Filler agglomeration, segregated CNT structure, field-driven graphene rotation, and pressure sensing in graphene- and CNT- nanocomposites
Creators: name: Du, Han
Publisher: name: No Publisher Supplied
Year: 2023

description: In this thesis, we conducted a theoretical investigation into the mechanical and electrical properties of graphene- and CNT-based nanocomposites. Due to their high stiffness and flexibility as well as their superior thermal, electrical, and optical properties, graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) are increasingly used as nano-additives to improve the performance of nanocomposites. In the electrical setting, one can turn an insulating composite into a conductive one by adding a small volume of GNPs or CNTs into the polymer matrix, a process known as percolation phenomenon. In the mechanical context, both elastic modulus and yield strength of the composite can be markedly improved. With this perspective, we focus on four main topics: evolution of graphene agglomeration, influence of segregated CNT networks, electric field-driven GNP rotation, and pressure sensing of hybrid graphene/CNT/elastomer nanocomposites. The backbone of the computational strategy is a percolation threshold-embedded effective-medium approximation (EMA) under perfect interface. Mori-Tanaka’s (MT) approach with ellipsoidal inclusion and Hashin’s exact solutions with spherical inclusions are also called upon under various conditions. The additional interfacial phenomena, including imperfect mechanical bonding, electron tunneling between conductive fillers, formation of Maxwell-Wagner-Sillars polarization at the filler/polymer interfaces, Dyre’s frequency-dependent electron hopping, and Debye’s frequency dependent dielectric relaxation, are also incorporated into the computation of effective conductivity and permittivity of the nanocomposites. Motivated by the observation that graphene agglomeration tends to increase with increasing graphene loading, we proposed an evolution equation in the first problem to account for such an increase, and examined how such evolution affects the electrical and mechanical properties of the nanocomposites. In an effort to find higher electric conductivity and dielectric permittivity, and lower percolation threshold for the nanocomposites, we constructed a segregated CNT network that exhibits higher CNT contents on the grain-boundary-like regions and very low CNT density in the interior in the second problem to meet the challenge. In a continued pursuit to find lower percolation threshold, we developed an overdamped dipole model to describe the kinetic motion of GNP rotation under various AC fields in the third. To explore broader applications of nanocomposites, we studied the issue of pressure sensing of hybrid graphene/CNT/elastomer nanocomposites under bending in the fourth one. Some remarkable results have been obtained in these four categories of research. Among several findings we have uncovered in the first part on the evolution of graphene agglomeration is that, in graphene/PP nanocomposites, the Young’s modulus is around 1 GPa without graphene agglomeration but it decreases to 0.6 GPa with agglomeration. The percolation threshold with agglomerates is approximately 70 times that of the percolation threshold without agglomerates. These results efficiently demonstrate the hindering effects of agglomerates on the overall effective mechanical and electrical properties. For the second part, we demonstrated that segregated CNT structure inside the CNT/PA nanocomposites can reduce the percolation threshold by 10 times and increase both conductivity and permittivity as compared to the composite with homogeneously dispersed CNTs. In the third part of investigation on the electric field-driven GNP rotation, we reported an ultra-low percolation threshold, which is around 0.03 vol% as compared with 0.75 vol% without application of the electric field. This obtained percolation threshold is - to the best of our knowledge - the lowest published in the literature and confirmed by Spratford and Shan’s experiment. Finally, in our last part on the pressure sensing study, we reported that the resistance of the hybrid graphene/CNT/elastomer nanocomposite can reduce by about 7 orders of magnitude as the bending pressure increases from 0 to 1 KPa, a phenomenon also validated by experiments. The outcomes of this investigation may benefit the future development of novel nanocomposites and fabrication of electronics devices.