Si 3 N 4 ceramics with high thermal conductivity and outstanding mechanical properties were prepared by adding β-Si 3 N 4 seeds and nanophase α-Si 3 N 4 powders as modifiers. The introduction of β-Si 3 N 4 seeds enhanced the growth of β-Si 3 N 4 grains. Owing to the interlocked structure induced by the β-Si 3 N 4 grains, the fracture toughness of Si 3 N 4 ceramics reached a high value of 7.6 MPa·m 1/2 ; also, the large-sized grains increased the contact possibility of Si 3 N 4 grains, improving the thermal conductivity of Si 3 N 4 ceramics (64 W/(m·K)). Because of the introduction of nanophase α-Si 3 N 4 , the flexural strength, fracture toughness, and thermal conductivity of the Si 3 N 4 ceramics increased to 754 MPa, 7.2 MPa·m 1/2 , and 54 W/(m·K), respectively. According to the analysis of the growth kinetics of Si 3 N 4 grains, the rapid growth of Si 3 N 4 grains was ascribed to the reduction in the activation energy resulting from the introduction of β-Si 3 N 4 seeds and nanophase α-Si 3 N 4 .
This study presents a scalable method for fabricating near-spherical silicon nitride (Si 3 N 4 ) particles with a high α phase content. Amorphous Si 3 N 4 powder was synthesized using the diimide synthesis method, mixed with graphite powder as a carbon source, and calcined at 1550°C to produce α-Si 3 N 4 powder. The effects of varying carbon contents on the phase composition and microstructure of the resulting products were investigated. In the presence of carbon, the Si 3 N 4 particles exhibit a near-spherical morphology, with a reduction in the fraction of whisker-like structures. The products show a complex, nonuniform structure with a broad particle size distribution and varying structural characteristics when excessive powder is added. Graphite powder not only provides nucleation sites but also plays a critical role in the phase transition from amorphous Si 3 N 4 to the α phase. The nucleation and growth of α-Si 3 N 4 are facilitated by the presence of SiO(g), which is reduced through interactions with C(s) or CO(g). It indicates that carbon accelerates the Si 3 N 4 phase transition through the vapor-solid mechanism, which is the key to inhibiting the whisker growth and controlling the particle size of α-Si 3 N 4 powder.
This study presents a cost-effective and environmentally friendly approach for fabricating large-area, low-defect Si₃N₄ ceramic substrate materials using tape casting combined with reaction-bonding sintering at 1900 °C. High-purity Si powder and a non-toxic Duramax™ B-1000 binder system were employed as raw materials. Orthogonal experiments (L9(3 4 ) array) optimized key parameters: 45 wt% solid content, 12.0 wt% binder, 0.4 wt% ammonium citrate dispersant, and a PEG-600 plasticizer to binder ratio (R) of 0.7. These conditions produced defect-free green tapes with smooth surfaces, minimal shrinkage, and balanced strength-flexibility properties. During re-sintering, the addition of 4 wt% Si 3 N 4 seeds maximized performance, yielding ceramics with 92.69% relative density, 15.6 GPa Vickers hardness, 6.6 MPa·m 1/2 fracture toughness, 636.1 MPa flexural strength, and 60.74 W·m −1 ·K −1 thermal conductivity. Microstructural analysis revealed elongated β-Si 3 N 4 grains, confirmed by XRD. This work demonstrates a scalable fabrication route for high-performance Si 3 N 4 substrates using economical and eco-conscious processing.
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