Abstract
Low-dimensional materials, such as one-dimensional (1-D) carbon nanotubes (CNTs) or two-dimensional (2-D) graphene, have attracted tremendous attention over the past few years for their exceptional electronic, mechanical and thermal properties underpinned by their extremely large specific surface area. However, a single piece of them is too delicate to be useful in most applications, for example, high-performance electrodes in energy storage, filters for wastewater/gas treatments in environmental systems, and lightweight structures. Crumpling and assembling them into three-dimensional (3-D) porous architectures, if achieved, will inherit the structural merits and unique properties of 1 and 2-D nanomaterials due to excellent aggregation-resistant properties and can also serve as building blocks to construct even large-scale bulk forms with unprecedented functions.
In this PhD dissertation, the fundamental deformation mechanism of 1 or 2-D nanomaterials that will be expected to experience large deformation and severe instability in the crumpling and assembling process will be ascertained to establish systematical mechanics models. Moreover, the proposed theoretical framework in mechanics will be used to guide a low-cost and high-throughput liquid evaporation-assisted manufacturing approach that can be readily realized by utilizing aerosol processing, dynamic spraying or 3-D printing techniques to build such a 1 and/or 2-D nanomaterial-based 3-D porous superstructure. More importantly, to highlight the leading role of mechanics in the explored manufacturing approach, we define underpinned mechanics mechanism as extreme mechanics.
Specifically, in Chapter 2 an energy-based theoretical framework is established to describe the liquid evaporation-driven deformation and self-folding of a single 1-D nanofiber and 2-D nanofilm suspended in a free-standing liquid droplet. The effect of cross-section shape of 1-D nanofiber and geometry shape and surface wettability of 2-D nanofilm on the folding process and final folded patterns are investigated. Comprehensive molecular dynamics simulation are performed and validate the theoretical predictions on both the energy variations and final folded morphology.
When there are multiple 2-D sheets suspended in a free-standing liquid droplet, the energy competition between self-folding of individual sheets and assembly of different sheets is elucidated by a rotational spring-mechanical slider network mechanics model in Chapter 3. A computational simulation method to unveil the synthesis mechanism and deformation of 2-D sheets during liquid evaporation is developed. The effect of overall pressure, the total area of graphene, graphene shape, number, and size distribution on the overall size, surface morphologies, and accessible areas of particles is discussed.
By mixing 1-D materials with 2-D materials in a free-standing liquid droplet, the assembling of CNTs-encapsulated, crumpled graphene assembled hybrid particles are obtained by liquid evaporation in Chapter 4. A theoretical framework is established to predict the overall size, surface morphologies, and inner structures of the particles. Molecular dynamics simulation is implemented to validate the theoretical predictions and illustrate details of the synthesis process. The effect of CNTs sizes and mass fraction on the morphologies and properties of assembled particles are investigated.
In Chapter 5, the crumpling and assembling of multiple 2-D sheets in the liquid droplet on a substrate by liquid evaporation is studied. The importance of curvature evolution of liquid surface and substrate properties on the assembling process of 2-D sheets is emphasized. A mechanics model incorporating the profile evolution of liquid droplet with the influence of substrate, comprehensive molecular dynamics simulation, and liquid evaporation experiments are conducted to explain the synergistic effect of liquid and substrate on the morphologies and properties of assembled particles.
The mechanics theories, manufacturing techniques, modeling skills, and characterization methods established in this thesis will provide a fundamental understanding of mechanics in large deformation, instability, and self-assembly of low-dimensional materials in liquid environments, and will also transform applications of the evaporation-assisted technique in the manufacturing of 3-D superstructures of a broad scope of nanomaterials such as lipid membranes, nanowires, nanotubes, nanofibers, and nanoparticles, to meet emerging needs in high-performance composites, filters, batteries, and sensors.
