Neuron is the basic computing unit in brain-inspired neural networks. Although a multitude of excellent artificial neurons realized with conventional transistors have been proposed, they might not be energy and area efficient in large-scale networks. The recent discovery of ferroelectricity in hafnium oxide (HfO₂) and the related switching phenomena at the nanoscale might provide a solution. This study employs the newly reported accumulative polarization reversal in nanoscale HfO₂-based ferroelectric field-effect transistors (FeFETs) to implement two key neuronal dynamics: the integration of action potentials and the subsequent firing according to the biologically plausible all-or-nothing law. We show that by carefully shaping electrical excitations based on the particular nucleation-limited switching kinetics of the ferroelectric layer further neuronal behaviors can be emulated, such as firing activity tuning, arbitrary refractory period and the leaky effect. Finally, we discuss the advantages of an FeFET-based neuron, highlighting its transferability to advanced scaling technologies and the beneficial impact it may have in reducing the complexity of neuromorphic circuits.

Spontaneous Raman spectroscopy (SR) is a versatile method for analysis and visualization of ferroelectric crystal structures, including domain walls. Nevertheless, the necessary acquisition time makes SR impractical for in situ analysis and large scale imaging. In this work, we introduce broadband coherent anti-Stokes Raman spectroscopy (B-CARS) as a high-speed alternative to conventional Raman techniques and demonstrate its benefits for ferroelectric domain wall analysis. Using the example of poled lithium niobate, we compare the spectral output of both techniques in terms of domain wall signatures and imaging capabilities. We extract the Raman-like resonant part of the coherent anti-Stokes signal via a Kramers–Kronigbased phase retrieval algorithm and compare the raw and phase-retrieved signals to SR characteristics. Finally, we propose a mechanism for the observed domain wall signal strength that resembles a Cerenkov-like behavior, in close analogy to domain wall signatures obtained by secondharmonic generation imaging.We, thus, lay here the foundations for future investigations on other poled ferroelectric crystals using B-CARS.

In dieser Arbeit werden ferroelektrische Phasenübergänge in verspannten (K,Na)NbO3-Schichten erstmals mit Hilfe temperaturabhängiger dreidimensionaler Röntgenbeugung untersucht. Der Fokus liegt auf stark anisotrop verspannten Dünnschichten, die bei Raumtemperatur ein geordnetes Fischgräten-Domänenmuster mit einer periodischen Anordnung von monoklinen a1a2/MC-Phasen aufweisen. Bei Erhöhung der Temperatur durchlaufen die (K,Na)NbO3-Dünnschichten einen ferroelektrischen Phasenübergang in die orthorhombische Hochtemperaturphase, die sich durch regelmäßige, alternierenden a1/a2-Streifendomänen mit ausschließlich lateraler Polarisation auszeichnet. In-plane Röntgenmessungen zeigen, dass die Filmeinheitszellen eine kleine Verzerrung in der Ebene erfahren, was zur Bildung von vier verschiedenen Einheitszellvarianten und damit vier verschiedenen (Super-)Domänenvarianten führt. Durch den Vergleich von Röntgenbeugungsmessungen verschiedener Bragg-Reflexe an Filmen mit unterschiedlicher Schichtdicke ist es möglich, die spezifischen Beugungsmerkmale zu unterscheiden und sie den einzelnen Phasen zuzuordnen. Mit Hilfe von in situ temperaturabhängiger Röntgenbeugung ist es daher möglich, die Details des Phasenübergangs vom Fischgräten in das Streifen-Domänenmuster aufzudecken. Es zeigt sich, dass dieser sich über einen großen Temperaturbereich erstreckt und in mehreren Schritten vollzieht. Die Beobachtung von Phasenkoexistenz innerhalb des Übergangs und einer thermischen Hysterese in der Phasenübergangstemperatur lassen auf einen Phasenübergang erster Art schließen. Zudem hängt die Phasenübergangstemperatur stark von der Kaliumkonzentration x in der KxNa1-xNbO3-Dünnschicht ab und kann durch eine Änderung von x=0,95 (stärker kompressiv verspannt) auf x=0,8 (stärker tensil verspannt) um etwa 60 K erhöht werden. Darüber hinaus ist dies die erste Studie, die experimentell beobachtete dreidimensionale Domänenanordnungen direkt mit Berechnungen aus Phasenfeldsimulationen vergleicht. / In this work, ferroelectric phase transitions in strained (K,Na)NbO3 films are studied for the first time using in situ temperature-dependent three-dimensional X-ray diffraction. The focus lies on strongly anisotropically strained thin films, which exhibit a well-ordered herringbone domain pattern with a periodic arrangement of monoclinic a1a2/MC phases at room temperature. Upon increasing temperatures, the (K,Na)NbO3 thin films undergo a ferroelectric phase transition to the orthorhombic high-temperature phase, which is characterized by a regular pattern of alternating a1/a2 stripe domains with pure lateral polarization. In-plane X-ray measurements show that the film unit cells undergo a small in-plane distortion, leading to the formation of four different unit cell variants and thus four different (super)domain variants. By comparing X-ray diffraction measurements of different Bragg reflections of films with different film thicknesses, it is possible to distinguish the specific diffraction features and assign them to the individual phases observed at the different temperatures. Using in situ temperature-dependent X-ray diffraction, it is therefore possible to reveal the details of the phase transition from the a1a2/MC herringbone to the a1/a2 stripe domain pattern. It is shown to extend over a wide temperature range and to occur in several steps. The observation of phase coexistence within the transition and a thermal hysteresis in the phase transition temperature suggests a first-order type phase transition. Moreover, the phase transition temperature strongly depends on the molar concentration of potassium x in the KxNa1-xNbO3 thin film and can be increased by about 60 K by changing x=0.95 (more compressively strained) to x=0.8 (more tensile strained). Furthermore, this is the first study to directly compare experimentally observed three-dimensional domain arrangements with calculations from phase field simulations.

Les matériaux ferroélectriques suscitent beaucoup d’intérêt du fait de leurs propriétés physiques telles que la piézoélectricité, la ferroélectricité ou encore leur permittivité élevée. Ainsi, on cherche à les intégrer dans les micro- et nano-systèmes dans lesquels on les retrouve généralement sous forme de couche mince dans une configuration de type capacité plane. En particulier, l’oxyde de plomb, titane et zirconium (PZT) est un matériau très attractif pour les applications RF capacitives du fait de sa grande permittivité. Son intégration sur des électrodes métalliques, i.e. les lignes coplanaires constituant le guide d’onde, implique de maîtriser sa croissance en film mince. L’application d’une tension dans un dispositif RF actif impose également de contrôler les propriétés électriques : nature des courants de fuite et comportement ferroélectrique du PZT. Dans ce contexte, les couches minces de PZT sont déposées par ablation laser (PLD) sur un empilement La₀.₆₇Sr₀.₃₃MnO₃ (LSMO) / Pt (111) déposé sur un substrat monocristallin de saphir. La couche d’accroche conductrice LSMO est nécessaire afin d’éviter la formation d’une phase pyrochlore paraélectrique. Le contrôle de l’orientation cristalline de la couche de LSMO permet de contrôler la texturation de la couche de PZT. Les courants de fuite au travers de l’empilement Pt/PZT/LSMO/Pt ont ensuite été étudiés dans l’intervalle de température 220-330K de façon à déterminer les mécanismes de conduction. Une transition a été mise en évidence entre, autour de la température ambiante, un mécanisme contrôlé par la diffusion des charges en volume et, à basse température, un mécanisme contrôlé par l’injection des charges aux interfaces électrode/PZT. Un mécanisme par sauts a été identifié au-dessus de 280K en cohérence avec la présence de défauts étendus et la structure colonnaire du PZT. Afin de contrôler ces courants de fuite, différentes stratégies ont été utilisées. La première consiste à insérer une couche d’oxyde isolante à l’interface supérieure Pt/PZT modifiant ainsi l’injection des charges et permettant de réduire les courants de fuite. La seconde stratégie consiste, quant à elle, à modifier la structure de la couche de PZT en volume en élaborant des composites diélectrique/PZT multicouches ou colonnaires. Ainsi, une couche d’oxyde isolante a été insérée au milieu de la couche de PZT et a permis de réduire les courants de fuite. Le contrôle de la nucléation du PZT a également permis par nanofabrication d’élaborer un composite colonnaire pérovskite PZT/pyrochlore. La densité de piliers de pyrochlore dans la phase ferroélectrique permet de moduler la densité de courant dans la structure. Le PZT et les hétérostructures permettant de réduire les courants de fuite ont ensuite été intégrés dans une structure RF capacitive avec des lignes coplanaires d’or. Les performances RF en termes d’isolation et de pertes par insertion ainsi que la compatibilité de ces différents matériaux ont été étudiées et ont montré que les solutions développées dans le cadre du contrôle des courants de fuite sont prometteuses pour être intégrées dans les dispositifs RF capacitifs. En outre, on a cherché à extraire la permittivité à haute fréquence du PZT lorsque celui-ci est inséré dans une structure capacitive. Cette étude a notamment permis de mettre en évidence les points techniques à modifier concernant la structure du dispositif afin de parvenir à exploiter les propriétés physiques du PZT à haute fréquence. / Ferroelectric materials are raising a lot of interest due to their physical properties such as piezoelectricity, ferroelectricity or high dielectric constant. Thus, they are generally integrated in micro- and nano-systems as thin films in a capacitive configuration. Especially, the lead zirconate titanate oxide (PZT) is an attractive material for capacitive RF applications due to its high dielectric constant. The growth of the PZT thin film has to be controlled on metallic electrodes for its integration on coplanar transmission lines. Moreover, electrical properties such as leakage current and ferroelectric behavior of PZT have to be monitored upon application of a dc voltage bias for RF device operation. In this context, PZT thin films were grown by the pulsed laser deposition technique (PLD) on a La₀.₆₇Sr₀.₃₃MnO₃ (LSMO) / Pt (111) electrode on a monocrystalline sapphire substrate. The LSMO buffer layer is mandatory to avoid the formation of the paraelectric pyrochlore phase. The control of the crystalline orientation of the LSMO layer allows for the control of the PZT layer texture. Leakage currents through the Pt/PZT/LSMO/Pt stack were then studied in the 220-330K temperature range to determine the conduction mechanisms. A transition is evidenced between a bulk-controlled mechanism near room temperature and an interface-controlled mechanism at low temperature. A hopping mechanism is identified above 280K in line with the presence of extended defects and the columnar structure of the PZT layer. Several strategies were tested to control leakage currents. The first one consists in inserting an insulating oxide layer at the top Pt/PZT interface. In this way, charge injection was modified and leakage currents were reduced. The second strategy consists in changing the PZT layer bulk structure by elaborating a layered or columnar dielectric/PZT composite. Thus, an insulating oxide layer was inserted in the middle of the PZT layer and permitted to reduce leakage currents. Moreover, the control of the PZT nucleation allowed for the elaboration of a columnar PZT/pyrochlore composite. The leakage currents in this composite can be tuned through the pyrochlore pillars density among the ferroelectric matrix. Then, PZT and the heterostructures for leakage current control were integrated in a capacitive RF structure with gold coplanar transmission lines. RF performances in terms of isolation and insertion loss of these materials were studied and gave good results. In particular the heterostructures developed to control the leakage currents are promising for their integration in capacitive RF devices. Besides, I tried to extract the permittivity of PZT at high frequency with the PZT layer in a capacitive configuration. This study highlighted the essential modifications of the capacitive structure that have to be made in order to be able to exploit PZT properties at high frequency.

Les matériaux ferroélectriques sont caractérisés par l'existence d'une polarisation électrique spontanée, dont l'orientation peut être inversée par l'application d'un champ électrique adéquat. Permettant de coder l'information sous la forme d'un domaine ferroélectrique, i.e. une région du matériau avec une certaine orientation de la polarisation, les ferroélectriques ouvrent la voie au stockage de masse de très haute densité (>10 Tbit/in ²). Dans ce contexte, nous avons employé la Piezoresponse Force Microscopy (PFM), un mode particulier de Microscope à Force Atomique (AFM), permettant la manipulation et la détection des domaines ferroélectriques à l'échelle du nanomètre. Avec pour objectif d'étudier les mécanismes de formation des domaines par l'intermédiaire d'une pointe AFM, nos travaux ont mis en valeur la cinétique de croissance des domaines dans des films minces monocristallins de LiTaO3, avec une approche complémentaire de celle thermodynamique, dépendante du champ électrique et soulignant le rôle de l'humidité dans une possible conduction de surface. En parallèle, les films de LiTaO3 ont permis d'appréhender davantage la nature électro-mécanique de la réponse PFM, pour notamment relier l'amplitude du signal mesuré à la géométrie du domaine sous pointe. PFM et domaines ferroélectriques se sont en effet révélés tour à tour, objet d'étude et outil de caractérisation. / Ferroelectric materials are characterized by their spontaneous polarization, whose direction can be reversed by the application of a suitable electric field. Using domains, i.e. regions of uniform polarization orientation, as information bits, ferroelectrics opens the pathway towards ultrahigh storage densities (>10 Tbit/in²). In this respect, Piezoresponse Force Microscopy (PFM), a technique derived from Atomic Force Microscopy (AFM), was used to manipulate and detect ferroelectric domains on the nanometer scale. Our study was focused on the domains formation mechanism in the local electric field of a nanosized tip. Within an approach complementary to the thermodynamic one, we underlined the kinetics of domains growth in single-crystal LiTaO3 thin films, and the role of humidity in a possible surface conduction. In parallel, the LiTaO3 thin films were used to better understand the PFM response, in particular the relation between the measured signal and the geometry of the domain below the tip. This way, PFM and ferroelectrics domains alternately appeared as object of study and characterization tool.

Die Ferroelektrische Lithografie ist ein im letzten Jahrzehnt entwickeltes Verfahren zur gezielten Steuerung des Aufbaus von Nanostrukturen auf ferroelektrischen Oberflächen. Hierbei wird ausgenutzt, dass die unterschiedlich orientierte Spontanpolarisation des Materials in den einzelnen Domänen zu einer charakteristischen Variation der Oberflächenchemie führt. Die vorliegende Dissertation behandelt die Umsetzung dieses Ansatzes zur gezielten und steuerbaren Deposition von Nanostrukturen aus Edelmetallen oder organischen Molekülen. Diese Deposition erfolgte mittels einer nasschemischen Prozessierung unter UV-Beleuchtung auf magnesiumdotierten, einkristallinen Lithiumniobat-Substraten. Als typisches Ergebnis zeigte sich sowohl für in Wasser gelöste Silber-, Gold- und Platinsalze als auch für wässrige Lösungen des organischen Fluoreszenzfarbstoffs Rhodamin 6G eine bevorzugte Abscheidung des Materials an den 180°-Domänenwänden auf der Substratoberfläche. Dabei beginnt die Abscheidung in Form einzelner Nanopartikel innerhalb eines 150−500 nm breiten Streifens parallel zur Domänenwand. Bei fortgesetzter Beleuchtung erfolgt ein weiteres Wachstum der Kristallite bis zur ihrer gegenseitigen Berührung. Damit ermöglicht dieser Abscheideprozess den Aufbau organischer oder metallisch polykristalliner Nanodrähte mit Abmessungen um 100 nm in Breite und Höhe. Die Länge ist lediglich durch die Probenabmessungen begrenzt. Die so erzeugten Strukturen wurden im Rahmen der experimentellen Arbeiten topografisch, elektrisch und optisch charakterisiert. Am Beispiel einzeln kontaktierter Platindrähte konnte dabei deren annähernd ohmsches Leitfähigkeitsverhalten nachgewiesen werden. Zudem reagiert der Widerstand eines solchen Platin-Nanodrahtes sehr sensitiv auf Änderungen des umgebenden Gasmediums, was die Eignung solcher Strukturen für die Integration in künftige Sensorbauelemente unterstreicht. Weitergehende Untersuchungen beschäftigten sich mit der Klärung der Ursachen dieser sogenannten Domänenwanddekoration. Hierzu wurde die Lage der abgeschiedenen Strukturen mit dem zu Grunde liegenden Domänenmuster verglichen. Bis auf wenige Ausnahmen wurde dabei eine auf die Domänengrenze zentrierte, symmetrische Bedeckung nachgewiesen. Als Erklärungsansatz wird die Trennung der photoinduzierten Elektron-Loch-Paare durch das elektrostatische Feld der Polarisations- und Abschirmladungen diskutiert. Diese führt zur Ladungsträgerakkumulation und erhöhten chemischen Reaktivität an den Domänengrenzen. / Ferroelectric lithography is a method for a controlled assembly of nanostructures on ferroelectric surfaces, which has has been established throughout the last decade. It exploits the characteristic variations in surface chemistry arising from the different orientations of the spontaneous polarisation within the separate domains. The scope of this thesis is the application of that approach for the directed and controlled deposition of nanostructures consisting of noble metals or organic molecules. For this deposition, a wet chemical processing under UV illumination was carried out on magnesium doped lithium niobate single crystals. As a typical result, the decoration of 180° domain walls was observed for aqueous solutions of silver, gold and platinum salts as well as for the dissolved organic fluorescent dye Rhodamine 6G. The deposition starts within a stripe of 150−500 nm in width parallel to the domain wall. Under continuing illumination, the crystallites grow further until they finally touch each other. Using this technique, organic or metallic polycrystalline nanowires with dimensions in the range of 100nm in width and height can be assembled. Their length is only limited by the sample size. These nanostructures were characterised in respect of their topographical, electrical and optical properties. In the case of contacted single platinum wires an electrical conduction was measured, which showed approximately ohmic behaviour. It was also shown that the resistance of such a platinum nanowire is very sensitive to changes in the surrounding gas medium. This emphasises the suitability of such structures for integration in future sensor devices. Further experiments were carried out to investigate the physical background of the observed domain wall decoration. For this, the positions of the deposited structures were compared with the underlying domain structure. Apart from few exceptions, a symmetric deposition centered at the domain wall was observed. As a starting point for explanation, the separation of electron-hole-pairs by the electrostatic field from polarisation and screening charges is discussed. This process leads to charge carrier accumulation at the domain boundaries, thus enhancing the local chemical reactivity.

Semiconductor quantum dots (QDs) are fascinating systems for potential applications in quantum information processing and communication, since they can emit single photons and polarisation entangled photons pairs on demand. The asymmetry and inhomogeneity of real QDs has driven the development of a universal and fine post-growth tuning technique. In parallel, new growth methods are desired to create QDs with high emission efficiency and to control combinations of closely-spaced QDs, so-called "QD molecules" (QDMs). These systems are crucial for the realisation of a scalable information processing device after a tuning of their interaction energies. In this work, GaAs/AlGaAs QDs with low surface densities, high optical quality and widely tuneable emission wavelength are demonstrated, by infilling nanoholes fabricated by droplet etching epitaxy with different GaAs amounts. A tuning over a spectral range exceeding 10 meV is obtained by inducing strain in the dot layer. These results allow a fine tuning of the QD emission to the rubidium absorption lines, increasing the yield of single photons that can be used as hybrid semiconductor-atomic-interface. By embedding InGaAs/GaAs QDs into diode-like nanomembranes integrated onto piezoelectric actuators, the first device allowing the QD emission properties to be engineered by large electroelastic fields is presented. The two external fields reshape the QD electronic properties and allow the universal recovery of the QD symmetry and the generation of entangled photons, featuring the highest degree of entanglement reported to date for QD-based photon sources. A method for controlling the lateral QDM formation over randomly distributed nanoholes, created by droplet etching epitaxy, is demonstrated by depositing a thin GaAs buffer over the nanoholes. The effect on the nanohole occupancy of the growth parameters, such as InAs amount, substrate temperature and arsenic overpressure, is investigated as well. The QD pairs show good optical quality and selective etching post-growth is used for a better characterisation of the system. For the first time, the active tuning of the hole tunnelling rates in vertically aligned InGaAs/GaAs QDM is demonstrated, by the simultaneous application of electric and strain fields, optimising the device concept developed for the single QDs. This result is relevant for the creation and control of entangled states in optically active QDs. The modification of the electronic properties of QDMs, obtained by the combination of the two external fields, may enable controlled quantum operations.

Ferroic materials play an increasingly important role in novel (nano-)electronic applications. Recently, research on domain walls (DWs) received a big boost by the discovery of DW conductivity in bismuth ferrite (BiFeO3 ) and lead zirconate titanate (Pb(Zrx Ti1−x )O3) ferroic thin films. These achievements open a realistic and unique perspective to reproducibly engineer conductive paths and nanocontacts of sub-nanometer dimensions into wide-bandgap materials. The possibility to control and induce conductive DWs in insulating templates is a key step towards future innovative nanoelectronic devices [1]. This work focuses on the investigation of the charge transport along conductive DWs in ferroelectric single crystals. In the first part, the photo-induced electronic DC and AC charge transport along such DWs in lithium niobate (LNO) single crystals is examined. The DC conductivity of the bulk and DWs is investigated locally using piezoresponse force microscopy (PFM) and conductive AFM (c-AFM). It is shown that super-bandgap illumination (λ ≤ 310 nm) in combination with (partially) charged 180° DWs increases the DC conductivity of the DWs up to three orders of magnitude compared to the bulk. The DW conductivity is proportional to the charge of the DW given by its inclination angle α with respect to the polar axis. The latter can be increased by doping the crystal with magnesium (0 to 7 mol %) or reduced by sample annealing. The AC conductivity is investigated locally utilizing nanoimpedance microscopy (NIM) and macroscopic impedance measurements. Again, super-bandgap illumination increases the AC conductivity of the DWs. Frequency-dependent measurements are performed to determine an equivalent circuit describing the domains and DWs in a model system. The mixed conduction model for hopping transport in LNO is used to analyze the frequency-dependent complex permittivity. Both, the AC and DC results are then used to establish a model describing the transport along the conductive DW through the insulating domain matrix material. In the last part, the knowledge obtained for LNO is applied to study DWs in lithium tantalate (LTO), barium titanate (BTO) and barium calcium titanate (BCT) single crystals. Under super-bandgap illumination, conductive DWs are found in LTO and BCT as well, whereas a domain-specific conductivity is observed in BTO.

Si l'utilisation des transformateurs piézoélectriques se bornait jusqu'alors à l'alimentation ou la protection de dispositifs électriques, ils sont aujourd'hui envisagés pour la génération de décharges plasma directement à leur surface. Les propriétés remarquables de ces générateurs piézoélectriques en font une alternative intéressante aux dispositifs conventionnels, notamment par la simplicité de mise en œuvre. La surface du transformateur constitue à la fois le support de décharge et l'élément élévateur de tension réduisant significativement l'encombrement des dispositifs. En outre les gains en tension de ces transformateurs sont remarquablement élevés et permettent d'obtenir des décharges pour des tensions d'alimentation n'excédant pas quelques volts. Ces avantages peuvent répondre avantageusement à certains problèmes rencontrés dans les procédés plasmas dont l'implantation dans les processus industriels, bien qu'elle soit en constante amélioration, est parfois confrontée à des problèmes de mise en œuvre d'enceintes complexes, rendant le procédé couteux ou inadapté aux conditions opératoires. L'objet de cette thèse porte sur l'étude fondamentale d'un transformateur piézoélectrique de type Rosen dédié à la génération de décharges électriques. Plus particulièrement, ce travail s'attèle au développement d'un modèle analytique permettant de mieux appréhender les limites de ce procédé innovant, ainsi qu'une meilleure compréhension du comportement des décharges plasma face aux spécificités de ce transformateur et de son matériau. Pour ce faire l'étude se consacre en première partie à la caractérisation du transformateur piézoélectrique hors décharge à partir de ses bornes, puis à l'extension d'un modèle analytique afin d'appréhender la distribution du potentiel électrique à sa surface. Le développement d'un dispositif expérimental permettra la mesure du potentiel ainsi que la discussion du modèle. Dans un second temps l'étude s'attache au comportement du transformateur piézoélectrique en décharge. La distribution de potentiel à présent connue constitue une donnée d'entrée nécessaire à l'étude de la dynamique de décharges dans ces différentes configurations. Les phénomènes à l'œuvre dans ce processus de génération étant complexes, l'étude est conduite suivant différentes étapes. Tout d'abord en passant par l'étude des propriétés des céramiques ferroélectriques au travers d'une décharge à barrière diélectrique plan-plan. Ensuite la dynamique des décharges est abordée par modélisation numérique suivant trois configurations différentes. Ces cas d'études conduisent à des régimes de décharges différents pouvant faire l'objet de mise en application future. Bien que le problème soit sous l'hypothèse d'un couplage faible, les résultats ont corroborés les observations expérimentales et ont permis de mieux comprendre l'influence des hautes permittivités et de la distribution du potentiel sur l'évolution spatio-temporelle de ce procédé. / Nowadays piezoelectric transformers are not only used to supply or protect electrical devices, but also to generate plasma discharges directly on their surface. The remarkable properties of these piezoelectric generators make them an interesting alternative to conventional devices, especially the simple implementation. The surface of the transformer constitutes both the discharge support and the voltage elevator component reducing significantly the bulk of the devices. Besides the transformers' gain voltage are remarkably high and permit to generate discharges for low voltage supply not exceeding a few volts. These advantages respond to some problems met in the plasma processes of which the establishment in industrial processes - in constant improvement - is sometimes confronted to problems of chambers implementations, making this process expensive and not adequate to the operating conditions. The purpose of this thesis focuses on the fundamental studies of a Rosen piezoelectric transformer dedicated to the generation of electrical discharges. In particular, this work tackles the development of an analytical model allowing to improve the understanding of the limits of this innovating process, as well as a better comprehension of the plasma discharges behavior face with transformer and material features. In order to do this the first part of the study is devoted to the characterization of the piezoelectric transformer without discharge, then the extension of the analytical modeling in order to comprehend the distribution of surface electrical potential. The development of an experimental device will allow the potential measurement and the discussion of the model. In a second part the study focuses on behavior of the piezoelectric transformer in discharge. The potential distribution known today constitutes a necessary input data for the study of the discharge dynamic in different configurations. The complexity of the phenomena implemented in this process of generation requires to conduct the study following different steps. First of all, by the study of ferroelectric ceramic features through a dielectric-barrier discharge. Then the discharges dynamic is approached by numerical modeling following three different configurations. This cases conduct to different discharge regimes that can be the subject of future application. Even if the problem is under the hypothesis of a weak coupling, the results confirmed the experimental observations and permitted to understand better the influence of high permittivity and of the potential distribution on the saptio-temporal evolution of this process.

Made available in DSpace on 2016-06-02T20:16:51Z (GMT). No. of bitstreams: 1 5339.pdf: 3033412 bytes, checksum: 3e684081236853aacfacccef70bd4635 (MD5) Previous issue date: 2009-10-02 / Financiadora de Estudos e Projetos / Lead titanate zirconate modified with lanthanum, or PLZT, is one the most ferroelectric compounds utilized in electronic devices, due to its versatility and low production costs in comparison with single-crystalline materials. When adequately prepared, this system presents good optical (high optical transmission) properties, in the visible and near infrared range, and can be electro-optically characterized. Recently, in the end of 90 s, it was verified its high potential as host of photoluminescent ions, as the lanthanide (rare-earth) family. The possibility to use its electro-optic properties (due to its ferroelectric characteristics) and its photoluminescent properties (achieved by the doping process) together ,enlarges the range of application of this system. In this way, the electro-optical characterization of doped PLZT ceramics becomes essential, besides the photonic characterization. In this work, the Senarmont compensator method for electro-optical characterization, or dynamic method, was instrumented, and the values of the induced (due to the quadratic electrooptic effect, Kerr) and permanent (due to the linear electro-optic effect, Pockels) birefringence were determined as a function of the temperature, wave-length and electric-field frequency, of the rare-earth (Nd2O3, Ho2O3, Er2O3, Tm2O3 e Yb2O3) doped PLZT, with La/Zr/Ti=9/65/35, ceramics. The results shown a relationship between of the electro-optic (electro-optic coefficients, or birefringence values) and the dielectric, ferroelectric and structural properties (studied in others works) of the ceramics, that were related with the site occupancy and the structural defects due to the aliovalent dopant. It also can be identified two distinct birefringence dependence as a function of the electric field, for the same electro-optic effect (Kerr, or Pockels), identified as a function of the doping process. In the case of electro-optical characterizations in function of the variable frequency, was observed an agreement with the characterization ferroelectric results made in other works in GCFerr, being evidenced the reduction of electro-optical properties with increasing frequency, where if it observed the occurrence of anomalies in the Pockels response with direct influence on the response Kerr. The characterization as a function of wavelength showed the occurrence of two types of behavior depending on the dopant ion used, being one of them the reduction of the values of birefringence with increasing wavelength (the samples pure and doped ions neodymium (Nd) and ytterbium (Yb)), with a tendency to expected behavior in the literature, however, in the second was seen irregular increase birefringence with increase wavelength (for samples doped with ions holmio ( Ho), erbium (Er) and thulium (Tm),not existing relation with to the theoretical models adopted. In relation the characterization as a function of temperature, this was carried through in a temperature interval that understood the characteristic temperatures of systems relaxores (freezing temperature (TF), the maximum dielectric permittivity (TM (e)) and Burns (TB)), except for the sample doped with neodymium ions, whose freezing temperature is below interval worked. By the curve of birefringence (Δn) as a function of temperature was possible to determine the temperature of maximum birefringence for each of the samples, correlated them with each other. Through the curve (d Δn / dt) vs. T was possible to identify a relationship between the maximum variations, positive and negative birefringence with the temperature characteristics TF and TB. / Entre os sistemas ferroelétricos, o sistema titanato zirconato de chumbo modificado com lantânio (PLZT) é um dos mais amplamente utilizados em dispositivos eletrônicos, dada sua versatilidade em aplicações e relação de custo quando comparado aos materiais monocristalinos utilizados nessa mesma área. Quando preparado pelo devido método de síntese, apresenta excelentes propriedades óticas (altos valores de transmissão ótica) desde a região do visível ao infravermelho próximo, possibilitando uma adequada caracterização de suas propriedades óticas e eletro-óticas. Recentemente, a partir do final da década de 90, foi verificada sua alta potencialidade como matriz hospedeira para íons fotoluminescentes, como os da família dos lantanídeos (ou terras-raras). A possibilidade do uso conjunto das propriedades eletro-óticas (dado seu caráter ferroelétrico) e de suas propriedades luminescentes (devido à incorporação de dopantes laser-ativos) aumentou ainda mais a possibilidade de aplicação desses materiais. Desse modo, a caracterização eletro-ótica das cerâmicas de PLZT dopado torna-se indispensável, além de sua caracterização fotônica. Sendo assim, neste trabalho foi instrumentado um sistema de caracterização eletro-ótica, utilizando o método do compensador Senarmont, também conhecido como método dinâmico, para determinar os valores da birrefringência induzida (devido ao efeito eletro-ótico quadrático, Kerr) e a permanente (devido ao efeito eletro-ótico linear, Pockels) em função da temperatura, comprimento de onda e frequência do campo elétrico de prova para composições cerâmicas de PLZT na razão La/Zr/Ti=9/65/35, dopadas com os óxidos terras-raras Nd2O3, Ho2O3, Er2O3, Tm2O3 e Yb2O3, na quantidade de 1,0% em peso. Os resultados mostraram que há uma relação entre as propriedades eletro-óticas encontradas (seja na forma de valores dos coeficientes eletro-óticos, ou na variação da birrefringência) com as propriedades dielétricas, ferroelétricas e estruturais (já observadas em outros trabalhos do grupo de pesquisa no qual esta dissertação foi realizada) das cerâmicas que, por sua vez, foram relacionadas com o tipo de ocupação e de defeitos gerados devido à incorporação dos dopantes. Além disso, foi possível observar que para uma mesma composição pode ocorrer a presença dos dois tipos de efeitos eletro-óticos - Kerr e Pockels - com proporções distintas em função do tipo de dopante. Através desse método, para esse conjunto de amostras, também foi possível identificar dois tipos distintos de variações da birrefringência em função do campo elétrico para um mesmo efeito eletro-ótico (Kerr, ou Pockels), que também puderam ser associados com o tipo de ocupação dos dopantes. Em se tratando das caracterizações eletro-óticas em função da variável frequência, foi observada uma concordância com os resultados da caracterização ferroelétrica, realizada em outros trabalhos no GCFErr, sendo evidenciada a redução das propriedades eletro-óticas com o aumento da frequência, em que se observou a ocorrência de anomalias na resposta Pockels com influência direta na resposta Kerr. A caracterização como uma função do comprimento de onda mostrou a ocorrência de dois tipos de comportamentos, dependendo do íon dopante utilizado, sendo um deles a redução dos valores da birrefringência com o aumento do comprimento de onda (caso das amostras pura e dopadas com os íons neodímio (Nd) e itérbio (Yb)), havendo certa tendência ao comportamento previsto em literatura, no entanto, no segundo caso foi constatado o aumento irregular da birrefringência com o aumento do comprimento de onda (caso das amostras dopadas com os íons holmio (Ho), érbio (Er) e túlio (Tm), não havendo relação com os modelos teóricos adotados. Quanto à caracterização em função da temperatura, esta foi realizada em um range de que compreendeu as temperaturas características de sistemas relaxores (de freezing (TF(e)), máxima permissividade dielétrica (TM(e)) e Burns (TB(e))), exceto para a amostra dopada com o íon neodímio, cuja TF(e) estava abaixo do intervalo considerado. Através da curva de birrefringência (Δn) em função da temperatura foi possível determinar a temperatura de máxima birrefringência para cada uma das amostras, correlacionado-as entre si. Através da curva de (dΔn/dt) vs. T, foi possível constatar uma relação entre as máximas variações, positiva e negativa, da birrefringência com as temperaturas características TF e TB.