Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet Oxide formation mechanism and its efect on the microstructure and thermoelectric properties of p-type Bi 0.5 Sb 1.5 Te 3 alloys May Likha Lwin a , Peyala Dharmaiah a , Babu Madavali a , Chul-Hee Lee a , Dong-won Shin a , Gian Song a , Kap-Ho Lee b , Soon-Jik Hong a,∗ a Division of Advanced Materials Engineering, Kongju National University, 275, Budae-dong, Cheonan City, Chungcheongnam-do, 330-717, Republic of Korea b Department of Materials Science & Engineering, Chungnam National University, Daejeon, Republic of Korea ARTICLEINFO Keywords: Oxygen contamination p-type Bi 0.5 Sb 1.5 Te 3 Heat treatment Thermoelectric properties Microstructure Gas atomization ABSTRACT Bismuth antimony telluride based thermoelectric (TE) materials have been intensively developed and synthe- sized using diferent mechanisms, for the room temperature TE applications. In particular, bismuth antimony telluride based TE alloys are very sensitive to deviations in their composition, and to contamination during the materials synthesis. Oxygen contamination during Bi-Sb-Te based materials synthesis is one of the critical factors that alters or diminishes thermoelectric-transport properties. Thus, in this study, how the oxide formation me- chanism on the powder surface and bulks of p-type Bi 0.5 Sb 1.5 Te 3 alloys afected the microstructural features and thermoelectric properties were elucidated quantitatively. While applying heat treatment (HT) to Bi 0.5 Sb 1.5 Te 3 powder, the constituent elements interacted with the ambient atmosphere and formed a new oxide phase which acted as a barrier to carrier transport. At the initial stage (300 °C) of heat treatment, only the powder surface was oxidized due to the reaction of outer surface atoms with atmospheric air and moisture. While increasing in temperature during HT, this surface oxygen contamination difused further inside the powder through the grain boundaries. More difusion and spreading occurred throughout the matrix at 450 °C. The increment of oxygen content from 0.05 to 0.82 wt% drastically decreased the electrical conductivity by 67%, and thermal con- ductivity by 7% at the heat treatment temperature of 450 °C. This reduction behavior is mainly due to severe scattering of the carriers/phonons at the new formation of oxide (Sb 2 O 3 ) phase near grain boundaries and within the matrix. At a glance, a small increase in the oxygen content wouldn't signifcantly infuence the thermoelectric properties; however, at a certain level of oxide formation (0.82 wt%), severe efects could occur due to the intensifed scattering or trapping of carriers by the oxide barrier formation at the grain boundaries. 1. Introduction Thermoelectric materials (TE) have become very attractive in rela- tion to their power generation (Seebeck efect) and active refrigeration (Peltier efect) [1]. The efciency of TE materials can be deduced by the dimensionless fgure of merit (ZT), which is defned as the ratio of the power factor (σα 2 ) to the thermal conductivity (κ). Thus, a good TE material should exhibit high electrical conductivity with a high Seebeck coefcient, and low thermal conductivity. It should also minimize the Joule heating efect, to ensure the large potential/thermo-voltage across the junction and create a steep temperature gradient [1,2]. Managing all these parameters within a certain material can be achieved by doping [3], scattering from nanoscale endotaxial pre- cipitation and mesoscale grain boundaries, atomic-scale alloy scattering [4], nanostructuring [5], quantum confnement, superlattices [6], and creating nanocomposites [7]. As mentioned above, improving the thermoelectric properties of thermoelectric materials using powder metallurgy processes has given promising results. On the other hand, the thermoelectric materials fabricated by powder metallurgy techniques such as gas-atomization [8,9], mechan- ical alloying [10], hot extrusion method, and hot-pressing [11,12] are commonly used for the fabrication of TE elements. However, there is a possibility of excessive oxidation of the fnal powder during fabrication or in the next process. To maintain high purity powder, the particles must be isolated from atmospheric oxidation or any contamination. It is known that reducing the powder using hydrogen can upgrade the powder characteristics because of a lower oxygen concentration. Even so, the reduction of powder using hydrogen or ideal gases does not markedly or perfectly afect the TE properties. In particular, the control of the oxides in materials, or the oxygen https://doi.org/10.1016/j.intermet.2018.09.015 Received 14 August 2018; Received in revised form 27 September 2018; Accepted 28 September 2018 ∗ Corresponding author. E-mail address: hongsj@kongju.ac.kr (S.-J. Hong). Intermetallics 103 (2018) 23–32 0966-9795/ © 2018 Elsevier Ltd. All rights reserved. T