Abstract

This research investigates the oxidation behavior of nanocrystalline/amorphous Si3N4 compounds in atmospheres with different oxygen pressures. High purity nanocrystalline/amorphous Si3N4 and MgAl2O4 mixtures were used to investigate oxidation behavior of Si3N4. Si3N4-xMgAl2O4 (0 < x < 90 wt.%) samples were prepared with mixing in a jar mill, followed by pressing and heating in a high vacuum furnace. Specimens were characterized using FE-SEM, EDS, XRD, FT-IR, and ICP. Thermal analysis (TG/DSC) was also employed to investigate the thermal behavior of samples in different atmospheres. Results showed that phase transformation in pure Si3N4 is controlled by holding time and oxygen pressure in the atmosphere. Results also showed an increase in the weight of pure Si3N4 by 5.3 and 15.2 % for 0.5 and 2 h in a vacuum atmosphere, respectively, inferring that crystallization reaction proceeds with holding time. In the temperature range 1000–1400 °C, a noticeable exothermic peak has appeared for pure Si3N4 in the air atmosphere which was attributed to the formation of cristobalite (SiO2) phase. Besides, thermal analyses of Si3N4-xMgAl2O4 (0 < x < 90 wt.%) samples in the nitrogen atmosphere showed that the maximum weight change was attained in the Si3N4-50% MgAl2O4 sample. Mechanisms and implications of oxidation in all cases are thoroughly discussed.

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