波形碳纳米管增强复合材料的有效刚度和局部应力的宏微观均质化数值模拟

采用分子动力学方法简化的碳纳米管等效纤维模型,利用具有精确周期性边界条件的均质化理论和宏微观均质化法分析正弦波形非连续碳纳米管的有效刚度和局部应力分布规律.结果表明,纳米增强复合材料的有效刚度和局部应力对碳纳米管的波形非常敏感,碳纳米管稍有弯曲就会导致复合材料有效刚度降低和应力传递能力的下降,为揭示复合材料中碳纳米管的增强机制和改善增强效果提供理论依据.     

Development of numerical scheme for phase field crystal deformation simulation

The phase field crystal (PFC) method is anticipated as a new multiscale method, because this method can reproduce physical phenomena depending on atomic structures in metallic materials on the diffusion time scale. Although the PFC method has been applied to some phenomena, there are few studies related to evaluations of mechanical behaviors of materials by appropriate PFC simulation. In a previous work using the PFC method, tensile deformation simulations have been performed under conditions where the volume increases during plastic deformation. In this study, we developed a new numerical technique for PFC deformation simulation that can maintain a constant volume during plastic deformation. To confirm that the PFC model with the proposed technique can reproduce appropriate elastic and plastic deformations, we performed a series of deformation simulations in one and two-dimensions. In one- and two-dimensional single-crystal simulations, linear elastic responses were confirmed in a wide strain rate range. In bicrystal simulations, we could observe typical plastic deformations due to the generation, annihilation and movement of dislocations, and the interaction between the grain boundary and dislocations. Moreover, the deformation behaviors of a nanopolycrystalline structure at high temperature were simulated and the intergranular deformations caused by grain rotation and grain boundary migration were reproduced.     

Excitation density dependence of luminescence spectrum of electron-hole plasma in diamond

Time-resolved band edge luminescence spectrum in IIa diamond has been measured with the 5th harmonics of a pulsed YAG laser (5.82 eV) and an ICCD image intensifier of 5 ns gate width at 290 K. The time-resolved luminescence spectrum is decomposed into three components of free exciton (FE), excitonic complex (EC) and electron-hole plasma (EHP). The decay times of the FE and EC luminescence are 45 and 27 ns, respectively and that of the EHP luminescence has been seen to be shorter than the gate width, 5 ns. The low energy onset of the EHP luminescence spectrum has been observed to decrease with increasing excitation density and attains the onset of the electron-hole drop luminescence spectrum at the excitation density of 0.6 J/cm², at which the electron-hole pair density is 1.2 × 10²⁰ cm^? 3. Furthermore, the excitation density dependences of the FE, EC and EHP luminescence intensities are explained with the percolation theory.     

Analysis of polymer electrolyte fuel cell performance by electrode polarization model

For development of polymer electrolyte fuel cell (PEFC) lifetime estimation method, a high accuracy PEFC electrode polarization model is required. An electrode polarization model which was previously proposed was verified. However, accuracy of the electrode polarization model was not enough to estimate PEFC performance under various conditions. A new high accuracy PEFC electrode polarization model has been developed based on electrochemical consideration and data observed at elevated pressures. In the cathode polarization model, effects of O₂ diffusion and H₂O plugging have to be considered to obtain high accuracy for long-term operation. In addition, PEFC performance degradation was analyzed by the electrode polarization model. Main factors of PEFC performance degradation are OCV drop, the cathodic activation polarization, voltage drops by O₂ diffusion and H₂O plugging.     

Launched pulse-shape dependence of the power spectrum of the spontaneous brillouin backscattered light in an optical fiber

We theoretically analyze the relationship between the electric field envelope shape of an optical pulse launched into an optical fiber and the power spectrum of the spontaneous Brillouin backscattered light it produces. The electric field envelope is characterized by the pulse width, leading-trailing time, and steepness. The peak power of the launched, pulsed-light power spectrum is proportional to the square of the pulse width regardless of the pulse leading-trailing time and steepness, and the power spectrum broadens in inverse proportion to the pulse width. The peak power of the spontaneous Brillouin backscattered light produced by the launched, pulsed light is proportional to the pulse width when it is above approximately 100 ns and is proportional to the square of the pulse width when it is below approximately 1 ns. The power spectrum of the spontaneous Brillouin backscattered light also broadens rapidly corresponding to the pulse width, when the pulse width falls below approximately 30 ns. As the pulse leading-trailing time is shortened or the pulse leading-trailing part becomes steep, the Brillouin backscattered-light power spectrum broadens greatly, even if the launched pulse width remains constant. Our analysis showed that an optical pulse with a triangular-shaped electric field envelope forms the Brillouin backscattered-light power spectrum with the narrowest profile and consequently gives the minimum error in measuring the peak-power frequency, when the pulse width is below approximately 50 ns. The measurement error with the triangular-shaped pulsed light is 1/ radical2 times smaller than that for a rectangular-shaped pulsed light, when the pulse width falls below several nanoseconds. By contrast, the rectangular-shaped envelope gives the minimum error when the pulse width exceeds ~50 ns.     

Transient response of cylindrical shells to localized heat sources

An exact analysis of the dynamic problem on the thermal displacements and stresses of cylindrical shells subjected to instantaneous heating is carried out. Equations of motion for cylindrical shells, including the thermal effects, are reduced and solved by using the Fourier and Laplace transformation methods, after obtaining the temperature distribution determined from the nonstationary equation of heat conduction for a shell under a prescribed temperature field. Numerical results are presented for the thermal displacement and stresses due to the time variation together with the quasistatic ones.     

Studies on synthesis and permeability of special polymer membranes

Summary The permeation characteristics of polymer blend membranes from poly(vinylidene fluoride) and poly-(ethylene glycol) to aqueous polymer solution were investigated under carefully controlled conditions. The permeation characteristics were influenced significantly by the blend ratio, the temperature and time of heat treatment, which changed the structure of the resulting membranes.     

Low temperature deposition of hexagonal BN films by chemical vapour deposition

Transparent hexagonal BN films were deposited onto copper substrates from the reactant gas BCl₃-NH₃-H₂ at temperatures in the range 250–700°C. The lowest deposition temperature of the films was about 250°C. The films deposited at temperatures below 450°C were unstable in moist atmosphere and devitrified; a 20%–30% decrease in weight was observed when these films were heated above 600°C in an argon atmosphere. In contrast, the films deposited at temperatures above 600°C were very stable, decreased in weight by 1%–2% on heating and were stable in air at temperatures below 750°C.