Electrospinning of pure polymer-derived SiBCN nanofibers with high yield

In this paper, we have discussed the synthesis of nanoscale silicoboron carbonitride (SiBCN) nanofibers by electrospinning of novel polyborosilazane precursor solutions, which were synthesized from boron trichloride, tris(dichloromethylsilylethyl)borane and hexamethyldisilazane using a one-pot method, followed by pyrolysis at 1000 °C under nitrogen atmosphere. The influence of the processing procedure (solution concentration, feeding rate and spinning voltage) of the electrospum fibers was also investigated. TGA and FT-IR experiments were employed to study the polymer-to-ceramic conversion and SEM was used to characterize the final materials. Combining the precursor derived ceramics route and electrospinning process enables to product SiBCN nanofibers for industrial applications.     

Dimethylaminoborane-modified copolysilazane as a novel precursor for high-temperature resistant SiBCN ceramics

A novel polyborosilazane (PBSZ) precursor was synthesized by the reaction of copolysilazane (CPSZ) with dimethylaminoborane (DMAB). The resultant PBSZs were characterized by FT-IR and NMR spectroscopy. It was found that both, BH and NH bonds of DMAB, react with CPSZ leading to boron containing copolysilazanes. The polyborosilazanes were pyrolyzed at 900 °C in argon and the precursor-to-ceramic transformation was studied by TG-MS and FT-IR spectroscopy. The modification of CPSZ with DMAB enhances the cross-linking of the resulting PBSZ, which increases the final ceramic yield from 57.8% to 77.5–80.0%. Finally, the ceramics obtained at 900 °C were subsequently annealed at different temperatures ranging from 1200 to 1800 °C. The heat-treated products were characterized by X-ray powder diffraction and electron microscopy. Accordingly, the resulting SiBCN ceramics exhibit significantly enhanced high-temperature-resistance with respect to decomposition and crystallization as compared with boron-free CPSZ-derived SiCN ceramics. TEM results support that the thermal stability is due to the segregation of a BN(C) phase as interlayer between Si₃N₄ nanocrystals formed during heat-treatment of SiBCN at T > 1500 °C.     

Synthesis and ceramic conversion of a novel processible polyboronsilazane precursor to SiBCN ceramic

Polyboronsilazane (PBSZ) precursors for SiBCN ceramics were prepared by using 9-borabicyclo-[1,3,3] nonane (9-BBN) and copolysilazanes (CPSZ) as starting materials, involving the hydroboration reaction between vinyl groups of PSZ and BH groups of 9-BBN under mild conditions. The as-synthesized PBSZ was obtained as a soluble liquid, which was characterized by FT IR and NMR. The polymer-to-ceramic conversion of PBSZ at a ceramic yield of 62.2–79.9% was investigated by means of FT IR and TGA. The crystallization behavior and microstructures of PBSZ-derived SiBCN ceramics were studied by XRD, SEM and HRTEM. The SiBCN ceramic began to crystallize at 1600 °C. Further heating at 1800 °C induced partial crystallization to give mixed XRD patterns for SiC, Si₃N₄, and BN(C). It is observed that the introduction of boron improves the thermal stability of SiBCN ceramics, especially under high temperatures of 1600–1800 °C. In addition, the introduction of boron significantly improves the ceramic density while inhibits the SiC crystallization.     

A dense amorphous SiBCN(O) ceramic prepared by simultaneous pyrolysis of organics and inorganics

Utilizing active function groups and radicals of polymers (polycarbosilane) to react with a mixture of organics (urea) and inorganics(boric acid) during polymer to ceramic conversion process, a novel SiBCN(O) ceramic was successfully prepared via hot-pressing from 380 to 700 °C combined with following pyrolysis at 1400 °C for 2 h in nitrogen atmosphere. The prepared SiBCN(O) ceramic with a low open porosity of ~4%, which maintained amorphous structure still. Experimental results confirmed that the inorganic network of SiBCN(O) amorphous ceramic was constructed by Si–C, Si–N, Si–O, B–N, and B–O bonds. Pyrolysis product BN served as active filler in densification process of the SiBCN(O) ceramic. Additionally, the introduced oxygen from boric acid could be well controlled and thus effectively improved the densification process of the ceramic.     

Effect of boron content on the microstructure and electromagnetic properties of SiBCN ceramics

Electromagnetic wave (EMW) absorbing materials have excellent potential for various applications in civil engineering and the military. In this study, siliconboron carbonitride (SiBCN) ceramics with excellent EMW absorption capability and oxidation resistance were obtained by adjusting the boron content. The results revealed that the graphite crystallite size in the SiBCN ceramics increased from 3.42 to 3.78 nm, whereas the thickness of the oxide layer decreased from 16.6 to 8.2 μm. The highest electrical conductivity and permittivity for the SiBCN ceramics were obtained when the boron content was 5%. The minimum reflection loss was ?35.25 dB at 10.57 GHz and a ceramic thickness of 2.0 mm. At a temperature of 600 °C, the SiBCN ceramic exhibited excellent EMW attenuation ability; particularly, the minimum reflection loss reached ?29.18 dB at 9.65 GHz and a ceramic thickness of 2.5 mm. The superior EMW absorption properties of the SiBCN ceramics at high temperatures can be ascribed to the synergistic effect of relaxation and conductivity. The results suggest that boron could enhance the transformation of amorphous carbon into crystalline graphite and increase the number of heterointerfaces and conductive paths. This work provides a method for obtaining SiBCN ceramics with excellent EMW absorption properties.     

Adjustable iron-containing SiBCN ceramics with high-temperature microwave absorption and anti-oxidation properties

Quaternary siliconboron carbonitride (SiBCN) ceramics show excellent high-temperature stability and oxidation resistance, indicating great potential as high-temperature electromagnetic (EM) wave absorbing materials. In this contribution, an efficient and facile method was developed to prepare bulk iron-containing SiBCN (Fe–SiBCN) ceramics with remarkable EM wave absorption at high temperature by pyrolyzing boron and iron containing precursors (PSZV-B–Fe). The introduction of boron and iron not only improves the high-temperature stability but also influences the complex permittivity and EM wave absorption. The minimum reflection coefficient (RC_min) is −61.05 dB, and the effective bandwidth absorption (EAB) is 3.35 GHz (9.05–12.4 GHz). The RC_min will be decreased to −52.3 dB at 600°C as well as the EAB covers more than 67% of the X band (2.8 GHz). The high-temperature stable Fe–SiBCN ceramics with adjustable dielectric properties can be utilized as high-performance EM wave-absorbing materials in high-temperature and harsh environments.     

Mechanical properties and thermal stability of SiBCN films prepared by ion beam assisted sputter deposition

The high hardness, exceptional high temperature stability, and oxidation resistance of bulk Si–B–C–N ceramics have led to the expectation that these materials will be good candidates for superior coating materials in high-temperature applications. In this study, SiBCN films were prepared using ion beam assisted sputter (IBAS) deposition, and the mechanical properties and thermal stabilities of the films at 600, 700, and 800 °C in air were investigated. In particular, the effects of the ion beam assist on the properties of the SiBCN films were examined. The SiBCN films were deposited on Si plates by sputtering a target composed of Si + BN + C using a 2-keV Ar⁺ ion beam. A low-energy N₂⁺ and Ar⁺ mixed ion beam irradiated the samples during the sputter deposition. The Si content in the SiBCN films was controlled by changing the Si/(BN + C) ratio of the target. BCN films were also deposited for comparison. The composition and chemical bonding structure of the prepared films were investigated by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. We found that c-BN bonds were formed in the ion-assisted BCN film. The oxide layer thickness on the SiBCN films after thermal annealing decreased due to the IBAS deposition and an increase in the Si content. Ion-assisted SiBCN films annealed at 800 °C showed the highest hardness of 20 GPa.     

A simple and efficient method for the synthesis of SiBNC ceramics with different Si/B atomic ratios

The high-temperature durability of SiBNC ceramics is significantly influenced by Si/B ratios and the synthetic procedures. Single-source synthetic routes can yield homogeneous ceramics at the atomic level, but the Si/B ratio cannot be efficiently adjusted. In this paper, a simple and efficient method for the synthesis of SiBNC precursor polyborosilazanes (PBSZs) with different Si/B ratios has been established via a one-pot reaction involving boron trichloride, dichloromethylsilane and hexamethyldisilazane in different molar ratios. The Si/B ratios of the derived SiBNC ceramics were consistent with that of the precursor PBSZs. When pyrolysed at 1000 °C, PBSZs with 0.52, 0.94 and 2.12 Si/B ratios transformed into SiB_2.6N₅C_2.2, SiB_0.9N_2.7C_1.3 and Si₂BN₃C_1.4 ceramics respectively. The polymer-to-ceramic process was also studied and featured ceramic yields of 43.2 wt%, 50.1 wt% and 62.2 wt%, respectively. The derived ceramic SiB_0.9N_2.7C_1.3 resisted crystallization up until 1700 °C, whereas the SiB_2.6N₅C_2.2 and Si₂BN₃C_1.4 could remain amorphous up to 1600 °C only. Using the precursor with 0.94 Si/B ratio, the SiBNC ceramic fibres were also obtained.     

Enhanced high-temperature resistance and magnetic property of SiFeC nanocomposites containing a small amount of boron

In this paper, the boron-modified polyferrocenylcarbosilanes (namely HBPFCS-Bs) with hyperbranched structure, as single-source-precursors, were successfully prepared by using 9-borabicyclo [3, 3, 1] nonane (9-BBN) and polyferrocenylcarbosilanes (HBPFCSs) as starting materials. The obtained HBPFCS-Bs were characterized using Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC), providing that the boron element can be introduced into the HBPFCS chains by the hydroboration (B-H/C = C) reaction. Then, the structural evolution of HBPFCS-Bs and the boron-containing SiFeC ceramics at different temperatures (1100–1500°C) was further investigated by thermogravimetric analysis, X-ray diffraction, energy dispersive spectroscopy, scanning electron microscope, and transmission electron microscopy. The results show that the incorporation of a small amount of boron (1 wt%) into precursors significantly improves the properties of the final ceramics, including the significant increase of the ceramic yield, enhanced high-temperature resistance, as well as the higher ceramic densification. Finally, the saturation magnetization of the HBPFCS-Bs-derived ceramics was also enhanced by the introduction of boron.     

Synthesis and Characterization of a Novel Preceramic Polymer for SiBNC-Ti Ceramics

A ceramic precursor (PBSZ-Ti) with different Ti content for preparing SiBNC-Ti ceramics was synthesized by the polymer-derived method, with dicyclopentadienyltitanium dichloride, trichlorosilane, hexamethyldisilazane and boron trichloride as starting materials. FT-IR, NMR, EA and XPS were employed to characterize the compositions and structures of PBSZ-Ti and SiBNC-Ti ceramics, which indicated that PBSZ-Ti contained Si–N–B and B–N six-membered ring structures, and Ti element was successfully introduced into the polymeric structure. The micro-morphology and high temperature crystallization behavior of SiBNC-Ti ceramics were studied by XRD, Raman and SEM. The results show that the surface of the SiBNC-Ti ceramic is smooth and dense. The amorphous state of SiBNC-Ti ceramics could be maintained to 1500 °C in a N₂ atmosphere and 1300 °C in air.

     

Microstructure and electromagnetic wave absorption property of reduced graphene oxide-SiCnw/SiBCN composite ceramics

In this account, RGO-SiC_nw/SiBCN composite ceramics were fabricated using polymer derived ceramic (PDC) combined with chemical vapor infiltration (CVI) technology. Dielectric property of as-obtained RGO-SiC_nw/SiBCN composite ceramics was significantly enhanced thanks to established conductive pathway through overlapped nanoscale SiC_nw and micro-sized RGO. The minimum RC of composite ceramics with 0.5 wt% GO and 2.29 wt% SiC_nw at thickness of 3.6 mm reached -42.02 dB with corresponding effective absorption bandwidth (EAB) of 4.2 GHz. As temperature rose from 25 to 400 °C, permittivity increased with enhanced charge carrier density and then it decreased due to oxidation process of RGO from 400 to 600 °C. The minimum reflection coefficient (RC) was recorded as -39.13 dB and EAB covered the entire X-band at 600 °C. EMW absorption ability was evaluated after high-temperature oxidation experiment under protective effect of wave-transparent Si₃N₄ coating. RGO-SiC_nw/SiBCN composite ceramics maintained outstanding EMW absorption ability with minimum RC of -10.41 dB after oxidation at 900 °C, indicating RGO-SiC_nw/SiBCN composite ceramics with excellent EMW absorption characteristic even at high temperatures and harsh environments.     

Synthesis,microstructure and electromagnetic properties of Hf-based SiBCN ceramics

In this work, two kinds of ceramic polymers, polyborosilazane and polyhafnoxane, were mixed by a precursor blending method, followed by curing and pyrolysis to obtain Hf-based SiBCN ceramics. Through FTIR and NMR characterization of cured product, it can be found that the two precursors underwent cross-linking reaction during the curing process, resulting in the increase of crystallization temperature of HfO₂ and β-SiC, and the formation of new components (HfB₂ and HfN) during pyrolysis. When the pyrolysis temperature increases, the average values of the real part and imaginary part of the dielectric constant of Hf-based SiBCN ceramics increased from 7.2 to 4.9 to 9.0 and 6.6, respectively, resulting from the precipitation of HfB₂ and HfC(N) with high dielectric constants. The effective absorption width decreased from 3.4 to 2.5 GHz, and the minimum value of reflection coefficient increased from −14.9 to −12.2 dB, which is caused by poor impedance matching. After being oxidized at 500 °C for 50 h in air, the free carbon basically disappears, and the full X-band absorption can be realized for the Hf-based SiBCN ceramic pyrolyzed at 1500 °C with a thickness of 2.8 mm.     

SiBCN ceramic aerogel/graphene composites prepared via sol-gel infiltration process and polymer-derived ceramics (PDCs) route

The SiBCN ceramic aerogel/graphene composites were synthesized by combining a simple sol-gel infiltration process with CO₂ supercritical drying technology and polymer-derived ceramics route. In order to select the best preceramic sample for sintering, the micromorphology of PSNB aerogel/graphene composites fabricated with different graphene oxide solution concentrations were investigated. The microstructure evolution of the prepared SiBCN ceramic aerogel/graphene composites and phase composition were studied by SEM, TEM and XRD, the pore structure of the preceramic composites pyrolyzed at 1200 °C was tested by specific surface area and pore size analyzer. Furthermore, the compressive strain-stress curve and toughening mechanisms of composites were also investigated in detail. The results showed that all the preceramic composites and obtained ceramic aerogel composites possessed the mesoporous structure. The basic structure of SiBCN aerogel network changed from the initial spherical particles accumulation to the nanowires lapping with the sintering temperature increased from 800 °C to 1200 °C. After pyrolyzing at 1200 °C, the specific surface area and pore volume for the sample were 101.61 m² g⁻¹ and 1.43 cm³ g⁻¹, respectively, and a small amount of β-SiC crystalline phases were formed in amorphous ceramic matrix and had an relatively uniform distribution. Moreover, the paepared ceramic aerogel composites possessed a certain degree of toughness, the toughening mechanisms of composite samples mainly included the crack deflection, graphene pull-out, graphene bridging and graphene crumpling.     

Synthesis and inorganic conversion of polyborodiphenylsiloxane

Polyborodiphenylsiloxane (PPBSO) was reported to play significant roles in the preparation of advanced SiC ceramics as a precursor initiator, sintering binder and boron introducer. However, neither the effect of this important raw material on the pyrolysis process nor the evolution of boron has been clarified. This study synthesized PPBSO as a preceramic polymer and thoroughly investigated the constitutional and structural evolutions during organic-to-inorganic conversion. Boron was found to transform into the B(OSi)₃ structure fully at 1300 °C, and this structure played an important role in increasing the Si content (from 18.51%_wt to 35.84%_wt) by forming a viscous fluid barrier that reduced the gaseous release, which led to an increase in the vapor pressure and a reduction in the Si–O–C phase decomposition according to the Le Chatelier principle. The dominant B–O–C phase that was observed at 1000 °C transformed into a B(OSi)₃ structure, and high-pressure-formed BC₄, detected via Raman spectroscopy, was the result of the partial pressure increase. A crystallization-promoting effect of free carbon in the presence of boron was also detected via Raman analysis. This study extensively describes the role of boron in the silicon carbide ceramic conversion process and will be of substantial benefit for the fabrication of high-performance ceramic materials.     

Densification behavior and microstructure evolution of hot-pressed SiC–SiBCN ceramics

Densification behavior and microstructure evolution of hot-pressed SiC–SiBCN ceramics were studied between 1660 °C and 1830 °C. Polyborosilazane was chosen as the SiBCN precursor and pyrolyzed at 1000 °C in inert atmosphere before use. Samples with SiBCN contents of 10% and 20% in weight were prepared. During the sintering, at temperatures <1660 °C, the density of all the samples showed a minor increase because of solid state particles rearrangement. Above 1660 °C, the density increased rapidly because of the grain boundary sliding with a non-Newtonian viscous boundary phase. After grain boundary sliding, grain-boundary diffusion enhanced by B and C elements from the SiBCN material was responsible for the further densification. The microstructure of the samples hot pressed at 1660 °C appeared particle packing state. The two samples can achieve almost full density when they were hot pressed at 1830 °C/40 MPa for 90 min.     

Polymer precursor synthesis of novel ZrC–SiC ultrahigh-temperature ceramics and modulation of their molecular structure

In this study, ZrC–SiC composite ceramics were prepared with varying Zr/Si molar ratios using sol–gel method. Composites were characterized by Fourier-transform infrared spectroscopy (FT–IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDS). FT–IR analysis confirmed macromolecular network structure of composites, in which the precursor is composed of polyvinyl butyral (PVB) as main chain, silane molecules are interlinked via –OH moieties in PVB side chains, and Zr atoms are crosslinked with Si in corresponding proportion. Ceramic precursor begins to decompose at a temperature exceeding 1300 °C and is completely transformed into ZrC–SiC composite ceramics with corresponding Zr/Si molar ratio at 1600 °C. Raman spectroscopy and TEM results reveal that after annealing at 1600 °C, ZrC powder uniformly covers surface of SiC ceramics, and high-crystallinity graphite carbon covers ZrC powder.     

Diffusion bonding of Ti-coated Cf-SiCf/SiBCN composites to Nb using Ag–Pd interlayer

The Ti-coated C_f-SiC_f/SiBCN ceramic was diffusion-bonded to Nb using an Ag–Pd interlayer. At a ceramic/Ag–Pd interface, it is found that Ti first reacted with C and Pd to form a TiC_x/Ti(Pd) double-layer structure, and then Ti(Pd) completely converted into TiC_x with the increase of holding time or temperature. Meanwhile, the infiltration of Pd along TiC_x grain boundary reacted with Si to form brittle Pd₂Si compound. By contrary, only a simple NbPd₃ layer was formed at the Nb/Ag–Pd interface during the whole process. A maximum shear strength of 16 ± 3 MPa is obtained for the joint prepared at 900°C for 30 min. The plastic deformation of the Ag–Pd interlayer and pullout of fibers inside ceramic contributed to the superior performance. Nevertheless, as the holding time and temperature reached 90 min and 950°C, the high chemical affinity of the Pd–Si system and enhanced atomic diffusion led to the massive formation of Pd₂Si, which increased the joint brittleness and degraded the ceramic performance.     

Construction of multiple heterogeneous interface and its effect on microwave absorption of SiBCN ceramics

Iron-containing siliconboron carbonitride (SiBCN) ceramics with multiple heterogeneous interfaces were fabricated using the microstructural design and polymer-derived ceramics (PDC) approach. The characterization results revealed the in-situ generation of nanocrystals, including graphite, belt-like silicon nitride (Si₃N₄), and silicon carbide (SiC) whiskers, in amorphous SiBCN matrix after annealing. At the same time, these dielectric lossy phases successfully constructed multiple heterogeneous interfaces and three-dimensional network structures. Consequently, the conductivity of the ceramics increased from 4.49 × 10⁻⁹ (annealed at 800 °C) to 0.67 × 10⁻⁴ S cm⁻¹ (annealed at 1600 °C). The real part of permittivity improved from 4.57–3.36 (annealed at 800 °C) to 10.90–8.38 (annealed at 1600 °C) in the frequency range of 2–18 GHz. The formation of multiple heterogeneous interfaces caused interfacial polarization and increased the multiple relaxations, which ultimately led to a superior microwave absorption property with a minimum reflection loss (RL_min) of −34.28 dB and an effective absorption bandwidth (EAB) of 3.76 GHz (8.64–12.4 GHz).     

Long-term oxidation behaviors and strength retention properties of self-healing SiCf/SiC-SiBCN composites

The self-healing SiC_f/SiC-SiBCN composites with various boron contents in SiBCN were prepared, and their long-term oxidation behaviors and strength retention properties were investigated. The 100 h oxidation at 1200–1350 °C leads to parabolic mass gain of the obtained composites. With the oxidation temperature increased from 1200 °C to 1350 °C, the oxidation rate constants increase from 5.91 × 10^?8 mg²/(mm⁴ h) to 9.31 × 10^?7 mg²/(mm⁴ h) for the boron-lean (3.14%) composites, and from 2.57 × 10^?7 mg²/(mm⁴ h) to 6.04 × 10^?7 mg²/(mm⁴ h) for the boron-rich (7.18 wt%) composites. Correspondingly, the oxidation activation energy decreases from 363 kJ/mol to 112 kJ/mol due to the low initial oxidation temperature of boron-rich SiBCN. All the composites exhibit the higher strength retention rates after 1350 °C oxidation due to the enhanced self-healing performance. The boron-rich composites show a high strength retention rate of up to 104% due to the good self-healing capacity of the boron-rich SiBCN as well as the high CVI-SiC content.     

Preparation of self-healing Cf/SiBCN(O) composite using a novel polyborosilazane

In this study, novel polyborosilazane-derived SiBCN(O) ceramic was used as self-healing component in self-healing C_f/SiBCN(O) composite, which was prepared by polymer infiltration and pyrolysis (PIP) process. Molecular-level structure design of boron-containing ceramic precursors was utilized to achieve uniform dispersion of boron-containing self-healing components in prepared composites. No elemental diffusion was observed at the interface of ceramic matrix and carbon fibers, which resulted in stable SiBCN(O) structure. In addition, boron was uniformly distributed in C_f/SiBCN(O) composite ceramic matrix, which was beneficial for self-healing of cracks. Cracks and indentations were able to heal at high temperatures in air. The best crack-healing behavior occurred in air atmosphere at 1000 °C, with nearly complete crack healing. This excellent self-healing behavior was achieved because silicon and boron atoms in SiBCN(O) ceramic reacted with available oxygen at high temperatures to form SiO₂(l), B₂O₃(l), and B₂O₃·xSiO₂ liquid phases, which effectively filled cracks. In general, as-prepared C_f/SiBCN(O) composite exhibited excellent self-healing properties and shows great application potential in high-temperature environment applications such as aviation, aerospace, and nuclear power.