Ni 3C Research Articles

Thermal and shock history of Almahata Sitta meteorite inferred from structure refinement of pyroxene and Mössbauer spectroscopy of Fe‐Ni metal

AbstractThe crystal structures of orthopyroxene (En86.3Fs8.6Wo5.1, space group Pbca) and pigeonite (En81.7Fs8.8Wo9.5, space group P21/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single‐crystal X‐ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single‐crystal diffraction data. The results indicate a rather disordered intracrystalline Fe2+‐Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe‐Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si‐bearing kamacite, and arise from two different nearest‐neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)3C], schreibersite [(Fe,Ni)3P], and an Fe‐oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.

The site preference and doping effect on mechanical properties of Ni3Al-based γ′ phase in superalloys by combing first-principles calculations and thermodynamic model

The fundamental aspects of site preference of alloying elements on sublattice of the strengthen γ′ phase with L12 structure have not been well understood, which hinders the optimized design of advanced Ni-based high-temperature alloys. In this contribution, the temperature- and composition-dependent site occupying preferences of the binary, ternary, and quaternary of Ni3Al-based γ′ phase alloyed with Mi where Mi represents the additional transitional metals Co, Cr, Cu, Fe, Mn, Mo, Re, Ta, Ti, V, or W atoms (arranged in alphabetical order) chosen frequently, were studied using a two-sublattice thermodynamic model (Ni, Al, Mi)1a(Ni, Al, Mi)3c. The site occupying fractions (SOFs) were calculated based on a thermodynamic database established in this work, where the thermodynamic data of the end-members involved were obtained using first-principles calculations and phonon spectrum calculations. The calculated SOFs results show that there is an obvious site preference for stoichiometry binary Ni3Al, and its site configuration changes from (Al)1a(Ni)3c at room temperature to (Al0.9984Ni0.0015)1a (Al0Ni0.9994)3c at 1273 K. For the γ′ phase with the composition 78Ni-26Al-4Mi (atom ratio and xNi/xAl = 3:1), Mo atoms always preferred to occupy the 1a sublattice (Al site), Co, Mn, and Ti atoms always prefer the 3c sublattice (Ni site) in the whole temperature range, while the site preference of Cr, Cu, Fe, Re, Ta, V, or W atom is affected by temperature. For example, when the heat treatment temperature is lower than 700 K, Cr, Cu, Fe, Ta, V, and W atoms occupy the 1a and 3c sublattice randomly, and Re atoms prefer to 3c sublattice, while when the heat treatment temperature is higher than 1273 K, Cr, Cu, and W atoms prefer 3c sublattice, Fe and Ta atoms prefer to 1a sublattice, while all Re atoms occupy the 3c sublattice exclusively, and all V atoms occupy the 1a sublattice exclusively, respectively. Likewise, the site preference of the quaternary system with selective compositions 78Ni-26Al-2 M1-2 M2 was also predicted. Based on calculated SOFs results, the mechanical and thermodynamic properties were studied at the ground state. It has been revealed that Cr, Re, and V doping can improve the microhardness of Ni3Al alloys; in particular, the effect of Cr is extraordinary; and all elements, except Mn, Mo, and Ti, would enhance the bulk modulus of Ni3Al-based γ′ phase, in which Mn have the greatest influence on reducing the bulk and shear modulus, respectively. Furthermore, all the B/G ratios of the computed Ni3Al-based γ′ phase are >1.75, showing inherent ductility. Only Cr doping significantly enhances the Debye temperature of the Ni3Al-based γ′ phase.

Diamond Crystallization at High Pressure: The Relative Efficiency of Metal-Graphite and Metal-Carbonate Systems

Data on the interaction of the Fe–Ni melt with CaCO3 and graphite at 5 GPa and 1400°С under the thermogradient conditions used in experiments on the growth of diamond on the BARS high-pressure apparatus are presented. The phase composition and component composition of the fluid captured by diamonds in the form of inclusions were studied by gas chromatography–mass spectrometry (GC–MS). Diamonds were synthesized from graphite. During the interaction of the Fe–Ni melt with CaCO3, Ca–Fe oxides and (Fe, Ni)3C carbide were formed. The stability of heavy hydrocarbons under the experimental conditions was confirmed. It was established that the composition of the fluid in synthesized diamonds is close to the composition of the fluid from inclusions in some natural diamonds. Nevertheless, it was concluded that crystallization of large diamonds under natural conditions is hardly possible due to the filling of the main crystallization volume with refractory oxide phases.

First-principles calculations for alloyed cementite (Fe–Ni)3C

First-principles calculations for the electron structure of cementite doped with Ni are performed. The obtained data for magnetization, local magnetic moments, and parameters of hyperfine interaction at Fe nuclei are compared with those for pure cementite and used to interpret experimental magnetic and Mossbauer data for the alloyed cementite obtained through mechanical alloying with Ni.

Structure and magnetic properties of mechanically synthesized (Fe1–x Ni x )75C25 nanocomposites

X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements have been used to study the phase formation in (Fe1–xNix) (with х = 0–0.20) nanocomposites upon severe plastic deformation in a planetary ball mill and subsequent annealings. It has been shown that the mechanical synthesis results in the formation of mainly a nickel-alloyed nanocrystalline (Fe, Ni)3C cementite with a distorted crystal lattice and an amorphous Fe–Ni–C phase. During heating above 300°С, the amorphous phase crystallizes with the formation of cementite, which is characterized by a higher Ni content compared to that in mechanically synthesized cementite. The mechanically synthesized samples exhibit low coercive force (equal to several tens ampere per centimeter). In the course of annealing at temperatures of up to 500–550°С, crystal lattice distortions are removed; this results in reliving the magnetic anisotropy constant and high-coercivity state of cementite. At the same time, Ni-rich cementite areas decompose with the formation of γ-(Fe, Ni, C) phase (austenite); as a result, the average nickel content in the cementite substantially decreases. Annealings at higher temperatures cause the complete decomposition of cementite and lead to an abrupt decrease in the coercive force (Н с) of samples. Alloying with nickel leads to an increase in the Curie temperature of cementite and a decrease in its specific saturation magnetization, coercive force, and thermal stability.

Neutron diffraction study of metal-matrix composite with fullerite

Interaction of amorphous fullerite C60 with austenitic Fe-33.2 wt. % Ni alloy at pressures 0-8 GPa and temperatures 600-1100 °C was studied by neutron diffraction. The amorphous fullerite was obtained by ball milling and mixed with the powder of the crystalline alloy. The interaction at sintering led to the dissolution of carbon in fcc Fe-Ni solid solution and the formation of carbide (Fe, Ni)3C, but the Fe-Ni-C alloy did not undergo phase transformations and preserved the original fcc structure. As a result, the alloy hardened, we could also witness a clear barometric effect: at the pressure of 2 GPa the amount of the dissolved carbon and the microhardness turned out to be significantly higher than those at 8 GPa. During sintering amorphous fullerite is undergoing phase transitions and its microhardness is higher than the microhardness of the metal component. At high temperatures of interaction graphite appears. The presence of Fe-Ni alloy in the composite reduces the temperature of graphite formation in comparison with transformations in the pure amorphous fullerene.

Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

Polycrystalline aggregates of cementite (Fe3C) and (Fe,Ni)3C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1 0 0] and [0 0 1] type, [1 0 0] being the most frequent. [1 0 0] dislocations are dissociated and glide in the (0 1 0) plane. [0 0 1] dislocations glide in (1 0 0) and (0 1 0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.

Laser cladding of high Co–Ni secondary hardening steel on 18Cr2Ni4WA steel

Laser cladding of the high Co–Ni secondary hardening steel on the 18Cr2Ni4WA steel using the laser direct metal deposition (LDMD) technology was investigated. The hardness was measured with a Vickers indenter, and the microstructural evolution was analyzed using a SEM equipped with an EDS and X-ray diffraction. The results indicated that the wear resistance of the 18Cr2Ni4WA steel substrate could be greatly improved by the coating of laser cladded high Co–Ni secondary hardening steel. After laser cladding, the microstructure of the heat-affected zone in the 18Cr2Ni4WA steel substrate was typical martensitic structure transformed from the initial granular bainitic structure. Besides, Fe-based ferrite, martensite, lath retained austenite, and two types of precipitations dispersed in the matrix were observed in the laser cladded high Co–Ni secondary hardening steel coating. They were determined to be the needle-shaped Fe(Cr, Co, Ni)3C cementite with size of approximately 1–3 μm and rod-shaped fine-scale (less than 0.1 μm in length) M2C carbides, respectively.

Interaction of amorphous fullerene C60 with austenite Fe–Ni alloy at high temperatures and pressures

Interaction of amorphous fullerite C60 with austenitic Fe-33.2 wt.% Ni alloy at pressures 0–8 GPa and temperatures 600–1100 °C was studied by neutron diffraction. The amorphous fullerite was obtained by ball milling and mixed with the powder of the crystalline alloy. The interaction at sintering led to the dissolution of carbon in fcc Fe–Ni solid solution and the formation of carbide (Fe, Ni)3C, but the Fe–Ni–C alloy did not undergo phase transformations and preserved the original fcc structure. As a result, the alloy hardened, we could also witness a clear barometric effect: at the pressure of 2 GPa the amount of the dissolved carbon, hardness at micro- and nanoindentation turned out to be significantly higher than those at 8 GPa.During sintering amorphous fullerite is undergoing phase transitions and its microhardness is higher than the microhardness of the metal component. At high temperatures of interaction graphite appears. The presence of Fe–Ni alloy in the composite reduces the temperature of graphite formation in comparison with transformations in the pure amorphous fullerene.

Synthesis, characterization, and ethylene polymerization behaviors of late-transition metal complexes coordinated with chlorinated bis(arylimino)pyridine ligand

A series of late-transition metal complexes supported with chlorinated bis(arylimino)pyridine ligands [2,6-(ArNCCl)2C5H3N]MtCl2 (Ar = 2,4,6-Me3C6H2, Mt = Fe (1a), Co (2a), Ni (3a); Ar = 2,6-iPr2C6H3, Mt = Fe (1b), Co (2b); Ar = 2,6-Et2C6H3, Mt = Co (1c), Ni (3c); Ar = 2,6-Me2C6H3, Mt = Fe (1d), Co (2d); Ar = 4-Cl-2,6-Me2C6H2, Mt = Fe (1e), Co (2e); Ar = 4-Br-2,6-Me2C6H2, Mt = Fe (1f), Co (2f)) were synthesized. At the presence of methylaluminoxane (MAO), Fe(II)- and Co(II)- based complexes were highly active towards ethylene polymerization, affording polymers with bimodal and unimodal molar mass distributions, respectively, while the Ni(II)-based complexes gave no polymer products. Moreover, the obtained polyethylenes were predominately saturated and vinyl-terminated for Fe(II)- and Co(II)-based complexes, respectively. Changing the ligand environment, polymerization parameters also posed a great influence on the catalytic activities and the properties of the resulting polymers.

The stability of Fe–Ni carbides in the Earth's mantle: Evidence for a low Fe–Ni–C melt fraction in the deep mantle

The Earth's mantle contains significant amounts of carbon and is at depths greater than ∼250–300 km potentially so reducing that the Fe–C redox couple determines the nature of the reduced phase(s), which may be diamond, metal and carbides. Carbides will be Fe-rich but their stability also depends on the presence of Ni. We thus have experimentally investigated the Fe–Ni–C subsolidus ternary at 10 GPa, and secondly determined eutectic melting temperatures in this system. At subsolidus, the Fe-rich side of the ternary has two of the phases: diamond, Fe7C3 (to a molar XNi=Ni/(Fe+Ni)=0.11), Fe3C (to XNi=0.24) and metal stable, depending on bulk C-contents. At higher Ni-contents, (Fe, Ni)3C coexists with diamond and metal while at XNi⩾0.53, carbides are absent and diamond coexists with metal.Because Ni is more noble than Fe, it partitions strongly into the reduced phases such that at low metal fractions the metal phase reaches XNi>0.5 (at a bulk Ni-content of 1800 ppm for the mantle). Thermodynamic calculations at subsolidus conditions suggest that the mantle contains 50–700 ppm Fe, Ni metal at ∼300 km depth. Adopting bulk C contents of 50 to 500 ppm in the mantle would result in the phase association (Fe, Ni)3C + metal + diamond (at 10 GPa).An unexpected finding of this study is that eutectic temperatures in the Fe–Ni–C system are very low, 1210 °C at the Fe–C side, decreasing to 1125 °C at an XNi of 0.5 in the reduced phase. Hence we postulate that most of the deep reducing mantle will contain a small Fe–Ni–C melt fraction. These melts should be ubiquitous in the mantle, only those mantle regions where C-contents are less than what can be dissolved in the solid metal (50 ppm at 400 km depth) would not contain such a melt phase. However, the presence of a metal–carbon melt phase is probably of little long term consequence to mantle geochemistry as this melt is expected to remain in isolated pockets.

Nonstoichiometry and growth of some Fe carbides

Iron carbides containing from 31 to 17 atomic % carbon, with cohenite XRD structure and optical properties, were grown in experiments in Fe–Ni–S–C, Fe–Ni–C, and in Fe–C at 1, 6, and 7 GPa. X-ray cell volumes increase with C content. Compositions listed above vary considerably outside the nominal (Fe,Ni)3C stoichiometry of cohenite/cementite. Cohenites coexisting with Fe–C liquid are carbon poor. The Eckstrom-Adcock carbide, nominally Fe7C3, was found to show compositions from 29 to 36 atomic % C at 7 GPa in Fe–C. Both these materials are better regarded as solutions than as stoichiometric compounds, and their properties such as volume have compositional dependencies, as do the iron oxides, sulfides, silicides, and hydrides. The fraction of C dissolved in cohenite-saturated alloy is found to become smaller between 1 and 7 GPa. If this trend continues at higher pressures, the deep mantle should be easier to saturate with carbide than the shallow mantle, whether or not carbide is metastable as at ambient pressure. At temperatures below the cohenite-graphite peritectic, cohenite may grow as a compositionally zoned layer between Fe and graphite. The Eckstrom-Adcock carbide joins the assemblage at 7 GPa. Phases appear between Fe and C in an order consistent with metasomatic interface growth between chemically incompatible feed stocks. Diffusion across the carbide layer is not the growth rate limiting step. Carbon transport along the grain boundaries of solid Fe source stock at 1 GPa, to form C-saturated Fe alloy, is observed to be orders of magnitude faster than the cohenite layer growth. Growth stagnates too rapidly to be consistent with diffusion control. Furthermore, lateral variations in carbide layer thickness, convoluted inert marker horizons, and variable compositional profiles within the layers suggest that there are local transport complexities not covered by one-dimensional diffusive metasomatic growth. In contrast to many transport phenomena which slow with pressure, at 7 GPa and 1,162 °C, carbide growth without open grain boundaries is faster than at 1 GPa with fast grain boundary channels, again suggesting C transport is less of a constraint on growth than C supply. C supply at 7 GPa is enhanced by graphite metastability and the absence of fast grain boundary channels to divert C into the Fe instead of growing carbide. At both 1 and 7 GPa, the growth rate of carbide is found to systematically vary depending on which of two stock pieces of graphite are used to form the growth couple, suggesting that some property of each specific graphite, like C release rate, possibly from amorphous binder material, may influence the cohenite growth process. At temperatures near and above the cohenite-graphite peritectic at 1–1.5 GPa, complex intergrowths involving Fe–C liquids and extensive thermal migration transport were encountered, eroding the organized spatial resolution, and the range of cohenite compositions found grown below this peritectic from growth couples of crystalline Fe and graphite. The migration of graphite to a position in the metasomatic sequence between liquid and cohenite demonstrates that the solubility of graphite in liquid increases with temperature above the peritectic, whereas the solubility of graphite in cohenite below the peritectic decreases with temperature. The variable solubility of graphite in cohenite, shown by thermal migration, emphasizes that cohenite does have compositional variations.

Surface-modification of TiO2 with new metalloporphyrins and their photocatalytic activity in the degradation of 4-notrophenol

A new mono-functionalized porphyrin derivative, 5-mono-[4-(2-(4-hydroxy)-phenoxy)ethoxy]-10,15,20-triphenylporphyrin (3) and its Cu(II) (3a), Zn(II) (3b) and Ni(II) (3c) metalloporphyrins were synthesized and characterized by using various spectroscopic techniques. The corresponding 3a, 3b, 3c-TiO2 photocatalysts were then prepared and characterized by means of FT-IR and diffused reflectance spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The photocatalytic activities of 3a, 3b, 3c-TiO2 were investigated by testing the photodegradation of 4-nitrophenol (4-NP) in aqueous solution under the halogen lamp irradiation. The results indicated that all the 3a, 3b, 3c enhanced the photocatalytic efficiency of bare TiO2 in photodegrading the 4-NP, and 3a-TiO2 exhibited the highest photocatalytic activity. The result is considered a combined action of potential match of 3a with TiO2 CB and effective impregnated of 3a onto the surface of TiO2.

Novel synthesis of metastable HCP nickel by water quenching

It is shown for the first time that water quenching from 1100 °C could yield the metastable HCP Ni phase. The lattice parameters of HCP Ni obtained by water quenching are surprisingly smaller than those reported for either Ni 3C, NiCl 2 or Ni synthesized by chemical routes. Our current findings may partially clarify discrepancies around stability of HCP Ni. In addition, NiO was not formed when quenching from 1100 °C but only when quenching from 950 °C. It is therefore logical to assume that the metastable Ni HCP phase is less prone to oxygen attack as compared to the expanded FCC phase.

Effect of nickel incorporation on structural, nanomechanical and biocompatible properties of amorphous hydrogenated carbon thin films prepared by low energy biased target ion beam deposition

Nickel incorporated amorphous hydrogenated carbon (Ni/a:C–H) thin films were deposited onto the Si substrates via biased target ion beam sputtering of nickel combined with reactive ion beam deposition of a:C–H using CH 4/Ar gas mixture. The effects of Ni doping and target bias voltage on the microstructure and mechanical properties of the as-deposited films were investigated by means of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, transmission electron microscopy and nanoindentation. It was found that the Ni content in the films gets increased with increasing target bias voltage, and most of the Ni atoms react with C atoms to form NiC x phases in the Ni/a:C–H films. Moreover, the nickel carbide nanoparticles attain crystallinity even at low deposition temperature and get embedded in the cross-linked carbon matrix. It was found that the presence of Ni 3C nanoparticles tends to increase the content of sp 2 carbon, thus decreasing the hardness of Ni/a:C–H films as compared with that of a:C–H films. Additionally it was found that the nickel incorporated films do not show any adverse effect on the osteoblast cellular adhesion. Overall, these carbidic nanocrystals initiate direct graphitization and intend to change diamond-like to graphite-like carbon structure in Ni/a:C–H films with promising biocompatibility.

Structural Determinants for Improved Stability of Designed Ankyrin Repeat Proteins with a Redesigned C-Capping Module

Designed ankyrin repeat proteins (DARPins) that specifically bind to almost any target can be obtained by ribosome display or phage display from combinatorial libraries. Although DARPins are already very stable molecules, molecular dynamics simulations, equilibrium denaturation experiments, structural studies, and recent NMR experiments suggested that the unfolding of the original C-terminal capping repeat (C-cap), taken from a natural ankyrin repeat protein, limits the stability of the initial DARPin design. Several point mutations had been introduced to optimize the C-cap and were shown to indeed further increase the stability of DARPins. We now determined crystal structures of DARPins with one or three full-consensus internal repeats (NI 1C or NI 3C) between an N-terminal capping repeat and mutants of the C-cap. An NI 1C mutant, in which the C-cap was only extended by three additional helix-forming residues, showed no structural change but reduced B-factors in the C-cap. An NI 3C C-cap mutant carrying five additional mutations in the interface to the preceding repeat, previously designed by using the consensus sequence as a guide, showed a rigid-body movement of the C-cap towards the internal repeat. This movement results in an increased buried surface area and a superior surface complementarity and explains the improved stability in equilibrium unfolding, compared to the original C-cap. A C-cap mutant with three additional mutations introducing suitably spaced charged residues did not show formation of salt bridges, explaining why its stability was not increased further. These structural studies underline the importance of repeat coupling for stability and help in the further design of this protein family.

Residue-Resolved Stability of Full-Consensus Ankyrin Repeat Proteins Probed by NMR

We investigated the stability determinants and the unfolding characteristics of full-consensus designed ankyrin repeat proteins (DARPins) by NMR. Despite the repeating sequence motifs, the resonances could be fully assigned using 2H, 15N, 13C triple-labeled proteins. To remove further ambiguities, we attached paramagnetic spin labels to either end of these elongated proteins, which attenuate the resonances of the spatially closest residues. Deuterium exchange experiments of DARPins with two and three internal repeats between N- and C-terminal capping repeats (NI 2C, NI 3C) and NI 3C_Mut5, where the C-cap had been reengineered, indicate that the stability of the full-consensus ankyrin repeat proteins is strongly dependent on the coupling between repeats, as the stabilized cap decreases the exchange rate throughout the whole protein. Some amide protons require more than a year to exchange at 37 °C, highlighting the extraordinary stability of the proteins. Denaturant-induced unfolding, followed by deuterium exchange, chemical shift change, and heteronuclear nuclear Overhauser effects, is consistent with an Ising-type description of equilibrium folding for NI 3C_Mut5, while for native-state deuterium exchange, we postulate local fluctuations to dominate exchange as unfolding events are too slow in these very stable proteins. The location of extraordinarily slowly exchanging protons indicates a very stable core structure in the DARPins that combines hydrophobic shielding with favorable electrostatic interactions. These investigations help the understanding of repeat protein architecture and the further design of DARPins for biomedical applications where high stability is required.

Hexagonal close-packed nickel or Ni 3C?

I compare the lattice spacings, synthesis conditions, and magnetic properties of hexagonal close-packed (hcp) Ni with Ni 3C reported recently in the literature. The reported lattice spacings and synthesis conditions of hcp Ni and Ni 3C are almost indiscernible. In addition, the magnetic properties of hcp Ni and Ni 3C show similar characteristics. Based on my experimental investigations on Ni 3C and the reported density functional theory calculations on the magnetic properties of hcp Ni, I present an explanation for understanding the discrepancies in the experimental results regarding hcp Ni reported in the literature.

Decomposition of metal carbides as an elementary step of carbon nanotube synthesis

The role of catalyst components in catalysts containing molybdenum, Mo/M/MgO (M Ni, Co, and Fe), as well as Mo-free catalysts, M/MgO (M Ni, Co, and Fe), for carbon nanotube (CNT) synthesis have been investigated by TEM, XRD, and Raman spectroscopy. CNT synthesis by the catalytic decomposition of CH 4 over M/MgO catalysts can proceed at reaction temperatures higher than the decomposition temperature of the metal carbides (Ni 3C, Co 2C, and Fe 3C), which indicates that carbon in the CNT originates from the graphitic carbon formed on the catalyst surface by the decomposition of metal carbides. For all catalysts containing Mo, thin CNT formation starts at an identical temperature of 923 K, corresponding to the decomposition temperature of MoC 1− x into Mo 2C. The significant effect of the addition of Mo is concerned with the formation of Mo 2C in a catalyst particle during CNT synthesis at high reaction temperatures. The presence of a stable Mo 2C phase leads to the formation of thin CNT with better crystallinity at high reaction temperatures. The role of Ni, Co, and Fe in the Mo/M/MgO catalysts is ascribed to the dissociation of CH 4.

Microstructure characterization of nanocrystalline Ni 3C synthesized by high-energy ball milling

Preparation of a metal carbide by a conventional route requires a huge instrumentation regarding melting the metal and graphite under vacuum at a very high temperature. However, mechanical alloying process is a very useful method for the fabrication of high melting point materials like metal carbides, which additionally produce nanocrystalline structure with improved properties. This article reports the microstructure characterization of nanocrystalline Ni 3C synthesized at room temperature by mechanical milling of Ni and graphite powders under inert atmosphere. The stoichiometric Ni 3C phase is formed after 5 h of milling. Microstructure of the prepared materials is characterized in terms of lattice imperfections, primarily by analyzing the X-ray powder diffraction data, employing both Warren–Averbach's method of line profile analysis and Rietveld's structure refinement method. The phase transformation kinetics studied by the Rietveld method reveals that the graphite powder becomes amorphous at the early stage of milling and then diffuses into Ni matrix and a Ni–C solid solution is formed within 1 h of milling. The high-resolution electron microscopy reveals that the Ni 3C phase is initiated as a thin shell on Ni–C core within 2 h of milling and becomes stoichiometric in composition within 5 h of milling. Electron microscopy observations agree well with the X-ray measurement of particle size (∼9 nm) of Ni 3C phase in 5 h milled sample and Ni–Ni 3C core shell structure in 3 h milled sample.