Characterization of mechanical properties of carbon nanotube turfs

by Zbib, Ali Ahmad.

Chair: David F. Bahr by Ali Ahmad Zbib, M.S. Washington State University December 2007 CNT turfs, whether vertically arrayed or randomly grown, have different properties than those of a single tube. Since their discovery in 1991, several researchers have attempted to measure the mechanical properties and behavior of a single carbon nanotube, it being a multi-walled carbon nanotube or a single-walled carbon nanotube. However, with the exception of a few special geometries, little has been done to characterize the mechanical properties of an assemblage of CNTs, and their mechanical behavior under applied loading. Their characteristic properties arise from their complex nano-geometry, and the van der Waals driven interactions between individual CNT segments. This work is therefore dedicated to measuring the mechanical properties, and understanding the mechanical behavior of these intriguing nano-structures under applied loading and also to predicting the buckling mode and critical stress of the turf under compressive loading. The general mechanical behavior of vertically aligned carbon nanotubes grown by chemical vapor deposition (CVD) was studied using nanoindentation techniques. Loaddepth curves extracted from nanoindentation were used to calculate the elastic properties iv of the turfs. Turfs exhibited non-linear elastic loading, with energy dissipation evident even with no permanent deformation. An average elastic modulus of CNT turfs was measured to be 14.9 MPa ± 5.8 MPa, and the hardness was on the order of 5.5 MPa. Permanent deformation occurred at an applied compressive stress of 2.53 MPa for these turfs, and was explained in terms of mechanical tube locking within the turfs, and was dependent on the CNT morphologies and density. Adhesive forces present between CNTs and the diamond tip affected the turf behavior under nanoindentation and increased the initial slope of the unloading segments, thus resulting in slightly higher elastic property measurements. Carbon nanotubes turfs proved to exhibit coordinated and oriented buckling under applied compressive loading. A micromechanical model was developed describing the coordinated buckling phenomena of CNT turfs. Buckling stresses were found to be highly dependent upon the turf’s shear modulus and height. To verify the results predicted by the model, several turfs with heights varying between 25 and 204 ?m were buckled under applied compressive loading, and critical compressive values ranging between 4.3 and 0.2 MPa were measured respectively. The intriguing results developed within this thesis are expected to affect the design of several applications that would use CNT turfs, such as thermal switches and composite materials. v