Materials Science Forum Vol. 750 (2013) pp 134-137
© (2013) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.750.134
Improvement of Thermoelectric Properties of CuAlO2
by Excess Oxygen Doping in Annealing
Yun Lu1, a,Kazunari Maeda2,Katauhiro Sagara2,b,
Liang Hao2,c and Yingrong Jin3,d
1
Graduate School & Faculty of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku,
Chiba,263-8522, Japan
2
Graduate School, Chiba University,1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan
3
School of Materials Science and Engineering, Xihua University, 610039 Chengdu, Sichuan, P. R.
China
a
[email protected], [email protected], [email protected],
d
[email protected]
Keywords: CuAlO2, Annealing, Excess Oxygen Doping, Decomposition, Thermoelectric Property
Abstract: The reaction evolution of CuAlO2 during annealing at high temperatures in air was
investigated. The relationship between thermoelectric properties including electrical resistivity,
excess oxygen doping and decomposition of CuAlO2 was discussed. The reaction process of
CuAlO2 compact during the annealing mainly included excess oxygen doping, decomposition of
CuAlO2 into CuAl2O4 and CuO and complete decomposition of CuAlO2 as following evolution:
CuAlO2 → CuAlO2+x → CuAlO2+CuO+CuAl2O4 → CuAl2O4+CuO. Electrical resistivity of the
CuAlO2 compact was decreased with the excess oxygen doping, but turned to increase when CuO
and CuAl2O4 were formed. Thermoelectric performance of the CuAlO2 compact was improved due
to the excess oxygen doping.
Introduction
In recent years, thermoelectric materials have attracted much attention due to their potential
applications in conversion between electrical power and heat such as electrical power generation
from waste heat [1]. Especially, oxides have gained wide interest for future thermoelectric
applications since they have many advantages such as thermal stability, non-toxicity, and high
oxidation resistance, among others [2-4]. Delafossite CuAlO2, which has a hexagonal structure and
can be viewed as a lay- structured oxide, has been expected to be used as a transparent conductive
oxide and a p-type thermoelectric oxide [5-11]. CuAlO2 has a large power factor as high as
6.62×10-5 Wm-1K-2 [8]. In our previous work [11], the reaction behavior during the formation of
CuAlO2 from CuO and Al2O3 powders was studied. It was clarified that an atmosphere of low
oxygen pressure promoted formation of CuAlO2. It also indicated that a series of reaction as
CuAlO2→CuAlO2+x→CuAlO2+CuO+CuAl2O4→CuAl2O4+CuO happened during heating and
holding CuAlO2 compact at high temperatures. Thermoelectric properties can be increased by
increasing the x value of CuAlO2+x.
In the present work, the excess oxygen doping and decomposition of CuAlO2 compact at high
temperatures were investigated. Thermoelectric properties including electrical resistivity and
Seebeck coefficient were examined. The influence of the excess oxygen doping and the
decomposition of CuAlO2 compact on thermoelectric properties was discussed.
Experimental
Fabrication of CuAlO2 compact
CuO powder (purity: 99.5%, average diameter: 3 µm) and Al2O3 powder (purity: 99.98%, average
diameter: 1 µm) were used as the source materials. To obtain CuAlO2 compact, CuO powder and
Al2O3 powder were weighed respectively with a molar ratio of Cu:Al=1:1. The weighed CuO and
Al2O3 powders were mixed well by a magnetic stirrer. The green compact with the dimensions of
70×30×2 mm was prepared with a pressure of 312 MPa. CuAlO2 compact was fabricated by
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Materials Science Forum Vol. 750
135
sintering the green compact at 1373 K for 0.5 h in air. The sintering conditions have been confirmed
in order to obtain single phase CuAlO2 [11]. The microstructure and the crystal type were analyzed
by SEM and XRD respectively.
TG of CuAlO2 compact
To examine the excess oxygen doping and decomposition of CuAlO2 compact, thermogravimetry
(TG) was performed by a thermal balance (TG-DTA 2000S, MAC science Co., Ltd.). Specifically,
50 mg sample of the compact was heated to high temperatures from 973 to 1173 K at 5 K/min in air
and holding for 5~100 h. The samples of heated compact were analyzed by SEM and XRD.
According to the results of the excess oxygen doping, CuAlO2 compact was annealed at 1023 K for
5~20 h to improve thermoelectric properties.
Measurement of thermoelectric properties
The plate samples of CuAlO2 compact with the dimensions of 40×5×2 mm, which were annealed at
1023 K for 5~20 h, were used to measure thermoelectric properties. The measurements were carried
out in air from approximately 323 K up to 923 K. Seebeck coefficient was measured by the static
method. The plate sample was heated, and held at the measurement temperatures. It was given a
temperature difference of ±6 K in the present work, in other words, a temperature gradient between
the two sides of the sample. The negative temperature difference implies a reverse temperature
gradient. The electrical resistivity at the elevated temperatures was measured by the 4-prob method
when the temperature difference was 0 K.
Results and discussion
Excess oxygen doping and decomposition of CuAlO2 compact
TG of the compact increased with holding time at high temperatures in air as shown in Fig.1. Also,
the higher the heating temperature, the larger the TG increase was. Therefore, the oxygen in air was
introduced into the compact at high temperatures. XRD patterns of the compacts after heating high
temperatures for 5 h in air is shown in Fig. 2. It can be seen that the obtained compact is single
phase of CuAlO2 as shown in Fig.2 (a). In the cases at and below 1023 K, the compact still kept
CuAlO2 single phase ((b) and (c) in Fig.2), probably formed CuAlO2+x due to the introduction of
oxygen as shown in Fig.1 (a) and (b). Besides, in the cases above 1073 K, CuO and CuAl2O4
appeared, the compact had a composition of CuAlO2, CuO and CuAl2O4, further was decomposed
and formed two phases of CuO and CuAl2O4 as Eq. 1 in the case of 1173 K. Thus, TG of the
compact at 1023 K for 5~100 h in air was examined as shown in Fig.3. The TG kept to increase
with increase of holding time till approximately
2 CuAlO2 ＋ (1/2) O2 → CuAl2O4 + CuO
(1)
Intensity (a. u.)
Temperature / K
TG / mg
80 h, after which, kept constant. When holding time was no more than 15 h, single phase of CuAlO2
was reserved as
CuO
CuAlO2
CuAl2O4
3 (a) 973 K
1500
(b) 1023 K
shown in Fig.4
(c) 1073 K (d) 1123 K
(e)
(b), (c) and (d),
(e) 1173 K
(f) 1173 K
probably
formed
2
(e) 1123 K
CuAlO2+x due to
1000
(d)
(d) 1073 K
the introduction
1
(c)
(c) 1023 K
of oxygen as
(b) 973 K
(b)
shown in Fig.3
500
0
(a)
(a) CuAlO compact
(a).
When
30
40
50
holding
time
0
2
4
6
8
10
2θ /deg
Time / h
came to above
Fig.2 XRD patterns of CuAlO2 compact after
Fig.1 Thermogravimetry of CuAlO2 compact
heating
to
973~1173
K
for
5
h
in
air.
20 h, CuO and
during heating and holding for 5 h in air.
CuAl2O4 phases
appeared, the compact had a composition of CuAlO2, CuO and CuAl2O4. CuAlO2 was completely
decomposed and formed CuO and CuAl2O4 as Eq. 1 when holding time reached 100 h (Fig.4 (g)).
2
136
Advanced Materials Science and Technology
From the above results and analysis, with the increase of TG at high temperatures, the compact
of CuAlO2 formed CuAlO2+x due to excess oxygen doping firstly. Then it was decomposed into
CuO and CuAl2O4 and finally was completely decomposed. A series of reaction can be given by Eq.
2.
CuAlO2 → CuAlO2+x → CuAlO2+CuO+CuAl2O4 → CuAl2O4+CuO
(2)
5 h
10 h
Temperature
2
1000
2
1
1
0
500
0
4
8
Temperature / K
Temperature
TG / mg
TG / mg
1500
3
3
0
(b) 40~100 h
40 h
100 h
(a) 5~20 h
15 h
20 h
12
16
Time / h
20
24
1000
Temperature / K
4
1500
4
500
0
20
40
60
Time / h
80
100
Fig.3 Thermogravimetry of CuAlO2 compact during heating to 1023 K and holding for 5~100 h in air.
CuO
CuAlO2
CuAl2O4
Intensity (a. u.)
(g) 100 h
(f) 40 h
(e) 20 h
(d) 15 h
(c) 10 h
(b) 5 h
(a) Sintered CuAlO2
30
40
50
2θ /deg
Fig.4 XRD patterns of CuAlO2 compact after
heating at 1023 K for 5~100 h in air.
state. New phases, CuO and
CuAl2O4
were
formed
beyond the critical value,
further the CuAlO2 was
completely decomposed and
CuO and CuAl2O4 were
formed.
It is expected to decrease electrical resistivity by introducing
the excess oxygen into CuAlO2 compact [12],
simultaneously avoiding formation of CuAl2O4 and CuO.
They have high electrical resistivity [13,14].
The mass gain (∆WCO) was calculated by Eq.1, assuming
that the CuAlO2 compact sample of W0 weight decomposed
completely. Mean while, assuming that all the introduced
oxygen is the excess oxygen for forming CuAlO2+x, the x
value was calculated by the measured mass gain (∆W).
These calculation results and the relevant data were listed in
Table 1. There was the critical x value, 0.02 (∆W /∆WCO:
4.18%), below which the introduced oxygen was in excess
Table 1 Mass gain, doped excess oxygen and compounds when CuAlO2 compact
was heated at 1023 K for 5~100 h in air.
⊿
Holding time
at 1023 K (h)
W0
(mg)
W
(mg)
⊿W
C
O
(mg)
⊿W/⊿W
(%)
C
O
x
Compounds
(by XRD)
5
50.09
0.07
3.27
2.12
0.01
CuAlO2+x
10
49.99
0.16
3.26
4.81
0.02
CuAlO2+x
15
49.95
0.36
3.26
11.08
0.06 CuAlO2+CuO+CuAl2O4
20
40
100
50.20
49.84
50.17
0.94
1.39
3.42
3.28
3.25
3.28
28.82
42.71
104.27
0.14 CuAlO2+CuO+CuAl2O4
0.21 CuAlO2+CuO+CuAl2O4
0.52
CuAl2O4+CuO
Doped
excess
oxygen
20
doping
and
Sintered CuAlO
105
thermoelectric
Annealed at 1023 K for
CuAlO2+ CuO+CuAl2O4
→ CuAl2O4+CuO
5h
104
properties
10 h
15
Electrical resistivity of
15 h
103
20 h
the compacts was
decreased largely by
102
10
annealing at 1023 K
for 5 h and 10 h than
101
that of the compact
50
0.05
0.1
0.15
100
without annealing, but
Doped excess oxygen, x
400
600
800
Measurement temperature / K
turned to increase after
Fig.6 Relationship between the doped excess oxygen
Fig.5
Electrical
resistivity of CuAlO2 compact
and electrical resistivity at 923 K of CuAlO2
annealing at 1023 K
annealed at1023 K.
compact annealed at 1023 K for 5~20 h in air.
for 15 h and 20 h in the
whole measured temperature range as shown in Fig.5, furthermore, firstly decreased and then turned
CuAlO2+x
Electrical resistivity, ρmin / Ωmm
Electric resistivity /Ωmm
2
Materials Science Forum Vol. 750
Conclusions
Seebeck coefficient / µVK-1
Sintered CuAlO2
600
Annealed at1023 K for
5h
10 h
15 h
20 h
500
400
300
400
600
800
Measurement temperature / K
Fig.7 Seebeck coefficient of CuAlO2 compact
annealed at1023 K.
Power factor / Wm-1 K-2
increased with the increase of the doped excess oxygen as
shown in Fig.6. It is consistent with the results in Fig.3,
Fig.4 and Table 1. In other words, electrical resistivity of
the compacts was controlled by the state of the introduced
oxygen in annealing. The compacts annealed at 1023 K,
which have semiconductor's behavior as shown in Fig.5,
were the thermal active in the temperature range from 100
K to 600 K, and then reached the saturation according to
ln σ − 1/T plots. The active energy was about 0.273~0.298
eV.
Seebeck coefficient of the compact annealed at 1023 K
decreased firstly and then tended to increase, also, kept
large values over 300 µVK-1 in the whole measured
temperature range as shown in Fig.7. It is related to the
states (the thermal active range or the saturation range) of
the compact. When holding time was 15 h, Seebeck
coefficient had large values over 450 µVK-1 in the
whole measured temperature range (Fig.7). Power factor
of the compact annealed at 1023 K for 5~15 h was
greater than that of the compact without annealing in the
whole temperature range, however, was close to that
without annealing due to formation of CuO and
CuAl2O4 when holding time was 20 h as shown in Fig.8.
137
10-5
Sintered CuAlO2
10-6
Annealed at 1023 K for
5h
10 h
15 h
20 h
400
600
800
The reaction process of CuAlO2 compact during the
Measurement temperature / K
annealing mainly included excess oxygen doping,
Fig.8 Power factor of the CuAlO2 compact
decomposition of CuAlO2 into CuAl2O4 and CuO and
annealed at1023 K.
complete decomposition of CuAlO2 as following
evolution : CuAlO2 → CuAlO2+x → CuAlO2+CuO+CuAl2O4 → CuAl2O4+CuO. Electrical
resistivity of the CuAlO2 compact was decreased with the excess oxygen doping, but turned to
increase when CuO and CuAl2O4 were formed. Power factor of the CuAlO2 compact was increased
due to the excess oxygen doping, and reached 2.82×10-5 W m-1 K-2 at 923 K.
References
[1] H. Scherrer, S. Scherrer, et al., in : Thermoelectrics Handbook, edited by D.M. Rowe, Section
III Thermoelectric Materials, Taylor & Francis Group (2006).
[2] I. Terasaki, Y. Sasago and K. Uchinokura: Phys. Rev. B, R12685(1997).
[3] R. Funahashi, I. Matsubara, H. Ikuta and T. Takeuchi: JPN. J. Appl. Phys., 39 (2000), L1127.
[4] Y. Lu, K. Sagara, L. Hao, Z. W. Ji and H. Yoshida: Mater. Trans., 53-7 (2012), 1208.
[5] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, et al.: Nature, 389-6654 (1997), 939.
[6] M.S. Lee, T.Y. Kim and D. Kim: Appl. Phys. Lett., 79-13 (2001), 2028
[7] T. Koyanagi, H. Harima, et al.: J. Phys. Chem. Solid. 64 (2003), 1443.
[8] K. Park, K. Y. Ko, and W.S. Seo: J. Euro. Ceam. Soci., 25 (2005), 2219.
[9] K. Koumoto, H. Koduka and W. S. Seo: J. Mater. Chem., 11-2 (2001), 251.
[10] A.N. Banerjee, R. Maity and K.K. Chattopadhyay: Mater. Lett., 58-1/2 (2004), 10.
[11] Y. Lu, K. Maeda, Y. R. Jin and M. Hirihashi: Mater. Sci. Tech. JPN., 48 (2011), 302.
[12] A.N. Banerjee, C.K. Ghosh and Chattopadhyay: Solar Energy Mater. & Solar Cells, 89(2005),
75.
[13] L.C. Leu, D.P. Norton, G.E. Jellison Jr., et al.: Thin. Solid Films, 515 (2007), 6938.
[14] Y.K. Jeong and G.M. Choi: J. Phys. Chem. Solids. 57 (1996), 81.