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Chapter 6. Conclusions
Advanced TE materials with excellent performance in various applications have been
constantly developed due to rapidly developing TE science and technology. It is found that
effective ZnO dopants can achieve higher electrical conductivity and lower thermal
conductivity. Therefore, the first objective is to investigate TE parameters such as electrical
conductivity, Seebeck coefficient, thermal conductivity, and ZT for the ZnO TE materials
before and after doping, and their relation with these parameters changing. The final objective
is to apply the new TS-SPS processes with achieving a critical density of > 75% theoretical
density at the first heating step and densification while preventing grain-growth at the second
heating step to improve PF, strongly suggesting that the carrier doping to TS-SPS ZnO
achieves much higher PF and, therefore, higher ZT values. The following are outlined
conclusions of each chapter.
Chapter 2. Thermoelectric Properties of In- and Ga-doped Spark Plasma Sintered ZnO
Ceramics
In- and Ga-doped consolidated ZnO, [Zn(1−x−y)GaxIny]O, ceramics were fabricated via SPS,
and the solubility of dopants and the correlation between crystal structure and various
compositions on the TE properties were investigated. Crystal structure and microstructural
analyses indicated the presence of a spinel secondary phase (ZnGa2O4) with a high relative
density, and ZnGa2O4 disappeared as In increased from y = 0 to 0.005. It can be concluded that
the addition of In contributed to the increase in the solubility of Ga because doping with In and
Ga (x = 0.02; y = 0.005) enabled higher doping of Ga. Consequently, doping with In and Ga
could achieve the highest PF of 0.99 mW K−2 m−1 at 773°C, and reduce the thermal
conductivity, attaining ZT = 0.003–0.015 from 67 to 257°C.
139
Chapter 3. Thermoelectric Performance of In and Ga Single/Dual-doped ZnO Ceramics
Fabricated by SPS
For the comparison of In single, and In and Ga dual doping effect on ZnO, the sintered
bodies of In single-doped, and In and Ga dual-doped ZnO ceramics were consolidated via
SPS. The microstructural and chemical composition analyses indicated that the secondary
phases of Zn7In2O10 and ZnGa2O4 existed in the single/dual doped ZnO. Through the TE
characterization, it was revealed that the segregation of ZnGa2O4 phase affected to decrease
electrical conductivity. Although the presence of secondary phases, the highest PF and the
lowest thermal conductivity were achieved for dual-doped ZnO. Hence, the substitution of Zn
by In and Ga is a promising approach to increase the values of PF of ZnO by tuning its carrier
concentration. Therefore, the In and Ga dual doping of ZnO was beneficial in improving its TE
performance.
Chapter 4. Characterization of ZnO Thermoelectric Ceramics and Their Microstructures
Consolidated by Two-Step Spark Plasma Sintering
To investigate the effect of control of grain sizes on TE properties, the density, grain size,
and TE performance of pure ZnO ceramics were studied by applying OS/TS-SPS processes
with temperature profiles. High relative densities of 91.4–94.9% were achieved except for OS1150 (85.6%). The TS-SPS process thus proved a significant advantage for increasing the
relative density. Moreover, a noticeable suppression of grain-growth (56%, reduced from 8.1
to 3.6 μm) for the TS-1100-120 sample proved the effect of fine-grained microstructures. The
value of PF, 7.8 × 10–5 W K−2 m−1 at 775°C (TS-1100-120) was higher than that of the OSSPS pure ZnO sample in chapter 3, indicating that the TS-SPS process is effective to improve
PF.
140
Chapter 5. Effects of Two-Step Spark Plasma Sintering on the Microstructures and
Thermoelectric Properties of Pure/In-doped ZnO
Pure and In-doped ZnO ceramics were successfully prepared by SPS with OS/TS
temperature profiles to optimize the TS-SPS process. High relative densities of 94.6–98.3%
were achieved without significant grain-growth. It was revealed that a noticeable suppression
of grain-growth (21.8%, reduced from 7.8 to 6.1 μm) in OS-SPS and TS-SPS samples with T1
= 1150°C and T2 = 950°C. The highest PF, 1.0 mW K−2 m−1, was achieved for the TS-SPS
sample, attaining an increased ZT (55.2%, from 0.125 to 0.194 compared with I-OS-1150) at
773°C. The ZT value was the highest value of In and Ga dual-doped ZnO in this thesis, proving
that the TS-SPS process is an effective strategy for improving PF, strongly suggesting that the
carrier doping to TS-SPS ZnO achieves much higher PF and, therefore, higher ZT values.
The main challenge was to improve the TE performance of In and Ga dual-doped ZnO.
Although the optimization of doping with In and Ga in ZnO was successfully conducted, the
reduction of thermal conductivity could not be significantly improved. As an alternative to the
fine-grained microstructure, TS-SPS processes were applied to non-doped and In-doped ZnO
for the first time. Through the temperature profiles, the TS-SPS process was divided into two
kinds of steps to achieve a high critical density and densification while preventing grain-growth
at the first and second heating steps, respectively. As a result, it was possible to achieve the
fine-grained microstructure that a noticeable suppression of grain-growth (21.8%, compared
with In-doped OS-SPS) with a relative density of 96%.
The meaningful enhancement of the ZnO TE ceramics in TE properties was achieved by
doping and applying the TS-SPS process. Consequently, it is expected that the new TS-SPS
strategy can provide a new opportunity for significant contribution to ZnO-based TE materials.
141
Research Achievements
1. List of published papers
[1] A. Jeong and B.K. Jang, “Effects of two-step spark plasma sintering on the microstructures
and thermoelectric properties of Pure/In-doped ZnO”, Materials Science and Technology
of Japan, 59 (2022) 33–39.
[2] A. Jeong, M. Ohtaki, and B.K. Jang, “Characterization of ZnO thermoelectric ceramics
and their microstructures consolidated by two-step spark plasma sintering”, Journal of the
Ceramic Society of Japan, 130 (2022) 889–894.
[3] A. Jeong, M. Ohtaki, and B.K. Jang, “Thermoelectric performance of In and Ga
single/dual-doped ZnO ceramics fabricated by spark plasma sintering”, Ceramics
International, 48 (2022) 14414–14423.
[4] A. Jeong, K. Suekuni, M. Ohtaki, and B.K. Jang, “Thermoelectric properties of In- and
Ga-doped spark plasma sintered ZnO ceramics”, Ceramics International, 47 (2021)
23927–23934.
- 解説
[1] ジョン アロン, 大瀧 倫卓, 張 炳國, “酸化物系熱電材料の研究開発動向”, 材料
の科学と工学, 58 (2021) 18–21.
142
2. List of international academic conferences
[1] A. Jeong, M. Ohtaki, and B.K. Jang, “Effect of Doping with Indium and Gallium on
Thermoelectric Properties in Zinc Oxide”, at the ICACC 2022 (The 46th International
Conference and Exposition on Advanced Ceramics Composites), Online conference,
01/23-28 (2022).
[2] A. Jeong and B.K. Jang, “Thermoelectric Performance of ZnO Ceramics Co-doped with
Ga and In Fabricated by Spark Plasma Sintering”, at the 23rd CSS-EEST (Cross Straits
Symposium on Energy and Environmental Science and Technology), Online conference,
12/02-03 (2021).
[3] A. Jeong and B.K. Jang, “Thermoelectric Properties and Microstructure of In- and Gadoped ZnO fabricated by Spark Plasma Sintering”, at the IEICES-2021 (International
Exchange and Innovation Conference on Engineering & Science), Online conference,
10/21-22 (2021).
3. List of domestic academic conferences
[1] ジョン アロン, 大瀧 倫卓, 張 炳國, “Thermoelectric Characterization of In and Ga
Single/Dual-doped ZnO Consolidated by Spark Plasma Sintering”, 日本熱電学会学術
講演会 (TSJ2022), アオーレ長岡, 長岡, 08/08-10 (2022).
[2] ジョン アロン, 大瀧 倫卓, 張 炳國, “Synthesis and Thermoelectric Characterization
of In and Ga co-doped ZnO by Spark Plasma Sintering”, 日本材料科学会, 2022 年度学
術講演大会, オンライン開催, 05/18-19 (2022).
[3] ジョン アロン, 大瀧 倫卓, 張 炳國, “Microstructure and Thermoelectric Properties
of In- and Ga-doped ZnO consolidated by Spark Plasma Sintering”, 日本セラミックス
協会 2022 年年会, オンライン開催, 03/10-12 (2022).
[4] ジョン アロン, 末國 晃一郎, 大瀧 倫卓, 張 炳國, “Effect of Doping Concentration
on the Thermoelectric Properties of In and Ga doped ZnO by Spark Plasma Sintering”, 日
本材料科学会, 2021 年度学術講演大会, オンライン開催, 05/20-21 (2021).
[5] ジョン アロン, 末國 晃一郎, 大瀧 倫卓, 張 炳國, “Thermoelectric Performance of
In and Ga Co-doped ZnO Fabricated by Spark Plasma Sintering”, 日本セラミックス協
会 2021 年年会, オンライン開催, 03/23-25 (2021).
143
Acknowledgments
I would like to appreciate my esteemed supervisor Prof. Byung-Koog Jang for his
invaluable supervision, full support, effort in research, and guidance of daily during the
doctoral course at Kyushu University. During the short term of two and a half years for the Ph.
D. course, he made me grow up through his outstanding guidance on the way how to set up
originality, research direction, experimental procedures, and methods of solving problems for
thermoelectric research. He gave me a roadmap on the TS-SPS process with doping on
thermoelectric materials. Based on the guidance, I learned how to be a self-directed learner and
to prepare academic papers professionally as the most important task so I would like to show
respect to Prof. Jang. His continuous supervision with plentiful experience has encouraged me
in all the time of my academic research. The guidance of my professor that I had never received
anywhere else, and it was by far the best. I have been extremely lucky to have a supervisor who
cared so much about my research. Consequently, I have achieved significant research
achievements, leading to a post-doctoral fellowship.
I am extremely grateful to Prof. Michitaka Ohtaki and Prof. Koichiro Suekuni for their
immense knowledge, experimental support, and academic discussion regarding thermoelectric
materials. I would like to express my gratitude to Prof. Michitaka Ohtaki, Prof. Koichiro
Suekuni, Prof. Ken Watanabe, and Prof. Koji Miyazaki for their informative comments and
guidance as the thesis committee, and Prof. Kiichi Hamamoto for dealing with the process of
the early graduation although their schedule is quite tight.
I would like to express appreciation to Prof. Kyuhyoung Lee during the research on the
preparation of sheet Al-doped ZnO by hydrothermal synthesis at Yonsei University. He helped
me to plan my research theme and gave me valuable discussion as a supervisor. Also, I am
grateful to Prof. Hiromichi Ohta during the research on thermoelectric performance of the
144
epitaxial thin films of In and Ga dual-doped ZnO at Hokkaido University. Thanks to the
opportunities, I learned a lot in the thermoelectric research field at different universities.
Moreover, I am tremendously appreciative of the sincere advice and mentorship of Prof. Jondo
Yun and Prof. Si-Young Choi and their encouragement helped me to endure a rough time
abroad.
On campus, I would like to thank Ms. Rumiko Ide and Ms. Kyoko Nozoe, staff of the
major office for their help with administrative processes without any problems during the Ph.D.
course. Also, my appreciation also goes out to our lab mates Ji-Hwoan Lee and Seung-Hyeon
Kim for team spirit. I could not have done it without our lab members. Particularly, I am really
grateful to my friends, Seungwon Lee, Kayoung Park, Hyungrok Jo, Taeyoung Kim, Kwangsu
Kim, Shoya Iwanami, Junyoung Heo, Katsuaki Hashikuni, Nhat Quang Minh Tran, Maneeyom
Sasikan, and Yuki Miwa. Thanks for having my back and helping for the research. My gratitude
extends to Rotary Yoneyama Memorial Foundation and Mr. Takayuki Yamada for the
scholarship opportunity to undertake my studies at Kyushu University.
First and foremost, I must express my very profound gratitude to my beloved parents for
their wholehearted support throughout my years of study, especially Ph. D. course. My parents
always suggested to me that I should do what I hope to do and achieve it without regret. I will
never forget their devoted care and trust. This accomplishment would not have been possible
without them. Thank you for always believing in me. I will remember forever what you have
guided me.
Finally, I am convinced that the valuable research and academic activities will be my
biggest asset during two and a half years on Chikushi Campus at Kyushu University. Thank
you for your goodness in my life.
January, 2023
ジョン
アロン
JEONG AHRONG
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