Sintering process andnonstoichiometry of NaCo2O4 layered thermoelectric oxide


M. Ohtaki, K. Shouji, T. Hayashida


Interdisciplinary Graduate Schoolof Engineering Sciences, Kyushu University

Kasuga, Fukuoka 816-8580, Japan


Layeredoxide NaCo2O4 is the best p-type oxide candidate forthermoelectric material to date, sintered sample having attained ZT = 0.78 [1].Although the previous report on the single crystal data of the oxide estimatedmuch superior performance (comparable to Bi2Te3 at roomtemperature), the reported performance of polycrystalline samples has beenstaying far below the expectation. We have already reported that sinteringconditions strongly affect the thermoelectric properties of the oxide,revealing that repeated sintering leads to improvement in both conductivity andthermopower. The important effects of the repeated sintering are considered asreducing structural imperfection within crystallites, and eliminatingnon-crystalline Na species that are undetectable by XRD measurements butseriously deteriorate the electrical conduction at the grain boundaries.  In this study, we have investigatedin-situ monitoring and further controlling the Na partial pressure in theatmosphere during sintering the oxide. The in-situ monitoring of the Na partialpressure was carried out by an electrochemical Na sensor using Na ionconducting solid electrolytes. Influence of bulk Na nonstoichiometry and localcompositional deviation particularly at the grain boundaries will be discussed.

[1]M. Ohtaki et al., Proc. 19th Int. Conf.Thermoelectrics, pp. 190-195 (2000).