Thermal Modelling and Experimental Evaluation of Lignin-Based Films as Potential Precursors for Laser-Induced Graphene

Sammanfattning

The increasing demand for sustainable and electrically conductive materials for electronics and energy storage applications has created a need for alternative fabrication routes based on renewable carbon sources. Laser-induced graphene (LIG) is a promising technique in which localized laser irradiation converts carbon-rich precursors into porous and electrically conductive carbon structures. In this study, lignin-based films were investigated as precursors for laser-induced conductive carbon formation, with a focus on the influence of coating thickness, substrate material, and heat transfer during CO₂ laser processing. Lignosulfonate-based coatings were deposited on silicon carbide (SiC) and silicon (Si) substrates with three different coating thicknesses for each substrate. The samples were laser engraved using four different scan speeds and eight laser power settings. The electrical response of the laser-treated rectangles was evaluated using two-point probe screening and four-point probe sheet resistance measurements, while scanning electron microscopy (SEM) was used to analyze the surface morphology. A simplified COMSOL Multiphysics heattransfer model was also developed to support the interpretation of localized heat accumulation during laser irradiation. The results showed that coating thickness had a clear influence on electrical performance. Thicker coatings generally produced a broader processing window, more measurable conductive rectangles, and lower sheet resistance values, while thinner coatings resulted in a higher number of non-measurable rectangles marked as Inf. This indicates that a larger amount of precursor material and improved heat retention within the coating may promote the formation of a more continuous conductive carbon network. The substrate material may have influenced the process through heat dissipation. However, within the investigated samples, coating thickness showed a clearer association with electrical response than substrate type. SEM observations supported the electrical measurements by showing that conductive rectangles generally exhibited a more continuous and interconnected porous morphology. In contrast, rectangles for which no measurable sheet resistance was obtained often showed flaking, delamination, or disrupted surface networks. This suggests that carbonized or graphene-like structures may still have formed locally in some non-measurable rectangles, but that the conductive pathways were not sufficiently connected for measurable sheet resistance. Overall, this study demonstrates that lignin-based coatings can be converted into electrically conductive carbon structures using CO₂ laser irradiation. Within the samples investigated, coating thickness showed the clearest association with electrical response, while the influence of substrate type was less conclusive. Further work should include Raman spectroscopy to confirm the formation of graphene-like structures, additional coating thicknesses, controlled environmental conditions, and validation of the COMSOL thermal model with experimental data.