283 research outputs found
How to tune the absorption spectrum of chlorophylls to enable better use of the available solar spectrum
Photon capture by chlorophylls and other chromophores in light-harvesting complexes and photosystems is the driving force behind the light reactions of photosynthesis. Excitation of photosystem II allows it to receive electrons from the water-oxidizing oxygen-evolution complex and to transfer them to an electron-transport chain that generates a transmembrane electrochemical gradient and ultimately reduces plastocyanin, which donates its electron to photosystem I. Subsequently, excitation of photosystem I leads to electron transfer to a ferredoxin which can either reduce plastocyanin again (in so-called “cyclical electron-flow”) and release energy for the maintenance of the electrochemical gradient, or reduce NADP+ to NADPH. Although photons in the far-red (700–750 nm) portion of the solar spectrum carry enough energy to enable the functioning of the photosynthetic electron-transfer chain, most extant photosystems cannot usually take advantage of them due to only absorbing light with shorter wavelengths. In this work, we used computational methods to characterize the spectral and redox properties of 49 chlorophyll derivatives, with the aim of finding suitable candidates for incorporation into synthetic organisms with increased ability to use far-red photons. The data offer a simple and elegant explanation for the evolutionary selection of chlorophylls a, b, c, and d among all easily-synthesized singly-substituted chlorophylls, and identified one novel candidate (2,12-diformyl chlorophyll a) with an absorption peak shifted 79 nm into the far-red (relative to chlorophyll a) with redox characteristics fully suitable to its possible incorporation into photosystem I (though not photosystem II). chlorophyll d is shown by our data to be the most suitable candidate for incorporation into far-red utilizing photosystem II, and several candidates were found with red-shifted Soret bands that allow the capture of larger amounts of blue and green light by light harvesting complexes.info:eu-repo/semantics/publishedVersio
Surface interactions and deposit growth in fouling of heat exchangers
Fouling of heat exchangers is a costly problem in industJ)'. There is a need for a better
understanding of the phenomena involved in the build up of deposits on surfaces.
The paper reviews the basic mechanisms of fouling, emphasizing their rdle in determining the
overall fouling rate. Mass transfer, adhesion, chemical or biological reactions can be the rate
limiting processes depending on the design and operating conditions of the heat exchanger.
Reference is made to the most common types of fouling, particularly in water systems, and to
their interaction with corrosion of metallic surfaces.
Special attention is devoted to adhesion, since the formation of fouling layers is primarily a
surface process. The main forces of adhesion and the methods fa predicting adhesion tendencies
are reviewed.
The paper presents and discusses data obtained in laboratory studies on fouling caused by
inorganic particles, by microorganisms, and also by both of these types of foulants acting
simultaneously. Synergistic phenomena (positive and negative) seem to occur when two oc
more types of foulants are present in the fluid, but the final result is still unpredictable. Data on
the effects of fluid velocity, temperature and surface material are also presented and discussed.(undefined
Interaction between different fouling agents in water systems
Publicado em "Heat transfer 1990 : proceedings of the Ninth International Heat Transfer Conference", Jerusalem, Israel, Vol. 5 (1990)Most of the fouling studies have been up to now focused
on the build up of deposits caused by only one foulant.
Although this type of research allows a more clear
identification of the mechanisms involved in the
formation of deposits, the study of more realistic situations
is now needed.
The paper reports the results of fouling tests where
different fouling agents were present in water: kaolin plus
magnetite particles, and bacteria (Pseudomonas
jl.uorescens) plus kaolin particles. Deposit formation was
monitored through heat transfer measurements. The tests
were performed with water flowing at several velocities and
the data were compared with previous results obtained
using a single foulant. in an attempt to determine possible
changes in the processes controlling the fouling rate.
Increased fouling resistances and rates were observed
when kaolin particles were suspended in the bacteria -
- water system.
The kaolin - magnetite interaction seems to decrease the
final amount of deposit and, at the same time, to increase
its mechanical strength.
Modifications in the metabolic processes or in the
adhesion forces may account for the observed changes
Assinaturas isotópicas de Sr em águas minerais de Portugal e suas relações com o ambiente geológico
Caracterização hidrogeoquímica e radiológica das águas subterrâneas da região de Ervedosa (Vinhais - Norte de Portugal)
Effect of metallic ions on the adhesion of biofilms formed by Pseudomonas fluorescens
Data on the adhesion of biofilms formed by Pseudomonas fluorescens to aluminium, copper and brass surfaces are reported in this paper. Biofilm thicknesses after 48 h were lower on the brass plates than on the other two metals. The results are compared with predictions of bacterial adhesion obtained by the method of van Oss et al. (C.J. van Oss, M.K. Chaudhury and R.J. Good, Chem. Rev., 88 (1988) 927), based on the evaluation of the change in the free energy of adhesion of the interacting systems. The presence of metallic ions released by the surfaces into the environment seemed to affect bacterial metabolism and adhesion and were taken into account to explain the discrepancies between experimental data and thermodynamic predictions. The possible effects of the extracellular polymers excreted by the bacteria on their attachment abilities are also stressed.(undefined
Human thermo-physiological sensation control based in the adaptive comfort philosophy
In order to improve the building thermal efficiency, increase the human thermal comfort level and to reduce the building energy consumption, in this work, the human thermo-physiological sensation control, based in the adaptive comfort philosophy, is applied. In this control methodology the occupants, during a lesson activity, can choose among the clothing level, the activity level and the natural ventilation system. The PMV and PPD indexes are used in the numerical simulation. This work is made in a classroom school building, in the Algarve region, with Mediterranean characteristics, in Spring conditions, based in air temperature and relative humidity measurements. The comparison of the human thermal comfort level without and with control strategies is made. The obtained results are used to define an applicable activity, clothing and ventilation strategy, during the day
A 5GHz1.8V low power CMOS low-noise amplifier
Wireless local-area networks (WLANs) have been deployed as office and home communications infrastructures worldwide. The diversification of the standards, such as IEEE 802.11 series demands the design of RF front-ends. Low power consumption is one of the most important design concerns in the application of those technologies. To maintain competitive hardware costs, CMOS has been used since it is the best solution for low cost and high integration processing, allowing analog circuits to be mixed with digital ones.
In the receiver chain, the low noise amplifier (LNA) is one of the most critical blocks in a transceiver design. The sensitivity is mainly determined by the LNA noise figure and gain. It interfaces with the pre-select filter and the mixer. Furthermore, since it is the first gain stage, care must be taken to provide accurate input match, low-noise figure, good linearity and a sufficient gain over a wide band of operation.
Several CMOS LNAs have been reported during the last decade, showing that the most research has been done at 802.11/b and GSM standards (900-2400MHz spectrum) and more recently at 802.11/a (5GHz band). One of the more significant disadvantages of 802.11/b is that the frequency band is crowded and subject to interference from other technologies, as is 2.4GHz cordless phones and Bluetooth. As the demand for radio-frequency integrated circuits, operating at higher frequency bands, increases, the IEEE 802.11/a standard becomes a very attractive option to wireless communication system developers.
This paper presents the design and implementation of a low power, low noise amplifier aimed at IEEE 802.11a for WLAN applications. It was designed to be integrated with an active balun and mixer, representing the first step toward a fully integrated monolithic WLAN receiver. All the required circuits are integrated at the same die and are powered by 1.8V supply source. Preliminary experimental results (S-parameters) are shown and promise excellent results.
The LNA circuit design details are illustrated in Section 2. Spectre simulation results focused at gain, noise figure (NF) and input/output matching are presented in Section 3. Finally, conclusions and comparison with other recently reported LNAs are made in Section 4, followed by future work
A 5GHz/1.8V CMOS active balun integrated with LNA
The development of high performance monolithic RF front-ends requires innovative RF circuit design to make the best of a good technology. A fully differential approach is usually preferred, due to its well-known properties. Although the differential approach must be preserved inside the chip, there are cases where the input signal is single-ended such as RF image filters and IF filters in a RF receiver. In these situations, a stage able to convert single-ended into differential signals (balun) is needed. The most cited topology, which is capable of providing high gain, consists on a differential stage with one of the two inputs grounded. Unfortunately, this solution has some drawbacks when implemented monolithically.
This work presents the design and simulated results of an innovative high-performance monolithic single to differential converter, which overcomes the limitations of the circuits.The integration of the monolithic active balun circuit with an LNA on a 0.18μm CMOS process is also reported. The circuits presented here are aimed at 802.11a. Section 2 describes the balun circuit and section 3 presents its performance when it is connected to a conventional single-ended LNA. Section 4 shows the simulated performance results focused at phase/amplitude balance and noise figure. Finally, the last section draws conclusions and future work
Automated design of microwave discrete tuning differential capacitance circuits in Si-integrated technologies
A genetic algorithm used to design radio-frequency
binary-weighted differential switched capacitor arrays (RFDSCAs) is
presented in this article. The algorithm provides a set of circuits all
having the same maximum performance. This article also describes the
design, implementation, and measurements results of a 0.25 lm
BiCMOS 3-bit RFDSCA. The experimental results show that the circuit
presents the expected performance up to 40 GHz. The similarity between
the evolutionary solutions, circuit simulations, and measured results
indicates that the genetic synthesis method is a very useful tool for
designing optimum performance RFDSCAs
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