Free vibration analysis of a polymer electrolyte membrane fuel cell

A free vibration analysis of a polymer electrolyte membrane fuel cell (PEMFC) is performed by modelling the PEMFC as a 20 cm × 20 cm composite plate structure. The membrane, gas diffusion electrodes, and bi-polar plates are modelled as composite material plies. Energy equations are derived based on Mindlin's plate theory, and natural frequencies and mode shapes of the PEMFC are calculated using finite element modelling. A parametric study is conducted to investigate how the natural frequency varies as a function of thickness, Young's modulus, and density for each component layer. It is observed that increasing the thickness of the bi-polar plates has the most significant effect on the lowest natural frequency, with a 25% increase in thickness resulting in a 17% increase in the natural frequency. The mode shapes of the PEMFC provide insight into the maximum displacement exhibited as well as the stresses experienced by the single cell under vibration conditions that should be considered for transportation and stationary applications. This work provides insight into how the natural frequencies of the PEMFC should be tuned to avoid high amplitude oscillations by modifying the material and geometric properties of individual components.     

Lateral cyclic behavior of zipper braced frames-considering connection details

During Northridge earthquake in USA in 1994, a variety of failures occurred in welded steel connections. Studying these structural failures has led to development of more reliable moment resisting connections and new ways of using braced frames as seismic load resisting systems. This article investigates through numerical simulations, the lateral capacity and seismic behavior of two of these newly-thought braced frames, zipper braced frames and suspended zipper braced frames. The overall seismic behavior of these frames is investigated through displacement-based pushover analyses considering the effect of connection elements such as gusset plate and shear tab. To study the efficiency of these two types of concentrically braced frames, a numerical investigation on their behaviors for low-, mid- and high-rise buildings was conducted. Three zipper braced frames and three suspended zipper braced frames with different number of stories have been modeled using OpenSEES software. For each simulation, frame maximum strength, maximum drift capacity, and weight are determined and compared with each other. It is concluded that connection modeling has significant effects on the lateral behavior of these frames. Furthermore, the suspended zipper braced frames show higher ductility when compared with the ductility of zipper braced frames. Finally, the suspended zipper braced frames are recommended to be used in high-rise buildings, however, for the lowand mid-rise buildings it is recommended to use zipper frames due to economic efficacy.     

Effect of mechanical vibrations on damage propagation in polymer electrolyte membrane fuel cells

Vibrations and impact loads are common sources of mechanical damage in transportation applications; however, their impacts on polymer electrolyte membrane fuel cells (PEMFCs) have yet to be fully investigated. In this work, the damage propagation in the membrane electrode assembly (MEA) is investigated under vibration conditions with a focus placed on the interface between the membrane and catalyst layer at the cathode. A numerical model based on the cohesive element approach is developed, and a parametric study is performed to investigate the effects of amplitude and frequency of applied vibrations as well as initial delamination length on damage propagation. Non-linear relationships were found between the damage propagation and these parameters, with the frequency of vibration having the dominant effect on damage propagation at larger amplitudes. This work provides insight into the importance of considering mechanical damage to the MEA under vibration conditions in transportation applications.     

Ultralow-voltage field-ionization discharge on whiskered silicon nanowires for gas-sensing applications

Several hundred million volts per centimetre of electric-field strength are required to field-ionize gas species. Such fields are produced on sharp metallic tips under a bias of a few kilovolts. Here, we show that field ionization is possible at dramatically lower fields on semiconductor nanomaterials containing surface states, particularly with metal-catalysed whiskers grown on silicon nanowires. The low-voltage field-ionization phenomena observed here cannot be explained solely on the basis of the large field-amplification effect of suspended gold nanoparticles present on the whisker tips. We postulate that field penetration causes upward band-bending at the surface of exposed silicon containing surface states in the vicinity of the catalyst. Band-bending enables the valence electron to tunnel into the surface states at reduced fields. This work provides a basis for development of low-voltage ionization sensors. Although demonstrated on silicon, low-voltage field ionization can be detected on any sharp semiconductor tip containing proper surface states.     

Modified triglycine sulphate (TGS) single crystals for pyroelectric infrared detector applications

Effect of caesium and cerium,l-alanine and caesium +l-alanine impurities are investigated on ferroelectric and pyroelectric properties of TGS crystals. Dielectric constant and loss, pyroelectric coefficient, spontaneous polarization and coercive field measurements of these modified crystals, as a function of temperature are reported. Caesium and cerium did not affect the electrical properties of TGS crystals significantly, whereasl-alanine- and, especially, Cs +l-alanine-doped TGS crystals exhibited promising improvements in pyroelectric properties, up to 48 °C, as compared to pure TGS crystals.     

The effects of rehabilitation objectives on near optimal trade-off relation between minimum weight and maximum drift of 2D steel X-braced frames considering soil-structure interaction using a cluster-based NSGA II

A cluster-based non-dominated sorting genetic algorithm (NSGA) II has been considered to investigate the effects of rehabilitation objectives on multi-objective design optimization of two-dimensional (2D) steel X-braced frames in the presence of soil-structure interaction. The substructure elasto-perfect plastic model has been adopted for modeling of the soil-structure interaction and the nonlinear pushover analysis is used to evaluate the performance level of the frames for a specified hazard level. Cross-sections of grouped elements of the frames are considered to be discontinuous design variables of the problem. Via implementing some of the constraints, which are independent of doing the time-consuming nonlinear analysis, input population of the optimization technique has been clustered. By using the nonlinear analysis technique in conjunction with the cluster-based NSGA II, near optimal trade-off relation between minimum weight and maximum story drifts of the frames are obtained. The allowable rotations, geometry, and resistance constraints of the structural elements are considered in the optimization design of the frames. The effects of the enhanced basic safety and limited selective rehabilitation objectives on optimum design of the frame are studied. The results show differences between the optimum results of the three mentioned rehabilitation objectives and effects of soil types.

     

A new PSO-based algorithm for multi-objective optimization with continuous and discrete design variables

This paper presents a new multi-objective optimization algorithm called FC-MOPSO for optimal design of engineering problems with a small number of function evaluations. The proposed algorithm expands the main idea of the single-objective particle swarm optimization (PSO) algorithm to deal with constrained and unconstrained multi-objective problems (MOPs). FC-MOPSO employs an effective procedure in selection of the leader for each particle to ensure both diversity and fast convergence. Fifteen benchmark problems with continuous design variables are used to validate the performance of the proposed algorithm. Finally, a modified version of FC-MOPSO is introduced for handling discrete optimization problems. Its performance is demonstrated by optimizing five space truss structures. It is shown that the FC-MOPSO can effectively find acceptable approximations of Pareto fronts for structural MOPs within very limited number of function evaluations.