Enhanced thermoelectric cooling properties of Bi 2 Te 3Àx Se x alloys fabricated by combining casting, milling and spark plasma sintering Seung Tek Han a , Pradip Rimal a , Chul Hee Lee a , Hyo-Seob Kim b , Yongho Sohn c , Soon-Jik Hong a, * a Division of Advanced Materials Engineering, Kongju National University, 331-717, South Korea b Metals Development, Ames Laboratory, Iowa State University, 5001, USA c Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816, USA article info Article history: Received 25 March 2016 Received in revised form 17 May 2016 Accepted 12 August 2016 Keywords: Bi 2 Te 3Àx Se x alloys High energy ball milling Amount of Se Thermoelectric properties Figure of merit abstract Bi 2 Te 3Àx Se x alloys are extensively used for thermoelectric cooling around room temperature, but, pre- vious studies have reported peak thermoelectric efficiency of the material at higher temperature around 450 K. This study presents the casting followed by high energy ball milling and spark plasma sintering as a thriving methodology to produce efficient and well-built Bi 2 Te 3Àx Se x material for the thermoelectric cooling around room temperature. In addition, changes in electrical and thermal transport properties brought up by amount of Se in the Bi 2 Te 3Àx Se x material for this methodology are measured and dis- cussed. Although Seebeck coefficient and electrical conductivity showed irregular trend, power factor, thermal conductivity and figure of merit ZT gradually decreased with the increase in amount of Se. A maximum ZT value of 0.875 at 323 K was obtained for x ¼ 0.15 sample owing to its higher power factor. This value is 17% and 38% greater than for x ¼ 0.3 and x ¼ 0.6 samples respectively. At 323 K, herein reported ZT value of 0.875 is higher than the state of art n-type Bi 2 Te 3 based thermoelectric materials produced by the time consuming and expensive methodologies. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Numerous investigations are searching for more efficient Ther- moelectric (TE) materials for TE cooling. TE coolers are of great interest because they are environment friendly, free of noise and vibration, easy to maintain, and more reliable compared to existing refrigeration systems. They also can operate in a wider range of operation by using a wide variety of TE materials [1e4]. Efficiency of TE materials, is given by the dimensionless figure of merit, ZT ¼ [(sa 2 ) T]/k, where s, a, T and k are electrical conductivity, Seebeck coefficient, absolute temperature and thermal conductiv- ity, respectively. N-type Bi 2 Te 3 based alloys are mostly used in the TE coolers at near room-temperature, and for the low-temperature electricity generation with their p-type counterpart [5]. Generally single crystal n-type Bi 2 Te 3Àx Se x or preferentially oriented polycrystalline alloys are suitable for the TE performance. But lower mechanical strength due to lamellar microstructure and susceptibility to cleavage along basal planes have limited their application potentials [6,7]. To improve the mechanical behavior of Bi 2 Te 3 based alloys, powder metallurgy methods have shown promising results by fabricating polycrystalline TE materials. Pri- mary powder fabrication processes such as atomization, ball mill- ing, melt spinning, and subsequent consolidation via hot extrusion, hot pressing, spark plasma sintering, or hot deformation have been explored to produce efficient and strong TE materials [8e10] by varying the critical processing parameters (e.g., temperature, pressure, etc.). Although there are many choices, it is worthwhile to note that a superior TE properties of n-type polycrystalline materials are typically obtained by the combination of rocking furnace and zone melting; rocking furnace and prolonged ball milling; rocking furnace, ball milling and repeated consolidation techniques. In- efficiency in operation, challenges in scale-up production and high cost of these processes have limited the commercial realization. More importantly, despite the use of inefficient and costly pro- cesses, ZT values below 0.8 near room-temperature were reported [11e 14]. In this study, we employed (1) casting, (2) high energy ball * Corresponding author. E-mail address: hongsj@kongju.ac.kr (S.-J. Hong). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet http://dx.doi.org/10.1016/j.intermet.2016.08.006 0966-9795/© 2016 Elsevier Ltd. All rights reserved. Intermetallics 78 (2016) 42e49