Melt-textured YBCO Research Articles

Scaling of nonlinear complex susceptibility applied to the study of type-II superconductors

Different χ measurements can be collapsed into a single universal curve if plotted against the scaling variable h( ω ̃ /ω) 1/σ or, more generally, h/ Jd, where J( T, B, ω) is a frequency dependent shielding current density, d is the sample half-width and h is the AC field amplitude. We have tested these scaling laws for two Nb samples, one melt textured YBCO sample, and one Er123 single crystal. Further, we found that J( T, B, ω) for Er123 displays a behavior closely related to the second magnetization peak ( fishtail effect), observed in M× H curves. Our results are satisfactorily interpreted in terms of vortex dynamic models.

New permanent magnets

Permanent magnets play an important role and are widely spread in daily-life applications. Due to their very low costs, large availability of the row materials and their high chemical stability, hard ferrites are still dominant in the permanent magnet market although their relatively poor magnetic properties are a distinct disadvantage. Today's high-performance magnets are mostly made from Nd 2Fe 14B. The aim of research is to combine the large spontaneous magnetization of 3d metals with strong anisotropy fields known from rare-earth transition-metal compounds and, at the same time, to maintain a high value of the Curie temperature. However, the number of iron-rich rare-earth intermetallics is very limited and, consequently, not much success can be noted in this field for the last 10 years. One alternative concept is to use magnetic fields trapped in type II superconductors where much higher fields can be achieved compared to conventional rare-earth magnets. Very recently, we obtained a trapped field as high as 14.4 T in a melt-textured YBCO bulk sample of a few centimeters in diameter. This is the highest value ever achieved in a bulk superconductor. The trapped field of a superconductor is not governed by the Laplace equation and, therefore, levitation works without any additional (active) stabilization. The disadvantage of these magnets is their low working temperature (of liquid nitrogen and below).

Investigation of high-Tc bulk material for its use in resistive superconducting fault current limiters

Superconducting fault current limiters (SCFCL) offer an attractive means to limit short-circuit currents in power systems. Comparisons of available HTS material led to the derision to investigate melt cast processed (MCP) BSCCO and melt textured polycrystalline YBCO for use in resistive SCFCLs. As a great length of a superconductor per unit element is preferred, the BSCCO samples are shaped as bifilar coils prepared from tubes and the YBCO samples are manufactured in a meander shaped geometry. Test results concerning contact resistances, E-J curves, AC losses as well as the quench behaviour of the samples are presented. The experiments reveal the requirement of a high material homogeneity as the main challenge.

Test results of 2-kWh flywheel using passive PM and HTS bearings

Toward demonstrating the potential of flywheel energy storage systems that use high-temperature superconductors (HTSs) and permanent magnets (PMs) as passive rotor bearings, a flywheel system was developed and tested with a 165-kg cylindrical carbon- and glass-fiber rotor to rim speeds of 400 m/s (19,000 rpm) and stored energies of >2.25 kWh. The main bearing's internal stack of PM rings was passively stabilized by HTS bearings at each end of the rotor. The stator portion of the HTS bearing consisted of an array of melt-textured YBCO pellets bathed in liquid nitrogen inside a nonconducting cryochamber. The motor/generator (M/G) was based on an internal-dipole Halbach array and could produce 1.5 Nm of torque. Each bearing and the M/G included multipiece banded PM rings secured to the rotor inside diameter with flexible urethane rings. In a vacuum enclosure at 10/sup -4/ Pa pressure, rotational drag on the rotor was hysteretic and at low speeds the coefficient of friction was well below 10/sup -6/.

Study of tin + cerium doping in YBCO bulk top seeded pellets

The effect of tin+cerium doping in top seeded melt textured YBCO has been studied. The compositions correspond to 0.5 wt.% CeO 2 with varying tin amounts from 0.25 to 1 wt.% in steps of 0.25 wt.%. In order to increase the bulk pellet density, the influence of pre-sintering on each composition has also been investigated. In spite of a slight decrease of T c, the best superconducting properties were obtained for the 0.25 wt.% SnO 2 doped sample without pre-sintering, with a critical current density reaching higher than 90 kA/cm 2 in self-field and 50 kA/cm 2 under 1 T at 77 K. Higher tin additions appear to damage the whole sample homogeneity and also the superconducting properties. The pre-sintering step, although beneficial to the pellet density, decreases the pinning under field.

Uranium fission fragment pinning centers in melt-textured YBCO

In the U/n process YBCO powders are doped with uranium, textured, and then irradiated with thermal neutrons. During texturing almost spherical deposits of (U 0.6Pt 0.4)YBa 2O 6 (called Y-5), 300nm in diameter, are formed. Prior to irradiation Y-5 deposits increase J c about 10% 0.1% M U, where M U is the percentage by weight of admixed uranium which contains an intimate mixture of both the 235U and the 238U isotopes. We find the number of Y-5 deposits ∝ M U. Hence, inter-deposit spacing, S ∝ M U −1 3 . Subsequent irradiation with thermal neutrons produces fission fragment defects that also act as pinning centers. The effective length for pinning, δ, of these pinning centers was estimated theoretically to be 2μm to 4μm. The effect of the chemical and irradiation pinning centers is multiplicative and increase J c, depending on applied field and temperature, by factors of 14 to 40. The value of F n at which J c peaks depends only on the product of F n and M U(235), where F n is the numberof neutrons per square centimeter. We find that optimum F n is inversely proportional to M U(235). M U was varied from 0.0375% to 1.0%, with M U(235) held constant at 0.025%, thus varying S by a factor of 3. The enhancement of trapped field, R M, is 500% at 0.15%<M U<0.30% and drops to 400% for o.375% M U, indicating δ ∼ 2.7 μm. Anisotropy of (3mm) 3 samples decreases monotonically from 3 to 2 with increasing F n. Residual radioactivity six months after irradiation is <200 nCi/g or <7400 Bq/g for samples with 0.025% M U(235) at optimum F n.

Microstructure and superconductivity of the melt-textured YBCO grown with the addition of SnO 2

Large domain YBa 2Cu 3O 7−δ/Y 2BaCuO 5 (YBCO) samples were prepared by top-seeded melt-textured process, with the addition of SnO 2 and CeO 2 or PtO 2. It was found that, in comparison to that in the CeO 2-doped sample, the YBa 2Cu 3O 7−δ (123) matrix was twisted in the PtO 2-doped sample showed a more obvious second-peak effect, and higher values of a-b plane critical current density ( J c a b ) and irreversibility fields ( B irr ). The enhancement of J c a b is related to the microstructure inhomogeneity and the core surface Δκ pinning.

Flux line depinning in a magnet-superconductor levitation system

The AC loss characteristics of a magnet-superconductor system were studied with the magnet fixed to the free end of an oscillating cantilever located near a stationary melt-textured YBCO pellet. Below a threshold AC field amplitude ≈2 Oe, the dissipation of the oscillator is amplitude-independent, characteristic of a linear, non-hysteretic regime. Above threshold, dissipation increases with amplitude, reflecting the depinning and hysteretic motion of flux lines. The threshold AC field is an order of magnitude higher than that measured for the same YBCO material via AC susceptometry in a uniform DC magnetic field. A partial lock-in of flux lines between YBCO ab planes is proposed as the mechanism for the substantial increase of the depinning threshold.

FABRICATION AND CHARACTERIZATION OF LARGE GRAINS GROWN ON YBCO PELLETS BY SEEDED MELT TEXTURING

In this work, the top-seeding technique was applied to the fabrication of both melt-textured YBCO and YBCO/Ag pellets. Both REBCO single crystals and single grains of REBCO are used as seeds. The effects of metal additive and seed nature on texture quality of the samples are discussed.

Flux dynamics and critical state in YBCO by AC harmonic susceptibility

The temperature dependences of the third harmonics of the AC susceptibility, χ 3′( T) and χ 3″( T), have been used to study the vortex dynamics in Melt Textured YBCO. The experimental curves measured at different AC field amplitudes and frequencies, have been compared with the simulations of χ 3′( T) and χ 3″( T) obtained by considering the diffusion of the magnetic flux in the presence of different dynamic regimes. The comparison made as function of the AC field frequency shows that, at low frequencies, the magnetic response is mainly described in terms of a Bean critical state model. As the frequency is increased, the contributions coming from the flux dynamic regimes become predominant. Nevertheless, at the highest frequencies, contrary to what was expected, the curves appear again very similar to the ones simulated in terms of a pure Bean critical state model.

Developing melt-textured YBCO

Melt-textured yttrium-barium-copper-oxide is suitable for many applications. By testing demonstrators where these elements are used, it can be learned which material properties must be optimized for a specific application field. For example, critical-current density must be increased for use in electric motors. Mechanical properties are very important for trapped-field magnets. Homogeneous properties on large monoliths seem necessary for bearings in fly-wheel systems.

Jc Enhancement in Melt Textured YBCO with Different 211 Content by Fast Neutron Irradiation

ABSTRACTMelt textured YBCO samples with and without 211 inclusions were irradiated by fast neutrons. The enhancement of the critical current density was studied as a function of the 211 content by means of SQUID magnetometry at temperatures from 5 to 77 K and magnetic fields from 0 to 8 T. The highest enhancement factor, found at 77 K and 1.5 T for a 211-free sample, is 27. The enhancement is less effective for samples with 211 inclusions. However, the critical current densities after irradiation show a similar temperature and field dependence in all samples. The role of the initial microstructure on the radiation effects is discussed.

STUDY ON THE TECHNOLOGY AND PROPERTY FOR Y2BaCuO5 PARTICLE S ARTIFICIAL DOPING TO OBTAIN SINGLE-CRYSTAL DOMAIN MELT-TEXTURED YBCO BULK SUPE RCONDUCTOR

This paper describes the method of artificial doping Y-211 phase and melt-textur ed growth with a top-seeded technique to prepare the quasi-single crystal domain YBCO bulk materials with various Y-211 particle contents.The specimens obtained have high density (more than 62g/cm3) and good mechanical toughnes s.The Jc of specimens reaches 123×106A/cm2 u nder 06T at 30K and 135×104A/cm2 under 2 T at 70K.The scanning electron microscope observation of specimens indicates that Y-211 part icles can improve the quality of melt-textured YBCO superconductor and reduce th e microcrack of specimens.Combining the microstructures with Jc measu rements shows that the smaller the particle size,the more homogeneous the partic le distribution,the higher the pinning effectiveness.The best ratio of Y-123 to Y-211 for the superconductor specimens is 1∶05 in moles,in which the Y-211 pa rticles are uniformly distributed and their size is about 200nm.

An investigation on the critical current density of the melt-textured YBCO with Sm211 addition

Sm 2BaCuO 5 (Sm211) addition shows an improved critical current performance and the peak effect in the critical current density J c vs. applied field H curve at 77.3 K. The post-annealing at 950°C reduces the critical current density and eliminates the peak effect. The followed oxygenation cannot recover them completely, but it seems that a weak peak appears again after oxygenating at 450°C for 120 h. The peak behavior of J c may be connected with the formation and disappearance of the spatial inhomogeneous (Y,Sm)123 matrix, in which a local low- T c (Y,Sm)(Ba,Sm) 2Cu 3O y phase forms owing to the substitution of Sm for Ba sites, especially in Sm-rich regions.

Current-voltage characteristics of melt-textured YBCO obtained from the field-sweep rate dependence of magnetization

We investigated current-voltage (I-V) characteristics in melt-textured YBCO sheets by measuring the field-sweep rate /spl beta/ dependence of magnetization M. We used a previously developed method that corrects for the current density distribution in the specimen [Y. Mawatari et al., Appl. Phys. Lett., vol. 70, 2300 (1997)]. For a wide temperature and field range (60-80 K, 0.2-5.0 T) the I-V curves followed a power-law behavior that often has been observed in high-T/sub c/ superconductors. We also investigated the relaxation of magnetization, flux creep. The time dependence of M also followed a power-law behavior, which was as expected because of the power-law I-V characteristics.

Measurements of misorientation angles over a large distance in strongly coupled grain boundaries

Grain boundaries in polycrystalline melt-textured YBCO samples fabricated by the liquid phase removal method have been analyzed by transmission electron microscopy (TEM). These samples contain a number of grains separated by high-angle grain boundaries with lengths up to several millimeters and J/sub c/ values up to 18000 A/cm/sup 2/ at 77 K. The results of TEM studies show that the high misorientation angles of these boundaries are maintained over a large distance by certain intra-grain features such as platelet boundaries and low-angle grain boundaries. Measurements on adjacent platelets as well as at locations inside single grains show that the presence of these boundaries maintains the misorientation of the high-angle grain boundary in these samples within 6 degrees. This indicates that high-angle grain boundaries in these samples do not dissociate into low-angle grain boundaries and the high superconducting current is carried by the entire length of the boundary.

High current transport test of a YBCO bulk conductor up to 25 kA

A melt-textured YBCO bulk material was employed to obtain high current transport characteristics of a bulk high temperature superconductor. A disk-shaped YBCO bulk material 65 mm in diameter and 15 mm thick was fabricated by a modified quench and melt growth (QMG) process. An "H" shaped bulk conductor was cut from the YBCO disk. The cross sectional area and length of the conductor were 7 mm/spl times/7 mm and 20 mm, respectively. The bulk conductor was mounted in a high current bus line with a conductor-holder which allows high current feed and mechanical support against the thermal stress and electromagnetic force. Both ends of the conductor were soldered to electrodes of the conductor-holder. At 4.2 K, high current transport experiments were carried out up to 25 kA.

Intragranular and intergranular superconducting properties of bulk melt-textured YBCO

We have investigated the superconducting properties of bulk melt-processed YBCO using different experimental techniques. First, single domain materials (i.e. containing no grain boundaries) have been studied. Electrical resistivity, current-voltage characteristics, M-H loops, and A.C. susceptibility have been measured for different field and current directions. The intragranular properties are characteristic of high-quality melt-processed YBCO material, with a critical current density J/sub c/(T=77 K, B=1 T//c)>10/sup 4/ A/cm/sup 2/. The anisotropy ratio J/sub c/(ab)/J/sub c/(c) is found to be close to 3. Both magnetic and transport measurements show that defects in the microstructure do not significantly impede the current flow inside the single domain. These results were compared to those measured on samples containing one "natural" single grain boundary which sometimes appears during the grain growth process. The intergranular properties show a much stronger current and magnetic field dependence than that measured within the grain and these differences are discussed.

Fabrication of HTS monoliths for a bearing system in a cryogenic vessel

Melt-textured YBCO monoliths were prepared by the top-seeded-melt-growth (TSMG) method with quadratic shape and dimensions of 38 mm/spl times/38 mm/spl times/12 mm. Generally, 16 samples of that size are prepared in one batch by carefully adjusting the processing parameters. Typical values of the maximum frozen-in field vary from 600-700 mT. A remarkably high value of 865 mT was recorded for a sample with dimensions of 17 mm/spl times/17 mm/spl times/12 mm. The levitation forces of these samples typically amount to 60-70 N. A demonstration model of a cryogenic vessel using arrangements of TSMG-YBCO monoliths and permanent magnets showed an improved performance compared to that of a conventional vessel.

Rf impedance of sintered BSCCO and melt-textured YBCO superconductors

The authors present the temperature and field dependence of radio frequency response of sintered BSCCO 110-K phase and melt-textured YBCO with Ag and 211 inclusions. The rf resistance and reactance measurements are carried out using a marginal oscillator operating at 11.9 MHz. Melt-textured samples are found to exhibit insignificant low-field losses and hence are potential candidates for applications at radio frequencies. The field induced loss in sintered BSCCO is described by invoking two kinds of weak links. The rf penetration depth of the materials measured is found to be much larger than the London penetration depth and represents the effective screen depth. More importantly, the authors observed an anomaly in the penetration depth of BSCCO which is explained in terms of an interplay between intragranular and Josephson penetration depths.