Carbon Nanotube Microarchitectures for Mechanical Metamaterials

Abstract

Metamaterials use ordered internal structure to exhibit properties uncommon or nonexistent in natural materials. To design a metamaterial with target performance, hierarchical specification of geometry and properties of the constituent elements is essential. Vertically aligned growth of carbon nanotubes (CNTs) is an attractive means to achieve such control because it is a scalable fabrication technique that can produce bulk thick films and patterned microstructures over a large area. CNTs also possess attractive properties such as high stiffness, strength, and electrical and thermal conductivities at low mass density. Therefore, the motivation of this dissertation is to develop methods to manipulate CNT growth and modification at the nano- and microscales, toward the realization of scalable CNT mechanical metamaterials. First, it is shown that CNT microstructures having complex three-dimensional shapes can be manufactured by controlling the CNT growth rate locally within each microstructure using a growth retardant layer patterned underneath the CNT growth catalyst film. Microstructures with complex trajectories are achieved by understanding the mechanical coupling among CNTs and designing the catalyst and offset patterns accordingly. The geometry of the strain-engineered microstructures is predicted using both an analytical model and the finite element method. Next, it is shown that the mechanics of CNT microstructures can be tuned by conformal coating at the nanoscale, via atomic layer deposition (ALD) of alumina. Using vertical cylindrical CNT micropillars, a stiffness tuning from 7 MPa to 50 GPa is demonstrated. The coating thickness also changes the dominant deformation behavior of the CNT microstructures, from buckling to brittle fracture. In the buckling regime, the coated CNT forests can withstand and fully recover compressive strain of up to 75%. Last, fabrication methods are developed toward application of the 3-D CNT microarchitectures. ALD, polymer infiltration, and lamination are used to fabricate a CNT microtruss nanocomposite having high stiffness and damping. Then, microstructure arrays with geometry mimicking the scales of a butterfly wing are fabricated and determined to exhibit superhydrophobic and directional wetting behaviors. Further work on 3-D CNT microarchitectures with engineered geometry, mechanics, and surface functionality may realize multifunctional materials with targeted combinations of mechanical, electrical, thermal, and/or optical properties.