Gradient Structure Design of Flexible Waterborne Polyurethane Conductive Films for Ultraefficient Electromagnetic Shielding with Low Reflection Characteristic.

Highly efficient electromagnetic shielding materials entailing strong electromagnetic wave absorption and low reflection have become an increasing requirement for next-generation communication technologies and high-power electronic instruments. In this study, a new strategy is employed to provide flexible waterborne polyurethane composite films with an ultra-efficient electromagnetic shielding effectiveness (EMI SE) and low reflection by constructing gradient shielding layers with a magnetic ferro/ferric oxide deposited on reduced graphene oxide (rGO@Fe3O4) and silver-coated tetraneedle-like ZnO whisker (T-ZnO/Ag) functional nanoparticles. Because of the differences in density between rGO@Fe3O4 and T-ZnO/Ag, a gradient structure is automatically formed during the film formation process. The gradient distribution of rGO@Fe3O4 over the whole thickness range forms an efficient electromagnetic wave absorption network that endows the film with a strong absorption ability on the top side, while a thin layer of high-density T-ZnO/Ag at the bottom constructs a highly conductive network that provides an excellent electromagnetic reflection ability for the film. This specific structure results in an "absorb-reflect-reabsorb" process when electromagnetic waves penetrate into the composite film, leading to an excellent EMI shielding performance with an extremely low reflection characteristic at a very low nanofiller content (0.8 vol % Fe3O4@rGO and 5.7 vol % T-ZnO/Ag): the EMI SE reaches 87.2 dB against the X band with a thickness of only 0.5 mm, while the shielding effectiveness of reflection (SER) is only 2.4 dB and the power coefficient of reflectivity ( R) is as low as 0.39. This result means that only 39% of the microwaves are reflected in the propagation process when 99.9999998% are attenuated, which is the lowest value among the reported references. This composite film with remarkable performance is suitable for application in portable and wearable smart electronics, and this method offers an effective strategy for absorption-dominated EMI shielding.