A multiscale model is proposed to study the influence of interfacial interactions at the nanoscale in carbon nanotube(CNT)-polymer nanocomposites on the macroscale bulk elastic material properties. The efficiency of CNT reinforcement in terms of interfacial load transferring is assessed for the non-functionalized and functionalized interfaces between the CNTs and polymer matrix using force field based molecular dynamic simulations at the nanoscale. Polyethylene (PE) as a thermoplastic material is adopted and studied first because of its simplicity. Characterization of the nanoscale load transfer has been done through the identification of representative nanoscale interface elements for unfunctionalized CNT-PE interface models which are studied parametrically in terms of the length of the PE chains, the number of the PE chains and the "grip" position. Referring to the non-functionalized interface, CNTs interact with surrounding polymer only through weakly nonbonded van der Waals (vdW) forces in our study. Once appropriate values of these parameters are deemed to yield sufficiently converged results, the representative interface elements are subjected to normal and sliding mode simulations in order to obtain the force-separation responses at 100K and 300K for unfunctionalized CNT-PE interfaces. To study the functionalization effects, atomistic interface representative elements for functionalized CNT-PE interface are built based on non-functionalized interface models by grafting functional groups between the PE matrix and the graphene sheet. This introduces covalent bonding forces in addition to the non-bonded vdW forces. A modified consistent covalent force field (CVFF) and adaptive intermolecular reactive empirical bond order (AIREBO) potentials, both of which account for bond breaking, are applied to investigate the interfacial characteristic of functionalized CNT-PE interface in terms of the force-separation responses at 100K in both normal opening and sliding mode separations. In these studies, the focus has been on the influence of the functionalization density on the load transfer at the nanoscale interface. As an important engineering material, Epon 862/DETDA epoxy polymer,a thermoset plastic, has also been used as the polymer matrix material in order to see the difference in interfacial load transfer between a network structured polymer and the amorphous entangled structure of the PE matrix. As for thermoset epoxy polymer, emphasis has been put on investigating the effects of the crosslink density of the epoxy network on the interfacial load transfer ability for both non-functionalized and functionalized CNT-Epoxy interface at different temperatures(100K and 300K) and on the functionalization effect influenceing the interfacial interactions at the functionalized CNT-Epoxy interface. Cohesive zone traction-displacement laws are developed based on the force-separation responses obtained from the MD simulations for both non-functionalied and functionalized CNT-PE/epoxy interfaces. Using the cohesive zone laws, the influence of the interface on the effective elastic material properties of the nanocomposites are observed and determined in continuum level models using analytic and computational micromechanics approaches, allowing for the assessment of the improvement in reinforcement efficiency of CNTs due to the functionalization. It is found that the inclusion of the nanoscale interface in place of the perfectly bonded interface results in effective elastic properties which are dependent on the applied strain and temperature in accordance with the interface sensitivity to those effects, and which are significantly diminished from those obtained under the perfect interface assumption for non-functionalized nanocomposites. Better reinforcement efficiency of CNTs are also observed for the nanocomposites with the functionalized interface between CNTs and polymer matrix, which results in large increasing for the effective elastic material properties relative to the non-functionalized nanocomposites with pristine CNTs. Such observations indicates that trough controlling the degree of functionalization, i.e. the number and distribution of covalent bonds between the embedded CNTs and the enveloping polymer, one can tailor to some degree the interfacial load transfer and hence, the effective mechanical properties. The multiscale model developed in this study bridges the atomistic modeling and micromechanics approaches with cohesive zone models, which demonstrates to deepen the understanding of the nanoscale load transfer mechanism at the interface and its effects on the effective mechanical properties of the nanocomposites. It is anticipated that the results can offer insights about how to engineer the interface and improve the design of nanocomposites. / Ph. D.

Pillared-graphene structure (PGS) is a novel three-dimensional structure consists of parallel graphene sheets that are separated by carbon nanotube (CNT) pillars that is proposed for efficient thermal management of electronics. For microscale simulations, finite element analyses were carried out by imposing a heat flux on several PGS configurations using a Gaussian pulse. The temperature gradient and distribution in the structures was evaluated to determine the optimum design for heat transfer. The microscale simulations also included conducting a mesh-independent study to determine the optimal mesh element size and shape. For nanoscale simulations, Scienomics MAPS software (Materials And Processes Simulator) along with LAMMPS (Large-scale Atomic/ Molecular Massively Parallel Simulator) were used to calculate the thermal conductivity of different configurations and sizes of PGS. The first part of this research included investigating PGS when purely made of carbon atoms using non-equilibrium molecular dynamics (NEMD). The second part included investigating the structure when supported by a copper foil (or substrate); mimicking production of PGS on copper. The micro- and nano-scale simulations show that PGS has a great potential to manage heat in micro and nanoelectronics. The fact that PGS is highly tunable makes it a great candidate for thermal management applications. The simulations were successfully conducted and the thermal behavior of PGS at the nanoscale was characterized while accounting for phonon scattering the graphene/CNT junction as well as when PGS is supported by a copper substrate.

Im Gegensatz zu herkömmlichen Dehnungsmessstreifen können Carbon nanotube (CNT)-basierte Komposite zusätzlich eine ausgeprägte Druck-abhängigkeit des Widerstandes aufweisen. Deshalb können Drucksensoren aus CNT-Nanokomposite ohne den Einsatz von Verformungskörpern wie z. B. Biegebalken aufgebaut werden. Die möglichen Anwendungsgebiete für diese direkt messenden Sensoren wurden in der vorliegenden Arbeit bei drei industriellen Anwendungen wie z. B. bei Robotergreifarmen gezeigt. Die Zielstellung dieser Arbeit ist die Entwicklung und Charakterisierung eines neuartigen Sensors aus CNT-Nanokomposite. Unter Verwendung von Multi-walled carbon nanotube (MWCNT)-Epoxidharz und interdigitalen Elektroden soll der Sensor auf wenigen Quadratzentimetern Drücke im Megapascal-Bereich und somit Kräfte im Kilonewton-Bereich messen können. Durch die Auswahl geeigneter Werkstoffe und die Modellierung mit der Finite Element Methode wurde der Sensorentwurf durchgeführt sowie der Messbereich abgeschätzt. Die Herstellung der MWCNT-Epoxidharz-Dispersion erfolgte durch mechanische Mischverfahren. Anschließend wurden aus der Dispersion druckempfindliche Schichten mit der Schablonendrucktechnik hergestellt. Dabei wurden die Herstellungs-parameter und besonders der Füllstoffgehalt der MWCNTs variiert, um deren Einflüsse auf das mechanische, thermische und elektrische Verhalten zu untersuchen. Die Charakterisierung der mechanischen Kenngrößen erfolgte mit Zugversuchen und dynamisch-mechanischer Analyse. In den Untersuchungen zeigen die MWCNT-Komposite eine signifikante Steigerung der Zugfestigkeit und eine Erhöhung der Glasübergangstemperatur gegenüber reinem Epoxidharz. Die Abhängigkeiten der Druckempfindlichkeit und der Temperaturempfindlichkeit vom Füllstoffgehalt wurden untersucht. Eine besonders hohe Druckempfindlichkeit, aber auch Temperaturempfind-lichkeit wurde bei Proben mit geringem Füllstoffgehalt (1 wt% und 1,25 wt%) festgestellt. Es ist also wichtig, die richtige Materialkombination für diese Art Sensor zu finden. Die realisierten Sensoren liefern zuverlässige Antwortsignale bei wiederholten Belastungen bis zu einer Belastung von 20 MPa (entspricht 2 kN). Zusätzlich wurde der Temperatureinfluss in einem Bereich von −20 °C bis 50 °C durch eine Wheatstonesche Brückenschaltung kompensiert. Die vorliegende Arbeit zeigt, dass eine zuverlässige Druckmessung mit einer Temperaturmessabweichung von 0,214 MPa/10 K gewährleistet werden kann. / In contrast to conventional metallic strain gauges, carbon nanotube (CNT) composites have an additional pressure sensitivity. Therefore, deformation elements such as bending beam is not needed by using pressure sensors, which are based on CNT nanocomposite. The possible areas of application for these pressure direct measured sensors were showed in three industrial application such as robot gripper. The focus of this work is the development and characterization of a new sensor manufactured from CNT nanocomposite. By using multi-walled carbon nanotube (MWCNT) epoxy and interdigital electrodes the sensor, which has a dimension of few square centimetre, should measure a pressure in mega Pascal range and hence a force in kilo newton range. By the selection of suitable materials and the modelling using finite element method, the sensor design as well as the measurement range were carried out. The MWCNT epoxy dispersion is manufactured by using a mechanical mixing process. Subsequent, the dispersion is used to fabricate pressure sensitive layers by stencil printing methods. Thereby, the fabrication parameters and especially the filler content of the MWCNTs were varied for the mechanical, thermal and electrical investigation. The characterization of the mechanical characteristic values were carried out by using tensile test and dynamic mechanical analysis. The results show a significant increasing of the tensile strength and glass transition temperature in comparison to neat epoxy. Additionally, the influence of the filler content to the pressure and thermal sensitivity were investigated. A highly pressure sensitivity but also a highly thermal sensitivity are obtained for samples with lower filler contents (1 wt% and 1.25 wt%). Therefore, a suitable material combination has to be chosen. The fabricated sensors show reliable response signals by repeated excitations up to 20 MPa (meets to 2 KN). Moreover, the temperature influence ranged from -20 °C to 50 °C was compensated with a Wheatstone bridge. This work demonstrate a direct pressure sensitive sensor with reliable response signals by a thermal deviation of 0.214 MPa/10K.

Carbon nanotubes (CNTs) have been recognized as potential candidates for mainstream device fabrication and technologies. CNTs have become a topic of interest worldwide due to their unique mechanical and electrical properties. In addition, CNTs possess high aspect ratio and low density that make them an important material for various technological applications. The field of carbon nanotube devices is rapidly evolving and attempts have been made to use CNTs in the fabrication of devices like field emitters, gas sensors, flow meters, batteries, CNT-field effect transistors etc. These molecular nanostructures are proposed to be an efficient hydrogen storage material. CNT cylindrical membranes are reported to be used as filters for the elimination of multiple components of heavy hydrocarbons from petroleum and for the filtration of bacterial contaminants of size less than 25 nm from water. Recently, CNT bundles have been proposed to be a good material for low-temperature sensing. CNTs have also been considered as promising filler materials due to extraordinary characteristics mentioned above. Fabrication of nanocomposites using CNTs as reinforcing material has completely renewed the research interest in polymer composites. The conductive and absorptive properties of insulating polymer doped with conducting filler are sensitive to the exposure to gas vapors and hence they can be used in monitoring various gases. The application of fiibre reinforced polymer composites in aeronautic industry are well known due to their high mechanical strength and light weight. Also, the conductive composite materials can be used for electromagnetic shielding. Desired properties in CNT-composites can be attained by adding small amount of CNTs in comparison to traditional filler materials. Due to high aspect ratio and low density of CNTs, percolation threshold in CNT-polymer composites can be achieved at 0.1 vol % as compared to ~16 vol. % in case of carbon particles. The research work ׽0.1 vol. %, as compared to reported in this thesis includes the preparation of multiwall carbon nanotube (MWNTs) and MWNT-polystyrene composites, experimental investigations on low temperature charge transport, and magnetic properties in these systems. This thesis contains 7 chapters. Chapter 1 provides an overview of CNTs and CNT-polymer composites. This chapter briefly describes the methods for synthesizing CNTs and fabricating CNT-polymer composites, charge transport mechanisms in CNTs and composites, and their magnetic properties as well. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in the present work. An adequate knowledge of the strengths and limitations of experimental equipment can help in gathering necessary information about the sample, which helps in studying and interpreting its physical properties correctly. Chapter 3 describes the synthesis of MWNTs and their use as filler material for the fabrication of composites with polystyrene (PS). The characterization results of as-prepared MWNT and composites show that MWNTs possess high aspect ratio (~4000), and are well dispersed in the composite samples (thickness ~50-70 µm). The composite samples are prepared by varying the MWNT concentration from 0.1 to 15 wt %. The as¬fabricated composites are electrically conductive and expected to display novel magnetic properties since MWNTs are embedded with iron (Fe) nanoparticles. Chapter 4 presents the study of charge transport properties of aligned and random MWNTs in the temperature range 300-1.4 K. The low temperature electrical conductivity follows the weak localization (WL) and electron-electron (e-e) interaction model in both samples. The dominance of WL and e-e interaction is further verified by magneto-conductance (MC) measurements in the perpendicular magnetic field up to 11 T at low temperatures. The MC data of these samples consists of both positive and negative contributions, which originates from WL (at lower fields and higher temperatures) and e-e interaction (at higher fields and lower temperatures). Chapter 5 contains the results of charge transport studies in MWNT-PS composite near the percolation threshold (~0.4 wt %) at low temperatures down to 1.4 K. Metallic-like transport behavior is observed in composite sample of 0.4 wt %, which is quite unusual. In general, the usual activated transport is observed for systems near the percolation threshold. The unusual weak temperature dependence of conductivity in MWNT-PS sample at percolation threshold is further verified from the negligible frequency dependence of conductivity, in the temperature range from 300 to 5 K. Chapter 6 accounts on the experimental results of magnetization studies of MWNTs and MWNT-PS composites. The observation of maxima in coercivity and squareness ratio at 1 wt % of Fe-MWNT in a polymer matrix show the dominance of dipolar interactions among the encapsulated Fe-nanorods within MWNTs. The hysteresis loop of 0.1 wt % sample shows anomalous narrowing at low temperatures, which is due to significant contribution from shape anisotropy of Fe-nanorods. Chapter 7 presents brief summary and future perspectives of the research work reported in the thesis.

Im Gegensatz zu herkömmlichen Dehnungsmessstreifen können Carbon nanotube (CNT)-basierte Komposite zusätzlich eine ausgeprägte Druck-abhängigkeit des Widerstandes aufweisen. Deshalb können Drucksensoren aus CNT-Nanokomposite ohne den Einsatz von Verformungskörpern wie z. B. Biegebalken aufgebaut werden. Die möglichen Anwendungsgebiete für diese direkt messenden Sensoren wurden in der vorliegenden Arbeit bei drei industriellen Anwendungen wie z. B. bei Robotergreifarmen gezeigt. Die Zielstellung dieser Arbeit ist die Entwicklung und Charakterisierung eines neuartigen Sensors aus CNT-Nanokomposite. Unter Verwendung von Multi-walled carbon nanotube (MWCNT)-Epoxidharz und interdigitalen Elektroden soll der Sensor auf wenigen Quadratzentimetern Drücke im Megapascal-Bereich und somit Kräfte im Kilonewton-Bereich messen können. Durch die Auswahl geeigneter Werkstoffe und die Modellierung mit der Finite Element Methode wurde der Sensorentwurf durchgeführt sowie der Messbereich abgeschätzt. Die Herstellung der MWCNT-Epoxidharz-Dispersion erfolgte durch mechanische Mischverfahren. Anschließend wurden aus der Dispersion druckempfindliche Schichten mit der Schablonendrucktechnik hergestellt. Dabei wurden die Herstellungs-parameter und besonders der Füllstoffgehalt der MWCNTs variiert, um deren Einflüsse auf das mechanische, thermische und elektrische Verhalten zu untersuchen. Die Charakterisierung der mechanischen Kenngrößen erfolgte mit Zugversuchen und dynamisch-mechanischer Analyse. In den Untersuchungen zeigen die MWCNT-Komposite eine signifikante Steigerung der Zugfestigkeit und eine Erhöhung der Glasübergangstemperatur gegenüber reinem Epoxidharz. Die Abhängigkeiten der Druckempfindlichkeit und der Temperaturempfindlichkeit vom Füllstoffgehalt wurden untersucht. Eine besonders hohe Druckempfindlichkeit, aber auch Temperaturempfind-lichkeit wurde bei Proben mit geringem Füllstoffgehalt (1 wt% und 1,25 wt%) festgestellt. Es ist also wichtig, die richtige Materialkombination für diese Art Sensor zu finden. Die realisierten Sensoren liefern zuverlässige Antwortsignale bei wiederholten Belastungen bis zu einer Belastung von 20 MPa (entspricht 2 kN). Zusätzlich wurde der Temperatureinfluss in einem Bereich von −20 °C bis 50 °C durch eine Wheatstonesche Brückenschaltung kompensiert. Die vorliegende Arbeit zeigt, dass eine zuverlässige Druckmessung mit einer Temperaturmessabweichung von 0,214 MPa/10 K gewährleistet werden kann. / In contrast to conventional metallic strain gauges, carbon nanotube (CNT) composites have an additional pressure sensitivity. Therefore, deformation elements such as bending beam is not needed by using pressure sensors, which are based on CNT nanocomposite. The possible areas of application for these pressure direct measured sensors were showed in three industrial application such as robot gripper. The focus of this work is the development and characterization of a new sensor manufactured from CNT nanocomposite. By using multi-walled carbon nanotube (MWCNT) epoxy and interdigital electrodes the sensor, which has a dimension of few square centimetre, should measure a pressure in mega Pascal range and hence a force in kilo newton range. By the selection of suitable materials and the modelling using finite element method, the sensor design as well as the measurement range were carried out. The MWCNT epoxy dispersion is manufactured by using a mechanical mixing process. Subsequent, the dispersion is used to fabricate pressure sensitive layers by stencil printing methods. Thereby, the fabrication parameters and especially the filler content of the MWCNTs were varied for the mechanical, thermal and electrical investigation. The characterization of the mechanical characteristic values were carried out by using tensile test and dynamic mechanical analysis. The results show a significant increasing of the tensile strength and glass transition temperature in comparison to neat epoxy. Additionally, the influence of the filler content to the pressure and thermal sensitivity were investigated. A highly pressure sensitivity but also a highly thermal sensitivity are obtained for samples with lower filler contents (1 wt% and 1.25 wt%). Therefore, a suitable material combination has to be chosen. The fabricated sensors show reliable response signals by repeated excitations up to 20 MPa (meets to 2 KN). Moreover, the temperature influence ranged from -20 °C to 50 °C was compensated with a Wheatstone bridge. This work demonstrate a direct pressure sensitive sensor with reliable response signals by a thermal deviation of 0.214 MPa/10K.

Various nano-fillers can introduce specific functions into polymer and expand their application areas. Myriad properties, such as mechanical, electrical, thermal, or magnetic properties can be combined with original polymer characteristics, including flexible, light weight, and ease of use. These composites can be used to produce multi-functional fibers as the next generation textile or fabrics. In this research, Polyacrylonitrile (PAN) is adopted as the main polymer with different nano-fillers, such as carbon nanotube (CNT), iron oxide nanoparticle, and graphene oxide nanoribbon (GONR). Using gel-spinning technology, PAN-based composite fibers are fabricated in single- or bi-component fibers. Fibers are also characterized for their structure, morphology, mechanical properties, as well as for their electrical, thermal, or magnetic properties. For example, bi-component fibers with polymer sheath and polymer-CNT core as well as polymer-CNT sheath and polymer core are processed. With electrical and thermal conductivity introduced by CNT, such bi-components fibers can be applied for wearable electronics or for thermal management. Joule-heating effect owing to applied electrical current on single component PAN/CNT fibers is also investigated. With controllable electrical conductivity and fiber temperature, this active functional fiber can be applied for temperature regulation fibers or new carbon fiber manufacturing process. Another example is magnetic fiber with superparamagnetic iron oxide nano-particles. These novel magnetic fibers with high strength can be used for actuator, inductors, EMI shielding, or microwave absorption. GONR is also discussed and used to reinforce PAN-based fibers. Several theoretical models are considered to analyze the observed results.

Ce travail présente, tout d'abord, l'étude des propriétés électriques (conductivité, permittivité) d'une matrice thermoplastique contenant du noir de carbone. Nous mettons en évidence les facteurs influençant ces propriétés en continu, basses fréquences et en micro-ondes à l'aide des techniques de spectroscopie diélectrique dynamique. L'influence de déformations mécaniques uniaxiales sur la permittivité en micro-ondes a été étudiée. Ensuite, les effets de la nature de la matrice et de l'arrangement spatial des nanotubes de carbone multiparois NTC sur les propriétés électriques des composites époxyde/NTC et latex /NTC ont été abordés. Les théories de la percolation et du milieu effectif sont utilisées pour décrire les mécanismes de transport électronique dans ces composites.

Downscaling of the contact length Lc of a side-contacted carbon nanotube field-effect transistor (CNTFET) is challenging because of the rapidly increasing contact resistance as Lc falls below 20–50 nm. If in agreement with existing experimental results, theoretical work might answer the question, which metals yield the lowest CNT–metal contact resistance and what physical mechanisms govern the geometry dependence of the contact resistance. However, at the scale of 10 nm, parameter-free models of electron transport become computationally prohibitively expensive. In our work we used a dedicated combination of the Green function formalism and density functional theory to perform an overall ab initio simulation of extended CNT–metal contacts of an arbitrary length (including infinite), a previously not achievable level of simulations. We provide a systematic and comprehensive discussion of metal–CNT contact properties as a function of the metal type and the contact length. We have found and been able to explain very uncommon relations between chemical, physical and electrical properties observed in CNT–metal contacts. The calculated electrical characteristics are in reasonable quantitative agreement and exhibit similar trends as the latest experimental data in terms of: (i) contact resistance for Lc = ∞, (ii) scaling of contact resistance Rc(Lc); (iii) metal-defined polarity of a CNTFET. Our results can guide technology development and contact material selection for downscaling the length of side-contacts below 10 nm.

The thesis deals with a study of the various carbon materials for negative electrode of lithium ion accumulators. The object of the work is to gain the most knowledge about these materials and to project their possible improvements. In the first part of the work is described the latest knowledge about carbon materials and their application. In the following parts of the work is handled about manufacture,optimization of composition of electrode and the choice of carbon material for liquid and gel electrolyte.

Dye-sensitized solar cells (DSSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Electron transport is one of the prominent processes in the cell and it is further a complex process because the transport medium is a mesoporous film. The gaps in the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, the electron transport in these so called state-of-the-art systems has a practical limit because of the low electron diffusion coefficient (Dn) in this mesoporous film photoanode. This work focuses on the influence of the advanced core-shell nanoarchitecture geometry on electron transport and also on the influence of electron-ion interactions. In order to achieve the proposed goals, DSSCs based on ordered, highly aligned, 3D radial core-shell Au-TiO2 hybrid nanowire arrays were fabricated, using three different approaches. J-V, IPCE, and EIS characteristics were studied. The efficiency, light scattering and charge transport properties of the core-shell nanowire based devices were compared to TiO2 nanotube as well as TiO2 mesoporous film based DSSCs. The Au nanowires inside the crystalline TiO2 anatase nanoshell provided a direct conduction path from the TiO2 shell to the TCO substrate and improved transport of electrons between the TiO2 and the TCO. The optical effects were studied by IPCE measurement which demonstrated that Au-TiO2 nanowires showed an improved light harvesting efficiency, including at longer wavelengths where the sensitizer has weak absorption. The metal nanostructures could enhance the absorption in DSSCs by either scattering light enabling a longer optical path-length, localized surface plasmon resonance (LSPR) or by near-field coupling between the surface plasmon polariton (SPP) and the dye excited state. Rapid, radial electron collection is of practical significance because it should allow alternate redox shuttles that show relatively fast electron-interception dynamics to be utilized without significant sacrifice of photocurrent. A combination of improved electron transport and enhanced light harvesting capability make Au-TiO2 core-shell nanowire arrays a promising photoanode nanoarchitecture for improving photovoltaic efficiency while minimizing costs by allowing thinner devices that use less material in their construction.