Controllable Construction Of Ultrafine Fiber-Based Elastic Sponges For Noise Absorption

With the rapid development of transportation and industry,the resulting environmental noise problem has become increasingly serious,making it one of the three major environmental pollution problems together with air pollution and water pollution.The harm of noise pollution is very serious,which can damage people’s hearing nerves,causing neurasthenia,cardiovascular/cerebrovascular diseases,deafness,and even lead to death.Therefore,effective control of noise pollution has become a worldwide problem that needs to be solved urgently.Fiber sound-absorbing materials have the advantages of high channel tortuosity and good channel connectivity,showing good high-frequency sound-absorbing performance,and have become the most widely used sound-absorbing materials.However,the existing microfiber sound-absorbing materials generally have the disadvantages of large diameter and low porosity,which make them poor at absorbing low-frequency sound waves with strong penetrating power.Compared with traditional microfibers,electrospun fibers with the features of small diameter,small pore size,and high specific surface area,have shown significantly improved sound absorption performance.However,due to the anisotropic deposition properties,electrospun fibers usually assemble into densely packed 2D membrane structures,which greatly shortens the dissipation paths of acoustic energy.Newly developed electrospun three-dimensional（3D）fiber material combines the characteristics of high porosity,controllable thickness,large specific surface area,and high channel tortuosity,showing great application advantages in the field of noise reduction.However,due to the single,uncontrollable,and highly connected macroporous structure,the existing 3D fiber materials still face the bottleneck problems of insufficient low-frequency sound absorption and poor mechanical properties,thus materials would be damaged when subjected to vibration and external force during use.Therefore,the accurate design of the microstructure of 3D fiber materials aiming at the features of different frequency noise has important guiding significance for improving the mechanical and noise-absorbing properties of materials.In this article,aiming at the bottleneck problem of existing porous sound-absorbing materials and according to the specific characteristics of noise in different frequencies,we prepared dual-network fiber sponges with good low-frequency sound absorption performance by humidity-assisted blend electrospinning technology.On this basis,a new type of composite fiber sponge with nanosheet vibration structures was prepared in one step by combining humidity-assisted electrospinning and electrospray technology.Furthermore,nanofiber-based sponges with hierarchical fine structures were prepared by combining foaming and freeze-drying techniques;and the structure-activity relationships between porous sound-absorbing structure/resonant sound-absorbing structure of nanofiber sponges and their noise absorption performance in different frequencies were analyzed.Meanwhile,the resonance absorption,vibration dissipation,and friction dissipation mechanism of fiber sponges were investigated,which provided a theoretical and practical basis for the design and development of novel high-efficiency sound absorption materials.The specific research work is summarized as follows:（1）In view of the poor low-frequency sound absorption performance of existing fiber materials,dual-network fiber sponges with PVDF nanofibers and PSU microfibers were prepared by using humidity-assisted blend electrospinning technology and in situ cross-linking method.The fluffy mechanism of fiber sponges based on phase separation was analyzed.Then,the effect of crosslinking agent content on the mechanical properties of composite fiber sponges was investigated,and the low-frequency sound absorption performance of fiber sponges was explored.The research results show that the fiber sponges could withstand the tensile force 10,000 times its weight without deformation,and there was almost no plastic deformation after 100 times of compression at 60%strain.Moreover,the good fluffiness of the PSU microfiber network and the high acoustic wave contact area of the PVDF nanofiber network endow fiber sponges with excellent low-frequency noise reduction performance,their sound absorption coefficient at 1000 Hz was up to 0.93（noise reduction coefficient of 0.49）,which is significant better than most existing sound absorption materials,showing broad application prospects in the field of traffic noise absorption.（2）To improve the low-frequency sound absorption of materials while maintaining the high-frequency sound absorption performance of materials,by regulating the synchronous occurrence of humidity-assisted electrospinning and electrospray,ultrathin GO nanosheets were in situ introduced into PSU/PU fiber sponges,and composite fiber sponges with thin sheet vibration structures were prepared in one step.The influence of ultrathin nanosheets on the vibration structure of the sponge was studied,and the effect of the thin sheet vibration structures on the mechanical and sound absorption properties of the fiber sponge was further investigated.The results show that the physical entanglement between the nanosheets and the fibers enabled fiber sponges to withstand 6000 times their weight in the tensile stress without deformation,which also can withstand 1000 compression and cyclic shear without damage.Moreover,thanks to the vibration effect of ultrathin GO nanosheets and the viscous friction effect of porous fiber network,the obtained composite sponges both have good low-frequency and high-frequency noise absorption performance.Furthermore,the noise reduction coefficient of the fiber sponge was as high as 0.52 when the volume density was only 7.00 mg cm^-3,which shows a huge application advantage in the field of noise absorption.（3）Aiming at the problem that fiber porous materials are easy to transmit sound waves,flexible GO/Si O₂ nanofiber sponges with hierarchical entangled structures were prepared by directional freeze-drying technology,taking flexible Si O₂ nanofibers as the building unit,taking GO nanosheet as the cross-linking agent and pore blocking agent.The influence of the ratio of nanofibers and nanosheets on the sound absorption and mechanical properties of sponges was analyzed.The research shows that the prepared composite fiber sponge exhibited excellent mechanical stability only under physical crosslinking,which could bend 1000 times under 60%large strain without damage,and the plastic deformation was only 4.3%after 1000 times of compression.Moreover,the resulting sponge could maintain temperature-independent stability in the range of–100 to 500°C.On this basis,the sandwich structure was further constructed in the thickness direction of the sponge to achieve ultrawide band noise absorption,which had high-efficiency sound absorption performance（noise reduction coefficient reached 0.56）while maintaining lightweight(9.33 mg cm^-3),and they could reduce automobile engine noise from 95.5 d B to 76.1 d B,possessing great practical value in traffic noise absorption.（4）To effectively absorb the high-temperature noise of aero-engine,flexible ceramic nanofiber sponges with cascaded resonant cavities were prepared by in situ introducing microbubble templates into the dispersion using freeze-casting and calcination techniques,taking Si O₂ nanofibers and montmorillonite nanosheets as the building blocks and silica sol as the nano glue.The effects of foaming parameters on the internal cavity structure of the nanofiber sponges were studied,and the strengthening mechanism of mutually hinged resonant cavities on the mechanical and sound absorption properties of the nanofiber sponges was explored.The results show that the stably hinged cavity structure enables the sponges to withstand 1000 bending cycles and 1000 compression cycles while maintaining their structural integrity.Benefiting from the cascaded resonant cavity structure and the interconnected nanofiber porous structure,the sponges exhibited three obvious resonance peaks in the range of 63–6300 Hz,and the noise reduction coefficient was as high as 0.66.In addition,the obtained sponges could effectively absorb noise at high temperatures up to 1100°C,showing great application potential in the field of high-temperature noise reduction.