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

Abstract

β-Si 3 N 4 rodlike seed crystallites were successfully produced by single-step heat treatment of commercial α-Si 3 N 4 powder at 1900 • C for 20 h under an N 2 gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α-⇀ β-Si 3 N 4 phase transformation proceeded mainly at 1900 • C, and this temperature was lower than the theoretical α-Si 3 N 4 dissociation temperature (1933 • C) under N 2 gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α-Si 3 N 4 crystal lattice was suggested as the driving force for the present oxide additive-free α-⇀ β-Si 3 N 4 phase transformation. β-Si 3 N 4 ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd 2 O 3 . Starting α-Si 3 N 4 powder with 10 vol% rodlike β-Si 3 N 4 seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950 • C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β-Si 3 N 4 ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m −1/2 and 109.3 ± 0.4 W m −1 K −1 , respectively, retaining a high fracture strength of about 1 GPa.

Thermal conductivity improvement in silicon nitride ceramics via grain purification

“…The starting powders used in this study are listed in Table 1. α-Si 3 N 4 mixed with sintering additives of 1 wt% MgO-3 wt% Re 2 O 3 and 10 vol% of β-Si 3 N 4 seed crystallites 17 were sintered at 1950 • C for 5-40 h under N 2 pressure of 784 kPa according to the procedures shown in our previous report. 17 The evaluation of density (ρ), fracture strength (σ f ), fracture toughness (K 1c ), and thermal conductivity (λ) of the sintered specimens, as well as the SEM microstructure observation and the subsequent SEM image analysis for the quantification of β-Si 3 N 4 grain size were conducted under the same manner as described in our previous report.…”

“…α-Si 3 N 4 mixed with sintering additives of 1 wt% MgO-3 wt% Re 2 O 3 and 10 vol% of β-Si 3 N 4 seed crystallites 17 were sintered at 1950 • C for 5-40 h under N 2 pressure of 784 kPa according to the procedures shown in our previous report. 17 The evaluation of density (ρ), fracture strength (σ f ), fracture toughness (K 1c ), and thermal conductivity (λ) of the sintered specimens, as well as the SEM microstructure observation and the subsequent SEM image analysis for the quantification of β-Si 3 N 4 grain size were conducted under the same manner as described in our previous report. 17 In this study, TEM, HRTEM, and STEM-EDS analyses were intensively performed on the sintered specimens to characterize the grain boundary glassy phase and glassy nanoprecipitates within the β-Si 3 N 4 matrix grains.…”

“…17 The evaluation of density (ρ), fracture strength (σ f ), fracture toughness (K 1c ), and thermal conductivity (λ) of the sintered specimens, as well as the SEM microstructure observation and the subsequent SEM image analysis for the quantification of β-Si 3 N 4 grain size were conducted under the same manner as described in our previous report. 17 In this study, TEM, HRTEM, and STEM-EDS analyses were intensively performed on the sintered specimens to characterize the grain boundary glassy phase and glassy nanoprecipitates within the β-Si 3 N 4 matrix grains. TEM specimens were prepared by cutting the test specimens after fracture strength measurement into thinner samples of approximately 10 × 10 mm 2 and 2 mm in thickness, and grinding them to a thickness of 100 μm, followed by dimpling and argon ion beam milling.…”

“…They concluded that the nearly 50 % increase in the thermal conductivity achieved for the post-sintered specimen was attributed to the removal of the lattice defects as well as the chemical purification of β-Si 3 N 4 grains in terms of the reduction in the lattice oxygen concentration. 14 In our previous works, [15][16][17] we proposed a facile method to produce rod-like β-Si 3 N 4 seed crystallites for the fabrication of high-performance β-Si 3 N 4 ceramics having a bi-modal structure composed of fine matrix grains and large elongated grains originating from the seed crystallites. As our initial result, bi-modal structure construction was successfully achieved for the liquid phase-sintered β-Si 3 N 4 -MgO (1 wt%)-Gd 2 O 3 (3 wt%) ceramics by the addition of 10 vol% rod-like β-Si 3 N 4 seeds produced at our laboratory, and the resulting ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPam −1/2 and 109.3 ± 0.4 Wm −1 K −1 , respectively, retaining high fracture strength of about 1 GPa.…”

“…As our initial result, bi-modal structure construction was successfully achieved for the liquid phase-sintered β-Si 3 N 4 -MgO (1 wt%)-Gd 2 O 3 (3 wt%) ceramics by the addition of 10 vol% rod-like β-Si 3 N 4 seeds produced at our laboratory, and the resulting ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPam −1/2 and 109.3 ± 0.4 Wm −1 K −1 , respectively, retaining high fracture strength of about 1 GPa. 17 In this study, a series of the bi-modal structurecontrolled β-Si 3 N 4 -MgO (1 wt%)-Re 2 O 3 (3 wt%, Re = Gd, La, Y, and Yb) ceramics are fabricated by high-temperature sintering at 1950 • C for an extended time of 40 h under N 2 pressure at 784 kPa. The relationships between the microstructure, fracture strength, fracture toughness and thermal conductivity of the 1950 • C-sintered specimens with the pseudo ternary β-Si 3 N 4 -MgO-Gd 2 O 3 system are investigated by a set of microstructure characterizations such as β-Si 3 N 4 grain size distribution, the area fraction of large β-Si 3 N 4 grains (≥ 2 μm in diameter) and grain boundary phases in terms of the thickness of two-grain boundary and chemical composition change at the multiple-grain…”