The Thermal Conductivity, Thermal Diffusivity, and Heat Capacity of Gaseous Argon

This paper presents new absolute measurements for the thermal conductivity and thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. Six isotherms were measured in the supercritical dense gas at temperatures between 296 and 423 K and pressures up to 61 MPa. A new analysis for the influence of temperature-dependent properties and residual bridge unbalance is used to obtain the thermal conductivity with an uncertainty of less than 1% and the thermal diffusivity with an uncertainty of less than 4%. Isobaric heat capacity results were derived from measured values of thermal conductivity and thermal diffusivity using a density calculated from an equation of state. The heat capacities presented here have a nominal uncertainty of 4% and demonstrate that this property can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be useful when applied to fluids which lack specific heat data.     

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature‐dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂Se, Cu₂S, Ag₂S, and Ag₂Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.     

Thermal Conductivity,Thermal Diffusivity,and Heat Capacity of Gaseous Argon and Nitrogen

Low-pressure thermal conductivity and thermal diffusivity measurements are reported for argon and nitrogen in the temperature range from 295 to 350 K at pressures from 0.34 to 6.9 MPa using an absolute transient hot-wire instrument. Thermal conductivity measurements were also made with the same instrument in its steady-state mode of operation. The measurements are estimated to have an uncertainty of 1% for the transient thermal conductivity, 3% for the steady-state thermal conductivity, and 4% for thermal diffusivity. The values of isobaric specific heat, derived from the measured thermal conductivity and thermal diffusivity, are considered accurate to 5% although this is dependent upon the uncertainty of the equation of state utilized.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom     

The Thermal Conductivity, Thermal Diffusivity and Isobaric Heat Capacity of Toluene and Argon

This paper presents absolute measurements for the thermal conductivity and thermal diffusivity of toluene obtained with a transient hot-wire instrument employing coated wires over the density interval of 735 to 870 kgm^–3. A new expression for the influence of the wire coating is presented, and an examination of the importance of a nonuniform wire radius is verified with measurements on argon from 296 to 323 K at pressures to 61 MPa. Four isotherms were measured in toluene between 296 and 423 K at pressures to 35 MPa. The measurements have an uncertainty of less than ±0.5% for thermal conductivity and ±2% for thermal diffusivity. Isobaric heat capacity results, derived from the measured values of thermal conductivity and thermal diffusivity, using a density determined from an equation of state, have an uncertainty of ±3% after taking into account the uncertainty of the applied equation of state. The measurements demonstrate that isobaric specific heat determinations can be obtained successfully with the transient hot wire technique over a wide range of fluid states provided density values are available.     

Simultaneous Measurement of Specific Heat Capacity, Thermal Conductivity, and Thermal Diffusivity by Thermal Radiation Calorimetry

Thermal radiation calorimetry has been applied to measure the thermal diffusivity of a solid specimen, along with simultaneous measurements of specific heat capacity and thermal conductivity. In this calorimeter, a disk-shaped solid specimen whose surfaces are blackened is heated and cooled slowly on one face by irradiation in a vacuum chamber. A quasi-steady-state approximation in which a linear temperature gradient within the specimen was assumed is considered in the analysis. The validity of this approximation was confirmed by the results of computer simulation based on the control-volume method. Measurements of Pyroceram 9606 and Pyrex 7740 by use of thermocouples in the temperature range between 250 and 400°C gave values consistent with those obtained by previous authors, within experimental error, for all three thermophysical properties.     

New Transient Hot-Bridge Sensor to Measure Thermal Conductivity, Thermal Diffusivity, and Volumetric Specific Heat

A high sensitivity thermoelectric sensor to measure all relevant thermal transport properties has been developed. This so-called transient hot bridge (THB) decidedly improves the state of the art for transient measurements of the thermal conductivity, thermal diffusivity, and volumetric specific heat. The new sensor is realized as a printed circuit foil of nickel between two polyimide sheets. Its layout consists of four identical strips arranged in parallel and connected for an equal-ratio Wheatstone bridge. At uniform temperature, the bridge is inherently balanced, i.e., no nulling is required prior to a run. An electric current makes the unequally spaced strips establish an inhomogeneous temperature profile that turns the bridge into an unbalanced condition. From then on, the THB produces an offset-free output signal of high sensitivity as a measure of the properties mentioned of the surrounding specimen. The signal is virtually free of thermal emf’s because no external bridge resistors are needed. Each single strip is meander-shaped to give it a higher resistivity and, additionally, segmented into a long and short part to compensate for the end effect. The THB closely meets the specific requirements of industry and research institutes for an easy to handle and accurate low cost sensor. As the key component of an instrument, it allows rapid thermal-conductivity measurements on solid and fluid specimens from 0.02 to 100 W· m⁻¹·K⁻¹ at temperatures up to 250°C. Measurements on some reference materials and thermal insulations are presented. These verify the preliminary estimated uncertainty of 2% in thermal conductivity.     

The Thermal Conductivity, Thermal Diffusivity, and Specific Heat of Liquid n-Pentane

The thermal conductivity and thermal diffusivity of liquid n-pentane have been measured over the temperature range from 293 to 428 K at pressures from 3.5 to 35 MPa using a transient hot-wire instrument. It was determined that the results were influenced by fluid thermal radiation, and a new expression for this effect is presented. The uncertainty of the experimental results is estimated to be better than ±0.5% for thermal conductivity and ±2% for thermal diffusivity. The results, corrected for fluid thermal radiation, are correlated as functions of temperature and density with a maximum uncertainty of ±2% for thermal conductivity and ±4% for thermal diffusivity. Derived values of the isobaric specific heat are also given.     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.     

Measurements of Thermal Conductivity and Thermal Diffusivity of CVD Diamond

The thermal conductivity of natural, gem-quality diamond, which can be as high as 2500 Wm^–1 K^–1 at 25°C, is the highest of any known material. Synthetic diamond grown by chemical vapor deposition (CVD) of films up to 1 mm thick exhibits generally lower values of but under optimal growth conditions it can rival gem-quality diamond with values up to 2200 Wm^–1 K^–1. However, it is polycrystalline and exhibits a columnar microstructure. Measurements on free-standing CVD diamond, with a thickness in the range 25–400 m, reveal a strong gradient in thermal conductivity as a function of position z from the substrate surface as well as a pronounced anisotropy with respect to z. The temperature dependence of in the range 4 to 400 K has been analyzed to determine the types and numbers of phonon scattering centers as a function of z. The defect structure, and therefore the thermal conductivity, are both correlated with the microstructure. Because of the high conductivity of diamond, these samples are thermally thin. For example, laser flash data for a 25-m-thick diamond sample is expected to be virtually the same as laser flash data for a 1-m-thick fused silica sample. Several of the techniques described here for diamond are therefore applicable to much thinner samples of more ordinary material.     

热电材料中的晶格热导率

随着可再生能源及能源转换技术的快速发展, 热电材料在发电及制冷领域的应用前景受到越来越广泛的关注。发展具有高热电优值材料的重要性日益突出, 如何获得低晶格热导率是热电材料的研究重点之一。本文阐述了热容、声速及弛豫时间对晶格热导率的影响, 介绍了本征低热导率热电材料所具有的典型特征, 如强非谐性、弱化学键、本征共振散射及复杂晶胞结构等, 并分析了通过多尺度声子散射降低已有热电材料晶格热导率的方法, 其中包括点缺陷散射、位错散射、晶界散射、共振散射、电声散射等多种散射机制。此外, 总结了几种预测材料最小晶格热导率的理论模型, 对快速筛选具有低晶格热导率的热电材料具有一定的理论指导意义。最后, 展望了如何获得低热导率热电材料的有效途径。     

Specific Heat Capacity and Thermal Conductivity of Foam Glass (Type 150P) at Temperatures from 80 to 400 K1

Low-temperature specific heat capacities of foam glass (Type 150P) have been measured from 79 to 395 K by a precision automated adiabatic calorimeter. Thermal conductivities of the glass foams have been determined from 243 to 395 K with a flat steady-state heat-flow meter. Experimental results have shown that both the specific heat capacities and thermal conductivities of the 150P foam glass increased with temperature. Experimentally measured specific heat capacities have been fitted by a polynomial equation from 79 to 395 K: C _p /J · g⁻¹· K⁻¹=0.6889+0.3332x− 0.0578x ²+0.0987x ³+0.0521x ⁴− 0.0330x ⁵− 0.0629x ⁶, where x=(T/K − 273)/158. Experimental thermal conductivities as a function of temperature (T) have been fitted by another polynomial equation from 243 to 395 K: λ/ W · m⁻¹· K⁻¹=0.14433+0.00129T − 2.834 × 10⁻⁶ T ²+2.18 × 10⁻⁹ T ³. In addition, thermal diffusivities (a) of the form glass sample were calculated from the specific heat capacities and thermal conductivities and have been fitted by a polynomial equation as a function of temperature (T): a/m² · s⁻¹=−1.68285+0.01833T − 5.84891 × 10⁻⁵ T ²+8.11942 × 10⁻⁸ T ³ − 4.24975 × 10⁻¹¹ T ⁴.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

具有低热导率的Skutterudite类新型热电材料

结合声子散射机制介绍了近来报道的几种降低skutterudite类热电材料热导率的途径，为研究和发展skutterudite类热电材料提供一些思路和方法。     

Cu2S相变过程中热扩散系数的精确测量和解析

材料发生相变时, 其结构和物理性能可能会发生剧烈的变化。采用激光闪射法测量热扩散系数时, 激光照射样品可能会伴随有光吸收/发射现象以及温度的显著升高, 导致其测量值偏离真实值。本工作以Cu₂S为研究对象, 发现激光照射样品后, 光吸收/发射的影响很小可以忽略, 但样品温度的升高则会明显影响热扩散系数的测量。通过构建具有不同石墨层厚度的石墨/Cu₂S双层结构, 利用石墨层减弱激光照射时Cu₂S样品的温度增加幅度, 成功使热扩散系数出现显著降低的起始温度接近采用DSC测量材料发生相变的起始温度。本研究进一步建立了石墨/Cu₂S双层结构样品的热流输运模型, 从石墨/Cu₂S双层结构样品的实验测试热扩散系数中解析出了Cu₂S在相变区间的本征热扩散系数。本工作对于理解和精确表征具有相变特征的离子导体热电材料、光敏、热敏材料的热扩散系数具有重要的意义。     

Conventional Simultaneous Measurement of Specific Heat Capacity and Thermal Conductivity by Thermal Radiation Calorimetry

Thermal radiation calorimetry has been applied to measure the thermal conductivity and the specific heat capacity of an isolated solid specimen simultaneously. The system, in which a disk-shaped specimen and a flat heater are mounted in a vacuum chamber with the specimen heated on one face by irradiation, is presented. A theoretical formulation of the simultaneous measurement at quasi-steady state is described in detail. Noncontact temperature measurement of both specimen surfaces has been performed using pyrometers and a thermocouple set in the gap between the heater and the specimen. Pyroceram 9609 specimens, whose surfaces were blackened with colloidal graphite, were used in the measurement. The largest error involved in the noncontact temperature measurement is ±2°C in the range from 450 to 650°C. The resultant values of the specific heat capacity and the thermal conductivity deviate by about 10% from the recommended values for the Pyroceram specimen.     

A Nonstationary Method and Experimental Equipment for Measuring the Thermal Conductivity of Heat Insulators

A method of measuring the thermal conductivity of solid heat-insulating materials based on the integral form of Fourier's equation, obtained by an integro-interpolation method is described. The theoretical basis for the calculation formula of the method, the results of investigations of the formula using a thermal model, the circuit of a device which realizes the theoretical formula, and the accuracy and time characteristics of the proposed method are presented. __________ Translated from Izmeritel'naya Tekhnika, No. 8, pp. 38–43, August, 2005.     

Effect of Heat Treatment on Thermal Conductivity of U-Mo/Al Alloy Dispersion Fuel

The molybdenum content of fuel core whose matrix is aluminium 1060, was varied to be 7, 8, and 10 wt% and the volume fraction of U-Mo fuel powders was varied to be 10, 30, and 40 vol%. In this work, thermal conductivities were calculated from measured thermal diffusivities, specific heat capacities, and densities, which were determined using the laser flash, DSC, and Archimedes methods, respectively. The thermophysical properties were measured over a temperature range from room temperature to 500°C. The U-Mo alloy was annealed at between 525 and 550°C for 1 to 36 hours. At high temperature, the U-Mo particles were reacted with aluminium matrix as forming layers of (U-Mo)Al _x . These reaction layers have been affected adversely by the thermal conductivity of fuel core. The thermal conductivities of annealed samples appeared to decrease with increasing volume fraction of the reaction layers.     

多掺杂协同调控碲化锡热导率和功率因子提升热电性能

碲化锡(SnTe)是一种碲化铅的无铅替代物, 在热电领域有广阔的应用前景。但是, 纯相碲化锡样品具有较高的热导率与较低的塞贝克系数, 导致热电性能较差。本研究通过多重掺杂可以显著降低热导率, 提升塞贝克系数, 从而提升热电性能。SnTe热压样品的晶格热导率随着Se和S的引入明显降低,比如SnTe_0.7S_0.15Se_0.15室温下晶格热导率仅为0.99 W•m^-1•K^-1。透射电子显微镜显示, SnTe掺杂样品内存在大量的纳米沉淀相与晶格位错。在此基础上, 掺杂In在价带顶引入共振态大幅提高了样品的塞贝克系数。实验表明通过多重掺杂可以有效提升碲化锡的热电性能, 其中样品Sn_0.99In_0.01Te_0.7S_0.15Se_0.15在850 K时峰值ZT值达到0.8, 这说明碲化锡的确是一种有应用前景的中温区热电材料。     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.     

Simultaneous Measurements of the Thermal Conductivity and Thermal Diffusivity of Molten Salts with a Transient Short-Hot-Wire Method

A transient short-hot-wire technique has been successfully used to measure the thermal conductivity and thermal diffusivity of molten salts (NaNO₃, Li₂CO₃/K₂CO₃, and Li₂CO₃/Na₂CO₃) which are highly corrosive. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a short wire with the same length-to-diameter ratio and boundary conditions as those used in the actual experiments. In the present study, the wires are coated with a pure Al₂O₃ thin film by using a sputtering apparatus. The length and radius of the hot wire and the resistance ratio of the lead terminals and the entire probe are calibrated using water and toluene with known thermophysical properties. Using such a calibrated probe, the thermal conductivity and thermal diffusivity of molten nitrate are measured within errors of 3 and 20%, respectively. Also, the thermal conductivity of the molten carbonates can be measured within an error of 5%, although the thermal diffusivity can be measured within an error of 50%.     

Two-Wire Solution for Measurement of the Thermal Conductivity and Specific Heat Capacity of Liquids: Experimental Design

After a brief review of the hot-wire method, the design of an experiment that employs a two-wire technique is proposed. Several uncertainty sources are considered in order to define the optimal experimental conditions and evaluate the advantages of the two-wire technique. Convection and radiation effects and finite properties of the wires are discussed. The measurement uncertainties of the temperature rise, the heat flux generated by the hot wire, the time of the measurements, and the radial position of the second wire are considered. The influence of the uncertainty sources on the simultaneous estimation of thermal conductivity and specific heat capacity is analyzed for the hot-wire and two-wire techniques.