Microstructure Controlling And Properties Of Graphene Oxide Aerogels

Single-layer graphene oxide is a two-dimensional material with unusually rich functional groups,which can be suspended in aqueous solution for a long time and can be easily assembled into three-dimensional ultra-light graphene oxide aerogel.On the one hand,graphene oxide aerogel is a porous material with extremely high porosity,The arrangement of two-dimensional graphene oxide in aerogel is closely related to its performance,so adjusting the arrangement of graphene oxide can effectively adjust the macro performance of aerogel.On the other hand,the oxygen content of graphene oxide can be effectively regulated by post-treatment such as reduction or heat treatment,Therefore the properties of aerogel,such as mechanical properties,thermal properties and electrical properties,can be controlled,and the research in this area has been very mature.Based on this,this paper puts forward different preparation strategies to control the temperature field distribution during freezing,and controls the arrangement of graphene oxide by controlling the crystallization direction of ice.As to obtain graphene oxide aerogels with different microstructures,and strive to establish the relationship between microstructures and properties.The main research contents of this paper are as follows:Highly oriented graphene oxide aerogel was prepared by continuous droplet method.The aqueous dispersion of graphene oxide flowed in the mold continuously and unidirectionally,and at the same time,the freeze casting was carried out,so that the liquid flow was used as the driving force to comb the arrangement of graphene,and the liquid flowing through the supercooled source provided a continuous gradient temperature field,which improved the orientation of graphene sheets in the aerogel.The thickness of the prepared highly oriented graphene aerogel is 30 mm,which is 600%of that of 5 mm reported at present.The maximum stress value of graphene aerogel with density of 10 mg/cm³ in different compression directions can be 1.7 times different,and the compression strength can reach 160 k Pa.The residual strain is only 1.2%after 20 cycles of normal compression,and the maximum stress attenuation is only 15%,The 4 mg/cm³ low density sample has a very low thermal conductivity of 0.013 W m^-1K^-1 in air,Graphene oxide aerogel prepared by freeze casting with discontinuous dropping method has gradient laminated structure,excellent compression elasticity in axial direction,compression strength of 160 k Pa and cyclic compression characteristics.The special gradient laminated structure has excellent thermal insulation performance,and its thermal conductivity is as low as0.014 W m^-1K^-1.We have prepared graphene oxide aerogel with twin-like symmetric structure by using symmetric temperature field,and its internal graphene lamellae also show highly oriented structure.The orientation angle of graphene lamellae can be adjusted by changing the mold wall plates with different thermal conductivities,and the obtained angles are 18.4°,45°and 78.7°,At the same time,its micro-pore size distribution and specific surface area can be controlled,which endows it with anisotropic compressive stress-strain characteristics of graphene aerogel.During the compression process,it shows special mechanical response modes of symmetric negative Poisson’s ratio,asymmetric one-sided positive Poisson’s ratio,contralateral negative Poisson’s ratio and near zero Poisson’s ratio in Y direction.In 20 cycles of50%compressive strain,Residual strain is only 0.5%,and the thermal conductivity is also anisotropic.The thermal conductivity can be reduced to 0.010 W m^-1K^-1 in the air environment with Y upward direction.Based on the above research results,the asymmetric structure of graphene oxide aerogel is realized by introducing asymmetric temperature field,which makes the same aerogel block have different mechanical properties at different positions,and the upper position and the lower position have different maximum stress and plastic deformation characteristics when compressed in X,Y and Z directions.At the same time,the direction of the lowest thermal conductivity in one sample changes from Y to X with the change of sampling position.The thermal conductivity in Y increases from 0.012 W m^-1K^-1 to 0.023 W m^-1K^-1,and the thermal conductivity in X decreases from 0.023 W m^-1K^-1 to 0.014 W m^-1K^-1.The wave-absorbing performance of aerogel with asymmetric graded structure under specific placement mode is related to the incident angle of electromagnetic wave,and the reflection loss is below-10 d B in the range of 11-18 GHz.The strongest wave absorbing effect can reach-16.5 d B at 17.3 GHz.