Effect of Oxygen Impurities and Synthesis Temperature on the Phase Composition of the Products of Self-Propagating High-Temperature Synthesis of Si3N4

A facile method to produce rodlike β‐Si3N4 seed crystallites for bimodal structure controlling

“…According to our previous thermodynamic calculation, 28 the Si 3 N 4 dissociation temperature was suggested as 1933°C under the N 2 pressure of 980 kPa (the same N 2 pressure applied in this study). The α‐ ⇀ β‐Si 3 N 4 phase transformation experimentally observed for the E10 and E03 powder samples proceeded mainly at 1900°C (Figures 1 and 6), that is, 33°C lower than the theoretical α‐Si 3 N 4 dissociation temperature, and much higher than the Si 2 N 2 O formation–deformation temperatures ranging from 1610 to 1750°C (Equations ) 24 . Moreover, the O impurity contents in the E10 and E03 powders after the heat treatment at 1900°C for 5 h were found as low as 0.64 and 0.41 wt%, respectively (Figure S3).…”

“…It is generally accepted that the α‐ ⇀ β‐Si 3 N 4 phase transformation proceeds through the dissolution–precipitation process in the oxynitride liquid phase formed in situ at the heat‐treatment temperature 18,19 . The kinetics of the dissolution–precipitation process have been discussed based on several factors, such as the liquid phase composition, amount of the liquid phase, and the amount of preexisted β phase, that is, β‐phase content of the starting Si 3 N 4 powder 20–24 . However, the initial SiO 2 contents of the E10 and E03 powder samples are essentially low and measured to be 5.24 and 2.81 mol%, respectively, and according to the phase diagrams of the binary SiO 2 –Si 3 N 4 system, 25,26 possible phases suggested for the E10 and E03 powder samples at 1600–1800°C are Si 3 N 4 and silicon oxynitride (Si 2 N 2 O), whereas the liquid phase formation can be excluded at such SiO 2 contents far below 50 mol% 25,26 .…”

“…Recently, Zakorzhevskii et al 24 . reported that the β‐phase content of the Si 3 N 4 powders produced by the self‐propagating high‐temperature synthesis (SHS) increased consistently with the O content of the starting Si powder and the synthesis temperature.…”

“…Recently, Zakorzhevskii et al 24 reported that the βphase content of the Si 3 N 4 powders produced by the selfpropagating high-temperature synthesis (SHS) increased consistently with the O content of the starting Si powder and the synthesis temperature. The β-Si 3 N 4 formation by the SHS using ammonium chloride (NH 4 Cl) and ammonium fluoride (NH 4 F) was suggested to involve the following steps 24 :…”

“…The effective amount of the O impurity to promote the β-Si 3 N 4 formation through the Si 2 N 2 O formation and decomposition was suggested at 2.2-2.5 wt%. 24 On the other hand, at low O impurity content (0.5-0.7 wt%), an alternative mechanism for the β-Si 3 N 4 formation was suggested: The impurity O dissolves into the crystal lattice of α-Si 3 N 4 and forms a metastable solid solution 27 that is the driving force of α-⇀ β-Si 3 N 4 phase transformation below the theoretical α-Si 3 N 4 dissociation temperature 24 :…”