Structural Regulation Of Graphene Aerogel By Regulating The Crystallization Behavior Of Ice Template And Exploration Of Its Functional Performance

Graphene aerogel（GA）is a new type of three-dimensional porous graphene material.It possesses not only the intrinsic properties of graphene,including high conductivity of electron and phonon,but also has structural features like abundant pore structure and high specific surface area.Thus,GA has broad application prospects in the frontiers such as catalysis,energy conservation and environmental protection.The structural and functional characteristics of GA are affected by the constructing elements and methods.Taking graphene oxide（GO）as the constructing elements and the ice template method as preparation means,the GA can be massively synthesized with advantages of facility and low cost,which makes the ice template method one of the most promising industrialization methods.However,it is difficult to control the pore structure of GA due to the difficulties in controlling the shape,size and distribution of the ice crystal templates,and the movement and form of GO sheets.Therefore,it is of practical significance to establish the experimental method and theoretical system of regulating GA pore structure by controlling the ice template.In view of this,the following work has been carried out in this thesis for the regulation of GA pore structure,regulation mechanism and its applications expansion:（1）By changing the surface property of GO sheets to control the interaction between the crystallization front and the GO sheets,and to establish new method of controlling the pore structure of partially reduced graphene aerogel（pr GA）based on the control of the reducing process.Combining experimental and simulation calculation results,the regulation mechanism of reaction time on pr GA pore structure is proposed.The experiment found that when the oxidation degree is high,the pore walls of pr GA is curly,and the internal GO sheets are stacked disorderly.With the increase of reaction time,the pore walls of pr GA are gradually flattened,and the internal GO sheets are stacked tightly and orderly.Molecular dynamics（MD）simulation calculation results further reveals that the number of surface functional groups related to the reaction time is the key factor affecting the interaction between GO and the crystallization front,which determines the process of its movement in the front of the crystallization.The longer the reaction time,the smaller the number of surface functional groups,and the weaker the interaction between GO and the crystallization front.（2）Further established a new method of controlling the pore structure of pr GA by introducing ethanol as the regulator of ice template.The mechanism of regulating the pore structure of pr GA was put forward based on both the experimental and simulation calculation results.The experimental results show that the change of ethanol volume fraction results in the change of the size and shape of the template crystals,leading to regulation of the pore structure of pr GA.With the increase of ethanol volume fraction,the pore size of pr GA increases first and then decreases.The MD simulation results show that the internal factor for the change of crystal behavior is the change of interaction between ethanol and water molecules,which causes different formation of cluster structures in different template solution.In addition,the addition of ethanol significantly affects the volume expansion rate and crystallization ability of template solution.The properties of solution including viscosity,diffusion coefficient,nucleation rate and crystal growth rate also changes significantly with the addition of ethanol,resulting in changes in crystal size,crystal forms and distribution.（3）Graphene aerogel/epoxy（GA/epoxy）composites were prepared by using GA with different pore structures as the thermally conductive percolation network,and the effect of the pore structure characteristics on the thermal conductivity of the composites was explored.The experimental results show that the thermal conductivity of the composite material is mainly related to the stacking form of the graphene sheets inside the GA skeleton,while the thermal conductivity of the epoxy matrix and the interface thermal resistance between GA and epoxy have little effect on it.The relationship between the thermal conductivity of the composite material and the oxidation degree of GO and the graphene stacking structure inside the GA framework was concluded.As the reaction time increases,the graphene sheets in the GA pore walls are stacked more tightly and orderly,and the thermal resistance inside the skeleton decreases,the thermal conductivity of the composite increases.When the reaction time is 24h,the thermal conductivity of the composite material reaches 2.69 W/（m·K）.In addition,as the ethanol volume fraction increases,the GA pore size first increases and then decreases,and the number of pores first decreases and then increases,which leads to the GA pore wall thickness increasing first and then decreasing.When the more number of holes and the thinner hole wall is,the heat dissipation at the interface is more serious,resulting in a decrease in thermal conductivity,and vice versa.When the ethanol volume fraction is 40%,the thermal conductivity of the composite material reaches 2.81 W/（m·K）.（4）Expanding the application of GA in the field of intelligent microwave absorption,and the GA composite with temperature-sensitive responsiveness is proposed,which for the first time gives the GA-based composite the ability to change microwave absorption band.Both in-situ structural characterization methods and MD simulation results have confirmed that the skeleton of GA could delaminate and restack with temperature changes.This structural change makes the composite exhibit frequency selectivity for microwave absorption.Below the critical phase transition temperature,the molecular chain of poly（N-isopropylacrylamide）（PNIPAAm）stretches,and the GA framework delaminates,resulting in a decrease in the conductivity of the composite(10^-5 S/m).The loss mechanism dominated by polarization relaxation makes microwave absorption mainly acts in the X-Ku frequency band.Above the critical temperature,the PNIPAAm molecular chain shrinks,driving the graphene sheet layer restacking,the conductivity of the composite material is recovered(10^-1 S/m),and the increase in conductivity loss makes the absorption frequency band shifts to the C and X frequency bands.（5）The new MXene@GO hybrid aerogel microspheres were prepared by combining the electrospinning technology and the ice template method.By optimizing the structural parameters,the high-performance absorption of microwaves was realized,which further expanded the application of GA in microwave absorption field.Based on the two-dimensional nanostructure with hydrophilic properties,MXene can spontaneously assemble with GO in a face-to-face form in an aqueous solution to form a layered hybrid,which forms three-dimensional aerogel microspheres with porous structure by electrospinning and the ice template method.The experimental results show that MXene/GO solutions with different ratios can form hybrids with different stacked structures.The introduction of MXene significantly improves the absorbing performance of aerogels,and the best performance is reached when the content is 30 wt%.When its proportion in paraffin wax is 10 wt%and the sample thickness is only 1.2 mm,the reflection loss reaches-49.1 d B at 14.2 GHz,and the effective absorption（<-10 d B）bandwidth is 2.9 GHz.In addition,when the thickness is less than 5 mm,the hybrid aerogel has excellent absorption performance in the S band.The electromagnetic parameter analysis results show that the introduction of MXene changes the dielectric properties of the hybrid aerogel,bring about a variety of new polarization mechanisms,and improve its polarization loss and conductivity loss capabilities.