Aging kinetics in the Cu-8 wt.% Al alloy with Ag additions

The influence of additions of 2, 4, 6, 8, 10 and 12 wt.% Ag in the isothermal aging kinetics of the Cu-8 wt.% Al alloy was studied using microhardness measurements, differential scanning calorimetry, optical and scanning electron microscopy and X-ray diffractometry. The results indicate that the presence of silver is responsible for the shift of the equilibrium concentration to higher Al contents, allowing the formation of the gamma(1) phase (Al4Cu9) in this alloy. For Ag additions up to 6% the dominant kinetic process is Ag precipitation and for additions from 8 to 12% Ag the nucleation of the perlitic phase dominates. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Ag precipitation and dissolution reactions in the Cu-3 wt.% Al-4 wt.% Ag alloy

The Ag precipitation and dissolution reactions in the Cu-3 wt.% Al-4 wt.% Ag alloy were studied using isothermal and non-isothermal analyses. The activation energy values, obtained for the Ag precipitation reaction indicated that, when the Kissinger, Ozawa and Johnson-MehI-Avrami methods are compared, the Kissinger method is the most appropriate. Although the Johnson-Mehl-Avrami equation often does not fit precipitation data, the energy values obtained for Ag precipitation kinetics are in agreement with what was experimentally observed. For the dissolution reaction of Ag precipitates the activation energy values obtained from the Kissinger and Ozawa methods are higher than that found in the literature for the Ag dissolution in Cu. This discrepancy seems to be related to the fact that the activation energy is influenced by the heating rate. (c) 2006 Elsevier B.V. All rights reserved.

Diffusion and interface controlled reactions in alpha-(Cu-Al-Ag) alloys

The isothermal kinetics of Ag precipitation was studied in Cu-Al-Ag alloys with concentrations ranging from 2 to 8 wt.%Al and 2 to 12 wt.%Ag, using scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDX) and microhardness measurements. The results indicated a change in the precipitates growing mechanism from diffusion to interface controlled process, probably due to a change in the nature of the interface with the Ag and Al enrichment of the precipitates. (C) 2006 Elsevier B.V. All rights reserved.

Influence of Ag additions on the activation energy for the reverse eutectoid reaction in Cu-Al alloys

The eutectoid transformation may be defined as a solid-state diffusion-controlled decomposition process of a high-temperature phase into a two-phase lamellar aggregate behind a migrating boundary on cooling below the eutectoid temperature. In substitutional solid solutions, the eutectoid reaction involves diffusion of the solute atoms either through the matrix or along the boundaries or ledges. The effect of Ag on the non-isothermal kinetics of the reverse eutectoid reaction in the Cu-9 mass%Al, Cu-10 mass%Al, and Cu-11 mass%Al alloys were studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The activation energy for this reaction was obtained using the Kissinger and Ozawa methods. The results indicated that Ag additions to Cu-Al alloys interfere on the reverse eutectoid reaction, increasing the activation energy values for the Cu-9 mass%Al and Cu-10 mass%Al alloys and decreasing these values for the Cu-11 mass%Al alloy for additions up to 6 mass%Ag. The changes in the activation energy were attributed to changes in the reaction solute and in Ag solubility due to the increase in Al content.

Comparison of the dehydration kinetics of solid state compounds of 2-methoxybenzylidenepyruvate with some divalent metal ions

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Kinetic parameters for thermal decomposition of supramolecular polymers derived from flunixin-meglumine adducts

Meglumine, (2R,3R,4R,5S)-6-methylaminohexane-1,2,3,4,5-pentol, is a carbohydrate derived from sorbitol in which the hydroxyl group in position one is replaced by a methylamine group. It forms binary adducts with substances having carboxyl groups, which have in common the presence of hydrogen bonding as the main force in the stabilization of these species. During melting, adducts of meglumine with flunixin (2-[[2-methyl-3-(trifluoromethyl)phenyl]amino]pyridine-3-carboxylic acid) polymerize or self-assemble in amorphous supramolecular structures with molecular weights around 2.0 x 10(5) kDa. DSC curves, in a first heating, show isomorphic transitions where the last one at 137 A degrees C for the flunixin-meglumine adduct originated the supramolecular amorphous polymers with glass transition around 49.5 A degrees C. The kinetic parameters for the thermal decomposition step of the polymers were determined by the Capela-Ribeiro non-linear isoconversional method. From data for the TG curves in nitrogen atmosphere and heating rates of 5, 10, 15, and 20 A degrees C min(-1), the E (alpha) and B (alpha) terms could be determined and, consequently, the pre-exponential factor, A(alpha), as well as the kinetic model, g(alpha).

Decomposition kinetics of iron (III)-diclofenac compound

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Kinetic parameters for thermal decomposition of microcrystalline, vegetal, and bacterial cellulose

Cellulose can be obtained from innumerable sources such as cotton, trees, sugar cane bagasse, wood, bacteria, and others. The bacterial cellulose (BC) produced by the Gram-negative acetic-acid bacterium Acetobacter xylinum has several unique properties. This BC is produced as highly hydrated membranes free of lignin and hemicelluloses and has a higher molecular weight and higher crystallinity. Here, the thermal behavior of BC, was compared with those of microcrystalline (MMC) and vegetal cellulose (VC). The kinetic parameters for the thermal decomposition step of the celluloses were determined by the Capela-Ribeiro non-linear isoconversional method. From data for the TG curves in nitrogen atmosphere and at heating rates of 5, 10, and 20 A degrees C/min, the E(alpha) and B(alpha) terms could be determined and consequently the pre-exponential factor A(alpha) as well as the kinetic model g(alpha). The pyrolysis of celluloses followed kinetic model g(alpha) = [-ln(1 - alpha)](1.63) on average, characteristic for Avrami-Erofeev with only small differences in activation energy. The fractional value of n may be related to diffusion-controlled growth, or may arise from the distributions of sizes or shapes of the reactant particles.

Approximations for the generalized temperature integral: a method based on quadrature rules

The generalized temperature integral I(m, x) appears in non-isothermal kinetic analysis when the frequency factor depends on the temperature. A procedure based on Gaussian quadrature to obtain analytical approximations for the integral I(m, x) was proposed. The results showed good agreement between the obtained approximation values and those obtained by numerical integration. Unless other approximations found in literature, the methodology presented in this paper can be easily generalized in order to obtain approximations with the maximum of accurate.

Thermal Study in Solid State of Zn(II)-Diclofenac Complex: Behavior Kinetic of the Dehydration, Transition Phase and Thermal Decomposition

The dehydration, thermal decomposition and transition phase stage of Zn(II)-diclofenac compound were studied by simultaneous TG-DTA and DSC techniques. The TG and DSC curves of this compound were obtained with the mass of sample of 2 and 5 mg. Additionally, DSC curves were carried out in opened and closed alpha-alumina pans under static and nitrogen atmosphere. The DTA and DSC curves show that this compound possesses exothermic transition phase between 170-180 degrees C, which it is irreversible (monotropic reaction) The kinetics study of this transition phase stage was evaluated by DSC under non-isothermal conditions. The obtained data were evaluated with the isoconversional method, where the values of activation energy (E(a) / kJ mol(-1)) was plotted in function of the conversion degree (alpha). The results show that due to mass sample, different activation energies were obtained From these curves a tendency can be seen where the plots maintain the same profile for closed lids and almost run parallel to each other.

Crystallization study of molybdate phosphate glasses by thermal analysis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Thermal-biological aspects on the seed germination of Cucumis anguria L.: influence of the seed coat

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Aplicação de revestimento comestível em abacaxis processados por métodos combinados: isoterma de sorção e cinética de desidratação osmótica

A desidratação osmótica é uma etapa essencial na elaboração de produtos de fruta através da Tecnologia dos Métodos Combinados ou de Obstáculos (Hurdle Technology), pois reduz a atividade de água para níveis que, combinando um ou mais obstáculos, aumentam a estabilidade do produto. Neste processo há uma perda de água da fruta para a solução e incorporação de sólidos solúveis pelo produto. Este último fluxo é considerado uma desvantagem do processo pois pode alterar o sabor do produto. Neste trabalho estudou-se a ação de revestimentos comestíveis a base de alginato e gelatina aplicados em abacaxis, previamente à desidratação osmótica, como barreira à incorporação de sólidos solúveis. Os abacaxis com e sem revestimento (controle) foram desidratados em solução de sacarose sob condições isotérmicas. Foram determinadas as isotermas de dessorção de abacaxis revestidos com gelatina, alginato e sem revestimento e os parâmetros cinéticos do processo de desidratação osmótica. Abacaxis revestidos com alginato apresentaram menor velocidade de ganho de sólidos, sem alterar a velocidade de perda de água, quando comparado ao controle.

Influência do uso do forno de microondas ou convencional na síntese de ZrO2

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Dynamical scaling properties of nanoporous undoped and Sb-doped SnO2 supported thin films during tri- and bidimensional structure coarsening

The coarsening of the nanoporous structure developed in undoped and 3% Sb-doped SnO2 sol-gel dip-coated films deposited on a mica substrate was studied by time-resolved small-angle x-ray scattering (SAXS) during in situ isothermal treatments at 450 and 650 degrees C. The time dependence of the structure function derived from the experimental SAXS data is in reasonable agreement with the predictions of the statistical theory of dynamical scaling, thus suggesting that the coarsening process in the studied nanoporous structures exhibits dynamical self-similar properties. The kinetic exponents of the power time dependence of the characteristic scaling length of undoped SnO2 and 3% Sb-doped SnO2 films are similar (alpha approximate to 0.09), this value being invariant with respect to the firing temperature. In the case of undoped SnO2 films, another kinetic exponent, alpha('), corresponding to the maximum of the structure function was determined to be approximately equal to three times the value of the exponent alpha, as expected for the random tridimensional coarsening process in the dynamical scaling regime. Instead, for 3% Sb-doped SnO2 films fired at 650 degrees C, we have determined that alpha(')approximate to 2 alpha, thus suggesting a bidimensional coarsening of the porous structure. The analyses of the dynamical scaling functions and their asymptotic behavior at high q (q being the modulus of the scattering vector) provided additional evidence for the two-dimensional features of the pore structure of 3% Sb-doped SnO2 films. The presented experimental results support the hypotheses of the validity of the dynamic scaling concept to describe the coarsening process in anisotropic nanoporous systems.