Printability and performance of 3D conductive graphite structures

Direct ink writing (DIW) of graphite-epoxy composites has gained significant importance in a number of applications in fabricating highly conductive free-standing 3D structures. Processing of the composite inks, which consist of highly loaded graphene nanoplatelets, first involves a detailed understanding of the underlying rheological properties. However, little is known about the effect of processing/print parameters, e.g., print speed has on the orientation of such 2D particles during the printing process and how this subsequently influences the macroscopic properties of the final cured composite. In this work, inks with solid loadings of 7 18 wt% were dispersed into a low viscosity epoxy resin (EPON 862) to form a shear thinning, viscoelastic material. The optimal GNP loading for printing is determined through rheological measurements, and the electrical properties are measured as a function of particle concentration and print speed. The results show a sharp increase in conductivity by a factor of ten as the print speed is increased from 5 to 40 mm/s, and all printed samples had conductivities higher than 10(-3) S/cm. We attribute this change in conductivity to the shear stresses generated during the deposition of the ink, resulting in a shift in the orientation of the 2D platelet-like fillers. Such results showcase the ability to tune the electrical properties of a printed structure with a constant loading of filler. The present work helps develop design rules for processing of graphene-based 3D structures with enhanced properties (electrical) using additive manufacturing. We envision the use of such structures in a number of applications such as thermal interface materials, shielding materials for electronic devices, and light-emitting devices, to name a few.