The study of novel properties of materials in nanometer length scales has been an extensive area of research in the recent past. The field of nanosciece and nanotechnology deals with such studies and has gained tremendous importance because of the potential applications of these nanosystems in devices. Many of the bulk properties tend to change as a function of size, be it particle size in case of nanoparticles, or thickness in case of very thin films. Not only is it important to study these changes from the point of view of applications, but also the interesting physics behind such changes prompts further research and exploration in this area. In this thesis we try to see how changes in the length scales affect the properties of nanoparticles and how change in thickness affects the properties of thin films, along with making an effort towards measurements of conductivity in the nanoscale using the technique of electron magnetic resonance (EMR) signal shape analysis. Electron magnetic resonance is a general term used to combine both electron paramagnetic resonance (EPR) and ferromagnetic resonance (FMR). This thesis deals with mainly two kinds of systems viz., nanoparticles of doped rare earth manganites and thin films of the conducting polymer, vapor phase polymerized polyethylendioxythiophene (VPP PEDOT). The general formula for doped manganites is A1-xBxMnO3 where A is a rare earth trivalent cation like La3+, Pr3+, Nd3+..., and B is an alkaline earth divalent cation like Sr2+, Ca2+, Ba2+... These together with Mn and O form the distorted perovskite structure to which manganites belong. The phase diagram of doped manganites involves many interesting phases like ferromagnetic metallic, antiferromagnetic insulating and charge ordered insulating phases. The magnetic properties of the manganites are governed by exchange interactions between the Mn ion spins. These interactions are relatively large between two Mn spins separated by an oxygen atom and are controlled by the overlap between the Mn d-orbitals and the O p-orbitals. The changing Mn-O-Mn bond lengths and bond angles as a function of the radius of the A and B cations [1, 2], and the different magnetic interactions among the Mn3+ and Mn4+ ions together are responsible for the different phases that we see in manganites as a function of temperature and magnetic field. Manganites have potential applications in the field of spintronics because of their colossal magnetoresistance (CMR) [3] and half-metallic [4] properties. Studies on nanoscale manganites have shown that as size reduces, their electrical and magnetic properties change significantly[5]. By changing the morphology and grain size, the properties of CMR manganites can be tuned [6-9]. Phase separation seems to disappear in nanoparticles compared to bulk [10]. In the charge ordered manganites, size reduction is known to bring about suppression of charge order [11], emergence of ferromagnetism [12, 13] and even metallicity in some nanostructures [12]. The conducting polymer under study viz., VPP PEDOT is in a semiconducting phase at room temperature and becomes more insulating as temperature reduces. It is a technologically important polymer which has cathodically coloring property, can be used as a highly conducting electrode in organic solar cells and organic LEDs [14-16]. In the following we give a summary of the results reported in the thesis chapter by chapter. Chapter 1: This chapter of the thesis consists of an introduction to the physics of manganites and the technique of EMR. This includes a detailed account of previous EMR studies done on manganites, in particular nano manganites. There is a section about different line shapes observed in EMR of manganites, their origin and how to fit them to an appropriate lineshape function [17]. There is an introduction to the transport properties of conducting polymers, including how magnetic fields can affect the transport and the mechanism behind variable range hopping transport which is the dominant kind of transport in such polymeric systems. There is also a description of the different experimental methods and instruments used to study the systems in the thesis and their working principles. They are: EPR spectrometer, SQUID magnetometer, Janis cryostat with superconducting magnet, atomic force microscope (AFM) and transmission electron microscope (TEM). Chapter 2: This chapter deals with the method of contactless conductivity of nanoparticles using EMR lineshape analysis. It is difficult to measure the conductivity of individual nanoparticles by putting contacts. Other methods tend to include the contribution of grain boundaries which mask the grain characteristics [5]. We have introduced a new contactless method to measure the conductivity of nanoparticles in a contactless manner [18]. Metallic nanoparticles in which the skin depth is less than the size of the particles, exhibit an asymmetric EMR signal called the Dysonian [19]. Dysonian lineshape is an asymmetric lineshape with the so-called A/B ratio >1, where, A is the amplitude of the low field half of the derivative and B is the amplitude of the high field half. In a ferromagnetic conducting sample, the lineshape has contributions from the Dysonian part and also a part which arises due to magnetocrystalline anisotropy [20]. We have developed a method of deconvoluting the signals from conducting nanoparticles to take out the Dysonian part from them and measure the A/B ratio as a function of temperature. The A/B ratio thus determined can then be used to find out the ratio of the sample size to the skin depth using the work by Kodera [21]. The skin depth can be used to determine the conductivity by using the relationship  = (1/)1/2, where,  is the measuring frequency,  is the conductivity and  is the permeability. This technique has been used to determine the conductivity as a function of temperature (from 60 K to 300 K) of La0.67Sr0.33MnO3 (LSMO) nanoparticles of average size 17 nm. The method has been cross-checked by measuring the conductivity of bulk LSMO particles at 300 K by EMR lineshape analysis method and by standard four-probe method, which give conductivity values close to each other within experimental error. Chapter 3: In this chapter, we report a novel phenomenon of disappearance of electron-hole asymmetry in nanoparticles of charge ordered Pr1-xCaxMnO3 (PCMO). In bulk PCMO there is asymmetry in electric and magnetic properties seen on either side of x = 0.5. In the samples with x = 0.36 (hole doped: called PCMH) and x = 0.64 (electron doped: called PCME), the bulk sample has opposite g-shifts as observed in EPR signals [22]. PCME sample shows g-value less than and PCMH sample shows g-value greater than the free electron g-value at room temperature. This is explained using the opposite sign of the spin-orbit coupling constant for the two different kinds of charge carriers. But when the size of PCMH and PCME is reduced to nanoscale (average size ~ 20 nm), the g-shift was seen on the same side i.e., positive and almost equal g-shift in both cases. This points towards a disappearance of electron-hole asymmetry at nanoscale. This positive g-shift is analyzed in the two cases in the light of disappearance of charge ordering and emergence of ferromagnetism in these systems, since emergence of ferromagnetic hysteresis is noticed at low temperatures in both nano PCMH and nano PCME. In nano PCMH, charge ordering completely disappears and in nano PCME it weakens. Exchange bias is seen in both the systems, suggestive of core-shell structure [23] in the nanoparticles. Other competing factors include spin-other orbit interactions and size reduction induced metallicity [12] which can average out the anisotropies in the system, causing the asymmetry to disappear. Chapter 4: This chapter deals with thickness induced change in transport mechanism in VPP PEDOT thin films. Two samples were studied with average thickness of 120 nm (VP-1) and 150 nm (VP-2). The average room temperature conductivity of VP-1 was found to be 126 Scm-1 and VP-2 was 424 Scm-1. The transport mechanism in VP-1 is seen to be 2-dimensional variable range hopping (VRH) [24]. However, as the thickness increases by 30 nm, the transport mechanism in VP-2 is found to be 3-dimensional VRH. The low temperature magnetotransport is analyzed in the two systems and it shows that there is wavefunction shrinkage in both the systems at 1.3 K [24]. The DC transport results are cross checked with AC transport data at 5 different temperatures in the frequency range of 40 Hz to 110 MHz. The data can be analyzed by using the extended pair approximation model [25]. The AC transport shows the presence of a critical frequency 0 which marks the transition from the frequency independent to a frequency dependent region. The value of 0 decreasing with decreasing temperature suggests that the system is becoming more insulating and it supports the DC transport model of VRH. The morphological studies were done using AFM which revealed higher grain size for VP-2, confirming the direct correlation of the average grain size with the conductivity of the sample. Chapter 5: summarizes the main conclusions of the thesis, also pointing out some future directions for research in the field.

In der vorliegenden Arbeit werden durch den inversen piezoelektrischen Effekt kontrolliert Dehnungen in Lanthanmanganatschichten eingebracht und ihr Einfluss auf die Eigenschaften der Schichten untersucht. Dazu wird im ersten Teil der Arbeit ein Zweischichtsystem bestehend aus einer Manganatschicht aus La0,7Sr0,3MnO3, La0,8Ca0,2MnO3 oder La0,7Ce0,3MnO3 und einer piezoelektrischen Schicht aus PbZr0,52Ti0,48O3 untersucht. Der epitaktisch auf Einkristallsubstraten abgeschiedene Aufbau entspricht einer Feldeffekt-Transistor-Struktur. Neben den Effekten der Dehnung auf den elektrischen Widerstand der Manganatschicht wird auch der elektrische Feldeffekt untersucht. Durch mechanische Klemmung des Substrats können nur kleine Dehnungen in die Manganatschichten eingebracht werden. Um größere und homogene Dehnungen steuerbar in Manganatschichten einzubringen, werden im zweiten Teil der Arbeit La0,7Sr0,3MnO3 - Schichten auf piezoelektrischen Einkristallsubstraten der Verbindung (1-x)Pb(Mg1/3Nb2/3)O3 - xPbTiO3 mit x = 0,28 epitaktisch abgeschieden. Der Einfluss von mechanischen Dehnungen von bis zu 0,1% auf den elektrischen Transport, die ferromagnetische Übergangstemperatur und die Magnetisierung kann so eingehend untersucht werden. Es wird ein außergewöhnlich großer Einfluss von Dehnungen auf die Eigenschaften von La0,7Sr0,3MnO3 gefunden. / In this work, strain arising from the inverse piezoelectric effect is induced into lanthanum manganite thin films in order to change and control their properties. In the first part of this work, manganite films of the compositions La0.7Sr0.3MnO3, La0.8Ca0.2MnO3 or La0.7Ce0.3MnO3 are combined with a piezoelectric layer of the composition PbZr0.52Ti0.48O3 in a bilayer system. This structure is grown epitaxially on single crystal substrates and corresponds to a field-effect transistor setup. Besides effects of strain on the electrical resistance of the manganite layers, field effects are observed. Due to clamping of the substrate, only small strains can be induced to the manganite films. In order to apply larger and homogeneous controllable strain to the manganite layers, thin films of La0.7Sr0.3MnO3 are grown epitaxially on piezoelectric single crystal substrates of the composition (1-x)Pb(Mg1/3Nb2/3)O3 - xPbTiO3, x = 0.28. Strain levels up to 0.1% are reached. The influence of the strain on electrical transport, ferromagnetic transition temperature and magnetization is analyzed. A remarkably large influence of the strain on the properties of La0.7Sr0.3MnO3 is found.

Complex oxides are an intriguing field of solid-state research, as they can exhibit a wide variety of functional properties, such as ferroelasticity, ferroelectricity, ferro- and antiferromagnetism or an even more complicated type of magnetic ordering, the combination or interaction of those ferroic properties (multiferroicity), high spin polarization, or high-temperature superconductivity. Thus they are prospective candidates for future materials in microelectronics. It is a matter of fact that the performance of such oxide-based devices depends mainly on transport properties, which in turn depend on the distribution and density of intrinsic or extrinsic electronic interface states across the device structure. The present thesis focuses on the identification and characterization of such electronic properties by two different photoassisted spectroscopy techniques: surface photovoltage spectroscopy and photoelectron spectroscopy. This work especially deals with perovskite oxides, namely with the model perovskite strontium titanate (SrTiO3) as a substrate and three differently doped lanthanum manganite thin films (10-15 nm thickness) grown by pulsed laser deposition (PLD) on the SrTiO3 substrate(La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). The first part aims at the identification of electronic surface and interface states at the free SrTiO3 surface as well as at the three different lanthanum manganite/SrTiO3 interfaces. For that purpose three different experimental realizations of the surface photovoltage spectroscopy technique were implemented and employed: photoelectron spectroscopy under additional optical excitation, the capacitive detection of the photoinduced displacement current in a parallel-plate capacitor geometry under modulated optical excitation, and the classical Kelvin probe technique. The methods are evaluated comparatively with respect to their suitability to analyze the given oxidic interfaces. The main result of this first part is a map of the energetic positions and relaxation time constants of the surface states at the SrTiO3 surface as well as of the interface states at the lanthanum manganite/SrTiO3 interfaces within the SrTiO3 bandgap. The interface states were classified into film- and substrate-induced states and it could be demonstrated that an appropriate annealing procedure can dramatically decrease their densities. The second part tackles the problem of the manganese valence and the doping type of di- and tetravalent-ion-doped LaMnO3. The question whether the insulating parent compound LaMnO3 becomes an electron-doped semiconductor after doping with tetravalent cations such as Ce4+ - which would be in analogy to the well-established hole doping after partial substitution of La3+ by divalent cations such as Sr2+ or Ca2+ - has been discussed controversially in the literature so far. Due to the physics of the manganite crystal lattice the question can also be formulated in a different way: Can part of the manganese ions be driven from the Mn3+ state towards the Mn2+ state without any crystal instabilities or phase separation phenomena? In order to contribute to the clarification of this question, an extensive X-ray- and UV-photoelectron spectroscopy (XPS/UPS) investigation was performed. The three differently doped lanthanum manganite thin films were comparatively studied considering the exchange splitting of the Mn 3s core level line, which is a linear function of the Mn valence, as measured by XPS and the work function as extracted from UPS. All measurements were performed at different states of deoxygenation after heating in ultrahigh vacuum and reoxidation after heating in a pure oxygen atmosphere. Strong evidence for electron doping of the La0.7Ce0.3MnO3 film after deoxygenation was found. Furthermore, the reversible tunability of the Mn valence by variation of the oxygen content could be demonstrated for both tetravalent- and divalent-ion-doped lanthanum manganite films. / Oxidische Komplexverbindungen können eine Vielzahl an funktionellen Eigenschaften, wie z.B. Ferroelastizität, Ferroelektrizität, Ferro- und Antiferromagnetismus sowie kompliziertere magnetische Ordnungen, die Kombination und Interaktion solcher ferroischer Eigenschaften (Multiferroizität), hohe Spinpolarisation oder Hochtemperatursupraleitung aufweisen und gelten daher als aussichtsreiche Materialien für die zukünftige Mikroelektronik. Entscheidend für die Funktionsfähigkeit oxidischer Bauelemente sind deren elektronische Transporteigenschaften, die in äußerst sensibler Weise von der Verteilung und Dichte von ex- oder intrinsischen elektronischen Defektzuständen an Grenz- und Oberflächen innerhalb der Bauelementstruktur abhängen. Die vorliegende Arbeit beschäftigt sich mit der Spektroskopie solcher elektronischer Eigenschaften mittels photonenbasierter Methoden. Im Fokus stehen dabei perowskitische Oxide , speziell das Modellperowskit Strontiumtitanat (SrTiO3) als Substrat und darauf mittels gepulster Laserdeposition (PLD) abgeschiedene dünne Filme (10-15 nm Dicke) dotierter Lanthanmanganate (La0.7Sr0.3MnO, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). Im Rahmen einer halbleiterphysikalischen Interpretation widmet sich der erste Teilder Identifikation elektronischer Ober- und Grenzflächenzustände an der SrTiO3-Oberfläche sowie an verschiedenen Lanthanmanganat/SrTiO3-Grenzflächen mittels dreier unterschiedlicher experimenteller Methoden zur Vermessung der Oberflächenphotospannung: der Photoelektronenspektroskopie unter zusätzlicher optischer Anregung, einer kapazitiven Detektionsmethode in Plattenkondensatorgeometrie unter modulierter optischer Anregung und der optischen Kelvin-Sonde. Neben einem auf die bei oxidischen Ober- und Grenzflächen auftretenden besonderen Herausforderungen zugeschnittenen Methodenvergleich werden Grenzflächenzustände bezüglich ihrer energetischen Position in der Bandlücke des SrTiO3 und ihres Relaxationsverhaltens analysiert, als substrat- oder filminduziert klassifiziert, und die Verringerung ihrer Dichte nach geeigneter Ausheilprozedur wird nachgewiesen. Der zweite Teil der Arbeit befasst sich mit der in der Literatur bisher kontrovers diskutierten Frage, ob sich die isolierende Stammverbindung LaMnO3 durch Dotierung mit tetravalenten Kationen, wie z.B. Ce4+, in einen elektronendotierten Halbleiter verwandeln lässt - analog zur Herstellung lochdotierter Lanthanmanganate durch Dotierung mit divalenten Kationen, wie z.B. Sr2+ oder Ca2+. Die Frage ist äquivalent zur Betrachtung, ob unter Beibehaltung der Stabilität des Kristallgitters ein Teil der Manganionen vom Mn3+-Zustand in den Mn2+-Zustand übergehen kann. Um einen Beitrag zur Klärung dieses Problems zu leisten, wurden als elektronisch sensitive Methoden die Röntgen- und UV-Photoelektronenspektroskopie (XPS/UPS) gewählt. Die oben genannten Lanthanmanganatfilme wurden dazu hinsichtlich der Austauschaufspaltung der Mangan-3s-Linie im XP-Spektrum, die in linearer Weise von der Manganvalenz abhängt, und der anhand der Breite des UP-Spektrums ermittelten Austrittsarbeit jeweils nach Reinigung der Oberfläche im Ultrahochvakuum (UHV) vergleichend untersucht. Die Messungen wurden nach unterschiedlich starker Desoxidation durch Heizen im UHV und Reoxidierung durch Heizen in Sauerstoffatmosphäre durchgeführt. Es konnte nachgewiesen werden, dass eine Elektronendotierung des La0.7Ce0.3MnO3-Films bei geeigneter Einstellung des Sauerstoffgehalts tatsächlich möglich ist. Außerdem wurde gezeigt, dass sich sowohl in di- als auch in tetravalent dotierten Lanthanmanganatfilmen die Manganvalenz und damit der Dotierungstyp reversibel durchstimmen lässt.

No description available.

Tesis por compendio / [ES] Debido a los rápidos avances tecnológicos de las últimas décadas han surgido nuevas necesidades que requieren la generación de nuevos materiales a la carta que cumplan con ciertas funcionalidades o que adquieran características concretas. A este diseño de materiales a la carta hay que sumar la problemática de los procesos de conformado, que en el caso de los materiales cerámicos son procesos largos que requieren altas temperaturas y conllevan un gran consumo energético. Materiales cerámicos como la circona (ZrO2) han atraído la atención de los investigadores debido a sus excelentes propiedades mecánicas, térmicas y de alta resistencia química. Además, hay que mencionar que se trata de un material altamente biocompatible y que no reacciona en ambientes húmedos, lo cual hace que pueda utilizarse en una amplia gama de sectores transversales, que van desde el aeroespacial hasta la medicina protésica. Una de las características importantes de la circona, dada su naturaleza de sólido cristalino blanco, es que sea sencilla de colorear, pudiendo obtener tonalidades muy similares a las de la dentina humana con pequeños dopajes de óxidos metálicos, es por ello que su uso se ha extendido ampliamente en la odontología. Otra de las propiedades significativas que hacen de esta cerámica un material excepcional es su alta conductividad iónica, que le permite ser utilizada como electrolito en las pilas de combustible de óxido sólido (SOFC). El conjunto de todas las propiedades mencionadas hace de la circona un material muy versátil con un amplio abanico de posibles aplicaciones, que abarca desde celdas de combustible sólido, recubrimientos para hélices de turbinas, intercambiadores de calor, así como para medicina protésica, odontología y otras aplicaciones. El propósito de esta tesis doctoral es la obtención de materiales cerámicos nanoestructurados basados en circona que puedan ser empleados en la fabricación de nuevos composites con propiedades a carta para los diversos sectores anteriormente mencionados. Para ello, se utilizará la tecnología de sinterización no convencional basada en microondas, la cual nos permite consolidar materiales altamente densificados a temperaturas relativamente bajas y ciclos muy cortos de tiempo, a la vez de que se trata de un procesado limpio, ecológico y medioambientalmente sostenible. En esta investigación se ha planteado el estudio de distintos composites base circona estabilizada con ytria: circona dopada con óxido de hierro (Fe2O3), composites de circona con manganita de lantano dopada con estroncio (LSM) y composites circona-circón (ZrSiO4). Los resultados obtenidos de esta investigación permitirán, por una parte, determinar si la técnica de sinterización rápida no-convencional empleada mejora las propiedades tanto mecánicas, eléctricas, magnéticas y químicas de los materiales en comparación con la sinterización convencional, y por otra parte, avanzar en el diseño y fabricación de materiales cerámicos avanzados. / [CA] A causa dels ràpids avanços tecnològics de les últimes dècades han sorgit noves necessitats que requereixen la generació de nous materials a la carta que complisquen amb unes certes funcionalitats o que adquirisquen característiques concretes. A aquest disseny de materials a la carta cal sumar la problemàtica dels processos de conformat, que en el cas dels materials ceràmics són processos llargs que requereixen altes temperatures i comporten un gran consum energètic. Materials ceràmics com la circona (ZrO2) han atret l'atenció dels investigadors a causa de les seues excel·lents propietats mecàniques, tèrmiques i d'alta resistència química. A més, cal esmentar que es tracta d'un material altament biocompatible i que no reacciona en ambients humits, la qual cosa fa que puga utilitzar-se en una àmplia gamma de sectors transversals, que van des de l'aeroespacial fins a la medicina protètica. Una de les característiques importants de la circona, donada la seua naturalesa de sòlid cristal·lí blanc, és que siga senzilla d'acolorir, podent obtindre tonalitats molt similars a les de la dentina humana amb xicotets dopatges d'òxids metàl·lics, és per això que el seu ús s'ha estés àmpliament en l'odontologia. Una altra de les propietats significatives que fan d'aquesta ceràmica un material excepcional és la seua alta conductivitat iònica, que li permet ser utilitzada com a electròlit en les piles de combustible d'òxid sòlid (SOFC). El conjunt de totes les propietats esmentades fa de la circona un material molt versàtil amb un ampli ventall de possibles aplicacions, que abasta des de cel·les de combustible sòlid, recobriments per a hèlices de turbines, bescanviadors de calor, així com per a medicina protètica, odontologia i altres aplicacions. El propòsit d'aquesta tesi doctoral és l'obtenció de materials ceràmics nanoestructurats basats en circona que puguen ser emprats en la fabricació de nous compòsits amb propietats a carta per als diversos sectors anteriorment esmentats. Per a això, s'utilitzarà la tecnologia de sinterització no convencional basada en microones, la qual ens permet consolidar materials altament densificats a temperatures relativament baixes i cicles molt curts de temps, alhora de què es tracta d'un processament net, ecològic i mediambientalment sostenible. En aquesta investigació s'ha plantejat l'estudi de diferents compòsits base circona estabilitzada amb ytria: circona dopada amb òxid de ferro (Fe2O3), compòsits de circona amb manganita de lantani dopada amb estronci (LSM) i compòsits circona-circón (ZrSiO4). Els resultats obtinguts d'aquesta investigació permetran, d'una banda, determinar si la tècnica de sinterització ràpida no-convencional empleada millora les propietats tant mecàniques, elèctriques, magnètiques i químiques dels materials en comparació amb la sinterització convencional, i d'altra banda, avançar en el disseny i fabricació de materials ceràmics avançats. / [EN] Due to the rapid technological advances of recent decades, new needs have arisen that require the generation of new on demand materials that meet certain functionalities or acquire specific characteristics. In addition to the design of tailor-made materials, there is the problem of forming processes, which in the case of ceramic materials are long processes that require high temperatures and entail high energy consumption. Ceramic materials such as zirconia (ZrO2) have attracted the attention of researchers due to their excellent mechanical, thermal and high chemical resistance properties. In addition, it should be mentioned that it is a highly biocompatible material that does not react in humid environments, which makes it suitable for use in a wide range of cross-cutting sectors, ranging from aerospace to prosthetic medicine. One of the important characteristics of zirconia, given its nature as a white crystalline solid, is that it is easy to color, being able to obtain shades very similar to those of human dentin with small doping of metal oxides, which is why its use has been widely extended in dentistry. Another significant property that makes this ceramic an exceptional material is its high ionic conductivity, which allows it to be used as an electrolyte in solid oxide fuel cells (SOFC). All these properties together make zirconia a very versatile material with a wide range of possible applications, ranging from solid fuel cells, coatings for turbine propellers, heat exchangers, as well as for prosthetic medicine, dentistry and other applications. The purpose of this doctoral thesis is to obtain zirconia-based nanostructured ceramic materials that can be used in the fabrication of new composites with tailored properties for the various sectors mentioned above. For this purpose, the non-conventional microwave-based sintering technology will be used, which allows us to consolidate highly densified materials at relatively low temperatures and very short cycle times, as well as being a clean, ecological and environmentally sustainable process. In this research, the study of different zirconia-based composites stabilized with yttria has been proposed: zirconia doped with iron oxide (Fe2O3), zirconia composites with lanthanum manganite doped with strontium (LSM) and zirconia-zirconia composites (ZrSiO4). The results obtained from this research will allow, on the one hand, to determine if the non-conventional fast sintering technique used improves the mechanical, electrical, magnetic and chemical properties of the materials in comparison with conventional sintering, and on the other hand, to advance in the design and fabrication of advanced ceramic materials. / Moratal Ruiz, S. (2023). Desarrollo de cerámicas mediante tecnología de microondas basadas en circona y manganitas de lantano [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/200904 / Compendio

Os filmes cerâmicos de manganito de lantânio dopado com estrôncio (LSM) e de manganito de lantânio dopado com estrôncio/zircônia estabilizada com ítria (LSM/YSZ) são utilizados como catodos das células a combustível de óxido sólido de temperatura alta (CaCOSTA). Estes filmes cerâmicos porosos foram depositados sobre o substrato cerâmico denso de YSZ, utilizado como eletrólito, componente estrutural do módulo, assim conferindo uma configuração de meia-célula denominada auto-suporte. O estudo da meia-célula é fundamental, pois na interface catodo/eletrólito ocorre a reação de redução do oxigênio, conseqüentemente influenciando no desempenho da CaCOSTA. Neste sentido, o presente trabalho contribui para a síntese de pós de LSM e LSM/YSZ e para o processamento de filmes finos, utilizando a técnica de pulverização de pó úmido, adotada para a conformação dos filmes cerâmicos por permitir a obtenção de camadas porosas com espessuras variadas na ordem de micrômetros. Os pós de LSM foram sintetizados pela técnica de citratos e os pós de LSM/YSZ pela técnica de mistura de sólidos. Na etapa de conformação foram preparadas suspensões orgânicas de LSM e LSM/YSZ alimentada por gravidade em um aerógrafo manual. Para a conformação do substrato de YSZ utilizou-se uma prensa uniaxial hidráulica. Foram possíveis a obtenção das meia-células de óxido sólido catodo/eletrólito de estruturas cristalinas hexagonal para a fase LSM e cúbica para a fase YSZ. E as micrografias das meia-células mostram que o substrato YSZ é denso, suficiente para ser utilizado como eletrólito sólido, e os filmes de LSM e LSM/YSZ apresentam-se porosos com espessura de aproximadamente 30 μm e com boa aderência entre os catodos e o eletrólito. A presença do catodo compósito entre o catodo LSM e o substrato YSZ, possibilitou um aumento no desempenho eletroquímico na reação de redução do oxigênio. / The ceramic films of strontium-doped lanthanum manganite (LSM) and strontiumdoped lanthanum manganite/yttria-stabilized zirconia (LSM/YSZ) are used as cathodes of the high temperature solid oxide fuel cells (HTSOFC). These porous ceramic films had been deposited on the YSZ dense ceramic substrate, used as electrolyte, structural component of the module, thus conferring a configuration of half-cell called auto-support. The study of the half-cell it is basic, therefore in the interface cathode/electrolyte occurs the oxygen reduction reaction, consequently influencing in the performance of the HTSOFC. In this direction, the present work contributes for the processing of thin films, using the wet powder spraying technique, adopted for the conformation of the ceramic films for allowing the attainment of porous layers with thicknesses varied in the order of micrometers. The LSM powders were synthesized by the citrate technique and the LSM/YSZ powders synthesized by the solid mixture technique. In the stage of formation were prepared organic suspensions of LSM and LSM/YSZ fed by gravity in a manual aerograph. For the formation of the YSZ substrate was used a hydraulical uniaxial press. The attainment of solid oxide half-cells cathode/electrolyte was possible of crystalline structures hexagonal for phase LSM and cubic for phase YSZ. The half-cells micrographs show that the YSZ substrate is dense, enough to be used as solid electrolyte, and the LSM and LSM/YSZ films are presented porous with approximately 30 μm of thickness and good adherence between the cathodes and the electrolyte. The presence of composite cathode between the LSM cathode and YSZ substrate, presented an increase in the electrochemical performance in the oxygen reduction reaction.

Os filmes cerâmicos de manganito de lantânio dopado com estrôncio (LSM) e de manganito de lantânio dopado com estrôncio/zircônia estabilizada com ítria (LSM/YSZ) são utilizados como catodos das células a combustível de óxido sólido de temperatura alta (CaCOSTA). Estes filmes cerâmicos porosos foram depositados sobre o substrato cerâmico denso de YSZ, utilizado como eletrólito, componente estrutural do módulo, assim conferindo uma configuração de meia-célula denominada auto-suporte. O estudo da meia-célula é fundamental, pois na interface catodo/eletrólito ocorre a reação de redução do oxigênio, conseqüentemente influenciando no desempenho da CaCOSTA. Neste sentido, o presente trabalho contribui para a síntese de pós de LSM e LSM/YSZ e para o processamento de filmes finos, utilizando a técnica de pulverização de pó úmido, adotada para a conformação dos filmes cerâmicos por permitir a obtenção de camadas porosas com espessuras variadas na ordem de micrômetros. Os pós de LSM foram sintetizados pela técnica de citratos e os pós de LSM/YSZ pela técnica de mistura de sólidos. Na etapa de conformação foram preparadas suspensões orgânicas de LSM e LSM/YSZ alimentada por gravidade em um aerógrafo manual. Para a conformação do substrato de YSZ utilizou-se uma prensa uniaxial hidráulica. Foram possíveis a obtenção das meia-células de óxido sólido catodo/eletrólito de estruturas cristalinas hexagonal para a fase LSM e cúbica para a fase YSZ. E as micrografias das meia-células mostram que o substrato YSZ é denso, suficiente para ser utilizado como eletrólito sólido, e os filmes de LSM e LSM/YSZ apresentam-se porosos com espessura de aproximadamente 30 μm e com boa aderência entre os catodos e o eletrólito. A presença do catodo compósito entre o catodo LSM e o substrato YSZ, possibilitou um aumento no desempenho eletroquímico na reação de redução do oxigênio. / The ceramic films of strontium-doped lanthanum manganite (LSM) and strontiumdoped lanthanum manganite/yttria-stabilized zirconia (LSM/YSZ) are used as cathodes of the high temperature solid oxide fuel cells (HTSOFC). These porous ceramic films had been deposited on the YSZ dense ceramic substrate, used as electrolyte, structural component of the module, thus conferring a configuration of half-cell called auto-support. The study of the half-cell it is basic, therefore in the interface cathode/electrolyte occurs the oxygen reduction reaction, consequently influencing in the performance of the HTSOFC. In this direction, the present work contributes for the processing of thin films, using the wet powder spraying technique, adopted for the conformation of the ceramic films for allowing the attainment of porous layers with thicknesses varied in the order of micrometers. The LSM powders were synthesized by the citrate technique and the LSM/YSZ powders synthesized by the solid mixture technique. In the stage of formation were prepared organic suspensions of LSM and LSM/YSZ fed by gravity in a manual aerograph. For the formation of the YSZ substrate was used a hydraulical uniaxial press. The attainment of solid oxide half-cells cathode/electrolyte was possible of crystalline structures hexagonal for phase LSM and cubic for phase YSZ. The half-cells micrographs show that the YSZ substrate is dense, enough to be used as solid electrolyte, and the LSM and LSM/YSZ films are presented porous with approximately 30 μm of thickness and good adherence between the cathodes and the electrolyte. The presence of composite cathode between the LSM cathode and YSZ substrate, presented an increase in the electrochemical performance in the oxygen reduction reaction.

Nous présentons la dépendance en température de la réponse infrarouge-visible des plans ab de Pr2-xCexCuO4. Dans l'état normal, en variant le dopage de couches minces de x=0.11 (non-supraconductrice) à x=0.17 (supraconductrice surdopée), on observe l'ouverture d'un gap partiel jusqu'au dopage optimal x=0.15. Un modèle d'onde de densité de spin reproduit qualitativement des données. En extrapolant l'amplitude de ce gap, celui-ci s'annule pour x=0.17 indiquant la présence d'un point critique quantique ainsi que la coexistence de magnétisme et de supraconductivité. En dessous de Tc, pour x=0.15 et x=0.17, la modification des spectres optiques à des énergies inférieures à 2Delta=4.7kBTc est interprétée comme la signature de l'ouverture du gap supraconducteur. La réponse optique de La2/3Ca1/3MnO3 mesurée est analysée par un modèle de milieu effectif. L'absorption supplémentaire proche de la transition isolant/métal est interprétée comme la signature d'une percolation dans le système.

Les demi-métaux, du fait de leur forte polarisation en spin jouent un rôle clef dans l'électronique de spin. Nous avons choisi d'étudier l'un d'eux : le manganite La0.7Sr0.3MnO3 (LSMO). Nous avons étudié les propriétés de transport dépendant du spin dans des hétérostructures à base de couches épitaxiales de LSMO. Il était ainsi requis d'étudier aussi l'épitaxie, la relaxation des contraintes et les modification de l'anisotropie magnétique du LSMO sous contraintes. L'étude des propriétés de magnétotransport de jonctions formées à partir de superréseaux de LSMO/SrTiO3 (STO) et à partir de tricouches de LSMO/STO/Co0.88Fe0.12 a montré des dépendances thermiques rapides des effets magnétorésistifs en température. Ces effets étant liés au magnétisme de surface du LSMO, nous avons donc effectué des mesures de magnétisme et de stoechiométrie en oxygène spatialement résolues à l'échelle du nanomètre en utilisant la réflectométrie de neutrons polarisés et la réflectivité X.

Les manganites de terres rares hexagonaux RMnO3 sont multiferroïques à basse température : ils sont simultanément ferroélectriques (TC ~ 900 K) et antiferromagnétiques (TN ~ 80 K) sous forme massive. Nous avons élaboré par MOCVD des hétérostructures RMnO3 et (RMnO3/R'MnO3)n, avec R = Y, Ho, Er, Dy et Tb, sur des substrats de ZrO2(Y2O3) (111) et Pt (111)/Si. L'axe c est perpendiculaire au substrat dans tous les cas. Les effets de l'épaisseur et de la composition des films sur leur structure ont été étudiés. La diffraction neutronique a mis en évidence une transition antiferromagnétique dans les films, dont la température TN diminue avec l'épaisseur. La taille des domaines magnétiques est de l'ordre de 20 à 50 nm. Des mesures optiques par génération de second harmonique ont montré que les films sont ferroélectriques à la température ambiante. Un effet magnétocapacitif géant, d'origine extrinsèque, a été observé sur une multicouche (YMnO3/HoMnO3)15.