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 .

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