Formation mechanism of porous reaction-bonded silicon nitride with interconnected pores in the presence of MgO

Abstract

In porous reaction bonded silicon nitride, whiskers normally grow in globular clusters as the dominant morphology and deteriorate the pore interconnectivity. However, the ceramic microstructure was significantly transformed with the addition of MgO; specifically, the morphology was modified to a combination of matte and hexagonal grains. Microstructural observation along with thermodynamic studies suggest that MgO interfered with the presence and nitridation of SiO(g). Consequently, rather than being involved in the whiskers' formation, surface silica instead reacted with volatile MgO to form intermediate products. Through these reactions, whisker formation was blocked, and a porous interconnected structure formed which was confirmed by 3D tomography. After heat-treatment at 1700 o C, β-Si3N4 crystallized in a glassy matrix containing magnesium. Resulting samples had an open-pore structure with porosity of 74-84 vol. %, and density of 0.48-0.75 g.cm -3 . Combination of high porosity and pore size of <40 μm led to compressive strengths of 1.1 to 1.6 MPa.

A comparative study on nitridation mechanism and microstructural development of porous reaction bonded silicon nitride in the presence of CaO, MgO and Al2O3

“…Under low p(O 2 ) (<5 × 10 −3 atm), nitridation of SiO and thus the formation of α-Si 3 N 4 whiskers will continue by reduction of SiO 2 [46]. Hence, as illustrated in Figure 1 and discussed in the literature [26,38,47,48], in the absence of additive and under ultra-high purity nitrogen gas, formation of α-Si 3 N 4 whisker clusters was observed during the nitridation of a highly porous RBSN ceramics fabricated via wet processing of Si-PMMA suspensions. The result of XRD analysis in Figure 2 shows the formation of both α-Si 3 N 4 (36.7%) and β-Si 3 N 4 (63.3%) phases with the α/β ratio of 0.5 after heating for 2 hr at 1425°C.…”

“…Besides the two morphologies of α-matte grains and elongated β-rods, Si 3 N 4 may form whiskers randomly distributed throughout the pore channels [24,25]. These interlocked whiskers with high aspect ratio improve the mechanical strength and fracture toughness of the load-bearing materials while decreasing the interconnectivity of channels and permeability and size of pores in filtration membranes [22,26,27]. Growth of granular grains on the pore walls in contrast leads to the formation of large cavities and enhanced channel interconnectivity suitable for interpenetrating ceramic-metal composites [22,26,27].…”

“…Many detailed studies have explained the growth mechanism of Si 3 N 4 grains in porous RBSN [26,27]; nevertheless, achieving a designed grain morphology and thus a controlled pore microstructure is the major challenge of this route as multiple processing variables are simultaneously involved [28][29][30][31]. As an illustration, porosity provides more reactive area per unit mass and encourages surface reactions such as dissociation and silicon oxidation.…”

Increased microwave absorption property of porous Si3N4 ceramics by loading polymer-derived SiCN for a multifunctional design

“…Porous silicon nitride (Si 3 N 4 ) ceramic, a new structurefunction integrated ceramic material developed from Si 3 N 4 ceramic and a porous ceramic, possesses low density, high mechanical strength, and excellent thermal-chemical corrosion resistance, which can be advantageous in extreme environments, such as high temperatures, high speed, and strong corrosive conditions. [8][9][10] Furthermore, owing to the excellent thermal shock resistance, low dielectric constant, and low dielectric loss, as well as good electromagnetic wave (EMW) transmission performance, porous Si 3 N 4 ceramic is a candidate material for the construction of radomes or antenna windows. 10,11 However, with the increasing complexity of the electromagnetic confrontation environment, constructing a radome with wave-transparent in-band and stealth out-offband characteristics is rather urgent; therefore, adding EMW absorption ability in the porous Si 3 N 4 ceramic is very meaningful.…”