YBCO Bulk Samples Research Articles

High Levitation Force and Trapped Field of Large Grain YBCO at 77K by Ga Doping

ABSTRACTLevitation force and magnetic field trapping are the most important properties of YBCO large grain materials. They are dependent on the pinning force exerting on the magnetic flux lines by various crystal defects in YBCO. In this paper, Ga-doped bulk YBCO samples were fabricated by SmBCO seeded melt-growth process. The levitation force and trapped field were enhanced by the introduce of nanometer scale week superconducting regions which are formed by the Ga partially substituting for Cu on the Cu-O chain.

“Electromaglev” (“active-maglev”) – magnetic levitation of a superconducting disk with a DC field generated by electromagnets. Part 4: theoretical and experimental results on supercurrent distributions in field-cooled YBCO disks

We present Part 4 results of a comprehensive theoretical study of an “electromaglev” system, in which a high-temperature superconducting bulk YBCO sample is levitated stably in a DC magnetic field generated by magnet system underneath the floating object. An electromagnetic analysis, based on a three-dimensional finite element technique (FEM) applied to the current vector potential method, has been developed to determine the supercurrent distribution in a field-cooled (and hence trapped-flux) YBCO disk that levitates stably in a magnetic field generated by the magnet system. The supercurrent distribution thus determined was in turn used to compute trapped-flux-induced field profiles of the disk and predict a “levitation current” in the magnet system at which the disk, initially resting on a support plate, begins to levitate. Agreement between computed field profiles and levitation currents and those measured in the experiment was excellent, validating the analysis itself and the method used to derive solutions. The analysis demonstrates that the supercurrent distribution within a trapped-flux disk is far more complicated than that derived from the Bean model for a long cylinder under a uniform axial magnetic field. It is used for a parametric study of the effects of disk dimensions (radius, thickness, radius/thickness ratio) and trapped-flux strength on supercurrent distribution and lift-to-weight ratio. The magnitude of the B r component generated by the magnet system is very important for lift and it is shown that thinner disks rather than thicker disks can improve lift-to-weight ratio. Because accuracy of the analysis is disk-size independent, small disks are time-efficient for performing the analysis.

Neutron Texture Analysis of Melt-Textured YBCO Bulk Samples

Neutron texture measurements on YBCO bulk samples show a very sharp texture of the superconducting phase YBa2Cu3O7-x with half-widths of less than 5°. Even with a rather coarse measurement grid of only 722 points per complete pole figure, satisfactory results for the recalculated (002) pole figures could be obtained. However, for a reliable calculation of a complete ODF, finer grids will have to be used. Due to the importance of a good alignment of the c-axes in the material, a quantitative analysis of the (002) pole figures, including an error estimation due to measurement grid and counting statistics, was made. An outline for the determination of a reliable background estimate is given.

`Electromaglev' (`active-maglev')—magnetic levitation of a superconducting disk with a DC field generated by electromagnets: Part 3. Theoretical results on levitation height and stability

We present Part 3 results of a comprehensive theoretical study of an `electromaglev' (`active-maglev') system, in which a high-temperature superconducting bulk YBCO sample is levitated stably in a DC magnetic field generated by a magnet system. Field solutions have been obtained numerically to compute levitation height and define stability criteria for the superconducting disk sample. Our analysis assumes that the disk, which otherwise obeys the Bean critical-state model, traps flux when cooled in the presence of a field from the normal state to the superconducting state. Indeed it is shown that the trapped flux makes subtle and crucial changes in field distribution (and thus current density distribution) in the disk, which differ from those in a disk strictly obeying the Bean model used in the zeroth-order theory. The analysis confirms a key experimental finding that the trapped flux is another essential element for determining levitation height and ensuring tilt-free stable levitation. For stability, trapped flux is in addition to at least two degrees of freedom for spatial supercurrent flow and the profile conditions imposed on the field generated by the magnet system. Procedures to produce stable, tilt-free levitation are described. Agreement between experiment and analysis on dependence of levitation height on magnet current is quite good. The analysis also shows that to achieve stable levitation, a YBCO ring sample requires a radial build that is sufficiently thick to permit the supercurrent to flow in the radial direction. The minimum radial build required, Δ R min, for a YBCO ring of outside radius 12.5 mm operating at 77 K is typically ∼50 μm. An analytical expression that gives approximate values of Δ R min has also been derived; Δ R min depends inversely on the square of the critical current density of the superconductor.

`Electromaglev' (`Active-Maglev') — magnetic levitation of a superconducting disk with a DC field generated by electromagnets: Part 2 Theoretical and experimental results on lift-to-weight ratio and lateral stiffness

We present Part 2 results of a comprehensive study, both theoretical and experimental, of an electromaglev (`active-maglev') system, in which a YBCO bulk sample is levitated stably in a DC magnetic field generated by a magnet system located underneath the floating sample. The 0th-order theory presented in Part 1 is used to interpret experimental results of levitation stability, lift-to-weight ratio, and lateral stiffness for four YBCO samples: 1) disk; 2) annulus; 3) the same annulus with a permanent magnet disk placed in the hole; and 4) ring. Also presented is a procedure to induce in a sample a trapped flux that is shown experimentally to be essential for the sample to levitate tilt-free. Trapped flux is another requirement for levitation stablity; it is in addition to at least two degrees of freedom required on spatial supercurrent flow and certain spatial profiles imposed on the field generated by the magnet system.

Study of the local magnetic relaxation in melting-textured-growth YBa2Cu3O7−δ bulk sample with Bi Hall probe

Using Bi Hall probe, the time dependence of the local inductionB at different locations on the surface of a melting-textured-growth (MTG) YBCO bulk sample was measured at liquid nitrogen temperature. The behavior of the Hall voltage VH(t) shows a marked fluctuation and is qualitatively similar for most geometrically equivalent points on the surface. But for the places with different distances from the center on the surface the VH(t) behavior can be quite different.

Influence of the geometrical aspect-ratio on the surface impedance of Y-based superconducting samples

Ac surface impedances of screen printed thick films and bulk YBCO samples having a wide range of geometrical aspect-ratio are compared. It is shown that very thin (d ∼ 45 μm) samples cut from bulk pellets behave in ac magnetic fields as bulk materials with low aspect-ratio. This demonstrates that the widely accepted explanation for the observation that in ac magnetic fields applied normal to surface of a superconductor film vortex depinning occurs much more easily than in bulk superconductors is a consequence of a high aspect-ratio of the film geometry is not correct. We also show that the ac surface impedance measurement on films is equivalent to the complex susceptibility measurement on bulk samples.

Resistivity and thermoelectric power of [formula omitted] superlattice with 2-unit-cell-thick YBa 2Cu 3O 7−δ layer in one period

Measurements of the resistivity and thermoelectric power (TEP) for a series of YBa 2Cu 3O 7−δ PrBa 2Cu 3O 7−δ ( YBCO PrBCO ) superlattice samples with 2-unit-cell-thick YBCO layer and 2, 4, 6 or 8-unit-cell-thick PrBCO layer in one period are reported. For the sample with 2-unit-cell-thick PrBCO layers, the resistivity of the YBCO layers is larger than that of bulk YBCO samples. The contribution of stoichiometric diffusion and interface scattering is estimated. Its TEP is positive with positive temperature coefficient, different from the typical negative one in bulk YBCO samples with positive TEP values. The property of the YBCO layers changes with increasing thickness of the PrBCO layers. For the sample with 8-unit-cell-thick PrBCO layers, the contribution of the PrBCO layers has to be considered.

Trapped magnetic fields in melt-textured YBCO

The surface of melt-textured bulk YBCO samples, cooled at liquid-nitrogen temperature, was mapped with an automatic Hall probe-based system to study the spatial distribution of trapped magnetic fields. Such a magnetic characterization offers detailed information about the granular structure of differently prepared samples. Both axial and tangential trapped magnetic-field components were measured. the axial component of the trapped magnetic field allows one to obtain the number of grains, their shapes, their dimensions and their locations, while measurements of the tangetial component of the trapped magnetic field give complementary information on the grain positions. Intragranular critical current densities were estimated by comparing experimental trapped magnetic-field profiles with theoretical ones calculated using the critical-state model.

Microdefects in relatively fast cooling MTG YBCO specimen

We have carried out a microstructure analysis of an YBCO bulk sample prepared using a simplified version of the melt texture growth process. Using only a 200 kV transmission electron microscope and diamond knife to section our sample, we obtain micrographs showing clear lattice fringes with interference spacing around 13 Å. The boundaries of the bright and dark regions, which are believed to arise from variations in crystal composition, coincide with the occurrence of stacking faults, twin boundaries and other dislocations. This sample, being full of microdefects, carries a bulk critical current of 1.75 × 10 4 A cm −2.

A method for determination of microwave surface impedance of high-T/sub c/ thick film and bulk superconductors

A resonator method is presented which enables R, and X/sub 8/ to be determined as a function of temperature with the exclusion of the thermal expansion effect. Results for R, and X, are given for a bulk YBCO sample at 13 GHz over the range 15-190 K. Good agreement with the "enhanced" two-fluid model is found for /spl lambda/(T), and /spl lambda/=657 nm at 77 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Anomalous magnetoresistance in bulk polycrystalline Y 1Ba 2Cu 3O 7−d

The nature of the resistive transition of bulk YBCO samples in low d.c. magnetic fields was investigated. Just below the phase transition onset, the under-field resistance becomes notably higher than the resistance prior to field application. In addition a difference between the resistance after field removal and the resistance prior to its application is also observed. Over part of the temperature range, this resistance exhibits anomalous negative remanent magnetoresistance. A qualitative interpretation is proposed.

Thermoremanence and Meissner effect in QMG and single-crystal YBCO

We compare flux pinning in a single crystal (twinned and detwinned) of YBa 2Cu 3O 7− x (YBCO) with that in a quench-melt-growth (QMG) bulk YBCO sample using measurements of the thermoremanent magnetization and Meissner effect. For temperatures up to 50 K, there is practically no difference in the thermoremanent magnetization of the crystal in the twinned and detwinned states, while above 50 K the remanent flux in the twinned state is larger. The rate of decrease of thermoremanent magnetization with temperature is much lower for the QMG material. The QMG material has practically no Meissner effect over a wide range of applied fields, whereas a considerable difference in Meissner effect is seen between the twinned and detwinned states of the single crystal.

High Tc superconductor voltage-current simulator and the pulse method of measuring critical current

A passive voltage-current ( V-I) simulator has been developed and tested using conventional direct current (d.c.) and pulse current methods. The simulator was designed to generate the extremely non-linear V-I characteristic of a superconductor and has a similar power-law behaviour. It is intended to be used to test various components of the measurement system such as instrumentation, measurement method and data analysis software, to determine the transport critical current, I c, or critical current density, J c, of a superconductor. This simulator does not emulate all the subtle effects of flux flow in a superconductor. However, it approximates a superconductor's major static and dynamic electrical characteristics. The simulator is therefore a necessary but incomplete test of a measurement system. Critical currents obtained on the simulator using the pulse and d.c. methods are compared and show close agreement. Also, preliminary comparisons of methods using bulk and thin film YBCO samples are given.

Gamma radiation effects on some properties of YBCO

Radiation effects of polycrystalline YBCO bulk sample irradiated by 60Co γ-rays, dose of 1×106 up to 7.5×108 rad, at room temperature on critical temperature and critical current were investigated. IR spectrum was also used to study the mechanism of the irradiation. A considerably strong dependence of these parameters upon the irradiation dose was observed. No significant effects on the critical temperature were found, but the critical current in zero magnetic field changed greatly. It shows a tendency to decrease with the increase of the irradiation dose except for a slight increase with the dose less than about 2×107 rad and no simple relations between critical currents and irradiation doses was found. A typical case is that the critical current is reduced to about 60% when the dose reaches 5×109 rad, but the dependence of critical currents on the magnetic field shows that the critical currents are higher than those of the unirradiated one in the range of magnetic field higher than 100 G and decrease more slowly in a magnetic field compared with the unirradiated one. The results indicate that the defects produced by γ-ray irradiation are beneficial to flux pinning in higher fields. IR spectra analysis reveals that the intensity of the peak responsible for the Cu(1)− O(1) chain vibration is decreased, indicating that the bond of the Cu(1)−O(1) may be partly broken through collision process of the Compton electron produced by the γ-ray. This effect probably gives rise to a decrease of the critical currents.

Structure characteristics of melt-processed YBCO

SEM, TEM, SAM and X-ray diffraction analyses have suggested that the bulk YBCO samples sintered with the co-precipitation powders are almost free of second phrases. However, a layer with non-123-stoichiometric compositions is always present near the grain boundaries, which forms the dominant zone of the weak links in these samples. The improved critical current density with reducing field dependence can be obtained by the melt-process. It has produced a multiphase microstructure with highly aligned plate-like 123 grains up to several mm in length and several mu m in thickness, which are parallel to the a-b plane, and the grain plate sometimes is the same width as the sample, and boundaries between parallel plates are usually discontinuous. Such structure is very favourable for the improvement of Jc. However, there are still various structure defects, such as second phases (CuO, BaCuO2, BaCO3 and 211 phase), cracks and some interfaces between adjacent superconducting regions with different c axis direction in the melt-processed samples. They would limit further improvement of the Jc of melt-processed samples.

Gamma Radiation Effects on Some Properties of YBCO

ABSTRACTRadiation effects of polycrystalline YBCO bulk sample irradiated by 60Co γ-rays from a dose of 1x106 up to 7.5×l09 rad at room temperature on critical temperature and critical current were investigated. IR spectrum was also used to study the mechanism of the irradiation. A considerable dependence of these parameters upon irradiation dose was observed. No significant effects on critical temperature were found, but the critical current in zero magnetic field changed greatly. It shows a tendency to decrease with the increase of the irradiation dose except for a slight increase with the dose less than about 2×107 rad and no simple relations between critical currents and irradiation doses was found. A typical case is that the critical current is reduced to about 60% when the dose reaches 5×109 rad, but the dependence of critical currents on magnetic field shows that the critical currents are higher than that of the unirradiated one in the range of magnetic field higher than 100 Gauss and decrease more slowly in magnetic field compared with the unirradiated one. The results indicate that the defects produced by γ-ray irradiation are benefit to flux pinning in higher fields. IR spectra analysis reveals that the intensity of the peak responsible for the Cul-O1 chain vibration is decreased, indicating that the bond of the Cul-O1 may be partly broken through collision process of the Compton electron produced by the γ-ray. This effect probably gives rise to decrease of the critical currents.

Two hole SQUID behaviour in a bulk YBCO sample at 77K

We describe the development of a 2-hole radio frequency SQUID made from a single disc of YBa 2Cu 3O 7−y. The device displays single flux quantum sensitivity and an estimated noise level of 1 × 10 −3φ o/Hz above 50Hz.