Electromechanical transducers based on elastomer capacitors are presently considered for many soft actuation applications, due to their large reversible deformation in response to electric field induced electrostatic pressure. The high operating voltage of such devices is currently a large drawback, hindering their use in applications such as biomedical devices and biomimetic robots, however, they could be improved with a careful design of their material properties. The main targets for improving their properties are increasing the relative permittivity of the active material, while maintaining high electric breakdown strength and low stiffness, which would lead to enhanced electrostatic storage ability and hence, reduced operating voltage. Improvement of the functional properties is possible through the use of nanocomposites. These exploit the high surface-to-volume ratio of the nanoscale filler, resulting in large effects on macroscale properties. This thesis explores several strategies for nanomaterials design. The resulting nanocomposites are fully characterized with respect to their electrical and mechanical properties, by use of dielectric spectroscopy, tensile mechanical analysis, and electric breakdown tests. First, nanocomposites consisting of high permittivity rutile TiO2 nanoparticles dispersed in thermoplastic block copolymer SEBS (poly-styrene-coethylene-co-butylene-co-styrene) are shown to exhibit permittivity increases of up to 3.7 times, leading to 5.6 times improvement in electrostatic energy density, but with a trade-off in mechanical properties (an 8-fold increase in stiffness). The variation in both electrical and mechanical properties still allows for electromechanical improvement, such that a 27 % reduction of the electric field is found compared to the pure elastomer. Second, it is shown that the use of nanofiller conductive particles (carbon black (CB)) can lead to a strong increase of relative permittivity through percolation, however, with detrimental side effects. These are due to localized enhancement of the electric field within the composite, which leads to sharp reductions in electric field strength. Hence, the increase in permittivity does not make up for the reduction in breakdown strength in relation to stored electrical energy, which may prohibit their practical use. Third, a completely new approach for increasing the relative permittivity and electrostatic energy density of a polymer based on 'molecular composites' is presented, relying on chemically grafting soft π-conjugated macromolecules to a flexible elastomer backbone. Polarization caused by charge displacement along the conjugated backbone is found to induce a large and controlled permittivity enhancement (470 % over the elastomer matrix), while chemical bonding, encapsulates the PANI chains manifesting in hardly any reduction in electric breakdown strength, and hence resulting in a large increase in stored electrostatic energy. This is shown to lead to an improvement in the sensitivity of the measured electromechanical response (83 % reduction of the driving electric field) as well as in the maximum actuation strain (250 %). These results represent a large step forward in the understanding of the strategies which can be employed to obtain high permittivity polymer materials with practical use for electro-elastomer actuation. / Die Palette von elektro-mechanischen Aktuatoren, basierend auf dem Prinzip weicher dehnbarer Kondensatoren, scheint besonders für Anwendungen in der Medizin und für biomimetische Applikationen unbegrenzt. Diese Wandler zeichnen sich sowohl durch hohe Reversibilität bei großer mechanischer Deformation als auch durch ihre Flexibilität aus, wobei die mechanischen Deformationen durch elektrische Felder induziert werden. Die Notwendigkeit von hoher elektrischer Spannung zur Erzeugung dieser mechanischen Deformationen verzögert jedoch die technisch einfache und breite Markteinführung dieser Technologie. Diesem Problem kann durch eine gezielte Materialmodifikation begegnet werden. Eine Modifikation hat das Ziel, die relative Permittivität zu erhöhen, wobei die Flexibilität und die hohe elektrische Durchbruchsfeldstärke beibehalten werden sollten. Durch eine Materialmodifikation kann die Energiedichte des Materials bedeutend erhöht und somit die notwendige Betriebsspannung des Aktuators herabgesetzt werden. Eine Verbesserung der funktionalen Materialeigenschaften kann durch die Verwendung von Nanokompositen erzielt werden, welche die fundamentalen Eigenschaften der Nanopartikel, d.h. ein gutes Verhältnis von Oberfläche zu Volumen nutzen, um eine gezielte makroskopische Materialmodifikation zu bewirken. Diese Arbeit behandelt die Anwendung innovativer Strategien für die Erzeugung von Nanomaterialien mit hoher Permittivität. Die so erzeugten Materialien und deren relevante Aktuatorkenngrößen werden durch elektrische und mechanische Experimente vollständig erfasst. Mittels der klassischen Mischansätze zur Erzeugung von Kompositmaterialen mit hoher Permittivität konnte durch nichtleitendes Titaniumdioxid TiO2 (Rutile) in einem Thermoplastischen-Block-Co-Polymer SEBS (poly-styrene-co-ethylene-cobutylene-co-styrene) die Permittivität bereits um 370 % erhöht und die elektrische Energiedichte um 570 % gesteigert werden. Diese Veränderungen führten jedoch zu einem signifikanten Anstieg der Steifigkeit des Materials. Aufgrund der positiven Rückkopplung von elektrischen und mechanischen Eigenschaften des Kompositmaterials ermöglicht bereits dieser einfache Ansatz eine Verbesserung der Aktuation, bei einer 27 %-igen Reduktion der Aktuatorbetriebsspannung. Eine direkte Verwendung von leitfähigen Nanopartikeln kann ebenso zu einem Anstieg der relativen Permittivität beitragen, wobei jedoch die Leitfähigkeit dieser Nanopartikel bedeutende Wechselwirkungen verursacht, welche somit die Energiedichte des Materials negativ beeinflusst und die praktische Verwendung dieses Kompositsystems ausschließt. Als ein völlig neuer Ansatz zur Steigerung der relativen Permittivität und Energiedichte und abweichend vom klassischen Mischverfahren, wird die Herstellung eines "Molekularen Komposits", basierend auf einem chemischen Propfverfahren, präsentiert. In diesem Ansatz wird ein π-konjugiertes leitfähiges Polymer (PANI) an die Hauptkette des Elastomers der Polymermatrix gebunden. Die daraus resultierende Ladungsverteilung entlang der Elastomerhauptkette bewirkt eine 470 %-ige Steigerung der Permittivität des "Molekularen Komposits" im Vergleich zur Permittivität des unbehandelten Elastomermaterials. Aufgrund der Verkapselung der chemischen Bindungen der PANI-Kette entstehen kaum negative Rückwirkungen auf die elektrischen und mechanischen Eigenschaften des so erzeugten Komposits. Diese Materialeigenschaften resultieren in einem signifikanten Anstieg der Energiedichte des Materials. Das mittels dieses Verfahrens erzeugte Komposit zeigt sowohl eine Steigerung der Sensitivität der elektromechanischen Antwort (Reduktion des elektrischen Felds um 83 %) als auch eine bedeutende Steigerung der maximalen Aktuation (250 %). Die Ergebnisse und Ideen dieser Arbeit stellen einen wesentlichen Sprung im Verständnis zur Permittivitätssteigerung in Polymermaterialien dar und werden deshalb in der Erforschung und Entwicklung von Elastomeraktuatoren Beachtung finden.

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption.

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption.

Ce travail de thèse a pour objectif la caractérisation des propriétés physico-chimiques descéramiques de composition (1-x) BaTi0.8Zr0.2O3-x Ba0.7Ca0.3TiO3 préparées par frittage conventionnelet frittage flash (SPS). Les études structurales réalisées au voisinage du point triple (x≈ 0.32) à l’aidede la diffraction des RX de haute résolution (synchrotron) sur poudre ont introduit des modificationsmajeures sur le diagramme de phase température-composition déjà proposé. La réponseélectromécanique géante mesurée est alors corrélée à la dégénérescence du profil de l’énergie libreinduite par les instabilités structurales. De plus, la flexibilité de la polarisation sous contraintesthermique et électrique est couplée à un assouplissement de la maille cristalline. Ces deuxcaractéristiques contribuent ensemble à une réponse électromécanique colossale via une forteactivité des murs de domaine. La dégradation des propriétés diélectriques, ferroélectriques etpiézoélectriques pour les céramiques BCTZ (x=0.32 et 0.5) élaborées par frittage flash estessentiellement attribuée aux fluctuations importantes de composition et à la stabilisation de laconfiguration des murs de domaines avec la diminution de la taille des grains. / The aim of this work is to make a full characterization of the structural, microstructural, dielectric,ferroelectric and piezoelectric properties of the perovskite-structured oxides (1-x) BaTi0.8Zr0.2O3-xBa0.7Ca0.3TiO3 prepared by a conventional solid-state reaction method (conventional sintering) andSPS fabrication technique. Using high-resolution synchrotron x-ray powder diffraction, the structuralinvestigations carried out close to the triple point (x≈ 0.32) have introduced significant corrections tothe previously published composition-temperature phase diagram. The colossal electromechanicalresponse was then correlated to a strongly degenerate free energy landscape caused by structuralinstabilities. Furthermore, the coupling between the high polarization flexibility under electric andthermal stresses and the ‘lattice softening’ gives rise to a giant electromechanical response due tohigh domain wall activities. The decrease of the dielectric, ferroelectric and piezoelectric propertiesof BCTZ ceramics (x=0.32 and 0.5) processed by SPS was essentially attributed to the largecompositional fluctuations and stable domain wall configurations as the grain size decreased.