The major risk of using carbon nanotubes (CNTs) to modify proton exchange membranes (PEMs) in fuel cells is possible short-circuiting due to the excellent electrical conductivity of CNTs. In this article, silica-coated CNTs (SiO 2 @CNTs) were successfully prepared by a simple sol-gel process and then used as a new additive in the preparation of sulfonated poly (ether ether ketone) (SPEEK)-based composite membranes. The insulated and hydrophilic silica coated on the surface of CNTs not only eliminated the risk of short-circuiting, but also enhanced the interfacial interaction between CNTs and SPEEK, and hence promoted the homogeneous dispersion of CNTs in the SPEEK matrix. Moreover, compared to the methanol permeability of the pure SPEEK membrane (3.42 × 10 À7 cm 2 s À1 ), the SPEEK/SiO 2 @CNT composite membrane with a SiO 2 @CNT loading of 5 wt% exhibits almost one order of magnitude decrease of methanol crossover, while the proton conductivity still remained above 10 À2 S cm À1 at room temperature. The obtained results expose the possibility of SPEEK/SiO 2 @CNT membranes to be served as highperformance PEMs in direct methanol fuel cells.

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Flexible rotor systems supported by angular contact ball bearings display very complicated nonlinear behavior due to clearance and nonlinear Hertzian contact force. In this paper, a nonlinear vibration theoretical model for a flexible rotorbearing system is derived in which the dynamic model of the shaft element, disk, coupling, nonlinear contact deformations and forces of the ball bearing are established. Nonlinear responses and orbits analysis of a flexible rotor angular contact ball bearing system are studied using the numerical integration method and maximal Lyapunov exponent. Bifurcation and stability of periodic motion of the flexible rotor-bearing system are studied using the continuationshooting method and Floquet multipliers. Influences of initial contact angles, unbalance, damping factor, axial preloads, bending moments, stiffness ratio and rotating speed on stability and the bifurcation of periodic motion of the rotor-bearing system are analyzed. Finally, the theoretical model for a flexible rotor-bearing system is experimentally verified.

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The electromagnetic relay, which has mechanical structure and break/make contacts, is affected by vibration and shock environment greatly. Contact wear and failure will be result from serious mechanical conditions. The samples used in this study were relays consisting two spring sets connected with armature component. Simulation results for a variety of vibration levels, frequencies, and shock levels are presented. Thereafter main factors and threshold conditions which influencing contact reliability were determined by the modal analysis and the harmonic response analysis and transient response analysis. The vibration and shock characteristics of movable parts within relay were also obtained by using high speed camera PhantomV7.3 10000fps , thus the mechanism of contact failure is revealed clearly. In addition, the characteristics of contact resistance were investigated when relay working in the mechanical environment. The results are valuable in modeling characteristics degradation and contact failure evaluation caused by mechanical environment for electromagnetic relay.