Porous silicon nitride-based ceramics fabricated from silicon and carbon powders

Porous ceramic materials are widely used as fluid flow filters, separation membranes, lightweight structural components, crystal supports, electrodes, bioreactors, catalysts, and so on, since they show superior properties such as high temperature stability, lower Young’s modulus, enhanced thermal shock resistance, erosion/corrosion resistance, catalytic activity etc. [1, 2]. Porous silicon nitride-based ceramics have also been studied. Recently, Kawai et al. successfully fabricated high strength porous silicon nitride with rod-shaped grains, which formed a three-dimensional network structure [3]. The silicon nitride with the relative density of 61.7% exhibited a high bending strength of 455 MPa. This high strength was achieved due to the microstructure of elongated rod-shaped grains. Based on their strategies, Yang et al. studied optimum fabrication conditions to achieve higher porosity and higher strength [4]. It is needless to say that a low-cost process is essentially important when production of such silicon nitride-based porous ceramics are put into commercial application. In this study, low-cost silicon and carbon powders are used as the starting material to fabricate silicon nitride-based porous ceramics through their reactions in a nitrogen atmosphere. The advantages of using silicon-carbon mixed powder are as follows. I. The nitridation reaction of silicon is difficult to control, since it is an exothermic reaction [5]. On the other hand, the reaction can be well controlled by adding SiC or carbon powder [6, 7]. II. A substantial amount of silicon is consumed for SiC formation from silicon and carbon, which is advantageous to control the porosity. Combining the following two stages can be considered appropriate to fabricate porous silicon nitridebased ceramics [5, 8]. The first stage is the reaction one. The silicon-carbon mixed raw powder with sintering additives is reacted to form silicon nitride and silicon carbide through reactions 3Si+ 2N2→ Si3N4 and Si+ C→ SiC at 1200–1400 ◦C in nitrogen atmosphere. The second stage is the microstructure control stage. The reacted body is heat-treated at 1600–1900 ◦C to form rod-shaped grains. Using this procedure, porous silicon nitride-based ceramics are fabricated under several conditions, and the mechanical properties are evaluated in relation to their microstructures. Silicon powder (High Purity Chemetals Laboratory, 99.9%, 1 μm), carbon black (Mitsubishi Chemical, #750, 18 nm), Y2O3 powder (Shinetsu, UF) and Al2O3 powder (Sumitomo, AKP 30) were first mixed. The weight ratio of Si : C : Y2O3 : Al2O3 was 0.8084 : 0.0782 : 0.0709 : 0.0425. This mixture would ideally form 73.6 wt.% Si3N4− 18.4 wt.% SiC− 5 wt.% Y2O3− 3 wt.% Al2O3 (the weight ratio of Si3N4 : SiC was 0.8 : 0.2), if reactions 3Si+ 2N2→ Si3N4 and Si+C→ SiC were completed. First, in order to evaluate the reaction condition, the powder compacts were heated at 1250 and 1350 ◦C for various periods of time in 0.1 MPa nitrogen atmosphere. The mixed powder was cold isostatically pressed (CIP) under a pressure of 490 MPa to form columnar green bodies, 14 mm in diameter and 10 mm in height. The degree of nitridation was calculated from the weight change of the specimen. Then, post-heat-treatment condition was studied for platelet-shaped specimens. The mixed powder of 25 g was compacted with a uniaxial pressure of 2.5 MPa in a carbon die with a base of 45 × 45 mm. A reaction was conducted at 1250 ◦C for 16 h in 0.1 MPa nitrogen atmosphere, subsequently followed by a heat treatment at 1650 ◦C for 3 h in 0.1 MPa nitrogen atmosphere, or by that at 1850 ◦C for 3 h in 1 MPa nitrogen atmosphere. Hereafter, the former and the latter are designated as specimens A and B, respectively. Note that 1650 ◦C is the highest temperature for heat treatment in atmospheric nitrogen pressure, while 1850 ◦C is presumably high enough to enhance grain growth of β-silicon nitride and form elongated grains. X-ray diffraction (XRD) analysis was performed to examine the phases of the specimens. The microstructures of the specimens were examined by scanning electron microscopy (SEM). The porosity and the pore-size distribution of the porous composites were determined by the mercury porosimetry. The three-point bending strength was measured using specimens of 3 × 4 × 40 mm cut from the sintered bodies. The measurement was conducted under the condition with a span of 30 mm at a cross-head speed of 0.5 mm ·min−1 at room temperature. A reaction study was conducted using the CIP columnar powder compact. Its initial density was 1.36× 103 kg ·m−3. The degree of nitridation is shown in Fig. 1. Note that the calculation was conducted assuming that the SiC formation occupied about 70% of the total reaction, which was estimated from our previous study [8]. Melting behavior was not observed at either temperature. A very short reaction time was actualized by adding carbon powder (and formation of SiC [6, 7]);