Figure 1.
SEM images of cold fractured surfaces for samples with 20, 40, and 60-wt% SDBS (a, b, and c, respectively).
These samples contain 20 wt% CNTs (99% purity). Fractures become sharp and disorganized as less surfactant is included, eventually forming heterogeneous structures as seen in (a). Scale bars indicate 10 µm.
Figure 2.
Electrical conductivity (a) and thermopower (b) for Series 1 samples containing 20-wt% CNT, 10-wt% PEI, and a varied amount of SDBS.
Correlation is positive for thermopower magnitude and electrical conductivity.
Figure 3.
Schematic representation of PEI-functionalized CNT bundles in the composite and the structure of branched PEI.
The amine groups are responsible for the electron donation which converts CNTs into n-type semiconductors. When the bundles are smaller, more tubes come in direct contact with PEI, which increases the n-type thermopower values. On the other hand, composites with large bundles have fewer junctions necessary for electron transport across the composite.
Figure 4.
Effect of PEI wt% on the electrical properties for the samples containing 20-wt% (a and b: Series 2:) and 40-wt% (c and d: Series 3) SDBS.
Thermopower sign changed with PEI whose concentrations between 0.1 and 1 wt%, as shown in (d), wherein it is seen that values for thermopower do not change significantly beyond 5-wt% PEI.
Figure 5.
Change in electrical conductivity (a) and thermopower (b) as a function of time.
The elapsed time was measured after the vacuum annealing (last process for fabricating samples) for Series 4 samples.
Table 1.
List of samples with the concentrations of CNT, SDBS, PEI, PVAc as well as the purity of CNT.