M. J. Diánez

Synthesis of needle-like BaTiO3 particles from the thermal decomposition of a citrate precursor under sample controlled reaction temperature conditions

Needle-like BaTiO3 particles with controlled microstructure have been prepared from the thermal decomposition of a metal-organic precursor under sample controlled reaction temperature (SCRT) conditions. In a first step, a mixed barium and titanium citrate precursor consisting of a single phase compound with controlled morphology and stoichiometry have been prepared. The precursor has been converted isomorphologically into BaTiO3 by thermal treatment under SCRT conditions. The so-obtained needle-like particles are constituted of self-assembled nanoparticles. The size of the assembled subunits has been tailored by controlling the reaction rate and the partial pressure of the gases generated during the thermal decomposition of the barium titanium citrate. This microstructural control accomplished by the SCRT method can not be achieved by conventional thermal decomposition of the citrate precursor. Finally, the set of samples obtained with different microstructures has been investigated in terms of stabilization of the cubic phase at room temperature.

Direct mechanosynthesis of pure BiFeO3 perovskite nanoparticles: reaction mechanism

In this work, a mechanochemical procedure is proposed as a simple and fast method to synthesize the pure BiFeO3 perovskite phase as a nanostructured material without the need for purification treatments, while the mechanochemical reaction mechanism has been investigated and correlated with that of the conventional solid-state reaction. Thus, different milling conditions have been used as a tool for tailoring the crystallite size of the resulting BiFeO3 nanoparticles. The materials prepared by the mechanochemical reaction could be annealed or sintered without the formation of undesirable phases. Both the ferroelectric and ferromagnetic transitions were observed by DSC. Finally, the dielectric constants of the prepared material at different frequencies as a function of the temperature have been measured, showing that the material is clearly an isolator below 200 °C, characteristic of a high quality BiFeO3 material.

Direct use of the mass output of a thermobalance for controlling the reaction rate of solid-state reactions

Sample controlled thermal analysis equipment has been developed constituted by an electrobalance in which the mass output (TG signal) is directly used for monitoring the temperature of thermal decomposition reactions under constant rate thermal analysis (CRTA) or stepwise isothermal analysis (SIA) control. The sample weight is programmed to follow a preset linear decrease as a function of the time by means of a conventional controller, that at the time control a second conventional temperature programmer. The CRTA control is achieved by controlling the temperature is such a way that if the mass input is higher than the setpoint, the temperature increases at a predefined heating rate; while if the mass input is lower than the setpoint, the temperature decreases at a predefined cooling rate. The SIA control is achieved by selecting the run–hold command from the menu of the digital input of the temperature programmer. In such a case, the programmed linear heating schedule is in progress while the sample weig...

Correlation between synthesis conditions, coherently diffracting domain size and cubic phase stabilization in barium titanate

Abstract The Constant Rate Thermal Analysis method has been used under different experimental conditions to obtain barium titanate powder of very small grain size. Both a crystallographic and a dielectric study ham been carried out on samples sintered at different temperatures. It is deduced that the formation of small coherently diffracting domains is improved by the generation of oxygen vacancies and the cubic phase stabilization depends on the coherently diffracting domain size rather than on the particle size.

High-Temperature Stability of a Nanostructured Cu-Al2O3 Alloy

A nanostructured oxide dispersion strengthened (ODS) copper alloy powder, containing an alumina dispersion (about 1% vol.) was prepared by mechanical alloying and subsequently consolidated by hot extrusion. The high temperature stability of the nanostructured and consolidated powders was studied. The stability of the nanostructure was checked with 1 hour heat treatments in the range 400-800oC. Powders were vacuum encapsulated in copper cans and extruded at temperatures in the range 500-600oC. The crystallite size of both the as-milled and the heat treated powders was determined with X-ray diffraction (XRD). The line broadening was used to calculate grain (crystallite) size. Two methods were used: the Scherrer equation (SE) and the Williamson-Hall plot (WH). The microstructures of both loose powders and extruded powders were characterised with Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The mechanical properties were assessed with microhardness measurements. The results obtained via XRD with both methods are within the same order of magnitude. A reasonable agreement is also obtained with SEM and TEM observations. The X-ray determination of grain size and the microhardness measurements indicate negligible grain coarsening up to 800oC. On the other hand, the consolidated powders showed lower hardness values than those of the as-milled and annealed powders. The nanostructured copper alloy studied shows good thermal stability at temperatures up to 800oC. Introduction To produce high strength materials by powder metallurgy it is desired to have small grain size fully to dense material, because strength depends on both grain size and porosity. To inhibit grain growth thermally stable dispersoids of a second phase can be used, for example ceramic particles. The resulting behaviour of such materials depends on the technique that has been used in their preparation [1,2]. It is well known that the nanostructure resulting from mechanical alloying/synthesis gives rise to an increase in strength and hardness [3]. However, softening is seen when this structure is subjected to heat treatments above 400oC. Extrusion processing induces strong shear deformation and is thus ideal for consolidation of ductile materials. Hot extrusion has yielded good results for the consolidation of rapidly solidified copper alloys [4]. However, the deformation process combined with the high temperature needed to extrude at moderate pressures promotes rapid coarsening of the grain structure. The aim of this work is to investigate the thermal stability of the nanostructured copper alloy powder consolidated by hot extrusion. Experimental procedures A nanostructured oxide dispersion strengthened (ODS) copper alloy powder was produced commercially by MBN srl, under the trade name CA11, containing a nominal 1 vol. % alumina. The chemical composition of the CA11 nanostructured powder prepared by mechanical alloying was assessed by a semi-quantitative method based on energy dispersive spectroscopy (EDS). The results obtained are shown in table 1. The iron content detected is probably due to contamination during the milling. A more precise method of analysis, using X-ray fluorescence, yielded 1.2 % Fe. Assuming that all oxygen is bound with aluminium to form Al2O3 then about 1.5 % vol. of alumina is expected. Table 1 Chemical composition of CA11 powders Al wt% O wt% Fe wt% Cu wt% 0.4 0.32 1.65 97.63 Key Engineering Materials Online: 2002-10-25 ISSN: 1662-9795, Vols. 230-232, pp 652-655 doi:10.4028/www.scientific.net/KEM.230-232.652

Lengthening of the carbon chain of sugars by the Ch(NO2·CH(OEt)2 fragment a route to higher 2-amino-2-deoxyaldoses☆

Abstract Nitroaldol (Henry) reactions promoted by trialkylsilyl chlorides have been applied to the preparation of chain-extended nitro and amino sugars. The one-pot reaction of 1,2:3,4-di O -isopropylidine-α- d - galacto -hexodialdo-1, 5-pyranose ( 1 ) with 1,1-diethoxy-2-nitroethane in the presence of Et 3 N, Bu 4 NF ·3H 2 O, and t BuMe 2 SiCl yielded 7-deoxy-1,2:3,4-di- O -isopropylidene-7-nitro- l - threo-α- d -galacto -octodialdo-1, 5-pyranose diethyl acetal ( 3a ) and a diastereoisomer in the ratio 4:1. The structure of 3a was established by a single crystal X-ray diffraction study. Hydrogenation of 3a (Ni, H 2 ) afforded the corresponding 7-amino-7deoxyl-1,2:3,4-di- O -isoprpylidene- l - threo-α- d -galacto -octodialdo-1,5-pyranose diethyl acetal 4a ). Conversion of the latter compound into a trans -oxazoline derivative demonstrated that the l - threo configuration had been retained. Likewise, reaction of the aldehydo sugar 1 with 1-nitropropane was carried out in order to compare the efficiency of the new nitroaldol procedure here applied with that reported using trialkylsilyl nitronic esters as intermediates.

A novel highly diastereoselective synthesis of chiral dihydrothiophenes from mesoionic compounds

Thioisomunchnones undergo 1,3-dipolar cycloaddition with chiral nitroalkenes to afford stereoselectively 4,5-dihydrothiophenes by means of an unprecedented fragmentation of cycloadducts; the structures of these products have been established by single crystal X-ray structure analyses.

A new synthesis of 6-oxopyrimidinium-4-olates. Theoretical study of the regioselective cycloaddition of arylisocyanates with a 1,3-thiazolium-4-olate system

Abstract A new and regioselective synthesis of 6-oxopyrimidinium-4-olate systems ( 13–21 ) is described, involving the 1,3-dipolar cycloaddition of the 1,3-thiazolium-4-olate 12 with arylisocyanates. MNDO and AM1 calculations rationalise the reactivity and regioselectivity experimentally observed.

Studies on sugar nitro-olefins. Part 7. Synthesis of 3-(alditol-1-yl)-1,2,3,5,6,7-hexahydro- and -1,5,6,7-tetrahydro-indol-4-ones. X-Ray molecular structure of (3S)-3-(1,2,3,4,5-penta-O-acetyl-D-galacto-pentitol-1-yl)-1,2,3,5,6,7-hexahydroindol-4-one

Raney nickel reduction of (3R)-2-hydroxyimino-3-(1,2,3,4,5-penta-O-acetyl-D-galacto- or -D-gluco-pentitol-1-yl)-3,5,6,7-tetrahydrobenzofuran-4(2H)-ones (5) gives (3S)-3-(1,2,3,4,5-penta-O-acetyl-D-galacto- or -D-galaco-pentitol-1-yl)-1,2,3,5,6,7-hexahydroindol-4-ones (6) in high yields. Deacetylation of compounds (6) with catalytic sodium methoxide in methanol affords the corresponding (pentitol-1-yl)indolones (7). Compounds (6) can be transformed into the 3-(1,2,3,4,5-penta-O-acetyl-D-galacto- or -D-gluco-pentitol-1-yl)-1,5,6,7-tetrahydroindol-4-ones (8) by treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The structure of (3S)-3-(1,2,3,4,5-penta-O-acetyl-D-galacto-pentitol-1-yl)-1,2,3,5,6,7-hexahydroindol-4-one (6a) was confirmed by an X-ray structure determination.

Constant Rate Thermal Analysis (CRTA) as a Tool for the Synthesis of Materials with Controlled Texture and Structure

Some examples of the use of the CRTA method for the synthesis of materials with controlled texture and structure are given. BaTiO3 has been obtained from the thermal decomposition of Barium Titanyl Oxalate (BTO) and Barium Titanyl Citrate (BTC) by controlling the reaction temperature in such a way that the partial pressure of the gases generated in the reaction was maintained constant at a value close to 10-2 mbar. It has been shown that this method allows getting BaTiO3 with crystal sizes considerably lower than those obtained by decomposing the same precursors by conventional methods. This small crystal sizes lead to the stabilisation of the metastable cubic phase with regards to the tetragonal phase.It has been also shown that the control of the CO generated in the carbothermal reduction of silica allows tailoring the phase composition of the silicon nitride obtained as final product.