High-temperature Thermal Insulation Research Articles

Study on the thermal insulation property of lanthanum–cerium oxide coatings

Functional fillers with low thermal conductivity and high near-infrared reflectance play a vital role in high temperature thermal insulation coatings and make an outstanding contribution to achieving efficient comprehensive thermal insulation. In this work, the thermal insulation and bonding properties of lanthanum–cerium oxide (LCO) coatings were regulated by composition design. The effects of the La/Ce mole ratio on phase composition, the thermal conductivity and near-infrared reflectivity of LCO coatings were investigated systematically. The results showed that coating with a La/Ce mole ratio of 1:1 possess the excellent thermal insulation properties (ΔT ≈ 115°C) due to the formation of a LCO solid solution and a lanthanum phosphate mesophase with low thermal conductivity and high near-infrared reflectivity. In addition, the mortise and tenon structure was formed between the coating and the substrate, which led to the remarkable bonding properties. The LCO coating offers excellent thermal insulation properties and broad application prospects in the fields of high temperature and energy conservation.

Experimental study on creep characteristics of glazed hollow beads-cement/sodium silicate grouting materials

This study introduces a new thermal insulation grouting material designed for high-temperature mine environment. The thermal insulation effect of cement/sodium silicate grouting materials can be enhanced by adding glazed hollow beads with outstanding thermal insulation properties. The ratio with the best comprehensive performance was selected through orthogonal tests, and the long-term creep mechanical characteristics of glazed hollow beads-cement/sodium silicate grouting materials were analyzed. Graded equal incremental loading with different loading durations was used to test the specimens' instantaneous strain, creep strain, creep rate, critical stress, and microscopic structures. For each loading stage, the Burgers model was used to fit the creep curve. The results show that graded loading intensity surpasses uniaxial loading across different loading durations. The different loading durations significantly affect the creep parameters, and stress concentration damage initially manifests on the surface of glazed hollow beads. Finally, the theoretical curve obtained by the Burgers model fits well with the creep test curve. This study has good practical significance for high-temperature roadway thermal grouting insulation technology.

Phase transformation inhibition and improved thermo-mechanical properties of rare earth niobates for thermal barrier coatings

Rare earth niobate Re3NbO7 (Re=Rare earth element) possesses ultra-low thermal conductivity for application as thermal barrier coatings (TBCs). However, Re3NbO7 with large rare earth ion radius (Re =La, Nd, Sm) exhibits both low temperature phase transition and poor facture toughness which is detrimental to the durability of the coatings. In this work, an effective method was proposed to solve those problems by doping. Considering the low-temperature phase transition at about 340K, La3NbO7 is selected as representative material. In order to suppress phase transition, Y3+ with relatively smaller ionic radius is used as a doped ion, and (La1-xYx)3NbO7 (x=0.0,0.1,0.3,0.5) were prepared and investigated. It is shown that, low temperature phase transition of La3NbO7 is effectively suppressed due to Y3+ doped. Facture toughness is increased significantly from 0.5 to 2.3 MPa.m1/2 with the increase of doping concentration. Furthermore, the Vickers hardness is also improved by doping. Considering the phase stability and ultra-low thermal conductivity, (La1-xYx)3NbO7 solid solution (x=0.5) maybe promising candidates for high temperature thermal insulation materials such as TBCs, thermoelectric materials, etc.

High Temperature Thermal Insulation Ceramic Aerogels Fabricated from ZrC Nanofibers Welded with Carbon Nanoparticles

High Temperature Thermal Insulation Ceramic Aerogels Fabricated from ZrC Nanofibers Welded with Carbon Nanoparticles

Emerging fabrication of ceramic nanofiber aerogel with the application in high-temperature thermal insulation, environment, and electromagnetic wave absorption

Emerging fabrication of ceramic nanofiber aerogel with the application in high-temperature thermal insulation, environment, and electromagnetic wave absorption

Robust, thermally insulating, and high-temperature resistant phosphate-enhanced mullite fiber porous ceramic composites

Porous mullite ceramics with high porosity and low thermal conductivity are urgently required as high-temperature thermal insulators in harsh environments. How to improve the strength of Porous mullite ceramics dramatically under the premise of no sacrificing its thermal insulating properties has remained an extremely challenging. Inspired by the freezing rain phenomenon in nature, high-strength and high-temperature resistant phosphate coatings were applied to the surface of mullite fibers, which significantly improved the mechanical properties of porous mullite ceramics without significantly increasing their thermal conductivity. The Al(H2PO4)3 solution concentration significantly affected the mechanical and thermal properties of the composite material, where the mechanical performance improved with an increasing Al(H2PO4)3 solution concentration. The phosphate coating phase stabilized with an increasing calcination temperature. The mechanical properties of the ceramics were improved by the excellent interfacial bonding strength between the coating and mullite fiber, and the forms of phosphate in the porous ceramics. The optimum composite preparation process increased its compressive strength by approximately nine-fold to 4.1 MPa and resulted in a thermal conductivity of only approximately 0.09 W/(m·K), demonstrating that the composite exhibited a significantly higher comprehensive performance than other previously reported porous ceramics and aerogels. Such new composites with excellent mechanical and thermal insulation properties exhibit excellent potential for high-strength thermal insulation in the chemical, metallurgical, pharmaceutical, aerospace, and other fields.

Fire-resistant and thermal insulation performance of SiC nanofiber batt fabricated from cotton fiber batt

The cotton fiber batt is one of the most commonly used thermal insulator in our lives at room temperature because of its flexible and porous structure. The SiC nanofibers have excellent mechanical, physical, and chemical properties such as high mechanical strength and modulus, low density, high-temperature resistance, which is a promising material for high temperature structural materials or thermal insulator. In this work, a flexible SiC nanofiber batt was fabricated from the cotton fiber batt by sol-gel carbothermal reduction reaction. The fluffy carbonized porous cotton batt was used as a template for the fabrication of sponge-liked SiC nanofiber batt composed of single crystalline β-SiC fibers. It exhibits good compressive flexibility, ultra-high porosity (a specific surface area was 30.90 m²/g), excellent high-temperature stability (ranging from −196 °C in liquid nitrogen to 1300 °C under the flame of butane blowtorch) and low thermal conductivity (0.02 W/(m·K) at room temperature). The SiC nanofiber batt, which has low density, excellent high-temperature resistance and oxidation resistance, is a promising material for high-temperature structural materials or thermal insulators under extreme conditions. The facile fabrication method of SiC nanofiber batt from common cotton batt provides a novel technique for economic industrial production of high temperature thermal insulation material.

Synthesis of Environmentally Friendly Nanoporous Monolithic Carbon Aerogels via Ambient Pressure Drying for High-Temperature Thermal Insulators

Synthesis of Environmentally Friendly Nanoporous Monolithic Carbon Aerogels via Ambient Pressure Drying for High-Temperature Thermal Insulators

Fabrication of thermally resistant and mechanically flexible polyimide foams with anisotropic pore structures for thermal insulation and irradiation tolerance applications

Flexible polyimide foams (PIFs) demonstrate a great potential for applications in the fields of thermal insulation, flame retardancy and irradiation tolerance. In this work, mechanically flexible PIFs with anisotropic pore structure were prepared by adopting the copolymerization strategy and microwave-assisted foaming process. The copolymerization reaction was conducted to synthesize polyester ammonium salt (PEAS) precursors using 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (BTDA) as dianhydride, 4,4′-diaminodiphenyl ether (ODA) and 4,4′-diaminodiphenyl sulfone (DDS) as the copolymerized diamines; afterwards, PEAS powders with particle sizes ranging from 100 to 300 μm were obtained by drying, crushing and sieving treatments, which were used as the feedstock for “bottom-up” microwave-assisted foaming and thermal imidization process, thereby leading to the formation of PIFs with regular three-dimensional near-spherical pores in vertical direction (parallel to the pore growth direction) and the aligned ellipsoidal strip-like pore structure in horizontal direction. The directional growth of foam pores resulted in anisotropic mechanical flexibility and thermal insulation performance. Moreover, PIFs possessed excellent thermal stability and flame retardancy with the initial thermal degradation temperature higher than 545 °C and limited oxygen index up to 58.8%. The PIFs exhibited exceptional thermal insulation performance with thermal conductivity values ranging from 0.0276 to 0.0517 W/(m·K) between 25 and 300 °C. Additionally, the PIFs demonstrated excellent irradiation tolerance, with the mechanical and thermal stability retention rate being above 96% after the irradiation experiment (radiation dosage: 1.15 × 105 Gy) using 60Co source. Thus, lightweight PIFs with superior high temperature thermal insulation performance, flame retardancy and irradiation tolerance were successfully prepared, which showed potential applications in aerospace, medical and nuclear power sectors.

Gas-solid coupling in a randomly distributed ceramic nanofibrous aerogel

Ceramic nanofibrous aerogel (CNA) has received tremendous interest recently due to its advantageous properties and potential applications. However, modeling its gas-contributed thermal conductivity is quite difficult due to the intricate gas-solid coupling heat transfer mechanism and complex structure. Herein, based on an equivalent method used for a gas in free space, a model for predicting gas-contributed thermal conductivity in its nanopores is built to provide a straightforward solution to the above-mentioned issues. We study both gas and gas-solid coupling effects on the effective thermal conductivity of the randomly distributed Al2O3-carbon core-shell nanofiber aerogel. The coupled term demonstrates low gas-contributed and effective thermal conductivity of 0.022 and 0.065 W m−1 K−1 at 1500 K, respectively. It is shown that the coupling effect of gas-solid interaction is noticeable and should not be ignored when the gas pressure pg ≤ 0.1 bar. The predicted effective thermal conductivity demonstrates good consistency with available experimental data, which could provide direction for quick and accurate design and optimization of high-temperature thermal insulation materials.

Preparation and thermal insulation performance of Al2O3[sbnd]Y2O3[sbnd]SiO2 ternary composite aerogels with high specific surface area and low density

Against the background of global energy shortages and the need for long-term flight safety of aerospace vehicles, traditional silica aerogels have the problems of insufficient temperature resistance and poor thermal insulation at high-temperatures, and are no longer able to meet the demand. In this study, a series of monolithic Al2O3Y2O3SiO2 ternary aerogels were successfully synthesized via synchronous sol–gel and ethanol supercritical drying. Both the prepared aerogels and the calcined samples at 1000 °C exhibited a complete three-dimensional network structure and a high specific surface area (up to 675.7 m2/g and 214.4 m2/g), which was attributed to the supporting effect of the Al2O3 component on the overall skeleton and the inhibiting effect of the Y2O3 component on the crystal transformation. Mullite fiber reinforced Al2O3Y2O3SiO2 aerogel composites were synthesized by vacuum impregnation using mullite fiber felt as a matrix. The prepared sample exhibit very low thermal conductivity (as low as 0.079 W⋅m−1·K−1 at 1000 °C). In summary, the good structural stability and high temperature thermal insulation performance make Al2O3Y2O3SiO2 ternary aerogel have the potential of efficient service in more application scenarios. This study provides a product optimization reference idea for the thermal insulation material industry.

Reusable, high specific surface areas, and excellent thermal stability Al2O3–SiO2 aerogel composites as high-temperature thermal insulators for radome applications

In this study, monolithic Al2O3–SiO2 aerogels with excellent thermal stability and high specific surface area were synthesized using boehmite nanorods (L=20–30 nm, D=3–5 nm). The high specific surface area of aerogels was maintained after calcination at 800 °C (663.54 m2/g), 1000 °C (486.91 m2/g), and 1100 °C (230.76 m2/g). The anti-sintering mechanism of the Al2O3-SiO2 aerogels was discussed. Thanks to the composite of Al2O3-SiO2 aerogel and mullite fibers, the disadvantages of brittleness and low strength of aerogel are overcome. The thermal conductivity of the composite is extremely low at 0.034 W/(m·K)-0.089 W/(m·K) (25 °C-1000 °C). Five muffle cycle back-temperature tests at 1000 °C and a butane blowtorch back-temperature test at 1350 °C also proved that the composites have excellent thermal insulation properties. Composites with excellent thermal insulation properties combined with lightweight, thinness, machinability, and good dielectric properties meet the need for high-temperature thermal insulating-wave transmitting integrated materials for hypersonic missile radomes.

Preparation and characterization of novel lightweight Y2Si2O7 fibrous porous ceramics for high temperature thermal insulation

Fibrous porous ceramics play a vital role in the field of thermal protection systems. In this work, novel lightweight Y2Si2O7 fibrous porous ceramics were successfully prepared based on Y2Si2O7 fibrous membranes by molding method. The effects of mechanical dispersion speed and heat treatment temperature on the properties of Y2Si2O7 fibrous porous ceramics were characterized. The results revealed that benefiting from the appropriate fiber length, the ceramics had a better performance at a mechanical dispersion speed of 4000 revolutions per minute. The density, shrinkage, and compressive strength of the ceramics increased, but the porosity decreased slightly with the increasing heat treatment temperature. The prepared Y2Si2O7 fibrous porous ceramics exhibited favorable properties like low density (0.37 g/cm3), high porosity (90.56%), good high-temperature resistance (∼1350 °C), and low thermal conductivity (∼0.12 W m−1 K−1 at 27 °C, ∼0.566 W m−1 K−1 at 1000 °C). These properties indicate that Y2Si2O7 fibrous porous ceramics could be a promising candidate as thermal insulation materials in high-temperature environments.

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.

Thermal-insulating ceramic fiber aerogels reinforced fusing knots of overlapping fibers for superelasticity and high compression resistance

Ceramic fiber aerogels, a unique feature of mesopores and/or macropores formed by overlapping fibers, are currently one of the most applicable materials in high-temperature thermal insulation. However, they usually exhibit...

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.