ISSN 1318-0010
KZLTET 32(3-519)165(1998)
CORRELATIONS BETWEEN FATIGUE AND VARIATIONS IN MATERIAL CONSTANTS OF PLZT
CERAMICS
KORELACIJE MED UTRUJANJEM IN SPREMEMBAMI MATERIALNIH KONSTANT PLZT-KERAMIKE
ADRIJAN LEVSTIK, V. BOBNAR, Z. KUTNJAK, M. KOSEC
Jožef Stefan Institute, P.O. Box 3000, 1001 Ljubljana, Slovenia
Prejem rokopisa - received: 1998-10-05; sprejem za objavo - accepted for publication: 1998-12-14
Correlations between fatigue and material constants of bulk 8/65/35 PLZT ceramics were investigated by a simultaneous measurement of fatigue and piezoelectric resonance as a function of switching cycles. It was found that as remanent polarization, also the real and imaginary parts of the complex dielectric and piezoelectric constants decrease with increasing number of switching cycles. However, real and imaginary parts of the complex elastic compliance and the electrical, mechanical, and electromechanical quality factors remain unchanged during fatigue, which suggests that there is no correlation between the fatigue and the mechanical degradation of the sample.
Key words: PLZT, fatigue, piezoelectric resonance, electromechanical properties
Korelacije med utrujanjem in materialnimi konstantami 8/65/35 PLZT-keramike so bile raziskane s simultano meritvijo utrujanja in odvisnosti piezoelektrične resonance od števila preklopnih ciklov. Enako odvisnost od števila preklopnih ciklov kot remanentna polarizacija imajo tudi realni in imaginarni deli kompleksnih dielektrične in piezoelektrične konstante. Realni in imaginarni del kompleksne elastične konstante ter električni, mehanski in piezoelektrični faktor kvalitete pa se med utrujanjem ne spreminjajo, kar pomeni, da med utrujanjem ni prišlo do mehanske degradacije vzorca. Ključne besede: PLZT, utrujanje, piezoelektrična resonanca, elektromehanske lastnosti
1	INTRODUCTION
Ferroelectric switching in thin films and bulk ferroelectric ceramics enables the application in memory elements and electrographic printing1. One of the material characteristics playing an important role in this application is fatigue - decreasing of the remanent polarization with increasing number of switching cycles. While the shape of the dielectric hysteresis loop and the value of the remanent polarization influence the quality of printing characteristics, the fatigue effect constrains the maximum number of rewriting cycles in PLZT material. Fatigue and its consequences for commercial applications of ferroelectrics have been recently extensively studied2-5. Electromechanical properties such as elastic, piezoelectric, and dielectric constants have been studied by the piezoelectric resonance method in various ceramic materials6-8, however, their dependence on fatigue has not yet been reported.
In this work we present measurements of the fatigue and piezoelectric properties of 8/65/35 PLZT ceramics carried out simultaneously in order to assure equal conditions during cycling procedure for all measuring quantities. Such an experiment makes possible to identify existence of correlations between fatigue and variations in material constants.
2	EXPERIMENTAL METHODS
Bulk 8/65/35 PLZT ceramics used in these studies was prepared as described elsewhere9. The sample was
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cycled with an a.c. electric field at the frequency of 50 Hz and the amplitude of 15 kV/cm. During cycling, hysteresis loops were measured by Sawyer-Tower technique. After each measurement of the hysteresis loop, the sample was poled by the d.c. electric field of 15 kV/cm. Then, the piezoelectric resonance was studied by measurements of admittance IY I and phase angle 8 in the frequency interval 1kHz - 1MHz.
From piezoelectric resonance data complex piezoelectric and dielectric constants and elastic compliance can be determined, since the admittance of the piezoelectric bar resonator is a function of the complex dielectric £33* = £33^33" and piezoelectric d31* = d31'-d31'' constants and complex elastic compliance su* = sn'-sn". The treatment of a piezoelectric bar resonator can be in the vicinity of the resonant frequency simplified by using the equivalent electric circuit, which was introduced by Damjanovi}8. Following his approach, the parameters of the equivalent circuit R, C, L, C0, R2, and x were determined by simultaneous fitting the experimental data to the expressions for the conductance G = I YI cos8 and susceptance B = I YI sin8 of the equivalent electrical circuit, which are expressed as
G
w2 RC2
(1-w2 LC)2 + w2R2C2
wC - w3 LC2
- x
1
+ —
(1-w2 LC)2 + w2R2C2 R;
(1)
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A. LEVSTIK ET AL.: CORRELATIONS BETWEEN FATIGUE AND VARIATIONS..
Figure 1: The remanent polarization Pr and the coercive field Ec versus switching cycles. Solid lines are guides to the eye Slika 1: Remanentna polarizacija Pr in koercitivno polje Ec kot funkciji preklopnih ciklov. Polne krivulje so vodilo očem
B =
wC - w3 LC2
(1-W LC)2 + w2R2C2
w2 RC2
+ x-
(1-w2 LC)2 + w2R2C2
+ wC0
(2)
E i 4
CdMuduiaa (ffld)
Figure 2: Susceptance B versus conductance G obtained at four different numbers of switching cycles. Dots represent experimental points. Solid curves represent fits to eqs (1) and (2) Slika 2: Susceptanca B kot funkcija prevodnosti G pri štirih različnih vrednostih preklopnih ciklov. Točke predstavljajo eksperimentalne rezultate. Polne krivulje so rezultat prilagajanja enačb (1) in (2)
the circles starts to decrease rapidly after 104 switching cycles, indicating suppression of the piezoelectric resonance amplitude which occurs together with the decreasing of the remanent polarization.
The real and imaginary parts of the complex material constants as a function of switching cycles are shown in Figure 3. At the same number of switching cycles as in the case of the remanent polarization dependence, also real and imaginary parts of the complex dielectric and piezoelectric constants start to decrease. Contrary to this, the elastic compliance is nearly independent on switching cycles. A similar behavior of the piezoelectric constant d33 was recently observed in PZT thin films5. As can be seen in Figure 3, the electrical Qe = £33'^'', mechanical Qm = sii'/sii'', and electromechanical Qm = d3i'/d3i'' quality factors are all independent on switching cycles. Namely, the real part of the particular constant has the same dependence on the increasing number of switching cycles as its imaginary part. This suggests that there is no correlation between the fatigue and the mechanical degradation of the sample, like for instance the
The set of complex material constants was then calculated from the parameters of the equivalent circuit.
3 RESULTS
The ferroelectric fatigue is shown in Figure 1. The remanent polarization Pr decreases steeply above 2 x 104 switching cycles, above which also the coercive field Ec shows a weak change in the slope.
Figure 2 shows experimentally determined conductance G and susceptance B in a complex plane and the results of the simultaneous fitting to eqs. (1) and (2) at four different numbers of switching cycles. The radius of
Figure 3: Various material properties as a function of switching cycles: (a) the real £33' and imaginary £33" parts of the complex dielectric constant, (b) elastic compliances su' and su" and (c) piezoelectric constants d3j' and d3j". Solid lines are guides to the eye Slika 3: Materialne konstante v odvisnosti od števila preklopnih ciklov: (a) realni £33' in imaginarni £33'' del kompleksne dielektrične konstante, (b) elastični konstanti su' and su" in (c) piezoelectrični konstanti d3j' in d3j". Polne krivulje so vodilo očem
520
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A. LEVSTIK ET AL.: CORRELATIONS BETWEEN FATIGUE AND VARIATIONS.
appearance of microcracks. It is interesting to note that the thermal annealing of the fatigued sample at 590°C nearly completely restored the remanent polarization to its initial value, pointing out that the pinning of domain walls due to entrapment of space charges at the electrode or domain interface is most likely responsible for the fa-tigue4.
The simultaneous measurement of piezoelectric properties and the fatigue has an important advantage in comparison to the case where measurements are performed in separate runs on different samples. Separate measurements would introduce an additional data scattering due to the known problems with cleaning procedure and electrode deposition, but the most important problem is onshelf aging - variations in dielectric properties with time in zero-field conditions. This is particularly important in PLZT ceramics where decrease in the dielectric constant for more than 20% can be observed within few hours after annealing. This phenomenon makes almost impossible to bring two different (or one annealed) samples in exactly the same state since their histories will never be exactly the same after thermal annealing.
4 CONCLUSIONS
The fatigue in bulk 8/65/35 PLZT ceramics reflects itself only in dielectric and piezoelectric properties, but not in elastic compliances. On the basis that the electrical, electromechanical and mechanical quality factors are independent on switching cycles, it was concluded that there is no correlation between the ferroelectric fatigue and the mechanical degradation of the sample.
ACKNOWLEDGEMENTS
This work was supported by Man Roland Druckmaschinen AG and by the Ministry of Science and Technology of Slovenia.
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