Changes in the potential energy upon the formation of vacancies in the volume and in the cores of crystallite-conjugation regions in cubic polycrystalline metals

Changes in the potential energy of atoms that constitute the nearest neighborhood of vacancies formed in the bulk of d transition and precious cubic metals have been determined. These changes agree with the available first-principles calculations of changes in the potential energy of atoms of the nearest neighborhood of vacancies. In the cores of crystallite-conjugation regions (CCRs) of bcc polycrystalline d transition metals, the formation of vacancies is accompanied by positive changes in the potential energy of atoms of their nearest neighborhood. The absolute magnitudes of these changes are several times less than the changes in the potential energy of atoms of the nearest neighborhood of vacancies in the bulk of these metals, in accordance with the relationship between the enthalpies of formation of vacancies in these regions of polycrystals. The changes in the potential energy of atoms of the nearest neighborhood of vacancies formed in the cores of CCRs of fcc polycrystalline metals are negative because of the split structure of vacancies in the CCR cores of such metals.     

Formation of Ge-Sn nanodots on Si(100) surfaces by molecular beam epitaxy

The surface morphology of Ge_0.96Sn_0.04/Si(100) heterostructures grown at temperatures from 250 to 450°C by atomic force microscopy (AFM) and scanning tunnel microscopy (STM) ex situ has been studied. The statistical data for the density of Ge_0.96Sn_0.04 nanodots (ND) depending on their lateral size have been obtained. Maximum density of ND (6 × 10¹¹ cm^-2) with the average lateral size of 7 nm can be obtained at 250°C. Relying on the reflection of high energy electron diffraction, AFM, and STM, it is concluded that molecular beam growth of Ge_1-x Sn _x heterostructures with the small concentrations of Sn in the range of substrate temperatures from 250 to 450°C follows the Stranski-Krastanow mechanism. Based on the technique of recording diffractometry of high energy electrons during the process of epitaxy, the wetting layer thickness of Ge_0.96Sn_0.04 films is found to depend on the temperature of the substrate.     

Studying the rate of data transfer through an optical communication channel with a detector based on a photon counter

The rate of data transfer through an optical communication channel with a photon counter based on an avalanche photodiode being used as a detector in this channel is studied. The rate of data transfer through the optical channel is found as a function of the reverse bias voltage of the photodiode, the optical signal power, and the threshold level of detection.     

Mechanism of metal and alloy wearing in hydrogen-containing media

The results of theoretical and experimental investigations into the mechanism of hydrogen wearing of machine friction units exposed to microbiological media are presented. It is shown that reactions of biochemical synthesis, activated by friction, speed up considerably the dissociation of hydrogen molecules on a metal surface. This also intensifies adsorption and diffusion of hydrogen in a metal since in these conditions, the rate of dissociation of hydrogen molecules into atoms is much higher than that in a gas medium. This fact is very important because saturation and penetration of hydrogen into metals is determined by the rate of surface processes under friction on the phase boundary, and this influences the selection of techniques and materials to be used for developing protective coatings.     

Brittle fracture toughness—Experimental estimation of RPV materials and their welds containing shallow cracks

This paper presents brittle fracture resistance results for types 15X2MFA, 15X2NMFA steels and their weldments, produced by submerged arc welding as applied to nuclear reactor VVER-440 and VVER-1000 pressure vessels production and maintenance conditions. The fracture toughness/temperature dependence of these materials was determined for cracks with the depth 0·1 (small) and 0·5 (standard) from specimen thickness. The crack shape (through, semi-elliptical); material state (beginning and end of service life), and the specimen production processing (presence or absence of anticorrosive cladding) were also varied. The consideration of real crack size enables one to sufficiently increase the brittle fracture resistance of reactor materials and their welded joints. This fact should be used in the performance of control calculations.