 | LOCAL STRUCTURE AND CHEMICAL BONDING IN PRECURSOR-DERIVED AMORPHOUS CERAMICS
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| | | 1) Ceramics Superplasticity Project, ICORP, Japan Science and Technology Corporation |
| | The structural and chemical properties of the precursor-derived amorphous materials were studied using the advanced methods and full power of the analytical electron microscopy, including quantitative electron energy-loss spectroscopy (EELS) analysis, energy-loss near-edge structures (ELNES) analysis, high-resolution electron microscopy (HREM) imaging, high-angle annular dark-field (HADF) Z-sensitive imaging and finally EELS/ELNES spectrum-imaging technique. Three materials systems were investigated in details: (a) Si-B-C-N system, (b) Si-C-N system and (c) Si-C-O system. It is always found that the chemical composition and bonding are not homogeneous at the scale of 1 to several nanometers, even the amorphous matrix appears uniform down to atomic level in Si-(B)-C-N systems. In Si-B-C-N system, clusters of 1-3nm size were observed by HADF imaging directly. These clusters are rather stable at temperatures at least up to 1600°C. EELS quantification reveals that the composition in the amorphous monolith has little changed by high temperature deformation. ELNES results indicated not only that these clusters are embedded n graphite-like carbon “matrix” but Si-N bonds dominate in the clusters. It is believed that the addition of boron prohibited the Si-N clusters to grow at high temperature. In Si-C-N system the situation is very similar except the crystallization took place at temperature several 100°C lower. The clusters are very much like Si3N4 in chemical bonding from ELNES result, and EELS quantification indicated the same thing. But their structure remained amorphous. The amount of carbon is substantially less than in Si-B-C-N monolith, it is more like amorphous carbon than graphite-like, according also to ELNES. In Si-C-O system crystalline SiC clusters were always found regardless of the oxygen content and of the pyrolysing (>=1300°C so far) and annealing temperatures. Instead, their size and abundance vary with these parameters. Graphite was also observed everywhere. ELNES analysis clearly identified that the remaining amorphous structures are made from SiO2 and carbon. EELS and ELNES profiles exhibited clearly the spatial variation of composition and chemical bonding in consistent completely with such picture. The common observation of clusters, amorphous or crystalline, reflects fundamental properties of precursor-derived ceramic materials. It is phase separation in amorphous state at 1nm scale. This remarkable property is a result of absence of atom diffusion in amorphous monolith. Such a new and stable structure is based not only on zero diffusivity but also on the inflexibility of covalent bonds in amorphous state, in stark contrast with the flexibility of amorphous SiO2. This observation raises the prospect of forming uniform covalent amorphous that should be more elastic and fracture resistant. |
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