High-temperature Thermal Insulation Research Articles

Flexible and high-temperature resistant ZrO2/SiC-based nanofiber membranes for high temperature thermal insulation

Silicon carbide (SiC) fibers have excellent high temperature resistance and chemical stability in extreme environment. However, the practical application of SiC fibers have been limited by their high intrinsic thermal conductivity. Herein, the novel ZrO2/SiC-based nanofiber membranes with high temperature resistance and low thermal conductivity were prepared for the first time using simple blending method via electrospinning technique combined with subsequent curing and pyrolysis. The microstructure, composition, high temperature resistance and thermal insulation mechanisms were analyzed in detail. The results showed that the introduction of Zr could significantly improve the high temperature resistance of the nanofibers up to 1400 °C, at the same time, the multi-phase microstructure (SiOxCy, ZrO2, β-SiC and residual carbon) endows it with ultralow thermal conductivity at high temperature (0.217 W m-1 K-1 at 1300 °C). Compared with the reported thermal insulation materials, this work not only prepares thermal insulation materials with higher temperature resistance, but also provides a new perspective for the research of SiC-based nanofibers in the field of thermal insulation.

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si3N4 ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si3N4 ceramic foams had excellent thermal insulation properties (<0.15 W m−1 K−1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si3N4 ceramic foams was greatly improved.

Thermo-Hydraulic Analysis of a Horizontal HTS ZFC Levitating Bearing Concerning Its Autonomy Safety Service Time

Electromagnetic heating of high-temperature superconductors (HTSs) and non-perfect thermal insulation of cryostats cause liquid nitrogen (LN2) vaporization and consumption. This paper presents results from the thermo-hydraulic analysis of the maximum safety service time of the prototype of a horizontal axis radial levitation bearing with zero-field cooled (ZFC) HTS bulks. It first studies the temperature sensitivity of yttrium barium copper oxide (YBCO) bulks to the LN2 level, particularly when partially immersed. This, when the main component of heat flux is according to the c-axis of the bulk crystal structure, that is the case of the studied bearing prototype and in most of all cryostats of maglev vehicles. Measurements used two platinum resistance thermometers leaning the top and side surfaces of a non-magnetized ZFC YBCO bulk. Correlations between the temperatures measured by the two platinum resistors are made to determine the average temperature of YBCO bulk. The thermal-time constant associated with a YBCO bulk losing contact with LN2 is also determined for two conditions: 1) initial ZFC and 2) initial field-cooled (FC) under the influence of typical magnetic field values generated by the studied bearing permanent magnet (PM) rotor. Results indicate that the YBCO temperature evolution is not notably affected by Joule losses from initial magnetization currents for this bearing. Due to continuous PM rings, the magnetic field perceived by the YBCO bulks does not change in the ideal case of spinning with no vibrations. Another experiment shows that the heating by induction from vibration dynamics does not contribute notably to LN2 consumption. Finally, the safety service time is verified from measured LN2 mass and level evolution in the HTS bearing's stator, due mainly to defective thermal insulation from the room temperature. The process consisted of calculating the times that different leveled YBCO bulks experiments reached the critical temperature and using the conclusions taken from the initial study about the temperature sensitivity of single bulks to the LN2 level. 3D finite element simulations reinforce our experimental results.

Theoretical modeling of thermal conductivity of alumina aerogel composites based on real microstructures

As compared with silica aerogels, alumina aerogels have excellent high-temperature stability, and hence, are regarded as a promising candidate for high-temperature thermal insulation materials. However, heat transfer mechanisms for such materials remain insufficiently explored. In this study, based on real material microstructures, an analytical thermal conductivity model for alumina aerogel-fiber-opacifier composites is developed via a modified parallel-series equivalent electrical circuit approach. The model takes into account solid, gaseous, and solid-gas coupling heat conduction in aerogels, solid heat conduction in both fibers and opacifier particles, and radiation in aerogels, fibers, and opacifier particles. The model is validated by comparing its predictions with measured thermal conductivity data. The model predictions reveal that there are two kinds of solid-gas coupling heat transfer mechanisms, which occur in the corner regions between interconnected secondary particles and in the slit regions between adjacent chains, respectively. Because of this coupling, the contribution of gaseous heat conduction is comparable to that of solid heat conduction and radiation of aerogels, fibers, and opacifier particles, which is well supported by experimentally measured data of thermal conductivity. The model is also used to analyze effects of various parameters on the thermal insulation performance of the composites, such as diameters and volume fractious of both fibers and opacifier particles.

Calcium aluminate cement in castable alumina: From hydrate bonding to the in situ formation of calcium hexaluminate

Calcium hexaluminate (CA6) is an intrinsically densification-resistant material, therefore, its porous structures are key materials for applications as high-temperature thermal insulators. This article reports on the combination of calcined alumina and calcium aluminate cement (CAC) in castable aqueous suspensions for the in situ production of porous CA6. The CAC content (10–34 vol%) and the curing conditions ensure structural integrity prior to sintering and maximize the development of hydrated phases. Changes in physical properties, crystalline phases, and microstructure were investigated after isothermal treatments (120–1500 °C), and three sequential porogenic events were observed. The hydration of CAC preserved the water-derived pores (up to 120 °C), and the dehydroxylation of CAC hydrates (250–700 °C) generated inter-particles pores. Moreover, the in situ expansive formation of CA2 and CA6 (900–1500 °C) hindered densification and generated intra-particle pores. Such events differed from those observed with other CaO sources, and resulted in significantly higher pores content and lower thermal conductivity.

A novel flexible, layered, recoverable SiO2 fiber skeleton and aerogel composites material prepared by papermaking process

The anisotropic ceramic fiber skeleton and its aerogel composite materials have important applications in the fields of high temperature insulation and aerospace. However, the traditional ceramic fiber skeleton has been limited in its application due to the complexity of its preparation process. Here, referencing the traditional papermaking technology, a flexible and recoverable SiO2 fiber skeleton with anisotropic layered structure was prepared through papermaking process and low-temperature sintering (170 °C) technology. Complete heteromorphic skeleton could be prepared by this simple process. On this basis, we impregnated the fiber skeleton with silica aerogel to further improve the composite properties of the pure fiber block. Compared with SiO2 fiber skeleton, the SiO2 aerogel/fiber skeleton composite exhibited lower thermal conductivity (0.023 W/(m·k)), higher compressive strength (2.19 MPa) and excellent high temperature thermal insulation. The recoverable and excellent thermal insulation properties of anisotropic layered composite materials provide an effective way for the manufacture of low-cost aerogel composite materials. At the same time, it has potential application value in high-temperature thermal insulation materials.

ВОЛОКНА ДИОКСИДА ЦИРКОНИЯ КАК КОМПОНЕНТ ВЫСОКОТЕМПЕРАТУРНОЙ ТЕПЛОИЗОЛЯЦИИ (обзор)

Based on the analysis of recent publications of scientific and technical literature, data on the production of zirconium oxide fibers used for the manufacture of high-temperature thermal insulation materials are presented. Information is provided on various methods of obtaining zirconium oxide fibers (methods of impregnation of the template and molding of the mixture, sol-gel method of spinning a fiber-forming precursor solution), as well as on the technique of fiber molding (manual pulling, dry and wet spinning, blowing and electrospinning). The use of such fibers for the production of thermal insulation materials (felts, cords and blocks) instead of currently existing materials made of aluminum oxide-based fibers can significantly increase the operating temperatures of the thermal protection systems.

Multi-phase SiZrOC nanofibers with outstanding flexibility and stability for thermal insulation up to 1400 °C

Ceramic fibers are one of the most important high-temperature thermal insulating materials for application in the harsh environment. However, the practical use of traditional ceramic fibers has been largely impeded by their poor thermal stability or high thermal conductivity at high-temperatures. Here, multi-phase SiZrOC (SiOxCy, SiO2, ZrO2, ZrSiO4 and C) nanofiber (NF) was rationally designed and prepared according to the intrinsic factors of these phases with lower heat transfer effects (significant phonon scattering and infrared shielding) that contributes to the high-temperature thermal insulation performance in ceramic fibers. Owing to the unique multi-phase microstructure, the obtained SiZrOC NFs exhibit ultralow thermal conductivity (~0.043 W m−1 K−1 at 25 °C and ~0. 233 W m−1 K−1 at 1400 °C), high flexibility, good mechanical strength, excellent fire resistance and thermal stability even up to 1400 °C, which makes it an ideal high-temperature thermal insulator especially for applications under extreme conditions. This work not only presents with the fabrication of a prosperous material for high-temperature thermal insulation but also offers a novel strategy for developing a wider range of high-temperature thermal insulating materials.

Designing high-temperature thermal insulators based on densification-resistant in situ porous spinel

In situ formation of magnesium aluminate spinel (MgAl2O4) based on Al2O3 and MgO sources is an expansive reaction that hinders its densification. Although its side effects have been extensively investigated for dense materials, its potentially useful role to produce refractory porous structures remains unexplored. In this study, spherical clusters of brucite (Mg(OH)2) nanoparticles were synthesized and combined with Al2O3 in different ratios for producing porous alumina-spinel-MgO structures. The evolution of their microstructure and physical properties was followed in the 300−1500 °C range. Different porogenic mechanisms related to particles’ packing, dehydroxylation of brucite, MgO consumption in the reaction with alumina (Wagner’s mechanism leading to the formation of Kirkendall voids) and expansive spinel formation were identified during initial heating. Variations in Mg(OH)2-Al2O3 ratio and sintering temperature resulted in structures of tailored porosity, compression strength, thermal conductivity and pore sizes in the 1−5 μm range, suitable for materials to be used at high temperatures.

Synthesis and thermal evolution of polysilazane-derived SiCN(O) aerogels with variable C content stable at 1600 °C

Ceramic aerogels possess intriguing thermophysical properties which make them excellent candidates for high temperature thermal insulators. However, their properties can degrade at high temperature because of crystallization phenomena or because of densification (causing a sensible reduction of their specific surface area and porosity).The polymer derived ceramic (PDC) route is a relatively new way of developing ceramic aerogels. Several aspects influence the properties of the final product when dealing with preceramic polymers, among them their chemical composition and molecular architecture.In this work, we investigated the possibility of producing aerogels belonging to the SiCN system from polysilazanes mixtures, namely perhydropolysilazane (PHPS) and a methyl/vinyl-containing polysilazane, namely Durazane 1800®, thus changing the C/Si ratio of the amorphous pyrolyzed products. It is shown that the chemical composition of the ceramic aerogel affects the main properties of the porous materials, such as thermal stability and specific surface area (SSA). Results show that the presence of carbon in the aerogels inhibits crystallization of Si3N4 up to 1600 °C in N2 and allows to maintain a SSA of ~90 m2/g up to this temperature.

Structure and properties of lightweight magnesia refractory castables with porous matrix

To improve the energy-saving capacity of magnesia refractory castables for working lining of high-temperature kilns, this study presents the researches on microstructure and properties of lightweight magnesia refractory castables with porous matrix fabricated by direct foaming method. The results show that formation of closed-pores in the matrix significantly enhanced high-temperature thermal insulation performance of castables with minor changes of slag corrosion resistance. The thermal conductivity of the lightweight magnesia castables at 1000 °C was below 1.2 W/m·K, which is 47.8% lower than that of the referenced magnesia castable. The increasing content of SDS (foaming agent, over 0.02 wt%) led to increments of size and number of large-sized pores, resulting in the significantly decreased density and mechanical performances. The slag resistance mechanism reveals that, in addition to intergranular penetration, the accumulation of slag and penetration between adjacent pores were the major ways of slag mass transfer in lightweight magnesia castables. In conclusion, controlling the size (below 53.2 μm), number and distribution of closed-pores in the matrix is effective to realize the coupling of high thermal insulation, mechanical properties and slag resistance for lightweight magnesia castables used in the metallurgical field.

Ultrastrong, Superelastic, and Lamellar Multiarch Structured ZrO2-Al2O3 Nanofibrous Aerogels with High-Temperature Resistance over 1300 °C.

Advanced ceramic aerogel materials with a performance combining sufficient mechanical robustness and splendid high-temperature resistance are urgently needed as thermal insulators in harsh environments. However, the practical applications of ceramic aerogel materials are always limited by poor mechanical performance and degradation under thermal shock. Here, we report the facile creation of lamellar multiarch structured ceramic nanofibrous aerogels that are simultaneously ultrastrong, superelastic, and high temperature resistant by combining ZrO2-Al2O3 nanofibers with Al(H2PO4)3 matrices. The resulting ZrO2-Al2O3 nanofibrous aerogels exhibit the integrated properties of rapid recovery from a strain of 90%, high compression strength of more than 1100 kPa (at a strain of 90%), high fatigue resistance, and temperature-invariant superelasticity. Moreover, the all-ceramic component feature also provides the ceramic nanofibrous aerogels with high-temperature resistance up to 1300 °C and thermal insulation performance with low thermal conductivity (0.0322 W m-1 K-1). These superior performances make the ceramic aerogels ideal for high-temperature thermal insulation materials in extreme conditions.

Micro-nano Ceramic Fibers for High Temperature Thermal Insulation

Micro-nano Ceramic Fibers for High Temperature Thermal Insulation

Directing near-infrared photon transport with core@shell particles

Directing the propagation of near-infrared radiation is a major concern in improving the efficiency of solar cells and thermal insulators. A facile approach to scatter light in the near-infrared region without excessive heating is to embed compact layers with semiconductor particles. The directional scattering by semiconductor@oxide (core@shell) spherical particles (containing Si, InP, TiO2, SiO2, or ZrO2) with a total radius varying from 0.1 μm to 4.0 µm and in an insulating medium at a low volume fraction is investigated using Lorenz–Mie theory and multiscale modeling. The optical response of each layer is calculated under irradiation by the Sun or a blackbody emitter at 1180 K. Reflectance efficiency factors of up to 83.7% and 63.9% are achieved for near-infrared solar and blackbody radiation in 200 µm thick compact layers with only 1% volume fraction of bare Si particles with a radius of 0.23 µm and 0.50 µm, respectively. The maximum solar and blackbody efficiency factors of layers containing InP particles were slightly less (80.2% and 60.7% for bare particles with a radius of 0.25 µm and 0.60 µm, respectively). The addition of an oxide coating modifies the surrounding dielectric environment, which improves the solar reflectance efficiency factor to over 90%, provided it matches the scattering mode energies with the incident spectral density. The layers are spectrally sensitive and can be applied as a back or front reflector for solar devices, high temperature thermal insulators, and optical filters in gradient heat flux sensors for fire safety applications.

Designing Colloidal Silica-Bonded Porous Structures of In-situ Mullite for Thermal Insulation

Abstract: Colloidal silica (CS) is a promising raw material for refractory castable ceramics. It consists of stable suspensions of synthetic amorphous silica nanoparticles that behave simultaneously as liquid medium and binder for ceramic particles and as a porogenic agent and highly reactive source of silica to promote in-situ reactions. The setting mechanism of CS balances two opposite effects. Adding more CS to a suspension increases the bonding potential for gelling reactions and strengthening; on the other hand, it also introduces more water into the system, enhancing pore content. Such effects can be advantageously employed in the preparation of porous structures from aqueous suspensions and applied as high-temperature thermal insulators. The present study addresses the production of porous structures of in-situ mullite attained from aqueous suspensions of highly porous transition alumina particles bonded with colloidal silica. Different grades of CS and transition aluminas were combined to present suitable workability (flowability and gelling time) and to generate stoichiometric mullite or mullite-alumina porous structures after sintering.

High-temperature resistant Y2SiO5–TiO2 aerogel composite for efficient thermal insulation

Ternary mullite fibrous felt/opacifier/aerogel composites (MYTC) were prepared by impregnating the TiO2 opacifier/Y2SiO5 aerogels into mullite felts. The effect of the TiO2 opacifiers on the thermal insulation property of aerogel composite was investigated by adjusting the TiO2 opacifier contents from 0 to 15 wt%. The as-prepared MYTC exhibited low density of 0.195 g/cm3 and low thermal conductivity of 0.042 W/m/K. The high-temperature resistance of MYTC was tested using a self-designed thermal insulation test system. It was found that the addition of Y2SiO5 aerogels and TiO2 opacifiers significantly improved the thermal insulation performance of aerogel composites. The results demonstrated that MYTC showed more efficient heat insulation property as the back surface temperature of the MYTC was 157 °C lower than that of the mullite fiber felts as the front surface temperature of both was about 1200 °C. The as-prepared MYTC exhibited excellent high-temperature thermal insulation performance, making the materials potential application to high-temperature thermal insulation.

Non-silica fiber and enabled stratified fiber doping for high temperature aerogel insulation

Spherical opacifier doping aerogels for thermal insulation have been investigated in the past decades. However, the cylindrical fiber doping is seldom studied, in particular, there are no thermal analysis on non-silica fiber-loaded aerogels which can satisfy thermal insulation and mechanical requirements in harsh thermal environment simultaneously. The radiation inhibition property and overall insulation performance of non-silica fiber doping method are numerically investigated in the present work. Our results reveal that the optimal diameter of non-silica fiber dopant is significantly lower than that of opacifier doping at identical temperature. Carbon fiber has incomparable infrared radiation suppression ability of about 4 times that of carbon opacifier in the whole temperature range. Compared with the opacifier doping, the non-silica fiber doping method is demonstrated to provide stronger light shielding effect, lower thermal conductivity at a certain doping fraction, and much higher mechanical properties. Moreover, a stratified core-shell design for oxidizable carbon fiber is proposed to extend its excellent radiation suppression ability to high temperature. These findings provide useful guidance for the application of fiber-reinforced aerogels for high temperature thermal insulation.

Geometric optimization of aerogel composites for high temperature thermal insulation applications

Silica aerogel has attracted great interest in thermal insulation applications due to its ultralow thermal conductivity. However, silica aerogel is transparent to the infrared radiation in the range of 3-8 µm, making them be not suitable for high temperature thermal insulation applications. Here, we developed an optimization method considering both thermal radiation and heat conduction to design the geometric structures of aerogel composites with minimized thermal conductivity. The results show that when the ambient temperature is lower than ~ 600 K, the additives with low thermal conductivity are preferred. When the ambient temperature is higher than ~ 600 K, the additives with high extinction coefficients are needed. The additives with a broad size distribution could enable the aerogel composites to have an optimal thermal insulation performance in the environment with a changing temperature. The work provides a guideline for the geometric design of aerogel composites for high temperature thermal insulation applications.

Double-cross-linked aerogels towards ultrahigh mechanical properties and thermal insulation at extreme environment

Aerogels with low density and excellent thermal insulation are seriously plagued by poor mechanical properties with low compressive moduli in the kPa range, especially in extreme conditions such as high temperatures and fires. In this work, a new double chemical cross-linking and physical cross-linking strategy is developed to synthesize ultrahigh-modulus aerogels with extreme-condition resistance from low-cost polyvinyl alcohol (PVA), bio-based phytic acid (PA) and montmorillonite (MMT). The resultant aerogels consist of interpenetrating networks with chemically cross-linked domains between PVA and PA and physically cross-linked domains among PVA, PA and MMT, leading to an ultrahigh compressive modulus of 41.9 MPa and a specific modulus of 455.4 MPa·cm3·g−1, which can even withstand a car crush. Benefitting from the formation of ultra-strong network backbones containing phosphorus and clay, the aerogels amazingly show excellent fire resistance and high-temperature thermal insulation features. This robust porous material can maintain its structural integrity and high modulus (~2 MPa) after exposure to extremely high-temperature flame (~1300 °C), and also exhibits high mechanical modulus of ~4 MPa after heating treatment at 900 °C. These special characteristics make the aerogel a promising insulation candidate in the fields of aviation, aerospace and other applications.

Light and strong nano-SiC foam with extremely low thermal conductivity

We herein report a porous nano-SiC foam fabricated by foam-gelcasting-freeze drying method with ultra-high porosity (96%). Porous nano-SiC sintered at 1300 °C and1400 °C exhibit excellent balanced high compressive strength (1.44 MPa) and low thermal conductivity (0.064 W/(m·K)). The porous nano-SiC can be invoked as a candidate for high temperature thermal insulation. The low thermal conductivity mainly originates in the high porosity and the strong scattering of phonons by the nanostructure of the skeleton.