Simulations of Carbon Nano-Structures for High-Strength Composites, Thermal Transport, and Hydrogen Storage
Yakobson, Boris I.
Doctor of Philosophy
Computational simulations and theoretical analysis are keys to understand and predict chemical or physical behaviors of carbon nano-materials. Carbon nano-materials such as carbon nanotubes (CNT) and graphene are claimed to have high strength, high thermal conductivity and light weight, which leads to extensive search about their novel properties and potential applications during the past two decades and still ongoing. Compared to the multi-scale simulations with different accuracies and computational cost from quantum mechanics to large scale industry processes, this work focuses on molecular-scale which employs classical force field on each atom and has system size in the nanometer scale. Three following examples demonstrate how the molecular dynamics simulations and modeling can be used to investigate and predict carbon nanomaterials’ thermal properties, gas adsorption in carbon nano-foams, and to design high-strength SiCNT-glass composites. In the first example, carbon nanotubes are used as reinforcement to enhance the mechanical strength of glass composites. Recently there has been a surging demand for glass materials on electric device such as a capacitive touch screen on Smartphones and tablets. The obstacle to designing a CNT-glass composite is how to make CNTs connect with a glass matrix due to the smooth and rigid nature of pristine CNTs. With computer-aided molecular dynamics simulation, the structures of CNTs, glass and their interface can be obtained with sufficient details in atomic level, which helps the understanding of the weakness of CNT-glass composites. An effective way to enhance the strength by doping 1-10% of silicon atoms on CNTs is proposed. The new structures show promising improvements of glass’s mechanical property and fracture resistance. The simulation results provide a guideline for experimentalists to synthesize CNT composites and high strength glass materials. The second example focuses on theoretical analysis of thermal properties and heat transfer in carbon nano-structures at such a nano-size where traditional mechanism for heat transfer-Fourier’s Law fails. A novel thermal rectification phenomenon that the heat flux is dependent on the temperature gradient and has preference in a certain direction was observed in some specific designed systems. The third example is about gas adsorption in carbon porous material, where the novel nano-porous structures were designed with light weight and high surface area. Grand Canonical Monte Carlo (GCMC) method was used to calculate the gas uptakes. Simulation results show that the carbon nano-foam structures have higher hydrogen storage capacities than traditional materials under normal temperature and pressure condition, and thus they can serve as a promising storage media of hydrogen and can be put in the car as a fuel cell to solve the energy challenge. The same materials and method can be also applied for CO2 capture.