1574 N. Bunjes et al. Journal of Non-Crystalline Solids 353(2007)1567-1576 25m Fig 10. HRTEM micrographs of: (a)3 c and(b)5e both thermolyzed at 1400C/2 h/Ar and subsequently annealed at 2000C/5 h/Ar organic-to-inorganic transformation takes place which is could not be identified exactly up to now. The elemental accompanied by the loss of hydrogen and hydrocarbon maps and EDPs of 5c agree with this interpretation. The car molecules and atomic rearrangement. Important properties bon concentration in Sic is 50 at. % whereas in the matrix of the resulting materials depend on thermolysis conditions phase with a composition B13.0N135C47 3, it is 64 at. % and can be controlled by subsequent annealing. At higher Therefore, in the carbon map, areas corresponding to Sic temperatures, the structural evolution is increasingly grains are darker than areas of the matrix phase. affected by thermodynamics leading to predominantly crys- An estimation of the crystal size from bright field images talline materials gave the following approximate results for material 5c: Thermolysis of 3 and 5 at 1050C yields inorganic mate- 1400C: 20-30 nm, average 26 nm, 1600C: 10-50 nm, rials 3c/1050 and 5c/1050 in an amorphous state. The ele- average 26 nm, 1700C: 25-125 nm, average 38 nm, ments Si, C, n and b are distributed on an atomic level 1800 C: 38-63 nm, average 43 nm, 2000 C: 25-300 nm. without middle or long-range order as could be shown by These numbers suggest that nucleation and initial grair X-ray(Fig. 2)and electron diffraction(Fig 3). After ther- growth proceed fast at low temperature(between 1050 molysis at 1400C nanometer-sized crystallites of Sic were and 1400C)and subsequent crystal growth is slow up to distributed within a matrix phase. Unfortunately, it was 1800C. Significantly accelerated grain growth was not possible to determine whether phase separation into observed between 1800 and 2000C. Corresponding obser Sic and a BNCx phase is complete at this stage. The matrix vations can be made for the evolution of the matrix phase phase might still contain silicon as amorphous SiC or even Up to 1800C, BNCx is mainly characterized by SiCxNy. If this is the case, the concentration and size of amorphous features: HRTEM indicates the presence of these amorphous areas must be small because they are sub-nm-sized areas with seemingly parallel or turbostratic not'visible' in the boron map orientation of atomic layers. Between 1800 and 2000C Annealing of different samples at highe er temperatur es diffusion processes become significantly faster; the forma produced microstructures with increasing grain size. As tion of middle-range ordered turbostratic layers can be shown by ESI and EDP(Figs. 5-9)SiC crystals are embed- observed indicating progressive evolution of BNC ded into a BNCx matrix phase. Whereas, the grains repre- towards the thermodynamically stable crystalline phases sent bright or grey areas in the silicon and carbon map, These changes of the matrix phase, however, are signif- the matrix contains boron and carbon which are homoge- icantly more pronounced in polysilazane-derivee neously distributed. In the carbon map, the amorphous compared to polysilylcarbodiimide-derived 5c(Fig. 10) phase appears brighter than the grains. To interpret these Corresponding differences are also observed in the electron results the overall chemical composition of material 5c has diffraction patterns of 3c/2000 and 5c/2000. For better to be taken into account. After thermolysis at 1400C, the comparison, the EDPs of materials after different heat um formula was determined to be Sil3 C60.4B130N135 by treatments are collocated in Fig. 11. On the left side hemical analysis [16]. During the heat treatments elemental (Fig. 11(a), difraction rings of 3e after thermolysis at concentrations did not change significantly as was shown by 1400C and after annealing at 2000C are presented. Sig- high-temperature thermogravimetric analysis [16]. There- nals due to the presence of graphite (or bn or a bNC fore, crystallization should lead to the formation of phase) are indicated. Obviously, the heat treatment at osite material consisting of 26.0 at. %BN, 0.8 at. %Si3 N4, higher temperatures initiated a clear structural evolution 25.5 at. SiC, and 47.7 at. %C. Earlier TEM studies on of the matrix phase. Whereas, diffraction signals of the polymer-derived Si/B/C/N ceramics indicated that boron as-thermolyzed material 3e are weak, broad, and diffuse nitride and'free carbon'tend to form a mixed matrix phase their intensity and sharpness were significantly increased (BNC)during thermolysis [20, 12-14]the structure of which after annealing at 2000C. This phenomenon was notorganic-to-inorganic transformation takes place which is accompanied by the loss of hydrogen and hydrocarbon molecules and atomic rearrangement. Important properties of the resulting materials depend on thermolysis conditions and can be controlled by subsequent annealing. At higher temperatures, the structural evolution is increasingly affected by thermodynamics leading to predominantly crys￾talline materials. Thermolysis of 3 and 5 at 1050 C yields inorganic mate￾rials 3c/1050 and 5c/1050 in an amorphous state. The ele￾ments Si, C, N and B are distributed on an atomic level without middle or long-range order as could be shown by X-ray (Fig. 2) and electron diffraction (Fig. 3). After ther￾molysis at 1400 C nanometer-sized crystallites of SiC were distributed within a matrix phase. Unfortunately, it was not possible to determine whether phase separation into SiC and a BNCx phase is complete at this stage. The matrix phase might still contain silicon as amorphous SiC or even SiCxNy. If this is the case, the concentration and size of these amorphous areas must be small because they are not ‘visible’ in the boron map. Annealing of different samples at higher temperatures produced microstructures with increasing grain size. As shown by ESI and EDP (Figs. 5–9) SiC crystals are embed￾ded into a BNCx matrix phase. Whereas, the grains repre￾sent bright or grey areas in the silicon and carbon map, the matrix contains boron and carbon which are homoge￾neously distributed. In the carbon map, the amorphous phase appears brighter than the grains. To interpret these results the overall chemical composition of material 5c has to be taken into account. After thermolysis at 1400 C, the sum formula was determined to be Si13.1C60.4B13.0N13.5 by chemical analysis [16]. During the heat treatments elemental concentrations did not change significantly as was shown by high-temperature thermogravimetric analysis [16]. There￾fore, crystallization should lead to the formation of a com￾posite material consisting of 26.0 at.% BN, 0.8 at.% Si3N4, 25.5 at.% SiC, and 47.7 at.% C. Earlier TEM studies on polymer-derived Si/B/C/N ceramics indicated that boron nitride and ‘free carbon’ tend to form a mixed matrix phase (BNCx) during thermolysis[20,12–14] the structure of which could not be identified exactly up to now. The elemental maps and EDPs of 5c agree with this interpretation. The car￾bon concentration in SiC is 50 at.%, whereas in the matrix phase with a composition B13.0N13.5C47.3, it is 64 at.%. Therefore, in the carbon map, areas corresponding to SiC grains are darker than areas of the matrix phase. An estimation of the crystal size from bright field images gave the following approximate results for material 5c: 1400 C: 20–30 nm, average 26 nm, 1600 C: 10–50 nm, average 26 nm, 1700 C: 25–125 nm, average 38 nm, 1800 C: 38–63 nm, average 43 nm, 2000 C: 25–300 nm. These numbers suggest that nucleation and initial grain growth proceed fast at low temperature (between 1050 and 1400 C) and subsequent crystal growth is slow up to 1800 C. Significantly accelerated grain growth was observed between 1800 and 2000 C. Corresponding obser￾vations can be made for the evolution of the matrix phase. Up to 1800 C, BNCx is mainly characterized by amorphous features; HRTEM indicates the presence of sub-nm-sized areas with seemingly parallel or turbostratic orientation of atomic layers. Between 1800 and 2000 C diffusion processes become significantly faster; the forma￾tion of middle-range ordered turbostratic layers can be observed indicating progressive evolution of BNCx towards the thermodynamically stable crystalline phases. These changes of the matrix phase, however, are signif￾icantly more pronounced in polysilazane-derived 3c compared to polysilylcarbodiimide-derived 5c (Fig. 10). Corresponding differences are also observed in the electron diffraction patterns of 3c/2000 and 5c/2000. For better comparison, the EDPs of materials after different heat treatments are collocated in Fig. 11. On the left side (Fig. 11(a)), diffraction rings of 3c after thermolysis at 1400 C and after annealing at 2000 C are presented. Sig￾nals due to the presence of graphite (or BN or a BNCx phase) are indicated. Obviously, the heat treatment at higher temperatures initiated a clear structural evolution of the matrix phase. Whereas, diffraction signals of the as-thermolyzed material 3c are weak, broad, and diffuse, their intensity and sharpness were significantly increased after annealing at 2000 C. This phenomenon was not Fig. 10. HRTEM micrographs of: (a) 3 c and (b) 5c both thermolyzed at 1400 C/2 h/Ar and subsequently annealed at 2000 C/5 h/Ar. 1574 N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576