SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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ALTERNATIVE STRATEGIES TO CONTROL ELECTRICAL
FATIGUE IN PIEZOELECTRIC CERAMICS
METIN OZGUL1, SERHAT TIKIZ1,
SUSAN TROLIER-MCKINSTRY2 AND CLIVE A. RANDALL2
1-Department of Materials Science and Engineering, Afyon Kocatepe University,
Afyonkarahisar, 03200,Turkey
2- Materials Research Institute, Pennsylvania State University,
University Park, PA, 16802, USA
ABSTRACT
In most applications electroceramics are exposed to repeated electrical cycles forcing
the material to repeatedly deform (piezoelectricity) or reverse its spontaneous
polarization (ferroelectricity) both causing property degradations, such as fatigue or
aging, limiting use of the electroceramics for many applications. Main focus of this
study is investigating strategies to control or eliminate electrical fatigue in various
piezoelectric systems. The influence of various factors (i.e., composition of the
ferroelectric material, choice of electrode, and the characteristics of the external
electric field) on fatigue has been studied in bulk PbZrTiO3 (PZT) ceramics.
Improvement of ferroelectric fatigue properties is observed when PZT is donor doped
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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with 2 and 4 at.% Nb. Compositional studies for the improvement of fatigue were
also extended to include various other ferroelectric systems such as Pb-free
compositions.
Key Words: PZT, Lead-free piezoelectrics, ferroelectricity, electrical fatigue
1. INTRODUCTION
Piezo/ferro-electric ceramics are among the most attractive and useful materials for
electronic devices, and various other high-technology applications. Both bulk
ceramics (single crystal or polycrystalline) and thin films are used in a broad variety
of applications ranging from sensors of many types to non-volatile memories [1-5].
One of the most useful properties of piezo/ferroelectric ceramics is their hysteretic
polarization and strain behavior under bipolar electric and mechanical stress.
Extensive research has been done on the study of polarization switching from one
state to another under the application of an external field in various ferroelectric
materials [6,7]. The concept of utilizing the reversible spontaneous polarization as a
memory state was one of the motivations for this extensive work from the early days
of ferroelectric research [5]. These fundamental studies established the ground work
for electronic devices which utilize the repeated reversal of spontaneous polarization.
When exposed to repeated alternating electric fields, the amount of switchable
polarization and/or strain of piezo/ferroelectrics are suppressed. For applications, i.e.,
ferroelectric memories, that utilize hysteretic behavior this type of degradation of
properties as called fatigue would be a major reliability problem [8-11]. Indeed for
most applications piezo/ferroelectric ceramics are exposed to repeated electrical
cycles forcing the material to repeatedly deform (piezoelectricity) or reverse its
spontaneous polarization (ferroelectricity) both causing property degradations due to
“electrical fatigue”. Technically it will result a reduction of many key electrical
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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property coefficients limiting use of the electroceramics for many applications. In
recent studies several mechanisms have been proposed to explain electrical
degradation issues and also certain strategies have been developed to control each of
them[12]. Various studies were done on the influence of composition on the electric
fatigue [13,14]. Reduced oxygen vacancy concentration and easier domain
reorientation have been presented as the key for the largely improved electrical
properties in donor doped ferroelectric bulk ceramics [15-17] and thin films [18-22].
Utilization of oxide electrodes or using layered type ferroelectrics instead of widely
used perovskites were two other main strategies that provide fatigue-free behavior in
certain systems. As the positive effect of donor doping on ferroelectric properties and
fatigue was attributed to the reduced oxygen vacancy concentration, another idea was
use of oxide electrodes to suppress the detrimental effects of oxygen vacancies.
These conducting oxides included RuO2,[23,24] IrO2,[25-27] SrRuO3,[28,29]
YBa2CuO7-,[30,31] and (La,Sr)CoO3[32-34]. When these oxide electrodes were used
in place of metal (mainly platinum) electrodes, polarization fatigue could be reduced
or eliminated. Compared to oxide electrodes, metals still have the advantage of their
much lower resistivity. The resistivity of the electrode material should be as low as
possible for the optimum device speed. Early in 1992, certain layered type perovskite
ferroelectrics (SrBi2Ta2O9 and similar compounds) were reported to show negligible
polarization fatigue (up to 1012 switching cycles) with Pt electrodes[35]. From the
application point of view, this was a most interesting achievement and has been
studied in several layered perovskite ferroelectrics with Pt electrodes, e.g.,
SrBi2Ta2O9 or SrBi2Nb2O9[36,37]. All of the experimental results briefly discussed
above led to the development of many theoretical models for the fatigue
phenomenon. An up-to-date review is presented by Tagantsev et al.[12] The
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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presently considered fatigue mechanisms in ferroelectrics include; surface layer
formation, damage of electrode and/or electrode/ferroelectric interface, pinning of the
domain walls by defects segregated in the wall region, clamping of polarization
reversal by volume defects, suppression of nucleation of oppositely oriented domains
at the surface. These mechanisms will modify either the applied electric field or the
switching process itself. Main focus of this study is investigating strategies to control
or eliminate electrical fatigue in various piezo/ferroelectric systems. The influence of
composition of the ferroelectric material, choice of electrode, and the characteristics
of the external electric field on fatigue has been studied in bulk PbZrTiO3 (PZT)
ceramics. Improvement of ferroelectric fatigue properties is observed when PZT is
donor doped with 2 and 4 at.% Nb. Compositional studies for the improvement of
fatigue were also extended to include various other ferroelectric systems such as Pbfree
compositions.
Reliability comparisons
were made between different
compositional systems and also different crystallographic directions in a system to
better understand fatigue related issues.
2. EXPERIMENTAL PROCEDURE
The flow sheet for the entire sample preparation process for polycrystalline PZT
ceramics is shown in Figure 1. Pure and Nb5+ doped PZT powders were made by
conventional solid-state reaction based on the formula of Pb1-0.5x 0.5x [(ZryTi1-y)1-x
Nbx]O3 where x=0, 0.02, 0.04, 0.06 and y=0.52 using appropriate amounts of
reagent-grade raw powders of lead carbonate (PbCO3), zirconium dioxide (ZrO2),
titanium dioxide (TiO2), and niobium pentaoxide (Nb2O5). Doped powders were
prepared in three different compositions to investigate the effect of dopant
concentration on the fatigue properties. Dry pressed pellets were exposed to binder
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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burnout process which was performed at 550 C for 2 hours. The green pellets were
put on platinum foil in alumina crucibles with 3 g of PbZrO3 (equimolar mixture of
PbO and ZrO2) as the lead source in a small crucible to minimize lead volatilization.
Sintering was performed at 1250 C for 2 hours with 10 C/min. ramp rates. X-ray
diffraction measurements were performed to determine the phase purity of calcined
powders and sintered ceramic samples. Scans were performed from a 2 of 20 to
80 at a scan rate of 4/min to determine primary phases present. Scanning electron
microscopy (SEM) study has been done to observe sintered microstructure of
samples. The ultimate goal of this study is to investigate polarization fatigue behavior
of PZT samples. To compare the results obtained in PZT, other perovskite type
samples of Na1/2Bi1/2TiO3 (NBT) ceramics and 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3
(PZN-4.5PT) single crystals were also tested for fatigue performance. Surface
preparation of samples is considered crucial in the fatigue process which is affected
by electrochemical parameters. For this reason, a great attention has been paid to
cleanness and homogeneity of surfaces. For electrical characterization, samples were
electroded on the two circular faces with sputter deposited gold (Au), silver (Ag), and
platinum (Pt) metals. Polarization and bipolar strain hysteresis (P-E and S-E)
measurements were performed simultaneously by using a modified Sawyer-Tower
circuit and linear variable differential transducer (LVDT) driven by a lock-in
amplifier (Stanford Research Systems, Model SR830). A high voltage amplifier
(Trek Model 609C-6) was used in both poling and polarization fatigue
measurements. Fatigue tests were performed under ac fields with a triangular wave
form. The dielectric properties of all samples were measured at room temperature.
Before applying any electric field on the samples for polarization fatigue test,
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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PbCO3
ZrO2
TiO2
(Nb2O5)
-DI water
-NH4OH
dispersant
Milling (~20 h)
Drying (150 0C, 10 h)
CALCINATION
(850 0C, 4 h)
-DI water
-NH4OH
dispersant
Calcined Powder
Milling (~24 h)
Drying (80 0C, 15 h)
PZT/PZNT Powder
acetone
Ground Powder
binder
XRD
Dry Pressing
Binder Burnout (550 0C, 2 h)
Sintering
(1250 0C, 2 h)
Figure 1. Preparation of undoped (PZT) and Nb-doped (PNZT) samples.
capacitance (C) and loss (D) were measured using an LCR meter (Model SR715,
Stanford Research Systems).
3. RESULTS AND DISCUSSION
3.1 Microstructural Characterization
The room temperature XRD pattern shown in Figure 2 indicates formation of
perovskite phase for PZT powder (JCPDS card number: 33784). After sintering,
densities of samples were measured and found to be consistent with the well-known
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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Figure 2. XRD pattern of PZT powder.
PZT characteristics. It is seen that the density of Nb-doped ceramics was higher than
those of undoped samples and densities increased with increasing Nb percent at the
same sintering conditions (1250 C, 2 hr.). In undoped PZT ceramics, the average
density is 7.51 g/cm3. On the other hand, the maximum density was achieved in 6%
Nb-doped PZT samples as 7.95 g/cm3. Figure 3 illustrates the Scanning electron
microscope (SEM) pictures of sintered samples for undoped, and 2% Nb-doped PZT.
Finer grain size was observed for dense Nb-doped samples. With the consistency of
lower densities of undoped samples grain size is larger and the grain size distribution
range is wider. The observed results can be explained with defect chemistry changes
in the system. The addition of Nb to the PZT solid solution impedes grain growth
while increasing density. This is explained by the effect of impurities on graingrowth mechanism [38]. It is believed that the donor doping ions concentrate near
the grain boundaries and extensively reduce boundary mobility. When the boundary
moves, it must drag the excess impurities with it. It has been reported [39] that
doping with aliovalent additives such as Nb5+ affects the densification rate of the
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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a)
b)
Figure 3. SEM pictures of undoped (a), and 2% Nb doped PZT ceramics sintered at
1200 oC for 2 h.
undoped PZT system, resulting from metal vacancies associated with doping ion for
compensating charge valence. Similar results were found in La-doped PZT system by
Haertling [40]. Weight loss values are decreasing with Nb doping. The maximum
and minimum weight loss values were observed in undoped and 6% Nb doped PZT
disks, respectively as 2.97 and 1.1%.
3.2 Electrical Characterization
For devices that use ferroelectric ceramic materials such as memory applications,
polarization fatigue refers to decrease of polarization with the increase of switching
cycles under an applied electric field, usually accompanying a change in coercive
field. Polarization fatigue behavior is most conveniently observed with the
measurements through hysteresis loops. Remanent polarization (Pr) which remains
after a material has been fully polarized and then had the field removed and coercive
field (EC), a specific field which results in zero net polarization, are marked on P-E
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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hysteresis loops. In our experiments, both undoped and doped ceramics showed
remarkable polarization fatigue when exposed to bipolar AC cycling. The main
approach in this study is to characterize polarization fatigue by controlling the
stoichiometry, metal type, application of electric field conditions. All the results were
normalized to make comparisons and emphasize differences in properties for
different samples. Normalized values represent the percentages of the measured
properties, such as remanent polarization, coercive field etc., with respect to the
initial values of those obtained at a few switching cycles.
Undoped and aged PZT samples exhibited “pinched” type hysteresis loops as shown
in Figure 4. The shape of hysteresis loop resumed normal shape after some cycling
under AC driving as shown in Figure 5.a. Changes in P-E Hysteresis loops for
undoped (a) and 2% Nb-doped (b) PZT samples before and after extensive electrical
cycling is shown in Figure 5. As seen in Figure 5.a, continuing driving increased the
remanent polarization (Pr) and coercive field (EC) in undoped PZT. This increase in
60
Undoped
Nb-doped
2
Polarization (C/cm )
40
20
0
-20
-40
-60
-40
-30
-20
-10
0
10
20
30
40
Electric Field (kV/cm)
Figure 4. P-E Hysteresis loops for undoped and 2% Nb-doped PZT ceramics. (Note
pinched shape for undoped PZT)
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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polarization behavior was attributed to the “de-aging”[39]. Aging is a process which
causes a steady decrease in the polarization values. During the cycling restoration of
decreased ferroelectric properties indicates de-aging in undoped PZT ceramics. One
other explanation of aging in PZT is that when even very little amount of Nb5+ ( 2%
) added as donor dopant, the hysteresis behavior changed totally as seen in Figure
5.b. This is consistent with the results obtained by other researchers [41,42].
Substitution of a donor species for a B-site cation reduces the Vo¨ concentration that
result from intrinsic acceptor impurity incorporation and/or PbO volatilization and
superoxidation [42]. Consistent with this picture, it is found that all well-known
donor dopants, such as Nb5+, Ta5+ , and W6+, reduce the thermally induced aging
[41,43]. In terms of polarization fatigue behavior, it is difficult to explain the
phenomena in undoped PZT samples due to the competition between de-aging and
polarization fatigue processes. It seems that in undoped PZT, aging is dominant over
polarization fatigue. For this reason, undoped PZT samples will be ignored in the
comparison of fatigue behavior in PZT samples with different Nb compositions as
seen in Figure 6. Donor doping with 2,4, and 6 atomic percent Nb5+ eliminated the
aging problem in PZT. However, Nb modification has a complex effect on
polarization fatigue rate. It appeared that when PZT is doped with 4% Nb, fatigue
rate becomes slower to compared to 2 and 6 % Nb contents. The generally accepted
philosophy of donor doping PZT to enhance fatigue resistance should therefore be
restated. There is an improvement in the fatigue behavior of PZT with Nb donor
additions but, above a critical amount Nb is detrimental to fatigue resistance. The
effects of introducing negative charge should be re-examined to understand the
reason behind it. It is quite possible that electronic charge trapping can lock domain
walls and lead to the suppression of the switchable polarization in PZT subjected to
electrical fatigue [44]. Some Nb doping can compensate the effects of positively
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SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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30
2 cyc.
5
8.10 cyc.
2
Polarization (C/cm )
20
10
0
-10
-20
-30
-40
-30
-20
-10
0
10
20
30
40
20
30
40
Electric Field (kV/cm)
(a)
60
2 cyc.
5
3.10 cyc.
2
Polarization (C/cm )
40
20
0
-20
-40
-60
-40
-30
-20
-10
0
10
Electric Field (kV/cm)
(b)
Figure 5. P-E Hysteresis loops for undoped (a) and 2% Nb-doped (b) PZT before and
after extensive electrical cycling.
charged impurities, such as holes and oxygen vacancies. The improvement of
properties in 4% Nb-doped samples over 2% doped samples can be explained with
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Figure 6. Remanent polarization (Pr) change for PZT samples with different amount
of Nb as a function of switching cycles.
the insufficient negative charge supply to compensate all positively charged impurity
effects. Increased rate of degradation of remanent polarization with higher Nb doping
can be due to excess negatively charged species that possibly cause again charge
trapping. It might be also necessary to consider the effect of doping on the other
factors in terms of defect migration affecting polarization fatigue.
Polarization fatigue has been considered as an interface-initiated problem in recent
models [45]. In this study, three different electrode materials were sputtered on 2%
Nb-doped PZT samples and investigated for their effects on polarization fatigue
behavior of the samples. PZT samples electroded with silver (Ag), gold (Au), and
oxide (SrRuO2) materials were cycled under 32 kV/cm a.c. field at 100 Hz up to
3.105 cycles, and the results are plotted in Figure 7.a. Gold electroded 2% Nb doped
PZT samples were driven under different (32, 40, and 45 kV/cm) electric fields. The
results are reported in Figure 7.b.
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Remanent Polarization Change (%)
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0
SrRuO2
Ag
Au
-5
-10
-15
-20
-25
10
100
1000
10000
100000
Switching Cycles
Remanent Polarization Change (%)
(a)
0
25 kV/cm
32 kV/cm
40 kV/cm
-10
-20
-30
-40
-50
10
100
1000
10000
100000
Switching Cycles
(b)
Figure 7. Remanent polarization (Pr) change as a function of switching cycles for
PZT ceramics with different electrode materials (SrRuO2, Au, Ag) at a
fixed 32 kV/cm (a) and samples with gold (Au) electrode exposed to
different electrical field strengths of 25, 32, and 45 kV/cm.
Results indicate a weak effect of both factors on fatigue for the chosen materials and
electric field range in this study. An explanation that focuses on vacancy migration
and accumulation near the electrodes creating lower capacitance regions may be
considered for fatigue. It has been suggested by Warren et al.[44] that the high field
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60
50
40
30
20
virgin
fatigued
10
0
200
400
600
800
1000
Depth from surface (Å)
Figure 8. Oxygen concentration change relatively from surface through bulk in
fatigued PZT sample.
during voltage cycling injects, or creates, mobile carriers in the ferroelectric. It was
proved in this study by XPS results as presented in Figure 8 that in fatigued samples
defect concentration increased from core to the surfaces under electrodes.
Charge trapping phenomena consider grain boundaries as trapping sites. To eliminate
the effect of charge trapping in grain boundaries on fatigue, single crystal samples
were used in fatigue experiments. Figure 9 shows crystallographic orientation
dependence of polarization fatigue in 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 (PZN4.5PT) single crystals oriented along with <001> and <111> directions [46]. This
fundamental observation was also tested in Na1/2Bi1/2TiO3 (NBT) ceramics with
randomly oriented and textured grains. Similar to the PZN-PT, polycrystal NBT
ceramics [47] also demonstrated fatigue anisotropy. This would provide new
opportunities for “fatigue-free” devices. With quality improvements and better
texturing better and more stable material properties would be beneficial for new
devices utilizing polarization reversal in these ferroelectric ceramics.
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Figure 9. Crystallographic orientation dependence of polarization fatigue in
0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 (PZN-4.5PT) single crystals
oriented along with <001> and <111> directions.
4. CONCLUSIONS
The study of polarization fatigue in PZT ceramics through fatigue induced changes
as a function of different switching conditions has been intended in this study. It is
found that polarization fatigue can be controlled by optimizing the conditions both
related with design and working parameters. Undoped PZT samples showed very
strong aging effects. It is seen that aging process is dominant over fatigue in pure
PZT. Even very small amount (2 at.%) of Nb (donor) doping completely eliminated
the aging in PZT. The best fatigue performance was gained in 4% Nb doped samples.
However, when PZT is doped with 6% Nb, an increased fatigue rate was observed
indicating excessive doping does not necessarily impede fatigue rates. The possible
mechanism to explain this result might be related with charge trapping phenomena
considering excess negatively charged species in a highly donor doped PZT ceramic.
Relatively weak influence of differing electrode materials and applied electric field
strengths was also observed. High fatigue rate is observed in single crystals clarifying
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the grain boundary effects on fatigue. Similar to the observations in single crystals,
textured ceramics also indicate orientation dependence of fatigue. This may provide
2
Polarization (C/cm )
potentials for the control of fatigue in polycrystal ceramics.
40
2 cycles
3
5.10
20
0
-20
NBT random
-40
-120 -90
-60
-30
0
30
60
90
120
Electric Field (kV/cm)
2
Polarization (C/cm )
40
2 cycles
3
5.10
20
0
-20
NBT<001> textured
-40
-100 -80 -60 -40 -20
0
20 40 60 80 100
Electric Field (kV/cm)
(a)
(b)
Figure 10. P-E hysteresis loops for Na1/2Bi1/2TiO3 (NBT) ceramics with randomly
oriented (a) and textured (b) grains. (Solid lines and dashed lines
showing polarization behavior after 2 and 5x103 electrical cycles,
respectively.)
16
SERES’09 I. International Ceramic, Glass, Porcelain Enamel, Glaze and Pigment Congress
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ACKNOWLEDGEMENT
This work was partially supported by The Center for Dielectric Studies at Pennsylvania
State University. Crystals used for this study were provided by Dr. T. R. Shrout, and Dr.
S. Zhang. One of the authors (M. Ozgul) would like to acknowledge Ministry of
National Education of Turkey for the financial support provided by.
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