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- Author or Editor: M. Silva x
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Forest biomes have expanded and contracted in response to past climate fluctuations, but it is not clear how they will respond to human-induced atmospheric change. We provide a review of the literature, describing historical links between biogeographical and atmospheric patterns, comparing characteristics of forest biomes and describing expected changes in climate forcings from observed range shifts. Over the geological history, climate fluctuations prompted changes in forest distribution that, in turn, stabilized the atmosphere. Over the past century, warming-induced stress has caused widespread declines of mature forests, but new forests have expanded into open areas of boreal, tropical and temperate regions. Historically, forest expansion happened at much faster rates in cold than in warm regions. Across biomes, species interactions control the use of limiting resources, regulating community dynamics and expansion rates in response to climate variability. Modern impacts of land use change on the distribution of forest biomes are well understood, but the expansion of new forests and their role in stabilizing the atmosphere are yet to be accounted for in global models. Expansion of tropical and temperate forests would yield a negative climate forcing through increased carbon sequestration and evaporative cooling, but in the boreal region forest expansion could amplify climate warming due to changes in albedo. Although qualitative descriptions of forest-atmosphere interactions are possible based on existing records, the net climate forcing from forest range shifts remains uncertain. Three critical gaps in knowledge hinder rigorous evaluations of causality necessary to probe for linkages between climatic and biogeographical patterns: (i) reconstructions of vegetation dynamics have not sufficiently represented warm biomes; (ii) climate and vegetation dynamics are typically assessed at non-comparable scales; and (iii) single-proxies are normally used to simultaneously infer changes in climate and vegetation distribution, leading to redundancy in interpretation. Addressing these issues would improve our ability to decipher past and predict future outcomes of forest-atmosphere interactions.
Abstract
Cis-norbixin isomer obtained by hydrolysis of cis-bixin and isolated by solvent extraction from annatto seeds. The thermal decomposition data of the cis-norbixin samples were analyzed by thermogravimetric analysis at different heating rates in the 25–900C temperature range. DSC curves showed that thermal decomposition reactions for cis-norbixin occurred in the solid phase. The kinetic parameters, such as activation energy and pre-exponential factor were determined using integral and approximate methods: Coats–Redfern, Madhusudanan, Horowitz–Metzger and Van Krevelen. F1 mechanism describes well the first stage of the thermal decomposition.
Abstract
The preparation, and thermal characterization and ion exchanger behavior of hydrous tungsten oxides obtained from purified tungsten trioxide, by processing of a Brazilian scheelite ore were described. By an alkaline fusion of tungsten trioxide with an excess of sodium carbonate at 900C an intermediate tungstate product is obtained. Acid lixiviation of the tungstate intermediary in different experimental conditions produces two different hydrous tungsten oxides I and II respectively. A third material was produced by thermal treatment of II, yielding the material IIA, a monohydrate species. The materials prepared were characterized by TG/DTG, X-ray diffractometry and by surface area measurements. Their ion exchange behavior was studied too, using Na+ and K+ ions as the exchanged species. The inorganic exchanger materials were partially regenerated and may be used consecutively.
Abstract
Differential thermal analysis, optical microscopy and ionic conductivity studies have been carried out on polymer electrolyte films prepared by deposition of solutions of neodymium trifluoromethanesulphonate and poly(ethylene oxide). A wide range of electrolyte concentrations were examined and a partial pseudo-equilibrium phase diagram of the system was determined. From the results obtained it is evident that the presence of relatively high concentrations of ionic guest species in the polymer host results in an inhibition of the growth of crystalline material (polymer spherulites or a polymer—salt coordination complex).
Abstract
The standard (p 0=0.1 MPa) molar enthalpy of formation, Δf H 0 m, for crystalline N-phenylphthalimide was derived from its standard molar enthalpy of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb-combustion calorimetry, as –206.03.4 kJ mol–1. The standard molar enthalpy of sublimation, Δg cr H 0 m , at T=298.15 K, was derived, from high temperature Calvet microcalorimetry, as 121.31.0 kJ mol–1. The derived standard molar enthalpy of formation, in the gaseous state, is analysed in terms of enthalpic increments and interpreted in terms of molecular structure.
Abstract
Samples of the composition of 10Fe2O3·10CaO·80SiO2 were prepared by the sol-gel method and heat-treated in different atmospheres. They were investigated by X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. In the heat-treated samples in air iron is present up to 1000 °C in form of hematite and as Fe3+ in the tetrahedral sites. A wide range of hematite particle sizes was observed, the average size increased with heating temperature. At 1000 °C wollastonite was observed, at 1200 °C tridymite was formed and all the iron was incorporated in hematite. A heat-treatment at 500 °C under reducing conditions led to poorly crystallized maghemite and at 700 °C to metallic iron and fayalite formation.
Abstract
Fluoride glasses have been extensively studied due to their high transparency in the infrared wavelength. The crystallization kinetics of these systems has been studied using DTA and DSC techniques. Most of the experimental data is frequently investigated in terms of the Johnson-Mehl-Avrami (JMA) model in order to obtain kinetic parameters. In this work, DSC technique has been used to study the crystallization of fluorozirconate glass under non-isothermal conditions. It was found that JMA model was not fit to be applied directly to these systems, therefore, the method proposed by Mlek has been applied and the Šestk-Berggren (SB) model seems to be adequate to describe the crystallization process.
