Echanced thermoeletric properties of P-Type calcium cobalt oxide via aluminum, al-ni and porous al-ni co-dopings

Abstract

Oxide-based thermoelectric (TE) materials are considered as one of the most eco-friendly materials for power generation applications. They seem more stable and less toxic compared to the conventionally used metals and semiconductors such as PbTe. Among the TE materials, the p-type Ca3Co4O9 is the most commonly investigated due to its high potential to be used in high temperature power generation. Many attempts have been carried out to further increase the generating power of this TE material which includes doping and co-doping with other elements. However, most of the literature reported that their figure of merit (ZT) is still lower than 0.12. This research aimed to enhance the thermoelectric properties of Ca3Co4O9 via doping and co-doping with metal elements and increasing its bulk porosity. The stoichiometric mixtures of Ca(NO3)2.4H2O, Co(NO3)2.6H2O and starch were first dissolved in the distilled water. The resultant gel was decomposed at temperature 673K until it changed into a black precursor. The black precursor was then calcined in the furnace at 773, 873, 973, and 1073 K for 4, 6, 8, 10, 12 and 14 h to obtain Ca3Co4O9 powder. The Ca3Co4O9 pellet was prepared using a 5 tons hydraulic press, then sintered in air at 1173K for 20h. The best properties of Ca3Co4O9 powder was then doped with Aluminum (Al). The amount of doping was varied at x = 0.1, 0.2, 0.3 and 0.4 on Ca3- xAlxCo4O9 powder. The figure of merit, ZT values were measured on doped Ca3Co4O9 powders and compared. The highest ZT value of the doped Ca3Co4O9 was selected for the next co-doping with Nickel (Ni) varied at y = 0.05, 0.1, 0.15 and 0.2. The highest ZT value obtained from Ni co-doped powder was selected and then their porosity was varied by adding starch at different weight percentages of 3, 5 and 7 wt%. Ca3Co4O9 synthesis and treatments were analyzed using a thermogravimetric analyzer (TGA), scanning electron microscope equipped with energy dispersive spectrometry (SEM- EDS), x-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). The changes in thermoelectric properties were measured from 300 to 700K. The calcination process that produced the best lattice parameters (b1/b2 = 1.613) and crystal size (458 °A) were observed at 12h. The highest ZT for Al-doped was 0.139 for x = 0.1 at 700 K. While the highest ZT for Ca3- xAlxCo4-yNiyO9 was 0.145 at x = 0.1 and y = 0.05. An investigation was conducted on the increasing structure porosity with Al-Ni co-doping. The highest figure of merit ZT was 0.161 at 700K, for the Al-Ni co-doped sample with 5wt%. The results revealed that the doping elements have a significant effect on the microstructure properties and morphology of polycrystalline of Ca3Co4O9, such as grain size reduction up to 1.76 µm for the best co-doped. These changes improve the overall thermoelectric properties.