Vol. 59 No. 6

Material Science and Technology of Japan, 59（2022）197～203

33

Regular Paper
Effects of Two-step Spark Plasma Sintering on the Microstructures
and Thermoelectric Properties of Pure/In-doped ZnO
Ahrong JEONG* and Byung-Koog JANG*Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen,
Kasuga-shi, Fukuoka 816-8580, Japan
Received Oct. 11, 2022; accepted Nov. 9, 2022
Abstract
Pure and In-doped zinc oxide thermoelectric ceramics [Zn(1−x)Inx]O (x = 0, 0.005) were prepared by spark plasma
sintering (SPS) with one-step/two-step (OS/TS) temperature profiles. The strategy of the TS temperature profile wassintering at a higher temperature (T1) followed by prolonged sintering at a lower temperature (T2) for densification
while suppressing grain growth. In the first step, the temperature was 1150 °C for 15 min, and the second step was
kept at 900, 950, 1000, 1050, and 1100 °C for 30 min. Through a combination of TS and SPS processes (TS-SPS),
high relative densities of 94.6–98.3% were achieved without significant grain growth. Electron backscatterdiffraction (EBSD) indicated noticeable grain growth suppression of 14.0 and 21.5% for the pure/In-doped TS-SPS
samples as compared with OS-SPS samples, respectively. The highest power factor of 1.0 mW K−2 m−1 was achieved
for the In-doped ZnO, attaining a dimensionless figure of merit (ZT) of 0.194 at 773 °C.
Keywords: Thermoelectric, ZnO, Doping, Two-step sintering, Spark plasma sintering

1.

Introduction

maximizing the power factor (S2σ, PF) and reducing the thermalWith increasing global concern for energy and environmental

conductivity.

problems, thermoelectric (TE) technology has attracted attention

The zinc oxide (ZnO) ceramic [6–10], which is low-cost,because it can convert waste heat into electrical power [1].

abundant, and environmental friendly, has attracted considerable

Intermetallic compounds such as Bi2Te3 and PbTe can give theattention as a new TE material for high-temperature applications

best performance as TE materials [2–4]. However, their

due to its excellent thermal and chemical stability in air.

operating temperatures (27–227 °C and below 627 °C,Nowadays, most researchers are focused on decreasing therespectively) are inherently limited by their melting points,

thermal conductivity of ZnO-based TE materials [11–13].

which are 580 °C and 924 °C, respectively. Even below theNevertheless, the enhancement of the PF is also important for

melting points, these materials are not attractive because surface

optimizing the ZT values. The undoped ZnO ceramics exhibit aoxidation and vaporization can easily occur at such highhigh S (~340 V K−1); however, the σ (~10 S m−1) is still too lowtemperatures. Furthermore, their constituting heavy metal

to be used [14]. One obstacle to increasing σ is the limited

elements are mostly toxic, scarce, and expensive. The efficiency

solubility of donor dopants such as aluminum (Al), indium (In),[5] of a TE material is defined by a dimensionless figure of merit,and gallium (Ga), which is too low to achieve a sufficient carrier

ZT = S2σT / κ, where S, σ, T, and κ are the Seebeck coefficient,concentration for maximizing the PF of ZnO-based TE materials

electrical conductivity, absolute temperature, and thermal[14–17]. Therefore, increasing the solubility of donor dopants isconductivity, respectively. The ZT values can be enhanced byneeded to optimize the PF. However, the enhancement of thecarrier concentration should be carefully designed because there
is a trade-off between S and σ. A higher carrier concentration*Corresponding author: jang.byungkoog@kyudai.jp(197)