Fabrication And Performance Of Silicon Nitride Related Porous Ceramics And Their Co-continous Composites

To meet the urgent requirement for Si₃N₄ related porous ceramics and their co-continuous composites,Si₃N₄ foams,micro-porous Si₃N₄-SiC and sub micro-porous Si₃N₄-SiC were fabricated by using protein foaming method,reaction bonded method and sintered reaction bonded method,respectively.The properties of these Si₃N₄ related porous ceramics were also investigated.Furthermore,these Si₃N₄ related porous ceramics were used as the reinforcement to fabricate Si₃N₄/EP co-continuous composite,Si₃N₄/Al co-continuous composite and Si₃N₄-SiC/Al co-continuous composite,respectively.The mechanical properties and the thermal-physical properties of these three kinds of co-continuous composites were investigated.A planetary ball-milling foaming route was employed by the protein foaming method for preparing Si₃N₄ foams.In comparison with the commonly used tumbling foaming route of protein foaming method,pores of the Si₃N₄ foams fabricated by this new foaming route were more uniform in size distribution.Morever,the pores packed more compact and the open porosities were much higher.The speed of rotation had little influence on the porosity,open porosity,density and flexural strength of the as-prepared Si₃N₄ foams.Pore size in the Si₃N₄ foams foamed at different speed of rotation was statistically analyzed.It was indicated that the pore size in the Si₃N₄ foams foamed at different speed of rotation fitted well with the logarithmic normal distribution.The influences of protein addition,solid content and sintering temperature on the pore structure and properties of the Si₃N₄ foams were studied.It was indicated that the protein addition and sintering temperature had little influence on the porosity,open porosity,density and compressive strength of the Si₃N₄ foams.Moreover,the wall became more compact and the closed porosity decreased at increasing temperature.In comparison with the protein addition and sintering temperature,the influence of the solid content on these properties was more prominent.The influence of air fraction in the foamed slurry on the pore structure and properties the Si₃N₄ foams was investigated.The porosity of Si₃N₄ foams linearly ascent with the air fraction in foamed slurry.The closed porosity of Si₃N₄ foams also ascent with the air fraction in foamed slurry.Si₃N₄ foams with three dimensional interconnected pore structure were prepared by a combination of protein foaming method and sacrificial template method via using hydrophobic EP particles as the sacrificial template.The results indicated that the windows in the Si₃N₄ foams fabricated with hydrophobic pore formers were derived from these hydrophobic pore formers and the addition of hydrophobic pore formers significantly increased the amount of windows on the cell walls.Owing to there were numbers of windows on the wall,the Si₃N₄ foams fabricated with hydrophobic pore formers exhibited non-catastrophic failure during the compression test.The effect of PF on the nitriding process of silicon powder was investigated.Below1350?,the weight increasing ratio during the nitriding process increased with the PF addition.However,the increasing ratio during the nitriding process decreased with the PF addition above 1350?.It was speculated that the heat released by the reaction between the silicon and the carbon derived from the pyrolysis of PF promoted the nitridation of silicon below 1350?.The porosity and pore size distribution of micro-porous Si₃N₄-SiC could be changed via adjusting the PF addition.With the increase of PF addition,the open porosity and pore size of micro-porous Si₃N₄-SiC increased.The influence of sintering additive addition of the property of micro-porous Si₃N₄-SiC was investigated.With the increase of sintering additive from 0wt%to 9wt%,the nitriding ratio increased firstly and then decreased.The highest nitriding ratio was97.1%.The sintering additive addition had limited influence on the open porosity and density of micro-porous Si₃N₄-SiC.However,it could markedly improve the flexural strength of the micro-porous Si₃N₄-SiC.The influence of ZrC/TaC addition on the phase composition of the nitrided product was investigated.It was indicated that the addition of ZrC/TaC could significantly increased the?-Si₃N₄ ratio in the nitrided product.The influence of ZrC/TaC addition on the pore structure and the properties of submicro-porous Si₃N₄-SiC were also investigated.It was found that the mechanical properties of submicro-porous Si₃N₄-SiC could be increased via adding ZrC/TaC.Submicro-porous Si₃N₄-SiC with more uniform pore structure could be acquired by increasing the addition of ZrC/TaC.The element Zr and Ta in the submicro-porous Si₃N₄-SiC were in the form of face-centered cubic Zr-Ta-C phase.In addition,there was no amorphous interfacial phase between the Zr-Ta-C phase and the Si₃N₄ grain.In the Zr-Ta-C phase,the atomic ratio of element Zr decreased from the core to the edge,while the element Ta was contrary.Si₃N₄/EP co-continuous composite was fabricated by using the Si₃N₄ foams as reinforcement.Furthermore,the mechanical property,the thermal-physical property and the dielectric property were investigated.The thermal conductivities of those as-fabricated Si₃N₄/EP co-continuous composites were in the range of 4.921⁶.718W·m^-1·K^-1,which were mainly determined by the Si₃N₄ foams.The highest thermal conductivity was about 30.8 times higher than the neat epoxy.The flexural strength and flexural modulus of the Si₃N₄/EP co-continuous composites were in the of 54.3⁷6.3MPa and 25.5³4.0 GPa,respectively.The Si₃N₄/EP co-continuous composites exhibited non-brittle fracture mode during the bending test and the highest fracture toughness was 4.80 MPa·m^1/2.The dielectric constant of Si₃N₄/EP co-continuous composite was much higher than those of the neat epoxy,whereas the loss tangent was much lower than that of the neat epoxy.Si₃N₄/Al co-continuous composites were fabricated via using the Si₃N₄ foams as reinforcement and their thermal-physical property and mechanical properties were also investigated.The thermal conductivity of the Si₃N₄/Al co-continuous composite increased with the open porosity of Si₃N₄ foam and the highest thermal conductivity was 97.917 W·m^-1·K^-1,corresponding to the CTE of 15.3×10^-6?RT¹50??,which was lower than that of the neat Al matrix(22.6×10^-6).The flexural strength and the flexural modulus of the Si₃N₄/Al co-continuous composites were in the range of172.0²68.7MPa and 43.7⁷5.6 GPa,respectively.The Si₃N₄/Al co-continuous composites also exhibited non-brittle fracture mode during the bending test.Si₃N₄-SiC/Al co-continuous composites were fabricated via using the micro-porous Si₃N₄-SiC as reinforcement and their thermal-physical property and mechanical properties were investigated.The lowest CTE the Si₃N₄-SiC/Al co-continuous composites was 4.64×10^-6?RT¹50??,which was further lower than that of the neat Al matrix.The highest thermal conductivity of the Si₃N₄-SiC/Al co-continuous composite was 91.548 W·m^-1·K^-1.The flexural strength and the flexural modulus of the Si₃N₄-SiC/Al co-continuous composites were in the range of330.9³91.3 MPa and 112.4¹18.4 GPa,respectively.