Investigation into the Mechanism of Oxidation on the Surface of Lead Selenide Semiconductor Structures

The oxidation of lead selenide samples prepared in the form of ground powders, pressed pellets, polycrystalline films, and faceted crystals is investigated. All samples are produced from a lead selenide batch heat treated under dynamic vacuum. The bulk and surface structural and phase compositions of the samples upon oxidation in dry air at temperatures of 298–773 K are studied using electron microscopy, X-ray diffractometry, and Auger electron probe microanalysis. The dependence of the resistivity of polycrystalline films and pressed pellets is analyzed in the temperature range 298–498 K in dry air and argon. It is demonstrated that the specific features in the temperature dependences of the resistivity of oxidized PbSe samples can be associated with their microstructure and environmental conditions. It is revealed that the PbSeO₃ phase is formed on the surface of all the PbSe samples starting from room temperature, whereas no phase transformations are observed in the bulk of samples.     

Investigation of fibres based on rigid-chain polyamides by the wide-line nmr and ultrasonic acoustics

Conclusions -- A dependence of the second moment of the NMR line on the angle between the fibre axis and the magnetic field has been given; the structural coefficient C ₀, C ₂, and C ₄ have been calculated for fibres based on poly-p-phenyleneterephthalamide and poly-p-benzamide at temperatures of –196 and +20°C and for Kevlar-49, from –196 to +400°C.-- The intramolecular structural coefficients C ₀ ^im, C ₂ ^im, and C ₄ ^im have been calculated theoretically, and the orientation parameters and and the acoustic orientation factor f have been obtained, values for which indicate a high degree of orientation of the macromolecules in fibres.-- The smaller values of the second moment of the NMR line for PABI as compared with fibres based on PPTA in the temperature region 250–380°C indicate a more mobile structure for the PABI.-- In the range from –196 to +20°C thermal action leads to a more important change in the coefficient C ₀ for Kevlar-29 and Kevlar-49 as compared with PABI; this is apparently connected with thermal expansion in planes having a predominant concentration of van der Waals bonds for Kevlar-29 and Kevlar-49.Translated from Khimicheskie Volokna, No. 4, pp. 23–26, July–August, 1990.     

Mesh-ray model and method for calculating the laser radiation absorption

The present paper puts forward a mathematical model of laser radiation absorption in a laser target, which combines approximations of geometrical and wave optics and the corresponding numerical algorithm. This model depends on the principles of geometrical optics in the range of weak variation of the plasma refractive index on the scale of the wave length. This enables one to describe the refraction of the radiation. A transition to wave approximation is carried out near the surface of critical density, where the approximation of the geometrical optics is a fortiori inapplicable. For this aim, the plasma medium is approximately represented as a set of plane layers, on which the one-dimensional Helmholtz equation is solved. This makes it possible to construct a simple and relatively accurate method for calculating the absorption and reflection of laser radiation near the critical density surface in order to effectively take into account the dependence of the interaction of radiation with matter on the polarization direction, etc. The proposed model is adapted for implementation in the radiation gas dynamics (RGD) code. A numerical computation subroutine is presented based on the analytical solution of the differential equations corresponding to the optical ray model of the laser radiation energy flux. This solution is obtained under the assumption that the squared gradient of the refractive index is constant in any cell of the mesh. The convergence rates of the proposed algorithms are estimated using the data obtained in the numerical experiments.     

Neuronet data processing algorithms in laser interferometer circuit at detecting object’s low oscillations and vibrations

A neuronet algorithm of interferometer data processing was developed for detection of microoscillations of the object. The algorithm parameters analysis has been carried out and sensitivity of the technique has been evaluated using experiment and simulation data. The article is published in the original.     

Expansion shock waves in numerical solutions of gasdynamic problems

The properties of diverse difference schemes of gas dynamics (nonconservative, conservative, and completely conservative) were studied with reference to the example of the classical problem of a piston. It is shown that in the case in which the piston is pulled out of a gas with the formation of an expansion wave, the application of schemes without a sufficient stability margin can lead to the appearance of discontinuities in the form of expansion shock waves. The explanation of this effect and possible ways for eliminating it are proposed     

Echelles: scalar, electromagnetic, and real-groove properties

For lack of alternatives, echelle-grating diffraction behavior has in the past been modeled on scalar theory, despite observations that indicate significant deviations. To resolve this difficulty a detailed experimental, theoretical, and numerical study is performed for several echelles that work at low (18-13), medium (35-55), high (84-140), and very-high (to 660) diffraction orders. Noticeable deviations from the scalar model were detected both experimentally and numerically, on the basis of electromagnetic theory: (1) the shift of the observed blaze position was shown to decrease with the wavelength-to-period ratio, and it tends to zero more rapidly than the decrease of the maximum width, so that the TE- and TM-plane responses tend to merge into each other; (2) cut-off effects (Rayleigh anomalies) were found to play a significant role for high groove angles, where passing-off orders are close to the blaze order. A possibility for evaluation of the blaze angle from angular, rather than from spectral, measure nts is discussed.Several reasons for the differences between real and ideal echelles (material-index deviations, profile deformations, and groove-angle errors) are analyzed, and their effects on the performance of echelles is studied.     

Theoretical and numerical investigations of the polarization properties of a lidar signal scattered by a set of oriented ice plates

The relationships for the backscattering cross sections are derived within the scope of the physical-optics method for the case in which polarized optical radiation interacts with an oriented semitransparent circular plate. Theoretical investigation of the depolarization relation is carried out for the polarization signal backscattered from a set of oriented circular plates. An algorithm is suggested for the calculation of the orientation angles of the plates in the scattering volume.     

Evolution of gypsy-group retrotransposons: phylogenetic analysis of domains included in the pol-polyprotein

Transposons of gypsy group are assigned to LTR-containing retrotransposons present in the genomes of invertebrates, fungi, and plants. In this work, a theoretical analysis of the potential products of ORFs of these retrotranposons was conducted. Alignments were obtained and trees of similarity were constructed for domains of the POL region. On the basis of the obtained data, two hypothetically monophyletic subgroups of transposons were distinguished within the framework of the gypsy group, settling the genomes of taxonomically related organisms (the subgroup of "true" gypsy of insects and the subgroup of gypsy-like transposons of plants and fungi). A number of peculiarities of the topology of these trees hypothetically indicate cases of genetic conversion and recombination of domains accompanying the evolution of this group. The amino acid substitution fixation rate was evaluated on the basis of comparison of sequences of the protein products of ORFs. Estimates of the time of divergence of subgroups of gypsy-group transposons are significantly less than estimates of the times of divergence of their host species. One explanation for this discrepancy might be the hypothesis of settlement by transposons of the genomes of isolated host species.     

Body thiamine level in acute alcohol psychoses

PURPOSE: To compare admission data and academic performances of medical students younger and older than 25, and to qualify older students' experiences and perceptions in medical school. METHOD: The authors reviewed 1988-1991 data for applications to the McGill University Faculty of Medicine. Data included GPAs and MCAT scores, as well as ratings for reference letters, autobiographical statements, and interviews. For those same years, the authors measured students' academic performances in the preclinical and clinical years. The authors compared the data by students' age: "younger" students, aged 17 to 24; and "older" students, aged 25 and above. All enrolled students took the Derogatis Stress Profile, and the older students participated in focus groups. RESULTS: The older applicants had lower GPAs and MCAT scores, but higher interview and reference letter ratings. For older accepted students, basic science course scores were lower than those of younger students, but clinical scores did not differ significantly between the groups. The two groups had similar stress levels, although older students tested lower in driven behavior, relaxation potential, attitude posture, and hostility. In focus groups, the older students spoke of learning style differences, loss of social support, and loss of professional identity. CONCLUSION: Different scores in admission criteria suggest that McGill uses different standards to select older medical students. Older students admitted under different criteria, however, do just as well as do younger students by their clinical years. A broad-based study of admission criteria and outcomes for the older student population is warranted.