Study of Flux-Creep in Bulk Melt-Textured YBa2Cu3Ox in the Regime of Individual Pinning

Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa₂Cu₃O_x bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = U _M was calculated from experimental data and compared to U = U ₀ ln(J _c /J), where U, is a constant, J _c is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of U _M on J was found, and the data indicated that a scaling function of U _M , g(T/T _c ) = (1–T/T _c )^1/2, was appropriate for describing the single-vortex pinning regime.     

Flux-Pinning and Irreversibility Line in Melt-Textured YBa2Cu3Oy Crystals of Different Sizes of Pinning Centers

Flux pinning force and irreversibility lines have been investigated for melt-textured YBa ₂ Cu ₃ O _y crystals containing Y ₂ BaCuO ₅ (211) particles with an average size ranging from 20 to 0.02 microns. We found that the irreversibility lines are increasingly improved and that the pinning force as well as the J _c is greatly enhanced as the average size of the 211 particles decreases. The F _p -H curve exhibits two peaks in the samples containing very fine 211 particles (nanometer size particles), which suggests that an addition pinning mechanism is involved in this type of samples. The experimental results can be well explained with combining the interface-pinning mechanism and the match effect.     

Improved Flux Pinning in Melt-Textured YBa2Cu3O7 − δ and ErBa2Cu3O7 − δ Through Chemical Additions

The effects of Ce-based additions (CeO₂ and BaCeO₃) in combination with MgO additions on the magnetic properties of melt-textured YBa₂Cu₃O_{7 ? δ} and ErBa₂Cu₃O_{7 ? δ} have been investigated. The additions lead to improvements in the magnetic properties of both RE-123 compounds compared to samples with either addition alone or with no additions. The Ce–Mg addition combination produces a “peak effect” in the magnetic hysteresis loop without significant T _c degradation. This is postulated to be due to the formation of a new type of pinning center. Both Ce and Mg ions are thought to substitute in the RE-Ba₂Cu₃O_{7 ? δ} lattice, creating defects that produce a “peak effect” in the magnetic hysteresis loop. Mg additions alone lead to a reduced T _c, while Ce additions restore the T _c and enhance the magnitude of the peak.     

Bond-Stretching Anomalies in YBa2Cu3Ox: a DFT Study

In the past few years, there has been growing evidence for unusual behavior of phonon modes related to the Cu–O bond stretching vibrations in several members of the high-T _c cuprates, which suggest a peculiar interaction between electronic and lattice degrees of freedom. Here, we analyze some of these anomalies in the light of density-functional calculations of phonon dispersions for optimally doped YBa₂Cu₃O₇ and for underdoped YBa₂Cu₃O_6.5 in the ortho-II structure with the goal to clarify which of the observed properties can not be accounted for by a local-density type screening, and thus may be seen as fingerprints for more subtle electron-correlation effects.     

Recrystallization of YBa2Cu3Ox superconductors in vacuum

YBa₂Cu₃O _x ceramics were recrystallized in vacuum at high temperatures. Recrystallized layers consisting of small grains were observed near the surfaces of the original large-grain 1 2 3 ceramics. The small grains consisted of transformation twins and were identified to orthorhombic 1 2 3 using X-ray diffraction. As vacuum annealing time increased, the thickness of the recrystallized layer increased. The relationship between the thickness and the annealing time showed a linear relationship and an effective diffusion coefficient of 6.25 × 10^–10cm²s^–1. The recrystallized layer showed a critical temperature of 90 K.     

Microstructure of plasma-sprayed YBa2Cu3Ox high-temperature superconductors

The arc plasma spraying process has been applied to deposit YBaCuO superconductors on to steel substrates. Detached, as-sprayed coatings were heat treated at 950 °C in air in order to restore superconductivity. Oxygen annealing at temperatures 400–500 °C has been used as the final stage in material preparation. The changes in the microstructure of the material during heat treatment have been followed with the aid of scanning electron and optical microscopy. The phase transformations resulting from heat treatment have been studied by differential thermal analysis and X-ray diffraction.     

Variations of internal friction in YBa2Cu3Ox superconductors

The internal friction (Q ^–1) spectrum of YBa₂Cu₃O_x superconducting ceramic exhibits several peaks. It has been confirmed that the high-temperature peak (around 240 K) depends on structural changes and varies during subsequent cycles of cooling and heating.Q ^–1 conductivity, X-ray spectra and the shielding effect have been measured on several samples having different superconducting properties obtained by various thermal treatments. Splitting is a characteristic feature of the high-temperature internal friction peak of the sample which exhibits good superconducting properties. In the case of the specimen exhibiting the worst properties the peaks decrease and overlap. In both cases an increase can be observed of this peak with the number of thermal cycles. After ageing at 470 K, the high-temperature peak disappears. Subsequent thermal cycles slightly recover it. Hysteresis of the Young modulus is also observed. The results are interpreted as transition of the 04 oxygen atom between two energy minima in the O4-Cu-O4 chain.     

Polymer coating for filamentary YBa2Cu3Ox superconductors

Polymer coating for filamentary YBa₂Cu₃O _x superconductors was examined. The precursor filaments were prepared by solution spinning through aqueous poly(vinyl alcohol) solution containing mixed acetates of Y, Ba and Cu. The as-drawn filaments were heated to remove volatile components and to generate a superconducting phase. The synthesis of polyamide acid for the polymer coating was made. The filamentary superconductors were coated with the polyamide acid and cured to convert to the polyimide. The surface of the sample was an insulator. The critical current (I _c) value of the filamentary sample at 77 K was preserved by the polyimide coating. Moreover, the I _c for the polyimide-coated sample was maintained after holding at room temperature for more than 6 months. After thermal cycling between 77 K and room temperature 10 times, the I _c for the coated sample did not deteriorate, whereas the I _c for the sample without polymer coating disappeared. Polymer coating using epoxy resin applied to the filamentary superconductors was also examined.     

Thermodynamics of oxygen uptake for the YBa2Cu3Ox superconductor

Thermogravimetric analysis and pressure (p) monitoring in known volume systems are used to give phase diagram information (e.g. vant Hoff plots) for roughly 6<x<7 in YBa₂Cu₃O_x. As an example we find for O₂ desorption for the important composition x = 6.8, beyond which superconductivity with transition temperature, T_s > 90 K becomes essentially independent of x, ΔH^o = 43.4 ±1 kcal/mo (O₂) and ΔS^o = 48.6 ±1 cal/Kmo (O₂). ΔS^o stays roughly constant for x > 6.7 but decreases for x < 6.7 indicating increased disorder on the O substructure. p (x) curves do not show phase separation which corresponds to above critical behavior, but given indications for limiting compositions near x = 6 and x = 7. Accordingly a model is proposed in which these compositions correspond to valencies of Cu⁺ and Cu³⁺ respectively for the symmetric Cu site (Ba coordinated) while the asymmetric Cu (Ba and Y coordinated) is taken to be 2+ over this whole range in x.     

Preparation and densification of superconducting YBa2Cu3Ox ceramics

The solid-state reaction method to form the superconducting oxide YBa₂Cu₃O _x was studied. It was found that the starting cupric and yttrium components accelerated the decomposition of the BaCO₃ component. At a constant heating rate of 10 ° Cmin^–1 in thermogravimetric analysis, the temperature of complete decomposition,T _f, was lowered from greater than 1000 ° C in pure BaCO₃ to between 915 and 985 ° C. The effectiveness in decreasingT _f can be ranked in the order of oxalate, carbonate and oxide. The highest sintered density achieved in this study was 6.03 g cm^–3 (/_th = 95%) at 990 ° C and 5.85 g cm^–3 (/_th = 92%) at 960 ° C. The source of cupric ion had the largest effect on densification. The use of cupric carbonate resulted in a consistently high Archimedes density of about 6.00gcm^–3 and large dimensional shrinkage of about 20% at 990 ° C for 12h. In contrast, the use of cupric oxide gave the lowest density and smallest shrinkage. Within the same powder lot, higher sintered density and smaller dimensional shrinkage were observed in samples with higher initial green density and compaction pressure. However, the data suggested that the enhanced densification and higher density achieved by the use of cupric carbonate and oxalate cannot be accounted for by the different physical characteristics of the powders and the mechanics of powder compaction, measured collectively by the green density.     

Compositional variations in the bulk of YBa2Cu3Ox thick films

Thick films of YBa₂Cu₃O_x (123) prepared on a number of substrate materials, namely Si, Al₂O₃, SrTiO₃, MgO and YSZ, have been investigated for the compositional variations/impurity phases present, microstructure, grain orientation etc. Energy-dispersive X-ray and X-ray diffraction analyses for as-prepared films of two different thicknesses, 2–3 m and 10–15 m, and after their partial and complete etching, are reported. The results show that the film-substrate reaction dominates up to 2–3 m thickness of a film. The decomposition of 123 material at the processing temperature and the substrate-film reaction together are seen to govern the crystallization behaviour, compositional variations and the superconducting properties of the films. In various cases, the films have been found to undergo superconducting phase transitions at temperatures between 66 and 89.5 K.     

Texture Analysis of Melt-Textured and Polycrystalline YBa2Cu3O y Using EBSD

We applied an electron backscatter diffraction (EBSD) technique to determine the texture and individual crystallite orientations on melt-textured and various polycrystalline YBa₂Cu₃O _y (YBCO) samples, doped with several Alkali metal additions (KClO₃, K₂CO₃ and Rb₂CO₃). The EBSD analysis on melt-textured YBCO reveals a direct interplay between the crystal orientations of YBCO and the Y₂BaCuO₅ (Y-211) particles. The Y-211 particles do not show any texture, but certain orientations of the Y-211 particles enable an homogeneous growth of YBCO. From the measured data, individual misorientations and the orientation distribution functions are obtained for all samples studied.     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.     

Transformation of the YBa2Cu3Ox superconducting phase to an insulator-nonsuperconducting phase

Three YBa₂Cu₃O _x samples (A, B, and C) were prepared under similar conditions. The starting materials were Y₂O₃ (99.99%), CuO (99.999%), and BaCO₃ (99.999%). All the specimens demonstrated a Meissner effect at liquid-nitrogen temperature. From temperature-dependent resistivity measurements, theT _c for the three samples was found to be aroundT _c =93 K, in agreement with other reports [1].     

Hydrostatic and uniaxial pressure effect on Tc of YBa2Cu3Ox

The Variation of the transition temperature T_c of YBa₂Cu₃O_x with hydrostatic He-gas pressure depends on the oxygen content x. The pressure effect dT_c/dp increases from small negative values at x=7 to dT_c/dp=7.4 K/GPa at x=6.7. For oxygen contents below x=6.7 dT_c/dp drops to 3 K/GPa and remains nearly constant¹. The charge transfer model² cannot explain the drop at x=6.7.Thermal expansion measurements on YBa₂Cu₃O_x indicated that the uniaxial pressure effects along the three crystal axes are different^3,4 To investigate the uniaxial pressure effects inductively an experimental setup was constructed. The T_c-change of several YBa₂Cu₃O_x single crystals with different oxygen contents has been investigated under pressure along the c-axis. To avoid oxygen ordering processes the samples were held below 105 K during the measurements. The results of uniaxial pressure measurements in c-axis direction fit to former uniaxial pressure data^3,4,5 and are explained within the charge transfer model. Hydrostatic pressure data¹² of overdoped samples fit to the same curve. However, this is not the case for underdoped samples. From this we conclude that only a part of the hydrostatic pressure effect can be explained by charge transfer in the underdoped region. The remaining part can be ascribed to uniaxial pressure effects along the a- and b-axis.     

Photoinduced Changes of the In-Plane and Out-Of-Plane Electronic Properties in YBa2Cu3Ox

We report on the photoinduced changes of the electronic properties of YBa₂Cu₃O_x (x 6.6) thin films, focussing on the electrical anisotropy in the normal and the superconducting state. The latter was determined from magnetoconductivity measurements, performed before and after light excitation, and fitting the experimental results to the Aslamazov–Larkin theory of superconducting order-parameter fluctuations. The normal-state anisotropy _ab/_c was calculated from resistance measurements carried out on samples grown on vicinal substrates. We observed that the photodoping process enhanced superconducting coherence lengths _c(0) and _ab(0), but reduced the superconducting anisotropy _ab(0)/_c(0) at all studied temperatures. In contrast, _ab/_c was enhanced by photodoping at low temperatures, but reduced at high temperature. Our results strongly suggest that two different mechanisms contribute to the photodoping process, with one affecting only the normal-state properties.     

Synthesis of a phase-pure orthorhombic YBa2Cu3Ox under low oxygen pressure

Reaction of Y₂O₃, BaCo₃ and CuO for 4 h at 800 °C in flowing O₂ with a total pressure of about 2.7 × 10² Pa, followed by cooling in O₂ at ambient pressure, has produced phase-pure orthorhombic YBa₂Cu₃O_x. Keeping the ratio of O₂ to evolved CO₂ above 50 was necessary to ensure phase purity. The resultant powder yielded pressed and inserted pellets with improved superconducting properties.     

Investigation of the dominant point defects in tetragonal YBa2Cu3Ox at elevated temperatures

The manner in which oxygen is incorporated into YBa₂Cu₃O _x (YBCO) at ～800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O _i ^x . The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O₂), as a function of compositionx and temperatureT (5.99≤x≤ 6.35, 825≤T≤1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O _i ^x are noninteracting forx ? 6. (b) The stoichiometric composition of YBCO isx ? 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O₂. (d) The Fermi level changes by ? ?0.2 eV asx increases from 6.05 to 6.35.     

Effects of Y2BaCuO5 on the directional growth of YBa2Cu3Ox superconductor

The effects of Y₂BaCuO₅ on the microstructure and superconducting properties of YBa₂Cu₃O_x samples, textured using the directional growth of Y₂BaCuO₅, BaCuO₂ and CuO powder mixtures, were studied. Y₂BaCuO₅ moved downward as the sample passed up through the hot zone, and this downward movement affected the superconducting properties of the directionally grown samples. The critical temperature increased, and the resistance transition width decreased as the excess 211 content in the powder mixture increased up to a value, beyond which the temperature decreased and the width increased. The critical current densities of the directionally grown samples were scattered because of the presence of uncontrollable severe cracks perpendicular to the growth direction. As the hot-zone temperature increased, the critical current density of the directionally grown sample increased up to a maximum value (>6000 A cm^–2) beyond which the critical current density decreased. Compared to data from other studies, the hot-zone temperature required to produce a maximum critical current density was lower.     

500–600 K Dilatation Anomaly and Tc Increase in YBa2Cu3Ox

YBa₂Cu₃O _x bulk ceramics have been simultaneously investigated by the thermal expansion and the acoustic emission methods in the 500–600 K temperature range. An error of the differential dilatometer does not exceed 10^?7. At T = 555 K, the dilatation anomaly accompanied by the acoustic emission has been observed and measured. Based on this measured anomaly, a value of Δc = ?0.003 μm has been determined. And based on this Δc size the stoichiometric coefficient x = 7.01 has been calculated too. The possible mechanism for T _c increase in YBa₂Cu₃O _x as a result of closing b axis by the absorbed oxygen atoms during the samples' heating through 500–600 K has been discussed.