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Mufundirwa A.,Hokkaido University | Fujii Y.,Hokkaido University | Kodama N.,Hakodate National College of Technology | Kodama J.-I.,Hokkaido University
Cold Regions Science and Technology | Year: 2011

In this paper, natural rock slope deformation across fractures predominantly in a chert rock mass was monitored using six surface fracture displacement sensors, and the deformations arising from thermal stresses were predicted using (5. m × 5. m) two-dimensional (2D) finite element (FE) plane strain analysis coupled with a model for rock mass expansion due to freezing of pore water. A new and simple method to minimize displacement proportional to temperature (due to thermal response of chert rock mass and sensor) was proposed. By applying the method, the corrected displacement, u', can be well recognized. Under u', clear rock mass movement, which could be related to fracture growth, was observed. In addition, progressive fracture opening and closure were noted. Results from this study indicate insignificant influences of weather conditions on fracture/rock mass movement. Furthermore, under numerical analysis (FE), in the rock mass model (with 1-m deep fracture), tensile stresses that were large enough to induce fracture growth appeared at the fracture tip when temperature lowered. And in the rock slope model (with 1-m deep fracture), small tensile stresses, which were sufficient to cause fracture growth along the planes of weakness, were observed. This research suggests that freezing effects on deformation of chert rock mass are insignificant, and we tentatively suggest that thermal fatigue predominantly caused the permanent fracture deformations. © 2010 Elsevier B.V. Source

Kotobuki M.,Hakodate National College of Technology
Advanced Materials Research | Year: 2013

Lithium ion battery prefers an electrode consist of small particles so as to allow giving short Li+ diffusion pathway. In order to obtain small particles of LiCoO2, which has been commonly used as cathode for lithium battery, the ball-milling is applied to LiCoO2 prepared through a sol-gel method. By the ball-milling, the particle size of LiCoO2 can be reduced from 400 to 250 nm. The discharge capacity of milled LiCoO2 under high current density of 5C is about 100 mA h g-1, that is much higher than non-milled LiCoO2 (60 mA h g-1). It is concluded that the small LiCoO2 prepared by the ball-milling possess superior performance due to short Li+ diffusion length. © (2013) Trans Tech Publications, Switzerland. Source

Kimura A.,Hakodate National College of Technology
Electrical Engineering in Japan (English translation of Denki Gakkai Ronbunshi) | Year: 2011

In inverter-converter driving systems for AC electric cars, the DC input voltage of an inverter contains a ripple component with a frequency that is twice the line voltage frequency, due to the use of a single-phase converter. The ripple component of the inverter input voltage causes pulsations in the torque and current of driving motors. To decrease the pulsations, a beatless control method, which modifies the slip frequency depending on the ripple component, is applied to the inverter control. In the present paper, the beatless control method is analyzed in the frequency domain. In the first step of the analysis, transfer functions which revealed the relationship among the ripple component of the inverter input voltage, the slip frequency, the motor torque pulsation, and the current pulsation were derived with a synchronous rotating model of induction motors. An analytical model of the beatless control method was then constructed using the transfer functions. The optimal setting of the control method was obtained according to the analytical model. The transfer functions and the analytical model were verified by simulations. © 2010 Wiley Periodicals, Inc. Source

Kotobuki M.,Hakodate National College of Technology
International Journal of Energy and Environmental Engineering | Year: 2013

LiCoPO4 has been recognized as a promising cathode material for lithium batteries due to its high stability and high operation voltage. However, poor electronic conductivity of LiCoPO4 prohibits its practical use. A carbon coating can improve electronic conductivity of LiCoPO4. A hydrothermal synthesis is a very convenient method because it allows us easy preparation of small particles of carbon-coated LiCoPO4; however, an effect of precursor for LiCoPO4 preparation on the performance of the synthesized LiCoPO4 has yet to be cleared. In this paper, the effect of Co source for carbon-coated LiCoPO4 (LiCoPO4/C) preparation on performance as a cathode material for Li-ion battery is investigated. The Co source strongly affects the pH value in the starting solution and final products. The single-phase LiCoPO4 is obtained only when CoSO4 or CoCl2 are used as the Co sources. A quality of carbon layer on the LiCoPO4 is also affected by the Co source. The carbon layer on the LiCoPO4 synthesized from CoSO4 contains graphite carbon with high concentration which provides high electronic conductivity compared with that from CoCl2. Accordingly, the LiCoPO4/C synthesized from CoSO4 shows a superior performance than that from CoCl2 due to high-quality carbon layer. © 2013 Kotobuki. Source

Yanagisawa T.,Japan National Institute of Advanced Industrial Science and Technology | Miyazaki M.,Hakodate National College of Technology
EPL | Year: 2014

In this study, we investigate the metal-insulator transition of charge transfer type in high-temperature cuprates. We first show that we must introduce a new band parameter in the three-band d-p model to reproduce the Fermi surface of high-temperature cuprates such as BSCCO, YBCO and Hg1201. We present a new wave function of a Mott insulator based on the improved Gutzwiller function, and show that there is a transition from a metal to a charge-transfer insulator for such parameters by using the variational Monte Carlo method. This transition occurs when the level difference Δdp = εp-εd between d and p orbitals reaches a critical value (Δdp)c. The energy gain .E, measured from the limit of large Δdp, is proportional to 1/Δdp for Δdp > (Δdp)c: ΔE∞ -t2 dp/Δdp. We obtain (Δdp)c ≃ 2tdp using the realistic band parameters. © EPLA, 2014. Source

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