High-temperature Thermal Insulation Research Articles

“Finger coral-like” ceramic fiber aerogels with enhanced high-temperature thermal insulation, anti-oxidation, and mechanical performance

Ceramic fiber aerogels are widely used in high temperature insulation. Thermal insulation under extreme conditions requires materials that can withstand sharp thermal shocks and extended high-temperature exposure. However, the oxidation resistance of traditional ceramic aerogels under a high temperature environment(above 1000 °C) is significantly reduced. Moreover, the strength degradation and structural collapse easily occurred, restricting their application. In this study, “Finger Coral” ceramic fibers aerogels were obtained using precursor infiltration pyrolysis (PIP) and electrospinning technology. The chemical composition, morphology, thermal insulation, mechanical properties and isothermal oxidation properties of SiCBN/SiC/disilicates nanoparticles (IP) ceramic aerogels were tested. The experimental results showed that the aerogel exhibited excellent mechanical and antioxidant properties over a wide range of temperatures. SiCBN/SiC/IP aerogel with a thickness of 2cm can achieve the heat insulation effect of the hot surface temperature from 1600 to 75 °C, and the thermal conductivity at 1600 °C was only 0.036 W·m−1·K−1. After 12 h of cyclic isothermal oxidation, the weight gain was only 5.17%. The material is expected to be used as a thermal insulation material in the hypersonic vehicles.

(Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Si2O7: A novel high-entropy rare earth disilicate porous ceramics with high porosity and low thermal conductivity

Herein, a novel quinary high-entropy rare earth disilicate (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Si2O7 ((5RE1/5)2Si2O7) porous ceramics were synthesized through a sol-gel/vacuum freeze-drying method with high-temperature calcination process. The microstructure, phase composition, thermal conductivity, compressive strength, and thermal stability of the as-prepared (5RE1/5)2Si2O7 porous ceramics were systematically investigated. The results indicated that the samples possess high porosity of 76.83%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low thermal conductivity of 0.94 W/(m·K) at 1273 K and high compressive strength of 6.55 MPa. These excellent properties indicated that as-prepared (5RE1/5)2Si2O7 porous ceramics has a great potential application value in the field of high-temperature thermal insulation materials.

Aluminum borate whisker-based lattices with a hierarchical pore structure obtained via digital light processing

To satisfy the ever-increasing demand for aluminum borate porous ceramics with complex shapes and tunable pore structures in a diverse set of fields, aluminum borate whisker-based lattices with hierarchical pore structures were fabricated by a combination of in situ reaction and digital light processing three-dimensional (3D) printing. The optimal dispersant concentration and exposure parameters for 3D printing were determined based on analyses of the rheological properties and working curves of the Al2O3–B2O3 photosensitive slurries. The effects of the B2O3/Al2O3 molar ratio on the morphology and properties of aluminum borate lattices were investigated. The results showed that the addition of an excess of B2O3 was beneficial to the growth of aluminum borate whiskers. When the B2O3/Al2O3 molar ratio was set to 6:9, the resultant aluminum borate lattices exhibited a typical hierarchical pore structure, including inherent large pores in the lattices and small pores formed by interlocked aluminum borate whiskers generated in situ within the struts. This unique hierarchical pore structure endowed the ceramic lattices with a high compressive strength (1.18 MPa) and porosity (82.58%), as well as non-brittle fracture characteristics. Owing to these outstanding properties, aluminum borate whisker-based lattices are promising candidates for high-temperature thermal insulation, catalyst supports, acoustic absorption, and particle filtration.

Hierarchically porous Al2TiO5 ceramics via freeze-gel casting

Porous Al2TiO5 ceramics have drawn increasing attention in diverse applications owing to their low thermal expansion coefficient, and good resistance to thermal shock; however, Al2TiO5 ceramics prepared in previous studies still exhibited a relatively high density, which limited their further application in the field of high-temperature thermal insulation. Herein, we reported the successful fabrication of hierarchically porous Al2TiO5 ceramics with a unique three-dimensional skeleton structure formed by overlapped ceramic nanofibers. Results show that the introduction of SiO2 into Al2TiO5 nanofibers could significantly improve their thermal stability and meanwhile reduce their formation temperature. Al2TiO5 nanofibers with 10 mol% SiO2 exhibited a superior microstructure and excellent thermal stability, and were selected as the raw material in the subsequent preparation of corresponding porous ceramics. Porous Al2TiO5 ceramics prepared by the freeze-gel casting exhibited a unique hierarchical pore structure with large pores originating from the sublimation of ice crystal and small pores formed by the overlapped of fibers. This unique microstructure endowed porous ceramics with an ultra-high porosity (97.59–95.59%), ultra-low thermal conductivity (0.0456–0.0754 W m−1 K−1) and excellent thermal stability. Owing to these outstanding characteristics, the resulting porous Al2TiO5 ceramics are expected to be employed as the high-temperature thermal insulation material in a wide spectrum of advanced applications.

Flexible SiC nanowire aerogel with excellent thermal insulation properties

Aerogels are excellent thermal insulation materials with extremely low thermal conductivity. However, the practical applications of traditional oxide aerogels, such as SiO2 and Al2O3 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.

Porous Refractory Ceramics for High-Temperature Thermal Insulation - Part 2: The Technology Behind Energy Saving

Porous Refractory Ceramics for High-Temperature Thermal Insulation - Part 2: The Technology Behind Energy Saving

Monolithic carbon aerogels within foam framework for high-temperature thermal insulation and organics absorption

Carbon aerogels exhibit high porosity, good electrical conductivity, and low thermal conductivity, but their practical applications are greatly hindered by their tedious preparation and inherent structure brittleness. Herein, monolithic carbon aerogels (MCAs) with low density and large size are prepared via a facile sol-gel polymerization of phenolic resin within melamine foam (MF), followed by ambient pressure drying and co-carbonization. During ambient pressure drying process, the MF matrix can deliver supporting force to counteract against the solvent evaporation surface tension, thus inhibiting volume shrinkage and shape deformation. Upon co-carbonization process, the MF matrix and organic aerogel could pyrolyze and shrink cooperatively, which could effectively prevent the brittle fracture of monolith. Therefore, large-sized MCAs (up to 250 × 250 × 20 mm) with low densities of 0.12–0.22 g·cm−3 are obtained. The as-obtained MCAs possess high compressive strength (2.50 MPa), ultra-low thermal conductivity (0.051 W·m−1·K−1 at 25 °C and 0.111 W·m−1·K−1 at 800 °C), and high-volume organic absorption capability (77.3–88.0%, V/V). This facile and low-cost method for the fabrication of large-sized monolithic carbon aerogels with excellent properties could envision enormous potential for high-temperature thermal insulation and organics absorption.

Proton Donor-Regulated Mechanically Robust Aramid Nanofiber Aerogel Membranes for High-Temperature Thermal Insulation

High-performance thermal insulators are urgently desired for energy-saving and thermal protection applications. However, the creation of such materials with synchronously ultralow thermal conductivity, lightweight, and mechanically robust properties still faces enormous challenges. Herein, a proton donor-regulated assembly strategy is presented to construct asymmetric aramid nanofiber (ANF) aerogel membranes with a dense skin layer and a high-porous nanofibrous body part. The asymmetric structure originates from the otherness of the structural restoration of deprotonated ANFs and the resulting ANF assembly due to the diversity of available proton concentrations. Befitting from the synergistic effect of the distinct architectures, the resulting aerogel membranes exhibit excellent overall performance in terms of a low thermal conductivity of 0.031 W·m-1·K-1, a low density of 19.2 mg·cm-3, a high porosity of 99.53%, a high tensile strength of 11.8 MPa (16.5 times enhanced), high heat resistance (>500 °C), and high flame retardancy. Furthermore, a blade-scraping process is further proposed to fabricate the aerogel membrane in a continuous and scalable manner, as it is believed to have potential applications in civil and military fields.

Organic/inorganic double-precursor cross-linked alumina aerogel with high specific surface area and high-temperature resistance

Alumina aerogel has drawn a great research interest in the aerogel community owing to its great high-temperature heat resistance. The typical preparation can be divided into two approaches depending on the type of precursors. However, the alumina aerogel derived from organic aluminum alkoxides suffers from poor monolithic integrity, whereas the one from inorganic aluminum salts presents unsatisfied thermal stability. In this work, we develop a novel organic/inorganic double-precursor cross-linking method to prepare alumina aerogels. Aluminum chloride hexahydrate is used to provide an acid condition for the hydrolysis and condensation reaction of aluminum sec-butoxide (ASB), and serves as a reactant to co-condensate with the hydrolyzed ASB to form an interpenetrating chain structure. The resulting alumina aerogel exhibits a high specific surface area (SSA) of 667 m2/g and good high-temperature thermal insulation performance. Moreover, it can still retain SSA of 224 and 105 m2/g after heating at 1000 and 1300°C, respectively.

An analytical and numerical investigation into conductive-radiative energy transfers in evacuated honeycombs. Application to the optimisation and design of ultra-high temperature thermal insulation

Ultra-high temperature thermal and latent energy storage technologies offer a potential solution to the decarbonisation of the energy sector. However, uneconomical thermal energy losses are a barrier to their development and where significant, their minimisation requires knowledge about the different modes of thermal transport and their interaction in a given structure. In this paper, evacuated rectangular honeycomb structures are investigated as candidates for efficient ultra-high temperature thermal insulation.First, a theory is laid out along with its numerical implementation for the modelling of coupled non-linear conduction and radiation in three-dimensional cuboid multi-media structures with rarefied gaseous and opaque or semi-transparent solid media. The model is then applied to the study, optimisation and design of efficient ultra-high temperature thermal insulation based on rectangular honeycomb structures.A dimensionless number Nrc is defined and an analytical thermal analysis of rectangular honeycombs is developed which correlates Nrc to the optimal geometry and equivalent thermal conductivity of a honeycomb insulator. A numerical optimisation procedure is then presented and the correlations are validated, and thus constitute simple practical design tools for honeycomb insulators. Finally, it is shown that with an appropriate choice of geometry and materials, thin-walled honeycombs have the capacity to outperform existing high-temperature thermal insulation technologies with thermal conductivities lower than 0.01 W/m/K at 1600 K. In particular, a wall thickness of 50μm on a titanium alloy honeycomb would suffice to outperform ceramic fibre insulation over its entire range of operating temperatures.

Preparation and characterization of lightweight SiC frameworks by connecting SiC fibers with SiC joints

The increasing energy challenges and the rapidly developing astronautic technology require thermal insulators with low density and thermal conductivity, high–temperature thermal, chemical and mechanical stabilities. Herein, highly porous SiC frameworks were fabricated through procedures of assembling SiC fibers and formation of SiC joints by carbothermal reduction, which were named as SiC–bonded SiC fiber (SBSF) composites. The porosities were controlled in the range of 76–93% by varying aspect ratios of the SiC fibers. The SBSF composites exhibited outstanding corrosion resistance, oxidation resistance, and high–temperature thermal stability, as the inherent advantages of SiC ceramics. High strength and reversible compressibility were obtained in the out–of–plane compression load tests, and low thermal conductivity of 0.036–0.102 W/m · K at 25–800 °C was achieved. The results of this study suggest that the SBSF composites are a good candidate for high–temperature thermal insulators.

Characterization of novel high-entropy (La0.2Nd0.2Sm0.2Dy0.2Yb0.2)2Zr2O7 electrospun ceramic nanofibers

We present a simple way to fabricate high-entropy (La0.2Nd0.2Sm0.2Dy0.2Yb0.2)2Zr2O7 (HE-RE2Zr2O7) ceramic nanofibers using the electrospinning and annealing processing in this work. The microstructure of nanofibers was characterized by thermal gravity-differential scanning calorimetry (TG-DSC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Meanwhile, the diameter and grain growth of HE-RE2Zr2O7 nanofibers under 1000 °C was analyzed. Results indicate that HE-RE2Zr2O7 nanofibers can be prepared at temperatures above 400 °C and the crystallite size can be controlled by annealing temperature. Both diameter and the grain growth of HE-RE2Zr2O7 nanofibers are lower than that of La2Zr2O7 nanofibers, attributed to the sluggish diffusion effect. The advantages of HE-RE2Zr2O7 nanofibers can further enlarge the application of nanofibers in the aspect of high-temperature thermal insulation materials.

Design and preparation of hollow SiO2 nanospheres/SiC-based nanocomposite fiber membrane for high temperature thermal insulators

Silicon carbide (SiC) fibers are attractive for their thermal stability and chemical erosion resistance. However, the practical application of SiC fiber in the fields of thermal insulation is limited owing to its high intrinsic thermal conductivity. Herein, a novel closed cell structured SiC-based nanocomposite fiber (NCF) using hollow SiO 2 nanospheres (HSNSs) as pore forming agent were designed by precursor conversion method combined with electrospinning technology. According to the content and size of HSNSs, NCFs with different pore size and porosity could be obtained. When the content of HSNSs reached 15%, a large number of closed cells were constructed inside the fiber, which broke the continuity of solid heat transfer and endowed it with excellent thermal insulation performance (∼0.105 W m −1 K −1 at 1000 °C). Meanwhile, the NCFs showed excellent high-temperature resistance and mechanical properties, which makes it have a wide application prospect as a high temperature thermal insulator. This research also offers a new insight for the development of SiC-based fiber with multiple functions to meet various applications, such as catalyst supporters and microwave absorbers. • A new type of porous insulation material with closed cell structure is designed. • Multiple phases enhance interface thermal resistance and infrared shielding performance. • Hollow SiO 2 spheres significantly reduces the thermal conductivity of the fibers. • The nanocomposite fibers show great application prospects as high temperature thermal insulators.

Preparation of a carbon fibre-reinforced carbon aerogel and its application as a high-temperature thermal insulator.

Carbon aerogels (CAs) have attracted attention in thermal insulation. However, the traditional sol–gel method for preparing them involves time-consuming solvent exchange and rigorous supercritical drying processes, and the obtained CAs are brittle and crumble easily. To address these problems, a carbon fibre-reinforced carbon aerogel (CF/CA) was prepared via combining a resorcinol–furfural (RF) gel containing a salt (ZnCl2) with polyacrylonitrile (PAN) fiber felt. The CF/CA not only has low thermal conductivity (0.6904 W m−1 K−1) even at an ultra-high temperature of 1800 °C in an argon atmosphere but also exhibits relatively high compressive strength (6.10 MPa, 10% ε) and a low density of 0.68 g cm−3. The CF/CAs can be used as ultrahigh-temperature thermal insulators (under inert atmospheres or vacuum) in thermal protection systems such as space vehicles or industrial high temperature furnaces. Our novel strategy may lead to lower-cost and large scale industrial processes of CF/CAs.

Design, fabrication, microstructure, and properties of highly porous alumina whisker foam ceramic

In this study, a new type of highly porous ceramic was successfully fabricated from α-Al 2 O 3 whiskers and nano-sized Al 2 O 3 particle slurry by vacuum infiltration with the subsequent annealing in air. It was found that with an increase in the alumina slurry amount or sintering temperature, the porosity of the foam ceramic decreased while its compressive strength increased. The optimal alumina slurry content and sintering temperature were equal to 4 g, and 1350 °C, respectively. The as-synthesised porous foam ceramic exhibited a low density of 0.65 g/cm 3 , high porosity of 83.51%, high compressive strength of 3.84 MPa, and low thermal conductivity of 1.37 W/m⋅K at room temperature, suggesting its potential applications in high-temperature gas filtration and thermal insulation materials.

Novel nanometer alumina-silica insulation board with ultra-low thermal conductivity

Advanced nano-porous super thermal insulation materials are widely used in spacecraft, soler-thermal shielding, heat exchangers, photocatalytic carriers due to their low thermal conductivity. In this work, adopting dry preparation technology, nano-Al2O3, nano-SiO2, SiC and glass fibers as raw materials, novel nanometer alumina-silica insulation board (NAIB) were prepared. The preparation process was simple, safe, and reliable. In addition, the NAIB exhibited a high porosity (91.3–92.3%), small pore size (39.83–44.15 nm), low bulk density (0.22–0.26 g/cm3), better volumetric stability, and low thermal conductivity (0.031–0.050 W/(m·K) (200–800 °C)), respectively. The as-prepared NAIB could render them suitable for application as high-temperature thermal insulation materials.

Ultrahigh-strength carbon aerogels for high temperature thermal insulation

Carbon aerogels with nanoporous structure are attractive for thermal insulation under extreme conditions, but their practical applications are usually plagued by the inherent brittleness and easy-oxidation characteristic at high temperature. Herein, silica-modified carbon aerogels (SCAs) with extraordinarily high strength are prepared via a facile sol-gel polymerization of phenolic resin and siloxane, followed by ambient pressure drying and carbonization. The resulting SCAs possess medium-high density of ∼0.5 g·cm−3 and mesoporous structure with the mean pore size of 33 nm. During carbonization process, the siloxane could be gradually transformed into the amorphous SiO2 particles and crystalline SiC particles, which are coated on the surface of carbon nanoparticle and consequently improve the oxidation-resistance of carbon aerogels. Due to the density-porosity trade-off, the SCAs have high compressive strength of 10.0 MPa and satisfied thermal conductivities of 0.118 W·m−1·K−1 at 25 °C and 0.263 W·m−1·K−1 at 1000 °C. Furthermore, needled carbon fiber-reinforced SCAs (CF-SCAs) with ultrahigh compressive strength of 210.5 MPa are prepared, which exhibit good thermal conductivities of 0.207 W·m−1·K−1 at 25 °C and 0.407 W·m−1·K−1 at 1000 °C. The ultrahigh mechanical strength, good oxidation-resistance, good thermal insulation as well as the facile preparation make the SACs great promising in high-temperature insulations especially under harsh conditions.

Highly Stretchable, Crack-Insensitive and Compressible Ceramic Aerogel.

Ceramic aerogels are attractive candidates for high-temperature thermal insulation, catalysis support, and ultrafiltration materials, but their practical applications are usually limited by brittleness. Recently, reversible compressibility has been realized in flexible nanostructures-based ceramic aerogels. However, these modified aerogels still show fast and brittle fracture under tension. Herein, we demonstrate achieving reversible stretch and crack insensitivity in a highly compressible ceramic aerogel through engineering its microstructure by using curly SiC-SiOx bicrystal nanowire as the building blocks. The aerogel exhibits large-strain reversible stretch (20%) and good resistance to high-speed tensile fatigue test. Even for a prenotched sample, a reversible stretch at 10% strain is achieved, indicating good crack resistance. The aerogel also displays reversible compressibility up to 80% strain, ultralow thermal conductivity of 28.4 mW m-1 K-1, and excellent thermal stability even at temperatures as high as 1200 °C in butane blow torch or as low as -196 °C in liquid nitrogen. Our findings show that the attractive tensile properties arise from the deformation, interaction, and reorientation of the curly nanowires which could reduce stress concentration and suppress crack initiation and growth during tension. This study not only expands the applicability of ceramic aerogels to conditions involving complex dynamic stress under extreme temperature conditions but also benefits the design of other highly stretchable and crack-resistant porous ceramic materials for various applications.

Correction to: Porous Refractory Ceramics for High-Temperature Thermal Insulation - Part 1: The Science Behind Energy Saving.

Correction to: Porous Refractory Ceramics for High-Temperature Thermal Insulation - Part 1: The Science Behind Energy Saving.

Robust and Multifunctional Porous Polyetheretherketone Fiber Fabricated via a Microextrusion CO2 Foaming.

Fabrication of multifunctional porous fibers with excellent mechanical properties has attracted abundant attention in the fields of personal thermal management textiles and smart wearable devices. However, the high cost and harsh preparation environment of the traditional solution-solvent phase separation method for making porous fibers aggravates the problems of resource consumption and environmental pollution. Herein, a microextrusion process that combines environmentally friendly CO2 physical foaming with fused deposition modeling technology is proposed, via the dual features of high gas uptake and restricted cell growth, to implement the continuous production of porous polyetheretherketone (PEEK) fibers with a production efficiency of 10.5cm s-1 . The porous PEEK fiber exhibits excellent stretchability (234.8% strain) and good high-temperature thermal insulation property. The open-cell structure on the surface is favorable for the adsorption to achieve superhydrophobicity (154.4°) and high-efficiency photocatalytic degradation of rhodamine B (90.4%). Moreover, the parameterized controllability of the cell structure is beneficial to widening the multifunctional window. In short, the first porous PEEK physical foaming fiber, which opens up a new avenue for the application expansion, especially in the medical field, is realized.