Magnetically Rotating Arc Plasma Vapor Phase Synthesis Of Graphene And Doped Graphene

Graphene,a two-dimensional sheet of sp2-hybridized carbon,has excellent mechanical,electrical,thermal,optical,and other properties.However,its very stable structure leads to constraints in the practical application of graphene,so improvement of the properties of graphene by doping with suitable elements is necessary.Yet highquality graphene and doped graphene are challenging to prepare on a large scale,which limits their extensive industrial applications.Magnetically rotating arc plasma gasphase synthesis is a new process for the preparation of carbon nanomaterials in recent years.Plasmas have unique chemical activity and can be used to grow nanostructures in gas phase without catalysts,which has excellent potential in carbon nanomaterials production.Therefore,it is important for the production to investigate the relationship between process parameters and product properties and clarify the pathway of product formation.A magnetically rotating arc plasma system was used for the synthesis of graphene flakes,and the effects of thermal history(input power and feedstock injection position)and buffer gas type on product physicochemical properties were investigated.Results showed that the graphene flakes were produced under high gas temperature and long thermal history,while the opposite conditions favored the spherical particles.The buffer gas also played a decisive role in the morphology and elemental composition of the products.Spherical particles,semi-graphitic particles,and graphene flakes coexisted in products under an Ar atmosphere.Under a He atmosphere,all products were graphene flakes.Using H2 and N2 as buffer gases could produce a hydrogen-rich environment that terminated the dangling carbon bonds;on the other hand,the high enthalpy gases could effectively increase the reaction temperature which suppressed the formation of curved or closed structures,leading to the production of graphene flakes with high crystallinity.Based on the above research,the effects of injection position,reaction temperature,and carbon precursors on nitrogen-doped graphene properties were systematically explored.Combined with experiments and calculations,it was speculated that HCN was the primary reaction precursor through which N atoms entered the product.Increasing the reaction temperature and the H/C ratio of the carbon precursor was beneficial to the HCN production,which led to a high doping level of products.On this basis,a link between experimental conditions and microwave absorption performance of the products was established.By adjusting the doping level and modifying the morphological structure,the saturation magnetization was significantly enhanced,and the impedance matching was improved.Results indicated that when the product was subjected to 13.5 kW,the optimal reflection loss approached-50.0 dB at 6.7 GHz with an absorber thickness of 3.1 mm,and the maximal effective absorption region(Reflection Loss<-10 dB)was wider than 3.5 GHz,This one-component absorber could be an excellent candidate for an ultra-lightweight,highly efficient electromagnetic wave absorber.To further expand the application in microwave absorption,CS2 was used as the sulfur source,and N2 was used as the nitrogen source for the one-step synthesis of sulfur and nitrogen co-doped graphene.The result showed that the sulfur and nitrogen doubleatom doping had a significant synergistic effect,with an exponential increase in doping levels compared to single-atom doping at the same reaction temperature,reaching a maximum doping content of 15.0 and 6.9 at%for the sulfur and nitrogen atoms,respectively.The co-doping gave the sample a richer loss pattern,greater attenuation capability,and better impedance matching.At an input power of 15 kW,good microwave absorption was shown,with an optimum reflection loss of-51.4 dB at 16.3 GHz and an effective absorption bandwidth of up to 6 GHz(13.3-16.3 GHz).To clarify the generation mechanism of graphene and its doped derivatives,the plasma vapor phase synthesis process was imaged by Reaxff MD.The results showed that the formation of carbonaceous nanoparticles could be broadly divided into three phases:the first phase was chain growth and early macrocycle formation,mainly from C2 addition and polyene/polyalkylene ring closure;the second phase was PAHs formation and crosslinking,which implied stable nodule formation;and the third phase was rapid growth and graphitization,in which carbon atoms underwent rearrangement,and heterocycles were transformed into aromatic rings,leading to a shift towards the graphite phase.The addition of N2 to the system inhibited the formation of carbonaceous nanoparticles.In the fuel cracking phase,a high H/C ratio of the feedstock produced more HCN species,which on the one hand trapped more carbon atoms and reduced the utilization of carbon in the feedstock,and on the other hand,the high concentration of carbon and nitrogen species facilitated the entry of nitrogen atoms into the graphene skeleton and increased the doping level of nitrogen atoms.This process was accelerated by the high reaction temperature.During the growth phase,the carbon and nitrogen species continuously underwent dynamic bonding and bond breaking with the active sites at the edges of the graphene,occupying the active sites for the growth of carbon atoms and limiting the growth of graphene.When the sulfur and nitrogen atoms were co-doped,both elements bonded to the fatty chains at the edges of the carbon skeleton in the form of short chains,increasing the doping efficiency of the heteroatoms within the same reaction frequency,resulting in a significant increase in the doping level of the co-doped graphene.The magnetically rotating arc plasma process developed in this thesis is simple to operate,easy to obtain raw materials,can be prepared continuously,and has low energy consumption.The relationship between process conditions and product properties was clarified,and the formation pathway of the products in the plasma process was briefly discussed,which has important guiding significance for the preparation of high-quality graphene and doped graphene.