Tesi sul tema "Dynamic dielectric properties"

The dynamic relaxation characteristics of Matrimid® (BTDA-DAPI) polyimide and several functionalized aromatic polyimides have been investigated using dynamic mechanical and dielectric methods. The functionalized polyimides were thermally rearranged to generate polybenzoxazole membranes with controlled free volume characteristics. All polyimides have application in membrane separations and exhibit three motional processes with increasing temperature: two sub-glass relaxations (ƴ and β transitions), and the glass-rubber (α) transition. For Matrimid, the low-temperature ƴ transition is purely non-cooperative, while the β sub-glass transition shows a more cooperative character as assessed via the Starkweather method. For the thermally rearranged polyimides, the ƴ transition is a function of the polymer synthesis method, thermal history, and ambient moisture. The β relaxation shows a dual character with increasing thermal rearrangement, the emerging lower-temperature component reflecting motions encompassing a more compact backbone contour. For the glass-rubber (α) transition, dynamic mechanical studies reveal a strong shift in Tα to higher temperatures and a progressive reduction in relaxation intensity with increasing degree of thermal rearrangement. The dynamic relaxation characteristics of poly(ether imide) and poly(methyl methacrylate) nanocomposites were investigated by dynamic mechanical analysis and dielectric spectroscopy. The nanoparticles used were native and surface-modified fumed silicas. The nanocomposites display a dual glass transition behavior encompassing a bulk polymer glass transition, and a second, higher-temperature transition reflecting relaxation of polymer chain segments constrained owing to their proximity to the particle surface. The position and intensity of the higher-temperature transition varies with particle loading and surface chemistry, and reflects the relative populations of segments constrained or immobilized at the particle-polymer interface. Dielectric measurements, which were used to probe the time-temperature response across the local sub-glass relaxations, indicate no variation in relaxation characteristics with particle loading. Nanocomposite studies were also conducted on rubbery poly(ethylene oxide) networks crosslinked in the presence of MgO or SiO2 nanoparticles. The inclusion of nanoparticles led to a systematic increase in rubbery modulus and a modest positive offset in the measured glass transition temperature (Tα) for both systems. The sizeable increases in gas transport with particle loading reported for certain other rubbery nanocomposite systems were not realized in these crosslinked networks.

The principal objective of this paper is to describe how complex impedance, obtained from frequency dependent dielectric sensing technique, can be used as an effective diagnostic tool for in situ real-time investigation in the fabrication of thermoset structures.;Two epoxy systems (Pr500 from 3M and My720 from Ciba Geigy) with different functionality are characterized in terms of their dielectric, thermal, and rheological behavior. It is observed that there is a one to one relationship between Tg and conversion which is independent of cure temperature. The chemical kinetics of the reaction is satisfactorily described by an autocatalyzed reaction mechanism. The chemical rate constant has the usual Arrhenius form, whereas the diffusion rate constant is assumed to be given by a modified form of the Williams-Landel-Ferry (WLF) equation. The overall reaction rate constant is modeled by a combination of the chemical rate constant and the diffusion rate constant. The ability of the frequency dependent dielectric sensing technique to monitor the progress of curing reaction, build up in glass transition temperature, viscosity and to detect the time of occurrence of gel during thermoset cure are explored. The temperature dependence of the dielectric relaxation time, ionic conductivity and viscosity are described by the modified WLF equation. This approach provides a common framework for describing and comparing different related properties.

In recent years the continuous and rapid development of the electronic industry together with the need for more efficient electric energy harvesting have notably increased the demand for: (i) high dielectric constant and breakdown strength materials for high energy density capacitors and (ii) piezoelectric flexible materials, with the ability to bend into diverse shapes, for powering low-power portable devices and self-powered electronic systems. Polymer-based composites and nanocomposites with inclusions of a ceramic active phase are very attractive for these applications because they combine materials with different characteristics, allowing the possibility to tune and optimize the dielectric and piezoelectric properties in the ensuing composite systems. In particular, many parameters can affect the material performance: (i) the nature of the polymer matrix and active component; (ii) the phases connectivity; (iii) the filler concentration, shape and dimensions; (iv) the filler/matrix interactions; (v) the preparation technique and processing. All this variability expands the possible applications of polymer-composites for energy-related purposes but also increases the difficulty in realistically predicting their ultimate properties. The design of polymer composites thus requires a rational selection of components, good interface engineering and proper processing optimization. To achieve this, a thorough comprehension of the process-structure-properties correlations is very important. This is the principal aim of this thesis work, which focus on the preparation of poly(vinylidene fluoride) homopolymer (PVDF) or poly(vinylidene fluoride-co-hexafluoropropylene) copolymer (PVDF-HFP) based composites with 0-3 connectivity containing different perovskite fillers, namely, BaTiO3 (BT), Pb(Zr,Ti)O3 (PZT) and Na0.5Bi0.5TiO3 - BaTiO3 (BNBT). The filler particles were used as prepared or properly surface modified and several techniques were employed for the composites preparation (i.e., solvent casting, melt blending, hot-pressing, compression moulding). Initially, a study of the neat polymer matrices was performed, by using, for the first time in literature, the compression moulding technique to tune the polymorphism of PVDF. A principal component analysis was performed on the infrared spectra of the moulded films to validate the equation usually employed for determining the electroactive phase amount (FEA) then multiple linear regression was applied to better understand how the processing parameters affect the FEA value. A double-step procedure was proven fundamental in inducing the formation of PVDF β phase and improving the dielectric properties of the ensuing polymer films. After this preparatory investigation, the study of the process-structure-properties correlations was extended to PVDF-based composites, addressing three main issues: (i) the influence of processing on the ultimate properties of the prepared samples; (ii) the influence of particles dimensions and surface modification on the dielectric behaviour of the composite materials; (iii) the response of flexible piezoelectric composites. The preparation technique affects the microstructure at different levels, but it was found that not always a flawless particles dispersion necessary leads to the best final performance of the composite. Whereas, a proper moulding method, by affecting the polymorphism of the polymer matrix and the compactness of the film, can improve significantly the dielectric response. The presence of an inorganic shell around BT particles allows a modulation of the effective permittivity of the composites; if intrinsic factors (i.e., the permittivity of the components) prevail on extrinsic ones (i.e., interfacial polarization), the composites response can be predicted by FEM calculations. However, in these conditions, the reduction in the dielectric constant compensates for the increase of the breakdown strength promoted by the shell and, as a whole, the stored energy decreases. It is worth noting that the composites containing core-shell particles are characterized by low tunability, a condition which is important for application as dielectric capacitors. The functionalization of the ceramic particles with the tested coupling agents, despite decreasing to a certain extent the dielectric permittivity of the ensuing composites (due to the intrinsic low permittivity of the silane moieties), increases the maximum electric field, thus leading to an energy recovering capability comparable or slightly higher than that of the composite containing pristine BT particles. The dielectric response of the composites is affected by the particles dimensions even though the films containing pristine BT and those containing TiO2-coated particles exhibit a different trend of dielectric permittivity with filler size; this suggests a not negligible contribution of the interfaces, which varies with the method of particles synthesis. As concerns the piezoelectric composites, the piezoelectric coefficient (d33), in general, increases if the filler dimensions increase significantly. The higher response of the samples containing sintered and crushed PZT or BNBT particles (with respect to simply calcined powders) probably derived from the higher particles connectivity inside the agglomerates, which in turn leads to higher local stresses inside the material. As far as we know, the piezoelectric properties of composites made of fluorinated polymer matrices and BNBT filler had not been studied yet. The obtained d33 are in line with those of many flexible lead-free composites made with particles different from BNBT, suggesting the potentiality of these composites in the field of energy harvesting. As principal achievements, I obtained: (i) an alternative and smart method to tune the polymorphism of PVDF homopolymer and its copolymers, by exploiting a simple and easily-scalable processing technique; (ii) solvent-free fabrication of polymer-based composites with dielectric properties improved by the moulding process; (iii) a better comprehension about the role of the interfaces, useful to tune the final performance of the dielectric composites; (iv) flexible lead-free polymer-based composites with a good piezoelectric response for potential application as safe energy harvesting devices.

There is great interest in the development of new polar dielectric ceramics and multiferroic materials with new and improved properties. A family of tetragonal tungsten bronze (TTB) relaxors of composition Ba₆M³⁺Nb₉O₃₀ (M³⁺ = Ga³⁺, Sc³⁺ and In³⁺, and also their solid solutions) were studied in an attempt to understand their dielectric properties to enable design of novel polar TTB materials. A combination of electrical measurements (dielectric and impedance spectroscopy) and powder diffraction (X-ray and neutron) studies as a function of temperature was employed for characterising the dynamic dipole response in these materials. The effect of B-site doping on fundamental dipolar relaxation parameters were investigated by independently fitting the dielectric permittivity to the Vogel-Fulcher (VF) model, and the dielectric loss to Universal Dielectric Response (UDR) and Arrhenius models. These studies showed an increase in the characteristic dipole freezing temperature (T[subscript(f)]) with increase B-cation radius. Crystallographic data indicated a corresponding maximum in tetragonal strain at T[subscript(f)], consistent with the slowing and eventual freezing of dipoles. In addition, the B1 crystallographic site was shown to be most active in terms of the dipolar response. A more in-depth analysis of the relaxor behaviour of these materials revealed that, with the stepwise increase in the ionic radius of the M³⁺ cation on the B-site within the Sc-In solid solution series, the Vogel-Fulcher curves (lnf vs. T[subscript(m)]) are displaced to higher temperatures, while the degree of relaxor behaviour (frequency dependence) increases. Unfortunately, additional features appear in the dielectric spectroscopy data, dramatically affecting the Vogel-Fulcher fitting parameters. A parametric study of the reproducibility of acquisition and analysis of dielectric data was therefore carried out. The applicability of the Vogel-Fulcher expression to fit dielectric permittivity data was investigated, from the simple unrestricted (“free”) fit to a wider range of imposed values for the VF relaxation parameters that fit with high accuracy the experimental data. The reproducibility of the dielectric data and the relaxation parameters obtained by VF fitting were shown to be highly sensitive to the thermal history of samples and also the conditions during dielectric data acquisition (i.e., heating/cooling rate). In contrast, UDR analysis of the dielectric loss data provided far more reproducible results, and to an extent was able to partially deconvolute the additional relaxation processes present in these materials. The exact nature of these additional relaxations is not yet fully understood. It was concluded application of the Vogel-Fulcher model should be undertaken with great care. The UDR model may represent a feasible alternative to the evaluation of fundamental relaxation parameters, and a step forward towards the understanding of the dielectric processes in tetragonal tungsten bronzes.

Les polysaccharides sont l'une des principales options à retenir pour progresser dans l'utilisation ou la conception de polymères renouvelables. Depuis les années cinquante, le développement industriel de ce type de polymères s'était considérablement réduit du fait de l'avènement des polymères synthétiques. Cependant, cet intérêt a cru considérablement ces dernières années en raison de la sensibilisation du public sur la limite des ressources fossiles. Ces biopolymères sont donc devenus un sujet d'importance, tant sur le plan industriel que sur celui de la recherche fondamentale. Toutefois, les systèmes à base de polysaccharides sont le plus souvent transformés via l'utilisation d'importantes quantités de solvants (y compris l'eau), ce qui globalement pénalise le procédé associé en l'affligeant d'une charge environnementale supplémentaire. Par la voie ‘fondue', le développement de polymères thermoplastiques à base de dérivés de la cellulose est un véritable défi, qui concerne autant le mode de transformation de ces systèmes que le niveau des propriétés du matériau final. Pour exemple, la température de dégradation de l'acétate de cellulose (CA) (dont le degré de substitution 2,5 est développé par le Groupe Solvay) est si proche de sa température de fusion que son procédé de mise en oeuvre ne peut être envisagé qu'avec l'ajout d'une quantité importante de plastifiants externes (entre 20 et 30 en poids selon le type d'additif). Le comportement d'un mélange CA-plastifiant est principalement régi par un «réseau» de très fortes interactions polaires, dont la force et la densité dépendent de 3 paramètres spécifiques: le degré de substitution de CA, la typologie de plastifiant et la quantité de plastifiant. Pour expliquer les différents mécanismes de plastification, il est donc important pour nous d'étudier et de comprendre les propriétés dynamiques (en ce qui concerne les phénomènes de relaxation) de ce type de systèmes et comment les trois leviers que nous avons identifiés peuvent influencer ou moduler les différentes interactions échangées dans les mélanges
Polysaccharides are one of the main options to the on-going move towards the use of renewable polymers. The industrial interest in this type of polymers drastically shrunk by the advent of synthetic polymers in the fifties, but is currently reviving due to the public awareness on the limit of fossil resources. These biopolymers are nowadays offering a challenging and industrially profitable playground for researchers. However, current polysaccharides based materials are mostly processed with extensive use of solvents (including water) making the total process an environmental burden despite the advantage of the starting material. Development of thermoplastic cellulose-based materials is very challenging regarding both final material properties and polymer processing. The degradation temperature of Cellulose Acetate (CA) (degree of substitution 2.5) is so close to its melting temperature that it can only be processed with a significant amount of external plasticizers (between 20 et 30 wt.% depending on the type of the additive). Behavior of a CA-plasticizer blend is mainly governed by a ‘network’ of high polar interactions, the strength and the density of which clearly depend of 3 specific parameters: the CA’s degree of substitution, the typology of the plasticizer, the amount of plasticizer. In an attempt to explain the different plasticization mechanisms, it is thus of utmost importance for us to study and understand the dynamic properties (regarding the relaxation phenomena) of this kind of systems and how the three levers that we identified can influence or modulate the different interactions exchanged within the blends

Dans le domaine des sciences des matériaux, la corrélation entre les propriétés diélectriques et mécaniques des nanocomposites polymères revêt une importance significative. Cette approche permet de prédire les comportements diélectriques à partir des caractéristiques mécaniques, et inversement, offrant ainsi une méthode alternative pour la caractérisation des matériaux, notamment lorsque les essais expérimentaux sont restreints. L'intégration de modèles fractionnaires pour établir cette corrélation représente une avancée innovante, permettant d'optimiser les performances des matériaux dans diverses applications. Dans ce contexte, l'attention accrue portée aux enjeux environnementaux a conduit à une exploration des matériaux issus de la biomasse, en raison de leur biodégradabilité et de leur impact réduit sur l'environnement. Cette recherche s'est donc également concentrée sur l'utilisation de charges cellulosiques, développant et étudiant des nanoparticules de cellulose comme renforts dans une matrice polymérique biodégradable. Les travaux ont examiné les propriétés thermiques, structurales, diélectriques et mécaniques des nanocomposites obtenus, avec pour objectif central de corréler ces deux dernières propriétés à l'aide de modèles fractionnaires. Cette corrélation vise à prédire les réponses diélectriques à partir des résultats mécaniques, et inversement. Les résultats expérimentaux mettent en évidence l'influence des biorenforts sur les caractéristiques de la matrice PVA/CS, avec une analyse détaillée des propriétés diélectriques et mécaniques dynamiques. Une corrélation classique entre ces propriétés a également été établie. Le cœur de l'étude explore la sensibilité des modèles fractionnaires et valide l'approche en établissant une corrélation entre les réponses diélectriques et mécaniques dynamiques de l'acide polylactique (PLA), puis en appliquant cette corrélation au mélange PVA/CS, démontrant ainsi la capacité à prédire la permittivité à partir du module de conservation, et réciproquement
In materials science, correlating the dielectric and mechanical properties of polymer nanocomposites is of significant importance. This correlation allows for predicting dielectric behaviors from mechanical characteristics and vice versa, offering an effective alternative for material characterization, particularly when experimental testing is limited. The application of fractional models to establish this correlation is a significant innovation, enabling the optimization of material performance across various applications. With growing environmental awareness, this research also addresses the use of biomass-derived materials due to their biodegradability and lower environmental impact. Specifically, it explores the incorporation of cellulosic fillers, developing and studying cellulose nanoparticles as reinforcements in a biodegradable polymer matrix. The research investigates the thermal, structural, dielectric, and mechanical properties of the resulting nanocomposites. The primary focus is on correlating dielectric and mechanical properties using fractional models. This correlation enables the prediction of dielectric responses from mechanical results and vice versa. Experimental results demonstrate the impact of bio-based reinforcements on the PVA/CS matrix, with a detailed examination of dynamic dielectric and mechanical properties. The study establishes a traditional correlation between these properties and further explores the sensitivity of fractional models. It validates the approach by correlating the dynamic dielectric and mechanical responses of polylactic acid (PLA) and then applies this correlation to the PVA/CS blend, showing the capability to predict permittivity from the storage modulus and vice versa

Une corrélation entre l’intensité et la nature des interactions intermoléculaires et les propriétés physiques, comme la conductivité thermique, a été rapportée pour des polymères amorphes. En particulier, une augmentation de la conductivité thermique a été associée à l’ajout d’interactions plus fortes par rapport aux liaisons de Van der Walls faibles, c’est-à-dire des liaisons hydrogène et ioniques. Dans ce travail, une tentative d'adapter la conductivité thermique dans les polymères amorphes a été réalisée par ingénierie des interactions intermoléculaires. Le poly(méthylméthacrylate) PMMA a été utilisé comme modèle et des copolymères poly(méthylméthacrylate-co-acide méthacrylique) (PMMA-co-MAA) ont été synthétisés par copolymérisation radicalaire afin d'introduire des liaisons H inter-chaînes et, après neutralisation, des liaisons ioniques. Des copolymères ont été obtenus avec succès jusqu'à 30 % en poids de MAA et caractérisés. Différents comonomères ont été utilisés pour évaluer l'influence d'une unité flexible apportant des liaisons H, le 2-hydroxyéthylméthacrylate (HEMA) ou le 2-carboxyéthylacrylate (CEA). La conductivité thermique a légèrement augmenté en augmentant la teneur en MAA et HEMA, tandis que pour les copolymères CEA, la présence de défauts a empêché la mesure. Le copolymère PMMA-co-MAA a été utilisé comme matrice pour les nanocomposites à base de cellulose afin d'adapter la compatibilité des charges, grâce à la présence de liaisons H entre l'unité MAA et la surface de la cellulose. Des nanofibres de cellulose (CNF) jusqu'à 15 % en poids ont été efficacement dispersées par coulée de solvant dans un mélange de deux solvants (THF/MeOH). La conduction thermique n'a montré aucun changement significatif après l'introduction des CNF. L'analyse mécanique dynamique (DMA) et la spectroscopie diélectrique à large bande (BDS) ont été utilisées en combinaison pour caractériser pleinement la dynamique moléculaire du PMMA-co-MAA copolymère suite à l'introduction de liaisons H inter-chaînes et à l'ajout ultérieur de CNF. Une relaxation β’ supplémentaire, caractérisée par une énergie d’activation (Ea) presque quatre fois plus élevé que le Ea(β), a été trouvée pour les nanocomposites et attribuée à l’établissement de liaisons H entre les groupes -COOH de la matrice et les groupes hydroxyles du CNF. Ainsi, une étude plus approfondie de la relaxation α a permis de révéler l’influence du CNF confirmant la présence de liaisons H interfaciales. En effet, malgré les Tgs similaires caractérisant la matrice et les nanocomposites, un décalage de leurs temps de relaxation vers des températures plus élevées a été observé suite à l’ajout de CNF. Les résultats rapportés dans cette thèse ont mis en évidence que les améliorations de la conductivité thermique obtenues par la modification des interactions entre les chaînes dans les matrices amorphes restent un défi extrêmement complexe
A correlation between the strength of the intermolecular interactions and physical properties has been reported for amorphous polymers. In particular, an increment of thermal conductivity has been associated to the addition of stronger interactions compared with weak van der Walls, i.e. hydrogen and ionic bonds. In this work, an attempt to tailor thermal conductivity in amorphous polymers has been made by engineering intermolecular interactions. Poly(methylmethacrylate) PMMA was used as standard and poly(methylmethacrylate-co-methacrylic acid) (PMMA-co-MAA) copolymers were synthesised by free radical copolymerization in order to introduce inter-chain hydrogen bonds and, after neutralisation, ionic bonds. Copolymers were successfully obtained up to 30wt% of MAA and characterized. Also, different comonomers were used to evaluate the influence of a flexible unit bringing H-bonds, 2-hydroxyethylmethacrylate (HEMA) or 2-carboxyethylacrylate (CEA). Thermal conductivity slightly increased increasing MAA and HEMA content, while for CEA copolymers the presence of defects prevented the measurement. Later, PMMA-co-MAA was used as a matrix for cellulose-based nanocomposites to tailor filler compatibility, thanks to the presence of H-bonds between MAA unit and cellulose surface. Cellulose nanofibers (CNF) up to 15wt% were efficiently dispersed by solvent casting in a mixture of two solvents (tetrahydrofuran/methanol). Thermal conduction showed no significant changes following the introduction of CNF. Dynamic mechanical analysis (DMA) and broadband dielectric spectroscopy (BDS) were used in combination to fully characterize the macromolecular mobility of PMMA-co-MAA following the introduction of inter-chain H-bonds and the subsequent addition of CNF. An additional β’-relaxation, characterized by an activation energy (Ea) nearly four times higher than the Ea(β), was found for the nanocomposites and ascribed to the establishment of H-bonds between the -COOH groups of the matrix and the hydroxyl groups of CNF, as confirmed by the lower values found for the thermal expansion coefficient of the free volume and the fragility of the material. A deeper investigation about the α-relaxation was able to reveal the influence of CNF confirming the presence of interfacial H-bonds. Indeed, despite the similar glass transition temperatures characterising the matrix and the nanocomposites, a shift of their relaxation times to higher temperatures was observed following the addition of CNF. Results reported in this thesis evidenced that the enhancements of thermal conductivity obtainable by the modification of the interchain interactions between chains in amorphous matrices remains an extremely complex challenge

Etude en régime dynamique et en régime statique. L'addition de titanate augmente la constante diélectrique tout en gardant un niveau faible des pertes diélectriques. Essais d'extrusion difficiles compte tenu des temps de relaxation et viscosité élevées de la matrice

Le but de cette thèse était tout d'abord de comprendre les théories physiques qui décrivent la dynamique des polymères à l'échelle macroscopique. Le modèle de Rouse et la théorie d'enchevêtrement de De Gennes décrivent la dynamique des polymères non enchevêtrés et enchevêtrés, respectivement. Nous avons étudiés les différentes transitions entre ces deux régimes en utilisant deux techniques expérimentales: Broadband Dielectric Spectroscopy (BDS) et rhéologie. Les effets d'enchevêtrement sur les spectres diélectriques ont été discutés. Un test complet du modèle de Rouse à été effectué sur en comparant les prédictions de ce modèle pour la dépendance en fréquence de la permittivité diélectrique et du module de cisaillement aux données expérimentales. Ensuite nous avons développés des méthodes bas"s sur la microscopie à force électrostatique afin d'étudier les propriétés diélectriques locales. En utilisant la simulation numérique de la Méthode des Charges Equivalentes la constante diélectrique a été quantifiée à partir de la mesure du gradient de force crée par un potentiel statique entre une pointe et un diélectrique. Cette méthode permet d'imager la constante diélectrique avec une résolution spatial de 40 nm. Le retard de phase de la composante en 2ω de la force ou du gradient de force crée par un voltage alternatif est relié aux pertes diélectriques. En mesurant cette quantité nous avons montré que la dynamique était plus rapide proche d'une interface libre et nous avons développé un mode d'imagerie des pertes diélectriques. Cette méthode simple pourrait être appliqué en biologie ou matière molle en générale afin d'étudier des variations locales de constantes diélectriques
The aim of this thesis was first to understand the physical theories that describe the dynamics of linear polymers at the macroscopic scale. Rouse and the reptational tube theory describe the large scale dynamics of unentangled and entangled polymers respectively. Using Broadband Dielectric Spectroscopy (BDS) and rheology we have studied the different transition between these two regimes. Effects of entanglement on dielectric spectra will be discussed (Rheologica Acta. 49(5):507-512). Avoiding the segmental relaxation contribution and introducing a distribution in the molecular weight we have been able to perform a comparison of the Rouse model with experiment dielectric and rheological data (Macromolecules 42(21): 8492-8499) Then we have developed EFM-based methods in order to study the local dynamics. Using the numerical simulation of the Equivalent Charge Method, the value of the static dielectric permittivity has been quantified from the measurement of the force gradient created by a VDC potential between a tip and a grounded dielectric (Journal of Applied Physics 106(2):024315). This method allows a quantitative mapping of dielectric properties with a 40 nm spatial resolution and is therefore suitable for the study of nano-defined domains (Physical Review E 81(1): 010801). The electrical phase lags in the 2ω components of the force or force gradient created by VAC voltage, ΔΦ2ω, are related with dielectric losses. Measuring the frequency dependence of ΔΦ2ω Crieder et al (Applied Physics Letters 91(1):013102) have shown that the dynamics at the near free surface of polymer films is faster than the one in bulk. We have used this method in order to visualize the activation of the segmental relaxation with temperature and frequency (Applied Physics Letters 96(21): 213110). All this measurements can be achieved using standard Atomic Force Microscope (and a lock-in) for VAC measurements

The electrical properties of proteins in solution are important for their structure and function. Computational biophysics studies of proteins need accurate parameters to ensure that numerical simulations match physical reality. Past work in this eld has compared the electrical properties of proteins obtained from dielectric spectroscopy to numerical simulations of proteins in water with adjustment of pKa values to try to capture the inevitable changes in electrical conformation that will occur in a complex structure such as a folded protein. However, fundamental veri cation of the charge parameters of the amino acid building blocks in common molecular dynamics software packages with electrical experiments needs to be performed to have increased con dence in the results from numerical simulations. The aim of this thesis is to start from a fundamental building block, the single amino acid alanine, and to compare numerical simulations of this amino acid in water using parameters from commonly used charge structures in CHARMM, GROMOS, and OPLS, with electrical parameters obtained from dielectric spectroscopy experiments in the GHz range. To this end, multiple molecular dynamics simulations were performed to accurately determine how these different charge structures yield different dielectric increments. Additionally, a commercial RF dielectric measurement probe was modi ed to perform measurements on solutions containing alanine at different concentrations. Using regression, the dielectric increment of alanine is readily determined and compared with the numerical simulations. The results indicate that the CHARMM and OPLS parameters seem to adequately capture the charge con guration of alanine in solution, while the GROMOS parameters produce a dielectric increment but do not seem to adequately capture the charge con guration of alanine in solution. These studies lay the foundation for future studies of additional amino acids in solution as well as a stepping stone for larger simulations of the electrical properties of fully solvated proteins in solution.

This master´s thesis deals about insulation and dielectric properties of water. It describes the structure of water, covalent and hydrogen bonds. It outlines primary chemical and physical properties of water and then it describes parameters of water - dynamic viscosity, conductivity, surface tension, pH and also describes dielectric properties - dielectric loss, loss factor, permittivity, polarization. Part of the work is practical section, which includes measurements of deionized water, destilled water, tap water and shows primary dependence such as dependence loss factor on frequency or relative permittivity on frequency.

In the dissertation AVBVICVII type compounds are analyzed theoretically and experimentally. Theoretical studies were carried out using Density Functional Theory (DFT), along with the Full Potential Linearized Augmented Plane Wave (FP-LAPW) method and the Generalized Gradient Approximation (GGA). For calculations Wien2k and PHONON comp. packages were used. Experimental studies were performed using spectroscopic ellipsometry method and measuring permittivity (electrical capacitance) as a function of temperature. The study discusses AVBVICVII type compounds’ inter-atomic chemical bonding, the electronic structure, optical properties, lattice dynamics, vibrational thermodynamic functions and dielectric properties in the paraelectric, ferroelectric and antiferroelectric phases.
Disertacijoje teoriškai ir eksperimentiškai nagrinėjami AVBVICVII tipo junginiai. Teoriniai tyrimai atlikti naudojantis tankio funkcionalo teorija kartu su pilno potencialo tiesinių padidintų plokščių bangų metodu ir apibendrinto gradiento aproksimacija. Skaičiavimams naudoti Wien2k ir PHONON komp. paketai. Eksperimentiniai tyrimai buvo atliekami naudojantis spektroskopinės elipsometrijos metodais bei matuojant dielektrinės skvarbos (elektrinės talpos) priklausomybes nuo temperatūros. Darbe nagrinėjamas AVBVICVII tipo junginių tarpatominis cheminis ryšys, elektroninė struktūra, optinės savybės, gardelės dinamika, virpesių termodinaminės funkcijos ir dielektriniai pokyčiai paraelektrinėje, feroelektrinėje ir antiferoelektrinėje fazėse.

Organic molecular crystals form a condensed solid phase offering a rich vein of physical phenomena which are open to investigation. The desire to harness these properties for technological and biological purposes has led to extensive experimental and theoretical investigations. The naturally occurring ɑ-amino acids form molecular crystals in the solid state; to date there have been very few studies of these systems. The work in this thesis is concerned with attempting to understand the relationship between the properties of the molecular crystal, and how these relate to the properties of the constituent molecules in isolation. To this end, density functional calculations of the structural and electronic properties of amino acids in both the crystalline and gaseous states are performed, and the results reported. The bonding mechanisms responsible for the crystal being stable are elucidated, and used to explain the zwitterionisation of the molecules upon formation of the solid state. In order to investigate the lattice dynamical and dielectric properties, the implementation of a variational density functional perturbation theory (DFPT) scheme within the plane wave pseudopotential formalism is described in detail. This scheme is fully self-consistent, and its computational cost is comparable to that of a single-point self-consistent total energy calculation. The long wave method is used to alleviate well-known problems associated with the application of homogeneous fields to crystal systems, viz. that such fields break the crystal symmetry, and the adequate treatment of electronic screening. Calculation of the first order perturbed wavefunctions and the second order change in the Kohn-Sham functional allows properties such as the polarisability, dielectric matrix, dynamical matrix and Born effective charge tensors to be determined. The treatment of crystalline symmetries is described in detail. The DFPT formalism is extended to allow IR absorption spectra to be obtained. The lattice dynamical and dielectric behaviour of the isolated molecules and the molecular crystals are obtained; calculation of the IR spectra facilitates an insight into the effects of the crystalline environment and zwitterionisation upon the lattice dynamics. Results indicate the importance of the molecular shape and structure upon the intermolecular interactions, and hence the crystal structure formed. It is these intermolecular interactions that are found to play the major part in modification of the lattice dynamical and dielectric behaviour.

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

L'objectif de ce travail est d'apporter un éclairage nouveau à la compréhension des mécanismes physico-chimiques qui contrôlent la croissance des trois oxydes d'Aluminium, de Zirconium, de Hafnium. Ces matériaux sont considérés comme les meilleurs candidats pour remplacer la silice en tant qu'oxyde de grille dans le futur composant MOS. La précision et la fiabilité de la méthode DFT associé à la fonctionnelle B3LYP, ont été testées à l'aide des résultats expérimentaux et des méthodes ab initio les plus précis telles que CCSD(T) et CISD(T), en utilisant différents ensembles des fonctions de bases. Nos résultats montrent que et la méthode hybride DFT peut prédire de façon précise les propriétés statistiques et dynamiques de la famille d'organométalliques (AlxCyHzOt) et des systèmes moléculaires à base de métaux de transition (Zr/HfxClyOzHt). Les premières études systématiques des surfaces d'énergie potentielle de TMA ont été présentes et les caractéristiques des rotors constitués des groups méthyles ont été rapportées avec une grande précision. Les mécanismes réactionnels, à plusieurs étapes, entre les molécules précurseurs de trois oxydes et les molécules d'eau résiduelle phase gazeuse ont été étudies en détail. Les mouvements internes fortement anharmoniques, des espèces moléculaires présentes touts au long du processus d'hydrolyse ont été déterminés. Les effets qualitatifs sur les cinétiques des réactions ont été discutés. La forte exothermicité de la réaction TMA/H2O a été démontrée, alors que la réaction avec les tétrachlorures de Zirconium et Hafnium ont montré un caractère plutôt endothermique. Nous avons aussi étudié les mécanismes réactionnels de la vapeur d'eau avec d'espèces moléculaires chimisorbés en surface. Les réactions interviennent dans les cycles initiales d'ALD sur en substrat de Si(001)-2x1 légèrement oxydé. Les mécanismes que nous proposons sont qualitativement proches des mécanismes d'hydrolyse discutés dans la phase gaz euse : confirmation de la forte réactivité exothermique avec les hydroxyméthyliques d'Aluminium, endothermicité des réactions avec hydroxychlorures de Zirconium et Hafnium. Les composés avec le Zirconium et le Hafnium ont des comportements similaires. Enfin, les effets de coopérativité, à la fois au niveau des molécules de vapeur d'eau, qu'au niveau des complexes en surface, sur les réactions ont été mis en évidence et discutés. Ils montrent des comportements tout à fait intéressants dans le cas des hydroxychlorures des Zirconium et Hafnium. Par contre, ces effets sont de moindre importance dans les cas de l'oxyde d'aluminium, qui est actuellement considéré comme le matériau le plus compatible avec la croissance par ALD
This work attempts to bring a new light on the understanding of some critical aspects of the physicochemical processes that control Alumina, Zirconia and Hafnia ALD growth, yet not sufficiently understood. These materials are addressed as potentially best candidates to replace gate dielectric SiO2 in the near future electronic applications. Most accurate ab initio correlated methods, like couple-cluster CCSD(T) and CISD(T), with different basis sets functions, as well as the available experimental data have been used for testing by a systematic study the accuracy and the reliability of DFT B3LYP functional. Our results have claimed this hybrid-DFT method to be chosen in predicting of high accurate static and dynamic properties throughout the family of organometallic-like (AlxCyHzOt) and transition metal-based (Zr/HfxClyOzHt) molecular systems. First systematic study of torsional potential surfaces of TMA has been performed and the related features of the hindered rotors of the methyl groups revealed with high accuracy. Laying on these accurate results we have also proposed least-squared fit methods to determine frequency scaling factors subject to different thermodynamic properties and/or thermal conditions. Many-step reaction mechanisms of ALD gas phase precursors of each of the three oxides with residual water, or regime of low pressure H2OÓALD pulses, have been studied in detail. Strong anharmonic internal movements of molecular species throughout the hydrolysis reactions have been observed and qualitatively discussed in relation with their possible effects on the reactions' kinetics. TMA/H2O reactions have been validated as strongly exothermic, while Hafnium and Zirconium tetrachlorides have founded to react endothermically with single H2O molecule. We have also studied in detail reaction mechanisms of the related on-surface ALD-complexes with water vapors. Our theoretical investigations address to the initial stage of ALD growth, more s pecifically on SiO2/Si(001)-2x1 like surfaces. The proposed many-step mechanisms, similar to those discussed for the gas phase, confirmed again the strong reactivity of H2O molecule with on-surface Aluminum hydroxymethylides, and responds strong endothemically as for the hydroxylation of Zirconium and Hafnium on-surface hydroxychlorides. The last two proved a very similar surface chemistry. Finally the cooperative effects of H2O molecules have been considered in our models of reactions, and have revealed dramatic influences on the reactivity Zirconium- and Hafnium hydroxychlorides surfaces. Our results proved the importance of both cooperative interactions of on-surface complexes and H2O molecules in the case of the Zirconia and HafniaÓALD growth, while for Aluminum oxide, presently considered ideal for ALD growth, these effects seem of secondary importance

L’originalité de ce travail repose sur une approche physique et physico-chimique des propriétés de mélanges de polylactides de chiralité différentes (poly L-lactique acide et poly D-lactique acide) et sur l’influence de cette chiralité sur les propriétés des phases amorphes de ces matériaux. Les matériaux sont des mélanges élaborés à partir de deux homopolymères (PLLA et PDLA) selon deux méthodes ; coulée en solution ou par extrusion. Nous avons étudié des matériaux totalement amorphes et cristallisés de façon isotrope afin de générer une phase amorphe plus ou moins confinée. Il est montré que le mélange pouvant donner naissance à une phase cristalline stéréocomplexe ne peut être obtenu que dans certaines conditions. Les résultats des analyses thermiques et de perméation ont montré que le mélange PLLA/PDLA améliore certaines propriétés du matériau, notamment des propriétés barrières à l’eau et aux gaz plus élevées par rapport aux homopolymères parents. Afin d’étudier la mobilité moléculaire des phases amorphes, du vieillissement physique et de la relaxation structural (relaxation α et β), le concept de Région de Réarrangement Coopératif (CRR) a été appliqué. Il a été montré que les phases amorphes des homopolymères et du mélange ont exactement les mêmes propriétés à la transition vitreuse et dans l’état vitreux lorsque les matériaux sont totalement amorphes
The originality of this work is based on analysis of physical and physicochemical properties of polylactide mixtures of different chirality (poly L-lactic acid and poly D-lactic acid) and on the influence of the chirality on the amorphous phase’s properties. The materials mixtures are elaborated from two homopolymers (PLLA and PDLA) according to two methods; solution casting or extrusion. Totally amorphous and isotropically crystallized materials with more or less confined amorphous phase were studied. It is shown that a stereocomplex crystalline phase can be obtained only under certain experimental conditions. The results of the thermal and permeation analyzes showed that the PLLA / PDLA mixture improved certain properties of the material, namely higher barrier properties towards liquid water and gases were obtained compared to parent homopolymers. In order to study the molecular mobility of amorphous phases, physical aging and structural relaxation (α and β relaxation), the Cooperative Rearrangement Region (CRR) concept has been applied. It has been shown that the amorphous phases of the homopolymers and the mixture have exactly the same properties at the glass transition and in the vitreous state when the materials are totally amorphous

The aim of the experiments described in this thesis was the investigation of different aspects of dielectric resonators with microwave experiments. Dielectric cavities have received much attention in the last years due to applications of microlasers and -cavities in telecommunications, integrated optics or as sensors. A key issue in current research is the correspondence between the ray– and the wave–dynamics in these systems, that is their semiclassical description. Therefore, different semiclassical approaches to dielectric resonators have been tested experimentally with macroscopic flat dielectric microwave resonators. The results can be directly applied to microcavities working in the infrared up to optical frequency regime via scaling as long as the ratio between wavelength and resonator dimensions is similar. Both quasi two–dimensional setups consisting of a dielectric plate squeezed between two metal plates and three-dimensional setups with “levitating” dielectric plates surrounded by air have been investigated. Both the frequency spectra and near field distributions were measured. In the first part of the thesis, a two-dimensional approximation for three-dimensional flat dielectric resonators has been tested quantitatively with two different circular Teflon disks. The approximation is based on the projection of the three–dimensional ray-dynamics onto two dimensions and the introduction of a so-called effective index of refraction. Comparison of the model calculations with the measured resonance frequencies and widths reveal that they predict the correct order of magnitude, but significant deviations remain. It was thus shown that the model of an effective index of refraction is too imprecise for the detailed understanding of measured frequency spectra, and that furthermore the systematic error of the model calculations is not under control. In the second part of the thesis, the localization of resonance states on certain periodic orbits was investigated. The existence of such so-called superscars in polygonal metal cavities is well known and was recently predicted also for dielectric polygonal resonators. Therefore, the field distributions of a square ceramic resonator were measured experimentally. The measurements confirmed the existence of superscarred states, but also showed localized states with unexpected character. The physical origin of these states is unclear so far, motivating further investigations. In the third part of the thesis, a trace formula connecting the resonance density of two-dimensional dielectric resonators with the periodic orbits of the corresponding classical billiards was investigated with quasi two-dimensional resonators of circular and square shape. The length spectra deduced from the measured frequency spectra reveal contributions of the periodic orbits to the resonance density, but also show significant deviations from the trace formula since only a part of the expected resonances could be observed experimentally. The results demonstrate that the systematics of the observed states must be taken into account for an understanding of the experimental length spectra. A connection between the most long-lived resonances of the cavities and the most strongly confined periodic orbits of the corresponding classical billiards was established. In the fourth part of the thesis, the applicability of the trace formula for twodimensional dielectric resonators to flat three-dimensional resonators was investigated. An approach combining the trace formula with the effective index of refraction model investigated in the first part was tested for two flat circular Teflon disks. Preliminary results show good qualitative agreement between the model and the experimental data, but also that additional effects due to the dispersion of the effective index of refraction and due to the systematic error of the model must be taken into account.

Today, nanotechnology has added a new dimension to materials technology by creating opportunities to significantly enhance the properties of existing conventional materials. Polymer nanocomposites belong to one such class of materials and even though they show tremendous promise for dielectric/electrical insulation applications (“nanodielectrics” being the buzzword), the understanding related to these systems is very premature. Considering the desired research needs with respect to some of the dielectric properties of polymer nanocomposites, this study attempts to generate an understanding on some of the existing issues through a systematic and detailed experimental investigation coupled with a critical analysis of the data. An epoxy based nanocomposite system is chosen for this study along with four different choices of nano-fillers, viz. TiO2, Al2O3, ZnO and SiO2. The focus of this study is on the properties of nanocomposites at low filler loadings in the range of 0.1 - 5% by weight and the properties under investigation are the permittivity/tan delta behaviors, DC volume resistivity, AC dielectric strength and electrical discharge resistant characteristics. Significant efforts have also been directed towards addressing the interface interaction phenomena in epoxy nanocomposites and their subsequent influence on the dielectric properties of the material. The accurate characterization of the dielectric properties for polymer nanocomposites depends on the dispersion of nanoparticles in the polymer matrix and to achieve a good dispersion of nanoparticles in the epoxy matrix for the present study, a systematic design of experiments (DOE) is carried out involving two different processing methods. Consequently, a laboratory based epoxy nanocomposite processing methodology is proposed in this thesis and this process is found to be successful in dispersing nanoparticles effectively in the epoxy matrix, especially at filler concentrations lower than 5% by weight. Nanocomposite samples for the study are prepared using this method and a rigorous conditioning is performed before the dielectric measurements. The dielectric properties of epoxy nanocomposites obtained in the present study show interesting and intriguing characteristics when compared to those of unfilled epoxy and microcomposite systems and few of the results are unique and advantageous. In an unexpected observation, the addition of nanoparticles to epoxy is found to reduce the value of nanocomposite real permittivity below that of unfilled epoxy over a wide range of frequencies. Similarly, it has been observed that irrespective of the filler type, tan delta values in the case of nanocomposites are either same or lower than the value of unfilled epoxy up to a filler loading of 5% by weight, depending on the frequency and filler concentration. In fact, the nanocomposite real permittivities and tan delta values are also observed to be lower as compared to the corresponding microcomposites of the same constituent materials at the same filler loading. In another significant result, enhancements in the electrical discharge resistant characteristics of epoxy nanocomposites (with SiO2/Al2O3 nanoparticles) are observed when compared to unfilled epoxy, especially at longer durations of discharge exposures. Contrary to these encouragements observed for few of the dielectric properties, the trends of DC volume resistivity and AC dielectric breakdown strength characteristics in epoxy nanocomposites are found to be different. Irrespective of the type of filler in the epoxy matrix, it has been observed that the values of both AC dielectric strengths and DC volume resistivities are lower than that of unfilled epoxy for the filler loadings investigated. The results mentioned above seem to suggest that there has to be an interaction between the nanoparticles and the epoxy chains in the nanocomposite and therefore, glass transition temperature (Tg) measurements are performed to characterize the interaction phenomena, if any. The results of Tg for all the investigated nanocomposites also show interesting trends and they are observed to be lower than that of unfilled epoxy up to certain nanoparticle loadings. This lowering of the Tg in epoxy nanocomposites was not observed in the case of unfilled and microcomposite systems thereby strongly confirming the fact that there exists an interaction between the epoxy chains and nanoparticles in the nanocomposite. Considering the variations obtained for the nanocomposite real permittivity and Tg with respect to filler loading, a dual nanolayer interface model is utilized to explain the interaction dynamics and according to the model, interactions between epoxy chains and nanoparticles lead to the formation of two epoxy nanolayers around the nanoparticle. Analysis shows that the characteristics of the interface region have a strong influence on the dielectric behaviors of the nanocomposites and the suggested interface model seems to fit the characteristics obtained for the different dielectric/electrical insulation properties rather well. Further investigations are performed to understand the nature of interaction between nanoparticles and epoxy chains through FTIR studies and results show that there is probably an occurrence of hydrogen bonding between the epoxide groups of the epoxy resin and the free hydroxyl (OH) groups present on the nanoparticle surfaces. The results obtained for the dielectric properties of epoxy nanocomposites in this study have widened the scope of applications of these functional materials in the electrical sector. The occurrence of lower values of real permittivity for nanocomposites is definitely unique and unexpected and this result has huge potential in electronic component packaging applications. Further, the advantages related to tan delta and electrical discharge resistance for these materials carry lot of significance since, electrical insulating materials with enhanced electrical aging properties can be designed using nanocomposite technology. Although the characteristics of AC dielectric strengths and DC volume resistivities are not found to be strictly advantageous for epoxy nanocomposites at the investigated filler loadings, these properties can be optimized when designing insulation systems for practical applications. In spite of all these advantages, serious and systematic research efforts are still desired before these materials can be successfully utilized in electrical equipment.

Today, nanotechnology has added a new dimension to materials technology by creating opportunities to significantly enhance the properties of existing conventional materials. Polymer nanocomposites belong to one such class of materials and even though they show tremendous promise for dielectric/electrical insulation applications (“nanodielectrics” being the buzzword), the understanding related to these systems is very premature. Considering the desired research needs with respect to some of the dielectric properties of polymer nanocomposites, this study attempts to generate an understanding on some of the existing issues through a systematic and detailed experimental investigation coupled with a critical analysis of the data. An epoxy based nanocomposite system is chosen for this study along with four different choices of nano-fillers, viz. TiO2, Al2O3, ZnO and SiO2. The focus of this study is on the properties of nanocomposites at low filler loadings in the range of 0.1 - 5% by weight and the properties under investigation are the permittivity/tan delta behaviors, DC volume resistivity, AC dielectric strength and electrical discharge resistant characteristics. Significant efforts have also been directed towards addressing the interface interaction phenomena in epoxy nanocomposites and their subsequent influence on the dielectric properties of the material. The accurate characterization of the dielectric properties for polymer nanocomposites depends on the dispersion of nanoparticles in the polymer matrix and to achieve a good dispersion of nanoparticles in the epoxy matrix for the present study, a systematic design of experiments (DOE) is carried out involving two different processing methods. Consequently, a laboratory based epoxy nanocomposite processing methodology is proposed in this thesis and this process is found to be successful in dispersing nanoparticles effectively in the epoxy matrix, especially at filler concentrations lower than 5% by weight. Nanocomposite samples for the study are prepared using this method and a rigorous conditioning is performed before the dielectric measurements. The dielectric properties of epoxy nanocomposites obtained in the present study show interesting and intriguing characteristics when compared to those of unfilled epoxy and microcomposite systems and few of the results are unique and advantageous. In an unexpected observation, the addition of nanoparticles to epoxy is found to reduce the value of nanocomposite real permittivity below that of unfilled epoxy over a wide range of frequencies. Similarly, it has been observed that irrespective of the filler type, tan delta values in the case of nanocomposites are either same or lower than the value of unfilled epoxy up to a filler loading of 5% by weight, depending on the frequency and filler concentration. In fact, the nanocomposite real permittivities and tan delta values are also observed to be lower as compared to the corresponding microcomposites of the same constituent materials at the same filler loading. In another significant result, enhancements in the electrical discharge resistant characteristics of epoxy nanocomposites (with SiO2/Al2O3 nanoparticles) are observed when compared to unfilled epoxy, especially at longer durations of discharge exposures. Contrary to these encouragements observed for few of the dielectric properties, the trends of DC volume resistivity and AC dielectric breakdown strength characteristics in epoxy nanocomposites are found to be different. Irrespective of the type of filler in the epoxy matrix, it has been observed that the values of both AC dielectric strengths and DC volume resistivities are lower than that of unfilled epoxy for the filler loadings investigated. The results mentioned above seem to suggest that there has to be an interaction between the nanoparticles and the epoxy chains in the nanocomposite and therefore, glass transition temperature (Tg) measurements are performed to characterize the interaction phenomena, if any. The results of Tg for all the investigated nanocomposites also show interesting trends and they are observed to be lower than that of unfilled epoxy up to certain nanoparticle loadings. This lowering of the Tg in epoxy nanocomposites was not observed in the case of unfilled and microcomposite systems thereby strongly confirming the fact that there exists an interaction between the epoxy chains and nanoparticles in the nanocomposite. Considering the variations obtained for the nanocomposite real permittivity and Tg with respect to filler loading, a dual nanolayer interface model is utilized to explain the interaction dynamics and according to the model, interactions between epoxy chains and nanoparticles lead to the formation of two epoxy nanolayers around the nanoparticle. Analysis shows that the characteristics of the interface region have a strong influence on the dielectric behaviors of the nanocomposites and the suggested interface model seems to fit the characteristics obtained for the different dielectric/electrical insulation properties rather well. Further investigations are performed to understand the nature of interaction between nanoparticles and epoxy chains through FTIR studies and results show that there is probably an occurrence of hydrogen bonding between the epoxide groups of the epoxy resin and the free hydroxyl (OH) groups present on the nanoparticle surfaces. The results obtained for the dielectric properties of epoxy nanocomposites in this study have widened the scope of applications of these functional materials in the electrical sector. The occurrence of lower values of real permittivity for nanocomposites is definitely unique and unexpected and this result has huge potential in electronic component packaging applications. Further, the advantages related to tan delta and electrical discharge resistance for these materials carry lot of significance since, electrical insulating materials with enhanced electrical aging properties can be designed using nanocomposite technology. Although the characteristics of AC dielectric strengths and DC volume resistivities are not found to be strictly advantageous for epoxy nanocomposites at the investigated filler loadings, these properties can be optimized when designing insulation systems for practical applications. In spite of all these advantages, serious and systematic research efforts are still desired before these materials can be successfully utilized in electrical equipment.

碩士
國立成功大學
材料科學及工程學系碩博士班
101
Strontium barium niobate ( SBN ) ceramic is a good electro-optic material. It has Tetragonal Tungsten Bronze (TTB) structure and SBN is a non centro-symmerty ferroelectrics. It has very high linear electro-optical coefficient、high pyroelectric coefficient and good photorefractive effect, SBN ceramic uses CZ method to growth now. Though the properties of the SBN single crystal has been intensively studied, its high cost and difficult fabrication have limited its practical use, such as shape, size or mechanical strength，hence it is necessary to develop polycrystal SBN ceramic. We take advantage of Cole-Cole plot to study the dielectric properties and domain wall dynamics of pure SBN50 and co-doping Cr2O3,CeO2 under ac field, and analyze effects of different temperature and doping ratio, then we use function to fit the region of relaxation. Finally, we observe the effect of co-doping in SBN50 by the fitting parameters.

碩士
國立中山大學
機械與機電工程學系研究所
102
We combine coarse grained (CG) molecular dynamics simulation and density functional theory (DFT) calculation to predict the phase behavior, the dielectric and the optical properties of liquid crystal molecule system. 4-Cyano-4′-pentylbiphenyl (5CB) is the earliest synthesized and the most popular liquid crystal molecule, which have prosperous related bibliographies and available experimental and theoretical data, so 5CB molecule is chosen to be the benchmark molecule. According to the Maier-Meier and Vuks equations, we have to obtain some parameters such as the dipole moment, the polarizability, the polarizability anisotropy, the order parameter and the molecular density to calculate dielectric and optical properties of liquid crystal molecule system. The CG model for the 5CB molecules is constructed, and the bond length, bending angle and nonbonded interaction parameters between CG beads, and then applied to the CGMD simulation to obtain the order parameter and molecular density. In addition, the polarizability and dipole moment of 5CB were obtained by DFT; therefore, the order parameter, the molecular density, the polarizability, and the dipole moment of 5CB were used to determine the dielectric constant, the dielectric anisotropy, refractive indices, and optical anisotropy by means of the Maier-Meier theory. Particularly, the calculation results from Maier-Meier theory can be improved by using effective dipole moment of the Dunmur-Palffy-Muhoray theory. Finally, our results show a good agreement with the experimental results, and it indicates that our simulation method is feasible and the results are accuracy.