Thermodynamics of a gas-coal massif and a nonuniform gas distribution in a coal bed

The thermodynamic potential of a coal bed is calculated, and the gas pressure in the crack-pore system of the bed is shown to be nonuniformly distributed. This pressure is high in “weak“ regions in the bed, i.e., at the sites of a high porosity, low elastic moduli, and low solubility, as well as at high bearing pressure sites. All these phenomena are caused by the elastic interaction of a gas with a solid-state coal frame and the possibility of the formation of a solid solution in the coal. The relative increase in the gas pressure at these sites is estimated. The results obtained can be used to predict the localization of explosive and outburst regions in coal beds.     

Coupling effects on a cantilever subjected to a follower force

In this investigation a solution methodology is presented for studying the stability of a uniform cantilever having a translational and rotational spring at its support, carrying two concentrated masses, one at the support and the other at its tip, and subjected to a follower compressive force at its free end. The analysis is based on Timoshenko's beam theory by considering the cantilever as a continuous elastic system. The coupling effects on the flutter load are fully assessed for a variety of parameters such as translational and rotational springs at the support, translational and rotational inertia of the concentrated masses, and cross-sectional shape, as well as transverse shear deformation and rotatory inertia of the mass of the column.     

Study of a bulk-Micromegas with a resistive anode

The design and performance of a Micromegas with a resistive anode are presented in this paper. A thin resistive sheet with volume resistivity of 10¹² Ω·m cm is glued onto the readout electrode surface and its performance is investigated by using a ⁵⁵Fe X-ray radioactive source in the operation gas of argon and isobutene mixtures (Ar/Iso=95/5). The gas gain at different mesh high voltage, counting rate and working time are given. Energy spectra at different working voltages are measured and the results are discussed. We have oberved that a Micromegas with a resistive anode can be operated at higer gain than a standard Micromegas without sparks.     

Casimir Interaction between a plate and a cylinder

We find the exact Casimir force between a plate and a cylinder, a geometry intermediate between parallel plates, where the force is known exactly, and the plate sphere, where it is known at large separations. The force has an unexpectedly weak decay approximately L/[H3 ln(H/R)] at large plate-cylinder separations H (L and R are the cylinder length and radius), due to transverse magnetic modes. Path integral quantization with a partial wave expansion additionally gives a qualitative difference for the density of states of electric and magnetic modes, and corrections at finite temperatures.     

Terahertz spectroscopy with a Josephson oscillator and a SINIS bolometer

The voltage response of a thin-film normal-metal hot-electron bolometer based on a SINIS (superconductor-insulator-normal metal-insulator-superconductor) structure to the radiation of a high-temperature Josephson junction in the terahertz frequency region was measured. Bolometers were integrated with planar log-periodic and double-dipole antennas, and Josephson junctions were integrated with log-periodic antennas. Measurements showed that the Josephson junction at a temperature of 260 mK was overheated by the transport current, so that its electron temperature exceeded 3 K at a bias voltage of 1 mV. The maximum response of a bolometer with a double-dipole antenna was observed at a frequency of 300 GHz, which agreed well with the calculated value. The Josephson radiation was observed at frequencies up to 1.7 THz. The voltage response of a bolometer reached 4×10⁸ V/W, and the total noise-equivalent power reached 1.5×10^?17 W/Hz^1/2.     

Formation of ball streamers at a subnanosecond breakdown of gases at a high pressure in a nonuniform electric field

The formation of a diffuse discharge plasma at a subnanosecond breakdown of a “cone–plane” gap filled with air, nitrogen, methane, hydrogen, argon, neon, and helium at various pressures has been studied. Nanosecond negative and positive voltage pulses have been applied to the conical electrode. The experimental data on the dynamics of plasma glow at the stage of formation and propagation of a streamer have been obtained with intensified charge-coupled device and streak cameras. It has been found that the formation of ball streamers is observed in all gases and at both polarities. A supershort avalanche electron beam has been detected behind the flat foil electrode in a wide range of pressures in the case of a negatively charged conical electrode. A mechanism of the formation of streamers at breakdown of various gases at high overvoltages has been discussed.     

Dynamics of a cavitation bubble near a solid wall

The cavitation bubble dynamics, the variation of pressure and velocity fields of the surrounding liquid in the process of the bubble axisymmetric compression near a planar solid wall are considered. It is assumed that the liquid is at rest at the initial moment of time, and the bubble has a spheroidal shape. The liquid is assumed inviscid and incompressible, its motion being potential. The bubble surface deformation and the liquid velocity on the surface are computed by the Euler scheme using the boundary element method until the moment of the collision of some parts of the bubble surface with one another. The influence of the distance of the bubble from the wall and its initial nonsphericity on the liquid pressure and velocity fields, the bubble shape, and the pressure inside the bubble at the end of the time interval under consideration are studied. The maximum pressure in liquid is shown to realize at the bottom of the cumulative jet arising at the bubble collapse with direction to the wall. In the upper part of this jet, the velocity and pressure are practically constant, and the pressure in the jet is approximately equal to the pressure in the bubble.     

The flow of a model suspension fluid past a sphere

The paper is concerned with the flow of a dilute suspension of monosized spheroids past a sphere placed at the centerline of a cylindrical tube. The suspension is modeled by the transversely isotropic fluid model and the numerical solution is obtained by a time-dependent boundary element method. No steady-state solution to the problem was found. However, the amplitude of the long-time oscillations in the drag force on the sphere is only about 1% of its mean value at a solid volume concentration of 0.01 and an aspect ratio of 10 for the spheroids. The initial orientation of the microstructure has a small effect on the drag force: it can give rise to a ±1 % variation in the drag force at large time.     

Response of a Bloch oscillator to a THz-field

In this paper we report on the observation of response of a Bloch oscillator at room temperature to a THz-field of a frequency larger than the Bloch frequency. The oscillator consisted of a semiconductor superlattice structure, with an applied dc voltage giving rise to a dc electron drift current. Submitting the oscillator to a field at a frequency of 3.3 THz caused a sizeable reduction of the current; the THz-field was generated by use of intense THz-radiation pulses focused on an antenna coupled to the superlattice. We attribute the THz-field induced reduction of the current to a frequency modulation of the Bloch oscillations of electrons at the frequency of the THz-field, leading to reduction of the electron drift velocity and, consequently, of the current.     

Quantum diffusion of a particle in a periodic potential with ohmic dissipation

The motion of a particle in a periodic potential is studied at low temperatures where transitions between the potential wells are caused by quantum tunnelling. The theory accounts for the dissipative interaction with an environment which for a wide range of parameters leads to incoherent tunnelling at a rate with a nonanalytic temperature dependence. The influence of an external force is determined and a nonanalytic response is found at T = 0. The case of a biased double-well system is treated too.     

Isolation effect of a dynamic damper and a trench on ground vibration caused by a construction machine

In this study, a simple isolation method of ground vibration is proposed by using a dynamic damper and a trench, which is feasible for temporary use during a construction period. The ground is modeled as a two-dimensional plane model. Periodic impulsive excitation acts at one point on the ground surface, and the vibrations are measured at several evaluation points on the ground surface. A simple dynamic damper composed of a weight and a restoring element is set up at the ground surface or in the shallow trench, and the vibration isolation effect is examined. The simulation shows that the ground vibration can be isolated when the dynamic damper is set near the excitation point on the ground surface, and setting the dynamic damper in a shallow trench has almost the same isolation effect as that in a deep trench. The results indicate that the proposed isolation method is feasible for actual application.     

平面上方分层小球的光散射计算

本文基于扩展的Mie理论方法求解平面上方分层小球的散射问题。通过建立小球和平面的模型,解决小球和平面的边界条件问题,并利用矢量波函数展开的方法求得了散射场。强调了小球与表面的相互作用。利用Mie理论方法得到了分层小球的散射场系数,通过计算平面上小球的散射模型,得到了平面上分层小球的散射场分布。结论给出了分层介质小球的微分散射截面图。     

Geometric phase of a central qubit coupled to a spin chain in a thermal equilibrium state

The geometric phase of a central qubit coupling to the surrounding XY chain in a transverse field at finite temperature is studied in this Letter. An explicit analytical expression of the geometric phase for coupled qubit is obtained in the weak coupling limit when the surrounding spin chain is in a thermal equilibrium state. It is shown that the GP displays dramatic change around the quantum phase transition points of the spin chain at zero and a finite range of temperature by numerical analysis. The result reveals that the GP can be used as a tool to detect QPT when the spin chain system is at finite temperature.     

Snaking states on a cylindrical surface in a perpendicular magnetic field

We calculate electronic states on a closed cylindrical surface as a model of a core-shell nanowire. The length of the cylinder can be infinite or finite. We define cardinal points on the circumference of the cylinder and consider a spatially uniform magnetic field perpendicular to the cylinder axis, in the direction South-North. The orbital motion of the electrons depends on the radial component of the field which is nonuniform around the circumference: it is equal to the total field at North and South, but vanishes at the West and East sides. For a strong field, when the magnetic length is comparable to the radius of the cylinder, the electronic states at North and South become localized cyclotron orbits, whereas at East and West the states become long and narrow snaking orbits propagating along the cylinder. The energy of the cyclotron states increases with the magnetic field whereas the energy of the snaking states is stable. Consequently, at high magnetic fields the electron density vanishes at North and South and concentrates at East and West. We include spin-orbit interaction with linear Rashba and Dresselhaus models. For a cylinder of finite length the Dresselhaus interaction produces an axial twist of the charge density relative to the center of the wire, which may be amplified in the presence of the Rashba interaction.     

Relaxation of a dislocation in a nanocrystallite

Experimental studies of dislocations in nanoparticles are just beginning. The corresponding theoretical models are still lacking. In this context, the author analyzes relaxation of a dislocation in a nanoparticle. Mechanistically, this process is considered to occur primarily via dislocation drift induced by the stress-related image forces. Elementary dislocation displacements include the formation of a kink at one of the sides of the dislocation line, its diffusion along this line, and annihilation at the opposite side. For this mechanism, the dependence of the time of dislocation disappearance on the nanoparticle size has been identified.     

Nonlinear dynamics of a cavitation bubble pair near a rigid boundary in a standing ultrasonic wave field

The dynamics of a micrometer-sized bubble pair in water near a rigid boundary under standing ultrasonic wave excitation is investigated in this study. The viscous effect in the boundary layer at the air-water interface is considered following the viscous correction model. The evolution of the bubble surface at the collapsing stage of the bubble pair is presented for different parameter sets. The field pressure near the rigid boundary, which is induced by the oscillating bubble pair, and the high-speed water jet at the collapse stage, form the main focus of the analysis. This reveals that a horizontal configuration of the bubble pair retards the strength of the bubble jet towards the boundary, whilst a vertical configuration, especially with differently-sized bubbles, can enhance the bubble collapse. This study may help to understand the interaction of multiple bubbles in an acoustic field and its application to surface cleaning.     

Theoretical analysis of the formation of a layered ice structure at the surface of a plate placed in a flow of a supercooled water aerosol

The motion of the front of crystallization and the growth of a film at the surface of a plate are analyzed in the case of a laminar and in the case of a turbulent flow mode. Conditions are determined under which there occurs a transition from a matt inhomogeneous structure to a transparent homogeneous structure of ice. It is shown that, for a film to be steadily preserved at the plate surface, the film thickness must be larger than a critical equilibrium-thickness value h _b.c, in which case a transparent homogeneous structure of ice is formed. Otherwise, the film at the plate surface is unstable and disappears in the course of time. The icing of aircrafts is the most important application of the results obtained in this study.     

Resolution enhancement at a large convergence angle by a delta corrector with a CFEG in a low-accelerating-voltage STEM

Resolution reduction by a diffraction limit becomes severe with an increase in the wavelength of an electron at a relatively low accelerating voltage. For maintaining atomic resolution at a low accelerating voltage, a larger convergence angle with aberration correction is required. The developed aberration corrector, which compensates for higher-order aberration, can expand the uniform phase angle. Sub-angstrom imaging of a Ge [1 1 2] specimen with a narrow energy spread obtained by a cold field emission gun at 60 kV was performed using the aberration corrector. We achieved a resolution of 82 pm for a Ge–Ge dumbbell structure image by high angle annular dark-field imaging.     

Quantum spin model of a cubic ferromagnet in a magnetic field

The zero temperature phase diagram of a one-dimensional ferromagnet with cubic single ion anisotropy in an external magnetic field is studied. The mean-field approximation and the density-matrix renormalization group method are applied. Two phases at finite magnetic fields are identified: a canted phase with spontaneously broken symmetry and a phase with magnetization along the magnetic field. Both methods predict that the canted phase exists even for the single-ion anisotropy strong enough to destroy the magnetic order at zero magnetic field. In contrast to the mean-field theory, the density-matrix renormalization group predicts a reentrant behavior for the model. The character of the phase transition at finite magnetic field has also been considered and the critical index has been found. Received 9 May 2000 and Received in final form 5 July 2000     

微液滴在超疏水表面的受迫振动及其接触线的固着-移动转变

利用高速摄影技术对超疏水表面液滴振动的动态行为进行观测,研究液滴在不同频率下的振动特性. 实验发现,液滴的共振频率满足Rayleigh方程,微液滴在超疏水表面具有自由液滴的振动性质. 在80–200 Hz的驱动频率范围内,接触线出现了明显的固着-移动现象,液滴的振动频率是驱动频率的一半,液滴振动时的形变较大. 当驱动频率大于200 Hz时,接触线基本固着,液滴的振动频率近似等于驱动频率,液滴共振时的形态边缘始终有节点存在. 分析表明,液滴对外界驱动的不同响应与接触线的振荡行为和变形程度密切相关 关键词： 超疏水表面 受迫振动 共振 接触线