High Temperature Thermal Conductivity Measurements of Quasicrystalline Al 70.8 Pd 20.9 Mn 8.3
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High Temperature Thermal Conductivity Measurements of Quasicrystalline Al70.8Pd20.9Mn8.3 Philip S. Davis and Peter A. Barnes, Auburn University, Auburn AL 36849 Cronin B. Vining, ZT Service Inc., 2203 Johns Circle, Auburn, AL 36830 Amy L. Pope, Robert Schneidmiller, Terry M. Tritt and Joseph Kolis, Clemson University, Clemson, SC 29634 ABSTRACT We report measurements of the thermal conductivity on a potential high temperature thermoelectric material, the quasicrystal Al70.8Pd20.9Mn 8.3. Thermal conductivity is determined over a temperature range from 30 K to 600 K, using both the steady state gradient method and the 3ω method. Measurements of high temperature thermal conductivity are extremely difficult using standard heat conduction techniques. These difficulties arise from the fact that heat is lost due to radiative effects. The radiative effects are proportional to the temperature of the sample to the fourth power and therefore can lead to large errors in the measured thermal conductivity of the sample, becoming more serious as the temperature increases. For thermoelectric applications in the high temperature regime, the thermal conductivity is an extremely important parameter to determine. The 3ω technique minimizes radiative heat loss terms, which will allow for more accurate determination of the thermal conductivity of Al70.8Pd20.9Mn 8.3 at high temperatures. The results obtained using the 3ω method are compared to results from a standard bulk-thermalconductivity-technique on the same samples over the temperature range, 30 K to 300 K. INTRODUCTION Thermoelectric devices are typically used in two distinct ways, either as a refrigerator or as an electric generator. For example, thermoelectric refrigerators can be used to cool electronics at room temperature, while thermoelectric generators are used to generate electricity at high temperatures, ~ 700 – 800 K, on deep space probes. These demands for thermoelectric devices require materials that are “thermoelectrically efficient” at the temperatures of use. Currently, Bi2Te3 and Si 1-xGex are the “thermoelectrically efficient” materials of choice in these respective applications. A “thermoelectrically efficient” material is one in which the dimensionless figure of merit, ZT, is a maximum, where
ZT =
S2
T
(1)
Z5.4.1
Here S is the Seebeck coefficient ,σ is the electrical conductivity and κ is the thermal conductivity. In order to increase the figure of merit, the numerator, which is also called the power factor, S2σ, of Equation 1 should be made as large as possible and the denominator should be made as small as possible. We will mainly be concerned with measuring and minimizing the denominator or the thermal conductivity. QUASICRYSTALS (POSSIBLE THERMOELECTRIC MATERIALS?) Quasiperiodic structures, or quasicrystals, are non-crystalline materials with perfect longrange order, but with no three-dimensional periodicity ingredient, not even the underlying lattice of the incommensurate structures.1 Theoretically, the results of quasiperiodicity could lead to intere
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