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

Trends on aerogel-based biosensors for medical applications: an overview

Producción CientíficaAerogels are unique solid-state materials composed of interconnected 3D solid networks and a large number of air-filled pores. This structure leads to extended structural characteristics as well as physicochemical properties of the nanoscale building blocks to macroscale, and integrated typical features of aerogels, such as high porosity, large surface area, and low density, with specific properties of the various constituents. Due to their combination of excellent properties, aerogels attract much interest in various applications, ranging from medicine to construction. In recent decades, their potential was exploited in many aerogels’ materials, either organic, inorganic or hybrid. Considerable research efforts in recent years have been devoted to the development of aerogel-based biosensors and encouraging accomplishments have been achieved. In this work, recent (2018–2023) and ground-breaking advances in the preparation, classification, and physicochemical properties of aerogels and their sensing applications are presented. Different types of biosensors in which aerogels play a fundamental role are being explored and are collected in this manuscript. Moreover, the current challenges and some perspectives for the development of high-performance aerogel-based biosensors are summarized.Fundación Portuguesa para la Ciencia y la Tecnología (FCT) - (grant UIDB/04029/2020)Fundación Portuguesa para la Ciencia y la Tecnología (FCT) - (grant LA/P/0112/2020)Fundación Portuguesa para la Ciencia y la Tecnología (FCT) - (grant SFRH/BD/150790/2020)Junta de Castilla y León, Ministerio de Ciencia e Innovación y Unión Europea NextGenerationEU/PRTR,Unión Europea (UE)- Fondo Europeo de Desarrollo Regional (FEDER) - (grant MA2TEC

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

Optical Properties of Polyisocyanurate–Polyurethane Aerogels: Study of the Scattering Mechanisms

Highly transparent polyisocyanurate–polyurethane (PUR–PIR) aerogels were synthesized, and their optical properties were studied in detail. After determining the density and structural parameters of the manufactured materials, we analyzed their optical transmittance. It was demonstrated that the catalyst content used to produce the aerogels can be employed to tune the internal structure and optical properties. The results show that the employment of lower catalyst amounts leads to smaller particles forming the aerogel and concomitantly to higher transmittances, which reach values of 85% (650 nm) due to aerogel particles acting as scattering centers. Thus, it was found that the lower this size, the higher the transmittance. The effect of the sample thickness on the transmittance was studied through the Beer–Lambert law. Finally, the scattering mechanisms involved in the light attenuation were systematically evaluated by measuring a wide range of light wavelengths and determining the transition between Rayleigh and Mie scattering when the particles were larger. Therefore, the optical properties of polyurethane aerogels were studied for the first time, opening a wide range of applications in building and energy sectors such as glazing windows

Improving the Insulating Capacity of Polyurethane Foams through Polyurethane Aerogel Inclusion: From Insulation to Superinsulation

A novel synthesis of polyurethane foam/polyurethane aerogel (PUF–PUA) composites is presented. Three different polyurethane reticulated foams which present the same density but different pore sizes (named S for small, M for medium, and L for large) have been used. After the characterization of the reference materials (either, foams, and pure aerogel), the obtained composites have been characterized in order to study the effect of the foam pore size on the final properties, so that density, shrinkage, porous structure, mechanical properties, and thermal conductivity are determined. A clear influence of the pore size on the density and shrinkage was found, and the lowest densities are those obtained from L composites (123 kg/m3). Moreover, the aerogel density and shrinkage have been significantly reduced through the employment of the polyurethane (PU) foam skeleton. Due to the enhanced mechanical properties of polyurethane aerogels, the inclusion of polyurethane aerogel into the foam skeleton helps to increase the elastic modulus of the foams from 0.03 and 0.08 MPa to 0.85 MPa, while keeping great flexibility and recovery ratios. Moreover, the synthesized PUF–PUA composites show an excellent insulating performance, reducing the initial thermal conductivity values from 34.1, 40.3, and 50.6 mW/(m K) at 10 °C for the foams S, M, and L, to 15.8, 16.6, and 16.1 mW/(m K), respectively. Additionally, the effect of the different heat transfer mechanisms to the total thermal conductivity is herein analyzed by using a theoretical model as well as the influence of the measurement temperature

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

Improving the Insulating Capacity of Polyurethane Foams through Polyurethane Aerogel Inclusion: From Insulation to Superinsulation

A novel synthesis of polyurethane foam/polyurethane aerogel (PUF–PUA) composites is presented. Three different polyurethane reticulated foams which present the same density but different pore sizes (named S for small, M for medium, and L for large) have been used. After the characterization of the reference materials (either, foams, and pure aerogel), the obtained composites have been characterized in order to study the effect of the foam pore size on the final properties, so that density, shrinkage, porous structure, mechanical properties, and thermal conductivity are determined. A clear influence of the pore size on the density and shrinkage was found, and the lowest densities are those obtained from L composites (123 kg/m3). Moreover, the aerogel density and shrinkage have been significantly reduced through the employment of the polyurethane (PU) foam skeleton. Due to the enhanced mechanical properties of polyurethane aerogels, the inclusion of polyurethane aerogel into the foam skeleton helps to increase the elastic modulus of the foams from 0.03 and 0.08 MPa to 0.85 MPa, while keeping great flexibility and recovery ratios. Moreover, the synthesized PUF–PUA composites show an excellent insulating performance, reducing the initial thermal conductivity values from 34.1, 40.3, and 50.6 mW/(m K) at 10 °C for the foams S, M, and L, to 15.8, 16.6, and 16.1 mW/(m K), respectively. Additionally, the effect of the different heat transfer mechanisms to the total thermal conductivity is herein analyzed by using a theoretical model as well as the influence of the measurement temperature

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

Nanoparticles Addition in PU Foams: The Dramatic Effect of Trapped-Air on Nucleation

To determine the effect of nanoclays and trapped air on the formation of rigid polyurethane foams, three different production procedures were used. To study the influence of mixing at atmospheric pressure, two approaches were carried out employing either an electric or a magnetic stirrer. The third approach was executed by mixing under vacuum conditions with magnetic stirring. The samples thus obtained were characterized, and the effect of trapped air into the reactive mixtures was evaluated by analyzing the cellular structures. Different levels of trapped air were achieved when employing each manufacturing method. A correlation between the trapped air and the increase in the nucleation density when nanoclays were added was found: the cell nucleation density increased by 1.54 and 1.25 times under atmospheric conditions with electric and magnetic stirring, respectively. Nevertheless, samples fabricated without the presence of air did not show any nucleating effect despite the nanoclay addition (ratio of 1.09). This result suggests that the inclusion of air into the components is key for improving nucleation and that this effect is more pronounced when the polyol viscosity increases due to nanoclay addition. This is the most important feature determining the nucleating effect and, therefore, the corresponding cell size decreases