Elastic And Strong Zirconia Nanofibrous Based Aerogels: Construction And Thermal Insulation Application

Ceramic aerogels have become attractive materials due to the advantages of low density,large specific surface area,and low thermal conductivity,which with great application potential in the fields of thermal insulation,catalyst carriers,and filters.However,traditional ceramic aerogels consist of a pearl necklace-like network structure,which has inherent brittleness and unstable temperature resistance,have been limited in the practical applications.Prior efforts on improving the mechanical properties of aerogels,including chemical cross-linking with reactive molecules or polymers and reinforcement by one-dimensional fibers or two-dimensional networks,have enhanced their mechanical properties.But the organic components would significantly reduce the temperature resistance,and the weak interaction between the reinforcing fibers and aerogel would lead to the release of dust particles.In recent years,researchers have developed a series of methods to prepare elastic ceramic aerogels using one-dimensional nanofibers as raw materials,including direct spinning,template,layer-by-layer stacking and freeze-drying.But the poor oxidation resistance of non-oxide ceramics and the crystal embrittlement at high temperature of amorphous ceramics have obstruct their application at extreme temperatures.Moreover,the fiber reinforced aerogels are difficult to recover or bend,while elastic ceramic fiber aerogels are usually too weak,which cannot resist strong mechanical or thermal shocks.Therefore,the preparation of ceramic nanofibrous aerogels with good elasticity and mechanical strength at the same time is of great significance for their applications in extreme environments such as thermal insulation.In this paper,we have carried out a series of researches on the theme of elastic construction of Zr O₂-based nanofibrous aerogels and the improvement of mechanical and thermal insulation properties.Firstly,flexible Zr O₂-based nanofiber membranes was prepared by sol-gel and electrospinning,which will be used as building blocks for Zr O₂-based nanofibrous aerogels.We designed a structure with layer multi-arched microstructure with a face-to-face mode to support the stress,endowing the final Zr O₂-based nanofiber aerogels with excellent elasticity and compressive strength.The energy loss mechanism of the aerogel was further studied,and its fatigue resistance was significantly improved.On this basis,the nanofiber-granular aerogels binary synergistic structure was obtained by introducing the granular aerogels with nano-scale pore size,which significantly improved its thermal insulation performance.The main study results achieved are summarized as follows:（1）By combining sol-gel and electrospinning methods to introduce inorganic components into crystalline Zr O₂ nanofibers,Zr O₂-based nanofiber membranes with good flexibility and mechanical properties were prepared.Through adjusting the ratio of precursor solution and the calcination temperatures,Zr O₂-Si O₂ nanofiber membranes with significantly improved toughness(324J m^-3)maintaining good flexibility,which is rarely reported at present.And the as prepared Zr O₂-based nanofiber membrane can maintain its excellent resilience even in high temperature flame,which will recover automatically after being bent in flame.In the quantitative buckling cycle tests,90%of the original stress has been maintained after 500 buckling cycles at 50%buckling strain.（2）Distinguish from the typical preparation approaches of ceramic fibrous aerogels,our synthetic strategy with the simplicity and efficiency to fabricate elastic ceramic aerogels by combining impregnation stacking and freeze-drying.We have fabricated elastic and strong Zr O₂-Al₂O₃nanofibrous aerogel by combining the flexible Zr O₂-Al₂O₃ nanofiber membranes with a high temperature resistant Al（H₂PO₄）₃ matrix.The result aerogels exhibit lamellar multi-arch microstructures,which convert the point-to-point contact force mode between single nanofibers into the face-to-face mode between the layers.The results show that the formation of the arched cells of the elastic geometric structure endows the aerogel with excellent elasticity and high compressive stress simultaneously.The large compression recovery strain is up to 90%,and the stress reaches 1100k Pa,which can withstand more than 60,000 times of its own weight.Moreover,the aerogels presented good cyclic compression resilience（12.5%irreversible deformation after 1000 cycles）,high temperature resistance and mechanical stability at extreme temperatures（-196～1100°C）.In addition,the maximum operating temperature of the aerogels is as high as 1300°C.（3）The lamellar multi-arched structure Zr O₂-Si O₂nanofibrous aerogels with thinner nanofibrous cell walls were designed and fabricated by regulating the ultrasonic-assisted freeze forming method,which facilitate the nanofiber layer fluffy and maintain fibrous networks.The results show that the method can significantly reduce the bulk density and increase the porosity of the aerogel while ensuring high compressive performance.The fatigue resistance is significantly improved with the plastic deformation of almost zero after 1000 cycles,and the energy loss coefficient during the compression recovery process is as low as 0.28,which is rarely reported in the existing aerogel materials.In addition,the aerogels exhibit a low thermal conductivity of 0.027 W m^-1 K^-1,indicating its promising application as a high-temperature thermal insulation material.（4）We designed and synthesized a nanofibrous-granular binary synergistic structured all-ceramic aerogel by using hierarchical cellular structured Zr O2-Si O2nanofibrous network as the supporting frame and silica granular aerogels as the filler and the silicon/boron sol as the"nano glue"binder.Utilizing the lamellar multi-arched structure and nanoscale pores of silica aerogels,a combination of excellent mechanical properties and thermal insulation was obtained.The as prepared aerogels show the characteristics of light weight（23 mg/cm~3）,superelasticity and excellent fatigue resistance,only 1.2%plastic deformation occurs after 1000 compression-recovery.And the aerogels can bear the weight greater than 8000 times its own weight for up to 24h and return to the original shape.Moreover,stable dynamic mechanical properties can still be maintained in the temperature range of-100～500°C of the aerogels.The nanofibrous-granular binary synergistic structure can not only maintain the excellent mechanical properties,but also endow the composite aerogels low thermal conductivity(0.024 W m^-1 K^-1)and exhibits excellent thermal insulation performance.