Mechanism and Kinetics of the Carbothermal Nitridation Synthesis of α‐Silicon Nitride

Abstract

The carbothermal nitridation synthesis of ␣-Si 3 N 4 is studied using electron microscopy techniques (FEG/SEM and TEM) and chemical composition analysis to characterize the reaction at various degrees of conversion. The reaction follows a nucleation-growth mechanism. Without ''seed'' ␣-Si 3 N 4 in the precursor, the reaction rate is controlled by the formation of nuclei which are associated with a Si-O-C intermediate phase. In the presence of ''seed,'' the limiting step is growth of ␣-Si 3 N 4 onto the ''seed'' nuclei. Growth appears to follow a gas-phase route and is characterized by an irregular porous layer which grows onto the ''seed.'' The porous structure is the result of reaction around carbon particles which are consumed during the process. The presence of admixed ''seed'' Si 3 N 4 in the precursor formulation increases the reaction rate since the nucleation step is eliminated. An activation energy of E = 457 ± 55 kJ/mol for the overall reaction closely approximates that previously reported for the formation of SiO. This result, along with the finding that residual crystalline SiO 2 is present at all stages of the reaction, indicates that the overall reaction rate is controlled by the reduction of SiO 2 . Since reaction at the carbon and SiO 2 contact points is fast, the rate-limiting step is most likely the gas-phase carbon reduction of SiO 2 with CO.

On the fabrication of ZrC x N y from ZrO 2 via two-step carbothermic reduction–nitridation

“…Similar observations have been noted with the carbothermal reduction-nitridation of silica where Si 3 N 4 nucleates and grows a point separate to the SiO 2 bulk particles and grows from that point to a larger Si 3 N 4 particle. [28,29] This mechanism however, is in contrast to the mechanism of carbothermal reduction-nitridation of UO 2 reported by Mukejee et al [19] and Bardelle and Warin [18], whereby an initial layer of carbonitride forms at the surface of UC, which then becomes UN on top of UC with carbon diffusing to the reaction layer being removed by reaction with hydrogen followed by the diffusion of nitrogen into the particle until a UN particle exists.…”

Synthesis and characterization of SiC and SiC/Si3N4 composite nano powders from waste material

“…TEM images of SiC prepared from coarse (a) and hand-ground (b) rice husk ashes pyrolysized at 1550 • C in nitrogen atmospheres. range that nanophase SiC/a-Si 3 N 4 composite powders can be synthesized [36,39]. In the present study, due to the relatively short dwell time for composite formation (2 h) and the relatively higher temperature (1550 • C), the amount of formed SiC was higher than the amount of Si 3 N 4 as indicated from XRD (Figs.…”

Preparation of Si3N4 Form Diatomite via a Carbothermal Reduction-Nitridation Process

“…Thus, b-Si 3 N 4 phase content reaches its maximum. For a longer soaking time, according to Vlasova et al 23 and Weimer et al, 24 the partial pressure of CO increased with the soaking time, which promoted the formation of a-Si 3 N 4 . This is the reason why a soaking time of 2 h was employed, as the relative content of a-Si 3 N 4 phase increased.…”