Carbon Nanotube/Polyimide Film Based Strain Microsensor for Carbon Fiber/Epoxy Composites

Abstract

The work reports the development of strain sensors based on a 5 µm thick piezoresistive polyimide film. A direct mixing process of carbon nanotubes with polyimide precursor was proposed for the material fabrication. As a proof of concept, the sensor was embedded within carbon fiber composites for strain measurements.

For the development of the sensors, the following steps were performed: - The adhesion between polyimide layers was investigated using surface treatments based on oxygen plasma and alkaline solutions. - Modeling of the electrical percolation of films was proposed to predict the amount of nanoparticles required to achieve electrical conductivity of the polyimide films. - The fabrication of a nanotube/polymer film was performed using ultrasonication and direct mixing approaches. As an alternative, graphene nanoplatelets were also tested. - The piezoresistive characterization of the nanotube/polyimide films was performed to evaluate the material sensitivity under applied strain. - The nanotube content of films and the minimum size for sensor structures were determined considering photolithography for the microfabrication of the sensors. The surface modification of the polyimide indicated that oxygen plasma is the proper route for ensuring the adhesion between the sensor layers. The direct mixing proposed for this work was the most efficient approach to produce polyimide films with 1.5 - 3 % nanotube contents. This particle content agreed with the predictions of the material percolation. On the other hand, graphene was not a suitable material due to its higher percolation threshold (>10 %). The sensors were fabricated as meander structures, with an aspect ratio of length/width = 10 and a minimum linewidth of 50 µm. The embedded devices showed a gauge factor K = 10.7, which is five times higher than the sensitivity of commercial strain gauges (K = 2). These sensors could be used as integrated sensors in different structural parts for online monitoring of forces or strain. The proposed material has additional potential applications such as pressure and temperature sensing, or polymer cure monitoring.