Electronic transport of co-doped misfit-layered cobaltites Ankam Bhaskar 1 • Zong-Ren Yang 1 • Chia-Jyi Liu 1 Received: 4 March 2015 / Accepted: 22 June 2015 / Published online: 1 July 2015 Ó Springer Science+Business Media New York 2015 Abstract We have fabricated two series of co-doped misfit-layered cobaltites Ca 3 Co 4 O 9?d , Ca 3-x Yb x Co 4-y Ag y O 9?d with (x = 0.05, y = 0.05), (x = 0.05, y = 0.10), and (x = 0.05, y = 0.10), and Ca 3-x Eu x Co 4-y Ag y O 9?d with (x = 0.05, y = 0.05), (x = 0.05, y = 0.10), and (x = 0.05, y = 0.10) using conventional solid state reac- tion. The electrical resistivity and thermopower were measured between 300 and 700 K. For all the samples, the temperature dependence of electrical resistivity exhibits broad maximum, indicating disappearance of quasiparticle resonance. Unlike the level-off thermopower behavior between 200 and 300 K, which is often observed for the misfit-layered cobaltites, the thermopower of all the sam- ples increases with increasing temperature up to 700 K, which could be associated with the strong temperature dependence of quasiparticle resonance. Considering that both the sublattices of CoO 2 and Ca 2 CoO 3 could make contribution to the electronic transport, it is plausible to explain the variation of the size and temperature depen- dence of electrical resistivity and thermopower in the framework of two-carrier system. Among the samples, Ca 2.95 Eu 0.05 Co 3.95 Ag 0.05 O 9?d exhibits the highest power factor of 3.36 lW cm -1 K -2 at 700 K. This value repre- sents an improvement of about 110 % compared to the undoped Ca 3 Co 4 O 9?d . Ca 2.95 Eu 0.05 Co 3.95 Ag 0.05 O 9?d had the highest dimensionless figure of merit of 0.037 at 300 K, representing an improvement of about 84 % compared to the undoped Ca 3 Co 4 O 9?d . 1 Introduction Thermoelectric materials have recently drawn much attention because of their potential application to clean energy conversion. The performance of thermoelectric materials is expressed by the dimensionless figure of merit, ZT = S 2 T/qj, where S is the thermopower, q the electrical resistivity, j the total thermal conductivity, and T the absolute temperature. In order to obtain high ZT, the material must have large S, low q and low j [1]. Ther- moelectric materials are required to be stable at high temperatures. Thermoelectric oxide materials such as Na x CoO 2 , Bi 2 Sr 2 Co 2 O 9 , and Ca 3 Co 4 O 9?d have gained great attention due to their good thermal and chemical stability at high temperature [2]. However, the sodium and bismuth are volatile at high temperature in Na x CoO 2 and Bi 2 Sr 2 Co 2 O 9 . Therefore, calcium cobaltites Ca 3 Co 4 O 9?d have been investigated extensively as potential thermo- electric material because it has large S, low q, and low j [3–5]. The crystal structure of Ca 3 Co 4 O 9?d consists of two interpenetrating subsystems: a CdI 2 -type [CoO 2 ] subsystem and a distorted NaCl-type [Ca 2 CoO 3 ] subsystem. The two monoclinic subsystems have common crystallographic a and c axes and b angle but are incommensurate along the b axis and are therefore referred to as ‘‘misfit-layered’’. The resulting structural formula is [Ca 2 CoO 3 ] P CoO 2 where p is called the misfit-layered parameter, given by the b axis ratio b 1 /b 2 & 0.62 of the two subsystems [6]. Polycrys- talline bulk Ca 3 Co 4 O 9?d are still at a relatively low level for industrial applications. Many attempts have been made to optimize the thermoelectric performance of Ca 3 Co 4 O 9?d by either partially substituting cations or using appropriate fabrication methods such as hot pressing (HP) or spark plasma sintering (SPS) techniques. Partial replacement of cations in Ca 3 Co 4 O 9?d has been carried out on either the & Chia-Jyi Liu liucj@cc.ncue.edu.tw 1 Department of Physics, National Changhua University of Education, Changhua 500, Taiwan, China 123 J Mater Sci: Mater Electron (2015) 26:9463–9469 DOI 10.1007/s10854-015-3401-9