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Present air conditioner systems are based on traditional vapor compression technology with usage of HCFC (Hydro Chloro Fluoro Carbon). The Kyoto Protocol has designated HCFC as one of the gases whose emissions are to be reduced. This requires the development of HCFC free systems or the usage of substances which have little greenhouse effect. Under these conditions, magnetic refrigeration technology, which has the potential for high efficiency without Freon gases, has become a focus of attention. The aim of this study is to develop a large-scale magnetic refrigerator which has a maximum kilowatt-class cooling power for air-conditioners mountable on railway vehicles to enable them to be free from Freon gases.

The aim of this study is to put a cooling technology using a magnetocaloric effect to practical use for air conditioners of railway vehicles in order to save energy. New magnetcaloric materials which have larger a magnetocaloric effect compared with Gd have been developed in the region of room temperature. Nevertheless, little discussion exists about how much the new materials affect the properties of a magnetocaloric cooling system. Therefore, the characteristics of the cooling system with Gd and La(Fe₀.₈₄Co₀.₀₆Si₀.₁₀)₁₃ have been studied experimentally and analytically.

The resonance modes of the on-board superconducting magnet for the JR Maglev system actually obtained at the durability test were two (propulsion and levitation modes). To reduce the durability test period of the specimen by half, it was investigated in this study how those two resonance modes were simultaneously reproduced. It appears that those two resonance modes are simultaneously reproducible if the three accelerating points to reproduce its levitation mode are used, and if proper beams to shift the resonance frequencies of its propulsion and levitation modes are added.

We have investigated the E–J characteristics for melt-textured NdBa2Cu3O7−δ (Nd123) with the secondary peak and Y123 with no secondary peak over a wide range of the electric field by measuring the ac susceptibility and the dc magnetization. In the case of Y123, the E–J characteristics exhibited normal behavior in that the convex curve changed to the concave with increasing magnetic field. In contrast the E–J characteristics of Nd123 were complicated and the convex curve changed to the concave with decreasing electric field near the secondary peak field. This shows that the E–J characteristics are affected by the field-induced pinning.

Nd–Ba–Cu–O sample fabricated by the oxygen-controlled-melt-growth method exhibits large J c values in a high field region and also shows a secondary peak effect. In the present study, the Nd–Ba–Cu–O sample was characterized by using the flux profile technique, in which the χ ′ signal is plotted against the amplitude of AC field. J c values determined with the flux profile technique also showed a clear secondary peak effect. In addition, such J c values were higher than those measured with DC susceptibility, presumably due to a difference in the electric field to determine J c .

We investigate the complex ac susceptibility of the melt-processed (Nd, Eu, Gd)- Ba-Cu-O and Nd-Ba-Cu-O. We observe the anomalous structure in the temperature dependence of the complex ac susceptibility. This anomaly may be related to the superconducting transition of the second phase.

Nd1+xBa2-xCu3Oy (Nd123ss) single crystals were isothermally grown under controlled oxygen partial pressure of 1% O2 at various growth temperatures (Tg) from 1000 to 1020 °C with a flux method. The flux composition of 20 mol% NdBa2Cu3Oy (Nd123) and 80 mol% Ba3Cu5O8 (035) was employed. Nd123ss single crystals grown at Tg≥1010 °C exhibited a transition temperature Tc≈95 K with transition width ≈1 K, the peak field (Bpk)≈2-3 T and the irreversibility field (Birr) at 77 K much higher than the experimental limit of 7 T. As the growth temperature is lowered, both the Bpk and Birr were reduced, which can be explained in terms of the chemical fluctuation which can occur at lower Tg.

Large Nd1+xBa2-xCu3Oy (Nd123ss) single crystal cubes of side 3 mm were fabricated isothermally by the flux method. The crystals exhibited a Tc of 95 K with a sharp superconducting transition. One crystal was cut into small pieces, and its magnetic properties were measured to study the spatial distribution of the superconducting properties. It was found that although Tc values were almost constant, a small but significant intrinsic difference in the Jc-B properties and irreversibility field was observed depending on the sample position. This suggests that the distribution of Nd-rich Nd123ss clusters are spatially different even within the same single crystal. Such differences may be caused by the time variation of Nd concentration in the liquid during crystal growth.

Superconducting transitions are measured in field-cooled cooling (FCC) and field-cooled warming (FCW) modes in external magnetic fields ranging between 1 mT and 7 T on various high-T c superconductors (single crystal and melt-processed YBa2-Cu3O7-δ, single crystals and melt-processed NdBa2Cu3O7-δ, and on various ternary 123-superconductors). Our measurements reveal that a two step transition is observed if the sample exhibits also the secondary peak effect in the magnetization loops.

The magnetic flux dynamics of Nd-Ba-Cu-O (NdBCO) bulk superconductors produced by the Oxygen-Controlled-Melt-Growth (OCMG) method has been investigated using the Magneto-Optical (M-O) technique at various low temperatures for an external magnetic field applied both parallel (H//c) and perpendicular (H⊥c) to the c -axis of the sample. While for H//c the flux motion was isotropic, for H⊥c anisotropic penetration and expulsion of the flux lines was observed directly. In the configuration H⊥c the flux lines were observed to move in and out the sample predominantly parallel to the a-b planes. The flux behaviour has also been correlated to the microstructure. Particularly, microcracks along the basal plane were found to provide a low resistance channel for flux motion.

We investigated the magnetic flux dynamics of NdBaCuO (NdBCO) bulk superconductors produced by the Oxygen-Controlled-Melt-Growth (OCMG) method using the Magneto-Optical (M-O) technique at various low temperatures. An external magnetic field was applied both parallel ( H t | c ) and perpendicular ( H ⊥ c ) to the c -axis of the sample. While for H t | c the flux motion was isotropic, for H ⊥ c anisotropic penetration and expulsion of the flux lines were directly visualized. In the configuration H ⊥ c the flux lines were observed to move in and out of the sample predominantly parallel to the a - b planes. The flux behavior was also correlated to the microstructure. Particularly, the microcracks along the basal plane were found to provide a channel for easy flux motion. The anisotropic behavior of the flux front reflects the anisotropy in the critical current density J c . The different components of J c were independently calculated from the M-O data directly analyzing the magnetic induction profiles. These J c values were consistent with those estimated interpreting the magnetization hysteresis measurements within the framework of the extended Bean model.

The a.c. magnetization characteristics of NdBa 2 Cu 3 O y (Nd123) bulk superconductors were investigated using a.c. susceptometry. For the present study, an OCMG-processed Nd123 bulk sample (OCMG-Nd123; T c = 93.9 K ) and a flux-grown Nd123 single crystal (Nd123SC; T c = 93.2 K ) were used. An MPMG-processed Y123 bulk sample (MPMG-Y123; T c = 91.6 K ) was also studied for comparison. While the fundamental responses are basically the same for all samples, the third harmonic responses by sweeping the bias field, x 3 ( H dc ), show an interesting feature in that the x 3 ( H dc ) curves for Nd123 samples indirectly reflect the fishtail peak of their d.c. magnetization hysteresis ( M-H ) curves. The boundary lines of Nd123 samples, determined from both the peak temperature ( T p ) of the x 1 ″( T ) maxima and the offset of the x 3 ( H dc ), are commonly located at higher field-temperature region relative to that of the Y123. Using the scaling relationship of H ∞(1 − t ) n , where t = T / T c , all the boundary lines from the x 3 ( H dc ) and the x 1 ″( T ) maxima could be best fit with a power n of ~ 1.2 and ~ 1.4, respectively.

Electric field ( E )–current ( J ) characteristics of the NdBa 2 Cu 3 O 7− δ (Nd-123) bulk superconductor have been investigated by measuring the third harmonic ac susceptibility. They were compared with E–J characteristics determined with the dc magnetization measurements and magnetic relaxation methods. The n value was estimated on the assumption that E follows the power law: E ∼ J n . We found that the sample, which has the larger secondary peak on the J c – B curve, exhibits the more remarkable peak structure on the magnetic field dependence of the n value.

Nd-Ba-Cu-O samples grown by the OCMG method exhibit large Jc values and they also exhibit a secondary peak effect. It is interesting to study how the secondary peak effect affects the results of ac susceptibility measurements. Therefore, we measured the complex ac susceptibility of an OCMG processed Nd-Ba-Cu-O sample in the presence of a static dc field with the ac field amplitude of 20 Oe and the ac frequency of 125 Hz. To compare with the obtained ac susceptibility we tried numerical simulation based on a simple Bean model.

The magneto-optical (M-O) technique, based on the Faraday rotation in an indicator film, is a useful method to understand the magnetization behavior of superconducting materials because it allows a direct, local and dynamic observation of flux distribution. In this work, the magnetic properties of Nd-Ba-Cu-O bulk superconductors have been investigated using the M-O technique. The field has been applied perpendicular to the caxis (H//ab) at various temperatures. It has directly been observed that flux preferentially penetrates into the sample along the a-b plane. The two components of the critical current density in the a-c plane (J c c parallel and J c ab perpendicular to the c-axis) have been independently deduced from the analysis of the microscopic flux distribution.

To identify the relationship between the flux pinning properties and the process conditions in the Nd1+x Ba2-x CU3O7-d (Ndl23SS) superconductors, single crystals were flux-grown in 1%O2-Ar atmosphere using an isothermal crystallization procedure at various undercoolings (∆T) from 10 to 35 °C The flux was composed of 20 mol% NdBa2Cu3O7-d (Ndl23) and 80 mol% Ba3gCu5O8 (035). For ∆T ≤ 25 °C, flux-grown Ndl23 single crystals exhibited aspect ratio w/t smaller than 1, where w and t correspond to the lengths along the a- (or b-) and c-axis respectively, transition temperature T c ≈ 95 K with transition width ≈ 1 K, the peak field (B pk) ≈ 2 – 3 T and the irreversibility field (B in) much higher than the experimental limit 7 T. In contrast, for ∆T ≥ 30 °C, Ndl23 single crystals exhibited a slight increase in w/t, a slight decrease in T c with broader transition width, and a reduction in B pk and B in.

Critical current density J c of Nd1+xJBa2-xCu3Oy (Nd123) single crystals flux-grown under the Oxygen-Controlled-Melt-Growth (OCMG) process has been investigated using DC magnetization measurements. The superconducting transition temperature T c of a Nd123 single crystal flux-grown using a 0.1% O2 — Ar gas in a zirconia crucible was 94 K. Using a magneto-optical observation method, the sample was found to be free of weakly-linked regions. DC magnetization hysteresis curves exhibited an anomalous peak effect in the high temperature range. This behavior is attributable to a field-induced pinning by finely-distributed low T c phases in the high T c matrix. J c calculation using the extended Bean model resulted in high J c of about 4 × 104 A/cm2 under 2 T and over 1 × 104 A/cm2 under 4 T at 77 K.

Two-dimensional distribution of flux density over the surface of a BiPbSrCaCuO superconducting plate was measured at 77 K, when a magnetic field of up to 100 mT was applied perpendicular to the sample surface. The distribution of flux density indicates a magnetic shielding effect when an external magnetic field is being applied. Simulation curves of a distribution of flux density over a sample surface were also obtained. The distribution of flux density over the sample surface changing the gap length between the sample surface and the Hall probe agreed with that of simulation qualitatively.

Flux pinning properties were studied for NdBa 2 Cu 3 O 7−d (Nd123) single crystals flux-grown in low oxygen partial pressures. In the high temperature ( T ) region roughly above 70 K, apparent fishtail peaks were commonly observed on the magnetization hysteresis ( M-H ) curves or the critical current density vs magnetic field ( J c -B curves). For this high T region, while J c (B,T ) curves in low B followed an empirical equation from Matsushita et al. [1], those in high B obviously deviated from the equation, however, those could be well scaled to a single curve by plotting the normalized flux pinning force density f p vs the reduced B/B irr where B irr is the irreversibility field, indicating that dominant pinning mechanisms are different between low and high B region.