Synthesis, characterization and structure-properties relationship of aerogels and aerogel-based composites

During the last few years, cellular materials research has pointed to aerogels as one of the most promising materials. This is due to the combination of properties, such as their low density, large specific surface area, excellent thermal and acoustic insulation, and high porosity. These properties are the consequence of their interesting structures characterized by pores and particles in the nanometric scale. This thesis focuses on the synthesis and characterization of aerogels and aerogel-based composites. The main objective has been the development of polyurethane-based organic aerogels obtained by sol-gel technology followed by supercritical drying. These materials present interesting optical properties (transparency to visible light), a thermal conductivity below that of the aerogels previously described for this matrix (included in the superinsulating range), achieving values of 12 mW/mK, and superior mechanical properties than the widely investigated silica aerogels. Additionally, the effect of the addition of infrared blockers on its thermal conductivity has been studied. On the other hand, the incorporation of polyurethane foams with reticulated structures during the synthesis of silica aerogels has allowed the notable improvement of their mechanical properties, providing them with great stiffness and flexibility. In addition, these materials have excellent thermal insulation. As a result of this research, the relationship between formulation-process-structure-properties has been understood, so that, through the analysis of different parameters, their influence on the final properties has been identified. Thus, certain synthesis strategies are established allowing to obtain promising aerogels with potential application in the construction, automotive, and aeronautics sectors, among others. This thesis, supervised by Prof. Dr. Miguel Ángel Rodríguez-Pérez and Prof. Dr. Fernando Villafañe González, constitutes the opening of a new research line at CellMat Laboratory, Department of Condensed Matter Physics of the University of Valladolid, allowing the development of numerous lines of future work.Durante los últimos años, la investigación en campo de los materiales celulares ha señalado a los aerogeles como uno de los materiales más prometedores. Esto es debido a la combinación de propiedades entre las cuales se encuentran su baja densidad, gran superficie específica, excelente aislamiento térmico y acústico, y alta porosidad. Estas propiedades son la consecuencia directa de sus interesantes estructuras caracterizadas por tamaños de poro y partículas que se encuentran en la escala nanométrica. Esta tesis se centra en la síntesis y caracterización de aerogeles y materiales compuestos basados en aerogel. El principal objetivo ha sido el desarrollo de aerogeles orgánicos en base poliuretano obtenidos mediante la tecnología sol-gel seguida de secado supercrítico. Estos materiales presentan propiedades ópticas de interés (transparencia a la luz visible), una conductividad térmica por debajo de los aerogeles previamente descritos para esta matriz, que están comprendidos en el rango del superaislamiento consiguiendo valores de tan solo 12 mW/mK, y unas propiedades mecánicas superiores a los aerogeles de sílice ampliamente investigados. Adicionalmente, se ha estudiado el efecto de la adición de partículas bloqueadoras de la radiación infrarroja en su conductividad térmica. Por otro lado, la incorporación de espumas de poliuretano con estructuras reticuladas durante la síntesis de aerogeles de sílice ha permitido la notable mejora de sus propiedades mecánicas, proporcionándoles una gran resistencia a la compresión y flexibilidad. Además, estos materiales presentan un aislamiento térmico excelente. Como resultado de esta investigación, se ha comprendido la relación entre formulación- proceso-estructura propiedades, de forma que, mediante el análisis de diferentes parámetros, se ha identificado su influencia en las propiedades finales. Así, se establecen ciertas estrategias de síntesis que permiten la obtención de aerogeles prometedores con potencial aplicación en el sector de la construcción, automoción, aeronáutica, entre otros. Esta tesis, supervisada por el Prof. Dr. Miguel Ángel Rodríguez-Pérez el Prof. Dr. Fernando Villafañe González, constituye la apertura de una nueva línea de investigación en el Laboratorio CellMat del Departamento de Física de la Materia Condensada de la Universidad de Valladolid, permitiendo el desarrollo de numerosas líneas de trabajo futuro.Escuela de DoctoradoDoctorado en Físic

Thermal conductivity of nanoporous materials: Where Is the limit?

Producción CientíficaNowadays, our society is facing problems related to energy availability. Owing to the energy savings that insulators provide, the search for effective insulating materials is a focus of interest. Since the current insulators do not meet the increasingly strict requirements, developing materials with a greater insulating capacity is needed. Until now, several nanoporous materials have been considered as superinsulators achieving thermal conductivities below that of the air 26 mW/(m K), like nanocellular PMMA/TPU, silica aerogels, and polyurethane aerogels reaching 24.8, 10, and 12 mW/(m K), respectively. In the search for the minimum thermal conductivity, still undiscovered, the first step is understanding heat transfer in nanoporous materials. The main features leading to superinsulation are low density, nanopores, and solid interruptions hindering the phonon transfer. The second crucial condition is obtaining reliable thermal conductivity measurement techniques. This review summarizes these techniques, and data in the literature regarding the structure and thermal conductivity of two nanoporous materials, nanocellular polymers and aerogels. The key conclusion of this analysis specifies that only steady-state methods provide a reliable value for thermal conductivity of superinsulators. Finally, a theoretical discussion is performed providing a detailed background to further explore the lower limit of superinsulation to develop more efficient materials.Ministerio de Ciencia, Innovación y Universidades - (grant FPU17/03299)Ministerio de Ciencia, Innovación y Universidades - (project RTI2018-098749-B-I00)Junta de Castilla y León y Ente Público Regional de la Energía de Castilla y León (EREN) - (Grant VA202P20)Junta de Castilla y León y Fondo Europeo de Desarrollo Regional (FEDER) - (grant CLU-2019-04

Complejos heteropolimetálicos con el fragmento tricarbonilrenio(I) y piridilpirazolato puente

En este trabajo se ha completado la caracterización de una serie de precursores catiónicos y neutros sintetizados durante el Trabajo de Fin de Grado mediante estudios por RMN de desprotonación y de sus propiedades fotofísicas. Se han empleado como compuestos de partida para lograr la síntesis de tres complejos heteropolimetálicos con el fragmento tricarbonilrenio(I), de los que hay escasos precedentes bibliográficos. Estos complejos se han caracterizado mediante las técnicas de RMN, IR en disolución y estado sólido, y sus estructuras cristalinas se han determinado mediante difracción de rayos-X. También se ha optimizado la síntesis del ligando pypzH y de varios complejos metálicos de Mn, Re y Ru que se utilizan como productos de partida en éste y en otros trabajos de nuestro grupo de investigación empleando un reactor microondas.Departamento de Química Física y Química InorgánicaMáster en Química Sintética e Industria

Complejos de renio(I) con el ligando 3-(2-piridil)pirazol

Este trabajo consiste en la síntesis de complejos tricarbonilrenio (I) que contengan coordinado el ligando quelato 3-(2-piridil) pirazol (pypzH) y otros lignados de tipo pirazol en la sexta posición de coordinación. El trabajo se completa con una pequeña introducción al estudio de su reactividad, más concretamente en procesos de desprotonación. Los complejos en los que el fragmento tricarbonilrenio (I) se encuentra unido a ligandos bidentados quelato se caracterizan por sus interesantes propiedades fotofísicas. El objetivo último de este trabajo sería estudiar posteriormente la luminiscencia y su variación en función de los sustituyentes presentes en los complejos. En el presente trabajo se describe la síntesis y la caracterización de cuatro complejos catiónicos del tipo fa-[Re(CO)3(pypzH)L]+ (L = pzH, dmpzH, indzH, pypzH). Posteriormente, se ha desprotonado uno de los hidrógenos ácidos presentes por reacción con una base débil obteniéndose así los correspondientes complejos neutros. Los complejos sintetizados se caracterizan mediante RMN y en cuatro casos por difracción de rayos-X de monocrital. Ninguno de los compuestos había sido descrito previamente en la bibliografía.Grado en Químic

Transparent polyisocyanurate-polyurethane-based aerogels: key aspects on the synthesis and their porous structures

Producción CientíficaThe effect of the catalyst concentration on the synthesis and textural properties of polyisocyanurate-polyurethane aerogels is analyzed. The use of different catalyst amounts allows obtaining low-density aerogels (0.10–0.16 g/cm3) with high porosities (85–91%). Their porous structures were analyzed by scanning electron microscopy and nitrogen adsorption–desorption isotherm. A noticeable decrease in the size of the scattering centers, particles and pores, was achieved when reducing the catalyst amount. In some samples, the small size of these features, much smaller than the wavelength of visible light, causes a bare light dispersion, leading to the first transparent polyisocyanurate-polyurethane aerogels. Light transmittance measurements at 532 nm have been made showing high values (ca. 76% for 1 mm thick samples) for the formulations with the smallest particle and pore sizes. These aerogels presenting optical transparency have many potential applications such as solar collectors, glazing systems for insulating windows, and sensors, among others.Ministerio de Ciencia, Innovación y Universidades (grant RTI2018-098749-B-I00)Junta de Castilla y León (grant VA202P20

Reaching a near zero radiative heat transfer by the inclusion of modified multiwalled-carbon nanotubes (MWCNTs) in polyurethane-polyisocyanurate aerogels

Producción CientíficaThis article studies the effect of adding different contents of surface-modified multiwalled carbon nanotubes (CNTs) on the structure and final properties of polyisocyanurate-polyurethane aerogels. The produced samples were characterized in terms of density, porosity, shrinkage, textural properties, mechanical behavior, ultrasonic behavior, and thermal conductivity. Low-weight aerogels were obtained with densities between 89 and 95 kg/m3. The inclusion of homogeneously dispersed carbon nanotubes protected the aerogel structure during supercritical drying decreasing the final shrinkage of these samples and, therefore, increasing their porosity. The porous structure of the aerogels containing CNTs was modified, and slightly larger pores were formed. The structural modifications contribute to decrease the stiffness of the samples while improving their resilience and elasticity. Finally, a significant enhancement on the insulating performance of the aerogels has been found when CNTs were added reaching values as low as 12 mW/mK. These fillers were proved to act as infrared opacifiers by absorbing and scattering the infrared radiation, a relevant contribution in these lightweight materials. In fact, the radiation contribution was strongly reduced when the content of CNTs reached 3 wt%, being this contribution near zero. Thus, this content was considered as the optimum for the final properties-balance in these aerogels.Ministerio de Ciencia e Innovación y Ministerio de Universidades (grant FPU17/03299)Ministerio de Ciencia e Innovación y Ministerio de Universidades (PID2021-127108OB-I00, TED2021-130965B–I00 y PDC2022-133391-I00)Junta de Castilla y León - EU-FEDER (CLU-2019-04 y VA202P20

Methodology for measuring the thermal conductivity of insulating samples with small dimensions by heat flow meter technique

Producción CientíficaNowadays, developing advanced, highly insulating materials for minimizing heat losses in buildings is of utmost relevance. Thus, there is a constant research activity focused on developing new and enhanced solutions for thermal insulation. However, characterizing the behavior of new thermal insulation materials, usually produced at lab-scale with small dimensions, by a steady-state approach is a challenge. The reason is that commercial heat fow meters require large samples (hundred on mm side) to provide accurate results of thermal conductivity because they are based on international standards. In this work, a new methodology to measure the thermal conductivity of small prototypes of thermal insulating materials (as low as 50×50 mm2 ) is developed by using an external heat fow sensor placed into a standard heat fow meter apparatus. Four diferent thermal insulators were used to validate the developed methodology by performing measurements in the heat fow meter with and without the external sensor. From these results, a calibration curve that relates both methods was calculated. Furthermore, the efect of the sample size was studied to explore the limits of the technique. Results show that the self-developed method is an accurate procedure to determine the thermal conductivity of samples with small dimensions via a steady-state condition.Ministerio de Ciencia, Innovación y Universidades (FPU Grant FPU17/03299)Ministerio de Ciencia, Innovación y Universidades (RTI2018- 098749-B-I00 y PTQ2019-010560Fondo de Desarrollo Regional de la Unión Europea y de la Junta de Castilla y León (ICE): R&D PROJECTS IN SMEs: PAVIPEX. (04/18/VA/008) and M-ERA.NET PROJECT: FICACEL. (11/20/VA/0001)Publicación en abierto financiada por el Consorcio de Bibliotecas Universitarias de Castilla y León (BUCLE), con cargo al Programa Operativo 2014ES16RFOP009 FEDER 2014-2020 DE CASTILLA Y LEÓN, Actuación:20007-CL - Apoyo Consorcio BUCL

Enhanced nitrates-polluted water remediation by polyurethane/sepiolite cellular nanocomposites

Producción CientíficaNanocomposite hydrophilic flexible polyurethane foams were prepared by in situ polymerization using an organomodified sepiolite as filler. The sepiolite affinity by nitrates dissolved in water was increased by treating them with quaternary ammonium salts (S-QAS), leading to remarkable nitrates maximum adsorption capacity (qmax = 23.30 mg/g). The Langmuir model well described the adsorption process of the nitrates by the modified sepiolite, and more remarkably, the sepiolite suffers no decrease of their nitrates removal capacity after being imbibed in the flexible polyurethane foam. The proper selection of the polyurethane chemistry allows significant contact between the polluted-water and the sepiolite, even when enclosed inside the cell walls. Accordingly, the as-prepared hydrophilic polyurethane/sepiolite foams provided a facile removal of nitrates-pollution from water, even actual nitrates-polluted water with a competitive adsorption environment, without the need to control or modify the pH and temperature of the polluted water neither to add further filtration steps to collect the active particles from the water.MAT2015-69234-RRTC-2015-3340-3RTI2018-097367-A-I00VA275P1

Whole microwave syntheses of pyridylpyrazole and of Re and Ru luminescent pyridylpyrazole complexes

Producción CientíficaThe synthesis of 3-(2pyridyl)pyrazole (pypzH) by conventional methods requires refluxing at high temperatures during more than 16 h, but this is reduced to two consecutive steps of 2 h at 100 °C, and of 10 min at 50 °C in a microwave (MW) assisted synthesis. Its coordination as chelating ligand to the “fac-ReIBr(CO)3” and “RuII(bipy)2” fragments also occurs MW assisted in 5 or 10 min, respectively. A new MW assisted synthetic method is also described for the synthesis of the starting material [Ru(bipy)2Cl2]·2H2O. Both pypzH complexes are characterized by X-ray crystallography, and the study of their photophysical properties support their phosphorescence. The electrochemistry of the Ru complex indicates that electrochemical oxidation is followed by a chemical process.This research was supported by the Spanish MINECO (Project CTQ2013-41067-P). E. C. thanks the UVa for her grant. The authors in Burgos gratefully acknowledge financial support from the Spanish MINECO, Spain (Project CTQ2015-71353-R) and Junta de Castilla y León, Consejería de Educación y Cultura y Fondo Social Europeo (Project BU051U16)