Abstract

This PhD thesis focuses on the synthesis and characterization of multifunctional polymer-derived SiOCN ceramics. The main objective is to synthesize N-doped silicon oxycarbides (SiOCs), to characterize their structure and their properties (mainly electrical conductivity) and correlate them with the presence of N in the structure. We also aim to understand how the architecture of the starting polymer can influence the retention of N into the SiOC structure. First, N-doped SiOC polymer precursors were synthesized via hydrosilylation reaction between Si-H groups present in a commercial polysiloxane (PHMS) and –CH=CH2 groups of three different commercial N-containing compounds. The structural characterization of as-synthesized preceramic polymer precursor was investigated by FT-IR and NMR. Thermal degradation was studied by TGA. The results show that the architecture of the polymer precursors plays an important role on the pyrolythic transformation. Then, SiOCN ceramics were obtained by pyrolysis of the as-synthesized polymer precursors in nitrogen atmosphere at various temperatures for 1h using a tubular furnace. Subsequently, high temperature structural evolution was studied using combined techniques such as XRD, FT-IR, NMR, Elemental analysis, and XPS. The obtained results show that the type of N-containing compounds impacts on the crystallization behavior of the final ceramics. Elemental analysis clearly indicates that N is present in the SiOC matrix and the degree of N retention after pyrolysis is related to the type of N-containing starting compounds. XPS data indicate that N-C bonds are present in the SiOC ceramic samples even if only N-Si bonds exist in the starting N-containing precursor. However, a larger fraction of N-C bonds is present in the final SiOCN ceramic when N atoms form bonds with sp2 carbon atoms in the pre-ceramic polymer. We have also studied electrical and optical properties of the SiOCNs. Electrical conductivity of the powdered ceramic samples was determined using powder-solution-composite technique. The results show an increase in room temperature AC conductivity of three orders of magnitude, from ≈10-5 (S/cm) to ≈10-2 (S/cm), with increasing pyrolysis temperature from 1000 to 1400 °C. Furthermore, the electrical conductivity of the SiOCN ceramic derived from N-C bond bearing precursor is three to five times higher than that of the sample derived from N-Si containing precursor at each pyrolysis temperature. The combined structural study by Raman spectroscopy and chemical analysis suggests that the increase of electrical conductivity with the pyrolysis temperature is due to the sp3-to-sp2 transition of the amorphous carbon phase. The higher conductivity of the amine-derived SiOCN is also discussed considering features like the volume % of the free-carbon phase and its possible N-doping. Fluorescence of the SiOCN samples treated at low temperatures, 400 and 600 °C, has been studied. The spectra show that the heated precursors fluoresce in the visible range with a dominant blue emission. Since the non-heated polymer precursors do not fluoresce, emitting centers must be formed during the polymer-to-ceramic transformation and associated with the structural changes. The origin of the luminescence could be originated from defects related to C, O and/or Si. Finally, we investigated the gas sensing behavior of the SiOCNs pyrolyzed at low and high temperatures. Regarding the electrical gas sensing of the SiOCN ceramics pyrolyzed at 1400 °C, the response to two target gases NO2 and H2 was tested by in situ DC conductance measurements at operating temperatures from 200 to 550 °C. The SiOCN ceramics are sensitive to NO2 at temperatures below 400 °C and to H2 at temperatures above 400 °C. In addition, the response observed for the studied SiOCN ceramics is higher than that reported in the previous studies for SiOC ceramic aerogels. With regard to the optical gas sensing of the SiOCNs obtained from the heat treatment of the polymer precursors at 400 and 600 °C, fluorescence spectra in the presence of organic vapors such as acetone and hexane were recorded. The results show that these tested vapors quench the fluorescence of the studied SiOCN. In conclusion, the SiOCN ceramics can be promising materials for the gas sensing application.