Synthesis of multi-functional porous graphene nanostructures and their engineering applications

Abstract

Due to its unique electrical and mechanical properties, graphene is emerging as one of the ideal materials for nanotechnology as well as for mechanical engineering. Depending on the physical energy source, it is possible to synthesize graphene into various structures from nanoscale to macroscopic scale. In this thesis, synthesis and engineering applications of multifunctional porous graphene are proposed to be applied as functional structures to next generation vehicles such as electric vehicles, hydrogen vehicles, and long-life unmanned aerial vehicles. Based on the engineering application of hydrogen storage, sound absorption and vibration damping, graphene-based 2D porous nanostructures and three-dimensional porous architectures were synthesized. Herein, this study presented the synthesis of four types of porous graphene nanostructures based on different physical energy sources: (1) nanohole-structured three-dimensional porous graphene by microwave irradiation method, (2) directionally antagonistic graphene foam by liquid nitrogen freezing method, (3) structural ordered/disordered-lattice graphene-polyurethane foam by liquid nitrogen freezing method and boiling water heating method, and(4) auxetic graphene foam by compression-hot pressing heating method. In detail, in Chapter 2, a defect engineered self-assembly synthesis has been developed to fabricate a three-dimensional nanohole-structured and palladium-embedded 3D porous grpahene hetero-nanostructure having multi-functionalities of super-high hydrogen storage and CO oxidation. In the multistage microwave method, agglomerated palladium nanoparticles having diameters of ~10nm produce physical nanoholes in the base plane of the graphene layer. The results of this study show that the defect-engineered hetero-nanostructure has a hydrogen storage capacity of ~ 5.4wt% at 7.5MPa and a CO oxidation catalyst activity at $190^\circ C$. In Chapter 3, a directionally antagonistic graphene-polyurethane aerogel graphene foam with aligned pores and facesheets was synthesized by liquid nitrogen freezing method and freeze drying method, and applied research into the field of sound attenuation proceeded. A ice template freezing method has been developed to arrange a functional graphene porous graphene sound absorbing material having different physical properties along the direction by arranging a grayscale plate of a microscale. This graphene porous sound absorbing material exhibits adjustable stiffness and improved sound absorption performance in a microscale bulk structure. By controlling the internal cell size and shape structure of the porous structure, the incidence and reflection at the surface and the air viscous resistance damping in the interior, In this paper, the sound absorption performance improvement by the thermal energy conversion is studied. The development of an antagonistic structure that shows the performance difference between one structure with superior sound absorption performance or superior reflection performance along the direction has various engineering applications in the future. In Chapter 4, a unique structure was synthesized by using a polyurethane foam with a large pore as a support and a grid array of ordered or disordered graphenes inside, and applied to engineering applications in the field of sound attenuation. As a reducing agent, l-ascorbic acid and KOH were used. Depending on the order and type of addition, macro-structures arranged in a wavy pattern are formed, and disordered lattice graphene foams irregularly entangled in a spider-like shape can be made. The new polyurethane foam-based ordered / disordered grid graphene foam thus developed exhibits different sound absorption performance depending on the arrangement. Finally, in Chapter 5, we have implemented a polyurethane foam-based graphene-wrapped auxetic structure and propose the possibility of engineering application of vibration damping which can be brought about by negative Poisson's ratio behavior. Auxetic graphene-polyurethane foam has an irregular porous structure and has structural properties that can effectively absorb stiffness and resistance to local action loads due to the increased density of compression sites due to negative Poisson's ratio behavior and impact energy. The polyurethane was fabricated by compression-heating method to form an auxetic structure, coating the surface of the graphene, and forming a graphene film between the polyurethane wires. Through this, the graphene damper pad was applied to the surface of the metal beam, and the vibration damping characteristic was confirmed by measuring the FRF response by impact hammer test.