Preparation Of Carbon-based Materials By Helicon Wave Plasma

Carbon-based functional materials are considered as the most promising materials to solve energy problems because of their low cost,sustainability and versatility.There are many kinds of carbon-based materials.One is the isomer composed of a single carbon element,such as graphene,carbon nanotubes and fullerene.The three-dimensional graphene nanosheets and multi walled carbon nanotubes made of graphene have become the new stars in the field of environmental protection and new energy;The other is the formation of covalent bonds with other elements.Among many carbon compounds,amorphous silicon carbide has attracted much attention due to its excellent properties such as high strength,high density and high hardness.However,the controlled growth of functional carbon-based materials for specific applications is still a challenge.There are many plasma methods for preparing functional carbon-based materials,such as ICP,CCP,Hot Wire CVD,ECR,MWCVD and DC arc CVD.However,these methods usually require high temperature,high pressure and other harsh preparation conditions.Helicon wave plasma(HWP)is an ideal method for the preparation of controllable carbon-based nanostructures because of its higher plasma density(>1019 m-3)and higher ionization rate,and its independent control of electron energy distribution,electron density,particle energy and electron temperature.By controlling the discharge parameters of helicon wave plasma chemical vapor deposition(HWP-CVD),it is possible to control the structure and morphology of carbon based nanomaterials.The applications of functional carbon-based materials in electrochemical energy storage devices,catalytic devices,electronic devices and sensors are broadened.To develop the application of HWP technology in the field of material processing and preparation,a variety of carbon-based functional film materials(Vertical graphene nanosheets(VGs),Multi wall carbon nanotube arrays(MWCNTs),amorphous silicon carbide(a-SiCx))were prepared by HWP-CVD technology.The influence of HWP-CVD process parameters on HWP characteristics was studied;The influence of structural characteristics of carbon-based materials on the properties of carbon-based materials was analyzed.The physical mechanism of the influence of the characteristic parameters of HWP on the structure formation of different carbon-based materials was analyzed.The physical model of the interaction between the HWP and the surface of carbon-based materials was established to provide the necessary reference and parameter selection basis for the actual plasma process.The specific process is as follows:(1)Diagnostic study on HWP discharge of carbon precursor.In order to study the influence of plasma parameters(electron density,electron energy distribution,electron temperature,gas phase composition,particle flux,etc.)on the growth of carbon-based films.The diagnostic of OES,Langmuir probe,mass spectrum and energy spectrum were used to diagnose the discharge characteristics of HWP under different process parameters.Chemical reaction process and the formation process of film-forming materials in the mixture of precursor gases were analyzed.The effect of the type and content of gas phase materials on the film growth was analyzed.The dynamic process of HWP-CVD preparation of carbonbased films was revealed.(2)Research on structure control of VGs by HWP-CVD.Based on the diagnostic study of HWP discharge characteristics under different process parameters,the flow rate of methane,the introduction of hydrogen,the selection of different substrate materials and the growth time were adjusted.The growth model of VGs prepared by HWP-CVD was established.The effect of VGs physical structure on capacitance characteristics was analyzed.The size and shape of graphene nanosheets in VGs can be controlled and the crystallization quality of the nanosheets can be improved by using HWP discharge with different flow ratios of argon and methane.Hydrogen was introduced into the Ar/CH4 discharge system as the diluent gas.The introduction of hydrogen promoted the physicochemical reaction in the plasma,increased the content of CH and C2 groups in the film-forming material,increased the growth rate of VGs,and formed a micro cavity structure with large wall spacing by atomic hydrogen etching of the amorphous phase on the surface of the film.Metal conductors with different resistivity were selected as substrates,and the morphology and structural characteristics of VGs grown on different substrates were studied.The results show that VGs grown on copper substrate have the strongest hydrophilicity,the weakest disorder and the largest number of graphene layers.By changing the growth time of VGs,the physical structure of VGs was controlled and the electrochemical performance was improved.(3)Preparation of MWCNTs by hydrogen modulated Ar/CH4 HWP.The effects of hydrogen and growth time on the growth of carbon nanotubes were discussed.The results show that the density of active radicals in HWP is increased with the addition of hydrogen.The growth rate,specific surface area,defect density and oxygen-containing functional group content of MWCNTs are improved.Higher specific capacitance(1.9 mF/cm-2)and rate performance(82%)are obtained.(4)Effect of substrate electric field strength on the growth of hard coated amorphous silicon carbide on Si substrate and 316 L substrate.High mechanical hardness amorphous silicon carbide films were prepared by changing substrate negative bias voltage at room temperature with TMS organic source as precursor.The hardness of the films grown on Si substrate can reach 18.5 GPa,and the hardness of the films grown on 316 L stainless steel can reach 33.6 GPa.