The structure and properties of silica glass nanostructures are examined using computational methods in this work. Standard synthesis methods of silica and its associated material properties are first discussed in brief. A review of prior experiments on this amorphous material is also presented. Background and methodology for the simulation of mechanical tests on amorphous bulk silica and nanostructures are later presented. A new computational system for the accurate and fast simulation of silica glass is also presented, using an appropriate interatomic potential for this material within the open-source molecular dynamics computer program LAMMPS. This alternative computational method uses modern graphics processors, Nvidia CUDA technology and specialized scientific codes to overcome processing speed barriers common to traditional computing methods. In conjunction with a virtual reality system used to model select materials, this enhancement allows the addition of accelerated molecular dynamics simulation capability. The motivation is to provide a novel research environment which simultaneously allows visualization, simulation, modeling and analysis. The research goal of this project is to investigate the structure and size dependent mechanical properties of silica glass nanohelical structures under tensile MD conditions using the innovative computational system. Specifically, silica nanoribbons and nanosprings are evaluated which revealed unique size dependent elastic moduli when compared to the bulk material. For the nanoribbons, the tensile behavior differed widely between the models simulated, with distinct characteristic extended elastic regions. In the case of the nanosprings simulated, more clear trends are observed. In particular, larger nanospring wire cross-sectional radii (r) lead to larger Young's moduli, while larger helical diameters (2R) resulted in smaller Young's moduli. Structural transformations and theoretical models are also analyzed to identify possible factors which might affect the mechanical response of silica nanostructures under tension. The work presented outlines an innovative simulation methodology, and discusses how results can be validated against prior experimental and simulation findings. The ultimate goal is to develop new computational methods for the study of nanostructures which will make the field of materials science more accessible, cost effective and efficient.

Diatoms are microscopic algae featuring porous cell wall structures (frustules). Due to the wide variety of diatom shapes and intricate architectures, diatom frustules are viable prototypes to guide the design and fabrication of nanodevices for applications ranging from sensors to nanotemplates. The porous structure and mechanical behavior of biosilica diatom frustules were evaluated through ambient nanoindentation, AFM-based nanoindentation and finite element method (FEM) analysis. Two diatom species, Coscinodiscus sp. (centric) and Synedra sp. (pennate), were investigated through ambient nanoindentation and FEM simulation. High-resolution microscopy unveiled the diatom species structures. These diatom frustule dimensions varied largely depending on diatom species (diameters from 70-250 μm and lengths from 100-300 μm) with pore diameters ranging from 0.1-3.0 μm. Young’s modulus E and hardness H of the diatom frustules were obtained via ambient nanoindentation. These values varied also depending on diatom species: E from 1.1-10.6 GPa, H from 0.10-1.03 GPa for the Coscinodiscus sp.; and E from 13.7-18.6 GPa, H from 0.85-1.41 GPa for the Synedra sp. Predictive FEM simulations were performed on well-validated 3D frustule models, to correlate the mechanical response with specific morphology variables such as pore sizes. A correlation between mechanical properties and porosity was established for selected frustules. Furthermore, AFM investigation and FEM simulation involving 3D frustule models with hierarchical geometries were achieved to investigate the structure-property relationship upon compressive loads. AFM imaging and AFM-based nanoindentation of the centric and pennate frustules provided surface morphology and associated load-displacement curves, which were used to further characterize the diatoms. The calculated average E was 21.5 GPa and 26 GPa for the Coscinodiscus sp. and Synedra sp. respectively, which are higher than the values obtained from ambient nanoindentation. FEM compression simulations were also performed on centric models only to study the role of the hierarchical structures on their mechanical response. Displacement and stress distributions of a three-layered hierarchical frustule model under varied loads were systematically investigated. Results were compared with a stacking of three porous layers for further discussion. Suggestions for future experiments and modeling efforts are provided to enhance the ability to map strategies for the fabrication of 3D nanostructures and nanodevices.