Unidirectional sensing via dual-sensitivity bi-axial flexible sensors. Based on electrically aligned carbon nanotube nanocomposites

Abstract

Imagine a regular flexible pressure sensor that can be integrated inside a host material (e.g. an industrial sealing) to measure the applied pressure. With pressure, the sensor changes its vertical dimensions and changing its intrinsic resistance. However, aside from the vertical change, the pressure also causes lateral strain in the sensor, changing its outer dimensions and affecting the measurement value of the sensor. A possibility to minimize this cross-sensitivity is by creating unidirectional sensitive sensors. A sensor that only response to a force from a single direction, and is insensitive to all other. This research presents the fabrication and characterization of flexible polymer-based sensors that can measure externally applied pressures and forces. A polymer substrate is implemented as a substrate to ensure its flexibility. Carbon nanotubes (CNTs) are integrated in the polymer to achieve a piezoresistive polymer. The working principle follows the fact that with an externally applied pressure, the polymer volume decreases, effectively decreasing the distance between individual CNTs. This in turn increases the composites conductivity, which can be measured. This work is divided into two major parts: flexible sensors with integrated carbon nanotubes that are either randomly dispersed within the polymer or aligned by means of an electrical field. The dissertation goes through all fabrication steps of dispersing the carbon nanotubes in the polymer, and the different available methods. Afterwards, the first strain and pressure results are shown with the two chosen polymers: Polydimethylsiloxane (PDMS) and Polyimide (PI). The advantages of both polymers and sensor types are discussed, followed by the introduction of the possibility to decrease the cross-sensitivity of the sensors by aligning the carbon nanotubes in the polymer before it is cured. The principle of the induced dipole moment within the CNT, causing the rotation in the presence of an electric field is highlighted. Numerical and analytical models are included to support the experimental results found in this research. The possibility of creating highly sensitive piezoresistive sensors based on aligned carbon nanotubes is attributed due to the electron tunneling effect. Carbon nanotubes that are within a few nanometer can ‘tunnel’, jumping over a short distance of non-conductive material. The resistance of such connections increases exponentially with distance, giving rise to highly sensitive sensors with Gauge factors up to 10^3 found in this research. The aligned PDMS-based sensors show a high degree of alignment, which can be even improved further by utilizing functionalized carbon nanotubes. Experiential results show a greater strain region for the sensor, and more controllable and connected carbon nanotube alignment. Finally, the randomly CNT-oriented and stable polyimide pressure sensors are integrated into rubber sealings in order to monitor the condition and applied pressure on the sealings.