## 3 Results and Discussion
### Effect of BST Content
Herein, 20 wt.% and 30 wt.% BST correspond to 4.75 vol.% and 7.87 vol.% BST, respectively. By taking the individual electrical conductivities measured at 130 \({}^{\circ}\)C for the pure counterparts, i.e., PANI and BST, and their relative volume fractions in the composites, the theoretical values of the electrical conductivity for the composites were calculated using Equation (3). These were found to be \(\sigma\) = 24.48 S/cm for 20 wt.% BST and \(\sigma\) = 37.85 S/cm for 30 wt.% BST. By comparing the experimental values with the theoretical ones (considering a parallel connection), the former were lower in all cases since they did not exceed 15 S/cm. This inconsistency between the theoretical and experimental values may be attributed to inhomogeneity with respect to the dispersion of the inorganic additive within the polymer matrix and crack formation in the composite materials produced during material preparation via mechanical mixing and post-processing. The electrical conductivity was also calculated for a series connection. However, the theoretical values were very close to those of the pure polymer. Figure 8 provides the thermal conductivity values as a function of the processing temperature for the PANI-BST composites. The obtained experimental data suggest that the composites' thermal conductivity is entirely dictated by the polymer matrix, and hence it is not affected by the increase in BST content. Consequently, the presence of a polymer matrix in polymer-based thermoelectric composites is considered to be advantageous with respect to the materials' thermoelectric performance since it limits the significantly higher thermal conductivity deriving from the inorganic additives, thus enhancing ZT [39]. ZT was estimated for all samples, and the optimum value measured at 130 \({}^{\circ}\)C was 2.93 \(\times\) 10\({}^{-3}\), with \(\kappa\) = 0.2800 W/Km, \(\sigma\) = 12.937 S/cm and S = 39.67 \(\mu\)V/K. The experimental processing conditions employed in the preparation of the best-behaved PANI-BST composite were the following: processing temperature: 80 \({}^{\circ}\)C; processing pressure: 2 tons; processing time: 75 min; and BST content: 30 wt.%. By comparing these results with those reported by [PERSON] et al. (ZT = 4.3 \(\times\) 10\({}^{-3}\), \(\kappa\) = 0.1096 W/mK, \(\sigma\) = 11.626 S/cm and S close to 38 \(\mu\)V/K at room temperature), for a structurally ordered cable-like polyaniline-bismuth telluride nanocomposite [31], it becomes clear that the higher ZT recorded in the above-mentioned literature is mainly attributed to the lower thermal conductivity value, since both \(\sigma\) and S are comparable to the ones reported herein. This difference might be attributed to the nanostructured nature of the composites prepared in ref. [31], in contrast to the present study, where the produced PANI-BST composites consisted of agglomerated BST particles dispersed  
Figure 7: Graphs of electrical conductivity (\(\sigma\)) versus processing temperature (T) measured at 130 \({}^{\circ}\)C (403K) at 4.5 tons (**left**) and 2 tons (**right**). The effect of the additive content (pure PANI—blue, 20 wt.% BST—green and 30 wt.% BST—red) and processing times (15, 45 and 75 min) can be seen in the graphs. In legend, X denotes the processing temperature, which is either 30, 80 or 100 \({}^{\circ}\)C.  
within the PANI matrix. Nevertheless, our approach involves a simple and easily scalable preparation process that can result in equally effective composite materials exhibiting good thermoelectrical performance under appropriate experimental and processing conditions.