182 research outputs found

    The modelling of network polymers

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    This thesis considers the modelling of two and three dimensional molecular networks with a view to being able to predict how the geometry of a network will affect the elastic constants and specifically the Poisson's ratios of the network. Materials with negative Poisson's ratios have much better engineering properties then those with positive Poisson's ratios. Theory states that a network polymer, with negative Poisson's ratios at a molecular level, would have much better properties than most materials with negative Poisson's ratios made to date. Molecular modelling has been used to examine the elastic constants of those two and three dimensional network polymers which are most likely to be synthesised in the near future. Such networks have been predicted to have either large positive or large negative Poisson's ratios depending on the molecular arrangement of the network. Poisson's ratios varying between 0.96 and -0.86 for the three dimensional cases and between -0.9 and 1.26 for the two dimensional cases have been calculated. Young's moduli in the order of 1 GPa have been observed for the three dimensional networks as compared to Young's moduli in the order of 20 - 400 kPa which have been experimentally measured for foam materials. Comparison with local density functional calculations for two 2-D networks with the molecular modelling have confirmed the negative Poisson's ratio in these networks and shown that it is not a function of the molecular modelling packages or force field used. The off-axis properties for both the two and three dimensional networks have been calculated. These show that whilst the networks with a positive Poisson's ratio in the principal axis directions always have a positive Poisson's ratio, those networks with a negative Poisson's ratio in the principal axis directions have off-axis Poisson's ratios that vary between large and positive and large and negative. In general the networks with positive Poisson's ratios are much more isotropic than those with negative Poisson's ratios. Analytical models which model the networks using simple beam theory have been produced for various two and three dimensional networks. These models can be used to predict the elastic constants of a network without the need to do time consumingmolecular modelling calculations to a first approximation. Comparison of the molecular models and analytical models has led to the development a library of force constants for two dimensional networks which can be used to more accurately predict the elastic constants of a network based on a knowledge of the geometry of the network and the constituent `sub-units' from which it is mad

    Time-resolved second harmonic generation study of buried semiconductor heterointerfaces using soliton-induced transparency

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    The transient second harmonic generation and linear optical reflectivity signals measured simultaneously in reflection from GaAs/GaSb/InAs and GaAs/GaSb heterostructures revealed a new mechanism for creating self-induced transparency in narrow bandgap semiconductors at low temperatures, which is based on the dual-frequency electro-optic soliton propagation. This allows the ultrafast carrier dynamics at buried semiconductor heterointerfaces to be studied

    Vegetation in urban streets, squares, and courtyards

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    One of various ways in which vegetation cover used in the greening of urban areas can help improve the health and well-being of people is in how it changes the acoustic environment. This chapter presents findings of computer simulations and scale modelling to examine and quantify the effectiveness of green roof and green wall (vertical garden) systems in reducing road traffic noise for streets, squares, and roadside courtyards. Noise reduction by sound absorption in reflected and diffracted (over roofs) sound paths is investigated. Particular attention is paid to the importance of vegetation placement relative to the receiver/listening positions. Because the soil substrate used for the vertical walls has good sound absorption properties, it also can be used for green barriers. In this chapter, the effects of a low barrier made of green wall substrate are studied for an installation on the ground and on the top of buildings surrounding a courtyard

    For public (and recontextualized) sociology: The promises and perils of public engagement in an age of mediated communication

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    This article argues for the analysis of public engagement as an essentially mediated activity. Although recent studies note that academic knowledge is increasingly available for consumption by nonacademic audiences, they tell us little about how it gets recontextualized while passing through the hands of media professionals on its way toward such audiences. In Burawoy’s (2005) influential call for the rebirth of public sociology, as in the debates his work provoked, the media is treated solely as a means for the transportation of knowledge. But as this article demonstrates, the media does not simply transport knowledge; it also, and at the same time, translates that knowledge in various, rhetorically consequential ways. Focusing on the mediated trajectory of an attempt by a group of academics to connect with audiences beyond academia, their initial contribution is compared to its subsequent translation(s) across various British newspapers. A discursive analysis reveals the techniques via which a classic form of public sociology came to be recontextualized such that, remarkably, these authors were left appearing to voice nothing but their own petty prejudices. The article concludes by noting that where public engagement involves mediation, public sociology should pay more attention to the recontextualizing affordances of media discourse
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