High Thermal Insulation Research Articles

Nanofibrillar self-reinforced cyclic olefin copolymer composite foam with high toughness and thermal insulation

Thermally insulating and sustainable polyolefin foams play an important role in alleviating heat waste. Cyclic olefin copolymer (COC) has remarkable heat resistance and low thermal conductivity, which makes it an excellent candidate as a thermal insulative material. However, COC has critical drawbacks in its foam processing due to low melt strength, which limits its effective application as a thermal insulative material. Herein, we exploit nanofibrillar processing technology to enhance melt strength to develop highly tough and thermally insulating self-reinforced COC composites. The nanofibril network generated by drawing from the COC blends provides dramatic enhancements in the mechanical, rheological and final foam and thermal insulation properties. In particular, this process increases tensile toughness of COC composites by up to twenty-folds higher than neat COC. The results of foam process reveal the presence of COC fibrils in COC matrix improved volume expansion ratio as well as the cell density of the COC composites. Specifically, the expansion ratio of the foams reaches up to 11 with a cell density of 107 cell/cm3, two orders of magnitude higher than that of neat COC foams and thermal conductivity decreased from 0.062 to 0.033 W m−1K−1. As a proof of concept, this work provides a new insight of a self-reinforced approach to develop recyclable, highly tough, and thermally insulative foam.

Experimental Study on the Thermal Conductivity of Three Natural Insulators for Industrial Fishing Applications

Ecological materials have been implemented in different industrial sectors due to their good performance as thermal insulators and the fact that they are 100% natural, recyclable, and biodegradable, contributing to environmental sustainability. The main objective of this article is to compare the thermal conductivity coefficients of three natural insulators with that of expanded polystyrene (a non-biodegradable material). Expanded polystyrene is one of the materials which is most often used to maintain cold temperatures in containers built for this purpose in the fishing industry; it is used for this purpose because of its properties, including a light weight and a high thermal insulation capacity and resistance. Almost all insulators have the ecological disadvantage of being environmentally unfriendly materials because they are made up of oil particles, which are not recyclable and are harmful to ecosystems. The natural insulator materials were evaluated and subjected to a drying process to reduce the humidity coefficient; then, the containers were built with an adequate insulation thickness of 25 mm. Three filling tests were carried out (at 100, 70, and 50%) to evaluate the thermal conductivity, using the Mann–Whitney U statistical analysis process to determine insulator differences. The results show that the expanded polystyrene had the lowest thermal conductivity of 0.032 W/m K, followed by the rice husk, which had a value of 0.036 W/m K. Finally, a comparative study of conservation costs was carried out in the different containers built with the natural insulators; the lowest value found was for the expanded polystyrene (COP 159.57 around USD 0.040). This allowed to conclude that rice husk is the material that comes closest to the insulating characteristics of expanded polystyrene.

Multilayered mullite ceramics with anisotropic properties

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

The impact of thermal treatment on the mechanical properties and thermal insulation of building materials enhanced with two types of volcanic scoria additives

The need to substitute cement with eco-friendly building materials has increased in recent years, and attempts to enhance the mechanical and thermal insulation properties of these materials are ongoing. Therefore, the present study aims to use two different types of scoria as substitutes for cement in building materials and investigate the impact of thermal treatment on improving mechanical characteristics and thermal insulation. Black and red volcanic scoria, both before and after thermal treatment at different temperatures (600, 700, 800, and 900 °C), were utilized as cement substitutes in concrete specimens. Concrete specimens with different proportions (0–30 % wt.%) of black and red scoria were cured for different durations (14, 21, 28, and 91 days), and then tested for compressive strength. The compressive strength of specimens increased with increasing curing time, but decreased when scoria content exceeded wt.10 %. Furthermore, thermal treatment positively impacted the compressive strength of concrete specimens with red scoria, but negatively affected those with black scoria, owing to the increase in crystalline phases with increasing temperature. The specimen containing 10 % red scoria thermally treated at 900 °C and cured for 91 days yielded the highest compressive strength (60 ± 1.22 MPa). Further, the thermal insulation analysis of the concrete specimens with each type of thermally treated scoria was performed on day 28 of curing. The thermal insulation increased as the proportions of black and red scoria increased, which involved increasing the thermal treatment temperature of both scoria from room temperature to 900 °C on day 28 of curing. Additionally, the thermal insulation of concrete specimens treated with red scoria was more optimized than that of concrete treated with black scoria, particularly at high thermal treatment temperatures, and more than seven times as much as that of ordinary concrete. The lowest thermal conductivity value of the specimen was 0.157 ± 0.01 W m−1 K−1. Based on the findings, concrete with thermally treated red scoria is a suitable eco-friendly building material with high compressive strength and efficient thermal insulation properties.

A flexible double network aerogel reinforced by SiO2/ZrO2 fibers paper with excellent thermal insulation at high-temperature

Silica aerogel composites reinforced by fibers (SRF) have been used as thermal insulation walls to suppress lithium-ion batteries (LIBs) thermal runaway in electric vehicles. Increasing energy density of lithium-ion batteries brings out the requirement of higher temperature resistance and better thermal insulation performance of silica aerogel composites. Herein, we report a simple strategy for preparing aerogel reinforced by ZrO2/SiO2 fibers papers (AZSP) with double network structure for thermal insulation at high-temperature by conventional papermaking process and sol-gel method. The papermaking process made long fibers oriented perpendicular to thickness and opacifier ZrO2 fibers dispersed evenly in composites and mesoporous silica aerogels were filled in nanofibers skeleton. AZSP exhibited an excellent thermal insulation property at high-temperature with thermal conductivity of 0.039 W/(m·K) at 500 °C. The nanofibers reinforcement had been found to impart several desirable attributes to the material, including high flexibility, smooth surface suitable for stamping, painting, writing, or printing, and tensile stress of 0.74 MPa, thus rendering it an attractive option for diverse applications. Furthermore, the exceptional thermal stability of AZSP was demonstrated by the absence of any significant shrinkage deformation upon exposure to a butane flame. These remarkable properties, in conjunction with its high flexibility and thermal insulation performance, made AZSP a promising candidate for utilization as an insulation material for electric vehicle thermal management systems to mitigate the risk of thermal runaway.

Preparation of calcium silicate board from tobermorite-rich residue for energy conservation in buildings

The tobermorite-rich residue was derived from coal fly ash after alumina extraction via a hydrothermal process, which was used to prepare calcium silicate board for energy conservation in buildings. By employing a formulation comprising 94 wt% tobermorite-rich residue and 6 wt% pulp, the resultant calcium silicate board exhibited notable characteristics when subjected to a compression pressure of 20 MPa, including a bending strength of 6.75 MPa, a bulk density of 0.63 g/cm3, and a thermal conductivity of 0.14 W/(m·K). Tobermorite within the residue predominantly exhibited a fibrous morphology and formed porous aggregations, which proved advantageous in preparing calcium silicate board of high bending strength and good thermal insulation performance. Furthermore, the tobermorite retained its fibrous morphology even at elevated temperatures exceeding 650 °C, this thermal stability conferred excellent fire resistance performance upon the resulting calcium silicate board. Conclusively, the calcium silicate board prepared from tobermorite-rich residue demonstrated superior performances like good thermal insulation, exceptional fire resistance, light weight and high strength, etc., which would show great potential for use in buildings.

Investigation of novel expandable polystyrene/alumina aerogel composite thermal insulation material

Expandable polystyrene (EPS) foam is one of the most commonly used building thermal insulation materials due to its low thermal conductivity, apparent density as well and low cost. However, its high flammability and the release of a large number of toxic gases during combustion hinder its application. In this paper, using EPS foam and alumina aerogel (Al2O3) as insulation components, epoxy resin (EP) as a binder, and expandable graphite (EG) and ammonium polyphosphate (APP) as composite flame-retardant, a novel organic-inorganic composite thermal insulation material (EPS-AEA) was designed and fabricated. Experiment results showed that adding Al2O3 to EPS could further reduce the thermal conductivity of the obtained composite, and improve its mechanical properties. The addition of EG and APP composite flame-retardant could markedly enhance flame-retardant performance. With the addition of 4.17% Al2O3, and 50% EG/APP (1/1 mass ratio), the obtained EPS-AEA-1/1 composite showed superior fire resistance (reaching UL-94 V-0 rating and limiting oxygen index (LOI of 40.6%), high thermal insulation (thermal conductivity of 0.0487 W/(m·K)and applicable mechanical properties. In addition, EPS-AEA had excellent anti-hygroscopic properties (moisture ab-desorption rate within 0.5%) and fairly low toxic gas emissions during combustion, indicating its potential in practical application.

Constructing a highly vertically aligned network of h-BN/CF in silicone rubber composites: Achieving superior through-plane thermal conductivity and electrical insulation

Polymer composites with both high through-plane thermal conductivity (TC) and electrical insulation are desperately needed for the efficient thermal management of modern electronic devices. Herein, a highly vertically aligned network of hexagonal boron (h-BN) nitride/carbon fiber (CF) was constructed in silicone rubber composites via the combination of multistage stretching extrusion, layer-by-layer stacking and hot-pressing procedures. The composites exhibited superior through-plane TC and electrical insulation, simultaneously. The through-plane TC of the composites reached 9.5 W/(m·K) when the content of h-BN and CF was 120 phr and 5 phr, respectively. Meanwhile, the volume resistivity and breakdown voltage of the composites were 1.89 × 1013 Ω cm and 7.7 kV/mm, respectively, since the low content of CF was insufficient to form electrically conductive pathways. Furthermore, both simulation and experimental results indicated that the composites show outstanding heat dissipation performance. The composites with such hybrid networks are very promising for efficient heat dissipation in high-performance electronic devices.

Fabrication of organic based thermal resistance material using pyrolyzed cellulose nanofiber and carbon fiber for high performance thermal insulation

Herein, polymer-based thermal insulating composites are fabricated using pyrolyzed cellulose nanofiber (p-CNF) and carbon fiber (CF) in order to overcome the existing limitations of organic-based thermal resistance materials. The p-CNF is prepared by iodine treatment and calcination. The pyrolyzed CNF/CF composites are then fabricated via the freeze casting of two-dimensional (2D) CFs to obtain a low thermal conductivity of 0.058 W/mK, along with a high thermal stability, high tensile stress. Therefore, the p-CNF/CF composite provides an efficient thermal insulation performance via low thermal conductivity along with a high thermal degradation temperature, thereby demonstrating the potential for the development of thermal resistance materials.

Bioinspired Thermochromic Textile Based on Robust Cellulose Aerogel Fiber for Self-Adaptive Thermal Management and Dynamic Labels.

Aerogel fiber has emerged recently for incorporation in personal thermal management textiles due to its flexibility, scalability, and ultrahigh porosity, which allows the body to keep warm via thermal isolation without energy consumption. However, the functionalization and intellectualization of cellulose-based aerogel fibers have not yet been fully developed. Herein, we propose a biomimicking design inspired by polar bear and Siamese cat hair that combines porous cellulose aerogel fiber (CAF) with reversible thermochromic microcapsules to mimic biological sensory and adaptive thermoregulation functions. The produced CAF has a controllable pore structure, a large specific surface area (230 m2/g), and excellent mechanical strength (∼15 MPa). Low-temperature darkening microcapsules have been incorporated into the robust CAF to spontaneously adjust color by perceiving the ambient temperature. The functional aerogel fiber fabric achieves high thermal insulation and photothermal modulation simultaneously at temperatures below 18 °C. The temperature of the thermochromic fabric was higher by 6 °C than that of the sample without the microcapsules at a light intensity of 0.2 W/cm2. In addition, the aerogel fibers mixed with two types of thermochromic microcapsules exhibit three color switches with fast response, a color-control precision of 0.2 °C, and good cycling performance. This smart aerogel fibers hold great promise for self-adaptive thermal management, temperature indication, information transfer, and anticounterfeiting in textiles.

High thermal conductivity electrical insulation composite EP/h‐BN obtained by DC electric field induction

AbstractEpoxy resin, as an adhesive for the main insulation system of large generators, has excellent insulation properties. But its poor thermal conductivity limits the capacity enhancement of large generators. Forming thermally conductive pathways by using electric field to induce high thermal conductivity fillers in the polymer can effectively improve the thermal conductivity of the composite at low loading condition. In this article, a DC electric field is used in epoxy resin (EP) to induce two‐dimensional inorganic fillers h‐BN deflection and head‐to‐tail lap, which can make the heat flow transfer along the in‐plane of the particles and contribute to the more efficient thermal conductivity network. A high thermal conductivity of 0.544 W/(m K) was achieved at a low load of 10 wt%, which has an improvement of 289% compared to pure EP (0.14 W/[m K]) and 130% compared to the randomly dispersed composite with the same load. The composites have excellent thermal conductivity as well as insulating properties, among which volume resistivity is 1.46 × 1012 Ω/cm, increasing by 61.4% compared to pure EP. This method is easy to be carried out in engineering and has great possibilities for the main insulation of large generators and even for other applications with high thermal conductivity electrical insulation.Highlights The structure of composites was adjusted by direct current electric field. The high in‐plane thermal conductivity of h‐BN is fully exploited. The relationship between deflection angle of BN and thermal conductivity of composites was built.

High-performance flexible thermoelectric generator based on silicone rubber and cover with graphite sheet

Harvesting thermal energy through a flexible thermoelectric generator (FTEG) offers an excellent micro-power solution for energizing node sensors in the realm of Internet of Things (IoT) and wearable electronics. Nonetheless, current FTEG suffer from drawbacks including low efficiency, significant thermal resistance, and complex manufacturing procedures. In this study, a high-performance FTEG using silicone rubber was designed and fabricated using a straightforward process. The finite-element method was used to optimize the copper electrode thickness, and the bendable substrate layers with various thermal conductivity were studied for the first time. The copper electrode thickness of 0.1 mm was selected because it offers high flexibility and bendability while still providing a relatively high-power output. The 5 × 5 cm2 FTEG device was fabricated and covered with a bendable substrate. Silicon rubber (0.08 Wm−1K−1), silicon rubber added 5% graphene (0.14 Wm−1K−1), and graphite sheets (15 Wm−1K−1) were used as bendable substrates. The FTEG cover with graphite sheets has a maximum output voltage of 1.1 V under a temperature difference (ΔT) at 65 °C. Its maximum output power is 162.4 mW, corresponding to a power density of 6499.1 µW/cm2 under the same above ΔT. The experimental findings indicated that integrating a bendable substrate with high thermal conductivity and electrical insulation properties enhances the performance of the FTEG.

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

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

Investigation on the thermal insulation and evaporative resistance of outdoor backpacks based on a sweating thermal manikin

The thermal insulation and evaporative resistance of backpacks are vital for users’ physical performance and safety. In hot humid weather, users carrying a backpack with high thermal insulation and evaporative resistance may face the risk of heat stress, such as heat stroke. However, few studies have investigated the thermal insulation and evaporative resistance of backpacks. In this study, a sweating thermal manikin was employed to evaluate the thermal insulation and evaporative resistance of backpacks. Four backpack samples with different back panel designs were tested under two load conditions (0 and 5 kg), two wind speeds (0.3 and 1.5 m/s), and two body movement levels (inactive 0 km/h and walking at a 2.5 km/h speed). Test results showed that the back panel designs fundamentally determined the thermal insulation and evaporative resistance. Both the air-permeable design of the back panel’s filling material and the ventilation design of the back panel structure could improve the heat dissipation performance of the backpack. However, the ventilation designs that formed tunnels with openings between the backpacks and the human body showed more efficient heat dissipation. In practical application, the resultant thermal insulation and evaporative resistance were affected by the load condition, wind speed, and body movement. However, backpacks with better ventilation were less affected by external conditions. The findings of this study provide fundamental knowledge to improve the heat dissipation property of outdoor backpacks.

Effects of carbonation on the compressive strength of autoclaved aerated concrete with different Ca/Si ratios

Autoclaved aerated concrete (AAC) is widely used for its lightweight, high thermal insulation, and superior machinability. However, its loose and porous microstructure can cause a significant decrease in compressive strength after carbonation and the mechanism of the deterioration remains uncertain. This study delved into the alteration trends of AAC's compressive strength and dry density within carbonation environments. Additionally, a variety of testing techniques were employed to elucidate the microscopic mechanisms. The results showed that when the calcium/silicon ratio (Ca/Si ratio) stood at 0.75, AAC exhibited its highest degree of resistance against carbonation. Microscopic analysis revealed a progression in hydration product characteristics as the Ca/Si ratio increased. The transformation journey encompassed a shift from the initial structure to a needle-like tobermorite configuration and, subsequently, to a flaky structure. The study offers invaluable insights that furnish guidance for enhancing the durability of AAC and broaden the horizons of its potential applications.

Tuning the Degree of Protonation to Prepare Ultra-light and High Thermal Insulation Mullite Nanofiber Aerogels

Ceramic fiber aerogels play an important role in thermal insulation due to their high specific surface area and porosity. They have great potential in aircraft, the military industry, and metal smelting. Although traditional ceramic fiber has the advantages of high-temperature resistance, its brittleness, tensile resistance, and high production cost, greatly limit the application of ceramic aerogels. Using the sol-gel method and electrostatic spinning technique, we controlled the fibers’ random winding and mechanical properties by controlling the protonation of the spinning fluid. We prepared ultralightweight, flexible, and highly insulated mullite nanofiber aerogels with a volume density as low as 7 mg / cm3, thermal conductivity as low as 27 mW m−1 K−1, excellent thermal stability at 1,300 °C, high thermal insulation and retention of up to 50% of the ceramic fiber after initial compression. This material’s fabrication process and excellent insulation properties propose new opinions on the lightweight and durable insulation materials needed for aerospace and military insulation.

Facile synthesis of hollow SiC/C nanospheres for high-performance electromagnetic wave absorption

Hollow silicon carbide/carbon (SiC/C) nanospheres have found wide applications owing to their excellent dielectric properties, high surface area, thermal insulation, and low effective density at high temperatures. However, their hollow structure is typically obtained by removing the templates using strong acids, which can lead to massive waste and severe pollution. Herein, a three-layer nanospheres containing resorcinol-formaldehyde (RF) and SiO2 (RF@SiO2@RF) were first prepared by a simple one-step method and used as precursors. Subsequently, monodispersed hollow SiC/C nanospheres (90 ± 5 nm in diameter) prepared in-situ through carbon thermal reduction. The hollow structure was formed by pyrolysis, eliminating the extra procedure to remove templates. The structure, size, and composition of the nanospheres were precisely controlled via synthesis temperatures (1400, 1450, 1500 °C) and using different raw material ratios. The obtained hollow SiC/C nanospheres demonstrated remarkable resistance to high temperature resistance. The specific surface area of the optimised hollow SiC/C nanospheres reached 301.9 m2 g−1. Their reflection loss (RL) value achieved −41.34 dB at 14.58 GHz, and the effective absorption bandwidth(EAB)was 4.0 GHz (12.77–16.78 GHz) with a thickness of 1.70 mm. The results of this study confirm that the proposed method allows for an in-situ synthesis of hollow SiC/C nanospheres for high temperature microwave absorption with low matching thickness.

Fabrication of flexible polyimide/rGO composite foams and their derived carbon foams for thermal insulation and EMI shielding applications

In this work, a facile microwave-assisted foaming and thermal imidization treatment method was proposed to fabricate flexible polyimide (PI)/rGO composite foams with anisotropic pore structures. The PI/rGO composite foams (PIF/rGO) were used as the starting derivatives to prepare carbon foams that demonstrated excellent anisotropic thermal insulation and electromagnetic interference (EMI) shielding applications. The directional growth of pore structures during the foaming period resulted in the formation of aligned ellipsoidal strip-like pore structure in the horizontal direction (i.e., perpendicular to the pore growth direction), which was attributed to the escape of foaming gases such as tetrahydrofuran, methanol and H2O. The homogeneous dispersion of rGO in the PI-derived carbon skeleton endowed corresponding carbon foams with excellent mechanical and thermal stability, high thermal insulation and electrical conductivity (σ) as well as absorption dominated EMI shielding performance. The σ and EMI shielding effectiveness (SE) of carbon foams showed an increasing trend with increasing rGO content. The σ, EMI SE and specific EMI SE of the carbon foams reached up to 33.0 S/m, 49.0 dB and 8792.8 dB/(g/cm2) in the horizontal direction, respectively. Therefore, this work provided a facile strategy to fabricate lightweight and anisotropic pore structure rGO-containing PIFs and carbonized derivatives, which demonstrated promising applications in high-end industrial fields.

A polar polyimide as multifunctional flame retardant for epoxy resin through constructing intimate 3D interpenetrating polymer network

With the rapid development and application of 5G technology, design and fabrication of lightweight polymer materials with excellent flame retardancy, low dielectric constant, and high thermal insulation is a worthy but great challenge. In this study, a polar polyimide (PI) contain amide group was designed and incorporated into epoxy resin (EP) to fabricate multifunctional materials combing a small molecule named phenoxycycloposphazene (HPCTP). Due to the synergistic effect of PI and HPCTP in flame retardant, EP blend passed UL-94 V-0 rating only needing 3 wt% PI and 1 wt% HPCTP, meanwhile the limited oxygen index (LOI) obviously increased from 23.8% to 35.0%. More importantly, the dielectric constant obviously decreased at a wide range from 10−1 to 107 Hz and the thermal conductivity also reduced by more than 45% from 0.236 to 0.127 W m−1 K−1. Due to the good compatibility and reactivity between PI and EP, PI was uniformly dispersed and connected EP to construct intimate 3D interpenetrating polymer network, and thus the tensile and flexural properties were slightly enhanced. Moreover, combustion parameters, including peak heat release rate (PHRR), total heat release (THR), smoke produce rate (SPR) and total smoke production (TSP), all indicated that EP composites significantly reduced fire risk. Overall, this study offers a promising strategy to fabricate flame retardant EP composites with outstanding comprehensive performances.

Improved creep resistance of AZ91 magnesium alloy after the micro arc oxidation process

AZ91 Mg alloy has a wide range of applications in the automotive industry, although its use is restricted to powertrain applications due to its low creep resistance. In this study, the effect of the micro arc oxidation (MAO) coating on the creep resistance of an AZ91 Mg alloy was investigated to take advantage of the coating layer with high thermal insulation properties. In this context, the MAO process was applied to AZ91 Mg alloy using a bipolar pulsed DC power supply. The creep tests were conducted at different temperatures (150–200 °C) and stresses (25–90 MPa) for the bare and coated samples, and the minimum creep rates were determined. It has been shown that the MAO coating reduces the creep rate of the bare alloy by 35%–84% depending on the temperature and the stress due to the stress-reducing effect and thermal barrier properties of the MAO coating. Based on the calculated creep activation energy and stress exponents, creep mechanisms were proposed for the bare and coated alloys. Effective activation energy was also calculated and lattice diffusion-controlled dislocation climb was determined to be the effective creep mechanism for both samples at lower stresses, while pipe diffusion-controlled dislocation climb was effective at higher stresses.