Systematic Structural Characterization of the High-Temperature Behavior of Nearly Stoichiometric Silicon Oxycarbide Glasses

Abstract

Three silicon oxycarbide glasses (SiCO) with increasing C content were obtained through pyrolysis in inert atmosphere at 1000 °C of sol-gel derived siloxane networks containing Si-CH 3 and Si-H bonds. The glasses were further annealed at 1200, 1400, and 1500 °C to follow their evolution at high temperature. Quantitative information concerning the structure of glasses before and after annealing at high temperature was collected with a wide range of techniques (some of them used for the first time in this field) with the aim of probing the following: (i) the short-range order and chemical composition ( 29 Si and 1 H MAS NMR, RDF derived from X-ray and neutron scattering, inelastic neutron scattering, FT-IR, and elemental analysis), and (ii) the long-range order (X-ray and neutron diffraction) and microstructural features (HR-TEM combined with electron diffraction, Raman, porosity, and surface area measurements). This extensive collection of data, carried out on the same set of specimens, provided detailed and sound structural information on nearly-stoichiometric SiCO glasses and their high-temperature behavior.

Thermal evolution of ladder-like silsesquioxanes during formation of black glasses

“…It is also worth to mention that during this step intensity of Si-O-Si band at 1100 cm -1 decreases, while the 440 cm -1 (Si-O bending mode) is starting to be more visible [18]. This confirms that during this step T/D-oligomers and D-oligomers, which are not involved into creation of glass structure, are being evaporated from the tested samples [19,21]. Similar situation is being observed for the samples fired at 600°C.…”

“…It can be attributed to the release of cyclic D-oligomers that formed during the synthesis of ladder-like silsesquioxanes as a by-product. In addition, a mass loss associated with the release of residual solvent and EtOH/ H 2 O from ongoing polycondensation reaction should be observed at this temperature [11,14,21,44]. High impact on loss of mass in this area is the number of D groups in the starting xerogel-it increases with the quantity (Fig.…”

“…550 cm -1 [7,8]. The Si-C 3 vibration from the Si-CH 3 molecules can be observed at 700 cm -1 [21,27,30]. Based on the XRD analysis (Fig.…”

“…Minor mass loss at approx. 300°C (290°C for 1/1, 340°C for 2/1 and 350°C for 4/1) most likely corresponds to elimination of branched T/D-oligomers formed during the synthesis [19,21,22]. Next loss is registered at 410 and 440°C for 2/1 and 1/1 mix accordingly.…”

“…Mass loss at approx. 600-700°C can be assigned to further evolution of D units and gases originating from redistribution reaction of Si-O/Si-C bondings [19,[21][22][23][24][25]44]. Release of T units (oligomers), which based on TG and dTG curves is rising along the decrease in terminal D groups, should still be present in this temperature range [22,23].…”