Nanoporous Aerogel Research Articles

Theoretical modeling and experimental validation for the effective thermal conductivity of moist silica aerogel

In this paper, the influence of water content on insulating performance of moist silica aerogel nano-porous composite material is studied theoretically and experimentally. A new fractal-intersecting sphere with inhomogeneous water film structural model is proposed. It considers the microstructures in aerogel and analysis of condensation process in moist environment based on surface physics theory. The effective thermal conductivity of composite aerogel material has been also derived using the equivalent circuit method. The nano-scale heat transfer effect and the influence of micron-scale additives such as opacifiers and reinforced fibers are involved in the present theoretical model. The theoretical model is semi-empirical because some parameters are introduced to better predict the effective thermal conductivity based on partial experimental data at a reference temperature. Experiments were conducted to measure thermal conductivities of nano-porous aerogel composites within temperature range of 15–45 °C and water uptake range of 0–110 mg/g by the transient plane source method. Validation study for both current samples and material in Zhang et al. (2015) was carried out, where the relative errors between experimental thermal conductivity and theoretical calculations are less than 7% and 8%, respectively. The prediction accuracy of present model for this super insulating material is much higher than that of Bjurström’s model which are widely used to calculate effective thermal conductivity for common porous media containing moisture. In addition, the results show that the increase of thermal conductivity with water uptake is linear at the same temperature. Lower porosity would result in a greater sensitivity to moisture content for aerogel material. Comparing to the influence of various additives, the properties of silica aerogel matrix is the most dominant factor in determining the effective thermal conductivity under different water content.

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.

All-printed 3D hierarchically structured cellulose aerogel based triboelectric nanogenerator for multi-functional sensors

The forthcoming ubiquitous innovations driven by flexible electronics and Internet of Things have inspired the relentless pursuit of advanced environmental-friendly power source for multi-functional sensors. Here, an all-printing method is utilized to fabricate high-performance biocompatible cellulose based triboelectric nanogenerators (TENGs) with 3D micro/nano hierarchically patterned structure. The printed triboelectric micro-3D pattern and the nano-porous aerogel structure in the all-printed triboelectric nanogenerator (AP-TENG) could significantly enhance the effectively utilizing of structure and contribute to the contact area, surface roughness and mechanical resilience of the device, which consequently contributes to improve the triboelectric response. The hierarchical micro/nano 3D structure of the AP-TENG results in higher voltage output than the traditional molding TENG counterpart. AP-TENGs with different contact angles are readily fabricated and systematically calculated and investigated. The AP-TENGs can efficiently harvest mechanical energy to drive 88 light-emitting diodes and act as self-powered mechanical sensor and humidity sensor. This work provides a new strategy to design and tailor high-performance 3D TENG that will be very useful for diverse multi-functional electronic applications.

Lattice Boltzmann method simulation of the gas heat conduction of nanoporous material

In order to deeply investigate the gas heat conduction of nanoporous aerogel, a model of gas heat conduction was established based on microstructure of aerogel. Lattice Boltzmann method was used to simulate the temperature distribution and gas thermal conductivity at different size, and the size effects of gas heat conduction have had been obtained under micro-scale conditions. It can be concluded that the temperature jump on the boundary was not obvious and the thermal conductivity remained basically constant when the value of Knudsen number was less than 0.01; as the value of Knudsen number increased from 0.01 to 0.1, there was a clear temperature jump on the boundary and the thermal conductivity tended to decrease and the effect of boundary scattering increased drastically, as the value of Knudsen number was more than 0.1, the temperature jump increased significantly on the boundary, furtherly, the thermal conductivity decreased dramatically, and the size effects were significantly.

A theoretical model for gas-contributed thermal conductivity in nanoporous aerogels

A theoretical model for gas-contributed thermal conductivity in aerogel is proposed by considering the coupling effects of both the local solid-gas interaction (heat crossing particles and adjacent gaseous pore) and the thermal-bridge interaction (heat crossing adjacent particles and the gas confined in the gap). The predictions of present model demonstrate a good agreement with experimental data of aerogels with different microstructures and thermophysical properties, exhibiting its good potential of application. The proposed model is then employed to investigate the influence mechanism of the above two solid-gas coupling effects in aerogels. It is shown that the coupling effect of local solid-gas interaction is in a dominant position, while the coupling effect of thermal-bridge interaction becomes noticeable and should not be neglected when the pressure exceeds about 1 atm. The effects of porosity, particle size and thermal conductivity of particles are also examined according to the present model, which could provide guidance for improving heat-insulating capacity of aerogels in engineering.

Preparation and optimization of ultra-light and thermal insulative aerogel foam concrete

Lightweight and thermal insulative foam concrete has gathered increasing interests in buildings for energy efficiency. However, their density (∼400 kg/m3) and thermal conductivity (0.10 W/(m·K)) need to be further decreased for high performance applications in green and nearly zero energy buildings. A novel type of aerogel foam concrete with a low density of 198 kg/m3 and thermal conductivity of 0.049 W/(m·K) was successfully fabricated by adding super-insulative and Nano-porous aerogels into micro-porous foam concrete. The microstructure and thermal conductivity of the aerogel foam concrete were experimentally investigated. A model was established for predicting the thermal conductivity of the aerogel foam concrete based on a modified Maxwell model. The calculated results were validated with experimental data, with an error of less than 5%. By using the model, the effects of the ternary ratios on the thermal conductivity were investigated, and the optimization aiming at the minimum thermal conductivity was then implemented. Accordingly, a ternary aerogel foam concrete sample with the optimal ratios was experimentally prepared, whose thermal conductivity and density were approximately 50% lower than those of existing foam concrete. Therefore, the novel type aerogel foam concrete, yielding high performance, could be extensively used for potential applications in green and nearly zero-energy consumption buildings.

Rotational dependence of line half-windth for fundamental band of CO2 confined in nanoporous aerogel

Rotational dependence of line half-windth for fundamental band of CO2 confined in nanoporous aerogel

A new type of microphotoreactor with integrated optofluidic waveguide based on solid-air nanoporous aerogels.

In this study, we developed a new type of microphotoreactor based on an optofluidic waveguide with aqueous liquid core fabricated inside a nanoporous aerogel. To this end, we synthesized a hydrophobic silica aerogel monolith with a density of 0.22 g cm−3 and a low refractive index of 1.06 that—from the optical point of view—effectively behaves like solid air. Subsequently, we drilled an L-shaped channel within the monolith that confined both the aqueous core liquid and the guided light, the latter property arising due to total internal reflection of light from the liquid–aerogel interface. We characterized the efficiency of light guiding in liquid-filled channel and—using the light delivered by waveguiding—we carried out photochemical reactions in the channel filled with aqueous solutions of methylene blue dye. We demonstrated that methylene blue could be efficiently degraded in the optofluidic photoreactor, with conversion increasing with increasing power of the incident light. The presented optofluidic microphotoreactor represents a versatile platform employing light guiding concept of conventional optical fibres for performing photochemical reactions.

Preparation of sponge-reinforced silica aerogels from tetraethoxysilane and methyltrimethoxysilane for oil/water separation

Polyurethane sponge-reinforced silica aerogels based on tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMS) were fabricated by a facile method through sol-gel reaction followed by ambient pressure drying. In sponge-reinforced silica aerogels, nanoporous aerogel aggregates fill in the pores of polyurethane sponge. The sponge-reinforced aerogels are hydrophobic and oleophilic and show extremely high absorption for machine oil (10.6 g g−1 for TEOS-based aerogel and 9.2 g g−1 for MTMS-based aerogel). In addition, the sponge-reinforced aerogel composites exhibit notable improvements with regards to mechanical properties. The compressive strength was enhanced obviously up to about 349 KPa for TEOS-based aerogel and 60 KPa for MTMS-based aerogel. Specially, sponge-reinforced silica aerogels based on MTMS drastically shrank upon loading and then recovered to the original size when unloaded. The property differences of the sponge-reinforced silica aerogels caused by the two precursors were discussed in terms of morphologies, pore size distributions and chemical structure.

Impact of thermal coupling effects on the effective thermal conductivity of aerogels

Nanoporous aerogels are excellent thermal insulation materials with thermal conductivities down to about 0.012 W m−1 K−1 at ambient conditions. So far, it was assumed that the total thermal conductivity of aerogels can be described by a simple superposition of the different individual heat transport contributions. However, recent investigations reveal that thermal coupling effects can result in a gas pressure dependent contribution that may be up to three times higher than expected from just a gas phase thermal conductivity, which is predicted by the Knudsen equation at given porosity and pore size. In this study, we use data from previous publications covering a gas pressure range from 10−5 to 10 MPa and analyze systematically the impact of pore size as well as solid phase and gas phase thermal conductivity on the coupling effect. The goal is to evaluate the data with respect to practical implications for aerogels in general. This means using the gas pressure dependence of the thermal conductivity of aerogels to determine their average pore size as well as allowing for a targeted optimization of aerogel-based insulations for applications at given gas pressure and temperature.

Efficient gaseous thermal insulation aerogels from 2-dimension nitrogen-doped graphene sheets

Gaseous thermal conductivity takes up a great proportion of the thermal conductivity of aerogels. In this work, we present a new strategy to restrain the gaseous thermal conductivity, using 3-dimension nanoporous aerogels from 2-dimension sheets. Experimental results show that the N-doped graphene aerogels, typical nanoporous aerogels from 2-dimension sheets, exhibit very low gaseous thermal conductivity (12.58mWm−1K−1 at room temperature) at extremely low bulk density (0.0320gcm−3), suggesting that the nanoporous aerogels from 2-dimension sheets can restrain the gaseous thermal conductivity more efficiently than that from 0-dimension pearls. This finding may significantly contribute to the next generation of thermal insulation materials with lower thermal conductivity and much lower density.

Synthesis and characterization of rigid and thermostable polyimide aerogel crosslinked with tri(3-aminophenyl)phosphine oxide

Polyimide (PI) aerogels cross-linked with a three amino compound tri(3-aminophenyl)phosphine oxide (TAPO) were synthesised by 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA) and 4,4′-diaminodiphenyl ether (4,4′-ODA) through chemical imidization method. Supercritical CO2 was used for drying the PI gels to fabricate nanoporous aerogels with tunable densities ranging from 0.09 to 0.32 g/cm3, and the specific surface areas between 198 and 340 m2/g. To enhance the thermal stability of the BPDA-ODA-based PI aerogels, PMDA was introduced into the formulating oligomer chain. The results showed that with the increase of PMDA concentration, the 5 wt% thermal degradation temperature of PI aerogels could rise to around 600 °C and the glass transition temperature could increase from 269 to 306 °C, which makes them ideal insulation materials for aerospace and other applications.

Preparation of aerogel-eicosane microparticles for thermoregulatory coating on textile

Aerogel/eicosane microparticles were prepared by dispersing nanoporous silica aerogel powder in a liquid of phase change material (PCM), eicosane. Melt-infiltration (M), solvent-dissolving (D) and combined melt-dissolving (MD) for particle dispersion were investigated. Differential Scanning Calorimetric thermograph indicated that microparticle M has maximum heat capacity of 198J/g whereas the heat capacity of microparticles D and MD was found to be 139J/g and 144J/g, respectively. FT-IR and SEM analyses of the microparticles revealed that the eicosane infiltrated and coated the fine aerogel particles but the microparticles still remained in the powder form. The nanoporous aerogel structure held the infiltrated eicosane while the ultra-high specific surface area of silica aerogel kept the surrounding eicosane in situ by surface tension. Infrared thermal imaging showed that the eicosane did not drip out from the microparticles even at 120°C, whereas the pure PCM completely changed phase to fluid when heated. The developed microparticles provide ease of coating application in room temperature for thermal protective textiles. The coated fabric showed significantly improved thermal resistance over the speculated phase transition period of the PCM. This opens up the opportunity for the developed microparticles to be utilised as a thermal protective coating additive in a wide temperature range without melting or dripping out from the applied substrates.

An advantageous technique to load drugs into aerogels: Gas antisolvent crystallization inside the pores

Over the past few years, both organic and inorganic nanoporous aerogels have shown a great promise as drug delivery vehicles. Different methods are utilized to load drugs into aerogels such as the addition of the drug to the reaction mixture in one of the steps before the gel formation or by supercritical deposition to the aerogels. These techniques have disadvantages such as possible reactions of pharmaceutical compounds with reactants used to form gels and low solubility of pharmaceutical compounds in supercritical carbon dioxide (scCO2). An alternative technique is to load the drug after gel formation by contacting the gel with a solution of the drug. The drug diffuses into the liquid inside the pores. When this drug-loaded gel is subjected to supercritical drying, scCO2 not only removes the solvent from the pores but also acts as an antisolvent, which causes the precipitation of the drug in the pores of the aerogel. This is similar to the gas antisolvent crystallization (GAS) process but in this case the process takes place inside the pores. In this study, this technique was used to load paracetamol into silica aerogels. The factors affecting the amount and distribution of the drug inside the aerogel matrix were investigated and a mathematical model to account for the movement of paracetamol inside the pores during supercritical drying leading to a varying drug concentration in the matrix with position was developed. It was concluded that high initial concentrations resulted in more homogeneous drug distributions. Moreover, XRD analysis demonstrated that paracetamol was in crystalline form. The process enables higher amount of loadings than conventional systems and also offers an advantage as it combines two processes such as drying and loading in a single one reducing the time and the operating expenses.

Formation of nanoporous aerogels from wheat starch

Biodegradable nanoporous aerogels were obtained from wheat starch using a simple and green method based on supercritical carbon dioxide (SC-CO2) drying. Effects of processing parameters (temperature, wheat starch concentration and mixing rate during gelatinization; temperature, pressure, and flow rate of CO2, during SC-CO2 drying) on the aerogel formation were investigated, and optimized for the highest surface area and smallest pore size of the aerogels. At the optimized conditions, wheat starch aerogels had surface areas between 52.6–59.7m2/g and densities ranging between 0.05–0.29g/cm3. The average pore size of the starch aerogels was 20nm. Starch aerogels were stable up to 280°C. Due to high surface area and nanoporous structure, wheat starch aerogels are promising carrier systems for bioactives and drugs in food and pharmaceutical industries.

Nanoporous aerogel as a bacteria repelling hygienic material for healthcare environment

Healthcare-associated infections (HAIs) caused by pathogenic bacteria are a worldwide problem and responsible for numerous cases of morbidity and mortality. Exogenous cross-contamination is one of the main mechanisms contributing to such infections. This work investigates the potential of hydrophobically modified nanoporous silica aerogel as an antiadhesive hygienic material that can inhibit exogenous bacterial contamination. Nanoporous silica aerogels were synthesized via sol–gel polymerization of tetraethyl orthosilicate and hydrophobized using trimethylsilyl chloride. Bacterial adhesion characteristics were evaluated via dip-inoculation in suspensions of Gram-negative Escherichia coli O157:H7 and Gram-positive Staphylococcus aureus. The attachment of E. coli O157:H7 and S. aureus to hydrophobic nanoporous silica aerogel (HNSA) was found to be significantly lower than that to hydrophilic and hydrophobic nonporous silica materials: 99.91% (E. coli O157:H7) and 99.93% (S. aureus) reduction in comparison to hydrophilic nonporous silica, and 82.95% (E. coli O157:H7) and 84.90% (S. aureus) reduction in comparison to hydrophobic nonporous silica. These results suggest that the use of HNSA as surfaces that come into contact with bacterial pathogens in the healthcare environment can improve bacterial hygiene, and therefore may reduce the rate of HAIs.

The thermal protection and comfort properties of aerogel and PCM-coated fabric for firefighter garment

The study evaluates the simultaneous use of aerogel and phase change material (PCM) on the face cloth of thermal liner in firefighter’s protective garment. Aerogel is commonly used to resist incoming heat flux in the field of high heat protection and to prevent the loss of body heat in the cold environment clothing. In high heat protection clothing, aerogel not only resists the incoming heat fluxes but also blocks the outbound body heat. As a result the wearer suffers from internal increase of body temperature. Previous studies identified the potential use of aerogel in firefighter’s protective clothing. However there was no clear approach to resolve the problem associated with body heat release. Current study focuses on the problem by applying PCM along with aerogel on fabric. The ambient-side of a thermal liner face cloth was coated with silica aerogel particles; meanwhile, the next to skin side was coated with PCM/aerogel composite powder. The new thermal liner revealed superior thermal protection and comfort. It extended the time to reach pain threshold and increased the pain alarm time. The Fourier transform infrared analysis of the aerogel/PCM composite powder showed the presence of PCM in nanoporous aerogel particles while the differential scanning calorimeter quantified the heat absorbing capacity of the new composite powder. Scanning electron microscope, air permeability tester, and jPOR macro of ImageJ software were used for the surface characteristics and porosity analysis of coated liner. The thermal stability of the composite powder was investigated through an infrared thermal camera. No dripping or form deterioration was observed when the composite powder was heated over a temperature three times above the melting temperature of the pure PCM.

Quantitative analysis of silica aerogel-based thermal insulation coatings

Abstract A mathematical heat transfer model for a silica aerogel-based thermal insulation coating was developed. The model can estimate the thermal conductivity of a two-component (binder-aerogel) coating with potential binder intrusion into the nano-porous aerogel structure. The latter is modelled using a so-called core–shell structure representation. Data from several previous experimental investigations with silica aerogels in various binder matrices were used for model validation. For some relevant cases with binder intrusion, it was possible to obtain a very good agreement between simulations and experimental data with shell thickness and/or thermal conductivity of the shell as adjustable parameters. However, the experimental data was not sufficiently detailed to allow a separation of the effects of the two parameters. In the ideal case of no aerogel binder intrusion, a comparison with a coating containing intact hollow glass or polymer spheres showed that silica aerogel particles are more efficient in an insulation coating than hollow spheres. In a practical (non-ideal) comparison, the ranking most likely cannot be generalized. A parameter study demonstrates how the model can be used, qualitatively, to get an indication of the effect of important model parameters on the thermal conductivity of an insulation coating. With relevant data available for service life exposure conditions and raw material costs, the model can also be used as an optimization algorithm.

Cellulose aerogels based on a green NaOH/PEG solution: Preparation, characterization and influence of molecular weight of PEG

A green cost-effective NaOH/polyethylene glycol (PEG) aqueous solution was adopted to fabricate nanoporous cellulose aerogels, and three kinds of PEG with different molecular weights (200, 4000, and 20000) was carried out to investigate the effect of PEG molecular weights on the morphology, specific surface area and pore structure, crystal structure, and thermal stability of the resulting aerogels. The results show that the PEG with the higher molecular weight promotes the dissolution of cellulose, the formation of pore structure, and the transformation of crystal structure from cellulose I to cellulose II. After being modified by methyltrichlorosilane (MTCS), the aerogels are hydrophobic, and exhibit the oil adsorption ratios of approximately 13–18 g/g, much higher than those of the untreated cellulose aerogels (10–15 g/g). Besides, the oil adsorption ratio has a positive correlation with the pore characteristic parameters (specific surface area and pore volume), revealing the facile controllability.

Fourier spectroscopy of water vapor in the volume of aerogel nanopores. Part 2. Calculation of broadening and shift of spectral lines by adsorbed molecules

The model for simulating the profile of water vapor confined in nanoporous aerogel [2] has been improved. In the improved model, we take into account a possibility of loss of rotational degrees of freedom for water molecules adsorbed on pore walls, collisions with which significantly contribute to line broadening and shifting. The half-widths and shifts calculated for this model are in a good agreement with experimental data.