Mesógenos alquiloxicianobifenilos : aspectos teóricos y experimentales en las transiciones de fase SmA-N y N-I

Tesi amb diferents seccions retallades per drets d'autor.The work presented here has been focused, basically, on the study of the SmA-N and N-I phase transitions of a group of liquid crystals, the alkyloxycyanobiphenyls (nOCB), in which, for some of them, as it is the case of 9OCB, there was practically no information. This study has been performed for the pure compounds (6OCB, 7OCB, 8OCB, 9OCB, 10OCB) and also for certain binary mixtures (8OCB+10OCB, 7OCB+9OCB). From an experimental point of view, there were used various techniques, as MDSC, dielectric spectroscopy, X ray diffraction, optical microscopy, densitometry and pressure calorimetry. These techniques allow us to obtain the evolution of various physical magnitudes of the materials used throughout their different phase transitions. The theoretical study centered on the application of the different theories about the order of the phase transitions with the aim of evaluating their critical behavior, based, generally, on thermodynamic and structural considerations. At the same time, using purely thermodynamic considerations, there has been developed an extension of the Oonk's method, which, makes possible the calculation of reentrant nematic domains and the estimation of the tricritical temperatures.Esta tesis de desarrolla básicamente en cuatro partes: En la primera describiremos y clasificaremos los cristales líquidos de una manera general; se describirán las transiciones de fase de éstos y las teorías relacionadas con las mismas. En la segunda parte se describirán las técnicas experimentales estudiadas, así como el instrumental y los métodos que se aplicaron para los datos experimentales. Asimismo, se presentarán los conceptos de estabilidad y el análisis termodinámico basado en el método de Oonk. En la tercera parte se presentan las características generales de los compuestos puros estudiados con un análisis detallado de los compuestos 6,7, y 9OCB, con sus respectivos resultados de mediciones experimentales. En la cuarta parte, se demuestra un nuevo procedimiento de estimación de dominios de nemática reentrante y determinación de puntos tricríticos en sistemas biocomponente aplicado en algunos sistemas binarios. Finalmente, en la última parte se presentan las conclusiones generales, donde se explican de una manera global los resultados de este trabajoPostprint (published version

Sintering of mixed powders

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Study of the critical behavior and scaling relationships at the N-to-I phase transition in hexyloxycyanophiphenyl

An exhaustive analysis of the critical behavior of the nematic to isotropic (N-to-I) phase transition on the liquid crystal hexyloxycyanobiphenyl (6OCB) has been performed. To do so, the accurate evolution of various physical magnitudes (static dielectric permittivity data together with specific heat and volumetric determinations) around the N-to-I transition has been required. The specific heat data with the isobaric thermal expansion coefficient and the derivative of the static dielectric permittivity with temperature have been proven to be related to each other by a scaling relationship. However, some discrepancies have been observed for the dielectric data in relation to such a scaling relationship and the critical behavior of the N-to-I phase transition. All information has been used to get some insight on the strength of the first-order N-to-I phase transition of the 6OCB in relation to the other counterparts in the nOCB series of compounds