2,191 research outputs found
Measuring Nonequilibrium Temperature of Forced Oscillators
The meaning of temperature in nonequilibrium thermodynamics is considered by
using a forced harmonic oscillator in a heat bath, where we have two effective
temperatures for the position and the momentum, respectively. We invent a
concrete model of a thermometer to testify the validity of these different
temperatures from the operational point of view. It is found that the measured
temperature depends on a specific form of interaction between the system and a
thermometer, which means the zeroth law of thermodynamics cannot be immediately
extended to nonequilibrium cases.Comment: 8 page
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The Characteristics and Applications of Ceramic Laser Fusion and Ceramic Laser Sintering
The aim of present study is to investigate the possible application of the ceramic parts
which are fabricated with the process of Ceramic Laser Fusion or Ceramic Laser Sintering.
The experimental results reveal: (1) CLF can lead to a reduction in the porosity of the
ceramic part but also can induce micro-cracks. Therefore, this process cannot produce a part
with the required strength by a post-process of infiltration; (2) CLS is capable of fabricating a
ceramic part with high porosity. By adjusting the slurry formulation and varying the scanning
energy, the open porosity can be over 90vol% of the total porosity. After a post-process of
infiltration, the density can be increased to 95%; therefore, CLS can apply to produce a part
with high strength. Because the high open porosity leads to a good permeability, the process
of CLS is suitable for the fabrication of ceramic shell mold.Mechanical Engineerin
Diffuse-interface model for rapid phase transformations in nonequilibrium systems
A thermodynamic approach to rapid phase transformations within a diffuse
interface in a binary system is developed. Assuming an extended set of
independent thermodynamic variables formed by the union of the classic set of
slow variables and the space of fast variables, we introduce finiteness of the
heat and solute diffusive propagation at the finite speed of the interface
advancing. To describe the transformation within the diffuse interface, we use
the phase-field model which allows us to follow the steep but smooth change of
phases within the width of diffuse interface. The governing equations of the
phase-field model are derived for the hyperbolic model, model with memory, and
for a model of nonlinear evolution of transformation within the
diffuse-interface. The consistency of the model is proved by the condition of
positive entropy production and by the outcomes of the fluctuation-dissipation
theorem. A comparison with the existing sharp-interface and diffuse-interface
versions of the model is given.Comment: 15 pages, regular article submitted to Physical Review
Maximal independent sets in graphs with at most one cycle
AbstractIn this paper, we determine the largest number of maximal independent sets among all connected graphs of order n, which contain at most one cycle. We also characterize those extremal graphs achieving this maximum value. As a consequence, the corresponding results for graphs with at most one cycle but not necessarily connected are also given
The effects of nonlocality on the evolution of higher order fluxes in non-equilibrium thermodynamics
The role of gradient dependent constitutive spaces is investigated on the
example of Extended Thermodynamics of rigid heat conductors. Different levels
of nonlocality are developed and the different versions of extended
thermodynamics are classified. The local form of the entropy density plays a
crucial role in the investigations. The entropy inequality is solved under
suitable constitutive assumptions. Balance form of evolution equations is
obtained in special cases. Closure relations are derived on a phenomenological
level.Comment: 16 pages, 1 figur
Vortex length, vortex energy and fractal dimension of superfluid turbulence at very low temperature
By assuming a self-similar structure for Kelvin waves along vortex loops with
successive smaller scale features, we model the fractal dimension of a
superfluid vortex tangle in the zero temperature limit. Our model assumes that
at each step the total energy of the vortices is conserved, but the total
length can change. We obtain a relation between the fractal dimension and the
exponent describing how the vortex energy per unit length changes with the
length scale. This relation does not depend on the specific model, and shows
that if smaller length scales make a decreasing relative contribution to the
energy per unit length of vortex lines, the fractal dimension will be higher
than unity. Finally, for the sake of more concrete illustration, we relate the
fractal dimension of the tangle to the scaling exponents of amplitude and
wavelength of a cascade of Kelvin waves.Comment: 12 pages, 1 figur
Array concepts for solid-state and vacuum microelectronics millimeter-wave generation
The authors have proposed that the increasing demand for contact watt-level coherent sources in the millimeter- and submillimeter-wave region can be satisfied by fabricating two-dimensional grids loaded with oscillators and multipliers for quasi-optical coherent spatial combining of the outputs of large numbers of low-power devices. This was first demonstrated through the successful fabrication of monolithic arrays with 2000 Schottky diodes. Watt-level power outputs were obtained in doubling to 66 GHz. In addition, a simple transmission-line model was verified with a quasi-optical reflectometer that measured the array impedance. This multiplier array work is being extended to novel tripler configurations using blocking barrier devices. The technique has also been extended to oscillator configurations where the grid structure is loaded with negative-resistance devices. This was first demonstrated using Gunn devices. More recently, a 25-element MESFET grid oscillating at 10 GHz exhibited power combining and self-locking. Currently, this approach is being extended to a 100-element monolithic array of Gunn diodes. This same approach should be applicable to planar vacuum electron devices such as the submillimeter-wave BWO (backward wave oscillator) and vacuum FET
ONSAGER RELATIONS AND EXTENDED IRREVERSIBLE THERMODYNAMICS
We show that when the thermodynamic fluxes are included as independent thermodynamic state variables of a generalized entropy. the original ONSAGER formulation may be directly used in the space of the fiuxes. Therefore, the ONSAGER relations may be derived either in the space of the classical (slow) variables, by using a spatial FOURIER transformation, or in the space of the non-classical (fast) variables such as the physical fiuxes, without need of any Fourier transform. Furthermore. we analyse the question of non-linear ONSAGER relations by studying one particular set of evolution equations of the fiuxes, and considering the fluctuations of the fluxes around a non-equilibrium steady
state. Comparison with kinetic theory is not completely conclusive, because of several open questions which we comment in the concluding remarks
Attenuation and damping of electromagnetic fields: Influence of inertia and displacement current
New results for attenuation and damping of electromagnetic fields in rigid
conducting media are derived under the conjugate influence of inertia due to
charge carriers and displacement current. Inertial effects are described by a
relaxation time for the current density in the realm of an extended Ohm's law.
The classical notions of poor and good conductors are rediscussed on the basis
of an effective electric conductivity, depending on both wave frequency and
relaxation time. It is found that the attenuation for good conductors at high
frequencies depends solely on the relaxation time. This means that the
penetration depth saturates to a minimum value at sufficiently high
frequencies. It is also shown that the actions of inertia and displacement
current on damping of magnetic fields are opposite to each other. That could
explain why the classical decay time of magnetic fields scales approximately as
the diffusion time. At very small length scales, the decay time could be given
either by the relaxation time or by a fraction of the diffusion time, depending
whether inertia or displacement current, respectively, would prevail on
magnetic diffusion.Comment: 21 pages, 1 figur
Knudsen Effect in a Nonequilibrium Gas
From the molecular dynamics simulation of a system of hard-core disks in
which an equilibrium cell is connected with a nonequilibrium cell, it is
confirmed that the pressure difference between two cells depends on the
direction of the heat flux. From the boundary layer analysis, the velocity
distribution function in the boundary layer is obtained. The agreement between
the theoretical result and the numerical result is fairly good.Comment: 4pages, 4figure
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