THE MECHANICS OF
INORGANIC NANOTUBES

Nanotubes are arrangments of atoms on a geometric lattice that possess extremely high strength and stiffness properties. Carbon nanotubes have seen an explosion of study in the past several years for use as reinforcing elements in composite structures and numerous other applications. Our work focuses on the much less-studied inorganic class of nanotube. Though neither as stiff or as strong as carbon, these tubes possess numerous unusual properties in their mechanical and electrical structures that could be exploited to great effect when these molecules act either as an individual element (such as in sensor applications) or as one of many elements (such as the reinforcement or actuating element in a composite material).

Our goal in this study is to characterize the effective continuum properties of the materials and forge methods of analysis that are consistent from the atomic scale through the level of a continuum. Our work uses molecular dynamics models, lattice represenations using discrete-elements, and finite element continuum approximations combined with micromechanics theories to aid in the prediction of these elements as structural components.

Collaborators on this study include Professor Anthony Rappe and Mr. Ian Rousom (Department of Chemistry, CSU), Dr. An Tran, Mr. Fernando Ramirez, Mr. Jason Crownholm, and Professor Paul Heyliger (Civil Engineering Department, CSU).

An end view of an aluminum-nitrogen-methyl nanotube, with capping atoms at the end of the tube visibile in the polygon interior. Unlike carbon and boron-nitride nanotubes, this is an example of how nanotubes with an irregular lattice can be formed, leading to unusual mechanical properties as well as potential piezoelectric effects. Also at issue is the role of the exterior atoms outside the usual structural lattice and the behavioral differences in mechanical response that can arise in the nanotube between flexure and pure extensional strain.
Three views of inorganic nanotubes, including end views of aluminum-oxygen-methyl (left) and armchair molybdenum sulfide (right), with a boron-nitride tube in the center. The BN tube has nearly the same structure as the much-studied carbon nanotube (with different properties because of the differing force constants). Our calculations show an effective continuum shell thickness of 0.269 nanometers for the carbon tube based on matching of the energies of MD and continuum simulations. This value is compared with a thickness of 0.302 nanometers for a molecular graphene sheet, yielding a decrease in effective thickness when going from sheet to tube, a decrease that matches ab initio calculations and indicates the potential changes in properties moving from sheet to tube.