Role of additives in the sintering of silicon nitride: A29Si, 27Al, 25Mg and 89Y MAS NMR and X-ray diffraction study

Abstract

Multinuclear MAS NMR in conjunction with X-ray diffraction (XRD) has been used to study the role of Al₂O₃, Y₂O₃ and MgO, both singly and in combination, in the sintering of silicon nitride at 1500–1800 °C. Under the present experimental conditions, Al₂O₃ enters the silicon nitride to form a low-z β′-SiAlON, whereas MgO reacts both with the oxidised surface SiO₂ layer to form forsterite (Mg₂SiO₄), and with the Si₃N₄ to form an X-ray amorphous Mg–Si–0–N phase characterised by a broad ²⁵Mg signal at about –50 ppm and a fast ²⁹Si relaxation time. Y₂O₃ forms an yttrium-rich Y–Si–O–N phase at 1500 °C which progressively becomes silicon-rich at higher temperatures. The ⁸⁹Y spectra of these phases are broad and could be detected only in samples containing added Yb₂O₃ to shorten the relaxation time. When used in combination, the Al₂O₃/MgO and Y₂O₃/MgO pairs behave similarly to the separate components, in terms of intergranular phase formation, but Al₂O₃/Y₂O₃ forms Y₁₀Al₂Si₃O₁₈N₄, for which the ²⁹Si and ²⁷Al MAS NMR spectra are reported.