Available online at www.sciencedirect.com
Journal of the European Ceramic Society 33 (2013) 3065–3075
The influence of different niobium pentoxide precursors on the solid-state
synthesis of potassium sodium niobate
Jitka Hreˇ sˇ cak
a,b,∗
, Andreja Bencan
a
, Tadej Rojac
a
, Barbara Maliˇ c
a,b
a
Joˇ zef Stefan Institute, Electronic Ceramics Department, Jamova cesta 39, 1000 Ljubljana, Slovenia
b
Joˇ zef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, Slovenia
Received 10 January 2013; received in revised form 4 July 2013; accepted 8 July 2013
Available online 1 August 2013
Abstract
Two batches of K
0.5
Na
0.5
NbO
3
were prepared from the orthorhombic and monoclinic Nb
2
O
5
polymorphs and potassium and sodium carbonates.
The influence of the different Nb
2
O
5
precursors on the solid-state synthesis of K
0.5
Na
0.5
NbO
3
was studied. To reduce the particle size, both types
of Nb
2
O
5
were milled prior to use. XRD and TEM analyses showed that the milled orthorhombic Nb
2
O
5
was single phase; however, after milling
the monoclinic Nb
2
O
5
consisted of large monoclinic particles and orthorhombic nanocrystals. The latter reacted with the carbonates to form
(K
x
Na
1-x
)NbO
3
solid solutions with varying K/Na molar ratios, while the orthorhombic Nb
2
O
5
reacted to form a homogeneous solid solution
of K
0.5
Na
0.5
NbO
3
. Sintering of the two powder compacts resulted in different densification behavior and microstructure. This study shows the
important influence of the Nb
2
O
5
precursor phase and the particle size distribution on the homogeneity and further densification of the potassium
sodium niobate solid solution.
© 2013 Elsevier Ltd. All rights reserved.
Keywords: Potassium sodium niobate; Niobium pentoxide; Solid-state synthesis
1. Introduction
Solid solutions of sodium potassium niobate with the com-
position K
0.5
Na
0.5
NbO
3
have been some of the most studied
lead-free piezoelectric materials over the past few years.
1,2
A
high electromechanical coupling factor and a low dielectric per-
mittivity make potassium sodium niobate ceramics interesting
for ultrasonic applications.
1
However, despite there being many
reports on the preparation and properties of this material, prob-
lems with densification and grain growth control remain. In
addition, the reproducibility of the solid-state synthesis is still
an issue, based on different authors reporting a variety of prop-
erties for the K
0.5
Na
0.5
NbO
3
product prepared using the same
procedure.
3–5
Very limited data can be found on the actual reaction
mechanism of K
0.5
Na
0.5
NbO
3
prepared using the classic
solid-state synthesis route. Maliˇ c
6
studied the solid-state
reaction of K
0.5
Na
0.5
NbO
3
from alkaline carbonates and
∗
Corresponding author.
E-mail address: jitka.hrescak@ijs.si (J. Hreˇ sˇ cak).
niobium pentoxide by diffusion couples. They found that at
600
◦
C, K
0.5
Na
0.5
NbO
3
is formed via an intermediate phase
that best corresponds to the solid solution (K
x
Na
1-x
)
2
Nb
4
O
11
.
The reaction proceeds by the diffusion of K
+
, Na
+
and O
2-
ions through the reaction layer of the intermediate phase and
K
0.5
Na
0.5
NbO
3
toward Nb
2
O
5
. The reaction rate is determined
by the diffusion of the slower species, i.e., K
+
.
A similar mechanism was previously reported for a better-
known system, i.e., BaTiO
3
. The solid-state reaction from
BaCO
3
and TiO
2
was studied by Templeton at al.
7
They reported
that at first, a small amount of BaTiO
3
is formed on the contact of
the reagents. The subsequent reaction is diffusion controlled and
in addition to the BaTiO
3
, Ba
2
TiO
4
is produced in a prevailing
amount until all the BaCO
3
is reacted. Finally, BaTiO
3
is formed
at the expense of the Ba
2
TiO
4
intermediate product through a
reaction with the residual TiO
2
. Later, Buscaglia et al.
8
studied
the influence of the particle size distribution (PSD) of BaCO
3
on
the solid-state synthesis of the BaTiO
3
. They reported that when
nanocrystalline TiO
2
and nanocrystalline BaCO
3
were used as
precursors, a single-phase BaTiO
3
was obtained after 10 h of
calcination at 800
◦
C, while in the case of the coarse BaCO
3
annealing for 4 h at 1000
◦
C was necessary.
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jeurceramsoc.2013.07.006