UDC 538.945:539.893
T. A. Prikhna, A. V. Vlasenko, S. N. Dub, V. S. Melnikov, Y. M. Savchuk, V. E. Moschil, N.V. Sergienko, P. A. Nagorny, V. B. Sverdun (Kyiv, Ukraine) Ks. Shaw (Grenoble, France) V. Gavalek, D. Litzkendorf, T. Habisreuter (Jena, Germany) J. Rabier, A. Jola (Poitiers, France)
Improving the superconducting and mechanical properties of bulk and thin-walled PT-YBCO ceramics by oxygen saturation
The possibility of saturation of the YBa2Cu3O7-δ (Y123) structure with oxygen up to 7–δ ≈ 6.9 atoms (which provides the maximum temperature of the transition to the superconducting state of this compound) is shown for the first time at high (800 °C) temperature and relatively low (16 MPa) oxygen pressure. This differs from the generally accepted ideas about the equilibrium in this system. It was found that oxygen saturation of PT-YBCО at elevated pressures and high temperatures leads to a decrease in the saturation time, as well as to a decrease in the number of cracks and an increase in the twin density in the material, which has a positive effect on the critical current density and mechanical properties of the ceramics. It was experimentally shown that twins are largely responsible for achieving a high density of critical currents jc and irreversibility fields in PT-YBCО ceramics. In the case of a high (1012 cm-2) dislocation density, but a low (0-1 μm-1) twin density in the Y123 structure, the critical current turned out to be an order of magnitude lower than in the case of a high twin density (22 μm-1) and the absence of dislocations and stacking faults. It was found that the density of twins and microcracks (parallel to the ab plane) in the structure of the YBa2Cu3O7-d phase depends on the size of the Y2BaCuO5 inclusions and the nature of their distribution, which, in turn, is determined by the starting materials used. A process for oxygen saturation of thin-walled (honeycomb) PT-YBCO ceramics under controlled pressure from 0.5 kPa to 16 MPa at temperatures of 900–800 °C has been developed, allowing record high jc values to be achieved and the magnitude of the trapped magnetic field to be doubled, both in comparison with bulk PT-YBCO ceramics saturated with oxygen under the same conditions and in comparison with thin-walled PT-YBCO saturated with oxygen at atmospheric pressure at the optimum temperature.
Keywords: superconductivity, oxygen saturation, twins, dislocations, pinning.
UDC 546.26-162
V. Yu. Dolmatov (St. Petersburg)
On the mechanism of detonation synthesis of nanodiamonds
All existing theories of detonation nanodiamond synthesis have been assessed.
Keywords: detonation nanodiamonds, synthesis, explosives.
UDC 544.722.2:539.26
V. P. Kazimirov, A. S. Roik, V. M. Perevertaylo, O. B. Loginova, S. A. Lisovenko (Kiev)
The nature of atomic ordering in the melt and surface properties of simple eutectic systems
Using models reconstructed from experimental curves of the structure factor using the inverse Monte Carlo and Voronoi-Delaunay methods, an analysis of the local atomic structure of simple eutectic systems Sn-Ge, Ag-Ge and Ni-C was performed. It was found that the nature of atomic ordering in the melt determines not only its volumetric but also its surface properties (surface tension, wetting). Clusters of single-sort atoms formed in melts, the binding energy inside which is higher than the binding energy between the atoms of the solvent and the cluster, exhibit surface activity in the melt, which explains the appearance of extremes on the density and surface tension isotherms of the melts. Clusters with chemical ordering of atoms according to the type of electron bonding Ме3С, found in the Ni—C and Ag—Ge systems, indicate that the equilibrium state diagrams of these systems at high pressures change from diagrams of a simple eutectic type to diagrams with a bond, i.e. an increase in pressure promotes the metallization of bonds in the melt.
Keywords: melt structure, ordering, clusters, surface properties.
UDC 621.744
V. V. Ivzhenko, I. P. Fesenko, N. V. Novikov, T. A. Prikhna, V. A. Popov, G. F. Sarnavskaya (Kyiv)
Study of the influence of injection molding parameters on the physical and mechanical properties of aluminum nitride ceramics
The results of a study of injection molding of thermoplastic masses based on aluminum nitride and paraffin are presented. The influence of casting parameters on the physical and mechanical properties of aluminum nitride-based materials with the addition of yttrium oxide was studied. A dense material with a crack resistance of 3.0-3.1 MPa m1/2 and a flexural strength of 280-320 MPa was obtained using injection molding. It is shown that the level of physical and mechanical properties of AlN-based ceramics obtained by injection molding technology depends on the ratio of the main parameters of the casting process: pressure, temperature and viscosity of the thermoplastic mass.
Keywords: aluminum nitride, injection molding, thermoplastic binder, density, flexural strength, crack resistance.
UDC 531.651:(546.26+666.233)
Yu. A. Klyuev, A. M. Naletov (Moscow) Influence of some coloring centers on the color of natural and synthetic diamonds
The mechanisms of the formation of diamond color caused by the absorption of light by the main impurity defects optically active in the visible region are investigated. Analytical dependences of the main color tone and color saturation of the diamond on the relative content of each of the defects were obtained. A comparative analysis of the effectiveness of the influence of the research centers on the color of diamonds was carried out.
Key words: color, absorption spectra, color centers, optical defects.
UDC 669.25/.27:539.217.1
M. M. Prokopiv (Kyiv)
Study of the microporosity of alloys of the WC-Co group
With the use of scanning electron microscopy, the presence of micropores located at intergranular and interphase boundaries, commensurate with the interlayer of the cobalt phase, was established in the structure of WC-Co alloys. The change in their size and shape is shown when the sizes of the cobalt interlayer and tungsten carbide grains change.
Key words: hard alloy, sintering, structure, tungsten carbide, cobalt, microporosity, grain, interlayer.
UDC 621.623
IN. IN. Rogov, N. D. Rublev, T. L. Krotenko, A. IN. Troyan (Kyiv)
Studies of the intensity of the tribochemical contact interaction of the polishing composition and sapphire during its processing
The results of the study of the intensity of the tribochemical contact interaction between sapphire (α-Al2O3) and the polishing composition based on colloidal silica SiO2, which occurs when polishing precision parts of electronic equipment and optical systems, are presented. It was established that the main factor intensifying this interaction is the acceleration of the chemical reaction between Al2O3 and SiO2 when the temperature in the contact zone increases, which experimentally confirms the chemical nature of the process of tribochemical polishing and allows to significantly reduce its labor intensity.
Keywords: tribochemistry, polishing, sapphire, silica.
UDC 621.77
AT. AND. Rosenberg, E. AND. Pashchenko, A. P. Maidanyuk, O. IN. Mikhailov, M. B. Stern (Kiev) E. Olevsky (San Diego, USA)
Evolution of density distribution, accumulated deformation and topological features of powder cylindrical blanks under the conditions of deforming stretching. Communication 2. Evolution of the internal topology of hollow cylindrical porous blanks during hardening of their internal surfaces with simultaneous compaction by the stretching method
The peculiarities of the structure of the pore space of powder bodies obtained by the method of deforming stretching and conventional axial pressing are analyzed. On the basis of experimental data, the possibility of distinguishing a separate component of the pore volume associated with the area of interparticle contacts is discussed. The topological inhomogeneity of the pore space of powder compacts is considered.
Key words: pore space, powder compact, probing, structure.
UDC 621.922.34
M. N. Sheiko, A. P. Maksimenko (Kyiv)
Grooving with diamond bars in the light of mechanical and statistical methods is presented about abrasive diamond machining. The established actual depth feed for multiple passes
The theoretical study of straightening with diamond bars continued. The phenomenon of accumulation of unremoved allowance of the cut abrasive wheel in the process of multiple passes of the table with a bar was investigated. The mechanical and statistical nature of the phenomenon is established. A mathematical model has been built that predicts the growth and subsequent stabilization of the cutting parameters, including the main value — the actual depth feed. Theoretical conclusions are confirmed experimentally.
Key words: mortise grinding, diamond bar, abrasive-diamond processing, abrasive wheel, accumulation of unremoved allowance, actual feed to depth, mechanical and statistical nature of the phenomenon.