Development of a high-pressure thermobalance working under constant rate thermal analysis

Journal of Thermal Analysis and Calorimetry - A thermogravimetric instrument that works at high pressure of different gases has been designed and assembled. The instrument has been devised to work...     

An improved model for the kinetic description of the thermal degradation of cellulose

In spite of the large amount of work performed by many investigators during last decade, the actual understanding of the kinetics of thermal degradation of cellulose is still largely unexplained. In this paper, recent findings suggesting a nucleation and growth of nuclei mechanism as the main step of cellulose degradation have been reassessed and a more appropriate model involving chain scission and volatilization of fragments has been proposed instead. The kinetics of cellulose pyrolysis have been revisited by making use of a novel kinetic method that, without any previous assumptions regarding the kinetic model, allows performing the kinetic analysis of a set of experimental curves recorded under different heating schedules. The kinetic parameters and kinetic model obtained allows for the reconstruction of the whole set of experimental TG curves.     

Combined kinetic analysis of solid-state reactions: a powerful tool for the simultaneous determination of kinetic parameters and the kinetic model without previous assumptions on the reaction mechanism

The combined kinetic analysis implies a simultaneous analysis of experimental data representative of the forward solid-state reaction obtained under any experimental conditions. The analysis is based on the fact that when a solid-state reaction is described by a single activation energy, preexponetial factor and kinetic model, every experimental T-alpha-dalpha/dt triplet should fit the general differential equation independently of the experimental conditions used for recording such a triplet. Thus, only the correct kinetic model would fit all of the experimental data yielding a unique activation energy and preexponential factor. Nevertheless, a limitation of the method should be considered; thus, the proposed solid-state kinetic models have been derived by supposing ideal conditions, such as unique particle size and morphology. In real systems, deviations from such ideal conditions are expected, and therefore, experimental data might deviate from ideal equations. In this paper, we propose a modification in the combined kinetic analysis by using an empirical equation that fits every f(alpha) of the ideal kinetic models most extensively used in the literature and even their deviations produced by particle size distributions or heterogeneities in particle morphologies. The procedure here proposed allows the combined kinetic analysis of data obtained under any experimental conditions without any previous assumption about the kinetic model followed by the reaction. The procedure has been verified with simulated and experimental data.     

Low-nickel stainless steel passive film in simulated concrete pore solution: A SIMS study

Low-nickel and AISI 304 austenitic stainless steel (SS) passive films were studied using secondary ion mass spectrometry (SIMS). An alkaline Ca(OH)₂ saturated test solution containing different chloride additions was used at room temperature. The passive film formed consists mainly of an inner chromium-rich oxide layer and an outer iron-rich oxide layer. The chemistry of the passive film depends strongly on the chloride content in the alkaline solution. Under these exposure conditions nickel was detected in the outer part of the oxide, whereas chloride ions were not found in the passive film for either the low-nickel or AISI 304 SS alloys.     

Choosing a leader on a complex network

In many real life applications a group of people interact through a communication network, mathematically modelled as a connected graph linking each element of the group. These participants may have diverse objectives and play very different roles depending on their knowledge and privileges. We focus on a particular scenario, in which a certain node is absolutely essential for completing the intended task. Moreover, if a technical failure results in disconnection of a participant to this leader node, this participant can no longer take part in the group's performance.     

Diphosphines and pyrazole/pyrazolate-type ligands as building blocks in luminescent Au(I) complexes

A series of gold(I) complexes containing diphenylphosphine bridging ligands, dppm, dppe, dpephos, dbfphos and biphep and co-ligands of the type pyrazole have been synthesized. The X-ray crystal structures of [Au₂(μ-dpephos)(μ-pz^2CH3)][PF₆], [Au₂(μ-dbfphos)(μ-pz^2CH3)][PF₆], and of the starting compound [Au₂Cl₂(μ-biphep)] indicate that the structural and stoichiometric characteristics of the new complexes depend on the diphosphine ligand. The three complexes show Au?Au contacts between 3.27 Å and 3.30 Å, with that of the biphep compound being the shortest. Digold (I)-diphosphine derivatives with a bridging pyrazolate ligand are obtained in all cases, except when [Au₂Cl₂(μ-biphep)] is used as starting material. Surprisingly, in this case, two monodentate neutral pyrazole ligands are attached to the gold atoms. The new complexes are luminescent in the solid state at 77 K and in solution both at room temperature and 77 K. Low energy emission bands related to the presence of Au?Au interactions have been identified in some of the compounds in the solid state and/or in solution.     

Evolutionary games defined at the network mesoscale: the Public Goods game

The evolutionary dynamics of the Public Goods game addresses the emergence of cooperation within groups of individuals. However, the Public Goods game on large populations of interconnected individuals has been usually modeled without any knowledge about their group structure. In this paper, by focusing on collaboration networks, we show that it is possible to include the mesoscopic information about the structure of the real groups by means of a bipartite graph. We compare the results with the projected (coauthor) and the original bipartite graphs and show that cooperation is enhanced by the mesoscopic structure contained. We conclude by analyzing the influence of the size of the groups in the evolutionary success of cooperation.     

Ionization Energies in Dithiocarbamate Derivatives of α-Amino Acids: A Spectroscopic Investigation on the Effect of the Side Chain

Ionization energies of dithiocarbamate derivatives of α-amino acids (glycine, alanine, valine, leucine) have been determined by X-Ray Photoelectron Spectroscopy. The values determined increase as the side chain in these amino acids becomes longer, a fact probably related to the different donor ability of these radicals. A parallel effect has been also observed in the precise positions of the IR absorption bands for these compounds.     

The NMR reciprocity theorem for arbitrary probe geometry

It is shown that the NMR reciprocity theorem is a variant of a problem considered by Lorentz in 1895. This formulation is quite general and applies to electric-dipole-based as well as coil-based or resonator-based magnetic resonance probes. The reasoning is related to, but different from, the proof of the reciprocity theorem for radiofrequency networks and for transmit/receive antenna systems in telecommunications. The signal-to-noise ratio of the NMR experiment is also discussed in very general terms.     

Direct observation of elemental segregation in InGaN nanowires by X‐ray nanoprobe

Using synchrotron radiation nanoprobe, this work reports on the elemental distribution in single In_x Ga_1–xN nanowires (NWs) grown by molecular beam epitaxy directly on Si(111) substrates. Single NWs dispersed on Al covered sapphire were characterized by nano‐X‐ray fluorescence, Raman scattering and photoluminescence spectroscopy. Both Ga and In maps reveal an inhomogeneous axial distribution inside sin‐ gle NWs. The analysis of NWs from the same sample but with different dimensions suggests a decrease of In segregation with the reduction of NW diameter, while Ga distribution seems to remain unaltered. Photoluminescence and Raman scattering measurements carried out on ensembles of NWs exhibit relevant signatures of the compositional disorder. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)