Doped Silicon Carbonitride: Synthesis, Characterization and Properties
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605 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
(B[NMe 2 ]3 ) or tris(dimethylamino)phosphine (P[NMe 2 ]3 ) was added dropwise and the reaction mixture is heated under reflux for 144 h under an argon atmosphere. Dimethyl amine which formed via polycocondensation was observed by reaction with hydrogen chloride yielding dimethyl ammonium chloride (eq.(1)). F• ,,,R' •01
NR2
Si•ZN
+ R2N-lV
H
NR2
Ru
-R2NH
/
R=CH3, R'= H RR R•
+
R2N-M
R'
•NeSiNA R2N-M
H
N,
-R2 NH
M
= B,
P
The solvent was evaporated under argon. Subsequently, the reaction product was dried in vacuum at room temperature for 2 h in order to remove residual solvent. Chemical analysis of the obtained polymers indicated the presence of the appropriate elements B and P. The pyrolysis products were subjected to chemical analysis. Nitrogen and oxygen were analysed by hot gas extraction with a heat conducting cell and an IR- absorption cell detecting CO 2 serving as the analyser, respectively, whereas carbon was oxidized to CO2 and quantified in an IR-absorption cell. B, P and Si were detected by means of OES-ICP after dissolving the samples in concentrated H2 SO 4 /HF [4]. The results are shown in Table I: Table I: Chemical analysis data [wt.%] of the boron and phosphorus doped silicon carbonitrides obtained after pyrolysis of the modified polysilazanes and pure silicon carbonitride at 1000°C under Ar molar ratio of B[N(CH 3 )2 ]3 or P[N(CH 3 )2 ]3 versus polysilazane ceramic yield
B-Si-C-N
P-Si-C-N
Si-C-N
4.44
3.39
0
74
74
75
B P
1.1 -
0.5
-
Si N C
52.9 28.0 14.6
57.8 27.4 11.7
57.8 26.0 14.3
606
-
The oxygen content of the obtained ceramics due to handling of the samples in air during chemical analysis was below I wt.%. Concerning the phosphorus and the boron content at higher temperatures chemical analysis reveals that samples held at 1350°C in N2 for 50 h still contain B and P while phosphorus is no longer present in the corresponding samples heat-treated at 1500°C. However, boron can be detected even in samples annealed at 1800°C. 4. Fabrication of ceramic monoliths Figure 1 provides an overview of the preparation of pure, boron or phosphorus doped silicon carbonitrides.
Fig.1: Schematic flow diagram of the production of pure, B- or P-doped amorphous silicon carbonitride monoliths In order to obtain ceramic monoliths consisting of pure, B- or P-doped silicon carbonitride it was necessary to crosslink the polymers at 400°C for 3 h in argon and subsequently for 1 h in vacuum in order to remove low boiling oligomers. The degree of crosslinking was monitored by thermomechanical analysis as the polymer should
607
not melt or soften during pyrolysis yielding a bloated or cracked body. Then, the polymer was crushed in a planetary ball-mill with zirconia balls under argon for 30 min. The resulting polymeric powder was then passed through a 28 micron screen in a glove box providing a fine graded powder which was transferred to rubber forms and subsequently cold isostatically pressed at 625 MPa fo
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