# Epoxy/Glass Fiber Nanostructured p- and n-Type Thermoelectric Enabled Model Composite Interphases
## 4 Discussion
### Evaluation of the Model Composites Thermoelectric Efficiency
The property of the electrical resistivity highly depends on carrier concentration and carrier mobility, while the electrical resistance depends on the resistor shape geometry and the specific material. Electrical resistivity is an intrinsic property of a conductive film which is directly proportional to the total resistance R, an extrinsic quantity that depends on the length and cross-sectional area of the corresponding film. Larger cross section areas present lower values of electrical resistance. In the case of SWCNTs, changes within the conductive network due to thickness optimization are able to positively impact the electrical conductivity. Both cases are true until the bulk resistance value is reached, thereby no further increments in electrical conductivity can be achieved. There also exists an optimal area that produces the highest possible thermoelectric values, as it is able to host the highest possible electron flow/current under a fixed \(\Delta\)T. Therefore, since the Seebeck coefficient is a material property, the power factor of the coated GFs can be optimized by combining the most appropriate substrate dimensions and coating thickness. This is of the utmost importance for the exploitation of the nanomaterial coated reinforcements as thermoelectric elements in composite structures. the errors associated with the handling of single fiber filaments, which typically requires extremely careful manipulation due to their small size and brittle nature [47; 48]. Several trials were performed with different specimen designs in order to consider all the different aspects that are involved in the particular test. Since the tow pull out process is not a standard test [49; 50], preliminary tests were performed to optimize parameters such as the embedded length and the resin pre-impregnation process before insertion in the matrix. Fracture morphologies of the pulled-out tows obtained via optical microscopy reveal a cohesive type of failure in all cases, since epoxy material remained on the tow surface after the pull-out process. In general, the GF tow was covered by a cone of resin and as such it was detached from the matrix. In the case of the n-type tows, a mixed type of failure mode was observed, whereby fiber/matrix-to-matrix failure was also present. The fiber-to-matrix failure, whereby the GF tow was completely detached from the resin, was not observed in any of the tested specimens. As should be noted, the persistent presence of cohesive failure within the matrix suggests that measured values were an underestimate of the true IFSS. This was also suggested during the process of experimental optimization where it was observed that small embedded length and low values of pull out force correlated more with fiber to matrix failure than cohesive failure. In conclusion, it can be assumed that the presence of surfactant for the dispersions of both SWCNTs inks affected negatively the IFSS when a commercial epoxy resin was used as a matrix. Further studies could be performed oriented to optimize the coating process and simultaneously assess the effect of the surfactant existence on the interfacial shear strength [51; 52]. As is well known, a strong interface provides efficient stress transfer but promotes brittle failure, whereas a weaker may promote energy dissipation and thus improve toughness. Balancing these conflicting requirements by engineering composite interphases to improve optimal strength and toughness values remains a great challenge.