L. Gama

Síntese e caracterização de nanopartículas de TiO2

Several methods of chemical synthesis have been developed and used to obtain powder for production of ceramic membranes. Amongst the alternative methods used in laboratory scale, the Pechini method has been used successfully for the preparation of several types of materials. The objective of this work is to synthesize and characterize TiO2 powders obtained by this method, aiming the preparation of ultra-filtration ceramic membranes. The powder has been characterized by gravimetric and differential thermal analysis, X-ray diffraction, infrared spectroscopy, nitrogen adsorption by BET, and scanning electron microscopy. The X-ray diffraction of the powders showed the presence of the anatase crystalline phase, with crystallite size 15 nm. The particle size calculated from the surface area was 19 nm and the powder morphology shows the presence of soft agglomerates. These results evidence that the Pechini method is interesting for the production of nanometric TiO2 appropriate for ceramic membranes preparation.

Preparation and Characterization of Spinel MCr2O4 (M = Zn, Co, Cu and Ni) by Combustion Reaction

The ceramic industry applies many natural and synthetic pigments as coloring agents in glasses, enamels and unglazed bodies. The purpose of the present work was to prepare a series of chromium spinels MCr2O4 (M = Zn, Co, Cu and Ni) by combustion reaction, using urea as fuel and characterizing the resulting powders. The compositions were prepared by a single step solution combustion reaction using nitrates and urea as fuel. The fired pigments and enameled samples were characterized by X-ray diffraction, BET, scanning electron microscopy (SEM), UV-VIS-NIR spectroscopy, and CIE-L*a*b* color-measurements. The results demonstrated that synthesis by the combustion reaction was very fast and safe, resulting in crystalline spinel with nanoparticles in all the compositions under study. The pigments obtained by combustion reaction displayed better solubility in the molten glazes than pigments obtained by mechanical mixture of oxide precursors. The results demonstrate the viability of using the powders obtained as ceramic pigments. Introduction Pigments are defined as particulate solids, which may be organic or inorganic, white, black, colored or fluorescent, that are insoluble in the substrate in which they become incorporated and that do not react chemically or physically with this substrate [1, 2]. Pigments should present well defined optical and physical properties. These properties, in turn, depend directly on the pigment’s crystalline structure, on its physical (particle shape, distribution, and degree of agglomeration) and chemical characteristics (purity and stability of the composition). The most important characteristic to be considered in a pigment is its capacity to develop color (its pigmentary capacity) [3]. The control of these properties is closely dependent on obtaining nanometric scale particle sizes, i.e., obtaining molecular structures at the atomic level. The most common form of obtaining pigments is the conventional one of mixing oxides, although several other chemical methods allow one to obtain nanometric material with highly controlled purity and chemical homogeneity. Some of the most wellknown methods are sol-gel [4], micro-emulsion [5], co-precipitation [6], supersonic radiation [7], Pechini’s method [8], hydrothermal synthesis [9], freeze-drying [10], and combustion synthesis [1113]. Among the aforementioned methods of chemical synthesis, combustion reaction synthesis stands out as an alternative and promising method for obtaining after-ceramics on a nanometric scale. This synthesization method, also known as auto-propagating synthesis, allows for particles to be obtained (without pre-sintering) in sizes in the order of 30nm [14]. Compared to other synthesization methods, the combustion reaction process offers the advantages of being simple, fast, without requiring subsequent intermediary calcination stages, and consuming less energy during the synthesis [11]. Moreover, using the non-conventional method of combustion reaction results in the synthesis of highly pure, chemically homogeneous powders, which usually generate products with the desired structures and composition due to the high homogeneity aided by the solubility of the salts in water, allowing Journal of Metastable and Nanocrystalline Materials Vols. 20-21 (2004) pp 325-332 online at http://www.scientific.net

Evaluation of the Cu Doping Effects in CeO2 Catalytic Supports Obtained by Combustion Reaction

This work has for aim to synthesize CeO2 catalytic supports doped with Cu2+ by combustion reaction method. Thus were obtained catalytic supports with the composition Ce1-xCuxO2 and the effect caused by doped element in the structure of the CeO2 was evaluated. The concentration value (x) of the Cu doped took over the values of 0.0; 0.3 and 0.5 mol. The catalytic supports developed were submitted to the structural characterization by X-ray diffraction, morphologic analysis by SEM and textural analysis by means of adsorption-dessorption of N2 by BET method. The results showed that the doped element (Cu) was not completely incorporated in the CeO2 structure and was evidenced that how much larger the quantity of Cu larger maid the disorder of the atomic structure of the obtained material. The analyzed supports present mesoporous nature structure.

Avaliação da microestrutura e das propriedades magnéticas de ferritas Ni-Zn dopadas com cobre

An evaluation was made of the microstructure and magnetic properties of Ni0.5-xCuxZn0,5Fe2O4 ferrites with x = 0.1, 0.2, 0.3 and 0.4 prepared by combustion reaction with particle sizes 23 to 29 nm. The resulting powders were uniaxially compacted at 385 MPa into pellets and sintered at 1000 oC/2 h. The samples were characterized by apparent density and porosity measurements, X-ray diffraction, scanning electron microscopy and M-H magnetic measurements. The X-ray diffraction revealed crystalline inverse spinel as the major phase in all the specimens and a small amount of hematite (a-Fe2O3) as the secondary phase in the x = 0.1 and 0.4 samples. An increase in the amount of copper caused a slight increase in grain size (0.65 to 0.68 µm), a decrease in the apparent porosity (33.7% to 6.6%) and a reduction in saturation magnetization from 69 to 54 emu/g.

The Importance of Natural Gas Reforming

Natural gas is a fossil fuel found in nature reserves, associated or not with petroleum. Its composition is a mixture of light hydrocarbons, generally alkanes, which are normally gaseous at room temperature. Methane is the most abundant gas, accounting for more than 85% of the natural gas, and the other constituents are light alkanes such as ethane, propane, butane, among others. The percentages of each constituent of natural gas vary depending on factors such as geological formation of the reservoir rock, as well as the type of organic matter that gave rise to the natural gas found. Recently, natural gas has attracted the interest of many researchers and the large amount of methane contained in natural gas has been considered an input in the production of other high-value products such as syngas and high purity hydrogen. Considering the global trend toward environmental preservation, which emphasizes clean and sustainable energy generation, it can be said that the interest of researchers for natural gas will increase significantly from now on (Odell & Rosing, 1983). The interest in natural gas is directly related to the search for alternatives to replace petroleum-based fuels and for generating energy from sources less aggressive to the environment. This behavior resulted in the intensification of research and exploration, particularly among developing countries. The result was not only the increase in proven oil reserves but also in its geographic expansion (the existence of reserves and the possibility of their exploitation must be proven by tests). Until 1970, these reserves were concentrated in a few regions of the world, like North America and the former Soviet Union (ANEEL, 2008). Awareness of the imminent scarcity of oil in the next decades is stimulating the search for a fuel that can partially replace petroleum-based fuels. Worldwide reserves of natural gas are under-exploited because they are not as valuable as the petroleum reserves. In some cases, when natural gas is associated with oil reserves, while the valuable petroleum is fully exploited, the associated gas of the same reserves is considered undesirable, volatilized into the atmosphere or burnt in the platform’s flare. Fortunately, though, this situation is gradually changing, and natural gas is getting more attention, due to the growing need to produce hydrogen from hydrocarbons. Among the fossil fuels, natural gas is the most suitable for this application (Fishtik et al., 2000). Energy generation is fundamental to the socioeconomic development of a country or region. Somehow, it is present in the entire chain of production, distribution and comsumption of goods and services. Equally important is the role of technology in the balanced and sustainable development of various economic sectors, especially power generation. The 3

Evaluation of NiFe2O4 Spinel, Synthesized by Combustion Reaction, as a Catalyst for Selective CO Oxidation

The selective reaction of CO oxidation (PROX) was named as the most attractive way to reduce the CO concentration, thereby purifying the hydrogen. The aim of this work is to make the structural and morphologic characterization of the NiFe2O4 spinel synthesized by combustion reaction, using glycine as fuel, and to evaluate as catalyst in the reaction of selective oxidation of carbon monoxide in the presence of hydrogen, oxygen and carbon monoxide. The powder was prepared by using a vitreous silica crucible on a hot plate at 480°C and according to stoichiometry established by theory of propellants and explosive. The powder was characterized by X-ray diffraction (XRD), FTIR, textural analyses, transmission electron microscopy (TEM) and catalytic measurements. The results from XRD show characteristic peaks of spinel phase without presence of secondary phases. The morphologic results show surface area of 3.1 m2/g and particle size calculated by TEM of 21.72 nm. The catalyst was active and selective for O2, reaching 100% of conversion.

Nanosize Nickel Ferrite Particles Synthesized by Combustion Reaction: Evaluation of Two Synthesization Routes

Ultrafine magnetic nickel ferrite particles have a significant potential for use in many applications such as magnetic recording media, ferrofluids, microwaves, catalysis and radar-absorbing coatings [1, 2]. Nickel ferrite powders with a nominal NiFe2O4 composition were synthesized by combustion reaction and an evaluation was made of the effect of two different conditions of synthesis on the nanostructural and magnetic characteristics of the resulting powders. Two synthesization routes were studied. The first, NFB, involved the preparation of the powder using a Pyrex beaker heated directly on a hot plate at 480°C until self-ignition occurred. By the second route, NFC, the powder was obtained under the same synthesization condition as the NFB route, but a vitreous silica basin was used. The resulting powders were characterized by X-ray diffraction (XRD), nitrogen adsorption by BET and scanning electron microscopy (SEM). The first route, NFB, proved more favorable to obtain powders with high surface area and, hence, smaller crystalline sizes (5.70 nm) and a superparamagnetic behavior. The NFC route confirmed the feasibility of obtaining powders with a crystalline size of 18.00 nm and a magnetic behavior. Saturation magnetization was 33.18 emu/g and the coercivity field was 25.63 Oe for powders obtained by the NFC route.

Combustion synthesis of α-Al2O3 powders

Among ceramic materials, the alumina has high importance because of its characteristics of resistance and refractory. The possibility to improve the final characteristics of this material, open possibilities for new applications. The aim of this work is to synthesize (α-Al2O3) alumina powders by combustion reaction and to evaluate the effect of the urea content in the final characteristics of the α-Al2O3 powders. Three compositions were studied : a) stoichiometric, b) with 10% of urea reduction and c) with 20% of urea reduction. The initial solution composition was based on the total valence of the reagents by using chemical concepts of the propellant. This powders were characterized by X-ray diffraction (XRD), granulometric determination by laser diffraction, nitrogen adsorption by BET, scanning electron microscopy (SEM), infrared spectroscopy (IR) and helium picnometer. The results showed that the urea reduction change the temperature of the reaction from 525 to 463oC , the characteristics of the powders, and principally the reduction of the particles size.

Nickel Ferrite: Combustion Synthesis, Characterization and Magnetic Properties

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.

Comparison of Ni-Zn Ferrite Powder Preparation by Combustion Reaction Using Different Synthesization Routes

Ni-Zn ferrite powders with a nominal Ni0.5Zn0.5Fe2O4 composition were synthesized by combustion reaction and the effect of three different conditions of synthesis on the characteristics of the resulting powders was evaluated. Three synthesization routes were studied. The first, RCB, involved the preparation of the powder using a Pirex-type beaker heated directly on a hot plate at 480oC until self-ignition occurred. By the second route, RCC, the powder was obtained under the same synthesization condition as the RCB route, but a vitreous silica basin was used. The third route, RCCM, involved the same procedure as the RCC route, but the powder was subjected to attritive milling for 4 hours. The resulting powders, which were characterized by X-ray diffraction (XRD), BET and Horiba, demonstrated the viability of obtaining crystalline and nanosized Ni-Zn ferrite powders by the three synthesization routes. The first route, RCB, proved to be the most favorable one to obtain powders with high surface area and, hence, smaller particle sizes.