Quantum Tunnelling Composites (QTC) have attractive mechanical-electrical properties and have found applications in a number of advanced technologies such as the touch-sensitive finger-tips on the gloves of the NASA space suits. There was a demand to apply QTC materials to consumer products such as credit cards, via a conventional printing process. The challenge, however, was that QTC materials only functioned properly at high solid loadings (thus very high viscosity) and as thick films. Hence, the aim of the study described in this thesis was to develop a series of QTC inks that would be printable via conventional printing technologies such as screen printing, meanwhile, these inks would be potentially applied as an electrical switch in the RFID card, sportswear, games and etc. To date, several screen printable functional inks, each containing a combination of conductive spiky-like nickel particles, three grades of semiconducting titanium dioxides particles and a range of water-based binders, have been formulated and used to study the electrical properties of the QTC inks. Relevant experimental designs, interpretation of the property-composition relationships, modelling of the electrical-mechanical properties and a strategy for the optimisation of the QTC inks, are all presented in this thesis.
When QTC materials are quiescent, the metal particles are well separated from each other. This leads to the QTC material behaves as an insulator. An increase in applied compression on the QTC material results in a lower electrical resistance through it. This is because the electrons from the metal particles can “hop” from one to the other with or without touching. This is known as “quantum tunnel effect”, arising because the nickel particles can build a high electric charge field on the spike tips.
Detailed in this thesis, are a series of comprehensive investigations of the pressure-sensitive response of a printed film, and the contribution of each component to the formulation in terms of printability, electrical property, rheology, thermal stability and the mechanical properties. It was found that the printed film behaved as an insulator in the absence of external compression, even when the nickel filler content was well above the expected percolation threshold. The electrical resistance of the printed film decreased, up to 10 orders of magnitude, with an increase in external compression. This dramatic resistance variation was explained by the quantum tunnelling mechanism and percolation mechanism, which were mainly dependent on the nickel loading, the distribution of the nickel particles and nickel aggregates, the morphology of the nickel particles and the elasticity of a polymer matrix. Furthermore, it was found that at least one grade of titanium dioxide could be used to reduce the sensitivity of electrical resistance of the printed film.
Moreover, a model of the response of the electrical properties an external compression of the printed QTC films has been successfully developed. This permits quantification of the relationship between the electrical properties and the structure of the composite. This model was most applicable to the printed composite film for the prediction of the electrical-mechanical properties of the nickel particles randomly dispersed in a polymeric matrix.
