| Noise pollution endangers human health and the environment globally;especially low-and middle-frequency noise can induce a wide range of influence and great actual harm and is difficult to control using sound-absorbing materials.Currently,the use of porous materials to absorb sound is effective for controlling noise pollution.Porous ceramic materials offer many advantages,such as good weather resistance,high fire resistance,safety,nontoxicity,and long service life.They also possess great potential for application in the field of noise pollution control.However,the sound absorption performance of porous ceramic materials is lower than that of other porous sound-absorbing materials（especially in the middle-and low-frequency range）.This is the main reason why it is difficulty to use widely.Therefore,it is important to use the advantages of ceramics to create porous sound-absorbing ceramics with high sound absorption performance in the low-and middle-frequency.In this paper,considering of designing internal pore structure to improve the noise reduction coefficient of porous ceramics,the high-performance sound absorption ceramics were fabricated and the effect of internal pore structure on noise reduction performance of porous ceramics was analyzed.In order to strengthen the low frequency sound absorption effect and extend the sound absorption mechanism to improve the low-frequency sound absorption performance of porous ceramics without reducing the noise reduction coefficient,Mg2B2O5 whisker porous ceramics,graphene oxide/styrene-butadiene rubber（GO/SBR）porous composite materials,and reduced graphene oxide/polyvinyl alcohol（rGO/PVA）porous composite sound-absorbing ceramics were designed and fabricated by in situ whisker growth or introducing GO composites.Further,the properties and sound absorption mechanism of the porous composites were systematically analyzed.By combining the twice-foaming and pore-forming agent methods,porous ceramic skeleton structures with highly connected pore was successfully prepared using the gel-casting method.The pore structures were characterized and the results demonstrated that the porosity of porous ceramics was more than 80%,and the pore size exhibited a multimodal distribution from the nanometer to the millimeter level although they were mainly concentrated at the submillimeter level.The connectivity was more than 99%,and the thermal conductivity was in the range of 0.0931 to 0.1081 W/m·K.Based on the characterization parameters of the porous ceramics,the effects of porosity,average pore size,standard deviation of the pore size,and sample thickness on the sound absorption performance of porous ceramics were simulated using finite element analysis for further guidance in their pore structure design.The standing wave tube method was used to testing the sound absorption performance of porous ceramics.The results demonstrated that the porous ceramics exhibited broadband sound absorption performance.The maximum sound absorption coefficient reached 1 at 1000 Hz,the maximum noise reduction coefficient was 0.67,and the maximum sound absorption frequency was 630Hz,which was better than that of commercial polyurethane sound-absorbing materials.In addition,when a 50-mm cavity was created at the back of the porous ceramic,the sound-absorption coefficient exceeded 0.73 in the frequency range of 250-800 Hz,and the noise reduction coefficient reached 0.79.Magnesium borate whisker was in-situ grown in the porous ceramic matrix using vacuum-impregnation and high-temperature solid-phase synthesis methods.The effects of firing temperature and holding time on the growth of the magnesium-borate whiskers and the whisker-growth mechanism were analyzed using multiple characterization methods.The results demonstrated that magnesium borate whiskers with a large length-to-diameter ratio and uniform distribution were successfully grown in situ on the matrix pore wall after holding at 800°C for6 h.Compared with the porous ceramic matrix,the fluid-solid interface of the whisker porous ceramics increased,and whiskers caused more reflection,refraction,and scattering of sound waves in the porous ceramic matrix,thereby resulting in an enhanced thermal-viscosity effect.The average sound absorption coefficient increased by 28.90%from 200 to 2000 Hz and 81.88%from 200 to 800 Hz.The sound absorption curve and peak values relocated to the low frequency.Further,the compressive strength of the porous ceramic matrix with 85.5%porosity increased from 1.33±0.13 MPa to 3.50±0.63 MPa,and the maximum of the thermal conductivity increased 56.26%.To further improve the low-frequency noise reduction performance,the enhancement effect was investigated by introducing rubber matrix composites into the porous ceramic matrix.From the interface interaction between graphene oxide（GO）and styrene-butadiene rubber（SBR）,the GO sheet dispersed stably in rubber latex,and the tensile properties and thermal conductivity of the rubber were improved to a certain extent.A GO/SBR composite film was coated on the pore wall of the porous ceramic using the atmospheric vulcanization method.The thermal conductivity and compressive strength of the porous ceramic matrix increased slightly,but the low-frequency sound absorption coefficient increased considerably.The average sound absorption coefficient increased by 15.4%in the 200-2000 Hz range and 30.4%in the 200-800Hz range.The overall sound absorption curve moved to the low-frequency range.The sound absorption performance improved mainly because of the transmission path of the sound wave became tortuous after the film coating and sound wave became more dissipated during the transmission process.The enhancement in the damping effect after surface modification was also a reason for the improved average sound absorption coefficient.An rGO/polyvinyl alcohol（PVA）（rGO/PVA）composite porous material with a noise reduction coefficient of 0.59 and thermal conductivity of as low as 0.024 W/m·K was prepared using the freeze-drying method.By adjusting the concentration of the rGO/PVA suspension and rGO content,the rGO/PVA network structure further modified the pore structure of the porous ceramic matrix,and rGO/PVA porous composite ceramics were obtained.Compared with those of the porous ceramic matrix,the average sound absorption coefficients of the rGO/PVA porous composite ceramics increased by 102.3%and 127.2%in the ranges of 200-2000 Hz and 200-1000 Hz,respectively.The reflection,refraction,and scattering of the sound waves in the rGO/PVA network structure,as well as its own vibration,made the 15-mm rGO/PVA porous composite ceramics achieve noise reduction performance for a 25-mm thick porous ceramic matrix,and the low-frequency sound absorption performance was better than that of 25-mm porous ceramics.In addition,the maximum thermal conductivity of composite porous ceramics was 0.0960 W/m·K,which exerted a certain effect on the heat insulation. |
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