S. A. Palomares-Sánchez

Synthesis of Fe2O3 Species Embedded in a Silica Xerogel Matrix: A Comparative Study

In this work we report composites of γ-Fe2O3/SiO2 and α-Fe2O3/SiO2 starting from three different iron precursors: iron nitrate, iron chloride and nanometric Fe particles prepared by aqueous chemical reduction. The composite samples were prepared by the sol-gel method and were ground in order to produce powders, heat treated at different temperatures under atmospheric conditions. The powder samples were characterized by X-Ray diffraction, IR absorption, and UV-visible spectroscopy. A specific composite can be produced depending on the temperature used for reaction.

Magnetic properties enhancement of M-Ba ferrites embedded in a SiO2 matrix

Abstract M–Ba ferrite powders prepared by the coprecipitation method were embedded, using the sol–gel technique, in an inert silica xerogel matrix. Samples were analysed by X-ray diffraction and vibrating sample magnetometry. We compare the magnetic and structural properties of pure ferrite samples and those embedded in the inorganic matrix. The mixed samples had a proportion of 30% BaFe 12 O 19 :70% SiO 2 in weight. We observed an enhancement of the coercive force in ferrite powders, embedded in the SiO 2 matrix, accompanied by a decrease of the saturation magnetization, with respect to pure samples.

Infrared Spectroscopy Analysis of Oxyhydroxides as Intermediate Species in the Formation of Iron Oxides-Silica Xerogels

A detailed infrared spectroscopy analysis in the 2500 to 3800 cm−1 region has been used to study the formation of species in samples of iron oxides embedded in silica xerogel matrix. We report the presence of α, β, and γ forms of iron oxyhydroxides as intermediate species in the formation of α-Fe2O3, γ-Fe2O3 and ε-Fe2O3 starting from three different iron precursors: iron nitrate, iron chloride and nanometric Fe particles prepared by chemical reduction. Our results show that under thermal dehydration α and γ forms of iron oxyhydroxides transform into hematite and maghemite, respectively, whereas the β form transform to the ε-Fe2O3 without going through an intermediate iron oxide phase.

Structural Effects of Heat-Treated Silica Xerogel Induced by Incorporation of Chlorophyll Species

Composites containing chlorophyll aggregates dispersed in amorphous silica are of interest because of their optical attractive properties. The silica powders added with chlorophyll species, prepared by the sol-gel method, were studied using X-ray diffraction, IR spectroscopy and differential temperature analysis. Silica xerogel samples were prepared using an ethanol/H2O/TEOS molar ratio of 4:11.6:1 and loaded with extracts from frozen spinach leaves. The silica xerogel microstructure of the powders was studied as a function of the annealing temperature. We found in our samples partial crystallization of the glass matrix in form of tridymite and cristobalite phases and quenching centers or nonfluorescing aggregates due to denaturation of photosystem promoted by chlorophyll decomposition after 400∘C.

Optimal target–substrate distance in the growth of oxides thin films by pulsed laser deposition

Abstract A simple model to investigate the formation of the oxide phase during the growth of ternary oxides films by laser ablation deposition has been proposed. Since the crystallinity properties of the oxide films are subject to the oxygen stoichiometry, the model is based on the balance between the density of the oxygen and a characteristic density of the system, δ =3 N 0 / L 0 3 , in order to scale the optimal target–substrate distance, L 0 . The interaction plume–laser is not considered. The model was applied for growing PbFe 12 O 19 hexaferrite thin film, and it shows that the theoretical conditions found are near to the optimal ones for the film growth. According to the model, for a fixed oxygen pressure of 1.0 mbar and a substrate temperature of 700 °C, a target–substrate distance, L 0 ≈3.1 cm, corresponding to a critical density of oxygen, n g =2.74×10 15 cm −3 , has been determined. The films show a high degree of c -axis orientation, high crystallographic quality and a proper stoichiometry.

First-Principles Study of Thermoelectric Properties of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are new emerging functional porous materials. Strong covalent bonds result in molecular building blocks that can be arranged in layered two-dimensional (2D) or three-dimensional (3D) periodic networks. However, to the best of our knowledge, there have been no reports on experimental and theoretical studies of thermoelectrical properties of COFs to date. Therefore, density functional theory (DFT) and the Boltzmann transport equation have been applied in this work to calculate the semiclassical transport coefficients for phthalocyanine (Pc)-based NiPc, NiPc-benzothiadiazole (BTDA), and Pc COFs. Owing to the well-ordered stacking of the phthalocyanine units and linkers in these compounds, charge-carrier transport is facilitated in the stacking direction. In all studied compounds, the highly directional character of π-orbitals provides band-structure engineering and produces a type of low-dimensional hole transport along the stacking direction. All studied compounds are indirect semiconductors. The low-dimensional transport of holes and the localized states in both valence and conduction bands prevent the electron–hole compensation effect in the Seebeck coefficients, correlating with the large Seebeck coefficients of the studied compounds. Insertion of the electron-deficient building block benzothiadiazole in the NiPc-BTDA COF leads to positive Seebeck coefficients along the a-, b-, and c-directions. The relaxation time was estimated in our investigations from DFT band-structure calculations and the experimentally defined mobility, leading to determination of the electrical conductivity and electronic contribution to the thermal conductivity, as well as figure of merit (ZT) estimation. Ni atom provided greater electrical conductivity along the c-direction in comparison with metal-free Pc COF, and NiPc COF showed the highest thermoelectric performance among the studied COFs.

Structures of nitrato-(2-hydroxybenzaldehydo) (2,2′-bipyridyl)copper and nitrato-(2-hydroxy-5-nitrobenzaldehydo)(2,2′-bipyridyl)copper

Nitrato-(2-hydroxy-5-nitrobenzaldehydo)(2,2′-bipyridyl)copper (I) and nitrato-(2-hydroxybenzaldehydo)(2,2′-bipyridyl)copper (II) were synthesized and characterized by X-ray diffraction. The coordination polyhedron of the central copper atom in complex I can be described as a distorted tetragonal pyramid whose base is formed by the phenol and carbonyl oxygen atoms of the monodeprotonated 2-hydroxy-5nitrobenzaldehyde molecule and the nitrogen atoms of the 2,2′-bipyridyl ligand and whose apex is occupied by the oxygen atom of the nitrato group. In the crystal structure, complexes I are linked by the acido ligands and the NO2 groups of the aldehyde molecule into infinite chains. In complex II, the central copper atom is coordinated by 2-hydroxybenzaldehyde, 2,2′-bipyridyl, and the nitrato group, resulting in the formation of centrosymmetric dimers. The coordination polyhedron of the central copper atom can be described as a bipyramid (4 + 1 + 1) with the same base as in complex I. The axial vertices of the bipyramid are occupied by the oxygen atom of the nitrato group and the bridging phenol oxygen atom of the adjacent complex related to the initial complex by a center of symmetry. In the crystal structure, complexes II are hydrogen bonded into infinite chains.

Coesite Formation at Ambient Pressure and Low Temperatures

Partial crystallization of silica xerogel in the form of coesite has been obtained at low-pressure conditions and temperatures of , in samples containing chlorophyll aggregates dispersed in amorphous silica. Silica xerogel samples were prepared by the sol-gel method using an ethanol::TEOS molar ratio of 4:11.6:1 and loaded with extracts from frozen spinach leaves. The silica xerogel microstructure of the powders was studied as a function of annealing temperature. It was found that partial crystallization of the glass matrix in the form of coestite was obtained at lower pressure than those specified by the phase diagram. Chlorophyll aggregates were added to the starting solutions which, upon thermal treatments, form small colloidal particles in the glass matrix. The presence of coesite is corroborated by the Rietveld refinement method.

Caracterización magnética y estructural de hexaferritas de (Ba, Sr)

Results on magnetic and structural characterization of ferrimagnetic compounds of BaxSrj.xFei20i9 (x=0.0, 0.25, 0.50, 0.75 and 1.0) prepared by the conventional ceramic method are reported. The samples were systematically examined using atomic forcé microscopy (AFM), X-ray diffraction and vibrating sample magnetometer. Structural and magnetic differences among the specimens were observed. The relations between structural features and magnetic properties are discussed. The presintering temperatures of the samples were 800°C and 1,000°C. The specimens were sintered at 1,200°C for one hour.

Topographic characterization of barium-hexaferrite powders using AFM

Abstract We report results on topographic characterization of ferromagnetic compounds of BaFe 12 O 19 prepared by chemical coprecipitation and conventional ceramic methods. Since it is desirable to use lower sintering temperatures without degrading the magnetic properties of the ferrites prepared by the ceramic method, they were doped with SiO 2 to enhance the densification. The samples have been examinated by atomic force microscopy (AFM) and a difference in the grain size between both kind of specimens was observed. The samples prepared by the coprecipitation method were sintered at 1373 °K and the oxide powders prepared by mechanical milling at 1453 °K, during 1 hour.