St Petersburg Polytechnic University Journal-Physics and Mathematics最新文献

In the paper, the changes in axial and contour lengths of skeletal interatomic bonds in the chain molecules of polyethylene nanocrystals have been measured using X-ray diffractometry and Raman spectrometry. In the course of the measurements the samples were subjected to stretching and heating (mechanical and thermal actions). The measured force and temperature dependences were analyzed and the calculated description of the polymer nanocrystal strain was inferred from them. In so doing the original results were obtained for the thermal action. The potential energy components related to both the skeletal bond stretching and the chain molecule bending were determined for the strained polymer crystal. The sharp distinction between the ratios of these components for the object under mechanical and thermal actions was found.

In this paper, an approach to creating corpuscular-optical devices for transportation and transformation of charged particle beams has been elucidated. These devices are able to optimize and create the most convenient configuration of ionic or electron paths. The approach relies upon the inverse dynamics problem formulated on the basis of the Hamilton-Jacobi equation. The motion in the symmetry plane of a three-dimensional (3D) field was considered. The problem was solved by analytical methods. An algorithm for constructing electric fields providing the particle motion on the desired trajectories was described. А key to this algorithm lies with a concept of conformal transformation from the theory of complex-valued function. This procedure was illustrated by examples. Quadratic potential was chosen as a basis. Three functions of conformal transformation were considered, providing the rotation of the focused charged particle beam at a fixed angle, the transformation of divergent flow to parallel one. The calculated two-dimensional potentials were extended into 3D-space by power series expansion on transverse coordinate. Device embodiments were suggested on the basis of the calculated field structures.

The article is dedicated to the study of bifurcations of stationary convection regimes in a closed, heated from below and tilted square cylinder filled with air for cases of heat-insulated and perfectly heat-conducting sidewalls. The temperature and velocity fields were obtained using grid method for inclinations from a horizontal position up to 30 degrees in the range of Rayleigh numbers up to 20-fold excess of its critical value. The limit angle of anomalous-flow existence in the cylinder with the heat-insulated walls was established to be about 3 times greater than that in the cylinder with the heat-conducting ones. In the case of the heat-conducting walls the maximum angle of the anomalous-flow existence reached 7.7 degrees at a 3.3-fold excess of the critical value of Rayleigh number.

The peculiarities of interfacial boundary diffusion where the boundary goes between nonmiscible metals with body-centered cubic (BCC) and face-centered cubic (FCC) lattices have been studied taking, as a case in point, the Cu/Nb system, and using the molecular dynamics method. The diffusion atomic displacements were shown to occur mainly near the mismatch dislocations and their intersections. The diffusion of the high-melting component was found to be characterized by high anisotropy with the predominant atomic displacement along the dense-packed direction in the interfacial boundary plane being common to FCC and BCC lattices with the Kurdyumov–Sachs mutual orientation.

Features of the formation of angular dependences of electrons emitted from a disordered solid and experiencing inelastic scattering have been considered. Such fine details of the dependences are formed by the processes of quantum transport of emitted particles. We took the cases of two-particle and multi-particle inelastic processes. Qualitative and quantitative assessments of the relative contributions of different groups of particles were carried out. The effects related to quantum electron transport were shown to be generally more pronounced in the case of registration of electrons generated inside the solid in the inelastic scattering of particles of the primary beam. This is true both for the electrons generated by ionization processes and Auger electrons. The obtained results point to the possibility of using this effect in applied electron spectroscopy.

The article analyzes the correspondence between the actually occurring physical and chemical processes and the concept of the greenhouse effect. The absorption of the solar radiation by the gases existing in the Earth's atmosphere has been examined. It was demonstrated that, despite the absorption of radiation from the Earth's surface in the middle and long-wave infrared (IR) regions, there is a strong absorption of the overtones and combined frequencies of water vapor in the solar radiation (visible and near IR regions), i.e., the transmittance bandwidth of ‘the glass greenhouse’. Thus, the Earth atmosphere does not really function as a greenhouse, and the terms ‘greenhouse effect’ and ‘greenhouse gases’ lost their original meaning and have remained symbolical.

Thin films based on CuTPP, ZnTPP and FeClTPP (ТРР – TetraPhenylPorphyrin) complexes and manufactured by vacuum deposition under quasi-equilibrium conditions have been investigated. The hot-wall epitaxy method was chosen for the preparation of samples. The structure of objects and their composition were examined by scanning electron microscopy (SEM) and energy dispersive microanalysis (EDX). The CuTPP film was established to form whiskers about 20 nm in diameter and 5 µm in length. The structural features and morphology of ZnTPP and FeClTPP films’ surface were revealed. The theoretical interpretation of obtained results was proposed that made it possible to relate the film structure with its composition.

In this paper, we have considered the issue of effectively forming a representative sample for training the neural network of the multilayer perceptron (MLP) type. The main problems arising in the process of the factor space division into the test, validation and training sets were formulated. An approach based on the use of clustering that allowed to increase the entropy of the training set was put forward. Kohonen's self-organizing maps (SOM) were examined as an effective clustering procedure. Based on such maps, the clustering of factor spaces of different dimensions was carried out, and a representative sample was formed. To verify our approach we synthesized the MLP neural network and trained it. The training technique was performed with the sets formed both with and without clustering. The approach under consideration was concluded to have an influence on the increase in the entropy of the training set and (as a result) to lead to the quality improvement of MLP training with the small dimension of the factor space.

We study the analyticity of a characteristic function of a process defined by means of SDEs. Namely, starting with the simple case of a scalar Ito SDE we show that the corresponding characteristic function is entire. The proof is based on Grönwall's inequality technique and the classic analyticity criterion in terms of moments. Further, we extend this criterion and derive a handy sufficient condition of analyticity in the multidimensional case, which is used to prove the corresponding general result. We assume that the drift vector obeys the linear growth condition and the diffusion matrix is time-only-dependent but possibly degenerate. The approach used in this article can be extended to more general types of SDEs.

In this paper, we present binding energies between hydrogen (H), carbon (C), nitrogen (N) and oxygen (O) atoms and a vacancy in the hexagonal closed-packed (HCP) lattice of titanium (Ti) and the face centered cubic (FCC) lattice of aluminum (Al), calculated using the density functional theory (DFT). We have also investigated the trapping of up to five hydrogen atoms by a vacancy and the reduction of the vacancy formation energy, due to the formation of a hydrogen–vacancy complex. We used the molecular-dynamics modeling with consecutive relaxation at 0 K to obtain an atomic configuration of the vacancy–impurity complex, corresponding to the global energy minimum. According to our calculations, C–V, H–V, C– (H–V), N–(H–V) complexes are stable in the Al lattice with only H–V complex being stable in Ti. The formation of C–(H–V) and N–(H–V) complexes in the Al lattice results in the negative vacancy formation energy. The formation of H–V complex decreases the vacancy formation energy by 0.26 eV in the Ti lattice. A vacancy in the Ti lattice can trap up to four hydrogen atoms.