Prussian blue as a single precursor for synthesis of Fe/Fe3C encapsulated N-doped graphitic nanostructures as bi-functional catalysts

We report a unique, single source precursor Prussian blue (iron(III) ferrocyanide (Fe-4(III)Fe-II(CN)(6)](3))) for the synthesis of Fe/Fe3C nanoparticle encapsulated N-doped graphitic layers and bamboo-like graphitic nanotubes. Hollow N-doped graphite (N-HG) nanostructures are obtained when the encapsulated nanostructures are treated with an acid. Both the encapsulated nanostructures and N-HG are shown to be applicable as bi-functional electrocatalysts for oxygen reduction (ORR) and oxygen evolution reactions (OER). The ORR activity is shown to be improved for N-HG and is comparable to commercial Pt/C. On the other hand, encapsulated nanostructures exhibit OER activity with long-term stability comparable to commercial RuO2.

Prussian blue as a single precursor for synthesis of Fe/Fe3C encapsulated N-doped graphitic nanostructures as bi-functional catalysts

We report a unique, single source precursor Prussian blue (iron(III) ferrocyanide (Fe-4(III)Fe-II(CN)(6)](3))) for the synthesis of Fe/Fe3C nanoparticle encapsulated N-doped graphitic layers and bamboo-like graphitic nanotubes. Hollow N-doped graphite (N-HG) nanostructures are obtained when the encapsulated nanostructures are treated with an acid. Both the encapsulated nanostructures and N-HG are shown to be applicable as bi-functional electrocatalysts for oxygen reduction (ORR) and oxygen evolution reactions (OER). The ORR activity is shown to be improved for N-HG and is comparable to commercial Pt/C. On the other hand, encapsulated nanostructures exhibit OER activity with long-term stability comparable to commercial RuO2.

Mossbauer effect studies of the thermal decomposition of iron(II) oxalate in hydrogen

Mössbauer-effect and X-ray studies were carried out on the product samples of the thermogravimetric analysis (TGA) and of the isothermal decomposition of iron(II) oxalate in flowing H2. Two types of sample configurations were employed for isothermal studies between 280 to 420°C for various periods of heating. Low temperature Mossbauer measurements at liquid nitrogen temperature were carried out to examine the superparamagnetic (SPM) contributions. From the spectra of samples decomposed at 340°C, in vertical experiments, the percentage SPM and percentage ferromagnetic (FM) area of Fe3O4 were estimated and an average size (˜167Å) for Fe3O4 was derived. Mossbauer measurements (at high temperatures) were carried out on Fe3C formed in horizontal experiments, for two samples decomposed at ˜320°C for 1 hr and 2 hr. An estimate of SPM and FM Fe3C was obtained by calculating KV, the anisotropy energy for the Fe3C in these two samples and values of 5.07 × 10−16 and 7.02 × 10−16 erg/sec, respectively, were obtained.

Size-dependent magnetic properties of iron carbide nanoparticles embedded in a carbon matrix

Here we report on the magnetic properties of iron carbide nanoparticles embedded in a carbon matrix. Granular distributions of nanoparticles in an inert matrix, of potential use in various applications, were prepared by pyrolysis of organic precursors using the thermally assisted chemical vapour deposition method. By varying the precursor concentration and preparation temperature, compositions with varying iron concentration and nanoparticle sizes were made. Powder x-ray diffraction, transmission electron microscopy and Mossbauer spectroscopy studies revealed the nanocrystalline iron carbide (Fe3C) presence in the partially graphitized matrix. The dependence of the magnetic properties on the particle size and temperature (10 K < T < 300 K) were studied using superconducting quantum interference device magnetometry. Based on the affect of surrounding carbon spins, the observed magnetic behaviour of the nanoparticle compositions, such as the temperature dependence of magnetization and coercivity, can be explained.

An investigation of carbon nanotubes obtained from the decomposition of methane over reduced Mg1− xM xAl2O4 spinel catalysts

Carbon nanotubes produced by the treatment of Mg1−xMxAl2O4 (M = Fe, Co, or Ni; x = 0.1, 0.2, 0.3, or 0.4) spinels with an H2–CH4 mixture at 1070 °C have been investigated systematically. The grains of the oxide-metal composite particles are uniformly covered by a weblike network of carbon nanotube bundles, several tens of micrometers long, made up of single-wall nanotubes with a diameter close to 4 nm. Only the smallest metal particles (<5 nm) are involved in the formation of the nanotubes. A macroscopic characterization method involving surface area measurements and chemical analysis has been developed in order to compare the different nanotube specimens. An increase in the transition metal content of the catalyst yields more carbon nanotubes (up to a metal content of 10.0 wt% or x = 0.3), but causes a decrease in carbon quality. The best compromise is to use 6.7 wt% of metal (x = 0.2) in the catalyst. Co gives superior results with respect to both the quantity and quality of the nanotubes. In the case of Fe, the quality is notably hampered by the formation of Fe3C particles.

Morphological Evolution in Air-Stable Metallic Iron Nanostructures and Their Magnetic Study

Iron nanostructures with morphology ranging from discrete nanoparticles to nearly monodisperse hierarchical nanostructures have been successfully synthesized using solvated metal atom dispersion (SMAD) method. Such a morphological evolution was realized by tuning the molar ratio of ligand to metal. Surface energy minimization in confluence with strong magnetic interactions and ligand-based stabilization results in the formation of nanospheres of iron. The as-prepared amorphous iron nanostructures exhibit remarkably high coercivity in comparison to the discrete nanoparticles and bulk counterpart. Annealing the as-prepared amorphous Fe nanostructures under anaerobic conditions affords air-stable carbon-encapsulated Fe(0) and Fe3C nanostructures with retention of the morphology. The resulting nanostructures were thoroughly analyzed by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Raman spectroscopy. TGA brought out that Fe3C nanostructures are more robust toward oxidation than those of a-Fe. Finally, detailed magnetic studies were carried out by superconducting quantum interference device (SQUID) magnetometer and it was found that the magnetic properties remain conserved even upon exposure of the annealed samples to ambient conditions for months.

Air Stable Iron/Iron Carbide Magnetic Nanoparticles Embedded in Amorphous Carbon Globules

We have synthesized Fe/Fe3C magnetic nanoparticles embedded in an amorphous carbon globule by pyrolysing of benzene, ferrocene and hydroboric acid. The diameter of the globules is similar to 1 mu m and that of Fe/Fe3C magnetic nanoparticles is similar to 40 nm. The globules exhibit ferromagnetic like behavior and the magnetization as well as the coercivity is found to increases with decreasing temperature.

Reconciling results of MOCVD of a CNT composite with equilibrium thermodynamics

Composition and microstructure of the composite films can be tailored by controlling the CVD process parameters if an appropriate model can be suggested for quantitative prediction of growth. This is possible by applying equilibrium thermodynamics. A modification of such standard modeling procedure was required to account for the deposition of a hybrid film comprised of carbon nanotubes (CNTs), metallic iron (Fe), and magnetite (Fe3O4), a composite useful for energy storage. In contrast with such composite nature of the deposits obtained by inert-ambient CVD using Fe(acac)3 as precursor, equilibrium thermodynamic modeling with standard procedure predicts the deposition of only Fe3C and carbon, without any co-deposition of Fe and Fe3O4. A modification of the procedure comprising chemical reasoning is therefore proposed herein, which predicts simultaneous deposition of FeO1-x, Fe3C, Fe3O4 and C. At high temperatures and in a carbon-rich atmosphere, these convert to Fe3O4, Fe and C, in agreement with experimental CVD. Close quantitative agreement between the modified thermodynamic modeling and experiment validates the reliability of the modified procedure. Understanding of the chemical process through thermodynamic modeling provides potential for control of CVD process parameters to achieve desired hybrid growth. (C) 2016 Elsevier B.V. All rights reserved.

The phase of iron catalyst nanoparticles during carbon nanotube growth

We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For α-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron-carbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe3C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes. © 2012 American Chemical Society.

Synthesis Gas Generation by Chemical-Looping Reforming Using Ce-Based Oxygen Carriers Modified with Fe, Cu, and Mn Oxides

Chemical-looping reforming (CLR) is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. It involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from combustion air to the fuel. Composite oxygen carriers of cerium oxide added with Fe, Cu, and Mn oxides were prepared by co-precipitation and investigated in a thermogravimetric analyzer and a fixed-bed reactor using methane as fuel and air as oxidizing gas. It was revealed that the addition of transition-metal oxides into cerium oxide can improve the reactivity of the Ce-based oxygen carrier. The three kinds of mixed oxides showed high CO and H-2 selectivity at above 800 degrees C. As for the Ce-Fe-O oxygen carrier, methane was converted to synthesis gas at a H-2/CO molar ratio close to 2:1 at a temperature of 800-900 degrees C; however, the methane thermolysis reaction was found on Ce-Cu-O and Ce-Mn-O oxygen carriers at 850-900 degrees C. Among the three kinds of oxygen carriers, Ce-Fe-O presented the best performance for methane CLR. On Ce-Fe-O oxygen carriers, the CO and H-2 selectivity decreased as the Fe content increased in the carrier particles. An optimal range of the Ce/Fe molar ratio is Ce/Fe > 1 for Ce-Fe-O oxygen carriers. Scanning electron microscopy (SEM) analysis revealed that the microstructure of the Ce-Fe-O oxides was not dramatically changed before and after 20 cyclic reactions. A small amount of Fe3C was found in the reacted Ce-Fe-O oxides by X-ray diffraction (XRD) analysis.

激光拼焊成套装备定位方法及焊接工艺研究

激光拼焊由于具有能量密度高，焊缝深宽比大，变形和热影响区小，焊接速度快，满足工件不同部位对材料各自性能的需求，焊接质量好，容易实现自动化等众多优点，使其广泛应用于工业中各个领域。激光拼焊生产线是利用高能激光作为焊接热源，将两块或多块汽车板材一次焊接成形，实现高效全自动化生产，年产可达百万片，以满足汽车行业的需求。而我国作为世界制造加工大国，目前尚不能完全独立自主开发激光拼焊成套装备。 针对自主开发激光拼焊成套装备的几个关键技术点，本文以中国科学院知识创新工程重要方向性项目“全自动激光拼焊成套装备关键技术研究与示范应用”为课题背景，结合项目实际开发中的具体要求，在对现有激光拼焊技术深入分析的基础上，对激光拼焊定位夹紧、最优化激光拼焊工艺和焊接接头机械性能、金相组织等方面进行了深入研究，得出了激光拼焊最优工艺规范和相应的焊接质量变化规律，为激光拼焊成套装备开发提供参考。 第一部分激光拼焊定位夹紧对中方法研究。重点分析了由于激光光束本身条件限制、激光拼焊生产线高速高效的要求、激光焊接热变形和目前国内汽车板材本身理化性能一致性差以及激光焊接过程中的诸多不确定因素，总结出了激光拼焊生产线工装夹具设计影响因素，并在此基础上指出了激光拼焊定位夹紧结构所应具备的功能特征。针对激光拼焊板材介于刚体和弹性体之间的柔性体特征，结合传统的定位原理“3-2-1”，提出了“N-2-1”的定位方式，给出了激光拼焊夹具定位原理及设计准则，初步建立了激光拼焊夹具参数化零件库和相应准则，并对激光拼焊夹紧机构进行了柔性分析，给出了拼焊夹具的柔性评价方案。 第二部分激光拼焊工艺研究。首先分析了激光拼焊工艺特征，重点研究了在大功率固态激光器条件下，激光拼焊焊接工艺参数主要包括激光功率、焊接速度、离焦量和侧吹保护气体的喷嘴高度、倾斜角度及气体流量等因素变化对焊接质量的影响，得出了变化规律曲线，为激光拼焊工艺规范微调方向提供参考依据;同时也为激光工艺库开发提供推理机制。最后系统全面地研究了目前汽车常用板材全厚度系列激光拼焊工艺，采用叠代寻优的方法获得到了适用于全自动激光拼焊生产线的最优化工艺规范。 第三部分激光拼焊工艺库开发。针对激光拼焊成套装备项目开发面临的问题：在中国市场中，激光拼焊成套装备销售必须附带汽车行业常用工艺规范库的现实问题。在大量试验的基础上，借助于VB6.0集成开发环境，建立了激光拼焊工艺库系统，主要包括激光拼焊示范应用模块、工艺参数查询模块、经济评估模块、数据库维护模块、管理查询模块等，初步实现了系统演示、查询、评估、管理等功能，基本满足激光拼焊生产线要求。 第四部分激光拼焊焊接接头金相组织性能研究。结合金属Fe3C相图，分析了在激光焊接快速冷却、大过冷度条件下，分别从合金性质、成分、固/液界面上的表面能、均相成核速度和固相生长速度等角度，探讨了金相组织结构变化特征。然后通过试验分别研究了汽车碳钢板和不锈钢板激光拼焊时，板材厚度、激光功率、焊接速度等工艺参数变化，对激光拼焊焊接接头的抗拉强度、显微组织硬度和金相组织结构影响。 第五部分针对激光拼焊钢板实际生产中，存在着焊接性能随焊接工艺变化差异大的问题，利用激光拼焊生产线实际得到的数据，建立了基于BP神经网络激光拼焊焊接性能预测模型，初步实现了根据工艺参数的改变，直接预测焊接力学性能的目的，克服了激光拼焊初期需大量试验性研究的缺点，为工艺参数优化研究提供了一种有效的手段。