Dependence of the Structural, Electrical and Magnetic Properties of YBa2Cu3O7?δ Bulk Superconductor on the Ag Doping

The effects of Ag dopants on the superconducting properties of YBCO bulk samples, prepared under solid-state reaction method, have been studied by resistivity?Ctemperature (???CT), X-ray diffraction (XRD), scanning electron microscope (SEM), bulk density, AC susceptibility, DC magnetization measurements, and theoretical analysis. Small Ag substitutions (x??0.10) do not effect T _c of pure YBa₂Cu_3?x Ag _x O_7??? , while we observed small decreases of 1.5?K for larger doping levels (x??0.15). AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of DC bias field. The critical current densities (J _c ) have been estimated as a function of magnetic field from the magnetization data employing Bean??s critical state model. The increase in Ag amount (x) in YBa₂Cu_3?x Ag _x O_7??? (x??0.10) system effectively decreases the intragrain critical current density, which is attributed to the reduced connectivity between the grain boundaries. In the case of small additions (x=0.05), Ag atoms fill partly the pores, leading better crystallization of the grains.     

Magnetic Field Effects on Surface Morphology and Magnetic Properties of Co–Ni–N Thin Films Prepared by Electrodeposition

The surface morphology and magnetic properties of Co?CNi?CN thin films electrodeposited under an external magnetic field were investigated. The films were electroplated on Al substrates using the same electrodeposition parameters (temperature and pH) for all experiments, with an external magnetic field of 107?Oe applied to the cathode surface. The films were compared with similar samples obtained in the absences of magnetic field. The magneto-induced modifications in the Co?CNi?CN morphology can be explained by the specific local convection of ions at the interface cathode-electrolyte, which promotes changes both in the electrical charge of the double layer and in the thickness of the diffusion layer. From the magnetic measurements, we found that the coercivity varied between H _c =(14÷27)?kA/m depending on the direction of the applied magnetic field and on the sodium nitrate content in the plating bath. It was observed that an induced anisotropy appeared in the Co?CNi?CN films due to the preferential orientation of the easy axis of magnetization in the magnetic field direction. In addition, the Co?CNi?CN alloy films showed good magnetic property, which is considered that not only the smaller grain size of the films, but also more uniform surface of the films than that deposited in absence of applied magnetic field.     

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.     

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.     

The Influence of Magnetron Sputtering Conditions on the Structure of Zr–Pd Coatings

Technical Physics Letters - Using the method of DC magnetron sputtering, nonstructured amorphous metal coatings of Zr75Pd25 composition were obtained with an average deposition rate of 1.3 nm/s at...     

Effect of Growth-Induced Linear Defects on High Frequency Properties of Pulse-Laser Deposited YBa2Cu3O7?δ Films

Growth-induced linear defects are shown to strongly affect the microwave surface resistance, R_s, of highly biaxially oriented high temperature superconductor (HTS) YBa₂Cu₃O_7– (YBCO) films. Measured R_s(77 K) turned out to be 4–5 times higher than in single crystals. The films were deposited by modified pulse-laser technique, J_c(77 K) = (3-6) × 10⁶ A/cm², onto LaAlO₃ substrates. R_s(T) was measured at 134 GHz and 20–100 K. TEM/HREM study of YBCO films deposited at T_s = 750°C–780°C revealed a reduction of edge dislocation density with T_s increase (from 2 · 10¹¹ to 10¹⁰ lines/cm²). YBCO films deposited at T_s = 780°C exhibited the smallest R_s(77 K, 134 GHz) < 120 m and the lowest density of dislocations detected by HREM and X-ray analysis. A nature of the dislocation effect is discussed within a model of local anisotropic elastic deformation in a vicinity of dislocation cores, where T_c variation and an enhancement of normal quasiparticle density are significant.     

rf Field Dependence of Surface Resistance for a–b Plane-Textured YBa2Cu3O7–δ Films Deposited on Copper Substrate

Low rf loss at high rf field levels should be realized in order to apply high-T_c films to accelerator cavities. It is well known that c-axis perpendicular to the surface is essential to reduce R_s and its field dependence However, the effect of a–b plane texturing on R_s is not so clear because lack of experiments, especially for films deposited on metallic substrates. We developed a deposition technique that enables a–b plane texturing as well as c-axis orientation of YBCO films on a copper substrate. We prepared four samples with c-axis normal to the surface: two of them were a–b plane well textured films and the other two were a–b plane weakly textured films. The a–b plane well-textured films in low rf field exhibited a R_s higher than that of weakly textured films below 80 K. The rf field dependence measurements showed that the increase of R_s with increasing rf field for the a–b plane well-textured films was slower as compared with that of the weakly textured films.     

The Influence of the Interface on the Magnetic State in Two-Layer Films of the Fe–Bi System

Technical Physics Letters - Results of the experimental investigation of two-layer films in the Fe–Bi system are presented. It is found that the order of sequential deposition of the magnetic...     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

Pore Structure and Transport Properties in Bulk YBa2Cu3O7?δ Doped with Sb2O3

In this work we have studied the pore structure and electrical transport properties of superconducting YBa₂Cu₃O_7?y polycrystalline samples doped by the addition of different Sb₂O₃ concentrations, i.e. resulting in (YBa₂Cu₃O_7?y )_1?x (Sb₂O₃) _x . The samples were prepared through the solid-state reaction method. Rietveld analyses of X-ray diffraction data were used to investigate how the lattice parameters are modified by doping. Specific superficial area measurements identified the principal characteristics of the pore structure of the samples and how these properties change with doping. The superconducting properties were studied by using zero field cooling magnetization and transport critical current measurements. The critical temperature of the samples does not depend on the doping level, but their transport critical current density strongly decreases as the Sb₂O₃ concentration is increased. Our experimental results suggest that for the samples studied here there is not a direct correlation between the modification by doping of both, the pore structure and the transport critical current density.     

Effect of Micro-Phase Separation on Magnetic Response in Pr-Doping YBa2Cu3O7−δ Single Crystals

Pr-doping Y_0.928Pr_0.072Ba₂Cu₃O_7−δ superconducting single crystal was studied systemically by the magnetic response measurement below T _c . DC magnetization shows a possible existence of micro-phase separation caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The magnetic field dependence of critical current density J _c and pinning force density F _p had been obtained from the magnetization hysteresis loop. Flux pinning mechanisms in different magnetic fields had been discussed using the scaling function as well as the ratio of the pinning force field at H _on and H _peak. We find that the peak effect is influenced by the micro-phase separation which is caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The interface between the different T _c superconductivity areas can be the effective pinning center which is useful to the appearance of peak effect. The value of the ratio of the maximum pinning force field to the irreversibility field is discussed. The conclusion is consistent with the model constructed by Wen et al.; we finally draw out the thermomagnetic phase diagram for our samples.      

An Approach of the Sintering YBa2Cu3O7−δ System

The abnormal glow discharge applied to the sintering process is a recent technique used for processing both metallic and ceramic materials. In this paper, we use the abnormal glow discharge as an alternative method for the sintering step of the YBa₂Cu₃O_7?δ superconductor material. The physical properties of the sintered samples by glow discharge were compared with the properties of sintered samples in resistive furnace, which is commonly used for production of high temperature superconductor ceramics. The structural analysis of the YBa₂Cu₃O_7?δ samples was carried out by the X-ray diffraction technique, microstructural analysis by Scanning Electron Microscopy and electrical resistivity by the four probes method (resistance measurements as a function of temperature). The experimental results permitted to establish the similar structure and morphology for all samples: produced by plasma and resistive furnace sintering. The superconducting behavior was corroborated for both sintering processes.     

Effect of Oxygen Partial Pressure on Phase Formation and Stability of LuBa2Cu3O7?x Compounds

Among the several R-123-type (R = rare-earth element) superconductors, it has been reported that LuBa₂Cu₃O_7?x (Lu-123) could not be prepared as single-phase superconducting ceramics at temperatures between 900 and 1000?°C both in air and in pure O₂. We present here the results of our investigations of the synthesis and structural stability of nearly phase-pure ceramic Lu-123 compounds prepared by a solid-state reaction route. It is found that the phase-pure Lu-123 compound can be formed within a narrow range of oxygen partial pressure in the temperature range of 880?C900?°C. It is also found that the Lu-123 phase becomes unstable due to the structural decomposition at temperature above 800?°C in N₂ and in pure O₂ of 1 atmosphere, and above 830?°C in air.     

Correlation Between the Morphology of Ag and the Contact Resistivity of the Ag/YBa2Cu3O7?δ Thin Film Contact

The morphology of the silver films deposited and annealed on laser ablated YBa₂Cu₃O_7– thin films and the corresponding contact resistivity have been systematically investigated. A minimum contact resistivity of 6 × 10^–8 cm² was reached at 77 K by annealing Ag/YBa₂Cu₃O_7– contact at the optimum temperature. The effect of the annealing temperature on the contact resistivity was explained by considering the morphology of the silver films and the diffusion of silver into YBa₂Cu₃O_7– film, etc. The difference of the contact resistivity for Ag contact to polycrystalline, single crystal and thin film of YBa₂Cu₃O_7– were also explained.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

Crystallization Characteristics of YBa2Cu3O7?δ Film during TFA-MOD

The phase formation and film shrinkage during the crystallization process of YBa₂Cu₃O_7−δ (YBCO) film prepared by the trifluoroacetate organic deposition (TFA-MOD) were investigated through a series of quenching experiments. X-ray diffraction patterns show that the YBCO grains initially form following the formation of the Ba-O-F phase at the heating ramp between 720 °C and 770 °C in the case of a high heating rate of 20 °C/min. It is identified that the crystallization process is involved three stages, i.e., the first one characterized by the formation of the Ba-O-F phase, the second one by the YBCO grain nucleation together with the significant shrinkage of films, and the last one by continuous YBCO grain growth. Moreover, a heterogeneous Cl element is believed to arise from the flowing water vapor, and it may move into the intermediate pyrolyzed film. It is observed that the densification process existing in the secondary stage can be destroyed by the exterior Cl element, leading to a more porous structure in contrast to their surrounding.     

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.     

Influence of Boron Doping on Transport Properties of?YBa2Cu3O7?y HTS

The nominally pure and boron-doped YBa₂Cu₃B _x O_7?y samples with B-doping level x varying between 0 and 0.15 were prepared by the solid state reaction. X-ray diffraction analysis shows that all the obtained compounds are single YBa₂Cu₃O_7?y phase. The small additives of boron in YBa₂Cu₃B _x O_7?y (x=0.025 and 0.05) do not essentially affect the critical temperature T _c and it remains near 90 K. The higher-level boron doping causes degradation of T _c and tail remains on the ??(T) curve for x=0.15 at 65 K. Lowest-level boron doping applied leads to a significant improvement in J _c compared to the undoped sample (from 100 A?cm^?2 for a control sample to 147 A?cm^?2 for a B-doped sample with x=0.025). Grain boundary critical current density and superconducting volume fraction decrease with increasing amount of added B₂O₃. The obtained results indicate possibility of boron dopant being entered into the lattice-sites.