459 research outputs found
Non-local permittivity from a quasi-static model for a class of wire media
A simple quasi-static model applicable to a wide class of wire media is
developed that explains strong non-locality in the dielectric response of wire
media in clear physical terms of effective inductance and capacitance per unit
length of a wire. The model is checked against known solutions and found to be
in excellent agreement with the results obtained by much more sophisticated
analytical and numerical methods. Special attention is given to suppression of
the spatial dispersion effects in wire media.Comment: 22 pagees, 4 figure
Effects of Spatial Dispersion on Reflection from Mushroom-type Artificial Impedance Surfaces
Several recent works have emphasized the role of spatial dispersion in wire
media, and demonstrated that arrays of parallel metallic wires may behave very
differently from a uniaxial local material with negative permittivity. Here, we
investigate using local and non-local homogenization methods the effect of
spatial dispersion on reflection from the mushroom structure introduced by
Sievenpiper. The objective of the paper is to clarify the role of spatial
dispersion in the mushroom structure and demonstrate that under some conditions
it is suppressed. The metamaterial substrate, or metasurface, is modeled as a
wire medium covered with an impedance surface. Surprisingly, it is found that
in such configuration the effects of spatial dispersion may be nearly
suppressed when the slab is electrically thin, and that the wire medium can be
modeled very accurately using a local model. This result paves the way for the
design of artificial surfaces that exploit the plasmonic-type response of the
wire medium slab.Comment: submitted for publication, under revie
Portmanteau tests for linearity of Stationary Time Series
This paper considers the problem of testing for linearity of stationary time series. Portmanteau tests are discussed which are based on generalized correlations of residuals from a linear model (that is, autocorrelations and cross-correlations of different powers of the residuals). The finite-sample properties of the tests are assessed by means of Monte Carlo experiments. The tests are applied to 100 time series of stock returns
Collaborative research: Accomplishments & potential
Although a substantial part of scientific research is collaborative and increasing globalization will probably lead to its increase, very few studies actually investigate the advantages, disadvantages, experiences and lessons learned from collaboration. In environmental epidemiology interdisciplinary collaboration is essential and the contrasting geographical patterns in exposure and disease make multi-location projects essential. This paper is based on a presentation given at the Annual Conference of the International Society for Environmental Epidemiology, Paris 2006, and is attempting to initiate a discussion on a framework for studying collaborative research. A review of the relevant literature showed that indeed collaborative research is rising, in some countries with impressive rates. However, there are substantial differences between countries in their outlook, need and respect for collaboration. In many situations collaborative publications receive more citations than those based on national authorship. The European Union is the most important host of collaborative research, mainly driven by the European Commission through the Framework Programmes. A critical assessment of the tools and trends of collaborative networks under FP6, showed that there was a need for a critical revision, which led to changes in FP7. In conclusion, it is useful to study the characteristics of collaborative research and set targets for the future. The added value for science and for the researchers involved may be assessed. The motivation for collaboration could be increased in the more developed countries. Particular ways to increase the efficiency and interaction in interdisciplinary and intercultural collaboration may be developed. We can work towards "the principles of collaborative research" in Environmental Epidemiology
Removal of ammonium from wastewater with geopolymer sorbents fabricated via additive manufacturing
Geopolymers have been recently explored as sorbents for wastewater treatment, thanks to their mechanical and chemical stability and to their low-energy manufacturing process. One specific application could be the removal of ammonium (NH4+) through exchange with Na+ ions. Additive manufacturing (AM) represents an especially interesting option for fabrication, as it allows to tailor the size, distribution, shape, and interconnectivity of pores, and therefore the access to charge-bearing sites. The present study provides a proof of concept for NH4+ removal from wastewater using porous geopolymer components fabricated via direct ink writing (DIW) AM approach. A metakaolin-based ink was employed for the fabrication of a log-pile structure with 45\ub0 rotation between layers, producing continuous yet tortuous macropores which are responsible for the high permeability of the sorbents. The ink consolidates in an amorphous, mesoporous network, with the mesopores acting as preferential sites for ion exchange. The printed sorbents were characterized for their physicochemical and mechanical properties and the NH4+ removal capacity in continuous-flow column experiments by using a model effluent. The lattices present high permeability and high cation exchange capacity and maintained a high amount of active ions after four cycles, allowing to reuse them multiple times
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