Large Fe Particles Research Articles

Understanding how single-atom site density drives the performance and durability of PGM-free Fe–N–C cathodes in anion exchange membrane fuel cells

One of the most important needs for the future of low-cost fuel cells is the development of highly active platinum group metal (PGM)-free catalysts. For the oxygen reduction reaction, Fe–N–C materials have been widely studied in both acid and alkaline media. However, reported catalysts in the literature show quite different intrinsic activity and in-cell performance, despite similar synthesis routes and precursors. Here, two types of Fe–N–C are prepared from the same precursor and procedure – the main difference is how the precursor was handled prior to use. It is shown that in one case Fe overwhelmingly existed as highly active single-metal atoms in FeN4 coordination (preferred), while in the other case large Fe particles coexisting with few single metal atoms were obtained. As a result, there were drastic differences in the catalyst structure, activity, and especially in their performance in an operating anion exchange membrane fuel cell (AEMFC). Additionally, it is shown that catalyst layers created from single-atom-dominated Fe–N–C can have excellent performance and durability in an AEMFC using H2/O2 reacting gases, achieving a peak power density of 1.8 W cm−2 – comparable to similar AEMFCs with a Pt/C cathode – and being able to operate stably for more than 100 h. Finally, the Fe–N–C cathode was paired with a low-loading PtRu/C anode electrode to create AEMFCs (on H2/O2) with a total PGM loading of only 0.135 mg cm−2 (0.090 mgPt cm−2) that was able to achieve a very high specific power of 8.4 W mgPGM−1 (12.6 W mgPt−1).

Simultaneous Selective Chlorination and Carbothermic Reduction of High-Iron Manganese Ore for the Recovery of Manganese Chloride and Metallic Iron

Metallurgical processing of low-grade manganese ore with high iron content is gaining increasing attention due to the gradual depletion of high-grade Mn ores, amid the difficulties in its efficient extraction for both Mn and Fe values in an environmentally-friendly manner. Attempting to tackle the difficulties, this paper describes an innovative process for selectively chlorinating and reducing the high-Fe manganese ore in a simultaneous manner, aiming to produce water-soluble MnCl2 and metallic Fe. After pre-mixing with carbonaceous reductant, CaCl2 and MgCl2 as the chlorinating agent, the Mn ore was heated at 1000 °C. As much as 89.4% Mn can be chlorinated in its water-soluble form, with dissolution of only 3.0% Fe. The presence of CaCl2 during carbothermic reduction resulted in significant promotion in both the Fe reduction rate and formation of large metallic Fe particles due to the segregation effect, facilitating subsequent separation. Selective Mn chlorination by MgCl2 took place with or without the involvement of SiO2, forming MgSiO4 or MgO, respectively.

Preparation of Fe2P and FeP catalysts for the hydrotreating reactions

Highly loaded iron phosphides were prepared by the phosphidation of unsupported Fe, 80%Fe/C and 60%Fe/C with Fe metal particle sizes of 38, 35 and 29nm, respectively, using triphenylphosphine in liquid phase. The large Fe particles such as those in the unsupported sample could not be phosphided completely, while the smaller ones in the 60%Fe/C could be completely phosphided into Fe2P at 543K for 18h and FeP at 593K for 33h, respectively. The FeP/C was found to be significantly more stable and thus generally more active than the Fe2P/C for the hydrotreating reactions.

Heating behavior of ferromagnetic Fe particle-embedded thermoplastic polyurethane adhesive film by induction heating

The heating behavior of micrometer-sized Fe ferromagnetic particle-embedded thermoplastic polyurethane adhesive (TPU) under induction heating is examined in this study. The effects of particle size and content, TPU film thickness, and output power of the induction heater were considered. According to this study, heat generation, including the heating rate and maximum temperature, is proportional to the size and content of Fe particles. Greater film thickness and output power also led to higher heat generation. The permeability of the micrometer-sized Fe particles is the most important parameter for higher heat generation of TPU films with larger Fe particles.

Cu redistribution during sintering of Fe–2Cu and Fe–2Cu–0·5C compacts

The effective use of alloying elements in powder metallurgical steels requires a deep understanding of their redistribution kinetics during sintering. In this work, interrupted sintering trials of Fe–2Cu and Fe–2Cu–0·5C compacts were performed. Moreover, diffusion simulations of Cu in γ-Fe using Dictra were performed. It is found that transient liquid phase penetrates the Fe interparticle and grain boundaries in less than 3 min of holding time. However, C addition limits the penetration of liquid Cu, particularly into grain boundaries of large Fe particles. The results also show that the mean diffusion distance of Cu in γ-Fe in the C added system is slightly lower than that in the C-free system at 3 min of holding time; however, after 33 min, the mean diffusion distance is similar in both systems. The diffusion distances of Cu in γ-Fe, predicted by Dictra, are in good agreement with the measured values.

What is below the support layer affects carbon nanotube growth: an iron catalyst reservoir yields taller nanotube carpets

Here we demonstrate an approach to enhance the growth of vertically aligned carbon nanotubes (CNTs) by including a catalyst reservoir underneath the thin-film alumina catalyst underlayer. This reservoir led to enhanced CNT growth due to the migration of catalytic material from below the underlayer up to the surface through alumina pinholes during processing. This led to the formation of large Fe particles, which in turn influenced the morphology evolution of the catalytic iron surface layer through Ostwald ripening. With inclusion of this catalyst reservoir, we observed CNT growth up to 100% taller than that observed without the catalyst reservoir consistently across a wide range of annealing and growth durations. Imaging studies of catalyst layers both for different annealing times and for different alumina support layer thicknesses demonstrate that the surface exposure of metal from the reservoir leads to an active population of smaller catalyst particles upon annealing as opposed to a bimodal catalyst size distribution that appears without inclusion of a reservoir. Overall, the mechanism for growth enhancement we present here demonstrates a new route to engineering efficient catalyst structures to overcome the limitations of CNT growth processes.

Effect of hydrogen pretreatment on the spin-capability of a multiwalled carbon nanotube forest

The authors report the effect of hydrogen (H2) pretreatment on the spin capability of vertically aligned multiwalled carbon nanotubes (MWCNTs) grown on an iron (Fe) catalyst by using a chemical vapor deposition system. Variations in H2 pretreatment time had significant effects on the distribution, morphology, diameter, and height of MWCNTs. A longer H2 pretreatment time to the Fe film increased the number of large Fe particles as well as crystallization of Fe3O4 phase. As a result, the areal density of carbon nanotubes (CNTs) was largely reduced, whereas the mean CNT diameter slightly increased. This worsened the alignment of the CNTs and reduced the spin-capability of MWCNTs.

Continuous synthesis of controlled size carbon-encapsulated iron nanoparticles

Carbon-encapsulated iron nanoparticles were continuously and selectively synthesised in a thermal plasma jet from ethanol (carbon source) and Fe powders with different grain sizes. The grain size of the Fe powder influenced the size distribution of the as-produced carbon encapsulates. The products obtained from large Fe particles (50–78 μm) were comprised of small encapsulates with diameters between 5 and 10 nm. Larger carbon encapsulates with a broad diameter distribution (10–100 nm) were synthesised from the finest Fe particles (18 μm). It was also found that Fe particle size was the most crucial parameter for determining the encapsulation yield. The encapsulation yield was also influenced by the carbon to iron ratio and the thermal conductivity of the plasma gas.

Reaction mechanisms of tridymite iron phosphate with lithium ions in the low-voltage range

The electrochemical reaction mechanisms of tridymite FePO 4 with Li ions were studied in the voltage range of 2.4–0 V. The lithiation/delithiation of FePO 4 was found to be similar to that of binary 3d-metal oxides, which involve the formation and decomposition of Li 2O along with the reduction and oxidation of nanoscale 3d-metal nanoparticles. In the first discharge, FePO 4 was reduced to metallic Fe nanoparticles dispersed in a lithium compound matrix consisting of Li 3PO 4 and Li 2O. In the subsequent charge, these metallic Fe nanoparticles were partially oxidized to FeO by the decomposition of Li 2O. The Li 3PO 4 matrix was inactive during the change of potential from 0 to 2.4 V. It is notable that the re-oxidation of metallic Fe occurred only in the case of small nanoparticles, and the large Fe particles remained electrochemically inert during the reversible reaction.

Characterization of Carbon Nanotubes Deposited in Microwave Torch at Atmospheric Pressure

Carbon nanotubes (CNTs) were prepared in microwave plasma torch at atmospheric pressure on the Si/SiO2 substrates with iron catalytic layer from the mixture of CH4/H2/Ar. The pretreatment phase for production of Fe nanoparticles was an integral part of the deposition. It took only few seconds and the substrate temperature reached 1 020–1 170 K. The stable temperature of the deposition was 870–950 K. Due to a spatial non-uniformity of the discharge, the prepared samples exhibited several different regions of CNTs. A blackish transition region close to the iron margins contained thin CNTs with the average diameter below 18 nm and much larger Fe particles. Most of the black deposit was a continuous film of straight standing CNTs with an average diameter of 38 nm. Iron particles of the approximately the same size were found at the nanotube tips. HRTEM showed the presence of triple-walled nanotubes with outer diameter below 5 nm. Raman spectra in the low frequency region consisted of several weak peaks assigned to RBM of thin MWNTs whereas first-order Raman peaks in the high frequency region were attributed to D, G, and D′ bands.

XRD and HRTEM analyses of the ball milled nanocrystalline FeCo alloy

The nanostructures of the ball milled FeCo particles were characterized as functions of the ball milling time (t) using quantitative X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) analysis techniques. The results show that the nanocrystalline bcc FeCo particles are available using carbonyl iron and cobalt powders as the start materials during the high-energy ball milling. At the early stage of ball milling, Co powders are easily mashed into nanocrystalllites, by which the surface of the larger Fe particles of about 80–150 nm is coated. With t increasing, the refinement of grain size and the incorporation of defects including dislocations, disclinations and grain boundaries happen, and then FeCo alloy with a certain layered structure is formed, finally the layered structure disappears with the formation of isotropic grains having a steadystate grain size in the nanometer regime after a certain period of t.

Numerical Analysis of Deformation and Damage in Copper Alloy Sheet

By using the finite element method and large strain two-dimension plane strain model, the flaking damage of Cu-Fe-P lead frame sheet is investigated. The characterization of microstructure under surface flaking is the larger Fe particles embedded in the copper matrix. The numerical analysis reveals that at the interface of Cu and Fe there are greater strain mutation and intense stress concentration that is increased with particle diameter and the extent of deformation. This strain and stress concentration makes the interface initiate crack and develop crack easily. The micro-crack around the Fe particle is also attributed to surface flaking damage under the finish rolling deformation. The larger Fe particles should be avoided in the production of Cu-Fe-P alloy.

Experimental and computational investigation of the magnetic susceptibility of composite soft materials

The magnetic properties of densely packed composite granular materials are strongly affected by interactions between the grains. This paper addresses the influence of the interactions, as a function of the packing density, on the initial susceptibility. Our calculations are based on systems with realistic physical microstructures which are free from an artificially imposed lattice. The introduction of particle elongation via demagnetizing factors due to prolate spheroids is shown to enhance the susceptibility and give results which are in good agreement with experimental observations of composite soft materials prepared by dispersing large Fe particles in a non-magnetic matrix.

Synthesis of carbon nanotubes–Fe–Al 2O 3 powders. : Influence of the characteristics of the starting Al 1.8Fe 0.2O 3 oxide solid solution

Al 1.8Fe 0.2O 3 amorphous solid solution have been calcined at temperatures between 1025 and 1100°C and the obtained powders have been reduced in a H 2–CH 4 gas mixture (12 mol% CH 4) at 1050°C in order to prepare carbon nanotubes (C NTs)–Fe/Fe 3C–Al 2O 3 nanocomposite powders. The calcination at 1100°C leads to powders containing an α-Al 2−2xFe 2xO 3 solid solution (x < 0.1) and some traces of a Fe-rich phase (α-Fe 2−2yAl 2yO 3) that gives, under reduction, large Fe particles which catalyze the growth of ribbon like filaments. An α-Al 1.8Fe 0.2O 3 monophased solid solution with a higher specific surface area is obtained after a calcination at only 1050 or 1025°C. In the corresponding composite powders, all filaments are isolated C NTs or bundles of C NTs and the high specific area is beneficial to both the quantity and the quality parameters which are greatly enhanced in comparison with previous results. About 20% of the observed C NTs are single wall nanotubes (SWNTs). The majority of the observed multiwall nanotubes (MWNTs) have only two walls and the inner diameters are distributed between 0.8 and 6 nm, with a peak at 2 nm. The in-situ catalysis by selective reduction of a solid solution allows for production of nanocomposite powders containing very large quantities of high quality C NTs.

Carbon nanotubes–Fe–alumina nanocomposites. Part I: influence of the Fe content on the synthesis of powders

Oxides based on α-alumina and containing various amounts of Fe (2, 5, 10, 15 and 20 cat%) were prepared by decomposition and calcination of the corresponding mixed-oxalates. Selective reduction of the oxides in a H 2–CH 4 atmosphere produces nanometric Fe particles which are active for the in-situ nucleation and growth of carbon nanotubes. These form bundles smaller than 100 nm in diameter and several tens of micrometers long. However, the carbon nanotubes–Fe–Al 2O 3 nanocomposite powders may also contain Fe carbide nanoparticles as well as undesirable thick, short carbon tubes and thick graphene layers covering the Fe/Fe carbide nanoparticles. The influence of the Fe content and the reduction temperature on the composition and micro/nanostructure of the nanocomposite powders have been investigated with the aim of improving both the quantity of nanotubes and the quality of carbon, i.e. a smaller average tube diameter and/or more carbon in tubular form. A higher quantity of carbon nanotubes is obtained using α-Al 1·8Fe 0·2O 3 as starting compound, i.e. the maximum Fe concentration (10 cat%) allowing to retain the monophase solid solution. A further increase in Fe content provokes a phase partitioning and the formation of a Fe 2O 3-rich phase which upon reduction produces too large Fe particles. The best carbon quality is obtained with only 5 cat% Fe (α-Al 1·9Fe 0·1O 3), probably because the surface Fe nanoparticles formed upon reduction are a bit smaller than those formed from α-Al 1·8Fe 0·2O 3, thereby allowing the formation of carbon nanotubes of a smaller diameter. For a given Fe content (≤ 10 cat%), increasing the reduction temperature favours the quantity of nanotubes because of a higher CH 4 sursaturation level in the gas atmosphere, but also provokes a decrease in carbon quality.

Stress-induced martensitic transformation of boundary eta -Fe particles in a Cu-Fe alloy

The eta alpha martensitic transformation in large coherent Fe particles on a small-angle boundary in Cu-Fe alloy bicrystals has been induced by applying external stress at 77 K without activating lattice dislocations. Depending on the sense (tension or compression) of the applied stress, some particular variants of martensite are preferentially formed. The preferential formation of the variants can be explained by postulating that the applied stress helps the shear movement of atoms to change the lattice from fcc to bcc.

Growth kinetics and martensitic transformation of large Fe particles in a Cu-1.5 mass% Fe alloy

The growth kinetics and martensitic transformation induced by simple cooling of large Fe particles in Cu-1.5 mass % Fe single crystals aged at 973 K for up to 60 days are examined by transmission electron microscopy and magnetic measurement. The growth kinetics of large Fe particles can be described by the Lifshitz-Slyozov-Wagner theory and the interfacial energy between the Fe particles and the Cu matrix is estimated as 0.24 to 0.40 J m−2. The volume fraction of α-Fe particles increases with the mean particle size and with lowering of the cooling temperature. The critical sizes below which the transformation by simple cooling does not occur, are found to be 290 and 350 nm for specimens cooled to 77 K and room temperature, respectively. The saturation magnetization of the Cu-Fe alloy containing large α-Fe particles increases by further annealing of the cooled specimens. These facts imply that α-Fe particles are not produced during the prolonged ageing but are formed martensitically during the cooling process.

Hydrocarbon synthesis from COH 2 on supported iron: Effect of particle size and interstitials

Rates of hydrocarbon synthesis (HS) from COH 2 (syngas) were measured for Fe supported on MgO ( Fe MgO ) while Mössbauer effect spectroscopy (MES) was used for monitoring the structure of the metal. Three effects were observed as particle size d of H 2 prereduced Fe changed between 1 and 17 nm. First, the site-time yield of CH 4 synthesis (STY CH 4 , the number of CH 4 molecules produced per CO-titrated site per second) increased by over an order of magnitude. Second, under conditions used ( 525K, H 2 CO = 3 , atmospheric pressure), large Fe particles produced a lower fraction of C + 2 in the hydrocarbon product (C + 2 selectivity) and showed a greater tendency for deactivation and loss of C + 2 selectivity than small particles. Third, as reaction in syngas proceeded, the dominant phase shifted from x-carbide to ε′-carbide as d decreased, and further decrease of d produced a decrease in the rate of ε′-carbide formation. The three trends for HS were the same qualitatively and similar quantitatively when the catalysts were converted to ε-nitride before reaction, instead of being used directly after reduction in H 2. However, MES showed that the ε-nitride structure persisted even after 20 h in reacting syngas although measurements of CH 4 evolved during reduction of used samples indicated that the surface of all catalysts was thoroughly carbided. Thus, for HS, particle size was more important than interstitials in determining activity and selectivity of supported Fe.

On the Sintering Process of Iron and Aluminum Mixed Powder Compacts

Influence of Al powder addition on the sintering process of Fe powder compact was made by means of dilatometric and microscopic examinations, X-ray diffraction, E.P.M.A. and D.T.A. The results were summarized as follows.(1) An intermediate phase (Fe2Al5) was produced by the interaction of molten Al and solid Fe at the melting point of Al at 660°C, where an expansion and an exothermic phenomenon were observed in the Fe-Al mixed powder compacts.(2) This expansion was due to the penetration of molten Al into the interfaces between Fe particles, leaving pores at the place where the solid Al had been located.In this stage, the cracks were generated at these interfaces in the case of ore reduced Fe-Al mixed powder compacts. It was very difficult to eliminate the large pores and the cracks in more advanced stages of the sintering process. The exothermic phenomenon was thought to be caused by the formation of Fe2Al5.(3) It was proved that for the carbonyl Fe-Al mixed powder compacts of composition over y-loop, the sintering was remarkably accelerated by the process stabilizing the a-Fe phase with the diffusion of Al into the Fe matrix from Fe2Al5 at high temperature.On the other hand, the sintering of ore reduced Fe-Al mixed powder compacts was disturbed by the difficulties to uniformly diffuse Al into larger Fe particles and to repair the cracks.