Samples Of YBa2Cu3O7 Research Articles

Study of the frequency and low-field dependence of ac susceptibility in YBaCuO

The frequency dependence of the ac susceptibility in polycrystalline samples of YBaCuO has been studied at the ac field range 0.06–1.2 Oe. The curve of the real part of the susceptibility χ′ exhibits two drops, the first one close to Tc and the other well below Tc which could correspond to screening behavior of the grains and grain boundaries, respectively. The imaginary part of the susceptibility shows only a peak below Tc which is related to hysteresis losses at the grain boundaries. The peak corresponding to intragrains hysteresis losses is very small and appears masked by the losses of the intergrains. In the range of 1–20 kHz, the onset temperature of χ′ showed no frequency dependence. However, the width of the transition decreases lightly and there is a small shift in the peak of χ″ to higher temperature as the frequency increases. This behavior could be explained in terms of the thermally activated Anderson flux creep. The shift depends on the amplitude of the measuring field. The activation energy for flux creep ranges from 6.4 eV at 0.02 Oe to 3.23 eV at 1.2 Oe in the zero-field-cooled measurements and from 3.41 eV at 0.02 Oe to 1.41 eV at 1.2 Oe in the field-cooled measurements.

Remanent magnetization of layered and isotropic superconductors (abstract)

There are contradicting data on the anisotropy of remanent magnetization of high-temperature superconductors. To clarify the mechanism of this anisotropy we performed a comparative study of thermoremanent (TRM) and isothermal remanent (IRM) magnetizations. We report on the results obtained on layered single crystals of LaCrSuO and RBaCuO (R=Y,Gd) families as well as on isotropic soft alloy Bi:Pb:Sn and nontextured ceramic YBaCuO samples. The experiments were done by means of a vibrating-sample magnetometer. The external magnetic field up to 8 kOe was applied to magnetize the samples at arbitrary directions. It was found that all the studied plate-like samples show the effect of ‘‘easy remanent magnetization axis:’’ the vectors of IRM and TRM tend to point along the normal of the plate at any magnetizing or cooling angle. In the case of TRM this effect can be well described considering the influence of the sample shape. This fact was proved by measurements on samples with substantially different aspect ratios. Thus, we argue that the anisotropy of TRM is not related to anisotropic flux trapping. The anisotropy of IRM is a more complex phenomenon, as it involves inhomogeneous flux distribution in the sample. Nevertheless, it was found that the dependence of IRM direction on the magnetizing angle is also governed mainly by the sample shape. The corrections to the effective demagnetization factors were calculated, which arise due to the flux distribution. The analysis of the absolute value of IRM on the magnetizing angle allows one to distinguish between the influence of the critical current anisotropy and the sample shape.

Critical current density and percolation in YBaCuO ceramic

Theoretical and experimental investigations of supercurrent percolation in granular YBaCuO ceramic have been carried out. In the mean field approximation the expression for critical current density j c= j c0 (p s− > 1 3 ) has been derived. j c0 is the critical current density of an infinite skeleton cluster, p s= kϱ/ ϱ id, where k is the share of the superconducting phase, ϱ id=6.35 g/ cm 3 is the calculated value of the maximal density. Three series of YBaCuO samples with ϱ in the range 3.5-5.4 g/cm 3 and different values of k=0.7, 0.9 and 0.95 have been investigated experimentally. Linear dependences of j c(ϱ) and j c( p s) are obtained. The experimental value of the percolation threshold is close to value of 0.33 predicted by theory.

Magnetization studies of near-room-temperature flux-trapping phenomenon in CuO-based materials

DC magnetization studies on a variety of ceramic and crystalline YBaCuO samples indicate a superconducting-like flux-trappinig behavior occurring at similar temperatures as zero-resistance transition temperatures observed near room temperature. The flux-trapping behavior is exemplified by diamagnetic-like deviations from a positive magnetic background in the zero-field-cooled magnetization, by a divergence between the zero-field-cooled and field-cooled magnetization data, or by a positive Meissner effect in the field-cooled magnetization. These behaviors are consistent with a high-Tc (>250 K) superconducting phase in an inhomogeneous superconductor where the superconducting regions exist as defect structures or surface coatings and whose volume fraction is less than a few percent.

Determination of carbon dioxide contents in cuprate superconductors of the Y 2Ba 4Cu 6+ nO 14+ n family

Thermoanalytical measurements (thermogravimetry and differential thermal analysis (DTA) coupled with mass spectroscopy) are used to determine the carbonate content in YBaCuO samples. Up to 0.1 mol carbon dioxide per formula unit, depending on the synthesis procedure, was found in the samples. Simultaneous evolution of oxygen and carbon dioxide from the samples was observed for all compounds (YBa 2Cu 3O 7−χ, YBa 2Cu 3.5O 7.5−χ and YBa 2Cu 4O 8) investigated.

Low-dose irradiation effects produced by 14 MeV neutrons in different quality Y- and Tl-based high-T c ceramics

Various changes in the microwave absorption signals of irradiated samples of different quality YBa2Cu3O7 and Tl2CaBa2Cu2O8 target materials by lowdose (1014 n/cm2) monoenergetic neutrons have been observed and studied. The irradiation induced changes of the shape of microwave absorption lines together with transport measurements show that the low-dose irradiation in poor quality samples increases, while in good quality ones, decreases the superconducting volume fraction. Low quality (non-percolative) samples are more sensitive to irradiation than the high quality ones. These observations indicate that the consequences of irradiation are determined mainly by the preirradiation structure of the samples.

Vortex flux creep and magnetic hysteresis in a type-II superconductor (abstract)

Upon reversal of the external magnetic field H during any hysteretic cycling of a type-II superconductor in its vortex state, the rate at which the magnetic flux density B varies with H is typically seen to start from zero and then change gradually. Measurements have now been made of the time dependence of B (the vortex flux creep) at many fixed-H points on several hysteresis loops of a grain-oriented YBa2Cu3O7 sample at 4.2 K (with H and B along the c axis), and it is observed that the size of the logarithmic dB/dt drops abruptly to zero, before proceeding to change in sign, whenever H is reversed. This curious similarity between the hysteretic behavior of dB/dt at fixed H and that of the nearly instantaneously measured dB/dH can be understood qualitatively in terms of the vortex pinning. As rotational magnetization measurements have recently shown,1 the unpinning and repinning of moving vortices is manifested macroscopically as a frictional process. Thus, the reversal of vortex motion involves the reversal of frictional forces, during which the vortex population in the superconducting sample (which is proportional to B) does not change with time or field.

BCS temperature-dependent superconducting energy gap in domain boundaries of melt-textured YBa2Cu3O7

Attenuation and velocity measurements in low magnetic fields in the superconducting state of a melt-textured sample of YBa2Cu3O7 are reported. Changes in both attenuationα and relative velocitydv/v are observed at the penetration fieldH′ c1 of its domain boundaries. The change indv/v atH′ c1 appears to be proportional to the repulsive force between vortices. The change inα atH′ c1 may be produced by a relaxation process involving superconducting quasiparticles in the vicinity of the normal core of the vortices. Analysis of the relaxation time associated with this process yields a temperature-dependent energy gap which follows the BCS dependence. The relaxation time associated with the process is about 10−11 sec.

Fabrication of an HTSC electrical switch

The possibility and apparent ease of transferring superconductors from the superconducting (i.e., ideally conducting) into the normal (i.e., well insulating) state and back has attracted interest in utilizing this specific feature of superconductors in the field of electrical switches. The discovery of high-temperature superconductors (HTSCs) in 1986 opened up new prospects. Due to the possibility of liquid-nitrogen cooling, devices made up of HTSC materials may be more simple and economical. The problem of the application of HTSC materials in electrical switches has been formulated by a Hungarian power engineering company. The investigation under this project is aimed at the creation of a prototype controlled HTSC switch. The use of the proposed switch would offer a novel way of suppressing the arcs developing in heavy current-switching equipment. This paper describes the results of the first stage of the project. This stage includes the fabrication of silver-sheathed bulk YBaCuO samples and that of metal contacts.

Ultrasonic determination of the superconducting energy gap in domain boundaries of melt-textured YBa2Cu3O7

Abstract Ultrasonic attenuation and velocity measurements as a function of magnetic field in the superconducting state of a melt-textured sample of YBa2Cu3O7 with Tc = 91 K are reported. Sharp rises in both attenuation α and relative velocity dv/v are observed at the penetration fields H′c1 of its domain boundaries. The change in dv/v at H′c1 is proportional to the repulsive force between vortices. The change in a at H′c1 is produced by a relaxation process involving the excitation and decay of superconducting quasiparticles in the vicinity of the normal core of the vortices. The relaxation time associated with this process yields the Burdeen-Cooper-Schrieffer temperature-dependent energy gap.

Novel approaches in the melt-texturing of YBa2Cu3O7−y

Electrochemically pre-textured samples have been subjected to melt-processing in order to produce dense and highly textured bulk samples of YBa2Cu3O7−y (YBCO). Full oxygenation of these samples has been achieved at high processing temperatures of 720 °C by electrochemical titration, in order to increase the superconducting transition temperature to > 89 K. These samples show a large magnetic hysteresis, and the value of Jc calculated using the Bean model is in the range 4000–6000 A cm−2 at 77 K and 1 tesla magnetic field, and is independent of the applied field in that range. In another variation of the melt-processing technique — referred to as “isothermal melt-textured growth” — highly textured samples have been produced by the movement of the solidification front at a constant temperature in an oxygen activity gradient.

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT c , on a number of YBa2Cu3O7 samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu2+ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.

Magnetic bearings using high Tc superconductorsconfigurations and first results

The development achieved with superconducting oxides makes possible the design of magnetic bearings in association with permanent magnets. The inherent properties of these superconductors ensure passive stability in opposition to classical systems which need at least one actively controlled direction. This paper reports some results about levitation performances between permanent magnets and YBaCuO samples at 77 K. The force-displacement measurements are corroborated by magnetization curves. Several structures are proposed including different magnet configurations and hybrid system consisting in magnets providing high levitation pressure stabilized by superconductors. In order to investigate optimum configurations some first numerical modellings have been developed.

Irreversibility line and non-linearity in the AC response caused by flux pinning in high- Tc superconductors

We have studied the irreversibility lines (IL) obtained by AC magnetic measurements in the region with notable dependence of the AC susceptibility on the AC field amplitude. The commonly used procedure of taking temperatures of maxima in the imaginary part of AC susceptibility χ″ is shown to be consistent with the definition of the IL as the line of constant critical current density. The data obtained in experiments on melt-grown YBaCuO samples were used to compare this method with two other ones: (a) searching for the appearance of flux pinning in a non-linear flux-flow regime, (b) construction of current-voltage characteristics from AC magnetic measurements. We demonstrated that it is possible to reach good agreement between irreversibility lines acquired under the conditions of a non-linear AC regime by different methods if the current densities and the electrical fields in the sample are comparable.

Oxygen re-arrangement and desorption in 57Fe:Y-Ba-Cu-O

We report on in situ high temperature 57Fe Mössbauer measurements on several Fe:YBaCuO samples with different Fe concentrations under controlled oxygen partial pressures. The results are discussed in terms of the temperature dependent binding of oxygen in Fe-rich clusters which differs from that in their more Cu-rich surrounding.

Experimental study on magnetic relaxation in YBa2Cu3O7 by means of a simple inductive technique

The time dependence of the field-cooled susceptibility in polycrystalline samples of YBa2Cu3O7 has been studied in the temperature range 77–95 K. Field-cooled and zero-field-cooled susceptibility measurements and its variation with the ac magnetic field are also reported. Measurements have been carried out by means of an inductive technique. Our fully automatic system allows us to measure the first data points of field-cooled susceptibility versus time from the beginning of the magnetic relaxation just after removing the field.

Critical current density and activation energy in melt textured growth YBaCuO from magnetic measurements

Textured YBaCuO samples have been prepared using a modified melt textured growth (MTG) method. AC susceptibility data are presented showing the high quality of the textured samples. Static hysteresis loop measurements are realized for magnetic fields up to 5 T in a large temperature range (4.2 to 100 K). The anisotropic behavior is studied, the magnetic field being applied successively in the ( ab) planes and along the c-axis. The critical current densities are obtained using the Bean model. Dynamic hysteresis loop measurements are performed at T ≥ 77 K and over the frequency range from 0.2 to 10 Hz using a field of 800 Oe. The magnetization relaxation effect appears to be significantly lower in the textured sample than in a single crystal. The characteristic activation energies calculated in the flux creep model are presented.

X-ray texture analysis in YBaCuO ceramics and films

The technological applications of high temperature superconductors will be possible only for bulk samples or thin films exibiting high critical current density. It has been confirmed that the superconducting properties of polycrystalline materials are strongly orientation-dependent, these properties being affected by the orientation of the crystallites. The simplest method to analyse the preferred orientation or “texture” of a crystalline material is to compare qualitatively the intensities of ( hkl) reflexions of an X-ray diffraction pattern. This allows, however, only a rough estimation of the main features of the texture. Another method is the pole figure determination which gives the crystallographic orientation of the crystallites with respect to a specific orientation of the sample. In YBaCuO samples, generally the (001) pole figures define the c-axis alignment, while the ( hkl) pole figures give information concerning the rotation about the c-axis i.e. the “in plane texture”. The pole-figure technique allows one to compare different samples, to analyse the effects of processing parameters and to study epitaxial relationships. Texture analysis of YBaCuO oriented ceramics and uni and multi-layer films are presented.

Investigation of collective creep in YBaCuO

Time dependent decay of the magnetic moment M(t) and magnetization measurements were used to study flux creep in melt textured YBaCuO samples. From both measurements voltage current characteristics were reconstructed between 10 −1 and 10 −7 μV/cm up to the region of the irreversibility field. The exponent μ of the power law dependence of the pinning barrier was obtained from M(t) using the time constant of the initial stage of relaxation and from E(j) analysis. The current dependence of μ shows a maximum, negative values on both sides as well as the field dependence are not in agreement with theoretical predictions.

The angular dependence of the critical current density of textured YBaCuO

Highly textured bulk YBaCuO samples consisting of dense highly oriented platelets were prepared by a liquid phase process. Critical current density measurements at 77 K and at 6 T were made as the angle between the transport current and the externally applied magnetic field was varied in the ab-plane. At all times, the c-axis was perpendicular to both the DC transport current and the external magnetic field. The critical current density values were found to achieve a maximum of 7000 A/cm/sup 2/ when the magnetic field was parallel to the transport current and a minimum of 2000 A/cm/sup 2/ when the magnetic field was close to an orientation perpendicular to the transport current, with one notable exception: a small increase to approximately 3000 A/cm/sup 2/ from the minimum value of 2000 A/cm/sup 2/ was observed when the magnetic field was perpendicular to the transport current. Ignoring this exception, the angular variation of critical current density is qualitatively consistent with the variation expected from the Lorentz force acting upon the vortices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>