@article{xu_rezvanian_zikry_2013, title={Electro-mechanical modeling of the piezoresistive response of carbon nanotube polymer composites}, volume={22}, ISSN={["1361-665X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876921770&partnerID=MN8TOARS}, DOI={10.1088/0964-1726/22/5/055032}, abstractNote={A coupled electro-mechanical FE approach was developed to investigate the piezoresistive response of carbon nanotube polymer composites. Gauge factors (GFs) and resistance variations of CNT–polymer composite systems were obtained by coupling Maxwell equations to mechanical loads and deformations through initial piezoresistive coefficients of the CNTs, the epoxy, and the tunnel regions, for different arrangements, percolated paths, tunnel distances, and tensile, compressive, and bending loading conditions. A scaling relation between GFs and applied strains was obtained to understand how variations in loading conditions and CNT arrangements affect sensing capabilities and piezoresistive carbon nanotube polymer composite behavior. These variations in GFs were then used to understand how the coupled strains, stresses and current densities vary for aligned and percolated paths for the different loading conditions, CNT arrangements, and tunnel distances. For the percolated path under tensile loading conditions, elastic strains as high as 16% and electrical conductivities that were four orders in magnitude greater than the initial matrix conductivity were obtained. Results for the three loading conditions clearly demonstrate that electrical conductivity and sensing capabilities can be optimized as a function of percolation paths, tunneling distance, orientation, and loading conditions for piezoresistive applications with large elastic strains and conductivities.}, number={5}, journal={SMART MATERIALS AND STRUCTURES}, author={Xu, S. and Rezvanian, O. and Zikry, M. A.}, year={2013}, month={May} }
 @article{xu_rezvanian_zikry_2013, title={Electrothermomechanical Modeling and Analyses of Carbon Nanotube Polymer Composites}, volume={135}, ISSN={["1528-8889"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84888342251&partnerID=MN8TOARS}, DOI={10.1115/1.4023912}, abstractNote={A new finite element (FE) modeling method has been developed to investigate how the electrical-mechanical-thermal behavior of carbon nanotube (CNT)–reinforced polymer composites is affected by electron tunneling distances, volume fraction, and physically realistic tube aspect ratios. A representative CNT polymer composite conductive path was chosen from a percolation analysis to establish the three-dimensional (3D) computational finite-element (FE) approach. A specialized Maxwell FE formulation with a Fermi-based tunneling resistance was then used to obtain current density evolution for different CNT/polymer dispersions and tunneling distances. Analyses based on thermoelectrical and electrothermomechanical FE approaches were used to understand how CNT-epoxy composites behave under electrothermomechanical loading conditions.}, number={2}, journal={JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Xu, S. and Rezvanian, O. and Zikry, M. A.}, year={2013}, month={Apr} }
 @article{xu_rezvanian_peters_zikry_2013, title={The viability and limitations of percolation theory in modeling the electrical behavior of carbon nanotube-polymer composites}, volume={24}, ISSN={["1361-6528"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84875677113&partnerID=MN8TOARS}, DOI={10.1088/0957-4484/24/15/155706}, abstractNote={A new modeling method has been proposed to investigate how the electrical conductivity of carbon nanotube (CNT) reinforced polymer composites are affected by tunneling distance, volume fraction, and tube aspect ratios. A search algorithm and an electrical junction identification method was developed with a percolation approach to determine conductive paths for three-dimensional (3D) carbon nanotube arrangements and to account for electron tunneling effects. The predicted results are used to understand the limitations of percolation theory and experimental measurements and observations, and why percolation theory breaks down for specific CNT arrangements.}, number={15}, journal={NANOTECHNOLOGY}, author={Xu, S. and Rezvanian, O. and Peters, K. and Zikry, M. A.}, year={2013}, month={Apr} }
 @article{rezvanian_zikry_2011, title={Continuum modeling of large-strain deformation modes in gold nanowires}, volume={26}, ISSN={["2044-5326"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84864699682&partnerID=MN8TOARS}, DOI={10.1557/jmr.2011.148}, abstractNote={Abstract}, number={17}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Rezvanian, Omid and Zikry, Mohammed A.}, year={2011}, month={Sep}, pages={2286–2292} }
 @article{shanthraj_rezvanian_zikry_2011, title={Electrothermomechanical Finite-Element Modeling of Metal Microcontacts in MEMS}, volume={20}, ISSN={["1941-0158"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79953737151&partnerID=MN8TOARS}, DOI={10.1109/jmems.2010.2100020}, abstractNote={Three-dimensional fractal representations of surface roughness are incorporated into a finite-element framework to obtain the electrothermomechanical behavior of ohmic contacts in radio frequency (RF) microelectromechanical systems (MEMS) switches. Fractal surfaces are generated from the Weierstrass-Mandelbrot function and are representatives of atomic force microscope surface roughness measurements of contact surfaces in fabricated RF MEMS switches with metal contacts. A specialized finite-element scheme is developed, which couples the thermomechanical asperity creep deformations with the electromechanical contact characteristics to obtain predictions of contact parameters and their evolution as a function of time and loading. A dislocation-density-based crystal plasticity framework is also used to investigate microstructure evolution at microcontacts and its effects on contact parameters. Using this approach, simulations are made to investigate how surface roughness, initial residual strains, and operating temperature can affect asperity contact behavior. Based on these predictions, tribological design guidelines can be obtained to increase the lifetime of low-contact-resistance RF MEMS switches by limiting stiction and electrical resistance increase.}, number={2}, journal={JOURNAL OF MICROELECTROMECHANICAL SYSTEMS}, author={Shanthraj, Pratheek and Rezvanian, Omid and Zikry, Mohammed A.}, year={2011}, month={Apr}, pages={371–382} }
 @article{rezvanian_zikry_2009, title={Inelastic contact behavior of crystalline asperities in rf MEMS devices}, volume={131}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77955373765&partnerID=MN8TOARS}, DOI={10.1115/1.3026545}, abstractNote={Microelectromechanical systems (MEMS), particularly those with radio frequency (rf) applications, have demonstrated significantly better performance over current electromechanical and solid-state technologies. Surface roughness and asperity microcontacts are critical factors that can affect contact behavior at scales ranging from the nano to the micro in MEMS devices. Recent investigations at the continuum level have underscored the importance of microstructural effects on the inelastic behavior of asperity microcontacts. Hence, a microstructurally based approach that accounts for the inhomogeneous deformation of the asperity microcontacts under cyclic loading and that is directly related to asperity physical scales and anisotropies can provide a detailed understanding of the deformation mechanisms associated with asperity microcontacts so that guidelines can be incorporated in the design and fabrication process to effectively size critical components and forces for significantly improved device durability and performance. A physically based microstructural representation of fcc crystalline materials that couples a multiple-slip crystal plasticity formulation to dislocation densities is used in a specialized finite-element modeling framework. The asperity model and the loading conditions are based on realistic service conditions consistent with rf MEMS with metallic normal contacts. The evolving microstructure, stress fields, contact width, hardness, residual effects, and the localized phenomena that can contribute to failure initiation and evolution in the flattening of single crystal gold asperity microcontacts are characterized for a loading-unloading cycle. It is shown that the nonuniform loading conditions due to asperity geometry and contact loading and the size effects due to asperity dimensions result in significant contribution of the geometrically necessary dislocation densities to stress, deformation, and microstructural evolution of crystalline asperities. This is not captured in modeling efforts based on von Mises continuum plasticity formulations. Residual strains and stresses are shown to develop during the cyclic loading. Localized tensile stress regions are shown to develop due to stress reversal and strain hardening during both loading and unloading regimes. Hardness predictions also indicate that nano-indentation hardness values of the contact material can overestimate the contact force in cases, where a rigid flat surface is pressed on a surface roughness asperity.}, number={1}, journal={Journal of Engineering Materials and Technology}, author={Rezvanian, O. and Zikry, Mohammed}, year={2009}, pages={0110021–01100210} }
 @article{brown_rezvanian_zikry_krim_2009, title={Temperature dependence of asperity contact and contact resistance in gold RF MEMS switches}, volume={19}, number={2}, journal={Journal of Micromechanics and Microengineering}, author={Brown, C. and Rezvanian, O. and Zikry, M. A. and Krim, J.}, year={2009} }
 @article{rezvanian_zikry_rajendran_2008, title={Microstructural modeling in f.c.c. crystalline materials in a unified dislocation-density framework}, volume={494}, ISSN={["0921-5093"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-49849087562&partnerID=MN8TOARS}, DOI={10.1016/j.msea.2007.10.091}, abstractNote={A unified physically based microstructural representation of f.c.c. crystalline materials, has been developed such that evolving microstructural behavior at different physical scales can be accurately predicted. This microstructural framework is based on coupling a multiple-slip crystal plasticity formulation to three distinct dislocation densities, which pertain to statistically stored dislocations, geometrically necessary dislocations, and grain boundary dislocations. This interrelated dislocation-density formulation is then used with specialized finite-element modeling techniques to predict the evolving heterogeneous microstructure and the localized phenomena that can contribute to failure initiation as a function of inelastic deformation.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Rezvanian, O. and Zikry, M. A. and Rajendran, A. M.}, year={2008}, month={Oct}, pages={80–85} }
 @article{rezvanian_brown_zikry_kingon_krim_irving_brenner_2008, title={The role of creep in the time-dependent resistance of Ohmic gold contacts in radio frequency microelectromechanical system devices}, volume={104}, number={2}, journal={Journal of Applied Physics}, author={Rezvanian, O. and Brown, C. and Zikry, M. A. and Kingon, A. I. and Krim, J. and Irving, D. L. and Brenner, D. W.}, year={2008} }
 @article{rezvanian_zikry_rajendran_2007, title={Statistically stored, geometrically necessary and grain boundary dislocation densities: microstructural representation and modelling}, volume={463}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-36348999315&partnerID=MN8TOARS}, DOI={10.1098/rspa.2007.0020}, abstractNote={A unified physically based microstructural representation of f.c.c. crystalline materials has been developed and implemented to investigate the microstructural behaviour of f.c.c. crystalline aggregates under inelastic deformations. The proposed framework is based on coupling a multiple-slip crystal plasticity formulation to three distinct dislocation densities, which pertain to statistically stored dislocations (SSDs), geometrically necessary dislocations (GNDs) and grain boundary dislocations. This interrelated dislocation density formulation is then coupled to a specialized finite element framework to study the evolving heterogeneous microstructure and the localized phenomena that can contribute to failure initiation as a function of inelastic crystalline deformation. The GND densities are used to understand where crystallographic, non-crystallographic and cellular microstructures form and the nature of their dislocation composition. The SSD densities are formulated to represent dislocation cell microstructures to obtain predictions related to the inhomogeneous distribution of SSDs. The effects of the lattice misorientations at the grain boundaries (GBs) have been included by accounting for the densities of the misfit dislocations at the GBs that accommodate these misorientations. By directly accounting for the misfit dislocations, the strength of the boundary regions can be more accurately represented to account for phenomena associated with the effects of the GB strength on intergranular deformation heterogeneities, stress localization and the nucleation of failure surfaces at critical regions, such as triple junctions.}, number={2087}, journal={Philosophical Transactions of the Royal Society of London. Series A, Mathematical, Physical and Engineering Sciences}, author={Rezvanian, O. and Zikry, Mohammed and Rajendran, A. M.}, year={2007}, pages={2833–2853} }
 @article{rezvanian_zikry_brown_krim_2007, title={Surface roughness, asperity contact and gold RFMEMS switch behavior}, volume={17}, ISSN={["1361-6439"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34748865043&partnerID=MN8TOARS}, DOI={10.1088/0960-1317/17/10/012}, abstractNote={Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. Three-dimensional surface roughness profiles were generated, based on a Weierstrass–Mandelbrot fractal representation, to match the measured roughness characteristics of contact bumps of manufactured RF MEMS switches. Contact asperity deformations due to applied contact pressures were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating and contact forces. This modeling framework was used to understand how contact resistance evolves due to changes in the real contact area, the number of asperities in contact, and the temperature and resistivity profiles at the contact points. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on integrated modeling and experimental measurements, which can be used in the design of reliable RF MEMS devices with extended life cycles.}, number={10}, journal={JOURNAL OF MICROMECHANICS AND MICROENGINEERING}, author={Rezvanian, O. and Zikry, M. A. and Brown, C. and Krim, J.}, year={2007}, month={Oct}, pages={2006–2015} }
 @article{rezvanian_zikry_rajendran_2006, title={Microstructural modeling of grain subdivision and large strain inhomogeneous deformation modes in f.c.c. crystalline materials}, volume={38}, ISSN={["0167-6636"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33746933462&partnerID=MN8TOARS}, DOI={10.1016/j.mechmat.2005.12.006}, abstractNote={In this study, evolution equations related to a heterogeneous microstructure that is physically representative of the densities and dimensions of dislocation-cells and walls have been formulated and coupled to a multiple-slip crystal plasticity formulation. Specialized finite-element methodologies have then been used to investigate how an imbalance in shear-strain amplitudes can result in deformation band formation in a cube-oriented aluminum single crystal subjected to strains of up to 30% under rolling deformation. It has been shown that a change in the microstructural morphology from matrix to transition bands occurs as the dislocation-cell size increases with decreases in the stored dislocation density and as a function of slip-system structure and orientation. Comparisons with experimental measurements and observations clearly indicate that the transition and matrix bands can occur in cube orientations as a consequence of shear strain imbalance on active slip-systems.}, number={12}, journal={MECHANICS OF MATERIALS}, author={Rezvanian, O. and Zikry, M. A. and Rajendran, A. M.}, year={2006}, month={Dec}, pages={1159–1169} }