Full text of DOI:10.1111/j.1151-2916.2003.tb03497.x

J. Am. Ceram. Soc., 86 [9] 1464–67 (2003)
journal
Synthesis of Bismuth Sodium Titanate Nanosized Powders by
Solution/Sol–Gel Process
Chang Yeoul Kim,† Tohru Sekino,* and Koichi Niihara*
The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
(Bi1/2Na1/2)TiO3 (BNT) is a prominent candidate for a lead-
free piezoelectric material. In this study, BNT was synthesized
using the solution/sol–gel method, in which a solution of Bi2O3
and Na2CO3 was dissolved in HNO3 as starting materials. The
solution then was mixed with ethylene glycol and titanium
tetraisopropoxide. The obtained BNT powder was analyzed
using FT-IR, DTA-TG, Raman spectroscopy, and high-
temperature XRD. Results showed that BNT crystallization
occurred above 600°C. TEM investigation showed that 100–
200 nm BNT particles were formed by heat-treating the
sol–gel-derived BNT sol at 700°C for 6 h.
Synthesis of BNT powder has been approached using a modified
sol–gel method, which is called the solution/sol–gel (SSG)
method. This study intends to show that BNT sol synthesis is
possible using bismuth oxide (Bi2O3) and sodium carbonate
(Na2CO3) solution in HNO3, titanium tetraisopropoxide, and
ethylene glycol as precursors. Use of metal oxide or carbonate
solution in acids is helpful to decrease costs of ceramic powder
synthesis using the sol–gel method. Crystallization of BNT by
heat-treating the prepared sol is studied using differential thermal
analysis and thermogravimetry (DTA-TG) and high-temperature
X-ray diffractometry (XRD) analysis. Morphology and BNT
powder size is observed using transmission electron microscopy
(TEM).
I. Introduction
EAD-FREE piezoelectric and pyroelectric ceramics have at-
tracted attention recently because PbO is volatile and detri-
II. Experimental Procedure
L
mental to human health and the environment. Bismuth sodium
titanate ((Bi1/2Na1/2)TiO3, BNT) is considered to be an excellent
candidate for lead-free piezoelectric materials: BNT is ferroelec-
tric, with remnant polarization (Pr) of 38 ␮C/cm2, Curie temper-
ature (TC) of 320°C, and coercive field (Ec) of 73 kV/cm at room
temperature.1–4 The crystal structure of BNT is rhombohedral, of
which the solid solution with tetragonal perovskite can have a
rhombohedral and tetragonal morphotropic phase boundary
(MPB).5,6 Ferroelectric ceramics with compositions approximat-
ing the MPB are known to have high piezoelectricity. Takenaka et
al.6 have reported that BNT–BaTiO3 has a piezoelectric constant
(d33) of 125 (10Ϫ12 C/N). There are reports of BNT superstruc-
ture,7,8 (Bi1/2K1/2)TiO3–(Bi1/2Na1/2)TiO3 system phase relations,9
(Bi1/2Na1/2)TiO3–PbTiO3 system,10 and BNT single crystals and
Bismuth oxide (Bi2O3, 99.9%, 1–2 ␮m, Kojundo Chemical
Laboratory Co., Ltd., Saitama, Japan) and sodium carbonate
(Na2CO3, 99%, anhydrous, Kojundo Chemical Laboratory) were
dissolved in nitric acid (HNO3, 69%–70%, Wako Pure Chemical
Industries, Ltd., Osaka, Japan). They then were added to ethylene
glycol (HOCH2CH2OH, Ն99.5%, Wako Pure Chemical Indus-
tries). The solution was evaporated at 80°C to remove water
contained in nitric acid. After the soluntion was distilled, the color
of the remaining solution was pale yellow. A stoichiometric
amount of titanium tetraisopropoxide (Ti(OCH(CH3)2)4, Ն95%,
Wako Pure Chemical Industries) then was added to the solution,
and the mixture was stirred at 70°C for 2 h. The color then became
transparent yellow. Viscosity of this clear sol was ϳ50 cP.
High-temperature XRD analysis (Scintag, Inc., Santa Clara, CA)
with CuK␣ radiation was conducted to determine whether BNT
crystal could be formed from the sol and at what temperature BNT
crystallized. BNT sol was first dried at 170°C, then put on a
platinum plate and analyzed using high-temperature XRD at room
temperature, 300°, 500°, 700°, and 900°C after holding at each
temperature for 30 min in air. DTA and TG (DTA50, Shimazu
Corp., Kyoto, Japan) were performed to determine thermodynamic
reaction of the sol; DTA and TG measurements were conducted at
10°C/min heating rate in air.
For further investigation, the obtained BNT sol was dried at
170°C and heat-treated at 500°, 600°, and 700°C for 6 h,
Fourier-transform infrared spectroscopy (FT-IR; Model WIN-
SPEC100, JEOL, Tokyo, Japan) was used to measure the heat-
treated powders. Raman spectroscopy (Model SPEX1482D, Spex
Industries, Inc., Edison, NJ) of the obtained gel and BNT powders,
which were heat-treated at 600°, 700°, and 900°C, was also
measured using an argon laser (514.5 nm). TEM investigation
(Model H-8100T, Hitachi Co., Ltd., Tokyo, Japan) was performed
at 200 kV to distinguish crystal shape and size of powders
heat-treated at 700°C for 6 h.
(Bi1/2Na1/2 (1Ϫ1.5x)
LaxTiO3.11,12
)
For aspects of practical application, BNT is considered to be a
prominent candidate for surface acoustic wave (SAW) substrates,
ultrasonic generators, ferroelectric random access memory
(FRAM), and so on. For these applications, it is necessary to
fabricate a thin-film form of BNT, although few studies have been
reported on film synthesis. The sol–gel method is thought to be
one of the prominent techniques for this purpose. Therefore,
detailed investigation on sol–gel processing and powder synthesis
of BNT might be required.
This study specifically addresses BNT nanosized powder syn-
thesis using a solution/sol–gel process. Potential advantages of
sol–gel-derived powders over conventional powders (i.e., physical
mixing of minerals and chemicals) are controlled size and shape,
molecular scale homogeneity, and lower processing temperature.
P. Davies—contributing editor
Manuscript No. 187726. Received April 25, 2001; approved March 28, 2003.
Supported in part by the Japan Society for Promotion of Science (JSPS) Joint
Research Projects under Bilateral Programs between Japan and Korea (Core Univer-
sity Program).
III. Results and Discussion
(1) Infrared Spectroscopy
Infrared spectra for dried gel and calcined powders are shown in
Fig. 1. The bending mode of H–O–H appears at 1635 cmϪ1, and
*Member, American Ceramic Society.
†Present address: Korea Institute of Ceramic Engineering and Technology, Seoul,
Korea.
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September 2003
Synthesis of Bismuth Sodium Titanate Nanosized Powders by Solution/Sol–Gel Process
1465
Fig. 3. High-temperature XRD patterns of BNT powders after drying at
170°C. Platinum peaks marked by arrows correspond to platinum specimen
holder and heating stage. HNO3 was used for dissolving Bi2O3 and
Na2CO3.
Fig. 1. FT-IR spectra of BNT powders (a) dried at 170°C and heat-treated
at (b) 500°, (c) 600°, and (d) 700°C for 6 h.
the stretching mode of O–H is at 3415 cmϪ1. The sharp and
intensive peak at 1384 cmϪ1 is due to presence of nitrate,13,14
which is added as HNO3 as a solvent for Na2CO3 and Bi2O3. Two
peaks at 2854 and 2921 cmϪ1 indicate the C–H stretching mode.
Peaks at 1076, 908, and 790 cmϪ1 are assigned to CO vibration.15
When the specimen is heat-treated at 500°C, C–H and CO peaks
disappear and the H–O–H bending mode peak diminishes. The
nitrate peak remains, although it also becomes smaller. The peak at
1429 cmϪ1 is assigned to COO vibration. Therefore, carbon
remains when the specimen is heat-treated below 600°C. When the
specimen is heat-treated at 700°C, nitrate and carbon peaks
disappear. However, water is thought to exist in the form of OH
because of remaining O–H bending and stretching peaks. Music et
al.13 have assigned 650–550 cmϪ1 to Ti–O vibration, which is
also observed in our experiment (Fig. 1). In addition, it has been
confirmed that heat-treated powder at 600°C simultaneously con-
tains yellow and black regions, which show there are two types of
powders coexisting at this temperature.
are four steps of weight degradation. There is ϳ20% weight loss
up to 230°C, ϳ64.8% weight loss to 338°C, 80.8% weight loss to
484°C, and 94% weight loss to 600°C. This implies that BNT
formation might undergo multiple stages of water and solvent
evaporation as well as organics decomposition and polycondensa-
tion, which are similar to a previous report on barium titanium
citrate thermal decomposition.16
BNT crystallization is thought to begin at ϳ600°C or higher,
because no weight loss is confirmed. However, it is difficult to
determine crystallization temperature from this thermal analysis,
because there is no DTA peak above 600°C.
(3) High-Temperature XRD Analysis
High temperature XRD profiles of dried BNT gel, which were
prepared using HNO3 for dissolving Bi2O3 and Na2CO3, are
shown in Fig. 3. No peaks are evident until 500°C; BNT crystal
peaks first appear at 700°C. Peak intensity increases with increas-
ing test temperature up to 900°C. At 700°C, there is a broad peak
around 2␪ ϭ 30°, showing the remaining amorphous phase.
Crystallization of BNT derived from the solution/sol–gel method
thus is found to start below 700°C and finish below 900°C.
(2) DTA and TG Analyses
Figure 2 shows DTA and TG curves for dried BNT precursor.
Complicated exothermic peaks and accompanied weight loss
appear between 200° and 600°C. There is no weight loss above
600°C. Exothermic peaks and weight loss are thought to be caused
by residual solvent and organics volatilization and BNT polymer-
ization. More careful analysis for TG curves indicates that there
Fig. 4. High-temperature XRD patterns of dried (170°C) BNT powder
prepared using HCl for dissolving Bi2O3 and Na2CO3 ((Œ) Bi2Ti2O7, (f)
Bi4Ti3O12, and (F) BiOCl).
Fig. 2. DTA and TG curves of BNT dried at 170°C in air at a heating rate
of 10°C/min.

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Journal of the American Ceramic Society—Kim et al.
Vol. 86, No. 9
For comparison, sintered BNT ceramic was prepared using con-
ventional powder mixing and a solid-state sintering method. The
Raman spectrum of this sintered specimen was also measured at
room temperature: A1(TO) absorption bands of rhombohedral
BNT appear at 130, 269, and 541 cmϪ1 and an E(TO) band
appears at 52.5 cmϪ1 19
In the case of the specimen heat-treated
.
below 500°C, there is no Raman band, indicating that there is no
Raman active mode. When the specimen is heat-treated at 600°C,
A1(TO) Raman peaks begin to appear. As the heat-treatment
temperature increases, Raman scattering intensity increases.
Therefore, it is thought that rhombohedral crystals begin to form at
600°C and that BNT crystallization is almost completed at 900°C.
Although there exists differences of each experimental condition,
these Raman results agree well with DTA and high-temperature
XRD investigations. Raman scattering intensity for the powders
calcined at 900°C is almost the same as that of the sintered BNT
specimen that is prepared using a conventional solid-state reaction
sintering method.
Fig. 5. Raman spectra of BNT powders that were heat-treated at 600°,
700°, and 900°C for 6 h. Spectrum of the BNT ceramic body that was
sintered at 1150°C for 4 h using solid-state reaction and sintering method
is also indicated for comparison.
(5) TEM Observation of BNT Powder
TEM images and a corresponding electron diffraction pattern
for calcined powder at 700°C are shown in Fig. 6. Figure 6 shows
that BNT crystal size is on the nanometer scale (ϳ100–200 nm)
with a polygonal shape when the specimen is heat-treated at 700°C
for 6 h. The electron diffraction pattern (Fig. 6(b)) shows a spotty
ring pattern, indicating that the powder is crystallized. Figure 6(c)
shows the BNT crystal surface is covered with small particles and
amorphous phase.
When BNT synthesis was conducted by dissolving Bi2O3 and
Na2CO3 in HCl, mixing with ethylene glycol, and adding titanium
tetraisopropoxide, BNT crystallization was not confirmed, as
shown in Fig. 4. Figure 4 shows that BiOCl is first formed at
300°C, Bi2Ti2O7 appears at 500°C, and Bi4Ti3O12 begins to
appear at 700°C. Prasada Rao et al.17 have reported Bi4Ti3O12
synthesis from a coprecipitation route using TiCl4 and Bi(NO3)3;
however, Bi4Ti3O12 obtained from TiCl4 in their study shows
contamination of BiOCl, which subsequently leads to formation of
Bi2Ti4O11. Mao et al.18 have reported that they have succeeded in
solution/sol–gel synthesis of BiPbSrCaCuO powder by dissolution
of metal oxides (Bi2O3, PbO, and CuO) and carbonates (SrCO3
and CaCO3) in HNO3. Therefore, the type of acid used as a solvent
of oxide and carbonate is very important when their solutions are
used as precursors for sol–gel synthesis. In the present investiga-
tion for BNT synthesis, we therefore infer that dissolution of Bi2O3
and Na2CO3 in HNO3 enables a yield of an appropriate BNT
single-phase compound, because HNO3 forms no salt that affects
formation of impure crystals, such as BiOCl, in the products.
IV. Conclusions
In this study, solutions of Bi2O3 and Na2CO3 in HNO3 are used
as precursors for BNT sol–gel synthesis. Nanosized BNT powders
(100–200 nm) are first synthesized using the sol–gel method.
Sol–gel synthesis of BNT is possible without using high-cost
alkoxide. Results of high-temperature XRD, DTA and TG analy-
ses, and Raman spectroscopy indicate that BNT crystallization
occurs above 600°C. Moreover, DTA-TG and IR spectroscopy
show that BNT crystals seem to be synthesized in multiple stages
of solvent evaporation, organics volatilization, and condensation.
TEM photography shows that 100–200 nm sized crystals of BNT
are synthesized using the present sol–gel method when heat
treatment at 700°C for 6 h is used. This method is considered to be
useful to prepare a lead-free piezoelectric BNT material. This
result implies that the present sol–gel process has advantages for
(4) Raman Spectroscopy
Raman spectroscopy of BNT powders heat-treated at 600°,
700°, and 900°C was measured at room temperature (see Fig. 5).
Fig. 6. (a) TEM photograph and (b) corresponding selected-area electron diffraction pattern for BNT powder that was heat-treated at 700°C for 6 h. (c) BNT
crystal surface is covered with fine particles and amorphous phase.

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Synthesis of Bismuth Sodium Titanate Nanosized Powders by Solution/Sol–Gel Process
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The authors thank Dr. Doh-Hyung Riu of Korean Institute of Ceramic Engineering
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