Silicon nitride: Enthalpy of formation of the {alpha}- and {beta}-polymorphs and the effect of C and O impurities
- Thermochemistry Facility, Chemistry Building, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616 (United States)
- National Institute of Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305 (Japan)
High-temperature oxidative drop solution calorimetry was used to measure the enthalpy of formation of {alpha}{endash} and {beta}{endash}Si{sub 3}N{sub 4}. Two different solvents, molten alkali borate (48&hthinsp;wt&hthinsp;{percent} LiBO{sub 2}{center_dot}52&hthinsp;wt&hthinsp;{percent} NaBO{sub 2}) at 1043 and 1073 K and potassium vanadate (K{sub 2}O{center_dot}3V{sub 2}O{sub 5}) at 973 K, were used, giving the same results. Pure {alpha}{endash} and {beta}{endash}Si{sub 3}N{sub 4} polymorphs have the same molar enthalpy of formation at 298 K of {minus}850.9{plus_minus}22.4 and {minus}852.0{plus_minus}8.7 kJ/mol, respectively. The unit cell dimensions of impure {alpha}{endash}Si{sub 3}N{sub 4} samples depend linearly on the O and C impurity contents, and so does the molar enthalpy of formation. The energetic stability of the {alpha}{endash}Si{sub 3}N{sub 4} phase decreases when the sample contains O and C impurities. The experimental evidence strongly suggests that the impurities dissolve into the {alpha}{endash}Si{sub 3}N{sub 4} structure to form a (limited) isostructural solid solution series but that this solid solution series is energetically less stable than a mechanical mixture of pure ({alpha} or {beta}) Si{sub 3}N{sub 4}, SiO{sub 2}, and SiC. Thus, the {alpha}-phase is not stabilized by impurities and is probably always metastable. {copyright} {ital 1999 Materials Research Society.}
- OSTI ID:
- 353697
- Journal Information:
- Journal of Materials Research, Vol. 14, Issue 5; Other Information: PBD: May 1999
- Country of Publication:
- United States
- Language:
- English
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