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Concept
Silicon nitride
Summary
Silicon nitride is a chemical compound of the elements silicon and nitrogen. Si3N4 (Trisilicon tetranitride) is the most thermodynamically stable and commercially important of the silicon nitrides, and the term ′′Silicon nitride′′ commonly refers to this specific composition. It is a white, high-melting-point solid that is relatively chemically inert, being attacked by dilute HF and hot H3PO4. It is very hard (8.5 on the mohs scale). It has a high thermal stability with strong optical nonlinearities for all-optical applications.
Silicon nitride is prepared by heating powdered silicon between 1300 °C and 1400 °C in a nitrogen atmosphere:
3 Si + 2 N2 → Si3N4
The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen. Without an iron catalyst, the reaction is complete after several hours (~7), when no further weight increase due to nitrogen absorption (per gram of silicon) is detected.
In addition to Si3N4, several other silicon nitride phases (with chemical formulas corresponding to varying degrees of nitridation/Si oxidation state) have been reported in the literature. These include the gaseous disilicon mononitride (Si2N), silicon mononitride (SiN) and silicon sesquinitride (Si2N3), each of which are stoichiometric phases. As with other refractories, the products obtained in these high-temperature syntheses depends on the reaction conditions (e.g. time, temperature, and starting materials including the reactants and container materials), as well as the mode of purification. However, the existence of the sesquinitride has since come into question.
It can also be prepared by diimide route:
SiCl4 + 6 NH3 → Si(NH)2 + 4 NH4Cl(s) at 0 °C
3 Si(NH)2 → Si3N4 + N2 + 3 H2(g) at 1000 °C
Carbothermal reduction of silicon dioxide in a nitrogen atmosphere at 1400–1450 °C has also been examined:
3 SiO2 + 6 C + 2 N2 → Si3N4 + 6 CO
The nitridation of silicon powder was developed in the 1950s, following the "rediscovery" of silicon nitride and was the first
large-scale method for powder production.
Official source
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