Carbon nanotube bearings in theory and practice

Abstract

Carbon Nanotubes (CNTs) are attractive elements for bearings in Micro-Electro-Mechanical Systems (MEMS), because their structure comprises nested shells with no bonding and sub-nanometer spacing between them, enabling relative motion with low friction and wear. A few demonstrations of CNT bearings have been reported in the literature, and atomistic simulations have been used to probe the properties of these bearings. This thesis extends the state of knowledge about these bearing systems, by building on these prior works in both the experimental and simulation domains. The prototype CNT rotor device presented in this thesis, and accompanying fabrication process, improve on existing CNT bearing demonstrators by establishing a vertical bearing orientation (enabling superior rotor balance and speed, and flexibility of placement for drive mechanisms) and a more manufacturable process (employing CNTs grown in place by chemical vapor deposition, and evaluating trade-offs in growth parameters). The device consists of a silicon rotor, supported on a cantilevered CNT shaft, and actuated by impingement of air jets on blades around its perimeter. For the fabrication development, extensive and consistent studies on the compatibility of CNTs with a suite of standard MEMS process were conducted, yielding valuable information for future CNT-based device designers on the effects of these processes on CNTs. Additionally, manual manipulation and placement of loose CNTs into the required vertical alignment was demonstrated, providing an alternate fabrication route, as well as a useful research technique for development of CNT devices. Simulation of friction in a CNT bearing system has been a popular topic, yet many questions remain open. For example, the quantitative estimates of this friction reported to date range by as much as eight orders of magnitude, and simulation techniques employ a variety of disparate simulation paradigms and parameters. This thesis presents a new suite of consistently implemented but complementary and independent simulations, which span the approaches reported to date, yet agree quantitatively within the error margin. Furthermore, the quantitative relationships between friction and sliding speed, temperature, geometry, and simulation implementation parameters are determined, and a description of the causes of friction based on phonon analyses is offered.