Synthesis and Rietveld analysis for CoSb 3 compounds prepared by Sb self-flux method Takeshi Souma 1,* and Michitaka Ohtaki 1, 2 1 Japan Science and Technology Agency, 3-8-34 Momochihama, Sawara-ku, Fukuoka 814-0001, Japan 2 Faculty of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka 816-8580, Japan * kfd01657@nifty.ne.jp, +81-92-851-8213 (phone), +81-92-851-8224 (fax) Abstract High purity CoSb 3 bulk crystals have been successfully synthesized by Sb self-flux method and a relation between reaction conditions and chemical composition on the method has been systematically analyzed by powder XRD study using the Rietveld analysis. Sb self flux method at 923 K using 100 mesh Co elements can directly provide a single phase CoSb 3 bulk crystal in a brief time of 10 h. Advantages of Sb self-flux methods will be discussed compared with other preparation methods. Introduction The Sb rich region of Co-Sb phase diagram re- investigated by Feschotte et al. [1] is shown in Fig.1. The Co-Sb system has three intermetallic compounds; CoSb, CoSb 2 and CoSb 3 . Among them, CoSb 3 has been identified as a promising candidate for the thermoelectric conversion in the intermediate temperatures, and a number of studies have been carried out to investigate the physical properties and to improve the performance [2-8]. As shown in Fig. 1, CoSb 3 is an incongruent-melt compound which has a peritectic point at 1146 K. Therefore, most samples have been synthesized by quenching from liquid phase or by mechanical alloying below the peritectic point to prevent formation of impurity phases, CoSb 2 and/or CoSb. However, an additional heating process necessary to eliminate the impurity phases requires more time and apparatus. Bridgemann-solution method is one of the best technique for the direct preparation of crack free and highly pure bulk crystals of CoSb 3 [2]. However, the Bridgemann-solution method is usually time consuming process, and is unsuitable for mass production. In order to familiarize CoSb 3 compounds as practical thermoelectric materials such as Bi-Te compounds, more convenient preparation technique is strongly desired. Shimozaki et al. studied the formation kinetics of solid/liquid/vapor Sb and solid Co between 723 and 1123 K, and found that small grain (100-200 µm) of CoSb 3 formed in the boundary between liquid Sb and solid Co plate with non- stoichometric composition at 1023 K after 49 h [9]. However, a thin film (30-50 µm) of CoSb simultaneously formed at the same boundary. These results suggest that Sb self-flux technique is possible for preparing CoSb 3 compounds. In addition, an optimization in the reaction condition, e.g. Co particle size and reaction time, is necessary to develop the technique as a practical preparation method for CoSb 3 . In this study, we have tried to synthesize CoSb 3 using Sb self-flux method with diffrerent reaction conditions and carried out the XRD study using the Rietveld method to analyze the chemical compositions. Our goal is to optimize the reaction conditions for Sb self-flux method in order to synthesize high purity CoSb 3 bulk crystals as practical thermoelectric materials. 40 50 60 70 80 90 100 500 600 700 800 900 1000 1100 1200 1300 1400 800 900 1000 1100 1200 1300 1400 1500 1600 CoSb CoSb 2 CoSb 3 1112 929 ±2 873±5 621±2 L+CoSb at% Sb L+CoSb 2 CoSb+CoSb 2 L+CoSb 3 (CoSb) CoSb 3 +Sb CoSb 2 +CoSb 3 Temperature [ ℃] Liquid 1215 S.F. Temperature [K] Fig.1. Sb rich region of Co-Sb phase-diagram [1]. The temperatures in the figure are expressed in Celsius. The symbol denoted as ‘S.F.’ represents the chemical composition and reaction temperature in the present Sb self- flux method. Experimental details In this study, we have used 3N grade Co with 3 different particle size (325 mesh, 100 mesh and 1-2 mm) and adopted 6 different reactioin times of 0.3, 1.0, 3.0, 10, 30 and 100 h. Therefore, 18 sets of Co powders were weighted with 6N grade Sb lump (3-10 mm) at the stoichometric ratio of CoSb 3 . The weight of the 18 sets of elements were adjusted to approximately 2 g. The elements were introduced into quartz ampoules with 8/10 mm in inner/outer-diameters, and the ampoules were well evacuated and back-filled with 0.01 MPa of high-purity (99.9999%) He gas to ensure the heat conduction. The ampoules were heated at 923 K, which temperature is below the peritectic point of CoSb 3 and slightly higher than the melting point of Sb (see the ‘S. F.’ point in Fig.1). The 18 bulk crystals were successfully obtained and the average weight loss between before and after the reaction was 1.5 %. Powder XRD samples were prepared from the 18 bulk crystals and ground by using a Nitto ANM-150 automatic mortar. The powder X-ray diffraction patterns were measured from 20 to 80 deg. at 300 K on a Rigaku RINT- 2500V diffractometer with Cu-Kα radiation. The beam 0-7803-9552-2/05/$20.00 ©2005 IEEE 121 2005 International Conference on Thermoelectrics