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.