Fabrication And Property Research Of Porous Composites With Low Thermal Expansion And High Strength Used For Particulate Filters

The regulations for emission of vehicles become increasingly stringent due to increasingly serious environmental pollution. To meet emission limitations on matter and number of particles, particulate filters (PF) are widely installed on diesel and gasoline vehicles. At present, most PF is made from cordierite which is vulnerable to high temperatures. Aluminum titanate is well-known for its low thermal expansion coefficient and high melting point, therefore is very suitable for PF application. However, the intrinsic low strength of porous aluminum titanate impedes its practical application in PF. In this dissertation, composite strengthening in aluminum titanate was studied with different reinforcement materials such as mullite fiber, mullite particles, strontium feldspar(SAS), silicon carbide whisker, calcium dialuminate(CA2). Porous composites with low thermal expansion and high strength were synthesized by reaction sintering method. The effects of the reinforcement materials, pore forming agent type and content, sintering temperature and matrix system on the microstructure, pore size distribution, porosity, thermal expansion and mechanical properties of the aluminum titanate porous composites were systematically studied. Besides, the reinforcement mechanism and pore formation mechanism were discussed. The results are as follows:(1)Porous mullite-aluminum titanate composites with low thermal expansion and high strength were prepared. The incorporation of mullite (fibers or particles) decreased sintering shrinkage and bulk density, while increased porosity and flexural strength of aluminum titanate. The sample containing mullite fibers had a larger pore size than the one containing mullite particles. Excellent overall performance can be obtained by adding 5 wt%-10 wt% mullite into aluminum titanate. A low thermal expansion coefficient, less than 1.5×10-6/℃, can be achieved by adding 5 wt% mullite fiber and sintering between 1350℃ and 1450℃. The thermal expansion coefficient and strength of composites containing 5 wt% mullite particles reached 1×10-6/℃ and 5 MPa, respectively.(2)Porous strontium feldspar-aluminum titanate composites with low thermal expansion and high strength were prepared. The sintering shrinkage, flexural strength and thermal expansion coefficient increased with the increasing content of strontium feldspar. The sample containing 10 wt% strontium feldspar and sintered at 1450℃, showed a flexural strength of 5 MPa and a low thermal expansion coefficient of 0.7×10-6/℃. A high flexural strength of 30 MPa can be achieved in the composite with 40 wt% strontium feldspar.(3)Sintering properties of the composites decreased when silicon carbide whisker was added. Porosity of the composites changed slightly with increasing content of silicon carbide whisker. The sample with 3 wt% silicon carbide wisker sintering at 1450℃ showed a flexural strength of 22 MPa. Optimum mechanical performance was obtained in samples with 1 wt%-6 wt% silicon carbide whisker.(4)Calcium dialuminate-aluminum titanate (AT-CA2) composites with low thermal expansion and high strength were prepared. The well sintered AT-CA2 composites exhibited exceptionally low thermal expansion coefficient and high strength. The AT-40w%CA2 sample sintered at 1400 C showed a bending strength of 35 MPa, which was about two orders of magnitude higher than that of the pure AT sample, and the thermal expansion coefficient was ultralow approaching zero.(5)Adding pore-forming agent remarkably increased the porosity and pore size’and decreased the strength and bulk density of the composites. The shape and size of the pore-forming agents critically influenced the morphology and pore size of the porous composites. High porosity porous composites can be achieved by adding flake graphite, polyacrylic resin and starch. I.arge pore size also can be prepared by adding flake graphite, starch, mullite fiber and strontium feldspar. The pore formation mechanism was primarily influenced by particle accumulation and pore forming agent.These findings have important guidance on the production and application of aluminum titanate based honeycomb composite ceramics with superior properties for next generation particulate filters.