Crystal Structure and Thermal Expansion Coefficient of Cordierite Honeycomb Ceramics

Cordierite honeycomb ceramics are sintered under a range of sintering atmosphere and temperature conditions. The linear thermal expansion coefficient (CTE) is measured along the extrusion direction. The samples sintered in an enclosed crucible have a much lower CTE than those sintered in open air. The CTE decreases with increasing sintering temperature up to 1450 degrees C. The polymorphic phases, texture, amorphous phase, impurity crystals, and porosity are analyzed using scanning electron microscopy, HR-TEM, porosimeter, and Rietveld refinement method. The sintered samples consist of two polymorphic phases: indialite (hexagonal symmetry) and cordierite (orthorhombic symmetry). Both the volume % of the cordierite phase and the crystal texture increase with increasing sintering temperature. The orthorhombic distortion of cordierite from the hexagonal lattice (indialite) increases with increasing sintering temperature. The characteristic six-membered ring of the (Si,Al)-O4 tetrahedra in the cordierite structure shows substantial changes in the bond length due to Al-Si ordering. Amorphous phase and impurity crystal phases such as silimanite and spinel remain at temperatures as high as 1450 degrees C. The effect of the polymorphic phases, the degree of texture, amorphous phase, impurity crystal phases, and the orthorhombic distortion is discussed in relation to the CTE characteristics of honeycomb.