[a] 1IntroductionIron oxide-based materials have been of greatt echnological importance for al ong time. Even though, they still attract attention, especially in the form of nanoparticleso r nanocomposites, due to their unique magnetic, optical and catalytic properties [1][2][3].A lthough al arge number of crystalline phases are knownf or iron oxides,h ydroxides and oxy-hydroxides [4],a morphous iron oxide nanoparticles are more active than theirr elated nanocrystalline polymorphs, due to the highers urface-to-volume ratio and high surface energy in the amorphous phase [3,5],b eing much more interesting for catalytic applications.In composite solid propellants,A Pi st he most used oxidizer and its thermald ecomposition is strongly affected by the presence of metal oxide additives. In particular, Fe 2 O 3 is believed to act as catalyst in the gas-phase reactions that take place during the decomposition process of AP.T hus, the catalyst'sc oncentration, its specific surface area, particle size and state of agglomeration will be of great influence over AP thermald ecomposition [6,7].Many papers have demonstrated that nanoparticles of different natures show higher catalytic efficiency than microsizedp articles [6,[8][9][10].T herein authors associate this efficiency to both size and morphology of the catalytic particles, but few of them either determine or compare the textural properties of the studied catalysts. Nonetheless, due to theirh igher surface energy,p articles in nanometric scale are pronet oa gglomeration and, unless this process is completelya voided, it is not always true that the size and Abstract:M etal oxide nanoparticles have beenu sed as burning rate catalysts for ammonium perchlorate (AP) decomposition in composite solid propellants. Though most papers point to the efficiencyo fd ifferent sizes, shapesa nd compositions,t he texture of the agglomerated particles plays an important role in the catalytic efficiency,b ut this aspect is not always discussed. In this paper,i ron oxide and composite iron oxide/silica powders were synthesized in microemulsion systems and their effect on the decomposition of AP was investigated.X -ray diffraction (XRD) analysis and Fourier transformed infrared spectroscopy (FT-IR) showed that the synthesizedp owders havea na morphous to nanocrystalline pattern,w ith Fe 2 O 3 composition.T he use of different FT-IR spectroscopic techniques -t ransmission, diffuse reflectance (DRIFT)a nd universal attenuated total reflectance (UATR) -a llied to electron microscopy analysis allowed the characterizationo ft he samples' surface, indicating that silicon oxide formsathickm atrixt hat covers the iron oxide nanoparticles.A dsorption of N 2 ,l ight scattering and electron microscopy pointed that all samples are formedb ym esoporous agglomerated nanoparticles containing micropores indicating that silicon oxide forms at hick matrix that covers the iron oxide nanoparticles. Adsorption of N 2 ,p ointed that all samples show different microstructures and light scattering indicated re...
Recebido em 4/5/09; aceito em 1/2/10; publicado na web em 3/5/10 THE INFLUENCE OF AMMONIUM QUANTITY ON THE SYNTHESIS OF IRON OXIDE NANOPARTICLES IN MICROEMULSION. Iron oxide nanoparticles were synthesized in microemulsion systems composed by Triton X-100/hexyl alcohol/ cyclohexane/aqueous solution. The nanoparticles were synthesized in microemulsions containing different amounts of ammonium, in order to evaluate the influence of this parameter on the size of the nanoparticles and on the phase transformation after heat treatment. Powder materials were obtained after centrifugation, washing and drying, and they were analyzed as synthesized and after heating at 350, 500 and 1000 °C. It was observed that the higher amount of ammonium induced smaller particles and minor phase transformation, possibly due to a preferential nucleation process.Keywords: nanoparticle; nucleation effect; iron oxide phase transformation. INTRODUÇÃOO desenvolvimento científico atual busca materiais em escala nanométrica, em função da possibilidade de obtenção de novas propriedades físicas e químicas, determinadas pelo reduzido tamanho (confinamento quântico) e pelo aumento da razão área/volume (efeitos de superfície). sonoquímico 17 e microemulsão. 18 Cada um destes métodos tem vantagens e desvantagens que resultam em materiais mais ou menos homogêneos em relação à composição química, tamanho e forma das partículas. Neste trabalho, as nanopartículas de óxido de ferro foram obtidas por microemulsão, cujo diferencial está no fato de que as partículas formadas apresentam dimensões dentro de uma faixa de tamanhos estreita, o que limita o número de variáveis a serem consideradas no trabalho. Outra vantagem do método está relacionada ao baixo custo da aparelhagem empregada para a preparação do material. Como as condições de síntese influenciam fortemente as características físicas e químicas dos materiais e, portanto, as propriedades dos mesmos, a síntese dos óxidos de ferro foi realizada estabelecendo como parâmetro de controle a quantidade de hidróxido de amônio no sistema e avaliando a influência deste no tamanho das partículas e na evolução das transições de fase do material, submetido a tratamento térmico até 1000 °C. Síntese de nanopartículas por microemulsãoAs microemulsões são sistemas coloidais, assim como o sol-gel e o aerosol, cuja característica básica é a dispersão de uma fase em outra. No caso da microemulsão, estas fases são líquidas, sendo uma aquosa e uma orgânica. Para manter a estabilidade deste tipo de sistema, é necessária a presença de uma substância anfifílica ou surfatante e, muitas vezes, de um coadjuvante para a surfatação, chamado de cosurfatante, formando sistemas ternários e pseudoternários, respectivamente.Devido a interações intermoleculares, moléculas da fase aquosa tendem a se isolar da fase orgânica e isto ocorre com a interposição de moléculas de surfatante, originando os nanorreatores, dentro dos quais pode ocorrer uma reação química promovendo a geração de nanopartículas. Para tanto, os materiais de interesse ...
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