Modified CNTS for thermoelectric application

• Main Author: Zhao, Weiyun
• Other Authors: Boey Yin Chiang, Freddy
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials
• Online Access: http://hdl.handle.net/10356/55439
• Institution: Nanyang Technological University

With over 60% of the energy generated from primary energy sources wasted as dissipated heat, conversion of this huge amount of waste heat back to useful energy has been identified to play a significant role in improving sustainability. Thermoelectric materials, which are able to achieve this goal, have attracted many concerns. Although theoretical calculation indicated that CNT may have good thermoelectric performance with very high ZT values (i.e. ZT > 2), typical experimental results show rather small ZT values for CNTs (10-3 to 10-2), which is too low for thermoelectric applications. In this thesis, we studied the thermoelectric properties of random networks of SWCNT and then modified flexible CNT sheets via plasma irradiation and nanoparticles (i.e.: Sb and Ag2Te) decoration for thermoelectric applications. With these modifications, thermoelectric performance (both p-type and n-type) of CNT sheets could be significantly enhanced. For pristine random network CNT, the ZT value is low due to the low Seebeck coefficient. After plasma irradiation, p-type plasma treated CNT had obtained a maximum ZT value of 0.4 which is around 40 times larger than the pristine one. This great improvement on thermoelectric behavior is mainly enhanced by much higher Seebeck coefficient and lower thermal conductivity. It is also found that the plasma irradiation have the effects of tuning carrier concentration and changing electrical conduction behavior. In an attempt to further enhance the power factor of the system and shift the Seebeck coefficient peak position, plasma treated CNTs can be decorated by Sb nanoparticles for further carrier concentration tuning and electronic structure modification. With Ag2Te decoration as a large amount of electron doping, n-type CNT is obtained. The decorated sample is found to have large Seebeck coefficient, high electrical conductivity and also low thermal conductivity. The large enhancement of Seebeck coefficient is due to increased impurity levels. The electrical conductivity is improved due to higher mobility. The maximum ZT value is around 0.44. Such improvements in both p-type and n-type CNT based materials’ thermoelectric performance expand the application range of CNT and make it possible to fabricate flexible CNT based thermoelectric devices.