Quench And Melt Growth Research Articles

Imaging of magnetic flux distribution in vicinity of insulating particles in high-Tc superconductor by electron holography

REBaCuO (RE: rare earth) bulk superconductors, whose microstructures can be controlled by quench and melt growth (QMG) processing, have a high critical current density Jc, which makes them well suited for use in practical applications. Through a combination of electron holography and a focused ion-beam technique, we succeeded in visualizing a fluxon pinned by an insulating particle in a QMG bulk Y-Ba-Cu-O superconductor. We also clarified the magnetic flux distribution in the vicinity of the insulating particles at different temperatures in a uniform external magnetic field.

Design and fabrication of 5 GHz band pass filter using circle-type HTS bulk resonator

We designed and fabricated a transmit band pass filter (BPF) using circle-type high temperature superconductor (HTS) bulk resonators. A Dy–Ba–Cu–O bulk was fabricated using a modified quench and melt growth (QMG) process and cut into specimens of 8.40 and 8.44mm in diameter and 0.5mm thick for use as the HTS bulk resonators. A three-pole stripline (SL) BPF was designed based on a Chebyshev function and the frequency response and electromagnetic field of the filter were simulated using a three-dimensional electromagnetic field simulator. From the results of the simulation, the center frequency, bandwidth, insertion loss, and ripple of the designed filter were 4.97GHz, 100MHz, 0.03dB, and 0.048dB, respectively. In the experimental results on the actual fabricated filter, the filtering response was clearly observed; however, the center frequency of 5.46GHz was higher than that of the simulation. The simulated maximum surface current in the resonators of the SL filter was approximately 86% smaller than that of a conventional hairpin filter. Furthermore, the measured response of the Dy–Ba–Cu–O bulk filter at an input power of 20dBm was almost the same as that at 0dBm. These results mean that an SL filter using a Dy–Ba–Cu–O bulk resonator may be practicable as a high-power transmit BPF.

Surface resistance measurement of modified QMG superconducting bulks

We investigated the microwave properties of high-temperature superconductor (HTS) bulks for high-power microwave applications. The HTS bulk samples of Dy–Ba–Cu–O and Gd–Ba–Cu–O, were prepared using the modified quench and melt growth (QMG) process. The relationship among the bulk surface mirror polishing process, oxygen annealing process, and surface resistance (Rs) was also investigated. The Rs of the HTS bulks was measured using the dielectric resonator method at 21.8GHz for the TE011 mode. The measurement temperature was varied from 20 to 95K. We found that the annealing after surface mirror polishing was more effective to obtain low Rs than polishing after annealing. Although the Rs value of the optimized Dy–Ba–Cu–O bulk sample at 20K was twofold larger than that of the high-quality YBa2Cu3O7−δ thin film fabricated by THEVA GmbH, the Rs value was even low enough to be used for microwave devices. These results indicate that Dy–Ba–Cu–O bulk superconductors are feasible for applications to high-power transmit filters for mobile communication systems.

Tensile and bending mechanical properties of bulk superconductors at room temperature

Tensile and 3-point bending tests of specimens cut from an Y123 and a Gd123 bulk superconductors fabricated by using the modified QMG (Quench and Melt Growth) process were carried out at room temperature. The mechanical properties obtained by two testing method were compared. The average values of the Young's modulus obtained by tensile tests, 122 GPa for Y123 and 109 GPa for Gd123, were lower than those obtained by bending tests, 132 and 117 GPa, respectively. The average values of the tensile strength, 39 MPa for Y123 and 42 MPa for Gd123, were also lower than those of the bending strength, 74 MPa and 71 MPa, respectively. There was no appreciable difference between the Weibull coefficient for the tensile strength and that for the bending strength, 6.9, in Gd123. On the other hand, the coefficient for the bending strength, 17.6, was significantly higher than that for the tensile strength, 6.8, in Y123.

Correlation between vortex distribution and temperature in YBCO films and YBCO-QMG samples

We observed vortices with a modified Bitter method by evaporating nickel as the medium of decoration, in film and QMG (which are made with the quench-and-melt-growth method) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) samples. The temperature range of observation was from 25 K to 90 K, and the external magnetic field was 2 mT. Consequently, we found that the vortex structures of these samples were stable from 25 K to near the critical temperature (/spl sim/90 K). The vortex density was almost three times larger than that estimated from the external magnetic field. This suggests that the vortex motion may be hopping during nickel evaporation.

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.

Ettingshausen and Nernst Effects of QMG-YBa2Cu3O7-δ in Magnetic Fields up to 14 T

The dissipation behavior in the mixed state of a high-T c oxide superconductor YBa2Cu3O7-δ has been investigated by measuring thermomagnetic effects, namely the Ettingshausen and Nernst effects. These two effects were measured for the same sample prepared by the quench and melt growth (QMG) method without Pt addition. The Ettingshausen effect was also measured for a sample with 0.5% Pt. The temperature and magnetic field dependences of the temperature gradient produced by the Ettingshausen effect and the electric field induced by the Nernst effect are presented. Combining these data with the electrical resistivity and the thermal conductivity, the transport energy of a vortex line is obtained. The effect of the Pt addition on the Ettingshausen effect is also discussed.

酸化物超伝導バルクマグネットの作製と特性

Oxide-superconductor bulk materials that can act as magnets after field cooling were made by a modified quench-and-melt-growth (QMG) process. To enlarge QMG crystals having a fine 211 phase in the matrix of a crystalline 123 phase, a modi tied QMG process was developed by adding a seeding technique and a Pt addition technique to the original QMG process. Enlarged QMG bulk superconductors were operated as high-field permanent magnets. A bulk magnet 70 mm in diameter and 35 mm in thickness had trapped maximum values of 4.5 T, 2.5 T, and 0.99 T at 40 K, 63 K, and 77 K, respectively. Thermornagnetomechanical instability leading to quenching phenomena was not observed above 40 K.

Construction and Performance Test of a Magnetically Levitated Transport System in Vacuum Using High-Tc Superconductors

We have developed a magnetically levitated transport system in vacuum using the strong pinning force of high-T c superconductors. This transport system consists of superconductors prepared by the quench-and-melt-growth (QMG) method and permanent magnets. The superconductors move within the cooling jacket inserted into the transport chamber. A transfer arm moves in the transport chamber without any contact or sliding, because stable levitation is brought about by magnetic interaction between the magnet attached to the arm and the superconductor. In this paper, structure and performance test results are described.

Processing and properties of QMG materials

Recent progress of Quench and Melt Growth (QMG) process is reviewed. QMG process is one of the melt processes for Y based superconductor. A pseudo single crystal (QMG crystal) which includes finely dispersed 211 phase in the matrix of 123 phase was produced by the original QMG process. The critical current density of the QMG crystal that has no high angle grain boundaries has been exceeding 10 4A/cm 2 at 77K and 1T. The original QMG process was improved to make larger QMG crystals of which crystallographic orientation was controlled. Modified QMG process can be characterized by Pt addition in precursor preparation and seeding technique in growth process. Crystallinity of the QMG crystal made by this modified process was investigated. It appeared that there were two kinds of domain structures that depend on the growth direction. QMG crystals enlarged by this modified process are applicable to various fields. For instance, QMG crystal can operate as a bulk magnet. The maximum value of flux density was 4.5T at 40K on the surface. It was found that QMG crystal is thermally stable above 40K at least.

Radio-frequency acoustic powder echoes in high-T c YBaCuO ceramics around the H-T phase boundary

Intensity of rf acoustic powder echoes are studied around the H-T phase boundary in the superconducting state for powdered samples of YBaCuO prepared by the quench and melt growth (QMG) process and for conventional powder of YBa 2Cu 3O 7−δ. The echoes are fond to vanish at the critical field H C2 rather than the irreversibility field H irr for both of the samples. The intrinsic pinning rather than the pinning of fluxoids to defects seems to be important for the observation of rf acoustic powder echoes in the high-T c YBaCuO ceramics.

Weak link property in superconducting Y-Ba-Cu-O prepared by QMG process

The weak link property in a QMG (quench and melt growth)-processed single-grain Y-Ba-Cu-O specimen was investigated by measuring the dependence of the critical current density on temperature, history of magnetic field application, and its enhancement under a longitudinal field geometry. The results show that weak-link regions still remain in the specimen. It is speculated that these link regions, which can transport superconducting currents at low temperatures, are degraded with increasing temperature and lose superconductivity around 20 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

酸化物高温超電導体を用いた超電導軸受構造の動特性

We have studied a superconducting bearing system using high-Tc superconductors with a strong pinning force. The superconducting bearing system consists of a superconductor prepared by quench and melt growth (QMG) method and a set of alternating-polarity magnets. This paper describes the dynamic characteristics of the superconducting bearing system with small damped vibrations. An impulse force was applied to the set of alternating-polarity magnets levitated over the superconductor of the system. The vibrations of the set of magnets were measured using a laser displacement meter. Stiffness, damping coefficient, and energy loss of the superconducting bearing system were discussed. The results show that the dynamic characteristics of the bearing system are closely related to the static characteristics. It is necessary to evaluate the dynamic characteristics of the superconducting bearing system in applying the system to a radial bearing with a high rotation speed.

Magnetic Shielding by Superconducting Y-Ba-Cu-O Prepared by the Modified Quench and Melt Growth (QMG) Process

Magnetic shielding capability has been investigated on Y-Ba-Cu-O crystals prepared by the modified quench and melt growth (QMG) process at 77 K and 4.2 K. Overall critical current densities in large single-grained specimens were estimated to be about 104 A/cm2 based on the Bean model in order to predict shield effect. At 77 K, a single grain cylinder shields the inside up to about 0.2 T of the external field, and this value is the highest among all the values reported so far for tubes fabricated of oxide superconductors. A comparison between shielding characteristics of a single grain and those of a polygrain demonstrated that weaklinking high-angle grain boundaries should be eliminated for high field shielding. Lorentz force, flux creep, and flux jump are discussed.

High field trapping and flux creep of large QMG(Quench and Melt Growth) crystal

Abstract Magnetic flux density trapped in large QMG (Quench and Melt Growth) crystal was measured at 77K and 63K. QMG crystals which had no high-angle grain boundaries were enlarged by seeding. Distribution of magnetic flux density was measured on the surface of the QMG crystal using several Hall probes. Field cooling was performed to investigate trapping of the magnetic field. According to the critical state model, it is expected that trapped flux density increases with increasing sample size. A QMG crystal which size was 45mm in diameter 15mm thick trapped 0.72T at the center of the surface. Double piled QMG crystals trapped 1.35T at the center at 77K. The overall critical current density which was estimated by Bean type critical state model was about 7×10 3 A/cm 2 . Flux creep rate (normalized decay rate) was about 2.5×10 −2 at 77K. Moreover, a QMG crystal trapped 1.07T in triplet point of nitrogen (63K). Flux jump was not observed during these measurements. From these results, it is found that QMG crystal could stably trap high magnetic field without stabilization at 77K and 63K unlike conventional metal superconductors at 4.2K.

Critical current density of a YBa2Cu3Ox by melt (QMG) process

We investigated the macroscopic and microscopic critical current densities of a melt processed materials by a magnetic measurement. By the Quench and Melt Growth (QMG) process, large YBa2Cu3Ox crystals without a large angle grain boundary can be obtained. Several types of powders less than 100ωm were prepared by pulverizing and classifying QMG crystals. These powders were aligned in a magnetic field and the magnetization was measured. The magnetization hysteresis of the prepared powders was proportional to their average diameter. The critical current density of the powders estimated from the Bean model is higher than that of the bulk crystal. This indicates that the current in the grain is not uniform. A current is probably prevented by cracks mostly parallel to the ab-plane or subgrain boundaries.

Scanning tunneling spectroscopic studies of quench and melt growth (QMG) YBaCuO crystals at 4.2K

Microscopic properties of high temperature superconductors (HTS) were investigated by a low temperature scanning tunneling microscope at 4.2K. We have obtained a topographic image of YBa 2Cu 3O 7 at 4.2K and were able to observe twin boundaries on the cleaved surface for a large area scan (153×153 nm 2). Spectroscopic studies were carried out on the same cleaved surface. A superconducting energy-gap (2Δ) of 20–35 meV at 4.2K measured in the present study.

Magnetization Properties of YBaCuO Prepared by Quench and Melt Growth Process

DC magnetization measurements were carried out on YBaCuO prepared by the quench and melt-growth (QMG) process using a vibrating sample magnetometer and a SQUID magnetometer below 100 K. It was found that the DC magnetization behavior of QMG-grown YBaCuO at 4.2 K can be quantitatively explained in terms of Bean's critical state model. This sample also can become a strong bulk permanent magnet with an energy product of more than 80 MGOe at 5 K because of its strong pinning force.

Rotational decay torques of superconducting magnetic bearing

A superconducting magnetic bearing is proposed as one of the applications of a high-critical-temperature (T/sub c/) oxide superconducting material. New models of bearings with a free rotation axis were assembled. In these models, the superconductor, YBa/sub 2/Cu/sub 3/O/sub x/, prepared by the quench and melt growth (QMG) method, was applied to the bearing section and a permanent magnet was used for a rotor. This development originated from the knowledge that YBa/sub 2/Cu/sub 3/O/sub x/ prepared by the QMG method has a stronger pinning force for magnetic flux than that of conventional superconductors. A study of the forces and the damping for magnet rotation above the QMG superconductor was also made. It showed that levitation force and lateral force due to flux pinning were extremely large and damping for magnet rotation was small.

A new domain structure in YBa 2Cu 3O 7− x prepared by the quench and melt growth (QMG) process

Crystallographic analyses have revealed a new “domain structure” in YBa 2Cu 3O 7− x (123) prepared by the quench and melt growth (QMG) process. The analyses were carried out by X-ray diffractometry: a kind of pole figure method and an improved technique of microarea X-ray diffraction. The “domain structure” is composed of crystallographic units which are aligned in two-dimensions; the c-axes of them are parallel to each other and there is a slight deviation in directions of their a-and b-axes. The degree of deviation of them differs in scale: macrograins (∼ 10 mm), subgrains (∼ mm) and crystallites (∼ 100 μm) and it is about a few degrees in scale of crystallites. The “domain structure” in QMG-processed materials is closely related to their high J c values, J c > 10 4 A/cm 2 at 77 K under a magnetic field of 1 T. Crystallographic units in the “domain structure” are more strongly linked in superconducting than those in conventionally sintered specimens. The control of the “domain structure” is one of the most important factors for improving properties of superconductors.