Mechanism of active and passive oxidation of reaction-bonded Si3N4-SiC refractories

Abstract

Abstract The mechanism of the active and passive oxidation of reaction-bonded Si 3 N 4 -SiC refractories used for six-month in silicon nitridation shuttle has been analyzed. The change of morphology and component of reaction-bonded Si 3 N 4 -SiC block of saggar have been analyzed. The results show that in Si 3 N 4 -SiC, Si 3 N 4 has a higher activity and is more easily oxidized than SiC in both active and passive oxidation modes. All the fibrous α-Si 3 N 4 which exists in a substantial amount in Si 3 N 4 -SiC before use has transformed to β-Si 3 N 4 or was oxidized. The burning side of Si 3 N 4 -SiC block, which operates in high oxygen partial pressure, is oxidized in passive mode generating SiO 2 , Si 2 N 2 O and carbon. The working side of Si 3 N 4 -SiC block, who works in low oxygen partial pressure, is oxidized in the active mode generating gaseous SiO. In another stage of service, SiO generated by silicon nitridation in the saggar infiltrates into the block and reacts with nitrogen forming small rods of Si 3 N 4 that fill pores, which leads to lower apparent porosity. In the middle section of the block, two different oxidation modes coexist, since there is a gradually variation of oxygen partial pressure.