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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.
1

Control of Nanoscale Thermal Transport for Thermoelectric Energy Conversion and Thermal Rectification

Pal, Souvik 18 December 2013 (has links)
Materials at the nanoscale show properties uniquely different from the bulk scale which when controlled can be utilized for variety of thermal management applications. Different applications require reduction, increase or directional control of thermal conductivity. This thesis focuses on investigating thermal transport in two such application areas, viz., 1) thermoelectric energy conversion and 2) thermal rectification. Using molecular dynamics simulations, several methods for reducing of thermal conductivity in polyaniline and polyacetylene are investigated. The reduction in thermal conductivity leads to improvement in thermoelectric figure of merit. Thermal diodes allow heat transfer in one direction and prevents in the opposite direction. These materials have potential application in phononics, i.e., for performing logic calculations with phonons. Rectification obtained with existing material systems is either too small or too difficult to implement. In this thesis, a more useful scheme is presented that provides higher rectification using a single wall carbon nanotube (SWCNT) that is covalently functionalized near one end with polyacetylene (PA). Although several thermal diodes are discussed in literature, more complex phononic devices like thermal logic gates and thermal transistors have been sparingly investigated. This thesis presents a first design of a thermal AND gate using asymmetric graphene nanoribbon (GNR) and characterizes its performance. / Ph. D.
2

Experiments on the Thermal, Electrical, and Plasmonic Properties of Nanostructured Materials

Myers, Kirby 29 June 2018 (has links)
Nanofabrication techniques continue to advance and are rapidly becoming the primary route to enhancement for the electrical, thermal, and optical properties of materials. The work presented in this dissertation details fabrication and characterization techniques of thin films and nanoparticles for these purposes. The four primary areas of research presented here are thermoelectric enhancement through nanostructured thin films, an alternative frequency-domain thermoreflectance method for thin film thermal conductivity measurement, thermal rectification in nanodendritic porous silicon, and plasmonic enhancement in silver nanospheroids as a reverse photolithography technique. Nanostructured thermoelectrics have been proposed to greatly increase thermopower efficiency and to bring thermoelectrics to mainstream power generation and cooling applications. In our work, thermoelectric thin films of SbTe, BiTe, and PbTe grown by atomic layer deposition and electrochemical atomic layer deposition were characterized for enhanced performance over corresponding bulk materials. Seebeck coefficient measurements were performed at temperatures ranging from 77 K to 380 K. Atomic composition was verified by energy-dispersive X-ray spectroscopy and structures were imaged by scanning electron microscopy. All thin films measured were ultimately found to have a comparable or smaller Seebeck coefficient to corresponding materials made by conventional techniques, likely due to issues with the growth process. Frequency-domain thermoreflectance offers a minimally invasive optical pump-probe technique for measuring thermal conductivity. Like time-domain thermoreflectance, the version of frequency-domain thermoreflectance demonstrated here relies on a non-zero thermo-optic coefficient in the sample, but uses moderate cost continuous wave lasers modulated at kHz or MHz frequencies rather than a more expensive ultrafast laser system. The longer timescales of these frequency ranges enables this technique to take measurements of films with thicknesses ranging from 100 nm to 10 um, complimentary to time-domain thermoreflectance. This method differentiates itself from other frequency-domain methods in that it is also capable of simultaneous independent measurements of both the in plane and out of plane values of the thermal conductivity in anisotropic samples through relative reflective magnitude, rather than phase, measurements. We validated this alternate technique by measuring the thermal conductivity of Al2O3 and soda-lime and found agreement both with literature values and with separate measurements obtained with a conventional time-domain thermoreflectance setup. Thermal rectification has the potential to enhance microcircuit performance, improve thermoelectric efficiency, and enable the creation of thermal logic circuits. Passive thermal rectification has been proposed to occur in geometrically asymmetric nanostructures when heat conduction is dominated by ballistic phonons. Here, nanodendritic structures with branch widths of ~ 10 nm and lengths of ~ 20 nm connected to ~ 50 um long trunks were electrochemically etched from <111> silicon wafers. Thermal rectification measurements were performed at temperatures ranging from 80 K to 250 K by symmetric thermal conductivity measurements. No thermal rectification was ultimately found in these samples within the margin of thermal conductivity measurement error 1%. This result is consistent with another study which found thermal rectification with greater conduction in the direction opposite to what ballistic phonon heat conduction theories predicted. Plasmonic resonance concentrates incident photon energy and enables channeling of that energy into sub-wavelength volumes where it can be used for nanoscale applications. We demonstrated that surface plasmon polaritons induced in silver nanosphereoid films by 532 nm light defunctionalize previously photocleaved ligands adsorbed onto the films, to yield a reverse photolithographic technique. In this method, gold nanosphere conjugation were conjugated to a photocleaved ligand, however conjugation could be inhibited by exposing the cleaved ligand to 532 nm light and consequently yield a reversal technique. This defunctionalizion effect did not occur on gold films or nanoparticles conjugated with the ligand in IR spectroscopy, and was observed to have a reduced effect in silver films relative to silver nanospheroid film. As silver nanospheroid films and gold nanospheres of the size used in this study are known to have plasmon resonance in the green wavelengths, while gold and silver continuous films do not, this defunctionalization likely results from plasmonic effects. / Ph. D.
3

Interplay of Finite Size and Strain on Thermal Conduction

Majdi, Tahereh January 2019 (has links)
Since strain changes the interatomic spacing of matter and alters electron and phonon dispersion, an applied strain ϵ can modify the thermal conductivity κ of a material. This thesis shows how the strain induced by heteroepitaxy is a passive mechanism to change κ in a thin film and how the film thickness is key to the functional form of κ(ϵ). Molecular Dynamics simulations of the physical vapor deposition and epitaxial growth of ZnTe thin films provide insights into the role of interfacial strain on the thermal conductivity of a deposited film. ZnTe films grown on a lattice mismatched CdTe substrate exhibit ~6% in-plane biaxial tension and ~7% out-of-plane uniaxial compression. In the T=700 K to 1100 K temperature range, the conductivities of strained ZnTe layers that are 5 unit cells thick decrease by ~ 35%, a result that is relevant to thermoelectric devices since strain can also enhance charge mobility and increase their overall efficiency. The resulting understanding of dκ/dT shows that strain engineering can also be used to create a thermal rectifier in a material that is partly strained and partly relaxed, like at the junction of an axial nanowire heterostructure. To better isolate the role of strain, the study is extended to free-standing ZnTe films with thicknesses between 116 Å to 1149 Å under the application of both uniform and biaxial strain between -3% to 3% at 300 K. Since the boundaries of the film are diffuse, κ becomes size dependent when the film thickness approaches the order of the mean free path of the phonons. As this thickness is decreased, the magnitude of κ decreases until boundary scattering dominates so that κ(ϵ) depends on v_g (ϵ). This conclusion is important as it can be generalized to other materials and potential functions; it suggests that if a film is thin enough for boundary scattering to dominate, then the behavior of κ(ϵ) can be predicted based on the bulk dispersion curve alone, which should greatly simplify strain-based device design. / Thesis / Doctor of Philosophy (PhD) / Since strain changes the interatomic spacing of matter and alters electron and phonon dispersion, an applied strain ϵ can modify the thermal conductivity κ of a material. This thesis shows how the strain induced by heteroepitaxy is a passive mechanism to change κ in a thin film and how the film thickness is key to the functional form of κ(ϵ). Molecular Dynamics simulations of the physical vapor deposition and epitaxial growth of ZnTe thin films provide insights into the role of interfacial strain on the thermal conductivity of a deposited film. The result is relevant to thermoelectric devices since strain can also enhance charge mobility and increase their overall efficiency. The resulting understanding of dκ/dT shows that strain engineering can also be used to create a thermal rectifier in a material that is partly strained and partly relaxed, like at the junction of an axial nanowire heterostructure.
4

Propriedades tecnológicas da madeira termorretificada de três espécies de Eucalyptus / Technological properties of thermally modified wood from three Eucalyptus species

Cademartori, Pedro Henrique Gonzalez de, Cademartori, Pedro Henrique Gonzalez de 17 December 2012 (has links)
Made available in DSpace on 2014-08-20T13:34:25Z (GMT). No. of bitstreams: 1 Dissertacao Pedro Henrique Gonzalez de Cademartori.pdf: 2886719 bytes, checksum: 352a362f9db748d742e87be22ddaa1b0 (MD5) Previous issue date: 2012-12-17 / This study aimed to evaluate physicomechanical and chemical behaviour of Eucalyptus grandis, Eucalyptus saligna e Eucalyptus cloeziana thermally modified wood. To achieve this, six trees of each species were randomly selected and samples measuring 1.6 x 1.6 x 25cm were prepared. The samples were thermally modified through exposure to nine treatments in an oven and in an autoclave steriliser. The process conditions in the oven were: temperatures between 180 and 240ºC and time of exposure of 4h, whereas for the treatments in autoclave a constant temperature (127°C) and pressure (1.5kgf/cm²) were used for 1h. The evaluation of the physical properties was done through the performance of weight loss, equilibrium moisture content, stability dimensional and specific gravity tests. Colour changes and wettability were measured by colorimetric and contact angle techniques, respectively. Regarding mechanical properties, thermally modified wood was evaluated through the performance of static bending tests and non-destructive ultrasonic tests. Chemical modifications were qualitatively measured through infrared spectroscopy (ATR-IR). The main results showed that thermal treatments significantly influenced weight loss, equilibrium moisture content and dimensional stability. On the other hand, specific gravity did not show a great behaviour in order to explain the influence of thermal treatments on wood. A gradual darkening in both radial and tangential sections was observed, mainly due to a high reduction of L* after treatment 2 (180°C), since a* and b* showed distinct behaviour. Generally, stiffness remained constant when related to the control treatment, whereas mechanical strength decreased significantly, mainly after the treatments 6, 7, 8 and 9. The use of the non-destructive ultrasound technique obtained coefficients of determination between 0.66 and 0.80 for the relationship MOE x Ed. The wettability of thermally modified wood presented the best results in the treatments 2, 3, 4 and 5 for all the species, with the contact angle increasing and remaining with high stability as a function of time of exposure. Infrared spectroscopy showed modifications in different peaks, mainly related to the hemicelluloses and crystallinity of the structure, representing thermal degradation of the material as a function of temperature of treatment. Therefore, the thermal treatments reduced hygroscopicity and modified the colour of the wood. The mechanical strength was reduced significantly and the ultrasound was efficient in order to estimate the modulus of elasticity. Wettability decreased significantly as a function of the thermal treatments and infrared spectroscopy (ATR-IR) demonstrated to be an excellent tool for the qualitative analysis of chemical modifications due to thermal treatments. / O presente estudo objetivou avaliar o comportamento físico, mecânico e químico das madeiras de Eucalyptus grandis, Eucalyptus saligna e Eucalyptus cloeziana submetidas a diferentes tratamentos de termorretificação. Para tal, selecionaram-se ao acaso seis árvores de cada espécie para a confecção de corpos de prova com dimensões de 1,6 x 1,6 x 25cm. Submeteram-se os corpos de prova a nove tratamentos de termorretificação a partir da utilização de uma estufa e uma autoclave de esterilização. As condições de processo para os tratamentos em estufa foram temperatura entre 180 e 240°C e tempo de exposição de 4h, enquanto que a temperatura dos tratamentos em autoclave permaneceu constante em 127°C e pressão de 1,5 kgf/cm² durante 1h. A avaliação das propriedades físicas foi realizada por meio de ensaios de perda de massa, teor de umidade de equilíbrio, estabilidade dimensional e massa específica. Verificaram-se as alterações na coloração e molhabilidade por meio das técnicas de colorimetria e ângulo de contato, respectivamente. Mecanicamente, o material foi avaliado por meio de ensaios de flexão estática e ultrassom. As modificações químicas foram avaliadas qualitativamente por espectroscopia no infravermelho (ATR-IR). Os principais resultados mostraram significativa influência da termorretificação na perda de massa, teor de umidade de equilíbrio e estabilidade dimensional. Por outro lado, a massa específica não apresentou comportamento capaz de explicar a influência da termorretificação. Observou-se escurecimento gradual na madeira das três espécies em ambas as seções, em que se ressalta uma forte redução do L* após o tratamento 2 (180°C), diferentemente do a* e b* que apresentaram comportamento distinto. Verificou-se que, enquanto a rigidez da madeira termorretificada manteve-se inalterada quando relacionada ao tratamento de referência, a resistência do material foi significativamente reduzida, principalmente após os tratamentos 6, 7, 8 e 9. Já a utilização da técnica não destrutiva de ultrassom permitiu a obtenção de coeficientes de determinação entre 0,66 e 0,80 para a relação MOE x Ed. A molhabilidade da madeira termorretificada apresentou os melhores resultados para os tratamentos 2, 3, 4 e 5 na madeira das três espécies, em que o ângulo de contato elevou-se e manteve-se com maior estabilidade ao longo do tempo. A técnica de ATR-IR identificou modificações nos picos relacionados, principalmente as hemiceluloses e também à cristalinidade da estrutura, incitando a degradação térmica do material conforme o aumento da temperatura. Dessa maneira, concluiu-se que os tratamentos empregados foram suficientes para reduzir a higroscopicidade, bem como modificar os tons de cor para cada temperatura utilizada. A resistência mecânica foi significativamente reduzida e, ao mesmo tempo, o ultrassom mostrou-se eficiente quanto à predição do módulo de elasticidade. A molhabilidade da madeira reduziu significativamente conforme o emprego dos tratamentos. A ATR-IR mostrou-se como uma excelente ferramenta de análise qualitativa das modificações químicas ocorridas durante a termorretificação.
5

Efeito do tratamento térmico nas propriedades químicas, físicas e mecânicas em elementos estruturais de Eucalipto citriodora e Pinus taeda / Thermal rectification effect on the mechanical, chemical and physical properties in structural elements of Eucalyptus citriodora and Pinus taeda

Silva, Marcio Rogério da 21 November 2012 (has links)
O tratamento térmico da madeira (ou termorretificação) é realizado com o aquecimento da madeira em faixas de temperaturas inferiores à carbonização, até 280ºC. O objetivo desta tecnologia é melhorar as propriedades da madeira e, desta forma, proporcionar a obtenção de produtos com maior valor agregado. Neste trabalho, avaliou-se o comportamento em peças estruturais das espécies Eucalipto citriodora (Corymbia citriodora) e Pinus taeda, tratadas termicamente nas temperaturas de 160, 180, 200, 220 e 240ºC e, para o pínus, até a temperatura de 260ºC. Os ensaios experimentais realizados para a madeira tratada termicamente foram de classificação visual, obtenção de suas propriedades químicas, físicas, mecânicas (resistência e rigidez), entomológicas e com fungos. Quanto à análise química, foi identificado redução nos teores de extrativos e hemiceluloses, após termorretificação das amostras, e aumento no teor de celulose e lignina. A densidade aparente e a propriedade de inchamento das madeiras reduziram com o aumento da temperatura, e a permeabilidade a gás aumentou para o eucalipto. As mudanças nas propriedades colorimétricas das madeiras levaram ao escurecimento, devido à degradação dos seus principais componentes químicos, principalmente as hemiceluloses. De uma maneira geral, as propriedades de resistência (à compressão, à flexão, à tração e ao cisalhamento paralelo às fibras) das madeiras diminuiram e as propriedades de rigidez (módulo de elasticidade na compressão Eco e o MOE) aumentaram ao longo do tratamento com calor. A determinação dos valores característicos de resistência para cálculos estruturais foram adequados quando comparados à norma brasileira ABNT NBR 7190/97. O tratamento térmico proporcionou a proteção da madeira de Eucalipto citriodora quando ela foi exposta ao ataque de cupins subterrâneos Nasutitermes sp., de acordo com os ensaios de alimentação forçada, e das madeiras de E. citriodora e Pinus taeda, quando expostas ao fungo de podridão mole (Paecilomyces variotti). / Heat treatment of wood (or thermal rectification) is performed by heating wood below carbonization temperature, up to 280ºC. The aim of this technology is to improve the wood properties and thus to obtain products with higher added value. In this study, we evaluate the behavior of structural pieces of two species: Eucalyptus citriodora (Corymbia citriodora) and Pinus taeda, thermally treated at temperatures of 160, 180, 200, 220 e 240ºC, and pine up to the temperature of 260ºC. The experimental analyses to thermally treated wood were visual grading, chemical, physical, mechanical (strength and stiffness) and durability. Regarding chemical analysis, there was a reduction in the extractive and hemicellulose contents, while and the cellulose and lignin content increased; density and swelling properties decreased with increasing temperature, and the air permeability increased to eucalyptus. Changes in the colorimetric properties of wood caused the species in the study to become darker due to degradation of chemical components, mainly hemicellulose. In general, the strength properties (compression, bending, tension and shear parallel to the grain) of wood reduced and the stiffness properties (elasticity modulus by compression (Eco) and MOE) improved along thermal treatment. The determination of characteristic values of strength to structural calculations was adequate when compared to the Brazilian norm ABNT NBR 7190/97. It was observed that thermal treatment provided protection of wood of Eucalyptus citriodora from subterranean termites attack Nasutitermes sp., in the force-feeding test and Eucalyptus citriodora and Pinus taeda on soft rot fungi Paecilomyces variotti.
6

Fenômenos de transporte em sistemas fora do equilíbrio / Transport Phenomena in Out-of-Equilibrium Systems

Santos, Pedro Henrique Guimarães dos 04 July 2017 (has links)
Fenômenos de transporte constituem um dos grandes desafios teóricos da mecânica estatística fora do equilíbrio, uma vez que a compreensão dos mecanismos microscópicos que regem tais fenômenos não está completamente estabelecida. Conduzidos, portanto, pela motivação de melhor compreender esses mecanismos, propomos nesta tese o estudo dos fenômenos de transporte através de dois modelos microscópicos em dois contextos distintos: clássico e quântico. No contexto clássico, consideramos como modelo uma cadeia de osciladores harmônicos acoplados, sujeita a um potencial local (pinning) anarmônico quártico (conhecido como modelo phi4). A cadeia está em contato, através de suas extremidades, com dois reservatórios térmicos mantidos a temperaturas distintas, e sua dinâmica é dada por um sistema de equações de Langevin. Além disso, consideramos a inclusão de um ruído conservativo que inverte aleatoriamente o sentido da velocidade de cada partícula. Nesse sistema, estudamos dois fenômenos de transporte associados à condução de calor: a Lei de Fourier e a retificação térmica. Os resultados foram obtidos numericamente através da simulação do sistema usando-se métodos de dinâmica estocástica. A partir desses resultados pudemos concluir que, tanto a validade da Lei de Fourier, quanto a presença de uma retificação finita no limite termodinâmico, estão associadas à presença do ruído conservativo na dinâmica do sistema. No contexto quântico, utilizamos como modelo de trabalho uma cadeia de spins do tipo XX posta em contato, através de suas extremidades, com dois reservatórios mantidos a diferentes temperaturas e potenciais químicos. A interação com os reservatórios foi feita através de dissipadores de Lindblad presentes na equação mestra quântica que fornece a dinâmica do sistema. Esses dissipadores são acoplados aos modos normais do hamiltoniano do modelo de forma que, no equilíbrio, o sistema termaliza corretamente para o estado de Gibbs. Além de resultados numéricos, obtivemos através de um método perturbativo, expressões analíticas para os fluxos de energia e de partículas ao longo da cadeia, verificando que ambos possuem a estrutura da fórmula de Landauer. No regime em que o acoplamento com os reservatórios é fraco, verificamos ainda que as relações de reciprocidade de Onsager entre esses fluxos são satisfeitas. / Transport phenomena are one of the great theoretical challenges of out-of-equilibrium statistical mechanics since the understanding of the microscopic mechanisms governing such phenomena is not yet fully established. To better understand these mechanisms, we propose in this thesis the study of transport phenomena through two microscopic models in two distinct contexts: classical and quantum ones. In the classical context, we considered as a working model a chain of coupled harmonic oscillators, subject to a quartic anharmonic pinning (known as the phi4 model). The chain is in contact, through its ends, with two thermal reservoirs kept at different temperatures, and its dynamics is given by a system of Langevin equations. In addition, we considered the inclusion of a conservative noise that randomly reverses the direction of the velocity of each particle. In this system, we studied two transport phenomena associated with heat conduction: the Fourier Law and the thermal rectification. The results were obtained numerically by simulating the system using stochastic dynamics methods. From these results we concluded that both the validity of the Fourier Law and the presence of a finite rectification in the thermodynamic limit are associated with the presence of the conservative noise in the system dynamics. In the quantum context, we used as a working model the XX spin chain that was put in contact, through its ends, with two reservoirs kept at different temperatures and chemical potentials. The interaction with the reservoirs was modeled through Lindblad dissipators included in the quantum master equation that describes the system dynamics. These dissipators are coupled to the normal modes of the model Hamiltonian so that, in equilibrium, the system thermalizes correctly to the Gibbs state. In addition to numerical results, we obtained through a perturbative method, analytical expressions for the energy and particle fluxes along the chain, verifying that both have the structure of the Landauer formula. In the regime where the coupling with the reservoirs is weak, we also verified that the Onsager reciprocal relations between these fluxes are satisfied.
7

Rectifications optique et thermique générées à l'aide de jonctions tunnel planaires électromigrées / Optical and thermal rectifications with planar-electromigrated tunnel junctions

Gourier, Marie-Maxime 12 December 2017 (has links)
Les travaux de cette thèse consistent à étudier le phénomène de rectification optique au sein de dispositifs plasmo-électroniques. L’adressage optique de ces composants, de taille extrêmement réduite et présentant un temps de réponse ultra-rapide, induit une conversion du champ incident en un courant statique mesurable. L’intégration monolithique d’éléments plasmoniques et électroniques requiert une connaissance détaillée des mécanismes de transport thermique et électrique à l’échelle du nanomètre. Ces travaux visent donc également à discuter l’ensemble des effets thermiques inhérents à l’excitation optique de ces dispositifs connectés dans le but d’identifier les différentes contributions entrant en jeu dans la génération d’un courant photo-assisté. / The work described in this manuscrit consists in studying the optical rectification within plasmo-electronic devices. These ultra-compact optically adressed components with an ultra-fast time response induces a conversion of the incident field into a static current. The monolithically-integrated electronically optical antenna requires a detailed knowledge of nanoscale thermal and electrical transport mechanisms. This work also aims to discuss all thermal effects inherent in the optical excitation of these connected devices, in order to identify the different contributions in the generation of a photo-assisted current.
8

Fenômenos de transporte em sistemas fora do equilíbrio / Transport Phenomena in Out-of-Equilibrium Systems

Pedro Henrique Guimarães dos Santos 04 July 2017 (has links)
Fenômenos de transporte constituem um dos grandes desafios teóricos da mecânica estatística fora do equilíbrio, uma vez que a compreensão dos mecanismos microscópicos que regem tais fenômenos não está completamente estabelecida. Conduzidos, portanto, pela motivação de melhor compreender esses mecanismos, propomos nesta tese o estudo dos fenômenos de transporte através de dois modelos microscópicos em dois contextos distintos: clássico e quântico. No contexto clássico, consideramos como modelo uma cadeia de osciladores harmônicos acoplados, sujeita a um potencial local (pinning) anarmônico quártico (conhecido como modelo phi4). A cadeia está em contato, através de suas extremidades, com dois reservatórios térmicos mantidos a temperaturas distintas, e sua dinâmica é dada por um sistema de equações de Langevin. Além disso, consideramos a inclusão de um ruído conservativo que inverte aleatoriamente o sentido da velocidade de cada partícula. Nesse sistema, estudamos dois fenômenos de transporte associados à condução de calor: a Lei de Fourier e a retificação térmica. Os resultados foram obtidos numericamente através da simulação do sistema usando-se métodos de dinâmica estocástica. A partir desses resultados pudemos concluir que, tanto a validade da Lei de Fourier, quanto a presença de uma retificação finita no limite termodinâmico, estão associadas à presença do ruído conservativo na dinâmica do sistema. No contexto quântico, utilizamos como modelo de trabalho uma cadeia de spins do tipo XX posta em contato, através de suas extremidades, com dois reservatórios mantidos a diferentes temperaturas e potenciais químicos. A interação com os reservatórios foi feita através de dissipadores de Lindblad presentes na equação mestra quântica que fornece a dinâmica do sistema. Esses dissipadores são acoplados aos modos normais do hamiltoniano do modelo de forma que, no equilíbrio, o sistema termaliza corretamente para o estado de Gibbs. Além de resultados numéricos, obtivemos através de um método perturbativo, expressões analíticas para os fluxos de energia e de partículas ao longo da cadeia, verificando que ambos possuem a estrutura da fórmula de Landauer. No regime em que o acoplamento com os reservatórios é fraco, verificamos ainda que as relações de reciprocidade de Onsager entre esses fluxos são satisfeitas. / Transport phenomena are one of the great theoretical challenges of out-of-equilibrium statistical mechanics since the understanding of the microscopic mechanisms governing such phenomena is not yet fully established. To better understand these mechanisms, we propose in this thesis the study of transport phenomena through two microscopic models in two distinct contexts: classical and quantum ones. In the classical context, we considered as a working model a chain of coupled harmonic oscillators, subject to a quartic anharmonic pinning (known as the phi4 model). The chain is in contact, through its ends, with two thermal reservoirs kept at different temperatures, and its dynamics is given by a system of Langevin equations. In addition, we considered the inclusion of a conservative noise that randomly reverses the direction of the velocity of each particle. In this system, we studied two transport phenomena associated with heat conduction: the Fourier Law and the thermal rectification. The results were obtained numerically by simulating the system using stochastic dynamics methods. From these results we concluded that both the validity of the Fourier Law and the presence of a finite rectification in the thermodynamic limit are associated with the presence of the conservative noise in the system dynamics. In the quantum context, we used as a working model the XX spin chain that was put in contact, through its ends, with two reservoirs kept at different temperatures and chemical potentials. The interaction with the reservoirs was modeled through Lindblad dissipators included in the quantum master equation that describes the system dynamics. These dissipators are coupled to the normal modes of the model Hamiltonian so that, in equilibrium, the system thermalizes correctly to the Gibbs state. In addition to numerical results, we obtained through a perturbative method, analytical expressions for the energy and particle fluxes along the chain, verifying that both have the structure of the Landauer formula. In the regime where the coupling with the reservoirs is weak, we also verified that the Onsager reciprocal relations between these fluxes are satisfied.
9

Efeito do tratamento térmico nas propriedades químicas, físicas e mecânicas em elementos estruturais de Eucalipto citriodora e Pinus taeda / Thermal rectification effect on the mechanical, chemical and physical properties in structural elements of Eucalyptus citriodora and Pinus taeda

Marcio Rogério da Silva 21 November 2012 (has links)
O tratamento térmico da madeira (ou termorretificação) é realizado com o aquecimento da madeira em faixas de temperaturas inferiores à carbonização, até 280ºC. O objetivo desta tecnologia é melhorar as propriedades da madeira e, desta forma, proporcionar a obtenção de produtos com maior valor agregado. Neste trabalho, avaliou-se o comportamento em peças estruturais das espécies Eucalipto citriodora (Corymbia citriodora) e Pinus taeda, tratadas termicamente nas temperaturas de 160, 180, 200, 220 e 240ºC e, para o pínus, até a temperatura de 260ºC. Os ensaios experimentais realizados para a madeira tratada termicamente foram de classificação visual, obtenção de suas propriedades químicas, físicas, mecânicas (resistência e rigidez), entomológicas e com fungos. Quanto à análise química, foi identificado redução nos teores de extrativos e hemiceluloses, após termorretificação das amostras, e aumento no teor de celulose e lignina. A densidade aparente e a propriedade de inchamento das madeiras reduziram com o aumento da temperatura, e a permeabilidade a gás aumentou para o eucalipto. As mudanças nas propriedades colorimétricas das madeiras levaram ao escurecimento, devido à degradação dos seus principais componentes químicos, principalmente as hemiceluloses. De uma maneira geral, as propriedades de resistência (à compressão, à flexão, à tração e ao cisalhamento paralelo às fibras) das madeiras diminuiram e as propriedades de rigidez (módulo de elasticidade na compressão Eco e o MOE) aumentaram ao longo do tratamento com calor. A determinação dos valores característicos de resistência para cálculos estruturais foram adequados quando comparados à norma brasileira ABNT NBR 7190/97. O tratamento térmico proporcionou a proteção da madeira de Eucalipto citriodora quando ela foi exposta ao ataque de cupins subterrâneos Nasutitermes sp., de acordo com os ensaios de alimentação forçada, e das madeiras de E. citriodora e Pinus taeda, quando expostas ao fungo de podridão mole (Paecilomyces variotti). / Heat treatment of wood (or thermal rectification) is performed by heating wood below carbonization temperature, up to 280ºC. The aim of this technology is to improve the wood properties and thus to obtain products with higher added value. In this study, we evaluate the behavior of structural pieces of two species: Eucalyptus citriodora (Corymbia citriodora) and Pinus taeda, thermally treated at temperatures of 160, 180, 200, 220 e 240ºC, and pine up to the temperature of 260ºC. The experimental analyses to thermally treated wood were visual grading, chemical, physical, mechanical (strength and stiffness) and durability. Regarding chemical analysis, there was a reduction in the extractive and hemicellulose contents, while and the cellulose and lignin content increased; density and swelling properties decreased with increasing temperature, and the air permeability increased to eucalyptus. Changes in the colorimetric properties of wood caused the species in the study to become darker due to degradation of chemical components, mainly hemicellulose. In general, the strength properties (compression, bending, tension and shear parallel to the grain) of wood reduced and the stiffness properties (elasticity modulus by compression (Eco) and MOE) improved along thermal treatment. The determination of characteristic values of strength to structural calculations was adequate when compared to the Brazilian norm ABNT NBR 7190/97. It was observed that thermal treatment provided protection of wood of Eucalyptus citriodora from subterranean termites attack Nasutitermes sp., in the force-feeding test and Eucalyptus citriodora and Pinus taeda on soft rot fungi Paecilomyces variotti.

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