Hisayuki Imamura

Oxide additive-free α- → β-Si₃N₄ phase transformation of a high-purity commercial α-Si₃N₄ powder was investigated at 1600 to 1900 °C under a nitrogen pressure of 980 kPa. The XRD analysis revealed that the α- → β-Si₃N₄ phase transformation proceeded mainly at 1900 °C, and was completed by the extensive 1900 °C heat treatment for 20 h. This phase transformation temperature was 33 °C lower than the theoretical α-Si₃N₄ dissociation temperature and was confirmed as completely different from that often discussed for the liquid-phase sintering of α-Si₃N₄ powder by direct comparison with the phase transformation behavior of a reference powder, α-Si₃N₄ powder doped with 1 mol% Y₂O₃. The unique α- → β-Si₃N₄ phase transformation was further studied by a set of characterization techniques including elemental analysis, HAADF-STEM and STEM-EDS analyses. The results strongly suggested that the oxide additive-free α- → β-Si₃N₄ phase transformation was governed by the formation of a metastable solid solution between α-Si₃N₄ and impurity oxygen of approximately 0.6 wt%, which promoted the dissociation below the theoretical α-Si₃N₄ dissociation temperature to afford thermodynamically favorable β-Si₃N₄. Along with the β-Si₃N₄ formation, the impurity oxygen concentrated at the grain boundaries was released from the sample via the grain boundary diffusion to afford high-purity β-Si₃N₄.

Oxide additive-free α- ⇀ β-Si3N4 phase transformation of a high-purity commercial α-Si3N4 powder was investigated at 1600 to 1900 ℃ under nitrogen pressure of 980 kPa. The XRD analysis revealed that the α- ⇀ β-Si3N4 phase transformation proceeded mainly at 1900 ℃, and was completed by the extensive 1900 ℃-heat treatment for 20 h. This phase transformation temperature was 33 ℃ lower than the theoretical α-Si3N4 dissociation temperature and confirmed as completely different from that often discussed for the liquid phase sintering of α-Si3N4 powder by direct comparison with the phase transformation behavior of a reference powder, α-Si3N4 powder doped with 1 mol% Y2O3. The unique α- ⇀ β-Si3N4 phase transformation was further studied by a set of characterization techniques including elemental analysis, HAADF-STEM and STEM-EDS analyses. The results strongly suggested that the oxide additive-free α- ⇀ β-Si3N4 phase transformation was governed by the for-mation of metastable solid solution between α-Si3N4 and impurity oxygen of approximately 0.6 wt%, which promoted the dissociation below the theoretical α-Si3N4 dissociation temperature to afford thermodynamically favorable β-Si3N4. Along with the β-Si3N4 formation, the impurity oxygen was concentrated at the Si3N4 crystal grain boundaries and subsequently released from the sample via the grain boundary diffusion.

β‐silicon nitride (β‐Si ₃ N ₄ ) ceramics with an additive oxide system of 1 wt% MgO and 3 wt% Re ₂ O ₃ (Re = Gd, La, Y, and Yb) were fabricated by liquid phase sintering at 1950°C under nitrogen pressure of 748 kPa. Starting α‐Si ₃ N ₄ powder with 10 vol% rod‐like β‐Si ₃ N ₄ seed crystallites and an extended sintering time from 5 to 40 h resulted in the formation of bimodal microstructure composed of fine matrix grains and large grains. The 40 h‐sintered specimens of pseudo ternary β‐Si ₃ N ₄ ‐MgO‐Gd ₂ O ₃ system exhibited enhanced thermal conductivity of 127.2 ± 2.5 W m ⁻¹ K ⁻¹ associated with a degradation of the fracture strength from 1008.0 ± 38.0 to 491.7 ± 32.0 MPa, which was due to the formation of coarse‐grained aggregates that acted as both fracture origin as well as a thermal conductive pathway. The theoretical thermal conductivity was predicted for the sintered specimens by using equations based on a mean‐field micromechanics model to estimate the effective thermal conductivity of each component of binary composites. The calculation results suggested that the thermal conductivity of the large β‐Si ₃ N ₄ grains (≥ 2 μm in diameter) was relatively high and estimated to be in the range of 175 to 191 Wm ⁻¹ K ⁻¹ . The improved thermal conductivity of the 40 h‐sintered specimens was further discussed for the series of β‐Si ₃ N ₄ ‐MgO‐Re ₂ O ₃ systems based on the nanostructure characterization results obtained by the high‐resolution transmission electron microscopy and scanning transmission electron microscopy‐energy dispersive X‐ray spectrometry analyses.