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A misalignment of anisotropic crystallites causes small values of anisotropy and decreases the critical current density of textured polycrystalline superconductors. To relate the crystallite misalignment and out-plane anisotropy, the magnetic properties of the textured Bi2223 polycrystalline superconductor were investigated. A distribution of orientation angles of crystallites was determined using different data: scanning electron microscopy images and hysteresis magnetization loops when an external magnetic field was applied at different angles with respect to the texturing plane of the sample. It was demonstrated that the standard deviation of the distribution and the magnetic disorder angle of crystallites in textured samples can be determined from the magnetization data in perpendicular directions. These data may be either the irreversible magnetization measured for two different orientations of the sample or the simultaneously measured magnetization projections parallel and perpendicular to the magnetic field., 11 pages, 6 figures

The article describes the issues of the operation of semiconductor regulators-stabilizers of alternating voltage for distribution networks (PRSPN). The relevance of using this class of devices is formulated. An approach to the PRSPN construction based on the use of a booster transformer and a thyristor commutator is indicated. The functional capabilities of the implemented PRSPN are shown on the example of a commercial prototype. The results of the PRSPN experimental-industrial operation performed in the electrical power networks of MRSK Tsentra PJSC — Belgorodenergo (subsidiaries and associates of Rosseti PJSC) confirmed the efficiency of the device and its high technical and economic indicators. The effects of the introduction of the device, as well as the prospects for its development and application are demonstrated.

Polycrystalline Y1−xNdxBa2Cu3O7−δ (x=0.02, 0.11, and 0.25) superconductors are synthesized. Nd atoms are uniformly distributed over grains. The magnetization loops of the samples have a pronounced second peak in a wide temperature range. The magnetization data are analyzed using the extended critical state model. It is found that the order-disorder transition of the vortex lattice is affected by doping with neodymium and temperature; the second-peak field and width decrease monotonically with increasing x value. The undoped polycrystalline YBa2Cu3O7−δ compound is assumed to exhibit the peak effect in higher magnetic fields.

Copper oxide nanoparticles were produced by direct plasmachemical synthesis in a plasma arc discharge of low pressure. The formation of CuO nanoparticles with an average size of 12 nm and narrow size distribution intervals was determined by using the x-ray diffraction analysis and TEM microscopy methods. It was defined by using a vibration magnetometer and a SQUID magnetometer, that the magnetic properties of CuO nanoparticles with such size were extremely different from the magnetic properties of bulk antiferromagnetic CuO. Structural defects caused the formation of a ferromagnetic state, remaining at least up to the room temperature. The temperature of corresponding antiferromagnetic ordering was significantly decreased (down to ∼ 100 K). Meanwhile, some of the copper surface spins showed a spin-glass behavior at low temperatures.

The influence of a pressure of gas mixture (10 vol% O 2 + 90% N 2 ) on an average size of copper oxide nanoparticles, produced in the plasma of low pressure arc discharge, has been studied as a basic process variable. A correlation between the dependence of average particle size on gas mixture pressure and the dependence of discharge gap voltage on product of interelectrode distance by a gas mixture pressure, has been found. The estimation was carried out by means of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). A mathematical model of the cathode region, which shows the applicability of the similarity theory to the low pressure arc discharge, has been represented.

The temperature and time dependences of the magnetization of copper oxide nanoparticles 8, 13, and 18 nm in size have been investigated. Specific features of the behavior of CuO nanoparticles formed by vacuum plasma-arc synthesis as compared to other antiferromagnetic particles have been revealed. It has been shown that the bifurcation of magnetization curves upon cooling in the zero (ZFC) and nonzero (FC) magnetic fields occurs above the Neel temperature, while the usual peak of the magnetization curve is absent in the ZFC mode. The problems associated with the nonequilibrium behavior of synthesized CuO nanoparticles have been discussed.

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi1.8Pb0.3Sr1.9Ca2Cu3Ox have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.

The potentiometric titration of water extracts from the upper horizons of taiga-zone soils by salt solutions of heavy metals (Pb, Cu, and Zn) showed that their addition is an additional source of the extract acidity because of the involvement of the metal ions in complexation with water-soluble organic substances (WSOSs). At the addition of 0.01 M water solutions of Al(NO3)3 to water extracts from soils, Al3+ ions are also involved in complexes with WSOSs, which is accompanied by stronger acidification of the extracts from the upper horizon of soddy soils (with a near-neutral reaction) than from the litter of bog-podzolic soil (with a strongly acid reaction). The effect of the Al3+ hydrolysis on the acidity of the extracts is insignificantly low in both cases. A quantitative relationship was revealed between the release of protons and the ratio of free Cu2+ ions to those complexed with WSOSs at the titration of water extracts from soils by a solution of copper salt.

Y 0.75 Nd 0.25 Ba 2 Cu 3 O 7−d superconductor has the peak effect with pronounced fishtail feature on magnetization hysteresis. The magnetic field of the peak effect maximum is in ~4 times higher than one of NdBa 2 Cu 3 O 7−d . The magnetization hysteresis is tilted anticlockwise due to paramagnetic Nd 3+ ions occurred on the surface of grains and in the vortex cores.

This article presents indicators of the influences of salts of iron, mercury, and copper on the acid- base properties of aqueous extracts from the upper soil horizons of taiga zones. Their acidification is estab- lished; under the influence of metals, the processes responsible for elevated levels of protons in the extracts are identified, viz., the hydrolysis of metal ions and their inclusion in the complexation with water-soluble organic substances. We discovered the effects on these processes of organic materials of the extracts, as well as of the properties of metals and the type and amount of salts, in whose composition the metals are included in the extracts.

The current-voltage characteristics of a porous superconductor Bi2Sr2Ca2Cu3Ox (Bi2223) have been measured at temperatures in the range from 10 to 90 K in magnetic fields of 0–80 kOe. The experimental dependences have been analyzed within the model allowing for pinning by clusters of a normal phase with fractal boundaries, as well as the model taking into account phase transformations of vortex matter. It has been found that the electrical resistance of the superconductor material significantly increases at temperatures of 60–70 K over the entire range of magnetic fields under consideration without changing in the sign of the curvature of the dependence R(I). It has been assumed that this behavior is associated with the specific feature of the pinning in a highly porous high-temperature superconductor, which lies in the fractality of the distribution of pinning centers in a wide range of self-similarity scales. The studied material at the aforementioned temperatures is characterized by the melting of the vortex structure.

The influence of thermomagnetic prehistory on the behavior of a resistive transition R(T) in external magnetic fields of polycrystalline YBa2Cu3O7 and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature supercon-ductors and the Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag texture has been investigated. It has been found that, for YBa2Cu3O7, the thermomagnetic prehistory exerts a substantial influence on the dissipation in the subsystem of grain boundaries in magnetic fields up to ∼103 Oe, and this effect becomes insignificant in fields higher than ∼104 Oe. This behavior has been explained by the influence of magnetic moments of high-temperature superconductor grains on the effective magnetic field in the intergranular medium. For bismuth high-temperature superconductors, no influence of thermomagnetic prehistory on the resistive transition has been observed; however, this effect manifests itself in current-voltage characteristics at high transport current densities. There is also a radical difference in the behavior of isotherms of the magnetoresistance R(H) for the yttrium and bismuth systems. For YBa2Cu3O7, there is a clear separation between the dissipation regimes in the intergranular medium and in grains, which manifests itself even at low transport current densities as a change of sign in the curvature of the dependence R(H). For a texture based on the bismuth high-temperature superconductor, this feature has been observed only at high current densities (comparable to the critical current density at H = 0). This difference in the behavior of magnetoresistive properties of the classical high-temperature superconductor systems under investigation has been explained by relatively low irreversibility fields of the bismuth high-temperature superconductors. In these materials, simultaneous processes of dissipation can occur in an external magnetic field both in the subsystem of grain boundaries between crystallites and in the crystallites themselves.

In order to clarify the mechanisms in charge of broadening of resistive transition R(T) in magnetic fields of bismuth-based polycrystalline high-T C superconductor (HTSC), a comparative study of Bi1.8Pb0.3Sr1.9Ca2Cu3O x (BSCCO) and YBa2Cu3O7−δ (YBCO) have been performed. Magnetoresistive effects and irreversibility line obtained from magnetic measurements have been studied. It was established that (1) for YBCO, the smooth part of R(T) dependence unambiguously corresponds to dissipation in the intergrain boundaries for arbitrary magnetic fields; (2) for polycrystalline BSCCO, the smooth part of R(T) dependences correspond to dissipation within intergrain boundary subsystem in the field range H 102 Oe reflect the dissipation processes occurring both in intergrain boundary and HTSC grain subsystems; (3) for the high-field range, the contribution from intergrain boundaries of BSCCO can be distinguished from magnetoresistance R(H) dependences obtained at high enough current density on textured samples. It is proposed that various magneto-resistive properties of these classical HTSC systems are due comparatively weak pinning in BSCCO.

The effect of magnetically driven high-frequency rectification in a polycrystalline La0.7Ca0.3MnO3 manganite has been measured at different frequencies of microwave radiation. The magnetic field dependence of a rectified voltage has a broad peak resembling an absorption line, whose shape and position are determined by the radiation frequency. The rectification effect in a polycrystalline manganite sample is related to a ramified network of magnetic tunnel junctions, which is formed by ferromagnetic conducting grains with insulator boundaries. The results of measurements are consistent with a model for the magneto-dependent rectification effect based on the interplay between a spin-polarized current through the tunnel junctions and magnetic resonance induced in the grains forming the junctions.

The transport properties (R(T) and R(H) dependences at various values of the transport current in magnetic fields up to 65 kOe) and low-temperature heat capacity in magnetic fields up to 90 kOe of the BaPb0.75Bi0.25O3 superconductor (TC ≈ 11.3 K) are investigated with the goal of clarifying the mechanisms determining the nonmonotonic behavior and hysteresis of its magnetoresistance R(H). The type of R(H) hysteretic dependences for BaPb0.75Bi0.25O3 is analogous to that observed in granular high-Tc superconductors (HTSCs); however, unlike classical HTSC systems, the field width of the magnetoresistance hysteresis loop for polycrystalline BaPb0.75Bi0.25O3 depends on the transport current. This means that although the mechanisms responsible for the magnetoresistance hysteresis (the influence of the magnetic flux trapped in superconducting regions on the effective field in Josephson interlayers) are identical in these objects, the transport current in BaPb0.75Bi0.25O3 may considerably affect the diamagnetic response of the superconductor. A considerable effect of transport current on the field in which the R(H) dependences have a peak and exhibit hysterestic properties is observed. Such a behavior can be adequately interpreted using the model of the spatially inhomogeneous superconductor-insulator state proposed by Gorbatsevich et al. [JETP Lett. 52, 95 (1990)]. The nonmonotonic dependence of quantity C/T (C is the heat capacity) on the magnetic field discovered in the present study also agrees with the conclusions based on this model.

Magnetoresistance of bulk textured Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag ceramics has been studied in the magnetic fields applied parallel and perpendicular to a–b planes of Bi2223 crystallites. Besides well known anisotropy of magnetoresistance of textured superconductors (RH || c > RH || a–b), anisotropic hysteresis of R(H) dependences was investigated. Parameters characterizing hysteretic R(H) curves differ for the cases H || c and H || a–b. This behavior is explained within the model of a granular superconductor where the total magnetic induction in the intercrystallite boundaries is superposition of the external field and the magnetic field induced by dipole magnetic moments of neighbor crystallites.

Low-density polycrystalline Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature superconductors with a foamlike microstructure and composites consisting of this superconductor and silver in an amount of 20, 25, and 30 vol % are synthesized. The microstructure, as well as the temperature and field dependences of the magnetization, M(T) and M(H), are studied. It is found that, in Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature superconductors and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag composites, the diamagnetic response is enhanced and the screening properties are improved compared with high-temperature polycrystalline superconductors with the same composition that are prepared by the standard technology. The observed effect is explained by the features of magnetic flux penetration into a porous medium.

The field dependences of the magnetoresistance of Bi1.8Pb0.3Sr1.9Ca2Cu3Ox samples with different densities, which have a foam microstructure and exhibit different diamagnetic responses, were studied at 77.4 K to identify the mechanism responsible for the formation of a negative magnetoresistance region in granular high-temperature superconductors. A region with negative magnetoresistance was found to exist in samples with magnetizations highest in absolute magnitude. This behavior finds a reasonable interpretation as due to the effect exerted by dipole moments of high-temperature superconductor grains on the effective intergranular field. The strength of this effective field has been estimated.

The methods of Mossbauer spectroscopy, magnetic measurements, chemical analysis, and thermodynamic calculation of the phase composition were used in systematic studies of oxide iron–aluminosilicate (SiO 2 –Al 2 O 3 –Fe 2 O 3 ) system in a wide range of its macrocomponent composition: 0.6–65.6 wt.% SiO 2 ; 0.05–20.5 wt.% Al 2 O 3 ; 4.6–92.5 wt.% Fe 2 O 3 . These correspond to the composition of magnetic microspheres and cenospheres from fly ashes originated from powdered-coal combustion of five varieties of energy coals. It was established that the main phase governing the magnetic properties is magnetite with a defective structure. The microspheres saturation magnetization increases with the iron content range of 4.6–80.0 wt.% Fe 2 O 3 is caused by the increase of the spinel phase content. The magnetization reduction in ferruginous microspheres with the high iron content (83.0–92.5 wt.% Fe 2 O 3 ) is the result of the hematite–magnetite displacement.

The hysteretic behavior of the magnetoresistance R(H) of granular high-temperature superconductors (HTSCs) of the Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and La-Sr-Cu-O classical systems is investigated for transport current densities lower and higher than the critical density (at H = 0). All systems exhibit universal behavior of the width of the magnetoresistance hysteresis loop: independence of transport current under identical external conditions. This means that flux trapping in HTSC grains is the main mechanism controlling the hysteretic behavior of the magnetoresistance of granular HTSCs, while pinning of Josephson vortices in the intragranular medium makes no appreciable contribution to the formation of magnetoresistance hysteresis (when transport current flows through the sample). Experimental data on relaxation of residual resistance after the action of a magnetic field also confirm this conclusion.

(abridged) In this paper we derive observed and modelled shape parameters (apparent ellipticity and orientation of the ellipse) of 650 Galactic open clusters identified in the ASCC-2.5 catalogue. We provide the observed shape parameters of Galactic open clusters, computed with the help of a multi-component analysis. For the vast majority of clusters these parameters are determined for the first time. High resolution ("star by star") N-body simulations are carried out with the specially developed $\phi$GRAPE code providing models of clusters of different initial masses, Galactocentric distances and rotation velocities. The comparison of models and observations of about 150 clusters reveals ellipticities of observed clusters which are too low (0.2 vs. 0.3), and offers the basis to find the main reason for this discrepancy. The models predict that after $\approx 50$ Myr clusters reach an oblate shape with an axes ratio of $1.65:1.35:1$, and with the major axis tilted by an angle of $q_{XY} \approx 30^\circ$ with respect to the Galactocentric radius due to differential rotation of the Galaxy. Unbiased estimates of cluster shape parameters require reliable membership determination in large cluster areas up to 2-3 tidal radii where the density of cluster stars is considerably lower than the background. Although dynamically bound stars outside the tidal radius contribute insignificantly to the cluster mass, their distribution is essential for a correct determination of cluster shape parameters. In contrast, a restricted mass range of cluster stars does not play such a dramatic role, though deep surveys allow to identify more cluster members and, therefore, to increase the accuracy of the observed shape parameters., Comment: 13 pages, 12 figures, accepted for publication in Astronomy and Astrophysics

Tidal tails of star clusters are not homogeneous but show well defined clumps in observations as well as in numerical simulations. Recently an epicyclic theory for the formation of these clumps was presented. A quantitative analysis was still missing. We present a quantitative derivation of the angular momentum and energy distribution of escaping stars from a star cluster in the tidal field of the Milky Way and derive the connection to the position and width of the clumps. For the numerical realization we use star-by-star $N$-body simulations. We find a very good agreement of theory and models. We show that the radial offset of the tidal arms scales with the tidal radius, which is a function of cluster mass and the rotation curve at the cluster orbit. The mean radial offset is 2.77 times the tidal radius in the outer disc. Near the Galactic centre the circumstances are more complicated, but to lowest order the theory still applies. We have also measured the Jacobi energy distribution of bound stars and showed that there is a large fraction of stars (about 35%) above the critical Jacobi energy at all times, which can potentially leave the cluster. This is a hint that the mass loss is dominated by a self-regulating process of increasing Jacobi energy due to the weakening of the potential well of the star cluster, which is induced by the mass loss itself., 14 pages, 17 figures; accepted by MNRAS

The time evolution of the magnetoresistance of bulk YBCO + CuO composites at T = 4.2 K in constant applied magnetic fields was studied to clarify the mechanism of hysteretic behavior of magnetoresistance R ( H ) of granular HTSC. The composites represent “model” granular HTSC with weakened Josephson coupling between superconducting (YBCO) crystallites. It was found for the first time that on the ascending branch of R ( H ) dependence, the resistance at H = const decreased with time while on the descending branch, the resistance increased with time in an applied constant magnetic field. In the range of low magnetic fields (below the minimum point of the descending branch of the R(H) dependence), the resistance at H = const decreased again. Similar measurements performed on pure polycrystalline YBCO at T = 77.4 K have shown that the behavior of evolution of resistance with time is similar to that observed for the composite. This proves the peculiarity of time evolution of magnetoresistance to be a common feature of granular HTSCs. The behavior revealed is well described by the model of granular HTSC, where the intergrain media is in an effective magnetic field which is the superposition of the external field and the field induced by superconducting grains. The time evolution of resistance reflects the time relaxation of magnetization of HTSC grains due to the intragrain flux creep processes.

The hysteresis of magnetoresistance R(H) and relaxation of the remanent resistance R rem with time after magnetic field treatment of HTSC (Y-Ba-Cu-O) + CuO composites are studied. Such a composite constitutes a network of Josephson junctions wherein the nonsuperconducting component (CuO) forms Josephson barriers between HTSC grains. By comparing the experimental R rem(t) and R(H) dependences, it is shown that the relaxation of the remanent resistance is caused by the decreased magnetic field in the intergrain medium due to relaxation of magnetization. The reason is uncovered for the differences between the published values of pinning potentials determined by measuring the relaxation of magnetization or resistance and fitting them by the Anderson law.

The broadening of the resistive transition in magnetic field and isotherms of magnetoresistance of bulk composites Y–Ba–Cu–O + CuO have been studied. These composites exhibit large magneto-resistive effect in a wide temperature range below TC due to weakening of Josephson coupling in this system. The broadening of the resistive transition and magnetoresistance are explained well by the Ambegaokar–Halperin (AH) model for phase slip in Josephson junctions. The magnetic field dependence of pinning potential in the intergrain boundaries deduced from AH model found out to be similar to that of critical current of an array of Josephson junctions. The values of pinning energy point out that the large magneto-resistive effect observed in the composites results from flux flow-like processes at the intergrain boundaries.

Hysterestic behavior of the magnetoresistance of granular HTSCs and its interaction with the magnetic hysteresis are studied by measuring magnetoresistance R(H) and critical current Ic(H) of composites formed by HTSC Y0.75Lu0.25Ba2Cu3O7 and CuO. A network of Josephson junctions is formed in such composites, in which the nonsuperconducting component plays the role of barriers between HTSC grains. Hysteretic dependences R(H) of magnetoresistance are studied in a wide range of transport current density j and are analyzed in the framework of the two-level model of a granular superconductor, in which dissipation takes place in the Josephson medium and the magnetic flux can be pinned both in grains and in the Josephson medium. The interrelation between the hysteresis of critical current Ic(H) and the evolution of the hysterestic dependence R(H) of the magnetoresistance upon transport current variation is demonstrated experimentally. The effect of the magnetic past history on the hysteretic behavior of R(H) and the emergence of a segment with a negative magnetoresistance are analyzed. It is shown for the first time that the R(H) dependences are characterized by a parameter that is independent of the transport current, viz., the width of the R(H) hysteresis loop.

A new method of fabricating textured ceramics with a composition of Bi1.8Pb0.3Sr1.9Ca2Cu3Ox is described, according to which the initial low-density ceramics is subjected to uniaxial pressing in a liquid medium followed by drying and annealing. Analysis of the X-ray diffraction and scanning electron microscopy data show evidence for a high degree of texture in high-temperature superconductor ceramics fabricated using the proposed method. The results of magnetic measurements indicate that the obtained material possesses anisotropy of the magnetic moment for the magnetic field H oriented parallel and perpendicular to the direction of uniaxial pressing. The textured ceramics also show high diamagnetic response for H parallel to the c axis of grains, which makes these ceramics promising materials for practical applications.

The magnetoresistive properties of bulk YBCO + CuO and YBCO+BaPb0.75Sn0.25O3 composites for different orientations of external magnetic field H and macroscopic transport current j have been measured. These composites exhibit large magnetoresistance in weak magnetic fields (

The transport properties of a novel foamed superconductor Bi1.8Pb0.3Sr1.9Ca2Cu3Ox have been studied. As the analysis of resistive transitions in a magnetic field shows, the dissipation follows a thermally activated flux creep model with the temperature-independent pinning potential in S–N–S-type Josephson junctions.

The temperature dependences of the critical current of Y3/4Lu1/4Ba2Cu3O7 polycrystalline high-temperature superconductors subjected to annealing for different times and those of Y3/4Lu1/4Ba2Cu3O7 + BaPbO3 composites were measured. The results obtained were analyzed in terms of the Gunsenheimer-Schussler-Kummler theory considering the Andreev reflection of carriers in a “superconductor-normal metal-superconductor” (S-N-S) junction. Good agreement between the experimental and theoretical temperature dependences of the critical current over a wide temperature range for polycrystals with both natural and artificially created boundaries (composites) made it possible to estimate the effective length of grain boundaries in the high-temperature superconductors. It was revealed that the critical current density measured at 4.2 K is an exponential function of the length of grain boundaries in samples of both types.

The application of magnetic cenospheres including ferrospinel nanoparticles as a “transport container” in medicine and biology is quite attractive. Their nanoscale size, high porosity, and well pronounced magnetic properties provide good consumer choice. In this contribution, we report on the magnetic properties of microspheres from energetic ashes of Ekibastuz coals.

Phase composition of six narrow size fractions of microspheres extracted from fly ashes of Ekibastuz coals has been determined by Mossbauer spectroscopy. The basic phase is defect magnetite substituted by Al, Mg, and Ti ions. The magnetic properties of microspheres depend on the content of iron and its distribution over spinel positions.

The preliminary results of the investigation of magnetic properties of a novel low-density superconducting material Bi1.8Pb0.3Sr2Ca2Cu3Ox with a low density and microfoam structure are presented. The temperature, magnetic field, and time dependences of magnetization M have been measured. The experimental hysteresis loops M(H) are well described in the framework of the Val’kov-Khrustalev theory developed for type II granular superconductors.

This paper reports on the results of an investigation into the influence of magnetic fields (0–60 kOe) on the temperature dependences of the electrical resistance R(T) of the Y3/4Lu1/4Ba2Cu3O7 + CuO composites. The structure of these composites is considered to be a network of tunnel-type Josephson junctions in which a nonsuperconducting component (CuO) forms boundaries (barriers) between high-temperature superconducting crystallites. The temperature dependence R(T) of the composites has two steps characteristic of granular superconductors: (i) an abrupt change in the electrical resistance at the critical temperature of high-temperature superconducting crystallites and (ii) a smooth transition to the superconducting state under the influence of the boundaries between the crystallites. The experimental dependences R(T) are analyzed within the Ambegaokar-Halperin model of thermal fluctuations in Josephson junctions and the flux creep model. An increase in the magnetic field leads to a crossover from the Ambegaokar-Halperin mechanism to the flux creep mechanism. The temperature dependences R(T) in the range of weak magnetic fields (from 0 to 102 Oe) are adequately described by the relationship following from the Ambegaokar-Halperin model. In the range of strong magnetic fields (from 103 to 6 × 104 Oe), the dissipation obeys the Arrhenius law R ∼ exp(−U(H)/T)], which is characteristic of the flux creep model with a temperature-independent pinning energy U(H). The effective Josephson coupling energies and the pinning energies corresponding to the Ambegaokar-Halperin and flux creep mechanisms are determined.

The experimental results of the effect of the magnetic field (up to 60 kOe) on the broadening of the resistive transition of bulk composites Y 3/4 Lu 1/4 Ba 2 Cu 3 O 7 (YBCO) + CuO are presented. These composites represent the network of the tunnel-type Josephson junctions where the copper oxide acts as a material forming barriers between YBCO crystallites. The mechanisms responsible for broadening of the resistive transition under magnetic field are discussed. The analysis of experimental R ( T ) dependences have shown that in the low field range 0–10 2 Oe, the R ( T ) dependences are described well by the Ambegaokar–Halperin (AH) model. In the range 10 3 –6 × 10 4 Oe, the dissipation follows Arrhenius law R ∼ exp(− U ( H )/ k B T ) characteristic for thermally activated flux creep model. In the range H ∼ 10 2 –10 3 , the crossover from AH to flux creep dissipation mechanisms occurs.

We study the effect of the structure on critical currents and current–voltage characteristics (CVC’s) of foamed bismuth-based polycrystalline high-temperature superconductors (HTSC). The fractal cluster structure of superconducting foams has been observed and the fractal dimension of boundaries between superconducting and normal clusters has been determined. Based on the magnetic and transport properties of the foamed polycrystalline superconductors, we have shown that the initial parts of CVC’s of the superconducting foams are described well by the model that accounts the magnetic flux trapping in fractal clusters of a normal phase.

HTSC Bi 1.8 Pb 0.3 Sr 2 Ca 2 Cu 3 O x of low density has been synthesized and its structural and physical properties have been studied. Scanning electron microscopy (SEM) image shows that the material obtained has porous “foam” structure. The levitation force measured for the porous Bi 1.8 Pb 0.3 Sr 2 Ca 2 Cu 3 O x is appeared to be approximately the same as for polycrystalline YBCO of high density. These “foam” structures may be an ideal material for some practical applications of HTSCs in bulk form.

The phase composition of magnetic microspheres isolated from Ekibastuz coal fly ash in six narrow size ranges is determined by Mossbauer spectroscopy. The major phase in the microspheres is nonstoichiometric, cation-substituted (Al, Mg, and Ti) magnetite. Their magnetic properties are shown to depend on the Fe content and cation distribution over the spinel sites.

We have studied the magnetoresistive effect in bulk 1-2-3 YBCO + CuO and 1-2-3 YBCO + BaPb1−xSnxO3 composites prepared using the fast backing technique. We have found that the composites exhibit large magnetoresistance in low magnetic fields (

The results of the study of transport properties (temperature dependences of resistivity and critical current, current-voltage characteristics) of two-phase composites on the basis of Y 3/4 Lu 1/4 Ba 2 Cu 3 O 7 high-temperature superconductor (HTSC) produced by fast baking technique and modelling networks of Josephson-type weak links are reported. Non-superconducting ingredients with different magnetic structures, insulators and normal metals, function as barriers between HTSC crystallites. Besides strong suppression of superconducting properties in HTSC composites with magnetic barriers, anomalous behavior of temperature dependences of resistivity below T c of HTSC grains have been observed and investigated.

Bulk Y3/4Lu1/4Ba2Cu3O7 + BaPb1−xFexO3 composites representing a model of a network of superconductor–normal metal–superconductor (SNS) Josephson junctions with paramagnetic impurities (Fe) in the N layers have been prepared. The transport properties of the composites have been studied. An exponential decay of the critical current with increasing Fe content in the non-superconducting ingredient of the composite has been observed. Such a behaviour is accounted for in terms of the de Gennes proximity effect theory.

The temperature evolution of the current-voltage (I-U) characteristic of a contact of the break-junction type with direct conduction is investigated on a polycrystalline HTSC of the Y-Ba-Cu-O system. The experimental I-U characteristics possessing a hysteresis are correctly described in the framework of the Kummel-Nicolsky theory for an S-N-S contact (S stands for a superconductor; N, for a normal metal) in which the Andreev reflection of quasiparticles from the N-S interface is considered. It is shown that the shape of the I-U curve, as well as the existence of a hysteresis, is determined by the ratio of the number of “long” and “short” intergranular boundaries in the polycrystal under investigation. The coincidence of the calculated and experimental I-U curves made it possible to estimate the effective length of “natural” intergranular boundaries in polycrystalline HTSC materials. The estimate is obtained from the experimental temperature dependence of the critical current in the sample under investigation.

Composites representing a network of random Josephson junctions and characterized by the compositions 92.5 at. % Y3/4Lu1/4Ba2Cu3O7+7.5 at. % NiTiO3 and 92.5 at. % Y3/4Lu1/4Ba2Cu3O7+7.5 at.% MgTiO3 are synthesized, and their magnetoresistance properties are studied. The temperature dependence of the resistance R(T) measured for the composite that contains the paramagnetic NiTiO3 compound exhibits a characteristic feature below the superconducting transition temperature Tc of the high-Tc superconductor, namely, a region where R is independent of the current j and weakly depends on the magnetic field H. Below a certain temperature Tm, a strong dependence of R on j and H is observed, which is peculiar to a network of Josephson junctions. The dependences R(T, j, H) obtained for the “reference” samples with the nonmagnetic MgTiO3 compound exhibit no such features. The anomalous behavior of the HTSC + NiTiO3 composite is explained by the effect produced by the magnetic moments of Ni atoms in the insulating barriers on the transport current.

The preparation, processing and characterization of the composite materials on high-T c superconductor (HTSC) basis, YBa 2 Cu 3 O 7 /Ag and Y 0.75 Lu 0.25 Ba 2 Cu 3 O 7 /BaPbO 3 , is reported. The initial components YBa 2 Cu 3 O 7 and BaPbO 3 were prepared by the standard solid state reaction technique. The microstructures of the samples were observed by scanning electron microscopy, whilst the XRD paterns of the composites HTSC+Ag and HTSC+BaPbO 3 revealed the 123 superconducting phase and the Ag or BaPbO 3 structure. The chemical compositions of the powders and the interface zone were found by employing energy dispersive spectrometry. The resistive and magnetic measurements of the composites indicated the transition temperature 93.5 K. Application of such composite materials in the construction of a superconducting fault current limiter model is reported.

A new concept for a device for protecting an alternating-current network is offered. It is based on a combination of a superconducting limiter of a short-circuit current and a circuit breaker. A high-temperature superconductor in the form of a pile of plane rings is used as the active element of the limiter. The test results of a model of such a limiter are obtained in the steady-state short-circuit regime. The characteristics of composite silverless materials used for design of the breaker are given.