Exploration and Optimization of Tellurium‐Based Thermoelectrics

V∙K ‐1 at 592 K. As the bismuth content increases, so does the Seebeck coefficient, which by nominal
composition Tl9Sn0.2Bi0.8Te6, displays values of 89 V∙K ‐1 at room temperature and 151 V∙K ‐1 at 587 K.
This general trend can be seen in Figure 12.2 increasing with increasing Bi‐content. For ease of reference,
Tl9SnTe6 from the previous chapter has been included in the plot. As expected it appears as the lowest
magnitude, showing 39 V∙K ‐1 at room temperature – a mere 5 V∙K ‐1 below Tl9Sn0.8Bi0.2Te6. Hot‐
pressed measurements published on Tl9BiTe6 state that it has S = ~150 V∙K ‐1 at room temperature [249]
showing not only does this data follow an appropriate trend, but is nestled between the two extreme
cases: Tl9SnTe6 and Tl9BiTe6.
The electrical conductivity, σ, for all trials shows a gentle decrease in the slope values from
room temperature (~295 K) to the higher temperatures measured herein (Figure 12.2 (dashed)). The
straight decreasing line is reminiscent to that of heavily‐doped semiconductors. With an excess of
carriers, increasing T disrupts and decreases σ. The trend is appropriately inversed to that of the
Seebeck coefficient due to their (more‐or‐less) inverse relationship. In the case of Tl9Sn1‐xBixTe6, the
smallest σ can be found in Tl9Sn0.2Bi0.8Te6 with 530 ‐1 cm ‐1 at room temperature to 357 ‐1 cm ‐1 at 587 K.
The highest of this system can be found in Tl9Sn0.8Bi0.2Te6 with 1224 ‐1 cm ‐1 at room temperature to 821
‐1 cm ‐1 at 592 K. The intermediate compositions of x = 0.6 and x = 0.4 also follow this trend. σ of
Tl9BiTe6 is reported to be almost one quarter of Tl9Sn0.2Bi0.8Te6 showing ~125 ‐1 cm ‐1 for zone‐refined [222]
and ~200 ‐1 cm ‐1 for hot‐pressed [249] (~300 K).
Figure 12.2 ZEM measurements for Tl 9Sn 1‐xBi xTe 6:
solid Seebeck coefficient and dashed electrical conductivity (left), and power factor (right).
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