| In this work, ceramic hollow sphere structures(CHSS), a novel porous sound absorbing material, were studied for noise reduction in the cabin of space station, which should satisfy lightweight, environmentally friendly, fire-retardant and broadband sound absorption properties. However, common porous materials could not fulfil the above requirements. Moreover, the sound absorption of porous materials is generally poor at low frequency band. In present work, the sound absorotion characteristics of CHSS materials and acoustical structures were studied to improve the sound absorption at low frequency band.Geometric model of hollow sphere structures, fabricated by bonding methods, was built. Moreover, the optimal open pore structures for broadband sound absorption were designed as follows: open cell sizes d were 50-200 μm, open porosity was 50%, bonding angles were 20o-30o. The porous materials, with 100 μm open cell size, had good sound absorption at low frequency band, which were discussed by theory. The diameter distributions 2R of hollow spheres were approximately 100-400 μm, by which the above open pore structures were achieved. The CHSS materials were fabricated using fly ashes and aluminum phosphate adhesive throug invented pre-bonding and curing process.The compositions and interfacial microstructure of CHSS were studied. It was observed by SEM that the porous aluminum phosphate coating with approximate 10 μm thickness was formed on the surface of fly ashes. Neck regions between fly ashes were generated by aluminum phosphate adhesive, and the CHSS was formed as the connection of fly ashes through the neck regions. The XRD results showed that the main phase structures of fly ashes were aluminosilicate glass phase, mullite crystal and quartz crystal, the main phase structures of aluminum phosphate adhesi ve after curing were glass aluminum phosphate polymer and α-Al2O3 crystal, and no new crystal phases were generated during the formation of CHSS. Furthermore, it was characterized by TEM that compact and smooth interfacial microstructures between fly ashes and aluminum phosphate were formed, and element diffusion s at the interface regions were observed, therefore, the interfacial bonding was not simple physical or mechnical bonding. Similar to the cellular materials, three typical regions were observed in the compressive process of CHSS: elastic region, plateau region and fracture region. In the elastic region, the peak stress was approximately 5.8 MPa with strain of 0.6% and the elastic modulus was 1.25 GPa. Furthermore, in the plateau region, the stress increased by 9%, and the strain range was 0.6%-4%, showing excellent energy absorption characteristics.Finally, in the fracture region, it was showed that the stress declined gradually, not collapsed.The characterizations of pore structures of CHSS were studied through multiple methods. The 3-D and semi-quantitative characterizations of closed pore structures of CHSS were obtained by combination of μ-CT and laser confocal microscopy. The closed cell sizes were approximately 140-220 μm, and followed near normal distribution and bimodal distribution. Moreover, it was observed that fly ashes distributed randomly in CHSS. The visualization and quantitative characterizations of open pore structures of CHSS on micro-scale and nano-scale were conducted by combination of SEM, TEM and MIP. The results showed that the size distributions of micro-sized open cell followed near normal distribution and bimodal distribution, and the peak values were 100 μm and 10 μm, respectively. The porosity of micro-sized open cell was 48% approximately, which was amended by the factor c=0.1 in the geometric model. The pore structure factors χ decreased as the increasing open cell sizes. The experimental results agreed with the calculation results based on the geometric model, indicating that the open pore structures of CHSS could be designed easily. The nano-scale open cells were formed in aluminum phosphate polymer. The size distributions of nano-scale open cell were approximately 10-100 nm, and followed near normal distribution. The CHSS materials were porous material with high flow resistivity, which could achieve good absorption properties on conditions of lightweight and thin(10 mm and 20 mm mainly) samples. Moreover, the sound absorption properties of CHSS were greater than typical rigid porous materials: aluminum foam, and similar grade with the fiber porous materials and polymer foams. The influences of pore structure parameters on the sound absorption of CHSS were discussed. It is showed that the propagation constant k and characteristic impedance Zc of CHSS could be tailored easily. The improvement of poor sound absorption of porous materials at low frequency band would be achieved through acoustic reactance ratio x, however, the acoustic resistance ratio r had little influence. The effects of open porosity of CHSS on its sound absorption were remarkable at high frequency band, however, were small at low frequency band. The obvious size effects of open cell on s ound absorption of CHSS were observed, and the sound absorption at low frequency band could be improved through decreasing the open size. Especially, outstanding sound absorption at low frequency for porous materials with open cell size of 100 μm was verified through experimental and calculation results.The semi-phenomenological sound absorption model of CHSS with randomly packed hollow spheres was built, and the sound absorption mechanisms were studied. On microscopic scale, the semi-phenomenological equations of effective density r were obtained to represent the viscous effect through Pride model, and those of bulk modulus K were obtained to represent the thermal effect through Allard model, basing on the open pore structures of CHSS. The sound absorption mechanisms of CHSS were viscous effect and thermal effect in the 3-D connected open pores. On macroscopic scale, the CHSS materials with open porosity f were equivalent to the free fluid with effective density r f and bulk modulus Kf. Therefore, the semi-phenomenological sound absorption model of CHSS was obtained through the combination of microscopic mechanisms and macroscopic open pore parameters. The open pore parameters were revised. The calculated values and tested results agreed well, which validated the accuracy of emi-phenomenological model.The sound absorption characteristics of acoustical impedance gradient structure nearly agreed with that of CHSS, and improved approximately 3% or 6%. The perforated plate structures were fabricared by the combination of perforated CHSS facing with 2 mm diameter through pores and air cavity. The perforated CHSS facing increased the resistance induced by acou stic deformation layer, and the air cavity decreased the resistance. Therefore, the sound absorption coefficient increased by approximate 43.8% at frequency band of 1500-4000 Hz, however, little effect at low frequency band. The micro-perforated plate structures were fabricared by the combination of CHSS facing and air cavity. The perforated CHSS facing greatly increased the resistance, and the air cavity decreased the resistance. Therefore, the sound absorption coefficient increased by approximate 110% at low and medium frequency band of 100-2500 Hz, which was the optimal sound absorption structure. Moreover, the machining operation problem of normal micro-perforated plate structures was solved. The acoustical honeycomb structure was studied with facing pla te of CHSS and NOMEX honeycomb core. The curve of sound transmission loss of honeycomb structure could be divided into three stages: elastic stage, resonance stage and mass stage. The influence factors were stiffness, damping and mass. The noise reduction performance could be improved as the increasing core thickness. The sound transmission loss was 12 d B when the core thickness was 15 mm. |
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