ATOMISTIC MODELS TO STUDY MECHANICAL AND FRACTURE BEHAVIOUR OF DRY AND WATER SUBMERGED h-BN NANOSHEETS

Abstract

Due to superior water permeability, structural stability, and mechanical properties, h-BN nanosheets are emerging as an efficient membrane for many applications including ion separation and water desalination. Despite the improved performance of h-BN nanosheets for water desalination, their commercialization is still a challenging task as the overall dimensions of these nanosheets are small, limiting the production rate achieved in desalination. To address the low production rate, nanosheets with larger dimensions are required. Generally, large size nanosheets are synthesized by the chemical vapor deposition (CVD) technique, but multiple nucleation sites inadvertently introduce geometrical defects, viz. adatoms, vacancies, Stone–Thrower–Wales (STW), dislocations and grain boundaries (GBs). In order to make these nanosheets permeable to water molecules, slits, holes are usually created, which effects the mechanical as well as fracture behaviour of these nanosheets. Herein this thesis, the effects of functionalization and geometrical defects on the mechanical and fracture properties of h-BN nanosheets were investigated under dry and water submerged states. Atomistic modeling techniques are viable alternatives to the costly and time-consuming experimental techniques. They are accurate enough to predict the mechanical properties and fracture toughness of dry and water submerged h-BN nanosheets. This thesis encompasses different types of interatomic potentials that can be used for capturing the atomistic interaction in h-BN, water, and the interface between the two, and further elaborate on developments and challenges associated with the classical mechanics based approach. Initially, molecular dynamics (MD) based simulations were performed to study the effect of functionalization and GBs on the mechanical and fracture properties of h-BN nanosheets under the dry state; followed by simulations with water submerged state. It was predicted from the initial simulations that edge and crack edge atom passivation with hydroxyl group have positive effect on the mechanical and fracture behaviour of h-BN nanosheet, whereas the covering of nanosheet with functional group proved to have detrimental effects on the properties of h-BN nanosheet. Also, MD based simulations were performed to investigate the effect of GB on the trans-granular and inter-granular fracture toughness of h-BN nanosheets under dry state. The fracture toughness of bicrystalline h-BN nanosheet is significantly dependent on the misorientation angle, GB configuration in terms of homo elemental bonds (B-B or N-N) and orientation of loading. Atomistic simulations help in predicting a positive effect of GB plane in near vicinity to the crack tip on the trans-granular fracture toughness. Distance of GB plane from the crack tip, and limited hydrogen functionalisation of GB atoms, further helps in improving the trans-granular fracture toughness.