Effect of Microstructure on the Thermal Expansion Coefficient of Sintered Cordierite Prepared from Sol Mixtures

Abstract

Dense cordierite ceramics were prepared from a sol mixture of alumina, silica, and magnesia, and the relationship between microstructure and thermal expansion was clarified for sinters with relative density greater than 97%. In the dense cordierite ceramics, submicrometer-sized primary cordierite crystals aligned in the same crystal orientation and constituted the domain structure. We discovered that these domain structures could be easily observed by optical polarizing microscopy and quantified by digital image analysis of the photographs. The occurrence of microcracks between domains larger than 40 lm was induced by the thermal expansion anisotropy of the cordierite crystal axes. As a result, the mean thermal expansion coefficient of the cordierite ceramics decreased to 0.4 3 10 −6 K −1 from the average value of the crystal axes of 1.7 3 10 −6 K −1 . This lower thermal expansion coefficient could be theoretically explained by partial microcracking.

Ferroelectric domain structures in <001>-oriented K0.15Na0.85NbO3 lead-free single crystal

“…Referring to Another interesting phenomenon in this crystal is that, an area shows diffraction pattern with superlattice spots close to √ 2ax6a supercells, as well as their incommnesrate structure have been reported in the KNN ceramics. 21 NaNbO 3 has an antiferroelectric phase with space group Pbcm at room temperature (300 K). The antiferroelectric phase consists of two or more sublattice polarizations of antiparallel nature, which in turn give rise to superlattice reflections in the diffraction patterns.…”

Microstructure–Property Connections for Porous Ceramics: The Possibilities Offered by Micromechanics

“…Microcracking is also responsible for the nonlinearity of the stress–strain curves, especially in porous ceramics . In comparing the lattice and macroscopic thermal expansions, Ohya et al could only calculate the microcrack volume, whereas Kobayashi et al, estimated the fraction (and crystal orientation) of microcracked grains; none of them attempted at calculating stresses. Modeling the thermally induced microstresses, and the resulting microcracking, has been attempted with varying level of geometrical complexity, mostly by considering 2‐D hexagonal grains and calculating stresses generated by the thermal expansion mismatch between them.…”

Effect of particle size of tabular talc powders on crystal orientation and sintering of cordierite ceramics

“…When cordierite grains of c-axis perfectly aligned in one plane, CTE in this plane is an average value of ¡ a and ¡ c . From the thermal expansion measurement of lattice parameter of cordierite crystal by authors, 14) 0.88 © 10 ¹6 K ¹1 could be obtained. On the other hand, CTE in vertical direction corresponds to that of a-axis 3.36 © 10 ¹6 K ¹1 .…”