Showing papers by "Masato Murakami published in 1997"

Effect of mixing three rare-earth elements on the superconducting properties of

Abstract: We report the effect of mixing three rare-earth (RE) elements in the RE site on the superconducting properties of melt-processed , in which RE elements are selected from the group of Nd, Sm, Eu and Gd. The heat treatment profiles for melt processing were determined according to the peritectic decomposition temperatures obtained from the thermal analysis measurements. Microstructural observations and compositional analysis showed that the main phase was (, in which the ratio of three RE elements (R1, R2, R3) was equal to that of the nominal composition, indicating that mixing of RE elements was uniform. All the samples showed relatively high in the range 93.1 - 95.3 K and exhibited the secondary peak effect in the magnetization curves at 77 K with the irreversibility field higher than that of melt-processed Y - Ba - Cu - O. A high value of at 77 K and 2 T was achieved in (Nd, Eu, Gd) for field parallel to the c-axis.

Melt processing of high-Tc Nd-Ba-Cu-O superconductors in air

Abstract: We have succeeded in fabricating Nd-Ba-Cu-O bulk superconductors, which exhibit high T c and a sharp superconducting transition, by the melt process in air without additional high temperature annealing in a reduced oxygen atmosphere The key to this achievement is the use of Ba-rich Nd 4−2 z Ba 2+2 z Cu 2− z O 10− δ as a precursor Fabricated bulk samples exhibit onset T c values of 94–95 K and the secondary peak effect in the M-H loop like the case of Nd-Ba-Cu-O superconductors produced by melt processing in a reduced oxygen atmosphere The peak position and the irreversibility field for the best sample were about 12 T and 5 T at 773 K, respectively, and the estimated J c at the peak field was about 23 × 10 4 A cm −2

Improved superconducting properties of melt-textured Nd123 by additional heat treatment

Abstract: We have investigated the effect of additional heat-treatment on the superconducting transition and the flux pinning properties of NdBaCuO melt-textured in air. After the heat-treatment at high temperatures, >900°C, under low oxygen partial pressure, P(O2) = 0.001 atm, the superconducting transition became sharper accompanied by an increase of Jc. However, the increase of Jc was very small and the secondary peak effect commonly observed in NdBaCuO melt textured in low P(O2) could not be observed. Transmission electron microscopic observations and energy dispersive X-ray analyses show that the spatial variation of the Nd/Ba ratio is reduced after high-temperature heat-treatment, which indicates that an improvement in Tc and Jc is attributed to a suppression of Nd substitution on the Ba site.

Effect of high-temperature heat-treatment on the pinning properties of melt-textured Nd123

Abstract: We have investigated the effect of high-temperature annealing on the superconducting transition and the flux pinning properties of Nd123 melt-textured in 1% O 2 Ar. After the high-temperature annealing, a decrease in J c was observed in most samples even though no appreciable change was observed in T c . TEM observations and EDX analysis show that the spatial variation of the Nd/Ba ratio almost disappears after high-temperature annealing. Therefore, the observed change in J c - B properties can be understood by a reduction in the field-induced pinning due to homogenization of Nd/Ba substitution regions. These results strongly support the idea that the secondary peak effect and high pinning performance in Nd123 can be ascribed to the presence of Nd/Ba spatial variation.

Effects of oxygen content on the superconducting properties of melt-textured LRE123 superconductors

Abstract: The effects of oxygen content on the superconducting transition temperatures, T c , and magnetic properties of melt-textured LRE123 (LRE  Nd, Sm) superconductors were studied. The oxygen content could be varied by changing the annealing temperature, T a , in that the oxygen deficiency, δ, monotonically decreased with decreasing the T a from 600°C to 300°C. A further decrease of T a to 275°C showed no appreciable change. In both systems the highest T c was obtained for the samples annealed at 300°C. Magnetic properties were also strongly dependent on the oxygen content, showing that the control of oxygen content is critical for achieving good pinning properties.

Microstructure of NdBaCuO superconductors prepared by the oxygen-controlled-melt-growth method

Abstract: Microstructure analyses by polarized light microscopy show that Nd4Ba2Cu2O10 (Nd422) particles in oxygen-controlled-melt-grown NdBaCuO bulk superconductors have similar influence on the NdBa2Cu3O7−x (Nd123) microstructure as Y211 particles in the Y123–211 system. The twin spacing decreases with shortening of mean distance between Nd422 particles. The a–b microcracking in the 123 matrix was ascribed to thermal stresses around Nd422 particles set up in the matrix during cooling due to the different thermal expansion of Nd123 and Nd422 phases. The relaxation of thermal stresses in the 〈100〉 directions by twin boundary motion was related to the observed rosette of detwinned regions or regions with predominance of one twin variant formed around Nd422 particles. The level of thermal stresses was estimated. The elongated c- and a-subgrains with rectangular cross section were shown to be a result of cellular growth, i.e. the subgrains in melt-processed REBaCuO (RE: rare earth) are growth related crystal defects.

Flux pinning in melt processed REBaCuO

Abstract: REBaCuO (RE: rare earth element selected from Nd, Sm, Eu, Gd) superconductors exhibit high T c and J c values when they are melt processed in a reduced oxygen atmosphere (oxygen-controlled-melt-growth: OCMG process) Large J c values in these systems can be attributed to finely dispersed RE rich 123 clusters, which can act as field-induced pinning centers, and thus leading to the second peak effect in M - H loops In this paper, we first propose the mechanism how RE rich clusters are formed during melt processing Second we show that flux pinning depends on oxygen annealing conditions, because the optimum oxygenation conditions are different depending on the range of NdBa substitution We also report that the addition of ND422 is effective in raising J c , although the mechanism is rather complicated owing to the fact that ND422 also has a REBa solid solution toward Ba rich direction

Application of oxygen controlled melt growth (OCMG) in ternary RE123 systems

Abstract: The detail investigations on growth and superconducting properties of ternary RE 123 systems have been done employing XRD, TEM, SEM, optical microscopy and DC magnetization techniques. XRD results confirmed that in all these systems the main phase was 123 with inclusions of small amount of second phase. The DTA, TEM, SEM and optical microscopy investigations revealed that even in the presence of three rare-earths the matrix RE123 was very uniform. The SQUID measurements on the post annealed samples showed that superconducting transitions T c (onset) are in the range of 93–95.3K. The highest J c value obtained in this series of systems was about 50000 A/cm 2 at 2T.

Comparative study of the flux pinning by columnar defects in high-Tc superconductors

Abstract: Columnar defects are introduced to single crystals of Nd123, La214 and Bi2212 by means of 5.8- GeV Pb- ion irradiation. After the irradiation, all samples showed the enhancement of the hysteresis width in a low field range, B < BΦ. However, there are some differences in the pinning characteristics. Both irradiated Nd123 and La214 samples exhibit ‘peak effect’ in the magnetization curve, which is not observed in irradiated Bi2212 sample. The observed peak effect is likely to orginate from the ‘matching effect’.

Refinement of Nd-422 Phase Trapped in a Nd1+δBa2−δCu3O7−y Superconducting Matrix

Abstract: The possibility of controlling Nd-422 phase dimension and morphology, both in the partially melted state and in the final superconducting matrix, was investigated Nd{endash}Ba{endash}Cu{endash}O samples were prepared by the melt powder melt growth (MPMG) technique Precursor materials containing Nd as either Nd{sub 2}O{sub 3} or Nd-422, formed during precursor quenching, were used The role of Pt in Nd-422 refining was analyzed The effect on Nd-422 nucleation and growth, of stoichiometries with increasing Nd-422 molar excesses (0{percent}, 10{percent}, 20{percent}, and 40{percent}) combined with different heating processes, up to the partially melted state, were examined Uniformly distributed needle-like particles (1 {mu}m diameter and 10{endash}15 {mu}m length) were obtained at 1120{degree}C and resulted in a further refined distribution trapped in completely processed samples

Flux pinning properties of NdBa2Cu3O7−d single crystals flux-grown in low oxygen partial pressures

Abstract: 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.

Mutual conversion between ultrasonic and electromagnetic waves via compressional distortion of vortices in a c-axis aligned YBa2Cu3O7

Abstract: An RF magnetic field applied to a high- T c superconductor YBa 2 Cu 3 O 7 under a parallel DC magnetic field generates a longitudinal ultrasonic wave, associated with compressional distortion of vortices. Conversely, a longitudinal ultrasonic wave generates an RF magnetic field. In order to utilize the mutual conversion for the study of pinning and dynamics of vortices, the nature has been investigated comprehensively with respect to superconducting layers and the magnetic field direction in a c -axis-aligned YBa 2 Cu 3 O 7 . The temperature and field dependences of the conversion are discussed in relation to the pinning characteristics based on the presented model.

Numerical evaluation of magnetization properties of a bulk HTSC with a hole

Abstract: Field-cooled and pulsed field magnetizations of a bulk high T/sub c/ superconductor (HTSC) with a hole are evaluated by using thin plate multi layers numerical codes. Dynamic effects of the pulsed field magnetization is analyzed with the flux creep-flow model. Merits and demerits of the HTSC with and without a hole are discussed from their practical applications as a trapped field magnet.

Melt pressing and properties of lre-ba-cu-o bulk superconductors

Abstract: Light rare earth (LRE)-Ba-Cu-O bulk superconductors melt-processed in a reduced oxygen atmosphere are very promising for high field application as a superconducting permanent magnet with the liquid nitrogen refrigeration. Compared to a good quality melt-grown Y-Ba-Cu-O bulk, LREBCO bulks exhibit larger critical current density in high magnetic field and much improved irreversibility field at 77 K, implying that more effective flux pinning can be realized in a commercially feasible way. This paper reports melt processing for LREBCO bulk superconductors and their characteristic superconducting properties.

Steam generator for station service of nuclear power station

Abstract: PROBLEM TO BE SOLVED: To enhance deaeration effect of a deaerator at all supply water flow rates within a wide range of supply water flow rates, suppress disturbance occurring at the time of switching a boiler and a flash tank and reduce pressure in the flash tank at the time of stopping a station service steam generator. SOLUTION: A water supply valve for controlling water supply rate to a deaerator 5 is provided on at least one of a plurality of the deaerators 5 for the purpose of effecting spray water supply to the deaerators 5 and the water supply valve 21 is opened and closed by an interlock circuit 26 according to quantity of steam generated by a flash tank 8 and discharge pressure of a water supply pump. An exhaust line 27 is provided for discharging station service steam within the flash tank 8 into atmospheric air or to a device capable of recovering heat at the time of changing a boiler 20 and the flash tank 8, and the interlock circuit 28 opens and closes an exhaust valve 24 according to manual operation or operation status signals of the boiler.

Residual Microstresses, a-b Microcracking and Twin Structure in Oxygen Controlled Melt Grown NdBaCuO Superconductors

Abstract: Microstructure analysis by polarized light microscopy has shown that Nd4Ba2Cu2O10 (422) particles in oxygen controlled melt grown NdBaCuO bulk superconductors have similar influence on the NdBa2Cu3O7–x (123) microstructure as 211 particles in the Y123–211 system. The twin spacing is narrowed by decreasing the mean distance between 422 particles. a-b microcracking in the 123 matrix was ascribed to the thermal stresses around 422 particles set up in the 123 matrix during cooling due to the differential thermal expansion of 123 and 422 phases. The relaxation of thermal stresses in the directions by twin boundary motion was evidenced by the observed rosette of detwinned regions or regions with a predominance of one twin variant formed around 422 particles. The level of thermal stresses was estimated.