On mechanical behavior and in-plane modeling of constrained PEM fuel cell membranes subjected to hydration and temperature cycles

Currently, ionomer membranes are used in a variety of specialized applications. Such applications include, but are not limited to, dialysis, electrolysis, membrane separators, reaction catalysts and the most promising application: polymer electrolyte fuel cells. Although their use is widespread, significant gaps in understanding the mechanical behavior of these materials still remain. Many ionomer membranes change their structure, and in turn, their mechanical properties in response to applied thermal and moisture conditions that are functions of position. It has been observed that constrained materials subjected to changing environmental conditions can exhibit unusual behavior, e.g., in some cases, mechanical failure is seen in the absence of external applied mechanical loads. This condition is especially important in polymer membranes (specifically Nafion^®) used in polymer electrolyte membrane (PEM) fuel cells and is the major motivation of the present work. Laboratory characterization has been conducted to determine the mechanical properties of a proton exchange membrane with respect to temperature and relative humidity. Data recovered in these tests along with properties from literature have been used in finite element models to predict the behavior of membranes used in certain applications and geometries. The overall goal of this investigation was to characterize the mechanical response of ionomer membranes in in-plane constraint configurations subjected to variable hygro-thermal environments. Expansion/contraction mechanical response of the constrained membrane as a result of change in hydration and temperature is studied in uniform and non-uniform geometries and environments. With this information, mechanical failure modes can be analyzed which is necessary for durability modeling and life prediction. The present work concentrates on defining and understanding the basic mechanical behavior of ionomeric membranes clamped in a rigid frame, and subjected to changes in temperature and humidification.     

Optimum Shunt Capacitor for Power Factor Correction at Busses with Lightly Distorted Voltage

The goal of this paper is to determine the effect of time variation of system impedances and voltage harmonics on the value of optimum capacitor for power factor correction at busses with nonsinusoidal voltage. Two types of 24 hours time-variation of voltage harmonices and Thevenin impedance are assumed. The equivalent load impedance is also considered time variable and assumed to contain a large proportion of induction motors. The daily energy losses are computed and graphed in function of the shunt capacitance used for power factor correction. The results of this study indicate that in order to avoid resonances and to find the optimum capacitor the time variation of harmonics and system impedances must be known as precise as possible.     

Development of a full range multi-scale model to obtain elastic properties of CNT/polymer composites

The main goal of this study was to develop a full range multi-scale modeling technique to extract Young??s modulus and Poisson??s ratio of carbon nanotube reinforced polymer (CNTRP) composites covering all nano, micro, meso and macro scales. The developed model consists of two different phases as top-down scanning and bottom-up modeling. At the first stage, the material region will be scanned from the macro level downward to the nano-scale. Effective parameters associated with each scale will be identified through this scanning procedure. Taking into account identified effective parameters of each specific scale, the suitable representative volume elements (RVE) will be defined for all nano, micro, meso and macro scales, separately. In the second stage of the modeling procedure, a hierarchical multi-scale modeling approach is developed. This modeling strategy would analyze the material at each scale and obtained results that were fed to the upper scale as input information. Due to involved random parameters, the developed modeling technique is implemented stochastically. It has been shown that the developed modeling procedure provides a clear insight to the properties of CNTRP and it is a very efficient tool for simulation of mechanical behavior of CNTRP composites. A sensitivity analysis was conducted to quantify the influence of the identified random parameters on the overall behavior of CNTRP.     

Investigation of nanotube length effect on the reinforcement efficiency in carbon nanotube based composites

The main goal of this research is to study the tensile behavior of embedded short carbon nanotubes (CNTs) in a polymer matrix in presence of van der Waals (vdW) interaction as inter-phase region. A 3D finite element model of a unit cell consisting of capped carbon nanotubes, inter-phase and surrounding polymer is built. The unit cell is subjected to tensile load case to obtain longitudinal Young’s modulus of the investigated cell. A parametric study is carried out to investigate the effect of CNT’s length on reinforcement. It is observed that improvement in the Young’s modulus of CNT-composite is negligible for lengths smaller than 100 nm and saturation takes place in larger lengths on the order of 10 μm. Furthermore, a comparison between results obtained for short carbon nanotubes and long carbon nanotube is presented. The efficient length of CNT in form of (10, 10) is obtained at the order of 10 μm. Finally, it was shown that direct use of micromechanics equations for short fibers will overestimate the stiffness. However, employing effective stiffness of equivalent fiber comprising of CNT and its inter-phase instead of high modulus of CNT will lead us to more appropriate results, which are in an acceptable agreement with conventional semi-empirical micromechanics equations.     

Wireless amperometric neurochemical monitoring using an integrated telemetry circuit

An integrated circuit for wireless real-time monitoring of neurochemical activity in the nervous system is described. The chip is capable of conducting high-resolution amperometric measurements in four settings of the input current. The chip architecture includes a first-order Delta Sigma modulator (Delta Sigma M) and a frequency-shift-keyed (FSK) voltage-controlled oscillator (VCO) operating near 433 MHz. It is fabricated using the AMI 0.5 microm double-poly triple-metal n-well CMOS process, and requires only one off-chip component for operation. Measured dc current resolutions of approximately 250 fA, approximately 1.5 pA, approximately 4.5 pA, and approximately 17 pA were achieved for input currents in the range of +/-5, +/-37, +/-150, and +/-600 nA, respectively. The chip has been interfaced with a diamond-coated, quartz-insulated, microneedle, tungsten electrode, and successfully recorded dopamine concentration levels as low as 0.5 microM wirelessly over a transmission distance of approximately 0.5 m in flow injection analysis experiments.     

Retracted: HYDRO‐SORTING OF APPLE USING TERMINAL VELOCITY IN WATER

Retracted : The following article from Journal of Food Process Engineering, “Hydro‐sorting of Apple Using Terminal Velocity in Water” by Kamran Kheiralipour, Ahmad Tabatabaeefar, Hossein Mobli, Shahin Rafiee, Ali Rajabipour, and Ali Jafari published online on 22 February 2010 in Wiley Online Library ( http://www.onlinelibrary.wiley.com ), has been retracted by agreement between the authors, the Journal's Co‐Editors, M. Elena Castell‐Perez and Rosana Moreira, and Wiley Periodicals, Inc. The retraction has been agreed due to overlap between this article and the following article published in the Transactions of the American Society of Agricultural Engineers (now called Transactions of the American Society of Agricultural and Biological Engineers), “Sorting of Kiwifruit for Quality Using Drop Velocity in Water” by Robert B. Jordan and Christopher J. Clark. Vol. 47:6 (2004): 1991–1998.     

Theoretical study of failure in composite pressure vessels subjected to low-velocity impact and internal pressure

A theoretical solution is aimed to be developed in this research for predicting the failure in internally pressurized composite pressure vessels exposed to low-velocity impact. Both in-plane and out-of-plane failure modes are taken into account simultaneously and thus all components of the stress and strain fields are derived. For this purpose, layer-wise theory is employed in a composite cylinder under internal pressure and low-velocity impact. Obtained stress/strain components are fed into appropriate failure criteria for investigating the occurrence of failure. In case of experiencing any in-plane failure mode, the evolution of damage is modeled using progressive damage modeling in the context of continuum damage mechanics. Namely, mechanical properties of failed ply are degraded and stress analysis is performed on the updated status of the model. In the event of delamination occurrence, the solution is terminated. The obtained results are validated with available experimental observations in open literature. It is observed that the sequence of in-plane failure and delamination varies by increasing the impact energy.     

Investigation of chirality and diameter effects on the Young’s modulus of carbon nanotubes using non-linear potentials

The main goal of this research is to predict Young’s modulus of carbon nanotubes using a full non-linear finite element model. Spring elements are used to simulate molecular interactions in atomic structure of carbon nanotube. All interactions are simulated non-linearly. A parametric study is performed to investigate effects of chirality and diameter on the Young’s modulus of single walled carbon nanotubes. Unlike the results of presented linear finite element models, the results of current model imply on independency of Young’s modulus from chirality and diameter. Obtained results from this study are in a good agreement with experimental observations and published data.     

A Wireless IC for Wide-Range Neurochemical Monitoring Using Amperometry and Fast-Scan Cyclic Voltammetry

An integrated circuit for real-time wireless monitoring of neurochemical activity in the nervous system is described. The chip is capable of conducting measurements in both fast-scan cyclic voltammetry (FSCV) and amperometry modes for a wide input current range. The chip architecture employs a second-order DeltaSigma modulator (DeltaSigmaM) and a frequency-shift-keyed transmitter operating near 433 MHz. It is fabricated using the AMI 0.5-mum double-poly triple-metal n-well CMOS process, and requires only one off-chip component for operation. A measured current resolution of 12 pA at a sampling rate of 100 Hz and 132 pA at a sampling rate of 10 kHz is achieved in amperometry and 300-V/s FSCV modes, respectively, for any input current in the range of plusmn430 nA. The modulator core and the transmitter draw 22 and 400 muA from a 2.6-V power supply, respectively. The chip has been externally interfaced with a carbon-fiber microelectrode implanted acutely in the caudate-putamen of an anesthetized rat, and, for the first time, extracellular levels of dopamine elicited by electrical stimulation of the medial forebrain bundle have been successfully recorded wirelessly using 300-V/s FSCV.     

A time and hydration dependent viscoplastic model for polyelectrolyte membranes in fuel cells

Ionomers are co-polymers with ionic groups. One of the interesting applications of ionomer membranes is as electrolytes in proton exchange membrane (PEM) fuel cells. The most commonly used membranes in PEM fuel cells are perfluorosulfonic acid (PFSA) membranes, e.g., Nafion^® from DuPont^TM. Besides its dependency on temperature and hydration due to phase inversion and cluster formation, Nafion^® as a polymer, exhibits strong time and rate effects. In this work, the stress–strain behavior of Nafion^® at different strain rates has been obtained in an environmental chamber for various temperatures and hydrations. After a certain strain was reached in each test, stress relaxation was performed for an hour to observe the relaxation behavior of Nafion^®. We attempted to use a nonlinear, time-dependent constitutive model to predict the hygro-thermomechanical behavior of Nafion^®. Because a substantial component of the response is unrecoverable, a viscoplastic model was employed. The proposed two-layer viscoplasticity model consisted of an elastoplastic network that was in parallel with an elastic-viscous network (Maxwell model) which separates the rate-dependent and rate-independent behavior of the material. After obtaining the necessary parameters for different hydrations, this model showed reasonably accurate success in predicting the stress–strain behavior at different strain rates, and matched the relaxation test results. Finite element simulations based on the proposed two-layer viscoplasticity model were in good agreement with test results and can be used to study the stress–strain state of the ionomer membranes in fuel cell configurations.