EffectsofcalcinationtemperatureandtimeontheCa3Co4O9puritywhensynthesizedusingstarch-assistedsol–gelcombustionmethod

Ca3Co4O9 is a p-type semiconducting material that is well-known for its thermoelectric（TE）, magnetic, electronic, and electro-optic properties. In this study, sol–gel autoignition was used to prepare Ca3Co4O9 at different calcination temperatures（773, 873, 973, and 1073 K） and time（4, 6, 8, 10, 12, and 14 h） using starch as a fuel. The phase and microstructure of the prepared Ca3Co4O9 powder were investigated. Thermogravimetry–differential thermal analysis（TGA） confirms that the final weight loss occurred at 1073 K to form Ca3Co4O9 stable powder. The variable-pressure scanning electron microscopy（VP-SEM） images show that the size of powder particles increases from 1.15 to 1.47 μm as calcination time increases from 4 to 12 h, and the size remains almost constant thereafter. A similar pattern is also observed on the increment of the crystallite size and percentage of crystallinity with X-ray diffraction（XRD） analysis. The highest crystallinity is found about 92.9% when the powder was calcinated at 1073 K for 12 and 14 h with 458 and 460 ? crystallite size, respectively. Energy dispersive X-ray spectroscopy（EDS） analysis demonstrates that the calcinated powder has a high intensity of Ca, Co, and O with uniform distribution. High-resolution transmission electron microscopy（HRTEM） images prove that there is no distinct lattice distortion defect on the crystal structure.