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1.
Reactivity of several oxide materials (OM) with BSCCO powder and oxygen diffusion through OM layer has been tested at temperature ≈840 °C in air. The OM (e.g.: BaZrO3, SrCO3, MgO and ZrO2) showing the low or no reactivity with BSCCO have been mixed (10 wt.%) with precursor powder and used for single-core tapes. Bi-2223/Ag/OM/Ag single-core tapes with oxide barriers made of BaZrO3, SrCO3, ZrO2 and Al2O3 have been also prepared by a standard powder-in-tube technique. The used OM in the direct contact with BSCCO influences the electrical properties of Bi-2223 phase differently. These is because the oxides react with BSCCO during the heat treatment and simultaneously affect the 2212→2223 phase transformation, the Bi-2223 grain growth and so also grain connectivity. SrCO3 powder has been evaluated as the best material from the point of no destructive effect on 2223 phase transport current property. The oxide barrier controls the oxygen diffusion during the tape heat treatment and simultaneously the HTS phase formation kinetics, its purity and content within the superconducting core. For single-core Bi-2223/Ag/OM/Ag tapes, the highest current density was measured for Al2O3 due to only slightly reduced oxygen diffusion through the barrier.  相似文献   

2.
Roles of the blocking layer that we named the layer which separates Cu–O2 layers more than 6Å have been studied by comparing two kinds of highT c copperoxide superconductors such as Bi(2212) and La(2126) compound. These following results have been obtained. The hole concentration decreases when Sr is substituted by La and increases when Bi is substituted by Pb in Bi(2212), and it can be optimized by these substitution. In La(2126)T c becomes up to 43K and the hole concentration (p;[Cu–O]+p) increases to 0.09 by the substitution of Ca for La and heat treatment under high oxygen partial pressure. The distance between Cu–O2 layers in both Bi(2212) and La(2126) are not changed by these substitution and heat treatment. We have found that the blocking layer has not direct roles for the maximumT c value of the material though by supplying carriers to Cu–O2 layers, it affects the actualT c value.  相似文献   

3.
In continuous magnetic fields H up to 28 T, we have studied the out-of-plane transport properties and tunneling characteristics of high-quality nondoped single crystals of the Bi-cuprate family: Bi2Sr2CuO6+δ (Bi2201), Bi2Sr2CaCu2O8+δ (Bi2212) and Bi2Sr2Ca2Cu3O10+δ (Bi2223) grown by an identical method. For all compounds the out-of-plane magnetotransport ρc(H) is negative in the temperature region where ρc(T) shows in the normal state a semiconducting-like temperature dependence. The negative magnetoresistance of ρc corresponds to the suppression of the semiconducting temperature dependence of ρc(T) which is found to be isotropic. For the Bi2201 compound, where the normal state can be reached in the available magnetic fields (28 T), a nearly complete suppression of the low-temperature upturn in ρc(T) is observed in the highest magnetic fields with a tendency towards a metallic behavior down to the lowest temperatures (0.4 K). Using the break-junction technique, especially for the Bi2212 and Bi2232 compounds, a clear superconducting gap structure can be observed. Both for temperatures above the critical temperature and for magnetic fields above the upper critical field, a pseudogap structure remains present in the tunneling spectra. The applied magnetic fields yield a stronger suppression of the superconducting state compared to that of the normal-state gap structures as manifested in ρc(T) transport and tunneling.  相似文献   

4.
A systematic study of the intergranular properties of (Bi,Pb)2Sr2Ca2Cu3Oy (Bi2223) polycrystalline samples has been done using the electrical resistivity and Ac susceptibility techniques. In this project, we have prepared a series of Bi2223 samples with different sintering temperature. The XRD results show that by increasing sintering temperature up to 865 °C the Bi2212 phase fraction decrease. It was found that the Bi2212 phase on the grain boundaries is likely to play the role of weak links and consequently reduces the intergranular critical current densities. Analysis of the temperature dependence of the Ac susceptibility near the transition temperature (Tc) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (Jc) with temperature indicates that the weak links are changed from superconductor–normal metal–superconductor (SNS) for well-coupled sample to superconductor–insulator–superconductor (SIS) type of junctions for the sample with high Bi2212 phase fraction.  相似文献   

5.
We present measurements of the specific heat and of the thermal expansion of the most prominent phases of the Bi and Tl high temperature superconductors (Bi-2212, Bi-2223, Tl-2212, Tl-2223) in the neighbourhood ofT c . In all these systems we observe small but sharp anomalies which have very little similarity with the mean field jump found in the conventional superconductors. The analysis of these anomalies shows clear evidence for the presence of strong fluctuations of the order parameter. For temperatures more than 5 K away fromT c ,2D Gaussian fluctuations are found, while within ±5 K ofT c the fluctuation contribution is best fitted by critical fluctuations. The shape of the thermal expansion anomalies is similar to that of the specific heat. Combination of both measurements predicts dT c /dp+0.2 K/kbar for Bi- and Tl-2223 systems.  相似文献   

6.
A crystallization study has been carried out for rapidly solidified Bi2Pb0.5Sr2Ca4Cu5Ox glass. Glass transition temperature T g, crystallized superconducting phases and microstructural changes were measured and analysed by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The crystallization mechanism of the three superconducting phases — (2201) 20 K phase, (2212) 80 K phase, and (2223) 110 K phase — has been discussed, and a time-temperature-transformation diagram for the glass has been constructed.  相似文献   

7.
The effect of both the chemical composition and the nature of the chemical bonding of amorphous alloyed samples of GexInySe100−xy prepared under vacuum with x=20, 4y15 on the efficiency of the structural modification Δφ is analysed using a simple consideration based on the coordination number Z, and the number of topological constraints Nco. The previously obtained parameters have been used for the determination of the number of continuous deformation (i.e. zero-frequency modes f ), and for the estimation of the cohesive energies of these glasses, assuming simple additivity of bond energies. A trial has been made to correlate the results of this paper with the previously published data of glass transition temperature Tg, activation energy 0 and the shift of the K-absorption edge (ΔEK) of the composition of vacuum prepared GexInySe100−xy. It was found that there is a correlation between the lone-pair electrons and the stability of the vitreous state. According to the criterion of Liang, the above correlation has been interpreted in terms of Δφ.  相似文献   

8.
Fine structures were observed in the tunneling conductance of a sintered Bi 2223-SnO2 junction. The structures are weaker than those of a single crystalline Bi2212. They correspond to the Raman spectrum of Bi2223 and approximately to the phonon density of states of Bi2212. The structures must therefore be phonon structures, and phonons may contribute substantially to highT c superconductivity. Multiphonon structures are scarcely discernible. Hence we propose a new model, in place of the prior multiphonon model, to explain the rapid increase inT c with the CuO2 layer number. 2 (21 K)=68±4 meV inT c=98±5 K. 2 (0)/k B T c is 8.1±0.9. The temperature dependence of the gap was also observed and discussed.  相似文献   

9.
Superconducting Tl-based films were prepared on a LaAlO3 single crystal substrate. Spray pyrolysis of Ba, Ca and Cu nitrate solutions was used for deposition of the precursor films. They were subsequently ex-situ thallinated in flowing oxygen (open system). While the superconducting Tl-2212 phase formed at an annealing temperature of 880°C, thallination at 900°C led to the formation of a Tl-2223 /Tl-2212 mixture. The amount of Tl-2223 increased with prolonged thallination, whereas the Tl-2212 phase progressively disappeared. Films prepared in such a manner were c-oriented and contained only low amounts of non-superconducting impurities. The resulting samples were characterized by XRD and SEM and their T C values were determined by resistive four-point measurements. They showed critical temperatures in the range of T ON = 125–135 K, T C0 = 91–93 K. Differences between the composition and properties of the obtained films and those thallinated in closed systems under 50 kPa of oxygen are discussed in this paper. Results show that the Tl-2212 → Tl-2223 transformation proceeds at a slower rate under flowing oxygen than in a closed system. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia.  相似文献   

10.
The dispersion of light in Bi4Ge3O12 and Bi12GeO20 single crystals and thin Bi2O3 films with a monoclinic structure was investigated in the visible spectral region. The parameters of a single-oscillator approximation have been found. It is established that in Bi4Ge3O12 crystals the absorption band caused by the O2p–Bi6p transitions makes the main contribution to the dispersion curve in the visible region, whereas in Bi12GeO20 crystals this is made by transitions from the hybrid O2p–Bi6p states to the conduction band. The dispersion energy, the degree of the ionicity of binding, and the coordination number of the first coordination sphere of the Bi3+ cation have been determined.  相似文献   

11.
Flow propagators, used for the study of advective motion of brine solution in porous carbonate and sandstone rocks, have been obtained without the influence of Nuclear Magnetic Resonance (NMR) relaxation times, T1 and T2. These spin relaxation mechanisms normally result in a loss of signal that varies depending on the displacement ζ of the flowing spins, thereby preventing the acquisition of quantitative propagator data. The full relaxation behaviour of the system under flow needs to be characterised to enable the implementation of a true quantitative measurement. Two-dimensional NMR correlations of ζ − T2 and T1 − T2 are used in combination to provide the flow propagators without relaxation weighting. T1 − ζ correlations cannot be used due to the loss of T1 information during the displacement observation time Δ. Here the moments of the propagators are extracted by statistical analysis of the full propagator shape. The measured displacements (first moments) are seen to correlate with the expected mean displacements for long observation times Δ. The higher order moments of the propagators determined by this method indicate those obtained previously using a correction were overestimated.  相似文献   

12.
The p(O2)–Tδ diagram of perovskite-type SrCo0.85Fe0.10Cr0.05O3−δ was determined by the coulometric titration technique in the temperature range 770–1250 K at oxygen partial pressures from 8 10−10 to 0.5 atm. Stability of the cubic perovskite phase of SrCo0.85Fe0.10Cr0.05O3−δ, existing down to the oxygen pressures of 10−3–10−5 atm, was found to be slightly higher than that of SrCo0.80Fe0.20O3−δ, probably due to stabilization of oxygen octahedra neighboring Cr4+ cations. When the oxygen nonstoichiometry of the Cr-containing perovskite decreases from 0.47 to 0.38, the partial molar enthalpy and entropy for overall oxygen incorporation reaction vary in the ranges −165 to −60 kJ mol−1 and 90 to 150 J mol−1 K−1, respectively. Within the stability limits of the single perovskite phase, the p(O2)–Tδ diagram can be adequately described by equilibrium processes of oxygen incorporation, cobalt disproportionation and interaction of cobalt and iron cations, with the thermodynamic functions independent of defect concentrations. Increasing grain size in SrCo0.85Fe0.10Cr0.05O3−δ ceramics from submicron size to 100–200 μm has no effect on the oxygen thermodynamics. The two-electrode coulometric titration technique, based on the alternate use of electrodes for oxygen pumping and e.m.f. measurements, is described and verified by studying oxygen nonstoichiometry of La0.3Sr0.7CoO3−δ and PrOx.  相似文献   

13.
We have studied the phase composition, alignment, and bonding at the Ag/Bi-Sr-Ca-Cu-O (BSCCO) interface for silver-sheathed tapes of both the 2212 and 2223 compositions which have been processed under different conditions. Strong texturing of the (001) planes of the BSCCO phases parallel to the Ag interface was seen, regardless of the Ag orientation. Short facets of non-(001) BSCCO planes accommodate local misalignments between the Ag and the (001) planes of the BSCCO phases. Whether the 2212 composites are processed by complete melting or by sintering below the melting point, there is always a residual half-layer of 2201 at the interface. By contrast, 2223 composites exhibit 2223 layers right up to the interface without any transition phases. Both compounds bond strongly to Ag and cracks, when present, occur preferentially away from the interface.  相似文献   

14.
It is clear that many of the cuprate superconductors are truly superlattices, composed of sheets whose individual superconducting critical temperatures may approach bulk values. Such a cuprate is Bi2Sr2CaCu2O8, often referred to as BSCCO-2212. Scanning tunneling spectroscopy (vacuum tunneling) applied toa-bBiO cleavage planes ofTc≈90 K BSCCO single crystals under liquid helium simultaneously provides topography and local dI/dVspectra (superconducting density of states: DOS). The spectra, which are similar to those obtained from angle-resolved photoemission spectroscopy, confirm a large gap parameter Δ(x,y) on the uppermost layer. The dI/dVspectra do not unequivocally select order parameter symmetry, but are probably consistent with d-wave or anisotropic s-wave states. Spatial variations of Δ on a 100 Å scale are attributed to variation in BiO metallicity, originating in oxygen stoichiometry variations. A model is presented to explain the different dI/dVspectra which are seen, and associated with different local oxygen concentrations. This model, based on the superconducting proximity effect, assumes that in some regions the BiO uppermost layer is insulating and in other regions it is metallic, in the latter case induced superconductivity by proximity to the CuO2planes. Our STM measurements appear to sample only the uppermost half cell of the crystal, and contain no obvious superlattice features. Recent measurements have confirmed Josephson radiation from voltage biasedc-axis pillars of BSCCO. From the point of view of the present work, the superconducting systems which weakly couple along thec-direction to create Josephson junctions are probably half-cell slabs of height 15.4 Å, each containing two CuO2and two BiO layers, which act as single composite electrodes for the Josephson junctions.  相似文献   

15.
BixY3−xFe5O12 thin films have been grown on GGG (Gd3Ga5O12) (1 1 1) substrates by the combinatorial composition-spread techniques under substrate temperature (Tsub) ranging from 410 to 700 °C and O2 pressure of 200 mTorr. In order to study the effect of substrates on the deposition of BixY3−xFe5O12 thin films, garnet substrates annealed at 1300 °C for 3 h were also used. Magneto-optical properties were characterized by our home-designed magneto-optical imaging system. From the maps of Faraday rotation angle θF, it was evident that the Faraday effect appears only when Tsub = 430-630 °C. θF reaches to the maximum value (∼6°/μm, λ = 632 nm) at 500 °C, and is proportional to the Bi contents. XRD and EPMA analyses showed that Bi ions are easier to substitute for Y sites and better crystallinity is obtained for annealed substrates than for commercial ones.  相似文献   

16.
We have studied the stationary Josephson effect on YBa2Cu3O7−δ (Tc=90 K) and Bi2Sr2Ca1Cu2 O8 (Tc=80 Kand 87 Kfortwosamplesofdifferentorigin) ceramic based junctions. The temperature dependence of the critical current near Tc has been found as Ic≈(Tc-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi2Sr2Ca1Cu2O8, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/kBTc3.5.  相似文献   

17.
Ba(Ti1−x,Nix)O3 thin films were prepared on fused quartz substrates by a sol–gel process. X-ray diffraction and Raman scattering measurements showed that the films are of pseudo-cubic perovskite structure with random orientation and the change of lattice constant caused by Ni-doping with different concentrations is very small. Optical transmittance spectra indicated that Ni-doping has an obvious effect on the energy band structure. The energy gap of Ba(Ti1−x,Nix)O3 decreased linearly with the increase of Ni concentration. It indicates that the adjusting of band gap can be achieved by controlling the Ni-doping content accurately in Ba(Ti1−x,Nix)O3 thin films. This has potential application in devices based on ferroelectric thin films.  相似文献   

18.
Thin carbon films have been deposited in vacuum (∼10−4 Pa) on Si substrates by pulsed laser ablation of a graphite target using a Nd:YAG laser operating in the near infrared region (λ=1064 nm). The samples have been deposited at different substrate temperatures (T sub) ranging from room temperature (RT) to 800°C. X-ray diffraction analysis established the progressive formation of nanosized graphene structures as T sub increased. In fact, film structure evolves from almost amorphous to nanostructured phase characterized by graphene layers oriented perpendicularly to the film plane. The film density, evaluated by X-ray reflectivity measurements, is strongly affected by T sub. At RT the film density is similar to the graphite one, while it decreases at higher T sub. The electrical properties of the samples have been characterized by field emission measurements. The parameters describing the emitter properties (threshold field E th and field enhancement factor β) have been evaluated using variable anode-to-cathode distance method. Samples deposited at low T sub have shown the best emission properties, presenting lower E th and larger β values than those deposited at higher T sub. This is mainly attributed to the sensible density variation, which is in competition with the slighter augment of mean nanoparticle size.  相似文献   

19.
Molybdenum oxide thin films were produced by magnetron sputtering using a molybdenum (Mo) target. The sputtering was performed in a reactive atmosphere of an argon-oxygen gas mixture under varying conditions of substrate temperature (Ts) and oxygen partial pressure (pO2). The effect of Ts and pO2 on the growth and microstructure of molybdenum oxide films was examined in detail using reflection high-energy electron diffraction (RHEED), Rutherford backscattering spectrometry (RBS), energy-dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) measurements. The analyses indicate that the effect of Ts and pO2 on the microstructure and phase of the grown molybdenum oxide thin films is remarkable. RHEED and RBS results indicate that the films grown at 445 °C under 62.3% O2 pressure were stoichiometric and polycrystalline MoO3. Films grown at lower pO2 were non-stoichiometric MoOx films with the presence of secondary phase. The microstructure of the grown Mo oxide films is discussed and conditions were optimized to produce phase pure, stoichiometric, and highly textured polycrystalline MoO3 films.  相似文献   

20.
The millimeter-wave spectrum of 2,3-dihydrofuran in the ground and five ring-puckering excited states has been measured in the frequency range 100–250 GHz. The ground and first ring-puckering excited states have been fitted to a two-state Hamiltonian including Coriolis coupling interaction. The determined energy difference of 18.684(7) cm−1between these states and theaandbtype coupling parameters are consistent with the ring-puckering potential function and the previously observed dependence of the centrifugal distortion constants ΔJK, ΔK, and δK. A small ring-puckering dependence of the quartic centrifugal distortion constants ΔJand δJhas been also observed. This dependence is well accounted for in terms of the ring-puckering potential function and the vibrational dependence of the rotational constants.  相似文献   

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