Effects of Cation Substitution on the Thermoelectric Properties in Ca-Co-O
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Effects of Cation Substitution on the Thermoelectric Properties in Ca-Co-O Ichiro Matsubara, Ryoji Funahashi, Masahiro Shikano National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka 563-8577, JAPAN Kei Sasaki, Hiroyuki Enomoto Osaka Electro-Communication University, Neyagawa, Osaka572-8530, JAPAN ABSTRACT We have prepared (Ca1-x-yMxBiy)3Co4Oz (M = Mg, Sr, and Ba) thin films by a combinatorial approach using a solution process. In the systems of (Ca1-x-yMxBiy)3Co4Oz (M = Mg, Sr, and Ba), solid solution range was determined to be x < 0.8 (M = Sr, y = 0), x < 1.0 (M = Mg, y = 0), x = 0.0 (M = Ba, y = 0), and x < 0.4 (M = Bi, x = 0). No solid solution range was obtained for the substitution of Ba for Ca site. The in-plane compressive stress in the CoO2 sublattice is controllable by the cation substitution for Ca in the (Ca2CoO3) sublattice. With increasing in-plane stress, the magnitude of thermoelectric power and resistivity increased. INTRODUCTION Since the discovery of a high thermoelectric (TE) performance in NaCo2O4 [1, 2], a lot of works on Co-based oxides with a layered structure have been carried out. Recently, another layered cobaltite has been discovered as potential TE material, (Ca2CoO3)xCoO2 [3-8] (x ≅ 0.61: 349 phase), which has the same CdI2-type CoO2 structural component as that in NaCo2O4. The structure of misfit layered (Ca2CoO3)xCoO2 has been reported as triple rock salt-type Ca2CoO3 layers and single CdI2-type CoO2 layers are stacked alternately. Sr- and Bi-doped 349 single crystal whiskers have a figure of merit ZT of over 1.2 at T > 600 °C in air [5]. Due to the high ZT value of the whiskers, this type of material is recognized as good candidate for high temperature application. In this study, we have investigated the effects of cation substitution on the thermoelectric properties in the 349 phase. We have adopted a combinatorial approach to synthesize the cation doped 349 thin films rapidly. Combinatorial process involves the design and synthesis of high density libraries aimed at efficiently exploring large numbers of structurally or compositionally diverse compounds thought to be of interest as a result of an understanding of their chemical, physical, and structural properties [9]. The combinatorial chemistry has firstly developed in the pharmaceutical industry, and it is now being applied to the discovery of more efficient catalysts and materials. We have adopted a solution technique to synthesize 349 thin films by a combinatorial approach. Solution processing offers molecular level mixing of constituents leading to excellent chemical homogeneity and composition control. Shorter diffusion distances for reactants lead to lower reaction temperatures for crystallization. Moreover, the process has a potential to form films on large size substrates in a short time without any high vacuum apparatus. We have synthesized (Ca1-x-yMxBiy)3Co4Oz (M = Mg, Sr, and Ba) films and determined the solid solution range. We also discuss the effects of cation substitution on the TE properties
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