A Crystalline Si3N4/Amorphous Si3N4 Composite

A composite consisting of elongated α-Si₃N₄ crystallites (5–50 (Am in diameter) embedded in an amorphous Si₃N₄ matrix was synthesized by chemical vapor deposition. The hardness and indentation fracture toughness of the amorphous matrix and of the composite have been evaluated at temperatures from ambient to 1200°C. It was found that the crystallites have relatively little influence on the hardness and indentation fracture toughness when the surrounding matrix is amorphous. However, a 1400°C heat treatment of the material results in a matrix consisting of small crystals (100 nm in diameter) surrounded by carbon-containing regions which appear to be amorphous in the TEM; TEM and EELS in nearby triple points revealed the presence of amorphous carbon. After heat treatment, the indentation fracture toughness at ambient and at 1200°C is increased due to extensive microcracking. The Vickers hardness at 1200°C also increased significantly as a result of the heat treatment. The relationship between the mechanical properties and the microstructure is discussed.     

Microstructure and mechanical properties of Si3N4f/Si3N4 composites with different coatings

The Si₃N₄ coating and Si₃N₄ coating with Si₃N₄ whiskers as reinforcement (Si₃N_4w-Si₃N₄) were prepared by chemical vapor deposition (CVD) on two-dimensional silicon nitride fiber reinforced silicon nitride ceramic matrix composites (2D Si₃N_4f/Si₃N₄ composites). The effects of process parameters of as-prepared coating including the preparation temperature and volume fraction of Si₃N_4w on the microstructure and mechanical properties of the composites were investigated. Compared with Si₃N₄ coating, Si₃N_4w-Si₃N₄ coating shows more significant effect on the strength and toughness of the composites, and both strengthening and toughening mechanism were analyzed.     

流延法在Si3N4块体及Si3N4/BN层状材料制备中的应用

本采用流延法制备了Si3N4块体及Si3N4/BN层状材料,流延法已经在陶瓷的制备工艺中得到了广泛的应用,但是很少用于Si3N4体系,尤其是水基流延法。用流法制备Si3N4/BN层状材料时,可以较为容易地控制坯片的厚度,得到性能稳定的层状材料。     

Si3N4/Si3N4扩散连接的力学性能研究

采用热压法进行氮化硅陶瓷材料的扩散连接.结果表明:在1520℃,15MPa,60min条件下,氮化硅连接体的最高强度为448.6MPa,超过母材强度;平均连接强度为401.5MPa,为母材强度的96%.     

氮化硅及其微粉的制备

本文介绍了氮化硅的性能、应用范围以及其微粉的制备方法。     

氮化硅及其微粉的制备

介绍了氮化硅的性能、应用范围以及其微粉的制备方法。     

Sn/Si3N4复合镀层中纳米Si3N4含量测定方法的比较

采用电熔解法溶解Sn/Si3N4复合镀层,以紫外可见分光光度法测定了镀层中纳米Si3N4的含量,并与传统酸溶解及重量法对比。结果表明,电解法在10 min内使镀层完全溶解且在测定波长448 nm处纳米Si3N4溶液的吸光度值与其浓度在10～200 mg/L范围保持良好的线性关系,该方法简单,省时,误差小,回收率98.5%～100%。     

Effect of long-term oxidation on creep and failure of Si3N4 and Si3N4/SiC nanocomposites

The high-temperature mechanical behaviour of an Si₃N₄/SiC nanocomposite and its monolithic Si₃N₄ reference material was studied after long-term oxidation treatments intended to simulate future operating conditions in a severe environment. Creep and failure at elevated temperature were significantly affected, in the direction of increased brittleness. The transition stress between the ductile range present at low stresses and the brittle range existing at high stresses was shifted to distinctly lower values. The creep resistance in the low-stress range was increased by the oxidation treatment. The failure time under a given stress was drastically reduced; this was attributed to an increased sensitivity to subcritical crack growth. The failure stress for a given failure time was decreased by about half. The phenomena are explained in terms of a purification of the intergranular phase and by the formation of surface defects and of a uniformly distributed pore population.     

SiC/Si3N4陶瓷前驱体的合成

通过在六甲基环三硅氮烷中加入部分甲基氢二氯硅烷的氨解产物（ＣＨ３ＨＳｉＮＨ）ｘ,经氢化钾催化制备了聚硅氮烷;用热重分析法研究了其裂性能,考察了反应原料比例、溶剂对陶瓷影响;结果表明;溶剂和原料比例不仅影响硅氮烷的性能,且对陶瓷产率有重要影响。以四氢呋喃做溶剂可得到较高的陶民产率。     

Nitrogen-doped-CNTs/Si3N4 nanocomposites with high electrical conductivity

Novel highly electrically conducting nanocomposites consisting of a silicon nitride (Si₃N₄) ceramic matrix containing up to 13.6 vol.% of nitrogen-doped multi-walled carbon nanotubes (CNx) were fabricated. As-synthesized CNx were treated with hydrogen peroxide in order to efficiently detach/isolate the nanotubes from bundles, then they were mixed with the ceramic powders and fully densified using the spark plasma sintering (SPS) technique. Composites containing 13.6 vol.% CNx reached an electrical conductivity of 2174 S m⁻¹ that is the highest value reported hitherto for carbon nanotubes/Si₃N₄ nanocomposites. The nitrogen doping also favored a strong mechanical interlocking between the nanotubes and the Si₃N₄ matrix; when compared to the undoped carbon nanotubes. These novel nanocomposites could be used in devices associated to power generation or telecommunications.     

Interfacial microstructure of the Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was jointed with itself by active brazing with a Cu–Pd–Ti filler alloy. Interfacial microstructure of the Si₃N₄/Si₃N₄ joint was analyzed by EPMA, TEM and X-ray diffract meter. The results indicate that a TiN reaction layer with a thickness about 5 μm is formed at the interface between Si₃N₄ ceramic and filler alloy. The TiN reaction layer is composed of two zones: one next to the Si₃N₄ ceramic with grains of 100 nm and the other zone that connects with the filler alloy and has grains of 1 μm. The microstructure of the joint can be described as: Si₃N₄ ceramic/TiN layer with fine grains/TiN layer with coarsen grains/Cu[Pd] solid solution. Some new phases, such as Pd₂Si, PdTiSi, Ti₅Si₃ and TiN, were formed in the Cu[Pd] solid solution interlayer. With increasing brazing temperature from 1100 °C to 1200 °C, the thickness of the TiN reaction layer is not changed. Meanwhile, the amount and size of the TiN and Pd₂Si phases in the Cu[Pd] solid solution increase, while, the amount of the PdTiSi phase decreases.     

Formation mechanism of Si3N4 in reaction-bonded Si3N4-SiC composites

The formation mechanism and thermodynamics of Si₃N₄ in reaction-bonded Si₃N₄-SiC materials were analyzed. There are two kinds of Si₃N₄, fibroid α-Si₃N₄ and columnar β-Si₃N₄, which are formed by different processes in Si₃N₄-SiC materials. Silicon reacts with oxygen, forming gaseous SiO and reducing oxygen partial pressure. SiO(g) diffuses from central to peripheral sections of blocks and reacts with nitrogen, thus forming Si₃N₄, mainly in peripheral sections. The reaction between silicon and oxygen causes the consumption of oxygen and leads to low oxygen partial pressure in the sintering system, which allows silicon to react with nitrogen directly generating Si₃N₄in situ. SiO(g) reacts with nitrogen forming Si₃N₄ at both central and peripheral sections of block. The non-uniform distribution of Si₃N₄ and uneven microstructure is caused by the generation process, indicating that it is unavoidable in Si₃N₄-SiC composites.     

Structural correlations at Si/Si3N4 interface and atomic stresses in Si/Si3N4 nanopixel-10 million-atom molecular dynamics simulation on parallel computers

We have combined first-principle calculations of charge transfer at the Si/Si₃N₄ interface with the interaction potential models for bulk Si and Si₃N₄ to produce a model for the Si/Si₃N₄ interface. Using these interatomic potentials, million atom molecular dynamics simulations have been performed to characterize the structure of Si(111)/Si₃N₄(0001) and the Si(111)/a-Si₃N₄ interfaces. Ten million-atom simulations are performed using multiresolution molecular-dynamics method on parallel computers. Atomic stress distributions are determined in a 54 nm nanopixel on a 0·1 μm silicon substrate. Effects of surfaces, edges, and lattice mismatch at the Si(111)/Si₃N₄(0001) interface on the stress distributions are also investigated. Stresses are found to be highly inhomogeneous in the nanopixel—the top surface of silicon nitride has a compressive stress of +3 GPa and the stress is tensile, −1 GPa, in silicon below the inter-face. These simulation methods can also be applied to other semiconductor/ceramic interfaces as well as to metal/ceramic and ceramic/ceramic interfaces.     

Cup-stacked carbon nanotubes hybridized Si3N4/Si3N4 composite ceramics for high-effciency microwave absorption with excellent thermal stability

Hybridization between carbon nanotubes (CNTs) and Si₃N₄ is a promising strategy for developing high-temperature microwave absorption (MA) materials for military application. Toward long-life services, it's important to achieve strong MA at a filler loading as low as possible on account of antioxidant protection against CNTs wastage. Herein, cup-stacked CNTs (CSCNTs) have been prepared in porous Si₃N₄ ceramics by chemical vapor deposition (CVD) and then CVD Si₃N₄ has been coated on them, forming CSCNT-Si₃N₄/Si₃N₄ composite ceramics. Results show that CSCNTs possess abundant exposed atomic edges on the outer surface and in the inner channel. Such unique defects not only benefit the impedance match but also cause considerable conductive loss, which helps CSCNT-Si₃N₄/Si₃N₄ with a filler content of only 0.79 wt% to achieve an effective absorption bandwidth (EAB) of 3.74 GHz in the X band at a thickness of 3.5 mm coupled with a minimum reflection loss of ?43.3 dB and an EAB covering the entire Ku band at a thickness of 2.25 mm. The ultralow filler loading generates a high efficiency of CVD Si₃N₄ in protecting CSCNTs against high-temperature oxidation, leading to a steady MA performance for CSCNT-Si₃N₄/Si₃N₄ during 23–1200 °C thermal shock tests in air.     

Effects of heat treatment on mechanical properties of 3D Si3N4f/BN/Si3N4 composites by PIP

The fabrication of three-dimensional silicon nitride (Si₃N₄) fiber-reinforced silicon nitride matrix (3D Si₃N_4f/BN/Si₃N₄) composites with a boron nitride (BN) interphase through precursor infiltration and pyrolysis (PIP) process was reported. Heat treatment at 1000–1200 °C was used to analyze the thermal stability of the Si₃N_4f/BN/Si₃N₄ composites. It was found after heat treatment the flexural strength and fracture toughness change with a pattern that decrease first and then increase, which are 191 ± 13 MPa and 5.8 ± 0.5 MPa·m^1/2 respectively for as-fabricated composites, and reach the minimum values of 138 ± 6 MPa and 3.9 ± 0.4 MPa·m^1/2 respectively for composites annealed at 1100 °C. The influence mechanisms of the heat treatment on the Si₃N_4f/BN/Si₃N₄ composites include: (Ⅰ) matrix shrinkage by further ceramization that causes defects such as pores and cracks in composites, and (Ⅱ) prestress relaxation, thermal residual stress (TRS) redistribution and a better wetting at the fiber/matrix (F/M) surface that increase the interfacial bonding strength (IBS). Thus, heat treatment affects the mechanical properties of composites by changing the properties of the matrix and IBS, where the load transfer efficiency onto the fibers is fluctuating by the microstructural evolution of matrix and gradually increasing IBS.     

Effects of interface bonding properties on cyclic tensile behavior of unidirectional C/Si3N4 and SiC/Si3N4 composites

In this paper, the effect of fiber/matrix interface bonding properties on the cyclic loading/unloading tensile stress?strain hysteresis loops of 2 different ceramic‐matrix composites (CMCs), ie, C/Si₃N₄ and SiC/Si₃N₄, has been investigated using micromechanical approach. The relationships between the damage mechanisms (ie, matrix multicracking saturation, fiber/matrix interface debonding and fibers failure), hysteresis dissipated energy and internal frictional damage parameter have been established. The damage evolution processes under cyclic loading/unloading tensile of C/Si₃N₄ and SiC/Si₃N₄ composites corresponding to different fiber/matrix interface bonding properties have been analyzed through damage models and interface frictional damage parameter. For the C/Si₃N₄ composite with the weakest fiber/matrix interface bonding, the composite possesses the lowest tensile strength and the highest failure strain; the hysteresis dissipated energy increases at low peak stress, and the stress?strain hysteresis loops correspond to the interface partially and completely debonding. However, for the SiC/Si₃N₄ composite with weak interface bonding, the composite possesses the highest tensile strength and intermediate failure strain; and the hysteresis dissipated energy increases faster and approaches to a higher value than that of composite with the strong interface bonding.     

Effects of heat treatment on mechanical and dielectric properties of 3D Si3N4f/BN/Si3N4 composites by CVI

In this study, three-dimensional silicon nitride fiber-reinforced silicon nitride matrix (3D Si₃N_4f/BN/Si₃N₄) composites with a boron nitride (BN) interphase were fabricated through chemical vapor infiltration. Through comparing the changes of microstructure, thermal residual stress, interface bonding state, and interface microstructure evolution of composites before and after heat treatment, the evolution of mechanical and dielectric properties of Si₃N_4f/BN/Si₃N₄ composites was analyzed. Flexural strength and fracture toughness of composites acquired the maximum values of 96 ± 5 MPa and 3.8 ± 0.1 MPa·m^1/2, respectively, after heat treatment at 800 °C; however, these values were maintained at 83 ± 6 MPa and 3.1 ± 0.2 MPa·m^1/2 after heat treatment at 1200 °C, respectively. The relatively low mechanical properties are mainly attributed to the strong interface bonding caused by interfacial diffusion of oxygen and subsequent interfacial reaction and generation of turbostratic BN interphase with relatively high fracture energy. Moreover, the Si₃N_4f/BN/Si₃N₄ composites also displayed moderate dielectric constant and dielectric loss fluctuating irregularly around 5.0 and 0.04 before and after heat treatment, respectively. They were mainly determined based on the intrinsic properties of materials system and complex microstructure of composites.     

Linking microstructure evolution and impedance behaviors in spark plasma sintered Si3N4/TiC and Si3N4/TiN ceramic nanocomposites

We proposed a novel approach to investigate the three-dimensional microstructures and sintering behaviors of Si₃N₄-based ceramic nanocomposites by electrochemical impedance spectroscopy. Si₃N₄/TiC and Si₃N₄/TiN with various weight percentages of conductive phases were prepared by spark plasma sintering (SPS) at different temperatures and dwell times. The incorporation of TiC and TiN into β-Si₃N₄ provides pulse current paths inside the ceramics due to their higher conductivity. These paths enable the localized Joule heating and mass transport, facilitating the densification and grain growth of ceramic compact. The electrochemical study of such nanocomposites has revealed three-dimensional information of the evolution of their microstructures, and the capacitive and resistive characteristics of the nanocomposites reflect the densification, grain growth, and element distribution in the compact. The impedance model presented in this work suggests isolated distribution of TiN in Si₃N₄ while Si₃N₄/TiC of the same amount of additives at the same sintering conditions formed conductive network. This impedance analysis further explained the differences in densification mechanism of SPS in Si₃N₄/TiN and Si₃N₄/TiC.     

Fabrication of Si3N4 preforms from Si3N4 produced via CRN technique

Ceramic preforms with randomly distributed particles as reticulated porous structure which are generally used for metal infiltration as reinforcement, membranes, catalyst supports etc. Preforms are characterized by open porosity making possible their infiltration by liquid metal alloys. In this work, quartz powders using carbon black as a reducing agent were used for alpha Si₃N₄ powders synthesis through a carbothermal reduction and nitridation (CRN) process. The CRN process was carried out under nitrogen flow at 1,450 °C for 4 h. At high temperatures, carbon as reducing agent reacts with the oxygen of SiO₂, and the resulting metallic silicon compounds with nitrogen gas to obtain silicon nitride powder. The reacted powders were used to obtain reticulated ceramic by replica method. The powders containing various bentonite ratios were mixed in water to prepare slurry. The slurry was infiltrated into a polyurethane sponge. A high porous ceramic foam (preform) structure was achieved after burn out of the sponge. All ceramic preforms were sintered to increase stiffness (in the temperature range 900–1,350 °C). The sintered ceramic foams were subjected to compressive tests. The scanning electron microscopy was used to examine the reticulated ceramic foam structure, and X-ray diffraction analysis was performed to determine phases.     

聚氨酯/微米Si_3N_4复合材料的性能研究

对聚氨酯/微米Si3N4复合材料的性能进行了研究,实验表明：聚氨酯/Si3N4复合材料具有优异的性能,聚氨酯/微米Si3N4复合材料,其硬度、力学性能和抗冲蚀磨损性能比纯聚氨酯优异;在微米Si3N4含量为3%时,其硬度、力学性能最佳;在微米SiN含量为5%时,其抗冲蚀磨损性能最佳。