Volume 24 № 2 2022
Influence of Sacrificial Mg2Si Layers and Kinetic Parameters on the Growth, Structure and Optical Properties of Thin Ca2Si Films on Silicon Substrates P.145-166. Download
The conjugation of crystal lattices of two-dimensional Mg2Si layers with atomically clean Si(001)2×1 and Si(110)"16×2" surfaces was simulated. Thick films were grown by the MBE method by the formation of Ca2Si templates. It is shown that for a Si(001) substrate at a temperature of T=300 °C and at the ratio of Ca and Si deposition rate of 4.7, three different silicides with comparable contributions are formed in a 140 nm thick film: Ca2Si, CaSi, and hR3-CaSi2. At the temperature up to 250 °C, with a decrease in the MBE growth and at the ratio of Ca and Si deposition rates equal to 8.4, a polycrystalline Ca2Si film is formed on Si(110) with a minimal contribution from CaSi. Polycrystalline and epitaxial Ca2Si films with thicknesses from 22 nm to 114 nm were grown on Si(111) substrates by the sacrificial-template method and at the MBE growth at a temperature of 250 °C and different ratios of Ca and Si deposition rates (4.0 – 20.0) on Si(111) substrates. The minimum deposition rate ratio ensures single-phase growth with the Ca2Si(100)/Si(111) epitaxial ratio; as it increases from 7.3, on Si(111) polycrystalline films grow with the following three orientations: Ca2Si(100), Ca2Si(110) and Ca2Si(111). It has been found that in Ca2Si films, regardless of their structure, the fundamental transition is masked by the Urbach edge in the photon energy range of 0.78 – 1.0 eV, and then the second direct interband transition is identified, E2d = (1.095±0.1) eV.
Ignition of Al- and B-Based Dispersed Fuels by a Radiant Heat Flux P.167-178. Download
The efficiency of using boron in high-energy materials and solid propellants for rocket propulsion systems significantly depends on the presence of an oxide coating on particles, ignition and combustion conditions, particle structure and size distribution. The present paper outlines the results of an experimental study of the ignition of Al conglomerate particles and dispersed fuels based on Al, B, and AlBn under radiant heating by a continuous-wave CO2 laser in the range of the heat flux density q = 65 – 190 W/cm2. On the basis of the results obtained, the dependences of the ignition delay time on the heat flux density are established, and the mechanism of ignition of an individual particle of aluminum and dispersed metal fuels in the air is described. It was found that the ignition delay times of micron aluminum boride powders are 1.9 – 2.3 (AlB2) and 3.2 – 3.5 (AlB12) times shorter than those for Al powder under the same heating conditions. The ignition delay times of amorphous boron are minimal (3 – 10 times shorter than that of Al). In this case, the ignition temperatures of the AlB2 and AlB12 powders are 110 – 130 °C higher than the ignition temperature of the micron aluminum powder. With an increase in the mass concentration of boron in AlB12 particles, the rate of heat released during heating and oxidation of the powder increases.
Dynamics of the Flow Pattern of a Two-Component Single-Phase Flow in a Channel P.179-187. Download.
In the present work the hydrodynamics of a two-component water-oil emulsion – is studied by numerical methods. The dynamics of the flow pattern of the water-oil mixture flow in a channel is investigated. The current study is carried out on the basis of the mathematical modeling of the flows of a two-component incompressible viscous liquid in a mixing T-shape channel. The length of the channel mixing section is 5 meters, and its height is 0.25 meters. The solution of the basic equations of a two-component medium is carried out using the openFoam algorithms in a non-stationary formulation. The solution is based on the SIMPLE algorithm and uses the volume of fluid (VOF) approach including the effect of surface tension. The following assumptions have been made. Firstly, the density and viscosity of water and oil are constant. Secondly, the flow is two-dimentional. Thirdly, the surface tension on the phase boundary between water and oil is constant. And, fourthly, a one-speed model of the emulsion motion is used. The detailed distributions of the main flow parameters (velocity, pressure, volume fraction) have been obtained. The simulation results show a continuous restructuring of the flow throughout the entire channel. The flow structure changes from a smooth stratified flow to a slug flow. Briefly, the scenario of the flow-structure development is as follows: smooth stratified flow → wave stratified flow → mixed stratified flow → slug flow. It is shown that a thin oil layer with variable thickness is formed on the bottom wall of the channel. The change of the velocity profile along the channel was studied. It transforms due to the significant difference between the water and oil viscosity and correlates with the local flow pattern. Also, the wall stress tension and pressure distribution along the bottom channel wall and the channel middle line, respectively, have been obtained. Mixed stratified and slug modes of emulsion introduce pulsations into the wall shear stress and pressure plots. In addition, the slug flow regime is characterized by a decrease in the rate of the pressure drop.
Mathematical Modeling of the Process of Fibre-Reinforced Polymer Bar Heating in a Curing Oven Taking into Account the Binder Curing Kinetics P.188-203. Download.
The mathematical model of the process of fibre-reinforced polymer bar (FPB) heating in a curing oven taking into account the binder curing kinetics is proposed. The mathematical model is based on the differential equation of non-stationary heat conduction written in a two-dimensional formulation. In the model the phenomena of radiant heat transfer from infrared emitters to the FPB and free convection are taken into account. The radiant heat transfer heats the FPB surface and free convection can both heat and cool the FPB surface. The phase change that occurs during the binder curing also is considered in the model. The density, mass specific heat capacity and thermal conductivity coefficient of the FPB are considered as average values of the constituent components. An assessment of the influence of the spatiotemporal grid parameters on the temperature field of the FPB is presented. A relative change of no more than 1 % in the FPB temperature field will provide the following parameters of the spatiotemporal grid: Δr – from 5·10-5 to 10-5 m; Δx – from 10-3 to 5·10-4 m; Δt – from 0.1 to 0.01 c. The calculated temperature values obtained by the developed mathematical model have been experimentally verified. For the case when the first FPB in the batch 8 mm in diameter was needletruded through a curing oven 6 m long at speeds of 1 m/min and 3 m/min and the temperature of the oven air still remained equal to room temperature, the temperature fields and the degree of the FPB binder curing have been calculated. It has been established that at the speed 3 m/min the curing of the fibre-reinforced polymer bar is not completed. The temperature of the FPB has been estimated in two-dimensional and one-dimensional formulations. For the same initial data and spatiotemporal grid parameters, the temperature values according to the one-dimensional model are on average 2 – 4 % higher than the temperature values according to the two-dimensional model, which is from 4 to 8 °С at a final FPB temperature of 200 °С.
Formation and Stability of the Crystalline Structures in the Two-mode Phase-field Crystal Model P. 204-217. Download.
The two-mode Phase-Field Crystal (PFC) model is an extended classical PFC model which allows one to describe hexagonal and other complex lattices. In such model the order parameter is periodic in crystalline state and constant in disordered or liquid phase. The PFC model allows linking mesoscopic and microscopic spatio-temporal scales and to implement in the results of the molecular dynamics method into phase-field models. In present work, the regimes of crystallization of two-dimensional structures were studied using the phase-field crystal (PFC) method. A quasicrystalline anisotropic striped phase was found; the phase qualitatively corresponds to the phases obtained in colloidal solutions. The stability of the structures was investigated during phase transitions between crystals with various symmetries. A structure diagram was constructed, and the melting curve was presented. A numerical approach for the two-mode PFC model was developed and the simulations were carried out using finite-element method in direct space. The dependence of the types of crystalline structures on the control parameters was investigated. It is shown that the scale parameters q0, q1 allow to control the lattice symmetry type, and the shift parameters r0, r1 affect the position of the structures existence regions and the melting curve. Also the shift parameters permit to control the formation of the quasicrystalline structures. The stability of structures with hexagonal and quadratic symmetries was investigated. During the phase transitions the change in near- and far-order symmetry occurs sequentially. The possible existence of a crystal with 5-fold symmetry as an intermediate phase was figured out. The sequence of transitions from triangular lattice to the liquid phase through honeycomb and square lattice is shown. It is shown that the transitions themselves occur through the mixed glassy phase. The proposed model and methodare applicable for modeling transitions between three-dimensional hexagonal (HCP) and cubic (FCC/BCC) lattices in metals and alloys.
Strengthening of Thick-Walled Pipes by High-Temperature Thermomechanical Treatment with Gradient Tempering in Wall Thickness P.228-237. Download.
The possibility of improving the characteristics of the structural strength of tubular parts made of structural steels due to the formation of a variable complex of mechanical properties in the wall thickness during high-temperature thermomechanical processing and gradient tempering with induction heating is considered. Plastic deformation during high-temperature thermomechanical treatment was carried out by screw compression (radial compression and torsion in two mutually opposite directions). A variable set of mechanical properties was provided by gradient tempering. Tempering was carried out at a negative temperature gradient from the outer surface of the pipe to the inner one. The temperature of the outer and inner surfaces was ~650 °C and ~200 °C, respectively. The scheme of the gradient induction tempering is as follows: a workpiece is given translational and rotational motion; the workpiece is heated successively and continuously in the inductor; simultaneously the inner surface of the workpiece is cooled with the use of an internal sprayer; then the workpiece is completely cooled with water by an external sprayer. The level and nature of the distribution of mechanical properties (HRC, σB, σ0.2, δ, ψ, KCU, K1C) in the thickness of the pipe wall is determined. In the wall thickness of 11 mm of the pipe blanks, the difference in HRC hardness from the inner surface to the outer is 11.5 – 12 units. The distribution of other properties is determined by the level of hardness and by the investigated dependences of the mechanical properties of the workpiece material on the temperature of the furnace tempering. The characteristics of the structural strength of tubular samples are evaluated when tested for internal dynamic pressure (the pressure of the elastic resistance limit and the fracture pressure). It is shown that compared with induction hardening after gradient tempering high-temperature thermomechanical treatment provides more improved mechanical properties of the material and characteristics of the structural strength of pipes.
About Assessment of the Wear Resistance of Thin Coatings P.238-245. Download.
Modern methods of hardening contacting surfaces operating in difficult operational conditions (high contact pressures, aggressive environments, severe temperature conditions, etc.) are based on technologies for obtaining sufficiently thin protective coatings of small thickness, not exceeding the level of 5-10 μm. This, in particular, greatly complicates the monitoring of performance characteristics in assessing the wear resistance of such coatings. Studies of tribological and mechanical properties using modern control methods are a scientific task, without the solution of which it is impossible to create new materials. An important performance characteristics of materials is their wear resistance. As a rule, the reduction in the service life of parts is due to the destruction of the surface layers of the working surfaces. In practice, an increase in the wear resistance of structural materials is achieved with the use of protective (wear-resistant) coatings and various methods of hardening and modification of surface layers. Here we consider the features of tribological tests of thin coatings (wear on a fixed or monolithic abrasive, on an abrasive cloth). Wear is estimated from the loss of the mass of the tested sample (mass wear) at a given friction path, fixed load and the sample movement speed across the abrasive surface. The objects of the tests can also be various structures and inclusions of small thickness (units of micrometers); in particular, the described approach can be adapted for layer-by-layer study of the wear resistance of transition zones of hardened materials of different nature (metals, composite materials, etc.). In a simplified version, it is also applicable to the assessment of the wear of massive materials with increased wear resistance. In the present paper, a method with advanced functionality is proposed for testing wear resistance applicable to coatings of small thickness due to the introduction of an artificial "increase in the total wear area of the sample". It is important that the proposed technique allows testing without the use of high-precision equipment and qualified personnel while ensuring high accuracy of measurements.
Voltammetric Behaviour of Vanadium in Sulfuric Acid Dimethylsulfoxide-Dimethylformamide Electrolytes P.246-255. Download.
The voltammetric behavior of metal ions in mixed mineral-organic electrolytes has a number of features, including such as the dependence of the nature and intensity of the analytical signal on the qualitative composition of organic and inorganic components, as well as on their concentration ratios in a background medium. The change in the ratios has a significant effect on the values of the electroreduction potentials of metal cations, which makes it possible to increase the selectivity of cathode electrochemical reactions, which is important in the analysis of multicomponent systems. The electrochemical behavior of vanadium ions has been studied mainly in the background solutions of various inorganic electrolytes, while mixed mineral-organic media containing organic solvents with high solvating ability have not been sufficiently studied, which does not allow to assess the prospects for their use in voltammetry for the analysis of natural and technical materials. The voltammetric behavior of vanadium ions in dimethylsulfoxide-dimethylformamide sulfuric acid solutions has been studied, and the dependences of electroreduction potentials and vanadium limit currents on the composition of the mixed electrolyte have been established. It is shown that under these conditions it is possible to form vanadyl VO2+ complexes, which include dimethylsulfoxide and dimethylformamide molecules in the coordination sphere and the areas of the linear dependence of vanadium limit currents on its concentration in solution are found. The prospects of using dimethylsulfoxide-dimethylformamide sulfuric acid electrolytes for voltammetric determination of vanadium content in petroleum raw materials are evaluated.
Kinetics of Gamma-Alpha-Transformation of Retained Austenite in Maraging Steel after Double Annealing at Sub-Zero Temperatures and Loads P.256-264. Download.
The paper studies the influence of mechanical tensile loads in the range of 0-300 MPa on the nature of gamma-alpha-transformation in cold-rolled maraging steel in the temperature range of -20 for -60 оС. Based on the kinetic curves of the electrical resistance the critical temperatures Mbeg and Mend were determined, the diagrams of the isothermal decomposition of supercooled retained austenite annealed in the two-phase region after 2 cycles at a temperature of 650 °C for 6 hours were plotted. It has been established that mechanical loads in the range of 20-300 MPa affect the parameters of the gamma-alpha-transformation ambiguously, shifting the isothermal diagrams in temperature and time and significantly changing the stability characteristics of austenite. The resistance of austenite to supercooling varies from -25 to -55 °С; the incubation period – from 2 min to 8 min; the decay period – with two extremes (50 MPa and 100 MPa) from 51 min to 518 min depending on the load level. The recorded maximum duration of the annealing austenite decomposition at low loads (20 – 100 MPa) is explained by the structural and chemical inhomogeneity of the phases, as well as the stress level of the initial state of the samples under study. The threshold stress is determined, loading up to which leads to a decrease by about 15 °С in the beginning of the gamma-alpha-transformation in the interval of sub-zero temperatures (from -40° to -55 °C). It can be assumed that the shift of the diagrams to the range of low temperatures is due to a change in the mechanism of the gamma-alpha-transformation. It is shown that cyclic heat treatment in the two-phase region at a temperature of 650 °C for 6 hours at loads up to 100 MPa delays the isothermal decomposition of supercooled retained austenite in the steel under study.
Investigation of Nickel Hydroxide Precipitates Formed During the Disposal of Spent Galvanic Solutions P.265-272. Download.
Despite a number of advantages, chemical nickel plating has a significant drawback - the formation of spent solutions of chemical nickel plating (SSCNP) and creates a threat of environmental pollution and the loss of valuable raw materials – nickel compounds. The aim of our study was to research poorly soluble nickel compounds obtained during the disposal of SSCNP. The object of the study was the spent solution of chemical nickel plating of one of the industrial enterprises of Udmurtia. To obtain a precipitate of nickel hydroxide, a 10 % NaOH solution was used, the amount of which was determined from the potentiometric titration curves. The identification of the precipitates obtained was carried out by infrared spectra taken on a Fourier-transform IR-spectrometer FSM 1202. Nickel hydroxide precipitates obtained in the pH range from 9 to 12 were identified using IR-spectroscopy. Broad absorption bands in the region of wave numbers 3400 and 1576 cm-1 refer to the vibrations of the OH-groups of water molecules. A broad peak in the range of 1100-1200 cm-1 corresponds to the stretching vibrations of sulfate ions, and in the region of 1430 and 1576 cm-1 – to carbonate ions. The spectra show deformation vibrations δ(Ni-O-H) at 515 and 643 cm-1, which are characteristic of α-Ni(OH)2. The results of differential thermal analysis indicate the formation of β-modified nickel hydroxide with an increase in precipitation pH. The heat treatment of hydroxide precipitates leads to the formation of nickel oxide; at different pH values the IR-spectra of nickel oxide are characterized by the similarity of the position and intensity of the characteristic absorption bands of the Ni-O bond, sulfate and carbonate ions, as well as water molecules. Experimental data confirm the possibility of using sodium hydroxide as a precipitator for SSCNP when the pH value of precipitation is varied taking into account the economic feasibility of the production of in- demand products and also the reduction of the negative environmental impact of the galvanic production.