J. R. Martínez

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.

Quantitative analysis of iron oxide particles embedded in an amorphous xerogel matrix

Using DTA, X-ray diffraction, vibrating sample magnetometry, and Rietveld refinement, we analyze the structural evolution under thermal treatment, and its relation to magnetic properties, of the compound formed by iron oxide particles embedded in a silica xerogel matrix. A variation of the Rietveld refinement method is used to calculate the amorphous and crystalline content in these composites by considering an initial crystalline model assumed to be the same composition as the material to be modelled. We refined the structure of compounds using the Maud program and we found that this program can be used to determine the amorphous and crystalline fractions of composites. Structural changes are correlated with magnetic properties.

Preparation of (Ni–B)/SiO2, Ni/SiO2 and NiO/SiO2 nanocomposites

Abstract In this work we report composites of nickel–boron (Ni–B)/SiO 2 , Ni 3 B/SiO 2 , Ni/SiO 2 and NiO/SiO 2 . As precursor of the nickel compounds, we used nanometric amorphous Ni–B alloy particles. The nanometric Ni–B particles were prepared by aqueous chemical reduction and studied by transmission electron microscopy (TEM), differential thermal analysis (DTA) and X-ray diffraction (XRD) before their incorporation into the SiO 2 xerogel. The composite samples were prepared by the sol–gel method. The produced samples were ground to form powders, heat treated at different temperatures under atmospheric conditions and characterized by TEM, XRD and DTA techniques. A specific composite (Ni 3 B/SiO 2 , NiO/SiO 2 , Ni/SiO 2 , (Ni–B)/SiO 2 ) can be produced depending on the temperature used for reaction. Composites were stable at room temperature and there were no significant phase transformations under aging.

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.

Formation and characterization of γ-Fe2O3@SiO2@Ag

Fe2O3@SiO2@Ag composites were synthesized, using the coprecipitation method to obtain maghemite nanoparticles coated with SiO2 using the Stöber method to avoid the oxidation, and finally silver nanoparticles were incorporated by a chemical method. The samples were characterized by X-ray diffraction, transmission electron microscopy, IR spectroscopy and vibrating sample magnetometry.

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.

Infrared Ellipsometry Analysis of Heritage Photographic Prints

ABSTRACT Focusing on the photographic archive of Julian Carrillo (Mexico), we study and characterize the photographic processes of a set of 13 photographs dated between 1884 and 1925. By using infrared spectroscopic ellipsometry, we classified a selected set of photographs according to its kind of binder. Thus, we recognized for each photograph, the presence of proteins, and therefore, the particular photographic process. Furthermore, we have identified the presence of baryta layer, the use of plasticizer, and the eventual coating utilized to protect the photograph, whose composition was based in natural organic components, mainly shellac, beeswax, or camphor.

Preparation and Characterization of Nickel Ferrite-SiO2/Ag Core/Shell Nanocomposites

Magnetic composites with silver nanoparticles bonded to their surface were successfully prepared using a simple chemical method. By means of a sol-gel technique, nickel ferrite nanoparticles have been prepared and coated with silica to control and avoid their magnetic agglomeration. The structural and magnetic properties of the nanoparticles were studied in function of the annealing temperature. Then, silver nanoparticles were incorporated by hydrolysis-condensation of tetraethyl orthosilicate, which contains silver nitrate on the surface of the nickel ferrite-SiO2 core/shell. Samples were characterized using X-ray diffraction, IR spectroscopy, SEM, and magnetometry. Results show that the silica covered the nickel ferrite nanoparticles and the silver nanoparticles remain stable in the surface of the composite.