Role of oversized dopant potassium on the nanostructure and thermoelectric performance of calcium cobaltite ceramics

Abstract

The impact of the non-stoichiometric addition of potassium (K) on the nanostructure and thermoelectric performance of misfit layered calcium cobaltite (Ca₃Co₄O₉) ceramics is reported. The samples were prepared with the designed nominal composition of Ca₃Co₄O₉K_x (x = 0, 0.05, 0.1, 0.15, and 0.2). The K addition promoted the crystal growth and improved the crystal texture. The nanostructure and chemical analysis revealed the segregation of K at the Ca₃Co₄O₉ grain boundaries, while the Ca₃Co₄O₉ grain interior was free of K. At the optimal doping level, the dopant K grain boundary segregation reduced the electrical resistivity and simultaneously increased the Seebeck coefficient, resulting in a large increase in the power factor. At 320 K, the sample Ca₃Co₄O₉K_0.1 achieved the power factor of 930 μW K⁻² m⁻¹, which is 2.25 times higher than 412 μW K⁻² m⁻¹ from pristine Ca₃Co₄O₉ and by far, the highest power factor at room temperature regime for the Ca₃Co₄O₉ ceramics. The impact of the dopant segregation on the ionic diffusion along the grain boundaries and its resultant thermoelectric performance enhancement of Ca₃Co₄O₉ ceramics are discussed.