° 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, T< software 1400C:10°C/min;1400%C<T<2150°C:5°/min)in The elemental mapping was acquired using the three n argon atmosphere. Heat treatment experiments of window method(Edgerton; two for the background and obtained ceramics were performed in graphite furnaces one to acquire the chosen element signal). The energy slit using graphite crucibles in an argon atmosphere. X-ray dif- width was adjusted to 20 eV for silicon and boron and fraction diagrams were obtained with a Siemens D5000 30 eV for the elements carbon and nitrogen. This allowed Kristalloflex unit( Cu-Ko radiation) equipped with a to use exposure times between 2 and 10 s 2-fold binning quartz primary monochromator and a position-sensitive of the CCD was used to increase the signal-to-noise ratio proportional counter. nd to minimize drift during acquisition time as much as possible. The collection semiangle B was 12.5 mrad and 2. 2. Sample preparation onvergence angle Ceramic materials were obtained by thermolysis of Unprocessed (i.e. as-obtained after synthesis) polymeric Si 2.4.2 HRTEM measurements High-resolution images analyses were conducted in a /C/N precursors synthesized according to the literature JEOL 1250 with LaB cathode transmission electron [15]. The polymer-to-ceramic transformation was per formed in a flowing argon atmosphere at 1050C(heating microscope (JEOL, Japan) operated at 1250 kv. Images rate, 25C/h; dwell time, 4 h)or 1400C(heating rate were taken on Kodak Electron image film SO-163(8.3x 60C/h; dwell time, 2 h) in alumina Schlenk tubes. The 10.2 cm)which were scanned with an HP400 flatbed neg- resulting ceramic particles were of uneven size and shape. atve scanner working with a maximum pixel resolution of For further investigations, bulk mm-sized grains of the sar 1200×2400 ples thermolyzed at 1400C were chosen and heat treated at different temperatures Tmax(1600-2000C)for 5 h(heating 3. Results tes,T<1400%C,10%/min;T>1400°,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: 10 C/min; 1400 C < T < 2150 C: 5 C/min) in an argon atmosphere. Heat treatment experiments of asobtained ceramics were performed in graphite furnaces using graphite crucibles in an argon atmosphere. X-ray diffraction diagrams were obtained with a Siemens D5000 Kristalloflex unit (Cu-Ka radiation) equipped with a quartz primary monochromator and a position-sensitive proportional counter. 2.2. Sample preparation Ceramic materials were obtained by thermolysis of unprocessed (i.e. as-obtained after synthesis) polymeric Si/ B/C/N precursors synthesized according to the literature [15]. The polymer-to-ceramic transformation was performed in a flowing argon atmosphere at 1050 C (heating rate, 25 C/h; dwell time, 4 h) or 1400 C (heating rate, 60 C/h; dwell time, 2 h) in alumina Schlenk tubes. The resulting ceramic particles were of uneven size and shape. For further investigations, bulk mm-sized grains of the samples thermolyzed at 1400 C were chosen and heat treated at different temperatures Tmax (1600–2000 C) for 5 h (heating rates, T < 1400 C, 10 C/min; 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