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

Visible quantum cutting (QC) has been observed in green emitting BaF₂ co-doped with Gd³⁺, Tb³⁺ phosphor via down-conversion (DC) synthesized by wet chemical method. Powder X-ray diffraction (XRD) analysis shows structural purity of synthesized phosphors. The excitation (PLE) and PL spectra in the vacuum ultraviolet (VUV) or UV region were measured with the help of 4B8-VUV spectroscopy experimental station of the Beijing Synchrotron Radiation Facility (BSRF). In the QC process, one VUV–UV photon absorbed is cut into more than one visible photons emitted by Tb³⁺ through cross relaxation (CR) and direct energy transfer (DET) between Tb³⁺ and Tb³⁺ or Tb³⁺ and Gd³⁺, depending on the excitation wavelength. From the emission spectra monitored at different wavelength excitation, the two-step energy transfer process is investigated and theoretically calculated quantum efficiency observed 148% and 177% at the excitation wavelength of 174 nm and 219 nm respectively.

The albedo contribution to the total hard X-ray solar-flare emission has been considered. First, the distribution of primary hard X-ray photons localized along the flare loop was found, and finally, the Monte Carlo calculations of the Compton scattered hard X-ray photons were performed. The albedo for photons was shown to contribute to the total X-ray flux only in the energy range from 30 to 100 keV. The backscattered photon flux depends on the loop's position and on the localization of the source of primary radiation along the loop. For an isotropic distribution of primary photons, the backscattered photons’ contribution to the total flux is maximal for the loop in the center of the Sun and reduces when shifting to a limb. In the case of an anisotropic source the angle at which the contribution is maximal depends on the degree of anisotropy and on X-ray directivity in the source.

In the paper, the effect of the choice of equations of state (EOSs) depicting the states of metal plasma and water on the simulated results obtained for an underwater electrical explosion of conductors has been analyzed. In order to compare various EOSs, a one-dimensional, cylindrically symmetrical, magnetohydrodynamic model of an underwater wire explosion was employed. The simulated results were compared with the experimental data on both micro- and nanosecond explosions of aluminum and copper wires. The right choice of EOSs and the model of transportation coefficients allowed us to improve the agreement between the experimental and simulated data and to replicate the thermodynamic evolution of the system more closely. The comparison revealed the most appropriate EOSs for application to simulation of an electrical explosion.

The study of temperature evolution of KNO₃ (NOP) structure in ferroelectric (1−x)KNO₃ + (x)BaTiO₃ composites with BaTiO₃ concentrations х = 0.25, 0.50 and 0.53 has been carried out on cooling with the use of the neutron diffraction technique. It was shown that, on cooling, the phase transition temperature (T_c) from the high-temperature paraelectric phase into the ferroelectric one did not depend on barium titanate concentration and practically coincided with T_c for pure NOP. Moreover, it was found that the admixture of BaTiO₃ essentially enlarged the temperature range of NOP ferroelectric phase stability in the composites with BaTiO₃ concentrations x = 0.25 and 0.50. The suppression of the ferroelectric phase was observed for the composite with x = 0.53.

The calculation results on dynamic characteristics of a geometrically imperfect ring turning out of shape in its plane have been exemplified by a simpler computational model for a ring resonator of a gyroscope based on elastic waves in solids. The specific malconformations were shown to be responsible for a splitting of the bending frequency spectrum of such rings. In so doing the spectral mismatch may appear in cases different from the ideas of modern theory.

The splitting of the bending frequency spectrum was established to occur not only in the cases when the number of formative waves being equal to that of malconformation waves of the ring (as it is commonly believed at present) but also in the cases when the number of formative waves being two, three, four and so on times more than that of malconformation waves.

The algorithm of stage-by-stage qualitative modeling of the mechanism of a semiconductor–metal phase transition in vanadium dioxide has been proposed. The basis for the model is a statement that the transition is complex in character and consists of the anhysteretic, purely electronic Моtt transition occurring over a wide temperature range, and the temperature-abrupt structural Peierls transition having a thermal hysteresis. The initial stage of the model is based on the solution of a quantum-mechanical problem of an electronic spectrum of a linear vanadium-ion's chain. The model is completed by consideration of correlation effects and a martensitic character of the structural transition through taking consecutively account of results obtained by X-ray, spectroscopic, impedansmetric and magnetic resonance methods.

In this paper, an effective numerical method to achieve the numerical solution of nonlinear Riccati differential equations of arbitrary (integer and fractional) order has been developed. For this purpose, the fractional order of the Chebyshev functions (FCFs) based on the classical Chebyshev polynomials of the first kind have been introduced, that can be used to obtain the solution of these equations. Also, the operational matrices of fractional derivative and product for the FCFs have been constructed. The obtained results illustrated demonstrate that the suggested approaches are applicable and valid.

Several schemes of the second-order approximation worked out in the literature for unstructured-grid-based computations of gasdynamic flows are described. The convective fluxes on the control-volume's faces are evaluated using Roe's approximate Riemann solver. The MUSCL approach with the use of various quasi-one-dimensional schemes of reconstruction of gasdynamic variables and limiters making the solution monotonic is applied in order to improve the approximation accuracy. Comparative analysis of the working capacity of the schemes under consideration has been carried out through solving two problems of inviscid gas flow. Namely, the transonic NACA-0012 airfoil flow and the superpersonic flow in a duct with a central ramp were computed. The smoothness of solution, obtained with different schemes, dissipativity features of the schemes and computational process stability were evaluated.

The commonly employed highly efficient and robust Q-estimate of the scale parameter proposed by Rousseeuw and Croux has been approximated using computationally fast Huber M-estimates. The suggested M-estimates were shown to be robust and highly efficient for an arbitrary underlying data distribution due to correctly choosing the approximation parameters. The following indicators of the efficiency and robustness of M-estimates of scale were computed: their asymptotic variances, influence functions and breakdown points. Special attention was given to the particular cases of the Gaussian and Cauchy distributions. It is noteworthy that for the Cauchy distribution, the suggested robust estimate of scale coincides with the maximal likelihood estimate. Finally, the computation time of these highly efficient and robust estimates of scale is 3–4 times less than for the corresponding Q-estimates.