N. Bunjes et al. Journal of Non-Crystalline Solids 353(2007)1567-1576 with short-range ordered turbostratic layers mixed with circular shape, whereas with growing crystal size, contours cemingly amorphous domains. The grain boundary become more irregular. The aspect ratio of the largest crys- appears to be clean and smooth. The orientation of tur- tals was determined to be 1: 2. A common feature of these bostratic units at grain boundaries was found to be arbi- largest grains is the formation of polytypes; stripes in the trary, ranging from parallel to perpendicular directions bright field image are always parallel to the longer grain axis. The electron diffraction pattern of this material 3.3. Samples annealed at 2000C (Fig. 9(b)) reveals polycrystalline silicon carbide and dif- fraction rings and spots which can be assigned to partially a Annealing treatments of 3e and 5c at 2000C initiated crystallized carbon(schwarzite). Besides, the presence of a her grain growth of the SiC crystals. A typical micro. BNCx phase is signaled by two or three rings which can be structure of 3c/2000 is shown as a bright field image in attributed to(0002),(1010)/(1011)and(1120)reflec 300 nm. Small grains(25-75 nm)appear to be of nearly A comparison of the bright field images of 3c/2000 and results(Fig. 2(b)), however, it is obvious that structural variations of the matrix phases are to be expected. There- fore, high-resolution TEM was used concentrating on the matrix phase. In Fig. 10, microstructural features of the BNC phase in 3c/2000 and 5c/2000 are shown. Crystallin- ity of the matrix phase is clearly more advanced in ceramic 3c/2000 compared to 5c/2000. In the former material, the elements B, N, and C form layers with a distinct short range order. Up to 5-8 layers are oriented parallel forming long ribbons which assemble an interweaved structure. The orientation of these ribbons is arbitrary. Small areas in between seem to remain amorphous. In contrast to this, structural characteristics in 5c/2000 vary in a wide range. Parallel layer ribbons are also observed, but for the most part they are less broad and less planar forming closed or open shells (larger curvature) The starting materials 3 and 5(Fig. 1)are highly cross- linked organometallic polymers building an amorphous Fig8 HRTEM micrograph of 5c thermolyzed at 1400C/2 h/Ar and network of the constituting elements Si, C, N, B and H subsequently annealed at 1800C/5 h/Ar with a distinct short-range order. Upon thermolysis, the a b CL =580 mm Fig 9.(a) Bright field image and(b) EDP of 3e thermolyzed at 1400C/2 h/Ar and subsequently annealed at 2000C/5 h/Arwith short-range ordered turbostratic layers mixed with seemingly amorphous domains. The grain boundary appears to be clean and smooth. The orientation of tur￾bostratic units at grain boundaries was found to be arbi￾trary, ranging from parallel to perpendicular directions. 3.3. Samples annealed at 2000 C Annealing treatments of 3c and 5c at 2000 C initiated further grain growth of the SiC crystals. A typical micro￾structure of 3c/2000 is shown as a bright field image in Fig. 9(a). Crystal diameters are ranging from about 25 to 300 nm. Small grains (25–75 nm) appear to be of nearly circular shape, whereas with growing crystal size, contours become more irregular. The aspect ratio of the largest crys￾tals was determined to be 1:2. A common feature of these largest grains is the formation of polytypes; stripes in the bright field image are always parallel to the longer grain axis. The electron diffraction pattern of this material (Fig. 9(b)) reveals polycrystalline silicon carbide and dif￾fraction rings and spots which can be assigned to partially crystallized carbon (schwarzite). Besides, the presence of a BNCx phase is signaled by two or three rings which can be attributed to (0 0 0 2), (1 0 1 0)/(1 0 1 1) and (1 1 2 0) reflec￾tions of graphite. A comparison of the bright field images of 3c/2000 and 5c/2000 reveals no significant differences. From XRD results (Fig. 2(b)), however, it is obvious that structural variations of the matrix phases are to be expected. There￾fore, high-resolution TEM was used concentrating on the matrix phase. In Fig. 10, microstructural features of the BNCx phase in 3c/2000 and 5c/2000 are shown. Crystallin￾ity of the matrix phase is clearly more advanced in ceramic 3c/2000 compared to 5c/2000. In the former material, the elements B, N, and C form layers with a distinct short￾range order. Up to 5–8 layers are oriented parallel forming long ribbons which assemble an interweaved structure. The orientation of these ribbons is arbitrary. Small areas in between seem to remain amorphous. In contrast to this, structural characteristics in 5c/2000 vary in a wide range. Parallel layer ribbons are also observed, but for the most part they are less broad and less planar forming closed or open shells (larger curvature). 4. Discussion The starting materials 3 and 5 (Fig. 1) are highly cross￾linked organometallic polymers building an amorphous network of the constituting elements Si, C, N, B and H with a distinct short-range order. Upon thermolysis, the Fig. 8. HRTEM micrograph of 5c thermolyzed at 1400 C/2 h/Ar and subsequently annealed at 1800 C/5 h/Ar. Fig. 9. (a) Bright field image and (b) EDP of 3c thermolyzed at 1400 C/2 h/Ar and subsequently annealed at 2000 C/5 h/Ar. N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576 1573