Separation of electric and thermal transport with in-situ precipitates matrix in Ca3Co4O9+δ

Suppressing the thermal conductivity of materials through the reduction of the particle sizes has long been applied to enhance the thermoelectric performance of nanostructured materials. Maintaining a high electrical conductivity in the as-formed nanostructure that is comparable to its bulk materials, is equally important in determining the final thermoelectric behavior of the nanostructure. However, this is not always attainable since the phononic and electronic transport in solids is often interdependent on each other. Naturally occurring with a complex layered structure, bulk polycrystalline Ca3Co4O9+δ has been selected to investigate the subtle correlation between electronic and phononic subsystems, which is still challenging for complex thermoelectrics. By combining high-energy ball milling and spark plasma sintering (SPS), we generate a novel structure of CCO matrix with in-situ precipitates, in which the bar-shaped CCO grains form a three-dimensional network, and the Co3O4 polyhedrons are found to be attached to the CCO grain surfaces with CaCO3 nano-precipitates filling the network space. The CCO grains serve as electrical paths to maintain the high electrical conductivities, while the in-situ precipitates, especially CaCO3, have significantly blocked the phonon transport in the bulk CCO samples with nanocrystalline grains, thus greatly reducing the thermal conductivity. Electrochemical analysis and the modified Debye-Callaway calculation provide a substantial understanding for the transport of charge carriers and phonons. A record zT value of 0.5 with ultra-low thermal conductivity of 0.496 Wm−1K−1 at 973 K are achieved in the CCO-66 sample. To date, this is the highest reported zT value in undoped polycrystalline CCO material systems.