Elaborate Size-Tuning of Silica Aerogel Building Blocks Enables Laser-Driven Lighting

Transparent Silica Aerogels: Optical and Chemical Design, Controlled Synthesis, and Emerging Applications

Transparent silica aerogel, serving as one typical porous and transparent material, possesses various unique features (e. g., large amounts of pores and interfaces, super-lightweight, super thermal insulation, low refractive index similar to gas), and it has attracted great attention in the fields of science, technology, engineering, art, and others. Transparency is one important evaluation index of transparent silica aerogel, and it was influenced by various factors such as raw materials, sol-gel reactions, phase separation, and drying methods. The structure design and fabrication of transparent silica aerogel is one huge and fine engineering. In this review, the optical/chemical guidance and design for the preparation of transparent silica aerogels are discussed, and typical applications, such as Cherenkov detectors, solar energy collection, lighting systems, and transparent fabric, were also discussed. Finally, a future outlook on the opportunities and challenges of transparent silica aerogels was proposed.

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.

High-Energy Laser Protection Performance of Fibrous Felt-Reinforced Aerogels with Hierarchical Porous Architectures

Continuous-wave lasers can cause irreversible damage to structured materials in a very short time. Modern high-energy laser protection materials are mainly constructed from ceramic, polymer, and metal constitutions. However, these materials are protected by sacrificing their structural integrity under the irradiation of high-energy lasers. In this contribution, we reported multilayer fibrous felt-reinforced aerogels that can sustain the continuous irradiation of a laser at a power density of 120 MW·m-2 without structural damage. It is found that the exceptional high-energy laser protection performance and the comparable mechanical properties of aerogel nanocomposites are attributed to the unique characteristics of hierarchical porous architectures. In comparison with various preparation methods and other aerogel materials, multilayer fibrous felt-reinforced aerogels exhibit the best performance in high-energy laser protection, arising from the gradual interception and the Raman-Rayleigh scattering cycles of a high-energy laser in the porous aerogels. Furthermore, a near-zero thermal expansion coefficient and extremely low thermal conductivity at high temperature allow the lightweight felt-reinforced aerogels to be applied in extreme conditions. The felt-reinforced aerogels reported herein offer an attractive material that can withstand complex thermomechanical stress and retain excellent insulation properties at extremely high temperature.

Mismatched Refractive Index Strategy for Fabricating Laser-Driven Wood Diffusers from Bulk Wood for Illumination Applications

Laser-diode-based solid-state lighting is primarily used in state-of-the-art illumination systems. However, these systems rely on light-converting inorganic phosphors, which have low quantum efficiencies and complex manufacturing conditions. In this study, a mismatched refractive index strategy is proposed to directly convert natural bulk wood into a laser-driven wood diffuser using a simple delignification and polymer infiltration method. The resulting material has the potential to be used in laser-driven diffuse illumination applications. The optical performance of the laser-driven wood diffuser is optimized by changing the density of natural wood. The optimal coefficient of illuminance variation of the wood diffuser is as low as 17.7%, which is significantly lower than that of commercial diffusers. The illuminance uniformity is larger than 0.9, which is significantly higher than the ISO requirements for indoor workplace lighting. The laser damage threshold is 7.9 J cm-2, which is considerably higher than those of the substrates of commercially available phosphors. Furthermore, the optimized wood diffuser exhibits outstanding mechanical properties, excellent thermal stability, tolerance to harsh environmental conditions, and low speckle contrast. These results show that the laser-driven wood diffuser is a promising laser-color converter that is suitable for indoor, long-distance outdoor, undersea, and other high-luminance laser lighting applications.

Regulating microstructures of aerogels by controlling phase separation mechanism for improving specific surface area and energy harvesting

Aerogels with 3D porous structures have been attracting increasing attention among functional materials due to their advantages of being lightweight and high specific surface area. Precise control of the porous structure of aerogel is essential to improve its performance. In this work, polylactic acid (PLA) aerogels with distinctly different microstructures were fabricated by precisely controlling the phase separation behavior of the ternary solution system. Rheological and theoretical analyses have revealed that the interactions between polymer molecules, solvents and non-solvents play a crucial role in determining the nucleation and growth of poor olymer and rich polymer phases. By adjusting the non-solvent type and the solution composition, aerogels with spider network structure, bead-like connected microsphere structure, and cluster petal structure were obtained. Ideal spinodal phase separation conditions were obtained to produce aerogels with a homogeneous fiber network structure. The optimum PLA aerogel achieved an extremely porosity of 96 % and a high specific surface area of 114 m2/g, which rendered it with excellent triboelectric generation performance. Thus, this work provides fundamental insights into the precise regulation of the phase separation behavior and the structure of the aerogel, which can help boost the performance and expand the applications of PLA aerogels.

Revealing Disparities in Porous Networks Between Yttria Aerogel Assemblies with Nanosheets and Nanoparticles and Their Ultrathermal Insulation and Optical Properties

Recent advancements have introduced anisotropic structures, particularly 2D nanosheets, into aerogels, resulting in unique morphologies and exceptional properties that differ from those assembled by isotropic nanoparticles. However, exploration of the distinct porous networks and the resulting properties is limited. We focus on rare earth yttria (Y2O3) aerogels as a case in point and demonstrate the synthesis of aerogels with nanosheet and nanoparticle assemblies using elaborative sol-gel chemistry. With the aid of X-ray computed tomography, three-dimensional visualizations of the aerogels provide relative compressive views of the porous network, revealing that the Y2O3 aerogel assembled by nanosheets possesses a hierarchical pore structure characterized by uneven pore distribution, particularly the presence of macropores throughout; in contrast, these consist of nanoparticles exhibiting a relative uniform pore distribution. High-temperature examinations indicate that the nanosheet aerogels are much more stable with a specific surface area of 64 m2·g-1 after being exposed at 1300 °C; meanwhile, the aerogels present durable and efficient thermal insulation performances. The exceptional thermal properties are attributed to the synergistic effects of the nanosheets' crystalline nature and the hierarchical porous network. The nanosheet Y2O3 aerogel also exhibited superior luminescent emission characteristics, further enhancing its potential for various applications. Our findings provide further insights into optimization of the microstructures in nanoporous aerogels, particularly through the utilization of anisotropic nanosheets.

Ratio-Tuning of Silica Aerogel Co-Hydrolyzed Precursors Enables Broadband, Angle-Independent, Deformation-Tolerant, Achieving 99.7% Reflectivity

The super-white body might be defined as its reflectivity exceeding 98% at any angle in the visible light spectrum, which can be used in a variety of emerging fields including optics, energy, environment, aerospace, etc. However, elaborate synthesis of a light-weight, highly reflective super-white aerogel body remains a great challenge. In this work, fine-tuning of silica aerogel co-hydrolyzed precursor ratios, 99.7% reflectivity with angle-independence in the visible light spectrum has been successfully achieved when the areal density is only 0.129 g cm-2 , which breaks through the theoretical bandwidth limit of photonic crystals as well as the measured reflectivity limit of conventional porous materials. Furthermore, the reflectivity of super-white silica aerogel remains unchanged after various harsh deformations including compression and bending 1000 times, solar (≈800 W m-2 ), ultraviolet (≈0.68 W m-2 ), and humidity (100%) aging for 100 days, liquid nitrogen (-196 °C) and high-temperature (300 °C) thermal shock 100 times. As proofs of performance, the resulting super-white silica aerogels have been used as the novel standard white plate  for better spectrum calibration, as the flexible projector curtains for optical display, as well as the transmitted light reflective layer in the photovoltaic cell for improving the relative power conversion efficiency of 5.6%.

Smart Energy-Absorbing Aerogel-Based Honeycombs with Selectively Nanoconfined Shear-Stiffening Gel

Aerogels, shaped as fibers, films, as well as monoliths, have demonstrated a plethora of applications in both academia and industry due to charming properties including ultralow density, large specific surface area, high porosity, etc., however studies on more complicated aerogel forms (e.g., honeycombs) with more powerful applications have not been fully explored. Herein, the Kevlar aerogel honeycomb is firstly constructed through a dry ice-assisted 3D printing method, where the Kevlar nanofiber ink is printed directly in dry ice freezing atmosphere, followed by supercritical fluid drying. The subsequent 3D Kevlar/shear-stiffening gel (SSG) honeycomb (3D-KSH) can be obtained by selective nanoconfining of SSG into nanopores of the aerogel skeleton wall (with the loading amount of 93 wt%) rather than into open honeycomb channels, solving the leakage, creep deformation, and shape design infeasibility of the SSG. Combining the advantages of Kevlar, honeycomb and SSG, the fabricated 3D-KSH shows obvious smart responsive behavior to external stimulus. Additionally, the 3D-KSH has high strain rate sensitivity (sensitivity factor of 4.16 × 10^-4 ) and excellent impact protection performance (energy absorption value up to 176 J g^-1 at the strain rate of 6300 s^-1 ), which will significantly broaden application prospect in some intelligent protection fields.

The Rising Aerogel Fibers: Status, Challenges, and Opportunities

Aerogel fibers garner tremendous scientific interest due to their unique properties such as ultrahigh porosity, large specific surface area, and ultralow thermal conductivity, enabling diverse potential applications in textile, environment, energy conversion and storage, and high-tech areas. Here, the fabrication methodologies to construct the aerogel fibers starting from nanoscale building blocks are overviewed, and the spinning thermodynamics and spinning kinetics associated with each technology are revealed. The huge pool of material choices that can be assembled into aerogel fibers is discussed. Furthermore, the fascinating properties of aerogel fibers, including mechanical, thermal, sorptive, optical, and fire-retardant properties are elaborated on. Next, the nano-confining functionalization strategy for aerogel fibers is particularly highlighted, touching upon the driving force for liquid encapsulation, solid-liquid interface adhesion, and interfacial stability. In addition, emerging applications in thermal management, smart wearable fabrics, water harvest, shielding, heat transfer devices, artificial muscles, and information storage, are discussed. Last, the existing challenges in the development of aerogel fibers are pointed out and light is shed on the opportunities in this burgeoning field.

3D Printed Hybrid Aerogel Gauzes Enable Highly Efficient Hemostasis

Hemostatic materials have played a significant role in mitigating traumatic injury by controlling bleeding, however, the fabrication of the desirable material's structure to enhance the accumulation of blood cells and platelets for highly efficient hemostasis is still a great challenge. In this work, directed assembly of poly(vinyl alcohol) (PVA) macromolecules covering the rigid Kevlar nanofiber (KNF) network during 3D printing process is utilized to fabricate hydrophilic, biocompatible, and mechanically stable KNF-PVA aerogel filaments for effective enriching blood components by fast water absorption. As such, KNF-PVA aerogel gauzes demonstrate remarkable water permeability (338 mL cm-2 s-1 bar-1 ), water absorption speed (as high as 9.64 g g-1 min-1 ) and capacity (more than ten times of self-weight), and ability to enrich micron-sized particles when contacting aqueous solution. All these properties favor efficient hemostasis and the resulting KNF-PVA aerogel gauzes significantly outperform the commercial product Quikclot Gauze (Z-Medica) during in vivo experiments with the rat liver laceration model, reducing the hemostasis time by half (60 ± 4 s) and the blood loss by two thirds (0.07 ± 0.01 g). These results demonstrate a robust strategy to design various aerogel gauzes for hemostasis applications.

Digital Light Processing 3D-Printed Silica Aerogel and as a Versatile Host Framework for High-Performance Functional Nanocomposites

Vat-photopolymerization-based 3D printing enables on-demand construction of customized objects with scalable production capacity and high precision. Herein, the sol-gel process for aerogels with digital light processing 3D printing to produce advanced functional materials possessing hierarchical pore structures and complex shapes is combined. It has revealed the temporal evolution of the photorheological behavior of acrylate-modified silica sols in an acid-base catalytic procedure, and confirmed that silica aerogels can be fabricated with very low acrylate content. The resulting aerogels are thermostable with intrinsic silica contents, skeletal densities, and physical characteristics similar to those of commercial silica aerogels yet distinct mechanical behaviors. More importantly, the printed silica aerogels can be used as a versatile nanoengineering platform to produce high-performance and multifunctional interpenetrating phase nanocomposites with complex shapes through programmable post-printing processes. Epoxy-based nanocomposites possessing excellent mechanical performance, ionogel-based conductive nanocomposites with decoupled electrical and mechanical properties, and anti-swelling hydrogel-based nanocomposites are demonstrated. The results of this study offer new guidelines for the design and fabrication of novel materials by additive manufacturing.

Preparation and Properties of Highly Transparent SiO2 Aerogels for Thermal Insulation

SiO2 aerogels have attracted extensive attention due to their unique structural characteristics, which exhibit many special properties, especially good optical transparency. As far as we know, the sol-gel stage during the synthesis of aerogel plays an important role in the construction of the gel skeleton. In this study, we adjusted the amount of silicon source and catalyst to explore the best scheme for preparing highly transparent SiO2 aerogels, and further clarify the effects of both on the properties of SiO2 aerogels. Results indicated that the pore size distribution was between 10 and 20 nm, the thermal conductivity was between 0.0135 and 0.021 W/(m·K), and the transmittance reached 97.78% at 800 nm of the aerogels, better than most studies. Therefore, it has the potential to be used in aerogel glass for thermal insulation.

Bioinspired Multilayer Structures for Energy-Free Passive Heating and Thermal Regulation in Cold Environments

Passive thermal regulation has attracted increasing interest owing to its zero-energy consumption capacity, which is expected to alleviate current crises in fossil energy and global warming. In this study, a biomimetic multilayer structure (BMS) comprising a silica aerogel, a photothermal conversion material (PTCM), and a phase change material (PCM) layer is designed inspired by the physiological skin structure of polar bears for passive heating with desirable temperature and endurance. The transparent silica aerogel functions as transparent hairs and allows solar entry and prevents heat dissipation; the PTCM, a glass plate coated with black paint, acts as the black skin to convert the incident sunlight into heat; and the PCM composed of n-octadecane microcapsules stores the heat, regulating temperature and increasing endurance. Impressively, outdoor and simulated experiments indicate efficient passive heating (increment of 60 °C) of the BMS in cold environments, and endurance of 157 and 92 min is achieved compared to a single aerogel and PTCM layer, respectively. The uses of the BMS for passive heating of model houses in winter show an increase of 12.1 °C. COMSOL simulation of the BMSs in high latitudes indicates robust heating and endurance performance in a -20 °C weather. The BMS developed in this study exhibits a smart thermal regulation behavior and paves the way for passive heating in remote areas where electricity and fossil energy are unavailable in cold seasons.

Transparent thermal insulation ceramic aerogel materials for solar thermal conversion

Thermal management in energy-efficient solar thermal energy conversion and transparent windows requires advanced materials with low thermal conductivity and high transparency, such as transparent silica aerogel materials. However, the large scatter domains in porous silica materials would deteriorate their optical transparency. Herein, we report transparent silica aerogels by controlling hydrolyzation and meanwhile silylation modification to enhance the integrity of the microstructure under ambient pressure drying. The transparent silica aerogel materials show a broad-spectrum transparency of 70% from 400 nm and 800 nm, showing promising applications in transparent windows and solar thermal energy conversion systems. The scalability for transparent windows could be achieved with a composite material by incorporating transparent polymeric materials. The solar receiver coupled with a transparent silica aerogel could reach 122 °C within 12 min at a solar irradiance of 1 Sun, ∼200% higher than that in the ambient atmosphere. The engineered structure of the transparent porous silica backbone provides a pathway for solar thermal systems and transparent window applications.