Enthalpy of Formation of Carbon-Rich Polymer-Derived Amorphous SiCN Ceramics Riham Michelle Morcos, z Gabriela Mera, y Alexandra Navrotsky, z Tamas Varga, z Ralf Riedel, w,y Fabrizia Poli, J and Klaus Mu¨ller J z Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, California 95616 y Institut fu¨ r Materialwissenschaft, Technische Universita¨ t Darmstadt, Darmstadt, Germany z Argonne National Laboratory, Materials Science Division, Argonne, Illinois 60439 J Department of Chemistry, University of Stuttgart, Stuttgart, Germany Carbon-rich silicon carbonitride (SiCN) ceramics derived from polysilylcarbodiimides represent a novel class of materials where the incorporation of a high amount of carbon was demonstrated to be beneficial for ultrahigh-temperature resistance against crystallization. Calorimetric measurements of heat of oxidative dissolution in a molten oxide solvent show that these amorphous SiCN ceramics produced at 10001 or 11001C possess a small positive or near zero enthalpy of formation relative to their crystalline constituents, namely silicon nitride, silicon carbide, and graphite. The enthalpy of formation does not change strongly with increasing SiC mole fraction. Because the enthal- pies of formation from crystalline constituents are at most slightly positive, and the entropies of formation are expected to be significantly positive because of disorder in the amorphous phase, it is likely that the free energies of formation from silicon carbide, silicon nitride, and graphite are negative and the high- temperature persistence of amorphous SiCN ceramics may originate from thermodynamic stabilization. However, this sta- bilization is less pronounced than that for SiCO polymer-derived ceramics studied earlier. I. Introduction S INCE the 1970s, polymer-derived ceramics (PDCs) have been synthesized directly by the pyrolysis of polymer structures. 1 The success of synthesizing PDCs can be attributed to the strong bonding between silicon and carbon in the polymer that pre- vents carbon from volatilizing as a hydrocarbon during pyroly- sis in a controlled inert environment. 2 PDCs can be synthesized by a relatively low-temperature route where complete pyrolysis occurs at or below 11001C. 3,4 The ternary PDC system, 3 namely silicon carbonitrides (SiCN) has unique properties, such as high-temperature persis- tence, 4–10 oxidation, and corrosion resistance, 5,11 as well as en- hanced thermomechanical, 4 electrical, 11,12 magnetic, and optical properties. 11–14 A characteristic of polymer-derived SiCN ce- ramics is their possibility to incorporate significant amount of free carbon into the Si, N-containing microstructure. The thermal stability and resistance to crystallization of PDCs are enhanced when the ceramics are fabricated with a high content of excess carbon as reported for carbon-rich SiCN 15 and published recently also in the case of SiCO materials. 16–18 Cross et al. 19 reported that the presence of carbon segrega- tions helps to reduce coarsening of SiC and Si 3 N 4 particles in polysilazane-derived polycrystalline ceramics. In a comprehen- sive study related to the thermal stability of SiCN ceramics, Iwamoto et al. 10 discussed the specific crystallization behavior of different preceramic silicon polymers, namely polysilazanes and polysilylcarbodiimides. They used solid-state NMR and X-ray diffraction (XRD) to show that carbon segregation found in polysilylcarbodiimide-derived SiCN hinders the crystallization of amorphous Si 3 N 4 . 10 Carbon-rich phases in PDCs may also assist in improved friction and wear performance by tailoring the microstructure and chemistry. 2 Recent determination of enthalpies of formation by high- temperature oxide melt solution calorimetry has shown that PDCs in the SiCO system are energetically stable with respect to silicon carbide, cristobalite, and graphite. Thus, their persistence at high temperature derives from thermodynamic as well as from kinetic factors. 20,21 To better understand these unusual properties of PDCs in the SiCN system, it is necessary to evaluate their stability and to correlate the energetics with their nanostructure. The motivation of the present work is to compare for the first time the ternary SiCN system in terms of their energetics with the SiCO system studied recently. 20,21 This investigation includes four C-rich SiCN samples synthesized from preceramic silylcarbodiimide- based polymers. The carbon content of the SiCN was changed by varying one substituent attached to the silicon atoms in the starting polymer. Earlier studies have suggested that the micro- structure of PDCs depends on precursor synthesis, polymeriza- tion, degree of cross-linking, and the pyrolysis temperature, as well as on the atmosphere applied for the pyrolysis process. 19 In this study, the enthalpy of formation obtained from calorimetric measurements is used to quantify the thermodynamic stability of PDCs in the SiCN system synthesized from polysilylcarbodiimides of the general composition [Ph(R)Si– N 5 C 5 N–] n with R 5 H, CH 3 , and Ph. II. Experimental Procedure (1) Material Preparation and Characterization The ceramic compositions S1–S4 were synthesized from prece- ramic polymers, namely phenyl-containing polysilylcarbodiimides. Thermolysis of the polymers resulted in the formation of ceramics comprised of amorphous Si 3 N 4 and amorphous C as determined T. Rouxel—contributing editor This work was supported by grant from the Ceramics Program of the Division of Ma- terials Research of the National Science Foundation DMR-0502446 at the University of California, Davis. These grants are funded under the MWN (Materials World Network) Program between the National Science Foundation and the Deutsche Forschungsgemeinsc- haft (DFG). The research at Stuttgart is supported by the DFG under grant Mu 1166/12-1 and the work at TU Darmstadt is supported by the DFG under grant Ri 510/33-1. R. R. also thank Fonds der Chemischen Industrie, Frankfurt, Germany for the support provided. w Author to whom correspondence should be addressed. e-mail: riedel@materials. tu-darmstadt.de Manuscript No. 24524. Received April 9, 2008; approved June 23, 2008. J ournal J. Am. Ceram. Soc., 91 [10] 3349–3354 (2008) DOI: 10.1111/j.1551-2916.2008.02626.x r 2008 The American Ceramic Society 3349