Rapid Manufacturing
Carbon nanomaterials have been shown to rapidly evolve heat in response to electromagnetic fields. Initial studies focused on the use of microwaves, but more recently, it was discovered that carbon nanomaterial systems heat in response to electric fields in the radio frequency range (RF, 1–200 MHz). This is an exciting development because this range of radio frequencies is safe and versatile compared to microwaves. Additional RF susceptor materials include other carbonaceous materials such as carbon black, graphite, graphene oxide, laser-induced graphene, and carbon fibers. Such conductive fillers can be dispersed in matrices such as polymer or ceramics; these composites heat rapidly when stimulated by electromagnetic waves. These findings are valuable for materials processing, where volumetric and/or targeted heating are needed, such as curing composites, bonding multi-material surfaces, additive manufacturing, chemical reactions, actuation, and medical ablation. By changing the loading of these conductive RF susceptors in the embedding medium, material properties can be customized to achieve different heating rates, with possible other benefits in thermo-mechanical properties. Compared to traditional heating and processing methods, RF heating provides faster heating rates with lower infrastructure requirements and better energy efficiency; non-contact RF applicators or capacitors can be used for out-of-oven processing, allowing for distributed manufacturing.
Radio Frequency Heating and Fabrication of Fiber Composites
Commercial carbon fiber composites are fabricated using pre-impregnated fibers with epoxy or “pre-pregs”, that are B-Staged (partially cured) using large ovens to ensure partial cross-linking for easy handling. The impetus for this work came from the exciting discovery that carbon fiber itself may show rapid heating in response to applied RF fields. Heating rates up to 6°C/s can be achieved at input RF powers as low as 5 W, and as high as 70°C/s at 25 W.
We developed a closed-loop welding machine that employs the recent discovery of radio-frequency (RF) heating of carbonaceous materials. The machine is successfully able to weld polylactic acid (PLA) coupons with graphitic RF susceptors at the bondline in less than 2 min and using less than 50 W of input RF power. We found that a higher areal density of the graphitic paint lowers the mechanical properties of the weld because the carbonaceous materials hinder polymer chain diffusion. A significant change was not observed in weld properties for welding pressure ranges between 0 and 0.3 MPa. However, increasing out-of-plane welding displacement increased the modulus and strength of the weld. This work provides an interesting new automated system for welding polymer composites using RF fields, with potential applications in various manufacturing industries.
Composites repair using Plasma
This research presents a unique approach for repairing carbon fiber reinforced composites (CFRCs) using Dielectric Barrier Discharge (DBD) generated plasma for heating and curing. An epoxy-impregnated carbon fiber patch was applied to the damaged area, and upon exposure to DBD-generated plasma, the conductive carbon fibers heated up, thereby curing the surrounding epoxy. This cured patch notably improved the mechanical properties of the damaged part, extending its lifespan. Using X-ray Photoelectron Spectroscopy (XPS) and Differential Scanning Calorimetry (DSC), we studied the surface functionality of the fibers and how the curing process could be controlled. Tensile testing showed a significant increase (approximately 78%) in the ultimate tensile strength of the repaired samples. This study highlights DBD as a resource-efficient, out-of-oven method for CFRC damage repair.