N. Bunjes et al I Journal of Non-Crystalline Solids 353(2007)1567-1576 b (1010),(1011) T=1400c T=2000°c T=1400°c T=2000°c Fig ll. EDPs of: (a)3c and(b)5c after thermolysis at 1400C or after annealing at 2000C/5 h/Ar, respectively. observed for material 5c. Here, the evolution of graphite would be very interesting to determine the elemental distri- diffraction signals from the as-thermolyzed to the annealed bution of carbon and boron nitride within the matrix phase state is less pronounced. Diffraction signals of the bNcx to see if the elemental concentrations are the same in crys- process of the matrix phase is slower than crystallization tallized and in amorphous areas. Unfortunately, this was phase remained indistinct and diffuse an in 3e. By con- not possible with the equipment that was used. trast. the changes of Sic reflections asso treatment of 3c and 5e are very simle ciated with thermal The evolution of the matrix phase in 3c or 5c can bly be a consequence of the polymer structure Materials 3c Special microstructural features of polymer-derived and Se were derived from different polymeric precursors. ceramics can easily be visualized using(high-resolution) Whereas, 3e was obtained by thermolysis of a polyborosi- transmission electron microscopy. Two ceramic materials lazane containing Si-NH, units bridged by(C2H4)3B obtained by thermolysis of different polymers were ana- groups, the precursor for 5c was a boron-containing poly- lyzed. They were chosen because of ylcarbodiimide with ESi-N=C=N-Si] and con- necting( C2H4)3B groups. Bond cleavage and bond different structural units in the polymeric state, formation during the ceramization process of different pre- similar elemental composition before and after ursor-derived Si/B/C/N ceramics was thoroughly investi- thermolysis, gated and intermediates formed at different temperatures ceramic compositions located within the three-phase were characterized. Spectroscopic analyses of such interme- field SiC t Bn+C (i.e. no 'complications' by Si3N4 diate structures indicated that the carbodiimide unit partly withstands thermal decomposition up to high tempera-. high-temperature mass stability up to 2000C (i.eno tures. According to NMR measurements the formation compositional changes during annealing of bn during thermolysis of polysilazanes and polysilylcar bodiimides occurs between 400 and 500C [21]. The carbo- Thermolysis of the polymers at 1050C in an argon diimide groups N=C=N, however, do not decompose atmosphere yielded amorphous materials with a homoge completely in this temperature range since C=N vibrations neous elemental distribution on a sub-nanometer scale were detected in IR spectra up to 800C [21]. On this When thermolysis was performed at 1400oC, the samples account the formation of Bn and 'free carbon' should be contained Sic crystals of about 7.5 nm(3c) or 20-30 nm retarded in polysilylcarbodiimides compared to polysila-(5c) diameter embedded in a non-crystalline matrix. Addi zanes. If BN and C are preformed in an early stage during tional annealing at 1600C led to Sic grain coarsening thermolysis as in silazanes, crystallization of the BNCx with crystallites dispersed in a matrix phase consisting of phase at higher temperatures will possibly proceed faster ron. nitro gen, and carbon. Up to 1800C, no distinct The elemental composition of the matrix phase may evolution of the matrix phase structure was observed. After play a minor role. BN crystallization was shown to be heat treatment at 2000C for 5 h, the BNCr matrix of the retarded in the presence of carbon [22]. Therefore, it seems polysilazane-derived ceramic appeared to be structured in a possible that an increase of the carbon content within the short-or middle-range order forming large planar or tur matrix phase from 60 at in 3e to 65% in 5c may lead bostratic ribbons around small seemingly amorphous to structural changes but the effect should be small. It areas. In the polysilylcarbodiimide-derived material theobserved for material 5c. Here, the evolution of graphite diffraction signals from the as-thermolyzed to the annealed state is less pronounced. Diffraction signals of the BNCx phase remained indistinct and diffuse i.e. the crystallization process of the matrix phase is slower than in 3c. By con￾trast, the changes of SiC reflections associated with thermal treatment of 3c and 5c are very similar. The evolution of the matrix phase in 3c or 5c can possi￾bly be a consequence of the polymer structure. Materials 3c and 5c were derived from different polymeric precursors. Whereas, 3c was obtained by thermolysis of a polyborosi￾lazane containing „SiANH2 units bridged by (C2H4)3B groups, the precursor for 5c was a boron-containing poly￾silylcarbodiimide with [„SiAN@C@NASi„] and con￾necting (C2H4)3B groups. Bond cleavage and bond formation during the ceramization process of different pre￾cursor-derived Si/B/C/N ceramics was thoroughly investi￾gated and intermediates formed at different temperatures were characterized. Spectroscopic analyses of such interme￾diate structures indicated that the carbodiimide unit partly withstands thermal decomposition up to high tempera￾tures. According to NMR measurements the formation of BN during thermolysis of polysilazanes and polysilylcar￾bodiimides occurs between 400 and 500 C [21]. The carbo￾diimide groups N@C@N, however, do not decompose completely in this temperature range since C@N vibrations were detected in IR spectra up to 800 C [21]. On this account the formation of BN and ‘free carbon’ should be retarded in polysilylcarbodiimides compared to polysila￾zanes. If BN and C are preformed in an early stage during thermolysis as in silazanes, crystallization of the BNCx phase at higher temperatures will possibly proceed faster. The elemental composition of the matrix phase may play a minor role. BN crystallization was shown to be retarded in the presence of carbon [22]. Therefore, it seems possible that an increase of the carbon content within the matrix phase from 60 at.% in 3c to 65% in 5c may lead to structural changes but the effect should be small. It would be very interesting to determine the elemental distri￾bution of carbon and boron nitride within the matrix phase to see if the elemental concentrations are the same in crys￾tallized and in amorphous areas. Unfortunately, this was not possible with the equipment that was used. 5. Summary Special microstructural features of polymer-derived ceramics can easily be visualized using (high-resolution) transmission electron microscopy. Two ceramic materials obtained by thermolysis of different polymers were ana￾lyzed. They were chosen because of • different structural units in the polymeric state, • similar elemental composition before and after thermolysis, • ceramic compositions located within the three-phase field SiC + BN + C (i.e. no ‘complications’ by Si3N4 formation), • high-temperature mass stability up to 2000 C (i.e. no compositional changes during annealing). Thermolysis of the polymers at 1050 C in an argon atmosphere yielded amorphous materials with a homoge￾neous elemental distribution on a sub-nanometer scale. When thermolysis was performed at 1400 C, the samples contained SiC crystals of about 7.5 nm (3c) or 20–30 nm (5c) diameter embedded in a non-crystalline matrix. Addi￾tional annealing at 1600 C led to SiC grain coarsening with crystallites dispersed in a matrix phase consisting of boron, nitrogen, and carbon. Up to 1800 C, no distinct evolution of the matrix phase structure was observed. After heat treatment at 2000 C for 5 h, the BNCx matrix of the polysilazane-derived ceramic appeared to be structured in a short- or middle-range order forming large planar or tur￾bostratic ribbons around small seemingly amorphous areas. In the polysilylcarbodiimide-derived material the Fig. 11. EDPs of: (a) 3c and (b) 5c after thermolysis at 1400 C or after annealing at 2000 C/5 h/Ar, respectively. N. 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