Synthesis of nanosized bismuth ferrite (BiFeO3) by a combustion method starting from Fe(NO3)3·9H2O-Bi(NO3)3·9H2O-glycine or urea systems

Two bismuth ferrite potential precursors systems, namely Fe(NO₃)₃·9H₂O-Bi(NO₃)₃·9H₂O-glycine/urea with different metal nitrate/organic compound molar ratios have been investigated in order to evaluate their suitability as BiFeO₃ precursors. The presence into the precursor of both reducing (glycine and urea) and oxidizing (NO₃⁻) components, modifies dramatically their thermal behaviour comparative with the raw materials, both from the decomposition stoichiometries and temperature occurrence intervals points of view. Also, the thermal behaviour is dependent on the fuel nature but practically independent with the fuel content. The fuel nature influences also some characteristics of the resulted oxides (phase composition, morphologies). In the case of the oxides prepared using urea as fuel, a faster evolution toward a single phase composition with the temperature rise is evidenced, the formation of the BiFeO₃ perovskite phase being completed in the temperature range of 500–550°C.     

Studies of the rates of thermal decomposition of glycine,alanine, and serine

Rates of thermal decomposition of glycine, alanine, and serine are described by the equation of first order reaction in the temperature range 200–300°C. Apparent rate constants and apparent activation energies of decomposition of α-amino acids were evaluated. It was found that the main gaseos reaction product is carbon dioxide.     

On the sol–gel preparation of different tungstates and molybdates

The preparation and characterization of the M′–M′′–O nitrate–tartrate (M′ = Ca, Ba, Gd and M′ = W, Mo) precursor gels synthesized by simple, inexpensive, and environmentally benign aqueous sol–gel method is reported. The obtained gels were studied by thermal (TG/DSC) analysis. TG/DSC measurements revealed the possible decomposition pathway of synthesized M′–M′′–O nitrate–tartrate gels. For the synthesis of different metal tungstates and molybdates, the precursor gels were calcined at different temperatures (650, 800, and 900 °C). According to the X-ray diffraction (XRD) analysis data, the crystalline compounds CaMo_1-x W _x O₄ doped with Ce³⁺ ions, BaMo_1-x W _x O₄ doped with Eu³⁺ ions and Gd₂Mo₃O₁₂ were obtained from nitrate–tartrate gels annealed at 650–900 °C temperatures. The XRD data confirmed that the fully crystalline single-phase powellite, scheelite, or Gd₂(MoO₄)₃ structures were formed already at 650 °C. Therefore, the suggested sol–gel method based on the complexation of metal ions with tartaric acid is suitable for the preparation of mixed tungstates–molybdates at relatively low temperature in comparison with solid-state synthesis.     

Carbothermal reduction of the YSZ–NiO solid oxide fuel cell anode precursor by carbon-based materials

The thermal behavior of the yttria-stabilized zirconia (YSZ) and nickel oxide (YSZ–NiO) composite mixtures with the addition of graphite, multiwall carbon nanotubes and functionalized multiwall carbon nanotubes was studied. The YSZ–NiO composite is the precursor of the YSZ–Ni anode of solid oxide fuel cells. The anode exhibits a porous structure, which is usually obtained by the addition of carbon containing pore formers. Thermal analysis and X-ray diffraction evidenced that the properties of carbonaceous materials (C) and atmosphere have a strong influence on the thermal evolution of the reactions taking place upon heating the anode precursor. The dependence of both the carbon content and the chemical nature of the ceramic matrix on the thermal behavior of the composite were investigated. The discussed results evidenced important features for optimized processing of the anode.     

Oxidative degradation and pyrolysis of polyvinyl chloride in binary mixtures of the system K,Na, Ca ‖ NO3, NO2, OH

Oxidative degradation and pyrolysis of polyvinyl chloride in the temperature range 200–500°C in binary mixtures containing sodium, potassium, and calcium nitrates and nitrites and calcium hydroxide were studied. A scheme of oxidative degradation of polyvinyl chloride and of binding of chlorine and carbon present in the polymer in the reactions of the degradation products with the mixture components was suggested.     

Investigation of Solid-Solid Interactions between Pure and Li2O-Doped Magnesium and Ferric Oxides

The results obtained showed that the addition of small amounts of LiNO₃ to the reacting mixed solids, consisting of equimolar proportion of Fe₂O₃ and basic MgCO₃ much enhanced the thermal decomposition of magnesium carbonate. The addition of 12 mol% LiNO₃ (6 mol% Li₂O) decreased the decomposition temperature of MgCO₃ from 525.5 to362°C. MgO underwent solid–solid interaction with Fe2O3 at temperatures starting from800°C yielding MgFe₂O₄. The amount of ferrite produced increased by increasing the precalcination temperature of the mixed solids. However, the completion of this reaction required prolonged heating at elevated temperature above 1100°C. Doping with Li₂O much enhanced the solid–solid interaction between the mixed oxides leading to the formation of MgFe₂O₄ phase at temperatures starting from 700°C. The addition of 6 mol% Li₂O to the mixed solids followed by precalcination at 1050°C for 4 h resulted in complete conversion of the reacting oxides into magnesium ferrite. The heat treatment of pure and doped solids at 900–1050°C effected the disappearance of most of IR transmission bands of the free oxides with subsequent appearance of new bands characteristic for MgFe₂O₄ structure. The promotion effect of Li2O towards the ferrite formation was attributed to an effective increase in the mobility of the various reacting cations. The activation energy of formation (ΔE) of magnesium ferrite was determined for pure and variously doped solids and the values obtained were 203, 126, 95 and 61 kJ mol⁻¹ for pure mixed solids and those treated with 1.5, 3.0 and 6.0 mol% Li₂O, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterization and preparation of nanocrystalline MgCuZn ferrite powders synthesized by sol–gel auto-combustion method

Nanocrystalline Mg–Cu–Zn ferrite powders were successfully synthesized through nitrate–citrate gel auto-combustion method. Characterization of the nitrate–citrate gel, as-burnt powder and calcined powders at different calcination conditions were investigated by using XRD, DTA/TG, IR spectra, EDX, VSM, SEM and TEM techniques. IR spectra and DTA/TGA studies revealed that the combustion process is an oxidation–reduction reaction in which the NO₃ ⁻ ion is oxidant and the carboxyl group is reductant. The results of XRD show that the decomposition of the gel indicated a gradual transition from an amorphous material to a crystalline phase. In addition, increasing the calcination temperature resulted in increasing the crystallite size of Mg–Cu–Zn ferrite powders. VSM measurement also indicated that the maximum saturation magnetization (64.1 emu/g) appears for sample calcined at 800 °C while there is not much further increase in M _s at higher calcination temperature. The value of coercivity field (H _c) presents a maximum value of 182.7 Oe at calcination temperature 700 °C. TEM micrograph of the sample calcined at 800 °C showed spherical nanocrystalline ferrite powders with mean size of 36 nm. The toroidal sample sintered at 900 °C for 4 h presents the initial permeability (μ _i) of 405 at 1 MHz and electrical resistivity (ρ) of 1.02 × 10⁸ Ω cm.     

Dynamic study of ammonium dawsonite doped with yttrium transformation at elevated temperatures

Co-precipitation of alumina/YAG precursor from aluminum and yttrium nitrates solution with ammonium carbonate results in dawsonite (NH₄Al(OH)₂CO₃). Its crystallographic parameters differ from the compound precipitated without the yttrium additive. It indicates that yttrium ions become incorporated into the dawsonite structure. The DSC/TG and X-ray measurements show decomposition of dawsonite at elevated temperature resulting in γ-Al₂O₃ which transforms to δ and θ modifications at still higher temperatures. The full transformation to α-Al₂O₃ and YAG occurs at temperatures higher than 1,230 °C.     

Thermal study of TiO<Subscript>2</Subscript>–CeO<Subscript>2</Subscript> yellow ceramic pigment obtained by the Pechini method

TiO₂–CeO₂ oxides for application as ceramic pigments were synthesized by the Pechini method. In the present work the polymeric network of the pigment precursor was studied using thermal analysis. Results obtained using TG and DTA showed the occurrence of three main mass loss stages and profiles associated to the decomposition of the organic matter and crystallization. The kinetics of the degradation was evaluated by means of TG applying different heating rates. The activation energies (E _a) and reaction order (n) for each stage were determined using Horowitz–Metzger, Coats–Redfern, Kissinger and Broido methods. Values of E _a varying between 257–267 kJ mol^–1 and n=0–1 were found. According to the kinetic analysis the decomposition reactions were diffusion controlled.     

TG,DTA and electrical conductance properties of some Cu(II) AND Mn(II) bisazo- dianils complexes

The TG and DTA of a new series of Mn(II) and Cu(II) complexes with a number of newly prepared bisazo-dianil ligands were studied in the temperature range (20-700°C). The TG and DTG curves display to main steps, the first one within the temperature range (25-330°C) correspond to the elimination of water or and ethanol from the complexes. The second step within the range (350-625°C) is due to the decomposition of the complexes yielding the metal oxides as the final product. The rate constants of the dehydration and decomposition reactions were determined, from which some kinetic parameters were evaluated. The DTA curves show that the dehydration of the metal complexes is an endothermic reaction. In all cases the anhydrous metal complexes undergo exothermic decomposition reactions to give the metal oxide. The thermodynamic parameters (ΔE, ΔH, ΔS, ΔG) for the occurring processes are calculated. The electrical conductivities of the solid complexes were measured and the activation energy of the complex and its free ligand are also calculated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of the modifier ion on the properties of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> and ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> pigments

Magnesium and zinc ferrites have been prepared by the polymeric precursor method. The organic material decomposition was studied by thermogravimetry (TG) and differential thermal analysis (DTA). The variation of crystalline phases and particle morphology with calcination temperature were investigated using X-ray diffraction (XRD) and scanning electronic microscopy (SEM), respectively. The colors of the ferrites were evaluated using colorimetry. Magnesium ferrite crystallizes above 800°C, presenting a yellow- orange color with a reflectance peak at the 600–650 nm range, while zinc ferrite crystallizes at 600°C, with a reflectance peak between 650–700 nm, corresponding to the red-brick color.     

Isothermal mass crystallization of ammonium and potassium nitrates from Tergitol NP-4-based water-in-oil microemulsions

A procedure is elaborated for preparing powders of ammonium and potassium nitrates with microsized particles and stable sols with particle sizes of 7–16 nm based on isothermal mass crystallization from Tergitol NP-4-stabilized water-in-oil microemulsions in decane. The crystallization process is studied by turbidimetry and photon-correlation spectroscopy. The isolated powders are characterized by scanning electron microscopy, while sols and microemulsions are studied by photon-correlation spectroscopy. Evaporation of water from microemulsion droplets upon stirring water-in-oil micellar solutions at 25–45°C is shown to be the reason for the salt crystallization. It is ascertained that the time of the onset of crystallization and the morphology of resulting particles depend on temperature, content of an aqueous pseudophase, and the nature of a salt. A desolubilization-emulsion hypothesis is proposed for explaining the formation of nanoparticle powders and organosols.     

Dilatometry and high temperature x-ray diffractometry study of LaCrO<Subscript>3</Subscript> prepared using microwave heating

The phase transitions in the LaCrO₃ were studied using bulk dilatometry and high temperature X-ray diffractometry from room temperature to 1050 and 1200°C, respectively. LaCrO₃ was prepared at 500°C from oxalate precursor employing microwave heating technique. Bulk shrinkage measurements on LaCrO₃ pellets were carried out using dilatometer designed and fabricated in our own laboratory. Dilatometric curves of LaCrO₃ showed two peaks in ΔL/L vs. temperature curves in the range 200–400 and 800–1000°C, respectively. These phase transitions have been confirmed using high temperature X-ray diffractometry. The role of simple technique like bulk dilatometry in detecting and monitoring the polymorphic transformations in solids is discussed for lanthanum chromates.     

Pyrolysis of biocollagenic wastes of vegetable tanning. Optimization and kinetic study

Three solid wastes generated from the vegetable tanning of bovine skin in the Leather Industry (shavings, trimmings and buffing dust) were mixed together in the same proportions in which they were produced and the mixture was then used as a pyrolysis precursor for this research study. The optimal pyrolysis conditions for obtaining energy from the generated fractions (char, tar and gas fraction), and the preparation of activated carbons from the carbonaceous material (char), were established. The final conditions were chosen from two different points of view, the thermogravimetric results (TG/DTG) obtained at different heating rates (2–20 °C/min) and an optimization of the pyrolysis parameters by means of experiments carried out in a conventional furnace. The pyrolysis conditions finally selected were: heating rate (5 °C/min), final temperature (750 °C), and time at final temperature (60 min) and inert gas flow (N₂ 150 ml/min). The carbonaceous material (char) obtained exhibits good properties as a solid fuel due to its high calorific value and relatively low ash content. It also shows suitable characteristics as a precursor for the preparation of activated carbons. The condensable fraction has a predominantly phenolic nature and contains significant amounts of nitrogen compounds (nitriles, diketopiperazines, etc.), alkanes, alkenes, acids and esters, derived from the decomposition of tannins and collagen, with possible industrial applications in the preparation of chemical products. The gaseous phase is rich in carbon monoxide and carbon dioxide, and also contains a certain amount of methane and hydrogen. The syngas content increases with the pyrolysis temperature. A kinetic study of the pyrolysis was carried out using a model of independent reactions. The variation in the heating rate produced a slight shift to higher temperatures of the decomposition peaks, although this did not significantly affect either the kinetic parameters of the degradation processes or the percentage weight losses.     

CuZnAl mixed-metal oxides prepared by a novel sol–gel route and the application for synthesis of 2- methylpyrazine

Metal (Cu, Zn, Al) nitrates and chlorides were used for preparing CuZnAl xerogels by a sol–gel route with propylene oxide as gelation initiator. The CuZnAl mixed-metal oxides were further obtained by thermal treatment the xerogels at 500 °C for 5 h in air. Thermal decomposition behavior of the CuZnAl xerogels, the microstructures and the reducibility of the calcined xerogels were investigated by thermogravimetry (TG), scanning electron microscope (SEM), powder X-ray diffraction (XRD), N₂ adsorption/desorption (BET, BJH) and temperature-programmed reduction (TPR) techniques. The catalytic activity in dehydrocyclization of ethylenediamine (ED) with 1, 2-propyleneglycol (PG) to 2-methylpyrazine (2-MP) was carried out at 380 °C. The results displayed that the CuZnAl mixed-metal oxides prepared using nitrates as the metal precursors had a higher metallic Cu dispersion and a superior low-temperature reducibility than those prepared by chlorides, which results in a higher catalytic activity for the synthesis of 2-methylpyrazine. Especially when the molar ratio of Cu/Zn/Al = 2:1:2, the catalyst using nitrates as the metal precursors improved the selectivity of 2-MP to 87.5%.     

Adsorption and DSC study of mineral–carbon sorbents obtained from coal tar pitch–polymer compositions

The purpose of this study was to determine the possibility of producing hydrophobic mesoporous mineral–carbon sorbents from aluminum hydroxide and compositions of coal tar pitch–polymers on carbonization at 600 °C in a nitrogen atmosphere. Blends of the products of co-precipitation of aluminum hydroxide in the carbonaceous substances medium were subjected to carbonization process. The extent of porous structure development was evaluated using low temperature nitrogen adsorption, adsorption of benzene vapors, and adsorption of iodine from aqueous solution. The highest value of BET surface area of about 370 m²/g was achieved for the carbonization product obtained from co-precipitated raw components with 10 wt% compositions coal tar pitch–polymer. These materials demonstrated high capacity to reduce organic pollutions from sewage. Pitch–polymer composition containing poly(ethylene terephthalate) or phenol–formaldehyde resin was studied by the means of DSC method in order to determine the high-temperature transformations taking place under the conditions of carbonization. DSC method enables to determine i.a. the decomposition temperatures of carbonizates produced from pitch–polymer compositions and the evaluation of their sorption abilities. The additive of poly(ethylene terephthalate) and phenol–formaldehyde resin caused the increase of thermal resistance of the pitch expressed by higher decomposition temperatures.     

Use of emanation thermal analysis to characterize thermal reactivity of brannerite mineral

Emanation thermal analysis (ETA) was used to characterize the thermal reactivity of amorphous brannerite mineral of general formula U_1–xTi_2+xO₆ (locality El Cabril, near Cordoba, Spain). It was demonstrated that on sample heating up to 880°C microstructure changes taking place in the sample were accompanied by the formation of new radon diffusion paths, followed by their closing up during the final transformation of amorphous to crystalline brannerite in the range 900–1020 °C. Relative changes in structure irregularities that served as radon diffusion paths during heating and subsequent cooling of the sample to temperatures of 300, 550, 750, 880, 1020 and 1130°C, respectively, were determined from the ETA results. Mass losses in temperature ranges of 230–315, 570–760 and 840–1040°C were observed by thermogravimetry. Mass spectrometry indicated the release of CO2 mainly due to the decomposition of minor carbon amount in the brannerite mineral sample.     

The kinetics of tobacco pyrolysis

When tobacco is pyrolysed under non-isothermal flow conditions in an inert atmosphere, variation of the inert gas or its space velocity has only a minor effect on the profiles of formation rate versus temperature for seven product gases. Thus, mass transfer processes between the tobacco surface and the gas phase are very rapid, and the products are formed at an overall rate which is determined entirely by that of the chemical reactions.The effect of radical chain inhibitors (nitrogen oxides) on the pyrolysis is complex because of the resultant oxidation. Nevertheless, no evidence was found for the occurrence of radical chain reactions in the gas phase. A small proportion (less than 10%) of all the gases monitored are formed by homogeneous decomposition of volatile and semi-volatile intermediate products, in the furnace used.At temperatures above about 600°C the reduction of carbon dioxide to carbon monoxide by the carbonaceous tobacco residue becomes increasingly important. However, when tobacco is pyrolysed in an inert atmosphere, only a small amount of carbon dioxide is produced above 600°C and consequently its reduction to carbon monoxide contributes only a small proportion to the total carbon monoxide formed above that temperature. The rate of the tobacco/carbon dioxide reaction is controlled by chemical kinetic rather than mass transfer effects. Carbon monoxide reacts with tobacco to a small extent.When the tobacco is pyrolysed in an atmosphere containing oxygen (9–21% v/v), some oxidation occurs at 200°C. At 250°C the combustion rate is controlled jointly by both kinetic and mass transfer processes, but mass transfer of oxygen in the gas phase becomes increasingly important as the temperature is increased, and it is dominant above 400°C. About 8% of the total carbon monoxide formed by combustion is lost by its further oxidation.The results imply that inside the combustion coal of a burning cigarette the actual reactions occurring are of secondary importance, the rate of supply of oxygen being the dominant factor in determining the combustion rate and heat generation. In contrast, in the region immediately behind the coal, where a large proportion of the products which enter mainstream smoke are formed by thermal decomposition of tobacco constituents, the chemistry of the tobacco substrate is critical, since the decomposition kinetics are controlled by chemical rather than mass transfer effects. tobacco substrate is critical. In addition, the heat release or absorption due to the pyrolytic reactions occurring behind the coal will depend on the chemical composition of the substrate. Thus, together with the differing thermal properties of the tobacco, the temperature gradient behind the coal should depend on the nature of the tobacco.     

Synthesis of nanosized BaSnO3 powders from metal isopropoxides

Nanocrystalline BaSnO₃ with a primary particle size of 40–60 nm was prepared through hydrolysis of a barium tin isopropoxide and following crystallization. The thermal decomposition, the crystallization and the microstructure of the obtained powders were investigated with the help of TG-DTA, IR, XRD, HRSEM and HRTEM. The organic rest groups in the as-prepared powder decompose thermally at 350°C, which is accompanied by the building of BaCO₃ that disappear again at 600°C. The crystallization of BaSnO₃ takes place at 500–600°C. Single-phase BaSnO₃ powders have been obtained at a temperature as low as 600°C. The amorphous as-prepared powder shows a cluster structure. Nucleation of BaSnO₃ beginning at 350°C was observed under HRTEM, and the spherical nano-particles of BaSnO₃ calcined at 760°C crystallize well and are strongly aggregated. The presented results indicate a heterogeneous nucleation and growth mechanism by the formation of BaSnO₃.     

Application of catalysts to coal gasification with carbon dioxide

The effects of the additives of iron, nickel, zinc, and their oxides on the process of coal gasification with carbon dioxide at temperatures of 550–800°C was studied. It was found that the additives used did not exhibit noticeable activity upon mixing with coal. Impregnation with nickel was found most efficient; it caused a notable increase in the rate of the process and a higher degree of CO₂ conversion, as compared with the thermodynamically equilibrium conversion for a given temperature. Thus, impregnation with 5% nickel made it possible to decrease the reaction temperature by 80°C with the retention of the yield of CO at about 100%. Kinetic models for describing noncatalytic and catalytic gasification reactions were considered and a mechanism of the process in the presence of nickel was proposed.