Conference Programme

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Overview
Session
DD-03: Wearables and Flexible Thermoelectrics
Time:
Tuesday, 20/Jun/2017:
10:30am - 12:00pm

Session Chair: Xun Shi, Shanghai Institute of Ceramics, Chinese Academy of Sciences
Session Chair: Qiang Zhu, Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR)
Location: Rm 335

Wearables and Flexible Thermoelectrics


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Presentations
10:30am - 11:00am
Invited

Variations in Device Architecture for Low $/W and Wearable System

Woochul KIM

Yonsei University, South Korea

We propose a way to lower the $/W value, while maintaining a decent power output of a thermoelectric device by changing the device architecture. We demonstrated that the $/W value can be reduced to around 10% while maintaining ~65–70% of the maximum possible power output with a given zT. A proof of concept experiment is shown as well. The device architecture we proposed should be useful to recover low quality waste heat, which is abundant and could be harvested as long as the $/W value is low enough in general. In the second part of the talk, we present two types of wearable thermoelectric devices based on bulk inorganic materials; one is bracelet type and the other is mat style. Utilizing high performance that can be extracted from the bulk inorganic materials, the devices based on the materials are bendable and flexible. Performance of the device attached on a human body is compared with theoretical analysis based on a human thermoregulatory model.


11:00am - 11:15am
Oral

High Power Density Thermoelectric Generators

Dongyan XU

The Chinese University of Hong Kong, Hong Kong S.A.R. (China)

Thermoelectric generators (TEGs) are promising for harvesting waste heat from the environment to power wireless sensor networks in smart buildings. In this work, we developed both non-flexible and flexible TEGs by integrating pulsed electroplating with microfabrication processes. The device consists of a total of 127 pairs of n-type Bi2Te3 and p-type Sb2Te3 thermoelectric pillars embedded in a SU-8 matrix. Both thermoelectric pillars and interconnectors are formed by electroplating, which is advantageous because of facile and low cost fabrication and low parasitic electrical resistances. The non-flexible TEG demonstrates a maximum power of 3 mW at a temperature difference of 52.5 K, corresponding to a power density as high as 9.2 mW cm-2. The power density of our TEG is more than two times the highest value reported for the electroplated TEGs in the literature, which can be attributed to the low internal resistance and high packing density of thermoelectric pillars. We also developed flexible TEGs by modifying the fabrication process. The flexible TEG we developed achieves a power density of 4.5 mW cm-2 at a temperature difference of 50 K. Compared to the non-flexible TEG, the lower power density of the flexible device is mainly due to the larger thermal resistance of the flexible substrate and thus smaller effective temperature drop across the thermoelectric pillars.


11:15am - 11:30am
Oral

Integrated Materials to Structural Design of Thermoelectric Fabrics using Carbon Nanotubes for Wearable Energy Harvester

Masakazu NAKAMURA1, Mitsuhiro ITO1, Takuya KOIZUMI1, Hirotaka KOJIMA1, Takeshi SATO2, Ichiro YAMASHITA3

1Nara Institute of Science and Technology, Japan; 2National Institute of Advanced Industrial Science and Technology, Japan; 3Osaka University, Japan

Thermoelectric generators (TEGs) are promising energy harvesters for independent, small circuits in sensor networks and wearable electronics. Toward such an energy-harvesting use, there has been an effort in recent years to fabricate flexible, wearable TEGs from organic and organic/inorganic hybrid materials. Their thermoelectric figure of merit (ZT) is increasing year by year. However, in operating conditions where ambient air is the only medium for heat dissipation, the efficiency is restricted not only by the ZT value but also by the thickness and thermal conductivity of the device, which influence the temperature difference used for power generation. An integrated design of materials, structure and fabrication process for these devices are therefore important to satisfy difficult requirements; thickness must be more than a few millimeters, but the device must be flexible.

In this presentation, we propose two novel approaches to use carbon nanotubes (CNT) for wearable energy harvesters.

The first one is a promising device design for thickness-controllable, flexible, and thermally insulating TEGs, namely “thermoelectric fabrics.” CNTs are spun into threads with binding polymers and p/n-doped to form striped patterns. By sewing the CNT thread into a felt fabric, operation of a prototype thermoelectric fabric by a finger touch is demonstrated.

The second one is a novel material design to dramatically suppress the thermal conductivity of CNT composites. Specially designed cage-shaped protein molecules are inserted into CNT junctions, which allow electrons to tunnel through but forbid phonons to propagate. As a result, thermal conductivity is suppressed to approximately 1/1000 of that of neat CNT solids while their high electrical conductivity is maintained.


11:30am - 11:45am
Oral

Thermoelectric Power Factors of Conducting Polymer Nanowire/Inorganic Nanowire Hybrid Films

Kun ZHANG

Donghua University, China

Nanostructured polymers have attracted more and more attentions for promising thermoelectrics owing to their highly ordered molecular chains and potentially high carrier mobility. In this presentation, we carefully explore the electrical transport in poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires and systematically study thermoelectric (TE) properties of freestanding PEDOT nanowire films by optimizing the doping level and doping agents. In addition, p-type and n-type inorganic nanowires were synthesized and incorporated to fabricate novel p-type and n-type PEDOT-nanowire-based nanocomposites, respectively. Their power factors were thoroughly investigated and potential TE enhancement mechanisms are scrutinized.


11:45am - 12:00pm
Oral

Growth of Flexible Ca3Co4O9 Thin Films for Wearable Thermoelectric Applications

Biplab PAUL, Jun LUB, Per EKLUNDC

Linköping University, Sweden

Inorganic materials, because of their inherent rigidity and brittleness, have always been underestimated for flexible thermoelectric applications. However, for high output power density and reliable and consistent performance over many cycles the use of inorganic materials is inevitable. So, challenge is find the novel technique to induce flexibility in inherently rigid thermoelectric materials. In this study, we demonstrate a novel nanostructural tailoring approach to induce flexibility in inherently rigid Ca3Co4O9 thin films. The present approach being simple and low cost is potentially suitable for industrial upscaling. Flexible Ca3Co4O9 thin films have been grown by thermally induced phase transformation from CaO-CoO thin films to final phase of Ca3Co4O9 on mica substrate. CaO-CoO thin films is deposited by rf-magnetron reactive co-sputtering from elemental targets of Ca and Co. The film growth is found not to be influenced by the substrate as due to the weak interactive force between the film and the mica substrate, i.e. mica substrate act as semiwettable substrate. It is influenced by the surface energies and relative arrangements of the initial CaO and CoO phases. The nanostructural evolution of Ca3Co4O9 can be controlled by controlling different deposition parameters, e.g. substrate bias voltage, substrate temperature, partial pressure of reactive gas during deposition, hence allowing for nanostructural tailoring of the materials properties. These flexible Ca3Co4O9 films are bendable to a radius of 14 mm without any deterioration of thermoelectric performance or mechanical properties, even for several hundred times of repeated bending. Power factors above 0.1 mWm-1K-2 is achieved in a wide temperature range, from room temperature to 300 °C, and thus opening avenues for low-temperature use of this material. The additional advantage is that the films are transferable onto other flexible platform by mechanical dry transfer method without compromising their functionalities, and thus it opens new opportunity in the area of transferable thermoelectrics.



 
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