51 Mater. Res. Soc. Symp. Proc. Vol. 1490 © 201 Materials Research Society DOI: 10.1557/opl.2012. Enhancement in Thermoelectric Figure-of-merit of n-type Si-Ge Alloy Synthesized Employing High Energy Ball Milling and Spark Plasma Sintering Sivaiah Bathula 1, 2 , M. Jayasimhadri 2 , Ajay Dhar 1 , M. Saravanan 1 , D. K. Misra 1 , Nidhi Singh 1 , A. K. Srivastava 1 , R. C. Budhani 1 1 CSIR-Network of Institutes for Solar Energy, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi -110012, India 2 Department of Applied Physics, Delhi Technological University, Delhi, India ABSTRACT In the present study, we report the enhancement in figure-of-merit (ZT) of nanostructured n-type Silicon-Germanium (Si 80 Ge 20 ) thermoelectric alloy synthesized using high energy ball milling followed by spark plasma sintering (SPS). After 90 h of ball milling of elemental powders of Si, Ge and P (2 at.%), a complete dissolution of Ge in Si matrix has been observed forming the nanostructured n-type Si 80 Ge 20 alloy powder. X-ray diffraction analysis (XRD) confirmed the crystallite size of the host matrix (Si) to be ~7 nm and also indicated the formation of an additional phase of SiP nano-precipitates after SPS. HR-TEM analysis revealed that the nano-grained network was retained post-sintering with a crystallite size of size of 9 nm and also confirmed the SiP precipitates formation with a size of 4 to 6 nm. As a result, a very low thermal conductivity of ~2.3W/mK at 900°C has been observed for Si 80 Ge 20 alloy primarily due to scattering of phonons by nanostructured grains and nano- scaled SiP precipitates which further contribute to this scattering mechanism. Electrical conductivity values of SiGe sintered alloy are slightly lower to that of reported values in literature. This was attributed to the formation of SiP which creates a compositional difference between the grain boundary region and the grain region, leading to a chemical potential difference at interface and the grain region. Figure-of-merit (ZT) of n-type Si 80 Ge 20 nanostructured alloy was found to be ｧ1.5 at 900°C, which is the highest reported so far at this temperature. Keywords: Mechanical alloying, Electron Microscopy, Si-Ge thermoelectric alloy, Spark Plasma Sintering, Seebeck coefficient, thermal conductivity, Nanostructured Interfaces INTRODUCTION The direct conversion of heat energy into electricity based on thermoelectric effect without moving parts is attractive alternative for many applications in power generation [1]. The performance of a thermoelectric device depends on its figure-of-merit (ZT), a dimensionless quantity of the material defined as ZT = (α 2 σT/κ), where α, σ, T and κ are Seebeck coefficient, electrical conductivity, temperature and thermal conductivity respectively. The optimization of ZT clearly demands high thermoelectric power and electrical conductivity, but a small thermal conductivity. Improving the performance of a thermoelectric materials or devices mainly involve in controlling the motion of phonons, which carry most of the heat, and electrons, which carry the electric current and some of the heat [2]. Silicon-Germanium (Si-Ge) alloys have been extensively used in deep space missions conducted by NASA. Synthesis of polycrystalline silicon germanium alloys was 1 Corresponding author: adhar@nplindia.org , rcb@nplindia.org Tel.: +91-11- 4560 9455, Fax.: +91-11-45609310 1731 3