Electron/Proton Radiation Effects And Its Mechanisms Of Polyimide Aerogel

The aerogel materials have excellent performance due to structure characteristics of high porosity and large specific surface area.It is commonly used in catalytic filtration,energy storage,heat/sound insulation and other fields.Owing to ultralow density and excellent heat insulation properties,aerogels show great potential in the aerospace activities.Based on the application of polyimide aerogel in the space environment,the polyimide aerogel is taken as the research object in this paper.The Monte Carlo method is used to systematically study the transport behavior of charged particles in nanoporous materials.The effects and mechanisims of polyimide aerogel under proton/electron irradiation have been systematically investigated in this paper by nanoindentation,ultraviolet-visible spectrophotometer,Fourier transform infrared spectrometer,X ray photoelectron spectrometer and electron paramagnetic resonance analysis spectrometer.The simulation results indicate that the range and FWHM（Full Width at Half Maxima）of the particle（proton/electron）increases with porosity.The distribution of energy deposition moves to the deeper position and become wider as porosity increasing.The nanoporous structures magnify the straggling effect of the interaction between particles and materials.Moreover,the distribution of particle and energy deposition show obvious inhomogeneity in local area:a large number of particles will deposit on the nanoskeleton while a small proportion of particles deposited on the center position of the voids.When the porosity is same,the pore size would affect the transport behavior.The bigger of the pore size,the larger of the proton average range and FWHM.The viriation of pore size has no obvious influence on the overall distribution of electron.But the peaks and valleys would appear on the distribution curves and the number of peaks/valleys is inversely proportional to the pore size.The different characteristics of proton and electron make the transport behavior showing obvious diversity.Although both of them show inhomogeneity in local area,the overall distribution of protons is relatively concentrated with a typical Bragg peak,while the distribution of electron is more dispersed and has a larger range.The polyimide aerogel would produce significant discharge phenomenon under electron irradiation.The materials on the discharge channel have been transformed to carbonized structure,while the surrounding materials still remained pristine.The more content of residual gas,the more times of discharge,and more damage produced in materials.The flux of electron would affect the charge accumulation rate and discharge times.The higher of the flux,the faster of the charge accumulation rate and the more discharge times.The electron energy affected the location of discharge and the amount of internal charge deposition.The results indicate that the internal discharge path in polyimide aerogel is dendritic,showing obvious orientation,and the orientation is related to the scanning mode of electron irradiation.The high-yield secondary electrons induced by incident electron deposite on the polyimide aerogel skeleton,showing obviously inhomogeneity.The extreme high potential difference is formed in local area,causing the discharge in polyimide aerogel.The internal discharge types in aerogel are related to residul gas,including gas discharge and surface flashover.The formation of dendritic micron channels inside the aerogel also affects the thermal insulation performance,and the thermal conductivity of the aerogel decreases after electron irradiation.Because the channels in the homogeneous material reduce the heat transfer section,and the dendritic structure increases the heat transfer path.After proton irradiation,the molecular chains,the three-dimensional skeleton structure and the macroscopic properties of the polyimide aerogel changed accordingly.The three-dimensional nano-skeleton densified gradually with fluence and transformed to the layered structure.The layered structure became wider and moved to the surface when the fluence was reached 1×10¹⁶cm^-2.The degradation behavior of polyimide aerogel skeleton materials is similar to the bulk polyimide:the scission of C-N bond and depolymerization of imide ring.But it is difficult to detect the presence of free radicals after proton irradiation.The reason is related to its large specific surface area.The highly active free radical is driven by surface force,diffused and recombination on the surface of the nanoskeleton.After proton irradiation,the surface optical absorption increased while the specific heat capacity decreased,which was related to the degradation of molecular chain and surface carbonization.The obvious Pop-In phenomenon appears in the displacement-load curve,which is related to the densification of the irradiated area.Based on the transformation process of the three-dimensional skeleton,the layered structure formation mechanisms are proposed:The near-equiaxed voids of polyimide aerogel became shrinkage at the place of maximum energy deposition location,where the local tensile stress perpendicular to the direction of the proton beam is generated,causing the near-equaxial voids to elongate until the skeleton is welded.The tensile stress promotes the formation of layered structure.