Thermophysical Properties Research Laboratory, Inc.
Phone : 765-463-1581
The Kohlrausch method involves the determination of the product of the thermal conductivity
(λ) and the electrical resistivity (ρ). Since the electrical resistivity is also measured at the
same time, conductivity 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 of λ and ρ:
The top and bottom temperatures, T1+T3 are summed and the middle, T2,
is the middle temperature. Since the electrical resistivity can be obtained during measurements
from the following equation:
where V is the voltage drop between the top and bottom thermocouples, A is the cross sectional area,
I is the current, and L is the distance between the top and bottom thermocouples, λ 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. For a new temperature point, additional current is used,
a new set of equilibrium conditions is obtained, and the process repeated.
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West Lafayette, IN 47906
Last updated 1/5/2012