Investigation of thermopower waves based energy sources

Miniaturisation of energy sources is critical for the development of the next generation electronic devices. However, reduction in dimensions of none of the commonly used energy generation technologies including batteries, fuel cells, heat engines and supercapacitors have resulted in efficient and reliable energy sources with high specific powers (power-to-mass ratio). Recently, the new concept of energy generation based on thermopower waves has shown promise for miniaturization. In such sources, exothermic chemical reactions of a reactive fuel are coupled to charge carriers of a thermoelectric (TE) material in its affinity, resulting in an intense thermal wave that self-propagates along the surface of the TE materials. This wave simultaneously entrains charge carriers, resulting in a large current. If the TE material also has a high Seebeck coefficient, a large output voltage and subsequently large specific power output are obtained. As the thermal wave results in a power output, it is called a thermopower wave.<br><br>In the first stage of the PhD research, the author demonstrated thermopower wave systems based on thin films of Bi₂Te₃. Bi₂Te₃ was implemented due to its high S (~ –200 μV/K) and σ (10⁵ S/m). As Bi₂Te₃ exhibits a low <i>κ</i>, the author devised a novel strategy by placing it on thermally conductive alumina (Al₂O₃) substrate to compensate for this deficiency. The Bi₂Te₃ based thermopower wave sources generated voltages and oscillations higher (at least 150 %) than the previously reported multi-walled carbon nanotube (MWNT) based thermopower wave sources, while maintaining a high specific power in the order of 1 kW/kg.<br><br>In the second stage, the author implemented a novel combination of p-type Sb₂Te₃ and n-type Bi₂Te₃ as the core TE materials with complimentary semiconducting properties, to show the generation of voltage signals with alternating polarities. In the third stage, the author implemented zinc oxide (ZnO), which is a TE transition metal oxide (TMO), for the first time as the core material in thermopower wave sources. It was shown that both <i>S</i> (~ –500 μV/K at 300 °C) and<i> σ</i> (~ 4×10³ S/m at 300 °C) of ZnO increased at elevated temperatures. By incorporating ZnO as the core TE material, the PhD candidate obtained voltages and oscillation amplitudes at least 200 % higher than any previously demonstrated thermopower wave systems (in the order of > 500mV), while maintaining a high specific power (~ 0.5 kW/kg). In the final stage, in order to exceed voltages larger than 1 V, the PhD candidate identified that manganese dioxide (MnO₂), which is another TE TMO, can exhibit exceptionally large <i>S</i> and moderate <i>σ</i> at elevated temperatures. As a result, the author implemented MnO₂ as the core TE material. It was shown that the <i>S</i> of MnO₂ increased dramatically with temperature, exhibiting a peak value of approximately –1900 μV/K at 350 °C. Consequently, voltages large enough (~1.8 V) to drive small electronic circuits were obtained, while maintaining high specific powers in the order of 1 kW/kg.