Fabrication Of Al₂O₃ Matrix Porous Graded Ceramics By Precursor Pyrolysis

Porous graded ceramics is one of important porous ceramics. It is also one of functionally graded materials (FGM), which with graded porous structures and is required for several applications, such as insulation materials, piezoelectric materials, and biomaterials. However, it is difficult to fabricate porous graded ceramics which limit its wide application.Firstly, in this thesis, fabrication of Al2O3 matrix porous ceramic by precursor pyrolysis was studied. The effects of silicon resin content, pyrolysis temperature, shaping pressure on characterizations of as-fabricated porous ceramics were investigated. The structures and properties of as-fabricated porous ceramics were tested and the reasons were analyzed. On this basis, fabrication of high porosity Al2O3 matrix porous ceramic by precursor pyrolysis with PMMA as pore-forming agent was studied. The effects of pyrolysis temperature on characterizations of as-fabricated porous ceramics were investigated. The relation between PMMA content and structure and properties of high porosity Al2O3 matrix porous ceramic was studied and analysis. Finally, fabrication of Al2O3 matrix porous graded ceramic with silicone as binder by laminating process was studied, combining with XRD and SEM to analyze the phase and microstructure. The thermal properties of Al2O3 matrix porous graded ceramic were analysis.Through studing the different porosity of Al2O3 matrix porous ceramic by precursor pyrolysis, successfully fabricating Al2O3 matrix porous ceramic with open porosity changing from 33% to 74%. The results showed that:the Si-O-C content increased with the increasing of the pyrolysis temperature. The amorphous Si-O-C filled into the pores between Al2O3 particles, which enhanced the bending strength. Because of the amorphous Si-O-C filled and the volume of gas increasing with pyrolysis, the open porosity was first increased and the decreased. When pyrolysis temperature was 1100℃, the amorphous Si-O-C structure was achieved to the most stable state with 33% porosity and higher bending strength. When pyrolysis temperature was 1200℃, the amorphous Si-O-C structure was destroyed. The SiC and SiO2 crystals began to precipitate.The structures and properties were only related to SR content and pyrolysis temperature, when not to add pore-forming agent. The amorphous Si-O-C content was increasing with SR volume content. When the SR volume content was 60vol.%, the porosity up to 33% and the bending strength up to 94MPa which are all the highest. Because the Al2O3 particles were dispersively distributed in the amorphous Si-O-C ceramics to enhanced bending strength to a maximum. The addition of PMMA significantly increased porosity. The highest porosity of Al2O3 matrix porous ceramic was 74%, when the maximum content of PMMA was 60vol.%. Because of the poro structure, compressive strength was index decreasing with porosity increasing. The thermal conductivity was rised with the porosity. When porosity was more than 58%, the thermal conductivity of Al2O3 matrix porous ceramic was less than 0.7W·(m·k)-1, which can meet insulation requirements.Successfully fabricating Al2O3 matrix porous graded ceramic with silicone as binder by laminating process, whose porosity changed from 33% to 74%. The layers were binded in good condition with no cracks. Al2O3 matrix porous graded ceramic overcome the contradiction between porosity and compressive strength and showed good thermal performance. The thermal conductivity was only 0.6 W·(m·k)-1 by calculating.