Crystallization Mechanism of CVD Si3N4–SiCN Composite Ceramics Annealed in N2 Atmosphere and Their Excellent EMW Absorption Properties

To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si₃N₄–SiCN composite ceramics were prepared from SiCl₄–NH₃–C₃H₆–H₂–Ar system and then annealed at the temperatures of 1400–1700°C in N₂ atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave‐absorbing properties of the ceramic were investigated. Results showed that the CVD Si₃N₄–SiCN ceramics gradually crystallized into nanosized SiC grains, Si₃N₄ grains and graphite (T ≤ 1600°C), and then the grains grew up at T = 1700°C. The permittivity, dielectric loss, and electrical conductivity of as‐annealed CVD Si₃N₄–SiCN ceramics (T ≤ 1600°C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700°C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as‐annealed CVD Si₃N₄–SiCN ceramic at 1600°C was ?41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si₃N₄–SiCN ceramic (T = 1600°C) had the superior microwave‐absorbing ability.     

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

A kind of chemical vapor infiltration (CVI) Si₃N₄–BN–SiCN composite ceramic with excellent electromagnetic wave (EMW) absorbing properties is obtained by CVI BN interface and SiCN matrix on porous Si₃N₄ ceramics, and then annealed at high temperatures (1200°C‐1500°C) in N₂ atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI SiCN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si₃N₄–BN–SiCN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient (RC) and maximum effective absorption bandwidth (EAB) of the composite ceramics annealed at 1300°C were ?47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m^1/2, respectively.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

From design to characterization of zirconium nitride/silicon nitride nanocomposites

ZrN/Si₃N₄ nanocomposites have been prepared by chemically crosslinking two polysilazanes with a zirconium-based compound and subsequent heat-treatment at temperatures ranging from 1000 to 1600 °C. The polymer synthesis has been systematically investigated using FT-IR, solid-state NMR, and elemental analyses. Then, the pyrolysis under ammonia at 1000 °C trigering the thermo-chemical polymer-to-ceramic conversion was examined, leading to X-ray amorphous ceramics with yields governed by the chemistry of the neat polysilazane. Investigations of the structural evolution of the single-phase amorphous ceramic network above 1000 °C by X-ray diffraction and Raman spectroscopy pointed out that the ZrN phase already segregated at 1400 °C and formed highly crystalline ZrN/Si₃N₄ nanocomposites at 1600 °C. HRTEM investigations validated the unique nanostructural feature of the nanocomposites made of ZrN nanocrystals distributed in α- and β-Si₃N₄ phases. Our preliminary investigations of the optical properties showed that these structural changes allowed tuning the optical properties of ZrN/Si₃N₄ nanocomposites.     

Silicon Oxynitride Ceramics Prepared by Plasma Activated Sintering of Nanosized Amorphous Silicon Nitride Powder without Additives

Si₂N₂O ceramics were prepared by plasma activated sintering using nanosized amorphous Si₃N₄ powder without sintering additives within a temperature range of 1400°C–1600°C in vacuum. A mixed Si–N_4?n–O_n (n = 0, 1…4) amorphous structure was formed in the process of sintering, and Si₂N₂O crystals were nucleated where the local structure was similar with Si₂N₂O. After sintering at 1600°C, the Si₂N₂O ceramic was composed of elongated plate‐like Si₂N₂O grains and amorphous phase. The Si₂N₂O grains showed a width of less than 100 nm and a very high aspect ratio.     

Polymer-derived SiHfN ceramics: From amorphous bulk ceramics with excellent mechanical properties to high temperature resistant ceramic nanocomposites

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.     

Polymer-derived Co2Si(Co)/SiCN composite ceramics with tunable microwave absorption properties

In this paper, Co₂Si(Co)/SiCN composite ceramics were synthesized by simple precursor-derived ceramics method. The phase composition, morphology, and microwave absorption properties of Co₂Si(Co)/SiCN composite ceramics at different pyrolysis temperatures (1000–1400°C) were studied. When pyrolysis temperature was 1300°C, carbon nanowires (CNWs), Co₂Si, Si₂N₂O, SiC and Si₃N₄ were in situ generated and the best electromagnetic wave (EMW) absorption performance was obtained. The minimum reflection loss reached−50.04 dB at 4.81 mm, and the effective absorption bandwidth broadened to 3.48 GHz (14.52–18 GHz) at 1.31 mm. The excellent EMW absorption performance mainly comes from the coexistence of multiple loss mechanisms, including the magnetic loss of Co₂Si, the conduction loss of CNWs, and the heterogeneous interfaces polarization between varieties of nanocrystals and amorphous ceramic matrix. By adjusting the sample thickness from 1 to 5 mm, the effective absorption of S1300 can cover the entire X and Ku bands, from 3.36 to 18 GHz. This study provides a simple way to synthesize high performance ceramic-based microwave absorbing materials.     

Single-source-precursor synthesis and high-temperature evolution of a boron-containing SiC/HfC ceramic nano/micro composite

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. B_yC). High temperature annealing of B_yC/SiHfC(N,O) leads to the precipitation of HfC_xN_1-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/HfC_xN_1-x/HfB₂/C ceramic nano/micro composite.     

Heat treatment of silica-fiber-reinforced BN-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> matrix composite

A new wave-transparent composite reinforced by silica fibers with a hybrid matrix comprising BN and Si₃N₄ was prepared by precursor infiltration and pyrolysis, and it was heat-treated at elevated temperatures. The variations of the composite during heat treatments were characterized and investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as-received composite exhibits good mechanical properties, and it is almost amorphous. When treated at 1600°C, it turned brittle, and silica fibers in it were fused; the composite showed a good crystalline form. When treated at 2100°C, the composite broke into pieces, and the composition showed only BN. Si₃N₄ was decomposed, and silica fibers were volatilized. The presence of BN probably affected the phase transitions of silica fibers. __________ Translated from Novye Ogneupory, No. 8, pp. 49–52, August 2007.     

Advanced pressurelessly prepared adhesive composites based on modified preceramic polymer for the joining of amorphous SiBON ceramics

A series of polysilazane (PSZ)-based adhesives, which could be employed for high temperature environment in aerospace industry, were prepared to bind amorphous SiBON ceramics. It was found that the addition of dopants had a significant influence on the binding strength of samples, and physicochemical properties of adhesives were also closely correlated to their kinds and quantities. The fracture mechanism and phase transformation in joints were characterized. New phases including C₃N₄, Si₃N₄, SiC, BN, Si, ZrB₂ and amorphous SiCN, were formed with the increase of treatment temperature, influencing the binding strengths of samples. The maximum binding strength of joints could reach 16 MPa after heating at 1100 °C in N₂. The macroscopic and microscopic images of the fractured surface of joints and finite element analysis of the joint indicated that the failure of the joint occurred not only at the binding layer but also at the ceramic substrate. This work can serve as a guide for binding amorphous ceramic for high temperature environment.     

Single-source-precursor synthesis and high-temperature evolution of novel mesoporous SiVN(O)-based ceramic nanocomposites

Mesoporous SiVN(O) ceramics were prepared from a mixture consisting of VO(acac)₂-modified perhydropolysilazane and polystyrene. The resulting amorphous single-phase SiVN(O) ceramics remained amorphous in nitrogen atmosphere up to 1400 °C. The as-prepared materials consist of nanoscaled vanadium nitride dispersed in amorphous Si₃N₄; exposure to 1600 °C leads to the crystallization of VN and Si₃N₄. The specific surface area (SSA) and the pore size of the SiVN(O)-based ceramics can be easily controlled by the temperature of thermal treatment and by the amount of polystyrene. The average pore size of the prepared SiVN(O) ceramics was 4–10 nm and their largest SSA values, 642 and 506 m²/g, were achieved upon ammonolysis at 800 and 1000 °C, respectively. The combination of metal-modified single-source precursors and encapsulated porogens provides a convenient one-pot synthesis process to prepare mesoporous ceramic nanocomposites with controllable phase compositions and morphology.     

Synthesis and thermal evolution of polysilazane-derived SiCN(O) aerogels with variable C content stable at 1600 °C

Ceramic aerogels possess intriguing thermophysical properties which make them excellent candidates for high temperature thermal insulators. However, their properties can degrade at high temperature because of crystallization phenomena or because of densification (causing a sensible reduction of their specific surface area and porosity).The polymer derived ceramic (PDC) route is a relatively new way of developing ceramic aerogels. Several aspects influence the properties of the final product when dealing with preceramic polymers, among them their chemical composition and molecular architecture.In this work, we investigated the possibility of producing aerogels belonging to the SiCN system from polysilazanes mixtures, namely perhydropolysilazane (PHPS) and a methyl/vinyl-containing polysilazane, namely Durazane 1800®, thus changing the C/Si ratio of the amorphous pyrolyzed products. It is shown that the chemical composition of the ceramic aerogel affects the main properties of the porous materials, such as thermal stability and specific surface area (SSA). Results show that the presence of carbon in the aerogels inhibits crystallization of Si₃N₄ up to 1600 °C in N₂ and allows to maintain a SSA of ~90 m²/g up to this temperature.     

Effects of boron content on the microwave-transparent property and high-temperature stability of continuous SiBN fibers

Owing to their high-temperature stability and microwave-transparent properties, SiBN fibers are promising reinforcement materials for microwave windows or radomes in harsh environments. This work investigates the high-temperature properties of continuous SiBN fibers with different boron contents. Compared with Si₃N₄ fibers, the SiBN fibers demonstrated superior high-temperature stability, retaining their original tensile strength up to 1600°C with a smooth surface and amorphous structure. The boron-containing microstructure improved the microwave-transparent properties of the SiBN fibers by decreasing their dielectric constant and loss. The values were stabilized at temperatures up to 1200°C. The high-temperature stability and microwave transparency of the SiBN fibers were likely contributed by the synergistic effect of the amorphous BN phase and the Si–N–B networks (the main existing state of the boron atoms).     

The influence of pyrolysis temperature on the oxidation resistance of carbon-rich SiCN ceramics derived from reaction of silazanes with acrylonitrile

The influence of pyrolysis temperature on the oxidation resistance of carbon-rich SiCN ceramics derived from reaction of acrylonitrile with a commercially available oligosilazane (HTT1800) and its subsequent pyrolysis in nitrogen atmosphere was evaluated. The investigation of the oxidation behavior reveals that the final pyrolysis temperature plays an important role in the oxidation resistance of the developed SiCN/C nanocomposites. Increasing the pyrolysis temperature promotes an ordering of the free-carbon phase and consequently, an improvement of the oxidation resistance is noticed. However, starting phase separation during annealing at 1500 °C reduces slightly the protection effect provided by the oxidation resistant Si₃N₄ phase.     

High-purity stoichiometric Si3N4 ceramics through trimethylsilyl-substituted polysilazanes

Trimethylsilyl-substituted polysilazanes were designed and successfully synthesized. They were used to fabricate high-purity stoichiometric Si₃N₄ ceramics through pyrolysis process. Trimethylsilyl groups improved the stability of polysilazanes and easily escaped during pyrolysis, which effectively reduced oxygen and carbon content in the final polymer-derived Si₃N₄. The C content of Si₃N₄ ceramic was below 0.06 wt%, and the O content was below 1.2 wt%. The Si₃N₄ ceramics remained amorphous up to 1400°C, yet they were completely transformed into α-Si₃N₄ at 1500°C. Synergistic effect from low oxygen and carbon content contributed to highly stable amorphous state of Si₃N₄ till high temperatures. This amorphous Si₃N₄ ceramics could be used in cutting-edge technology where high purity is compulsory.     

Joining of Si3N4/Si3N4 with in-situ formed Si-Al-Yb oxynitride glasses interlayer

Si₃N₄ ceramic was successfully joined to itself with in-situ formed Yb-Si-Al oxynitride glass interlayer. The joints were composed of three parts: (I) Si₃N₄ matrix, (II) oxynitride glass interlayer in which hexagonal or fine elongated β-sialon grains and a few ball-like β-Si₃N₄ grains exist, and (III) diffusion zone in Si₃N₄ matrix containing a thin dark layer and a ~ 25?µm thick bright layer. The seam owned similar microstructure to matrix and was inosculated with the matrix as a whole. The strength of the joint tended to increase with the increase of bonding temperature and reached the value of 225?MPa, when the joints were prepared at 1600?°C for 30?min under a pressure of 1.5?MPa. The high-temperature strength remained 94.7% and 75.2% of R.T. strength when the joints were tested at 1000?°C and 1200?°C, respectively. It may be contributed to the high softening temperature of the Yb-Si-Al oxynitride glass phase formed in the seam. Even suffered to the air exposure for 10?h at 1200?°C, the residual strength of the joints was still 143?MPa, attributed to the existence of YbAG phase.     

Assessment of the high temperature elastic and damping properties of silicon nitrides and carbides with the impulse excitation technique

The impulse excitation technique (IET), based on resonant vibration analysis, was used to determine the high temperature elastic and damping properties of hot-pressed silicon nitride (Si₃N₄) and silicon carbide (SiC) materials with Al- and Y-additives. The Si₃N₄ materials were doped with 2 wt.% of TiN to suppress the crystallization of intergranular glassy pockets. Near 1000 °C the investigated materials display a characteristic damping peak, which is essentially unaffected by temperature excursions up to 1400 °C. Two existing models which aim at linking elastic and damping properties with microstructural and micromechanical details, are considered. One of the models is used to provide an estimate of the amount of amorphous intergranular pocket phase. This type of information is of particular relevance since the high temperature deformation resistance of silicon based ceramics is severely dependent on the presence and amount of amorphous multiple grain pockets.     

Enhanced hydrogen evolution reaction catalyzed by carbon-rich Mo4.8Si3C0.6/C/SiC nanocomposites via a PDC approach

In this study, mesoporous carbon-rich Mo_4.8Si₃C_0.6/C/SiC ceramic nanocomposites were successfully prepared via a single-source precursor route, starting from allylhydridopolycarbosilane (AHPCS, SMP-10), bis(acetylacetonato) dioxomolybdenum (VI) [MoO₂(acac)₂], and divinylbenzene (DVB). Besides, polystyrene (PS) was used as a pore former. The obtained carbon-rich single-source precursor/PS mixtures were pyrolyzed at 1100°C, and then annealed at 1350°C-1600°C to fabricate a series of carbon-rich Mo_4.8Si₃C_0.6/C/SiC ceramics comprised of high carbon content above 50 wt%. In comparison to the carbon-poor materials, the carbon-rich samples retain the higher specific surface area up to 214.6-304 m²/g at higher annealing temperatures (1350°C-1600°C) due to the enhancement of carbothermal reaction. The carbon-rich samples synthesized at 1500°C, denoted as SM/Mo/PS/DVB 2-1-4-2 1500 exhibit enhanced electrocatalytic performance with ultra-low overpotentials of 119 mV vs reversible hydrogen electrode at a current density of 10 mA cm⁻² in acidic media, which is superior to that of the Mo_4.8Si₃C_0.6/C/SiC ceramic (138 mV) with lower carbon content reported in our previous study. Therefore, our porous materials comprised of high carbon content and Nowotny phase (Mo_4.8Si₃C_0.6, NP) are considered as promising catalysts for the hydrogen evolution reaction (HER).     

Dielectric and microwave absorption properties of polymer derived SiCN ceramics annealed in N2 atmosphere

Porous SiCN ceramics were successfully fabricated by pyrolysis of a kind of polysilazane. The effects of annealing temperature on the microstructure evolution, direct-current electrical conductivity, dielectric properties, and microwave absorption properties of SiCN in the frequency range 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, SiC, Si₃N₄ and free carbon nanodomains are gradually formed in the SiCN. Both the SiC and free carbon nanodomains lead to the increases of the complex relative permittivity and loss tangent of SiCN. With the increase of the annealing temperature, the average real permittivity, imaginary permittivity and loss tangent increase from 4.4, 0.2 and 0.05 to 13.8, 6.3 and 0.46, respectively. The minimum reflection coefficient and the frequency bandwidth below −10 dB for SiCN annealed at 1500 °C are −53 dB and 3.02 GHz, indicating good microwave absorption properties.     

Hierarchically structured polymer-derived ceramic fibers by electrospinning and catalyst-assisted pyrolysis

Hierarchically structured polymer-derived ceramic fibers were successfully produced by electrospinning a commercially available preceramic polymer to which a cobalt-based catalyst precursor was added, followed by pyrolysis in nitrogen at temperatures ranging from 1250 to 1400 °C. The nanowires formed via the vapor–liquid–solid (VLS) mechanism, involving the reaction of SiO and CO gases, generated from the decomposition of the polymer-derived-ceramic at high temperature, with the heating atmosphere assisted by the presence of nano-sized CoSi droplets. The main crystalline phase for the nanowires was Si₃N₄ below 1350 °C, and Si₂N₂O at 1400 °C, and the amount of nanowires increased with increasing heating temperature. Hierarchically structured fiber mats possessed a higher specific surface area (14.45 m²/g) than that of a sample produced without the cobalt catalyst (4.37 m²/g).