No 2 (2023)

The paper presents the results of a detailed study of the process and products of combustion during self-propagating high-temperature synthesis (SHS) of ZnO zinc oxide powder from mixtures of such common reagents as oxidizer zinc nitrate and reducing agent (fuel) glycine, as well as the application of synthesized highly dispersed submicron and nanosized ZnO powder for the phenol photocatalytic decomposition under the action of ultraviolet irradiation. An aqueous solution of a mixture of reagents (the SHS-S process or Solution Combustion Synthesis – SCS) and the gel from a mixture of initial dry reagents formed when they were moistened due to hygroscopicity (the SHS-G process or Gel Combustion Synthesis – GCS) were combusted. The authors studied the phase and chemical compositions, the structure of the combustion product, and the effect of calcination in an oxidizing air medium and grinding in drum ball and planetary-centrifugal mills, as well as in mortar, on them and their photocatalythic activity. The study showed that calcination considerably increases the photocatalytic activity of combustion products due to a significant decrease in carbon impurity in the unburned fuel remains, and grinding in mills reduces the photocatalytic activity due to iron contamination and coarsening of ZnO particle agglomerates. The difference between the photocatalytic activity of the SHS-G and SHS-S products in the phenol decomposition is evident only at the initial stage of ultraviolet irradiation, after which this difference disappears. The authors discuss the direction of further research to increase significantly the photocatalytic activity of zinc oxide synthesized during combustion to use it effectively for the phenol decomposition under the action of visible light.

Polycrystalline composite materials made of carbon fiber reinforced plastics have more and more widespread application in mechanical engineering and become the main material for the production of modern types of high-speed transport. Thus, their share has already reached 35–45 % in the structural design of passenger aircrafts. However, the technology of machining surfaces of parts made of these materials, in particular, holes, is characterized by insufficient knowledge, the absence of regulatory standards for cutting modes and is most often based on the production experience of enterprises. When changing the processing conditions and the material, the pre-production engineering duration causes a significant increase in the cost of manufacturing parts due to the need for experimental selection of the cutting mode rational elements. To exclude the empirical selection of rational elements of the machining equipment cutting mode, the authors considered the possibility of using digital twins for studying the processes of drilling holes in the blanks made of composite materials, including those with the ultrasonic field energy introduction into the new surface shaping zone (to improve the processing quality and productivity). When modeling, the LS-DYNA program was used. The authors prepared the models and processed the results using the LS-PrePost 4.8 program. During the study, an explicit modeling method was used with preliminary validation and calibration of the results of tests of composites. The authors carried out calibration on test operations of tension, three-point bending, and interlaminar shear of the ВКУ-39 polymer composite material based on carbon fibers (carbon fiber reinforced plastic) widely used in domestic engineering. The developed finite element computer models allow simulating drilling procedures without carrying out rather complicated and expensive field tests. As a result of modeling, a simulation file was obtained, which reflects the process of drilling holes in a polymer composite material blank, as close as possible to the real-life situation with chip removal.

Due to low electrical resistivity, the Cu–Pd and Cu–Pd–Ag system alloys can be used as corrosion-resistant conductors of weak electrical signals. The paper deals with a comparison of the structure and physical-mechanical properties of Cu, Cu–3Pd and Cu–3Pd–3Ag (at. %) alloys after deformation at room or cryogenic temperature followed by annealing. The authors studied specimens in different initial states: quenched, deformed at room and cryogenic temperatures. To study the processes of structure rearrangement and the evolution of properties, annealing was carried out in the temperature range from 100 to 450 °C, followed by cooling in water. The duration of heat treatments was 1 h. The dependences of the yield strength and elongation to failure on the annealing temperature showed that cryodeformation significantly increases the thermal stability of the structure of both pure copper and the Cu–3Pd–3Ag ternary alloy. According to the temperature dependence of specific electrical resistivity of the deformed Cu–3Pd–3Ag alloy during heating at a rate of 120 deg./h, it was found that the decrease in electrical resistance caused by recrystallization begins at above 300 °C. The dependences of specific electrical resistivity on true strain showed that the structure rearrangement mechanisms during deformation are different for pure copper and the Cu–3Pd–3Ag alloy. The results of mathematical processing of the peaks in the diffraction patterns established that two phases appear in the Cu–3Pd–3Ag alloy after cryodeformation and annealing, one of which is silver-enriched, and the other is depleted. The study showed that during annealing of the deformed (especially after cryodeformation) Cu–3Pd–3Ag alloy, an anomalous increase in strength properties is observed. It was identified that alloying copper with palladium and silver leads to an increase in the recrystallization temperature. Thus, copper alloys with small palladium and silver additives are obviously attractive for practical applications, since they have improved strength properties, satisfactory electrical conductivity, and a higher recrystallization temperature compared to pure copper.

The paper analyzes the features of the acoustic emission (AE) signal generation during plasma-electrolytic oxidation (PEO) of the AMg6 aluminum alloy in a bipolar (anode-cathode) pulsed mode within each cycle of voltage application. The authors studied the range of PEO modes that almost completely covers all standard technological modes for processing aluminum alloys by the current densities (6–18 A/dm²) and current ratio in half-cycles (0.7–1.3), which allowed fixing and studying the AE accompanying the formation of oxide layers for various purposes. For the first time, due to AE registration, a new PEO stage was identified, in which there was no microarc breakdown to the substrate, but which was accompanied by an increase in the layer thickness, and the nature of which has not yet been determined. According to the known features of the oxidation stages, the authors systematized the repetitive forms of AE manifestation in the cycles of exposure and identified their five types and three subtypes. The study shows that the approach used to establish the PEO stages by the “acoustic emission amplitude” parameter has poor accuracy, since it does not take into account the form of signals and the half-period of their registration. Therefore, the authors developed and tested a new approach for analyzing AE frames synchronously with the cycles of change in the forming voltage during PEO, and proposed a new “acoustic-emission median” parameter, which allows identifying the main types and subtypes of signals accompanying the oxidation stages. An experimental study of the proposed AE parameter was carried out to identify these PEO stages, which confirmed the operability, high accuracy and sensitivity of the proposed parameter to the subtypes of AE signals recorded at the cathode stage of “soft sparking”. The latter is of particular interest, since it is a means of studying a given oxidation stage with a resolution equal to the exposure cycle.

In the modern literature, there are practically no data on the electrical characteristics of bituminous binders modified with carbon nanotubes and graphene nanoplates, while they are necessary for the design and development of innovative asphalt pavement compositions sensitive to the super-high-frequency microwave radiation. Contemporary bituminous binders are multi-component systems that may contain polymers, rubbers, synthetic or natural resins, inorganic salts, and even fragrances. As a result of application of modifying additives, bitumen acquires high performance characteristics. A special class of modifiers are micro- and nano-sized electrically conductive fibers and particles (steel wool, carbon fibers, carbon black, carbon nanotubes, graphene nanoplates), the use of which makes it possible to ensure the sensibility of bituminous binders to super-high-frequency microwave radiation and the implementation of the process of healing cracks in an asphalt pavement with its subsequent regeneration. As part of the study, the authors developed an original technique to produce bituminous binders modified with carbon nanotubes and multilayer graphene. Modified bituminous compositions in the concentration range from 0.2 to 6 and from 0.2 to 11 wt. % for multi-walled carbon nanotubes (MWCNT) and multilayer graphene nanoplates (MG), respectively were experimentally obtained. For the first time, the dependence of the specific volume electrical conductivity of bitumen-based nanocomposites on the concentration of nanostructured carbon filler (MWCNT and MG) was researched. The maximum values of electrical conductivity were 4.76×10⁻⁴ S/cm and 3.5×10⁻⁴ S/cm for nanocomposites containing 6 wt. % MWCNT and 11 wt. % MG, respectively. The study determined the filler volume fractions at the percolation threshold for nanocomposites containing MWCNT and MG. They amounted to 0.22 and 2.18, respectively. The formation of a percolation contour in nanocomposites containing MWCNT occurs at significantly lower filler concentrations compared to bituminous compositions containing MG.