Polycrystalline YBa Research Articles

The Circulation Radius and Critical Current Density in Type II Superconductors

A method is proposed for estimating the length scale of currents circulating in superconductors. The estimated circulation radius is used to determine the critical current density on the basis of magnetic measurements. The obtained formulas are applicable to samples with negligibly small demagnetizing factors and to polycrystalline superconductors. The proposed method has been verified using experimental magnetization loops measured for polycrystalline YBa$_2$Cu$_3$O$_{7-d}$ and Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$ superconductors.

Циркуляционный радиус и плотность критического тока в сверхпроводниках второго рода

AbstractA method is proposed for estimating the length scale of currents circulating in superconductors. The estimated circulation radius is used for determining the critical current density on the basis of magnetic measurements. The obtained formulas are applicable to samples with negligibly small demagnetizing factors and to polycrystalline superconductors. The proposed method has been verified using experimental magnetization loops measured for polycrystalline YBa_2Cu_3O_7 – δ and Bi_1.8Pb_0.3Sr_1.9Ca_2Cu_3O_ x superconductors.

A study of the dynamics of penetration and trapping of magnetic flux in polycrystalline YBa2Cu3O7−xsamples

In-phase and quadrature components of the first harmonic response of polycrystalline YBa2Cu3O7−x, were measured in a dc magnetic field. For small field modulation amplitude of 0.005 Oe, in-phase component of the first harmonic response of a sample was found to be proportional to the differential magnetic susceptibility. It was found that there is no pinning of Josephson vortices in a weakly coupled medium in magnetic fields below Hc2J, where Hc2J is the upper critical field of Josephson weak links. The obtained initial magnetization curve allowed us to describe the nonlinear part of the magnetization arising due to the weak bonds in ceramics. The size of a Josephson vortex and the critical parameters Hp, Hc2J and Hc1g were evaluated.

INHOMOGENEITY INDUCED CONDUCTIVITY FLUCTUATION IN YBa2Cu3O7-δ/BaTiO3–CoFe2O4 COMPOSITE

Polycrystalline (1-x) YBa 2 Cu 3 O 7-y + x BaTiO 3– CoFe 2 O 4(x = 0.0, 0.2, 0.4, 0.6 wt. %) superconductors were prepared by solid state route. XRD analysis reveals no significant change in "b" parameter and increase in "a" and "c" parameters. SEM micrographs show no change in grain size of the samples. With the increase of BaTiO 3– CoFe 2 O 4 (BTO–CFO) addition it has been analyzed that the superconducting transition temperatures (Tc) determined from standard four-probe method was decreased and dropped sharply with higher wt.% addition. Excess conductivity fluctuation analysis using Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above Tc. The decrease in 2D–3D crossover temperature T LD (Lawerence–Doniach temperature) in the mean field region has been observed as a consequent dominance of 3D region to increase in wt.% in the composite. The increasing value of ρwl and ρ0 and the decreasing trend in the value of zero-resistance critical temperature (Tc0) indicates that the connectivity between grains decreases gradually with the addition of magneto–electric composite BTO–CFO.

Magnetoresistivity and microstructure of YBa 2Cu 3O y prepared using planetary ball milling

We have studied the microstructure and the magnetoresistivity of polycrystalline YBa 2Cu 3O y (YBCO or Y-123 for brevity) embedded with nanoparticles of Y-deficient YBCO, generated by the planetary ball milling technique. Bulk samples were synthesized from a precursor YBCO powder, which was prepared from commercial high purity Y 2O 3, Ba 2CO 3 and CuO via a one-step annealing process in air at 950 °C. After planetary ball milling of the precursor, the powder was uniaxially pressed and subsequently annealed at 950 °C in air. Phase analysis by X-ray diffraction (XRD), granular structure examination by scanning electron microscopy (SEM), microstructure investigation by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDXS) were carried out. TEM analyses show that nanoparticles of Y-deficient YBCO, generated by ball milling, are embedded in the superconducting matrix. Electrical resistance as a function of temperature, ρ( T), revealed that the zero resistance temperature, T co , is 84.5 and 90 K for the milled and unmilled samples respectively. The milled ceramics exhibit a large magnetoresistance in weak magnetic fields at liquid nitrogen temperature. This attractive effect is of high significance as it makes these materials promising candidates for practical application in magnetic field sensor devices.

How the substitution of Zn for Cu destroys superconductivity in YBCO system?

Understanding the mechanism of suppression of superconductivity in perovskite cuprates is as important as to discover the cause of its evolution. Substituting Zn for Cu is known to rapidly suppress the superconductivity in these perovskites but the mechanism is still not understood. To throw some light on this, polycrystalline YBa 2(Cu 1− x Zn x ) 3O 7− δ samples ( x = 0.0–0.06) were synthesized and investigated using X-ray diffraction, titration, resistivity, magnetization, X-ray absorption and photoemission measurements. Results show that their oxygen stoichiometry ( δ) changes on Zn substitution affect their normal state resistivity as well as their T c . However, the observed changes cannot be accounted for solely on the basis of changes in δ. Zn cation is found to act as strong scattering centre in the lattice that causes local lattice distortion (LLD). It also induces local magnetic moment as evidenced from our magnetization results. It is concluded that a combination of Δ δ, LLD and magnetic pair breaking effect is responsible for the observed rapid suppression of superconductivity.

Fluctuation induced magneto-conductivity studies in YBa 2Cu 3O 7−δ + xBaZrO 3 composite high- T c superconductors

Fluctuations on the electrical conductivity of polycrystalline YBa 2Cu 3O 7−δ + xBaZrO 3 ( x = 1.0, 2.5, 5.0 and 10.0 wt.%) superconductors were investigated from the resistivity vs. temperature data for zero field and 8 T (Tesla) external magnetic fields. Attempts have been made to identify the optimum inclusion of BaZrO 3 (BZO) in YBa 2Cu 3O 7−δ (YBCO) superconductors. The phase formation, texture and grain alignments were analyzed by XRD and SEM techniques. Then the effects of superconducting fluctuations on the electrical conductivity of granular composite superconductors were studied for zero field and 8 T external magnetic fields. Though inclusions of BZO sub-micron particles are not expected to influence superconducting order-parameter fluctuation (SCOPF) much, the transition from 2D to 3D of the order parameter in the mean-field region depends on the BZO content in the composites. It has been observed that BZO residing at the grain boundary of YBCO matrix influences the tailing region without having significant change in the mean-field critical temperature. In the present work, attention has been focused mostly in the experimental domain relatively above the T c . It reveals that, 1 wt.% composite exhibits a better superconducting property in comparison with pure YBCO.

The study on the ZnO and Zn 0.95Mn 0.05O added YBCO system: Investigation of microstructure and transport properties

The effect of nano-size Zn 0.95Mn 0.05O and ZnO (30 nm) addition on the microstructure and the normal state transport properties of polycrystalline YBa 2Cu 3O y (YBCO) was systematically studied. Samples were synthesized in air using a standard solid state reaction technique by adding nano-sized particles up to 10 wt.%. When Zn 0.95Mn 0.05O and ZnO are added to the YBCO the orthorhombic structure maintained even at the highest concentration. TEM and EDS analyses show the presence of inhomeginities embedded in the superconducting matrix. To interpret the normal state properties of the samples, the percolation theory based on localized states is applied. A cross-over between variable-range hopping and Coulomb gap mechanisms is observed as a result of increasing the nano-particles concentration. The ZnO addition modifies the electrical behavior of samples from metallic to insulating with a much lower concentration comparatively to Zn 0.95Mn 0.05O addition. The calculated values of the localization length, d, are greater in the case of Zn 0.95Mn 0.05O addition. This result can be interpreted by the internal structure defects.

DEVELOPING A TOOL FOR A QUALITATIVE MEASUREMENT OF PINNING FORCES USING ROTATIONAL MAGNETIZATION EXPERIMENTS

We have investigated the behavior of vortices in a polycrystalline YBa 2 Cu 3 O 7-δ disk sample using rotational magnetization-vector (RMV) measurements. The measurements were conducted in the field-cooled (FC) and the zero-field cooled (ZFC) states for several values of applied magnetic field parallel to the sample surface at 4.2 K and for various angles of rotation (θ rot ) up to 360°. The total magnetic flux density, B, in the sample at any rotational angle was considered to be composed of two types of vortices: weakly pinned vortices that rotate frictionally in the opposite direction of sample rotation and strongly pinned vortices that rotate rigidly in the same direction of sample rotation. From the rate of change of these two types of vortices and the rate of change of B, we were able to develop a tool to qualitatively describe the average strength of pinning centers in high-temperature superconductors.

Magnetic field dependence of the relative critical current density in γ‐irradiated polycrystalline YBa2Cu3O7

AbstractThe effect of different doses of γ‐rays on the behavior of the critical current density, Jc in an YBa2Cu3O7 polycrystalline sample has been investigated at high temperatures. The samples were irradiated at room temperature by a 60Co γ‐ray source at a dose rate of 0.5 MR/h. Jc was found to increase significantly with after irradiation dose of 10 MR. Further irradiation dose of 50 MR produced a slight and field dependent enhancement of Jc above its values at 10 MR. The most interesting result is that the relative change in the critical current density was found to have a non‐monotonic behavior with the applied magnetic field. These results are discussed in terms of the roles of several mechanisms created by γ‐rays in the regions of the grain boundaries combined with the effect of the magnetic field on these mechanisms. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermoelectric figure of merit of oxygen-deficient YBCO perovskites

We have performed a study of temperature dependence of thermal conductivity κ ( T ) , Seebeck coefficient S ( T ) and electrical resistivity ρ ( T ) on oxygen-deficient polycrystalline YBa 2 Cu 3 O 7 - δ ( 0 < δ < 1 ) samples. The measurements of S ( T ) , ρ ( T ) and κ ( T ) were carried out as a function of oxygen content in the temperature range between 77 and 300 K, the influence of oxygen content on the transport properties and the dimensionless figure of merit ( ZT) was studied. As the oxygen content is reducing the Seebeck coefficient and the electrical resistivity show a systematic increase while κ ( T ) decreases. ZT ( T ) exhibits an enhancement reaching values close to 0.3 in the samples with low oxygen levels. This behavior suggests that the charge carrier density decreases while the oxygen level decreases and it opens the possibility to use this kind of materials as active thermoelements, which could work at room temperature and below it.

Uniaxial textured (001) metal film formation via YBa 2Cu 3O 7/SrTiO 3/metal trilayers

The formation of (001)-oriented uniaxially textured metal layers on randomly oriented surfaces using textured oxide buffer layers is described. The initial formation of uniaxial texture is through the deposition of polycrystalline YBa 2Cu 3O 7 − x films, which has a strong tendency to form in the c-axis perpendicular orientation due to strong surface energy anisotropy. An epitaxial SrTiO 3 cap layer on the (001) textured YBa 2Cu 3O 7− x , layer retards decomposition of the oxide surface under high temperature, low pressure conditions that are suitable for metal epitaxy. The resulting metal surfaces have a strong uniaxial texture and may prove useful in applications where (001) uniaxial texture in metal films is desired.

Elastic properties of polycrystalline YBa 2Cu 3O 7− δ: Evidence for granularity induced martensitic behavior

In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa 2Cu 3O 7− δ prepared by the sol– gel method. The quality of all samples was checked by X-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse– echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval ( T < 200 K). The obtained results show a strong hysteretic behavior in the ultrasonic attenuation (in addition to usually observed hysteretic behavior for the velocity) which strongly supports the existence of a martensitic-like phase above the superconducting critical temperature T c. We argue that this peculiar behavior can be attributed to the granularity present in the samples.

Temperature dependence of vortex flux pinning in polycrystalline YBa 2Cu 4O 8

Magnetization measurements versus the magnetic field were done on a polycrystalline YBa 2Cu 4O 8 sample over temperature range from 4.2 K up to T C = 80 K. Within the applicability of the Bean critical state model, the magnetization remanence Δ M R represents the pinning forces at H = 0. We found that Δ M R decreases sharply with the temperature T for T/ T C < 0.4 and for T/ T C > 0.4 square root behavior exists. We found that Δ M R scales according to the empirical formula Δ M R (emu/cm 3) = 13(1 − T/ T C) 6 + 2(1 − T/ T C) 1/2. The two distinct behavior of Δ M R with T implies an existence of different pinning mechanisms.

AC susceptibility study of YBCO thin film and BSCCO bulk superconductors

The temperature dependence of AC susceptibility of polycrystalline YBa 2Cu 3O y thin film and (Bi,Pb) 2Sr 2Ca 2Cu 3O y bulk samples were measured as a function of frequency and AC field amplitude. Analysis of the temperature dependence of the AC susceptibility near the transition temperature ( T c) has been done employing Bean's critical state model. The observed variation of intergranular critical current densities ( J c) with temperature indicates that the intergrain coupling in these samples can be modeled in terms of superconductor–normal metal–superconductor junctions. The frequency effect on AC susceptibility was also measured. As the frequency increases, the peak temperature ( T p) shifts to higher temperature. This effect can be interpreted in terms of flux creep. The field dependence of activation energy obtained from the Arrhenius plot for the frequency ( f) and ( T p), can be described as U( H AC)∼ H AC − β for both samples.

Magnetic hysteresis of the magnetoresistance and the critical current density in polycrystalline YBa 2Cu 3O 7− δ–Ag superconductors

A systematic study of the magnetic hysteresis in transport properties of polycrystalline YBa 2Cu 3O 7− δ –Ag compounds has been made based on two kinds of measurements at 77 K and under applied magnetic fields up to 30 mT: critical current density J c( B a) and magnetoresistance R( B a). The R( B a) curves show a minimum in their decreasing branch occurring at B= B min which was found to be both the excitation current I ex and the maximum applied magnetic field B am dependent. In addition, for a certain value of B am>5 mT, we have observed that B min increases with increasing I ex and reaches a saturation value. The J c( B a) curves show a maximum in decreasing applied magnetic fields occurring at B= B max. We have also found that B max increases with increasing B am and reaches a saturation value. The minimum in the R( B a) and the maximum in J c( B a) curves were found to be related to the trapped flux within the grains. All the experimental results are discussed within the context of the flux dynamics and transport mechanisms in these high- T c materials.

The freezing of spin and charge at low temperature in YBa 2Cu 3O 6+ x

We present the results of an extensive μSR reinvestigation of the doping dependence of the AF order in polycrystalline YBa 2Cu 3O 6+ x samples (0< x<0.4), prepared by a topotactic-like technique which yields oxygen equilibrated and ordered pairs of YBCO samples of well defined oxygen order conditions. For x>0.2, we detected a spin freezing correlated to the charge freezing of the holes in the CuO 2 planes. The data may be fitted to a model of size-dependent reduction of the staggered magnetization implied in a stripe scenario. The hole concentration dependence of these freezing phenomena is mapped onto the phase diagram of YBa 2Cu 3O 6+ x .

Electron backscatter diffraction study of polycrystalline YBa 2Cu 3O 7− δ ceramics

The electron backscatter diffraction (EBSD) technique was applied to polycrystalline YBa 2Cu 3O 7− δ ceramics. To obtain good quality EBSD Kikuchi patterns on the ceramic superconductors, a new sample polishing route was established. Samples with and without KClO 3-doping were investigated; and the crystallographic orientations of the grains were analyzed using pole figures and EBSD orientation mapping. On the polycrystalline KClO 3-doped YBa 2Cu 3O 7− δ samples we found two major orientations (0 0 1) and (1 0 0) normal to the sample surface, while the undoped sample did not exhibit any orientation maxima. We ascribe this observation to the effect of a liquid phase due to the KClO 3-additive during the preparation process.

Effect of grain orientation and local strain on the quality of polycrystalline YBa 2 Cu 3 O 7 superconductive films

The critical current densities of superconducting thin films and their dependence on the film structural characteristics has been a major research interest for more than a decade. Controlling this relationship is crucial if large-scale high-quality YBa 2 Cu 3 O 7 (YBCO) tapes are to be produced. Two major keystones of information have been established in this field. Firstly, there is a direct relationship between the critical current density and the grain-boundary angle in polycrystalline YBCO films. Grain boundaries with a mismatch angle higher than 5° usually result in reduced critical current densities. This detrimental effect of large-angle grain boundaries to the quality of YBCO films has been attributed to strain fields resulting from such grain boundaries. Secondly, the quality of the YBCO film can be enhanced by straining its lattice in specific direction. Here, we report, for the first time, direct experimental results coupling local grain orientation and local strain maps of thin YBCO films deposited on a (001) biaxially textured nickel substrate. These results were correlated to the quality of the film and showed how grain structure in the nickel substrate affects the grain structure in the YBCO films even in the presence of several buffer layers. More importantly, the data show that highquality films with high critical current densities can be produced, in spite of large-angle grain boundaries, if the film is compressed in the range of 0.5% strain normal to the a axis.

Effects of transverse magnetic fields on pinned vortices in polycrystalline YBa 2Cu 3O 7− δ

The effects of a field H applied perpendicular to a previously established saturation remanence M R were investigated by magnetization-vector ( M ) measurements on a polycrystalline YBa 2Cu 3O 7− δ sample at 4.2 K. It was found that M T, the M component transverse to H , decreases steadily with increasing H. The fact that M T does not stay constant (at value M R) over an initial range of H indicates that the pinning torques τ P on the vortices comprising M R have a broad distribution in strength. Calculated model curves for M T versus H give the closest data fits when the τ P distribution function is assumed to diminish with increasing τ P. Our experiments also revealed that when the applied H is reduced from various values to zero, the magnitude of M T changes very little, signifying that the shifts in vortex orientation produced by H are essentially irreversible.