Superplastic Deformation in Silicon Nitride–Silicon Oxynitride In Situ Composites

Starting with a mixture of ultrafine β-Si₃N₄ and a SiO₂-containing additive, a superplastic Si₃N₄-based composite was developed, using the concept of a transient liquid phase. Significant deformation-induced phase and microstructure evolutions occurred in the nonequilibrium, fine-grained Si₃N₄ material, which led to the in situ development of a Si₃N₄–22-vol%-Si₂N₂O composite and strong texture formation. The unusual ductility of the composites with elongated Si₂N₂O grains was attributed to the fine-grained microstructure, the presence of a transient liquid phase, and the alignment of the elongated Si₂N₂O grains. The mechanical properties of the resultant composite were enhanced rather than impaired by superplastic deformation and subsequent heat treatment; the resultant composite exhibited both high strength (957 MPa) and high fracture toughness (4.8 MPa·m^1/2).     

Texture Development in Silicon Nitride–Silicon Oxynitride In Situ Composites via Superplastic Deformation

Silicon nitride–silicon oxynitride (Si₃N₄–Si₂N₂O) in situ composites have been fabricated via either the annealing or the superplastic deformation of sintered Si₃N₄ that has been doped with a silica-containing additive. In this study, quantitative texture measurements, including pole figures and X-ray diffraction patterns, are used in conjunction with scanning electron microscopy and transmission electron microscopy techniques to examine the degree of preferred orientation and texture-development mechanisms in these materials. The results indicate that (i) only superplastic deformation can produce strong textures in the β-Si₃N₄ matrix, as well as Si₂N₂O grains that are formed in situ ; (ii) texture development in the β-Si₃N₄ matrix mainly results from grain rotation via grain-boundary sliding; and (iii) for Si₂N₂O, a very strong strain-dependent texture occurs in two stages, namely, preferred nucleation and anisotropic grain growth.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.     

Processing, Mechanical Properties, and Oxidation Behavior of Silicon Oxynitride Ceramics

Silicon oxynitride ceramics were prepared by hot-pressing an equimolar Si₃N₄+ SiO₂ mixture with 3 mol% CeO₂. The Ce₂O₃/SiO₂ ratio of intergranular phase (liquid phase) increased as the formation of Si₂N₂O proceeded. The intergranular liquid remained as a glass on cooling until the Ce₂O₃/SiO₂ ratio exceeded a certain value, at which point the liquid crystallized. There were great differences in thermal and mechanical properties and oxideation behavior between the specimen containing intergranular glassy phase and the one containing intergranular crystalline phase (Ce₅(SiO₄)₃N–Ce_4.67(SiO₄)₃O). The specimen containing the intergranular glassy phase showed excellent hightemperature strength and oxidation resistance.     

常压化学气相沉积法氮氧化硅薄膜性能的研究

我们用常压化学气相沉积法(APCVD),以SiH4和NH3为先驱体,在较低的温度(＜700℃)下制备氮氧化硅(Si-O-N)薄膜,并对其性能进行了研究.研究结果表明氮氧化硅薄膜能使改性后平板玻璃硬度提高40%,并具有良好的抗高温氧化性,这种薄膜在材料表面改性领域有着广阔的应用前景.     

Y—La—Si—O—N系统氧氮玻璃形成与性能

本研究利用溶胶－凝胶技术制备Ｙ－Ｌａ－Ｓｉ－Ｏ－Ｎ氧氮玻璃配合料，在１大气压Ｎ２气氛中１６００℃左右温度熔制，制备出表观呈灰黑色透明的Ｙ－Ｌａ－Ｓｉ－Ｏ－Ｎ系统氧氮玻璃。实验结果表明氧氮玻璃含氮量为６．７－１０．９ａｔ％；显微硬度Ｈｖ为３．５－６．４７ＧＰａ；断裂韧性为０．６４－１．３８ＭＰａｍ＾１／２，Ｔｇ在１０００℃以上，膨胀系数为１．０５×１０＾－６℃＾－１．     

Formation of Ring-Like Si–O–Zr Bonds at Intergranular Interfaces in Silica-Doped Zirconia

Possible cluster models of the intergranular interfaces phase for SiO₂/ZrO₂ binary oxides were optimized by the density-functional theory (DFT/B3LYP). New results on the formation of the interfacial ring-like Si–O–Zr bonds are validated by the analyses of reaction Gibbs free energies and computational infrared vibration spectra. Moreover, the Mulliken charge population of interfacial Si–O–Zr structures shows that the increased temperature and critical size of the tetragonal-to-monoclinic phase transformation for SiO₂-doped ZrO₂ mainly come from the reduced charge of oxygen atoms located at the interface and coordinated with the neighboring zirconium atoms, which accords well with the previous theoretical results.     

Properties and Crystallization of Rare-Earth Si–Al–O–N Glasses Containing Mixed Trivalent Modifiers

Thirteen glasses of the general formula (M1, M2)_9.33Si₁₄Al_5.33O_41.5N_5.67 where M1=La or Nd and M2=Y or Er have been prepared with M1/(M1+M2) fractions of 1, 0.75, 0.5, 0.25, and 0. Data for molar volume (MV), glass compactness ( C ), Young's modulus ( E ), microhardness ( H ), glass transition temperatures ( T _g), and dilatometric softening temperatures ( T _d) have been recorded. In addition, temperatures at which crystallization exotherms arise have also been determined as well as crystalline phases present after the glasses had been heat treated to 1300°C in nitrogen. The results clearly demonstrate that glass properties vary linearly with effective cation field strength (CFS) of the combined modifiers (M1, M2), which is calculated from the atomic fractions of M1 and M2 and their associated CFSs. Glass stability in both the La–Y and La–Er systems reaches a maximum at M1 and M2 fractions of 0.5 because of the relative stability of different oxynitride and disilicate phases with changes in ionic radius. Furthermore, La appears to stabilize the α polymorph of yttrium disilicate because of combined La–Y ionic radius effects.     

Mass Spectrometric Identification of Si–O–H(g) Species from the Reaction of Silica with Water Vapor at Atmospheric Pressure

A high-pressure sampling mass spectrometer was used to detect the volatile species formed from SiO₂ at temperatures between 1200° and 1400°C in a flowing water vapor/oxygen gas mixture at 1 bar total pressure. The primary vapor species identified was Si(OH)₄. The fragment ion Si(OH)₃⁺was observed in quantities 3 to 5 times larger than the parent ion Si(OH)₄⁺. The Si(OH)₃⁺ intensity was found to have a small temperature dependence and to increase with the water vapor partial pressure as expected. In addition, SiO(OH)⁺, believed to be a fragment of SiO(OH)₂, was observed. These mass spectral results were compared to the behavior of silicon halides.     

Synthesis of Silicon Nitride by a Combustion Reaction under High Nitrogen Pressure

Fine Si₃N₄ powders were prepared by the combustion reaction of an Si powder compact undez 10 MPa nitrogen pressure. Addition of Si₃N₄ powder to the starting Si promoted conversion of the reactants to homogeneous Si₃N₄ particles. Submicrometer SisN₄powders with a uniform size distribution around 0.5 μm were obtained from a 1.8Si-0.4Si₃N₄ mixture (molar ratio); they were free of residual Si.     

氮氧化硅研究现状与前景

本文在笔者研究的基础上，综述了氮氧化硅的研究现状及前景，指出了在研究氮氧化硅过程中普遍存在的问题。     

改造高低压系统 实现高压开车

因低压开车弊端多,对高压系统及低压系统进行部分改造,改低压开车为高压开车,也使高压低压系统操作时方便、安全、环保。     

Molecular Dynamics Simulations of Amorphous Si–C–N Ceramics: Composition Dependence of the Atomic Structure

We have performed classical molecular dynamics simulations of amorphous Si–C–N materials. The dependence of the local order and of the microstructure on the chemical composition was investigated. Our simulations show that for a stoichiometric nitrogen/silicon ratio equal to or higher than 4/3, the amorphous ceramic separates into different amorphous domains, namely C-rich, SiN-rich, and SiC-rich phases. Below this ratio, the material is composed of mixed structures, homogeneously spread within the material. For a very particular composition range, we found that carbon atoms crystallize into monoatomic graphitic layers surrounding the SiN-rich domains.     

Influence of Crystalline Secondary Phases on the Densification Behavior of Reaction-Bonded Silicon Nitride During Postsintering under Increased Nitrogen Pressure

The influence of reaction-bonded silicon nitride (RBSN) characteristics, especially the crystalline secondary phases formed during the nitridation process, on the densification behavior of RBSN and the resulting microstructure of the sintered material during postsintering under increased nitrogen pressure is discussed. The type and amount of the secondary phases are controlled by the chemical characteristics of the Si starting powder and the additive composition. Three different commercially available Si powders which reveal particular differences in the Fe and Al content were investigated, and a correlation between the microstructural characteristics of RBSN and of the postsintered materials is presented. It is shown that besides micropore size distribution the crystalline secondary phases play an important part in the densification behavior during postsintering and the resulting microstructure of postsintered reaction-bonded silicon nitride.     

Effects of High Water-Vapor Pressure on Oxidation of Silicon Carbide at 1200°C

The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air + 15 vol% H₂O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water-vapor pressure on oxidation rates and microstructural development. At a water-vapor pressure of 1.5 atm (150 kPa), distinct SiO₂ scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO₂ layer, which remained constant in thickness with time as the crystalline SiO₂ continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water-vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO₂ volatility is rate controlling at high gas-flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO₂ to porous, nonprotective SiO₂.     

高硅高铁含铜渣氧压酸浸过程

对火法炼铅产高硅高铁含铜渣进行了氧压硫酸浸出过程研究. 结果表明,浸出时间、氧分压、酸量、浸出温度和搅拌速度对Cu的浸出率和浸出液中Fe含量有显著影响,溶液初始含Cu2+量对Cu浸出率影响不明显. 最佳工艺条件为：时间2 h,氧分压0.8 MPa,硫酸浓度46.6 g/L,温度(150±2)℃,搅拌速度600 r/min,复合絮凝剂A用量30~100 g/m3. 在该条件下,Cu浸出率>95%,浸出液中Fe<5 g/L,硫转化率20%~30%,料浆过滤速度约0.8 m3/(m2×h).     

Mechanistic Analysis of Time-Dependent Failure of Oxynitride Glass-Joined Silicon Nitride below 1000°C

Time-dependent failure at elevated temperatures currently governs the service life of oxynitride glass-joined silicon nitride. Creep, devitrification, stress-aided oxidation-controlled slow crack growth, and viscous cavitation-controlled failure are examined as possible controlling mechanisms. Creep deformation failure is observed above 1000°C. Fractographic evidence indicates cavity formation and growth below 1000°C. Auger electron spectroscopy verified that the oxidation rate. Time-to-failure is independent of oxygen concentration. Reasonable agreement is found between the observed time-to-failure data and those predicted using the Tsai and Raj, and Raj and Dang viscous cavitation models. It is concluded that viscous relaxation and isolated cavity growth control the rate of failure in oxynitride glassfilled silicon nitride joints below 1000°C. Several possible methods are also proposed for increasing the service lives of these joints.     

In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composite from Silicon Nitride–Boron Oxide–Carbon

This work proposes a new approach, based on the reaction Si₃N₄+ 2B₂O₃+ 9C → 3SiC + 4BN + 6CO, to synthesize an SiC–BN composite. The composite was prepared by reactive hot pressing (RHP), at 2000°C for 60 min at 30 MPa under an argon atmosphere, following a 60 min hold at 1700°C without applied pressure before reaching the RHP temperature. TG-DTA results showed that a nitrogen atmosphere inhibited denitrification somewhat and retarded the reaction rate. The chemical composition of the obtained material was consistent with theoretical values. FE-SEM observation showed that in situ -formed SiC and BN phases were of spherical morphology with very fine particle size of ∼100 nm.     

Impurity-Enhanced Intergranular Cavity Formation in Silicon Nitride at High Temperatures

The effect of trace impurities on high-temperature strength is examined in Si₃N₄ sintered without additives. Strength degradation above 1000°C occurs only in the low-purity material, which also exhibits an intergranular slow crack growth (SCG) rate 2 orders of magnitude faster than that of the high-purity material at 1400°C. A relaxation peak of internal friction is observed only in the low-purity material, at ≅1200°C, and the origin of this peak is ascribed to the initial stage of SCG—that is, the cavity-nucleation stage, enhanced by impurities. Based on the present results, a model for impurity-enhanced SCG is proposed.