| Due to their special pore structure and outstanding high temperature and chemicalstability, ceramic foams have great potentials in the applications of catalyst supports,hot gas filters, biomaterials and sound absorbing materials. Based on the demand onsound absorbing materials used in high temperature conditions, Si3N4foams wereprepared and their acoustical properties were investigated. Firstly, the preparationprocess of protein foaming method were optimized. Secondly, Si3N4foams wereprepared using egg white protein, whey protein isolate, a mixture of them or a mixtureof egg white protein and fish coallegn. Furthermore, the pore structures and propertiesof Si3N4foams were studied. Then SiC nanowires and CNTs (carbon nanotubes) weregrown in the foams and their morphologies and microstructure were studied. Finally, thesound absorption properties of the ceramic foams before and after the introduction ofCNTs or SiC nanowires were investigated.A special mould was designed for the consolidation of foamed slurry, in which airpressure could be controlled. By using this special mould, the influence of air pressureduring consolidation process on pore structures and properties of Si3N4foams werestudied. With the increase of air pressure, thickness of the wall decreased. Openporosity and average pore size peaked at78.6%and210μm, respectively, when foamedslurry consolidated at “constant-pressure (103.997kPa)”. A novel consolidation processwas also designed, by which the formation of cracks during the consolidation of foamedslurry could be avoided. Moreover, superiority of this novel process in fabricatingceramic foams of complex shape had been confirmed. Protein burnout temperature wasset to600℃according to DSC-TGA curve of egg white protein.When using egg white protein as foaming agent, influences of protein addition andsolid content on pore structures and properties of ceramic foams were investigated.With the increase of protein addition, open porosities slightly decreased, while densitiesincreased. With the increase of solid content, densities and compressive strengthincreased, while open porosities decreased. The combinational use of egg white proteinand fish collagen resulted in dramatic increase in average pore size, compared withsingle use of egg white protein.Layered structure was only found in ceramic foam prepared by whey proteinisolate. Upper part of ceramic foam prepared by whey protein isolate was foam, whilethe bottom was a dense part. Pores in the foam part were generally open, with bigwindows on the wall. However, pores of ceramic foam prepared by egg white protein were generally closed and sizes of them were evidently smaller than that of wheyprotein isolate. Open porosity of the foam part in ceramic foam prepared by wheyprotein isolate were evidently higher than that of egg white protein, and density andcompressive strength of the former were significantly lower than the latter one.Thickness of the dense parts decreased with the increasing ratio of egg white proteinaddition. Moreover, size and number of windows on the wall also decreased, whichresulted in decreasing open porosity and increasing density and compressive strength.When addition ratio of egg white protein and whey protein isolate was constant, withthe increase of solid content, size and count of windows on the wall decreased, whichresulted in decreasing open porosity and increasing density and compressive strength.With the increase of consolidation temperature, both size and count of windows on thewall increased, while shape of pore became irregular. Open porosity showed no evidentchange, while compressive strength indicated a significant change. Compressivestrength of ceramic foam consolidated at70℃was the highest.CNTs were grown in Si3N4foams by CVD (Chemical Vapor Deposition) methodusing C2H2as carbon source. When flowing rate of C2H2was high, ceramic foams wassplit after the growth of CNTs. CNTs grown at C2H2flowing ratio of20sccm exhibitedthe best appearance. Diameters of CNTs largely depended on the size of catalysts.CNTs prepared by small catalyst were generally in small diameters, and carbon atomson the walls were generally in ordered array. However, diameters of CNTs prepared bylarge catalyst were also large, and carbon atoms on the wall were generally indisordered array.SiC nanowires were successfully grown in Si3N4foams by PCS pyrolysis routewithout using catalyst and PCS solution infiltration step. The products of PCS after theheat treatment at300℃,400℃,500℃and600℃were used to grow SiC nanowires inSi3N4foam. It was indicated that the small molecules released before600℃during thepyrolysis of PCS were crucial to the growth of SiC nanowire. The as-prepared SiCnanowire with a fluctuating diameter along the length direction was grown along the<111> direction, and a layer of amorphous SiO2shell was found on the surface of it.Moreover, some nano-crystals were found in this amorphous SiO2shell, which had thesame crystal lattice space with that of SiC nanowire. SiC nano-crystal was found tohave some joints with a SiC nanowire, and tended to grow to be a part of the SiCnanowire, suggesting that this amorphous SiO2shell promoted the radial growth of SiCnanowire.Ceramic foams prepared in this paper showed excellent sound absorption properties. With the increase of the frequency, sound absorption coefficients of theceramic foams fluctuated slightly. From the contrast studies on sound absorptioncoefficients of the prepared foams, high porosity was found to be essential for highsound absorption properties, while size and count of windows on the walls also had aninfluence. The growth of SiC nanowires resulted in significant decrease of soundabsorption properties. However, the growth of CNTs only led to slight decrease ofsound absorption coefficient. |
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