Mechanism of high-temperature heat capacity of tungsten: Study of relaxation processes

It is experimentally shown that, in addition to fast relaxation, a slow relaxation process controlled by vacancy diffusion contributes to the high-temperature heat capacity C_p of tungsten. To account for the existence of contributions from two relaxation processes to C_p, it is suggested to consider the finite time required for the equilibrium density of transient complexes to set in.     

石墨烯的声子热学性质研究

声子是石墨烯导热过程中的主要载体,而声子的弛豫时间又是其中最基本、最重要的物理量.本文采用简正模式分解法研究了石墨烯声子的弛豫时间,并且借此分析了不同声子在导热过程中的贡献.该方法通过平衡分子动力学模拟实现,首先通过模拟得到单个声子的能量自相关函数衰减曲线,并进一步采用拟合和积分两种方法得到单个声子的弛豫时间.然后,研究了弛豫时间与波矢、频率和温度的关系.结果发现,弛豫时间随波矢的变化与对应的色散关系相近,弛豫时间与频率和温度的关系符合理论模型:1/τ=νnTm,其中声学支的n为1.56,而光学支结果较为发散,指数m对于不同声子支结果略有不同.最后,还研究了不同频率声子对导热的贡献,发现低频声子在态密度上占有绝对优势,并且其弛豫时间整体高于高频声子,所以低频声子对导热的贡献占据主导地位.     

Solution of the Boltzmann equation by expanding the distribution function with several time and coordinate scales in the enskog series in knudsen parameter

A method to solve the Boltzmann equation is analyzed in the case when the distribution function depends on slow and fast time and coordinate scales. Basic relationships for calculating the nonequilibrium multiscale distribution function are shown to differ substantially from those found in the framework of the Chapman-Enskog method: the transfer equations are complemented by the contributions of relaxation processes. The heat and momentum transfer equations derived from the general solution to the Boltzmann equation involve additional terms accounting for relaxation effects. The relaxation effects included in the energy equation result in both a hyperbolic heat conduction equation and a finite rate of heat transfer. In the viscous stress tensor, the Newtonian term of the transfer equation turns out to be supplemented by relaxation terms.     

Thermal relaxation times and heat conduction in β-cristobalite and α-quartz silica structures

Several studies have shown the key role of the rigid unit modes, i.e. rotational motions between neighbouring SiO₄ tetrahedra, in glass and in β-cristobalite due to the disorder generated by the large atomic amplitudes. They do not however reach a conclusion concerning heat conduction. To determine the nature of the heat carriers in the amorphous phase, we carry out an ultrashort timescale analysis of heat transfer in β-cristobalite; the behaviour is comparable to that of amorphous silica, as shown by our density of states and heat flux autocorrelation function calculations. We prove that rigid unit modes can be identified as the heat carriers by showing that the thermal relaxation time is in very good agreement with the rigid unit mode relaxation time predicted in the literature. Finally, we provide thermal conductivity data for β-cristobalite which are not available in the literature.     

Heating and Cooling Transients in the DyPO<Subscript>4</Subscript> Nanocrystals under Femtosecond Laser Irradiation in the NIR Spectral Range

We compared the heating and cooling theoretical and experimental transient times of DyPO₄ nanoparticles as a result of their heating by themultiphonon relaxation after femtosecond laser excitation in the near IR spectral range into different absorption spectral lines of the Dy³⁺ ion. We have shown that the relaxation of the heat flux to a stationary value occurs according to an exponential law. Depending on the value of the Biot number, two different relaxation mechanisms can be realized, in one of which the relaxation time depends on the thermal conductance of the interface, and in the other on the thermal diffusivity. It is shown that, after averaging over the ensemble of nanoparticles, the kinetics of the relaxation of the heat flux in these limiting cases has a substantially different character. The results might be helpful for assessing the prospects of the dielectric crystalline nanoparticles doped with rare-earth ions in the local hyperthermia treatment of cancer cells.     

Self-induced avalanches in relaxing media

A theory of boson avalanches (self-induced waves) has been constructed with allowance for longitudinal relaxation. It is shown that inclusion of longitudinal relaxation requires a completely different approach to the theory of superradiance than that taken previously. For the first time an equation is derived for an avalanche with allowance for longitudinal and transverse relaxation. Numerical solution of this equation makes It possible to establish all the distinctive features of a coherent self-induced wave under the conditions of the action of relaxation mechanisms. An inversion effect is found to occur in the action of longitudinal and transverse relaxation on the delay parameter of a self-induced wave, i.e., the superradiance delay time. Analytic expressions are found of the ensemble-averaged delay time of an avalanche and the relative fluctuations of the delay time as a function of irreversible phase relaxation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 81–86, September, 1986.     

Relaxation model of the orientational phase transition in fullerite C60

The model of thermal behavior of a thermoelastic medium is developed in the context of the Landau theory of phase transitions. In the framework of this model, two different problems are considered with allowance for order parameter relaxation: the problem of relatively slow uniform heating (cooling) of the medium under external hydrostatic pressure and the problem of order parameter relaxation at thermal isolation. A finite value of the relaxation constant τ of the order parameter is demonstrated to bring about the heating (cooling) rate dependence of the physical quantities, such as specific heat. The relaxation time of the order parameter is shown to be twice larger than the temperature relaxation time, as a consequence of the Landau expansion of the free energy.     

Comment on: An intermolecular potential for nitrogen from a multi-property analysis

A paper by Cappelletti et al. (1998, Molec. Phys. 93, 485) utilizes classical trajectory effective cross-sections to predict the rotational collision number for nitrogen gas. It is suggested that use of the rotational heat capacity, rather than the total internal heat capacity, will give a more meaningful conversion of the cross-sections into relaxation time. In addition, an error in the formula for first-order collision number is corrected.     

On the long-range detection and study of undamped directed temperature waves generated during the interaction between a cavitating water jet and targets

The features of the propagation of undamped thermal (temperature) waves in air are investigated. The presence of these waves is a consequence of solution of the heat equation taking into account the relaxation of local thermal perturbation. It is shown that such waves can exist only in media with a finite (nonzero) time of local thermal relaxation, and their frequencies are determined by this time. The time of relaxation in air depends on the gas composition, its temperature and increases with a decrease in pressure. Under normal conditions, the minimum frequency of undamped waves in air corresponds to 70–80 MHz. One of the methods for exciting these waves is associated with pulsed heating of the surface of a medium bordering air. Pulsed heating on account of the application of shock waves generated during water jet cavitation is used. It is shown for the first time that these waves with frequencies in the range of 70–500 MHz can propagate in air without damping over a distance of up to 2 m.     

多元可激发气体声弛豫频率的环境影响分析

推导多元可激发气体中声弛豫频率和环境温度、压强的解析关系.理论分析和仿真计算表明：声弛豫频率线性反比于主弛豫过程的弛豫时间,正比于主弛豫过程的振动耦合热容,反比于外自由度热容;温度升高导致振动耦合热容增加、内外自由度能量转移速率增大引起弛豫时间减少,进而造成声弛豫频率正比于环境温度;压强增加使得分子碰撞速率增加引起弛豫时间减少,进而使得声弛豫频率线性正比于环境压强.     

关於流体的容变粘滞弹性理论及其在声吸收现象中的应用

在这篇论文里我们证明,与某些作者所了解的不同,我们的流体的容变粘滞弹性理论不仅仅是在结构弛豫的情况中有效,而是对所有三种弛豫——热弛豫、结构弛豫与化学弛豫——都是同样有效的。从我们的容变不可逆性方程可以推到Herzfeld与Rice原为热弛豫所假定的热不可逆性方程。又可以证明的是在化学弛豫的情况中我们的容变不可逆性方程也包含着Liebermann由分子运动理论考究所得到的化学不可逆性方程。关于这理论在声吸收及速变现象中的应用,我们证明利用适当的热力学考究便可以从我们的压缩性理论所给的结果直接推到在热弛豫情况下有效的Bourgin-Kneser方程及在化学弛豫情况下可以有效的Liebermann方程。这个推导并揭发出来Liebermann的声吸收方程仅对液体说可以是一个良好的近似。我们附带地指出,在气体的情况中,某些已发表的声吸收及速变的实量结果的准确度已可以使我们从这些结果来算定气体的静态与立刻两压缩系数β₀及β_∞,从而确定热容量的两比值γ₀及γ_∞与外态内态两热容量C^(e)及C⁽ⁱ⁾。这样我们又能从这些结果取得一些有关分子构造及分子碰撞过程中能量转移的结论。最后,我们讨论了我们的容变粘滞系数的定义的适当性。     

The development and investigation of a strongly non-equilibrium model of heat transfer in fluid with allowance for the spatial and temporal non-locality and energy dissipation

The differential equation of heat transfer with allowance for energy dissipation and spatial and temporal nonlocality has been derived by the relaxation of heat flux and temperature gradient in the Fourier law formula for the heat flux at the use of the heat balance equation. An investigation of the numerical solution of the heat-transfer problem at a laminar fluid flow in a plane duct has shown the impossibility of an instantaneous acceptance of the boundary condition of the first kind — the process of its settling at small values of relaxation coefficients takes a finite time interval the duration of which is determined by the thermophysical and relaxation properties of the fluid. At large values of relaxation coefficients, the use of the boundary condition of the first kind is possible only at Fo → ∞. The friction heat consideration leads to the alteration of temperature profiles, which is due to the rise of the intervals of elevated temperatures in the zone of the maximal velocity gradients. With increasing relaxation coefficients, the smoothing of temperature profiles occurs, and at their certain high values, the fluid cooling occurs at a gradientless temperature variation along the transverse spatial variable and, consequently, the temperature proves to be dependent only on time and on longitudinal coordinate.     

Metrological aspects of thermal relaxation technique by radiation loss for volumetric heat capacity measurements

One of the best-known methods to measure the heat capacity of solids consists in the illumination of the sample and the analysis of the thermal relaxation when the illumination is stopped. In this work, the energy balance equation with heat losses due to radiation is solved exactly. This is used to establish the limits of the usual approximations used to obtain the heat capacity from the experimental data. It is shown that large temperature changes, induced by the heat source during the experiment can generate errors in the calculation of heat capacity when the traditional approach is used.     

Microscopic theory of the long-wavelength modes of two-component plasmas and ionic liquids: II. The longitudinal modes

The modification of the damping rate of the sound modes by Coulomb phenomena is demonstrated from first principles. The heat modes of one- and two-component systems of charged particles are shown to differ by a factor c_p/c_v. Microscopic expressions for the interspecies energy and longitudinal momentum relaxation frequencies are provided. The charge relaxation modes are shown to reduce in the limit of weak-coupling to a pair of plasma oscillations occurring slightly below the plasma frequency while being slightly damped even at infinite wavelength. In the opposite limit of strong-coupling the same pair of charge relaxation modes is shown to split into an interspecies momentum relaxation mode and an approximate hydrodynamic diffusion mode. An Einstein relation between the diffusion constant and the electric conductivity is also demonstrated. All expressions are obtained for arbitrary density and coupling.     

基于小负荷换热单元保护的松弛策略优化换热网络

WCE算法优化换热网络时,固定投资费用的存在易造成小负荷换热单元难以产生和保留,使部分尚未完全进化的结构过早被淘汰。本文建立一种固定投资费用的松弛处理方法,将固定投资费用与换热单元热负荷进行因变处理,根据换热单元热负荷的大小实时调整优化过程中的松弛力度,引导并促进小负荷换热单元的顺利产生和进化,从而增强换热网络的结构进化能力。将松弛策略应用于9SP和15SP算例,验证松弛策略促进结构进化的有效性,提升优化质量,获得的最优结果(2 903 528 ＄ ·a^-1、5 115 061 ＄ ·a^-1)优于文献结果。     

Quantum–Classical Correspondence Principle for Heat Distribution in Quantum Brownian Motion

Quantum Brownian motion, described by the Caldeira–Leggett model, brings insights to the understanding of phenomena and essence of quantum thermodynamics, especially the quantum work and heat associated with their classical counterparts. By employing the phase-space formulation approach, we study the heat distribution of a relaxation process in the quantum Brownian motion model. The analytical result of the characteristic function of heat is obtained at any relaxation time with an arbitrary friction coefficient. By taking the classical limit, such a result approaches the heat distribution of the classical Brownian motion described by the Langevin equation, indicating the quantum–classical correspondence principle for heat distribution. We also demonstrate that the fluctuating heat at any relaxation time satisfies the exchange fluctuation theorem of heat and its long-time limit reflects the complete thermalization of the system. Our research study justifies the definition of the quantum fluctuating heat via two-point measurements.     

Beta relaxation versus high frequency wing in the dielectric spectra of a binary molecular glass former

The binary molecular glass former 2-picoline in tri-styrene is investigated by means of broadband dielectric spectroscopy with the aim of understanding the role of secondary relaxation processes that emerge during the glass transition. It is shown that the "high frequency wing," which is seen in neat picoline, becomes a separate process in the mixture and exhibits all the features of a Johari-Goldstein relaxation. In particular, the previously found relation between activation energy and Tg is recovered. In addition, below Tg the width parameter of this secondary relaxation is shown to be governed by a common temperature dependence, and the time scale is characterized by an isokinetic point. Above Tg pronounced deviations from an Arrhenius behavior are observed.     

Effects of mass transfer on MHD second grade fluid towards stretching cylinder: A novel perspective of Cattaneo–Christov heat flux model

The aim of this research is to analyze the effects of mass transfer on second grade fluid flow subjected to the heat transfer incorporated with the relaxation time to reach the state of equilibrium on or after the state of upheaval. A new heat model namely Cattaneo–Christov heat flux comprising the relaxation time is employed instead of very commonly used mundane model based on classical theory of heat flux. Flow is considered towards stretching cylinder in the existence of external magnetic field. Suitable transformations are first used to deduce the momentum, heat and concentration equations and are then solved analytically. The effects of physical quantities such as fluid parameter, magnetic field, Schmidt number, relaxation time, curvature parameter, Prandtl number and chemical reaction on momentum, temperature and concentration profile are examined graphically whereas for validation of results convergence analysis along with residual error are obtained numerically. A comparison of obtained results is also given with the existing literature as a limiting case of reported problem and are found an excellent agreement. The temperature profile indicates thinning effect for higher values of Prandtl number and relaxation time. It is also noted that the velocity increases with increasing values of fluid parameter whereas it declines for the case of magnetic field. This study can be used an application of central heating system and to measure the fast chemical reactions rates.