Structure and Functionality of Novel Nanocomposite Granules for a Pressure-Sensitive Ink with Applications in Touchscreen Technologies

Abstract

Tactile sensors are now ubiquitous within human-computer interactions, where mouse and keyboard functionality can be replaced with a trackpad or touchscreen sensor. In most technologies the sensor can detect the touch location only, with no information given on the force of the touch. In this thesis, functional components of a novel nanocomposite ink are developed, which when printed, form a pressure-sensitive interface which can detect both touch location and touch force. The physical basis of the force-sensitive response is investigated for the touchscreen sensor as a whole, as well as the intrinsic force-sensitivity of the ink components. In an earlier form the nanocomposite ink, that was the starting point of this study, contained agglomerates of conductive nanoparticles which were formed during blending of the ink, and provided the electrical functionality of the sensor. Here, novel nanocomposite granules were pre-fabricated prior to inclusion in the ink. The granules were designed such that they exhibited well-defined size, structure and strength. Control of these parameters was achieved through selection of the granule constituents, as well as the energy and duration of the granulation process. When incorporated into the ink and screen-printed to form a pressure-sensitive layer in a touchscreen test device, the functional performance could be assessed. Sensors containing pre-formed granules showed improved optical transmission, compared to sensors containing the same mass loading of nanoparticles forming spontaneous agglomerates. Agglomerates tend to create a larger number of small scattering centres which scatter light to larger angles. The spatial variation in the force-resistance response, as well as the sensitivity of this response, was also linked to the distribution of the granules within the pressure-sensitive layer. The physical basis of the force-resistance response is two-fold. Firstly, mathematical simulations showed that deflection of the upper electrode increased the number of granules contacted with increasing applied force and therefore decreased the resistance through the sensor. Secondly, a force-sensitive resistance of the granules themselves was also observed at high forces. Analysis of the non-linear current-voltage characteristics suggested the presence of non-linear conduction pathways within the granules. Using a random resistor network model, the non-linear current contribution decreased after approximately 0.7 N force. To understand this effect, a model based on the physical basis of quantum tunnelling mechanisms was also applied, however this provided a poor fit to the data and no further understanding could be gained.