Core–shell spherical graphite@SiC attenuating agent for AlN-based microwave attenuating ceramics with high–efficiency thermal conduction and microwave absorption abilities

A core–shell structured spherical graphite (SG)@SiC attenuating agent with a tunable silicon carbide (SiC) shell thickness was synthesized via in-situ solid-liquid reaction of SG and Si. Then, fully dense 10 wt%SG@SiC/AlN microwave attenuating composite ceramics were prepared through hot-pressing sintering, and the morphology of SG@SiC particle was well maintained. By moderately modulating the thickness of the SiC shell with relatively low complex permittivity and thermal conductivity, an effectively inhibited solid solution of SiC into AlN, weakened dipole and electron polarization, enhanced conduction loss, and an improved impedance matching, thermal conductivity and microwave loss capacity were simultaneously achieved. Thus, the SG@SiC/AlN composite exhibit excellent and impressive thermal conductivity of 63.92 W m⁻¹·K⁻¹ and minimum reflection loss of −34.2 dB. The outstanding performance of SG@SiC/AlN composite indicates that the composite is promising microwave attenuating ceramic with excellent thermal conduction and microwave absorption ability. This work opens up a new core–shell structure strategy for designing and developing a high-efficiency attenuating agent and microwave attenuating ceramic.     

Facile synthesis of MgO–Mg2SiO4 composite ceramics with high strength and low thermal conductivity

The recycling of solid waste is a win-win solution for humans and nature. For this purpose, magnesite tailings and silicon kerf waste were employed to prepare MgO–Mg₂SiO₄ composite ceramics by solid-state reaction synthesis in the present work. Then, effects of sintering temperature and raw material ratio on as-prepared ceramics were systematically studied. As-prepared ceramics showed improvement in their relative density (from 47.55%–68.12% to 90.96%–95.25%) and cold compressive strength (from 7.34–118.66 MPa to 303.39–546.65 MPa) with the increase in sintering temperature from 1300 to 1600 °C. In addition, it was found that Si promoted synthesis process of Mg₂SiO₄ phase through transient liquid phase sintering and Fe₂O₃ accelerated sintering process through activation sintering. Consequently, the presence of Mg₂SiO₄ phase effectively improved the density and strength of MgO–Mg₂SiO₄ composite ceramic, while reducing its thermal conductivity. This work provides a potential reutilization strategy for magnesite tailings, and as-prepared products are expected to be applied in fields of construction, metallurgy, and chemical industry.     

Porous ZnO@C core–shell nanocomposites as high performance electrode materials for rechargeable lithium-ion batteries

A novel porous spherical ZnO@carbon (C) nanocomposite based on zeolitic imidazolate frameworks (ZIFs-8)-directed method was prepared for lithium-ion batteries (LIBs). In this strategy, spherical ZnO nanoparticles were firstly prepared, then 2-methylimidazolate and Zn²⁺ were added alternately under ultrasound to fabricate ZnO@ZIF-8. Finally, the novel porous spherical ZnO@C nanocomposites were obtained via pyrolyzing the corresponding ZnO@ZIF-8. The novel porous spherical ZnO@C nanocomposites were characterized with different analysis techniques such as scanning electron microscopy, transmission electron microscopy and X-ray powder diffraction. The resulted spherical ZnO@C nanocomposites exhibited a high reversible capacity of 932 mA h g^?1 at 0.1 A g^?1 after 100 cycles, which is much higher than that of the pure ZnO nanoparticles. The porous structure, high specific surface area and good electrical conductivity eventually contribute to the good performance of the resulted ZnO@C nanocomposites for LIBs should be ascribed to the proous structure and high BET surface area derived from ZIFs, as well as the good electrical conductivity of the amourphous carbon derived from ZIFs.     

Core–shell CNT–Ni–Si nanowires as a high performance anode material for lithium ion batteries

Core–shell carbon nanotube (CNT)–Si heterogeneous nanowires have been identified as one of the most promising candidates for future anode materials in lithium ion batteries. However, stress in these nanostructures, is the long-existing bottleneck, rendering severe fading of the capacities and even failure of the batteries. We prove that the interfaces between CNT cores and Si shells play a critical role in the stress engineering. With rationally engineered interfaces, our core–shell nanowires with CNT–Ni–Si structure are able to offer excellent capacity retention and rate performance. Introduction of the rough Ni interfacial layer and utilization of the CNT cores lead to reinforced stability of the structure, well accommodated stress and enhanced charge transfer, which are responsible for the improved performance. This core–shell CNT–Ni–Si nanostructure provides a simple but effective pathway towards realization of long lifetime and high performance lithium ion batteries.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

CaMn0.05Zr0.95O3–NiMn2O4 composite ceramics with tunable electrical properties for high temperature NTC thermistors

A series of novel high temperature negative temperature coefficient (NTC) composite ceramics based on (1-x)CaMn_0.05Zr_0.95O₃-xNiMn₂O₄ (x = 0, 0.1, 0.2, 0.3) were fabricated by using the solid-state method. The Rietveld refinement method and backscattered electron (BSE) image confirmed the absence of any other phase. The conduction mechanism of the composite ceramics was determined by analyzing the complex impedance and resistance-temperature characteristics, which could be related to the formation of a continuous Mn³⁺-O-Mn⁴⁺ network. The effect of spinel content on the high temperature stability was analyzed using aging tests. The ρ_400°C, ρ_700°C and B_400/700 values of the NTC thermistors were determined to be 4.9 × 10⁶–1.2 × 10⁴, 1.4 × 10⁵–0.8 × 10³ Ω cm and 12739-5712 K, respectively. This indicated that the electrical properties could be tuned by adjusting the weight ratio of CaMn_0.05Zr_0.95O₃ and NiMn₂O₄. The findings obtained in this study reveal that the composite NTC thermistor exhibits a good application potential at high temperatures and under harsh environments.     

Preparation of the novel B4C–SiC composite aerogel with high compressive strength and low thermal conductivity

Journal of Porous Materials - A novel B4C–SiC composite aerogel is synthesized using nano boron carbide suspension, 3-aminopropyltriethoxysilane (APTES), and resorcinol–formaldehyde...     

The high temperature reaction of ammonia with carbon and SiC–C ceramics

This contribution aimed at developing a treatment under ammonia in order to eliminate free carbon from the surface of SiC-based fine ceramics like fibers or coatings. The reaction of NH₃ with graphitic and non-graphitic carbon was first investigated through kinetic measurements, in situ gas phase analysis and physicochemical investigations of the solid. The carbon etching rate is controlled by heterogeneous reactions involving active sites arising from bulk structural defects and the formation of HCN. A selection of SiC-based fibers and coatings with various carbon contents and (micro)structures was treated in ammonia in favorable conditions. The analyses of the tested SiC–C specimens revealed a reduction of the free carbon content and, simultaneously, a nitridation of the initial Si–C–(O) continuum over a reaction layer. The growth rate, composition and the volume change of this layer vary with the initial microstructure. The ammonia treatment is able to restore the adhesion of carbon-contaminated surfaces.     

Porous pyridine based polyimide–silica nanocomposites with low dielectric constant

Highly porous polymer–silica hybrid materials were prepared based on the organo-soluble polyimides of four various dianhydride and 2,5-diaminopyridine. 3-Aminopropyltriethoxysilane (APS) was used to increase the intrachain chemical bonding and interchain hydrogen bonding between the polyimide and silica moieties, respectively. The chemical interaction would significantly affect the morphologies and properties of the prepared films. The produced polyimide–silica composites were investigated by X-ray diffraction analysis, scanning electron microscope and thermal analysis tecniques. The effect of silica modified with functional group of 3-aminopropyltriethoxy silane on the porous structure and dielectric properties as well as the thermal stability of films were investigated. Capacitances were determined with a HP4294A at a frequency between 1 kHz and 1 MHz. The dielectric constant was significantly reduced with increasing silica modified with APS. The result indicates that the composite materials are potentially useful in low dielectric materials.     

Optimization of SiC–ZrC high emissivity composite flexible coating for thermal protection with high interfacial bond strength and temperature resistance

Aluminum silicate fiber fabric (ASFF) has been widely used in the outer surface of flexible insulation felt on the leeward side of aerospace vehicle. In order to improve the temperature resistance of ASFF, a kind of SiC–ZrC composite coating was prepared on the surface of fiber fabric via spraying method with SiC as emittance agent and ZrC as additive. The surface morphology and mechanical properties of the coating were studied. Compared with the single-component SiC coating, the composite coating could effectively avoid coating spalling and improve the surface integrity at high temperature. After thermal treatment at 1100 °C for 2 h, the interface bond strength of the composite coating/substrate was 52.41% higher than that of SiC coating/substrate. The tensile strength of fiber fabric with SiC–ZrC composite coating could reach 91.75 MPa, which was 101.76% higher than that of raw ASFF. Therefore, the SiC–ZrC coating could greatly improve the temperature resistance of ASFF, and has an attractive application prospect in the field of thermal protection system.     

A facile way via integrating sol–gel and Grignard reaction to prepare siloxane/carbosilane hybridized benzocyclobutene resins with hyperbranched structure,low dielectric constant,and high thermal stability

The hybridization of siloxane- and carbosilane-based structures in molecular scale could combine their advantages of high thermal resistance and low dielectric constant. However, the hybridization still remained a big challenge in lacking convenient synthesis method. Herein, in this work, we developed a new method via simultaneous Grignard and sol–gel reactions of (chrolomethyl)trimethoxysilane to directly generate siloxane/carbosilane hybridized oligomers with hyperbranched structure. The oligomers were subsequently functionalized by benzocyclobutene and cured to produce crosslinked resins. As-resulted resins have low dielectric constant (~2.60) as compared with silica and higher thermal stability compared with polycarbosilane. The integrated high performance and facile preparation for these resins make them potentially used as low dielectric materials in integrating circuits insulating and wave transmitting.     

Preparation and characterization of cordierite glass–ceramics for bonding solar thermal transmission pipelines

TiO₂ was employed to develop cordierite glass–ceramics for thermal transmission pipeline binders by a melt-quenching method. The effects of TiO₂ on the phase composition, microstructure, and physical properties of glass–ceramics were studied. In addition, the thermal shock resistance of the glass–ceramics based binder was investigated. The results showed the formation of α cordierite could be increased by adding 1.0 wt% TiO₂, thereby improving bending strength and decreasing the coefficient of thermal expansion. However, a 3-5 wt% TiO₂ additive resulted in massive generation of µ cordierite, which exhibited a negative effect on the above performances. After crystallization at 1000°C for 2 h, sample B1 (1 wt% TiO₂ additional) displayed the best overall properties. It was demonstrated that cordierite glass–ceramics were satisfactory materials as heat transmission pipeline binders when the C2 binder (40 wt% frit, 60 wt% as-prepared sample B1) was applied, which had a good thermal shock resistance.     

Relaxation degree and defect dipoles-controlled resistivity and leakage current in BF–BT-based ceramics

The low resistivity and high leakage current of bismuth ferrite–barium titanate-based ceramics greatly limit its development. In this work, the resistivity and leakage current have been regulated by changing the relaxation degree and defect dipoles. With the introduction of Sb, the ceramics endure from weak relaxors with typically ferroelectric behavior to relaxor ferroelectrics and then to relaxors, where nearly 20 times improved resistivity can be obtained in relaxor ferroelectrics with chemical-homogeneous grain structure and strip-shaped/submicron-sized coexisted domains, whereas deteriorated properties appear in relaxors with core–shell structure and tweed-like ferroelectric domains. The decreased concentration of oxygen vacancies and Ti³⁺ is responsible for the improved insulation in relaxor-ferroelectrics, whereas the emergence of core–shell structure leads to electrical inhomogeneity and the conductive channels result in decreased resistivity. In addition, the leakage current mechanism is transformed from Ohmic conduction to space-charge-limited current as the electric field increases, which is induced by different domain switchings in ceramics with varied relaxation degrees.     

Core–shell adsorbents by electrospun MOF-polymer composites with improved adsorption properties: Theory and experiments

A new class of core–shell adsorbents has been created by electrospun metal–organic framework (MOF) particles embedded in polymer nanofibers, which have provided many unique properties compared to the existing MOF coating technologies. For the first time, we demonstrate the improved adsorption selectivity of CO₂ over N₂ using electrospun polymer/ZIF-8 adsorbents in experiments. Furthermore, an analytical model based on the assumption that the diffusivity in core is 10 times higher than that in shell is developed to describe the theory of improved selectivity for core–shell adsorbents that is validated against a more accurate finite element model developed in COMSOL. Our model shows three regimes including exclusive shell uptake, linear core uptake, and asymptotic core uptake. These regimes are related to material properties and uptake times, which could be used as design criteria to balance core stability, maximum selectivity, and maximum uptake. An advanced HAADF STEM tomography (Movie S1 ) shows that the shell thickness in the case of polymer/ZIF-8 is on the order of 10 nm, allowing the regime of maximum selectivity to be realized. Kinetically limited adsorption tests at 45°C demonstrate that these composite fibers can perform in a regime of selectivity and uptake for the separation of CO₂ and N₂ that is unobtainable by either the MOF or fiber independently, showing a great potential for postcombustion CO₂ capture.     

ZrSi2–MgO as novel additives for high thermal conductivity of β-Si3N4 ceramics

A novel ZrSi₂–MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900°C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi₂ in the range of 0-7 mol%, the effect of ZrSi₂ addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi₂–MgO were effective for the densification of Si₃N₄ ceramics. XRD observations demonstrated that ZrSi₂ reacted with native silica on the Si₃N₄ surface to generate ZrO₂ and β-Si₃N₄ grains. TEM and in situ dilatometry confirmed that the as formed ZrO₂ collaborated with MgO and Si₃N₄ to form Si–Zr–Mg–O–N liquid phase promoting the densification of Si₃N₄. Abnormal grain growth was promoted by in situ generated β-Si₃N₄ grains. Consequently, compared to ZrO₂-doped materials, the addition of ZrSi₂ led to enlarged grains, extremely thin grain boundary film and high contiguity of Si₃N₄–Si₃N₄ grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W·(m·K)⁻¹ when ZrO₂ was substituted by ZrSi₂.     

Effect of Cu and Fe addition on electrical properties of Ni–Mn–Co–O NTC thermistor compositions

The effect of addition of Cu and Fe on the electrical properties of Ni–Mn–Co–O based NTC thermistor compositions is studied. Compositions with very low resistivity were prepared by co-doping of very small amounts of Cu along with Co in NiMn₂O₄, without affecting the sensitivity very much. Compositions with high resistivity and good sensitivity were achieved by co-doping Fe with Co in NiMn₂O₄. The effect of sintering temperature on electrical properties was investigated and it was found that the resistivity and material constant increase with sintering temperature. The reliability characteristics of the compositions were studied by the accelerated thermal aging method. All the compositions exhibited very good reliability.     

Design and magneto-optical characteristics of Ge–Sb–S–PbI2 chalcogenide glasses with a high Verdet constant

To develop high-performance magneto-optical chalcogenide glasses and clarify the mechanisms of the Verdet constant, a series of GeS₂–Sb₂S₃–PbI₂ chalcogenide glasses were designed and fabricated, and their Faraday effects were investigated at a wavelength of 980 nm. A new parameter, that is, average polarizability, was proposed, and the results show that the Verdet constant has a good linear relationship with average polarizability, meaning that the Verdet constant of a chalcogenide glass can be directly estimated by its chemical constituents. The Verdet constant is as large as 0.200 min G⁻¹ cm⁻¹ at 980 nm for 22.5GeS₂–67.5Sb₂S₃–10PbI₂ composition glass, which is the largest value reported thus far for sulfide glasses; this glass also possesses good thermal and optical properties and therefore might be an attractive candidate for mid-infrared magneto-optical device applications.     

Fabrication of Al2O3–ZrO2 ceramics with high porosity and strength

This study describes the synthesis of ceramics, in which a micrometre-sized Al₂O₃–ZrO₂ nanopowder was used as an oxide base for the hardening of the materials. To a suspension of this mixed metal oxide, the pore-forming crystallisation additives camphor and carbamide were added to produce ceramics with thin permeable pores. Camphor crystallised in the oxide suspension in the form of single pentagonal stars and сarbamide crystallised in the form of thin elongated needles. The use of the different crystallisation additives allowed the formation of ceramics after sintering that have both permeable and complex pore morphologies, where anisotropic properties were observed using carbamide as an additive but not when camphor was used. The total porosity of the resulting ceramics was 51.3%, with a compressive strength in the range of 17.3–92.3 MPa.     

Preparation and thermal stability investigation of Al2O3–mullite–ZrO2–SiC composite ceramics for solar thermal transmission pipelines

To obtain composite ceramics with excellent thermal shock resistance and satisfactory high?temperature service performance for solar thermal transmission pipelines, SiC additive was incorporated into Al₂O₃?mullite?ZrO₂ composite ceramics through a pressureless sintering process. The effect of the SiC additive on thermal shock resistance was studied. Also, the variations in the microstructure and physical properties during thermal cycles at 1300 °C were discussed. The results showed that both thermal shock resistance and thermal cycling performance could be improved by adding 20 wt% SiC. In particular, the sample with 50 wt% Al₂O₃, 35 wt% Coal Series Kaolin (CSK), 15 wt% partially yttria?stabilized zirconia (PSZ), and 20 wt% SiC additional (denoted as sample A2) exhibited the best overall performance after firing at 1600 °C. Furthermore, the bending strength of sample A2 increased to 124.58 MPa, with an increasing rate of 13.63% after 30 thermal shock cycles. The increase in thermal conductivity and the formation of mullite were the factors behind the enhancement of thermal shock resistance. During the thermal cycles, the oxidation of SiC particles was favorable as it increased the microstructure densification and also facilitated the generation of mullite, which endowed the composite ceramics with a self?reinforcing performance.