Abstract
Boron carbide, one of the third hardest materials known in nature, is distinguished from other materials by its appealing physical and mechanical properties. Particularly, boron carbide at reduced dimensions, especially one-dimensional, often exhibits novel properties such as large specific surface area and high mechanical strength close to its theoretical value. These properties make the boron carbide nanowire a promising material for numerous applications, such as high-strength lightweight material applications in which they are exposed to extreme chemical and thermal conditions, including applications in thermo-electric energy conversion systems, neutron absorbers, and ceramic armors. To date, however, the growth mechanisms of boron carbide nanowires remain, to a large extent, unknown due to the complex crystal structure and widely varied composition of boron carbide. The lack of knowledge of boron carbide nanowire growth mechanisms largely hinders the development of nanowire synthesis techniques and increases the production cost. Furthermore, the outstanding mechanical properties make the multiscale structural design and hierarchical morphology control of boron carbide difficult, thereby restricting the exploration of boron carbide nanowire-based novel devices.
This dissertation studies the growth mechanism, properties, and applications of biomass-derived boron carbide nanowires. Specifically, in Chapter 2, it is demonstrated that the transformation between polytypic boron carbide phases promotes boron migration in the nanowire growth. An atomistic mass transport model was developed to explain such volume-diffusion-induced nanowire growth which cannot be explained by the conventional surface diffusion model alone. With the proposed growth mechanism, various biomass materials were explored as raw materials to synthesize boron carbide nanowires. In Chapter 3, we report that shear-mixing enabled graphene wrapped B4C nanowires (graphene@B4C-NWs) empowered exceptional dispersion of nanowires in polymer matrix and superlative nanowire-matrix bonding. The graphene@B4C-NW reinforced epoxy composites exhibited simultaneous enhancements in strength, elastic modulus, and ductility. Tailoring the composite interfaces with graphene enabled effective utilization of the nano-fillers, resulting in 2 times increase in load transfer efficiency. Molecular dynamics simulations unlocked the shear-mixing graphene/nanowire self-assembly mechanism. In Chapter 4, we demonstrate multiscale structural design and hierarchical morphology control of boron carbide. A new type of micro/nano hybrid filler was synthesized by an unconventional cotton aided method, which has boron carbide microplatelet as the core and radially aligned boron carbide nanowires as the shell. Such multiscale reinforcement design remarkably enhanced the load carrying efficiency of boron carbide. In Chapter 5, an unusual cathode configuration for lithium-sulfur (Li-S) batteries was constructed, employing boron carbide nanowires (BC-NWs) as a skeleton, porous activated textile (ACT) as a flexible carbon scaffold, and reduced graphene oxide (rGO) as a self-adaptive protective shell. This BC-NW@ACT/S/rGO cathode achieved superlative sulfur confinement and exceptional electrochemical performance. These findings significantly advance our understanding of biomass-derived boron carbide nanowire growth processes and provide new guidelines for the design of nanowire-structured composites and energy storage devices.
