Effect of spark plasma sintering temperature on thermoelectric properties of Bi 2 S 3 polycrystal

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Jing-Feng Li State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China (Received 15 June 2011; accepted 4 August 2011)

Bismuth sulfide (Bi2S3) polycrystalline bulks with a high relative density (.90%) were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). The microstructural and thermoelectric (TE) properties were investigated with a special emphasis on the influence of SPS temperature. The components (Bi/S ratio), grain size, and relative density of Bi2S3 bulk are sensitive to the SPS temperature, which all affect the electrical transport properties of samples. Elevating SPS temperature results in grain growth, densification, and spontaneous texturing, which benefit to enhance TE properties, whereas overheating leads to severe volatilization of both S and Bi and formation of porous microstructure. The highest figure of merit value reaches 0.22 at 573 K for the Bi2S3 sample obtained by applying SPS at 673 K for 5 min, which is the maximum value reported so far in bulk Bi2S3 system. The enhanced TE property of cheap and environmental friendly Bi2S3 material indicates a great promise in TE devices. I. INTRODUCTION

Thermoelectric (TE) material is a functional semiconductor which could directly interconvert thermal energy to electrical energy with each other. TE devices are silent, reliable, and scalable, making them ideal for small, distributed power generation.1–3 The efficiency of TE devices is determined by the dimensionless figure of merit (ZT), defined as ZT 5 a2T/qj, where a, q, T, and j are Seebeck coefficient, electrical resistivity, absolute temperature, and thermal conductivity, respectively.4 Bismuth telluride (Bi–Te)-based compounds show the best TE properties near room temperature. Although Te-based materials usually exhibit good TE properties and represent a dominant market share of TE materials, it is necessary to develop alternative materials to replace the rare and toxic tellurium. Bismuth sulfide (Bi2S3) as a layered semiconductor belongs to the same VI–VA group compound and has a direct band gap Eg 1.3 eV. Because of its high electrical resistivity, which is two orders higher than that of Bi2Te3,8 a few attention has been paid to Bi2S3 in TE development. Recent attempts reveal the potential of the Bi2S3 as a promising TE material because of the high Seebeck coefficient and low thermal conductivity.5–7 Decreasing the electrical resistivity is still a priority to improve the TE property for Bi2S3 material. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.273 J. Mater. Res., Vol. 26, No. 21, Nov 14, 2011

Lots of efforts have been performed on decreasing electrical resistivity.6,7,9,10 Mizoguchi et al.6 successfully reduced the electrical resistivity of Bi2S3 crystal whisker and bulk samples by vacuum annealing, which indicated that S defect could reduce electrical resistivity of Bi2S3. Liufu et al.7 prepared the Bi2S3 self-assembly nanorod