Effect of Synthesis Conditions on Formation, Electrical Properties, and Seebeck Coefficient of p-Type Ca3Co4O9±δ Thermoelectric Ceramics

Ca3Co4O9±δ ceramic powders have been prepared by a solid-state method. The calcination and sintering temperatures and reaction conditions were varied to achieve highly dense materials for thermoelectric applications. The optimized calcination temperature and reaction conditions were derived. X-ray diffraction patterns showed formation of secondary phases for longer calcination duration. The density of the ceramics ranged from 3.2 g cm−3 to 3.4 g cm−3, not varying greatly with the calcination/sintering conditions. The electrical properties and Seebeck coefficient reveal that the density and nonstoichiometry greatly influenced the achievement of good thermoelectric properties. Transmission electron microscopy (TEM) images showed fine particles with nanosize, strongly bound together to form metal-rich particle aggregates. Tubular morphology below 50 nm to 100 nm scale was observed in TEM images of as-prepared solid-state Ca3Co4O9±δ . As-prepared samples showed improved electrical conductivity and Seebeck coefficient, suitable for thermoelectric applications.