PL - PTCer

MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 65, 1, (2013), 50-54
www.ptcer.pl/mccm
Basic Properties of Lead-Zirconate-Titanate
Ceramics at Morphotropic Boundary
DIONIZY CZEKAJ1,*, MIROSàAW M. BUûKO2,*
University of Silesia, Department of Materials Science, 2 Sniezna St., 41-200 Sosnowiec, Poland
AGH - University of Science and Technology, Faculty of Materials Science and Ceramics, al. A. Mickiewicza 30, 30-059
Kraków, Poland
*e-mail: [email protected], [email protected]
1
2
Abstract
Investigations into lead zirconate-titanate (PZT) ceramics with the chemical composition from the morphotropic region are reviewed.
It is shown that to improve the properties of the basic PZT ceramic material almost always a dopant, modi¿er, or other composition in the
solid solution is included. In order to achieve the appropriate set of properties more than one type of additive (donor, acceptor or isovalent)
is included to the given chemical composition. It is indicated that PZT ceramics exhibits high sensitivity to technological conditions, temperature and electric ¿eld applied. Quality of ceramics can be checked by means of structural analysis and thus temperature dependencies
of electrophysical parameters of PZT-type ceramics and their changes caused by changes of PbTiO3 content in PZT-type solid solution
can be explained.
Keywords: PZT, Morphotropic boundary, Dielectric properties, Piezoelectric properties
PODSTAWOWE WàAĝCIWOĝCI CYRKONIANU-TYTANIANU OàOWIU NA GRANICY MORFOTROPOWEJ
Praca przedstawia syntetyczną analizĊ wáaĞciwoĞci cyrkonianu-tytanianu oáowiu (PZT) w zakresie skáadów chemicznych odpowiadających przemianie morfotropowej. Wykazano, Īe praktycznie w kaĪdym przypadku poprawa wáaĞciwoĞci PZT związana jest z mody¿kacja
strukturalną spowodowaną obecnoĞcią domieszki i utworzeniem roztworu staáego. OsiągniĊcie optymalnych wáaĞciwoĞci związane jest
zazwyczaj z wprowadzeniem do roztworu staáego wiĊcej niĪ jednego rodzaju domieszki: donorowej, akceptorowej lub izowalencyjnej.
Stwierdzono, Īe na wáaĞciwoĞci tego typu materiaáów istotny wpáyw mają zarówno sposób ich wytwarzania, jak i obecnoĞü pola elektrycznego. Temperaturowe zmiany wáaĞciwoĞci PZT mają Ğcisáy związek ze skáadem chemicznym roztworów staáych, a w konsekwencji z ich
skáadem fazowym w pobliĪu granicy morfotropowej.
Sáowa kluczowe: PZT, granica morfotropowa, wáaĞciwoĞci dielektryczne, wáaĞciwoĞci piezoelektryczne
1. Introduction
In our present highly technological age, it is not possible
to ignore ceramics, regardless of how complex they are. More
and more applications are being found for ceramic materials in devices, affecting our everyday life in such areas as
electronic components, environmental sensors, gas igniters,
intrusion alarms, loudspeakers, ultrasonic cleaners, and
medical diagnostic equipment. In fact, many of these applications have come about as a result of the special nature
and properties of ceramics, which set them apart from other
materials such as metals or plastics.
Ceramic materials prepared on the basis of Pb(Zr,Ti)O3
solid solution are called PZT-type materials for short. PZTtype ceramic materials are commonly used in engineering
among others as electromechanical transducers. Increasing
in application possibilities of these materials is connected
with both the selection of chemical composition and the
improvement of structure and microstructure (decreas-
50
ing in porosity, increasing in density, decreasing in grain
dimensions) by means of choice of suitable technological
conditions. Ferroelectric materials based on Pb(Zr,Ti)O3 solid
solutions are commonly used in practice owing to high values
of dielectric permittivity (İ), spontaneous polarisation (PS),
piezoelectric coef¿cients (dij) and coupling coef¿cient (kp). In
addition, these properties can be continuously modi¿ed over
speci¿ed ranges by changing the Zr/Ti concentration ratio
and the presence of admixtures called modi¿ers.
2. Phase stabilities in the Pb(Zr,Ti)O3 system
Lead zirconate-titanates of the formula Pb(Zr1-xTix)O3, or
generally ABO3, constitute one of the most important families
of ferroelectric materials used in the preparation of ceramic
components of industrial piezoelectric transducers. The
perovskite structure shown in Fig. 1 is cubic with a primitive
lattice, the Oh (m3m) point group, and the Oh1 (Pm3m) space
group [1]. CaTiO3 is actually the prototype perovskite material
BASIC PROPERTIES OF LEAD-ZIRCONATE-TITANATE CERAMICS AT MORPHOTROPIC BOUNDARY
Fig. 1. Cubic cell of the ABO3 perovskite-type structure.
Rys. 1. Regularna komórka elementarna ABO3 typu perowskitu.
and has symmetry lower than the cubic one. A2+ ions at the
apices, a B4+ ion at the centre, and O2- ions at the face centres
(Fig. 1) can represent the cell, cubic at higher temperatures.
The phase diagram of a PbTiO3-PbZrO3 solid solution
is shown in Fig. 2 where it can be seen that the boundary
between tetragonal and rhombohedral forms is nearly independent of temperature. Substitution of Zr4+ for Ti4+in PbTiO3
reduces the tetragonal distortion [2], and ultimately causes
the appearance of another ferroelectric phase of rhombohedral R3m symmetry. In the PZT system it occurs close to the
composition where the PbTiO3:PbZrO3 ratio is (53-52):(4748) by mole fraction. It is worth noting that the morphotropic
phase boundary (MPB) denotes an abrupt structural change
with composition at the constant temperature in the solid
solution range [3]. However, in the latter discussion the MPB
will be referred to a speci¿c composition. It is required that
this is a two-phase zone and this has been experimentally
con¿rmed. The MPB is considered as that composition where
the two phases are present in equal quantity [2].
The addition of more Zr4+ causes the appearance of the
orthorhombic antiferroelectric PbZrO3 phase (A0). The A0phase exhibits the small ¿eld of stability of the tetragonal antiferroelectric phase near the Curie point (AT). Cooling a PZT
solid solution with the composition near the morphotropic
phase boundary below its Curie-temperature (TC) induces the
phase transformation from the paraelectric cubic (PC) to the
800
510
500
PC
FR(HT)
490
700
480
470
Temperature [ K ]
PC
AT
0
600
AO
1
500
2
3
4
FR(HT)
FT
400
300
FR(LT)
AO
0
10
20
30
40
50
60
70
80
90
Molar fraction PbTiO3 [%]
Fig. 2. The phase diagram for the PbTiO3-PbZrO3 system [2].
Rys. 2. Diagram fazowy ukáadu PbTiO3-PbZrO3 [2].
100
ferroelectric rhombohedral (FR) and ferroelectric tetragonal
(FT) PZT modi¿cation. Simultaneously with the transformation distortion of the unit cells occurs, which is of different
crystallographic directions for the FT – ({001} direction) and
for the FR – phase ({111} direction) [4].
Recently, high-resolution synchrotron X-ray powder diffraction measurements on extremely homogeneous samples
with 0.42 d x d 0.52 showed that an intermediate monoclinic
phase exists between the rhombohedral and tetragonal PZT
phases [5-8]. The observation of this monoclinic phase in different PZT compositions at closely-spaced intervals of mole
fraction x gave the opportunity for the preliminary modi¿cation
of the phase diagram [6, 8].
At ambient temperature the samples exhibited tetragonal
symmetry and transformed below T | 250 K into a phase
with monoclinic symmetry (aM = 0.5717 nm, bM = 0.5703
nm, cM = 0.4143 nm, ȕM = 90.53° at T = 20 K). At T = 20 K
the monoclinic phase was stable in the range of 0.46 d x d
0.51, narrowing with the temperature increase. A ¿rst-order
phase transition from tetragonal to rhombohedral symmetry
was observed only for x = 0.45 [8]. Therefore, the MPB corresponded not to the tetragonal-rhombohedral phase boundary, but instead to the boundary between the tetragonal and
monoclinic phases for 0.46 d x d 0.51 [8].
3. Compositional modi¿cations
It is well known that the ABO3 structure permits large
variations in composition by substitution on A or B sites
by radius-compatible ions. A great deal of compositional
modi¿cations has been undertaken [e.g., 2, 9]. In case of
isovalent substitution of Sr+2 for Pb+2 in Pb(Zr,Ti)O3 solid
solution the Curie point lowers about 9.5 K for every atomic
percentage added, and thus the dielectric constant at room
temperature raises [2]. Since the coupling factor and elastic
modulus are not greatly altered, this has the effect of raising
in piezoelectric charge coef¿cient d33.
Many compensating valence solid solution systems are
possible in Pb(Zr,Ti)O3, most of them with substantial solid
solubility. If one substitutes combinations instead of individual
ions in such a manner that stoichiometry is maintained, so
that:
∑X
A
⋅ VA +
∑X
B
⋅ VB = 6
(1)
where X is concentration and V is the valence of ions entering the A and B positions, a great many modi¿cations are
possible [e.g., 9].
When the valence of the substituent ions differs from
that of the substituted ions, charge compensation results
from the following mechanisms: (i) oxidation of lead ions
on A sites from the +2 to the +3 or +4 valence state [10];
(ii) reduction of titanium ions on B sites from the +4 to the
+3 valence state; (iii) creation of neutral (or singly ionised)
anionic vacancies. These mechanisms affect the physical
properties of the material [9, 11, 12]. According to the desired
technical applications, the composition is modi¿ed by two
main groups of substitutions.
The ¿rst group includes substituent ions of a valence
lower than that of the substituent ion (i.e., the “acceptor”
substituent) e.g., K+, Na+ on A sites and Zn2+, Mg2+, Sc3+ or
Fe3+, Mn, Ni on B sites. In this ¿rst group, the transition ions
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 65, 1, (2013)
51
D. CZEKAJ, M.M. BUûKO
Rhombohedral
phase (R)
Coexistence of
R and T phases
Tetragonal
phase (T)
1.2
1.0
0.8
0.6
0.4
ε33
ε33
38
ε for hot deposited electrodes
ε for cold deposited electrodes
40
42
44
46
Molar fraction PbTiO3 [%]
a)
Rhombohedral
phase (R)
Coexistence of
R and T phases
Tetragonal
phase (T)
1.2
4. Piezoelectric properties and structural
parameters of PZT-type ceramics
0.8
Results of investigation of structural parameters and
piezoelectric properties in the ternary PZT-based system:
xPbTiO3(1-x)PbZrO30.1Pb(W0.5Cd0.5)O3 [9, 16, 17] in the
vicinity of the MPB region are given below. Measurements
of electric permittivity of the non-polarized ceramics (İ) and
the polarized ones (İ33) were performed by the Sawyer-Tower
method at a frequency Ȟ = 1·103 Hz. The sample was polarized in the electric ¿eld of intensity E = 4·106 V/m at 413 K for
50 min. It was found that the dielectric permittivity increases
with polarization of the samples in agreement with [2] (Fig. 3).
The electromechanical coupling coef¿cients (kp) for the
same samples are given in Fig. 4.
Dependencies of structural parameters on the PbTiO3
content for PZT-type ceramic samples sintered by the hotpressing method are given in Figs. 5 and 6 [17]. They show
that the phase transition takes place in a continuous way,
and that it is a non-linear function of PbTiO3 content within
0.4
ε33
ε33
38
40
ε for disc sample
ε for plate sample
42
44
46
Molar fraction PbTiO3 [%]
b)
Fig. 3. Dependence of dielectric permittivity on PbTiO3 content for
the ternary xPbTiO3·(1-x)PbZrO3–0.1Pb(W0.5Cd0.5)O3 system [16].
Measurements for different electrodes (a) and for different shapes
of a sample (b).
Rys. 3 ZaleĪnoĞü przenikalnoĞci dielektrycznej materiaáów z ukáadu
xPbTiO3–(1-x)PbZrO3–0,1Pb(W0,5Cd0,5)O3 od formalnej zawartoĞci
PbTiO3 [16]. Pomiary dla róĪnych elektrod (a) i róĪnych ksztaátów
próbek (b).
Coupling coefficient
0.8
Rhombohedral
phase (R)
Coexistence of
R and T phases
Tetragonal
phase (T)
0.6
R - phase
for hot deposited electrodes
for cold deposited electrodes
for plate sample
for disc sample
0.4
0.2
38
40
42
44
46
41.0
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 65, 1, (2013)
R
89.75 phase
R+T
phases
89.70
89.65
0.44 0.45 0.46 0.47 0.48
Molar fraction PbTiO3 [%]
T - phase
40.0
0.42
Fig. 4. Dependence of electromechanical coupling factor
(k P) on PbTiO 3 content for the ternary xPbTiO3–(1-x)PbZrO 3–
–0.1Pb(W0.5Cd0.5)O3 system [16]. Measurements for different electrodes and for different shapes of a sample.
Rys. 4 ZaleĪnoĞü wspóáczynnika sprzĊĪenia elektromechanicznego
(kP) materiaáów z ukáadu xPbTiO3–(1-x)PbZrO3–0,1Pb(W0,5Cd0,5)O3
od formalnej zawartoĞci PbTiO3 [16]. Pomiary dla róĪnych elektrod
i róĪnych ksztaátów próbek.
R+T
phases
89.80
40.5
Molar content PbTiO3 [%]
52
Lattice angle αR [deg]
Dielectric constant [103·ε]
1.6
with multiple valence (especially those of Mn) promote ¿xation of the domain con¿guration and are called „stabilisers”
[e.g., 11, 13]. These substitutions result in “hard materials”
with low losses (tanį # 0.4%) and are mainly used as power
transducers.
The second group includes ions of a valence higher than
that of the substituted ion, e.g., La3+, Nd3+ on A sites and
Nb5+, Sb5+ on B sites. Such materials, used at low electrical
¿elds, stresses, and strains, are termed „soft materials”.
A soft doping ion (or the “donor” substituent) may “soften”
properties of PZT-type piezoelectric ceramic. In other words,
such parameters like elastic compliance, dielectric constant,
electromechanical coupling factor and the bulk resistivity are
enhanced while the coercive ¿eld, mechanical and electrical quality factors are reduced [12]. Dielectric losses at low
electrical ¿elds are about 2%. It is worth noting that some
soft and hard compositions use dual substitution [14]. For
instance, the dual cationic substitution (M+, Nb5+) to stabilise
soft ceramics used in stroke igniters was investigated in [15].
Lattice constants [nm]
Dielectric constant [103·ε]
1.4
aT
<aT>
0.44
0.46
cT
aR
0.48
0.50
0.52
0.54
0.56
Molar fraction PbTiO3 [%]
Fig. 5. Dependence of the tetragonal (aT, cT, <aT>) and the rhombohedral (aR) lattice constants on PbTiO3 content for the ternary
xPbTiO3–(0.98-x)PbZrO3–0.02Pb(W0.5Cd0.5)O3 system [17].
Rys. 5. ZaleĪnoĞü parametrów sieciowych roztworów staáych w
ukáadzie xPbTiO3–(0.98-x)PbZrO3–0.02Pb(W0.5Cd0.5)O3 od formalnej
zawartoĞci PbTiO3 [17].
R - phase
60
1.5
T - phase
40
XT
XR
1.0
δT
δR
20
0
0.42
0.44
0.46
0.48
0.50
0.52
0.54
0.5
0.56
Molar fraction PbTiO3 [%]
the entire morphotropic phase boundary region (MPB). Also
the homogeneous parameters of deformation, įR and įT, of
the elementary cell given by relations:
įR | cos(ĮR)
(2)
(where ĮR is an angle of the rhombohedral cell) and:
⎞
2 ⎛c
δ T = ⋅ ⎜⎜ T − 1⎟⎟
3 ⎝ aT
⎠
(3)
vary in a non-linear way within MPB (Fig. 6).
The easiness of structural changes at low values of įR
and įT is conductive to an increase in mobility of the domain
walls, reconstruction of the domain structure, and hence
switching of ceramics in external electric ¿eld. This agrees
well with the minimum of coercive ¿eld (EC) and the maximum of remnant polarisation (PR) (Fig. 7), relative dielectric
constant (İ/İ0) (Fig. 8), piezoelectric change coef¿cient (d33)
Coercive field [105·V/m]
50
R - phase
40
T - phase
30
20
8
10
7
0.44
0
0.46
0.48
Remanent polarisation [102·C/m2]
60
R+T - phases
9
R - phase
T - phase
1.2
1.0
R+T - phases
0.8
0.6
0.44
0.46
0.48
0.50
0.52
0.54
0.56
Molar content PbTiO3 [%]
Fig. 6. Dependence of the homogeneous deformation parameters
for the rhombohedral (įR) and tetragonal (įT) lattice and content
of the rhombohedral (XR) and tetragonal (XT) phases on PbTiO3
content for the ternary xPbTiO3–(0.98-x)PbZrO3–0.02Pb(W0.5Cd0.5)
O3 system [17].
Rys. 6. ZaleĪnoĞü wspóáczynników deformacji sieci romboedrycznej
(įR) i tetragonalnej (įT) oraz zawartoĞci obu faz (XR, XT) roztworów
staáych w ukáadzie xPbTiO3–(0.98-x)PbZrO3–0.02Pb(W0.5Cd0.5)O3
od formalnej zawartoĞci PbTiO3 [17].
10
1.4
0.50
Molar fraction PbTiO3 [%]
Fig. 7. Dependence of coercive ¿eld (EC) and remnant polarisation
(PR) on PbTiO3 content for the ternary xPbTiO3–(0.98-x)PbZrO3–
–0.02Pb(W0.5Cd0.5)O3 system [17].
Rys. 7. ZaleĪnoĞü pola koercji (EC) oraz polaryzacji szczątkowej
(P R) roztworów staáych w ukáadzie xPbTiO 3–(0.98-x)PbZrO 3–
–0.02Pb(W0.5Cd0.5)O3 od formalnej zawartoĞci PbTiO3 [17].
Fig. 8. Dependence of the relative dielectric constant (İ/İ0) on PbTiO3
content for the ternary xPbTiO3–(0.98-x)PbZrO3–0.02Pb(W0.5Cd0.5)
O3 system [17].
Rys. 8. ZaleĪnoĞü wzglĊdnej staáej dielektrycznej (İ/İ0) roztworów
staáych w ukáadzie xPbTiO3–(0.98-x)PbZrO3–0.02Pb(W0.5Cd0.5)O3
od formalnej zawartoĞci PbTiO3 [17].
140 R - phase
70
120
T - phase
100
60
50
80
R+T - phases
40
60
30
40
0.44
0.46
0.48
0.50
0.52
0.54
0.56
Electromechanical coupling factor [a.u.]
80
Piezoelectric coefficient [1012·C/N]
Phase content XT , XR [%]
2.0
Relative dielectric constant [103 a.u.]
R+T - phases
100
Deformation parameters δT , δR [107 a.u.]
BASIC PROPERTIES OF LEAD-ZIRCONATE-TITANATE CERAMICS AT MORPHOTROPIC BOUNDARY
Molar content PbTiO3 [%]
Fig. 9. Dependence of piezoelectric charge coef¿cient (d33) and
electromechanical coupling factor (kP) on PbTiO3 content for the
ternary xPbTiO3–(0.98-x)PbZrO 3–0.02Pb(W 0.5Cd 0.5)O 3 system.
Measurements at room temperature [17].
Rys. 9. ZaleĪnoĞü wspóáczynnika piezoelektrycznego (d33) oraz
sprzĊĪenia elektromechanicznego (kP) dla roztworów staáych w ukáadzie xPbTiO3–(0.98-x)PbZrO3–0.02Pb(W0.5Cd0.5)O3 od formalnej
zawartoĞci PbTiO3 [17].
and coupling factor (kp) (Fig. 9).
5. Conclusions
Lead zirconate titanate ceramics are almost always used
with a dopant, modi¿er, or other composition in the solid solution. This is commonly done to improve on the properties
of the basic PZT ceramic material for speci¿c applications.
It should be mentioned here that it is common practice in
formulating compositions to include more than one type of
additive (donor, acceptor or isovalent) to the given composition in order to achieve the appropriate set of properties.
It results from the investigations reviewed above that
PZT-type ceramics with chemical composition from the morphotropic region exhibits high sensitivity to technological conditions, temperature, and is inÀuenced with the electric ¿eld.
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 65, 1, (2013)
53
D. CZEKAJ, M.M. BUûKO
Acknowledgements
[9]
This work was supported by the Ministry of Science and
Higher Education under the grant nr R 015 0005 04.
[10]
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i
Received 17 July 2012, accepted 27 October 2012