Fumed Silica-Derived, Ambient Dried, and Low-Cost Nanoporous Aerogel-like Monoliths for Thermal Insulation

Abstract

Nanoporous silica aerogels have great potential in the application of thermal insulation. However, the commonly used supercritical drying technology of nanoporous silica aerogels has disadvantages such as high cost, high complexity, and harsh drying conditions. Herein, a template orientation strategy has been proposed for fabricating nanoporous aerogel-like silica monoliths (ASMs) based on the low-cost, rapid ambient drying technology without additional treatments of surface modification and solvent exchanging. The successful implementation of the template orientation strategy depends on the silica slurry, which is composed of commercial fumed silica, hydrolyzed methyltrimethoxysilane (MTMS), and sole water solvent. The solidification of silica slurry has been achieved by a controlled polycondensation reaction catalyzed by ammonia originating from the thermolysis of urea. Owing to the skeleton enhancement of fumed silica powders and the hydrophobic effect of methyl groups, ASMs maintain well monolithic formability during ambient drying. Meanwhile, low-cost fumed silica powders have been seen as template orientation agents for inducing MTMS molecules to generate nanoparticle-based network skeletons, endowing ASMs with typical nanopore features (specific surface area as high as 113 m2/g and pore diameter distribution concentrated at ∼40 nm). The resulting ASMs exhibit low density (0.36 g/cm3), high compressive strength (0.92 MPa), and low thermal conductivity [0.056 W/(m·K)]. This work would provide significant guidelines for the fabrication and thermal insulation applications of nanoporous ASMs derived from ceramic powders. This simple and low-cost preparation strategy would promote the large-scale applications and industrial production of nanoporous silica for thermal insulation materials.

Biomass-derived lightweight SiC aerogels for superior thermal insulation

“…Typically, the gas phase heat conduction is mainly affected by the porosity of the material. 31–33 Air in the pores can be regarded as the dispersed phase in the aerogel, and the thermal conductivity ( K ) can be calculated according to the following equation: 34 K = K c (1 − φ d )/(1 + φ d /2) ≈ K c (1 − φ d ),where φ d is the gas volume fraction and K c is the thermal conductivity of the crystal continuous phases. Clearly, the excellent thermal insulation properties of SNWAs are mainly attributed to the abundant air in the porous structure with lower thermal conductivity.…”

“…36 In addition, from the perspective of solid-phase heat conduction, the decrease in thermal conductivity of SNWAs-3 is assigned to its high density of stacking faults at the junctions of the SiC nanowires. Generally, the solid-phase thermal conductivity of a material is mainly determined by its phonon and electron thermal conductivity, 31–33,37 according to the following equations: 34 K = K Ph + K e ,where K Ph is the phonon thermal conductivity, K e is the electron thermal conductivity, C ( ν ) is the specific heat capacity of the sample, ν is the phonon vibration frequency of the sample, l ( ν ) is the average free path of phonons and V is the volume of the sample. Given that the SiC nanowires are covalently bonded by Si–C in SNWAs-3, only scarce numbers of free electrons can impact its solid-phase thermal conductivity.…”

“…Typically, the gas phase heat conduction is mainly affected by the porosity of the material. [31][32][33] Air in the pores can be regarded as the dispersed phase in the aerogel, and the thermal conductivity (K) can be calculated according to the following equation: 34…”

“…36 In addition, from the perspective of solid-phase heat conduction, the decrease in thermal conductivity of SNWAs-3 is assigned to its high density of stacking faults at the junctions of the SiC nanowires. Generally, the solidphase thermal conductivity of a material is mainly deter-mined by its phonon and electron thermal conductivity, [31][32][33]37 according to the following equations: 34…”

Nanoporous Silica–Chitosan Aerogels for Thermal Insulation and Flame Retardancy

“…For example, the traditional silicon sources for preparing SA are mainly methyl orthosilicate and ethyl orthosilicate, which are not only costly but also possess toxicity concerns. Furthermore, the conventional drying method of SA is supercritical drying, which suffers from drawbacks such as prolonged duration, excessive energy consumption, and poor safety. These limitations significantly impede the industrial-scale production of SA.…”