304奥氏体不锈钢低温盐浴氮化处理及其耐蚀性

采用低温(430°C)盐浴对304奥氏体不锈钢进行氮化处理,研究了氮化时间对渗氮层组织、显微硬度及耐蚀性的影响。分别用X射线衍射仪(XRD)、表面显微硬度计、光学显微镜分析了渗氮层的相组成、显微硬度、截面形貌和厚度。结果表明,304不锈钢表面的渗氮层厚度和显微硬度都随处理时间的延长而增大。氮化处理1h得到的渗氮层由单一的S相组成。经盐浴渗氮处理的304不锈钢,其耐Cl-点蚀性能得到改善,430°C下氮化4h得到的渗氮层耐蚀性能最好。     

硅粉氮化输送床内气固反应过程数值模拟

以单个硅颗粒氮化反应缩核模型为基础,本文建立了硅颗粒在输送床内反应、辐射与对流传热耦合的数学模型,并借助CFD软件FLUENT对输送床内能质传输过程进行了数值模拟,分析了输送床壁面温度、氮气流量、预热温度、硅粉粒径等因素对输送床内温度场和硅粉氮化率的影响。在数值计算域内将单个颗粒反应过程转化为颗粒群整体反应过程,实时监测颗粒粒径及未反应硅颗粒粒径,为数值模拟颗粒流反应提供一种新思路。当壁面温度高于1723K时,输送床内会出现一高温区加速硅粉氮化反应;反应温度越高、颗粒粒径越小,氮化过程越剧烈,硅粉到达完全氮化所需时间越短。模型表明为使粒径为2.5μm的硅粉达到完全氮化且输送床内最高温度不超过氮化硅的分解温度2173K,应控制输送床壁面温度在1773K,氮化时间在170s以上,预热温度在1273K,粉气质量比为0.2,稀释剂比例为0.5~1。     

Influence of additives content on the high temperature oxidation of silicon nitride based composites

A life prediction tool for mechanical and electrical applications of electroconductive structural ceramics is essential in order to know the limit for engineering uses. The aim of this work was to study the influence of additives content on the oxidation behaviour, in pure oxygen between 900 and 1400 °C, of two fully dense Si₃N₄–35 vol.% TiN composites. For this purpose, a hot pressed material (HP), containing 3.7 wt.% of Y₂O₃ and Al₂O₃ as sintering aids, was compared to an hipped material (HIP), containing 0.4 wt.% of the same additives. Up to a temperature T<∼1200 °C, where the oxidation of the composite is mainly governed by the preferential oxidation of TiN, the two materials exhibit paralinear kinetics with very close oxidation resistance. Contrarily, the hipped material shows a better oxidation resistance at T>∼1200 °C, when the oxidation of the Si₃N₄ matrix takes place. The formation of a compact silica sub-scale acts as an efficient diffusion barrier leading to asymptotic kinetics, with final weight gains exhibiting a negative temperature dependence. In the case of the HP material, i.e. in presence of a higher content of additives, a deterioration of the protective nature of the scale is provoked by the increased mobility of the impurity cations (Y³⁺, Al³⁺) linked to a decrease of the viscosity of the secondary glassy phase. The kinetics have a paralinear shape up to 1400 °C, with final weight gains increasing as a function of temperature. Therefore, this study confirms the deleterious influence on oxidation resistance of additives used for a better sintering of powders and the beneficial effect of hot isostatic pressing for which lower amounts of aids are necessary in comparison with hot pressing.     

Influence of temperature and concentration on tribological properties of silicon nitride in glycerol aqueous solution

Friction and wear behaviors of self-mated Si₃N₄ in glycerol aqueous were investigated by varying the temperature (30 °C, 50 °C, and 70 °C) and concentration (pure water, 5 vol%, 20 vol%, and 50 vol%) of glycerol aqueous solution. Friction tests were conducted on a ball-on-disk apparatus. Normal load and sliding velocity were fixed at 30 N and 0.5 m/s, separately. After each tests, friction coefficients and wear rates were measured to evaluate friction and wear behavior. The results showed that the period of running-in process reduces with the increase of concentration and decrease of temperature. Increase of temperature could intensify wear behavior, and when concentration is larger than 20 vol%, wear rate of glycerol aqueous solution is one order less than that of pure water. Our findings could also guide for the use of glycerol aqueous solution as lubricant at different temperature. At 30 °C, the higher the concentration was, the smaller wear volume and total wear rate were. However, at 50 °C and 70 °C, total wear rates of disk were the largest when concentration is 5 vol%, a concentration of glycerol larger than 20 vol% must be added into water to reduce the wear rate. Wear regimes at different conditions were also given in this paper based on lubrication state number.     

Influence of sintering temperature and pressure on the 3C-6H transition of silicon carbide

The influence of Spark Plasma Sintering pressure and temperature on the evolution of the phase composition of two sources of β-SiC ceramic were studied based on X-ray diffraction and Raman scattering measurements. A commercial powder and a powder derived from a preceramic polymer precursor were chosen, and the investigation was undertaken in the [1850-2200 °C] temperature range and in the [17–127 MPa] pressure range. Analyses revealed that the 3C-6H phase transition depends on sintering pressure, which is shown to stabilize the cubic phase. The stability domains of the cubic phase as a function of sintering P and T were determined for both powdery precursors and revealed that the cubic phase stability is also linked to the nature of powdery precursor and in particular to its crystallinity state. The improvement of cubic phase stability at higher temperature by applying pressure allows sintering β-SiC ceramic with high density.     

Nanostructured particles of boron,aluminum, and silicon nitrides by thermally coupled SHS reactions of nitriding

The paper shows the investigation results of structure and phase formation of boron, aluminum and silicon nitride particles in gaseous nitrogen of high pressure (10–40 MPa) under the terms of thermocoupled high- and low-exothermic reactions (so-called chemical furnaces). In this case the high-exothermic reaction is an additional heat source for the low-exothermic one. The reactions are separated in space and chemically independent. The nitriding terms which are close to extreme ones (high burning temperatures of aluminum, boron and silicon, high heating rates, etc.) provide the reagent evaporation and promote gastransport reactions with the combustion mechanism transition from the solid-phase to gas-phase or vaporliquid due to volatile intermediate products of nitriding and impurities. As a result, the SHS products contain not only well-known forms of nitride particles (nano-fibers, nano-filaments, nano-plates, nano-whiskers, etc.) but such structures as BN nano-tubes with various types of morphology and cubic modifications of BN and AlN as well. It is to be studied additionally.     

Low temperature densification of silicon nitride materials

Thin layers of oxides, corresponding to additions of up to 5 w/o Na₂O have been deposited on the surface of grains of a commercial silicon nitride powder using alcoholic solutions containing appropriate amounts of the metal alkoxide. The resulting powders have been densified by hot pressing and pressureless sintering techniques, and their sintering characteristics identified in comparison with equivalent materials produced by adding the oxide in particulate form. In every case, a better sintering performance was observed at all temperatures for the oxide-coated materials, with ∼97% dense sintered materials being obtained at temperatures as low as 1400–1500 °C. Microstructures were observed using a S-2400 Hitachi Scanning Electron Microscope (SEM) and final microstructure was more uniform than that obtained by conventional method. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder.     

反应烧结SiC材料的高温氧化行为研究

研究了反应烧结SiC材料在 110 0℃空气中的高温氧化行为。结果表明 :反应烧结SiC在110 0℃的氧化动力学曲线符合抛物线规律 ;材料的氧化受O2 和CO在玻璃态硅酸盐中的扩散所控制 ;材料中的杂质元素降低了SiO2 氧化膜的粘度 ,促进了O2 和CO在氧化膜中的扩散     

Fabrication and characterization of magnetic porous silicon with curie temperature above room temperature

In this study, we demonstrate a completely novel synthesis route for producing magnetic porous silicon. The magnetic properties of this material are induced by manganese atoms. The Mn-doping in Si is achieved by ion implantation. A subsequent anodization of the substrate is done to turn it into porous silicon. Several characterization techniques, such as transmission electronic microscopy, atomic force microscopy and photoluminescence are combined to probe the structural and the optical properties of this material. Furthermore, temperature and magnetic field dependent magnetization is analyzed using superconducting quantum interference device. In addition to the well-reported structural and optical properties of the porous silicon, our Mn-doped porous silicon samples exhibit a magnetic behavior with a curie temperature (T_C) higher than room temperature. These results indicate that the magnetic porous silicon can be integrated with microelectronics and photonics technologies to produce new devices, such as magnetophotonic crystals and polarized emitting diodes.     

Influence of filler content,particle size and temperature on thermal diffusivity of polypropylene‐iron silicon composites

The filler fraction, particle size and temperature dependence of thermal diffusivity in polypropylene‐iron silicon composites were examined by laser flash method. Results show that raising the filler content raises the thermal diffusivity of the composite material only slightly. Nevertheless, the higher the filler content the higher the influence of the filler material expressed by a altered course of the thermal diffusivity versus temperature. Measurement values are compared with selected existing mathematical models whereas a model, originally developed for magnetic permeability, by Hashin and Shtrikman shows the best congruence. Further measurements show that the particle size of the filler does not have an influence on the composite's thermal diffusivity at the examined filler content. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

304不锈钢低温盐浴氮化处理后的微观结构及耐冲刷腐蚀性

在430℃下对304奥氏体不锈钢进行低温盐浴氮化处理,并用X射线衍射(XRD)、光学显微镜(OM)、能谱(EDS)、扫描电镜(SEM)和显微硬度计研究了氮化时间对渗氮层厚度、组织结构、显微硬度和耐冲刷腐蚀性能的影响.结果表明,渗氮层厚度和表面显微硬度均随渗氮时间的延长而增加.氮化时间为1h时,氮化层仅为单一的S相;氮化16h时,氮化层由CrN和S两相混合.氮化层中的CrN随渗氮时间延长而增多,氮化40 h时氮化层析出大量CrN.盐浴渗氮处理后,304不锈钢的耐冲刷腐蚀性能得到了一定的改善.在430℃氮化16h,其耐蚀性能最好.随着渗氮时间的继续增加,304不锈钢的耐冲刷腐蚀性能降低.     

High temperature strength and fractography of sintered silicon nitride

Influences of the sintering liquid system, temperature, microstructure and post sintering heat treatment of high temperature (30–1250°C) strength, Young's modulus and fracture toughness of sintered silicon nitride (SSN) have been studied. Based on quantitative fractography, typical fracture origin statistics has been presented for SSN. The measured strength of the SSN is in good agreement with the fractographically predicted strength.     

Growth and high temperature performance of semi-insulating silicon carbide

The problems of obtaining insulating properties in bulk single-crystal silicon carbide by vanadium doping under the LETI method growth process are considered. The prime novelty of this work is the growth of a semi-insulating bulk single-crystal n-4H-SiC:V by the LETI method in vacuum from a vanadium and aluminium-containing source. The obtained 4H-SiC:V material possesses resistivity >10⁷ Ω cm at 20°C and activation energy ∼1.6 eV at 20–800°C and can be applied as a semi-insulating substrate material for extreme electronics based on silicon carbide or nitrides.     

Effect of pyrolyzation temperature on wood-derived carbon and silicon carbide

The structure of carbon and silicon carbide produced through the pyrolyzation of wood and the subsequent melt-infiltration with silicon was studied as a function of initial carbon pyrolyzation temperature. Scanning electron microscopy, transmission electron microscopy, mercury intrusion porosimetry, X-ray diffraction and Raman spectroscopy were used to characterize material derived from initial carbon pyrolyzation temperatures in the range of 300–2400 $°$C. It was determined that, although structural differences abound in carbon pyrolyzed at different temperatures, the resulting silicon carbide is independent of the initial temperature of carbon pyrolyzation.     

Determination of silicon carbide fiber electrical resistivity at elevated temperature

The resistance of Sylramic‐iBN and Hi‐Nicalon SiC fiber tows was measured at elevated temperature in air. Resistivity could not be directly measured, since the fibers passed through a furnace with varying temperature along the length. The resistivity of the isothermal section of the fiber tow was modeled by a series circuit of finite elements. Existing data for Hi‐Nicalon resistivity vs temperature was used to verify the model and then extend it to Sylramic‐iBN, for which there is no literature data readily available. The model matched experimental values with low overall error (<±14%). Fiber resistivity decreased by more than two orders of magnitude when heating from 25°C to 1400°C. Sylramic‐iBN tow resistance was also measured during a 500 hour hold at 1315°C. The resistance increased by more than 140% during heat treatment. The resistance change correlated well with the decrease in SiC fiber diameter that resulted from oxidation.     

Elevated temperature instrumented Charpy impact of a sintered silicon carbide

A commercially available, densely sintered alpha silicon carbide was tested in air from room temperature to 1700°C using a modified instrumented pendulum unit and standard Charpy size test specimens. The resistance heated silicon carbide specimens exhibited only elastic behavior, even at 1700°C. A compliance analysis of the test revealed a linear elastic decrease of the absorbed impact energy and also a decrease of the fracture stress of the silicon carbide with increasing temperature.     

Synthesis of graphene on silicon carbide substrates at low temperature

A method for the synthesis of millimeter-scaled graphene films on silicon carbide substrates at low temperatures (750 °C) is presented herein. Ni thin films were coated on a silicon carbide substrate and used to extract the substrate’s carbon atoms under rapid heating. During the cooling stage, the carbon atoms precipitated on the free surface of the Ni and formed single-layer or few-layer graphene. The result shows that the number of graphene layers might be further controlled by appropriate process conditions. In contrast to the epitaxial graphene synthesis on single crystal silicon carbide, the graphene prepared here are continuous over the entire Ni-coated area, and can be stripped from the substrate much more easily for further characterization. The large-scaled, low temperature and transferable features of our method suggest the potential for future graphene-based applications.     

Effect of silicon and heat-treatment temperature on the morphology and mechanical properties of silicon - substituted hydroxyapatite

The precipitation method was used to synthesize silicon-substituted hydroxyapatite with different Si contents of 0.4, 0.8 and 1.6 wt.% (0.4, 0.8 and 1.6Si-HA) using silicon acetate [Si(OCOCH₃)₄] as a Si source. As-synthesized hydroxyapatite (HA) and Si-HA powders/bulks were heat-treated at different temperatures of 1150, 1200 and 1250 °C for 1 h. Pure 0.4Si-HA and 1.6Si-HA were obtained after heat-treatment at all temperatures, whilst α-TCP phase was formed in the 0.8Si-HA sample after heat-treatment at 1250 °C. SEM observation clearly showed that the substitution of Si in HA inhibited the grain growth of Si-HA even at high heat-treatment temperatures (1200 or 1250 °C). The highest diametral tensile strength (DTS) of 15.93 MPa was obtained in the 1.6Si-HA sample after heat-treatment at 1250 °C.     

Effects of nitridation temperature on properties of sintered reaction-bonded silicon nitride

We prepared sintered reaction-bonded silicon nitride ceramics by using yttria and magnesia as sintering additives and evaluated effects of the nitridation temperature on their microstructure, bending strength, fracture toughness, and thermal conductivity. The effects of the nitridation temperature were large, but different depending on the property. The ratio of β-phase in the nitrided compacts significantly increased with increasing the nitridation temperature, whereas their microstructures had no clear difference. Although the bending strength varied, it maintains a high value of 800 MPa. Fracture toughness was almost constant regardless the temperature. The thermal conductivity improved as the β-phase in the nitrided compact increases. This resulted in a decrease of the lattice oxygen content and increase of the thermal conductivity. Therefore, elevating the nitridation temperature and consequently the β-phase ratio should be a promising strategy for achieving compatibly high strength and high thermal conductivity, which are generally known to be in a trade-off relationship.     

Effect of temperature on the pre-creep mechanical properties of silicon nitride

The elasto-plastic properties and contact damage evolution of a commercial polycrystalline silicon nitride are evaluated as a function of temperature up to 1000 °C, using a recently developed method combining Hertzian indentation and FEM simulation. The results of the study are compared to existing data for other ceramic materials such as alumina and zirconia. Silicon nitride is found to exhibit an excellent combination of elasto-plastic properties in the pre-creep temperature range and good contact damage resistance. These qualities make this material ideal for high temperature applications in general, and in particular to be used in spherical indenters for the evaluation of mechanical properties of other materials at elevated temperature using the procedure applied in this work.