## 3 Results and Discussion
### Effect of BST Content  
Concerning the effect of the inorganic additive (i.e., BST) on the thermoelectric properties of the resulting PANI-BST composites, the experimental data suggest that by increasing  
Figure 4: SEM (SE) images of sample 30-T30-P2-D75 (processed at 30 \({}^{\circ}\)C) with (**a**) 2 k and (**c**) 10 k magnification and corresponding BSE images with (**b**) 2 k and (**d**) 10 k magnification. Figure 5: SEM images and EDS analysis of sample 20-T80-P2-D75: (**a**) SE image with 300 magnification, (**b**) BSE image with 300 magnification and EDS mapping, (**c**) S element from PANI (emeraldine salt), (**d**) Sb and Te elements from BST, (**e**) Bi element from BST and (**f**) contrast of S and Bi elements from PANI (emeraldine salt) and BST. the percentage of the BST additive in PANI, there is an increase in the Seebeck coefficient (Figure 6) that is further enhanced with the increase in processing temperature. Interestingly, in the electrical conductivity measurements (Figure 7), it appears that an increase in the additive content has no significant effect. More precisely, from the measurements performed in the entire temperature range, it can be concluded that, by increasing the additive content, the electrical conductivity of the composites increases only slightly compared to the pristine polymer. More precisely, the maximum value recorded was about 15 S/cm, which is significantly lower than that of the pure inorganic additive, which ranges between 650 and 350 S/cm for temperatures in the range of 300 K-500 K. This may be due to the fact that the inorganic additive is not homogeneously distributed within the polymer matrix and severe aggregation phenomena are observed in many areas, as shown by the SEM analysis. Furthermore, crack formation (which was also confirmed via SEM) may also contribute to the composites' reduced electrical conductivity. Hence, it is expected that, by improving the dispersion of the inorganic additive within the polymer matrix, the electrical conductivity will be enhanced. This is in line with previous literature findings reporting that, in highly ordered organic composites, a better dispersion of the additive phase results in a higher Seebeck coefficient and electrical conductivity [36; 37; 38]. Furthermore, the contact area is another important influencing factor since interface interactions may affect (either negatively or positively) the materials' properties and, hence, their performance [39; 40]. In order to better evaluate the results obtained from the electrical measurements, the electrical conductivity of the composites was theoretically calculated based on the general mixture model. According to this model, for particles dispersed within a matrix, interparticle interactions can be approached through a simple ohmic interaction binary system, considering either a parallel or a series connection. This model has already been applied in the case of SWCNT/PANI hybrid films for well-dispersed SWCNTs and CSA-doped PANI [37; 41]. Considering parallel connections, in the case of the PANI-BST composites, the following equation (Equation (3)) applies:  
\[\sigma_{\text{tcl}}=\sigma_{\text{PANI}}V_{\text{f\,PANI}}+\sigma_{\text{BST}} V_{\text{f\,BST}} \tag{3}\]  
where V\({}_{\text{f}}\) is the volume fraction and \(\sigma\) is the electrical conductivity of the pure components (PANI and BST), while \(\sigma_{\text{tcl}}\) is the electrical conductivity of the composite material. In order  
Figure 6: Graphs of Seebeck coefficient (S) versus processing temperature (T) measured at 130 \({}^{\circ}\)C (403 K) 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.  
to apply this model, the weight percentage must be converted to a volume percentage.