Levitation Properties Research Articles

Influence of surface doping on sintering and microstructure of a superconducting oxide

The influence of surface halogen doping on the sintering and microstructure of Y 1Ba 2Cu 3O 7− x (YBCO) has been investigated. It is seen that exposing the starting YBCO powder to Br gas prior to sintering results in secondary recrystallization and formation of long columnar grains. The chemical composition and structure of the overall material is unchanged indicating that it is only the microstructure that is effected by the bromine. Since large domain sizes and preferred orientations are key to electrical and levitation properties of these materials, this effect of increased grain growth may prove to be very useful in controlling the texture of these materials. Even in randomly mixed composites of Ag and YBCO, Br is seen to increase the grain size and porosity of the YBCO regions, indicating that grain growth outpaces pore elimination. It is likely that Br, as a surface additive, enhances liquid phase formation and/or increased diffusion at the interfaces that lead to these effects. The atomistic mechanism of the process and the quantitative optimization of the doping concentration need to be investigated.

Control of the growth rate of YBa 2Cu 3O x single-domained crystal by applying two-step undercooling in top-seeded melt growth

Parameters affecting the nucleation and growth of a Y123 single-domained crystal were studied in top-seeded melt processing. Based on the parametric study, the two-step undercooling technique was developed. The key points techniques are the controlled nucleation of Y123 at the primary undercooling step and the accelerated growth at the secondary undercooling temperature. Optimization of the process parameters in the two-step undercooling technique considerably reduced the processing time required for the growth of the Y123 single crystal without degradation of the levitation property.

High- Tc superconductor-polymer composites: YB 2Cu 3O 7− x-polyester polymer and YBa 2Cu 3O 7− x-Teflon

The YBa 2Cu 3O 7− x (YBCO) superconductor powder-polyester polymer (Polimal 109) and YBCO-Teflon composites were investigated. The volume fraction of YBCO powder ranged from 0.05 to 0.8. D.c. electrical conductivity of the composites increased exponentially with increase of YBCO volume content. The I–V characteristics showed transition from ohmic to highly nonohmic behaviour at electric field intensities of about 100 V/cm. Although these composites do not show zero resistance below the superconducting transition temperature T c of YBCO, the intrinsic diamagnetic properties of superconductor are observed. The magnetic field shielding effect indicates the superconducting transition at 90 K. The investigated composites have, at liquid nitrogen temperature, levitation properties similar to those of ceramic superconductors. The levitation force is proportional to the YBCO volume content.

Improvement of the Force Relaxation in HTSC Magnetic Bearing.

In the present study, we described a method for improving the influence of flux creep on the levitation property in HTSC magnetic bearing. The scenario of this method is to induce reverse flow shielding currents in two regions of the HTSC bulks through varying the distance between the permanent magnets and the HTSC bulks. The magnetic forces produced in both these regions also take opposite directions. In this case, the difference between the two forces will give the net magnetic force in the HTSC magnetic bearing. Obviously, the actual force decay observed in the HTSC magnetic bearing is determined by the difference between the force decay in these two regions, therefore being resultantly reduced. This phenomenon can be called as 'Hysteresis Hardening Effect'. For the evaluation of the mentioned approach, numerical simulations were carried out with the configuration of a practical radial-type HTSC magnetic bearing. The numerical results show the validity of the present method. Also some discussions were added with concerning of its practical application.

Characterization and properties of batch-processed melt-textured YBCO

High-temperature superconductor bulk parts are batch processed using a semi-automated processing technique based on the melt-texturation process. Levitation properties under static and dynamic load levels were analysed using a test bench with a three-dimensional force sensor unit. Measurements of levitation force give no detailed suggestions on texture, secondary domains or cracks. Therefore other measurements to control homogeneity of the bulk were performed. Texture on full-size pellets (FWHM ) was verified by elastic neutron scattering. To study the influence of local texture on properties a pellet was divided into nine segments. Preliminary results indicate that a correlation between neutron data and levitation force needs further investigations. Flux maps of samples magnetized by permanent magnets or by a pulsed magnetization unit were used to verify the homogeneity and domain structure of the material and to evaluate macroscopic critical currents.

Processing technique for fabrication of advanced YBCO bulk materials for industrial applications

We present a processing technique for fabrication of advanced semi-finished parts and products based on the high-temperature superconductor (HTSC) YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO). High-quality YBCO bulk components are produced powder metallurgically by sintering and partial melting of precursor powders. Depending on size, shape, tolerances and levitation requirements, parts are either die-pressed or precision machined from isostatically pressed blocks. The oxygenation process can be monitored in-situ by a macro-thermogravimetric analyzer. The texture of the bulk of full-size pellets was verified by elastic neutron scattering. Levitation properties under static and dynamic load levels were analyzed using a test bench with a three dimensional force sensor unit. Flux mapping was used to verify the homogeneity of the material, to evaluate macroscopic critical currents and to investigate the field trapping capability. Several HTSC frictionless bearing modules have been built. They are manufactured in various sizes, ranging from O 40 mm to O 140 mm. Lifting forces are up to 200 N per bearing module. Applications for the bearings are seen for mechanical components in high speed rotating machinery, in cryogenic systems, gyroscopes, transportation systems and vibration isolation.

高温超電導浮上型リング形状フライホイールの浮上特性

In this paper we propose a new combination of high Tc superconducting levitation and ring-shaped flywheel energy storage systems. Superconducting levitation is appropriate for rotating a ring-shaped flywheel which has neither shaft nor hub, because it is a non-contact and automatically stable levitation without any control systems. The levitation properties such as static and dynamic lateral stiffnesses, lateral damping, and lateral vibration during rotation have been investigated using a small-scaled experimental machine consisting of 16 bulk superconductors 46mm in diameter and a ring-shaped flywheel about 300mm in diameter. The spring constant increased as the levitation gap height decreased, and the dynamic spring constant was slightly higher than the static constant. The damping coefficient increased as the gap height decreased and the vibration amplitude increased. The experimental critical speed was in good agreement with the calculated one using a one-degree of freedom model. Finally, the possibility of large-scaled practical systems is discussed from the viewpoint of superconducting levitation.

Optimized configurations of autostable superconducting magnetic bearings for practical applications

In order to establish an optimized bearing design for a flywheel for energy storage, we have studied model bearing configurations involving bulk YBCO pellets and double-dipole magnet configurations. We were interested to see what the correlation is between the maximum attainable levitation force, measured for a typical bearing gap of 3 mm, and the separation between the magnetic poles. Equal polarity (north-north) and alternative polarity (north-south) configurations were investigated. The maximum levitation force was obtained with the alternate polarity arrangement for a separation between the magnetic poles of 6 mm. It represents an increase of 19% compared to a nonoptimized configuration. Our experiments demonstrate that configurations of superconducting magnetic bearings can be optimized to obtain better levitation properties.

Enhanced levitation properties in melt-textured YBCO samples grown without temperature gradient

We investigated different techniques for magnetic-levitation property enhancement of the melt-textured YBCO sample grown without temperature gradient. No improvement in microstructural properties could be achieved by increasing the excess Y-211 phase percentage above 20% (molar) or by adding a few weight percent of Ag. Large grain dimensions (up to 20 mm) were reached in samples undergoing both a presintering treatment at 950°C and a stage of several hours at 1000°C during the growth slow-cooling phase. The obtained samples are characterized at 77 K by a levitation force of 26.9 N and a magnetic critical current density up to 10 4 A/cm 2 at 1 T. Furthermore a single-grain sample could be obtained placing a Nd 2O 3 powder seed onto the sample surface.

Microstructure and levitation properties of floating zone melted YBCO samples

Radiation zone melting has been used to produce texture in sintered YBCO cylindrical samples. Microstructural analysis by electron microscopy and pole figure measurements reveals that the production process gives rise to a preferential orientation within large domains. DC transport measurements show that changes in alignment orientation can result in the inability to carry a transport current. Both AC magnetic field shielding and levitation properties are substantially improved by the floating zone melting, the levitation force being increased with the textured domain size growth. >

Magnetic bearings using high-temperature superconductors: some practical considerations

An experimental study of Meissner effect levitation involving high-temperature superconductors is presented. Two simple apparatuses are described for quantifying the static and dynamic characteristics of levitated systems. The levitation properties of Y-Ba-Cu-O in conventional pressed ceramic form and in insulating matrix composites are compared. Temperature-dependent force measurements are correlated with SQUID magnetometry data. A preliminary assessment of the practical potential of magnetic levitation using high-temperature superconductors is provided.

Levitation properties of the YBa2Cu3Ox and Tl-Ba-Ca-Cu-O superconducting systems

A torsion balance is used to measure the levitation force on a magnet as a function of height above bulk samples of the YBa2Cu3Ox (Tc∼90 K) and Tl-Ba-Ca-Cu-O (Tc∼110–120 K) superconducting systems. Measurements of magnetic shielding and trapped magnetic fields are also made.