Thermophysical Properties Research Laboratory, Inc.

Modified Kolrausch Method

The Kohlrausch method involves the determination of the product of the thermal conductivity "lambda" and the electrical resistivity "rho". Since the electrical resistivity is also measured at the same time, lambda can be calculated. The method involves passing constant direct current through the specimen to heat the sample while the ends are kept at constant temperature. Radial heat losses are minimized by an external heater maintained at the sample's midpoint temperature. With these provisions, at steady state a parabola-like axial temperature profile is obtained. Thermocouples are placed at the center and one centimeter on each side of the center. The thermocouples also act as voltage probes. Numbering the center thermocouple as the "2" position and the other positions as "1" and "3", it is possible to get the product lambda and rho:

where V3 - V1 is voltage drop between the third and first themocouple, T1 + T3 is the sum of the temperatures at the outside thermocouples, and T2 is the center temperature. Since electrical resistivity is also measured simultaneously (rho=(V3 - V1) A/IL where A is the cross-sectional area, I is the current and L is the distance between positions 1 and 3), lambda can be calculated. The data collection (T1, T2, T3, V3 - V1, I) are computerized and the results calculated for a set of measurements performed while the sample is under vacuum and the heater temperature matched to that of T2. Then additional current is used, a new set of equilibrium conditions is obtained, and the process repeated.

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