Multifunctional composite materials : Design, manufacture and experimental characterisation

Abstract: The use of lightweight materials in structural applications is ever increasing. Today, lightweight engineering materials are needed to realise greener, safer and more competitive products. A route to achieve this could be to combine more than one primary function in a material or component to create multifunctionality, thus reducing the number of components and ultimately the overall weight. This thesis presents approaches towards realising novel multifunctional polymer composites, which simultaneously can carry mechanical loads and store electrical energy. For this purpose, structural capacitor and battery materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. In papers I and II, structural capacitors have been realised using different papers and polymer films as dielectric separator and employing carbon fibre/epoxy pre-pregs as structural electrodes. Plasma treatment was used as a route for improved epoxy/polymer film adhesion. The manufactured materials were evaluated for mechanical performance by interlaminar shear strength (ILSS) and tearing tests and electrical performance by measuring capacitance and dielectric breakdown voltage.In paper III the concept was extended in a parametric study using the most promising approach with a polymer film as dielectric separator. Three thicknesses of PET (50, 75 and 125 μm) were used as dielectric separator with carbon fibre/epoxy pre-pregs as structural electrodes. Plasma treatment was used to improve the PET/epoxy adhesion. The capacitor materials were evaluated for mechanical performance by tensile and ILSS tests and for electrical performance by measuring capacitance and dielectric breakdown voltage. The multifunctional materials show good potential for replacing steel and other materials with lower specific mechanical properties but cannot match the high specific mechanical performance of monofunctional materials.Paper IV explores the effects of matrix cracking on the structural composite capacitormaterials performance. The structural capacitor materials were made from carbonfibre/epoxy pre-pregs as structural electrodes with thermoplastic PET as dielectricseparator as done in paper III. A method to induce and to measure the effect of matrixcracks on electrical properties was developed and used. The method is based on asimple tensile test and proved to be quick and easy to perform with consistent results.The structural capacitor material was found to maintain its capacitance even aftersignificant intralaminar matrix cracking in the CFRP electrodes from high tensilemechanical loads.Paper V explores another possible route for electrical energy storage in structural composites in the form of structural composite batteries. A laminated design approach would result in too long distances for ion mobility to give any useful energy storage with very low power density. Therefore, the work in this paper was focused on making each individual carbon fibre in a tow into a battery. Thus, realising a large number of batteries connected in parallel within a composite material. This is done by electro polymerisation of a solid polymer electrolyte onto the surface of the carbon fibres. The resulting sleeve of polymer is typically 500 nm thick making it thin enough to achieve useful electrical performance even with the relatively low ion conductivities of the employed solid polymer electrolytes. This paper demonstrates a new way forward to realise intrinsic multifunctional composite battery materials.