The Construction Of Three-dimensional Anisotropic Graphene Aerogels By Ice-Templating Method And Their Applications

Graphene has shown preeminent performances and been widely applied in many fields because of its unique properties since its discovery.Recently,further design and engineering on the structure of three-dimensional(3D)graphene-based materials have enable them a series of unprecedented properties.The anisotropic microstructure of 3D graphene aerogel is the key to determine its performances and applications.Therefore,on the basis of ice-templating method and using graphene oxide(GO)as precursor,three types of anisotropic graphene aerogels,including ultralight anisotropic graphene aerogel with vertically aligned structure and excellent resilience,3D lamellar-structured graphene aerogel,and high-quality anisotropic graphene aerogel with high density,are prepared by regulating the freezing method and freezing process to control the microstructures.Moreover,detailed researches are also studied on the application of different 3D graphene aerogels in high-performance sensors,removal of organic pollutants,and highly thermally conductive composites,respectively.The main innovative research contents are as fellows:1.The preparation of highly anisotropic graphene aerogels by calcium ion-assisted unidirectional freezing for highly sensitive sensors and efficient cleanup of crude oil spills:Aiming at the problem that direct freezing of GO dispersion could not endow graphene aerogel with oriented anisotropic structure,herein,a new calcium ion-assisted unidirectional-freezing approach coupled with thermal annealing reduction is designed and developed.Highly anisotropic graphene aerogels(GAs)are successfully constrated by unidirectionally freezing GO suspension with a trace amount of calcium ions,followed by freeze-drying and thermal reduction.The calcium ions could slightly crosslink adjacent GO sheets by complexing with hydroxyl and carboxyl groups,hence reducing the interactions between GO sheets and ice crystals,and promoting the vertical alignment of the GO sheets during freezing process.High temperature annealing and aligned porous structure endows GAs with excellent compression resilience.Therefore,a piezoresistive sensor based on obtained lightweight GA exhibits prominent sensitivity and durable reversibility of more than 6000 cycles.Furthermore,GA also shows excellent adsorption capacity for many organic pollutants from wastewaters.More importantly,the vertically aligned structure of GA enhances its adsorption of solar-light and solar-thermal energy conversion performance,making GA effecticvely convert solar energy to heat,hence decreasing the viscosity of crude oil and thus enhancing its adsorption capacity.2.3D lamellar-structured graphene aerogels for thermal interface composites with high through-plane thermal conductivity and fracture toughness:The constration of high-quality graphene aerogels with vertically aligned structures is important for the preparation of composites with high through-plane thermal conductivities.Herein,3D lamellar-structured graphene aerogels(LSGAs)with superior through-plane thermal conduction capacity are fabricated by bidirectional freezing of polyamic acid salt/graphene oxide(PAAS/GO)suspension followed by lyophilization,imidization and graphitization.The superior through-plane thermal conduction capacity of LSGA is derived from the vertically aligned and closely stacked high-quality graphene lamellae as well as the 3D continuous lamellar structure.After vacuum-assisted impregnation and compounding with epoxy,the resultant epoxy composite exhibits an outstanding through-plane thermal conductivity of as high as ?20.0 W m-1 K-1 at a graphene content of ?2.30 vol%,100 times of that of epoxy.Furthermore,it also shows a high specific thermal conductivity enhancement of?4310%.In addition,compared with epoxy resin,the lamellarstructured graphene aerogel also improves the fracture toughness of obtained composite by?1.71 times.3.High-quality and high-density anisotropic graphene aerogels for highly thermally conductive epoxy composites:The microstructures and properties of the skeleton walls of anisotropic high-quality graphene aerogels(AHGAs)effectively influence its own thermal conduction capacity.In addition,the lower apparent dendities of AHGAs also cause that the thermal conductivities of AHGAs-based composites are still at a low level.Herein,three types of AHGAs with different apparent densities and skeleton walls are fabricated by directional freezing,lyophilization,imidization and graphitization,using three types of suspension,including PAAS/GO,PAAS/GNP and PAAS/GO/GNP,as precursors.The effects of different AHGAs on the thermal conductivities of composites were also investigated.Among them,AHGA prepared by using PAAS/GO as precursor shows excellent thermal conduction capacity because GO could better promote the orientation of PI macromolecules and improve the graphization extent to form a smooth and dense high-quality skeleton wall together with the thermally reduced GO.Subsequently,this highquality AHGA is repeatedly coated by PAA,followed by imidization and graphitization to increase the apparent density of graphene aerogel while maintaining its high-quality.Consequently,the apparent density of high-quality graphene aerogel is greatly increased to as high as?228.1 mg cm-3.After impregnation with epoxy,the resultant epoxy composite with-9.69 vol%of graphene shows a prominent through-plane thermal conductivity of as high as?36.5 W m-1 K-1.