Disordered Vortex Phases in YBa2Cu3O7?δ

The disordered vortex phases induced by line and point pinning in YBa₂Cu₃O_7– are explored. At high defect densities, only a single disordered solid separated from the liquid phase by a melting line is observed. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of the first-order melting line separate the lattice and the disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.     

Oxygen Exchange in YBa2Cu3O7 – δ

The oxygen exchange in YBa₂Cu₃O_{7 –} materials of different densities was studied by thermogravimetry over wide ranges of temperatures and oxygen pressures. The results were correlated with x-ray diffraction (b – a), T _c (inductance method), and T _c data. According to polarized-light microscopy and coulometric titration results, the annealing conditions have a strong effect on the uniformity of oxygen distribution in the basal plane.     

YBa2 Cu3 O7-x单晶的制备

介绍了制备高温超导体YBa2Cu3O7-x单晶的主要方法,分析了在制备过程中存在的问题,指出液相的性质是影响生长速率的重要因素.在此基础上,着重探讨了通过控制氧压和元素替代等方法来改变液相的性质,从而达到提高YBCO单晶生长速率的目的.     

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.     

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.     

Enhanced Critical Current Density of FeF2 Added YBa2Cu3O7?δ

The effect of nanosized FeF₂ (??120?nm) addition on the critical current density, J _c, of YBa₂Cu₃O_7??? was studied. YBa₂Cu₃O_7??? superconductor was prepared by the solid-state reaction method with addition of 0 to 0.10 wt% FeF₂ additions. The magnetization critical current density, J _cm (at 77?K), was estimated from the extended Bean model by using vibrating sample magnetometer (VSM) measurements. The transport critical current density, J _ct, was measured using the four-point probe method with the 1???V/cm criterion. The onset transition temperature, $T_{\mathrm{c}\mbox{-}\mathrm{onset}}$ showed a maximal value at 92?K for 0 to 0.03?wt% additions. J _c increased with FeF₂ addition up to 0.03?wt%. Addition of FeF₂ of more than 0.03?wt% suppressed $T_{\mathrm{c}\mbox{-}\mathrm{onset}}$ and J _c. The inter-grain current density was enhanced by two orders of magnitude on 0.03?wt% FeF₂ addition. In addition to the condensation energy associated with their core, the full vortex magnetic energy by addition of nanosized FeF₂ enhanced J _c in the system.     

YBa2Cu3O7—x陶瓷氧扩散动力学

采用动态电导率的测量方法，研究了ＹＢａ２Ｃｕ３Ｏ７－ｘ陶瓷的氧扩散动力学过程，在５００－８５０℃温度范围内，氧分压为９６．８７ｋＰａ下而验证。     

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.     

Plasma-enhanced fluorination of YBa2Cu3O7−δ ceramics

The radiofrequency plasma technique involving mixtures of CF₄+O₂ gases has been applied to the surface treatment of high T_c superconducting oxides (YBa₂Cu₃O_7–). Investigation of the various experimental parameters of the process has shown that the improvement of the critical current density, J _c, mainly depends on the inlet precursor composition CF₄+%O₂, on the total pressure and on the reaction time. The presence of fluorine in the bulk of the ceramics has been observed from electron microprobe analysis, together with an increase of the Cu³⁺ content. The plasma-enhanced fluorination (PEF) treatment improves the superconducting properties of the materials: both values of the resistivity in the normal state and of the superconducting transition width are reduced and the critical transition temperature is improved by about 1 K.     

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.     

High-Frequency Fluctuational Conductivity in YBa2Cu3O7−δ

We present measurements of the fluctuational excess conductivity at 48 GHz and 24 GHz in YBa₂Cu₃O_7– films above the transition temperature. The measurements depart from the gaussian prediction for finite-frequency fluctuational conductivity. We focus on the region not too close to T _c, where the real part of the excess conductivity drops much faster than the prediction of the gaussian model. We calculate the dynamic excess conductivity within a Ginzburg–Landau approach. In the calculation we insert a short-wavelength cutoff of the order of the inverse coherence length, in order to suppress high-momentum modes. The excess conductivity of all samples measured can be described very well by the modified model.     

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.     

Enhancement of Vortex Pinning in YBa2Cu3O7?δ -BaHfO3 Superconductor-Insulator System

A systematic study on phase evolution, bulk density, resistivity, critical transition temperature (T _C), critical current density (J _C) and in-field transport critical current density (J _C?CB) of pure and nano particles of BaHfO₃ added YBa₂Cu₃O_7??? (YBCO) samples of different weight percentages has been carried out. The result shows a significant improvement in the electrical transport properties of the superconductor, namely the J _C, J _C?CB characteristics and flux pinning force (F _p) of the BaHfO₃ added samples. The transition temperature also was found to increase marginally. A maximum transport J _C of 742?A/cm² at 77?K (for x=2?wt%) was observed for BaHfO₃ added samples against 147?A/cm² for pure YBCO sample. The enhancement of the critical current density in the YBCO?CBaHfO₃ sample is attributed to the formation of an insulating and non-reacting YBa₂HfO_5.5 phase, acting as artificial pinning centers in the matrix. The introduction of nano particles of BaHfO₃ in bulk YBCO increases the pinning force density from 30×10³?N/m³ to 424×10³?N/m³ at 77 K. The improvement in sintering density due to an optimum value 2?wt% BaHfO₃ addition in bulk YBCO indicates its potential use in electrical devices and technology. The enhancement in the superconducting properties, particularly in the J _C?CB characteristics due to BaHfO₃ addition, seems to have great technological significance.     

Superconductivity Enhancement in Fe3O4 Doped YBa2Cu3O7−δ

Flux pinning can lead to the enhancement of J _c in superconductor. In order to study the effect of magnetic particles on superconductivity, the composites of (YBa₂Cu₃O_7?δ )_0.98(Fe₃O₄)_0.02 and (YBa₂Cu₃O_7?δ )_0.98(α-Fe₂O₃)_0.02 are synthesized with former sintered at different temperatures. The field-cooling measurements show that the T _c of the samples has not changed. The superconducting properties are enhanced when the samples are sintered at high temperature (750 ^°C), but suppressed for the samples without sintering or sintered at low temperature (350 ^°C). Scanning electron microscopy images show that the doped Fe₃O₄ are diffused into intergrain sites. With the increase of sintering temperatures, the grain size of the sample grows up and the flux pinning, which leads to the enhancement of J _c is formed by the addition of Fe₃O₄ nanoparticles. The extended X-ray absorption fine structure and X-ray absorption near edge structure experiments prove that after sintered at high temperatures the addition of Fe₃O₄ can induce disorder of local structures of YBa₂Cu₃O_7?δ . Also, the transition from the tetragonal phase to the orthorhombic phase is observed by the X-ray diffraction spectra. By the above analysis of experimental data, the enhancement of diamagnetic signal, which represents the superconducting properties can be attributed to the disorder enhancement of local structures and structural phase transition of the cuprates induced by the addition of Fe₃O₄.     

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.     

SmBa2Cu3O6.5 seed fabrication for seeded peritectic solidification of YBa2Cu3O7−δ

The availability of high quality SmBa2Cu3O6.5 seeds of known orientation is essential for the fabrication of a large grain YBa2Cu3O7– (YBCO) superconductor via a seeded peritectic solidification technique. The suitable seed must have a close lattice parameter match to YBCO and a relatively high melting temperature. We report a melt texturing process for the fabrication of SmBa2Cu3O7– (Sm-123) seeds using SrTiO3 to control grain nucleation. The physical and structural properties of the seeds were confirmed by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Seeds prepared in this study were used to grow large single grain YBCO superconductors of up to 2 cm in diameter.     

Sb2O3掺杂对YBa2Cu3O7—δ超导性质的影响

研究了Ｓｂ２Ｏ３掺杂的多晶ＹＢａ２Ｃｕ３Ｏ７－δ超导体的超导转变温度和输运临界电流密度Ｊｃ（Ｂ）在磁场中的行为。测量了超导性能对该掺杂浓度的依赖关系。适当浓度的掺杂可使Ｊｃ至少提高一个数量级，对掺杂样品进行了扫描电镜微结构观测和Ｘ射线结构分析。     

Superconducting YBa2Cu3O7?δ Thin Film Detectors for Picosecond THz Pulses

Ultra-fast THz detectors from superconducting YBa₂Cu₃O_7?δ (YBCO) thin films were developed to monitor picosecond THz pulses. YBCO thin films were optimized by the introduction of CeO₂ and PrBaCuO buffer layers. The transition temperature of 10 nm thick films reaches 79 K. A 15 nm thick YBCO microbridge (transition temperature—83 K, critical current density at 77 K—2.4 MA/cm²) embedded in a planar log-spiral antenna was used to detect pulsed THz radiation of the ANKA storage ring. First time resolved measurements of the multi-bunch filling pattern are presented.