Mechanism and kinetics of the carbothermal nitridation synthesis of {alpha}-silicon nitride
- Dow Chemical Co., Midland, MI (United States)
The carbothermal nitridation synthesis of {alpha}-Si{sub 3}N{sub 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`` {alpha}-Si{sub 3}N{sub 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 {alpha}-Si{sub 3}N{sub 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{sub 3}N{sub 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{sub 2} is present at all stages of the reaction, indicates that the overall reaction rate is controlled by the reduction of SiO{sub 2}. Since reaction at the carbon and SiO{sub 2} contact points is fast, the rate-limiting step is most likely the gas-phase carbon reduction of SiO{sub 2} with CO.
- Sponsoring Organization:
- USDOE Assistant Secretary for Energy Efficiency and Renewable Energy, Washington, DC (United States)
- DOE Contract Number:
- AC05-84OR21400
- OSTI ID:
- 556440
- Journal Information:
- Journal of the American Ceramic Society, Vol. 80, Issue 11; Other Information: PBD: Nov 1997
- Country of Publication:
- United States
- Language:
- English
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