Structure and electrical properties of [(Na_0.825K_0.175)_0.5Bi_0.5]_1+xTiO₃ piezoceramics with A-site nonstoichiometry

ABSTRACT

A type of nonstoichiometric piezoceramics, [(Na_0.825K_0.175)_0.5Bi_0.5]_1+x TiO₃ (BKNT-x, x = 0–0.009), was synthesized by the conventional sintering method. The effects of Bi³⁺, Na⁺ and K⁺ content on the structure and electrical properties of 0.825Bi_0.5Na_0.5TiO₃-0.175Bi_0.5K_0.5TiO₃ piezoceramics were investigated.The XRD patterns demonstrated that BKNT-x piezoceramics form a solid solution without a secondary phase. Meanwhile, SEM micrographs indicated that excess [(Na_0.825K_0.175)_0.5Bi_0.5]²⁺ suppresses grain growth and that BKNT-x ceramics become considerably denser. BKNT-0.002 ceramics offer the following enhanced properties: d₃₃ = 154 pC/N, k_p = 33%, ε_r = 1087, tanδ = 0.036, and Q_m = 116. This improvement of properties is most likely related to the dense structure and the maintenance of morphotropic phase boundaries through the introduction of a proper A-site cation excess. For ceramics containing volatile elements, stoichiometric control is an efficacious method of enhancing performance.

KEYWORDS:

• Structure
• X-ray diffraction
• piezoelectric properties
• Bi_0.5Na_0.5TiO₃
• ceramics
• oxygen vacancy

1. Introduction

To date, lead-free piezoceramics have received a great amount of attention from researchers due to their environmental friendliness. Smolenski et al [1]. reported Na_0.5Bi_0.5TiO₃ (BNT) in 1960. At room temperature, rhombohedral BNT ceramic shows a high Curie point and relatively good electrical properties [2,3], which have made BNT ceramics become one of promising lead-free ceramics. BNT ceramics have a large coercive field E_c (73kv/cm), however, that makes ceramic poling difficult [4,5]. A great deal of modification work has been conducted since the early 1990s. Other approaches have been proved to be helpful in the poling of the ceramics, as well as, in improving their electrical properties, include the formation of new solid solutions, such as, Bi_0.5(K_xNa_1-x-yLi_y)_0.5TiO₃ [6], BaTiO₃(BT)-BNT [3,7], BNT-Na NbO₃ [8], K_0.5Bi_0.5TiO₃(BKT)-BNT [5], BT- BNT-BKT [9], BNT-BaTiO₃-NaNbO₃ [10], BKT-BNT-SrTiO₃ [11], Bi_0.5(K_0.20Na_0.70Li_0.10)_0.5TiO₃ [12], Bi_0.5(K_xNa_1-x-Ag_y)_0.5 TiO₃ [13], and BNT-BT-(Na_0.5K_0.5)NbO₃ [14], and doping with metal oxides, such as, Dy₂O₃-doped (Na_0.7K_0.3)_0.5Bi_0.5TiO₃ [15], Gd₂O₃-doped (Na_0.5 Bi_0.5)_0.94Ba_0.06TiO₃ [16], Li₂O-doped0.825BNT-0.175BKT [17], La₂O₃-doped0.92BNT-0.08BT [18]. Among these systems, BKT-BNT (BNKT) was observed by Sasaki et al [5]. They found that it has relatively strong piezoelectric performance owing to the existence of morphotropic phase boundaries (MPB, x=0.16-0.2). However, properties of BNT-based piezoceramics are usually affected by the volatilization of A-site cations for the sintering process. Zuo R et al [19]. revealed that the sintering behavior of (Na_0.5Bi_0.5)_1+xTiO₃ ceramics is sensitive to variants of A-site cations that contribute to enhancing its electrical properties(x=0.01, d₃₃=83pC/N). Sung et al [20,21]. revealed that the effect of Bi nonstoichiometry on d₃₃ and T_d of BNT is different from the results of Na nonstoichiometry. Wu et al [22]. found that a proper Na/K excess shows good electrical properties in 0.85 BNT-0.15BKT systems (P_r=13.6 μC/cm², d₃₃* = 56 pm/V). Prasertpalichat [23] demonstrated that changes in Bi/Na stoichiometry have a significant influence on the electrical properties of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics. Chu et al [24]. also observed that changes in the A-site cations can effectively enhance the electrical properties of nonstoichiometric (Bi_0.5Na_0.5)_0.92Ba_0.08TiO₃ systems (d₃₃=149 pC/N, ε= 1230). Ni et al [25]. reported that Bi_0.505(K_0.18Na_0.82)_0.485TiO₃ ceramics show relatively good piezoelectric properties when tuned with A-site vacancies (d₃₃=151pC/N, P_r=29.5μC/cm²). In order to clarify nonstoichiometry effects, therefore, it is significant to investigate the connection between the properties and the amount of A-site cations in BNT-based piezoceramics.

In this case, a type of lead-free [(Na_0.825K_0.175)_0.5Bi_0.5]_1+xTiO₃ piezoceramics was synthesized using a conventional mixed-oxide method. The influence of [(Na_0.825K_0.175)_0.5Bi_0.5]²⁺ nonstoichiometry on the microstructure, crystal phase and electrical properties of (Na_0.825K_0.175)_0.5Bi_0.5TiO₃ (BKNT) piezoceramics was investigated in detail.

2. Experimental procedure

Piezoceramics [(Na_0.825K_0.175)_0.5Bi_0.5]_1+xTiO₃ (x = 0, 0.002, 0.004, 0.006, 0.009) were synthesized via a conventional solid state process. Bi₂O₃ (Chenguang Chemical Co., Ltd, Qishan, China, AR, 99.9%), K₂CO₃ (Xilong Chemical Co., Ltd, Shantou, China, AR, 99.8%), Na₂CO₃ (Xilong Chemical Co., Ltd, Shantou, China, AR, 99.9%), TiO₂ (Zhongxing Electronic Materials Co., Ltd, Xiantao, China, AR, 99.5%) powders were used as raw materials with their water removed at 120 ^oC/24 h. According to the desired stoichiometry, all the powders were blended with anhydrous ethanol, and then ball-milled at a rate of 350 rotations/min for 7–8 h. When the powders become dry under 75 ^oC conditions, the mixture was calcined at 850 °C/2 h. The heating rate was 3 min/^oC. Re-milled samples were blended with polyvinyl alcohol (5 wt%), and then pressed into discs measuring 10 mm in diameter and 1.2 mm in thickness under 20 Mpa pressure. The obtained samples were sintered at 1180 ^oC/2 h at a heating rate of 5 min/^oC. In order to form electrodes, a coating of silver conducting paste (PC-Ag-8080, Xizhi Electronic Materials Co., Ltd, China) was applied to two sides of polished compositions, which were fired at 600 °C. In a silicone oil bath (80 ^oC), the annealed compositions (9 mm in diameter, 1 mm in thickness) were poled with a DC electric field (4.5 kV/mm) for 45 min.

The sintered compositions had been determined by XRD (Cu Kα radiation, Rigaku Dmax/RB). The lattice parameters of the BKNT-x ceramics were refined by the Rietveld refinement method [26] and MAUD software (version 2.94) [27–29]. The weighted profile residual factor (R_wp) was less than 15%, indicating good agreement of refinement. The density of the compositions was calculated by the Archimedes method. The microstructures of the samples were characterized by SEM (JSM-6510, Japan). The average grain size was obtained by measuring the number of grains at the intersection lines. The measurement of piezoelectric properties was carried out with a d₃₃ meter (ZJ-3A, Institute of Acoustics, China). The dielectric properties of the BKNT-x piezoceramics were measured with an impedance analyzer (Agilent 4294A,1 KHz) at 25–500 °C. The k_p and Q_m were calculated by use of Onoe’s formula [30], employing the resonance and antiresonance frequencies method as a basis.

3. Results and discussion

Figure 1(a) displays the XRD patterns of BKNT-x ceramics. All the compositions possess typical diffraction peaks displayed by an ABO₃-type structure. There is no impurity phase in these solid solutions. Ni et al [25]. observed that when x$\ge$0.02, second phase Bi₂Ti₂O₇ forms in (K_0.18Na_0.82)_0.5–3xBi_0.5+xTiO₃ ceramics. Babu et al. [31]

Figure 1. (a)XRD Patterns of various compositions in the BKNT-x ceramics; (b) and (c) The expanded XRD patterns of BKNT-x ceramics in the range of 39–47°

also demonstrated that excess K₂CO₃ (x ≤ 0.8 mol%) makes up for the volatility of K⁺ to maintain the phase purity of (Na_0.8K_0.2)_0.5Bi_0.5TiO₃ piezoceramics. Seo et al [32]. observed, moreover, that Ba_0.06(Bi_0.5Na_0.5)_0.94+xTiO₃ (−0.01 ≤ x ≤ 0.02) piezoceramics possess a homogeneous structure with phase purity. Our results are consistent with previous reports, which showed that excess Bi³⁺, Na⁺, and K⁺(x ≤ 0.009) may keep (Na_0.825K_0.175)⁺/Bi³⁺ at 1:1 and maintain the phase purity of BKNT-x ceramics after the A-site cations of BKNT-x ceramics partly volatilize in the sintering process.

Sasaki et al [5]. reported that (Na_1-xK_x)_0.5Bi_0.5TiO₃ ceramics (BNKT) exhibit high performance due to the R-T MPB (0.16 ≤ x ≤ 0.2), which has a similar pseudocubic structure [33]. To investigate the phase structure of BKNT-x ceramics, the magnified XRD patterns are given in the 2ɵ range of 39–47°, as shown in Figure 1(b,c). According to the R3 c space group (COD ID: 2103295) and P4bm space group (COD ID: 2102068) [34,35], the structure could be designated by the following peaks: rhombohedral (003) and tetragonal (201) peaks at 40°, as well as rhombohedral(202) and tetragonal (002) peaks at 46°, repectively. The peak splitting reveals that BKNT-x ceramics are still in the rhombohedral(R)-tetragonal(T) phase transition range. In addition, the (002) peak moves toward lower angles with increases in x. The lattice parameters (a_T, c_T) and cell volume of the tetragonal phase of BKNT-x ceramics increase slightly with changes in the content of A-site cations, as shown in Table 1. In the sintering process, oxygen vacancies of BKNT ceramics are usually generated due to the volatilization of A-site cations. The addition of excess A-site cations (Bi³⁺, Na⁺, and K⁺, hereinafter BKN) balances the effect of BKN loss, which leads to a reduction in the number of oxygen vacancies. It is generally known that large numbers of oxygen vacancies can cause unit cells to shrink and lead to a shift of the diffraction peaks to high angles [36–39]. Therefore, the low-angle movement of (002) peaks for BKNT-x ceramics indicates a decrease in the number of oxygen vacancies.

Table 1. Lattice Parameters and lattice volume of BKNT-x ceramics

x	aT /Å	cT /Å	VT/Å^3
0	5.51298	3.89562	118.39938
0.002	5.51347	3.89552	118.41739
0.004	5.51477	3.89960	118.59732
0.006	5.51489	3.89980	118.60856
0.009	5.51685	3.89855	118.65484

Figure 2 displays surface micrographs of BKNT-x ceramics sintered at 1180 ^oC. The crystal grains of BKNT-x piezoceramics show a regular square shape. With increases in BKN, the average grain size decreases slightly. The mean crystallite sizes are about 1.28, 1.25, 0.98, 0.76 μm, respectively. This can probably be ascribed to the collection of excess BNK at the A-site lattice of ABO₃ perovskite structure due to volatilization of A-site cations (Bi³⁺, Na⁺, and K⁺) at high sintering temperatures.This behavior can reduce oxygen vacancies, while the existence of the defects is conducive to mass transport, accelerating the grain growth [40]. As a result, crystallite growth is restrained. This is similar to a previous report on (Bi_0.5+xNa_0.5)_0.94Ba_0.06TiO₃ systems with A-site nonstoichiometry [41]. It can be observed, from Figure 3, that with the concentration of BKN, the relative densities of BKNT-x piezoceramics reach a maximum, and then descend, as was achieved by the theoretical density of (Na_0.8K_0.2)_0.5Bi_0.5TiO₃ (5.889 g/cm³) [42]. Appropriate BKN excesses are able to decrease the evaporation of volatile A-site cations, and contribute to an improvement in the density of BKNT-x ceramics [43]. Bi₂O₃ with its melting point of 825°C usually decreases sintering temperatures by forming a liquid phase [44]. With further increases in BKN(x > 0.004), the content of bismuth oxide goes up, which leads to lower sintering temperatures of BKNT-x piezoceramics. For this reason, the ceramics can’t be fully sintered at the same sintering temperature (1180 ^oC), resulting in decreased compactness. The relative densities of BKNT-x piezoceramics are more than 95% of the theoretical values when 0.002$\le$x$\le$0.004. A dense microstructure is particularly advantageous to improvement of the electrical properties of BKNT-x ceramics.

Figure 2. SEM surface micrographs of BKNT-x ceramics with various compositions sintered at 1180 ^oC for 2 hours

Figure 3. Relative density of BKNT-x ceramics sintered at 1180 ^oC / 2 h

Figure 4 displays the temperature dependance of ε_r and tanδ for unpoled BKNT-x ceramics under different frequencies. BKNT-x ceramics have two dielectric anomalies: a relaxor antiferroelectric-antiferroelectric phase transformation and an antiferroelectric-paraelectric phase transformation corresponding to T_RE and T_m, respectively, which are consistent with NBT-based piezoceramics [5,45–48]. A phenomenon of mergence is exhibited by the different ε_r-T curves of BKNT-x piezoceramics, where the temperature is described as T_RE. Frequency has a great effect on ε_r when T< T_RE. Ma et al. reported that nanodomains with a short-range order stimulate the relaxor behavior of BNT-based ceramics and suggested a new term, “relaxor antiferroelectric,” to describe the phenomenon of frequency dispersion [46]. With further increments in temperature, the relative permittivity ε_r reaches a maximum value at the second dielectric anomaly, where the temperature is expressed as T_m. Ma et al. identified a long-range antiferroelectric(AFE) order presents in BNT-based ceramics at between T_RE and T_m, and showed that the paraelectric phase is located at temperatures above T_m [46]. As soon as the temperature rises above T_m, tanδ increases greatly due to the considerable leakage conduction [49]. In addition, pure BKNT ceramics exhibit larger values for dielectric constants and loss tangents than BKNT-x ceramics at high temperatures (T$>$T_m) and low frequencies, which can be ascribed to the fact that A-site vacancies and oxygen vacancies form at preparation process and lead to the appearance of frequency dispersion for pure BKNT ceramics [46].

Figure 4. Temperature dependence of relative permittivity and dielectric loss for BKNT-x ceramics (1180°C/2 h)

Figure 5 shows the electrical properties of BKNT-x piezoceramics. When the content of BKN is 0.2 mol%, the maximum values of the piezoelectric constant d₃₃ and electromechanical coupling factor k_p reach 154 pC/N and 33%, respectively. The comprehensive performance of BKNT-x piezoceramics is slightly superior to those of NBT-based ceramics with nonstoichiometry (see Table 2). This can be ascribed to the following reasons: a proper BKN excess improves the compactability of BKNT-x piezoceramics which makes the polarization of ceramics more completely under a broad electrical field [17]. Moreover, an excess amount of BKN can make up for the volatilization of A-site cations and maintain the R-T phase fractions of compositions around MPB, which improves the piezoelectricity of BKNT-x ceramics. Analogous to the close dependence of BKN content and d₃₃, the maximum value of the relative dielectric constant ε_r (1087) occurs at x=0.002. Meanwhile, the dielectric loss tanδ initially decreases (x<0.002) and reaches a minimum value (0.0362), and then, finally increases with the concentration of BKN. Q_m gradually decreases with the increases in BKN. This can be ascribed to the decrement of oxygen vacancies caused by the compensation of excess BKN for the loss of Bi³⁺, Na⁺ and K⁺.

Table 2. Comparison of the properties of different NBT-based ceramics with nonstoichiometry

systems	d33(pC/N)	kp	Td(^oC)	References
0.94(Na0.5Bi0.52)TiO3-0.06BaTiO3	118	0.24	29	[41]
0.8(Na0.5Bi0.5)TiO3-0.2(K0.5Bi0.5)TiO3 + 0.8 mol% K2CO3	196	-	-	[31]
Bi0.51(Na0.82K0.18)0.5TiO3	145	0.31	~125	[25]
Bi0.505(Na0.82K0.18)0.485TiO3	151	0.298	101	[25]
[(Na0.825K0.175)0.5Bi0.5]1.002TiO3	154	0.33	137	This work

Figure 5. Piezoelectric and dielectric properties of BKNT-x ceramics

The temperature dependence of the k_p of BKNT-x ceramics is demonstrated in Figure 6. The k_p of BKNT-x ceramics shows a decreasing trend with increases in the annealing temperature. The k_p changes significantly as the annealing temperature increases up to 134 ^oC (x=0.004, k_p=0.03), and then decreases as the annealing temperature increases further. When the annealing temperature is 134-140 ^oC, the piezoelectric responses of BKNT-0.004 ceramics almost vanish. In other words, the depolarization temperature (T_d) of BKNT-0.004 ceramics is about 134 ^oC, while pure BKNT ceramics have a high T_d of around 140 ^oC, as reported by Sapper [50]. As a result, the addition of excess BKN has a negative effect on T_d. On the other hand, variations in T_d can reflect changes in the oxygen vacancies. The depolarization temperature of ceramics increases at a high oxygen vacancy concentration owing to a strong coaction between the oxygen vacancies and the domains [41]. Therefore, use of moderate amounts of BKN to compensate for the loss of Na⁺, Bi³⁺ and K⁺ during the sintering step contributes to a decrease in oxygen vacancies that causes T_d to shift to a low temperature.

Figure 6. The temperature dependency of k_p for BKNT-x ceramics

4. Conclusions

The influence of BKN on the structure, crystal phase and electrical properties of (Na_0.825K_0.175)_0.5Bi_0.5TiO₃ piezoceramics, which were synthesized by conventional sintering, were systematically researched. All the BKNT-x piezoceramics have typical diffraction peaks with pure perovskite structures. Excess BKN compensates for the loss of the A-site cations, and leads to a decrease in oxygen vacancies which restrain the growth of crystallites. All the compositions are well sintered with dense microstructure at 1180 ^oC/2 h when 0.002$\le$x$\le$0.004. BKNT-x piezoceramics with BKN compensation achieve high d₃₃ (154 pC/N) and k_p (33%). The electrical properties of BKNT-x ceramics have been enhanced by the introduction of excess BKN, which maintains the structural density and the R-T phase fractions of the samples at around MPB. As a result, in order to acquire the desirable electrical properties, it is essential to control the content of volatile A-site cations sufficiently.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was sponsored by the Natural Science Foundation of Department of Education of Guizhou Province, China [No. KY [2018]253]; Science and Technology Fund of Guizhou Province, China [[2020]1Y204]; and Doctor Start-up Foundation of Guizhou Institute of Technology [No.XJGC20190920].