Fiber Reinforced Polyimide Aerogel Composites with High Mechanical Strength for High Temperature Insulation

Glass fiber/polyimide aerogel composites are prepared by adding glass fiber mat to a polyimide sol derived from diamine, 4,4′‐oxydianiline, p‐phenylene diamine, and dianhydride, 3,3′,4,4′‐biphenyltetracarboxylic dianhydride. The fiber felt acts as a skeleton for support and shaping, reduces aerogel shrinkage during the preparation process, and improves the mechanical strength and thermal stability of the composite materials. These composites possess a mesoporous structure with densities as low as 0.143–0.177 g cm^?3, with the glass fiber functioning to improve the overall mechanical properties of the polyimide aerogel, which results in its Young's modulus increasing from 42.7 to 113.5 MPa. These composites are found to retain their structure after heating at 500 °C, in contrast to pure aerogels which decompose into shrunken ball‐like structures. These composites maintain their thermal stability in air and N₂ atmospheres, exhibiting a low thermal conductivity range of 0.023 to 0.029 W m^?1 K^?1 at room temperature and 0.057to 0.082 W m^?1 K^?1 at 500 °C. The high mechanical strengths, excellent thermal stabilities, and low thermal conductivities of these aerogel composites should ensure that they are potentially useful materials for insulation applications at high temperature.     

Synthesis,microstructure and property characterization of Mo4Y2Al3B6 ceramic fabricated by spark plasma sintering

Polycrystalline Mo₄Y₂Al₃B₆ ceramic (92.84 wt% Mo₄Y₂Al₃B₆ and 7.16 wt% MoB) was prepared by spark plasma sintering at 1250 ℃ under 30 MPa using Mo, Y, Al, and B as starting materials. The dense sample obtained has a high relative density of 96.6 %. The average thermal expansion coefficient is 8.38 × 10^?6 K^?1 in the range of 25–1000 ℃. The thermal diffusivity decreases from 6.50 mm²/s at 25 °C to 4.33 mm²/s at 800 °C. The heat capacity, thermal conductivity, and electrical conductivity are 0.30 J·g^?1·K^?1, 11.73 W·m^?1·K^?1, and 0.66 × 10⁶ Ω^?1·m^?1 at 25 °C, respectively. Vickers hardness with increasing load in the range of 10–300 N at room temperature decreases from 10.82 to 9.49 GPa, and the fracture toughness, compressive strength, and flexural strength are 5.14 MPa·m^1/2, 1255.14 MPa, and 384.82 MPa, respectively, showing the promising applications as structural-functional ceramics.     

Functionalized few-layered graphene nanoplatelets for superior thermal management in heat transfer nanofluids

The superior thermal conductivity and lightweight of graphene flakes make them materials of choice for advanced heat transfer applications, especially for transport of electricity from sustainable power stations such as concentrating solar power plants. In view of the excellent thermal conductivity of graphene or graphene-like nanomaterials (3000–5000 W m⁻¹ K⁻¹), their dispersion into conventional host fluids such as water (0.613 W m⁻¹ K⁻¹) or ethylene glycol (0.25 W m⁻¹ K⁻¹) can significantly improve fluid heat transfer characteristics. The two-dimensional structure and high surface area as well as the cost-efficient carbon-based material make graphene nanoplatelets (GNPs) suitable for large-scale applications in colloidal thermal conductive fluids. For an efficient dispersion of GNPs in base fluids, intrinsically hydrophobic GNPs were acid treated to obtain highly concentrated (4 wt.%) graphene-based nanofluids. Investigations on various GNP sizes and reaction parameters showed significant influences on the resulting thermal conductivity values of the nanofluid. After 14 h measurements in a dormant system, the most efficient nanofluid reached a thermal conductivity of 0.586 W m⁻¹ K⁻¹ (the base fluid of 0.391 W m⁻¹ K⁻¹) and a low viscosity of 6.39 cP resulting in an overall efficiency improvement of 77%, when compared to the base fluid without particles.     

Polar bear hair inspired ternary composite ceramic aerogel with excellent interfacial bonding and efficient infrared transmittance for thermal insulation

Highly porous, heat resisting ceramic aerogels are considered as promising materials for high-temperature insulation. However, the general structural characteristics of ceramic aerogel, such as poor mechanical strength and transparency to infrared radiation, pose a major obstacle to their practical application. In this paper, we report a general strategy to prepare hollow mullite fiber (HMF) structures by coaxial electrostatic spinning and grow TiO₂ nanorods (TiO₂/NAs) in situ on HMF. The ternary composite ceramic aerogel material was prepared by filling the pores of HMF-TiO₂/NAs with SiCN aerogel. The TiO₂/NAs increased the fiber/aerogel interfacial bonding of the composite (0.392 MPa, 30% strain) and improved the IR transmittance (∼0%, 1200 ℃) without sacrificing their low density and thermal conductivity. In addition, low thermal conductivity (0.041 W/(m·K), 1200 °C) and excellent high-temperature insulation properties allow the composite aerogel to meet the urgent need for lightweight, high-strength, high-temperature insulation systems for spacecraft.     

Infrared radiation and thermal properties of Al-doped SrZrO3 perovskites for potential infrared stealth coating materials in the high-temperature environment

The Al-doped SrZrO₃ perovskite powder with low infrared emissivity at high temperatures was prepared. The infrared radiation performance and thermophysical properties of the perovskites at high temperatures were discussed. As a result, the infrared emissivity of the Al-doped SrZrO₃ perovskite powder is associated with Al³⁺-doping content, phase composition and particle morphology. The flaky particles SrZr_0.85Al_0.15O_2.925 formed by heat treatment at 1000 °C for 6 h have the lowest infrared emissivity of 0.245 in 3–5 μm wavebands at 590 °C. The perovskite powder's infrared emissivity is positively correlated with its electrical resistivity and has no apparent change after heating over 800 °C for long-term. The SrZr_0.85Al_0.15O_2.925 perovskite ceramic formed by pressureless sintering still maintains ideal heat insulation performance with the thermal conductivity from 1.17 to 2.21 W m^?1 K^?1 below 1400 °C. The Al-doped SrZrO₃ perovskite tablet exhibits significant weak radiation intensity due to its characteristics of both low infrared emissivity and thermal conductivity at high temperatures.     

Synchronously enhanced thermoelectric and mechanical properties of Ti doped NbFeSb based half-heusler alloys by carrier regulation and phonon engineering

NbFeSb based half-heusler thermoelectric materials have great potential in high-temperature energy harvesting. However, their conversion efficiency is limited by the low figure of merit zT, which is attributed to poor electronic performance and high thermal conductivity. Here, it is demonstrated that enhanced power factor of 46 μW cm^?1 K^?2 and zT value of 0.9 at 973 K is achieved in spark plasma sintered Nb_0.8Ti_0.2FeSb. It is depicted that a tremendous increase in the hole concentration owing to Ti substitution, results in a significant promotion of electrical performances. In addition, numerous phonon scattering centers, such as point-defects, nano precipitates as well as electro-acoustic coupling, collectively suppress the lattice thermal conductivity from 7.4 W m^?1 K^?1 to 3.3 W m^?1 K^?1 at 973 K. Furthermore, NbFeSb based alloys exhibit excellent mechanical property and the Vickers hardness reaches 10.8 GPa, which would be beneficial for TE devices fabrication.     

Fiber of bioinspired columnar cactus prickle-like structure for reinforced SiC aerogel: Thermal insulation and mechanical properties

The inspiration was obtained from the unique morphology of the columnar cactus prickle-like structure. Biomimetic fiber with a "double network" capability of reflecting heat radiation was prepared by electrospinning and hydrothermal methods. Among them, the first reflective network was TiO₂ nanorods grown in-situ on the fiber surface, and the second reflective network was TiO₂ particles embedded in the fiber. The bionic fiber was added to SiC aerogel as reinforcement to build composite aerogel. The results displayed that the "double network" reflecting ability of the bionic fiber can effectively shield the heat radiation conduction and thus reduce the effective thermal conductivity of the composite aerogel. The TiO₂ nanorods of the fiber surface not only reflected the heat radiation but also enhanced the fiber/aerogel interface bonding. In addition, the composite aerogel exhibited low thermal conductivity, high ablation resistance and high compressive strength.     

Rare-earth-tantalate high-entropy ceramics with sluggish grain growth and low thermal conductivity

A series of rare-earth-tantalate high-entropy ceramics ((5RE_0.2)Ta₃O₉, where RE = five elements chosen from La, Ce, Nd, Sm, Eu and Gd) were prepared by conventional sintering in air at 1500 ^°C for 10 h. The (5RE_0.2)Ta₃O₉ high-entropy ceramics exhibit an orthogonal structure and sluggish grain growth. No phase transition occurs in the test temperature of 25–1200 ^°C. The thermal conductivities of all (5RE_0.2)Ta₃O₉ ceramics are in the range of 1.14–1.98 W m^?1 K^?1 at a test temperature of 25–500 ^°C, approximately half of that of YSZ. The sample of (Gd_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)Ta₃O₉ exhibits a low glass-like thermal conductivity with a value of 1.14 W m^?1 K^?1 at 25 ^°C. The thermal expansion coefficient of (5RE_0.2)Ta₃O₉ ceramics ranges from 5.6 × 10^?6 to 7.8 × 10^?6 K^?1 at 25–800 ^°C, and their fracture toughness is high (3.09–6.78 MPa·m^1/2). The results above show that (5RE_0.2)Ta₃O₉ ceramics could be a promising candidate for thermal barrier coatings.     

Rapid fabrication of Se-modified skutterudites obtained via self-propagating high-temperature synthesis and pulse plasma sintering route

Selenium is an effective dopant in skutterudite-based thermoelectric materials. It strongly influences thermal transport properties due to effective phonon scattering. This study proposes a short-term fabrication route to Se-modified CoSb₃-based materials. Alloy synthesis was conducted via self-propagating high-temperature synthesis. Subsequently, pulse plasma sintering consolidated all materials. As a result, thermoelectric materials with high electrical properties homogeneity were obtained. Seebeck potential mapping showed the measured deviation of the Seebeck coefficient for all fabricated samples was between 5 and 7%. A very low thermal conductivity (1.59 W m^?1 K^?1, at 573 K) was achieved for the highest doped sample, and one of the lowest reported results obtained for bulk skutterudite-based thermoelectric materials ever. This resulted in a low lattice thermal conductivity (1.51 W m^?1 K^?1, at 573 K). This led to the highest ZT (0.27 at 623 K) for the highest doped sample.     

High thermal conductive shape-stabilized phase change materials based on water-borne polyurethane/boron nitride aerogel

Phase change materials (PCMs) applied in energy storage and temperature control system are important for energy conservation and environmental protection. In this work, structure-adjustable water-borne polyurethane (WPU)/boron nitride (BN) aerogels were synthesized via directional freeze-drying method, and used as supporting scaffolds to confine paraffin wax (PW) and obtain composite phase change materials. The three-dimensional (3D) porous thermal conductivity network of BN was derived by the in-situ ice crystal mound in aerogel, which endows the PW/WPU/BN composite PCM-2.5 with high thermal conductivity (0.96 W m^?1 K^?1) and high energy storage density (140.04 J/g). Shape-stabilized PCMs with high thermal conductivity and excellent electrical insulation prepared by the simple method have great potential for the thermal management of electronic products.     

Fabrication and thermal insulation properties of ceramic felts constructed by electrospun γ-Y2Si2O7 fibers

Ceramic fiber felts are attractive candidates for high temperature insulation due to their lightweight, high porosity and low thermal conductivity. In this work, ceramic felts constructed by γ-Y₂Si₂O₇ fibers were prepared by a facile method of Y–Si–O/PVB sol-gel electrospinning combined with subsequent high-temperature calcination. Effects of calcination temperature on the phase composition, microstructure and thermal insulation properties of ceramic felts were systematically studied. Results indicated that the organic components in the sol-gel fibers were removed after high temperature calcination, while the fibers kept the original continuous microtopography with high aspect ratios. Ceramic fiber felts of pure γ-Y₂Si₂O₇ phase could be obtained after calcinated at 1300 °C. The as-prepared paper-like γ-Y₂Si₂O₇ fiber felt presented low density of ～120 mg/cm³ and a high porosity up to 97.03%. Combined with the inherent high temperature stability and low thermal conductivity of γ-Y₂Si₂O₇, this light ceramic felts possessed high-temperature resistance and thermal insulating property (low thermal conductivity of 0.052 W m^?1 K^?1). The successful preparation of this ceramic fiber felt may provide a perspective for insulation materials used in harsh environments.     

Thermophysical properties and cyclic lifetime of plasma sprayed SrAl12O19 for thermal barrier coating applications

About 6-8 wt% yttria-stabilized zirconia (YSZ) is the industry standard material for thermal barrier coatings (TBC). However, it cannot meet the long-term requirements for advanced engines due to the phase transformation and sintering issues above 1200°C. In this study, we have developed a magnetoplumbite-type SrAl₁₂O₁₉ coating fabricated by atmospheric plasma spray, which shows potential capability to be operated above 1200°C. SrAl₁₂O₁₉ coating exhibits large concentrations of cracks and pores (~26% porosity) after 1000 hours heat treatment at 1300°C, while the total porosity of YSZ coatings progressively decreases from the initial value of ~18% to ~5%. Due to the contribution of porous microstructure, an ultralow thermal conductivity (~1.36 W m⁻¹ K⁻¹) can be maintained for SrAl₁₂O₁₉ coating even after 1000 hours aging at 1300°C, which is far lower than that of the YSZ coating (~1.98 W m⁻¹ K⁻¹). In thermal cyclic fatigue test, the SrAl₁₂O₁₉/YSZ double-ceramic-layer coating undertakes a thermal cycling lifetime of ~512 cycles, which is not only much longer than its single-layer counterpart (~163 cycles), but also superior to that of YSZ coating (~392 cycles). These preliminary results suggest that SrAl₁₂O₁₉ might be a promising alternative TBC material to YSZ for applications above 1200°C.     

Flexible Electrospun strawberry-like structure SiO2 aerogel nanofibers for thermal insulation

Herein, a novel flexible SiO₂ aerogel composite nanofiber membrane with strawberry-like structure and excellent thermal insulation properties, in which SiO₂ aerogel particles act as thermal insulation filler, was prepared by electrospinning technology. With the addition of nano-pore structure SiO₂ aerogel particles, the heat transfer path of the fibers inside the membrane became discontinuous, endowing the as-prepared membrane an ultra-low thermal conductivity of 30.3 mW/(m?K) and large surface area of 240 m²/g. Moreover, the nanofibers membrane also possesses the combined merits of excellent fire resistance, high-temperature stability, and temperature-invariant flexibility, rendering it a promising in the application of insulation and gas adsorption. The successful preparation of this flexible nanofiber membrane paves a new way to design materials with excellent thermal insulation and adsorption properties.     

Thermal expansion performance and intrinsic lattice thermal conductivity of ferroelastic RETaO4 ceramics

Ferroelastic RETaO₄ ceramics are promising thermal barrier coatings (TBCs) because of their attractive thermomechanical properties. The influence of crystal structure distortion degree on thermomechanical properties of RETaO₄ is estimated in this work. The relationship between Young's modulus and TECs is determined. The highest TECs (10.7 × 10⁻⁶ K⁻¹, 1200°C) of RETaO₄ are detected in ErTaO₄ ceramics and are ascribed to its small Young's modulus and low Debye temperature. The intrinsic lattice thermal conductivity (3.94-1.26 W m⁻¹ K⁻¹, 100-900°C) of RETaO₄ deceases with increasing of temperature due to an elimination in thermal radiation effects. The theoretical minimum thermal conductivity (1.00 W m⁻¹ K⁻¹) of RETaO₄ indicates that the experimental value is able to be reduced further. We have delved deeply into the thermomechanical properties of ferroelastic RETaO₄ ceramics and have emphasized their high-temperature applications as TBCs.     

High entropy (LaCeSmEuNd)2Zr2O7 ceramic aerogel with low thermal conductivity and excellent structural heat resistance

Dimensions and thermal insulation properties of nanoporous ceramics are unstable at high temperatures due to structural disruptions. This work prepared high entropy (LaCeSmEuNd)₂Zr₂O₇ ceramic aerogel via non-alkoxide sol-gel, supercritical drying, and calcination. XRD and EDS analysis showed that the (LaCeSmEuNd)₂Zr₂O₇ ceramic existed as a single phase. SEM images demonstrated the successful synthesis of aerogel structure. After two hours of annealing at 1200 °C, the cylindrical sample pressed from (LaCeSmEuNd)₂Zr₂O₇ ceramic aerogel had a compressive strength of 58.75 MPa, and its diameter shrinkage was 0.56%, whereas the La₂Zr₂O₇ reached 13.68%. The thermal diffusivity of annealed (LaCeSmEuNd)₂Zr₂O₇ was as low as 0.119 mm² s^?1, and its thermal conductivity at room temperature was 0.073 W·m^?1 K^?1, which was attributed to lattice disorder, stable porous structure, and abundance of grain boundaries caused by high entropy effects. Extending the high entropy effect to ceramic nano-insulation products is beneficial for enhancing their thermal stability.     

Potential thermal barrier coating materials: RE3NbO7 (RE=La, Nd,Sm, Eu,Gd, Dy) ceramics

In this work, RE₃NbO₇ ceramics are synthesized via solid‐state reaction and the phase structure is characterized by X‐ray diffraction and Raman spectroscopy. The relationship between crystal structure and thermophysical properties is determined. Except Sm₃NbO₇, each RE₃NbO₇ exhibits excellent high‐temperature phase stability. The thermal expansion coefficients increase with the decreasing RE³⁺ ionic radius, which depends on the decreasing crystal lattice energy and the maximum value reaches 11.0 × 10^?6 K^?1 at 1200°C. The minimum thermal conductivity of RE₃NbO₇ reaches 1.0 W m^?1 K^?1 and the glass‐like thermal conductivity of Dy₃NbO₇ is dominant by the high concentration of oxygen vacancy and the local structural order. The outstanding thermophysical properties pronounce that RE₃NbO₇ ceramics are potential thermal barrier coating materials.     

Enhanced thermoelectric performance of p-type sintered BiSbTe-based composites with AgSbTe2 addition

Bismuth telluride-based materials have been widely used in the field of thermoelectric cooling near room temperature. However, the material utilization and device conversion efficiency were limited by the low thermoelectric performance and poor mechanical properties of commercial zone-melting materials. With an aim to optimize the comprehensive properties, we prepared the composite samples of Bi_0.48Sb_1.52Te₃ (BST)-x wt% AgSbTe₂ (x = 0, 0.05, 0.1, 0.2) via the hot pressing method. It was found that the AgSbTe₂ addition can effectively increase the carrier concentration and improve the power factor to 46 μW cm^?1 K^?2 at 300 K. Due to the introduction of dislocations, stress and Te inhomogeneities, the lattice thermal conductivity of the composite was significantly reduced to 0.69 W m^?1 K^?1 at 325 K. As a result, a maximum ZT of 1.15 at 325 K is obtained for the x = 0.1 sample. Interestingly, BST-0.1 wt% AgSbTe₂ exhibits roughly isotropic thermoelectric performance perpendicular to and parallel to the pressing direction. Our study suggests that the BST-AgSbTe₂ composite is very promising for the application of thermoelectric refrigeration near room temperature.     

Improving specific stiffness of silicon carbide ceramics by adding boron carbide

A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²?s^?2 to ~153 × 10⁶ m²?s^?2 when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 W?m^?1?K^?1 when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 25 wt% B₄C were ~690 MPa and ~95 W?m^?1?K^?1, respectively.     

Study on the properties of periclase-forsterite lightweight heat-insulating refractories for ladle permanent layer

The heat dissipation of the ladle permanent layer is urgent to be tackled. Periclase has good compressive strength, high fire resistance, and high chemical stability. The thermal conductivity of forsterite is only 1/3–1/4 of periclase, which is low. Thus, the periclase-forsterite lightweight heat-insulating refractories for a permanent layer of steel ladle prepared from low-grade talc powder and light burned magnesia powder made by low-grade magnesite can well satisfy the requirements of the permanent layer. In this study, the effect of raw material ratio on as-prepared refractories was systematically investigated. The results demonstrated that with the increase in talc, the bulk densities of the samples decreased from 1.73 g/cm³ to 1.46 g/cm³, the porosities increased from 42% to 58%, and the compressive strengths decreased from 49 MPa to 28 MPa; besides, the thermal conductivities decreased from 1.062 to 1.22 W m^?1 K^?1 to 0.496–0.61 W m^?1 K^?1 with the increase in forsterite. Additionally, talc promoted the formation of intercrystalline pores, and the synthesis of forsterite in the experiment was dominated by the “Template growth” mechanism.     

Lightweight,robust, porous heterocyclic para-aramid aerogel hollow fibers for multifunctional applications

In nature, many fibers with warmth-retention properties, such as the hair of polar bears and rabbits, both have a hollow cross-section structure. The static air in fiber cavities can effectively inhibit heat conduction and serve as an effective thermal insulator. In this work, the high-performance heterocyclic para-aramid polymer was selected as the spinning solution, and aerogel hollow fiber was prepared by coaxial wet spinning and freeze-drying techniques. The effects of spinning solution concentration and lyophilized solvent on the micromorphology, mechanical properties, and specific surface area of heterocyclic para-aramid aerogel hollow fiber (HPAAHF) were systematically studied. The produced HPAAHF possessed excellent mechanical properties (tensible strength ~3.85 MPa), high specific surface area (~ 260.90 m² g⁻¹), and lightweight advantages. The thermal conductivity of HPAAHF was only 0.0278 W m⁻¹ K⁻¹, indicating its excellent thermal insulation properties. The aerogel fabric exhibited outstanding flame retardancy properties, with a total heat release of only 0.7 MJ m⁻² in the cone calorimetric experiment, making it a self-extinguishing fabric. In addition, phase change material was injected into the hollow structure to obtain aerogel-phase change material composite fibers, which exhibited great energy storage prospects. As a result, the high-performance heterocyclic para-aramid polymer-based aerogel hollow fiber was successfully prepared and had multifunctional applications in thermal insulation, flame retardancy, and heat energy storage fields.