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The University of Southampton

Research project: Investigation into the Switching Characteristics of Gold Coated Carbon Nanotubes under Low Current Conditions

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Multi-walled carbon nanotubes (MWCNTs) are a novel material to be used as an electrical contact surface due to their unique and excellent electrical and mechanical properties. They are used to improve the switching performance of micro-electromechanical systems (MEMS) switches since they can provide a compliant layer in the switching contacts. The lifetime of a gold coated multi-walled carbon nanotube (Au/MWCNTs) composite has been shown to sustain over 70 million switching cycles at a current level of 20 mA, with a load voltage of 4 V and a contact force of 1 mN. However the characteristics of Au/MWCNT composites have not been widely studied, therefore this research investigates the switching and failure behaviour.

Fig 1
Fig 1

MWCNTs are synthesized on a silicon wafer and sputter coated with a gold film. The planar surfaces are mounted on the tip of a piezoelectric actuator and mated with a gold coated hemispherical surface to form the electrical contact. These switching contacts are tested under conditions typical of MEMS relay applications; 4 V applied load, with a static contact force of 1 mN, and a load current of between 20 to 50 mA.

During the switching behaviour investigation, at the opening event, the thermodynamic processes associated with the contact interface, known as a molten metal transfer are evaluated. At the closing process, a bounce characteristic is evaluated. In the failure behaviour investigation, two parameters have been considered, the contact resistance and the number of bounces. They show the same trend which can divided into four stages, unstable, stable, rising and failure stage. Both the contact resistance and the number of bounces can be used to predict the onset of failure of the switch contact. One of the important parameters used to calculate the constant (k) in fine transfer model is the number of cycles to failure (Nf). Here, the fine transfer model from previous research has been further developed by multiplying the Nf with the number of bounces to obtain the actual cycles to failure. Therefore this research has collected experimental data to show the empirical relationship of the fine transfer mechanism more accurately. However the Au/MWCNT surface to use this model is only valid for the MWCNTs with 30 µm in height, a load voltage of 4 V and contact force 1 mN. It should also be noted that the wear mechanism needs to be dominated by the fine transfer process and not a delamination process.

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