Nanotweezers are electromechanical nanosized devices with versatile applications. Due to the challenges for experiments on the nanoscale, the support of models and simulations is crucial to get a better insight into the operational mode of nanotweezers. In the current work, armchair and zigzag individual carbon nanotubes (CNTs) as well as nanotweezers are investigated with regard to their behavior under various prescribed electrostatic loads. The applied model is a combination of the model by Cox and Hill for the geometry, the modified molecular structural mechanics (MSM) model for the covalent bonds and the charge-dipole-model by Mayer for the electric field. Compared to the models based on the solution of the Schrdinger equation, the computational cost of the MSM model is much lower. A weak coupling is given between the mechanical and the electrical field through the application of the Coulomb forces on the mechanical model. The finite element software ABAQUS was used for the numerical simulation. It was shown that (i) the resulting strains and closing gaps are nearly identical for armchair and zigzag CNTs with the same diameter and that (ii) an increase in CNT length leads to greater strains for individual CNT and narrower closing gaps between the arms of the nanotweezers.

In engineering-especially in mechanics, lightweight engineering, aerospace engineering, electrical engineering as well as bioengineering-there is a desire of developing materials enabling an excellent performance with respect to mechanical, thermal, and electrical properties. One of the most promising materials are carbon nanotubes (CNTs), as they show excellent mechanical and electrical properties. To improve the understanding of the electrical behavior, i.e., the charge distribution in single-walled carbon nanotubes, both open ended armchair and zigzag types are investigated. In the present research, three different modeling approaches, a classic electrostatic model, the model by Li and Chou and the model by Mayer, are analyzed and compared with respect to their further applicability. In the numerical investigations, different test cases are performed: (i) the carbon nanotubes are charged with an overall charge, (ii) the CNTs are exposed to an external electric field, and (iii) the test cases (i) and (ii) are combined. Furthermore, the influence of different geometric parameters is investigated. It is shown that the charge applied to the CNTs distributes over the whole CNT having maxima at the ends of the tubes. These maxima can be influenced by both, the geometric parameters and the electric field strength.

In the present research, a numerical modeling approach of the initial stage of consolidation during spark plasma sintering on the microscopic scale is presented. The solution of a fully coupled thermo-electro-mechanical problem also accounting for grain boundary and surface diffusion is found by using a staggered way. The finite-element method is applied for solving the thermo-electro-mechanical problem while the finite-difference method is applied for the diffusion problem. A Lagrange-based non-linear formulation is used to deal with the detailed description of plastic and creep strain accumulation. The numerical model is developed for simulating the structural evolution of the involved particles during sintering of powder compacts taking into account both the free surface diffusion of the particles and the grain boundary diffusion at interparticle contact areas. The numerical results obtained by using the two-particle model—as a representative volume element of the powder—are compared with experimental results for the densification of a copper powder compact. The numerical and experimental results are in excellent agreement.

The mechanisms of densification in spark plasma sintering (SPS) were investigated both analytically and numerically for a model system of two spherical metallic powder particles. From the microscopic temperature distribution, the possibility of a micro-local overheating of the particle-particle contacts was analysed for different particle sizes, contact geometries, materials, and electrical loads. It is shown that, for particles below the size of one millimetre, local overheating is below one Kelvin. Subsequently, the material transport by thermomigration, electromigration, and diffusion driven by surface curvature and external pressure was derived from microscopic field distributions obtained from analytical calculations and finite-element simulations. The results show that, while the mechanical pressure accelerates material transport by orders of magnitude, the electrical current and the temperature gradients do not. It is also shown that pulsing the current has no significant influence on the densification rate.

Due to their outstanding mechanical properties, carbon nanotubes (CNTs) are very promising materials for further applications in the field of lightweight construction. Carbon nanotube fibers, whose structure consists of a multitude of load-bearing carbon nanotube bundles interconnected by threads, are an excellent possibility to utilize these properties as engineering material. In the present research, a new method for the prediction of the mechanical properties of carbon nanotube bundles is presented. Within this, the complex structure is transformed into a simplified model based on suitable assumptions. Several parameters of the bundle are taken into account such as different types of nanotubes and various nanotube lengths. The model is applied to different configurations of carbon nanotube bundles by using a molecular mechanics approach. The interactions between the nanotubes are investigated by analyzing the Lennard–Jones potential in a virtual tensile loading test. For different configurations, the resulting forces and stresses are obtained. The results give a clear insight into the influencing parameters and demonstrate their effect on the mechanical behavior. In conclusion, the present approach is an excellent method to analyze the mechanical behavior of CNT bundles.

Molecular mechanics models like the molecular structural mechanics (MSM) model have been proven to be a valuable tool for the investigation of carbon nanotubes. This class of models is based on a chemical force field representing the bonds present in the structure. Up to now, the underlying chemical force field applied in the MSM model is usually based on harmonic potentials. This simple approach comes with major drawbacks limiting the abilities of the MSM approach. In order to overcome these limitations, the present work integrates the 2nd generation reactive bond order potential (REBO), which is a sophisticated chemical force field, into the MSM approach. The development of this advanced MSM model is given in detail. As a demonstration of its capabilities, the elastic properties of different armchair and zig-zag CNTs are investigated numerically. It was found, that the results for the elastic properties differ significantly with the choice of the underlying chemical force field. The arising differences are discussed and reasons for the occurring behavior are given. The developed advanced MSM approach on the basis of the REBO potential strongly enhances the capabilities of the MSM approach while maintaining its excellent accessibility and applicability together with a low computational effort.

Contact growth and temperature behavior in time during a single high-current pulse representing the initial stage of field-assisted/spark plasma sintering (FAST/SPS) have been studied experimentally and numerically. The measured evolution of the electrical resistance and of the neck formation process in two-particle systems is compared to the results obtained from the fully coupled thermo-electro-mechanical finite element simulations. The results of simulations with various models of inelastic deformation show that the viscoelastic/viscoplastic material model provides a realistic contact growth in initial stage of FAST/SPS. The impact of electrical and mechanical loads, material parameters and particle size on temperature, on inelastic strain distribution and on densification has been studied by finite element simulations for copper, stainless steel and nickel particles.

The outstanding properties of carbon nanotubes (CNTs) keep attracting the attention of researchers from different fields. CNTs are promising candidates for applications e.g. in lightweight construction but also in electronics, medicine and many more. The basis for the realization of the manifold applications is a detailed knowledge of the material properties of the carbon nanotubes. In particular for applications in lightweight constructions or in composites, the knowledge of the mechanical behavior of the CNTs is of vital interest. Hence, a lot of effort is put into the experimental and theoretical determination of the mechanical material properties of CNTs. Due to their small size, special techniques have to be applied. In this research, a modified molecular structural mechanics model for the numerical determination of the mechanical behavior of carbon nanotubes is presented. It uses an advanced approach for the geometrical representation of the CNT structure while the covalent bonds in the CNTs are represented by beam elements. Furthermore, the model is specifically designed to overcome major drawbacks in existing molecular structural mechanics models. This includes energetic consistency with the underlying chemical force field. The model is developed further to enable the application of a more advanced chemical force field representation. The developed model is able to predict, inter alia, the lateral and radial stiffness properties of the CNTs. The results for the lateral stiffness are given and discussed in order to emphasize the progress made with the presented approach.

The initial stage of contact formation during field-activated/assisted sintering of powders is investigated experimentally and numerically for spherical particles of nickel, steel, and copper. The results of experimental studies of the contact resistance and the neck formation process due to melting/sintering in two- to three-particle systems under a single high-current pulse from a capacitor discharge are compared with results of fully coupled thermo-electrical finite-element simulations. The impact of particle size, contact geometry, and electrical load on the temperature and temperature gradient distribution during the pulse discharge is analyzed numerically for nickel spheres. The influence of the material itself is compared for nickel, steel, and copper systems.

A new, modified molecular structural mechanics model for the determination of the elastic properties of carbon nanotubes is presented. It is designed specifically to overcome drawbacks in existing molecular structural mechanics models, which are not consistent with their underlying chemical force fields in terms of energy. As a result, modifications are motivated, developed and implemented in order to create a new, energy consistent molecular structural mechanics model. Hence, the new model leads to a better prediction of the material parameters for single wall carbon nanotubes, while the simple applicability of the approach is maintained. The results calculated for the elastic constants (Young's modulus, Poisson ratio) of armchair and zig-zag CNTs are given and discussed. Both elastic constants were found to be dependent on the chirality as well as on the carbon nanotube diameter. An asymptotic value of approximately 800 GPa was obtained for the Young's modulus and a value of approximately 0.28 for the Poisson ratio. (C) 2015 Elsevier Ltd. All rights reserved.

Since many years, carbon nanotubes (CNTs) have been considered for a wide range of applications due to their outstanding mechanical properties. CNTs are tubular structures, showing a graphene like hexagonal lattice.Our interest in the calculation of the mechanical properties is motivated by several applications which demand the knowledge of the material behavior. One application in which the knowledge of the material behavior is vital is the CNT based fiber. Due to the excellent stiffness and strength of the individual CNTs, these fibers are expected to be a promising successor for state of the art carbon fibers. However, the mechanical properties of the fibers fall back behind the properties of individual CNTs. It is assumed that this gap in the properties is a result of the van-der-Waals interactions of the individual CNTs within the fiber. In order to understand the mechanical behavior of the fibers we apply a molecular mechanics approach.The mechanical properties of the individual CNTs are investigated by using a modified structural molecular mechanics approach. This is done by calculating the properties of a truss-beam element framework representing the CNT with the help of a chemical force field.Furthermore, we also investigate the interactions of CNTs arranged in basic CNT assemblies, mimicking the ones in a simple CNT fiber. We consider the van-der-Waals interactions in the structure and calculate the potential surface of the CNT assemblies.

The present work describes the modeling techniques applied for the investigation of assemblies of Carbon Nanotubes (CNTs) and Multi Wall Carbon Nanotubes (MWNTs). Motivated from possible future applications, for instance in new composite materials, the investigations of the interaction behavior of individual Carbon Nanotubes within an assembly of Carbon Nanotubes is mandatory in order to understand the behavior of larger scale structures made from CNTs, e.g. Carbon Nanotube fibers. The present research applies the Lennard‐Jones potential for modeling the van‐der‐Waals interaction occurring between the nanotubes realizing the interaction behavior. The individual Carbon Nanotubes within the assembly are assumed to be rigid. All van‐der‐Waals bonds are taken into account in order to calculate an overall potential surface describing the behavior of the investigated structure. An example consisting of a Multi Wall Carbon Nanotube made of three armchair Single Wall Carbon Nanotubes (SWNTs) is investigated (with and without additional outer force) with respect to equilibrium positions of the structure. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carbon nanotubes are due to their outstanding mechanical properties destined for a wide range of possible applications. Since the knowledge of the material behavior is vital regarding the possible applications, experimental and theoretical studies have been conducted to investigate the properties of this promising material. The aim of the present research is the calculation of mechanical properties and of the mechanical behavior of single wall carbon nanotubes (SWCNTs). The numerical simulation was performed on basis of a molecular mechanics approach. Within this approach two different issues were taken into account: (i) the nanotube geometry and (ii) the modeling of the covalent bond. The nanotube geometry is captured by two different approaches, the roll-up and the exact polyhedral model. The covalent bond is modeled by a structural molecular mechanics approach according to Li and Chou. After a short introduction in the applied modeling techniques, the results for the Young's modulus for several SWCNTs are presented and are discussed extensively. The obtained numerical results are compared to results available in literature and show an excellent agreement. Furthermore, deviations in the geometry stemming from the different models are given and the resulting differences in the numerical findings are shown. Within the investigation of the deformation mechanisms occurring in SWCNTs, the basic contributions of each individual covalent bond are considered. The presented results of this decomposition provide a deeper understanding of the governing deformation mechanisms in SWCNTs.