Superconducting cluster in YBa2Cu3O7−x ceramic

The method of point laser heating of a sample is used to perform experimental investigations of the electrical conductivity of YBa₂Cu₃O_7−x ceramic in the vicinity of its critical state. It is found that, in the vicinity of transition to the superconducting state, the electrical conductivity of the ceramic exhibits a clusterlike behavior substantially nonuniform over the sample cross section. The topology of a superconducting cluster is investigated, as well as its spatial localization in the sample. A model of the formation of a superconducting cluster in a ceramic superconductor is suggested.     

Shock wave working of high-TC superconductors

Abstract

The results of investigating the processes of explosive compaction of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O powdered ceramic high temperature superconducting materials and the shock-induced variation of their properties are presented. The possibility for obtaining metal-ceramic cylinder and spiral-shaped articles is illustrated. The shock wave loading of sintered ceramics is shown to result in degrading its superconducting properties up on R(T), which are subsequently restored by low-temperature annealing. The loss of superconducting properties is demonstrated to correlate with the X-ray lines width. In case of Bi-4334 ceramic, we have observed the improving SP-properties on susceptibility just after shocking it. The effect of the compaction pressure, initial particle size, atmosphere (air, O₂, N₂), initial temperature on material properties is studied.     

当前功能陶瓷发展动向

简要介绍了功能陶瓷包括电容介质，陶瓷封装，压电瓷，透明铁电陶瓷，铁电薄膜，变阻器，热敏电阻，超导瓷及传感器瓷的最近进展，讨论了陶瓷学和固体物理以及工艺之间的相互作用和渗透，它们将对功能陶瓷的未来发展产生深远的影响。     

Interphase phenomena in superconductive polymer-ceramic nanocomposites

Dynamic mechanical properties (elastic moduli, phase angle) for superconducting (SC) polymer–ceramic composites based on Y₁Ba₂Cu₃O_7?x SC oxide ceramic and superhighmolecular polyethylene have been investigated. The analysis of the obtained data shows a strong interaction of the polymeric binder with the surface of the ceramic grains. It is concluded that changes of packing and structure of the macromolecules occur at the ceramic–polymer interface. This is confirmed by melting enthalpy measurements of SC polymer–ceramic composites of different filler content. Scanning electron microscopy studies of the high temperature SC composites showed that the ceramic grains are evenly covered by the binder for both amorphous and crystalline polymers. EPR (electron paramagnetic resonance) spectra of polymer–ceramic composites have shown that the intensity of the EPR signals of Cu²⁺(1) depends on the nature and the content of binder. The Mn, Co, Zn, Ni containing superconducting composites have been obtained by frontal polymerization.     

Probing the ground state properties of iron-based superconducting pnictides and related systems by muon-spin spectroscopy

In this short review, we attempt to give a comprehensive discussion of studies performed to date by muon-spin spectroscopy (more precisely the relaxation and rotation technique, also know as μSR) on the recently discovered layered iron-based superconductors. On one side, μSR has been used to characterize the magnetic state of different families of layered iron-based systems. Similarly the subtle interplay of the magnetic state and the structural transition present in some families has been investigated. We will also discuss the information provided by this technique on the interaction between the magnetic state and the superconducting phase. Finally the μSR technique has been used to investigate the magnetic penetration depth of the superconducting ground state. The study of its absolute value, temperature and magnetic field dependence provides crucial tests for investigating possible unconventional superconducting states in such systems.     

Specific features of magnetic field penetration into granular superconductors and possible sensitivity of magnetic sensors based on them

The results of experimental and theoretical studies of the magnetic properties of a superconducting ceramic with the composition YBa₂Cu₃O_{7 ? x} are presented. The limiting sensitivity of magnetic field sensors on the basis of a high-temperature superconducting ceramic is discussed.     

Modification of several physicochemical properties of a high-temperature superconducting yttrium ceramic upon doping with gold

The influence of additions of gold (5 and 10 at. %) on the lattice parameters, the superconducting transition temperature, and the diamagnetic susceptibility of the high-temperature superconducting ceramic YBa₂Cu₃O_7−δ is investigated. The influence of gold on the kinetics of the release of weakly bound oxygen during heating of the ceramic in a vacuum is also studied. It is shown that doping the ceramic with gold causes uniaxial expansion of the lattice of the material, a significant decrease in the diamagnetic susceptibility, and nonmonotonic variation of the kinetic parameters of the oxygen release, while the superconducting transition temperature remains nearly unchanged. Zh. Tekh. Fiz. 67, 31–34 (February 1997)     

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi_3.9Pb_0.1Sr₃Ca₃Cu₄O _x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E _g) has also been made from the linear fit ofα ² vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.     

Transport properties of NdBa2Cu3O6+x ceramics at the edge of the superconducting region upon a decrease in carrier density as a result of oxygen disordering in the Cu-O x planes

An initially nonsuperconducting ceramic sample with the composition NdBa₂Cu₃O_6+x is brought, by means of pressure and quenching, to a state with a high carrier density and a superconducting transition, after which it is returned to the initial state by gradual annealing in several steps. The evolution of the magnetoresistance of the sample showed that even in the most resistive state realized in the experiment the superconducting interaction influences the resistance of the sample at fields all the way up to 5–6 T. In an 8 T field the change in resistance in this state in the temperature interval from 0.4 K to 20 K is described well by a logarithmic law ΔR∝ logT. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 475–480 (25 March 1997)     

The effect of heat treatment on explosively compacted (Y---Ba---K---Cu---O) superconductive powders

A ceramic powder mixture corresponding to the stoichiometric ratio of YBa_1.95K_0.05Cu₃O_y superconducting ceramic with 10% w/w Ag was heat treated at 950°C for 3 h in flowing oxygen. The resultant superconducting powder was consolidated in a silver tube by the explosive compaction technique (pre-heat treated ceramic). The same fabrication technique, i.e. explosive compaction, was employed to the initial stoichiometric powder mixture, without prior heat treatment and the green product was subsequently subjected to a series of heat-treating cycles (920°C, 70 h), in order to obtain the required superconducting properties (post-heat treated ceramic). The microstructure and physical properties of the pre- and post-heat treated ceramics were analyzed and compared using various characterization techniques. X-ray diffraction performed on the pre-heat and post-heat treated ceramics indicated that maximum 123-phase content retained in the former case (pre-heat treated ceramic), whilst DC magnetic susceptibility measurements have shown the characteristic Meissner shielding effect at 99 and 92 K, respectively. Furthermore, the pre-heat treated compacted billet was characterized by a critical current density, J_c equal to 2800 A/cm² at 77 K after suitable annealing at 850°C for 10 h in oxygen stream.     

Interesting normal state and superconducting properties of the intermediate valence compound CeRu2

Superconductivity in CeRu₂ was discovered 40 years ago, and was extensively studied because alloying with magnetic elements showed the coexistence of superconductivity and magnetic order. The normal state of CeRu₂ has been of interest because of its intermediate valuence character. The superconducting state has been studied extensively because of its paramagnetic nature and anomalous pinning properties. This review presents the present status of knowledge, and discusses the puzzling features of CeRu₂.     

The effect of heat treatment on explosively compacted (YBaKCuO) superconductive powders

A ceramic powder mixture corresponding to the stoichiometric ratio of YBa_1.95K_0.05Cu₃O_y superconducting ceramic with 10% w/w Ag was heat treated at 950°C for 3 h in flowing oxygen. The resultant superconducting powder was consolidated in a silver tube by the explosive compaction technique (pre-heat treated ceramic). The same fabrication technique, i.e. explosive compaction, was employed to the initial stoichiometric powder mixture, without prior heat treatment and the green product was subsequently subjected to a series of heat-treating cycles (920°C, 70 h), in order to obtain the required superconducting properties (post-heat treated ceramic). The microstructure and physical properties of the pre- and post-heat treated ceramics were analyzed and compared using various characterization techniques. X-ray diffraction performed on the pre-heat and post-heat treated ceramics indicated that maximum 123-phase content retained in the former case (pre-heat treated ceramic), whilst DC magnetic susceptibility measurements have shown the characteristic Meissner shielding effect at 99 and 92 K, respectively. Furthermore, the pre-heat treated compacted billet was characterized by a critical current density, J_c equal to 2800 A/cm² at 77 K after suitable annealing at 850°C for 10 h in oxygen stream.     

Studies of irreversible magnetization in superconductors—a review

Magnetic measurements in the superconducting state of the high temperature superconductors have been characterized by the feature of irreversibility. Similar effects have been known in the conventional type II superconductors for about three decades now, and have been studied in great detail during the last few years. Recent studies of magnetic irreversibilities, in both conventional and high temperature superconductors, will be reviewed here. Thermally-activated relaxation accompanies such irreversibilities, and studies on flux-creep will also be reviewed. This review shall cover the measurement of isothermal magnetization curves, of ac susceptibility, of thermo-magnetic history effects in the magnetization at a particular field and temperature, and of flux creep. An understanding of these in terms of Bean’s celebrated macroscopic model shall be discussed. We shall also cover measurements that confirm the existence of weak links in ceramic high-temperature, as well as in conventional multifilamentary, superconductors.     

Local moment spin resonance in a superconductor

The ESR of Gd in CeRu₂ and LaRu₂ has been observed in the normal and superconducting state. The temperature dependence of the linewidth in the superconducting state follows a trend expected from NMR measurements in superconductors.     

Effect of silver content on the mechanical and electrical properties of the YBaCuO/Ag ceramic

Doping the YBaCuO superconducting ceramic with 3–5 wt % silver is shown to substantially increase its microplasticity and strength. Temperature spectra of microplastic strain rates for YBaCuO/Ag ceramics with 0, 1.6, 2.9, 4.2, and 7.6 wt % silver were obtained. The structure of the spectra and the relation of one of the peaks to the superconducting transition are discussed. The specific features of the I–V characteristics of samples cooled in a magnetic field are considered. It is shown that the position of the rising branch of a I–V characteristic depends on its origin; this observation can be used to fix two or more stable states of an HTSC-based memory cell.     

Thermodynamic properties of superconducting nuclear matter

Phase diagrams of superconducting nuclear matter are calculated by solving a set of finite temperature gap equations, using several Skyrme effective interactions. Our results indicate that nuclear matter may have a superconducting phase in a small region with density near one half of the normal nuclear matter density and temperature k_BT ≲ 1.4 MeV. Our calculation is based on a finite temperature Green's function method with an abnormal pair cutoff approximation. The same approximation is employed in deriving the internal energy, entropy and chemical potential of superconducting nuclear matter. In this way, its equation of state is obtained, and compared with that of normal nuclear matter. The energy gap of superconducting nuclear matter is found to depend rather sensitively on both density and temperature. This dependence is analysed in terms of the Skyrme interaction parameters. The correlation effect on chemical potential is found to be important at high density, and its inclusion is essential in determining the equation of state of superconducting nuclear matter.     

Microwave surface resistance measurement in superconducting pellets

Microwave surface resistance measurement constitutes one of the most advantageous procedures for the investigation and characterization of the new superconducting oxides. Keeping in mind that the bulk ceramic materials usually have cylindrical symmetry, a new automatic measuring system, based on a resonant cavity, has been developed with special emphasis on superconducting pellets. The resolution capability of our system is pointed out by measuring the surface resistance of YBCO and BSCCO samples.     

Current-voltage characteristics of quasi-one-dimensional superconductors: an S-shaped curve in the constant voltage regime

Applying a constant voltage to superconducting nanowires we find that its I-V characteristic exhibits an unusual S behavior. This behavior is the direct consequence of the dynamics of the superconducting condensate and of the existence of two different critical currents: j(c2) at which the pure superconducting state becomes unstable and j(c1)相似文献   

Thermal stabilization of the resistive states of superconducting composites: Quasilinear approximation

The conditions of the occurrence and development of thermal instabilities in the composite superconductor with a continuously increasing current-voltage characteristic, which is described by the power equation, have been studied. The conditions for thermal stabilization have been analyzed in the general form using dimensionless variables that keep their invariance when varying. For the local temperature disturbance, the critical energies and velocities of its irreversible propagation have been calculated. It has been proved that composites superconductors can have stable states, when the ultimate currents can be higher or lower of the conventionally preset critical current of the composite. Furthermore, superconductivity destruction at supercritical currents takes place not in the form of a stepwise transition from the superconducting to normal state, but due to the formation of thermal and electric switching waves that propagate along the composite superconductor with a constant speed. The condition for full thermal stabilization has been formulated for the superconducting composites with a power current–voltage characteristic. The results of the numerical experiments have proved that the existing theory of thermal stabilization, which assumes a stepwise superconducting–normal transition, leads to the considerable limitation of the range of the stable currents, at which a superconducting state can be kept.     

Investigation of the high-temperature superconductor Tl−Ca−Ba−Cu−Fe−O in a wide range of temperature and iron concentration

A series of the composition Tl₂Ca₂Ba₂Cu_3?x Fe _x O_δ with 0≦x≦0.1 were prepared by the usual ceramic method. The superconducting behaviour was investigated by electrical conductivity and Mössbauer effect. X-ray diffraction on the samples was also used to study the different phases. The study revealed a superconducting transition temperature about 100 K for samples withx=0.00 and above 80 K forx=0.02 and 0.04. From the Mössbauer effect spectra measured for different compositions and at different temperatures, it is evident that the Fe ions substituted Cu into different sites. A clear evidence of vibration anisotropy is observed, together with a low spin state for each position. These results are discussed on the basis of the structure and electronic state of the compounds.