No description available.

L'objectif de ces recherches est l'amélioration du procédé de placement de fibre par dépose laser, appliqué à des composites Poly(EtherKetoneKetone) / fibre carbone continue. L'optimisation des paramètres de dépose implique l'étude de la stabilité de la matrice, ainsi que l'analyse du comportement thermique des composites. La conductivité et diffusivité thermiques, ainsi que l'émissivité sont les paramètres clés pour comprendre le comportement des transferts volumiques et surfacique dans ces matériaux. Des composites PEKK/particules de carbone ont été élaborés afin d'étudier l'effet de la morphologie du carbone sur les paramètres thermiques. L'émissivité des composites PEKK / fibres continues a été mesurée en comparant les radiations émises par le matériau et celle émises par un corps noir à la même température. La conductivité et la diffusivité thermiques, qui sont des paramètres intrinsèques au composite, augmentent avec le taux de charge et la température. Ces deux paramètres ont été étudiés en fonction de la Température, dans le cas de composite PEKK / fibre carbone continue et PEKK / particules de carbone pour plusieurs taux de charge. / : The aim of this research is the improvement of the laser depositing fiber placement process, applied to Poly (EtherKetoneKetone) / continuous carbon fiber composites. The optimization of deposit parameters implies the investigation of the stability of the matrix and of the thermal behavior of composites. Thermal conductivity, diffusivity, and optical emissivity, are the key parameters for understanding the behavior of volume and surface conduction of composites. PEKK / carbon particle composites have been processed to study the effect of carbon morphology on thermal parameters. The emissivity of the PEKK / continuous fiber composites was measured by comparing the radiation emitted by the material with the one emitted by a black body at the same temperature. Thermal conductivity and diffusivity, which are intrinsic parameters of the composite, increase with the rate of charge and the temperature. These two parameters have been studied as a function of temperature, in the case of PEEK / continuous fibers composites and PEKK / carbon particles composites for various charge rates.

Growth Mechanism and Infrared Detection of High-temperature Superconducting and Colossal Magnetoresistance Films Department of Electrical Engineering, National Sun Yat-Sen University Meng-Tsong Hong* Ying-Chung Chen**, Hsiung Chou** -------------------------------------------------- Abstract---- The growth mechanism of YBa2Cu3O7-d (YBCO) films grown by RF sputtering has been investigated. When growing films by RF sputtering, the shape of the plasma and the degree of resputtering effect were varied by setting different relative positions of the heater to the gun. As the substrate was near the plasma, the negative oxygen ions resputtered part of the mobile atoms from the surface of film back into the plasma, which caused the composition distortion, delayed the merge of grains and left uncovered holes. Setting a longer relative distance, the resputtering effect was suppressed and the precipitates appeared on the surface of films resulting in a rough surface. At an optimum relative position between heater and gun, the function of resputtering produced a polishing effect on the surface of films. This polishing effect suppressed the growth of precipitates without slowing down the growth of grains, a smooth and precipitate-free YBCO film might obtain. We also found that the film with smooth and precipitate-free morphology exhibited suppressed superconductivity. The most direct way to enhance the photoresponse of a bolometer is by modifying the microbridge from a single straight bridge to a meander or change the thermal coupling configuration between bolometer and heat sink. In the study of high-temperature superconducting (HTSC) bolometers, it is found that the geometry and thermal coupling configuration play very important roles on the behavior of heat conduction, which alter the thermal conversion efficiency, DT/WD. Actually, DT/WD is a matter of the absorption of the AC thermal irradiation and the dissipation of both the irradiation and the DC joule heat generated by the bias current. The competition between the capability of heat dissipation and the thermal generation determined the magnitude of DT/WD. The La0.67Ca0.33MnO3-y (LCMO) thin films with epitaxial structure and smooth surface morphology have been deposited. A LCMO thin-film microbridge was fabricated into a microbridge by conventional photolithography and dry etching for optical detection. The measured photoresponse, DV, of this LCMO thin-film microbridge reveals that it is bolometric in nature. The photoresponse is linearly proportional to the bias current Ib and the power density of irradiation WD, which strongly suggests the applicability of an LCMO thin-film microbridge to a linear optical detector. The ratio of the photoresponse to the irradiated power density, DV/WD, is independent of the incident-light wavelength l from 0.633 to 3.5 mm. The dependence of the photoresponse on modulated frequency f, follows the DV µ f -0.21 relation. Under Ib = 100 mA and f = 5 Hz at an operating temperature Top = 223 K, the responsivity S and noise voltage Vn are 685 V/W and 20 nV¡ÑHz -0.5, respectively, for this LCMO thin-film microbridge. From the measured S and Vn, the noise equivalent power (NEP) and detectivity D* were be calculated to be 2.92´10-11 W¡ÑHz -0.5 and 2.76´109 cm¡ÑHz 0.5¡ÑW -1, respectively, for this LCMO thin-film microbridge. The experimental results from this LCMO thin-film microbridge show the practical applicability of this new detector system compared to other established detectors. *Student **Advisor

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.

The problem of heating the solar corona requires the conversion of magnetic energy into thermal energy. Presently, there are two promising mechanisms for heating the solar corona: wave heating and nanoflare heating. In this thesis, we consider nanoflare heating only. Previous modelling has shown that the kink instability can trigger energy release and heating in large scale loops, as the field rapidly relaxes to a lower energy state under the Taylor relaxation theory. Two distinct experiments were developed to understand the coronal heating problem: the avalanche effect within a multiple loop system, and the importance of thermal conduction and optically thin radiation during the evolution of the kinked-unstable coronal magnetic field. The first experiment showed that a kink-unstable thread can also destabilise nearby threads under some conditions. The second experiment showed that the inclusion of thermal conduction and optically thin radiation causes significant change to the internal energy of the coronal loop. After the initial instability occurs, there is continual heating throughout the relaxation process. Our simulation results show that the data is consistent with observation values, and the relaxation process can take over 200 seconds to reach the final relaxed state. The inclusion of both effects perhaps provides a more realistic and rapid heating experiment compared to previous investigations.

According to classical linearized resistive magnetohydrodynamics theory, pressuredriven modes are unstable in the reversed-field pinch (RFP) due to unfavorable magnetic field line curvature. The result is based on the assumption of an adiabatic energy equation where anisotropic thermal conduction effects are ignored as compared to convection and compression. In this thesis the effects of heat conduction in the energy equation have been studied. We have examined these effects on the linear stability of pressure-driven resistive modes using boundary value theory (Δ´ ) and a novel initial-value full resistive MHD code employing the Generalized Weighted Residual Method (GWRM). In the Δ´ method, a shooting technique is employed by integrating from the resistive layer to boundaries. The GWRM method, on the other hand, is a time-spectral Galerkin method in which the fully linearized MHD equations are solved. For detailed computations, efficiency requires the temporal and spatial domains to be divided into subdomains. For this purpose, a number of challenging test cases including linearized ideal MHD equations are treated. Numerical and analytical investigations of equilibria reveal that thermal conduction effects are not stabilizing for reactor relevant values of Lundquist number, S0, and normalized pressure, βθ, for tearing-stable plasmas. These studies show that growth rate scales as  γ~_ S0−1/5 , which is weaker than for the adiabatic case, γ~_ S0−1/3. A numerical study of optimized confinement for an advanced RFP scenario including ohmic heating and heat conduction, is also part of this thesis. The fully nonlinear resistive MHD code DEBSP has been employed. We have identified, using both Δ´ and GWRM methods, that the observed crash of the high confinement is caused by resistive, pressure-driven modes. / <p>QC 20130503</p>

En mécanique des matériaux et des structures, l’interface entre deux composants matériels ou deux éléments structuraux est traditionnellement et le plus souvent supposé parfaite. Au sens mécanique, une interface parfaite est une surface à travers laquelle le vecteur de déplacement et le vecteur de contrainte sont tous les deux continus. L’hypothèse des interfaces parfaites est inappropriée dans de nombreuses situations en mécanique. En effet, l’interface entre deux corps ou deux parties d’un corps est un endroit propice aux réactions physico-chimiques complexes et favorable à l’endommagement mécanique. L’intérêt pour les interfaces imparfaites devient depuis quelques années grandissant avec le développement des matériaux et structures nanométriques dans lesquels les interfaces et surfaces jouent un rôle prépondérant. A partir de la configuration de base où une interphase de faible épaisseur sépare deux phases, ce travail établit trois modèles d’interface imparfaite généraux qui permettent de remplacer l’interphase par une interface imparfaite dans les cas de la conduction thermique, de l’élasticité linéaire et de la piézoélectricité sans perturber les champs en questions à une erreur fixée près. La dérivation de ces modèles est basée sur le développement de Taylor et sur une approche originale de géométrie différentielle indépendante de tout système de coordonnées. Les trois modèles généraux permettent non seulement de mieux appréhender certains modèles phénoménologiques d’interface imparfaite mais aussi de décrire les effets d’interface que les modèles existants ne sont pas en mesure de prendre en compte. Les modèles d’interface imparfaite établis sont appliqués dans la détermination des propriétés effectives thermiques, élastiques et piézoélectriques d’un matériau composite constitué d’une matrice renforcée par des particules ou fibres enrobées d’une interphase. La méthode utilisée pour rendre compte des effets des interfaces imparfaites sur les propriétés effectives repose sur une condition d’équivalence énergétique qui ramène un matériau hétérogène avec interfaces imparfaites à un matériau hétérogène avec interfaces parfaites / In mechanics of materials and structures, the interface between two material components or two structural elements is traditionally and the most often assumed to be perfect. In mechanics, a perfect interface is a surface through which the displacement and stress vectors are continuous. The assumption of the perfect interfaces is inappropriate in many situations in mechanics. Indeed, the interface between two bodies or two parts of a body is a place propitious to complex physicochemical reactions and vulnerable to mechanical damage. The interest in imperfect interfaces has become for a few years growing with the development of nanometric materials and structures in which the interfaces and surfaces play a preponderant role. Starting from the basic configuration where an interphase of thin thickness separates two phases, this work establishes three general models of imperfect interface which make it possible to replace the interphase by an imperfect interface in the cases of thermal conduction, linear elasticity and piezoelectricity without disturbing the fields in questions to within a fixed error. The derivation of these models is based on the development of Taylor and an original coordinate-free approach of differential geometry. The three general models make it possible not only to get a better understanding of certain phenomenological models of imperfect interface but also to describe the effects of interface which the existing models are not able to take into account. The established models of imperfect interface are applied to determining the thermal, elastic and piezoelectric effective properties of composite materials consisting of a matrix reinforced by particles or fibers coated with an interphase. The method used to account for the effects of imperfect interfaces on the effective properties rests on an energy equivalency which brings back a heterogeneous material with imperfect interfaces to a heterogeneous material with perfect interfaces

Les modèles linéaires non entiers ont prouvé leur efficacité dans la modélisation de la diffusion thermique pour de faibles variations de température. Cependant, pour de grandes variations de température, les paramètres thermiques dépendent de la température. Par conséquent, la diffusion thermique est régie par un modèle non linéaire non entier. Dans cette thèse, une classe de modèles non linéaires non entiers, basée sur les séries de Volterra non entières, est proposée. Les paramètres non linéaires, tels que les s^n-pôles et l’ordre commensurable, sont estimés au même titre que les coefficients linéaires. Ensuite, le comportement thermique d’un échantillon de fer ARMCO est modélisé pour de grandes variations de température. / Linear fractional differentiation models have proven their efficacy in modeling thermaldiffusive phenomena for small temperature variations. However, for large temperature variations,the thermal parameters are no longer constant but vary along with the temperatureitself. Consequently, the thermal system could be modeled by non linear fractional differentialmodels. Volterra series are first extended to fractional derivatives. Volterra seriesare then used for modeling a non linear thermal system, constituted of an ARMCO iron sample,for large temperature variations.

Ce manuscrit porte sur l’étude de cuprates de basse dimensionnalité, les systèmes à chaînes de spins SrCuO₂ et Sr₂CuO₃. Un des intérêts de ces deux composés est qu’ils présentent des conductions thermiques fortement anisotropes. Celles-ci comportent une contribution magnétique due au transport de la chaleur via les excitations de spinons qui se manifeste uniquement dans la direction des chaînes de spins. Notre étude a pour objectif la mise en évidence des mécanismes qui gouvernent ces propriétés de transport, notamment à travers l’étude des interactions entre les spinons, les phonons et les défauts. Les interactions spinons (phonons)-défauts ont été sondées par l’introduction intentionnelle de dopants (1-2%) non-magnétiques sur le site du cuivre : Mg²⁺, Zn²⁺, Pd²⁺ ou Ni²⁺, ou encore par l’introduction d’éléments possédant des degrés d’oxydation différents sur le site du strontium : La³⁺ ou K⁺. Les composés ont été synthétisés sous leur forme monocristalline par la méthode de fusion de la zone solvante. Des caractérisations structurales, magnétiques et thermiques des composés purs et dopés ont été réalisées. Les spectres d’excitations magnétiques de ces cuprates ont été déterminés par diffusion inélastique de neutrons, spectroscopie RMN et spectroscopie de photoémission résolue en angle afin de révéler l’impact de la substitution. L’étude des spectres de phonons a également été réalisée par diffusion inélastique de neutrons. Les résultats de ces mesures sont corrélés aux propriétés de conduction thermique des composés purs et dopés.. / This manuscript deals with the study of low dimensional cuprates, namely, the spin chains systems SrCuO₂ and Sr₂CuO₃. These two compounds exhibit highly anisotropic thermal conduction properties along the spin-chains direction, where magnetic thermal conduction contributes to the heat transport process via spinon excitations. Our study aims to highlight the mechanisms that govern the heat transport properties, particularly through the study of the scattering channels involving spinon, phonon and defects. The spinon (phonon)–defect scattering was probed by the intentional introduction of nonmagnetic dopants (1-2%) on the copper site, by: Mg²⁺, Zn²⁺, Pd²⁺ or Ni²⁺, or by the introduction of elements carrying different oxidation level on the strontium site, by: La³⁺ or K⁺. Single crystals of the pure and doped materials have been grown by the travelling solvent floating zone method. The structural, magnetic and thermal characterizations of the pure and doped compounds were performed. The magnetic excitation spectra of the compounds were determined by inelastic neutron scattering, NMR spectroscopy, and angle resolved photoemission spectroscopy to reveal the impact of the substitution on the spin dynamics of the doped compounds. The study of phonon spectra has also been performed by inelastic neutron scattering. Results from inelastic neutron scattering have been correlated to the heat transport properties of the pristine and substituted materials.

Shape memory alloys when deformed can produce strains as high as 8%. Heating results in a phase transformation and associated recovery of all the accumulated strain, a phenomenon known as shape memory. This strain recovery can occur against large forces, resulting in their use as actuators. The goal of this project is to lower the operating temperature range of shape memory alloys in order for them to be used in cryogenic switches, seals, valves, fluid-line repair and self-healing gaskets for space related technologies. The Ni-Ti-Fe alloy system, previously used in Grumman F-14 aircrafts and activated at 120 K, is further developed through arc-melting a range of compositions and subsequent thermo-mechanical processing. A controlled atmosphere arc-melting facility and vertical vacuum quench furnace facility was commissioned to fabricate these alloys. The facility can create a vacuum of 10-7 Torr and heat treat samples up to 977 °C. High purity powders of Ni, Ti and Fe in varying ratios were mixed and arc-melted into small buttons weighing 0.010 kg to 0.025 kg. The alloys were subjected to solutionizing and aging treatments. A combination of rolling, electro-discharge machining and low-speed cutting techniques were used to produce strips. Successful rolling experiments highlighted the workability of these alloys. The shape memory effect was successfully demonstrated at liquid nitrogen temperatures through a constrained recovery experiment that generated stresses of over 40 MPa. Differential scanning calorimetry (DSC) and a dilatometry setup was used to characterize the fabricated materials and determine relationships between composition, thermo-mechanical processing parameters and transformation temperatures.