Synthesis of large-sized spherical Co-C alloys with soft magnetic properties though a high-pressure solid-state metathesis reaction.

In this work, we report a novel high-pressure solid-state metathesis (HSM) reaction to produce spherical bulk (diameters 2-4 mm) Co-C alloys (Co3C and Co1-xCx). At 2-5 GPa and 1300 °C, C atoms preferentially occupy the interstitial sites of the face-centered cubic (fcc) Co lattice, leading to the formation of metastable Pnma Co3C. The Co3C decomposes above 1400 °C at 2-5 GPa, C atoms infiltrate the interstitial sites of the fcc Co lattice, saturating the C content in Co, forming an fcc Co1-xCx solid solution while the C atoms in excess are found to precipitate in the form of graphite. The Vickers hardness of the Co-C alloys is approximately 6.1 GPa, representing a 19.6% increase compared to hexagonal close-packed (hcp) Co. First-principles calculations indicate that the presence of C atoms in the Pnma Co3C structure leads to a relative decrease in the magnetic moments of the two distinct Co atom occupancies. The Co-C alloys exhibited a soft magnetic behavior with saturation magnetization up to 93.71 emu g-1 and coercivity of 74.8 Oe; coercivity increased as the synthesis pressure rises.

High-Pressure Coupling Reactions to Produce a Spherical Bulk RexN/Fe3N Composite.

We report a novel high-pressure coupling (HPC) reaction that couples the nitridation of Re with high-pressure solid-state metathesis (HPSSM) of Fe3N to produce a spherical bulk RexN/Fe3N composite. Compared with conventional methods, upon coupling of the HPSSM reactions, the synthetic pressure for Re nitridation was successfully reduced from 13 to 10 GPa (for Re3N) and from 20 to 15 GPa (for Re2N). The product RexN species would be surrounded by product Fe3N, resulting in a spherical bulk RexN/Fe3N composite (x = 2 or 3). The composite exhibits a soft magnetic behavior, and the content of nitrogen in RexN (x = 2 or 3) was controlled by adjusting the P-T conditions. The HPC reaction establishes a new approach for the bulk synthesis of 5d transition metal nitride.

Controlling the quantity of γ-Fe inside multiwall carbon nano-onions: the key role of sulfur.

One of the most interesting structural features of multiwall carbon onions (MWCNOs) and nanotubes (MWCNTs) is the excellent chemical stability, which allows in situ encapsulation of chosen magnetic materials of interest and multifunctional applications. In this letter, we present an innovative chemical vapour synthesis (CVS) approach, in which the inclusion of small quantities of sulfur during the pyrolysis of ferrocene/dichlorobenzene mixtures allows for an important control in the relative abundance of FCC γ-Fe, up to a maximum value of ∼86.5% (structural- and phase-control). The variation in the relative percentage of the encapsulated Fe-based phases was estimated by employing X-ray diffraction (XRD) and Rietveld refinement analyses. The magnetic characterization was achieved by employing superconducting quantum interference device (SQUID) magnetometry, with zero field cooled (ZFC) and field cooled (FC) curves acquired at applied fields of 300 Oe and ∼50 000 Oe.

Probing electrical properties of individual carbon nanotubes filled with Fe3C nanowires.

The electrical properties of individual multiwall carbon nanotubes (CNTs) filled with Fe3C nanowires (Fe-CNTs) grown by chemical vapor deposition were investigated. The individual Fe-CNTs were measured by two-probe configuration in a scanning electron microscope, in which one probe was used to contact one end of the nanotubes and the other varied its contact position to measure the resistance along the Fe-CNTs. The data suggest that the ferromagnetic nanowires and the CNTs were well connected into a conduction network, and the resistance of the individual Fe-CNTs decreased as the filling rate increased. Analysis shows that the encapsulated ferromagnetic nanowires played a profound part in determining the electrical behavior of individual Fe-CNTs. The results may be useful for understanding of electronic transport of individual Fe-CNTs and applications based on individual Fe-CNTs.

Vacancy-induced interfacial ferromagnetic features in SmFeO3-filled graphitic carbon foam.

We report a novel structural and magnetic investigation of carbon foam (CFM) materials filled with SmFeO3 crystals produced by (1) high temperature fusion between Sm2O3- and Fe3C-filled carbon onions and (2) annealing of iron filled CFM with nanosized Sm2O3. Presence of a defect-rich monolayer-like CFM arrangement characterized by sharp interfaces with a SmFeO3 single-crystal phase is demonstrated through TEM and HRTEM. Further, the presence of intense sp3-rich features with variable carbonate content is evidenced by XPS and Raman spectroscopy. Complementary VSM, SQUID and ESR show also presence of intrinsic magnetization features which appeared to be attributable to the interfacial vacancy-rich regions of the graphitic CFM layers, as confirmed by Raman spectroscopy. Together with these signals, possible ferromagnetic contributions from the SmFeO3 phase and α-Fe impurities are reported. These observations highlight therefore the presence of switchable interfacial magnetization features at the carbon/SmFeO3 interface due to the variable concentrations of vacancies at the CFM interface, opening new directions towards applications in magnetic and interfacial-driven ferroelectric devices.

Room temperature synthesis of ReS2 through aqueous perrhenate sulfidation.

In this study, a direct sulfidation reaction of ammonium perrhenate (NH4ReO4) leading to a synthesis of rhenium disulfide (ReS2) is demonstrated. These findings reveal the first example of a simplistic bottom-up approach to the chemical synthesis of crystalline ReS2. The reaction presented here takes place at room temperature, in an ambient and solvent-free environment and without the necessity of a catalyst. The atomic composition and structure of the as-synthesized product were characterized using several analysis techniques including energy dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and differential scanning calorimetry. The results indicated the formation of a lower symmetry (1T') ReS2 with a low degree of layer stacking.

Giant magnetic coercivity in Fe3C-filled carbon nanotubes.

One of the major challenges in the synthesis of ferromagnetically filled carbon nanotubes is the achievement of high coercivities. Up to now the highest coercivity has been shown to be 2200 Oe at 2 K ranging down to 500 Oe at temperatures of 300 K. Here we show that the anomalously large coercivity of 3440 Oe is observed in comparable samples. By comparing our result to those reported in previous studies no correlation is found between coercivity and the shape anisotropy or the crystal-diameter. Instead we suggest that the main parameter which controls the coercivity of these structures is the interplay of the grain size and shape anisotropy. We attribute the anomalous coercivity to the grain size being below the calculated single magnetic domain limit.

Cl-Assisted Large Scale Synthesis of Cm-Scale Buckypapers of Fe3C-Filled Carbon Nanotubes with Pseudo-Capacitor Properties: The Key Role of SBA-16 Catalyst Support as Synthesis Promoter.

We show a novel chemical vapour deposition (CVD) approach, in which the large-scale fabrication of ferromagnetically-filled cm-scale buckypapers is achieved through the deposition of a mesoporous supported catalyst (SBA-16) on a silicon substrate. We demonstrate that SBA-16 has the crucial role of promoting the growth of carbon nanotubes (CNTs) on a horizontal plane with random orientation rather than in a vertical direction, therefore allowing a facile fabrication of cm-scale CNTs buckypapers free from the onion-crust by-product observed on the buckypaper-surface in previous reports. The morphology and composition of the obtained CNTs-buckypapers are analyzed in detail by scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX), transmission electron microscopy (TEM), high resolution TEM (HRTEM), and thermogravimetric analysis (TGA), while structural analysis is performed by Rietveld Refinement of XRD data. The room temperature magnetic properties of the produced buckypapers are also investigated and reveal the presence of a high coercivity of 650 Oe. Additionally, the electrochemical performances of these buckypapers are demonstrated and reveal a behavior that is compatible with that of a pseudo-capacitor (resistive-capacitor) with better performances than those presented in other previously studied layered-buckypapers of Fe-filled CNTs, obtained by pyrolysis of dichlorobenzene-ferrocene mixtures. These measurements indicate that these materials show promise for applications in energy storage systems as flexible electrodes.

Fabrication of cm scale buckypapers of horizontally aligned multiwalled carbon nanotubes highly filled with Fe3C: the key roles of Cl and Ar-flow rates.

A key challenge in the fabrication of ferromagnetically filled carbon-nanotube buckypapers in the presence of Cl-radicals is the achievement of a preferential horizontal nanotube-alignment. We show that a horizontal-alignment can be achieved by tuning two main CVD parameters for a fixed dichlorobenzene concentration: the precursor-evaporation temperature and the flow rate.

Enhanced saturation magnetization in buckypaper-films of thin walled carbon nanostructures filled with Fe3C, FeCo, FeNi, CoNi, Co and Ni crystals: the key role of Cl.

We report an advanced chemical vapour deposition approach which allows the direct in situ synthesis of cm-length ultrathin buckypapers comprising carbon nanostructures filled with Fe3C, FeCo, FeNi, CoNi, Co and Ni by sublimation and pyrolysis of single or combined metallocenes with very low quantities of dichlorobenzene. As a result, extremely high saturation magnetizations of 117 emu g(-1), 90 emu g(-1) and 80 emu g(-1) are obtained for the specific cases of Fe3C, FeCo and FeNi, respectively, while variable saturation magnetizations of 70 emu g(-1), 58 emu g(-1) and 6.7 emu g(-1) are obtained for Co, CoNi and Ni respectively.

Boundary layer chemical vapour synthesis of self-organised ferromagnetically filled radial-carbon-nanotube structures.

Boundary layer chemical vapour synthesis is a new technique that exploits random fluctuations in the viscous boundary layer between a laminar flow of pyrolysed metallocene vapour and a rough substrate to yield ferromagnetically filled radial-carbon-nanotube structures departing from a core agglomeration of spherical nanocrystals individually encapsulated by graphitic shells. The fluctuations create the thermodynamic conditions for the formation of the central agglomeration in the vapour which subsequently defines the spherically symmetric diffusion gradient that initiates the radial growth. The radial growth is driven by the supply of vapour feedstock by local diffusion gradients created by endothermic graphitic-carbon formation at the vapour-facing tips of the individual nanotubes and is halted by contact with the isothermal substrate. The radial structures are the dominant product and the reaction conditions are self-sustaining. Ferrocene pyrolysis yields three common components in the nanowire encapsulated by multiwall carbon nanotubes, Fe3C, α-Fe, and γ-Fe. Magnetic tuning in this system can be achieved through the magnetocrystalline and shape anisotropies of the encapsulated nanowire. Here we demonstrate proof that alloying of the encapsulated nanowire is an additional approach to tuning of the magnetic properties of these structures by synthesis of radial-carbon-nanotube structures with γ-FeNi encapsulated nanowires.

The origin of long-period lattice spacings observed in iron-carbide nanowires encapsulated by multiwall carbon nanotubes.

Structures comprising single-crystal, iron-carbon-based nanowires encapsulated by multiwall carbon nanotubes self-organize on inert substrates exposed to the products of ferrocene pyrolysis at high temperature. The most commonly observed encapsulated phases are Fe3C, α-Fe, and γ-Fe. The observation of anomalously long-period lattice spacings in these nanowires has caused confusion since reflections from lattice spacings of ≥ 0.4 nm are kinematically forbidden for Fe3C, most of the rarely observed, less stable carbides, α-Fe, and g-Fe. Through high-resolution electron microscopy, selective area electron diffraction, and electron energy loss spectroscopy we demonstrate that the observed long-period lattice spacings of 0.49, 0.66, and 0.44 nm correspond to reflections from the (100), (010), and (001) planes of orthorhombic Fe3C (space group Pnma). Observation of these forbidden reflections results from dynamic scattering of the incident beam as first observed in bulk Fe3C crystals.With small amounts of beam tilt these reflections can have significant intensities for crystals containing glide planes such as Fe3C with space groups Pnma or Pbmn.