Determining Thermal Conductivity and Thermal Diffusivity of Low-Density Gases Using the Transient Short-Hot-Wire Method

The transient short-hot-wire method for measuring thermal conductivity and thermal diffusivity makes use of only one thermal-conductivity cell, and end effects are taken into account by numerical simulation. A search algorithm based on the Gauss–Newton nonlinear least-squares method is proposed to make the method applicable to high-diffusivity (i.e., low-density) gases. The procedure is tested using computer-generated data for hydrogen at atmospheric pressure and published experimental data for low-density argon gas. Convergence is excellent even for cases where the temperature rise versus the logarithm of time is far from linear. The determined values for thermal conductivity from experimental data are in good agreement with published values for argon, while the thermal diffusivity is about 10 % lower than the reference data. For the computer-generated data, the search algorithm can return both thermal conductivity and thermal diffusivity to within 0.02 % of the exact values. A one-dimensional version of the method may be used for analysis of low-density gas data produced by conventional transient hot-wire instruments.     

Thermal Diffusivity and Sound Velocity of Toluene Over a Wide Temperature Range

The thermal diffusivity and the sound velocity of toluene have been determined in a wide temperature range up to the critical point by dynamic light scattering. Measurements were performed for both the liquid and the vapor phase at saturation conditions. The results obtained corroborate the usefulness of the technique for the determination of thermophysical properties. From the lack of reference data for the thermal diffusivity or other properties from which the thermal diffusivity may be derived in the high-temperature range and from deviations of experimental sound velocity data compared to values derived from the currently established equation of state, the need for further experimental investigations on this important reference fluid was established.     

Anisotropic Thermal Diffusivity Measurements in Deforming Polymers and the Stress-Thermal Rule

In recent years, both experimental and theoretical research on energy transport in deforming polymeric materials has increased. Theoretical results indicate that the thermal conductivity in such systems is anisotropic, and support, analogous to the well-known stress-optic rule, the validity of a stress-thermal rule where the thermal conductivity and stress tensors are linearly related. In this study a method to measure the thermal diffusivity in deforming polymers has been developed. The method is based on an optical technique known as forced Rayleigh scattering. This sensitive and non invasive technique is shown to be capable of quantitative measurements of anisotropic thermal diffusivity in both static and dynamic (relaxing) polymers subjected to deformations. Results have been obtained for a polymer melt in step-shear strain flows and a cross-linked elastomer in uniaxial extension. Thermal diffusivity data are complemented by measurements of stress and birefringence so that evaluations of the stress-optic and stress-thermal rules can be made. Results show that the thermal diffusivity is enhanced in the flow (or stretch) direction compared to the equilibrium value and that the stress-thermal rule is valid for the modest deformations achieved in this study.     

Experimental Investigation on the Reliability of Thermal Wave Interferometry in the Thermophysical Characterization of Plasma Sprayed Coatings

The main focus of this work is to compare thermal diffusivity and effusivity data resulting from thermal wave interferometry (TWI) experiments on tungsten coatings of different thicknesses with those obtained using reference techniques, namely, the laser flash method and scanning electron microscopy (SEM). The deviations between TWI and the latter techniques are discussed in terms of lack of data in the low frequency range. The investigation shows that the lack of data at low frequencies does not affect diffusivity measurements, while it has a strong effect on effusivity measurements for thermally thick coatings. The conclusions of this experimental study are in good agreement with theoretical predictions resulting from a sensitivity analysis reported in a previous study.     

Thermal Diffusivity Mapping of Solids by Scanning Photoacoustic Piezoelectric Technique

Quantitative thermal diffusivity mapping of solid samples was achieved using the scanning photoacoustic piezoelectric (PAPE) technique. Based on the frequency-domain PAPE theoretical model, the methodology of the scanning PAPE thermal diffusivity mapping is introduced. An experimental setup capable of spatial and frequency scanning was established. Thermal diffusivity mapping of homogeneous and inhomogeneous samples was carried out. The obtained thermal diffusivity images are consistent with the optical images in image contrast and consistent with the reference values in thermal diffusivity. Results show that the scanning PAPE technique is able to determine the thermal diffusivity distribution of solids, hence providing an effective method for thermal diffusivity mapping.     

Determination of thermal diffusivity of solid materials near the melting point

The paper deals with the determination of the thermal diffusivity of solid materials near the melting point by studying the unidirectional propagation of a solidification front through the melts. The method is based on Neumann's thermal analysis of the liquid/solid interface during a solidification process. Measurements are given and the thermal diffusivity is determined for ice, mercury, and aluminum. The results are in fair agreement with reference data. An attempt is also made with data from the literature to calculate thermal diffusivity in very rapidly quenched metals, namely, tin, lead, and zinc. The calculated values are substantially lower than for the solid metals under normal conditions, a result that may reflect the glassy structure of the materials attained by quenching.     

Thermal Diffusivity Measurements of Candidate Reference Materials by the Laser Flash Method

The National Metrology Institute of Japan (NMIJ) in AIST has investigated the laser flash method in order to establish a thermal diffusivity standard for solid materials above room temperature. A uniform pulse-heating technique, fast infrared thermometry, and a new data analysis method were developed in order to reduce the uncertainty in thermal diffusivity measurements. The homogeneity and stability of candidate reference materials such as isotropic graphite were tested to confirm their qualification as thermal diffusivity reference materials. Since graphite is not transparent to both the heating laser beam and infrared light for thermometry, the laser flash method can be applied to graphite without black coatings. Thermal diffusivity values of these specimens with different thicknesses, were measured with changing heating laser pulse energies. A unique thermal diffusivity value can be determined for homogeneous materials independent of the specimen thickness, by extrapolating to zero heating laser pulse energy on the plot of apparent thermal diffusivity values measured with the laser flash method as a function of heating laser pulse energy.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22--27, 2003, Boulder, Colorado, U.S.A.     

The Laser Flash Method with Repeated Pulses—Optimal Experimental Design Analysis

The paper deals with analytical aspects of the laser flash method with repeated pulses, which is a photothermal experimental method for measurement of the thermal diffusivity of solids. It concentrates on the data reduction—an estimation of the thermal diffusivity from the experimental data. Special attention is given to the technique of correction of the width and shape of the heat pulses. Results of sensitivity and optimal experimental design analysis are discussed in detail. It focuses on questions of the influence of setting the experimental parameters, heat pulse period and the number of applied heat pulses, to the sensitivity of the method as well as the optimum time of duration of an experiment.     

Thermal Diffusivity of Sintered Steels with Flash Method at Ambient Temperature

Due to lack of reliable thermal diffusivity data of sintered steels in literature, experimental investigations were conducted on samples made of different powder types (based on prealloyed, or diffusion-bonded, or admixed powders) and under different process conditions. So the influence of pressing pressure and sintering temperature on thermal diffusivity was established. Thermal diffusivity was measured using the “flash method”: a sample in the shape of a slab is irradiated with a light pulse on one of the two surfaces, and temperature of the other surface is detected by an ambient temperature pyrometer. The value of the thermal diffusivity is obtained by a least squares regression on the entire trend of the temperature vs. time using the analytical solution of the heat conduction as regression model. Results show the increase of the thermal diffusivity with increasing density. This outcome can be explained from the mutual effect of thermal conductivity and density on thermal diffusivity in porous media. The experimental results have also permitted to verify the influence of the composition of the sintered materials and carbon contents on thermal diffusivity.     

Analytical study for the estimation of thermal properties of processed meat based on hyperbolic heat conduction model

This study proposes an analytical method in conjunction with existing experimental temperature to estimate the unknown relaxation time and thermal diffusivity of processed meat based on the hyperbolic heat conduction model. This analytical method is a combination of the Laplace transform and least squares methods. The thermal contact resistance at the interface between adjacent samples at different temperatures is assumed to be negligible. The relaxation time is estimated from the temperature jump at a specific measurement location. The thermal diffusivity is determined from the definition of the dimensionless spatial coordinate and the resulting relaxation time. The results show that the relaxation time and thermal diffusivity obtained are in good agreement with the existing results. The obtained dimensionless temperature history at a specific measurement location is close to the experimental temperature data. This means that the Cattaneo–Vernottee (CV) model can be suitable for this study. The proposed analytical inverse method can be applied to determine a more accurate estimate of such problems. A comparison of the estimate obtained from CV and dual phase lag models is made.     

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.     

A new data reduction method for pulse diffusivity measurements on coated samples

A new method, based on numerical solutions of the heat conduction equation, is presented for reducing flash diffusivity data to determine the diffusivity of a coating on a substrate of known thermal properties. Measurements are performed with a conventional thermal flash apparatus and the calculated curve from the numerical simulations is fitted to the measured temperature–time curve, the “thermogram”. The current work is a natural extension of previous work on single layer, homogeneous samples [1]. The main advantage of this new data reduction method, which incorporates nonlinear least-squares regression, is that both the thermal diffusivity of the coating and the thermal contact resistance can be determined. When the thermal contact resistance is small, the solution automatically converges to the perfect thermal contact case. To demonstrate its efficacy, the method is first implemented by analyzing a group of simulated data and then applied to a set of experimental data obtained from three different bilayer samples.     

Thermophysical Properties of Solid Phase Zirconium at High Temperatures

This paper presents experimental results on the thermophysical properties of relatively pure polycrystalline zirconium samples in the solid phase from room temperature up to near the melting point. The specific heat capacity and specific electrical resistivity were measured from 290 to 1970 K, the hemispherical total emissivity from 1400 to 2000 K, the normal spectral emissivity from 1480 to 1940 K, and the thermal diffusivity in the range from 290 to 1470 K. From these data, the thermal conductivity and Lorentz number were computed in the range from 290 to 1470 K. For necessary corrections the most recent values of the linear thermal expansion from the literature have been used. Subsecond pulse calorimetry for measuring heat capacity, specific electrical resistivity, and both emissivities and the laser flash method for measuring thermal diffusivity were applied. Samples in the form of a thin rod and in the form of a thin disk were used in the first and second methods, respectively. Measurement uncertainties were generally about 3% for heat capacity, 1.6% for specific electrical resistivity, 3–10% for the two emissivities, and from less than 1% up to 6% for thermal diffusivity. All the results are discussed in reference to available literature data.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

The thermal diffusivity of argon in the critical region

In this paper experimental results on the thermal diffusivity of argon in the supercritical region are reported. Five isotherms were investigated at 150.90, 153.16, 163.15, 173.14, and 188.14 K, in the pressure range from 2 to 13 MPa, corresponding to density variations from 90 to 800kg · m^–3. The experimental thermal diffusivity data are compared with theoretical predictions. The corresponding thermal conductivity coefficients are calculated and correlated with respect to the spinodal curve.     

Suggestions regarding thermal diffusivity measurements on pyrolytic graphite and pyrolytic boron nitride by the laser pulse method

The laser pulse method can be successfully applied to the measurement of thermal diffusivity of isotropic materials subject to some assumptions. For anisotropic materials, this method is applicable to the measurement of principal thermal diffusivity only on the condition that there is no difference in direction between the principal axis and that of the temperature gradient. After analyzing the heat conduction process in an anisotropic solid, it has been shown that large errors in the measurement of thermal diffusivity would exist if the direction of the principal axis deviates inconspicuously from that of the temperature gradient. The experimental results of thermal diffusivity of highly oriented pyrolytic graphite (HOPG) samples with various deviation angles have been compared with the analytical results. The laser pulse method is not applicable to measurements on semitransparent pyrolytic boron nitride (PBN). We adopted a two-layer composite sample to measure the thermal diffusivity of PBN in the c direction and a particular graphite-PBN composite sample has been prepared which has a very low thermal resistance at the interface. The thermal diffusivity and thermal conductivity of PG (below 2300°C) and PBN (below 1000°C) are given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Measurement of the Thermal Diffusivity of Solids with an Axisymmetrically Positioned Source of Heat Pulse

The pulse method of measurement of the thermal diffusivity of cylindrical samples is considered: an optimum version of normalization of the geometric parameters of a heat pulse, the thicknesses of a cylinder to the radius, and significance of the length of a heat pulse are discussed. The method is realized on an automated experimental setup with simultaneous recording of a thermal signal and the shape and length of a laser pulse. Nonlinear effects are eliminated by decreasing the energy density on the front surface of the sample. The setup presented allows measurement of the thermal diffusivity within a wide range of its values with an error not exceeding 5%. The obtained results of the determination of the thermal diffusivity of Al, Cu, and Fe are presented in comparison with the literature data.     

Measurement of the Thermal Diffusivity of Solids with an Improved Accuracy

A photothermal radiometry technique is being developed at the NPL with the goal of improving the accuracy of thermal diffusivity measurements. The principle is to perform a laser-induced thermal experiment while simultaneously making accurate measurements of the experimental boundary conditions. A numerical three-dimensional heat diffusion model based on thermal transfer functions has been developed to account for the measured boundary conditions. The thermal diffusivity is determined from the experimental data by a nonlinear, least-squares fit to the model. Experiments carried out on pure metals at 900 K demonstrate good agreement between the theoretical predictions and experimental data, and uncertainties of about 1.5% for the thermal diffusivities of platinum, titanium, and germanium were obtained.     

International Comparison on Thermal-Diffusivity Measurements for Iron and Isotropic Graphite Using the Laser Flash Method in CCT-WG9

The first international pilot study of thermal-diffusivity measurements using the laser flash (LF) method was organized by the working group 9 (WG9) of the Consultative Committee for Thermometry (CCT) of the Bureau International des Poids et Mesures (BIPM). Four National Metrology Institutes (NMIs) participated in this comparison. Thermal-diffusivity measurements on the Armco iron and the isotropic graphite IG-110 were carried out from room temperature to about 1200 K. The sample sets consist of five disk-shaped specimens of 10 mm in diameter and (1.0, 1.4, 2.0, 2.8, and 4.0) mm in thickness, each cut from the same block of material. These sample sets were specifically prepared for the comparison and sent to the participants. In the pilot comparison, the thermal diffusivity of each sample was estimated using the LF method with a specific extrapolating procedure. This procedure has the advantage of determining the inherent thermal diffusivity of the material. The extrapolated value in a plot of measured apparent thermal-diffusivity values versus the amplitude of the output signal corresponding to the temperature rise during each measurement is defined as the inherent thermal diffusivity. The overall results showed good agreement between independent laboratories, measurement equipment, and specimen thicknesses. The thermal diffusivities of the materials were determined using our measured results. A quantitative evaluation of the variability of the data obtained by the participants has been done, by evaluating the deviations from the reference value, the Z-value, and the En-number. Some data showed a large deviation from the reference value. It was concluded that these are caused by an insufficient time response of the measurement equipment and some difficulties with changing the pulsed heating energy. The effect of the thermal expansion on the thermal diffusivity was checked. It was found that the thermal-expansion effect was very small and negligible in this case.     

利用红外热成像技术测算碳纤维材料的热扩散系数

利用红外热成像技术测算了5种碳纤维材料的热扩散系数，并分析了影响测量准确度的因素。结果表明，利用红外热成像技术可以方便、快速地测算出碳纤维材料的导热性能参数值，从而为这类材料导热参数的测算提供了一种新颖、可行的方法。