Preparation,structure And Performance Of Nitrogen-doped Graphene Aerogel For Thermal Insulation

The restrain efficiency of gaseous heat conduction in present thermal insulation materials is low,which is a significant problem in thermal insulation field.To address this issue,the idea of restraining gaseous thermal conduction using three-dimension nanoporous materials from two-dimension?2D?sheets was come up with.And the graphene aerogel was introduced into thermal insulation field for the first time.The defects and doping atoms were introduced into the graphene to lower the solid thermal conductivity of graphene aerogel and the pore size was minimized to restrain the gaseous thermal conduction in the aerogel.The N-doped graphene aerogel with low density,low thermal conductivity and high gaseous thermal insulation efficiency was obtained.But,the tensile strength of N-doped graphene aerogel is weak to some degree,and its high-temperature resistance performance is also poor.To overcome the former problem,the idea of reinforcing N-doped graphene aerogel and orienting N-doped graphene using long fiber as director was come up with,by which not only its tensile strength is enhanced but also the anisotropy of 2D graphene is made the most of.To overcome the latter problem,the idea of conversing out-plane doping into in-plane doping was come up with,by which the problem that the doping atoms in N-doped graphene is easy to be eliminated at high temperature is overcome.?1?The N-doped graphene aerogel with low density,high defects and doping content,and small pore size was prepared with graphene oxide?GO?suspension as main raw materials,paraphenylene diamine?PPD?as doping and reducing agent,ammonium hydroxide?NH₃·H₂O?as reducing accelerant by gelling at 90?through chemical reduction one-step method,gradient exchanging and supercritical CO₂ drying.Its thermal conductivity?0.0232W/?m·K??is low,which is only 1/2 of that ever reported and even lower than that of static air.Compared with SiO₂ aerogel,the N-doped graphene aerogel shows similar thermal conductivity,but much lower density,whose restraining efficiency of gaseous thermal conduction is larger than 2 orders of that in SiO₂ aerogel.?2?The thermal insulation mechanisms of N-doped graphene aerogel was studied for the first time.The reasons for low solid thermal conductivity was studied by nonequilibrium molecular dynamics.Four facts contribute to this phenomenon:?1?The great amount of defects and doping;?2?The overlap,few connection and bridging.?3?The small size of N-doped graphene.?4?The low density and complicated structure.Among these reasons,inner thermal resistance takes the greatest part.The concept of effective pore size and statistic effective pore size were proposed.The effective pore size of N-doped graphene aerogel is larger than its practical value,but its statistic effective pore size is smaller than its practical value.Hence,the restraining efficiency of gaseous thermal conduction in the aerogel is high.Lower the thermal conductivity of2D materials,which contributes to the lowering of gas-solid coupling thermal conductivity in the aerogel,is the key to lower the thermal conductivity of this new 3D materials.The 2D structure of graphene,perfect opacity,and the repeated reflection,adsorption and radiation for the porous structure of N-doped graphene aerogel significantly contribute to the lowering of radiative thermal conductivity.?3?N-doped graphene aerogel composites with low density,high tensile strength,low thermal conductivity and anisotropy for thermal insulation were prepared with long fibers as director and reinforcement.The N-doped graphene in the composites orientes along the in-plane orientation.The mechanic behavior of N-doped graphene aerogel composites can be illustrated with model of composites reinforced by long fibers,in which the strength of fibers is much higher than matrix and the in-plane strength of matrix is higher than its through-plane strength.The solid thermal conductivity of N-doped graphene aerogel composites is similar with that of N-doped graphene aerogel.But the pore size of the composites increases,which increases their gaseous thermal conductivity.The through-plane thermal conductivity of N-doped graphene aerogel composites is lower than their in-plane thermal conductivity,which helps avoid overheating and contributes to their applications in thermal insulation field.The N-doped graphene aerogel distributes among the fibers in the composites,separating and enveloping the fibers.This prevents the effect of heat-bridge and lowers the thermal conductivity of the composites.Also,this stops the gaseous convection and restrains the gaseous thermal conduction.The perfect opacity of graphene depresses the radiative thermal conductivity.?4?The pyrolyzed N-doped graphene aerogel with low density,low thermal conductivity,high-temperature resistance and flame retarding was prepared by pyrolyzing N-doped graphene aerogel at high temperature.The pyrolyzing process is mainly composed of the evaporation of adsorbed water,the condensation of hydroxyl groups into epoxy groups,the elimination of carboxyl groups,the conversion of amino N into pyridinic N and pyrrolic N,the elimination of epoxy groups and partial amino N,the conversion of pyrrolic N into graphitic N by atomic arrangement,the elimination of carbonyl groups and residual hydroxyl groups,the elimination of pyridinic N and graphitic N,and the repair of defects in graphene by atomic arrangement.The high-temperature resistance of pyrolyzed N-doped graphene aerogel is improved.The aerogel keeps intact when being placed in the flame of alcohol lamp.With the increase of pyrolyzing temperature,the thermal conductivity increases after keeping at certain value.When the pyrolyzing temperature is 1000?,the density and thermal conductivity of pyrolyzed N-doped graphene aerogel are 0.030g/cm³ and 0.134W/?m·K?,respectively.Its thermal insulation efficiency is higher than that of other high temperature thermal insulation materials.N atoms in pyridinic state and graphitic state are observed in the Pyrolyzed N-doped graphene aerogel.Theses atoms also depress the thermal conductivity of graphene,but the depressing efficiency is lower than pyrrolic N and amino N.The other thermal insulation mechanisms of pyrolyzed N-doped graphene aerogel are similar with that of N-doped graphene aerogel except that the gas-solid coupling thermal conductivity obviously increases.