Aurelio Beltrán

Nanoparticulated Silicas with Bimodal Porosity : Chemical Control of the Pore Sizes

Nanoparticulated bimodal porous silicas (NBSs) with pore systems structured at two length scales (meso- and large-meso-/macropores) have been prepared through a one-pot surfactant-assisted procedure by using a simple template agent and starting from silicon atrane complexes as hydrolytic inorganic precursors. The final bulk materials are constructed by an aggregation of pseudospherical mesoporous primary nanoparticles process, over the course of which the interparticle (textural) large pore system is generated. A fine-tuning of the procedural variables allows not only an adjustment of the processes of nucleation and growth of the primary nanoparticles but also a modulation of their subsequent aggregation. In this way, we achieve good control of the porosity of both the intra- and interparticle pore systems by managing independent variables. We analyze in particular the regulating role played by two physicochemical variables: the critical micelar concentration (cmc) of the surfactant and the dielectric constant of the reaction medium.

Polymer solution processing of (Bi, Pb)SrCaCuO

Abstract A simple polymer processing route to the 110 K phase (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductor has been developed. The influence of the polymer to metal starting ratio, as well as the sintering temperature and time on the quality of the resulting superconducting powders has been studied by XRD, a.c. susceptibility and resistivity measurements. Microstructure of ceramic compacts has been studied by SEM, and qualitatively analysed by EDS for compositional homogeneity. The results indicate that the polymer synthesis route described here may offer a good alternative to the conventional solid state preparation methods towards attaining homogeneous 110 K superconductor powder within a reasonable processing time.

Large monolithic silica-based macrocellular foams with trimodal pore system

Silica-based materials with hierarchical pore systems at three different length scales (small mesopores-large mesopores-macropores) have been prepared through a nanotectonic approach by using mesoporous nanoparticles as building blocks; the resulting materials present a highly accessible foam-like architecture and can be prepared as large monoliths.

Biomimetic chitosan-mediated synthesis in heterogeneous phase of bulk and mesoporous silica nanoparticles

Both bulk and mesoporous silica nanoparticles can be obtained in the form of granular aggregates using chitosan flakes as additive under very soft biomimetic reaction conditions.

Silica-based powders and monoliths with bimodal pore systems

Porous pure and doped silicas with pore sizes at two length scales (meso/macroporous) have been prepared and shaped both as powders and monoliths through a one-pot surfactant assisted procedure by using a simple template agent and starting from atrane complexes as inorganic precursors.

A new approach to the synthesis of molybdenum bimetallic nitrides and oxynitrides

A simple processing route to molybdenum bimetallic nitrides and oxynitrides has been developed, based on the use of precursors resulting from freeze-drying of aqueous solutions of the appropriate common metal salts. Thermal decomposition of amorphous freeze-dried powders originates crystalline mixed oxides, which have been carefully characterized. Nitridation of the crystalline intermediate having Ni:Mo=2:3 metal ratio led to single-phased Ni 2 Mo 3 N. For the V-Mo-O-N system, nitridation of both amorphous and crystalline precursors results in bimetallic oxynitrides. In this way, not only the already known V 2 Mo(O x N y ) catalyst but also a new compound of different metal stoichiometry, V 3 Mo 2 (O x N y ), has been prepared. The materials have been characterized by X-ray powder diffraction, elemental analysis, scanning electron microscopy, thermogravimetry under air flow, and temperature programmed oxidation. The existence of a solid solution of stoichiometry V 1–z Mo z (O x N y ) (0≤z≤1) having rock-salt structure is discussed.

Chemistry of interstitial molybdenum ternary nitrides MnMo3N (M=Fe, Co, n=3; M=Ni, n=2)

Interstitial molybdenum ternary nitrides, MnMo3N (M=Fe and Co, n=3; M=Ni, n=2), can be obtained by ammonolysis of molybdate precursors, MMoO4nH2O. A study of the influence of the preparative variables on the outcomes from this procedure is presented. Fe3Mo3N and Co3Mo3N are prepared as nearly single phases at temperatures as low as 973K, but higher temperatures are required to obtain pure samples (1073 and 1173K for Fe and Co compounds, respectively). In the case of Fe3Mo3N, moreover, a slow cooling of the samples results in segregation of impurities. The nickel nitride derivative shows a different stoichiometry, Ni2Mo3N, which results in the systematic presence of impurities. The structures of M3Mo3N (M=Fe, Co) have been refined from X-ray powder diffraction data. These nitrides crystallize in the cubic system, space group Fd3m [a=11.07633(8) and 11.02396(8) for M=Fe and Co, respectively]. The structure of Ni2Mo3N has been determined in an ab initio manner from X-ray powder diffraction data. The cell is cubic, space group P4132 [a=6.63422(4)]. Starting positional parameters were obtained by direct methods, and the structure was refined by Rietveld analysis of the data. All three nitrides are prepared as phases constituted by submicrometer homogeneous particles. They show metallic behavior, and temperature programmed oxidation studies reveal an enhanced stability for the nickel derivative in an oxygen atmosphere.

Improving epoxide production using Ti-UVM-7 porous nanosized catalysts

Nanosized Ti-UVM-7 materials with a hierarchical system of pores at two different length scales have been prepared through a one-pot procedure by using a simple template agent; the catalytic activity and selectivity of the resulting materials in bulky olefin epoxidation by organic peroxides are the highest reported to date.

Procedures for synthesis of single-phase 2212 bismuth material☆

Abstract Benefiting from the features of the Bi2O3-CuO binary system, it is possible to isolate a single-phase Bi2Sr2CaCu2O8 high-Tc superconducting material starting from Bi2CuO4. The existence of (BiO2)n chains in the channel-containing open structure of this cuprate, which melts at relatively low temperature, makes it a suitable solid state precursor of the 2212 material.

Crystal fibers of BiSrCaCuO materials grown by the laser floating zone method

The technological importance of good-quality fibers of the new high-Tc superconducting oxide materials warrants their investigation through a variety of preparatory techniques. In particular, the laser floating zone (LFZ) growth method is very versatile for rapidly growing fibers of specific length, diameter and orientation. This report describes a study of the preparation, morphology and electrical properties of dense, oriented, LFZ-grown polycrystalline fibers derived from ceramic precursors in the BiCaSrCuO system. The features of the resulting materials are discussed on the basis of the procedural variables and chemical history of the polycrystalline precursors.