# Epoxy/Glass Fiber Nanostructured p- and n-Type Thermoelectric Enabled Model Composite Interphases
## 4 Discussion
### Evaluation of the Model Composites Thermoelectric Efficiency  
The electrical resistance as well as the electrical conductivity of the prepared model composites are presented in Figure 7b. The p-type SWCNTs coated GF model composites exhibited electrical resistance of 127.2 \(\pm\) 2.9 Ohm, which corresponds to electrical conductivity of 1.097 \(\pm\) 0.05 S/cm, while for the n-type SWCNTs coated GF model composites showed very similar values i.e., 95.6 \(\pm\) 1.1 Ohm and 1.561 \(\pm\) 0.023 S/cm, respectively. The high electrical conductivity of the coated GFs is a key parameter for thermoelectric performance, since it can facilitate the flow of the thermally induced charge carriers upon being exposed to a temperature difference. Additionally, as aforementioned, low thermal conductivity is also desirable for the sustainability of the thermoelectric effect. In the case of the SWCNT interphase, it may also be postulated that the infusion of the thermally insulating epoxy molecules reduces thermal conductivity, thus further enhancing thermoelectric performance. More analytically, this mechanism creates a barrier at the SWCNTs junctions and prevents the direct physical contact between them resulting in reduced thermal energy transport. [23; 29]. The Seebeck coefficient (\(S=\Delta V/\Delta T\)) was derived from the slope of \(\Delta V\) vs. \(\Delta T\) curves by linear fitting. Figure 7c shows the Seebeck coefficients of the p and the n-type model composites. For the p-type model composite, the Seebeck coefficient was found to be \(+16.2\pm 6\)\(\mu\)V/K corresponding to a power factor of 0.02 \(\mu\)W/m-K2 and for the n-type \(-28.4\pm 4\)\(\mu\)V/K corresponding to a power factor of 0.12 \(\mu\)W/m-K\({}^{2}\). As was observed, the p-type SWCNTs coated GF model composites exhibited slightly lower values of Seebeck coefficient compared to the corresponding bucky papers. This can be attributed to the high nitrogen amount of the hardener within the epoxy system, acting as a n-dopant [32]. On the other hand, for the n-type SWCNTs coated GF model composites the Seebeck coefficient values remain almost identical. Figure 7d depicts the generated thermovoltage upon exposure to various values of \(\Delta T\), for both p and n-type SWCNTs coated GF model composites. More specifically, the TE voltage of the in-series electrical and the in parallel thermal connection between the p-type and the n-type model composites creating a p-n junction are being displayed. As can be observed, within the studied temperature range, the generated voltage increased linearly with increasing temperature, \(R^{2}>0.98\) for all cases. The measured voltage output was \(4.46\pm 0.3\) mV with a device resistance of \(235.5\pm 2.5\) Ohm at 100 \({}^{\circ}\)C \(\Delta T\). The maximum output of the p-n model composites TEG device was calculated 0.02 \(\mu\)W at a temperature difference of 100 \({}^{\circ}\)C, according to the following equation \(P_{\rm max}=\Delta V^{2}/4\times RTEG\)[24]. The derived maximum output power density of the device (p-n model composites pair) was 0.016 \(\mu\)W/cm\({}^{2}\). Overall, the relatively high values of the electrical resistance were directly related to the area of the evaluated coated GF rows. It is expected that by increasing the area of the thermoelement (in this case the single coated GF tow), the electrical conductivity could be significantly increased while the Seebeck coefficient would not be affected as it is an inherent material property and is independent of the geometry of the evaluated specimen. To further elaborate on this statement, when the cross-sectional area of the coated substrate is increased, by increasing the thickness of the CNT coating on the GF substrate, its electrical resistance is reduced. Generally, the electrical resistivity of the CNTs films decreases as the thickness of the coatings is being increased.