Properties Of Carbon Nanotube Networks Research Articles

Aggregation of nanoparticles and their effect on mechanical properties of carbon nanotube networks

Nanoparticles (NPs) are widely used to tune the mechanical and physical properties of carbon nanotube (CNT) networks in many practical applications. However, the distribution of NPs in CNT networks and their effects on microstructural evolutions and mechanical properties remain elusive. We employed the coarse-grained molecular dynamics (CGMD) simulations to systematically investigate these issues. First, we found that compared to ambient temperature and the frequency of cyclic loads, the relative cohesion energy of NPs and strain amplitude of cyclic loads are more important factors for controlling the aggregation of NPs; it was also revealed that NPs with larger relative cohesion energy and in larger-strain cyclic loads would aggregate more severely in CNT networks. Second, we observed that the Young's modulus and the tensile strength of the material can be increased by up to ∼3.2 and ∼2.0 times, respectively, at the critical volume fraction of NPs (∼21.6%). Finally, the underlying mechanisms by which NPs strengthen the inter-tube joints and constrain the bundling of CNTs are unveiled. These results should be useful for the optimal design of CNT-based advanced materials.

Enhanced mechanical properties of carbon nanotube networks by mobile and discrete binders

The mechanical enhancement of carbon nanotube (CNT) networks by mobile and discrete binders was explored by performing coarse-grained molecular dynamics (CGMD) simulations. The underlying mechanism of modifying deformation affinity is elucidated. We perform tensile tests and dynamical mechanical analysis (DMA) to quantify the strength, energy dissipation capacity, and viscoelastic performance of the material. Simulation results show that binders are apt to aggregate and cluster at the junctions between CNTs, resisting the reorientation, bundling and sliding processes that lead to microstructural evolution of the network. The effects enhance the strength, energy dissipation capacity and storage modulus of the network generally. Their dependence on the interfacial adhesion energy and weight fraction of binders is discussed in detail based on the simulation results. The findings here pave the way for tuning properties of CNT networks for mechanical, electrochemical, and filtration applications.

Rheological model predicting compressive responses of carbon nanotube networks

Carbon nanotube (CNT) macrostructures like anisotropic CNT arrays and isotropic CNT networks have many unique mechanical properties. Among the various properties of CNT networks, their collective responses to compressive deformation are studied here. A rheological model is found to account for the mechanical properties of CNT networks. The computational results show that the compressive responses of CNT networks include the mechanical action of interconnected springs and dry friction elements. The availability of the model is validated by comparing the computed results to the measured data of different density CNT sponges. The good agreement between the computed and measured results suggests that the compressive responses of CNT segments and the sliding friction between CNTs are the major factors contributing to the compressive responses of CNT networks. The effectiveness of the model in describing the CNT networks also indicates that their collective responses are similar to that of the rheological model during the compression cycle.

Electromechanical properties of carbon nanotube networks under compression

The network of entangled multiwall carbon nanotubes and the composite consisting of a polystyrene filter-supported nanotube are introduced as conductors whose conductivity is sensitive to compressive stress both in the course of monotonic stress growth and when loading/unloading cycles are imposed. The testing has shown as much as a 100% network conductivity increase at the maximum applied stress. It indicates the favorable properties of the multiwall carbon nanotube network for its use as a stress-electric signal transducer. To model the conductivity–stress dependence, it is hypothesized that compression increases local contact forces between the nanotubes, which in turn leads to a decrease in the contact resistance between them. The lack of detailed knowledge of the mechanism as well as an unclear shift from individual contacts to the whole network conductance behavior is circumvented with a statistical approach. In this respect, the conductivity/compression data were fitted well using the Weibull distribution for the description of the nanotube contact resistance distribution.

Experimental characterization of dielectric properties of carbon nanotube networks

This paper explores the characterization of dielectric and conductive properties of carbon nanotube (CNT) networks. This is carried out by building planar transmission lines where conventional metallic traces are replaced by CNT networks. The proposed transmission lines with CNT networks are presented. Experimental realization and repeated two-port microwave measurements of proposed transmission lines enable the accurate extractions of their fundamental parameters showing percolation effects due to presence of CNT networks. The frequency-dependent phase velocity characteristics show a dramatic reduction compared to the speed of light in vacuum. The large magnitude of extracted complex permittivity for CNT networks also exhibits its percolation performance. The effects of CNTs' bulk density on measured and calculated parameters are explained. The results presented in this paper demonstrate the feasibility and the potential of building transmission lines and radio-frequency (RF) circuits elements using CNT networks.

Tailoring optical and electrical properties of carbon nanotube networks for photovoltaic applications

Maximizing the electrical conductivity and optical transmittance of a carbon nanotube (CNT) network is critical for using CNTs as electrodes of solar cells. In this paper, the effect of CNT network morphology on its electrical and optical properties is investigated both experimentally and theoretically, indicating that a network consisting of loosely-percolated small long nanotube bundles or individual CNTs showed outstanding transparency and electrical conductivity. Further removal of surfactant absorbed onto the CNTs or treating them with nitric acid significantly increased the electrical conductivity of CNT films without sacrificing the transparency. The optimized CNT network demonstrated a surface resistivity of 205 ohm/square with 65% optical transmittance. These results will significantly facilitate the application of CNTs in solar cells and other electronic devices.

Mechanical properties of carbon nanotube networks by molecular mechanics and impact molecular dynamics calculations

We report a theoretical investigation of the mechanical properties of idealized networks formed by single-walled carbon nanotubes showing crossbar and hexagonal architectures. The study was performed by using molecular mechanics calculations and impact dynamics simulations based on bond-order empirical potential. The studied networks were predicted to have elasticity modulus of $\ensuremath{\sim}10--100\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and bulk modulus of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The results show a transition from high to moderate flexibility during the deformation stages. This behavior was associated with the existence of two deformation mechanisms presented by the network related to the nanotube stretching and junction bending processes.