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Materials Today: Proceedings 4 (2017) 12333–12342 www.materialstoday.com/proceedings

EMRSH_2017 Recent progress of thermoelectric devices or modules in Japan

Yoshikazu Shinohara*

*National Institute for Materials Science, Tsukuba 305-0047, Japan

Abstract

The Japanese government has promoted the thermoelectric research and development as one possible technology for raising the energy usage efficiency since 1978. The 1st application of thermoelectric power generation in Japan was a candle radio using β-FeSi2 modules, which was commercialized in 1990. Development of thermoelectric devices or modules was reactivated in 2002, when the 5-year NEDO project titled by “Development of thermoelectric energy conversion system with high efficiency” started. The Bi-Te devices with high energy conversion efficiency of 7.2% under a temperature gradient of 303-553K was reported. Ministry of Economy, Trade and Industry (MITI) of Japan has launched the 10-year project of "Development on the innovative utilization technologies of unused heat energy" since 2013. The administrator of this project has been changed from MITI to NEDO since 2015. Prototype devices of Mg-Si, skutterudite and organic polymers have been developed for power generation. In this paper, the recent progress of thermoelectric devices or modules in Japan is reviewed.

© 2017 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or Peer-review under responsibility of EMRS Spring Meeting, symposium H.

Keywords: thermoelectric device; thermoelectric module;

* Corresponding author. Tel.: 81-29859-2649; fax: +81-29859-2601 E-mail address: [email protected]

2214-7853 © 2017 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or Peer-review under responsibility of EMRS Spring Meeting, symposium H. 12334 Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342

1. Introduction

Japan started thermoelectric research and development in 1960’s [1]. The government has promoted these activities as one possible technology to raise the energy usage efficiency since 1978 [2]. The 1st national project from 1978 to 1993 was called as Moon Light Project. st The 1 general application of thermoelectric power generation in Japan was a candle radio using β-FeSi2 devices that was commercialized by YOUTES Co. in 1990 [3]. The 2nd one was a thermoelectric wrist watch using Bi-Te modules by CITIZEN WATCH Co., Ltd. in 1999 [4]. Development of thermoelectric devices or modules had been activated since 2002, when the 5-year NEDO project titled by “Development of thermoelectric energy conversion system with high efficiency” started [5]. The so much activities on devices or modules, however, has not been reported in the scientific journals or on the English website, since Japanese private companies paid almost no attention to the foreign markets of power generation. Ministry of Economy, Trade and Industry (MITI) of Japan started the 10-year project of "Development on the innovative utilization technologies of unused heat energy" in 2013 [6]. The main target is automobile applications, and the research association consists of Furukawa Co., Ltd., Hitachi, Ltd. Fijifilm Co., Furukawa Electric Co., Ltd., Nippon Thermostat Co., Ltd. and National Institute of Advanced Industrial Science and Technology (AIST). The administrator of this project has been changed from MITI to New Energy and Industrial Technology Development Organization (NEDO) since 2015. Figure 1 shows the history of typical applications of thermoelectric power generation. The spacecrafts such as the Voyger and the Pionner are the 1st touch of power generation, followed by the power generator for gas pipe line monitoring [7-8]. Only the prototypes were manufactured for a codeless fun heater in 1988 and a liquid crystal projector in 2006, which were not in the market. The 3rd application in Japan is a thermoelectric pot using Bi-Te and oxide modules that was commercialized by TES New Energy Co. The most recent product is a cassette fan heater with Bi-Te modules by Iwatani Co.,[9]. There are many kinds of thermoelectric devices or modules developed by Japanese companies; β-FeSi2, Bi-Te, skutterudite, Heusler, half-Heusler, oxide, organic polymer, and so on. In this article, the recent progress of thermoelectric devices or modules in Japan is reviewed.

Fig.1 Typical thermoelectric power applications. Red circled ones are Japanese applications. Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342 12335

2. Showcase of Japanese device or modules

In this chapter, typical Japanese thermoelectric devices or modules are shown in chronological order.

2.1. In 1990

A candle radio was manufactured and commercialized by YOUTES Co. in 1990 [3]. A U-shape β-FeSi2 module was applied to the product. The top of the module is a joint of p-type Mn-doped β-FeSi2 and n-type Co-doped one. This type of module was developed by National Institute for Materials Science. The advantages of β-FeSi2 are 1) the common constituent elements that are abundant on earth, 2) high oxidation resistance, and 3) low price. The modules were fabricated by sintering techniques. The generated power by 5 pairs is 30mW directly heated by a candle flame, which is enough to work a radio.

2.2. In 1999

A thermoelectric wrist watch was developed and commercialized by CITIZEN WATCH Co., Ltd. in 1999 [4]. A small size of Bi-Te module shown in Fig.2 was applied as a body-temperature power generator. The number of p-n pairs is 1242 in the 7.5x7.5mm size module. The leg size of pairs is 0.09x0.11mm. The module was fabricated by the sintering and semiconductor processing techniques. The generated power s 14μW at ΔT=1K. This power is enough to μ work a watch movement, since the required power is 1 W. Fig.2 Small Bi-Te module installed in the wrist watch by CITIZEN WATCH 2.3. In 2004 Co., Ltd.

A high power density module named by Giga Topaz was developed and commercialized by Toshiba Co. in 2004 [10]. The material is half-heusler (Ti, Zr, Hf) Ni (Sn,Sb). The power density was 1W/cm2 at ΔT=480K (773K-293K) in 2004. The material was modified to reach 4.1W/cm2 at ΔT=644K (984K-339K) in 2012. The maximum ZT was over 1.5 at 673K. This module is a top runner of power density in Japan.

2.4. In 2006

An excellent Bi-Te module was developed as a power source of a liquid crystal projector, and commercialized by Yamaha Co. in 2006 [11]. The module dimensions are from 8mm to 14mm, and the ZT is ~1.2 at 300K. Rapid cooling technology was applied to preparation of the Bi-Te powders for sintering. The power density is 0.5W/cm2 at ΔT=100K (398K-298K). This module presents a high power density at ΔT=100K (398K-298K). The NEDO’s 5- year project from 2002 to 2007 contributed to development of this module. This company has been developing the sensing system of body information using this module as an energy harvester. The temperature sensing system for a human body and the temperature and humidity sensing system for factories were developed in 2013.

2.5. In 2009

A reliable common Bi-Te module for power generation was developed and commercialized by KELK Ltd. in 2009 [12]. This company is a leading company of the sintered Bi-Te modules in Japan. The dimensions are 50x50x4.2mm, and the weight is 47g. The maximum output power is 24W at ΔT=250K (553K-303K). The power density is 1W/cm2 and the efficiency is 7.2% at the maximum output power. This module is regarded as a benchmark in Japan regarding thermoelectric performance and reliability. This company has tested the power generation system of 10kW class at the iron continuous casting process in corporation with JFE Steel Co. A new NEDO project on demonstration of that system by KELK Ltd. and JFE Steel Co. started in 2017[13]. 12336 Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342

2.6. In 2011

A practical multilayer module shown in Fig. 3 was developed by Murata manufacturing Co. Ltd. in 2011 [14]. The p-type material is Ni doped with Mo, and the n-type one is SrTiO3 substituted by La. The grey layer in Fig.3 is p-type, and the black one is n-type. The manufacturing process is as follows; 1) Thin thermoelectric green sheets were fabricated by doctor blade method for p-type (0.03mm) and n-type (0.12mm). 2) The insulating layer 6μm thick of partially stabilized ZrO2 was formed on the green sheets by screen printing method. 3) The green sheets with the insulating layers were stacked layer by layer, and then sintered at 1573K in the air. This module consists of 50 pairs of the p-n junctions. The power Fig.3 Multilayer module by Murata μ 2 Δ density is 100 W/cm at T=10K (303K-293K). This company has manufacturing Co. Ltd. been developing the sensing system for factories and plants by applying this module. A prototype of the integrated wireless sensor node was developed in 2015.

A new concept of Bi-Te tubular module shown in Fig.4 was developed by Panasonic Co. in 2011 [15]. This is a one-leg module composed of Bi-Sb-Te and Ni. Ni is an electrode material. This module is made of a laminate structure of Bi0.5Sb1.5Te3/Ni. Power generation is achieved from the temperature difference between the outer and inner surfaces of the module by the transverse thermoelectric effect. Hot water flows inside the tube and the cold Fig.4 Tubular Bi-Te module by Panasonic water flows outside the tube to get a temperature difference in the Co. radial direction. The power is 8.2W/pipe at ΔT=91K (369K-278K). A feasibility study of waste heat recovery from hot water in the garbage incineration plant (Kyoto northeastern clean center) has been performed by applying this module. This company also developed another type of Bi-Te tubular module in 2015. The wall of this module consists of many π-shape devices of p-type and n- type of Bi-Te legs. The output power was modified by 2~3 times in comparison with the tubular one-leg module. An organic flexible module shown in Fig 5 a) was reported by AIST in 2011 [16]. The similar module in Fig.5 b)was developed by Fujifilm Co. in 2013 [17]. These are one-leg modules with p-type CNTs-organic mixtures. The ZT was 0.15 for the module by AIST, and over 0.3 for the module by Fujifilm Co. The AIST module was fabricated by printing the ink including the organic mixtures on thin plastic films. It can be bent into a circle with a curvature of 5mm. The Fujifilm module was fabricated by printing the mixtures of CNTs, conductive polymer and photo-acid-generating agent on PET films. The key point of these modules is dispersion of CNTs. High dispersion of CNTs was realized in the Fujifilm module by developing a new type conductive polymer. Both AIST and Fujifilm Co. have played a role of development of organic thermoelectric Fig.5 Flexible organic module by AIST materials in the MITI’s 10-year project of “Development on the (a) and Fujifilm Co.(b) innovative utilization technologies of unused heat energy" since 2013. Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342 12337

2.7. In 2012

A spin Seebeck device was developed by NEC Co. in cooperation with Tohoku University in 2012 [18]. In this device, the spin flow goes with the heat flow in the ferromagnetic material in the thickness direction. By the inverse spin Seebeck effect, the charge flow is generated in the paramagnetic metal in the orthogonal direction of the spin flow. The first developed device was composed of GGG (Gd3Ga5O12) as a substrate, Bi-doped YIG (BiY2Fe5O12) as a ferromagnetic material and Pt as a paramagnetic metal. The generated voltage is proportional to the thickness of the ferromagnetic material, and the power is proportional to the area of the paramagnetic metal. The company has been applying the common ferromagnetic material of iron oxides to the spin Seebeck device. The energy conversion efficiency is still less than 1/10 of thermoelectric energy conversion.

2.8. In 2013

A full Heusler module was developed by ATSUMITEC Co. Ltd. in cooperation with Nagoya Institute of Technology in 2013 [19]. The materials are Fe-V-Al for both p- and n-types. The power density of the module is 0.25W/cm2 at ΔT=280K (573K-293K). This company has applied this module to the sub-power generator of a motorbike from the exhausted gas heat. The main part of this power generator is SOFC using unburned fuel, and the thermo-module is set on the fuel cell. Total output power including the thermoelectric power is 400W. The power density of this module was improved by 1.5 times next year.

2.9. In 2015

An one leg module of n-Mg-Si and π-shape module of n-Mg-Si and p-Mn-Si shown in Fig.6 were developed by Yasunaga Co. and Nippon Thermostat Co., Ltd. in 2015[20]. The power density of one-leg module is 0.75 W/cm2 and 0.99 W/cm2 at ΔT=530K. It is interesting that one-leg Fig.6 a) one leg module of n-Mg-Si π module indicates relatively high performance in comparison with π- and b) -shape module of n-Mg-Si and shape module. This is an example of one-leg module. p-Mn-Si by Yasunaga Co. and Nippon Thermostat Co., Ltd.

A Pb-Te module with nano-precipitates of Mg- Te shown in Fig.7 was developed by AIST in 2015[21]. The ZT value of 1.8 at 823K was achieved by the nano-precipitation effect of passing carrier and blocking phonon. A segmented module of Bi-Te and Pb-Te was also reported. The power density of Pb-Te module and segmented module was 2.1 W/cm2 at ΔT=570K (873K-303K) and 1.4 W/cm2 at ΔT=590K (873K- 283K). The energy conversion efficiency was 8.8 and 11% for each module. The reason why the segmented module indicated lower power density Fig.7 a) TEM image of precipitation of Mg-Te in Pb-Te. and higher energy conversion efficiency is that b) Pb-Te module, and c) segmented module of Bi-Te and Pb-Te the length of leg was not optimized for segmentation of Bi-Te and Pb-Te.

12338 Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342

2.10. In 2016

A flexible Bi-Te module shown in Fig.8 was developed by E- thermogentech Co. in 2016[22]. The 250 pairs of p-type and n-type Bi-Te are mounted on the flexible substrate. The curvature is 20~25/m. The applicable temperature is till 423K. It is reported that the targeted power density 3 years later is 0.15 W/cm2 at ΔT=70K.

2.11. In 2017

KELK Ltd. developed three type of Bi-Te thermoelectric units named by ‘thermoelectric energy harvesting device’, ‘thermoelectric power source unit’ Fig.8 Flexible Bi-Te module by and ‘thermoelectric exhausted heat recovery unit’ in 2017[23]. E-thermogentech Co. ‘Thermoelectric energy harvesting device’ shown in Fig.9 a) is a wireless unit to send the measured temperatures using a Bi-Te thermoelectric power generator. The unite sizes are W60xD42 xH25~W100xD100xH30. The output power of ‘Thermoelectric power source unit’ is 2.5~4W for supplying to USB output at a hot temperature of 493K. The unit sizes are W95xD115xH90 and W150xD148xH81. The output power of ‘Thermoelectric exhausted heat recovery unit’ shown in Fig.9 b) is 240W under a temperature condition between cooling water and 523K. The Fig.9 a) ‘thermoelectric energy harvesting device’, dimensions is W290xD290xH85 and weight is 12kg. The and b) ‘thermoelectric exhausted heat recovery unit’ by KELK Ltd. application of this unit is exhaust heat recovery at factories and plants.

2.12. Other modules

There are other thermoelectric modules developed in Japan. Some of them are listed as follows;

1) SHOWA DENKO K.K.： Sukutterudite module (2.4W/ cm2 at ΔT=550K) [24] 2) TES NewEnergy Co.： Oxide+Bi-Te module (1.2W/ cm2 at ΔT=550K) 3) FURUKAWA Co. Ltd.：Sukutterudite module (1～1.2 W/ cm2 at ΔT=550K) [25] 4) AIST et al：Clathrate module (0.22 W/ cm2 at ΔT=300K) [26] 5) KYOCERA Co：Silicide module (0.33 W/ cm2), Bi-Te module (ZT~1) [27] 6) Toshiba Co.: Bi-Te module (Efficiency=3.6% at ΔT=100K) [28] 7) SWCC SHOWA HOLDINGS Co., Ltd: Oxide module (0.067 W/ cm2 at ΔT=528K)[29] 8) Lintec Co.: BiTe/PEDOT-PSS module (48m W/ cm2 at ΔT=65K)[30]

There are two types of general applications of thermoelectric power generation; independent power and retail power. The independent power is classified into two groups; energy harvesting group and exhausted heat recycling group [31]. The former group includes a wireless sensing device, wearable device, household devices and so on, and the latter group includes automobile, ship, bike and agricultural applications. The retail power needs big power generation of kW class or more. The applications of thermoelectric devices or modules have shifted slightly from exhausted heat recycling to energy harvesting in Japan. Research and development of thermoelectric devices or modules has been activated for μW class ~ several W class power generation since 2010. Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342 12339

3. Trend from publication

The number of published scientific papers was evaluated to clarify the state-of-the-art of devices and modules in the academic activities. Southeast Asian countries of Japan, China, Korea and India, and Western countries of USA, Germany, England & Ireland and France were selected. The date source was WEB OF SCIENCE. The web search keywords were “thermoelectric” and “thermoelectric & (device or module)”.

3.1. Asian countries

Figures 10 shows change in the number of published papers for Southeast Asian countries in this ten years from 2007 to 2016. There is a similar tendency of change in the number of published papers between keywords of “thermoelectric” and “thermoelectric & (device or module)”. Japan presents a wavy increase in both the keywords, which is corresponding to two big national projects of from 2002 to 2007 and from 2013 to2023. On the other hand, other three Southeast Asian countries show a monotonous increase year by year. The governments of China and India have supported thermoelectric research and development for almost ten years. More than 90% of published papers dealt not with Peltier cooling but with power generation. Peltier modules have already a certain world market of several hundred million USD in the fields of optical communication, refrigerator, military use, and so on, while power generation modules have only a little world market in the pole, military and space use. That is why the tendencies of Figure 10 are probable to be closely related to the financial support by the government of each country.

Fig.10 The number of published papers with key words of ‘thermoelectric’ and ‘thermoelectric & device or module) from 2007 to 2016 for Japan, China, Korea and India 12340 Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342

3.2. Western countries

Figures 11 shows change in the number of published papers for Western countries in this ten years from 2007 to 2016. There is a similar tendency of change in the number of published papers between keywords of “thermoelectric” and “thermoelectric & (device or module)” similarly to Fig.10. USA shows a period of almost stagnation for both key words in this five years from 2012 to 2016, and EU countries have a similar period in this three or four year. This tendency is also deeply related to the financial support by the government as well as Southeast Asian countries. Some big projects finished 3~5 years ago in USA and EU.

3.3. Activeness of development of device or module

The ratio of the number of published papers with both keywords “thermoelectric & (device or module)” to that with a keyword “thermoelectric” was evaluated in from 1990 to 2016 for Japan, USA, China and Germany. The result is shown in Fig.12. This ratio indicates the share of device or module research papers in the total thermoelectric ones. Japan, China and Germany increased the ratio monotonously from 0 to 0.25 in from 1990 to 2016, whereas USA revealed the ratio increased rapidly from zero to 0.5 in from 1990 to 2006 and decreased slightly from 0.5 to 0.3 in from 207 to 2016. NASA has operated space missions since 1960’, and the big space exploratory missions for outer planets finished in 2005. This is probably related to the ratio change in 2006. The NASA related number of published papers with “thermoelectric & (device or module)” in 2007 was decreased to 2/3 of that in 2006.

Fig.11 The number of published papers with key words of ‘thermoelectric’ and ‘thermoelectric & device or module) from 2007 to 2016 for USA, Germany, England & Ireland and France. Yoshikazu Shinohara / Materials Today: Proceedings 4 (2017) 12333–12342 12341

Fig.12 The ratio of the number of published papers with both keywords “thermoelectric & (device or module)” to that with a keyword “thermoelectric” from 1990 to 2016 for Japan, USA, China, Russia and Germany.

Summary The recent progress of thermoelectric devices or modules in Japan was introduced. Typical Japanese thermoelectric devices or modules are shown in chronological order from1990 to 2017. The applications of thermoelectric power generation has shifted slightly from exhausted heat recycling to energy harvesting in Japan. Research and development of thermoelectric devices or modules has been activated for μW class ~ several W class power generation since 2010. Research and development of thermoelectric devices or modules are affected by financial support of the government. Ministry of Economy, Trade and Industry (MITI) of Japan has started the 10-year project of "Development on the innovative utilization technologies of unused heat energy" since 2013. The research and development have been activated since 1990 in Japan as well as China, Korea and India. On the other hand, while there has been a period of stagnation in USA, Germany, England & Ireland and France since 2010. This tendency in Western countries are corresponding to the finish of some big projects 3~5years ago [32].

References

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