Ernest Karawacki

Transient hot-strip method for simultaneously measuring thermal conductivity and thermal diffusivity of solids and fluids

A transient hot-strip method has been developed for use with solids and fluids with low electrical conductivity. The hot strip (thin metal foil) is used both as a constant plane heat source and a sensor of the temperature increase. The accuracy of the method is so good that it might even be used for the measurement of the specific heat especially under difficult experimental conditions when the standard methods cannot be used or would be very inconvenient. This method has been tested in measurements on fused quartz, glycerine and Araldite at room temperature. The experimental conditions that cause deviations from the mathematical solution of the thermal conductivity equation are discussed and estimates for their maximum influence on the measured quantities are given.

Thermal transport studies of electrically conducting materials using the transient hot-strip technique

The transient hot-strip (THS) method has been used for thermal conductivity and thermal diffusivity studies of electrically conducting materials by introducing a thin electrically insulating layer between the hot strip and the metallic material under study. The insulating layer introduces a certain thermal contact resistance between the hot strip (heat source cum temperature sensor) and the surface of the sample to be studied. To account for this thermal resistance a theory has been developed which indicates how measurements on these kind of materials should be performed and how the reduction of data from transient recordings should be carried out to give reliable results. The new experimental approach, which should be applied whenever a thermal contact resistance is suspected, has been demonstrated by two series of measurements on a stainless steel at room temperature.

Determination of the thermal‐conductivity tensor and the heat capacity of insulating solids with the transient hot‐strip method

The recently described transient hot‐strip method has been developed for the study of solids with direction dependent thermal conductivity. By making three independent measurements, with the hot‐strip properly oriented, information can be obtained about the thermal conductivities along the principal directions and also about the specific heat capacity per unit volume. To demonstrate the versatility of the method, crystalline quartz was studied over the temperature range 230–340 K and the results are in good agreement with earlier reported data.

Thermal conductivity of a microporous particulate medium: moist silica gel

A systematic study of the thermal conductivity of beds of moist silica gel is presented. The influence of porosity, water content, total gas pressure and temperature is determined through measurements under transient conditions with the transient hot-strip (THS) method and under static conditions in a bench-scale reactor. The predictions of the effective thermal conductivity of the beds from four different simple models (Russell, geometric mean value, unit-cell model and stochastic model) agree reasonably well with the experimental results. The unit-cell model is extended in order to account for the water sorbed in the micropores and describes satisfactorily the dependency of the effective thermal conductivity on the water content.

Transient hot‐strip probe for measuring thermal properties of insulating solids and liquids

The transient hot‐strip (THS) method has been developed for experimental situations with two different materials kept against the sides of the strip. Based on this development THS probes, with known or separately measured thermal properties, have been designed and used successfully in a series of reported experiments. If the thermal diffusivity of the probe is less than that of the material under study and good contact is established between the probe and the sample, the accuracy of the measurements turns out to be as good as with the ordinary transient hot‐strip method.

Dynamic plane source technique for simultaneous determination of specific heat, thermal conductivity and thermal diffusivity of metallic samples

A technique is developed for the simultaneous determination of specific heat, thermal conductivity and thermal diffusivity of metals. The method is of a transient heat-flow type where the heating element serves both as a heat source and the temperature detector. The experiment is arranged in such a way that the temperature development in the sample is close to adiabatic. The method has been tested on samples of pure iron (99.95% of purity), cast iron (SIS 2140) and austenitic stainless steel (SIS 2343). The results obtained were in an excellent agreement with the literature data for these materials. The accuracy of determination of specific heat has been additionally tested on an Al sample.

Refractive Index Measurements in Fused NaNO 3 and KNO 3 by a Modified Thermooptic Technique

A thermooptic technique for studying the temperature dependence of refractive index of liquids at room temperature has been modified and applied to the study of molten NaNO(3) and KNO(3) within a temperature range of some 80 K above the melting point for five different wavelengths within the visible spectral range. The experiments show a temperature dependence of the polarizability for the two salts as calculated from the Lorentz-Lorenz formula, which cannot be explained by experimental inaccuracy.

An extension to the dynamic plane source technique for measuring thermal conductivity, thermal diffusivity, and specific heat of dielectric solids

The recently developed dynamic plane source (DPS) technique for simultaneous determination of the thermal properties of fast thermally conducting materials with thermal conductivities between 200 and 2 W/mK has now been extended for studying relatively slow conducting materials with thermal conductivities equal or below 2 W/mK. The method is self‐checking since the thermal conductivity, thermal diffusivity specific heat, and effusivity of the material are obtained independently from each other. The theory of the technique and the experimental arrangement are given in detail. The data evaluation procedure is simple and makes it possible to reveal the distortions due to the nonideal experimental conditions. The extension to the DPS technique has been implemented at room temperature to study the samples of cordierite‐based ceramic Cecorite 130P (thermal conductivity equal to 1.48 W/mK), rubber (0.403 W/mK), and polycarbonate (0.245 W/mK). The accuracy of the method is within ±5%.

Circuit design for transient measurements of electrical properties of thin metal films and thermal properties of insulating solids or liquids

A general circuit design is described for the transient hot‐strip method which is presently used for studies of electrical properties of thin metal films and for measurements of thermal properties of insulating solids and liquids. It is demonstrated that a low driving voltage can be used for thin films having comparably high resistance, and that the output of power in the strip source (being the nonlinear component of the circuit) can be kept constant throughout the transient recording. The experimental arrangement for constant power conforms with the assumptions of the theory for the method and extends its validity to substantially larger temperature increases of the nonlinear resistance.

Heat transfer and ion migration in the system Li2SO4–Na2SO4

Abstract The transient plane source technique has been used for simultaneous measurements of thermal conductivity and diffusivity in the lithium–sodium sulphate system starting near 300 K. Samples with 100, 77.5, or 50% Li2SO4 behave differently from pure Na2SO4. In the first case both the thermal conductivity and the diffusivity start to increase rapidly at about 640 K, while the crystal structure remains monoclinic for Li2SO4 and trigonal for LiNaSO4. Concerning pure Li2SO4, there is an additional discontinuous increase of the two thermal parameters at the structural transition from monoclinic to cubic (fcc) at 850 K, while the temperature gradients become significantly smaller in the cubic phase than in the monoclinic one. In contrast, both the thermal conductivity and the diffusivity of Na2SO4 decrease over the whole studied temperature range, which includes the phase transitions at 474 K and 520 K. Furthermore, a corresponding study was performed for silver iodide over the range 295–640 K, i.e. on both sides of the phase transition at 420 K. There is an indication of a small decrease of the thermal conductivity and the diffusivity at the phase transition. The high temperature phases fcc Li2SO4, bcc LiNaSO4 and bcc AgI are solid electrolytes, but it is characteristic for the two sulphate phases that a coupled rotation-like motion of the sulphate ions enhances the cation migration. Obviously, such motion is also of importance for heat transport.