S. G. Bogdanov

Structural state of expanded graphite prepared from intercalation compounds

The structural state of nanocrystalline samples of expanded graphite is investigated using X-ray diffraction and neutron diffraction analyses. The expanded graphite samples are prepared by a rapid thermal decomposition of intercalation compounds of oxidized graphite based on fluorinated graphite, graphite oxide, and graphite aminofluoride. It is established that the main phase of expanded graphite belongs to the hexagonal crystal system (space group P63/mmc) and that carbon atoms in the structure occupy the 2b and 2c positions. The unit cell parameters and the unit cell volume in the structure of expanded graphite samples are larger than those in the structure of massive graphite.

Size effect in nanocrystalline manganites La1−x A xMnO3 (A=Ag, Sr)

Nanocrystalline samples of the manganites La0.9Ag0.1MnO3, La0.7Ag0.3MnO3, and La0.7Sr0.3MnO3 were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.

Small-angle neutron scattering from samples of expanded carbon

The subatomic structure of expanded graphite has been investigated by small-angle neutron scattering. Samples were synthesized during quick thermal decomposition of intercalated compounds based on oxidized graphite. They had a low bulk density (up to 0.1 g/cm3) and were characterized by considerable small-angle scattering. It has been established that majority of the volume of expanded graphite samples is occupied by participles with characteristic sizes in two ranges: from 6 to 8 nm and from 20 to 30 nm. Small particles have properties of a surface fractal with the dimension Ds = 2.4–2.6, whereas the larger particles are mainly smooth and have the dimension Ds = 2.0–2.1. The specific surface of the samples studied was determined from the small-angle scattering data.

Multiple small-angle neutron scattering in ferromagnets

Abstract The conclusions of the multiple small-angle neutron scattering theory, developed recently by Maleyev, Pomortsev, and Skryabin (MPS) for the Born parameter α⪢1 are experimentally confirmed by magnetic scattering from domain structure of pure Fe and Ni, as well as Fe65Ni35 alloy. A critical thickness L0 is found at which the multiple refraction transforms to multiple Fraunhofer diffraction. This crossover is accompanied by variation of the neutron beam broadening w versus sample thickness L, from w ∼ √L to w ∼ L.

Atomic and Superatomic Structures of Solid Solutions of Zirconium and Calcium Oxides

Solid solutions of the ZrO2-CaO system with 7, 9.6, and 20 mol % of calcium oxide are analyzed by X-ray and neutron diffraction and small-angle neutron scattering. Samples are synthesized by the sol-gel method and annealed at the temperature T = 600°C for 4 h. All samples have a tetragonal structure (P42/nmc space group). The size of crystallites is 10–20 nm. Small-angle neutron scattering curves are characterized by a peak near the momentum transfer q = 0.5 nm−1. The existence of the peak is caused by correlation between solid nanoparticles in the samples. Density correlation functions are numerically computed from the experimental small-angle scattering cross-sections by the inverse Fourier transform. Correlation lengths, concentrations of scattering nanoparticles, and specific surface areas of the samples are determined.

Multiple small angle neutron scattering: Numerical analysis and experiment

Abstract The numerical calculations of the multiple small angle neutron scattering (SANS) cross section derived by Moliere for an arbitrary Born parameter α have been carried out. The results were compared with those obtained from approximate expressions at the time of using the diffraction and refraction regimes. It is found that the approximate formulas can be used within the whole range of α. The results of the numerical analysis have been applied for the discussion of the experimental SANS data on domain structure in ferromagnets: pure iron and nickel and invar alloys Fe65Ni35 and Fe3Pt.

Determination of Texture Degree of NdFeB-Magnets by Means of Neutron Diffraction

Six samples of Nd-Fe-B-permanent magnets, distinguished in the texture degree, have been measured by neutron diffraction in the reflection and transmission regimes. The texture in the samples was formed by application of impulses of external magnetic field 0H = 4 T. Analysis of the obtained neutron diagrams by means of the Fullprof program allowed quantitative determination of the texture degree and conclusion to be made that the texture degree grows with an increase in numbers of magnetic field impulses.

Spin-Reorientation Transition in Rare-Earth Intermetallics

The Tm2Fe17 compound has been studied by means of powder neutron diffraction. The compound undergoes a spin-reorientation transition (SRT) of the “easy axis – easy plane” type with increasing temperature from 75 K to 100 K. The SRT is accompanied by a distinct anomaly in the temperature dependencies of the Tm- and Fe-ion magnetizations, significative of large magnetization anisotropy. Using three sublattices model, the parameter of the Tm-Fe interaction and constants of magnetic anisotropy are determined.