° ScienceDirect JOURNAL OF NON-CRYSTALLINE SOLIDS ELSEVIER Journal of Non-Crystalline Solids 353(2007)1567-1576 www.elsevier.com/locate/jnoncrysol Crystallization of polymer-derived SiC/BN/C composites investigated by tem Natascha Bunjes, Anita Muller, Wilfried Sigle, Fritz Aldinger Max-Planck-Institut fir Metallforschung and Institut fuir Nichtmetallische Anorganische Materialien, Universitat Stuttgart, orium. 70569 Si Received 2 August 2006: received in revised form 19 January 2007 Available online 19 March 2007 Abstract The crystallization behavior of two polymer-derived Si/B/C/N ceramics with similar compositions lying close to the three-phase field BN+ Sic +C was investigated by (high-resolution) transmission electron microscopy. The materials were high-temperature mass stable up to T=2000C. During thermolysis at 1050C a homogeneous amorphous solid formed SiC crystallization started at about 1400C. Further annealing to higher temperatures up to 2000C led to formation of microstructures composed of Sic crystals embedded into a structured BNCr matrix phase. With increasing temperature, both the size of the crystallites and the ordering of the matrix phase increased C 2007 Elsevier B. v. All rights reserved Keywords: Crystallization; Ceramics; TEM/STEM; Microstructure: Nano-composites: Medium-range order 1. Introduction Thermolysis of preceramic Si/B/C/N/H polymers is usu- ally performed at es up to I400°C. Since the pioneering work of Verbeek and Winter [1, 2] process, cross-linking of the polymers takes place in the the synthesis of polymer-derived silicon-based ceramics range between 100 and 400C, whereas heating to has become a field of topical interest. In contrast to ceram- 400-800C initiates the organic-to-inorganic conversion ics obtained by the 'classic powder sintering route, poly- and an amorphous network is formed. At higher tempera mer-derived ceramics are free of sintering additives. They tures(800-1400C), most of the residual hydrogen evapo offer the advantage of high chemical purity, microstruc- rates. Further increasing the temperature leads to tural homogeneity, low processing temperatures and versa- crystallization of the thermodynamically stable phases tile fabrication potentials [3-5]. In 1990, Seyferth et al. [6, 7] and/or decomposition. The structural evolution during introduced boron into polymeric Si/C/N precursors. Upon crystallization(1300-2000oC)can be monitored by micro- thermolysis Si/B/C/N ceramics exhibiting very interesting scopic, spectroscopic and diffraction methods. Many Si/B/ properties were obtained. Since then, by variation of educts C/N ceramics with compositions lying in the four-phase and reaction pathways a constantly increasing number of field BN Si3 N4 SiC tC were analyzed by NMR, Ft- polymer-derived Si/B/C/N ceramics has been synthesized IR, XRD, and TEM. It has been shown that in materials with compositions varying in a wide range. Many of them revealing substantial high-temperature stability, a metasta were proven to be resistant against oxidation, decompo ble microstructure composed of nano-sized SiC and Si3n4 tion or creep at high temperature(see for example Refs. crystals embedded in a turbostratic BNCx matrix phase [8-11) formed at about 1800C [12-14. Few TEM studies have been performed on polymer-derived materials with compo- Corresponding author. Present address: HTW Aalen, Beethovenstr I, sitions located in the neighboring boron-containing three- 73430 Aalen. Germany phase fields BN+ Si3N4+ SiC, Bn+ Si3N4+C, and 0022-3093/S- see front matter 2007 Elsevier B v. All rights reserved doi: 10.1016/j-jnoncrysol. 2007.01.025
Crystallization of polymer-derived SiC/BN/C composites investigated by TEM Natascha Bunjes, Anita Mu¨ller *, Wilfried Sigle, Fritz Aldinger Max-Planck-Institut fu¨r Metallforschung and Institut fu¨r Nichtmetallische Anorganische Materialien, Universita¨t Stuttgart, Pulvermetallurgisches Laboratorium, 70569 Stuttgart, Germany Received 2 August 2006; received in revised form 19 January 2007 Available online 19 March 2007 Abstract The crystallization behavior of two polymer-derived Si/B/C/N ceramics with similar compositions lying close to the three-phase field BN + SiC + C was investigated by (high-resolution) transmission electron microscopy. The materials were high-temperature mass stable up to T = 2000 C. During thermolysis at 1050 C a homogeneous amorphous solid formed. SiC crystallization started at about 1400 C. Further annealing to higher temperatures up to 2000 C led to formation of microstructures composed of SiC crystals embedded into a structured BNCx matrix phase. With increasing temperature, both the size of the crystallites and the ordering of the matrix phase increased. 2007 Elsevier B.V. All rights reserved. Keywords: Crystallization; Ceramics; TEM/STEM; Microstructure; Nano-composites; Medium-range order 1. Introduction Since the pioneering work of Verbeek and Winter [1,2] the synthesis of polymer-derived silicon-based ceramics has become a field of topical interest. In contrast to ceramics obtained by the ‘classic’ powder sintering route, polymer-derived ceramics are free of sintering additives. They offer the advantage of high chemical purity, microstructural homogeneity, low processing temperatures and versatile fabrication potentials [3–5]. In 1990, Seyferth et al. [6,7] introduced boron into polymeric Si/C/N precursors. Upon thermolysis Si/B/C/N ceramics exhibiting very interesting properties were obtained. Since then, by variation of educts and reaction pathways a constantly increasing number of polymer-derived Si/B/C/N ceramics has been synthesized with compositions varying in a wide range. Many of them were proven to be resistant against oxidation, decomposition or creep at high temperature (see for example Refs. [8–11]). Thermolysis of preceramic Si/B/C/N/H polymers is usually performed at temperatures up to 1400 C. During this process, cross-linking of the polymers takes place in the range between 100 and 400 C, whereas heating to 400–800 C initiates the organic-to-inorganic conversion and an amorphous network is formed. At higher temperatures (800–1400 C), most of the residual hydrogen evaporates. Further increasing the temperature leads to crystallization of the thermodynamically stable phases and/or decomposition. The structural evolution during crystallization (1300–2000 C) can be monitored by microscopic, spectroscopic and diffraction methods. Many Si/B/ C/N ceramics with compositions lying in the four-phase field BN + Si3N4 + SiC + C were analyzed by NMR, FTIR, XRD, and TEM. It has been shown that in materials revealing substantial high-temperature stability, a metastable microstructure composed of nano-sized SiC and Si3N4 crystals embedded in a turbostratic BNCx matrix phase formed at about 1800 C [12–14]. Few TEM studies have been performed on polymer-derived materials with compositions located in the neighboring boron-containing threephase fields BN + Si3N4 + SiC, BN + Si3N4 + C, and 0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.025 * Corresponding author. Present address: HTW Aalen, Beethovenstr. 1, 73430 Aalen, Germany. www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 353 (2007) 1567–1576
N. Bunjes et al. Journal of Non-Crystalline Solids 353(2007)1567-1576 BN SiC +C. The first-mentioned are difficult to obtain order to avoid charging of the sample during TEM opera because most of the polymeric precursors deliver carbon- tion. Then the samples were milled with a Gatan hand pol rich materials. SiC-free composites are usually not high- isher down to 100 um, dimpled with a Cu wheel and I um temperature stable and decomposition will interfere with Gatan diamond paste down to 25 um and argon ion sput- rials of the BN Sic +C system were studied. Their syn- of 4.0 and an ion energy of 3.5 kek. an PiPS at an angle microstructure formation. In this paper, two ceramic mate- tered to electron transparency in a Gatan PIPS at an angle thesis was described in a previous paper [15, 16]. XRD results revealed some differences between these ceramics 24. TEM measurements garding the formation and structure of the matrix phase even though the compositions were very similar To inves- 2.4.1 EFTEM measurements tigate the crystallization process in more detail, we applied A Zeiss EM 912Q with LaBs cathode energy-filtered(EFTEM)and high-resolution transmission 120kV electron microscopy (HRTEM)as powerful tools for for he equipped with an imaging omega filter was used Itional TEM and EFTEM investigations. microstructure characterization The bright field images and the selected area energy-filtered diffraction patterns(SAED) were recorded at an emission 2. Experimental procedure current of 6 HA with a side-mounted SIS Mega View CCD camera with 1376x 1032 pixel resolution. For the elemen- 2. General comments tal mapping the data acquisition was performed with a water-cooled Gatan 1024 1024 slow scan CCD camera The thermal mass stability of the as-thermolyzed with the Gatan software Digital Micrograph 2.5 and for samples was investigated using a simultaneous thermal the electron energy loss data with the Gatan EL/P analyzer model Netzsch STA 501(heating rates, T1400°,5°C/min)in an argon atmosphere. The annealing schedule did not con The crystallization behavior of two polymer-derived tain intermediate dwell times, i.e. specimens annealed at ceramics was investigated by transmission electron micros- 1800C were not previously heat treated for 5 h at 1600 copy (TEM). Synthesis [15] and some high-temperature and 1700C, respectively, but were produced by annealing properties [16] of these materials are described in the litera of samples which were thermolyzed at 1400C, cooled ture. Ceramic 3c was derived from the boron-modified silaz down, and then directly heated to 1800C for 5h. ane 3 containing Si-NH2 units interconnected by B(C2H4)3 groups(Fig. 1). In contrast to this, the material 5c was 2.3. TEM sample preparation obtained by thermolysis of the polyborosilylcarbodiimide 5(Fig. 1)composed of cross-linked Si-N=C=N-Si units. For the transmission electron microscopic measure- Ceramization of the polymers 3 or 5 at 1400C yielded ments samples were cut to approximately 3 mm ceramics 3e or 5c with very similar overall composition and mounted on a copper ring for better grounding in Si133C568B158N141 for the polysilazane-derived material
BN + SiC + C. The first-mentioned are difficult to obtain because most of the polymeric precursors deliver carbonrich materials. SiC-free composites are usually not hightemperature stable and decomposition will interfere with microstructure formation. In this paper, two ceramic materials of the BN + SiC + C system were studied. Their synthesis was described in a previous paper [15,16]. XRD results revealed some differences between these ceramics regarding the formation and structure of the matrix phase even though the compositions were very similar. To investigate the crystallization process in more detail, we applied energy-filtered (EFTEM) and high-resolution transmission electron microscopy (HRTEM) as powerful tools for microstructure characterization. 2. Experimental procedure 2.1. General comments The thermal mass stability of the as-thermolyzed samples was investigated using a simultaneous thermal analyzer model Netzsch STA 501 (heating rates, T 1400 C, 5 C/min) in an argon atmosphere. The annealing schedule did not contain intermediate dwell times, i.e. specimens annealed at 1800 C were not previously heat treated for 5 h at 1600 and 1700 C, respectively, but were produced by annealing of samples which were thermolyzed at 1400 C, cooled down, and then directly heated to 1800 C for 5 h. 2.3. TEM sample preparation For the transmission electron microscopic measurements samples were cut to approximately 3 mm2 pellets and mounted on a copper ring for better grounding in order to avoid charging of the sample during TEM operation. Then the samples were milled with a Gatan hand polisher down to 100 lm, dimpled with a Cu wheel and 1 lm Gatan diamond paste down to 25 lm and argon ion sputtered to electron transparency in a Gatan PIPS at an angle of 4.0 and an ion energy of 3.5 keV. 2.4. TEM measurements 2.4.1. EFTEM measurements A Zeiss EM 912 X with LaB6 cathode operating at 120 kV equipped with an imaging omega filter was used for the conventional TEM and EFTEM investigations. The bright field images and the selected area energy-filtered diffraction patterns (SAED) were recorded at an emission current of 6 lA with a side-mounted SIS MegaView CCD camera with 1376 · 1032 pixel resolution. For the elemental mapping the data acquisition was performed with a water-cooled Gatan 1024 · 1024 slow scan CCD camera with the Gatan software Digital Micrograph 2.5 and for the electron energy loss data with the Gatan EL/P software. The elemental mapping was acquired using the threewindow method (Edgerton; two for the background and one to acquire the chosen element signal). The energy slit width was adjusted to 20 eV for silicon and boron and 30 eV for the elements carbon and nitrogen. This allowed to use exposure times between 2 and 10 s. 2-fold binning of the CCD was used to increase the signal-to-noise ratio and to minimize drift during acquisition time as much as possible. The collection semiangle b was 12.5 mrad and the convergence angle 1.6 mrad. 2.4.2. HRTEM measurements High-resolution images analyses were conducted in a JEOL 1250 with LaB6 cathode transmission electron microscope (JEOL, Japan) operated at 1250 kV. Images were taken on Kodak Electron image film SO-163 (8.3 · 10.2 cm2 ) which were scanned with an HP400 flatbed negative scanner working with a maximum pixel resolution of 1200 · 2400. 3. Results The crystallization behavior of two polymer-derived ceramics was investigated by transmission electron microscopy (TEM). Synthesis [15] and some high-temperature properties [16] of these materials are described in the literature. Ceramic 3c was derived from the boron-modified silazane 3 containing SiANH2 units interconnected by B(C2H4)3 groups (Fig. 1). In contrast to this, the material 5c was obtained by thermolysis of the polyborosilylcarbodiimide 5 (Fig. 1) composed of cross-linked SiAN@C@NASi units. Ceramization of the polymers 3 or 5 at 1400 C yielded ceramics 3c or 5c with very similar overall composition: Si13.3C56.8B15.8N14.1 for the polysilazane-derived material 1568 N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576
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 polysilylcarbodiimidederived 5c. Starting from the sum formulas, the amount of thermodynamically stable phases in completely crystallized samples can be calculated. According to CalPhaD predictions (calculation of phase diagrams [17]) the ceramic materials 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 differences 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 temperatures (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 transmission electron microscopy (HRTEM). Since the hightemperature 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 progressing crystallization with temperature from an amorphous 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 maximum 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. Consequently, the microstructure observed for the 1800 C material does not perforce issue from that of the 1700 C sample etc. The experiments described in this study were performed in order to gain some basic insight into the effects of thermal treatments on structurally different polymerderived ceramics. For a detailed discussion of crystallization 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 currently 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 without 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
N. Bunjes et al. Journal of Non-Crystalline Solids 353(2007)1567-1576 b Fig 3.(a) Bright field image and(b) EDP of 3c thermolyzed at 1050C/4 h/Ar. After thermolysis of the polymers at 1400C for 2 h in of the sample, the distribution seems to be almost an argon atmosphere the samples contained Sic crystals homogeneous. and a BNCx phase as was indicated by XRD(Fig. 2(a)) The EDP of 5c/1400( Fig 4(b) confirms this result. The 3. 2. Samples annealed at 1600-1800C observed diffraction rings can be attributed to Sic, while wo diffuse signals corresponding to the (0002) and toTo characterize materials obtained by annealing the(1010) /(1011)reflections of graphite are usually at higher temperatures, 3c and 5c, which had been assigned to the BNCx phase [20]. The TEM bright field thermolyzed at 1400C, were heat treated at different tem- image(Fig. 4(a)) taken from this sample shows homoge- peratures for 5 h in an argon atmosphere. A typical micro- neously distributed grains of nearly uniform size and shape structure obtained by TEM analysis of 5c annealed at with diameters of about 7.5 nm(3c)or 20-30 nm (5c) 1600C is shown in Fig. 5. From the bright field image which are embedded in a matrix phase. Since Sic and (BF) it is obvious that nano-sized grains are embedded in BNCx are the only phases present in significant amounts matrix phase Compared to the as-thermolyzed ceramic in this material and BNCx crystallization is usually hin- 5c/1400( Fig. 4)the grain size distribution is larger with dered, the grains most probably represent Sic crystals. diameters ranging from about 10 to 50 nm and an average Unfortunately, the elemental distribution images obtained grain size of 26 nm. by ESI(not shown here) cannot be interpreted unequivo- The chemical composition was analyzed by electron cally. Because of the small grain size and the thickness spectroscopic imaging(ESI). The results are presented in b CL=580 mn Fig 4.(a)Bright field image and(b)EDP of 5e thermolyzed at 1400C/ h/Ar
After thermolysis of the polymers at 1400 C for 2 h in an argon atmosphere the samples contained SiC crystals and a BNCx phase as was indicated by XRD (Fig. 2(a)). The EDP of 5c/1400 (Fig. 4(b)) confirms this result. The observed diffraction rings can be attributed to SiC, while two diffuse signals corresponding to the (0 0 0 2) and to the (1 0 1 0)/(1 0 1 1) reflections of graphite are usually assigned to the BNCx phase [20]. The TEM bright field image (Fig. 4(a)) taken from this sample shows homogeneously distributed grains of nearly uniform size and shape with diameters of about 7.5 nm (3c) or 20–30 nm (5c) which are embedded in a matrix phase. Since SiC and BNCx are the only phases present in significant amounts in this material and BNCx crystallization is usually hindered, the grains most probably represent SiC crystals. Unfortunately, the elemental distribution images obtained by ESI (not shown here) cannot be interpreted unequivocally. Because of the small grain size and the thickness of the sample, the distribution seems to be almost homogeneous. 3.2. Samples annealed at 1600–1800 C To characterize materials obtained by annealing at higher temperatures, 3c and 5c, which had been thermolyzed at 1400 C, were heat treated at different temperatures for 5 h in an argon atmosphere. A typical microstructure obtained by TEM analysis of 5c annealed at 1600 C is shown in Fig. 5. From the bright field image (BF) it is obvious that nano-sized grains are embedded in a matrix phase. Compared to the as-thermolyzed ceramic 5c/1400 (Fig. 4) the grain size distribution is larger with diameters ranging from about 10 to 50 nm and an average grain size of 26 nm. The chemical composition was analyzed by electron spectroscopic imaging (ESI). The results are presented in Fig. 3. (a) Bright field image and (b) EDP of 3c thermolyzed at 1050 C/4 h/Ar. Fig. 4. (a) Bright field image and (b) EDP of 5c thermolyzed at 1400 C/2 h/Ar. 1570 N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576
N. Bunjes et al. Journal of Non-Crystalline Solids 353 (2007)1567-1576 BF 100nm 100nm B c 100 Fig. 5. Bright field image and elemental distribution images of Si, C, and B of 5c thermolyzed at 1400C/2 h/Ar and subsequently annealed at 1600C/ 5 h/Ar elemental maps where bright areas show the presence and Fig. 6(a)shows different sets of diffraction rings which dark areas the absence of a particular element. In this can be assigned to SiC and a BNCx phase way, the distribution of Si, C and b within the sample Annealing of 5c at 1700C for 5 h in an argon atmo- could be visualized as shown in Fig. 5. The grains observed sphere leads to formation of larger SiC crystals compared in the bright field micrographs seem to correspond toto those observed after the heat treatment at 1600C Ring bright areas in the Si map. This indicates a high Si concen- patterns in the EDP (Fig. 6(b) become slightly sharper and tration within the grains. The surrounding matrix phase show the absence of further crystalline phases apart from appears dark in the Si map. Nevertheless, the presence of SiC and BNCx at this temperature. A typical microstructure Si cannot be excluded by this analysis because small con- and elemental maps of Si, C, and B are shown in Fig. 7 centrations of Si are difficult to detect. In the carbon map From the bright field image, the crystal size can be esti- the brightness is seemingly inverted. The grains(see BF) mated to range from about 25 to 125 nm with an average are clearly darker than the matrix. Therefore, the carbon diameter of 37.5 nm. The shape of the grains is mostly glob. concentration within the matrix must be higher than that ular. As was already observed for the 1600C sample, the within the grains. The boron map shows similar features crystal grains are composed of silicon carbide. In the Si or with dark areas at grain positions and brighter areas C map, they can be observed as bright or gray areas, respec between the grains. In contrast to the carbon distribution, tively. The larger crystals show striped contrast features however, grain boundaries are more distinct in the boron which are caused by stacking faults due to the formation map. In both C and B map, the distribution of the elements of Sic polytypes in the grains as can be shown by hrtEM within the matrix phase seems to be homogeneous and selected area diffraction. The surrounding matrix con- The phase content of this sample was furthermore ana- tains carbon, and boron(and nitrogen) with a C concentra- red by electron diffraction. The EDP presented in tion significantly higher than in SiC
elemental maps where bright areas show the presence and dark areas the absence of a particular element. In this way, the distribution of Si, C and B within the sample could be visualized as shown in Fig. 5. The grains observed in the bright field micrographs seem to correspond to bright areas in the Si map. This indicates a high Si concentration within the grains. The surrounding matrix phase appears dark in the Si map. Nevertheless, the presence of Si cannot be excluded by this analysis because small concentrations of Si are difficult to detect. In the carbon map the brightness is seemingly inverted. The grains (see BF) are clearly darker than the matrix. Therefore, the carbon concentration within the matrix must be higher than that within the grains. The boron map shows similar features with dark areas at grain positions and brighter areas between the grains. In contrast to the carbon distribution, however, grain boundaries are more distinct in the boron map. In both C and B map, the distribution of the elements within the matrix phase seems to be homogeneous. The phase content of this sample was furthermore analyzed by electron diffraction. The EDP presented in Fig. 6(a) shows different sets of diffraction rings which can be assigned to SiC and a BNCx phase. Annealing of 5c at 1700 C for 5 h in an argon atmosphere leads to formation of larger SiC crystals compared to those observed after the heat treatment at 1600 C. Ring patterns in the EDP (Fig. 6(b)) become slightly sharper and show the absence of further crystalline phases apart from SiC and BNCx at this temperature. A typical microstructure and elemental maps of Si, C, and B are shown in Fig. 7. From the bright field image, the crystal size can be estimated to range from about 25 to 125 nm with an average diameter of 37.5 nm. The shape of the grains is mostly globular. As was already observed for the 1600 C sample, the crystal grains are composed of silicon carbide. In the Si or C map, they can be observed as bright or gray areas, respectively. The larger crystals show striped contrast features which are caused by stacking faults due to the formation of SiC polytypes in the grains as can be shown by HRTEM and selected area diffraction. The surrounding matrix contains carbon, and boron (and nitrogen) with a C concentration significantly higher than in SiC. Fig. 5. Bright field image and elemental distribution images of Si, C, and B of 5c thermolyzed at 1400 C/2 h/Ar and subsequently annealed at 1600 C/ 5 h/Ar. N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576 1571
N. Bunjes et al. Journal of Non-Crystalline Solids 353(2007)1567-1576 B-sic b as B-sic C T=1600°c T=1700°c T=1800°c Fig. 6. Electron diffraction pattern of 5c samples thermolyzed at 1400C/2 h/Ar and subsequently annealed at: (a)1600C,( b)1700C,and (c)1800°C/5h/Ar BF Si B 100 mc Fig. 7. Bright field image and elemental distribution images of Si, C, and B of 5e thermolyzed at 1400C/5 h/Ar and subsequently annealed at 1700C/5 h/Ar. One sample of 5e was also heat treated at 1800C for Fig. 6(b)(1700C specimen) and Fig. 6(c)(1800C speci 5 h in an argon atmosphere. TEM investigations of the men) are very similar. More detailed information on material revealed no significant differences between this short-range ordering of the matrix phase was obtained by and the 1700C sample SiC crystal size was found to be high-resolution TEM(HRTEM). In Fig 8, a typical micro- between 37.5 and 62.5 nm with a change of aspect ratio structure is presented. In the left-hand part of the picture, for grain diameters exceeding 40 nm. An aspect ratio of the stacking faults of a polytype SiC grain are visI 1. 25 was observed for these particles. The EDPs shown in The right-hand side of the picture shows the matrix phas
One sample of 5c was also heat treated at 1800 C for 5 h in an argon atmosphere. TEM investigations of the material revealed no significant differences between this and the 1700 C sample. SiC crystal size was found to be between 37.5 and 62.5 nm with a change of aspect ratio for grain diameters exceeding 40 nm. An aspect ratio of 1.25 was observed for these particles. The EDPs shown in Fig. 6(b) (1700 C specimen) and Fig. 6(c) (1800 C specimen) are very similar. More detailed information on short-range ordering of the matrix phase was obtained by high-resolution TEM (HRTEM). In Fig. 8, a typical microstructure is presented. In the left-hand part of the picture, the stacking faults of a polytype SiC grain are visible. The right-hand side of the picture shows the matrix phase Fig. 6. Electron diffraction pattern of 5c samples thermolyzed at 1400 C/2 h/Ar and subsequently annealed at: (a) 1600 C, (b) 1700 C, and (c) 1800 C/5 h/Ar. Fig. 7. Bright field image and elemental distribution images of Si, C, and B of 5c thermolyzed at 1400 C/5 h/Ar and subsequently annealed at 1700 C/5 h/Ar. 1572 N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576
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/Ar
with short-range ordered turbostratic layers mixed with seemingly amorphous domains. The grain boundary appears to be clean and smooth. The orientation of turbostratic units at grain boundaries was found to be arbitrary, 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 microstructure 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 crystals 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 diffraction 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) reflections 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. Therefore, 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. Crystallinity 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 shortrange 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 crosslinked 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
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 not
organic-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 crystalline materials. Thermolysis of 3 and 5 at 1050 C yields inorganic materials 3c/1050 and 5c/1050 in an amorphous state. The elements 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 thermolysis 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 embedded into a BNCx matrix phase. Whereas, the grains represent bright or grey areas in the silicon and carbon map, the matrix contains boron and carbon which are homogeneously 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]. Therefore, crystallization should lead to the formation of a composite 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 carbon 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 observations 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 formation 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 significantly 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. Signals 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
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 the
observed 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 contrast, 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 possibly 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 polyborosilazane containing „SiANH2 units bridged by (C2H4)3B groups, the precursor for 5c was a boron-containing polysilylcarbodiimide with [„SiAN@C@NASi„] and connecting (C2H4)3B groups. Bond cleavage and bond formation during the ceramization process of different precursor-derived Si/B/C/N ceramics was thoroughly investigated and intermediates formed at different temperatures were characterized. Spectroscopic analyses of such intermediate structures indicated that the carbodiimide unit partly withstands thermal decomposition up to high temperatures. According to NMR measurements the formation of BN during thermolysis of polysilazanes and polysilylcarbodiimides occurs between 400 and 500 C [21]. The carbodiimide 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 polysilazanes. 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 distribution of carbon and boron nitride within the matrix phase to see if the elemental concentrations are the same in crystallized 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 analyzed. 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 homogeneous 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. Additional 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 turbostratic 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. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576 1575
ayers were less well-ordered with more variations of struc- [5]J. Bill, F Aldinger, Adv Mater. 7(1995)775 tural elements. These differences may be correlated to the [6]D Seyferth, H Am Ceram. Soc. 73(1990)2131 different polymer structures which in turn govern decom- U Bassie teret p p W.S. Rees Jr.. K. Buchner. in: A R (Eds ) Frontiers of position processes during thermolysis. Early formation ry, Royal Society of Chemistry, Cambridge, 1991, p. 15. and/or separation of the matrix phase in polysilazanes [8]E. Butchereit, K G. Nickel, A. Moller, J. Am. Ceram Soc. 84(2001) favors the development of well-ordered structures inducing crystallization during annealing experiments. By contrast, 9]R. Riedel, L M. Ruwisch, L. An, R. Raj, J. Am. Ceram. Soc. 81 the thermal stability of the carbodimide group most prob- (1998)3341 [OJR. Riedel, A. Kienzle, w. Dressler, L.M. Ruwisch, ably shifts BN formation to higher temperatures. As a J. Bill. F Aldinger, Nature 382(1996)796 result the matrix phase is less well-ordered. The observa- [11]H.. Baldus. M. Jansen, Angew. Chem. 109(1997)338 tion that even after 5 h at 2000C polymer-derived ceram Angew. Chem., Int. Ed. Engl. 36(1997)328 cs are not fully crystalline i.e. not thermodynamically [2JA Jalowiecki, J. Bill, F. Aldinger, J. Mayer, Composites 27A(1996) stable, and'remember'structural features of the polymeric state points to the fact that the stability of the amorphous 13]A. Jalowiecki, PhD thesis, Universitat Stuttgart, 1997(in German) state in these materials is essentially based on chemical [14]J. Bill. T.w. Kamphowe, A. Miller, T.Wichmann, A. Zern,A Jalowiecki, J. Mayer, M. Weinmann, J. Schuhmacher, K. Muller, J Peng, H.J. Seifert, F. Aldinger, Appl. Organomet. Chem. 15(2001) Acknowledgements [5]A. Moller, P. Gerstel, M. Weinmann, J. Bill, F. Aldinger, Chem. Mater.14(2002)3398 [6]A. Muller, J. Peng, H.J. Seifert, J. Bill, F. Aldinger, Chem. Mater. 14 The authors would like to thank horst Kummer(high (2002)3406. temperature annealing), Maria Sycha and Ute Bader for [7]H. J Seifert, F. Aldinger, Z. Metallkd 87(1996)84 the help with sample preparation, Rainer Hoschen for [18]H Schmidt, G Borchardt,AMiller,J.Bill,JNon-Cryst.Solids 341 assistance with the high-voltage microsco (2004)13 [9]H. Schmidt, w. Gruber, G. Borchardt, P. Gerstel, A. Muller, N Bunjes, J. Eur. Ceram Soc. 25(2005)227. References [20]A. Muller, A. Zern, P. Gerstel, J. Bill, F. Aldinger, J. Eur. Ceram. Soc.22(2002)1631. []W German Patent DE 2 218 960.1973 21]J Schuhmacher, PhD thesis, Universitat Stuttgart, 2000 (in German). [2]W G. winter. German Patent DE 2 236 068. 1974 [22]P. Gerstel, A. Muller, J. Bill, F. Aldinger, Chem. Mater. 15(26) [3]M T. Aahs. M. Bruck, Adv Mater. 2(1990)398. (2003)4980 [4]M p pillot H. Dunogues, Chem. rev 95 (1995)1443
layers were less well-ordered with more variations of structural elements. These differences may be correlated to the different polymer structures which in turn govern decomposition processes during thermolysis. Early formation and/or separation of the matrix phase in polysilazanes favors the development of well-ordered structures inducing crystallization during annealing experiments. By contrast, the thermal stability of the carbodiimide group most probably shifts BN formation to higher temperatures. As a result the matrix phase is less well-ordered. The observation that even after 5 h at 2000 C polymer-derived ceramics are not fully crystalline i.e. not thermodynamically stable, and ‘remember’ structural features of the polymeric state points to the fact that the stability of the amorphous state in these materials is essentially based on chemical design. Acknowledgements The authors would like to thank Horst Kummer (hightemperature annealing), Maria Sycha and Ute Ba¨der for the help with sample preparation, Rainer Ho¨schen for assistance with the high-voltage microscope. References [1] W. Verbeek, German Patent DE 2 218 960, 1973. [2] W. Verbeek, G. Winter, German Patent DE 2 236 068, 1974. [3] M. Peuckert, T. Vaahs, M. Bru¨ck, Adv. Mater. 2 (1990) 398. [4] M. Birot, J.-P. Pillot, H. Dunogue`s, Chem. Rev. 95 (1995) 1443. [5] J. Bill, F. Aldinger, Adv. Mater. 7 (1995) 775. [6] D. Seyferth, H. Plenio, J. Am. Ceram. Soc. 73 (1990) 2131. [7] D. Seyferth, H. Plenio, W.S. Rees Jr., K. Bu¨chner, in: A.R. Bassindale, P.P. Gaspar (Eds.), Frontiers of Organosilicon Chemistry, Royal Society of Chemistry, Cambridge, 1991, p. 15. [8] E. Butchereit, K.G. Nickel, A. Mu¨ller, J. Am. Ceram. Soc. 84 (2001) 2184. [9] R. Riedel, L.M. Ruwisch, L. An, R. Raj, J. Am. Ceram. Soc. 81 (1998) 3341. [10] R. Riedel, A. Kienzle, W. Dressler, L.M. Ruwisch, J. Bill, F. Aldinger, Nature 382 (1996) 796. [11] H.-P. Baldus, M. Jansen, Angew. Chem. 109 (1997) 338; Angew. Chem., Int. Ed. Engl. 36 (1997) 328. [12] A. Jalowiecki, J. Bill, F. Aldinger, J. Mayer, Composites 27A (1996) 717. [13] A. Jalowiecki, PhD thesis, Universita¨t Stuttgart, 1997 (in German). [14] J. Bill, T.W. Kamphowe, A. Mu¨ller, T. Wichmann, A. Zern, A. Jalowiecki, J. Mayer, M. Weinmann, J. Schuhmacher, K. Mu¨ller, J. Peng, H.J. Seifert, F. Aldinger, Appl. Organomet. Chem. 15 (2001) 777. [15] A. Mu¨ller, P. Gerstel, M. Weinmann, J. Bill, F. Aldinger, Chem. Mater. 14 (2002) 3398. [16] A. Mu¨ller, J. Peng, H.-J. Seifert, J. Bill, F. Aldinger, Chem. Mater. 14 (2002) 3406. [17] H.-J. Seifert, F. Aldinger, Z. Metallkd. 87 (1996) 841. [18] H. Schmidt, G. Borchardt, A. Mu¨ller, J. Bill, J. Non-Cryst. Solids 341 (2004) 133. [19] H. Schmidt, W. Gruber, G. Borchardt, P. Gerstel, A. Mu¨ller, N. Bunjes, J. Eur. Ceram. Soc. 25 (2005) 227. [20] A. Mu¨ller, A. Zern, P. Gerstel, J. Bill, F. Aldinger, J. Eur. Ceram. Soc. 22 (2002) 1631. [21] J. Schuhmacher, PhD thesis, Universita¨t Stuttgart, 2000 (in German). [22] P. Gerstel, A. Mu¨ller, J. Bill, F. Aldinger, Chem. Mater. 15 (26) (2003) 4980. 1576 N. Bunjes et al. / Journal of Non-Crystalline Solids 353 (2007) 1567–1576