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This research studies the effect of the heating rate at calcination temperature (2°C, 5°C, 10°C, and 20°C/min) on the Ca3Co4O9 phase and crystallite size, as well as the thermal conductivity (κ) and electrical resistivity (ρ) of the material. Co(NO3)2∙6H2O and CaCO3 synthesised by sol–gel combustion technique. The Rietveld refinement technique confirmed the structure and crystal size calculations were performed using the Williamson-Hall (WH) plot. Thermogravimetric-differential thermal analysis (TGA-DTA) showed differences in the exothermic energy produced (553–683 K), which related to the reaction rate and phase transitions affecting the crystal size and thermoelectric (TE) properties as the p-type TE material. The κ and ρ values tend to increase with increasing heating rate temperature, where κ and ρ values range from 1.69 to 1.90 W/mK and 82.73 to 118.57 mΩ cm respectively. [ABSTRACT FROM AUTHOR]