Poling electric field dependent domain switching and

Poling electric field dependent domain switching and piezoelectric properties of mechanically activated (Pb0.92La0.08) (Zr0.60Ti0.40)O3 ceramics Ajeet Kumar, V. V. Bhanu Prasad, K. C. James Raju & A. R. James

Journal of Materials Science: Materials in Electronics ISSN 0957-4522 J Mater Sci: Mater Electron DOI 10.1007/s10854-015-2899-1

1 23

Your article is protected by copyright and all rights are held exclusively by Springer Science +Business Media New York. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.

1 23

Author's personal copy J Mater Sci: Mater Electron DOI 10.1007/s10854-015-2899-1

Poling electric field dependent domain switching and piezoelectric properties of mechanically activated (Pb0.92La0.08)(Zr0.60Ti0.40)O3 ceramics Ajeet Kumar • V. V. Bhanu Prasad K. C. James Raju • A. R. James

•

Received: 24 November 2014 / Accepted: 28 February 2015 Ó Springer Science+Business Media New York 2015

Abstract (Pb0.92La0.08)(Zr0.60Ti0.40)O3 (PLZT 8/60/40) ceramics were prepared via a combinatorial approach of high energy mechanical ball milling (mechanical activation), followed by cold isostatic pressing. Electrical properties of the piezoelectric ceramics are greatly influenced by the poling conditions (poling electric field, poling time and poling temperature). In this present study, the effect of the poling electric fields on the piezoelectric properties of PLZT 8/60/40 ceramics was investigated. It is a common practice to subject piezoelectric ceramics to electric fields well beyond their coercive fields, in order to pole them but here the measured values of piezoelectric (d33 and g33) and electromechanical coupling coefficients (kp, k33 and k31) at different poling fields show that a ferroelectric material can be poled at *5 kV/cm (\0.5 Ec), far below the coercive field without compromising the induced high piezoelectricity. The results were discussed with the help of polarization, current and strain versus electric field curves.

1 Introduction From the time of its discovery, great attention has been paid to a ferroelectric material with perovskite structure, lanthanum A. Kumar (&) V. V. Bhanu Prasad A. R. James Ceramics and Composites Group, Defence Metallurgical Research Laboratory, Hyderabad 500058, India e-mail: [email protected] V. V. Bhanu Prasad e-mail: [email protected] A. Kumar K. C. James Raju School of Physics, University of Hyderabad, Hyderabad 500046, India e-mail: [email protected]

modified Pb(Zr1-xTix)O3 and its solid solutions for their ultrahigh piezoelectric and electromechanical coupling coefficients. Recently, there has been a spurt of research on lead free piezoelectric ceramics. However, at present, there are no commercially available lead-free ceramics to completely replace lead based ceramics. PLZT based ceramic systems, which embraces all compositional aspects of the dielectric, piezoelectric, pyroelectric, ferroelectric and electro-optic properties, are widely used for numerous types of sensors, actuators, nano-positioners and transducers [1]. PZT compositions show optimum electrical properties which are intensively used for technological applications (sensors and actuators, micro-electromechanical systems, and high frequency devices), near to the morphotropic phase boundary (MPB), a region separated by tetragonal (P4mm) and rhombohedral (R3m) phases. The coexistence of rhombohedral and tetragonal phases can be linked with the martensitic phase transition theory (reorientation of complex nanodomain structures) [2] or by the existence of a monoclinic Cm phase (the polarization rotation mechanism) [3, 4]. Piezoelectric properties of the PLZT ceramics depend on many factors, such as composition and processing techniques, MPB conditions [5, 6], grain sizes [7–10], poling conditions [11–13] etc. Studies show that at the vicinity of the MPB, different ceramic materials have outstanding piezoelectric properties, which can be attributed to the unique structural feature of the coexistence of tetragonal and rhombohedral phases. In the vicinity of the MPB region, the six domain states of the tetragonal structure along the (100) direction (90° and 180° domains) symmetry coexist with the eight domain states of the rhombohedral along (111) directions (71°, 90° and 180° domains) symmetry. This results in 14 possible available directions of spontaneous polarization. All of the aforementioned factors make the properties show a maximum

123

Author's personal copy J Mater Sci: Mater Electron

around the MPB [14]. PLZT 8/60/40 composition is near the MPB [15] and shows the highest strain [16] and better piezoelectric and coupling coefficients [17]. It is well known that in a piezoelectric polycrystalline ceramic, the ferroelectric domains are randomly reoriented and do not show any piezoelectric property and are piezoelectrically inactive. Piezoelectricity can be induced in ferroelectric ceramics by a process called poling. In this process, a direct current (dc) electric field with strength higher [18] than the coercive field strength is applied to the ferroelectric ceramic at a high temperature (higher poling temperature facilitates the motion of domains) which results in spontaneous polarization within each grain to get oriented towards the direction of the applied field. This leads to a net polarization in the direction of poling field. Hence, the process of poling, the alignment of domains to the external electric field, plays an important role in the piezoelectric ceramics. Jiang et al. [19] experimentally as well as theoretically calculated the field dependence of domain switching speeds in a wide temperature range using extended Merz’s equation. From classical domain-nucleation models with thermal fluctuations, an ultimate nucleation time (ps) was derived and studied the domain switching response for the ferroelectric capacitor Cf under a square pulse with the voltage V(t) which is measured through in series resistors with the total resistance Rt in the circuit. Domain switching time, current and field were estimated and matched with experimental values. Experimental observations using electrical pulses on plate like ferroelectric capacitors have shown that the time and space averaged speed, m, of domain wall motion under a field, E, follows Merz’s equation (1) [19–21] m ¼ m0 ed=E

ð1Þ

where d is the activation field and m0 is the ultimate nucleation speed. Nucleation of oppositely polarized domains at defects (dislocation or inclusion) helps the polarization switching [22]. There were very few reports about the effect of poling condition on the properties of PZT and PLZT ceramics. Available reports [11–13, 23] suggest that to get good piezoelectric properties, poling field should be higher than the coercive field. The purpose of this paper is to investigate the effect of poling electric fields on the piezoelectric properties of PLZT 8/60/40 electroceramics.

2 Materials and methods 2.1 Preparation of PLZT 8/60/40 electroceramics and characterization techniques PLZT 8/60/40 was prepared via a mechanical activation method. Stoichiometric quantities of commercially available

123

raw oxide powders of PbO (99.9 %), La2O3 (99.9 %), ZrO2 (99 %) and TiO2 (99.9 %) (Sigma Aldrich, USA) were weighed and subjected to the high energy mechanochemical (HEM) ball milling operation using zirconia vial and balls. Full details of ceramic preparation processing structural, morphological, ferroelectric and piezoelectric characterization techniques are available in previous publication [16]. 2.2 Poling of PLZT 8/60/40 ceramics High temperature conductive silver paste was applied to both surfaces of the disks as electrodes and then fired at 500 °C for 15 min for proper curing. Ceramic samples were electrically poled for 15 min and at 120 °C for different poling electric fields by immersing the samples in silicone oil (Dow Corning 704) bath to acquire all electromechanical and piezoelectric properties.

3 Results and discussion 3.1 Micro-structural and morphological studies (density, XRD, SEM) Before going to study the electrical properties of the ceramics, first micro-structure of the ceramic should be examined carefully. Ceramic should have a minimum density ([95 %) [24], fine grain sizes, single and pure perovskite phase. The electrical properties of such materials are significantly affected by the density of ferroelectric ceramics. Density of the PLZT 8/60/40 ceramics was calculated and found to be[98 % relative of the theoretical density. Grain size is an important factor that affects the dielectric constant along with tetragonality (c/a ratio), phase transition temperature (Tc) and polarization of the ferroelectric ceramics. Previous study [16] (SEM image is not shown here) of the sintered fractured surface of PLZT 8/60/40 composition shows a very dense structure, uniform grain sizes with clearly visible grain shapes indicating the existence of a polycrystalline microstructure with average grain size \1.5 lm. Fine grain size ceramics are found to be good compared to conventional ceramics [25]. The grain size dependence of the ferroelectric, dielectric and piezoelectric properties of PLZT 8/60/40 ceramic was studied by Kamel et al. [8]. Figure 1 shows the X-ray diffraction (XRD) pattern of (Pb0.92La0.08)(Zr0.60Ti0.40)O3 for the sintered compact of PLZT 8/60/40 ceramics. After the sintering, pure perovskite single phase was observed for PLZT ceramics free from any secondary phases that indicate the complete chemical reaction, confirmed by JCPDS 53-0698 file. The average crystallite size for PLZT 8/60/40 was calculated by using full width at half maxima from the Debye–Scherrer


formula and found to be 24 nm. Hence the PLZT 8/60/40 ceramics have fine grains, single-pure perovskite phase and good density and can be used for further electrical characterizations. 3.2 Ferroelectric studies (P–E hysteresis loop) Piezoelectric properties can be induced by aligning the randomly oriented domains inside each grain of ceramic materials in the direction of an applied external electric field, called as poling electric field, higher than the coercive field. The field necessary to bring the remnant polarization (Pr) to zero is called the coercive field (Ec) and can be evaluated from the polarization versus electric field (P– E) hysteresis loop. Current versus electric field (I–E) curves also can be used for the same. When an electric field applied to ferroelectric materials, two types of current signal was observed, leakage current and current due to domain switching phenomenon. A peak in the current signal before reaching the maximum electric field indicates that domain switching is taking place. The leakage current at the maximum applied electric field is very low compared to the peak current caused by domain switching. The coercive field Ec can be identified as the electric field corresponding to the domain switching current peak, from the saturated P–E hysteresis loop. Polarization reversal or switched domain fraction is described by uniform random nucleation of reversed domains according to the Kolmogordov–Avrami–Ishibashi (KAI) model with a single filed domain-switching time t0 under different voltages (V) [20, 26–28] pðtÞ ¼ 1 e

ðt=t0 Þn

ð2Þ

domain growth (n = 2 for 2D growth and n = 3 for 3D growth). The Landau–Ginzburg (LG) mean-field theory of ferroelectricity predicts the intrinsic coercive field for polarization reversal [29]. The experimentally measured value of the coercive field in real ferroelectrics is invariably much smaller than the intrinsic predicted value which is caused by localized nucleation of domains with reversed polarization, which then grow and coalesce by domain wall motion [30, 31]. The Gibbs free energy density G may be described by the Landau–Ginzburg (LG) expansion in the terms of electrical polarization P to derive ferroelectric equation of state: [29, 32] G ¼ F0 þ

a 2 b 4 c 6 P þ P þ P PE 2 4 6

ð3Þ

where a ¼ a0 ðT T0 Þ, T is the temperature, E is the electric field, and F0 is the free energy density of the paraelectric phase at zero electric field. The stable polar state exists at temperatures below the phase transition temperature Tc. The electric field E [30] is calculated from the minimum of free energy density G (Eq. 3), and inversion of this relation gives the intrinsic polarization hysteresis function P (E) for the ferroelectric state, at temperatures below TC. At the turning points (intrinsic coercive field Ec) in the P–E loops, polarization shows the bistable nature and reversed its direction. The magnitude of the intrinsic coercive field Ec [30] is calculated from the extrema of Eq. (4): Ec ¼ Ec0 f ðtÞ

qffiffi

ð4Þ 5=2

where t is time elapsed after the application of the external field E and n is the material parameter, dimension of

6 3 jbj where Ec0 ¼ 25 and 5 c3=2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi h qffiffiffiffiffiffiffiffiffiffi i 5 1 5 5 p1ffiffi 1 þ 1 t t t 1 þ 1 9 2 9 3 The 2

Fig. 1 X-ray diffraction pattern for the PLZT 8/60/40 sintered ceramic showing single phase perovskite structure

of the intrinsic coercive field at temperature T0 is EC0. As discussed above, both the I–E and P–E curves can be used to calculate the coercive field (a field responsible for domain switching current peaks in an I–E curve and for the polarization flipping in P–E hysteresis curve). Figure 2a to d shows the P–E hysteresis loops and I–E loops of PLZT 8/60/40 unpoled ceramics at 10, 12, 21 and 45 kV/cm, respectively. The appearance of a domain switching current peak (Fig. 2b), when the applied electric field exceeds 10 kV/cm confirms the ferroelectric nature of PLZT ceramics. The magnitude of domain switching current peak increases with an increase in applied electric field and finally gets saturated. Figure 2 shows the alignment of ferroelectric domains in the direction of applied electric field. Once the domains started switching (Fig. 2b), the corresponding P–E curve shows an improvement in squareness and symmetry compared to the previous Fig. 2a, that can be further improved by the application of a higher electric

f ðt Þ ¼ magnitude

123


Fig. 2 Evolution of polarization versus electric field (P–E) hysteresis curves and current peaks corresponding to the domain switching mechanism in current versus electric field (I–E) curves at different

electric fields a 10 kV/cm, b 12 kV/cm, c 21 kV/cm and d 45 kV/cm for the PLZT 8/60/40 unpoled ceramic sample at 1 Hz and 25 °C

field. P–E loops show saturation in polarization, symmetry and squareness, when most of the domains are aligned. Figure 2d shows the typical P–E hysteresis loop and I–E curve of PLZT 8/60/40 ceramics at room temperature. The saturated P–E hysteresis loop and the domain switching current peaks before the maximum applied electric field, both confirm the ferroelectric nature of PLZT 8/60/40 ceramic. Remnant polarization (Pr) and coercive field (Ec) values were found to be *34.03 lC/cm2 and 11.97 kV/cm, respectively from the saturated hysteresis P–E loop (Fig. 2d) of PLZT 8/60/40. Based on the coercive field value (From Fig. 2), PLZT 8/60/40 ceramics were poled at different poling fields to study the effect of poling field. Maximum remnant polarization (Pr) for the polycrystalline ferroelectric materials depends upon the available domain states. The remnant polarization, coercive field (Ec) and shape of the loops are affected by many factors such as charged defects, mechanical stresses, preparation conditions and thermal treatment [1, 14]. High remnant polarization, low coercive field, well saturated loop shape, high strain and low hysteresis loss can possibly be attributed to a uniform distribution of grain sizes, fewer defects and imperfections in the crystallites. Figure 3 shows the change in remnant polarization (Pr) and coercive field (Ec) as a function of applied electric field

for the PLZT 8/60/40 electroceramics. Geneko et al. [33] studied the effect of domain switching time as well as applied electric fields on the polarization reversal and their results for applied electric field were matched with Fig. 3. In this paper we studied only an applied electric field (not time) dependent polarization. In Fig. 3 it can be seen that the remnant polarization increases rapidly with an application of electric field and gets saturated due to the alignment of domains. The coercive field curve also follows the same path and shows the same saturation and switching field characteristics. Saturation of remnant polarization is related to the alignment of domains in the direction of the applied electric field. Switching of domains separated by 180° domain walls plays an important role and has a major contribution to the Pr value. The dependence of both Pr and Ec on the applied electric field and switching at the same electric field value suggests that both parameters depend on the 180° domain wall switching.

123

3.3 Poling field studies After the cooling process (paraelectric phase to ferroelectric phase), a grain may contain many randomly oriented domains and ceramics have no piezoelectricity. Polycrystalline


Fig. 3 Variation in remnant polarization and coercive field as a function of applied electric field for the PLZT 8/60/40 unpoled ceramic sample at 1 Hz and 25 °C

ferroelectric materials are brought into a polar state by applying a strong electric field (at times at elevated temperatures). With poling; reorientation of domains is possible within the individual grains in the direction of field for polycrystalline ferroelectric materials. Maximum remnant polarization for the polycrystalline ferroelectric materials depends on available domains state. For those ferroelectric electro ceramics having only 180° domains, after poling, maximum remnant polarization (maximum Pr) is 0.25 Ps. Maximum remnant polarization values for the tetragonal (six domain states), rhombohedral (eight domain states) and orthorhombic (twelve domain states) symmetries are 0.83 Ps, 0.87 Ps and 0.91 Ps respectively, assuming reorientations of all domains along the available associated direction by poling field [14]. However the actual polarization is always lower because many domains cannot reorient due to their complex set of internal stresses and electric fields in grains. Figures 4a, b and c represents the schematic orientations of domains for un-poled, poled (*0.5 Ec) and excess poled (*3 Ec) PLZT ceramics respectively. Figure 4a shows the randomly oriented domains in un-poled PLZT electroceramics in the absence of an electric field. When an electric field is applied to the un-poled ceramics, domains begin to align in the direction of applied electric field. After crossing the threshold electric field; most of the domains suddenly switch into the direction of applied electric field (Fig. 4b) that can be seen in Fig. 2b and for Figs. 5 and 6 (*5 kV/cm, *0.5 Ec). After domain switching, the piezoelectric properties do not show much change with increase in the electric field (0.5 Ec B E B 3Ec), when the remaining domains gets aligned (Fig. 4c). As discussed earlier, from the crystallographic point of view, there are 14 available domain orientation states in perovskite structured PLZT 8/60/40 ceramics with the

Fig. 4 Schematic representation of orientations of domains for a unpoled, b poled (*0.5 Ec) and c excess poled (*3 Ec) PLZT ceramics respectively

existence of 180° and non-180° (71° and 90°) domains. 180° domains can be reversed with only minimal structural strains, while the switching of non-180° domains requires significantly larger deformation. As a consequence, the reversal of 180° domains can be easier than the switching

123


Fig. 5 Variation in piezoelectric charge coefficient (d33) and piezoelectric voltage constant (g33) values as a function of poling field

Fig. 6 Change in electromechanical coupling coefficients (kp, k33 and k31) with respect to the poling electric field

of non-180° domains. Consequently, only the reversal of 180° domains and little switching of non-180° domains can be detected during lower poling field. Figures 5 and 6 shows the change in piezoelectric charge coefficient (d33), piezoelectric voltage constant (g33) and electromechanical coupling factors (kp, k33 and k31) of PLZT 8/60/40 ceramic as a function of poling electric field with poling time (15 min) and temperature (120 °C) being fixed as constants. It can be seen that the electrical properties (d33, g33, kp, k33 and k31) of PLZT 8/60/40, are significantly influenced by the poling electric fields. An abrupt increase in the piezoelectric coefficients and electromechanical coupling factor were observed at *5 kV/cm (EC * 11.97 kV/cm). This critical poling field is denoted as EP. It is noted that d33, g33, kp, k33 and k31 values only slightly increases and quickly saturates beyond EP. Thereafter, the d33 value of the ceramics increases slightly when the poling field beyond EP is applied. One possible

123

explanation for the results shown in Figs. 5 and 6 is that EP is determined under DC field while EC is under AC field and is known to be strongly frequency dependent [34–36]. Electric fields used for poling can cause domain switching and reorientation, and then make the ceramics exhibit piezoelectric properties. Therefore, the degree of domains oriented is very important to obtain higher piezoelectric properties. During the course of poling, 180° domains switching do not affect the spontaneous strain, only non180° domains can cause spontaneous strain, thereby, 180° domains can switch more easily than non-180° domains. Figures 5 and 6 shows that an increase in d33, g33, kp, k33 and k31 results from the switching of 180° domains at lower poling fields (\5 kV/cm) owing to their easier switching, than non-180° domains. The latter are the dominant factor affecting piezoelectric constants and electromechanical coefficients during higher poling electric field (5–40 kV/ cm). The values of these parameters first increases rapidly with poling field in the range of 0.8 to *5 kV/cm (less than half of the coercive field) for PLZT 8/60/40 ceramics. However, when the poling field exceeds *5 kV/cm, there is not much of a significant change was observed (values are saturate), only a small fraction of an increment in piezoelectric values were seen. Large piezoelectricity below the coercive field in (1 - x)(BiNa)TiO3–xBaTiO3 ceramics was also reported by Guo et al. This was interpreted on the basis of the polarization alignment of polar nano-domains in the non-ergodic relax or phase. This unusual poling behaviour was observed only for the compositions those were having P4bm phase not for the R3c and P4mm ferroelectric phase regions [34]. With the application of electric field, all the dipoles started to align in the direction of applied electric field. Such an orientation is opposed by the thermal agitation which tends to randomize the dipoles. Potential energy for the dipole in the applied electric field is found minimum, only when it aligns in the direction of electric field. By using Boltzmann statistics, Langevin [37] calculated the average dipole moment contribution per dipole (P|) within the range 0° to 180° Pj ¼ pLðaÞ

ð5Þ

where LðaÞ ¼ cothðaÞ 1a = Langevin function and a ¼ pE kT , E = applied electric field, T = temperature of the ceramic, p = dipole moment, k = Boltzmann constant. When ‘a’ is very large (high electric field and low temperature), L(a) approaches to 1, maximum alignment of the dipoles along the direction of the electric field. In the presence of high external applied electric filed, free electrons collide with each other and accumulate energy that leads to rise in temperature, fall in piezoelectric properties, and final thermal breakdown. Other possibility is electrons


may move from the forbidden band to the conduction band because of the tunnel effect of quantum mechanics. Therefore, the free electron is speeded up in a high field, which results in the electron impact ionization. The temperature of local region rises due to the increasing of electric current [11] result in the decline of poling dependent electrical properties and breakdown of ceramics. But in this case dielectric (electrical) and physical breakdown was observed for PLZT 8/60/40 ceramic. However, when the electric field exceeded 40 kV/cm, PLZT 8/60/40 ceramics were electrically broken with visible cracks on the surface of the ceramics albeit without decline in piezoelectric properties. This may be due to pores (not seen in SEM images), vacancy defects and other physical flaws in the samples, which easily move when the poling field exceeds 40 kV/cm, and results in an increase in electrical conductivity. In addition, because the coercive field Ec of PLZT 8/60/40 ceramic is 11.97 kV/cm, the poling electric field of 40 kV/cm can make PLZT 8/60/40 ceramics sufficiently polarized for a poling time of 15 min and temperature of 120 °C. Some reports are available on the optimum electric field for poling stating that it should be higher than coercive field [11–13, 23]. Therefore, it can be concluded for PLZT 8/60/40 ceramic that the optimum poling field should be *5 kV/cm, which is slightly less than half of the coercive field (Ec = 11.97 kV/cm) and six times lesser than the previous reports. Poling of ceramics at lower electric fields may cause stability issues. This problem can be solved by poling of ceramics at 2Ec at elevated temperatures that guarantee the most aligned domain configuration with good stability. The effect of temperature on the piezoelectric charge coefficient (d33) and remnant polarization (Pr) is shown in Fig. 7. The value of d33 increases and Pr value decreases with temperature from 30 °C to 170 °C. The value of piezoelectric charge coefficient (d33) was calculated by taking a slope from the S–E curve at different temperature. An in situ structural description of the microscopic origin of the macroscopic Ferro- and piezoelectric properties as a function of the applied electric field E for lead zirconate titanate (PZT) ferroelectric material was studied by Hinterstein et al. [6] by using Reitveld refinement method of synchrotron X-ray diffraction pattern. In situ XRD was performed to describe the structural evolution as a function of the applied electric field over a complete hysteresis cycle. Monoclinic-tetragonal phase coexistence and micro-strains [6, 38] were used for the structural model. Hinterstein et al. reported that the calculated monoclinic phase (%) and thermal (Biso) parameter of lead atom, shows the ferroelectric properties are highly electric field dependent, and hysteresis analogue to the polarization versus electric field hysteresis curves. Thermal (Biso)

Fig. 7 Change in piezoelectric charge coefficient (d33) and remnant polarization (Pr) as a function of temperature from 30 °C to 170 °C

parameter reaches a maximum value at 0.5 kV/mm less than coercive fields (0.8 kV/mm) for the PZT ceramics. This study shows that samples can be poled at significantly reduced poling voltages which could be very advantageous if the process has to be scaled up to industrial levels. Also it greatly simplifies the problem if the samples have poor resistivity. 3.4 Piezoelectric studies: unipolar strain versus electric field (S–E) hysteresisloops Piezoelectric properties of the PLZT ceramics depend on many factors, such as different compositions and processing techniques, MPB conditions, grain sizes, poling conditions etc. An improvement in electrical properties (Pr, Ec, d33, g33, kp, k33 and k31) of PLZT 8/60/40 electroceramics can be attributed to a combinational approach of high energy ball milling followed by CIP, resulting in better density [16]. When an electric field is applied to the ceramic sample; in addition to the polarization, strain is also developed due to the converse piezoelectric effect of the lattice and switching and movements of domain walls. An application of electric field results in the creation of a large strain linked to the piezoelectric effect odkij ¼

ogij oEk

ð6Þ

where dkij is the piezoelectric coefficient, gij is the strain, and Ek is the applied electric field. Electric field induced unipolar strain curves of PLZT 8/60/40 ceramics recorded at room temperature are shown in Fig. 8. The unipolar strain increased with increase in applied electric field. The PLZT 8/60/40 electroceramic exhibited a 0.18 % strain at 32 kV/cm, due to increase in alignment of domains. With increasing electric field, the alignment of domains saturates and the loss (represented by

123


Fig. 8 Strain versus electric field hysteresis curves for the PLZT 8/60/40 electroceramics at different applied electric field

Fig. 9 Piezoelectric charge coefficient (d33) calculated from a the slop of S–E curve and b maximum displacement divided by applied electric field versus electric field curve

the area of the loop) decreases and is almost negligible at high fields [16]. It is evident from Fig. 8 that the S–E curves do not exhibit a linear region at maximum applied electric fields (32 kV/cm). The maximum strain for this system was found to be *0.27 % at an electric field of 80 kV/cm [16]. The high strain values at high applied electric fields for PLZT 8/60/40 ceramic are due to their good dielectric strength. In this study our primary aim is to discuss the change in piezoelectric charge coefficient (d33), calculated from the strain measurement data in the low field region by taking the slope of the S–E curve, so the maximum value of maximum applied electric field was restricted up to 32 kV/cm. Piezoelectric charge coefficient (d33) can be calculated from the S–E hysteresis curves. S–E measurements were performed on the PLZT 8/60/40 electroceramic in unipolar direction. For unipolar measurements where the total displacement is of interest, an average d33 can be defined as the slope of the displacement versus electric field hysteresis loop. Figure 9a shows the calculated d33 value from the

123

S–E hysteresis loop as a function of applied electric fields. The normalized strain coefficient (Smax/Emax) was also used to calculate the d33 value for the PLZT 8/60/40 electroceramics as shown in Fig. 9b. Ferroelectric materials always contain electrical and elastic defects and imperfections that can affect the movement and switching of domain walls and polarization within individual grains. In ferroelectric materials domains form to minimise the electrostatic energy of the depolarising field, which arises due to surface charge and elastic energy associated with mechanical constraints. Both non-180° (90°) and 180° domain walls helps to reduce the effect of depolarisation electric fields but only the non-180° domain walls participate in minimising the elastic energy [14]. Ceramic samples usually contain a number of non-180° domains. In addition to the pure piezoelectric response, the movement and switching of non-180° walls involves a significant change in dimensions of the sample, resulting in increase in strain in the ceramics. The value of the high-field piezoelectric strain coefficient d33* (computed in the high-field region by determining the slope of the curve) was found to be 352 pC/N. At the high field region all the non-180° domain walls participate to minimise the elastic energy and these saturate and hence the change in strain value with further increase in electric field is negligible. So the calculated high-field piezoelectric strain coefficient d33* is always less at higher fields due to the saturation of all domains.

4 Conclusions Single and pure perovskite phase PLZT 8/60/40 compositions were successfully prepared. HEM ball milling helps to prepare ceramics without excess PbO, with reduced calcination and sintering temperatures & improved electrical properties. An improvement was found in ferroelectric properties such as shape of the hysteresis loop, remnant polarization, and coercive field. PLZT 8/60/40 electroceramics were poled at different poling electric fields. Poling fields study for the PLZT 8/60/40 ceramic suggests that ceramics can be poled below the coercive field without compromising the piezoelectric properties. Polarization versus electric field hysteresis loops were traced to evaluate the coercive field and domain switching characteristics. Strain versus electric field measurements were done to check the piezoelectric properties with the application of an electric field. Acknowledgments The authors acknowledge the financial support from DRDO for carrying out the research work and express their gratitude to the Director DMRL for his interest in this work.


References 1. G.H. Haertling, J. Am. Ceram. Soc. 82, 797 (1999) 2. Y.M. Jin, Y.U. Wang, A.G. Khachaturyan, J.F. Li, D. Viehland, J. Appl. Phys. 94, 3629 (2003) 3. B. Noheda, D.E. Cox, G. Shirane, J.A. Gonzalo, L.E. Cross, S.E. Park, Appl. Phys. Lett. 74, 2059 (1999) 4. R. Guo, L.E. Cross, S.E. Park, B. Noheda, D.E. Cox, G. Shirane, Phys. Rev. Lett. 84, 5423 (2000) 5. L. Pdungsap, N. Udomkan, S. Boonyuen, P. Winotai, Sens. Actuators, A 122, 250 (2005) 6. M. Hinterstein, K.A. Schoenau, J. Kling, H. Fuess, M. Knapp, H. Kungl, M.J. Hoffmann, J. Appl. Phys. 108, 024110 (2010) 7. G. Viola, K.B. Chong, M. Eriksson, Z. Shen, J. Zeng, Q. Yin et al., Appl. Phys. Lett. 103, 182903 (2013) 8. T.M. Kamel, G. deWith, J. Euro. Ceram. Soc. 28, 851 (2008) 9. B.M. Jin, J. Kim, S.C. Kim, Appl. Phys. A 65, 53 (1997) 10. C.A. Randall, N. Kim, J.P. Kucera, W. Cao, T.R. Shrout, J. Am. Ceram. Soc. 81, 677 (1998) 11. L. Zhang, Q. Sun, W. Ma, Y. Zhang, H. Liu, J. Mater. Sci.: Mater. Electron. 23, 688 (2012) 12. Y. Zhao, R. Huang, R. Liu, H. Zhou, Ceram. Int. 38, 6067 (2012) 13. S. Su, R. Zuo, S. Lu, Z. Xu, X. Wang, L. Li, Curr. Appl. Phys. 11, S120 (2011) 14. D. Damjanovic, Rep. Prog. Phys. 61, 1267 (1998) 15. G.H. Haertling, C.E. Land, J. Am. Ceram. Soc. 54, 1 (1971) 16. A. Kumar, V.V. BhanuPrasad, K.C. JamesRaju, A.R. James, J. Alloys Compd. 599, 53 (2014) 17. A.R. James, J. Paul Praveen, M. Premkumar, V.V. Bhanu Prasad, Mater. Res. Bull. 47, 3459 (2012) 18. T.M. Kamel, F.X.N.M. Kools, G. deWith, J. Eur. Ceram. Soc. 27, 2471 (2007) 19. A.Q. Jiang, H.J. Lee, C.S. Hwang, J.F. Scott, Adv. Funct. Mater. 22, 192 (2012) 20. W.J. Merz, Phys. Rev. 95, 690 (1954) 21. J.F. Scott, L. Kammerdiner, M. Parris, S. Traynor, V. Ottenbacher, A. Shawabkeh, W.F. Oliver, J. Appl. Phys. 64, 787 (1988)

22. S. Jesse, B.J. Rodriguez, S. Choudhury, A.P. Baddorf, I. Vrejoiu, D. Hesse, M. Alexe et al., Nat. Mater. 7, 209 (2008) 23. H. Du, F. Tang, F. Luo, W. Zhou, S. Qu, Z. Pei, Mater. Sci. Eng., B 137, 175 (2007) 24. L.B. Kong, J. Ma, T.S. Zhang, J. Mater. Res. 16, 1636 (2001) 25. W. Hackenberger, M.J. Pan, V. Vedula, P. Pertsch, W. Cao, C. Randall, T. R. Shrout, in Proceedings of the SPIE Conference Smart Materials Technology, San Diego, CA, vol. 3324, pp. 28–36, March, 1998 26. A. Gruverman, D. Wu, J.F. Scott, Phys. Rev. Lett. 100, 097601 (2008) 27. A.K. Tagantsev, I. Stolichnov, N. Setter, J.S. Cross, M. Tsukada, Phys. Rev. B 66, 214109 (2002) 28. J. Li, B. Nagaraj, H. Liang, W. Cao, C.H. Lee, R. Ramesh, Appl. Phys. Lett. 84, 1174 (2004) 29. V.L. Ginzburg, Zh Eksp. Teor. Fiz. 15, 739 (1945) [J. Phys. X 107 (1946)] 30. S. Ducharme, V.M. Fridkin, A.V. Bune, S.P. Palto, L.M. Blinov, N.N. Petukhova et al., Phy. Rev. Lett. 84, 175 (2000) 31. W.J. Merz, J. Appl. Phys. 27, 938 (1956) 32. M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectrics and Related Materials, Ch. 11 (Clarendon Press, Oxford, 1977) 33. Y.A. Genenko, S. Zhukov, S.V. Yampolskii, J. Schu¨trumpf, R. Dittmer, W. Jo et al., Adv. Funct. Mater. 22, 2058 (2012) 34. H. Guo, C. Ma, X. Liu, X. Tan, Appl. Phys. Lett. 102, 092902 (2013) 35. M. Ozgul, S. Trolier-McKinstry, C.A. Randall, J. Appl. Phys. 95, 4296 (2004) 36. D. Viehland, Y.H. Chen, J. Appl. Phys. 88, 6696 (2000) 37. M.A. Wahab, Solid State Physics: Structure and Properties of Materials, Ch. 14 (Narosa publishing house, New Delhi, 2013) 38. J. Frantti, S. Eriksson, S. Hull, V. Lantto, H. Rudlof, K. Kakihana, J. Phys.: Condens. Matter 15, 6031 (2003)

123