| Graphene with outstanding thermal, electrical and mechanical properties has immense potential for a broad range of applications, including high speed electronics, thermal management materials in high power electronics, mechanical reinforcement materials, and energy storage. In this thesis, several approaches have been developed to assemble atomic thin graphene sheets into highly ordered macroscopic structures, e.g., electrospray deposition process for layer-by-layer film structure; wet-spinning of highly aligned sheets into fibers; and microfluidics enabled fabrication of graphene belts and hollow tubes. Graphene structures have been used into various applications, such as heat spreader in thermal management, highly conductive film, thermal energy storage, fast responding actuator and mechanically strong fibers/belts for reinforcement materials.;Targeting the fabrication of large area free standing graphene paper with highly ordered internal structure, a layer-by-layer electrospray deposition process integrated with roll-to-roll line has been developed. The deposited graphene film on highly hydrophilic substrates has been separated by a novel water exfoliation method. The internal microstructure of graphene paper has been well controlled by in situ electrospray deposition, post mechanical pressing and heat treatment. The well stacked and defect-free graphene paper with superior thermal and electrical properties shows outstanding heat diffusive performance as heat spreader in removing hot spots.;Phase change composites with superior heat transport performance has been produced following two strategies. Upon thermal annealing, defect-free graphene sheets obtained have been applied as filler to synthesize phase change composite. The incorporation of defect free graphene sheets into a phase change materials drastically enhance the heat transport performance and meanwhile minimize the loss of phase change enthalpy, resulted from the complete removal of phonon scattering centers for heat transport. Highly thermally conductive graphene tube has been fabricated by wet spinning through a special designed co-axial nozzle. The study of graphene oxide rheology and fluid dynamic assists the wet spinning process optimization to produce graphene tubes with sheets well aligned inside tube wall to ensure the high heat transport capability. Graphene tube has been applied to encapsulate phase change materials to increases the thermal conductivity up to a record value of 94.01W/mK at 12.1wt. % graphene loading, representing two orders of magnitudes improvement; meanwhile, the crystallization and melting behaviors of phase change materials have been well maintained due to the less interaction between phase change materials and graphene sheets, insuring a small subcooling and high phase change enthalpy.;Highly thermally/electrically conductive, high strength and high modulus graphene fibers with an interlaced structure of large-sized/small-sized graphene sheets have been developed. Upon simultaneous thermal reduction of graphene oxide fibers by wet spinning and elimination of residual defects, the graphene fibers obtained achieve thermal conductivity up to 1290 W/mK, over ~30% greater than the best carbon fibers. The highest tensile strength of GFs approaches 1080 MPa, Young's modulus up to 135 GPa, and electrical conductivity up to 2.2x105 S/m. A thermal bimorph actuator has been fabricated using the highly thermally conductive and mechanically robust GFs showing a sub-second response, exceptional actuation capability and cyclic stability.;In addition, microfluidics-enabled assembling highly ordered multiscale graphene structures have been developed. The mechanistic study of rheology of graphene oxide fluid has been assisted through fine design of microfluidic channels. The motion of graphene oxide sheets during assembling process has been well controlled to optimize molecular orientation and macroscopic ordering of the graphitic domains in the final structures. During flow, two dimensional graphene oxide sheets show higher order of orientation in low aspect ratio microfluidic channels than circular channels. The obtained graphene belts from low aspect ratio microfluidic channels show perfect graphitic crystalline structure, achieving thermal conductivity up to 1600 W/mK and electrical conductivity of 9.3x106 S/m. |
