Additieve Vervaardiging van Polymeer Nanocomposieten voor Elektromagnetische Afscherming

The exponential growth of wireless telecommunications, driven by the rollout of 5G networks and more and more wireless appliances, has resulted in a dense electromagnetic (EM) environment. This proliferation necessitates advanced EM shielding solutions to protect sensitive electronics, human health and ensure signal integrity. Traditional metallic shields, while effective, suffer from high density, susceptibility to corrosion and challenging processing into complex shapes that limit their integration into modern, lightweight and flexible devices. Furthermore, metals primarily shield EM waves via reflection, which contributes to secondary EM pollution, spreading the EM energy back into the environment. This thesis investigates the development of conductive polymer nanocomposites processed via additive manufacturing (AM) techniques as a transformative alternative. By combining the tunable electrical properties of carbon-based nanocomposites with the geometric freedom and inherent layer-by-layer structuring of 3D printing, this research aims to fabricate absorption-dominant EM shields via advanced fused deposition modeling (FDM) and direct ink writing (DIW) techniques. Current AM techniques still face challenges, one of which is precise spatial control over the composition of the printed object. This work uses the inherent layer-by-layer nature of AM while also developing new methods to further enhance this local compositional control, such as the implementation of static mixers inside the extrusion nozzle. A major cornerstone of this work is the design of a framework to optimize the absorption-dominant shielding performance of layered stacks. We demonstrate this for 10 layer stacks of 4 mm total thickness printed via FDM of in-house made and characterized polylactic acid (PLA) composites filled with multi-walled carbon nanotubes (MWCNTs). The framework consists of learning the effective EM properties of each manufactured type of layer in a stack. Knowing these, an optimization algorithm based on this gray-box modeling is used to find the optimal stacking sequence to maximize absorption while still delivering a certain minimal shielding effectiveness. The optimized stacks were manufactured and measured in a waveguide setup, showing excellent agreement with the model predictions. The optimized stacks showed a significant increase in absorption-dominant shielding performance compared to non-optimized reference stacks of uniform composition with a comparable total amount of filler. This work illustrates that more filler is not always better and that the spatial distribution of filler within a shield is crucial to maximize absorption while minimizing reflection. Building further on this framework, the implementation of holey patterns in an EM shield are investigated. The influence of the pattern is simulated for a commercial conductive PLA filament and for a 10 layer multilayer structure which shows a minimum in reflection at 10 GHz. Aiming to combine the effect of multilayers with the effect of holey patterns, the hypothesis is that the combination of both will lead to a further increase in absorption-dominant shielding performance. The simulations show that the combination of both effects indeed leads to a further increase in absorption via a minimum in transmission on top of the minimum in reflection. The holey patterns are then implemented in the FDM printed multilayer stacks and measured via two larger-scale techniques, an absorption box and a shielded room setup. To produce samples of the required size, a new printing procedure, based on puzzle pieces, is developed. Individual puzzle pieces are printed and then assembled to form a larger shield. The measurements confirm the simulation results in the case of the commercial conductive PLA filament, showing a significant increase in absorption-dominant shielding performance when implementing the holey pattern. However, for the in-house made PLA-MWCNT composites, the results are less conclusive, which is attributed to the high sensitivity of the in-house made composites to processing variations. The setups used for EM characterisation of these large samples can also introduce measurement inaccuracies which make the interpretation of the results less straightforward. Overall, this work demonstrates that the combination of multilayer stacking and holey patterns could be a promising strategy to further enhance the absorption-dominant shielding performance of 3D printed EM shields. Shifting the focus to DIW, this thesis presents a high throughput method to determine the electrical percolation threshold (EPT) of conductive inks via continuous gradient printing. Many applications of conductive polymer composites, including EM shielding, rely on achieving electrical percolation. The determination of the EPT requires in general the preparation and measurement of multiple discrete samples with varying filler content, which is material and time intensive. To address this, our method involves printing a single specimen with a continuous gradient in filler concentration along the printed path. This is achieved with an in-house designed mixing nozzle for DIW, allowing for the fabrication of samples that span a wide range of compositions in one print. A gradient path of a starch-based ink is printed from the pure matrix to a highly loaded conductive composite, containing a loading of carbon black (CB) well above percolation. The conductivity along the path is measured and the effective CB loading is determined via thermogravimetric measurements at discrete points. The results are fitted with a Boltzmann sigmoidal function, in which the inflection point corresponds to the EPT. This method significantly reduces material consumption and experimental scatter compared to traditional approaches. Since the processing method can influence the EPT, especially for anisotropic fillers, this method can be employed to more accurately serve applications where conductivity is critical, such as, but certainly not limitied to, EM shielding. Finally, composited based on polydimethylsiloxane (PDMS) filled with MWCNTs are developed as printable flexible EM shields. However, the shielding performance of these materials is insufficient for practical use. Alternatively, these materials are used as strain sensors via DIW. A dual syringe setup for DIW is designed to handle two inks in parallel or alternate between different compositions in the same printed object. This allows to print stretchable strain sensors consisting of different cells of varying MWCNT loading. Three PDMS-MWCNT inks are formulated with varying MWCNT loadings, of which the lowest is just above the EPT. The strain sensing performance of single composition sensors is evaluated, showing that the resistance change upon stretching is the highest for the composition just above the EPT, due to the pronounced disruption of the conductive network upon deformation. Combining cells of different loadings in series in the same sensor allows to tune the effective strain in the different cells, leading to a tunable strain sensing response via strain amplification in cells close to depercolation. This work demonstrates the potential of DIW to fabricate multifunctional devices with tunable sensing capabilities. In conclusion, this thesis presents several strategies to enhance the absorption-dominant EM shielding performance of 3D printed conductive polymer nanocomposites via FDM and DIW. In particular, the developed optimization framework for multilayer stacks and the implementation of holey patterns are shown to significantly enhance the absorption-dominant shielding performance of FDM printed EM shields. Furthermore, the high throughput method for EPT determination and the fabrication of tunable strain sensors contribute to advancing the field of additive manufacturing for conductive polymer composites which can enable the next generation of smart, lightweight and efficient electromagnetic shielding and sensing devices.