Materials Chemistry and Physics 99 (2006) 329–332
Lead-free (K0.5Bi0.5)TiO3 powders and ceramics
prepared by a sol–gel method
Mankang Zhu, Lei Hou, Yudong Hou ∗ , Jingbing Liu, Hao Wang, Hui Yan
The Key Laboratory of Advanced Functional Materials of China Education Ministry, Beijing University of Technology, Beijing 100022, China
Received 23 February 2005; received in revised form 2 September 2005; accepted 31 October 2005
Abstract
A lead-free ferroelectric material, (K0.5 Bi0.5 )TiO3 (KBT), was prepared by a sol–gel process. Thermal analysis and X-ray diffraction showed
that perovskite KBT powders with a grain size of 100–200 nm were obtained by calcining dried gels above 700 ◦ C. When the calcining temperature
was lower than 650 ◦ C, only the Bi2 Ti2 O7 phase was found in the X-ray diffraction patterns. Moreover, KBT ferroelectric ceramics was fabricated
by traditional sintering of as-prepared KBT powders. The KBT ceramics sintered at 1050 ◦ C showed a high density (91.2% of theoretic density)
and low dielectric loss, better than that prepared by a solid-state reaction method. The temperature dependence of dielectric permittivity of KBT
ceramics showed a Curie temperature at 385 ◦ C.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Ferroelectric materials; Phase transformation; Sol–gel method
1. Introduction
Recently, lead-free piezoelectric ceramic materials have
attracted attention from the viewpoint of environmental protection. The increasing demand for environmentally benign
materials points in the direction of lead-free ceramic materials for different electronic applications, such as ceramic filters and resonators. Among them, (K0.5 Bi0.5 )TiO3 (KBT) is a
typical lead-free ferroelectric material of perovskite structure
[1–3]. It has a Curie temperature TC of 380 ◦ C higher than that
of (Na0.5 Bi0.5 )TiO3 , which is beneficial for the application as
piezoelectric filters, resonators and micro-electro-mechanical
systems. However, research on KBT has rarely been reported
possibly because it is difficult to fabricate high-density KBT
ceramics due to the serious volatilization of the K and Bi components at sintering temperatures [4,5].
KBT was synthesized by a mixed-oxide reaction method
[6–9]. However, the powders prepared by this method usually
feature a high agglomeration and an inhomogeneous particle
size as a result of the high-temperature treatment. In comparison
with other techniques, a sol–gel process has shown considerable
advantages, including excellent chemical stoichiometry, compo-
∗
Corresponding author. Tel.: +86 10 6739 2733; fax: +86 10 6739 2412.
E-mail address: ydhou@bjut.edu.cn (Y. Hou).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2005.10.031
sitional homogeneity, and lower crystallization temperature due
to the mixing of liquid precursors on the molecular level [10,11].
In the present work, the sol–gel method was used to prepare KBT
ferroelectric powders of perovskite structure. Through analysis
of the thermal behaviour and phase evolution during calcination
of dried gels, pure perovskite KBT powders were fabricated
at 700–900 ◦ C. KBT ceramic samples were fabricated and its
dielectric performances were investigated.
2. Experimental details
Reagent grade bismuth nitrate pentahydrate (Bi(NO3 )2 ·5H2 O), potassium
nitrate (KNO3 ), tetrabutyl titanate (Ti(OC4 H9 )4 ), acetic acid (CH3 COOH), and
ethanol (CH3 CH2 OH) were adopted as the raw materials. In the experiment,
bismuth nitrate pentahydrate was dissolved in acetic acid, and potassium nitrate
dissolved in CO2 -free distilled water, respectively. The two solutions were mixed
and then introduced into a prepared ethanol solution of tetrabutyl titanate with a
stoichiometric amount. After stirring vigorously for 2 h, a slight yellow sol was
formed. Then, the sol was heated to 70 ◦ C for 12 h to prepare dried gels. Finally,
white powders were obtained by calcining the dried gels for 2 h at 700–900 ◦ C.
The obtained white powders were pulverized, and pressed into pellets with
a diameter of 11.5 mm under a uniaxial stress of 10 MPa, adding 0.5 wt.%
polyvinyl alcohol (PVA) as binder. Conventional sintering was performed at
a temperature interval of 25 ◦ C between 1000 and 1100 ◦ C for 2 h in a sealed
alumina crucible. The sintered specimens were then lapped and electroded with
a silver paste for the dielectric measurements.
The thermal behaviour of the dried gels were investigated by differential
scanning calorimetry (DSC) and thermogravimetry (TG) using a Netzsch 439C
thermoanalyzer in ambient condition at a heating rate of 10 K min−1 . The X-ray
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M. Zhu et al. / Materials Chemistry and Physics 99 (2006) 329–332
diffraction (XRD) analysis was performed in the θ–2θ mode using an X-ray
diffractometer (Bruker D8 Advance) with Cu K␣ radiation. The Raman spectra
were recorded from a Spex 1403 Raman spectrometer in a backscattering
geometry with 488 nm radiation of an Ar+ laser and 100 mW output power. The
dielectric measurements were carried out on an automated system consisting
of a temperature control unit and a precision LCR meter (Agilent 4284A).
3. Results and discussion
Thermal analysis was carried out to investigate the decomposition and phase transformation of the KBT dried gels. Fig. 1
shows the TG–DSC curves of the dried gels in the range
50–900 ◦ C. The TG data show a stepwise weight loss in two primary stages. The first stage appeared in the range 200–300 ◦ C,
causing a weight loss of 64%. Correspondingly in DSC curve,
exothermal peaks at 200–300 ◦ C were observed, which can be
attributed the evaporation of the solvents and the decomposition
of metal organics [12,13]. The second stage appeared in a broad
range from 400 to 700 ◦ C, leading to a weight loss of 24%. This
weight loss may result from the oxidation of the residual organic
groups. However, some weak endothermic and exothermic peaks
appeared in this temperature region, which may be attributed to
the low heat change of the crystallization of Bi2 Ti2 O7 and KBT
as observed in the XRD patterns.
XRD analysis was carried out to study the phase evolution
of the dried gels in the calcinations process. Fig. 2 shows the
XRD patterns of dried gels calcined at 400, 500, 600, 650,
680, 700, 800, and 900 ◦ C, respectively. As can be seen in the
figure, an amorphous phase was formed at sintering temperatures lower than 400 ◦ C. When the temperature arised to 500 ◦ C,
some diffraction peaks appeared. All of them can be assigned to
Bi2 Ti2 O7 structure as reported in the JCPDS File No. 32-0118
[14,15]. When the temperature increased to 600 and 650 ◦ C, the
height of the diffraction peaks belonging to Bi2 Ti2 O7 increased,
while the full width at half maximum decreased due to crystal
growth and the improvement of crystallinity. It means that the
powders calcined in the temperature region 500–650 ◦ C were
cubic pyrochlore Bi2 Ti2 O7 . However, for the powders heated at
Fig. 1. TG and DTA curves of the KBT precursor gel dried at about 70 ◦ C.
Fig. 2. XRD data of the dried gel heated at different temperatures.
680 ◦ C, obvious change was observed in the XRD patterns. The
characteristic peaks of perovskite KBT crystals (JCPDS File No.
36-0339) appeared at 22.56◦ , 31.96◦ , and 39.44◦ , respectively.
After further increasing of the temperature to 700 ◦ C and above,
only diffractions belonging to perovskite KBT was observed and
there was no evidence of a second phase. Besides, the full width
at half maximum of the (1 0 1) diffraction peak of KBT powders
changed from 0.94◦ at 700 ◦ C to 0.75◦ at 800 ◦ C and to 0.61◦
at 900 ◦ C, respectively. It means that the grain size increased
from 100 to 150 nm according to Scherrer’s equation, indicating
nanoscale dimensions of the powders.
The Raman spectra of powders calcined at 600 and 700 ◦ C
are shown in Fig. 3. The Raman spectra of powders calcined at
700 ◦ C show three scattering bands. Through fitting by Gaussian function, nine peaks at 145, 204, 265, 326, 433, 536, 634,
740, and 845 cm−1 , respectively, could be separated out. The
results are in good agreement with perovskite KBT as reported
by Jones et al. [16] and Kreisel et al. [17]. However, the Raman
spectra of powders calcined at 600 ◦ C showed a different pattern.
There appeared peaks at 152, 226, 281, 326, 418, 562, 622, and
Fig. 3. Raman spectra of the powder heated at 700 ◦ C.
M. Zhu et al. / Materials Chemistry and Physics 99 (2006) 329–332
331
Table 1
Dielectric constant εr and dielectric loss tan δ of the KBT ceramics sintered at
1000–1100 ◦ C
Sintering temperature (◦ C)
εr
tan δ
Relative density (%)
1000
1025
1050
1075
1100
1652
856
690
570
536
1.020
0.678
0.052
0.132
0.360
81.2
85.0
91.2
88.5
86.7
726 cm−1 , respectively. As reported by Kojima et al. [18] and
Wang et al. [19], there is only one mode in the 600–800 cm−1
range for Bi2 Ti2 O7 . So the pattern of powders calcined at 600 ◦ C
present characteristic of cubic Bi2 Ti2 O7 . It affirmed that the
powder obtained by calcining the dried gels at 700 ◦ C is a perovskite KBT phase.
KBT ceramics was fabricated using the powders heat-treated
at 700 ◦ C by a traditional sintering process. The dielectric constant, εr , and dielectric loss, tan δ, of the KBT ceramics sintered
at 1000–1100 ◦ C were measured (Table 1). The KBT ceramics
sintered at 1000 ◦ C shows a εr value of 1652. However, due to
the high value of tan δ (1.02), the large εr value is not reliable.
As the sintering temperature increased to 1050 ◦ C, the εr value
of KBT ceramics was about 690, which is in agreement with the
value of 682 of material prepared by a conventional solid-state
reaction method [8]. Meanwhile, tan δ of KBT ceramics sintered
at 1050 ◦ C was significantly lowered (0.052), only 10% of the
tan δ value (0.490) for the samples prepared by a conventional
solid-state reaction method [8]. The temperature dependence of
εr of the ceramics sintered at 1150 ◦ C was measured at a frequency of 10 kHz (Fig. 4). It shows that the Curie temperature
(385 ◦ C) is very close to that of reported data [1–3]. The superior dielectric properties of the KBT ceramics prepared by the
sol–gel method can be attributed to the higher relative density
of 91.2%, compared to the low density (70%) of a ceramics
prepared by conventional solid-state reaction method. However,
increasing the sintering temperature above 1075 ◦ C decreased
Fig. 5. XRD data of the KBT ceramics sintered at different temperatures.
the density of the ceramics and increased the dielectric loss due
to the volatilization of the K and Bi components. Fig. 5 shows
the XRD patterns of KBT ceramics sintered at different temperatures. It can be seen that, when the sintering temperature is below
1075 ◦ C, only diffractions corresponding to perovskite KBT
are observed; however, diffraction peaks belong to K4 Ti3 O8
(JCPDS File No. 41-0167) appeared when the sample was sintered at 1100 ◦ C. The appearance of a second phase may result
from the high volatilization of the bismuth element. Thus, the
dielectric properties were seriously deteriorated.
4. Conclusions
KBT powders and ceramics have been prepared by a sol–gel
technique. The gels converted to pure perovskite phase of KBT
at 700 ◦ C from pyrochlore Bi2 Ti2 O7 phase at lower temperatures. The obtained KBT powders had a smaller size and even
shape than those prepared by a traditional solid-state reaction
method. Besides, high-density KBT ceramics can be prepared
from the as-prepared KBT powders, and ceramic samples with
a relative density of more than 90% were prepared at sintering
temperature of 1050 ◦ C. It means that the powders prepared by
the sol–gel method can be sintered at a lower temperature, which
is beneficial to controlling the volatilization of the K and Bi components. The dielectric properties of the powders are better than
those of powders prepared by a conventional solid-state reaction
method.
Acknowledgement
The authors are grateful to the Science & Technology Development Project of the Beijing Education Committee for financial
support.
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Fig. 4. Dielectric constant with temperature at various frequencies for the KBT
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