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Enhanced piezoelectric performance of donor La3+-doped BiFeO3–BaTiO3 lead-free piezoceramics
Journal Pre-proof Enhanced piezoelectric performance of donor La piezoceramics
3+
-doped BiFeO3–BaTiO3 lead-free
Muhammad Habib, Myang Hwan Lee, Da Jeong Kim, Hai In Choi, Myong-Ho Kim, Won-Jeong Kim, Tae Kwon Song, Kyu Sang Choi PII:
S0272-8842(19)33392-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.199
Reference:
CERI 23568
To appear in:
Ceramics International
Received Date: 11 October 2019 Revised Date:
18 November 2019
Accepted Date: 22 November 2019
Please cite this article as: M. Habib, M.H. Lee, D.J. Kim, H.I. Choi, M.-H. Kim, W.-J. Kim, T.K. Song, 3+ K.S. Choi, Enhanced piezoelectric performance of donor La -doped BiFeO3–BaTiO3 lead-free piezoceramics, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.199. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Enhanced piezoelectric performance of donor La3+-doped BiFeO3-BaTiO3 lead-free piezoceramics Muhammad Habib1, Myang Hwan Lee1, Da Jeong Kim1, Hai In Choi1, Myong-Ho Kim1, Won-Jeong Kim2, Tae Kwon Song*1, and Kyu Sang Choi3 1
School of Materials Science and Engineering, Changwon National University, Changwon Gyeongnam 51140, South Korea 2
Department of Physics, Changwon National University, Changwon Gyeongnam 51140, South Korea 3 Department of Physics Education, Sunchon National University, Sunchon, Jeonnam 57922, South Korea Abstract Lead-free 0.70Bi1.03FeO3-0.30Ba(1-x)LaxTiO3 piezoelectric ceramics (with x = 0.000, 0.005, 0.010, 0.015 and 0.035 abbreviated as 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa) were prepared through the conventional solid-state reaction route followed by water quenching process. The X-ray diffraction profile shows that the substitution of Ba2+ = 1.61 Å with a donor ion, La3+ = 1.36 Å, has a profound impact on the crystal symmetry as results the crystal structure transform from dominant rhombohedral (R) to tetragonal (T) phase. A large remnant polarization (Pr = 30.3 µC/cm2) and an enhanced direct piezoelectric coefficient, (d33 = 297 pC/N) together with a high Curie temperature (TC = 530
) was obtained near to the
morphotropic phase boundary (MPB) of the R and T phases. A maximum strain of (Smax = 0.188%) with corresponding converse piezoelectric coefficient (d33*= 340 pm/V) and low strain hysteresis (≈ 20%) was found for 1.5BaLa ceramic. Additionally, the water quenching effect was more prominent for 1.0BaLa ceramic as observed from the high thermal hysteresis in heating/cooling results of the dielectric constant (εr), leading to the enhancement of ferroelectric switching behavior. Hence, a small amount of La3+-substitution for Ba2+-site is more effective for the enhancement of ferroelectric and piezoelectric properties, similarly to the donor La3+-doped Pb(Zr,Ti)O3 ceramic. Keywords: Lead-free, Piezoelectric, La-doping, BiFeO3-BaTiO3, Water quenching *Corresponding author email: [email protected]
1
1. Introduction Piezoelectric ceramics exhibited electromechanical coupling which responds mechanically to the applied electric field and vice versa [1]. Nowadays the demand for piezoelectric ceramic is still an open issue that must be work under the harsh environment (> 300
) [2]. Lead zirconate titanate (PbZr(1-x)TixO3 or PZT) is one of the most widely studied
materials for the manufacturing of piezoelectric devices such as sensors, actuators, and transducers because of their excellent piezoelectric performance [3]. Electromechanical properties of PZT can be tailored by the substitution of different dopants on A or B-site in perovskite structure [4-6]. Among the doping elements, La3+-substitution on A-site is more effective for the improvement of ferro/piezoelectric properties [7, 8]. However, the toxic nature of Pb-based material has serious apprehensions on human health and the environment [9]. Hence, it is necessary to search for appropriate lead-free electroceramics to replace Pbbased materials. Many efforts have been devoted before for the development of lead-free piezoelectric ceramics such as (Ba,Ca)(Zr,Ti)O3, (K,Na)NbO3 and Bi0.5Na0.5TiO3 [10-12]. However, none of the above-mentioned materials have the full potential to replace the PZT ceramic for the high-temperature commercial applications. On the other hand, the solid solution of BiFeO3-BaTiO3 (BF-BT) ceramic is an exceptional material that exhibited a high Curie temperature (TC ≥ 400
) with outstanding
piezoelectric properties near to the morphotropic phase boundary (MPB) composition [13]. However, there are still some serious issues related to BF-based ceramics that impediment in the path of its usage for real applications [14]. During the high-temperature process, Bi2O3 volatility creates charge defects as a result piezoelectric property degraded [15]. Hence, some excess amount of Bi2O3 powders were used for the compensation of Bi-volatility. Zhu et al. [16] made an effort to improve the piezoelectric properties (direct piezoelectric coefficient, d33 = 208 pC/N and converse piezoelectric coefficient, d33* = 256 pm/V) of BF30BT ceramic by optimizing the sintering conditions. Unfortunately, due to the phase instability of BF during the slow cooling process, some non-ferroelectric secondary phases (Bi-rich, Fe-rich) induces which obstruct the high ferro/piezoelectric performance [17]. Hence, the thermal quenching process effectively avoids the formation of non-ferroelectric phases and improved the electrical properties of BF-based ceramics [18]. Recently, Lee et al. achieved very high piezoelectric properties, (d33 = temperature (TC = 451
454 pC/N and d33* = 471 pm/V) with a high Curie
) through the water quenching process in BiGaO3-doped BF33BT
ceramic [17]. In the same way, many researchers have tried to improve the piezoelectric 2
property of BF-BT ceramics through the La3+-substitution on Bi3+-site [19-21]. Actually, in BF-BT ceramic the ferro/piezoelectricity mostly originates from the Bi3+ (6s2) lone pair [22]. Nevertheless, the substitution of Bi3+ with a non-lone pair element (La3+) will weaken this lone pair activity [23, 24]. Additionally, Bi3+(1.34 Å with 12 coordination number) is an isovalent element and almost equivalent ionic radius with La3+(1.36 Å with 12 coordination number), which is not so much effective for the improvement of the functional property of BF-BT ceramic [25, 26]. However, a small amount of La3+ as a donor doping on Ba2+-site (1.61 Å with 12 coordination number) has a profound impact on the crystal structure and defect chemistry of BF-BT ceramic [24]. In fact, the donor-doping control the oxygen vacancies (
••
) in the host material and create a soft ferroelectric effect, leading towards the
high piezoelectric performance [7]. Compared to the other reare-earth element La siginficantly reduce TC of BaTiO3 and induce ferroelectric effect [27]. From the material design point of view, the based composition of the present work was selected on the rhombohedral side of the MPB had a hard ferroelectric behavior. Hence, La3+ was incorporated on Ba2+-site as donor dopant according the chemical formula 0.70Bi1.03FeO30.30(Ba(1-x)Lax)TiO3 with x = 0.000, 0.005, 0.010, 0.015 and 0.035 (hereafter 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa, and 3.5BaLa). All ceramics were synthesized via a conventional solid-state ceramic process and subsequently quenched in water. The crystal structure, microstructure, dielectric and piezoelectric properties were studied systematically. 2. Experimental procedure All ceramics were synthesized by a conventional solid-state reaction route from initial starting materials i.e. Bi2O3 (99.9 %), La2O3 (99.99 %), Fe2O3 (≥ 99 %), BaCO3 (≥ 99 %), and TiO2 (≥ 99.9 %) obtained from Sigma Aldrich. The powders were weighed and 3 mol% excess bismuth was added for the compensation of Bi3+ volatility during heat treatment. Weighed powders were mixed with ethanol and milled for 24 h with zirconia balls on an epicycle rotating polyethylene jar. After milling, the dried powders were calcined twice in order to make the chemical homogeneity. Polyvinyl alcohol (PVA) was added as a binding agent and dried at 120
. All powders were passed through 100 m sieve and uniaxially
pressed into 10 mm diameter. The optimized sintering temperature was 970 and 0.5BaLa, while 980 10
for 0.0BaLa
for 1.0BaLa, 1.5BaLa and 3.5BaLa composition at heating rate of
/min. After the completion of the sintering time, all pellets were thermally quenched to
room temperature in water. The crystal structure of the investigated ceramics was determined by X-ray diffraction (XRD, MiniFlex II, Rigaku). A thin line of Si powder (99.999%) pasted 3
on the surface of the pellet as a reference peak to calibrate the tilting and height error during XRD measurement [28]. The surface morphology of the as-sintered ceramics was studied from the images taken by scanning electron microscopy (SEM, JEOL, JSM-6510). The 0.4 mm ceramics were electrically sputtered with Pt and silver electrodes were pasted on both sides. The edges of ceramics were polished to make the capacitor-like specimens for electrical characterization. Ferroelectric polarization-electric field (P-E) hysteresis loops were measured at 10 Hz by a ferroelectric measurement system (Radiant, RT6000 HVS) at room temperature. The electric field-induced strain (S-E) measurements were performed with a linear variable differential transformer (LVDT, Millimar 1240, Mahr) probe at 50 mHz. The electrical poling process was performed in silicone oil bath at room temperature by applying 50 kV/cm for 30 minutes and direct piezoelectric constant (d33) was measured by using piezod33-meter (IACAS, ZJ-6B) at 0.25 N and 110 Hz. Temperature-dependent dielectric behavior was studied by using an impedance analyzer (HP4194A, Agilent) connected with a computer program controlled furnace system. Leakage current was measured with an electrometer (Keithley 6514) and high voltage source (Keithly 248). 3. Results and discussions The XRD pattern of all ceramics shows a pure perovskite phase without an indication of secondary phases as shown in Fig. 1(a). For convenience, all peaks initially indexed according to the pseudocubic symmetry. However, the ionic radii difference and valency mismatch of the chemical modifier (Ba2+ = 1.61 Å and La3+ = 1.36 Å) definitely bring distortion in the unit cell of BF-BT ceramic and leads to the high piezoelectric performance [13, 24]. Mostly in perovskite ceramics, the reflection from (111) and (200) peaks are used as a fingerprint for the identification of the rhombohedral (R) and tetragonal (T) phases [13]. From the first glance, a predominant R phase can be suggested for both 0.0BaLa and 0.5BaLa compositions because of the clearly splitting of (111) peak and a single (200) peak. As the La3+ content increased to 0.010 or 0.015 only a small shoulder in the (111) peak and a plateau appears at the (200) peak strongly suggest the vicinity of the MPB between the R and T phases. This compositionally-driven phase MPB leading towards the enhancement of piezoelectric performance [18]. Finally, for 3.5BaLa composition the doublet (111) peak changed into a single peak indicating the involvement of the high symmetry tetragonal phase [29]. A similar sequential crystal structure phase transformation from R to T was observed before in the La3+-doped PZT (PLZT) and La3+-doped BF30BT ceramic [19, 30].
4
For further confirmation of the crystal structure, Rietveld refinement analysis was performed with best fits being obtained using the R (R3c) and T (P4mm) phases as shown in Fig. 2(a-e). A mixed-phase of R (60.34%) and T (39.66%) polymorphs were obtained for the undoped BF30BT ceramic yielding excellent goodness of factor (χ2 = 1.45) which is consistent with the other BF30BT ceramic [31]. As the La3+-content increase, the R-phase monotonously decreases and T-phase gradually dominating as shown in Fig. 2(f). Particularly, for 1.0BaLa composition, a typical MPB was formed where R and T phases almost coexisted with each other. Therefore, high piezoelectric performance can be expected due to the flattened thermodynamic profile in the vicinity of the MPB region [32]. However, for x ≥ 0.015 the crystal structure transforms to the T-phase with the phase volume fraction of 77% for 3.5BaLa composition. Hence, a small amount of donor La3+-incorporation in BF-BT ceramic stabilizes the T-phase similarly to the PLZT ceramic, where the MPB shifted towards R-phase [33]. Fig. 3(a) shows the lattice parameters (such as
,
and
) and unit cell volume for
the rhombohedral (VR) and tetragonal (VT) phases that were found from the crystal structure refinement. As the composition approaches the MPB, the stabilization of the T-phase as a result
/
lattice constant increases due to
ratio rises and reached to 1.008 for 1.0BaLa
composition. The lattice constant elongation along the c-axis also increases the lattice cell volume. The lattice constant and rhombohedral distortion (90°− αR) of the R-phase deduced from the hexagonal unit cell [34]. The 90°− αR continuously reduced from 0.26° to 0.11° upon the La3+-doping and show consistency with the suppressing of the R-phase fraction as shown in Fig. 2(f). However, for x ≥ 0.015 the declining of the lattice anisotropy (such as /
ratio and 90°− αR) represents the involvement of high symmetry tetragonal or
pseudocubic phase as result unit cell volume decreases. A similar increasing and decreasing trend in the lattice constant and unit cell volume was observed before in the Nd(Li0.5Nb0.5)O3 doped BF30BT ceramic [31]. The SEM micrographs of the as-sintered ceramics (for 0.0BaLa, 1.0BaLa and 3.5BaLa) along with the average grain size are provided in Fig. 4(a-d). It is evident that the average grain size decreased as La3+-content increases. According to the linear intercept method, the average grain size was calculated to be 5.2 µm, 4.2 µm, and 2.8 µm for 0.0BaLa, 1.0BaLa, and 3.5BaLa compositions, respectively. The reduction of grain size may be associated with the heterogeneity induced from chemical modifier as observed earlier in BF5
BT ceramic [35]. Actually, in ferroelectric bulk ceramic, the domain size strongly depends on the grain size and grain morphology [36]. High activation energy is required for the switching of large size domains with a big grain size [37]. Meanwhile, in the case of fine-grain the ferroelectric domain density increases, as a result, the response comes from domain become weak [38]. Therefore, a better ferro/piezoelectric response can be obtained for a specific composition near to the MPB with appropriate grains size and grains configuration. Room temperature P-E hysteresis loops, bipolar and unipolar electric-field-induced strain (S-E) curves are shown in Fig. 5(a-e). For 0.0BaLa and 0.5BaLa ceramics, the hard ferroelectric P-E loops and butterfly-shaped S-E curves were obtained. However, the 3.5BaLa ceramic showed a soft P-E hysteresis loop with sprout-shaped S-E curves and behave like a relaxor ferroelectric. Particularly, the 1.0BaLa sample gives a well-saturated and squareshaped P-E hysteresis loop with the highest polarization. In BaTiO3, the Ba2+ substitution by La3+ change its defects structure and affect the interaction of the domain walls with the charged defects as given below [39, 40]. La2O3 + 2TiO2
(1/2)O2 + 2La • + 2Ti × + 2
+ 6O×
(1)
On the other hand, the possible defect reaction in BiFeO3 due to Bi2O3 volatilization may be written as following relation [41]. 2BiFeO3 → 2Fe× + 3O × + 2
+3
••
+ Bi2O3↑
(2)
The electronic charge of Eq. (1) will compensate the
••
which is induced due to the Bi2O3
evaporation as shown in Eq. (2) [42]. Actually the charge defects, such as (Fe
"
are making complex defect dipoles (Fe
#$
"
−
••
) or (
"
−
••
#
"
)′,
••
and
) which restrict the
domain reorientations under the applied electric field [43]. According to the previous reports, a small amount of off-valent donor-doping (La3+) in PZT ceramic compensate the
••
and
resulting well-saturated P-E hysteresis loops with high ferroelectric polarization [4, 7]. Hence, there is a possibility that in BF-BT ceramic about ≈1 mol% Ba2+-site donor La3+doping can suppress the
••
concentration and avoide the formation of defect dipoles, as a
result, the ferro/piezoelectric properties will be improved. However, for the high amont of La3+-content the electric charge defects ( ) become dominent according to Eq. (1) and increase the leakage current.
6
The composition-dependent remnant polarization (Pr) and electric field-induced maximum strain (Smax) values are shown in Fig. 5(f). A colossal increase was noted in Pr from 16.8 µC/cm2 to 30.3 µC/cm2 after the 1 mol% of La3+ substitution. Meanwhile, the Smax level reached to 0.184% for 1.0BaLa and 0.188% for 1.5BaLa composition. The different parameters related to the piezoelectric properties are summarized as a function of La3+)* content are given in Fig. 6(a,b). The increased in negative strain (%&'( ) mainly attributed to
the enhancement of domains switching phenomenon caused by the donor La3+ ion [44]. The .&* ) of the unipolar curves initially decreased with a internal strain hysteresis (Hyst = ∆Suni/%+,-
small amount of La3+-doping (1.0BaLa and 1.5BaLa) and again increase for 3.5BaLa sample as given in Fig. 5(a), where ∆Suni is the largest difference of strain in unipolar electric field driving. BNT-based ceramics have a high strain response, with a large unwanted strain hysteresis, Hyst ≈ 50 – 80% [45-47]. However, in this work a large converse piezoelectric coefficient, d33* = 340 pm/V with very low Hyst ≈ 20% obtained for 1.5BaLa specimen. At the same time, a very high direct piezoelectric coefficient, d33 = 297 pC/N obtained for 1.0BaLa, ceramic. The good ferro/piezoelectric properties for 1.0BaLa ceramic may be attributed to the MPB between R and T phases as evident from the XRD results in Fig. 1(b) and Fig. 2(f). Electrical poling has a significant impact on electrical polarization in hard ferroelectric and almost no effect on the soft ferroelectric ceramics [48]. Therefore, ferroelectric P-E hysteresis loops of the poled ceramic were compared with unpoled ones (Fig. 7) in order to clear that La3+ donor doping on Ba2+-site really creates the soft ferroelectric effect. The Pr values for both unpoled and poled ceramics were plotted as a function of La3+-content as shown in Fig. 7(f). A remarkable change was noticed in Pr from 16.8 µC/cm2 to 36.2 µC/cm2 and 18.4 µC/cm2 to 36.5 µC/cm2 for 0.0BaLa and 0.5BaLa specimens respectively. Such improvement in Pr after poling related to the irreversible domain switching phenomenon and indorsing the hard ferroelectric ceramics [49]. A similar trend was observed in PLZT electroceramic, where a prototypical P-E hysteresis loop was obtained for a poled ceramic [8]. In contrast, the poling effect becomes weak as the La3+content increased above the 0.5 mol%. Hence, the weakening of the poling effect on the ferroelectric polarization attributed to the soft ferroelectric effect [50, 51]. According to our previous work, the isovalent doping (La3+ on Bi3+-site) also induces the soft ferroelectric effect above 3.5 mol% of La3+-content [52]. However, the donor doping (La3+ on Ba2+-site) more easily creates the soft ferroelectric effect even at the low level (1 mol%) of La3+-doping. 7
From the above results, it should be noted that high ferroelectric and piezoelectric performance (Pr = 30.3 µC/cm2, d33 = 297 pC/N and d33* = 335 pm/V as shown in Fig. 5 and Fig. 6) for 1.0BaLa ceramic related to governing of soft ferroelectric effect. The temperature dependence of dielectric constant (εr) and dielectric loss (tanδ) was measured for heating as well as cooling runs at 10 kHz as shown in Fig. 7(a-f). It can be seen that, TC decreased with increasing of La3+ concentration. The 0.0BaLa, 0.5BaLa, and 1.0BaLa ceramics exhibited a sharp peak around 560
, 548
and 530
, respectively. However, for
the high amount of La3+-doping (1.5BaLa and 3.5BaLa) a completely different dielectric characteristic was observed with a diffused phase transition, which is the characteristic of a relaxor-ferroelectric. Such relaxor-like behavior can be ascribed to the local structure heterogeneity on the atomic scale or domain miniaturization induced from the La3+-donor doping [53]. A similar relaxor behavior in the εr was noted before in PLZT and BF-BT ceramics [30, 54]. Moreover, the magnitude of the εr also strongly depends upon the grain size and decreases with the reduction of grain size, as observed earlier in PZT ceramics [55]. Additionally, the high thermal hysteresis (δTC) in heating and cooling runs around TC for 1.0BaLa specimens reflect the tendency of the quenching effect [17, 43]. Such behaviors in the vicinity of phase transition is a key factor for a good switchable dielectric material [56]. Temperature-dependent stability of tanδ increased as increasing of La3+ content and persisted over a wide temperature range. However, tanδ loss rises sharply above the 300
indicating
an increase in electrical conductivity at higher temperatures [23]. The tendency of the dielectric properties shows a complete agreement with the ferroelectric and piezoelectric properties of the current work as mentioned above (Fig. 5 and Fig. 6). According to our previous research work, the Bi3+-site 3.5 mol% La3+ substituted sample behaves like a normal ferroelectric and showed very high d33 = 232 pC/N with a sharp phase transition peak (εr = 39000) [52]. On the other hand, 3.5 mol% Ba3+-site substituted ceramic gives three times lower dielectric and piezoelectric properties (εr =12000 and d33 = 83 pC/N) with a diffused phase transition and behave like a relaxor ferroelectric. Hence, La3+-doping concentration on Ba2+-site is more constrained due to donor doping and easily creation of relaxor ferroelectric character in BF-BT ceramic. In Table 1, the ferroelectric and piezoelectric properties were summarized along with TC of the current work and recently reported Nd3+ and La3+-doped BF30BT ceramics. The Bi3+-site doping effect of BF30BT ceramic was compared in terms of the functional 8
properties with Ba2+-site doping. Here we found that excellent piezoelectric performance (d33 = 297 pC/N, d33* = 335 pm/V) and good ferroelectricity (Pr = 30.3 µC/cm2) with a high (TC = 530
) for 1.0BaLa ceramic mainly attributed to MPB and high lattice anisotropy. Hence, a
small amount of La3+ as a donor doping on Ba2+-site is more effective relative to the Bi3+-site isovalent doping.
Mostly reported BF-BT ceramics exhibited poor ferro/piezoelectric properties due to the high leakage current caused by charge defects [57, 58]. These charge defects strongly interact with polarization switching and easily breakdown the sample during electric poling [17]. Therefore, the leakage current density was measured at room temperature as shown in Fig. 9(a). A very small variation occurred in the leakage current density as a function of La3+content, which first decrease and then increase at 20 kV/cm and 40 kV/cm as shown in Fig. 9(b). All samples showed a quite low leakage current density (≈5.3 × 10-6 A/cm2) which is far less than another La3+-doped BF-BT ceramic at an applied electric field of 40 kV/cm [25]. Hence, the low leakage current of our samples further supports the well-saturated P−E hysteresis loops with high piezoelectric performance as given in Table 1. 4. Conclusions 0.70Bi1.03FeO3-0.30Ba(1-x)LaxTiO3 (with x = 0.000, 0.010, 0.015 and 0.035) bulk ceramics were successfully synthesized via a conventional solid-state sintering route. A small amount of La3+ incorporation (i.e., x = 0.010) significantly increased the Pr from 16.8 µC/cm2 to 30.3 µC/cm2 due to the coexistence of R and T phases. Electrical poling has a profound impact on the polarization of P-E hysteresis in the hard ferroelectric with R-phase and the poling effect gradually becomes weak as La3+ concentration increased. Interestingly, the optimum composition (i.e. 1.0BaLa) exhibited a large direct piezoelectric coefficient (d33 = 297 pC/N) with a high Curie temperature (TC = 530
). Moreover, a maximum strain of (Smax
= 0.188%) with a corresponding dynamic piezoelectric coefficient (d33* = 340 pm/V) simultaneously with low strains hysteresis (≈ 20%) was found for 1.0BaLa ceramic. In fact, such excellent piezoelectric performance mainly was attributed to the MPB between the R and T phases. Here we found that a small amount of La3+ donor doping on Ba2+-site is much more effective, compared to the Bi3+-site doping. However, the La3+-doping concentration on
9
Ba2+-site is much more constrained due to the easy formation of the relaxor ferroelectric character from the charges defect of the donor ions.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grants funded by
the
Korea
government
(2016R1A2B2016348,
2018R1A5A6075959, 2019R1F1A1059292).
10
2017R1D1A3B03030165,
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15
Doping site
Bi3+-site
Ba2+-site
Composition
Pr
d33
d33*
TC
BF30BT
(µC/cm2)
(pC/N)
(pm/V)
( )
La3+ = 0.75%
25.6
223
350
530
[21]
La3+ = 1.0%
19.5
274
303
532
[52]
La3+ = 1.0%
22.3
170
-
477
[20]
La3+ = 2.0%
≈ 25
121
-
≈ 540
[19]
Nd3+ = 2.0%
≈ 24
≈ 150
333
≈ 420
[23]
La3+ = 0.0%
16.8
251
246
560
La3+ = 1.0%
30.3
297
335
530
La3+ = 1.5%
27.6
278
340
540
Reference
This work
Figure. 1 (a) Room-temperature XRD pattern for Ba2+-site La3+doped BF30BT ceramics in the 2 ranging from 10o to 60o and (b) expended pattern of (111) and (200) XRD peaks. Figure. 2 (a-e) Rietveld refinement of 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa ceramic using rhombohedral (R3c) and tetragonal (P4mm) phases and (f) the phase fraction as a function of La3+-content. Figure. 3 (a) Lattice parameters ( ( /
,
, and
) and lattice cell volume (b) tetragonality
) and rhombohedral distortion (90°− αR) as a function of La3+-content.
Figure. 4 (a-c) SEM images of 0.0BaLa, 1.0BaLa and 3.5BaLa as-sintered ceramics and (d) average grain size. Figure. 5 (a-e) Room temperature P-E hysteresis loops, and S-E curves of 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa ceramic (f) Pr and Smax as a function of La3+-content. Figure. 6 (a) Composition dependence unipolar strain hysteresis (Hyst) and bipolar negative strain (
) (b) direct and converse piezoelectric coefficient (d33 and d33*) as function of La-
content. Figure. 7 (a-e) The ferroelectric P-E hysteresis loops of the virgin and poled ceramics 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa composition (f) Pr values of the unpoled and poled ceramic as a function of La3+-content. Figure. 8 (a-e) Temperature dependence dielectric constant (εr) for heating and cooling measured from room temperature up to 700 ℃ at 10 kHz, (f) denotes the corresponding tanδ loss on heating runs for BF3BT ceramic as a function of La3+-content. Figure. 9(a) The leakage current density measured at room temperature and (b) leakage current density at 20 kV/cm and 40 kV/cm applied electric field.
1
Figure. 1
2
Figure. 2
3
Figure. 3
4
Figure. 4
5
Figure. 5
6
Figure. 6
7
Figure. 7
8
Figure. 8
9
Figure. 9
10
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
3+
-doped BiFeO3–BaTiO3 lead-free
Muhammad Habib, Myang Hwan Lee, Da Jeong Kim, Hai In Choi, Myong-Ho Kim, Won-Jeong Kim, Tae Kwon Song, Kyu Sang Choi PII:
S0272-8842(19)33392-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.199
Reference:
CERI 23568
To appear in:
Ceramics International
Received Date: 11 October 2019 Revised Date:
18 November 2019
Accepted Date: 22 November 2019
Please cite this article as: M. Habib, M.H. Lee, D.J. Kim, H.I. Choi, M.-H. Kim, W.-J. Kim, T.K. Song, 3+ K.S. Choi, Enhanced piezoelectric performance of donor La -doped BiFeO3–BaTiO3 lead-free piezoceramics, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.199. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Enhanced piezoelectric performance of donor La3+-doped BiFeO3-BaTiO3 lead-free piezoceramics Muhammad Habib1, Myang Hwan Lee1, Da Jeong Kim1, Hai In Choi1, Myong-Ho Kim1, Won-Jeong Kim2, Tae Kwon Song*1, and Kyu Sang Choi3 1
School of Materials Science and Engineering, Changwon National University, Changwon Gyeongnam 51140, South Korea 2
Department of Physics, Changwon National University, Changwon Gyeongnam 51140, South Korea 3 Department of Physics Education, Sunchon National University, Sunchon, Jeonnam 57922, South Korea Abstract Lead-free 0.70Bi1.03FeO3-0.30Ba(1-x)LaxTiO3 piezoelectric ceramics (with x = 0.000, 0.005, 0.010, 0.015 and 0.035 abbreviated as 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa) were prepared through the conventional solid-state reaction route followed by water quenching process. The X-ray diffraction profile shows that the substitution of Ba2+ = 1.61 Å with a donor ion, La3+ = 1.36 Å, has a profound impact on the crystal symmetry as results the crystal structure transform from dominant rhombohedral (R) to tetragonal (T) phase. A large remnant polarization (Pr = 30.3 µC/cm2) and an enhanced direct piezoelectric coefficient, (d33 = 297 pC/N) together with a high Curie temperature (TC = 530
) was obtained near to the
morphotropic phase boundary (MPB) of the R and T phases. A maximum strain of (Smax = 0.188%) with corresponding converse piezoelectric coefficient (d33*= 340 pm/V) and low strain hysteresis (≈ 20%) was found for 1.5BaLa ceramic. Additionally, the water quenching effect was more prominent for 1.0BaLa ceramic as observed from the high thermal hysteresis in heating/cooling results of the dielectric constant (εr), leading to the enhancement of ferroelectric switching behavior. Hence, a small amount of La3+-substitution for Ba2+-site is more effective for the enhancement of ferroelectric and piezoelectric properties, similarly to the donor La3+-doped Pb(Zr,Ti)O3 ceramic. Keywords: Lead-free, Piezoelectric, La-doping, BiFeO3-BaTiO3, Water quenching *Corresponding author email: [email protected]
1
1. Introduction Piezoelectric ceramics exhibited electromechanical coupling which responds mechanically to the applied electric field and vice versa [1]. Nowadays the demand for piezoelectric ceramic is still an open issue that must be work under the harsh environment (> 300
) [2]. Lead zirconate titanate (PbZr(1-x)TixO3 or PZT) is one of the most widely studied
materials for the manufacturing of piezoelectric devices such as sensors, actuators, and transducers because of their excellent piezoelectric performance [3]. Electromechanical properties of PZT can be tailored by the substitution of different dopants on A or B-site in perovskite structure [4-6]. Among the doping elements, La3+-substitution on A-site is more effective for the improvement of ferro/piezoelectric properties [7, 8]. However, the toxic nature of Pb-based material has serious apprehensions on human health and the environment [9]. Hence, it is necessary to search for appropriate lead-free electroceramics to replace Pbbased materials. Many efforts have been devoted before for the development of lead-free piezoelectric ceramics such as (Ba,Ca)(Zr,Ti)O3, (K,Na)NbO3 and Bi0.5Na0.5TiO3 [10-12]. However, none of the above-mentioned materials have the full potential to replace the PZT ceramic for the high-temperature commercial applications. On the other hand, the solid solution of BiFeO3-BaTiO3 (BF-BT) ceramic is an exceptional material that exhibited a high Curie temperature (TC ≥ 400
) with outstanding
piezoelectric properties near to the morphotropic phase boundary (MPB) composition [13]. However, there are still some serious issues related to BF-based ceramics that impediment in the path of its usage for real applications [14]. During the high-temperature process, Bi2O3 volatility creates charge defects as a result piezoelectric property degraded [15]. Hence, some excess amount of Bi2O3 powders were used for the compensation of Bi-volatility. Zhu et al. [16] made an effort to improve the piezoelectric properties (direct piezoelectric coefficient, d33 = 208 pC/N and converse piezoelectric coefficient, d33* = 256 pm/V) of BF30BT ceramic by optimizing the sintering conditions. Unfortunately, due to the phase instability of BF during the slow cooling process, some non-ferroelectric secondary phases (Bi-rich, Fe-rich) induces which obstruct the high ferro/piezoelectric performance [17]. Hence, the thermal quenching process effectively avoids the formation of non-ferroelectric phases and improved the electrical properties of BF-based ceramics [18]. Recently, Lee et al. achieved very high piezoelectric properties, (d33 = temperature (TC = 451
454 pC/N and d33* = 471 pm/V) with a high Curie
) through the water quenching process in BiGaO3-doped BF33BT
ceramic [17]. In the same way, many researchers have tried to improve the piezoelectric 2
property of BF-BT ceramics through the La3+-substitution on Bi3+-site [19-21]. Actually, in BF-BT ceramic the ferro/piezoelectricity mostly originates from the Bi3+ (6s2) lone pair [22]. Nevertheless, the substitution of Bi3+ with a non-lone pair element (La3+) will weaken this lone pair activity [23, 24]. Additionally, Bi3+(1.34 Å with 12 coordination number) is an isovalent element and almost equivalent ionic radius with La3+(1.36 Å with 12 coordination number), which is not so much effective for the improvement of the functional property of BF-BT ceramic [25, 26]. However, a small amount of La3+ as a donor doping on Ba2+-site (1.61 Å with 12 coordination number) has a profound impact on the crystal structure and defect chemistry of BF-BT ceramic [24]. In fact, the donor-doping control the oxygen vacancies (
••
) in the host material and create a soft ferroelectric effect, leading towards the
high piezoelectric performance [7]. Compared to the other reare-earth element La siginficantly reduce TC of BaTiO3 and induce ferroelectric effect [27]. From the material design point of view, the based composition of the present work was selected on the rhombohedral side of the MPB had a hard ferroelectric behavior. Hence, La3+ was incorporated on Ba2+-site as donor dopant according the chemical formula 0.70Bi1.03FeO30.30(Ba(1-x)Lax)TiO3 with x = 0.000, 0.005, 0.010, 0.015 and 0.035 (hereafter 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa, and 3.5BaLa). All ceramics were synthesized via a conventional solid-state ceramic process and subsequently quenched in water. The crystal structure, microstructure, dielectric and piezoelectric properties were studied systematically. 2. Experimental procedure All ceramics were synthesized by a conventional solid-state reaction route from initial starting materials i.e. Bi2O3 (99.9 %), La2O3 (99.99 %), Fe2O3 (≥ 99 %), BaCO3 (≥ 99 %), and TiO2 (≥ 99.9 %) obtained from Sigma Aldrich. The powders were weighed and 3 mol% excess bismuth was added for the compensation of Bi3+ volatility during heat treatment. Weighed powders were mixed with ethanol and milled for 24 h with zirconia balls on an epicycle rotating polyethylene jar. After milling, the dried powders were calcined twice in order to make the chemical homogeneity. Polyvinyl alcohol (PVA) was added as a binding agent and dried at 120
. All powders were passed through 100 m sieve and uniaxially
pressed into 10 mm diameter. The optimized sintering temperature was 970 and 0.5BaLa, while 980 10
for 0.0BaLa
for 1.0BaLa, 1.5BaLa and 3.5BaLa composition at heating rate of
/min. After the completion of the sintering time, all pellets were thermally quenched to
room temperature in water. The crystal structure of the investigated ceramics was determined by X-ray diffraction (XRD, MiniFlex II, Rigaku). A thin line of Si powder (99.999%) pasted 3
on the surface of the pellet as a reference peak to calibrate the tilting and height error during XRD measurement [28]. The surface morphology of the as-sintered ceramics was studied from the images taken by scanning electron microscopy (SEM, JEOL, JSM-6510). The 0.4 mm ceramics were electrically sputtered with Pt and silver electrodes were pasted on both sides. The edges of ceramics were polished to make the capacitor-like specimens for electrical characterization. Ferroelectric polarization-electric field (P-E) hysteresis loops were measured at 10 Hz by a ferroelectric measurement system (Radiant, RT6000 HVS) at room temperature. The electric field-induced strain (S-E) measurements were performed with a linear variable differential transformer (LVDT, Millimar 1240, Mahr) probe at 50 mHz. The electrical poling process was performed in silicone oil bath at room temperature by applying 50 kV/cm for 30 minutes and direct piezoelectric constant (d33) was measured by using piezod33-meter (IACAS, ZJ-6B) at 0.25 N and 110 Hz. Temperature-dependent dielectric behavior was studied by using an impedance analyzer (HP4194A, Agilent) connected with a computer program controlled furnace system. Leakage current was measured with an electrometer (Keithley 6514) and high voltage source (Keithly 248). 3. Results and discussions The XRD pattern of all ceramics shows a pure perovskite phase without an indication of secondary phases as shown in Fig. 1(a). For convenience, all peaks initially indexed according to the pseudocubic symmetry. However, the ionic radii difference and valency mismatch of the chemical modifier (Ba2+ = 1.61 Å and La3+ = 1.36 Å) definitely bring distortion in the unit cell of BF-BT ceramic and leads to the high piezoelectric performance [13, 24]. Mostly in perovskite ceramics, the reflection from (111) and (200) peaks are used as a fingerprint for the identification of the rhombohedral (R) and tetragonal (T) phases [13]. From the first glance, a predominant R phase can be suggested for both 0.0BaLa and 0.5BaLa compositions because of the clearly splitting of (111) peak and a single (200) peak. As the La3+ content increased to 0.010 or 0.015 only a small shoulder in the (111) peak and a plateau appears at the (200) peak strongly suggest the vicinity of the MPB between the R and T phases. This compositionally-driven phase MPB leading towards the enhancement of piezoelectric performance [18]. Finally, for 3.5BaLa composition the doublet (111) peak changed into a single peak indicating the involvement of the high symmetry tetragonal phase [29]. A similar sequential crystal structure phase transformation from R to T was observed before in the La3+-doped PZT (PLZT) and La3+-doped BF30BT ceramic [19, 30].
4
For further confirmation of the crystal structure, Rietveld refinement analysis was performed with best fits being obtained using the R (R3c) and T (P4mm) phases as shown in Fig. 2(a-e). A mixed-phase of R (60.34%) and T (39.66%) polymorphs were obtained for the undoped BF30BT ceramic yielding excellent goodness of factor (χ2 = 1.45) which is consistent with the other BF30BT ceramic [31]. As the La3+-content increase, the R-phase monotonously decreases and T-phase gradually dominating as shown in Fig. 2(f). Particularly, for 1.0BaLa composition, a typical MPB was formed where R and T phases almost coexisted with each other. Therefore, high piezoelectric performance can be expected due to the flattened thermodynamic profile in the vicinity of the MPB region [32]. However, for x ≥ 0.015 the crystal structure transforms to the T-phase with the phase volume fraction of 77% for 3.5BaLa composition. Hence, a small amount of donor La3+-incorporation in BF-BT ceramic stabilizes the T-phase similarly to the PLZT ceramic, where the MPB shifted towards R-phase [33]. Fig. 3(a) shows the lattice parameters (such as
,
and
) and unit cell volume for
the rhombohedral (VR) and tetragonal (VT) phases that were found from the crystal structure refinement. As the composition approaches the MPB, the stabilization of the T-phase as a result
/
lattice constant increases due to
ratio rises and reached to 1.008 for 1.0BaLa
composition. The lattice constant elongation along the c-axis also increases the lattice cell volume. The lattice constant and rhombohedral distortion (90°− αR) of the R-phase deduced from the hexagonal unit cell [34]. The 90°− αR continuously reduced from 0.26° to 0.11° upon the La3+-doping and show consistency with the suppressing of the R-phase fraction as shown in Fig. 2(f). However, for x ≥ 0.015 the declining of the lattice anisotropy (such as /
ratio and 90°− αR) represents the involvement of high symmetry tetragonal or
pseudocubic phase as result unit cell volume decreases. A similar increasing and decreasing trend in the lattice constant and unit cell volume was observed before in the Nd(Li0.5Nb0.5)O3 doped BF30BT ceramic [31]. The SEM micrographs of the as-sintered ceramics (for 0.0BaLa, 1.0BaLa and 3.5BaLa) along with the average grain size are provided in Fig. 4(a-d). It is evident that the average grain size decreased as La3+-content increases. According to the linear intercept method, the average grain size was calculated to be 5.2 µm, 4.2 µm, and 2.8 µm for 0.0BaLa, 1.0BaLa, and 3.5BaLa compositions, respectively. The reduction of grain size may be associated with the heterogeneity induced from chemical modifier as observed earlier in BF5
BT ceramic [35]. Actually, in ferroelectric bulk ceramic, the domain size strongly depends on the grain size and grain morphology [36]. High activation energy is required for the switching of large size domains with a big grain size [37]. Meanwhile, in the case of fine-grain the ferroelectric domain density increases, as a result, the response comes from domain become weak [38]. Therefore, a better ferro/piezoelectric response can be obtained for a specific composition near to the MPB with appropriate grains size and grains configuration. Room temperature P-E hysteresis loops, bipolar and unipolar electric-field-induced strain (S-E) curves are shown in Fig. 5(a-e). For 0.0BaLa and 0.5BaLa ceramics, the hard ferroelectric P-E loops and butterfly-shaped S-E curves were obtained. However, the 3.5BaLa ceramic showed a soft P-E hysteresis loop with sprout-shaped S-E curves and behave like a relaxor ferroelectric. Particularly, the 1.0BaLa sample gives a well-saturated and squareshaped P-E hysteresis loop with the highest polarization. In BaTiO3, the Ba2+ substitution by La3+ change its defects structure and affect the interaction of the domain walls with the charged defects as given below [39, 40]. La2O3 + 2TiO2
(1/2)O2 + 2La • + 2Ti × + 2
+ 6O×
(1)
On the other hand, the possible defect reaction in BiFeO3 due to Bi2O3 volatilization may be written as following relation [41]. 2BiFeO3 → 2Fe× + 3O × + 2
+3
••
+ Bi2O3↑
(2)
The electronic charge of Eq. (1) will compensate the
••
which is induced due to the Bi2O3
evaporation as shown in Eq. (2) [42]. Actually the charge defects, such as (Fe
"
are making complex defect dipoles (Fe
#$
"
−
••
) or (
"
−
••
#
"
)′,
••
and
) which restrict the
domain reorientations under the applied electric field [43]. According to the previous reports, a small amount of off-valent donor-doping (La3+) in PZT ceramic compensate the
••
and
resulting well-saturated P-E hysteresis loops with high ferroelectric polarization [4, 7]. Hence, there is a possibility that in BF-BT ceramic about ≈1 mol% Ba2+-site donor La3+doping can suppress the
••
concentration and avoide the formation of defect dipoles, as a
result, the ferro/piezoelectric properties will be improved. However, for the high amont of La3+-content the electric charge defects ( ) become dominent according to Eq. (1) and increase the leakage current.
6
The composition-dependent remnant polarization (Pr) and electric field-induced maximum strain (Smax) values are shown in Fig. 5(f). A colossal increase was noted in Pr from 16.8 µC/cm2 to 30.3 µC/cm2 after the 1 mol% of La3+ substitution. Meanwhile, the Smax level reached to 0.184% for 1.0BaLa and 0.188% for 1.5BaLa composition. The different parameters related to the piezoelectric properties are summarized as a function of La3+)* content are given in Fig. 6(a,b). The increased in negative strain (%&'( ) mainly attributed to
the enhancement of domains switching phenomenon caused by the donor La3+ ion [44]. The .&* ) of the unipolar curves initially decreased with a internal strain hysteresis (Hyst = ∆Suni/%+,-
small amount of La3+-doping (1.0BaLa and 1.5BaLa) and again increase for 3.5BaLa sample as given in Fig. 5(a), where ∆Suni is the largest difference of strain in unipolar electric field driving. BNT-based ceramics have a high strain response, with a large unwanted strain hysteresis, Hyst ≈ 50 – 80% [45-47]. However, in this work a large converse piezoelectric coefficient, d33* = 340 pm/V with very low Hyst ≈ 20% obtained for 1.5BaLa specimen. At the same time, a very high direct piezoelectric coefficient, d33 = 297 pC/N obtained for 1.0BaLa, ceramic. The good ferro/piezoelectric properties for 1.0BaLa ceramic may be attributed to the MPB between R and T phases as evident from the XRD results in Fig. 1(b) and Fig. 2(f). Electrical poling has a significant impact on electrical polarization in hard ferroelectric and almost no effect on the soft ferroelectric ceramics [48]. Therefore, ferroelectric P-E hysteresis loops of the poled ceramic were compared with unpoled ones (Fig. 7) in order to clear that La3+ donor doping on Ba2+-site really creates the soft ferroelectric effect. The Pr values for both unpoled and poled ceramics were plotted as a function of La3+-content as shown in Fig. 7(f). A remarkable change was noticed in Pr from 16.8 µC/cm2 to 36.2 µC/cm2 and 18.4 µC/cm2 to 36.5 µC/cm2 for 0.0BaLa and 0.5BaLa specimens respectively. Such improvement in Pr after poling related to the irreversible domain switching phenomenon and indorsing the hard ferroelectric ceramics [49]. A similar trend was observed in PLZT electroceramic, where a prototypical P-E hysteresis loop was obtained for a poled ceramic [8]. In contrast, the poling effect becomes weak as the La3+content increased above the 0.5 mol%. Hence, the weakening of the poling effect on the ferroelectric polarization attributed to the soft ferroelectric effect [50, 51]. According to our previous work, the isovalent doping (La3+ on Bi3+-site) also induces the soft ferroelectric effect above 3.5 mol% of La3+-content [52]. However, the donor doping (La3+ on Ba2+-site) more easily creates the soft ferroelectric effect even at the low level (1 mol%) of La3+-doping. 7
From the above results, it should be noted that high ferroelectric and piezoelectric performance (Pr = 30.3 µC/cm2, d33 = 297 pC/N and d33* = 335 pm/V as shown in Fig. 5 and Fig. 6) for 1.0BaLa ceramic related to governing of soft ferroelectric effect. The temperature dependence of dielectric constant (εr) and dielectric loss (tanδ) was measured for heating as well as cooling runs at 10 kHz as shown in Fig. 7(a-f). It can be seen that, TC decreased with increasing of La3+ concentration. The 0.0BaLa, 0.5BaLa, and 1.0BaLa ceramics exhibited a sharp peak around 560
, 548
and 530
, respectively. However, for
the high amount of La3+-doping (1.5BaLa and 3.5BaLa) a completely different dielectric characteristic was observed with a diffused phase transition, which is the characteristic of a relaxor-ferroelectric. Such relaxor-like behavior can be ascribed to the local structure heterogeneity on the atomic scale or domain miniaturization induced from the La3+-donor doping [53]. A similar relaxor behavior in the εr was noted before in PLZT and BF-BT ceramics [30, 54]. Moreover, the magnitude of the εr also strongly depends upon the grain size and decreases with the reduction of grain size, as observed earlier in PZT ceramics [55]. Additionally, the high thermal hysteresis (δTC) in heating and cooling runs around TC for 1.0BaLa specimens reflect the tendency of the quenching effect [17, 43]. Such behaviors in the vicinity of phase transition is a key factor for a good switchable dielectric material [56]. Temperature-dependent stability of tanδ increased as increasing of La3+ content and persisted over a wide temperature range. However, tanδ loss rises sharply above the 300
indicating
an increase in electrical conductivity at higher temperatures [23]. The tendency of the dielectric properties shows a complete agreement with the ferroelectric and piezoelectric properties of the current work as mentioned above (Fig. 5 and Fig. 6). According to our previous research work, the Bi3+-site 3.5 mol% La3+ substituted sample behaves like a normal ferroelectric and showed very high d33 = 232 pC/N with a sharp phase transition peak (εr = 39000) [52]. On the other hand, 3.5 mol% Ba3+-site substituted ceramic gives three times lower dielectric and piezoelectric properties (εr =12000 and d33 = 83 pC/N) with a diffused phase transition and behave like a relaxor ferroelectric. Hence, La3+-doping concentration on Ba2+-site is more constrained due to donor doping and easily creation of relaxor ferroelectric character in BF-BT ceramic. In Table 1, the ferroelectric and piezoelectric properties were summarized along with TC of the current work and recently reported Nd3+ and La3+-doped BF30BT ceramics. The Bi3+-site doping effect of BF30BT ceramic was compared in terms of the functional 8
properties with Ba2+-site doping. Here we found that excellent piezoelectric performance (d33 = 297 pC/N, d33* = 335 pm/V) and good ferroelectricity (Pr = 30.3 µC/cm2) with a high (TC = 530
) for 1.0BaLa ceramic mainly attributed to MPB and high lattice anisotropy. Hence, a
small amount of La3+ as a donor doping on Ba2+-site is more effective relative to the Bi3+-site isovalent doping.
Mostly reported BF-BT ceramics exhibited poor ferro/piezoelectric properties due to the high leakage current caused by charge defects [57, 58]. These charge defects strongly interact with polarization switching and easily breakdown the sample during electric poling [17]. Therefore, the leakage current density was measured at room temperature as shown in Fig. 9(a). A very small variation occurred in the leakage current density as a function of La3+content, which first decrease and then increase at 20 kV/cm and 40 kV/cm as shown in Fig. 9(b). All samples showed a quite low leakage current density (≈5.3 × 10-6 A/cm2) which is far less than another La3+-doped BF-BT ceramic at an applied electric field of 40 kV/cm [25]. Hence, the low leakage current of our samples further supports the well-saturated P−E hysteresis loops with high piezoelectric performance as given in Table 1. 4. Conclusions 0.70Bi1.03FeO3-0.30Ba(1-x)LaxTiO3 (with x = 0.000, 0.010, 0.015 and 0.035) bulk ceramics were successfully synthesized via a conventional solid-state sintering route. A small amount of La3+ incorporation (i.e., x = 0.010) significantly increased the Pr from 16.8 µC/cm2 to 30.3 µC/cm2 due to the coexistence of R and T phases. Electrical poling has a profound impact on the polarization of P-E hysteresis in the hard ferroelectric with R-phase and the poling effect gradually becomes weak as La3+ concentration increased. Interestingly, the optimum composition (i.e. 1.0BaLa) exhibited a large direct piezoelectric coefficient (d33 = 297 pC/N) with a high Curie temperature (TC = 530
). Moreover, a maximum strain of (Smax
= 0.188%) with a corresponding dynamic piezoelectric coefficient (d33* = 340 pm/V) simultaneously with low strains hysteresis (≈ 20%) was found for 1.0BaLa ceramic. In fact, such excellent piezoelectric performance mainly was attributed to the MPB between the R and T phases. Here we found that a small amount of La3+ donor doping on Ba2+-site is much more effective, compared to the Bi3+-site doping. However, the La3+-doping concentration on
9
Ba2+-site is much more constrained due to the easy formation of the relaxor ferroelectric character from the charges defect of the donor ions.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grants funded by
the
Korea
government
(2016R1A2B2016348,
2018R1A5A6075959, 2019R1F1A1059292).
10
2017R1D1A3B03030165,
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15
Doping site
Bi3+-site
Ba2+-site
Composition
Pr
d33
d33*
TC
BF30BT
(µC/cm2)
(pC/N)
(pm/V)
( )
La3+ = 0.75%
25.6
223
350
530
[21]
La3+ = 1.0%
19.5
274
303
532
[52]
La3+ = 1.0%
22.3
170
-
477
[20]
La3+ = 2.0%
≈ 25
121
-
≈ 540
[19]
Nd3+ = 2.0%
≈ 24
≈ 150
333
≈ 420
[23]
La3+ = 0.0%
16.8
251
246
560
La3+ = 1.0%
30.3
297
335
530
La3+ = 1.5%
27.6
278
340
540
Reference
This work
Figure. 1 (a) Room-temperature XRD pattern for Ba2+-site La3+doped BF30BT ceramics in the 2 ranging from 10o to 60o and (b) expended pattern of (111) and (200) XRD peaks. Figure. 2 (a-e) Rietveld refinement of 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa ceramic using rhombohedral (R3c) and tetragonal (P4mm) phases and (f) the phase fraction as a function of La3+-content. Figure. 3 (a) Lattice parameters ( ( /
,
, and
) and lattice cell volume (b) tetragonality
) and rhombohedral distortion (90°− αR) as a function of La3+-content.
Figure. 4 (a-c) SEM images of 0.0BaLa, 1.0BaLa and 3.5BaLa as-sintered ceramics and (d) average grain size. Figure. 5 (a-e) Room temperature P-E hysteresis loops, and S-E curves of 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa ceramic (f) Pr and Smax as a function of La3+-content. Figure. 6 (a) Composition dependence unipolar strain hysteresis (Hyst) and bipolar negative strain (
) (b) direct and converse piezoelectric coefficient (d33 and d33*) as function of La-
content. Figure. 7 (a-e) The ferroelectric P-E hysteresis loops of the virgin and poled ceramics 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa composition (f) Pr values of the unpoled and poled ceramic as a function of La3+-content. Figure. 8 (a-e) Temperature dependence dielectric constant (εr) for heating and cooling measured from room temperature up to 700 ℃ at 10 kHz, (f) denotes the corresponding tanδ loss on heating runs for BF3BT ceramic as a function of La3+-content. Figure. 9(a) The leakage current density measured at room temperature and (b) leakage current density at 20 kV/cm and 40 kV/cm applied electric field.
1
Figure. 1
2
Figure. 2
3
Figure. 3
4
Figure. 4
5
Figure. 5
6
Figure. 6
7
Figure. 7
8
Figure. 8
9
Figure. 9
10
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.