Preparation And Acoustic Performance Of Sound Decoupling Porous Ceramics

For underwater vehicles,sound decoupling materials are important components that affect their invisibility and detectability.With the continuous improvement of navigation water depth,excellent acoustic performance and mechanical strength of sound decoupling materials are required.That is,the sound decoupling materials are required to possess lower density,sound velocity and acoustic impedance,as well as high sound absorption coefficient and compressive strength.In general,silicon nitride ceramics have high mechanical properties,while silica ceramics have excellent acoustic properties,making them candidates for sound decoupling materials.Generally,the acoustic properties of sound decoupling materials require ceramics to possess high porosity.The introduction of porous structures can effectively reduce the density of ceramics,thereby improving acoustic performance,but to a large extent,it will reduce mechanical strength.Therefore,it is necessary to adjust the pore structure during the preparation process of porous ceramics to balance the contradiction between the mechanical properties and acoustic properties of the porous ceramics.In this paper,silicon nitride and fused silica porous ceramics with higher porosity are prepared respectively,and the influence of experimental parameters on pore structure parameters is systematically studied.The effect of porosity,average pore size,pore morphology and other pore structure factors on the performance of sound decoupling porous ceramics is also systematically investigated.The short-chain surfactant n-propyl gallate（PG）was used to modify silicon nitride particles,and silicon nitride ceramic foams were prepared by direct foaming.The results showed that PG can adsorb on the surface of silicon nitride particles to make them hydrophobic,which benefit the formation and stability of the foams in the ceramic slurries,and the porosity and microstructures of the ceramic foams are tailored by adjusting the PG content,solid content and ball milling speed.It is found that the increase of porosity of ceramic foams prompt the decrease of flow resistance rate,which is conducive to the improvement of sound absorption performance.The preparation of the ceramic foams with the highest porosity of 96.4%is achieved,and the sound velocity and acoustic impedance are 1204 m/s and 0.1390·10⁶ kg/m²s,respectively,with an average sound absorption coefficient NRC of 0.622.In order to further improve the integrity of the pore structure,a mixture of long-chain surfactant was used to modify the hydrophobicity of silicon nitride particles.The results showed that:cetyltrimethylammonium bromide（CTAB）and sodium lauroyl sarcosinate（SLS）behave differently in the adsorption on ceramic particles.By adjusting the ratio of CTAB/SLS,different microstructures of silicon nitride ceramics can be achieved.It is found that when the same amount of CTAB/SLS is added,the ceramic foams have complete pore walls,uniform pores,and smallest pore size,therefore,compared with short-chain surfactants,silicon nitride ceramic foams have higher mechanical properties,and the porosity of 92.1%-96.8%prepared by adjusting the experimental parameters.Silicon nitride ceramic foams owns compressive strength of 4.85 MPa-0.86 MPa,NRC of 0.436-0.647,sound velocity of 1615 m/s-1162 m/s,and acoustic impedance of less than 0.4·10⁶ kg/m²s.In order to realize the adjustment of the pore size of the ceramic foams.silicon nitride ceramic foams with similar porosity and different pore diameters were prepared,by adjusting the chain length,content and powder particle size of the surfactant in the slurry.The results show that:the viscosity and surface tension of the material are the key factors to control the pore diameter.The average pore diameter and the ratio of the surface tension of the slurry to the viscosity are linear related,and it is found that the reduce of the ceramic foams pore diameter reduces the stress concentration and enhances the resonance effect of the pores,which can be beneficial to the improvement of sound absorption performance and compressive strength at the same time,and has little effect on its acoustic impedance.When the minimum pore size with a porosity of94%reaches 113μm,its compressive strength reaches 3.51 MPa and the average sound absorption coefficient NRC reaches 0.673.The pore wall morphology of the foam ceramic was modified,and the results showed that the sintering aids type,sintering aids content and powder particle size gradation in the powder raw material was adjusted to achieve the design of pore wall of the silicon nitride ceramic foams.The length,aspect ratio,mesophase and grain gradation ofβ-Si₃N₄ grains are tailored,and it is found that the increase in the length and aspect ratio ofβ-Si₃N₄ grains and the bimodal distribution of grains are very conducive to the mechanical properties.After the pore wall of the foam ceramic is modified by resin and carburizing,both of them can effectively improve the mechanical properties of the foam ceramic on the basis of a small reduction in the acoustic performance.In general,the overall performance of ceramic foams prepared by carburizing the pore wall is better.When the density of the final foam ceramic is 0.288g/cm3,its compressive strength reaches 4.97 MPa,the sound velocity reaches 1422 m/s,the acoustic impedance is 0.399·10⁶ kg/m²s,and the NRC is 0.471.The fused silica particles are hydrophobic modified with a composite long-chain surfactant,silicon nitride is used as sintering aid,and direct foaming method is used to prepare high-porosity fused silica ceramic foams.The results show that silicon nitride can inhibit the crystallization of fused silica and promote the sintering and densification of its pore walls.By controlling of the solid content and surfactant content,the porosity and average pore size of the fused silica ceramic foams can be tailored.And the lowest sound velocity and acoustic impedance of the ceramic foams reaches 1039 m/s and0.198·10⁶ kg/m²s,respectively.At the same time,it is found that compared with silicon nitride ceramic foams,fused silica ceramic foams have comparable comprehensive properties at a lower porosity.performance.Using tert-butanol（TBA）as the freezing vehicle,fused silica porous ceramics were prepared by directional freeze-drying method.The results show that the directional pore diameter and porosity fused silica porous ceramics can be controlled by adjusting the freezing temperature and solid content.The porosity of the ceramic is78.2%-84.59%,the average pore diameter range is 8.4μm-64.2μm,and the corresponding compressive strength range is 5.51 MPa-1.38 MPa;the lowest sound velocity and acoustic impedance reached 1401m/s and 0.481·10⁶ kg/m²s,respectively.At the same time,it is found that under the same porosity,compared with ceramic foams.oriented porous ceramics have better sound absorption performance.