Nanotechnologies / Materials
Nanotechnologies, Materials, Energy

Development of ecofriendly nanostructured materials withhigh thermoelectric performance
Division of Advanced Electronics and Optical Science, Graduate school of Engineering ScienceProfessor Yoshiaki Nakamura
Researchmap https://researchmap.jp/research_nakamura

Nanotechnologies / Materials

Abstract
As one of the clean power sources, thermoelectric conversion
is highly attractive. To enhance the thermoelectric performance,
lowering thermal conductivity and enhancing thermoelectric
power factor are both required. Unlike heavy element-based materials with inherently-high thermoelectric performance, light element-based materials, which are ecofriendly, originally exhibit low
thermoelectric performance. We succeeded in simultaneous realization of low thermal conductivity and high thermoelectric power
factor by using nanostructuring technique. For example, connected
Si nanodot system exhibited ultralow thermal conductivities beyond
amorphous limit. Using nanodots with low thermal conductivity, in
the Au-SiGe system including Ge-rich nanodots, higher power factor was achieved.

▍ Significance of the research and Future perspective
Our nanostructuring technique overcome the long-standing issue of thermoelectric conversion: simultaneous realization of low
thermal conductivity and high thermoelectric power factor. This
success shows us that even eco-friendly light element-based materials can have a potential as thermoelectric materials. Nowadays,
the sensor network society is coming, which changes our lifestylelargely. If our nanostructured thermoelectric materials meet the requirement of social use as thermoelectric generator, they will play
an important role in supplying electrical energy to the sensor.

▍ Background & ResultsThermoelectric conversion from wasted heat is drawing considerable attention as one of the clean power sources. For thermoelectric conversion, materials must have higher power factor andlower thermal conductivity simultaneously. Therefore, conventional
thermoelectric materials are composed of heavy elements because of their inherently low thermal conductivity although they are
generally toxic and expensive. On the other hand, ecofriendly lightelement-based materials, suitable to the social use, exhibit low
thermoelectric performance in general. Using our nanostructuring
technique, we have established a pronounced methodology toovercome the long-standing issue of thermoelectric conversion:
simultaneous realization of low thermal conductivity and high thermoelectric power factor in ecofriendly materials. We first developed
epitaxial connected Si nanodots (CSNs) with ultrasmall diameters,
where nanodots worked as phonon scattering centers. The CSNs
exhibited ultralow thermal conductivities beyond amorphous limit
because of ultimate confinement of phonon. Recently, we expanded the nanostructure technique to development of transparent thermoelectric materials for reuse of heat on windows using nanowires.
Using this information about ultralow thermal conductivity nanostructures, we recently formed SiGe with a small amount of Au,
where Ge-rich nanodots with low thermal conductivity exist around
Au. Therein, due to the large contrast of thermal conductivity, large
temperature difference is dominantly applied to Ge-rich nanodots(or SiGe) that has high Seebeck coefficient, not to Au with small
Seebeck coefficient. Furthermore, resonant scattering occurs byAu impurity level in SiGe, which enhances Seebeck coefficient. Bythis strategy: thermal distribution control and resonant level scattering, we achieved twice enhancement of power factor.
Thus, we succeeded in enhancing thermoelectric performanceof light element-based materials, opening an avenue of ecofriendly
thermoelectric materials suitable to social application.

High performance thermoelectric materials including well-controlled nanodot
crystals

Transparent thermoelectric material including nanowires.

Future vision

P a t e n t Japanese Patent Application No. 2019-220283, No. 2012-124940 PCT／JP2013／063580
Nakamura, Yoshiaki; Watanabe, Kentaro; Fukuzawa, Yo, et al. Observation of the quantum-confinement effect in individual Ge nanocrystals on oxidized Si substrates using scanning tunneling spectroscopy. Appl. Phys. Lett. 2005; 87,133119-1-3.
Nakamura, Yoshiaki; Isogawa Masayuki; Ueda Tomohiro, et al. Anomalous reduction of thermal conductivity in coherent nanocrystal architecture for silicon thermoelectric material. Nano Energy 2015; 12, 845-851.
Nakamura, Yoshiaki, Nanostructure design for drastic reduction of thermal conductivity while preserving high electrical conductivity. Science and Technology of Advanced Materials 2018; 19, 31-43.
T r e a t i s e Ishibe, Takafumi; Tomeda, Atsuki; Watanabe, Kentaro, et al. Methodology of thermoelectric power factor enhancement by controlling nanowire interface. ACS Applied Materials & Interfaces 2018; 10, 37709-37716.
Taniguchi, Tatsuhiko; Ishibe, Takafumi; Naruse, Nobuyasu, et al. High Thermoelectric Power Factor Realization in Si-rich SiGe/Si Superlattices by Super-Controlled Interface. ACS Applied Materials & Interfaces 2020; 12, 25428-25434.
Taniguchi, Tatsuhiko; Terada, Tsukasa; Komatsubara, Yuki, et al. Phonon transport in nano-system of Si and SiGe films with Ge nanodots and approach to ultralow thermal conductivity. Nanoscale 2021; 13, 4971-4977.
Sakane, Shunya; Ishibe, Takafumi; Mizuta, Kosei et al. Anomalous enhancement of thermoelectric power factor by thermal management with resonant level effect. J. Mater. Chem. A 2021; 9, 4851-4857.
https://resou.osaka-u.ac.jp/ja/research/2018/20181031_2
https://www.jst.go.jp/pr/announce/20141210/
https://engineer.fabcross.jp/archeive/181101_nanowire.html
UR
L http://www.adv.ee.es.osaka-u.ac.jp/
http://www.optronics-media.com/news/20181031/53791/
https://resou.osaka-u.ac.jp/ja/research/2021/20210129_1
https://resou.osaka-u.ac.jp/ja/research/2014/20141210_1
K e y w o r d thermoelectric material, nanocrystal, ubiquitous element, crystal orientation, ecofriendly material