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Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journal homepage: http://ees.elsevier.com

Enhancement in dielectric, ferroelectric, and piezoelectric properties of BaTiO ₃ - modified Bi _0.5 (Na _0.4 K _0.1 )TiO ₃ lead-free ceramics

Le Dai Vuong

^a,∗

, Phan Dinh Gio

^b

aHue Industrial College, Viet Nam

bUniversity of Sciences, Hue University, Viet Nam

A R T I C L E I N F O

Article history:

Received 30 July 2019

Received in revised form 15 October 2019 Accepted 23 October 2019

Available online xxx Keywords Ceramics Ferroelectrics Piezoelectricity Energy storage materials Crystal structure

A B S T R A C T

In this paper, the effect of BaTiO3on dielectric, ferroelectric, and piezoelectric properties was observed in (1-x)[Bi0.5(Na0.4K0.1)TiO3]–xBaTiO3ceramics (BNKT-BT, withx=0; 0.01; 0.02; 0.03 and 0.04). The phase for- mation and microstructure of BNKT-BT ceramics were examined by X-ray diffraction (XRD), the Raman scattering spectra, and scanning electron microscopy (SEM). A pure perovskite phase was observed in the ceramic samples, proving the dissolution of BaTiO3into the lattice structure of the BNKT to form a homogeneous solid solution (x≤0.04). Atx=0.02, the best physical properties of the BNKT-BT ceramics, such as density,ρ=5.86g/cm³ (98.5% of the theoretical density); electromechanical coupling factors (kp), 0.30; (kt), 0.31; remanent polarization (Pr), 17.2μCcm⁻²; dielectric constant (ε_r), 1354; and highest dielectric constant (ε_max), 4037, were obtained. Fur- ther, the energy storage properties of 0.98Bi0.5(Na0.4K0.1)TiO3–0.02BaTiO3ceramics were studied and compared with the textured ceramics (the same composition), which were formed due to the high degree of orientation obtained by the template grain growth method. The textured ceramics showed lower remnant polarization, while the piezoelectric coefficient (d31) and electromechanical coupling coefficients (kpandkt) of the textured ceramics were found to increase by about 8%, 13.3%, and 19.4%, respectively.

© 2019

1. Introduction

In recent years, there has been a considerable increase in the inter- est on the application of Pb(Zr(1-x)Tix)O3(PZT)-based ceramics owing to their excellent piezoelectric properties and applications in piezoelectric actuators and transformers [1–7]. However, the use of the lead-based ceramics poses serious environmental concerns owing to the high tox- icity of lead oxide. Therefore, it is necessary to develop lead-free ce- ramics with good ferroelectric and piezoelectric properties to replace the lead-based ceramics [8–10]. Nonetheless, lead-free materials still cannot completely replace lead-based materials in all applications espe- cially high-pressure piezoelectric materials, piezoelectric transformers, or ultrasonic motors [3,6,11]. Thus it became highly intriguing to de- velop new lead-free materials with improved properties that are suit- able for the desired industrial applications [11]. Among the various lead-free materials available today, Bi_0.5Na_0.5TiO₃(BNT) [12–17], Ba- TiO3[11,18–20] and K0.5Na0.5NbO3(KNN) [8,19,21] based ceramics have been considered as most promising candidates owing to their excel- lent electromechanical properties near the morphotropic phase bound- ary (MPB) [18]^. Kamnoy et al. [11] added BaTiO3 as starting ma- terials in Bi0.5(Na0.8K0.2)0.5TiO3 ceramic by using solid-state

∗Corresponding author.

E-mail address:ledaivuongqb@gmail.com (L. Dai Vuong)

reaction method. BaTiO3,at a content of 8mol%, resulted in the high- est value of the dielectric constantε_maxof 6700. However, in this tra- ditional ceramic material, the orientation of the particles is random, and the exhibited properties are the average values of all particles in the material. Therefore, texture engineering, the technology used to de- velop crystallographically textured materials, seems to be promising in the fabrication of ceramic materials with desired properties. Dong et al. [18] reported a high degree of orientation (96%) in the textured 0.88Na0.5Bi0.5TiO3-0.08K0.5Bi0.5TiO3-0.04BaTiO3ceramics with optimal properties, at 4wt% of NaNbO3 templates. The optimum properties, such as piezoelectric coefficient (d₃₃) of 185pC/N, high remanent po- larization (Pr) of 26.4μC/cm², low coercive field (Ec) of 31kV/cm, and the dielectric constant (ε_max) of 3950, were obtained. Recently, Fan et al. [16] prepared (1-x)Bi1/2(Na0.82K0.18)1/2TiO3–xBi4Ti3O12ceramics using conventional solid-state reaction method and found that the nor- mal ferroelectric-to-ergodic relaxor (FE-to-ER) transition occurs at a low temperature (TF-R≈0°C). At a Bi4Ti3O12content ofx=9wt%, ceramics exhibited best electrical properties such as a large signal piezoelectric coefficient,d*₃₃of 485 pm/V, and a small hysteresis,ηof 23%.

In this study, the synthesis of (1-x)[Bi0.5(Na0.4K0.1)TiO3]–xBaTiO3

(BNKT-BT) lead-free textured ceramics was carried out by the conven- tional solid-state reaction method and template grain growth method with pure-phase Bi4Ti3O12 templates. The textured samples with pre- ferred orientation showed an enhanced electrical property. The goal of texturing was to form BNKT-BT lead-free piezoelectric ceramic with

https://doi.org/10.1016/j.jallcom.2019.152790 0925-8388/© 2019.

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(1-x)[Bi_0.5(Na_0.4K_0.1)TiO₃]–xBaTiO₃, wherex=0.0, 0.01, 0.02, 0.03, and 0.04. The following reagents were used: Bi₂O₃, TiO₂, Na₂CO₃, K₂CO₃ and BaCO₃ (Merck, purity≥99.5%). In particular, Bi_0.5(Na_0.4K_0.1)TiO₃ceramics based on our previous research formula [12,14].

2.1. Synthesis of BNKT-BT ceramics by the conventional solid-state reaction method

The process of fabrication technology for Random ceramics, as shown in Fig. 1. Starting raw materials of Bi2O3, Na2CO3, K2CO3, TiO2

and BaCO₃(Fig. 1(a)) were weighed and milled with ultrasound treat- ment with an electric power of 100W and fixed frequency of 28kHz in ethanol medium for 1h (Fig. 1(b)) [12].

After that, the powders were dried and calcined at 850°C for 2h to synthesize BNKT-BT compounds (Fig. 1(c)). The calcined BNKT-BT powder milled for 20h (Fig. 1(d)). The ground materials were pressed into disk 12mm in diameter and 1.5mm in thick under 100MPa (Fig.

1(e)). The BNKT-BT samples were sintered at temperature 1100°C for 2h (Fig. 1(f)). Then, the sample is treated, covered with electrodes

Fig. 1.The process of fabrication technology for random ceramics.

demonstrated in Fig. 2. The microstructure study revealed a characteris- tic plate-like shape for the Bi4Ti3O12template after sintering (Fig. 2(a)).

The revelation of particle size length in the range of 4–18μm and ap- proximate width of 0.5–1μm (Fig. 2(b)), which originated due to the high anisotropy of Bi4Ti3O12crystal structure (Fig. 2(c)), suggested that the molten salt method was useful not only in producing particles with anisotropy but also for reducing the aggregation of the particles [22].

As the last step, according to the abovementioned stoichiometric ra- tio for the reaction, a predetermined amount of Bi4Ti3O12powder (Fig.

2(a)) and calcined BNKT-BT powder (Fig. 3(a)) were mixed together and milled in a planetary milling machine (PM400/2-MA-Type) for 2h (Fig. 3(b)). Then, this mixture was continuously mixed with a binder solution and was subsequently tape cast to form thin sheets with a thick- ness of about 0.1mm (Fig. 3(c)) in which the plate-like Bi₄Ti₃O₁₂parti- cles were arranged such that the faces of the plates were parallel to the sheet surface (Fig. 3(d)). As the next step, the sheets were cut into disks of 12mm (Fig. 3(e)), stacked in 30–40 layers (Fig. 3(f)), and pressed at room temperature under a pressure of 100MPa for 3min to form green compacts with a thickness of about 2mm, as shown in Fig. 3(g). At the end of this process, it reached textured microstructure as shown in Fig.

3(h).

The properties of the samples were studied using various analytical methods. In elaboration, X-ray diffraction (XRD) analysis (Rigaku RINT 2000) at room temperature was employed to explore the crystallinity, while field emission scanning electron microscopy (Nova NanoSEM 450-FEI-HUS-VNU) was used to examine the morphology [23]. Fur- thermore, the density and ferroelectric loops, used to identify ferro- electricity, of the samples, were measured by the Archimedes and Sawyer-Tower method, respectively [24]. Moreover, the grain size was determined by using a mean linear intercept method [3]. The mea- surement of dielectric properties was achieved by measuring the tem- perature

Fig. 2.Characteristics of Bi4Ti3O12template: a) Microstructure; b) The distribution of particle size length; c) Crystal structure.

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Fig. 3.The process of fabrication technology for textured ceramics.

dependencies of capacitance and phase angle (HIOKI 3532). Lastly, in other to study piezoelectric responses, the pellets were poled in a 120°C silicone oil bath by applying a DC electric field of 35kV/

cm for 20min then cooling down to room temperature. The pellets were aged for 24h prior to testing. The coupling factors (kp,kt) and

Fig. 4.The values ofρ,εr, and tanδobserved for the BNKT-BT ceramics as a function of BaTiO3contents.

piezoelectric constant (d31) were determined using a resonance method (HIOKI 3532, HP4193A) and calculated by the IEEE standard [24].

3. Results and discussion

Fig. 4 shows the variations in the density of (1-x)BNKT-xBT sam- ples at different concentrations of BaTiO3. All the sintered samples ex- hibit high relative density more than 96%, indicating that dense mi- crostructures are obtained. With the increase in the BaTiO3content up to 0.02mol, the density of BNKT ceramics was increased. It achieved the maximum value (ρ=5.86g/cm³, 98.5% of the theoretical density) at a BaTiO3content of 0.02mol and then decreased. This could be ex- plained by the fact that when BaTiO₃content in the ceramic system was <0.02mol, a large number of pores were present, indicating in- sufficient densification of the samples (Fig. 5). As the BaTiO3content was increased, the ceramics became denser, and the sample was al- most completely dense at a BaTiO₃content of 0.02mol. In general, it has been observed that the addition of BaTiO3enhances the density of BNKT ceramics. This result is consistent with that observed by Man- otham et al. [25]. The effect of BaTiO3concentration on the dielectric constant (ε_r) and dielectric loss (tanδ) of the BNKT ceramics at 1kHz is also illustrated in Fig. 4. When the concentration of BaTiO₃ was

Fig. 5.Typical SEM images of the BNKT-BT ceramics at different contents of BaTiO3.

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creased. The smallest average grain size of 0.83μm was observed for the sample without BaTiO3(x=0.0). This suggests that the microstructure can be improved by a BaTiO₃additive. According to the research re- sults of Khanal et al. [26] the conventionally prepared BaTiO₃ceramics sintered at 1350°C for 5h exhibited a grain size of 15.9μm. The large grains observed with increasing concentration of seed content may have resulted from the large particles (BT) diffusing into the BNKT ceramics, thus increasing the grain growth. This relationship can be used to em- pirically determineG_susing Eq. (1):

Gs= (1-x)(0.83μm) +x(15.9μm) (1)

It is evident from the data that Eq. (1) gives a reasonable approxi- mation of the average grain size with x=0 to 0.02. However, when the addition of BaTiO₃is higher than 0.02mol, there appears a large differ- ence between the experimental data and theoretical calculations (Fig.

5). This can be explained by the fact that the diffusivity of solute atoms is normally different from that of host atoms. This result indicates that the addition of BaTiO₃exceeding the solubility limit in BNKT ceramics inhibits grain growth due to the solute drag effect.

Fig. 6(a) depicts the XRD patterns of BaTiO3-modified BNKT ceram- ics. All samples exhibited a pure perovskite phase, and no trace of sec- ondary phase was detected in the investigated region. In order to illus- trate the effect of BaTiO3on the structure of the materials, a magni- fied view of the peaks in the region from 39° to 41°, and 46°–47° are shown in Fig. 6(b) and (c), respectively. It can be seen that the pure BNKT sample possesses a rhombohedral symmetry characterized by a pure (200) peak [12]. However, the large size Ba²⁺(1.61Å) ions dif- fuse into the BNKT lattice to replace Bi³⁺(1.17Å), Na⁺(1.39Å), and K⁺(1.64Å) [12,27,28] resulting in the enlargement of lattice constant and a shift in the XRD peak position toward a lower diffraction an- gle direction. Though, a significant shift in the peak position of the re- flection towards a higher angle side has been observed for the samples for samples containing BaTiO3content higher than 0.03mol. Similar

diffused into the 0.92(Bi_0.5Na_0.42K_0.08)TiO₃-0.08(BaNb_0.01Ti_0.99)O₃. The sample also showed a tetragonal perovskite (P4mm) structure in the ce- ramic. The phase diagram of the binary system (1-x)Bi0.5Na0.5TiO3– xBaTiO3[31] showed morphotropic phase boundary regions between the ferroelectric rhombohedral and tetragonal phases in thexrange of 0.06–0.08. According to Kwei et al. [32] The long-range structure of BaTiO3is can be described by the P4mm space group with lattice pa- rameters of a=3.99095Å and c=4.0352Å with a c/a ratio of 1.011.

With increasing molar fraction of BaTiO3, the crystal symmetry of the (1-x)BNKT-xBaTiO3ceramics should change due to the tetragonal dis- tortions of BaTiO3. Therefore, in order to understand the degree of disor- der and phase transformation behavior, the tolerance factor (t) [14,33]

was calculated for these ceramics based on the ABO3structure as shown in Fig. 7(a).

Therefore, in the pure Bi0.5(Na0.4K0.1)TiO3structure,

(3) and in the pure BaTiO3(BT),

(4) From Eqs. (3) and (4), the tolerance factors of BNKT (t=0.9492), which is lower than the tolerance factor of BT (t=1.0554), was calcu- lated by considering the ionic radii ofRBa2+=1.61Å,RBi3+=1.17Å, R_Na⁺=1.39Å, R_K⁺=1.64Å, R_Ti⁴⁺=0.61Å and R_O²⁻=1.42Å [27,33]. It can be shown that the incorporation of BT into BNKT slightly increased the tolerance factor in the investigated region (Fig. 7(b)), corresponding to the decrease in lattice distortion and a trend toward slightly stable structure of tetragonal symmetry, similar to the study of Ghosh et al. [27] for Bi0.5(Na0.8K0.2)0.5TiO3ceramics.

Fig. 6.The XRD patterns of BaTiO3-modified BNKT ceramics with 2θranging from (a) 20°–70°, (b) 39°–41° and (c) 46°–47°.

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Fig. 7.a) The perovskite structure illustrated for ABO3; b) The tolerance factors of BNKT-BT ceramics.

(2)

In order to provide convincing evidence for the phase structure evo- lution of BNKT-BT ceramics at different BaTiO₃contents, Raman spec- troscopy in the range of 80–1000cm⁻¹ was carried out (Fig. 8(a)).

It was applied to further characterize the surface groups and chemi- cal bond states of materials [29,34–36]. As shown in Fig. 8(a), all of the bands were relatively broad, which is typical for relaxor based perovskite ferroelectrics [37]. The regions can be split into four cate- gories according to the wavelength as above 700cm⁻¹, between 400 and 700cm⁻¹, between 200 and 400cm⁻¹and below 200cm⁻¹. In the pre- sent study, the spectral deconvolution was performed according to six Gaussian-Lorentzian modes by using a best-fitting algorithm to illustrate the effect of composition on the changes in peak characteristics [29,38].

In the first band region, the peak centered between 100 and 129cm⁻¹was assigned to A1(TO) mode, which could be associated with the A site vibrations of the Bi–O, Na–O, K–O, and Ba–O bonds [16,39]

and is sensitive to phase changes in the crystal structure of the per- ovskite [39]. It is interesting to note that the A-site mode exhibited a slight decrease in the wavenumber and intensity with the increasing Ba- TiO3contents (Fig. 8(a)). It can be explained by the fact that the Ba- TiO3could dissolve into the BNKT lattice thus affecting the vibrations of the perovskite A-site and may induce a phase transition [40,41]. As mentioned above, the radius of Bi³⁺ion (~1.61Å) is similar to those of the Bi³⁺(~1.17Å), Na⁺(~1.39Å) and K⁺(~1.64Å) ions, suggest- ing that Ba²⁺ions having a smaller ionic radius enter the A site of the perovskite unit cell to replace Bi³⁺, Na⁺and K⁺ions, which changes

the lattice structure. Similar observations have been reported by Man-

otham et al. [41] in other

(1-x)Bi0.5(Na0.84K0.16)0.5TiO3-xBa(Nb0.01Ti0.99)O3ceramics.

The second region is located in the range of 200–400cm⁻¹and is attributed to the vibration of the Ti–O bond. It is localized at around 266cm⁻¹(x=0) and associated with the tetragonal phase in the per- ovskite structure [37,42,43]. However, with the increasing BaTiO3con- tents, the peak splits into two bands at ~254 and ~306cm⁻¹. This proves that the (1-x)BNKT-xBaTiO₃ceramics with 0–2mol% of BaTiO₃ exhibited the rhombohedral phase, which changed to tetragonal, as the BaTiO3content was increased further. Similar kinds of behavior have been observed by Kreisel et al. [44] in the Raman spectra of the Bi0.5(Na1−xKx)0.5TiO3ceramics, where the system changes its symmetry from rhombohedral R3c to tetragonal P4bm through an MPB between those two phases. Hayashi et al. [45] suggested that the bands around 305cm⁻¹, which is characteristic of the tetragonal BaTiO3phase, is as- signed to the B1mode.

Meanwhile, the two overlapping bands associated with the vibra- tions of the TiO6octahedra within 450–650cm⁻¹range become increas- ingly distinct with the increasingx content demonstrating the emer- gence of phonon behavior in the structure [46]. The modes of the TiO6octahedra localized at around 601cm⁻¹exhibited an increase in the wavenumber with the increasing contents of BaTiO₃(Fig. 8(b)).

It can be seen that the peaks shifted slightly, suggesting a distortion in the TiO6octahedral and the presence of internal stresses that could possibly stabilize dielectric and ferroelectric properties at the room temperature after BaTiO₃ addition [41]. The broad band observed

Fig. 8.Raman spectra of BNKT-BT ceramics.

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x=0–0.02mol, the maximum value of the dielectric constant (ε_max) in- creases from 3296 to 4037 (Fig. 9(b)). On the other hand, ε_maxde- creases with the increasing BaTiO₃concentration beyond 0.02mol con- comitant with the increase in the grain size as shown in Fig. 5.

The diffuseness (γ) was evaluated by plotting ln(1/ε- 1/εmax) versus ln(T - Tm) [14] at 1kHz and temperatures greater thanTm, as demon- strated in Fig. 9(c). The increase inγin this range is higher than the increase in the case when the content of BaTiO3was from 0.0 (γ=1.60) to 0.02mol (γ=1.87) (Fig. 9(b)). Then, it rapidly decreases with in- creasing x. Considering from a structural viewpoint, we can explain that the BaTiO₃get dissolved into the BNKT lattice, which affects the vibrations of the perovskite A-site and may induce a phase transition due to the difference in radius ions as discussed above. In other words, when the BaTiO3addition is above the limit, the excessive Ba²⁺ions would segregate at the grain boundary, inhibit the grain growth, and lead to the heterogeneity of the structure causingγto decrease. These results are in good agreement with a study of Kamnoy et al. [11] on lead-free BNKT-based ceramics. Thus, it can be concluded that the ad- dition of BaTiO3into BNKT ceramics increases the value of γ, which means that the paraelectric-ferroelectric phase transition is of diffuse type, and the ceramics exhibit high disorder [14]. This is also the rea- son for the decrease ofTF-R from 120°C to 85°C and the decrease of Tmfrom 269°C to 231°C when increasing BaTiO3content is gradual (Fig. 9(d)). These results are consistent with the literature [27]. Ac- cording to Ghosh et al. [27], the decrease inT_F-Ris related to non-cubic distortion with the incorporation of BaTiO3into BNKT ceramics, con- sequently destabilizing the long range ordering state. According to Lu

cating that this ceramic is a well behaved complete solid solution. This relationship can be used to empirically determineT_musing Eq. (5) [50]:

Tm= (1-x)(269 °C) +x(120°C) (5)

The variation in the calculatedTmas a function of compositionxis shown in Fig. 9(d). It is evident from the data that Eq. (5) gives a rea- sonable approximation of the transition temperatureT_mwithx=0.0 to 0.02. This result suggests thatTmof (1-x)Bi0.5(Na0.8K0.2)0.5TiO3–xBa- TiO3ceramics can be varied over a wide range from 120°C to 269°C by controlling the content of BaTiO3 in the composition. However, when the addition of BaTiO3is higher than 0.02mol, there appears a large difference between the experimental data and theoretical calcu- lations. This can be explained by the solubility limit of Ba²⁺ions in Bi0.5(Na0.8K0.2)0.5TiO3ceramics near 0.02mol. However, if the addition of BaTiO3is above 0.02mol, excess Ba²⁺ions accumulate at the grain boundaries, which has little influence on the average energy of the oxy- gen octahedron; thus, the temperatureTmwill be abnormally influenced (not according to rules) by the excess Ba²⁺ions.

Fig. 10 shows the changes in electromechanical coupling factors in planar mode (k_p), thickness mode (k_t), the piezoelectric constant (d₃₁), and mechanical quality factorQmas a function of the BaTiO3content in BNKT ceramics sintered at 1100°C for 2h. When the content of Ba- TiO3is≤0.02mol, the values of kp, kt, d31, and Qmincrease rapidly with an increase in the BaTiO3content, then decrease. The optimum values fork_pof 0.30, k_tof 0.31,d₃₁of 99pC/N andQ_mof 189 were obtained atx=0.02, which is near the solution limit. In other words,

Fig. 9.Temperature dependence of dielectric characteristics of BNKT-BT ceramics.

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Fig. 10.kp,kt,d31, andQmas a function of BaTiO3content (x) in BNKT ceramic.

the improvement in the piezoelectric properties of the samples can be partially ascribed to the enhancement in the density and the in- creasing grain size effect. Similar observations were made for Ba-doped Bi0.5(Na0.85K0.15)0.5TiO3ceramics [51]. It can be found in Fig. 4 that all the relative densities are larger than 95%, indicating that the effect of pores on the piezoelectric properties is negligible [52]. Therefore, the improvement of piezoelectric properties of ceramics can only be ex- plained by the particle size increase effect and density.

Qaiser et al. [53] show that the dense structure not only improves the piezoelectric response but also increases theQmvalue by reducing the dissipated energy. As a result of Fig. 5 presents the ceramic sam- ples have surfaces with very few pores and exhibit good density with sufficient grain growth. In which, the 0.98Bi_0.5(Na_0.8K_0.2)_0.5TiO₃- 0.02- BaTiO3samples have the best modification of microstructure with great density. It is believed to be associated with the larger grain comparing to other samples, where far fewer grain boundaries will make the ferro- elastic domain wall reversal easier [18]. On the other hand, according to Hayati et al. [54] the improvement of the dielectric and piezoelectric properties are assumed to be related to the formation of a homogenous microstructure with optimum grain size in piezoceramics.

The P-E hysteresis loops of the BaTiO₃ doped BNKT sintered at 1100°C were measured at room temperature and are shown in Fig.

11(a–e); the remanent polarization (Pr), coercive field (Ec) and the squareness of the hysteresis loop (Rsq) were also determined and are shown in Fig. 11(g). Based on the increase ofPrand decrease ofEc, there is improvement of ferroelectric properties of BaTiO₃doped BNKT ceramics by doping BaTiO3up to 0.02mol. However, when the BaTiO3

content of the samples exceeded 2mol%, the BNKT-BT ceramics have a small grain size, which results in large grain surface area, and internal stress which will reduce the orientation of polarization along the electric field, and strongly influence ferroelectric properties of ceramics [25].

On the other hand, the 0.98Bi_0.5(Na_0.4K_0.1)TiO₃- 0.02BaTiO₃ceramics shows good ferroelectric properties, with high remanent polarization of Pr=17.3μC/cm²and rather low coercive electric field ofEc=27.2kV/

cm. Cao et al. [55] derived an empirical equation to measure not only the deviation in the polarization axis but also that in the electric field axis like this:

(6) whereR_sqis the squareness of the hysteresis loop,P_ris the remnant po- larization,P_sis the saturation polarization, andP_1:1Ecis the polarization at an electric field equal to 1.1 times of coercive field. When the BaTiO3

content varied from 0 to 0.04mol, the values ofRsqchanged to 0.96, 0.99, 1.04, 0.89 and 0.73, respectively (Fig. 11(g)). Similar observa- tions were made for Ba(Nb_0.01Ti_0.99)O₃-doped Bi_0.5(Na_0.84K_0.16)_0.5TiO₃ ceramics [41]. Thus, the samples showed relaxor dielectric relations and this observation is consistent with the dielectric analysis (Fig. (9)).

Fig. 11(f) showsP–Ehysteresis loops at room temperature for tex- tured 0.98BNKT-0.02BT ceramics with 10wt% of Bi₄Ti₃O₁₂templates using the process of fabrication technology for textured ceramics (Fig.

3). The Bi4Ti3O12templates were well-aligned parallel to the casting di- rection and seeded the formation of the BNKT phase around the oriented template particles (Fig. 12). Almost a single layer of BNKT particles was formed on the (100) surface of Bi₄Ti₃O₁₂templates, similar to the study of Chang et al. for Sr0.61Ba0.39Nb2O6ceramics [56]. As can be seen that the textured sample exhibited low remnant polarization (Pr=15.2μC/

cm²) and coercive field (Ec=22.6kV/cm) compared with the random sample (Fig. 11(c),P_r=17.3μC/cm²,E_c=27.2kV/cm) due to the crys- tal-like characteristics of the textured ceramic). It is known that the co- ercive field of a single crystal is usually lower than that of polycrys- talline ceramic with the same composition [19]. Textured BNKT-BT ce- ramic exhibitedRsqof 1.01, which is lower than that of the random sam- ples (Rsq=1.04).

The energy storage densityW₁ (Fig. 11(f), marked in blue area) was obtained by integrating the area between the polarization axis

Fig. 11.(a–f)P–Ehysteresis loops and (g, h) the calculated the ferroelectric parameters of ceramics.

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Fig. 12.Microstructure of Textured 0.98BNKT-0.02BT ceramics.

and the discharge curve of the unipolarP–Ehysteresis loops using Eq.

(7) [48,57]:

(7) The energy loss density (W2) (Fig. 11(f), marked in the green area) caused by the domain reorientation was obtained by integrating the area between the charge and the discharge curve. The energy storage effi- ciency (η) of the material can be calculated by Eq. (7) [48,57]:

(8) The varying trend in the energy storage density (W1) of the ceram- ics at different BaTiO3contents, is similar to the trend of the energy storage efficiency (η), as shown in Fig. 11(h). BothW1andηincrease almost linearly with the increment of BaTiO3content at x<0.03 and reaches a maximum value of 0.30J/cm³ and 24.6%, respectively at x=0.04. While, as the BaTiO₃content increased from 0 to 0.04mol, the values ofW2 of samples increased from 1.0 to 1.16J/cm³ reach- ing the highest value of 1.16J/cm³atx=0.02, upon which it then de- creased. At this content, the textured 0.98BNKT-0.02BT ceramic exhib- itedW₁of 0.14J/cm³andW₂of 0.88J/cm³, which are lower than that of the random samples (W1=0.15J/cm³,W2=1.16J/cm³) at 42.3kV/

cm as illustrated in Fig. 11(h). These values are comparable to the pre- viously reported of BNKT-based bulk ceramics studied by Manotham et al. [41] which showed an energy density value of ~0.09J/cm³ and an energy storage efficiency value of ~9% (measured at 25°C).

kt) of the textured ceramics were found to increase by about 8%, 13.3%, and 19.4%, respectively. This finding is in a good agreement with the degree of orientation of the ceramic samples (Fig. 12). The derivations of the plate-like square-shaped particles with a greater length of 3μm and thickness of 1.5μm from the template growth at the expense of the matrix powders after sintering can be seen in Fig. 12. This affirms the effectiveness of the template grain growth method in aligning the tem- plate particles. In short, the BaTiO₃-modified BNKT ceramics show im- proved electrical properties, which are maximum at a BaTiO₃content of 0.02mol. Besides, the electrical properties of BNKT - BT ceramics can be improved by the process of fabrication technology for textured ceramics (Fig. 3). As shown the inset in Fig. 12, the non-split (100) peak at the 2θof around 22° and the (200) peak at the 2θof around 46° proved that textured 0.98BNKT-0.02BT ceramic has a rhombohedral structure [14].

The intensity of the (110) reflections was found to decrease with an in- crease in the intensity of the (100) and (200) reflections at 2θangles of 22.76° and 46.61°, respectively, which indicates the development of tex- ture in the samples [59].

To confirm the achievements of the present study, the characteris- tic parameters of the BNKT-BT ceramics sintered at 1100°C as such: the sintering temperature (Ts), Curie temperature, (Tm), electromechanical coupling coefficient (k_p), the piezoelectric coefficient (d₃₁), the coercive field (E_c), and the remnant polarization (P_r) were extracted and com- pared with those of other Bi-based lead–free ceramics [16–19,60,61] as listed in Table 2. Our results indicated that the BaTiO3-modified BNKT ceramics shows a lower sintering temperature, while the electrical prop- erties of the material ceramics are well maintained.

4. Conclusions

In this work, the BaTiO3doped Bi0.5(Na0.4K0.1)TiO3ceramics were successfully synthesized by a solid-state mixed oxide method. The results indicate that BaTiO3enhances the dielectric, ferroelectric, and piezo- electric properties of BNKT-based ceramics. The best physical proper- ties of the BNKT-BT ceramics, such as density,ρ=5.86g/cm³, 98.5% of the theoretical density); electromechanical coupling factors (k_p), 0.30;

(k_t), 0.31; remanent polarization (P_r), 17.2μCcm⁻²; dielectric constant (ε_r), 1354; and highest dielectric constant (ε_max),

Table 1

The ferroelectric and piezoelectric properties of the textured and random ceramics.

Samples Textured/Random kp kt

d31

pC/N Pr

μC/cm² Ps

μC/cm² EC(kV/cm) W1

J/cm³ W2

J/cm³ η(%) Rsq

BNKT Random 0.27 0.19 63 9.1 12.5 29.1 0.08 1.00 7.4 0.96

0.99BNKT-0.01BT Random 0.29 0.21 88 12.4 16.46 28.16 0.11 1.1 9.1 0.99

0.98BNKT-0.02BT Random 0.3 0.31 99 17.3 22.7 27.2 0.15 1.16 11.5 1.04

0.97BNKT-0.03BT Random 0.24 0.28 78 16.1 23.64 18.8 0.19 1.05 15.3 0.89

0.96BNKT-0.04BT Random 0.21 0.22 74 12.7 22.4 13.9 0.3 0.92 24.6 0.73

0.98BNKT-0.02BT Textured 0.34 0.37 107 15.2 22.3 22.6 0.14 0.88 13.5 1.01

BNKT [41] Random – – – 29 42 16–42 0.09 – 9 1.25

UNCORRECTED

PROOF

Table 2

The comparison of the characteristic parameters of the BNKT-BT ceramics and other ceramics.

Ceramics Textured or Random TS(ºC) Tm(ºC) kp

d31

pC/N EC(kV/cm) Pr(μC/cm²) Ref.

Bi0.5(Na0.82K0.18)0.5TiO3 Random 1100 300 – – 10.5–32.4 9–30 [16]

(1−x)Bi1/2Na1/2TiO3–xSrTiO3 Random 1175 200 – 20–135 5–22 2–12 [17]

0.88Na0.5Bi0.5TiO3-0.08K0.5Bi0.5TiO3-0.04BaTiO3 Random 1165 290 – 150 35 35.2 [18]

0.88Na0.5Bi0.5TiO3-0.08K0.5Bi0.5TiO3-0.04BaTiO3 Textured 1165 270 – 185 31 26.4 [18]

Na0.5Bi0.5TiO3–BaTiO3 Random 1200 280 0.24 128 30 39.6 [19]

Na0.5Bi0.5TiO3–BaTiO3 Textured 1200 294 0.34 299 24.2 34.1 [19]

(1-x)Bi0.5(Na0.84K0.16)0.5TiO3-xBa(Nb0.01Ti0.99)O3 Random 1125 300 – – 16–42 20–29 [41]

(Ba0.7Ca0.3)Ti1-xCuxO3-x Random 1350 110 0.24 120 5–20 6–10 [60]

Bi3.25La0.75Ti2.97V0.03O12 Random 1350 110 0.22 17 46 11 [61]

Bi3.25La0.75Ti2.97V0.03O12 Textured 1100 110 0.28 33 46 26.2 [61]

Bi0.5(Na0.4K0.1)TiO3–BaTiO3 Random 1100 262 0.30 99 27.2 17.3 This work

Bi0.5(Na0.4K0.1)TiO3–BaTiO3 Textured 1100 – 0.34 107 22.6 15.2 This work

4037, were obtained at 0.02mol of BaTiO₃. The dielectric curve exhib- ited broad transition peaks aroundTF-RandTm, which showed the char- acteristics of a diffuse phase transition.

In addition, the ferroelectric, piezoelectric properties and the energy storage efficiency in textured 0.98Bi_0.5(Na_0.4K_0.1)TiO₃–0.02BaTiO₃ce- ramics using the process of fabrication technology for textured ceramics were studied and compared with the random ceramics.

In general, two methods are considered very effective in this study:

development of (1-x)Bi0.5(Na0.4K0.1)TiO3 – xBaTiO3 ceramics through compositional modification of BaTiO3and fabrication of textured ceram- ics having a uniform grain orientation. The first, the enhancement of the piezoelectric and dielectric properties of the BNKT-BT ceramics by the process of fabrication technology for textured ceramics. This leads to achieving significant enhancements in the piezoelectric response of ce- ramics suitable for applications to fabricate an ultrasonic sensor, ultra- sonic cleaners, and hydroacoustic equipment. The second, the improved ferroelectric properties may be contributed by the increase in the Ba- TiO3addition in BNKT ceramics. This leads to achieving significant en- hancements in the energy storage properties of the material for dielec- tric capacitor applications. Note that high efficiency is also crucial for dielectrics because it means less waste heat, better reliability and longer lifetime of capacitors in practical applications [58].

Acknowledgement

This research is funded by Vietnam National Foundation for Sci- ence and Technology Development (NAFOSTED) under grant number 103.02-2017.308.

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