Structural and Electrical Properties of NaXCOO2 Thermoelectric Synthesized via Citric-Nitrate Auto-Combustion Reaction

Abstract

A series of NaxCoO2 thermoelectric has been successfully prepared from nanoparticles powder synthesized via citric-nitrate auto combustion reaction. In this work, the aqueous Co nitrate was mixed with a controlled concentration of aqueous Na nitrate to obtain the mixture solution with Na molar concentration, x from = 0.1 to 0.9. A calculated amount of citric acid and ammonia solutions were added to the mixture as a fuel and ignition agent for the combustion reaction respectively. The drying process at 300 °C on the hotplate with an infrared lamp at the top has turned the mixture solution into precursor gels which yielded fine ashes throughout the auto combustion reaction. The ashes are calcined at 700 °C for 1 h, pelletized by 14.2 MPa of compressive force and sintered at 900 °C for 1 h. The TG/DT analysis of the gel samples show that the decomposition reaction of the Na and Co nitrates occurred in a highly exothermic single-step reaction at around 250 °C to 280 °C and left the desirable mass. The XRD analysis revealed that the calcination process has transformed the mass into pure NaxCoO2 phase. The atomic percentage obtained from the EDX indicated that the Na in NaxCoO2 yielded with expected x in each sample. The FESEM images of the calcined samples depicted two distinct microstructures appeared in samples with x ≤ 0.3 which was recognized as NaxCoO2 and CoO2 phases. Whereas for the NaxCoO2 samples with x ≥ 0.5 a single, dense and plate-like microstructure is developed during the calcination process. The pellet sample with x = 0.7 exhibits the highest temperature dependence electrical conductivity around 690 S·cm−1 at 330 K compared to other samples due to effective phonon scattering center provided by Na ions by this concentration.