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Lightweight phenolic matrix composites are highly potential in ablative thermal protective materials for future aerospace vehicles, and hybrid modification is considered as one of effective ways to improve the oxidation and ablation resistance of phenolic matrix. Herein, the co-continuous silica zirconium hybrid phenolic resin (SZRx) was synthesized, which can be used to prepare lightweight ablation resistant aerogel composites by the sol-gel method. The SZRx aerogel showed stable porous structure and excellent hydrophobicity, and the silicon zirconium hybrid greatly improved the high temperature carbon residue and oxidation resistance of phenolic resin. The SZRx aerogel composites also possessed excellent integrated characteristics of heat protection and insulation, with a linear ablation rate as low as 0.032 mm/s and a back temperature below 200 °C. The synergistic effect of silicon and zirconium formed the multiphase ceramic layer (C, SiO 2 , SiC and ZrO 2) with the sea-island-like structure on the surface of the composite material, which can effectively resist high-temperature ablation and aerodynamic exfoliation. This can indicate that silicon zirconium dual element hybrid phenolic resin is the highly competitive matrix for lightweight and ablation resistant composite materials. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, a type of red mud/slag-based cenospheres/geopolymer foam was directly fabricated using the direct foaming method at room temperature. The effects of high-temperature exposure (700–1100 °C) on the phase evolution, pore structure, and properties of the foams were investigated. The results showed that the untreated foam was amorphous structure. The porosity of the foams increased from 36.4 ± 0.6% (1 wt%) to 71.9 ± 0.1% (5 wt%) with the addition of H 2 O 2. The compressive strength of the foams was 1.20–18.15 MPa, which was attributed to the good bond between the cenospheres and red mud/slag-based geopolymer matrix, the fractured spheres, and the crack deflection. After this high-temperature exposure, the foam crystallized mainly into the leucite and mullite phases. The density and compressive strength of the foams decreased as the exposure temperature increased. The thermal conductivity of the foams reached the lowest value of 0.169 W/(m⋅K) and 0.108 W/(m⋅K) with 5 wt% H 2 O 2 before and after the exposure, respectively. These foams have demonstrated good potential for application in the building insulation field. [ABSTRACT FROM AUTHOR]

The porous SiC-Mullite porous ceramics with different pore size were prepared by SiC particles, kaolin, Al(OH) 3 , and flake graphite, V 2 O 5 and AlF 3 were used as mineralizers, the pore size was controllable by adjusting the size of flake graphite. Then the improved empirical formula and grey theory were used to study the effect of the change in pore size on the material properties. The results showed that with the increase of mean pore size from 1.1 μm to 8.5 μm, the cold modulus of rupture and cold crushing strength decreased by 20.8 MPa and 13.7 MPa, respectively. And the thermal conductivity increased from 1.213 W⋅m−1⋅K−1 to 1.721 W⋅m−1⋅K−1 at 800 °C. The <5 μm pores were helped enhance the strength and decreased the thermal conductivity. [ABSTRACT FROM AUTHOR]

Si 3 N 4 -TiN composites have extensive manufacturing applications because of their first-rate mechanical and thermal properties. Conversely, they are very demanding on machines that use conventional practices because of their superior strength and hardness. Electrical discharge machining is a viable substitute for machining of Si 3 N 4 -TiN composites through electrical erosion. However, very few studies have been conducted on the machinability and surface characteristics of electrical-discharge-machined composites. This study focuses on the materials processing characteristics of a Si 3 N 4 -TiNcomposite using rotary EDM and die-sinking EDM performed with a Cu electrode. In addition, the surface and cross section of the subsurface of the composite were examined, along with energy dispersive X-ray analysis, to identify the elements present on the machined surface. It was found that the high level of hardness and low level of toughness of the Si 3 N 4 -TiN composite contributed to the presence of microcracks, globules, pores, recast layers, and craters on the EDMed surface. The recast layer thicknesses of the die-sinking EDM (optimum hole, 106 μm; best hole, 151 μm; worst hole, 166 μm) and rotary EDM (optimum hole, 53 μm; best hole, 55 μm; and worst hole, 58 μm) were examined. In addition, the presence of pores and ablation increased significantly during the rotary EDM process. Rotary EDM parameters capable of producing a minimum white layer thickness were also investigated. Finally, the porosity levels were estimated according to the number of pores using ImageJ software. For the die-sinking EDM, the estimated porosity values for the optimum, best, and worst holes were 14.989%, 19.525%, and 21.706% respectively and for the rotary EDM, these values were 24.873%, 33.04%, and 37.99% respectively. [ABSTRACT FROM AUTHOR]

The porous silicon carbide ceramics with multilevel directional pores were prepared by chemical vapor infiltration (CVI) combined with sacrificial template method. Carbon fiber preform with specific structure was used as template, and silicon carbide was deposited by CVI process. Then, the carbon fiber template was oxidized and removed, so that the porous silicon carbide ceramics were obtained. The influence of deposition time on pore structure, mechanical properties and thermal properties was studied. Compared with the traditional pore-making process, the pores formed by sacrificial template method are more regular, which brings excellent mechanical properties and thermal insulation properties. The porosity and flexural strength of the sample deposited for 72 h can reach 66.00% and 67.52 MPa, respectively, the thermal conductivity can be kept below 1 W/(m·K) from room temperature to 1200 °C. [ABSTRACT FROM AUTHOR]

This article aims to illustrate the influence of the composite micro-powder of α-Al 2 O 3 and zirconium basic carbonate (ZBC) on the microstructure, mechanical and thermal properties, and slag resistance of corundum castable. The results show that the addition of the composite micro-powder to corundum castable sintered at 1550 °C optimized its pore structure, which presented an apparent porosity of 16.8% and a slump in pore size from 10.80-45.72 μm to 0.36–1.83 μm. The ZrO 2 dissolving from ZBC combined with corundum matrix to form "intergranular" and "intragranular" submicron structures, which promoted the densification of matrix structure and improved the strength of castable by 25.8%–42.9%. In terms of slag resistance, castable with composite micro-powder saw an absorption of liquid slag in micropores and a Cr-rich complex spinel blocking layer with high melting point during slag erosion. Moreover, the in-situ ZrO 2 could enter slag to boost its viscosity, thus inhibiting the penetration capacity. Consequently, the penetration and erosion ratios of castable reduced by 6.32% and 83.99%, respectively. [ABSTRACT FROM AUTHOR]

Flexible pressure sensors embody outstanding mechanical and sensing performance, particularly in the fields of personal health monitoring and artificial intelligence, while catering to multifunctional applications resilient against harsh weather conditions. Among these, Ti 3 C 2 T x MXene-based aerogel materials with high surface area and excellent conductivity have attracted a lot of attention. However, there are big challenges in improving the mechanical properties at low densities and designing multi-level pore structures. Herein, the prepared silane monomers (MP) are firstly synthesized by 3-(Trimethoxysilyl) propyl methacrylate (MPTES) and polyethylene glycol diacrylate (PEGDA), then polydopamine-modified silicon dioxide (PDA-SiO 2) nanofibers employed with MXene nanosheets form covalent cross-linking nanofiber aerogels. The multi-level stable structure improves both mechanical strength and elasticity. The assembled MXene/MP/PDA-SiO 2 pressure sensors exhibit a high sensitivity of −0.83 kPa−1, a rapid response time of 48 ms, a minimum 1 % strain detection limit, and reliable stability over 10,000 cycles. Furthermore, these sensors can be used for human motion detection and health diagnostics, and show good personal thermal management performance. This work holds a promising candidate for the next generation of multifunctional flexible electronic devices. [ABSTRACT FROM AUTHOR]

BaZrO 3 with the ABO 3 perovskite structure can be doped at the A or B sites to obtain the corresponding properties. In this study, BaZrO 3 was doped with Ca2+, Sr2+, Ti4+, and Ce4+. The structure, phase composition, and mechanical and thermal properties of the composites were investigated. The sintered Ba 1-x Ca x ZrO 3 samples with x = 0.2 and 0.4 exhibited relatively superior comprehensive behaviours; the cold modulus of rupture was increased by 127.36% and 134.59%, while that for BaZr 0 · 8 Ti 0 · 2 O 3 and BaZr 0 · 8 Ce 0 · 2 O 3 was increased by 99.30% and 112.37%, respectively. Further increases in strength and density were obtained in the Ba 0 · 8 Ca 0 · 2 Zr 0 · 8 Ti 0 · 2 O 3 and Ba 0 · 6 Ca 0 · 4 Zr 0 · 8 Ti 0 · 2 O 3 co-doped samples, and the corresponding apparent porosities were greatly decreased from 30.6% to below 1.0%. The thermal conductivities of the doped samples were generally low, achieving a minimum of 0.361 W/(m·K). The combination of high strength and low thermal conductivity demonstrate the significant application potential of BaZrO 3 -based composite materials. [ABSTRACT FROM AUTHOR]

Silver nanoparticles (AgNPs) are grown uniformly on the surface of white porous boron nitride nanofibers (BNNFs) via a thermal reduction method to overcome the issue of nanoparticle aggregation. The optical characteristics of the AgNPs are retained, with different colors present depending on reduction temperature. This mainly depends on the white appearance of BNNFs. In addition, compared to boron nitride (BN) nanosheets and nanospheres, abundant pore structure and high specific surface area of the BNNFs enable superior AgNPs loading without agglomeration. The prepared AgNPs-loaded BNNFs (Ag-BNNFs) are then incorporated into poly (vinyl alcohol) (PVA) hydrogels fabricated through cyclic freeze-thaw techniques to develop antibacterial Ag-BNNFs/PVA composite hydrogels. The addition of 3 wt% Ag-BNNFs increases the tensile strength by 73.1% and compressive strength by 92.8% relative to the unmodified PVA hydrogels. Thermal stability also improves considerably, with the maximum PVA decomposition rate shifting from 280.9°C to 323.6°C in the composite hydrogels. Importantly, a potent bactericidal effect against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) are achieved, with 99.5% and 100% inhibition after 24 h exposure to the Ag-BNNFs/PVA composite hydrogels, respectively. This work demonstrates a new method of dispersing AgNPs with BNNFs, and the resulting Ag-BNNFs effectively enhance the mechanical properties, thermal stability and antibacterial activity of PVA hydrogels. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Spherical powders containing Si with tailorable porosity were originally obtained. • Prepared micro-size porous samples consisted of nano-sized channels for reaction. • The energetic kinetics of prepared fuels outperformed Al nanoparticles. • Discrepant reaction mechanism for AlSi@pSi at different circumstances was found. Energetic porous fuels are attractive for their high reactivity. In this study, porous silicon/aluminum–silicon (pSi/AlSi@pSi) powders were prepared via dealloying Al-Si alloy. Results demonstrate that pSi and AlSi@pSi with evenly distributed pores and tailorable porosity were obtained. AlSi@pSi shows a core–shell structure with Al-Si alloy core and porous Si shell. Thermal properties indicate that consistent with its core–shell structure, the oxidation of AlSi@pSi can be considered as a combination of pSi and Al-Si alloy. The main oxidation is kicked in by the reaction of the Al-Si alloy core, followed by the intense reaction of the porous Si shell. The combustion characterization demonstrates that the dramatically improved reactivity of porous samples is attributed to the ultra-high kinetics of porous Si outperforming nano-sized aluminum (n-Al). The shorter ignition delay and combustion duration of porous samples than Al-Si alloy and n-Al result from the lower apparent activation energy (E 0) and the low initiation oxidation temperature for porous Si. The different oxidation processes for AlSi@pSi under thermal oxidation and combustion owe to the high oxidation peak temperature and low E 0 of porous Si. Above all, the prepared porous fuels with tailorable porosity and superior reactivity than n-Al are promising fuels in energetic materials. [ABSTRACT FROM AUTHOR]

Monolithic polymer-derived SiC is highly desirable for high temperature applications because of its unique thermal properties in extreme conditions. The current study focuses on the formation, evolution, and stability of polymer-derived SiC at different high temperatures. SiC-based monoliths were pre-pyrolyzed at 1200 °C and then pyrolyzed at 1600 - 2500 °C in a flowing argon atmosphere. The porosity, density, compositions, and phases of the SiC samples were studied. Dense SiC monolith was obtained from 1200 °C to 2200 °C. SiC crystallized at 1900 °C and formed a near stoichiometric composition. β-SiC remained stable even at 2200 °C. However, the SiC monolith experienced continuous mass loss above 1900 °C and disintegrated at 2500 °C with ∼89.5% mass loss. This work opens a pathway for preparing near stoichiometric and dense SiC monoliths through polymer precursor pyrolysis; it also provides fundamental understanding of polymer-derived SiC at different high temperatures. • Dense SiC monolith was prepared by crosslinking and pyrolysis of SMP-10 precursor. • SiC formed at 1900 °C was near stoichiometric. • Pyrolysis at 2200 °C initiated the sublimation of 'Si' and led to excessive carbon. • Pyrolysis at 2500 °C led to disintegration of SiC and a porous graphitic structure. • The work offers a complete map of polymer-derived SiC formation and decomposition. [ABSTRACT FROM AUTHOR]

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/cm3, 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. [ABSTRACT FROM AUTHOR]

Pore structure is one of the major factors influencing the failure mechanisms of thermal barrier coatings (TBCs) during high-temperature service. Powders exhibiting different pore structures have been produced using the plasma spheroidization technique. To improve the strain tolerance of an aircraft engine's hot sections, the preparation method of the TBCs material—8-wt% yttria-stabilized zirconia powders—has been continuously optimized. In this paper, the constrained effects of pores on the ceramic top coating of plasma-sprayed lamellar TBCs were investigated. Furthermore, the crystal phase structures, thermophysical properties, porosity, microstructures, and synthesis mechanism of powders and coatings were analyzed. Results revealed that the top ceramic coating, which was made from powders cooled by air and distilled water, can influence the coating porosity. Moreover, thermal insulation was substantiated by the finite element analysis method. Consequently, dense powders cooled by water can be used on the top surface in direct contact with high temperature flame flow to resist the diffusion of oxides, and the porous powder cooled by air can be applied in the middle coating to decrease the thermal conductivity. Hence, TBCs with various layers of different porosities can prolong the service life of thermal barrier coatings. [ABSTRACT FROM AUTHOR]

KLS-1 Lunar regolith simulant was microwave sintered to explore its potential applicability in future lunar construction. The effects of sintering temperature on linear shrinkage, density, porosity, and microstructural, mechanical, and thermal properties were investigated. As the sintering temperature increased, linear shrinkage and density increased and porosity decreased. Structural evolution in the sintered samples was characterized by scanning electron microscopy and X-ray diffraction. Unconfined compressive strength testing showed that mechanical strength increased significantly with increasing sintering temperature, with 1120 °C giving the highest strength of 37.0 ± 4.8 MPa. The sintered samples exhibited a coefficient of thermal expansion of approximately 5 × 10−6 °C−1, which was well-maintained even after cyclic temperature stress between −100 and 200 °C. Therefore, this microwave processing appears promising for the fabrication of building material with sufficient mechanical strength and thermal durability for lunar construction. [Display omitted] [ABSTRACT FROM AUTHOR]

In this investigation, we use pulsed thermography and a multilayer analytical model to critically evaluate the thermal diffusivity and inherent porosity in cold-sprayed Ti-6Al-4V coatings on Al6061-T6 substrates. These coatings are of importance in deploying cold spray as an additive manufacturing technique for the restoration of high-cost alloy components. Our study draws upon experimental data obtained at spray angles from 50°to 90°, illustrating the relationship among the spray angle, porosity within the coating, and the resultant thermal diffusivity. Our results reveal a decrease in porosity and an increase in thermal diffusivity as the spray angle increases from 50°to 90°. In conjunction with our experimental approach, a numerical model was developed, incorporating binary images derived from optical micrographs to simulate transient heat diffusion during pulsed thermography experiments. The congruence between our empirical findings and simulation data affirms the effectiveness of the applied methodology, shedding light on the complex interplay between processing parameters and the resulting thermal properties. This study explores the effects of porosity and thermal gradients introduced during pulsed thermography, providing a profound understanding of the thermal dynamics of cold-sprayed coatings. [Display omitted] • Spray angle affects the porosity and thereby the thermal trait • Microscopic images-based FEM models deepen heat diffusion understanding • Porosity insights enhance cold-sprayed coating thermal dynamics [ABSTRACT FROM AUTHOR]

Silver (Ag) paste sintering is used as a die-bonding technology for the next generation of power electronic modules as Ag offers a high melting temperature as well as excellent thermal and electrical conductivities. Ag paste sintering is performed using specific conditions depending on the type of paste and the users' specifications (temperature, time, pressure), ending up in a porous joint. As a result, the properties of those joints are heavily influenced by their densities (i.e. porosity). Despite the numerous studies reported in the literature, the relationship between the density of the joint and its thermal conductivity remains an issue since access to the density of an embedded thin joint is very challenging. Furthermore, little is known on the evolution of the thermal conductivity in operating conditions. In this study, these issues were investigated by developing self-standing specimens with microstructures representative of those of real joints. In order to study a wide range of porosity, sintering was performed using a single time/temperature under various pressures. The thermal conductivity was measured using 3D flash method, consisting in applying a short non-uniform laser excitation on the surface of the sample, leading to three-dimensional heat transfer. The relationship between porosity and thermal conductivity is established for the as-sintered state and after aging up to 500 h at temperatures ranging from 150 °C to 350 °C and is compared to existing models. The evolution of the thermal properties during thermal aging is discussed considering both the elaboration conditions and the microstructure evolution. [Display omitted] [ABSTRACT FROM AUTHOR]

Two types of eco-friendly shape-stabilized phase change materials (SSPCMs) were developed in this study. Biobased PCMs were impregnated into the pore structure of recycled expanded glass granules and coated with silica fume particles using a 4-step vacuum impregnation method. The morphological and thermal properties of the fabricated SSPCMs along with their effects on the hydration, microstructural, mechanical, and thermal properties of cement-based composites were characterized through a set of multiphysics tests including isothermal calorimetry, compressive and tensile strength, capillary water absorption, 3D image analysis of μCT scans, and thermoregulating tests. It was found that coating PCM-integrated EG granules with porous silica fume particles can further mitigate the risk of leakage and increase the impregnation capacity of SSPCM composites, thus attaining desirable latent heat capacity. Furthermore, the incorporation of the developed environmentally friendly SSPCMs into concrete can maintain desirable mechanical strength, while achieving considerable thermal energy storage capacity, which can moderate indoor temperature fluctuations, thus yielding substantial HVAC energy savings. • Leak-free ecofriendly biobased SSPCMs with high latent heat capacity successfully fabricated. • TES cement-based composites incorporating SSPCMs developed. • SSPCM inclusion can increase thermal inertia and reduces indoor temperature fluctuation. • SSPCM-integrated cementitious composites can yield significant energy savings in buildings. [ABSTRACT FROM AUTHOR]

A very large amount of fly ash is generated from coal combustion, and the high value-added and efficient utilization conversion of this solid waste into a resource remains an issue to be solved. Among the many methods of waste resource utilization, the preparation of porous geopolymers is an effective way to improve the added value of fly ash. In this review, the processes for obtaining fly ash-based porous geopolymers are divided into two categories: bulk-type (direct foaming, sacrificial template, additive manufacturing, and fast microwave foaming method) and sphere-type (suspension solidification, direct molding, and calcium chloride solution gel method). The synthesis routes, main characteristics (pore structure, porosity), and performance (mechanical and thermal properties as well as adsorption etc.) of fly ash-based porous geopolymers are reviewed, and examples of applications such as heat insulation, filtration, sound absorption, and pH buffering are presented. Moreover, the future development prospects of this field are discussed. [ABSTRACT FROM AUTHOR]

We performed numerical heat conduction analysis for the new proposed functionally graded porosity (FGP) media. Fourier, Cattaneo-Vernotte (C V), dual-phase lag (DPL) and time-fractional heat conduction responses for a porous solid-gas media are compared with the different flux pulse duration experimental results. Four configurations of the FGP domain in which the porosity gradations are controlled by the power-law volume fractions, are introduced. Accordingly, the porosity-dependent effective thermal properties of the domain are estimated and the temperature responses are discussed for different heat conduction models. Even a slight change in the porosity, leads to a different temperature-time history and its gradation affects the temperature distributions through the medium; therefore, by tuning the porosity gradations the desired temperature distributions can be achievable. Finally, the diffusion-like DPL and the classical Fourier models in descending configuration predict about 35% and 41% higher peak temperatures than the constant porosity ones, respectively. Furthermore, these models demonstrate the same temperature distributions in constant porosity; that is, in the case of graded porosity the maximum difference is less than 4% due to not taking into account the time laggings as the function of porosity gradations. • The transient and steady-like DPL model proves the highest accuracy in comparison with experimental data. • Four functionally graded porosity configurations have been proposed. • Fourier, C V, DPL temperature responses have been compared for the FGP media. • The diffusion-like DPL and Fourier models are identical for the constant porosity. [ABSTRACT FROM AUTHOR]

Lightweight and multiscale nanocomposites are considered to be the most promising materials applied in the field of advanced aerospace and modern building industry, owing to their outstanding mechanical, thermal and flame-resistant properties served in high-temperature oxidizing environments. Herein, a novel siliconoxycarbide (SiOC) modified needle quartz fiber felt reinforced phenolic resin nanocomposite (SiQF/PR) with a hierarchical macro-/micro-/nano-scaled structure is reported. The as-prepared highly porous SiQF/PR nanocomposite exhibits a multiscale three-dimensional network and the micro-sized SiOC particles, derived from the hydrolysis of methyltrimethoxysilane (MTMS) and dimethyldiethoxysilane (DDS), are uniformly incorporated into the phenolic aerogel. The resultant nanocomposite has a bulk density of 0.30–0.35 g/cm3, and the maximum compressive strength reaches 4.20 MPa and 3.34 MPa in xy and z directions, respectively. Thermal stability and oxidation resistance experiments show that the SiQF/PR multiscale composite has a good ablation and heat insulation performance under oxyacetylene test condition, and the backside temperature of SiQF/PR is only 55.5 °C under 1.8mw/m2 for 120 s, and line ablation rate of SiQF/PR multiscale nanocomposite decreases by 20.1% at 1.8 MW/m2 and 59.5% at 0.4 MW/m2 compared to non-SiOC modified composite. These results indicate that the lightweight SiQF/PR nanocomposite fabricated by this method is very suitable for architectural or aerospace applications as high-performance insulation material. [Display omitted] [ABSTRACT FROM AUTHOR]