Thermal properties of compressed expanded graphite: photothermal measurements

The thermal diffusivity and the thermal conductivity of compressed expanded graphite (CEG) samples were investigated by photothermal measurements in two geometries differing by a place of temperature disturbance detection. This disturbance can be detected on a surface opposite to the one at which the disturbance was generated (rear detection) or on the same surface (front detection). A measurement based on the rear detection allowed us to determine the effective thermal diffusivity of the sample, while the method with front detection gives the possibility of analysis of homogeneity of the sample. It is shown that the thermal diffusivity of CEG strongly depends on its apparent density. Moreover, CEG samples reveal anisotropy of the thermal properties. The thermal diffusivity in the direction parallel to the compacting axis is lower than the one in the direction perpendicular to it. The parallel thermal diffusivity decreases with growing apparent density, while the perpendicular thermal diffusivity significantly grows when the apparent density grows. The perpendicular thermal conductivity exhibits the same behavior as the perpendicular thermal diffusivity. The parallel thermal conductivity slightly grows with growing density and then reaches a plateau. The anisotropy of CEG samples grows with growing apparent density and vanishes for low-density samples. The photothermal measurement with front signal detection revealed that the CEG samples are non-homogeneous in the direction of the compacting axis and can be modeled by a two-layer system.     

冰盘管排列密度对蓄冷速率影响的实验研究

设计了蒸发器盘管排列密度可以改变的直接蒸发冰盘管蓄冷装置,在高、中、低三种盘管排列密度情况下,进行了蓄冷槽内载冷剂在自然对流和强制对流下的静态和动态蓄冷实验,研究了盘管排列密度和蓄冷状态对蓄冷速率的影响。结果显示:高密度系统平均蓄冷速率较低密度系统提高27.9%以上,动态系统平均蓄冷速率较静态系统提高2%左右。     

液态硝基甲烷热分解行为及压力效应的第一性原理研究

采用基于密度泛函理论的第一性原理分子动力学方法对液态硝基甲烷的热分解行为进行了模拟,结合各产物布居数随时间的演化,讨论了热分解初期可能发生的3种反应,即分子内/分子间的质子迁移反应和C—N键的断裂.在长时间（30 ps）的模拟过程中,H₂O是主要产物.研究了液态硝基甲烷在不同密度（压力）条件下热分解的动力学行为.发现不同密度（压力）条件下液态硝基甲烷热分解呈现明显不同的变化趋势,并给出了解释. 关键词： 硝基甲烷 分子动力学 热分解 压力效应     

Determination of Plasma Density Profile and Other Parameters of Cylindrical Plasma Columns with an Electroacoustic Probe

A new diagnostic method based on the excitation of thermal resonances using an electroacoustic probe has been developed to determine the electron density profile and other plasma parameters of a cylindrical warm-plasma column. A coaxial electroacoustic probe is used to excite and detect the dipole and thermal resonances as the plasma density is varied. The electron density profile is expressed functionally in terms of unknown profile parameters. The profile and other parameters of the plasma column are then determined numerically based on some theoretical formulas and experimental results.     

采用非均匀防热材料进行高速飞行器防热设计的防热效果研究

本文提出采用非均匀防热材料进行高超声速飞行器防热设计的设想,设计出外层部分为抗烧蚀、抗剪切的紧密结构材料,内层为密度轻、隔热效率高的稀松材料,中间为逐渐过渡层的非均匀防热材料,数值模拟了非均匀防热材料的烧蚀防热情况,探讨了非均匀防热材料在飞行器防热设计中应用的可行性和防热效果,通过本文研究发现采用非均匀防热材料可以降低原始材料质量消耗,大大减轻防热结构重量,减少向内部结构传导的热量,降低内部结构的温升,这些优点显示了非均匀防热材料具有很高的防热效率和广阔的应用前景.     

A Thermal Approach to Model Laser Damage in KDP and DKDP Crystals

A thermal model is considered in order to better understand the mechanism of laser induced damage in KDP and DKDP crystals. We demonstrate that the expressions of pinpoint density and damage probability, predicted by the thermal model, are consistent with the experimental data. We also discuss the effect of particle interaction on the thermal model.     

Study of thermal strain relaxation in GaAs grown on Ge/Si substrates

A detailed analysis of the thermal strain relaxation in GaAs/Ge/Si structures by means of crack formation is presented. The study was performed through optical microscopy and photoluminescence measurements. The as grown epilayers were found to be in a metastable state with respect to crack formation. Repeated thermal cycling increased the crack density up to an asymptotic limit. The thermal strain is not fully relaxed even near to the asymptotic crack density.     

热辐射中存在偶极相互作用的原子量子态保真度研究

建立了两个两能级原子置于热辐射场环境中的模型,当计及原子偶极间的相互作用时,利用密度矩阵方法,得到两能级原子密度矩阵元随时间的演化规律.针对三种不同的初始状态,分析置于强热辐射场中原子量子态保真度.结果表明:两个原子初始处于不同量子叠加态,量子信息在传输过程中可能发生部分失真,也可能不失真.初始状态对量子信息失真的快慢程度有明显的影响.     

Phase diagram of a wide current-carrying superconducting film

A model of current heating of a wide superconducting film is suggested assuming nonlinearity of the film conductance. Within this model, the parameters of the film can be characterized by a single dimensionless coefficient of thermal balance which includes both thermal and resistive parameters of the system. The stability of the state of the current-carrying film is analyzed. A phase diagram of the film is constructed in terms of the coefficient of thermal balance and the average current density. The propagation velocity of a stationary nonuniform temperature distribution in the film is calculated as a function of current density at various values of the coefficient of thermal balance.     

A density matrix approach to the Goldanskii-Karyagin effect

A density matrix formalism has been introduced to describe the perturbation of the Mössbauer line intensities caused by the anisotropic recoilless fraction in random absorbers. General properties of the matrix elements are discussed and relationships between thermal tensors and density matrices are investigated with the attention focused on the possibility to extract as much information about the thermal tensors as possible from the observed line intensities. It is shown, that quartic (anharmonic) anisotropy may be observable in quadrupolarly dominated transitions under favourable conditions.     

固体吸附式制冷中热波循环的分析研究

1引言由Tehernev博士和S．V．Shelton教授提出的热波循环，是吸附式制冷中引起广泛兴趣的一种循环方式。其特点是高效回热，Shelton采用斜波法［‘］和方波法［‘］分析了热波循环，回热率达70％，热泵工况COP超过1．6。其它学者作了改进研究［‘并热波循环的模拟效果很好，但实验方面进展相当缓慢。采用螺旋板式换热器作吸附器，也发现热波循环的运行效果很不理想问。目前，相关的文献主要是系统模拟，而对其关键，热波的形成、特性研究较少。另外，研究侧重于系统性能（COP），对能量密度（SPD）考虑较少。本文将从传热的角度分析热…     

Fluctuations and stability of stationary non-equilibrium systems in detailed balance

A generalized thermodynamic potential for Markoffian systems with detailed balance and far from thermal equilibrium has been derived in a previous paper. It was shown that the principle of detailed balance is equivalent to a set of conditions fulfilled by this potential (“potential conditions”). The properties of this potential allow us to extend the validity of a number of thermodynamic concepts well known for systems in or near thermal equilibrium to stationary states far from thermal equilibrium. The concept of symmetry breaking phase transitions for these systems is introduced in analogy to thermal equilibrium systems by considering the dependence of the stationary probability density of the system on a set of externally controlled parameters {λ}. A functional of the time dependent probability density of the system is defined in close analogy to the Gibb's definition of entropy. This functional has the properties of a Ljapunov functional of the governing Fokker-Planck equation showing the stability of the stationary probability density. The Langevin equations connected with the Fokker-Planck equation are considered. It is shown that, by means of the potential conditions, generalized “thermodynamic” fluxes and forces may be defined in such a way that the smoothly varying part of the Langevin equations (kinetic equations) constitutes a linear relation between fluxes and forces. The matrix of coefficients is given by the diffusion matrix of the Fokker-Planck equation. The symmetry relations which hold for this matrix due to the potential conditions then lead to the Onsager-Casimir symmetry relations extended to systems with detailed balance near stationary states far from thermal equilibrium. Finally it is shown that under certain additional assumptions the generalized thermodynamic potential may be used as a Ljapunov function of the kinetic equations.     

Thermal lensing of diode side-pumped solid-state lasers

The pump energy distribution in a diode side-pumped solid-state laser, is an overlap of propagating Gaussian beams. A simple model has been developed to calculate the thermal focal length of a diode side-pumped solid-state laser, which is based on a thermal model with a Gaussian heat density in any cross section of a laser rod. It can be seen that as the waists of pump beams increase, the energy distribution tends to be uniform and the thermal focal length tends to be long, which means a smaller thermal focusing.     

Effects of Spatial Variation of Thermal Electrons on Whistler-Mode Waves in Magnetosphere

A ray-tracing method is developed to evaluate the wave growth/damping and specifically propagation trajectories of the magnetospherically reflected Whistler-mode waves. The methodology is valid for weak wave growth/damping when plasma is comprised of a cold electron population and a hot electron population, together with background neutralizing ions, e.g. protons. The effect of anisotropic thermal electrons on the propagation of Whistler-mode waves is studied in detail. Numerical results are obtained for a realistic spatial variation model of plasma population, including the cold electron density distribution, and the thermal electron density and temperature distribution. It is found that, analogous to the case of the typical cold plasma approximation, the overall ray path of Whistler-mode waves is insensitive to the thermal electron density and temperature anisotropy, and the ray path reflects where wave frequency is below or comparable to the local lower hybrid resonance frequency flhr. However, the wave growth is expected to be influenced by the thermal electron population. The results present a first detailed verification for the validity of the typical cold plasma approximation for the propagation of Whistler-mode waves and may account for the observation that the Whistler-mode waves tend to propagate on a particular magnetic shell L where the wave frequency is comparable to fthe.     

Pion production as a probe of the nuclear matter equation of state

It is shown that thermal and chemical equilibrium are approached during the high density stage in central nucleus-nucleus collisions and that the yield of produced pions is determined at that time. A chemical model with Rankine-Hugoniot compression is used to extract a nuclear matter equation of state from the observed pion yield assuming a partition of the internal energy per nucleon into thermal and compressional energy fractions, with only the former part contributing to particle production.     

Effect of storage medium on thermal properties of packed beds

The present paper is concerned with studying the effects of storage medium properties on the thermal behaviour of packed beds during charging. A transient one-dimensional two-phase model is used to describe the temperature fields in the air and solid media constituting the bed. The numerical solution of the resulting two-coupled partial differential equations is obtained, and the variation of thermal energy stored with time is computed. Solutions are obtained for a wide range of storage media (the solid phase) properties, namely density, thermal conductivity and specific heat. The results show that increasing either the density or specific heat increases the rate and capacity of energy storage, and decreases the rate of temperature rise throughout the storage medium. On the other hand, increasing the thermal conductivity is found to markedly increase the rate of temperature rise and energy stored inside the bed up to a certain time during charging, beyond which this trend completely reverses. In addition, the thermal storage properties of aluminum, steel and rock-packed beds are computed and compared.     

Expected thermal deformation and wavefront preservation of a cryogenic Si monochromator for Cornell ERL beamlines

Cornell energy‐recovery linac (ERL) beamlines will have higher power density and higher fractional coherence than those available at third‐generation sources; therefore the capability of a monochromator for ERL beamlines has to be studied. A cryogenic Si monochromator is considered in this paper because the perfect atomic structure of Si crystal is needed to deliver highly coherent radiation. Since neither the total heat load nor the power density alone can determine the severity of crystal deformation, a metric called modified linear power density is used to gauge the thermal deformation. For all ERL undulator beamlines, crystal thermal deformation profiles are simulated using the finite‐element analysis tool ANSYS, and wavefront propagations are simulated using Synchrotron Radiation Workshop. It is concluded that cryogenic Si monochromators will be suitable for ERL beamlines in general.     

Time-Evolution of Photon-Number Distribution and Density Operator of Squeezed Thermal State in the Thermal Environment

By virtue of the thermal entangled state representation, we analytically study time-dependent evolution of photon-number distribution and density operator of squeezed thermal state (STS) in the thermal environment. It is found that the initial density operator of STS still keeps squeezing and thermal within the thermal environment. At long times, such a state decays to thermal, a Gaussian classical state, as a result of decoherence. Moreover, the oscillations of photon-number distribution slowly disappear with increasing t, but the change of oscillations is completely different from that of STS in amplitude dissipative channel.     

Phonon scattering and thermal conductance properties in two coupled graphene nanoribbons modulated with bridge atoms

The phonon scattering and thermal conductance properties have been studied in two coupled graphene nanoribbons connected by different bridge atoms by using density functional theory in combination with non-equilibrium Green's function approach. The results show that a wide range of thermal conductance tuning can be realized by changing the chemical bond strength and atom mass of the bridge atoms. It is found that the chemical bond strength (bridge atom mass) plays the main role in phonon scattering at low (high) temperature. A simple equation is presented to describe the relationship among the thermal conductance, bridge atom, and temperature.