Fabrication of Na0.5Bi0.5TiO3-BaTiO 3-textured ceramics templated by plate-like Na0.5Bi 0.5TiO3 particles

Article abstract of DOI:10.1111/j.1551-2916.2009.03043.x

Textured 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO ₃ (NBT-6BT) ceramics were fabricated by templated grain growth (TGG) using anisotropically shaped Na_0.5Bi_0.5TiO₃ (NBT) templates. Platelet N

Full text of DOI:10.1111/j.1551-2916.2009.03043.x

J. Am. Ceram. Soc., 92 [7] 1607–1609 (2009)
DOI: 10.1111/j.1551-2916.2009.03043.x
r 2009 The American Ceramic Society
ournal
J
Fabrication of Na Bi TiO –BaTiO -Textured Ceramics Templated
0.5 0.5
3
3
by Plate-Like Na Bi TiO Particles
0.5 0.5
3
w
Wei Zhao, Jing Ya, Ying Xin, Lie E and Dan Zhao
Department of Materials Science and Engineering, Tianjin Institute of Urban Construction, Tianjin 300384, China
Heping Zhou
State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China
Textured 0.94Na0.5Bi0.5TiO –0.06BaTiO (NBT–6BT) ceram-
3
limit the improvement of textured NBT-based lead-free piezo-
electric ceramics.
Recently, material researchers are attracted by soft chemical
3
ics were fabricated by templated grain growth (TGG) using an-
isotropically shaped Na0.5Bi0.5TiO (NBT) templates. Platelet
3
6
,7
NBT was synthesized by the topochemical technique, using pre-
cursor Na0.5Bi4.5Ti O (NBIT). The NBT particles have an
methods. The molten-salt method and topochemical micro-
crystal conversion method offer potentially powerful tools for
controlling thermodynamically inaccessible structures and mor-
phological features at the kinetic level. Topochemical synthesis
involves replacing or modifying the interlayer cations but re-
taining the morphological and structural features of plate-like,
layered perovskite precursors by ion exchange and intercalation
4
15
average length of 10–15 lm and a thickness of 1 lm, which are
suitable templates for obtaining textured ceramics (especially
NBT-based ceramics) by the TGG process. This study revealed
that the NBT templates are effective in inducing grain orienta-
tion in NBT–6BT ceramics. For NBT–6BT ceramics textured
with 5 vol% NBT templates, a Lotgering factor of 0.87 and a
d33 of 299 pC/N are given.
8
reactions at low temperatures. Saito et al. introduced a method
to synthesize platelet NaNbO templates with a perovskite struc-
3
ture by topochemical conversion. Our group used a similar
method to successfully synthesize plated BaTiO last year. Ap-
plying this method, many more aimed products with desired
shapes and structures can be possibly designed and synthesized.
Unlike perovskite NBT, bismuth layer-structured ferroelec-
9
3
I. Introduction
ECENT concern for the environment requires the develop-
ment of lead-free piezoelectric materials. Na0.5Bi0.5TiO3
NBT) is one of the candidates for lead-free piezoelectric ceram-
R
ics, but the piezoelectric properties of NBT-based ceramics are
(
4
trics, such as Na0.5Bi4.5Ti O15 (NBIT), can be easily obtained as
large plate-like particles by the molten-salt process. Because the
structures of NBIT and NBT are similar, the layer-structured
NBIT can be transformed into a perovskite NBT in the topo-
1
not large enough for practical applications. There are two ap-
proaches for enhancing the electrical properties of polycrystal-
line ceramics. One is to search for new compositions and the
other is to control the microstructure of the ceramics with a
known composition. Texture control of polycrystals is an im-
portant approach for enhancing piezoelectric properties without
drastically changing the composition of the ceramics. Among
various methods (such as hot pressing, hot forging, and mag-
1
0
chemical reaction. Professor Zeng et al. have been synthesized
the NBT platelets by two steps, while the desired morphology of
the final product could not be precisely controlled. In the present
work, NBT templates were synthesized from plate-like NBIT
particles by the topochemical method, and textured (Bi0.5
3
Na0.5)0.94Ba0.06TiO (BNT–6BT) ceramics using NBT templates
2
netic orientation ), templated grain growth (TGG) is one of the
were fabricated by the tape casting method. The d33 of the syn-
thesis of NBT–6BT textured ceramics reaches 299 pC/N, which
is 2.4 times that of conventional ceramics.
most convenient processes, in which textured ceramics are pre-
pared by the sintering of a green compact matrix, where particles
with an anisotropic shape are aligned.
3
Anisotropic particles or templates play a crucial role in the
grain orientation process. In a previous research, plate-like
II. Experimental Procedure
3 4 3
SrTiO and Bi Ti O12 have been used to texture NBT-based
Plate-like NBIT particles were synthesized by the molten-salt
materials by TGG and reactive template grain growth (RTGG);
however, the piezoelectric properties are not improved notably
because of the different composition between the matrix and the
method using platelet Bi
4
Ti
et al. ), then NBIT platelets and Na
Na CO /NBIT molar ratio of 3:4 and mixed with a magnetic stir
3
O
12 (synthesized according to Zhao
11
2
CO were weighed in a
3
2
3
4
,5
templates. In principle, plate-like NBT particles are the most
suitable templates for fabricating textured NBT-based ceramics.
The resulting textured ceramics would have a high degree of
grain orientation and the depolarization temperature would not
be decreased. However, NBT has a perovskite structure with
high symmetry, so it is difficult to obtain large anisotropic NBT
particles by conventional methods, such as hydrothermal or
solid-state reaction processes. The difficulties mentioned above
bar in an ethanol medium, and an equal weight of NaCl salt was
added. The slurry was dried and subsequently heated at 9501C for
4
h. NBT seeds were obtained that retained the morphology of the
NBIT precursor. These were separated from the salts by washing
with hot deionized water. The by-product Bi was removed by
using HCl. The corresponding chemical reaction is as follows:
2 3
O
4
Na0:5Bi4:5Ti4O15 þ 3Na2CO3 !
6Na0:5Bi0:5TiO3 þ 5Bi2O3 þ 3CO2
(1)
1
G. Messing—contributing editor
The synthesized NBT templates were used to fabricate tex-
tured NBT–6BT ceramics using the TGG method. Analysis-
grade metal oxides and carbonate powders of Bi O , Na CO ,
Manuscript No. 25692. Received December 29, 2008; approved February 17, 2009.
Author to whom correspondence should be addressed. e-mail: zhaowei04@gmail.com
2
3
2
3
w
BaTiO , and TiO were first mixed and calcined at 8001C to
3 2
1
607

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Communications of the American Ceramic Society
Vol. 92, No. 7
form a perovskite structure. The calcined powders were then
mixed thoroughly with the same weight solvents (azeotropic
mixture of ethanol and butanone) and dispersant castor oil with
3
wt% of the powders by ball milling for 12 h using zirconia
balls, and the plasticizers dibutyl phthalate (DBP, 2 wt% of the
powders) and polyethylene glycol (PEG, 2 wt% of the powders)
and the binder polyvinyl butyral (PVB, 8 wt% of the powders)
were added and mixed for another 12 h. The plate-like NBT
3
template particles with a high aspect ratio and BaTiO fillers
were added and mixed into the slurry obtained above with a stir
bar. The slurry was tape-cast with a blade opening of 150 mm.
The viscosity of the slurry was about 300 mPa ꢀ s. The tapes ob-
tained were dried, cut, and laminated into a multilayer sheet
approximately 2 mm thick under a pressure of 40 MPa at 801C
for 0.5 h. The laminated green compacts blanked into discs with
a diameter of 12 mm. The green samples obtained were heated in
air at 6001C for 15 h to remove organic additives, then the sam-
ples were pre-reacted at 8001C for 2 h and sintered at 12001C for
Fig. 2. X-ray diffraction patterns of NBT-6BT ceramics. (a) Non-
textured, (b) textured.
2
–10 h to get the final product.
The crystalline phases and the degree of textured develop-
by the Lotgering factor (f). The f value of the NBT–6BT tex-
tured ceramics is 0.87, indicating a high degree of grain orien-
tation. This also suggests that the NBT templates were efficient
in inducing grain orientation in the NBT–6BT ceramics, and
because the composition of NBT templates is so similar to
NBT–6BT, only a few templates (5 vol% in this work) will
lead to a high degree of texture, and the textured ceramics will be
well-sintered and much denser than those composed of much
templates. As shown in Fig. 3, the NBT–6BT textured ceramics
are mostly oriented by the NBT templates, and the large
grains grow at the expense of the finer matrix grains. Some re-
searchers refer to this process as a homoeptaxial process.
r
Figure 4 shows the variation of the dielectric constant e and
dissipation factor tan d of the unpoled textured and non-
textured NBT–6BT ceramics sintered at 12001C. A frequency-
dependent behavior is observed with a hump between room
ment were determined by X-Ray Diffraction (Model Rigaku
500, Tokyo, Japan) and evaluated using the Lotgering factor
f). The microstructures of the sintered samples were observed
2
(
by a scanning electron microscope (SEM Model JMS 6460LV,
JEOL, Tokyo, Japan). Silver paste was fired on both faces of
the discs at 8001C as electrodes. The samples for the measure-
ment of the piezoelectric properties were poled in silicone oil at
room temperature under 4 kV/mm for 15 min. The piezoelectric
constant d33 was measured by a ZJ-3A (Beijing, China) quasi-
static d33 meter. The temperature dependence of the dielectric
constant and the dissipation factor of the unpoled samples
were measured between 251 and 4501C using an impedance
analyzer (HP-4192A). The planar-mode and thickness mode
12
p t
electromechanical coupling coefficients (k and k , respectively)
were determined by the resonance method according to the
IEEE Standard 176, using an impedance analyzer (HP 4194A).
temperature and B1401C (T ) and a diffusion of the relative
d
dielectric constant at higher than B2801C (T ), which
m
d
demonstrates the relaxor characteristic. Here, T refers to the
temperature of the transition between the ferroelectric phase and
anti-ferroelectric phase, which is also called the depolarization
III. Results and Discussion
Figure 1(a) is the SEM image of NBIT, which shows a micro-
structure of plate-like morphology with a length of about 10–15
mm and a thickness of 1 mm. NBIT with a high aspect ratio and
regular shape was a suitable precursor for the next step of syn-
thesis. Because the topochemical reaction involves only the
changes in chemical composition and structure, the plate-like
morphology of the NBIT templates can be retained in the NBT
templates. Figure 1(b) displays the SEM micrographs of the
NBT templates prepared from the NBIT particles by the topo-
chemical reaction at 9501C for 4 h. Similar to NBIT particles,
most of the BNT templates are large and of plate-like shape,
with an average size of 15 mm and a thickness of 1 mm.
temperature, and T
to the maximum value of the dielectric constant, which is also
m
stands for the temperature corresponding
1
3
called the Curie temperature. The textured materials exhibit
similar temperature dependences of e compared with the ran-
dom ones, but with a smaller T
temperature, the dissipation factor tan d increases more than in
the non-textured ceramics. With the BaTiO fillers in the NBT–
r
d m
and higher T and at a higher
3
6
BT powders, the textured composition is the same as the non-
textured one, so the NBT–6BT textured ceramics still maintain a
similar relaxivity. As is accepted for the dissipation factor, the
maximum dielectric constant and transition temperature will be
suppressed for the textured ceramic relative to the random ma-
terials due to the porosity and internal stresses. Here, the relative
The XRD patterns of the textured and nontextured NBT–
6
BT ceramics are shown in Fig. 2. Both the ceramics exhibit a
single-phase perovskite structure. Because the nontextured ce-
ramic is composed of grains with random orientation, it exhibits
a strong (110) diffraction peak. For the textured ceramic, the
(200) diffraction peak becomes the strongest, indicating that a
large fraction of grains is aligned with their a-axis normal to the
sample surface. The degree of grain orientation can be estimated
Fig. 1. Microstructure of the plate-like particles (a) NBIT and
(
Fig. 3. Microstructure of NBT-6BT ceramics templated by
NBT platelets.
b) NBT.

July 2009
Communications of the American Ceramic Society
1609
Table I. Piezoelectric and Dielectric Properties of the
Textured and Non-Textured NBT–6BT Ceramics
d
33
P
r
E
c
Tan d
T
c
2
(
pC/N)
k
p
k
t
(mC/cm ) (kV/cm) e (251C) (251C) (1C)
Nontextured 128 0.24 0.39 39.6 30.0 1834 4.8% 280
Textured 299 0.34 0.54 34.1 24.2 1828 5.2% 294
should be noted that the observed d33 for the textured ceramic is
99 pC/N, which is indeed very high as compared with the other
2
NBT-based ceramics. With the development of the homoepit-
axial TGG process, NBT-based ceramics will partially replace
commercial lead piezoelectrics in the near future.
IV. Conclusion
Fig. 4. Temperature dependence of the dielectric constant and the dis-
sipation factor of NBT-6BT ceramics.
The NBT templates were synthesized from NBIT particles by
the topochemical reaction. These were efficient in inducing grain
orientation in NBT–6BT ceramics. Textured NBT–6BT ceram-
ics have been fabricated by the TGG method using an aniso-
tropically shaped NBT template. The resulting ceramics were
dense and sintered well, in which a considerable amount of
grains were oriented by the NBT templates. The Lotgering fac-
tor of the textured ceramics is 0.87. After texturing, the piezo-
electric properties were improved while the depolarization
temperature decreased slightly. For the NBT–6BT ceramic tex-
tured with 5 vol% NBT templates, the d33 reaches 299 pC/N,
density of textured NBT–6BT is 0.94. This is a common prob-
lem in the TGG process, because it is difficult for a compact with
strong anisotropy and a large difference in grain size to obtain a
high density of the product. In the present work, the Tm of tex-
tured NBT–6BT ceramics is a little higher than those prepared
by the conditional technique.
P–E hysteresis loops of ceramics were recorded at room tem-
perature and are shown in Fig. 5. After texturing, the remanent
and the values of k
p
t
and k are 0.34 and 0.54, respectively.
r c
polarization P and E decreased from 39.6, 30.0 to 34.1, 24.2,
respectively, which may be caused by the crystal-like character-
istics of the textured ceramic. It is known that the coercive field
of a single crystal is usually lower than that of a ceramic with the
same composition.
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t
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Fig. 5. P–E hysteresis loops of the textured and nontextured
NBT-6BT ceramics.
&