Effect of double annealing on the critical parameters of highly textured YBa2Cu3O6.9

The effect of low-temperature treatment (200°C) and subsequent annealing at 930°C on the critical parameters of highly textured YBa₂Cu₃O_6.9 is studied. The structural defects that are formed during the low-temperature decomposition of this compound into phases with different oxygen contents and during interaction with atmospheric moisture are shown to deteriorate the superconducting properties. After annealing at T = 930°C and subsequent oxidation, superconductivity is restored and the formed defects are partly retained and serve as effective pinning centers, including the case of high magnetic fields. The stresses induced by the low-temperature treatment lead to primary recrystallization at T = 930°C, which results in the disappearance of texture and an isotropic state of the material in high fields.     

Effect of low-temperature treatment and subsequent high-temperature annealing on the critical current density of YBa2Cu3O y

The field dependences of the critical current density of the HTSC compound YBa₂Cu₃O _y recovered at T = 920?C950°C after the low-temperature treatment have been investigated. At T = 200°C, structural defects are formed in a wet environment, which are capable of initiating pinning of magnetic vortices. A short-term (1?C3 h) recovery annealing performed at T = 930?C950°C leaves in the samples a fairly large amount of structural defects formed during the low-temperature treatment, which results in a substantial increase in the critical current density in magnetic fields of ??2 T as compared to the ceramics not subjected to double annealing. A longer high-temperature treatment removes the structural defects formed and brings the electrophysical properties of YBa₂Cu₃O _y to the level characteristic of the ceramics produced by standard technology.     

Effect of structural water on the critical characteristics of highly textured YBa2Cu3O6.9

The effect of low-temperature treatment (200°C) in a humid argon atmosphere and subsequent annealing (930°C) on the critical parameters of a highly textured YBa₂Cu₃O_6.9 has been studied. During annealing at T = 200°C, the absorbed water is incorporated into the structure of the compound, which is accompanied by the deterioration of its superconducting properties. However, after the recovery annealing at T = 930°C and subsequent oxidation, the superconducting characteristics (j _c , B _1c , and F _p ) are improved. This is explained by the formation of 124-type planar defects, which are effective pinning centers, especially in high fields applied perpendicular to the c axis (⊥ c). The optimum conditions of double annealing substantially increasing the critical current density (j _c ≥ 10⁴ A/cm²) in an external magnetic field up to 10 T and also the first critical fields have been found. In fields up to ～3 T, the critical current density j _c is isotropic despite the conservation of high texture in the samples.     

Effect of vapors of the simplest alcohols on the structure and properties of YBa2Cu3Oy compounds

With low-temperature treatments (200–300°C), the effect of alcohol vapors on the structural and electrophysical properties of YBa₂Cu₃O_y (123) having different oxygen content has been studied. It has been established that CuO and YBa₂Cu₃O_y are catalysts for the dehydration of alcohols to form water and aldehydes. Similar to the action of water vapors, the treatment in vapors of the simplest alcohols leads to hydration of 123 and its transformation into the pseudo-124 phase. In this case, planar stacking faults are formed to be centres of pinning and can improve the critical characteristics of high-temperature superconductors (HTSС) in magnetic fields. The influence of the alcohol vapors is more ‘mild’ as compared to the treatment in water vapors. At t?=?300°C, the alcohols are reducing agents of copper, followed by the decomposition of 123 into simple compounds.     

Low-Field Nuclear Magnetic Resonance Relaxometry as a Tool in Monitoring the Aging of Coating Solutions (Case Study: Barium Propionate Precursor Coating Solution)

In the present study, we investigate the relationship between the aging of a barium coating solution used in the synthesis of the precursors for YBa₂Cu₃O_7−x -coated conductors or any oxide compound containing BaO and the transverse relaxation time measured in a nuclear magnetic resonance (NMR) experiment. The barium propionate precursor solution sample was prepared by the stepwise addition of an excess of propionic acid and ammonia to a barium acetate dispersion in methanol. All NMR relaxation experiments were performed at 20°C and a proton resonance frequency of 20 MHz. The experimental results are compared with a proposed model that assumes the reduction of the relaxation centers during the aging process. The model allows us the extraction of a characteristic aging time for the monitored sample.     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

Increased critical current density and pinning in thick Ag/YBa2Cu3O7−x multilayers

We have investigated the superconducting properties of Ag/YBa₂Cu₃O_7−x thick multilayers grown by Pulsed Laser Deposition, and found that the artificial pinning centres induced by Ag nanodots lead to a significant increase in critical current, especially in high applied magnetic fields. Transmission Electron Microscopy showed a columnar growth of YBa₂Cu₃O_7−x induced by Ag nanodots, while angle-dependent transport measurements revealed the existence of strong, both isotropic and c-axis correlated, artificial pinning centres.     

Effects of tin-compounds addition on Jc and microstructure for YBCO films

Tin-compounds were doped into YBa₂Cu₃O_7−δ (YBCO) films as pinning centers to enhance J_c in magnetic fields. YBCO films were grown by a metal organic deposition process using tri-fluoroacetates starting solutions. Tin-acetylacetonate salts were then dissolved into the starting solution as pinning centers. J_c of the YBCO films with tin-acetylacetonates were enhanced in all magnetic field angles. Transmission electron microscopy (TEM) and TEM-EDS (Energy Dispersive X-ray Spectroscopy) observations revealed the existence of tin-compound particles with the size of about 30 nm in the YBCO film. These nano particles were distributed randomly in the film and were considered to act as 3-dimentional pinning centers.     

Determination of the critical current density and the flux pinning force in single crystals YBa2Cu3O7-δ from the magnetization hysteresis cycles

In this study, the magnetization measurements have been performed on high-temperature superconductor's single crystals YBa₂Cu₃O_7-δ at large ranges of temperature T (15-85 K) and in magnetic fields up to 6T at different values of the angle θ between the applied magnetic field and c-axis. The critical current density Jc deduced from the magnetic hysteresis loops by the Bean formula for H parallel to the c-axis (θ=0°), our results have shown that the critical current density Jc was strongly dependent on the applied magnetic field. The pinning force Fp=Jc×μ₀H was determined from magnetization for H//c, however, a plot of the normalized pinning force density fp= Fp/Fpmax as a function of the reduced magnetic field h= H/Hirr at different temperatures have shown good scaling with the form fp ~h^p(1-h)^q, where p and q are scaling parameters. We also found that the point pinning is more dominant than surface pinning under high temperatures.     

Study of magnetic flux pinning in granular YBa2Cu3O7 − y /nanoZrO2 composites

In this work, the effect of ZrO₂ nanoparticles prepared in a low-pressure arc discharge plasma on magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites has been studied. It has been shown that the ZrO₂ nanoparticles do not change the superconducting transition and the microstructure of superconductors. At a temperature of 5 K, the addition of 0.5 and 1 wt % of ZrO₂ nanoparticles may lead to the additional effect of magnetic flux pinning and the increase in the critical current density J _c. The J _c value for composites with 1 wt % is two times larger than that for the reference sample. The fishtail effect is observed for YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites at the temperatures of 20 and 50 K. The problems associated with the additional effect of magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites and the appearance of the fishtail effect have been discussed.     

Setup for studying nonlinear magnetic properties of high-temperature superconductors with the aid of magnetization harmonics

A setup for measuring cophasal and quadrature components of higher harmonics of an electromotive-force signal of the response of a high-temperature superconductor makes it possible to study nonlinear magnetic properties of superconductors in variable magnetic fields of up to 1 kOe and constant magnetic fields of up to 10 T in the temperature range of 5?C300 K. This setup was used to measure the temperature dependences of the absolute values of the real and imaginary parts of the first and third harmonics of the magnetization of textured Yba₂Cu₃O_{7 ? x} polycrystalline samples in the temperature range of 77?C220 K at various values of variable and constant magnetic fields. An analysis of resulting data made it possible to reveal the presence of different dynamical modes of the magnetic flux in YBa₂Cu₃O_{7 ? x} that were dominant in different temperature ranges. The nonlinearity of the magnetization of YBa₂Cu₃O_{7 ? x} (the appearance of higher harmonics) was observed up to temperatures in the range of T = 103?C112 K, which were substantially higher than the temperature of the transition of this compound to a superconducting state. The observed feature in the magnetization of YBa₂Cu₃O_{7 ? x} was associated with the emergence of a pseudogap state in this compound.     

Influence of chemical composition on the stability of YBa2Cu3O y in a humid atmosphere

The influence of replacements of yttrium and barium in YBa₂Cu₃O _y and doping of this compound with CeO₂, ZrO₂, and Pr₂O₃ on the capability of absorbing water from the annealing atmosphere at T = 200°C has been studied. The complication of the chemical composition of the 123-type compounds leads to an enhancement of their stability to aggressive components of the gaseous phase (H₂O, CO₂). These compounds have a variable chemical composition both in oxygen and water and can exist in three modifications: the oxygen-ordered ortho-phase, the oxygen-disordered tetra-phase, and the pseudocubic phase with a variable water content, which at T = 200°C separates into an oxygen-deficient water-enriched phase and an oxygen-rich anhydrous phase.     

Andreev states and the Josephson effect in superconducting heterojunctions on thin YBa2Cu3Ox films

Conductance anomalies at low bias voltages and superconducting currents in Au/YBa₂Cu₃O_x and Nb/Au/YBa₂Cu₃O_x heterojunctions in which the c axis of the YBa₂Cu₃O_x (YBCO) epitaxial film is rotated in the (110) YBCO plane through 11° with respect to the normal to the substrate plane were studied experimentally. The films were prepared by laser deposition onto (7 2 10)-oriented NdGaO₃ substrates. The current-voltage characteristics of the heterojunctions exhibit conductance anomalies at low voltages. The behavior of these anomalies is studied at various temperatures and in various magnetic fields. The critical current and Shapiro steps observed in the current-voltage characteristics of Nb/Au/YBa₂Cu₃O_x were evidence of the Josephson effect in these heterojunctions. The experimental results are analyzed in terms of the model of the arising of bound states caused by Andreev reflection in superconductors with d-type symmetry of the superconducting order parameter.     

Effect of the microstructure of epitaxial YBa2Cu3O7 − x films on their electrophysical and nonlinear microwave properties

The local nonlinear microwave response of YBa₂Cu₃O_{7 ? x} films was measured by near-zone field microscopy with a spatial resolution of 50 μm, and YBa₂Cu₃O_{7 ? x} film microbridges were locally studied by low-temperature scanning microscopy with a spatial resolution of 4 μm. The microstructure of epitaxial YBa₂Cu₃O_{7 ? x} films was examined using x-ray diffraction and electron microscopy. A correlation is detected between the average crystallite size and the half-widths of the temperature dependences of the third-harmonic power (W _TH) and the electron-beam-induced voltage (W _EBIV). The experimental results are described in terms of a model of a two-phase medium taking into account the nonlinear I-V characteristic of the superconductor. For large crystallites, the nonlinear microwave response is shown to be caused by intracrystallite vortex pinning. As the average crystallite size decreases, an additional contribution to the nonlinear response appears due to the pinning of a magnetic flux by the Josephson network of crystallite boundaries. Calculations show that a three-fold increase in the crystallite size decreases the nonlinearity coefficient of YBa₂Cu₃O_{7 ? x} films by two orders of magnitude.     

Interaction of YBa2Cu3O6.8 with atmospheric moisture during low-temperature annealing

The interaction of YBa₂Cu₃O_6.8 (123) with water vapors at T = 200°C and the water influence on the structure and electrophysical properties of the compound have been studied using Raman spectroscopy, magnetometry, and X-ray diffraction. It has been found that the penetration of water into the 123 structure leads to its transition to a hydride-oxyhydroxide H_{2x ? z} YBa₂Cu₃O _{y + x ? z} (OH) _z containing fragments of the 124-type structure and exhibiting the spectrum of two-magnon scattering characteristic of antiferromagnetic 123 á compositions; in this case, the superconducting properties of the material as a whole are conserved. After short-time recovery annealing and subsequent oxidation, the water is removed from the compound structure, which leads to the disappearance of the spectrum of spin fluctuations. A possible mechanism of change in the 123 structure upon hydrogen and water intercalation has been discussed.     

Enhancement of the critical current density in YBCO/Ag composites

YBa₂Cu₃O_7 + xAg?(x = 0.0, ?5.0, ?15.0 and 20 wt%) composite samples have been prepared by the solid state reaction method. The changes in structure are confirmed from the X-ray diffraction analysis and SEM measurements. The critical current density is calculated using Bean's formula from the magnetization measurement. We find that the addition of silver in YBCO enhances the critical current density (?J_C) by a factor of nearly six (for 15% Ag) in comparison to pure YBCO. Enhancement of the pinning force (F_P) by a factor of ten is also reported. The enhancement in ?J_C is investigated over a wide range of magnetic fields. These significant changes in ?J_C and F_P are attributed to the presence of Ag particles as efficient artifical pinning centers in YBCO.     

Ca‐doped Ba2Cu3O7–δ bicrystal junctions fabricated on asymmetric SrTiO3 substrates

Bicrystal grain–boundary Josephson junctions of Ca–doped YBa₂Cu₃O_7–δ that is Y_0.7Ca_0.3Ba₂Cu₃O_7–δ were fabricated on three bicrystal SrTiO₃ (001) substrates with asymmetric 30°, 40° and 45° orientations. An enhancement of the critical current density in these Ca–doped junctions was observed when compared with normal YBCO grain–boundary junctions with similar angular orientations. The observed increase in the critical current density is large for the junctions fabricated on the asymmetric 30° bicrystal substrate and small or negligible, for those on the asymmetric 45° bicrystal substrate. The critical current was modulated by a magnetic field applied in the plane of the junctions. However, the Fraunhofer pattern observed due to the applied magnetic field deviates from the ideal one.     

Peak effect at microwave frequencies in swift heavy ion irradiated YBa2Cu3O7-δ thin films

The vortex dynamics at microwave frequencies in YBa₂Cu₃O_7-δ (YBCO) films have been studied. We observe a peak in the microwave (4.88 and 9.55 GHz) surface resistance in some films in magnetic fields up to 0.8 T. This is associated with the ‘peak-effect’ phenomenon and reflects the order-disorder transformation of the flux line lattice near the transition temperature. Introduction of artificial pinning centers like columnar defects created as a result of irradiation with 200 MeV Ag ion (at a fluence of 4×10¹⁰ ions/cm²) leads to the suppression of the peak in films previously exhibiting ‘peak effect’.     

Transport studies of YBa2Cu3Oy/YxPr1−xBa2Cu3Oy and YBa2Cu3Oy/R0.7M0–3MnO3 superlattices,in which R=La or Nd,and M=Ca or Sr

We report the transport studies of YBa₂Cu₃O_y/Y_xPr_1−xBa₂Cu₃O_y and YBa₂Cu₃Oy/R_1−xM_xMnO₃ superlattices in magnetic fields in which R=La or Nd, and M=Ca or Sr. The X-ray diffraction of samples shows superlattice structure. The resistive transition in a magnetic field shows thermal activated behavior. The flux pinning is reduced when the coupling strength between YBCO layers is decreased. The angular dependence of the critical current of YBa₂Cu₃O_y/PrBa₂Cu₃O_y superlattices reveals the dimensionality of superlattices. The magnetoresistance ratio (MR), |Δρ(H=7 T)−Δρ(H=0)|/Δρ(H=7 T), of YBa₂Cu₃O_y/R_1−xM_xMnO₃ superlattices is affected by the layer coupling of R_1−xM_xMnO₃ layers. The enhancement of the MR ratio in the tri-layer YBa₂Cu₃O_y/La_0.7M_0.3MnO₃/YBa₂Cu₃O_y film in the low temperature regime is significant and has a value of 33650% at T=75 K. We attribute this enhancement of the MR to the ordering of magnetic moment in ferromagnetic layers in magnetic fields. The results are discussed in terms of existing theories.     

Analyzing anode and cathode products obtained by the electrolysis of Y-Ba-Cu-O and Y-Ba-Cu-K-O systems

Energy dispersive X-ray spectroscopy (EDX) and electron diffraction and high-resolution electron microscopy are used to study the composition and structure of anode and cathode deposits formed during the high temperature (950°C) electrolysis of the Y-Ba-Cu-O system and low temperature (450°C) electrolysis of the Y-Ba-Cu-K-O system. It is found for the first time that an oxide with the YBa₂Cu₃O _y structure (123 phase) is synthesized during the high temperature electrolysis of Y_0.02Ba_0.30Cu_0.70O _y and Y_0.02Ba_0.25Cu_0.75O _y melts. The 123 phase is not synthesized during low temperature electrolysis, where melts are formed from Y₂O₃, BaO₂, CuO, and KOH. Two new Pt-containing oxides with hexagonal structure are found in the products of high temperature electrolysis.