High-temperature Thermal Insulation Research Articles

Design of hollow glass fiber/silica aerogel composites for high-temperature thermal insulation applications

Pure silica aerogels are not suitable for high-temperature (500 ∼ 2000 K) thermal insulation applications due to their transparency to infrared thermal radiation. The addition of solid glass/ceramic fibers, such as aluminum oxide and silicon oxide, into the silica aerogel matrix can remarkably suppress the radiative heat transfer. However, the introduced solid fibers increase the structural density and enhance heat conduction in silica aerogel composites. Herein, we propose adopting hollow fibers (HFs) to simultaneously minimize thermal radiation and heat conduction through silica aerogels in a high-temperature environment. The results show that increasing fiber hollowness improves the short-wavelength (<5 µm) extinction efficiency, indicating that using HFs is an effective way to lower the thermal conductivity (TC) at high temperatures of silica aerogels. To achieve minimal radiative TC, the fiber hollowness needs to increase with the external diameter of the fiber at a specific temperature, for example, at 1500 K. This study contributes to the understanding of heat transfer mechanisms in porous materials, particularly those with hollow structures, and offers valuable insights for the strategic design of thermal insulation structures effective at high temperatures.

Porous MgO-based ceramics synthesized by the volume expansion effect: Designing and performance

High-quality porous ceramics are extremely attractive in terms of energy conservation and consumption reduction, and thus are one of the research hotspots pursued in recent years. For this purpose, porous MgO-based ceramics with outstanding thermal shock resistance and thermal insulation performance were designed and prepared by using the direct firing method and introducing alumina with different paticle sizes as additives. The results reveal that the in-situ formed MgAl2O4 phase by alumina additives simultaneously improves the apparent porosity, thermal insulation, and thermal shock resistance of the prepared MgO-based ceramics. Specifically, the increase in the apparent porosity can be attributed to the volume expansion effect of MgAl2O4 phase, so the industrial alumina with larger particle size is more effective (20 wt% addition: the apparent porosity from 43.5 % to 55.4 %, the thermal conductivity from 0.977 to 0.389 W m−1 K−1 at 800 °C); moreover, the enhancement in thermal shock resistance can be ascribed to the residual stress effect caused by the MgAl2O4 particles, so the micron alumina with smaller particle size is more efficient (10 wt% addition: the residual strength ratio from 93.48 % to 65.13 %–113.92 % and 94.47 % after one and five air-quenching). Therefore, this work demonstrates a novel synthetic strategy for porous ceramics with great potential in the field of high-temperature thermal insulation.

A robust silicone aerogel via copolymerization of a difunctional organoalkoxysilane and polymethylmethoxysiloxane for high-temperature thermal insulation

A robust silicone aerogel for high-temperature thermal insulation prepared by a sol–gel process using polymethylmethoxysiloxane as the framework and a difunctional organoalkoxysilane as the crosslinker.

Facile preparation of lightweight and robust carbon aerogel monoliths for high-temperature thermal insulation and oil-water separation

Lightweight carbon aerogels (CAs) are urgently required for thermal insulation and organics absorption. However, a facile preparation method of ambient pressure drying (APD) remains a challenge since the large surface tension causes severe shrinkage, collapse or even cracking of porous network. Herein, a series of lightweight, robust and large-sized CA monoliths (0.240–0.327 g/cm3) are prepared through sol-gel polymerization of resorcinol-formaldehyde (RF) followed by solvent exchange-free APD using cotton fibers as reinforcement precursor. The deliberate tailoring of synthesis parameters and the appropriate introduction of cotton fibers jointly bring the advantages of facilitating sol-gel of RF solution.reducing drying shrinkage of hydrogels, and improving compatibility of carbonization shrinkage between fibers and matrix. Consequently, for instance, the CA composite with a density of 0.283 g/cm3, have high compressive strength (3.66 MPa), low thermal conductivity (0.070 W/(m·K)), excellent hydrophobicity (water contact angle as high as 145°), and high-volume organic absorption capability (74.8–95.7%). The facile preparation of CA composites through solvent exchange-free APD with monomer polymerization should be a promising approach to their further application in thermal insulation and oil-water separation.

Heat transfer mechanism of TiO2-doped silica aerogel

The remarkable thermal insulation properties of silica aerogel stem from its distinctive three-dimensional nano-network structure, rendering it a promising candidate for diverse applications such as energy-efficient construction and aerospace engineering, among others. Nonetheless, the application of pure silica aerogels at high temperatures is limited due to their nearly transparent nature in the wavelength range of 2–8 µm. To overcome this limitation, the incorporation of opacifiers into pure silica aerogels can effectively augment the overall extinction coefficient of the material, consequently minimizing radiative heat transfer. In this paper, titanium dioxide is used to improve the high-temperature thermal insulation performance of silica aerogels. In order to fully understand the internal heat transfer properties of silica aerogel-based composites theoretically and to provide guidance for the structural design of high-temperature insulation materials, a theoretical model is established. This model accounts for the actual microstructure of the materials as well as the porous structural properties of the secondary particles in silica aerogel. Furthermore, it considers the intricate nature of heat transfer in multiphase hybrid materials. This paper compares the predictions of the model with experimental data. The results show that the theoretical model can quickly and accurately predict the thermal conductivity of TiO2-doped silica aerogels, and the error is controlled within 8 %, with a minimum error of 1.1 %.

Structural, thermophysical, mechanical and electrical properties of equimolar five-principal element fluorite-structured high-entropy oxides

Four kinds of high-entropy oxides (HEOs) with compositions of Y0.2Zr0.2Hf0.2Ce0.2X0.2O2−δ (X=La, Nd, Gd, Dy, abbreviated as HEO-La, Nd, Gd, Dy, respectively) are synthesized by solid-state-reaction method combined with conventional sintering in air atmosphere at 1600 °C for 10h. These HEOs have a fluorite-structured phase with homogeneous element distribution. The thermal conductivity (κ) of these four samples is in 1.68–2.18W·m-1·K-1 in 25–900 °C. At 800 °C and 900 °C, the κ value of HEO-Gd is lower than those of the HEO-La, Nd, Dy, yttria-stabilized zirconia (YSZ) and some other HEOs with various crystal structures ((Gd0.2Dy0.2Ho0.2Er0.2Tm0.2)Ta3O9, (La0.25Eu0.25Gd0.25Yb0.25)2Zr2O7, (Sm0.2Dy0.2Ho0.2Er0.2Yb0.2)NbO4, Hf0.25Zr0.25Ce0.25Gd0.125Ca0.125O2-δ and Sr(Y0.2Sm0.2Gd0.2Dy0.2Yb0.2)O4). The sample of HEO-Gd has the highest thermal expansion coefficient (CTE) (12.1×10-6K-1) and fracture toughness (KIC) (2.5MPa·m1/2) among HEO-La, Nd, Gd, Dy. Its KIC value is higher than those of other HEOs mentioned above. In addition, HEO-Gd exhibits a better oxygen barrier property compared with YSZ in 450–700 °C. These results indicate that HEO-Gd is a promising candidate as high-temperature thermal insulation ceramic.

Analysis of heat transfer characteristics of sandwich cylindrical shell with an aerogel-metal-rubber composite core

Metal-rubber sandwich cylindrical shell displays a broad range of potential applications in the field of thermal insulation due to its advantages of lightweight, high-temperature stability, and superior high-temperature thermal insulation properties. To optimize the thermal insulation performance of a sandwich cylindrical shell with a metal-rubber core, this study proposes the idea of filling the pores of the metal-rubber with aerogel, thereby forming a composite structure of aerogel and metal-rubber (AMR). Through finite element analysis, the law of internal temperature changes in the AMR composite structure and the distribution laws of the velocity field, viscosity field, and pressure field of the sandwich cylindrical shell under varying wind speeds are investigated. These findings are further verified through heat transfer and forced convection tests. The results reveal that the thermal conductivity of the AMR composite structure at the same density is lower than that of the metal-rubber, and the thermal insulation performance of the AMR composite structure is directly proportional to its density. Meanwhile, through the analysis of forced convection heat transfer in the cylindrical shell, it is found that wind speed has a direct relationship with the pressure field, velocity field, and viscosity field. In addition, there is a relatively high viscosity and a low flow rate near the wall of the cylindrical shell, with a higher flow rate at the axis of the cylindrical shell. At the same wind speed, the temperature rise of the sandwich cylindrical shell shows a positive correlation with an increase in density, and the maximum stable temperature also increases correspondingly. Moreover, at the same density, the thermal conductivity demonstrates a negative correlation with increasing temperature.

Fabrication of mullite micro/nano fiber-based porous ceramic with excellent mechanical and thermal insulation properties

Mullite fiber-based porous ceramics is one of the most commonly used high-temperature insulation materials. However, how to further reduce the thermal conductivity and improve the strength of such materials has been a challenge in the field of high-temperature insulation. In this paper, mullite micro/nano fiber-based porous ceramic was prepared by introducing mullite nanofibers into mullite microfiber-based porous ceramics and the effect of microfiber/nanofiber weight ratio on the samples properties were characterized and analyzed. Results showed that the compressive strength and high-temperature thermal insulation properties of the porous ceramics increased significantly with an increase in the nanofiber content. For the porous ceramics with similar density (about 0.50 g/cm3), the compressive strength of the sample with a microfibers/nanofibers weight ratio of 1:2 was 1.43 MPa, which was much higher than that of the porous ceramics prepared with only microfibers (0.26 MPa). Meanwhile, the backside thermal equilibrium temperature of the sample with a microfiber/nanofiber weight ratio of 1:2 at 900 °C environment was 355.8 °C, which was much lower than that of the porous ceramic prepared with only microfibers (476.5 °C). In addition, in order to investigate the temperature resistance of mullite micro/nano fiber-based porous ceramics, the sintering temperatures was increased from 1400 °C to 1500 °C. Results shows that the high-temperature thermal insulation performance of mullite micro/nano fiber-based porous ceramics with high mullite nanofiber content decays more seriously. The back-temperature test showed that the thermal equilibrium temperature of the samples with a mullite micron/nanofiber weight ratio of 1:1 in 900 °C environment increased rapidly from 386.3 °C to 454.2 °C when the sintering temperature was increased from 1400 °C to 1500 °C. The preparation of mullite micro/nanofiber-based porous ceramics provides a new strategy to improve the performance of fiber-based porous ceramics.

Mullite porous ceramics with high strength for high-temperature thermal insulation

To address the low thermal efficiency and serious energy consumption of industrial kilns, the gel casting method is used herein for the successful preparation of porous mullite thermal insulation materials with good thermal and mechanical properties. The effects of poly-γ-glutamic acid (γ-PGA) content (0.6–2.2 wt%) on the performances of the slurries, green bodies, and sintered bodies are comprehensively investigated based on the gelation mechanism. The mullite slurries show good flowability and stability, and the powders are evenly dispersed. Furthermore, the green bodies have sufficiently high mechanical strengths to meet the conditions of processing into complex shapes. The sample open porosity is shown to increase, and the pore size gradually becomes more uniform, as the γ-PGA content is increased. At a γ-PGA content of 1.8 wt%, the open porosity reaches 75.85 %, the compressive strength is 14.02 MP. The thermal conductivity increases from 0.263 W/(m⋅K) to 0.544 W/(m⋅K) as the temperature increases from 25 °C to 1000 °C. At the same time, the Gong model accurately describes the relationship between the thermal conductivity and porosity. In addition, infrared thermographic analysis and alcohol lamp ablation experiments macroscopically demonstrate the excellent thermal insulation performance of the porous mullite at high temperatures.

Aggregation-induced microfilaments enabled cotton cellulose aerogels with highly compressive and fatigue resistance, greatly thermal insulation and fireproofing

Green and sustainable cellulose aerogels have attracted extensive attention, but are severely limited by unsatisfied elasticity, fatigue resistance as well as fire hazard for functional applications. Herein, we proposed a novel and feasible strategy to fabricate highly elastic, thermal insulating and fireproof phytic acid/cellulose (PA/CE) aerogel via constructing microfilament substructure derived from PA-induced aggregation and assembly of cellulose chains. Benefiting from the interconnected fibrous cellular walls integrating with abundant microfilaments, the resultant PA/CE aerogels demonstrated excellent compressibility and resilience with recovery rate of 91.26% after 100 compressive cycles at 50% strain. With the hierarchical porous structure, PA/CE aerogels exhibited low thermal conductivity of 0.0340–0.0352 W/m·K, could maintain its structural integrity and high temperature thermal insulation after heat treatment for 30 min at 400 ℃. Moreover, the integration of PA enabled aerogel high fireproofing with limiting oxygen index (LOI) of 42.6% and UL-94 V-0 rating. The peak of heat release rate (PHRR) and total heat release (THR) of PA/CE aerogels were 79.97 kW/m2 and 1.75 MJ/m2, reduction by 43.0% and 46.5% compared with cellulose aerogels. Thus, the elastic cellulose aerogels proposed by this strategy provide an insight and feasibility for high performance and multifunctional applications.

Layered Inorganic Silicate Aerogel Pillared by Nanoclusters for High Temperature Thermal Insulation.

Layered inorganic material, with large-area interlayer surface and interface, provides an essential material platform for constructing new configuration of functional materials. Herein, a layered material pillared with nanoclusters realizing high temperature thermal insulation performance is demonstrated for the first time. Specifically, systematic synchrotron radiation spectroscopy and finite element calculation analysis show that ZrOx nanoclusters served as "pillars" to effectively produce porous structures with enough boundary defect while maintaining the layered structure, thereby significantly reducing solid state thermal conductivity (≈0.32Wm-1 K-1 , 298-573K). Moreover, the layered inorganic silicate material assembled aerogel also exhibits superior thermal insulation performance from room temperature (0.034Wm-1 K-1 , 298K, air conditions) to high temperature (0.187Wm-1 K-1 , 1073K, air conditions) and largely enhanced compressive strength (42kPa at 80% compression), which is the best layered material-based aerogel that has achieved synergistic improvement in thermal and mechanical performance so far. Layered inorganic silicate aerogel pillared by nanoclusters will pave a new avenue for the design of advanced thermal insulation materials under extreme conditions.

Facile Preparation of a Novel HfC Aerogel with Low Thermal Conductivity and Excellent Mechanical Properties.

Aerogels emerge as captivating contenders within the realm of high-temperature thermal resistance and thermal insulation. Nevertheless, their practical applications are usually constrained by their inherent brittleness when subjected to rigorous conditions. Herein, employing hafnium dichloride oxide octahydrate (HfOCl2·8H2O) as the hafnium source and resorcinol-formaldehyde (RF) as the carbon precursor, hafnium carbide (HfC) aerogels are fabricated via the sol-gel method complemented with carbothermal reduction reaction. Investigations are conducted into the effects of various molar ratios, duration, and temperatures of calcination on the microstructural features and physico-chemical characteristics of the as-prepared HfC aerogel. The aerogel shows a high BET-specific surface area (601.02 m2/g), which is much larger than those of previously reported aerogels. Furthermore, the HfC aerogel exhibits a low thermal conductivity of 0.053 W/(m·K) and a compressive strength of up to 6.12 MPa after carbothermal reduction at 1500 °C. These excellent thermal insulation and mechanical properties ensure it is ideal for the utilization of high-temperature thermal resistance and thermal insulation in the fields of aerospace.

Structure-stabilized SiO2 packed beds based on sawdust-doped for high-temperature thermal insulation

The utilization of nanoparticles for thermal insulation has attracted many interests. However, due to their low melting point at high temperatures, the volume shrinkage of the materials leads to a significant increase in thermal conductivity. In this work, sawdust was employed as an additive to constrain the thermal shrinkage of SiO2-based thermal insulation materials. In the sintering treatment, sawdust will be converted into carbon, while the structure will become more porous. Due to the large thermal contact resistance between carbonized sawdust and SiO2, as well as the abundant pores, which could further reduce thermal conductivity. Results show that adding 20 % of sawdust in SiO2 nanoparticles could lead to a significant decrease of 15 % in density and a substantial reduction of 22 % in thermal conductivity. Simultaneously, adding sawdust could improve the volume shrinkage resistance. Besides, the addition of 20 % sawdust is the optimal choice because of the lowest thermal conductivity of 0.037 W/(mK). Excessive addition of sawdust could lead to an increase in water adsorption, compromising the thermal insulation performance. The thermal cycle stability of 20 % carbonized-sawdust/SiO2 nanoparticle bed under high humidity conditions was also investigated, and the bed shows a stable thermal conductivity of 0.04 W/(mK). This work provides a facile method to inhibit the structural shrinkage of thermal insulation materials, which is of great significance for the application in high-temperature.

Lightweight and large-scale rGO reinforced SiBCN aerogels with hierarchical cellular structures exposed to high-temperature environments

SiBCN ceramic aerogel is an ideal potential candidate for ultra-high temperature thermal insulation due to its unique microscopic pore structure combined with the excellent thermal stability of SiBCN ceramic. Here, reduced graphene oxide (rGO) modified SiBCN aerogels (rGO/SiBCN) were prepared through solvothermal, freeze-casting and pyrolysis, and the dimension of the aerogel is up to Φ130 mm × 28 mm. The density of the rGO/SiBCN aerogel is as low as 0.024 g/cm3 and the microstructural regulation is achieved by controlling the rGO content in the aerogel. The hierarchical cellular structure endows the aerogel with a high specific surface area (148.6 m2/g) and low thermal conductivity (0.057 W m−1 K−1). The 10 mm-thick sample exhibits excellent thermal insulation and ablation resistance, as evidenced by its ability to reduce the temperature from ∼1100 °C to ∼180 °C under the intense heat of a butane flame. Moreover, benefiting from the ultrahigh-temperature stability of SiBCN, the rGO/SiBCN aerogel exhibits good thermal stability up to 1200 °C in argon and short-oxidation resistance at 800 °C in air. Therefore, the rGO/SiBCN aerogel with superior overall performance could expand its practical application in high-temperature thermal insulation under extreme environments.

Lightweight, strong, and thermally insulating polybenzoxazine aerogel thermal protection composites for antioxidant ablation long to 1800 s

The lightweight ablation materials that have been successfully applied in thermal protection systems (TPS) are the focus of attention owing to the urgent demand for aerospace vehicles to have efficient thermal insulation materials with lightweight, long-term antioxidant, and micro-ablation. However, improving the antioxidant ablation properties in a long-term aerobic atmosphere is still a significant challenge. Herein, a novel strategy to fabricate needle quartz fiber (NQF) reinforced SiO2 aerogel interpenetrating polybenzoxazine (PBO) aerogel thermal protection composites (NQF/SiO2–PBOs) with a binary network structure is proposed. The as-prepared NQF/SiO2–PBOs perfectly inherited their porous nanostructure and captivating properties, including lightweight (0.53 g/cm3), high mechanical strengths, and low thermal conductivity of 0.048 W/(m·K) at 25 °C and 0.079 W/(m·K) at 1100 °C. Moreover, the NQF/SiO2–PBOs exhibited outstanding high-temperature thermal insulation and long-time antioxidant ablation with low linear and mass ablation rates. The cold surface temperature peaked at approximately 307.2 °C within 1800 s when the hot surface temperature exceeded 1100 °C. The ablation/thermal insulation mechanism was also discussed through the analysis of the microstructure, chemical structure, and crystal structure. This research provides a meaningful reference for the development and exploitation of new advanced lightweight, long-term antioxidant, and micro-ablative thermal protective materials.

Foam-gelcasting preparation of porous SiC ceramic for high-temperature thermal insulation and infrared stealth

Foam-gelcasting preparation of porous SiC ceramic for high-temperature thermal insulation and infrared stealth

Multilayered mullite ceramics with anisotropic properties

Anisotropic layered porous ceramics are crucial to satisfy the demand for devices with directional functions. However, challenges, such as complicated preparation processes and difficulties in regulating the oriented structure, severely limit their application. Here, multilayered mullite ceramics (MLMCs) with specific porosities and strongly anisotropy properties, were prepared by designing porous thin-layered units and an interlayer layout, in combination with a simple gel-casting. Benefiting from the suitable slurry properties, the samples did not show obvious defects, and a perfect bonding was observed between the layers. The optimized MLMCs exhibited a porosity of 65.12%, the differences in compressive and flexural strengths of 14 (±0.7) and 5 (±0.2) MPa in different pressure directions respectively, and the anisotropy factor of thermal conductivity up to 0.98, as well as exhibited good high temperature thermal insulation properties. Ultimately, the MLMCs formed transverse heat conductor and longitudinal heat insulation heat-transfer patterns, with promising applications in thermal insulation.

High thermal insulation and high strength SiBCN@SiC double network ceramic composite aerogel

Silicon carbide (SiC) fiber materials prepared by precursor conversion method have attracted extensive attention in recent years due to their excellent high temperature resistance and thermal insulation properties. Herein, a novel SiBCN/SiC dual network ceramic composite aerogel with the three-dimensional (3D) porous SiC fiber network obtained by direct electrospinning as the first network and SiBCN aerogel as the second network was prepared by combining electrospinning with sol-gel technology and supercritical drying technology. The microstructure, phase composition, pore size distribution, compressive resistance, thermal insulation and related mechanisms of the 3D SiC fiber network and composite aerogel at different heat treatment temperatures were investigated in detail. The results show that the constructed double-network structure can make up for the heat transfer problem of a single SiC fiber network and further reduce its thermal conductivity, so that the prepared composite aerogel exhibits a lower thermal conductivity (∼0.0316 W m−1 K−1). In addition, the unique double network structure endows the composite aerogel with excellent compressive properties (2.56–2.89 MPa).

Phenyl-modified silica aerogels with high-temperature hydrophobicity for self-cleaning and thermal insulation under extreme circumstances

Silica aerogels tend to be employed as thermal insulating materials, however their properties are inevitable to deteriorate in humid conditions owing to the hydrophilic nature, especially above 200 °C. Herein, phenyl modified silica aerogels with superhydrophobicity, self-cleaning capacity, and ultra thermal insulation are prepared. Phenyltriethoxysilane (PTES) working as the modifier condenses with tetraethyl orthosilicate (TEOS), forming a three-dimension network containing ladder polyphenylsilsesquioxane structure, where the introduction of bulk phenyl groups decreases free energy and fabricates hierarchical regime, leading to distinctive water repellency. The water contact angles are larger than 150° with low hysteresis (less than 2°), rendering droplet bouncing and self-cleaning capacities. As the molar ratio of PTES to TEOS rises to 1, the superhydrophobicity can be preserved at temperatures as high as 500 °C. In addition, hot-steam resistance and excellent thermal insulation are achieved. When subjected to a direct impact of 200 °C steam for 5 h, the hybrid aerogel demonstrates a reversible rise of thermal conductivity around 34%. The maximum back-surface temperature keeps 55.9 °C after exposure to hot steam for 10 min. The modification can broaden the application area of silica aerogels in high-temperature thermal insulation.

Fabrication of Lightweight Polyimide Foams with Exceptional Mechanical and Thermal Properties.

Lightweight polyimide foams (PIFs) with exceptional thermal resistance and compressive properties arefabricated by heating polyester ammonium salts (PEASs) which areprepared by copolymerizing 4, 4'-diaminobenzanilide (DABA), 4, 4'-diaminodiphenyl methane (MDA) and 3, 3', 4, 4'-benzophenone tetracarboxylic dianhydride (BTDA). Hydrogen bonds areformed between CONH and CO in the PI chains due to the addition of DABA and the melt viscosity of PEAS precursors increase with increasing content of DABA, which isadvantageous to bind the foaming gases for cell expansion. The expansion ratio of PEAS precursors isincreased from 633% to 1133% when the molar ratio of MDA/DABA ischanged from 10:0 to 6:4. The compressive strength and modulus of PIFM9D1 (i.e., the molar ratio of MDA/DABA is9:1, foam density: 120.8 kg m-3 ) reach as high as 0.59 and 15.0MPa, respectively. The PIFs possess prominent thermal performance with the initial thermal degradation temperatures (under both nitrogen and air atmosphere) and glass transition temperatures (as assessed by DSC and DMA) exceeding 511 and 292°C, respectively. The thermal conductivity of PIFs islower than 0.049 W m-1 K-1 , which exhibits promising applications for serving as high-temperature thermal insulation materials in the fields of aerospace, marine, and nuclear sectors among others.