Journal of Alloys and Compounds 403 (2005) 308–311
Mechanical properties of Ag-doped Na
1.5
Co
2
O
4
Tosawat Seetawan
a,∗
, Vittaya Amornkitbamrung
a
, Thanusit Burinprakhon
a
,
Santi Maensiri
a
, Ken Kurosaki
b
, Hiroaki Muta
b
, Masayoshi Uno
b
, Shinsuke Yamanaka
b
a
Department of Physics, Khon Kaen University, 123 Mittraparb Road, Muang District, Khon Kaen 40002, Thailand
b
Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaka 2-1, Suita, Osaka 565-0871, Japan
Received 24 April 2005; accepted 6 May 2005
Available online 2 August 2005
Abstract
The polycrystalline samples of Na
1.5
Co
2-x
Ag
x
O
4
(x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized from powder precursors prepared by the
polymerized complex (PC) method. Various mechanical properties of the samples, including the elastic modulus, Debye temperature and
Vickers hardness, were evaluated. The elastic moduli and Debye temperature increase with increasing Ag content. The relationship between
the Vickers hardness and Young’s modulus indicates a metallic characteristic. The microstructure of the non-doped sample shows coarse
grains of about 10–20 m. On the other hand, the Ag-doped samples show fine grains with Ag particles around 10 m.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Sodium cobalt oxide; Mechanical properties; Young’s modulus; Debye temperature; Heat capacity; Thermal barrier coating
1. Introduction
Sodium cobalt oxide (Na
x
Co
2
O
4
) was first synthesized
in 1974 by Von Jansen and Hoppe [1]. The crystal struc-
ture of the -phase of this material is hexagonal, which can
be pictured as a layer structure comprising CoO
2
conduct-
ing layers which are made of edge-shared CoO
6
octahedra
and inter-layers of Na
+
ions alternatively stacked along the c-
axis. The sodium ions are intercalated in trigonal prismatic or
octahedral coordination of oxygen atoms [2]. An important
feature in this structure is that sodium ions randomly occupy
the available sites by 50%. In this sense, NaCo
2
O
4
should
be written as Na
x
CoO
2
(x = 0.5). Nevertheless, we will call it
NaCo
2
O
4
, because the best thermoelectric properties are real-
ized near the 50% Na occupancy [3]. The -phase Na
x
Co
2
O
4
shows a large Seebeck coefficient despite its metallic conduc-
tivity. Recently, Terasaki et al. [4] reported the outstanding
thermoelectric properties of a single crystal of this oxide, i.e.,
an unusual Seebeck coefficient of 0.1 mV K
-1
accompanied
by a low resistivity of 0.2 m cm, at 300K. To improve the
∗
Corresponding author. Tel.: +81 6 6879 7905; fax: +81 6 6879 7889.
E-mail address: seetawan@nucl.eng.osaka-u.ac.jp (T. Seetawan).
thermoelectric efficiency of Na
x
Co
2
O
4
, Na or Co is substi-
tuted by other metals. For instance, the Seebeck coefficient of
100–150 VK
-1
and the dimensionless figure of merit (ZT)
of 0.03–0.05 have been obtained for NaCo
2-x
T
x
O
4
(T = Mn,
Ru, Pb, Cu, Pd, Rh), at 300 K [5–9].
In the present study, the thermoelectric materials of
Na
1.5
Co
2-x
Ag
x
O
4
(x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were pre-
pared from the powder precursors obtained from the poly-
merized complex (PC) method. The mechanical properties
of these materials were measured and calculated. The rela-
tionship between the hardness and Young’s modulus of the
materials was studied.
2. Experimental procedure
Polycrystalline samples of Na
1.5
Co
2-x
Ag
x
O
4
(x = 0, 0.1,
0.2, 0.3, 0.4, 0.5) were synthesized from powder precursors
prepared by the PC method [10]. Firstly, citric acid (99.7%)
was dissolved in ethylene glycol (99.5%) by heating and stir-
ring at 473 K for 1 h. Secondly, Co(NO
3
)
2
·6H
2
O (99.95%),
NaNO
3
(99%) and AgNO
3
(99.9%) corresponding to the
nominal composition of NaCo
2-x
Ag
x
O
4
were added to this
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.05.021