554 research outputs found
Non-pollen palynomorphs from surface sediments along an altitudinal transect of the Venezuelan Andes
Breakdown of the mean-field approximation in a wealth distribution model
One of the key socioeconomic phenomena to explain is the distribution of
wealth. Bouchaud and M\'ezard have proposed an interesting model of economy
[Bouchaud and M\'ezard (2000)] based on trade and investments of agents. In the
mean-field approximation, the model produces a stationary wealth distribution
with a power-law tail. In this paper we examine characteristic time scales of
the model and show that for any finite number of agents, the validity of the
mean-field result is time-limited and the model in fact has no stationary
wealth distribution. Further analysis suggests that for heterogeneous agents,
the limitations are even stronger. We conclude with general implications of the
presented results.Comment: 11 pages, 3 figure
La Termogènesi als calorÃmetres per conducció: caracterÃstiques dinà miques i possibilitats deconvolutives
Es descriuen les caracterÃstiques generals dels dispositius calorimètrics, les possibilitats de tractament
mitjançant models i els mètodes per aproximar-se a la termogènesi o dissipació instantà nia al si de la cèl•lula laboratori.
La descripció dinà mica es fa dins de l'espai freqüencial,
que permet introduir de manera natural diferents lÃmits
de freqüència segons les caracterÃstiques dels dispositius
i del procés que vol estudiar-se.
Per al tractament global dels calorÃmetres s'utilitza
una escala relativa de temps i de freqüència. Això permet
tractar sistemà ticament les condicions imposades per les
tècniques deconvolutives, l'aparellatge experimental i els
propis fenòmens fÃsics.General features of flow or conduction calorimeters are
briefly described together with several methods to obtain the thermogenesis or instantaneous power dissipated inside the laboratory cell. The possibi l ities of solvable models are also discussed. The dynamic behaviour of the calorimeter is readily described in frequency space, thus allowing the intrcduction of several frequential limits depending on the characteristics both of the device and of the phenomenon under study. A systematic treatment of conduction calorimeters may be attained if relative scales, i n time (t/τ1) and frequency
(θ τ1), are considered. Now, the efficiency of a given deconvolutive technique and the limits imposed by the experimental device itself or by the physical phenomena studied are easily evaluated
Electrochemical characterization of organic coatings for protection of historic steel artefacts
Figuras en el archivo zipElectrochemical techniques are mainly known in the field of cultural heritage conservation as a
tool for the elimination of corrosion layers or the removal of chlorides. However, these techniques are also
a valuable tool for assessing the anti-corrosive efficiency of protective coatings. The aim of this study was
to evaluate the performance of different coatings for their use in metallic heritage conservation using
polarization resistance (Rp) and electrochemical impedance spectroscopy (EIS). Carbon steel samples were
prepared to simulate the surface composition and morphology of historic steel artefacts, and coated by a
conservator-restorer following the common practices in conservation treatments. Three commercial organic
coatings have been studied: a microcrystalline wax (RenaissanceTM) and a methyl acrylate/ethyl
methacrylate copolymer resin (ParaloidTM B-72) dissolved in acetone –both them commonly used in
conservation and restoration treatments– and a ethylene copolymer wax emulsion in water (PoligenTM ES-
91009), that has not been used so far for this purposes. Four commercial corrosion inhibitor additives were
added to the ParaloidTM B-72 resin and PoligenTM ES-91009 wax. The additives were commercial
preparations with the following known active components: a blend of triazoles (M435), an ammonium salt
of tricarboxylic acid (M370), a calcium sulphonate (M109), and a bis-oxazoline (Alkaterge-TTM). Rp and
EIS results showed that the best protection of the steel specimens was afforded by PoligenTM ES-91009
when applied in thick layers. None of the additives have shown a clear improvement of the protection
properties of the coatings, and one of them impaired the barrier effect of the coating.Acknowledgements The authors express their gratitude to the Sixth
Framework Programme of the European Commission for financial
support of PROMET Project (Contract 509126). D.M. Bastidas
expresses his gratitude to the CSIC of Spain for his contract under
the I3P Programme, co-financed by the European Social Fund.Peer reviewe
Nd3+-doped TeO2–PbF2–AlF3 glasses for laser applications
A study of the optical properties of Nd3+ ion in TeO2–PbF2–AlF3 glasses has been carried out for different
Nd3+ concentrations. Based on the Judd–Ofelt theory, intensity parameters and radiative properties were
determined from the absorption spectra. Focusing on the suitability of this host for laser applications, the
spectroscopic quality factor v was obtained with a value of 1.07, a value of the order of other
compositions proposed as laser hosts. For the most intense emission corresponding with the
4F3/2?4I11/2 transition (1.06 lm), the absorption and emission and have been calculated with values
of 1.20 10 20 cm2, 2.08 10 20 cm2. A positive value for the gain cross-sections has been found for a
population inversion factor c of 0.4 in the spectral range from 1060 to 1110 nm. All these results suggest
the potentially use of this system as a laser host
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