<p>This thesis presents studies of the X-ray linear dichroism (XLD) in individual single-walled (SW) and multi-walled (MW) carbon nanotubes (CNT) measured by a scanning transmission X-ray microscope (STXM). The C 1s spectra of CNT showed a large XLD at the C 1s→π* transition. The magnitude of the XLD was found to be related to the quality of CNT such that in high quality CNT, it was fairly large and as the quality lowered it decreased. This dichroic effect was used to map defects along individual CNT. In addition, STXM was employed to map chemical components in pristine, purified, and dodecyl functionalized SWCNT bundles to investigate the changes occurring in them due to chemical functionalization.<br />STXM has limited spatial resolution. Thus, electron energy loss spectroscopy (EELS) in a transmission electron microscope (TEM) was used to obtain similar information about CNT, but at much higher spatial resolution. The measurements performed in the scanning transmission electron microscopy (STEM) mode produced signals analogous to the XLD when the orientation of the momentum transfer (q) was resolved. This was achieved by displacing the pattern of electron scattering from CNT relative to the EELS entrance aperture. TEM-EELS was also utilized to map defects in pristine and focused ion beam (FIB) modified CNT.</p> / Doctor of Philosophy (PhD)

At present, structural health monitoring efforts focus primarily on the sensors and sensing systems for detecting instances and locations of damage through techniques such as X-ray, micro CT, acoustic emission, infrared thermography, lamb wave etc., which only detect cracks at relatively large length scales and rely heavily on sensors and sensing systems which are external to the material system. As an alternative to conventional commercially available SHM techniques, the current work explores processing-structure-property relationships starting from carbon nanotube (CNT) based nanocomposites to particulate composites with nanocomposite binder/matrix materials, i.e. hybrid particulate composites to investigate deformation and damage sensing capabilities of inherently sensing materials and structures through their piezoresistive (coupled electro-mechanical) response. Initial efforts focused on controlling the dispersion of CNTs and orientation of CNT filaments within nanocomposites under dielectrophoresis to guide design and fabrication process of nanocomposites by tuning CNT concentration, applied AC electric field intensity, frequency and exposure time. It is observed that a combination of exposure time to AC electric field and the AC field frequency are the key drivers of filament width and spacing and that the network for filament formation is much more efficient for pristine CNTs than for acid treated functionalized CNTs. With the knowledge obtained from controlling the morphological features, AC field-induced long range alignment of CNTs within bulk nanocomposites was scaled up to form structural test coupons. The morphology, electrical and mechanical properties of the coupons were investigated. The anisotropic piezoresistive response both for parallel and transverse to CNT alignment direction within bulk composite coupons under various loading conditions was obtained. It is observed that control of the CNT network allows for the establishment of percolation paths and piezoresistive response well below the nominal percolation threshold observed for random, so called well-dispersed CNT network distributions. The potential for use of such bulk nanocomposites in SHM applications to detect strain and microdamage accumulation is further demonstrated, underscoring the importance of microscale CNT distribution/orientation and network formation/disruption in governing the piezoresistive sensitivities. Finally, what may be the first experimental study in the literature is conducted for real-time embedded microscale strain and damage sensing in energetic materials by distributing the CNT sensing network throughout the binder phase of inert and mock energetic composites through piezoresistive response for SHM in energetic materials. The incorporation of CNTs into inert and mock energetic composites revealed promising self-diagnostic functionalities for in situ real-time SHM applications under quasi-static and low velocity impact loading for solid rocket propellants, detonators and munitions to reduce the stochastic nature of safety characterization and help in designing insult tolerant energetic materials. / Ph. D. / At present, structural health monitoring (SHM) efforts focus primarily on the sensors and sensing systems for detecting instances and locations of damage, which only detect cracks at relatively large length scales and rely heavily on sensors and sensing systems which are external to the material system. As an alternative to conventional commercially available SHM techniques, the current work explores the incorporation of carbon nanotubes (CNTs) into nanocomposites and particulate composites to investigate deformation and damage sensing capabilities of inherently sensing materials and structures through their coupled electromechanical response. Initial efforts focused on controlling the dispersion of CNTs and orientation of CNT filaments within nanocomposites to guide design and fabrication process of nanocomposites. With the knowledge obtained from controlling the morphological features, long range alignment of CNTs within bulk nanocomposites was scaled up to form structural test coupons. The potential for use of such bulk nanocomposites in SHM applications to detect strain and microdamage accumulation is further demonstrated. Finally, what may be the first experimental study in the literature is conducted for real-time embedded deformation and damage sensing in inert and mock energetic composites to reduce the stochastic nature of safety characterization and help in designing insult tolerant solid rocket propellants, detonators and munitions.

We numerically analyze three mechanics problems described below. For each problem, the developed computational model is verified by comparing computed results for example problems with those available in the literature. Effective utilization of single wall carbon nanotubes (SWCNTs) and single layer graphene sheets (SLGSs) as reinforcements in nanocomposites requires their strong binding with the surrounding matrix. An effective technique to enhance this binding is to functionalize SWCNTs and SLGSs by covalent attachment of appropriate chemical groups. However, this damages their pristine structures that may degrade their mechanical properties. Here, we delineate using molecular mechanics simulations effects of covalent functionalization on elastic moduli of these nanomaterials. It is found that Young's modulus and the shear modulus of an SWCNT (SLGS), respectively, decrease by about 34% (73%) and 43% (42%) when 20% (10%) of carbon atoms are functionalized for each of the four functional groups of different polarities studied. A shell theory that gives results close to the solution of the corresponding 3-dimensional problem depends upon the shell geometry, applied loads, and initial and boundary conditions. Here, by using a third order shear and normal deformable theory and the finite element method (FEM), we delineate for a doubly curved shell deformed statically with general tractions and subjected to different boundary conditions effects of geometric parameters on in-plane and transverse stretching and bending deformations. These results should help designers decide when to consider effects of these deformation modes for doubly curved shells. Composite laminates are usually fabricated by curing resin pre-impregnated fiber layers in an autoclave under prescribed temperature and pressure cycles. A challenge is to reduce residual stresses developed during this process and simultaneously minimize the cure cycle time. Here, we use the FEM and a genetic algorithm to find the optimal cycle parameters. It is found that in comparison to the manufacturer's recommended cycle, for a laminate with the span/thickness of 12.5, one optimal cycle reduces residual stresses by 47% and the total cure time from 5 to 4 hours, and another reduces the total cure time to 2 hours and residual stresses by 8%. / Ph. D. / We analyze using computational techniques three mechanics problems described below. In the last three decades, two carbon nanomaterials (i.e., allotropes of carbon having length-scale of 10^-9 m), namely, single wall carbon nanotubes (SWCNTs) and single layer graphene sheets (SLGSs) have evolved as revolutionary materials with exceptional properties per unit weight that are superior to conventional engineering materials. A composite (i.e., a material made by combining two or more materials to attain desired properties which cannot be achieved by any of its constituents alone) made by using either of these carbon nanomaterials as reinforcements in a polymer could be a potential candidate for applications requiring high strength and light weight. However, the effective utilization of these composites for an application requires the strong binding between their constituents. An effective technique to enhance this binding is to modify the surface properties of SWCNTs and SLGSs by covalently bonding to them suitable chemical group that is usually called covalent functionalization. However, this damages their pristine structures that may degrade their mechanical properties. Here, it is found that the functionalization reduces elastic moduli of carbon nanomaterials, the reduction increases with an increase in the amount of functionalization and is essentially independent of the functionalizing chemical group. This study should help engineers interested in utilizing these materials to design novel nanocomposites. Composite laminates, made by stacking and binding together layers of fiber-reinforced composites, are widely used in aircraft, aerospace, marine, automobile, power generation, chemical and ballistic applications due to their high strength and stiffness per unit weight compared to that of conventional metallic materials. Shell theories are widely used to analyze deformations of composite laminates which reduces a 3-dimensional (3-D) problem to an equivalent 2-D problem by making certain assumptions related to the deformations of the laminate. This approach requires less computational effort to find a numerical solution (i.e., an approximate solution obtained using a computational technique) of the problem as compared to that needed for solving the full 3-D problem. However, the accuracy of the results predicted by a shell theory depends on the problem being studied, i.e., the shell geometry, applied loads, initial conditions (i.e., the motion of the laminate at the start of application of the load) and boundary conditions (i.e., constraints imposed on the deformations of the edges of the laminate). Here, we analyze effects of geometric parameters of the laminated shells on their deformations for different types of applied loads and various boundary conditions specified on the edges. The results should help designers find an optimal geometry of the composite laminates for a given mechanical application. Fiber-reinforced composite laminates are usually fabricated by curing (which involves heating and cooling in a prescribed manner under application of the pressure) resin preimpregnated fiber layers under prescribed temperature and pressure cycles. However, during this cure process the laminate deforms and the final product is not stress-free. Here, we find optimal parameters of the cure cycle that minimize stresses developed during the cure process as well as the time required to cure the laminate. It is found that for a laminate studied these optimal parameters reduce the stresses by 47% and the cure time from 5 to 4 hours in comparison to the standard cure cycle recommended by the laminate manufacturer. This study will provide manufacturing engineers with an approach to fabricate composite laminates of desired quality.

Silicon intracortical microelectrodes arrays (MEA) provide some of the best performance for brain computer interfaces but suffer from lifetime limitations that prevent clinical adoption. A primary contributor of lifetime limitations is the chronic foreign body response (FBR) which remains active in response to the brain tissue strain caused by implanted probes during brain micromotion. Designing probes with softer materials can reduce the chronic FBR and improve device lifetime. A high-aspect ratio CNT MEA could provide several advantages as an intracortical MEA but has yet to be mechanically characterized. CNT MEAs are infiltrated with 3 degrees of carbon infiltration and mechanically tested to determine their Young’s modulus. A novel dual deflection (DD) test is designed to measure the cantilever bending of the CNT electrodes and determine their modulus. The CNT diameters of the microelectrodes are measured with SEM as a quantification method of carbon infiltration. A total of 64 CNT microelectrodes (MEs) are tested with the DD test and the modulus is calculated for a subset of 59 CNT MEs. The modulus results are grouped by infiltration level for statistical calculations. The DD test measured a mean modulus of 19.6 ± 14.5 MPa, 67.7 ± 22.7 MPa, and 168 ± 62.3 MPa for arrays fabricated with 0 second, 15 second, and 30 second infiltrations, respectively. A finite element analysis (FEA) model is developed to examine the brain tissue strain under a static, 10 µm, micromotion deflection of penetrating probes with the maximum (1.7 GPa), median (72 MPa), and minimum (3.9 MPa) CNT moduli we measured, as well as the modulus of silicon (165 GPa) for comparison. The max CNT and Si probes induced similar strain in the brain model at the probe tip, while the minimum and median CNT probes showed almost no strain at the tip. The model implies that for the CNT MEA probe geometry, significant reductions in the brain tissue strain can be achieved by CNT MEs with moduli in tens of MPa range, which can be fabricated with 15 seconds of carbon infiltration. Microelectrodes with 15 seconds of infiltration also demonstrated increased resilience compared to the other infiltration time groups, indicating that CNT MEAs with 15 seconds of infiltration might be a viable candidate material for a neural probe array.

Ce travail a porté sur la caractérisation et la quantification en 3D de la répartition de charges de différents types (nanoparticules, nanotubes, etc.) dans des matrices polymères. Nous nous focalisons sur les techniques de tomographie en microscopie électronique. Une approche multiple en tomographie électronique a été réalisée : la tomographie en FIB/MEB (faisceau d’ions focalisé/microscope électronique à balayage), la tomographie en MEB et la tomographie en MET (microscope électronique en transmission). Les nanocomposites polymère sont généralement élaborés aux fins d’améliorer les propriétés physiques (mécanique, électrique, etc.) du matériau polymère constituant la matrice, grâce à une addition contrôlée de charges nanométriques. La caractérisation de tels matériaux, et l’établissement de corrélations précises entre la microstructure et les propriétés d’usage, requièrent une approche tri-dimensionnelle. En raison de la taille nanométrique des charges, la microscopie électronique est incontournable. Deux systèmes de nanocomposite polymère ont été étudiés par une approche multiple de tomographie électronique : P(BuA-stat-S)/MWNTs (copolymère statistique poly (styrène-co-acrylate de butyl) renforcé par des nanotubes de carbone multi-parois), et P(BuA-stat-MMA)/SiO2 (copolymère statistique poly(butyl acrylate-co-methyl methacrylate) renforcé par des nanoparticules de silice). Par combinaison de divers techniques, la caractérisation et la quantification des nanocharges ont été possibles. En particulier, la taille, la fraction volumique et la distribution des charges ont été mesurées. Cette étude a ainsi fourni des informations en 3D qui contribuent à mieux comprendre les propriétés des nanocomposites. Une attention particulière a été portée aux artefacts et causes d’erreur possibles durant l’étape de traitement 3D. Nous avons également essayé de comparer les différentes techniques utilisées du point de vue de leurs avantages et inconvénients respectifs, en dégageant des possibilités d’amélioration future. / This work is focused on the characterization and quantification of the 3D distribution of different types of fillers (nanoparticles, nanotubes, etc.) in polymer matrices. We have essentially used tomography techniques in electron microscopy. Multiple approaches to electron tomography were performed: FIB-SEM (focused ion beam/scanning electron microscope) tomography, SEM tomography and TEM (transmission electron microscope) tomography. Polymer nanocomposites are basically synthesized in order to improve the physical properties (mechanical, electric, etc.) of the pure polymer constituting the matrix, by a controlled addition of fillers at the nanoscale. The characterization of such materials and the establishment of accurate correlations between the microstructure and the modified properties require a three-dimensional approach. According to the nanometric size of the fillers, electron microscopy techniques are needed. Two systems of polymer nanocomposites have been studied by multiple electron tomography approaches: P(BuA-stat-S)/MWNTs (statistical copolymer poly(styrene-co-butyl acrylate) reinforced by multi-walled carbon nanotubes) and P(BuA-stat-MMA)/SiO2 (statistical copolymer poly(butyl acrylate-co-methyl methacrylate) reinforced by silica nanoparticles). By combining various techniques, the characterization and the quantification of nanofillers were possible. In particular, statistics about size, distribution and volume fraction of the fillers were measured. This study has then provided 3D information, which contributes to a better understanding of properties of the nanocomposites. Attention has been paid to analyze carefully original data, and artifacts and causes of errors or inaccuracy were considered in the 3D treatments. We also attempted to compare benefits and drawbacks of all techniques employed in this study, and perspectives for future improvements have been proposed.

The main problem addressed by this dissertation is the accurate and automated determination of electron microscope imaging conditions. This enables the restoration of the object wave, which confers direct structural information about the specimen, from sets of differently aberrated images. An important member in the imaging chain is the image recording device, in many cases now a charge-coupled device (CCD) camera. Previous characterisations of these cameras often relied on the unjustified assumption that the Modulation Transfer Function (MTF) also correctly describes the spatial frequency dependent attenuation of the electron shot noise. A new theory is therefore presented that distinguishes between signal and noise transfer. This facilitates the evaluation of both properties using a detailed Monte-Carlo simulation model for the electron and photon scattering in the scintillator of the camera. Furthermore, methods for the accurate experimental determination of the signal and noise transfer functions are presented. In agreement with the Monte-Carlo simulations, experimental results for commercially available CCD cameras show that the signal transfer is significantly poorer than the noise transfer. The centrepiece of this dissertation is the development of new methods for determining the relative aberrations in a set of images and the absolute symmetric aberrations in the restored wave. Both are based on the analysis of the phase information in the Fourier domain and give each Fourier component a weight independent of its strength. This makes the method suitable even for largely crystalline samples with little amorphous contamination, where conventional methods, such as automated diffractogram fitting, usually fail. The method is then extended to also cover the antisymmetric aberrations, using combined beam tilt and focal series. The applicability of the new method is demonstrated with object wave restorations from tilt and focal series of complex inorganic block oxides and of carbon nanotubes filled with one-dimensional inorganic crystals. The latter specimens allowed for the first time a direct comparison between the phase shift in the restored object wave of a specimen with precisely known thickness and the value predicted by simulations.

L'émergence des nanotubes de carbone a ouvert de nouveaux champs d'application dans le domaine des matériaux polymères. L'ajout de ces charges carbonées au sein de polymères permet la mise en œuvre de composites aux propriétés électriques optimisées. La conductivité de ces matériaux dépend en grande partie de l'organisation des charges dans la matrice, notamment de la présence de réseaux percolants. L'objectif du présent travail de thèse est de comprendre les mécanismes de structuration des nanotubes de carbone au sein de différents milieux. L'architecture de ces réseaux de charges a principalement été révélée par le biais de mesures électriques et diélectriques. L'originalité de nos travaux réside dans l'utilisation de matrices liquides, notamment des huiles de silicone, afin de s'affranchir des contraintes présentes dans les plastiques d'une part, et de simplifier les processus de mise en œuvre d'autre part. Le manuscrit de thèse est articulé autour de six chapitres. Une première partie bibliographique aborde les propriétés des nanotubes de carbone ainsi que les phénomènes que sont la percolation et la percolation dynamique. Le second chapitre, matériel & méthode, présente les matériaux employés ainsi que les différentes techniques de caractérisation utilisées au cours de la thèse. Le troisième chapitre de la thèse aborde, à travers des mesures de conductivité, la percolation dynamique des nanotubes de carbone sein d'huiles de silicone. Le chapitre 4 propose une modification la loi de puissance de Kirkpatrick, afin de décrire la conductivité en fonction du temps et du taux de charge. L'exposant critique de percolation, caractérisant la transition isolant conducteur, se révèle être un indicateur de l'état de dispersion des nanotubes à travers la matrice. Le chapitre 5 démontre la possibilité de contrôler l'organisation des charges par l'application d'un champ électrique. L'application d'un champ élevé permet une augmentation de plusieurs ordres de grandeur de la conductivité ainsi qu'une diminution des charges nécessaire à la formation d'un réseau percolant. Nous avons notamment déterminé des seuils de percolation de l'ordre de 0.005% massique en nanotube de carbone. Enfin, l'influence des propriétés intrinsèques de la matrice, telles la viscosité et la tension de surface, est étudié dans le chapitre 6. La dispersion des nanotubes de carbone s'avère être favorisée au sein de liquides ayant des tensions de surface proches de celle des tubes. Au contraire, une agrégation de charge est rapidement observée dans le cas ou la différence de tension de surface charge-matrice est importante. Nous avons également observé que la percolation des nanotubes est défavorisée au sein de milieux visqueux. / The rise of carbon nanotube has open possibility for composites polymers. Mixing this carbonaceous filler with polymer medias leads to an optimization of the electrical properties. Then, conductivity mainly depends of the filler architecture, especially the presence of percolating networks. The objective of this work is to understand the percolation mechanisms of the carbon nanotubes in different media. During this study, filler network has been revealed by the mean of electrical and dielectrical measurements. The originality of our work lies in the use of liquid matrices, such as silicone oils, in order to overcome the stresses in the plastic on the one hand, and to simplify the processing in other hand. This thesis is organized around six chapters. The first bibliographic part discusses the carbon nanotubes properties as well as percolation and dynamic percolation phenomena. The second chapter, matériel & méthode, presents the materials used and the different characterization techniques employed. The third chapter of the thesis talks about dynamic percolation of carbon nanotubes in silicone oil, probed by conductivity measurements. Chapter 4 provides a change of the power law Kirkpatrick to describe the conductivity as a function of time and filler content. The critical exponent of percolation is proving to be an indicator of the dispersion state of nanotubes throughout the matrix. In the Chapter 5, electric field is depicted as a tool to control the organization of fillers. The application of a high field increases the conductivity of several orders of magnitude and decreases the percolation threshold. Percolation thresholds close to 0.005 wt % have been determined. At last, the influence of the intrinsic properties of the matrix, such as viscosity and surface tension, is discussed in Chapter 6. Carbon nanotubes dispersion appears to be favored if the difference of surface tension between filler and liquid is low. In contrast, a filler aggregation is rapidly observed in the case where the difference in surface tension is important. We also observed that the percolation of the nanotubes is favored in viscous media.

Ce travail de thèse a porté sur l’élaboration de nanomatériaux hybrides de type nano-câbles fonctionnels, composés de polymères covalents et de molécules auto-assemblées. L’approche «bottom-up» adoptée repose sur des processus uniquement physiques, à savoir la nucléation hétérogène, la cristallisation et la gélification thermo réversible. Deux systèmes hybrides ont été élaborés et étudiés: le premier est composé de molécules de tetra-2-éthylhexanoate de bicuivre (CuS8) auto-assemblées en filaments, lesquels sont encapsulés au sein des fibrilles de polystyrène isotactique (iPS). Nous avons montré au travers de différentes études (DSC, DNPA,SQUID, EXAFS et IR-TF) que leur encapsulation permet non seulement de les stabiliser mais également de modifier leur comportement antiferromagnétique. Le deuxième système a consisté à des fibrilles de poly(alkylthiophène)s (P3AT), emmaillotées au moyen de molécules diamides (BHPB-10) capables de s'assembler en nanotubes. En plus des études de la morphologie et de la structuration par TEM et UV-Vis,nous avons étudié les propriétés de conductivité du système hybride P3BT/BHPB-10 en C-AFM. Nous avons montré qu’il est effectivement possible de réaliser des nano-câbles semi-conducteur gainés. / This thesis deals with new hybrid nanomaterials of functional nanocable-like structures, consisting of covalent polymers and self-assembled molecules. The «bottom-up» approach adopted for the elaboration is based only on physical processes such as heterogeneous nucleation, crystallization and thermoreversible gelation. This original approach allowed us to easily prepare two functional nanocables: the first consisted of bicopper tetra-2-ethylhexanoate (CuS8) molecules self-assembled on filaments which are encapsulated within isotactic polystyrene (iPS) fibrils. We proved throughout different studies (DSC, SANS, SQUID, EXAFS and FT-IR) that the encapsulation allows one to get stable filaments, and particularly to modify their antiferromagnetic behavior as well. The second system constituted of poly(alkylthiophene)s fibrilles (P3AT), sheathed by diamides molecules (BHPB-10) self-assembled on nanotubes. Besides the morphological and the structuration studies (TEM and UV-Vis), we investigated the conductivity of the hybrid system P3BT/BHPB-10 by C-AFM. Results showed the possibility to obtain sheathed semi-conducting nano-cables.

Afin de produire des champs magnétiques intenses (100 T), les fils conducteurs utilisés dans les bobines pulsées doivent présenter une contrainte à la rupture élevée et une très faible résistivité électrique. Le LNCMI et l'équipe NNC du CIRIMAT explorent des solutions originales basées sur l'élaboration de fils de cuivre nanostructuré et de fils nanocomposites nanotube de carbone - cuivre (NTC-Cu) par la combinaison originale du spark plasma sintering (SPS) et de l'étirage à température ambiante. Des barreaux de cuivre ont été élaborés par SPS à partir de poudres commerciales micrométriques. La croissance cristalline est très faible et la taille des grains de cuivre est 10 fois plus petite que celle des précurseurs de fils classiques. Les barreaux ont été étirés, sans rupture, sous forme de fils de diamètre décroissant (jusqu'à 0,198 mm) et de plusieurs mètres de long. Les grains ultrafins de Cu sont fortement allongés dans la direction de l'étirage. Aucune macle n'a été observée. Tous nos fils de cuivre présentent une résistance à la rupture en traction (à 293K et 77K) supérieure à celle des fils préparés à partir d'un précurseur de cuivre OFHC classique, ce qui pourrait résulter de la combinaison de l'écrouissage et des mécanismes d'Orowan. La résistivité électrique des fils est environ 12% plus élevée que celle des fils de cuivre OFHC. Pour les composites NTC-Cu, une adaptation de la méthode de mélange (fonctionnalisation des NTC biparois et à huit parois, mélange, cryogénisation, lyophilisation, réduction sous H2) a permis de produire des lots de poudre de 14 g en ayant une dispersion homogène des NTC. Du fait de la très faible teneur en carbone (= 1%), la préparation des barreaux puis des fils par les méthodes employées pour le cuivre pur est possible sans modification. La contrainte maximale à la rupture des fils NTC-Cu est supérieure (10-25%) à celle des fils de cuivre correspondants. Les NTC ont peu d'influence sur la microstructure du cuivre et leur probable alignement permet de bénéficier de leur grande résistance en traction. La résistivité est légèrement supérieure à celle des fils de cuivre correspondants (environ 12% à 77K). Le dernier chapitre est consacré à la préparation d'éprouvettes " os-de-chien " (Cu et NTC-Cu) directement par SPS " near-net-shape ". Nous avons mis en évidence l'influence de la nature du matériau dans lequel est usinée la matrice (graphite ou WC-Co) sur la microstructure, la microdureté et la contrainte à la rupture, pour un même cycle de frittage. / In order to produce high magnetic fields (100 T), the conducting wires used in pulsed coils must show both a high tensile strength and very low electrical resistivity. The LNCMI and NNC team of CIRIMAT explore creative solutions based on the development of nanostructured copper wires and carbon nanotube - copper (CNT-Cu) nanocomposite wires by the original combination of spark plasma sintering (SPS) and room-temperature wire-drawing (WD). Copper cylinders were prepared by SPS of micrometric commercial powders. Crystal growth is very low and the copper grains size is 10 times lower than for conventional wire precursors. The cylinders were wire-drawn, without breaking, into wires of decreasing diameter (down to 0.198 mm) and several meters long. The ultrafine Cu grains are highly elongated in the WD direction. No twinning was observed. Our copper wires show an ultimate tensile strength (UTS) at 293K and 77K higher than those for wires prepared from conventional OFHC copper, which could result from the combination of strain hardening and Orowan mechanisms. The electrical resistivity is about 12% higher than those for the OFHC wires. For the CNT-Cu nanocomposites, an adaptation of preparation route (functionalization of double-walled and eight-walled CNTs, mixing, freeze-drying, H2 reduction) resulted in the production of 14 g powder batches with a homogeneous dispersion of the CNTs. Due to the very low carbon content (= 1%), the preparation of the cylinders and wires by the methods used for pure copper is possible without modification. The UTS of the CNT-Cu wirers is 10-25% higher than for the corresponding copper wires. The CNTs have little influence on the Cu microstructure and their probable alignment allows one to benefit from their high tensile strength. The electrical resistivity is only moderately higher than for the corresponding copper wires (about 12% at 77K). The last chapter was devoted to the preparation of "dog-bone" Cu and CNT-Cu test samples by "near-net-shape" SPS. We have brought to the fore the influence of the nature of the die (graphite or WC-Co) on the microstructure, microhardness and tensile strength, for the same sintering cycle.

The Thesis reports charge transport studies on conducting polymers, polymer carbon nanotube composites and organic semiconductor devices. Conducting and semiconducting polymers consisting of π-conjugated chains have attracted considerable attention as they combine the optoelectronic properties of semiconductors with mechanical properties and processing advantages of plastics. The chemical/electrochemical/photodoping of these semiconducting polymers can tune the Fermi levels and conductivity in a controlled way, and hence the properties of devices can be easily tailored to suit in several applications. Carbon nanotube (CNT) is another another novel promising material for electronic/optoelectronic applications. Lately there has been a great interest in developing composites of polymer and CNTs to utilize the advantages of both CNTs and polymers. The inclusion of CNTs in polymers improves the mechanical, electrical and thermal properties since the aspect ratio (ratio of length to diameter) is very large, as well its density is rather low. The Thesis consists of 6 chapters. First chapter is a brief introduction of general and transport properties of conducting polymers and polymer-carbon nanotube composites. In Chapter 2, the sample preparation and experimental techniques used in this work are discussed. The charge transport in poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) is presented in Chapter 3. Chapter 4 focuses on the transport measurements in the polymer-CNT composite samples. Chapter 5 elaborates the ac and dc characterization of organic field-effect transistors (OFETs). And chapter 6 presents the conclusion and future directions of the work that has been presented in the Thesis. Chapter 1: In the scientific and technological revolution of the last few years, the study of high performance materials has been steadily increasing including the study of carbon-based materials. Conducting polymers have special properties that are interesting for this new technology. The charge transport in conjugated polymers is important to optimize the performance of devices. The discovery of CNTs with exceptional thermal, mechanical, optical, electrical and structural properties has facilitated the synthesis of new type of nanocomposites with very interesting properties. Nanocomposites represent a guest-host matrix consisting of easily processible functionalized conjugated polymer as host, incorporating CNTs as fillers with versatile electronic and magnetic properties, which provide a wide range of technological applications. To optimize their electrical properties it is essential to understand the charge transport mechanism in detail. Chapter 2: The multi-wall carbon nanotubes (MWNTs) grown by thermal chemical vapor deposition (CVD) are mixed with a 1:1 mixture of 98% H2SO4 and 70% HNO3 to produce sulfonic acid functionalized multi-wall carbon nanotubes (s-MWNTs). The s-MWNTs are dispersed in a solution of Nafion by ultrasonication and then cast on a glass substrate and slowly dried by moderate heating to obtain the composite films. Polyaniline (PANI)-MWNT composites were obtained by carrying out the chemical synthesis of nanofibrilar PANI in the presence of CNTs. This water dispersible PANIMWNT composite contains well segregated MWNTs partially coated by nanofibrilar PANI. The ac and dc charge transport measurements suggest hopping transport in these materials. OFETs are fabricated with pentacene, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)(PBTTT) and poly(3-hexylthiophene) (P3HT) as active materials. A novel technique is used to characterize the acphotoresponse of these OFETs. Chapter 3: Charge transport studies on PEDOT-PSS have been carried out and found that it correlates with the morphology. The dc conductivity of PEDOT–PSS shows enhanced delocalization of the carriers upon the addition of dimethyl sulfoxide (DMSO) and this is attributed to the extended chain conformation. PEDOT-PSS is known to form a phase-segregated material comprising highly conducting PEDOT grains that are surrounded by a sea of weakly ionic-conducting PSS and a wide variation in the charge transport properties of PEDOT-PSS films is attributed to the degree of phasesegregation of the excess insulating polyanion. The magnetotransport and temperature dependent ac transport parameters across different conducting grades of PEDOT-PSS processed with DMSO were compared. Depending on the subtle alterations in morphology, the transport at low temperatures is shown to vary from the hopping regime (Baytron P) to critical regime of the metal-insulator transition (Baytron PH510) There is a significant positive magnetoresistance (MR) for P–films, but this is considerably less in case of PH510-film. From the low temperature ac conductance it is found that the onset frequency for PH510 is nearly temperature independent, whereas in P type it is strongly temperature dependent, again showing the superior transport in PH510. The presence of ‘shorter network connections’ together with a very weak temperature dependence down to ~ 5 K, suggest that the limitation on transport in PH510 arises from the connectivity within the PEDOT-rich grain rather than transport via the PSS barriers. Chapter 4: DC and AC charge transport properties of Nafion s-MWNT and PANI-MWNT composites are studied. Such a detailed investigation is required to optimize the correlation among morphology and transport properties in these composites towards applications in field-effect transistors, antistatic coating, electromagnetic shielding, etc. The conductivity in Nafion s-MWNT shows a percolative transport with percolation threshold pc = 0.42 whereas such a sharp percolation is absent in PANI-MWNT composite since the conduction via PANI matrix smears out the onset of rapid increase in conductivity. Three-dimensional variable range hopping (VRH) transport is observed in Nafion s-MWNT composites. The positive and negative MR data on 10 wt. % sample are analyzed by taking into account forward interference mechanism (negative MR) and wave-function shrinkage (positive MR), and the carrier scattering is observed to be in the weak limit. The electric-field dependence, measured to high fields, follows the predictions of hopping transport in high electric-field regime. The ac conductivity in 1 wt. % sample follows a power law: ( )  A s , and s decreases with increasing temperature as expected in the correlated barrier hopping (CBH) model. In general, Mott’s VRH transport is observed in PANI-MWNT samples. It is found that the MWNTs are sparingly adhered with PANI coatings, and this facilitates inter-tube hopping at low temperatures. The negative MR of MWNT-PANI composites suggest that the electronic transport at low temperatures is dominated by MWNT network. AC impedance measurements at low temperatures with different MWNT loading show that ac conductivity become temperature independent as the MWNT content increases. The onset frequency for the increase in conductivity is observed to be strongly dependent on the MWNT weight percentage, and the ac conductivity can be scaled onto a master curve given by  ( )  0[1 k( 0 )s ]. Chapter 5: Organic field-effect transistors (OFETs) based on small molecules and polymers have attracted considerable attention due to their unique advantages, such as low cost of fabrication, ease of processing and mechanical flexibility. Impedance characterization of these devices can identify the circuit elements present in addition to the source-drain (SD) channel, and the bottlenecks in charge transport can be identified. The charge carrier trapping at various interfaces and in the semiconductor can be estimated from the dc and ac impedance measurements under illumination. The equivalent circuit parameters for a pentacene OFET are determined from low frequency impedance measurements in the dark as well as under light illumination. The charge accumulation at organic semiconductor–metal interface and dielectric semiconductor interface is monitored from the response to light as an additional parameter to find out the contributions arising from photovoltaic and photoconductive effects. The shift in threshold voltage is due to the accumulation of photogenerated carriers under SD electrodes and at dielectric–semiconductor interface, and also this dominates the carrier transport. Similar charge trapping is observed in an OFET with PBTTT as the active material. This novel method can be used to differentiate the photophysical phenomena occurring in the bulk from that at the metal-semiconductor interface for the polymer. Chapter 6: The conclusions from the various works presented in the thesis are coherently summarized in this chapter. Thoughts for future directions are also summed up.