Overview of properties of 'metallic' Na_xCo_2O_4 thermoelectric materials

摘要

Since the thermoelectric properties of transition-metal oxide NaCo_2O_4 were first reported by Terasaki in 1997, extensive research work has been conducted on this system of ceramic oxide thermoelectric materials. Efforts to improve the thermoelectric properties through doping or new synthesis approaches have been investigated. More recently, efforts have been expended in theoretical investigations in order to explain the coexistence of small resistivity values and relatively large thermopower values simultaneously in these materials, which is not apparent from conventional free-electron theory. Current studies indicate the strong electron correlation and spin entropy may be the main reasons behind the enhanced thermopower of this "metallic-like" ceramic oxide, Na_xCo_2O_4. Recently, superconducting behavior under 5K was reported in the hydrated form of this oxide Na_xCo_2O_4·yH_2O. In this paper a review of the newest research developments are presented, and as well our own results and future research directions are also discussed.