Single-Source-Precursor Synthesis of SiC-Based Ceramic Nanocomposites for Energy-Related Applications

Abstract

The present work deals with the synthesis, characterization, and fabrication of Si-M-C-based ceramic nanocomposites (M = B and V). These were produced by the thermal transformation of tailor-made single-source-precursors, which were synthesized by the chemical modification of an allyl-hydrido polycarbosilane with suitable precursors (i.e., borane dimethylsulfide, allyl-functionalized carboranes, vanadium acetylacetonate and vanadium oxytriisopropoxide). The typical approach to this synthesis consists of a pyrolytic ceramization of the precursors, which converted into amorphous single-phase SiMC(O)-based materials. They are further subjected to high temperature treatment for phase separation and crystallization processes to furnish SiC-based ceramic nanocomposites. The preceramic polymer allyl-hydrido polycarbosilane (commercial name SMP-10) and derived SiC-based ceramics were thorougly investigated with respect to cross-linking behavior, polymer-to-ceramic transformation as well as high-temperature phase composition and microstructure. This knowledge served to optimize the processing of the preceramic polymeric precursor to produce dense and crack-free SiC-based monolithic ceramics by pressureless technique. The obtained ceramic monoliths have been shown to exhibits residual porosity of 15-25 vol%, which however can further be reduced by the use of polymer-infiltration and pyrolysis (PIP) to about 0.5 vol%. Boron-containing single-source-precursors were synthesized upon reactions of SMP-10 with borane dimethylsulfide complex (BMS) or with allyl-functionalized carboranes (AFC). In case of BMS-modified SMP-10 (BMS-SiBC), a detailed structural characterization has been done by means of various spectroscopic techniques. The main aspects addressed in case of BMS-modified SMP-10 (BMS-SiBC) are the fate of boron in the prepared SiBC ceramics, which was not been clarified unambiguously so far, and the role of boron in terms of densification of SiC. X-ray diffraction data, corroborated with X-ray photoelectron spectrocopy, Attenuated total reflectance-Fourier transform infrared spectroscopy, and Raman spectroscopic results indicate that in the SiBC ceramic prepared from the BMS-SiBC, boron preferably gets incorporated within the segregated carbon phase. Moreover, it was shown that the incorporation of boron has a positive effect on the densification behavior of SiC; so monolithic SiC ceramics with residual porosity below 5 vol% could be produced with pressureless processing. The SiBC material prepared from the AFC-modified SMP-10 shows a different phase composition, indicating the presence of a boron-rich boron carbide phase, which was not detected in BMS-SiBC. The results shows the crucial effect of the molecular architecture and chemism of the single-source-precursors on the phase composition and consequently properties of the resulting ceramic materials. Vanadium-containing single-source-precursors were obtained upon chemical modification of SMP-10 with vanadyl acetylacetonate or vanadium oxytriisopropoxide. High temperature treatment of the resulting single-source-precursors in argon atmosphere initially led to an amorphous single-phase ceramic (SiVCO) which was then converts into ceramic nanocomposites consisting of a non-stoichiometric vanadium carbide phase (V8C7) finely dispersed in a polycrystalline β-SiC matrix. In this context, the first investigation was carried out on biomorphic and the template-assisted processing of single-source-precursor to form porous monolithic samples. In addition, preliminary results of the catalytic activity of SiVC(O) show that the nanocomposites are active for the decomposition of the ammonia. The maximum ammonia conversion efficiency was found to be 35 % at around 650 oC which is higher than that of pure V8C7 reported in the literature (13 %). The results of this study show that the processing of ceramics starting from suitable preceramic polymer is a versatile technique for the production of SiC-based ceramic nanocomposites with tailored phase composition, microstructure, and property profiles. Moreover, the single-source-precursor technique used for the preparation of ceramic nanocomposites allows flexibility with respect to processing. Thus it is possible, starting from preceramic precursors to prepare ceramic powder, crack-free and dense monoliths as well as materials/components with tailored porosity which can be used flexibly for different applications.