Thermal conductivity measurements at low temperatures

We describe briefly the experimental facilities developed for the measurement of thermal conductivity of solids in the temperature range 10K–300K. Different techniques have been used for the determination of thermal conductivity, depending on the relaxation time of the system under investigation. Measurements on stainless steel 304, using steady state and non-steady state methods are presented. Values of thermal conductivity obtained by both these methods agree to each other and are consistent with those reported earlier. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

A transient hot-wire instrument for thermal conductivity measurements in electrically conducting liquids at elevated temperatures

This paper describes a novel type of transient hot-wire cell for thermal conductivity measurements on electrically conducting liquids. A tantalum wire of 25 m. diameter is used as the sensing element in the cell, and it is insulated from the conducting liquids by an anodic film of tantalum pentoxide, 70 nm thick. The cell is suitable for measurements on conducting liquids at elevated temperatures. The results of test measurements on liquid water at its saturation vapor pressure are reported in order to confirm the correct operation of the thermal conductivity cell. The data, which have an estimated accuracy of ±3%, depart by less than ±1.8% from the correlation proposed by the International Association for the Properties of Steam. Results are also presented for concentrated aqueous solutions of lithium bromide, which are frequently used in absorption refrigerator cycles.     

Establishment of accuracy limits and standards for comparative thermal conductivity measurements

New techniques have been developed for reducing thermal conductivity data from thermal comparative measurements. The first of these techniques is based on making a Taylor-series expansion of the stack centerline temperature profile. The result is an expression giving the ratio of sample to reference conductivities at any temperature as a function of measured quantities, the stack thermocouple readings and stack element thicknesses. The conventional formula presently used to reduce comparative conductivity data is shown to be a special result of the general analysis. A second technique involves the use of linear least-squares (LS) techniques to derive both the sample and the reference conductivities from the measured data. The LS technique provides the coefficients for a polynomial temperature expansion of the reference and sample conductivities directly. Use of the new techniques is illustrated in a reduction of some comparative data on the conductivities of Pyrex 7740 and Pyroceram 9606. It is shown that a highly self-consistent pair of conductivity functions can be derived for these two commonly used reference materials if the conductivity vs temperature relation for Pyrex is modified slightly from its recommended value. The Pyroceram conductivity results from the comparative measurements are in good agreement with a conductivity derived from pulse diffusivity and differential scanning calorimetry measurements and also in good agreement with the recommended Pyroceram conductivity function.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

An apparatus for noncontact measurement of thermal conductivity by thermal radiation heating

Thermal radiation calorimetry was applied to measure the thermal conductivity of insulating solid specimens. We consider 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. A temperature difference between two faces was observed at elevated temperatures under steady-state conditions. An apparatus was developed using a thin graphite sheet as the heater element. Disk-shaped Pyrex glass and Pyroceram specimens, whose surfaces were blackened with colloidal graphite, were used in the measurements. Noncontact temperature measurement was performed using pyrometers and a thermocouple set in the gap between the heater and the specimen. Deviations of the estimated thermal conductivities from the recommended values were about 5% in the temperature range 250 to 800°C. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Transient hot-wire measurements of the thermal conductivity of gases at elevated temperatures

The paper describes a new instrument for the measurement of the thermal conductivity of gases over a wide range of thermodynamic states. The instrument operates on the transient hot-wire principle and the design of the cells that is necessary to secure an accuracy of ±0.3% in the thermal conductivity is considered in some detail. A selection of the results on ten pure gases is presented. The data for the monatomic gases in the limit of zero density are employed to confirm the accuracy of the measurements, whereas the results at higher densities for these and other gases are used to examine the concept of a temperature-independent excess thermal conductivity.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

An improved correlation for the calculation of liquid thermal conductivity

Eighteen correlations appearing in the literature for the prediction of thermal conductivity, , of liquids are critically analyzed, and their reliability is checked using coherent input data and selected experimental values. The best results are obtained using the Reid, Sherwood, and Prausnitz correlation with a mean deviation of about 8% between predicted and experimental values. An improved correlation is proposed starting from the Viswanath equation, chosen because of its simplicity and convenience. The values of thermal conductivity obtained by this new correlation agree with the experimental values within 1%.     

Thermal conductivity of ceramic fibrous insulators at high temperatures

The temperature and bulk density dependence of the thermal conductivity of commercial aluminosilicate fibrous insulators were studied by using the transient hot wire method. The thermal conductivity of ceramic fibrous insulators in both air and helium gas atmosphere increased with increasing bulk density. At high temperatures, however, the insulators with lower bulk density showed a higher thermal conductivity because of heat radiation. The following experimental relation between thermal conductivity and temperature was obtained for aluminosilicate fibruous insulators: =a exp(b). Relationships are given between the constants a and b and the bulk density. From the relation, the optimum bulk density of ceramic fibrous insulators can be calculated for each working temperature.Paper presented at the Fourth Japan Symposium on Thermophysical Properties, October 20–22, 1983, Yokohama, Japan.     

Determination of the thermal conductivity of xenon-helium mixtures at high temperatures by the shock-tube method

The thermal conductivity of gases at high temperatures has been measured by the shock-tube method, which is uniquely suited to measure thermal conductivities of gases at high temperatures above 2000 K. A consistent set of thermal-conductivity data over a wide range of temperatures has been obtained from optimum combinations of shock-tube experiments at high temperatures, previously published data at lower temperatures, and a theoretical correlation of the temperature dependence. In the present study, the thermal conductivity of xenon-helium mixtures has been determined at compositions of 10 and 30 mol% xenon over the temperature range from 300 to 4800 K. Even though there is a large difference between the thermal conductivity of pure xenon and that of helium, it is interesting that the dependences of the thermal conductivity of the mixture on temperature and composition are linear. The experimental results are in good agreement with the predicted values based on the corresponding-states principle and the mixing rule. From these experimental results, interpolating the corresponding-states correlation data, we represent the equation of xenon-helium gas mixtures for thermal conductivity in terms of temperature and composition.     

探针法测量低温下猪主动脉导热系数研究

低温下导热系数测定对生物器官的低温保存、低温外科医学及数值模拟计算至关重要.在分析探针法测量原理的基础上,用探针法对低温下猪主动脉的导热系数进行了测量研究.实验表明探针在用甘油和蒸馏水进行标定后,可以方便准确地测量-90～-35℃温区下猪主动脉的导热系数.     

Thermal conductivity and diffusivity measurements at cryogenic temperatures by double specimen technique

A double specimen technique is used in measuring the thermal conductivity and diffusivity of low thermal conductivity materials. In this technique good thermal contact is maintained between the heat source and sink and two geometrically similar specimens. A thin-copper heater plate is compressed between the two specimens and the temperature difference is measured between the heat source and the temperature controlled heat sink. Thermal conductivity is determined at steady state conditions by the differential method while the diffusivity is determined from transient measurements combined with an analytical solution to the one dimensional solution of the diffusion equation.     

A method for accurate thermal conductivity measurements of small samples at ultra-low temperatures

A specific experimental arrangement has been developed for low temperature measurements of thermal conductivity of small samples such as single crystals of magnetic insulators with a typical length of a few millimeters. A frame of low conductance, serving as a mechanical support for ruthenium thermometers recording the temperature gradient on a sample, has been tested in the temperature range from 150 mK to 5 K by using commercial 99.95% purity polycrystalline non-annealed molybdenum. The applicability of the setup is discussed for the samples with the thermal conductance in the range 10⁻⁵-10⁻³ W/K.     

Experimental apparatus for measuring the thermal diffusivity of pure fluids at high temperatures

Dynamic light scattering represents a suitable method for measuring the thermal diffusivity of optically transparent fluids. The classic application of the method is the immediate vicinity around the critical point due to its dependence upon the intensity of scattered light and its high sensitivity to undesired light scattering. By means of subsequent modifications of the experimental setup, we have been able to expand this region of applicability over the last 12 years and could systematically investigate numerous substances and their binary mixtures within a temperature range of 280 K<T<350 K. Our planned investigation of fluids suitable for ORC-HP-technology necessitates performing measurements at higher temperatures and pressures. The experimental apparatus newly designed for this purpose is capable of sustaining a relatively high temperature constance at temperatures up to 700 K. Factors restricting the measurable range of state and their influence on the design of the sample cell are discussed.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Reduction of lattice thermal conductivity by point defects at intermediate temperatures

The lattice thermal conductivity is reduced by point defects because they scatter phonons. An analytic expression can be derived only in the limit of high temperatures; at lower temperatures one must have recourse to numerical calculations. Because the conductivity is due mainly to phonons of low frequencies when point-defect scattering is strong, the high-temperature approximation can be used at temperatures above half the Debye temperature. Numerical calculations, using the Ge-Si system as an example, show that the error incurred by using the high-temperature approximation is less than 10%.     

An improved correlation for the thermal conductivity of HCFC123 (2,2-dichloro-1,1,1-trifluoroethane)

An updated survey of the existing thermal conductivity data for HCFC123 is presented. In addition, new wide-ranging thermal conductivity measurements, which have been carried out at NIST, are summarized. These results supplement the existing database and are used for an improved correlation of the thermal conductivity of HCFC123. The correlation covers the temperature range from 180 to 480 K with pressures up to 67 MPa or densities up to 1900 kg m⁻³. The correlation includes an empirical critical enhancement term of a form suitable for industrial use and represents the NIST dataset within ±2.22% at the 95% confidence level.     

制冷机做冷源的低温热导率测量装置研制

设计研制了1套以4.2 K制冷机为冷源的低温热导率快速测量装置.该装置采用可拆卸的具有独立真空环境的样品测试杆,使得样品测试部件与低温冷源部件分离,从而实现不破坏制冷机冷源真空环境及制冷机不复温的情况下快速更换样品.高真空绝热恒温样品杆通过高导热柔性热桥与制冷机冷头良好热耦合.热导率测试采用稳态绝热纵向热流法.除快速测...     

A new method for the evaluation of thermal conductivity and thermal diffusivity from transient hot strip measurements

The standard straight-line fit to data of a transient hot strip (THS) experiment to determine the thermal conductivity and thermal diffusivitya suffers from two major drawbacks: First, due to the statistical nature of the estimation procedure, there is no relation between the uncertainty of the measured value on one hand and the transport properties obtained on the other. Second, in order to account for he heat capacity of the strip and outer boundary conditions, two intervals of the plot must he rejected before analyzing it. So far, these intervals are selected arbitrarily. We now treat the THS working equation as a function of the four parameters concerned. a.U ₀ (initial voltage), andt ₀ (time delay). Chi-square fittings. following the Levenberg-Marquardt algorithm. are performed separately for several overlapping time intervals of the entire plot to find and a with minimal standard deviation. In the course of subsequent iterations an individual weighting factor is applied to each point to account for systematic errors. This procedure yields the "best" values of anda along with their individual errors. comprising the systematic and the statistical errors. Experimental results on Pyrex glass 7740 were taken to verify the new procedure.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder. Colorado, U.S.A.     

Viscosity and thermal conductivity of alkali metal vapors at temperatures up to 2000 K

New experimental data were obtained on transport coefficients of alkali metals in gaseous phase at high temperatures and within the pressure range from about 10 to about 100 kPa: lithium—thermal conductivity, T= 1400–1800 K, and viscosity, T=1600–2000 K; sodium-viscosity, T= 1100–1500 K; and cesiumviscosity, T=900–1250 K. Viscosity of the alkali metal vapors has been measured using a stationary-technique viscometer with an annular gap. Thermal conductivity was measured by the method of the nonstationary monotonous heating. Experimental data were used as a basis for computing effective atomatom and atom-molecule collision cross section, the values obtained from data on viscosity being in good agreement with those derived from thermal conductivity data. In the case of lithium, the atom-atom cross sections yielded by experiments are fairly consistent with the results of calculations with exact formulae of kinetic theory on the basis of quantum-mechanical potential curves for atom-atom interactions. This has enabled the authors to compile consistent tables of viscosities and thermal conductivities for lithium in a gaseous phase within the temperature range from 800 to 2500 K and pressures from 0.5 to 800 kPa, including the saturation curve.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of liquids under microgravity conditions

A transient short-hot-wire technique is proposed and used to measure the thermal conductivity and thermal diffusivity of liquids simultaneously. The method is based on the numerical evaluation of unsteady heat conduction from a wire with the same length diameter ratio and boundary conditions as those in the experiments. To confirm the applicability and accuracy of this method. Measurements were made for five sample liquids with known thermophysical properties and were performed under both normal gravity and microgravity conditions. The results reveal that the present method determines both the thermal conductivity and the diffusivity within 2 and 5%. respectively. The microgravity experiments clearly indicate that even under normal gravity conditions, natural-convection effects are negligible for at least l s after the start of heating. This method would be particularly suitable for a valuable and expensive liquid, and has a potential for application to electrically conducting and or corrosive liquids when the probe is effectively coated with an insulating and anticorrosive material. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Comments on the measurement of thermal conductivity and presentation of a thermal conductivity integral method

A discussion is presented regarding the significance of the spatial temperature gradient approximation normally used in thermal conductivity measurement. Examples are presented illustrating the magnitude of temperature differences allowed for conductivity integral (TCI) method of analysis is presented as an alternative method which totally eliminates the need to impose temperature difference restrictions on the measurement process, so long as other errors, such as radiative heat losses, do not become excessive.