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Ceramic aerogels are promising candidates for high-temperature thermal resistance and thermal insulation materials. However, their application can be restricted by their brittleness when exposed to harsh conditions. Herein, this study has developed a novel synthesis process based on a sol-gel method and carbothermal reduction-nitridation to produce a porous TiN composite aerogel from an organic/inorganic hybrid. The resulting TiN displays a polyhedral particle size of around 30–60 nm with nanoscale pores. In addition, the TiN aerogel demonstrates a high BET-specific surface area of 653.42 m2/g and a very low thermal conductivity of 0.046 W/(m·K). Furthermore, the TiN aerogel composite displays a good thermal difference with no shrinkage and cracks of the morphology during the butane blazing torch at 1300 °C for 10 min under air environment, demonstrating its excellent thermal insulation property and anti-oxidation characteristics. The gas-solid reaction mechanism of TiN aerogel based on the Gibbs free energy calculations is also proposed. [ABSTRACT FROM AUTHOR]

Flexible SiC nanowire aerogel felt has excellent foldability, high-temperature resistance, and thermal insulation properties. However, the large-scale and high-efficiency production of SiC nanofiber aerogel felt remains a challenge. In this work, a three-dimensional network-structured short-chopped carbon fiber felt was used as a template for preparing SiC nanowire aerogel felt. The short-chopped carbon fibers were combed and dispersed using an airflow in a carding machine to obtain the desired felt. This method enables rapid and uniform preparation of carbon fiber felt, enhancing the efficiency of SiC nanowire aerogel felt production. The experimental results showed that the aerogel felt was mainly composed of β-SiC nanowires with a density of 5.29 g/cm3. Additionally, the SiC nanofiber aerogel felt exhibited remarkable superelasticity and low thermal conductivity (0.013 W/m·K). This research contributes to the practical application of flexible SiC nanowire aerogel felt in the field of thermal insulation. • Carding machine dispersed carbon fiber. • Fabrication of large size SiC nanofiber aerogel felt. • SiC nanofiber aerogel felt has flexibility and low thermal conductivity. [ABSTRACT FROM AUTHOR]

Lightweight ablative materials owing to their low density, high efficiency heat insulation and excellent designability, which are considered as the most promising candidate materials in the field of aerospace ablative thermal protection. In this work, a polycarbosilane (PCS) in-situ reinforced carbon felt/phenolic (PR/C f) aerogel composite is reported. The surface of aerogel and carbon fiber was coated with PCS protective layer by sol gel method and secondary vacuum impregnation process. The aerogel composite material possesses low density (0.264–0.307 g/cm3), hydrophobic, low thermal conductivity (0.121–0.178 W/(m·K)) and significant high compressive strength (12.89 MPa). When the oxygen acetylene flame with high heat flow density ablates for 20 s, it shows high efficiency in heat protection and insulation, and the linear ablation rate is as low as 0.089 mm/s. The PL/PR/C f aerogel composite shows competitive and broad application prospects in the field of ablation heat protection in the future. [ABSTRACT FROM AUTHOR]

In this study, we investigated the tunable porous structure of SiCN(O)-derived ceramic aerogels by changing the molecular structure of the polysilazanes from linear to branched. We also studied the effect of molecular structure on the thermal insulation properties of ceramic aerogels. As the percentage of branched molecular structure in the polysilazane precursor increased, the internal microscopic pore structure of the aerogels changed from macroporous to mesoporous. The specific surface areas and pore volumes of the ceramic aerogels prepared with different precursors increased after pyrolysis at 1000 °C, ranging from 3.7 to 255.9 m2/g and 0.01–0.36 cm3/g, respectively. The corresponding thermal conductivities increased slightly as the aerogels contracted after pyrolysis. The low thermal conductivity (0.046 W/(m·K) at minimum) can be attributed to the decrease in pore size caused by adjusting the precursor structure, which limits the thermal conduction of gas in the porous aerogel materials. [ABSTRACT FROM AUTHOR]

High-performance thermally insulating aerogel with low density, high porosity, and low thermal conductivity characteristics was widely used in heat insulation. However, the large-scale application of aerogel was still limited by its brittleness and infrared radiation transparency at high-temperature. Fiber composite aerogel had achieved significant progress, but its anti-oxidation ability was poor, and its thermal insulation required further improvement at ultra-high temperatures. Herein, inspired by the structure of elytra, nanoparticle fiber (NF) was prepared by electrospinning of coaxial fiber loaded with opacifier and antioxidant nanoparticles. The NF was incorporated into the SiBCN aerogel to prepare NF/SiBCN ceramic fiber aerogel. The mechanical properties were improved by fiber networks. The shell structure increased the antioxidant properties. Heat conduction and heat convection were suppressed by the aerogel, while heat radiation was reduced by the coaxial fiber. The results showed that the ceramic fiber aerogel exhibited superior mechanical, antioxidant, and ultra-low thermal conductivity properties. • A novel 'particle/fiber/aerogel' composites were constructed for thermal insulation. • The composites exhibited fire resistance and ultralow thermal conductivity at 1300 °C. • The composites had low density and excellent mechanical properties. • Coaxial fiber had self-healing and anti-oxidation properties at ultra-high temperatures. [ABSTRACT FROM AUTHOR]

Aerogels are excellent thermal insulation materials with extremely low thermal conductivity. However, the practical applications of traditional oxide aerogels, such as SiO 2 and Al 2 O 3 aerogels, are limited by the disadvantages of easy sintering and intrinsic brittleness. SiC is a good candidate for new aerogel due to its superior stability and good properties from room to high temperatures. However, it is generally supposed that SiC cannot be employed as thermal insulation material because of its high intrinsic thermal conductivity. We herein developed SiC nanowire aerogel with quite low thermal conductivity using freeze-drying method. The fabricated SiC nanowire aerogel exhibits ultra-high porosity (∼99.5%), excellent high-temperature stability (∼1700 °C), extremely low thermal conductivity (0.035 W/(m⋅K)) and good compressive flexibility. The low thermal conductivity is resulted from ultra-high porosity and multi-level nanostructures. Flexible compression behavior is attributed to the capability of moving and buckling of SiC nanowires, originating from both ultra-high porosity and strong connection between nanowires. This exploration highlights the flexible SiC nanowire aerogel as a promising high temperature thermal insulation material. [ABSTRACT FROM AUTHOR]

In the field of building energy conservation, the development of biodegradable biomass aerogels with excellent mechanical performance, flame retardancy and thermal insulation properties is of particular importance. Here, a directional freeze-drying method was used for fabricating composite sodium alginate (SA) aerogels containing functionalized ammonium polyphosphate (APP) flame retardant. In particular, APP was coated with melamine (MEL) and phytic acid (PA) by a supramolecular assembly process. Through optimizing the flame retardant addition, the SA-20 AMP sample exhibited excellent flame retardant and thermal insulation properties, with the limiting oxygen index of 38.2 % and the UL-94 rating of V-0. Such aerogels with anisotropic morphology demonstrated a low thermal conductivity of 0.0288 (W/m·K) in the radial direction (perpendicular to the lamellar structure). In addition, as-obtained aerogels displayed remarkable water stability and mechanical properties, indicating significant potential for practical applications. [Display omitted] • A high-efficiency flame retardant was synthesized. • AMP has a good interface compatibility with sodium alginate. • SA-20 AMP reached V-0 rating and the LOI value was 38.2 %. • Directional freezing endows SA-20 AMP with outstanding thermal insulation properties. [ABSTRACT FROM AUTHOR]

[Display omitted] • A method is developed for the regulation of secondary unstable dendrites during ice crystal growth. • The flexible polyimide (PI) aerogels possesses a micro/nano multi-scale porous skeleton. • The PI aerogels are characterized by ultra-low density and exceptional thermal insulation. The freeze-casting is widely utilized as a tool for regulating micro-scale pore structures in the fabrication of flexible polyimide (PI) aerogels for applications in adsorption, filtration, and fire-retardant. Herein, a confinement-controlled freeze-casting method is developed to fabricate PI aerogels with excellent thermal insulation properties by designing multi-scale structural adaptability. In a confinement-controlled system, the enhanced solid/solid and solid/liquid interface interactions in the solution dispersion system induce dendritic evolution of ice crystals. Under the directional freezing, grown dendritic ice crystals squeeze the loaded particles to form polyamic acid salt (PAAs) cryogels. Followed by template removal and thermal imidization, PI aerogels with a micro/nano-scale slit-like porous structure were obtained. The PI aerogels are characterized by ultra-low density (as low as 9.82 mg/cm3) and exceptional thermal insulation performance (25.0 mW/(m·K) at 25 °C). [ABSTRACT FROM AUTHOR]

Gas absorption capability of material proposes a dominant role in the energy storage, selective gas absorption/capture. Aerogels with ultra-high porosity, ultra-low thermal conductivity have been widely used in gas absorption. However, a persistent limitation of these materials is their static and unalterable gas absorption capacity. In this work, a wrinkled graphene structure is proposed to adjust the gas capability. A series of molecular dynamics (MD) simulations are carried out to propose an atomic-level understanding of the effects of wrinkled structure, including effects of pressure, surface wettability, number of gas molecules, thermal insulation, and mechanical properties. It is found that the gas capability can be reduced by 20% with wrinkle structure. Notably, the linear response between pressure and gas absorption capability is observed. Moreover, wrinkle structure functions as a thermal resistance to restrict solid heat transfer. The mechanical property of the aerogel composite is improved by a doped graphene sheet with a high wrinkle degree. This work will pave the way for the development of aerogel with high and selective gas absorption capability. • MD model of wrinkled graphene doped silica aerogel is built. • A linear response between pressure and gas absorption capability is found. • Wrinkle graphene damages the gas absorption. • High degree of wrinkle graphene improves the mechanical properties. [ABSTRACT FROM AUTHOR]

Laser technology has made great progress over the past decades. However, corresponding defense systems have lagged slightly behind. Therefore, there is significant room for research with respect to the principles and bases of existing laser protective materials. In this paper, a kind of silica aerogel material is prepared with the acid-base sol-gel process. Considering the influence of thermal conductivity and the porosity of the aerogel materials on the laser irradiation results, the best laser resistance times of the materials were obtained. The concept of the maximum energy transfer radius is proposed to build a preliminary laser irradiation model for porous materials. It indicates that under the irradiation of a continuous wave laser with a power of 7 kW·cm−2, the silica aerogel materials with the thickness of 3, 6, and 9 mm can withstand the laser for maximum times of 26, 61, and 100 s, respectively. After 7 s of irradiation, the surface temperature of 3 mm from the laser incident point is 226.9 °C. Aerogels, especially silica aerogels, should be considered as future laser protection materials. The extremely low thermal conductivity and the strong scattered ability for laser of these materials will provide a good defensive effect against large-energy laser weapons. [ABSTRACT FROM AUTHOR]

Aluminosilicate aerogels are promising thermal insulation materials for high temperature applications. In this study, aluminosilicate aerogels were effectively synthesised using affordable precursors: dispersible boehmite powder and waterglass solutions, with subsequent drying achieved by supercritical CO 2 processing. According to the evaluation of their properties, the Al–Si aerogels exhibited low densities (0.11 g/cm3), low thermal conductivities (<0.021 W/m·K) and high specific surface areas (SSA) (516 m2/g for an Al:Si molar ratio of 3:1). When waterglass-based aerogel and aluminosilicate samples were treated at high temperature (1200 °C), the formation of mullite was demonstrated for the aluminosilicate aerogel with a molar ratio of Al:Si 3:1, which showed a higher value for SSA compared to the waterglass-based aerogel. [Display omitted] • Low-cost waterglass and boehmite precursors were employed in aerogel's preparation. • Aluminosilicate aerogels present low densities and low thermal conductivity values. • Aluminosilicate aerogel materials exhibit uniform structure and high specific surface area. • Porosity is retained after heat treatment at 1200 °C for the aerogel with an aluminium: SiO2 molar ratio equal to 3. [ABSTRACT FROM AUTHOR]

In this study, a multi-functional guar gum aerogel with the oriented lamellar structure, which introduced sodium silicate (Na 2 O·nSiO 2) and phytic acid (PA) as thermal insulation additives and flame-retardant agents, respectively, was fabricated via freeze drying. Our aerogel's chemical structure, morphology, and thermal and mechanical properties were analyzed. The oriented lamellar structure was attributed to the orientated growth of ice crystals, which was induced by the "silicate-guar, guar-phytate, and phytate-silicate" multiple hydrogen bonds formed between Na 2 O·nSiO 2 , PA, and guar gum. The density of the sample with 2 wt% PA could reach 0.0335 g·cm−3, and the porosity was 5 %, along with a specific pore volume of 0.8144 cm3·g−1. The mechanical properties and thermal insulation performed significant differences in the radial and axial direction of the oriented lamella (nearly 100 % resilience while 50 % deformation quantity and 0.0235 W/(m*K) of thermal conductivity in the radial direction, up to 0.079 MPa of compressive strength in the axial direction). The presence of PA attached a good flame-retardant ability to our aerogel (The Limiting Oxygen Index (LOI) was 30.77 %). This work provides a novel and promising method for developing anisotropic aerogel with excellent potential in building energy efficiency and flame-retardant. [ABSTRACT FROM AUTHOR]

The anisotropic cellulose nanofiber (CNF)/carbon nanotube (CNT) aerogels hold a great promise in directional applications due to their distinct xylem-like aligned penetrating pore structures. The aspect ratio of CNF plays a crucial role in the pore structures of aerogels, directly dominating the final macroscopic properties of materials. Herein, three types of CNF with different aspect ratios were extracted through the 2,2,6,6-tetrmethylpiperidine-1-oxyl radical (TEMPO) oxidation process by changing the doses of oxidant. The corresponding anisotropic CNF/CNT aerogels were prepared by the unidirectional freeze-drying method and then their pore morphologies and properties were investigated in detail. The resulting aerogel with the shortest aspect ratio of CNF exhibited the densest porous structure, thereby obtaining the highest compressive strength of 110 kPa and elastic modulus of 383 kPa, while that containing the longest CNF possessed the highest thermal conductivity coefficient of 0.17 W m−1 K−1 and the worst thermal insulation. This research explored the relationship between the properties of the CNF/CNT aerogels and devisable pore structures caused by various aspect ratios of CNF, thus providing a new insight into the development of CNF/CNT aerogels with tunable performances. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel type of PI/SIO 2 composite aerogel was prepared by a confined filling strategy. • The PI/SiO 2 composite aerogels exhibit ultra-low density and super thermal insulation. • The sound insulation and mechanical properties are significantly enhanced owing to the "honeycomb-like" structure. Aerogels have enormous potential in integrated thermal insulation and noise reduction applications. According to the knowledge of the favorable mechanical and acoustic properties of honeycomb structures. We employed anisotropic "honeycomb-like" polyimide (PI) aerogels as the matrix and utilized a confined filling strategy to directionally filled continuous SiO 2 aerogels into the PI aerogel matrix. This resulted in the creation of an exceptional organic–inorganic composite aerogel (PISis) with outstanding thermal and acoustic insulation performance. The radial thermal conductivity of PISis ranged from 22.37 to 27.65 mW·m−1·K−1, and the initial decomposition temperature was approximately 524–557 °C. In the frequency range of 500–6300 Hz, the average sound transmission loss (STL) of the 18 mm thick composite material reached 30.67 ∼ 31.89 dB, and the compressive strength at 10 % strain is 0.416 MPa. The PISis exhibits a density ranging from 0.1012 to 0.1302 g/cm3. These make it an ideal lightweight, high-strength, and efficient material for thermal and acoustic insulation. [ABSTRACT FROM AUTHOR]

• Silica aerogel granules were replaced by quartz sand of 0 to 4.5 wt% to produce lightweight cement-based composite mortars. • SAG has increased the water absorption and porosity of the mortar, while reduced compressive strength and hardened density. • Pores increased with increase in aerogel, SAG tend to agglomerate, and pores accumulate around the aerogel particles. • SAG significantly decreased thermal conductivity from 0.772 to 0.155 W/mK. • SAG also reduced thermal diffusivity and heat storage, and increased specific heat. It has been discovered in recent years that the use of silica aerogel in cement-based materials contributes to the thermal insulation properties of the materials, and studies on this subject have gained importance. However, it was determined that the studies mostly remained at the level of the thermal conductivity coefficient and the main physical and mechanical properties of the materials. In this study, a comprehensive research was carried out with the replacement of micro-sized aerogel particles with quartz sand in different ratios (0, 1, 1.6, 2.4, 3.2, 4 and 4.5 wt%) in lightweight cement-based composite mortar (LCCM) combinations. Specifically, the effect of silica aerogel granules (SAG) on flowability, fresh and hardened densities, porosity, water absorption, compressive strength, macro/microstructural properties (by optic microscope, SEM, XRD analysis), thermal conductivity, specific heat, thermal diffusivity and heat storage capability of LCCMs were investigated. While SAG could increase the flowability (up to 8.4 %) at low using rates, it decreased it at higher rates. It is very effective in reducing fresh and hardened densities. However, it highly increased porosity and water absorption, and greatly decreased compressive strength. On the other hand, generally the poor interfacial properties of the cement matrix and the aerogel were moderately improved in this study. The lowest thermal conductivity was determined as 0.155 W/mK in the highest SAG usage. SAG increased specific heat, decreased thermal diffusivity and decreased heat storage capability of LCCMs up to 45.23 %, 63.49 % and 46.32 %, respectively. As a result of the study, aerogel has been identified as a very suitable material that can be used to highly improve thermal properties in LCCMs. [ABSTRACT FROM AUTHOR]

• Properties of alkali-excited fly ash aerogel foam concrete were studied. • The microstructure of alkali-excited fly ash aerogel foamed concrete was analyzed. • Test variable included aerogel particles, fly ash, foam and polypropylene fiber. • Developed concrete exhibited excellent thermal performance and mechanical strength. • Aerogel used in concrete had excellent thermal insulation performance. With increasing focus on low-carbon energy conservation and environmental protection, there is a need to consider all aspects of life in future developments. To meet the concept and requirements of energy conservation and environmental protection in the building industry, we have developed alkali-excited aerogel-foamed concrete that exhibits excellent thermal insulation performance and mechanical strength. The variables involved in its production include fly ash, foam, aerogels, and polypropylene fibres. In this study, the microstructure and mechanical and thermal properties of alkali-excited fly ash aerogel foam concrete was analysed. The results indicated that fly ash can replace a portion of the cement, resulting in a reduction in the thermal conductivity of the concrete. The addition of aerogels and foams to the concrete can significantly decrease its thermal conductivity. Specifically, when the aerogel content reaches 12 % and the foam content reaches 75 %, the thermal conductivity of the concrete decreases from 0.498 W/m·K to 0.183 W/m·K and 0.19 W/m·K, respectively. However, this decrease in thermal conductivity comes at a cost of approximately 50 % reduction in the flexural and compressive strengths of the concrete. The findings reveal that the aerogel has excellent thermal insulation performance similar to that of the foam, but it has little influence on the mechanical properties of the concrete. Additionally, an increase in the polypropylene fibre content significantly improves the mechanical properties of alkali-excited fly ash aerogel concrete, but the thermal properties of the concrete remain mostly unaffected. [ABSTRACT FROM AUTHOR]

[Display omitted] • Porous coral-like Co/CoO/RGO hybrid aerogels are successfully developed. • The special structure brings multiple loss modes and good impedance matching. • Achieving high EM wave loss, hydrophobicity and thermal insulation properties. • DFT simulations are adopted to verify the interface polarization of interfaces. Addressing the growing issue of electromagnetic radiation calls for urgent exploration of effective electromagnetic wave absorbers. Herein, a series of coral-like Co/CoO nanoparticles/reduced graphene oxide (Co/CoO/RGO) hybrid aerogels were synthesized via a freeze drying-thermal reduction strategy. Their advantages involving porous interconnected framework, abundant heterogeneous interfaces, and integration of dielectric/magnetic units could bring multiple loss modes. By manipulating the Co(CO 3) 0.5 (OH)·0.11H 2 O (CCOH) addition and reduction temperature, the electromagnetic parameters could be tuned, ultimately resulting in optimal absorption capability with maximal reflection loss of –32.4 dB and effective bandwidth of 4.2 GHz. More importantly, excepting the high electromagnetic wave loss, Co/CoO/RGO hybrid aerogels also exhibit good hydrophobicity and thermal insulation properties, making them suitable for application in harsh environment. Besides, the special structure and multiple components lead to multiple heterointerfaces in aerogels, the resultant interface polarization is deemed to contribute to electromagnetic wave loss. Here, Density Functional Theory (DFT) simulation based on first-principles is adopted to verify the contribution of interface polarization from the theoretical level. According to the simulation results, the electron-occupied states near the Fermi level are significantly different among the different phases comprising the heterointerfaces, and there is a noticeable accumulation of charge at these interfaces, successfully affirming the interface polarization behavior at different interfaces. [ABSTRACT FROM AUTHOR]

Naturally-derived aerogels have attracted considerable attention owing to their good biocompatibility, biodegradability and sustainability, but their weak mechanical properties largely limit their applications in various fields. Herein, we proposed the use of a directional freeze-drying method to prepare an anisotropic honeycomb three-dimensional porous aerogel with water-soluble chitosan (CS) as a rigid skeleton and water-soluble biobased epoxy resin as cross-linked hard segments, which had low volume shrinkage and density of 13.9 % and 34.3 mg/cm3, respectively. The resultant aerogel had anisotropic mechanical properties, such as rigidity in the axial direction with a maximum axial modulus of 6.71 MPa, which was 51.6 times larger than that of the pure chitosan aerogel, demonstrating a good compressive elasticity in the radial direction. It also had anisotropic thermal management properties, with a lower thermal conductivity in the radial direction than in the axial direction, down to 0.029 W/mK. The introduction of biobased epoxy resin improved the overall thermal stability, flame retardancy, and increased the biomass content in the aerogel, reducing the carbon footprint of the material. This study paves the way for the construction of a special graded porous, structurally and functionally integrated thermal insulation aerogel, which is of great significance for the development of new thermal insulation materials. [Display omitted] • Anisotropic honeycomb bio-based aerogels have been prepared by directional freeze-drying. • The introducing of bio-based epoxy resin improves the comprehensive properties of aerogel. • The aerogel exhibits low thermal conductivity of 0.029 W/mK in the radical direction. • The aerogel shows low volume shrinkage (13.9 %) and density (34.3 mg/cm3). [ABSTRACT FROM AUTHOR]

• Polyimide aerogels with nano-porous structure were prepared by ambient drying. • SiO 2 nanoparticles were used as skeleton enhancers and shrinkage inhibitors. • The aerogels exhibit robust mechanical properties and efficient thermal insulation. Ambient drying of polyimide (PI) aerogel is rarely reported due to the strong cross-linking network leading to large shrinkage. In this work, the hydrophobic SiO 2 nanoparticles are used as skeleton enhancers and shrinkage inhibitors to maintain the nano-porous network structure and achieve ambient drying of PI/SiO 2 aerogels. The results show that the PI/SiO 2 aerogel prepared by ambient drying has similar thermal insulation and mechanical properties as the PI aerogel prepared by supercritical drying. The PI/SiO 2 aerogel prepared by ambient drying is low-cost and simple to operate, which is expected to realize the industrialization and wide engineering application. [ABSTRACT FROM AUTHOR]

• Lightweight, flexible aerogels using renewable curdlan and wood waste chips as raw materials. • Low thermal conductivity (30.1 × 10−3 W/(m·K)) and high thermal stability up to 300 °C. • Hydrophobic and good compression-resilience behavior. Developing flexible and lightweight materials with elevated thermal insulation performance from renewable biomass is significant for green and sustainable energy. In this work, a multifunctional biomass aerogel using wood waste as a raw material is proposed. The composite aerogel consists of curdlan as a structural framework, wood fibers and nano-hydroxyapatite as reinforcing phases. By employing thermogravimetry (TG), infrared thermal imager and thermal conductivity tester, the thermal stability and thermal insulation performance of the aerogel were evaluated. The consequent thermal conductivity reaches 30.1 × 10−3 W/(m·K). As the strength and toughness of the aerogel is improved by introducing cellulose into the matrix, it shows a maximum compressive modulus of 1.65 MPa. The hydrophobicity of the aerogel is achieved by an alkylation method and the contact angle is over 120°. This nontoxic aerogel composite with high thermal and mechanical performance indicates a potential utilization in the thermal management of food and electronic industry. [Display omitted] [ABSTRACT FROM AUTHOR]

• Phenolic resin aerogel is synthesized from linear and boron phenolic resin. • The ultralow shrinkage attributes to the strong B-O bond in boron phenolic resin. • Thermal conductivity of aerogel with a density of 0.105 g/cm3 is 0.0352 W/(m·K). • Aerogel shows excellent mechanical stability and resilience capacity. Lightweight phenolic resin aerogel with excellent thermal insulation and mechanical properties is an attractive material for thermal engineering applications. However, to synthesize such material, generally, a significant volume shrinkage would occur during the aerogel formation and the processing time is long. To conquer such issues, the lightweight phenolic resin aerogel with excellent thermal insulation and mechanical properties was synthesized by an elaborately designed process, where linear thermoplastic phenolic resin (LPR) and boron-modified thermosetting phenolic resin (BPR) were used as the raw materials. It was found that the shrinkage rate and the processing time of the phenolic resin aerogel were significantly reduced. By optimizing the synthesis process, the aerogel with a density of 0.105 g/cm3 and a thermal conductivity of 0.0352 W/(m·K) was achieved. [ABSTRACT FROM AUTHOR]

• The chemical grafting of ammonium polyphosphate onto the surface of HGM was reported for the first time. • Composites aerogel have excellent thermal insulation properties. • Composites aerogel have excellent flame retardancy, and the LOI value is as high as 100. • Composites aerogel have good mechanical properties. The development of high-performance flame-retardant materials with better thermal insulation properties is of great significance to modern building energy conservation. In this study, we report the facile and scalable fabrication of novel aerogels (SA-HGM-APP) based on the building blocks of ammonium polyphosphate (APP) coated hollow glass microspheres (HGM) and matrix material sodium alginate (SA) by freeze drying method. The as-prepared composite aerogel exhibits better thermal stability (decomposition temperature over 210°C). In addition, SA-HGM-APP composite aerogels have good thermal insulation properties and low thermal conductivity, which is 0.035 W m−1 K−1. More importantly, the peak heat release rate (pHRR) is 14.14 kW m−2, the limiting oxygen index (LOI) value is as high as 100%, and the aerogel composite material can reach UL-94 V-0 grade, so it shows excellent flame retardancy. These results attributed to a synergistic effect, which combines the rich porous structure derived from HGM, so that SA and APP have better thermal insulation and excellent non flammability, thus providing excellent flame retardancy. SA-HGM-APP aerogel composite has many advantages, such as simple preparation method and high mechanical strength. It has great potential in future thermal insulation and flame retardancy applications. [ABSTRACT FROM AUTHOR]

• Graphene/Fe3O4 hybrid aerogel were prepared by self-assembly synthesis. • The effects of concentration of nano-Fe3O4 in the aerogel were studied. • Mechanical and thermal properties of the hybrid aerogels were investigated. [Display omitted] Graphene aerogel and graphene-magnetite hybrid aerogel were prepared using hydrothermal method followed by a freeze-drying process. Magnetite nanoparticles were encapsulated by graphene sheets in an electrostatic self-assembly process. The morphology and properties of aerogels were characterized by several techniques including UV-Vis spectroscopy, Raman spectroscopy, FESEM, TEM and XRD. The mechanical and thermal behavior of the aerogels were evaluated at different concentration levels of Fe 3 O 4 nanoparticles (0, 10, 20 and 30 wt%). The compressive strengths of the hybrid aerogels at the strain of 80% for graphene aerogels containing 0, 10, 20 and 30 wt% Fe 3 O 4 nanoparticles were determined to be 5.93, 12.96, 6.85, and 6.15 kPa, respectively. The highest mechanical properties were achieved for the sample containing 10 wt% Fe 3 O 4 nanoparticles (FGA10). Thermal analysis revealed that FGA10 sample had a 15% less weight loss when heated up to 600 °C compared with that for the bare graphene aerogel, indicating the formation of a more thermally stable structure. Furthermore, adding Fe 3 O 4 nanoparticles improved the thermal insulation performance of the graphene aerogel, since the dynamic heat transfer rate and heat dissipation rate of FGA10 were lower than those for the bare graphene aerogel. [ABSTRACT FROM AUTHOR]

[Display omitted] • Low cost and high performance. • Silica aerogel fabricated from incineration bottom ash. • Thermal insulation paint with silica aerogel additive. • 12 °C surface temperature reduction improvement on the heated plate at 60 °C. • 1 °C in-chamber temperature reduction with external temperature of 44.6 °C. In this work, silica aerogel (SA) powder is fabricated through a sustainable method from incineration bottom ash. The fabricated SA has a low thermal conductivity of 0.025 W/mK, high surface area of 786 m2/g, and high porosity of 96.36%. The thermal analysis of SA powder shows it has a stable hydrophobicity up to 418 °C. The SA powder has been mixed into water-based paints to develop a high efficient thermal insulation paint with inproved thermal insulaton properties. Different percentage of SA were mixed into the paint samples to prepare the silica aerogel thermal insulation paint (SATIP) and their physical properties were studied. The SATIP samples were then coated on cement board substrates. The thermal conductivity, water resistance, durability and insulation performance of the samples were investigated and evaluated systematically. The scrub resistance of SATIP can be improved significantly by 219% after adding 5 vol% SA powder. And by adding 20 vol% of SA into the SATIP, it achieves up to 12 °C surface temperature reduction improvement compare to the one without SA on the heated plate at 60 °C, and 1 °C in-chamber temperature reduction with external temperature of 44.6 °C. The spectral reflectance analysis demonstrates that the improvement of the thermal insulation performance of the SATIP is attributed to the thermal insulation property of SA and proves that the developed SATIP is color insensitive and has wider applications. The results demonstrated a low-cost and scalable process fabricates SA with superor properties. The developed SATIP has a great potential for application in sustainable and green buildings for energy saving. [ABSTRACT FROM AUTHOR]

• Unidirectional anisotropic PIAs have been prepared by freeze-drying. • The PIA shows excellent thermal stability and stable mechanical behavior. • The u -PIA-6 exhibits great strength, which is over twice as high as that of the TEEK-HH at 177 °C with a similar density. • The u -PIA shows great thermal insulation properties in anisotropic. Polyimide aerogels (PIAs) are the new type of high-performance lightweight material that combines the excellent thermal and mechanical behavior of polyimides with the highly porous and lightweight properties of aerogels. PIAs can meet the increasing material requirements of high-tech fields such as the aerospace and transportation industries. This study reports a series of all-aromatic anisotropic polyimide aerogels (p-phenylenediamine (PDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA)) with unprecedented behavior. These PIAs were synthesized using a random or unidirectional freeze-drying method followed by thermal imidization of the precursor solution. Their mechanical and thermal conductivity performance were anisotropic (u -PIA-8 strength: 1.86 and 0.14 MPa, thermal conductivity: 62.9 and 50.1 mW·m−1·K−1 along the axial and radial direction, respectively). Furthermore, the PIAs possessed excellent compression behavior in a wide temperature range from 25 °C to 316 °C, with a compressive strength over twice that of the commercial polyimide foam TEEK-HH (u -PIA-6 strength: 0.83 and 0.32 MPa at 177 and 316 °C, TEEK-HH strength: 0.31 MPa at 177 °C). These aerogels also demonstrated good thermal insulation performance after being held at 300 °C for 20 min (u -PIA-8: 76.3 °C at 25 mm). These highly anisotropic aerogels exhibit flexible thermal protection properties that extend the potential use of PIAs to applications such as high-temperature-resistant rocket fairings, sandwich structures, and arc-shaped structures for aerobats or the automobile industry. [ABSTRACT FROM AUTHOR]

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. • Fully bio-based aerogel was prepared with recyclable cellulose fiber addition. • WPCF addition changed the aerogel structure including pore shape and pore size. • The optimum thermal insulation property was achieved at WPCF addition of 0.5 wt%. • The thermal insulation mechanism was interpreted through the pore structure changes. [ABSTRACT FROM AUTHOR]

In this paper, large-sized covalent organic framework (COF) aerogels were prepared using high-efficiency catalysis under room temperature combined with rapid gel and freeze-drying methods for the first time. Two kinds of COF aerogels with rigid and semi-rigid building blocks were prepared using this method, having good mechanical strength (126 and 106 kPa), elasticity (aerogels structure remains intact after 10 cycles), thermal insulation properties (low thermal conductivity of 0.03 wm−1K−1), low dielectric constant (1 kHz of 1.21 and 1.51), thermal stability and solvent stability, and ultra-light characteristics (densities of 0.029 and 0.040 g cm−3). Compared to the traditional COF methods that generally result in small particles, the high-efficiency catalysis method developed in this paper can be used to prepare large-sized COF aerogels under mild reaction temperatures and within less time conducive to industrial production. This method is expected to greatly broaden the application field and application scope of COF materials. [Display omitted] • Two COF aerogels were prepared combined with rapid gel and freeze-drying methods for the first time. • The COF aerogels behave good mechanical properties, thermal insulation properties, and ultra-light characteristics. • The method in this paper can be used to prepare large-sized COF aerogels under mild reaction temperatures and less time. [ABSTRACT FROM AUTHOR]

Lightweight and multiscale nanocomposites are considered to be the most promising materials applied in the field of advanced aerospace and modern building industry, owing to their outstanding mechanical, thermal and flame-resistant properties served in high-temperature oxidizing environments. Herein, a novel siliconoxycarbide (SiOC) modified needle quartz fiber felt reinforced phenolic resin nanocomposite (SiQF/PR) with a hierarchical macro-/micro-/nano-scaled structure is reported. The as-prepared highly porous SiQF/PR nanocomposite exhibits a multiscale three-dimensional network and the micro-sized SiOC particles, derived from the hydrolysis of methyltrimethoxysilane (MTMS) and dimethyldiethoxysilane (DDS), are uniformly incorporated into the phenolic aerogel. The resultant nanocomposite has a bulk density of 0.30–0.35 g/cm3, and the maximum compressive strength reaches 4.20 MPa and 3.34 MPa in xy and z directions, respectively. Thermal stability and oxidation resistance experiments show that the SiQF/PR multiscale composite has a good ablation and heat insulation performance under oxyacetylene test condition, and the backside temperature of SiQF/PR is only 55.5 °C under 1.8mw/m2 for 120 s, and line ablation rate of SiQF/PR multiscale nanocomposite decreases by 20.1% at 1.8 MW/m2 and 59.5% at 0.4 MW/m2 compared to non-SiOC modified composite. These results indicate that the lightweight SiQF/PR nanocomposite fabricated by this method is very suitable for architectural or aerospace applications as high-performance insulation material. [Display omitted] [ABSTRACT FROM AUTHOR]

Biomass-derived aerogels have received extensive attention as potential thermal management materials for energy-efficient buildings. However, it remains a huge challenge to fabricate a fully bio-based aerogel with excellent mechanical property, flame retardancy, and low thermal conductivity. Herein, we demonstrate a novel and facile strategy to manufacture a fully biomass-based aerogel from naturally abundant ammonium alginate (AL) and phytic acid (PA), in which PA acting as both flame retardant and cross-linking components constructs a strong network with AL matrix. Consequently, the resultant biomass aerogel with a low density of 0.052 g/cm3 exhibits ultrahigh mechanical modulus (25.1 ± 3.1 MPa) and specific modulus (440.4 ± 54.4 MPa cm3·g−1), much superior to those of biomass aerogels ever reported. Due to the existence of the uniform three-dimensional porous network, the biomass aerogels exhibit low thermal conductivity (34–38 mW/m·K) and excellent thermal insulation performances. Further, the introduction of PA endows the aerogel with high flame retardancy (limiting oxygen index value of 57%, UL-94 V-0 rating, and extremely low heat release), while the biodegradability of the materials keeps at a high level with a biodegradation rate of 91.43%. Combining with the advantages of mechanically strong property, high flame retardancy, excellent thermal insulation, and biodegradation, the aerogel of this work provides a new strategy to fabricate thermal insulation materials with high environmental safety. [Display omitted] [ABSTRACT FROM AUTHOR]

Advanced composite aerogel materials have attracted much attention due to the unique structure and properties. However, the efficient manufacturing process and structural stability were still a challenge. Herein, a novel hydroxyapatite nanowires/polyimide (HAnws/PI) composite aerogel was synthesized by the ultralong HAnws combined with polyamic acid via directional freezing and thermal imidization. Among them, the HAnws and PI intertwined to form a three-dimensional network structure, which decreased the volume shrinkage of aerogel in thermal imidization from 42.9% to 12.6%, resulting in high porosity (>96%) and low density (0.0329–0.0431 g cm−3). In addition, the HAnws/PI aerogel with layered porous structure exhibits obvious anisotropic mechanical properties. The axial specific modulus as high as 78.05 kN m kg−1 and radial high elasticity can withstand 100 cycles of fatigue resistance. Moreover, the lowest radial thermal conductivity at 32.21 mW m−1 K−1 and the maximum of the anisotropy factor was 2.2, which was very suitable for insulation pipes to prevent additional heat loss. Therefore, the novel nanofibers reinforced organic aerogel was expected to provide more abundant functional applications in energy efficiency. [Display omitted] • HAnws can effectively inhibit the volume shrinkage of HAnws/PI aerogel. • The composite aerogel with tubular pores was prepared by directional freezing. • HAnws/PI aerogel exhibited anisotropic mechanical properties and thermal insulation. • HAnws/PI aerogel showed a low radial thermal conductivity and the maximum of the anisotropy factor was 2.2. [ABSTRACT FROM AUTHOR]

• Poor solvents were introduced into the reagent during the synthesis of PI precursor. • Submicron-scale pores are produced in aerogels owing to the poor solvents. • Aerogels in this work have low density (~91 mg cm−3) and low thermal conductivity. The sol–gel process is essentially a process of phase separation between micelles and solvent, so the influence of solvent composition on the result of phase separation will further affect the wet gel and the structure of the aerogel made by it. In this study, during the preparation of polyimide (PI) aerogels, poor solvents were introduced into the reagent during the synthesis of PI aerogel precursor, polyamic acids. The pore structure and thermal insulation performance changes of the PI aerogels caused by this behavior were studied. With the addition of poor solvent, submicron-scale pores were found inside the aerogel. In addition, the introduction of a small amount of poor solvent improved the thermal insulation performance of the prepared PI aerogel. [ABSTRACT FROM AUTHOR]

Excellent thermal insulating aerogels have broad application prospects in pipeline transportation, aerospace, construction and other fields. However, defects in thermal stability and mechanical are the main factors limiting their use to special environments. In this work, a novel polyimide composite aerogel is successfully synthesized through 3-Aminopropyltriethoxysilane (APTES) functionalized clay halloysite nanotubes (FHal) as inorganic enhancement phase and uniformly embedded in polyimide (PI) aerogels. Functionalization can increase the interfacial compatibility between clay inorganic phase and polyimide acid. The results indicated that FHal with high aspect ratio interacts with PI to form a three-dimensional network structure. The FHal/PI composite aerogels had low shrinkage (21.9%–30.79%), low bulk densities (65.285–71.464 mg/cm3), low thermal conductivities (39.18–42.57 mW/(m·K)), high thermal stability (onset of decomposition above 520 °C) and excellent mechanical property (1.14 MPa, 2.62 times of pure PI aerogel). Therefore, FHal/PI composite aerogels can be used as a promising thermal management material for practical and complex insulation applications. [Display omitted] • Hal can effectively inhibit the shrinkage of PI aerogel during preparation. • The functional process enhances the interaction between Hal and PI. • FHal/PI aerogels have excellent thermal insulation and mechanical properties. [ABSTRACT FROM AUTHOR]

To reduce energy losses due to the insufficient thermal insulation is one of the current "hot" topics. Various commercial porous materials are used with the best conductivity around 0.03–0.04 W/(m·K). Aerogels are the only known materials with "intrinsic" thermal superinsulating properties, i.e. with thermal conductivity below that of air in ambient conditions (0.025 W/(m·K)). The classical thermal superinsulating aerogels are based on silica and some synthetic polymers, with conductivity 0.014–0.018 W/(m·K). Aerogels based on natural polymers are new materials created at the beginning of the 21st century. Can bio-aerogels possess thermal superinsulating properties? What are the bottlenecks in the development of bio-aerogels as new high-performance thermal insulationing materials? We try to answer these questions by analyzing thermal conductivity of bio-aerogels reported in literature. • Thermal conductivity of bio-aerogels is analysed. • Correlations between thermal conductivity and aerogel density and surface area are built. • A special focus is made on thermal conductivity below that of air. • Prospects of using bio-aerogels for thermal insulation are discussed. [ABSTRACT FROM AUTHOR]

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 (λ s). 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 (λ s) 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. [ABSTRACT FROM AUTHOR]