Thermoelectric Properties of Ca 3x Dy x Co 4 O 9+d with x = 0.00, 0.02, 0.05, and 0.10 ANKAM BHASKAR, 1 C.-S. JHANG, 1 and CHIA-JYI LIU 1,2 1.—Department of Physics, National Changhua University of Education, Changhua 500, Taiwan, ROC. 2.—e-mail: liucj@cc.ncue.edu.tw Polycrystalline samples of Ca 3x Dy x Co 4 O 9+d (x = 0.00, 0.02, 0.05, and 0.10) have been prepared by conventional solid-state synthesis. The x-ray diffrac- tion (XRD) results revealed that all the samples are single phase. The ther- moelectric properties were measured at 25 K to 300 K. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity of all the samples exhibited the nonmetal-to-metal transition at below 75 K. The electrical resistivity decreased and the thermopower increased with increasing Dy 3+ content. Among all the samples, Ca 2.9 Dy 0.10 Co 4 O 9+d had the highest dimen- sionless figure of merit of 0.044 at 300 K. Key words: Thermoelectric materials, sintering, thermoelectric, thermal conductivity, x-ray diffraction, figure of merit INTRODUCTION Thermoelectric energy conversion can be used to generate electricity from waste heat. The efficiency of thermoelectric materials in this process is deter- mined by their dimensionless thermoelectric figure of merit, ZT = S 2 rT/j, where S, r, T, and j are the Seebeck coefficient, the electrical conductivity, the absolute temperature, and the thermal conductivity, respectively. 1 Wide attention has been focused on exploration of thermoelectric materials recently. Good thermoelectric materials require large ther- mopower (S) to generate a large thermal voltage, low electrical resistivity (q) to minimize Joule heating, and low thermal conductivity (j) to retain heat at the junctions. 2 Besides, thermoelectric materials are re- quired to be stable at high temperatures. In recent years, layered cobalt oxides have gained great attention since NaCo 2 O 4 single crystal was found to exhibit good thermoelectric properties. 3 Recently, the misfit cobalt oxides (Ca 3 Co 4 O 9+d ) have been investigated extensively as potential thermoelectric materials because of their large S, low q, and low j. 4–9 The crystal structure of the Ca 3 Co 4 O 9+d system con- sists of two subsystems, viz. the distorted NaCl-type (Ca 2 CoO 3 ) sublattice and the CdI 2 -type (CoO 2 ) sub- lattice, alternately stacking along the c-axis. 10 Poly- crystalline bulk Ca 3 Co 4 O 9+d samples are still at a relatively low level for industrial applications. Many attempts have been made to optimize the thermo- electric performance of Ca 3 Co 4 O 9+d by either par- tially substituting cations or using appropriate fabrication methods such as hot pressing (HP) 11 or spark plasma sintering (SPS) techniques. 12 Partial replacement of cations in Ca 3 Co 4 O 9+d has been car- ried out on either the Ca site 13–24 or the Co sites. 5,8,25–28 It has been reported that partial substitution for Ca by heavier ions with trivalence such as Eu 3+ , 13 Nd 3+ , 14 Bi 3+ , 15 Y 3+ , 21 or Yb 3+ , 22 is effective in improving the thermoelectric properties. There are a few reports regarding the high-temper- ature ( >300 K) thermoelectric properties of the Ca 3x Dy x Co 4 O 9+d system with higher doping level (x ‡ 0.10). 9,29,30 Therefore, it is interesting to inves- tigate the low-temperature ( <300 K) thermoelectric properties of the Ca 3x Dy x Co 4 O 9+d system. We report herein the low-temperature thermoelectric properties of the Ca 3x Dy x Co 4 O 9+d (0 £ x £ 0.10) system. EXPERIMENTAL PROCEDURES Polycrystalline samples of Ca 3x Dy x Co 4 O 9+d (x = 0.00, 0.02, 0.05, and 0.10) were synthesized by (Received March 11, 2013; accepted May 2, 2013; published online June 11, 2013) Journal of ELECTRONIC MATERIALS, Vol. 42, No. 8, 2013 DOI: 10.1007/s11664-013-2634-8 Ó 2013 TMS 2582