Ruth Herta Goldsmith Aliaga Kiminami

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 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.

Al2O3/mullite/SiC powders synthesized by microwave-assisted carbothermal reduction of kaolin

Al2O3/mullite/SiC powders have been synthesized by microwave-assisted carbothermal reduction of kaolin. Kaolin, a low cost aluminosilicate, was mixed to carbon black to achieve the precursor blend which was dried, broken in mortar, sieved then reacted in a kitchen microwave oven for 45 min under reducing atmosphere. In order to compare the characteristics of the powders, conventional carbothermal reduction reaction was performed at 1450°C for 2 h. The as-reacted powders were characterized by X-ray diffraction and scanning electron microscopy to confirm the phase formation. Results obtained show Al2O3/mullite/SiC phase in XRD for both reaction routes, products which can be achieved only in temperature ranges of about 1450°C.

Use of Ni-Zn ferrites doped with Cu as catalyst in the transesterification of soybean oil to methyl esters

The purpose of this work is to evaluate the performance of Ni0.5Zn0.5Fe2O4 ferrite doped with 0.1 and 0.4 mol of Cu as a catalyst for the transesterification of soybean oil to biodiesel, using methanol. The samples were characterized by X-ray diffraction, nitrogen adsorption and scanning electron microscopy. The reaction was performed for 2 hours at a temperature of 160 °C, using 10 g of soybean oil, a molar ratio of oil: alcohol of 1:20, and 4% (w/w) of catalyst. The product of the reaction was characterized by gas chromatography, which confirmed conversion to methyl esters. The diffraction patterns showed the presence only of Ni0.5Zn0.5Fe2O4 ferrite phase with a crystallite size of 29 nm. The samples doped with 0.1 and 0.4 mol of Cu showed a surface area and particle size of 22.17 m2g - 1 and 50.47 nm; and 23.49 m2g - 1 and 47.64 nm, respectively. The morphology of both samples consisted of brittle block-shaped agglomerates with a wide particle size distribution. A comparative analysis of the two catalysts indicated that the catalyst doped with 0.4 mol of Cu showed the better performance, with a conversion rate of 50.25%, while the catalyst doped with 0.1 mol of Cu showed 42.71% conversion.

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

Ceramic system based on ZnO·CuO obtained by freeze-drying

Compositions of powders containing ZnO and copper(II) acetate monohydrate [(CH3COO)2Cu·H2O, denoted CuAcH2O] equivalent to ZnO+x mol% Cu (x=0, 0.05, 0.5 and 5.0) were obtained by freeze-drying. The powders were characterised by thermogravimetry (TG) up to 700 °C and differential scanning calorimetry (DSC) up to 1150 °C, in static air with heating rate of 5 °C/min. Pellets of the freeze-dried powders were compacted without pressing additives and sintered in air at 950 °C/1 h. After crushing, the crystalline phases of the sintered samples were characterised by powder X-ray diffractometry (XRD). Microstructure and Cu-element mapping of sintered ZnO+5.0 mol% Cu samples were characterised by scanning electron microscopy (SEM) with microprobe apparatus. TG and DSC indicated the presence of three important thermal events. Event I: (100–190 °C) corresponds to dehydration of CuAcH2O, giving rise to copper(II) acetate [(CH3COO)2Cu, denoted CuAc], with an endothermic peak at 125 °C. Event II: (190–350 °C) corresponds to thermal decomposition of CuAc giving rise to CuO, with two exothermic peaks at 237 and 261 °C. In air, above 350 °C, the ceramic system is ZnO·CuO. Event III: (800–1000 °C) corresponds to a solid-state reaction involving ZnO·CuO, with an exothermic peak at 895 °C. DSC, XRD and SEM indicated that, after sintering at 950 °C/1 h, the resulting ceramic matrix is composed by Cu-doped ZnO grains (ZnO/Cu) for ZnO+x mol% Cu (x≤1) and by ZnO/Cu grains with CuO-inclusions (ZnO/Cu·CuO) for ZnO+x mol% Cu (x>1).

Physical changes of sintered ceramics obtained from freeze-dried ZnO+(CH3COO)2Cu·H2O powders

Abstract Freeze-drying was used to produce highly homogeneous mixtures of powders containing ZnO and (CH3COO)2Cu·H2O (copper(II) acetate monohydrate, denoted by CuAcH2O). Mixtures of ZnO+CuAcH2O, in specific concentrations of Cu2+, i.e., ZnO+x mol% Cu (0.01≤x≤5.0) were obtained by freeze-drying. Due to the polymeric characteristic of CuAcH2O, pellets of those mixtures were compacted without any pressing additives. After sintering at temperature range 750–1150 °C in air for 1 h, physical changes such as mass loss, shrinkage and density were evaluated. The results showed that all physical changes increased as the concentration of CuAcH2O increased. The mass loss was nearly independent of the sintering temperature, especially from 750 to 1050 °C, and directly proportional to the concentration of CuAcH2O. The mass loss was mainly related to dehydration and thermal decomposition of CuAcH2O, although the presence of water excess or sublimation of copper(II) acetate (CuAc) could not be neglected. In addition, contributions to mass loss were supposed to be related to sublimation of ZnO or reduction of CuO to Cu2O, for sintering temperature above 1050 °C. Shrinkage and density were nearly independent of the sintering temperature above 850 °C, which indicates that sintering (grain growth) may occur in that temperature range. Shrinkage and densification increased as the concentration of CuAcH2O increased, especially for ZnO+x mol% Cu (x>1.0). The presence of CuO particles on the ZnO surface, originated from thermal decomposition of CuAcH2O during heating, is believed to promote shrinkage and densification.

Influence of Fuel in the Synthesis of ZnAl2O4 Catalytic Supports by Combustion Reaction

The aim of this work is to evaluate the influence of fuel in the synthesis of ZnAl2O4 catalytic supports by combustion reaction. For this, it was used the fuels: urea, carbohidrazide, glycine and aniline. The total amount of reagents was calculated according to the theory of propellants and explosive using urea in the stoichiometric proportion (Φe = 1). The structural and morphological characteristics of the powders were evaluated by XRD, FTIR, TEM, SEM and particle size distribution. The results from XRD showed the formation of the normal cubic spinel structure. The powders presented nanosized particles with narrow agglomerates size distribution. The powders prepared with urea showed better value of surface area and smaller crystallite size.

Ni-Zn Nanoferrites Synthesized by Microwave Energy: Influence of Exposure Time and Power

This paper describes the synthesis of Ni-Zn nanoferrites by combustion reaction using microwave energy as a heating source, and evaluates the performance of these materials as absorbers of electromagnetic energy at frequencies between 4 - 12 GHz. The influence of the synthesis conditions on the structure, morphology and absorption characteristics was investigated. The powders were characterized by DRX, BET, AGM and reflectivity measurements in the frequency bands of 8 to 12 GHz. The XRD results show the formation of Ni-Zn ferrite phase and Fe2O3 and Ni as secondary phases. The crystallite sizes ranged from 32 to 42 nm. The parameters of exposure time and power of the microwave oven changed the final characteristics of the resulting powders. The morphology of all the powders consisted of soft nanoparticle agglomerates. The best saturation magnetization and attenuation results were 70 emu/g and -4.1 dB in the frequency of 10 GHZ.

Ni-Zn Ferrite Nanoparticles Prepared by Combustion Reaction

The combustion reaction is an important powder preparation process by which several hundred compounds may be prepared. Ni-Zn ferrite nanoparticles (Ni 1-x Zn x Fe 2 O 4 , x = 0,3; 0,5 and 0,7 mol) have been prepared by a single step solution combustion reaction using nitrates and urea as fuel. XRD, BET, SEM, and helium pycnometer characterized the as-prepared powders. Ni-Zn ferrite nanoparticles prepared by the present method can easily form the well-crystallized particles with a large surface area in the range of 42-64 m 2 /g. The resulting powders showed extensive XRD line broadening and the crystallite sizes calculated from the XRD line broadening were in the nanometer range (19-25 nm). The results evidenced the influence of different compositional variables on the powders characteristic.