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Abstract  

Thermal analysis was done to determine the temperatures of thermal reactions, phase transformations or melting reaction during continuous heating. These reactions are a direct response to the steel composition and to the sintering atmosphere. Simultaneous thermal analysis TG-DTA (STA) shows up the sintering behaviour of sintered low prealloyed chromium steels and their peculiarities. Given the high oxygen affinity of chromium, graphite additions can modify their thermal reactions, and hence the sintering behaviour of the steel. Evidence is given of the effect of carbon on the sintering process and the nature of the oxides.

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Journal of Radioanalytical and Nuclear Chemistry
Authors:
Hector Vega-Carrillo
,
M. Martinez-Blanco
,
Victor Hernandez-Davila
, and
Jose Ortiz-Rodriguez

Abstract  

Artificial neural networks have been applied to unfold the neutron spectra and to calculate the effective dose, the ambient equivalent dose, and the personal dose equivalent for 252Cf, 241Am–Be, and 239Pu–Be neutron sources. The count rates that these neutron sources produce in a Bonner Sphere Spectrometer with a 6LiI(Eu) were utilized as input in both artificial neural networks. Spectra and the ambient dose equivalent were also obtained with BUNKIUT code and the UTA4 response matrix. With both procedures spectra and ambient dose equivalent agrees in less than 10%. The Artificial neural network technology is an alternative procedure to unfold neutron spectra and to perform neutron dosimetry.

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Abstract

In this study, a systematic study of the effect of the temperature on the density and surface tension of HMT (hexamethylentetramine) in water was developed. The density and surface tension were determined at temperatures of 288.15, 293.15, 298.15, 303.15, and 308.15 K. Precise data of surface tension have not been reported previously in literature. From the density measurements, the apparent molar and partial molar volumes were calculated. The apparent molar volume decreases with concentration, the molar partial volume increases with temperature. The surface tension of the aqueous solutions of HMT decreases with concentration. The excess surface concentration was calculated, the values increase with concentration, indicating that the amount of HMT that goes to the interface gas liquid increases at higher concentrations of HMT.

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The question addressed in this paper is whether plant traits and plant functional types related to forage selection by grazers are also related to those expressing short-term community response after grazing. Vegetation of natural campos grassland in south Brazil was examined for species composition and locally described for seven morphological traits before and after a controlled grazing period by bovine cattle. An optimization algorithm was used for the identification of plant functional effect types (PF ef T) and plant functional response types (PF ef T)-in this case, groups of plants similar in a given set of traits (assessed before and after one grazing short period, respectively) and in their association to grazing intensity. The results have shown that plant traits optimally defining plant types related to forage selection (PF ef Ts) were the same traits optimally defining short-term community response to grazing (PF re Ts); also similar trends of plant morphological variation were observed among populations before and after grazing, based on the traits’ correlation structure. However, at the community level the correlation vanished, since similar communities described by the performances of PF ef Ts were not as similar when described by PF re Ts. Hence, whether plant functional types related to forage selection (effect types) are also related to community response to grazing may depend on the level of organization considered. The paper advances on the operational definition of possible overlaps between effect and response plant functional types.

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Abstract  

The kinetics of a poly(methyl methacrylate) (PMMA)-modified epoxy resin cured with different functionalities amine mixtures was analyzed using differential scanning calorimetry (DSC) in both isothermal and dynamic conditions. A delay in the reaction rate was observed which increased with PMMA content. An approach of kinetic features involved in curing was carried out. A linear dependence of preexponential factors of neat systems with modifier content was considered. The approach shows the contribution of other factors including the dilution effect of the functional groups to the observed delay. Fourier transform infrared spectroscopy (FTIR) indicated a noticeable change in the interactions present in neat systems due to the presence of PMMA. On the other hand, a significant influence of the ratio between each amine in the epoxy/amine mixtures on the final physical appearance was observed. At constant curing conditions, materials from completely opaque (phase separated) to transparent (miscible) were obtained with the increase in monoamine content.

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Journal of Thermal Analysis and Calorimetry
Authors:
M. López
,
M. Blanco
,
A. Vazquez
,
J. Ramos
,
A. Arbelaiz
,
N. Gabilondo
,
J. Echeverría
, and
I. Mondragon

Abstract  

The curing kinetics of nanocomposites based on phenolic resol cured with triethylamine (TEA) containing different amounts of organic montmorillonite was analyzed by differential scanning calorimetry. Kissinger-Akahira-Sunose (KAS) model-free kinetics has been applied to correlate the dynamic cure behaviour in the presence of modified montmorillonite. The effect in the curing of the use of different clay modifiers has also been studied. A commercial clay with hydroxyl groups (Cloisite 30B) and a customized montmorillonite (PheMMT) whose reactive groups induce condensation reactions with the resol matrix have been used. Strong dependency of activation energy on apparent conversion has been observed for all compounds.

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Abstract  

Thermogravimetric analysis of pyridine adsorption was applied to study the acidity at different temperatures of clays pillared with Al pillars and mixed Al-Ga pillars, in relation to the starting montmorillonite. These results were compared with those obtained by means of a pulse-chromatographic technique. The pillaring process produces a large number of acid centers in the samples. Al-Ga-PILC has a higher acidity than Al-PILC.

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Abstract

Seven variously substituted derivatives of polyhedral oligomeric silsesquioxanes (POSSs) with general formula R7R′1 (SiO1.5)8, where R- and R′- were a cyclopentyl and a substituted phenyl group, respectively, were prepared in this study, and their compositions were checked by elemental analysis, 1 H NMR and 13 C NMR spectroscopy. The compounds obtained were studied by TG and DTA techniques, in both flowing nitrogen and static air atmospheres, to draw useful information about their resistance to thermal degradation. Experiments, performed in the 35–700 °C temperature range, showed different behaviours between the two used atmospheres. The formation of volatile compounds only, with a near-complete mass loss, was observed under nitrogen; by contrast, in oxidative environment, a solid residue (≈50% in every case) was obtained because of the formation of SiO2 as indicated by the FTIR spectra performed. The results obtained for the various compounds investigated were discussed and compared with each other, and heat resistance classifications in the studied environments were made.

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Journal of Thermal Analysis and Calorimetry
Authors:
F. Fraga
,
V. Soto
,
J. Blanco-Méndez
,
A. Luzardo-Alvarez
,
E. Rodríguez-Núñez
,
J. Martínez-Ageitos
, and
M. Pérez

Abstract  

Knowledge of the the kinetic study of chitosan/genipin allow to know the different effects that time and temperature have on the cure reaction of the material. The total enthalpy of reaction, the glass transition temperature and the partial enthalpies have been determined using DSC in dynamic mode. Two models, one based on chemical kinetics and the other accounting for diffusion were used. The incorporation of the diffusion factor in the second model allowed for the cure kinetics to be predicted the whole range of conversion.

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