Materials Science Forum Vols. 498-499

Abstract: Nanosized spinel nickel ferrite particles have attracted considerable attention and efforts continue to investigate them for their technological importance to the microwave industries, high speed digital tap or disk recording, repulsive suspension for use in levitated railway systems, ferrofluids, catalysis and magnetic refrigeration systems. Nanosize nickel ferrite powders (NiFe2O4) have been prepared by combustion reaction using nitrates and urea as fuel. The resulting powders were characterized by X-ray diffraction (XRD), BET, and transmission electron microscopy (TEM). The results showed nanosize nickel ferrite powders with high specific surface area (55.21 m2/g). The powders showed extensive XRD line broadening and the crystallite size calculated from the XRD line broadening was 18.0 nm. The samples were uniaxially compacted by dry pressing, sintered at 1200°C/2h and characterized by bulk density, SEM and magnetic properties measurements. The samples showed uniform microstructures with grain size of 4.45 μm, maximum flux density of 0.18T, field coercive of the 488 A/m, and hysteresis loss of 47.58 W/kg.

Abstract: Powders of Ce0.8Gd0.2O1.9 were synthesized by co-precipitation and amorphous citrate routes and their compositions and structures were compared. Co-precipitation process was carried out at room temperature, while amorphous citrate synthesis was performed at 60-70 °C. The powders obtained were calcined at 700 °C for 1 h. X-ray diffraction analyses showed that a single fluorite structure was formed by both synthesis techniques. Pellets from these powders were prepared by compaction at 30 kN/cm2 and sintered at 1593 °C in air. WDS analysis confirmed the homogeneity of the pellets. The sintered pellets obtained by amorphous citrate and by coprecipitation routes showed relative densities over 97 % of the theoretical density. These results indicated that the citrate amorphous route is an interesting and simple technique to prepare gadolinium-doped ceria powders with high sinterability.

Abstract: La0.8Sr0.2MnO3±d (LSM) powders were prepared by a Pechini-type polymerizable complex process and by amorphous citrate technique. Both processes involve the complexation of the LSM cations from their nitrates or carbonates. The aim of this work is to determinate the differences between the powers obtained by these techniques. The powders synthesized by the Pechini-type process were calcined between 500 °C and 1100 °C. The powders prepared by amorphous citrate technique were calcined at 900 °C. No contamination of either of the powders was observed by X ray fluorescence analysis. X ray diffraction results showed that a perovskitetype structure phase was obtained. BET results showed that the specific surface areas of the powders prepared by the polymerizable complex process and the amorphous citrate route were 6.6 m2g-1 and 5.7 m2g-1, respectively.

Abstract: In this work tin dioxide (SnO2) doped with nickel is studied with the aim of develop integrated gas sensors for some important pollution gases into environment, principally sulphur dioxide (SO2). SnO2 powders doped with Nickel were prepared by Pechini`s method [1] and characterized morphologically by X-Ray diffraction, measurement of specific surface area (SBET) and transmission electron microscopy (TEM), which revealed grain sizes around 5 to 10 nm. Gas sensors were fabricated by spin-coating deposition of Ni doped SnO2 films over alumina substrate with gold contacts. Electrical resistance of such device is characterized, at room temperature, as function of SO2 concentration into environment, showing a sensitivity around 0.0058 ppm-1 for relative resistance variation, with good linearity for SO2 concentrations up to 30 ppm. Also, FTIR analysis shows the desorption of SO2 from SnO2 surface, after cleaning into fresh air.

Abstract: The complex perovskite compound 0.9PbMg1/3Nb2/3O3-0.1PbTiO3 is one of the most promising relaxor ceramic because the addition of lead titanate increases Tm by about 5°C/mol% from intrinsic Tm value for pure PMN (near –7 to -15°C). A Ti-modified columbite precursor was used to prepare PMN-PT powders containing single perovskite phase. This variation on columbite route includes Ti insertion in MgNb2O6 orthorhombic structure so that individual PT synthesis becomes unnecessary. Furthermore, effects of Li additive on columbite and PMN-PT structures were studied by XRD to verify the phase formation at each processing step. XRD data were also used for the structural refinement by Rietveld method. The additive acts increasing columbite powders crystallinity, and the amount of perovskite phase was insignificantly decreased by lithium addition. By SEM micrographs it was observed that Li presence in PMN-PT powders leads to the formation of rounded primary particles and for 1mol% of additive, the grain size is not changed, different from when this concentration is enhanced to 2mol%.

Abstract: One possible route for the production of nanometric powders is the reactive high-energy milling. For a variety of systems of highly exothermic reactions, the milling can lead to self-sustaining reactions, with the reaction being observed after an induction or ignition time, which produces a temperature increase in the reactants. In this work, WC powder was obtained by reactive high energy-milling, performed in a SPEX 8000 shaker/mill. During milling the highly exothermic displacement reaction of reduction of the WO3 by Mg was performed in presence of carbon to produce WC and MgO. The material to ball mass ratio was fixed in 4:1 and the ignition time of the reaction was determined. In order to characterize the transformations from reactant powders to reaction products, the milling was stopped at given times before, immediately after and after the reaction; the powders obtained were characterized by X-ray diffraction, scanning electron microscopy and specific surface area. Depending on the amount of carbon, W and the W2C were also observed as reaction products. The complete formation of WC was achieved with addition of an excess of carbon.

Abstract: The solid material from electroplating wastewater treatment was used as raw material in synthesis of ceramic pigments, once the formation of ZnCr2O4 and FeCr2O4 crystalline phases can be considered as a process of heavy metals inertization, such as chromium, zinc and iron. To reach the purpose of this paper, the techniques of elementary chemical analysis (Atomic Absorption Spectroscopy and X-Ray Fluorescence, thermal analysis (DTA/TGA) and X-Ray Diffraction were applied in samples of waste with the aim of physical and chemical characterization. After the characterization of residue were incorporate metallic oxides for correction of composition and obtaining of inorganic pigment. Pigment formation was followed through X-ray diffraction, where the formation of spinel phases containing the metals Fe, Cr and Zn were observed. The pigment was characterized through scanning electron microscopy, differential thermal analysis and thermogravimetry. The 8 hours cycle calcinated pigments formed crystalline spinel phases, where we can say that the present metals in the sludge became inert. It was possible to incorporate up to 20% of the galvanic waste in the production of Fe, Cr and Zn based inorganic pigments.

Abstract: The aluminum monohydroxide pseudoboehmite is a common precursor for catalysts and ad/absorbents. Fibrillar pseudoboehmite powders were synthesized by the same preparation method from aqueous sols but aged in different times: 24, 48 and 168 hours. The sol particles were from fibrillar pseudoboehmite; the average length of the microfibrils changed sample to sample, the sizes increasing with the thermal aging time. The sols were dried by different methods: at room temperatures, forming a solid membrane; spray dried; dried at 60°C and 110°C from a gel from the sol aged 168 hours; all were fired between 200°C and 1000°C in oxidative atmosphere to produce activated transition aluminas. The phases formed was identified by XRD. The aim of the present paper is to present data showing that method of drying the sol has an effect on the temperature of formation of alpha-alumina due to the anisodiametric shape of the pseudoboehmite crystals.

Abstract: Transition-metal spinels are efficient catalysts in a number of heterogeneous processes, such as CO oxidation, catalytic combustion of hydrocarbons and oxychlorination of methane. The properties of catalytic materials are highly dependent on the synthesis route. Spinels are often produced at high temperatures by the calcination of precursors such as powder mixtures, slurries or resins. Combustion synthesis is a cost-efficient method used to produce homogeneous and fine particles with high reproducibility. Cu0.8Ni0.2Cr2O4 spinel was obtained by the combustion of metallic nitrates using urea as fuel. The resulting powders were calcinated at different temperatures and characterized by thermogravimetric and particle size analyses, X ray diffraction, and scanning electron microscopy. The effect of urea on the control of the process and particle morphology was investigated. The results revealed the formation of porous powders with increasing crystallinity as the calcination temperature increased. Crystallization of spinel started at 700 oC.

Abstract: The solid solution 0.9PbMg1/3Nb2/3O3-0.1PbTiO3 is one of the most widely investigated relaxor ceramic, because of its high dielectric constant and low sintering temperatures. PMN-PT powders containing single perovskite phase were prepared by using a Ti-modified columbite precursor obtained by the polymeric precursor method. Such precursor reacts directly with stoichiometric amount of PbO to obtain pyrochlore-free PMNPT powders. The structural effects of K additive included in the columbite precursor and 0.9PMN-0.1PT powders were also studied. The phase formation at each processing step was verified by XRD analysis, being these results used for the structural refinement by the Rietveld method. It was verified the addition of K in the columbite precursor promotes a slight increasing in the powder crystallinity. There was not a decrease in the amount of perovskite phase PMN-PT for 1mol% of K, and the particle and grain size were reduced, making this additive a powerful tool for grain size control.