The thermomechanical properties of monolayer graphene and the interfacial interactions between graphene and an SiO₂ substrate are investigated in this dissertation using a multiscale approach. The temperature dependent mechanical behavior of graphene with thermal fluctuations is studied by statistical mechanics analysis under harmonic approximation, which is then compared to molecular dynamics simulations. It is found that the amplitude of thermal fluctuation depends nonlinearly on the graphene size due to anharmonic interactions between bending and stretching modes, but a small positive pre-strain could suppress fluctuation amplitude considerably and results in very different scaling behavior. The thermal expansion of graphene depends on two competing effects: positive expansion due to in-plane modes and negative expansion due to out-of-plane fluctuations. The in-plane stress-strain relation of graphene becomes nonlinear even at infinitesimal strain due to the entropic contribution. Consequently, the modulus of graphene depends on strain non-monotonically, with strain stiffening followed by intrinsic softening. Moreover, it is found that the thermomechnical behavior of graphene is dependent on its interactions with environment such as supporting substrate. The interfacial interactions between graphene and SiO₂ substrate is investigated in terms of three perspectives. Firstly, the interaction mechanisms between graphene and SiO₂ substrate are studied by density functional theory (DFT). The dispersion interaction is found to be the predominant mechanism, and the interaction strength is strongly influenced by changes of SiO₂ surface structures due to surface reactions with water. The adhesion energy is reduced when the reconstructed SiO₂ surface is hydroxylated, and further reduced when covered by a monolayer of adsorbed water molecules. Next, we study the interfacial interactions between graphene and a wet substrate that is covered by a liquid-like water film. During the separation of graphene from the wet substrate, MD simulations show evolution of the water from a continuous film to discrete islands. The water bridging effects are further described by continuum models. Finally, a continuum model is developed to predict how the surface roughness may affect the adhesion between graphene membranes and their substrate. / text

The purpose of this investigation is to make more detailed studies of transformations. Fourteen compounds have been examined by high temperature X-ray diffraction for this purpose. The investigations have been carried out in such a way as to reveal: 1. the existence of transformations, 2. the influence of polarizability on thermal expansion, 3. the anisotropy of expansion, and 4. the discontinuity of thermal expansion.

La cotation fonctionnelle doit garantir la montabilité et le bon fonctionnement d'un mécanisme en imposant les spécifications fonctionnelles à respecter sur les pièces. Ces spécifications sont exprimées avec les normes ISO de cotation et doivent être vérifiées à 20°C. Pour les mécanismes soumis à de fortes températures, il est nécessaire de cumuler l'influence des tolérances et des dilatations aux différents régimes thermiques. Après avoir formulé des hypothèses de comportement dans les liaisons avec contact ou avec jeux affectés par les déformations thermiques et l'influence des incertitudes sur les températures, la méthodologie proposée permet de séparer le calcul thermique et le tolérancement. Le bureau de calcul thermique détermine les champs de température et les déplacements des mailles par la méthode des éléments finis à partir des modèles nominaux des pièces. Le cumul des tolérances et des dilatations est basé sur la méthode des droites d'analyse. Pour chaque exigence, la surface terminale est discrétisée en différents points d'analyse. Dans chaque jonction, les relations de transfert déterminent les points de contact et l'influence des dilatations et des écarts thermiques en ces points sur l'exigence. Une application à un mécanisme industriel démontre l'intérêt d'optimiser les dimensions nominales des modèles afin de maximiser les tolérances tout en respectant l'ensemble des exigences. / Functional dimensioning should guarantee assembly feasibility and proper functioning of a mechanism giving functional specifications on parts to be met. Those specifications are defined with ISO standard and may be considered at 20°C. For high heated mechanisms, impacts of tolerances and thermal expansion for all thermal stages have to be combined. After giving behavior assumption into links with clearance or not while considering thermal expansion and uncertainty of temperature, the proposed method makes it possible to separate thermal calculations and tolerancing. Thermal calculations office determines temperature field and displacements from nominal parts by finite elements method. Integration of tolerancing and thermal expansion is based on analysis direction method. For each requirement, final surface is discretized in various points. In every link, transfer relations determine contact points and impact of thermal expansion on these analysis points on the requirement. A study case shows the interest of nominal dimension optimization in order to maximize tolerances while respecting all mechanism requirements.

There has been significant interest in BiFeO₃ over the past decade. This interest has focused on the magnetic and electrical properties, which in the long term may prove useful in device applications. This thesis focuses on the synthesis, electrical characterisation, and structural origin of the electrical properties of rare earth doped bismuth ferrite. Two systems have been studied: BiFeO₃ doped with lanthanum and neodymium (Bi₁₋ₓREₓFeO₃ RE= La, Nd). Specific examples have been highlighted focusing on a detailed structural analysis of a lanthanum doped bismuth ferrite, Bi₀.₅La₀.₅FeO₃, and a neodymium analogue, Bi₀.₇Nd₀.₃FeO₃. Both adopt an orthorhombic GdFeO₃-type structure (space group: Pnma) with G-type antiferromagnetism. Structural variations were investigated by Rietveld refinement of temperature dependent powder neutron diffraction using a combination of both conventional “bond angle/bond length” and symmetry-mode analysis. The latter was particularly useful as it allowed the effects of A-site displacements and octahedral tilts/distortions to be considered separately. This in-depth structural analysis was complemented with ac-immittance spectroscopy using the multi-formulism approach of combined impedance and modulus data to correlate structural changes with the bulk electrical properties. This approach was essential due to the complex nature of the electrical response with contributions from different electroactive regions. The structural variations occur due to a changing balance between magnetic properties and other bonding contributions in the respective systems. This results in changes in the magnitude of the octahedral tilts, and A-site displacements giving rise to phenomena such as negative thermal expansion and invariant lattice parameters i.e., the invar effect. More specifically, analysis of Bi₀.₅La₀.₅FeO₃ highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO₆ octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behaviour is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi³⁺ and La³⁺ along the a-axis. This contrasts with the neodymium analogue Bi₀.₇Nd₀.₃FeO₃ in which a combination of increasing A-site displacements in the ac-plane and decrease in both in-phase and anti-phase tilts combine with superexchange giving rise to negative thermal expansion at low temperature. The A-site displacements correlate with the orthorhombic strain. By carefully changing the synthesis conditions, a significant change in bulk conductivity was observed for a number for Bi₁₋ₓLaₓFeO₃ compositions. A series of Bi₀.₆La0.₄FeO₃ samples are discussed, where changes in the second step of the synthesis result in significantly different bulk conductivities. This behaviour is also observed in other compositions e.g. Bi₀.₇₅La₀.₂₅FeO₃. Changes in the electrical behaviour as a function of temperature are discussed in terms of phase composition and concentration gradients of defects. Activation energies associated with the conduction process(es) in Bi₁₋ₓLaₓFeO₃ samples, regardless of composition, fall within one of two broad regimes, circa. 0.5 eV or 1.0 eV, associated with polaron hopping or migration of charge via oxygen vacancies, respectively. The use of symmetry-mode analysis, in combination with conventional crystallographic analysis and electrical analysis using multi-formulism approach, presents a new paradigm for investigation of structure-property relationships in rare earth doped BiFeO₃.

[pt] A síntese de pós nanométricos do Al2Mo3O12 para otimização de seu coeficiente de expansão térmica na forma maciça tem como objetivo principal aproximar o comportamento térmico intrínseco e extrínseco do material. A expansão térmica intrinseca de escala atomica é medida por difração de raios-X a partir do aumento dos parametros de rede, por outro lado, a tecnica de dilatometria mede ambos os efeitos tanto intrinsecos quanto extrinsecos provenientes da microestrutura. Materiais anisotropicos apresentam coeficientes de expansão termica diferentes ao longo dos eixos cristalograficos, e com isso são encontradas maiores diferenças entre as propriedades intrinseca e maciça da expansão termica. Dessa forma a aplicação desses materias anisotropicos na forma maciça é comprometida devido a formação de microtrincas. O Al2Mo3O12 foi obtido na forma nanometrica pela síntese por coprecipitação e na forma micrométrica pela síntese de sol-gel assistido com álcool polivinilico e por reação em estado solido. Dessa forma o resultado de CET maciço obtidos pelos três métodos foram comparados entre si e também comparados aos existentes na literatura para comportamento intrínseco e maciço. Os resultados mostraram que o Al2Mo3O12 na forma nanometrica possui resultado de CET maciço muito próximo ao intrínseco, diferente do obtido para o micrométrico e também do já reportado na literatura,o que confirma que a partir de um tamanho de cristal critico não seria mais possível obter um mesmo CET intrínseco e maciço para um mesmo material. / [en] Optimization of the bulk thermal expansion coefficient of the Al2Mo3O12 using nanometric powder in order to approximate the intrinc and the extrinsic thermal properties.When a solid body is exposed to temperature variation, a change of dimensions will occur due to emergence of different effects originating at atomic (intrinsic) or microstructural (extrinsic) scales. The intrinsic thermal expansion is measured by X-ray diffraction from lattice parameters increase, on the other hand, the technique of dilatometric measures both the intrinsic as both extrinsic effects may then be defined as their CTE solid (bulk). Cubic materials exhibit isotropic behavior during thermal expansion, and thus may be insignificant variations between intrinsic and CTE s massive. Anisotropic materials have different coefficients of thermal expansion along the crystallographic axes, and presents major differences between the intrinsic properties and thermal expansion of the bulk, being mostly a bulk CTE smaller than the intrinsic one. The application of these anisotropic materials is difficult because bulk CTE massive changes expected due to formation of microcracks. The Al2Mo3O12 was obtained by three routes :coprecipitation (nanometric way) , sol-gel assisted with polyvinyl alcohol (PVA) and by solid state reaction (micrometric ways). Thus the result of bulk CET obtained by the three methods were compared and also compared with those found in the literature for intrinsic behavior and bulk. The nanometric Al2Mo3O12 showed a bulk linear CTE close to the intrinsic value, whereas micrometric one showed a negative bulk CTE ,which confirms that from a critical cristal size it is no possible to obtain bulk CTE close to the intrinsic one.

A dilatação térmica é um problema com o qual os engenheiros de tubulação frequentemente precisam lidar, já que parte do papel destes prossionais é controlar as dilatações lineares totais e minimizar tensões e forças associadas a este fenômeno físico. O projeto de sistemas de tubulação é guiado por normas, sendo a ASME B31.3 (2010) certamente a mais utilizada no Brasil e nos Estados Unidos. Para a referida norma o sistema de tubulação, do ponto de vista térmico, é avaliado com base em uma temperatura de projeto constante e uniforme denida pelos critérios desta norma, temperatura esta normalmente estabelecida com base na temperatura de regime permanente. Estes critérios são sucientes para garantir a integridade estrutural da tubulação em virtude da forma como as tensões admissíveis e atuantes estão estabelecidas, contudo, a norma é omissa em relação ao transiente térmico e a dilatação linear da tubulação durante esse período. Tal dilatação poderá estar associada a forças transmitidas pela tubulação a equipamentos e estruturas, forças essas negligenciadas pela ASME B.31.3 e a literatura em geral. Esse trabalho apresenta as equações envolvidas no problema do transiente térmico de tubulações e, baseando-se nos resultados de simulações numéricas e na mecânica classicamente adotada pela Engenharia de Tubulação para computar forças, faz uma discussão a respeito das forças associadas à dilatação no período do transiente térmico. Vericou-se que quanto maior a velocidade do escoamento, maior o número de Nusselt e maior a difusividade térmica do material do tubo, maior será a taxa de aquecimento da tubulação e que, quanto maior essa taxa de aquecimento e o coeciente de dilatação, maior será a taxa de dilatação linear do tubo. Além disso, a força associada à dilatação linear passa a ser transmitida ao ponto xo (ancoragem ou trava) de forma abrupta e aumenta de forma intermitente até o seu máximo valor, para então cair ao seu mínimo valor de forma extremamente abrupta, sendo este valor mínimo o obtido nas análises usuais de sistemas de tubulação. / Thermal expansion is a problem the pipe engineers often have to deal with, since it is important to control the total linear thermal expansion and minimize stresses and forces associated with this physical phenomenon. The design of pipe systems is guided by standards, being ASME B31.3 (2010) certainly the most used in Brazil and the United States. For this standard the pipe system, from the thermal standpoint, is evaluated based on a constant and uniform design temperature, being this normally referenced by the steady state value. These criteria are sucient to ensure the pipe structural integrity due to the way the admissible and acting stresses are established; however, the standard is silent regarding the thermal transient and the pipe linear thermal expansion during this period. Such thermal expansion may be associated with forces transmitted by the pipe to equipment and structures, neglected by ASME B.31.3 and the literature. This work presents the equations involved in the pipe thermal transient problem. Based on the results of numerical simulations and the procedures normally used by Pipeline Engineering to compute forces, a discussion is made about the forces associated with the thermal expansion in the period of the thermal transient. It was found that the higher the ow velocity, the Nusselt number and the thermal diusivity of the pipe material, the higher the pipe heating rate. Besides, it was found that the higher the heating rate and the thermal expansion coecient, the higher the linear thermal expansion rate of the pipe. Moreover, the force associated with the linear thermal expansion starts to be transmitted to the pipe xed point (anchor or stop) in an abrupt way and increases intermittently until its maximum value, then falls to its minimum value in an extremely abrupt way, being this minimum value the load obtained in the usual pipe load analysis.

Existe atualmente uma tendência de se utilizar em revestimento das fachadas de grandes edificações placas cada vez maiores, separadas por juntas de dilatação com dimensões menores possíveis, a fim de proporcionar efeitos arquitetônicos agradáveis e ao mesmo tempo dificultar a infiltração de agentes deterioradores na estrutura das edificações. Desta forma, há a necessidade de se dimensionar precisamente estas juntas, cuja função é absorver as variações do comprimento de placas produzidas pela dilatação térmica linear (&#946) de rochas graníticas e gnáissicas extraídas nos Estados de São Paulo, Mato Grosso e Espírito Santo, usadas comercialmente na forma de placas para revestimento de edificações. Os coeficientes de dilatação térmica linear (&#946) foram determinados em dilatômetro de quartzo fundido e correlacionadas com o conteúdo em quartzo, granulação, porosidade aparente e estrutura das rochas estudadas. Conclui-se que, nas rochas gnáissicas, os valores de &#946 são muito maiores quando determinados paralelo à gnaissificação. Nas graníticas, o &#946 aumenta com o conteúdo em quartzo e diminui com os aumentos da porosidade aparente e do tamanho dos grãos minerais. / There is a great tendency of using larger plaques of rocks on the covering of the faces of buildings, separated by expansion joints of very small dimensions as possible, in order to provide agreeable architectural effects and at the same time to make difficult the infiltration in the building structure of deterioration agents. Therefore, there is a great need of dimensioning precisely these joints whose function is to absorb the plates length variation produced by thermal expansion. This research presents values of plates length variation produced by thermal expansion. This research presents values of thermal linear expansion coefficient (&#946) of granite and gneiss rocks extracted from São Paulo, Mato Grosso and Espírito Santo states, used commercially in plate shapes for covering of buildings. The thermal linear expansion coefficients (&#946) were determined by cast quartz dilatometer and correlated with the quartz content, granulation, apparent porosity, and the studied rock structures. It can be concluded that, in gneiss rocks, the &#946 values are much bigger when determined in parallel with the structure. On the granite rocks, the &#946 value increases with the quartz content and it decreases with the increases of apparent porosity and the size of mineral grains.

In order to investigate the thermal stress and expansion as well as the associated strain effect on material properties caused by high temperature and large temperature gradient, a two-way thermo-mechanical coupling solver is developed. This solver integrates a new structural response module to the Kentucky Aerothermodynamics and Thermal response System (KATS) framework. The structural solver uses a finite volume approach to solve either hyperbolic equations for transient solid mechanics, or elliptic equations for static solid mechanics. Then, based on the same framework, a quasi-static approach is used to couple the structural response and thermal response to estimate the thermal expansion and stress within Thermal Protection System (TPS) materials. To better capture the thermal expansion and study its impacts on material properties such as conductivity and porosity, a moving mesh scheme is also developed and incorporated into the solver. Grid deformation is transferred among different modules in the form of variations of geometric parameters and strain effects. By doing so, a bi-direction information loop is formed to accomplish the two-way strong thermo-mechanical coupling. Results revealed that the thermal stress experienced during atmospheric re-entry concentrates in a banded area at the edge of the pyrolysis zone and its magnitude can be large enough to cause the failure of the TPS. In addition, thermal expansion causes the whole structure to deform and the changes in material properties. Results also indicated that the impacts coming from structural response should not be ignored in thermal response.

Corrosion of steel reinforcement in concrete structures results in significant repair and rehabilitation costs. In the past several years, new fiber reinforced polymer (FRP) reinforcing bars have been introduced as an alternative to steel reinforcing bars. Several national and international organizations have recently developed standards based on preliminary test results. However, limited validation testing has been performed on the recommendations of these standards. High variability of the tensile properties, degradation of tensile strength, direct shear capacity, predicted deflections due to creep, cracking behavior of FRP-reinforced concrete flexural members, bond behavior and development length, and effects of thermal expansion on cracking of FRP reinforced concrete have all been reported, but are areas that need further investigation and validation. The objective of this study is to evaluate the characteristics of glass FRP reinforcing bars and provide recommendations on the design and construction of concrete structures containing these bar types with regard to the areas described. The recently developed ACI 440 design guidelines were analyzed and modifications proposed.

The work presented in this thesis detail the measurement of the thermal expansion coefficient of three magnetic spin materials. Thermal expansion coefficient values were measured by capacitive dilatometry in several key low (T < 250 K) temperature regions specific to each material. This thesis is separated into several key parts. The first part establishes the theory behind observing phase transitions through the thermal expansion coefficient. Beginning with the classical definitions of the specific heat, compressibility and thermal expansion coefficient, the three properties are related using a property known as the Grüneisen parameter. To first order, the parameter allows phase transitions to be observed by the thermal expansion coefficient. The second part introduces capacitive dilatometry; a technique used to measure the thermal expansion coefficient. Three capacitive dilatometer devices are presented in this section. The silver compact dilatometer, the fused quartz dilatometer and the copper dilatometer. Each device discusses merits and weaknesses to their designs. Particular focus is made on the fused quartz dilatometer which was built during the duration of this thesis. The third part presents research on three magnetic spin materials; LiHoF4, Tb2Ti2O7 and Ba3NbFe3Si2O14. These materials are studied individually focusing on specific aspects. LiHoF4, a candidate material for the transverse field Ising model, provides insight to quantum phase transitions. Thermal expansion coefficient and magnetostriction along the c-axis for T ≈ 1.3-1.8 K and transverse field Ht ≈ 0-4 T were measured extracting critical points for a Ht-T phase diagram. Existing thermal expansion coefficient measurements had evidence of possible re-entrant behaviour. With a high density of low transverse field critical points it was established that LiHoF4 showed no evidence of re-entrant behaviour. The highly debated material Tb2Ti2O7 has a rich, controversial low temperature behaviour. Originally believed to be a spin liquid, specific heat results propose a scenario involving a sample composition dependent ordered state. Still under considerably attention, thermal expansion coefficient measurements were performed for T < 1 K. The results are interpreted to either fit into the proposed scenario or provide evidence for an alternate scenario. The material Ba3NbFe3Si2O14 exhibits a magnetoelectric multiferroic phase below TN ≈ 27 K; a phase where magnetic and electric order simultaneously exist. The formation of this phase is believed to have a similar structural shift observed in hexagonal perovskite multiferroic materials. The ferroelectric ordering in those materials are brought about through a centrosymmetric to non-centrosymmetric structural shift. The thermal expansion and thermal expansion coefficient coefficient along the a and c axis are measured for T > TN searching for a displacive structural phase transition.