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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.

The aim of this work consists in the use of cashew gum (Anacardium occidentale), a naturally occurring tropical specie from the Brazilian northeastern coast, for the synthesis of CoFe2O4 (CF) and NiFe2O4 (NF) nanoparticles. The structural, morphological and vibrational properties of nanoparticles were characterized by analytical and spectroscopic techniques such as X-ray diffraction (XRD), FTIR, Raman spectroscopy, TEM, SAED and TG. Magnetic properties were investigated through Mossbauer spectroscopy and DC magnetometry. The XRD results showed single phase nanoparticles with space group Fd-3m and crystallite size of 7.4 and 6.0 for CF and NF, respectively. TEM images showed agglomerated particles with mode sizes of 5.0 and 6.5 nm for CF and NF. SAED confirmed the crystalline spinel structure. The TGA and FTIR showed the presence of a carbonaceous material in the samples. FTIR and Raman spectroscopy demonstrated vibrational modes characteristic of metal‑oxygen bonds in the tetrahedral and octahedral sites. Magnetization measurements showed that both samples are superparamagnetic at 300 K. The Mossbauer spectra at 90 K showed the presence of single-phase CF and NF.

Nanocrystalline of Co sillenite-Co3O4 composites were prepared by combustion and sol-gel methods. The synthesized materials were further investigated by XRD, FT-IR, Raman spectroscopy, DC magnetometry, FESEM and EDX. The XRD characterization revealed that the sillenite phase is nanostructured with a crystallite size in the range from 48.4 to 64.4 nm. Both syntheses methods yielded between 75 and 80 wt% of sillenite. The FESEM micrographs showed that the particles had sphere like and a nanosheets - sphere like shapes when prepared by combustion and sol-gel methods, respectively. Confirmation of the chemical structure of composites was determined by the FT-IR and Raman spectra through the vibrational modes of bismuth-oxygen and cobalt-oxygen bonds. The M-H and M-T magnetic measurements showed ferromagnetic like behavior at 6 K. An antiferromagnetic transition due to Co3O4 was observed at TN = 36 K. Both samples showed a antiferromagnetic Curie-Weiss behavior with a large -θCW, (θCW » TN) their calculated frustration degrees were of f ≈ 7.5 and 15.1 for samples prepared by sol-gel and combustion methods, respectively. The sample prepared by combustion method showed a blocking temperature at Tb = 148 K, Tb decreased when applied a higher field, it was ascribed to CoO clusters like phase. These results show that the combustion method allows to produce a multiphasic sample with a high degree of geometric magnetic frustration, while, the sol-gel method allows to synthesize a sample with a high crystallinity and purity.