硅铁闪速燃烧合成氮化硅铁

利用闪速燃烧合成新技术 ,以粒度≤ 0 .0 88mm的FeSi75硅铁细粉为原料 ,在 0 .2MPa的低氮气压力与 14 0 0℃的燃烧温度条件下 ,制备了细蜂窝状氮化硅铁。XRD和SEM分析结果表明 ,这种氮化硅铁主要由短柱状β Si3N4 相和Si3Fe相组成 ,其结构特征是以Si3Fe形成核心 ,并被Si3N4 包裹。同时 ,还用热力学原理探讨了由硅铁闪速燃烧合成氮化硅铁的工艺条件、形成产物的形式、反应的中间产物和残留金属的形态。热力学研究结论和实验检测结果相一致 ,从而在理论上阐明了闪速燃烧合成是制备氮化硅铁的一种理想工艺     

Mechanism of growth of silicon nitride crystals in combustion of ferrosilicon in nitrogen

The process of nitride formation in burning of iron-silicon melt in gaseous nitrogen was investigated. It was found that silicon nitride is synthesized at temperatures (2100°C) which are much higher than the temperature of appearance of the liquid phase (1206°C). Silicon is in the liquid state during synthesis in the form of an iron-silicon melt and a gaseous melt. The electron-microscopic studies showed that silicon nitride crystals grow according to two mechanisms: vapor-liquid-crystal and crystallization from iron-silicon melt. The ratio of the contributions of these mechanisms to structural formation of silicon nitride is determined by the conditions of synthesis. __________ Translated from Steklo i Keramika, No. 8, pp. 18–21, August, 2007.     

Effect of the diluent on combustion synthesis of silicon nitride

Si₃N₄ powders were prepared by combustion synthesis with 1- and 3-μm α-Si₃N₄, β-Si₃N₄ diluent and BN inert diluent. The maximum temperatures of samples with boron nitride (BN) as a diluent are about 1500–1600°C lower than that of samples with α-Si₃N₄ and β-Si₃N₄ as diluents are about 1600–1800°C. Moreover, the newly formed α-Si₃N₄ contents in the synthesized products with BN as diluent over 90 wt% are much higher than those with α-Si₃N₄ and β-Si₃N₄ as diluent about 20–40 wt%. The strip-like α-Si₃N₄, rod-like β-Si₃N₄ grains, and radiative shaped grains can be observed in the synthesized products. Finally, the effect of the diluent on the α-phase content of combustion synthesized Si₃N₄ is discussed, which provides key guidance for preparing Si₃N₄ powders with high α-phase content.     

稀土硅铁合金中钛含量测定

研究了用二安替比林甲烷分光光度法测定稀土硅铁合金中钛含量时试样的熔融方法,即采用无水碳酸钠和硼酸混合熔剂在高温下快速熔融试样,并探索了试样熔融的最优条件。试验结果表明：溶样完全,测试结果稳定,误差在允许范围内,满足分析要求;该方法能快速分解矿样,测定结果准确,与采用硝酸和氢氟酸溶解矿样相比,具有环保、快速、适应批量分析等优点。熔融稀土硅铁合金试样的最优条件是混合碱与试样比6∶1,碳酸钠与硼酸比2∶1,熔融温度1000 ℃,熔融时间20 min;各因素影响的主次顺序为混合碱与试样比>熔融时间>熔融温度>碳酸钠与硼酸比。     

Determining the oxygen content in partly oxidized silicon nitride

Conclusions The vacuum-extraction method is suitable for determining with adequate accuracy the content of oxygen (oxynitride) in partly oxidized silicon nitride and in silicon-carbide refractories with a nitride and silicon oxynitride bond.     

Cost-effective manufacture and synthesis mechanism of ferrosilicon nitride porous ceramic with interlocking structure

To realize cost-effectively manufacture of high-performance Si₃N₄ porous ceramic, a ferrosilicon nitride porous ceramic with an optimized interlocking structure was synthesized by flash combustion synthesis using FeSi75 powder as raw material. And the technology has been improved in many ways to ensure stable industrial production. The theoretical combustion temperature of FeSi75 in N₂(g) is up to 4608K, while Si₃N₄ is unstable. Both adding diluent and designing the preheat temperature of nitrogen are taken to control synthesis temperature below 1600 °C. During synthesis, the Fe–Si liquid phase and SiO(g), which are essential for the selective growth of elongated columnar β-Si₃N₄ and whisker α-Si₃N₄ respectively, are formed firstly. Then, nitriding proceed in multiple ways. N diffuses through Fe–Si(l) and reacts with Si to form β-Si₃N₄, and the growth of elongated β-Si₃N₄ in Fe–Si liquid follows the dynamic ripening model, which is very fast and effective. Thus, an interlocking structure composed of elongated β-Si₃N₄ with an aspect ratio above 20 is reached. There is also an indirect nitridation reaction, that is, FeSi75 preferentially reacts with trace O₂ in atmosphere to form SiO(g), which is further nitrided to form needle-like α-Si₃N₄. Needle-like α-Si₃N₄ is interspersed in the well-developed columnar β-Si₃N₄, making the structure stronger. Fe finally exists in the form of Fe₃Si, which binds the surrounding elongated Si₃N₄ to form a sea-urchin like unit, making the structure more stable and strengthened. Through control of these reactions, optimizations in microstructure are reached, and the annual output of has reached 25,000 tons. The reaction model is established.     

Mechanism of structure formation of silicon nitride with combustion of silicon in nitrogen

Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 26, No. 1, pp. 45–52, January–February, 1990.     

铋磷钼蓝分光光度法测定硅铁中磷的研究

研究用抗坏血酸还原,磷与钼酸铵、硝酸铋形成蓝色的显色反应。分析了酸度、显色时间的影响,确定了最佳测定条件。结果表明,加标回收率达97．88％。此方法简单、快速且稳定,能获得满意的分析结果。     

Effects of nitrogen pressure 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 the effects of nitrogen pressure (0.1–1.0 MPa) on their microstructure, bending strength, fracture toughness, and thermal conductivity. The ratio of β phase in the nitrided compacts varied with the pressure and increased with increasing it. Since many β grains in the nitrided compacts were formed and interlocked each other with a stable three-dimensional structure which restricted the shrinkage during the sintering procedure, many pores remained in the sintered body. Under the middle pressure (0.3–0.5 MPa), the grains grew large because the number of formed nuclei was small. On the other hand, under the high pressures (0.8–1.0 MPa), the grains were relatively fine and uniform because of a large number of nuclei. Since the porosity and grain length depended on the nitridation mechanism, which was affected by the nitrogen pressure, the properties largely varied accordingly. The nitridation at 0.1 MPa gave the best properties in this study.     

Effect of oxygen on the strength of reaction-bonded silicon nitride

Conclusions When obtaining reaction-bonded Si₃N₄, drying of the silicon powder or the charge at 400°C leads to an increased oxygen content in it and can cause deterioration of the mechanical properties of the ceramics after sintering.Addition of small quantities of SiO₂ to the reaction-bonded Si₃N₄ improves the high-temperature strength to some extent although it affects the room temperature strength adversely.Translated from Ogneupory, No. 5, pp. 13–14, May, 1991.     

Effect of monomer content on physical properties of silicon nitride ceramic green body prepared by gelcasting

The gelcasting technique was employed to prepare Si₃N₄ green body. The monomers used in the research were acrylamide (AM) and N,N′-methylenebisacrylamide (MBAM). The influences of the monomer content (AM and MBAM) and the ratio of monomers (AM/MBAM) on the warpage rate, shrinkage rate, and the flexural strength of Si₃N₄ ceramics green body were investigated. Both warpage rate and shrinkage rate of green body were found to decrease with the increase of monomer content, and monotonically increase with the ratio of monomers after drying. The variation of warpage rate with the ratio of monomers is evident when monomer content is 20 wt.%, but the variations are not evident when monomer contents are 40 and 55 wt.%. The flexural strength of the green body is highest at an optimum value of the monomers ratio, and increases with increasing monomer content, reaching 50–90 MPa when monomer contents are 40 and 55 wt.%.     

Controlled nitrogen content synthesis of hafnium carbonitride powders by carbonizing hafnium nitride for enhanced ablation properties

In this study, a new method of carbonizing hafnium nitride was proposed to synthesize ultrahigh-temperature hafnium carbonitride (HfC_xN_y) powders. The new method helps to maintain both the purity of phases and control the content of nitrogen in the HfC_xN_y. The results show that the as-prepared HfC_xN_y powders have a single phase, with an average particle size of approximately 2 $\mu m$, and Hf, C and N are evenly distributed. Moreover, the microstructures, phase compositions, ablation properties and mechanism of the HfC_0.62N_0.38 composites under a plasma ablation environment were studied in detail. The results show that the HfC_0.62N_0.38 composites exhibited excellent ablation resistance at 3073 K for 60 s and the ablation mechanism of HfC_0.62N_0.38 can be identified as HfC_0.62N_0.38→HfC_xO_y→HfO₂. The mass ablation rate of the HfC_0.62N_0.38 composite is evaluated to be 1.36 mg/cm²∙s, which is lower than that of HfC ceramics. Our work is intended to provide new insight regarding the development of ultrahigh-temperature ceramics and widen their applications.     

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.     

Effect of seeding on the thermal conductivity of self-reinforced silicon nitride

Thermal conductivity of Si₃N₄ containing large β-Si₃N₄ particles as seeds for grain growth was investigated. Seeds addition promotes growth of β-Si₃N₄ grains during sintering to develop the duplex microstructure. The thermal conductivity of the material sintered at 1900 °C improved up to 106 W m⁻¹ K⁻¹, although that of unseeded material was 77 Wm⁻¹ K⁻¹. Seeds addition leads to reduction of the sintering temperature with developing the duplex microstructure and with improving the thermal conductivity, which benefits in terms of production cost of Si₃N₄ ceramics with thermal conductivity. ©     

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.     

硅锰合金中锰含量测定的不确定度评定

采用高氯酸氧化滴定法测定硅锰合金中锰,应用现代统计学理论对其分析结果不确定度的产生原因进行分析,同时对硅锰合金中锰测定结果的不确定度分量进行评定,确定了分析结果的置信区间为：[0．6566,0．6598]。     

Using ferrosilicon nitride of nitro-fesil grade in gate and spout components

Development results are presented for the new NITRO-FESIL strengthening additive, which is intended for gate and spout parts in blast furnace production. Researches on the high-temperature process in the ferrosilicon-nitrogen system have provided a new industrial technology for making materials based on silicon nitride. The technology is characterized by the absence of energy consumption, complete ecological safety, and a product distinguished by good working properties. Translated from Novye Ogneupory, No. 9, pp. 45–50, September, 2008.     

燃烧合成氮化硅陶瓷晶须研究进展

燃烧合成法是一种高效能、低消耗的陶瓷材料合成方法。简要介绍了燃烧合成方法特点,评述了近年来燃烧合成氮化硅陶瓷晶须的研究进展,详细总结了原料选择、多种添加剂（如铁、稀土氧化物、铵盐等）对氮化硅晶须最终形貌和性能的影响, 总结了工艺参数,尤其是氮气压力和堆积密度对晶须生长的影响,并详细讨论了在燃烧合成过程中晶须的生长机理。