E. P. Elsukov

Determination of nanoparticle sizes by X-ray diffraction

Different procedures for analysis of particle sizes by the X-ray diffraction method are compared by the example of nanoparticles of nickel and iron(3+) oxide (Fe2O3). A modified Warren-Averbach method is proposed for the analysis of the X-ray diffraction line profile based on the approximation by the Voigt function, which yields stable solutions, and the efficiency of the method is shown. The analysis within the frame-work of the Warren-Averbach method makes it possible to restore the distribution function of nanoparticles (crystallites) over true diameters, which satisfactorily correlates with electron microscopy data. The applicability of the Warren-Averbach method to the estimation of crystallite sizes by the analysis of a single diffraction line is substantiated. The range of the applicability of the Scherrer, Williamson-Hall, Warren-Averbach, and modified Warren-Averbach methods to the substructure analysis by the X-ray diffraction is determined as depending on the method of nanostructure formation.

Structure, magnetostatic properties, and microwave characteristics of mechanoactivated nanocrystalline Fe and Fe-Si powders

The effect of the chemical composition and structural and morphological features of Fe and Fe87Si13 powders milled in an inert atmosphere of Ar and a 3% solution of oleic acid in heptane (H + OA) on the magnetostatic and microwave properties has been studied in this work. Irrespective of the milling medium, all the samples were nanocrystalline. The powder particles obtained in Ar have a stonelike shape; those obtained in H + OA, have a platelike (flaky) shape. The effect of the particle shape manifested itself in the processes of magnetization and in the qualitative and quantitative type of the frequency dependences of the dielectric permittivity and magnetic permeability in a frequency range from 0.1 to 3 GHz.

The role of cementite in the formation of magnetic hysteresis properties of plastically deformed high-carbon steels: I. Magnetic properties and structural state of cementite

Magnetic properties of cementite after strong plastic deformations and subsequent annealing in a broad range of temperatures are studied. The plastically deformed cementite is shown to exist in a soft (Hc ≈ 80 A/cm) state; the annealed cementite, in a hard (Hc ≈ 240 A/cm) state. The nature of the cementite’s soft and hard states is discussed. The field dependence of the cementite’s magnetostriction is measured. The longitudinal magnetostriction of the polycrystalline cementite’s saturated state is shown to be negative and approximately four times smaller than iron’s magnetostriction in the saturated state.

Microwave absorbing properties of Fe powders milled in various media

Principal factors determining the microwave-absorption material parameters (shape, size, and chemical and phase compositions of the particles) and their dispersion relations in a range from 0.1 to 3 GHz were determined for composites containing milled Fe particles as the filling agent. The basic physical mechanisms of the effect of the aforementioned factors were assumed to be the domain-wall resonance and ferromagnetic resonance.

On the analysis of the mechanisms of the strain-induced dissolution of phases in metals

The relationship between the processes of the nanostructure evolution and the strain-induced dissolution of phases upon severe plastic deformation is studied. The known mechanisms of these phenomena are analyzed, and new ones are proposed. The extended metastable and equilibrium phase diagram used for the analysis of the deformed nanostructured solid solution allows for the relationships of the equilibrium states of the system of bulk phases with the equilibrium and metastable states of the systems of planar, linear, and point defects.

On the magnetic structure of the ground state of ordered Fe-Al alloys

Based on a comparison of the results of low-temperature magnetic and Mössbauer studies, a model of the magnetic structure of the ground state (T = 0) of ordered (D03 and B2 types) Fe-Al alloys is suggested. The model is based on the dependence of both the magnitude and sign of the local magnetic moment at an Fe atom on the number of Al atoms in its nearest neighborhood. It has been established that in terms of this model at Al concentrations exceeding 25 at. % there are formed magnetic inhomogeneities (regions with a magnetic moment oriented opposite to the total magnetization) on a nanometer scale (2–5 nm in size).

Formation of chromium carbides in copper matrix during mechanical alloying in carbon-containing media

Structural and phase transformations that occur during mechanical alloying (MA) and subsequent annealing of nanocrystalline Cu-Cr-C alloys obtained from copper and chromium powders and graphite or xylene as the source of carbon have been studied. It is shown that, when using graphite, a supersaturated Cu(Cr) solid solution and an X-ray amorphous Cr-C phase are formed during MA. Heat treatment leads to their decomposition and the appearance of Cr3C2 in the nanocrystalline copper matrix. When xylene is used as the source of carbon, no strongly supersaturated Cu(Cr) solid solution and no X-ray amorphous Cr-C phase are formed, but the same volume fraction of chromium carbide, i.e., 20–24 vol %, appears. When graphite is used, the carbide is formed after shorter times of MA.

Structural state and magnetic properties of cementite alloyed with manganese

Using X-ray diffraction analysis, Mössbauer spectroscopy, and magnetic measurements, the structure, parameters of hyperfine interactions, localization of Mn atoms in the lattice, coercive force, and specific saturation magnetization have been investigated in the mechanically alloyed and annealed cementite (alloyed with manganese) of compositions (Fe1 − xMnx)3C (x = 0–0.12). It has been shown that strongly deformed cementite resides in the low-coercivity state and, after annealing in the vicinity of 500°C, in the high-coercivity state. Alloying with manganese reduces the coercive force, the specific saturation magnetization, and the Curie temperature of cementite. Inhomogeneities of the distribution of manganese atoms indicate the temperature dependence of the coercive force of mechanically alloyed and annealed cementite samples.

Influence of shape and chemical and phase compositions of iron-containing particles on microwave performance of composites with an insulating matrix

The structure and microwave magnetic properties of Fe powders grounded in argon or acetone and also of Fe-Si-C and amorphous Fe-Co-Si-C powders mechanically alloyed in argon are studied using X-ray diffraction, Mössbauer spectroscopy, granulometric and microscopic analyses, magnetostatic measurements, and microwave spectroscopy. The goal of investigation is to determine the influence of factors (shape, size, and chemical and phase compositions of grains) governing the microwave material parameters of composites based on these alloys in the frequency range 0.1–3.0 GHz. It is shown that the difference in the grain shape is the basic reason for the difference in the microwave permeability at low frequencies (3 GHz or lower). At higher frequencies, the magnetic properties are related to the skin effect and depend largely on the grain size. The differences in the microwave properties of the composites are not significant and are concealed by the above effects.

Initial stage of mechanical alloying in a binary system with composition Si70Fe30

Mössbauer spectroscopy and X-ray diffraction have been used to study the sequence of solidstate reactions that occur upon the mechanical alloying of mixtures of Si and Fe powders taken in an atomic ratio of 70: 30 in a planetary ball mill. In the course of the formation of a nanocrystalline state, the interpenetration of Si atoms into Fe particles and of Fe atoms into Si particles occurs. In the Si particles, clusters with a local neighborhood of Fe atoms that is characteristic of the deformed α-FeSi2 phase are formed. In the Fe particles, clusters of the ɛ-FeSi and the β-FeSi2 type arise. With increasing time of mechanical treatment, second phases of α-FeSi2 in Si particles and of ɛ-FeSi and β-FeSi2 in Fe particle are formed. In the latter case, a reaction ɛ-FeSi + Si → β-FeSi2 occurs up to the complete disappearance of the ɛ-FeSi phase if the mixture under study is not contaminated by the material of the vessel (Fe) and balls.