HYDROGEN CRACKING IN α-Fe

用正电子湮没技术和透射电子显微方法研究了α-Fe 中氢致裂纹的机制;结果表明阴极充氢后氢-空位复合体促进沿晶界微空洞的形成,并且这些微空洞的聚集和长大导致微裂纹的萌生。此外,还研究了这种微裂纹的热稳定性。     

Dynamic recrystallization during hot compression of α-Fe

Specimens of high purity -Fe were deformed in the GLEEBLE-1500 at temperatures of 550°C, 700°C, 800°C and 900°C at strain rates ranging from 0.001 to 10 s^–1. The microstructural changes, which occur during the hot compression, have been investigated by optical microscopy and related to the true stress-true strain curves. The experimental results show that the dynamic recrystallization is accelerated with increase of deformation temperature and decrease of strain rate. The relation between the dynamic recrystallization and Z-parameter has been investigated. Dynamic recrystallization takes place approximately in a certain range of Z parameter, i.e., 25 < lnZ < 37.     

Modeling the Vibrational Spectrum of Vacancy-Containing α-Fe Crystals

The local atomic structure and vibrational spectrum of vacancy-containing -Fe crystals are calculated using a many-particle interatomic potential. The phonon spectrum is evaluated by a recursion method, and force-constant matrices are calculated using sparse matrix technology.     

Hydrogen hardening effect in heavily deformed single crystal α-Fe

Molecular dynamics simulations were performed to investigate the hydrogen interaction with edge dislocations during deformation in α-Fe. In particular, uniaxial tensile tests of a single crystal iron were conducted after the single crystal was plastically deformed to introduce high density of edge dislocations and was doped with different levels of lattice hydrogen concentration. During system relaxation, hydrogen atoms have tendency to diffuse and stay around dislocation line – a well-known hydrogen trap-site in α-Fe. Our simulations show that the yield strength of the bcc iron is very sensitive to the presence of hydrogen within edge dislocations, i.e., it increases as hydrogen concentration increases. After yielding, hydrogen atoms are de-associated with the moving dislocations, suggesting that the yield strength enhancement is caused by the hydrogen pinning effect. This direct observation of hydrogen hardening effects has confirmed the experimental findings by Matsui, etc. Additional simulations also indicate that hydrogen interaction with edge dislocations is sensitive to temperature as well as vacancies around dislocation core.     

Evidence for intergranular tunnelling in polyaniline passivated α-Fe nanoparticles

Nanoparticles are of immense importance both from the fundamental and application points of view. They exhibit quantum size effects which are manifested in their improved magnetic and electric properties. Mechanical attrition by high energy ball milling (HEBM) is a top down process for producing fine particles. However, fineness is associated with high surface area and hence is prone to oxidation which has a detrimental effect on the useful properties of these materials. Passivation of nanoparticles is known to inhibit surface oxidation. At the same time, coating polymer film on inorganic materials modifies the surface properties drastically. In this work a modified set-up consisting of an RF plasma polymerization technique is employed to coat a thin layer of a polymer film on Fe nanoparticles produced by HEBM. Ball-milled particles having different particle size ranges are coated with polyaniline. Their electrical properties are investigated by measuring the dc conductivity in the temperature range 10-300?K. The low temperature dc conductivity (I-V) exhibited nonlinearity. This nonlinearity observed is explained on the basis of the critical path model. There is clear-cut evidence for the occurrence of intergranular tunnelling. The results are presented here in this paper.     

Top-down synthesis of mesocrystalline α-Fe2O3 submillimeter-sized rhombohedrons

ABSTRACT

Here, we focus on the obtaining of mesocrystalline submillimeter-sized (150/50 µm) rhombohedral hematite (α-Fe₂O₃) by thermal treatment in air of single crystalline submillimeter-sized (150/50 µm) rhombohedrons of ferrous carbonate (FeCO₃). Mass spectrometer-coupled thermogravimetric analysis and TGA-MS revealed the chemical reactions occurring during the thermal treatment of ferrous carbonate sample. The X-ray Diffraction (XRD) data sustain that the final product is hematite. The XRD line-profile analysis indicates that the resulted hematite is built of individual ordered crystallites with 66 ± 5 nm average sizes, confirmed by scanning electron microscopy and transmission electron microscopy images. Small-angle x-ray scattering investigation of hematite sample was presented. The log-log plot of scattering intensity decay showed the same slope, α = ?3.76, corresponding to both high and low scattering vector regions; the fractal surface is D_s = 2.24. This fractality is extended over a range of sizes and can touch high molecular dimensionality. The internal morphology and the synthesis mechanism of the obtained hierarchical superstructure were described.     

Surface Modification of α-Fe Metal Particles by Chemical Surface Coating

The structure of α-Fe metal magnetic recording particles coated with silane coupling agents have been studied by TEM, FT-IR, EXAFS, Mossbauer. The results show that a close, uniform, firm and ultra thin layer, which is beneficial to the magnetic and chemical stability, has been formed by the cross-linked chemical bond Si-O-Si. And the organic molecule has chemically bonded to the particle surface, which has greatly affected the surface Fe atom electronic structure. Furthermore, the covalent bond between metal particle surface and organic molecule has obvious effect on the near edge structure of the surface Fe atoms.     

Anisotropy of electrical properties in α-Fe2O3 ceramics

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in -Fe₂O₃ single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of -Fe₂O₃ and limit the mobility of n type carriers.     

Mechanical processing,compaction, and thermal processing of α-Fe powder

High-energy ball milling is a proven technique for both mechanical alloying and for the production of nanocrystalline powders. In this U.S. Bureau of Mines study, it is found that mechanical processing pure α-Fe powders in a nitrogen environment results in nanocrystalline powders with nitrogen concentrations that increase linearly with time. In addition, processing in nitrogen results in a much smaller ultimate crystallite size than obtained in identical powders processed in argon. Consolidation of these powders by explosive compaction results in dense bodies that retain both their nanostructure and nitrogen content, whereas consolidation by HIPing results in retained nitrogen levels but a loss of nanostructure. Both the nanostructure and the high nitrogen levels of the explosively compacted materials are retained up to 863K.     

Synthesis and Characterization of Nanocapsules of α-Fe(NiCoAl) Solid-solution

α-Fe(NiCoAl) solid-solution nanocapsules were prepared with pure powders of Fe, Ni, Co and Al by the plasma arc-discharging using a copper crucible. The shapes of the nanocapsules are in polyhedrons with the core/shell structure. The body centered cubic (BCC) phase is formed in the core. The size of the nanocapsules is in the range of 10-120 nm and the thickness of the shell is 4-11 nm. Saturation magnetization JS=150 Am2/kg and coercivity iHC=24.3 kA/m are achieved for the nanocapsules.     

Green synthesis of α-Fe2O3 nanoparticles for photocatalytic application

In this study, the preparation of α-Fe₂O₃ nanoparticles using curcuma and tea leaves extract are reported. The curcuma and tea leaves are acted as a reductant and stabilizer. The crystal structure and particle size of the as-synthesized materials were measured through X-ray diffraction. X-ray diffraction patterns revealed that the as-prepared samples were α-Fe₂O₃ nanoparticles with well-crystallized rhombohedral structure and the crystallite sizes of the α-Fe₂O₃ nanoparticles are 4 and 5 nm. Scanning electron microscopy images showed that the prepared samples have spherical shape. The purity and properties of the as-synthesized α-Fe₂O₃ nanoparticles were measured by Raman spectroscopy. The chemical compositions of the as-prepared α-Fe₂O₃ nanoparticles have been analyzed by Fourier transform infrared spectroscopy. The absorption edge of the α-Fe₂O₃ nanoparticles are 561 and 551 nm. The photocatalytic activity of the α-Fe₂O₃ nanoparticles was measured by degradation of methylene orange and the α-Fe₂O₃ nanoparticles showed the excellent photocatalytic performance.     

Synthesis and Characterization of Nanocapsules of α-Fe(NiCoAl) Solid-solution

α-Fe(NiCoAl) solid-solution nanocapsules were prepared with pure powders of Fe, Ni, Co and Al by the plasma arc-discharging using a copper crucible. The shapes of the nanocapsules are in polyhedrons with the core/shell structure. The body centered cubic (BCC) phase is formed in the core. The size of the nanocapsules is in the range of 10～120 nm and the thickness of the shell is 4～11 nm. Saturation magnetization Js=150 Am2/kg and coercivity iHC=24.3 kA/m are achieved for the nanocapsules.     

球磨法制备α′-Fe(N)超细粉末

通过球磨α－ Ｆｅ 和脲的混合粉末，制得α′－ Ｆｅ（Ｎ） 超细粉末．使用Ｘ 射线衍射仪（ＸＲＤ） 、透射电镜（ＴＥＭ） 和热失重（ＴＧＡ） 分析了在真空条件下经１３０ ｈ 和２００ ｈ 球磨后的粉末样品的结构和性能．结果表明，经１３０ ｈ 球磨后开始得到α′－ Ｆｅ（Ｎ） 相；热分析表明，α′－ Ｆｅ（Ｎ） 相在４３０℃以下是稳定的；经球磨２００ ｈ 后，得到α′－ Ｆｅ（Ｎ） 超细粉末（０ ．０３～０ ．０６ μｍ） ，它是ｒ（Ｎ） 为１３ ．４９％ 的过饱和固溶体．     

高饱和磁化强度α″-Fe16N2的研究

详细介绍了有关α″-Fe16N2相的研究结果,对α″-Fe16N2相的"巨磁矩"特性进行了全面评述.     

Exchange bias and suppression of superparamagnetism of α-Fe nanoparticles in NiO matrix

Nanocomposites of Fe-NiO are synthesized by the mechanical milling technique. The phase purity of the sample was checked by X-ray diffraction (XRD), which shows only lines of α-Fe and NiO. Transmission electron microscopy (TEM) measurements confirmed a homogeneous dispersion of α-Fe nanoparticles in NiO matrix. Magnetic hysteresis measurements showed an enhancement in coercivity at room temperature with increasing milling durations and presence of exchange bias field. With increasing milling durations, the exchange field is found to increase while blocking temperature decreased. Presence of defects in Fe-NiO nanocomposites ball-milled for prolonged durations is confirmed by positron annihilation lifetime spectroscopy (PALS) measurements. Mössbauer spectroscopic measurements reveal that Fe nanoparticles are in the blocked state though their size is less than the critical size for becoming single domain and superparamagnetic. This is attributed to strong exchange coupling between the ferromagnetic and antiferromagnetic moments.     

Preparation and characterization of α-Fe2O3–CeO2 composite

In our previous study we attempted to see the effect of cerium doping (Ce/Fe ratio 0.015 to 0.074) on goethite matrix and conversion of doped goethite to hematite. In the present communication, nano-structured α-Fe₂O₃–CeO₂ composite with Fe/Ce weight ratio as 1.1 has been synthesized by calcination of goethite-cerium hydroxide precursor prepared by co-precipitation method. It was observed that co-precipitation of cerium along with iron in hydroxide medium resulted in hindering the formation of crystalline order as the precursor formed showed poorly crystallized goethite and almost no crystallinity in Ce(OH)₄. Calcination of the precursor at 400 °C showed the formation of hematite together with a broad peak corresponding to cerium oxide whereas at 800 °C, two distinct phases of α-Fe₂O₃ and CeO₂ were observed. The Mössbauer spectra showed the presence of a paramagnetic component both for the precursor as well as for the sample calcined at 400 °C but on raising the calcination temperature to 800 °C, the paramagnetic component disappeared and the spectrum corresponding to pure α-Fe₂O₃ phase was observed. The microstructure of the product obtained by calcining at 800 °C showed rod like structure (30 to 50 nm width and 300 to 500 nm length) of α-Fe₂O₃ having equi-dimensional CeO₂ particles on and around the surface. Besides the rods, equi-dimensional particles and agglomerates corresponding to CeO₂ were also observed. The results show that co-precipitation followed by calcinations gives nanorods hematite with CeO₂ particles bonded to its surface.     

Thermal decomposition synthesis of 3D urchin-like α-Fe2O3 superstructures

Urchin-like α-Fe₂O₃ superstructures have been deposited on Si substrate using thermal decomposition FeCl₃ solution at 200–600 °C in the oven. The morphologies and structures of the synthesized urchin-like superstructures have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that urchin-like α-Fe₂O₃ superstructures were a polycrystal with the rhombohedral structure and typical diameters of 16–20 nm and lengths up to 1.0 μm. The as-prepared α-Fe₂O₃ superstructures have a high Brunauer–Emmett–Teller (BET) surface area of about 60.24 m²/g. The photoluminescence spectrum of the urchin-like α-Fe₂O₃ superstructures consists of one weak emission peak at 548 nm (2.26 eV). A possible new mechanism for the formation of the urchin-like superstructures was also preliminarily discussed.     

Dislocations in the Crystallites of Nanocrystalline α-Fe —A Molecular Dynamics Study

Molecular dynamics simulations are carried out in order to Study the atomic structure of crystalline component of nanocrystalline α-Fe when it is consolidated from small grains. A two-dimensional computational block is used to simulate the consolidation process. All the preset dislocations in the original grains glide out of them in the consolidation process, but new dislocations can generate when the grain size is large enough. It shows that dislocations exist in the consolidated material rather than in the original grains. Whether dislocations exist in the crystalline component of the resultant model nano-material depends upon grain size. The critical value of grain size for dislocation generation appears to be about 9 nm. This result agrees with experiments qualitatively.