The ultimate success in the drive towards a greener and sustainable global environment can only be achieved if a significant portion of the 70 % waste energy that is lost as heat from various industrial and commercial devices is recovered and is converted into useful applications. This ambitious goal requires more efficient and affordable thermoelectric devices that operate in the lower midrange (150 − 300 ℃) temperatures. This work focuses on contributing to the development of non-toxic semiconductor and conducting polymer composite thermoelectric device arranged in mixture form and in a layered structure (superlattice-like) to meet the desired goal. Through morphology manipulation at nanoscale and conducting polymer addition, the figure of merit (ZT) of thermoelectric devices was manipulated to produce high performing devices that operate in the lower midrange temperature where most of the commercial devices are used. This study achieved this goal by using CuO since it has high Seebeck coefficient (S), and it is a cheap and environmentally friendly material. The figure of merit of CuO was enhanced by lowering thermal conductivity through: (1) nano-structuring, by varying the precursor concentration from 1: 0.2 M to 1: 0.8 M and (2) morphology manipulation by varying the decomposition temperature from 95 ℃ to 135 ℃, and (3) combining CuO and polyaniline to form a composite material. CuO powders were synthesized using hydrothermal method and structural properties were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterization analysis, which showed that crystalline size increased with temperature and decreased with increase in precursor concentration. Surface morphology changed from ellipsoidal to nanorods when decomposition temperature was increased. Furthermore, the synthesized CuO powders were mixed with 5.6wt% polyaniline (PANI) and were sintered into pellets using spark plasma sintering and ultimately the thermoelectric properties of pellets were studied. The influence of morphology manipulation on the thermoelectric parameters of CuO nanoparticles was investigated. Thermal conductivity of CuO was lowered when morphology changed from nanorods to ellipsoidal. Thermal conductivity below 10.4 vii W/mK was reached for CuO with ellipsoidal morphology. Crystal size increased slightly from 16.05 nm to 18.60 nm when decomposition temperature increased from 100 ℃ to 135 ℃. This change in crystal size affected the electrical conductivity of the metal oxide. High values of electrical conductivity were recorded for CuO with crystal size of 16.05 nm. The influence of adding PANI to the CuO was also investigated. Thermal conductivity reduced to below 8.8 W/mK, and electrical conductivity was improved by 106 times when PANI was mixed with CuO, however the Seebeck coefficient reduced by a factor of 102. Maximum ZT of ~5 × 10−4 was achieved at 200 ℃ for CuO mixed with PANI compared to maximum ZT of ~2 × 10−5 for CuO without PANI. This study has opened opportunities for development of CuO based polymer composites at low temperature for thermoelectric applications.

Thesis (MSc of Physics and Astronomy sciences)---Botswana International University of Science and Technology, 2022