(Bi13Co11)Co2O40-Co3O4 nanocomposites: Approach to different fuels in sol-gel combustion synthesis using the Box-Behnken design

The sol-gel combustion method was applied using three types of fuels (urea, glycine, and ethylene glycol) to synthetize Co sillenite. A Co sillenite - Co3O4 nanocomposite was obtained as result of the synthesis. The effects of initial pH, fuel type, and amount of fuel on the relative mass concentration of Co Sillenite phase present in the nanocomposite, obtained by Rietveld refinement of XRD data, were evaluated by the Box-Behnken design of experiments. Statistical analysis allowed the quantification of each synthesis parameter and the choice of the samples with higher relative mass concentration of Co Sillenite for each type of fuel. The selected samples were analyzed by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (FESEM), energy dispersive x-ray spectroscopy (EDX), x-ray fluorescence (XRF) and dielectric open-ended coaxial probe techniques. Rietveld refinements showed that Co sillenite and Co3O4 have cubic structure, and Co sillenite structure belongs to I 23 space group. The Co sillenite has crystallite sizes of 43.6 nm (urea), 40.3 nm (glycine) and 44.1 nm (ethylene glycol). The FESEM images displayed a significant change in samples morphology, presenting sphere, flakes and flowers-like shapes. The samples purity was confirmed by the presence of Bi, Co and O elements, identified through EDX. Functional groups and vibrational modes of Co sillenites (Bi-O, Co-O) were identified by FT-IR technique and Raman spectroscopy. Dielectric properties measurements displayed accordance with structural characterizations of each sample. Low losses could also be achieved above 4.5 GHz, leading to a composite suitable for highly efficient microwave systems.