Textured Bulk Research Articles

The property of the vortex glass in the vicinity of the pinned vortex liquid for textured bulk YBa2Cu3O7

We have performed AC susceptibility measurement and shown the property of the vortex glass in the region near the pinned vortex liquid, where it is proposed that the velocity of flux diffusion, ν=ν0(j/jc)exp(-U/T), instead of ν=ν0exp(-U/T) in the region far away from the glass melting line for providing a gradual crossover to the viscous flux flow regime (ν ∝j, where j is current density). It is shown that the additional factor (j/j0) in the velocity is necessity to explain the experimental finding of the dependence of time scale t0 on the ac field amplitude.

Influence of the sinter-forging conditions on the mechanical properties of textured bulk (Bi,Pb)-2223 ceramics

(Bi,Pb)2Sr2Ca2Cu3O10+x high critical temperature superconductive oxide have been synthesised and densified into bulk and textured ceramics by a sinter-forging process. Three point bending tests were carried out, both at room temperature (manipulation temperature) and at 77 K (application temperature), to investigate the mechanical characteristics of such ceramics, relatively to their coarse microstructure and structural anisotropy.

Peak effect in Jc and sub-micron structures of Y-Ba-Cu-O superconducting bulks melt-grown under varying oxygen pressure

Abstract A novel melt-growth technique, or a melt-growth under varying oxygen pressure (MG-VOP) method in which the oxygen partial pressure is increased to solidify the initial melt, has been employed to obtain textured bulk samples consisting of a superconducting phase, YBa2Cu3O7−δ (Y-123), and a secondary (non-superconducting) phase, Y2BaCuO5 (Y-211). The Jc values of such samples reached as high as 40,000 A/cm2 (at 65 K) even under a high magnetic field of H=3 T, to exhibit a peak in the Jc- vs -H relation, i.e. a peak effect. To understand the mechanism of such a peak effect and the high values of Jc at high magnetic fields, sub-micron structures of the MG-VOP processed samples were examined by means of transmission electron microscopy (TEM), and the distributions of both Y-211 particles and twin boundaries were studied. In order to collect microstructural data for statistical analyses, TEM specimens were prepared using a focused ion-beam (FIB) thinning technique. TEM-observable area of such a specimen was as wide as 20×20 (μm)2. A statistical analysis of the TE micrographs yielded the fraction of the Y-211 phase, the average size of Y-211 grains and the inter-twin-boundary distance for each sample. It is concluded that a sample that has the higher twin-boundary density possesses the higher Jc value under a high magnetic field in the vicinity of the peak, while fine Y-211 particles are likely to contribute to the Jc value only at very low fields.

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.

Effects of Pulling Direction on Directional Growth of YBa2Cu3O7-x Superconductors

The effects of the pulling direction (upward and downward) on the microstructure and the superconducting properties of the textured bulk YBa2Cu3Ox high-temperature superconductor samples fabricated using extrusion and directional growth of the superconductor grains reacted from Y2BaCuO5, BaCuO2 and CuO powder mixtures were studied. Compared to the pulled-down sample, the pulled-up sample showed relatively dense and well-textured microstructure, high critical temperature and high critical current density.

Scaling properties of the anisotropic critical current density in bulk textured YBaCuO. Evidence toward a 3D flux line lattice

We have measured dc transport critical current densities of melt texture grown and magnetically melt textured bulk YBaCuO at 77 K and in magnetic field. A maximum value of over 31,000 A/cm2 is obtained with a field of 7 teslas applied parallel to the (a, b) planes. Over the rest of the angular range the critical current is shown to be determined mainly by thec-axis component of the applied field. Although this dependency is expected in the presence of two-dimensional vortices, in fact the data are shown to correspond better to the behavior expected of an anisotropic three-dimensional superconductor. These results are compared to magnetization measurements on the same samples. We show that when the field is directed close to the c-axis, superconducting transport currents flow at fields well above the field at which the irreversible magnetization disappears.

POINT CONTACT SPECTROSCOPY IN ORIENTED YBa2Cu3O7−δ

We report point contact spectroscopy measurements on YBa 2 Cu 3 O 7−δ thin films and textured bulk ceramics. The I(V) curves measured along the Copper-Oxygen (ab) plane direction are characteristic of the Andreev reflection phenomenon, with a sharp break at the gap edge. This allows a rather precise determination of the (in plane) gap, Δ = 20 mev . Subgap structure suggests the existence of a smaller gap for some orientations in the (ab) plane, of the order of 12 mev. The data also enabled us to set a lower bound to the inplane Fermi velocity in YBa 2 Cu 3 O 7−δ: v F ≥ 7 × 107 cm/sec . The value of v F has a crucial importance in the determination of the theoretical explanation for superconductivity in the High Temperature Superconductors. For out of plane measurements, only a weak zero bias anomaly is observed at energies up to 5 mev.

Magnetic field dependence of d.c. critical current for large samples of textured bulk YBaCuO

Large samples of textured YBaCuO were prepared at the Texas Center for Superconductivity using a continuous process. These were cut into a rectangular shape with typical dimensions 4.0 cm × 0.5 cm × 0.25 cm. The large physical size of these samples and their ability to carry high currents particularly in zero magnetic field required low resistance contacts to avoid Joule heating at the sample current - electrode interface. Accordingly, low resistance metal contact pads for current and voltage electrodes were applied at NIST Boulder, using a sputter etch, sputter deposition, thermal evaporation and oxygen anneal procedure. Transport critical current density measurements at 77 K, and in magnetic fields up to 6 T were performed at NRL. A holder was designed to hold the samples securely in such a way that would allow for contraction during cool-down and to support them against the anticipated large Lorenz forces. The highest critical current measured with no external field was 393 A. This corresponds to a critical current density of 1871 A cm −2. At 1 T, these values decreased to 229 A or 1090 A cm −2. These critical current values, found in material that was fabricated using a technique capable of being scaled up for larger sizes, demonstrate a potential usefulness in large scale applications.

Sharp gap edge and determination of the fermi velocity in Y1Ba2Cu3O7?? by point contact spectroscopy

We report point contact spectroscopy measurements on Y1Ba2Cu3O7−δ thin films and textured bulk ceramics. The I(V) curves measured along the copper-oxygenab plane direction are characteristic of the Andreev reflection phenomenon, with a sharp break at the gap edge. This allows a rather precise determination of the (in-plane) gap,Δ = 20 meV. Subgap structure suggests the existence of a smaller gap for some orientations in theab plane, of the order of 12 meV. The ratio of the zero bias conductance to the normal state one, in theab direction measurements, enabled us to set a lower bound to the Fermi velocity in Y1Ba2Cu3O7−δ in this direction:υF>-7 × 107 cm/sec. The value ofυF has a crucial importance in the determination of the theoretical explanation for superconductivity in the high-temperature superconductors. For out-of-plane measurements, only a weak zero bias anomaly is observed at energies up to 5meV.

Critical Current Densities JC(H,T) and Current-Voltage Characteristics in Melt Processed Bi-2212 with and without Preferential Orientation

ABSTRACTIn contrast to sintered Bi2Sr2Ca1Cu2Oy (Bi-2212), melt processed material exhibits nonzero intergrain supercurrents at T < 77 K. By means of partial melting and long term annealing we achieved nonoriertted material with Jc(77) - 3500 A/cm2 (1μV/cm criterion) and Tc = 96 K. Zone melting leads to well textured bulk material with the c-axis of the crystallites oriented perpendicular to the pulling direction R, and thus to an anisotropie critical current density. The anisotropy factor is between 2 and 5. At low temperatures the melt textured ceramic exhibits higher critical currents than the nonoriented material. This is not true at high temperatures, since the latter has a higher critical temperature.At 77 K and small fields both oriented and nonoriented ceramics exhibit no transition in their current-voltage characteristics, but obey the power law E = Eo(j/jo(H)) α(H) over a large range in E (10”−3-10”−3 V/cm). Measurements on the textured ceramic with magnetic fields parallel and perpendicular to the pulling direction demonstrate that the superconducting properties of these materials are only affected by the field components parallel to the c-axes of the crystallites. For fields parallel to the pulling direction the measured values of α(H), Jo(H) and of the reversible magnetization Mrev can be attributed to the misorientation of the Bi-2212 platelets which is in average about 14°.

Impurities and defects of high-J c YBa2Cu3Ox superconductor with flux jumps

The microstructures of a highly textured bulk YBa2Cu3 (123) superconductor, with the high-transport Jc (≥ (R18)3.7×104 A/cm2 at 77 K) and flux jumps (T<15 K), were characterized by x-ray diffraction, polarized optical microscopy, scanning electron microscopy, and scanning transmission electron microscopy. It was found that the grains of the 123 superconductor are preferentially oriented along the a-b plane of the 123 phase. The Y2BaCuO5 and CuO particles are dispersed uniformly in the 123 matrix, and the highly dense twins run through the matrix everywhere. Some irregular-shaped BaCuO2-CuO mixtures were observed at the 123 grain boundaries. The aligned 123 crystals and microstructural defects in the 123 matrix are considered to be responsible for the high-Jc value and flux jumps.

Grain orientation and anisotropic superconducting properties of textured bulk YBa 2Cu 3O x

Intensive X-ray diffraction (XRD) analysis technique along mutually orthogonal surfaces, has been used to study various structural details (average grain size, unit cell parameter, microdistortion, mean oxygen concentration and anisotropy) of preferred grain oriented (textured) bulk YBa 2Cu 3O x. The estimated anisotropy agrees well with the critical current determination from ac-susceptibility measurements.

Highly oriented Bi system bulk samples prepared by a decomposition-crystallization process

A decomposition-crystallization method for preparing highly oriented Bi system bulk samples is reported. The effects of processing parameter, decomposition temperature, cooling rate, and post-treatment conditions on texture and superconductivity have been investigated. The method has been used to prepare highly textured Bi system bulk samples. High-temperature anealing does not destroy the growing texture, the cooling rate has some effect on texture and superconductivity, and annealing in N2-O2 atmosphere can improve the superconductivity of the textured sample.

Microstructure of a textured YBa2Cu3Ox superconductor with flux jumps

Abstract The microstructure of a highly textured bulk YBa 2 Cu 3 O x (123) superconductor, with high transport J c (≥ 3.7×10 4 A/cm 2 at 77 K) and flux jumps (T ≤ 10K), were characterized by XRD, polarized optical microscopy, SEM and STEM. It was found that the grains of the 123 superconductor are preferentially oriented along the a-b plane of the 123 phase. The Y 2 BaCuO 5 and CuO particles are dispersed in the 123 matrix and the highly dense twins run through the matrix everywhere. Some irregular-shaped BaCuO 2 CuO mixtures were observed in the grain boundaries. The aligned 123 crystals and microstructural defects in the 123 matrix are considered to be responsible for the high J c value and flux jumps.

Homopolar DC motor and trapped flux brushless DC motor using high temperature superconductor materials

Two motors have been designed and built for use with high-temperature-superconductor (HTSC) materials. They are a homopolar DC motor that uses HTSC field windings and a brushless DC motor that uses bulk HTSC materials to trap flux in steel rotor poles. The HTSC field windings of the homopolar DC motor are designed to operate at 1000 A/cm/sup 2/ in a 0.010-T (100-G) field. In order to maximize torque in the homopolar DC motor, an iron magnetic circuit with small air gaps gives maximum flux for minimum Ampere turns in the field. A copper field winding version of the homopolar DC motor has been tested while waiting for 575 Ampere turn HTSC coils. The trapped flux brushless DC motor has been built and is ready to test melt textured bulk HTSC rings that are currently being prepared. The stator of the trapped flux motor will impress a magnetic field in the steel rotor poles with warm HTSC bulk rings. The rings are then cooled to 77 K to trap the flux in the rotor. The motor can then operate as a brushless DC motor.

Study on the magnetization of 107K textured BiPbSrCaCuO bulk material

Abstract We have studied the magnetization of the cold-pressed bulk Bi 1.8 Pb 0.34 Sr 1.86 Ca 2.0 Cu 3.0 O y sample, which is compared with the sample of the same composition by conventional solid state reaction method. Cold pressing results in the orientation of the grains and good connectivity, on the other hand, it also leads to the formation of many pinning centers in intergrains. These are reasons of the improvement in transport critical current density Jc. The difference of magnetic relaxation between the two samples may come from the distinct pinning mechanism. The pinning potential energy Uo for the cold-pressed samples also has been discussed.

Manufacture of bulk superconducting YBa2Cu3O7-δ by a continuous process

THE high critical current densities (Jcs) measured in thin films and within grains of the high-transition-temperature (high-MTc) oxide superconductors indicate that low Jc should not prevent the use of high-Tc materials in high-current devices1. Indeed, a Jc of ∼7.5×104 A cm−2 has been obtained in highly textured bulk YBa2Cu3O7 ('123') at 0 T, processed by a melt-textured2 or liquid-phase3 growth technique. Unfortunately, textured materials with this improved Jc comprised only a very small portion (< 1 cm) of the bulk precursor, which had random grain orientations on a larger scale. For high-current applications, one needs bulk materials fabricated in usable shapes of large dimensions with controlled grain orientation to carry a high total current, not just a high current density. Here we report the development of a continuous process for fabricating large bulk superconductors with a pre-determined grain orientation. A bar of the 123 compound with dimensions 5 x 0.5 x ∼0.3 cm with excellent grain alignment (a-b plane along the longest sample dimension) has been fabricated continuously. The bulk 123 thus obtained has magnetically determined Jcs of ∼3 x 104 and 1.2 x 104 A cm−2 at 0 and 1 T, respectively and transport Jcs of 2 x 104, 1.1 x 104 and 7.5 x 103 A cm2 at 0, 0.54 and 0.83 T.