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991.
Porous silicon (PS) presents efficient photoluminescence and electroluminescence with potential applications in the optoelectronic industry, in particular in photovoltaic devices. It is now generally accepted that the interesting optical properties of PS are due to two combined aspects: on the one hand, the quantum confinement and on the other, the surface states. Although there has been a great effort to study PS, its transport properties are still not well understood. Due to the complex structure of PS an averaging theory to describe its effective properties is justified. In this work the effective dielectric function, effective absorption coefficient and effective refractive index are calculated using the volume averaging method for a model of periodic columns with different surface coatings simulating porous silicon. This approach allows analytical results within certain approximations and the analysis of surface contributions. The method uses parameters to characterize the bulk and the surface. We choose for the bulk c-Si, and cover it with three different possible surfaces: siloxane, a-Si : H and SiO2. The results are compared with experimental data and other theoretical approaches for silicon wires. We obtain good agreement with some experimental results showing the important role of the surface in the effective response of porous silicon. 相似文献
992.
It is difficult to deny that comparison between recommender systems requires a common way for evaluating them. Nevertheless, at present, they have been evaluated in many, often incompatible, ways. We affirm this problem is mainly due to the lack of a common framework for recommender systems, a framework general enough so that we may include the whole range of recommender systems to date, but specific enough so that we can obtain solid results. In this paper, we propose such a framework, attempting to extract the essential features of recommender systems. In this framework, the most essential feature is the objective of the recommender system. What is more, in this paper, recommender systems are viewed as applications with the following essential objective. Recommender systems must: (i) choose which (of the items) should be shown to the user, (ii) decide when and how the recommendations must be shown. Next, we will show that a new metric emerges naturally from this framework. Finally, we will conclude by comparing the properties of this new metric with the traditional ones. Among other things, we will show that we may evaluate the whole range of recommender systems with this single metric. 相似文献
993.
Using the principle of geochemistry of fluoride, green and cost effective anion adsorbents were developed for the removal of F? from water systems. The scheme was further applied for the removal of NO3? also. Carboxymethylated starch functionalized through network formation with acrylamide was used as adsorbent, and the resultant hydrogels were loaded with Fe2+ ions to generate anchorage for the anions. Sorption of Fe2+ was studied as a function of different factors such as time, temperature, pH, and ion strength. The network having the highest Fe2+ uptake was loaded with the Fe2+ ions under optimum conditions and used for the sorption of F? and NO3?. High efficiency has been observed for F?, as even up to 100% uptake has been observed within just 10 minutes. The support shows high selectivity for NO3?, which was used as anion reference. Thermodynamics of sorption confirms low order and low energy processes. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 相似文献
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996.
Helmus Stärker ÖVE Richard Eltschka ÖVE Gerhard Bartak Günther Brauner ÖVE 《e & i Elektrotechnik und Informationstechnik》2000,117(4):298-301
Ohne Zusammenfassung 相似文献
997.
998.
Tungsten trioxide and two types of tin dioxide were compacted at different pressures and then sintered at different temperatures
in order to study how the microstructure formed is dependent on these parameters. The structures were studied using dilatometric
and porosimetric techniques and also using electron microscopy. X-ray diffraction was used to ascertain the completeness of
the decomposition reaction of SnSO4 being converted to SnO2. Tin dioxide, which was prepared from tin sulphate by calcining, was characterised by a large specific surface area. For
all three materials studied the mean pore size was found to increase and the specific surface area was found to decrease with
increasing sintering temperature. In the case of WO3 the total pore volume decreased markedly as the sintering temperature was increased from 900 to 1000° C. However, in the
case of SnO2, the total pore volume remained almost constant in the sintering temperature range of 600 to 1500° C. In the case of SnO2 derived from SnSO4 the sintered density was about 45% of the theoretical value while for the conventionally made SnO2 the sintered density was about 55% of the theoretical value. The sintered density of WO3 was found to show a marked increase, from 55% to 70% of the theoretical value, as the sintering temperature was raised from
1000 to 1100° C. 相似文献
999.