Thermal Conductivity Of Silica Aerogel Research Articles

Elevating high-temperature insulation performance of silica aerogels enabled by innovative surface-structured opacifiers

Silica aerogel, recognized as an exceptional material for thermal insulation, has gained considerable attention in thermal protection. However, its inherent infrared transparency compromises its insulating performance at high temperatures. To address this limitation, incorporating micron-sized opacifier particles has emerged as a crucial strategy. This study introduces a novel opacifier particle, distinguished by surface protrusions inspired by the ’structural absorption’ mechanism, and assesses its extinction capabilities. Then, a thorough analysis was conducted to understand how the shape, volume, and numbers of these surface protrusions influence the radiative heat transfer properties of silica aerogels. The findings reveal that the presence of protrusions on the opacifier surface significantly enhances its extinction capacity, consequently reducing the radiative thermal conductivity of silica aerogels. Notably, cylindrical protrusions demonstrate the most effective extinction properties compared to spherical protrusions and rectangular protrusions. Additionally, for opacifier particles with cylindrical protrusions, there exists an optimal number of 12 protrusions that minimize the radiative thermal conductivity of the aerogel. At 1300 K, the radiative thermal conductivity of the silica aerogel doped with opacifiers featuring 12 protrusions is 22.6 % lower than that of the silica aerogel doped with opacifiers lacking protrusions. Moreover, it was observed that maintaining a constant diameter of cylindrical protrusions while increasing their height diminishes the radiative thermal conductivity of silica aerogel. However, the effect on the radiative thermal conductivity becomes less significant beyond a certain protrusion height. The outcomes of this study provide significant insights into the innovation of opacifiers.

Theoretical study on the effective thermal conductivity of silica aerogels based on a cross-aligned and cubic pore model

Abstract Aerogel nanoporous materials possess high porosity, high specific surface area, and extremely low density due to their unique nanoscale network structure. Moreover, their effective thermal conductivity is very l1ow, making them a new type of lightweight and highly efficient nanoscale super-insulating material. However, predicting their effective thermal conductivity is challenging due to their uneven pore size distribution. To investigate the internal heat transfer mechanism of aerogel nanoporous materials, this study constructed cross-aligned and cubic pore model (CACPM) based on the actual pore arrangement of SiO2 aerogel. Based on the established cross-aligned and cubic pore model (CACPM), the effective thermal conductivity expression for aerogel was derived by simultaneously considering gas-phase heat conduction, solid-phase heat conduction, and radiative heat transfer. The derived expression was then compared with available experimental data and the Wei structure model. The results indicate that, according to the model established in this study for the derived thermal conductivity formula of silica aerogel, for powdery silica aerogel under the conditions of T=298K, a 2=0.85, D 1=90μm, ρ=128kg/m3, within the pressure range of 0-105Pa, the average deviation between the calculated values and experimental values is 10.51%. In the pressure range of 103-104Pa, the deviation between calculated values and experimental values is within 4%. Under these conditions, the model has certain reference value in engineering verification. This study also makes a certain contribution to the research of aerogel thermal conductivity heat transfer model and calculation formula.

The Influence of Reinforced Fibers and Opacifiers on the Effective Thermal Conductivity of Silica Aerogels.

Fiber-particle-reinforced silica aerogels are widely applied in thermal insulation. Knowing their effective thermal conductivity (ETC) and radiative characteristics under high temperatures is necessary to improve their performance. This article first analyzes the radiation characteristics of silica aerogels doped with opacifier particles and reinforced fibers, and then a universal model is established to predict the ETC. Furthermore, the impacts of different parameters of opacifier particles and reinforced fibers on the thermal insulation performance of silica aerogels are investigated. The results indicate that SiC exhibits comparatively strong absorption characteristics, making it a good alternative for opacifiers to improve thermal insulation performance under high temperatures. For the given type and volume fraction of opacifier particles, there exists an optimal diameter and volume fraction to achieve the best insulation performance of silica aerogel under a certain temperature. Considering that SiO2 fibers exhibit a limited extinction capability and higher conductive thermal conductivity under high temperatures, for fiber-particle-reinforced silica aerogels, it is beneficial for their insulation performance to reduce the fiber volume fraction when the required mechanical properties are satisfied.

Thermal transport properties of gas-filled silica aerogels.

Silica aerogel (SA), recognized as an efficient insulating material, is characterized by its extremely low thermal conductivity (TC) and high porosity, presenting extensive application potential in aerospace and building energy conservation. In this study, the thermal transport properties of gas-filled SA are explored using molecular dynamics (MD) methods. It is found that an increase in porosity leads to a significant decrease in TC, primarily due to enhanced phonon scattering and reduced material stiffness. Additionally, the TC of SA influenced by gas exhibits a pattern of initial decrease, followed by an increase, and then a decrease again, driven by complex interactions between gas molecules and pore walls, phonon localization, and scattering mechanisms. At a gas concentration of 80%, the TC in confined spaces is significantly increased by nitrogen, attributed to enhanced intermolecular interactions and increased collision frequency. The impact of gases on the TC of gas-solid coupled composite materials is also investigated, revealing that gas molecules serve as a "bridge" for phonons, playing a crucial role in reducing interfacial scattering and enhancing low-frequency vibrational modes, thus further enhancing heat transfer efficiency. The TC of these composite materials is primarily regulated by the gas-phase TC in response to temperature, while the response to strain is predominantly governed by variations in the solid-phase TC. These results provide essential theoretical support and design guidelines for the development and design of new high-efficiency insulating materials.

Radiative properties of non-spherical opacifiers doped in silica aerogels for high-temperature thermal insulation

Silica aerogel, a super-insulating material, has garnered significant attention due to its extensive thermal insulation applications. However, the high-temperature performance of silica aerogels is compromised due to their infrared transparency. This study tackles this issue by investigating the influence of opacifier shape on the insulation properties of silica aerogel composites. Employing the Discrete Dipole Approximation (DDA) theory, the radiative properties of opacifiers of varying shapes are investigated. The study also employs a theoretical model to ascertain the effective thermal conductivity of silica aerogels doped with non-spherical opacifiers. To counter oxidation at ultra-high temperatures, an innovative ellipsoidal [email protected] core–shell opacifier is introduced. Non-spherical opacifiers are further optimized to identify the ideal particle diameters and volume fractions for specific temperature ranges. The investigation uncovers that non-spherical carbon black opacifiers with low shape parameter ratios considerably enhance the insulating performance at elevated temperatures. [email protected] core–shell opacifiers, distinguished by high inner/outer radius ratios, offer superior thermal stability and great infrared radiation extinction. In summary, this study offers a pragmatic approach to fabricating aerogels with enhanced insulation properties at high temperatures.

Thermal Insulation Performance of Monolithic Silica Aerogel with Gas Permeation Effect at Pressure Gradients and Large Temperature Differences

ABSTRACT Silica aerogel is an excellent thermal insulator for high-speed aircraft, but there is little research on it in a high-temperature and complex-pressure environment. This research aims to evaluate the thermal insulation performance of silica aerogel monoliths with different porosities under large temperature differences and pressure gradients. We established an experimental system to measure and analyze the hot surface temperature response by fixing the heat flux and the cold surface temperature at transient pressure conditions. An unsteady-state heat transfer model considering gas flow is developed. The effective thermal conductivity of silica aerogels with 79.55 ~ 90.91% porosity is measured at different temperature differences between cold and hot surfaces (127 ~ 512 K), near-vacuum (<10 Pa), and transient pressure conditions. The results demonstrated that silica aerogel with 90.91% porosity showed the best thermal insulation performance when the temperature differences were over 500 K, while the aerogel with 79.55% porosity became the best when the temperature differences were less than 500 K. In addition, both the temperature and pressure difference affect the thermal insulation performance: the energy transport caused by gas flow affects the dynamic temperature response when gas permeability is of the order of 10−15 m2; the thermal insulation performance is improved by increasing gas permeability and pressure difference when gas flow and heat transfer directions are opposite.

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.

Thermal Insulation Performance of Silica Aerogel Composites Doped with Hollow Opacifiers: Theoretical Approach.

Silica aerogels demonstrate great promise in thermal insulation applications, such as energy-efficient buildings, cold-chain transportation, and aerospace engineering. However, the application of pure silica aerogels is limited in high temperature applications (>500 K) due to their transparency in the wavelength of 2–8 µm. The conventional strategy is to dope silica aerogel with solid spherical opacifiers (e.g., SiC, TiO2, and ZrO2) to increase their extinction coefficient; however, incorporating solid opacifiers into silica aerogel matrix improves the structural density of silica aerogel composites. Herein, we propose to improve the extinction coefficient of the silica aerogel by using hollow opacifiers. A theoretical model was developed to investigate the parameters including the outer diameter, shell thickness, and mass fraction on both the radiative thermal conductivity and total thermal conductivity of the silica aerogel composite doped with hollow opacifiers. Our results indicate that doping hollow opacifiers can enable the silica aerogel matrix to achieve lower radiative thermal conductivity when compared to matrices doped with optimally sized solid opacifiers. The total thermal conductivity of silica aerogel doped with hollow opacifiers could be lower than that of the silica aerogel doped with optimally sized solid opacifiers. This work contributes to the understanding of heat transfer within porous materials and guides the structural design of high-temperature thermally insulating materials.

Sorbitol cross-linked silica aerogels with improved textural and mechanical properties

It is well known that although organically modified silica aerogels have enhanced mechanical properties, the specific surface area decreases due to the larger pore size. However, cross-linking with sorbitol enhances the mechanical properties while maintaining the highly textural structure, low density, and the thermal conductivity of silica aerogel. Herein, we report the silica aerogels reinforced with sorbitol via facile sol-gel polymerization. The sorbitol improved mechanical properties while maintaining the textural structure of the silica aerogels. Sorbitol with surface hydroxyl groups was covalently cross-linked with either tetraethoxysilane (TEOS) or methyl trimethoxysilane (MTMS) precursor. The two possible combinations of sorbitol-TEOS or sorbitol-MTMS aerogels were systematically prepared by varying mol% of the precursor and aerogels to obtain different properties. The sorbitol-MTMS aerogel with a 90:1 M ratio of methanol to precursor attained a large surface area (1193 m2/g), good mechanical strength (205.9 kPa) during compression testing, a small pore volume (2.2 cm3/g), and low thermal conductivity (0.041 Wm−1K−1). The thermal stability of sorbitol-TEOS and sorbitol-MTMS cross-linked silica aerogels in air was up to ∼418 °C, as ascertained from their thermo-gravimetric profiles. The results indicate that using small linear organic molecules for cross-linking with an inorganic silica precursor is highly useful for obtaining aerogels with a high surface area and improved mechanical properties.

Elastic modulus prediction based on thermal conductivity for silica aerogels and fiber reinforced composites

The speed of sound is a critical parameter in the test of mechanical and thermal properties. In this work, we proposed a testing method to obtain the elastic modulus of silica aerogel from the sound speed formulas. The solid thermal conductivity of the silica aerogel is experimentally measured for predicting the sound speeds, and then the elastic modulus is calculated based on the elasticity sound speed model. The experimental data of the solid thermal conductivity of silica aerogels with different densities are employed and the obtained elastic modulus is fitted as a power-law exponential function of the density. Two existing sound speed models and three groups of available experimental data are also employed to validate the present fitting relation, and good agreement is obtained for the silica aerogel in the density range of 150–350 kg/m3. The fitting formula can also be extended to estimate the elastic modulus of the glass fiber-reinforced silica aerogel composite. The results show that the elastic modulus of the aerogel composite is sensitive to the glass fiber volume fraction, while the thermal conductivity is weakly dependent on the glass fiber volume fraction at room temperature in the studied range of fiber volume fraction.

Water molecular bridge undermines thermal insulation of Nano-porous silica aerogels

It is challenging to quantitatively predict the insulation degradation of porous materials caused by moisture absorption. A multiscale solution is proposed in this work. The adsorption intensity on connecting nanoparticles is evaluated at the nanoscale by defining an effective contact radius, which is detected and measured numerically. The thermal conductivity of connecting nanoparticles with adsorptive water is assessed by molecular statistics as the thermal conductivity of backbones. The up-scaling from nanoparticles to porous materials is built by mathematical modeling based on geometry structures. A simulation-based theoretical model is proposed for the thermal conductivity of silica aerogels with moisture. The results predicted by the present model agree well with experimental data. An in-depth analysis of experimental data using the proposed model demonstrates why a simple linear dependence is observed between the effective thermal conductivity and water fraction. In addition, the linearity can be attributed to the adsorbed water effect on the heat transfer through the silica-silica interface. The proposed model can deserve more credits considering that all the parameters are introduced explicitly and/or measured reliably by experimental measurement, mathematical derivations, or molecular dynamics simulations. It can demonstrate the molecular origin of the degeneration of thermal insulators under moisture. The present methodology may stimulate more cross-scale modeling and promote more understandings of heat transfer levels in nanoparticle-based materials.

Atmospheric drying preparation and microstructure characterization of fly ash aerogel thermal insulation material with superhydrophobic

In this paper, silica aerogel is prepared cheaply by atmospheric drying method using coal fired fly ash, a solid waste produced by power generation. By means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), pore size and surface area analysis, X-ray diffraction (XRD), Fourier transform infrared spectrum(FT-IR), laser confocal Raman spectrum(LRS), differential scanning calorimeter(DSC), thermogravimetric analyser(TG) and coefficient of thermal conductivity meter and other modern characterization methods, to explore the surface morphology, microstructure, chemical bond state, thermal stability, chemical stability and thermal conductivity of silica aerogels, and the effect of acid amount on its performance are analyzed. The results show that fly ash can prepare silica aerogel with high purity, high specific surface area, low thermal conductivity, and good chemical stability. When the volume ratio of sulfuric acid to calcining mixture is 10, the silica aerogel prepared has the best performance, the thermal conductivity is as low as 0.0234 W/m·K, the surface is super hydrophobic, the hydrophobic angle can reach 146°, the limit service temperature is 400°C, and the specific surface area is 809.923 m2/g.

Numerical Study of Thermal Properties of a Silica Aerogel Material

AbstractSilica Aerogel is material with a special structure consisting of interconnected solid particles of SiO2 forming the skeleton that enclose nanopores filled with confined air and occupying more than 90% of the volume. It is characterized by a thermal conductivity that can reach lower values than any other material. Our aim was to explain the causes of the super-insulation of this material with the use of a calculating method of conductive heat transfer flux in an aerogel structure and determine the equivalent thermal conductivity. For this purpose, numerical specific software was developed to generate random structure of silica aerogel with predefined concentration of solid particles and properties of both skeleton and confined air. Calculation of the conductivity at any point in the confined gas domain shows variable values as a function of the pore size and the location of the point in the pore. A new developed numerical method was used to calculate the equivalent thermal conductivity of the whole structure. Concentration of solid particles has proved not to be the only parameter in which determine the thermal conductivity of silica aerogel and the influence of tortuosity has been demonstrated. A correlation linking thermal conductivity to both concentration of solid particles and tortuosity of the material was suggested.

Comparative study of a cubic, Kelvin and Weaire-Phelan unit cell for the prediction of the thermal conductivity of low density silica aerogels

Super insulating porous materials such as silica aerogels have remarkable thermal insulating properties as demonstrated, experimentally. However, unravelling the underlying heat transfer phenomena is difficult because of the complex multiscale 3D structure of aerogels. For densities higher than 150 kg.m−³, there is a good correlation between experimental and predicted thermal conductivity values based on cubic unit cells. However, below 150 kg.m−³, large discrepancies between measured and predicted thermal conductivity values still exist. This numerical study tackles this issue by predicting thermal conductivities of silica aerogels including solid, gaseous and radiative heat transfers using more representative shaped unit cell types (Kelvin and Weaire – Phelan cells). The effects of the pore size distribution and their shape could then be analyzed. A parametric study was carried out including the skeleton and neck size, the gas void dimension and the contact length between the beads with respect to the density and was benchmarked against available literature. For the investigated range of densities, good agreement was found between the predicted results obtained using these newly applied unit cell geometries and previously measured experimental data.

Thermal Conductivity of Silica-aerogel (SA) and Autoclave Aerated Concrete (AAC) Composites

Improving the thermal insulating performance of porous building materials is of great practical significance for building energy conservation. In this work, silica aerogels (SA) with ultralow thermal conductivity were proposed as an appropriate candidate to be integrated with autoclaved aerated concrete (AAC) to produce novel SA-AAC composites with higher thermal insulating performance by physical solution impregnation method. The pore-structures, mechanical and thermal properties of the SA-AAC composites were probed by various experimental tests. According to the microscopy and porosimetry results, SA were observed to adhere to the surface walls of the AAC holes, thus reducing the amount of macro-sized pores. In addition, the improved thermal insulating performance of AAC was successfully achieved with the relative improvement depending on the porosity of the pristine AAC. At the mass fraction of SA of ~7%, the highest relative improvement was found to be ~30% The results of this work exhibited a great potential of this novel SA-AAC composite in engineering applications.

A novel method to predict the thermal conductivity of nanoporous materials from atomistic simulations

The low density (smaller than 10% of the bulk density) and the nanostructured porosity of silica aerogels provide their extremely low thermal conductivities but also impact their poor mechanical properties. Atomic scale simulation is the appropriate tool to predict the thermal and mechanical properties of such materials. For such simulations, the interatomic potential should be carefully chosen to ensure result validity but also reasonable computational times. A truncated BKS potential has been used for aerogels as it fairly reproduces the nanostructure. It allows reducing the computational time by a 3000 gain factor on the CPU time per atom per step compared to the original BKS interatomic potential while predicting correctly the mechanical properties. However, when it comes to skeletal thermal conductivity of nanoporous silica, the associated computation times are too large for a representative volume. This is due to the low thermal diffusivity of the material. Here, a new method that takes advantage of the amorphous structure of silica and the diffusive nature of phonon heat transfer at the scale of an aerogel aggregate is proposed. The time dependent temperature profile in the system obtained from Non-Equilibrium Molecular Dynamics simulations is compared to the classical solution of the thermal diffusion equation and an identification procedure is used to determine the thermal conductivity of silica aerogels.

A theoretical model for the effective thermal conductivity of silica aerogel composites

In the present paper, a complete engineering calculation procedure for fast evaluations of the effective thermal conductivity of the silica aerogel composites is developed as follows. First, the spherical hollow model is adopted to calculate the conductive thermal conductivity of pure silica aerogels. Then, a mixing model is used to consider the conduction enhancement effects of doped opacifiers and fibers. Next, the Mie theory is adopted to calculate the temperature-dependent Rosseland mean extinction coefficient of the aerogel composites, and the radiative thermal conductivity is obtained according to the Rosseland equation. Finally, the superposition of the conductive thermal conductivity and radiative thermal conductivity gives the total effective thermal conductivity of aerogel composites. To validate the accuracy of the present model, some corresponding experimental measurements are conducted based on the Hot Disk thermal constant analyzer, and the experiment data agree well with the theoretical calculation values. The maximum deviation is about ±10%. The influences of the temperature, pressure and the structure parameters on the prediction of the effective thermal conductivity are then investigated. To determine the dominant factor of the effective thermal conductivity, the contributions of gas, solid and radiation to the total effective thermal conductivity are investigated individually and some significant results are obtained.

Synthesis of composite insulation materials—expanded perlite filled with silica aerogel

The aim of this paper is to synthesis a new type of insulation material (EPA), which fill the aerogels into the expanded perlite (EP). EP is a kind of lightweight filler, its application is constrained by the character of absorbing water easily. The silicic acid is inhaled into the expanded perlite at −0.1 MPa, aging 24 h, it becomes aerogel after solvent exchanging/surface modifying and drying. The pores of EP are filled with aerogel which affects thermal conductivity of expanded perlite little and makes it hydrophobic. EPA has a wider use than EP for its hydrophobic character. A new recipe to synthesize aerogel, filled into EP, with the thermal conductivity of 0.034 W/m K in ambient pressure drying is found in this experiment. The time and reagents dosage for synthesizing EPA are less than large block of aerogel, while the thermal conductivity is close to it. The scanning electron microscopy is used to analyze EPA’s micro structure. The thermal conductivity tester is used for testing the thermal conductivity of silica aerogel, expanded perlite and EPA.

A multi-level fractal model for the effective thermal conductivity of silica aerogel

In the present paper, the fractal analysis on silica aerogel is performed based on the statistical self-similarity of porous media. A fractal model for the effective thermal conductivity of silica aerogel is proposed in consideration of the tortuosity of heat transfer path and the microstructure of secondary particle. The proposed model assumes that silica aerogel consists of two levels of microstructures: the secondary particles symbolized by improved 2-level Menger sponge and the cluster formed by secondary particles, which can be expressed as a function of density, the contact ratio, and the fractal dimensions. Validation with experimental data and predictions of other theoretical models shows that the present fractal model is reliable and accurate.

Thermal Properties of Silica Aerogel Formula

The thermal conductivity of silica aerogel developed in this research program was measured using the transient hot-wire technique. The thermal conductivity of monolithic samples drops significantly from 9.3 mW/m · K to 3.2 mW/m · K with modest pressure reduction from 1 atm to 0.1 atm. The same aerogel in granular form has a thermal conductivity of 15.0 mW/m · K at ambient gas pressure with a modest compression applied to compact the granules and reduce the air void sizes. Radiation heat transfer in the hot-wire test may not be representative of its contribution in large scale applications. Measurements of the monochromatic extinction coefficient over the wavelengths of interest resulted in a Rosseland mean extinction coefficient of 2400 m−1 at 300 K. The small thermal penetration distance during the hot-wire measurements suggest that in actual use radiation could contribute approximately 2.5 mW/m · K with a possible upper limit of 3.0 mW/m · K to the effective thermal conductivity over that measured using the transient hot-wire method.