N. Bunjes et al. Journal of Non-Crystalline Solids 353 (2007)1567-1576 BB小 phous to a predominantly crystalline state. For annealing experiments at T=1600, 1700, 1800, or 2000C specimens thermolyzed at 1400C were directly heated to the maxi- 一N=C=N-Si mum temperature without intermediate dwell times at lower temperature, i.e. a sample heat treated at 1800C was not previously annealed at 1600 and 1700C. Conse- quently, the microstructure observed for the 1800C mate- [B(C H43SiNH2]: 3 5 rial does not perforce issue from that of the 1700C sample Fig. 1. Sche esentations of the precursor polymer structures etc. The experiments described in this study were per- and 5 befo nre herm olssen formed in order to gain some basic insight into the effect of thermal treatments on structurally different polymer derived ceramics. For a detailed discussion of crystalliza- 3e and Si131C604B13 oN135 for polysilylcarbodiimide- tion mechanisms, isothermal annealing of the materials derived 5c. Starting from the sum formulas, the amount of investigating crystal growth with time will be necessary thermodynamically stable phases in completely crystallized In this area, Schmidt et al. [18] published some results on amples can be calculated. According to CalPhaD predic- the formation kinetics of Si, N/SiC composites(boron-free tions(calculation of phase diagrams [17] the ceramic mate- Si/C/N ceramics) fitting the data into classic crystallization rials should consist of 28. 1 at BN, 2.2% B4C, 26.6% SiC, models. First experiments on boron-containing 5c and 43. 1%C( 3c), or 260% BN, 0.8% Si3 N4, 25.5% SiC, and 1400C indicated that crystallization of nano-crystalline 47.7%C(5c), respectively. Even though these compositions SiC proceeds very fast(within 15 min) and is followed by after heat treatment at 1400 C were very similar, some dif- grain coarsening [19). more detailed investigations are cur ferences were indicated by the X-ray patterns(Fig. 2(a))[16]. rently in progress The reflections at about 26 and 420 which can be attributed to BN, C, or a BNCx phase are clearly more intense and 3. 1. As-thermolyzed samples broader in diffractograms of ceramic 5c than in those of 3e. This phenomenon was also observed in the diffraction Thermolysis of the polymeric silazan 3 at 1050C for 4 h diagrams of 3c and 5c samples annealed at higher tempera- leads to the formation of an inorganic material 3e/1050 ures(1600-2000C, Fig 2(b)). To gain more insight into which is X-ray amorphous(Fig. 2(a)). A TEM micrograph the crystallization process, as-thermolyzed and annealed of the sample(Fig 3(a) reveals a one-phase material with samples were analyzed by energy-filtered transmission out distinct features. The distribution of the constituting electron microscopy(EFTEM)and high-resolution trans- elements silicon, carbon, boron, and nitrogen was analyzed mission electron microscopy (HRTEM). Since the high- by electron spectroscopic imaging(ESI, not shown here) temperature behavior of 3c and 5c is comparable in many and was found to be homogeneous over the whole area respects, common features will be presented exemplarily Furthermore, the electron diffraction pattern(EDP)was on samples of 3c or 5e measured( Fig 3(b). The absence of sharp diffraction rings Here, it should be emphasized that the micrographs or spots clearly indicates an amorphous material. Taking shown in this study do not necessarily represent the pro- into account the resolution of the micrograph, the grain gressing crystallization with temperature from an amor- size must be below I nm ■+。 3c(1400 Fig. 2. XRD diagrams of: (a) as-thermolyzed ceramic materials 3c and 5e and (b) after annealing at 2000C for 5 h in an argon atmosphere [161.3c and Si13.1C60.4B13.0N13.5 for polysilylcarbodiimide￾derived 5c. Starting from the sum formulas, the amount of thermodynamically stable phases in completely crystallized samples can be calculated. According to CalPhaD predic￾tions (calculation of phase diagrams [17]) the ceramic mate￾rials should consist of 28.1 at.% BN, 2.2% B4C, 26.6% SiC, and 43.1% C (3c), or 26.0% BN, 0.8% Si3N4, 25.5% SiC, and 47.7% C (5c), respectively. Even though these compositions after heat treatment at 1400 C were very similar, some dif￾ferences were indicated by the X-ray patterns (Fig. 2(a)) [16]. The reflections at about 26 and 42 which can be attributed to BN, C, or a BNCx phase are clearly more intense and broader in diffractograms of ceramic 5c than in those of 3c. This phenomenon was also observed in the diffraction diagrams of 3c and 5c samples annealed at higher tempera￾tures (1600–2000 C, Fig. 2(b)). To gain more insight into the crystallization process, as-thermolyzed and annealed samples were analyzed by energy-filtered transmission electron microscopy (EFTEM) and high-resolution trans￾mission electron microscopy (HRTEM). Since the high￾temperature behavior of 3c and 5c is comparable in many respects, common features will be presented exemplarily on samples of 3c or 5c. Here, it should be emphasized that the micrographs shown in this study do not necessarily represent the pro￾gressing crystallization with temperature from an amor￾phous to a predominantly crystalline state. For annealing experiments at T = 1600, 1700, 1800, or 2000 C specimens thermolyzed at 1400 C were directly heated to the maxi￾mum temperature without intermediate dwell times at lower temperature, i.e. a sample heat treated at 1800 C was not previously annealed at 1600 and 1700 C. Conse￾quently, the microstructure observed for the 1800 C mate￾rial does not perforce issue from that of the 1700 C sample etc. The experiments described in this study were per￾formed in order to gain some basic insight into the effects of thermal treatments on structurally different polymer￾derived ceramics. For a detailed discussion of crystalliza￾tion mechanisms, isothermal annealing of the materials investigating crystal growth with time will be necessary. In this area, Schmidt et al. [18] published some results on the formation kinetics of Si3N4/SiC composites (boron-free Si/C/N ceramics) fitting the data into classic crystallization models. First experiments on boron-containing 5c at 1400 C indicated that crystallization of nano-crystalline SiC proceeds very fast (within 15 min) and is followed by grain coarsening [19]. More detailed investigations are cur￾rently in progress. 3.1. As-thermolyzed samples Thermolysis of the polymeric silazan 3 at 1050 C for 4 h leads to the formation of an inorganic material 3c/1050 which is X-ray amorphous (Fig. 2(a)). A TEM micrograph of the sample (Fig. 3(a)) reveals a one-phase material with￾out distinct features. The distribution of the constituting elements silicon, carbon, boron, and nitrogen was analyzed by electron spectroscopic imaging (ESI, not shown here) and was found to be homogeneous over the whole area. Furthermore, the electron diffraction pattern (EDP) was measured (Fig. 3(b)). The absence of sharp diffraction rings or spots clearly indicates an amorphous material. Taking into account the resolution of the micrograph, the grain size must be below 1 nm. Si NH2 B B B Si N=C=N B B B Si B B B [B(C2H4)3SiNH2]: 3 [{B(C2H4)3Si}2NCN]: 5 Fig. 1. Schematic representations of the precursor polymer structures 3 and 5 before thermolysis. 10 20 30 40 50 60 70 80 SiC C BN 5c (1400 ˚C) 3c (1400 ˚C) 3c (1050 ˚C) 2θ/˚ 10 20 30 40 50 60 70 80 5c (2000 ˚C) 3c (2000 ˚C) 2θ/˚ SiC C BN Fig. 2. XRD diagrams of: (a) as-thermolyzed ceramic materials 3c and 5c and (b) after annealing at 2000 C for 5 h in an argon atmosphere [16]. N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576 1569