Thermal Conductivity Of Aerogels Research Articles

A Review of High-Temperature Aerogels: Composition, Mechanisms, and Properties.

High-temperature aerogels have garnered significant attention as promising insulation materials in various industries such as aerospace, automotive manufacturing, and beyond, owing to their remarkable thermal insulation properties coupled with low density. With advancements in manufacturing techniques, the thermal resilience of aerogels has considerable improvements. Notably, polyimide-based aerogels can endure temperatures up to 1000 °C, zirconia-based aerogels up to 1300 °C, silica-based aerogels up to 1500 °C, alumina-based aerogels up to 1800 °C, and carbon-based aerogels can withstand up to 2500 °C. This paper systematically discusses recent advancements in the thermal insulation performance of these five materials. It elaborates on the temperature resistance of aerogels and elucidates their thermal insulation mechanisms. Furthermore, it examines the impact of doping elements on the thermal conductivity of aerogels and consolidates various preparation methods aimed at producing aerogels capable of withstanding temperatures. In conclusion, by employing judicious composition design strategies, it is anticipated that the maximum tolerance temperature of aerogels can surpass 2500 °C, thus opening up new avenues for their application in extreme thermal environments.

Construction of the Aerogel Thermal Conductivity Model based on k×k Sierpinski Carpet Fractal Units

With the characteristics of low density, high specific surface area, and high porosity, aerogel boasts prominent advantages in the field of thermal protection. The thermal insulation performance of aerogel has a significant relationship with its internal microstructure. In this study, the thermal conduction model of Sierpinski aerogel filled with solid in ga
 With the characteristics of low density, high specific surface area, and high porosity, aerogel boasts prominent advantages in the field of thermal protection. The thermal insulation performance of aerogel has a significant relationship with its internal microstructure. In this study, the thermal conduction model of Sierpinski aerogel filled with solid in gas is established based on the equivalent circuit method. We calculated the optimal fractal unit structure of the aerogel via its porosity, applied it to the thermal conductivity calculation of four types of aerogels, and revealed the average relative error of less than 11.58%, which is lower than the calculation results of the thermal conductivity model of the aerogel with the fractal unit structure of , indicating the effectiveness and reliability of the proposed thermal conductivity prediction model.
 s is established based on the equivalent circuit method. We calculated the optimal fractal unit structure of the aerogel via its porosity, applied it to the thermal conductivity calculation of four types of aerogels, and revealed the average relative error of less than 11.58%, which is lower than the calculation results of the thermal conductivity model of the aerogel with the fractal unit structure of , indicating the effectiveness and reliability of the proposed thermal conductivity prediction model.

Microscopic revelation of the solid–gas coupling and Knudsen effect on the thermal conductivity of silica aerogel with inter-connected pores

As a star insulation material, aerogel plays a significant role in saving energy and meeting temperature requirements in industry due to its extremely low thermal conductivity. The prediction of aerogel’s thermal conductivity is of great interest in both research and industry, particularly because of the difficulty in measuring the separated gas conductivities directly by experiment. Hence, the proportions of separated gas conduction and solid–gas coupling conduction are debatable. In this work, molecular dynamics simulations were performed on porous silica aerogel systems to determine their thermal conductivities directly. The pore size achieved in the present study was improved significantly, making it possible to include the gas phase in the investigation of aerogel thermal conductivity. The separated solid conductivity {lambda }_{s} and the separated gas thermal conductivity {lambda }_{g} as well as the effective solid conductivity {lambda }_{s}^{e} and the effective gas conductivity {lambda }_{g}^{e} were calculated. The results suggest that the solid–gas coupling effect is negligible in rarefied gas because the enhancement of thermal conduction due to the short cut bridging effect by gas between gaps in the solid is limited. The gas pressure is the most significant factor that affects the solid–gas coupling effect. The large differential between the prediction and the actual value of the thermal conductivity is mainly from the underestimate of {lambda }_{g}, and not because of ignoring the coupling effect. As a conclusion, the solid–gas coupling effect can be neglected in the prediction of silica aerogel’s thermal conductivity at low and moderate gas pressure, i.e., decreasing the gas pressure is the most efficient way to suppress the coupling effect. The findings could be used in multi-scale simulations and be beneficial for improving the accuracy of predictions of aerogel thermal conductivity.

Optimization Calculation and Analysis of Thermal Conductivity of Aerogels

Optimization Calculation and Analysis of Thermal Conductivity of Aerogels

The use of cellulose fiber from office waste paper to improve the thermal insulation-related property of konjac glucomannan/starch aerogel

The development of eco-friendly thermal insulation material composed of recyclable wastes contributes to enhancing environmental sustainability with less pollution. In this study, polysaccharide-based aerogels with improved thermal insulation property were obtained by using konjac glucomannan, starch, and office waste paper cellulose fiber (WPCF) via sol-gel and freeze-drying method. The obtained aerogels had a bulk density of 30.1–44.9 mg/cm3, a porosity of 95.1–97.0%. Results revealed that increased WPCF addition did not impact the thermal stability, but improved the sol stability, reduced the pore size, strengthened the mechanical property, and lowered the hygroscopicity of the aerogels, benefiting practical application. Although WPCF solids showed a high thermal conductivity, increased WPCF addition at 0.1–0.5 wt% reduced the aerogel thermal conductivity, which was explained by the longer heat path and less heat convention. Further higher WPCF addition at 0.5–1.5 wt% increased the aerogel thermal conductivity as the heat conduction of solid matrix was increased and the heat convection was strengthened. Therefore, the lowest thermal conductivity (0.0335 Wm−1 K−1) of the aerogel was reached at the optimized WPCF addition of 0.5 wt%. This work showed the high potential of the WPCF to be used to strengthen the thermal insulation property of polysaccharide-based aerogels.

Examination on aerogel based on the heat transfer characteristics of nanotechnology

With the rapid development of global industrialization, the research and development of new thermal insulation materials has great social and economic significance for national development and social progress. Due to the porous network structure of nanotechnology, the porosity of aerogel is 80–99.8%, the thermal conductivity can be as low as 0.013 w/(m k) at room temperature. It is the best insulation material by far, it has important value in the fields of aerospace, petrochemical, electric metallurgy, shipbuilding, precision instruments and so on. This paper is based on aerogel nanotechnology materials at home and abroad. In order to solve the problem of the study on heat transfer characteristics based on nanotechnology is not perfect, and the calculation model of solid phase thermal conductivity coefficient and total thermal conductivity of complex nanotechnology structure is not accurate enough. Through a heat transfer calculation method suitable for nanotechnology to conduct more detailed and in-depth research on these issues, introduces and analyzes the development of the equivalent thermal conductivity calculation model of Novel Aerogel nanotechnology insulation material in recent years. And analyzed The heat transfer characteristics and thermal insulation principle of nanotechnology aerogel by using equivalent thermal conductivity calculation model of aerogel nanotechnology insulation material. Research indicates: For aerogel nanotechnology materials, Use heat transfer calculation method aerogel nanotechnology insulation materials can be well calculated using a heat transfer calculation method that suitable for nanotechnology can calculate the equivalent thermal conductivity of aerogel nanotechnology insulation material very well; nanotechnology particles and aerogel meshes and continuous spatial network structures are reason for the very low thermal conductivity of aerogels; The heat transfer of the material is mainly achieved by convection, radiation and heat transfer. So it is possible to reduce the thermal conductivity from the heat transfer path of the material.

Dense and strong, but superinsulating silica aerogel

Silica aerogel is the ultimate thermal insulator thanks to its record-breaking low thermal conductivity (λ), open porosity and hydrophobicity. Silica aerogel's thermal conductivity is lowest at intermediate densities (ρ ≈ 0.11 g/cm3) and, because of the strong, power-law dependence of the E modulus on density, this rather low density so far led to low E moduli. Even with polymer reinforcement, increasing stiffness is only possible at higher density, thus higher conductivity. This paper explores the synthesis of silica aerogel granules using ambient pressure drying to provide enhanced mechanical stiffness whilst maintaining thermal conductivities well below 20 mW m−1 K−1. The aging and drying conditions affect the interplay between mechanical and thermal properties, and are varied to optimize the physical properties. The dense (ρ≤0.29 g/cm3), but superinsulating (λ≈15 mW m−1 K−1) silica aerogels presented in this paper challenge the community's understanding of heat transport in aerogels, and do not rely on polymer reinforcement. The underlying microscopic structural parameters affecting the mechanical and thermal transport properties are investigated by modelling and simulation of the aerogel back-bone. Short aging times reduce the cross-section of, and heat transport through, inter-particle necks, leading to an overall decrease in thermal conductivity through the solid skeleton (λs). In addition, short-aged gels undergo a partial pore collapse during ambient pressure drying of the pore fluid due to less aged, hence weaker network structures. The resulting denser structure contains additional point contacts that increase stiffness, by up to an order of magnitude. However, heat transport through these newly formed point-contacts is limited and the gas phase conduction (λs) is further suppressed due to the even smaller pore sizes. Strong and superinsulating particles are ideal fillers for aerogel composites, concrete and renders. The optimized APD aerogels, available as granules, are finally compiled in a composite thermal insulation board with an effective thermal conductivity down to 20 mW m−1 K−1 with improved strength: a 2-fold increase for E, compared to a board produced from classical silica aerogel granulate. The possibility to improve mechanical properties of pure silica aerogels can help aerogels to break into new high-strength, superinsulating structural applications needed to reduce carbon emissions of the built environment.

Preparation of nano-SiO<sub>2</sub> building aerogel thermal conductivity coatings and study on the factors of thermal conductivity

Preparation of nano-SiO<sub>2</sub> building aerogel thermal conductivity coatings and study on the factors of thermal conductivity

Novel GO/silica composite aerogels with enhanced mechanical and thermal insulation properties prepared at ambient pressure

ABSTRACTNovel graphene oxide (GO)/silica composite aerogels were fabricated to strengthen the nanostructure and overcome inherent fragility of silica aerogels by a sol-gel method using tetraethyl orthosilicate (TEOS) and low concentration of graphene oxide solution as the main materials, followed by an environmentally friendly ambient pressure drying (APD). The interaction between GO nanosheets and silica matrix was investigated by using FTIR and SEM. Meanwhile, the mechanical properties and thermal conductivity of aerogels were also studied. The results showed that GO were uniformly dispersed in the silica aerogel, and there is a good physico-chemical combination between GO and silica matrix. The addition of GO can enhance the mechanical of the composite aerogels, and when the GO content is increased from 0 to 3wt%, the mechanical strength is improved from 0.04 MPa to 0.65 MPa. Meanwhile, the thermal conductivity of composite aerogels first decreases and then increases with increasing GO content, and the optimal value is 0.018 W/m K. The composites with outstanding properties prepared by APD are potential for insulation applications.

Starch Aerogels: A Member of the Family of Thermal Superinsulating Materials.

Starch aerogels were prepared via dissolution in water (thermomechanical treatment), retrogradation, solvent exchange, and drying with supercritical CO2. Amylose content in starches was varied from 0 to 100%. The aerogels' bulk density, morphology, specific surface area, thermal conductivity, and mechanical properties under compression were investigated. Pea starch aerogels had one of the highest specific surface area and lowest density and thermal conductivity (0.021-0.023 W/m·K), with the latter indicating that a new thermal superinsulation material was obtained. A detailed study of the influence of processing parameters on pea starch aerogels properties showed the importance of retrogradation time which decreases specific surface area and increases mechanical properties and thermal conductivity. Finally, a comparison of starch aerogel thermal conductivity with that of other bioaerogels is performed.

A general strategy for improving the thermal insulation performance of aerogels by multiple impregnation

A new strategy is proposed to make aerogels possessing both low thermal conductivity and high density by a multiple impregnation approach to the as-prepared silica aerogel monoliths using hydrolyzed sol. The result demonstrates that the thermal conductivity of aerogels after multiple impregnations does not increase compared with that of pristine aerogels and presents a minor decrease from 0.0090 to 0.0081W/(mK) with the density increment from 0.074 to 0.218g/cm3. A mechanism has been revealed that the decrease of macropores by multiple impregnation approach could be beneficial for improving the thermal insulation performance of silica aerogels in wide density range.

Novel three-dimensional halloysite nanotubes/silica composite aerogels with enhanced mechanical strength and low thermal conductivity prepared at ambient pressure

Novel three-dimensional halloysite nanotubes (HNTs)/silica composite aerogels were fabricated to strengthen the nanostructure and overcome inherent fragility of silica aerogel by a sol–gel method using tetraethyl orthosilicate (TEOS) and 1,4-Phenylene diisocyanate (PPDI)-modified HNTs (p-HNTs) as the main materials, followed by an environmentally friendly ambient pressure drying (APD). The modification and dispersion of HNTs were investigated by using FTIR, SEM, TEM. Meanwhile, the morphology, mechanical properties, and thermal conductivity of aerogels were also studied. The results showed that the p-HNTs were uniformly dispersed in the silica aerogel. This resulted in composites with unique interpenetrating network microstructure that could give an enhanced mechanical properties to aerogels. The compressive strength was found to vary from 400 kPa to 1.45 MPa and thermal conductivity from 0.025 to 0.038 W/m K. In addition, randomly oriented p-HNTs can obviously reduce both the linear shrinkage and density of composite aerogels. The composites with outstanding properties prepared by APD are potential for building applications.

Cellulose Cork/phenolic Aerogel Nanocomposites as a Lightweight Thermal Insulator

Cork-based composites are one of the well-known insulators. In order to improve the properties of these composites, especially thermal conductivity, aerogel nanostructures that have low thermal conductivity, thermal diffusivity, density and high compressive strength can be used as fillers. Because of the low thermal conductivity of aerogels, thermal conductivity of final nanocomposite products can reduce. In this work, phenolic aerogels were first synthesized through sol-gel procedure. Then, prepared organic aerogels were dried at elevated temperatures and used to prepare nanocomposite of phenolic aerogels and cellulose cork. Thermal measurements and scanning electron microscope (SEM) technique were conducted on prepared nanocomposites. Through loading of small amounts of phenolic aerogel thermal conductivity and thermal diffusivity of nanocomposites were reduced about 38.8 and 44.8%, respectively.

Joule Heating Characteristics of Emulsion‐Templated Graphene Aerogels

The Joule heating properties of an ultralight nanocarbon aerogel are investigated with a view to potential applications as energy‐efficient, local gas heater, and other systems. Thermally reduced graphene oxide (rGO) aerogels (10 mg cm−3) with defined shape are produced via emulsion‐templating. Relevant material properties, including thermal conductivity, electrical conductivity and porosity, are assessed. Repeatable Joule heating up to 200 °C at comparatively low voltages (≈1 V) and electrical power inputs (≈2.5 W cm−3) is demonstrated. The steady‐state core and surface temperatures are measured, analyzed and compared to analogous two‐dimensional nanocarbon film heaters. The assessment of temperature uniformity suggests that heat losses are dominated by conductive and convective heat dissipation at the temperature range studied. The radial temperature gradient of an uninsulated, Joule‐heated sample is analyzed to estimate the aerogel's thermal conductivity (around 0.4 W m−1 K−1). Fast initial Joule heating kinetics and cooling rates (up to 10 K s−1) are exploited for rapid and repeatable temperature cycling, important for potential applications as local gas heaters, in catalysis, and for regenerable of solid adsorbents. These principles may be relevant to wide range of nanocarbon networks and applications.

Heat conduction modeling in 3-D ordered structures for prediction of aerogel thermal conductivity

A modified three-dimensional (3-D) numerical model is proposed to study heat conduction in nanoporous aerogel by simulating the representative ordered structures. We introduce size effects of the aerogel particle and the pore into the numerical model to improve prediction accuracy for the effective thermal conductivity of aerogel. The numerical model is examined by comparing with available experimental data and 2-D numerical results. The coupled heat transfer between the aerogel solid and gas phases is investigated, and the effect of gas pressure on prediction accuracy of the present model is also discussed. The results show that the present model can accurately predict the effective thermal conductivity of aerogel when the aerogel density is above 120kg/m3 and the gas pressure is in the range of 104–105Pa.

Experimental and analytical analyses of the thermal conductivities and high-temperature characteristics of silica aerogels based on microstructures

An analytical heat transfer model based on scanning electron microscopy, Brunauer–Emmett–Teller and pycnometry measurements and a 3D random diffusion-limited cluster–cluster aggregation structure is proposed to calculate the temperature-dependent microstructural parameters and thermal conductivities of silica aerogels. This model is a pure prediction model, which does not need experimentally fitted empirical parameters and only needs four measured structural parameters as input parameters. This model can provide high-temperature microstructural and thermophysical properties as well as theoretical guidelines for material designs with optimum parameters. The results show that three stages occur during the thermal evolution processes of the aerogel structure with increasing temperature from 300 to 1500 K. The current analytical model is fully validated by experimental data. The constant structure assumptions used in previous heat transfer models are found to cause significant errors at higher temperatures as the temperature-dependent structure deformation significantly increases the aerogel thermal conductivity. The conductive and total thermal conductivities of silica aerogels after high-temperature heat treatments are much larger than those with no heat treatment.

Lignin–phenol–formaldehyde aerogels and cryogels

Highly porous organic aerogels have been prepared for the first time from a mixed lignin–phenol–formaldehyde (LPF) resin. Six different P/L weight ratios and two (L+P)/F weight ratios have been tested, leading most of times to nice and reproducible hydrogels which were subsequently either supercritically of freeze-dried. A broad family of aerogels and cryogels, respectively, was thus obtained. These materials were thoroughly investigated in terms of porous structure, based on pycnometry, adsorption and electron microscopy studies. The pore-size distributions were found to depend strongly on the initial composition, but not on the method of drying. The thermal conductivity of aerogels and cryogels has been measured and found to be minimal in materials combining both high mesopore volume and ideal pore sizes.

Experimental Investigations on the Thermal Conductivity of Silica Aerogels by a Guarded Thin-Film-Heater Method

A thin-film-heater method is setup to measure the thermal conductivity of super insulating materials such as silica aerogels. The experimental setup is purposely designed for insulating materials and allows direct measurement of the thermal conductivity. Few experimental data are available in the literature concerning thermal conductivity of aerogels even though these materials are of major interest in thermal insulation. More data are necessary in order to understand thermal transport and to validate existing models. Monolithic and granular silica aerogels are investigated. Our experimental technique enables to quantify the influence of important parameters, such as air pressure and distribution of grain sizes, on the insulating performance of this material.

On non-extensive nature of thermal conductivity

In this paper we study non-extensive nature of thermal conductivity. It is observed that there is similarity between non-extensive entropic index and fractal dimension obtained for the silica aerogel thermal conductivity data at low temperature.