Misao Hashimoto

Processing and properties of QMG materials

Abstract 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 104A/cm2 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.

Comparison of the levitation forces of melt-processed YBaCuO superconductors for different magnets

Abstract It is difficult to compare the levitation force of superconductors for different magnets. A new parameter, α, is proposed in this paper, which is defined as the ratio of a superconducting levitation force to the limitation force for a given magnet. Using this parameter, the comparison between different superconducting levitation forces in melt-processed YBaCuO superconductors (QMG) has been done, such as a levitation force of 269 kN/m2 (=2.74 kgf/cm2) for a superconductor 77.3 mm in diameter and a magnet of NdFeB 50 mm in diameter, and a levitation force of 260 kN/m2 (= 2.65 kgf/cm2) for a superconductor 46.5 mm in diameter and a magnet of PrFeBCu 36 mm in diameter. The former is α = 0.67 and the latter is α = 0.81.

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.

Magnets Made of QMG Crystals

Magnetic flux density trapped in large QMG (Quench and Melt Growth) crystal was measured at 85K, 77K and 63K. QMG crystals which had no high-angle grain boundaries were made. A sample which size was 50mm in diameter 24mm thick was used. Distribution of magnetic flux density was measured on the suface of the QMG crystal using several Hall probes. Field cooling was performed to investigate trapping of the magnetic field. Temperature of coolant was varied by controlling the pressure over the nitrogen vessel. At 63K, flux density was roughly double of that at 77K. Maximum value of trapped field was 1.72T. Flux creep rates were 2.4x10−3, 2.2x10−3 and 0.9x10−3 at 84K, 77K and 63K respectively. Since flux creep is still not negligeable at these high temperatures, we tried to prevent it by cooling the QMG material down to 77K after trapping at 84K and also cooling down to 63K after trapping at 77K the flux allows to save a lot of time for the field stability to be achieved.

Observation of Flux Movement in Quench-and-Melt-Grown YBCO Generated by a Pulsed Magnetic Field

The dynamics of the quantized flux in a ring-shaped YBa2Cu3O7 sample has been studied through the observation of the magnetic flux motion generated by a pulsed magnetic field. The flux speed is obtained as a function of the rise rate of the external field and as a function of temperature. It is shown that the flux speed is not intrinsic as determined by the creep theory but is determined by the external field and the Jc value. To interpret the slow rise of the observed signal, a distribution of different kinds of pinning centers in a sample is suggested.

Imaginary AC susceptibility in superconducting Y-Ba-Cu-O

Abstract The imaginary part of the AC susceptibility was measured as a function of temperature under various AC field amplitudes for a powder of sintered Y-Ba-Cu-O and melt-processed bulk Y-B-Cu-O with and without cracks. Its peak value was found to reduce from the prediction of the critical state model with a shift of the position to higher temperature for powder and cracked specimens, in agreement with the AC field amplitude being decreased. This deviation from the model is attributed to reversible fluxoid motion. This motion is prominent for superconductors with an effective size comparable to or smaller than Campbells Ac penetration depth. The experimental results agree with the theoretical analysis based on Campbells model for the reversible fluxoid motion. This analysis is useful to estimate the effective size of superconducting specimens and hence, to infer causative defects for percolative superconducting currents.

Flux motion in quench-and-melt-grown YBa2Cu3O7 under pulsed magnetic field

Abstract Motion of magnetic flux in a ring-shaped quench-and-melt-grown YBa 2 Cu 3 O 7 sample has been studied using several search coils coaxially set on a surface of the ring. It is shown that the flux does not obey the critical state model but moves freely under the Lorentz force induced by external pulsed magnetic field.

Trains of Flux Jumps Observed in a Quench-and-Melt-Grown YBa2Cu3O7 under Pulsed Magnetic Field

The motion of quantized fluxes in ring-shaped YBa2Cu3O7 samples has been observed under pulsed magnetic field in the 15 K to critical temperature (T c) range. Successive flux jumps have been observed in a low-J c sample, where J c is the transport critical current density. All these jumps are determined by the Lorentz force, and not by the thermal avalanche conditions. It is suggested that the trains of jumps are caused by the weakened pinning force in a degraded sample.

Study of YBa2Cu3O7 Superconductor by Low-Temperature Scanning Tunneling Microscopy and Spectroscopy

The electronic structure of YBa2Cu3O7 (YBCO) superconductor was studied by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS), since the electronic states of CuO2 planes in the superconducting state may shed light on the mechanism of high-Tc superconductivity. Experiments were performed on the cleaved surface of a bulk single-grain YBCO specimen at 4.2 K. Two different STS conductance spectra were obtained, a BCS-like one and a metalliclike one. These spectra were observed alternately with a periodicity of 0.8±0.2 nm, suggesting that YBCO has a two-dimensional structure in which CuO2 planes are strongly superconducting and CuO chains are metallic-like.

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.