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Ceramics International 40 (2014) 15439–15445
www.elsevier.com/locate/ceramint

Electrical Properties of Ca-modified Na0.5Bi0.5TiO3–BaTiO3 ceramics

Saeed ullah Jan, Aurang Zeb, Steven J. Milnen
Institute for Materials Research, University of Leeds, Leeds LS2 9JT, UK
Received 21 May 2014; received in revised form 20 June 2014; accepted 21 June 2014
Available online 30 June 2014

Abstract

Ceramics in the system (1
x)Na0.5Bi0.5TiO3–xBa0.80Ca0.20TiO3 demonstrate a phase boundary between pseudo-cubic and tetragonal at 0.1
o x o0.2. There was a gradual convergence in the temperatures of a frequency-dependent inflection and peak in relative permittivity-temperature
plots with increasing x: for x ¼0.5 and 0.6, normal relaxor-like behaviour was observed, with Tm 150 1C (1 kHz). Constriction in the
polarisation-electric field hysteresis loops for the phase boundary composition x ¼0.1 was consistent with an electric field-induced phase
transition, giving rise to an electromechanical strain, 0.25 % at 40 kV/cm and high-field d33n 640 pm/V.
& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: C. Dielectric properties; C. Piezoelectric properties; D. Perovskites; D. BaTiO3 and titanates

1. Introduction below the peak temperature of  300 1C [12–14]. An inflection in

ɛr–T plots is common to a number of NBT solid solutions.
Investigations of pervoskite solid solutions are of relevance Examples of NBT solid solutions systems include: Na0.5-
in the search for new dielectric and piezoelectric ceramics. The Bi0.5TiO3–BaTiO3 [NBT–BT] [17,21,22]; Na0.5Bi0.5TiO3–
strong piezoelectric activity of lead-based perovskites such as BaTiO3–K0.5;Na0.5NbO3 [NBT–BT–KNN] [18]; Na0.5Bi0.5-
lead zirconate titanate, PZT, is attributed to the presence of a TiO3–K0.5Bi0.5TiO3 [NBT–KBT] [19] and Na0.5Bi0.5TiO3–
morphotrophic phase boundary (MPB) between tetragonal/ BaTiO3–CaZrO3 [NBT–BT–CZ] [20].
monoclinic/rhombohedral phases which facilitates polarisation For the system, (1
y)Na0.5Bi0.5TiO3–yBaTiO3, a MPB
rotation [1–3]. However, the potential health risks associated between rhombohedral and tetragonal crystal systems is
with lead oxide has led to interest in alternative lead-free reported across the compositional range 0.06r y r 0.10
piezoelectric materials [4–7]. Examples include solid solutions [1,29,33,35,39], with most reports locating the MPB at
based on Na0.5Bi0.5TiO3 (NBT) [8,9]. The crystallography of x=0.06. Typical properties at the MPB are a piezoelectric
NBT is complex [10,11], having originally been assigned as charge coefficient, d33  125 pC/N, coupling factor, k33  0.5
rhombohedral at room-temperature, high resolution diffraction and dielectric peak temperature  290 1C [1,31].
has suggested NBT to be monoclinic [15,16]. In the present work, the effects of Ca modification on the
Other workers have observed inter-growth of tetragonal plate- properties of NBT–BT have been examined. Formulations
like structures within a rhombohedral matrix in NBT by transmis- were prepared assuming Ca substation on Ba sites at the 20
sion electron microscopy; these change to an orthorhombic at% level for the compositional series (1
x)Na0.5Bi0.5TiO3–
structure on heating which was discussed in terms of the origins
xBa0.8Ca0.20TiO3. Calcium substitution on the Ba sites of
of an inflection in relative permittivity–temperature plots well
BaTiO3 widens the temperature range of the tetragonal
n
Corresponding author.
pervoskite phase field, principally by decreasing the tetrago-
E-mail address: s.j.milne@leeds.ac.uk (S.J. Milne). nal–orthorhombic phase transition temperature [23,24].

http://dx.doi.org/10.1016/j.ceramint.2014.06.107
0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
15440 S.u. Jan et al. / Ceramics International 40 (2014) 15439–15445

2. Experimental procedure For dielectric measurements, the sintered ceramics were

ground using 400 and 600 grade silicon carbide papers and
A series of solid solutions (1
x)Na0.5Bi0.5TiO3–xBa0.80- electroded using silver paint (Agar Scientific, Stansted, UK);
Ca0.20TiO3 (0 r xr 1.0) were synthesised by a mixed oxide pellets were fired at 600 1C for 10 min to form the electrodes.
route, using starting reagents: Na2CO3 (Sigma Aldrich The relative permittivity εr and loss tangent (tan δ) were
4 99.5%,), BaCO3 (Sigma Aldrich Z 99%), Bi2O3 (Aldrich measured as a function of temperature using an impedance
99.9%) TiO2 (Aldrich 99.9%) and CaCO3 (Sigma Aldrich analyser (HP Agilent, 4192A Hewlett packed, Santa Clara,
Z 99%). All the raw materials were dried at 200 1C for 24 h CA). The polarisation–electric field (P–E) and bipolar strain
and cooled to room temperature in a desiccator. The powders
were weighed according to the desired ratios. Combined
powders were ball milled in isopropanol using zirconia
grinding media for 24 h, dried and sieved using a 300 mm
nylon mesh. The powders were calcined at 850 1C for 4 h. A
binder (Ciba Glascol HA4; Ciba Speciality Chemicals, Bradford,
UK) was introduced and the powders were ball milled again for
24 h. The powders were dried and compacted at 100 MPa in a
10 mm diameter steel die.
The compacted pellets were embedded in a previously
calcined powder of the same composition in an alumina
crucible, and a lid fitted to minimise loss of volatile compo-
nents during sintering, which was carried out in the tempera-
ture range 1130–1200 1C depending on composition, at
heating and cooling rates of 300 1C/h for 4 h. The densities
of the ceramics were determined geometrically. Phase analysis
of the sintered pellets was performed on powders of crushed Fig. 2. Variation of lattice parameters as a function of x in (1
x)NBT–xBCT;
pellets using an X-ray diffractometer (XRD Bruker; D8, compositions x o0.1 are pseudocubic; 0.1o xo0.2 are mixed phase pseudo-
Karlsruhe, Germany, CuKα λ  1.5406 Å, scan speed 11/min). cubic and tetragonal; compositions xZ 0.2 are tetragonal.

Fig. 1. (a) Room temperature XRD and (b) 12θ region highlighting cabic/pseudocubic 111 and 002/200 reflections for (1
x)NBT–xBCT ceramics (crushed
sintered pellets).
 S.u. Jan et al. / Ceramics International 40 (2014) 15439–15445 15441

Fig. 3. Relative permittivity versus temperature plots for (1
x)NBT–xBCT ceramics. (a) x¼0; (b) x¼ 0.1; (c) x ¼0.15; (d) x¼ 0.4; (e) x ¼0.5; (f) x¼ 0.6; and
(g) x¼ 0.7.
15442 S.u. Jan et al. / Ceramics International 40 (2014) 15439–15445

electric field response (S–E) of the samples were determined at boundary in the (1-y)NBT-yBaTiO3 system occurs at a slightly
applied fields r 40 kV/cm (1 kHz) using a Precision LC more NBT-rich composition 0.06rxr0.10 than for the present
analyser (Radiant Technologies Inc.). Microstructural analysis Ca modified system [26].
was carried out using scanning electron microscopy, SEM Plots of relative permittivity (εr) and dielectric loss tangent
(LEO Gemini 1530 Oxford instruments). (tan δ) versus temperature for (1
x)NBT–xBCT (0rxr1) at
1 kHz–1 MHz are shown in Fig. 3. The data relate to ceramic
3. Results and discussion samples 92–95 % of theoretical density with grain sizes r5 mm
for x¼ 0 (BNT), decreasing slightly for the solid solutions with
X-ray powder diffraction patterns for the (1
x)Na0.5Bi0.5TiO3– BCT, Fig. 4. For the BNT end member (x¼ 0) an inflection in εr
xBa0.8Ca0.20TiO3 system (0rxr1) are shown in Fig. 1. All the occurred at  220 1C (temperature of onset of inflection on
ceramic compositions showed perovskite XRD patterns, with no heating, 1 kHz) with a broad dielectric peak at  330 1C, in
secondary phases. The diffraction pattern of NBT (x¼ 0) appeared close agreement with previous reports [27]. There is debate as to
cubic, but with slight broadening and asymmetry to the 111c peak the origin of the lower temperature anomaly in the parent NBT–
consistent with poorly resolved peak splitting of a lower symmetry BT and NBT–KBT systems, with TEM suggesting a biphasic
structure (rhombohedral and monoclinic systems have been structure as described in Section 1 [13,28].
reported for NBT [25]). Hereafter the NBT pattern is referred to The incorporation of BCT at the level x=0.1 led to a more
as pseudocubic (pc). The XRD pattern of the x¼ 0.1 sample was pronounced frequency-dependent inflection than for NBT.
similar to NBT, but with a sharpening of the 111pc peak and an There was a decrease in the temperature of the ɛr inflection,
increase in apc lattice parameter, Fig. 2. Three peaks were observed to  140 1C and a decrease in the main peak temperature to
in the region of the 200 pc peak ( 4712θ) for sample x¼ 0.12,  270 1C. A progressive convergence in temperatures of the
consistent with an additional phase of tetragonal symmetry main peak and inflection in εr–T plots occurred for x4 0.1,
coexisting with the pc phase. The x¼ 0.15 sample was similar to Fig. 3, such that a normal relaxor response with broad
x¼ 0.12, but with a reduced intensity of the middle peak frequency dependent ɛr peak ( at Tm  150 1C (1 kHz)) was
suggesting a reduced proportion of psuodocubic phase. For observed for samples 0.4o x r 0.6. The associated frequency–
compositions xZ0.2, only tetragonal peaks were evident. Thus a dispersion in tan δ (at T
Tm) was also characteristic of a
mixed phase region of pseudocubic and tetragonal phases is relaxor dielectric. A change in εr–T plots from relaxor to
restricted to a narrow range of compositions, 0.1oxo0.2. Within diffuse, non-frequency dependent peaks occurred at x4 0.7
the tetragonal phase field, the a and c lattice parameters increased with a gradual sharpening of the εr peak and a lowering in peak
linearly with composition. High temperature phase analysis in this temperature consistent with normal ferroelectric character as x
region of the NBT–BCT system would be required before labelling increased to 1.0. Trends in dielectric parameters are sum-
the region as a true MPB. The corresponding morphotropic phase marised in Table 1.

Fig. 4. SEM micrographs of (1
x)BNT–xBCT ceramics. (a) x ¼0; (b) x ¼0.2; and (c) x ¼0.7.
 S.u. Jan et al. / Ceramics International 40 (2014) 15439–15445 15443

Table 1
Summary of dielectrc properties.

Composition (x) ɛr (25C) ɛr max Td (0C) Tm(1C)

0.0 330 1240 220 330

0.1 1360 4070 140 270
0.15 1370 5360 – 230
0.4 530 2900 – 180
0.5 380 3130 – 155
0.6 310 2870 – 150
0.7 180 1410 – 155
1.0 850 10900 – 125 (Tc)

Fig. 6. Polarisation–electric field response for (1
x)BNT–xBCT ceramics.

x¼ 0.0, 0.1, and 1.0.

Fig. 7. Bipolar electromechanical strain S versus electric field E for (1
x)

NBT–xBCT, x¼ 0.1 phase boundary composition.

compositions except x ¼ 0.1 gave broad ellipsoid P–E loops

typical of lossy dielectrics. The values of polarisation for
BCT and for x ¼ 0.1, Fig. 6, are in the range reported for
Fig. 5. Plots of Inð1=ε
1=εm Þ vs InðT
T m Þ for compositions 0 r xr1
(using 100 kHz data). BaTiO3 and NBT based dielectrics respectively [32,33]. A
non-conventional P–E loop similar to a distinctive pinched
P–E loop was observed for sample x=0.1, with a coercive
The relative permittivity of typical ferroelectrics above the field  þ Ec=14 kV/cm. This composition lies very close to
paraelectric transition temperature obey the Curie Weiss law (1/ɛ¼ the pseuodocubic-tetragonal phase boundary (Fig. 1). One
(T
T0)/C, where T0 is the Curie Weiss temperature and C is the other Ca modified NBT–BT composition is reported in the
Curie Weiss constant. A modified expression has been suggested literature, containing a higher Ca content 30 at%, but these
for relaxors (1/ɛ
1/ɛm)¼ ðT
T m Þγ=C; ð1 r γ r 2Þ, where C samples showed no such constriction in P–E loops [27].
and γ are material constants [29,30]. For classical ferroelectrics However similar P–E behaviour is reported in the parent
γ=1; for typical relaxors γ=2. Dielectric data for the NBT–BCT (1
y)NBT–yBT system for compositions close to its
samples are considered in terms of the modified Weiss law in morphotrophic phase boundary. Guo et al. observed pinch-
Fig. 5. The value of γ reached a maximum of  2 at composition ing in room-temperature P–E loops for a y ¼ 0.07 composi-
x¼ 0.15. tion in poled samples, and ascribed this behaviour to a
Polarisation–electric field loops are shown in Fig. 6. The coexistence of ferroelectric and anti-ferroelectric phases
NBT end member x ¼ 0 showed no evidence of ferroelectric [34]. Tan et al. subsequently showed by electric field in
domain switching at the fields tested o 40 kV/cm [31]. The situ TEM that phase stability around the MPB in NBT–BT
BCT end-member (x ¼ 1) gave a well-saturated ferroelectric was dependent on applied electric field [35]. In the
hysteresis loop with polarisation values in the range reported NBT–KBT system electric field in situ synchrotron X-ray
for other BaTiO3 samples. Intermediate solid solution diffraction demonstrated that the position of the
15444 S.u. Jan et al. / Ceramics International 40 (2014) 15439–15445

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