Papers by Keyword: Aerogel

Abstract: Materials with relative permittivity or dielectric constant near to that of air (εr~1) are known as ‘ultra-low k’ materials. They find a number of applications in inter-connects of micro-electronic circuits, antennae, high-speed communication substrates etc. Among the inorganic materials, porous silica is the widely studied candidate. Porous silica can be of many types depending upon the extent of porosity and size and connectivity of pores. This paper presents the details of measurement of permittivities and the results of silica beads and silica aerogels. Silica beads, prepared by microwave heating of silica gels, are spherical beads of average 1mm size. Hydrophobic silica aerogels, prepared by ambient pressure drying of silica gels, are irregular chunks of 5-10 mm size. Both are potential bulk fill insulation materials and hence the permittivity can be measured as an aggregate filling a definite volume. The permittivities of these have been measured upto 1 MHz by 3-terminal method using a precision LCR meter and a powder-paste cell as per ASTM-D150-11. The εr values of silica aerogels and silica beads in 20 Hz-1 MHz range could be measured and are less than 1.6 at 1 MHz.

Abstract: Silica aerogels have been prepared through sol-gel process by polymerization of TEOS in the presence of NH₄F and NH₄OH as catalysts. The solvent present in the gel is replaced by ethanol followed by a non-polar solvent such as n-hexane prior to solvent modification step. Gels are made hydrophobic by treating them with HMDZ to prevent rupture during drying, which has been confirmed by FTIR. Gels are then washed and dried carefully in a PID controlled oven at atmospheric pressure. The ageing duration and solvent exchange combinations are optimized to yield crack-free gels prior to drying. Aerogels are characterized for density, specific surface area, pore volume, pore size, thermal stability and contact angle. Hydrophobic, high surface area (570 m2/g), low density (0.07 g/cm3) silica aerogels are synthesized by using optimized mole ratio of precursors and catalysts. Silica aerogel granules (1-3 mm) as well as monoliths (Ф~35 mm) could be produced through ambient pressure drying of gels.

Abstract: The LiCl/DMAc was used as solvent system to dissolve meta-aramid short fiber, the aramid fibrid was prepared by injecting the aramid solution into the high shearing coagulant at room temperature and the mixture of water and N, N-dimethylacetamide (DMAc) was used as coagulant. The aramid fibrid and aramid short fiber were mixed at differents weight proportion, wet paper was dried and aramid paper was prepared by heat pressing. The aramid paper has excellent mechanical properties when the weight proportion of the aramid fibrid and aramid short fiber was 60:40,and it had a good thermal bonding between the fibrid and fiber under heat pressing. The hydrophobic SiO₂ was mixed with aramid fibrid and aramid short fiber (60:40) in ethanol, and aramid/aerogel composite was prepared by drying and hot compressing. The composite had a good heat-resisting, however, the mechanical property decreased with the content of aerogel increasing.

Abstract: The incorporation of lightweight aggregates in cement-based coating mortars contributes to a better performance of some of their physical properties, influencing their hygrothermal behaviour. This paper analyses the effect of some lightweight aggregates (expanded clay, granulated cork and silica aerogel) on the following mortars’ characteristics: porosity, bulk density, capillary water absorption coefficient, drying index and water vapour permeability. The experimental results showed that low percentages of incorporation of lightweight aggregates led to more porous mortars with lower bulk density, higher capillary water absorption coefficient and greater drying facility, relative to the reference cement-based mortar. The incorporation of silica aerogel in cement-based mortars resulted in major differences in terms of water resistance, with significant benefits in terms of the drying process due to their porous structure.

Abstract: Installation of insulation materials in buildings can reduce the usage of air conditioners by retarding heat flow into the building. Aerogel is one of the best insulation materials with distinctive properties that can replace existing building insulation materials such as fibre glass and polyurethane. However, brittleness of Aerogel makes it difficult to handle and disqualifies its viability as a building insulation material. Reinforcement of Aerogel with binding materials can improve its mechanical and thermal properties to overcome its brittleness. However, only a few studies have been carried out on this area. Furthermore, from the few existing studies, vital information such as thermal conductivity and specific application of the reinforced Aerogel studied were not considered. As an initiative to fill in this research gap, a review on reinforcement of Aerogel is presented.

Abstract: Aerogels are porous materials with potential applications in fields ranging from thermal insulation, catalyst support, filters, electrical storage, components in optical devices, mechanical damping all the way to drug release. However, careful reliable characterization is the base for both, understanding of fundamental structure - property relationships as well as a directed development of materials and composites for specific applications. The review therefore addressed severe problem upon aerogel characterization that have been identified in the past and presents reliable non-destructive alternatives and novel methods that can be applied for the characterization of aerogels as well as their gel precursors.

Abstract: With the increasing attention towards energy-efficient and zero emission buildings, improvement to concrete properties is becoming more and more significant in construction and building sectors. One such area is to enhance the thermal properties, while maintaining maximum strength of the material. Here, attempts were made to address this challenge by formulating mortar composites with low thermal conductivity while targeting a minimum compressive strength of 20 MPa at 28 days. For this purpose, aerogel was utilized in an ultrahigh performance concrete (UHPC) formulation to create new aerogel-incorporated mortar (AIM). It was found that AIMs possessing 50 vol% aerogel registered a compressive strength of 20 MPa, while displaying a thermal conductivity of ~ 0.55 W/(mK). By adding more aerogel to reach 70 vol%, while the thermal conductivity of the concrete decreased by ~ 20 %, a sharp decrease in strength to 5.8 MPa was observed. This represents only 1/30 of the original strength of the UHPC mortar. Further addition of aerogel till 80 vol% showed negligible compressive strength, attributing to the imbalance of the particle-matrix ratio in the mortar system, causing a decrease in adhesion of the binder-aggregates.

Abstract: Magnesium-based aerogels were prepared by sol-gel technology and drying with supercritical fluid dry process in 245°C. The structure and properties of the magnesium-based aerogels were featured by TEM, XRD and BET. The densities of the prepared samples were about 0.34~0.59 g/m³. The results show that the MgO aerogel has a high surface area.

Abstract: Silica aerogel with extremely low thermal conductivity has great potential to be used as thermal insulating material. Opacification using carbon black is normally applied to reduce radiative heat loss in silica aerogel. This work attempted to replace carbon black with activated carbon as opacifer. Both the silica aerogel and activated carbon were synthesized via bamboo leaf. Effects of carbon loading and temperatures on the thermal conductivity of opacified aerogel were studied. The results show that an optimal carbon loading that minimized the thermal conductivity present at different temperatures. Such optimal loading increased as temperature applied to the opacified aerogel increased. Properties of aerogels opacified with activated carbon were also compared with aerogels opacified with carbon black.

Abstract: Cellulose hydrogel was prepared from cellulose/NaOH aqueous solutions. Silica was added into cellulose hydrogel by TEOS hydrolysis method. Finally aerogel was obtained by the freeze-drying technique. The influence of silica on the properties of cellulose aerogel was studied in detail. With cellulose content 3%, the water absorption of cellulose-SiO₂ composite aerogel was only about 50% of the water absorption of cellulose aerogel. The result showed that silica could effectively reduce the water absorption of cellulose aerogel significantly and reduce the extent of structural damage that resulted from highly water absorption of cellulose aerogel. At the same time, with the increase of cellulose content, density of cellulose aerogel increased, while porosity decreased. It led to the water absorption of aerogel decrease with the increasing of cellulose content.