Structural Design Of Graphene Aerogels Via Ice-templating Method And Their Applications For Phase Change Energy Storage And Force Sensors

Thanks to their light-weight,porous,both electrically and thermally conductive features,three-dimensional(3D)graphene aerogels are appropriate candidates for both phase change materials and force sensors.However,isotropic graphene aerogels present randomly oriented pore structure,which is detrimental to their applications in these two fields.In terms of phase change materials,the extraordinary thermally conductive ability of graphene cannot be fully adopted.In terms of sensing materials,isotropic structure of graphene aerogels contributes to their high mechanical strength,leading to low sensitivity.Meanwhile,junctions between pore walls act as stress concentration points,which easily yield and break when graphene aerogels are subjected to pressure,leading to plastic deformation.Hence,it is essential to design and build specific pore structures for fully utilizing superb properties of graphene.By using ice templating method,designing different molds and adjusting temperature gradients,pore structures of graphene aerogels can be rationally designed and realized.Anisotropic aerogels present outstanding performance for phase change materials and force sensing materials.The main research contents are as follows.(1)Anisotropic high-quality graphene aerogels by directional-freezing method for phase change composites featuring fast solar energy storage.Graphene aerogels act as exceptional thermally conductive filler to boost the thermal conductivities of phase change materials.However,superb in-plane thermal conductivity of graphene cannot be fully utilized due to isotropic pore walls.Therefore,anisotropic graphene aerogels with vertically aligned pore walls are fabricated by directional-freezing graphene oxide/poly(amic acid)salt(GO/PAAS)suspension,followed by lyophilization,imidization and graphitization.During the directional freezing process,ice crystals grow from bottom to top,expelling particles in the suspension,leading to vertically aligned pore walls.After graphitization,GO and PAAS can be converted to graphene.GO have three positive effects on resulting graphene aerogels.Firstly,GO leads to morphological lamellar-to-cellular transition and further decrease the equivalent diameter of cellular pores.Secondly,GO can induce the graphitization of polyimide to produce graphene with higher quality.Thirdly,GO prohibits excess thermal shrinkage of graphene aerogels during graphitization.Ascribed to these three positive effects of GO,the resulting phase change materials exhibit excellent shape stability,high thermal conductivity of 8.87 W m^-1 K^-1 along vertical direction,and extraordinary latent heat retention of 98.7%.With its high thermal conductivity along vertical direction,the resulting phase change material presents fast heat storage and low temperature gradient when it serves as solar-to-thermal energy conversion device.(2)Lamellar graphene aerogels by bidirectional freezing for ultrasensitive piezoresistive and bending sensors.Electrically conductive graphene aerogels can serve as piezoresistive sensing materials due to high elasticity.However,isotropic pore structure of graphene aerogels leads to high compressive strengths,resulting in low sensitivity.Moreover,stress concentration points do harm to the elasticity of graphene aerogels.Hence,lamellar graphene aerogels are designed for ultrasensitive piezoresistive materials.By using specific mold,two orthogonal temperature gradients can be generated through GO suspension,leading to the growth of lamellar ice crystals parallel to these two directions.GO can be expelled and arranged to lamellar pore walls.After thermal annealing,GO can be converted to graphene.Compared to isotropic and unidirectional-freezing graphene aerogels,lamellar aerogels show highest sensitivity of-3.69 k Pa^-1.Thanks to its ultrasensitivity,lamellar graphene aerogel can detect subtle pressure as low as 0.15 Pa,with applicability when immersed in liquid nitrogen.Besides,lamellar graphene aerogels can be employed to detect dynamic forces with frequencies up to 2000 Hz,and sound waves.Ascribed to feeble junction between graphene lamellae,the lamellar graphene aerogels can be easily cut into slices for bending sensors.These bending sensors can detect wrist pulse and human motion with a detection limit of 0.29^o.(3)Both highly compressible and stretchable snake-mimicking graphene aerogels by geometry-confined ice-templating method for force sensors.Graphene aerogels exhibit high elasticity against pressures.However,they are easily broken when tensile forces are applied.This drawback limits their application as force sensors.Snake-mimicking graphene aerogels are fabricated by freezing GO/PAAS suspension by geometry-confined ice-templating method.The macro-size snake-mimicking structure leads to tensile elasticity for snake-mimicking graphene aerogels while micro-size aligned pore walls can be generated because of the ice crystals growth directed by temperature gradient between copper fingers,leading to exceptional compressive elasticity.Moreover,this unique structure provides ultrahigh sensitivity,-58.0 k Pa^-1 for compressive sensitivity and 2150.6 k Pa^-1 for tensile sensitivity.Thanks to the ultrahigh sensitivity,snake-mimicking graphene aerogels-based force sensor can be applied to detect bending direction of bendable robots.