Conference Programme

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Overview
Session
DD-01: Novel approaches to semiconductor thermoelectrics
Time:
Monday, 19/Jun/2017:
1:30pm - 3:30pm

Session Chair: Ajay Soni, Indian Institute of Technology Mandi
Location: Rm 335

Novel approches to semiconductor thermoelectrics


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Presentations
1:30pm - 2:00pm
Invited

New Routes for Bottom-up Nanostructuring and Utilization of Magnetic Semiconductors to Enhance Thermoelectric Materials

Takao MORI1,2

1National Institute for Materials Science, Japan; 2University of Tsukuba, Japan

Efforts worldwide to find viable thermoelectric (TE) materials are intensifying [1]. We have achieved selective scattering of phonons and thereby critical enhancement of TE properties through nanostructuring: both a) mechanical and b) bottom-up methods. For example, previously, quick, inexpensive, bottom-up wet processes were found to fabricate nanosheets of telluride thermoelectric materials leading to enhanced ZT [2]. We have also recently discovered a bottom-up nanostructuring method leading to a 100% enhancement (i.e. ZT~1.6) in “empty” rare earth-free skutterudites [3]. This is achieved by co-doping the Sb cages of CoSb3 and utilizing phase diagrams to create surprising controlled and effective porosity in the materials. Fabrication of nanocomposites with partial metallic networks in borides has been shown a route to overcome the traditional trade-off between electrical conductivity and Seebeck coefficient leading to large enhancement in power factor [4]. Characterization of thermal properties on the nanoscale will also be presented; focused picosecond laser thermoreflectance and in-situ TEM thermal probe development. We have discovered that magnetic semiconductors can have enhanced thermoelectric properties, indicated to be derived from magnetic and carrier-magnon interaction [5,6] This work is supported by CREST, JST.

References:

[1] Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, (Springer, Heidelberg, 2013).

[2] C. Nethravathi et al., J. Mat. Chem. A, 2, 985 (2014).

[3] A. U. Khan et al., Nano Energy, 31, 152 (2017).

[4] T. Mori and T. Hara, Scripta Mater., 111, 44 (2016).

[5] N. Tsujii and T. Mori, Appl. Phys. Express, 6, 043001 (2013).

[6] R. Ang et al., Angew. Chem. Int. Ed., 54, 12909 (2015).


2:00pm - 2:15pm
Oral

Doping and Nanostructure Control to Enhance Thermoelectric Properties In Bulk CoSb3 Skutterudites

Raghavan GOPALAN, Manjusha BATTABYAL

International Advanced Research Centre for Powder Metallurgy and New Materials, ARCI, India

Thermoelectric (TE) device converts heat directly into electricity when a temperature gradient is created across the device. The TE materials required for the device must have figure of merit (ZT) ~ 1.5 and above at the operation temperature. The efficiency of TE materials depends on the figure of merit (ZT), which is defined as ZT=S2sT/k (where S is the Seebeck co-efficient, s is the electrical conductivity and k is the thermal conductivity of the material). Worldwide, significant research is going on to improve the ZT of the thermoelectric materials by nanostructuring, mesostructuring, band engineering and synergistically defining the key strategies to improve the ZT.
Nanostructuring and doping in the binary CoSb3 skutterudites are considered as promising candidates for waste heat recovery between the temperature range 573K to 773K in automotive application due to their stable thermoelectric properties, flexibility of fabrication and cost effectiveness. We have investigated doped and filled n-type skutterudite (NixCo4-xSb12Te0.1 (x=0, 0.3, 0.5)) materials fabricated by powder metallurgy route. Microstructure investigations were carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microstructural studies reveal that nanostructured single-phase skutterdite could be obtained and the phase remains stable till 773 K. Temperature dependent thermoelectric properties measurements show substantial enhancement of the electronic properties and decrease in thermal conductivity in the Ni doped materials. Power factor of ~ 5mW/mK2 has been achieved in Ni0.5Co4Sb12Te0.1 which appears to be the highest value in n-type skutterdites reported so far. Detailed investigations of microstructure and electron-phonon scatterings on the thermophysical properties of Ni doped filled skutterudites will be discussed. Our analysis shows that decrease in lattice thermal conductivity is mainly due to increased phonon anharmonic scattering. We will also present efforts put in fabricating the TE module and testing.


2:15pm - 2:45pm
Invited

Good Thermoelectric Half-Heusler and Zintl Materials

Zhifeng REN

University of Houston, United States

Significant progress has been made on searching for good thermoelectric materials. Recently significant advances have been made in some half-Heusler and Zintl materials. These materials may find their potential applications in the mid-high temperature heat sources.


2:45pm - 3:15pm
Invited

CuX-based Thermoelectric Materials

Xun SHI

Shanghai Institute of Ceramics, Chinese Academy of Sciences, China

Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation and offers the prospect for novel thermal-to-electrical energy conversion technology that could lead to significant energy savings by generating electricity from waste industrial heat. The key to the development of advanced TE technologies is to find highly efficient TE materials. Recently, several novel concepts have been proposed to enhance the efficiency of TE materials and laboratory results suggest that high zT values can be realized in several families of bulk materials. In this presentation, we show the study on the thermoelectric properties of Cu2X-based materials. We will show these materials possess interesting thermoelectric properties with ultralow thermal conductivity and good thermoelectric figure of merit. The physical mechanisms behind these abnormal thermoelectric properties will also be discussed. Finally, the stability of these compounds will also be presented and discussed.


3:15pm - 3:30pm
Oral

Optimization of Thermoelectric Properties for Rough Nano-Ridge GaAs/AlAs Superlattice Structure

Chao-Wei WU, Yuh-Renn WU

National Taiwan University, Taiwan

The optimization of thermoelectric(TE) properties with rough surface at both sidewalls of the nano-ridge GaAs/AlAs superlattice(SL) structures is studied. Different from the traditional SL structures, we proposed the nano-ridge featured with rough surface at both sides of the SL structure, where the modification of the phonon spatial confinement and phonon surface roughness scattering are considered. The elastic continuum model is adapted to calculate the phonon dispersion relation and the related phonon group velocity. Reported experimental results with SL structures were used to verify our model. The electrical conductivity, Seebeck coefficient, electronic thermal conductivity, and the lattice thermal conductivity are calculated by Boltzmann transport equations and relaxation time approximation. We consider two and four layers nano-ridge GaAs/AlAs SL structures to find the optimal configurations of lattice thermal conductivity kph , where the lateral confined nano-ridge width is W=25nm. Our calculations show that kph is 2.17W/mK for two layers nano-ridge GaAs(4nm)/AlAs(1nm) SL structures. Moreover, we find that the lowest lattice thermal conductivity kphis 1.60W/mK for the optimal configuration of four layers GaAs(4nm)/AlAs(1nm)/GaAs(4nm)/AlAs(2nm) SL structure. However, we fell that it is not enough to design a high performance TE device. Therefore, if we make the surface rough between the GaAs/AlAs SL structure and air boundary intentionally, we obtain the much lower lattice thermal conductivity, which is around 0.10W/mK for the surface roughness characteristics with auto-covariance length L=6.0nm and roughness degree RMS Δ=1.5nm. At the nano-ridge GaAs/AlAs SL and air surface, the electrons would be pushed away from the surface to the center of the nano-ridge SL structures due to the Fermi level pinning. Our simulation results show that the optimal configuration of electron doping density at T=300K is ND=3.46×1019 cm−3 , and at T=1000K is approximately ND=2.81×1019 cm−3. The highest ZT is 1.285 at 300K and 3.04 at 1000K.



 
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