Volume 11, Nº 6 (2017)

Various zone plates: amplitude, amplitude-phase, and phase ones (including Fresnel and Gabor zone plates, Laue- and Bragg-Fresnel lenses, photon sieves, and aperiodic zone plates), as well as their properties and X-ray focusing possibilities, are described and analyzed.

The growth of high-temperature AlN and AlGaN layers on (0001) sapphire substrates by ammonia based molecular-beam epitaxy is studied. Factors affecting the formation of inverted domains in high-temperature AlN films are examined. The density of inverted domains is found to correlate with the density of nucleation islands during the initial stages of growth. The denser coverage of a surface by nucleation islands suppresses the formation of inverted domains. It is possible to increase the density of surface coating at the nucleation growth stage by increasing the degree of substrate nitriding, reducing the deposition temperature, and using intense ammonia fluxes during deposition of the initial layers. The kinetic model in the mean field approximation is developed to explain the observed effects of growth parameters on the density of nucleation islands. The growth features of AlN and its structure are taken into account. The obtained results are used to grow AlN/AlGaN layers with improved structural quality. The grown films have a root-mean-square surface roughness of 2 Å and 120 arc s FWHM of X-ray diffraction peaks for the AlN 0002 reflection. The density of inverted domains is decreased to below 10⁵ cm^-2. Improvement in the quality of the AlN films is achieved by using two-step growth and by the application of gallium as a surfactant.

Ceramic-like coatings with a thickness of up to 40 μm are formed on aluminum composites without additives and with copper additives (1 and 4.5%) in a silicate-alkaline electrolyte by microarc oxidation. The composites are prepared by powder metallurgy (cold pressing and sintering in forevacuum). An increase in the copper concentration in the composites to 4.5% leads to the retardation of anode voltage growth on the initial stage of oxidation corresponding to the formation of a barrier layer. The coatings are studied by scanning electron microscopy, X-ray microanalysis, X-ray photoelectron spectroscopy, and X-ray diffraction. The morphology of their surface corresponds to the morphology of the surface of coatings on compact aluminum alloys. According to X-ray photoelectron spectroscopy, a thin 1-μm layer forms on the surface. It consists predominantly of electrolyte components. X-ray diffraction analysis shows that the coatings mainly consist of γ-Al₂O₃ oxide as well as the η-Al₂O₃ phase, the peaks of which are broadened. This broadening is characteristic of the amorphous component and may be due to the presence of nanocrystalline regions in the coating structure. In the coatings on the composite Al + 4.5% Cu, mullite Al₂SiO₅ and copper oxide CuO are also found. The excess aluminum content may be associated with residual unoxidized aluminum inclusions in the structure of the coatings.

The influence of electron-beam parameters on the thickness and phase composition of a hardened layer formed upon the nitriding of austenitic stainless steel 12Cr18Ni10Ti in plasma produced by a beam in a low-pressure (3 Pa) nitrogen-argon mixture is studied. The results obtained in the DC and pulse-periodic modes of beam generation with the same mean current and electron energy are compared. In this case the negative bias voltage applied to the samples is 100 V. The nitriding temperature of 400°C is maintained at a mean beam current of 2.6 A and various combinations of frequency (100–500 Hz) and current pulse durations (0.1–0.3 ms) with an amplitude of 80 A. The mean ion-plasma current densities in the DC and pulsed modes are close in magnitude (2–3 mA/cm² at 400°C). The high pulsed ion-current density (35–70 mA/cm²) creates conditions under which the surface sputtering rate during the pulse exceeds the growth rate of the nitrided layer. The nitriding of steel in the pulsed and DC modes over four hours gives the same result. Hardened layers with a thickness of 7–8 μm and a microhardness of the surface component of 15 ± 1 GPa in which the main phase is a supersaturated nitrogen solid solution (expanded austenite) are formed. A possible explanation is that nitriding in an electron-beam plasma proceeds mainly under the action of long-lived active neutral nitrogen particles rather than as a result of ion bombardment.

Diamond-like coatings with a total thickness of ~0.6 μm are obtained by physical vapor deposition with plasma separation and a pulsed carbon arc source with a cooled cathode and laser arc ignition; the substrates are titanium alloy (VT4), stainless steel (12Cr18N10T), and copper (M1). Scanning electron microscopy and profilometry are used to study the coatings surface and structure. The composition of the coatings and the fraction of sp³ bonds are studied using Raman spectroscopy. A wide peak in the 1580 cm^-1 region is observed characteristic of diamond-like coatings. The coatings have a dense, nonporous structure. The tribological properties of the coatings are evaluated by the ball-on-disk method using a friction pair with WC and technical diamond. The strength characteristics are determined using linear scratch testing and nanoindentation measurements. The strength characteristics of the coatings vary and depend on the substrate materials. The friction coefficient of a diamond-like coating on VT4 alloy is ~0.1 in a friction pair with WC and ~0.01 with technical diamond.

The results of comparative studies of the rate of mechanical polishing of the surface of synthetic diamond in various directions and different crystallographic surface orientations are presented. A free abrasive on a cast-iron disk is used for polishing. A method is proposed for numerical simulation of the polishing of an arbitrarily oriented diamond surface, based on an estimation of changes in the internal energy of the diamond crystal lattice under the action of the abrasive. The method enables determination of the direction of polishing, which ensures the maximum processing rate of a diamond surface of any crystallographic orientation. The validity of the proposed method is confirmed experimentally.

Structure of smooth hydrocarbon CD_x films with a high deuterium ratio x ~ 0.5 redeposited from T-10 tokamak D-plasma discharges (NRC Kurchatov Institute, Moscow) has been studied. For the first time, small and wide angle X-ray scattering technique using synchrotron radiation and neutron diffraction have been employed. A fractal structure of CD_x films is found to consist of mass-fractals with rough border, surface fractals (with rough surface), plane scatterers and linear chains forming a branched and highly cross-linked 3D carbon network. The found fractals, including sp² clusters, are of typical size ~1.60 nm. They include a C₁₃ fragment consisting of three interconnected aromatic rings forming a minimal fractal sp² aggregate 9 × C₁₃. These graphene-like sp² clusters are interconnected and form a 3D lattice which can be alternatively interpreted as a highly defective graphene layer with a large concentration of vacancies. The unsaturated chemical bonds are filled with D, H atoms, linear sp² C=C, C=O, and sp³ structural elements like C-C, C-H(D), C-D_2,3, C-O, O-H, COOH, C_xD(H)_y found earlier from the infrared spectra of CD_x films, which are binding linear elements of a carbon network. The amorphous structure of CD_x films has been confirmed by the results of earlier fractal structure modeling, as well as by researches with X-ray photoelectron spectroscopy which allow finding a definite similarity with the electron structure of their model analogues — polymeric a-C:H and a-C:D films with a disordered carbon network consisting of atoms in sp³ + sp² states.

The chemical structure of Fe₇₈B₁₃Si₉ alloy in the solid and liquid states and local atomic environment are studied in situ by X-ray photoelectron spectroscopy (XPS). The chemical bonds between elements in the melt are analyzed during a temperature increase. Two temperature regions are identified. The liquid surface in the first temperature region is shown to contain clusters of Fe-Si and (Fe-O_x)-Si types. In the second one, clusters of Fe-B and (Fe-O_x)-B types dominate. It is impossible to determine the composition of the clusters definitively using XPS data only. A jump-like change in the composition of the surface layers of the melt is detected, which is interpreted as structural transformations within the liquid state.

The possibility of chemical metallization of the surface of poly(ethylene terephthalate) (lavsan) fibers in solutions using hydrazine sulfate as a reducing agent is shown. As a result, a conductive material is obtained. The surface of lavsan fibers is activated by treating the fibers in caustic ammonia with ultrasound treatment and processed in alkaline solutions. The elemental composition and surface morphology of the metalized fibers are characterized using X-ray diffraction, microprobe analysis, and scanning electron microscopy. The surfaces of metallized fibers treated with various copper-plating solutions are shown to be coated by copper layers of 4 μm thickness, having good adhesion. The coating is uniform and defect-free over the entire length of the fibers. The fiber electrical resistivity is 0.4–0.9 Ω/cm.

The motion of relativistic electrons in an ideal three-dimensional magnetic undulator field satisfying the stationary Maxwell equation is considered. The system of nonlinear differential equations of the electron motion is solved analytically using perturbation theory rather than the method for averaging fast oscillations of the electron trajectory (the focusing approximation), as was done in a series of previous studies. The obtained analytical expressions for the trajectories describe the behavior of particles in a three-dimensional magnetic undulator field much more accurately than the formulas obtained within the framework of the focusing approximation. The analysis of these expressions shows that the behavior of electrons in a three-dimensional undulator field is much more complicated than that described by equations obtained using the averaging method. In particular, it turns out that the electron trajectories in the undulator have a cross dependence; in this case, variations in the initial trajectory parameters in the vertical plane cause changes in the horizontal trajectory components, and vice versa. The results of calculations of the trajectories carried out using analytical expressions are close to those of numerical calculations using the Runge-Kutta method.

A new universal function f(x, z) describing electron energy losses over depth ϕ(z) and the lateral distribution of electron energy losses ψ(x) in a scanning electron microscope is developed in the context of a multiple electron scattering model at energies of 1–50 keV in condensed matter, where two groups of backscattered primary electrons are taken into account for the first time.

We made software to simulate Larmor precession in a setup for SESANS with adiabatic/RF flippers in magnets, existing at PNPI. The final polarisation of a divergent “gribbon beam” of height 2 cm is calculated as a function of λ. For λ = 6 Å, flippers 56 cm apart and RF frequency 1 MHz we find spin-echo length δ = 0.9 δm. We show numerically, how λ is converted to δ. Extension to δ = 20 δm is realistic.

The boundary-value problem for determining the transmission function by means of the transport equation is solved using the invariant embedding method. The obtained solution makes it possible to find the angular distributions and energy spectra of atomic particles that have passed through a solid layer if the differential elastic and inelastic scattering cross sections are known. Exact numerical solutions to equations and analytical solutions to equations in the small-angle approximation are found. For problems of electron transport under conditions that are characteristic for the most actively used electron-spectroscopy procedures, the results obtained in the transport approximation are compared with the exact numerical solution and the solution in the small-angle approximation. It is shown that, within the framework of the transport approximation, it is impossible to describe a phenomenon such as “brightness-body rotation”. The calculation of various cases within the framework of the small-angle approximation is in good agreement with the exact solutions. The calculation error in the small-angle approximation does not exceed 10%. The results were approbated using DOM, MDOM, and DISORT codes applied in remote sensing of the Earth atmosphere.

The distributions of backscattered ions over paths in the target are calculated numerically for different ion energies and different ratios of masses and target atoms. The calculations are performed by solving the one-velocity transport equation with a scattering cross section for a truncated Coulomb potential. The problem is reduced to the Chandrasekhar integral equation, which is solved using the method of successive approximations. The path distribution of backscattered ions obtained as a result of the inverse Laplace transformation has a characteristic cupola-shaped form with a maximum. Within the framework of the model of continuous energy losses, the path distributions are recalculated to obtain the ion reflection coefficients and compared with the SRIM simulation results.

Martensitic Fe-based metal was achieved by argon arc surface cladding using self-developed low-temperature phase transformation (LTT) powders. Residual stress (RS), retained austenite (RA) content, hardness and wear resistance of the cladding layer under different powder compositions were tested. Meanwhile, the phase composition, microstructure and wear mechanism of the cladding metal were also analyzed. The results show that under suitable process parameters, the cladding layer and the substrate are metallurgical bonding without crack and porosity. The microstructure of the cladding layer is mainly lath martensite with less RA distributed on. Due to the volume expansion during phase transformation from austenite to martensite at about 200°C degree, large compressive RS is produced within the cladding metals, which has effectively compensated the tensile RS result from thermal shrinkage. The compressive RS reaches maximum of -361 MPa, and the average RA in the cladding layer is 10.85% for powder with composition of 10% Cr and 8% Ni. The hardness of the cladding layer is 2.6 times higher than that of the matrix material, which could reach up to 557.2 HV. The wear resistance of the surface cladding layer is nearly 57 times higher than that of the base material, which has the most excellent comprehensive performance.

Using methods of modern material science the structure-phase states and tribology properties of coating surfaced on martensite low carbon Hardox 450 steel by powder Fe-C-Ni-B wire and modified by following electron-beam treatment are studied. It is shown that electron-beam treatment of layer leads to the formation of multiphase state with the main phases: α-phase, iron boride FeB, boron carbide B₄C. The surfaced structure formed by intensive electron beam irradiation is characterized by high value of wear resistance. It is in 20 times more that wear resistance of steel and in 11 times more than wear resistance of surfaced layer without of electron beam treatment, friction coefficient decreased in 3.5 and 2.2 times, correspondently.