Enhanced thermoelectric performance of compacted Bi 0.5 Sb 1.5 Te 3 nanoplatelets with low thermal conductivity

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Alan B. Kaiser MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington, New Zealand (Received 14 December 2010; accepted 4 May 2011)

We report fabrication of compacted Bi0.5Sb1.5Te3 nanoplatelets using hydrothermal methods followed by cold pressing and sintering in an evacuated ampoule at various temperature of 300–380 °C. The compacted Bi0.5Sb1.5Te3 sintered at 340 °C has the highest power factor of 11.6 lW/cmK2 and its thermal conductivity is 0.37 W/mK at 295 K, which is very low as compared to the typical value of 1 W/mK. The resulting dimensionless figure of merit ZT is 0.93 at 295 K.

I. INTRODUCTION

Thermoelectric materials can produce electrical power via the Seebeck effect and provide cooling capability via the Peltier effect without using coolants. Therefore, they can be considered as energy and environmentally friendly materials. The figure of merit (Z) of thermoelectric materials is determined by three transport parameters and can be expressed as Z 5 rS2/j, where r, S, and j are the electrical conductivity, thermopower, and thermal conductivity, respectively. Bismuth alloys show the highest dimensionless figure of merit ZT (where T is the absolute temperature) around room temperature and are one of the most commercialized thermoelectric materials. The crystal lattice of (Bi,Sb)2(Te,Se)3 has the symmetry of R 3m. The layers of atoms A(Bi,Sb) and B(Te,Se) are arranged in the order of -B(1)-A-B(2)-A-B(1)- along the c axis. Turning the currently best thermoelectric materials into nanoscale grain boundaries or nanostructure helps improve thermoelectric figure of merit by increasing the thermoelectric power (TEP) as a result of an enhanced density of states and by reducing thermal conductivity as a result of an increased phonon scattering.1,2 Recently, the theoretical concept has been fulfilled by a significant enhancement of ZT value for nanostructured bulk of Bi2-xSbxTe3.3,4 Several groups reported bottom-up chemical synthesis of Bi2Te3 nanocrystals at low temperatures of 140–150 °C using either organic solvent or water as reacting medium.5–10 It is well known that the carrier concentration needs to be optimized for thermoelectric materials to achieve the best electronic transport properties due to its opposite trends with respect to electrical conductivity a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.158 J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011

and thermopower. There exist complex lattice defects in Bi2-xSbxTe3. The ionized defects could change the carrier concentration and hence the electronic transport properties.11 Furthermore, lattice defects are known to be sensitive to sintering or annealing temperature12 and play an important role to obtain good thermoelectric properties of Bi2-xSbxTe3. Although bottom-up chemical synthesis of Bi2-xSbxTe3 nanocrystals is feasible at low temperatures, it is, therefore, desirable to optimize the thermoelectric properties by finding suitable sintering conditio