Synthesis of low density and high temperature resistant Y2O3 doped silica aerogels

Commercialization of aerogels has been slow down due to high cost and manufacturability issues [1]. Therefore, in the present paper, we manufactured silica aerogels at low cost and low risk. In this paper, we report the experimental results on the synthesis of low density and high temperature resistant Y2O3 doped silica aerogels. Silica sols were prepared by keeping the molar ratio of TEOS: ethanol (EtOH): water ( 0.01 M HCl as acid catalyst): Ammonium Flu-oride (0.5 M NH4F as base catalyst) was kept constant at 1:15:7:0.6 respectively, while the weight percent of Y2O3 powder was varied from 0.1 to 4%. The aerogels have been produced by two-stage sol-gel process followed by supercritical CO2 drying. The best quality silica aer-ogel in terms of low high-temperature thermal conductivity (0.097 W.mK-1 at 1000 °C), low density (26 mg/cc) and high optical transmission (about 87% in the red region) have been obtained with molar ratio of 1TEOS: 15EtOH:7water:0.6 NH4F and 3% of Y2O3 powder. The best quality Y2O3 doped silica aerogel is as shown in figure (Fig 1). The Y2O3 doped aerogels have been characterized by SEM (Scanning Electron Microscopy), FTIR (Fourier Transform Infra-Red Spectroscopy), Optical transmittance, TG/DTA (Thermogra-vimetry/Differential Thermal Analysis, Thermal conductivity and BET (Brunauer-Emmett-Teller) analysis. The experimental results on the physical and thermal properties of Y2O3 doped silica aerogels under normal and high temperature have been discussed by taking into account the chemistry and porosity of aerogels

Carbon aerogels - promising materials for fuel cell applications

Carbon aerogels, first introduced by Richard Pekala in 1989, are three-dimensional, open porous solid materials produced via carbonization of organic aerogels based on e.g. resorcinol-formaldehyde, phenol-formaldehyde or melamine-formaldehyde polymers. The synthesis parameters of organic aerogels such as pH-value, the amount of catalyst, the molar ratios of educts, temperature influence the formation of the microstructure of aerogels significantly. The microstructure, in turn, is reflected in properties of aerogels. An increase in pH during stirring leads to the formation of small particles. Differences in the molar ratios of the reactants affect the connectivity between the particles and have a corresponding direct effect on the electrical properties of the aerogel. The final structure of carbon aerogel depends very critically on carbonization process, e.g. temperature, duration, gases. Unique properties of carbon aerogels such as well-controlled porosity and pore size, large specific surface area about 500-2000 m²/g, high electrical conductivity, and low envelope density make them promising material for application in adsorption, catalysis, supercapacitors, fuel cells or as a cathode host in metal-sulfur cells. Their remarkable electrical conductivity is one of the key factors for electrochemical applications. The open-pore network with adjustable microstructure offers a high level of freedom in material design to specifically adapt the carbon matrix to the requirements of the electrode. Within the presentation we will report on the properties of carbon aerogel materials and dependences of the synthesis and carbonization routines

Effect of diffusive and ballistic aggregation on properties of gels

Effect of diffusive and ballistic aggregation on structural and fractal properties of gels was demonstrated in the poster

Silylation of Monolithic Resorcinol-Formaldehyde Xerogels

Hydrophobic materials are useful for many applications where liquids are involved. Phenolics represent a class of materials that is intrinsically hydrophilic due to the presence of free aromatic hydroxy groups. Due to their high porosity, phenolic aerogels and xerogels are particularly prone to absorb water and other polar liquids. This behavior includes the most prominent organic gels, based on resorcinol-formaldehyde (RF). However, few strategies to overcome the inherent hydrophobicity of RF gels are reported, and these are often limited to powders. Literature-known examples include the trimethylsilylation of powdered RF resulting in material that retain hydrophobic for a limited time. To this end, we report on the silylation of monolithic RF xerogels, using sterically demanding silyl reagents. Our synthetic studies include the variation of amine base and electronic nature of the silyl counterion. Characterization of the resulting gels by means of elemental analysis, X-ray photoelectron spectroscopy, pycnometry, sorption analysis, and scanning electron microscopy with electron-dispersive X-ray spectroscopy is presented. Furthermore, the wetting behavior of the silylated gels exemplified by their contact angle to water after exposure of the gels to ambient air and various liquids (water, acid and base) is discussed. Though apparently silylated to little extent, these gels retained significant hydrophobic behavior over the course of several months. Moreover, monoliths bearing sterically demanding silyl groups displayed higher stability towards aqueous acid than trimethylsilylated RF gels

Long-lasting hydrophobization of monolithic phenolic gels via silylation

The nature of a material’s surface is of great importance for industrial applications, and various physical and chemical methods are used to adapt it to the respective requirements. Hydrophobic materials are particularly suitable for applications where the surface needs to be protected from moisture or corrosion. Due to the presence of free aromatic hydroxyl groups, phenolics are naturally hydrophilic materials. Phenolic aerogels and xerogels are particularly sensitive to the absorption of water and other polar liquids due to their high porosity. This behavior is also typical for organic gels based on resorcinol-formaldehyde (RF). Among the few literature-known methods for the chemical modification of RF is its silylation using trimethylsilyl reagents. However, the method described is restricted to powdered specimen and provides the material with a temporary hydrophobicity of a few weeks. The research objective of this work was to develop methods for the hydrophobization of RF aerogels using silylation processes that offer longer-lasting protection. For the purpose of increasing reactivity of silyl reagents, electronically activated reagents (solutions of silyl chlorides and triflates) were employed as well as suitable amine bases. In order to decrease lability of the resulting silyl ethers towards hydrolysis, reagents based on sterically demanding silyl moieties were investigated. Elemental analysis, X-ray photoelectron spectroscopy, pycnometry, absorption analysis and scanning electron microscopy using electron scattering X-ray spectroscopy were used to structurally characterize the resulting gels. Furthermore, the wetting behavior of the modified gels was investigated on the basis of the contact angle. Despite only slight incorporation of the silylation reagents, highly hydrophobic gels were formed that maintained the wetting behavior with contact angles of >130° over the course of several months. The hydrophobic behavior of the monoliths was also retained over several months in acidic or base environments. As envisioned, monoliths with sterically demanding silyl groups showed a higher stability to aqueous acid than trimethylsilylated RF gels

Synthesis of Cellulose Aerogel Fibers from Agricultural Residues and Considerations on its Continuous Production

Academia and industry have been developing ways to produce novel materials from renewable bio-based sources. We have been developing strategies to produce cellulose aerogel fibers from waste hemp fibers and study its continuous production using the LabLineCompact® technology. Alkali hydrolysis and bleaching established a simple method to extract high-grade cellulose from hemp waste fiber fibers. Cellulose solution was prepared by dissolution using a mixture of NaOH, urea, and water as solvent. Optimal parameters such as cellulose concentration and operational conditions of the LabLineCompact® were analyzed, and the production of highly porous aerogel fibers was accomplished from commercial and extracted cellulose. After selection of the best parameters, up-scaling experiments were performed using a Fourné "Lab-to-Pilot" wet-spinning machine. Besides the valorization of waste biomass resources and the production of highly porous aerogel fibers, we developed a continuous system for producing, washing, and exchanging the solvent of the fibers before drying

Electrical conductivity of monolithic and powdered carbon aerogels and their composites

The electrical conductivity of powdered carbon aerogels is one of the key factors required for electro-chemical applications. This study investigates the correlation between the structural, physical, mechanical and electrical properties of pure and activated carbon aerogels, as well as aerogel-composites. The thermal activation with carbon dioxide led to higher electrical conductivity and a decrease in density and particle size. Furthermore, the influence of applied force, compressibility of aerogels and aerogel composites on electrical conductivity was studied. A number of different carbonaceous powdered additives with various morphologies, from almost spherical to fiber- and flake-like shaped, were investigated. For two composites, theoretical values for conductivity were calculated showing the great contribution of particle shape to the conductivity. The results show that the conductive behavior of composites during compression is based on both the mechanical particle arrangement mechanism and increasing particle contact area

Electrical conductivity of monolithic and powdered carbon aerogels and their composites

Carbon aerogels are three-dimensional, open-porous, amorphous materials introduced by R. Pekala 1989. Starting from organic aerogels, carbon aerogels exhibit unique properties such as well-controlled pore size distribution, high porosity, large specific surface area, high electrical conductivity, and low envelope density. This make them promising material for applications in adsorption, catalysis, supercapacitors or as a sulfur hosting material in cathodes of metal-sulfur battery cells. One of the key factors for electrochemical applications is the electrical conductivity. For amorphous carbon materials it is related to their electronic structure, the size of graphitic lattices or graphitic character, heteroatoms and so-called bulk electrical conductivity. In most electrochemical applications, the carbon materials are used as powders for e.g. electrode materials. Therefore, the measurement of the electrical conductivity of powder materials is of great importance. For powders, conductivity consists of: 1) the conductivity of individual grains and 2) the conductivity of the powder. The conductivity of individual grains depends only on monolithic conductivity of the material. In contrast, the conductivity of powder depends on several factors e.g. the shape of grains, their packing, compressibility, and the contact between the grains. Measurements of the electrical resistivity of powders are usually performed on the bed of grains under pressure. Within this presentation, we will report on our recent studies showing the correlation between structural, physical, mechanical and electrical properties of pure and activated carbon aerogels, as well as aerogel-composites. For this purpose, the influence of applied force, compressibility of aerogels and composites, and particle shape were investigated using the four-pin method to measure electrical conductivity. Monoliths and powders were measured at room temperature, and for powders the resistivity was determined in the force range from 1 to 20 kN. For structural and physical characterization nitrogen sorption, scanning electron microscopy, and pycnometry were used

A real-time estimator of electrical parameters for vector controlled induction motor using a reduced order extended Kalman filter

This paper presents an application of the extended Kalman filter (EKF) to the simultaneous on-line estimation of the dq rotor flux components and all the electrical parameters of a vector controlled induction motor. A time-discrete reduced order model structure is deduced and presents a simple and reduced state equation and a scalar output equation. This approach, combined with the use of the rotor reference frame, offers advantages for real-time identification, compared with full order models, because it reduces the computational cost. The proposed new approach requires the measurement of motor speed, stator voltages and currents signals. Simulation and experimental studies presented in this paper highlight the improvements produced by this new approach based on the extended Kalman filter and a new discretization technique, under real operation conditions