Memoria para optar al título de Químico / Se estudiaron mezclas de tres monómeros líquidos cristalinos con fullereno: dos de ellos son azo compuestos A6OA12 y A6A12, y el tercero una imina, A11R6. No se apreció una buena miscibilidad entre dichos compuestos y fullereno utilizando técnicas de microscopia de luz polarizada y Análisis Térmico Diferencial. Un mejor resultado se obtuvo al utilizar un compósito del tipo polímero:monómero, el cual posee comportamiento antiferroeléctrico. Se obtuvieron mezclas homogéneas que fueron posteriormente dispuestas en celdas de 6,8 μm. Las celdas fueron sometidas al proceso de poling y estudiadas utilizando la técnica piroeléctrica. Las mezclas polímero:monómero con fullereno no perdieron sus propiedades antiferroeléctricas. Se observó un considerable aumento en el valor del coeficiente piroeléctrico. Este aumento se atribuye a un aumento en la absorción de luz y por tanto térmica por parte del compósito en longitudes de onda en el infrarrojo aportadas por el fullereno. Los incrementos registrados en las curvas piroeléctricas fueron más de dos veces amplificados con respecto al composito sin fullereno, a concentraciones tan baja como 2,1% en la mezcla. Es posible considerar aplicaciones de este material en sensores o para el desarrollo de celdas fotovoltaica de alta eficiencia / Mixtures of three liquid crystalline fullerene monomers were studied: two of them are azo compounds A6OA12 and A6A12, and the third one an imine, A11R6. Good miscibility between these fullerene and the monomeric compounds was not appreciated using techniques of polarized light microscopy and differential thermal analysis. A better result was obtained by using composite of polymer:monomer, which had been probed to exhibit antiferroelectric behavior. Homogeneous mixtures with fullerene were then arranged into cells of 6.8 microns. The cells were subjected to the poling process and studied using the pyroelectric technique. The mixtures polymer:monomer and fullerene did not lose its antiferroelectrics properties. A considerable increase in the value of the pyroelectric coefficient was observed. This increase is attributed to an increase in light absorption and thus heat in the composite provided by the fullerene inclusion at the near infrared. The pyroelectric curves were more than two times amplified compared with the composite without fullerene, at concentrations as low as 2.1% in the mixture. Applications as sensors of this material or for the development of high efficiency photovoltaic cells may be expected for these materials

The study of the elastic, dielectric and piezoelectric non-linear behavior in PZT-based and KNN-based piezoelectric ceramics represent the main goal of this work. Both systems present a perovskite-like structure. Several experimental set-ups were designed, mounted and automated to be used in this study. First, the elastic non-linear response, in resonance, is measured. The purpose is to study the variation of the elastic coefficient of the material when it is submitted to high stress. A model is formulated based on the variation of the coefficients due to the increase in amplitude of the high stress applied. The correspondence between the model and the experimental observations is verified. The results show a good agreement in soft PZT case but not in hard PZT, confirming that is not possible to assume a non-linear -like dependence for hard PZT coefficients. Secondly, we take a view of the dielectric, piezoelectric and elastic response over the temperature. The purpose of the study is to evaluate the contribution of the extrinsic effect to piezoceramics response. Anomalous behavior of the macroscopic properties of the hard PZT around the Room Temperature is observed. A preliminary model is proposed to explain such behavior. The contribution of the extrinsic effect to the macroscopic properties in a KNN compound is evaluated and the results suggest that the anomaly observed around Room Temperature would be associated, in this case, with the presence of a structural transition between two ferroelectric phases. Temperature dependent non-linear dielectric response is also observed. The study was carried out by measuring the dielectric permittivity at low frequencies and at high applied electric fields in a temperature range from 20K till 400K. The results have been analyzed using the Rayleigh model. The change in the hard PZT dynamic walls is shown and explained through an intuitive model. The results obtained in the KNN system display a soft PZT like behavior at Room Temperature. Nevertheless, the anomaly observed in the dielectric proper ties around Room Temperature was associated to the above mentioned structural transition between two ferroelectric phases. Finally, temperature dependent mechanical non-linear response in soft PZT ceramics is studied. The variations of the coefficients with the stress and with the temperature are studied and the similarities between the linear and non-linear regime are assessed. It is observed that the change of the extrinsic contribution with the temperature is qualitatively similar that the variation observed by the application of the high mechanical stress, thus indicating that the effect is due to a common phenomenon.

Esta tesis versa en la síntesis, mediante métodos de baja temperatura(sol-gel y solvotermal), la caracterización y el estudio de las propiedades eléctricas, mediante Espectroscopia de Impedancia, de dos tipos de materiales ferroeléctricos: el óxido mixto de titanio y bario, BaTiO3(BT), dopado con diferentes iones (con estructura perovsquita), y la familia de compuestos Ba2LnTi2Nb3O15 (Ln = ión lantánido), con estructura de bronce de tungsteno tetragonal (TTB). Los principales resultados en el primer sistema estudiado (BT) es el descubrimiento de un nuevo fenómeno en materiales dopados con iones aceptores relacionado con la respuesta no-óhmica de su resistencia al aplicar un pequeño voltaje dc, tanto de la frontera como del interior de grano, lo que los hace prometedores para aplicaciones tales como dispositivos de memoria (similares a los memristores), de protección o sensores. Respecto al segundo sistema estudiado (TTB), el principal resultado fue la inusual histéresis térmica en la transición ferroeléctrica y la gran diferencia entre la temperatura de transición y la temperatura de Curie-Weiss. En todos los materiales se ha analizado en detalle la relación existente entre estructura, composición y propiedad.