High performance Bi
0.5
Na
0.5
TiO
3
-BiAlO
3
-K
0.5
Na
0.5
NbO
3
lead-free pyroelectric
ceramics for thermal detectors
Zhen Liu, Weijun Ren, Ping Peng, Shaobo Guo, Teng Lu, Yun Liu, Xianlin Dong, and Genshui Wang
Citation: Appl. Phys. Lett. 112, 142903 (2018); doi: 10.1063/1.5020424
View online: https://doi.org/10.1063/1.5020424
View Table of Contents: http://aip.scitation.org/toc/apl/112/14
Published by the American Institute of Physics
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High performance Bi
0.5
Na
0.5
TiO
3
-BiAlO
3
-K
0.5
Na
0.5
NbO
3
lead-free pyroelectric
ceramics for thermal detectors
Zhen Liu,
1,2,a)
Weijun Ren,
2
Ping Peng,
2
Shaobo Guo,
2
Teng Lu,
1
Yun Liu,
1,a)
Xianlin Dong,
2
and Genshui Wang
2,a)
1
Research School of Chemistry, The Australian National University, Canberra, Australian Capital
Territory 2601, Australia
2
Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China
(Received 22 December 2017; accepted 25 March 2018; published online 3 April 2018)
Both high pyroelectric properties and good temperature stability of ferroelectric materials
are desirable when used for applications in infrared thermal detectors. In this wo rk, we report lead-
free ternary 0.97(0.99Bi
0.5
Na
0.5
TiO
3
-0.01BiAlO
3
)-0.03K
0.5
Na
0.5
NbO
3
(BNT-BA-KNN) ceramics,
which not only exhibi ts a large pyroelectric coefficient (p 3.7 10
8
Ccm
2
K
1
) and figures of
merit (F
i
, F
v
, and F
d
) but also shows excellent thermal stable properties. At room temperature, F
i
,
F
v
, and F
d
are determined as high as 1.32 10
10
m/V, 2.89 10
2
m
2
/C, and 1.15 10
5
Pa
1/2
at 1 kHz and 1.32 10
10
m/V, 2.70 10
2
m
2
/C, and 1.09 10
5
Pa
1/2
at 20 Hz, respectively.
During the temperature range of RT to 85
C, the achieved p, F
i
, F
v
, and F
d
do not vary too much.
The high depolarization temperature and the undispersed ferroelectric-ergodic relaxor phase transi-
tion with a sharp pyroelectric coefficient peak value of 400 10
8
Ccm
2
K
1
are suggested
to be responsible for this thermal stability, which ensures reliable actual operation. The results
reveal the BNT-BA-KNN ceramics as promising lead-free candidates for infrared thermal detector
applications. Published by AIP Publishing. https://doi.org/10.1063/1.5020424
Pyroelectric materials have been of strong interest for
the development of thermal based sensors, accelerators, and
energy harvesters.
1–5
The utilization of pyroelectric materials
for uncooled infrared thermal detectors has recently gained a
rapid growth of investigations since they promise a broad
wavelength response, high sensitivity, low cost, and elimina-
tion of cooling systems. It is general to evaluate pyroelectric
ceramics though the figures of merit (FOMs):
5
F
i
¼
p
c
v
,
F
v
¼
p
c
v
e
0
e
r
, and F
d
¼
p
c
v
ffiffiffiffiffiffiffiffiffiffiffiffiffi
e
0
e
r
tan r
p
, where c
v
is the volume heat
capacity and F
i
, F
v
, and F
d
represent the current responsivity,
voltage responsivity, and detectivity, respectively. During
the last few decades, lead-based ferroelectric ceramics such
as Pb(Zr,Ti)O
3
(PZT),
6–8
Pb(Zr,Sn,Ti)O
3
(PZST),
9
Pb(Sc,Ta)O
3
(PST),
10–12
and Pb(Mg,Nb)O
3
-PbTiO
3
(PMN-
PT)
13
were widely investigated for infrared detection due to
their superior pyroelectric properties. However, environmen-
tal concerns and global restrictions present high demands to
eliminate the pollution of lead-containing compounds and
develop high-performance lead-free substitutions. By far,
many investigations have been carried out on lead-free ferro-
electric ceramics for possible pyroelectric based applica-
tions. For example, the enhanced pyroelectric coefficient
(p) of 1.24 10
8
Ccm
2
K
1
and the figure of merit of
detectivity (F
d
) of 0.61 10
5
Pa
1/2
were realized on
Ca
0.2
Sr
0.1
Ba
0.7
Nb
2
O
6
ceramics by Chen et al.
14
Moreover,
Jiang et al.
15
studied the pyroelectric properties of Mn-doped
0.97K
0.5
Na
0.5
NbO
3
-0.03(Bi
0.5
K
0.5
)TiO
3
ceramics and
achieved p and F
d
values of 2.18 10
8
Ccm
2
K
1
and
0.571 10
5
Pa
1/2
, respectively. Srikanth et al.
16
also
reported 0.6(Ba
0.9
Ca
0.1
)TiO
3
-0.4Ba(Sn
0.2
Ti
0.8
)O
3
pyroelec-
tric ceramics with an optimized p value of 2.05 10
8
C
cm
2
K
1
and an F
d
value of 0.41 10
5
Pa
1/2
. Despite
that many progresses have been reported in lead-free pyro-
electric materials, they still exhibit far inferiority to these
lead-based materials.
Sodium bismuth titanate (Bi
0.5
Na
0.5
TiO
3
, BNT)
ceramics exhibit large remanent polarization (P
r
¼38 lC/
cm
2
) and high Curie temperature (T
C
¼320
C), exhibiting
favorable characteristics for pyroelectric applications.
However, its large leakage current and high coercive electric
field (7.3 kV/mm) make them difficult to be poled.
Forming BNT based solid solutions with other perovskites
such as BaTiO
3
, BiAlO
3
, and (Bi,K)TiO
3
not only improves
the poling effectiveness but also offers great opportunities
for tuning their ferroelectric properties.
17–20
To fit for real
pyroelectric applications, the BNT family must satisfy two
aspects of requirements. One is high pyroelectric perfor-
mance at room temperature, including pyroelectric coeffi-
cients and various figures of merit. On the other hand, the
good temperature stability of p and FOMs is also inevitable
to ensure their reliable operation. Recently, Sun et al.
21
reported that Mn doped 0.946Bi
0.5
Na
0.5
TiO
3
-0.054BaTiO
3
(BNT-BT) single crystals with the (111) orientation possess
high pyroelectric coefficients and large figures of merit.
The value of p can reach as high as 5.88 10
8
Ccm
2
K
1
and does not increase too much with temperature varying
from 20
Cto85
C. However, for their ceramics counter-
parts, although a large p value of 5.7 10
8
Ccm
2
K
1
was realized by Guo et al.
22
on 0.93Bi
0.5
Na
0.5
TiO
3
-
0.07Ba(Zr
0.055
Ti
0.945
)O
3
(BNT-BZT) pyroelectric ceramics
near the morphotropic phase boundary (MPB), the value of p
a)
Authors to whom correspondence should be addressed: zhenliumse@163.
com; yun.liu@anu.edu.au; and genshuiwang@mail.sic.ac.cn.
0003-6951/2018/112(14)/142903/5/$30.00 Published by AIP Publishing.112, 142903-1
APPLIED PHYSICS LETTERS 112, 142903 (2018)
soared to 20.6 10
8
Ccm
2
K
1
with temperature increas-
ing to 50
C. Quite recently, Balakt et al.
23
obtained even
higher p of 7.42 10
8
Ccm
2
K
1
at room temperature
on La doped 0.94Bi
0.5
Na
0.5
TiO
3
-0.06BaTiO
3
ceramics.
However, the great enhancement of p was realized at the
expense of bringing the depolarization temperature (T
d
)
down to near room temperature, which will also result in the
temperature instability. Moreover, the ceramics will experi-
ence polarization loss during the heat-involved manufacture
procedures, thus affecting their reusability, which limits their
applicability as well. Therefore, BNT based ceramics pos-
sessing both high pyroelectric properties and favorable ther-
mal stability necessarily deserve further research.
In present work, a potential lead-free pyroelectric
ceramics was presented for thermal detectors. A large
pyroelectric coefficient of 3.9 10
8
Ccm
2
K
1
and
enhanced FOMs were achieved at room temperature in
0.97(0.99Bi
0.5
Na
0.5
TiO
3
-0.01BiAlO
3
)-0.03K
0.5
Na
0.5
NbO
3
(BNT-BA-KNN) ceramics. Moreover, the obtained p and
FOMs demonstrate good temperature stability over a wide
temperatu re range of RT to 85
C. The high depolarization
temperatu re and undisp ersed ferroelectric-relaxor phase
transition of BNT-BA-KNN are suggested to be responsible
for the improvement. Our results reveal the great potential
of BNT-BA-KNN ceramics for in frared thermal detector
applications.
The BNT-BA-KNN ceramics were prepared through a
conventional solid-state reaction method. BiAlO
3
(BA) was
chosen due to its ability to improve the polarization of
BNT.
18
K
0.5
Na
0.5
NbO
3
(KNN) was added since it was
widely used to tune the depolariz ation temperature of BNT-
based materials.
24
Bi
2
O
3
(99.9%), TiO
2
(99.8%), Na
2
CO
3
(99.8%), Al
2
O
3
(99%), K
2
CO
3
(99%), and Nb
2
O
5
(99.9%)
were used as the starting raw materials. The mixed oxides
were calcined at 850
C for 2 h and sintered at 1180
C for
2 h in a covered alumina crucible. To minimize the evapora-
tion of the volatile elements Bi, Na, and K, the disks were
embedded in atmospheric powder of the same composition.
The samples were ground to disks with a thickness of
0.5 mm and a diameter of 8 mm. Both sides of the disks were
coated with a thin layer of silver paste with a diameter of
7.8 mm through screen printing and were fired at 700
C for
30 min.
The crystal structure of the as-sintered ceramics was
characterized using an X-ray diffractometer (XRD, D8
Advance, Bruker, Karlsruhe, Germany), operated with Cu
Ka radiation at room temperature ( 30
C). The microstruc-
ture of the ceramics was taken using a field emission scan-
ning electron microscope (FE-SEM, S-4800, Hitachi, Japan).
For dielectric and pyroelectric measurements, the samples
were poled in silicone oil under an electric field of 7 kV/mm
at 100
C for 30 min and then cooled to room temperature
without electric field removal to make sure that the samples
were fully poled. The dielectric constant (e
r
) and dielectric
loss (tan d) were measured using a Hewlett Packard LCR
meter with a heating rate of 2
C/min at 100 Hz, 1 kHz,
10 kHz, 100 kHz, and 1 MHz. The frequency dependent e
r
and tand were measured at room temperature from 20 Hz to
10 kHz. The polarization-electric field (P-E) loops were
characterized at 1 Hz using an aix ACCT TF 2000 analyzer
ferroelectric measuring system (aix ACCT Co., Aachen,
Germany). The pyroelectric coefficient was measured by the
Byer-Roundy method as a function of temperature.
25
The
pyroelectric current was recorded with a Keithley 6517A
electrometer/high resistance meter for poled samples both on
heating and cooling with a rate of 2
C/min.
Figure 1 shows the XRD pattern of BNT-BA-KNN
ceramics, and the inset illustrates the surface microstructure
of the as-sintered samples. The XRD result confirms that the
ceramics are crystallized into a perovskite structure. A small
trace of the bismuth-deficient Bi
2
Al
4
O
9
secondary phase was
detected at around 2h ¼29
, which was also observed previ-
ously in BNT-BA ceramics.
18
The amount of the impurity
phase is calculated to be around 0.32 mol. % through inte-
grating the related peaks. The samples exhibit a dense and
void-free microstructure, and the grain sizes vary from 1 to
6 lm. This is consistent with the high relative density
(>97%) determined by the Archimedes method.
Figure 2(a) presents the temperature dependent dielectric
constant (e
r
) and dielectric loss (tan d) of pre-poled BNT-BA-
KNN ceramics measured at different frequencies from room
temperature to 400
C. A general frequency-dispersion is
clearly visible for both e
r
and tan d, which indicates their
relaxor characteristic.
26
As temperature increases, three
anomalies can be detected, similar to previous reports.
27
The
first characteristic temperature T
F-R
canbedeterminedbythe
frequency-independent dielectric loss peak and dielectric con-
stant inflection point at 118
C, which denotes the transition
from ferroelectric to the ergodic relaxor (FE-ER) phase with
increasing temperature, as supported by the temperature
dependent polarization hysteresis measurements in Fig. 2(b)
and the TEM study.
28,29
The FE-ER phase transition will
result in reoriented dipoles and a significant reduction in rem-
anent polarization. Thus, T
F-R
had also been regarded as the
depolarization temperature T
d
.
30,31
The second transition tem-
perature is characterized by the frequency dispersion vanish-
ing point at 225
C. The third transition temperature T
m
happens at the maximum dielectric constant, which provides
the evolution to the paraelectric phase. Although many works
have been performed with regard to the temperature depen-
dent evolutions of the BNT family, the structural variations
during these phase transitions still remain controversy.
FIG. 1. The XRD pattern of as-sintered BNT-BA-KNN ceramics. The inset
illustrates the surface microstructure of the sample.
142903-2 Liu et al. Appl. Phys. Lett. 112, 142903 (2018)
For pyroelectric applications, it is important to evaluate
the dielectric properties at lower frequencies of <100 Hz.
However, previous studies performed on the BNT-based
ceramics usually report the dielectric properties at higher fre-
quencies. Figure 3 shows the f dependent dielectric constant
and loss of poled BNT-BA-KNN ceramics at room tempera-
ture during the f range of 20–10 kHz. It can be seen that as f
decreases from 10 kHz to 20 Hz, e
r
slightly increases and
tan d decreases slightly first and then increases slightly. To
be specific, e
r
is 514, 536, and 551 and tan d is 0.029, 0.028,
and 0.030 at 1 kHz, 100 Hz, 20 Hz, respectively. The rela-
tively stable dielectric parameters at lower frequencies are
favorable for their potential pyroelectric applications.
Figure 4 shows the temperature dependent pyroelectric
coefficient on heating calculated according to the following
definition:
22
p ¼
dP
r
dT
¼
IðTÞ
A ðdT=dtÞ
; (1)
where I(T) represents the pyroelectric current, A is the elec-
trode area of the sample, and dT/dt is the heating rate. At
room temperature, a large p value of 3.9 10
8
Ccm
2
K
1
upon heating is achieved, which is comparable to the com-
mercially used PZT pyroelectric ceramics.
5
The room tem-
perature p upon cooling is also measured to be as large as
3.5 10
8
Ccm
2
K
1
. It should also be noted that due to
the higher T
d
, the p value obtained here is slightly lower than
that of BNT-BZT
22
and La-doped BNT-BT.
23
However, a
high T
d
is necessary because it defines the upper limit tem-
perature of real pyroelectric-based applications. From the
peak of the pyroelectric curve, T
d
is determined as 118
C,
which equals the value of T
F-R
recorded from the e
r
-T and
tan d-T curves. This is different from the situation of BNT-
BT, for which T
d
measured from the pyroelectric current
curve locates 10
C lower than T
F-R
recorded from dielectric
measurements.
32
At T
d
, the peak value of the pyroelectric
coefficient reaches as high as 400 10
8
Ccm
2
K
1
.
Although this large p itself is unusable for practical pyroelec-
tric applications due to the irreversibility of the depolariza-
tion transition during cooling, the sharp pyroelectric
coefficient indicates the undispersed ferroelectric-ergodic
relaxor phase transition process, which is helpful for stabiliz-
ing the pyroelectric properties when T is lower than T
d
.
The inset of Fig. 4 presents the calculated remanent
polarization (P
r
) of poled BNT-BA-KNN ceramics as a func-
tion of temperature. P
r
is achieved by integration of depolari-
zation current according to the following equation:
P
r
¼
ð
T
2
T
1
pdT ¼
ð
T
2
T
1
IðTÞ
A ðdT=dtÞ
dT; (2)
FIG. 2. (a) Temperature dependent dielectric constant and dielectric loss of poled BNT-BA-KNN ceramics at 100 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz. (b)
P-E loops of BNT-BA-KNN ceramics at different temperatures at 1 Hz.
FIG. 3. The frequency dependent dielectric constant and loss at room tem-
perature during the range of 20 Hz–10 kHz.
FIG. 4. The pyroelectric coefficient of the poled BNT-BA-KNN ceramics
with the increasing temperature rate of 2
C/min. The inset shows the rema-
nent polarization as a function of temperature.
142903-3 Liu et al. Appl. Phys. Lett. 112, 142903 (2018)
where T
1
and T
2
represent the lower and the upper tempera-
ture, respectively. At room temperature, P
r
is determined to
be 34.6 lC/cm
2
, which is consistent wit h the values recorded
from P-E loops in Fig. 2(b). Also, it can be seen that P
r
slightly decreases from 34.6 lC/cm
2
to 29.5 lC/cm
2
, when
temperature increases from 5
Cto100
C. Note that the vari-
ation of P
r
is less than 15% during this temperature scope.
This shows that the ceramics can maintain a stable poled state
in a wide temperature window. This is desirable for pyroelec-
tric materials to be used as infrared thermal detectors. When
T further increases to near 120
C, a sudden decline of P
r
occurs due to the temperature induced ferroelectric-relaxor
phase transition, accompanied by the above-discussed sharp
pyroelectric peak.
Figure 5 displays the temperature dependent p and
FOMs of BNT-BA-KNN ceramics on heating at 1 kHz in the
range of RT to 85
C. The p and calculated FOMs of BNT-
BA-KNN ceramics at 1 kHz, 100 Hz, and 20 Hz are also
given in Table I. Note that an average p value of 3.7 10
8
Ccm
2
K
1
on heating and cooling is used in this table
to calculate the FOMs at different frequencies. Besides
BNT-BA-KNN, pyroelectric parameters of other lead-free
materials and lead-containing pyroelectric ceramics are also
summarized in Table I for comparison. It is obviously seen
that the overall pyroelectric properties of BNT-BA-KNN
ceramics are much better than those of other lead-free
FIG. 5. Pyroelectric coefficient p and figures of merit F
i
, F
v
, and F
d
as a
function of temperature on heating at 1 kHz during the range of RT to 85
C.
TABLE I. A comparison of pyroelectric parameters of the BNT-BA-KNN ceramics, other lead-free materials, and lead-containing ceramics.
Material e
r
tan d Freq. (Hz) T
d
(
C) T
m
(
C) p (10
8
Ccm
2
K
1
) F
i
(10
10
m/V) F
v
(10
2
m
2
/C) F
d
(10
5
Pa
1/2
) Reference
BNT-BA-KNN
a
514 0.029 1k 118 282 3.7 1.32 2.89 1.15 This work
BNT-BA-KNN
a
536 0.028 100 118 282 3.7 1.32 2.78 1.14 This work
BNT-BA-KNN
a
551 0.030 20 118 … 3.7 1.32 2.70 1.09 This work
CSBN
b
328.7 0.033 1k 200 218 1.24 0.60 2.03 0.61 14
SBN
c
1024 0.033 100 … 90 2.05 0.98 1.08 0.56 33
SBN
d
1700 … … … 60 11 … … … 34
KNN-based
e
980 0.035 100 … 360 2.18 0.994 1.14 0.571 15
KNN-based
f
1520 0.018 100 … … 1.90 0.931 0.7 0.598 35
BCT-BST
g
3500 0.025 1k … 75 2.05 1.00 0.32 0.41 16
BNT-BZT
h
1052 0.040 1k 87 … 5.7 2.03 2.18 1.05 22
BNT-BT-ST
i
1278 0.109 1M … 184 5.7 2.08 1.8 0.589 36
BNT-BT
j
671 0.042 1k 54.9 … 12.92 4.61 0.078 2.76 37
BNT-BT
k
835 0.010 100 120 280 3.80 1.315 2.0 1.56 21
TGS
l
55 0.025 1k … 49 5.5 2.12 43 6.1 5
LiTaO
3
m
47 0.005 … … 665 2.3 0.72 17 35.2–4.9 5
PZT-PMN
n
196 0.014 33 118 258 3.16 … 7.3 4.33 8
PZ-based
o
290 0.0027 1590 … 230 3.7 … 6 5.8 7
a
0.97(0.99Bi
0.5
Na
0.5
TiO
3
-0.01BiAlO
3
)-0.03K
0.5
Na
0.5
NbO
3
ceramics.
b
Ca
0.2
Sr
0.1
Ba
0.7
Nb
2
O
6
ceramics.
c
Sr
0.5
Ba
0.5
Nb
2
O
6
ceramics.
d
Sr
0.67
Ba
0.33
Nb
2
O
6
crystals.
e
Mn-doped 0.97K
0.5
Na
0.5
NbO
3
-0.03(Bi
0.5
K
0.5
)TiO
3
ceramics.
f
[(K
0.5
Na
0.5
)
0.96
Li
0.04
](Nb
0.84
Ta
0.1
Sb
0.06
)O
3
ceramics.
g
0.6(Ba
0.9
Ca
0.1
)TiO
3
-0.4Ba(Sn
0.2
Ti
0.8
)O
3
ceramics.
h
0.93(Bi
0.5
Na
0.5
)TiO
3
-0.07Ba(Zr
0.055
Ti
0.945
)O
3
ceramics.
i
0.71(Bi
0.5
Na
0.5
)TiO
3
-0.065BaTiO
3
-0.22SrTiO
3
ceramics.
j
La,Ta-0.94(Bi
0.5
Na
0.5
)TiO
3
-0.06BaTiO
3
ceramics.
k
Mn-doped 0.946Na
0.5
Bi
0.5
TiO
3
-0.054BaTiO
3
crystals.
l
TGS crystals.
m
LiTaO
3
crystals.
n
Mn-doped 0.975Pb(Zr
0.875
Ti
0.125
)O
3
-0.025Pb(Mg
1/3
Nb
2/3
)O
3
ceramics.
o
Fe, Nb, Ti, U-doped PbZrO
3
ceramics.
142903-4 Liu et al. Appl. Phys. Lett. 112, 142903 (2018)
