Littérature scientifique sur le sujet « Electric transport mechanisms »
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Articles de revues sur le sujet "Electric transport mechanisms"
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Guillon, Olivier, Roger A. De Souza, Tarini Prasad Mishra et Wolfgang Rheinheimer. « Electric-field-assisted processing of ceramics : Nonthermal effects and related mechanisms ». MRS Bulletin 46, no 1 (janvier 2021) : 52–58. http://dx.doi.org/10.1557/s43577-020-00008-w.
Texte intégralRésumé :
AbstractField-assisted processing methods, such as spark plasma sintering and flash sintering, have considerably expanded the toolbox of ceramic engineering. Depending on the conditions, substantial electric currents may flow through the material resulting in fast heating rates due to Joule heating. Here, we focus on nonthermal effects induced by electric fields during processing of fluorite- and perovskite-based ceramics. The fundamentals of how a field can directly modify defect formation and migration in crystals are discussed. In addition, the interplay of ion transport and electrical conductivity is considered, this interplay being crucial to understanding nonthermal effects caused by electric fields (as in memristive switching). Electrochemical reactions leading to new phases or reduction are also described, as are densification rates and sintering parameters that are significantly affected even though the sample temperature is held constant. Finally, as grain-boundary properties and segregation are changed by ion transport, we describe how both retardation and acceleration of grain growth can be achieved including graded microstructures.
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Kim, Sang-il, et Hyun-Sik Kim. « Calculated Electric Transport Properties of Thermoelectric Semiconductors Under Different Carrier Scattering Mechanisms ». Korean Journal of Metals and Materials 59, no 2 (5 février 2021) : 127–34. http://dx.doi.org/10.3365/kjmm.2021.59.2.127.
Texte intégralRésumé :
The widespread application of thermoelectric devices in cooling and waste heat recovery systems will be achieved when materials achieve high thermoelectric performance. However, improving thermoelectric performance is not straightforward because the Seebeck coefficient and electrical conductivity of the materials have opposite trends with varying carrier concentration. Here, we demonstrate that carrier scattering mechanism engineering can improve the power factor, which is the Seebeck coefficient squared multiplied by electrical conductivity, by significantly improving the electrical conductivity with a decreased Seebeck coefficient. The effect of engineering the carrier scattering mechanism was evaluated by comparing the band parameters (density-of-states effective mass, non-degenerate mobility) of Te-doped and Te, transition metal co-doped <i>n</i>-type Mg2Sb3 fitted via the single parabolic band model under different carrier scattering mechanisms. Previously, it was reported that co-doping transition metal with Te only changed the carrier scattering mechanism from ionized impurity scattering to mixed scattering between ionized impurities and acoustic phonons, compared to Te-doped samples. The approximately three times enhancement in the power factor of Te, transition metal co-doped samples reported in the literature have all been attributed to a change in the scattering mechanism. However, here it is demonstrated that Te, transition metal co-doping also increased the density-of-states effective mass. Here, the impact of the scattering mechanism change on the electric transport properties of <i>n</i>-type Mg2Sb3 without an effective mass increase was studied. Even without the effective mass increase, carrier scattering mechanism engineering improved the power factor, and its effect was maximized by appropriate carrier concentration tuning.
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Hoang, Mai Quyen, Thi Thu Nga Vu, Manh Quan Nguyen et Severine Le Roy. « Modelling the conduction mechanisms in low density polyethylene material using finite element method ». Ministry of Science and Technology, Vietnam 63, no 1 (30 janvier 2021) : 27–33. http://dx.doi.org/10.31276/vjst.63(1).27-33.
Texte intégralRésumé :
Polymers used as insulating materials are increasingly popular in many different fields. In electrical engineering - electronics, polymers are used in high-voltage transmission cables, capacitors, transformers, or as part of an embedded system in the IGBT module thanks to its superior thermal and electrical insulation properties. One of the disadvantages of polymers is the possible accumulation of space charge in the material volume for a long time, leading to an increase in the electric field compared to the original design value. Charge transport models in polymer materials have been increasingly developed to predict the conduction mechanisms under thermal-electrical stress. In this study, from a finite volume method (FVM), the authors developed a charge transport model in low density polyethylene (LDPE) based on the finite element method (FEM). The simulation results of this model are also compared to experimental results and to the FVM model under different electric fields for LDPE.
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Kokourov, D. V., et B. V. Malozyomov. « Algorithm for improving energy efficient wheel motor for electric vehicles ». Journal of Physics : Conference Series 2061, no 1 (1 octobre 2021) : 012049. http://dx.doi.org/10.1088/1742-6596/2061/1/012049.
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Abstract In accordance with the tendency to reduce the number of mechanical assemblies in electric driven machines and mechanisms, attempts are made to bring the electric motor and the actuator of the mechanism into a single whole. Thereby increasing the quality and productivity of the machines. A motor-wheel is a kind of a driving wheel, an actuator of a traction electric drive system of a pneumatic-wheeled transport vehicle. The work is devoted to the modernization of urban electric transport by equipping it with high-energy efficiency motor wheels, based on the frequency converter system - asynchronous motor. This paper describes the improvement algorithm and technological features.
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Sinyukova, T. V., V. N. Meshcheryakov et A. V. Sinyukov. « Research of control systems for lifting and transport mechanisms ». Power engineering : research, equipment, technology 23, no 3 (20 juillet 2021) : 47–61. http://dx.doi.org/10.30724/1998-9903-2021-23-3-47-61.
Texte intégralRésumé :
THE PURPOSE. To investigate the existing methods of frequency control and their influence on the characteristics of the asynchronous motor, including the energy parameters. Consider new circuit solutions of the control type under study. Create a mathematical model of each of the methods of frequency control. To analyze the results obtained from the point of view of the behavior of mechanical, electromechanical and energy components. METHODS. When solving this problem, the method of computer simulation modeling, implemented by means of Matlab Simulink, was used. RESULTS. In this paper, various aspects of electric drive systems based on asynchronous motors with the use of cascade switching are studied and described, existing connection schemes for such systems are analyzed, and several new options with improved characteristics are proposed. A comparative analysis of various connection schemes is made, the most interesting results of such analysis are presented, and conclusions are drawn about the future prospects of certain circuit solutions.Electric drive systems were modeled in the Simulink MATLAB environment using software tools to demonstrate the operation parameters of the considered circuits. CONCLUSION. Various schemes for switching on the asynchronous motor in the Simulink Matlab environment were investigated. The results of the study revealed the potential usefulness of using a circuit with a transformer in the rotor circuit, as well as the construction of a multi-motor electric drive with a common transformer and a common frequency converter circuit. The efficiency of parallel connection of rotary circuits of a two-motor electric drive was demonstrated. The methods of returning the sliding energy to the network are also compared, and their effectiveness in a comparative analysis with the operation in the closed-loop rotor mode is demonstrated.
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Li, Zhen, Yongsen Han, Ji Liu, Daomin Min et Shengtao Li. « Investigation of temperature-dependent DC breakdown mechanism of EP/TiO2 nanocomposites ». Applied Physics Letters 121, no 5 (1 août 2022) : 052901. http://dx.doi.org/10.1063/5.0097351.
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In dielectric science, the electrical breakdown strength of a polymeric material significantly decreases with elevated temperatures, which restricts the development of advanced electrical and electronic applications toward miniaturization. In the present study, to clarify the temperature-dependent DC breakdown mechanisms of epoxy resin (EP)/TiO2 nanocomposites, the effects of nanoparticle incorporation and temperature on charge transport and molecular chain dynamics were studied. The results indicate that space charge accumulation and electric field distortion are reduced by nanoparticle incorporation to enhance the deep trap level, while space charge accumulation, electric field distortion, and molecular displacement are all accelerated as temperature increases. To further investigate the influence of carrier traps and molecular chain dynamics on temperature-dependent breakdown, a DC breakdown simulation model that involves bipolar charge transport, molecular chain dynamics, and breakdown criterion equations is established. The calculated breakdown strengths of EP/TiO2 nanocomposites show great accordance with the experimental results, which indicates that temperature-dependent DC breakdown mechanisms are dominated by the synergetic effects of carrier traps and segment chain dynamics. Through the analysis of the breakdown model, a transition of the dominant mechanism (from segment chain to backbone dynamics) near the glass-transition temperature for DC breakdown of EP/TiO2 nanocomposites is discovered.
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Cola, Adriano, et Isabella Farella. « Electric fields and dominant carrier transport mechanisms in CdTe Schottky detectors ». Applied Physics Letters 102, no 11 (18 mars 2013) : 113502. http://dx.doi.org/10.1063/1.4795942.
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Dai, Xiying, Fuqiang Tian, Fei Li, Shuting Zhang, Zhaoliang Xing et Jinbo Wu. « Investigation on Charge Transport in Polypropylene Film under High Electric Field by Experiments and Simulation ». Energies 14, no 16 (4 août 2021) : 4722. http://dx.doi.org/10.3390/en14164722.
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The charge transport in polypropylene was studied under DC electric fields at different temperatures. By the experimental measurement and simulation of the BCT model, we studied conduction currents, breakdown strength, and space charge distribution. In particular, the conduction characteristics under high temperature and high field, especially the conduction characteristics before the breakdown, were studied by systematic experiments, and the conduction characteristics and the breakdown mechanism were further studied by simulation. The results show that in the process of measuring conduction currents until breakdown, both high temperature and high electric field will promote charge transport. However, the free volume will increase at high temperature, which will easily lead to faster charge transport and breakdown. In the breakdown process at different temperatures, there are different breakdown mechanisms. At 20–60 °C, the electric breakdown process has mainly occurred in polypropylene film, and the breakdown strength is almost unchanged. At 80 °C, electric breakdown and thermal breakdown act together, and the charge transport is faster, and the breakdown field becomes smaller. Finally, we conclude that thermal stress plays a very important role in charge transport. In a high-temperature environment, the volume expansion of polypropylene will promote charge transport, and the insulation of polypropylene capacitor films will be damaged.
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Fedotov A. K., Movchan S. A., Apel P. Yu., Fedotova J. A. et Pashkevich A. V. « Electron Transport Mechanisms in Polyethylene Terephthalate Membranes ». Physics of the Solid State 64, no 10 (2022) : 1564. http://dx.doi.org/10.21883/pss.2022.10.54251.269.
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This paper describes carrier transport mechanisms in polyethylene terephthalate (PET) films and porous PET-based membranes (PMs) obtained by irradiating pristine PET film with swift heavy ions, with subsequent chemical etching in an alkali (NaOH) solution. The obtained PMs had through nanochannels (pores) with an average diameter of 720-750 nm. We observed that in the temperature range 240-300 K, the current-voltage characteristics I(V) of the initial Cu|PET|Cu structure obeyed the improved Mott--Gurney law, which is based on the Mark--Helfrich model for a space-charge-limited current (SCLC) mechanism for electron transport. It was found for the first time that creation of nanochannels in PMs resulted in a significant increase in the electric current density (by about three orders of magnitude) while maintaining the SCLC mechanism. The enhanced current density is explained by the formation of a highly conductive layer along the inner surface of the walls of the nanochannel that are covered with carboxyl end groups, which are created by alkaline hydrolysis. According to the model, the surface states formed by these groups enable the drift of additional electrons injected from the copper electrodes under the action of the bias voltage. Keywords: polyethylene terephthalate, electron transport, space-charge-limited current mechanism, Mark--Helfrich injection model.
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Suchanicz, J., K. Kluczewska-Chmielarz, D. Sitko et G. Jagło. « Electrical transport in lead-free Na0.5Bi0.5TiO3 ceramics ». Journal of Advanced Ceramics 10, no 1 (18 janvier 2021) : 152–65. http://dx.doi.org/10.1007/s40145-020-0430-5.
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AbstractLead-free Na0.5Bi0.5TiO3 (NBT) ceramics were prepared via a conventional oxide-mixed sintering route and their electrical transport properties were investigated. Direct current (DC, σDC) and alternating current (AC, σAC) electrical conductivity values, polarization current (first measurements) and depolarization current, current–voltage (I–U) characteristics (first measurements), and the Seebeck coefficient (α) were determined under various conditions. The mechanism of depolarization and the electrical conductivity phenomena observed for the investigated samples were found to be typical. For low voltages, the I–U characteristics were in good agreement with Ohm’s law; for higher voltages, the observed dependences were I–U2, I–U4, and then I–U6. The low-frequency σAC followed the formula σAC–ωs (ω is the angular frequency and s is the frequency exponent). The exponent s was equal to 0.18–0.77 and 0.73–0.99 in the low- and high-frequency regions, respectively, and decreased with temperature increasing. It was shown that conduction mechanisms involved the hopping of charge carriers at low temperatures, small polarons at intermediate temperatures, and oxygen vacancies at high temperatures. Based on AC conductivity data, the density of states at the Fermi-level, and the minimum hopping length were estimated. Electrical conduction was found to undergo p–n–p transitions with increasing temperature. These transitions occurred at depolarization temperature Td, 280 ℃, and temperature of the maximum of electric permittivity Tm is as typical of NBT materials.
Thèses sur le sujet "Electric transport mechanisms"
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Costantino, Mauro. « Low-frequency noise spectroscopy as an effective tool for electric transport analysis ». Doctoral thesis, Universita degli studi di Salerno, 2018. http://elea.unisa.it:8080/xmlui/handle/10556/4375.
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2016 - 2017In this work, several experiments and analyses performed by means of noise spectroscopy, on a broad typology of materials and compounds, are presented. Structural, DC electrical transport and noise properties are exposed for each investigated sample, and theoretical models and possible explanations of the experimental results are given to unravel physical phenomena. In particular, two distinct types of iron-chalcogenide superconductors are investigated, in their pristine and aged state, suggesting the more likely mechanism which generates the resistance fluctuations and resorting to Weak Localization theory. In the case of the polymer/carbon nanotubes composites, the fluctuation-induced tunneling model is introduced to explain the measured temperature dependence of the electrical conductance and the I-V curve behaviors. Then, noise measurements prove the existence of a structural phase transition occurring around 160 K within the perovskite compound and highlight the correlation between electronic defect states distribution and device performance. The variety of investigated devices and materials validates the soundness of the noise spectroscopy as an effective tool for electric transport analysis. [edited by author]
XXX ciclo
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Wachtmeister, Marcus. « Driving towards more flexibility ? : China's environmental and climate policy in the automotive sector ». Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:a525684b-072d-4288-9216-52a9321051ee.
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This doctoral dissertation examines the mode and efficacy of environmental and climate policy in China’s automotive sector. The ascent of China’s automobile market to the largest worldwide has detrimental effects on the country’s energy security situation, worsens environmental pollution, and increases greenhouse gas emissions. Environmental and climate policy measures to ameliorate these repercussions are the most apt tools available to the Chinese government. The objective of this dissertation is to identify the dominant mode of environmental and climate policies in China’s automotive industry and to assess the efficacy of select policy instruments. It does so by asking whether a uniform national approach to policy instrument adoption can be discerned that reflects China’s institutional and administrative history or whether modal exceptions exist. Secondly, if modal differences exist, to what extent do different instruments confirm the current understanding of the advantages and pitfalls of individual policy instrument types? And finally, how do Chinese instruments compare to those in other ambits in terms of policy mode and instrument efficacy? The literature on policy instruments holds that, due to their alleged efficiency advantages, incentive-based instruments dominate the political agenda of industrialised countries and international organisations (environmental consensus). This favouring of flexible instruments in academic and political circles contrasts with an evident lack of incentive-based instruments in practice and an observed lack of efficiency of some of those instruments actually implemented. Moreover, the policy mode adopted in developing countries and emerging markets has not yet received sufficient academic attention despite significant differences in institutional design, enforcement capacities, resources, and development paths that may imply reason for modal deviation. Applying a blend of qualitative and quantitative social sciences research methods, I add the case of China to the comparative literature and show that command-and-control regulation indeed forms the backbone of environmental and climate policy in China’s automotive industry. At the same time, modal differences exist between national regulation and local/ municipal incentive-based policy as well as in the electric vehicle sector, which shows a trend towards more incentive-based instruments and flexibility mechanisms in conventional regulation. Compared to other ambits, China has established a relatively flexible policy regime, at least for the case of vehicle efficiency standards. For the time being, incentive-based instruments remain comparatively ineffective and flexibility mechanisms in conventional regulation have an erosive effect on instrument stringency.
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Thomson, Susmita. « Local feedback regulation of salt & ; water transport across pumping epithelia : experimental & ; mathematical investigations in the isolated abdominal skin of Bufo marinus ». University of Western Australia. Dept. of Physiology, 2003. http://theses.library.uwa.edu.au/adt-WU2003.0022.
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[Truncated abstract] This study describes the results of a four and a half year investigation examining local regulation of ion transport through pumping epithelial cells. The study focussed on the standard isolated toad skin preparation, made famous by Hans Ussing. Originally, the objective was to perform some simple manipulations on the isolated toad skin, a standard and well-tested epithelial layer, which, according to the literature, was a well-behaved and stable preparation. The purpose of doing these toad skin experiments was to gain familiarity with the experimental techniques, such as measuring the open-circuit voltage (Voc) and the short-circuit current (Isc) across an epithelium. In the process, the experimental information that was obtained was to assist in the development and refinement of a mathematical model of a single pumping epithelial cell . . . Finally, it should be emphasised the toad skin was a convenient tissue model for exploring more general issues such as: (i) how pumping epithelial cells may adjust to changes in the extracellular environment by locally regulating their membrane conductances; (2) how the topology of a cell can influence its function (i.e. the topology can determine whether a cell is optimised for salt transport or water transport). (3) how different cells, with different functions, may be positioned in apposition in a pumping epithelial tissue so that gradients generated by one cell type can be utilised by another. From a broader perspective, it is likely that such issues are also applicable to other pumping epithelia, and ultimately, may assist in understanding how these epithelia function.
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Grange, Michael. « Integrative imaging and electron cryo-tomography of viral transport mechanisms ». Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:704e3459-053a-4a1c-b95a-9aaeca809cb2.
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To comprehensively understand a biological process or mechanism from molecule to cell it requires the combination of multiple techniques across a range of resolutions and scales. We currently rely on a variety of different approaches to highlight given aspects of biology that are then interlinked through interpretation. Intracellular transport is a process whereby vesicles and organelles are transported to different parts of cells with a precise spatial and temporal accuracy. This process is often hijacked by viruses, which utilize cellular machineries for active transport to and from the nucleus during their entry and egress. In this thesis I focus on the intracellular transport of adenoviruses, which are able to recruit and bind to the cytoskeletal motor, dynein, and use it to travel to the nucleus. I apply electron cryo-microscopy and fluorescence microscopy to elucidate the basis for this process, and link this to the behaviour of adenoviruses as they are recruited to microtubules. I then apply electron cryo-tomography to directly observe proteins in their native cellular environment. I discuss the ability of this technique to be utilized in the in situ determination of the structure of viruses as they infect a cell. I show that (i) in situ sub-volume averaging from single tomograms can guide and complement segmentation of biological features and (ii) that novel, transient processes can be imaged with high levels of detail. As a second example of transport in cells, I study the nuclear egress of herpesvirus capsids, utilizing a multi-modal approach in an attempt to characterize this process from cellular to molecular level, and ask how herpesviruses modulate the nuclear membrane during capsid egress into the cytosol. Finally, I discuss an emergent technique in the field of electron cryo-microscopy, the Volta phase plate, its potential application in molecular tomography, and how it may enhance our current ability to discern contextual molecular mechanism.
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Zhang, Tong. « Electrical conduction transport mechanisms of barium titanate- based multilayer ceramic capacitors ». Thesis, Virginia Tech, 1988. http://hdl.handle.net/10919/43074.
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dThe major objectives of this study were to examine electrical conduction properties of BaTiO3-based multilayer ceramic ( MLC ) capacitors in order to gain a better understanding of the conduction transport mechanisms inside the devices. The experiments involved mainly leakage current versus time measurements under both low temperature-low voltage stress and high temperature-high voltage stress.
It was established that leakage current conduction in a MLC capacitor under temperature-voltage stress can be divided into three different conduction regions due to different mechanisms. Those regions are polarization current, DC conduction current and degradation current. The polarization current decreases with time as a power law relation, i.e. Ic(t) t-m where the exponent value m is strongly dependent on the type of capacitor and temperature, but is only weakly dependent on the applied voltage.
It has been proposed that two degradation models ( a charge carrier concentration model and a reduction of grain boundary barrier height model ) can explain the degradation behavior for the Z5U devices tested. Degradation measurements indicate that the lifetime for Z5U capacitors can be described by Minford's expression. However, these models account only partly for X7R degradation. X7R behavior, is characterized by an early power law time dependence, followed by exponential voltage dependence.
The most probable conduction transport mechanism in X7R capacitors is small polaron hopping, while grain boundary transmission may be the predominant conduction transport mechanism in Z5U capacitors.
Master of Science
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Storti, Mattia. « Adaptation of mechanisms for regulation of photosynthetic electron transport upon land colonization ». Doctoral thesis, Università degli studi di Padova, 2018. http://hdl.handle.net/11577/3425893.
Texte intégralRésumé :
In eukaryotic organisms, oxygenic photosynthesis is catalyzed by 4 multiprotein complexes present on the thylakoid membranes inside the chloroplasts: Photosystem (PS) II, Cytochrome b6f (Cyt b6f), PSI and chloroplast ATP synthase (ATPase). The three former mediate a Linear Electron Flow (LEF) from H2O to NADP+ to synthetize NADPH. LEF also generate a proton gradient across the thylakoid membrane, supporting ATP synthesis. NADPH and ATP are exploited by Calvin Benson cycle to fix inorganic CO2 in sugars available for the metabolism.
Photosynthetic process is accompanied by the formation of unstable molecules that can generate reactive oxygen species (ROS) harmful for the cell. Particular regulatory mechanisms prevent the formation of these molecules. Among these, the Alternative Electron Flow (AEF) divert electrons from photosynthetic processes preventing over-reduction of electron transporters and ROS production. AEF mechanisms can be divided in Cyclic Electron Flow (CEF) and Pseudo-CEF (PCEF) according to the acceptors of the pathways, respectively the plastoquinone and O2. Two major CEF are known and mediated by PGR5/PGRL1 or by NDH-1 complex. PCEF is instead mediated by Flavodiiron proteins (FLV). In this thesis I focused on the early land plant Physcomitrella patens, this moss presents all the main AEF identified in algae and vascular plants and is thus a suitable model to directly compared their activity in the same organisms.
FLV proteins were lost in Angiosperms, one of the plausible explanations to this loss is that another AEF mechanism replaces their function, making FLV dispensable. FLV were shown to be fundamental to respond to light fluctuation that occurs in the natural environments, a function that in angiosperms was replaced by PGR5/PGRL1.
To evaluate the hypothesis that PGRL1/PGR5 CEF mediated pathway can replace FLV, in chapter 3 we isolated a P. patens mutant defective for both FLV and PGRL1. Our experiments highlighted a strong synergy between the 2 pathways especially when plants were exposed to stressful light regimes, suggesting a functional overlap and the possible replacement of one pathway over the other during evolution. Further analysis allowed us to establish that FLV and PGRL1 are particularly important for protection of PSI from over-reduction, thus preventing photodamage.
In chapter 4, we characterized NDH-1 complex in P. patens isolating a line defective for NDH-1 function. NDH-1 defective mutants showed a small defect in PSI functionality in the first moment after light exposure. Reiterating cycle of dark and light, the defect observed increased significantly in NDH-1 defective plants suggesting that this complex may operates in first seconds after light exposure to avoid PSI over-reduction. However, this defect did not impact the plant growth not even in fluctuating light conditions, probably because of the compensation from other mechanisms present in the chloroplast.
In order to entangle this possible compensation in chapter 5 NDH-1 was depleted in plants knock out for both PGRL1 and FLV, by isolating a double mutant lacking both CEF pathways (pgrl1-ndhm) and a triple mutant defective for both CEF and PCEF (flva-pgrl1-ndhm). Both exhibited a drastic decrease in growth and photosynthetic performances showing that NDH-1 depletion has a much larger effect if other mechanisms for AEF are also absent, eliminating compensating effects. The triple mutant showed growth defects even in non-stressing conditions suggesting these mechanisms are important also for steady state photosynthesis. Spectroscopic and biochemical characterization detected extensive damage to PSI, correlated with the growth defects.Negli organismi eucarioti la fotosintesi ossigenica è catalizzata da 4 complessi localizzati sulle membrane tilacoidali dei cloroplasti: il fotosistema (PS) II, il citocromo b6f, il PSI e l’ATPasi cloroplastica. I primi tre mediano un trasporto lineare degli elettroni (LEF) dall’H2O al NADP+ per formare NADPH. L’ATPasi invece sfrutta il gradiente protonico generato dal LEF per sintetizzare ATP. NADPH e ATP sono utilizzati dal ciclo di Calvin per fissare la CO2 in zuccheri disponibili per il metabolismo. Durante la fotosintesi si formano molecole instabili che possono generare molecole dannose per la cellula quali le specie reattive dell’ossigeno (ROS). Particolari meccanismi regolatori prevengono la formazione di queste molecole, tra questi i trasporti elettronici alternativi (AEF) rimuovono elettroni dal LEF prevenendo l’over-riduzione dei traportatori e a formazione di ROS. Gli AEF possono essere divisi in trasporti ciclici (CEF) e pseudo-ciclici (PCEF) a seconda dell’accettore finale della via, rispettivamente il plastochinone o l’O2. I due principali CEF conosciuti sono mediati dalle proteine PGR5/PGRL1 e dal complesso NDH-1. Il PCEF è invece mediato dalle proteine FLV. In questa tesi è stato studiato Physcomitrella patens, un muschio che presenta tutti i principali AEF identificati in alghe e piante vascolari ed è quindi un buon modello dove poter comparare l’attività dei diversi AEF presenti. Le proteine FLV non sono presenti nelle angiosperme, una possibile spiegazione alla loro perdita è che un altro AEF abbia sostituito il loro ruolo rendendole dispensabili. Le FLV sono importanti nella risposta alla luce fluttuante, ruolo che nelle angiosperme è ricoperto da PGR5/PGRL1. Per valutare l’ipotesi che PGR5/PGRL1 abbia rimpiazzato le FLV, nel capitolo 3 di questa tesi abbiamo isolato un mutante di P. patens difettivo per entrambe le vie. Abbiamo individuato una forte sinergia tra le 2 vie, in particolare quando i mutanti sono stati esposti a regimi luminosi restrittivi, suggerendo una sovrapposizione funzionale e un possibile sostituzione di uno con l’altro nel corso dell’evoluzione. Inoltre, abbiamo stabilito che le FLV e PGR5/PGRL1 sono importanti per proteggere il PSI dalla sovra-riduzione, prevendendo cos’ danni ossidativi. Nel capitolo 4, abbiamo caratterizzato il complesso NDH-1 di P. patens, grazie all’isolamento di una linea difettiva per la sua funzione. Questi mutanti hanno evidenziato solo una piccola differenza nella funzionalità del PSI al momento in cui le piante sono state esposte ad una luce. Reiterando cicli di buio è luce, il difetto osservato aumentava significativamente suggerendo che NDH-1 possa essere attivo nei primi secondo dopo l’esposizione ad una fonte luminosa, proteggendo il PSI. Tuttavia, anche cresciute in una condizione di luce fluttuante le piante difettive di NDH-1 non hanno mostrato fenotipi di crescita, probabilmente a causa di un meccanismo di compensazione durante il processo di acclimatazione a questa condizione. Per identificare questa possibile compensazione, nel capitolo 5 abbiamo rimosso la funzione NDH-1 in piante già mancanti di PGRL1 e FLV, isolando un doppio mutante difettivo per entrambi CEF e un triplo mutante mancante sia di CEF che di PCEF. Per Entrambi abbiamo riscontrato ridotta crescita e capacità fotosintetica, evidenziando che la funzione di NDH-1 è molto più evidente quando gli altri AEF sono assenti, eliminando l’effetto di compensazione. Il triplo mutante inoltre ha mostrato una ridotta crescita anche in condizioni non stressanti dimostrando che i AEF sono indispensabili per una corretta attività fotosintetica. Analisi spettroscopiche e biochimiche hanno mostrato che i difetti individuati sono dovuti a evidenti danni a carico del PSI in queste piante.
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Tzamos, Christos. « Mechanism design : from optimal transport theory to revenue maximization ». Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/112000.
Texte intégralRésumé :
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 191-196).
A central problem in Economics and Algorithmic Game Theory is the design of auctions that maximize the auctioneer's expected revenue. While optimal selling of a single item has been well-understood since the pioneering work of Myerson in 1981, extending his work to multi-item settings has remained a challenge. In this work, we obtain such extensions providing a mathematical framework for finding optimal mechanisms. In the first part of the work, we study revenue maximization in single-bidder multi-item settings, connecting this problem to a well-studied problem in measure theory, namely the design of optimal transport maps. By establishing strong duality between these two problems, we obtain a characterization of the structure of optimal mechanisms. As an important application, we prove that a grand bundling mechanism is optimal if and only if two measure-theoretic inequalities are satisfied. Likewise, we obtain necessary and sufficient conditions for the optimality of any mechanism in terms of a collection of measure-theoretic inequalities. Using our machinery we derive closed-form solutions in several example scenarios, illustrating the richness of mechanisms in multi-item settings, and we prove that the mechanism design problem in general is computationally intractable even for a single bidder. In the second part of the work, we study multi-bidder settings where bidders have uncertainty about the items for sale. In such settings, the auctioneer may wish to reveal some information about the item for sale in addition to running an auction. While prior work has focused only on the information design part keeping the mechanism fixed, we study the combined problem of designing the information revelation policy together with the auction format. We find that prior approaches to this problem are suboptimal and identify the optimal mechanism by connecting this setting to the multi-item mechanism design problem studied in the first part of the work.
by Christos Tzamos.
Ph. D.
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Philipp, Martin. « Electrical Transport and Scattering Mechanisms in Thin Silver Films for Thermally Insulating Glazing ». Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-70920.
Texte intégralRésumé :
Thin silver films are widely used in low-emissivity coatings for building glazing due to their high reflectance in the infrared and high transmittance in the visible spectrum. The determining parameter for the infrared reflectance is the electrical conductance of the layer stack - the better the conductance the higher the reflectance. Electrically conductive films of thicknesses smaller than the electron mean free path exhibit a strong increase in the residual resistivity proportional to the inverse of the film thickness. Despite intensive discussions, which have extended over tens of years, it is not understood yet if this conductive behavior originates from electron scattering at interfaces (Fuchs-Sondheimer model) or grain boundaries (Mayadas-Shatzkes model). To achieve a fundamental understanding of the prevailing electron scattering mechanisms, aluminum-doped zinc oxide (ZnO:Al) / Ag / ZnO:Al layer stacks produced by magnetron sputtering were investigated concerning their electronic structure and electrical transport properties. The electronic structure of the layer stacks was probed and analyzed by electron energy-loss spectroscopy. By this technique, plasmonic excitations are observed, which can be categorized into excitations of the electrons in the bulk silver and excitations at the ZnO:Al / Ag interface. The plasmons were analyzed with respect to their dispersion and the peak width, and brought into relation with electrical conductivity measurements by calculating the plasmon lifetime and the electron scattering rate. The difficulty in determining the relative contributions of the interface and grain boundary scattering in experimental conditions is due to the fact that the way in which these scattering mechanisms depend on the film thickness, is very similar. Understanding the electron transport in thin films is of paramount importance, because the differentiation between the scattering mechanisms is a key issue for the improvement of the coatings. In the present work, the solution came from the expected difference in the temperature-dependent behavior of the resistivity between electron scattering at interfaces and electron scattering at grain boundaries. Hence, the resistivity was measured as a function of the temperature on layer stacks with different silver film thickness varying in the range of 4 to 200 nm. The data were analyzed using the extended Mayadas-Shatzkes model involving both electron scattering at interfaces (Fuchs-Sondheimer model), and electron scattering at grain boundaries. The results demonstrate that electron scattering at grain boundaries dominates for all film thicknesses. The basic layer stack was compared to more sophisticated systems, obtained either by adding a thin titanium layer in between silver and ZnO:Al, or by exposing the growing silver film to an oxygen partial pressure (oxidizing the film). Furthermore, the effect of annealing at 250°C was studied for all these systemsDünne Silberfilme werden aufgrund ihres hohen Reflexionsvermögens im infraroten Spektrum und ihres hohen Transmissionsvermögens im Spektrum des Sonnenlichtes als Wärmeschutzbeschichtungen für Fensterglas verwendet. Der entscheidende Parameter für das Reflexionsvermögen der Schicht ist die elektrische Leitfähigkeit - je höher die Leitfähigkeit, desto stärker wird Infrarotlicht reflektiert. Elektrisch leitende Schichten mit Schichtdicken dünner als die mittlere freie Weglänge der Elektronen weisen einen starken Anstieg des spezifischen Widerstandes auf, der sich proportional zur inversen Schichtdicke verhält. Trotz ausführlicher Diskussionen während der letzten Jahrzehnte, ist noch nicht geklärt ob dieses Verhalten auf Streuung von Elektronen an Grenzflächen (Fuchs-Sondheimer-Modell) oder an Korngrenzen (Mayadas-Shatzkes-Modell) zurückzuführen ist. Um ein grundlegendes Verständnis der vorherrschenden Streumechanismen zu erlangen, wurden Schichtstapel der Struktur Aluminium-dotiertes Zinkoxid (ZnO:Al) / Ag / ZnO:Al, welche mittels Magnetron-Sputtern hergestellt wurden, hinsichtlich ihrer Transporteigenschaften und elektronischen Struktur untersucht. Die elektronische Struktur der Schichtsysteme ist mittels Elektronen-Energieverlust-Spektroskopie untersucht und bezüglich ihrer plasmonischen Anregungen analysiert wurden. Diese können in Anregungen der Volumenelektronen des Silbers und Anregungen der Elektronen aus der ZnO:Al / Ag Grenzfläche unterteilt werden. Die Plasmonen wurden hinsichtlich ihrer Impulsabhängigkeit und Anregungsbreite analysiert und durch Berechnung der Plasmonenstreurate mit den Messungen der elektrischen Leitfähigkeit verglichen. Aufgund der Tatsache, dass Genzflächen- und Korngrenzstreuung eine ähnliche Schichtdickenabhängigkeit aufweisen, gestaltet sich die Bestimmung der relativen Beiträge beider Streumechanismen als schwierig. Diese Problem kann durch die Untersuchung der Temperaturabhängigkeit der Streumechanismen, die sich für Grenzflächen- und Korngrenzstreuung unterscheidet, gelöst werden. Der spezifische Widerstand wurde in Abhängigkeit von der Temperatur an mehreren Proben unterschiedlicher Silberschichtdicke (im Bereich von 4 bis 200 nm) gemessen. Die Daten wurden anhand des erweiterten Mayadas-Shatzkes-Modells, welches sowohl Streuung an Grenzflächen (Fuchs-Sondheimer-Modell) als auch an Korngrenzen berücksichtigt, evaluiert. Die Ergebnisse zeigen eindeutig, dass für alle Schichtdicken die Elektronenstreuung an Korngenzen der dominierende Streumechanismus ist. Die Ergebnisse der Analyse des fundmentalen Schichtsystems wurden mit denen komplexerer Systeme verglichen, bei denen zum einen durch Hinzufügen einer dünnen Titanschicht die Grenzfläche zwischen Silber und ZnO:Al modifiziert wurde und zum anderen der Silberfilm durch einen erhöhten Sauerstoff-Partialdruck während der Beschichtung oxidiert wurde. Des Weiteren wurde der Effekt einer Temperung bei 250°C an allen Systemen untersucht
Les vitrages bas-émissifs sont fréquemment élaborés par dépôts de revêtements dont la couche active est un film mince d'argent. Le paramètre qui détermine la réflexion dans l'infra-rouge est la conductance électrique de l'empilement. La résistivité électrique résiduelle de films dont l'épaisseur est inférieure au libre parcours moyen des électrons croît fortement en fonction de l'inverse de l'épaisseur. En dépit d'intenses recherches menées pendant des dizaines d'années, l'origine de cet accroissement de résistivité - réflexion des électrons par les interfaces (modèle de Fuchs-Sondheimer) ou par les joints de grains (modèle de Mayadas-Shatzkes). Pour comprendre les mécanismes à l'œuvre dans le transport des électrons, des couches ZnO dopé aluminium (ZnO:Al) / Ag / (ZnO:Al) produites par pulvérisation plasma ont été étudiée concernant leur structure électronique et propriétés de transport électrique. Les empilements ont été examinés par spectroscopie de pertes d'énergie d'électrons. Les spectres font apparaître les excitations des électrons de volume de l'argent et les excitations à l'interface ZnO:Al / Ag. Les excitations ont été analysés concernant leur dispersion. En outre, la durée de vie moyenne des plasmons déterminée d'après la largeur du pic de plasmon d'interface se compare bien à la l'inverse de la fréquence de diffusion des électrons qui se déduit de l'application du modèle de Drude aux données relatives à la résistivité. La difficulté dans la détermination des contributions relatives des modèles de Fuchs-Sondheimer et Mayadas-Shatzkes dans les conditions expérimentales est due au fait que ces deux modèles présentent des variations très similaires en fonction de l'épaisseur des films. D'importance primordiale pour la compréhension du transport dans les films minces, la question est une clé pour l'amélioration des revêtements bas-émissifs. La solution a été apportée ici par la différence de comportement en fonction de la température des diffusions des électrons aux interfaces et aux joints de grains. D'après cela, la résistance d'empilements comportant des films d'argent d'épaisseurs comprises entre 4 et 200 nm a été mesurée en fonction de la température. Les données ont été analysées au moyen de la version du modèle de Mayadas-Shatzkes qui inclut à la fois la diffusion des électrons aux interfaces (modèle de Fuchs-Sondheimer) et la diffusion des électrons aux joints de grains. Il a té démontré que, pour toutes les épaisseurs, la diffusion des électrons aux joints de grains constitue l'effet dominant. Les résultats de l'analyse du système fondamental ont été comparées avec les résultats de systèmes plus sophistiqués, obtenus soit en intercalant une couche additionnelle de titane entre l'argent et le ZnO (méthode communément utilisée pour améliorer le mouillage du ZnO par l'argent), soit par exposition à une pression partielle du film d'argent encours de croissance (pour oxyder le film). En outre, l'effet du recuit à 250°C a été étudié pour tous ces systèmes
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Sivakumar, Praveen. « Analysis of Electron Transport and Luminance Mechanisms in SrS Based Blue Emitting ACTFEL Devices ». UKnowledge, 2003. http://uknowledge.uky.edu/gradschool_theses/243.
Texte intégralRésumé :
The purpose of this thesis is to contribute to the understanding of SrS based ACTFEL devices. Better understanding of the processes in the host phosphor will give us the possibility to design more efficient blue emitting ACTFEL devices. Towards this aim, a physical model, that describes the optoelectronic processes taking place in the phosphor, was developed and analytical equations were written. The analytical model was numerically simulated and the plots of flux flowing through the device and luminance output by the device were obtained. Experiments were performed to obtain the plots of current flowing through the device and luminance output by the device. These plots were then qualitatively compared and the results of comparisons are presented. The numerical simulations qualitatively verify the accuracy of the model. The drive parameters were varied in order to study its effect on the VIL characteristics of the device. On varying the voltage applied to the device and its rise and fall times, a good insight was obtained into device behavior. Simulations were also performed to obtain responses to qualitatively match the experimentally obtained responses. Various What-If scenarios have been studied by varying the device parameters. These studies have indicated the importance of these parameters in determining device performance.
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Downs, Christopher Stephen Charles. « A route to strain-engineering electron transport in graphene ». Thesis, University of Exeter, 2015. http://hdl.handle.net/10871/18897.
Texte intégralRésumé :
Graphene, a single atomic layer of graphite, has many exciting electronic and mechanical properties. On a fundamental level, the quasi-relativistic behaviour of the charge carriers in graphene arises from the honeycomb-like atomic structure. Deforming the lattice changes the lengths of the carbon-carbon bonds, breaking the hopping symmetry between carbon sites. Mathematically, elastic strain in a graphene membrane can be described by additional terms in the low-energy effective Hamiltonian, analogous to the vector potential of an external magnetic field. Hence, certain non-uniform strain geometries produce so-called `pseudo-magnetic fields', leading to a predicted zero-field quantum Hall effect. These fictitious magnetic fields are distinct from an external magnetic field in that they are only observed by charge carriers within the membrane, and have opposing polarity for electrons in the K and K' valleys, preserving time-reversal symmetry of the lattice as a whole. Deforming graphene in the non-uniform manner required to produce a homogeneous pseudo-magnetic field has proven to be a huge technological challenge, however, restricting experimental evidence to scanning tunnelling spectroscopy measurements on, for example, highly deformed nanobubbles formed by the thermal expansion of an epitaxially grown sheet on a platinum substrate. These results stimulated a large amount of interest in strain-engineering electron transport in graphene, partly due to the extreme magnitude of the observed pseudo-magnetic field, a direct consequence of the strain components strongly varying over the space of a few nanometres, but the formation of nanobubbles is a highly stochastic process which cannot be reliably reproduced. Subsequent research found a way to fabricate nanobubbles with a high degree of consistency, but the measurements were still limited to local-probe techniques due to the nanoscale size of the devices. As such, a method to reliably induce a homogeneous pseudo-magnetic field within a micron-sized membrane would be an attractive proposition, and is the basis for the work presented within this thesis. The non-uniform strain required precludes a simple bending or elongation of the substrate, hence a more local method is required. A novel nanostructure consisting of suspended gold beams surrounding a graphene membrane will deform upon cooling to cryogenic temperatures, and crucially, the actuation mechanism can be designed to produce any configuration of strain, including uniaxial strain, triaxial strain and a fan-shaped deformation, the latter two of which are predicted to create homogeneous pseudo-magnetic fields within a membrane. Strain patterns which are predicted to produce experimentally significant pseudo-magnetic fields (~1 T) may be generated with complex actuation beams that are physically achievable. Furthermore, the actuation mechanisms may be utilised as electrical contacts to the membrane, allowing its conductivity to be measured in the context of a two- or multi-terminal measurement, in conjunction with an external magnetic field. The design of the devices was developed using finite-element analysis, and the behaviour verified by low-temperature imaging of prototypes. While, after careful annealing, some conventional two-terminal suspended devices exhibited quantum Hall features at very low fields, the fabricated strain-inducing devices did not display pseudo-Landau quantisation, nor Landau quantisation, due to the difficulties of using current annealing to clean devices post-fabrication. The presented work, however, could pave the way towards observing signatures of pseudo-magnetic fields in a range of experimental measurements, as well as creating alternative strain geometries.
Livres sur le sujet "Electric transport mechanisms"
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Charge transfer in physics, chemistry, and biology : Physical mechanisms of elementary processes and an introduction to the theory. Luxembourg : Gordon and Breach Publishers, 1995.
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Hans-Achim, Wagenknecht, dir. Charge transfer in DNA : From mechanism to application. Weinheim : Wiley-VCH, 2005.
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Wim, Schoenmaker, dir. Quantum transport in submicron devices : A theoretical introduction. Berlin : Springer, 2002.
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Abad, Enrique. Energy Level Alignment and Electron Transport Through Metal/Organic Contacts : From Interfaces to Molecular Electronics. Berlin, Heidelberg : Springer Berlin Heidelberg, 2013.
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International Workshop on Quantum Effect Physics, Electronics and Applications (1992 Luxor, Egypt). Quantum effect physics, electronics, and applications : Proceedings of the International Workshop on Quantum Effect Physics, Electronics and Applications. Bristol : Institute of Physics Pub., 1992.
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Fukuzumi, Shunichi. Electron Transfer : Mechanisms and Applications. Wiley-VCH Verlag GmbH, 2013.
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Fukuzumi, Shunichi. Electron Transfer : Mechanisms and Applications. Wiley & Sons, Incorporated, John, 2020.
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Fukuzumi, Shunichi. Electron Transfer : Mechanisms and Applications. Wiley & Sons, Incorporated, John, 2020.
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Fukuzumi, Shunichi. Electron Transfer : Mechanisms and Applications. Wiley & Sons, Incorporated, John, 2020.
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(Foreword), Harry Gray, et Hans-Achim Wagenknecht (Editor), dir. Charge Transfer in DNA : From Mechanism to Application. Wiley-VCH, 2005.
Trouver le texte intégralChapitres de livres sur le sujet "Electric transport mechanisms"
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Bartolf, Holger. « Static Electronic Transport Measurements ». Dans Fluctuation Mechanisms in Superconductors, 185–208. Wiesbaden : Springer Fachmedien Wiesbaden, 2015. http://dx.doi.org/10.1007/978-3-658-12246-1_11.
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Baldewijns, L., et R. Valcke. « Photosystem II Electron Transport in Transgenic Tobacco ». Dans Photosynthesis : Mechanisms and Effects, 1173–76. Dordrecht : Springer Netherlands, 1998. http://dx.doi.org/10.1007/978-94-011-3953-3_280.
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Volovik, O. I., T. T. Nosenko et S. K. Sytnik. « Control of Chloroplast Electron Transport by Phosphoproteins ». Dans Photosynthesis : Mechanisms and Effects, 2063–66. Dordrecht : Springer Netherlands, 1998. http://dx.doi.org/10.1007/978-94-011-3953-3_483.
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Jacoboni, Carlo. « Fundamentals of Quantum Mechanics ». Dans Theory of Electron Transport in Semiconductors, 15–26. Berlin, Heidelberg : Springer Berlin Heidelberg, 2010. http://dx.doi.org/10.1007/978-3-642-10586-9_2.
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Büttiker, M., et M. Moskalets. « Scattering Theory of Dynamic Electrical Transport ». Dans Mathematical Physics of Quantum Mechanics, 33–44. Berlin, Heidelberg : Springer Berlin Heidelberg, 2006. http://dx.doi.org/10.1007/3-540-34273-7_5.
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Balberg, Isaac. « Electrical Transport Mechanisms in Ensembles of Silicon Nanocystallites ». Dans Silicon Nanocrystals, 69–104. Weinheim, Germany : Wiley-VCH Verlag GmbH & Co. KGaA, 2010. http://dx.doi.org/10.1002/9783527629954.ch4.
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Dennig, Hans-Jörg, Adrian Burri et Philipp Ganz. « BICAR—Urban Light Electric Vehicle ». Dans Small Electric Vehicles, 157–66. Cham : Springer International Publishing, 2021. http://dx.doi.org/10.1007/978-3-030-65843-4_12.
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AbstractThis paper describes the technical features of the light electric vehicle (L2e-category) named BICAR. This specially designed vehicle is an all-in-one emissions-free micro-mobility solution providing a cost-effective and sustainable mobility system while supporting the transition towards a low carbon society (smart and sustainable city concept). The BICAR represents part of a multimodal system, complementing public transport with comfort and safety, relieving inner-city congestion and solving the “first and last mile” issue. The BICAR is the lightest and smallest three-wheel vehicle with weather protection. Due to the space-saving design, six to nine BICARS will fit into a single standard parking space. Safety is increased by an elevated driving position and a tilting mechanism when cornering. The BICAR achieves a range of 40–60 km depending on the battery package configuration in urban transport at a speed of 45 km/h. It features a luggage storage place and exchangeable, rechargeable batteries. The BICAR can be driven without a helmet thanks to the safety belt system, which is engineered for street approved tests. The BICAR has an integrated telematic box connected to the vehicle electronics and communicating with the dedicated mobile application, through which the BICAR can be geo-localised, reserved, locked/unlocked and remotely maintained.
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Kukushkin, Alexander, et Sophia Khuznetsova. « Pathways of Electron Transport, Regulation and the Role of Plastoquinone ». Dans Photosynthesis : Mechanisms and Effects, 1153–56. Dordrecht : Springer Netherlands, 1998. http://dx.doi.org/10.1007/978-94-011-3953-3_275.
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Johnson, Giles N., et Thomas Ott. « Feedback Regulation of Higher Plant Photosynthetic Electron Transport - a Physiological Phenomenon ? » Dans Photosynthesis : Mechanisms and Effects, 2537–40. Dordrecht : Springer Netherlands, 1998. http://dx.doi.org/10.1007/978-94-011-3953-3_595.
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Padmasree, K., et A. S. Raghavendra. « Mitochondrial Oxidative Electron Transport Optimizes Photosynthesis in Mesophyll Protoplasts of Pea ». Dans Photosynthesis : Mechanisms and Effects, 3727–30. Dordrecht : Springer Netherlands, 1998. http://dx.doi.org/10.1007/978-94-011-3953-3_869.
Texte intégralActes de conférences sur le sujet "Electric transport mechanisms"
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Usinin PhD, Uriy, Sergey P. Gladyshev, Maxim Grigoryev PhD, Anatoliy Shishkov et Andrey Bychkov. « The Losses in Control Electric Drives of Transport Mechanisms at Different Controlled Laws ». Dans SAE 2011 World Congress & Exhibition. 400 Commonwealth Drive, Warrendale, PA, United States : SAE International, 2011. http://dx.doi.org/10.4271/2011-01-0039.
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Kabelac, S., et M. Siemer. « Thermodynamic View on the Loss Mechanisms in PEM-Fuel Cells ». Dans ASME 2005 3rd International Conference on Fuel Cell Science, Engineering and Technology. ASMEDC, 2005. http://dx.doi.org/10.1115/fuelcell2005-74071.
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The fuel cell, which is a highly promising candidate for high efficiency energy conversion, is not reaching expected conversion efficiencies of η > 0,5 yet. Parallel to standard explanations of loss mechanisms by means of overvoltages, a thermodynamic view of addressing irreversibilities by calculating local entropy production rates is helpful. Entropy production rates are calculated by multiplying local transport fluxes with appropriate driving forces, i.e., gradients of temperature, chemical potentials and electric potentials. These gradients have to be calculated by solving the set of constitutive balance equations. Before this tedious task is done, simplified model equations have to be used. The reversible fuel cell is the starting point of analysis. Results for a one-dimensional PEMFC are shown.
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Goncharov, Kirill. « MATHEMATICAL MODELING OF THE MECHANICAL CHARACTERISTICS OF ELECTRIC MOTORS WITH A PHASE ROTOR UNDER STABLE MOTION OF MECHANISMS OF LIFTING AND TRANSPORTATION MACHINES ». Dans CAD/EDA/SIMULATION IN MODERN ELECTRONICS 2021. Bryansk State Technical University, 2021. http://dx.doi.org/10.30987/conferencearticle_61c997ee8d81c4.71775363.
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A method of mathematical synthesis of the equations of the mechanical characteristics of an electric motor with a phase rotor, corresponding to various resistance steps in the rotor circuit and approximated by straight lines, when simulating the unsteady movement of the mechanisms of hoisting-and-transport machines, is presented.
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Moerner, W. E., C. Poga, Y. Jia et R. J. Twieg. « Photorefractive Polymers for Holographic Optical Storage ». Dans Organic Thin Films for Photonic Applications. Washington, D.C. : Optica Publishing Group, 1995. http://dx.doi.org/10.1364/otfa.1995.wgg.1.
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In the past few years, a new class of polymeric materials for photonic applications has appeared called photorefractive (PR) polymers. Photorefractivity is defined as modulation of the index of refraction in an electro-optic material by internal electric fields produced by optical redistribution of charge carriers; hence it must not be confused with the more standard local mechanisms of index change such as photochromism, excited state population, heating, etc. When a material shows the required properties of charge generation, transport, trapping, and dependence of the index of refraction upon the internal electric field, it can be tested for photorefractivity by observation of asymmetric energy transfer (two-beam coupling) between two laser beams in the material.
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Moerner, W. E., S. M. Silence, G. C. Bjorklund, D. M. Burland, R. D. Miller, J. J. Stankus et R. J. Twieg. « Science and Applications of Photorefractive Polymeric Materials ». Dans Spectral Hole-Burning and Related Spectroscopies : Science and Applications. Washington, D.C. : Optica Publishing Group, 1994. http://dx.doi.org/10.1364/shbs.1994.fa1.
Texte intégralRésumé :
In the past few years, a new class of polymeric materials for photonic applications has appeared called photorefractive (PR) polymers. Photorefractivity is defined as modulation of the index of refraction in an electro-optic material by internal electric fields produced by optical redistribution of charge carriers; hence it must not be confused with local mechanisms of index change such as photochromism, excited state population, heating, etc. When a material shows the required properties of charge generation, transport, trapping, and dependence of the index of refraction upon the internal electric field, it can be tested for photorefractivity by observation of asymmetric energy transfer (two-beam coupling) between two laser beams in the material.
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Wang, Yu, Jianzhu Cao, Feng Xie, Xiaobao Yang, Peng Li, Jie Ma et Xianbao Duan. « Transport Behavior of Silver in High-Temperature Gas-Cooled Reactors ». Dans 2021 28th International Conference on Nuclear Engineering. American Society of Mechanical Engineers, 2021. http://dx.doi.org/10.1115/icone28-63484.
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Abstract High-temperature gas-cooled reactor (HTGR) is characterized by inherent safety, superior proliferation resistance, and high efficiency, with a promising prospect on electric power product and high-temperature heat and hydrogen production. The enhanced safety mostly relies on the good retention of tristructural-isotropic (TRISO) coated fuel particles for fission products. Most radioactive nuclides are retained in the fuel element dispersed with a large amount of TRISO coated fuel particles during normal and off-normal transients. However, silver can pass through the intact TRISO coated fuel particle and this has been reported by researchers worldwide. Numerous studies have focused on the migration of silver and tried to interpret its unique behavior. Additionally, several theories have been proposed and abnormal occurrences have been ascribed to excess silicon, chemical degradation of SiC, grain boundaries, long columnar grains, nano-pores, nano-cracks, re-wetting process, lattice diffusions, and so on. However, this problem has not been completely solved so far. This paper introduces the general diffusion theory for fission products in the spherical particle, reviews recent theoretical models and mechanisms on silver migration and discusses the diffusion features of silver under different mechanisms. Based on the current investigations, several suggestions have been posed to end debate on silver migration in SiC. These suggestions can lay a good foundation for further studies on migration behavior of silver.
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Fellman, Batya A., Muataz Atieh et Evelyn N. Wang. « Carbon-Based Electric Double Layer Capacitors for Water Desalination ». Dans ASME 2010 8th International Conference on Nanochannels, Microchannels, and Minichannels collocated with 3rd Joint US-European Fluids Engineering Summer Meeting. ASMEDC, 2010. http://dx.doi.org/10.1115/fedsm-icnmm2010-30696.
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In capacitive deionization (CDI), salt water is passed through two polarized electrodes, whereby the salt is adsorbed onto the electrode surface and removed from the water stream. This approach has received renewed interest for water desalination due to the development of new high-surface area carbon-based nanomaterials. However, there is currently limited understanding as to how electrode geometry, surface properties, and capacitance affect ion capture. In this work, we experimentally investigate various standard carbon-based electrode materials, including activated carbon and carbon cloths, as well as microfabricated silicon structures for CDI. Electrochemical characterization through cyclic voltammetry was used to determine the electrochemical properties of each material. In addition, a mini-channel test cell was fabricated to perform parametric studies on ion capture. By controlling electrode geometry and chemistry in these studies, the work helps elucidate transport mechanisms and provide insight into the design of optimal materials for capacitive deionization.
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Hossan, Mohammad Robiul, Matthew J. Benton, Prashanta Dutta et Robert Dillon. « Parametric Study of Dielectrophoretic Interactive Motion of Particles ». Dans ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels collocated with the ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems. American Society of Mechanical Engineers, 2015. http://dx.doi.org/10.1115/icnmm2015-48343.
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Dielectrophoresis (DEP) has become one of the most popular mechanisms for label free particle manipulations and transport in microfluidics. The efficacy of this mechanism is greatly dependent on the understanding and control of DEP interactive motion among particles. In this study, we performed a systematic investigation to understand the effect of particles size and electrical properties on DC DEP interactions among particles using in-house hybrid immersed boundary – immersed interface numerical method. Immersed boundary method is employed to predict flow field and immersed interface method is used to simulate electric field. The numerical model utilizes Maxwell’s stress tensor to obtain DEP forces, while solving transient Navier-Stokes equation it determines the hydrodynamic interaction between each of the particles and the fluid containing them. By varying the number of particles as well as the particles’ size, electrical properties and initial orientations, a number of possibilities were considered. Results indicate that the particles with similar electrical conductivities attract each other and tend to align themselves parallel to the external electric field regardless of sizes. If electrical conductivity of particles is lower than that of the fluid medium then the particle-particle interactions is caused by the negative DEP. If electrical conductivity of particles is higher than that of the fluid medium then the interactive motions of particle is attributed to the positive DEP. On the other hand, electrically dissimilar particles still attract each other but tend to align perpendicular to the electric field. Both negative and positive DEP contributes in interactions between electrically dissimilar particles. Numerical simulation also shows that the identical sized particles move at the same speed during interaction. In contrast, smaller particles moves faster than the larger particle during the interactions. This study explains the effect of size and electrical properties on DEP interactive motions of particles and can be utilized to design microfluidic devices for DEP particle manipulations.
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Pop, Eric. « Electron-Phonon Interaction and Joule Heating in Nanostructures ». Dans ASME 2008 3rd Energy Nanotechnology International Conference collocated with the Heat Transfer, Fluids Engineering, and Energy Sustainability Conferences. ASMEDC, 2008. http://dx.doi.org/10.1115/enic2008-53050.
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The electron-phonon energy dissipation bottleneck is examined in silicon and carbon nanoscale devices. Monte Carlo simulations of Joule heating are used to investigate the spectrum of phonon emission in bulk and strained silicon. The generated phonon distributions are highly non-uniform in energy and momentum, although they can be approximately grouped into one third acoustic (AC) and two thirds optical phonons (OP) at high electric fields. The phonon dissipation is markedly different in strained silicon at low electric fields, where certain relaxation mechanisms are blocked by scattering selection rules. In very short (∼10 nm) silicon devices, electron and phonon transport is quasi-ballistic, and the heat generation domain is much displaced from the active device region, into the contact electrodes. The electron-phonon bottleneck is more severe in carbon nanotubes, where the optical phonon energy is three times higher than in silicon, and the electron-OP interaction is entirely dominant at high fields. Thus, persistent hot optical phonons are easily generated under Joule heating in single-walled carbon nanotubes suspended between two electrodes, in vacuum. This leads to negative differential conductance at high bias, light emission, and eventual breakdown. Conversely, optical and electrical measurements on such nanotubes can be used to gauge their thermal properties. The hot optical phonon effects appear less pronounced in suspended nanotubes immersed in an ambient gas, suggesting that phonons find relaxation pathways with the vibrational modes of the ambient gas molecules. Finally, hot optical phonons are least pronounced for carbon nanotube devices lying on dielectrics, where the OP modes can couple into the vibrational modes of the substrate. Such measurements and modeling suggest very interesting, non-equilibrium coupling between electrons and phonons in solid-state devices at nanometer length and picoseconds time scales.
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Zenyuk, Iryna, et Shawn Litster. « The Effect of Electric Double Layers on Ionic Conductivity in the Agglomerates of PEM Fuel Cell Anodes ». Dans ASME 2010 8th International Conference on Fuel Cell Science, Engineering and Technology. ASMEDC, 2010. http://dx.doi.org/10.1115/fuelcell2010-33299.
Texte intégralRésumé :
We present a theoretical analysis of ionic transport inside the catalyst particle agglomerates that form the electrodes of proton exchange membrane (PEM) fuel cells. The electrodes continue to be the subject of intense research and development because they are still the largest cost and source of performance degradation in PEM fuel cells. The advancement of electrodes requires proper understanding of the electrode structure and the relevant transport processes. However, the details of the electrode microstructure and the micro-scale and nano-scale transport mechanisms are still not well understood. A common hypothesis, supported by recent coarse-grained molecular dynamics simulations, is that the primary pores (the pores inside the agglomerates) are void space and not filled with Nafion electrolyte. Instead, it has been postulated that the primary pores are saturated with liquid water during operation. Here, we report on the effect of the electric double layers (EDLs), which form at the interface between the water and the carbon catalyst supports, on the ionic transport within the agglomerates. The multi-scale model addresses phenomena at two length scales: (1) the nano-scale EDL thickness and (2) the microscale agglomerate radius. We model the EDL using the Gouy-Chapman-Stern model, which provides a pore average conductivity for the spherical conduction-reaction model of the agglomerate. We use a spherical agglomerate model to calculate an effectiveness factor for the electrochemical reactions. Here we present the application of the model to the anode, where the low activation overpotential allows linearizations and convenient analytical solutions. A key finding of this work is the important role the EDLs have in establishing the effectiveness of the platinum catalyst utilization. In addition, we resolve the dependence of the agglomerate effectiveness factor on the activation overpotential and agglomerate radius. We observe a significant nonmonotonic dependence of the catalyst effectiveness factor on the overpotential and dramatic improvement in effectiveness of catalyst utilization with smaller agglomerates.
Rapports d'organisations sur le sujet "Electric transport mechanisms"
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Sullivan, J. P., T. A. Friedmann, R. G. Dunn, E. B. Stechel et P. A. Schultz. The electronic transport mechanism in amorphous tetrahedrally-coordinated carbon films. Office of Scientific and Technical Information (OSTI), février 1998. http://dx.doi.org/10.2172/634073.
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Nilsson Lewis, Astrid, Kaidi Kaaret, Eileen Torres Morales, Evelin Piirsalu et Katarina Axelsson. Accelerating green public procurement for decarbonization of the construction and road transport sectors in the EU. Stockholm Environment Institute, février 2023. http://dx.doi.org/10.51414/sei2023.007.
Texte intégralRésumé :
Public procurement of goods and services contributes to about 15% of global greenhouse gas emissions. In the EU, public purchasing represents 15% of its GDP, acting as a major influencer on the market through the products and services acquired by governments from the local to national levels. The public sector has a role to play in leveraging this purchasing power to achieve the best societal value for money, particularly as we scramble to bend the curve of our planet’s warming. Globally, the construction and transport sectors each represent about 12% of government procurements’ GHG emissions. Furthermore, these sectors’ decarbonization efforts demand profound and disruptive technological shifts. Hence, prioritizing these sectors can make the greatest impact towards reducing the environmental footprint of the public sector and support faster decarbonization of key emitting industries. Meanwhile, the EU committed to achieving 55% reduction in GHG emissions by 2030 compared to 1990 levels. Drastic emissions reductions are needed at an unprecedented speed and scale to achieve this goal. Green Public Procurement (GPP) is the practice of purchasing goods and services using environmental requirements, with the aim of cutting carbon emissions and mitigating environmental harm throughout the life cycle of the product or service. While the EU and many of its Member States alike have recognized GPP as an important tool to meet climate goals, the formalization of GPP requirements at the EU level or among local and national governments has been fragmented. We call for harmonization to achieve the consistency, scale and focus required to make GPP practices a powerful decarbonization tool. We surveyed the landscape of GPP in the EU, with a focus on construction and road transport. Through interviews and policy research, we compiled case studies of eight Member States with different profiles: Sweden, the Netherlands, France, Germany, Estonia, Poland, Spain and Italy. We used this information to identify solutions and best practices, and to set forth recommendations on how the EU and its countries can harmonize and strengthen their GPP policies on the path toward cutting their contributions to climate change. What we found was a scattered approach to GPP across the board, with few binding requirements, little oversight and scant connective tissue from national to local practices or across different Member States, making it difficult to evaluate progress or compare practices. Interviewees, including policy makers, procurement experts and procurement officers from the featured Member States, highlighted the lack of time or resources to adopt progressive GPP practices, with no real incentive to pursue it. Furthermore, we found a need for more awareness and clear guidance on how to leverage GPP for impactful societal outcomes. Doing so requires better harmonized processes, data, and ways to track the impact and progress achieved. That is not to say it is entirely neglected. Most Member States studied highlight GPP in various national plans and have set targets accordingly. Countries, regions, and cities such as the Netherlands, Catalonia and Berlin serve as beacons of GPP with robust goals and higher ambition. They lead the way in showing how GPP can help mitigate climate change. For example, the Netherlands is one of the few countries that monitors the effects of GPP, and showed that public procurement for eight product groups in 2015 and 2016 led to at least 4.9 metric tons of avoided GHG emissions. Similarly, a monitoring report from 2017 showed that the State of Berlin managed to cut its GHG emissions by 47% through GPP in 15 product groups. Spain’s Catalonia region set a goal of 50% of procurements using GPP by 2025, an all-electric in public vehicle fleet and 100% renewable energy powering public buildings by 2030. Drawing from these findings, we developed recommendations on how to bolster GPP and scale it to its full potential. In governance, policies, monitoring, implementation and uptake, some common themes exist. The need for: • Better-coordinated policies • Common metrics for measuring progress and evaluating tenders • Increased resources such as time, funding and support mechanisms • Greater collaboration and knowledge exchange among procurers and businesses • Clearer incentives, binding requirements and enforcement mechanisms, covering operational and embedded emissions With a concerted and unified movement toward GPP, the EU and its Member States can send strong market signals to the companies that depend on them for business, accelerating the decarbonization process that our planet requires.
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Armstrong, Andrew M., Sylvie Aubry, Eric Arthur Shaner, Michael P. Siegal, Qiming Li, Reese E. Jones, Tyler Westover et al. Impact of defects on the electrical transport, optical properties and failure mechanisms of GaN nanowires. Office of Scientific and Technical Information (OSTI), septembre 2010. http://dx.doi.org/10.2172/991531.
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Kirchhoff, Helmut, et Ziv Reich. Protection of the photosynthetic apparatus during desiccation in resurrection plants. United States Department of Agriculture, février 2014. http://dx.doi.org/10.32747/2014.7699861.bard.
Texte intégralRésumé :
In this project, we studied the photosynthetic apparatus during dehydration and rehydration of the homoiochlorophyllous resurrection plant Craterostigmapumilum (retains most of the photosynthetic components during desiccation). Resurrection plants have the remarkable capability to withstand desiccation, being able to revive after prolonged severe water deficit in a few days upon rehydration. Homoiochlorophyllous resurrection plants are very efficient in protecting the photosynthetic machinery against damage by reactive oxygen production under drought. The main purpose of this BARD project was to unravel these largely unknown protection strategies for C. pumilum. In detail, the specific objectives were: (1) To determine the distribution and local organization of photosynthetic protein complexes and formation of inverted hexagonal phases within the thylakoid membranes at different dehydration/rehydration states. (2) To determine the 3D structure and characterize the geometry, topology, and mechanics of the thylakoid network at the different states. (3) Generation of molecular models for thylakoids at the different states and study the implications for diffusion within the thylakoid lumen. (4) Characterization of inter-system electron transport, quantum efficiencies, photosystem antenna sizes and distribution, NPQ, and photoinhibition at different hydration states. (5) Measuring the partition of photosynthetic reducing equivalents between the Calvin cycle, photorespiration, and the water-water cycle. At the beginning of the project, we decided to use C. pumilum instead of C. wilmsii because the former species was available from our collaborator Dr. Farrant. In addition to the original two dehydration states (40 relative water content=RWC and 5% RWC), we characterized a third state (15-20%) because some interesting changes occurs at this RWC. Furthermore, it was not possible to detect D1 protein levels by Western blot analysis because antibodies against other higher plants failed to detect D1 in C. pumilum. We developed growth conditions that allow reproducible generation of different dehydration and rehydration states for C. pumilum. Furthermore, advanced spectroscopy and microscopy for C. pumilum were established to obtain a detailed picture of structural and functional changes of the photosynthetic apparatus in different hydrated states. Main findings of our study are: 1. Anthocyan accumulation during desiccation alleviates the light pressure within the leaves (Fig. 1). 2. During desiccation, stomatal closure leads to drastic reductions in CO2 fixation and photorespiration. We could not identify alternative electron sinks as a solution to reduce ROS production. 3. On the supramolecular level, semicrystalline protein arrays were identified in thylakoid membranes in the desiccated state (see Fig. 3). On the electron transport level, a specific series of shut downs occur (summarized in Fig. 2). The main events include: Early shutdown of the ATPase activity, cessation of electron transport between cyt. bf complex and PSI (can reduce ROS formation at PSI); at higher dehydration levels uncoupling of LHCII from PSII and cessation of electron flow from PSII accompanied by crystal formation. The later could severe as a swift PSII reservoir during rehydration. The specific order of events in the course of dehydration and rehydration discovered in this project is indicative for regulated structural transitions specifically realized in resurrection plants. This detailed knowledge can serve as an interesting starting point for rationale genetic engineering of drought-tolerant crops.
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Epel, Bernard L., Roger N. Beachy, A. Katz, G. Kotlinzky, M. Erlanger, A. Yahalom, M. Erlanger et J. Szecsi. Isolation and Characterization of Plasmodesmata Components by Association with Tobacco Mosaic Virus Movement Proteins Fused with the Green Fluorescent Protein from Aequorea victoria. United States Department of Agriculture, septembre 1999. http://dx.doi.org/10.32747/1999.7573996.bard.
Texte intégralRésumé :
The coordination and regulation of growth and development in multicellular organisms is dependent, in part, on the controlled short and long-distance transport of signaling molecule: In plants, symplastic communication is provided by trans-wall co-axial membranous tunnels termed plasmodesmata (Pd). Plant viruses spread cell-to-cell by altering Pd. This movement scenario necessitates a targeting mechanism that delivers the virus to a Pd and a transport mechanism to move the virion or viral nucleic acid through the Pd channel. The identity of host proteins with which MP interacts, the mechanism of the targeting of the MP to the Pd and biochemical information on how Pd are alter are questions which have been dealt with during this BARD project. The research objectives of the two labs were to continue their biochemical, cellular and molecular studies of Pd composition and function by employing infectious modified clones of TMV in which MP is fused with GFP. We examined Pd composition, and studied the intra- and intercellular targeting mechanism of MP during the infection cycle. Most of the goals we set for ourselves were met. The Israeli PI and collaborators (Oparka et al., 1999) demonstrated that Pd permeability is under developmental control, that Pd in sink tissues indiscriminately traffic proteins of sizes of up to 50 kDa and that during the sink to source transition there is a substantial decrease in Pd permeability. It was shown that companion cells in source phloem tissue export proteins which traffic in phloem and which unload in sink tissue and move cell to cell. The TAU group employing MP:GFP as a fluorescence probe for optimized the procedure for Pd isolation. At least two proteins kinases found to be associated with Pd isolated from source leaves of N. benthamiana, one being a calcium dependent protein kinase. A number of proteins were microsequenced and identified. Polyclonal antibodies were generated against proteins in a purified Pd fraction. A T-7 phage display library was created and used to "biopan" for Pd genes using these antibodies. Selected isolates are being sequenced. The TAU group also examined whether the subcellular targeting of MP:GFP was dependent on processes that occurred only in the presence of the virus or whether targeting was a property indigenous to MP. Mutant non-functional movement proteins were also employed to study partial reactions. Subcellular targeting and movement were shown to be properties indigenous to MP and that these processes do not require other viral elements. The data also suggest post-translational modification of MP is required before the MP can move cell to cell. The USA group monitored the development of the infection and local movement of TMV in N. benthamiana, using viral constructs expressing GFP either fused to the MP of TMV or expressing GFP as a free protein. The fusion protein and/or the free GFP were expressed from either the movement protein subgenomic promoter or from the subgenomic promoter of the coat protein. Observations supported the hypothesis that expression from the cp sgp is regulated differently than expression from the mp sgp (Szecsi et al., 1999). Using immunocytochemistry and electron microscopy, it was determined that paired wall-appressed bodies behind the leading edge of the fluorescent ring induced by TMV-(mp)-MP:GFP contain MP:GFP and the viral replicase. These data suggest that viral spread may be a consequence of the replication process. Observation point out that expression of proteins from the mp sgp is temporary regulated, and degradation of the proteins occurs rapidly or more slowly, depending on protein stability. It is suggested that the MP contains an external degradation signal that contributes to rapid degradation of the protein even if expressed from the constitutive cp sgp. Experiments conducted to determine whether the degradation of GFP and MP:GFP was regulated at the protein or RNA level, indicated that regulation was at the protein level. RNA accumulation in infected protoplast was not always in correlation with protein accumulation, indicating that other mechanisms together with RNA production determine the final intensity and stability of the fluorescent proteins.
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Izhar, Shamay, Maureen Hanson et Nurit Firon. Expression of the Mitochondrial Locus Associated with Cytoplasmic Male Sterility in Petunia. United States Department of Agriculture, février 1996. http://dx.doi.org/10.32747/1996.7604933.bard.
Texte intégralRésumé :
The main goal of the proposed research was to continue the mutual investigations into the molecular basis of CMS and male fertility restoration [MRF], with the ultimate goal of understanding these phenomena in higher plants. The experiments focused on: (1) dissecting apart the complex CMS - specific mitochondrial S-Pcf locus, in order to distinguish its essential parts which cause sterility from other parts and study its molecular evolution. (2) Studying the expression of the various regions of the S-Pcf locus in fertile and sterile lines and comparing the structure and ultrastructure of sterile and fertile tissues. (3) Determine whether alteration in respiration is genetically associated with CMS. Our mutual investigations further substantiated the association between the S-Pcf locus and CMS by the findings that the fertile phenotype of a population of unstable petunia somatic hybrids which contain the S-Pcf locus, is due to the presence of multiple muclear fertility restoration genes in this group of progenies. The information obtained by our studies indicate that homologous recombination played a major role in the molecular evolution of the S-Pcf locus and the CMS trait and in the generation of mitochondrial mutations in general. Our data suggest that the CMS cytoplasm evolved by introduction of a urs-s containing sublimon into the main mitochondrial genome via homologous recombination. We have also found that the first mutation detected so far in S-Pcf is a consequence of a homologous recombination mechanism involving part of the cox2 coding sequence. In all the cases studied by us, at the molecular level, we found that fusion of two different cells caused mitochondrial DNA recombination followed by sorting out of a specific mtDNA population or sequences. This sequence of events suggested as a mechanism for the generation of novel mitochondrial genomes and the creation of new traits. The present research also provides data concerning the expression of the recombined and complex CMS-specific S-Pcf locus as compared with the expression of additional mitochondrial proteins as well as comparative histological and ultrastructural studies of CMS and fertile Petunia. Evidence is provided for differential localization of mitochondrially encoded proteins in situ at the tissue level. The similar localization patterns of Pcf and atpA may indicate that Pcf product could interfere with the functioning of the mitochondrial ATPase in a tissue undergoing meiosis and microsporogenesis. Studies of respiration in CMS and fertile Petunia lines indicate that they differe in the partitioning of electron transport through the cytochrome oxidase and alternative oxidase pathways. The data indicate that the electron flux through the two oxidase pathways differs between mitochondria from fertile and sterile Petunia lines at certain redox states of the ubiquinone pool. In summary, extensive data concerning the CMS-specific S-Pcf locus of Petunia at the DNA and protein levels as well as information concerning different biochemical activity in CMS as compared to male fertile lines have been accumulated during the three years of this project. In addition, the involvement of the homologous recombination mechanism in the evolution of mt encoded traits is emphasized.
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Ohad, Itzhak, et Himadri Pakrasi. Role of Cytochrome B559 in Photoinhibition. United States Department of Agriculture, décembre 1995. http://dx.doi.org/10.32747/1995.7613031.bard.
Texte intégralRésumé :
The aim of this research project was to obtain information on the role of the cytochrome b559 in the function of Photosystem-II (PSII) with special emphasis on the light induced photo inactivation of PSII and turnover of the photochemical reaction center II protein subunit RCII-D1. The major goals of this project were: 1) Isolation and sequencing of the Chlamydomonas chloroplast psbE and psbF genes encoding the cytochrome b559 a and b subunits respectively; 2) Generation of site directed mutants and testing the effect of such mutation on the function of PSII under various light conditions; 3) To obtain further information on the mechanism of the light induced degradation and replacement of the PSII core proteins. This information shall serve as a basis for the understanding of the role of the cytochrome b559 in the process of photoinhibition and recovery of photosynthetic activity as well as during low light induced turnover of the D1 protein. Unlike in other organisms in which the psbE and psbF genes encoding the a and b subunits of cytochrome b559, are part of an operon which also includes the psbL and psbJ genes, in Chlamydomonas these genes are transcribed from different regions of the chloroplast chromosome. The charge distribution of the derived amino-acid sequences of psbE and psbF gene products differs from that of the corresponding genes in other organisms as far as the rule of "positive charge in" is concerned relative to the process of the polypeptide insertion in the thylakoid membrane. However, the sum of the charges of both subunits corresponds to the above rule possibly indicating co-insertion of both subunits in the process of cytochrome b559 assembly. A plasmid designed for the introduction of site-specific mutations into the psbF gene of C. reinhardtii. was constructed. The vector consists of a DNA fragment from the chromosome of C. reinhardtii which spans the region of the psbF gene, upstream of which the spectinomycin-resistance-conferring aadA cassette was inserted. This vector was successfully used to transform wild type C. reinhardtii cells. The spectinomycin resistant strain thus obtained can grow autotrophically and does not show significant changes as compared to the wild-type strain in PSII activity. The following mutations have been introduced in the psbF gene: H23M; H23Y; W19L and W19. The replacement of H23 involved in the heme binding to M and Y was meant to permit heme binding but eventually alter some or all of the electron transport properties of the mutated cytochrome. Tryptophane W19, a strictly conserved residue, is proximal to the heme and may interact with the tetrapyrole ring. Therefore its replacement may effect the heme properties. A change to tyrosine may have a lesser affect on the potential or electron transfer rate while a replacement of W19 by leucine is meant to introduce a more prominent disturbance in these parameters. Two of the mutants, FW19L and FH23M have segregated already and are homoplasmic. The rest are still grown under selection conditions until complete segregation will be obtained. All mutants contain assembled and functional PSII exhibiting an increased sensitivity of PSII to the light. Work is still in progress for the detailed characterization of the mutants PSII properties. A tobacco mutant, S6, obtained by Maliga and coworkers harboring the F26S mutation in the b subunit was made available to us and was characterized. Measurements of PSII charge separation and recombination, polypeptide content and electron flow indicates that this mutation indeed results in light sensitivity. Presently further work is in progress in the detailed characterization of the properties of all the above mutants. Information was obtained demonstrating that photoinactivation of PSII in vivo initiates a series of progressive changes in the properties of RCII which result in an irreversible modification of the RCII-D1 protein leading to its degradation and replacement. The cleavage process of the modified RCII-D1 protein is regulated by the occupancy of the QB site of RCII by plastoquinone. Newly synthesized D1 protein is not accumulated in a stable form unless integrated in reassembled RCII. Thus the degradation of the irreversibly modified RCII-D1 protein is essential for the recovery process. The light induced degradation of the RCII-D1 protein is rapid in mutants lacking the pD1 processing protease such as in the LF-1 mutant of the unicellular alga Scenedesmus obliquus. In this case the Mn binding site of PSII is abolished, the water oxidation process is inhibited and harmful cation radicals are formed following light induced electron flow in PSII. In such mutants photo-inactivation of PSII is rapid, it is not protected by ligands binding at the QB site and the degradation of the inactivated RCII-D1 occurs rapidly also in the dark. Furthermore the degraded D1 protein can be replaced in the dark in absence of light driven redox controlled reactions. The replacement of the RCII-D1 protein involves the de novo synthesis of the precursor protein, pD1, and its processing at the C-terminus end by an unknown processing protease. In the frame of this work, a gene previously isolated and sequenced by Dr. Pakrasi's group has been identified as encoding the RCII-pD1 C-terminus processing protease in the cyanobacterium Synechocystis sp. PCC 6803. The deduced sequence of the ctpA protein shows significant similarity to the bovine, human and insect interphotoreceptor retinoid-binding proteins. Results obtained using C. reinhardtii cells exposes to low light or series of single turnover light flashes have been also obtained indicating that the process of RCII-D1 protein turnover under non-photoinactivating conditions (low light) may be related to charge recombination in RCII due to back electron flow from the semiquinone QB- to the oxidised S2,3 states of the Mn cluster involved in the water oxidation process.
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Splitter, Gary A., Menachem Banai et Jerome S. Harms. Brucella second messenger coordinates stages of infection. United States Department of Agriculture, janvier 2011. http://dx.doi.org/10.32747/2011.7699864.bard.
Texte intégralRésumé :
Aim 1: To determine levels of this second messenger in: a) B. melitensiscyclic-dimericguanosinemonophosphate-regulating mutants (BMEI1448, BMEI1453, and BMEI1520), and b) B. melitensis16M (wild type) and mutant infections of macrophages and immune competent mice. (US lab primary) Aim 2: To determine proteomic differences between Brucelladeletion mutants BMEI1453 (high cyclic-dimericguanosinemonophosphate, chronic persistent state) and BMEI1520 (low cyclicdimericguanosinemonophosphate, acute virulent state) compared to wild type B. melitensisto identify the role of this second messenger in establishing the two polar states of brucellosis. (US lab primary with synergistic assistance from the Israel lab Aim 3: Determine the level of Brucellacyclic-dimericguanosinemonophosphate and transcriptional expression from naturally infected placenta. (Israel lab primary with synergistic assistance from the US lab). B. Background Brucellaspecies are Gram-negative, facultative intracellular bacterial pathogens that cause brucellosis, the most prevalent zoonosis worldwide. Brucellosis is characterized by increased abortion, weak offspring, and decreased milk production in animals. Humans are infected with Brucellaby consuming contaminated milk products or via inhalation of aerosolized bacteria from occupational hazards. Chronic human infections can result in complications such as liver damage, orchitis, endocarditis, and arthritis. Brucellaspp. have the ability to infect both professional and non-professional phagocytes. Because of this, Brucellaencounter varied environments both throughout the body and within a cell and must adapt accordingly. To date, few virulence factors have been identified in B. melitensisand even less is known about how these virulence factors are regulated. Subsequently, little is known about how Brucellaadapt to its rapidly changing environments, and how it alternates between acute and chronic virulence. Our studies suggest that decreased concentrations of cyclic dimericguanosinemonophosphate (c-di-GMP) lead to an acute virulent state and increased concentrations of c-di-GMP lead to persistent, chronic state of B. melitensisin a mouse model of infection. We hypothesize that B. melitensisuses c-di-GMP to transition from the chronic state of an infected host to the acute, virulent stage of infection in the placenta where the bacteria prepare to infect a new host. Studies on environmental pathogens such as Vibrio choleraeand Pseudomonas aeruginosasupport a mechanism where changes in c-di-GMP levels cause the bacterium to alternate between virulent and chronic states. Little work exists on understanding the role of c-di-GMP in dangerous intracellular pathogens, like Brucellathat is a frequent pathogen in Israeli domestic animals and U.S. elk and bison. Brucellamust carefully regulate virulence factors during infection of a host to ensure proper expression at appropriate times in response to host cues. Recently, the novel secondary signaling molecule c-di-GMP has been identified as a major component of bacterial regulation and we have identified c-di-GMP as an important signaling factor in B. melitensishost adaptation. C. Major conclusions, solutions, achievements 1. The B. melitensis1453 deletion mutant has increased c-di-GMP, while the 1520 deletion mutant has decreased c-di-GMP. 2. Both mutants grow similarly in in vitro cultures; however, the 1453 mutant has a microcolony phenotype both in vitro and in vivo 3. The 1453 mutant has increased crystal violet staining suggesting biofilm formation. 4. Scanning electron microscopy revealed an abnormal coccus appearance with in increased cell area. 5. Proteomic analysis revealed the 1453 mutant possessed increased production of proteins involved in cell wall processes, cell division, and the Type IV secretion system, and a decrease in proteins involved in amino acid transport/metabolism, carbohydrate metabolism, fatty acid production, and iron acquisition suggesting less preparedness for intracellular survival. 6. RNAseq analysis of bone marrow derived macrophages infected with the mutants revealed the host immune response is greatly reduced with the 1453 mutant infection. These findings support that microlocalization of proteins involved in c-di-GMP homeostasis serve a second messenger to B. melitensisregulating functions of the bacteria during infection of the host.