ENHANCED THERMOELECTRIC PERFORMANCE IN NANOWIRES
Renkun Chen
UC San Diego
Department of Mechanical and Aerospace Engineering
Wednesday, April 6, 2011
11:00 am – 12:00 pm
Fung Auditorium
Abstract:
Thermoelectric (TE) materials have been used to build solid-state power generators and refrigerators, and can potentially be used to recover waste heat and increase energy efficiency. However, more widespread use of thermoelectrics has been limited by low efficiency and high cost of the TE materials. The efficiency of a TE material is related to the TE figure of merit, defined as ZT=S2σ/k, where ‘S’, ‘σ’ and ‘k’ are the thermopower, electrical conductivity and thermal conductivity, respectively. The state-of-the-art bulk thermoelectric material is Bismuth Telluride (Bi2Te3) and its alloys with a ZT of ~ 1 at room temperature. Over the past five decades it has been challenging to increase ZT>1, since the parameters of ZT are generally interdependent. However, in the past few years, some novel nanostructured materials have emerged to hold the promise for large ZT (>2), which is primarily based on using nanostructures to reduce k contributed from lattice vibrations (or phonons). However, the materials and processes involved in these systems are too expensive for widespread applications. In this presentation, I will discuss our work on reducing k and improving ZT in Si based semiconductor nanowires, including pure Si and heterostructured Si/Ge nanowires. By exploiting various unique features possessed by the nanowires, including small diameter, large surface/volume ratio and nanoscale interfaces, we have shown that the thermal conductivities in these nanowires are approaching the lower limit of k in the corresponding bulk materials. In the case of Si nanowires, ZT has been improved by more than 50- fold, and is comparable to that of Bi2Te3.
Biosketch:
Renkun Chen received his B.S. degree in Thermal Engineering from Tsinghua University in 2004, and Ph.D. in Mechanical Engineering from UC Berkeley in 2008. Following a one year postdoctoral fellowship at Lawrence Berkeley National Laboratory, he began his faculty appointment as an assistant professor in the department of Mechanical and Aerospace Engineering at UC San Diego in November 2009. His research group at UCSD currently works on micro and nano- scale heat transfer and its applications for thermoelectric energy conversion and thermal management of electronic devices. His is a recipient of a R&D 100 award in 2010.