M. R. Morelli

Combustion synthesis of calcium aluminates

Commercial calcium aluminate cements for refractory use are known to contain various phases, the hydration behavior of which is interdependent and not fully understood. Hydration studies normally require that pure oxides be synthesized. Solid-state synthesis of single aluminates requires high temperatures and full conversion is not guaranteed. Moreover, it is difficult to obtain a compositionally homogeneous product with this method. The present work discusses a straightforward combustion synthesis technique to prepare submicron calcium aluminates using the corresponding metal nitrates-urea mixtures, at low temperature and short reaction times. The effect of the ratio oxidizer/fuel in the redox mixture was investigated, namely, to find out if urea contents below stoichiometry were enough to trigger the explosive combustion of the fuel and the subsequent decomposition of the salts, and how that ratio affected the phase formation and the characteristics, e.g., morphology and grain size, of the powder produced.

Combustion synthesis of aluminium titanate

Abstract Initial interest in aluminium titanate was due to its low thermal expansion coefficient and high thermal shock resistance, but further research was soon discouraged following the discovery of the expansion anisotropy and the instability of the compound over a specific range of temperatures. The development of a suitable active precursor powder could provide a possible solution to the fabrication difficulties (microcracking and decomposition). The scarce available thermodynamic data for the formation of Al2TiO5from its constituent oxides indicate that the reaction is endothermic and only possible at high temperature because of the titanate being entropy stabilised. The present work describes a straightforward combustion synthesis technique to prepare submicron Al2TiO5 powders, using the corresponding metal precursors-urea mixtures, at low temperature and short reaction times. A thermodynamic interpretation of the reaction is provided and the characteristics of the powder produced, like morphology, specific surface area and grain size, are discussed. The thermal behaviour of the combustion powder is compared with that of Al2TiO5 produced via the conventional ceramic solid state route.

Combustion synthesized ZnO powders for varistor ceramics

Abstract Commercial ZnO varistor ceramics are multicomponent, with minor amounts of added oxides that play important roles, both in the strict electrical sense and for the control of the microstructure. The present work describes the straightforward combustion synthesis of pure and doped ZnO powders from stoichiometric mixtures of the relevant water soluble metal nitrates as cation precursors and urea as fuel. The mixtures were ignited at 500°C resulting in a dry, very fine powder. The as-prepared combustion products, characterized by XRD, SEM, TEM and BET, show high specific surface area, have very small particle sizes and are crystalline, with atomic level homogeneity. Implications on sintering and electrical behaviour are discussed.

Effect of heating conditions during combustion synthesis on the characteristics of Ni0.5Zn0.5Fe2O4 nanopowders

Ni-Zn ferrite powders were synthesized by combustion reaction. The effect of external conditions of heating on the characteristics of the resulting powders was evaluated. Two synthesis routes were studied. The first involved preheating on a hot plate at 300°C and subsequently heating in a muffle furnace at 700°C (RCPM). In the second route the powders (RCP) were heated directly to 600°C on a hot plate until self-ignition occurred. The resulting RCP products were evaluated before and after attritor milling in order to reduce the size and increase the uniformity of particles and/or agglomerates. The resulting powders were characterized by X-ray diffraction (XRD), BET, scanning electron microscopy (SEM), helium pycnometry, sedimentation and transmission electron microscopy (TEM). The results showed that it was possible to obtain Ni-Zn ferrite powders using both routes and that the second route (RCP) was the most favorable in terms of obtaining powders with high surface area. The efficiency of the grinding was confirmed by the reduction of the size of the particles.

Combustion process in the synthesis of ZnO–Bi2O3

Abstract Combustion synthesis of homogeneous ZnO–Bi2O3 powder mixtures has been investigated, with emphasis on the reaction mechanism. The reaction uses salts containing the zinc and bismuth ion (oxidizer) and a fuel. The self-sustaining reaction leads to high temperatures that promote oxide formation and crystallization in a short period of time. The resulting ZnO–Bi2O3 powders show characteristics that favor applications in the field of electronic ceramics, such as varistors. Such characteristics, namely particle morphology and crystallinity degree, were accessed by X-ray diffraction and scanning electron microscopy, showing that combustion synthesis is a promising alternative method to produce adequate powders for the manufacture of varistors.

Combustion synthesis, sintering and magnetical properties of nanocristalline Ni-Zn ferrites doped with samarium

An investigation was made of combustion synthesis to uniformly incorporate small amounts of samarium additive into nanocrystalline Ni0.5Zn0.5Fe2 − xSmxO4 (0.0 ≤ x ≤ 0.1) nanopowders (≈26–20 nm particle size). The effect of the addition of the rare-earth ion samarium on the microstructure, relative density and magnetic properties of the Ni-Zn ferrite obtained by combustion reaction was studied. The samples were uniaxially compacted by dry pressing, sintered at 1200°C/2 h, and characterized by bulk and apparent density, XRD, SEM and magnetic properties. The nanopowder samples without additive displayed an average grain size of 2.87 μm, while the addition of 0.05; 0.075 and 0.1 wt% Sm was found to inhibit grain growth, decreasing the average grain size to 0.77; 0.68 and 0.62 μm, respectively. The relative density was found to increase with the addition of samarium (>98.00% of the theoretical density). The samples without additive showed higher hysteresis parameter values.

Electrical properties of polycrystalline ZnO : Cu obtained from freeze-dried ZnO+copper(II) acetate powders

Highly homogeneous mixtures of powders containing ZnO and monohydrated copper(II) acetate, (CH3COO)2CuċH2O, AcCuH2O were obtained by freeze drying. Thermal analysis and high-temperature X-ray diffraction showed that pure freeze-dried AcCuH2O powder decomposed to CuO, in air, at temperatures above 325 °C. Due to the polymeric nature of AcCuH2O, the powder mixtures were compacted without pressing additives. Pellets with ZnO+x mol % Cu (x=0.05, 0.5, and 5.0) compositions were sintered in air from 750 to 1150 °C for 1 h. After sintering, the density, shrinkage and mass loss increased as the concentration of AcCuH2O increased. A microstructural analysis of samples sintered at 950 °C for 1 h revealed Cu-doped ZnO grains with Cu-clusters of x=5.0 mol % Cu; the J–E curves showed that both the breakdown electric field, Ebr, and the nonlinearity coefficient, α, increased as x increased from 0.05 to 0.5. Complex acceptor defects involving Cu+Zn in the grain boundary are believed to have compensated for the n-type conductivity of the ZnO grains, giving rise to high Ebr values.

Potential use of natural red mud as pozzolan for Portland cement

Red mud, the main waste generated in aluminum and alumina production by the Bayer process, is considered hazardous due to its high pH, according to the Brazilian standard NBR 10004/2004, and worldwide generation of this waste exceeds 117 million tons/year. In this work, non-calcined red mud was used, thus requiring less energy and time and reducing costs, which is the ideal condition for reusing wastes. Mortars containing 30 wt. (%) of cement substituted by red mud showed higher strength of hardened products. The pozzolanic activity index was evaluated based on physical and mechanical parameters (Brazilian NBR 5751 and NBR 5752 standards) and on a chemical analysis (European EN 196-5 standard). A comparison of the reference mixture (without red mud) and the results obtained with red mud confirm the potential of non-calcined red mud for use a as pozzolanic additive in cementitious materials. The setting time (according to the MERCOSUL NM 65 standard) tends to increase but workability remains almost unchanged.

Sintering of Ni-Zn ferrite nanopowders by the constant heating rate (CHR) method

The constant heating rate method employed in sintering studies offers several advantages over the isothermal method, particularly the fact that all the parameters that describe the sintering phenomena can be obtained from a single sample. The purpose of this work is to determine the parameters of sintering kinetics of nanosized Ni-Zn ferrite powders synthesized by combustion reaction. The nonisothermal sintering method was studied using a constant heating rate (CHR). The Ni-Zn ferrite powders, with average particle size varying from 18 nm to 29 nm, were uniaxially pressed and sintered in an horizontal dilatometer at a constant heating rate of 5.0 °C/min from 600 °C up to complete densification, which was reached at 1200 °C. The compacts were characterized by scanning electron microscopy (SEM). Experimental results revealed three different sintering stages, which were identified through the Bannister Theory. The shrinkage and the shrinkage rate analyzed showed a viscous contribution in the initial sintering stage, which was attributed to the mechanism of structural nanoparticle rearrangement.

Bimevox type ionic conductors produced by melting process

Abstract BICUVOX ionic conductors with enhanced electrical conductivity were produced through a new route of synthesis, using powders obtained from melting. Compositions with the general formula Bi4V(2-x)MexO(11-x) with x varying from 0.07 to 0.20 were investigated using this new process, and high electrical conductivity was observed in the temperature range of 100 to 400°C with high density materials at 750°C.