Materials like skutterudite, plumb telluride, and bismuth telluride are increasingly being used for thermoelectric applications. The high efficiency of the thermoelectric system is essential for economic use in areas such as thermal power plants and automobiles, and is represented by the ZT value.
In addition to high electrical conductivity and a high Seebeck coefficient, a low thermal conductivity is also needed. The objective of the analyses is to minimize the phononic contribution and to extend the electronic contribution of the thermal conductivity. This can, for instance, be achieved by using doping or through the establishment of structural conditions such as targeted phonon scattering.
The LFA 457 MicroFlash from Netzsch was used to perform thermal conductivity measurements on disk-shaped samples that have a diameter of 12.6 mm and a thickness range of 2-3 mm. The samples’ front surfaces were plane-parallel.
Figure 1. LFA 457 MicroFlash for measurements between 125 °C and 1100 °C
The thermal conductivity, thermal diffusivity, and specific heat capacity of Bi0,5Sb1,5Te3 (P-38) are illustrated in Figure 2. There is only a slight increase in the specific heat with increasing temperature and there is a decrease in thermal diffusivity with increasing temperature in the low-temperature range. However, the thermal diffusivity significantly increases at elevated temperatures. The decrease in thermal diffusivity in the low-temperature range is characterized by the behavior of a mere phononic conductor with the well-known 1/T dependence. At elevated temperatures, the formation of more number of free electrons/holes in a semiconducting material plays the decisive role. This trend is followed by the thermal conductivity because of the low temperature dependence of the specific heat capacity.
Figure 2. Thermophysical properties of sample P-38
The thermal conductivity of the p- and n-conducting layers P-38 (Bi0,5Sb1,5Te3) and N38 (Bi2Se0,2Te2,8) is compared in Figure 3. Both materials have roughly the same thermal conductivity at -150 °C. Up to room temperature, the decline in thermal conductivity of N-38 is lower than that of P-38. This may be due to a significant decline in the phononic contribution of the thermal conductivity for P-38.
Figure 3. Thermal conductivity of P-38 and N-38
Both materials exhibit almost the same degree of increase in thermal conductivity at elevated temperatures. This means that the amount of contribution by the free electrons/holes is the same for both materials. Moreover, a comparatively low thermal conductivity was observed in both cases. The significant increase at elevated temperatures could represent a high electrical conductivity, assuming a high ZT value for these materials.
The thermo-physical properties of different thermoelectric materials were analyzed using a laser flash system. The results show that the laser flash technique is ideal for optimization of thermoelectric materials (high ZT values and low lattice conductivity) and direct measurement of the thermal conductivity, specific heat capacity, and thermal diffusivity. The use of the LFA 457 MicroFlash helps in arriving conclusions on the composition and optimum structure of thermoelectric materials.
This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.
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