La mise en place de nouvelles normes, de plus en plus contraignantes, est un défi pour l’élaboration de nouveaux matériaux résistants à haute température. Le premier objectif de l’étude est de mettre au point un panneau – coupe-feu 2h à base de plâtre. Il devra présenter à la fois de bonnes qualités d’isolant thermique et des propriétés mécaniques suffisantes pour maintenir l’intégrité d’un ouvrage d’art. Le second objectif est de comprendre, d’une part, l’influence de divers paramètres sur le phénomène de prise du plâtre et d’autre part, de déterminer les propriétés thermomécaniques du composite. Ce type de matériau est obtenu par l’association d’eau, d’une matrice céramique composée essentiellement de sulfate de calcium dihydraté et de charges utilisées en tant que renforts thermiques et/ou mécaniques.Dans une première partie, l’étude porte essentiellement sur la matrice pour laquelle une granulométrie permettant d’optimiser les propriétés mécaniques est déterminée. La matrice est ensuite caractérisée chimiquement. Une étude par calorimétrie isotherme de la réaction d’hydratation du sulfate de calcium semihydraté (plâtre) est réalisée afin de comprendre le mécanisme de prise du plâtre et de maîtriser les temps de prise. Pour cela, on étudie l’influence de la taille des grains, de la quantité d’eau, de la composition chimique du plâtre et de la présence ou non d’adjuvants sur la cinétique d’hydratation du plâtre.Dans une seconde partie, les renforts nécessaires à l’élaboration du composite sont sélectionnés. Les relations entre les quantités de charges et les propriétés thermomécaniques (conductivité thermique, module d’Young, dureté Shore C) du système sont étudiées. Ainsi, une modélisation du comportement du composite sous sollicitations thermique et mécanique est proposée. Cette étude a permis de définir une formulation de panneau présentant de très bonnes propriétés thermiques et des propriétés mécaniques suffisantes pour assurer l’intégrité d’un ouvrage d’art en cas d’incendie. La formulation mise au point a fait l’objet d’un dépôt de brevet (n° BIP207506FR00 en décembre 2010). Cette formulation est actuellement commercialisée par la société EXTHA sous forme de plaques. / The increase of prevention and the introduction of more and more restrictive standards are challenges for the development of new materials resistant to high temperatures. The aim of the study is to develop a fire panel with both good properties of thermal insulator (low thermal conductivity, fumes tightness) and mechanical properties sufficient to maintain a structure integrity in case of fire.That kind of material is composed of an inorganic matrix mainly composed of calcium sulfate dihydrate and of additives used as thermal and mechanical reinforcements. The first part of the study is focused on the matrix, especially on the determination of a particle size distribution for which the mechanical properties are optimized. Then, the matrix is chemically characterized. A study by isothermal calorimetry of the hydration reaction of the calcium sulfate hemihydrate (plaster) is conducted to understand the mechanism of hydration and to control setting times. For this, the influence of the grain size, of the quantity of water, of the chemical composition of plaster, of additives on the kinetics of hydration of the plaster is studied. The second part of this work resumes the different steps of the selection of additives. After that, the relation between the microstructure and thermo - mechanicals properties (thermal conductivity, Young modulus, Shore C hardness) of the system is studied.This study has permitted to establish a panel formulation having very good thermal and mechanical properties to ensure building integrity in case of fire. The formulation has been patented in December 2010 (No BIP207506FR00) and it is currently marketed as panels by Extha.

Thesis advisor: David A. Broido / A theory is presented for describing the lattice thermal conductivities of graphene and single-walled carbon nanotubes. A phonon Boltzmann transport equation approach is employed to describe anharmonic phonon-phonon, crystal boundary, and isotopic impurity scattering. Full quantum mechanical phonon scattering is employed and an exact solution for the linearized Boltzmann transport equation is determined for each system without use of common relaxation time and long-wavelength approximations. The failures of these approximations in describing the thermal transport properties of nanotubes is discussed. An efficient symmetry based dynamical scheme is developed for carbon nanotubes and selection rules for phonon-phonon scattering in both graphene and nanotubes are introduced. The selection rule for scattering in single-walled carbon nanotubes allows for calculations of the thermal conductivities of large-diameter and chiral nanotubes that could not be previously studied due to computational limitations. Also due to this selection rule, no acoustic-only umklapp scattering can occur, thus, acoustic-optic scattering must be included in order to have thermal resistance from three-phonon processes. The graphene selection rule severely restricts phonon-phonon scattering of out-of-plane modes. This restriction leads to large contributions to the total thermal conductivity of graphene from the acoustic, out-of-plane modes which have been previously neglected. Empirical potentials used to model interactions in carbon-based materials are optimized to better describe the lattice dynamics of graphene-derived systems. These potentials are then used to generate the interatomic force constants needed to make calculations of the thermal conductivities of graphene and carbon nanotubes. Calculations of the thermal conductivities of single-walled carbon nanotubes and graphene for different temperatures and lengths are presented. The thermal conductivities of SWCNTs saturate in the diffusive regime when the effects of higher-order scattering processes are estimated and correctly reproduce the ballistic limit for short-length nanotubes at low temperatures. The effects of isotopic impurity scattering on the thermal conductivities of graphene and SWCNTs are explored. Isotopic impurities have little effect in the low (high) temperature regime where boundary (umklapp) scattering dominates the behavior of the thermal conductivities. In the intermediate temperature regime, modest reductions in the thermal conductivities, 15-20%, occur due to impurities. The thermal conductivities of a wide-range of SWCNTs are explored. The thermal conductivities of successively larger-diameter, one-dimensional nanotubes approach the thermal conductivity of two-dimensional graphene. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thermal management in electronic devices is one of the biggest challenges faced by the semiconductor industry. Thermal Interface Materials (TIMs) are used in electronics to fill air gaps between the surfaces of integrated circuit (IC) chips to dissipate heat. Polymer-graphene composites, a very promising choice as TIMs also have a drawback of high interfacial thermal resistance and a low thermal conductivity of polymer. It is known from the theoretical models that application of pressure may affect the thermal conductivity in a desirable manner, but quantitative simulations were not available. In this paper, the pressure dependence of thermal conductivity of epoxy and interfacial resistance at epoxy-graphene interface is studied using non-equilibrium molecular dynamics (NEMD) simulations. The results show that the thermal conductivity of epoxy increases with increase in pressure, and they compare well with the predictions using a theoretical model. The interfacial thermal resistance at epoxy-graphene interface reduces with increase in pressure. The reduction is sharp in the beginning and slowly reaches saturation as pressure increases. At 10 GPa compressive pressure, a 90-95% decrease in interfacial thermal resistance is observed.

Most existing thermal conductivity measurement techniques of nuclear fuel only measure the overall effective thermal conductivity of the fuel, cladding, and gap, with low spatial-resolution. However, damage to nuclear fuel microstructure caused by neutron-irradiation can result in sharp, local changes of thermal conductivity. Additionally, extremely large temperature-gradients (~1600 K/cm) from the fuel centerline to the coolant result in similar gradients of thermal conductivity. Therefore, in pursuit of greater understanding of nuclear fuel performance, the objective of this study was to develop a non-contact thermal conductivity measurement technique to provide micron-sized spatial-resolution capability. Based on photothermal techniques and using both frequency and spatial-domain photothermal reflectance methods, an experimental measurement system was designed, built, and tested for measuring the thermal conductivity of a thin-film coated material with micron resolution. This hybrid method involves separate measurement of thermal diffusivity, D, and thermal effusivity, e, from which, thermal conductivity, k = (e2/D)1/2 is calculated. A detailed parametric analysis using analytical solutions and a numerical model has been performed to guide the experiment and optimize measurement conditions. The measurement system was validated using two calibration samples having thermal conductivities at both the upper and lower limit of the common range of nuclear fuels (~1 - 10 W/m/K). Sources of experimental errors are discussed qualitatively and the uncertainty of the measurement system for the thermal conductivity range of interest is quantified. The measured error is found to be about 10%, and up to close to 20% for the worst case (upper limit of k range). An extended application of the modulated laser excitation technique is explored to measure mechanical properties of solid materials. This technique involves obtaining the natural frequencies of different vibrational modes of a cantilever beam sample allowing for the extraction of the elasticity constants of the material. From Neumann's principle, the number of independent elasticity constants is dependent on the symmetry of the material structure. Specifically, symmetries of crystalline materials and composite materials are analyzed. Experimental results of two validation samples with cubic crystal system agreed well with the published values with experimental errors of ~10%.

The grain-size dependences of thermal conductivity and electrical resistivity of polycrystalline and nanocrystalline nickel were measured by the flash method and four-point probe method, respectively. Nanocrystalline nickel was made by the pulsed-current electrodeposition process, while polycrystalline nickel was commercially available Ni 200 in annealed condition. The grain sizes of the materials examined ranged from 28 nanometers to 57 micrometers. Noticeable changes in thermal conductivity and electrical resistivity with grain size were observed in particular for samples with grain sizes less than 100 nm. These results can be explained on the basis of the rapid increase in the intercrystalline grain boundary and triple junction volume fractions at very small grain sizes. The relationship between thermal conductivity and electrical resistivity of nanocrystalline nickel follows the classic Wiedemann-Franz law.

The grain-size dependences of thermal conductivity and electrical resistivity of polycrystalline and nanocrystalline nickel were measured by the flash method and four-point probe method, respectively. Nanocrystalline nickel was made by the pulsed-current electrodeposition process, while polycrystalline nickel was commercially available Ni 200 in annealed condition. The grain sizes of the materials examined ranged from 28 nanometers to 57 micrometers. Noticeable changes in thermal conductivity and electrical resistivity with grain size were observed in particular for samples with grain sizes less than 100 nm. These results can be explained on the basis of the rapid increase in the intercrystalline grain boundary and triple junction volume fractions at very small grain sizes. The relationship between thermal conductivity and electrical resistivity of nanocrystalline nickel follows the classic Wiedemann-Franz law.

Thermally enhanced greases made of dispersions of small conductive particles suspended in fluidic polymers can offer significant advantages when used as a thermal interface material (TIM) in microelectronics cooling applications. A fundamental problem which remains to be addressed is how to predict the effective thermal conductivity of these materials, an important parameter in establishing the bulk resistance to heat flow through the TIM. <br /><br /> The following study presents the application of two simple theorems for establishing bounds on the effective thermal conductivity of such inhomogeneous media. These theorems are applied to the development of models which are the geometric means of the upper and lower bounds for effective thermal conductivity of base fluids into which are suspended particles of various geometries. <br /><br /> Numerical work indicates that the models show generally good agreement for the various geometric dispersions, in particular for particles with low to moderate aspect ratios. The numerical results approach the lower bound as the conductivity ratio is increased. An important observation is that orienting the particles in the direction of heat flow leads to substantial enhancment in the thermal conductivity of the base fluid. Clustering leads to a small enhancement in effective thermal conductivity beyond that which is predicted for systems composed of regular arrays of particles. Although significant enhancement is possible if the clusters are large, in reality, clustering to the extent that solid agglomerates span large distances is unlikely since such clusters would settle out of the fluid. <br /><br /> In addition, experimental work available in the literature indicates that the agreement between the selected experimental data and the geometric mean of the upper and lower bounds for a sphere in a unit cell are in excellent agreement, even for particles which are irregular in shape.

Thermally enhanced greases made of dispersions of small conductive particles suspended in fluidic polymers can offer significant advantages when used as a thermal interface material (TIM) in microelectronics cooling applications. A fundamental problem which remains to be addressed is how to predict the effective thermal conductivity of these materials, an important parameter in establishing the bulk resistance to heat flow through the TIM. <br /><br /> The following study presents the application of two simple theorems for establishing bounds on the effective thermal conductivity of such inhomogeneous media. These theorems are applied to the development of models which are the geometric means of the upper and lower bounds for effective thermal conductivity of base fluids into which are suspended particles of various geometries. <br /><br /> Numerical work indicates that the models show generally good agreement for the various geometric dispersions, in particular for particles with low to moderate aspect ratios. The numerical results approach the lower bound as the conductivity ratio is increased. An important observation is that orienting the particles in the direction of heat flow leads to substantial enhancment in the thermal conductivity of the base fluid. Clustering leads to a small enhancement in effective thermal conductivity beyond that which is predicted for systems composed of regular arrays of particles. Although significant enhancement is possible if the clusters are large, in reality, clustering to the extent that solid agglomerates span large distances is unlikely since such clusters would settle out of the fluid. <br /><br /> In addition, experimental work available in the literature indicates that the agreement between the selected experimental data and the geometric mean of the upper and lower bounds for a sphere in a unit cell are in excellent agreement, even for particles which are irregular in shape.

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Cool roofs are the roofs that can deliver high solar reflectance and high thermal emittance. The benefits associated with cool roofs include reduced cooling energy load, reduced air pollution and greenhouse gas emission, and improved human health and comfort. This study attempts to develop standard measurement method for evaluating the reflectance and emmittance of a cool roof material. First, a literature survey was conducted to analysis the current programs promoting the use of cool roofs in the world, and then more than 2000 cool roof materials¡¦ data were collected in this study. In addition, the dynamic building energy load simulation by using eQuest was conducted to investigate the energy-saving benefits of cool roof applied in Taiwan. The results indicated that the reflectance, emmittance and thermal conductivity have a significant effect on the roof heat gain. The higher reflectance or emmittance of the roof, the less heat gain absorbed in the roof. But, reflectance has a larger effect on roof energy-saving than emittance does. The energy-saving effect by using cool roof on the flat-type roof is larger than on low-slope type roof.