Aqueous Processing of Textured Silicon Nitride Ceramics by Slip Casting in a Strong Magnetic Field

摘要

Recently, textured microstructure has received particular interest in the processing of advanced Si_3N_4 ceramics because of significant improvement in the mechanical properties and thermal conductivity. This work will report a highly textured β-Si_3N_4 ceramic by aqueous slip casting in a magnetic field and subsequent pressureless sintering, using commercial α-Si_3N_4 raw powder and a mixture of Y_2O_3 and Al_2O_3 as sintering aids. To obtain the well-dispersed Si_3N_4-Y_2O_3-Al_2O_3 slurries, polyethylenimine (PEI) was chosen as a dispersant. Effects of the sintering aids, PEI amount, pH and stirring time on the stability of the Si_3N_4 slurries were studied. It is shown that PEI is an effective dispersant for stabilizing the Si_3N_4-Y_2O_3-Al_2O_3 slurries that does not show a time-dependent behavior at an optimum pH ≈ 10, compared to the case in the absence of PEI. Using the 30 vol% Si_3N_4-Y_2O_3-Al_2O_3 slurries stabilized with 1.5 dwb% PEI at pH 10, the highly textured β-Si_3N_4 with 97 % relative density could be obtained by slip casting in a magnetic field of 12 T and subsequent sintering at 1800 °C for 1 h. The textured microstructure is featured by the alignment of c-axis of β-Si_3N_4 crystals perpendicular to the magnetic field, and the Lotgering orientation factor, f, is determined to be 0.8.