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Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions

Abstract

Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties1,2. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features3,4. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives1,2,5. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

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Figure 1: Atomic-resolution scanning transmission electron microscope (STEM) images of an intergranular film (IGF) in La-doped β-Si3N4.
Figure 2: Magnified HAADF-STEM images of the interface between the IGF and the prismatic surface of an β-Si3N4 grain.

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Acknowledgements

We thank R. L. Satet and M. J. Hoffmann for supplying the silicon nitride ceramics used in this study. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract with UT-Battelle, LLC. N.S. is a fellow of the Japan Society for the Promotion of Science (JSPS).

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Correspondence to Naoya Shibata.

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T.R.G. is chief scientist in Pixon LLC. The remaining authors declare that they have no competing financial interests.

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Shibata, N., Pennycook, S., Gosnell, T. et al. Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions. Nature 428, 730–733 (2004). https://doi.org/10.1038/nature02410

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