Vol 12, No 6 (2018)

Mössbauer and magnetic studies of Ca-doped lanthanum manganites La_1 – xCa_xMn_0.98Fe_0.02O_3 + δ (x = 0.05, 0.10, 0.20) of a nonstoichiometric composition are performed. An increase in the Ca concentration and a decrease in the interstitial oxygen quantity results in a decreasing electric-field gradient in the region of the Fe atomic nucleus due to the spatial rearrangement of the charge of surrounding atoms. The Mössbauer spectra of all studied lanthanum-manganite samples demonstrate a relaxation character at low temperatures, which can be associated with the presence of small-sized magnetic clusters. This finding correlates with the results of magnetic investigations. The values of the coercive force, saturation magnetization, and blocking temperature are determined.

The present work is aimed at discussing the possibility in principle of obtaining rhenium-silicide microcrystals in tin melt at temperatures of 200–250 K below those used in the synthesis of pure components. The formation of rhenium silicides is confirmed by X-ray diffraction and scanning electron microscopy data. According to phase diagrams, neither rhenium nor silicon is soluble in tin, which does not allow the process to be described in the context of classical solution-melt crystallization theory. In this case, Sn is assumed to be a catalyst in the high-temperature chemical reaction between rhenium and silicon, favoring a decrease in the silicide-formation temperature and opens up the possibility of producing polycrystalline layers of these materials with a temperature gradient.

Two-dimensional polymers are highly promising materials for microsystems technologies and fundamental science. Some of the most interesting, but relatively unstudied, materials of this kind are polyphthalocyanines. These are a unique class of organometallic polymers with a two-dimensional conjugated structure and variable electronic properties. Since polyphthalocyanines of a high degree of polymerization can be obtained only as thin films and not as a bulk crystal, their study and characterization is a complicated scientific challenge. In this work we demonstrate the potential of transmission electron microscopy as a key tool in characterizing thin films. In particular, transmission-electron-microscopy data provide a basis for interpreting the Raman spectra of polyphthalocyanines, and also make possible interpretation of the crystal structure of the two-dimensional polymer.

The regularities of the formation of the surface micromorphology and structure in stainless steel prepared via the selective laser melting of CL20ES powder are investigated. It is demonstrated that samples with optimal microgeometric characteristics possess the largest values of the density and microhardness and a low level of imperfection.

Different types of tread rubbers used in pneumatic tires of different applications are studied in the present work. The influence of various rubbers used in manufacturing treads on the rubber surface structure and its tribological, mechanical, and adhesive properties is estimated. A study and comparative analysis of the results of tribological tests and microscopic studies are performed. Microscopy of the sample surfaces is performed both before and after the tribological tests. To visualize the surface, the methods of scanning electron microscopy (combined with elemental analysis) and scanning probe microscopy are used. The character of changes occurring on the surface during friction, which is simulated on a tribometer with a rubber ring–disk contact circuit, is studied using these microscopy methods. It is established that the worn surfaces of all elastomers under study have a close structure and typical surface microrelief, but have different adhesion and mechanical properties depending on the type of rubber used in the rubber base and the used fillers. It is noted that high values of the sliding-friction coefficient and similarity of the rubber surface structures prove that their abrasive wear mechanisms are identical. The performed tribological laboratory tests show that the friction coefficient of the tread rubbers at high temperatures decreases with an increase in the normal load, while the opposite trend is observed at low temperatures. It is established that the change in the relaxation properties of rubbers due to temperature changes greatly affects the dependence of the friction coefficient on the sliding speed, while the energy loss due to friction is maximal in the temperature range of 15–20°C.

The processes of the formation and decay of vortex motion on the surface of water are studied experimentally. The vortex motion is formed by capillary waves, which are generated as a result of harmonic oscillations of a cell with water in the vertical direction. It is shown that, at oscillation amplitudes smaller than the critical value, a vortex lattice with a period that is equal to the excitation wavelength is formed. The characteristic time of the formation and decay of vortex motion is close to the time of viscous damping of the pumping wave. At pumping amplitudes that are greater than the critical value, a vortex lattice is initially formed on the surface; then it is destroyed gradually by large-scale movements. The energy distribution over the wave vector can be described by a power function that is close to k^–5/3. It is suggested that large-scale vortex motion arises as a result of the interaction of low-frequency waves appearing on the surface as a consequence of the nonlinear interaction of waves.

In this article the conducted experiments resulted in identifying a power nature of dependence of an average radius of bubbles in the model foam metastable samples on time with a value close to 0.5. The special experimental setup has been developed for studying the dependence of the average radius of gas bubbles in the model foam metastable samples on time. Within the assumption of a thermally-activated mechanism of the gas phase diffusion through bubble walls, the analysis of temperature influence on behavior of average radius dependences on time allowed to estimate molar energy of the process activation.

Quaternary compounds (Cu₂ZnGeSe₄ and Cu₂ZnSnSe₄) and solid solutions on their basis are prepared via single-temperature synthesis from Cu, Zn, Ge, Sn, and Se. The crystallographic characteristics of the synthesized compounds and Cu₂ZnGe_xSn_1 – xSe₄ solid solutions are determined using the X-ray diffraction method at room temperature. It is found that parameters a and c reduce with increasing Ge concentration.

The impact of the addition of a polymer (polyethylene glycol) with a molecular weight of 20 kDa on the structure of micellar systems of an anionic surfactant (sodium oleate) in aqueous solutions is investigated via small-angle neutron scattering. The structure and interaction parameters of micelles (micelle aggregation number, degree of ionization, axial ratio, average diameter, charge, inverse screening length, and surface potential) are compared in a solution with and without the addition of the polymer. Using the concentration dependences of the experimental data on surface tension, parameters such as the critical micelle concentration, area per molecule, surface activity, surface excess, and critical aggregation concentration are determined in the case of complex solutions. The observed effect of the addition of the polymer on the behavior of a micellar solution of sodium oleate can be related to a change in the surface activity of the surfactant in the presence of the polymer. As a result of comparison with previous experimental data, it is concluded that the weight of the polymer significantly affects the structural properties of sodium oleate–polyethylene glycol mixed solutions.

The magnetic properties of polycrystalline and amorphous cobalt- and nickel-ferrite films obtained by solid-phase synthesis are studied. Polycrystalline nickel-ferrite films are shown to be characterized by significant coercive fields at room temperature in the direction normal to the substrate plane. Compressive stresses that arise in the films during solid-phase oxidation and synthesis are assumed to contribute to the enhancement of the coercive fields. In polycrystalline films, a magnetic anisotropy of the “easy plane” type, characteristic of two-dimensional layers, is found. X-ray amorphous films have only paramagnetic properties.

The results of investigating the microstructure and phase composition of AlN/SiN_x multilayer films with alternating nanocrystalline aluminum nitride (nc-AlN) and amorphous silicon nitride (a-SiN_x) phases, which were formed via magnetron sputtering, are presented. The layer thickness varies from 2 to 10 nm, the total film thickness is 300 nm, and the grain size of the AlN phase corresponds to the nc-AlN layer thickness. By means of transmission electron microscopy, it is revealed that, due to 30-keV He⁺ ion irradiation with a dose of 5 × 10¹⁶ cm^–2, gas pores with an average size of 2.0–2.4 nm and localized mainly in a-SiN_x layers are generated within the projective range of helium ions. The appearance of such inclusions is due to the fact that implanted helium atoms migrate into the formed vacancy complexes. In this case, the structural state of AlN crystal layers remains unchanged.

The anomalous-diffraction method applied near the absorption edge of Hf is used to investigate the features of atomic ordering in the Eu₂Hf₂O₇ structure. Structural parameterization describing a continuous transition between ideally disordered (fluorite) and ideally ordered (chalcolamprite) structures is proposed. A comparison between the calculated and experimental results demonstrates qualitative coincidence and confirms the hypothesis that the sample under study exists in the intermediate phase. The method is sensitive to cation ordering, but anion ordering of the suggested model is not described.

The features of the surface structure of nanoporous and microporous polymer-membrane films promising for use in cleanrooms during the manufacturing of microelectronics products are examined. The degrees of asymmetry of selective and supporting surfaces are determined via scanning probe microscopy. The hydrodynamic characteristics of the membranes under study are analyzed.

In this work we propose and implement an algorithm for identifying field-emission images with the aim of determining the crystallographic orientation of an emitter and obtaining detailed information about the structural features of its surface. A threshold-type analysis of the brightness distribution in the image is found to be preferable for selecting information-containing areas and their contours. An original algorithm is proposed for selecting the mask of a “spot” of arbitrary shape. The proposed algorithm of processing the information area is also applicable to “skeletons”; however, the contour of an information fragment should not contain more than one gap. In contrast to the skeletons, the use of internal areas of emission “spots” by means of determining geometric moments improves the reliability of processing results due to an increase in sampled information.

In the present study the evolution of the electronic structure of the graphene/Fe/Ni(111) interface with increasing number of intercalated iron atoms is investigated. The E_σ(k) dependences and full and partial densities of states are calculated within density functional theory. It is shown that an increase in the number of iron layers does not affect the electronic structure, but greatly affects the magnetic properties of the interface.

A brief description of the properties of titanium carbide, its applications, and established methods for its industrial fabrication are provided. A novel procedure for creating titanium-carbide coatings by subjecting a titanium sample to ultrashort laser pulses in a liquid hydrocarbon medium is described. Details of the experimental procedure are provided and the used laser setup is described. The prepared samples are characterized by scanning electron microscopy and Raman spectroscopy. The specifics of titanium-carbide formation at the boundary of the area subjected to the irradiation of a femtosecond laser are investigated based on Raman-spectroscopy data, and a comparison with a sample of commercial titanium carbide is made.

The phenomenon of energy conversion by a thin layer of finely dispersed spontaneously polarized semiconductor sandwiched between two plane-parallel metallic electrodes (being in direct contact with them), upon exposure to hydrogen gas, is studied. When the device is incorporated in a closed electrical circuit, the powder layer acts as a source of direct current. We develop a theoretical model that presupposes that the heat absorbed by the powder layer from the surrounding environment is converted into electrical energy. The model is shown to be consistent with the experiment.

The migration processes occurring in soil are considered to be diffusion processes. At present, there is no sufficient information on the parameters of radionuclide migration processes in soils. This makes it difficult to solve issues of preventing the consequences of radioactive contamination. In existing models based on Gaussian (normal) distributions, a large number of important factors capable of affecting radionuclide migration in the soil is not taken into account. In turn, radionuclide migration processes occurring in the soil behave unusually. Hence, classical estimates can lead to significant differences from experimental data. Due to the application of nonstandard approaches to this issue, soil can be regarded to be an aggregate of fractal clusters with a characteristic property of self-similarity. The observed spatial and temporal correlations can lead to the appearance of memory effects and self-organization. The fractal and chaotic dynamics of radionuclide migration in soil must be understood as a unified holistic process of the formation of a fractal model. As a result, it is possible to eliminate the discrepancy between experimental data and theoretical models constructed according to classical laws.

The possibility of increasing the critical current of MgB₂ tapes containing carbon and oxygen additives under the action of shock waves produced by pulsed plasma in a Plasma Focus setup is demonstrated. The microstructure of the tapes and the chemical composition of superconducting layers in the initial state and after impact at different distances between the samples and the anode of Plasma Focus installation (from 20 to 45 mm) and the number of shocks (from three to five) are investigated. The regularities of the change in the critical current as a function of the magnitude of transverse and longitudinal magnetic fields in the range of 2–9 T are studied at 4.2 K. It is established that as a result of the impact and thermal action of the plasma, the superconducting interlayers become denser, the grains are fragmented, the composition is homogenized, and the chemical composition is changed. This promotes an increase in the critical current by 50–80 A in transverse magnetic fields of 2–3 T.

The forming system of wire and arc additive manufacture (WAAM) based on cold metal transfer (CMT) is a high build rate system for production of near-net shape components layer by layer, which is composed of industrial robot operation system and 3D path simulation software. In the 3D path simulation software, the working layout of the off-line virtual robot is carried for the imported three-dimensional model to screen the model wall thickness, correct process library, set process parameters, slice, layer, plan deposition path, form simulation and upload program to execution system. Among the whole system, the 3D path simulation software provides essential database for process control and an innovative planning path to coordinate industrial robot platform to build parts layer by layer. Furthermore, the experiment is also performed to study the stability of aluminum alloy forming using the WAAM-CMT system by establishing different experimental models and changing process parameters and process modes of industrial robot and Fronius digital welding machine embedded on robot operation platform.

To study the detecting radiosensitvie of (In₂O₃)_0.1(TeO₂)_0.9 thin films irradiated by gamma rays. Materials and Methods: The effects of gamma irradiation of various levels on the current-voltage characteristics for the (In₂O₃)_0.1(TeO₂)_0.9 thin films, prepared by thermal evaporation in vacuum. The current increases linearly with the gamma radiation dose up to certain dose and decreases thereafter. The sensitivity of these thin films, at different applied voltages in the range 0–4.8 V, has been found to be in the range 35–190 mA/cm ²/Gy. Correspondingly, the minimum measurable dose has been found to be in the range 0.05–0.26 mGy. The values of the sensitivity are reasonably high in comparison to the commercially available gamma radiation dosimeters, revealing high scope for further developments.

The effect of optical illumination on the resistive switching in ultrathin (~4 nm) ZrO₂(Y) films with embedded single-layer Au nanoparticle arrays 2–3 nm in size is studied via tunneling atomic force microscopy. The ZrO₂(Y) films with Au nanoparticles are grown by layerwise magnetron deposition onto glass substrates with a conductive indium-tin-oxide sublayer, followed by annealing at 450°C. An increase in hysteresis due to bipolar resistive switching in the ZrO₂(Y) films is observed on the cyclic current–voltage curves of the microscope probe-to-sample contact. The effect is found to manifest itself in a dense Au nanoparticle array (~660 nm) when the contact area is photoexcited through a transparent substrate exposed to the radiation of a semiconductor laser at the plasmon-resonance wavelength. The effect is attributed to the photon-assisted field emission of electrons from Au nanoparticles to the conduction band of ZrO₂(Y) in a strong electric field between the microscope probe and the indium-tin-oxide substrate under plasmon-resonance conditions.