Дисертації з теми "Molding materials Thermal properties"

This diploma thesis deals with thermal qualities of moulding mixtures of foundry mould of various types of sand grains and a computer simulation of the solidification of the cast. The aim of this work is to compare the values of thermal qualities of moulding materials bonded with water glass taken in experimental measurements to the values of thermal qualities of moulding materials connected with organic binding agent contained in simulation software. A sample cast will be moulded in order to evaluate cooling capacities of individual moulding mixtures according the shift of thermal axis. These results will consequently be compared to the results of the simulation.

Mathematical models of heat flow and elastic stress generation based on the finite-element method have been developed and utilized to analyze the Epic-3 Monofrax-S casting process (Monofrax-S is primarily composed of 47-57% A1₂O₃, 34-41% ZrO₂ and 10-15% SiO₂). The results of the mathematical analysis, in conjunction with information obtained from a comprehensive industrial study, has led to the development of mechanisms for the formation of the various crack types found in this casting process. Thermal stresses have been predicted to be generated early in the solidification process in association with rapid cooling of the refractory surface as it contacts the initially cool mould and again later in the solidification process in conjunction with the tetragonal-to-monoclinic phase transformation which occurs in the zirconia component of Monofrax-S. The mathematical analysis has also helped to identify indirectly a potential mechanism for the generation of mechanical stresses. Based on an understanding of the generation of tensile stresses, recommendations have been made for modifications to the moulding and casting procedures in order to reduce the propensity for the formation of cracks. The modifications have included changes to the mould construction and geometry to reduce the generation of mechanical stresses and changes to the moulding materials to impact on the flow of heat at key times during solidification and cooling. With the recommendations in place, the casting process has been re-examined with the mathematical models to verify the impact of the modifications. The predictions show that the modifications have acted to reduce tensile stresses associated with the formation of Type-A and -B cracks. Preliminary industrial trials with the modified mould have yielded blocks free of these defects.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

In the search of clean and efficient energy sources intermediate temperature solid oxide fuel cells are among the prime candidates. What sets the limit of their efficiency is the solid electrolyte. A promising material for the electrolyte is ceria. This thesis aims to improve the characteristics of these electrolytes and help provide thorough physical understanding of the processes involved. This is realised using first principles calculations. The class of methods based on density functional theory generally ignores temperature effects. To accurately describe the intermediate temperature characteristics I have made adjustments to existing frameworks and developed a qualitatively new method. The new technique, the high temperature effective potential method, is a general theory. The validity is proven on a number of model systems. Other subprojects include low-dimensional segregation effects, adjustments to defect concentration formalism and optimisations of ionic conductivity.

La gestió tèrmica és un problema crític en el disseny de dispositius nanoelectrònics. Les solucions de refredament avançades i la recol·lecció eficient d'energia són clau per mantenir la tendència de productes electrònics cada vegada més petits i ràpids. Aquesta tesi se centra en la gestió tèrmica i l'ús de calor dissipada en materials emergents per a l'electrònica. En particular, els materials bidimensionals (2DM) i heteroestructures d'aquests són dels candidats més interessants per al futur de l'electrònica i s'estan investigant intensament. En aquesta tesi, s'exploren dos temes principals: (i) el transport tèrmic de 2DMs suspesos, inclòs el grafè CVD, dicalcògens de metalls de transició (TMDC) i heteroestructures de TMDC amb nitrur de bor hexagonal (hBN); i (ii) les propietats tèrmiques i de termoelectricitat de les pel·lícules primes (Bi1-xSbx)2Te3 (BST). Aquests materials s'estan considerant per interconnexions i transistors fins als THz (grafè), per a electrònica digital (TMDCs) i per a aïllament elèctric (hBN) i són ben coneguts com a generadors termoelèctrics, com ho són també materials recentment identificats com a aïllants topològics (BST). En primer lloc, l'objectiu era mesurar la conductivitat tèrmica de 2DMs utilitzant el mètode d'espectroscòpia Raman de dos làsers, recentment desenvolupat. La implementació fou dificultada per l'ús de membranes relativament petites obtingudes dels materials investigats i la seva alta conductivitat tèrmica. Demostràrem que la conductivitat tèrmica del grafè CVD és d'aproximadament 300 W/(m·K). Encara que menor que en el grafè exfoliat, això podria ser degut a les fronteres de gra i al desordre en el grafè CVD. Demostràrem també que les conductivitats tèrmiques de MoS2 i MoSe2 exfoliats (dos TMDC) són de 12 a 24 W/(m·K) i 60 W/(m·K), respectivament. I que per a membranes primes (de poques monocapes) de MoS2, la conductivitat incrementa amb el gruix. Afegint una membrana de hBN exfoliada sobre una mostra de MoS2 prèviament caracteritzada demostràrem un augment notable de la conductivitat tèrmica en l'heteroestructura de hBN/MoS2, quan s'introdueix calor al MoS2. Aquesta presenta una conductivitat tèrmica de 185 W/(m·K), gairebé un ordre de magnitud més gran que el MoS2 sol. En segon lloc, capes primes de BST crescudes mitjançant epitàxia de feix molecular s'estudiaren amb l'objectiu de correlacionar-ne les propietats termoelèctriques amb el seu nivell de Fermi, que sintonitzaria el pes relatiu del transport de volum i dels estats topològics de superfície (TSS). Primer demostràrem que és possible dissenyar l'estructura de la banda i ajustar el nivell de Fermi des de la valència fins a la banda de conducció simplement controlant la concentració de Sb. La demostració s'aconseguí utilitzant espectroscòpia de fotoemissió amb resolució angular en combinació amb conductivitat elèctrica i mesures d'efecte Hall en pel·lícules relativament primes (10 nm). S'identificà la concentració de Sb a la qual els TSSs dominen el transport i es dugueren a terme experiments termoelèctrics en les mateixes capes. No es trobà una correlació clara entre l'energia termoelèctrica i la naturalesa dels portadors de càrrega quan els TSSs eren dominants. Això indica que el transport dels TSSs té una influència limitada en les propietats termoelèctriques d'aquest material, i que per tal d'observar els efectes de superfície es necessitarien capes encara més primes. Finalment, una caracterització de les capes primes de BST usant espectroscòpia Raman demostrà variacions específiques en el comportament associat a la concentració de Sb. En particular, l'augment de la potència del làser va donar lloc a l'aparició de pics Raman no actius d'origen indeterminat. Aquests pics poden indicar la ruptura de simetries estructurals, modes de fonó de superfície o altres efectes com ara ressonàncies plasmòniques que són d'alt interès. La inesperada resposta observada en l'espectre Raman hauria de motivar investigacions addicionals.
La gestión térmica es un problema crítico en el diseño de dispositivos nanoelectrónicos. Las soluciones de enfriamiento avanzadas y la recolección eficiente de energía son clave para mantener la tendencia de productos electrónicos cada vez más pequeños y rápidos. Esta tesis se centra en la gestión térmica y el uso de calor disipado en materiales emergentes para la electrónica. En particular, los materiales bidimensionales (2DM) y las heteroestructuras basadas en ellos son candidatos muy interesantes para el futuro de la electrónica y se están investigando intensamente. La tesis trata dos temas principales: (i) el transporte térmico de 2DMs suspendidos, incluido el grafeno CVD, dicalcogenuros de metales de transición (TMDC) y heteroestructuras de TMDC con nitruro de boro hexagonal (hBN); y (ii) las propiedades térmicas y de termoelectricidad de películas delgadas de (Bi1-xSbx)2Te3(BST). Estos materiales están siendo considerados para interconexiones y transistores hasta THz (grafeno), electrónica digital (TMDCs) y aislamiento eléctrico (hBN) y son bien conocidos como generadores termoeléctricos, como también lo son materiales recientemente identificados como aislantes topológicos (BST). En primer lugar, el objetivo fue medir la conductividad térmica de 2DMs utilizando el método de espectroscopia Raman de dos láser, recientemente desarrollado. El desafío fue el uso de membranas relativamente pequeñas obtenidas y su alta conductividad térmica. Demostramos que la conductividad térmica del grafeno CVD es de aproximadamente 300 W/(m·K). Aunque menor que en el grafeno exfoliado, esto podría deberse a los bordes de grano y al desorden en grafeno CVD. Demostramos también que las conductividades térmicas de MoS2 y MoSe2 exfoliados (dos TMDC) son 12 a 24 W/(m·K) y 60 W/(m·K), respectivamente. Y que para membranas delgadas (pocas monocapas) la conductividad incrementa con su grosor. Agregando una membrana de hBN exfoliada sobre una muestra de MoS2 previamente caracterizada nos permitió demostrar un notable aumento de la conductividad térmica en la heteroestructura de hBN/MoS2, cuando se introduce calor en MoS2. Esta presenta una conductividad térmica de 185 W/(m·K), casi un orden de magnitud mayor que para MoS2. En segundo lugar, se estudiaron películas delgadas de BST crecidas mediante epitaxia de haz molecular con el objetivo de correlacionar sus propiedades termoeléctricas con su nivel de Fermi, que sintonizaría el peso relativo del transporte de volumen y de los estados topológicos de superficie (TSS). Primero demostramos que es posible diseñar la estructura de la banda y ajustar el nivel de Fermi desde la valencia hasta la banda de conducción simplemente controlando la concentración de Sb. Para ello se utilizó espectroscopia de fotoemisión con resolución angular en combinación con conductividad eléctrica y mediciones de Hall en películas relativamente delgadas (10 nm). También se identificó la concentración de Sb a la que los TSSs dominan el transporte y se llevaron a cabo experimentos termoeléctricos en las mismas películas. No se encontró una correlación clara entre la energía termoeléctrica y la naturaleza de los portadores de carga cuando los TSSs eran dominantes, indicando que el transporte de los TSSs tiene una influencia limitada en las propiedades termoeléctricas de este material y que para observar los efectos de superficie se necesitarían películas más delgadas. Finalmente, una caracterización de las películas delgadas de BST usando espectroscopia Raman demostró variaciones específicas en el comportamiento asociado a la concentración de Sb. En particular, el aumento de la potencia del láser dio lugar a la aparición de picos Raman no activos de origen indeterminado. Estos picos pueden indicar la ruptura de simetrías estructurales, modos de fonón de superficie u otros efectos tales como resonancias plasmónicas que son de alto interés, una respuesta que debería motivar investigaciones adicionales.
Thermal management is becoming a critical issue in the packaging and design of nanoelectronics. Advanced cooling solutions and efficient energy harvesting are key aspects to help keep the trend for ever smaller and faster electronics. This thesis is focused on thermal management and the use of heat waste in emerging materials for electronics. In particular, two-dimensional materials (2DM), and related heterostructures, are amongst the most intriguing prospects for future electronics and are being intensively investigated. Here, two main subjects were explored. First, the thermal transport of suspended 2DMs, including CVD graphene, transition metal dichalcogenides (TMDCs) and heterostructures of TMDCs with hexagonal boron nitride (hBN) and, second, the thermal properties and thermoelectricity of (Bi1-xSbx)2Te3 (BST) thin films. These materials are being considered for interconnects and THz transistors (graphene), digital electronics (TMDCs) and electrical insulation (hBN) and are well known as thermoelectric generators, as are also materials that have recently been identified as topological insulators (BST). In the first part, the objective was to demonstrate the measurement of the thermal conductivity of 2DMs using the recently developed two-laser Raman spectroscopy method. Its implementation was rendered difficult by the relatively small exfoliated flakes of the materials investigated and their high thermal conductivity. The thermal conductivity of CVD graphene was found to be about 300 W/(m·K). Although smaller than exfoliated graphene, it is argued that this could be due to grain boundaries and disorder. Exfoliated MoS2 and MoSe2 (two well-known TMDCs) presented thermal conductivities of 12 to 24 W/(m·K) and 60 W/(m·K). Measurements on different membranes of MoS2 further showed that the conductivity increases with the thickness in thin membranes (few monolayers). Furthermore, stacking an exfoliated hBN membrane on top of a previously characterized MoS2 sample allowed us to demonstrate a notorious increase of the thermal conductivity in the hBN/MoS2 heterostructure, when heat is introduced on MoS2. Indeed, when compared with MoS2 alone the thermal conductivity is found to be almost one order of magnitude larger, 185 W/(m·K). For the second part, BST thin films were grown by molecular beam epitaxy. The main objective was to investigate the correlation of the thermoelectric properties of these materials with the Fermi level, which would tune the relative weight of bulk and topological surface state (TSS) transport. It was first demonstrated that controlling the concentration of Sb we could engineer the band structure and tune the Fermi level from the valence to the conduction band. Such demonstration was achieved by using angle-resolved photoemission spectroscopy in combination with conductivity and Hall measurements in relatively thin (10 nm) films. The Sb concentration at which TSS dominated the transport was also identified. Thermoelectric experiments on the same films were then carried out but no clear correlation between the thermopower and the carrier nature was found when the TSSs were dominant. These results indicate that TSS transport has limited influence on the thermoelectric properties. Further studies should be carried our using even thinner films. Finally, a side characterization of the BST thin films using Raman spectroscopy demonstrated specific variations in the behaviour associated to Sb concentration. An increase of the laser power showed the emergence of non-active Raman peaks of undetermined origin. However, they can indicate the presence of broken structural symmetries, surface phonon modes or other effects such as plasmonic resonances. This interesting response is worthy of for further investigation.
Universitat Autònoma de Barelona. Programa de Doctorat en Física

A programme of work was instigated to investigate and improve the efficiency, and stability of intra-cavity étalons for the Argon ion laser, especially in the far blue to near ultra-violet wavelength region, by using sol-gel prepared silica. The original program has been modified by experiences outlined below. We have attempted to differentiate between the sol-gel derived and the high temperature prepared silica. We confirm that relatively small changes in thickness of silica monoliths can give rise to proportionally much larger changes of refractive index. Our main target has been to assess whether it is possible to make a new optical material that shows insignificant changes in optical path length or refractive index with temperature.

Many modern technologies are enabled by the use of thin films and/or nanostructured composite materials. For example, many thermoelectric devices, solar cells, power electronics, thermal barrier coatings, and hard disk drives contain nanostructured materials where the thermal conductivity of the material is a critical parameter for the device performance. At the nanoscale, the mean free path and wavelength of heat carriers may become comparable to or smaller than the size of a nanostructured material and/or device. For nanostructured materials made from semiconductors and insulators, the additional phonon scattering mechanisms associated with the high density of interfaces and boundaries introduces additional resistances that can significantly change the thermal conductivity of the material as compared to a macroscale counterpart. Thus, better understanding and control of nanoscale heat conduction in solids is important scientifically and for the engineering applications mentioned above. In this dissertation, I discuss my work in two areas dealing with nanoscale thermal transport: (1) I describe my development and advancement of important thermal characterization tools for measurements of thermal and thermoelectric properties of a variety of materials from thin films to nanostructured bulk systems, and (2) I discuss my measurements on several materials systems done with these characterization tools. First, I describe the development, assembly, and modification of a time-domain thermoreflectance (TDTR) system that we use to measure the thermal conductivity and the interface thermal conductance of a variety of samples including nanocrystalline alloys of Ni-Fe and Co-P, bulk metallic glasses, and other thin films. Next, a unique thermoelectric measurement system was designed and assembled for measurements of electrical resistivity and thermopower of thermoelectric materials in the temperature range of 20 to 350 °C. Finally, a commercial Anter Flashline 3000 thermal diffusivity measurement system is used to measure the thermal diffusivitiy and heat capacity of bulk materials at high temperatures. With regards to the specific experiments, I examine the thermal conductivity and interface thermal conductance of two different types of nanocrystalline metallic alloys of nickel-iron and cobalt-phosphorus. I find that the thermal conductivity of the nanocrystalline alloys is reduced by a factor of approximately two from the thermal conductivity measured on metallic alloys with larger grain sizes. With subsequent molecular dynamics simulations performed by a collaborator, and my own electrical conductivity measurements, we determine that this strong reduction in thermal conductivity is the result of increased electron scattering at the grain boundaries, and that the phonon component of the thermal conductivity is largely unchanged by the grain boundaries. We also examine four complex bulk metallic glass (BMG) materials with compositions of Zr₅₀Cu₄₀Al₁₀, Cu_46.25Zr_44.25Al_7.5Er₂, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂, and Ti_41.5Zr_2.5Hf₅Cu_42.5Ni_7.5Si₁. From these measurements, I find that the addition of even a small percentage of heavy atoms (i.e. Hf and Er) into complex disordered BMG structures can create a significant reduction in the phonon thermal conductivity of these materials. This work also indicates that the addition of these heavy atoms does not disrupt electron transport to the degree with which thermal transport is reduced.
Ph. D.

Thesis (Ph.D.)--Chemical Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Prof. Sankar Nair; Committee Co-Chair: Prof. Samuel Graham; Committee Member: Prof. Amyn S. Teja; Committee Member: Prof. Mo Li; Committee Member: Prof. Peter Ludovice.

The thermal properties of nanocrystal arrays and large unit cell molecular crystals are studied using experimental and computational techniques. The major objective is to understand the mechanisms of thermal transport through three-dimensional organic-inorganic superstructured materials that are built from superatoms. The frequency domain thermoreflectance technique is applied to measure thermal conductivity in thin films and nanoliter sized single crystals. Molecular dynamics simulations, lattice dynamics calculations, and density functional theory calculations are employed to interpret the measurements and to explore experimentally-inaccessible nanoscale phenomena. A superatom is a cluster of atoms that behaves as a stable or metastable unit with emergent properties distinct from its elemental atoms. Superatoms can self-assemble into three-dimensional hierarchical materials with each superatom occupying a lattice site to form a periodic solid (i.e., a superlattice). By varying geometry and composition, the resulting solid can have tunable electrical and optical properties. This work presents the first investigation of the thermal properties of solids built from two classes of superatoms: (i) monodispersed nanocrystals that form a nanocrystal array (NCA) and (ii) inorganic-organic superatoms of precise stoichiometric composition that form a molecular crystal (LUMC). The thermal conductivity of NCAs was measured to be between 0.1 to 0.3 W/m-K. Experiments revealed that energy transport is mediated by the density and chemistry of the organic/inorganic interfaces as well as the volume fractions of nanocrystal cores and surface ligands. The NCA thermal conductivity trends upward then plateaus with increasing temperature suggesting elastic scattering events dominate transport at the organic-inorganic interfaces. The onset temperature of the plateau is dependent on the overlap of the vibrational states in the core and ligand. Atomistic computational analysis of the thermal transport explored experimentally inaccessible trends that provided new insights for controlling heat flow in NCAs. A decreasing interfacial thermal conductance trend for the organic-inorganic interface with increasing nanocrystal diameter was discovered. This trend can be related to the interfacial thermal conductance of a self-assembled monolayer (SAM) interface through a geometrical scaling law. Changing the atomic mass of the nanocrystal core to vary its vibrational states resulted in a non-monotonic trend in both the thermal conductivity and interfacial thermal conductance. Peaks in both properties occur at the same small atomic mass and are related to the overlap and coupling of the organic and inorganic vibrational states. Preliminary measurements of LUMCs indicate that the thermal conductivity is between 0.2 and 0.4 W/mK at the temperature of 300 K, comparable to that of an amorphous polymer. A slight increase in thermal conductivity is observed for the binary-species LUMCs that contain fullerene derivatives over their corresponding mono-species LUMCs composed of inorganic core superatoms. This increase may be attributed to the stronger ionic intermolecular bonds in the binary LUMCs. The presence of a larger chalcogenide element in the inorganic core superatom decreases the thermal conductivity of the LUMC. This decrease is consistent with lower frequency vibrational modes that have a lower group velocity in the crystal. The temperature dependent thermal conductivity of a mono-species CoSe LUMC has a crystalline-like behavior, unlike most low thermal conductivity materials. Nanoscale superstructured organic-inorganic materials self-assemble from solutions and can be scalable to replace single crystal semiconductors for many technologies. Arrays of ligand-stabilized colloidal nanocrystals with size-tunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, opto-electronic, and energy-related applications. The low thermal conductivity in the NCA presents a challenge for thermal management but a boon for thermoelectric waste heat scavenging. The class of large unit cell molecular crystals investigated here have low thermal conductivity and a moderate electrical conductivity, making them novel candidates for thermoelectricity.

In the presented thesis work, meshfree method with distance fields is applied to create a novel computational approach which enables inclusion of the realistic geometric models of the microstructure and liberates Finite Element Analysis(FEA) from thedependance on and limitations of meshing of fine microstructural feature such as splats and porosity.Manufacturing processes of ceramics produce materials with complex porosity microstructure.Geometry of pores, their size and location substantially affect macro scale physical properties of the material. Complex structure and geometry of the pores severely limit application of modern Finite Element Analysis methods because they require construction of spatial grids (meshes) that conform to the geometric shape of the structure. As a result, there are virtually no effective tools available for predicting overall mechanical and thermal properties of porous materials based on their microstructure. This thesis is a separate handling and controls of geometric and physical computational models that are seamlessly combined at solution run time. Using the proposedapproach we will determine the effective thermal conductivity tensor of real porous ceramic materials featuring both isotropic and anisotropic thermal properties. This work involved development and implementation of numerical algorithms, data structure, and software.

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.

Thesis (M.Sc.)--City University of Hong Kong, 2004.
At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Sept. 1, 2006) Includes bibliographical references.

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 62-72).
Solar thermal fuels utilize molecules that undergo reversible photo-isomerization to convert solar energy into stored thermal energy.¹ Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable energy source. However, it has remained a challenge to identify a suitable solar thermal fuel material that exhibits high energy density, high energy conversion efficiency, long energy storage lifetime, and can be produced at low cost. A recent proposal is a nanotemplate-photoisomer hybrid system, e.g. functionalized azobenzene, a well-known photoisomer molecule, attached to carbon nanostructure templates such as carbon nanotubes, graphene, pentacene or alkene chains. Such structures have been suggested and tested as candidate solar thermal fuel materials with high energy density and long storage time²⁻⁴ In this thesis work, we further investigated optical properties of functionalized azobenzene and geometry-modified azobenzene. We found the best structure that yields maximum optical isomerization rate for trans-azobenzene and minimum optical isomerization rate for cis-azobenzene, calculating the reaction rate based on overlap between the solar spectrum and optical spectra calculated using time-dependent density functional theory (TDDFT). We showed that energy-charged-state molecule (cis-isomer) content at the photostationary state can be improved from 73 percent for pure azobenzene to 83 percent and to 97 percent by functionalizing azobenzene and a designing different geometry for azobenzene, respectively. From this, a desired structure for nanotemplates-photoisomer hybrid system can be estimated and same calculation technique may be employed to calculate and optimize photostationary state of the nanotemplates-photoisomer hybrid system.
by Jee Soo Yoo.
S.M.
S.M. Massachusetts Institute of Technology, Department of Materials Science and Engineering

The present dissertation investigates the relationship between the structure and thermoelectric properties of ZnO based materials, with a focus on trivalent element doping on engineering the microstructure and altering the electrical and thermal transport properties. Within the solubility range, the addition of trivalent elements, such as In3+, Fe3+ and Ga3+, is observed to increase the electrical conductivity of ZnO and decrease the thermal conductivity.
Engineering and Applied Sciences

Current magnetic data storage technology is encountering certain fundamental limitations that present roadblocks to its scalability to areal densities of 1 Tbit/in^2 and beyond. Next generation magnetic storage technology is expected to use optical near field techniques to heat the magnetic film locally to write data bits. This requires experimental measurement of thermal conductivity of materials with sub--100 nm resolution. This is essential for the tailoring of the thin film stack to optimize the heat transfer of the process. This can be accomplished with a simple modification to a traditional atomic force microscopy (AFM) system. The modification requires the deposition of a thin metal film on the AFM cantilever thus creating a bimetallic cantilever. The curvature of a bimetallic cantilever is sensitive to temperature. Another modification is the use of a heating laser to raise the temperature of the cantilever so that when it scans across a sample with areas of varying thermal conductivity the bimetallic deformation of the heated cantilever is altered. The resulting system is sensitive to local variations in thermal conductivity with nanoscale resolution. Nanoscale thermal conductivity measurements can then be used to optimize the heat transfer properties of the materials used in a heat assisted magnetic recording system. AFM technology can also play a key role in the development of next generation solid-state chemical sensors. An AFM can be used to measure the workfunction of a material with near atomic resolution thus enabling the study of chemical reactions with high spatial resolution. Since chemical sensors typically use a chemical reaction at their front end to monitor the prescience of a gas, an AFM system can thus be used to understand and optimize the properties of the chemical reaction by monitoring the local workfunction. In this thesis, I explain the use of atomic force microscopy in measuring thermal and chemical properties of materials with applications towards the magnetic storage industry and chemical sensing.

Historically ceramic crystal laser material has had disadvantages compared to single crystal laser material. However, progress has been made in the last decade and a half to overcome the disadvantages associated with ceramic crystal. Today, because of the promise of ceramic crystal as a high power laser material, investigation into its properties, both physical and optical, is warranted and important. Thermal expansion was measured in this thesis for Nd:YAG (yttrium aluminum garnet) ceramic crystal using an interferometric method. The interferometer employed a spatially filtered HeNe at 633 nm wavelength. Thermal expansion coefficients measured for the ceramic crystal samples were near the reported values for single crystal Nd:YAG. With a similar experimental setup as that for the thermal expansion measurements, dn/dT for ceramic crystal Nd:YAG was measured and found to be slightly higher than the reported value for single crystal. Depolarization loss due to thermal gradient induced stresses can limit laser performance. As a result this phenomenon was modeled for ceramic crystal materials and compared to single crystals for slab and rod shaped gain media. This was accomplished using COMSOL Multiphysics, and MATLAB. Results indicate a dependence of the depolarization loss on the grain size where the loss decreases with decreased grain size even to the point where lower loss may be expected in ceramic crystals than in single crystal samples when the grain sizes in the ceramic crystal are sufficiently small. Deformation-induced thermal lensing was modeled for a single crystal slab and its relevance to ceramic crystal is discussed. Data indicates the most notable cause of deformation-induced thermal lensing is a consequence of the deformation of the top and bottom surfaces. Also, the strength of the lensing along the thickness is greater than the width and greater than that due to other causes of lensing along the thickness of the slab. Emission spectra, absorption spectra, and fluorescence lifetime were measured for Nd:YAG ceramic crystal and Yb:Lu2O3 ceramic crystal. No apparent inhomogeneous broadening appears to exist in the Nd:YAG ceramic at low concentrations. Concentration and temperature dependence effects on emission spectra were measured and are presented. Laser action in a thin disk of Yb:Y2O3 ceramic crystal was achieved. Pumping was accomplished with a fiber coupled diode laser stack at 938 nm. A slope efficiency of 34% was achieved with maximum output energy of 28.8 mJ/pulse.
Ph.D.
Department of Physics
Sciences
Physics PhD

Thesis (Ph.D.)--Mechanical Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Graham, Samuel; Committee Co-Chair: Nair, Sankar; Committee Member: Grover - Gallivan, Martha; Committee Member: McDowell, David; Committee Member: Schelling, Patrick; Committee Member: Zhang, Zhuomin

Graphite and graphite composite materials are of great importance in various applications; however, they have been widely used in nuclear applications. Primarily in nuclear applications such, as a moderator where its primary aim is to stop the fast neutrons to thermal neutron. The composite graphite (HTR-10) has potential applications as a moderator and other applications including in aerospace field. Structurally the composite shows stable hexagonal form of graphite and no traces of the unstable Rhombohedral patterns. Thermal conductivity indicates the same trends observed and known for nuclear graded graphite. The composite was made as a mixture of 64 wt% of natural graphite, 16 wt% of synthetic graphite binded together by 20 wt% of phenolic resin. The resinated graphite powder was uni-axially pressed by 19.5 MPa to form a disc shaped specimen. The disc was then cut and annealed to 1800 °C. The composite was further cut into two directions (parallel and perpendicular) to the pressing direction. For characterization the samples were cut into 2.5 x 2.5 x 10 mm3. There were exposed to proton irradiation for 3 and 4.5 hrs respectively and characterized both structurally and thermally. Through the study what we have observed was that as the composite is exposed to proton irradiation there is an improvement structurally. Thus, the D peak in the Raman spectroscopy has decreased substantially with the irradiated samples. XRD has indicated that there is no un-stable Rhombohedral phase pattern in both the pristine and the irradiated samples. However this was further confirmed with that thermal conductivity is also increasing with irradiation exposure. This is anomalous to irradiated graphite in which defects are supposedly induced. Looking into the electrical resistivity we have noted that pristine samples have higher resistivity as compared to the irradiated samples. Seebeck coefficient indicates that there is some form of structural perfection and the samples have a phonon drag dip at the known graphite temperature of 35 K. This has shown us there are no impurities induced by irradiation of the samples.
Dissertation (MSc)--University of Pretoria, 2013.
gm2014
Physics
Unrestricted

Malgré ces propriétés intéressantes et l'abondance du matériau brut dont on peut les extraire, les applications des CNC pour des produits commerciaux restent limitées en raison des limitations liées à l’utilisation de procédés industriels. En effet, les matériaux produits à base de CNC sont généralement préparés par la technique de coulée/évaporation. Ce procédé permet de bien contrôler les propriétés comme l’humidité, la dispersion des particules et aussi d’assurer la formation d'un réseau tridimensionnel des particules. Cependant, ce procédé n'est pas facilement transposable à l'industrie. Une alternative à la technique de coulée/évaporation serait l'application du procédé d'extrusion et/ou de moulage par injection. Ces n'utilisent pas de solvants et peuvent donc être considérés comme des procédés plus verts que la coulée/évaporation. Il y a cependant des points négatifs. Ils sont essentiellement associés à des problèmes de dispersion, de dégradation thermique et de propriétés mécaniques plus faibles. Dans ce contexte, la thèse développée ici présente quelques études qui cherchent à combler quelque unes des lacunes décrites ci-dessus.Le Chapitre 1 présente une revue de la littérature et des concepts de base utilisés pendant la thèse. Dans le Chapitre 2, les particules de caoutchouc présentes dans la suspension de latex ont été oxydées en surface par addition de KMnO4 en conditions contrôlées. L’objectif était d’induire l’oxydation des doubles liaisons présentes dans la structure du caoutchouc naturel (NR). Les essais de traction cycliques ont montré que pour ces échantillons, les interactions interfaciales entre les nanoparticules et le polymère sont plus fortes. Cependant, quand le taux d’oxydation augmente, l’hydrophilie des chaînes de caoutchouc est observable.Dans le Chapitre 3, des composites à base de polycarbonate (PC) et de CNC ont été obtenus par la technique d’extrusion. Ce procédé implique de hautes températures (200oC) et la dégradation thermique devient alors un problème pendant la préparation des matériaux. Pour limiter cette dégradation, un mélange maître (masterbatch) contenant environ 30% de CNC a été produit et utilisé comme base pour la préparation des films. La dispersion des CNC entre les chaînes de PC permet probablement d’éviter la dégradation en raison de la protection physique des nanoparticules. Malheureusement, la présence des CNC semble réduire la stabilité thermique du polymère et accélérer sa dépolymérisation. Le chapitre 4 est dédié aux systèmes à base de polybutyrate-adipate-téréphtalate (PBAT) et de nanocristaux de cellulose. Le chapitre est divisé en deux parties. La première partie est focalisée sur l’influence de la cristallisation que les nanocristaux peuvent induire sur la matrice polymère après le processus d’extrusion. Trois sources différentes de CNC ont été choisies pour permettre l’obtention de nanoparticules avec différents facteurs de forme L/d. Des nanocomposites ont été préparés par extrusion en utilisant un taux de particules correspondant au seuil de percolation. Cela a créé une sorte de compétition entre la taille de la particule et sa fraction volumique. Dans cette étude, les nanocristaux avec plus grand rapport longueur/diamètre et sont, également, responsables de la cristallisation la plus significative.Dans la deuxième partie, des nanocomposites ont également été préparés par extrusion/injection puis caractérisés en termes d’organisation structurale. Les résultats de SAOS ont montré un changement de viscosité et de la valeur de G’, ainsi que dans la pente de la courbe G’ x ω, suggérant une modification de la structure des particules après conditionnement. Grâce à les tests de rheologie 2D, il a été possible d’observer une modification de la structure des particules qui suggère une plus grande dispersion. Cependant, il est clair que les particules ne sont pas complètement réorganisées sous forme de réseau 3D
The preparation of composites based on cellulose nanocrystals (CNC) is normally performed using techniques such as melt processing or casting/evaporation. In the last one, impressive mechanical properties can be reached due to the creation of a particle 3D network that is based on new hydrogen bonds between the cellulose nanorods. This process of new H-bond formation normally takes time and is dependent of the nanoparticle size and its volume fraction. Besides, the quality of filler dispersion into the polymeric matrix is also an important parameter to provide the highest surface area and provides an ideal structure for the rigid structure. In this work, we tried to propose different preparation methods and characterizations to obtain nanocomposites with a simple preparation either by casting/evaporation or melt processing.First, we improve the compatibility between an hydrophobic matrix and CNC by the chemical modification of the former. This approach tried to be an alternative to standard modification process, normally performed on nanoparticle surface by –OH groups substitution. As a good model for the study of composite mechanical properties, a natural rubber matrix was used with double bonds oxidized by the use of a strong oxidant aiming to introduce new hydroxyl groups on the isoprene chains. These new groups seem to interact with the cellulose surface, creating new supramolecular interactions between particle and matrix. It was observed that the increase in the degree of oxidation can first increase the compatibility between the polymer and the filler, increasing the mechanical properties. Later, over-oxidation starts to cause the plasticization of the system.In sequence, we had use melt processing for produce nanocomposites at high temperatures by using amorphous and semi-crystalline polymers. In a first approach, CNC were coated with the matrix polymer (polycarbonate) by a system of dilution/precipitation in suitable solvents. The coated nanocrystals are extruded with polycarbonate at 230ºC thanks to the coating approach, that allows an increase in the processing temperature of CNCs. Also, this technique improves its dispersion in the matrix due to entanglement of the polymer chains and the individual nanocrystals. The thermal analyzes shows that the CNC presence and coating masterbatch preparation reduces the polycarbonate Activation Energy (Ea) causing an acceleration in its thermal degradation and a molecular weight (Mv) reduction. As consequence, an increase in the crystallinity of the material occurs. Mechanical characterizations (DMA) show that nanocrystals presence and Mv reduction incresed the mechanical properties of the materials. The modulus (E') values, after the Tg, are higher than theoretical values calculated by Halpin−Kardos model for all studied compositions.The last part of this work is focused on investigate the influence of extrusion and thermal history of the material on its mechanical and rheological properties.In a first approach, we investigated the role of CNCs with different sizes on the material’s final properties (i.e. crystallinity and mechanical properties). Different sources were used to obtain CNCs with different percolation volume fractions, aiming to investigate its capacity on crystallization of a semi crystalline thermoplastic matrix. The results showed that crystallinity and mechanical properties are, indeed, strongly affected by the aspect ratio of the nanorod. In fact, longer CNC particles seem to be more capable to create crystalline domains and reinforce the polymer despite the lower total number of particles. In a second step, the rheological properties of the materials were investigated to characterize the effect of particle micro-structure. The composite internal organization seems to be dependent of the system viscosity and filler volume fraction, what can bring strong impact on the mechanical properties of the material

Organic hybrid composites based on carboxylated nitrile rubber and nylon-12 reinforced with mercerized and diisocyanated lignocellulose residue (LCR) was prepared. The influence of the LCR on the viscoelastic properties of these organic hybrids was investigated by dynamic mechanical analysis and thermal analysis (differential scanning calorimetry (DSC)). It is found that either the position of the damping peak was shifted to higher values or the intensity of the damping peak was significantly increased with LCR. These results could imply that the LCR enhanced the damping properties of the composites. The thermal stability of the composites was evaluated with the mean values obtained using thermogravimetrical analysis. The decomposition rate was investigated using differential thermal gravimetry. The crystallization behavior of the prepared composites was checked by DSC.

Electrical and thermal conductivities of the yttria-stabilised zirconia/alumina (YSZ/Al2O3) composites and the yttria-zirconia-ceria (YSZ-CeO2) solid solutions are studied in this thesis. The electrical conductivity of the YSZ/Al2O3 composites decreases with an increase in the volume fraction of Al2O3 and exhibits typical percolation behaviour. The electrical conductivity of the YSZ/Al2O3 interface is higher than that of the YSZ grain boundary, but lower than that of the YSZ grains. The thermal conductivity of the YSZ/Al2O3 composites increases with an increase in the Al2O3 volume fraction, and it can be fitted well to the Maxwell theoretical model, which indicates the absence of obvious interfacial thermal resistances in the composites. The low interfacial thermal resistance of the YSZ/Al2O3 interface is due to the 'clean' and coherent nature of the YSZ/Al2O3 interface, along with the small difference between the elastic properties of YSZ and Al2O3. The electrical conductivity of the [(ZrO2)1-x(CeO2)x]0.92(Y2O3)0.08 (0 ≤ x ≤ 1) solid solutions has a 'V-shape' variation as a function of the mole ratio of CeO2 (x). In the ZrO2-rich region (x < 0.5), CeO2 doping increases the concentration of defect associates which limits the mobility of the oxygen vacancies; in the CeO2-rich region (x > 0.5), the increase of x increases the lattice parameter, which enlarges the free channel for oxygen vacancy migration. A comparison of the YSZ-CeO2 solid solutions with the YSZ-HfO2 series indicates the ionic radius of the tetravalent dopant determines the composition dependence of the ionic conductivity of the solid solutions.The thermal conductivity of the [(ZrO2)1-x(CeO2)x]0.92(Y2O3)0.08 (0 ≤ x ≤ 1) solid solutions also has a 'V-shape' variation as a function of the mole ratio of CeO2 (x), which indicates an incorporation of Zr4+ and Ce4+ can effectively decrease the thermal conductivity of the end members YSZ and yttria-doped ceria (YDC). In the ZrO2-rich region (0 ≤ x ≤ 0.5), the thermal conductivity is almost temperature independent; in the CeO2-rich region (0.5 ≤ x ≤ 1), it decreases obviously with increasing temperature. By calculating the phonon scattering coefficients, it is concluded that the composition dependence of the thermal conductivity in the ternary solid solutions is dominated by the mass difference between Zr and Ce at the cation sites, whereas the temperature dependence is determined by the order/disorder of oxygen vacancies at the anion sites.

In previous studies, evidence of thermal wave behavior was found in heterogeneous materials. Thus, the overall goal of this study was to experimentally verify those results, and develop a parameter estimation scheme to estimate the thermal properties of various heterogeneous materials. Two types of experiments (Experiments 1 and 2) were conducted to verify the existence or non-existence of thermal wave behavior in heterogeneous materials. In Experiment 1 sand, ion exchanger, and sodium bicarbonate were used as test materials, while processed meat (bologna) was used in Experiment 2. The measured temperature profiles of the samples were compared with the parabolic and hyperbolic heat conduction model results. The values of thermal diffusivity and thermal conductivity were obtained using the Box-Kanemasu parameter estimation method which is based on the comparison between temperature measurements and the solutions of the theoretical model. Overall, no clear experimental evidence was found to justify the use of hyperbolic heat conduction models rather than parabolic for the materials tested. Further comprehensive experimentation using different heating rates is warranted to definitely identify the accurate type of heat conduction process associated with such materials, and to describe the physical mechanisms which produce wave-like heat conduction in heterogeneous materials.
Master of Science

Thermal energy storage (TES) has a crucial role to play in conserving and efficiently utilising energy, dealing with mismatch between demand and supply, and enhancing the performance and reliability of our current energy systems. This thesis concerns TES materials and devices with an aim to establish a relationship between TES device level performance to materials properties. This is a multiscale problem. The work focuses on the use of carbonate-salt-based composite phase change materials (CPCMs) for medium and high temperature applications. A CPCM consists of a carbonate salt based phase change material (PCM), a thermal conductivity enhancement material (TCEM, graphite flake in this work) and a ceramic skeleton material (CSM, MgO in this work). Both mathematical modelling and experiments were carried out to address the multiscale problem. The wettability of carbonate salt and MgO system is first studied, followed by exploring the CPCMs microstructure characteristics and formation mechanism, and then the effective thermal conductivity of the CPCMs is carried out based on the developed microstructures. At the last part, heat transfer behaviour of CPCMs based TES at component and device levels is investigated.

The ultimate goal of this research was to determine the feasibility of measuring thermal conductivity of food materials using the bead thermistor with particular reference to high temperature. Feasibility was established by examining the effects of the input parameters and the measurement error associated with them on the ability to estimate the test medium thermal conductivity test medium. This study showed that estimation of effective radius and bead thermal conductivity, the probe parameters, had the most significant impact on the ability to estimate the thermal conductivity of food materials. The probe parameters were determined by standardizing the thermistor probe against materials of known thermal conductivity. The current lack of well defined thermal reference materials in the range of water and most food products is a primary source of error associated with the method. The accuracy and coefficient of variation of the Bead Thermistor Method were statistically documented in 10° increments over the temperature range of 25°C to 125°C. These results showed the method to have better than 10% accuracy across the entire temperature range. Distinct differences in accuracy between probes at a given temperature were also discovered. Standardization with water and castor oil resulted in a more accurate method than was achieved using water, castor oil, and glycerin. The minimum particle diameter necessary to maintain the infinite boundary condition assumption required by heat transfer theory was found to be >5 mm. The methodology was evaluated by examining the effects of temperature on the thermal conductivity of milk of different fat contents. A prediction equation for each product was attempted from the experimental data, but the data appear best fit by assuming a constant value across temperature. Heat altered the product physically which likely affected temperature dependence. Based on the results of this study, the bead thermistor method can be considered a practical method for determining thermal conductivity of food materials over the temperature range considered in this study.
Ph. D.

A major challenge of developing faster and smaller microelectronic devices is that high flux of heat needs to be removed efficiently to prevent overheating of devices. The conventional way of heat removal using liquid reaches a limit due to low thermal conductivity and limited heat capacity of fluids. Adding solid nanoparticles into fluids has been proposed as a way to enhance thermal conductivity of fluids, but recent results show inconclusive anomalous enhancements in thermal conductivity. A possible way to improve heat transfer is to increase the heat capacity of liquid by adding phase change nanoparticles with large latent heat of fusion into the liquid. Such nanoparticles absorb heat during solid to liquid phase change. However, the colloidal suspension of bare phase change nanoparticles has limited use due to aggregation of molten nanoparticles, irreversible sticking on fluid channels, and dielectric property loss. This dissertation describes a new method to enhance the heat transfer property of a liquid by adding encapsulated phase change nanoparticles (nano-PCMs), which will absorb thermal energy during solid-liquid phase change and release heat during freeze. Specifically, silica encapsulated indium nanoparticles, and polymer encapsulated paraffin (wax) nanoparticles have been prepared using colloidal method, and dispersed into poly-alpha]-olefin (PAO) and water for high temperature and low temperature applications, respectively. The shell, with a higher melting point than the core, can prevent leakage or agglomeration of molten cores, and preserve the dielectric properties of the base fluids. Compared to single phase fluids, heat transfer of nanoparticle-containing fluids have been significantly enhanced due to enhanced heat capacities. The structural integrity of encapsulation allows repeated uses of nanoparticles for many cycles.; By forming porous semi crystalline silica shells obtained from water glass, supercooling has been greatly reduced due to low energy barrier of heterogeneous nucleation. Encapsulated phase change nanoparticles have also been added into exothermic reaction systems such as catalytic and polymerization reactions to effectively quench local hot spots, prevent thermal runaway, and change product distribution. Specifically, silica-encapsulated indium nanoparticles, and silica encapsulated paraffin (wax) nanoparticles have been used to absorb heat released in catalytic reaction, and to mitigate the gel effect during polymerization, respectively. The reaction rates do not raise significantly owing to thermal buffering using phase change nanoparticles at initial stage of thermal runaway. The effect of thermal buffering depends on latent heats of fusion of nanoparticles, and heat releasing kinetics of catalytic reactions and polymerizations. Micro/nanoparticles of phase change materials will open a new dimension for thermal management of exothermic reactions.
ID: 029809237; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (Ph.D.)--University of Central Florida, 2011.; Includes bibliographical references (p. 164-191).
Ph.D.
Doctorate
Mechanical Materials and Aerospace Engineering
Engineering and Computer Science

Nanostructured materials are gaining an ongoing demand because of their exceptional chemical and physical properties. Due to complexities and costs of experimental studies at nanoscale, computer simulations are getting more attractive asexperimental alternatives. In this PhD work, we tried to use combination of atomistic simulations and continuum modeling for the evaluation of thermal conductivity and elastic stiffness of nanostructured materials. We used molecular dynamics simulations to probe and investigate the thermal and mechanical response of materials at nanoscale. The finite element and micromechanics methods that are on the basis of continuum mechanics theories were used to evaluate the bulk properties of materials. The predicted properties are then compared with existing experimental results.

Les matériaux nanostructurés suscitent un intérêt qui va croissant en raison de leurs propriétés chimiques et physiquesexceptionnelles. A cause de la complexité et du coût des développements expérimentaux à l’échelle nano, la simulationnumérique devient une alternative de plus en plus populaire aux études expérimentales. Dans ce travail de thèse, nous avons essayé de combiner des simulations à l’échelle atomique avec de la modélisation en milieu continu pour évaluer la conductivité thermique et la réponse élastique de matériaux nanostructurés. Nous avons utilisé des simulations de dynamique moléculaire pour calculer la réponse mécanique et thermique des matériaux sur des volumes à l’échelle nano. Des méthodes de micromécanique et la méthode des éléments finis, qui utilisent la mécanique des milieux continus, ont permis d’évaluer les propriétés mécaniques des matériaux à l'échelle macroscopique. Les résultats obtenus par ces simulations numériques ont été ensuite comparés avec ceux issus de l’expérience
Nanostructured materials are gaining an ongoing demand because of their exceptional chemical and physical properties. Due to complexities and costs of experimental studies at nanoscale, computer simulations are getting more attractive asexperimental alternatives. In this PhD work, we tried to use combination of atomistic simulations and continuum modeling for the evaluation of thermal conductivity and elastic stiffness of nanostructured materials. We used molecular dynamics simulations to probe and investigate the thermal and mechanical response of materials at nanoscale. The finite element and micromechanics methods that are on the basis of continuum mechanics theories were used to evaluate the bulk properties of materials. The predicted properties are then compared with existing experimental results

Plans for development of human colonies on the Moon, Mars or other planets will require the investigation of new structural materials. In order to foster self-sufficiency and to make the colonies economically feasible, materials must be developed from locally available resources when possible. In this investigation a material made from a lunar soil simulant has been developed and tested for its mechanical properties. The simulant was mixed with varying percentages of aluminum, stainless steel and carbon steel fibers and heated to 1100°C to form a solid material. Beam shaped samples were cut from these specimens for bending tests. From the intact portions of the tested beams, samples for compression testing were cut and tested. Analysis of the results includes bending strength, compressive strength, and investigation of elastic moduli. The material was found to have significant strength in bending and compression. Results indicate the presence of fibers significantly changes the behavior of the material.

This thesis is concerned with developing porous materials from tyre shred residue and polyurethane binder for acoustic absorption and thermal insulation applications. The resultant materials contains a high proportion of open, interconnected cells that are able to absorb incident sound waves through viscous friction, inertia effects and thermal energy exchanges. The materials developed are also able to insulate against heat by suppressing the convection of heat and reduced conductivity of the fluid locked in the large proportion of close-cell pores. The acoustic absorption performance of a porous media is controlled by the number of open cells and pore size distribution. Therefore, this work also investigates the use of catalysts and surfactants to modify the pore structure and studies the influence of the various components in the chemical formulations used to produce these porous materials. An optimum type and amounts of catalyst are selected to obtain a high chemical conversion and a short expanding time for the bubble growth phase. The surfactant is used to reduce the surface tension and achieve a homogenous mixing between the solid particulates tyre shred residue, the water, the catalyst and the binder. It is found that all of the components significantly affect the resultant materials structure and its morphology. The results show that the catalyst has a particularly strong effect on the pore structure and the ensuing thermal and acoustical properties. In this research, the properties of the porous materials developed are characterized using standard experimental techniques and the acoustic and thermal insulation performance underpinned using theoretical models. The important observation from this research is that a new class of recycled materials with pore stratification has been developed. It is shown that the pore stratification can have a positive effect on the acoustic absorption in a broadband frequency range. The control of reaction time in the foaming process is a key function that leads to a gradual change in the pore size distribution, porosity, flow resistivity and tortuosity which vary as a function of sample depth. It is shown that the Pade approximation is a suitable model to study the acoustic behaviour of these materials. A good agreement between the measured data and the model was attained.