Electromagnetic interference shielding properties of polymer derived SiC-Si_3N_4 composite ceramics

SiC-Si₃N₄ composite ceramics are successfully fabricated by pyrolysis of ferrocene-modified polycarbosilane (PCS) mixed with inert filler Si₃N₄ powders, followed by thermal treatment from 1100 °C to 1400 °C in Ar atmosphere. The porosity of SiC-Si₃N₄ ceramics decreases to 6.4% due to the addition of inert filler Si₃N₄. And the content and crystallization degree of free carbon and SiC derived from PCS are improved simultaneously with the increase of thermal treatment temperature. Finally, the free carbon and SiC interconnect, forming the conductive network. As a result, the electromagnetic interference (EMI) shielding performance of the as-prepared ceramic annealed at 1400 °C reaches up to 36 dB, meaning more than 99.9% of EM energy is shielded. The low porosity and high EMI shielding performance enable SiC-Si₃N₄ composite ceramics to be a promising electromagnetic shielding and structural material.     

Properties of porous Si3N4 ceramic electromagnetic wave transparent materials prepared by technique combining freeze drying and oxidation sintering

A simple and low-cost technique combining freeze drying and oxidation sintering is explored to prepare Si₃N₄ ceramics with high porosity and complex shape. The effects of sintering temperature and time on the phase composition, microstructure, porosity, pore size and dielectric constant of the porous Si₃N₄ ceramics are studied. Due to the variations of phase composition and microstructure, the porous Si₃N₄ ceramics sintered at different temperature possess characteristic in flexural strength. The porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h have the highest flexural strength of 71–74 MPa. The changes of porosity and composition have much effect on the dielectric constant of porous Si₃N₄ ceramics. Because of the high porosity and SiO₂ volume fraction, the porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h possess low dielectric constant of 3.4–3.6 and small pore size of 0.9 μm. The porous Si₃N₄ ceramics are good structural/functional and promising electromagnetic wave transparent material.     

Microwave dielectric properties of (1 − x)(Mg0.4Zn0.6)2SiO4–xCaTiO3 composite ceramics

Phase formation, microstructure and microwave dielectric properties of (1 ? x)(Mg_0.4Zn_0.6)₂SiO₄–xCaTiO₃ (MZS-C) composite ceramics synthesized by using the conventional solid-state method were systematically investigated. Three phase structure was observed in all samples by using X-ray diffraction and the back scattering electron images. Mg₂SiO₄ can form a solid solution with Zn₂SiO₄, which improved sinterability of the MZS-C composite ceramics. As the CaTiO₃ content was increased from 0.06 to 0.14, dielectric constant ε _r and temperature coefficient of resonant frequency τ _f values of the MZS-C ceramics sintered at 1,180 °C for 4 h increased from 6.74 to 8.35 and ?41.5 to ?6.46 ppm/°C, respectively. Zero τ _f value can be obtained by properly adjusting the x value of the (1 ? x)MZS–xC ceramics. With increasing content of CaTiO₃, densification temperatures of the composite ceramics were decreased. The composite ceramic with x = 0.14 sintered at 1,180 °C for 4 h exhibited excellent microwave dielectric properties of ε _r = 8.35, Q × f = 28,125 GHz and τ _f = ?6.46 ppm/°C.     

Mechanical and electromagnetic wave absorption properties of C_f-Si_3N_4 ceramics with PyC/SiC interphases

Aiming to obtain microwave absorbing materials with excellent mechanical and microwave absorption properties, carbon fiber reinforced Si_3N_4 ceramics(Cf-Si_3N_4) with pyrolytic carbon(PyC)/SiC interphases were fabricated by gel casting. The influences of carbon fibers content on mechanical and microwave absorption properties of as-prepared Si_3N_4 based ceramics were investigated. Results show that chemical compatibility between carbon fibers and Si_3N_4 matrix in high temperature environment can be significantly improved after introduction of Py C/SiC interphases. As carbon fibers content increases from 0 to 4 wt%, flexural strength of Si_3N_4 based ceramics decreases slightly while fracture toughness obviously increases. Moreover, both the real and imaginary parts of complex permittivity increase with the rising of carbon fibers content within the frequency range of 8.2–12.4 GHz. Investigation of microwave absorption shows that the microwave attenuation ability of Cf-Si_3N_4 ceramics with Py C/SiC interphases is remarkably enhanced compared with pure Si_3N_4 ceramics. Effective absorption bandwidth(-10 d B) of10.17–12.4 GHz and the minimum reflection less of-19.6 d B are obtained for Si_3N_4 ceramics with 4 wt%carbon fibers in 2.0 mm thickness. Cf-Si_3N_4 ceramics with Py C/SiC interphases are promising candidates for microwave absorbing materials with favorable mechanical property.     

NMR and X-ray diffraction studies of amorphous and crystallized pyrolysis residues from pre-ceramic polymers

²⁹Si MAS NMR and X-ray diffraction studies are presented of black and white pyrolysis residues obtained by initial 1100°C pyrolyses in N₂ and NH₃ atmospheres followed by 1550°C pyrolyses in Ar, N₂ or vacuum atmospheres of a polycarbosilane and four polysilazane precursors to SiC and Si₃N₄ ceramics. Amorphous white pyrolysis residues crystallized under the various conditions to give not only Si₃N₄ but also Si₂N₂O, SiC, SiO₂ and Si, while black amorphous pyrolysis residues crystallized to form only Si₃N₄ or SiC. In general, the crystalline ceramic products observed depended on a variety of factors, i.e. moisture sensitivity of polymer, the initial 1100°C pyrolysis gas (N₂/NH₃), the dryness of the 1100°C-NH₃ pyrolysis gas and the 1550°C pyrolysis atmosphere (N₂, Ar, vacuum). An additional factor of interest affecting product distribution was the choice of crucible (alumina/graphite) employed in the 1550°C pyrolysis. The combined studies suggest that the white amorphous pyrolysis residues are complex silicon oxycarbonitrides (Si_xN_yO_zC_a), while the amorphous black residues are silicon carbonitrides (Si_xN_yC_z). This revised version was published online in November 2006 with corrections to the Cover Date.     

Low temperature sintering and dielectric properties of Li2ZnTi3O8–TiO2 composite ceramics doped with CaO–B2O3–SiO2 glass

The effects of CaO–B₂O₃–SiO₂ (CBS) glass addition on the sintering temperature and dielectric properties of Li₂ZnTi₃O₈–TiO₂ (LZT) composite ceramics have been investigated. Due to the compensating effect of rutile TiO₂ (τ_f ≈ +450 ppm/ °C), the temperature coefficient of resonant frequency (τ_f) for Li₂ZnTi₃O₈ + 4 wt% TiO₂ with biphasic structure was adjusted to a value near zero. The pure LZT ceramics were usually sintered at high temperature of about 1,160 °C. It was found in our experiment that a small amount of CBS glass additives could effectively lower the sintering temperature of LZT ceramics to 900 °C. With increasing the content of CBS glass, both of dielectric constant (ε_r) and quality factor (Q × f) value decreased. Typically, the 1 wt% CBS glass added Li₂ZnTi₃O₈ + 4 wt% TiO₂ ceramic sintered at 900 °C for 4 h exhibited good microwave dielectric properties of ε_r = 26.9, Q × f = 23,563 GHz and τ_f = ?1.5 ppm/ °C, which made it promising for low temperature co-fired ceramics technology application.     

Manufacture of fibrous β-Si3N4-reinforced biomorphic SiC matrix composites for bioceramic scaffold applications

The manufacturing of the Si₃N₄ reinforced biomorphic microcellular SiC composites for potential medical implants for bone substitutions with good biocompatibility and physicochemical properties was performed in a two step process. First, wood-derived porous Si/SiC ceramics with various porosities were produced by liquid silicon infiltration (LSI) at 1550 °C with static nitrogen atmosphere protection (0.1 MPa), followed by subsequent partial removing of the Si in vacuo at 1700 °C for different periods of time. Secondly, the final porous Si₃N₄ fiber/SiC composite was obtained by further chemical reaction of nitrogen with the infiltrated residual silicon at 1400 °C for 4 h under high concentration flowing nitrogen atmospheres (0.5 MPa). The bending strengths of the porous Si₃N₄ fiber/SiC composite at axial and radial direction were measured as 180.03 MPa and 90 MPa respectively. The improvement in bending strength was primarily attributed to grain pull-out and bridging enhanced by the elongated β-Si₃N₄ grains cross-linked in the depth of the pore channels. The TG analysis showed an obvious improvement in oxidation resistance of the nitride specimens.     

Effect of starting PMMA content on microstructure and properties of gel casting BN/Si3N4 ceramics with spherical-shaped pore structures

By gel casting with polymethylmethacrylate microbeads (PMMA) as pore-forming agent, porous boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were successfully prepared. The obtained ceramic shows bimodal hierarchical structures that composed of spherical-shaped micro pores depending on PMMA content and irregular sub-micro pores formed by the stacking of ceramic particles. Porosity of the porous BN/Si₃N₄ ceramics can be well controlled from 53.0 to 60.6 % by the PMMA content from 10 to 40 wt%, as well as the mechanical and dielectric properties. Effect of PMMA content on phase composition and the relationship between microstructure and the basic properties of the porous BN/Si₃N₄ ceramics was discussed in detail. Microstructure analysis reveals that the sub-micro pores acted as channels between micro pores. BN particles have a relatively denser distribution on the wall of spherical-shaped micro pores with a window between micro and sub-micro pores, and resulting in a half-closed micro pore structure, which is meaningful for material design with concentration of BN particles on the wall of pore structure.     

Silicon nitride matrix composites with unidirectional silicon carbide whisker reinforcement

SiC whisker reinforced Si₃N₄ was fabricated by fiber extrusion and hot pressing. SiC whiskers were unidirectionally oriented in a carrier fiber. The fibers containing the oriented whiskers were hot pressed in Si₃N₄ powder to form a SiC_w/Si₃N₄ composite with approximately 5 volume% whiskers. SEM micrographs were image processed to quantify whisker orientations in the extruded fiber and the composite. Oriented whiskers contributed to nominal increase in fracture strength over monolithic samples before and after thermal shock testing from 500, 600 and 700°C.     

Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 μ and their mixture with Si of ∼ 4.83 μ particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si₃N₄ phase and blunting crack front. Formation of Si₃N₄ whisker was also observed. The strength of the mixture composition was maximum.     

Mechanical properties of Si3N4/BN fibrous monolithic ceramics at elevated-temperature

The mechanical properties at high temperature of Si₃N₄/BN fibrous monolithic ceramics were tested. The flexural strength of SiC whisker reinforced Si₃N₄/BN fibrous monolithic ceramics from 25°C to 1200°C were investigated. The strength degraded slowly from 1000°C to 1200°C which was different to Si₃N₄ monolithic ceramics. The creep behaviors of the material at different temperatures were characterized. Si₃N₄/BN fibrous monolithic ceramics possess high creep resistance. The chemical composition and microstructure of the composites were analyzed by XRD and SEM.     

Initial nucleation of amorphous Si–B–C–N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000?C followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200?C and dominate at 1500?C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400?C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si_3N_4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si_3N_4-like clusters at 1200–1400?C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si_3N_4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si_3N_4-like clusters as well as from the excessive oxygen to form the stable SiO_2.     

Si3N4/hBN复相陶瓷的研究现状及进展

Si3N4/hBN复相陶瓷凭借优良的综合力学性能,逐渐成为人们研究的热点。从制备工艺和性能出发阐述了国内外Si3N4/hBN复相陶瓷的研究现状,分析了各种制备方法的优缺点以及hBN含量对Si3N4/hBN复相陶瓷的可加工性能、力学性能、介电性能、摩擦学性能的影响,并指出简化工艺、降低烧结温度、hBN含量与性能定量表征等可能是今后的发展方向。     

Microstructure and microwave dielectric properties of low-temperature sinterable (1 − x)Ba3(VO4)2–xCaWO4 composite ceramics

Low-temperature-sintered Ba₃(VO₄)₂–CaWO₄ composite ceramics were prepared by cofiring mixtures of pure-phase Ba₃(VO₄)₂ and CaWO₄. The thermo-mechanical analysis revealed that the CaWO₄ in the composite ceramic can significantly promote the densification process and lower the sintering temperature to ~900 °C. The X-ray diffraction results indicated that Ba₃(VO₄)₂ and CaWO₄ phases coexist in the sintered ceramics, and no secondary phases can be detected in the composite, implying the good chemical compatibility between the two phases. The near-zero temperature coefficients of the resonant frequency (τ_f) could be achieved by adjusting the relative content of the two phases owing to their opposite τ_f values. The composite ceramics with 60 wt% CaWO₄ sintered at 900 °C exhibited desirable microwave dielectric properties of the quality factor Q × f ~ 37,000 GHz, dielectric constant ε_r ~ 12, and τ_f ~ ?1.4 ppm/°C.     

Effect of heat treatment temperature on microstructure and electromagnetic shielding properties of graphene/SiBCN composites

Three-dimensional (3D) graphene/SiBCN composites (GF/SiBCN) were prepared by depositing SiBCN ceramics in 3D graphene foam via the chemical vapor infiltration technique. The effect of the heat treatment temperature on the microstructure, phase composition, and electromagnetic properties of the GF/SiBCN composite was investigated. The SiBCN ceramics maintained an amorphous structure in the composite below 1400 °C above which the crystallinity of the free carbon phase gradually increased. While the Si₃N₄ and B₄C phases started to crystallize at 1500 °C and their crystallinity increased with temperature, SiC was observed at 1700 °C. The electromagnetic shielding effectiveness of GF/SiBCN increased with the heat treatment temperature.     

Effects of LiF addition on the sintering behavior and microwave dielectric properties of Li<Subscript>2</Subscript>Mg<Subscript>3</Subscript>SnO<Subscript>6</Subscript> ceramics

Li₂Mg₃SnO₆ (abbreviation for LMS) ceramics doped with 1–4 wt% lithium fluoride (LiF) were prepared by the conventional solid-state reaction method. The effects of LiF addition on the phase compositions, sintering behaviors and microwave dielectric properties of LMS ceramics were investigated. A small amount of LiF addition could effectively decrease the sintering temperatures due to the liquid phase in the sintering process and induced no apparent degradation of the microwave dielectric properties. The optimized quality factor values for each composition firstly increased and then decreased with the increase of the LiF content. Whereas, the optimized dielectric permittivity increased with increasing of the LiF content. Distinguished microwave dielectric properties with a dielectric constant (ε _r) of 11.13, a quality factor (Q·f) of 104,750 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?10.83 ppm/°C were obtained for LMS ceramics sintered at 950?°C doped with 3 wt% LiF, which showed that the materials were suitable for the low temperature co-fired ceramics applications (LTCC).     

Study on low temperature sintering and electrical properties of CuO-doped Ca0.28Ba0.72Nb2O6 ceramics

Ca_0.28Ba_0.72Nb₂O₆ (CBN28) ceramics with different content of CuO were prepared by the conventional ceramic fabrication technique. The effects of CuO content on the phase structure, microstructure, dielectric and ferroelectric properties of obtained CBN28 ceramics were investigated. XRD results showed that pure tungsten bronze structure was obtained in all ceramics and CuO additive could accelerate the phase formation at lower temperatures. The CuO aid was effective for the uniform grain size in CBN28 ceramics, as it could facilitate the sintering behavior and suppress the anisotropic growth behavior obviously. The dielectric and ferroelectric properties of CBN28 ceramics depended greatly on the CuO content. Curie temperature T _c and dielectric loss tanδ both shifted downward, whereas the maximum dielectric constant ε _m and the dielectric constant around room temperature ε _r all increased initially and then decreased as x increased from 0.1 to 0.4 wt%. Normal ferroelectric hysteresis loops could be observed in all compositions, and the remnant polarization P _r decreased gradually. It was found that the comprehensive electric performance was optimized in CBN28-0.2 wt% CuO ceramics: ε _r = 453, ε _m = 3,371, T _c = 226 °C, tanδ = 0.048, P _r = 4.72 μC/cm² and E _c = 13.81 kV/cm, showed that CuO sintering aid could not only ameliorate the sintering behavior but also improve the electrical properties.     

Dielectric properties in GHz range of porous Si3N4–BN–SiO2 ceramics with considerable flexural strength prepared by low temperature sintering in air

Abstract

Porous Si₃N₄–BN–SiO₂ ceramics with ultimate apparent porosities between 0·140 and 0·799 were fabricated in air at 1100°C by partial sintering using core starch as both consolidator and pore former in the green bodies. The pores were derived from burning off the starch, the partial oxidation of silicon nitride and the stack of particles of the start materials. Effect of retaining time on the microstructure of sintering bodies was analysed by SEM analysis. Reference intensity ratio (RIR) technique based on the X-ray diffractometry results demonstrated the phase components content of sintered bodies. Influence of porosity on the flexural strength of porous Si₃N₄–BN–SiO₂ ceramics was investigated. The ceramic with a porosity of 0·140 attained a maximal flexural strength of 60±4·11 MPa. In addition, the dielectric constants and loss tangents were presented for porous Si₃N₄–BN–SiO₂ triphase ceramics in the frequency range of 18–40 GHz, and the real part of dielectric constant of the materials reached as low as 2·67 at the porosity of 0·732 at a frequency of 20 GHz.     

SiCw/Al复合材料的一种界面结构

应用透射电镜技术研究了用高压凝固铸造法(Squeeze casting)制备的碳化硅晶须增强铝(SiCw/Al)复合材料界面两侧晶须与基体的位向关系。结果表明,碳化硅晶须与基体铝之间至少存在一种匹配关系,这种关系是:<110>_SiC//<110>_Al,<111>_SiC//<100>_Al(约差4°)。由此提出了SiCw/Al复合材料的半共格界面结构模型。      

Fatigue Hysteresis Behavior of Unidirectional SiC/Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Composite at Elevated Temperature under Tension-Tension Loading

In this paper, the fatigue hysteresis behavior of unidirectional SiC/Si₃N₄ ceramic-matrix composite at elevated temperature has been investigated. The hysteresis loops models considering interface friction between fibers and the matrix have been developed to establish the relationships between the fatigue hysteresis loops, fatigue hysteresis dissipated energy and the interface frictional coefficient. Using the experimental fatigue hysteresis dissipated energy, the interface frictional coefficient of SiC/Si₃N₄ composite at 1000 °C were obtained for different cycle numbers and fatigue peak stresses. The effects of fatigue peak stress, test temperature and cycle number on the evolution of fatigue hysteresis dissipated energy and interface frictional coefficient have been analyzed. It was found that the fatigue hysteresis dissipated energy can be used to monitor the interface debonding and damage evolution inside of the composite.