Gel-Casting Prepared Porous Si 3 N 4 Ceramics with Different Contents of Y 2 O 3 and Al 2 O 3 Additives
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Gel-Casting Prepared Porous Si3N4 Ceramics with Different Contents of Y2O3 and Al2O3 Additives Chunrong Zou, Shaojun Guo, Xiaosong Zhou, Song Li, Chunlei Yan, and Tongsheng Shen Submitted: 24 August 2020 / Revised: 7 October 2020 / Accepted: 13 October 2020 / Published online: 18 November 2020 Porous Si3N4 ceramics were prepared via a controlled gel-casting process followed by gas pressure sintering with 8 wt.% Y2O3 and Al2O3 additives. The gel-casting parameters including slurry pH value, AM: MBAM ratio, degassing procedure, drying condition and decarbonize process were optimized, and the effects of different Y2O3: Al2O3 ratios on the microstructure and properties of the Si3N4 ceramics were investigated. With an optimized gel-casting process, a high solid loading of 46.5 vol.% was achieved, and as the Y2O3 content decreased from 6 to 1 wt.%, the porosity of Si3N4 ceramics declined from 36.3 to 27.5% while the bSi3N4 phase content reduced from 38.9 to 15.0%. The bending strength and elastic modulus of the ceramics varied with different Y2O3: Al2O3 ratios and were influenced by the porosity as well as the tailored interlocking structure of elongated b-Si3N4 grains. The Si3N4 ceramics also exhibited low and stable dielectric constants. Keywords
dielectric properties, gel-casting, mechanical properties, microstructure, Si3N4 ceramics
1. Introduction Silicon nitride (Si3N4) ceramic has been widely used as structural and functional materials because of its excellent mechanical, thermal and electrical properties. With respect to wave-transparent applications, Si3N4 ceramic displays favorable mechanical strength, good oxidation resistance, low thermal expansion coefficient and stable dielectric constant at high temperatures (Ref 1, 2). However, dense Si3N4 ceramic usually demonstrates a relatively high dielectric constant (5.6 for a-Si3N4 and 7.9 for b-Si3N4), which exceeds the requirement of the microwave apparatus and missile radomes (Ref 3). Recently, intensive attentions have been paid to the preparation of porous Si3N4 ceramic that is capable of generating low dielectric constant while maintaining good mechanical properties (Ref 4, 5). The mechanism of high strength lies in the tailored microstructure, in which the anisotropic growth of columnar b-Si3N4 grains can be controlled to form an interlocked structure that strengthens the ceramic analogous to whisker-reinforced material (Ref 6, 7). Gel-casting is an innovative approach to the preparation of porous Si3N4 ceramics. The basic principle of the technique is forming a three-dimensional polymer structure through in situ polymerized organic molecules to tightly hold the ceramic powders (Ref 8). The advantages include dimensional accuracy, density uniformity, complex shaping capability and cost Chunrong Zou, Shaojun Guo, Xiaosong Zhou, Song Li, and Tongsheng Shen, Advanced Interdisciplinary Technology Research Center, National Innovation Institute of Defense Technology, Beijing 100071, China; and Chunlei Yan, Rocket Force Universi
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