Study On Massive Production Of Multilayer High-quality Graphene And Its Applications

Graphene is one of the allotropes of carbon with typical two-dimensional?2D?crystalline structure.The carbon atoms of graphene form a compact and regular hexagonal honeycomb lattice structure with only one atomic layer thickness by the SP²hybrid orbitals and large?bonds.Owing to this special structure,graphene exhibits excellent mechanical,thermal and electrical properties.Therefore,graphene has attracted great attentions and enthusiasm in industry.However,there is still a problem to be solved in the applications of graphene,i.e.,massive production of graphene with high quality and low cost.At present,the usual methods to fabricate graphene,e.g.,mechanical exfoliation,liquid exfoliation,chemical vapor deposition?CVD?,cannot meet the requirements of high quality and low cost,simultaneously.Some newly arisen fabrication methods such as Flash Graphene and bubbling CVD?B-CVD?also have some problems.Aiming on this problem,based on B-CVD method,more efficient molten nickel-based alloys and cheaper molten salts are used as catalysts to realize the conversion from low-cost methane to high-quality graphene in this dissertation.This work improves the efficiency and decreases the cost of B-CVD method.The main works and results are listed as follows:1.The molten nickel-based alloys are used to catalyze the pyrolysis of methane and growth of graphene growth using B-CVD method.In this way,the yield of graphene achieves 0.49 g/h and the conversion efficiency of methane reaches 77.05%when 200 ml molten alloy is used for graphene growth at the temperature of 1300^oC.Compared with the reported copper,the nickel-based alloy leads to much higher conversion efficiency at a lower growth temperature.In addition,the fabricated graphene has high quality and its thickness can be controlled by the growth parameters,typically 7.4 to 24.2 layers.The growth mechanism of graphene in the methane gas bubbles in molten nickel-based alloys has also been investigated and the isothermal precipitation of graphene on the surface of bubbles has been concluded.2.The molten salts with high boiling point,CaCl₂,is used as the catalyst to produce the high-quality graphene using the B-CVD method.At the condition,the yield of graphene and conversion efficiency of methane is lower than those of nickel-based alloy as catalyst,indicating that the molten salt has weaker catalytic power for the pyrolysis of methane,however,the quality of graphene is better.In addition,the impurities on the surface of graphene can be easily cleaned by water since CaCl₂ has high solubility in water.Therefore,the post-treatment cost of graphene is greatly decreased and this method is more environmentally friendly.The growth mechanism of graphene in the methane gas bubbles has also been investigated.Different from the nickel-based alloy,the decomposed carbon atoms assemble into graphene directly on the surface of bubbles instead of migrating into the molten salt and then isothermally precipitating.3.The fabricated graphene with good quality and wrinkled surface can be used in different fields.The large surface area resulted from the wrinkled surface and the hydrophobic and oleophilic properties of high-quality graphene lead to the good adsorption capability of graphene toward oil and organic solvents.The experimental results show that its adsorption capability can reach 65 to 160 g/g and can be used repeatedly.In addition,the graphene/PDMS composite has good mechanical performance when the graphene content is 7 wt%.For 0.4 mm thick composites,the electromagnetic shielding efficiency can reach 36 dB in X-band.4.A highly conductive three-dimensional?3D?graphene network?GN?was fabricated by chemical vapor deposition on a 3D nickel fiber network and subsequent etching process.Then a lightweight and flexible GN/polydimethylsiloxane?PDMS?composite was prepared by a vacuum infiltration method by using the graphene network as a template.The composite showed the superior electrical conductivity of 6100 S/m even at a very low loading level of graphene?1.2 wt%?.As a result,an outstanding electromagnetic interference?EMI?shielding effectiveness?SE?of around 40 and 90 dB can be achieved in the X-band at thicknesses of 0.25 and 0.75 mm,respectively,which are much higher than most of the conductive polymers filled with carbon.The 3D graphene network can also act as a mechanical enhancer for PDMS.