Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Thermoelectric characterization of multi-walled carbon nanotube/ Sodium cobalt oxide prepared by a low-cost flame sintering technique Chutima Oopathump a,b , On-Uma Kheowan a,b , Anek Charoenphakdee c , Adul Harnwunggmoung d , Siwaporn M. Smith b,e, ⁎ , Christopher B. Smith f, ⁎⁎ a Chemical Physics Postgraduate Program and Faculty of Science, Mahidol University, 272 Rama IV Rd., Rajathevi, Bangkok 10400, Thailand b Department of Chemistry, Faculty of Science, Mahidol University, 272 Rama IV Rd., Rajathevi, Bangkok 10400, Thailand c Faculty of Sciences and Liberal Arts, Rajamangala University of Technology Isan, 744 Suranarai Road Muang District, Nakhon Ratchasima 30000, Thailand d Faculty of Science and Technology, Rajamangala University of Technology Suvarnabhumi, 60 Moo 3, Asian Highway, Huntra Phranakhon Si Ayutthaya 13000, Thailand e Center of Sustainable Energy and Green Materials, Faculty of Science, Mahidol University, 999 Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand f Faculty of Science, Mahidol University, 999 Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand ARTICLE INFO Keywords: Multi-walled carbon nanotube Sodium cobalt oxide Composite Flame sintering Thermoelectric properties ABSTRACT Multi-walled carbon nanotubes/sodium cobalt oxide (MWCNTs/Na x CoO 2 ) composites were successfully ob- tained via a flame sintering method. Such sintering is accountable for a surface densification that can preserve the MWCNTs entity within the composites. Raman spectroscopy confirmed the presence of MWCNTs in the composites, with Na x CoO 2 being present as a single phase (γ) as revealed by X-ray diffraction. EDX results suggested non-uniform distribution of MWCNTs within the Na x CoO 2 material, although their presence resulted in increased electrical conductivity through enhancements in charge mobility. The effect of MWCNTs addition on the thermoelectric performance of Na x CoO 2 was examined. Addition of MWCNTs results in increased levels of phonon scattering in the composites, leading to decreased thermal conductivity. Although Seebeck coefficients decreased due to an increase in charge density, the thermoelectric efficiency of the composites reported as ZT values was enhanced in comparison to those in material without added MWCNTs, with optimum conditions found in composites derived from 0.25 wt%MWCNTs/Na x CoO 2 (with ZT enhancement by ca. 60% at 398 K). 1. Introduction The recovery of waste heat has gained more attention for its po- tential use in a power supply contributing to reduction of fuel con- sumption and low carbon footprint. Thermoelectric utility of low tem- perature (< 400.15 K, 127 °C) waste heat to generate electrical energy via the Seebeck effect may realize the potential of this underutilized resource. The efficiency of waste heat to be transferred through mate- rials depends on its electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ), with the transfer efficiency being de- termined using the Figure of Merit (ZT) [1] as described in Eq. (1). = ZT Sσ κ 2 (1) Thus, the efficiency of thermoelectric materials can be enhanced by increasing the electrical conductivity and Seebeck coefficient, or decreasing thermal conductivity. Metal oxides are promising thermo- electric materials having low toxicity, and high chemical stability at temperatures greater than 800 K, with Na x CoO 2 having the highest ZT in comparison to other p-type metal oxide materials at even lower (< 400 K) temperatures [1]. However, the thermoelectric performance of this metal oxide still requires improvements, and enhancements in ZT can be made by increasing phonon scattering and the mobility of charge carriers. Grain boundaries, band energy structure and Fermi levels are also critical factors. Suitable grain boundary sizes can be achieved through microstructure control; these affect the thermal conductivity, electrical conductivity and Seebeck coefficient [1,2]. Fabrication of composites is another strategy to derive materials exhibiting enhanced thermoelectric performance. This relates to the presence of micro- and nano-grain boundaries giving rise to decreased thermal conductivity. In addition, with particle size reductions to the nanoscale allows potential increases in ZT to be attained through lowered thermal conductivity, as http://dx.doi.org/10.1016/j.ceramint.2017.09.123 Received 21 August 2017; Received in revised form 14 September 2017; Accepted 15 September 2017 ⁎ Corresponding author at: Center of Sustainable Energy and Green Materials, Faculty of Science, Mahidol University, 999 Phuttamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand. ⁎⁎ Corresponding author at: Faculty of Science, Mahidol University, 999 Phuttamonthon 4 Rd, Salaya, Nakhon Pathom 73170, Thailand. E-mail addresses: siwaporn.smi@mahidol.edu (S.M. Smith), christopher.smi@mahidol.ac.th (C.B. Smith). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Oopathump, C., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.123