三聚氰胺对直接氮化法合成氮化铝纳米线的影响

由于氮化铝纳米线具有优异的导热性,国内外学者对其进行了广泛研究。本文以三聚氰胺和氟化钇为添加剂,采用直接氮化法制备了氮化铝纳米线。利用X射线衍射(XRD)、扫描电镜(SEM)、高分辨率透射电子显微镜(HRTEM)、能谱仪(EDS)等表征了氮化铝纳米线的晶体结构和微观形貌,分析了三聚氰胺和氟化钇对氮化反应的促进作用,研究了不同含量的三聚氰胺和不同反应温度对制备氮化铝纳米线的影响。结果表明:添加三聚氰胺可以提高氮化反应速率,促进纳米线的生成;当反应温度为1 200 ℃,铝粉和三聚氰胺质量比为1∶4,氟化钇掺量为5%(质量分数)时,成功制得了高长径比的氮化铝纳米线。     

界面类型对三维Nextel 720纤维增韧莫来石陶瓷基复合材料力学性能的影响

采用化学气相渗透工艺在Nextel 720纤维表面制备PyC和PyC／SiC两种涂层，然后以正硅酸乙酯和异丙醇铝作为先驱体，以先驱体浸渗热解法制备三维Nextd 720纤维增韧莫来石陶瓷基复合材料，比较分析了两种涂层复合材料的力学性能和断裂模式。结果表明：具预先涂覆PyC的复合材料中纤维与基体直接接触，发生烧结形成强结合界面，复合材料脆性断裂，三点抗弯强度仅56MPa。PyC／SiC涂层则演化为间隙／SiC复合界面层，SiC成为阻滞纤维与基体接触的阻挡层，间隙保证了纤维拔出，复合材料韧性断裂且三点抗弯强度高达267．2MPa。     

反应烧结氮化硅-碳化硅复合材料的氮化机理

为分析反应烧结氮化硅结合碳化硅(Si3N4-SiC)材料中微观结构和氮化硅分布不均匀的原因,对在隔焰燃气氮化梭式窑中应用反应烧结氮化方法制备的氮化硅结合碳化硅复合材料进行结构研究和热力学分析。结果表明:材料中的氮化硅以纤维状和柱状两种形状存在。Si的氮化机理为:Si首先被氧化成气态SiO,降低了体系的氧分压,当氧分压足够低时,Si与N2直接反应形成柱状Si3N4,气态SiO亦可与N2反应生成氮化硅,这是一个气-气反应,故生成的Si3N4为纤维状。氮化反应前SiO主要分布于材料孔隙和表面,因而生成的氮化硅分布不均匀,导致了反应烧结Si3N4-SiC材料结构的不均匀。     

CaZrO_3耐火材料的制备及其与钛合金熔体的界面反应

合成了一种CaO-ZrO_2系复合耐火材料,用模压法成型,分别在1 650℃和1 680℃烧结制成坩埚.使用此坩埚在感应炉中熔化钛合金(TiNi),在真空条件下, 1 500℃保温5min.对在1 680℃烧结成的坩埚进行了抗水化性能测试.用X射线衍射仪对CaZrO_3材料进行物相分析.用扫描电子显微镜观察钛合金与此材料的界面反应层的微观形貌,结合能谱仪进行微区成分分析.结果显示:1 680℃烧成的CaZrO_3材料具有良好的抗水化性;用此坩埚熔化TiNi合金后,液态钛合金和耐火材料的界面反应层厚度约为30μm,界面反应层中,Ca,Zr,Ti,Ni等元素无明显扩散.     

酯化反应耦联渗透汽化膜过程对平衡移动的影响

以乙酸正丁酯的合成为目标反应,系统地考察了渗透汽化－ 酯化反应耦联复合膜反应器中过程因素（进料中醇、酸摩尔比率、膜面积与反应混合物体积的比值、复合膜渗透通量的大小,反应、分离温度等）对酯化反应化学平衡移动的影响。实验发现反应（分离）温度是影响酯化反应化学平衡移动的最重要因素。     

Fe对硅粉氮化的影响

以硅粉为原料,Fe和尿素作为添加剂,用直接氮化法制备氮化硅。经XRD和SEM分析,结果表明:在1350℃/10h的氮化条件下,所制备的Si3N4以α相为主,颗粒尺寸大约在1.8～4.5μm范围,平均尺寸为3.3μm。添加剂Fe和尿素对硅粉的氮化有一定的促进作用。     

催化剂对酯化反应渗透汽化膜稳定性的影响

比较了酯化反应耦合渗透汽化膜反应器中不同的催化剂体系及其含量对ＰＰＶＡ／ＰＡＮ复合膜稳定性的影响。试验结果表明,固体超强酸催化剂体系对复合膜的稳定性没有影响。活性层酯化度为３．３％及６．２％的复合膜具有良好的稳定性。能够适合于酯化反应耦合渗透汽化过程研究及应用。     

Pressure infiltration of boron nitride preforms with molten aluminum

The infiltration of compacted cubic BN (cBN) with molten aluminum has been investigated as a potential route for a cheap and easy method of manufacturing cBN/metal composites. CBN compacts have been infiltrated with molten Al at a temperature between 670 and 800 °C and pressure of 15 MPa in vacuum. At these temperatures no pronounced interactions between hexagonal and cubic BN with Al was observed, allowing the complete infiltration of cBN with 12 μm mean grain size. After infiltration at 800 °C, the temperature was increased without pressure to convert aluminum into borides and AlN. The hardness of the resulting materials depends on the content of hexagonal, cubic BN and the rate of conversion of Al into borides and AlN. The infiltration height of less than 1 mm obtained from infiltrating the 3 μm cBN powder green compacts gave a hardness of 22.0 ± 0.6 GPa after heat treatment.     

Synthesis of cubic aluminum nitride by carbothermal nitridation reaction

Cubic aluminum nitride (AlN) was synthesized by the carbothermal nitridation reaction of aluminum oxide (Al₂O₃). The effects of Al₂O₃ particle size, reaction temperature and reaction time on the synthesis of cubic AlN were investigated, and the reaction mechanism was also analyzed. The results showed that cubic AlN could be formed at a lower temperature with fine Al₂O₃ powder than with coarse Al₂O₃ powder. The cubic AlN may be the product of Al₂₃O₂₇N₅ synthesized from Al₂O₃ and hexagonal AlN, and transforms into hexagonal AlN at temperatures above 1800°C.     

Pressure infiltration of molten aluminum for densification of environmental barrier coatings

Environmental barrier coatings(EBCs) effectively protect the ceramic matrix composites(CMCs) from harsh engine environments, especially steam and molten salts. However, open pores inevitably formed during the deposition process provide the transport channels for oxidants and corrosives, and lead to premature failure of EBCs. This research work proposed a method of pressure infiltration densification which blocked these open pores in the coatings. These results showed that it was difficult for al...     

Effect of NiS addition on nitridation of alumina to aluminum nitride using polymeric carbon nitride as a nitridation reagent

We investigated the effect of adding nickel(II) sulfide (NiS) on nitridation of alumina (Al₂O₃) to aluminum nitride (AlN) using polymeric carbon nitride (PCN), which was synthesized by polymerization of dicyandiamide at 500 °C. The product powders obtained from nitridation of a mixture of δ-Al₂O₃ and NiS powders (mole ratio of 1:0.01) at various reaction temperatures were characterized by powder X-ray diffraction, ²⁷Al magic-angle spinning nuclear magnetic resonance, and Raman spectroscopy. δ-Al₂O₃ began to convert to AlN at 900 °C and completely converted to AlN at 1300 °C. The as-synthesized sample powders contained nitrogen-doped carbon microtubes (N-doped CMTs) with a length of several tens of mm and thickness of ca. 3 µm. The addition of NiS to δ-Al₂O₃ resulted in the enhancement of the amount of N-doped CMTs and nitridation rate, which might be due to the catalytic action of Ni particles on the thermal decomposition of vaporized PCN. The change in Raman spectra with reaction temperatures indicated that the crystallinity of N-doped CMTs was increased by calcining at higher reaction temperatures.     

Study on rapid nitridation process of molten silicon by thermogravimetry and in situ Raman spectroscopy

The melt of silicon, hindering nitridation for its agglomeration, should be avoided in the direct nitridation of silicon to synthesize silicon nitride powders, although liquid phase facilitates nitridation. Therefore, we proposed a method to nitride molten silicon without agglomeration. Thermogravimetric and in situ Raman studies on the nitridation process of molten silicon were performed. The as-prepared silicon nitride samples were found to be micron clusters composed of submicron grains with high α-Si₃N₄ content. The nitridation of molten silicon at 1500°C was completed after 500 s and 109 times faster than the nitridation of solid silicon at 1350°C. β-Si₃N₄ is produced dominantly by α–β-phase transition. Less nitridation time and low temperature can decrease the β-Si₃N₄ content. The rapid nitridation was owning to core–shell structure Si@Si₃N₄, which was formed after the initial nitridation of silicon particles and hindered the agglomeration of molten silicon.     

Effect of the passivating coating type, particle size, and storage time on oxidation and nitridation of aluminum powders

Processes of nonisothermal oxidation, nitridation, and ageing of aluminum powders with different particle sizes (nano-sized powder, ASD-1 powder, and PAP-2 powder) are considered. Application of non-oxide coatings onto particles of aluminum nanopowders reduces their thermal stability. Owing to scale-shaped particles, the PAP-2 powder after long-time storage preserves high activity of oxidation and nitridation, which is commensurable with that of the aluminum nanopowder. The activity of the coarse ASD-1 powder consisting of spherical particles in terms of oxidation and nitridation is low and only slightly changes during ageing. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 2, pp. 61–69, March–April, 2006.     

Large-scale synthesis of AlN nanofibers by direct nitridation of aluminum

AlN nanopowders and nanofibers were synthesized by direct nitridation of Al and rice bran mixture compacts in a tube furnace up to 1300 °C in a nitrogen flow without addition of extra catalyst. The effect of the compaction pressure applied onto the green bodies on the morphology of the final AlN products was investigated. A green body compacting pressure in between 320 MPa and 480 MPa was found to be favorable for the synthesis of AlN fibers with aspect ratio up to 400, diameter in the range of 50–500 nm, and length up to tens of micrometers; for a lower pressure of 160 MPa and a higher pressure of 640 MPa, nano-sized AlN powders were the primary morphology in the final product. The AlN products were characterized by several techniques and the VLS growth mechanism was proposed as the main reason for the AlN fibers formation.     

氮化反应合成β''''-Sialon材料的工艺研究

研究了以硅粉、铝粉和氧化铝粉为原料氮化反应制备不同Z值 β’ Sialon材料的氮化工艺制度 :以 10℃·min- 1的升温速度升到 6 0 0℃ ,再以小于 2℃·min- 1的升温速度升到 70 0℃ ,保温 2 .5h ;以 10℃·min- 1的升温速度升到 12 0 0℃ ,再以 2℃·min- 1的升温速度升到 135 0℃ ,保温 6h。炉内气氛为流动氮气 ,流速小于 0 .2m3 ·h- 1。所有试样的氮化率都在 80 %以上 ,说明该工艺制度是合适的。物相分析表明 :氮化最终产物以 β’ Sialon为主 ,高Z值试样残余较多的α Al2 O3 ,并伴生少量15R相 ;烧结后试样的 β’ Sialon呈棱柱状。     

Effect of interfacial-active elements addition on the incorporation of micron-sized SiC particles in molten pure aluminum

Ceramic particles generally have poor wettability by liquid metal, leading to a major drawback in fabrication of cast metal matrix composites (MMCs). In this work,  the effect of 1 wt. % of Ca, Mg, Si, Ti, Zn and Zr interfacial-active alloying elements was studied on the incorporation of micron-sized SiC particles into the molten pure aluminum using the vortex casting method at 680 °C. The results indicated that Ti, Zr, Zn and Si were not positively effective in improving particulate incorporation, while Ca and especially Mg were very efficient at increasing the incorporation of ceramic particles into the molten Al. Also, it was revealed that Al₃Ti, and Al₃Zr intermetallic phases were formed for samples containing Ti and Zr, making hybrid MMCs with a higher amount of hardness. Finally, it was found that a reaction layer between Al and SiC particles was formed at the Al/SiC interface for all of the samples, expect for the ones containing Si and Ti, indicating that for most of the samples at 680 °C an exothermic reaction took place between the Al and SiC particles.