Yunus, Pg Hj Md Esa Al-Islam bin Pg Hj Md
Carbon nanotube surfaces for low force contact application.
University of Southampton, School of Engineering Sciences,
This thesis focuses on developing a testing method to estimate the mechanical and electrical characteristics of CNT-based contact surfaces. In particular the use of gold thin films deposited on a CNT forest has potential to offer a very effective contact surface. Two different pieces of experimental apparatus were used in this research to determine the mechanical and electrical properties of gold/multi-walled carbon nanotube (Au/MWCNT) composites: 1) a modified nano-indentation; and 2) a PZT actuator test rig. These apparatuses were used to mimic force-displacement and contact behaviour of the MEMS relay?s contact at a maximum load of 1 mN with dry-circuit and hot-switched conditions. The surfaces were compared with reference Au-Au and Au-MWCNT contact pairs studied under the same experimental conditions. In the modified nano-indentation experiment, tests of up to 10 cycles were performed. The results showed that the Au-MWCNT pair electrical contact resistance improved when the Au-Au/MWCNT pair was used. Additionally the Au-Au/MWCNT pair electrical contact resistance (Rc) was comparable with the Au-Au contact pair. When the Au-Au/MWCNT composite surface is in contact with the Au hemispherical probe it provides a compliant surface. It conforms to the shape of the Au hemispherical probe. For a higher number of cycles, a PZT actuator was used to support Au/MWCNT planar coated surfaces. This surface makes electrical contact with a gold coated hemispherical probe to mimic the actuation of a MEMS relay?s contact at higher actuation frequencies. This apparatus allows the performance of the contact materials to be investigated over large numbers of switching cycles. Different current loads were used in this experiment, 1mA, 10mA, 20mA and 50mA at 4V supply. The Rc of these surfaces was investigated as a function of the applied force under repeated cycles. Under current loads of 1mA and 10mA the Au/MWCNT composite surface provides a stable contact resistance of up to more than a million cycles and no degradation was observed on the surface. Compared with Au-Au contact pair, degradation occurred after 220 cycles. The Au-Au contact pair shows delamination of the Au surface on the probe. The possible reason is the softening or melting of the Au surface. Furthermore, under higher current loads of 20mA to 50mA, degradation had occurred after 50 million cycles (at 20mA) and degradation had occurred at around 45 to 150 cycles for 50mA to 30mA respectively. This is because of the softening or melting of the Au and Au fatigue after a large number of cycles. This study is the first step to show the potential of CNT surfaces as an interface in low force electrical contact applications. With this research, current trends in materials used on contacts and fabrication methods can be explored and even modified or adopted. The use of CNT?s and their composites for contacts can be tested using the available apparatus to look at their performance and reliability in terms of mechanical and electrical properties. This is useful for MEMS contacts that form part of MEMS relay devices.
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