A cooled amplifier with field effect transistors having a noise temperature of 1K

The noise characteristics of an amplifier based on KP307 FET's have been studied in the range 0.03 to 3.0 MHz. A minimum value of T_noise for an amplifier based on these transistors cooled to 78 K was obtained experimentally as 1 ± 1 K in the operating frequency range. Comparison is made with theory, and an estimate of the errors in the method of measurement used is given.     

On the quantum core of an optical vortex

An optical vortex is a line around which the phase increases by an integer multiple of 2π. It follows that the phase on the line itself is undefined and hence the field must have zero amplitude there. Berry and Dennis have suggested that this line of darkness is smoothed by a ‘quantum core’ with a radius proportional to ?^1/2 and have illustrated this idea by considering the competition between stimulated and spontaneous emission by an excited atom placed in the vicinity of the vortex. We show here that a similar phenomenon may be seen in absorption when the quantum state of motion of the absorbing atom is taken into consideration. There is, however, an underlying quantum singularity in which the absorption events for an atom centred on the vortex core can take place only if accompanied by a transfer of angular momentum to the atomic motion. The nature of this singularity relies on the evolution of an entangled state between the electronic and motional degrees of freedom of the trapped atom. We comment briefly on the effects of field quantisation on this quantum core of the optical vortex.     

On the static equilibrium of an elastic orthotropic medium with an arbitrarily oriented elliptical crack

We analyze the problem of the stress distribution in an elastic orthotropic medium with an arbitrarily oriented elliptical crack. To construct the problem solution, the Willis approach is used which is based on the triple Fourier transformation of spatial variables and Fourier-image of Green’s function for an infinite anisotropic space. The investigation results in special cases are compared with the data of other authors. The effect of the elliptical crack orientation in an orthotropic space on the distribution of the stress intensity factors along its contour is studied. __________ Translated from Problemy Prochnosti, No. 4, pp. 146–159, July–August, 2007.     

Compositional dependence of formation of an apatite layer on glass-ceramics in simulated physiological solution

In simulated physiological solution, an apatite layer is formed on the surface of apatite-containing glass-ceramics having the ability to bond to living bone. In this study, the influence of composition in the system CaO-P₂O₅-SiO₂-MgO, Al₂O₃ on apatite layer formation is investigated. On CaO-P₂O₅-SiO₂ glass-ceramics, an apatite layer was formed rapidly in simulated physiological solution. However, a solution containing an excess of Mg²⁺ prevented apatite layer formation. On glass-ceramics containing MgO, the amount of apatite formed on the surface decreased. An apatite layer was not formed on glass-ceramics containing Al₂O₃. The prevention of apatite layer formation on glass-ceramics containing MgO is attributed to an increase of Mg²⁺ concentration in the solution. It is thought that glass-ceramics containing Al₂O₃ form are Al₂O₃-rich layer, and that this layer prevents the formation of an apatite layer.     

A simple mesh deformation technique for fluid–structure interaction based on a submesh approach

This paper describes an improvement in techniques currently used for mesh deformations in fluid–structure calculations in which large body motions are encountered. The proposed approach moving submesh approach (MSA) is based on the assumption of a pseudo-material deformation applied on a triangular coarse mesh to significantly reduce the CPU time. The computation mesh is then updated using an interpolation technique similar to the finite element method. This method may be applied on structured as well as on unstructured meshes. An extension to complex boundaries undergoing large rigid-body motions is proposed combining the MSA and an encapsulation box. The influence of the coarse mesh on the quality mesh is discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

On the analysis of stress state in elliptical tensile neck

The tensile necking in cylindrical and plate specimens is described based on a modification of the Bridgman theory for an isotropic case. A method of determination of principal stress trajectories in an elliptical tensile neck is put forward, which involves transformation of the initial coordinate grid by means of conformal mappings. Expressions are derived for the principal stresses and the radius of curvature of stress trajectories. Based on assumptions that depart from the Bridgman theory, an approximate analytical solution is obtained for the stress distribution in specimens with an elliptical cross-section. The new solution belongs to the one-parameter family of solutions which includes the Bridgman’s and Davidenkov–Spiridonova’s solutions. Translated from Problemy Prochnosti, No. 4, pp. 19–28, July–August, 2009.     

A new mass lumping scheme for the mixed hybrid finite element method

Diffusion‐type partial differential equation is a common mathematical model in physics. Solved by mixed finite elements, it leads to a system matrix which is not always an M‐matrix. Therefore, the numerical solution may exhibit unphysical results due to oscillations. The criterion necessary to obtain an M‐matrix is discussed in details for triangular, rectangular and tetrahedral elements. It is shown that the system matrix is never an M‐matrix for rectangular elements and can be an M‐matrix for triangular an tetrahedral elements if criteria on the element's shape and on the time step length are fulfilled. A new mass lumping scheme is developed which leads to a less restrictive criterion: the discretization must be weakly acute (all angles less than π/2) and there is no constraint on the time step length. The lumped formulation of mixed hybrid finite element can be applied not only to triangular meshes but also to more general shape elements in two and three dimensions. Numerical experiments show that, compared to the standard mixed hybrid formulation, the lumping scheme avoids (or strongly reduce) oscillations and does not create additional numerical errors. Copyright © 2006 John Wiley & Sons, Ltd.     

The networking university

This article examines the subject of research collaboration, and elaborates on this subject on an institutional rather than an individual level. An empirical case-study is presented; the research collaboration of Umeå University in Sweden, during the period 1991–1993 is investigated. Institutional co-authorships based on the addresses of the departments are used as an indicator of this collaboration. The results are separated into three levels: the local level, the national level, and the international level. It is obvious that the research collaboration is most extensive. Finally the university's collaboration is discussed and a scheme is proposed with the purpose to understand research collaboration in a social as well as a cognitive context. The guiding terms here are access, visibility and attractiveness.     

Determining the effective fractal dimension of nanodimensional coatings with the aid of magnetic field

Method of the short-range potential that was originally constructed for a three-dimensional space is generalized to Euclidean spaces of arbitrary (including fractional) dimensions. Based on this method, a transcendental equation is derived for the electron binding energy in a superposition of the field of an n-dimensional δ-well and an external magnetic field. The possibility of experimental determination of the fractal dimension of a nanostructured coating using the character of the dependence of its surface work function on the magnetic field strength measured in an atomic force microscope is considered.     

Tangential friction on the wall of a container with a spherical charge and fluid motion through the charge

Viscous tangential stress on the interior wall of a container with a spherical charge, that develops in filtration of an incompressible fluid through the charge is investigated. Based on an analysis of an experimental dependence of the dimensionless stress on the Reynolds number, two critical Reynolds numbers are determined: the first number corresponds to the beginning of an abrupt drop in the stress, and the second number, to its reaching a regime that is self-similar in velocity. Comparison with the theory permits interpretation of the effects of pseudoturbulence and turbulence, respectively. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 1, pp. 50–54, January–February, 1997.     

A fast algorithm for expansion over spherical harmonics

Algorithms for expansion over spherical harmonics are often used in electrostatic field calculation, calculation of the density functions in quantum chemistry and calculation of molecular surfaces. It usually includes expansion over spherical harmonics of degrees to several dozens. The usual method is to use an integration method over some grid on the unit sphere and in fact is a multiplication of the matrix of values of spherical harmonics in the grid points by a vector of values of the expanding function in the set of points. This algorithm executes O(NL²) operations whereN is the number of the grid points andL is the maximal degree of the spherical harmonics involved. We provide an algorithm of complexity O(NLlog² L) for multiplication of the matrix of values of spherical harmonics in points of an arbitrary grid on the unit sphere. The algorithm is based on interrelation between spherical harmonics and Legendre polynomials and on a fast algorithm for expansion over Legendre polynomials.     

Stress intensity factor analyses of interface cracks between dissimilar anisotropic materials using the finite element method

Delamination along an interface between dissimilar materials is the primary cause of failure in microstructures like electronic packages, micro-electro-mechanical systems (MEMS), and so on. Fracture mechanics is a powerful tool for the evaluation of delamination. However, many materials used in microstructures such as composite materials and single crystals are anisotropic materials. Stress intensity factors of an interface crack between dissimilar anisotropic materials, which were proposed by Hwu, are useful for evaluating the reliability of microstructures. However, numerical methods that can analyze the stress intensity factors of an interface crack between anisotropic materials have not been developed. We propose herein a new numerical method for the analysis of an interface crack between dissimilar anisotropic materials. The stress intensity factors of an interface crack are based on the generalized plane strain condition. The energy release rate is obtained by the virtual crack extension method in conjunction with the finite element method for the generalized plane strain condition. The energy release rate is separated into individual modes of the stress intensity factors K_I, K_II, and K_III, using the principal of superposition. The target problem to be solved is superposed on the asymptotic solution of displacement in the vicinity of an interface crack tip, which is described using the Stroh formalism. Analyses of the stress intensity factors of center interface cracks between semi-infinite dissimilar anisotropic media subjected to concentrated self-balanced loads on the center of crack surfaces and to uniform loads are demonstrated. The present method accurately provides mode-separated stress intensity factors using relatively coarse meshes for the finite element method.     

Observation of metal on Si(0 0 1) by STM/STS and its consideration based on the first principles calculations

Local density of states (LDOS) is obtained by the first principles calculation based on the density functional theory on the Si(0 0 1)2 × 1 surface and on the surface with an Al dimer. At an Al dimer, LDOS has a high intensity in the conduction band region, which cannot be seen on the Si(0 0 1)2 × 1 surface. This tendency is observed in STS measurements as well. The possibility for a microelementary analysis is presented by applying this method to other metal atoms on the Si surface. Furthermore, it is pointed out that STS measurements should be always performed at the same tip-sample separation to obtain reproducible STS spectrums.     

On the aerodynamic forces acting on an airfoil section with a penetrable region

Strict formulas for calculating the aerodynamic forces acting on a section of an airfoil with a penetrable region, through which a part of an external flow of an ideal incompressible fluid is sucked in, and on an airfoil section, from which a jet stream is blown out, in the case where these sections are in a steady-state flow without separation have been derived. Formulas for calculating the power expended for the realization of suction of a part of an external flow through an airfoil section and simultaneous blow-out of a jet stream from it are proposed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 2, pp. 126–130, March–April, 2006.     

Effects of atmospheric conditions on mechanical mixed layer formation and wear behaviour in A356–15 vol.-%SiCp composite

Abstract

Dry sliding wear tests have been carried out on A356–15 vol.-%SiC_p metal matrix composite to evaluate the effects of argon and oxygen atmospheres on mechanical mixed layer (MML) formation and wear behaviour. The wear rate of the composite under an oxygen atmosphere is higher by one order of magnitude than that under an argon atmosphere. The formation of the MML depends strongly on the atmospheric conditions. The MML formed under argon is found to be more stable and results in a lower wear rate within the parametric regime, compared with that formed under an oxygen atmosphere. The mechanism of material removal under an argon atmosphere is by delamination. Under the oxygen atmosphere the MML is highly unstable, and material removal is by severe two and three body abrasion at low load and low sliding velocity. At high load and high sliding velocity, the material removal mechanism is by delamination.     

Effect of annealing and hydrogen plasma treatment on the luminescence of hydrothermally grown bulk ZnO

The optical properties of hydrothermally grown bulk ZnO is investigated by low temperature photoluminescence (PL) spectroscopy. The effects of annealing in an argon atmosphere, as well as the influence of hydrogen plasma exposure, on the PL of as-grown material are studied. The 11 K PL spectrum of the as-grown ZnO shows different excitonic lines in the NBE region: several bound exciton lines are clearly visible. The origin of these bound excitons is discussed, as well as the influence of annealing on these lines. A shift in the deep level emission (DLE) is also observed with an increase in annealing temperature: a red shift is detected when the annealing temperature is increased up to 650 °C, while a subsequent blue shift is observed upon an increase in the annealing temperature. The involvement of Li and Cu in this phenomenon will be discussed and the effect of hydrogen on this DLE will also be studied.     

提升上海国际航运中心综合竞争力的策略研究

在分析东亚各主要港口的竞争态势与我国的实际国情和国际物流发展趋势的基础上,从资源、组织、运行等方面提出了富有建设意义和前瞻性的建议和对策措施：以“轴辐”式立体网络发挥我国的腹地优势,积聚上海周边各港口资源以整合各方力量,以新型组织体系实现港口的一体化管理,以灵活的政策平台快速形成比较竞争优势,以兼容稳定的信息平台提升基于时间的竞争优势,以智能型运行机制赢得持续竞争优势等。除此之外,还必须强化人力资源管理、建立一体化现场监控调度系统,最终形成上海国际航运中心的特色,真正成为世界枢纽港。     

On the Cholesky algorithm with shifts for the eigensolution of real symmetric matrices

This paper presented an efficient computational algorithm for the eigensolution for real symmetric positive definite matrices. The algorithm is based on the Cholesky decomposition and will be referred to as the Cholesky algorithm. The efficiency of the algorithm is due to the combination of several iteration cycles into one, as well as to a judicious choice of shifts. The latter is accomplished by combining an approach based on Sturm's theorem for the initial stages of the process with another approach based on Gersch-gorin's theorem for the subsequent and final stages of the process. The efficiency of the proposed algorithm was tested against that of the QL method and was found to be superior. Moreover, its superiority increases with an increase in the order of the eigenvalue problem. For an eigenvalue problem of order 100, the proposed algorithm produced a solution in about one-half the time required by the QL method.     

Constitutive equations for the nonlinear elastic response of rubbers

Summary A constitutive model is derived for the elastic behavior of rubbers at three-dimensional deformations with finite strains. An elastomer is thought of as an incompressible network of flexible chains bridged by permanent junctions that move affinely with the bulk medium. The constraints imposed by surrounding macromolecules on configurations of an individual chain are introduced by combining the Flory–Erman and Erman–Monnerie approaches. To describe inter-chain interactions in a tractable way, the conventional picture of a tube where a chain is confined is replaced by geometrical restrictions on the positions of its ends and center of mass. The constraints on the chain ends are formulated within the traditional Flory concept, whereas those on the position of center of mass are described following the Ronca–Allegra scenario. Stress–strain relations for a network of constrained chains are derived by using the laws of thermodynamics. The constitutive equations involve four adjustable parameters with transparent physical meaning. The material constants are found by fitting experimental data on elastomers at uniaxial and equi-biaxial tensions and pure shear. It is demonstrated that (i) the model provides an acceptable prediction of stresses in a test with one deformation mode, when its parameters are found by matching observations in an experiment with another mode, and (ii) material constants are affected by chemical composition of elastomers in a physically plausible way.     

Rotation of the wavefront of an optical vortex in free space

It is shown experimentally and theoretically that when an optical vortex propagates in free space, its wavefront rotates through an angle numerically equal to the Gouy phase. It is found that both the energy maximum of the optical vortex light flux and the amplitude zero of the perturbed optical vortex field propagate along the ray surface. It is shown that the ray surface, which is a consequence of the relativistic constraints on the beam group velocity, forms an unparted hyperboloid of revolution and has various properties: 1) the circulation of the Poynting vector on the surface does not depend on the longitudinal coordinate z; 2) the evolution of the light flux and a pure screw dislocation takes place along straight lines of this surface; 3) the Poynting vector on the ray surface is always perpendicular to the wavefront surface. Pis’ma Zh. Tekh. Fiz. 25, 87–94 (March 12, 1999)