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.     

Thermal diffusivity, thermal conductivity, and specific heat of flax fiber–HDPE biocomposites at processing temperatures

There is increasing work on the use of flax fibers as reinforcement for manufacturing composites because of their lower cost and environmental benefit. During manufacturing of such natural fiber–plastic composites, heat transfer is involved, but information about the thermal conductivity and thermal diffusivity at the processing temperatures is not available. In this study, the thermal conductivity, thermal diffusivity, and specific heat of flax fiber–high density polyethylene (HDPE) biocomposites were determined in the temperature range of 170–200 °C. The fiber contents in biocomposites were 10%, 20%, and 30% by mass. Using the line-source technique, the instrumental setup was developed to measure the thermal conductivity of biocomposites. It was found that the thermal conductivity, thermal diffusivity, and specific heat decreased with increasing fiber content, but thermal conductivity and thermal diffusivity did not change significantly with temperature in the range studied. The specific heat of the biocomposites increased gradually with temperature.     

Thermophysical Properties of Plant Leaves and Their Influence on the Environment Temperature

It is known that the thermal properties of a material influence the temperature around it. Once heated, the rate at which a material transfers the absorbed heat into the surroundings is determined by the thermal effusivity (or thermal inertia) of the material, and it depends on the well-known thermal properties, thermal conductivity, and specific heat capacity. Since a direct measurement of these properties is rather difficult for thin biological specimens such as plant leaves, a photothermal technique is used to measure the thermal effusivity, thermal diffusivity, thermal conductivity, and specific heat capacity for a few representative species of plant leaves. Measurements have been carried out on fresh as well as dry leaves to estimate the differences in their properties. Thermal properties of plant leaves are compared with the corresponding properties of two materials abundant in the environment and discussed. The influence of thermal properties, particularly the thermal effusivity and specific heat capacity, of plant leaves on controlling the temperature of the environment around them is discussed.     

点热源热脉冲法测算生物流体的热物性参数

采用微珠状热敏电阻作为点热源和测温元件，在一维点源脉冲传热模型的基础上建立一种同时测量生物流体热扩散系数、导热系数和热容的瞬态方法。运用非线性参数拟合，直接从感温热敏电阻对热脉冲温度响应中同时获取待测的热物性参数。实验中设计了一个高灵敏度的温度测量电路，测试结果表明，本方法测量误差小于4％。此外，还讨论了测量数据的处理和影响测量的因素。     

Modeling of Effective Thermal Conductivity of Dunite Rocks as a Function of Temperature

The thermal conductivity, thermal diffusivity, and heat capacity per unit volume of dunite rocks taken from Chillas near Gilgit, Pakistan, have been measured simultaneously using the transient plane source technique. The temperature dependence of the thermal transport properties was studied in the temperature range from 303 K to 483 K. Different relations for the estimation of the thermal conductivity are applied. A proposed model for the prediction of the thermal conductivity as a function of temperature is also given. It is observed that the values of the effective thermal conductivity predicted by the proposed model are in agreement with the experimental thermal conductivity data within 9%.     

Transient Hot Wire (THW) Method: Uncertainty Assessment

The standard method for measuring thermal transport properties of dielectric solids such as ceramics and refractories is the transient hot wire (THW) technique. In its simplest arrangement, a thin wire is embedded between two sample halves, where it acts simultaneously as a resistive heat source and a thermometer. From its temperature signal, the thermal conductivity and the thermal diffusivity of the dielectric can be derived. Up to now, there is no uncertainty assessment for this technique strictly following the ISO Guide to the Expression of Uncertainty in Measurement. Here we analyze the ISO standard uncertainty of the THW technique in the same way as in a previous paper on the uncertainty of the closely related transient hot strip (THS) technique. The two papers provide a comprehensive comparison of the most important advantages and disadvantages of these two transient techniques. The results obtained here for the uncertainty (5.8% for the thermal conductivity and 30% for the thermal diffusivity) are nearly the same as those for the THS method. Experiments on a Pyrex standard-reference sample confirm the results.     

Modeling of the Effective Thermal Conductivity of Consolidated Porous Media with Different Saturants: A Test Case of Gabbro Rocks

The thermal conductivity and thermal diffusivity of porous consolidated gabbro rocks have been measured simultaneously by the transient plane source technique at normal temperature and pressure using air and water as saturants. The density and porosity are measured using American Society for Testing and Materials (ASTM) standards under ambient conditions. The mineral composition is obtained using a petrography technique. Data are presented for 12 specimens of gabbro, taken from Warsik near Peshawar, Pakistan. A recently proposed empirical model for the prediction of the thermal conductivity of porous consolidated igneous rocks is established using different fluids in pore spaces, under ambient conditions. An exponential decay formula is also proposed for the prediction of the thermal conductivity at room temperature and normal pressure. The results are compared with different existing empirical models. A simple correlation between density and porosity is also reported.     

The thermal conductivity and heat capacity of gaseous argon

This paper presents new absolute measurements of the thermal conductivity and of the thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. We measured seven isotherms in the supercritical dense gas at temperatures between 157 and 324 K with pressures up to 70 MPa and densities up to 32 mol · L^–1 and five isotherms in the vapor at temperatures between 103 and 142 K with pressures up to the saturation vapor pressure. The instrument is capable of measuring the thermal conductivity with an accuracy better than 1% and thermal diffusivity with an accuracy better than 5%. Heat capacity results were determined from the simultaneously measured values of thermal conductivity and thermal diffusivity and from the density calculated from measured values of pressure and temperature from an equation of state. The heat capacities presented in this paper, with a nominal accuracy of 5%, prove that heat capacity data can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be invaluable when applied to fluids which lack specific heat data or an adequate equation of state.     

Simultaneous measurements of thermal conductivity and diffusivity of Se80Te20-xInx (x = 2, 4, 6 and 10) chalcogenide glasses at room temperature

Measurements of thermal conductivity and thermal diffusivity of twin pellets of Se₈₀Te_20-xIn_x (x = 2, 4, 6 and 10) glasses, prepared under a load of 5 tons were carried out at room temperature using transient plane source (TPS) technique. The measured values of both thermal conductivity and diffusivity were used to determine the specific heat per unit volume of the said materials in the composition range of investigation. Results indicated that both the values of thermal conductivity and thermal diffusivity increased with the addition of indium at the cost of tellurium whereas the specific heat remained almost constant. This compositional dependence behaviour of the thermal conductivity and diffusivity has been explained in terms of the iono-covalent type of bond which In makes with Se as it is incorporated in the Se-Te glass.     

Measurements of Thermal Diffusivity for Thin Slabs by a Converging Thermal Wave Technique

The measurement of thermal diffusivity for thin slabs by a converging thermal wave technique has been studied. Temperature variation at the center of the heat source ring that is produced by a pulsed high-power laser is detected by an infrared detector. A computer program based on the finite difference method is developed to analyze the thermal diffusivity of the slabs. Materials of both high thermal diffusivity (CVD diamond wafer) and low thermal diffusivity (stainless-steel foil) have been used for the measurements. The measurements have been performed by varying the size and the thickness of specimen. The converging thermal wave technique has proved to be a good method to measure the thermal diffusivity of a CVD diamond without breaking the wafer into small specimens. The technique can be applied for a small slab if the diameter of the slab is two times larger than that of the heat source ring. The sensitivity of thickness in measuring the thermal diffusivity is low for ordinary CVD diamond. The use of the converging thermal wave technique for nonhomogeneous, nonuniform, and anisotropic materials has been accomplished by applying the finite difference method.     

Evaluation of Thermophysical Properties of Functionally Graded Materials

In this work, by considering four-layered functionally graded material (FGM) specimens of Cu/Ni and PSZ/NiCrAlY, the transient characteristics and homogeneity of heat conduction media have been studied. The thermal diffusivities of the considered specimens have been measured by the laser flash method. As the temperature response curve of a FGM is very similar to that of a homogeneous material, it is difficult to distinguish a FGM from a homogeneous material by the shape of the temperature responses. Therefore, the thermal diffusivity obtained from the half-time method is usually taken as the corresponding value of the thermal diffusivity. The apparent thermal conductivity, obtained from the corresponding value of the thermal diffusivity and the average of the heat capacity of each layer, is different from the effective thermal conductivity, obtained from the sum of the heat resistances of each layer. As the values of the heat capacity of materials exist over a certain range, and the heat capacity distribution can be predicted when the materials in a FGM are known, the amount of error that will be caused when the effective thermal conductivity is replaced by the apparent value can be determined. Also, the heterogeneity of a FGM, based on an evaluation of thermophysical properties, has been discussed.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui,P. R. China.     

The thermal diffusivity and conductivity of transformation-toughened solid solutions of alumina and chromia

The thermal diffusivity of a series of solid solutions of alumina and chromia transformation toughened with a dispersed phase of unstabilized zirconia was measured by means of the laser-flash method from room temperature to 1400° C. It was found, in general, that the thermal diffusivity could be decreased significantly by the combined effects of solid solution alloying, microcracking and by the presence of the low conductivity dispersed phase of zirconia. The decrease in thermal diffusivity by microcracking was found to be present in the solid solution with low chromia content which underwent extensive grain growth. The effectiveness of solid solution formation and microcracking on thermal diffusivity was found to be greatest at the lower and intermediate ranges of temperature. The decrease in the thermal diffusivity due to the zirconia inclusions was found to be effective over the total temperature range. A numerical example is presented for the thermal conductivity calculated from the thermal diffusivity multiplied by the volumetric heat capacity.     

Measurement of thermal diffusivity of solids using infrared thermography

We report measurement of thermal diffusivity of solid samples by using a continuous heat source and infrared thermal imaging. In this technique, a continuous heat source is used for heating the front surface of solid specimen and a thermal camera for detecting the time dependent temperature variations at the rear surface. The advantage of this technique is that it does not require an expensive thermal camera with high acquisition rate or transient heat sources like laser or flash lamp. The time dependent heat equation is solved analytically for the given experimental boundary conditions. The incorporation of heat loss correction in the solution of heat equation provides the values of thermal diffusivity for aluminum, copper and brass, in good agreement with the literature values.     

Effects of detector nonlinearity and specimen size on the apparent thermal diffusivity of NIST 8425 graphite

With the use of a graphite thermal conductivity standard it is demonstrated that optical detector non-linearity, coupled with excessive laser pulse energies, is primarily responsible for the anomalous specimen size dependence of the thermal diffusivity measured by the laser-pulse technique. High laser pulse energies also result in an anomalous positive temperature dependence for thin specimens near room temperature, in contrast to the expected negative temperature dependence. Using moderately thick specimens and attenuated laser pulses yields excellent agreement with thermal diffusivity calculated from standard thermal conductivity data.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of Se90?xTe5Sn5Inx (x?=?0, 3, 6, and 9) multi-component chalcogenide glasses

Measurements of thermal conductivity and thermal diffusivity of twin pellets of Se_90−x Te₅Sn₅In _x (x = 0, 3, 6, and 9) chalcogenide glasses were carried out at room temperature using transient plane source technique. The measured values of thermal conductivity and thermal diffusivity were used to determine the specific heat per unit volume of these glasses in the composition range of investigation. Results indicated that both values of thermal conductivity and thermal diffusivity were increased with addition of indium concentration at the cost of selenium, whereas the specific heat per unit volume was slightly decreases with increase of indium content. This compositional dependence behavior of the thermal conductivity and diffusivity can be explained in terms of the iono-covalent type of bonds, which In (indium) makes with Se as it is incorporated in the Se–Te–Sn glass.     

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.     

Stepwise and Pulse Transient Methods of Thermophysical Parameters Measurement

Stepwise transient and pulse transient methods are experimental techniques for measuring the thermal diffusivity and conductivity of solid materials. Theoretical models and experimental apparatus are presented, and the influence of the heat source capacity and the heat transfer coefficient is investigated using the experiment simulation. The specimens from low-density polyethylene (LDPE) and polymethylmethacrylate (PMMA) were measured by both methods. Coefficients of variation were better than 0.9 % for LDPE and 2.8 % for PMMA measurements. The time dependence of the temperature response to the input heat flux showed a small drop, which was caused by thermoelastic wave generated by thermal expansions of the heat source.     

Determination of the thermal conductivity and diffusivity of thin fibres by the composite method

It is suggested that the thermal conductivity of very fine fibres can be evaluated indirectly with the aid of composite theory using the experimental data for the heat transport properties of an appropriate composite which contains the fibres. The feasibility of this approach was investigated by determining the thermal conductivity and diffusivity of fibres of amorphous silicon carbide from 25° C to 1000° C contained within a lithium aluminosilicate glass-ceramic using the laser-flash technique for measurement of the thermal diffusivity of the composite. Due to the amorphous nature of the fibres, values for their thermal conductivity and diffusivity were found to be far less than the corresponding data for crystalline silicon carbide. The positive temperature dependence of the thermal conductivity, coupled with the independent observation of an increase in thermal conductivity with specimen thickness, suggests that radiative heat transfer makes a significant contribution to the total heat transferred. A number of advantages and limitations of the composite method for the evaluation of thermal transport properties of fibres are discussed.     

Experimental investigation and modeling of effective thermal conductivity and its temperature dependence in a carbon-based foam

The effects of test temperature and a graphitization heat treatment on thermal and thermo-mechanical properties of a carbon-based foam material called CFOAM^® are investigated experimentally. Thermal diffusivity is determined using a laser flash method, heat capacity via the use of differential scanning calorimetry, while (linear) thermal expansion is measured using a dilatometric technique. Experimental results are next used to compute the effective thermal conductivity and the coefficient of thermal expansion as a function of test temperature. The computed thermal conductivity results are then compared with their counterparts obtained using our recent model. The agreement between the experiment-based and the model-based results is found to be fairly good only in the case when the graphitization temperature is high relative to the maximum test temperature and, hence, CFOAM^® does not undergo a significant additional graphitization during testing. A potential use of CFOAM^® as an insulation material in thermal protection systems for the space vehicles is discussed.     

Modified Dynamic Plane Source Method for Measuring Thermophysical Parameters of Solids

The authors proposed and experimentally verified a new method of measurements of thermal conductivity and thermal diffusivity of homogeneous and isotropic materials with thermal conductivities of <2 W ·m^-1 ·K^-1{2\,{\rm W} \cdot{\rm m}^{-1} \cdot{\rm K}^{-1}} . The theoretical model and the experimental arrangement of the method, referred to as the modified dynamic plane source method, are described. The influence of the floating temperature of the heat sink was analyzed and included into the evaluation. The method was both theoretically and experimentally compared with other transient methods, namely, transient plane source and extended dynamic plane source methods. Two different polymethylmethacrylate materials were measured at laboratory temperature using all three methods, obtaining results with coefficients of variation of <2 % and 5 % for the thermal conductivity and thermal diffusivity, respectively. In addition, the sensor calibration and the sources of measurement uncertainty are outlined.