Enhanced piezoelectric and energy storage
performance of 0.96(K 0.48 Na 0.48 Li 0.04 )(Nb 0.95 Sb 0.05 )O 3 – 0.04Bi 0.5 (Na 0.82 K 0.18 ) 0.5 ZrO 3 ceramics using two-step sintering method
Phan Dinh Gio1, Le Dai Vuong2,* , and Le Tran Uyen Tu1
1University of Sciences, Hue University, Hue City, Thua Thien Hue Province, Vietnam
2Faculty of Natural Sciences, Thu Dau Mot University, Thu Dau Mot City, Binh Duong Province, Vietnam
Received:7 March 2021 Accepted:8 April 2021
Ó
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021ABSTRACT
The 0.96(K0.48Na0.48Li0.04)(Nb0.95Sb0.05)O3–0.04Bi0.5(Na0.82K0.18)0.5ZrO3(KNLNS–
BNKZ) lead-free ceramics were successfully synthesized by combining the conventional mixed-oxide method with the two-step sintering method. The influence of two-step sintering parameters (including sintering temperature,T2, and time, t2) on the phase structure, dielectric, ferroelectric, piezoelectric, and energy storage properties of KNLNS–BNKZ ceramics is studied. X-ray diffrac- tograms (XRD) revealed a pure perovskite structure, consisting of a mixed phase of rhombohedral–tetragonal (space group:R3m?P4/mbm) crystal structure in all the ceramics, irrespective of variation in sintering temperature and time.
However, the second phase (K6Nb10.8O30) appeared when the sintering time is C6 h. Corresponding to optimized two-step sintering parameters, electrical properties of KNLNS–BNKZ ceramics are best: the density (q) of 4.45 g/cm3, the electromechanical coupling factor (kp) of 0.51, (kt) of 0.55, the piezoelectric constant (d33) of 251 pC/N, and the remanent polarization (Pr) of 16.77lC/cm2 were obtained at T2= 1060°C and t2= 5 h, proving the efficacy of two-step sintering technique. Especially, the optimal sintering conditions for KNLNS–
BNKZ ceramics resulted in enhanced energy storage density of 200% and an energy storage efficiency of 51.8%, respectively.
1 Introduction
The engineering application of Pb(Zr,Ti)O3-based piezoelectric ceramics has increased tremendously over the last half-century [1–7]. The toxicity and high
vapor pressure of lead oxide cause serious environ- mental pollution and affect human health [7–9];
therefore, it is indispensable to develop lead-free ceramics to impede hazardous effects of lead-based ceramics. Among different lead-free piezoceramic
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systems with a perovskite structure [7, 10, 11], (Na,K)NbO3(KNN)-based ceramics have attracted considerable attention due to their strong ferroelec- tricity, high Curie temperature (about 420°C), eco- friendliness, and good piezoelectric properties.
However, the poor sinterability of these ceramics makes it difficult to produce a final ceramic part with high density by conventional solid-phase sintering method, which limits their application in several fields. Different methods, such as spark plasma sin- tering [12], flame-reaction [13,14], hot pressing [15], microwave heating [16], two-step sintering [17], templated grain growth method [18], and KNN- based solid solutions formation by combining KNN with other perovskite compounds to improve the properties of (Na,K)NbO3-based ceramics [19, 20]
have been studied. Currently, KNN-based ceramics with the addition of the Bi0.5(Na1-yKy)0.5ZrO3
(BNKZ) [21] or (Bi0.5K0.5)0.90Zn0.10ZrO3(BKZZ) have shown promising piezoelectric properties [22].
Tangsritrakul et al. [23] have prepared (1 -x)(K0.48-
Na0.52Nb0.95Sb0.05)–xBi0.5(Na0.82K0.18)0.5ZrO3 (KNNS–
BNKZ) ceramic systems by the mixed-oxide method withx = 0–0.05. The results showed that the addition of BNKZ into KNNS ceramics leads to changes in various phase shift temperatures. Curie temperature (TC) decreased from 302 to 214°C, TR–O increased, and theTO–T decreased from 125°C to below 50°C, leading to orthorhombic–tetragonal and rhombohe- dral–tetragonal phases coexist at room temperature, for compositions withx = 0.02 and 0.04, respectively.
Besides, KNN-based ceramics are focused on various advanced fabricating techniques that are adapted [7,15,16,24,25]. Among those, the two-step sintering technique is a simple and effective method for enhancing piezoelectric properties of ceramics, which is currently being used by several researchers exten- sively [22,26–28]. For example, by two-step sintering method, Zheng and Wu [29] have studied the fabri- cation of the 0.96(K0.4Na0.6)(Nb0.96Sb0.04)O3–0.04Bi0.5-
K0.5Zr0.9Sn0.1O3(KNNS–BKZS) ceramics, by varying two experimental parameters, namely, temperature (T) and time (t) (T1= 1120°C, t1= 5 min., T2= 700 °C, t2= 3 h). It has achieved a significant improvement in the piezoelectric constant of samples (d33*350–400 pC/N). However, there are very few research papers published on the effect of two-step sintering parameters on the properties of Bi0.5(- Na0.82K0.18)0.5ZrO3-modified K0.48Na0.48Li0.04)(Nb0.95-
Sb0.05)O3 ceramics. In one of the previous data
published on (1 - x)(Na0.5K0.5)NbO3–xLiNbO3
(KNLN) piezoceramic system [30], the optimal con- centration of LiNbO3was found as (4 mol%). In this work, 0.05 mol antimony (Sb) [31] is added to KNLN system to form K0.48Na0.48Li0.04)(Nb0.95Sb0.05)O3
(KNLNS) ceramics. In the present study, the influ- ence of two-step sintering parameters on the phase structure, dielectric, ferroelectric, piezoelectric, energy storage properties of 0.96(K0.48Na0.48Li0.04)(- Nb0.95Sb0.05)O3–0.04Bi0.5(Na0.82K0.18)0.5ZrO3 ceramics was investigated in detail.
2 Experimental procedure
Lead-free 0.96(K0.48Na0.48Li0.04)(Nb0.95Sb0.05)O3– 0.04Bi0.5(Na0.82K0.18)0.5ZrO3 (KNLNS–BNKZ) piezo- electric ceramics were synthesized by combining the conventional mixed-oxide method with the two-step sintering method. K2CO3, Na2CO3, and Li2CO3 car- bonates and Nb2O5, Bi2O3, Sb2O3, and ZrO2 oxides (purity C99%) were used as starting materials.
K2CO3and Na2CO3powders were dried in an oven at 200°C for 2 h. The mixed powder was milled for 8 h in ethanol medium by ZrO2 balls using a PM 400/2 milling machine. The dried powders were calcined at 850°C for 2 h to obtain a homogeneous composition. The calcined powders were further ball milled for 18 h and compressed into disks of 12 mm diameter and 1.5 mm thickness each by applying a pressure of 1.5 T/cm2. Finally, sintered by the fol- lowing two-step technique [26]: the temperature was raised at a rate of 5 °C/min to the first-step temper- ature (T1) of 1140°C, held at T1for a short socking time (t1) of 5 min, rapidly cooled to a lower different second-step temperature (T2) (T2= 1000, 1030, 1060, 1090, 1120°C for 30°C/min, and then held at T2for 4 h (t2). After determining the optimal sintering temperature, theT2temperature was fixed (1060°C), and sintering time was varied (2, 3, 4, 5, 6 h).
The crystal structures of the samples were exam- ined by X-ray diffraction (XRD; D8 ADVANCE) at room temperature under Cu-Ka radiation of wave- length 1.5405 A˚ . The structural analysis was carried out by the Rietveld refinement method in the FullProf program. A scanning electron microscope (SEM;
Hitachi S_4800) was used to reveal the surface mor- phologies of the as-prepared ceramics. The densities of the samples were measured by Archimedes’
principle, and ferroelectric hysteresis loops were
examined by the Sawyer–Tower method. In order to measure electrical properties, both surfaces of the samples were coated with silver paint and heated at 600°C for 15 min. The electrical properties of ceramics were determined by an impedance analyzer (HP 4193A and RLC HIOKI 3532). The ceramic samples were poled in a silicone oil bath at 60°C by applying an electric field of 40 kV/cm for 30 min and then aged for 24 h. Thed33piezoelectric constant was determined by a d33 meter (YE2730A, SINOCERA, China).
3 Results and discussion
XRD patterns of the KNLNS–BNKZ ceramics sin- tered under different conditions are shown in Fig.1.
As shown in Fig.1, a pure perovskite phase is observed in all the ceramic samples, indicating that BNKZ diffused into KNLNS ceramics, irrespective of the sintering temperature (T2), and formed a homo- geneous solid solution without any detectable second phase. However, a second phase (K6Nb10.8O30) appeared in the samples when sintering timet2= 6 h, as shown in Fig.1b. This is probably due to the evaporation of the alkali elements at a longer sinter- ing duration, leading to non-stoichiometry in the system. Besides, from Fig.1, it can be seen that the shapes of diffraction peaks did not change with variation in T2 and t2, indicating that the phase composition is nearly the same for all the samples [26,31].
To further analyze the effect of T2 and t2 on the crystalline phase transition of KNLNS–BNKZ ceramics, the XRD patterns of all samples (at 2h = 44747°) were enlarged and simulated based on the Lorentz fitting function (inset of Fig.1a, b). It is confirmed that the phase structure of all samples consists of rhombohedral and tetragonal (R–T) crystal structures, characterized by the overlap of (002)T, (200)T, and (200)R diffraction peaks. The presence of both rhombohedral (R3c) and tetragonal (P4/mbm) crystal symmetries was confirmed by the Rietveld refinement technique (Fig.1c, d). The fitting analysis of peak shape and peak position, structure, back- ground, and the percentage of weighted profile R- factor (Rwp) was performed with the Rwp values of samples which are less than 9% (Fig.1c, d). This is also in agreement with the results of the dielectric
constant of the prepared ceramics, which will be discussed in detail in the following section.
These XRD results are similar to the results repor- ted by Liu et al. [21] and Wang et al. [29]. Tangsri- trakul et al. [23] reported that the substitution of [Bi0.5(Na0.7K0.2Li0.1)0.5]2? or [Bi0.5(Na1-yKy)0.5]2? spe- cies decreasedTO–T, whereas Zr4?ions increased the TR–O of KNN-based ceramics; therefore, the coexis- tence ofO–T,R–O–T, andR–Tphases was observed near room temperature. Besides, the diffraction peaks are found to get shifted slightly to the higher angles with increasing sintering temperature (T2) (Fig.1b).
According to Bragg’s equation (Eq.1):
2dsinh¼nk ð1Þ
wheredis the interplanar distance,his Bragg’s angle, k is the wavelength of the incident X-ray beam, and nis an integer; the shift of diffraction peaks at higher angles suggests a decrease in the interplanar spacing, which eventually suggests a decrease in crystal lattice volume with an increase in temperature (T2). The decrease in crystal lattice volume may be attributed to the evaporation of the alkali elements at high temperatures, creating more vacancies at A-sites of the lattice, which eventually results in shrinkage of the unit cells [32].
Figure2a_e shows SEM micrographs and grain size distribution of the KNLNS–BNKZ ceramics sintered at different temperatures (T2) varying from 1000 to 1120°C for 4 h. It can be observed that all the ceramic samples had cuboidal grain morphology, which is the typical characteristic shape of KNN-based ceramics [33]. The results show that the microstructures of the ceramic samples are strongly dependent on the sin- tering temperature (T2). With low sintering temper- ature (T2 \ 1030°C), the ceramics had a porous microstructure with the distribution of discrete grains, which eventually suggests the lower density of the unsintered ceramics. However, when the temperature (T2) increases to 1060°C, pores are found to reduce significantly, showing a denser microstructure. The grain size was increased with the increasing the temperature of T2and reached to the maximum value of 1.76 lm at 1090°C and then decreased. Besides, the number of pores decreased significantly leads to the high ceramic density, espe- cially at T2= 1060°C, then tends to increase as the further increase of T2. This result is consistent with the two-step sintering mechanism, where densifica- tion increases via pore reduction. The measured
density values of the produced KNLNS–BNKZ ceramics, as a function of sintering temperature (T2), are shown in Fig.2f. It can be observed that with the increase in T2, the density of the ceramics also increased significantly and reaches a maximum value of about 4.40 g/cm3 at T2= 1060°C and then decreases with further increase in T2. The more probable cause of this phenomenon may be the par- tial evaporation of sodium and potassium at higher sintering temperatures [17]. This indicates that increasing the sintering temperature to an appropri- ate value during the second step can advance the densification of the ceramics, resulting in an increased ceramic density [34].
Variation in the microstructure of the ceramic samples, sintered at T2= 1060°C, with different sintering time (t2) viz., (a) 2 h, (b) 3 h, (c) 4 h, (d) 5 h, and (e) 6 h, as shown in Fig.3. The results show that the microstructure of the ceramic samples changes
with variation in sintering time (t2). It can be seen from Fig.3a–e, with the increasing of sintering time, the grain size of KNLNS–BNKZ ceramics increased from 0.91 to 1.83lm and the grains are more homo- geneous with increasing sintering time from 2 to 5 h (Fig.3f). Meanwhile, the measured density values of the produced KNLNS–BNKZ ceramics, as a function of sintering time (t2), are shown in Fig.3f. It can be observed that with the increase in t2, the density of the ceramics also increased significantly and reaches a maximum value of about 4.45 g/cm3att2= 5 h and then decreases with further increase in t2. Thus, the tendency to increase ceramic density is similar to the increase in the grain size and dense microstructure, and a clear grain boundary. In other words, initially, the ceramics turn into a dense structure gradually, which indicates that the increase of sintering time is effective in improving the density of ceramics.
However, the density of the ceramics would be Fig. 1 The XRD patterns of KNLNS–BNKZ ceramics prepared by two-step sintering process with various T2sintering temperatures (a) andt2times (b) in the 2hrange of 20780°; Rietveld refined XRD patterns of ceramics:T2= 1060°C (c) andt2= 5 h (d)
decreased as they are sintered for over 5 h. Research- ers have suggested an increase in grain size with an increase in the sintering time [17,35]. However, it is also observed that whent2C6 h, the grain size of the
samples slight decrease, and a higher amount of porosity content was also visualized. This might be possibly due to higher volatilization of potassium (K) and sodium (Na) during longer sintering time, Fig. 2 Effect of sintering temperature (T2) on microstructure and
grain size distribution of KNLNS–BNKZ ceramics sintered at differentT2temperatures: (a) 1000°C, (b) 1030°C, (c) 1060°C,
(d) 1090°C, (e) 1120°C for 4 h, and (f) the variation of the density, and grain size as a function ofT2
Fig. 3 Effect of sintering time (t2) on microstructure and grain size distribution of KNLNS–BNKZ ceramics sintered atT2= 1060°C for different sintering time (a) 2 h, (b) 3 h, (c) 4 h, (d) 5 h, (e) 6 h, and (f) the variation of the density, and grain size as a function oft2
leading to grain growth [36]. From the above results, it is evident that the two-step sintering parameters, T2= 1060°C andt2= 5 h, are found to be suitable for producing KNLNS–BNKZ ceramic with high density.
Figure4 presents the temperature-dependent dielectric constants (e) and dielectric loss (tand) of the KNLNS_BNKZ ceramics with different sintering tem- peratures (T2) measured at 10 kHz in the temperature range of 25–300°C. The variation of the ferroelectric–
paraelectric phase transition temperature (TC) with different sintering temperatures (T2) was mainly investigated. The (e–T) curves of the ceramics had two clear peaks: the first one at low temperatures (inset of Fig.4a) corresponding to ferroelectric phase transition temperatures TR–T (rhombohedral–tetragonal (R–T) ferroelectric phase boundary [22,26]) and the second one at higher temperatures corresponding to the fer- roelectric-paraelectric phase transition temperature (TC). As can be seen in Fig.4b, theTR–Ttemperature of ceramic samples is low in the range of 40–55°C. This result is consistent with the results of structural phase evolution observed through the XRD technique. The other peak appeared in the higher temperature range (188–250°C), and this may be attributed to the tran- sition of the tetragonal ferroelectric phase to the cubic paraelectric phase (TC). It can also be seen that, with increasing sintering temperature (T2), Curie tempera- ture (TC) was also changed slightly (Fig.4b), and their values fluctuated in the range of 2207225°C.
However, the maximum dielectric constant (emax) values of the samples were found to increase with increasing sintering temperature (T2) up to 1090°C (emax= 4200) and then found to decrease gradually
with further increase in T2. Meanwhile, it is very interesting to note that the sintering time (t2) also has a great influence on TC[17], as seen in Fig.5.
Figure5 shows the sintering time (t2) dependence of TC and TR–T of the KNLNS–BNKZ ceramic sam- ples sintered atT2= 1060°C. It can be observed that TCwas gradually increasing with an increase in sin- tering time from 2 to 6 h, when measured in the high temperature range (188–228°C), andTR–Twas found to vary slightly with sintering time (t2) when mea- sured in the low-temperature range of 35–45°C (Fig.5b). The maximum dielectric constant (emax) values of the samples were found to increase with increasing sintering time (up to 5 h) (emax= 4300) and then started to decrease with further increase in t2. These results indicate that the sintering time of 5 h is beneficial for improving the maximum dielectric constant. Hence, it can be summarized that variation in dielectric constant followed a similar pattern con- cerning sintering temperature and time.
To determine the piezoelectric properties of the ceramic samples produced under different sintering conditions, the resonant vibration spectra of the samples were measured at room temperature. Reso- nant vibration spectra of ceramic sample sintered at T2= 1060°C and t2= 5 h, as shown in Fig.6, show the frequency dependence of impedance Z and the phase angle h, in which the impedance reaches the minimum value at 210.5 kHz and maximum value at 237.5 kHz (Fig.6a), while thickness resonant spectra of ceramic sample sintered at T2= 1060°C and t2= 5 h are characterized by two peaksf1= 2.1 MHz and f2= 7 MHz, respectively (Fig.6b).
Fig. 4 Temperature-dependent dielectric constant e and dielectric loss tandof the KNLNS_BNKZ ceramics with different sintering temperatures (T2) measured at 10 kHz
Based on these vibration spectra, the piezoelectric parameters like electromechanical coefficients (kp,kt) of the ceramic samples were determined, and piezo- electric factor (d33) was determined by a d33 meter (Fig.7).
Variation in electromechanical coefficients (kp, kt) and piezoelectric factor (d33) (measured at room temperature) of the produced ceramic samples, as a function of sintering temperature (T2) and sintering time (t2), are shown in Fig.7. It was found that the variation of thed33, kp, and kt values with sintering temperature (T2) and sintering time (t2) followed a similar pattern. As seen in Fig.7a, thed33,kp,and kt
values of the ceramic samples increased with the increase inT2. Maximum values ofd33 = 231 pC/N, kp= 0.45, kt= 0.50 were obtained with sintering temperatureT2= 1060°C, and then dropped gradu- ally with further increase inT2. Similarly, thed33,kp,
and ktvalues of ceramics were also seen to increase rapidly with the increase in sintering time and reached a maximum (d33 = 251 pC/N,kp= 0.51,kt= 0.55) when t2= 5 h, and then slowly started to decrease with further increase in t2. From these results, it can be summarized that the optimum piezoelectric properties of the KNLNS–BNKZ ceramics were obtained atT2= 1060°C andt2= 5 h.
This is probably related to characteristics of the increasing grain size [3, 4]. As is well known, the increased grain size makes domain reorientation easier and severely promotes domain wall motion, which could increase the piezoelectric properties.
Thus, using the two-step sintering technique with suitable sintering parameters, the piezoelectric properties of the ceramics are found to improve sig- nificantly. This improvement in piezoelectric prop- erties can be attributed firstly to the rhombohedral–
Fig. 5 Temperature-dependent dielectric constanteand dielectric loss tandof the KNLNS_BNKZ ceramics with different sintering times (t2) measured at 10 kHz
Fig. 6 aRadial andbThickness resonant spectra of ceramic sample withT2= 1060°C andt2= 5 h
tetragonal (R–T) ferroelectric phase boundary [10,37], which was found to be formed at near room temperature in the KNLNS–BNKZ ceramic samples, and secondly, the high density (4.45 g/cm3in Fig.3f) and the large grain size (1.83lm in Fig.3d) com- bined dense microstructure and a clear grain boundary [38]. According to Tan et al. [36], this R–
Tphase transition near room temperature may cause a higher degree of polarization instability, which makes the direction of polarization rotate easily when ceramics are polarized under an external electric field and can achieve high piezoelectricity. Hence, it can be summarized that samples with maximum density (4.45 g/cm3) and enhanced dielectric and piezoelec- tric properties (d33= 251pC/N, kp = 0.51, kt = 0.55) are achieved by two-step sintering process, with optimized sintering parameters (T2= 1060°C and t2= 5 h).
Figure8a shows the shapes of P–E ferroelectric hysteresis loops measured at room temperature of the KNLNS–BNKZ ceramic samples sintered at 1060°C for different sintering time (t2). As shown in Fig.8a, saturatedP–Ehysteresis loops are observed, showing good ferroelectric properties. From the shape of these loops, the remanent polarization Pr
and the coercive field EC of ceramics were deter- mined, as shown in Fig.8b. With increasing of t2
sintering time, the value ofPrincreases and reaches a maximum value of 16.77 lC/cm2 at t2= 5 h, then decreases. This result is also consistent with the piezoelectric properties of the KNLNS–BNKZ ceramics as indicated above. Different from Pr, the observed Ec values fluctuate in the range of
5.68–7.13 kV/cm with the increase of sintering time from 2 to 6 h.
In practical applications, the properties of high piezoelectricity, good dielectricity, and high energy storage efficiency (g) are also expected. In this work, theg was calculated by Eq. (2) [39,40]:
g¼ W1
W1þW2 ð2Þ
In which, the energy storage density W1 (marked by the green area in Fig.8c) was calculated by Eq. (3):
W1¼ Z Pmax
Pr
EdP ð3Þ
The energy loss density W2 was marked by the gray area in Fig.8c. It, due to the domain reorienta- tion, was obtained by integrating the area between the charging and discharging curves.
As observed in Fig. 8d, the variation trend of the energy storage density at different sintering times was similar to the trend for energy storage efficiency.
Both W1 and g increased almost linearly with the increasing sintering time and reached the maximum values of 0.24 J/cm3and 51.8%, respectively, at 6 h.
Thus, the sintering time of 6 h improved the energy storage density of KNLNS–BNKZ ceramics by* 200% in comparison to that of 4 h. While the energy loss densityW2tends to decrease as the sintering time increases from 4 to 6 h, which is beneficial for improving energy efficiency. The result is an energy storage efficiency of 51.8% which is achieved at the sintering time of 6 h. This is very important for dielectrics because it means less waste heat, better Fig. 7 Measurement ofkp,ktandd33of KNLNS–BNKZ ceramics as a function ofasintering temperature (T2) andbsintering time (t2)
reliability, and a longer lifetime of capacitors in practical applications [1].
4 Conclusions
The 0.96(K0.48Na0.48Li0.04)(Nb0.95Sb0.05)O3–0.04Bi0.5(- Na0.82K0.18)0.5ZrO3 (KNLNS–BNKZ) lead-free cera- mic samples were prepared successfully using two- step sintering technique. The effect of sintering parameters on the structural phase transition, microstructure, dielectric, and piezoelectric proper- ties of ceramics was investigated. All the sintered samples had tetragonal–rhombohedral (R–T) mixed pure perovskite phases irrespective of sintering parameters. The diffraction peaks were found to shift slightly to the higher angles with increasing sintering temperature (T2), suggesting shrinkage in crystal volume due to evaporation of the alkali elements and creation of more vacancies at A-sites of the lattice at high temperature. Piezoelectric parameters like
electromechanical coefficients (kp, kt) and piezoelec- tric factor (d33) of the samples were found to increase with sintering temperature and time (up to T2= 1060°C andt2= 5 h) and then decreased grad- ually with further increase in both the parameters.
Improvement in piezoelectric properties is attributed to (i) rhombohedral–tetragonal (R–T) ferroelectric phase boundary, formed near room temperature, and (ii) fine-grained and highly dense microstructure of the samples. Sintering parameters also showed an eminent effect onTCandTR–Ttemperatures. Samples with maximum density (4.45 g/cm3), enhanced dielectric, piezoelectric, and ferroelectric properties (d33= 251pC/N, kp = 0.51, kt = 0.55, Pr= 16.77 lC/
cm2) were achieved with optimized sintering parameters (T2= 1060°C and t2= 5 h). This study concludes that with the optimized two-step sintering parameters, the piezoelectric properties of the KNLNS–BNKZ ceramics can be significantly improved. Besides, the optimal sintering conditions for KNLNS–BNKZ ceramics resulted in enhanced Fig. 8 aTheP–E hysteresis loops of ceramic samples with the
different sintering time;bThe valuesPrandEC, as a function of thet2sintering time;cThe energy storage densityW1(marked in
green area); d The calculated energy storage performance of ceramics (Color figure online)
energy storage density of 200% and an energy storage efficiency of 51.8%, which are useful for capacitors in practical applications.
Acknowledgements
This research is funded by Vietnam National Foun- dation for Science and Technology Development (NAFOSTED) under Grant Number 103.02-2019.08
Declarations
Conflict of interest The authors declare that they have no conflicts of interest.
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