Journal of Alloys and Compounds

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Enhancement in dielectric, ferroelectric, and piezoelectric properties of BaTiO ₃ - modi ﬁ ed 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 24 October 2019

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]exBaTiO3ceramics (BNKT-BT, withx¼0; 0.01; 0.02; 0.03 and 0.04). The phase formation 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 (x0.04). Atx¼0.02, the best physical properties of the BNKT-BT ceramics, such as density,r¼5.86 g/cm³(98.5% of the theoretical density); electromechanical coupling factors (kp), 0.30; (kt), 0.31; remanent polarization (Pr), 17.2m^{C cm}²; dielectric constant (εr), 1354; and highest dielectric constant (εmax), 4037, were obtained. Further, the energy storage properties of 0.98Bi0.5(Na0.4K0.1)TiO3e0.02BaTiO3ceramics 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.

1. Introduction

In recent years, there has been a considerable increase in the interest on the application of Pb(Zr_(1-x)Ti_x)O₃(PZT)-based ceramics owing to their excellent piezoelectric properties and applications in piezoelectric actuators and transformers [1e7]. However, the use of the lead-based ceramics poses serious environmental concerns owing to the high toxicity of lead oxide. Therefore, it is necessary to develop lead-free ceramics with good ferroelectric and piezoelectric properties to replace the lead-based ceramics [8e10]. None- theless, lead-free materials still cannot completely replace lead- based materials in all applications especially high-pressure piezoelectric materials, piezoelectric transformers, or ultrasonic motors [3,6,11]. Thus it became highly intriguing to develop new lead-free materials with improved properties that are suitable for the desired industrial applications [11]. Among the various lead-free materials available today, Bi_0.5Na_0.5TiO₃(BNT) [12e17], BaTiO₃[11,18e20] and K0.5Na0.5NbO3 (KNN) [8,19,21] based ceramics have been

considered as most promising candidates owing to their excellent electromechanical properties near the morphotropic phase boundary (MPB) [18]^.Kamnoy et al. [11] added BaTiO3as starting materials in Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ceramic by using solid-state reaction method. BaTiO3, at a content of 8 mol%, resulted in the highest value of the dielectric constantεmaxof 6700. However, in this traditional 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 develop 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 0.88Na_0.5Bi_0.5TiO₃- 0.08K_0.5Bi_0.5TiO₃-0.04BaTiO₃ceramics with optimal properties, at 4 wt% of NaNbO3 templates. The optimum properties, such as piezoelectric coefﬁcient (d33) of 185 pC/N, high remanent polarization (P_r) of 26.4m^C/cm², low coerciveﬁeld (E_c) of 31 kV/cm, and the dielectric constant (εmax) of 3950, were obtained. Recently, Fan et al. [16] prepared (1-x)Bi1/2(Na0.82K0.18)1/2TiO3 e xBi4Ti3O12ce- ramics using conventional solid-state reaction method and found that the normal ferroelectric-to-ergodic relaxor (FE-to-ER)

*Corresponding author.

E-mail address:[email protected](L.D. Vuong).

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j o u rn a l h o m e p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m

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transition occurs at a low temperature (TF-Rz0C). At a Bi4Ti3O12

content ofx¼9 wt%, ceramics exhibited best electrical properties such as a large signal piezoelectric coefﬁcient,d*33of 485 pm/V, and a small hysteresis,h^{of 23%.}

In this study, the synthesis of (1-x)[Bi_0.5(Na_0.4K_0.1)TiO₃]exBa- TiO3(BNKT-BT) lead-free textured ceramics was carried out by the conventional solid-state reaction method and template grain growth method with pure-phase Bi₄Ti₃O₁₂templates. The textured samples with preferred orientation showed an enhanced electrical property. The goal of texturing was to form BNKT-BT lead-free piezoelectric ceramic with improved physical properties. As such, this study will provide two important information: (i) theﬁrst, the studied results indicate that BaTiO3enhances the dielectric, ferroelectric, and piezoelectric properties in BNKT-based ceramics; (ii) the second, the BNKT-BT ceramics with the optimized content of BaTiO3 will be selected for fabrication by template grain growth method. Besides, based on the obtained results, (1-x) [Bi_0.5(Na_0.4K_0.1)TiO₃]exBaTiO₃ceramics are promising candidates for lead-free ceramics for dielectric, ferroelectric, piezoelectric and energy storage applications.

2. Experimental

The general formula of the studied material was (1-x) [Bi0.5(Na0.4K0.1)TiO3]exBaTiO3, wherex¼0.0, 0.01, 0.02, 0.03, and 0.04. The following reagents were used: Bi2O3, TiO2, Na2CO3, K2CO3

and BaCO3(Merck, purity99.5%). In particular, Bi0.5(Na0.4K0.1)TiO3

ceramics 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 inFig. 1. Starting raw materials of Bi₂O₃, Na₂CO₃, K₂CO₃, TiO₂ and BaCO3(Fig. 1(a)) were weighed and milled with ultrasound treatment with an electric power of 100 W andﬁxed frequency of 28 kHz in ethanol medium for 1 h (Fig. 1(b)) [12].

After that, the powders were dried and calcined at 850C for 2 h to synthesize BNKT-BT compounds (Fig. 1(c)). The calcined BNKT-BT powder milled for 20 h (Fig. 1(d)). The ground materials were pressed into disk 12 mm in diameter and 1.5 mm in thick under 100 MPa (Fig. 1(e)). The BNKT-BT samples were sintered at temperature 1100C for 2 h (Fig. 1(f)). Then, the sample is treated, covered with electrodes and examined for properties (Fig. 1(g and h)). The BNKT-BT ceramics with the optimized content of BaTiO3

will be selected for fabrication by template grain growth method.

2.2. Synthesis of BNKT-BT ceramics by template grain growth method

As theﬁrst step, Bi2O3and TiO2were weighed and milled under ultrasound (electric power of 100 W and a ﬁxed frequency of 28 kHz) in ethanol for 1 h. The as-prepared powders were then dried at 100C for 2 h and subsequently calcined at 500C for 2 h in order to obtain amorphous Bi₄Ti₃O₁₂precursor powder which was subsequently mixed with Na2CO3/K2CO3 eutectic mixture in a sealed alumina crucible and was sintered at 1050C for 2 h characteristics of Bi₄Ti₃O₁₂ template as demonstrated in Fig. 2. The microstructure study revealed a characteristic plate-like shape for the Bi4Ti3O12template after sintering (Fig. 2(a)). The revelation of particle size length in the range of 4e18mm and approximate width of 0.5e1mm (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 stoichio- metric ratio for the reaction, a predetermined amount of Bi₄Ti₃O₁₂ powder (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 2 h (Fig. 3(b)). Then, this mixture was continuously mixed with a binder solution and was subsequently tape cast to form thin sheets with a thickness of about 0.1 mm (Fig. 3(c)) in which the plate-like Bi₄Ti₃O₁₂particles 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 12 mm (Fig. 3(e)), stacked in 30e40 layers (Fig. 3(f)), and pressed at room temperature under a pressure of 100 MPa for 3 min to form green compacts with a thickness of about 2 mm, as shown inFig. 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, whileﬁeld emission scanning electron microscopy (Nova NanoSEM 450-FEI-HUS-VNU) was used to examine the morphology [23]. Furthermore, the density and ferroelectric loops, used to identify ferroelectricity, 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 measurement of dielectric properties was achieved by measuring the temperature dependencies of capaci- tance and phase angle (HIOKI 3532). Lastly, in other to study piezoelectric responses, the pellets were poled in a 120C silicone oil bath by applying a DC electricﬁeld of 35 kV/cm for 20 min then cooling down to room temperature. The pellets were aged for 24 h prior to testing. The coupling factors (k_p, k_t) and 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 samples at different concentrations of BaTiO₃. All the sintered samples exhibit high relative density more than 96%, indicating that dense microstructures are obtained. With the increase in the BaTiO₃content up to 0.02 mol, the density of BNKT ceramics was increased. It achieved the maximum value (r¼5.86 g/cm³, 98.5% of the theoretical density) at a BaTiO3content of 0.02 mol and then decreased. This could be explained by the fact that when BaTiO₃ content in the ceramic system was<0.02 mol, a large number of pores were present, indicating insufﬁcient densiﬁcation of the Fig. 1.The process of fabrication technology for random ceramics.

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samples (Fig. 5). As the BaTiO3content was increased, the ceramics became denser, and the sample was almost completely dense at a BaTiO₃content of 0.02 mol. In general, it has been observed that the addition of BaTiO3 enhances the density of BNKT ceramics. This result is consistent with that observed by Manotham et al. [25]. The effect of BaTiO₃concentration on the dielectric constant (εr) and dielectric loss (tand) of the BNKT ceramics at 1 kHz is also illustrated inFig. 4. When the concentration of BaTiO3was increased from 0.0

to 0.02 mol, the values ofεrwere also increased proportionally and reached a maximum of 1354 at 0.02 mol of BaTiO₃. A further increase inxresulted in a rapid decrease inεr. On the other hand, tand was decreased with increasing BaTiO3content. The minimum tand of 0.041 was obtained atx¼0.02, which increased further by the addition of BaTiO3. It could be explained on the basis of the large and homogeneous grain size [1,6] and dense composition of 0.98Bi_0.5(Na_0.8K_0.2)_0.5TiO₃e0.02BaTiO₃ceramic.

Fig. 5shows SEM micrographs and grain size distribution of the BNKT-BT ceramics at different BaTiO3contents. The grains in all the samples exhibited rectangular shapes with a clear grain boundary.

The grain size was increased with the increasing BaTiO₃ content and reached to the maximum value of 1.28mm at 0.02 mol of BaTiO3

and then decreased. The smallest average grain size of 0.83m^{m was} observed for the sample without BaTiO₃(x¼0.0). This suggests that the microstructure can be improved by a BaTiO3additive. According to the research results of Khanal et al. [26] the conventionally prepared BaTiO₃ ceramics sintered at 1350C for 5 h exhibited a grain size of 15.9mm. 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 empirically deter- mineGsusing Eq.(1):

G_s¼(1-x)(0.83m^m)þx(15.9m^m) ⁽¹⁾

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

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

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

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It is evident from the data that Eq. (1) gives a reasonable approximation of the average grain size with x¼0 to 0.02. How- ever, when the addition of BaTiO3is higher than 0.02 mol, there appears a large difference 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 BaTiO3

exceeding 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 ceramics. All samples exhibited a pure perovskite phase, and no trace of secondary phase was detected in the investigated region. In order to illustrate the effect of BaTiO3on the structure of the materials, a magnified view of the peaks in the region from 39to 41, and 46e47are shown inFig. 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^þ(1.61 Å) ions diffuse into the BNKT lattice to replace Bi³^þ (1.17 Å), Na^þ(1.39 Å), and K^þ(1.64 Å) [12,27,28] resulting in the enlargement of lattice constant and a shift in the XRD peak position toward a lower diffraction angle direction. Though, a significant shift in the peak position of the reflection towards a higher angle side has been observed for the samples for samples containing

BaTiO3content higher than 0.03 mol. Similar kinds of behavior have been observed by V. Pal et al. [29] in the XRD patterns of the (1- x)(Bi0.96La0.04)0.5Na0.5TiO3)x(Ba0.90Ca0.10TiO3) ceramics, where the system changes its symmetry from rhombohedral R3c to tetragonal P4bm through an MPB between those two phases, similar to the study of Sowmya et al. [30] for Bi0.5(Na0.4K0.1)TiO3 ceramics. It proves that Ba^2þ diffuses into the BNKT lattice to change the structure of the material. Ghosh et al. [27] observed that the structural changes occurred gradually from a rhombohedral phase in BNKT ceramic to tetragonal in (1-x)BNKT-xBaTiO3ceramics at x¼0.05. In the study of Kamnoy et al. [11], (1-x)BNKT-xBaTiO₃ ceramic with 0 and 2 mol% of BaTiO3 exhibited rhombohedral phase, which changed to tetragonal when the BaTiO3content of the samples exceeded 2 mol%. Recently, Manotham et al. [25] reported that Ba²^þwas successfully diffused into the 0.92(Bi_0.5Na_0.42K_0.08) TiO3-0.08(BaNb0.01Ti0.99)O3. The sample also showed a tetragonal perovskite (P4mm) structure in the ceramic. The phase diagram of the binary system (1-x)Bi_0.5Na_0.5TiO₃exBaTiO₃[31] showed morphotropic phase boundary regions between the ferroelectric rhombohedral and tetragonal phases in thexrange of 0.06e0.08.

According to Kwei et al. [32] The long-range structure of BaTiO₃is can be described by the P4mm space group with lattice parameters of a¼3.99095 Å and c¼4.0352 Å with a c/a ratio of 1.011. With Fig. 5.Typical SEM images of the BNKT-BT ceramics at different contents of BaTiO3.

Fig. 6.The XRD patterns of BaTiO3-modiﬁed BNKT ceramics with 2qranging from (a) 20e70, (b) 39e41and (c) 46e47.

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increasing molar fraction of BaTiO3, the crystal symmetry of the (1- x)BNKT-xBaTiO₃ ceramics should change due to the tetragonal distortions of BaTiO3. Therefore, in order to understand the degree of disorder and phase transformation behavior, the tolerance factor (t) [14,33] was calculated for these ceramics based on the ABO₃ structure as shown inFig. 7(a).

t_ABO₃¼ R_AþR_O ffiffiffi2

p ðR_BþR_OÞ (2)

Therefore, in the pure Bi_0.5(Na_0.4K_0.1)TiO₃structure,

t_BNKT¼0:5RBiþ ð0:4RNaþ0:1RKÞ þR_O ffiffiffi2

p ðR_TiþR_OÞ (3)

and in the pure BaTiO3(BT),

t_BT¼ R_BaþR_O ffiffiffi2

p ðR_TiþR_OÞ (4)

From Eqs.(3) and (4), the tolerance factors of BNKT (t¼0.9492), which is lower than the tolerance factor of BT (t¼1.0554), was calculated by considering the ionic radii of RBa2þ¼1.61 Å, R_Bi³^þ¼1.17 Å,R_Na^þ ¼1.39 Å,R_K^þ¼1.64 Å,R_Ti⁴^þ¼0.61 Å andR_O²¼1.42 Å [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)0.5TiO3

ceramics.

In order to provide convincing evidence for the phase structure evolution of BNKT-BT ceramics at different BaTiO3contents, Raman spectroscopy in the range of 80e1000 cm¹ was carried out (Fig. 8(a)). It was applied to further characterize the surface groups and chemical bond states of materials [29,34e36]. 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 categories according to the wavelength as above 700 cm¹, between 400 and 700 cm¹, between 200 and 400 cm¹ and below 200 cm¹. In the present study, the spectral deconvo- lution 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 theﬁrst band region, the peak centered between 100 and 129 cm¹was assigned to A1(TO) mode, which could be associated with the A site vibrations of the BieO, NaeO, KeO, and BaeO bonds [16,39] and is sensitive to phase changes in the crystal structure of

the perovskite [39]. It is interesting to note that the A-site mode exhibited a slight decrease in the wavenumber and intensity with the increasing BaTiO3contents (Fig. 8(a)). It can be explained by the fact that the BaTiO3 could 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^þion (~1.61 Å) is similar to those of the Bi^3þ(~1.17 Å), Na^þ(~1.39 Å) and K^þ(~1.64 Å) ions, suggesting that Ba²^þions having a smaller ionic radius enter the A site of the perovskite unit cell to replace Bi³^þ, Na^þ and K^þions, which changes the lattice structure. Similar observations have been reported by Manotham 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 200e400 cm¹and is attributed to the vibration of the TieO bond. It is localized at around 266 cm¹(x¼0) and associated with the tetragonal phase in the perovskite structure [37,42,43]. However, with the increasing BaTiO3 contents, the peak splits into two bands at ~254 and

~306 cm¹. This proves that the (1-x)BNKT-xBaTiO₃ceramics with 0e2 mol% 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(Na1xKx)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 305 cm¹, which is characteristic of the tetragonal BaTiO3phase, is assigned to the B₁mode.

Meanwhile, the two overlapping bands associated with the vibrations of the TiO6octahedra within 450e650 cm¹range become increasingly distinct with the increasingxcontent demonstrating the emergence of phonon behavior in the structure [46]. The modes of the TiO6octahedra localized at around 601 cm¹exhibited 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 TiO6 octahedral and the presence of internal stresses that could possibly stabilize dielectric and ferroelectric properties at the room temperature after BaTiO₃addition [41]. The broad band observed in the fourth region could be assigned to the A1(LO) and E(LO) overlapping bands, while the presence of broader peaks is related to the presence of oxygen vacancy [43].

The temperature dependence of dielectric characteristics of BaTiO3-modiﬁed BNKT ceramics measured at 1 kHz from room temperature to 330C is presented inFig. 9(a). The ceramics un- dergo two phase transitions in the measured temperature range.

Theﬁrst is the normal ferroelectric-to-ergodic relaxor (FE-to-ER) transition occurring at a low temperature (TF-R), and the second is

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

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the ferroelectriceparaelectric transition located at a higher temperature (Curie temperature, Tm). This exhibited relaxor characteristics by the phase transition occurring within a broad temperature range. This result is in a good agreement with the literature [30,41,47]. When the content of BaTiO3 changes from x¼0e0.02 mol, the maximum value of the dielectric constant (εmax) increases from 3296 to 4037 (Fig. 9(b)). On the other hand, εmaxdecreases with the increasing BaTiO3concentration beyond 0.02 mol concomitant with the increase in the grain size as shown inFig. 5.

The diffuseness (g) was evaluated by plotting ln(1/ε - 1/εmax) versus ln(T - T_m) [14] at 1 kHz and temperatures greater thanT_m, as demonstrated inFig. 9(c). The increase ingin this range is higher than the increase in the case when the content of BaTiO3was from 0.0 (g_¼1.60) to 0.02 mol (g_¼1.87) (Fig. 9(b)). Then, it rapidly de- creases with increasingx. 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 Ba^2þ ions would segregate at the grain boundary, inhibit the grain growth, and lead to the heterogeneity of the structure causing g to 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 addition of BaTiO3into BNKT ceramics increases the value ofg, which means that the paraelectric-ferroelectric phase transition is of diffuse type, and the ceramics exhibit high disorder [14]. This is also the reason for the decrease ofTF-Rfrom 120C to 85C and the decrease ofTm

from 269C to 231C when increasing BaTiO₃content is gradual (Fig. 9(d)). These results are consistent with the literature [27].

According to Ghosh et al. [27], the decrease inTF-Ris related to non- cubic distortion with the incorporation of BaTiO₃ into BNKT ceramics, consequently destabilizing the long range ordering state.

According to Lu et al. [48], the temperatureTF-Rcorresponds to the ferroelectricerelaxor transition, T_F-R should be lower than the temperature range of energy storage application to obtain a large energy density. Recent investigations by Liu et al. [49] have shown that the Raman modes vary nonmonotonically with distinctive abrupt changes aroundT_F-R, suggesting a thermal transition from theﬁeld-forced ferroelectric regime to disordered ergodic relaxor phase. As can be observed inFig. 7(c), the incorporation of BaTiO3

into BNKT decreasesT_min the investigated region. There is a difference between the phase transformation temperatures of BNKT (Tm~ 269C, at x¼0) and BaTiO3 (Tm~ 120C) [33], thus it is important to determine the dependence of phase transition temperature of the (1-x)Bi0.5(Na0.8K0.2)0.5TiO3 exBaTiO3ceramics on BaTiO3 content (Fig. 9(d)). There is a good linear relationship Fig. 8.Raman spectra of BNKT-BT ceramics.

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

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

T_m¼(1-x)(269C)þx(120C) (5)

The variation in the calculatedTmas a function of compositionx is shown inFig. 9(d). It is evident from the data that Eq.(5)gives a reasonable approximation of the transition temperatureT_mwith x¼0.0 to 0.02. This result suggests that Tm of (1-x) Bi0.5(Na0.8K0.2)0.5TiO3exBaTiO3ceramics can be varied over a wide range from 120C to 269C by controlling the content of BaTiO₃in the composition. However, when the addition of BaTiO3is higher than 0.02 mol, there appears a large difference between the experimental data and theoretical calculations. This can be explained by the solubility limit of Ba²^þions in Bi0.5(Na0.8K0.2)0.5-

TiO3ceramics near 0.02 mol. However, if the addition of BaTiO3is above 0.02 mol, excess Ba^2þions accumulate at the grain boundaries, which has little inﬂuence on the average energy of the oxygen octahedron; thus, the temperatureTmwill be abnormally inﬂu- enced (not according to rules) by the excess Ba^2þions.

Fig. 10shows the changes in electromechanical coupling factors in planar mode (kp), thickness mode (kt), the piezoelectric constant (d31), and mechanical quality factorQmas a function of the BaTiO3

content in BNKT ceramics sintered at 1100C for 2 h. When the content of BaTiO3is0.02 mol, the values ofkp,kt,d31, andQm

increase rapidly with an increase in the BaTiO3 content, then decrease. The optimum values fork_pof 0.30, k_tof 0.31,d₃₁of 99 pC/

N andQmof 189 were obtained atx¼0.02, which is near the solution limit. In other words, the improvement in the piezoelectric properties of the samples can be partially ascribed to the enhancement in the density and the increasing grain size effect.

Similar observations were made for Ba-doped Bi0.5(Na0.85K0.15)0.5-

TiO3 ceramics [51]. It can be found in Fig. 4that 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 explained by the particle size increase effect and density.

Qaiser et al. [53] show that the dense structure not only im- proves the piezoelectric response but also increases theQmvalue by reducing the dissipated energy. As a result ofFig. 5presents the ceramic samples 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 - 0.02BaTiO₃ samples 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 ferroelastic 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.

TheP-Ehysteresis loops of the BaTiO3doped BNKT sintered at 1100C were measured at room temperature and are shown in Fig. 11(aee); the remanent polarization (P_r), coercivefield (E_c) and the squareness of the hysteresis loop (Rsq) were also determined and are shown inFig. 11(g). Based on the increase ofPrand decrease of E_c, there is improvement of ferroelectric properties of BaTiO₃ doped BNKT ceramics by doping BaTiO3up to 0.02 mol. However, when the BaTiO3 content of the samples exceeded 2 mol%, 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 0.98Bi0.5(Na0.4K0.1)TiO3 - 0.02BaTiO3 ceramics shows good ferroelectric properties, with high remanent polarization of Pr¼17.3m^C/cm² ând ^rather low coercive electric field of E_c¼27.2 kV/cm. Cao et al. [55] derived an empirical equation to measure not only the deviation in the polarization axis but also that in the electricfield axis like this:

Rsq¼Pr

P_sþ P_1:1E_C

P_r (6)

whereRsqis the squareness of the hysteresis loop,Pris the remnant polarization,P_sis the saturation polarization, andP_1:1Ecis the polarization at an electric ﬁeld equal to 1.1 times of coerciveﬁeld.

When the BaTiO3content varied from 0 to 0.04 mol, the values of R_sq changed to 0.96, 0.99, 1.04, 0.89 and 0.73, respectively (Fig. 11(g)). Similar observations were made for Ba(Nb0.01Ti0.99)O3- doped Bi0.5(Na0.84K0.16)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) showsPeEhysteresis loops at room temperature for textured 0.98BNKT-0.02BT ceramics with 10 wt% of Bi₄Ti₃O₁₂templates using the process of fabrication technology for textured ceramics (Fig. 3). The Bi4Ti3O12templates were well-aligned parallel to the casting direction 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 Bi4Ti3O12 templates, similar to the study of Chang et al. for Sr_0.61Ba_0.39Nb₂O₆ceramics [56]. As can be seen that the textured sample exhibited low remnant polarization (Pr¼15.2m^C/cm²^{) and} coercivefield (Ec¼22.6 kV/cm) compared with the random sample (Fig. 11(c),P_r¼17.3m^C/cm²^,Êc¼27.2 kV/cm) due to the crystal-like characteristics of the textured ceramic). It is known that the coercive field of a single crystal is usually lower than that of poly- crystalline ceramic with the same composition [19]. Textured BNKT-BT ceramic exhibitedRsqof 1.01, which is lower than that of the random samples (Rsq¼1.04).

The energy storage densityW₁(Fig. 11(f), marked in blue area) was obtained by integrating the area between the polarization axis and the discharge curve of the unipolarPeEhysteresis loops using Eq.(7)[48,57]:

W₁¼ ð

Pmax

Pr

EdP (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 efﬁciency (h) of the material can be calculated by Eq.

(7)[48,57]:

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

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h

¼ W₁

W₁þ W₂ (8)

The varying trend in the energy storage density (W1) of the ceramics at different BaTiO₃contents, is similar to the trend of the energy storage efﬁciency (h), as shown inFig. 11(h). BothW1andh increase almost linearly with the increment of BaTiO3content at x<0.03 and reaches a maximum value of 0.30 J/cm³ and 24.6%, respectively at x¼0.04. While, as the BaTiO₃ content increased from 0 to 0.04 mol, the values ofW2of samples increased from 1.0 to 1.16 J/cm³ reaching the highest value of 1.16 J/cm³ atx¼0.02, upon which it then decreased. At this content, the textured 0.98BNKT-0.02BT ceramic exhibited W1 of 0.14 J/cm³ and W2 of 0.88 J/cm³, which are lower than that of the random samples (W₁¼0.15 J/cm³, W₂¼1.16 J/cm³) at 42.3 kV/cm as illustrated in Fig. 11(h). These values are comparable to the previously reported of BNKT-based bulk ceramics studied by Manotham et al. [41]

which showed an energy density value of ~0.09 J/cm³and an energy storage efficiency value of ~9% (measured at 25C). However, in the same composition (0.98BNKT-0.02BT), the energy storage efficiency (h) of textured BNKT is higher than that of the random ceramics, i.e., 13.5% and 11.5%, respectively (Table 1). Although the random 0.98BNKT-0.02BT ceramic possesses higherPmvalues, it also bears largerPr,ECprobably because of the larger FE hysteresis and higher leakage current at high electricfields. As a result, their energy densities are substantially limited, as also observed by Pan et al. [58]. In other words, the largeRsqalso causes a remarkable decline inhof random ceramics.

The ferroelectric and piezoelectric properties of the textured and random ceramics are compared inTable 1. As can be noticed that the textured ceramic shows a lower remnant polarization, while the piezoelectric coefficient (d₃₁) and electromechanical coupling coefficients (kp and 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 3mm and thickness of 1.5mm from the template growth at the expense of the matrix powders after sintering can be seen inFig. 12. This affirms the effectiveness of the template grain growth method in aligning the template particles. In short, the BaTiO₃-modified BNKT ceramics show improved electrical properties, which are maximum at a BaTiO3 content of 0.02 mol. 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 2qof around 22and the (200) peak at the 2qof around 46proved that textured 0.98BNKT- 0.02BT ceramic has a rhombohedral structure [14]. The intensity of the (110) reflections was found to decrease with an increase in the intensity of the (100) and (200) reflections at 2qangles of 22.76 and 46.61, respectively, which indicates the development of texture in the samples [59].

To confirm the achievements of the present study, the characteristic parameters of the BNKT-BT ceramics sintered at 1100C as such: the sintering temperature (T_s), Curie temperature, (T_m), electromechanical coupling coefficient (kp), the piezoelectric coefficient (d31), the coercivefield (Ec), and the remnant polarization Fig. 11.(aef)PeEhysteresis loops and (g, h) the calculated the ferroelectric parameters of ceramics.

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

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(P_r) were extracted and compared with those of other Bi-based leadefree ceramics [16e19,60,61] as listed inTable 2. Our results indicated that the BaTiO3-modiﬁed BNKT ceramics shows a lower sintering temperature, while the electrical properties of the material ceramics are well maintained.

4. Conclusions

In this work, the BaTiO₃ doped Bi_0.5(Na_0.4K_0.1)TiO₃ ceramics were successfully synthesized by a solid-state mixed oxide method.

The results indicate that BaTiO3enhances the dielectric, ferroelectric, and piezoelectric properties of BNKT-based ceramics. The best physical properties of the BNKT-BT ceramics, such as density, r¼5.86 g/cm³, 98.5% of the theoretical density); electromechanical coupling factors (k_p), 0.30; (k_t), 0.31; remanent polarization (P_r), 17.2m^{C cm}²; dielectric constant (εr), 1354; and highest dielectric constant (εmax), 4037, were obtained at 0.02 mol of BaTiO3. The dielectric curve exhibited broad transition peaks aroundTF-Rand T_m, which showed the characteristics of a diffuse phase transition.

In addition, the ferroelectric, piezoelectric properties and the energy storage efﬁciency in textured 0.98Bi0.5(Na0.4K0.1)TiO3 e 0.02BaTiO₃ceramics 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)TiO3exBaTiO3ceramics through compositional modification of BaTiO3 and fabrication of textured ceramics having a uniform grain orientation. Thefirst, 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 enhance- ments in the piezoelectric response of ceramics suitable for applications to fabricate an ultrasonic sensor, ultrasonic cleaners, and hydroacoustic equipment. The second, the improved ferroelectric properties may be contributed by the increase in the BaTiO₃addition in BNKT ceramics. This leads to achieving significant en- hancements in the energy storage properties of the material for

dielectric 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 Science and Technology Development (NAFOSTED) under grant number 103.02-2017.308.

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

Samples Textured/Random kp kt d31

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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 e e e 29 42 16e42 0.09 e 9 1.25

Table 2

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

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(1x)Bi1/2Na1/2TiO3exSrTiO3 Random 1175 200 e 20e135 5e22 2e12 [17]

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

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

Na0.5Bi0.5TiO3eBaTiO3 Random 1200 280 0.24 128 30 39.6 [19]

Na0.5Bi0.5TiO3eBaTiO3 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 e e 16e42 20e29 [41]

(Ba0.7Ca0.3)Ti1-xCuxO3-x Random 1350 110 0.24 120 5e20 6e10 [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]

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Journal of Alloys and Compounds - CSDL Khoa học

Enhancement in dielectric, ferroelectric, and piezoelectric properties of BaTiO 3 - modi ﬁ ed Bi 0.5 (Na 0.4 K 0.1 )TiO 3 lead-free ceramics

Le Dai Vuong

, Phan Dinh Gio

Journal of Alloys and Compounds

h

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