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Silicon Nitrides

Several nitrides of silicon appear to exist as white amorphous substances. They are:

SiN, density 3.17,
Si2N3, density 3.64, insoluble in HF,
Si3N4, density 3.44, soluble in HF, forming (NH4)2SiF6,

and are produced together when silicon is strongly heated in nitrogen. When silicon is heated in a crucible packed round with carbon, access of oxygen is almost prevented, and Si3N4 is formed together with the carbonitride Si2C2N (or Si3C3N), the carbide SiC and the carboxide SiCO.

According to Martignon, the nitride Si3N4 is formed when a mixture of silica and-carbon is heated in a current of nitrogen at 1400-1500° C., according to the equation:

3SiO2 + 6C + 2N2 = Si3N4 + 6CO.

From theoretical considerations the heat of formation of this nitride is calculated to be 159,300 calories.

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