Plasma-enhanced fluorination of YBa2Cu3O7−δ ceramics

Mechanisms of interaction between the reactive species of a (CF₄+O₂) plasma and YBa₂Cu₃O_7– ceramics have been proposed through detailed angle-resolved X-ray photoelectron spectroscopic analyses. At the surface of the outer grains, the plasma treatment removes (OH)^– and (CO₃)^2– species contained in the degradation layer and gives rise to a fluoride-rich layer, whereas in the bulk of the material the occurrence of metal-fluorine bonds in the superconducting phase has been assumed. An increase of the oxidation state of copper has been also detected, confirming the oxidizing effect of the plasma treatment. A comparison with the oxidation mechanisms obtained by fluorine gas treatment is proposed.     

Processing of bulk YBa2Cu3O7−δ ceramics prior to peritectic solidification

The influence of the physical and chemical properties of precursor green bodies on the properties of fully melt-processed YBa₂Cu₃O_7– ceramic (YBCO) have been investigated. Cold isostatic pressing has been found to allow better control of the size and distribution of Y₂BaCuO₅ phase inclusions in the melt-processed ceramic than a combination of die pressing and sintering. Astudy of the loss of liquid from YBCO during partial melting has revealed that the total percentage weight loss is sensitive to both the heating rate and proportion of excess 211 phase and is maximum at a temperature corresponding to the peak of the differential thermal analysis partial melting endotherm. Densification and expansion processes in the sample, which compete during melting, have been investigated in detail. The temperature where the expansion rate is maximum has been found to coincide with the maximum rate of oxygen desorption by the sample. The expansion process terminates on completion of partial melting whereas the densification process, which is dominated by the volume proportion of liquid, continues but at a reduced rate. The use of dense green bodies and an optimum heat-treatment process has been found to be essential for the fabrication of large-grain melt-processed YBCO if a fine distribution of 211 phase inclusions and the homogeneity and shape of the sample are to be retained.     

Investigation of Lu effect on YBa2Cu3O7−δ superconducting compounds

This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa₂Lu_xCu₃O_7?δ (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J_c) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 °C), and the best ambient for the sample fabrication is determined to be 970 °C for 20 h. Zero resistivity transition temperatures (T_c), critical current densities (J_c), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 °C for 20 h are compared with each other. T_c and J_c values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T_c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T_c value (92.5 K). However, beyond x = 0.1, the T_c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J_c values measured are found to reduce from 142 to 76 A/cm² with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.     

Long range coupling in YBa2Cu3O7−δ/Y1−xPrxBa2Cu3O7−δ multilayers

YBa₂Cu₃O_7–/(Y_1–xPr_x)Ba₂Cu₃O_7– multilayers have been used to probe coupling through (Y_1–x:Pr_x)Ba₂Cu₃O_7– alloys. We observe that the coupling between ultrathin YBa₂Cu₃O_7– layers, 12 or 24 Å thick, survives through several hundred Å of (Y_1–xPr_x)Ba₂Cu₃O_7– with x=0.4 and 0.55. T_c versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. T_c measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

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.     

Anisotropic Characteristics of In-Plane Aligned a-Axis YBa2Cu3O7−δ Thin Films

Highly in-plane aligned a-axis oriented YBa₂Cu₃O_7– (YBCO) thin films with unprecedented transition temperature (T _c 91.3 K) have been deposited on (100) LaSrGaO₄ substrates using a continuously grown PrBa₂Cu₃O_7– (PBCO) template layer by pulsed laser. The anisotropies between b- and c-axis were investigated by transport measurements and atomic force microscopy (AFM). Although, the onset T _c 's along b- and c-axis appeared approximately the same, a significant filing was observed in the resistive transition along c-axis. The temperature-dependent of the tailing effect, however, cannot be explained by thermally activated phase-slippage (TAPS) commonly encountered in grain-boundary weak links.     

Influence of preparation conditions on characteristics of the YBa2Cu3O7−δ compound

Synthesis of ceramic YBa₂Cu₃O_7– by solid-state reaction was performed under different conditions. Different values of cooling rate and oxygen flow were used, and no significant influence on superconducting characteristics of the samples was observed. A major influence on their mechanical properties was found.     

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.     

The superconducting state of YBa2Cu3O7−δ

Many low temperature properties of high T_c superconductors deviate significantly from the detailed predictions of BCS theory. Here we discuss whether these effects could be caused by either: (a) an unconventional pairing state, or (b) local randomness in the gap function due to the intrinsic disorder. We review recent experiments pertinent to these questions: Josephson effects in (001) oriented planar junctions between YBa₂Cu₃O₇- and classic superconductors and the temperature dependence of the a-b plane electro-magnetic penetration depth at low temperatures. We also calculate the density of states of s-wave superconductors with local quenched disorder in the gap function so as to determine whether s-wave pairing could be consistent with the low energy quasiparticle excitations seen in many experiments.     

Novel vortex dynamics in untwinned YBa2Cu3O7−δ single crystals

Magnetotransport measurements on a clean, untwinned YBa₂Cu₃O_7– single crystal show that the vortex dynamics at temperatures just below the vortex lattice melting transition are highly dependent on the type of modulation of the probing current. While in the case of DC current the flux flow is disordered, the shaking of the vortex lattice by a square-wave current leads to a more uniform vortex motion. A small asymmetry (10%) in the durations of the positive and negative parts of the square-wave period induces periodical oscillations of the voltage response amplitude. The period of oscillations (100 s) is the same order of magnitude as the time needed for vortices to cross the sample (transit time). We relate the observed voltage oscillations to a periodically ordered vortex motion.On leave from Moscow Institute of Radioengineering, Electronics and Automation, 117454 Moscow, Russia     

Characterization of YBa2Cu3O7−δ thin films deposited by dc magnetron sputtering

It was found from plan-view transmission electron microscopy (TEM) observations that c-oriented YBa₂Cu₃O_7– (YBCO) films on LaAlO₃, prepared by the magnetron sputtering technique had a single crystalline-like YBCO matrix, but always contained some precipitates of impurity phases. A large number of highly defective twin boundaries were also observed in all of these epitaxial films. Such highly defective twin boundaries are thought to be flux pinning centres and also paths for the diffusion of water into YBCO films. Therefore the films need to be passivated against attack from humid air.     

Fabrication of Micro-pipes of YBa2Cu3O7−δ Superconductor

Synthesis of micro-pipes of YBa₂Cu₃O_7−δ superconductor has been carried out for the first time by using chemical method. The shape, size and length of these micro-pipes depends upon dimensions of growth matrix and structure of geometry, i.e., circular, square, rectangular and hexagonal, etc., which could be uniformly coated with superconductor. The onset temperature of diamagnetism of the material is found to be 93 K, which is not shifted to lower values by the application of DC magnetic field; this is most likely due to large surface area provided by micro-pipes to the shielding currents. These micro-pipes behave as pinning centers, hence they can be used for the fabrication of devices that are capable of working in high magnetic fields.      

c-axis infrared reflectivity of magnetically aligned YBa2Cu3O7−δ as a function of doping

Dense polycrystalline samples of YBa₂Cu₃O_7– with a high degree of c-axis texture in the plane of the sample were prepared by magnetic alignment. The c-axis infrared reflectivity was measured between 300 and 6K on polished surfaces. Spectra compare well with measurements on single crystals indicating that magnetically aligned samples are a reliable alternative to single crystals for c-axis measurements.     

Electromagnetic properties of electrophoretic YBa2Cu3O7−δ films

In order to apply highT _c superconductivity to general microwave cavities and to shielding devices for quasistationary magnetic fields, we have developed an electrophoretic coating technique. This work reports about the continuation of our experiments. More than 60 samples of electrophoretic layers deposited on silver substrates were fabricated by systematically varying the processing parameters. The r.f. surface resistance measured at 21.5 GHz and 77 K at low excitation fields is found to be a sensitive measure of the quality of the samples and falls rapidly with increasing average grain size. Textured electrophoretic layers of large grain size show the smallest sensitivity ofR _s against the r.f. surface magnetic fieldH _s. The magnetic sensitivitydR _s /dH _s is found to be in direct proportion toR _s (77 K). After optimizing the sintering procedure the electrodes of the prototype cavity for a compact hydrogen maser were coated. The cavity was successfully operated at 1.42 GHz, and a surface resistance of 1 mΩ was achieved at 77 K. This compares to 4 mΩ for copper at the same temperature.     

Low-Field Phenomenology and Characteristic Lengths of Hydrogen- and Halogen-Doped YBa2Cu3O7−δ Compounds

We measure the diamagnetic response of YBa ₂ Cu ₃ O _y A _x (A=Cl, F, H) ceramics. We obtain low-field effective penetration depths of a few 10 m, first flux-penetration field H₁ distinguishable from the volume penetration field H_C1 and two characteristic temperatures T_s and T_c, T_cs, defined by the onset of the real and imaginary parts of the susceptibility, respectively. H ₁ (T) goes to zero as a power law H ₁ (T, x)=H ₁ (0, x) (T–T/T_c) ^2.7 near T_C. From the values of and H ₁ we infer the correlation length at T=4.2 K. A large range of values for (4.2) suggests that chemical disorder plays an essential role in the phenomenology of cuprates. We find (4.2)H ₁ (4.2) ^–1/3 . This, and the power law (4.2)H ₁ (4.2) ^–2/3 result from the scaling properties of a phase transition governed by disorder.     

Thermoelectric Power Studies on YBa2−xCaxCu3O7−δ System

The temperature dependence of the thermoelectric power of calcium substituted YBa_2?x Ca _x Cu₃O_7?δ pellets with 0≤x≥1.5 is presented between 60 K and 300 K. A metal–insulator transition was reported earlier by us in this system and was attributed to the ionic size-dependent localization effect. While the sign of thermoelectric power of all the calcium substituted samples was found to be positive, its magnitude increases significantly with calcium content in YBCO. The normal state thermoelectric power data of substituted YBa_2?x Ca _x Cu₃O_7?δ (0≤x≥1.5) are discussed in light of a two-band model originally proposed by Gottwick et al. for heavy fermion systems and later modified by Forro et al.     

Thermal annealing-induced ageing and structural effects in YBa2Cu3O7−δ superconductors

The YBa₂Cu₃O_7– superconductor loses part of its oxygen by thermal annealing, effectively at annealing temperaturesT _a 400 °C, under reduced air pressure. The release of the oxygen decreases monotonically as a function of time, at givenT _a, and presents two different slopes; fast at the beginning and slow later. The initial slopes (dM/dt) of the isothermal mass loss curves, measured at differentT _a, follow a straight line, as In (dM/dt) is plotted against theT _a ^–1 (K^–1), suggesting the release of the oxygen to be a kinetic process with an activation energyQ ₁=0.45 eV. The oxygen vacancies probably reside along the central-cage Cu-O chains, replacing O(4) ions, which break up spinal Cu-O linkages, and consequently the highT _c superconductivity properties suffer. A YBa₂Cu₃O₇ sample annealed for 30 min at 600 °C (at 10 mbar air pressure) thus gives a considerably reducedT _c at 50 K, compared to 91 K without annealing. However, annealing, peculiarly at a higherT _a 700 °C, for a short period of 10 min, has little effect on the highT _c appearing at 70 K. We believe that the oxygen vacancies possibly populate on O(1) sites along the side Cu-O chains, at this particular temperature, and become positively charged on trapping the hole, thus helping the superconductivity.