Progress in Researches on the Layered Cobalt Oxides for Thermoelectric Devices

The layered cobalt oxides as thermoelectric (TE) materials are introduced in detail on their developments, representative systems, some theories, unsolved problems and approaches for improving performances. TE performances of the layered cobalt oxides are discussed from structures, doped situations, processes and the dimensionless figure of merit. In contrast with other TE materials, the layered cobalt oxides show many promising applications. The theory of Seebeck coefficient and hopping conduction mechanism in the layered cobalt oxides are discussed. Heike formula explains that Seebeck coefficient origins from electronic spin states and proportions of different value states of Co ion. An unbalance of the spin and orbital degrees of freedom between Co3+ and Co4+ sites results in the large Seebeck coefficient. On the basis of the Boltzmann transport equation, Seebeck coefficient is decided by energy band structures. High state density near Fermi Energy band is responsible for the coexistence of large Seebeck coefficient and high electrical conductivity. Hopping conduction mechanism found in experiments is a main transport way of charge carriers at high temperature for the layered cobalt oxides. Through different materials systems and theories analysis, unsolved problems and new approaches for improving TE performances are put forward.