Energetics and mechanical properties of silica nanotubes

The energetically favorable structures and mechanical response to tensile and pure bending forces of single-wall and multi-wall cylindrical silica nanotubes of varying lengths and radii are predicted via classical molecular dynamics and checked, in part, by quantum mechanical studies. Two distinct parameterizations of a popular pair potential are used. One is adapted to bulk properties, the other to small nanoclusters. Predicted stable structures for single-walled tubes as a function of length at specified radii are dependent on potential parameterization. For the bulk-adapted parameterization, singlewalled tubes with large radii (12-membered rings) have an energetic preference to rearrangement into twinned parallel-column structures. Conversely, the nanobased parameterization puts such twinned structures slightly higher in energy than the corresponding single tubes. Both the mechanical properties and failure mechanisms in tension and pure bending of the single-walled tubes depend upon the nanorod dimensions. Predicted structures for the double-walled nanotubes exhibit qualitatively opposite trends for the two parameterizations. Though the potentials give different values for the tensile and bending elastic moduli for the various structures, the qualitative pictures of nanotube failure are quite similar. For comparison and insight, we also studied a small single-walled nanotube using both pure quantum forces and quantum- classical multi-scale simulations. Some distinctly different behaviors emerged.