The positive temperature coefficient of resistivity (PTCR) sensors is resistor materials that undergo a sharp change in resistivity at a designed Curie temperature due to its unique structure and chemical composition. This effect serves important control functions in a wide variety of electronic circuitry and similar applications. Conventional calcining of mixed oxides method (CMO) is used for fabricating lanthanum doped barium titanate (BaTiO3) for PTCR behaviour through solid-state-sintering route, at 1100°C, 1350°C. Two batches of samples were fabricated at low and high sintering temperatures of 1100°C, 1350°C respectively. The effect of different concentrations of donor dopant on BaTiO3 on the electrical properties of Ba(1-x)LaxTiO3 with x= 0.0005, 0.001, 0.002, 0.0025, 0.003 mol%, is investigated at low sintering temperature. The influence of lantanum doping with Al2O3+SiO2+TiO2 (AST) as sintering aids on the electrical properties of Ba(1-x)LaxTiO3 with x= 0.0005, 0.001, 0.003 mol%, is also investigated. The results of the electrical characterization for the first batch of samples showed an increase in room temperature resistance with increaisng donor concentration. Also the results of the electrical characterization for the second batch of samples also showed the same increase in room temperature resistance with increasing donor concentration. For first batch of sensors the high room temperature resistance keeps the jump small and these materials showed V-shaped NTCR-PTCR multifunctional cryogenic sensor behavior with a strong negative coefficient of resistance effect at room temperature.Where as the second batch of sensors showed few orders of magnitude rise in resistivity values. The La-doped BaTiO3 ceramics co-doped with Mn gives an enhanced PTCR effect which can be exploited for various sensor applications.

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

Fuel for nuclear reactors with an increased thermal conductivity offers the potential for lower fuel operating temperatures and reduced fission gas release rates. Uranium dioxide (UO2) based composites offer a method of achieving a higher thermal conductivity. Silicon carbide (SiC) and molybdenum (Mo) have been identified as potential candidates for use in a composite fuel material. Uranium dioxide composites were manufactured with the inclusion of whiskers and granules of SiC up to a 30 vol% loading. The manufacturing route used was based on the current process employed to commercially manufacture UO2 fuel, by reductive sintering. Composites containing Mo were manufactured via spark plasma sintering and included loadings of up to 10 vol% Mo. The composites were characterised on their microstructural properties and where appropriate the thermal conductivity was determined by laser flash analysis. The composites containing SiC achieved low densities, 95%TD. The microstructure contained channel like structures of Mo, due to the use of an agglomerated UO2 precursor powder. An increased thermal conductivity was determined for the molybdenum composites. At the maximum measurement temperature of 800°C the increase was found to be 68% in the 10 vol% composites compared to UO2.

Glaciers are important components of the Earth's environment and are mainly found in polar and high elevation areas. They are crucial for understanding the past, ongoing and upcoming environmental changes, relevant for fresh water supply, logistical and recreational purposes. Subsurface temperature of glaciers is an important parameter heavily influencing the fluxes of mass and energy. The project focuses on how the temperature changes inside glaciers and which factors contribute to the change. Thermal conduction is one of the key processes controlling the thermodynamics of glaciers. This defines how well heat is transferred inside glaciers and how well the temperature propagates. The process of heat conduction at Lomonosovfonna ice field, Svalbard, is described using numerical simulations constrained by measured initial and boundary conditions. Simulated subsurface temperature is in line with measurements before the onset of melt in summer. After that deviations increase as the used model does not consider the process of melt water refreezing. This makes the simulation only partially successful. / Glaciärer är viktiga komponenter i jordens omgivning och återfinns främst i polarområden och områden på hög höjd. De är viktiga för att förstå tidigare, pågående och kommande miljöförändringar, relevanta för färskvattenförsörjning, logistiska och återskapande ändamål. Temperaturen inom glaciärer är en viktig parameter som påverkar flödena av massa och energi. Projektet fokuserar på hur temperaturen förändras inom glaciärer och vilka faktorer som bidrar till förändringen. Värmeledning är en av nyckelprocesserna som kontrollerar termodynamiken hos glaciärer. Detta definierar hur väl värme förflyttas inom glaciärer och hur väl temperaturen sprider sig. Värmeledningsprocessen på isfältet Lomonosovfonna, Svalbard, beskrivs med hjälp av numeriska simuleringar begränsade av uppmätta initial- och gränsförhållanden. Simulering av temperaturen under ytan stämmer överens med mätningarna före smältningen på sommaren. Därefter ökar avvikelsen, eftersom modellen som använts inte tar hänsyn till processen av återfrysning av smältvatten. Detta gör att simuleringen endast är delvist lyckad.

Les matériaux composites à matrice métallique (CMM) sont actuellement étudiés pour être utilisés dans de nombreux domaines d’application. L’une des applications potentielles concerne leur utilisation en tant que drain thermique pour les modules de puissance. Pour cette application, deux conditions sont requises : une conductivité thermique (CT) élevée pour évacuer la chaleur générée par la puce électronique et un coefficient d’expansion thermique (CTE) proche du substrat céramique (2-8×10-6 /K) utilisé dans le module de puissance.Ainsi des matériaux composites à matrice aluminium (Al : CT de 217 W/m.K et CTE de 24×10-6 /K) et à renfort plaquette de graphite (GF : CT de 1000 W/m.K et CTE de -1×10-6 /K dans le plan de la plaquette) ont été élaborés. Ces matériaux composites ont été fabriqués par Métallurgie des Poudres (MP) conventionnelle mais aussi par un procédé original nommé Flake Powder Metallurgy (FPM). Ce procédé, qui consiste à utiliser une poudre métallique à morphologie plaquette, a permis d’optimiser l’orientation des renforts plaquette dans un plan perpendiculaire à la direction de densification sous l’action d’une pression uniaxiale. De plus, ce procédé a permis d’obtenir une meilleure adhésion entre la matrice et le renfort comparé aux matériaux composites élaborés par MP conventionnelle. Cela a abouti à une amélioration de la CT qui est passée de 400 W/m.K à 450 W/m.K pour un taux de renfort de 50%vol. Néanmoins, concernant la dilatation thermique, des CTE de 21,8×10-6 /K et 21,7×10-6 /K ont été obtenus par MP et FPM respectivement, ce qui est incompatible avec l’application visée.Pour surmonter cette problématique, des matériaux composites à renfort multiple ont été élaborés par frittage en phase liquide. Ainsi des fibres de carbone (FC) ont été rajoutées à l’aluminium et aux plaquettes de graphite. L’ajout de ce second renfort au graphite a permis de diminuer de manière importante le CTE des composites Al/(GF+FC) avec une faible proportion en FC tout en maintenant une haute CT. De plus les matériaux composites Al/(GF+FC) présentent des CTE nettement inférieurs aux composites Al/FC avec un %vol. de FC équivalent. Ainsi des matériaux composites Al/(GF+FC) ont été élaborés par frittage en phase liquide permettant d’obtenir une CT de 400 W/m.K (comparable à la CT du cuivre) et un CTE de 8×10-6 /K (comparable au CTE de l’alumine). De plus la légèreté de l’aluminium confère aux matériaux composites Al/C une faible densité (d=2,4). Par conséquent, les matériaux développés dans cette étude sont prometteur en tant que drain thermique léger, notamment dans le domaine de l’électronique embarquée. / Many carbon/metal composites are currently used in several applications. One of them concerns their use as heat sinks in microelectronics. Concerning this application, two conditions are required: a high thermal conductivity (TC) in order to evacuate the heat generated by the electronic chip and a coefficient of thermal expansion (CTE) similar to the used material type of the electronic device (2-8×10-6 /K).Therefore, graphite flakes (GF; TC: 1000 W/m.K and CTE: -1×10-6 /K in the graphite plane) reinforced aluminum matrix (Al; TC: 217 W/m.K and CTE: 25×10-6 /K) composites were fabricated. These composite materials were fabricated by Powder Metallurgy (PM) and Flake Powder Metallurgy (FPM). This process, which consist to use a flattened metallic powder, helped to improve the in-plane orientation (perpendicular to the pressure direction) of GF under uniaxial pressure. Moreover, this process provided a better Al-C interface thanks to a planar contact between the matrix and the reinforcements. This resulted in an improvement of the CT from 400 W/m.K to 450 W/m.K for a reinforcement content of 50 vol.%. Nevertheless, regarding thermal dilation, CTEs of 21.8×10-6 /K and 21.7×10-6 /K were obtained by MP and FPM respectively, which is incompatible with the intended application.To overcome this problem, composite materials with multiple reinforcement were developed by solid-liquid phase sintering. Then, carbon fibers (CF) have been added to aluminum and graphite flakes. The addition of CF to GF reinforcement reduced significantly the CTE of the Al/(GF+CF) composites with a small proportion of CF, while preserving a high TC. In addition, the Al/(GF+FC) composite materials have significantly lower CTEs than the Al/CF composites with a equivalent vol.% of CF. Therefore, Al/(GF+CF) composite materials were developed by solid-liquid phase sintering to obtain a TC of 400 W/m.K (comparable to the TC of copper) and a CTE of 8×10-6 /K (comparable to the CTE of alumina). In addition, the lightweight of aluminum gives composite materials Al/C a low density (d = 2.4 g/cm3). Therefore, the composite materials developed in this study are promising as a lightweight heat sink in microelectronic industries.

Les nanofils semiconducteurs sont aujourd’hui le sujet de nombreuses recherches pour leurs propriétés physiques intéressantes. S’appuyant plus spécifiquement sur les propriétés thermiques des nanostructures, l’objectif de cette thèse est de démontrer la faisabilité d’un capteur de conductivité thermique conçu à partir de nanofils de silicium pour des applications en tant que jauge Pirani ou détecteur de gaz. Le travail réalisé aborde les questions posées par la réduction de taille des objets telles que l’augmentation du bruit ou la conduction thermique en régime de raréfaction et élabore des solutions à ces problématiques. Le manuscrit aborde l’ensemble des étapes nécessaires à la réalisation d’un capteur, à savoir la conception des dispositifs, s’appuyant sur une étude détaillée du comportement physique des objets utilisés, la fabrication sur plaque 200mm de ces capteurs par la salle blanche du CEA-Leti en ayant recours aux techniques classiques de la microélectronique et enfin leur caractérisation en tant qu’instrument de mesure de pression (jauge Pirani) ou en tant que capteur de concentration de gaz. Le travail réalisé s’intègre dans un projet plus global de réalisation d’un système de détection de gaz portatif pour l’analyse de l’air ou de l’eau / Semiconducting nanowires are nowadays the topic of numerous research for their interesting physical properties. Relying more specifically on the thermal properties of nanostructures, the purpose of this thesis is to demonstrate the feasibility of a thermal conductivity detector based on silicon nanowires for pressure sensing (Pirani gauge) or gas detection. The work presented herein addresses the questions raised by the reduction of the objects size such as the increase of the noise or the thermal conduction in a rarefied gas and tries to bring a solution to those problematics. This work deals with all the steps required for the realization of such devices. That is, the design and simulation of the sensor, based on a detailed study of the physical behavior of the objects, the fabrication of such devices on 200mm wafers by the CEA-Leti cleanroom using standard microelectronics processes and finally their characterization as a pressure sensor and gas detector. The work presented here is part of a wider project that aims at developing of a portable gas detection system for air or water analysis.

Materials in proximity to quantum critical points (QCPs) experience strong fluctuations in the order parameter associated with the transition and often, as a result, display interesting properties. In this dissertation, we have used a variety of experimental probes such as Shubnikov-de Haas quantum oscillations, thermal conductivity and heat capacity, to better understand two such materials — $A_3T_4$Sn$_{13}$ and YFe$_2$Ge$_2$. $A_3T_4$Sn$_{13}$ ($A$ = Ca, Sr; $T$ = Ir, Rh) is a family of quasi-skutterudite superconductors with moderate $T_c$’s between 4 and 8 K. Although the superconductivity is believed to be phonon-mediated with s-wave pairing symmetry, an unusual second-order structural transition makes this material family fascinating to study. Whether this structural transition is a result of three distortions with perpendicular wavevectors resulting in a cubic-to-cubic transformation, or each wavevector acting independently giving rise to cubic-to-tetragonal transformations and formation of twinned domains is a disputed issue. We have measured quantum oscillations in the resistivity of Sr3Ir4Sn13 and compared it to density functional theory (DFT) calculations for both scenarios. Our results strongly suggest that the former interpretation is correct. The structural transition temperature $T^*$ in $A_3T_4$Sn$_{13}$ can be suppressed to zero by tuning with physical or chemical pressure. In (Ca$_x$Sr$_{1−x}$)$_3$Rh$_4$Sn$_13$, the quantum critical point can be accessed purely by chemical substitution at x ~ 0.9. In the vicinity of the QCP, we expect large fluctuations of the order parameter at low temperatures, which for a structural transition could manifest as a structural disorder. We have measured thermal conductivity at temperatures much lower than $T_c$ and found that it is well described by a single power law with suppressed exponents near the QCP. The heat capacity, however, remains ~ $T^3$. After excluding conventional phonon scattering mechanisms, we propose the possibility of intrinsic quasi-static spatial disorder that is related to the structural QCP. YFe$_2$Ge$_2$ is closely linked to the “122” family of iron-based superconductors like KFe$_2$As$_2$, although it has a significantly lower $T_c$ ~ 1 K. It has a rather three-dimensional Fermi surface which closely resembles that of KFe$_2$As$_2$ in the pressure-induced collapsed tetragonal phase. YFe$_2$Ge$_2$ is in proximity to several types of magnetic order which are predicted by DFT calculations to have lower energy than the non-spin polarised case. Even though YFe$_2$Ge$_2$ is non-magnetic, its superconductivity could be strongly affected by magnetic fluctuations. Through a collaboration with researchers at the University of Waterloo, we have measured the thermal conductivity of YFe$_2$Ge$_2$ down to millikelvin temperatures and up to 2.5 T in field. Our results suggest that YFe$_2$Ge$_2$ is a nodal superconductor. This result could assist in the explanation of the unconventional superconductivity in iron-based superconductors.

Ce travail porte sur l'étude du transport de chaleur dans le Silicium cristallin nanostructuré et l’effet de l’amorphisation. La conductivité thermique de diverses nanostructures est calculée à l'aide de deux méthodes numériques : la Dynamique Moléculaire et la résolution de l'équation de transport de Boltzmann par technique Monte Carlo. Les matériaux contenant des nanopores sphériques sont d'abord examinés et l'importance de la densité de surfaces de diffusion est mise en évidence. Puis des nanofilms à pores cylindriques périodiques, souvent appelés cristaux phononiques, sont étudiés. La densité d'états calculée par Dynamique Moléculaire ne montre pas de modifications majeures des propriétés des porteurs de chaleur (phonons). En revanche, les résultats montrent que l'orientation des surfaces, la disposition des pores ou la présence d’une couche de silicium oxydé ou amorphisé peuvent jouer un rôle important pour la dissipation de la chaleur. Ensuite, le transport de chaleur dans les nanofils est étudié, notamment l'évolution radiale de la conductivité thermique. Cette dernière est maximale au centre des nanofils et décroît en s'approchant de la surface du nanofil. Des structures composées de nanofils interconnectés, appelées réseaux de nanofils, sont également étudiées; elles possèdent des conductivités extrêmement basses. Enfin, l'effet de la rugosité et de l'amorphisation des surfaces sur le transport thermique est analysé pour différents types de nanostructures. Ces deux derniers phénomènes contribuent fortement à la réduction de la conductivité thermique, qui peut prendre des valeurs très basses en gardant une fraction cristalline importante. Cela ouvre de nouvelles perspectives pour le contrôle de cette propriété à travers le design des matériaux / This study deals with heat transport in crystalline nanostructured silicon and the impact of amorphization. The thermal conductivity of various nanostructures is computed with two numerical methods: Molecular Dynamics and Monte Carlo resolution of the Boltzmann transport equation. First, materials with spherical nanopores are investigated and the importance of the surface density is highlighted. Then, nanofilms with periodic cylindrical pores, often called phononic crystals, are studied. The density of states computed with Molecular Dynamics does not show major modifications of the heat carriers (phonons) properties. However, results show that the surfaces orientation, the pore distribution and the existence of native oxide or amorphous layers may have an important impact on the thermal conductivity. Then, heat transport in nanowires is studied, in particular the radial evolution of the thermal conductivity. The latter one is maximum at the center of the nanowire and decreases when approaching the nanowire surface. Structures made from interconnected nanowires, called nanowire networks, are also studied; they have an extremely low thermal conductivity. Finally, the impact of the roughness and amorphization of the surfaces on thermal transport is analyzed for different types of nanostructures. The two latter phenomena contribute strongly to the reduction of the thermal conductivity, which can reach very low values while keeping an important crystalline fraction.It opens new perspectives for the control of this property with material designing

A presente dissertação desenvolve um estudo numérico em duas direções espaciais com o objetivo de encontrar a configuração de geometrias acopladas a aletas de alta condutividade térmica em forma de “T” que resultam na menor resistência ao fluxo de calor utilizando o método Design Construtal. Como restrição as áreas de ambos os componentes, o corpo sólido onde há geração de calor e a aleta, são mantidas constantes. A equação diferencial da difusão do calor bidimensional, em regime permanente e propriedades constantes, com as condições de contorno, foram solucionadas pelo método dos elementos finitos utilizando o programa MATLAB ®, mais precisamente a ferramenta PDETOOL, Partial Differential Equations Tool. Em outras palavras, minimizar a resistência térmica ao fluxo de calor gerado para uma melhora na refrigeração, variando somente os comprimentos e larguras que formam o sólido de baixa condutividade térmica e a aleta composta por material de alta condutividade térmica. Para cada geometria proposta foram avaliadas todas as possibilidades geométricas dentro do domínio estabelecido Três geometrias foram propostas para os sólidos geradores de calor: retangular, trapezoidal e semicircular, todas acopladas com a aleta na forma de T. Além dos graus de liberdade, também foram avaliados o efeito dos seguintes parâmetros adimensionais: (condutividade térmica da aleta), (fração de área), (fração de área auxiliar) e ℎ . O melhor design encontrado é aquele que distribui melhor as imperfeições, ou seja, a geometria que distribui melhor os pontos de temperatura máxima. Os resultados reforçam, ainda, o entendimento de que sistemas multicomponentes devem ser estudados globalmente e não cada componente individualmente. Para a geometria retangular houve uma melhora de 66% no desempenho quando comparados os desempenhos da primeira para a última otimização. O melhor desempenho obtido para a geometria trapezoidal superou em aproximadamente 3,5% o desempenho da geometria retangular. Por fim a geometria semicircular atingiu o melhor desempenho entre as geometrias estudadas, superando em 40% o resultado atingido pela geometria trapezoidal. / This work used the method Construtal Design to develop a numerical study trying to find out the best configuration of geometries coupled to T-shaped materials of high thermal conductivity to improve the heat transfer between the heat generating body, which is a low heat conductor, and the environment. As a restriction, both areas are kept constant. The differential equations of heat diffusion, steady state and constant properties, and their boundary conditions were solved numerically using the MATLAB ® software, specifically the PDETOOL tool. The objective of this work is to improve the flux of heat through the Tshaped materials of high thermal conductivity, in other words, minimize the thermal resistance to improve the refrigeration, changing only the values of the lengths and widths that setup the solid of low thermal conductivity and the T-shaped materials of high thermal conductivity. All geometric possibilities were evaluated, respecting the domain. The optimal geometry was that which resulted in lower thermal resistance. Three geometries have been proposed for solid heat generators: rectangular, trapezoidal and semicircular. All coupled with the T-shaped materials of high thermal conductivity Besides the degrees of freedom were also evaluated the effect of the following dimensionless parameters: (thermal conductivity), (area fraction), (auxiliary area fraction) and ℎ . The best design found is that better distributes the imperfections, in other words, it is the geometry that better distributes the points of maximum temperature. The results reinforce also the understanding that multicomponent systems should be studied globally rather than each component individually. For the rectangular geometry there was an improvement of 66% in performance when comparing the performances of the first to the last optimization. The best performance obtained for the trapezoidal geometry exceeded by approximately 3.5% performance of the rectangular geometry. Finally the semicircular geometry achieved the best performance among the studied geometry, exceeding by 40% the result achieved by the trapezoidal geometry.

Main concerns in the building industry includes the development of alternative building materials that reduces the amount of energy spent during manufacturing process and easier to work with. Wood-crete is a composite material developed in this study, made up of wood waste (sawdust), paper, tradical lime and water. Wood-crete is developed to provide an alternative material in construction solving problems associated with the delivery of low-cost housing across all income earners, reducing the amount of energy spent during manufacturing process of construction materials and the ease with which these construction materials are developed and solve issues related to waste management. This thesis presents the processing technologies, factors which affect the performance and properties of wood-crete. Wood-crete properties were found to be closely related to the composition of the constituent elements though compressive strength and modulus of elasticity were low when compared to other building materials like concrete and steel. In a bid to improve the strength of the developed wood-crete, the properties were investigated based on the modification of sawdust by hot water boiling and alkaline treatments which help to modify cellulose fibre surface to reduce the hydrophilic nature of sawdust thereby improving the sawdust-matrix bonding. It was found that the surface modification, processing of cellulosic fibril and the extraction of lignin and hemi-cellulosic compounds with alkali had an effect on the compressive strength of wood-crete, with treating sawdust with 4% NaOH at 140mins of boiling time achieving the highest compressive strength and boiling sawdust from 100mins to 140mins had a gradual increase in compressive strength but reduced at higher boiling time. Furthermore, treating sawdust with NaOH more than 4% weakened the individual wood particles thus leading to poor strength of wood-crete. Additionally, the properties of wood-crete were investigated based on the type of wood sawdust – hardwood (beech and oak) and softwood (pine and cedar). Apart from individual wood density having a significant effect on the density of wood-crete, other factors such as lignin, cellulose, hemicellulose contents including fibre length of individual wood species affect the strength properties of wood-crete. The compressive strength of wood-crete was closely related to the wood species, with highest compressive strength of 3.93MPa recorded for hardwood wood-crete compared to 1.37MPa and 0.26MPa of wood-crete from softwood and mixed wood respectively. Results from thermal conductivity tests on wood-crete also show that wood-crete blocks can be produced with good insulating properties for building construction. Addition of different types of paper fibres to reduce the density of wood-crete and improve the insulating properties of composite developed also had a dominant influence on both strength and thermal conductivity, reflecting its effect on the structure of composite and contribution of self strength of paper fibres. The addition of various percentages of waste paper (de-fibred) had a significant influence on the thermal conductivity of wood-crete with 75% addition of waste paper achieving a thermal conductivity value of 0.046W/mK performed with the TCi thermal conductivity analyser. Thermal conductivity results for wood-crete made from hardwood and softwood sawdust was closely related to the chemical composition of various wood species, with softwood wood-crete having about 20% lower thermal conductivity compared to hardwood wood-crete. The developed wood-crete was able to withstand impact load and considered, like hempcrete, most suitable for wall panelling or other non- and semi-structural applications with good thermal insulating properties. Findings of this study provides an alternative new material for the construction industry and an important background for achieving better strength of wood-crete, choosing what type of sawdust to be used for development of wood-crete and for directing a better use of this potential material with very small embodied energy and carbon negative.