Abstract
The microstructure of segregated carbon in silicon oxycarbide (SiOC), hot-pressed at T = 1600 °C and p = 50 MPa, has been investigated by VIS Raman spectroscopy (λ = 514 nm) within the temperature range 25–1000 °C in air. The occurrence of the G, D’ and D bands at 1590, 1620 and 1350 cm−1, together with a lateral crystal size La < 10 nm and an average distance between lattice defects LD ≈ 8 nm, provides evidence that carbon exists as nano-crystalline phase in SiOC containing 11 and 17 vol % carbon. Both samples show a linear red shift of the G band up to the highest temperature applied, which is in agreement with the description of the anharmonic contribution to the lattice potential by the modified Tersoff potential. The temperature coefficient χG = −0.024 ± 0.001 cm−1/°C is close to that of disordered carbon, e.g., carbon nanowalls or commercial activated graphite. The line width of the G band is independent of temperature with FWHM-values of 35 cm−1 (C-11) and 45 cm−1 (C-17), suggesting that scattering with defects and impurities outweighs the phonon-phonon and phonon-electron interactions. Analysis of the Raman line intensities indicates vacancies as dominating defects.
Highlights
Phonons are of fundamental importance for crystalline materials
We present a temperature-dependent Raman study of silicon oxycarbide (SiOC) containing 11 and 17 vol % of segregated carbon
The D band is induced by any kind of disorder. It is forbidden in graphite, but becomes Raman active band is induced by any kind of disorder. It is forbidden in graphite, but becomes Raman active through a disorder-induced double resonance Raman process [34,35], which causes in-plane breathing through a disorder-induced double resonance Raman process [34,35], which causes in-plane vibrations of the aromatic structures (A1g symmetry)
Summary
The analysis of Raman active modes is well-suited to assessing the lattice dynamics of, e.g., carbon and its allotropes graphite [1,2], carbon nanotubes, [3,4,5] graphene, [6,7] amorphous carbon, [8,9] and diamond-like carbon [10,11]. We present a temperature-dependent Raman study of silicon oxycarbide (SiOC) containing 11 and 17 vol % of segregated carbon (labeled as C-11 and C-17 in the following). Due to its unique microstructure, SiOC possesses a variety of interesting properties, such as high temperature and corrosion stability [14,15,16], high creep resistance [17] and high piezoresistivity [18,19]. The mentioned properties seem to be determined by the segregated carbon phase randomly distributed within the SiOC matrix
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