Interface Driven Energy Filtering of Thermoelectric Power in Spark Plasma Sintered Bi 2 Te 2.7 Se 0.3 Nanoplatelet Composites Ajay Soni, † Yiqiang Shen, ‡ Ming Yin, ‡ Yanyuan Zhao, † Ligen Yu, § Xiao Hu, ‡ Zhili Dong, ‡ Khiam Aik Khor, § Mildred S. Dresselhaus, ∥ and Qihua Xiong* ,†,⊥ † Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ School of Material Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore § Division of Manufacturing Engineering, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore ∥ Department of Physics and Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ⊥ Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore ABSTRACT: Control of competing parameters such as thermo- electric (TE) power and electrical and thermal conductivities is essential for the high performance of thermoelectric materials. Bulk- nanocomposite materials have shown a promising improvement in the TE performance due to poor thermal conductivity and charge carrier filtering by interfaces and grain boundaries. Consequently, it has become pressingly important to understand the formation mechanisms, stability of interfaces and grain boundaries along with subsequent effects on the physical properties. We report here the effects of the thermodynamic environment during spark plasma sintering (SPS) on the TE performance of bulk-nanocomposites of chemically synthesized Bi 2 Te 2.7 Se 0.3 nanoplatelets. Four pellets of nanoplatelets powder synthesized in the same batch have been made by SPS at different temperatures of 230, 250, 280, and 350 °C. The X-ray diffraction, transmission electron microscopy, thermoelectric, and thermal transport measurements illustrate that the pellet sintered at 250 °C shows a minimum grain growth and an optimal number of interfaces for efficient TE figure of merit, ZT∼0.55. For the high temperature (350 °C) pelletized nanoplatelet composites, the concurrent rise in electrical and thermal conductivities with a deleterious decrease in thermoelectric power have been observed, which results because of the grain growth and rearrangements of the interfaces and grain boundaries. Cross section electron microscopy investigations indeed show significant grain growth. Our study highlights an optimized temperature range for the pelletization of the nanoplatelet composites for TE applications. The results provide a subtle understanding of the grain growth mechanism and the filtering of low energy electrons and phonons with thermoelectric interfaces. KEYWORDS: Thermoelectric figure of merit, Bi 2 Te 2.7 Se 0.3 nanoplatelet composites, spark plasma sintering, interfaces, grain boundaries, energy filtering R ecently, the bulk nanocomposite approach has been shown to be advantageous over their bulk counterparts to achieve an enhancement in the high thermoelectric (TE) figure of merit, primarily due to poor thermal conductivity or quantum confinement effects although the electrical con- ductivity of those highly disordered nanomaterials is usually compromised. 1,2 In this context, the designing and engineering of interfaces and preferential phonon scattering centers in nanocomposites have become an important field of research to improve the performance of future TE materials. 3,4 The efficiency of the TE materials is scaled as a dimensionless TE figure of merit, ZT = S 2 σT/(κ e + κ l ), where S is the Seebeck coefficient, T is the absolute temperature, σ is the electrical conductivity, and κ e and κ l are, respectively, the electronic and lattice contributions to the thermal conductivity. 5 Thus a good TE material should have a high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity. Combining all three physical parameters, doped semiconduc- tors are found to be the best TE materials. 5−7 Though nanostructure composites are promising candidates for Received: May 28, 2012 Revised: July 14, 2012 Published: July 23, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 4305 dx.doi.org/10.1021/nl302017w | Nano Lett. 2012, 12, 4305−4310