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Synthesis of Cu(Al,Fe)O2 Thermoelectric Ceramics by a

Reaction–Sintering Process

Yi-Cheng Liou*, Hong-Chou Tsai, Wen-Chou Tsai, Uang-Ru Lee

Department of Electronic Engineering, Kun Shan University, 949, Da Wan Rd., Tainan

71003, Taiwan, R.O.C.

*Corresponding author. Tel.: 886-6-2050521; Fax: 886-6-2050250

E-mail address: ycliou@mail.ksu.edu.tw

Abstract

Fabrication of CuAl0.9Fe0.1O2 ceramics via a reaction–sintering process was investigated.

Without any calcination involved, the mixture of raw materials was pressed and sintered

directly. Single phase CuAl0.9Fe0.1O2 ceramics were obtained. The shrinkage is < 3% at

1150°C and increases to 8.6–11.2% at 1200–1300°C. A density 4.73 g/cm3 was obtained

for pellets sintered at 1200°C for 4 h. Grains of ~30μmcould be seen in pellets sintered

at 1150°C and increases to ~50 μm in pellets sintered at 1300°C for 2 h. The

reaction–sintering process has proven a simple and effective method in preparing

CuAl0.9Fe0.1O2 ceramics for thermoelectric application.

Keywords: CuAl0.9Fe0.1O2, reaction–sintering process

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1. Introduction

Many oxide systems such as In2O3-MOx (M=Cr, Mn, Ni, Zn, Y, Nb, Sn) [1],

(Ca,Ln)MnO3 [2], (Zn,Al)O [3], (Ba,Sr)PbO3 [4], and (ZnO)5In2O3 [5] have been

investigated for thermoelectric application. The figure of merit Z=S2σ/κis used to

evaluate the performance of thermoelectric materials, where S, σ, and κ are the Seebeck

coefficient, electrical conductivity, and thermal conductivity, respectively. The figure of

merit Z for the above systems is lower than alloys and semiconductors [6]. A high figure

of merit 8.8×10-4K-1 in NaCo2O4 was found. However, its application is limited due to

the volatility of sodium above 800oC and hygroscopicity in air [7]. Ca3Co4O9+δ (δ=0.33)

was first synthesized in 1968 [8], and later in 1970 Woermann grew the single crystal of

Ca3Co4O9 [9]. Shikano and Funahashi obtained single crystals Ca3Co4O9 with a high

thermoelectric figure merit ZT~0.87 at 700oC. This makes Ca3Co4O9 a promising

material for practical application in thermoelectric power generation [10]. Recently,

CuAlO2 has received many attentions. Ishiguro and co-workers extensively studied the

crystalline structure of CuAlO2 [11]. Koumoto et al. first reported CuAlO2 with a power

factor (at 1073K) ~1.04×10−4 Wm−1K−2 for single crystal and ~2.0×10−5 Wm−1K−2 for

polycrystalline, respectively [12]. Park et al. obtained power factors 4.98×10−5 and

6.62×10−5 Wm−1K−2 at 1140K for the CuAlO2 ceramics sintered at 1433 and 1473K,

respectively [13]. They also found the substitution of Ca up to x = 0.1 for Al in the

CuAl1-xCaxO2 samples gave rise to an increase in both the electrical conductivity and

the Seebeck coefficient. On the other hand, the higher Ca substitution (x≥0.15)

decreased both the electrical conductivity and the Seebeck coefficient. The highest

value of power factor 7.82×10−5 Wm−1K−2 was attained for CuAl0.9Ca0.1O2 at 1140K

[14]. From these studies, CuAlO2 based ceramics are promising thermoelectric

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materials for energy conversion. Various methods have been used to prepare the CuAlO2

based ceramics including the conventional solid–state reaction [13–15], ion exchange

[16] and sol–gel process [17].

Our group had prepared many Pb–based complex perovskite ceramics via a simple

and effective reaction–sintering process [18–21]. This process was also used to produce

some microwave dielectric ceramics such as BaTi4O9, (BaxSr1-x)(Zn1/3Nb2/3)O3,

Ba5Nb4O15, Sr5Nb4O15, CaNb2O6 and NiNb2O6 successfully [22–26]. Recently, we used

this simple and effective process in preparing ceramics for solid oxide fuel cells and

thermoelectric devices applications. Sr0.995Ce0.95Y0.05O3-δ electrolyte ceramics with

98.4% of the theoretical density were obtained after being sintered at 1350oC for 2 h. A

total conductivity 1.42 mS/cm at 900oC could be obtained in Sr0.995Ce0.95Y0.05O3-δ

sintered at 1500oC for 4 h. BaCe0.9Nd0.1O3-δelectrolyte ceramics with 91.7% of the

theoretical density were obtained after being sintered at 1500oC for 2 h. A total

conductivity 11.54 mS/cm at 900oC could be obtained in BaCe0.9Nd0.1O3-δsintered at

1350oC for 6 h [27]. >99.5% of theoretical density was obtained for Ce0.9Gd0.1O1.95

electrolyte ceramics sintering at 1500–1600oC [28]. Some peaks of remained Al2O3

were detected in CuAlO2. CaAl4O7 and CuO were detected in CuAl0.9Ca0.1O2. Density

values 2.83-3.04 g/cm3 were found in CuAlO2 pellets sintered at 1350oC. In

CuAl0.9Ca0.1O2 ceramics, density values 3.10-3.31 g/cm3 were found in pellets sintered

at 1350oC. Thin polygonal grains were observed in CuAl0.9Ca0.1O2 [29].

In this study, synthesis of CuAl0.9Fe0.1O2 ceramics using a reaction–sintering process

was investigated.

2. Experimental procedure

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CuAl0.9Fe0.1O2 (CAF) ceramics in this study were prepared from reagent-grade

powders: CuO (99%, SHOWA, Japan), Al2O3 (99.5%, SHOWA, Japan), and Fe2O3

(100%, J.T. Baker, USA). Appropriate amounts of raw materials were milled in

de-ionized water with zirconia balls for 12 h. After the mixtures had been dried and

pulverized, they were formed into pellets 12 mm in diameter and 1–2 mm thick. The

pellets were then heated at a rate 10oC/min and sintered in a covered alumina crucible

for 2–6 h in air at temperatures ranging from 1100–1250oC. The reaction of the raw

materials happened during the heating up period and the calcination stage in the

conventional solid-state reaction was bypassed.

We analyzed the sintered pellets by X-ray diffraction (XRD) to identify the reflections

of various phases. Microstructures were analyzed by scanning electron microscopy

(SEM). The density of the sintered pellets was measured using the Archimedes method.

1324–1359

3. Results and Discussion

The XRD profiles of the CAF ceramics sintered at 1100–1200oC for 2 h are

illustrated in Fig. 1. The reflections match well with those of CuAlO2 in ICDD PDF #

00-035-1401 and no secondary phase detected. This implies that 1100oC is high enough

for a complete reaction of the reactants. Park et al. reported an endothermic reaction

occurs at 1069–1095oC for the mixed powders of CuAlO2 and 1051–1086oC for the

mixed powders of CuAl0.8Fe0.2O2 [30]. The reaction–sintering process is proven a

simple and effective process to obtain CAF ceramics. In our study of CuAlO2 ceramics

prepared via the reaction–sintering process, some peaks of remained Al2O3 were

detected. While in CuAl0.9Ca0.1O2 ceramics prepared via the reaction–sintering process,

CaAl4O7 and CuO were detected [29]. The addition of Fe2O3 seems to be effective to

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eliminate secondary phase in CuAlO2 ceramics. Park et al. observed CuAl1−xFexO2,

CuO, Fe2O3 and CuFeO2 phases in CuAl1−xFexO2 (x=0.05–0.2) ceramics calcined at

800oC for 2 h and sintered at 1200oC for 20 h [30]. The reaction–sintering process is

proven more effective than the conventional solid–state reaction method in preparing

single phase CAF ceramics.

Figure 2 indicates the linear shrinkage of the CAF ceramics sintered at various

temperatures. An expansion is observed for pellets sintered at 1100°C. The shrinkage is

< 3% at 1150°C and increases to 8.6–11.2% at 1200–1300°C. A similar trend is seen for

the density values as shown in Fig. 3. Maximum density values of 4.36–4.73 g/cm3

were observed at 1200–1300°C. The value 4.73 g/cm3 was obtained for pellets sintered

at 1200°C for 4 h. This reaches 92.8% of the theoretical density 5.097 g/cm3 of CuAlO2

ceramics. Park et al. obtained 85.2% of the theoretical density in CAF ceramics calcined

at 800oC for 2 h and sintered at 1200oC for 20 h [30]. Therefore the reaction–sintering

process is proven more effective than the conventional solid–state reaction method in

preparing dense CAF ceramics. In our study of CuAlO2 ceramics prepared via the

reaction–sintering process, density values 2.53–2.68 g/cm3 were observed in CuAlO2

pellets sintered at 1200oC [29]. The addition of Fe2O3 enhances the densification in

CuAlO2 ceramics.

SEM photographs of as-fired CAF ceramics sintered at 1150–1300°C for 2 h are

presented in Fig. 4. Grains of ~30 μmcould be seen in pellets sintered at 1150°C and

increases to ~50 μm in pellets sintered at 1300°C. In our study of CuAlO2 ceramics

prepared via the reaction–sintering process, grains of 3 μm could be found in the pellet

sintered at 1200oC for 2 h [29]. This implies that the addition of Fe2O3 enhances the

grain growth in CuAlO2 ceramics. Park et al. obtained grains of 2.08 μm in CAF

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ceramics calcined at 800oC for 2 h and sintered at 1200oC for 20 h [30]. Therefore the

reaction–sintering process is proven more effective than the conventional solid–state

reaction method for grain growth in CAF ceramics. The major difference between the

reaction–sintering process and the conventional solid–state reaction method is that the

mixed raw materials are pressed and sintered directly. The calcinations stage and the

following pulverization are bypassed. The clusters of calcined CAF powder via the

conventional solid–state reaction did not exist in the pressed pellets of mixed raw

materials via the reaction–sintering process. Sizes for clusters of calcined powder are

larger than the milled mixed raw materials without calcining. The driving force to break

the contact surface of calcined CAF clusters is much higher than the driving force to

joint the just reacted and nucleated small CAF particles in the pressed pellets of mixed

raw materials. On the other hand, CuO is often used as a sintering aid to lower the

sintering temperature in preparing ceramics. The sintering temperature is 200oC lowered

in Ba5Nb4O15 ceramics with the addition of 1 wt% CuO via the reaction–sintering

process [24]. Yang et al. [31] used the CuO–BaO mixture as a sintering aid in the

fabrication of BaTiO3 ceramics and studied its effects on the microstructure and

densities of BaTiO3 ceramics. An addition of 1 wt% CuO–BaO mixture to BaTiO3

significantly increased the sintering rate of BaTiO3 at temperature between 1000 and

1100°C. Some parts of mixed raw materials in the pressed pellets reacted into CAF first

and then nucleated and grow to larger size. It is possible that these firstly formed CAF is

surrounded with some CuO particles and a liquid phase sintering occurred during the

heating period. Therefore the reaction of raw materials and a liquid phase sintering are

supposed to occur at a same period. This may be the reason why much larger grains

formed in CAF ceramics via the reaction–sintering process. The liquid phases are

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clearly seen in Fig. 4–6. Grain growth is also found to be easier in CuAlO2 and

CuAl0.9Ca0.1O2 prepared via the reaction–sintering process than via the conventional

solid–state reaction [29].

4. Conclusions

The reaction–sintering process proved to be a simple and effective method for

obtaining single phase CuAl0.9Fe0.1O2 ceramics. CuO, Fe2O3 and CuFeO2 phases

appeared in CuAl0.9Fe0.1O2 ceramics via the conventional solid–state reaction method

were not detected. The shrinkage is < 3% at 1150°C and increases to 8.6–11.2% at

1200–1300°C. A density 4.73 g/cm3 was obtained for pellets sintered at 1200°C for 4 h.

Grains of ~30μmcould be seen in pellets sintered at 1150°C and increases to ~50μmin

pellets sintered at 1300°C for 2 h. The reaction–sintering process is more effective than

the conventional solid–state reaction method for grain growth in CuAl0.9Fe0.1O2

ceramics.

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Figure captions

Figure 1 XRD patterns of CAF ceramics sintered at 1100–1200oC for 2 h.

Figure 2 Shrinkage percentages of CAF ceramics sintered at various temperatures

for 2–6 h.

Figure 3 Density of CAF ceramics sintered at various temperatures for 2–6 h.

Figure 4 SEM photographs of as-fired CAF ceramics sintered at (A) 1150°C, (B)

1200°C, (C) 1250°C, and (D) 1300°C for 2 h.

Figure 5 SEM photographs of as-fired CAF ceramics sintered at (A) 1150°C, (B)

1200°C, (C) 1250°C, and (D) 1300°C for 4 h.

Figure 6 SEM photographs of as-fired CAF ceramics sintered at (A) 1150°C, (B)

1200°C, (C) 1250°C, and (D) 1300°C for 6 h.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6