Le graphène planaire se présente comme un des matériaux les plus prometteurs pour la nanoélectronique de demain, grâce particulièrement à sa conductivité thermique et sa mobilité électronique qui sont les plus élevées jamais mesurées dans un solide. Parmi ses allotropes, le graphène découpé en nanorubans est une des formes les plus intéressantes, notamment pour les possibilités d'ingénierie de bandes qu'il offre. Ses propriétés électroniques et vibrationnelles sont fortement influencées par la présence des bords et s’éloignent de celles du graphène planaire. A ce jour, ses propriétés thermiques et thermoélectriques ont été encore peu explorées. Dans ce travail de thèse, grâce à une modélisation atomistique du réseau cristallin, les modes de vibration caractéristiques de chaque type de ruban ont été identifiés et, dans le cadre du formalisme des fonctions de Green hors équilibre, le transport de ces modes a été simulé. On a ainsi évalué les propriétés thermiques des nanorubans en identifiant les types de rubans offrant la plus forte conductance thermique pour envisager une meilleure gestion de la chaleur dans les dispositifs du futur. Dans la direction opposée, des techniques de nanostructuration du ruban permettent de dégrader le transport des phonons et d’amplifier la figure de mérite thermoélectrique en bénéficiant simultanément d'un phénomène de transport électronique résonant. En exploitant ces techniques, un premier dispositif thermoélectrique basé sur les nanorubans de graphène a été conçu et ses performances ont été évaluées par une approche multi-échelle. La possibilité de très forte densité d’intégration du graphène fait l’intérêt d’un tel dispositif qui pourrait fournir des puissances électriques ou de refroidissement très supérieures à celles des dispositifs thermoélectriques actuels. / 2D graphene is one of the most promising materials for nanoelectronics; its thermal conductivity and electronic mobility are the highest ever measured in solids. Among its allotropes, graphene cut in nanoribbons (GNRs) is one of the most interesting structures because it offers possibilities of bandgap engineering. Electronic and vibrational properties of GNRs are strongly affected by the presence of the edges and can differ significantly from those of 2D graphene. Up to now, their thermal and thermoelectric properties have been rarely explored. In this thesis, using an atomistic model of crystal lattice, the vibrational modes associated to each type of ribbon have been identified and via the formalism of nonequilibrium Green’s functions, the transport of these modes has been simulated. We have evaluated the better ribbon structures in terms of thermal conductance for a better heat management in future devices and circuits. On the other side we have identified some particular nanostructured ribbons where the thermoelectric figure of merit is strongly amplified thanks to both the degradation of phonon conductance and the occurring of resonant electron transport. A first thermoelectric device based on such GNRs has been designed and its performance has been evaluated using a multi-scale approach. This device becomes interesting in the case of high integration density of GNRs.

Depuis les années 90, dans le contexte de la pénurie annoncée d'énergies fossiles et de modifications climatiques, un regain d'intérêt s'est fait sentir pour les énergies renouvelables et, parmi elles, pour la thermoélectricité. Cette dernière permettant la conversion directe entre énergies thermique et électrique devrait permettre la récupération de l'énergie perdue sous forme de chaleur, durant les processus industriels. Cependant, à l'heure actuelle, l'efficacité de la conversion thermoélectrique, évaluée à partir du facteur de mérite, ZT, n'est pas suffisante pour des applications à grande échelle. Cependant, pour évaluer l'impact potentiel de la thermoélectricité, il est nécessaire de prendre en compte d'autres critères que celui de l'efficacité relativement faible de la thermoélectricité, qui la cantonne dans des niches spécifiques, et tels que la faible toxicité des matières premières, leur abondance, leur faible coût et leur facilité de mise en œuvre. Diverses familles de matériaux répondent à ces critères parmi lesquelles les plus favorables sont les siliciures pour les matériaux inorganiques et les polymères conducteurs pour les composés organiques. Dans ce travail de thèse, nous nous sommes intéressés au disiliciure de chrome (CrSi2) et au polymère PEDOT:PSS.Parmi les siliciures, CrSi2 est un candidat prometteur mais sa grande conductivité thermique est un handicap par rapport aux matériaux conventionnels. Elle peut cependant être réduite par nanostructuration. Nous présentons une étude CrSi2 nanostructurés (10-15 nm) obtenus soit par four à arc suivie d'un broyage mécanique, soit par mécanosynthèse. Ces poudres sont chimiquement stables jusqu'à 1073 K. A plus hautes températures, des phases secondaires apparaissent, notamment CrSi. Grâce à la technique SPS, CrSi2 a pu être densifié jusqu'à 94% en maintenant une taille nanométrique (30-45 nm) ce qui a conduit à une réduction de la conductivité thermique d'un facteur 2 par rapport au composé CrSi2 massif. Malheureusement, le facteur de mérite ZT n'a pas été amélioré en raison de l'accroissement de la résistivité électrique.Guidés par des calculs DFT, nous avons explorés de nouvelles voies de dopage pour améliorer les propriétés thermoélectriques de CrSi2. Des alliages Cr1-xTixSi2, Cr1-xZrxSi2 et Cr1-xMoxSi2 ont ainsi été synthétisés et nous avons étudié leurs propriétés thermoélectriques. A Tamb, le dopage au Ti semblerait de ne pas significativement améliorier le facteur de puissance (PF) pour une porosité de 11% par rapport à nano-CrSi2. Par contre, pour 2% Zr, PF a été amélioré d'un facteur 1.7 pour une porosité de 30%. En plus, il semblerait que le PF de Cr0.98Zr0.02Si2 avec une porosité de 0% est amélioré d'un factuer 1.9 par rapport au CrSi2 massif. Néanmoins, à hautes températures il existe encore un potentiel d'amélioration pour le dopage au Ti. Bien qu'il soit métastable, l'alliage Cr1-xZrxSi2 a pu être obtenu jusqu'à 5% Zr. Une optimisation supplémentaire est encore nécessaire pour les alliages dopés au Ti et Zr. L'alliage Cr1-xMoxSi2 est le plus prometteur avec une amélioration significative du facteur de Mérite (ZTmax > 0.2) pour 10% et 20% de Mo entre 500 et 700 K. Ces valeurs sont supérieures à celle du massif bien que toujours du même ordre de grandeur que le meilleur ZT obtenu pour CrSi2.Enfin, nous avons élaboré des composites à base de CrSi2 associé à du Polypyrrole (PPy) ou du PEDOT:PSS. Aucune amélioration significative n'a été obtenue pour le composite PPy-CrSi2. Dans le cas du PEDOT:PSS, nous avons testé la fonctionnalisation de CrSi2 pour améliorer sa dispersion dans le polymère. Nous avons montré qu'un agent de fonctionnalisation avec un groupement phosphonique réagissant avec Cr est le plus efficace. Cette étape est très prometteuse car il s'agit de la première étude de ce type effectuée dans le cas de l'élaboration de composites à base de siliciures intermétalliques pour des applications thermoélectriques. / In the context of renewable energies, which is related to the shortage of fossil energies and climate change, thermoelectricity has regained interest in recent years (1990s). The concept of this technology is the direct conversion between thermal and electrical energies. This can contribute to a progress in the industrial sector. However, the efficiency of the thermoelectric materials, denoted ZT (figure of merit), is not sufficient for large scale applications. Nevertheless, thermoelectricity can be found in the niche market sector where other criteria are taken into account such as the abundance of the raw elements, their price and toxicity. Several families meet these criteria with the silicides as inorganics materials and conductive polymers as organics materials being the most favorable. The objective of this dissertation was to investigate the chromium disilicide (CrSi2) and PEDOT:PSS.Among the silicides, CrSi2 stands as a promising candidate for thermoelectric applications. However, its relatively high thermal conductivity compared to the conventional materials is considered as a drawback. Therefore, we were interested in nanostructuration in order to reduce this thermal conductivity. Nanostructured CrSi2 is synthesized by arc melting followed by mechanical milling or mechanical alloying with a grain size of 10-15 nm. These powders have a good chemical stability up to 1073 K. Above this temperature, secondary phases, such as CrSi, are formed. SPS processing permits to efficiently densify our pellets up to 94% while maintaining nanometric grain size (30-45 nm). This leads to a reduction of the thermal conductivity by a factor of 2 compared to bulk CrSi2. However, the Figure of merit (ZT) does not improve as the electrical resistivity increases.DFT calculations predict that some doping elements have potentiality to improve CrSi2 electronic structure. Therefore, Cr1-xTixSi2, Cr1-xZrxSi2 et Cr1-xMoxSi2 alloys are synthesized in order to enhance the thermoelectric properties of CrSi2. At RT, The power factor of Cr0.9Ti0.1Si2 seems not to be improved compared to nano-CrSi2 for a porosity of 11%. On the other hand, 2% Zr doping improved the power factor by 1.7 at RT for a porosity of 30%. It also seems that for 0% porosity, the power factor of Cr0.98Zr0.02Si2 is improved by a factor 1.9 compared to bulk CrSi2. Nevertheless, an improvement could be predicted at high temperatures for Cr0.9Ti0.1Si2. Although Cr1-xZrxSi2 alloy is metastable, we were able to synthesize it by mechanical alloying up to 5%. Further optimization is needed for improving the thermoelectric performances of Ti and Zr- doped CrSi2 . On the other hand, Cr1-xMoxSi2 alloys seem to be the most promising as an increase of the Figure of merit is observed with a ZTmax reaching values larger than 0.2 for Cr0.9Mo0.1Si2 and Cr0.8Mo0.2Si2 in the temperature range of 500-700 K. These values are higher compared to the bulk but still in the same order of magnitude as for the highest ZT value report for CrSi2.We then elaborated CrSi2 based composites with Polypyrrole (PPy) and PEDOT :PSS. No significant enhancement was observed for PPy-CrSi2 composites. In order to improve the dispersion of CrSi2 in the PEDOT :PSS, we have tested the functionnalization. Preliminary tests show that grafting agents with phosphonic acid group is the most efficient by reacting with Cr. This step is very promising as no study till date reported the functionnalization approach for the elaboration of silicides based composites, especially for thermoelectric applications.

Dans le contexte énergétique actuel, la thermoélectricité, basée sur la conversion directe de l'énergie thermique en énergie électrique, est en plein essor. Cette technologie, qui permet de récupérer l'énergie perdue sous forme de chaleur au cours de processus industriels, est basée sur l'utilisation de modules thermoélectriques. Ces modules sont constitués de la juxtaposition de branches thermoélectriques, constituées de semi-conducteurs n et p, reliées par des jonctions électriques. Dans la gamme de températures de 300 à 600°C, les matériaux de type skutterudite RM4X12 (R : terre rare, M : métal de transition, X : pnictogène) présentent des propriétés thermoélectriques intéressantes. Cependant, les propriétés aux interfaces entre les jonctions électriques et le matériau thermoélectrique jouent un rôle très important dans la performance des modules thermoélectriques. Il est nécessaire de minimiser les pertes électriques et thermiques au niveau de ces interfaces. De même, en zone chaude principalement, il est impératif de limiter l'inter-diffusion entre les éléments constituant le matériau thermoélectrique et le matériau utilisé pour les connexions électriques. La formation de composés aux interfaces peut, en effet, être à l'origine de la dégradation des propriétés du matériau thermoélectrique. Ces contraintes conduisent à introduire des barrières de diffusion entre le matériau thermoélectrique et les connexions électriques. C'est dans ce contexte que ce situe la présente étude, l'objectif étant d'étudier la potentialité de matériaux à base de nitrure de tantale en tant que barrières de diffusion. Ainsi, des couches minces à base de nitrure de tantale ont été déposées, par le procédé de pulvérisation cathodique, sur les substrats de type skutterudite, CeFe4Sb12, les connexions électriques étant en cuivre. L'efficacité de barrières monocouches, TaN, et tri-couches, TaN/Ta/TaN, a été étudiée, ces couches présentant une épaisseur totale de 1 ou 1,5 µm. La première étape de ce travail a consisté en l'étude de la stabilité thermique du matériau skutterudite afin de déterminer le domaine d'utilisation en température de ce matériau. Dans une seconde étape, la potentialité de monocouches, TaN, et multicouches, TaN/Ta/TaN, en tant que barrières de diffusion a été déterminée à partir d'une étude microstructurale. Les assemblages CeFe4Sb12/barrière/Cu ont été préalablement soumis à des traitements thermiques sous vide à des températures variant de 400 à 600°C. Enfin dans une dernière étape, l'étude théorique des mécanismes de migration a été menée à partir des calculs d'énergies d'incorporation et de migration des atomes étrangers, tels que le cuivre et l'antimoine, dans le nitrure de tantale massif. / Due to the current energy context, thermoelectricity based on the direct conversion of thermal energy into electrical energy is of great interest. Direct conversion of thermal energy to electrical energy requires the use of thermoelectric devices made of n- and p-type semiconductor couples connected by electrical junctions. RM4X12 (R: rare earth, M: transition metal, X: pnictogen) skutterudite compounds have been reported to be promising for thermoelectric applications in the [400-600]°C intermediate temperature range. However the performance of thermoelectric devices is strongly dependent on the joining of thermoelectric couples with metal electrodes as the conversion efficiency is greatly influenced by the contact resistance. High electrical and thermal conductivities are required associated with a high interfacial mechanical strength. Moreover the joining material has to be selected to avoid any interfacial reaction occurring during the device fabrication and use. Diffusion barriers are also needed to limit these interfacial reactions which may be detrimental to the thermoelectric device performance. The aim of this work is to study the efficiency of tantalum nitride based materials as diffusion barriers. TaN single layer and TaN/Ta/TaN multilayers barriers were deposited by sputtering between the CeFe4Sb12 skutterudite substrate and the Cu electrical junction. The inter-diffusion of elements was studied through these mono- and multi-layers, of 1 μm or 1.5 μm in thickness. In a first step, the thermal stability of the skutterudite has been investigated to determine the use temperature range of this material. In a second step the efficiency of TaN and TaN/Ta/TaN layers as diffusion barriers has been determined from a microstructural study. CeFe4Sb12/barrier/Cu stackings were previously annealed under vacuum in the [400-600]°C temperature range. Finally a theoretical study of the migration mechanisms was carried out from the calculations of the incorporation and migration energies of species, more specifically Sb and Cu, in the bulk tantalum nitride.

ii ABSTRACT Percolation phenomena involving the electrical conductivity, dielectric constant, Hall coefficient, magnetoconductivity, relative magnetoresistivity, 1/ f noise and thermoelectric power are investigated in graphite (G) and hexagonal boron-nitride (BN) powder mixtures. Two kinds of systems are used in the experiments: highly compressed discs and parallelepipeds, cut from these discs, as well as 50%G-50%BN and 55%G-45%BN powder mixtures undergoing compression. The measured DC conductivities follow the power-laws 0"( <p, 0) ex: (<p-<Pc)t (<p > <Pc) and O"(<p, 0) ex: (<Pc-<Pti (<p < <Pc), and the low frequency (lOOHz & 1000Hz) dielectric constant varies as c( <p, W ~ 0) ex: (<Pc - <P )-S( <P < <Pc), where <Pc is the percolation threshold, t and s are the conductivity exponents, and s is the dielectric exponent. Near the percolation threshold and at high frequencies, the AC conductivity varies with frequency as 0"( <p, w) ex: WX and the AC dielectric constant varies as c( <p, w) ex: w-Y, where the exponents x and y satisfy the scaling relation x + y = 1. The crossover frequency We scales with DC conductivity as Wc ex: O"q( <p, 0) (<p > <Pc), while on the insulating side, Wc ~ 1, resulting in q ~O for the three G-BN systems. The loss tangent tan t5( <p, w) (<p < <Pc) is found to have a global minimum, in contrary to the results of computer simulations. The Hall constant could not be measured using existing instrumentation. The measured magnetoconductivity and relative magnetoresistivity follow the power-laws - 6. 0" ex: (<p - <Pc)3.08 and 6.R/ R ex: (<p - <Pc)O.28 respectively. These two exponents, iii 3.08 and 0.28, are not in agreement with theory. The 1/ f noise was measured for the conducting discs and parallelepipeds. The normalized 1/ f noise power varies as Sv I V2 ex RW with the exponents w = 1.47 and 1.72 for the disc and parallelepiped samples respectively. Furthermore, the normalized noise power near the percolation threshold is, for the first time, observed to vary inversely with the square-root of sample volume. Based on the Milgrom-Shtrikman-Bergman-Levy (MSBL) formula, thermoelectric power of a binary composite is shown to be a linear function of the WiedemanFranz ratio. A scaling scheme for the Wiedeman-Franz ratio for percolation systems is proposed, which yields power-law behavior for the thermoelectric power. The proposed power-laws for the thermoelectric power can be written as (Sm - Md ex (<p - <Pc)h 1 for <P > <Pc and as (Sm - /~1d ex (<Pc - <p)-h2 for <p < <Pc, where Sm is the thermoelectric power for the composites, Afl is a constant for a given percolation system, and hI and h2 are the two critical exponents. The experimental thermoelectric power data for the G-BN conducting parallelepipeds was fitted to the above powerlaw for <p > <Pc. A least squares fit yielded the exponent hI = -1.13 and parameter MI =9.511l V I I< respectively.

Dans le contexte actuel de recherche d’efficacité énergétique, c’est-à-dire de maîtrise de l’énergie incluant la valorisation de l’énergie perdue, ce projet de thèse propose de synthétiser et caractériser les propriétés thermoélectriques de la polyaniline dopée. Nous avons ainsi synthétisé plusieurs échantillons de polyaniline et étudié l’influence des conditions de synthèse, du taux de dopage, du type de dopant, du taux de cristallinité ou de la mise en forme du matériau fini sur leurs propriétés thermoélectriques. Tout d’abord, nous avons mesuré les dépendances en température des conductivités électriques et thermiques ainsi que du coefficient Seebeck de la polyaniline dopée à l’acide chlorhydrique (Pani/HCl). Nous avons montré que lors de la synthèse, une température de polymérisation basse (223K) permet d’obtenir de meilleures conductivités électriques et donc des propriétés thermoélectriques améliorées. D’autre part, la valeur de la pression utilisée pour compresser le matériau et le mettre en forme doit être assez élevée (1.109GPa) pour optimiser les performances thermoélectriques. Afin de mieux comprendre les propriétés de transport de la polyaniline dopée au chlorure, nous nous sommes ensuite intéressés à ses propriétés magnétiques et calorifiques. Les mesures de la susceptibilité magnétique et de la capacité calorifique du polymère dopé en fonction de la température ont ainsi permis de déterminer la nature de ses états électroniques et de déduire le type de transport au sein du polymère. Enfin, nous avons synthétisé et étudié la polyaniline dopée au camphresulfonate en faisant varier le taux de dopage entre 30% et 90%. Les mesures de conductivités électriques, thermiques et du coefficient Seebeck en fonction de la température montrent qu’un optimum peut être trouvé pour un taux de dopage de 50%. / In the actual frame of energy efficiency research, namely, the energy management including wasted energy, this PhD project deals with the synthesis and characterization of thermoelectric properties of doped polyaniline. Consequently, polyanilines have been synthesized and the effect of the synthesis conditions, the doping level, the type of dopant, the crystallinity or the sample preparation of the final material on the thermoelectric properties have been studied. Firstly, we measured the thermal dependencies of electrical and thermal conductivities as well as Seebeck coefficient hydrochloric acid doped polyaniline (Pani/HCl). We showed that a low polymerization temperature (223K) lead to better electric conductivity and so improved thermoelectric properties. On the other side, the pressure used to compress powders should as high as 1.109GPa to optimize the thermoelectric performance. Then, in order to have a better understanding of the transport properties of chloride doped polyaniline, we investigate the magnetic and thermal properties. Measurements of the specific heat and the magnetic susceptibility of chloride doped polyaniline as a function of the temperature allowed to determine the electronic states and the mechanism of transport in the polymer chains. Finally, we synthesized and studied camphorsulfonate doped polyaniline by varying the doping level between 30% and 90%. The electrical, thermal conductivities and Seebeck coefficient as a function of the temperature show clearly on optimum of doping level at 50% with camphorsulfonate doping agent.

Thermoelectric materials play an important role in energy conversion as they represent environmentally safe and solid state devices with a great potential towards enhancing their efficiency. The ability to generate electric power in a reliable way without using non-renewable resources motivates many experimentalists as well as computational physicists to search and design new thermoelectric materials. Several classes of materials have been identified as good candidates for high efficient thermoelectrics because of their inherently low thermal conductivity. The complex study of the crystal and electronic structures of such materials helps to reveal hidden properties and give fundamental understanding, necessary for the development of a new generation of thermoelectrics. In the current thesis, ab-initio computational methods along with experimental observations are applied to investigate several material classes suitable for thermoelectric applications. One example are Bi-Sb bismuth rich alloys, for which it is shown how structural anomalies affect the electronic structure and how inclusion of the Spin-Orbit coupling is necessary for this type of materials. Another example are bournonite materials whose low thermal conductivity is attributed to distortions and interactions associated with lone-electron s^2 pair distributions. In addition, it is shown how doping with similar atoms can affects electronic structure of these materials leading to changes in their transport properties. Clathrate materials from the less studied type II Sn class are also investigated with a detailed analysis for their structural stability, electronic properties and phonons. These systems are considered with partially substituted atoms on the framework and different guests inside. The effect upon insertion of Noble gases into the cage network is also investigated. In addition, the newly synthesized As based cationic material is also studied finding novel structure-property relations. Another class of materials, quaternary chalcogenides, have also been studied. Because of their inherently low thermal conductivity and semiconducting nature their transport properties may be optimized in a favorable way for thermoelectricity. The present work provides an in-depth study of structural and electronic properties of several classes of materials, which can be used by experimentalists for input and guidance in the laboratory.

Thermoelectric devices are capable of providing both localized active cooling and waste heat power generation. This work will explore the possibility of embedding thermoelectric devices within electronic packaging in order to achieve better system performance. Intel and Nextreme, Inc. have produced thin-film superlattice thermoelectric devices that have above average performance for thermoelectrics and are much thinner than most devices on the market currently. This allows them to be packaged inside of the electronic package where the thermoelectric devices can take advantage of the increased temperatures and decreased thermal lag as compared to the devices being planted on the outside of the package. This work uses the numerical CFD solver FLUENT and the analog electronic circuit simulator SPICE to simulate activity of thermoelectric devices within an electronics package.

Performance of two different waste heat recovery systems (one based on Rankine cycle and the other one using thermoelectricity) combined with non-hybrid, mild-hybrid and full hybrid systems are investigated. The vehicle under investigation was a 440hp Scania truck, loaded by 40 tons. Input data included logged data from a long haulage drive test in Sweden.All systems (waste heat recovery as well as hybrid) are implemented and simulated in Matlab/Simulink. Almost all systems are modeled using measured data or performance curves provided by one manufacturer. For Rankine system results from another investigation were used.Regardless of practical issues in implementing systems, reduction in fuel consumption for six different combination of waste heat recovery systems and hybrid systems with different degrees of hybridization are calculated. In general Rankine cycle shows a better performance. However, due to improvements achieved in laboratories, thermoelectricity could also be an option in future.This study focuses on “system” point of view and therefore high precision calculations is not included. However it can be useful in making decisions for further investigations.

In this work, patterns in the behavior of different classes and types of thermoelectric materials are observed, and an alchemy that could help engineer a highly efficient thermoelectric is proposed. A method based on cross-correlation of Seebeck waveforms is also presented in order to capture physics of magnetic transition. The method is used to compute Curie temperature of LaCoO3 with an accuracy of 10K. In total, over 26 systems are analyzed, and 19 presented: Chalcogenides (PbSe, PbTe, Sb2Te3, Ag2Se), Skutterudites and Clathrates (CoSb3, SrFe4Sb12, Cd (CN)2, CdC, Ba8Ga16Si30*), Perovskites (SrTiO3, BaTiO3, LaCoO3, CaSiO3, Ce3InN*, YCoO3*), Half-Heuslers (ZrNiSn, NbFeSb, LiAlSi, CoSbTi, ScPtSb*, CaMgSi*), and an assorted class of thermoelectric materials (FeSi, FeSi2, ZnO, Ag QDSL*). Relaxation time is estimated from experimental conductance curve fits. A maximum upper bound of zT is evaluated for systems that have no experimental backing. In general, thermoelectric parameters (power factor, Seebeck coefficient and zT) are estimated for the aforementioned crystal structures. Strongly correlated systems are treated using LDAU and GGAU approximations. LDA/GGA/L(S)DA+U/GGA+U approach specific errors have also been highlighted. Densities of experimental results are estimated.

This project was basically exploratory in the electrical properties of carbon nanotube (CNT) based materials. The direct current (DC) conductivity of CNT/polymer composites was computed by using equivalent circuit method and a three dimensional (3-D) numerical continuum model with the consideration of tunneling conduction. The effects of the potential barrier of polymer and the tortousity of CNTs on the conductivity were analyzed. It was found that both of percolation threshold and DC conductivity can be strongly affected by the potential barrier and the tortousity. The influence of contact resistance on DC conductivity was also computed, and the results revealed that contact resistance and tunneling resistance had significant influences on the conductivity, but did not affect the percolation threshold. The microstructure-dependent alternating current (AC) properties of CNT/polymer composites were investigated using the 3-D numerical continuum model. It was found that AC conductivity and critical frequency of CNT/polymer composites can be enhanced by increasing the curl ratio of CNTs. In the mid-range CNT mass fraction, with increasing curl ratio of CNTs, AC conductivity, interestingly, became frequency-dependent in low frequency range, which cannot be explained by reference to the percolation theory. A proper interpretation was given based on the linear circuit theory. It was also found that the critical frequency can also be affected by the size of CNT cluster. Series numerical formulas were derived by using a numerical capacitively and resistively junction model. In particular, this work introduced an equivalent resistor-capacitor (RC) circuit with simple definitions of the values of contact resistance and average mutual capacitance for CNT/polymer nanocomposites. Theoretical results were in good agreement with experimental data, and successfully predicted the effect of morphology on the AC properties of CNT/polymer composites. DC and AC conductivities of multi-walled carbon nanotube (MWCNT)/graphene oxide (GO) hybrid films were measured for selected MWCNT mass fractions of 10%, 33.3%, 50%, 66.7%, and 83.3% using four-probe method. The experimental results were fitted using scaling law, and relatively high percolation threshold was found. This high percolation threshold was understood in terms of the potential energy and intrinsic ripples and warping in the freestanding graphene sheets. The capacitance of these hybrid films were measured using the voltmeter-ammeter-wattmeter test circuit with different voltages and heat treatments. The MWCNT/GO film showed relatively high specific capacitance (0.192F/cm3 for the mass fraction of 83.3%) and power factor compared to conventional dielectric capacitors. Both of measured capacitance and power factor can be enhanced by increasing testing voltages. The capacitance of MWCNT/GO films rapidly decreased after heat treatments above 160 ℃. This decrease was caused by redox reaction in the GO sheets. The capacitive behaviour of MWCNT/GO hybrid films was also interpreted by using the equivalent circuit model. Single-walled carbon nanotube (SWCNT) and SWCNT/Poly(vinyl alcohol) (PVA) films were used to form a piezoresistive strain sensor. Both of static and dynamic strain sensing behaviours of SWCNT and SWCNT/PVA films were measured. It was found that the sensitivities of these films decreased with increasing their thicknesses. The SWCNT film with a thickness of 1900 nm and SWCNT/PVA film exhibited viscoelastic sensing behaviour, because van der Waals attraction force allowed axial slippages of the smooth surface of nanotubes. A numerical model was derived based on the dynamic strain sensing behaviour. This model could be useful for designing CNT strain sensors. Finally, thermoelectric power (TEP) of deformed SWCNT films with various thicknesses was measured. It was observed that positive TEP of SWCNT films increased with increasing stain above the critical point. The experimental results were fitted by using a numerical model in terms of a variation of Nordheim-Gorter relation and fluctuation induced tunneling (FIT) model. From the numerical model, it was found that the increase of TEP above the critical strain resulted from the positive term of the contribution from the barrier region, and the effect of barrier regions decreases with increasing the thickness of the film.