Investigation into the switching characteristics of gold coated carbon nanotubes under low current conditions

摘要

Gold coated multi-walled carbon nanotubes (Au/MWCNT) are a novel composite material, which have been investigated for use as an electrical contact surface. The Au/MWCNT composite is a MWCNT forest synthesized on a silicon wafer and sputter coated with a gold film. The multi-walled carbon nanotubes (MWCNTs) have advantageous mechanical properties so that they can act as a compliant layer in the switching contacts. Moreover, gold is renowned as a useful contact material due to its high electrical conductivity. This new composite material is used to improve the switching performance of micro-electromechanical systems (MEMS) switches. However, as an emerging material for electrical contacts, the characteristics of Au/MWCNT composites had not been widely studied. To address these challenges this research focuses on the investigation of the switching behaviour, contact resistance performance and material transfer characteristics. Further to this, a model, based on fine transfer mechanisms, has been developed to describe the failure mechanism.The planar samples are mounted on the tip of piezoelectric actuators 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; with a fixed load voltage and contact force of 4 V and 1 mN respectively, the hot-switching performance of the Au/MWCNT composite has been evaluated over a range of load currents from 10 mA up to 200 mA. Further to this, the effect of current on the failure mechanism has been evaluated by performing dry switching experiments.The lifetime of Au/MWCNT composites have been shown to sustain over 800 million switching cycles at a load current of 20 mA, with a load voltage of 4 V and a contact force of 1 mN. Every cycle the switches experience two events: an opening event and a closing event. During the opening event, thermodynamic processes associated with the contact interface, known as a molten metal bridge transfer, occur. During the closing event, a bounce characteristic occurs. In the contact resistance performance investigation, two parameters have been considered, the contact resistance and the number of bounces. Over the lifetime of the contact, they show similar trends, which can be divided into four stages, initial, 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. Moreover, the wear process from the material transfer experiment shows the combination between a fine transfer process and a delamination process. This result is very significant and useful to understand the switching wear characteristics over the switch lifetime. Finally, this research has used the experimental data to create an empirical model of the material transfer mechanism to predict the switch lifetime.