Tribological behavior of nanostructured high velocity oxy-fuel (HVOF) thermal sprayed WC-17NiCr coatings

In this research, the nanostructured WC-17NiCr cermet coatings were developed using the high velocity oxy-fuel (HVOF) thermal spraying processes on ACI CD4MCu cast duplex stainless steel substrates, widely used in pump industry for abrasive wear protection of surfaces. The coatings, sprayed by both robotic and manual methods, had two different fuel (methane) to oxygen ratios (FTOR), namely 0.68 and 0.62. Using different analytical and microstructural techniques, the microstructural characteristics of the powder particles and mechanical, microstructural, and tribological properties of the coatings were determined. Different morphologies were assigned to sprayable particles, namely spherical, apple, donut, irregular, and mixed. It was revealed that the rate of WC decarburization had increased with increasing the FTOR. In contrast, the scanning electron microscopy and image analyses showed that the lowest porosity percentage was obtained for the robotically-sprayed coating with 0.68 FTOR. The Vickers microhardness increased along with fracture toughness, which can be attributed to the effect of the ‘duplex structure’ associated with the particle outer coating of Co and is a novelty in the research. The pin-on-disk reciprocal sliding wear tests at various loadings had shown different wear rates in the coatings. It was inferred that the wear performance was improved with the microstructural homogeneity, hardness, and the fracture toughness in the coatings. In all coatings, lower coefficient of friction (COF) was observed at higher loads. Finally, the wear mechanisms involved in the wear processes were identified as deformation and removal of the binder, fracture and pullout of the carbide particles, and delamination and spallation of the splats.     

激光重熔对Ni/WC涂层组织结构的改善机制研究

为了进一步探讨激光重熔等离子喷涂金属陶瓷涂层的组织与性能。本文通过等离子喷涂设备在45钢表面上制备了Ni/WC金属陶瓷涂层,再进行激光重熔处理,然后利用SEM、XRD、Photoshop软件及显微硬度测试仪等分析测试手段研究了该涂层在激光重熔前后的组织性能变化。结果表明:激光重熔前涂层为典型的层状结构,基体与涂层的结合面为机械结合,涂层内有大量未熔WC颗粒,且XRD检测其高温作用使得喷涂颗粒发生分解,分解出的C元素与其他元素发生反应生成新的化合物,丰富了涂层的硬质相;激光重熔后涂层中颗粒细化,分布均匀且能消除涂层中大部分孔隙和WC团聚。WC再次发生分解,生成新的硬质相,与周围的Ni形成“软基相+硬质点”的组合分布,基体与涂层的结合方式由机械结合转变为冶金结合。孔隙率由7.02%降到了3.08%,显微硬度也相应提高,且涂层显微硬度比基体高了255HV。     

激光熔覆镍基合金组织及磨损性能研究

利用CO2激光器在45钢基体上熔覆Ni基合金,分析激光熔覆层的微观组织,测试其显微硬度及磨损性能。结果表明,所制得熔覆层组织致密、无裂纹,与基体形成良好的冶金结合。从熔覆层表层到基体热影响区,组织呈现出由细小的树枝晶→胞状晶、树枝晶→平面晶过渡。激光熔覆层磨损量约为基体的1/3,熔覆层耐磨能力增强的主要原因在于熔覆层与基体良好的冶金结合,以及基体与涂层元素固溶强化和碳化物等析出相的强化作用所致。     

天然钛铁矿原位合成金属陶瓷材料的研究现状

钛铁矿在自然界中储量非常丰富,分布较广.本文结合笔者的研究工作,概述和分析了以天然钛铁矿为主要原料,碳热、铝热、镁热等原位合成金属陶瓷材料的还原反应机理,并预测了工业化应用的前景.充分利用钛铁矿中的钛和铁两种资源,原位合成制备金属陶瓷材料,将成为天然钛铁矿综合开发利用中的一个重要方面.     

基于镍基载氧剂的甲烷化学链燃烧机理与优化

基于吉布斯自由能最小化原理,在Aspen Plus软件上建立了Ni O-CH_4化学链燃烧模型。通过计算反应器内的化学平衡,分析了反应物摩尔比、反应温度、反应压力和气体流量等对反应器出口气相产物组成的影响,明确了Ni O-CH_4化学链燃烧的反应机理,进而对Ni O-CH_4化学链燃烧的反应条件进行了优化。实验结果表明,当n(Ni O)∶n(CH_4)≤1时,燃料的燃烧以甲烷重整反应为主;当1n(Ni O)∶n(CH_4)≤4时,逐渐以还原反应为主;当n(Ni O)∶n(CH_4)4时,全部发生还原反应,生成CO_2和H_2O。当n(Ni O)∶n(CH_4)=4、反应温度为1 000~1 100 K时,Ni O-CH_4化学链燃烧效率的效率最高;适当提高反应压力有助于提高CH_4化学链燃烧效率,抑制其他反应的发生,但反应压力不宜大于8 MPa;在满足燃烧器功率的要求下,应尽量降低气体流量。     

自蔓延高温合成结合铸造技术制备复合材料的研究

将自蔓延高温合成(SHS)、粉末冶金与铸造工艺结合的复合材料制备方法,可以制备出表面层厚3～20 mm的材料.其过程是将选配好的反应物料压制成块并将其粘在型腔内或直接将混合均匀后反应物料粉体粘接在所需部位,用熔融金属液浇注并引发SHS反应,同时金属液浸透反应后的毛细孔隙,自然凝固并与母材金属结为一体,在所需表面原位合成金属-陶瓷复合层.     

Recent advance in catalysis for solving energy and environmental problems

Recent advances in catalysis for solving the energy and environmental problems are summarized. For these purposes, rapid conversion and selective reaction even under conditions deviating extremely from reaction stoichiometry must be indispensable requisites. In order to realize these requisites, changes in the state of catalyst surface during the reaction were studied, and the catalyst structures on which the optimum reaction performance occurs were determined. An ultra-rapid reforming of methane to syngas with a space–time yield (STY) of 25 000 mol/l h was achieved by using a Rh-modified Ni–Ce₂O₃–Pt catalyst in which the Rh played the role of portholes for hydrogen spillover and prevents coke deposition on the catalyst surface. As a result, a stable state of the catalyst and the high reaction rate were exhibited. A new catalyst composed of Cu–Zn–Cr–Al–Ga oxides modified with supported Pd exerted a high activity with a high STY of methanol, 6700 g/l h. The catalyst components, Pd and Ga, controlled the reduction state of the catalyst surface by their role on normal and inverse spillover of hydrogen, respectively. The methanol thus produced was then totally converted selectively on a metallosilicate catalyst containing Ga or Fe into an aromatics-lean gasoline using an STY of 1860 g/l h. Finally, non-linear reaction mechanism is used to explain the elimination of NO on metallosilicate catalysts under O₂-excess conditions.     

Effect of anion exchange ionomer content on electrode performance in AEM water electrolysis

Anion exchange membrane water electrolysis (AEMWE) has acquired substantial consideration as a cost-effective hydrogen production technology. The anion ionomer content in the catalyst layers during hydrogen and oxygen evolution reaction (HER and OER) is of ultimate significance. Herein, an in-situ half-cell analysis with reference electrodes was carried out for simultaneous potential measurements and identification of the influence of the anion exchange ionomer (AEI) content on anode and cathode performance. The measured half-cell potentials proved the influence of AEI content on the catalytic activity of HER and OER, which was supported by the rotating disk electrode (RDE) measurements. Cathode overpotential of Ni/C was not negligible and more affected by the AEI content than anode with the optimized AEI content of 10 wt% while NiO anode OER overpotential was independent of the AEI content. For the same AEI content, PGM catalysts showed higher electroactivity than Ni-based catalysts for HER and OER and the cathode catalyst's intrinsic activity is of high importance in the AEM electrolysis operation. Post-mortem analysis by SEM mapping of both AEI and catalyst distributions on the electrode surface showed the effect of AEI loading on the catalyst morphology, which could be related to the electrode performance.     

Effect of mineralizers for preparing ZrO2 support on the supported Ni catalyst for steam-CO2 bi-reforming of methane

Supported Ni catalysts on ZrO₂ towards steam-CO₂ bi-reforming (SCBR) of methane for the production of synthesis gas were synthesized by the hydrothermal process with different mineralizers followed by l-arginine ligand-assisted incipient wetness impregnation (HT-LA-IWI) method. The effect of type and amount of mineralizers for preparing ZrO₂ supports on the nature of supports and supported Ni catalysts, as well as on the catalytic properties and structure–performance relationship were investigated. Results show that the catalytic performance is strongly dependent on the morphology and textural of ZrO₂ support notably affected by the type and amount of mineralizer. The supported Ni catalyst on the ZrO₂ prepared by using sodium acetate (molar ratio of sodium acetate/zirconium, N_SAc/Zr = 0.5) as mineralizer (Ni/ZrO₂ (SAc0.5)) shows much higher catalytic activity than the one on ZrO₂ prepared by using sodium carbonate (molar ratio of sodium carbonate/zirconium, N_SC/Zr = 0.5) as a mineralizer (Ni/ZrO₂ (SC0.5)), ascribed to higher Ni dispersion and smaller average crystallite size of Ni. With respect to both activity and stability, the sodium acetate can be selected as a suitable mineralizer for the preparation of excellent ZrO₂ support. Furthermore, the increasing N_SAc/Zr from 0.5 to 2.0 leads to an increase in surface area but a decrease in pore diameter and pore volume, which endows the Ni/ZrO₂ (SAc2.0) catalyst with much larger average crystallite size of Ni but much worse Ni dispersion than Ni/ZrO₂ (SAc0.5). As a result, Ni/ZrO₂ (SAc2.0) shows much lower catalytic activity than Ni/ZrO₂ (SAc0.5). Moreover, the Ni/ZrO₂ (SAc2.0) catalyst shows worse Ni sintering resistance than Ni/ZrO₂ (SAc0.5) owing to its weaker NiZrO₂ interaction confirmed by H₂-TPR results, which endows it with lower catalytic stability although it has higher coke deposition resistance.     

Thermocatalytic decomposition of methane over mesoporous nanocrystalline promoted Ni/MgO·Al2O3 catalysts

Thermocatalytic decomposition of CH₄ is an interesting method for the production of hydrogen. In this article, the catalytic and structural properties of the La, Ce, Co, Fe, and Cu-promoted Ni/MgO·Al₂O₃ catalysts were investigated in the thermal decomposition of CH₄. Mesoporous MgO·Al₂O₃ powder with the high BET area (>250 m²/g) was synthesized by a novel and simple sol–gel method. The different instrumental methods (XRD, BET, SEM, H₂-TPR and TPO) were used for evaluating the physicochemical characteristics of the samples. The addition of Cu to Ni/MgO·Al₂O₃ dramatically improved the catalytic performance and the Cu-promoted catalysts exhibited the highest CH₄ conversion and H₂ yields among the promoted and unpromoted catalysts. The Cu-promoted catalyst possessed the highest stability in CH₄ conversion during 10 h of reaction. The results also indicated that the Ni–Cu/MgO·Al₂O₃ catalyst with 15 wt.% Cu showed the highest catalytic activity and stability at higher temperatures (>80% CH₄ conversion).