A. Ye. Yermakov

Synthesis, structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe3O4 oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.

Nanocrystalline copper oxide for selective solar energy absorbers

The high-energy edge of the transparency window in nanocrystalline copper monoxide (CuO) can be shifted to 0.6 eV (against 1.45 eV in single crystals). This shift is related to a modification of the shape of the fundamental absorption edge of CuO without any change in the refractive index. The obtained nanocrystalline CuO is recommended for use as a selective solar energy absorber.

Carbon States in Carbon-Encapsulated Nickel Nanoparticles Studied by Means of X-ray Absorption, Emission, and Photoelectron Spectroscopies

The electronic structure of nickel nanoparticles encapsulated in carbon was characterized by photoelectron, X-ray absorption, and X-ray emission spectroscopies. Experimental spectra were compared with the density of states calculated in the frame of density functional theory. The carbon shell of Ni nanoparticles has been found to be multilayer graphene with a significant (about 6%) amount of Stone–Wales defects. Results of the experiments evidence protection of the metallic nanoparticles from environmental degradation by providing a barrier against oxidation at least for 2 years. Exposure in air for 2 years leads to oxidation only of the carbon shell of Ni@C nanoparticles with coverage of functional groups.

Heterogeneous magnetic state in hydrogen-amorphized GdFe2

Abstract Hydrogen-amorphized GdFe 2 H x were prepared for two different compositions of parent alloy and different hydrogenation temperature. The average hyperfine field grows from 22.5 T for the initial compound up to 33.4 T for the amorphized hydride. Samples amorphized at different temperatures show the same hyperfine field, whereas magnetic moment, compensation temperature and Curie temperature decrease with increasing the hydrogenation temperature. The non-Brillouin behavior of temperature dependence of magnetic moment of Gd sublattice and anomalous high-field magnetization curves were found in amorphous hydrides. The obtained results were interpreted in the assumption of heterogeneous structural state.

Characterization of nanoscaled heterogeneities in mechanically alloyed and compacted CuFe

Cu80Fe20 and Cu50Fe50 were mechanically alloyed from the pure elements by ball milling for 36 h. The alloy powder was compacted into tablets at room temperature by applying a pressure of 5 GPa. Characterization of the Cu80Fe20) and Cu50Fe50 alloys was carried out by high-resolution transmission electron microscopy (HREM), atom probe field ion microscopy and three-dimensional atom probe (3DAP). The grain size of the nanocrystalline microstructure of the ball-milled alloys observed with HREM varies between 3 and 50 nm. Atom probe and 3DAP measurements indicate that the as-prepared state is a highly supersaturated alloy, in which the individual nanocrystals have largely varying composition. Fe concentration in Cu was found to range from about 8 to 50 at%. It is concluded that by ball milling and compacting an alloy is produced which on a nanometer scale is heterogeneous with respect to morphology and composition.

Electronic structure and resonant X-ray emission spectra of carbon shells of iron nanoparticles

The electronic structure of carbon shells of carbon encapsulated iron nanoparticles carbon encapsulated Fe@C has been studied by X-ray resonant emission and X-ray absorption spectroscopy. The recorded spectra have been compared to the density functional calculations of the electronic structure of graphene. It has been shown that an Fe@C carbon shell can be represented in the form of several graphene layers with Stone-Wales defects. The dispersion of energy bands of Fe@C has been examined using the measured C Kα resonant X-ray emission spectra.

Atomic structure and magnetic properties of Cu80Co20 nanocrystalline compound produced by mechanical alloying

Abstract Direct observation of the atomic structure of the mechanically alloyed Cu80Co20 compounds has been made using the field ion microscope (FIM). Phase composition, defect structure and morphology of material on the atomic scale have been determined. It has been established that the studied material is chemically inhomogeneous, presenting a mixture of two main phases: heterogeneous solid solution of cobalt in copper, and pure cobalt. Phase volume ratios, particle and cluster sizes have been estimated. An evaluation of Co content in CuCo solid solution has been made. The width of interfaces in this mechanically alloyed material was revealed to be at least twice the width of phase boundaries in metals and alloys. Superparamagnetism of the compound studied at elevated temperatures and saturation magnetization deficit at low temperatures are discussed on the basis of the above-mentioned structural data.

Structural State of Mechanically Alloyed Cu–Co Compound with a Considerable Magnetoresistance Effect

The structural state of the Cu 80 Co 20 compound after mechanical alloying and a subsequent thermal treatment has been studied by NMR spectroscopy and X-ray diffraction as well as measurements of magnetic and magnetoresistive properties. The analysis of the data obtained suggests the presence of ultradispersed cobalt particles in the copper matrix. A partial dissolving of cobalt in copper is likely to take place in defect regions. Such a structure is responsible for superparamagnetism of the Cu 80 Co 20 compound at T > 150 K and for a considerable GMR effect (12% at T = 77 K). The shifting of X-ray peaks of the copper matrix can be related to a coherent conjugation of Cu and Co constituents.

Effects of mechanical grinding on magneto-structural properties of BaFe12O19 powders

Abstract The structural and magnetic properties of deformed powders of M-type Baferrite, obtained by mechanical grinding, have been investigated using X-ray diffraction, Mossbauer spectroscopy, thermomagnetic analysis, and magnetic measurements. The rapid decrease in the magnetization after short grinding times in air is attributed to the formation of a phase which is paramagnetic at room temperature but at 150 K the phase behaves like a spin-glass, with antiferromagnetic short-range order. From Mossbauer spectra this phase has at least five non-equivalent sites of iron with hyperfine interaction field values Hrf(77 K): H1=38.69; H2=37.01; H3=36.14; H4= 34.31 and H5=30.96 MA/m, and a magnetic moment μFe = 2.1μB. At longer grinding times the deformed BaFe12O19 samples were found to be thermodynamically unstable and decomposed partially into α-Fe2O3 and Fe3O4.

Low-energy charge transfer excitations in NiO

Comparative analysis of photoluminescence (PL) and photoluminescence excitation (PLE) spectra of NiO poly- and nanocrystals in the spectral range 2-5.5 eV reveals two PLE bands peaked near 3.7 and 4.6 eV with a dramatic rise in the low-temperature PLE spectral weight of the 3.7 eV PLE band in the nanocrystalline NiO as compared with its polycrystalline counterpart. In frames of a cluster model approach we assign the 3.7 eV PLE band to the low-energy bulk-forbidden p-d (t1g(π)-eg) charge transfer (CT) transition which becomes the allowed one in the nanocrystalline state while the 4.6 eV PLE band is related to a bulk allowed d-d (eg-eg) CT transition scarcely susceptible to the nanocrystallization. The PLE spectroscopy of the nanocrystalline materials appears to be a novel informative technique for inspection of different CT transitions.