Crystallization of Pyrolyzed Polysilazanes
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CRYSTALLIZATION OF PYROLYZED POLYSILAZANES
Roger L. K. Matsumoto, Hercules Incorporated, Research Center, Wilmington, DE 19894
ABSTRACT Polysilazanes are inorganic polymers which convert to a ceramic material when pyrolyzed. Initial pyrolysis results in a solid, amorphous material. Further heating transforms the amorphous structure to a crystalline material. In addition to the Si-N linkages in the polymer backbone, polysilazanes generally contain appreciable amounts of carbon in side groups. Consequently, pyrolysis can result in mixed Si3N4 - SIC crystalline materials. It will be shown that either Si3N4 or SiC could be selectively crystallized depending upon processing conditions. INTRODUCTION Polysilazane compounds are defined by Si-N linkages in the polymer backbone. Although polysilazanes containing no Si-C bonds have been shown to pyrolyze to silicon nitride exclusively (1), most polysilazanes of practical Interest have organic groups attached to silicon. Figure 1 shows a generalized polysilazane structure. These polysilazanes may produce mixed S 3N4 - SiC ceramics upon pyrolysis (2-4). EXPERIMENTAL PROCEDURE Polysilazanes
of
the
composition (Si1 0N 1. 0C1.4H.54)x were
CH
I pyrolyzed under either nitrogen or argon. Materials were pyrolyzed from 1200 0C to 1600°C in 100°C intervals under nitrogen I I and at 1200 0C and 1600 0C under argon. The polysilazanes were - X filled with an alpha silicon nitride powder (UBE SN-E110) to 20 weight percent (w/o) and were pyrolyzed under nitrogen to = 3 1200 0C and 1600°C. They were also filled with alpha silicon carbide powder (Starck A-10) to Figure 1 Generalized Polysilazane Structure 50 w/o and pyrolyzed under argon to 1200 0C and 16000C. A polysilazane filled with 50 w/o beta silicon carbide whiskers (American Matrix) was pyrolyzed under argon to 1600 0C. In all cases the heating rate used was 100C/min and the time at maximum temperature was 2 hours. All pyrolysis
Si
N
R
R, R'
H, CH
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
798
products were examined by X-ray diffraction (XRD). RESULTS The evolution of crystallinity In the pyrolyzed unfilled polysilazane fired under nitrogen is shown by XRD patterns in Figure 2. Up to 1400 0C, the pyrolysis product remains amorphous. By 1500 0C, it has crystallized to both the alpha and beta forms of silicon nitride. The relative intensities of the alpha and beta polymorphs do
not change upon further heating
to 16000C.
The XRD patterns of the
pyrolysis products
of unfilled
are polysilazanes under argon 0 shown in Figure 3. At 1200 C, the material Is
completely
0 )
aL E A
AA
o o
A
,,a D
. -
C E3
r _ 20
B ___ 28
____ 3S6
44
A 52
6b
DEGREES20
Figure 2 XRD Patterns of Polysilazanes
amorphous. It has 0 completely Pyrolyzed to; (A) 1200°C; (B) 13000C; (C) crystallized by 1600 C, however 14000C; (D) 15000C; and (E) 16000C Under this time mostly beta silicon Nitrogen. carbide (3C polytype) has (a) Denotes Alpha Silicon Nitride Peaks, (o) formed. Denotes Beta Silicon Nitride Peak
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