The Relationship Between High Thermal Conductivity and Grain Orientation in Ceramics

β-Si₃N₄ ceramics with highly oriented grains, show high thermal conductivity along the tape-casting direction. In order to clearly understand the relationship between microstructure and thermal conductivity in ceramics, it is important to understand degrees of orientation for each grain and the grain boundary.

In this study Japanese researchers, Hiromi Nakano, Hiroshi Nakano and Koji Watari, from Ryukoku University and the National Institute of Advanced Industrial Science and Technology Ceramic Research Institute tried to determine the orientation of each grain in β-Si₃N₄ ceramic with controlled grain orientation by two methods - electron back-scattered diffraction (EBSD) and standard electron diffraction. This work focuses on highly oriented β-Si₃N₄ ceramics obtained by tape-casting with seed particles, followed by hot-pressing and HIPing.

A whole sintered body was analyzed by electron back-scattered diffraction (EBSD) with a scanning electron microscope.  Detailed analysis was carried out by an electron diffraction method with a conventional transmission electron microscope (TEM).  The degrees of tilting angle of each grain from the basis zone axis was calculated from the selected area diffraction (SAD) patterns, and these were expressed by tone of color and color variation, respectively.  Furthermore, the microchemistry of the grain boundary was analyzed by energy dispersive spectroscopy (EDS).

Results indicated that each grain was oriented within 20 degrees tilting angle from the [0001] axis in the TEM observed specimen.  Furthermore, the relationship between the microstructural data and thermal data are compared and discussed for various types of β-Si₃N₄ ceramics.  In this case, the appearance of high anisotropic thermal conductivity was caused by crystallographic nature, grain features, and grain purity during a liquid-sintering process. These findings may lead to uses of silicon nitride in high thermal conductivity applications

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