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     

还原气氛下闪速燃烧合成氮化硅铁的高温行为

为模拟闪速燃烧合成氮化硅铁在工作环境中的高温行为,研究了高温还原气氛下氮化硅铁的存在状态。根据热力学计算,采用在空气气氛中埋碳升温的方法,控制氧气分压在较低水平,将氮化硅铁样品分别升温至1 300℃及1 500℃,保温300 min后迅速水冷,以保存高温下样品的微观结构。采用X射线衍射和扫描电子显微镜表征样品的物相组成...     

闪速燃烧合成氮化硅铁的氮化机理

以硅铁合金Fe Si75为原料,研究了闪速燃烧合成氮化硅铁的氮化机理。结果表明:在氮化过程中,首先是硅的活性氧化,硅被氧化生成气态Si O,使得体系氧分压降低,当氧分压p(O2)≤10–20 MPa(T=1 823 K)时,Si与N2(g)直接反应形成氮化硅,气态Si O最终与N2(g)发生反应生成氮化硅。硅铁合金Fe Si75中的Fe Si2与Fe0.42Si2.67没有促进氮化硅的形成,且与未参与氮化反应的硅反应形成Fe3Si。氮化硅铁的主要物相为氮化硅和Fe3Si,其中存在大量的柱状氮化硅,Fe3Si被柱状Si3N4包裹,呈孤立状态。     

合成工艺对氮化硅铁物相和结构的影响

以硅铁合金(FeSi75)为原料,分别采用闪速燃烧合成工艺和自蔓延高温合成工艺制备氮化硅铁样品,利用X射线衍射仪和扫描电镜对样品进行了表征,探讨了合成工艺对氮化硅铁物相和显微结构的影响。采用闪速燃烧工艺合成的氮化硅铁相组成为β-Si3N4,α-Si3N4,Fe3Si和少量SiO2;而采用自蔓延高温合成的氮化硅铁由β-Si3N4,α-Si3N4,Fe3Si和Si2N2O组成。闪速燃烧合成的氮化硅铁样品中存在大量长径比较高的柱状氮化硅晶体,Fe3Si位于柱状结晶所包裹材料的内部;自蔓延高温合成的氮化硅铁显微结构为致密的氮化硅块体,在块体表面覆盖有氧氮化硅膜,块体的间隙存在晶形细小的氮化硅晶体,含铁组分镶嵌在致密的块体中。闪速燃烧合成的氮化硅铁结构疏松,活性较强;自蔓延高温合成的氮化硅铁结构致密,性质稳定。     

铁元素在氮化硅铁中的存在状态

用化学分析、XRD，SEM，EDS等检测手段，首次对闪速燃烧工艺制备的新型合成原料——氮化硅铁(Fe-Si3N4)中铁元素的存在状态进行了研究。结果表明：以小于0．074mm的FeSi75颗粒为原料制备氮化硅铁时，FeSi75颗粒表面的硅原子氮化形成氮化硅包覆层，硅铁受热熔化；随着硅的持续氮化减少，铁含量相对增加，硅的氮化难度加大；最后，铁以Fe3Si和α-Fe两种形式保留下来，并且主要分布于氮化硅粉体颗粒的内部，并用热力学进行了分析。     

优势区相图在Al-TiO2-C-ZrO2燃烧合成体系热力学分析中的应用

以TiO2, Al, C和ZrO2为原料, 燃烧合成制备Al2O3-TiC-ZrO2纳米复相陶瓷是一种方法简单、节时省能的新工艺. 对Al-TiO2-C-ZrO2体系进行了热力学分析,计算出该体系的绝热燃烧温度, 并利用Al-O-N,Ti-O-N,Zr-O-N,C-O-N四个体系的叠加优势区相图, 分析了各相间反应进行的趋势和最终稳定存在的平衡相. 热力学分析表明：绝热燃烧温度为2327 K, 燃烧合成产物包括Al2O3,TiC,ZrO2三相. XRD检测未发现其它杂相, 证实热力学分析结果可信.     

氮化硅铁在空气气氛中的烧结试验

以闪速燃烧法合成的粒度≤0.074mm氮化硅铁细粉为原料,以胶水为临时结合剂,在250kN的压力下液压成型,干燥后于空气气氛中1500℃保温3h烧成,冷却后在试样内部钻取36mm×50mm的试样,检测其显气孔率、体积密度和常温耐压强度,同时对该试样和氮化硅铁原料进行了XRD分析。结果表明:烧成后试样的显气孔率、体积密度和常温耐压强度分别为39.6%、2.10g·cm-3和34.5MPa;与氮化硅铁原料相比,烧成后氮化硅铁中除Fe相及SiO2消失外,其余物相都存在。     

硅铁粉粒度对合成氮化硅铁的影响

采用FeSi75为原料,利用直接氮化合成法制备了氮化硅铁粉末,研究了中位径(d50)分别为13.41μm、8.023μm和5.229μm的3种硅铁粉分别在1150℃、1250℃和1350℃保温9h处理后的氮化规律。借助XRD、SEM等测试手段测定和观察了产物的物相组成和显微形貌。结果表明:较细的硅铁粉(d50=5.229μm)氮化时,反应快速、剧烈,导致烧结严重,氮化效果差,而较粗硅铁粉(d50=13.41μm)氮化效果较好;较细硅铁粉氮化后易于形成须状、纤维状和柱状氮化硅晶体,较粗硅铁粉氮化后易于形成球状氮化硅团聚体。制备的氮化硅铁中有大量充满氮化硅的孔洞,产物中的Fe3Si与FexSi被其包围,这种结构有利于体现氮化硅铁的优异性能。     

优势区相图在A l-T i O2-C-ZrO2燃烧合成体系热力学分析中的应用

以TiO2, Al, C和ZrO2为原料, 燃烧合成制备Al2O3-TiC-ZrO2纳米复相陶瓷是一种方法简单、节时省能的新工艺. 对Al-TiO2-C-ZrO2体系进行了热力学分析,计算出该体系的绝热燃烧温度, 并利用Al-O-N,Ti-O-N,Zr-O-N,C-O-N四个体系的叠加优势区相图, 分析了各相间反应进行的趋势和最终稳定存在的平衡相. 热力学分析表明:绝热燃烧温度为2327 K, 燃烧合成产物包括Al2O3,TiC,ZrO2三相. XRD检测未发现其它杂相, 证实热力学分析结果可信.     

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。     

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.     

Reaction and formation mechanism of Fe-Si3N4 composite prepared by flash combustion synthesis

In this paper, in order to improve the performance of Fe–Si₃N₄ composite synthesized by flash combustion, the detailed nitridation mechanism and formation process were discussed. In the process of high temperature nitriding, ξ phase rapidly melted to form Fe-Si melt and the cycle of rapid surface nitriding → rupture of nitridation shells → new melt exposing and nitriding occurred continually on the surface of Fe-Si melt. Since ions have different activities on the surface and internal, Fe ions near the surface migrated inwards and Si ions inside moved out; meanwhile, the Fe-Si melt kept shrinking. The nitriding reaction of the Fe-Si melt finished till the overall activity a_Si approached 0, leaving the atom ratio of [Fe] to [Si] at 3:1. During the falling of the formed Si₃N₄ and Fe₃Si melt in the N₂ flow, the surface of Si₃N₄ was oxidized to form a SiO₂ film. The nitridation product fell into the product pool, loosely stacking and adhering together by the SiO₂ film. α-Si₃N₄ dissolved and precipitated to form β-Si₃N₄ crystals, and the β-Si₃N₄ crystals kept growing to form radioactive elongated crystals. As the temperature decreased, the Fe₃Si melt cooled down; the Si-N-O melt, α-Si₃N₄ and the roots of elongated β-Si₃N₄ crystals formed the dense areas.     

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.     

Synthesis of nanocrystalline boron nitride by combustion process

Nanocrystalline boron nitride powders were synthesized by combustion process using urea as a fuel. Experiments were carried out by heating boric acid and urea in an N₂ atmosphere at 850°C. Boric acid was used as a source of boron while urea, as a source of nitrogen. The reactions were carried out in an autoclave with provisions for purging with nitrogen gas. The samples were characterized by powder X-ray diffraction, Fourier transform IR spectroscopy (FT-IR), FT-Raman spectroscopy, UV-VIS spectroscopy, and SEM. The article is published in the original.     

Combustion synthesis of fine aluminum nitride powders under micropositive N2 pressure with water additive

Fine aluminum nitride (AlN) powders were prepared by a facile and efficient way of combustion synthesis under micropositive N₂ pressure of 0.15 MPa and with 3 wt% water as additive. By this approach, the maximum combustion temperature was well regulated to a low value. The influence of water on the reaction rate, the phase composition, and crystal growth of the products was systematically investigated. The addition of water was crucial to the complete nitridation of Al. Furthermore, H₂O vapor played a two-sided role in the reaction. It could accelerate the reaction by promoting the diffusion of Al vapor and N₂ and restrain the nitridation rate by absorbing heat.     

Synthesis of boron nitride nanotubes by combining citrate-nitrate combustion reaction and catalytic chemical vapor deposition

High-quality boron nitride nanotubes were successfully synthesized via a novel two-step method, including citrate-nitrate combustion reaction and catalytic chemical vapor deposition. The composition, bonding features and microstructures of as-synthesized sample were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction techniques. The results show that the as-synthesized boron nitride nanotubes with smooth surface are relatively pure. The diameter ranges between 20 and 80?nm, while the length is about dozens of micrometers. During the synthesis process of boron nitride nanotubes, citric acid chelates the cobalt ions and reacts with nitrate to form the cobalt oxide, depositing on the surface of boron powder homogeneously. The catalyst content and annealing temperature have a significant impact on the composition and microstructures of the final products. Based on the experimental results and thermodynamic analysis, the possible chemical reactions are listed, and vapor-liquid-solid mechanism is proposed to be dominant for the formation of boron nitride nanotubes.     

Formation of metastable wurtzite phase boron nitride by emulsion detonation synthesis

Emulsion detonation synthesis (EDS) is a newly developed process to synthesize nano‐sized ceramic powders based on the detonation of 2 water‐in‐oil emulsions. The process provides high pressure and temperature along with rapid quenching. In this work, we report the formation of wurtzite phase BN (w‐BN) for the first time by EDS process, using hexagonal BN (h‐BN) as the precursor. Characterization studies demonstrated the formation of w‐BN with sizes varying from nanometer to micrometer scale either embedded in or grown from h‐BN matrix. These findings provide a new avenue to synthesize metastable and superhard BN phases.