° 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. 2007.01.025Crystallization 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 ceram￾ics obtained by the ‘classic’ powder sintering route, poly￾mer-derived ceramics are free of sintering additives. They offer the advantage of high chemical purity, microstruc￾tural homogeneity, low processing temperatures and versa￾tile 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, decomposi￾tion or creep at high temperature (see for example Refs. [8–11]). Thermolysis of preceramic Si/B/C/N/H polymers is usu￾ally 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 tempera￾tures (800–1400 C), most of the residual hydrogen evapo￾rates. 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 micro￾scopic, 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, FT￾IR, XRD, and TEM. It has been shown that in materials revealing substantial high-temperature stability, a metasta￾ble 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 compo￾sitions located in the neighboring boron-containing three￾phase 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