Thermal Conductivity of Ionic Liquids: Measurement and Prediction

This study reports thermal-conductivity data for a series of [EMIM] (1-ethyl-3-methylimidazolium)-based ionic liquids (ILs) having the anions [NTf₂] (bis(trifluoromethylsulfonyl)imide), [OAc] (acetate), [N(CN)₂] (dicyanimide), [C(CN)₃] (tricyanomethide), [MeOHPO₂] (methylphosphonate), [EtSO₄] (ethylsulfate), or [OcSO₄] (octylsulfate), and in addition for ILs with the [NTf₂]-anion having the cations [HMIM] (1-hexyl-3-methylimidazolium), [OMA] (methyltrioctylammonium), or [BBIM] (1,3-dibutylimidazolium). Measurements were performed in the temperature range between (273.15 and 333.15) K by a stationary guarded parallel-plate instrument with a total measurement uncertainty of 3 % (k = 2). For all ILs, the temperature dependence of the thermal conductivity can well be represented by a linear equation. While for the [NTf₂]-based ILs, a slight increase of the thermal conductivity with increasing molar mass of the cation is found at a given temperature, the [EMIM]-based ILs show a pronounced, approximately linear decrease with increasing molar mass of the different probed anions. Based on the experimental data obtained in this study, a simple relationship between the thermal conductivity, molar mass, and density is proposed for the prediction of the thermal-conductivity data of ILs. For this, also densities were measured for [EMIM][OAc], [EMIM][C(CN)₃], and [HMIM][NTf₂]. The mean absolute percentage deviation of all thermal-conductivity data for ILs found in the literature from the proposed prediction is about 7 %. This result represents a convenient simplification in the acquisition of thermal conductivity information for the enormous amount of structurally different IL cation/anion combinations available.     

Measurement and Prediction of the Thermal Conductivity of Tricyanomethanide- and Tetracyanoborate-Based Imidazolium Ionic Liquids

The thermal conductivity of ten ionic liquids (ILs) based on the anions \([\mathrm{C(CN)}_{3}]^{-}\) (tricyanomethanide) and \([\mathrm{B(CN)}_{4}]^{-}\) (tetracyanoborate) carrying a homologous series of the [alkyl-MIM] \(^{+}\) (1-alkyl-3-methylimidazolium) cations [EMIM] \(^{+}\) (ethyl), [BMIM] \(^{+}\) (butyl) [HMIM] \(^{+}\) (hexyl), [OMIM] \(^{+}\) (octyl), [DMIM] \(^{+}\) (decyl) was measured by a steady-state guarded parallel-plate instrument in the temperature range between (283.15 and 353.15) K at atmospheric pressure with a total uncertainty of 5 % ( \(k\,=\,2\) ). Furthermore, the refractive index required for data evaluation and the density, which is an important property in the developed prediction method for the thermal conductivity, were determined. In general, the measured thermal conductivities of the probed ILs decrease with increasing temperature and increasing alkyl-chain length of the cation. Regarding the influence of the anion, somewhat smaller values for the \([\mathrm{B(CN)}_{4}]^{-}\) -based ILs compared to the \([\mathrm{C(CN)}_{3}]^{-}\) -based ILs carrying the same cation are observed. Our previously developed simple prediction method for the thermal conductivity of ILs at 293.15 K using only information on the molar mass and the density could be improved. By the combination of this approach with the temperature dependence of the density, an extended empirical correlation additionally describing the temperature dependence of the thermal conductivity of ILs is recommended. This correlation represents all experimental thermal-conductivity data in the literature with a standard deviation of less than 7 %.     

瞬态热线法导热系数测量实验数据处理方法的研究

为克服瞬态热线法导热系数测量中实验数据处理一般方法的弊端,将数值模拟引入实验数据处理过程,并通过比较理论计算曲线与实验曲线的符合程度来获得最终的实验结果。通过不同方法对实验数据处理结果的比较分析表明,所使用的方法可以更好地处理瞬态热线法导热系数测量数据,同时,与传统方法相比,采用较少的数据点即可得到正确的结果。研究结果不仅可以改进瞬态热线法导热系数实验数据的分析方法,而且对实验系统的设计与搭建也有借鉴意义。     

瞬态热带恒功率法测量导热率

基于传统热桥法基本原理,提出了新的非对称结构模型--瞬态热带恒功率法用于测量材料的导热率。非对称结构模型将热源热带与温度传感热带电路彼此独立,消除了热带电阻自热以及环境温度变化带来的影响。通过短时间内快速测量恒流电路中两热带的电压差随时间的变化关系准确测量材料的导热率。依据此模型设计了热带加热片,建立了基于LabVIEW测试软件的实验装置自动测量平台,实验测量结果与参考材料导热率具有良好的一致性,相对偏差在3％左右,验证了该理论模型的有效性。     

热线法测量半透明固体材料的导热系数

建立了适用于热线半透明固体材料导热系数测试系统的简化解析模型。针对一种半透明固体材料—Ｋ９冕玻璃，在２９７～１２３０Ｋ温度范围内进行了测试研究。     

固化吸附剂导热系数测量参数选择

在开发高导热吸附剂过程中经常需要测量材料的导热系数,稳态平板法测量导热系数因方法简单直接而应用广泛.通过数值模拟和实验发现侧面散热对导热系数测量影响很大,采用大空间自然对流换热实验关联式对试验数据处理公式进行修正,可得到较满意的实验结果,并进一步研究了热源温度、样品厚度、试样导热系数对侧面散热的影响.为减少侧面散热以及热电偶波动对测量结果的影响,并考虑到试样加工难易程度,就所用实验装置而言,试样厚度在12mm较为适宜,热源温度选用55℃较为适宜.     

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.     

Measurement of the Thermal Conductivity and Thermal Diffusivity of Liquids. Part I: “Pure Conduction and an Example of a Solution”

The measurement of the thermal conductivity of liquids is rather complicated due to the nature of the fluid. To the conduction, which has to be characterized, are added the natural convection, the radiative transfer, and the perturbations caused by the presence of enclosure walls. The goal of this work, composed of two parts, is to implement an experimental bench allowing the measurement of the thermal diffusivity and thermal conductivity of liquids. The first part (Part I) presented here, is about pure conduction and focuses on several aspects involved in this measurement, which will lead one, based on theoretical and practical considerations, to choose a pulse method in a one-dimensional (1D) and cylindrical geometry to solve the problem. In the second section of this part, the problem of the parameters estimation is investigated with the presence of the walls of the measuring cell and this will allow us to define the characteristics of the walls (thickness and thermophysical properties). The entire problem is treated through the thermal quadrupoles method. Finally, in a last section, a setup at room temperature is described. The second part (Part II) of this work that is presented in another paper will show how it is possible to get rid of the convection by a judicious choice of the extension of the measuring cell and how the radiation effects can be taken into account to perform measurements at high temperatures (up to 500°C).Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Corrections for Thermal Expansion in Thermal Conductivity Measurement of Insulations Using the High-Temperature Guarded Hot-Plate Method

The anticipation of recently published European product standards for industrial thermal insulation has driven improvements in high-temperature thermal conductivity measurements in an attempt to obtain overall measurement uncertainties better than 5 % (k = 2). The two measurement issues that are focused on in this article are the effect of thermal expansion on in situ thickness measurement and on determining the metering area at high temperatures. When implementing in situ thickness measurements, it is vital to correct the thermal expansion of components in a high-temperature guarded hot plate (HTGHP). For example, in the NPL HTGHP this could cause 3.2 % measurement error for a 50 mm thick specimen at 800 °C. The thermal expansion data for nickel 201 measured by NPL are presented, and the effect of this on the metering area of NPL’s heater plate (nickel 201) is discussed.     

Thermal Conductivity Measurement of Semitransparent Solids by Hot-Wire Technique1

A new simple analytic model applicable to the measurement system of a hot wire and a semitransparent solid material is developed. An experimental study is carried out on a special glass sample, glass K9, in the temperature range of 297 to 1230 K, and the radiation-free thermal conductivity is reported.     

Determining Effective Thermal Conductivity of Fabrics by Using Fractal Method

In this article, a fractal effective thermal conductivity model for woven fabrics with multiple layers is developed. Structural models of yarn and plain woven fabric are derived based on the fractal characteristics of macro-pores (gap or channel) between the yarns and micro-pores inside the yarns. The fractal effective thermal conductivity model can be expressed as a function of the pore structure (fractal dimension) and architectural parameters of the woven fabric. Good agreement is found between the fractal model and the thermal conductivity measurements in the general porosity ranges. It is expected that the model will be helpful in the evaluation of thermal comfort for woven fabric in the whole range of porosity.     

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.     

Measurement of Thermal Conductivity of Anisotropic SiC Crystal

Silicon carbide (SiC) crystals with excellent heat conduction and thermal stability can be widely used in microelectronic devices and integrated circuits. It is important for the study of a functional type SiC material to have accurate thermal-conductivity and thermal-diffusivity values of SiC crystal. A 3ω technique is employed to determine the anisotropic thermal conductivity of SiC crystal. Three micrometal probes with different widths are deposited by chemical-vapor deposition on the surface of SiC crystal. Each micrometal probe is used as a heater, and also as a thermometer. The temperature fluctuation signals of a micrometal probe represent heat conduction in different directions in the specimen. Thermal conductivities both in the cross-plane and in-plane directions of SiC crystal are achieved through fitted values. The results indicate that thermal conductivities in three different directions of SiC crystal can be characterized using the metal heater construction.     

Measurement of the Thermal Conductivity of Nanodeposited Material

The small size of nanomaterials deposited by either focused ions or electron beams has prevented the determination of reliable thermal property data by existing methods. A new method is described that uses a suspended platinum hot film to measure the thermal conductivity of a nanoscale deposition. The cross section of the Pt film needs to be as small as 50 nm × 500 nm to have sufficient sensitivity to detect the effect of the beam-induced nanodeposition. A direct current heating method is used before and after the deposition, and the change in the average temperature increase of the Pt hot film gives the thermal conductivity of the additional deposited material. In order to estimate the error introduced by the one-dimensional analytical model employed, a two-dimensional numerical simulation was conducted. It confirmed the reliability of this method for situations where the deposit extends onto the terminals by (1 μm or more. Measurements of amorphous carbon (a-C) films fabricated by electron beam induced deposition (EBID) produced thermal conductivities of 0.61 W · m⁻¹ · K⁻¹ to 0.73 W · m⁻¹ · K⁻¹ at 100 K to 340 K, values in good agreement with those of a-C thin films reported in the past.     

Thermal Conductivity Measurement of an Electron-Beam Physical-Vapor-Deposition Coating

An industrial ceramic thermal-barrier coating designated PWA 266, processed by electron-beam physical-vapor deposition, was measured using a steady-state thermal conductivity technique. The thermal conductivity of the mass fraction 7 % yttria-stabilized zirconia coating was measured from 100 °C to 900 °C. Measurements on three thicknesses of coatings, 170 μm, 350 μm, and 510 μm resulted in thermal conductivity in the range from 1.5 W/(m·K) to 1.7 W/(m·K) with a combined relative standard uncertainty of 20 %. The thermal conductivity is not significantly dependent on temperature.     

Measurement of the Thermal Conductivity of Si and GaAs Wafers Using the Photothermal Displacement Technique

Thermal conductivity and thermal diffusivity of Si and GaAs wafers were measured using the photothermal displacement technique, and the temperature dependence of these two quantities was investigated. Thermal diffusivity was obtained from the phase difference between the heating source and the signal, and thermal conductivity was determined from the maximum value of the signal amplitude in the temperature range 80 to 300 K. It was verified that an increase in doping concentration gives rise to a decrease in thermal conductivity at low temperatures. The experimental results obtained on samples with different types and doping concentrations are consistent with those expected from theoretical considerations.     

热探针法测定含湿土壤的导热系数

根据线热源瞬态导热模型,设计制作了用于测定含湿土壤导热系数的热探针,研究了加热功率、加热时间、探针壁厚对导热系数测定的影响。对不同含水量的土壤导热系数进行测定,得出纯土、河沙含水量～导热系数拟合关系式。结果表明,热探针法测定含湿土壤的导热系数具有便捷、准确的特点。     

周期热流法测定金属导热系数

根据周期热流法的测试原理,采用理论分析、实验测定和数值仿真相结合的研究方法,设计了一套导热系数测试系统.利用先进的数据采集系统,通过自编程实现了数据的自动采集与处理.通过数值实验发现所需测试试样的最小长度远小于相关文献中的穿透深度.对几种试样进行了测试,发现导热系数最大误差不超过5%,经仿真修正后误差在3%以内.该测试系统具有简单、可行、数据可靠的特点.     

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.