Aerogel Granules Research Articles

Aerogel two ways: Towards ultralight noise absorbers

Aerogels are a class of synthetic materials that are derived by using supercritical drying to extract the liquid component of a gel and replacing it with gas. This process results in the formation of an ultra-lightweight nanoporous solid composed of up to 99.98% air by volume. In this talk, we explore the possibility of developing aerogel-based ultralightweight sound absorbers in two different forms. First, we focus on the acoustic characterization of aerogel granules. The absorption properties of four different aerogels with granule diameters ranging from 2 to 4000 μm and granule densities ranging 120 to 180 kg/m3 are presented. Next, we discuss the additive manufacturing of aerogel-based sound absorbers using a freeze casting method. This novel method allows the fabrication of solid aerogel structures with controlled microstructural properties. The sound absorption and transmission loss properties of 3-D printed aerogel samples are measured using an impedance tube setup. We demonstrate that such structures can provide significantly higher absorption and transmission loss properties as compared with current passive noise reduction materials. [Work Sponsored by NASA EPSCoR.]

Experimental and numerical investigation of the thermal transmittance of PVC window frames with silica aerogel

The poor thermal performance of windows can largely be attributed to their frames, which typically have higher thermal transmittance than the glazing portions. This study investigates an innovative polyvinyl chloride (PVC) window frame modified by inserting silica aerogel granules in the frame cavities. Experimental tests done in a hot box are described and compared with numerical modeling results. The direct object of the comparison was the difference in thermal transmittance of the frame, obtained by insulating frame cavities with aerogel. Both types of tests have confirmed that even with a simple PVC frame, it is possible to achieve at least 30% decrease of its thermal transmittance. The investigations allow discussing the importance of different frame cavities and of the frame orientation. Infrared imaging of the frame within the hot box enables analyzing the significance of the aerogel particle distribution and the edge effects. This paper supports experimentally the results obtained with numerical simulations. The research concludes that filling most of the existing cavities with aerogel granules would reduce the thermal transmittance of the case study frame to 0.80 W/m2K. Finally, this study confirms the feasibility of enhancing window frames with aerogel granules within the frame cavities.

Highly efficient and robust preparation of aerogel granules from nanoparticles and graphene oxide

Nanoparticle-based aerogels can combine the unique structures and properties from both nanoparticles and aerogels, and hence provide opportunities in a wide range of applications. The present liquid-dripping and liquid-extrusion methods to produce aerogels from nanoparticles have limited preparation efficiency and high condition requirements. Here, we report a new, highly efficient and robust liquid-granulating method to produce aerogel granules from nanoparticles and graphene oxide. The simple home-made set-up could produce 4 kg gel granules in 45 min, leading to an ever-high production efficiency and robustness.

Optimizing the thermal performance of window frames through aerogel-enhancements

Windows are often considered the weakest point in building envelopes, especially in buildings with high window-to-wall ratios, where the windows exhibit much higher thermal transmittance than the opaque portion of the walls. The poor thermal performance of windows can largely be attributed to their frames which have higher thermal transmittance than the glazing portions. This study investigates the thermal transmittance of 48 commercially-available high-performance aluminium, fibreglass, polyvinylchloride (PVC) and wood-composite window frames. Then, it focuses on the possible improvements of each frame by inserting aerogel in the frame cavities. Several modifications of the frame cavities are assessed through two-dimensional numerical modelling done according to the ISO 10077-2 standard. This research concludes that filling existing empty cavities of window frames with aerogel granules could reduce the frame thermal transmittance by 4–29% depending on the frame type. Moreover, the complete filling of the cavities with aerogel can further reduce the thermal transmittance by 35%. Finally, for each investigated material, window frames with a thermal transmittance as low as 0.5 W/m2 K are proposed.

Physical, mechanical, and microstructural characterisation of an innovative thermal insulating render incorporating silica aerogel

There has been an increasing need for innovative coating materials with high thermal insulation properties to achieve energy-efficiency on buildings’ façades. This can be accomplished through the incorporation of nanomaterials in new thermal renders’ formulations. However, it is also essential that these coatings systems have multiperformance to fulfil the current standard requirements. The studies already made lack a global performance approach, often focusing only on physical and mechanical performance. Therefore, within the scope of this research, it was possible to obtain a new super-insulating thermal render formulation with the Mediterranean climate zone in perspective, characterising its performance at fresh and hardened states, from the mechanical, physical, chemical, and microstructural points-of-view. With this work, it was possible to preserve the aerogel granules integrity while also presenting a homogeneous distribution within the paste, due to the incorporation of rheological agents. This allowed us to obtain a low thermal conductivity multifunctional render (~ 0.029 W⋅m−1⋅K−1), with potential for industrial development, leading to an increase of the energy-saving abilities of these materials, and broadening their scope of application to both new construction and rehabilitation.

Machine learning-based multi-objective optimisation of an aerogel glazing system using NSGA-II—study of modelling and application in the subtropical climate Hong Kong

Application of super-insulating materials in building glazing system shows promising prospects for low-energy buildings. In this research, the heat transfer, solar radiation transmission and indoor illuminance of an aerogel glazing system were characterized through an experimentally validated numerical model. Contribution ratios of multi-variables to multi-objectives were thereafter quantified, following the Taguchi standard orthogonal array. In respect to the application of aerogel glazing system in subtropical climates, an energy-related contradiction between indoor illuminance from solar and indoor heat gain, has been presented, discussed, together with effective solutions. In order to minimise the total heat gain and maximise the indoor illuminance transmitted through the aerogel glazing system, a generic multi-objective optimisation methodology, with high computational efficiency and accuracy, has been developed, to identify the optimal design. The results indicate that through the Pareto front from the multi-objective optimisation results, a significant reduction of total heat gain and an obvious increase of the indoor illuminance can be noticed. Compared to the optimal case in the standard orthogonal array, with the application of the proposed multi-objective optimisation methodology, the annual total heat gain could be reduced from 489305.5 to 333396.4 Wh by 31.9% and the annual indoor illuminance could be increased from 56786.6 to 172973.5 lux by 67.2%. The year-round performance indicates that, compared to the bi-objective optimisation (annual transmitted heat gain and annual indoor illuminance) with the annual total heat gain at 333.4 kWh/m2 and annual indoor illuminance at 162.3 klux, the bi-objective optimisation (annual total heat gain and annual indoor illuminance) shows a lower annual total heat gain at 322.4 kWh/m2 by 3.4%) and a higher annual indoor illuminance at 173 klux by 6.6%. This study proposes an overall framework and technical guidance of a new multi-objective optimisation methodology, which can automatically learn mechanisms of heat transfer and solar radiation transmittance through nanoporous aerogel granules, and identify the optimal multi-variables setting for the robust system design and operation.

Phase-separation induced synthesis of superhydrophobic silica aerogel powders and granules

A phase-separation induced (PSI) method of synthesizing superhydrophobic silica aerogel powders and granules is developed by direct hydrophobilization of silica alcogels in diluted trimethylchlorosilane organic solution and followed by ambient pressure drying (APD). It’s interesting to observed that after the successful hydrophobilization of the alcogels, phase-separation occurred and the gels are dispersed in the organic solvent layer while ethanol and HCl forming another layer. Therefore the gels could be facilely collected and dried under ambient pressure, without the essential solvent-exchange step that frequently employed in the traditional APD methods. The silica aerogel powders and granules synthesized by the PSI method possess specific surface areas up to 756.0 m2/g, thermal conductivity down to 0.0164 W/m·K, and variable densities from 47.6 to 120.0 mg/cm3. Besides, the appearance of the silica aerogels could be well-controlled to be highly transparent powders or granules. In sum, the PSI method provide a simple method to synthesize high quality silica aerogel powders and granules in an economic way, and the aerogels may find valuable applications in thermal insulation, environmental remediation and adsorption, etc.

Thermal insulating cementitious composite containing aerogel and phosphate-based binder

Thermal conductivity is an important material property in the energy design process of the buildings. While cement-based materials are the most consumed materials in the construction industry, thermal properties of these materials can play a significant role in energy efficiency of the buildings. Cementitious materials with low thermal conductivity can be desirable for using as a part of heat insulation or for thermal bridge calculations. They may be used in different forms such as cast concrete, on-site 3D printing or surface finishes. In this study, aerogel granules was used as aggregate and combination of phosphate based cement and fly ash was considered as binder in order to achieve a fast setting material with low thermal conductivity. Furthermore, the calcination process was eliminated for the phosphate cement, which led to a material with lower environmental impact. Thermal conductivity as low as 0.04 W/mK was measured using the proposed material composition.

Effect of Hydrophobic Aerogel Granules on Thermomechanical Properties of Cementitious Composites

Thermal conductivity is an important material property in the energy design process of the buildings. While cement-based materials are the most consumed materials in construction industry, thermal properties of these materials can play a significant role in energy efficiency of the buildings. Cementitious materials with low thermal conductivity can be desirable for using as a part of heat insulation or for thermal bridge calculations. In this study, hydrophobic aerogel granules were used as aggregate and combination of Portland cement and fly ash was considered as the binder in order to achieve a material with low thermal conductivity. Replacement of 50 vol% hardened cement paste (hcp) by aerogel granules led to about 70% reduction in thermal conductivity of air dried samples (from 0.67 W/mK to 0.20 W/mK). However, this reduction was nearly 50% in moist samples submerged in water for three days (from 0.97 W/mK to 0.50 W/mK) due to replacement of air by water in hcp pores, which can be avoided by using hydrophobic agents. The thermal conductivity can be reduced by increasing water-cement ratio as well as aerogel content. This type of composite can be used as cast concrete or on-site 3D printing of wall elements.

Organic solvent-saving preparation of water glass based aerogel granules under ambient pressure drying

An organic solvent-saving method of preparing silica aerogel is reported. The silica aerogel was made from water glass (WG) and used as the solvent. Na+ was removed through ion exchange; prior to this ion exchange, concentrated hydrochloric acid (HCl) was added to the WG solution. The sol pH was adjusted to ~1 to suppress the condensation to the maximum extent. Ethanol was added to the sol after ion exchange. Surface modification was conducted with the assistance of hexamethyldisiloxane, which was catalyzed by HCl. The entire process can be finished in 6 h. The overall consumption of organic solvent is much lower than that in the previous preparation route. Characterization results indicate that the resulted silica aerogel has a uniform mesoporous structure and low thermal conductivity (0.0237–0.0245 W/m·K at 1 atm).

Numerical Study on the Thermal and Optical Performances of an Aerogel Glazing System with the Multivariable Optimization Using an Advanced Machine Learning Algorithm

AbstractThe implementation of advanced materials in high‐efficient glazing system is important for green buildings. In this study, aerogel granules are implemented in the glazing system to form a translucent window with super‐insulating performance. An experimentally validated numerical modeling integrating both heat transfer model and optical model is developed to characterize the sophisticated heat transfer and solar radiation transmission mechanisms. Sensitivity analysis is presented with quantifiable contribution ratio of each parameter to the total heat gain. Instead of returning back to numerical modeling repeatedly, an advanced optimization engine implemented with a generic optimization methodology with competitive computational efficiency and accuracy is proposed by implementing the supervised machine learning and advanced optimization algorithms. The research results show that the developed artificial neural network modeling is more accurate and computational‐efficient than the traditional lsqcurvefit fitting methodology. In addition, the optimal case through the teaching‐learning‐based optimization is more robust and competitive than the optimal case through the particle swarm optimization in terms of the total heat gain. This study presents an in‐depth understanding of heat transfer and solar radiation transmission of nanoporous aerogel granules together with a robust optimal design, which is important for the promotion of green buildings with high‐energy performance.

Effects of the Heat Treatment in the Properties of Fibrous Aerogel Thermal Insulation

Nowadays, besides the use of conventional insulations (plastic foams and wool materials), aerogels are one of the most promising thermal insulation materials. As one of the lightest solid materials available today, aerogels are manufactured through the combination of a polymer with a solvent, forming a gel. For buildings, the fiber-reinforced types are mainly used. In this paper, the changes both in the thermal performance and the material structure of the aerogel blanket are followed after thermal annealing. The samples are put under isothermal heat treatments at 70 °C for weeks, as well as at higher temperatures (up to 210 °C) for one day. The changes in the sorption properties that result from the annealing are presented. Furthermore, the changes in the thermal conductivity are followed by a Holometrix Lambda heat flow meter. The changes in the structure and surface of the material due to the heat treatment are investigated by X-ray diffraction and with scanning electron microscopy. Besides, the above-mentioned measurement results of differential scanning calorimetry experiments are also presented. As a result of using equipment from different laboratories that support each other, we found that the samples go through structural changes after undergoing thermal annealing. We manifested that the aerogel granules separate down from the glass fibers and grow up. This phenomenon might be responsible for the change in the thermal conductivity of the samples.

Effects of heat exposure on the properties and structure of aerogels for protective clothing applications

Abstract In this paper, an attempt has been made to investigate the effects of heat and thermal radiation on the structure and properties of hydrophobic aerogels intended for use in textile materials for protective clothing. A novelty is a comprehensive approach to the subject by considering all aspects relevant to the using of aerogels in clothing protecting against heat and adaptation of test methods commonly used to assess the protective properties of clothing materials for the needs of aerogel testing. The weight changes of aerogels upon heat and radiation exposure was determined by the gravimetric method and thermogravimetric analysis (TGA). The nano- and micro-structure of untreated and heat-treated aerogels was characterized by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Additionally, Fourier transform infrared spectroscopy (FTIR) and contact angle technique was used to study the chemical structure and hydrophobicity of the aerogels, respectively. Textural characteristics (specific surface area, pore volume, pore size distribution) were determined based on adsorption/desorption isotherms of N2 at 77 K. It was found that the exposure of aerogels to heat led to a reduction in their weight, for example at 180 °C the weight loss was about 2% (irrespective of treatment time), whereas at 350 °C the loss was three times greater and amounted to approx. 7%. The weight loss at 350 °C indicates a slow removal of the surface –CH3 groups, which has been confirmed by the FTIR analysis. The oxidation of methyl groups provides also change the hydrophobic nature of the aerogels demonstrated on the basis of the results of water contact angles. From SEM and AFM images it can be observed that exposure to heat changed the structure and morphology of silica aerogel granules, while aerogel powder was resistant to temperature treatment. The scale of observed changes of the structure and weight of silica aerogels allows their potential use in the design of protective clothing.

A model and method to determine solar extinction coefficient of aerogel granules layer through experiment under real climatic condition

Solar extinction coefficient is an important parameter in simulating the thermal and energy performances of glazing, but it is difficult to obtain the solar extinction coefficient of aerogel granules layer (SECAL) due to its nano-porous network structure. In this study, an in-field test room is built to measure the irradiance at both sides of granular nano-porous silica aerogel glazing system under different climatic conditions. The SECAL is estimated through an optical model and mathematical methodology from the measurement data. The optical model is proposed on the base of interface energy balance principle, in which the incident angle and scattering phenomena happening in the nano-porous silica aerogel are taken into account. Through the optical model, the irradiation transmittance is calculated by an assumed SECAL value and indoor irradiance is predicted. The SECAL value is estimated by finding the minimal error between the predicted and measured indoor irradiance. The SECAL values for clear and overcast sky are respectively obtained, and the values are 0.0392/mm and 0.0296/mm respectively. Sky clearness factor is used as the index of weather condition to select the SECAL values. The optical model and methodology is valid to determine SECAL and the optical model is accurate to calculate solar irradiance through aerogel glazing system.

Three routes to superinsulating silica aerogel powder

Silica aerogel is the archetypal thermal superinsulator and commonly applied to improve the thermal performance of on- and off-shore industrial infrastructure and buildings. Hitherto, the main products on the market are silica aerogel based blankets fabricated by casting a silica sol into a porous or fiber matrix, followed by gelation, hydrophobization, and (supercritical) drying. Considering the diffusion efficiency of the reagents and solvents used in the preparation, a reduced size of the gel bodies may accelerate and simplify the sol-gel, hydrophobization, and drying processes. Thus, particle based products, derived from silica aerogel granulate and powder additives and semi-finished products, are an attractive solution towards inexpensive aerogel applications. Here, we optimized the process parameters for silica aerogel powder production from three common silica precursors: waterglass (WG), ion-exchanged waterglass, and tetraethoxysilane (TEOS), including gelation pH, hydrophobization procedure, solvent system, processing temperatures, and ambient pressure drying protocol. Successful hydrophobization is confirmed by elemental analysis, FTIR and quantitative solid-state NMR spectroscopy. All three routes lead to silica aerogel powders with similar type IV isotherms, and BET surface areas above 700 m2/g. Importantly, the thermal conductivity of the packed powder beds does not exceed 20 mW/(m⋅K) for all three routes, implying even lower thermal conductivities of the aerogel phase itself. Total processing time, including gelation, aging, surface modification, and ambient pressure drying is between 2 and 4 h, depending on the selected route. Given that high quality silica aerogel powders can be produced from all investigated silica precursors and hydrophobization agents, the process selection for industrial upscaling can be based entirely on engineering and economic considerations.

Effect of Low-Temperature Heating on the Properties of Graphene Oxide Aerogel

The paper describes the effect of low-temperature annealing in air on the composition and structure of graphene oxide aerogel. It has been found that the concentration of carbon in the aerogel sharply increases, i.e., its carbonization occurs, already at a temperature of 175°C. At the same time, both the outer shape of the aerogel granules and the internal porous structure are preserved.

Quantitative research on the influence of particle size and filling thickness on aerogel glazing performance

Silica aerogel has attracted increasing interest in building glazing for energy efficiency owing to its light weight, ultra-low thermal conductivity and high visual transmittance. In this study, four types of aerogel granules with various average particle sizes (viz. 0.41, 1.03, 1.64 and 2.54 mm) were prepared by using a low-cost ambient pressure drying method. Three gaps between two pieces of glass (viz. 6, 9 and 12mm) were filled with aerogel granules, yielding 12 pieces of aerogel filling glazing. The light transmittance and reflectance of the aerogel glazing samples were experimentally measured by using a spectrometer. Based on the experimental results, the correlation between the light transmittance performance and the geometrical parameter of the aerogel glazing were established based on a proposed dimensionless geometrical parameter. It revealed that the visible light transmittance and the shading coefficient of the aerogel glazing had a positive correlation with the particle size and a negative correlation with the filling thickness. In addition, the filling thickness had a greater influence on the properties of aerogel glazing than the particle size. This opens new possibility for a multiple-objective evaluation of the aerogel glazing aiming at building energy efficiency.

Impact of glass plates rigidity on load sharing in an aerogel composited glass unit

An aerogel composited glass unit consists of two glass sheets and an interlayer filled with aerogel granules. When it is subjected to a uniformly distributed load, part of the load is transferred to the leeward glass sheet through the interlayer. A numerical analysis method is employed to describe the effect of glass plates rigidity on load sharing of two glass plates and an experimental research is conducted to verify the validity of the numerical method. The experimental results suggested that the assumptions which were employed in the numerical method are valid when the difference of glass sheets rigidity is small, but it becomes inapplicable when the difference of glass sheets rigidity becomes much greater.

Enhancement Studies on Manufacturing and Properties of Novel Silica Aerogel Composites.

Silica Aerogel composites are ultra-low density, highly porous foam-like materials that exhibit excellent thermal insulation and high strain recovery characteristics. In the present work, environment-friendly silica aerogel composites are fabricated using silica aerogel granules with bio based porcine-gelatin as the binding agent dissolved in water and by further drying the mix at sub-zero condition. This article focuses on improvement studies carried on the mold design and the manufacturing process to achieve better geometric compliance for the silica aerogel composites. It also presents contact angle measurements, compressive behavior under different cycles of loading, time dependent behavior and flexural response of the composites. The influence of additives, such as fumed silica and carbon nanotubes on mechanical properties of the composites is also deliberated. Water droplet contact angle experiments confirmed the ultra-hydrophobic nature of the composites. The mechanical properties were characterized under cyclic loading-unloading compression and three-point flexure tests. On successive compression in three consecutive load cycles, the strain and thickness recovery were found to decrease by around 30%. The flexural properties of the aerogel composites were investigated using it as the core covered by thin carbon composite face sheets. It was found that the flexural strength and the failure strain of this aerogel sandwich composites is approximately half of the conventional nomex honeycomb sandwich equivalent.

Physical and mechanical properties of plasters incorporating aerogel granules and polypropylene monofilament fibres

Growing concern over global warming in recent years has required buildings to become significantly more energy efficient. One of the main ways of achieving this aim has been through the use of innovative materials to facilitate improvements in levels of building insulation. This paper describes the use of aerogel granules as an additive material for lime-based plasters, with the objective of improving the thermal efficiency of buildings whilst maintaining or improving vapour permeability.Five experimental lime composite mixes were prepared, with lime putty as the binder material and aggregate comprising differing proportions of standard sand and aerogel granules. Previous work had already confirmed very low strengths for plaster mixes containing aerogel granules alone as the aggregate material; therefore, polypropylene fibres were incorporated as a secondary additive material to improve the mechanical properties and reduce strength loss attributed to shrinkage and cracking.The flexural strength, compressive strength, thermal conductivity and water vapour permeability of lime composite mortars containing different volume fractions of aerogel were determined. Microstructures were examined using scanning electron microscopy and transmission electron microscopy. The results showed that aerogel granules can be successfully incorporated into lime plasters to improve thermal efficiency. The addition of aerogel was also found to improve moisture vapour permeability. The inclusion of polypropylene fibres in aerogel plasters was effective in reducing shrinkage and cracking to acceptable levels. Experimental mixes exhibited a slight reduction in strength compared to standard plaster mixes, although this was compensated for by a high level of flexibility and toughness.This work provides innovative information on utilising aerogel granules as an insulating plaster additive by addressing the issues of strength and flexibility, properties that are not normally associated with aerogel but which are of importance in a functional plaster material.