| Aerogel is a three-dimensional(3D)network porous solid material rich in gas dispersion medium.Owing to the excellent properties such as low density,high porosity,high specific surface area,low thermal conductivity,low dielectric and low acoustic impedance,aerogel is widely used in military and aerospace,industrial and building insulation,chemical catalysis,electrochemistry,sewage treatment and sound insulation.Nowadays,although inorganic aerogels represented by common silica have strong heat resistance and high specific surface area,their inherent brittleness limits their applications.The emergence of organic aerogels improves the toughness and strength of aerogels,but the thermal stability represented by polyvinyl alcohol is poor and cannot meet the application in some extreme environments.Therefore,it is of great significance to study and prepare aerogels with high temperature resistance,good toughness and high strength for the application.Polyimide(PI),as the material of the engineering plastic pyramid tip,has the reputation of "problem-solving expert".Its excellent properties such as high temperature resistance,corrosion resistance,radiation resistance,high mechanical strength,low thermal expansion coefficient,and low dielectric allow PI to be widely used in the fields of insulation,wearables,antenna substrates,and aero-power reducers.However,at this stage,PI products are mostly focused on the film field,and there are relatively few PI aerogels.Although PI aerogels have attracted more and more attention in recent years,and have been applied in some fields,they still have many shortcomings.For example,Rigid monomers and unknown process details lead to the PI aerogels brittle;lack of support in the layered PI aerogels,weak compressive strength and insufficient resilience;tedious and cumbersome construction processes make negative Poisson’s ratio polyimide aerogel extremely expensive;its poor thermal conductivity cannot meet the requirements of integrated circuit thermal management materials.In response to the above problems,this work starts from the three levels of molecule,process and network structure,and develops a series of high-performance PI aerogels,including high toughness PI aerogels,PI aerogels with high elasticity and high mechanical strength,ultra-negative Poisson’s ratio PI aerogels,and high thermal conductivity PI/boron nitride(BN)hybrid aerogel.The performance regulation and application expansion of PI aerogels are realized.The specific research contents are as follows:(1)Preparation of high-toughness polyimide aerogel and optimization of its process.The ultralight,high-strength and high-temperature resistance properties of PI aerogels have attracted much attention,but the rigidity of its monomers and unknown process details make the brittleness of PI aerogels confusing,which limits its scope of application.Molecular type,process determine the mechanical properties of PI aerogels.In order to find out the above influencing factors in detail and improve the toughness of PI aerogel,we started with the selection of monomers and analyze the influence of the flexibility of repeating units on the structure,energy dissipation coefficient,maximum stress and plastic deformation.Based on the optimal monomer selection,the process of PI aerogel was optimized in detail.Under the optimal conditions of PI molecular weight,TEA reaction amount,PAAC solution solid content and thermal imidization,through SEM observation of microstructure changes and cyclic compression tests,the conditions and rules for constructing low-cost,high-toughness PI aerogels were proposed.Among them,TPE and ODPA were polymerized at a molar ratio of 1:1.02 firstly,and then PAA and TEA were dissolved at a mass ratio of 1:0.5 to obtain a 5 wt.% PAAC aqueous solution,finally the obtained PI-TO aerogel after thermal imidization at 200 °C for 180 min had the best comprehensive mechanical properties.This study provides detailed design parameters and theoretical guidance for the construction of high-toughness PI aerogels.(2)Preparation of layered polyimide aerogels with high elasticity and high mechanical strength and their applications in harsh environments.PI aerogels possess various applications in harsh environment but are often limited by poor mechanical property.The ice template method to construct a layered aerogel has structural advantages,but there are still problems of insufficient compression recovery and anisotropy.We investigated the method of forming a spring-like bridge in layered structure,and verified the effect of bridges on the compression deformation,compressive strength,energy dissipation,and fatigue properties.Based on the structural observations,a corresponding force mechanism was proposed.After 10,000-cyclic compression tests at 50 % strain,PI aerogels had only 5.8 % plastic deformation,82 %stress retention and stable energy dissipation coefficient below 0.3.Interestingly,the compressive performance was isotropic in horizontal direction.Combined with low thermal conductivity,PI aerogels had high structural stability when repeatedly used at300 °C.This research may provide an available method for design of high-strength and high-toughness polymer aerogels suitable for harsh environment.(3)Preparation of ultranegative Poisson’s ratio polyimide aerogels and their impact-resistant applications.Although various developed materials with a negative Poisson’s ratio have been inspired by nature,the preparation of a negative-Poisson-ratio PI aerogel has been difficult because of its inherent structural limitations.These limitations have hindered the expansion of potential PI applications.We developed a tridirectional freezing method to prepare a superelastic PI aerogel with an ultranegative Poisson’s ratio.Through dynamic observation,the PI aerogel with a herringbone-tape frame structure exhibited an anisotropic negative Poisson’s ratio along three orthogonal axes.The maximum negative Poisson’s ratio was –0.79.Combined with a finiteelement simulation,the formation mechanism of the large ultranegative Poisson’s ratio was investigated.The special internal structure provided the PI aerogels with the high fatigue resistance under 10,000-cyclic compression at 50 % strain,and resilience under the strong impact of a metal ball.The intrinsic thermal stability of the PI provided the aerogel with a striking stability under cyclic compression at 130 °C or –100 °C ambient temperature.Our study provides a practical route for designing super-elastic,ultranegative-Poisson-ratio polymer aerogels for applications in strong-impact and high-or low-temperature environments.(4)Preparation of high thermal conductivity polyimide/boron nitride Hybrid aerogel and its application in chip heat dissipation.Low-dielectric,ultra-light PI aerogels are widely used in integrated circuit packaging materials.However,PI aerogels with low thermal conductivity cannot dissipate heat on high-power chips,which will accelerate chip aging and failure.Today,polymer hybrid materials with low dielectric and high thermal conductivity BN as fillers have significantly improved thermal conductivity.However,for PI mixed BN,the thermal conductivity is poor due to the difficulty of uniform mixing of the two and the random dispersion of BN.Therefore,there are few reports of highly thermally conductive PI/BN hybrid aerogels.This work constructed a top-down layered PI/BNNS mixed aerogel by a directional freezing method by using a uniform dispersion of water-soluble PAAC and BNNS.The BNNS nanosheets were uniformly distributed parallel to the layered structure,forming a high thermal conductivity path.By optimizing the doping amount of BNNS and combining with SEM structure observation,the formation mechanism of high thermal conductivity was proposed.At room temperature,the thermal conductivity of PI/BNNS-39.6 could reach 7.24 W/m·K.During chip application,the PI/BNNS hybrid aerogel exhibited a remarkable heat dissipation performance.This work provides a reliable route to construct high thermal conductivity polymer-based mixed thermal management materials. |
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