| Polyimide(PI)aerogel not only has the characteristics of low density,high porosity,and low thermal conductivity of ordinary aerogels,but also has the characteristics of flexible molecular design and can be made into flexible films.Therefore,researchers are trying to apply PI aerogels to the aerospace field,such as the flexible thermal protection system of hypersonic inflatable pneumatic reducers.However,the cross-linking agent required to prepare PI aerogels is costly and the synthesis method is complicated.These key technical problems have restricted the further development of PI aerogel.In view of this,an economical and effective amino-terminated crosslinking agent,namely melamine,was used in the study to prepare a melamine crosslinked PI(MPI)aerogel with excellent comprehensive properties.The density,shrinkage rate,chemical structure,thermal insulation and other performance of MPI aerogel have been deeply analyzed and studied.Research results show that MPI aerogel has the characteristics of low density,low thermal conductivity,good thermal stability and acceptable mechanical properties.Therefore,the use of melamine to prepare cross-linked PI aerogels is a feasible strategy.In addition,in order to avoid shrinkage deformation of MPI aerogels when they are used at high temperature,a series of aerogels,eg.MPI-E aerogels,were prepared by supercritical ethanol drying based on the chemical formula of MPI aerogels.MPI-E aerogels have the characteristics of low density,low thermal conductivity and high thermal decomposition temperature of most PI aerogels.Compared with MPI aerogels,MPI-E aerogels had better thermal stability under high temperature.And MPI-E aerogels did not show significant shrinkage deformation after 200 ℃ heat treatment.Finally,in order to further optimize the performance of the MPI aerogels and improve the resistance to shrinkage of the MPI-E aerogel during the drying process,the structure of the MPI-E aerogels were reinforced by carbon foam,and a carbon foam-enhanced MPI was prepared,that is,MPI-C aerogel.The research results show that the addition of carbon foam provides a certain degree of support for MPI-E aerogel,and the shrinkage of MPI-C aerogel during the drying process is significantly reduced.The final MPI-C aerogel sample has lower density and shrinkage rate and better thermal insulation performance.In addition,its thermal stability and mechanical properties have not been greatly affected,and it has the possibility of exerting its application value in high-temperature thermal protection. |
