Results 1 - 10 of 13058
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[en] The role of hybrid pairing originating from electron-phonon interaction has been investigated for a two band (f and d) superconductor at T = O. This type of pairing seems to be less important than f-f pairing for the case of half-filled f-band when the latter type of Cooper pairs plays the dominating role. On the other hand, for the case of rather filled (or rather empty) f-band, the superconducting properties of the system are mainly determined by the formation of conduction electrons d-d pairs. (author)
[en] We discuss theoretically the properties of an electromechanical oscillating system whose operation is based upon the cyclic conservative conversion between gravitational potential, kinetic and magnetic energies. The system consists of a superconducting coil subjected to a constant external force and to magnetic fields. The coil oscillates and has induced in it a rectified electrical current whose magnitude may reach hundreds of amperes. The design differs from that of most conventional superconductor machines since the motion is linear (and practically unnoticeable depending on frequency) rather than rotatory and it does not involve high speeds. Furthermore, there is no need for an external electrical power source to start up the system. We also show that the losses for such a system can be made extremely small for certain operational conditions, so that by reaching and keeping resonance the system's main application should be in the generation and storage of electromagnetic energy. (rapid communication)
[en] The criteria for the stabilization of a condensed Wigner phase are re-examined for a low-density free-electron gas (jellium) in a uniform magnetic field. By a new calculation of the Coulomb energy it is shown that below a critical density the lowest energy state has electrons in cigar-shaped charge distributions arranged on an elongated body-centred tetragonal lattice. The critical densities are computed as functions of magnetic-field strength for free electrons in astrophysical situations and for electrons of low effective mass in semiconductors. In the latter case, the results can be used to give a satisfactory interpretation of experimental results in heavily compensated InSb. (author)
[en] Calculations of ground-state and excited-state properties of materials have been one of the major goals of condensed matter physics. Ground-state properties of solids have been extensively investigated for several decades within the standard density functional theory. Excited-state properties, on the other hand, were relatively unexplored in ab initio calculations until a decade ago. The most suitable approach up to now for studying excited-state properties of extended systems is the Green function method. To calculate the Green function one requires the self-energy operator which is non-local and energy dependent. In this article we describe the GW approximation which has turned out to be a fruitful approximation to the self-energy. The Green function theory, numerical methods for carrying out the self-energy calculations, simplified schemes, and applications to various systems are described. Self-consistency issue and new developments beyond the GW approximation are also discussed as well as the success and shortcomings of the GW approximation. (author)
[en] Comparison of analysis results for regularities of a metallized condensed substance ignition by a single small-sized particle heated to high temperatures and a hot massive plate was carried out. Ignition delay time was defined for metallized condensed substance. Influence of local energy source heat content on integrated characteristics of process was established. It was shown that use of heat transfer mathematical model with perfect thermal contact conditions on surface of a condensed substance is impossible in case of its ignition by a local energy source with limited heat content
[en] Piezoelectric materials are excellent candidates for use in energy harvesting applications due to their high electromechanical coupling properties that enable them to convert input mechanical energy into useful electric power. The electromechanical coupling coefficient of the piezoelectric material is one of the most significant parameters affecting energy conversion and is dependent on the piezoelectric mode of operation. In most piezoceramics, the d_1_5 piezoelectric shear coefficient is the highest coefficient compared to the commonly used axial and transverse modes that utilize the d_3_3 and the d_3_1 piezoelectric strain coefficients. However, complicated electroding methods and challenges in evaluating the performance of energy harvesting devices operating in the shear mode have slowed research in this area. The shear deformation of a piezoelectric layer can be induced in a vibrating sandwich beam with a piezoelectric core. Here, a model based on Timoshenko beam theory is developed to predict the electric power output from a cantilever piezoelectric sandwich beam under base excitations. It is shown that the energy harvester operating in the shear mode is able to generate ∼50% more power compared to the transverse mode for a numerical case study. Reduced models of both shear and transverse energy harvesters are obtained to determine the optimal load resistance in the system and perform an efficiency comparison between two models with fixed and adaptive resistances. (paper)
[en] Thermoelectric power generation (TEG) represents one of the cleanest methods of energy conversion available today. It can be used in applications ranging from the harvesting of waste heat to conversion of solar energy into useful electricity. Remarkable advances have been achieved in recent years for various thermoelectric (TE) material systems. The introduction of nanostructures is used to tune the transport of phonons, while band structure engineering allows for the tailoring of electron transport. In this overview, top-down approaches to phonon engineering, such as atomic construction of new materials, will be reviewed. Bottom-up approaches to electron engineering, such as the formation of ordered nanostructures, will also be discussed. The assembly of TEG devices is still particularly challenging, and consequently, thermal-to-electric conversion utilizing these devices has been realized only in niche applications. In this review paper, we will discuss some of the challenges that must be overcome to enable widespread use of TE devices. These include thermal stability at the material level, and reliable contact at the device level
[en] Latent heat storage systems are an effective way of storing thermal energy. Recently, phase change materials were considered also in the thermal control of compact electronic devices. In the present work a numerical and experimental investigation is presented for a solid-liquid phase change process dominated by heat conduction. In the experimental arrangement a plane slab of PCM is heated from above with an on-off thermal power simulating the behaviour of an electronic device. A two-dimensional finite volume code is used for the solution of the corresponding mathematical model. The comparison between numerical predictions and experimental data shows a good agreement. Finally, in order to characterize this thermal energy storage system, the time distribution of latent and sensible heat is analyzed.
[en] A simple effective scheme to improve the self energy obtained by the dynamical mean field theory is proposed, in which a feedback of magnetic fluctuations is taken into account. We demonstrate effectiveness of the scheme for the two-dimensional periodic Anderson model by investigating the effect of the magnetic fluctuation in the formation of heavy quasiparticles. It is found that the spectral intensity near the Fermi level is strongly suppressed by the antiferromagnetic fluctuation slightly above the magnetic instability.
[en] An overview summary of recent Boeing work on high-temperature superconducting (HTS) bearings is presented. A design is presented for a small flywheel energy storage system that is deployable in a field installation. The flywheel is suspended by a HTS bearing whose stator is conduction cooled by connection to a cryocooler. At full speed, the flywheel has 5 kW h of kinetic energy, and it can deliver 3 kW of three-phase 208 V power to an electrical load. The entire system, which includes a containment structure, is compatible with transportation by forklift or crane. Laboratory measurements of the bearing loss are combined with the parasitic loads to estimate the efficiency of the system. Improvements in structural composites are expected to enable the operation of flywheels with very high rim velocities. Small versions of such flywheels will be capable of very high rotational rates and will likely require the low loss inherent in HTS bearings to achieve these speeds. We present results of experiments with small-diameter rotors that use HTS bearings for levitation and rotate in vacuum at kHz rates. Bearing losses are presented as a function of rotor speed.