20,311 research outputs found
Study of alkaline hydrothermal activation of belite cements by thermal analysis
The effect of alkaline hydrothermal activation of class-C fly ash belite cement was studied using thermal analysis (TG/DTG) by determining the increase in the combined water during a period of hydration of 180 days. The results were compared with those obtained for a belite cement hydrothermally activated in water. The two belite cements were fabricated via the hydrothermal-calcination route of class-C fly ash in 1 M NaOH solution (FABC-2-N) or demineralised water (FABC-2-W). From the results, the effect of the alkaline hydrothermal activation of belite cement (FABC-2-N) was clearly differentiated, mainly at early ages of hydration, for which the increase in the combined water was markedly higher than that of the belite cement that was hydrothermally activated in water. Important direct quantitative correlations were obtained among physicochemical parameters, such as the combined water, the BET surface area, the volume of nano-pores, and macro structural engineering properties such as the compressive mechanical strength
Absence of hot gas within the Wolft-Rayet Bubble around WR16
We present the analysis of XMM-Newton archival observations towards the
Wolf-Rayet (WR) bubble around WR16. Despite the closed bubble morphology of
this WR nebula, the XMM-Newton observations show no evidence of diffuse
emission in its interior as in the similar WR bubbles NGC6888 and S308. We use
the present observations to estimate a 3-\sigma upper limit to the X-ray
luminosity in the 0.3-1.5 keV energy band equal to 7.4x10^{32} erg s^{-1} for
the diffuse emission from the WR nebula, assuming a distance of 2.37 kpc. The
WR nebula around WR16 is the fourth observed by the current generation of X-ray
satellites and the second not detected. We also examine FUSE spectra to search
for nebular O VI absorption lines in the stellar continuum of WR16. The present
far-UV data and the lack of measurements of the dynamics of the optical WR
bubble do not allow us to confirm the existence of a conductive layer of gas at
T~3x10^5 K between the cold nebular gas and the hot gas in its interior. The
present observations result in an upper limit of n_e < 0.6 cm^-3 on the
electron density of the X-ray emitting material within the nebula.Comment: 6 pages, 3 figures, 1 table; To appear in Astronomy and Astrophysic
The Importance of Social and Government Learning in Ex Ante Policy Evaluation
We provide two methodological insights on \emph{ex ante} policy evaluation
for macro models of economic development. First, we show that the problems of
parameter instability and lack of behavioral constancy can be overcome by
considering learning dynamics. Hence, instead of defining social constructs as
fixed exogenous parameters, we represent them through stable functional
relationships such as social norms. Second, we demonstrate how agent computing
can be used for this purpose. By deploying a model of policy prioritization
with endogenous government behavior, we estimate the performance of different
policy regimes. We find that, while strictly adhering to policy recommendations
increases efficiency, the nature of such recipes has a bigger effect. In other
words, while it is true that lack of discipline is detrimental to prescription
outcomes (a common defense of failed recommendations), it is more important
that such prescriptions consider the systemic and adaptive nature of the
policymaking process (something neglected by traditional technocratic advice)
How to excite the internal modes of sine-Gordon solitons
We investigate the dynamics of the sine-Gordon solitons perturbed by
spatiotemporal external forces. We prove the existence of internal (shape)
modes of sine-Gordon solitons when they are in the presence of inhomogeneous
space-dependent external forces, provided some conditions (for these forces)
hold. Additional periodic time-dependent forces can sustain oscillations of the
soliton width. We show that, in some cases, the internal mode even can become
unstable, causing the soliton to decay in an antisoliton and two solitons. In
general, in the presence of spatiotemporal forces the soliton behaves as a
deformable (non-rigid) object. A soliton moving in an array of inhomogeneities
can also present sustained oscillations of its width. There are very important
phenomena (like the soliton-antisoliton collisions) where the existence of
internal modes plays a crucial role. We show that, under some conditions, the
dynamics of the soliton shape modes can be chaotic. A short report of some of
our results has been published in [J. A. Gonzalez et al., Phys. Rev. E, 65
(2002) 065601(R)].Comment: 14 .eps figures.To appear in Chaos, Solitons and Fractal
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