RESUMEN
Fine-tuning of grain sizes can significantly influence the interaction between different dielectric phenomena, allowing the development of materials with tailored dielectric resistivity. By virtue of various synthesis mechanisms, a pathway to manipulate grain sizes and, consequently, tune the material's dielectric response is revealed. Understanding these intricate relationships between granulation and dielectric properties can pave the way for designing and optimizing materials for specific applications where tailored dielectric responses are sought. The experimental part involved the fabrication of dense BCT-BZT ceramics with different grain sizes by varying the synthesis (conventional solid-state reaction route and sol-gel) and consolidation methods. Both consolidation methods produced well-crystallized specimens, with Ba0.85Ca0.15O3Ti0.9Zr0.1 (BCTZ) perovskite as the major phase. Conventional sintering resulted in microstructured and submicron-structured BCT-BZT ceramics, with average grain sizes of 2.35 µm for the solid-state sample and 0.91 µm for the sol-gel synthesized ceramic. However, spark plasma sintering produced a nanocrystalline specimen with an average grain size of 67.5 nm. As the grain size decreases, there is a noticeable decrease in the maximum permittivity, a significant reduction in dielectric losses, and a shifting of the Curie temperature towards lower values.
RESUMEN
A novel high-entropy perovskite powder with the composition Bi0.2K0.2Ba0.2Sr0.2Ca0.2TiO3 was successfully synthesized using a modified Pechini method. The precursor powder underwent characterization through Fourier Transform Infrared Spectroscopy and thermal analysis. The resultant Bi0.2K0.2Ba0.2Sr0.2Ca0.2TiO3 powder, obtained post-calcination at 900 °C, was further examined using a variety of techniques including X-ray diffraction, Raman spectroscopy, X-ray fluorescence, scanning electron microscopy, and transmission electron microscopy. Ceramic samples were fabricated by conventional sintering at various temperatures (900, 950, and 1000 °C). The structure, microstructure, and dielectric properties of these ceramics were subsequently analyzed and discussed. The ceramics exhibited a two-phase composition comprising cubic and tetragonal perovskites. The grain size was observed to increase from 35 to 50 nm, contingent on the sintering temperature. All ceramic samples demonstrated relaxor behavior with a dielectric maximum that became more flattened and shifted towards lower temperatures as the grain size decreased.
RESUMEN
Magnesium oxide (MgO) was synthesized by three different methods: the sol-gel (SG), microwave-assisted sol-gel (MW), and hydrothermal (HT) methods for comparing the influence of the preparation conditions on the properties of the products. The powders were annealed at 450 °C. The samples were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM/HRTEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDX), BET specific surface area and porosity, photoluminescence, and UV-Vis spectroscopy. The samples consisted mainly of periclase as a crystalline phase, and the MW and HT preparation methods generated particles with higher specific surface areas. The powders had less-defined morphologies and high levels of aggregation. The optical band gaps of the samples were determined from UV DRS, and the photocatalytic activities of the magnesium oxides obtained by the three methods towards the degradation of methyl orange (MO) under UV light irradiation was evaluated.
RESUMEN
Two new families of zinc/cobalt/aluminum-based pigments, with a unique composition, were obtained through the polyol method. The hydrolysis process of a mixture of Co(CH3COO)2, Zn(acac)2 and Al(acac)3 (acac- = acetylacetonate ion) in 1,4-butanediol afforded dark blue gels (wPZnxCo1-xAl), in the presence of a supplementary amount of water, and light green powders (PZnxCo1-xAl), respectively, for the water-free procedure (x = 0, 0.2, 0.4). The calcination of the precursors yielded dark green (wZnxCo1-xAl) and blue (ZnxCo1-xAl) products. XRD measurements and Rietveld refinement indicate the co-existence of three spinel phases, in different proportions: ZnxCo1-xAl2O4, Co3O4 and the defect spinel, γ-Al2.67O4. The Raman scattering and XPS spectra are in agreement with the compositions of the samples. The morphology of wZnxCo1-xAl consists of large and irregular spherical particle aggregates (ca. 5-100 mm). Smaller agglomerates (ca. 1-5 mm) with a unique silkworm cocoon-like hierarchical morphology composed of cobalt aluminate cores covered with flake-like alumina shells are formed for ZnxCo1-xAl. TEM and HR-TEM analyses revealed the formation of crystalline, polyhedral particles of 7-43 nm sizes for wZnxCo1-xAl, while for ZnxCo1-xAl, a duplex-type morphology, with small (7-13 nm) and larger (30-40 nm) particles, was found. BET assessment showed that both series of oxides are mesoporous materials, with different pore structures, with the water-free samples exhibiting the largest surface areas due, most likely, to the high percent of aluminum oxide. A chemical mechanism is proposed to highlight the role of the water amount and the nature of the starting compounds in the hydrolysis reaction products and, further, in the morpho-structural features and composition of the resulting spinel oxides. The CIE L*a*b* and C* colorimetric parameters indicate that the pigments are bright, with a moderate degree of luminosity, presenting an outstanding high blueness.
RESUMEN
Europium substituted bismuth ferrite powders were synthesized by the sol-gel technique. The precursor xerogel was characterized by thermal analysis. Bi1-xEuxFeO3 (x = 0-0.20) powders obtained after thermal treatment of the xerogel at 600 °C for 30 min were investigated by X-ray diffraction (XRD), scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectroscopy, and Mössbauer spectroscopy. Magnetic behavior at room temperature was tested using vibrating sample magnetometry. The comparative results showed that europium has a beneficial effect on the stabilization of the perovskite structure and induced a weak ferromagnetism. The particle size decreases after the introduction of Eu3+ from 167 nm for x = 0 to 51 nm for x = 0.20. Photoluminescence spectroscopy showed the enhancement of the characteristic emission peaks intensity with the increase of Eu3+ concentration.