Dielectric heating of carbon precursors

Carbon fibre (CF) composites are becoming more widespread in many applications such as automotive, aerospace, defence, wind turbines and pressure vessels. CF is produced from precursor fibre via a series of energy-intensive heat treatments. During conventional heating processes, the energy is transferred from the source through heat transfer. Microwave (MW) heating has been identified as a more energy-efficient alternative due to direct transfer of energy into the material via a susceptor structure. An alternative carbonisation method has been developed to reduce energy consumption and environmental impact during manufacture of CF utilising a susceptor assisted MW heating technique, which is achieved by a layer by layer approach at the nanoscale. The project goes beyond the state-of-the-art through the carbonisation of PAN and sustainable precursors via MW heating. Initially, the technology is applied for conversion of the most commonly used, petroleum-based precursor – polyacrylonitrile (PAN). The effects of susceptor coating procedure and MW power level are studied. It is found that addition of even a single layer of susceptor allows for a successful MW carbonisation. Addition of extra layers can increase temperatures. Application of susceptor before stabilisation leads to fusion of the layer and more stable heating. Susceptor layers applied after stabilisation allow higher temperatures and CF conversion rates, but tend to be more prone to damage during the process. MW carbonisation at high power enables carbonisation temperatures within seconds, but promotes development of defects via gas emission. MW carbonisation at progressive power levels limits overheating, leads to noticeable improvement of morphology and mechanical properties, especially for lignin-based CF. The latter approach is used to produce lignin-based CF with properties matching those of control samples prepared via conventional techniques. Finally, results of MW heating modelling and life cycle assessment are presented.