Parameter study of microwave assisted exfoliation of graphite and its application to large deformation strain sensors

Exfoliated graphite (EG), when blended into elastomer, is a promising material for the fabrication of large deformation strain sensors that can detect large deformation up to or more than 50%. Much, however, still remains uncertain and controversial about the exfoliating nature of EG and the conductivity of polymer mixed with fragmented EG (EG/polymer), which leads to the need for parameter study on EG and EG/polymer. Here, we characterize the expansion of graphite in the microwave-assisted exfoliation where KMnO₄ and HNO₃ are respectively used as oxidant and intercalation agent, therefore investigating the dependence of the expanding characteristics of graphite on: (i) mixing ratio (by weight) of graphite, KMnO₄, and HNO₃; (ii) mixing time; (iii) graphite form (natural vs. synthetic), type (lump vs. flake), and size (10 to 40 μm in diameter). The relation between the conductivity of polydimethylsiloxane (PDMS) mixed with fragmented EG (EG/PDMS) and the wt% of the composite is also measured. Based on the experimental results, 1-axis and 3-axis large deformation strain sensors are fabricated by airbrush stencil technique for the composites where pure PDMS and pure PDMS are respectively used as insulating polymer and conducting polymer. The sensors can be used for real-time structure health monitoring of engineering structures having free curved surfaces.