Sol–Gel Synthesis of Oxonitridoaluminosilicates (SiAlON)

Specific features of the synthesis of sialons (SiAlON) in the Si–Al–O–N system by firing at 1600–1700°C in a nitrogen atmosphere of mixtures of different initial compositions corresponding to the same composition of the final product Si3Al3O3N5 have been studied. β-Sialon was synthesized from reaction mixtures based on crystalline silicon and aluminum nitrides and amorphous highly dispersed hydrated oxides—(1) AlN + SiO2(sol), (2) Si3N4 + Al2O3(sol), and (3) AlN + Si3N4 + Al2O3(sol)—obtained by applying sols based on silicon or aluminum alkoxides (based on the β-sialon stoichiometry) to the surface of nitride powders by the sol-gel method. It has been demonstrated that β-Si3Al3O3N5 can form in the reaction at 1600°C of highly dispersed mixed xerogels made of silicon and aluminum alkoxides—(4) (SiO2 + Al2O3) (mixed sol)—with molecular nitrogen without the participation of the initial nitrides. The influence of the starting composition of xerogel mixtures and synthesis conditions on the phase composition and morphological features of sialons obtained in one stage of calcination in a nitrogen atmosphere has been studied by thermogravimetry, X-ray powder diffraction analysis, and electron microscopy. It has been shown that the proposed reactions of the formation of sialons involving crystalline nitrides, xerogels, and molecular nitrogen are related to reduction–nitridation reactions, in which the reducing agents are organic products of hydrolysis of silicon and aluminum alkoxides and their pyrolytic decomposition in a nitrogen atmosphere.