Les systèmes d'information actuels reposent fortement sur les technologies micro-ondes utilisées pour les communications hertziennes. L'amélioration des performances des MEMS radio fréquence aptes à fonctionner dans la bande X (8 GHz et 12 GHz) est un enjeu important pour des applications de télécommunications mais aussi pour les applications radar. Pour y parvenir l'intégration de matériaux ferroélectriques à haute constante diélectrique est requise. Les matériaux qui possèdent de telles propriétés et qui sont les plus adaptés, sont les composés qui dérivent de la structure pérovskite. Intégrer ce type de matériaux dans des commutateurs radio-fréquence (MEMS-RF) pose de nouveaux chalenges en termes de maîtrise du matériau et de compatibilité avec les technologies MEMS existantes. Cette thèse s'est portée sur le composé PMN-PT avec la composition 65/35 qui possède une permittivité relative supérieure à 10000 sous forme de matériau massif.Ce travail de thèse a été consacré à l'étude de l'intégration du composé PMN-PT dans des composants passifs que sont les commutateurs MEMS. Dans la gamme de fréquence d'intérêt, de 500 MHz jusqu'à 20 GHz, les propriétés de ces matériaux ont été peu étudiées sur les matériaux massifs et encore moins sous forme de films minces. L'objectif de cette thèse était de réaliser les couches minces ferroélectriques et de tester leur compatibilité dans l'ensemble du fonctionnement d'un composant MEMS mais aussi de mener une étude réciproque : l'analyse des FeMEMS (MEMS basé sur les ferroélectriques) permettant de compléter les connaissances de ces matériaux dans cette gamme de fréquence. Ce travail est d'intérêt pour l'industrie de la technologie MEMS mais aussi pour la science des matériaux ferroélectriques mais aussi par la compréhension des mécanismes physiques gouvernant aux propriétés diélectriques en termes de pertes notamment dans ce domaine de fréquences.Les caractérisations des MEMS-RF présentées dans cette thèse ont démontré la compatibilité du MEMS PMN-PT dans la gamme de fréquence entre 500MHz et 10 GHz avec de très bonnes performances. En utilisant cette adaptation, la technologie actuelle est ainsi capable de couvrir tous les bandes de fréquence les plus importantes : la bande de télécommunication civile de 1 GHz à 5 GHz en utilisant le PMN-PT, la bande X pour les satellites entre 5 GHz et 15 GHz avec PZT et la bande de haute fréquence de 15 GHz à 40 GHz pour la défense avec les diélectriques traditionnels (Si3N4).

Single self-assembled semiconductor quantum dots (QDs) are able to emit single-photons and entangled-photons pairs. They are therefore considered as potential candidate building blocks for quantum information processing (QIP) and communication. To exploit them fully, the ability to precisely control their optical properties is needed due to several reasons. For example, the stochastic nature of their growth ends up with only little probability of finding any two or more QDs emitting indistinguishable photons. These are required for two-photon quantum interference (partial Bell-state measurement), which lies at the heart of linear optics QIP. Also, most of the as-grown QDs do not fulfil the symmetries required for generation of entangled-photon pairs. Additionally, tuning is required to establish completely new systems, for example, 87Rb atomic-vapors based hybrid semiconductoratomic (HSA) interface or QDs with significant heavy-hole (HH)-light-hole (LH) mixings. The former paves a way towards quantum memories and the latter makes the optical control of hole spins much easier required for spin- based QIP. This work focuses on the optical properties of a new type of QDs optimized for HSA experiments and their broadband tuning using strain. It was created by integrating the membranes, containing QDs, onto relaxor-ferroelectric actuators and was quantified with a spatial resolution of ~1 µm by combining measurements of the µ-photoluminescence of the regions surrounding the QDs and dedicated modeling. The emission of a neutral exciton confined in a QD usually consists of two fine-structure-split lines which are linearly polarized along orthogonal directions. In our QDs we tune the emission energies as large as ~23meV and the fine-structure-splitting by more than 90 µeV. For the first time, we demonstrate that strain is able to tune the angle between the polarization direction of these two lines up to 40° due to increased strain-induced HH-LH mixings up to ~55%. Compared to other quantum emitters, QDs can be easily integrated into optoelectronic devices, which enable, for example, the generation of non-classical light under electrical injection. A novel method to create sub-micrometer sized current-channels to efficiently feed charge carriers into single QDs is presented in this thesis. It is based on focused-laserbeam assisted thermal diffusion of manganese interstitial ions from the top GaMnAs layer into the underlying layer of resonant tunneling diode structures. The combination of the two methods investigated in this thesis may lead to new QDbased devices, where direct laser writing is employed to preselect QDs by creating localized current-channels and strain is used to fine tune their optical properties to match the demanding requirements imposed by QIP concepts.

The present research work mainly focuses on fabrication of compositionally modulated multilayers of (l−x) Pb(Mgi/3N2/3)O3 - x PbTiO3 (PMNPT) through multi target pulsed laser ablation technique. Heterostructures like compositionally varying multilayers; multilayers with graded interface and a ferroelectric [PbTiO3 (PT)] and relaxor (PMN) superlattices of different periodicities were fabricated. Role of artificially enhanced chemical heterogeneity and strain on enhancement of physical property was studied. Dimensional dependent ferroelectric and antiferroelectric type of polarization behavior was observed in the case of both compositionally varying multilayers and the superlattice structures fabricated. The dimensional dependence of various ferroelectric interactions like long-range, short-range and interfacial coupling among the layers was studied. The phase transition behavior and dielectric studies were carried out on these heterostructures. An artificial superlattice of a relaxor ferroelectric with a ferromagnetic layer was also fabricated for magnetoelectric applications. Chapter 1 provides a brief introduction to ferroelectric (FE) heterostructures, their technological applications and the fundamental physics involved in ferroelectric heterostructures. Initially an introduction to the technological importance and advantages of ferroelectric heterostructures is provided. A brief introduction to relaxor ferroelectrics and their characteristic structural features are discussed. A brief review of the ferroelectric heterostructures both from fundamental science and technological point of view is provided. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, structural, micro structural and physical property measurements is discussed. Chapter 3 addresses the problem of phase formation of PMNPT over platinum substrates and the role of the template over the phase formation, micro structural evolution and polarization behavior. The surface modifications of bare Pt under the processing conditions used to fabricate PMNPT was also studied. An intermediate roughening mechanism was observed. The role of LSCO over the micro structural evolution of PMNPT, the minimum thickness of LSCO required for phase formation of PMNPT, role of LSCO on phase formation and its effect on the polarization behavior of PMNPT of constant thickness are discussed. Chapter 4 deals with fabrication of different types of relaxor based heterostructures studied in this work. Three different types of PMNPT based heterostructures was fabricated using a multi target laser ablation chamber. The first type of heterostructure is a compositionally modulated multilayer thin film with four different compositions of (1-x) PMN - x PT (x = 0.0, 0.1, 0.2, 0.3 at.%) and is represented as PMNPT multilayer (ML) further in this thesis. PMNPT ML with different individual layer thickness was fabricated (30, 40, 60, 80, 100 and 120 nm). The second type of heterostructure is the PMNPT ML of same dimensions, but associated with a post deposition annealing to achieve a graded interface between the multilayers present and will be named as PMNPT graded or simply graded, further in this thesis. The third type of heterostructure is an artificial superlattice of a simple relaxor ferroelectric (PMN) and a normal ferroelectric (PT), which will be named as PMN-PT superlattice (SL) further in this thesis. The crystallinity, micro structural features and the nature of the interface present in the fabricated heterostructures were studied using various experimental techniques. Chapter 5 deals with the FE studies of compositionally modulated PMNPT ML thin films and PMNPT graded thin films. The ML with individual layer thickness of 120nm exhibited a clear FE behavior but with a reduced remnant polarization and reduced non linear behavior in capacitance - voltage (C-V) characteristics. But on varying the dimensions of the individual layers (30, 40, 60, 80, 100 and 120nm) a large dielectric tunability of around 74% was observed at lOOnm. The polarization behavior of these ML exhibited an interesting size dependent polarization behavior. A FE behavior was observed at low dimensions of 40 and 30nm. An AFE type of loop was observed at 60 and 80nm of individual layer thickness and at lOOnm it showed a clear paraelectric kind of behavior both in polarization hysteresis (P-E) and C-V studies. Graded films exhibited clear FE behavior at all dimensions fabricated and hence the role of interface in developing a critical polarization behavior in the case of ML was confirmed. Apart from the fundamental physics these ML and graded films permits the tunability of their physical properties on just varying the individual layer thickness. The dimensional dependence of dielectric tunability of ML and graded films were studied and it was found that in the case of a ML the dielectric tunability was high at lOOnm individual layer thickness and at 40nm in the case of a graded film. Thus the interfacial strain, interfacial coupling and chemical heterogeneity give an opportunity to engineer the physical property depending on the requirements. Chapter 6 deals with ferroelectric studies (P-E, C-V) of PMN-PT superlattice structures with different periodicities. The dimensional range in which, the interfacial coupling dominates the overall polarization behavior of the system was analyzed. A dimensional dependent FE and AFE behavior was observed in the PMN-PT SL structures. The dimensional dependent tunability of physical properties was achieved. The different interactions like short range, long range and the interfacial coupling and their dimensional dependent behavior was studied. The dimensional dependent tunability of the P-E and C-V behavior was observed both in symmetric and asymmetric SL structures. Chapter 7 deals with the relaxor behavior of the fabricated PMNPT ML, graded and PMN-PT SL structures. The dielectric phase transition of a PMNPT ML exhibited local maxima in the real part of dielectric constant with temperature. The local maxima correspond to the temperature regime at which, the individual layer dielectric maxima dominates the phase transition behavior of the ML structure. In the case of graded films an averaged behavior of all the compositions, with an enhanced diffusivity was observed. All the characteristic features of a relaxor ferroelectric were observed in the phase transition behavior of a graded thin film. The dielectric maxima exhibited a Vogel-Fulcher type of behavior with frequency, A similar averaged behavior was observed in the phase transition behavior of PMNPT ML at low dimensions (< 40 nm) of the individual layer. The dielectric phase transition behavior of PMN-PT SL structures of different periodicities was studied. No characteristic of a relaxor ferroelectric was observed for the periodicities in the range of 10 to 50 nm. At 60 nm periodicity the individual layer dominance was observed in the phase transition behavior of the SL structure. The phase transition behavior was found to be insensitive to the interfacial coupling in both the PMNPT ML and PMN-PT SL. Chapter 8 deals with the dielectric response, impedance spectroscopy and the DC leakage characteristics of the relaxor heterostructures. All the relaxor heterostructures fabricated, exhibited low frequency dispersion, similar to that of the Jonscher's universal type of relaxation behavior. The anomalous dispersion common of a relaxor ferroelectric was observed in the imaginary dielectric constant at high frequencies. A.multi debye type of relaxation behavior was observed in the impedance analysis and the relaxation time was found to obey Vogel-Fulcher type of relation with temperature. The leakage current of all the heterostructures were found to be few orders less than the homogeneous single layer thin films. A space charge limited conduction was observed in all the heterostructures fabricated. Chapter 9 deals with an attempt of realizing the magnetoelectric effect in an artificial superlattice structure consisting ferromagnetic [Lao.6Sro.4Mn03 (LSMO)] and ferroelectric (PMNPT 70-30) layers. Both symmetric and asymmetric SL structures were fabricated and the asymmetric SL exhibited both room temperature ferromagnetic and ferroelectric behavior. A weak influence of magnetic field over the polarization behavior was observed. The magnetic behavior and its influence over electrical behavior were found to be dominated by the interface and were confirmed from the Maxwell-Wagner type of relaxation. Chapter 10 gives the summary and conclusions of the present study and also discusses about the future work that could give more insight into the understanding of the relaxor heterostructures.

Fabrication of artificially structured superlattices, when controlled on a nanoscale level, can exhibit enhanced dielectric properties over a wide temperature range. Possible fabrication of new functional devices based on the parametric values of dielectric constants of these heterostructures was the major motivation behind the work. Chapter 1 gives a brief overview of ferroelectrics; their defining features and their commercial importance to electronic industry. An introduction to ferroelectric superlattices, their technological application and fundamental physics that influence the behavior of superlattices are provided. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, structural, micro structural and physical property measurements is discussed. Chapter 3 involves fabrication of two-component ferroelectric superlattices consisting of Barium Titanate (BTO), and Strontium Titanate (STO) with nanoscale control of superlattice periodicities by high-pressure multi target pulsed laser deposition on Pt (111)/Ti/SiO2/Si (100) substrate. Superlattices with varying periodicities were fabricated and their compositional variation across the thin film and the interface width were studied using Secondary Ion Mass Spectrometry (SIMS). Fabrications of superlattice structure were supported by observation of satellite peaks in XRD corresponding to the coherent heterostructures. The microstructural analysis was carried out using cross-sectional scanning electron microscopy (SEM), and contact mode-AFM was used to image surface morphology and root-mean-square (rms) roughness of the thin film heterostructure. Chapter 4 deals with ferroelectric studies of BTO/STO superlattices. The size dependent polarization behaviors of the superlattices are shown. The experimental realization of the dimensional range in which, the long-range coupling interaction dominates the overall polarization behavior of the system was studied. The dependence of average spontaneous polarization on the individual layer thickness, temperature and the dimensional range of interaction are discussed. The enhanced non-linear behaviors of the films were measured in terms of tunability. The dielectric phase transition behavior of superlattice structures of different periodicities was studied. Chapter 5 focuses on fabrication of three-component ferroelectric superlattices consisting of Barium Titanate (BTO), Calcium Titanate (CTO) and Strontium Titanate (STO). The fabrications of superlattice structures were confirmed by the presence of satellite reflections in XRD analysis and a periodic concentration of Sr, Ba and Ca throughout the film in Depth profile of SIMS analysis. The microstructural analysis was carried out using cross-sectional scanning electron microscopy (SEM), and contact mode-AFM was used to image surface morphology and root-mean-square (rms) roughness of the thin film heterostructure. The dielectric characteristic and polarization properties of the system are discussed. Large variations of lattice distortion in the consisting layers were achieved by varying the stacking sequence and superlattice periodicity. The influence of interfacial strain on enhancement of ferroelectric polarization was studied. The size dependence and the role of interfaces in the observed enhancements of the dielectric behaviors were highlighted. The tunability of about 55% was achieved in these systems and was higher than any of the single polycrystalline thin film of the constituent materials reported till date. The enhanced dielectric properties were thus discussed in terms of the interfacial strain driven polar region due to high lattice mismatch and electrostatic coupling due to polarization mismatch between individual layers. Chapter 6 deals with the dielectric response, impedance spectroscopy and the DC leakage characteristics of the superlattice structures. All the heterostructures fabricated, exhibited low frequency dispersion, similar to that of the Jonscher’s universal type of relaxation behavior. The anomalous dispersion was observed in the imaginary dielectric constant at high frequencies. A Debye type relaxation behavior was observed in the impedance analysis at low temperatures, whereas, a departure from ideal ‘Debye’ type was noticed as the temperature was increased. The leakage currents of all the heterostructures were found to be a few orders less than the homogeneous single layer thin films. A space charge limited conduction was observed in al the superlattice structures fabricated. Chapter 7 summarizes the present study and discusses about the future work that could give more insight into the understanding of the ferroelectric perovskite heterostructures.

In this dissertation, BaTiO3 nanocrystals, Bi4Ti3O12 nanostructured microspheres, and cosubstituted Bi4Ti3O12 nanoparticles and ceramics were prepared using solvothermal, hydrothermal and citrate-gel methods. The ferroelectric properties of the prepared cosubstituted Bi4Ti3O12 ceramics were studied using P–E hysteresis loop, leakage, and polarization fatigue measurements. A two-phase solvothermal synthesis approach for the preparation of hydrophobic BaTiO3 nanocrystals was developed. The two-phase method is based on the growth of nanocrystals at the oil/water interface by the reaction between metal surfactant complexes in the oil phase and a mineralizer in the water phase. Three kind of organic solvents, hexadecene, toluene, and heptane were used as the oil phase and compared to each other with respect to the product quality. The BaTiO3 particles are crystalline with a mean size of 3.7 nm and can be dispersed in a variety of organic solvents forming highly transparent dispersions. A hydrothermal method was developed for the synthesis of Bi4Ti3O12 nanostructured microspheres consisting of granular nanoparticles and nano-platelets. The precursor powder was prepared using a diethylene glycol mediated coprecipitation method. Tailoring of the morphology was achieved by changing the precursor quantity, sodium hydroxide concentration, and reaction time. The formation mechanism of the nanostructured microspheres probably involves aggregation, followed by dissolution and recrystallization. Bi3.25Pr0.75Ti2.97V0.03O12 (BPTV) and Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ferroelectric nanoparticles and ceramics were synthesized using a modified citrate-gel method that has a crystallization temperature as low as 450 °C. The synthesized nanoparticles were spherical ranging from 30 to 100 nm. Except Nb5+, other donor cations were introduced using the corresponding oxides that have advantages in terms of high purity, low cost, and availability. The Bi3.25Pr0.75Ti2.97V0.03O12 ceramic is orthorhombic and its 2Pr and 2Ec values measured at 300 kV/cm were 35 μC/cm2 and 148 kV/cm respectively. The texture, microstructure, and ferroelectric properties of the prepared Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ceramics depend on x. The maximum 2Pr (30–32 μC cm−2) was achieved at an optimum cosubstitution level (x = 0.025 for M6+, x = 0.03 for M5+). The high remanent polarization, low leakage current, and low polarization fatigue render the prepared ceramics promising for practical applications.

This thesis reports the inspection and manipulation of thin ferroelectric films of lead titanate (PbTiO3 : PTO), lead zirconium titanate (Pb(Zr0.25Ti0.75)O3 : PZT), and barium titanate (BaTiO3 : BTO) by means of scanning force microscopy - specifically Piezoresponse and Kelvin-Probe. The film thickness of the investigated samples ranged between 50nm and 800nm. This experimental work focussed on the following issues: native domain structures, creation of domains by short voltage pulses and area-switching with the force microscope, local qualitative and quantitative measurements of the ferroelectric hysteresis loops, and investigations at the interface between film and platinum-electrode in the PZT/Pt-System. Lamellar domain structures were visualized with high lateral resolution of 5nm on the surface of the PTO-samples, whereas the PZT- and BTO-samples showed prepolarisation and no domains. In the switching experiments a pronounced thickness dependence was found and partly a good agreement to macroscopic measurements. For BTO-films of 50nm and 125nm thickness no stable switching of polarisation could be observed. Using appropriate preparation methods it was possible to provide evidence of a 200nm thick interface layer with reduced polarisation above the electrode in the PZT/Pt-system. / Gegenstand dieser Arbeit ist die Untersuchung dünner ferroelektrischer Schichten von Bleititanat (PbTiO3 : PTO), Bleizirkoniumtitanat (Pb(Zr0.25Ti0.75)O3 : PZT) und Bariumtitanat (BaTiO3 : BTO) und deren Manipulation auf der sub-µm-Skalamittels Rasterkraftmikroskopie. Die Dicke der Schichten lag im Bereich von 50nm bis 800nm. Zum Einsatz kamen die Meßmodi Piezoresponse und Kelvin-Sonde. Die experimentelle Arbeit erstreckte sich über die Abbildung von Domänenstrukturen, die Erzeugung von Domänen durch kurze Spannungspulse und flächiges Umschalten mit dem Kraftmikroskop, lokale qualitative und quantitative Messungen der ferroelektrischen Hysterese, sowie Untersuchungen an der Grenzschicht zwischen Film und Platin-Elektrode am PZT/Pt-System. Lamellenartige Domänenstrukturen konnten mit hoher lateraler Auflösung von 5nm auf der Oberfläche von PTO abgebildet werden. Die PZT- und BTO-Proben waren vorpolarisiert und es ließen sich keine Domänen nachweisen. Bei den Schaltversuchen wurde eine ausgeprägte Schichtdickenabhängigkeit der Koerzitivfeldstärken und teilweise gute Übereinstimmung mit makroskopischen Messungen gefunden. Für dünne BTO-Schichten von 50nm und 125nm Dicke konnte kein stabiles Umschalten der Polarisation gezeigt werden. Mittels geeigneter Präparation der PZT/Pt-Grenzschicht konnte durch direkte Messung eine Schicht von 200nm Dicke mit verminderter Polarisation oberhalb der Elektrode nachgewiesen werden.

Das Ziel dieser Arbeit ist die optische Untersuchung von ferroelektrischen Domänen und Domänenwänden auf lokaler Skala. Dafür wurden neuartige nichtinvasive Ansätze entwickelt, die auf der Anwendung optischer Rastersondenmikroskopie basieren. Die untersuchten Schlüsseleigenschaften umfassen den elektrooptischen Effekt für verschiedene Domänenorientierungen und die Brechungindexänderungen an Domänenwänden an Bariumtitanat-Einkristallen. Die lokale Messung der elektrooptischen Eigenschaften wurde mit räumlich stark begrenzten elektrischen Feldern durchgeführt, die mittels elektrisch leitfähigen Spitzen angelegt wurden. Dieser experimentelle Ansatz erlaubt nicht nur die Messung verschiedener elektrooptischer Koeffzienten, sondern auch die Unterscheidung von allen auftretenden, auch antiparallelen, Domänenausrichtungen. Durch Anlegen eines zusätzlichen elektrischen Feldes mittels der gleichen Spitze konnte auch das ferroelektrische Schalten mit dieser optischen Methode untersucht werden. Die Experimente wurden durch eine numerische Modellierung der elektrischen Feldverteilung und der resultierenden elektrooptischen Antwort begleitet. Die Ergebnisse der Modellierung sind dabei in sehr guter Übereinstimmung mit den experimentellen Ergebnissen. Dies erlaubt auch die Trennung von Beiträgen verschiedener elektrooptischer Koeffzienten und den entsprechenden Feldkomponenten. ür die experimentelle Untersuchung von den theoretisch vorhergesagten Brechungsindexprofilen einzelner Domänenwände, wurde die Sensitivität der optischen Sonde auf lokale Änderungen des Brechungsindex mittels Polarisations- und Positionsmodulation erhöht. Obwohl die Abbildung einer einzelnen Domänenwand nicht gelang, konnte damit zumindest eine obere Grenze für den optischen Effekt einer Domänenwand experimentell gewonnen werden, welche verträglich mit den theoretischen Vorhersagen ist. / The goal of this thesis is the optical investigation of ferroelectric domains and domain walls at the very local scale. For that, novel noninvasive approaches based on optical scanning probe microscopy are developed. The key properties investigated are the electrooptic effect for different domain orientations and refractive-index changes at single domain walls of barium titanate single crystals. The local probing of the electro-optic response is performed with strongly confined electric fields, applied via a conductive tip. With this approach we can not only probe different electro-optic coeffcients, but also identify all occurring domain orientations, even antiparallel ones. The application of additional bias fields by the same tip is used to investigate ferroelectric switching and domain growth by optical means. The experiments are supported by numerical modelling of the electric-field distribution and the resulting electro-optic response. The modelling shows excellent agreement with the measurements, and allows us to separate the contributions of different electro-optic coeffcients and their associated electric-field components. For the experimental observation of the theoretically predicted refractive-index profiles at single ferroelectric domain walls, polarization and position modulation of the optical probe is used to obtain high sensitivity to local modifications of the refractive index. An upper limit to the optical effect to the optical effect of a single domain wall is deduced from the experiment, which is compatible with the effect predicted by theory.

Στην παρούσα εργασία αναπτύχθηκαν και μελετήθηκαν πειραματικά σειρές σύνθετων υλικών πολυμερικής μήτρας, με παράμετρο τον τύπο και την περιεκτικότητα σε ενισχυτική φάση. Ως μήτρα χρησιμοποιήθηκε εποξειδική ρητίνη υψηλών προδιαγραφών. Ως ενισχυτική φάση χρησιμοποιηθήκαν μικροσωματίδια, νανοσωματίδια τιτανικού βαρίου και αποφλοιωμένα γραφιτικά νανοεπίπεδα (exfoliated graphite nanoplatelets). Η επιλογή των υλικών είχε ως στόχο να εκμεταλλευτούν σε κοινό σύνθετο σύστημα οι «θετικές» ιδιότητες των συστατικών του, όπως η θερμο-μηχανική σταθερότητα της μήτρας, η υψηλή διαπερατότητα και η σιδηροηλεκτρική συμπεριφορά του τιτανικού βαρίου και οι καλές μηχανικές ιδιότητες μαζί με την υψηλή ειδική αγωγιμότητα των αποφλοιωμένων γραφιτικών νανοεπιπέδων. Παρασκευάστηκαν και μελετήθηκαν τα παρακάτω συστήματα σύνθετων υλικών, για διάφορες περιεκτικότητες σε ενισχυτική φάση: (α) σύστημα μικροσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (β) σύστημα νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (γ) σύστημα αποφλοιωμένων γραφιτικών νανοεπιπέδων/εποξειδικής ρητίνης, (δ) υβριδικό σύστημα μικροσωματιδίων τιτανικού βαρίου/νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης, (ε) υβριδικό σύστημα αποφλοιωμένων γραφιτικών νανοεπιπέδων/ νανοσωματιδίων τιτανικού βαρίου/εποξειδικής ρητίνης. Την παρασκευή των δοκιμίων ακολούθησε πολύπλευρος χαρακτηρισμός τους. Για λόγους αναφοράς παρασκευάστηκε και μελετήθηκε και δοκίμιο μη ενισχυμένης ρητίνης. Η μορφολογία τους διερευνήθηκε με την τεχνική της ηλεκτρονικής μικροσκοπίας σάρωσης (scanning electron microscopy) και την τεχνική σκέδασης ακτίνων-Χ (x-ray diffraction scattering). Διαπιστώθηκε η επιτυχής διασπορά των νανο-εγκλεισμάτων αλλά και η ύπαρξη μικρών συσσωματωμάτων. Τα φάσματα σκέδασης ακτίνων-Χ πιστοποίησαν την παρουσία των πληρωτικών μέσων που χρησιμοποιήθηκαν σε κάθε κατηγορία σύνθετου συστήματος. Ακολούθησε θερμικός χαρακτηρισμός των σύνθετων υλικών, με στόχο τον προσδιορισμό της θερμοκρασίας υαλώδους μετάπτωσής τους. Η μελέτη της μηχανικής συμπεριφοράς των συνθέτων έγινε υπό στατικές και δυναμικές συνθήκες. Η στατική συμπεριφορά εξετάστηκε με την τεχνική κάμψης τριών σημείων σε θερμοκρασία περιβάλλοντος. Διαπιστώθηκε αύξηση του μέτρου ελαστικότητας με την περιεκτικότητα σε ενισχυτική φάση, σε όλες τις κατηγορίες σύνθετων συστημάτων. Παράλληλα, διαπιστώθηκε μείωση της μηχανικής αντοχής με τη συγκέντρωση πληρωτικού μέσου σε όλες τις κατηγορίες σύνθετων υλικών που μελετήθηκαν. Η δυναμική μηχανική απόκριση μελετήθηκε με την τεχνική της δυναμικής θερμικής ανάλυσης (dynamic mechanical thermal analysis) σε ευρύ φάσμα θερμοκρασιών. Τα ενισχυμένα συστήματα παρουσιάζουν αυξημένες τιμές του μέτρου αποθήκευσης, ενώ οι κορυφές της εφαπτομένης απωλειών επιτρέπουν τον προσδιορισμό της θερμοκρασίας υαλώδους μετάπτωσης (Tg). Η Tg φαίνεται να διαφοροποιείται ελαφρά με την περιεκτικότητα σε ενισχυτική φάση, άλλοτε προς μεγαλύτερες και άλλοτε προς μικρότερες τιμές. Οι διαφοροποιήσεις αυτές εκφράζουν τις αλληλεπιδράσεις μεταξύ των φάσεων και ίσως την πλήρη ή μη διαβροχή των εγκλεισμάτων από τη μήτρα. Η ηλεκτρική απόκριση των σύνθετων συστημάτων εξετάστηκε με τη μέθοδο της διηλεκτρικής φασματοσκοπίας ευρέως φάσματος, σε μεγάλο εύρος συχνοτήτων και θερμοκρασιών. Η ανάλυση των πειραματικών δεδομένων έγινε μέσω των φορμαλισμών της ηλεκτρικής διαπερατότητας, του ηλεκτρικού μέτρου και της ειδικής αγωγιμότητας εναλλασσομένου. Η χρήση και των τριών φορμαλισμών προσφέρει τη δυνατότητα εξαγωγής περισσότερων πληροφοριών για τις φυσικές διεργασίες που λαμβάνουν χώρα στο εσωτερικό των συνθέτων. Διαπιστώθηκε η παρουσία δύο διηλεκτρικών χαλαρώσεων που σχετίζονται με την πολυμερική μήτρα. Αυτές αποδίδονται, στη μετάπτωση από την υαλώδη στην ελαστομερική φάση της εποξειδικής ρητίνης (α-χαλάρωση) και στην επαναδιευθέτηση πλευρικών πολικών ομάδων (β-χαλάρωση). Η παρουσία των εγκλεισμάτων στο εσωτερικό της μήτρας εισάγει ηλεκτρική ετερογένεια με αποτέλεσμα την εμφάνιση του φαινομένου διεπιφανειακής πόλωσης (interfacial polarization). Μη δέσμια φορτία συσσωρεύονται στη διεπιφάνεια των φάσεων, όπου σχηματίζουν μεγάλα δίπολα που παρουσιάζουν αδράνεια ως προς τον προσανατολισμό τους, παράλληλα του εφαρμοζόμενου πεδίου. Η διεπιφανειακή πόλωση είναι η πλέον αργή διεργασία και παρατηρείται σε χαμηλές συχνότητες και υψηλές θερμοκρασίες. Το πραγματικό μέρος της ηλεκτρικής διαπερατότητας, όπως και η ειδική αγωγιμότητα παρουσίασαν αύξηση με την περιεκτικότητα σε ενισχυτική φάση, ιδιαίτερα στην περίπτωση των συστημάτων με γραφιτικά νανοεπίπεδα. Η δυνατότητα αποθήκευσης ενέργειας στα συστήματα διερευνήθηκε με χρήση της πυκνότητας ενέργειας υπό σταθερό ηλεκτρικό πεδίο. Διαπιστώθηκε αύξηση της αποθηκευόμενης ενέργειας με αύξηση της περιεκτικότητας σε ενισχυτική φάση. Τη βέλτιστη συμπεριφορά επέδειξε το σύστημα με τη μέγιστη περιεκτικότητα σε γραφιτικά νανοεπίπεδα. Η δυναμική των χαλαρώσεων μελετήθηκε μέσω διαγραμμάτων Arrhenius, από τα οποία προέκυψαν και οι τιμές της ενέργειας ενεργοποίησης. Η θερμοκρασιακή γειτνίαση των διεργασιών της α-χαλάρωσης και της διεπιφανειακής πόλωσης οδήγησε σε αλληλοεπικάλυψη των διεργασιών. Από τις ενέργειες ενεργοποίησης που υπολογίστηκαν φαίνεται πως στο δοκίμια της μη ενισχυμένης ρητίνης επικρατεί η συνεισφορά της α-χαλάρωσης, ενώ στα σύνθετα συστήματα επικρατεί η συνεισφορά της διεπιφανειακής πόλωσης. Τα σωματίδια του τιτανικού βαρίου υφίστανται δομικό μετασχηματισμό από την πολική τετραγωνική δομή (σιδηροηλεκτρική φάση) στην μη-πολική κυβική δομή (παραηλεκτρική φάση) σε μία κρίσιμη θερμοκρασία, πλησίον των 130οC. Η μετάβαση αποδείχθηκε μέσω των φασμάτων ακτίνων-Χ και είναι περισσότερο έντονη στην περίπτωση των μικροσωματιδίων. Η λειτουργική συμπεριφορά των συστημάτων σχετίζεται με τη θερμικά διεγειρόμενη δομική μετάβαση από τη σιδηροηλεκτρική στην παραηλεκτρική φάση των εγκλεισμάτων τιτανικού βαρίου, τη μεταβολή του προσήμου του θερμοκρασιακού συντελεστή ειδικής αγωγιμότητας και τη δυνατότητα αποθήκευσης ενέργειας. Η συνύπαρξη σε κοντινές θερμοκρασίες των διεργασιών α-χαλάρωσης και διεπιφανειακής πόλωσης μαζί με την κρίσιμη θερμοκρασία μετάβασης των σιδηροηλεκτρικών εγκλεισμάτων, δυσχεραίνει πολύ την διάκρισή τους. Με την εισαγωγή της διηλεκτρικής συνάρτησης ενίσχυσης (dielectric reinforcing function) έγινε δυνατός ο διαχωρισμός των φαινομένων. Επιπλέον, η συνάρτηση διηλεκτρικής ενίσχυσης προσφέρει τη δυνατότητα εξέτασης της λειτουργικής συμπεριφοράς και της δυνατότητας αποθήκευσης ενέργειας, ανεξάρτητα των γεωμετρικών διαστάσεων του υλικού. Τέλος, το σύνολο των αποτελεσμάτων έγινε αντικείμενο συγκρίσεων και συζήτησης. / In this study, series of polymer matrix composite materials were developed and experimentally studied, varying the reinforcing phase content. The employed matrix was a high tech epoxy resin, while reinforcing phase was micro- and/or nano-barium titanate particles, as well as exfoliated graphite nanoplatelets. The choice of the materials was targeting to take advantage in a common composite system of the thermo-mechanical stability of the matrix, the high dielectric permittivity and the ferroelectric behaviour of barium titanate and the enhanced mechanical properties in tandem with the high conductivity of the exfoliated graphite nanoplatelets. The following composite materials systems were fabricated and studied, for various filler contents: (a) barium titante micro-particles/epoxy resin composite system, (b) barium titante nano-particles/epoxy resin composite system, (c) exfoliated graphite nanoplatelets/epoxy resin composite system, (d) barium titante micro-particles/barium titante nano-particles /epoxy resin hybrid composite system, (e) exfoliated graphite nanoplatelets /barium titante nano-partcles /epoxy resin hybrid composite system. The fabrication of the composites was followed by a multiple characterization of the produced specimens. For reference reasons pure resin was also prepared and studied. Systems’ morphology was investigated by means of scanning electronic microscopy and x-ray diffraction scattering. It was ascertained the existence of fine nanodispersions, as well as of small clusters, within the composites. XRD spectra verified the presence of filler in each category of composite systems. Thermal characterization was conducted via differential scanning calorimetry aiming to determine the glass to rubber transition temperature of all studied systems. Mechanical behaviour was investigated under static and dynamic conditions. Static behaviour was determined via three point bending tests at ambient temperature. It was found that modulus of elasticity increases with filler content in all composite systems categories. On the other hand, mechanical strength decreases with filler content. Dynamic response was studied by means of dynamic mechanical thermal analysis in a wide temperature range. Reinforced systems exhibit higher values of storage modulus, while the loss tangent peaks allow the determination of the glass transition temperature Tg. Tg slightly varies with reinforcing phase content, to higher or lower values depending on the type and the amount of filler concentration. These variations express the interactions between the phases of the composites and possibly the uncompleted wetting of the inclusions in some cases. The electrical response of the composite systems was examined by means of broadband dielectric spectroscopy in a wide frequency and temperature range. The analysis of the experimental data was carried out via the dielectric permittivity, electric modulus, and ac conductivity formalisms. The usage of all three formalisms provides the opportunity to extract more information concerning the physical mechanisms occurring within the composites. It was found that two dielectric processes are related to the polymer matrix. These are attributed to the glass to rubber transition of epoxy resin (α-relaxation) and to the re-arrangement of polar side groups of the main polymer chain (β-relaxation). The presence of inclusions within the matrix introduces electrical heterogeneity resulting in the occurrence of interfacial polarization. Unbounded charges accumulate at the interface of the phases, forming large dipoles, which exhibit inertia in orienting themselves parallel to the applied field. Interfacial polarization is the slowest process in the systems and thus it is observed at low frequencies and high temperatures. The real part of dielectric permittivity, as well as, the conductivity increase with reinforcing phase content, especially in the case of the systems with graphite nanoplatelets. The energy storage efficiency was investigated via the density of energy, at constant electric field. It was found that the energy storage capability increases with filler content. Optimum behaviour is displayed by the system with maximum content in graphite nanoplatelets. The dynamics of the relaxations was studied via Arrhenius graphs, from which the values of activation energy were calculated. Interfacial polarization and α-relaxation appear in adjacent temperature ranges, leading in a superposition of both processes. From the calculated values of activation energy it is concluded that in the pure resin specimen the dominating contribution is related to the α-relaxation, while in the composite systems the contribution of interfacial polarization seems to prevail. Barium titanate particles undergo a structural transition from the polar tetragonal structure (ferroelectric phase) to the non-polar cubic structure (paraelectric phase) at a critical temperature closed to 130oC. This transition was proved via XRD spectra and is more intense in the case of barium titanate microparticles. Systems’ functional behaviour is related to the thermally stimulated structural transition from the ferroelectric to the paraelectric phase of barium titanate inclusions, to the change of sign of the temperature coefficient of conductivity, and their ability for energy storage. The coexistence at adjacent temperatures ranges of α-relaxation and interfacial polarization, as well as the critical transition temperature of ferroelectric inclusions, hampers the discrimination of the effects. By introducing the dielectric reinforcing function the discrimination of the processes became possible. Furthermore, the dielectric reinforcing function provides the possibility to examine the functional behaviour and the energy storage efficiency of the systems, neglecting the materials’ geometrical characteristics influence. Finally, experimental results and analysis are compared and discussed.

Piezokeramischen Werkstoffe auf der Basis von Bleizirkonat - Titanat (PZT) zeigen Extremwerte der elektromechanischen Eigenschaften im morphotropen Phasenübergangsbereich. Durch Modifikation des Verhältnisses von rhomboedrischer und tetragonaler Phase im Gefüge können die piezoelektrischen Eigenschaften des Werkstoffs entsprechend den jeweiligen Anforderungen angepasst werden. Es wurde eine Methode vorgestellt, einen mathematisch kohärenten Satz piezoelektrischer Kleinsignalkoeffizienten vollständig und mit hoher Genauigkeit über einen breiten Temperatur-(-200°C...+200°C) und Zusammensetzungsbereich (0...1 rh/tet) zu bestimmen. Desweiteren wurden die piezoelektrischen Eigenschaften dem Phasenanteil im Gefüge zugeordnet. / Piezoceramic materials based on Lead- Zirconate- Titanate (PZT) show extreme electromechanic properties in the area of morphotropic phase transition. PZT materials can be tailored to specific demands by modifying the ratio of volume of the rhombohedral and tetragonal phase within the micro structure. A method was introduced to accurately determine a complete and mathematically coherent set of piezoelectric small signal coefficients. This was done over a wide range of temperature (-200°C…+200°C) and phase composition (0…1 rh/tet). Additionally, the piezoelectric properties were correlated to the ratio of rhombohedral and tetragonal phases.

La technique d’optique ultra-rapide pompe-sonde, qui repose sur l’emploi de lasers à impulsion ultracourte(femtoseconde), permet de déclencher et étudier des processus ultrarapides dans la matière. L’acoustique picoseconde concerne pour sa part l’étude des processus de génération et détection de phonons acoustiques haute fréquence ainsi quel’analyse des nanomatériaux avec ces phonons (nanoéchographie). Les travaux de recherche de cette thèse avaient pourbut l’étude des couplages électronphonon acoustique dans le matériau ferroélectrique BiFeO3 par acoustique ultrarapide. Nous avons pu mettre en évidence que selon l’orientation du cristal photoexcité, l’émission des phonons acoustiques cohérents longitudinaux (LA) et transverses (TA) pouvait être modulée. De manière spectaculaire, nous avons purévéler un couplage électron-phonon acoustique transverse très efficace comme cela n’avait jamais été observé jusqu’alors dans les métaux, semiconducteurs ou nanostructures artificielles. Une étude détaillée indique que le mécanismepiézoélectrique inverse semble être le moteur de ce couplage électron-phonon (Lejman et al, Nature Communications, 2014). Dans une seconde partie, nous avons montré que BFO, ainsi qu’un autre ferroélectrique biréfringent LiNbO3 (LNO), peuvent être utilisés pour la conversion de mode ultra-rapide par processus acousto-optique (manipulation de la polarisation de la lumière à l’échelle de la picoseconde avec des phonons acoustiques). Cet effet, jamais mis enévidence jusqu’alors dans le domaine GHz, pourrait potentiellement être exploité dans de nouveaux dispositifs photoniques/phononiques pour des modulations acousto-optiques à haute cadence. / Ultrafast optical pump-probe technique, by exploiting ultrashort laser pulses (femtosecond), allows to initiate and monitor ultrafast processes in matter. Picosecond acoustics is a research field that focuses on the generation and detection mechanisms of high frequency coherent acoustic phonons in different media, as well as on their application in testing of nanomaterials and nanostructures. This PhDs research project was devoted to study of electron-acoustic phonon coupling in ferroelectric BiFeO3 (bismuth ferrite, BFO) by ultrafast acoustics. We have evidenced that depending on the BFO crystal orientation it was possible to tune the coherent phonons spectrum with in particular variable amplitude of longitudinal (LA) and transverse (TA) acoustic modes. In some grains with particular crystallographic orientations much stronger TA than LA signal was observed. Spectacularly, we have revealed an efficient coupling between electron and transverse acousticphonon. Such high ratio never reported before in any metal, semiconductor or nanostructure before, can be principally attributed to the photoinduced inverse piezoelectric effect (Lejman et al Nature Communications 2014). In a second part, we have shown that BFO as well as another birefringent ferroelectric LiNbO3 (LNO) can be used for ultrafast acousto-optic modeconversion (manipulation of light polarization at the picosecond time scale with coherent acoustic phonons). This effect, never reported at GHz up to now, can be potentially applied in photonics for ultrafast manipulation of light polarization bycoherent acoustic phonons in next generation photonic/phononic devices.