Preparation of Porous Si₃N₄ Ceramics with Controlled Porosity and Microstructure by Fibrous α‐Si₃N₄ Addition

Abstract

Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α‐Si3N4 powder with a various content ratios. The effects of the contents of the fibrous α‐Si3N4 powder (0%–100%) on the microstructure and mechanical properties of porous Si3N4 ceramics were studied. As the increase of the fibrous α‐Si3N4 powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si3N4 particle. XRD analysis proved the complete formation of single β‐Si3N4 phase. SEM analysis revealed that the microstructure of the low content of fibrous α‐Si3N4 porous ceramics was almost composed of fine elongated β‐Si3N4 grains with high aspect ratio while numerous coarse elongated β‐Si3N4 grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α‐Si3N4 powder increased. With the increase in content of the fibrous α‐Si3N4 powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties.

Permeability and corrosion resistance of the porous alumina doped by Y2O3 with monodispersed PMMA as pore former

“…The high β value means that the strength was decreased more drastically with the porosity increased. It can be seen that the β value of FA35, 14.67, is larger than the typical value (3‐9) as reported 23,24 . With the firing temperature increased from 1350 to 1550°C, the β value was decreased from 14.67 to 4.12, indicating that, the strength was decreased more gently with the increasing porosity.…”

“…It can be seen that the β value of FA35, 14.67, is larger than the typical value (3-9) as reported. 23,24 With the firing temperature increased from 1350 to 1550 • C, the β value was decreased from 14.67 to 4.12, indicating that, the strength was decreased more gently with the increasing porosity. This is due to the tighter alignment between grains and the better crystallinity, which can be concluded by observing the SEM micrographs of the samples (Figures 4 and 5).…”

“…[23][24][25] 𝜎 = 𝜎 0 exp(−𝛽𝑃), (6) where σ and σ 0 are the strengths of porous ceramics with porosity P and pore-free ceramics, β is the structural factor, which is a slope of a semi-log plot of σ against P, and the typical β value for σ would be 3-9. 23,24 Figure 7B presents the relationship between bending strength and porosity of the porous alumina obtained in this work. The β values of the samples with FA as raw powders fired at 1350 • C, 1450 • C, and 1550 • C are 14.67, 5.88, and 4.12, respectively.…”

Extrusion of highly porous silicon nitride ceramics with bimodal pore structure and improved gas permeability

“…In view of the similar development of β‐Si 3 N 4 grains, the β value for different samples can be thought to be the same. Based on Equation and the obtained strength, it can be calculated that the current structural factor is ~9, which is relatively large, considering that the typical reported β value is 3‐9 . The high β means a more dramatic degradation in strength with the increase in porosity, which should originate from existence of the large pores in the samples with petroleum coke.…”

“…Based on Equation (3) and the obtained strength, it can be calculated that the current structural factor is~9, which is relatively large, considering that the typical reported b value is 3-9. 17 The high b means a more dramatic degradation in strength with the increase in porosity, which should originate from existence of the large pores in the samples with petroleum coke. These large pores are usually thought as serious flaws from a mechanical strength perspective, which lead to stress concentration in the vicinity of the large pores.…”