• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 97
  • 40
  • 17
  • 17
  • 15
  • 12
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 249
  • 41
  • 40
  • 32
  • 30
  • 26
  • 23
  • 23
  • 18
  • 16
  • 15
  • 15
  • 15
  • 14
  • 14
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
211

Biodisponibilité des métaux et métalloïdes de particules micro- et nanométriques en relation avec leur phytotoxicité / Bioavailability of metal(loid)s from micro- and nanometric particles in relation with their phytotoxicity

Xiong, Tiantian 29 October 2015 (has links)
Des particules ultrafines y compris nanométriques, enrichies en métaux (PM) sont émises dans l'atmosphère en zones industrielles et urbaines, puis sont transférées vers les écosystèmes terrestres et aquatiques, avec des conséquences sur la qualité des plantes et la santé humaine. Dans un contexte socio-scientifique mondial où la réglementation sur l’impact (éco)toxicologique des substances et les pressions de l'espace public se renforcent, des études d'impacts environnement-santé sur tout le cycle de vie des PM sont indispensables. La thèse visait tout d'abord l’étude du devenir et de l’impact des métaux des PM: leur cinétique de transfert, les mécanismes de phytodisponibilité, phytotoxicité, et les risques pour la santé humaine lors de l’ingestion de végétaux pollués. Ensuite, à travers le cas de potagers urbains en Chine, une étude socio-scientifique a été réalisée afin de proposer des moyens de gestion durable des risques environnement-santé. Les légumes peuvent accumuler des quantités importantes de métaux par absorption foliaire lorsque des PM pénètrent par les stomates. Des PM de PbO et des nano-CuO induisent une forte phytotoxicité (réduction de la biomasse et des échanges gazeux, nécroses). La phytotoxicité n’est pas simplement régie par la concentration totale en métaux car des bio-transformations se produisent et modifient les formes chimiques des métaux. L’analyse par résonance paramagnétique électronique (EPR) a mis en évidence un changement de spéciation du cuivre dans les tissus des feuilles. Par ailleurs, une influence significative de la nature du métal, de l’espèce végétale et du type d’exposition (foliaire/racinaire) sur la bioaccessibilité gastro-intestinale des éléments a été démontrée. A proximité d’un incinérateur de déchets ou d’une autoroute, l’absorption foliaire des PM induit des concentrations élevée en métaux dans les plantes, en plus du transfert sol-plante. Une bioaccessibilité humaine relativement élevée (60-79%) a été mesurée, suggérant un risque potentiel pour la santé en cas de consommation régulière. Les jardins potagers étudiés présentent un risque sanitaire faible (cas de l’incinérateur) ou modéré (cas de l’autoroute) à l'égard de la consommation humaine des légumes étudiés. Mais, une exposition à différents polluants organiques en plus des métaux est souvent possible. Cette thèse souligne l'importance de prendre en compte l'influence de l'atmosphère en plus de la qualité du sol pour estimer la qualité des plantes consommées cultivées en zones anthropisées (fermes et jardins urbains), pour la gestion durable des agricultures urbaines / Ultrafine particles including nanosized enriched with metal(loid)s (PM) are emitted into the atmosphere of industrial or urban areas, these PM can transfer into soil and water ecosystems and have consequences on plant quality and human health. In a global socio-scientific context that regulation on (eco)toxicity of chemicals and public space pressures are recently increased, studies of environmental and health impacts throughout the life cycle of PM are of crucial sanitary concern. The PhD aims first to study metal(loid)s present in the PM: their transfer kinetic and mechanism of phytoavailability, phytotoxicity, and human health risks-ingestion bioaccessibility. Then, through the case of vegetable gardens near an incinerator and a highway in China, a socio-scientific study was performed in order to give suggestions for sustainable environmental and health risk management for these sites. Vegetables can significantly accumulate metal(loid)s by foliar uptake when PM directly enter into leaves through stomata apertures. Ultrafine PbO and nano-CuO particles caused serious phytotoxicity (reduced biomass and gaseous exchange, and necrosis) after interaction with leaf surface. Phytotoxicity of metal(loid)s is not simply governed by their total concentration, but also depended on the potential bio-transformation. Electron paramagnetic resonance (EPR) analysis clearly evidenced copper speciation change in leaf tissues. Moreover, a significant influence of the nature of metal, plant species and the exposure pathways (foliar/root) on gastro-bioaccessibility of metal(loid)s had been demonstrated. For a social-scientific study near waste incinerator and roadside, we found that atmosphere PM fallouts can induce significant metal foliar uptake in addition to soil-plant transfer. The relatively high human bioaccessibility of metal (60-79%) was measured, suggesting a potential health risk in the case of regular consumption of polluted vegetables. Vegetable gardens present a low (waste incinerator) or moderate (highway) health risk with respect to human consumption quantity of the investigated vegetables, but exposure to different organic pollutants in addition to metals is often possible. Our studies highlight the importance of taking atmosphere and soil quality into account for estimating the quality of consumed plants grown in anthropic areas (farms and kitchen gardens), and for sustainable management of urban agricultures.
212

Risk-averse periodic preventive maintenance optimization

Singh, Inderjeet,1978- 21 December 2011 (has links)
We consider a class of periodic preventive maintenance (PM) optimization problems, for a single piece of equipment that deteriorates with time or use, and can be repaired upon failure, through corrective maintenance (CM). We develop analytical and simulation-based optimization models that seek an optimal periodic PM policy, which minimizes the sum of the expected total cost of PMs and the risk-averse cost of CMs, over a finite planning horizon. In the simulation-based models, we assume that both types of maintenance actions are imperfect, whereas our analytical models consider imperfect PMs with minimal CMs. The effectiveness of maintenance actions is modeled using age reduction factors. For a repairable unit of equipment, its virtual age, and not its calendar age, determines the associated failure rate. Therefore, two sets of parameters, one describing the effectiveness of maintenance actions, and the other that defines the underlying failure rate of a piece of equipment, are critical to our models. Under a given maintenance policy, the two sets of parameters and a virtual-age-based age-reduction model, completely define the failure process of a piece of equipment. In practice, the true failure rate, and exact quality of the maintenance actions, cannot be determined, and are often estimated from the equipment failure history. We use a Bayesian approach to parameter estimation, under which a random-walk-based Gibbs sampler provides posterior estimates for the parameters of interest. Our posterior estimates for a few datasets from the literature, are consistent with published results. Furthermore, our computational results successfully demonstrate that our Gibbs sampler is arguably the obvious choice over a general rejection sampling-based parameter estimation method, for this class of problems. We present a general simulation-based periodic PM optimization model, which uses the posterior estimates to simulate the number of operational equipment failures, under a given periodic PM policy. Optimal periodic PM policies, under the classical maximum likelihood (ML) and Bayesian estimates are obtained for a few datasets. Limitations of the ML approach are revealed for a dataset from the literature, in which the use of ML estimates of the parameters, in the maintenance optimization model, fails to capture a trivial optimal PM policy. Finally, we introduce a single-stage and a two-stage formulation of the risk-averse periodic PM optimization model, with imperfect PMs and minimal CMs. Such models apply to a class of complex equipment with many parts, operational failures of which are addressed by replacing or repairing a few parts, thereby not affecting the failure rate of the equipment under consideration. For general values of PM age reduction factors, we provide sufficient conditions to establish the convexity of the first and second moments of the number of failures, and the risk-averse expected total maintenance cost, over a finite planning horizon. For increasing Weibull rates and a general class of increasing and convex failure rates, we show that these convexity results are independent of the PM age reduction factors. In general, the optimal periodic PM policy under the single-stage model is no better than the optimal two-stage policy. But if PMs are assumed perfect, then we establish that the single-stage and the two-stage optimization models are equivalent. / text
213

Assessment of Ambient Air Particulate Matter in the New Orleans Historic District

Portillo, Federico 19 December 2008 (has links)
Particulate matter in the ambient air of down-town New Orleans, LA, is not currently recorded by any government, state, or private organization. This research quantified particulate matter (PM) and its metal concentration in ambient air of the New Orleans Historic District. Both, PM10 (PM less than 10 micron in size) and PM2.5 (PM less than 2.5 micron in size) were quantified by gravimetric analysis using an Andersen Dichotomous Sampler (Series 240). Posterior to gravimetric analysis, the filters were analyzed with a fluorescent test method using an Innov-X Portable XRF Analyzer (Model A-6500). This study demonstrated that the ambient air of the New Orleans Historic District is in compliance with the National Ambient Ai Quality Standards regarding contamination of particulate matter and Lead.
214

Contribution à l'étude des machines électriques en présence de défaut entre-spires : modélisation - Réduction du courant de défaut / Contribution for study of electrical machines with enter-turn faults : modeling reduce of fault current

Vaseghi, Babak 03 December 2009 (has links)
Le principal objectif de nos travaux était l’établissement de modèles suffisamment précis pour étudier le comportement des machines électriques en présence d’un défaut de court-circuit entre spires et d’en déduire les signatures pertinentes pour la détection de ce type de défaut. L’autre objectif était de dimensionner des machines électriques à courant de court-circuit d’amplitude limitée pour réduire le risque de propagation du défaut. La première approche de modélisation consiste à effectuer une étude complète en utilisant la méthode d’éléments finis pas à pas dans le temps. Les résultats obtenus par ce modèle "éléments finis" concernant une MSAP et une MAS, saines et aussi en présence de plusieurs défauts "entre spires" de niveaux de sévérité différents, concordent avec ceux obtenus expérimentalement sur deux bancs d’essai. La seconde approche a consisté à mettre au point un modèle "circuits électriques" dont la complexité dépend du type de structure magnétique et du type de bobinage de la machine étudiée. Nous avons proposé deux méthodes de détermination des paramètres : 1- des méthodes numériques (éléments finis) ; 2- l’établissement des nouvelles expressions analytiques. Dans le dernier chapitre, une méthode basée sur la segmentation des aimants sous un pôle qui n’est en fait qu’une démultiplication du nombre de pôles au rotor sans modification du bobinage statorique est proposée est utilisé pour réduire le courant de défaut / The main objective of this research is to establish the sufficiently precise models to study the behavior of electrical machines in the presence of inter-turn short circuit fault and then find the relevant signatures to detect this type of fault. The other objective is to design a limited short-circuit current electrical machines to reduce the risk of fault development. The first modeling approach is a comprehensive study using the time stepping finite element method. The results obtained by this model "finite element" on a MSAP and MAS, healthy and faulty, for different levels of fault severity, are close with those obtained experimentally by two test benches. The second approach is to develop a model circuit electric, whose complexity depends on the type of magnetic structure and the type of machine winding. We have proposed two methods for determining the model parameters: 1 - numerical methods (FEM) which require long time bur very precise; 2 – establish new analytical expressions which is fast but less precise. In the last part, a method based on segmentation of the magnet is presented in order to reduce the short circuit current. The segmented PM motor contains the reduced fault current and can be used in the application which requires high degree of reliability
215

Contribution à la modélisation des préférences des consommateurs en fonction de dimensions sensorielles et subjectives par les modèles d'équations structurelles.Application aux préférences des sièges conducteurs de véhicules / Contribution to the modelling of consumers' preferences based on sensory and subjective dimensions by structural equations models Application to preferences for automotive driver's seat

Masson, Marine 03 April 2014 (has links)
En Analyse Sensorielle, les préférences des consommateurs sont généralement modélisées en fonction de données sensorielles par les méthodes de cartographie des préférences. L'objectif de cette thèse est de modéliser les préférences des consommateurs en intégrant, en plus des données sensorielles, de nouvelles variables relatives à leur perception des produits. Nous appellerons ces variables les dimensions subjectives. Elles recouvrent des dimensions pragmatiques liées à l'utilisation du produit et des dimensions plus symboliques telles que l'esthétisme, la modernité, l'originalité…Les problématiques relatives aux dimensions subjectives ont d'abord été étudiées lors d'une étude exploratoire sur des tasses à café. L'ensemble du travail a ensuite été réalisé sur 11 sièges de voitures. Dans un premier temps, des entretiens qualitatifs ont été réalisés auprès de 16 consommateurs d'une part et de 2 designers d'autre part. Ces entretiens ont permis d'identifier les dimensions subjectives caractéristiques des sièges. Une évaluation quantitative des dimensions subjectives et des préférences a ensuite été réalisée par 110 consommateurs. Enfin, les sièges ont été caractérisés sensoriellement par des experts. Les préférences des consommateurs ont été modélisées en fonction des données sensorielles et des dimensions subjectives par des modèles d'équations structurelles à variables latentes, plus précisément par Partial Least Square Path Modeling. Quatre modèles, fondés sur les groupes de préférences, ont été mis en place. Selon le groupe étudié, la contribution des deux jeux de données diffère et quatre profils de clients sont identifiés. D'un point de vue méthodologique, ce travail fournit des éléments de réponse sur l'intérêt des dimensions subjectives pour la modélisation des préférences. L'ensemble de la démarche est en cours d'application sur un produit alimentaire : le chocolat. / In Sensory Science, preference mapping is used to explain consumers' preferences with sensory data. This PhD aims to integrate not only sensory data but also new variables that are related to consumers' perception of the product in the modelling of consumers' preferences. These variables are labelled as subjective dimensions. They address the pragmatic dimensions that cover the context of use of the products and more symbolic dimensions, such as aesthetics, modernity, originality…An exploratory study based on coffee cups was a first mean to approach the issues related to subjective dimensions. Then, all the work was done on a study of 11 car seats. The first step consisted in qualitative interviews of 16 consumers and of 2 designers. These interviews allowed identifying the subjective dimensions that characterize car seats. 110 consumers then performed a quantitative evaluation of their preferences and subjective dimensions. Finally, the seats were characterized by experts with sensory descriptors. The consumers' preferences were modelled according to both sensory data and subjective dimensions, using structural equations: the Partial Least Square Path Modeling. Four models based on preferences clustering were established. The contribution of two kinds of data differed according to the considered cluster, which led to the identification of four customer profiles. From a methodological point of view, this work provides first elements about the benefit of subjective dimensions in preference modelling. The methodology is being implemented on a food product: chocolate.
216

Evaluation of the Influence of Non-Conventional Sources of Emissions on Ambient Air Pollutant Concentrations in North Texas

Lim, Guo Quan 08 1900 (has links)
Emissions of air pollutants from non-conventional sources have been on the rise in the North Texas area over the past decade. These include primary pollutants such as volatile organic compound (VOC) and oxides of nitrogen (NOx) which also act as precursors in the formation of ozone. Most of these have been attributed to a significant increase in oil and gas production activities since 2000 within the Barnett Shale region adjacent to the Dallas-Fort Worth metroplex region. In this study, air quality concentrations measured at the Denton Airport and Dallas Hinton monitoring sites operated by the Texas Commission on Environmental Quality (TCEQ) were evaluated. VOC concentration data from canister-based sampling along with continuous measurement of oxides of nitrogen (NOx), ozone (O3), particulate matter (PM2.5), and meteorological conditions at these two sites spanning from 2000 through 2014 were employed in this study. The Dallas site is located within the urban core of one of the fastest growing cities in the United States, while the Denton site is an exurban site with rural characteristics to it. The Denton Airport site was influenced by natural gas pads surrounding it while there are very few natural gas production facilities within close proximity to the Dallas Hinton site. As of 2013, there were 1362 gas pads within a 10 mile radius to the Denton Airport site but there were only 2 within a 10 mile radius to Dallas Hinton site. The Dallas site displayed higher concentrations of NOx and much lower concentrations of VOC than the Denton site. Extremely high levels of VOC measured at the Denton site corresponded with the increase in oil and gas production activities in close proximity to the monitoring site. Ethane and propane are two major contributors to the measured VOC concentration, suggesting the influence of fugitive emissions of natural gas. In Dallas, the mean and maximum values of ozone had decreased since 2000 by about 2% and 25%, respectively. Similarly NOx decreased by 50% and 18% in the mean and maximum values. However, the mean VOC value showed a 21% decrease while the maximum value increased by about 46%. In Denton, the change in percentage of ozone and NOx were similar to Dallas but the mean VOC concentration increased by about 620% while the max value increased 1960%. Source apportionment analysis confirmed the findings by identifying the production of natural gas to be the primary source of VOC emissions in Denton, while traffic sources were more influential near the Dallas site. In light of the recent proposal by EPA to revise the ozone standard, the influence of these new unconventional sources should be further evaluated.
217

Etude et réalisation d'une machine électrique à forte densité de couple et fort rapport de sur-couple pour des applications de traction automobile / Study and realization of an electric machine with high torque density and high transient torque capability for automotive applications

Benlamine, Raouf 07 July 2015 (has links)
Le contexte sociétal et environnemental en France et en Europe durant ces dernières années est largement favorable à l’électrification des moyens de transports, principalement les véhicules, afin de réduire les émissions de CO2. Les pouvoirs publics déploient d’importants efforts afin de rendre les véhicules électrifiés plus accessibles aux citoyens, en octroyant des primes conséquentes lors de l’achat de véhicules « propres ». De leur côté, les constructeurs automobiles cherchent à diminuer le coût des véhicules électriques et hybrides en proposant des solutions techniques pouvant réduire le coût de fabrication des organes, principalement les batteries, les moteurs thermiques et électriques, tout en assurant des performances élevés. L’objectif principal de cette thèse est d’étudier et de réaliser une machine électrique répondant à un cahier des charges très contraignant en termes d’encombrement axial et de densité de couple pour une application comme moteur de traction d’un véhicule hybride. De plus, cette machine doit être innovante, robuste et dont le procédé de fabrication est automatisable et à faible coût. Ainsi, un état de l’art élargi sur les différentes machines électriques a été effectué. En fonction des exigences liées à notre application, une machine à flux axial à aimants permanents enterrés à bobinage concentrique a été choisie.Initialement, un modèle analytique simplifié a été établi et couplé à un outil d’optimisation. Les paramètres géométriques et électriques obtenus ont été alors réajustés en utilisant un modèle numérique basé sur les éléments finis 3D. Différentes modifications ont été apportées à cette machine initiale en raison de l’évolution du cahier des charges des performances et des contraintes géométriques. Les grandeurs électromagnétiques telles que le couple et la puissance ont été analysées pour différents points de fonctionnement. Les pertes dans les aimants permanents ont été calculées en développant un modèle numérique hybride basé sur les différences finies 3D, réduisant ainsi le temps de calcul comparativement aux éléments finis 3D. Afin de valider les différents résultats obtenus, un prototype de la machine a été réalisé. Cette machine a également été modélisée en utilisant les Circuits Equivalents Magnétiques quasi-3D. Ce modèle semi-analytique est générique par rapport aux dimensions géométriques et électriques, avec une discrétisation adaptative. De plus, les effets de denture et de saturation sont pris en compte. Les inductions magnétiques, les flux magnétiques et le couple électromagnétique ont pu être obtenus avec ce modèle, en assurant une bonne précision et un temps de calcul réduit comparativement aux éléments finis 3D. / The societal and environmental context in France and Europe during the last few years has been largely favorable to the electrification of transportation means, mainly vehicles, in order to reduce CO2 emissions. The authorities are working hard to make the electrified vehicles more affordable, by providing substantial premiums during the purchase of “clean” vehicles. For their part, the automakers are looking to reduce the cost of electric and hybrid electric vehicles by offering technical solutions that can reduce the manufacturing cost of the car components, mainly batteries, thermal and electric motors, while ensuring high performances. The main objective of this thesis is to study and realize an electric machine, which satisfies very restrictive specifications in terms of axial size and torque density in order to be used as a traction motor for a hybrid electric vehicle. In addition, this machine must be innovative, with a high reliability, low cost and automated manufacturing process. Thus, a state of art about the various electric machines has been achieved. Depending on the requirements of our application, an axial flux machine with buried permanent magnets and concentrated winding has been selected. Initially, a simplified analytical model has been developed and coupled to an optimization tool. The obtained geometrical and electrical parameters have been adjusted using a numerical model based on the 3D finite element. Various modifications have been applied to the initial machine due to the modification of the geometrical and performance specifications. Electromagnetic performances such as torque and power have been analyzed for various operating points. Losses in the permanent magnets have been calculated using a hybrid numerical 3D model based on the finite difference and finite element, which allows to reduce the computation time compared to transient 3D finite element. In order to validate the different results, a prototype of the machine has been realized. This machine has also been modeled using a quasi-3D magnetic equivalent circuits. This semi-analytical model is generic regarding the geometrical and electrical parameters, with an adaptive discretization. Furthermore, the saturation and the slotting effects have been taken into account. The magnetic flux density, the flux linkage and the electromagnetic torque have been calculated with this model, ensuring high accuracy and reduced time computation compared to 3D finite element.
218

Highly efficient linear CMOS power amplifiers for wireless communications

Jeon, Ham Hee 20 February 2012 (has links)
The rapidly expanding wireless market requires low cost, high integration and high performance of wireless communication systems. CMOS technology provides benefits of cost effectiveness and higher levels of integration. However, the design of highly efficient linear CMOS power amplifier that meets the requirement of advanced communication standards is a challenging task because of the inherent difficulties in CMOS technology. The objective of this research is to realize PAs for wireless communication systems that overcoming the drawbacks of CMOS process, and to develop design approaches that satisfying the demands of the industry. In this dissertation, a cascode bias technique is proposed for improving linearity and reliability of the multi-stage cascode CMOS PA. In addition, to achieve load variation immunity characteristic and to enhance matching and stability, a fully-integrated balanced PA is implemented in a 0.18-m CMOS process. A triple-mode balanced PA using switched quadrature coupler is also proposed, and this work saved a large amount of quiescent current and further improved the efficiency in the back-off power. For the low losses and a high quality factor of passive output combining, a transformer-based quadrature coupler was implemented using integrated passive device (IPD) process. Various practical approaches for linear CMOS PA are suggested with the verified results, and they demonstrate the potential PA design approach for WCDMA applications using a standard CMOS technology.
219

Atributos do vinho, risco percebido e intenção de recompra online : análise do efeito moderador da qualidade do site

Sales, Emanuel das Chagas, 0000-0002-0788-2013 31 October 2017 (has links)
Made available in DSpace on 2018-08-01T23:40:25Z (GMT). No. of bitstreams: 1 tese_11544_Dissertação_Emanuel_Versão_Final20180131-115221.pdf: 2670227 bytes, checksum: 80557f8a62de4b4543889b19e3f60459 (MD5) Previous issue date: 2017-10-31 / Este estudo foi desenvolvido com o objetivo de analisar o efeito da qualidade do site na relação entre os atributos do vinho e o risco percebido em compras de vinhos online e, ainda, analisar o seu efeito na relação entre o risco percebido em compras de vinhos online e a intenção de recompra de vinhos online. O interesse por esse tema se deu, primeiramente, pelo fato de o consumo de vinhos no Brasil ter apresentado um substancial crescimento nos últimos anos, o que se permite supor tratar-se de um segmento de mercado em franca expansão e, ainda, pelo crescente uso da internet como um canal alternativo de compras deste produto, o qual apresenta complexidades peculiares, desde o processo de decisão de compra até o seu consumo. Com base na revisão da literatura, foi elaborado o modelo conceitual deste estudo, assim como o questionário utilizado para verificar se as relações teóricas observadas se confirmariam na amostra selecionada para este trabalho. A coleta de dados foi operacionalizada a partir do envio do questionário, por e-mail, à uma amostra composta por 5.200 possíveis respondentes, todos usuários de um dos maiores e-commerce de vinhos do mundo, tendo sido coletadas 376 respostas, durante o mês de setembro de 2017, as quais foram submetidas às análises multivariadas por meio da técnica da modelagem de equações estruturais, com o auxílio do pacote PLS-PM utilizado no software “R”. Os resultados demonstraram que os atributos do vinho não apresentaram significância estatística na relação com o risco percebido em compras de vinhos online. Por outro lado, foram encontradas evidências de que o risco percebido em compras de vinhos online impacta negativamente a intenção de recompra de vinhos online. Por fim, não foram encontradas evidências que permitissem afirmar o efeito moderador da qualidade do site nas relações observadas neste estudo. Os resultados desta pesquisa foram discutidos e, ao final, apresentaram-se as suas limitações e algumas sugestões para pesquisas futuras. / The purpose of this study was to analyze the effect of site quality on the relationship between wine attributes and perceived risk in online wine purchases, and to analyze its effect on the relationship between perceived risk in online wine purchases and the online wine repurchase intention. The interest in this theme came first because the consumption of wines in Brazil has shown a substantial growth in recent years, assuming that it is a rapidly expanding market segment and due to the increasing use of the internet as an alternative channel of purchases of this product, which presents peculiarities, from the purchase decision process to its consumption. Based on the literature review, the conceptual model of this study was elaborated, as well as the questionnaire used to verify if the theoretical relationships would be confirmed in the sample selected for this study. The data collection were done by sending the questionnaire, by e-mail, to a sample composed of 5,200 potential respondents, all users of one of the largest e-commerce wines in the world. Three hundred and seventy-six responses were collected in September 2017, which were subjected to multivariate analysis using the structural equation modeling technique in the PLS-PM package using software R. The results showed that the attributes of the wine did not present statistical significance in relation to the perceived risk in online wine purchases. On the other hand, there is evidence that the perceived risk in online wine purchases negatively affects the online wine repurchase intention. Finally, no evidence was found to support the moderating effect of site quality on the relationships observed in this study. The results of this research were discussed and, at the end, its limitations and some suggestions for future research are presented.
220

Fabricação de compósitos de matriz metálica da liga de alumínio AA1100 com reforço cerâmico de Óxido de Zinco através de técnicas de metalurgia do pó

LINS, André Emanoel Poroca 28 January 2015 (has links)
Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2017-04-04T13:36:40Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação_Mestrado_(Andre E. Poroca Lins)_Final.pdf: 64666922 bytes, checksum: 584e69803acfdd0b31e2b58094eb9b6c (MD5) / Made available in DSpace on 2017-04-04T13:36:40Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação_Mestrado_(Andre E. Poroca Lins)_Final.pdf: 64666922 bytes, checksum: 584e69803acfdd0b31e2b58094eb9b6c (MD5) Previous issue date: 2015-01-28 / Os materiais com óxido de zinco vêm passando por um rápido desenvolvimento devido as suas potenciais aplicações em uma ampla variedade de áreas tecnológicas, tais como eletrônica, catálise, cerâmica, fotodetectores, sensores, células solares, entre outras. Porém, torna-se fundamental o estudo das propriedades, condições de síntese e aplicações. Um material que vem se destacando devido as suas propriedades mecânicas, elétricas, magnéticas, ópticas e químicas é o óxido de zinco (ZnO). No óxido de zinco tais propriedades dependem principalmente do tamanho e morfologia de suas partículas. O avanço no desenvolvimento de materiais com óxido de zinco vem recebendo bastante destaque no meio científico e se tornando de fundamental importância devido à interdisciplinaridade entre vários campos da ciência, e por permitir a obtenção de novos materiais com melhores propriedades físicas e químicas. O objetivo principal desse trabalho é produzir um compósito de matriz de alumínio AA1100, reforçado com material cerâmico, o óxido de zinco (ZnO), utilizando o processo de metalurgia do pó e técnica de moagem de alta energia. Para tanto, utilizou-se a caracterização por microscopia ótica (MO), microscopia eletrônica de varredura (MEV), difração de raios X (DRX) e difração a laser para avaliar as características do compósito, além de verificar as propriedades mecânicas inerentes e constatar a superioridade em relação a materiais produzidos de forma convencional. No primeiro momento é feita a avaliação da mistura dos pós produzidos, segundo variação percentual do reforço e tempo de processamento, com o objetivo de obter dados iniciais. Em seguida, é feito o processamento das misturas dos pós para compactação e sinterização; visando obter pastilhas do compósito, nas quais serão feitas ensaios e caracterização microestrutural, e por fim avaliação de resultados e conclusões. / The zinc oxide composites materials are undergoing rapid development due to their potential applications in a wide variety of technological areas such as electronics, catalysis, ceramics, photodetectors, sensors, solar cells, among others. However, it is fundamental the studies of the properties, synthesis conditions and applications. A material that has been highlighted due to its mechanical, electrical, magnetic, optical and chemical properties is the zinc oxide (ZnO). In zinc oxide such properties mainly depend on the size and morphology of the particles. The technological progress in the development of materials with zinc oxide has been receiving a lot of attention in the scientific community and becoming of paramount importance due to several interdisciplinary fields of science, and for allowing the obtaining of new materials with improved physical and chemical properties. The main objective of this work is to produce matrix composites of aluminum alloy AA1100, reinforced with ceramic material, the zinc oxide (ZnO), the process of using powder metallurgy technique and high energy milling. For this we used the characterization by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD, and laser diffraction to evaluate and compare the features of composite, besides checking the mechanical properties and see the inherent superiority over conventionally produced material. At first assessment is made of the mixture of powders produced according to percentage change in the reinforcement and processing time, in order to obtain initial data. Then the processing is done mixtures of powders for compaction and sintering, to obtain tablets composite in which are made tests and microstructural characterization, and ultimately evaluating the results and conclusions.

Page generated in 0.0301 seconds