No.29(AUGUST 2010)

Lithium ion secondary batteries are applied in many fields including small mobile equipment, electric vehicles, and large power storage. In mobile applications, higher energy densities are demanded. Lithium alloying metal based anode materials such as Sn, Si have been extensively investigated for high energy density Li-ion batteries. However, they have poor cyclability because of their larger volume change compared to conventional graphite base anodes during the charge-discharge cycle. The large volume change causes a peeling and shedding of anode material from the copper foil collector as well as breakage into fragments. As the copper foil collectors suffer large stress from the anode materials expanding, the mechanical properties of the copper foil are a major factor relating to the degradation of electrode performance.
We therefore conducted a charge/discharge cycle test by using a unique cell to examine the deformation behavior of copper foil collectors. The behavior of copper foils was organized in terms of stresses generated due to the expanded volume of active materials and the 0.2% yield strength of the copper foils. Even when the same active material was involved, the low-yield strength copper foils underwent plastic deformation when charged, and deformation remained after discharge. High-yield strength copper foils, however underwent repeated elastic deformation during the charge-discharge cycle. Plastically deformed copper foils cease to follow the subsequent volumetric changes of the active materials, thereby promoting the omission of active materials. That is, a minimum yield strength is considered necessary for a specific copper foil according to the type and composition of active material. These high-yield strength copper foils are suited for Sn, Si, and other high-capacity active materials subject to large volumetric changes.

Railroads require low energy consumption and discharge relatively small amounts of carbon dioxide given the volume of transportation they provide, and consequently have attracted attention in recent years as a planet-friendly means of transportation. Overseas railroad projects are being announced one after another, and the rolling stock business overseas is expected to expand significantly in the future. However, in Europe where there is a highly developed network of railroad rolling stock, the application of regional uniform standards known as the EN standards is spreading. In response, we have recently developed rolling stock cables adaptable to EN standards.

Abstract We have been developing high-performance Nb3Al superconducting wires for the Large Hadron Collider (LHC). In this study, the cross-sectional structure and fabrication processes were improved for fabricating long Nb3Al wires with a Ta matrix, which is indispensable for wires that are to be used in the LHC, and to increase their critical current densities. Concerning the increase in length, we examined the factors that affect wire-drawing, thereby clarifying the four factors that are highly sensitive to the fabrication characteristics. As the result of a prototype made with these results reflected in the cross-sectional composition, we succeeded in manufacturing wires 330 m long (1.35 mm in diameter) without a wire break for the first time using Ta matrix wires, although there is still one remaining problem: the matrix ratio is higher than that of the Nb matrix. Moreover, regarding the increase in critical current density, the restacked wires currently being developed achieved 900 A/mm² (20% higher than that of conventional wires) at 4.2K and 15T. This demonstrated that Nb3Al wires are extremely promising as possible next-generation wires for the LHC.

Abstract In the mobile communication field, techniques are developing to make transmission capacity bigger are bigger, and multiple input multipul output (MIMO) antenna system is adopted as standard technique, and 2 × 2 MIMO system are now in practical use. We then built an evaluation environment for 4 × 4 MIMO system in advance, tried propagation channel experiments, and demonstrated the effects of MIMO by measurements. We also evaluated fading correlation coefficient-the key to MIMO antennas for base stations-by simulation, and by measurement of the three-dimensional radiation pattern using SG64, that is a system made by SATIMO in France. This propagation channel experiment and measurement of the fading correlation coefficient were conducted to establish a technology for evaluating MIMO antennas, thereby made it possible to derive specification conditions required for small MIMO antennas for base stations. In the future we will propose MIMO antennas having new structures according to these specifications.

I have considered how to expand the capacity and speed up calculation for the method of moments (MoM) applied in the automatic antenna design technology used in antenna commercialization and consideration. To speed up the calculation of impedance matrixes and admittance matrixes used in MoM, I examined the methods of calculation and reverse matrix calculation based on the regularity of a current point as defined by these methods. Moreover, these methods were applied in memory distribution and parallel calculations based on a PC cluster of a general, inexpensive configuration for expanding the capacity and speeding up calculation for MoM.