The Evolutionary Process during Pyrolytic Transformation of Poly(N‐methylsilazane) from a Preceramic Polymer into an Amorphous Silicon Nitride/Carbon Composite

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Abstract
The pyrolytic evolution of poly(N‐methylsilazane), –[H2SiN‐Me]x–, from preceramic polymer to ceramic product is followed by heating samples of the partially cross‐linked polymer, in 200°C increments, from ambient temperature to 1400°C. The intermediate products are characterized by chemical analysis, diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), Raman spectroscopy, and29Si and13C magic‐angle spinning (MAS) solid‐state NMR. Spectro‐scopic characterization indicates that the 1400°C pyrolysis products are amorphous silicon nitride mixed with amorphous and graphitic carbon (as determined by Raman spectroscopy), rather than silicon carbide nitride, as expected based on the presence of up to 20 mol% retained carbon. Efforts to crystallize the silicon nitride through heat treatments up to 1400°C do not lead to any crystalline phases, as established by transmission electron microscopy (TEM) and small‐area electron diffraction (SAD). It appears that the presence of free carbon, along with the absence of oxygen, strongly inhibits crystallization of amorphous silicon nitride. These results contrast with the isostructural poly‐(Si‐methylsilazane), –[MeHSiNH]x–, which is reported to form silicon carbide nitride on pyrolysis.