The effect of Cr substitution on the structure and properties of misfit-layered Ca 3 Co 4x Cr x O 9+d thermoelectric oxides Natkrita Prasoetsopha a , Supree Pinitsoontorn b,c,⇑ , Teerasak Kamwanna b,c , Vittaya Amornkitbamrung b,c , Ken Kurosaki d , Yuji Ohishi d , Hiroaki Muta d , Shinsuke Yamanaka d a Materials Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand b Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand c Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Khon Kaen University, Khon Kaen 40002, Thailand d Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan article info Article history: Received 27 September 2013 Received in revised form 6 November 2013 Accepted 6 November 2013 Available online 16 November 2013 Keywords: Ca 3 Co 4 O 9 Thermoelectric oxide Cr doping XANES XPS Figure-of-merit abstract Misfit-layered Ca 3 Co 4x Cr x O 9+d (0 6 x 6 0.2) powders were synthesized by a simple thermal hydro- decomposition method and then fabricated to form highly dense ceramics using spark plasma sintering. X-ray diffraction, and a field emission scanning electron microscope equipped with an energy dispersive X-ray spectrometer were used to verify the single phase of Ca 3 Co 4 O 9+d for the samples with x = 0, 0.05, 0.10 and 0.15. The characteristic plate-like grain structure was observed. The chemical composition of the fabricated samples was found to be close to nominal composition. The thermoelectric measurement showed that Cr doping has an influence on the thermoelectric properties. The resistivity and the Seebeck coefficient increased while the thermal conductivity was suppressed when Cr was added into the system. Explanation of the change in thermoelectric properties was discussed regarding the difference in the charge states and the ionic radii between Co and Cr ions. The highest ZT of 0.19 at 1073 K was obtained for the Ca 3 Co 3.85 Cr 0.15 O 9+d sample. In addition, the paramagnetic property was observed in all samples at room temperature. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction At present clean energy technologies are attractive due to envi- ronmental pollution problems and energy resource limitation. One of the candidate technologies is a thermoelectric (TE) material which can convert waste heat to electricity without environmental pollution. Generally, the performance of a TE material is defined in term of the dimensionless figure-of-merit, ZT = S 2 T/qj, where Z, T, S, q and j are the figure-of-merit, the absolute temperature, the Seebeck coefficient (or so called thermopower), the electrical resis- tivity and the thermal conductivity, respectively [1]. For practical application, ZT 1 is typically required [2]. Typically, TE materials are made of intermetallic compounds, such as Bi 2 Te 3 , PbTe. Although these intermetallic compounds are the state of the art TE materials and exhibit high performance, they are rare, highly oxidative, and toxic [3,4]. Recently, some transition metal oxide ceramics have been discovered for TE applications at high temper- ature [5]. They can be used as alternative TE materials because of their good TE properties and high heat resistance [3,4]. Misfit-layered Ca 3 Co 4 O 9+d is one of the attractive metal oxide materials, which is better expressed as [Ca 2 CoO 3 ][CoO 2 ] b1/b2 since the crystal structure of Ca 3 Co 4 O 9+d can be illustrated as two mono- clinic sublattices: NaCl-type [Ca 2 CoO 3 ] sublattice and CdI 2 -type [CoO 2 ] sublattice stacking along the c-axis direction [6,7], where b 1 and b 2 are the length of the sublattices along the b-axis. This incommensurate structure provides the anisotropic thermoelectric characteristics, such as the difference in resistivity along or per- pendicular to the ab-plane [6]. The single crystal form of Ca 3 Co 4 O 9+d shows high TE perfor- mance (ZT = 0.87 at 973 K) [8]. Nevertheless, it is expensive and difficult to fabricate in large quantity. Therefore, polycrystalline Ca 3 Co 4 O 9+d ceramics, though lower ZT, has been intensively inves- tigated. There are many attempts to improve TE properties of Ca 3- Co 4 O 9+d polycrystalline materials either by improving the fabrication methods, such as hot-pressing [9], spark plasma sinter- ing (SPS) [10], and thermoforging [11], or by doping elements [12– 25]. In the case of ions doping for TE improvement, the transition and other metals have been substituted either at the Ca-sites or the Co-sites. Examples for Ca-sites substitution are Y [17] and Gd [18] dopants which results in an increase in both Seebeck coeffi- cient and electrical resistivity, due to a decrease of carrier concen- trations. For elemental substitution at the Co-sites, a number of 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.034 ⇑ Corresponding author at: Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. E-mail address: psupree@kku.ac.th (S. Pinitsoontorn). Journal of Alloys and Compounds 588 (2014) 199–205 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom