Aluminum‐Containing Nilrogen Melilite Phases
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A significant solubility of Al in N-melilite phases (M) has been observed, and this results in the formation of a melilite solid solution (M{prime}{sub ss}) of general formula Ln{sub 2}Si{sub 3{minus}x}Al{sub x}O{sub 3 + x}N{sub 4{minus}x} (Ln = rare earth). Up to one Si can be replaced by Al without change of structure, and the M{prime} solid solution terminates at Ln{sub 2}Si{sub 2}AlO{sub 4}N{sub 3} in samarium SiAlON systems. M{prime}{sub ss} may appear as an intermediate phase during the sintering of SiAlONs, and its melting temperature is critical to the densification of the materials. For example, samarium M{prime}{sub ss} melts at a temperature lower than neodymium M{prime}{sub ss} and as a result, samarium oxide shows better densification behavior in the preparation of {alpha}-SiAlON ceramics than does neodymium oxide. Devitrification of M{prime}{sub ss} from an amorphous grain boundary phase occurs above 1500 C during post heat-treatment. The M{prime}{sub ss} is refractory and may offer better oxidation resistance than N-melilite because of the replacement of Al-O for Si-N in the structure. Therefore M{prime}{sub ss} is considered to be a most desirable grain boundary phase for {alpha} and {alpha}-{beta} SiAlON ceramics.
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