Carbon Based Nanocomposites
Experimental and theoretical studies on individual Carbon nanotubes (CNTs) point to their exceptional mechanical properties, strength as high as 100 GPa and modulus of ~1 TPa. However, macrostructures fabricated by assembling CNTs (e.g., in bundles or fibers), achieve a very small fraction of such strength and modulus. This is due to insufficient interactions between walls of individual CNTs as well as between neighboring nanotubes (bundles). In our lab, we use a multidisciplinary approach to modify the structure of CNTs and include binding molecules, aimed at developing high strength-high toughness fibers and films. The hierarchical structures of our studies range from multiwalled carbon nanotubes (MWNTs), to mats of chemical vapor deposition (CVD) grown CNT-based bundles fabricated by MER Corp, to macroscopic CNT-based fibers and yarns (see Fig. 1). The understanding of inter-shell and tube cross-linking gained through this work will have a direct impact on the aerospace industry (Boeing, NASA) and their efforts to scale up in size carbon nanotube-based fiber materials without loss in stiffness, strength and performance.
On the nanoscale, we are interested in understanding how different molecular bonds, which crosslink both innershells of MWNTs and shells of CNTs within bundles effect the mechanical behavior of the materials. Currently we are studying both the effects of electron irradiation induced crosslinking as well as polymer intermediaries between tubes. Our group provided the first direct correlation between failure stress and number of failed shells in multiwalled carbon nanotubes. Failure modes and carbon atomic structures (inter-shell cross-linking) were also identified as a function of exposure dose to electron or ion radiation. The reason for this phenomenon is that inter-shell cross-linking occurs during growth or during specimen radiation with electrons or ions possessing energy above a certain threshold (energy for vacancy formation in the carbon shell. While cross-linking was hypothesized prior to this work, there was no direct imaging of atomic structure or correlation to measurements of associated load-displacement curves (see Fig. 2). The fundamental understanding of load transfer and energy dissipation mechanisms in these nanoscale MWNTs (Fig. 2a) as well as CNT bundles (Fig. 2b) will guide the development of scaling the constituents to larger sizes. To develop this understanding we adopt a combined experimental-computational approach. In-situ scanning/tunneling electron microscope (SEM/TEM) mechanical testing methods are compared with DFT, MM/MD, and coarse-grain simulations to study CNTs and bundles of CNTs (see Fig. 3). Experimental methods also include novel methodologies developed in our group, such as MEMS based in-situ SEM/TEM tensile testing stages. To compliment experimental findings, we employ MM/MD models coupled with continuum shear lag models of CNT hierarchical structures. Such modeling helps us guide the design of next generation CNT based fibers and yarns. For modeling we have established collaboration with Prof. George Schatz’s group at Northwestern and Prof. Markus Buehler’s group at MIT.
On the macroscale, we incorporate CNT mats to spin yarns (see Fig. 3). The process of spinning is intended to reduce the average distance between CNT bundles and increase the interaction between them. In addition, this process induces CNTs alignment which can further enhance the load transfer in CNTs. Another advantage of this process, which is currently under investigations in our lab, is the possibility of inducing binding chemical agents that can not only improve the load transfer between neighboring CNT bundles, but also increase the energy to failure of CNT based structures. We investigate the effects of treatments on the mechanical behavior of the samples by performing in situ SEM tension tests with real time monitoring of the modes of the deformation of the sample using a miniaturized loading stage inside SEM.
As another direction of our research is on the application of CNTs to develop high performance materials, we are incorporating CNT mats to increase the shear toughness of laminar composites. Our preliminary in situ end notch flexure (ENF) experiments on CNT reinforced and nonreinforced samples demonstrate the efficiency of CNT mats in this regard.