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101.
Sol‐to‐Gel Transition in Fast Evaporating Systems Observed by in Situ Time‐Resolved Infrared Spectroscopy
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Prof. Dr. Plinio Innocenzi Dr. Luca Malfatti Dr. Davide Carboni Prof. Dr. Masahide Takahashi 《Chemphyschem》2015,16(9):1933-1939
The in situ observation of a sol‐to‐gel transition in fast evaporating systems is a challenging task and the lack of a suitable experimental design, which includes the chemistry and the analytical method, has limited the observations. We synthesise an acidic sol, employing only tetraethylorthosilicate, SiCl4 as catalyst and deuterated water; the absence of water added to the sol allows us to follow the absorption from the external environment and the evaporation of deuterated water. The time‐resolved data, obtained by attenuated total reflection infrared spectroscopy on an evaporating droplet, enables us to identify four different stages during evaporation. They are linked to specific hydrolysis and condensation rates that affect the uptake of water from external environment. The second stage is characterized by a decrease in hydroxyl content, a fast rise of condensation rate and an almost stationary absorption of water. This stage has been associated with the sol‐to‐gel transition. 相似文献
102.
Marco Bee Giuseppe Espa Diego Giuliani Flavio Santi 《Journal of computational and graphical statistics》2017,26(3):695-708
In this article, we use the cross-entropy method for noisy optimization for fitting generalized linear multilevel models through maximum likelihood. We propose specifications of the instrumental distributions for positive and bounded parameters that improve the computational performance. We also introduce a new stopping criterion, which has the advantage of being problem-independent. In a second step we find, by means of extensive Monte Carlo experiments, the most suitable values of the input parameters of the algorithm. Finally, we compare the method to the benchmark estimation technique based on numerical integration. The cross-entropy approach turns out to be preferable from both the statistical and the computational point of view. In the last part of the article, the method is used to model the probability of firm exits in the healthcare industry in Italy. Supplemental materials are available online. 相似文献
103.
We compare different notions of curvature on contact sub-Riemannian manifolds. In particular, we introduce canonical curvatures as the coefficients of the sub-Riemannian Jacobi equation. The main result is that all these coefficients are encoded in the asymptotic expansion of the horizontal derivatives of the sub-Riemannian distance. We explicitly compute their expressions in terms of the standard tensors of contact geometry. As an application of these results, we obtain a sub-Riemannian version of the Bonnet–Myers theorem that applies to any contact manifold. 相似文献
104.
Davide Giraudo 《Journal of Theoretical Probability》2017,30(1):196-211
We provide some sufficient mixing conditions on a strictly stationary sequence in order to guarantee the weak invariance principle in Hölder spaces. Strong mixing and \(\rho \)-mixing conditions are investigated as well as \(\tau \)-dependent sequences. The main tools are deviation inequalities for mixing sequences. 相似文献
105.
Models for the macroscopic behavior of Shape Memory Materials can be conveniently constructed within the Ziegler–Green–Naghdi
framework where all the constitutive information is encoded in two ingredients: the free energy and the dissipation function.
In a previous work, we have proposed various expressions for the basic functions suitable to model pseudoelasticity with complete
transformations cycles. In this work we consider additional effects due to Martensite reorientation and to transformation
reversal prior to transformation completion. The new constitutive model allows for the modeling of a variety of effects including:
shape memory associated with thermally induced transformation, internal pseudoelastic subloops and the determination of limit
cycles associated with repetitive stress cycling. 相似文献
106.
Some simple models for the macroscopic behavior of shape memory materials whose microstructure can be described as a mixture of two phases are derived on the basis of a free energy and a dissipation function. Keeping a common expression for the free energy, each model is based on a different expression for the dissipation function. Temperature-induced as well as isothermal, adiabatic and convective stress-induced transformations are studied. Attention is paid to closed form solutions, comparison among the models and parameter identification. 相似文献
107.
Gloria Tabacchi Ettore Fois Davide Barreca Alberto Gasparotto 《International journal of quantum chemistry》2014,114(1):1-7
First‐principles modeling can be a powerful tool for the understanding and optimization of bottom‐up processes for nanomaterials fabrication, such as chemical vapor deposition (CVD), a key technology for the development of advanced systems and devices. Molecule‐to‐material conversion by CVD involves complex chemical phenomena, which are often obscure and still largely unexplored. A proper modeling would require high level of accuracy, large sized models and should include both temperature effects and statistical sampling of reactive events. By presenting a few selected examples, this perspective surveys such problems and discusses currently available approaches for their solution. Possible strategies for future advances in the field are also highlighted. © 2013 Wiley Periodicals, Inc. 相似文献
108.
Effects of wall-based skin-friction drag reduction strategies on the statistical properties of large-scale motions in moderate Reynolds number turbulent flows have been investigated by exploiting Direct Numerical Simulation of turbulent channels. To educe large scales, a new efficient parallel distributed memory algorithm has been implemented which delivers data-driven modes of increasing characteristic lengthscales: the Fast and Adaptive Bidimensional Empirical Mode Decomposition (FABEMD). The influence of wall-based skin friction reduction on large scales is studied by comparing single point statistics, such as r.m.s. fluctuations, and two-point statistics, as cross-correlation functions in controlled and uncontrolled channel flow fields at constant friction Reynolds number. The traditional way of observing large-scale footprinting at the wall, as cross-correlation of the streamwise velocity components at different wall distances, has been found to be unreliable when comparing drag-reduced flows, due to the arbitrary choice of a reference plane in the logarithmic layer. A more sound way of observing the footprinting via the correlation of the streamwise velocity with the friction velocity is addressed and shows an increase of the footprinting in drag-reduced flows. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
109.
Mores D Kornatowski J Olsbye U Weckhuysen BM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(10):2874-2884
Coke formation during the methanol‐to‐olefin (MTO) conversion has been studied at the single‐particle level with in situ UV/Vis and confocal fluorescence microscopy. For this purpose, large H‐ZSM‐5 crystals differing in their Si/Al molar ratio have been investigated. During MTO, performed at 623 and 773 K, three major UV/Vis bands assigned to different carbonaceous deposits and their precursors are observed. The absorption at 420 nm, assigned to methyl‐substituted aromatic compounds, initiates the buildup of the optically active coke species. With time‐on‐stream, these carbonaceous compounds expand in size, resulting in the gradual development of a second absorption band at around 500 nm. An additional broad absorption band in the 600 nm region indicates the enhanced formation of extended carbonaceous compounds that form as the reaction temperature is raised. Overall, the rate of coke formation decreases with decreasing aluminum content. Analysis of the reaction kinetics indicates that an increased Brønsted acid site density facilitates the formation of larger coke species and enhances their formation rate. The use of multiple excitation wavelengths in confocal fluorescence microscopy enables the localization of coke compounds with different molecular dimensions in an individual H‐ZSM‐5 crystal. It demonstrates that small coke species evenly spread throughout the entire H‐ZSM‐5 crystal, whereas extended coke deposits primarily form near the crystal edges and surfaces. Polarization‐dependent UV/Vis spectroscopy measurements illustrate that extended coke species are predominantly formed in the straight channels of H‐ZSM‐5. In addition, at higher temperatures, fast deactivation leads to the formation of large aromatic compounds within channel intersections and at the external zeolite surface, where the lack of spatial restrictions allows the formation of graphite‐like coke. 相似文献
110.
Aramburo LR Karwacki L Cubillas P Asahina S de Winter DA Drury MR Buurmans IL Stavitski E Mores D Daturi M Bazin P Dumas P Thibault-Starzyk F Post JA Anderson MW Terasaki O Weckhuysen BM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(49):13773-13781
A combination of atomic force microscopy (AFM), high‐resolution scanning electron microscopy (HR‐SEM), focused‐ion‐beam scanning electron microscopy (FIB‐SEM), X‐ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron‐based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM‐5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol‐to‐olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM‐5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM‐5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5–50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity. 相似文献