Development of a thermal conductivity cell with nanolayer coating for thermal conductivity measurement of fluids

Various techniques and methodologies of thermal conductivity measurement have been based on the determination of the rate of directional heat flow through a material having a unit temperature differential between its opposing faces. The constancy of the rate depends on the material density, its thermal resistance and the heat flow path itself. The last of these variables contributes most significantly to the true value of steady-state axial and radial heat dissipation depending on the magnitude of transient thermal diffusivity along these directions. The transient hot-wire technique is broadly used for absolute measurements of the thermal conductivity of fluids. Refinement of this method has resulted in a capability for accurate and simultaneous measurement of both thermal conductivity and thermal diffusivity together with the determination of the specific heat. However, these measurements, especially those for the thermal diffusivity, may be significantly influenced by fluid radiation. Recently developed corrections have been used to examine this assumption and rectify the influence of even weak fluid radiation. A thermal conductivity cell for measurement of the thermal properties of electrically conducting fluids has been developed and discussed.     

Statistical mechanical theory for steady-state systems. III. Heat flow in a Lennard-Jones fluid

A statistical mechanical theory for heat flow is developed based upon the second entropy for dynamical transitions between energy moment macrostates. The thermal conductivity, as obtained from a Green-Kubo integral of a time correlation function, is derived as an approximation from these more fundamental theories, and its short-time dependence is explored. A new expression for the thermal conductivity is derived and shown to converge to its asymptotic value faster than the traditional Green-Kubo expression. An ansatz for the steady-state probability distribution for heat flow down an imposed thermal gradient is tested with simulations of a Lennard-Jones fluid. It is found to be accurate in the high-density regime at not too short times, but not more generally. The probability distribution is implemented in Monte Carlo simulations, and a method for extracting the thermal conductivity is given.     

Study of temperature profile and specific heat capacity in temperature modulated DSC with a low sample heat diffusivity

One important application of temperature modulated DSC (TMDSC) is the measurement of specific heat of materials. When the sample has very good thermal conductivity as in the case of metals, the temperature gradient is not normally an important factor and can be ignored most of the time. However, in the case of materials with poor heat transfer properties, for example, polymers, the thermal conductivity is only in the order of 1/1000 or so of that of metals. This could have a major effect on the test results. In this paper, a round analytical solution is given and a numerical model is used to analyze the effects of thermal diffusivity on temperature distribution inside the test sample and specific heat measurement by TMDSC, PET sample test results are presented to demonstrate the effects of material thermal diffusivity.     

超临界压力下国产航油RP-3导热系数的流动法测量

为了测量高温高压条件下航空燃料的导热系数,基于恒热流密度条件下,常物性不可压缩流体在圆管层流充分发展时的努塞尔数为常数原理,搭建了导热系数测量装置.通过牛顿冷却定律测量管内流体充分发展时的换热系数,可以在线测量流体的导热系数.经不确定度分析得到实验条件下最大相对不确定度为5.76％.使用乙苯和正十烷验对实验装置进行了标...     

Phonon thermal conductivity in nanolaminated composite metals via molecular dynamics

We use nonequilibrium molecular dynamics to characterize the phonon contribution to thermal conduction of Al nanostructures and the role of interfaces in metallic nanocomposites. We characterize the lattice thermal conductivity of pure Al samples as a function of size and temperature from which we obtain, using kinetic theory, the temperature dependence of the phonon mean free path. We also calculated the thermal conductivity of AlAl* and AlNi nanolaminate composites (where Al* differs from Al only in its mass) for various periodic sizes and compositions as well as the associated interfacial thermal resistivities (ITRs). We find that simple, additive models provide good estimates of the thermal conductivities of the nanocomposites in terms of those of the individual components and interfaces if size effects on the behavior of the individual components are considered. The additive models provide important insight to the decrease in thermal conductivity of the nanolaminates as their periodicity (thickness of a bilayer) is reduced to a size comparable with the phonon mean free path and break down when this characteristic size is reduced further. At this point the system can be regarded as homogeneous and the conductivity increases with decreasing periodicity of the laminates. We also observe that the ITR depends on the direction of the heat flux; this is the first molecular level characterization of such thermal diode behavior in a realistic three dimensional material.     

A computer-controlled apparatus for thermal conductivity measurement by the transient hot wire method

The aim of this paper is to review the transient hot wire method for measurement of thermal conductivity, which is based on the measurement of temporal history of the temperature rise caused by linear heat source (hot wire) embedded in a test material. If a current is passed through the wire, the rise in temperature will be dependent, among other factors, on the thermal conductivity of the medium, surrounding the wire. Here the mathematical basis, as well as main modifications of the hot wire method — cross technique, resistance modifications with potential and compensated lead methods; hot wire probe method and parallel wire technique, are described and discussed. A fully automated computer-controlled transient hot wire apparatus is presented and tested, which allows measurement of thermal conductivity of solid, powder and granular materials at high temperatures.     

Thermal-Conductivity-Enhancing Copper-Plated Expanded Graphite/Paraffin Composite for Highly Stable Phase-Change Materials

Paraffin (PA)/expanded graphite (EG) is an important composite phase change material with low cost, high heat storage, good thermal conductivity and cycling stability. Its thermal conductivity needs to be further improved for application in the thermal management system of power lithium-ion batteries. In this paper, copper plated expanded graphite (CPEG) with 3D porous structure was prepared by electroless copper plating method, which was used as thermal conductivity enhancing material to replace part of EG in PA/EG composite materials. For the optimized phase change material composed of 80 %PA-14 %EG-6 %CPEG, the copper content is very low (0.768 wt %), but its thermal conductivity can be significantly improved without loss of latent heat and thermal cycling stability. Its thermal conductivity is increased from 11 times to 16.5 times that of paraffin while compared with the copper-free composite material (80 %PA-20 %EG). The PA/EG/CPEG composite material exhibits good temperature control effect on power lithium-ion batteries.     

Thermophysical Properties of Granules of Instant Drinks in the Process of Structuring

In this article the research of the thermophysical properties of the material depending on its moisture content been represented. The interrelation of various physical properties with the content of moisture in the mass were detected. In addition, their dependence on the quantitative ratio and the properties of structural carcass and pore spaces were proved. The need to consider the volume phase composition of the mixture was justified. A formula to calculate the thermal conductivity coefficient of the product molded from disperse materials was obtained on the basis of Lykov criterion defining the property of drying material and volume phase concentrations. It is proved that the use of volume phase characteristics of disperse systems opens new possibilities for improving the methodologies for determining the those thermophysical properties of dry and wet dispersion materials, allows increasing the reliability results of these definitions, revealing the mechanism of anomalous heat conductivity of wet material.     

改性氧化石墨烯稳定Pickering乳液法制备高导热相变整体材料

采用表面引发原子转移自由基聚合法(SI-ATRP)改性氧化石墨烯(GO), 并用其稳定Pickering高内相乳液, 一步成型制得高导热氧化石墨烯/石蜡复合整体相变材料. 通过SI-ATRP方法, 在氧化石墨烯表面引入分子刷, 提高GO的分散性, 实现了低GO含量下优异的导热强化效果. 当GO添加量仅为相变复合材料整体的0.4%(质量分数)时, 其热导率(3.968 W?m^-1?K^-1)比纯石蜡的热导率(0.608 W?m^-1?K^-1)有较大提升. 通过测试发现, 在1000次循环后相变材料的泄漏率仅为1.1%~1.3%, 表现出良好的形状稳定性和热可靠性. 制备的新型形状稳定相变材料在温控、 储能应用中具有潜在的用途.     

石蜡/膨胀石墨/石墨烯复合相变储热材料的制备及性能

以石蜡(PA)作为相变储热材料、 膨胀石墨(EG)作为主导热材料和支撑材料, 石墨烯气凝胶(GA)作为导热增强材料和辅支撑材料制备了PA/EG/GA复合相变材料, 研究了GA添加量对复合相变材料相变温度、 相变潜热、 导热性能以及循环稳定性的影响. 结果表明, 所制备的80%PA-17%EG-3%GA复合相变材料导热性能良好, 循环稳定性出色. 与80%PA-20%EG复合材料相比, 该材料的相变温度、 相变潜热以及循环稳定性无明显变化, 但导热系数由4.089 W/(m·K)提升到了5.336 W/(m·K), 显示出良好的应用前景.     

Anisotropic heat transport in nanoconfined polyamide-6,6 oligomers: atomistic reverse nonequilibrium molecular dynamics simulation

While polymers are known as thermal insulators, recent studies show that stretched single chains of polymers have a very high thermal conductivity. In this work, our new simulation scheme for simulation of heat flow in nanoconfined fluids [H. Eslami, L. Mohammadzadeh, and N. Mehdipour, J. Chem. Phys. 135, 064703 (2011)] is employed to study the effect of chain ordering (stretching) on the rate of heat transfer in polyamide-6,6 nanoconfined between graphene surfaces. Our results for the heat flow in the parallel direction (the plane of surfaces) show that the coefficient of thermal conductivity depends on the intersurface distance and is much higher than that of the bulk polymer. A comparison of results in this work with our former findings on the heat flow in the perpendicular direction, with the coefficient of heat conductivity less than the bulk sample, reveal that well-organized polymer layers between the confining surfaces show an anisotropic heat conduction; the heat conduction in the direction parallel to the surfaces is much higher than that in the perpendicular direction. The origin of such anisotropy in nanometric heat flow is shown to be the dramatic anisotropy in chain conformations (chain stretching) beside the confining surfaces. The results indicate that the coefficients of heat conductivity in both directions, normal and parallel to the surfaces, depend on the degree of polymer layering between the surfaces and the pore width.     

石墨烯导热研究进展

石墨烯具有目前已知材料中最高的热导率,在电子器件、信息技术、国防军工等领域具有良好的应用前景。石墨烯导热的理论和实验研究具有重要意义,在最近十年间取得了长足的发展。本文综述了石墨烯本征热导率的研究进展及应用现状。首先介绍应用于石墨烯热导率测量的微纳尺度传热技术,包括拉曼光谱法、悬空热桥法和时域热反射法。然后展示了石墨烯热导率的理论研究成果,并总结了石墨烯本征热导率的影响因素。随后介绍石墨烯在导热材料中的应用,包括高导热石墨烯膜、石墨烯纤维及石墨烯在热界面材料中的应用。最后对石墨烯导热研究的成果进行总结,提出目前石墨烯热传导研究中存在的机遇与挑战,并展望未来可能的发展方向。     

Thermal properties measurement of cut tobacco based on TPS method and thermal conductivity model

Drying process of biomass porous media is widely involved in agricultural products processing. Accurate measurement of thermal properties and prediction of thermal conductivity variation at different conditions is the key of heat transfer simulation and optimization for drying process. The present work measured the thermal properties of cut tobacco in a constant temperature experimental platform by transient plane source method (TPS method), and developed a model to predict thermal conductivity of cut tobacco at different conditions. The results showed that there was a high test precision for thermal properties measurement of cut tobacco by TPS method. Thermal conductivity of cut tobacco increased significantly with the increase of temperature and moisture content at the range of 25–65 °C and 12.5–25 %. Volume heat capacities showed a similar trend. The model predictions of thermal conductivity showed strong correlation coefficient with experimental values. The deviation of model predictions is less than 10 %, which indicated that the established model had a good prediction precision for thermal conductivity of cut tobacco.     

The use of heat flow apparatus to find polymer transition points

As changes that occur at transition points, e.g. the glass temperature of polymers, involve factors such as thermal conductivity rather than exothermic or endothermic reactions it was considered that heat flow would be the most suitable technique to study the occurrence of these transitions. A simple heat flow apparatus is described and examples are given of its application to polymer films and to co-polymers.     

Thermal Properties of Polymers at Low Temperatures

Abstract

The existing measurements and theories of the low-temperature thermal properties, heat capacity, and thermal conductivity of polymers are reviewed with particular attention paid to the differences between partly crystalline and amorphous polymers. The most striking feature of the low-temperature heat capacity of polymers is that in the liquid helium temperature range the heat capacity does not depend upon the cube of the temperature as for other solids. Further, only well below 1°K does the heat capacity approach the value predicted on the basis of the sound velocity. This behavior indicates the presence of a small number of low-frequency modes of vibration in the frequency spectrum. The fact that such anomalous behavior seems linearly related to the crystallinity implies that this behavior is associated with the amorphous structure, perhaps with motions of pendent groups within cavities formed in the amorphous structure. The thermal conductivity of semicrystalline and amorphous polymers differs considerably. Semicrystalline polymers display a temperature dependence of the thermal conductivity similar to that obtained from highly imperfect crystals, the thermal conductivity having a maximum in the temperature range near 100°K which moves to lower temperatures and higher thermal conductivities as the crystallinity is increased. Amorphous polymers display a temperature dependence similar to that obtained for glasses with no maximum but a significant plateau region in the range between 5 and 15°K. The theoretical interpretation of the thermal conductivity of these materials is considered.     

Some elements in specific heat capacity measurement and numerical simulation of temperature modulated DSC (TMDSC) with R/C network

One important application of temperature modulated DSC (TMDSC) is the measurement of specific heat of materials. In this paper, a thermal resistance/capacitance (R/C) numerical model is used to analyze the effects of experimental parameters and calibration on the measurement of specific heat in TMDSC under isothermal conditions. The actual TMDSC experiments were conducted with sapphire and pure copper samples, respectively. Both simulation and experiments showed that in TMDSC, the measured sample specific heat is a non-linear function of many factors such as sample mass, the heat transfer properties of the TMDSC instrument, temperature modulation period, the heat capacity difference between calibration material and the test material, but modulation amplitude has very little effect on the results. The typical behavior of a heat flux type TMDSC can be described as a low pass filter in terms of specific heat capacity measurement when the instrument heat transfer properties are taken into account. At least for metallic materials, where the temperature gradient inside the sample can normally be ignored, the sample should be chosen in such a way that its total heat capacity (mass times specific heat) is close to that of the calibration material in order to get a more accurate result. Also, a large modulation period is beneficial to improving the test accuracy.     

导热膨胀石墨/聚醚酰亚胺复合材料的制备与性能

利用膨胀石墨(EG)经高温处理后比表面积大的特点, 以膨胀石墨作为导热填料, 通过球磨和热模压方法制备了膨胀石墨/聚醚酰亚胺(PEI)导热复合材料, 并对其加工过程、 微观形貌、 热性能和导热性能进行了研究. 结果表明, 球磨处理可以打破膨胀石墨的“泡沫”状态并减少石墨纳米片间的间隙, 热压可以诱使和促进石墨纳米片沿着水平方向排列和取向, 从而显著提升了复合材料的平面内导热性能. 当膨胀石墨在复合材料中的质量分数为20%时, EG/PEI复合材料的面内导热系数为2.38 W?m^?1?K^?1. 与PEI相比, 复合材料导热系数的增幅约为12倍. 所制备的EG/PEI复合材料均具有良好的散热能力、 较好的热稳定性和较高的储能模量, 是一种综合性能优异的导热材料.     

The gas temperature in and the gas expulsion from a graphite furnace used for atomic absorption spectrometry

A simplified model for heat transfer based on thermal conduction is used to calculate the radial gas temperature distribution inside a semi-enclosed, commercial graphite tube furnace used for atomic absorption spectrometry. In the absence of a forced convective flow of a purge gas, the gas temperature inside the graphite furnace during its heating is lower than the wall temperature. After the wall temperature has attained a steady-state value, the gas temperature approaches the wall temperature and the radial temperature gradient in the gas decreases. The difference between the wall temperature and the gas temperature depends on the temperature program used, the thermal properties of the purge gas, and the atomizer geometry. The residence time of relatively volatile analyte elements is largely controlled by expulsion when wall atomization at high heating rates and high atomization temperatures are used. Analytical sensitivities are often enhanced by vaporizing the analyte into a gas having an approximately constant temperature.     

Thermophysical properties of polypropylene/aluminum composites

This article is dedicated to the study of the thermal parameters of composite materials. A nonlinear least‐squares criterion is used on experimental transfer functions to identify the thermal conductivity and the diffusivity of aluminum‐polymer composite materials. The density measurements were achieved to deduce the specific heat and thereafter they were compared to values given by differential scanning calorimetry measurement. The thermal parameters of the composite material polypropylene/aluminum were investigated for the two different types of aluminum filler sizes. The experimental data were compared with several theoretical thermal conductivity prediction models. It was found that both the Agari and Bruggeman models provide a good estimation for thermal conductivity. The experimental values of both thermal conductivity and diffusivity have shown a better heat transport for the composite filled with large particles. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 722–732, 2004     

Applications of some thermo-analytical techniques to glasses and polymers

Thermal characterization of materials provides conclusions regarding the identification of materials as well as their purity and composition, polymorphism, and structural changes. Analytical experimental techniques for thermal characterization comprise of a group of techniques, in which physical properties of materials are ascertained through controlled temperature program. Among these techniques, traditional differential scanning calorimetry (DSC) is a well-accepted technique for analyzing thermal transitions in condensed systems. Modulated DSC (MDSC) is used to study the same material properties as conventional DSC including: transition temperatures, melting and crystallization, and heat capacity. Further, MDSC also provides unique feature of increased resolution and increased sensitivity in the same measurement. “Hot disk thermal constant analyzer”, based on Transient Plane Source (TPS) technique, offers simultaneous measurement of thermal transport properties of specimen, which are directly related to heat conduction such as thermal conductivity (λ) and thermal diffusivity (χ). This method enables the thermal analysis on large number of materials from building materials to materials with high thermal conductivity like iron. The temperature range covered so far extends from the liquid nitrogen point to 1000 K and should be possible to extend further. This review also presents some interesting results of phase transition temperature of miscible (CPI/TPI) and immiscible (PS/PMMA) polymeric systems carried out through dynamic mechanical analyzer along with the thermal transport properties obtained for cis-polyisoprene (CPI), trans-polyisoprene (TPI), and their blends determined by TPS technique.