TopPage学生の学会発表 > 06論文

2006 論文

2009/01/13(Tue) 10:06


1: Magnetic properties of Ce(Fe1-xCox)2 under high pressure
Okada, H; Koyama, K; Yamada, M; et al.

We have studied the magnetic properties of Ce(Fe1-xCox)2 with x = 0 and 0.05 by magnetization measurements under high pressure. In both compound, the remarkable suppression of the magnetization is observed below T* in the ferromagnetic (FM) state under 1.2 GPa. T* shifts to lower temperature with increasing the magnetic field, and the extrapolated value to the zero magnetic field is estimated to be 95 and 123 K. These high pressure and magnetic field effects are probably due to the enhancement and the suppression of the AFM spin fluctuation, respectively.

2: Specific heat measurement of the spinel compound CuV2S4
Okada, H; Koyama, K; Watanabe, K; et al.
AIP Conference Proceedings 850 (2006) 1317-1318

We have performed specific heat measurements of a high-quality polycrystalline sample of CuV2S4. Two anomalies associated with charge density wave (CDW) transitions are clearly observed at Tt1 = 92 K and Tt2h = 58 K. The anomalies at Tt1 and Tt2h show λ-type and spike-shaped behavior, indicating second and first order phase transitions, respectively. The electronic specific heat coefficient γ and the Debye temperature D are estimated to be 36 mJ/molK2 and 347 K, respectively, γ of CuV2S4 is much larger than that of CuTi2S4, whereas CuV2S4 exhibits a CDW gap at low temperature. This enhancement of γ is probably attributed not only to the electron-phonon interaction but also to electron-electron interactions.

3: Effect of prebending strain on CuNb/Nb3Sn superconducting coils
   using a react and wind method

Oguro, H; Nishijima, G; Awaji, S; et al.

We have found that superconducting properties such as a critical current for bronze route Nb3Sn superconducting wires were enhanced by prebending strain (epsilon(pb)), which is the repeated bending strain at room temperature. In this study, four kinds of react and wind (R&W) processed CuNb reinforced bronze route Nb3Sn coils with epsilon(pb) = 1.0, 0.8, 0.5 and 0% were prepared. We investigated the effect of prebending strain for the coils. In the electromagnetic compressive stress condition, a critical current (I-c) of the R&W processed coil was enhanced by the prebending strain. These I-c values are larger than those of a short sample wire without prebending strain. In the hoop stress condition, the I-c of four coils revealed a similar value. Therefore, it is considered that the I-c was limited by the large tensile stress for all coils in the hoop stress states. These results suggest that the I-c enhanced by the effect of prebending strain is applicable for the R&W coil design without degradation in large stress and strain states.

4: Design of an 8 MW Water-Cooled Magnet for a 35 T Hybrid Magnet at the HFLSM
K. Takahashi, S. Awaji, Y. Sasaki, K. Koyama and K. Watanabe
IEEE Trans. Appl. Supercond. 16 (2006) 2, 977-980.

A new water-cooled poly Bitter magnet for a hybrid magnet was designed under the fully utilization of the electric power source of 8 MW and the cooling system installed in the High Field Laboratory for Superconducting Materials, Tohoku University. the magnet consists of four axial water-cooled Bitter coils, which are electrically connected in series and all of cooling water flows from the bottom to upper side of coils. The designed poly Bitter magnet will produce 22.8 T by 7.5 MW in the room temperature bore of 16 mm and 34.3 T as a hybrid magnet with a backup field of 11.5 T. A magnetic force field, B(partial derivative B/partial derivative z), reaches to approximately -11, 100 T-2/m at the central field of 34.3 T, which is very large enough to levitate semiconductors such as silicon or germanium.

5: A New Model of Two Directional Jc Distributions for Bi2Sr2CaCu2 O8 Materials
K. Takahashi, S. Awaji, G. Nishijima and K. Watanabe
IEEE Trans. Appl. Supercond. 16,2 (2006) 1643021, 1019-1022.

E-J properties of the high-Tc materials are related to the distribution of the local critical current density Jc. From the previous results on Bi2212 thick films and practical Ag-sheathed Bi2212 tapes, we found that the distribution of the local Jc is much affected by microstructures, and the local Jc distribution becomes broad and asymmetric for well aligned plate-like Bi2212 grains. However, from a viewpoint of microstructures, the origin of the asymmetric Jc distribution remains unclear. In order to understand the relationship between the Jc distribution of Bi2212 materials and microstructure, we propose a "two directional Jc distribution model". In this model, the difference of the local Jc distributions for various kinds of Bi2212 samples was explained in terms of the two different directional current flows and the aspect ratio of the grains. Moreover, the possibility of the small n-value even for high Jc materials in Bi2212 is also shown.

6: NEG123およびY123バルクの強磁場輸送特性
難波雅史,淡路 智,磯野伸之,渡辺和雄,Miryala MURALIDHAR,et al.
低温工学 41 (2006), 8 259-364.

The bulk material comprised of (Nd, Eu, Gd)Ba2Cu3Ox(NEG123) shows a high irreversibility field over 14 T at 77.3 K. In order to study the origin of the high irreversibility field, transport measurements of NEG123 and Y123 bulk samples were performed as functions of temperature, high magnetic field and field angle, and transport properties were compared with each other. Dip structures for the angular dependence of resistivity are observed around B//c for all samples. These results indicate that c-axis correlated disorder exists. The NEG123 bulk sample with a larger dip shows a higher irreversibility temperature. In addition, we found that the differences of the irreversibility temperature between on and 12° off the dip are scaled universally as a function of the depth of the dip for NEG123 and Y123 bulk samples. It is considered that c-axis correlated disorder is important for high irreversibility fields.