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Within the present work, additive-free amorphous bulk SiHfN ceramics with excellent mechanical properties were prepared by a resource-efficient low-temperature molding method, namely warm-pressing. As densification mechanism viscous flow has been identified based on cross-linking reaction. The critical problems concerning gas evolution and crystallization inducing bloating and cracking are addressed through controlled thermolysis and pressure. The microstructural evolution of the SiHfN ceramics indicates that the incorporation of Hf in perhydropolysilazane not only increases the ceramic yield (97.4 wt%) and crystallization resistance (1300 °C), but also suppresses the transformation from α-Si3N4 to β-Si3N4 at high temperatures (1700 °C). Especially, HfN/α-Si3N4 nanocomposites converted by the SiHfN ceramics at 1500 °C show a slight weight loss of 3.13 wt%, indicating the high temperature resistance of the ceramic nanocomposites. The method proposed in this work opens a new strategy to fabricate additive-free polycrystalline Si3N4- and amorphous Si3N4-based (nano)composites.

SiBN ceramics are widely considered to be the most promising material for microwave-transparent applications in harsh environments owing to its excellent thermal stability and low dielectric constant. This work focuses on the synthesis and ceramization of single-source precursors for the preparation of SiBN ceramics as well as the investigation of the corresponding microstructural evolution at high temperatures including molecular dynamic simulations. Carbon- and chlorine-free perhydropolysilazanes were reacted with borane dimethyl sulfide complex at different molar ratios to synthesize single-source precursors, which were subsequently pyrolyzed and annealed under N2 atmosphere (without ammonolysis) to prepare SiBN ceramics at 1100, 1200, and 1300 °C with high ceramic yield in contrast to previously widely-used ammonolysis synthesis process. The obtained amorphous SiBN ceramics were shown to have remarkably improved thermal stability and oxidation resistance compared to amorphous silicon nitride. Particularly, the experimental results have been combined with molecular dynamics simulation to further study the amorphous structure of SiBN and the atomic-scale diffusion behavior of Si, B, and N at 1300 °C. Incorporation of boron into the Si—N network is found to suppress the crystallization of the formed amorphous silicon nitride and hence improves its thermal stability in N2 atmosphere.

Polyaluminocarbosilane (PACS) as a single-source-precursor of SiAlC(O) ceramic was prepared by reacting a hyperbranched allylhydridopolycarbosilane (AHPCS) and aluminum(iii) acetylacetonate (Al(acac)3), and the PACS was characterized using gel-permeation chromatography, Fourier-transform infrared spectroscopy, and nuclear magnetic-resonance spectroscopy. The polymer-to-ceramic transformation of the obtained PACSs was investigated by Fourier-transform infrared spectroscopy and thermogravimetric analysis. The ceramic yield of the PACS was approximately 15% higher than that of the original AHPCS at 1,200°C. The phase composition and microstructure of the final ceramics were studied by X-ray diffraction, energy-dispersive spectroscopy, and scanning electron microscopy. The introduction of aluminum to the SiC(O) ceramics suppressed the β-SiC crystal growth and improved the density of the ceramics that were annealed at 1,800°C, which is advantageous for high-temperature ceramics. The aluminum content of the SiAlC(O) ceramics can be readily controlled by the Al(acac)3 content in the PACS precursors.

Multicomponent CNTs/Fe3Si/Fe/SiOCN ceramic nanocomposites are fabricated by a single-source-precursor approach, exhibiting excellent electromagnetic wave absorbing properties.

Carbon supported metallic nanomaterials are of great interest due to their low-cost, high durability and promising functional performance. Herein, a highly active oxygen evolution reaction (OER) electrocatalyst comprised of defective carbon shell encapsulated metal (Fe, Co, Ni) nanoparticles and their alloys supported on in-situ formed N-doped graphene/carbon nanotube hybrid is synthesized from novel single-source-precursors (SSP). The precursors are synthesized by a facile one-pot reaction of tannic acid with polyethylenimine and different metal ions and subsequent pyrolysis of the SSP. Benefiting from the heteroatom doping of carbon and formation of well-encapsulated metal/alloy nanoparticles, the obtained FeNi@NC-900 catalyst possesses lowest overpotentials of 310 mV to achieve a current density of 10 mA cm

In this paper, W-containing SiC-based ceramic nanocomposites were successfully prepared by a polymer-derived ceramic approach using allylhydridopolycarbosilane (AHPCS) as a SiC source, WCl6 as a tungsten source, polystyrene (PS) as a pore forming agent as well as divinyl benzene (DVB) as a carbon rich source. High-temperature phase behavior of the W-containing SiC-based ceramics after heat treatment was studied, showing that excessive DVB content in the feed will inhibit the crystallinity of W-containing nanoparticles in the final ceramic nanocomposites. The high specific surface area (SSA) of 169.4–276.9 m2/g can be maintained even at high temperature in the range of 1400–1500 °C, due to the carbothermal reaction which usually occurs between 1300 and 1400 °C. All prepared W-containing SiC-based nanocomposites reveal electrocatalytic activity for the hydrogen evolution reaction (HER). In detail, compared with reversible hydrogen electrode (RHE), the ceramic sample PWA-2-1300 after heat treatment at 1300 °C has the smallest overpotential of 286 mV when the current density is 10 mA·cm−2 in acid medium, indicating the promising perspective in the water splitting field.

Severe volume expansion and inherently poor lithium ion transmission are two major problems of silicon anodes. To address these issues, we proposed a pomegranate-type Si/C composite anode with highly dispersed tiny silicon particles as the core assisted by small amount of SiC. Skillfully exploiting the high heat from magnesiothermic reduction, SiC can assist the good dispersion of silicon and provide good interface compatibility and chemical stability. The silicon anchored to the carbon shell provides multipoint contact mode, that together with the carbon shell frame, significantly promoting the transfer of dual charge. Besides, the pomegranate-type microcluster structure also improves the tap density of the electrode, reduces the direct contact area between active material and electrolyte, and enhances the electrochemical performance.

A boron-containing SiHfC(N,O) amorphous ceramic was synthesized upon pyrolysis of a single-source-precursor at 1000 °C in Ar atmosphere. The high-temperature microstructural evolution of the ceramic at high temperatures was studied using X-ray powder diffraction, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy. The results show that the ceramic consists of an SiHfC(N,O)-based amorphous matrix and finely dispersed sp2-hybridized boron-containing carbon (i.e. ByC). High temperature annealing of ByC/SiHfC(N,O) leads to the precipitation of HfCxN1-x nanoparticles as well as to β-SiC crystallization. After annealing at temperatures beyond 1900 °C, HfB2 formation was observed. The incorporation of boron into SiHfC(N,O) leads to an increase of its sintering activity, consequently providing dense materials possessing improved mechanical properties as compared to those of boron-free SiC/HfC. Thus, hardness and elastic modulus values up to 25.7 ± 5.3 and 344.7 ± 43.0 GPa, respectively, were measured for the dense monolithic SiC/HfCxN1-x/HfB2/C ceramic nano/micro composite.

Dielectric properties and electromagnetic (EM) wave absorbing performance of monolithic (SiC/HfC/C)/SiO2 nanocomposites (denoted as SHCOs) have been investigated in the X-band (8.2–12.4 GHz). The multiphase SHCOs are composed of insulating SiO2 and SiC/HfC/C nanocomposite fillers (SHC), which fillers composed of semiconducting β-SiC, conductive HfC-Carbon core-shell nanoparticles, and interconnected carbon nanoribbons. Dielectric response indicates that the increased SHC content results in an enhanced imaginary part of the permittivity and dielectric loss, leading to an improved EM absorbing performance. The unique microstructure with an EM wave-transparent SiO2 matrix is favorable for impedance matching and effective EM wave propagation. The enhanced interface polarization and conduction loss are considered as the key mechanisms for EM wave attenuation. The minimum reflection loss of the SHCOs achieves – 60.7 dB containing 20 vol% of SHC (at 9.98 GHz) with the sample thickness of 3.33 mm, and the effective absorbing bandwidth (EAB) covers ca. 72 % of the X-band. The monolithic (SiC/HfC/C)/SiO2 nanocomposites with outstanding EM wave absorbing performance are promising candidates for EM application at high temperatures.

Improving the electromagnetic wave (EMW) absorption performance of continuous SiC fibers is of great significance for the development of structure-function integrated composites. Doping magnetic metal elements into SiC fibers can effectively improve their dielectric/magnetic loss, which is expected to produce SiC fibers with wide frequency EMW absorption. At present, introducing metal into continuous SiC fibers generally has the disadvantages of (1) low metal content and (2) weak EMW absorption ability. In this paper, vinylferrocene (VF) with monofunctional groups was selected as the iron source to introduce Fe atoms into polycarbosilane (PCS) through chemical modification without changes in the molecular framework of PCS, which is beneficial for the melt spinning of the precursors. A series of continuous Fe-SiC fibers were prepared by polymer-derived ceramic approach, and their composition, structure and EMW properties were systematically studied. Besides SiC crystallization, the turbostratic carbon, core–shell structures of Fe5Si3@C and Fe3Si@C were identified for the continuous Fe-SiC fibers. It is worth noting that in the present work, (1) the introduction of Fe catalyzes the formation of turbostratic carbon, which can be observed at a low temperature of 900 °C and (2) the highest Fe content (7.70 wt%) was obtained among all the reported continuous Fe-SiC fibers. The unique microstructure of final Fe-SiC fibers demonstrated outstanding EMW absorbing performance. For the Fe-SiC-2-900 °C fiber, the minimum reflection loss (RLmin) reached −54.63 dB at 10.2 GHz. By adjusting the thickness, the effective absorption bandwidth of Fe-SiC-3-1200 °C can almost cover the X-band. The results indicate that the presented continuous Fe-SiC fibers exhibit excellent EMW absorbing performance.

In order to enhance dielectric properties of polymer-derived SiC ceramics, a novel single-source-precursor was synthesized by the reaction of an allylhydrido polycarbosilane (AHPCS) and divinyl benzene (DVB) to form carbon-rich SiC. As expected, the free carbon contents of resultant SiC ceramics annealed at 1600 °C are significantly enhanced from 6.62 wt% to 44.67 wt%. After annealing at 900–1600 °C, the obtained carbon-rich SiC ceramics undergo phase separation from amorphous to crystalline feature where superfine SiC nanocrystals and turbostratic carbon networks are dispersed in an amorphous SiC(O) matrix. The dielectric properties and electromagnetic (EM) absorption performance of as-synthesized carbon-rich SiC ceramics are significantly improved by increasing the structural order and content of free carbon. For the 1600 °C ceramics mixed with paraffin wax, the minimum reflection coefficient (RCmin) reaches –56.8 dB at 15.2 GHz with the thickness of 1.51 mm and a relatively broad effective bandwidth (the bandwidth of RC values lower than –10 dB) of 4.43 GHz, indicating the excellent EM absorption performance. The carbon-rich SiC ceramics have to be considered as harsh environmental EM absorbers with excellent chemical stability, high temperature, and oxidation and corrosion resistance.

To further improve the oxidation resistance of C/SiC–SiBCN at temperatures over 1200 °C, C/SiC–SiHfCN and C/SiC–SiHfBCN composites were prepared by a new method called chemical vapor infiltration (CVI) combined with polymer infiltration and on-line pyrolysis (PIOP) process. The weight change behavior, mechanical properties, and microstructure of C/SiC–SiHf(B)CN before and after oxidation in wet oxygen were studied in detail. Through the replacement of B by Hf, the weight loss of the composite in high-temperature wet oxygen environment has been significantly reduced. The strength retention of C/SiC–SiHfBCN is up to 75% after oxidation in wet oxygen at 1400 °C and the strength retention of C/SiC–SiHfBCN can reach 85% after oxidation in an atmosphere with high oxygen and water vapor content at 1300 °C. In addition, microscopic analysis showed that after oxidation at 1400 °C, only the fibers in the near-surface matrix were oxidized to some extent and hafnium has been enriched at the composite surface. Our work provides a reference for the study of the oxidation resistance of C/SiC composites in harsh environments.

Oxidation behavior of boron-containing HfC/SiC nanocomposites (SHBC) at temperatures up to 1500 °C and with exposure time up to 100 h was investigated. Two strategies to improve the oxidation resistance of the HfC/SiC ceramics are proposed. First concept involves the incorporation of a small amount of boron (ca. 0.6 wt.%) into the nanocomposite via a single-source-precursor approach, which contributes significantly to the enhancement of its oxidation resistance. Parabolic oxidation rate constants of 10−3 to 10-4 mg2/(cm4 h) at 1300−1500 °C were measured for SHBC and were several orders of magnitude lower than those recorded for boron-free HfC/SiC. The second improvement concept is realized via passivation of the samples upon short-term oxidation at 1400 °C, providing an excellent oxidation resistance over a wide temperature range. This is a crucial step especially when considering the poor oxidation behavior of HfC and the sluggish formation of protective silica scale at moderate temperatures.

In order to further expand the application of SiC/SiC composites in the hot-end components for advanced aerospace engines, a self-healing SiC/(SiC-SiBCN)x composite prepared by CVI + PIOP has been tested for their creep behavior at 1200 °C with the partial pressure of 12 kPa H2O: 8 kPa O2: 80 kPa Ar. For the samples tested under low stresses, the load is borne by a load-bearing region, so that the secondary stage of creep curves can be subdivided into several small steps. Besides, it is found that the number of defects in SiC/(SiC-SiBCN)x composites is predominantly related to the stress level while the size of the defects is related to the creep time. Our findings provide important analytical data related to the creep behavior of the SiC/(SiC-SiBCN)x composite material and contribute to the further development of CMCs with outstanding mechanical properties and corrosion resistance even under harsh environments.

A novel single-source-precursor for SiC-TaC-C nanocomposites was successfully synthesized by the chemical reaction between a polycarbosilane (allylhydridopolycarbosilane, AHPCS) and tantalum(V) chloride (TaCl5), which was confirmed by Fourier transform infrared spectra (FTIR) measurement. After pyrolysis of the resultant single-source-precursors at 900 °C, amorphous ceramic powders were obtained. The 900 °C ceramics were annealed at different temperatures in the range of 1200–1600 °C to gain SiC-TaC-C nanocomposites. The phase evolution of ceramic nanocomposites was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results indicate that the TaC starts to crystallize at lower temperature than the β-SiC. It is particularly worth pointing out that the unique core-shell structured TaC@C nanoparticles were in-situ formed and homogeneously distributed in the ceramic matrix after annealing at 1400 °C. Even at a high temperature of 1600 °C, the grain sizes of β-SiC and TaC are smaller than 30 nm, fulfilling the definition of nanocomposites. The present study related to SiC-TaC-C nanocomposites paves a new road for enriching ultra-high temperature ceramic family suitable for structural/functional applications in harsh environment.

For the first time, dielectric properties and electromagnetic wave (EMW) absorbing performance of single-source-precursor derived Mo4.8Si3C0.6/SiC/Cfree ceramic nanocomposites with a highly electri...

Ultra-light carboxylic functionalized multi-walled carbon nanotubes (CNTs-COOH) and Ti3C2 MXene hybrids modified sodium alginate (CNTs/Ti3C2-SA) based composite foams were prepared through ice-templated freeze-drying method. The microstructure of the synthesized CNTs/Ti3C2 hybrids and CNTs/Ti3C2-SA foams is characterized by the presence of CNTs inserted between MXene layers which prevents their restacking. The resultant CNTs/Ti3C2 hybrids exhibit a unique sandwich-like hierarchical structure. Scanning electron microscopy (SEM) images show that the CNTs/Ti3C2-SA foam exhibits a heterogeneous anisotropic microstructure and CNTs/Ti3C2 hybrids are homogeneously dispersed in the skeleton of the porous foam. In case that the content of the hybrids amounts 40 mg/cm3, the CNTs/Ti3C2-SA foam possesses excellent electromagnetic (EM) absorption performance with a minimum reflection coefficient (RCmin) as low as -40.0 dB. In case of a sample thickness of 3.95 mm, the RCmin reaches -24.4 dB and the effective absorption bandwidth covers the whole X band from 8.2 to 12.4 GHz. A control test shows that, with the same absorbent content, the CNTs/Ti3C2-SA foam exhibits a far better EM performance than that of CNT-free Ti3C2-SA foam.

The ternary Nowotny phase (NP), with a composition Mo3+2xSi3C0.6 (x = 0.9‐0.764), is found to be catalytically active in the field of electrochemical water splitting. The NP embedded in a porous SiC/C nanocomposite matrix is synthesized via a single‐source‐precursor approach which involves the reaction of allylhydridopolycarbosilane with MoO2(acac)2. Thermal treatment of the single‐source‐precursor up to 1400°C in a protective atmosphere results in the in situ formation of nanocrystalline Mo3+2xSi3C0.6 immobilized in a thermally and corrosion‐stable SiC/C matrix. The weight fractions of the observed crystalline phases Mo3+2xSi3C0.6 and SiC amount to ca. 28 (26) and 72 (74) wt%, respectively, when prepared at 1400°C (1350°C). The porosity of the formed nanocomposite is adjusted by the addition of polystyrene (PS) as a pore former to the single‐source‐precursor resulting in a specific surface area up to 206 m2/g. The electrocatalytic activity of the Mo3+2xSi3C0.6/C/SiC nanocomposite with respect to the hydrogen evolution reaction (HER) is characterized by low over potentials of 22 and 138 mV vs reversible hydrogen electrode (RHE) for applying 1 and 10 mA cm−2 of current density, respectively. The analyzed electrocatalytic performance exceeds that of most Mo‐based electrocatalysts and shows high stability (over 90%) during 35 hours.

For the first time, ZrC–ZrB2–SiC ceramic nanocomposites were successfully prepared by a single-source-precursor route, with allylhydridopolycarbosilane (AHPCS), triethylamine borane (TEAB), and bis(cyclopentadienyl) zirconium dichloride (Cp2ZrCl2) as starting materials. The polymer-to-ceramic transformation and thermal behavior of obtained single-source precursor were characterized by means of Fourier transform infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA). The results show that the precursor possesses a high ceramic yield about 85% at 1000 °C. The phase composition and microstructure of formed ZrC–ZrB2–SiC ceramics were investigated by means of X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Meanwhile, the weight loss and chemical composition of the resultant ZrC–ZrB2–SiC nanocomposites were investigated after annealing at high temperature up to 1800 °C. High temperature behavior with respect to decomposition as well as crystallization shows a promising high temperature stability of the formed ZrC–ZrB2–SiC nanocomposites.

For the first time, single-source-precursor synthesized dense monolithic SiC/HfCxN1−x/C ceramic nanocomposites with outstanding electromagnetic (EM) shielding performance at temperatures up to 600 °C are reported. The total shielding effectiveness (SET) of the SiC/HfCxN1−x/C monolith is >40 dB at 600 °C, which is superior than most of the reported EM shielding materials under the same conditions. Compared with a Hf-free SiC/C monolith, the SiC/HfCxN1−x/C monolith possesses superior EM shielding performance due to the presence of a highly conductive HfCxN1−x phase. Moreover, the HfCxN1−x-particles are covered by a carbon layer forming core–shell nanoparticles connected with graphite-like carbon ribbons, which result in electrically conductive networks within the semiconducting β-SiC matrix. In addition, the hardness, Young's modulus and flexural strength of the dense SiC/HfCxN1−x/C monolith are measured to be 29 ± 4 GPa, 381 ± 29 GPa and 320 ± 25 MPa, respectively. The outstanding EM shielding performance combined with excellent mechanical properties of the dense monolithic SiC/HfCxN1−x/C nanocomposites provides a novel strategy to fabricate EM shielding materials for applications in harsh environments and/or under high mechanical load.