu3O7−x Bulk Superconductors Diffused with Sn Nanoparticles

Mechanical behavior of YBa₂Cu₃O_7?x (Y123) superconductors exposed to Sn nanoparticles diffusion is determined by the way of Vickers microhardness (H _v ) conducted at different applied loads (0.245N≤F≤2.940 N). Load dependent microhardness, load independent microhardness, elastic modulus, and yield strength values are estimated from the microhardness curves. Unpredictably, the findings of the H _v values reveal that the undiffused sample and Sn diffused sample prepared at 500 ^°C exhibit reverse indentation size effect (RISE) behavior while the other samples obey indentation size effect (ISE) nature. Further, we extract the load independent (true) microhardness using the Meyer’s law, proportional specimen resistance (PSR), elastic/plastic deformation (EPD), Hays–Kendall (HK) approach and indentation-induced cracking (IIC) model, and compare the true hardness with the apparent hardness.     

Preparation of superconducting YBa2Cu3O7−x powders by a solution technique

Bulk superconducting YBa₂Cu₃O_7–x powder has been synthesized by a solution technique using a mixture of Ba-ethylenediaminetetra-acetic acid (EDTA) and [Y, Cu]-citric acid complexes. A light-blue, molecular-level, homogeneously mixed precursor was prepared, and transferred to powder form through vacuum drying. The vacuum-dried powder was decomposed at 800 °C for 4 h under flowing oxygen, then heat treated at high temperature from 850 to 950 °C for 6–12 h. The results ofT _c measurements and X-ray analysis show that the orthorhombic, superconducting phase can be formed at temperatures above 850 °C following low-temperature annealing. A sharp transition (T2 K) and high density can be achieved after 930 °C heat treatment. The 930 °C heat treated sample shows aJ _c value of 510 A cm^–2. It is concluded that this solution technique provides better stoichiometric control and lower reaction temperature than the conventional solid-state sintering process.     

A study on the stability of superconducting YBa2Cu3O7−x phase

The thermal stability of the superconducting phase of nominal composition YBa₂Cu₃O_7–x -sintered pellets has been studied with respect to different temperatures (ranging from 300 to 950° C), time (ranging from 1 to 72 h), oxygen partial pressure (from 4 Pa to 1 atm) and carbon dioxide partial pressure (from 10^–4 Pa to 1 atm). Annealed samples were characterized by X-ray diffraction analysis, optical microscopy, and resistive measurements of the superconductive transition temperature. A stability field of the orthorhombic and tetragonal phases was obtained, showing a region of coexistance. The decomposition of the 1 2 3 phase is found to be strongly influenced by the presence of a small amount of CO₂ (1 p.p.m.) in the sintering atmosphere. A sintering process is proposed to avoid the formation of by-products.     

Preparation and characterization of high-T c superconducting YBa2 Cu3 O7−x filaments

High-T _c superconducting filaments of the Y-Ba-Cu-O system were prepared using a low-cost suspension spinning method, where the solvent was removed by a phase inversion technique. The Y-Ba-Cu oxide precursor, containing polysulphone (PSF), was spun as a filament into a precipitating medium, removing the solvent by phase inversion and using water as a non-solvent. The green product filament was washed, dried and subjected to a heat treatment to remove the binding material and generate the oxide in, the appropriate superconducting phase. Stoichiometry, porosity and grain size were investigated by scanning electron microscopy, energy-dispersive X-ray analysis and electron probe micro analysis, while crystal structure was checked by X-ray diffraction analysis. Resistivity measurements as a function of temperature were performed by the four-point method and typicalT _c values of 88 K were observed, whileJ _c in the range of 125 A cm^–2 at 77 K and zero field.     

Normal state properties of lithiated YBa2Cu3O7−x samples

Measurements of the resistivity in the normal state of lithiated YBa₂Cu₃O_7−x are reported. At low x_Li-values (<0.06), lithium substitutes copper of the CuO₂ planes. The samples are metallic; a progressive localization of the charge carriers is observed. In the intermediate concentration range, lithium occupies both copper sites planes and chains which leads to a two-phased system; the slope dϱ/dT is nearly independent of temperature. In the concentrated range, lithium is in the chain site. The resistivity behaviour is well described by the simple relation with ρ₀T₀=10Ω.cm.K, a result which is consistent with a single YBCO₆-type phase.     

Directional Vortex Pinning at Microwave Frequency in YBa2Cu3O7−x Thin Films with BaZrO3 Nanorods

We investigate the effect of the anisotropy and of the directional pinning in YBa₂Cu₃O_7?x films grown by pulsed laser ablation from targets containing BaZrO₃ at 5 mol%. BaZrO₃ inclusions self-assemble to give nanorods oriented along the c-axis, thus giving a preferential direction for vortex pinning. The directionality of vortex response is studied at high ac frequency with the complex microwave response at 48 GHz, as a function of the applied field and of the angle θ between the field and the c-axis. The complex microwave response does not exhibit any angular scaling, suggesting that the structural anisotropy of YBa₂Cu₃O_7?x is supplemented by at least another preferred orientation. The pinning parameter r shows evidence of directional pinning, effective in a wide range of angles around the c-axis (thus ascribed to BZO nanocolumns).     

Synthesis of bulk and film YBa2Cu3O7−x high-temperature superconductor by the sol-gel method

Bulk YBa₂Cu₃O_7–x has been prepared by the sol-gel method using metal alkoxide and metal acetate as precursors and ammonia or ethylenediamine as complexing agent. The temperature of the transformation of the gel precursor to YBa₂Cu₃O_7–x depended on the type of precursor and the agent used for adjusting the pH. The gel synthesized from precursor alkoxide gives YBa₂Cu₃O_7–x at lower temperature (750C for 6 h) than with acetate. The gel synthesized by different metal acetates and ammonia produces YBa₂Cu₃O_7–x at a lower temperature than the sample prepared from acetate and ethylenediamine. From X-ray diffraction studies it appears that a relationship exists between the reactivity of the gel and the orientation of the corresponding pellet. The preliminary results from a film prepared by painting the gel on alumina and yttria-stabilized zirconia are presented. A high sintering temperature (940C for 1 h) was found to be necessary for obtaining superconductor film. The reactions between YBa₂Cu₃O_7–x and supports were reduced by using silver as buffer. The T _on of the film prepared on the modified alumina support was 91 K and T _off was 78 K.