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  • 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.
131

The Effect of Porous Concrete Paving on Underlying Soil Conditions and Growth of Platanus orientalis

Morgenroth, Justin January 2010 (has links)
Urbanisation is characterised by mass migration of people to urban areas and conversion of land from rural to urban land uses. Changes in population dynamics have led to half the world’s population living in urban areas; in developed countries, urban dwellers account for three-quarters of the total population. Though populations have shifted from rural to urban areas, people continue to rely on their environment, and trees in particular, for tangible and intangible benefits alike. A great deal of factual and anecdotal knowledge supports the role of trees for ecological, social, and economic well-being. In spite of this, during urbanisation, previously vegetated land is converted to housing, roads, or utility corridors, all of which are necessary to support growing populations. This thesis investigates tree growth in these modified urban landscapes, in particular, the effects of pavements on urban trees. Pavements are truly pervasive, covering more than half of all land in highly developed urban areas. Their durability and strength are of great importance to transportation, but large-scale soil sealing is not without consequence. Pavements affect the hydrologic cycle, soil and air temperature, and nutrient cycling. Because of their effect on the surrounding environment, pavements inherently affect remnant or planted trees. They are believed to negatively affect tree growth and survival, thereby compromising the ecological, social, and economic benefits otherwise derived from the urban forest. In recent times, porous pavements have been increasingly installed in favour of impervious pavements. Porous pavements are perceived to be an environmentally-sound alternative to standard impervious pavements. This thesis begins by reviewing the literature concerning porous pavement’s effect on underlying soil and urban vegetation, thus illustrating the scarcity of empirical data describing the effect of porous pavement on tree growth. A greater understanding of porous pavement’s impact on the surrounding environment is needed, if its installation is to continue. With this aim in mind, this thesis describes an experiment in Christchurch, New Zealand, which monitored the impacts of porous and impervious pavement on underlying soil conditions, and subsequent tree growth. The experiment comprised 50 Platanus orientalis trees planted in an augmented factorial design, which consisted of controls and four treatments. Trees were split evenly amongst plots, such that ten replicates existed per treatment. The pavement treatments measured 2.3m by 2.3m, and were based on the combination of pavement type (2 levels: porous, impervious) and pavement profile design (2 levels: +/- subbase compaction and gravel base). The resulting four treatments were impervious concrete pavement (IP), impervious concrete pavement with compacted subbase and gravel base (IP+), porous concrete pavement (PP), and porous concrete pavement with compacted subbase and gravel base (PP+). From December 2007 to March 2009, data were collected to determine the effect of these treatments on soil moisture, aeration, pH, and nutrient concentration. Final tree height, stem diameter, shoot and root biomass, and root distribution were also measured at the conclusion of the experiment. Results of this experiment indicated that the effects of pavement porosity on soil moisture and aeration were dynamic, varying with season and soil depth. Increased soil moisture beneath porous pavements resulted from rapid infiltration following precipitation. This decreased the duration of plant stress resulting from drought. Relative to bare soil, paved plots had consistently greater soil moisture, likely because pavements reduced evaporation. The inclusion of a gravel base in the profile design limited capillary upflow, which resulted in lower soil moisture under pavements designed with a gravel base. Soil aeration was significantly lower beneath pavements relative to unpaved plots. This is likely related to greater soil moisture beneath pavements. Finally, soil pH increased beneath pavements, in particular beneath porous pavements. Though all growth parameters increased for trees surrounded by porous, rather than impervious pavement, this occurred only in the absence of a compacted subgrade and gravel base. Evidently, the impact of the compacted subgrade superseded the impact of pavement porosity. Furthermore, root growth was relatively shallow beneath pavements, likely due to favourable soil moisture directly beneath pavements. This research highlights (i) the dramatic effect of pavements on underlying soil conditions; (ii) that pavements do not inherently limit tree growth; (iii) that porous pavements can conditionally improve tree growth; and (iv) that soil compaction limits potential benefits resulting from porous pavements.
132

PROCESSAMENTO DE CERÂMICAS COM POROSIDADE GRADUADA UTILIZANDO AS TÉCNICAS DE FREEZE CASTING E COLAGEM DE BARBOTINA

Carvalho, Gustavo Antoniácomi de 30 January 2018 (has links)
Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2019-02-27T11:49:16Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Gustavo Antoniacomi de Carvalho.pdf: 6310308 bytes, checksum: 8e1efd9d86bc5a1adf80b45bba2a3985 (MD5) / Made available in DSpace on 2019-02-27T11:49:16Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Gustavo Antoniacomi de Carvalho.pdf: 6310308 bytes, checksum: 8e1efd9d86bc5a1adf80b45bba2a3985 (MD5) Previous issue date: 2018-01-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Cerâmicas porosas vêm ganhando visibilidade devido a algumas aplicações tecnológicas interessantes, tais como a utilização em eletrólitos sólidos, ânodos de células a combustível, filtros cerâmicos e reposição óssea e dental. Dentre elas, há especial atenção ao estudo de materiais porosos com porosidade graduada, nos quais a quantidade de poros e a morfologia dos mesmos se alteram pelo volume do material. Nesse trabalho foi realizado o processamento e caracterização de materiais cerâmicos de alumina com porosidade graduada a partir das técnicas de freeze casting e colagem de barbotina, utilizando hidróxido de alumínio e amido de arroz como fases de sacrifício. Após a conformação das amostras por esses métodos, a porosidade foi caracterizada por microscopia eletrônica de varredura, pelas medidas de porosidade aparente feita pelo Princípio de Arquimedes e pela distribuição de tamanho de poros feita pela técnica de porosimetria de mercúrio. Foi avaliada também a resistência mecânica das amostras a partir de ensaio de compressão. Foi confirmada a relação entre as amostras processados isoladamente e suas respectivas camadas nas amostras graduadas. Foi observada também uma boa interação interfacial entre cada uma das camadas das amostras graduadas. A porosidade das amostras com porosidade graduada se manteve próxima do esperado, o valor esperado foi determinado a partir da média das amostras processadas isoladamente em relação às camadas do material com porosidade graduada. O ensaio mecânico demonstrou que não houve influência das interfaces dos materiais graduados na sua resistência à compressão. / Porous ceramics have been gaining visibility due to some interesting technological applications, such as its use as solid electrolytes, fuel cell anodes, ceramic filters and bone and dental reposition. Among them, there is special care in studying graded porosity materials, where the quantity of pores and pore morphology changes through the material volume. In this work the processing and characterization of alumina ceramic materials with functionally graded porosity by freeze casting and slip casting techniques using aluminum hydroxide and rice starch as sacrificial template was performed. After conformation, the porosity was characterized through electron scanning microscopy, apparent porosity through Archimedes method and median pore size through mercury porosimetry. The mechanical resistance was also obtained by compression testing. The analysis allowed to confirm the relation between each of the isolated samples’ microstructure and its respective layer in each of the graded materials, also, the graded materials shown good interfacial interaction between each of the layers. The porosity in graded materials kept close to the expected value, which was determined by the medium value of the porosities of the isolated samples respective to the graded material. Mechanical testing shown that there was no influence of the graded material interfaces in its compressive strength.
133

PI-equivalência em álgebras graduadas simples

Naves, Fernando Augusto 29 February 2016 (has links)
Submitted by Luciana Sebin (lusebin@ufscar.br) on 2016-10-11T13:29:51Z No. of bitstreams: 1 DissFAN.pdf: 767462 bytes, checksum: 05054cc8952eed4e120838068aee80d8 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2016-10-17T18:50:57Z (GMT) No. of bitstreams: 1 DissFAN.pdf: 767462 bytes, checksum: 05054cc8952eed4e120838068aee80d8 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2016-10-17T18:51:05Z (GMT) No. of bitstreams: 1 DissFAN.pdf: 767462 bytes, checksum: 05054cc8952eed4e120838068aee80d8 (MD5) / Made available in DSpace on 2016-10-17T19:06:01Z (GMT). No. of bitstreams: 1 DissFAN.pdf: 767462 bytes, checksum: 05054cc8952eed4e120838068aee80d8 (MD5) Previous issue date: 2016-02-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / This work aims to give a description, under certain hypothesis, of the graded simple algebras and prove that they are determined by their graded identities. For this, we study the papers [3] and [19]. More precisely we will show the following: Let G be a group, F an algebraically closed eld, and R = L g2G Rg a finite dimensional G-graded F-algebra such that the order of each finite subgroup of G is invertible in F. Then R is a G-graded simple algebra if and only if R is isomorphic, as graded algebra, to the tensor product C = Mn(F) F [H], where H is a nite subgroup of G, is a 2-cocycle in H, Mn(F) has an elementary G-grading, F [H] has a canonical grading and C has an induced G-grading by the tensor product. Based on this result, admitting the same assumptions and adding that G is an abelian group, we prove that two graded simple algebras satisfy the same graded identities if and only if they are isomorphic as graded algebras. / Este trabalho tem por objetivo dar uma descrição, sob certas hipóteses, das álgebras graduadas simples e demonstrar que elas são determinadas por suas identidades graduadas. Para isso, estudamos os artigos [3] e [19]. Precisamente mostraremos o seguinte: sejam G um grupo, F um corpo algebricamente fechado e R =Lg2GRg uma F-álgebra G-graduada de dimensão finita, tal que a ordem de todo subgrupo finito de G e invertível em F. Então R é uma álgebra G-graduada simples se, e somente se, R é isomorfa, como álgebra graduada, ao produto tensorial C = Mn(F) F[H], onde H e subgrupo finito de G, e um 2-cociclo em H, Mn(F) tem uma graduação elementar, F[H] tem uma graduação canônica e considera-se em C a G-graduação induzida pelo produto tensorial. Partindo deste resultado, admitindo as mesmas hipóteses e adicionando que G seja um grupo abeliano, provaremos que duas álgebras graduadas simples satisfazem as mesmas identidades graduadas se, e somente se, são isomorfas como álgebras graduadas.
134

The adoption of laser melting technology for the manufacture of functionally graded cobalt chrome alloy femoral stems

Hazlehurst, Kevin Brian January 2014 (has links)
Total Hip Arthroplasty (THA) is an orthopaedic procedure that is performed to reduce pain and restore the functionality of hip joints that are affected by degenerative diseases. The outcomes of THA are generally good. However, the stress shielding of the periprosthetic femur is a factor that can contribute towards the premature loosening of the femoral stem. In order to improve the stress shielding characteristics of metallic femoral stems, stiffness configurations that offer more flexibility should be considered. This research has investigated the potential of more flexible and lightweight cobalt chromium molybdenum (CoCrMo) femoral stems that can be manufactured using Selective Laser Melting (SLM). Square pore cellular structures with compressive properties that are similar to human bone have been presented and incorporated into femoral stems by utilising fully porous and functionally graded designs. A three dimensional finite element model has been developed to investigate and compare the load transfer to the periprosthetic femur when implanted with femoral stems offering different stiffness configurations. It was shown that the load transfer was improved when the properties of the square pore cellular structures were incorporated into the femoral stem designs. Factors affecting the manufacturability and production of laser melted femoral stems have been investigated. A femoral stem design has been proposed for cemented or cementless fixation. Physical testing has shown that a functionally graded stem can be repeatedly manufactured using SLM, which was 48% lighter and 60% more flexible than a traditional CoCrMo prosthesis. The research presented in this thesis has provided an early indication of utilising SLM to manufacture lightweight CoCrMo femoral stems with levels of flexibility that have the potential to reduce stress shielding in the periprosthetic femur.
135

Extensive Reading in Spanish: How Much Vocabulary Do Students Need To Know

Kirk, Sadler 03 May 2017 (has links)
This study analyzes ten beginning-level, Spanish-language graded readers in an attempt to answer the question of how much vocabulary do students need to know before being able to successfully participate in an extensive reading program. The study also analyzes the vocabulary taught in a beginning-level Spanish textbook to determine how well this vocabulary reflects the most common words in Spanish according to a frequency dictionary. It was found that a lexicon of 5,000 words was needed to obtain a sufficient level of comprehension in order for extensive reading to occur and that the textbook examined in this study did not present vocabulary that was extremely reflective of the most common words in the Spanish language.
136

Controle da fissuração em compósitos com fibras orgânicas aplicando conceito de materiais com gradação funcional. / Control of cracking in fiber cement apply concepts of functionally graded materials.

Giordano, Brunoro Leite 09 December 2011 (has links)
O objetivo deste trabalho é controlar a incidência de fissuras em fibrocimentos aplicando o conceito de materiais com gradação funcional através da protensão química gerada pela aplicação de silicato de sódio alcalino entre as camadas dos fibrocimentos. Atualmente é bastante comum os fibrocimentos apresentarem fissuras ao longo das bordas devido aos gradientes de umidade gerados durante a estocagem das pilhas de telhas no pátio das indústrias. O potencial da protensão química foi avaliado através da porosidade total, da quantificação das fases hidratadas, da retração por secagem e do desempenho mecânico. A aplicação de silicato de sódio alcalino no ligante CPII F provocou retração por secagem 1,5 vezes maior que a referência aos 91 dias. O módulo de ruptura (MOR) não sofreu alteração, mas o limite de proporcionalidade da matriz (LOP) aumentou em torno de 95%. O módulo de elasticidade dinâmico foi 13 % maior. O aumento da retração por secagem e o ganho de desempenho mecânico apontam o potencial da protensão química para o controle da fissuração em fibrocimentos produzidos pelo processo Hatschek. / The objective of this work is controlling the incidence of cracks in fiber cement, using the concept of functionally graded materials through the chemical prestressing, generated by application of alkaline sodium silicate among fiber cement layers. Currently, its very common the fiber cements present cracks along the edges due to moisture gradients, caused during storage of piles of tiles in the courtyard of the industry. The chemical prestressing potential was evaluated through of the total porosity, the quantification of hydrate phases, the drying shrinkage and the mechanical performance. The application of alkaline sodium silicate in the cement CPII F caused drying shrinkage 1,5 times greater than the reference to 91 days. The modulus of rupture (MOR) didnt suffer change, but the proportional limit of matrix (LOP) increased by around 95%. The dynamic modulus of elasticity was 13% higher. The increase of drying shrinkage and the mechanical performance gain indicate the chemical prestressing potential to control the cracking in fiber cement produced by the process Hatschek.
137

Proposta de classes de resistência para peças estruturais de madeira: Eucalyptus urograndis, Pinus taeda e Schizolobium amazonicum (Paricá) / Proposal for strength classes for structural timber: Eucalyptus urograndis, Pinus taeda e Schizolobium amazonicum (Paricá)

Moritani, Fabiana Yukiko 19 June 2018 (has links)
A madeira de floresta plantada se destaca no setor florestal e da construção civil, diante das restrições para o uso em larga escala das espécies tropicais. Devido ao rápido crescimento e as características inerentes às árvores, as propriedades mecânicas de espécies de floresta plantada são influenciadas pelas características de secagem e de crescimento. A norma brasileira ABNT NBR 7190: 1997 está em fase de revisão e, dentre os itens discutidos pela comissão científica estão os métodos de classificação visual e mecânica e a caracterização de resistência para peças em dimensões estruturais. O trabalho teve como objetivo propor classes de resistência de três espécies de madeira de floresta plantada (Eucalyptus urograndis, Pinus taeda e Schizolobium amazonicum (Paricá)) para uso estrutural. Foram realizados os ensaios de classificação visual e mecânica para peças estruturais e os ensaios de caracterização de resistência para que fosse possível estimar as propriedades mecânicas por meio da classe de resistência atribuída na classificação visual e mecânica. Os resultados foram testados estatisticamente para avaliar as propriedades mecânicas das classes de resistência de cada espécie e compará-las com as normas estrangeira e internacional. Com base nos resultados obtidos e nas análises estatísticas realizadas, foram propostas três classes de resistência para o Eucalyptus urograndis, duas classes de resistência para o Schizolobium amazonicum (Paricá) e para o Pinus taeda. / Timber from reforestation species are been used for civil construction, considering the restrictions on the large-scale use of tropical species. The mechanical properties of reforestation species are influenced by the drying and growth characteristics, due to the fast growth and the inherent characteristics of the trees. The Brazilian standard ABNT NBR 7190: 1997 is under review and among the items is being discussed by the scientific committee are the methods of visual and mechanical grading and the full-size testing for structural elements. The aim of this work was to propose strength classes of three timber species from reforestation species (Eucalyptus urograndis, Pinus taeda and Schizolobium amazonicum - Paricá) for structural use. Grading tests based on the visual and mechanical characteristics were performed and compared with the strength and stiffness properties making possible to estimate the mechanical properties by of the strength classes assigned in the visual and mechanical grading. The results were statistically tested to evaluate the mechanical properties for the strength classes of each species and to compare them with the foreign and international standards. According to the results obtained and the statistical analysis three strength classes were proposed for Eucalyptus urograndis, two strength classes for Schizolobium amazonicum (Paricá) and Pinus taeda.
138

Modelagem, simulação e fabricação de circuitos analógicos com transistores SOI convencionais e de canal gradual operando em temperaturas criogênicas. / Modeling, simulation and fabrication of analog circuits with standard and graded-channel SOI transistors operating at cryogenic temperatures.

Souza, Michelly de 16 October 2008 (has links)
Neste trabalho apresentamos a análise do comportamento analógico de transistores MOS implementados em tecnologia Silício sobre Isolante (SOI), de canal gradual (GC) e com tensão mecânica aplicada ao canal, operando em baixas temperaturas (de 380 K a 90 K), em comparação com dispositivos SOI convencionais. Este estudo foi realizado utilizando-se medidas experimentais de transistores e pequenos circuitos fabricados, bem como através da utilização de simulações numéricas bidimensionais e modelos analíticos. No caso dos transistores de canal gradual, inicialmente foi proposto um modelo analítico contínuo para a simulação da corrente de dreno em baixas temperaturas. Este modelo foi validado para temperaturas entre 300 K e 100 K e incluído na biblioteca de modelos de um simulador de circuitos. Foram analisadas características importantes para o funcionamento de circuitos analógicos, tais como a distorção harmônica de dispositivos operando em saturação e o descasamento de alguns parâmetros, como tensão de limiar e corrente de dreno, em diversas temperaturas. No caso da distorção, foi verificada uma melhora significativa promovida pela utilização da estrutura de canal gradual, ultrapassando 20 dB em 100 K. O descasamento apresentou piora em relação ao transistor convencional, devido a imperfeições de alinhamento que podem ocorrer no processo de fabricação, principalmente na etapa de definição da região fracamente dopada do canal. Foi observada uma piora de até 2,5 mV na variação da tensão de limiar e mais de 2% na corrente de dreno, em temperatura ambiente, em relação ao transistor uniformemente dopado. O impacto da utilização de transistores GC SOI em espelhos de corrente e amplificadores dreno comum também foi também avaliado. Os resultados experimentais mostraram que a estrutura de canal gradual é capaz de promover a melhora no desempenho destes dois blocos analógicos em comparação com transistores uniformemente dopados em todo o intervalo de temperaturas estudado. Amplificadores dreno comum com ganho praticamente constante e próximo do limite teórico e espelhos de corrente com precisão de espelhamento superior àquela apresentada por transistores convencionais, com maior excursão do sinal de saída e maior resistência de saída, foram obtidos. Foram também comparadas características analógicas de transistores SOI com tensão mecânica uniaxial e biaxial agindo sobre o canal em função da temperatura. Os resultados obtidos indicam que a tensão mecânica sobre o canal resulta em ganho de tensão melhor, ou no mínimo igual, àquele obtido com um transistor convencional com as mesmas dimensões e tecnologia. / In this work an analysis of the analog behavior of MOS transistors implemented in Silicon-on-Insulator technology, with graded-channel (GC) and mechanical strain applied to the channel, operating at low temperatures (from 380 K down to 90 K), in comparison to standard SOI devices is presented. This study has been carried out by using experimental measurements of transistors and small circuits, as well as through two-dimensional numerical simulations and analytical models. In the case of graded-channel transistors, an analytical model for the simulation of the drain current at low temperatures has been initially proposed. This model has been validated from 300 K down to 100 K and included to the models library of a circuit simulator. Important characteristics for analog circuits have been evaluated, namely the harmonic distortion of devices biased in saturation regime and the mismatching of parameters like the threshold voltage and the drain current, at several temperatures. Regarding the distortion, it has been verified a significant improvement due to the use of the graded-channel architecture, which reached more than 20 dB at 100 K. The matching has been worsened in comparison to standard transistor, due to misalignements that may take place in the devices processing, mainly in the definition of the lightly doped region in the channel. It has been observed a worsening of up to 2.5 mV in the threshold voltage variation and more than 2 % in the drain current, at room temperature, in comparison to the uniformly doped device. The impact of the application of GC transistors in current mirrors and commondrain amplifiers has been also evaluated. The experimental results showed that the graded-channel structure is able to provide improved performance of these analog blocks in comparison with uniformly doped transistors in the entire studied range of temperatures. Commom-drain amplifiers with virtually constant gain, close to the theoretical limit and current mirrors with improved mirroring precision in comparison to standard transistors, with increased output swing and output resistance have been obtained. Analog characteristics of SOI transistors with uniaxial and biaxial mechanical strain in the channel have been also compared as a function of the temperature. The analysis of experimental measurements indicates that the use of mechanical strain results in better or, at least, similar voltage gain than stardard transistors, for the same dimensions and technology.
139

Modelagem, simulação e fabricação de circuitos analógicos com transistores SOI convencionais e de canal gradual operando em temperaturas criogênicas. / Modeling, simulation and fabrication of analog circuits with standard and graded-channel SOI transistors operating at cryogenic temperatures.

Michelly de Souza 16 October 2008 (has links)
Neste trabalho apresentamos a análise do comportamento analógico de transistores MOS implementados em tecnologia Silício sobre Isolante (SOI), de canal gradual (GC) e com tensão mecânica aplicada ao canal, operando em baixas temperaturas (de 380 K a 90 K), em comparação com dispositivos SOI convencionais. Este estudo foi realizado utilizando-se medidas experimentais de transistores e pequenos circuitos fabricados, bem como através da utilização de simulações numéricas bidimensionais e modelos analíticos. No caso dos transistores de canal gradual, inicialmente foi proposto um modelo analítico contínuo para a simulação da corrente de dreno em baixas temperaturas. Este modelo foi validado para temperaturas entre 300 K e 100 K e incluído na biblioteca de modelos de um simulador de circuitos. Foram analisadas características importantes para o funcionamento de circuitos analógicos, tais como a distorção harmônica de dispositivos operando em saturação e o descasamento de alguns parâmetros, como tensão de limiar e corrente de dreno, em diversas temperaturas. No caso da distorção, foi verificada uma melhora significativa promovida pela utilização da estrutura de canal gradual, ultrapassando 20 dB em 100 K. O descasamento apresentou piora em relação ao transistor convencional, devido a imperfeições de alinhamento que podem ocorrer no processo de fabricação, principalmente na etapa de definição da região fracamente dopada do canal. Foi observada uma piora de até 2,5 mV na variação da tensão de limiar e mais de 2% na corrente de dreno, em temperatura ambiente, em relação ao transistor uniformemente dopado. O impacto da utilização de transistores GC SOI em espelhos de corrente e amplificadores dreno comum também foi também avaliado. Os resultados experimentais mostraram que a estrutura de canal gradual é capaz de promover a melhora no desempenho destes dois blocos analógicos em comparação com transistores uniformemente dopados em todo o intervalo de temperaturas estudado. Amplificadores dreno comum com ganho praticamente constante e próximo do limite teórico e espelhos de corrente com precisão de espelhamento superior àquela apresentada por transistores convencionais, com maior excursão do sinal de saída e maior resistência de saída, foram obtidos. Foram também comparadas características analógicas de transistores SOI com tensão mecânica uniaxial e biaxial agindo sobre o canal em função da temperatura. Os resultados obtidos indicam que a tensão mecânica sobre o canal resulta em ganho de tensão melhor, ou no mínimo igual, àquele obtido com um transistor convencional com as mesmas dimensões e tecnologia. / In this work an analysis of the analog behavior of MOS transistors implemented in Silicon-on-Insulator technology, with graded-channel (GC) and mechanical strain applied to the channel, operating at low temperatures (from 380 K down to 90 K), in comparison to standard SOI devices is presented. This study has been carried out by using experimental measurements of transistors and small circuits, as well as through two-dimensional numerical simulations and analytical models. In the case of graded-channel transistors, an analytical model for the simulation of the drain current at low temperatures has been initially proposed. This model has been validated from 300 K down to 100 K and included to the models library of a circuit simulator. Important characteristics for analog circuits have been evaluated, namely the harmonic distortion of devices biased in saturation regime and the mismatching of parameters like the threshold voltage and the drain current, at several temperatures. Regarding the distortion, it has been verified a significant improvement due to the use of the graded-channel architecture, which reached more than 20 dB at 100 K. The matching has been worsened in comparison to standard transistor, due to misalignements that may take place in the devices processing, mainly in the definition of the lightly doped region in the channel. It has been observed a worsening of up to 2.5 mV in the threshold voltage variation and more than 2 % in the drain current, at room temperature, in comparison to the uniformly doped device. The impact of the application of GC transistors in current mirrors and commondrain amplifiers has been also evaluated. The experimental results showed that the graded-channel structure is able to provide improved performance of these analog blocks in comparison with uniformly doped transistors in the entire studied range of temperatures. Commom-drain amplifiers with virtually constant gain, close to the theoretical limit and current mirrors with improved mirroring precision in comparison to standard transistors, with increased output swing and output resistance have been obtained. Analog characteristics of SOI transistors with uniaxial and biaxial mechanical strain in the channel have been also compared as a function of the temperature. The analysis of experimental measurements indicates that the use of mechanical strain results in better or, at least, similar voltage gain than stardard transistors, for the same dimensions and technology.
140

Lie infini-algébroides et feuilletages singuliers / Lie infinity-algebroids and singular foliations

Lavau, Sylvain 04 November 2016 (has links)
On dit qu'une variété est feuilletée lorsqu'il existe une partition de celle-ci en sous-variétés immergées. La théorie des feuilletages a des applications très profondes dans divers champs des Mathématiques et de la Physique, et il semble d'autant plus intéressant de pouvoir analyser le feuilletage à partir de ce qui semble être une donnée plus fondamentale : sa distribution de champs de vecteurs associée. C'est ainsi que nous avons observé que si le feuilletage est résolu par un fibré gradué, on peut relever le crochet de Lie des champs de vecteurs en une structure de Lie infini-algébroide sur ce fibré. D'autre part, cette structure est universelle dans le sens où toute autre résolution du feuilletage sera isomorphe à celle-ci dans un sens L_infini, mais seulement à homotopie près. Lorsqu'on se limite à l'étude au dessus d'un point, on observe que la cohomologie associée à la résolution devient potentiellement non triviale. La structure de Lie infini-algébroide universelle se réduit alors à une algèbre de Lie graduée sur cette cohomologie. Cette structure algébrique peut être transportée (non canoniquement) tout le long de la feuille, transformant la cohomologie au dessus d'une feuille en algébroide de Lie gradué. Cela nous permet de retrouver des résultats déjà connus par ailleurs et de déduire des avancées prometteuses / A smooth manifold is said to be foliated when it is partitioned into immersed submanifolds. Foliation Theory has profound applications in various fields of Mathematics and Physics, and it seems much more interesting to analyze the foliation from what seems to be a more fundamental point of view: its associated distribution of vector fields. Thus we have noticed that if the foliation is resolved by a graded fiber bundle, one can lift the Lie bracket of vector fields into a Lie infinity-algebroid structure on this fiber bundle. Moreover, this structure is universal in the sense that any other resolution of the foliation is isomorphic to it in the L_infinity setup, but only up to homotopy. When one restricts the analysis over a point, we observe that the cohomology associated to the resolution may become non trivial. The universal Lie infinity-algebroid structure hence reduces to a graded Lie algebra structure on this cohomology. This algebraic structure can be carried (non canonically) along the leaf, providing the cohomology over a leaf with a graded Lie algebroid structure. This enables us to retrieve former well-known results, as well as promising advances

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