ABSTRACT
Vibration-induced failure of devices has plagued the developers of mechanical systems since the development of early machinery. This problem has now manifested itself into design issues in small-scale mechanical devices. Since the first microelectromechanical device was reported in 1967, numerous issues with the longevity, energy lost, wear, instability and performance of these devices have been associated with parasitic vibration noises. This problem is especially acute now in the rapidly developing area of nanoelectromechanical systems because of the extreme surface-to-volume ratios these devices have. As such, numerous studies are currently devoted to improving the vibration damping in nano-devices. The traditional method for addressing these issues is to integrate dampers and low-friction materials together in a device. However, unlike macro environments, implementation of such protection on the sub-
micrometer scale is either unfeasible or extremely costly because of the complex fabrication involved. A more reasonable solution is to use an intrinsic damping material built directly into the device for energy dissipation and load recovery. One promising solution is the use of carbon nanotubes and their composites. This is one of the main driving factors in the study of carbon nanotube films and carbon nanotube-polymer composites for their mechanical strength and viscoelastic properties. This chapter thus focuses on the strength and weaknesses associated with using carbon nanotubes and their composites in damping and viscoelastic applications. 13.1 IntroductionCarbon nanotubes’ (CNTs) exceptional mechanical strength, low density, light weight and high elasticity make them ideal candidates as structural materials or for material reinforcements in many applications [1-6]. In many respects, CNTs are like ideal nanometersized strands of fiber or pillars. They are strong, resilient to repeated deformation and bending. Moreover, many of CNTs’ properties can be tuned by adjusting their geometric dimensions as well as their type (single wall (SWCNT), double wall, multiwall (MWCNT), etc.). It is this possibility of tailoring the properties of CNTs that makes them so exciting for many applications. Although individual CNTs are already remarkable in terms of mechanical strength, it is when they are group together that their true potential is realized [7]. Singly, CNTs have good prospects as nanowires or conducting filaments in nano-electronics but for current mechanical applications requiring high strain and high frequency loading-unloading cycles, a single strand of CNT is simply not suitable. However, by binding them together either as CNT-forest or CNT films, [8, 9] real practical mechanical applications can be realized. In fact, one of the most interesting mechanical properties of CNTs and their composites lies in their viscoelastic nature. Individual CNTs have recently been reported to exhibit negative structural stiffness leading to enhance overall microstructure stiffness and damping [10, 11]. This effect of CNTs has been verified by both molecular dynamics and finite element simulations [5, 12]. More recently, Yap et al. have also experimentally demonstrated this through atomic force microscopy (AFM) cantilever loading on
MWCNTs [10, 11] (more on this work will be discussed later). In addition to this intrinsic effect of individual tubes, energy dissipation can be further enhanced when large amount of CNTs interact with each other or other elements creating frictional loses. This effect happens in CNT films as well as CNT-based composites. This enhancement is due to the CNTs moving or slipping against each other or their surrounding matrix material. This slippage causes energy losses due to frictional interactions, which in turn leads to energy losses, which are translated into enhanced damping behavior. As there are two main types of CNT-based damping materials, this chapter is divided into two separate sections. The first part discusses pure CNT films while the next part focuses on CNT composites; mainly CNT/polymer composites. 13.2 Pure Carbon Nanotube FilmsCurrent conventional damping treatments are based on several different techniques and materials, including elastomers, actuating (magnetic, piezoelectric, etc.) materials and fluids, and electronic feedback circuits, etc. [13-15]. These techniques have generally been reliable for large systems. However, they are usually heavy, non-compact and may not be suitable in high temperature environments. Moreover, for smaller devices, the complexity involve in miniaturizing these techniques make their implementation either not feasible or simply too costly. A more direct approach through the use of viscoelastic materials to damp vibration would be better. One of the more promising materials to be used in this form are CNT films. To study the damping properties of CNT films, Koratkar et al. [16] used a piezo-silica sandwich beam with an embedded MWCNT film in a flat-wise bend testing experiment. The CNT film used in their setup was several tens of micrometers thick with tube diameters at ~30 nm. Chemical vapor deposition (CVD) using xylene-ferrocene precursors was used to synthesis their CNTs. [17, 18] Their sandwich films consisted of a top piezoelectric sheet with a bottom silica substrate encasing the MWCNT film between them. The Pb(Zr,Ti)O3 (PZT) and silica sheets were bonded using a special cyanoacrylate adhesive. The test setup is schematically shown in Figure 13.1b. A reference sample with no CNT reinforcement was also tested to compare it with the CNT sandwiched film.
