Effect of laser irradiation on the electrical properties of dysprosium doped LiCo-Ferrite

The effect of laser irradiation on the electrical properties of Li_0.5+z Co _z Dy _x Fe_2.5?2z?x O₄ ferrite (0.0 ≤ x ≤ 0.2, z = 0.1) has been studied in the temperature range 300 K ≤ T ≤ 750 K at frequencies of 10 kHz?5 MHz, using a LIMO-IR laser diode, at a wavelength of 808 nm. It was found that laser irradiation increases the polarization, the resistivity and the paramagnetic region. As the result of electronic rearrangement and lattice defects, small polorons and clusters were created. The doping of LiCo-Ferrite by Dy³⁺ increases both the AC and DC resistance of the investigated material. The variation of the AC and DC resistance with the Dy-content (x) obeys the following correlations R _ac/100 = 50x ²+4x+0.005 and R _dc/1000 = 31x ²+0.099x+0.09, respectively. A peculiar behaviour was obtained for the sample with Dy-content x = 0.075, as the resistance notably decreases. The applicable result is that laser irradiation increases the resistance of LiCo-ferrite by about 17% while its doping by dysprosium at x = 0.15 increases the resistance by about 23%. Its value is nearly stable for the temperature range from 340 to 480 K.     

Preparation and thermoelectric power of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and a stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ have been prepared and characterized by X-ray powder diffraction analysis. At room temperature the carrier concentration values are n?=?1.1?×?10¹⁹?cm^?3 for Bi₂Se₃ and n?=?0.53?×?10¹⁹?cm^?3 for SnBi₄Se₇. The thermoelectric power has been measured over the temperature range 90–420?K. The thermoelectric power of Bi₂Se₃ is higher than that for SnBi₄Se₇, which shows that the Sn impurity has an acceptor character. Therefore, doping Bi₂Se₃ with tin atoms does not improve thermoelectric properties of this material, due to decrease the value of the power factor σS ². Transport properties of the studied polycrystalline samples are characterized by a mixed transport mechanism of free carriers. It is necessary to add more than one Sn atom to the Bi₂Se₃ compound in order to suppress the electron concentration by one electron. Such behaviour of the dopant is explained by the formation of various structural defects. Besides the dominant substitutional defect, Sn_Bi, tin atoms also form uncharged defects, corresponding to seven-layer lamellae of the composition Se–Bi–Se–Sn–Se–Bi–Se which corresponds to the structure of the SnBi₂Se₄.     

Optimal control of infinite-dimensional uncertain systems

In this paper, we consider a minimax problem of optimal control for a class of strongly nonlinear uncertain evolution equations on a Banach space. We prove the existence of optimal controls. A nontrivial example of a class of systems governed by a nonlinear partial differential equation with uncertain spatial parameters is presented for illustration.This work was supported in part by the National Science and Engineering Research Council of Canada under Grant No. A7109 and The Engineering Faculty Development Fund, University of Ottawa.The authors would like to thank the anonymous reviewers for their valuable comments and suggestions.     

Vibration analysis of quasi-symmetric structures

An efficient computational procedure is presented for the free vibration analysis of structures with unsymmetric geometry. The procedure is based on approximating the unsymmetric vibrational response of the structure by a linear combination of a few symmetric and antisymmetric modes (global approximation vectors), each obtained using approximately half the degrees of freedom of the original model. The three key elements of the procedure are: (a) use of mixed finite element models having independent shape functions for the internal forces (stress resultants) and generalized displacements, with the internal forces allowed to be discontinuous at interelement boundaries, (b) operator splitting, or additive decomposition of the different arrays in the governing finite element equations to delineate the contributions to the symmetric and antisymmetric response vectors, and (c) use of a reduction method through successive application of the finite element method and the classical Bubnov-Galerkin technique. The finite element method is first used to generate a few symmetric and antisymmetric global approximation response vectors. Then, the classical Bubnov-Galerkin technique is used to substantially reduce the size of the eigenvalue problem.

An initial set of global approximation vectors is selected to be a few symmetric and antisymmetric eigenvectors, and their various-order derivatives with respect to a tracing parameter identifying all the correction terms to the symmetric (and antisymmetric) eigenvectors. A modified (improved) set of approximation vectors is obtained by using the inverse iteration procedure. The effectiveness of the proposed procedure is demonstrated by means of a numerical example.     

On the cosmological effects of thermal masses in the era 1011K⩾T⩾5xl09K

The cosmological effects of thermal masses of particles (masses induced via interactions at nonzero temperature) as well as ordinary masses are studied. These effects are shown to be negligible for photons. For electrons, however, they modify the dependence of the universe's radiusR and the timet on temperature.     

Fast Neutron Albedo Calculations for a Concrete Shield with Different Curvatures

The O5R Monte Carlo neutron transport Code had been used to calculate the neutron albedo for neutrons reflected from plane and curved concrete shields. The present calculations were performed to investigate the fast-neutron albedo in case of ordinary concrete shield, in order to perform comparative studies with the case of neutron reflection against a flat wall. The calculations were performed for three different neutron source energies of 1 MeV, 5 MeV and 15 MeV and at neutron incident angles of 5°, 30°, 45°, 60° and 90° and for surfaces with different curvatures (flat, 100, 50, 20 and 5 cm). The results obtained reveal that there will be an appreciable error on using the flat wall albedo value in the case of duct penetration calculations. The error was assumed to be due to the neglection of the curvature effect as well as to the improper choice of the neutron incident angle.     

Capillary instability of an annular liquid jet surrounding a solid cylinder

Summary The capillary instability of an annular liquid jet surrounding a solid cylinder is presented. A general dispersion equation is derived based on the linear-perturbation technique. The instability as well as stability characteristics of that model are identified analytically and confirmed numerically. The model is unstable only to the axisymmetric perturbation whose wavelengths are longer than the circumference of the liquid jet, while it is stable to all other perturbations. The maximum temporal amplification values prevailing on such a model are fairly lower than those of the full liquid jet. The thicker the solid cylinder, whether it is regular or irregular, the larger is its stabilizing effect.

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Riassunto Si presenta l’instabilità capillare di un getto anulare di liquido che circonda un cilindro solido. Si deduce un’equazione generale di dispersione basata sulla tecnica di perturbazione lineare. Le caratteristiche d’instabilità cosí come quelle di stabilità di quel modello si identificano analiticamente e sono confermate numericamente. Il modello è instabile solo rispetto alla perturbazione assisimmetrica, le cui lunghezze d’onda sono piú lunghe della circonferenza del getto di liquido, mentre è stabile rispetto a tutte le altre perturbazioni. I valori massimi dell’amplificazione temporale che prevalgono in tale modello sono molto piú bassi di quelli del pieno flusso di liquido. Piú spesso è il cilindro solido sia esso regolare o irregolare, maggiore il suo effetto stabilizzante.

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Резюме Анализируется капиллярная неустойчивость кольцевой жидкой струи. окружающей твердьй цилиндр. Выводится общее дисперсионное уравнение. основанное на линейной пертурбационной технике. Неустойчивость, а также характеристики устойчивости в этой модели определяются аналитически и подтверждаются численно. Предложенная модель оказывается неустойчнвой только для осесимметричных возмушений, длины волн которых превышают окружность жидкой струи, и является устойчивой для всех других возмущений. Максимальные значения временного увеличения в такой модели оказываются довольно низкими. Стабилизирующий эффект увеличивается с толщиной твердого цилиндра.

     

Artifact-free black-blood cine cardiac imaging in a single breath-hold

Fast imaging using the STimulated Echo Acquisition Mode (STEAM) sequence can produce cine images of the heart with black-blood contrast. Nevertheless, correction of deformation-related artifacts is required in order to maintain myocardial signal throughout the cardiac cycle. Recent work by our group has eliminated this artifact by combining two STEAM sequences acquired with two different demodulation gradients. Unfortunately, these two STEAM sequences were acquired on two separate breath-holds; thus, scan time doubled. In this work, we present a technique to reduce the total scan time by one half, without sacrificing image quality. The technique is based on interleaving two demodulations within one acquisition in order to obtain quality cine images of the heart in a single breath-hold. The technique was tested on animal models and human subjects, and the impact of interleaved acquisition on image quality was studied using quantitative and qualitative measures.     

MHD Flow and Heat Transfer Characteristics in a Casson Liquid Film Towards an Unsteady Stretching Sheet with Temperature-Dependent Thermal Conductivity

Theoretical and numerical outcomes of the non-Newtonian Casson liquid thin film fluid flow owing to an unsteady stretching sheet which exposed to a magnetic field, Ohmic heating and slip velocity phenomena is reported here. The non-Newtonian thermal conductivity is imposed and treated as it vary with temperature. The nonlinear partial differential equations governing the non-Newtonian Casson thin film fluid are simplified into a group of highly nonlinear ordinary differential equations by using an adequate dimensionless transformations. With this in mind, the numerical solutions for the ordinary conservation equations are found using an accurate shooting iteration technique together with the Runge-Kutta algorithm. The lineaments of the thin film flow and the heat transfer characteristics for the pertinent parameters are discussed through graphs. The results obtained here detect many concern for the local Nusselt number and the local skin-friction coefficient in which they may be beneficial for the material processing industries. Furthermore, in some special conditions, the present problem has an excellent agreement with previously published work.