We present a detailed study of static and dynamic magnetic behavior of Fe 3 O 4 nanoparticles with average particle sizes ͗d͘ ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, T V , shifts down to ϳ20 K for ͗d͘ϭ50 nm and is no longer observable for smaller particles. The smallest particles (͗d͘ϭ5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at T B ϳ45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization M S at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with ͗d͘ϭ5 nm, dynamic ac susceptibility measurements show a thermally activated Arrhenius-Néel dependence of the blocking temperature with applied frequency. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy. From the calculus of the magnetic anisotropy, it is inferred that there is no structural transition from cubic to triclinic symmetry for ͗d͘ϭ5 nm, in agreement with the absence of the Verwey transition. A value K 1 ϭ4.68ϫ10 5 erg/cm 3 is obtained for the magnetocrystalline anisotropy constant of the cubic phase.

We have performed a detailed characterization of the magnetic properties of Ni nanoparticles embedded in a SiO 2 amorphous matrix. A modified sol-gel method was employed which resulted in Ni particles with average radius ϳ3 nm, as inferred by TEM analysis. Above the blocking temperature T B Ϸ20 K for the most diluted sample, magnetization data show the expected scaling of the M /M S vs H/T curves for superparamagnetic particles. The hysteresis loops were found to be symmetric about zero field axis with no shift via exchange bias, suggesting that Ni particles are free from an oxide layer. For TϽT B the magnetic behavior of these Ni nanoparticles is in excellent agreement with the predictions of randomly oriented and noninteracting magnetic particles, as suggested by the temperature dependence of the coercivity field that obeys the relation H C (T)ϭH C0 ͓1Ϫ(T/T B) 1/2 ͔ below T B with H C0 ϳ780 Oe. The obtained values of H C0 , considering the temperature dependence of the magnetic anisotropy constant, are discussed within the scenario of isolated randomly oriented and noninteracting single-domain particles.

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Methane is the main constituent of natural gas and can be converted in higher value‐added products for electricity cogeneration. It could be used as a solid membrane reactor (SMR) for application in Alkaline Anion‐Exchange Membrane Fuel Cell (AAEMFC). The investigation for the conversion of methane was based on sodium borohydride (NaBH4) method using Pt/C Basf, Pd/C, Ni/C as catalysts. The electrocatalysts were prepared with 20 wt% of metals loading on carbon. The X‐ray diffraction (XRD) analysis revealed a face‐centered cubic structure (FCC) for Pt/C and Pd/C catalysts, was observed Ni/NiO phases for Ni/C electrocatalyst. The Transmission Electron Microscopy (TEM) exhibited a good dispersion of nanoparticles and some agglomerations on the support, with a mean size of 6.4 nm for Pd/C, 5.7 nm for Ni/C and near to 2 nm size for Pt/C. The experiments with AAEMFC showed that all materials can carry out the reaction spontaneously. Pt/C catalyst presents energy density higher than the other materials. FTIR data suggest that methane was converted into small products organic molecules such as methanol and formate in different potentials for Pt/C, Pd/C, and Ni/C. The products were quantified by Raman spectroscopy. The high conversion efficiency obtained was about 20% at 0.3 V using Pt/C catalyst, the maximum conversion over Pd/C was 17.5% at 0.15 V, associated with the formation of a thin layer of PdO on the catalytic surface. The highest conversion rate (13%) was observed in closed circuit potentials to the short circuit in the cell with Ni/C catalyst. The results suggest that for the selective conversion of methane to methanol are most promising using materials containing Pt or Pd.

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INTRODUÇÃOCélula a combustível é um dispositivo que converte eletroquimicamente combustíveis químicos em eletricidade; é, essencialmente, uma bateria que não para de fornecer corrente elétrica por causa da contínua alimentação externa de combustível. Em outras palavras, é uma bateria na qual os dois eletrodos não são consumidos durante a descarga, mas agem simplesmente como locais para a reação entre combustível e oxidante [1]. Células a combustível convertem energia química diretamente em energia elétrica com eficiência termodinâmica não limitada pelo ciclo de Carnot [2,3]. Essa vantagem das células a combustível depende, entretanto, de como os combustíveis que serão utilizados podem ser reformados para produzir hidrogênio e dióxido de carbono [4]. Toda célula a combustível é composta de uma seqüência de unidades, cada uma com quatro componentes: o eletrólito, o eletrodo para o ar (ar é o oxidante), o eletrodo para o combustível (o mais comum é o hidrogênio), e o interconector.Muitos tipos de células a combustível foram desenvolvidos, sendo as células classificadas geralmente de acordo com o tipo de eletrólito. Os cinco principais tipos são:1-célula a combustível de ácido fosfórico, operacional a 180 o C; 2-célula a combustível de membrana trocadora de prótons, ou célula a combustível de eletrólito de membrana polimérica, operacional na faixa de temperatura 60-80 o C; 3-célula a combustível de eletrólito alcalino, operacional a temperaturas relativamente baixas (80 o C). Tem sido usada no ônibus espacial como principal fonte de energia. Embora tenha operado confiável e eficientemente em missões espaciais por mais de 40 anos, não tem sido usada para outras finalidades, principalmente por causa do alto custo ResumoA partir da definição de células a combustível, é feita uma introdução sucinta dos tipos de células e dos materiais cerâmicos que são empregados em projeto e fabricação destes dispositivos geradores de energia elétrica. Tomando por base a ampla literatura científica disponível em publicações periódicas internacionais indexadas e arbitradas, bem como patentes, são relatados com detalhes os materiais cerâmicos com comportamento elétrico adequado para uso como eletrólitos, anodos, catodos, interconectores e selantes, que são os componentes básicos de células a combustível de óxidos sólidos. Por fim, é feita uma avaliação do estado da arte na pesquisa e desenvolvimento de materiais cerâmicos para uso em células a combustível de óxidos sólidos. Palavras-chave: célula a combustível, eletrólito sólido, anodo, catodo, interconector. Abstract Basic definitions of fuel cells and a brief introduction of different types of fuel cells

The proton transport properties of Nafion membranes were studied in a wide range of temperature by using an air-tight sample holder able to maintain the sample hydrated at high relative humidity. The proton conductivity of hydrated Nafion membranes continuously increased in the temperature range of 40–180 °C with relative humidity kept at RH = 100%. In the temperature range of 40–90 °C, the proton conductivity followed the Arrhenius-like thermal dependence. The calculated apparent activation energy Ea values are in good agreement with proton transport via the structural diffusion in absorbed water. However, at higher measuring temperatures an upturn of the electrical conductivity was observed to be dependent on the thermal history of the sample.

Our experiments disentangle the low and high frequency dispersions in perfluorosulfonate ionomer solutions and membranes, providing a reasonable model for understanding these relaxations. Dielectric spectroscopy (DS) and small angle x-ray scattering (SAXS) measurements revealed that the dielectric relaxations observed at low (α relaxation) and high (β relaxation) frequencies show typical features of the longitudinal and radial polarization, respectively, of rodlike polymeric aggregates. Such relaxations were attributed to counterion fluctuations in the vicinity of sulfonic acid groups, in resemblance with polyelectrolytes. Characteristic correlation lengths calculated from both DS and SAXS data are in good agreement adding further evidence to the proposed model. Such description provides insights for the understanding of the crossover from polyelectrolytes, dominated by charge repulsion, to ionomers, dominated by dipolar attraction.