SiAlOC glasses derived from sol-gel synthesized ladder-like silsesquioxanes

Abstract The main aim of this study was to describe both the formation process of SiOC glasses doped with Al3+ cations and the resulting differences in their structures in comparison with undoped silicon oxycarbide glasses. A novel material was formed by means of the sol-gel method with the application of alkyl-substituted alkoxysilanes and a butoxy derivative of aluminum (Al(OC4H9)3) as a source of Al3+ ions. After the synthesis of a sol, the material was treated with a two-step thermal processing including aging (70 °C) and annealing (800 °C). As a result, yellowish xerogel (aging) and black glasses (annealing) were obtained and subjected to structural investigation, including X-ray diffraction (XRD), middle infrared spectroscopy (MIR), Raman, electron dispersive spectroscopy (EDS), and nuclear magnetic resonance (NMR) methods. The obtained results were compared to corresponding findings for SiOC glasses, obtained with the same experimental procedure. An additional thermogravimetric analysis (TGA) study was carried out for the xerogel in order to determine its thermal behavior in a protective atmosphere up to 1000 °C. The resultant mass loss curve was much smoother for the Al-doped material, indicating the beneficial impact of Al3+ addition on the formation process. XRD and spectroscopic (MIR, Raman, magic angle spinning (MAS) NMR) research revealed the formation of an amorphous silsesquioxane xerogel with a slightly modified ladder-like structure, and silicon oxycarbide glasses successfully doped with Al3+ ions, after aging and annealing, respectively. Moreover, NMR studies confirmed the introduction of aluminum into the glass network, based on the presence of Q4(1Al) and [AlO4] species in the 29Si and 27Al MAS NMR spectra, respectively. Additionally, the results from EDS and Raman spectroscopic studies suggested the partial release of free carbon phase from the glass network because of cationic substitution and ordering influence of Al3+ cations on the glass structure.